Figure 3: HOR performances.

(a) Polarization curves of Ni/N-CNT, Ni/CNT, Ni (all of the three catalysts with a loading of 0.25 mg_Ni cm⁻²) and N-CNT (0.1 mg_Ccm⁻²) catalysts in H₂-saturated 0.1 M KOH at a scan rate of 1 mV s⁻¹ and rotating speed of 2,500 r.p.m. (b) Polarization curves of Ni/N-CNT in H₂-saturated 0.1 M KOH at a scan rate of 1 mV s⁻¹ and various rotating speeds. Inset is the Koutecky–Levich plot at an overpotential of 50 mV. (c) HOR/HER Tafel plots of the specific current density on Ni/N-CNT, Ni/CNT and Ni in H₂-saturated 0.1 M KOH. The dashed lines indicate the Butler–Volmer fitting. (d) Mass activity at 50 mV (unpatterned) and exchange current density (patterned) of the Ni/N-CNT, Ni/CNT and Ni, respectively.