Review of the Application of Metal-Supported Solid Oxide Fuel Cell in the Transportation Field

Review of the Application of Metal-Supported Solid Oxide Fuel Cell in the Transportation Field

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Zhaohuan Zhang¹, Haoyu Du¹, Kai Xu², Xiaoqing Zhang², Xiao Ma¹, Shijin Shuai²

Automotive Innovation | 2025, 8(2) : 443 - 471

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Automotive Innovation | 2025, 8(2): 443-471

Review of the Application of Metal-Supported Solid Oxide Fuel Cell in the Transportation Field

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Zhaohuan Zhang¹, Haoyu Du¹, Kai Xu², Xiaoqing Zhang², Xiao Ma¹, Shijin Shuai²

Affiliations

¹ Tsinghua University State Key Laboratory of Intelligent Green Vehicle and Mobility, School of Vehicle and Mobility Beijing 100084 China

² Tsinghua University Institute for Aero Engine Beijing 100084 China

doi: 10.1007/s42154-024-00316-w

Outline

Abstract

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As a promising clean energy conversion device, solid oxide fuel cells (SOFCs) could efficiently utilize multiple fuels and have been applied in stationary power generation and other fields. However, the high operating temperature limits the use of SOFCs in the transportation sector. Consequently, reducing the operating temperature is a key focus in SOFCs development. In recent years, metalsupported SOFCs (MSSOFCs) have received significant attention because of intermediate operating temperatures. MSSOFCs offer advantages such as simple and reliable sealing, rapid startup, and high power density, making MSSOFCs a promising option for transportation applications. However, for commercial applications, the methods to enhance performance, improve durability and mitigate degradation need to be further studied. This paper comprehensively reviews the state of the art approaches to performance improvement, durability under various conditions and different fuel reforming methods. Finally, it highlights the prospects and challenges for future advancements in this field.

Key words

Metal-supported / Solid oxide fuel cell / Fuel reform / Performance / Durability

Cite this Article

Zhaohuan Zhang, Haoyu Du, Kai Xu, Xiaoqing Zhang, Xiao Ma, Shijin Shuai. Review of the Application of Metal-Supported Solid Oxide Fuel Cell in the Transportation Field[J]. Automotive Innovation, 2025 , 8 (2) : 443 -471 . DOI: 10.1007/s42154-024-00316-w

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Abbreviations

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AFC	Alkaline fuel cell
AS	Anode-supported
ATR	Autothermal reformer
BSCF	Ba_0.5Sr_0.5Co_0.8Fe_0.5O_3-δ
CGO	Cerium gadolinium oxide
CTE	Coeffi cient of thermal expansion
DBL	Diff usion barrier layer
DIR	Direct internal reforming
ER	External reforming
ES	Electrolyte-supported
GDC	Gadolinium-doped ceria
GT	Gas turbine
ICE	Internal combustion engine
LSC	(La, Sr)CoO₃
LSCF	$\mathrm{La}_{x} \mathrm{Sr}_{1-x} \mathrm{Co}_{y} \mathrm{Fe}_{1-y} \mathrm{O}_{3-\delta}$
LSCFN	$\left(\mathrm{La}_{0.4} \mathrm{Sr}_{0.6}\right)_{1-x} \mathrm{Co}_{0.2} \mathrm{Fe}_{0.7} \mathrm{Nb}_{0.1} \mathrm{O}_{3-\delta}$
LSFSc	$\mathrm{La}_{0.6} \mathrm{Sr}_{0.4} \mathrm{Fe}_{0.9} \mathrm{Sc}_{0.1} \mathrm{O}_{3-\delta}$
LSGM	$\mathrm{La}_{0.9} \mathrm{Sr}_{0.1} \mathrm{Ga}_{0.8} \mathrm{Mg}_{0.2} \mathrm{O}_{3-\delta}-\mathrm{Sm}_{0.2} \mathrm{Ce}_{0.8} \mathrm{O}_{2}$
LST	$\mathrm{La}_{0.1} \mathrm{Sr}_{0.9} \mathrm{TiO}_{3-\delta}$
MCFC	Molten-carbonate fuel cell
MPD	Maximum power density
MS	Metal-supported
MSR	Methane steam reforming
PAFC	Phosphoric acid fuel cell
PEMFC	Proton exchange membrane fuel cell
PLD	Pulsed laser deposition
POX	Partial oxidation
PVD	Physical vapor deposition
SBSCO	$\mathrm{SmBa}_{0.5} \mathrm{Sr}_{0.5} \mathrm{Co}_{2} \mathrm{O}_{5+d}$
SBSC50	$\mathrm{SmBa}_{0.5} \mathrm{Sr}_{0.5} \mathrm{Co}_{2} \mathrm{O}_{5-d} / \mathrm{Ce}_{0.9} \mathrm{Gd}_{0.1} \mathrm{O}_{1.9}$
SDC	$\mathrm{Ce}_{0.8} \mathrm{Sm}_{0.2} \mathrm{O}_{2-\delta}$
SOFC	Solid oxide fuel cell
SR	Steam reforming
SSC	$\mathrm{Sm}_{0.5} \mathrm{Sr}_{0.5} \mathrm{CoO}_{3-\delta}$
SSZ	Scandia-stabilized-zirconia
TPB	Triple phase boundary
YSZ	Yttria-stabilized zirconia

1 Introduction

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Concomitant with economic advancement, the demand for fossil fuels is also increasing [1]. Over the past 2 decades, the application of fossil energy has not only caused serious environmental pollution but also faced strict energy policies. Meanwhile, carbon emissions caused by traditional power generation have emerged as a significant and pressing concern of paramount importance. At the same time, the number of vehicles equipped with internal combustion engines (ICEs) continues to grow, exacerbating carbon and other emissions issues. Therefore, the pursuit of new energy technologies has assumed a foremost position within the strategic agendas of nations seeking to transform their energy structure. The fuel cell is a high-efficiency and environment-friendly power generation device, which has attracted extensive attention.

The rapidly evolving fuel cell technologies are illustrated in Fig. 1 [2,3]. Different kinds of fuel cells exhibit significant variation in the operating principle, and operating temperature, alongside differing degrees of adaptability to hydrocarbon fuels. In general, fuel cells can convert chemical energy directly into electricity through the electrochemical oxidation of fuels. Thus the fuel cell can surpass the efficiency limitations of the Carnot cycle, achieving a maximum efficiency of 70% [4]. The oxidation reaction in a fuel cell takes place inside the anode and generates electrons. These electrons traverse the external circuit arriving at the cathode and participating in the reduction reaction. In addition to generating electricity and reaction products such as water and carbon dioxide, reactions in fuel cells also generate heat and thermal management should be considered at the system level. The operating principles of fuel cells are different because of charge carriers. The operating temperature of alkaline fuel cell (AFC), proton exchange membrane fuel cell (PEMFC), and phosphoric acid fuel cell (PAFC) are relatively low which requires pure hydrogen and precious metal catalysts. In contrast, benefitting from higher operating temperatures, the advantages of molten-carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC) are that both $\mathrm{{CO}}$ and ${\mathrm{H}}_{2}$ can be electrochemically oxidized inside the anode without precious metal catalyst, and the fuel reforming reaction can be completed inside the fuel cell stack. Compared with MCFCs, SOFCs have mature technology and excellent performance [5,6]. Additionally, the fuel adaptability of SOFCs allows for the utilization of a wide range of fuels, including hydrogen, ammonia [7], and hydrocarbon fuels like methane, methanol, and so on. When fueled by hydrocarbon fuels, the waste gases from SOFCs are water vapor and carbon dioxide. Furthermore, even when ammonia is utilized as a fuel source in SOFCs, there are no nitrogen oxide $\left({\mathrm{{NO}}}_{x}\right)$ emissions observed [8]. These inherent advantages enhance the competitiveness of SOFCs.

According to different charge carriers, SOFCs can be divided into proton conducting SOFCs and oxygen ion conducting SOFCs [9], as shown in Fig. 1. For oxide-ion conducting SOFCs, the fuel is oxidized inside the anode releasing electrons, and the oxygen is reduced inside cathode generating oxide ions. These oxide ions are conducted to the anode through the electrolyte, thus ${\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{{CO}}}_{2}$ are formed at the anode side. To achieve higher ionic conductivity of electrolyte, a higher operating temperature is required. As for proton conducting SOFCs, the only difference is that the protons are conducted from anode to cathode so that ${\mathrm{H}}_{2}\mathrm{O}$ is produced at the cathode side. The mechanism of proton conducting SOFC is reviewed in reference [3] in detail. The technology of oxygen ion conducting SOFCs is relatively more mature, the commercialization progress is faster and this paper will mainly introduce the content of oxygen ion conducting SOFCs.

The structure of SOFC contains an anode for the electrochemical oxidation of fuels, a dense electrolyte layer conducting carriers, and a cathode for air or oxidizing agents. To achieve high ionic conductivity, the materials of electrolyte are fragile ceramics that work at high temperatures. A thick substrate as supported layer is necessary for cell preparation. To make the structure simple and match the coefficient of thermal expansion (CTE) of materials, the components of SOFCs are always chosen as the substrate such as anode, electrolyte or cathode [10]. Among these substrates, anode is the most common choice, which is called anode-supported SOFC (AS-SOFC) technology route. However, the high operating temperature still impedes the advancement and implementation of SOFCs, thus reducing the operating temperature has emerged as the mainstream for further development. With the breakthrough of material science and the progress of preparation technology, SOFCs are able to work in the intermediate temperature range of ${500} -{700}{}^{ \circ }\mathrm{C}$. Thus, it is possible to use metals as substrate which is called metal-supported SOFC (MS-SOFC). MS-SOFC utilized cerium gadolinium oxide (CGO) as the electrolyte with the metal substrate at anode side [11], which is different from other SOFCs. Using metal as the supported layer, the process of heat transfer becomes faster which contributes to transient process such as start-up. In addition, it also helps to improve robustness owing to the metal welding sealing technology [12]. The development of the SOFC with different supported layers is shown in Fig. 2 [13].

In the transportation sector, PEMFC is the most widely used application among all fuel cell technologies [14-20]. However, its catalyst uses platinum group metals which are rare and expensive. The higher cost is the major disadvantage of PEMFC. Although the relatively low temperature $\left({{80}{}^{ \circ }\mathrm{C}}\right)$ makes the PEMFC system more convenient, it suffers from carbon monoxide poisoning. It is imperative for PEMFC to maintain a carbon monoxide concentration below 10 ppm against carbon monoxide poisoning [21], so it needs additional device to remove carbon monoxide, such as a water vapor transfer reactor and a preferential oxidation reactor [22]. Although PEMFC is relatively mature, it is less adaptable to fuels such as impure hydrogen, hydrocarbon fuels and even ammonia. The preparation, storage and transportation of high purity hydrogen will be an obstacle to its application. The government needs to invest significant resources in building the fuel supply infrastructure. It is worth mentioning that SOFCs require no additional carbon monoxide removal processes and can use a variety of fuels. Thus, SOFCs can take advantage of the existing fuel supply infrastructure such as gas stations and gas pipelines. One of the major problems for fuel cells in mobile applications is the lifetime under frequent start-up cycles [18], especially for traditional SOFCs using ceramic materials and working under high temperatures. However, MS-SOFC has a higher thermal conductivity and thus can be started quickly with less external energy input, which is one of the most significant advantages [23]. The characteristics of high power density and reliable sealing make MS-SOFCs more attractive. Therefore, MS-SOFC is a prospective contender for fuel cells in mobile applications such as vehicles, aircraft and ships. MS-SOFC is a feasible technical route of fuel cells to meet transportation requirements including high power density, high durability, high efficiency, reliability, and fast transient response.

Like AS-SOFC, researchers tried porous metal on the anode side as the substrate for MS-SOFCs. Plansee SE developed its MS-SOFC with porous ferrite steel as the substrate achieving a 5kW range extender for vehicles [24]. Ceres Power is one of the most advanced companies developing MS-SOFC technology. Ceres Power creatively uses laser-drilled ferrite steel as the substrate simplifying the preparation process of the substrate [25]. Cooperating with Ceres Power, Weichai Power Co. Ltd. launched the world’s first electric bus with a 30kW MS-SOFC range extender in 2019. Their commercial MS-SOFC achieved net power generation efficiency exceeding 60%, and the cogeneration efficiency reached 92.55% in 2023 [26]. These showed the prospects of commercial applications of MS-SOFCs.

In recent years, numerous researchers have worked on the technical advancement of MS-SOFC, including some exploration of mobile applications. However, the review of MS-SOFC research in the transportation sector is rare, which can’t provide valuable references for researchers. Therefore, this paper summarizes the key issues of MS-SOFC applications in the transportation field, including studies on materials and structures, durability and degradation, fuel adaptability as well as commercial applications. Besides, the challenges and prospects are also summarized. For the wide application of MS-SOFC in the transportation sector, it is necessary to explore methods to further improve the power density, shorten the start-up time and enhance durability. Novel materials, processes and structures of SOFC stacks need to be explored. The design and integration of system components, such as reformers, burners and heat exchang-ers, are also critical to the performance and lifetime of MS-SOFC systems. Section 2 will introduce the performance of MS-SOFC in materials and structures. Section 3 focuses on the lifetime of MS-SOFC for both steady-state durability and cyclic degradation. Section 4 mainly talks about the fuel adaptation and reforming processes. Section 5 discusses the performance of the advanced commercial products of MS-SOFC. The challenges and prospects are summarized in Sect. 6.

2 Advances in High-Performance SOFC Materials and Structures

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Increasing the power density continuously, especially for volumetric power density, will enable MS-SOFC to be applied in the transportation sector. The volumetric power density of MS-SOFC stacks could achieve 2kW/L meeting the requirement of mobile applications [27]. However, most of the research improving these was based on the button or single cell level focusing on structures, materials, and fabrication processes. Therefore, researchers made lots of attempts to augment the power per unit area in the last decade. In order to improve the power density, there are several factors to consider. Firstly, increasing the current density is the most direct way. The current density reflects the rate of electrochemical reactions which is mainly affected by the number of reaction sites and the activity of catalysts. Thus, infiltrating the electrodes with nanoparticles of catalysts and sintering the electrodes with finer powders are the effective processes. However, the output voltage goes down when increasing the current density. Thus, the second method is to reduce the polarization loss, such as increasing the conductivity of the electrolyte. Meanwhile, the electronic conductivity has a great influence on the open circuit voltage. All these methods are closely related to materials, preparation processes and cell structures. Section 2.1 will introduce the novel materials and fabrication methods of MS-SOFC. Section 2.2 will mainly talk about the novel structures including the symmetric substrate, micro-structured honeycomb, tubular and micro-tubular structures.

2.1 MS-SOFC with Novel Materials and Fabrication Methods

While the substrate is made of metal, the operating temperature cannot exceed ${800}{}^{ \circ }\mathrm{C}$ to avoid oxidation, melting, and other issues of the substrate. However, traditional electrolyte materials, such as yttria-stabilized zirconia (YSZ), have lower ionic conductivity when the temperature goes down. Meanwhile, the lower temperature also limits the temperature of the fabrication process [28],such as sintering. It is the lower operating temperature that drives the researchers to look for novel materials with fabrication methods.

2.1.1 Substrate of MS-SOFC

The structure of MS-SOFC is like that of AS-SOFCs. However, researchers replaced the supported layer of the anode with foamed metal of nickel, iron, copper, or their alloys. However, the primary problem to be solved is the CTE matching of metal and ceramic materials. As the substrate is next to the anode side and nickel is the catalyst, nickel is a potential substrate material. However, nickel’s high cost and CTE limit its application. In addition, nickel is sensitive to carbon and sulfur poisoning [29]. Researchers found that the 400 series stainless steel matches the ceramic material of electrolyte in CTE as shown in Table 1. Meanwhile, the 400 series stainless steel has a cheap price and a chromium oxide layer with high conductivity on its surface. For these advantages, ferritic stainless steel becomes the ideal choice of substrate material for MS-SOFCs. Further research on ferritic stainless steel in SOFC was summarized by Yang [30]. In 2015, researchers from the German Aerospace Center (DLR) used foamed metal of NiCrAl impregnated with ${\mathrm{{La}}}_{0.1}{\mathrm{{Sr}}}_{0.9}{\mathrm{{TiO}}}_{3 -\delta }$ (LST) as the substrate of MS-SOFCs [31]. Although the difference in sintering rate or CTE between stainless steel and anode materials is small [29], it still causes warping of the cell for large stacks.

In addition to foamed metals, the substrate can also be prepared using laser drilling. Compared with the foamed metal, laser-drilled stainless steel plate has more straight through pores which is more conducive to gas diffusion under the same porosity and thickness. Thus, the laser drilled substrate could achieve the same mechanical strength with lower porosity and thickness. According to the research of Leah et al. in 2005 [35], the diameter of the holes is 10-30 $\mu \mathrm{m}$, the spacing of the holes is 250 $\mu \mathrm{m}$, and the thickness of the stainless steel is 200-300 $\mu \mathrm{m}$. The porosity of the stainless steel is approximately 0.1-1.1% which is relatively small but sufficient compared to the supported layer of AS-SOFCs. Meanwhile, the distance between channels should match the spacing of the holes in order to make the flow and diffusion process more uniform between channels. Although it could be achieved by reducing the spacing of the holes, this will make the substrate more warped. Not only that, the substrate may be melted when using lasers to drill more holes.

2.1.2 Electrolyte of MS-SOFC

As the interlayer of SOFC, electrolyte should exhibit high ionic conductivity (0.01-0.1 S/cm [10]) and prevent electrons and gas transfer. Thus, to achieve better performance, novel materials and fabrication methods have been applied to the electrolyte. For novel materials, the properties of chemical and thermal compatibility with both cathodes, anodes, and gases are crucial. Towards the trend of lowering the working temperature, the next generation of electrolyte is about the proton-conducting materials that could work below ${600}^{ \circ }\mathrm{C}$. Researchers [3,36] have summarized the proton-conducting electrolytes which are far from commercialization. This paper focuses on cells with traditional oxygen ion-conducting electrolyte. The main materials for electrolytes are YSZ at high temperature $\left({{800} -{1000}{}^{ \circ }\mathrm{C}}\right)$ and gadolinium-doped ceria (GDC) at medium temperature $\left({{600} -{800}{}^{ \circ }\mathrm{C}}\right)$. The relationship between the conductivity and temperature of materials has been tested and illustrated in Fig. 3 [2, 37].

Despite doped bismuth oxide having higher ionic conductivities, it decomposes to Bi metal when exposed to a reducing atmosphere. All the materials show a decrease in ionic conductivity with decreasing temperature which is called the Arrhenius-type temperature dependence. It is suggested that decreasing the thickness of electrolyte is a good way to obtain lower resistance. However, GDC faces the challenge of electronic conductivity at low oxygen partial pressure especially for thin electrolytes, which brings about a significant drop in the voltage [38-40]. Electrons generated from the electrochemical reactions inside the anode could flow not only from the external circuit to the cathode, but also directly through the electrolyte to the cathode, which is called the internal short circuit. When the reaction current is small, this phenomenon is very significant. To mitigate the internal short circuit, the total electronic conductive resistance should be increased by increasing the thickness or reducing the electronic conductivity and the operation temperature.

In 2009, Kang et al. [41] tried fabricating MS-SOFC with $\mathrm{{Ni}} -\mathrm{{Fe}}$ alloy and GDC as substrate and electrolyte via the co-sintered method. The cell achieved a power density of ${0.23}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${600}^{ \circ }\mathrm{C}$. In 2013, Steffen Wolf et al. [42] from DLR explored the plasma sprayed manufacturing methods. The result showed that the reduction of nickel oxide caused extreme stress within the electrolyte which was enough to create cracks. In 2013, Metcalfe et al. [43, 44] from the University of Toronto tried the axial-injection plasma sprayed method fabricating two different MS-SOFCs. Differing in the sequence of electrode deposition, the anode-down cell with bi-layer anode achieved a power of ${0.63}\mathrm{\;W}/$ cm ${}^{2}$ at ${750}^{ \circ }\mathrm{C}$, while the performance of the cathode-down cell was poor due to the significant vertical cracks which resulted in large leak rate in the electrolyte. From their previous research [45], it was suggested that the thermal stress stemming from the different thermal conductivity of the ${\mathrm{{La}}}_{x}{\mathrm{{Sr}}}_{1 -x}{\mathrm{{Co}}}_{y}{\mathrm{{Fe}}}_{1 -y}{\mathrm{O}}_{3 -\delta } -{\mathrm{{Ce}}}_{0.8}{\mathrm{{Sm}}}_{0.2}{\mathrm{O}}_{2 -\delta }$ (LSCF-SDC) composite produced vertical cracks. Then, Metcalfe et al. [46] pioneered the method of solution precursor plasma spraying fabricating MS-SOFC with a maximum power density (MPD) of 0.52W/cm² at 750℃.

In 2013, Forschungszentrum Jülich (FZJ) [47] fabricated thin YSZ electrolytes via the Sol-Gel Spin-coating technique. Despite the thickness of the electrolyte film reaching ${500}\mathrm{\;{nm}}$ with a leak rate of 1-10×10^-4, the thin film could be contaminated by small particles from the environment, which resulted in defects. Due to the medium-low operation temperature of MS-SOFC, the conductivity of YSZ is not sufficient. One way to solve this issue is to obtain ultra-thin electrolyte, but the ultra-thin electrolyte and dense electrolyte show a trade-off relationship. However, GDC exhibits well electrolytic properties under medium-low temperatures [48]. By replacing the Ni-YSZ anodes with Ni-GDC anodes, the single cell’s performance was improved [49, 50], achieving MPD of 0.89 and ${0.55}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${800}{}^{ \circ }\mathrm{C}$ and ${700}^{ \circ }\mathrm{C}$, respectively. In 2014, Reolon et al. [28] fabricated the continuous, dense, and thin films of GDC electrolyte (thickness $< {5\mu }\mathrm{m}$ ) via spray pyrolysis under a relatively low temperature of ${600}^{ \circ }\mathrm{C}$. More information could be seen in reviews [9, 51]. In 2014, Plansee SE [52] developed MS-SOFC with the multi-layer thin-film electrolyte and graded anode. The electrolyte was obtained via a physical vapor deposition (PVD) gas flow sputtering process. Meanwhile, the thickness of electrolyte over ${4\mu }\mathrm{m}$ was sufficient for low leakage rates. Although the operating temperature of ${850}^{ \circ }\mathrm{C}$ was a bit high, the cell achieved MPD of 1.4W/cm².

2.1.3 Cathode of MS-SOFC

The traditional material for SOFC cathode is LSCF. However, its sintering temperature is a bit high for MS-SOFC. In 2011, Bae [53] from Korea Advanced Institute of Science and Technology (KAIST) tested the performance of ${\mathrm{{SmBa}}}_{0.5}{\mathrm{{Sr}}}_{0.5}{\mathrm{{Co}}}_{2}{\mathrm{O}}_{5 -d}/{\mathrm{{Ce}}}_{0.9}{\mathrm{{Gd}}}_{0.1}{\mathrm{O}}_{1.9}$ (SBSC50) and ${\mathrm{{Ba}}}_{0.5}{\mathrm{{Sr}}}_{0.5}{\mathrm{{Co}}}_{0.8}{\mathrm{{Fe}}}_{0.2}{\mathrm{O}}_{3 -\delta }$ (BSCF) as the in-situ cathodes which have lower sintering temperature of ${800}^{ \circ }\mathrm{C}$. The results showed MPD of 0.5 and ${0.65}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${800}{}^{ \circ }\mathrm{C}$, which implied that BSCF had better properties. Further research found that the optimal sintering temperature was over ${750}^{ \circ }\mathrm{C}$ [54]. Considering that the sintering temperature could be lower than the operating temperature, KAIST fabricated MS-SOFC with an unsintered BSCF cathode [55] and tried completing sintering cathode during operation. The cell achieved MPD of ${0.91}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${800}{}^{ \circ }\mathrm{C}$, which reached the performance of commercial AS-SOFC [56]. Although BSCF and ${\mathrm{{SmBa}}}_{0.5}{\mathrm{{Sr}}}_{0.5}{\mathrm{{Co}}}_{2}{\mathrm{O}}_{5 + d}$ (SBSCO) have well sinterability at a low temperature of ${800}^{ \circ }\mathrm{C}$, their CTEs are much larger than typical electrolytes.

In 2011, Trine Klemensø et al. [57] from the Technical University of Denmark (DTU) introduced GDC barrier layers fabricated by magnetron sputtering and $\left({\mathrm{{La}},\mathrm{{Sr}}}\right) {\mathrm{{CoO}}}_{3}$ (LSC) cathode to MS-SOFC achieving MPD of 1.14 W/cm ${}^{2}$ at ${650}^{ \circ }\mathrm{C}$. In 2017, researchers from FZJ [58-60] presented a novel sintering method for the LSCF cathode. To prevent oxidation of the substrate, the cathode was sintered at ${950}^{ \circ }\mathrm{C}$ under an argon atmosphere. With the characterization of the cathode microstructure, it was found that the adherence and mechanical stability were improved and the cell achieved a power density of ${0.67}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${750}^{ \circ }\mathrm{C}$. In 2019, to increase the stability of cathodes, FZJ developed LSC/GDC dual-phase cathodes [61]. The cell with a dual-phase cathode achieved 0.91 and ${0.38}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at 750 and ${650}^{ \circ }\mathrm{C}$, respectively.

Zhang et al. [62] obtained an LSCF cathode with nano-structure using the liquid precursor high-velocity oxygen fuel flame spraying process. The cell showed excellent performance with MPD of 0.65 and ${1.1}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at 600 and ${700}^{ \circ }\mathrm{C}$ respectively. Via the vacuum cold sprayed technique, Zhang et al. [63] constructed the nanostructured LSCF cathode. The cell’s power density was over $1\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${600}{}^{ \circ }\mathrm{C}$ with plasma-sprayed bilayer electrolyte of GDC and ${\mathrm{{La}}}_{0.9}{\mathrm{{Sr}}}_{0.1}{\mathrm{{Ga}}}_{0.8}{\mathrm{{Mg}}}_{0.2}{\mathrm{O}}_{3 -\delta } -{\mathrm{{Sm}}}_{0.2}{\mathrm{{Ce}}}_{0.8}{\mathrm{O}}_{2}$ (LSGM) as well as Ni-GDC anode. In 2016, researchers from University West tried an atmospheric plasma spraying technique fabricating anodes with dense and thin coatings [64,65]. The cell achieved power density up to almost ${0.6}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${800}{}^{ \circ }\mathrm{C}$ in the absence of a diffusion barrier layer between the electrolyte and cathode. The in-situ “exsolution & re-oxidation” of transition metal oxides could greatly reduce the polarization resistance of the cathode [66-68]. Ni et al. [69] tried the method of(Fe, Ni)exsolution from ${\mathrm{{Sr}}}_{0.95}{\mathrm{{Ti}}}_{0.3}{\mathrm{{Fe}}}_{0.6}{\mathrm{{Ni}}}_{0.1}{\mathrm{O}}_{3 -\delta }$, constructing the cathode with rather lower polarization resistance. With Ni-YSZ anode and YSZ electrolyte, the cell achieved MPD of ${0.74}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${750}^{ \circ }\mathrm{C}$.

In 2012, Young-Wan Ju et al. [70] studied Pr-doped ${\mathrm{{LaCrO}}}_{3}$ as the buffer layer in electrolyte to prevent the migration of La from LSGM to ${\mathrm{{Sm}}}_{0.2}{\mathrm{{Ce}}}_{0.8}{\mathrm{O}}_{2}$. Using the pulsed laser deposition (PLD) method fabricating the electrolyte, the cell showed an MPD of ${1.99}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${700}{}^{ \circ }\mathrm{C}$. Then, by introducing a double columnar shape SDC- ${\mathrm{{Sm}}}_{0.5}{\mathrm{{Sr}}}_{0.5}{\mathrm{{CoO}}}_{3 -\delta }$ (SSC) interlayer between LSGM-SDC electrolyte and SSC cathode layer, Ju et al. [71] achieved an MPD of ${2.2}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${700}^{ \circ }\mathrm{C}$. The double interlayer was fabricated with the PLD method. In 2019, Kang et al. [72] found the optimal composition ratio of SSC and SDC was 6:4 with the smallest overpotential of cathode resulting in the cell’s MPD over ${3.0}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${700}{}^{ \circ }\mathrm{C}$.

2.1.4 Anode of MS-SOFC

The traditional anode material is nickel cermet such as Ni-YSZ. In 2008, Xie et al. [73] tried pneumatic spray deposition and electrostatic spray deposition fabricating MS-SOFC with samaria-doped ceria electrolyte and NiO-SDC anode. The cell showed MPD of ${0.42}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${650}^{ \circ }\mathrm{C}$. Together with Karlsruhe Institute of Technology (KIT), FZJ tried plasma spray-physical vapor deposition (PS-PVD) fabricating the anode and electrolyte [74], achieving a power density of ${0.89}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${817}^{ \circ }\mathrm{C}$. In 2019, Plansee SE developed the three-layered anode [75]. In the direction of oxygen ion conduction, there were anode layer A of coarse Ni-8YSZ (65/35 wt%), anode layer B of ultrafine Ni-8YSZ (65/35 wt%), and anode layer $\mathrm{C}$ of $\mathrm{{Ni}} -\mathrm{{GDC}}{10}\left({{60}/{40}\mathrm{{wt}}\% }\right)$. By optimizing the structure, the cell achieved MPD of ${1.25}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${700}{}^{ \circ }\mathrm{C}$. After several iterations, in 2020, Plansee SE developed its third generation MS-SOFC [11], of which the power density was over ${1.96}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${650}^{ \circ }\mathrm{C}$. In 2016, Roehrens et al. [76] from FZJ developed MS-SOFCs with three-layer Ni/8YSZ anodes and an activated in situ LSCF cathode, achieving power density over $1\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${800}^{ \circ }\mathrm{C}$.

Improving the activity of electrodes constitutes a viable approach to increase the power density of SOFC. Researchers have tried infiltration and exsolution methods for anodes. In 2016, Ricardo Fernández-González et al. [77] infiltrated the Ni-YSZ anode with the CGO20 solution decreasing the polarization loss of the anode. The experimental results showed that infiltration could improve power density. In 2017, DTU studied the effect of infiltrated ${\mathrm{{La}}}_{0.4}{\mathrm{{Sr}}}_{0.4}{\mathrm{{Fe}}}_{0.03}{\mathrm{{Ni}}}_{0.03}{\mathrm{{Ti}}}_{0.94}{\mathrm{O}}_{3}$ (LSFNT) anodes [78]. At the benchmark point ( ${700}{}^{ \circ }\mathrm{C}$ and ${0.7}\mathrm{\;V}$ ), the single cell with ${16}{\mathrm{\;{cm}}}^{2}$ active area achieved a power density of ${0.65}\mathrm{\;W}/$ cm ${}^{2}$. The fuel utilization was ${31}\%$. Zhu et al. [79] exposed the ${\mathrm{{Sr}}}_{0.95}\left({{\mathrm{{Ti}}}_{0.3}{\mathrm{{Fe}}}_{0.63}{\mathrm{{Ni}}}_{0.07}}\right) {\mathrm{O}}_{3 -\delta }$ (STFN) anode to the fuel exsolving nanostructured ${\mathrm{{Ni}}}_{0.5}{\mathrm{{Fe}}}_{0.5}$ on the surface of perovskite, which leads to a huge decline in anode polarization resistance. In contrast to the STFN anode without exsolution, the cell’s MPD increased from 0.13 to ${0.4}\mathrm{\;W}/$ cm ${}^{2}$ at ${700}^{ \circ }\mathrm{C}$. Despite the lack of understanding of exsolution, it was suggested that the nanostructured $\mathrm{{Ni}}$ could promote the hydrogen adsorption process resulting in the decrease of polarization resistance. Then Zhu [80] tried ${\left({\mathrm{{La}}}_{0.4}{\mathrm{{Sr}}}_{0.6}\right) }_{1 -x}{\mathrm{{Co}}}_{0.2}{\mathrm{{Fe}}}_{0.7}{\mathrm{{Nb}}}_{0.1}{\mathrm{O}}_{3 -\delta }$ (LSCFN) as anode. Single cells were fabricated after ${\mathrm{{Co}}}_{1 -x}{\mathrm{{Fe}}}_{x}$ exsolved from the anode under humidified or dry hydrogen atmosphere. At ${850}^{ \circ }\mathrm{C}$, the cell’s MPD was up to ${1.2}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ with dry hydrogen.

2.2 MS-SOFC with Novel Structures

To avoid warping of the cell, Tucker et al. [81] from Lawrence Berkeley National Laboratory (LBNL) developed MS-SOFCs with symmetric structures as shown in Fig. 4. This button cell achieved maximum power density (MPD) of 0.124,0.313, and ${0.551}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at600,700, and ${800}{}^{ \circ }\mathrm{C}$ respectively. By replacing YSZ with ${10}\mathrm{{Sc}}1\mathrm{{CeSZ}}$ for electrolyte [82],the cell achieved MPD of about 0.31, 0.40, and ${0.57}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at 700,750, and ${800}^{ \circ }\mathrm{C}$. In 2017, Tucker et al. [83-85] introduced the MS-SOFC to direct flame SOFC for personal power use. The 5-cell stack could provide 2.7 W (${0.156}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$) which was able to charge the mobile phone or LED lighting when camping. Despite the small power, it showed a portable application demonstration. In 2020, Tucker et al. [86] scaled up the cell size to ${50}{\mathrm{\;{cm}}}^{2}$, and by improving the infiltration process, the cell achieved an MPD of ${0.30}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ which showed the equivalent performance to the button cell. Meanwhile, by increasing the infiltration times from 5 to 10, the MPD increased to ${0.52}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$. In 2022, Tucker [27] optimized the fabrication and structure of MS-SOFC, which showed that the optimal catalyst firing temperature, pore former volume ratio, metal substrate volume ratio, and metal substrate thickness were ${800}^{ \circ }\mathrm{C},{1.6} : 1$, and ${195\mu }\mathrm{m}$. The cell achieved MPD of ${0.90}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${700}{}^{ \circ }\mathrm{C}$.

In 2013, Zhou et al. [87] from the Shanghai Institute of Ceramics used ${\left({\mathrm{{Bi}}}_{2}{\mathrm{O}}_{3}\right) }_{0.7}{\left({\mathrm{{Er}}}_{2}{\mathrm{O}}_{3}\right) }_{0.3} -\mathrm{{Ag}}$ (ESB) composite cathode with YSZ electrolyte and 430L porous stainless steel fabricating an MS-SOFC which achieved MPD of ${0.568}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${750}^{ \circ }\mathrm{C}$. Inspired by Tucker’s research [88],Zhou [89] impregnated porous YSZ cathode backbones with ${\mathrm{{Sr}}}_{2}{\mathrm{{Fe}}}_{1.5}{\mathrm{{Mo}}}_{0.5}{\mathrm{O}}_{6 -\delta }$. The result showed MPD of 0.438 and ${0.221}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at 800 and ${700}^{ \circ }\mathrm{C}$ respectively. Then Zhou [90] impregnated the anode and cathode with redox-stable ${\mathrm{{La}}}_{0.6}{\mathrm{{Sr}}}_{0.4}{\mathrm{{Fe}}}_{0.9}{\mathrm{{Sc}}}_{0.1}{\mathrm{O}}_{3 -\delta }$ (LSFSc) based upon porous 430LIYSZlporous YSZ, achieving MPD of 0.65 and ${0.30}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at 800 and ${700}{}^{ \circ }\mathrm{C}$ respectively. When impregnated with nano-scale ${\mathrm{{SrFe}}}_{0.75}{\mathrm{{Mo}}}_{0.25}{\mathrm{O}}_{3 -\delta }$, the cell achieved MPD of 0.81 and ${0.40}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at 800 and ${700}^{ \circ }\mathrm{C}$ [91,92] respectively. Replacing the anode catalyst with nano-scale Ni increased the cell’s MPD to 0.907 and 0.418 $\mathrm{W}/{\mathrm{{cm}}}^{2}$ at 800 and ${700}{}^{ \circ }\mathrm{C}$ respectively, while the cathode catalyst was still LSFSs [93]. In 2014, Zhou [94] developed scandia-stabilized-zirconia (SSZ) as electrolyte and infiltrated the porous ${430}\mathrm{\;L} -\mathrm{{YSZ}}$ anode and SSZ cathode with SDC and SBSCO. The cell achieved MPD of 1.25 and ${0.61}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at 700 and ${600}^{ \circ }\mathrm{C}$ respectively.

Except for the planar MS-SOFC, there are many other structures of MS-SOFC under research. In 2014, Ricardo Fernández-González et al. [95] proposed a novel MS-SOFC with a microstructured honeycomb metal substrate. This design could reduce nearly 65% of the metallic material and achieve a power density of over ${0.3}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$. The cell showed excellent potential for mass power density. In 2016, Han et al. [96] fabricated tubular MS-SOFC with $\mathrm{{Ni}} -\mathrm{{Fe}}$ supported layer using phase-inversion, co-sintering, and in-situ reduction technologies. The cell achieved MPD of ${0.26}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${800}^{ \circ }\mathrm{C}$. In 2022, Wang [97] developed a novel micro-tubular MS-SOFC with the porous ${430}\mathrm{\;L}$ substratel porous 430L-SSZI SSZI porous SSZ sandwich structure. The cell showed MPD of ${0.271}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${800}{}^{ \circ }\mathrm{C}$ respectively. In 2017, Hirofumi SUMI et al. [98, 99] studied micro-tubular MS-SOFC with a ceria-based electrolyte, which achieved a power density 3 times larger than AS-SOFC in ${20}\%$ hydrogen at ${550}^{ \circ }\mathrm{C}$. As the thickness of the anode was thin, the polarization resistance was high in a hydrogen-rich environment. Although there are many types of MS-SOFC, they are under the lab stage and the basic planar type is mostly studied and used.

2.3 Summary

At present, materials and preparation processes are still the main research objects in the advancement of high-performance MS-SOFC. In the aspect of materials, composition optimization based on the existing material system is the main research direction. The fabrication process is the key from powder to power. By optimizing the material composition and preparation process, the power density of MS-SOFC with a small size has attained a power density of 1 $\mathrm{W}/{\mathrm{{cm}}}^{2}$ at ${600}{}^{ \circ }\mathrm{C}$. Besides, most of the MS-SOFC performance test results are based on the level of button cells and also have different test specifications. The research based on the unified standard and large size stack level remains to be strengthened. In addition, the large-scale application of MS-SOFC requires further optimization of stack design in terms of heat and mass transport based on materials and preparation technology.

3 Durability and Degradation of MS-SOFC Operation

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Apart from power density, application in the transportation sector also requires durability to be in accordance with the vehicles’ lifespan. Durability of fuel cell system in the transportation scenario includes both operation life as well as resistance to aging due to operation cycling, and is often evaluated by the rate of performance degradation [100]. This paper only discussed the durability and degradations of the cells, despite that durability of the balance of plant is also an important factor. With matched CTE and advanced sintering techniques, state-of-the-art SOFCs can avoid cracks and serious delamination. However, gradual performance degradation caused by electrochemical and thermomechani-cal factors remains a major restriction.

Compared to stationary applications where more emphasis is put on mitigating SOFC’s steady-state degradation, the transportation application of MS-SOFC equally requires performance stability during steady-state operation as well as cycling, since vehicles operate intermittently[101,102]. Unlike combined heat and power plants that rarely terminate, even commercial vehicles operating under tight schedules would experience multiple start-ups and stops daily. AS-SOFC stacks usually have a cycling life of around 100 thermal cycles. It’s anticipated that SOFC installed on vehicles should endure more than 1000 thermal cycles within a ${10},{000}\mathrm{\;h}$ lifetime [56,101]. In addition to more cycle numbers, the drastic working condition change during vehicle quick start-up also enhances cyclic degradation. During quick cold start of fuel cell vehicles, the rate of temperature change can exceed ${50}\mathrm{\;K}/\mathrm{{min}}$. Therefore, the prospects of MS-SOFC’s durability for successfully entering the mobile market are quite clear. Apart from the steep escalation in power density, the U.S. Department of Energy’s development target for SOFC mobile application also marked a ${0.2}\% /{1000}\mathrm{\;h}$ performance degradation rate by 2030 [103]. In AVL-List GmbH’s more ambitious project of SOFC range extender systems, the lifetime of the SOFC stack should exceed ${5000}\mathrm{\;h}$, whereas the time required for system startup should be within $5\mathrm{\;{min}}$ [104].

Among all three major SOFC configurations, MS-SOFC is believed to have the greatest potential in terms of thermal-cycling reliability [101]. A significant amount of research interest has been put into improving MS-SOFC’s durability. This section will focus on single cells, summarizing research progress and bench-scale tests from two different perspectives: Sect. 3.1 introduces state-of-the-art MS-SOFC steady-state durability and corresponding modifications concerning cell configuration. Section 3.2 looks into cyclic degradation with some extensive discussion on quick-start cycles.

3.1 MS-SOFC’s Durability Against Steady-State Degradation

Various factors can impose gradual performance loss on MS-SOFCs during steady-state operation. The rate of degradation is often measured by the percentage of performance loss per thousand-hour. A longer lifetime of SOFCs can be achieved through the reduction of the degradation rate. Major intrinsic degradation factors (related to materials and structure) include:

(1) Chromium poisoning of the cathode. Chromium depleted from the metal substrate and interconnec-tors can enter the cathode either through evaporation-deposition or migration. Two degradation mechanisms concerning $\mathrm{{Cr}}$ have been found: chromium deposition on cathode surface or cathode/electrolyte interface, and chromium reacting with certain elements in cathode materials such as Mn, Sr, etc. [105, 106]. MS-SOFC is more likely to suffer from chromium poisoning compared to AS-SOFC and electrolyte-supported SOFC (ES-SOFC), since stainless steel or other Fe-Cr alloy are often used as materials of the metal substrate, which contains a high concentration of chromium ( $\sim {10}\mathrm{{wt}}\%$ ). Under oxidative or humidity environment, chromium scale on the metal substrate can be oxidized to form gaseous species which might eventually depositing on the cathode surface or cathode-electrolyte interface [107] . To mitigate chromium poisoning of the cathode, recently developed MS-SOFC often had diffusion barrier layers (DBL) applied to the metal substrate surface and cathode/electrolyte interface to cut off the chromium migration route.
(2) Ni coarsening during fabrication (co-sintering) and operation. Driven by surface potential gradient, nickel particles exhibit a propensity to coalesce under high temperatures during fabrication and ${600} -{800}{}^{ \circ }\mathrm{C}$ operation [108] . Anode coarsening can cause Ni isolated from the ionic conductive phase and reduce the active triple phase boundaries (TPBs). The infiltration method instead of high-temperature sintering can mitigate microstructural change of $\mathrm{{Ni}}$ surface during cell fabrication, yet coarsening in steady-state operation is still the dominant anode degradation factor [93, 106, 109].
(3) Element diffusion. Fe and Cr can diffuse in the substrate (stainless steel) and anodes(Ni)as well as between cathodes and electrolytes. Fe and $\mathrm{{Cr}}$ are oxidized upon the anode surface to form chromia and ferrite, blocking active sites on the Ni surface [110] . Diffusion between $\mathrm{{Co}},\mathrm{{La}}$, and $\mathrm{{Mn}}$ in cathode materials like $\mathrm{{LSC}}$ or ${\mathrm{{La}}}_{0.8}{\mathrm{{Sr}}}_{0.2}{\mathrm{{MnO}}}_{3 -\delta }$ (LSM) and electrolyte material like zirconia leads to a reaction forming zirconates between cathode and electrode, which acts as a poor ion conductivity interphase [111, 112].
(4) Structural damage due to thermal stress accumulated through continuous operation, like electrodes/electrolyte delamination.

Extrinsic factors of cell degradation mainly refer to the poisoning of electrodes by contaminates in carbon fuel and air. Although SOFC has a wide range of fuel adaptability, the electrode still has a certain degree of poisoning risk, especially for utilizing highly concentrated carbon fuels directly. Experiments have shown that carbon deposits in the anode will erode and destroy the anode structure while using pure methane for a few hours [113]. In addition, the impurities in the fuel or air can also affect the anode or cathode. For example, sulfur can react with Sr or La in the cathode material, forming sulfides [106, 114]. Experimental results suggested that higher ${\mathrm{O}}^{2 -}$ potential driven by sulfur contamination would in turn facilitate Ni depletion [114]. $\mathrm{{Ni}}$ also reacts directly with hydrogen sulfide, blocking catalytic active sites. Researchers and institutes have performed long-term stability tests on MS-SOFC single cell or short stack at galvanostatic or potentiostatic conditions. However, as performance degradation is determined by specific cell configuration, material, fabrication methods, and experiment settings, direct comparison of degradation rates may not reflect all features of the durability.

The majority of research conducted galvanostatic tests and degradation was rated in terms of voltage loss, with 0.7 $\mathrm{V}$ as the often-chosen initial voltage. Tucker et al. conducted a series of research to promote both long-term durability and resilience against cyclical degradation of MS-SOFCs. Modifications including cell configuration, electrolyte materials, and electrode catalysts were tested against the constant operation and thermal cycles [115]. The novel porous steel|YSZ|porous steel dual-metal substrate structure with infiltrated anode and cathode catalysts showed satisfactory resistance to thermal stress under both long-term and cyclic operation. Single cells with different catalyst combinations were fabricated: LSM cathode/Ni-SDC anode and Ni-SDC cathode/Ni-SDC anode. Both cells underwent 1200 $\mathrm{h}$ continuous operation at ${700}{}^{ \circ }\mathrm{C}$ and ${0.7}\mathrm{\;V}$ and suffered from serious performance degradation. The results showed ${60}\% /\mathrm{{kh}}$ for LSM/Ni-SDC and ${54}\% /\mathrm{{kh}}$ for Ni-SDC/Ni-SDC. Chromium deposition and catalyst coarsening are dominant degradation factors as suggested by EIS and SEM inspection results [116]. In a successive work, ${\mathrm{{CoO}}}_{\mathrm{x}}$ protective atomic layer deposition coating was applied to the cathode on a single metal substrate cell to mitigate chromium deposition. Metal substrate was pre-oxidized to prevent damage against ${\mathrm{{CoO}}}_{\mathrm{x}}$ coating from chromia formation. Infiltrated catalysts were also pre-coarsened to increase stability. Combining all three modifications, single cell test for ${1200}\mathrm{\;h}$ yielded a ${2.3}\% /\mathrm{{kh}}$ degradation rate at ${700}{}^{ \circ }\mathrm{C}$ and ${0.7}\mathrm{\;V}$, much better than the results before. However, initial power density decreased by roughly ${35}\%$ compared to cells without ${\mathrm{{CoO}}}_{\mathrm{x}}$ coating and pre-treatments. Researchers suggested that the certain trade-off between performance and durability might be favorable considering MS-SOFC’s general operability [117].

As infiltrated catalysts suffered from coarsening of nano-structures, ceramet (combination of ceramics and metals) anodes were introduced to stabilize anode performance. The addition of GDC has been proved effective against anode coarsening. Ni-GDC anodes were either prepared using the infiltration method or tape casting, forming active anode containing low content $\mathrm{{Ni}}$ nanoparticles carried by GDC coating. Klemensø et al. [57] conducted a durability test on single cells with porous backbone and Ni-GDC anode at ${650}^{ \circ }\mathrm{C}$ and ${0.25}\mathrm{\;A}$, achieving a voltage degradation rate of $\sim 5\% /\mathrm{{kh}}$. GDC coating in the anode can also prevent element deposition. According to SEM results, ferrite and chromia formation were only found on the metal substrate surface. In the solid oxide electrolysis cell experiment infiltrating GDC upon pre-sintered Ni nanoparticles was also tested effective against coarsening. Research showed that GDC layers had certain resistance against Fe-Cr-Ni diffusion as well [118].

DBLs are also widely applied to anode/metal substrate interface and cathode/electrolyte interface, blocking element interdiffusion while maintaining electron/ion conductivity. Klemensø et al. [57] demonstrated durability improvement by the introduction of DBL (made of GDC) between cathode and electrolyte using spin coating and physical vapor deposition technique. LSClporous YSZINi-GDC configured single cells with and without DBL were fabricated. The cells then operated at ${650}^{ \circ }\mathrm{C}$ and ${0.25}\mathrm{\;A}/{\mathrm{{cm}}}^{2}$ for up to ${3000}\mathrm{\;h}$, where degradation rates of cells with DBLs were significantly lower (0.9-1.4%/kh) than those without DBLs (4.5%/kh).

Wang’s group proposed a configuration without DBLs. In 2014, Zhou et al. [93] infiltrated LSFS/Ni electrodes on porous 430L stainless steel|YSZ|porous YSZ backbone. 200 $\mathrm{h}$ single cell steady-state test at ${650}^{ \circ }\mathrm{C}$ and ${0.7}\mathrm{\;V}$ showed an ${11}\% /{100}\mathrm{\;h}$ degradation rate. It was implied by EIS measurement and symmetrical anode/cathode tests that anode active polarization increased dramatically due to $\mathrm{{Ni}}$ coarsening was the dominant degradation factor, and interdiffusion between $\mathrm{{Ni}}$ and $\mathrm{{Fe}} -\mathrm{{Cr}}$ alloy was a potential factor as well. The authors then fabricated an LSFS/Ni-SDC cell on porous steelISSZlporous SSZ backbone using the infiltration method. The single cell’s degradation rate of ${1500}\mathrm{\;h}$ experiment at ${650}^{ \circ }\mathrm{C}$ and ${0.9}\mathrm{\;A}/{\mathrm{{cm}}}^{2}$ was ${1.3}\% /\mathrm{{kh}}$. Though the degradation rate was higher compared to Klemensø’s work [57],the new configuration significantly increased power output, since the absence of DBL lowered the total resistance of the cell. Post-analysis showed no sign of metal interdiffusion in the anode, yet coarsening and cracking of the infiltrated Ni-SDC layer was still the dominant degradation factor [109].

Development and application of GDC coating, DBLs had extended MS-SOFC’s steady-state operation time by preventing or hindering element diffusion, chromium deposition, and coarsening. Recent publications have shown promising results in reducing steady-state degradation rates to a commercially applicable level. However, for cells using infiltrated electrodes, the coarsening of active nanoparticles remained a major degradation factor and seemed to lack exhaustive solutions without compromising performance.

3.2 Cyclic Degradation and Quick Start-Up

Drastic operating condition change during SOFC’s start-up and shut-down also induces performance loss, which is often referred to as cyclic degradation [101, 119]. Cyclic degradation is usually caused by the combination of thermal stress and structural damage because of redox reactions. Unlike steady-state degradation where thermal stress accumulation and nanostructure morph progressed rather gradually, performance loss during the thermal and redox cycle happens in a much faster period, which makes cyclic degradation harder to mitigate. Similar to long-term operation, SOFC start-up, and shut-down processes are also faced with thermal stability problems. Large temperature changes during cold start and turning off can result in quick expansion and contraction of cell components. Unequal volumetric change among different layers due to a mismatch of materials’ CTE henceforth induces thermal stress, causing delamination, cell fracture, or warp.

State-of-the-art MS-SOFCs generally use stainless steel metal substrate, which has a CTE matching with YSZ or GDC as well as satisfactory oxidative resistance [120]. Lab tests of MS-SOFC single cells exceeded 100 thermal cycles with small performance loss specifically related to cyclic degradation [121]. On the other hand, as shown in Fig. 5,in 2017, Ceres Power reported tolerance against over 1500 thermal cycles of MS-SOFC stacks as proof of MS-SOFC’s thermal cycling capability in industrial applications [25]. In addition, MS-SOFC still suffers from redox issues during operation cycles. The redox reaction is often induced by uneven fuel supplied across the anode surface, perturbation in fuel supply rate, lack of protective gas during shutdown, or oxygen exposure of the anode due to electrolyte or sealant leakage. As supplied reductants are insufficient, oxygen directly reacts with anode $\mathrm{{Ni}}$ to form $\mathrm{{NiO}}$, causing $\sim {70}\%$ volumetric expansion according to experimental results [122]. Such large volumetric change can generate detrimental mechanical strain across anode structure. Typical redox-oriented cell damage ranges from the decrease of TPB length to cell crack, as strain overwhelms the cell’s mechanical strength [123, 124].

Cyclic degradation has a more significant impact in mobile scenarios compared to stationary ones since the power unit in the former system starts and stops more frequently. When used as the main power source or as range extenders, MS-SOFC stack not only suffers performance loss from the accumulation of operation time but also from large temperature change rate and large spatial temperature gradient during the heating and cooling of the thermal cycles [125]. The operating condition more frequently excurses in the mobile scenario as well, which can induce detrimental redox reactions. The rapidness of the start-up is essential for MS-SOFC’s deployment in transportation applications. The simulation showed that during the cold start, the balance of both mass transportation and electrochemical reactions can be established in a short period of about ${10}^{2}\mathrm{\;s}$ [126, 127], much shorter than the typical start-up time $\left({ \geq 1\mathrm{\;h}}\right)$ of commercialized AS-SOFC or ES-SOFC stacks so far. For SOFCs with metal substrate, performance degradation due to thermal stress and redox during heating up was the main obstacle toward rapid start-up.

Researchers and companies performed “rapid cycling” tests on MS-SOFC single cells or stacks to evaluate resistance against the quick start-up. It is worth mentioning that as quick start criteria vary among projects, the actual heating rate as well as the range of temperature change through cycling could also be different. For example, during the button cell experiment, researchers from the University of Toronto performed 9 rapid cycle tests (400-800℃ range) with up to 20K/min temperature change rate [128]. AVL List’s Mestrex project set a target for the quick start-up capability of cold start within ${30}\mathrm{\;{min}}$, leading to the average heating rate being 20-25K/min [104]. The project aimed to develop a practical SOFC range extender system for vehicles. Thermal cycling on the early prototype of Ceres Power’s SteelCell imposed only 7-10K/min average temperature change rate on room-temperature to 600℃ cycles, while heating and cooling were speeded up to about 25K/min in experiments on later versions [129, 130]. On the other hand, heating rates exceeding ${50}\mathrm{\;K}/\mathrm{{min}}$ were often reported by LBNL [116, 131].

Some modifications in cell structure and configuration have been claimed effective to mitigate cyclic degradation. Tucker et al. tested a series of symmetrical MS-SOFCs with two metal substrates clamping ceramic backbone and infiltrated electrodes. The double metal substrates can enhance structural stability and better counter-restrain thermal/redox expansion during cycling. In 2008, thermal cycle tests on a tubular LSM/YSZ/Ni cell with porous metal substrate on both sides demonstrated resistance to extremely fast cycling, with peak and average heating rates that reached ${350}\mathrm{\;K}/$ min and ${100}\mathrm{\;K}/\mathrm{{min}}$, respectively [131]. SEM results showed no sign of structural damage; performance loss was mainly caused by Ni coarsening. Recently, with the Ni-SDC anode introduced to mitigate the anode coarsening issue, the planar symmetric structure MS-SOFC button cell completing 200 rapid thermal cycles and 20 redox cycles only suffered from minor degradation [116]. Apart from the effect of heating or cooling rate, Hagen et al. [102] investigated MS-SOFC’s tolerance against spatial temperature gradient during thermal cycling. The tested short stack was comprised of three stainless steel support cells, each had Ni-GDC anode, ScYSZ electrolyte, LSC cathode, and GDC barrier layers. Based on EIS results, cathode delamination as a result of thermal strain was the main degradation cause.

3.3 Summary

For MS-SOFC, the factors affecting its durability include intrinsic degradation factors related to cell fabrication methods, materials and configurations, as well as cyclic degradation induced by rapid start-stop in transportation scenarios, which is summarized in Fig. 6. The former includes sulfur poisoning of electrodes, chromium poisoning of the cathodes, carbon deposition, Ni coarsening during fabrication and operation, element diffusion and segregation, and structural damage. The development and application of GDC coating and DBLs can extend the steady-state operation time of MS-SOFC. However, the coarsening of active nanopar-ticles remains a major degradation factor for the fuel cells using infiltrated electrodes. The latter is usually caused by structural damage induced by thermal stress and redox reactions, which occurs rapidly. For this, it is necessary to further optimize the volumetric change among different layers due to the mismatch of materials’ CTE, the anti-oxidation performance of the metal substrate, and the structure of MS-SOFCs. The unified standard for durability testing also needs to be established to better compare the results of different research.

4 Fuel Adaptation and Reforming

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In the current transportation sector, a diverse spectrum of fuels is used, including hydrogen-fueled/fossil-fueled vehicles, planes powered by jet fuel and ships powered by bunker oil [56]. One energy conversion device often corresponds to only one fuel. Thus, fuel adaptability is one of the advantages of SOFCs, which makes SOFC popular as an energy conversion device. Although hydrogen is one of the most popular fuels for fuel cells, the production, storage, and transportation of hydrogen are still huge challenges that obstruct its further application [138]. Therefore, the feature of fuel adaptability can reduce the costs of SOFC in transportation applications. At the same time, SOFCs are free of contaminants in many fuel application scenarios. Some researches focus on the multi-fuel application in the SOFC system. The operating characteristics of ES-SOFC while changing the fuel among hydrogen, methane, and bioethanol were measured by Nobrega et al. [139]. Gupta et al. [140] presented a novel coke-resistant catalyst for a reformer supplied by methanol, gasoline, and diesel in the SOFC system. Teramoto et al. [141] experimentally investigated the methane-ammonia mixed fuel reforming performance of Ni-YSZ anode. As for MS-SOFC, the operating characteristics with methane, propane, ammonia, and dimethyl ether were tested by Welander et al. [142].

The relatively high operating temperature, typically ${500} -{700}^{ \circ }\mathrm{C}$, is the reason that confers MS-SOFC operability on a wide range of fuels. The materials of anode still require high electrocatalytic activity in the fuel oxidation along with adequate catalytic efficiency for diverse fuel reforming [10]. Additionally, the anode materials are supposed to be stable to avoid reacting with the interconnect or electrolyte at operating temperatures. Meanwhile, in the archetype of MS-SOFC, the electrolytes are YSZ [56] and GDC [143]. Taking these factors into consideration, Ni-YSZ is a widely used anode material of MS-SOFC [144]. At the same time, $\mathrm{{Ni}} -\mathrm{{GDC}}$ is also gaining attention because it can reduce the operating temperature and offers great physical and chemical properties [10]. Nickel at the anode can catalyze both electrochemical and reforming reactions, thus direct internal reforming (DIR) MS-SOFC is a hot research field. The reforming reaction occurs in the anode for a DIR-SOFC, while it takes place in a reformer for an external reforming (ER) SOFC. Researches about DIR-SOFCs involve a wide range of fuels, including hydrogen [145], hydrocarbon fuels [146, 147], oxygenated fuels [148-150], gasoline, diesel, and ammonia [151-154]. However, DIR has several challenges and an ER is necessary for the further development of the MS-SOFC powertrain system.

This section will focus on the reforming technologies. Section 4.1 introduces DIR technologies for hydrocarbon fuels, oxygenated fuels and ammonia and summaries its challenges. Section 4.2 summaries the ER technologies.

4.1 Direct Internal Reforming

The reaction principle of DIR in MS-SOFC is consistent with that in other types of SOFC. In a DIR-SOFC stack, the reforming reactions occur inside the anode producing hydrogen and carbon monoxide. Oxygen ions produced in the cathode travel through the electrolyte arriving at the TPB and oxidizing the hydrogen and carbon monoxide inside the anode. The electrochemical reactions inside the anode can be written as Eqs. (1)-(2) [155] :

(1)

$ {\mathrm{H}}_{2} + {\mathrm{O}}^{2 -} \rightarrow {\mathrm{H}}_{2}\mathrm{O} + 2{\mathrm{e}}^{ -} $

(2)

$ \mathrm{{CO}} + {\mathrm{O}}^{2 -} \rightarrow {\mathrm{{CO}}}_{2} + 2{\mathrm{e}}^{ -} $

Inside the cathode, ${\mathrm{O}}_{2}$ is reduced to oxygen ions, as shown in Eq. (3):

(3)

$ {\mathrm{O}}_{2} + 4{\mathrm{e}}^{ -} \rightarrow 2{\mathrm{O}}^{2 -} $

The reforming progress takes place inside the SOFC stack. Thus, the advantages of DIR-SOFC include a simplified system design and reducing the difficulty of thermal management because the heat generated by electrochemical reactions can be directly utilized for reforming reactions [156]. Hydrocarbon fuels, oxygenated fuels, and ammonia are the most commonly applied fuels in DIR-SOFC.

4.1.1 Internal Reforming of Hydrocarbon Fuels

SOFCs fueled by hydrocarbon fuels are anticipated to have greater commercial value because of the safety, convenience and economy of hydrocarbon fuels [157]. Methane is one of the most attractive hydrocarbon fuels because of its relatively simple manufacturing method of conversion from biomass and ${\mathrm{{CO}}}_{2}$. Moreover, methane can be obtained directly from other fuels such as natural gas and combustible ice. Methane reforming chemistry includes various reactions. Depending on the agent and reaction condition, it can be converted to ${\mathrm{H}}_{2}$ in the anode by steam reforming (SR), dry reforming (DR), partial oxidation (POX), or cracking [158]. Among these reforming processes, methane steam reforming (MSR) is the most extensively utilized and well-developed technique due to the similarity with industrial hydrogen production technique which produces about 75% of the ${\mathrm{H}}_{2}$ globally [146]. Therefore, most of the research used steam as the reforming agent. The MSR reaction is written as Eq. (4) [159]:

(4)

$ {\mathrm{{CH}}}_{4} + {\mathrm{H}}_{2}\mathrm{O} \rightarrow \mathrm{{CO}} + 3{\mathrm{H}}_{2} $

In addition to Eq. (2), the produced CO also reacts with water to produce ${\mathrm{{CO}}}_{2}$ and ${\mathrm{H}}_{2}$ through a reversible reaction which is called the water-gas shift reaction as shown in Eq. (5) [160]. This reaction is an important electrode chemical reaction, and it occurs in different reforming technologies of carbon-contained fuels.

(5)

$ \mathrm{{CO}} + {\mathrm{H}}_{2}\mathrm{O} \leftrightarrow {\mathrm{{CO}}}_{2} + {\mathrm{H}}_{2} $

In ${\mathrm{{CO}}}_{2}$ dry reforming,${\mathrm{{CO}}}_{2}$ takes the place of steam as an oxidant as shown in Eq. (6) [161]. Renewed interest in this technique appeared recently because SOFC could directly operate with biogas [162] which primarily comprises ${\mathrm{{CH}}}_{4}$ (50-70%) and ${\mathrm{{CO}}}_{2}$ (25-50%) accompanied by some minor components such as ${\mathrm{H}}_{2}\left({1 -5\% }\right)$ or ${\mathrm{N}}_{2}\left({{0.3} -3\% }\right)$ and impurities like ${\mathrm{{NH}}}_{3},{\mathrm{H}}_{2}\mathrm{\;S}$ or halides [163]. Besides, this method shows the potential of capturing and utilizing both ${\mathrm{{CO}}}_{2}$ and ${\mathrm{{CH}}}_{4}$. Nevertheless, there persist substantial challenges, including high energy demands and necessity for sources of ${\mathrm{{CO}}}_{2}$ with a high degree of purity.

(6)

$ {\mathrm{{CH}}}_{4} + {\mathrm{{CO}}}_{2} \rightarrow 2\mathrm{{CO}} + 2{\mathrm{H}}_{2} $

POX of methane generates heat and it is less applied in DIR-SOFC, as shown in Eq. (7) [164]. It is often combined with other methods and used in external reformers.

(7)

$ {\mathrm{{CH}}}_{4} + \frac{1}{2}{\mathrm{O}}_{2} \rightarrow \mathrm{{CO}} + 2{\mathrm{H}}_{2} $

For other hydrocarbon fuels, the reactions of steam reforming, dry reforming, and partial oxidation are written as Eqs. (8)-(10) [164] separately:

(8)

$ {\mathrm{C}}_{n}{\mathrm{H}}_{m} + n{\mathrm{H}}_{2}\mathrm{O} \rightarrow n\mathrm{{CO}} + \left({n + \frac{m}{2}}\right) {\mathrm{H}}_{2} $

(9)

$ {\mathrm{C}}_{n}{\mathrm{H}}_{m} + n{\mathrm{{CO}}}_{2} \rightarrow {2n}\mathrm{{CO}} + \frac{m}{2}{\mathrm{H}}_{2} $

(10)

$ {\mathrm{C}}_{n}{\mathrm{H}}_{m} + \frac{n}{2}{\mathrm{O}}_{2} \rightarrow n\mathrm{{CO}} + \frac{m}{2}{\mathrm{H}}_{2} $

Extensive researches focus on the methane-fueled DIR-SOFC, and the research on catalyst performance investigation, SOFC structure design, and operating parameters optimization have been jointly reviewed recently. Abdelkareem et al. [165] and Fan et al. [166] reviewed the technical challenges and application progress of DIR-SOFC fueled by biogas or syngas mainly composed of methane. The challenges are mainly thermal stress, carbon deposition, and sulfur poisoning which constrain the further development of DIR-SOFC. To decrease carbon deposition and enhance the stability and durability of internal reforming SOFC, a catalyst layer is added in some research to pre-reform the hydrocarbon fuels to ${\mathrm{H}}_{2}$ and $\mathrm{{CO}}$ before fuels encounter the anode material. This kind of SOFC is called gradual internal reforming SOFC, which is different from DIR and ER-SOFC. The catalyst layer should exhibit a pronounced catalytic activity toward hydrocarbon while maintaining exceptional stability under demanding conditions, and related studies were reviewed by Shi et al. [157]. However, an extra catalyst layer could decrease the electric efficiency of SOFC stacks. The technical challenges such as operational strategies and development of anode materials were reviewed by Fan et al. [158]. Other referable reviews of direct methane reforming in SOFC include overviews of mathematical models proposed by Faheem et al. [167], reviews of simulation and experimental studies of ${\mathrm{{CH}}}_{4}/{\mathrm{{CO}}}_{2}$ operated SOFC by Girona et al. [162], reviews of reaction principle of $\mathrm{{DR}}$ of ${\mathrm{{CH}}}_{4}$ and the application of perovskites catalyst by Wei et al. [168]. In addition, the DIR-SOFC using other hydrocarbon fuels, including propane [169] and liquid hydrocarbon, also attracts the attention of researchers. Regarding other hydrocarbon fuels, Sen-godan et al. [164] conducted a comprehensive examination of progress in the POX of hydrocarbons for hydrogen production and their applicability in fuel cell systems, while Cimenti et al. [170] reviewed the anode material development of the direct utilization of liquid fuels in SOFC for portable applications. The main application scenario of hydrocarbon-fueled DIR-SOFC is stationary power generation, while the application for mobile powertrain sources is rare.

4.1.2 Internal Reforming of Oxygenated Fuels

Except for hydrocarbon fuels, oxygenated fuels can also be reformed inside the SOFC stack. Among them, alcohols, especially methanol, and ethanol, have been extensively researched recently. Methanol is one of the optimal fuels because of its various features and properties. Firstly, methanol is the simplest oxygenated fuel, and its liquid state at ambient conditions brings convenience for its transportation and storage promoting its application in the portable SOFC system [171]. Besides, the application of methanol could reduce the demands for gas cleaning or pre-treatment because of the high degree of purity in commercial-grade methanol [172]. Meanwhile, methanol has a higher volumetric energy density than that of liquid hydrogen. The most attractive point is that methanol is a clean and renewable source. It can be produced from agricultural byproducts and biomass fermentation or gasification [173], and this makes it possible for SOFC systems to achieve carbon-neutral. Depending on the agent, methanol can be converted to ${\mathrm{H}}_{2}$ and ${\mathrm{{CO}}}_{2}$ via $\mathrm{{SR}}$ and $\mathrm{{POX}}$ just like methane, and the reforming reaction is written as Eqs. (11)-(12) respectively [174]. Methanol can also be thermally decomposed at the anode as shown in Eq. (13) [171].

(11)

$ {\mathrm{{CH}}}_{3}\mathrm{{OH}} + \frac{1}{2}{\mathrm{O}}_{2} \rightarrow {\mathrm{{CO}}}_{2} + 2{\mathrm{H}}_{2} $

(12)

$ {\mathrm{{CH}}}_{3}\mathrm{{OH}} + {\mathrm{H}}_{2}\mathrm{O} \rightarrow {\mathrm{{CO}}}_{2} + 3{\mathrm{H}}_{2} $

(13)

$ {\mathrm{{CH}}}_{3}\mathrm{{OH}} \rightarrow \mathrm{{CO}} + 2{\mathrm{H}}_{2} $

Ethanol is another popular fuel because it is also a renewable source for use in transportation sector. Biomass-derived ethanol (bio-ethanol) is produced mainly from the fermentation of saccharides [175]. The DIR reactions of ethanol can be divided into SR and POX, as shown in Eqs. (14)-(15) [174], and hydrogen production by SR of ethanol is an efficient energy production process [176].

(14)

$ {\mathrm{{CH}}}_{3}{\mathrm{{CH}}}_{2}\mathrm{{OH}} + 3{\mathrm{H}}_{2}\mathrm{O} \rightarrow 2{\mathrm{{CO}}}_{2} + 6{\mathrm{H}}_{2} $

(15)

$ {\mathrm{{CH}}}_{3}{\mathrm{{CH}}}_{2}\mathrm{{OH}} + \frac{3}{2}{\mathrm{O}}_{2} \rightarrow 2{\mathrm{{CO}}}_{2} + 3{\mathrm{H}}_{2} $

At present, researches are focusing on the application of methanol and ethanol in DIR-SOFCs. Sun et al. [177] reviewed the hydrogen production through methanol reforming for fuel cell applications and summarized different reforming processes and the selection of the catalyst to increase the conversion efficiency of methanol. Liu et al. [171] tested the performance of direct liquid methanol-fueled SOFCs experimentally. Shcheklein et al. [178] analysis of the energy efficiency of the installation with SOFC operating on the hydrogen-rich gas mixture derived from the catalytic process of methanol directly within a vehicle, and showed the possibility of using methanol for a vehicle powertrain with SOFCs. Besides, several modeling studies of methanol-fueled DIR-SOFC were carried out [179, 180], in which the multi-physics were simulated and the influence of operating parameters was examined. The difference in performance between methanol-fueled and ethanol-fueled DIR-SOFC is analyzed in a thermodynamic model by Leone et al., and the results were verified experimentally [149]. As for MS-SOFC, Dogdibegovic et al. [148] implemented an internal reforming experimental study of ethanol on the MS-SOFC for the application of vehicles. The MS-SOFC was optimized for high performance and extended operational longevity.

4.1.3 Internal Cracking of Ammonia

As a hydrogen carrier, ammonia is a promising fuel for SOFC. Firstly, ammonia has a relatively high hydrogen mass fraction of 17.7 wt% [181] and is a carbon-free fuel which differs from hydrocarbon or oxygenated fuels. Secondly, it has a high volumetric energy density. Besides, it is easy to be liquefied which contributes to transportation and storage. Meanwhile, ammonia ranks as the second most extensively produced chemical on a global scale making it relatively cheap. The infrastructures for the transportation and storage of ammonia have already been well established and distributed worldwide [182]. Finally, it is less flammable and its smell is worse than other fuels which makes it relatively safe because any leakage is difficult to burn and easy to detect [183]. The reaction of ammonia over the Ni-based anode at high temperatures is that it cracks into ${\mathrm{H}}_{2}$ and ${\mathrm{N}}_{2}$ directly according to Eq. (16). Thus, the design or simulation of direct ammonia SOFC is popular. There is no ${\mathrm{{NO}}}_{x}$ production, affirming that there is no risk of producing this undesired gas during operation [8].

(16)

$ {\mathrm{{NH}}}_{3} \rightarrow \frac{1}{2}{\mathrm{\;N}}_{2} + \frac{3}{2}{\mathrm{H}}_{2} $

Recently, scientific literature has shown a notable concentration of scholarly attention on SOFCs fueled by ammonia [184-186], including catalyst development, durability analysis, and numerical and experimental studies. Both stack and system investigations of direct ammonia SOFC were conducted to explore its application potential for power generation and mobile powertrain [183]. Performance comparisons of ammonia and other pure fuels or mixed fuels were also studied [141, 181]. Besides, the planar SOFC, tubular SOFC fed by ammonia was studied more than other fuels [187-189], but its vehicle use is restricted by small scale.

4.1.4 Challenges of DIR-SOFCs

For DIR-SOFC, the reforming reaction at the anode makes it face challenges of thermal stress, carbon deposition, poisoning due to impurities such as ${\mathrm{H}}_{2}\mathrm{\;S}$ and siloxanes, and nickel nitriding while using ammonia. Further application of DIR-SOFC is largely limited by these shortcomings.

Reforming reactions absorb heat, and electrochemical reactions release heat, resulting in severe thermal stress and even the destruction of the anode structure [190]. Therefore, for DIR-SOFCs, anode material with high stress resistance such as flexible material is studied [191], and effective methods to minimize thermal stress need to be introduced. For the latter, introducing oxidizing agents such as ${\mathrm{O}}_{2}$ is an efficacious approach for mitigating thermal stresses, which has been simulated and verified by experiments [165]. However, the additional air leads to the thorough oxidation of fuels and even the oxidation of the anode materials, which causes the drop in both efficiency and voltage output.

Carbon deposition of SOFC running on hydrocarbon or oxygenated fuels is another issue. Solid carbon intrusion may result in the degradation of the porous anode structure, obstructing the mass-transfer pathway and affecting reforming and electrochemical reactions negatively in anode. All these effects ultimately degrade the SOFC performance [192], especially for the conventional Ni-YSZ anode. Simultaneously, significant carbon deposition will cause stress damage to the cell’s structure, affect its durability, and even shorten its lifetime. Take hydrocarbon fuels as an example, the primary reactions contributing to carbon deposition are thermal cracking and reverse of boudouard reaction, as shown in Eqs. (17)-(18):

(17)

$ {\mathrm{C}}_{n}{\mathrm{H}}_{m} \rightarrow n\mathrm{C} + \frac{m}{2}{\mathrm{H}}_{2} $

(18)

$ 2\mathrm{{CO}} \rightarrow \mathrm{C} + {\mathrm{{CO}}}_{2} $

Steam has the capacity to remove deposited carbon and generate ${\mathrm{H}}_{2}$ and $\mathrm{{CO}}$, as shown in Eq. (19):

(19)

$ {\mathrm{H}}_{2}\mathrm{O} + \mathrm{C} \rightarrow \mathrm{{CO}} + {\mathrm{H}}_{2} $

From the view of thermodynamics, the mitigation of carbon formation can be achieved through the augmentation of the hydrogen and oxygen ratio within the fuel composition [193]. It is worth noting that methanol is less prone to cause severe coking thermodynamically compared to other hydrocarbons or ethanol, owing to the distinctive presence of the $\mathrm{C} -\mathrm{O}$ bond in methanol as opposed to the $\mathrm{C} -\mathrm{C}$ bond [179]. The deposition of carbon includes three stages which are dissociation, diffusion, and separation. For hydrogen fuels, the fuel molecules are adsorbed on the surface of $\mathrm{{Ni}}$ particles. Then, surface reactions occur producing chemisorbed carbon. Finally, through diffusion and deposition, carbon could grow in the shape of whisker carbon or encapsulating carbon. At present, there has been some research on the chemical formation mechanism of carbon deposition over the Ni-YSZ surface [194-196] and the in-situ or ex-situ observation of deposited carbon [197-199]. In 2023, through experiments combined with Focused ion beam-scanning electron microscope measurements [113], Shikazono et al. from the University of Tokyo found that serious carbon deposition occurs in the DIR of methane when the steam-to-carbon ratio is low. Carbon is first deposited on the surface of $\mathrm{{Ni}}$ and the metal dusting is formed. It was suggested that this change could be considered as a reversible process, because the microstructure was not strongly affected. However, as the carbon built up, there was irreversible damage. The carbon eroded and even crushed $\mathrm{{Ni}}$ causing some carbon depositing on the surface of YSZ.

The morphology of carbon generated inside the anode is related to the temperature [200]. The appearance of deposited carbon of MS-SOFC is different from other types of SOFC because of its characteristics of intermediate temperature $\left({{500} -{700}{}^{ \circ }\mathrm{C}}\right)$. For methane, the carbon fibers manifest a characteristic whisker-like carbon morphology at ${650}^{ \circ }\mathrm{C}$, while a significant quantity of encapsulating carbon was detected at ${800}^{ \circ }\mathrm{C}$. Therefore, the deposited carbon in MS-SOFC may mainly be whisker carbon which affects the anode in a different way from encapsulating carbon in the AS-SOFC or ES-SOFC, and the formation mechanism requires further investigation. It is also observed that the amount of carbon deposition under unsteady operating conditions is greater than that under steady operating conditions [201,202]. These studies indicate that the operating features of MS-SOFC exert a substantial impact on the carbon deposition process inside the anode. To develop MS-SOFC in the transportation field, methods such as air injection, steam injection, and catalyst development were proposed, which were summarized in reviews [158, 165].

Apart from thermal stress and carbon deposition, sulfur poisoning is another major challenge of DIR-SOFC, especially when using natural gas or biogas with ${\mathrm{H}}_{2}\mathrm{\;S}$ impurity. After the desulfurization process, there still remains about $1 -{10}\mathrm{{ppm}}$ of ${\mathrm{H}}_{2}\mathrm{\;S}$. Only a few ppm of ${\mathrm{H}}_{2}\mathrm{\;S}$, even less than ${10}\mathrm{{ppm}}$, may undergo chemisorption on the surface of Ni-based catalyst, occasionally leading to the formation of nickel sulfide which exhibits diminished catalytic activity [203]. Because of the reduction of the reforming activity, there is an augmented rate of oxidation of $\mathrm{{Ni}}$ at the anode-electrolyte interface. This phenomenon results in the expansion of the anode and subsequent detachment from the electrolyte [204]. The development of anode material or structure of DIR-SOFC is the main direction to solve problems [158]. As for MS-SOFCs, Welander et al. [205] tested the cell’s performance with composite Ni-SDC anode fed by reformed natural gas with $1\mathrm{{ppm}}$ and $5\mathrm{{ppm}}$ of sulfur which is consistent with pipeline natural gas. The results also demonstrated the utility of MS-SOFC using pipeline natural gas for applications requiring rapid start-up.

When a DIR-SOFC is operating with ammonia directly, the problem of nickel nitriding should be taken into consideration. Due to the reaction between $\mathrm{{Ni}}$ and ${\mathrm{{NH}}}_{3},\mathrm{{Ni}}$ nitrides are produced through Eq. (20) [206]. ${\mathrm{{Ni}}}_{3}\mathrm{\;N}$ is unstable at high temperatures and in a reducing environment, reactions of decomposition of ${\mathrm{{Ni}}}_{3}\mathrm{\;N}$ are written as Eqs. (21)-(22). There are local defects after ${\mathrm{{Ni}}}_{3}\mathrm{\;N}$ returns to $\mathrm{{Ni}}$, leading to the degradation of the cell [207]. Possible solutions include depositing a catalyst layer and introducing an external cracking reactor [185].

(20)

$ 6\mathrm{{Ni}} + 2{\mathrm{{NH}}}_{3} \rightarrow 2{\mathrm{{Ni}}}_{3}\mathrm{\;N} + 3{\mathrm{H}}_{2} $

(21)

$ 2{\mathrm{{Ni}}}_{3}\mathrm{\;N} \rightarrow 6\mathrm{{Ni}} + {\mathrm{N}}_{2} $

(22)

$ 2{\mathrm{{Ni}}}_{3}\mathrm{\;N} + 3{\mathrm{H}}_{2} \rightarrow 6\mathrm{{Ni}} + 2{\mathrm{{NH}}}_{3} $

To sum up, the direct utilization of different fuels results in various challenges impacting the durability and efficiency of SOFC. Although solutions are proposed from the aspects of materials, stack design, and operating parameters, problems can’t be eliminated. To promote the transportation application of MS-SOFC, the external reformer is indispensable.

4.2 External Reforming

In MS-SOFC systems for transportation, hydrogen is predominantly generated in a device called external reformer or pre-reformer. The catalytic reforming process is executed prior to introducing the gas to the SOFC stack. Although the addition of reformers increases system complexity and may reduce system efficiency, it is able to mitigate problems of thermal stress, carbon deposition, and nickel nitriding. According to the reforming pathway, reformers can be mainly classified into the steam reformer and POX reformer for hydrocarbon and oxygenated fuels, cracking reactor for ammonia, and autother-mal reformer (ATR). The reaction process of SR, POX, and thermal cracking is shown in Sect. 2.1. In an ATR for hydrocarbon and oxygenated fuels, SR reaction and POX reaction occur in a fixed proportion to achieve thermos-neutral, while oxidation and cracking reaction are combined for ammonia. Because of the characteristic of nearly zero enthalpies of the reaction, this kind of reformer requires no external heat source. The autothermal reforming reactions of methane, methano, and ammonia are shown in Eqs. (23)-(25) [151, 208]. Aqueous-phase reforming is another reforming process for oxygenated fuels to minimize the disposal of organic water, and the reaction of methanol is written as Eq. (26) [209].

(23)

$ {\mathrm{{CH}}}_{4} + \frac{1}{2}{\mathrm{H}}_{2}\mathrm{O} + \frac{1}{4}{\mathrm{O}}_{2} \rightarrow \frac{5}{2}{\mathrm{H}}_{2} + \mathrm{{CO}} $

(24)

$ {\mathrm{{CH}}}_{3}\mathrm{{OH}} + \frac{4}{5}{\mathrm{H}}_{2}\mathrm{O} + \frac{1}{10}{\mathrm{O}}_{2} \rightarrow {\mathrm{{CO}}}_{2} + \frac{14}{5}{\mathrm{H}}_{2} $

(25)

$ {\mathrm{{NH}}}_{3} + \frac{19}{200}{\mathrm{O}}_{2} \rightarrow \frac{1}{2}{\mathrm{\;N}}_{2} + \frac{131}{100}{\mathrm{H}}_{2} + \frac{19}{100}{\mathrm{H}}_{2}\mathrm{O} $

(26)

$\mathrm{CH}_{3} \mathrm{OH} \stackrel{\mathrm{H}_{2} \mathrm{O}}{\longleftrightarrow} \mathrm{CO}+2 \mathrm{H}_{2}$

The reforming subsystem consists of external reformers and SOFC stacks, and it exerts a significant influence on system reliability and performance. Specifically, the external reformer requires careful design, of which the operating parameters should be optimized as well to better distribute the proportion of internal and external reforming reactions. To improve the catalytic activity and resistance to carbon deposition and impurities poisoning of catalysts in the reformer, several researches focused on the catalysts development which has been fully reviewed. To match the MS-SOFC stacks, the start-up time of the reforming subsystem should be reduced and the load-following capacity requires improvement. Additional heat sources of reformers need to be avoided to simplify the system and energy management strategies. Therefore, the design of ATR which could achieve thermally self-sustaining operation attracted attention. The numerical models of ATRs for methane [210],methanol [211],and ammonia [181],and experiments of ATRs for propane [212] and methanol [213] were introduced. Heat balance can be achieved by selecting catalysts, for example, the heat released by a series of oxidation reactions of copper with oxygen and methanol can be used as the heat source of reforming reactions of methanol, and the cold start-up is realized [214]. However, these designs of external reformers targeted other types of SOFC, and dynamic characteristics of integration with MS-SOFC stacks need further study.

Biert et al. [215] assessed the performance of the reforming subsystem, contrasting combinations of either isothermal or adiabatic pre-reforming with water or anode off-gas recirculation, and there were certain differences in terms of efficiency. Bae et al. [22] investigated the rapid start-up strategy of a $1\mathrm{\;{kW}}$ diesel reformer integrated with MS-SOFC for transportation applications. The start-up time of the reformer was reduced from 22 to 9 min while utilizing the heat of the stack. Operating parameters of the reforming subsystem, such as fuel types and reforming pathways, inlet temperature and pressure of reformers or stacks, operating temperature, etc., also make much difference to the whole system. Ma et al. [12] developed an MS-SOFC power system for vehicles with bio-ethanol external reforming. Four different ethanol reforming processes were simulated and contrasted via the reformer and SOFC stack models, and the autothermal ethanol reforming showed a peak stack efficiency of ${50}\%$. A subsequent numerical analysis probed the impact of 4 reformed fuels on the temperature distribution of a counterflow tubular SOFC. The temperature gradient within the fuel channel reaches minimum when methanol is applied [216]. To meet the demands of the quick start-up, high integration level, and inhibition of thermal cracking and carbon deposition, the reforming subsystem needs further study.

4.3 Summary

The feature of fuel adaptability improves the potentiality of the transportation application of MS-SOFC. Table 2 summarizes the challenges for each reforming technology route with different fuels. Reforming reactions occur both in cells, stacks and the reformer. Benefiting from simple system configuration and high overall efficiency, DIR is studied in great demand. However, because of challenges such as thermal stress and carbon deposition, external reformers are necessary for mobility powertrain systems to improve the performance and reliability of the system. One of the most attractive points of external reformers is the ability of multi-fuel adaptability. Thus, to promote the application of MS-SOFC, further research on external reformers, stacks, system integration and operating conditions of the reforming subsystem are needed. Moreover, because of the relatively lower operating temperature and faster start-stop characteristics of MS-SOFC, the reforming process also faces challenges including new carbon deposition characteristics that require further study.

5 Stacks and System Configuration for Commercial Applications

Less

For commercial applications in the transportation sector, fuel cells are supposed to be integrated as stacks and the utility-oriented power density should be based on volume or mass rather than area. Despite the button or single cell showing great performance of MPD, the stack’s MPD may drop down a lot because of the inhomogeneity of current density, heat transfer, or mass transfer. So as to the durability and degradation processes. Section 5.1 will mainly focus on the stack design of MS-SOFCs. Meanwhile, the weight, volume and complexity of MS-SOFC system should be considered for the mobile application. In fact, there is less difference between MS-SOFC and AS-SOFC at the system level. External reformer, burner, heat exchanger and blower are necessary. Thus, as there are few relevant studies of MS-SOFC at the system level, Sect. 5.2 will introduce system configuration for all SOFC.

5.1 Commercial Stacks

In 2017, GE-Fuel Cells (GEFC) developed a large-scale MS-SOFC stack [217]. The stack showed a power density of ${0.2}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at high efficiency. With 1708-inch cells, GEFC produced ${14}\mathrm{\;{kW}}$ stacks and assembled 4 stacks into a ${50}\mathrm{\;{kW}}$ system. In 2017, Minh [218] proposed an advanced MS-SOFC stack. Via designing an egg-carton shaped interconnect with fuel and airflow fields arranged on both sides, as shown in Fig. 7,a reduction in the weight and volume resulted in an enhanced power density. Bae et al. [219, 220] studied MS-SOFC stacks with numerical methods. Because of the existence of the bonding layer, the mass transfer of MS-SOFC performed worse than that of AS-SOFC, causing 17% lower current density, which stressed the significance of channel designs. To increase the current density and decline the pressure loss, a dual-flow hybrid design was presented which showed a better comprehensive performance of 2.91 $\mathrm{A}/{\mathrm{{cm}}}^{2}$ with 750 Pa. In this dual-flow hybrid design, the ${\mathrm{H}}_{2}$ channel used a serpentine design for the highest current density, and the air channel used a parallel design to decline the pressure drop.

Owning to greater design and manufacturing capability, some companies have made remarkable progress in improving the durability of MS-SOFC stacks, which is key to practical applications. Ceres Power and Plansee SE are the two most promising companies in the field of MS-SOFC, each representing a different MS-SOFC type in terms of the morphology of the metal substrate [221]. Ceres Power developed laser drilled substrate and Plansee SE developed a sintered porous substrate. Meanwhile, because the laser-drilled holes are straight-through, the mass transfer process could perform better with lower porosity and higher mechanical strength.

Founded in 2001, Ceres Power developed its primitive version MS-SOFC cell based on fundamental research done by Imperial College London, with LSCF cathode, CGO/ GDC electrolyte, and Ni anode deposited top-down on stainless steel support [221, 223]. It is implied that MS-SOFC stacks could be compact by applying Ceres Power’s design of which the structure could be seen in Fig. 8. In 2005, Ceres Power’s cell could achieve ${0.25} -{0.35}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${550} -{600}{}^{ \circ }\mathrm{C}$, which predicted a mean power density of ${0.148}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ for a ${2.5}\mathrm{\;{kW}}$ stack [224]. In 2015, Ceres Power achieved a power-weight ratio of ${0.12}\mathrm{\;{kW}}/\mathrm{{kg}}$ for a repeating unit of a stack [222]. In 2017, Ceres Power published the technical parameters of a $1\mathrm{\;{kW}}$ stack with a stack power density of ${0.20}\mathrm{\;{kW}}/\mathrm{L}$ and ${0.091}\mathrm{\;{kW}}/\mathrm{{kg}}$. In 2018, with the aim of an electric vehicle range extender (EVRE), Ceres Power started developing a compact 5kW stack [225],which needs electrical efficiency, power density, and start-up time to reach ${50}\%,{0.4}\mathrm{\;{kW}}/\mathrm{L}$, and below 15 min, respectively estimated by Ceres Power. By the end of 2019, Ceres Power has achieved ${0.342}\mathrm{\;{kW}}/\mathrm{L}$ for its 5kW stack [226]. After assembling six 5kW stacks into a 30kW system, Ceres Power and Weichai demonstrated that MS-SOFC, as an EVRE, was commercially viable [130, 227]. In 2021, Ceres Power reported its ${4.2}\mathrm{\;{kW}}$ system had been put on sale in Japan, as the first-time commercialization of its MS-SOFCs [129].

Low operating temperature $\left({{550} -{600}{}^{ \circ }\mathrm{C}}\right)$ improved stack durability, as no significant performance degradation was observed after the ${2000}\mathrm{\;h}$ constant operation and there was only lower than 3% performance loss during 25 thermal cycles and 7 redox cycles. While the overall configuration of Ceres Power’s design had not changed much since then, iterations saw significant improvement concerning the durability of both steady-state operation and cycling. The newest version stacks achieved $< {0.2}\% /\mathrm{{kh}}$ degradation rate at 0.133 or ${0.166}\mathrm{\;A}/{\mathrm{{cm}}}^{2},{610}^{ \circ }\mathrm{C}$, and was nearly intact to thermal and redox cycles. The longest-running stack had a lifetime of more than 22000h [129, 130]. However, details of cell and stack fabrication and specific degradation mechanisms concerning the unique cell configuration as well as modifications done to mitigate them were merely mentioned in Ceres Power’s technical reports.

Plansee SE in close cooperation with FZJ, GmbH, and AVL List had developed AS-SOFCs with outstanding long-term durability. Had constantly operated for more than ${10},{000}\mathrm{\;h}$ [228]. Plansee SE also developed its own MS-SOFC design, as shown in Fig. 9. Plansee SE’s cell fabrication process did not involve infiltration procedures. Instead, both cathode and anode catalysts were ex-situ sintered with the rest of the cell in a reductive Ar atmosphere at ${950}^{ \circ }\mathrm{C}$ [229]. Plansee SE’s Gen 2 and Gen 3 MS-SOFC cells were comprised of the LSC cathode, YSZ electrolyte, multi-layer anode (Ni-GDC active layer and Ni-YSZ supported layer), porous Fe-Cr substrate, and two GDC DBLs at metal substrate/anode interface and electrolyte/cathode interface, respectively. The company argued that the cathode fitting of co-sintering had the advantage over infiltrated cathodes or cathodes that were in situ activated during the first start-up of the cell in terms of stability, as complete sintering enhanced adherence between active catalysts and the electrolyte. Stronger adherence might also slow down coarsening of both cathode and anode nanostructure, as argued by Thaler. Early-stage durability test of Gen 2 cells did not cause evident degradation after 1500-h continuous operation in dry hydrogen fuel and 292-h operation in humidified fuel at ${700}{}^{ \circ }\mathrm{C}$ and ${0.3}\mathrm{\;A}/{\mathrm{{cm}}}^{2}$ [229,230]. Post-mortem examination of the evaluated cell showed slight oxidation in substrate and anode, without any sign of coarsening or element interdiffusion. In Plansee SE’s recent report, Gen 3 MS-SOFC cells with thin-film electrolyte and thickness-increased anode achieved a remarkably high current density of ${2.8}\mathrm{\;A}/{\mathrm{{cm}}}^{2}$ at ${650}^{ \circ }\mathrm{C}$ and ${0.7}\mathrm{\;V}$. However, the durability of the newest cell, especially durability at its peak power output—had not been tested yet [11].

In 2012, researchers from Topsoe Fuel Cell A/S produced a planar MS-SOFC with advanced performance [231]. Operating at ${680}^{ \circ }\mathrm{C}$, the 25-cell MS-SOFC stack achieved MPD of ${0.275}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$. In 2018, DTU reduced the thickness of the metal substrate gradually from 313 to ${108\mu }\mathrm{m}$ [23] resulting in a dense of the metal substrate, which caused a decrease in performance. To avoid this, researchers endeavored to introduce gas channels and to modify the co-sintering profile, making the cell achieve a power density of ${1.01}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ and fuel utilization of ${48}\%$ at ${700}{}^{ \circ }\mathrm{C}$ and ${0.7}\mathrm{\;V}$ with a substrate thickness of ${243\mu }\mathrm{m}$. In 2022, DTU developed the monolithic fuel cell stack [232]. Compared with the advanced commercial AS-SOFCs, the single repeat unit showed equal performance with MPD over ${0.46}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${780}^{ \circ }\mathrm{C}$, while the stack was much more compact with a power density of ${5.6}\mathrm{\;{kW}}/\mathrm{L}$, which exhibited promise for mobile applications. In 2017, DLR compared the MS-SOFC and the commercial AS-SOFC at 750 °C [233]. Despite the MS-SOFC’s power density achieving ${0.448}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$, it was much lower than AS-SOFC.

Despite the performance of MS-SOFC stacks is not sufficient enough for the main power source of mobile applications, it reaches the requirement of an APU for mobile applications [234]. In 2012, cooperated with KIT, FZJ, and AVL List GmbH, Plansee SE established a prototype of a $3\mathrm{\;{kW}}$ SOFC APU system with an efficiency of ${35}\%,{75}\mathrm{\;L}$, and ${60}\mathrm{\;{kg}}$ [235]. In 2018, Bae [236] studied a 3-cell stack of MS-SOFC for APU applications. By a sinter-joining fabricating method, the $5 \times 5{\mathrm{\;{cm}}}^{2}$ single cell achieved MPD of ${0.433}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ at ${800}^{ \circ }\mathrm{C}$, while the ${12} \times 8{\mathrm{\;{cm}}}^{2}3$ -cell stack achieved MPD of ${0.1}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ and a maximum power of 23.1 W. The power density was much lower than that in their previous work. The key point could be the consistency of manufacture when scaling up the cell size from the button cell to the single cell, even to the stack. In 2019, S. Hashimoto et al. [237, 238] tried to re-design MS-SOFC stacks as APU for the airplane system. The main modification was increasing the operating current density from 0.2 to ${0.5}\mathrm{\;A}/$ cm ${}^{2}$ and using new materials.

Based on the above-mentioned progresses, though the overall commercialization of the MS-SOFC concept is less established compared to AS-SOFC companies like Bloom Energy or Ene-Farm, the former products’ lifetime and resistance to thermal/redox cycling had a certain potential to match the latter.

5.2 System Configuration

Hybrid systems composed of the SOFC and gas turbine (GT) or ICE can facilitate a more resilient and adaptable power generation system paving the way for renewable and sustainable mobility. SOFC-GT systems are widely used in distributed generation and aviation, while the stationary power generation and vehicular utilization of SOFC-ICE systems have been extensively studied. In a SOFC-GT system, the high temperature working requirements of SOFC can be satisfied by GT. The SOFC-GT hybrid system is regarded as one of the foremost technologies to address the imperative for high efficiency and reduced emissions [239]. It can also be utilized as the auxiliary power unit (APU) in large aircraft. Compared with the conventional aircraft APU, which consists of a diminutive gas turbine intended to cater to segments of the aircraft’s electrical and pneumatic demands [240],SOFC-GT APU can produce high electrical power and therefore significantly improve efficiency and save fuel. With desulphurization and external reforming, SOFC-GT system permits the employment of aviation fuel. However, apprehensions regarding the incremental weight of the system persist, including the reformer, compressor, heat exchanger, and other additional components [241]. In several researches on system design [241-243],external reformers are included to avoid carbon deposition and meet the high-reliability requirements of aviation power sources. Anode exhaust gas recirculation is a common strategy [242].

ICEs have been vastly employed for global transport with mature production technology and supply chain, which is conducive to the promotion of the application of SOFC-ICE. After the SOFC stack heats up to its operating temperature, the ICE could turn to rich combustion as a partial oxidation reformer while also functioning as a heater and compressor [244]. SOFC-ICE systems fed by various fuels for ships [245],vehicles [246],and unmanned aerial vehicles [247,248] have been simulated or actually tested. In typical configurations, the system integrates SOFC, reformer, heat exchanger and engine, etc., and the reformer is placed behind the engine to utilize both the exhaust gas and fresh fuels. Other configurations are also proposed, for instance, Lee et al. [249] proposed a double-reformer SOFC-ICE configuration where two reformers were fed by exhaust gas of anode and ICE respectively and had different heat sources, possessing a total system power of 5kW, and achieving an electrical efficiency of ${50.6}\%$.

With several attractive advantages, MS-SOFCs can act as the power source for vehicles without GT or ICE. It can play the role of range extender of battery-powered electric vehicles. Ceres Power proposed MS-SOFC with compact structures and high mechanical robustness, which is more suitable for vehicle application [25]. In June 2016, Plansee SE pioneered the world’s first SOFC-powered vehicle in collaboration with AVL and Nissan, using an MS-SOFC stack and fueled ${100}\%$ by ethanol [24]. The system configuration is shown in Fig. 10. The employed 5kW MS-SOFC stack has a start-up time of $9\mathrm{\;{min}}$ outpacing other SOFC applications. It has a specific power of ${0.0385}\mathrm{\;{kW}}/\mathrm{{kg}}$. Syngas is produced in the external reformer for SOFC, and the residual syngas undergoes combustion in the afterburner with heat recovered for reform, which promotes reducing start-up time [24].

6 Summary and Future Prospects

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In this review, the application of MS-SOFC in the transportation field is summarized. Due to the intermediate operating temperature, MS-SOFC could realize high power density, fast start-up, and high durability at the same time. Therefore, MS-SOFCs exhibit significant applicative promise in the transportation field. However, MS-SOFCs still face many challenges in power density, start-up time, durability and fuel reforming.

In terms of materials, the 400-series stainless steel is the best candidate for substrate because of well CTE matching with electrolyte materials. Due to the use of the metal substrate, the working temperature should be lowered which drives researchers to explore new materials for electrolyte, anode, and cathode. For electrolytes, GDC shows higher ionic conductivity than YSZ under medium temperature but suffers the issue of electron conduction. For the anode, the mostly used materials are nickel and the ceramic material contingent upon electrolyte’s compositions. For the cathode, LSCF is widely used and there are many novel materials under research. In recent years, increasing the activity of electrodes is an effective way of improving performance, such as the infiltration and exsolution methods. In terms of structures, numerous novel structures of MS-SOFC have been proposed such as symmetric, micro-structured honeycomb, tubular and micro-tubular structures. Despite the amount of research that has been done with these novel MS-SOFCs, the planar structure shows the greatest potential in the automotive field. At present, the state-of-art MS-SOFC with button size could achieve MPD of ${0.5} -{3.0}\mathrm{\;W}/{\mathrm{{cm}}}^{2}$ under ${600} -{800}^{ \circ }\mathrm{C}$.

In terms of durability, the intrinsic degradation factors are related to cell materials, fabrication methods and configuration. In addition, the cyclic degradation induced by rapid start-stop in transportation scenarios is another central factor that influences the durability of MS-SOFC. Although MS-SOFC has shown good durability in fixed application scenarios, it still cannot meet the needs of rapid and multiple start-stop in transportation scenarios. The matching of material CTE between different layers and the thermal stress reduction method based on the optimized design of the high-power stack structure still needs further research.

The advantage of fuel adaptability improves the potential of MS-SOFCs for mobile applications. Using direct internal reformers, the system configuration is simpler and system efficiency is higher. However, because of the challenges of thermal stress, carbon deposition, sulfur poisoning, and nickel nitriding, external reformers are necessary for mobility powertrain systems to gain better performance and reliability. To promote the transportation application of MS-SOFCs, studies of external reformers and the integration and operating conditions of the reforming subsystem are required. In order to improve the fuel adaptability, reformers for multi-fuel need to be further investigated. Moreover, as carbon neutrality continues to advance, fuel choices need to be targeted. Hydrogen, methanol, ammonia and sustainable aviation fuel are of great potential. A key constraint to the application of MS-SOFC in the transportation sector is the volume of the whole system. The future goal is to carry out a compact design of the system including cell stacks, reformer, burner, heat exchanger, blower and controller.

Although the operating temperature is lower than AS-SOFC, the operating temperature of MS-SOFC is still over ${500}^{ \circ }\mathrm{C}$, which limits its application in the field of transportation. Although the durability under thermal cycles does not yet meet commercial needs, there is an approach to mitigating the problem. As the system needs to be arranged in the hot box, users will not have to start the system from zero if the hot box insulation performance is good enough. The better insulation performance also helps reduce heat transfer loss and improve temperature uniformity. The disadvantage is that the size and weight of the system will increase, which is the second challenge of MS-SOFC applications. This paper mainly focuses on the power density of cells and stacks rather than systems, but the compact design of the whole system plays an important role in the commercialization. However, there is still a lack of relevant research. From the review above, it seems that methods improving power density and durability have a trade-off relationship. As SOFCs have been applied as range extenders for vehicles, which shows that the initial conditions for commercial application have been met. Therefore, the durability under a steady state is even more important for commercial applications. Both the power density and durability should be considered together. As the material has been basically constructed, using the existing materials to design cells and optimize preparation processes would be an ideal choice. The key is to fabricate cells with thin and dense electrolyte, highly active electrodes. The barrier layer preventing element diffusion and electron conduction is necessary and the preparation process should be stabilized for mass production.

In addition, the current testing standards for MS-SOFC in terms of performance and durability still need to be unified, and there are still obstacles in the comparison of different research results. Moreover, most of the research focused on button cells or small-sized single cells, the research on high-power stacks in transportation applications still needs to be further improved. Finally, research on MS-SOFC experimental testing for transportation scenarios still needs to be carried out. For example, the seismic performance of the cells has a great impact on their application in the field of transportation. Although it can be predicted that the seismic performance of MS-SOFC will be fine, it still needs experimental verification.

Acknowledgements This research is supported by National Key R&D Program of China (Grant No. 2021YFB2500404), Natural Science Foundation of China (Grant No. 51976100), China Postdoctoral Science Foundation (Grant No. 2023TQ0170) and Shuimu Tsinghua Scholar Program.

Declarations

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Conflict of interest On behalf of all the authors, the corresponding author states that there is no conflict of interest.

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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doi: 10.1007/s42154-024-00316-w

Receive Date：2023-11-04
Online Date：2025-07-21

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Received：2023-11-04
Revised：2024-07-11
Accepted：2024-07-11

Affiliations

¹ Tsinghua University State Key Laboratory of Intelligent Green Vehicle and Mobility, School of Vehicle and Mobility Beijing 100084 China

² Tsinghua University Institute for Aero Engine Beijing 100084 China

Corresponding:

Xiao Ma max@tsinghua.edu.cn

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Table 1 Summary of candidate support metals and ceramic materials

Materials	CTE (ppm/K)	Cost ($/kg)	References
$\mathrm{{Ni}}$	16.5 @ 435 °C	~31	[29]
$\mathrm{{Ni} -{Fealloy}}$	13.7	9	[13]
$\mathrm{{Ni}} -\mathrm{{Cr}} -\mathrm{{Fe}}$	15-16	63	[29]
YSZ	~10	~105	[29]
GDC	$\sim {11}$ @ ${500}{}^{ \circ }\mathrm{C}$	$\sim {197}$	[32]
LSM	~11.3	~131	[33]
Hastelloy-X	15.5-16	22	[29]
300-series stainless steel	18-20	2	[29]
400-series stainless steel	10-12	2	[34]

Table 2 Summary of reforming technology route and fuel adaptability

Technology		Hydrocarbon fuels	Oxygenated fuels	Ammonia
DIR	SR	Carbon deposition	Little carbon deposition	$\mathrm{{Ni}}$ may be
	DR	Severe carbon deposition	Carbon deposition	damaged
	POX	Oxygen may react with Ni	Oxygen may react with Ni	by nitrid-ing
ER	SR	System may be complicated	System may be complicated	Ammonia may cor-rode the reformer

Fig. 1 Compilations of fuel cell technologies [2,3]

Fig. 2 SOFCs with different supported layers [13]

Fig. 3 Conductivities of electrolyte materials

Fig. 4 The symmetric structure of MS-SOFC [81]

Fig. 5 Average cell voltage under load for a Ceres Power $1\mathrm{\;{kW}}$ e-class stack running continuous thermal cycles [25]

Fig. 6 The factors of durability and degradation. a Microstructure of anodes with carbon deposition reconstructed by FIB-SEM [113] . b Mechanism of sulfur poisoning inside anodes [132] . c Ni coarsening and depletion inside anodes [133] . d SEM micrograph and line scan of an anode coating deposited on metal support without the addition of barrier layer at ${850}^{ \circ }\mathrm{C}$ for ${720}\mathrm{\;h}$ [134] . e Segregation and reaction on the interface of LSCF or LSM cathode and YSZ electrolyte [135] . f Mechanism of chromium poisoning inside cathodes [107] . g Mechanism of sulfur poisoning inside LSCF or LSM cathodes [136] . h Cracks of electrolyte due to reduction and reoxidation cycles [137]

Fig. 7 An example of prime-surface interconnect design and the stack design concept [218]

Fig. 8 Structure of laser-drilled MS-SOFC [222]

Fig. 9 The structure of Plansee SE’s MS-SOFC with multi-layer electrolytes [11]

Fig. 10 System configuration of the Nissan SOFC vehicle [24]

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