A team of Chinese researchers presented a new method of efficient solar water separation. The approach demonstrated in the new paper by ACS Applied Materials involves modifying bismuth vanadate anodes to improve charge separation. The method provides a simple modification path and a promising design strategy.
Study: Efficient solar water separation through improved BiVO charge separation4 Photoanode. Image Credit: Fred Mantel/Shutterstock.com
Photoelectrochemical separation of water
The urgent need to phase out greenhouse gas-emitting fossil fuels has facilitated research into the production of alternative fuels. Hydrogen is an abundant, nearly carbon-neutral, renewable resource that can meet the energy needs of a decarbonizing society.
Photoelectrochemical water separation (PEC) is a green strategy for producing hydrogen from one of the most abundant, cleanest and most renewable sources on earth: water. Photoelectrochemical materials are used to harvest light energy from the sun and then dissociate water into its building blocks – hydrogen and oxygen. The hydrogen is then recovered to produce value-added products such as electricity or fuel, and the oxygen is released as a clean by-product of the reaction.
UV-vis scattered reflectance spectra of pure BiVO4 and Ni-NDAD/BiVO4. Image Credit: Wang, L et al., ACS Applied Energy Materials
In a photoelectrochemical water separation system, the selection of materials is essential to ensure the efficiency of the reactions. Over the years, many materials have been explored for photoanodes, with the first reported material being TiO2. Other photocatalysts that have been investigated for use in PEC devices include ZnO, Fe2O3WO3and BiVO4 (bismuth vanadate).
Among these, the monolithic BiVO4 proved to be an attractive semiconductor material, possessing a narrow bandgap and a suitable band edge location for reactions. However, this material has limitations, mainly its unfavorable recombination rate of electron holes and its slow water oxidation kinetics. These drawbacks cause the practical efficiency of the reactions to be lower than their theoretical efficiency.
The use of composite materials is a strategy to overcome the disadvantages of this photocatalytic material. Research has developed composites of functional materials including CoOOH/BiVO4 and co3O4/BiVO4. Incorporating functional materials into composites improves charge separation and anode kinetics, promoting more efficient water separation.
In recent years, researchers have studied the use of metal complexes such as cobalt, nickel and iron for PEC reactions. These materials have attracted attention due to their enhanced performance as catalysts, rich structures and diverse functionalities.
The TEM image of Ni-NDAD/BiVO4. Image Credit: Wang, L et al., ACS Applied Energy Materials
While there has been a growing body of research on the incorporation of metal complexes into BiVO4 photoanodes to make composite materials that have the effect of improving the efficiency of water splitting reactions, there has been little research on incorporating Ni-NDAD layers into BiVO4 composite materials. The team used the long-term hydrothermal method to produce the BiVO4/Ni-DAD composite photoanode.
Prepared material displays higher photocurrent density compared to pure BiVO4 in a potassium borate electrolyte. The figures obtained were more than three times those displayed by the pure BiVO4. The analysis revealed that the presence of high amounts of active sites and the ultra-thin structure of Ni-NDAD contributed to the improved charge separation in the BiVO composite.4/Ni-NDAD material.
Transmission electron microscopy revealed the presence of close interactions between BiVO4 and Ni-NDAD layers, which promoted efficient charge transfer. The authors noted that this could be beneficial for the design of highly efficient PEC systems. Ni-NDAD plays a key role in converting light energy into electrical energy.
Another key research finding was the increased catalytic activity of the composite material. The catalytic activity of PEC was evaluated on chlortetracycline, an antibiotic, in persulfate. The degradation activity of this antibiotic was promoted by the new photocatalyst presented in the research and reached 89.19% in thirty minutes. This activity is beneficial as there is growing interest in the release of antibiotics into the environment and the increase in drug resistant bacteria.
The authors studied the stability of the proposed material, which is crucial for practical applications. They performed three replicate experiments on the degradation of chlortetracycline, demonstrating no noticeable reduction in photocatalytic activity. SEM images and XRD patterns showed almost no change, indicating the stability of the material.
SEM images of Ni-NDAD/BiVO4 photoanodes after PEC degradation of the CTC test. Image Credit: Wang, L et al., ACS Applied Energy Materials
Research has demonstrated a promising photoanode for PEC water splitting applications that combines BiVO4 and Ni-NDAD. The prepared material presented in the research displays significantly improved charge separation and can provide high photocurrent density compared to pure BiVO4 photoanodes, improving current PEC water separation technologies.
Photoelectrochemical water separation has the potential to provide a clean and sustainable pathway to green hydrogen production, helping to achieve global net-zero emissions goals by 2050. Although there are still significant challenges in this field, this research paper presents an innovative and inexpensive solution. , and a highly effective approach to improving PEC water splitting technologies.
Wang, L et al. (2022) Efficient Solar Water Separation via Improved Charge Separation of BiVO4 Photoanode ACS Applied Energetic Materials [online] pubs.acs.org. Available at: https://pubs.acs.org/doi/10.1021/acsaem.2c00779