Raising the temperature on electrodes for anion exchange membrane electrolysis - activity and stability aspects

Abstract

The membrane electrode assembly is the powerhouse of the anion exchange membrane water electrolyser (AEMWE), thereby placing a great importance on the associated preparation conditions. This paper investigated how annealing temperature and time impacted activity and stability for both anode and cathode electrodes with catalyst-PTFE thin-films. The effect of annealing was thoroughly characterised through SEM/EDS, TEM, XRD, Raman spectroscopy and XPS. Moderate heat-treatment (T≤500°C) had a positive effect by improving morphology and enhancing reaction kinetics as seen through three-electrode measurements. Annealing temperature affected hydrogen adsorption, resulting in a change in the hydrogen evolution pathway as shown by hydrogen adsorption peaks and Tafel curves. These beneficial effects were further augmented by an enlarged surface area as shown in both three- and two-electrode measurements. Two electrode measurements revealed a staircase activity-trend, where the annealing temperature yielding the greatest activity declined with increasing annealing time. This resulted in efficient cathodes annealed at 2h-500°C, 3h-400°C and 4h-300°C. The aforementioned cell configurations reached approximately 500 mA cm at 1.73 V, 1.82 V and 2.04 V respectively. Stable electrodes were produced for temperatures ≤500°C, after which their mechanical integrity began to fail due to pyrolysed PTFE. Stability was meticulously characterised and a degradation pathway for carbon catalysts was proposed, where expansion of the carbon black onion layers ultimately lead to catalyst particle detachment.