(a) HAADF-STEM, and elemental mapping images of the hollow square Ni-CuO. (b) NH₃ yield rate and ${\text{FE}}_{{\text{NH}}_{\text{3}}}$ of Ni-CuO. (c) Selectivity of NH₃ of Ni-CuO. DMPO-involved EPR spectra of the Ni-CuO under (d) electrolyte without KNO₃ and (e) electrolyte with KNO₃. (f) TEM image of the hollow square Ni-doped CuO catalyst and schematic illustration of the synergistic mechanism for electrochemical nitrate reduction to ammonia on the Ni-CuO surface. Adapted from Ref. [34]. Copyright 2025, American Chemical Society. (g) TEM image of PdCuCo hollow NPs (inset: the corresponding size distribution histogram of HPdCuCo NPs). (h) HR-HAADF-STEM image and HAADF-EDS elemental mappings of HPdCuCo NPs. (i) Cycling stability test of HPdCuCo/RGO at −0.5 V in Ar-saturated 0.5 M K₂SO₄ electrolyte with 2000 ppm NO₃⁻. (j) The illustration of the HPdCuCo/RGO catalyst and the proton mediator boosted tandem electrocatalysis process. Adapted from Ref. [35]. Copyright 2025, Elsevier.