Benchmarking Nanosized Electrocatalysts for the Redox-Mediated Alkaline Hydrogen Evolution Reaction

Pinho, Luís and Toghill, Kathryn (2024) Benchmarking Nanosized Electrocatalysts for the Redox-Mediated Alkaline Hydrogen Evolution Reaction. In: 38th Topical Meeting of the International Society of Electrochemistry, 2024-09-08 - 2024-09-11, University of Manchester.

Abstract

Objective comparisons of the activity of nanosized electrocatalysts using standard methods and conditions are necessary to evaluate their performance in new decoupled electrochemical water splitting systems that are redox-mediated and enable green hydrogen production [1]. For these systems, numerous combinations of redox mediators (acting as charge and potential vectors) with electrocatalysts operating in alkaline pH are possible [2-3]. Here we outline a standardized protocol in which the mediators are conventionally charged but are then chemically discharged over the electrocatalyst in small volume glass vials to characterize and compare the electrocatalysts in terms of their activity. Specifically, we report on the activity of 11 nanosized electrocatalysts (5 noble metal based materials and 6 non-noble metal based) for the hydrogen evolution reaction (HER) in aqueous alkaline pH against 3 redox mediators: dihydroxyphenazinesulfonic acid (DHPS), iron triisopropanolamine (Fe-TIPA) and benzo[a]hydroxyphenazine-7/8-carboxylic acid (BHPC). For comparison, we also elucidate the behaviour of these catalysts at near-neutral (chromium propanediamine tetraacetate, CrPDTA) and acidic pH (V(II)). We also discuss the electrochemical properties and microstructure of all electrocatalysts together with an additional kinetic evaluation screening for the most active ones. For reactions with more sluggish HER kinetics in alkaline pH, Ni powder (58.4±5.1 %) and Pt over Ni foam (51.2 ± 2.3 %) are the non-noble metal based materials that show the highest values of Faradaic efficiency (FE), when operating together with DHPS. Most importantly, the FE values obtained are very close to those obtained for the noble metal based materials tested. In near neutral pH, the large driving force of CrPDTA yields high FE values for all non-noble metal catalysts. Unsurprisingly, for the reaction with more favourable HER kinetics provided by acidic pH, the highest values of FE were obtained for Mo2C and Ni powder with V(II) as a mediator. The overall activity of nanosized electrocatalysts is driven by the enhancement of the surface area and the higher availability of active sites. Further optimization will emerge through careful selection of redox mediators, together with tuning composition and morphology of the catalysts and their integration into conductive and structured porous supports.