Periodic Reporting for period 1 - H2O-SPLIT (Carbon-Oxynitride Coupled Artificial Photosynthesis System For Solar Water Splitting Beyond 600 nm)
Reporting period: 2019-05-01 to 2021-04-30
The depletion of fossil fuels and serious environmental problems have urged modern society to search for renewable energy by utilizing abundant solar energy. Although hydrogen is considered as a zero-emission energy carrier, its current worldwide production heavily relies on fossil fuels (95%). In nature, oxygenic photosynthesis splits water into oxygen, protons and electrons, which reduce carbon dioxide and generate carbohydrates. In analogy, artificial photosynthesis, pioneered by Fujishima and Honda in early 70’s, targets the splitting of water into oxygen and hydrogen on a semiconductor in the photoelectrochemical cell by applying an external bias. Although metal oxide-based photocatalysts have shown high photocatalytic activity and excellent chemical stability for water splitting for green hydrogen generation, they can only function under UV light due to their large optical band gap energy. It is well known that UV light makes up only a small portion (5%) of the total solar energy, making an insufficient use of solar energy. On the other hand, visible light makes up about 52% of the total solar energy. Therefore, it is necessary to develop visible-light-active photocatalysts to efficiently utilize solar energy. In this project, we aimed at developing novel carbon-oxynitride coupled artificial photosynthesis system for solar water splitting beyond 600 nm. This aim was met by setting four scientific objectives: (i) to engineer the band structure of BaTaO2N by doping; (ii) to study the dimensional effect of carbon allotrope on water splitting of BaTaO2N; (iii) to evaluate solar water splitting efficiency, photo-stability, and scalability of the carbon-BaTaO2N; and (iv) to design a monolithically integrated photocatalyst.
In this project, a novel carbon-oxynitride coupled artificial photosynthesis system for solar water splitting beyond 600 nm was developed by setting and achieving a set of scientific objectives: (i) Synthesis of doped BaTaO2N (to address scientific objective 1): first, a novel approach was developed to reduce the synthesis time and defect density in BaTaO2N by localizing an NH3 delivery system directly to the starting materials. Using such localized NH3 delivery system, we succeeded to synthesize the BaTaO2N crystals, within 4 hours, with less defect density, resulting in an enhanced photoelectrochemical performance. Then, the band structure and band gap of BaTaO2N were engineered by a B-site doping via involving various aliovalent cations and changing the dopant amount up to 15%. Particularly, Al was found to be the most efficient in improving the photocatalytic H2 and O2 evolution reactions compared to other dopants. Also, the flux method was found to be efficient over hydrothermal and solid state reaction for synthesizing the BaTaO2N crystals with less defect density. (ii) Development of carbon-BaTaO2N composite (to address scientific objective 2). To develop carbon-oxynitride composites for solar water splitting, BaTaO2N and various carbon allotropes with different dimensions were combined. Particularly, the concentration (0-30 wt%) of carbon allotropes on water oxidation activity of BaTaO2N was studied, and 20 wt% carbon allotrope was found to be the most efficient amount for enhancing the water oxidation activity. In electrode system, carbon allotrope was mixed with BaTaO2N particles and deposited on conductive FTO substrate, and their photoelectrochemical performance was studied. It was found that graphene-BaTaO2N composite exhibited the highest activity in comparison to BaTaO2N combined with other carbon allotropes. (iii) Evaluation of solar water splitting activity (to address scientific objective 3). The photoelectrochemical performance was characterized in a typical three-electrode electrochemical cell setup using the fabricated photoelectrode, an Ag/AgCl electrode in saturated KCl, and a Pt wire connected to a potentiostat as working, reference, and counter electrodes, respectively. The effect of various O2 and H2 evolution cocatalysts was investigated. It was found that CoOx and Pt cocatalysts were the most efficient for O2 and H2 evolution reactions, respectively. The kinetics and mechanisms of water splitting over the developed artificial photosynthesis systems were also studied by conducting time-dependent experiments and by using time-resolved absorption spectroscopy, respectively. (iv) Design of photoelectrochemical device (to address scientific objective 4). A novel carbon-oxynitride coupled artificial photosynthesis system in the form of panel was fabricated and its stability, efficiency and scalability was tested. Various characterizations and testing revealed that the developed carbon-BaTaO2N artificial photosynthesis system has good stability, efficiency and scalability. In order to provide further application opportunities, the stability and efficiency of the carbon-BaTaO2N for solar water splitting need to be further improved. The researcher will continue his active collaboration with academic and industrial partners for further enhancing the stability and efficiency and testing newly developed materials in the future. The project findings have been widely disseminated in the form of invited and oral presentations at nine international conferences and in the form of scientific articles in seven international peer-reviewed journals.
During this two-year project, the researcher in close collaboration with the host, academic partner (first secondment) and industrial partner (second secondment) successfully developed a novel carbon-oxynitride coupled artificial photosynthesis system for solar water splitting beyond 600 nm. The outcome of this project offers a simple approach to develop an inexpensive and efficient carbon-oxynitride coupled artificial photosynthesis system by involving various earth-abundant carbon allotropes and dopants. Particularly, the time-retrenched synthesis route not only reduces the defect density and enhances the efficiency of BaTaO2N but also saves energy in its production. The developed artificial photosynthesis system has strong potential to reduce CO2 emission during green hydrogen generation, having a significant social and environmental impact. The scientific findings of the project will be beneficial for pushing the boundaries of the field of advanced materials for solar-to-hydrogen conversion and paving the way for a sustainable production of renewable and green hydrogen. The current momentum on green hydrogen is unprecedented because of fast climate change (there is the urgency to radically lower emissions), and scientists, engineers, and policy-makers must continue working together to overcome the challenges associated with cost, efficiency, and application. This project contributes to the excellence and leadership of the European Union in the field of green hydrogen from renewable resources and influences the knowledge-based and low-carbon economy and climate action of the European Union.