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

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. (Co0.7 Fe0.3 )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 (Co1-x Fex )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.