Investigation of Practical Electro-catalysis using the Floating Electrode

Lead Research Organisation: Imperial College London
Department Name: Dept of Chemistry

Abstract

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.

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/P51052X/1 01/10/2016 30/03/2022
2135758 Studentship EP/P51052X/1 15/10/2017 14/01/2022 Xiaoqian Lin
 
Description We affirm that CO adsorption rather than Hupd should always be used to check the ECSA (electrochemically active surface area) of the Pt-based catalyst layer especially when the Pt loading is low, and hydrophobicity of the layer is key to facilitate high mass transport especially within the HCD (High Current Density) region of the ORR (Oxygen Reduction Reaction) which agrees with the work on MEA.

There are many different ways of making electrode hydrophobic. In the floating electrode system, we found that coating a very small amount of Teflon AF 2400 (a type of perfluoropolymer) on the electrode which can increase its HCD region performance by 100 times.
We compared three different perfluoropolymer materials with similar hydrophobicity and revealed that it is the gas permeability of perfluoropolymer (which acts as a barrier around the catalyst agglomerates) that mainly affects the maximum current density (power density) of both HOR (Hydrogen Oxidation Reaction) and ORR. With a
Koutecký-Levich type analysis, we saw that when using Teflon AF 2400 which has the highest gas permeabilities, there is barely any mass transport limitation due to the perfluoropolymer.

We provide insights into how Nafion ionomer content can affect the performance of the catalyst layer. We compared different loadings of Nafion ionomers on floating electrode and only saw suppression on HCD region of ORR, which disagrees with the results on RDE (Rotating Disk Electrode) where a significant suppression can be seen at high low overpotential region (0.9 V vs. RHE) but is closer to the observations on fuel cell electrodes. Comparing our results for Nafion 900 and Nafion 1100 of the same structure but different total acid capacity, we suggested that this suppression is due to the sulphonate groups of Nafion ionomers.
Exploitation Route In this work, we have studied the possible factors that can significantly affect the performance of catalysts when assessed using the floating electrode technique with a view to enabling the standardization of this technique to measure high-performance electrocatalysts at the gas/liquid interface. Thanks to the resemblance of the floating electrode to the fuel cell electrode, we use this assessment to provide some insights into the low PGM content catalyst layers for fuel cells.
Sectors Energy,Environment

 
Description Great Exhibition Road Festival 2019 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact We had fantastic opportunities for introducing our research to the public and we were thrilled to be flooded by all kinds of questions from people with great interests. The general public got more understanding of hydrogen fuel cell and how it is used in the automotive systems.
Year(s) Of Engagement Activity 2019
URL https://www.greatexhibitionroadfestival.co.uk/