Macau University of Science & Technology

Metal oxides are a promising catalyst for photoelectrochemical (PEC) water splitting to produce hydrogen as alternative energy. However, their effectiveness is restricted at low voltage. A research team has successfully mediated the poor charge carrier transport at low voltage by adding phosphorus to a metal oxide catalyst, which reduced energy losses during water splitting. The findings offer a potential option for achieving carbon neutrality. Metal oxides are a promising catalyst for photoelectrochemical (PEC) water splitting to produce hydrogen as alternative energy. However, their effectiveness is restricted at low voltage. A research team led by scholars from City University of Hong Kong (CityU), Australia and Germany successfully mediated the poor charge carrier transport at low voltage by adding phosphorus to a metal oxide catalyst, which reduced energy losses during water splitting. The findings offer a potential option for achieving carbon neutrality. The research was co-led by Professor Ng Yun-hau of CityU's School of Energy and Environment (SEE) and researchers from Australia and Germany. Their findings were published in the scientific journal Nature Communications, titled "Low-bias photoelectrochemical water splitting via mediating trap states and small polaron hopping." Bismuth vanadate (BiVO4) is a metal oxide semiconductor, which is responsive to both ultraviolet and visible light, and is regarded as a top-performing photocatalyst for PEC water splitting. "In the PEC water-splitting process, hydrogen and oxygen are produced from water, using sunlight and specialised semiconductors as photocatalysts, such as BiVO4. With light energy and an additional small voltage supply, the photocatalysts directly dissociate water molecules into hydrogen and oxygen," explained Dr Ng, an expert in PEC research. "However, if the voltage supply is too low, a large fraction of the photo-excited charge carriers cannot be extracted efficiently, leading to energy loss and affecting the water-splitting efficiency. This poor charge transport is due mainly to the trap states of charge carriers and small polaron formation." Native defects and polaron formation hinder charge carrier transport With solar energy, the electrons in the semiconductor are excited, and can bounce up and across the band gap from the valence band to the conduction band to make an electric current flow. But the native defects of the semiconductor introduce "trap states," which trap the photo-induced electrons and the positively charged holes until they recombine, preventing them from moving freely to become an electric current. Moreover, when an electron is excited within a semiconductor, its charge can induce lattice expansion, confining the electron within the lattice unit, and forming a small polaron, which can be regarded as a deep trap state that strongly traps the electron. It requires thermal vibration energy (known as polaron hopping activation energy) to hop from one site to another. Hence, the small polaron formation has a detrimental effect on charge mobility, which is common in transition metal oxides. The research team took on this challenge to find ways to enhance charge mobility. They found that by modifying the BiVO4 photoanodes with phosphorus doping, the charge mobility is 2.8 times higher than that of the pristine one. This also greatly increased the charge separation efficiency, up to 80% at 0.6V, which is about 1.43 times stronger than the pristine one, and up to 99% at 1.0V. Dr Wu Hao, the first author of the paper, then-postdoc in Professor Ng's group, and now an Assistant Professor in the Macao Institute of Materials Science and Engineering at Macau University of Science and Technology, shared one of the highlights of the study: "We discovered that the polaron hopping activation barriers of BiVO4 photoanodes were reduced upon incorporating phosphorus. This was proven by our combined theoretical and experimental studies." Synergistic effects of phosphorus doping The team's experiments and measurements also confirm that phosphorus doping passivated the trap states that are intrinsically formed on the BiVO4 surface, thereby increasing the open-circuit photovoltage for splitting water molecules. They showed that the charge transport in phosphorus-doped BiVO4 was improved by concurrently mediating the polaron hopping barrier and trap state, thus introducing efficient PEC water splitting for hydrogen production at low voltage. The synergistic effects allowed the phosphorus-doped BiVO4 to exhibit a record-high photon-to-current conversion efficiency of 2.21% at 0.6V. "We hope the mechanistic understanding of the enhancement of BiVO4 properties will provide key insights into trap state passivation and polaron hopping for many photoactive metal oxides, and more importantly, will offer a potential option for efficient hydrogen production to help achieve carbon neutrality," said Professor Ng. The first author of the research is Dr Wu, and the corresponding author is Professor Ng. Other collaborators included researchers from the Institute for Solar Fuels of Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) and Queensland University of Technology. The research was supported by the Hong Kong Research Grants Council, and the Science and Technology Innovation Committee of Shenzhen Municipality.

While there is some evidence that consuming omega-3 fatty acids in food or dietary supplements may reduce the risk of high blood pressure, the optimal amount to consume for this benefit is unclear. A review of dozens of studies suggests that the optimal amount of omega-3 fatty acids to consume to lower blood pressure is likely about 3 grams daily. Consuming higher amounts of omega-3 fatty acids may be beneficial for people at high risk for developing cardiovascular disease, though more research is needed researchers said. About 3 grams daily of omega-3 fatty acids, consumed in foods or supplements, appears to be the optimal daily dose to help lower blood pressure, according to a research review published today in the Journal of the American Heart Association, an open access, peer-reviewed journal of the American Heart Association. Omega-3 fatty acids docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) are typically found in fatty fish, such as salmon, tuna, sardines, trout, herring and oysters. Some people also take combined DHA and EPA in supplements. While some studies suggest that consumption of omega-3 fatty acids may lower blood pressure, the optimal dosage needed to lower blood pressure has not been clear. The National Institutes of Health has established an adequate intake of omega-3 fatty acids for healthy people at 1.1- 1.6 grams daily, depending on age and sex. "According to our research, the average adult may have a modest blood pressure reduction from consuming about 3 grams a day of these fatty acids," said study author Xinzhi Li, M.D., Ph.D., assistant professor and program director of the School of Pharmacy at Macau University of Science and Technology in Macau, China. Researchers analyzed the results of 71 clinical trials from around the world published from 1987 to 2020. The studies examined the relationship between blood pressure and the omega-3 fatty acids DHA and EPA (either individually or combined) in people aged 18 and older with or without high blood pressure or cholesterol disorders. There were nearly 5,000 participants combined, ranging in age from 22 to 86 years. Participants took dietary and/or prescription supplement sources of fatty acids for an average of 10 weeks. The analysis found: About 4-5 ounces of Atlantic salmon provide 3 grams of omega 3 fatty acids. A typical fish oil supplement contains about 300 mg of omega-3s per pill, but doses vary widely. "Most of the studies reported on fish oil supplements rather than on EPA and DHA omega-3's consumed in food, which suggests supplements may be an alternative for those who cannot eat fatty fish such as salmon regularly," Li said. "Algae supplements with EPA and DHA fatty acids are also an option for people who do not consume fish or other animal products." The U.S. Food and Drug Administration (FDA) announced in June 2019 that it did not object to the use of certain health claims that consuming EPA and DHA omega-3 fatty acids in food or dietary supplements may reduce the risk of hypertension and coronary heart disease. However, they noted that the evidence was inconclusive and highly inconsistent. "Our study supports the FDA guidance that EPA and DHA omega-3 fatty acids may reduce the risk of coronary heart disease by lowering high blood pressure, especially among people already diagnosed with hypertension," he said. "However, while our study may add a layer of credible evidence, it does not meet the threshold to make an authorized health claim for omega-3 fatty acids in compliance with FDA regulations." Limitations of the review include differences in how blood pressure was measured, and whether the studies examined omega-3 intake from supplements or diet, which may affect the strength of the conclusions. The American Heart Association recommends eating two servings (3-4 ounces cooked) of fish per week (particularly fatty fish such as salmon) as part of a heart-healthy diet.