Developing Reversal-Tolerant Anode for PEFCs by Introducing Iridium-Based Layered Structure

Abstract

On the anode side of the polymer electrolyte fuel cell (PEFC), hydrogen fuel can be blocked from reaching the catalyst layer due to anode water flooding which can occur at high humidity, low current density, and rapid load changing conditions [1-3]. This can cause the cell voltage reversal phenomena where anode potential increases relative to the cathode potential resulting in severe carbon corrosion, Platinum catalyst dissolution and agglomeration, and then causing performance loss and eventually cell failure. Currently, control based strategies such as voltage monitoring or flushing excess water have been suggested to overcome this issue [4,5], but unfortunately this increases the complexity and cost of the overall system. In order to greatly reduce system cost, more passive, material-based based solutions such asdevelopment of reversal tolerant anodes (RTAs) are necessary. RTAs include a water electrolysis catalyst such as Ir, Ru, IrO2, RuO2, etc, that can preferentially promote and sustain water electrolysis reaction during cell reversal in order to delay or even prevent the anode from reaching voltages higher than ~1.7 V, [6] resulting in a passive and hence cost-effective solution for the fuel starvation problem.The standard Pt/C+IrO2 RTA catalyst was first studied to investigate its cell reversal characteristics, and several current shortcomings were identified. We found that the RTA underwent significant carbon-based performance degradation even during the water electrolysis stage, and the oxygen evolution reaction (OER) catalyst, IrO2, was found to be deactivated after every subsequent cell reversal experiments. Furthermore, a comparative study of the standard Pt/C+IrO2 RTA with other structures such as Pt/C+Ir/C and Ir(IrOx)/Pt/C hybrid catalyst revealed a new cause of performance degradationbased on Platinum and Iridium particle alloying. Therefore, it is important to manage both the carbon-based degradation and alloying-based degradation in order to make RTAs feasible, and thus tackle the fuel starvation problem.Therefore introduction of layered structure was introduced in RTA catalyst, as explained in Fig. 1. In this study two types of layered RTA catalysts were explored towards addressing these problems. Layered RTA catalysts, LRC1, was made by spray-printing IrO2 (0.24 mgIrO2/cm2) first and then Pt/C (0.3 mgPt/cm2) on the Nafion 212 membrane as shown in Fig.1 (b). In LRC2 case, Cvulcan layer was spray-printed between IrO2 and Pt/C layers as described in Fig.1 (c). Compared to the standard RTA, LRC1 showed improved OER activity and higher performance after the fuel starvation durability test. The LRC2 structure was able to suppress alloying based degradation almost completely. Our findings contribute greatly to the understanding of the fuel starvation based degradation phenomena, and will help in designing better RTAs with improved performance and durability. Fig. 1 Schematic diagram showing (a) standard mixed RTA: Pt/C+IrO2, (b) LRC1: Pt/C -> IrO2, (c) LRC2: Pt/C -> Cvulcan -> IrO2 References [1] Hong, B. K., Mandal, P., Oh, J.-G. & Litster, S. On the impact of water activity on reversal tolerant fuel cell anode performance and durability. J. Power Sources 328, 280–288 (2016).[2] Knights, S. D., Colbow, K. M., St-Pierre, J. & Wilkinson, D. P. Aging mechanisms and lifetime of PEFC and DMFC. J. power sources 127, 127–134 (2004).[3] O’Rourke, J., Ramani, M. & Arcak, M. In situ detection of anode flooding of a PEM fuel cell. Int. J. Hydrog. Energy 34, 6765–6770 (2009). [4] Kurnia, J. C., Sasmito, A. P. & Shamim, T. Advances in proton exchange membrane fuel cell with dead-end anode operation: A review. Appl. Energy 252, 113416 (2019).[5] Chen, Y.-S., Yang, C.-W. & Lee, J.-Y. Implementation and evaluation for anode purging of a fuel cell based on nitrogen concentration. Appl. Energy 113, 1519–1524 (2014).[6] Ralph, Thomas R., Sarah Hudson, and David P. Wilkinson. "Electrocatalyst stability in PEMFCs and the role of fuel starvation and cell reversal tolerant anodes." ECS Transactions1.8 (2006): 67. Figure 1