(Invited) Modification, Activation and Stabilization of Copper(I) Oxide Photocathodes: Competition between Catalytic and Inhibiting Effects during Light-Induced Electoreduction of CO2

Abstract

Among representative examples of semiconducting materials for the selective reduction of carbon dioxide to desired small organic molecules (fuels, utility chemicals), there are transition metal oxides (p-type semiconductors), in particular Cu-based oxides, such as Cu2O, CuFeO2, CuBi2O4, CuNb2O6, CuO and Cu2O) that are capable of generating electrons with use of solar visible light. Selectivity of the catalytic systems largely depends on the activing adsorptive (CO2) phenomena and the affinity of catalytic centers to the adsorbed carbon monoxide intermediates leading to their protonation or hydrogenation. Here application of aqueous or water-containing electrolytes is generally preferred. Furthermore, a compromise must be reached between the activity of a catalyst toward hydrogen evolution (water reduction) and its ability to promote reductive adsorption of CO2 and to generate moderate coverages of H atoms capable of stabilizing subsequent reduced intermediates. Our interest concentrates on copper(I) oxides but their practical use requires means of stabilization without poisoning or deactivation. To improve stability against photocorrosion, to assure the sufficient electron-hole pair separation, as well as to increase population of electrons in the conduction band, mixed oxide systems combining the p-type semiconductor (e.g. Cu2O) with n-type semiconductors such as TiO2, SrTiO3 or KTaO3 have been proposed. To meet requirements for practical Cu2O-based photocathodes, not only the ability to absorb efficiently visible light and to assure fast interfacial electron transfers but also the stability issue needs to be addressed. Contrary to the poor performance of the pristine (bare) copper(I) oxide photocathode, the visible-light-illuminated photoelectrochemical reduction of carbon dioxide has been successfully performed using the p-type Cu2O-semiconductor (deposited onto the transparent fluorine-doped conducting glass electrode) over-coated with the ultra-thin films of various polymer (poly(4-vinyl pyridine, oligoaniline) or polynuclear type materials (titanium, zirconium and tungsten oxides).. In this respect, ultra-thin films of functionalized carbon nanostructures, supramolecular complexes (with nitrogen containing ligands and certain transition metal sites) or robust bacterial biofilms could also be advantageous. For example, the oligoaniline or tungsten oxide over-layers [1,2] have been generated on copper(I) oxide surfaces. Under such conditions, the copper(I) oxide photoelectrode does not change the oxidation state during photoelectrochemical diagnostic experiments. Thus certain robust (not necessarily thin) could exhibit the effect of stabilization of Cu2O against photocorrosion. Among important issues is the ability of CO2 to undergo adsorption (and activation toward reduction to CO) at both bare and modified surfaces of Cu2O. The proposed bi-layered photocathode has been demonstrated to produce such simple organic fuels as alcohols (mainly methanol and some ethanol). [1] Szaniawska E., Rutkowska I.A., Frik M., Wadas A., Seta E., Krogul-Sobczak A., Rajeshwar K., Kulesza P.J., Electrochim. Acta, 265, 400 (2018). [2] Rutkowska I.A., Szaniawska E., Taniewicz J., Wadas A., Seta E., Kowalski D., Kulesza P.J., J. Electrochem. Soc., 166, H3271 (2019).