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所需的高电流密度下的行为。该项目的主要内容将是PEM燃料电池中负责氧还原反应（ORR）的阴极催化剂层。ORR是PEM燃料电池中最困难的反应，提高该反应速率是降低膜电极组件（MEA）成本的主要途径，因为目前MEA阴极上所需的PGM负载过高。较低成本的mea将使燃料电池更广泛地渗透到现有和发展中市场，如汽车市场。目前，只有一种电化学方法可以探测高电流密度下MEA外气体扩散电极的电催化行为；这是帝国理工学院Kucernak教授小组开发的浮动电极（FE）方法。FE的原理如图1所示：电催化剂同时与气态反应物、水性电解质和良好的电子导体（溅射金）接触。该技术的一个关键特点是它能够弥合研究离散电催化剂团聚体和不同PGM负载的连续催化剂层之间的差距，同时避免了使用成熟的电池内MEA测试所遇到的复杂问题。虽然这种方法的可行性和实用性已经由Chris Zalitis证明，并在JMTC内部购买，但有限元的应用提出了许多新的问题，例如：1。为什么ORR催化剂（Pt/C）的质量活性随着电极负载的增加而明显降低？质子在催化剂层内和催化剂层接触的电解质（固体膜或含水酸）内的传导对ORR的限制有多大？是否可以明确地表明，Pt合金催化剂在高电流密度下实际上比纯Pt催化剂活性更低，尽管在低电流密度下正好相反？这些问题的解决将有助于FCR小组对mea行为的理解，而补充工作将侧重于将理解应用于改进的催化剂和催化剂层设计。一旦上述基本问题得到解决，该项目的后期阶段可以研究其他气相电催化剂系统。这些将包括氢的释放反应，在工业电化学过程的阴极上无处不在，如电氯化，以及使用PEM电解槽从可再生能源中产生氢的核心。此外，电化学系统的灵活性使得利用间歇性可再生能源来合成除氢以外的有价值的化学物质，包括氨和小链碳氢化合物。FE允许在更现实的条件下以基本的方式研究这种高速率的气体消耗或气体演化电化学反应。

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.