. Morphologies and element composition of samples FESEM images of (a) Cu₂O, (b) Cu₂S, (c) CuO-FO and (d) CuO-FS; (e) TEM image of CuO-FS; (f) HAADF image of the CuO-FS; (g, h) Corresponding EDS elemental mapping

. XRD patterns of Cu₂O, Cu₂S, CuO-FS and CuO-FO

. XPS spectra of CuO-FS and CuO-FO

. (a) Cathodic polarization curves of CuO-FO and CuO-FS in 0.1 mol/L KHCO₃ electrolyte saturated with CO₂/Ar, FE of all products and current density over (b) Cu₂S, (c) Cu₂O, (d) CuO-FO, and (e) CuO-FS, and (f) FE of all products of CuO-FS tested at different voltages Colorful figures are available on website

. (a) FE, (b) partial current densities, (c) Tafel plots of HCOOH over Cu₂S, Cu₂O, CuO-FO and CuO-FS; (d,e) CV curves of (d) CuO-FO and (e) CuO-FS at various test voltage scan rates, and (f) current density of four samples at various test voltage scan rates Colorful figures are available on website

. Long-term stability test at the voltage of -0.9 V for the CuO-FS and corresponding FE of different products

. N₂ adsorption isotherms of CuO-FS and CuO-FO

. XRD patterns of CuO-FS and CuO-FO after (a) 3 min and (b) 20 h test

. EPR spectra of CuO-FS and CuO-FO after electrochemistry test for 1 h