Novel design of TiO2 nanotube arrays as photoactive materials for water photo-electrolysis and CO2 reduction back to liquid fuels

Background Fossil fuel consumption and rapid increase in the level of CO2 are both matters of great concern. Several discussions have been made in the scientific community about the strategies to be adopted to replace fossil fuels and introduce renewables in the future energy scenario. In this context, the conversion of solar energy into H2 via water splitting process is one of the most attractive engineering solutions, as well as the photo-electrocatalytic conversion of CO2 to high chain hydrocarbons (C>2). The last is a good opportunity to diminish the level of CO2 in the environment, thus closing the CO2 cycle and preserving the large investments made for the existing infrastructures based on liquid fuel storage and transport [1]. Aims This contribution is aimed to the development of novel TiO2 thin films with characteristics suited to be used as electrodes in PhotoElectroChemical (PEC) reactors for H2 production by water splitting. The key aspect of manufacturing these kinds of materials is the capability to control their nanoscale morphology and structure, thus improving their performances in terms of H2 productivity. The same reactor configuration may be used to convert CO2, water and sunlight into liquid fuels (hydrocarbons and oxygenates), by feeding CO2 at the cathode side. Methods A dense array of TiO2 nanotubes is synthesized by controlled anodic oxidation of Ti foils, applying an electric field (15-60 V) in presence of a suited electrolyte [2]. The TiO2 nanotube arrays are then assembled with i) a proton membrane (Nafion®) and ii) a C-based electrocatalyst to form a Membrane Electrode Assembly (MEA), which is very similar to those used in fuel cells. Finally the MEA is located inside the PEC reactor for testing experiments in H2 evolution by water splitting. Results The solar PEC reactor was designed to produce H2 and O2 in two separate compartments, avoiding the fast back reaction to form water again. Results showed a H2 productivity of about 5 mol h-1 with a photogenerated current (measured in situ) of 0.2 mA. It is to notice that our experiments were carried out without applying any voltage and by using no sacrificial donors. Hydrogen productivity was about one order of magnitude higher with respect to a random assembly of TiO2 nanoparticles tested in a classical slurry photoreactor. Moreover, we are currently working on a new design of cell configuration, based on the direct use of TiO2 nanotubes as photoactive membrane, thus eliminating the Nafion® (a very expensive material) and solving the issues related to the presence of several interfaces which limit the mass/charge phenomena. Conclusion Even though the efficiencies of such solar materials/devices are still too low for an actual implementation, the recent advances in the design of PEC reactors together with the capability to "tailor" a specific nanoarchitecture in TiO2, are very promising.