Development of a TiO2 nanotube array-based photo-reactor for H2 production by water splitting

The issues regarding greenhouse gas emissions and fossil fuel depletion, together with the increase of the world population and growing energy demand, have raised the social and economic pressure to develop clean and safe energy conversion processes and introduce fundamental changes in the present energy supply systems. There is the need on a short-term of new technologies for energy saving and efficiency, and use of biomass. On a medium term, a more rational use of renewable resources is needed, including solving the issue of energy storage and transport, and find a sustainable solution to CO2 emissions, because the full transition to non-fossil fuels will require longer time. Finally, in a long-term, the renewable energy scenario, based in particular on solar energy, will become predominant [1]. The conversion of solar energy into H2 via water splitting process is one of the most attractive ways to obtain clean and renewable energy. The main work in photochemical water splitting, at present, concerns the increase of the efficiency and stability of the photoactive materials, in the way to achieve the required efficiency benchmark of 10% that will be viable for commercial implementation. In order to perform this aim, two parallel lines of research are pursued: the former refers to the synthesis of new materials with characteristics suited to be used as photo-catalysts (band gap near to the visible region, stability, non-toxicity, low cost etc.) [2]; the latter is the correct evaluation of the engineering aspects concerning the photochemical reactor, to achieve an improvement of the process efficiency (irradiation pattern, geometrical configuration, material of construction, heat exchange and mixing and flow characteristics) [3]. In this work we report on the development and realization of an innovative photoelectrochemical (PEC) cell based on the use of nano-engineered TiO2 array thin films as photo-anode for water splitting and/or ethanol photoreforming processes. The photo-reactor, built in Plexiglas and equipped with a quartz window, was realized in a highly compact configuration in order to reduce light scattering phenomena due to the water and maximize the photocatalytic efficiency. The separation of the photo-induced process in two physically distinct areas related to water oxidation (to form O2, protons and electrons) and proton reduction (to form H2) shows many potential advantages, by limiting charge recombination and avoiding the fast back reaction to water. For a practical use of the PEC solar cell, its design is quite different from that commonly used in literature: the anode and cathode are in the form of a thin film separated from a proton-conducting membrane and deposited over a porous conductive substrate, which allows the efficient collection/transport of the electrons over the entire film, as well as the diffusion of protons to/from the membrane and an efficient evolution of the gases. The particular configuration of the PEC cell allows also to measure in-situ the photocurrent generated between the electrodes. The highly ordered TiO2 nanotube arrays were synthesized by controlled anodic oxidation of Ti foils. The essence of the method may be described as a reconstruction of a thin TiO2 layer (formed initially by oxidation of a Ti foil) which occurs under the application of a constant voltage (in the range of 20-60 V) in presence of fluoride-based electrolytes. The surface characterization was performed by different techniques, such as FESEM, TEM, GAXRD, UV-Vis diffuse reflectance and current-time transients during the anodization process. Results in terms of hydrogen production and photo-generated current are very promising. [1] C. Ampelli, G. Centi, R. Passalacqua, S. Perathoner. Energy & Environmental Science, Vol. 3 (2010), pp. 292-301. [2] C. Ampelli, R. Passalacqua, S. Perathoner, G. Centi, D.S. Su, G. Weinberg. Topics in Catalysis, Vol. 50 (2008), pp. 133-144. [3] C. Ampelli, R. Passalacqua, S. Perathoner, G. Centi. Chemical Engineering Transactions, Vol. 17 (2009), pp. 1011-1016.