a, CV curves of ZrN NPs on a GDE or a glassy carbon disk electrode (GCDE) at a scan rate of 50 mV·s−1 in Ar- and O 2 -saturated 0.1 M KOH solutions. The GDE oxygen reduction peak at 0.81 V in the O 2 -saturated electrolyte is at a slightly higher potential than that of ZrN NPs on a GCDE. b, LSV curves of the ZrN NPs and Pt/C on a GDE or a GCDE at a scan rate of 10 mV·s−1 in O 2 -saturated 0.1 M KOH solution. Both the onset and half-wave potentials of ZrN NPs on the GDE are close to those for the GCDE, although the GDE has a higher limited diffusion current due to the difference between electrical conductivities of the two electrodes. c, RDE polarization curves of ZrN NPs on the GDE in O 2 -saturated 0.1 M KOH at rotation speeds ranging from 400 to 2025 rpm and a scan rate of 10 mV·s−1. d, Koutecky–Levich plots of ZrN NPs on the GDE at potentials from 0.4 to 0.6 V. On the basis of the slopes of the Koutecky-Levich plots, the number of transferred electrons is 4, which is same as that obtained using the GCDE. These results demonstrate that C from the glassy carbon electrode is not required for catalytic activity, indicating that the activity of ZrN is intrinsic.