Hybrid and Functionalized Interfaces for Photoelectrochemical Water Splitting and Electrocatalytic Carbon Dioxide Reduction

Abstract

The reduction of carbon dioxide is a very inert process that requires breaking the double C=O bond in the stable CO2 molecule. There is a need for conversion of carbon dioxide, a potent greenhouse gas and a contributor to the global climate change, to useful carbon-based fuels or useful chemicals. Our contribution addresses the low-temperature CO2-conversion processes based on electrocatalytic and photoelectrochemical approaches. In the realistic electrolysis cells, the reduction of CO2 (at cathode or photocathode) would have to be accompanied by water oxidation (at anode or photoanode). Our interest is in the PEM electrolyzers utilizing the proton exchange polymer electrolyte membranes (as separators between anode and cathode compartments) allowing electrolysis at low pH’s and, in principle, capable of yielding higher current densities than alkaline electrolyzers. Nevertheless, under such conditions, the parasitic hydrogen evolution reaction is likely to become predominant and complicate formation of any CO2-reduction products. Our preliminary results with the copper or ruthenium containing tungsten oxide nanorods as catalytic materials are promising with respect to the investigations in acid media. Recent advances (including our studies) in photoelectrochemical water splitting are based on semiconducting photoanodes (typically n-type semiconducting metal oxides) combined either with a metallic cathode (e.g. Pt) or with a p-type semiconductor as a photocathode should be mentioned here in a context of the development of the CO2-electrolyzers exploring photoelectrocatalytic properties and operating under illumination. Among representative examples of metal oxides with n-type semiconducting and promising photoelectrocatalytic properties, tungsten oxide (WO3) should be mentioned. The WO3-based semiconductors are highly stable in acid media, and they are characterized by an energy band gap of 2.5-2.7 eV thus allowing absorption of a reasonable fraction of the solar spectrum up to ca. 500 nm. It comes from our research that, upon illumination, the onset potential for photooxidation of water (in acid medium) is as low as 0.45 V (vs. RHE) what is of practical importance to the energy efficient photoelectrolysis. Further improvement of performance of photoactive metal oxides, e.g. through doping with certain anions or oxo-species will be discussed. Utilization of such systems in the CO2-electrolyzers would require, however, development of practical cathodes (CO2-reduction) operating in acid media that will be addressed during presentation. In our work, the solar energy will be absorbed by a suitable semiconductor to drive water oxidation to oxygen and protons, whereas the reduction of CO2 will be achieved electrochemically with use of a proper (selective) catalyst. By utilizing the aqueous acidic medium, no external hydrogen is required for the CO2-reduction process because the H+ ions not only exist at sufficiently large population but they are generated in situ during the anodic reaction. Coupling of the two processes in a single unit leading to one-step photoelectrochemical/electrocatalytic approach should be possible (e.g. with use of a tandem cell) but, in practice, physical separation of the two reactions in photoanodic (photoelectrochemical water oxidation) and cathodic (carbon dioxide electroreduction) compartments is necessary to increase efficiency and to limit charge recombination. Conversion of CO2 to CO (accompanied by H2-evolution) is an attractive target because syngas (CO + H2) can be utilized for manufacturing of many useful chemicals including synthetic fuels via the Fischer-Tropsch type process.