(241c) Molybdenum-Aminate Electrocatalysts for the Hydrogen Evolution Reaction Designed By Hybrid Molecular Layer Deposition

Achieving cost-effective water electrolysis is critical to realizing the future of clean hydrogen in a fully renewable energy economy. This pursuit demands hydrogen evolution reaction (HER) electrocatalysts that are composed of low-cost materials, driving immense research interest in the development of HER catalysts based on earth-abundant transition metals. Nitrides of these metals have attracted recent attention due to their high conductivity and corrosion resistance relative to other transition metal compounds. Molybdenum nitrides in particular stand out for their high catalytic activity and demonstrate morphological synergies that are unique among the transition metal compounds studied for HER electrocatalysis. Many of the best reported activities for molybdenum nitride catalysts feature composite architectures where crystalline MoN_x domains are embedded in an amorphous matrix of N-doped carbon.

This work utilizes the molecular-level control offered by hybrid molecular layer deposition (MLD) to design and study these catalytically desirable morphologies. Hybrid MLD involves the sequential exposure of a growth surface to vapor-phase precursors which saturate available surface sites through self-limiting reactions, leading to layer-by-layer deposition with precise control over the composition and structure of the resulting material. In this work, we report the first implementation of hybrid MLD for the synthesis of Mo-aminate films, using Mo(CO)₆ with a series of amine precursors—including ethylenediamine, p-phenylenediamine, and tris(2-aminoethyl)amine.

Harnessing the fine chemical control offered by hybrid MLD, this study examines how the catalytic properties of Mo active sites are affected by their local bonding environment. The choice of organic linker molecule is shown to induce variations in catalyst activity and stability, namely through structural effects like crosslinking and crystallinity. An inverse relationship is generally observed between the stability and activity of these Mo-aminates, and the effect of combining different amine precursors is explored. Higher nitrogen content in the catalysts is shown to decrease their HER activity in acidic media (0.5 M H₂SO₄) and improve film stability under electrochemical testing and in air. Films with N/Mo ratios above 2 demonstrate the best performance, with HER overpotentials lower than 300 mV at -10 mA/cm². The catalysts exhibit an increase in active site density induced by favorable morphological changes which evolve under HER testing conditions and through annealing. The rate of these changes is affected by the choice of organic precursor – occurring more slowly when aromatic and multifunctional amines are incorporated – as is the extent to which they occur. The potential of these new metal-organic aminate materials for electrocatalysis applications will be discussed.