In a search of the single-atom electrocatalysts for hydrogen production: The first sulfur-free mono- and diphenanthrenyl-terminated iron and cobalt(II) clathrochelates versus their thioanalogs

This paper reports on green chemistry approaches to the molecular design and synthesis of cheap, efficient and eco-friendly electrocatalysts of the hydrogen evolution reaction (HER). The title clathrochelates (including first those do not containing sulfur derivatives as “catalytic poisons”) were prepared using nucleophilic substitution of their chloroclathrochelate precursors and characterized by analytical, spectral and XRD methods. These complexes showed the HER 2 H + /H 2 electrocatalytic activity in the solutions. They form the Langmuir monolayers and possess a high physisorption on activated carbon (AC, up to 0.55 mmol·g –1 ) and reduced graphene oxide (RGO, up to 0.33 mmol·g –1 ). Contrary that on carbon paper (CP) is very low. Therefore, AC- or RGO-containing clathrochelate-immobilized components are suitable for preparation of hybrid CP-based cathodes, allowing to substantially increase a surface concentration of electrocatalytically active centers up to 0.5 μmol·m –2 . Cyclic voltammetry data suggest that the electrochemically generated cobalt(I) complexes, as the catalytically active intermediates, are stable and most prospective candidates for electrocatalytic hydrogen production. Clathrochelate-based single-atom catalysts were prepared in accordance with basic principles of green chemistry. They are derivatives of abundant and cheap 3d -biometals and low-toxic α -dioximes and possess an extremely high atomic utilization efficiency matching the “economy of atoms” principle. Nowadays, the carbon-supported metallic platinum is used as HER catalyst. The reserves of this noble and expensive metal on the Earth are limited and its replacement by such cheap and abundant HER materials will accelerate the development and implementation of green hydrogen-producing technologies. The recommendations on chemical structures of optimal molecular electrocatalysts were evaluated. • Iron and cobalt clathrochelates with terminal phenanthrenyl group(s) were prepared. • These complexes possess the relatively high physisorption on two carbon materials. • Electrochemically generated reduced Co complexes are stable in voltammetry time scale. • They are the most prospective candidates for electrocatalytic hydrogen production. • The obtained “single-atom catalysts” are derivatives of abundant 3d-biometals.