Electrochemical Nitrogen Fixation on Transition Metal Single-Atom Catalysts Supported on Nigerian Graphite-Derived Substrates Under Ambient Conditions

Electrochemical nitrogen fixation under ambient temperature and pressure conditions as a decentralised, renewable-energy-powered alternative to the Haber-Bosch process for ammonia synthesis represents one of the most significant unsolved challenges in heterogeneous catalysis and electrochemistry, and the development of highly active, selective single-atom catalysts on Nigerian natural graphite-derived carbon supports would simultaneously advance fundamental catalytic nitrogen reduction science and provide a strategic basis for future indigenous fertiliser industry development. This study synthesises and characterises molybdenum, iron, and ruthenium single-atom catalysts anchored on nitrogen-doped graphene and holey graphene substrates derived from natural graphite ore from Cross River State, South South Nigeria, and evaluates their electrochemical nitrogen reduction performance under ambient conditions. Graphite exfoliation employs electrochemical and liquid-phase ultrasonication methods to produce graphene sheets subsequently nitrogen-doped by thermal annealing with ammonia gas. Single-atom catalyst deposition employs atomic layer deposition and wet impregnation with low metal loading to maximise atom utilisation. Catalyst characterisation by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy confirms single-atom dispersion, while X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopy determine coordination environment and oxidation state of each metal centre. Electrochemical nitrogen reduction performance is evaluated in a custom three-compartment cell with proton exchange membrane separator, quantifying ammonia production by colourimetric indophenol blue method and ion chromatography with isotope labelling (15N2) to exclude contamination artefacts. Faradaic efficiency, ammonia yield rate, and nitrogen-to-hydrogen selectivity are benchmarked against literature single-atom catalyst performance. Density functional theory calculations elucidate the nitrogen reduction reaction pathway, activation energy profile, and the origin of selectivity over the competing hydrogen evolution reaction for each metal centre. An original descriptor-based catalyst design principle relating d-band centre position and coordination number to nitrogen reduction activity is proposed as the principal theoretical contribution. Keywords: electrochemical nitrogen fixation, single-atom catalysts, graphene, ambient conditions, Nigeria