Fig. 4. Structural characterization of electrocatalysts.

a, b Dark-field scanning transmission electron micrograph images and corresponding energy-dispersive X-ray spectroscopy mapping for Cu (blue dot) and Co (red dot). c, d High-resolution transmission electron micrograph images of the CoO-2.5 nm/Cu/PTFE catalysts before and after stability test. The scale bars in (a, b) and (c, d) are 200 and 5 nm, respectively. e Co 2p X-ray photoelectron spectroscopy spectra. f The k²-weighted Fourier-transform spectra from extended X-ray absorption fine structure and g the normalized X-ray absorption near-edge structure spectra at the Co K-edge of the CoO-2.5 nm/Cu/PTFE catalysts before and after stability test. Red and blue lines for before and after stability test, respectively. h Operando Co K-edge of X-ray absorption fine structure of CoO-2.5 nm/Cu/PTFE catalyst under an operating current density of 225 mA cm⁻² in a flow cell using 1 M KHCO₃ electrolyte. i Fourier-transform infrared spectra of the CoO-2.5 nm/Cu/PTFE catalysts before and after stability test, compared with the bare and ligand localized Cu/PTFE. Red and blue lines for CoO-2.5 nm/Cu/PTFE catalyst before and after stability test, respectively; light grey and dark grey lines for Cu/PTFE and ligand/Cu/PTFE controls, respectively. Red and black arrows for –CO– and –COO– in the poly(methacrylic acid) ligand, respectively. a.u. presents arbitrary unit.