Nickel–salen as a model for bifunctional OER/UOR electrocatalysts: pyrolysis temperature–electrochemical activity interconnection

Abstract

In this study, the nickel derivative of a salen-type Schiff base was pyrolyzed under controlled conditions to form a nano-sized Ni/NiO_x core–shell species that serves as a model for a proof-of-concept investigation of the relationship between pyrolysis temperature and electrocatalytic activity. The pyrolytic process was tracked via measurements of crystallinity, carbon defect, specific surface area, heterogeneous elemental doping and elemental distribution. The decomposition process was elucidated by TG-MS; the electrochemical characteristics of the residue were deduced from synchrotron XAFS combined with HRTEM, XRD and XPS (Ni(C₂₆H₂₈N₃O₄) → Ni/NiO_x + (C_xH_y, H₂, CO₂ + CH₃O) → Ni/NiO_x@NCs). An optimized specimen (coded S–Ni-800) provided 268 mV overpotential in OERs and 1.353 V in UORs, and was stable for 15 hours in an alkaline medium. This model study underscores the importance of thermal tracking in the design of Ni-derived carbon nanomaterial electrocatalysts that are associated with OERs/UORs at low potentials.