Fig. 5: Operando Raman and UV–vis spectra characterizations.

a 2D contour plots of operando Raman spectra of CoFe-NBs in 1.0 M KOH for the OER. b Representative normalized differential operando UV–vis spectra of CoFe-NBs in 1.0 M KOH for the OER. The differential absorption spectra were obtained from the difference between the spectra at different potentials and in the electrolyte without any applied potentials. c Absorbances of CoFe-NBs at 500 and 540 nm upon CV scan. d Calculated surface accumulated charges of Co-NBs and CoFe-NBs against the OER applied potentials.