In situ observation of water in a fuel cell catalyst using scanning electron microscopy

To visualize water in the catalyst of polymer electrolyte fuel cells (PEFCs), backscattered electron (BSE) imaging by means of scanning electron microscopy was employed. To confine a wet specimen of catalyst, an environmental wet cell was manufactured with a silicon nitride thin film (∼100 nm) as the beam window. By supplying humidified gas into the cell, a change in BSE brightness was detected in the catalyst attached to the silicon nitride window. As humidification proceeded, the BSE image became darker and returned brighter by switching to a dry gas. Monte Carlo simulations were performed to evaluate the energy and number of BSE obtained after passing through water with thickness d. Combining the results of the Monte Carlo simulation successfully converted the change in brightness to the change in thickness from d = 100 nm to d = 3 μm. This established method of evaluating water with a thickness resolution of the order of Δd = 100 nm can be applied to in situ observations of the catalyst in a PEFC during operation.