Enhanced Local Ion Transport in High-Performance Anion Exchange Membrane Water Electrolyzers Via Quaternary Ammonium-Functionalized Metal-Organic Framework Electrocatalysts

Abstract

Anion-exchange membrane water electrolyzers (AEMWEs) present a multitude of advantages, particularly in the utilization of non-platinum group (non-PGM) catalysts. While diverse catalysts have demonstrated notable electrocatalytic activity in alkaline conditions, the integration of the ionomer with the electrocatalyst at the active catalyst layer is crucial for enhancing overall AEMWE performance in practical device demonstrations by improving mass transport. However, the intrinsic instability of polymeric ionomers, leading to physical detachments or alkaline (chemical) degradation, poses a challenge that can potentially diminish performance. In this study, drawing inspiration from the aforementioned ionomer chemistry, we introduce non-PGM NiFe-based metal-organic frameworks (MOFs) incorporating quaternary ammonium (QA) cation moieties covalently functionalized to benzenedicarboxylic acid (BDC) ligands as promising electrocatalysts for the oxygen evolution reaction (OER) in alkaline environments. Three specifically designed functional groups with varying ion exchange capacities (IEC) and alkaline stability—trimethylamine (TMA), N-methylpyrrolidine (C4N), and N-methylpiperidine (C5N)—were directly tethered to BDC ligands. The alkaline OER activity of Ni2Fe1-QA-MOF was meticulously assessed using rotating disk electrodes and AEMWE devices. These directly tethered QA groups markedly facilitated improved ionic transport of OH- ions at the catalyst/electrolyte interfaces compared to BDC ligands, ultimately leading to a significant enhancement in half-cell activity and AEMWE performance. In-situ IR/RAMAN spectroscopy revealed that QA moieties highly promoted local ionic transport at the interfaces, as evidenced by characteristic peaks indicating a robust interaction between QA and OH- ions. Unexpectedly, QA moieties not only contributed to ionic transport in local environments but also induced oxidation states of Ni metal sites, while Fe sites exhibited no changes during the applied potential in the OER region, as comprehensively revealed by synchrotron X-ray absorption spectroscopy (XAS). In comparison with conventional NiFe layered double hydroxide (LDH) structures, we posit that the substantial structural transformation of Ni moieties can be attributed to an increased degree of M-O bond interaction, enhanced by the local interaction between QA and OH- ions. Density functional theory (DFT) calculations also validate the balanced adsorption of OER intermediates on QA-MOFs. Finally, QA-NiFe-MOF was directly synthesized on a porous transport layer (PTL), and its AEMWE performance was evaluated without the use of any ionomer. At 1.0 M KOH, C4N, C5N, and TMA-based NiFe MOFs exhibited current densities of 9.35 A/cm2, 9.11 A/cm2, and 7.6 A/cm2, respectively. These current densities surpassed those of bare BDC-NiFe MOF and commercial IrO2 anode catalysts, which exhibited only 7.39 A/cm2 and 6.4 A/cm2, respectively. Consequently, we emphasize that the facilitation of local ionic transport could yield remarkable AEMWE performance, even in the absence of conventional polymeric ionomers.