Abstract
Tailoring catalyst–ionomer and electrolyte interaction is crucial for the development of anion exchange membrane (AEM) water electrolysis. In this work, the interaction of Ni–MoO2 nanosheets with ionomers and electrolyte cations was investigated. The activity of Ni–MoO2 nanosheets for the hydrogen evolution reaction (HER) increased when tested in 1 M NaOH compared to 1 M KOH; however, it decreased when tested in 0.01 M KOH compared to 1 M KOH electrolyte. The capacitance minimum associated with the potential of zero free charge (pzfc) was shifted negatively from 0.5 to 0.4 V versus RHE when KOH concentration increased from 0.1 mM to 1 M KOH, suggesting a softening of the water in the double-layer to facilitate the OH– transport and faster kinetics of the Volmer step that lead to improved HER activity. The catalyst interaction with cationic moieties in the anion ionomer (or organic electrolytes) can also be rationalized based on the capacitance minimum, because the latter indicates a negatively charged catalyst during the HER, attracting the cationic moieties leading to the blocking of the catalytic sites and lower HER performance. The HER activity of Ni–MoO2 nanosheets is lower in benzyltrimethylammonium hydroxide (BTMAOH) than in tetramethylammonium hydroxide (TMAOH). Anion fumion ionomer and electrolytes with organic cations with benzyl group adsorption (such as BTMAOH) lead to decreased HER activity in comparison with TMAOH and Nafion. By utilizing Ni–MoO2 nanosheet electrodes as a cathode in a full non-platinum group metal (PGM) AEM electrolyzer, a current density of 1.15 A/cm2 at 2 V cell voltage in 1 M KOH at 50 °C was achieved. The electrolyzer showed exceptional stability in 0.1 M KOH for 65 h at 0.5 A/cm2.
Highlights
Hydrogen is a chemical feedstock for chemical synthesis and fuel for transportation and energy storage.[1]
Anion exchange membrane (AEM) electrolyzers aim to bring the merits of proton exchange membrane (PEM) electrolysis and liquid alkaline systems.[3−5] AEM electrolyzer systems can use cheaper catalysts/electrodes and a balance of plant components while having the potential for achieving high efficiency.[6]
The hydrogen evolution reaction (HER) kinetics is sluggish in an alkaline environment with 2 orders of magnitude slower than in an acidic environment for Pt-based catalysts.[7]
Summary
Hydrogen is a chemical feedstock for chemical synthesis and fuel for transportation and energy storage.[1]. DFT results by McCrum et al suggested that this effect is due to the interaction between the benzyl group and electrode surface.[59] TMAOH has been found to inhibit the HER by cation− hydroxide−water coadsorption under alkaline conditions.[51,60,61] Infrared reflection absorption spectroscopy (IRRAS) studies indicate that the adsorption of tetramethylammonium (TMA+) cation causes hydroxide and water coadsorption on the surface of Pt catalyst with higher hydroxide concentration compared to water.[51,60,61] The mobile QA+ cations from organic electrolytes can form a more compact double layer due to their mobility, and the blockage in the case of organic electrolytes will be in a similar manner as the anion ionomer as found in this study. Ni− MoO2 nanosheets’ activity and stability allow for active and cheap electrodes for AEM water electrolysis.[70]
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