Bi/Bi2O3 nanoparticles supported on N-doped reduced graphene oxide for highly efficient CO2 electroreduction to formate

Bi/Bi₂O₃ nanoparticles supported on N-doped reduced graphene oxide for highly efficient CO₂ electroreduction to formate

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Junjie Sun^a^,^b^,¹, Wanzhen Zheng^a^,^b^,¹, Siliu Lyu^b, Feng He^*^,^a, Bin Yang^b, Zhongjian Li^b, Lecheng Lei^b, Yang Hou^*^,^b^,^c

Chinese Chemical Letters | 2020, 31(6) : 1415 - 1421

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Chinese Chemical Letters | 2020, 31(6): 1415-1421

• Communication •

Bi/Bi₂O₃ nanoparticles supported on N-doped reduced graphene oxide for highly efficient CO₂ electroreduction to formate

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Junjie Sun^a^,^b^,¹, Wanzhen Zheng^a^,^b^,¹, Siliu Lyu^b, Feng He^*^,^a, Bin Yang^b, Zhongjian Li^b, Lecheng Lei^b, Yang Hou^*^,^b^,^c

Affiliations

^a College of Environment, Zhejiang University of Technology, Hangzhou 310014, China

^b Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China

^c Ningbo Research Institute, Zhejiang University, Ningbo 315100, China

Published: 2020-06-15 doi: 10.1016/j.cclet.2020.04.031

Outline

Abstract

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Electrocatalytic CO₂ reduction (CO₂ER) into formate is a desirable route to achieve efficient transformation of CO₂ to value-added chemicals, however, it still suffers from limited catalytic activity and poor selectivity. Herein, we develop a hybrid electrocatalyst composed of bismuth and bismuth oxide nanoparticles (NPs) supported on nitrogen-doped reduced graphene oxide (Bi/Bi₂O₃/NrGO) nanosheets prepared by a combined hydrothermal with calcination treatment. Thanks to the combination of undercoordinated sites and strong synergistic effect between Bi and Bi₂O₃, Bi/Bi₂O₃/NrGO-700 hybrid displays a promoted CO₂ER catalytic performance and selectivity for formate production, as featured by a small onset potential of -0.5 V, a high current density of -18 mA/cm², the maximum Faradaic efficiency of 85% at -0.9 V, and a low Tafel slope of 166 mV/dec. Experimental results reveal that the higher CO₂ER performance of Bi/Bi₂O₃/NrGO-700 than that of Bi NPs supported on NrGO (Bi/NrGO) can be due to the partial reduction of Bi₂O₃ NPs into Bi, which significantly increases undercoordinated active sites on Bi NPs surface, thus boosting its CO₂ER performance. Furthermore, a two-electrode device with Ir/C anode and Bi/Bi₂O₃/NrGO-700 cathode could be integrated with two alkaline batteries or a planar solar cell to achieve highly active water splitting and CO₂ER.

Key words

NrGO / Bi/Bi₂O₃ / CO₂ electroreduction / Formate / Hybrid catalysts

Cite this Article

Junjie Sun, Wanzhen Zheng, Siliu Lyu, Feng He, Bin Yang, Zhongjian Li, Lecheng Lei, Yang Hou. Bi/Bi₂O₃ nanoparticles supported on N-doped reduced graphene oxide for highly efficient CO₂ electroreduction to formate[J]. Chinese Chemical Letters, 2020 , 31 (6) : 1415 -1421 . DOI: 10.1016/j.cclet.2020.04.031

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Emissions of carbon dioxide (CO₂) from fossil fuel combustion during industrial processes are the main cause of global warming. In this regard, electrochemical CO₂ reduction to generate useful chemical fuels represents a significant way to decrease its atmospheric concentration [1]. However, it still suffers from the challenges of large catalytic overpotential required to activate CO₂ molecule, and low catalytic selectivity owing to the occurrence of competitive hydrogen evolution reaction (HER) in electrolyte [2].

In recent years, a wide variety of metal based catalysts (such as Ni [1f], Sn [3], In [4] and Cu [5]) have been explored to promote electrocatalytic CO₂ reduction reaction due to their superior catalytic performance and robust stability. However, the reduced products of electrocatalytic CO₂ reduction (CO₂ER) for above mentioned metal based materials are quite different. The noble metals and Ni based catalysts can usually selectively reduce the CO₂ into producing CO product [6], whereas the Sn and In based catalysts exhibit excellent catalytic selectivity for formate production [3, 4], while Cu based catalysts could achieve the transformation of CO₂ into producing hydrocarbon and alcohol [5]. Among all the reduced products from CO₂ER process, formate is easier to be yielded as liquid-phase product as compared to alcohol, since its lower theoretic potential and less reaction steps of proton coupled electron transfer process [7].

Recently, metal/metal oxide derived electrocatalysts have been displayed to boost catalytic activity for CO₂ER with respect to their fully reduced counterparts [1a, 8]. For instance, a Sn/SnO_x electrode exhibited greatly enhanced CO₂ER activity relative to a typical Sn electrode [8c]. It was reported that partial oxidation of the cobalt atomic could effectively increase their catalytic activity towards formate production relative to typical cobalt atomic layers [8a]. Obviously, the combination of undercoordinated sites from partial reduction of metal oxide, and the synergetic effect between metal oxide and metal can play an important role in boosting CO₂ER selectivity [8b]. Bismuth (Bi) composites have been widely reported to deliver high selectivity and catalytic activity for CO₂ER, crediting their strong adsorption of CO₂^·- intermediates [9]. However, some key problems still remain, such as low current density and poor catalytic stability [9c, 10]. It is thus quite vital to further probe into the feasibility of preparing Bi based catalysts with good catalytic activity and selectivity.

Herein, we developed a compound CO₂ER hybrid catalyst with Bi/Bi₂O₃ nanoparticles (NPs) supported on the surface of nitrogen-doped reduced graphene oxide (NrGO) via a combined hydrothermal treatment with calcination process to partially reduce Bi₂O₃ into Bi. The Bi/Bi₂O₃ NPs with average diameter of ~20 nm was supported on NrGO nanosheets, which was denoted as Bi/Bi₂O₃/NrGO. Owing to the strong synergistic effect between Bi and Bi₂O₃, as-prepared Bi/Bi₂O₃/NrGO-700 hybrid catalyst displayed an impressive CO₂ER activity and selectivity for formate production, featured by a Faradaic efficiency (F.E.) of 85%, a high electrolytic current density of -18 mA/cm² with an applied potential of -0.9 V, a small onset potential of -0.5 V, as well as a low Tafel slope of 166 mV/dec. Additionally, the higher catalytic activity of Bi/Bi₂O₃/NrGO-700 for CO₂ER than that of Bi/NrGO demonstrated that the bismuth-oxide-derived catalyst possessed a superior catalytic performance, which was attributed to the fact that partial reduction of Bi₂O₃ into Bi could increase the number of undercoordinated active Bi sites, thus resulting in increased CO₂ adsorption and accelerated electron transfer processes, highly boosting CO₂ER activity. We further combined the Bi/Bi₂O₃/NrGO-700 CO₂ER catalyst with an oxygen evolution reaction (OER) catalyst of commercial Ir/C as a two-electrode system to realize highly active CO₂ER and water splitting together when powered by planar silicon solar cell or two AA-size alkaline batteries.

The fabrication of Bi/Bi₂O₃/NrGO hybrid was depicted in Fig. 1a. The mixture precursor was prepared by mixing GO solution, dicyandiamide (DCDA), and Bi(NO₃)₃·5H₂O, and the obtained mixture was hydrothermally treated at 180 ℃ for 12 h, which resulted in the formation of the GO based precursor of (Bi₂O₂(OH)(NO₃)/Bi₂O₃/NGO) composite (Fig. S1 in Supporting information) [11]. After further calcination at different temperatures in the range of 600-900 ℃, final product of Bi/Bi₂O₃/NrGO was obtained (details in Experimental section in Supporting information), hereafter abbreviated as Bi/Bi₂O₃/NrGO-x (x = 600, 700, 800 and 900).

The morphology characterizations of as-prepared Bi/Bi₂O₃/ NrGO-700 hybrid were acquired by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images. The FESEM image (Fig. 1b) showed a nanosheet structure for Bi/Bi₂O₃/NrGO-700. The TEM images evidently proved the presence of large amounts of Bi/Bi₂O₃ NPs with diameter of ~20 nm, which were grown on the surface of NrGO nanosheets (Figs. 1c and d) [12]. Further high-resolution transmission electron microscopy (HRTEM) images clearly revealed that the Bi/Bi₂O₃ NPs were well confined into the NrGO nanosheets (Fig. 1e). The ordered lattice fringes with spaces of ~0.237 nm and ~0.319 nm, were associated with Bi (104) plane and Bi₂O₃ (201) plane, respectively. The corresponding selected area electron diffraction (SAED) pattern of Bi/Bi₂O₃/NrGO-700 showed four different diffraction rings and scattered dots, which could be attributed to crystal planes of Bi and Bi₂O₃(Fig. 1f) [13].

The crystal structure of Bi/Bi₂O₃/NrGO-700 hybrid was identified by X-ray diffraction (XRD) and Raman spectroscopy. One broad peak located at 26.5° in the XRD pattern (Fig. 1g) was associated to the graphitic carbon, while the peaks at 27.2°, 37.9°, 39.6°, 44.6°, 48.7°, 62.2° and 64.5° were associated with the (012), (104), (110), (015), (202), (116) and (122) planes of metallic Bi (JCPDS No. 85-1329) [14], and the other distinct and sharp diffraction peaks at 27.9°, 31.8°, 32.7°, 46.2°, 46.9°, 54.3° and 55.5° were associated to the (201), (002), (220), (222), (400), (203) and (213) planes of Bi₂O₃ (JCPDS No. 27-0050) [15], respectively. Fig. S1 showed the comparison of XRD results recorded on the GO based composites after heating treatment at different temperatures of 600-900 ℃. Obviously, the crystalline structure of Bi/Bi₂O₃/NrGO-700 was similar with that of Bi/Bi₂O₃/NrGO-600 and Bi/Bi₂O₃/NrGO-800. Yet, the XRD pattern of Bi/Bi₂O₃/NrGO- 900 only exhibited one peak located at 26.5° corresponding to graphitic carbon. The Raman spectrum of Bi/Bi₂O₃/NrGO-700 was shown in Fig. 2a, in which two Raman bands centered at 71 and 98 cm^-1 were associated with E_g and A_1g templates of metallic Bi [13, 16], while the peaks of oxidation phases were found at the location of 313 and 410 cm^-1. In the region above 500 cm^-1, three clear peaks of D, G, and 2D bands were displayed at the locations of 1356, 1583, and 2689 cm^-1, respectively. The I_D/I_G value of GO was determined to be 0.508 (Fig. S1), which was much smaller than that of Bi/Bi₂O₃/NrGO-700 with 0.891 (Fig. S2 in Supporting information), indicating the successful reduction of GO to rGO.

X-ray photoelectron spectroscopy (XPS) of Bi/Bi₂O₃/NrGO-700 proved the presences of Bi, O, and N elements, and the doped N content was determined to be 1.57 at% (Fig. S3 in Supporting information). In high resolution Bi 4f XPS spectrum, four clear peaks at binding energies of 157.0, 159.4, 162.4, and 164.7 eV could be attributed to Bi 4f_7/2, Bi³⁺ 4f_7/2, Bi 4f_5/2, and Bi³⁺ 4f_5/2, respectively [17] (Fig. 2b). The high resolution O 1s XPS spectrum exhibited two peaks located at 531.8 and 530.3 eV ascribed to the Bi-O bonds, and one low-intensity peak located at 533.3 eV corresponding to hydroxide group (Fig. 2c) [18, 19]. Moreover, it could be seen that the content of Bi₂O₃ was decreased with the increase of calcination temperatures, which could be indicated by XPS spectrum of Bi 4f (Fig. S4 in Supporting information). It could be deduced that the fracture of Bi-O bonds at high temperature, led to the reduction and agglomeration of Bi₂O₃. In this process, part of the Bi₂O₃ was reduced to metallic Bi. The most optimal carbonization temperature was identified as 700 ℃ throughout this work. The content of Bi₂O₃ phase in Bi/ Bi₂O₃/NrGO-600 was much higher than that of in Bi/Bi₂O₃/NrGO- 700. However, the Bi/Bi₂O₃/NrGO-600 had a low degree of graphitization and poor conductivity, resulting in poor electron transfer ability. Thus, Bi/Bi₂O₃/NrGO-700 catalyst displayed a superior catalytic activity for CO₂ER as compared to Bi/Bi₂O₃/NrGO-600. The N₂ adsorption desorption isothermal curve of Bi/ Bi₂O₃/NrGO-700 displayed a Brunauer-Emmett-Teller (BET) surface area of 93 m²/g and pore size distribution diameter of 2.4 nm (Fig. 2d), which were supposed to enhance the exposed numbers of active sites [20]. Furthermore, thermogravimetric analysis (TGA) of Bi/Bi₂O₃/NrGO-700 revealed that the molar percentage of Bi achieved value of 17.6 at% (Fig. S5 in Supporting information).

To investigate the electrocatalytic activity for CO₂ER, the asprepared samples were evaluated in a CO₂-saturated KHCO₃ electrolyte (0.5 mol/L) within a conventional H-type threeelectrode cell. In this work, all potentials were converted versus reversible hydrogen electrode (RHE). The gaseous products of Bi/ Bi₂O₃/NrGO for CO₂ER were measured by on-line gas chromatograph (GC), and liquid products were collected and analyzed by offline nuclear magnetic resonance (¹H NMR) (Fig. S6 in Supporting information). The F.E. of formate production over as-prepared samples with different potentials was displayed in Fig. 3a. For both Bi/Bi₂O₃/NrGO-700 and Bi/NrGO catalysts, in suitable range of applied potentials, formate F.E. was improved with the potential applied more negatively and achieved the highest F.E. value of 85% and 72% at a potential of -0.9 V, respectively. However, formate F.E. decreased at more negative potentials than -0.9 V, which might be attributed to improvement of HER catalysis [21]. It was noteworthy that Bi/Bi₂O₃/NrGO-700 catalyst always gave the higher formate F.E. than control Bi/NrGO catalyst, which confirmed the excellent CO₂ER activity of Bi/Bi₂O₃/NrGO-700. Fig. 3b showed Tafel slopes of Bi/Bi₂O₃/NrGO-700 and Bi/NrGO. It could be seen that the Tafel slope of 166 mV/dec for Bi/Bi₂O₃/NrGO-700 was lower than that of Bi/NrGO (182 mV/dec) under identical conditions, indicating that the reaction kinetics in Bi/Bi₂O₃/NrGO-700 catalyzed CO₂ER process was faster than Bi/NrGO catalyzed one [8a, 22]. Moreover, electrochemical active surface areas (ECSA) of samples were investigated using electrochemical double layer capacitance (C_dl), that were tested with cyclic voltammetry (Fig. 3c and Fig. S7 in Supporting information). The C_dl values of Bi/Bi₂O₃/NrGO-700 and Bi/NrGO were calculated to be 6.3 and 5.7 m F/cm², respectively. Since the ECSA was in direct proportional to the C_dl, one can deduce that the Bi/Bi₂O₃/NrGO-700 possessed a much larger ECSA than Bi/NrGO, which could help Bi/Bi₂O₃/NrGO-700 to expose more active sites during the CO₂ER process [7, 9d]. Further, the smaller charge transfer impedance of Bi/Bi₂O₃/NrGO-700 than that of Bi/ NrGO was observed via electrochemical impedance spectroscopy (EIS), suggesting that the charge transfer ability of Bi/Bi₂O₃/NrGO- 700 was superior to that of Bi/NrGO (Fig. 3d). Obviously, Bi/Bi₂O₃/ NrGO-700 catalyst exhibited an excellent electrochemical performance relative to Bi/NrGO catalyst. On the basis of these results, one can conclude that the synergetic effect between Bi and Bi₂O₃ played a critical role in CO₂ER [[8]].

To further investigate the influence of calcination temperatures on CO₂ER performance, polarization curves of as-prepared Bi/Bi₂O₃/NrGO samples at different temperatures were measured in a Ar- or CO₂- saturated KHCO₃ electrolyte within a H-type three-electrode cell. As shown in Fig. 4a, onset potential of Bi/ Bi₂O₃/NrGO-700 was measured to be -0.7 V in Ar-saturated electrolyte, originated from its HER activity. After injecting CO₂, onset potential of Bi/Bi₂O₃/NrGO-700 was increased to -0.5 V, along with the distinctly increased current density, demonstrating high CO₂ER catalytic performance of Bi/Bi₂O₃/NrGO-700. The onset potential of -0.5 V from CO₂ER catalyzed by Bi/Bi₂O₃/NrGO- 700 was lower than that of from the Bi/Bi₂O₃/NrGO-600 (-0.6 V), Bi/Bi₂O₃/NrGO-800 (-0.64 V), and Bi/Bi₂O₃/NrGO-900 (-0.68 V)(Fig. 4b), respectively, suggesting high CO₂ER activity of Bi/Bi₂O₃/ NrGO-700. As shown in Fig. 4c, the generated formate was the main CO₂ER product, together with minor amounts of H₂ and CO gases. Obviously, formate F.E. for Bi/Bi₂O₃/NrGO-700 achieved the highest value of 85%, which was about 1.1, 1.6, and 2.7 times higher than that of Bi/Bi₂O₃/NrGO-600 (77%), Bi/Bi₂O₃/NrGO-800 (53%), and Bi/Bi₂O₃/NrGO-900 (32%), respectively, further confirming the superior CO₂ER activity of Bi/Bi₂O₃/NrGO-700. Fig. 4d gave the comparison of recent representative CO₂ER catalysts with Bi/Bi₂O₃/NrGO-700 for producing formate. Although partially reported CO₂ER catalysts delivered a much higher formate F.E. than Bi/Bi₂O₃/NrGO-700, the applied potentials to achieve such high formate F.E. for these previously reported electrocatalysts were more negative than that for this Bi/Bi₂O₃/NrGO-700. To the best of our knowledge, such a high formate F.E. of 85% for Bi/ Bi₂O₃/NrGO-700 at a relatively positive potential of -0.9 V was superior to that of most of the previously reported CO₂ER electrocatalysts for formate production (Table S1 in Supporting information). The superior formate selectivity of Bi/Bi₂O₃/NrGO- 700 was attributed to a combination of the synergistic effect between Bi and Bi₂O₃ and increased number of undercoordinated sites. As a contrast, the Bi/Bi₂O₃, Bi/NrGO, and Bi₂O₃/NrGO exhibited the certain activity for CO₂ER (Fig. 4e). Nevertheless, the catalytic selectivity of these catalysts was far from satisfactory. The Bi/Bi₂O₃/NrGO-700 showed an obvious improved catalytic selectivity for formate production than the Bi/Bi₂O₃, Bi/NrGO, and Bi₂O₃/NrGO (Fig. 4f), proving the strong synergistic effect between Bi and Bi₂O₃.

(1)

(2)

(3)

It is noted that the CO₂ER mechanism is usually dominated for supported Bi or Bi compounds catalysts [7, 9b]. Although specific reaction mechanism of Bi/Bi₂O₃/NrGO-700 involved in CO₂ER to produce formate was still vague, the general reaction steps were illustrated as Eqs. (1–3). Firstly, dissolved CO₂ molecules were captured by metal active sites to form the CO₂_(ads), then electron (e^-) was transferred from the active sites to the CO₂_(ads) forming CO₂^·- intermediate. Next, the CO₂^·- intermediate was transformed into HCOO_(ads)^-intermediate throughone stepof cation coupledelectron transfer process on metal active sites. Finally, the HCOO_(ads)^- intermediate desorbed from metal active sites was transformed to form HCOO_(aq)^- ions dissolved in aqueous electrolyte.

Figs. 5a and b showed the partial current densities for formate and C1 (formate and CO) products of Bi/Bi₂O₃/NrGO samples, which reflected the CO₂ER efficiency. The Bi/Bi₂O₃/NrGO-700 demonstrated a much higher partial currentdensityof -26.7 mA/cm² than the Bi/Bi₂O₃/NrGO-600 (-19.5 mA/cm²), Bi/Bi₂O₃/NrGO-800 (-9.7 mA/cm²), and Bi/Bi₂O₃/NrGO-900 (-2.7 mA/cm²), respectively. The Bi/Bi₂O₃/NrGO-700 showed beneficial reaction kinetics with a Tafel slope of 166 mV/dec (Fig. 5c), which was lower than that of Bi/ Bi₂O₃/NrGO-600 (190 mV/dec), Bi/Bi₂O₃/NrGO-800 (201 mV/dec), and Bi/Bi₂O₃/NrGO-900 (209 mV/dec), indicating a beneficial kinetics of Bi/Bi₂O₃/NrGO-700 for the formation of CO₂^·- ntermediates [23]. Although the charge transfer impedance of Bi/Bi₂O₃/NrGO-800 and Bi/Bi₂O₃/NrGO-900 were smaller than that of Bi/Bi₂O₃/NrGO- 700 (Fig. 5d), the Bi/Bi₂O₃/NrGO-700 exhibited a superior catalytic activity for CO₂ER with respect with Bi/Bi₂O₃/NrGO-800 and Bi/Bi₂O₃/NrGO-900, which could be attributed to the higher content of Bi₂O₃ in Bi/Bi₂O₃/NrGO-700. The ECSA of Bi/Bi₂O₃/NrGO was investigated according to the C_dl, which were tested with cyclic voltammetry (Fig. 5e) [8a]. The C_dl value of Bi/Bi₂O₃/NrGO-700 was 6.3 mF/cm², higher than that of Bi/Bi₂O₃/NrGO-600 (5.3 mF/cm²), Bi/Bi₂O₃/NrGO-800 (4.7 mF/cm²), and Bi/Bi₂O₃/NrGO-900 (3.0 mF/cm²), indicating a remarkably large exposed numbers of active sites for Bi/Bi₂O₃/NrGO-700 [7, 9d]. Additionally, Bi/Bi₂O₃/ NrGO-700 maintained a stable current density of -18 mA/cm² at -0.9 V over 10 h associated with the average formate F.E. of 85%, suggesting a long-term stability of Bi/Bi₂O₃/NrGO-700 for CO₂ER (Fig. 5f).

Encouraged by the excellent CO₂ER performance of Bi/Bi₂O₃/ NrGO-700, we further pursued a two-electrode device that employed Bi/Bi₂O₃/NrGO-700 supported on carbon paper as cathode for CO₂ER and commercial Ir/C supported on carbon paper as anode for OER to explore practical application. As shown in Fig. 6a, polarization curve of CO₂ER-OER couple was measured to achieve a potential of ~3.77 V at 10 mA/cm² in 0.5 mol/L KHCO₃ electrolyte, and after adding N₂H₄, it displayed superior activity with a potential of ~2.51 V at 10 mA/cm². In addition, polarization curve of the two-electrode CO₂ER-OER couple showed an onset potential of ~1.9 V. After adding N₂H₄, the onset potential was decreased to ~0.6 V, suggesting more effective energy conversion efficiency of this couple system achieved with the adding of N₂H₄. Notably, overall catalytic performance of Bi/Bi₂O₃/NrGO-700-Ir/C was superior to that of Bi-Ir/C that was employed for CO₂ER-OER device (onset potential of ~2.1 V) [7]. Moreover, Bi/Bi₂O₃/NrGO- 700 was further combined with Ir/C to achieve CO₂ER-OER electrolysis powered by a plane silicon photovoltaic cell at impressive energy conversion efficiency. The CO₂ER-OER device showed a fast and repeatable dynamic photocurrent response following continuous on/off illumination circulation under simulated sunlight illumination (AM 1.5 G, 100 mW/cm²), indicating an excellent charge transmission capacity of this CO₂ER-OER device (Fig. 6b). Furthermore, we used two AA-size alkaline batteries (potential = 3.0 V) to drive the designed system for CO₂ER-OER electrolysis (Figs. 6c and d). The current density of batteries-driven CO₂ER-OER cell had a subtle variation after continues reaction over 2 h, and then tended to be stable. Notably, the CO₂ER-OER device could maintain a stable current density of ~ 4.3 mA/cm² over 10 h along with an average formate F.E. of 81.2%, suggesting practical feasibility of using two AA-size alkaline batteries to drive CO₂ER-OER device.

In summary, we developed a hybrid electrocatalyst composed of Bi/Bi₂O₃ NPs with an average diameter of ~20 nm supported on NrGO for CO₂ER. The Bi/Bi₂O₃/NrGO-700 catalyst displayed an impressive CO₂ER catalytic activity for formate production to achieve the highest formate F.E. of 85% at -0.9 V with a high current density of -18 mA/cm², a small onset potential of -0.5 V and a low Tafel slope of 166 mV/dec, which was competitive with the other reported Bi-based CO₂ER catalysts. The excellent catalytic activity and selectivity of Bi/Bi₂O₃/NrGO-700 could be attributed to the increased numbers of undercoordinated Bi active sites that originated from the reduction of Bi₂O₃, which accelerated the intermediate reaction steps during the CO₂ER catalysis. Additionally, the Bi/Bi₂O₃/NrGO-700 was combined with commercial Ir/C as a two-electrode system for efficient CO₂ER-OER electrolysis which was powered by planar silicon solar cell or two AA-size alkaline batteries. The Bi/Bi₂O₃ supported on NrGO hybrid designed in this work can pave a promising pathway for synthesizing other metal/metal oxide derived catalysts to enhance their CO₂ER catalytic activity and raise the possibilities that metal oxides may be involved in CO₂ER pathways on other metal catalyst.

Declaration of competing interest

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

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F. He thanks the support of the Natural Science Foundation of Zhejiang Province (No. LR16E080003). Y. Hou thanks the support of National Natural Science Foundation of China (Nos. 21922811, 51702284, 21878270), Zhejiang Provincial Natural Science Foundation of China (No. LR19B060002), the Fundamental Research Funds for the Central Universities, and the Startup Foundation for Hundred-Talent Program of Zhejiang University.

Appendix A. Supplementary data

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Supplementary material related to this article can befound, in the online version, at doi:https://doi.org/10.1016/j.cclet.2020.04.031.

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Year 2020 volume 31 Issue 6

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doi: 10.1016/j.cclet.2020.04.031

Receive Date：2020-03-28
Online Date：2026-01-31
Published：2020-06-15

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Received：2020-03-28
Revised：2020-04-12
Accepted：2020-04-16

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^a College of Environment, Zhejiang University of Technology, Hangzhou 310014, China

^b Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China

^c Ningbo Research Institute, Zhejiang University, Ningbo 315100, China

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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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Fig. 1 (a) Schematic fabrication process of Bi/Bi₂O₃/NrGO. (b) FESEM image and (c-e) TEM and HRTEM images of Bi/Bi₂O₃/NrGO-700. (f) SAED pattern and (g) XRD pattern of Bi/Bi₂O₃/NrGO-700.

Fig. 2 (a) Raman spectrum, (b) high resolution Bi 4f XPS spectrum, (b) high resolution O 1s XPS spectrum and (d) N₂ adsorption desorption isothermal curve and the distribution of pore size (inset) of Bi/Bi₂O₃/NrGO-700.

Fig. 3 (a) Formate F.E., (b) Tafel slopes, (c) C_dl values and (d) EIS Nyquist plots of Bi/ Bi₂O₃/NrGO-700 and Bi/NrGO-700.

Fig. 4 (a) Polarization curves of Bi/Bi₂O₃/NrGO-700 in Ar and CO₂-saturated 0.5 mol/L KHCO₃. (b) Polarization curves in CO₂-saturated 0.5 mol/L KHCO₃ and (c) F.E. for CO, H₂ and formate of Bi/Bi₂O₃/NrGO-600, Bi/Bi₂O₃/NrGO-700, Bi/Bi₂O₃/NrGO-800 and Bi/Bi₂O₃/NrGO-900. (d) Comparison of CO₂ER performance for formate production from Bi/Bi₂O₃/NrGO-700 and other previously reported CO₂ER electrocatalysts. (e) Formate F.E. of Bi/Bi₂O₃/NrGO-700, Bi/Bi₂O₃, Bi/NrGO and Bi₂O₃/NrGO. (f) F.E. for CO, H₂, and formate of Bi/Bi₂O₃/NrGO with different contents of Bi(NO₃)₃·5H₂O and DCDA.

Fig. 5 (a) Partial current densities for formate production, (b) partial current densities for C1 production, (c) Tafel slopes, (d) EIS Nyquist plots and (e) C_dl of Bi/Bi₂O₃/NrGO-600, Bi/Bi₂O₃/NrGO-700, Bi/Bi₂O₃/NrGO-800 and Bi/Bi₂O₃/NrGO-900. (f) Stability and formate F.E. of Bi/Bi₂O₃/NrGO-700 at -0.9 V.

Fig. 6 (a) Polarization curves of two-electrode CO₂ER-OER cell in 0.5 mol/L KHCO₃ with and without 0.5 mol/L N₂H₄. (b) Photocurrent densities of CO₂ER-OER device in 0.5 mol/L KHCO₃ at different applied potentials of 3.5, 4.0 and 4.5 V powered by solar panel under shredded simulated sunlight. (c) Chronoamperometric measurement and formate F.E. of batteries-driven CO₂ER-OER device. (d) Digital picture of CO₂ER-OER device driven by two AA-size alkaline batteries.

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