HER performance of nanoporous Cu/Cu5Zr electrode in alkaline electrolyte. (a, b) Polarization curves (a) and the corresponding Nyquist plots of EIS spectra (b) for nanoporous Cu/Cu5Zr catalyst electrode before and after 10,000 potential cycles. (c) Long-term stability tests of nanoporous Cu/Cu5Zr at different current densities. Inset: typical SEM image of nanoporous Cu/Cu5Zr electrode after stability measurement. (d) The production and Faradaic efficiency of theoretical hydrogen generation (dash line) and practical hydrogen (dot).