Understanding Performance and Durability with KOH and DI-Water Fed Anion Exchange Membrane Electrolyzers

Abstract

Anion Exchange Membrane electrolyzers (AEMELs) have increased in popularity in recent years due to their potential to combine the benefits from proton exchange membrane electrolyzers and traditional alkaline electrolyzers – namely, high current density operation, pressurized H2 discharge and lowering cost by utilizing low-cost earth abundant electrocatalysts and inexpensive component materials. In the AEMEL, the oxygen evolution reaction (OER) electrode plays a critical role dictating the overall efficiency of the cell due to sluggish OER activity and species transport. In our previous study, we optimized the OER electrode structure by investigating the effects of catalyst loading, catalyst type, the porous transport substrate and additive carbon [1] – resulting in an excellent performance of 1.55 V operation at 1.0 A/cm2.Though that study led to high performance, all of those gains were made using KOH electrolyte being fed to the cell, not deionized (DI) water. The use of DI water complicates AEMEL operation as the liquid phase can no longer be relied on to carry any of the ionic charge – this possibly can reduce the electrochemically active surface area (ECSA) [2]. It has also been stated that pure water operation may have negative consequences on cell durability. Therefore, to allow for AEMELs to operate efficiently on DI water, it is important to understand how the ion transport and charge transfer resistance change as cells are transitioned from KOH to DI water operation. It is also important to understand what effects even trace amounts of KOH can have on behavior. Lastly, the effect of the ionomer properties on the OER anode performance should be well-understood.In this study, we investigate the role of alkaline feed pH on the performance and durability of AEMELs. When shifting from alkaline feed to DI water, it will be shown that there is an increase in ohmic and charge transfer resistance, resulting in significant voltage loss. Because the alkaline feed enhances electrode conductivity by connecting catalyst active sites to the conductive ionomer and AEM [2-3], new electrode structures are needed that allow for enhanced ECSA and lower resistances with DI water feeds. In this work, that was accomplished by creating a four-layer electrode structure as well as manipulating the cell operating conditions. Lastly, our team enabled the use of a high IEC ionomer in the anode catalyst layer by introducing a cross-linker and introducing smaller, cryo-milled ionomer particles. This was important to overcome adhesion issues that can come from high water uptake and swelling in high-IEC ionomers [4-5]. This paper will focus not only on performance, but also longevity, with several cells being stably operated for more than 500 hours continuously.