Fig. 3: Electrochemical properties of nanoporous electrodes.

a OER polarization curves for self-supported nanoporous (NP) FeCo/CeO2−xNx (pink), FeCo/Ce-O (yellow), FeCo (blue), CeO2−xNx (green) electrodes, RuO2 electrodeposited on nickel foam (RuO2/NF, dark green) and commercially available RuO2 nanocatalyst immobilized on glassy carbon electrode (RuO2/GC, dark yellow) in O2-purged 1 M KOH electrolyte. Scan rate: 1 mV s−1. b Comparison of current densities at the overpotential of 360 mV for self-supported NP FeCo/CeO2−xNx, FeCo/Ce-O, FeCo, CeO2−xNx, RuO2/NF and RuO2/GC electrodes. c Tafel plots comparing the Tafel slopes of different catalysts according to the OER polarization curves in panel (a). d EIS spectra of NP FeCo/CeO2−xNx, FeCo/Ce-O, FeCo and CeO2−xNx electrodes, as well as RuO2/NF and RuO2/GC. Inset: The electrical equivalent circuit used for fitting EIS, where RI and RCT denote the intrinsic electrode and electrolyte resistance and the charge transfer resistance, respectively, RP is the pore resistance, CPE represents the constant phase elements. e Comparison of current density at overpotential of 300 mV and Tafel slope for NP FeCo/CeO2−xNx electrode with nonprecious metal-based OER catalysts previously reported (Supplementary Table S1).