Transient Distribution of Water in an Anion Exchange Membrane Fuel Cell Monitored by Operando Coherent Anti-Stokes Raman Scattering Spectroscopy (Supporting Information)

The hydration condition of an anion exchange membrane (AEM) in an operating fuel cell significantly affects its performance as well as its lifespan. In this paper, an in-house build coherent anti-Stokes Raman scattering (CARS) vibrational spectroscopy is used to establish the hydration of an AEM in an anion exchange membrane fuel cell (AEMFC) while it generates power. During steady-state operation, water on the anode side increased with current density. On the cathode side and at the center of the membrane, water initially decreased with current density and then started increasing at a slower pace than on the anode side. A deconvolution of the OH peak in the recorded CARS spectra into nine species revealed that only the H-bonded water species underwent variation. The rest of the species experienced a negligible change. A transient study revealed that maximum disturbance to the water distribution was achieved after 5 s of applying a current jump. The distribution of water became stable within 20 s after applying the current jump. The response to the current jump on the anode side was opposite to that on the cathode. These results open the way for a widespread dynamic study of water distribution in different AEMFCs. The technique could also be directly used to evaluate the dynamic degradation of AEMs.