Localized graphitization on transition-metal-chalcogenide-decorated carbon nanotubes for electrocatalytic OER

For successful commercialization of technologies for the sustainable production of green hydrogen, nickel (Ni)- and iron (Fe)-based materials are the most promising cheap and effective noble metal-free catalysts for alkaline OER catalysis. A fabrication strategy was adopted based on a cost-effective and high-yield synthesis of Ni-, Fe-, and Mo-doped ZIF-8 (Zn), e.g. NiFeMo-ZIF-8, as the precursor, in an aqueous solution at room temperature. Subsequently, the precursor was subjected to pyrolysis for carbonization in an inert atmosphere (800 °C), e.g. NiFeMo–C, before secondary thermal treatment in sulfur (S) and phosphorus (P)-rich atmospheres to produce highly thermodynamically-active and low concentration transition metal chalcogenide (TMC) nanoparticles in conductive and porous nitrogen (N)-doped multiwalled-carbon-nanotubes (N-MWCNTs), e.g. NiFeMo-C-PS. The results revealed that the wrapping of the metal derivatives (MDs) by the MWCNTs (∼10 nm diameter) resulted in considerably rapid electron transfer via the highly conductive MWCNTs, leading to accelerated OER performance through (1) minimal diffusion pathways which enabled efficient charge transfer and (2) the increased accessibility of metal derivatives, which formed varying active sites depending on the thermal atmosphere and conversion in the OER electrolyte at the operating voltage. The catalytic surface area was determined to be primarily NiFeOOH supported by Mo and S dopants with co-catalysis from phosphate ions. The best performing catalyst was Ni,Fe,Mo-doped ZIF-8 that was pyrolyzed and then heat treated in a P- and S-rich atmosphere to produce NiFeMo-C-PS; this showed a Tafel slope of 52 mV dec⁻¹ and overpotential of 437 mV at 1 A cm⁻².