Photoelectrocatalytic hydrogen production
The products of photosynthetic processes such as photocatalysis and CO2 reduction are motivating technological developments in artificial photosynthesis. If properly replicated, such photosynthetic processes could be used to efficiently and sustainably produce highly desirable fuels and chemicals such as hydrogen.
Photoelectrocatalysis and electrolysis
Photoelectrocatalytic processes offer the potential to use solar energy to split water into oxygen and hydrogen at low operating temperatures and with very high efficiency. While electrolysis technologies are rapidly advancing to produce green hydrogen, the large energy demand and operating costs are still limiting its viability. Investing into the research of photoelectrocatalytic hydrogen production therefore has the potential to provide an additional way to obtain green hydrogen – without the need for electrolysis
Photoelectrocatalytic hydrogen research and projects
A range of compounds are under investigation as potential catalysts to facilitate photoelectrocatalytic hydrogen generation. Many techniques utilize TiO2 based photocatalysts for such processes. As an example, one research group in China is using TiO2 nanotube arrays in conjunction with semiconducting nanoparticles to act as a water splitting catalyst to form H2.
The oil company Repsol is also keen to find more efficient means of green hydrogen production. As the largest producer and consumer of hydrogen in Spain, they are working on a photoelectrocatalytic hydrogen production plan with the help of several Spanish research institutes and Enagás, a company with several on and off shore electrolysis projects in Mallorca and Asturias.
Challenges with the scalability of photoelectrocatalytic hydrogen production
Due to the high energy requirements and the multi-electron transfer mechanism involved in water splitting, it is challenging to produce hydrogen with high efficiency – so researchers are getting creative. In 2020, Shinshu University developed an aluminum-doped SrTiO3 catalyst system which is nearly 100% efficient under very specific light and semiconductor conditions. This process will require optimization to become truly viable, but it is a significant step towards achieving efficient photocatalytic water splitting. A second study in 2021 found biomass can be used as the source of hydrogen to increase yields up to 70%.
But the challenge still remains to find a practical photocatalyst that can be formed from earth-abundant materials, doesn’t make toxic waste products, and is able to efficiently capture and convert solar energy. With that in mind, yet another study is experimenting with the use of charred wood as a substrate to increase the efficiency of the water splitting reaction. For now, though, photoelectrochemical solar-to-hydrogen conversion remains below 20% efficiency for practical, non-lab based systems.
Another challenge is finding a viable water source for photoelectrocatalytic hydrogen processes as they scale, as it is unlikely that high-purity, fresh water can be used as a reactant. Some researchers are examining the possibility of splitting sea water, although this requires specialized compounds. The use of Co3O4 is currently under investigation, as it is a stable, non-toxic, porous film that can act as a catalyst with the sea water. For now, hydrogen produced using this method only has 8% efficiency.
Photobiological hydrogen production
Trying to mimic the role of photosystem II, composed of 17 protein subunits and numerous cofactors, is proving to be quite a challenge. Therefore researchers are also investigating alternatives, such as the use of microalgae which exhibit natural photolytic and photofermentive hydrogen production. Hydrogen yield via this method is still low, but researchers are investigating how to improve efficiency and are finding creative ways to fuel their processes such as with wastewater.
Like many aspects of the hydrogen economy, the study of photoelectrocatalytic hydrogen production is still in its infancy and has quite a ways to go before it is commercially viable. It will certainly be interesting to see if all this research can provide solutions to current climate and technical challenges.