Cation Assisted H2 Evolution Reaction by Mo and W Bis(dithiolene) Complexes

Abstract

Hydrogen energy plays a vital role in addressing the climate crisis by offering a clean, versatile, and scalable solution for reducing greenhouse gas emissions, especially in sectors where direct electrification is challenging. Integrating hydrogen into the energy system can help accelerate the transition toward a low-carbon and sustainable future. The research is based on electrochemical hydrogen evolution reaction (HER) with a catalyst modeled from formate dehydrogenase (FDH). FDH is a metalloenzyme with an equatorial bis(dithiolene) ligand and an axial chalcogenide ligand of Mo/W central metal in nature. Dithiolene ligand participates in the redox process as it increases the electronic density and the overall basicity of the complex, thereby facilitating proton reduction. Unlike earth-abundant transition metals such as Ni, Co, and Fe with square planar structure, molybdenum and tungsten metals coordinated with bis(dithiolene) ligand have additional chalcogenide ligand (O, S, Se) on the axial site which makes the metal high-valent and plays an essential role in catalytic activity. Herein, we focus on the reactivity of the catalytic active site, molybdenum-oxo ligand, depending on the acidity of the proton source. A reversible shift of onset potential was observed by continuously applying low potential and high potential alternatively. Based on the experimental data, we have introduced a novel mechanism for the HER in high-valent metal dithiolene complexes, leading to facile HER process. Expanding on this concept, we hypothesized that the interaction between the cation and sulfur atom would enhance the HER efficiency. The impact of cations on homogeneous systems will be discussed, focusing on how cations can lower the onset potential under high pKa conditions where hydrogen bonding interactions are formed.