Fig. 4. (a) HER polarization curves of the catalysts in 0.5 M H₂SO₄ at a scan rate of 5 mV s⁻¹; (b) Tafel slope and overpotential at 10 mA cm⁻² and 50 mA cm⁻² of the catalysts; (c) comparison of the Tafel slope and overpotential for Rh-Co₃O₄ BNSs at different Rh loadings; (d) comparison of the Tafel slope and overpotential of Rh-Co₃O₄ BNSs and the reported catalysts; (e) EIS Nyquist plots of the catalysts; (f) C_dl values of the catalysts; (g) HER polarization curves of the catalysts before and after 2000 CV cycles; (h) chronopotentiometric curves of the catalysts at 10 mA cm⁻²; (i) Faraday efficiency of Rh-Co₃O₄ BNSs.