Synthesis of a Highly Efficient Oxygen‐Evolution Electrocatalyst by Incorporation of Iron into Nanoscale Cobalt Borides

Abstract

High-performance catalysts for the oxygen-evolution reaction in water electrolysis are usually based on expensive and rare elements. Herein, mixed-metal borides are shown to be competitive with established electrocatalysts like noble metal oxides and other transition-metal(oxide)-based catalysts. Iron incorporation into nanoscale dicobalt boride results in excellent activity and stability in alkaline solutions. (Co_0.7 Fe_0.3 )₂ B shows an overpotential of η=0.33 V (1.56 V vs. RHE) at 10 mA cm^-2 in 1 m KOH with a very low onset potential of ≈1.5 V vs. RHE, comparable to the performance of IrO₂ and RuO₂ . XPS shows that the original catalyst is modified under the reaction conditions and indicates that CoOOH and Co(OH)₂ are formed as active surface species, whereas the Fe remains in the catalyst, contributing to an improved catalyst performance. The nanoscale borides are obtained by a one-step solution synthesis, calcined, and characterized by XRD, energy-dispersive X-ray spectroscopy, and SEM. Single crystals of (Co_1-x Fe_x )₂ B grown under chemical transport conditions were used for an unambiguous specification of the nanostructured particles by relating the cobalt/iron ratio to the lattice parameters.