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Post-Treatment Technology Improves Fuel Cell Catalyst Stability

Yangyang Liu , Zigang Zhao , Hao Sun , Xianghui Meng , Guangjie Shao , Zhenbo Wang

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (4) : 973 -982.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (4) : 973-982 DOI: 10.7536/PC210429
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Post-Treatment Technology Improves Fuel Cell Catalyst Stability
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Abstract

Fuel cell is a kind of renewable new energy technology, which can directly convert the chemical energy of fuel into electric energy through the chemical reaction at the interface of electrode and electrolyte. Because energy conversion efficiency is high, no noise and pollution.Proton exchange membrane fuel cell (PEMFC) is one of the most widely used fuel cells, but PEMFC still has some problems to be solved, such as high cost, low power density and poor catalyst stability. Therefore, to achieve the large-scale application of proton exchange membrane fuel cell, research and development of high activity and high stability catalyst is the top priority. In order to meet the requirements of high activity and high stability of fuel cell catalysts, this paper reviews the research progress and performance improvement methods of catalysts for fuel cells. The methods to improve the stability of fuel cell were discussed from the perspectives of active components and carrier. The performance of catalyst was improved by reducing the diameter of active component particles, preparing platinum particles with specific orientation surface, alloying platinum with transition metals and the modification of carrier also had a significant impact on the stability of catalyst. Finally, the future development direction of fuel cell catalysts and the main problems in practical application are proposed.

Contents

1 Introduction

2 Fuel cell electrocatalyst

3 Post-processing technology

3.1 Active component angle improves stability

3.2 Carrier angle improves stability

4 Conclusion and prospect

Graphical abstract

Keywords

proton exchange membrane fuel cell / noble metal catalyst / stability / active component / carrier

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Yangyang Liu, Zigang Zhao, Hao Sun, Xianghui Meng, Guangjie Shao, Zhenbo Wang. Post-Treatment Technology Improves Fuel Cell Catalyst Stability. Progress in Chemistry, 2022, 34(4): 973-982 DOI:10.7536/PC210429

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1 Introduction

With the surge in global energy demand, the demand for clean and sustainable energy is also increasing dramatically. Countries around the world are actively investing in the development of new environmentally friendly energy conversion and energy storage technologies, such as solar cells, fuel cells, secondary batteries and hydrogen power generation. Among them, fuel cell has received close attention as an environmentally friendly energy source, which is considered to be one of the ideal energy sources in the future because of its environmental protection and high energy efficiency[1,2]. Fuel cell is an electrochemical device, which directly converts the chemical energy of fuel into electrical energy through the chemical reaction at the interface of electrode and electrolyte, including alkaline fuel cell (AFC), proton exchange membrane fuel cell (PEMFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFC)[3,4]. In particular, proton exchange membrane fuel cell (PEMFC) has been widely developed. Its advantages, including low operating temperature, high power density, and fast reaction rate, make PEMFC the most promising energy conversion device in automotive engines, distributed power generation, and portable electronic applications. The development of efficient and stable oxygen reduction catalysts is the key to obtain high performance PEMFC.
The durability of catalysts still makes it difficult for fuel cells to be commercialized globally. Catalysts accelerate the chemical reaction at the electrode-electrolyte interface of fuel cells, which affects the performance and service life of fuel cells. In order to extend the service life of fuel cells and reduce their cost, the current research focuses on improving the activity and stability of catalysts through post-treatment technologies such as alloying, polymer coating, and changing carbon-based materials, that is, improving the durability of catalysts while maintaining the high activity of catalysts[5,6][7,8].
In this paper, the post-treatment technology is described from the perspective of active component and carrier to improve the stability of the catalyst, so that the catalyst has both good catalytic activity and durability. At the same time, the relevant status of fuel cell catalysts and the upcoming challenges and development opportunities are summarized, which is conducive to the early realization of the commercial feasibility of fuel cells and the promotion of green and energy-saving development of society.

2 Fuel cell electrocatalyst

At present, many countries have done a lot of research work on PEMFC vehicles. At present, the least platinum consumption of fuel cell is Toyota (0. 17 G/kW), which is still far from the value specified by the U.S. Department of Energy (0. 125 G/kW), so the task of developing catalysts with low platinum loading, high activity and high stability is arduous[9].
Among the fuel cell catalysts, platinum-based catalysts are the only commercial catalysts. Platinum is supported in the form of nanoparticles and dispersed in conductive and corrosion-resistant carriers, which has high catalytic activity, but is prone to carbon monoxide poisoning. The aggregation or migration of platinum particles on the carbon support and the corrosion of the carbon support in the Pt/C catalyst will lead to the decrease of the activity and stability of the Pt/C catalyst. The electrochemical surface area (ECSA) of the catalyst is directly related to the activity of the catalyst, and the decrease of ECSA is the key to the decrease of the activity of Pt/C catalyst fuel cell[10]. There are three main reasons for the decrease of ECSA of Pt/C catalyst: (1) the migration of platinum particles on the carbon support; (2) the platinum particles are dissolved in the electrolyte; (3) Corrosion of carbon support.
At present, there are three main research focuses: reducing the diameter of Pt particles and increasing the dispersion of Pt to increase its active surface area; Developing a method for preparing Pt particles with special oriented surfaces; Different physical and chemical methods are applied to add other metal elements to Pt catalyst to alloy it or disperse Pt into other transition metals, metal alloys to form mixtures or alloys such as core-shell structure[11].

3 Reprocessing technique

Taking Pt/C catalyst as an example, the aggregation and migration of platinum particles on the carbon support or the dissolution of platinum particles in the electrolyte, as well as the corrosion of the carbon support, will lead to the decrease of the activity and stability of Pt/C catalyst[12]. The current study shows that the durability problem of platinum-based catalysts can be solved by the method of nanoscale interconnected structure, and having fully exposed active sites and an open structure conducive to mass transfer are the conditions for improving catalytic activity[13].
It is certain that the combination of interconnected structures with highly open structures and nanostructures such as porous structures, multi-dimensional structures, surface/framework surfaces, and ultra-thin layers on a platinum-based catalyst can meet the requirements of catalytic activity and stability of the catalyst at the same time. In order to meet the high catalytic activity and stability of the catalyst, the post-treatment technologies such as coating and doping can be used to modify and design the catalyst from the two aspects of the active component and the carrier. The study of electrocatalysts with high activity and durability is of great significance to the commercial development of fuel cells.

3.1 Active component angle elevation stability

3.1.1 Alloying treatment to control morphology and size to improve stability

Pt/C catalyst is not the first choice in the commercialization of fuel cells. Although the surface structure of carbon nanoparticles significantly improves the utilization of platinum, the high-index crystal planes and unsaturated atoms are thermodynamically unstable during fuel cell operation, and the deactivation of crystal planes and morphological deformation will lead to the reduction of catalytic activity. The stability of the alloy catalyst is improved by the synergistic effect of induced strain and ligand effect brought by the alloying of platinum and some transition metals, the use of platinum is reduced, and the cost is saved[14].
In the alloying process of PtM (M = Fe, Co, Cu, Ni) catalysts, improving the mass activity (MA) of platinum-based catalysts is a way to reduce the cost of catalysts and improve the durability of catalysts[15]. Other metal elements were added to synthesize PtM alloy to pursue lower platinum loading and higher stability. The common synthesis method is solvent-assisted thermal reduction, in which platinum metal salts and alloy metal salts are mixed in aqueous solution or organic solvents and reducing agents (such as sodium borohydride (NaBH4), 1,2-tetradecanol (TDD), tungsten hexacarbonyl) at a certain temperature to produce platinum-based alloys[16,17]. As shown in Fig. 1A and B, Herranz et al. Used NaBH4 as a reducing agent to prepare PtNi aerogel alloy in aqueous solution, and during the reduction process, the addition of surfactant could control the morphology of the alloy material[18]. As shown in Figure 1C, PtNi3 alloy materials with different sizes ranging from 3 to 10 nm were prepared by adjusting the concentrations of oleylamine and oleic acid[19].
In the Pt/C catalyst, the electrocatalytic oxidation of intermediates (such as CO) occurs on the platinum surface, which will occupy the catalytic active sites and reduce the catalytic performance of the platinum catalyst. The addition of a second metal can effectively enhance the desorption and oxidation of the intermediate, allowing the active site to be re-exposed[20]. The catalytic activity and CO tolerance of platinum are improved due to the electronic effect and synergistic effect of the alloy catalyst. Alloying platinum with other metals and controlling the structure and morphology of platinum-based nanoparticles are two basic methods to reduce the amount of platinum and improve the catalytic performance[21]. The platinum alloy catalyst changes the electronic state of the surface of platinum particles, which can effectively inhibit the dissolution of platinum particles, and the alloy elements also improve the oxidation resistance of platinum, thereby improving the stability of the catalyst. However, it is difficult to prepare the alloy catalyst, and the high temperature preparation condition will make the platinum particles more likely to agglomerate, resulting in the decrease of ECSA and the catalytic activity. In view of the above problems, metals with higher electrode potential, such as silver and copper, can be used in the selection of alloy components. In addition, the alloy catalyst can be subjected to core-shell processing to prepare the core-shell catalyst with the inner core of alloy or transition metal coated with platinum on the surface[22,23]. The catalyst has high catalytic activity and stability and low cost. However, the transition metal core in the catalyst will be lost during use, which is prone to structural differences and inconsistent catalytic performance. These problems need further study.
According to density functional calculations, Nørskov et al. Found that some platinum-early transition metal alloys (Pt-ETMAs) are as active as platinum-later transition metal alloys (Pt3Ni, Pt3Co, Pt3Fe) in ORR[24]. Their study further proved that it exhibited a higher heat of formation relative to Pt3Ni,Pt-ETMAs, which could effectively prevent surface metal migration. The experimental study shows that the formation enthalpy of Pt-ETMAs (about 4 orders of magnitude) is higher than that of platinum-late transition metal alloys, which further verifies the durability of Pt-ETMAs[25]. Despite the high stability of Pt-ETMAs, a major drawback is the difficulty in synthesis. Because platinum has a corrected standard reduction potential (Pt2+􀰙Pt,Eo=1.18 V) compared to early transition metals such as :T3+􀰙Y,Eo=2.37 V, the reduction of platinum precursors is faster than other transition and non-transition metals[26]. Mg is a cheap and abundant metal element, and its mass is only about 12% of Pt, which makes it possible for Mg to be used in PEMFC catalysts. Tetteh et al. Developed a new type of PtMg alloy catalyst, which uses magnesium powder as a precursor to overcome the oxygen affinity of magnesium, induce the alloying of magnesium and platinum, and enhance the activity and durability of the catalyst[27].

3.1.2 Alloying treatment selects crystal face morphology to improve stability

The rapid development of nanomaterials technology has promoted the research and development of highly active platinum alloy catalysts. The ORR activity of the platinum catalyst can be optimized by adjusting the structure by controlling the particle size, crystal morphology, etc[28]. Reducing the particle size can obtain higher ECSA and improve the utilization rate of platinum. The catalytic activity of Pt (111) is 10 times higher than that of Pt (110) and Pt (100) planes, and crystal engineering can realize the selective exposure of high-energy crystal planes favorable for ORR[29]. As shown in Fig. 2, the exposure of the modified local atomic environment can create more unsaturated coordination sites, which will enhance the ORR performance of platinum crystals. Specific nanostructure design can reduce particle agglomeration, spillage, or migration during electrochemical operation, thereby improving the activity and stability of the catalyst[30]; For example, thin platinum nanowires with isotropic properties show considerable ORR activity and high stability[31].
The activity of Pt3Ni(111)) is 90 times higher than that of commercial Pt/C catalyst, but the stability of Pt3Ni(111) is poor, so it has been proposed to improve the stability of Pt3Ni by changing its nanostructure. Changing the morphology and structure of the catalyst can also increase its active sites and further improve the catalytic activity and stability. As shown in figs. 3A and B, the charge transfer and electronic effects in the PtAu alloy catalyst greatly improve the activity and durability of the catalyst[32]; Moreover, the alloy element Au has the advantage of stabilizing the metal from oxidation and corrosion, thereby further ensuring the performance of the catalyst[33]. Zhang et al. Developed the method of microwave-assisted rapid heating and cooling, as shown in Fig. 3C, and successfully synthesized PtAu alloy nanocrystal catalyst with tunable composition[34]. The rapid heating and cooling strategy can synthesize PtAu alloy nanocrystal catalyst in a short time. Xie et al. Used a simple microwave-assisted strategy to synthesize a series of platinum/graphene (Pt-Au/GNs) catalysts with controllable Pt/Au ratio[35]. As shown in Fig. 3D, it was found that the Pt-Au/GNs catalyst with lower gold content (Pt: Au = 1: 0.1) simultaneously showed excellent electrocatalytic activity and stability in MOR and ORR because of the good dispersion of Pt-Au bimetallic and the change of platinum electronic structure after the addition of gold component.
Alloying is to adjust the local structure of platinum through the change of electron and lattice in the active component, so as to weaken the ability of chemical surface to adsorb oxygen. At the same time, the lattice contraction reduces the idle electronic States, and the d-band center moves downward relative to the Fermi level. These two factors determine that the alloy catalyst has good electrocatalytic performance and stability[36]. Therefore, the change in the local structure of the alloy catalyst improves the surface adsorption of oxygen species compared with pure platinum, which is a sign of enhanced ORR performance and durability of the catalyst[37].

3.1.3 Doping treatment improves stability

On the basis of the binary alloy catalyst, a multi-alloy catalyst can be formed by doping a third metal precursor salt, which will show higher ORR activity than platinum. Heteroatomic metal Mo and Rh doped PtNi alloys exhibit ultrahigh mass activity and specific activity[23,38].
Taking the platinum binary alloy catalyst as an example, when the third metal component is doped, the comprehensive surface strain will occur on the surface of PtM alloy[39]. The PtNi nano-alloy catalyst has better catalytic activity among the binary catalysts. Mo-doped Pt3Ni catalyst, electrochemical characterization revealed that the ORR activity of the catalyst was 6.98 A·mg-1[23]. Fine-tuning the chemical and electronic properties of the surface layer by the combination of Mo-Pt and Mo-Ni alleviates the dissolution of Pt and Ni during electrochemical operation, making Mo-Pt3Ni a highly active and durable catalyst. Highly ordered alloys or intermetallic compounds can adjust the structure of local platinum atoms, as shown in Figure 4A. Due to the Pt-M interaction in the alloy catalyst and the addition of new Pt-M bonds due to doping, the catalytic activity and durability of the catalyst are improved[40,41]. The Pt-Ni binary alloy catalyst was doped with Rh metal to improve the durability of the catalyst. The electrochemical characterization results showed that the initial ORR activity of the ternary alloy catalyst was lower than that of the binary catalyst, but the durability of the catalyst was significantly improved[38]. After 8000 cycles of stability test, the ORR mass activity of three-way catalyst was still 0.72 A·mg-1, but the ORR mass activity of Pt-Ni was 0.12 A·mg-1. The results of transmission electron microscopy show that the addition of Rh inhibits the migration of platinum atoms. It is found that the doping of Ga, Cu and other metals can also maintain the spatial structure of Pt-Ni catalyst to a certain extent, thus improving the durability of the catalyst[42][43]. Au is the best alloying element, because the introduction of gold clusters can improve the oxidation potential of Pt, reduce the migration and dissolution of Pt, and improve the stability of the catalyst. Such as Au-Pt3Ni (Figure 4B), Au-PtCo, etc., Au is the most promising among the strategies of alloying treatment to improve catalyst stability[44][45].
Doping is to introduce other metal components to further control or design the structure, size or morphology of the catalyst on the basis of alloying treatment, so as to improve the catalytic activity and stability of the catalyst.

3.1.4 Encapsulation, confinement, and spatial structural treatments to enhance stability

Due to the dissolution and agglomeration of the active component platinum in the fuel cell catalyst, the durability of the catalyst is greatly reduced. To solve these problems, polymer materials with unique molecular structures can be used as functionalization and anchoring agents. It can not only enhance the interaction between the active component and the carrier in the catalyst, but also limit the dissolution and agglomeration of the active component platinum, thus significantly improving the stability of the catalyst. A protective microporous coating is used, which is an inherently microporous polymer (PIM-EA-TB) with inertness and high rigidity (non-blocking)[46]. The rigid microporous PIM-EA-TB film was used here as a coating to encapsulate the Pt/C catalyst, while maintaining the effective diffusion channel diameter of 1 ~ 3 nm for water and reaction species. After accelerated aging tests (0. 541 ~ 0.941 V, 5000 times), the ECSA of the commercial platinum carbon catalyst was reduced to 66%, while the ECSA of PIM @ Pt/C was only reduced to 92%, and the average particle size of platinum nanoparticles in the new catalyst was significantly reduced, so the modification of Pt by PIM effectively improved the durability of the catalyst. A novel Pt-UTCL/C catalyst was prepared by coating Pt nanoparticles with ultrathin porous nanocarbon layer (UTCL)[47]. The experimental results show that UTCL effectively prevents the agglomeration and shedding of Pt nanoparticles and enhances the stability of the catalyst. Chen et al. Directly polymerized polyaniline (PANI) film on the carbon surface of Pt/C catalyst by in-situ chemical oxidative polymerization, and the stability of the prepared new catalyst was significantly improved[48]. The activity of the catalyst is determined by the number of active sites on the three-phase interface (TPB region) composed of gas, liquid and solid phases. Therefore, it is a hot research topic to select the appropriate polymer-coated catalyst to improve the stability while maintaining the activity of the catalyst.
Aiming at the problem of migration and shedding of Pt nanoparticles, we can also start from the perspective of confinement. Confinement is to achieve uniform spatial distribution by some porous materials such as molecular sieves, metal-organic frameworks, covalent organic frameworks (Fig. 5), etc., which act as cages to encapsulate and anchor the precursors of metals[49,50][51]. These precursors will be removed in subsequent processes, while their skeletons will be used to form individual atoms and prevent further reactions. Pt nanoparticles supported on graphitic carbon nitride (g-C3N4) are confined in the vacancies of carbon nanospheres (CNSs) scaffolds, which have a porous framework to ensure mass transfer and can also act as a conductive agent, compensating for the poor conductivity of g-C3N4. The movement, coalescence and detachment of Pt nanoparticles on the support can be effectively inhibited, and the confinement effect of CNSs on Pt nanoparticles contributes to the improvement of catalyst stability[52].
In contrast to disordered structural alloys, ordered intermetallic compounds have a specific arrangement with more precise lattice properties. It is found that different kinds of ordered alloys, such as binary PtCo, PtIn, CoPt and ternary PtNiCo, have better stability than their disordered alloys[53][54][55][56]. Most ordered intermetallic compounds are prepared by high temperature annealing. At high temperature, the active component particles are easy to agglomerate, so it is important to control the particle size in the intermetallic compound. The use of metal oxide or carbon coating can avoid particle aggregation or sintering. Chung et al. Coated 6 nm PtFe nanoparticles with dopamine, and the dopamine was carbonized in situ during annealing to form a nitrogen-doped carbon protective layer to protect the PtFe nanoparticles. At the same time, the PtFe nanoparticles themselves underwent a phase transition process to form L10-PtFe nanoparticles[57]. Compared with disordered PtFe and commercial Pt/C catalysts, L10-PtFe nanoparticles have better ORR activity and stability. Recently, ZnO has been used as a bifunctional precursor to provide atomic zinc diffusion into the platinum lattice to form ordered L10-PtZn. ZnO also acts as a physical barrier to prevent particle sintering and agglomeration during heat treatment. The strong ionic metal bonding between Zn and Pt, the shortened platinum lattice parameter, and the unique biaxial strain contribute to the enhanced ORR activity and catalyst durability[58]. Ordered intermetallic compound Pt-based nanocrystals have been shown to be a promising class of ORR catalysts, which have higher chemical stability and catalytic activity than ordinary disordered Pt-based alloys due to the highly uniform element distribution and alloying degree of the intermetallic compound nanocrystals.
In addition, the stability of the catalyst can also be improved by adjusting the spatial structure of the catalyst. Boone et al. Used a lower metal loading on Ptx-Pdy-Cuz/GNR nanocomposites (NCs) containing a porous dendritic alloy structure[59]. The existence of the metal alloy nano-dendritic structure on the Pt/Pd surface leads to the change of the active site caused by the contraction of the lattice, which improves the stability of the catalyst. Fang et al. Obtained PtCu alloy with nanowire network (NWN) structure by simple polyol synthesis method and self-assembly process in water/ethylene glycol (EG) mixed medium[60]. The ORR catalyst test showed that the activity and durability of PtCuNWNs/C were higher than those of PtNWNs/C and commercial Pt/C, because the NWN structure had a large number of surface defects and the electronic effect of Cu atom alloying. Yang et al. Prepared an octahedral Pt-Ni alloy catalyst with a size of 3 nm by a pulsed hydrothermal method. Compared with the conventional hydrothermal method, the pulse temperature can reduce the nucleation time, and the morphology of nanoparticles is different due to the growth temperature and time, which improves the stability of the catalyst[61]. Chen et al. Synthesized Pt-Co nanoframes from solid rhombic dodecahedra by chemical etching of Co (Fig. 6)[62]. During the preparation of the catalyst, the optimal bonding strength of the reaction intermediate improves the catalytic activity, and the unique solid structure and little electrochemical Co dissolution make the prepared Pt-Co nanoframes show excellent ORR mass activity in acidic electrolyte.

3.2 Carrier angle lift stability

3.2.1 Carbon carrier treatment improves stability

The support of the catalyst is also crucial for the overall activity and stability of the catalyst. Since the discovery of materials such as graphene and carbon nanotubes, carbon-based catalysts have developed faster and faster, and their stability has become better and better due to their unique structure. The dispersion, shape, size and structure of platinum nanoparticles can be adjusted by carrier materials, so the development of suitable carrier materials has become one of the effective strategies to achieve high-performance PEMFC[63]. The requirements for the catalyst support include: excellent conductivity; High corrosion resistance; High active specific surface area; High binding force with catalyst particles; The ability to effectively inhibit the migration and aggregation of catalyst particles. The catalyst particles also have a strong dispersion force on the support, that is, the catalyst has many mesopores. In the fuel cell working environment, platinum has a catalytic effect on the oxidation corrosion of carbon, and the metal particles prepared by chemical methods are deposited on the carbon support, and the weak interaction between them will lead to the migration and sintering of metal particles on the carbon support, thus reducing the active surface. As a result, catalyst stability is reduced during long-term operation. In order to solve the corrosion problem of carbon support, the development of highly graphitized carbon materials has been proposed, but after high temperature graphitization, the platinum loading of the catalyst prepared is low, which can be met by the research of new highly graphitized carbon-based materials, such as carbon nanotubes, graphene and so on.
Carbon nanotube (CNT) is a promising catalyst support material because of its high specific surface area and high electrical conductivity. However, carbon nanotubes have high chemical inertness, if they are directly used as carriers, the deposition of platinum nanoparticles will be very difficult and the dispersion will be poor, so special chemical treatment is needed to destroy the surface integrity of CNTs and introduce structural defects. In order to make CNTs better used in catalysts, many researchers have begun to use functional surface treatments, such as when CNTs are modified by thiol groups and conductive polymers as catalyst supports, which can not only serve as the deposition anchor sites of Pt nanocrystals, but also maintain the complete structure of CNTs, thus prolonging the service life of catalysts and increasing the stability of catalysts[64]. Graphene is a two-dimensional carbon material with only one carbon atom thickness, which is a hexagonal planar film composed of carbon atoms with sp2 hybrid orbitals, and has been widely studied as a new type of catalyst support material. Vinayan et al. Obtained N-doped graphene (NG) support by high temperature pyrolysis[63]. The doping of N atoms enhances the binding force between platinum nanoparticles and graphene, prevents the agglomeration and shedding of platinum particles, and improves the durability of the catalyst.
The commonly used carbon support (Vulcan xc-72) has the advantages of easy preparation, high conductivity and low cost, but its stability is poor, especially under high potential cathodic conditions, and it is very vulnerable to severe electrochemical oxidation corrosion. Wang et al. Prepared Pt/MnO2-CNF by electrospinning, showing higher ECSA and durability[65]. Nitrogen-doped porous carbon nanofibers were prepared by electrospinning. Pt nanoparticles were supported on the support, and it was observed that the uniformly sized Pt nanoparticles were well dispersed on the cross section of the catalyst, resulting in their ORR activity and durability significantly better than those of commercial platinum-carbon catalysts[66].
Heteroatom doping in the support can also improve the stability of the catalyst. Doping carbon support with heteroatoms (especially nitrogen) affects the catalyst system in three ways: (1) the nucleation and growth rate of the active species during catalyst synthesis; (2) chemical bonding between the metal active component and the support; (3) Electronic structure of the active component. The defects generated during the insertion of nitrogen into the carbon lattice increase the interaction between the carbon support and platinum nanoparticles, improving catalyst durability and ORR activity.

3.2.2 Non-carbon support treatment improves stability

Due to carbon support corrosion and the weak interaction between platinum nanoparticles and carbon support, carbon support materials are more susceptible to oxygen corrosion at high potential, resulting in the migration of metal particles and the overall performance of fuel cells. In addition to high active specific surface area and conductivity, the carrier also has the advantages of high corrosion resistance and strong interaction with platinum particles[67]. Some studies have been carried out on non-carbon supports, which not only need to meet the requirements of fuel cell catalyst supports, but also interact with catalyst metals so that metals can be adsorbed on catalyst supports. The strong interaction between the transition metal and platinum in the M-N-C system can also be utilized, which can not only improve the bonding strength between platinum and the carrier, but also improve the dispersion of platinum, so that the active component can be better attached to the carrier[68,69].
Recent studies have found that the main non-carbon support materials include nitrides, carbides, borides, mesoporous silica, conductive polymers and metal oxides[70]. It has been noticed that some metal oxides have suitable specific surface area, mechanical strength and electrochemical stability. Compared with carbon supports, the biggest advantage of metal oxides is that they have stronger electrochemical oxidation resistance and can significantly improve the stability of catalysts. Rare earth metals and their compounds (such as Ce and CeO2) have attracted much attention because they have more B and f orbital electrons. As shown in Fig. 7 a, Ma et al. Used CeOx to form hybrid metal alloy and rare earth metal oxide nanocrystals in FePt (i.e., the doping of PtFe-PtxFeyCezOj),Fe and Ce atoms can lead to lattice contraction, which changes the electronic structure of Pt atoms through nanohybrid structure and atomic interaction, and improves the stability of the catalyst[71]. Titanium dioxide has low cost, high chemical stability and environmental friendliness in the application of catalyst system; Moreover, titanium dioxide, as a semiconductor, has excellent stability of supported platinum particles (Pt/TiO2), but titanium dioxide has poor conductivity and activity, so it has attracted much attention to consider combining TiO2 with carbon matrix to synthesize TiO2/C composite as an electrocatalyst support[72]. As shown in Fig. 7 B, Kong et al., using a gel process and a high temperature annealing method, prepared a Pt/TiO2@CNT catalyst, which combines the advantages of TiO2 (high stability and strong interaction with supported Pt nanoparticles) and CNT (high specific surface area and larger conductivity)[73]. Wang et al. Designed a TiO2-C composite support (Fig. 7c),TiO2 was well dispersed on the carbon support, which enhanced the metal-support interaction, thus improving the catalytic activity of oxygen radicals[74]. After high temperature durability test, it also showed higher ORR activity compared with commercial 20 wt% Pt. Tungsten trioxide (WO3) is also considered as a promising support material for electrocatalysts because of its various special electrochemical properties.
At present, platinum and platinum-based alloys are the most effective catalysts for ORR in PEMFC, but the catalysts are easily deactivated by CO poisoning, high cost and limited platinum reserves, which hinder the large-scale application of catalysts. It is necessary to study non-noble metal catalysts working under acidic conditions, in which two main issues need to be considered: first, the dispersion of active sites and the transport properties of ORR-related species, which are determined by the surface area and pore structure of the catalyst; Second, the catalyst should have efficient intrinsic active sites and good durability, which is determined by the chemical composition and microstructure of the catalyst[75]. It has been shown that heteroatom doping (e.g., N, B, S, and P) can change the electronic structure of the doped catalyst, resulting in higher ORR activity. Especially for N atom doping, the electronegativity of N is higher than that of C, which can enhance the adsorption and reduction of oxygen, and there is electronic interaction between adjacent C atoms and N, which leads to the improvement of ORR activity and stability[76].

4 Conclusion and prospect

In order to realize the large-scale application of PEMFC, this paper summarizes the problems of high cost and poor durability of PEMFC from two perspectives. Firstly, from the perspective of active components, the post-treatment of catalysts can be improved from the aspects of alloying treatment, doping of other metal atoms and confined space structure of catalysts to enhance the stability of catalysts. Secondly, from the point of view of catalyst support, such as the development of new support materials and the research of non-precious metal catalysts, the support is doped with nitrogen atoms to enhance the force between the metal component and the support, and at the same time, the spatial structure of the support can be considered to increase the dispersion of the active component. At present, the most important problems of ORR catalysts are limited activity, poor durability and high cost, which seriously hinder the application of fuel cells. Platinum durability (oxidation corrosion, dissolution, migration, agglomeration), carbon corrosion and oxidation, lack of integration of the platinum support, and CO poisoning are the fundamental problems associated with poor performance of the membrane electrode assembly catalyst. Therefore, it is necessary to study the design principles of catalysts from the basic aspect, and to understand the core issues that hinder the conversion of half-cell performance to full-cell performance and practical applications from the membrane electrode assembly system.
At present, the performance of platinum-based catalysts has been significantly improved by different design strategies. By controlling the size, composition and alloying of the catalysts to improve the oxygen adsorption capacity, platinum-based alloys with different morphologies have shown excellent performance. However, for more rapid commercial applications, advanced and novel design and scale preparation strategies should be explored to minimize the use of platinum and improve the service life of the catalyst. There are the following expectations for the research of fuel cell catalysts: (1) The key to improve the stability of catalysts is to study the catalytic mechanism and active sites of catalysts; (2) The size, structure and morphology of the catalyst have a great influence on the stability of the catalyst. The synergistic effect between larger specific surface area, porous structure and defects can promote the ORR performance. During the continuous use of the electrode, the catalyst may suffer from particle separation, aggregation, support corrosion and Ostwald ripening, resulting in reduced catalyst stability. It is very important to prepare catalyst particles with high specific surface area and defects. Ensuring high activity and improving stability of catalysts is the most important thing for the overall performance of fuel cells, and further research and design of catalysts with high stability is the key to promote the commercialization of fuel cell catalysts.

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Funding

National Natural Science Foundation of China(21673064)

National Natural Science Foundation of China(51802059)

National Natural Science Foundation of China(21905070)

National Natural Science Foundation of China(22075062)

Heilongjiang Province “Hundred Million” Major Project of Engineering Science and Technology(2019ZX09A02)

Shandong Taishan Industry Leading Talent Project(2017TSCYCX-33)

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