Fig. 5From: Hollow Ni/NiO/C composite derived from metal-organic frameworks as a high-efficiency electrocatalyst for the hydrogen evolution reactiona Polarization curve of H–Ni/C, H–Ni/NiO/C, H–NiO/C, and NH–Ni/NiO/C. b Tafel slope of catalysts, c Nyquist plots of different samples recorded at a potential of − 0.2 V vs. the RHE. d Double-layer capacitances of various samples obtained using CVBack to article page