Dynamic Distribution of Chemical States of Water inside a Nafion Membrane in a Running Fuel Cell Monitored by Operando Time-Resolved CARS Spectroscopy

Abstract

The distribution and the chemical states of water in the electrolyte membrane of proton-exchange membrane fuel cells (PEMFCs) directly affect the performance and stability during cell operation. Coherent anti-Stokes Raman scattering (CARS) spectroscopy was used to investigate the transient behavior of water inside a Nafion membrane with a high time resolution of 0.5 s. After a current-density jump from 0.1 to 1.0 A cm-2, the cell voltage rapidly dropped, then increased, and reached equilibrium in 7 s, whereas the ohmic resistance immediately dropped due to the increase in the water species that contributed significantly to the proton conductivity. The number of water molecules per sulfonic acid group, λ, at the membrane surface of the cathode side initially overshot and reached equilibrium after 7 s. This synchronous change of the cell voltage and λ is suggested to be caused by the change in oxygen partial pressure near the catalyst layer. This CARS technique is expected to be useful for analyzing many transient phenomena in materials science.