| dc.description.abstract | The combination of solar energy and water splitting hydrogen production results in a feasible, prospective energy conversion and storage process—photoelectrochemical (PEC) water splitting. In this work, an all-solid-state PEC cell has been fabricated and its performance with different photoanodes has been studied. An attempt to move one step further to fabricate an even more compact solid-state PEC cell using a proton-electron mixed conducting membrane has also been carried out later. In the photoanodes preparation stage, titania with different morphologies, e.g., drop-cast P25 titania nanoparticles, thermally treated Ti foil and highly organized titania nanotubes (TNT), are prepared and tested for their intrinsic properties, such as the donor density, flatband potential, etc. In particular, TNTs are synthesized by a 2-step anodization method, with immersing pretreatment, long and stable TNTs can be grown on a thin Ti substrate. Three TNT samples synthesized for 5, 20 and 30 min in the 2nd anodization step have been studied, and later applied in the PEC cell. The Type 1 solid-state PEC cell is fabricated by simply attaching the photoanode and the cathode onto the two sides of a Nafion proton conducting membrane. In this way, water oxidation reaction and proton reduction reaction will take place in each side of the membrane, hence gases are separated as soon as they are generated. The cathode consists of the carbon paper as the substrate, and the platinum coated carbon nanoparticles (Pt-C) as the electrocatalyst, which is connected with the photoanode through an external circuit. Regarding the photo-to-current performance, PEC cells employed with TNT as their photoanodes perform better than the ones with other photoanodes. Furthermore, the ionic conductivity around the TNT photoanodes has a significant impact on the overall cell performance. With deionized water as the liquid environment, the cell employed with the TNT 5 min photoanode gives the highest efficiency, while replacing the deionized water with a 0.5 M sodium sulphate solution makes the cell with the TNT 30 min sample perform the best. The hydrogen production of the Type 1 solid-state PEC cell was confirmed by GC measurements. When it comes to the Type 2 cell, the external circuit was removed, and the Nafion membrane was integrated with the carbon paper to form a protonelectron mixed conducting membrane (MCM). Electrons and protons generated by the photoanode during the oxidation half reaction were expected to transport through the integrated membrane simultaneously. The electrocatalyst—Pt-C was deposited directly on one side of the MCM, so that proton and hydrogen can easily recombine into hydrogen molecules that side. The concept of producing hydrogen by this type of cell was confirmed by the detection of a hydrogen peak from GC measurements, in which a basic solution was introduced to the photoanode in order to enlarge the chemical potential difference between the two sides of the MCM. However, involving alkalies leads to the carbon corrosion, which results in the formation of carbonate ions. Consequently, a relatively large methane production was observed, since the reaction from carbonate ions to methane is energetically more favorable than hydrogen production. In conclusion, a compact, robust, solid-state PEC cell for water splitting hydrogen generation can be built through a simple process, and a novel nanostructure modification of titania as the photoanode can enhance the overall cell performance. Protons and electrons generated during the water oxidation can pass through a proton-electron MCM simultaneously, and get recombined into hydrogen where the electrocatalyst is present, which results in a more compact solid-state PEC cell. | eng |