Investigation of Practical Electro-catalysis using the Floating Electrode

本项目的目的是研究电催化剂在工业电催化应用中所需的高电流密度下的行为。该项目的主要内容将是负责PEM燃料电池中氧还原反应(ORR)的阴极催化层。在PEM燃料电池中，ORR反应是最困难的反应，提高该反应速度是降低膜电极组件(MEA)成本的主要途径，因为目前MEA阴极上所需的PGM负载量太高。成本更低的MEA将使燃料电池能够更广泛地渗透到现有和发展中的市场，如汽车。目前，只有一种电化学方法可以在高电流密度下探测MEA外部气体扩散电极的电催化行为，这是帝国理工学院Kucernak教授团队开发的浮动电极(FE)方法。FE的原理如图1所示：电催化剂同时与气态反应物、水溶液和良好的电子导体(溅射金)接触。该技术的一个主要特点是，它能够弥合研究不同PGM负载的离散电催化剂团聚和连续催化剂层之间的差距，同时避免使用全面的电池内MEA测试遇到的复杂问题。虽然Chris Zalitis已经证明了这种方法的可行性和实用性，并将其内部收购给JMTC，但FE的应用提出了许多新的问题，例如：1.为什么ORR催化剂(铂/碳)的质量活性随着电极负载的增加而明显降低？2.催化层内和催化层接触的电解液(固体膜或酸)内的质子传导对ORR有多大限制？3.能否明确地表明，在高电流密度下，铂合金催化剂的活性实际上低于只有铂的催化剂，尽管在低电流密度下情况相反--这些问题的解决将有助于FCR小组对MEAs行为的理解，补充工作将集中于将这种理解应用于改进的催化剂和催化层设计。一旦上述基本问题得到解决，该项目在后期阶段可以检查其他气相电催化剂系统。其中包括析氢反应，它普遍存在于电氯化等工业电化学过程的阴极上，也是使用PEM电解槽从可再生能源中产生氢气的核心。此外，电化学系统的灵活性使间歇性可再生能源能够用于合成除氢以外的有价值的化学品，包括氨和小链烃。有限元允许在更现实的条件下对这种高速率耗气或放气的电化学反应进行基本的研究。

The purpose of this project is to investigate the behaviour of electro-catalysts at the high current densities they need to operate at for industrial electro-catalysis applications. The main context of the project will be the cathode catalyst layer responsible for the oxygen reduction reaction (ORR) in PEM fuel cells. The ORR is the most difficult reaction in a PEM fuel cell and improvements to this reaction rate are the main route to lower cost membrane electrode assemblies (MEAs), because the currently required PGM loading on the MEA cathode is considered too high. Lower cost MEAs will enable wider penetration of fuel cells into existing and developing markets, such as automotive.At present, only one electrochemical method is available that can probe electro-catalysis behaviour of gas diffusion electrodes outside of an MEA at high current density; this is the Floating Electrode (FE) method developed at Imperial College in Prof. Kucernak's group . The principle of the FE is shown in Figure 1: the electro-catalyst is simultaneously in contact with gaseous reactant, aqueous electrolyte and a good electronic conductor (sputtered gold). A key feature of the technique is its ability to bridge the gap between studying discrete electro-catalyst agglomerates and continuous catalyst layers of different PGM loading, whilst avoiding the complicating issues encountered using full-blown in-cell MEA testing. Whilst the feasibility and utility of this method has been shown by Chris Zalitis and bought in-house to JMTC, application of the FE has thrown up many new questions, for example:1. Why does the mass activity of the ORR catalyst (Pt/C) apparently decrease significantly with increased electrode loading?2. How limiting to the ORR is the proton conduction within the catalyst layer and within the electrolyte that the catalyst layer contacts (solid membrane or aqueous acid)?3. Can it be definitively shown that Pt alloy catalysts are actually less active at high current densities than Pt-only catalysts, despite the converse being true at low current densities?Resolution of these questions will feed in to the FCR group understanding of the behaviour of MEAs and complimentary work will focus on applying the understanding to improved catalyst and catalyst layer design.Once the fundamental questions above have been resolved, the project in its later stages can examine other gas-phase electro-catalyst systems. These will include the hydrogen evolution reaction, ubiquitous at the cathodes of industrial electrochemical processes such as electro-chlorination and central to the generation of hydrogen from renewable energy using PEM electrolysers. Further, the flexibility of electrochemical systems enables the use of intermittent renewable energy to synthesise valuable chemicals other than hydrogen, including ammonia and small chain hydrocarbons. The FE allows such high rate gas-consuming or gas evolving electrochemical reactions to be studied in a fundamental way under much more realistic conditions.