Matter: the relationship between catalysts in fuel cells and metal air cells

The research on fuel cell catalyst has made great progress in understanding the reaction mechanism in the catalytic process, thus greatly improving the performance of fuel cell On the contrary, the performance of metal air battery (mAb), such as lithium air (Lab) and zinc air battery (Zab), still needs to be improved Although the metal anodes used in these systems are still far away from commercial applications, the research on the performance of catalysts for such batteries can not be ignored The purpose of this paper is to discuss the similarities and differences between metal air catalysts and catalysts used in water-based (alkaline / acidic) and non proton electrolyte fuel cells, hoping to apply the successful experience of fuel cell catalysts to metal air cells With the increasing demand for high performance energy storage systems, electrocatalysis has become a major topic of interest The discovery of catalysts related to oxygen reduction reaction is helpful to commercialize fuel cell based electric vehicles However, metal air cell, a technology closely related to fuel cell, has not yet found commercial application Similar to lithium-ion batteries, metal air batteries have the potential to use the power grid for charging without the need for hydrogen infrastructure In the past decade, lithium air battery and zinc air battery in metal air battery have aroused great interest Unfortunately, the performance of state-of-the-art metal air batteries is still a long way from practical application Therefore, it is valuable to apply the mature electrocatalyst in fuel cell to metal air cell 1 Water battery (1) acid medium water battery can be divided into acid medium and alkaline medium Among them, the most successful is acid medium hydrogen fuel cell, also known as proton exchange membrane (PEM) fuel cell In PEM, it is generally believed that the adsorbed oxygen will be reduced to the adsorbed OH * or ooh * before further reduction to H 2O In this process, the fracture of O-O bond is usually the limiting step of catalyst PEM fuel cells need to reduce the over potential associated with orr to operate with sufficient energy efficiency and power density The related catalytic process is shown in Figure 1a In the application of catalyst, it is expected that the surface of catalyst has the best adsorption energy for each reaction intermediate, so as to achieve the best catalytic activity However, the performance of catalysts in orr is controlled by linear proportional relationship (LSR) That is, a catalyst can not have the best adsorption energy for each reaction intermediate As a result, the volcanic map shown in Fig 1b also appears It is generally believed that the catalyst in the middle of the volcanic map has suitable adsorption energy for all kinds of reaction intermediates, so it has better catalytic performance In the volcanic map, Pt 3Ni (111) catalyst with Pt surface has the best catalytic activity, and its catalytic activity is 90 times that of commercial Pt / C catalyst Figure 1 The properties of oxygen reduction reaction (source: m atter) in addition to the expensive platinum group catalysts, the research of non noble metal catalysts has made great progress However, at present, the main catalyst used is platinum group catalyst, which also brings high cost to PEM In addition to the economy of catalysts used in PEM fuel cells, the construction of hydrogen storage and transportation infrastructure in the whole society will become a major obstacle In view of this, the acid renewable fuel cell (RFC) which will precipitate O 2 and H 2 during the charging process has a considerable development prospect, so the oxygen release reaction (OER) in acid medium has become an attractive research field However, most of the oer catalysts studied are not stable in acid medium Although IR based catalysts have excellent oer performance in acid medium, generally speaking, oer / orr process in acid medium is far more difficult than in alkaline medium, which limits the selection and design of many materials For systems where only orr is required and Pt is still available as a catalyst, acidic media may not be an issue However, for rechargeable battery systems such as mAb, there are few researches based on acid medium For Zab, the high chemical reactivity of zinc in acid brings another completely different engineering challenge, that is, the separation of anode and acid electrolyte (2) the most obvious fusion point between mAb and fuel cell may be between mAb and fuel cell For example, the process of Orr and oer of basic Zab is very similar to that of fuel cell in basic condition As we all know, many orr catalysts for alkaline fuel cells are considered suitable for Zab orr For example, α - MnO2 has been proved to be equally effective in basic Zab and basic RFC Similarly, water-based lithium air battery (a-lab) has the ability of 4 electron transfer, which can form water-soluble LiOH through O 2 This occurred through a series of reactions, almost the same as that in the orr of Zab and basic RFC Similarly, the activity of catalysts in alkaline media is also controlled by LSR and volcanic map However, this does not provide any means for researchers to reasonably design new and better catalysts In contrast, descriptors related to LSR and intrinsic catalytic properties of materials have always been an important topic in catalyst design Among all the proposed descriptors (oxidation state of transition metal, O 2p band center, 3-D electron number of B-site ion), it seems that e g filling can best describe the catalytic properties of the catalyst, which can be used to explain the properties of perovskite based catalysts and transition metal oxides (TMO) In terms of dual function Catalysis (oer / ORR), the design and selection of catalysts between alkaline fuel cell and water-based mAb are almost the same The performance of bifunctional catalysts considers both Orr and oer directions The performance index of bifunctional catalyst usually includes the value of charge potential (OER) minus discharge voltage (ORR) at a specific current density, rather than considering only the initial potential of a single reaction Among them, Pt / C-based catalysts have been proved to be very effective in water-based lab, which can enhance oer and orr However, because TMO catalyst is stable in alkaline condition, and orr process is relatively easy in alkaline condition, TMO materials with low cost dominate the catalysts of a-lab, Zab, and basic RFC These catalysts have strong orr / oer activity and corrosion resistance in alkaline electrolyte Among them, spinel type Co 3O 4 and its derivatives are considered as the core materials for the realization of dual-function catalysts In terms of design, many current dual-function catalyst designs focus on improving the circulation stability of these TMO materials The performance of oer / orr bifunctional catalyst will decrease in the long-term cycle The degradation of the performance of these catalysts is related to the oxidation of the carbon based conductive promoters in the circulation process In the absence of carbon, the degradation can be attributed to the aggregation of catalyst particles or the morphology change of TMO with high specific surface area A lot of work has been focused on the design of TMO with a large number of active sites and stable in the process of circulation, but the mechanism of bifunctional catalyst is less studied Bifunctional catalysts usually can not have optimal oer performance and orr performance at the same time An obvious solution is to use dedicated orr catalyst and oer catalyst respectively, but this will increase the total weight of catalyst Another promising direction may be to adjust the adsorption characteristics from the aspect of electrolyte design In the early stage, it was found that the adsorption properties of the reaction intermediates and the corresponding catalyst activity were changed by changing the properties of the electrolyte cation and the water solvated shell around the cation In addition, how to break the LSR relationship and the limit of volcanic map has been regarded as one of the most attractive and influential directions of catalyst research Although the catalyst design of a-lab may be similar to that of RFC and Zab, the main problem of a-lab is the protection of lithium metal Therefore, most of the work of lab is focused on the non proton lab system In the non proton lab, the non proton electrolyte reacts with lithium metal in a relatively mild way, and usually forms a quite stable passivation film in time Unfortunately, the use of aprotic electrolytes will significantly change the reaction path of lab, and in some ways change the function of Orr and oer catalysts 2 Some early studies on the reduction of oxygen in aprotic electrolyte (1) attempted to transfer the traditional heterogeneous orr catalyst to lab However, the use of this type of orr catalyst is a relatively small sub field in lab due to its low importance compared with oer, only a few studies have been carried out at present The early study of the orr of lab investigated the use of precious metals as catalysts A volcanic map similar to the orr of fuel cell has been found, in which PD has the highest activity (higher stable stage of discharge level), followed by Pt, Ru, Au and pure carbon In addition, Orr catalysts based on metal oxides were also studied Although the results are fascinating, the voltage obtained during discharge is only ~ 0.2V, which is almost less attractive than the reduction of oer over potential In addition, after short cycle, the capacity of the battery will change dramatically, which shows that lab and Zab, RFC have at least some different requirements for the design of catalyst system It can be said that the most influential application of heterogeneous catalysts to lab orr is the formation and stabilization of crystalline LiO2 Among them, iridium reduced graphene oxide has been proved to be an effective nucleation center to promote the formation and stabilization of crystalline LiO2 The stabilization of LiO2 can appropriately reduce the charging voltage in the subsequent cycle, thus improving the energy efficiency of the battery Fig 2 the orr process (source: matter) of aprotic lab with different donor numbers is shown in Fig 2 In aprotic lab, the reduction of O2 usually results in the formation of LiO2 in aprotic electrolyte, and then the formation of solid li2o2 Because the intermediate product LiO2 has higher solubility than li2o2 in the non proton electrolyte, the subsequent reduction or disproportionation of LiO2 leads to the formation of li2o2 Therefore, in this process, the catalyst can be used to stabilize and prolong the life of LiO2, making it diffuse, reduce and deposit in a non passivated way In contrast to RFC and Zab for catalyst design, for non proton lab, a lot of work has been devoted to the shape control and distribution of collecting fluid to prevent pore plugging The deposition of insoluble insulating Li 2O 2 can be controlled more effectively by the design of collecting fluid (2) The oxygen evolution reaction is opposite to the aqueous mAb and RFC, and the typical discharge product of lab is solid In addition, the release of oxygen from lab does not require the same catalyst as water decomposition oer In water decomposition, O-O is usually reformed by using precious metal as catalyst In the conventional reduction process of O 2 to Li 2O 2 at room temperature, the process is quite different due to the incomplete O-O bond fracture Direct application of popular fuel cell catalysts (such as Pt) to lab oer actually promotes the degradation of non proton electrolytes This not only leads to performance degradation, but also complicates any detection of oer processes The main source of Li 2O 2 charge overpotential is the ion and electron conductivity of Li 2O 2 difference Therefore, the main influence on oer process is the previous orr cycle (i.e Li 2O 2 formation process) and the resulting Li 2O 2 shape