Analysis of O2 Diffusion Resistance without Cathode Humidification in a PEMFC

Abstract

Fuel cells are power-generating devices from chemical energy by electrochemical reactions. Especially, proton exchange membrane fuel cells (PEMFCs) have a high-power density, even if they utilize polymer electrolyte and run at a low temperature. It is generally ascribed that oxygen transport to the cathode catalyst layer (CL) greatly affect a PEMFC performance. When molecular oxygen travels from the cathode flow channel to the surface of Pt, it is gradually in three diffusion regimes: molecular diffusion (MD), Knudsen diffusion, and ionomer film diffusion. Recently, much effort has been devoted to understanding oxygen diffusion resistance at the cathode, especially in the CL. Using the Stefan-Maxwell equation, Mashio et al.1 separated total oxygen diffusion resistance into three different oxygen diffusion that includes pressure-dependent resistance and two pressure-independent resistance, and they determined molecular diffusion resistance RMD and non-molecular diffusion resistance Rnon-MD from the experimentally measured limiting current with diluted gaseous oxygen. Similarly, in this study, we measured limiting current to study oxygen diffusion resistance at various RHs of the anode without the cathode humidification. Fig. 1a shows total oxygen diffusion resistance RMD with increasing total pressure Ptot at the cathode. RMD increases with increasing Ptot, while it decreases with higher RH at the anode. Fig. 1b gives Rnon-MD against pressure. The value of Rnon-MD decreases with increasing RH at the anode. It is inferred from the results that i) MD is dominant at the cathode with no humidification, depending on RHs at the anode that affect water content at the cathode and ii) smaller Rnon-MD with higher RH at the anode is associated with increased sites available for oxygen diffusion possibly due to ionomer swelling.2 References T. Mashio, A. Ohma, S. Yamamoto, K. Shinohara, ECS trans., 11(1) 529-540 (2007).M. Schalenbach, T. Hoefner, P. Paciok, M. Carmo, W. Lueke, and D. Stolten, J. Phys. Chem. C, 119, 25145-25155 (2015) Figure caption Fig. 1(a) Molecular oxygen diffusion resistance RMD and (b) non-molecular oxygen diffusion resistance Rnon-MD against total pressure at various relative humidity at the anode (No humidification at the cathode). Figure 1