Fig. 5. In situ Raman spectra of the catalysts and schematic of the tandem mechanisms of CuCoSP catalysts.

In situ Raman spectra of CuSP (a), CoSP (b) and CuCoSP (c) at different applied potentials in electrolytes containing 0.01 M NO₃⁻, 0.04 M K₂SO₄ and 0.01 M KOH. d A proposed reaction mechanism of CuCoSP tandem catalysis of NO₃RR at low overpotentials. In the middle of (d), the light blue region corresponds to the potential range for the reduction of NO₃⁻ to NO₂ species at the Cu/CuO_x phases, resulting in the etching of catalysts, while the light pink region shows the potential range for efficient tandem reduction of NO₃⁻ to NH₃.