The electrochemical and photoelectrochemical approaches are well-suited for the selective reduction of carbon dioxide to the desired small organic molecules. 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 is in photoelectrochemical approaches utilizing p-type semiconductors as the main components of the CO₂-reduction photocathodes capable of generating electrons with use of solar visible light.

Among representative examples of semiconducting materials, transition metal oxides (e.g. Cu-based oxides, such as Cu₂O, CuFeO₂, CuBi₂O₄, CuNb₂O₆, Cu/Cu₂O) could be considered. 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 or polynuclear type materials. In this respect, ultra-thin films of conducting polymers, supramolecular complexes (with nitrogen containing ligands and certain transition metal sites) or robust bacterial biofilms could be advantageous. For example, the oligoaniline over-layer [1] has been generated in the external methanol solution. Under such conditions, the copper(I) oxide photoelectrode does not change the oxidation state during photoelectrochemical diagnostic experiments. Thus the robust partially-polymerized aniline over-layer leads to 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 oligoaniline-modified Cu₂O. The proposed bi-layered photocathode has been demonstrated to produce such simple organic fuels as alcohols (mainly methanol).

[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)