Surface chemical microenvironment engineering of catalysts by organic molecules for boosting electrocatalytic reaction

Surface chemical microenvironment engineering of catalysts by organic molecules for boosting electrocatalytic reaction

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Xianxu Chu^a^,^*, Lu Wang^a, Junru Li^a, Hui Xu^b

Chinese Chemical Letters | 2024, 35(8) : 109105

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Chinese Chemical Letters | 2024, 35(8): 109105

• Review •

Surface chemical microenvironment engineering of catalysts by organic molecules for boosting electrocatalytic reaction

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Xianxu Chu^a^,^*, Lu Wang^a, Junru Li^a, Hui Xu^b

Affiliations

^a Henan Key Laboratory of Biomolecular Recognition and Sensing, College of Chemistry and Chemical Engineering, Shangqiu Normal University, Shangqiu 476000, China

^b Key Laboratory of Advanced Catalytic Materials and Technology, Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou 213164, China

Published: 2024-08-15 doi: 10.1016/j.cclet.2023.109105

Outline

Abstract

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Electrocatalysis is a surface-sensitive process, in which the catalytic activity of electrocatalyst highly relates to the surface adsorption/desorption behaviors of the reactants/intermediates/products on the catalytically active sites. Surface chemical microenvironment engineering via organic molecules functionalization is a promising strategy to tune the electrocatalytic activity since it can well modify the electrode/electrolyte interface and alter the reaction pathways. In this review, we summarize the recent progress of surface microenvironment engineering of electrocatalysts induced by organic molecules functionalization, with the special focus on the organic molecule-assisted growth mechanism and unique electronic effect. More importantly, the applications of organic molecule functionalized catalysts in various electrocatalytic reactions are also systematically summarized, along with a deep discussion on the conclusion and perspective. This work will open a new avenue for the construction and modification of advanced electrocatalysts based on organic molecule-mediated interface engineering.

Key words

Chemical microenvironment engineering / Organic molecules / Electrocatalysis / Functionalization Interface

Cite this Article

Xianxu Chu, Lu Wang, Junru Li, Hui Xu. Surface chemical microenvironment engineering of catalysts by organic molecules for boosting electrocatalytic reaction[J]. Chinese Chemical Letters, 2024 , 35 (8) : 109105 - . DOI: 10.1016/j.cclet.2023.109105

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1 Introduction

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The rapid consumption of conventional fossil fuels inevitably causes serious energy crisis and environmental pollution and seriously threatens human security and sustainable development [1–4]. To realize the green development of national economy, it is highly imperative to explore clean and renewable energy production, conversion, and storage technologies [5–7]. Fuel cells [8–12], water electrolysis [13–18], and metal-air batteries [19,20], are highly effective energy conversion technologies that can significantly reduce the dependence on fossil fuels and alleviate the energy shortage and environment crisis. However, the practical applications of these advanced technologies are still lack, due to the limitation of highly active and durable electrocatalysts.

Bearing these considerations into mind, enormous endeavors have been dedicated to the design and fabrication of advanced electrocatalysts to drive these electrocatalytic reactions. For example, noble metal Pt and Pd have been designed for boosting electrocatalytic oxygen reduction reaction (ORR) and fuel oxidation reaction in fuel cells [21,22] and hydrogen evolution reaction (HER) in water electrolysis [23], Ru and Ir-based catalysts have been fabricated for facilitating the electrocatalytic oxygen evolution reaction (OER) [24–27] and nitrogen reduction reaction (NRR) [28]. Although the great promise for future applications, some challenging issues should also be addressed due to a large gap between the existing catalytic activity and industrial standards. To promote the intrinsic activity of electrocatalysts, a lot of researchers are focusing on the regulation of particle size, geometric morphology, and chemical composition by taking the area effect, crystal plane effect, and synergistic effect between different components into consideration [29–33]. As a matter of fact, it is well accepted that the electrocatalytic reaction is a surface-sensitive process, in which the catalytic activity of the electrocatalyst highly relates to the surface adsorption/desorption behaviors of the reactants/intermediates/products on the catalytically active sites [34–39]. Rationally tailoring the surface properties of catalysts will endow them with extraordinary catalytic performance.

Recently, chemical functionalization and surface chemical microenvironment engineering by organic molecules and polymers are highly effective for tuning the surface properties of catalysts and improve their electrocatalytic performance [40]. It is demonstrated that the chemical functionalization can effectively modify the electrode/electrolyte interface structure, and thereby contributes to the substantial improvement in electrocatalytic performance [41–43]. In addition, it is also reported that surface chemical microenvironment engineering can induce the electronic effect to not only facilitate the electron transfer but also optimize the binding strength with intermediates [44]. Taking polyaniline (PANI) modified catalyst as an example, the PANI with long electron pairs on N atoms can effectively capture proton from hydronium ions and yield a positively charged surface [45]. Besides, the protonated amine groups in the PANI layer also possess positive charge density and can be electro-reduced to form adsorbed H atom. Furthermore, the electronic interactions between PANI and active species can significantly elevate the electronic conductivity and accelerate the electron transfer during electrochemical reactions. Owing to these distinctive advantages, chemical functionalized catalysts are demonstrated to be highly active and durable toward electrocatalytic reactions, and many interesting works regarding the surface chemical microenvironments of electrocatalysts have been reported.

To provide guidance for the future engineering in surface chemical microenvironment of electrocatalysts to boost catalytic reactions, we herein organize a review by systematically summarizing the recent achievements in this interesting field. As shown in Scheme 1, this review is started by illustrating the organic molecule-assisted growth of catalysts, and then the unique electronic effect caused by surface chemical functionalization is discussed in detail. Subsequently, the applications of surface chemical functionalized catalysts for electrocatalytic reactions and the underlying mechanisms are also discussed, which hope to guide the future regulation of surface chemical microenvironment. At last, the challenges and prospects are also proposed for expanding the research of this interesting field and beyond.

2 Strategies of organic molecules-assisted surface functionalization

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Generally, the strategies of organic molecules-assisted surface functionalization can mainly be divided into three categories. One is associated with the wet-chemical method, which has been widely employed for the synthesis of surface functionalized nanocatalysts. In a standard synthesis, metal precursors, surfactants, reductants, and organic molecules are dispersed in the solvents. After a solvothermal treatment, the surface functionalized noble metal crystals with well-defined nanostructures can thus be prepared. it is widely believed that synthesizing noble metal nanocrystals with uniform morphology in an aqueous system is more difficult than oil phase synthesis [46]. However, most of the organic molecule-assisted complexation reduction methods are realized in the aqueous phase, and no toxic organic solvents are involved, providing a relatively environmentally friendly synthetic route for the controllable synthesis of noble metal nanocrystals [47]. For example, a large number of amino groups (-NH₂) and/or imino groups (-NH-) in the polyamines can well interact with Pt^Ⅱ, Rh^Ⅲ, Pd^Ⅱ and Ag^Ⅰ to form complexes due to the coordination capability of lone pair electrons on the N atom, which can thus transform the growth process of noble metal nanocrystals from thermodynamic control to kinetics control [48]. As a consequence, the noble metals tend to generate nanospheres with minimal surface free energy. By varying the reaction conditions, various anisotropic nanostructures can be obtained.

Besides the conventional wet-chemical methods, the cyanogel-reduction method is a highly feasible strategy for the room-temperature synthesis of surface functionalized metal nanocrystals because of the solid and double-metal properties of the cyanogel intermediates. Taking Xu’s work as a good example, they have successfully synthesized the polyallylamine (PAA)-functionalized PdCo alloy nanonetworks (PdCo-NNW@PAA nanohybrids) by reducing K₂PdCl₄/K₃Co(CN)₆ cyanogel intermediate with a mixture of NaBH₄ and PAA at room temperature, which have been demonstrated to be highly active towards oxygen reduction reaction [49].

The electrodeposition of conducting polymers on small needle type microelectrodes is a subject of growing interest for realizing the surface functionalization of noble metal crystals. The conductive polymers were deposited on noble metal microelectrodes by repeatedly cycling the potential at a scan rate of 50 mV/s in an aqueous solution containing monomers. The precise choice of deposition conditions is important to achieve reproducible polymer films [50]. Therefore, electrodeposition is also an important strategy for the surface functionalization of noble metal crystals.

3 Organic molecule-assisted growth of catalysts

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Rationally tuning the size, morphology, and surface facet of catalyst is highly effective to improve their atomic utilization and electrocatalytic performance. As well known, the controlled growth of catalyst is greatly affected by the surfactants, which can serve as stabilizers during synthesis to shrink the nanoparticles to nanometer size and inhibit the aggregation via electrostatic stabilization mechanism and steric hindrance mechanism [51]. In addition, it is also reported that different adsorption states of the surfactants on crystal planes of metals can tailor the orientation growth of nanomaterials [52,53]. Therefore, surfactants play a crucial role in guiding the growth of nanomaterials with well-defined structures.

Organic molecules, such as polyethyleneimine, polyallylamine, and polyaniline are widely used as surfactants and agents to tailor the growth of nanocrystals, due to their good coordination capability, appropriate molecular weight, good hydrophilicity and electrostatic repulsion [54]. For instance, Chen et al. [55] reported a facile chemical reduction method for the synthesis of PAA-functionalized Pt tripods (Pt_tripods@PAA) with ultrathin and ultralong branches. It was revealed that the K₂PtCl₄ can interact with PAA to generate a water-soluble Pt^Ⅱ-PAA complex to slow the reduction of Pt^Ⅱ precursor and lead to the kinetically controlled synthesis of Pt nanocrystals. By collecting the intermediates at different reaction times (Figs. 1A-D), they found that the autocatalytic tip overgrowth was responsible for the growth of branches. The reaction-temperature-dependent branch length experiments confirmed the great significance of high Pt atom deposition rate on the fabrication of branch length (Figs. 1E and F). Moreover, the elemental mapping images also indicated the uniform distribution of Pt, C, and N element throughout the tripod structure (Figs. 1G and H), which further suggested the crucial role of PAA played in the controlled synthesis of Pt tripods.

Beside polyamines, other organic molecules are also demonstrated to be effective for the controllable synthesis of anisotropic nanostructures. For example, Bu et al. [56] synthesized the PtPb/Pt core/shell nanocatalysts with unique hexagonal nanoplates by using oleylamine (OAm)/octadecene (ODE) mixture as solvents and surfactants. Similar to the polyamines, OAm can also be pyrolyzed into N-involved species to possess unique coordination capability, which can thus control the growth dynamic of PtPb/Pt nanostructures (Fig. 2).

More importantly, it is also demonstrated that a molecular surface functionalization can effectively tune the metal nanoparticle catalysts to enable highly active electrocatalytic reactions. Cao et al. reported a molecular surface functionalization approach to modify gold nanoparticle (Au NP) electrocatalysts for the reduction of CO₂ to CO [57]. The N-heterocyclic (NHC) carbene-functionalized Au NP catalyst exhibits improved electrocatalytic performance towards CO₂ reduction reaction. As is well known to all, metal nanoparticles often possess surfactants with long alkylchains to stabilize the surfaces. However, these surfactants can block the catalytically active sites of metal nanoparticles and lower the catalytic activity. Furthermore, the mono-dentate nature of these surfactants can result in ligand detachment under the electrochemical reactions, causing particle aggregation and consequent activity decay. In this context, stabilization of metals surfaces while maintaining catalytic active sites through rational molecular design of surface capping ligands of nanoparticles is highly desired. Molecular functionalization is thus emerging as a promising strategy for addressing this issue. Inspired by this, Cao and coworkers explore the use of a tetradendate ligand that can chelate on the surface of Au nanoparticles that can tune the CO₂RR reactivity and improving the electrocatalytic stability [58].

4 Electronic effect of organic molecule functionalization

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According to the Sabatier principle, the adsorption of reaction intermediates on the catalyst surface should be neither too strong nor too weak [59]. Taking electrocatalytic HER as a good example, a hydrogen adsorption Gibbs free energy nearly zero enables Pt to be an ideal and promising candidate for serving as advanced HER catalyst [60,61]. Therefore, it is widely accepted that tailoring the surface adsorption energy of catalyst will be an effective strategy for achieving outstanding electrocatalytic performance. As is well known to all, electronic structure engineering lies at the heart of catalyst design, which not only affects the surface adsorption energy of catalyst with intermediates, but also poses great influence on the electron transfer capability [62–65]. Therefore, rationally engineering the electronic structure of catalyst will be promising for promoting their catalytic performance. As well known, many effective strategies have been proposed for the effective electronic structure engineering, such as heteroatom doping, defect engineering, and heterostructure engineering. Besides, surface chemical functionalization of catalyst via some organic molecules has recently been demonstrated to be a potential strategy for inducing strong electronic effects to optimize the binding strength with intermediates.

It is reported that the surface groups of some organic molecules possess strong coordination effect, which can adsorb on the surface crystals and tailor the electron transfer to affect the electronic effect. For example, the strong coordination of -NH₂ and -NH- in PAM enables a certain amount of PAM to be adsorbed on the surface of metal nanocrystals, leading to the formation of PAM-functionalized metal nanocrystals [55]. This functionalization is accompanied by N atoms donating electrons to the noble metal, thereby affecting the filling degree of the d orbital in the metal and resulting in changes of the interaction between reactant molecules and noble metals.

Owing to unique electronic effect, many advanced chemical functionalized catalysts have thus been well developed to exhibit superb electrocatalytic performance. For instance, Chen and coworkers [66] synthesized the polyallylamine (PA) functionalized frame-like concave RhCu bimetallic nanocubes (PA-RhCu cNCs) (Figs. 3A and B), which were demonstrated to be highly active towards the 8e reduction of nitrate to NH₃. Electrochemical measurements reported that the PA-RhCu cNCs could show remarkably high NH₃ production yield of 2.40 mg h⁻¹ mg_cat⁻¹ and a Faradaic efficiency as high as 93.7% (Fig. 3C). Upon the combination of experimental results and DFT calculations, it is uncovered that Cu and PA can coregulate the Rh d-band center, which can thus slightly weaken the adsorption energy of reaction-related species on Rh (Fig. 3D). More importantly, the surface PA modification can also accelerate the electron transfer and mass transport, thereby leading to notably enhanced electrocatalytic performance.

In addition, Wang et al. [67] also demonstrated that surface-functionalized organic molecules can modulate the electronic structure of catalyst to promote the electrocatalytic performance. They realized the successful synthesis of polyallylamine-encapsulated Ir metallene with defects and porous structure (Ir@PAH metallene) through a facile wet-chemical reduction method. The Ir metallene with ultrathin, layered, highly curved, porous, and defective structure can exhibit large electrochemically active surface area, good electronic conductivity, efficient mass and electron transport (Figs. 4A-E). As a result, such Ir@PAH metallene can exhibit superb catalytic performance towards HER, delivering an ultralow overpotential of merely 14 mV at 10 mA/cm² (Fig. 4F), and showing almost no activity decay after a long-time electrochemical cycle. Deep mechanistic study uncovers that the surface-functionalized PAH molecules can modulate the electronic structure through strong Ir-N interaction. More importantly, the -NH₂ groups of PAH can easily capture protons from the solution and are completely protonated, leading to the proton enrichment at the catalysts/solution interface, which is beneficial for the HER. Therefore, the surface-functionalized PAH molecules not only modifies the electronic structure to optimize the binding energy with intermediates, but also can captures proton to induce proton enrichment effect, thereby synergistically enhancing the electrocatalytic HER performance.

5 Applications of organic molecule functionalized catalysts

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5.1 Electrocatalytic oxygen reduction reaction

ORR is a crucial reaction of many significant energy conversion technologies, such as fuel cells and metal-air batteries. However, the sluggish reaction kinetic of oxygen reduction process seriously limited its reaction efficiency and rate [68–71]. Developing advanced electrocatalysts will not only accelerate the reaction rate, but also significantly enhance the reaction efficiency. In recent years, Pt-group metal nanocatalysts and Fe-based single-atom catalysts have been well demonstrated to be highly active and durable toward ORR, and some effective strategies have also been proposed for the further improvement in electrocatalytic ORR performance. For example, tailoring the coordination number or coordination environment of Fe single atom is an effective strategy for realizing the optimization of electrocatalytic performance of Fe-N-C catalysts [72]. For Pt-group metal catalysts, rational morphology, component, surface property, and electronic structure regulation are demonstrated to be favorable for promoting their catalytic performance [73–75].

Besides, surface chemical functionalization is also a highly demanded method for the optimization of surface and electronic properties. For instance, the PAM functionalization accompanied by N atoms donating electrons to the noble metal can affect the filling degree of the d orbital in the noble metal. For instance, Chen and coworkers [76] synthesized the polyethylenimine (PEI)-functionalized Pd nanowires (Pd-NWs@PEI), where the strong electron-donating effect of the -NH₂ group in the PEI can effectively generate the electron-rich Pd to feed back more electrons to the anti-bonding orbital of O₂ molecule, thereby facilitating the cleavage of O=O bonds (Figs. 5A and B). On the other words, the functionalization of PEI could alter the electronic structure of Pd nanowires, enabling them to be good ORR electrocatalysts. In addition, deep studies revealed that the formed PEI layers on the Pd nanowires not only effectively served as a "molecular window gauze" to physically block the access of alcohol molecules to Pd sites, but also allowed the access of oxygen molecules (Figs. 5C and D), resulting in outstanding ORR performance and good alcohol tolerance (Fig. 5E).

The surface chemical functionalization of noble metal by some conductive organic molecules can also form unique heterointerfaces to expose sufficient catalytically active sites and provide rich channels. As proved by Wang and coworkers [77], the PNAI-functionalized Pd metallenes possessed abundant active heterointerfaces to facilitate the electron transfer (Figs. 5F-H). In addition, the PANI functionalization can also offer sufficient catalytically active sites, tunable surface electronic structure, and enhanced electrical conductivity for ORR. As a result, the as-prepared Pd@PANI metallene could exhibit superb catalytic performance towards ORR, delivering ultrahigh mass and specific activities, with 10.9 and 8.3 folds higher than those of Pt/C catalyst (Fig. 5I). These works highlighted the great significance of polymer functionalization on the substantial improvement in electrocatalytic ORR performance.

5.2 Electrocatalytic hydrogen evolution reaction

Hydrogen production by electrochemical water splitting has been widely considered as a promising avenue to alleviate energy crisis and environment pollution. HER is an important half reaction of water splitting, developing highly efficient HER electrocatalysts will benefit for the substantial enhancement in hydrogen production efficiency. Since HER is sensitive to the topography and surface property of an electrocatalyst, many works regarding the morphology control, component regulation, and surface chemical functionalization of catalysts have been widely reported [78–81]. As previously discussed, surface polyamine functionalization of metal catalysts can affect their electrocatalytic activity and selectivity for proton-coupled HER. For instance, Chen et al. [82] reported the synthesis of PEI-functionalized NiP₂ nanosheets on carbon cloth (CC) nanohybrids to deliver outstanding electrocatalytic HER performance (Fig. 6A). Owing to the increase in the number of protons at the PEI-NiP₂–CC/electrolyte interface, the optimal PEI-NiP₂–CC nanohybrids afford 10 mA/cm² current density at 44 mV and 100 mV overpotentials for HER in 0.5 mol/L H₂SO₄ (Fig. 6B) and 1 mol/L PBS solution (Fig. 6C), respectively. Meanwhile, such nanohybrids can also exhibit superb long-term electrochemical stability for HER. They also synthesized the poly-ethyleneimine (PEI) functionalized Pt superstructures (Pt-SSs@PEI) with tetragonal, hierarchical, and branched morphologies [83]. The HER at the Pt-SSs@PEI has an unprecedented overpotential (64.6 mV) at 10 mA/cm² in strong acidic media. The superb activity of the Pt-SSs@PEI for the HER is attributed to the protonation of -NH₂ groups at the PEI adlayers on the Pt surface, which effectively elevates the local H⁺ concentration at the electrode/electrolyte interface.

More importantly, it is also demonstrated that the high conductivity of organic molecules can greatly boost the electron transfer to further improve the electrocatalytic HER performance. Wang and coworkers [84] have proved that the surface chemical functionalization of PANI can significantly enhance the electrical conductivity of catalyst to effectively facilitate the charge transfer (Figs. 6D and E). They synthesized the mRh@PANI and employed it as catalysts for HER. Benefitting from the unique 3D mesoporous structure, the high conductivity of PANI and strong synergistic effect between PANI and mRh, the prepared mRh@PANI showed enhanced HER catalytic activity and excellent stability both in alkaline and neutral electrolytes (Fig. 6F). As a result, to reach a current density of 50 mA/cm², low overpotentials of 37 mV and 69 mV were required in 1 mol/L KOH and 1 mol/L PBS solutions, respectively.

5.3 Electrocatalytic oxygen evolution reaction

OER is a crucial half reaction for water splitting, which directly determines the reaction rate and efficiency of hydrogen production. In recent years, many effective electrocatalysts have also been well developed for boosting electrochemical OER, including Ir/Ru oxides and transition metal compounds [85–87]. To further improve the electrocatalytic OER performance, many effective strategies have also been proposed. In recent years, surface functionalization is emerging as a promising strategy for enhancing the electrocatalytic performance towards OER. For one thing, the surface modification can substantially enhance the electrical conductivity to facilitate the electron transfer. For another, it can also modify the surface properties of catalyst to optimize the binding strength with oxygen-containing intermediated species. Inspired by this, You and coworkers [88] synthesized the Ni₅P₄@PANI, where the PANI coating can adaptively buffer the volume/phase changes of the Ni₅P₄ during oxidative reconstruction to restrain the structure corrosion (Figs. 7A-D). Additionally, the PANI interlayer strongly coupled with the reconstruction-formed nickel (hydr)oxides and positively charged them, which assured the optimal adsorption of OER intermediates such as *O (Figs. 7E and F). As a consequence, the as-prepared Ni₅P₄@PANi catalyst can exhibit superb electrocatalytic activity and durability toward OER. Deep mechanistic study revealed that the strong coupling between nickel (hydr)oxides and PANI could induce effective electron transfer from Ni²⁺ to PANI and results in Ni-based active sites with higher valance, thereby accounting for the enhanced electrocatalytic OER performance.

5.4 Electrocatalytic CO₂ reduction reaction

Electrocatalytic CO₂ reduction into value-added fuels or chemicals with renewable electricity is deemed to be a potential avenue to achieve a carbon-neutral energy cycle and alleviate atmospherically CO₂ emissions [89–92]. However, the high bond energy of the C=O double bond makes the CO₂ molecules thermodynamically stable and chemically inert, resulting in slow stress kinetics and large overpotentials [93,94]. In addition, the electrochemical CO₂ reduction reaction (CO₂RR) also poses a multiple pathway and a complex reaction process, resulting in poor product selectivity. Therefore, the development of electrocatalysts with high catalytic activity and selectivity is highly demanded. To date, a wide range of metal catalysts have been reported for electrocatalytic CO₂RR, including Pd, Sn, Pb, Bi, and In. Although these catalysts have been uncovered to be promising for the future applications, their catalytic activities and selectivities are still unsatisfied, which calls for the further improvement.

Fortunately, previous studies have indicated that modifying the surface chemical microenvironments of catalysts are expected to improve the electrocatalytic activity and selectivity of CO₂RR. For instance, amino groups of surface organic molecules could not only provide plenty of active sites for CO₂ adsorption based on Lewis acid and Base interactions and increase the area of active site exposed, but also greatly manipulate the reaction pathways by tailoring the electrochemical stability of intermediates to synergistically enhance the catalytic activity and selectivity. Xia and coworkers [92] reported the synthesis of In-based MOF electrocatalyst by amino-functionalization, which could display outstanding electrocatalytic activity for the CO₂RR to formate with a Faraday efficiency of 94%. It is revealed that the well-retained amino groups significantly improve the adsorption of CO₂ and promote the activation and hydrogenation of CO₂ by stabilizing CO₂^•− intermediate, thereby boosting the efficient conversion of CO₂ into formate. More recently, Xu et al. [95] synthesized the diethylenetriamine-functionalized mosaic Bi nanosheets (mBi-DETA NSs) to boost the selective electrocatalytic CO₂ reduction to formate (Fig. 8A). Mosaic nanosheet structure with rich lattice disordering defects could provide abundant under-coordinated Bi active sites and enhance the activation of CO₂ molecules (Figs. 8B-D). It is well recognized that the weak binding energy for OCHO^−* on bare Bi surface is a practical obstacle to generate HCOOH product. In the reconstructed mBi-DETA NSs, amino groups introduced by surface DETA molecules are capable to enhance CO₂ adsorption and regulate the product distribution by accelerating the formation of key intermediates (OCHO^−*), thus promoting the formate production (Fig. 8E). As a result, when subjected the mBi-DETA NSs as the catalyst for CO₂RR, it can deliver a significantly enhanced Faraday efficiency of 96.87% for HCOOH.

5.5 Electrocatalytic nitrogen reduction

Electrocatalytic NRR under ambient conditions provides a promising pathway for the sustainable and green production of NH₃, where the catalytic performance is strongly related to the properties of catalyst [96–98]. In recent years, many efficient electrocatalysts have been developed for boosting the electrochemical N₂ reduction into NH₃, however, the catalytic activity and selectivity are still unsatisfied. The electrochemical reaction is a surface-sensitive process, rationally optimizing the surface of catalyst is thus emerging as a promising strategy to enhance the adsorption between active site and the reactant and inhibit the rapid deactivation of nanocatalyst [99,100]. NRR electrocatalysts that modified with some functionalized organic molecules are also reported to be highly active and durable. For example, Sun et al. [101] synthesized the tannic acid (C₇₆H₅₂O₄₆, TA) surface-functionalized Pd nanoparticles toward improved NRR performance. It was uncovered that rich oxygen-containing phenolic hydroxyl functional groups could effectively modify the surface properties of the electrocatalysts to improve the NRR activity and stability. More recently, Wang and coworkers [102] also synthesized the TA surface-functionalized Au nanowires to boost electrocatalytic NRR (Figs. 9A-I). It is reported that TA-Au nanowires can exhibit a large NH₃ yield rate (15.71 µg h⁻¹ mg_cat⁻¹) and a high Faraday efficiency of 14.83% at −0.3 V with an excellent electrochemical durability (Figs. 9J and K).

5.6 Electrocatalytic nitrate reduction

Surface functionalization of catalysts can induce the distinctive electronic effect to optimize the binding strength with intermediates during electrocatalytic nitrate reduction since nitrate reduction is a complicated process [103,104]. In addition, it is also well documented that surface modification with highly conductive organic molecules is also favorable for the efficient electron transfer. In recent years, some advanced electrocatalysts that modified with conductive organic molecules are also prepared and demonstrated to be highly active and selective toward nitrate reduction reaction. For example, Wang et al. [105] synthesized the PANI-modified CuO nanowire arrays that supported on the surface of Cu foam and employed it as self-supported electrocatalyst for the selective nitrate electroreduction to ammonia. According to a series of characterizations, it is demonstrated that the PANI modification not only retains the nanowire array structure (Figs. 10A-E), but also significantly modulates the surface chemical microenvironment of catalyst. In addition, it is also revealed that the PANI with lone electron pairs on N atoms is capable to capture proton from hydronium ions and form a positively charged surface, which thus facilitate the enrichment and fixation of NO₃⁻ anions on the catalyst surface. Moreover, the protonated amine groups in the PANI possess positive charge density and can thus be electro-reduced to form adsorbed H atom, which is a key intermediate during nitrate reduction reaction. Furthermore, the strong interaction between PANI and CuO can enhance the electrical conductivity and facilitate the electron transfer. Benefitting from these advantages, the self-supported CuO@PANI/CF could exhibit outstanding electrocatalytic performance towards nitrate reduction reaction by achieving a Faraday efficiency of 93.88%, NH₃–N selectivity of 91.38% and NH₃ yield rate of 0.213 mmol h⁻¹ cm⁻² (Figs. 10F-J).

6 Conclusions and perspectives

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Since the electrocatalytic reaction is a surface-sensitive process, surface structure design of nanomaterials is key to the fabrication of catalysts with favorable properties. By taking full advantages of the strong coordination capability of polymer organic molecules, the catalyst can be generally tailored in many ways to achieve advanced surface structure, endowing them with good electrocatalytic activity and durability (Table 1). During the synthesis process, polymer organic molecules can tailor the growth kinetics and significantly reduce the redox potential of metal precursors and modify the morphology. More importantly, surface chemical functionalization by polymer organic molecules can also regulate the adsorption of reactants and intermediated species by varying the physical and chemical properties of active sites, such as the electronic configuration and coordination environment, so as to realize the substantial improvement in electrocatalytic performance. Meanwhile, some polymer organic molecules can also easily capture protons from the solution and are completely protonated, leading to the proton enrichment at catalysts/electrolyte interface. Although the great advantages, some issues in the related research work should also be addressed, and some important aspects should be considered seriously.

Despite some understandings on how polymer organic molecules effectively affect the catalytic performance have been gained, they are only focusing on tailoring the growth kinetics and modifying the electronic effect, other underlying influences and mechanisms should be further manifested.

Some reaction mechanisms, such as the transport mechanism of H⁺ in the polymer organic molecules on the surface of catalyst are still unclear. Therefore, theoretical calculations and molecular dynamics simulation should be employed to recognize and understand the detailed reaction mechanism. Meanwhile, some advanced characterizations such as in situ Raman and in situ FTIR should also be operated to monitor the intermediates in the electrocatalytic reaction process.

Most of the surface chemical functionalization treatments are focusing on the noble metals and some transition metal compounds, the works regarding the surface functionalization of single-atom catalysts are rarely reported. As well known, single-atom catalysts possess a high atom utilization efficiency that approaching to 100%, chemical functionalization of metal sites may pave a new way to substantially improve their electrocatalytic performance.

Update, functional molecules are only limited to PAM and tannic acid. In order to expand this interesting research field, more catalysts modified by other functional molecules should be extended. In addition, the relevant research on the as-synthesized catalysts in practical energy conversion devices is still lacking, more attention should be paid to boosting the practical applications.

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.

Acknowledgment

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This work was supported by the Key Research & Development and Promotion Projects in Henan Province (No. 232102230079).

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doi: 10.1016/j.cclet.2023.109105

Receive Date：2023-03-29
Online Date：2025-11-21
Published：2024-08-15

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Received：2023-03-29
Revised：2023-08-15
Accepted：2023-09-14

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^a Henan Key Laboratory of Biomolecular Recognition and Sensing, College of Chemistry and Chemical Engineering, Shangqiu Normal University, Shangqiu 476000, China

^b Key Laboratory of Advanced Catalytic Materials and Technology, Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou 213164, 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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Scheme 1 Schematically ullustrating the effect of surface microenvironment engineering induced by organic molecule functionalization and their applications in various electrochemical reactions.

Fig. 1 TEM images of the reaction intermediates collected at (A) 0.25, (B) 0.5, (C) 1, and (D) 2 h, respectively. TEM images of the Pt_tripods@PAA synthesized at different reaction temperatures: (E) 50 ℃ and (F) 80 ℃. (G) Large-area and (H) locally amplified EDX elemental maps of Pt_tripods@PAA. Reproduced with permission [55]. Copyright 2016, American Chemical Society.

Fig. 2 (A) HAADF-STEM image, (B) TEM image, (C) EDX spectrum and (D) XRD pattern of PtPb hexagonal nanoplates. (E) SAED and (F) HRTEM of individual hexagonal nanoplate. (G) Elemental mapping of PtPb hexagonal nanoplates. Reproduced with permission [56]. Copyright 2016, American Association for the Advancement of Science.

Fig. 3 (A) TEM image and HRTEM images, (B) elemental mapping images of the PA-RhCu cNCs. (C) Histograms of the NH₃ production yield and Faradaic efficiency. (D) Calculated d-band center values of Rh in Rh (111), RhCu (111), Rh (111)-ethylamine, and RhCu (111)-ethylamine surfaces. (E) Schematic diagram of PA induced enhanced interfacial mass transfer process by the electrostatic interaction during NO₃⁻-RR. Reproduced with permission [66]. Copyright 2022, Wiley-VCH.

Fig. 4 (A) TEM image, (B, C) HAADF-STEM images, (D) HRTEM image and (E) elemental mapping images of the Ir@PAH metallene. (F) HER polarization curves of Ir@PAH metallene other references. Reproduced with permission [67]. Copyright 2022, Wiley-VCH.

Fig. 5 (A) Configurations of adsorbed PEI on the Pd (111) surface. (B) HAADF-STEM image and elemental mapping images of the Pd-NWs@PEI. (C) Schematic illustration of the selectivity of window gauze for the fly and midge. (D) Schematic mechanism for the ORR selectivity of Pd-NWs@PEI in the presence of ethanol. (E) ORR polarization curves of the Pd-NWs@PEI and Pd black. Reproduced with permission [76]. Copyright 2015, Royal Society Chemistry. (F, G) HAADF-STEM image and (H) elemental mapping images of the Pd@PANI metallene. (I) ORR polarization curves of Pd@PANI metallene and other referenced catalysts. Reproduced with permission [68]. Copyright 2022, Elsevier.

Fig. 6 (A) HAADF-STEM image and elemental mapping images of the PEI-NiP₂–CC. HER polarization curves of PEI-NiP₂–CC in (B) 0.5 mol/L H₂SO₄ and (C) 1 mol/L PBS solution. Reproduced with permission [82]. Copyright 2019, Royal Society Chemistry. (D) Elemental mapping image, (E) TEM image, and (F) scheme for the hydrogen production by water electrolysis that driven by mRh@PANI. Reproduced with permission [84]. Copyright 2022, Elsevier.

Fig. 7 (A) Schematic diagram of the synthesis of Ni₅P₄@PANI and the in-situ reconstruction process. (B) TEM image, (C) HRTEM image, and (D) elemental mapping images of the Ni₅P₄@PANI. (E) Simplified configurations of the various reaction intermediates over rc-Ni₅P₄ and rc-Ni₅P₄@PANI along the OER pathway. (F) The calculated Gibbs free-energy diagram of the OER pathway on rc-Ni₅P₄ and rc-Ni₅P₄@PANI. Reproduced with permission [88]. Copyright 2022, Wiley-VCH.

Fig. 8 (A) The synthesis process of mBi-DETA NSs. (B) TEM image, (C) HRTEM image, and (D) elemental mapping images of the mBi-DETA NSs. (E) Schematic illustration of the proposed electrochemical CO₂RR mechanism over mBi-DETA NSs. Reproduced with permission [95]. Copyright 2023, Wiley-VCH.

Fig. 9 TEM images of (A) Au nanowires and (B, C) TA-Au nanowires. (D) HAADF-STEM image, (E-H) corresponding elemental mapping images and (I) line-scan profile of the TA-Au nanowires. (J) NH₃ yield and (K) Faradaic efficiency of Au nanowires and TA-Au nanowires. Reproduced with permission [102]. Copyright 2021, Elsevier.

Fig. 10 (A, B) SEM images, (C) TEM image, (D) HRTEM image, and (E) elemental mapping images of the CuO@PANI/CF. (F) NH₃ yield rate and (G) Faradaic efficiency of CuO@PANI/CF. (H) The ¹H NMR spectra of ¹⁵NH₄⁺ and ¹⁴NH₄⁺ calibration solutions, and ¹H NMR spectra of electrolytes after electrocatalytic nitrate reduction employing ¹⁵NO₃⁻ and ¹⁴NO₃⁻ as nitrogen sources. (I) The standard curve of integral area (¹⁵NH₄⁺−¹⁵N/C₄H₄O₄) against ¹⁵NH₄⁺−¹⁵N concentration. (J) Schematic illustrating the mechanism of electrocatalytic nitrate reduction to ammonia on CuO@PANI/CF. Reproduced with permission [105]. Copyright 2023, Elsevier.

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