Distribution of Water Molecules inside Proton Exchange Membrane in Transient State during Fuel Cell Operation Monitored By Operando Time-Resolved CARS Spectroscopy

Abstract

On the performance and stability of proton exchange membrane fuel cells (PEMFCs), the distribution of water inside the membrane during the power generation has a direct, profound influence, especially for the automotive applications. In this study, coherent anti-Stokes Raman scattering (CARS) spectroscopy was applied to investigating the distribution of water and its chemical states inside the membrane with high spatial (10 μm φ (area) ×1 μm (depth)) and time (0.5 s) resolutions during the power generation.Figure 1 shows a schematic illustration of the measurement system. Two laser lights, pump light (ω 1: 532 nm) and Stokes light (ω 2: 550-700 nm), were set coaxial and irradiated to the membrane inside the single fuel cell having a glass window in the middle on the cathode-side endplate1. The CARS light emitted was separated by filters and introduced into a spectrometer. Five measurement locations were set from the membrane surface (0 μm) of the cathode side through the membrane to that of the anode side separated by 5μm. Each spectrum was recorded every 0.5 s at each location during the current-density jump from 0.1 to 1.0 A cm−2. The number of water molecules per sulfonic acid group,λ, was quantified using spectral information from a calibration curve created in advance2.Figure 2 shows the number of water molecules per sulfonic acid group, λ, at five locations of the membrane with the ohmic resistance and the cell voltage during the current-density jump. Upon the jump, the cell voltage dropped abruptly but increased slowly and reached a steady state, whereas the ohmic resistance of the cell decreased abruptly and increased immediately to reach a steady state after the current density jump. Upon the jump, the λ value at 0 μm overshot with a peak at 4.5 s and reached a steady state at 7.0 s. At other locations, a gradual increase in λ was observed corresponding to the electro-osmosis drag and the back diffusion of water. The results of the analysis under conditions other than those in Fig. 2 will also be reported. Nishiyama et al. J. Phys. Chem. C 2020, 124, 18, 9703–9711Hara et al. Electrochim. Acta 2012, 82, 277-283 Figure 1