Hydrogen produced by splitting water using solar energy is also called “solar hydrogen” and is a possible future clean and low-cost fuel. Unfortunately, an efficient production system under irradiation with visible light has not yet been obtained. Therefore, for the future of solar hydrogen (which is crucial in decarbonization scenarios), the development of stable and efficient photosensitive materials is key.
The main objective of the research activity is the development of a hydrogen production system, separately from oxygen, by photoelectrocatalytic water splitting and optimization of the process to an efficiency of 6% with respect to incident radiation. In pursuit of the set goals, much attention and much of the experimental work has been focused on the development of photoelectrodes made from materials abundant in nature, with minimal environmental impact and low production cost, so as to facilitate faster and wider dissemination of this technology. During the reporting period, hydrogen could be separated from oxygen through water photoelectrolysis, achieving an efficiency value with respect to incident radiation of less than 1%; however, it is important to point out that the goal of containing the cost of production (which is very important for the technology to be an effective support to the decarbonization process) has strongly influenced the choice of materials and in fact prevented the expected efficiency value from being achieved. Indeed, in an attempt to limit the costs of said technology, efforts have been essentially directed toward the construction of thin-film photoelectrodes in which formulations capable of promoting reactions at the electrodes in the absence of platinum (the catalyst of the water splitting reaction) have been favored; the reaction would have resulted in a marked improvement in efficiency with respect to incident radiation, with the achievement of much higher values of at least 3%.
For the construction of the electrodes to be used in the production of hydrogen through the photocatalytic water splitting reaction, the research activity carried out in the reporting year was devoted to the optimization of the film deposition of the photo-active materials identified during the previous year.
Among the anodic materials identified while analyzing the bibliography, BiVO4 was selected, which was synthesized by a combustion procedure found in the literature and optimized to allow its deposition in the form of a crystalline, continuous film on a suitable conductive glass substrate.
For the cathodic material, the focus was on C3N4, whose performance as a photocatalyst was improved through the formation of heterojunctions with other semiconductor materials. As a co-catalyst, the perovskitic phase Cs3Bi2Br9 (already studied during the previous year) was used.
The materials were deposited by drop casting, using a polymer matrix to adhere the photocatalytic material in powder form to the substrate. The powdered materials were previously synthesized by pyrolysis in nitrogen flow (the C3N4) and precipitation from solution (the Cs3Bi2Br9).
For electrode optimization, the effect of deposition of different amounts of material was studied; in addition, in the case of cathodes, the effect of increasing the surface area of C3N4 (by heat treatment) and using a hydrogen ion conductor instead of an electron conductor as the polymer matrix was also studied.
Operation was evaluated by electrochemical characterization, using open-circuit potential readings, cyclic voltammetry, and constant potential measurements; material stability was assessed by scanning electron microscopy and X-ray diffraction.
The best-performing anode and cathode were used for the assembly of a complete cell through which hydrogen and oxygen were produced separately by photoelectrocatalytic water splitting.