Copper-doped ruthenium oxide as highly efficient electrocatalysts for the evolution of oxygen in acidic media

•Cu-doped RuO2 catalyst is synthesized via calcination of amorphous RuCu sample.•The catalytic activities of Cu-RuO2 for OER depend on the calcination temperature.•Cu-RuO2-300 shows an extremely low OER overpotential of 189 mV at 10 mA cm−2.•The activity is much higher than most OER catalysts reported in literature.•Large surface area and high-index facets are responsible for the high activity.