Elucidating the Role of Anion-Exchange Ionomer Conductivity within the Cathode Catalytic Layer of Anion Exchange Membrane Fuel Cells

Abstract

Anion exchange membrane fuel cells (AEMFCs) have seen a rapid increase in interest in recent years, among other things due to the impressive performance achieved by these devices. However, while reaching high cell performance is a valid and important goal, it is no less important to achieve cell performance stability. In order to achieve this, further research is needed to understand the factors affecting AEMFC operation lifetime. Unfortunately, the sources of performance loss over time are still poorly understood. The anion-exchange ionomer (AEI) incorporated into the cathode and anode of AEMFCs enables anion transport in the catalyst layers (CLs), thereby ensuring the delivery of hydroxide ions to and from the catalytic reaction sites. Using a one-dimensional model of AEMFC operation [1,2], we highlight and discuss the effect of ionomer conductivity (e.g., anion conductivity) on performance and its stability. In particular, we focus on the ionomer conductivity within the cathode CL and its effect on catalyst utilization and water management in the system. Next, the degradation of performance stability is analyzed in terms of voltage loss contributions from electrochemical kinetics, ohmic resistance, and mass transport. Our modeling results clearly show that improved AEI hydroxide conductivity within the cathode significantly increases AEMFC performance. More motivating is the positive impact on cell performance stability. While the cathode reaction kinetics is considered as a major factor restricting cell performance, the transport of water and hydroxide through the cathode CL is extremely crucial for the achievement of long-term AEMFC performance stability. We show that the main difference between the initial (0 h) and final states (3000 h) of the membrane electrode assembly is the increased voltage losses through the cathode CL and the anion exchange membrane (AEM). In addition, our findings suggest a two-step degradation mechanism where initially the voltage loss is controlled by the ionomer degradation within the cathode CL, and only in the second half of the cell lifetime AEM degradation becomes dominant leading to a significant voltage loss through this region. These results show, for the first time, the overpotential distribution across the cell during AEMFC performance stability simulations. This understanding is of crucial importance for future design and optimization of AEI towards the development of high AEMFCs performance stability (i.e. longevity).