Water splitting on 3D-type meso/macro porous structured photoanodes based on Ti mesh

The behaviour of photoanodes based on TiO2 nanotubes (TNTs) grown on Ti gauze (80 mesh) by controlled anodic oxidation was studied in a compact photo-electrocatalytic (PECa) device for H-2 production by water photo-electrolysis. The 3D-type structured TNTs-Ti meshes evidenced: i) a mesoporosity due to the presence of highly ordered TiO2 nanotube arrays grown on the round surface of the regularly woven wires of the Ti gauze and ii) a macroporosity due to the open area (36%) of the mesh. The resulting hierarchical porous 3D structure allowed the TNTs-Ti meshes to act both as photoactive materials for efficient light absorption and as porous substrates for fast mass diffusion of protons. The synthesis of TNTs was investigated by varying the applied voltage (in the range 40-60 V) and the anodization time (from 20 min to 7 h). Results showed that two main opposite aspects influenced their performances in the PECa cell: i) the inner diameter and ii) the length of TNTs. While the increase of the inner diameter with the anodization time should favour H-2 productivity, longer nanotubes negatively influence the catalytic activity due to increasing charge recombination phenomena. The result is an initial decreasing profile of H-2 evolution vs. anodization time, reaching a minimum at 3 h and then increasing again for the 5 h-anodized sample (H-2 production rate: 1.4 L m-(2) h(-1)). The maximum photo conversion efficiency (0.7%) was obtained, instead, for the 20 min-anodized sample. It is to remark that tests of water splitting were performed with no applied bias and without adding sacrificial donors, opening the route for a sustainable use of these 3D-type meso/macro porous structured photoanodes in the production of solar fuels.