Fig. 4. Mechanism investigations.

Operando XANES spectra of a Cu^δ+NCN and b CuNCN. c Fitted linear relationship between the energy position of the Cu K-edge in operando XANES spectra and the valence state of Cu. d Comparison of the EXAFS WTs of the Cu K-edge recorded during operando testing on Cu^δ+NCN. e Fourier-transformed k³-weighted EXAFS signals of the Cu K-edge recorded at different potentials on Cu^δ+NCN. f Changes of coordination number for the Cu–N, Cu–Cu and Cu–N/C coordination shells. g Changes of bond length for the Cu–N, Cu–Cu and Cu–N/C coordination shells. h Operando Raman spectra of Cu^δ+NCN. i Operando ATR-SEIRA spectra of Cu^δ+NCN, CuNCN and CuO.