Figure 2. Electrocatalytic performance of the CoN_x/C electrocatalysts.

(a) HER polarization plots of the CoN_x/C, N/C, Co/N and Pt/C catalysts in 0.5 M H₂SO₄. (b) RRDE measurements of hydrogen evolution from a 0.5 M H₂SO₄ solution on different catalyst-modified electrodes. The Pt-ring electrode was maintained at 0.7 V for the oxidation of the H₂ that was evolved on the disk electrode. (c) Tafel plots obtained from the polarization curves in a. (d) Initial and post-potential cyclic voltammograms of CoN_x/C (5000 cycles) and Co/N (100 cycles) in 0.5 M H₂SO₄. Potential sweeps were cycled between 0.2 and −0.25 V versus RHE (not iR-corrected). (e) HER polarization plots of CoN_x/C, N/C, Co/N and Pt/C catalysts in 1.0 M KOH. (f) Tafel plots obtained from the polarization curves in e. For all RDE and RRDE measurements, the catalyst loading is 2.0 mg cm⁻² for non-Pt materials and 0.2 mg cm⁻² (40 μg_Pt cm⁻²) for the Pt/C catalyst. For CoN_x/C catalysts, the cobalt loading is only 2.8 μg_Co cm⁻², based on the ICP-AES measurements. Electrode rotation speed: 1,600 r.p.m.; scan rate: 5 mVs⁻¹.