Approaching 2 W∙cm-2 AEMFCs through Electrode Engineering and Controlling the Cell Water Content and Balance
Authors: Omasta Travis J , Peng Xiong , Mustain William E 
Published: February 2020

In recent years, advances in alkaline exchange membrane fuel cells (AEMFCs) with anion exchange membrane (AEM) solid polymer electrolytes have gained traction due to their distinct – and potentially game-changing – advantages over proton exchange membrane fuel cells. However, AEMs and AEMFCs are at a significantly less mature stage in their developmental than proton exchange membrane fuel cells (PEMFCs), and have experienced limitations specifically in the area of stability, carbonation, and achievable current and power densities, exhibiting a sizable performance gap vs PEMFCs.1 This talk will focus on several fundamental and engineering advances that have enabled the creation of AEMFCs that are able to achieve up to 1.9 W∙cm-2 peak power (> 2 W∙cm-2 iR-corrected) and 100’s of hours of stable operation, bringing AEMFCs much closer to the incumbent PEMFC technology, and opening the way to overcome the cost barrier that has slowed the growth and large scale market implementation of fuel cells in the transportation sector.2

One of the largest contributors to the challenges faced in AEMFCs is the cell water content and balance, where the role of water has an impact on the anode (generation through the hydrogen oxidation reaction), cathode (consumption through the oxygen reduction reaction), and movement from the cathode to anode through electro-osmotic drag. This results in a significantly larger current-driven movement of water than PEMFCs, where less water required for the transport of the protons and there is no electrochemical consumption, and a need to provide adequate water to maintaining proper ionomer and membrane hydration while avoiding catalyst layer flooding or dry-out. Back diffusion of water from anode to cathode in the AEMFC, which is coupled to the membrane conductivity, is a promising way to decrease the extreme water gradient, and simplify the water balance problem in AEMFCs.3

In this study, we will discuss the influence of the membrane, ionomer and gas diffusion layer as well as the flow rate and dew points of the anode and cathode gases on AEMFC performance. Using a hydrophilic gas diffusion layer without a microporous layer increases membrane hydration, but also increases the possibility for flooding. Catalyst layer engineering – through adjusting the carbon/ionomer/catalyst loadings, adjusting hydrophilicity/hydrophobicity, manipulating electrode thickness and porosity, etc. – is investigated. We will also discuss controlling the total amount of water fed to the AEMFC through the gas feeds, as well as the water balance between the anode and cathode. Manipulating the dew points led to the counter-intuitive discovery that the cell performs better with the humidity higher at the anode than the cathode, despite water generation and electro-osmotic drag towards that electrode. In fact, removing too much water from the anode caused instability in the cell, while increasing the water at the anode decreased the membrane resistivity. Water back diffusion from the anode to the cathode likely plays an important role in membrane hydration and hydroxide transport through the membrane. Finally, we will also discuss the behavior of the AEM and powder anionomer in the presence of air both with and without CO2.

References

1. J. R. Varcoe, P. Atanassov, D. R. Dekel, A. M. Herring, M. A. Hickner, P. A. Kohl, A. R. Kucernak, W. E. Mustain, K. Nijmeijer, K. Scott, T. Xu and L. Zhuang, Energy Environ. Sci.,
7, 10 (2014).

2. L. Q. Wang, E. Magliocca, E. L. Cunningham, W. E. Mustain, S. D. Poynton, R. Escudero-Cid, M. M. Nasef, J. Ponce-González, R. Bance-Souahli, R. C. T. Slade, D. K. Whelligan and J. R. Varcoe, Green Chem.,
19(2017).

3. T. D. Myles, A. M. Kiss, K. N. Grew, A. A. Peracchio, G. J. Nelson and W. K. Chiu, J. Electrochem. Soc.,
158, 7 (2011).