Solar hydrogen production
Fossil fuels are energy sources that have developed within the earth over millions of years. Fossil fuels are hydrocarbon-containing natural resources that are not obtained from any animal or plant sources. Also, sometimes known instead as mineral fuels. The purpose of fossil fuels enables large-scale industrial development and largely replaced water-driven mills, as well as the combustion of wood or peat to heat.
Fossil fuel buried combustible geologic deposits of organic materials, formed from decomposing plants and animals that convert to crude oil, coal, natural gas, or heavy oil exposures to heat and pressure in the earth’s crust over hundreds of millions of years.
For several reasons, the age of fossil fuel is bound to end. In fossil fuels, hydrogen looks very attractive. The gas has a huge energy density, it can store or processed further, or directly provide clean electricity via a fuel cell. Hydrogen completely renewable with zero carbon emissions.
Like photosynthesis, sunlight can also use in artificial leaves, to split water into oxygen and hydrogen. Artificial leaves combine photoactive semiconductor materials and can reach efficiencies beyond 15%. However, the record efficiencies using expensive systems, which also decompose in aqueous solutions. For successful commercialization costs need to go down and stability needs to increase.
Complex metal oxide semiconductors (CMOS) are good applicants for artificial leaves. The semiconductors cheap and stable in aqueous solutions. For developing solar fuels, researchers focus their research on these materials. However, photo electrodes have shown moderate efficiencies because of their poor charge carrier mobility, up to 100,000 times lower than in classical semiconductors, such as gallium arsenide or silicon.
Charge carriers in metal oxides often short life spans of nanoseconds or even picoseconds. The charge carriers disappear before they can contribute to water splitting, Dr. Fatwa Abdi, from HZB-Institute, Germany.
metal oxide photo electrodes
To overcome this limitation a heat treatment under hydrogen atmosphere of the metal oxide layers after deposition. Researchers investigated this treatment to influence life spans, transport properties and lacks in bismuth vanadate (BiVO4), the most promising metal oxide photo electrodes.
The time resolved microwave conductivity (TRMC) technique reveals electrons and holes live more than twice as long in the bulk of the hydrogen-treated BiVO4, compared to the pristine BiVO4.
As a result, the overall photo current under sunlight largely improved. Further measurements provided that the presence of hydrogen in the metal oxide reduces or deactivates point defects in the bulk of BiVO4.
The results show that hydrogen treatment leads to less traps to charge carriers and fewer opportunities to change or lost. So, more charge carriers live for longer and may contribute to water splitting.
More information: [Advanced Energy Materials]