Investigating the Effect of KOH Feed Concentration on a Stainless-Steel Anode Using a 3-Electrode Anion Exchange Water Electrolyser Cell Set-up

Abstract

Anion-exchange membrane water electrolysers (AEM-WE) have the potential to lower the capital cost of hydrogen production compared to proton-exchange membranes water electrolysers (PEM-WE). The alkaline conditions of AEM-WE result in: (i) a kinetically more favourable oxygen evolution reaction (OER), (ii) the ability to use platinum group metal (PGM)-free metal catalysts, and (iii) reduced cell component costs because of the less corrosive operating conditions. However, AEM-WE is held back from commercialisation by limited current density and durability.1 Compared to conventional alkaline water electrolysis, AEM-WEs have the advantage of lower ohmic resistance thanks to a solid polymer electrolyte which provides intrinsic ionic conductivity instead of relying on 6 M KOH and a porous separator. Most studies show, however, that a dilute KOH feed (< 1 M), rather than pure water, is critical to reaching a high current density in AEM-WE. Varying the concentration of KOH in the feed solution impacts catalyst activity and membrane resistance thereby making performance measurements in AEM-WE more complex. Moreover, the overpotential for the alkaline hydrogen evolution reaction (HER) is significant due to its sluggish kinetics. This means full-cell measurements are insufficient for understanding the behaviour of OER and HER catalysts in AEM-WE. It is therefore crucial to make half-cell measurements in AEM-WEs to decouple the effect of the anode and cathode electrocatalysts from the overall cell performance.In this context, our group recently demonstrated a 3-electrode, 5 cm2 membrane electrode assembly electrolyser cell2 that uses a reference electrode to decouple the anode and cathode contributions to the overall water splitting reaction. The technique was demonstrated for AEM-WEs and is soon to be published by Malone et al. (Figure 1A)3. In this study, we aimed to investigate the effect of KOH concentration in the feed solution on OER by using this 3-electrode cell setup. We used a stainless steel (SS)-felt at the anode as both catalyst and porous transport layer (PTL) in different concentrations of KOH feed while maintaining Pt/C as the cathode catalyst. We also repeated the tests with various anode catalyst layers (IrOx, NiFeOx) in combination with the SS PTL. For each test, full and half-cell polarisation curves and impedance spectroscopy were recorded before and after a sixteen-hour durability test at 1 A cm-2 at 40 °C. The results revealed the catalytic significance of the SS-felt in 1 M KOH as recently shown by Chen et al.4 Additionally, we demonstrated how the SS masks the performance of an IrOx catalyst layer in 1 M KOH. Reducing the KOH concentration to 0.18 M resulted in ≈ 7 % increase in the overall cell voltage at 1 A cm-2 (from 1.88 V to 2.00 V at 40 °C in Figure 1B). Aside from the ≈ 35 % increase in membrane resistance (e.g. 180 to 240 mΩ cm2 at 0.9 A cm-2) in 0.18 M KOH, the higher overpotential arose from the anode (Figure 1C), which could be attributed to the higher OER overpotential of SS-felt in a lower pH solution. Meanwhile, there was no appreciable change in cathode overpotential when the KOH concentration was reduced. These findings demonstrate the importance of 3-electrode cell measurements for understanding the impact of each component in an AEM-WE cell. Furthermore, the results reveal that both the choice of PTL material as well as the concentration of feed solution should be considered when testing different catalysts in AEM-WE.