in-situ Raman Spectroscopic Investigation of Early CO2RR Surface Chemistry on CuP2

Abstract

In this study, we combined ex-situ structural/surface analyses with in-situ Raman spectroscopy to elucidate the very early interactions that occur on the surface of a phosphorus-rich copper phosphide, CuP2, during the CO2 electrochemical reduction reaction （CO2RR）. Whereas conventional CO2RR research aims at high current densities and high product yields, this work deliberately exploits the potential window in which the reaction is “not vigorously progressing” — a window that arises from the semiconducting nature of CuP2 — as a probe window for the initial reaction mechanism rather than as a drawback. XRD, SEM, and TEM analyses confirmed that the samples synthesized via the oleate route are flake- shaped particles with the monoclinic CuP2 crystal phase. Half-cell CV and LSV showed a cathodic feature near −0.1 V vs. RHE and a lower cathodic current density under CO2 than under N2, suggesting partial reduction of the surface phosphate/oxide layer together with blocking of HER-active sites by CO2-derived adsorbates. Pre/post-reaction XPS revealed a clear change in the Cu LMM line shape and enhanced exposure of the metal–P bond in the P 2p region, indicating that the CuP2 surface evolves from an initially phosphate-rich layer toward a reconstructed surface on which Cu- and oxygen-related species are more exposed. in-situ Raman spectra showed the characteristic peaks of monoclinic CuP2 （around 290, 430, and 460 cm-1） clearly at OCV and at positive potentials, but the overall spectral intensity decreased upon application of negative potentials under CO2. Under Ar the decrease was much smaller and the main peak positions remained essentially unchanged, which we interpret not as a bulk lattice deformation but as a reduction/reconstruction of the surface phosphate layer combined with interfacial changes induced by CO2-derived adsorbates. Comparison with computationally predicted Raman spectra of surface and bulk CuP2 models showed that the OCV and positive-potential spectra in our experiments matched the bulk-like model that contains both low- and high-frequency modes, and that this bulk-like region weakened uniformly at negative potentials. EC-AFM measurements likewise revealed surface morphology changes within the same potential range. Overall, this study provides in-situ spectroscopic evidence that the dynamic reconstruction of the CuP2 surface begins already in the near-onset region, before the reaction reaches full onset, and suggests that P sites or Cu–P interfaces may participate in the initial CO2 activation step.