Operando Electron Paramagnetic Resonance Studies of Electrocatalytic Hydrogen Evolution Reaction of MOS₂ Nanocrystals

Recently, there has been tremendous attention towards exploration of catalysts for hydrogen evolution reaction (HER). Replacing noble metal catalysts such as Pt with other materials have several advantages including cost, activity, stability, tunability, and environmental impact. Nanocrystalline Molybdenum disulfide (MoS₂) is one of the most explored catalysts due to its larger surface to volume ratio (1, 2). It demonstrates strong intrinsic catalytic properties, and numerous studies have explored various methods to enhance its catalytic performance. However, most attempts focused on tuning its electronic structure by employing complex processes, often without understanding which type of active sites prominently contribute to the catalytic activity. In this work, we designed in situ Electron Paramagnetic Resonance (EPR) methods to study the electrocatalytic activity of various sized MOS₂ nanocrystals in real-time conditions. Furthermore, the concentration of H₂ gas generated from each sample was measured and quantified for the first time using microsensors (UNISENSE). We analyzed spectral properties such as g-value and line width as a function of cell potential to track changes in the catalyst during HER at room -temperature. MOS₂ nanocrystals showed a decrease in overpotential compared to the control (Pt). The catalytic activity increased with a decrease in nanocrystalline size, consistent with an increase in the generation of H₂ gas generated corresponding to size of the nanocrystals. Our study provides insight into understanding chemical changes during HER from EPR spectral signatures offering an efficient approach for the scalable production of catalysts for hydrogen evolution reactions.