New Approach for Effective Water Splitting of Low-Voltage Hydrogen Generation Developed

Metal oxides are a promising photoelectrochemical (PEC) water-splitting catalyst for producing hydrogen as an alternative energy source.

Their usefulness, however, is limited at low voltage.

By adding phosphorus to a metal oxide catalyst, a study team was able to successfully mediate the poor charge carrier transport at low voltage, reducing energy losses during water splitting. The findings provide a possible path to carbon neutrality.

New method to introduce efficient water splitting for hydrogen production

(Photo : Appolinary Kalashnikova/Unsplash)

Bismuth vanadate (BiVO4) is a metal oxide semiconductor that responds to both ultraviolet and visible light and is widely used as a photocatalyst for PEC water splitting, as per ScienceDaily.

"In the PEC water-splitting process, sunlight and specialized semiconductors as photocatalysts, such as BiVO4, directly separate water molecules into hydrogen and oxygen," noted Dr. Ng, a PEC researcher.

If the voltage supply is too low, a large fraction of the photo-excited charge carriers cannot be extracted efficiently, resulting in energy loss and a reduction in water-splitting efficiency; this poor charge transport is primarily due to charge carrier trap states and small polaron formation.

The electrons in the semiconductor are energized by solar radiation and can bounce up and across the band gap from the valence band to the conduction band, causing an electric current to flow.

However, the semiconductor's natural imperfections produce trap states, which confine the photo-induced electrons and positively charged holes until they recombine, preventing them from freely flowing to form an electric current.

Furthermore, when an electron is excited within a semiconductor, its charge can cause lattice expansion, confining the electron within the lattice unit and generating a tiny polaron, which is a deep trap state that powerfully traps the electron.

To hop from one location to another, thermal vibration energy (also known as polaron hopping activation energy) is required.

As a result, the creation of tiny polarons has a negative influence on charge mobility, which is frequent in transition metal oxides.

Dr. Wu Hao, the paper's first author, then a postdoc in Professor Ng's group and now an Assistant Professor at Macau University of Science and Technology's Macao Institute of Materials Science and Engineering, shared one of the study's highlights.

Wu Hao discovered that the polaron hopping activation barriers of BiVO4 photoanodes were reduced upon incorporating phosphorus, as demonstrated by our combined theoretical and experimental studies.

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Synergistic effects of phosphorus doping

The team's studies and observations further show that phosphorus doping passivates the intrinsically generated trap states on the BiVO4 surface, boosting the open-circuit photovoltage for splitting water molecules, as per the Vervetimes.

They demonstrated that by concurrently mediating the polaron hopping barrier and trap state, they increased charge transport in phosphorus-doped BiVO4, bringing effective PEC water splitting for hydrogen generation at low voltage.

Because of the synergistic effects, the phosphorus-doped BiVO4 demonstrated a record-high photon-to-current conversion efficiency of 2.21% at 0.6V.

Professor Ng hoped that the mechanistic understanding of BiVO4 property enhancement 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.

Dr. Wu is the study's first author, while Professor Ng is the corresponding author. Researchers from the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) Institute for Solar Fuels and Queensland University of Technology were also involved.

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