(Invited) Quantitative Understanding of CO2 Behavior in Anion Exchange Membrane Fuel Cells

Abstract

The past two years have seen a tremendous enhancement in the performance and durability of anion exchange membrane fuel cells (AEMFCs) [1-2]. Multiple AEMFCs have been reported that can achieve 1.5-2.0 W/cm2 peak power, limiting current > 5 A/cm2 and >500 hour operation almost negligible voltage decay. These operation metrics suggest that AEMFCs are well on their way to competing with proton exchange membrane fuel cells (PEMFCs) in terms of performance, with the additional possible benefit of being lower cost. However, one limitation of all of this work is that it has been done with purified O2 or air fed to the cathode in order to avoid exposing the electrolyte to CO2, which leads to carbonation and a loss of cell performance. Most researchers believe that electrolyte carbonation simply leads to a loss of ionic conductivity, but we will show in this presentation that conductivity loss alone is only able to explain ~10% of the voltage loss once the CO2 is fed to the cathode, suggesting a different mechanism that will be explained. It has also been recently shown theoretically [3] that the level of carbonation in the AEMFC will be a function of the CO2 concentration and current density as the “self-purging” mechanism [4] acts to remove carbonates as they are formed. However, this behavior has not been confirmed experimentally and the carbonation dynamics are poorly understood. This presentation will present an extensive collection of carbonation data as a function of current density and CO2 concentration at the cathode. We will also link the level of carbonation to electrochemical performance, discuss the lower limits for CO2 tolerance in AEMFCs and discuss strategies to reduce make cells more tolerant to cathode CO2. Finally, one of the most critical items that has been overlooked in AEMFCS to date is the operation of realistic systems where the anode flowrate is preferably very low. In PEMFC systems, the low anode flowrate results in N2 accumulation on that side of the cell from diffusion. In AEMFCs, not only will this happen, but CO2 will accumulate on the anode side of the cell under operation on real air. This means that anodes will operate under very high relative humidity and very high CO2 concentration conditions. Therefore, the final topic discussed in this presentation will be the influence of accumulated anode CO2 on AEMFC performance. References Mustain, Current Opinion in Electrochemistry, DOI: 10.1016/j.coelec.2018.11.010Maurya, S. Noh, I. Matanovic, E.J. Park, C.N. Villarrubia, U. Martinez, J. Han, C. Bae and Y.S. Kim, Energy Environ. Sci., DOI:10.1039/C8EE02192AKrewer, C. Weinzierl, N. Ziv and D.R. Dekel, Electrochim. Acta, 263 (2018) 433-446.Ziv, W.E. Mustain and D.D. Dekel, ChemSusChem, 11 (2018) 1136-1150