Nano-Engineered Electrodes for the Generation of Solar Fuels: Benefits and Drawbacks of Adopting a Photo-Electrocatalytic (PECa) Approach

In this contribution, the advantages of adopting a photo-electrocatalytic (PECa) approach for the production of solar fuels were discussed through the analysis of the factors that can influence the performances of PECa cells. Particularly, there are some issues, conventionally not studied, that should be taken into account already at the initial stage of cell design, such as: i) light harvesting and charge separation, ii) electron conductivity and mass diffusion, iii) productivity and type of products formed. The choice of the electrodic materials and the preparation of nanostructured electrodes are of great importance to improve the above aspects, by enhancing the photo-catalytic activity and limiting the overpotential of the cell. The experimental data were obtained by preparing and testing two different types of electrodes, one based on highly ordered TiO2 nanotube arrays (the photo-anode) and the other based on Cu nanoparticles deposited on nano-carbons (the electro-cathode). The electrodes were assembled together and located in a compact device with two separate compartments for each half-reaction. Finally, they were tested in the process of water photo-electrolysis and/or photo-reforming of organic wastes for the production of H2, as well as in the process of CO2 reduction to liquid fuels. Results, given in terms of H2 evolution, solar-to-hydrogen (STH) efficiency and liquid fuel productivity, were very promising. Particularly, 195 |imol of H2 were produced in 4 h of light irradiation without adding sacrificial donors, being over 16 % of this production due to only solar irradiation. This attractive result (if considering that non-doped TiO2 was employed as the photo-anode) is to ascribe to the enhanced visible light absorption of TiO2 nanotube arrays (due to their structural resonance effect) and to the reduced charge recombination (for their improved electron transfer). On the other hand, CO2 electrocatalytic reduction allowed obtaining not only formic acid (as usually obtained in conventional cells) but also methanol, ethanol, acetic acid, isopropanol and acetone. The formation of these products is due to the catalytic sites located in the interface between the small copper particles and the carbon surface, favouring electron transfer and C-C bond formation.