The Ionomer As an Oxygen Evolution Reaction Promoter: Piperidinium's Impact on Mechanistic Pathways on NiO, IrO2, and Fe-NiO: Article No. 015026

The commercial viability of anion exchange membrane (AEM) electrolysis requires optimization of various stack components, with specific catalyst-ionomer combinations often yielding higher current densities, lowered Tafel slopes, and improved mass activity. In this joint theoretical-experimental study, theoretical calculations detail the impact of Versogen's piperidinium functional group on the complex, kinetically limiting oxygen evolution reaction, finding that the functional group can act as a promoter of specific steps (O*/O2* formation; H2O/O2 desorption with reaction enthalpies ranging between 0.2-0.6 eV at higher coverages of OxHy intermediates) on NiO and NiFeOx catalysts. In particular, Fe sites on the NiFeOx catalyst facilitate concerted mechanisms of O*/O2* formation and H2O desorption with a low enthalpy of 0.5 eV; O2 desorption alone requires only 0.3 eV. In contrast, Versogen-IrO2 results in stronger Ir-O bonds, where the enthalpies for bond breaking (Ir-OH2 and Ir-O2) are considerably higher (1.4 eV and 1.6 eV, respectively). Rotating disk electrode studies utilized commercially available NiO and IrO2 and synthesized 7.5 wt % Fe in NiFeOx catalysts in combination with Versogen, a common AEM ionomer, and Nafion, an alternative binder. Electrochemical testing validated the impact of these mechanistic changes on ionomer-catalyst combinations, finding that Versogen particularly activates NiO and NiFeOx compared to IrO2. Following a 13.5 h hold at 1.8 V, mass activities and Tafel slopes improved to 34 +- 13 A g-1 and 79 +- 2 mV dec-1 (NiO) and 82 +- 4.9 A g-1 and 72 +- 2 mV dec-1 (NiFeOx). In contrast, Versogen-IrO2 only reached 17 +- 2.9 A g-1 and 81 +- 3 mV dec-1. Optimization of the ionomer-catalyst can yield significant increases in performance from initial activity and after an electrochemical conditioning procedure: this enhancement to the mass activity resulted in a 200.9 +- 106.1% improvement for Versogen-NiFeOx and 1284.2 +- 260.5% for Versogen-NiO. In contrast, Nafion-NiFeOx and -NiO offered moderate improvements of 39.1 +- 30.5% and 120.9 +- 59.1%, respectively.