Next steps for producing green hydrogen from water and sunlight

To stop the climate crisis, we need to end our reliance on fossil fuels. Hydrogen fuel presents a possible renewable alternative, as it can be produced via photocatalytic water splitting—using the power of the sun to break water down to its component parts and siphon off hydrogen as fuel.

However, photocatalytic water splitting to produce fuel is currently not cost-effective or efficient enough to compete with fossil fuels. In their Frontiers in Science article, Hisatomi et al. discuss the advances needed to bring environmentally friendly hydrogen fuel into commercial production and show how water splitting could help solve our fuel dilemma.

This explainer summarizes the article’s main points.

What is photocatalytic water splitting?

Photocatalytic water splitting uses light-sensitive substances called photocatalysts to drive a chemical reaction that breaks water molecules (H₂O) into oxygen gas (O₂) and hydrogen gas (H₂). Depending on the method of photocatalytic water splitting, hydrogen and oxygen are obtained in a mixed state, and the hydrogen gas may need to be filtered for purity.

There are two kinds of systems:

• one-step excitation systems, where a single photocatalyst splits water into H₂ and O₂

• two-step excitation systems, where one photocatalyst reduces water into hydrogen and a second photocatalyst oxidizes water to oxygen. Electron mediators connect these two photocatalysts, allowing the water splitting reaction to proceed continuously.

One-step excitation systems usually have simple components which can be replicated over a large area. However, due to challenges producing photocatalysts that can handle the oxygen and hydrogen simultaneously, the solar-to-hydrogen energy conversion efficiency is too low for them to be commercially viable. Two-step excitation systems are more efficient because more species are involved in the reaction. However, such systems are somewhat complex and prone to unwanted reactions—so finding ways to control the direction of electron transfer and the reaction selectivity is key to building efficient two-step excitation systems.

How could water splitting photocatalysts be improved?

Future photocatalysts need to be more efficient. Photocatalysts currently only achieve around a 1% rate of solar energy to hydrogen fuel conversion, which is too low to be viable. The authors estimate that this rate will need to be at least 5% for reactors to be cost-effective.

Improving sustainability is also important. Many currently available photocatalysts are reliant on precious metals or hazardous materials. Thus far, more environmentally friendly alternatives don’t perform as well.

Finally, researchers are seeking to improve the safety of photocatalytic water splitting systems.

What are the safety concerns associated with photocatalytic water splitting?

Photocatalytic water splitting can produce highly explosive oxyhydrogen. The researchers recommend two different approaches to prevent industrial accidents. Firstly, two-step excitation systems produce the oxygen and hydrogen separately, eliminating the dangers of oxyhydrogen production and the need to filter the hydrogen separately.

Where oxyhydrogen production cannot be avoided, Hisatomi et al. identified ways to mitigate risk. These include:

• automating tasks close to areas where oxyhydrogen is produced

• capturing the gas in narrow spaces that minimize the risk of destructive explosion

• using soft materials like PVC, which won’t cause serious damage if the oxyhydrogen ignites.

The authors ran a reactor with these features for three years without related safety incidents.

Can photocatalysts produce other types of fuels?

Potentially. Artificial photosynthesis, inspired by plants’ consumption of carbon dioxide, can use sunlight and water to convert carbon dioxide into fuels like methanol—which are energy-dense and easy to store and transport. As a side-effect, processing this carbon dioxide alongside the hydrogen prevents the production of oxyhydrogen.

However, to achieve this alongside photocatalytic water splitting, a specific co-catalyst would need to be included in reactors. Many of today’s possible options are limited in their efficiency and reliant on expensive and unsustainable precious metals.

One potentially sustainable option would be to use microbes, which can synthesize carbon dioxide to produce complex compounds, as a co-catalyst. Unlike non-organic catalysts, they can potentially repair and reproduce themselves. Researchers are experimenting with “biohybrid” systems which include some of these microbes.

What are the next steps for advancing water splitting technology for green hydrogen fuel?

In addition to more efficient photocatalysts and safe hydrogen recovery, as discussed above, the authors outline two other developments for solar water splitting to become commercially viable.

• Low-cost, large-scale reactors. In order to provide a plentiful source of fuel, water-splitting photocatalysts will need to be deployed over large areas. Recently, photocatalyst sheets based on immobilized photocatalyst powder have been developed. These sheets are significantly easier to maintain, operate, and control for quality and consistency than the liquid slurry which was previously used. They can also be manufactured efficiently and cost-effectively on a large scale. The authors consider these sheets a “game-changer” for photocatalytic water splitting. Their prototype 100m² reactor showed that the technology works in real-world environments, even if the efficiency currently remains low. They recommend more large-scale experiments to determine how technologies perform and how they can be scaled up.

• Regulation, accreditation, and efficiency standards. In addition to ensuring reactor safety, laws and licensing procedures can help ensure smooth technology development and deployment. An accrediting organization composed of photocatalysis experts, for example, could standardize efficiency measurements—which would support both research and implementation.

Overall, the authors hope their article will inspire experts from various fields to participate in photocatalysis research and advance the development of photocatalytic water splitting systems.