In-Operando Insights on the Hydrogen Evolution Reaction Activity and Stability of Non-Noble Metal Electrocatalysts for Water Electrolysis

Abstract

Market penetration of proton exchange membrane water electrolysers (PEMWE) as energy conversion technologies in a renewable energy sector would imply, given the 2.3$/Kg H2 production cost aimed by the DoE,1 a drastic decrease in total noble metal electrocatalyst loading. For the state-of-the-art hydrogen evolution reaction (HER) catalyst, platinum, recent targets suggest this limit to be 0.05mgPt/cm2.2 In some applications, like direct photoelectrochemical water splitting, application of any amount of noble metal as a co-catalyst cannot be tolerated at all. Thus, given increasing scarcity of platinum, alternative materials based on earth-abundant non-noble metals represent a significant interest as promising HER catalysts. Among them, transition metal dichalcogenides such as molybdenum disulfide (MoS2) have provided excellent performances, particularly in their cluster-based allotropic structure [Mo3S13]2-.3 Whilst the role of sulfide atoms in the Mo-terminated edges as HER active sites in crystalline MoS2 is well-established, the HER mechanism of [Mo3S13]-based materials is still under debate. Preceded by an electrochemical activation step, operando spectroscopic studies ascribed the active sites to the electrochemically-cleaved S2 2-,4 but other reports have suggested undercoordinated Mo sites to be responsible for the HER activity.5 To date, no study has evaluated the in operando activity and stability of [Mo3S13]-based materials, shown to be intimately dependent on the electrochemical conditioning and electrolyte pH.6 This work devotes to the simultaneous detection of Mo and S species under HER operating conditions in pristine and anchored [Mo3S13]2- clusters to nitrogen-doped carbon nanotubes, by use of our dedicated on-line inductively-coupled plasma mass spectrometry (on-line ICP-MS). Our findings demonstrate, contrary to previous understanding, that the electrochemical activation step is mostly related to Mo dissolution rather than S loss, which is one order of magnitude lower. After such activation, Mo dissolution is significantly mitigated, whereas S dissolution is still observed, with magnitudes dependent on the anchoring material. The overarching picture obtained by such study enabled to shed light on the HER activation mechanism, and demonstrate that the stabilities of these materials are comparable to some Ir-based catalysts. The aforementioned results provide further guidelines to implement non-noble HER metal electrocatalysts in PEMWE.