Fig. 3a presents the polarization curves of CoFe-LDH/GF, RuO
2/GF and GF. As observed, bare GF has poor OER activity and RuO
2/GF is excellent in OER activity (
η = 211 mV at 10 mA/cm
2 and
η = 267 mV at 100 mA/cm
2). CoFe-LDH/GF is also highly active for the OER (
η = 252 mV at 10 mA/cm
2 and
η = 285 mV at 100 mA/cm
2). As shown in Figs. S2a and b (Supporting information), it also shows excellent electrochemical performance in 0.1 mol/L (
η = 283 mV at 10 mA/cm
2 and
η = 345 mV at 100 mA/cm
2) and 30 wt% KOH (
η = 231 mV at 10 mA/cm
2 and
η = 257 mV at 100 mA/cm
2). Our CoFe-LDH/GF compares favorably to the behaviors of most reported non-noble-metal OER catalysts in alkaline aqueous media, as listed in Table S1 (Supporting information). The reaction kinetic rate is evaluated by Tafel slope (
Fig. 3b). The Tafel slope of CoFe-LDH/GF (61 mV/dec) is close to RuO
2/GF (52 mV/dec), suggesting CoFe-LDH/GF also has fast OER kinetics. Moreover, stability is an important criterion for evaluating electrocatalyst performance. The polarization curve after 500 cyclic voltammetric (CV) scans is almost the same as the initial one, meaning CoFe-LDH/GF possesses high stability (
Fig. 3c).
Fig. 3d displays the multi-step chronopotentiometric curve of CoFe-LDH/GF with the applied current density gradually increased from 40 mA/cm
2 to 260 mA/cm
2. Under the starting current density, the corresponding potential immediately stabilized at 1.57 V and maintains unchanged for the remaining 500 s. The same results are also observed in other current densities, reflecting superior mass transportation properties, high mechanical robustness, and outstanding electrical conductivity of the CoFe-LDH/GF electrode [
35]. The time current density curve demonstrates that the CoFe-LDH/GF stably runs over 24 h at a current density of 30 mA/cm
2 (Fig. S3 in Supporting information). CoFe-LDH/GF still maintains its nanowire array morphology (Fig. S4 in Supporting information) after such a prolonged test. Moreover, there is almost no change in the XRD pattern of post-electrolysis CoFe-LDH (Fig. S5 in Supporting information), evidencing the high robustness against OER electrolysis. We further determined the initial pH values and after the durability test (Fig. S6 in Supporting information), pH values remain almost unchanged.