Enhancing the electron highway for solar-to-hydrogen power
Hydrogen is a promising green fuel, as it produces only water as a by-product, rather than the harmful carbon dioxide emitted by petrol vehicles. Hydrogen is abundantly available from splitting water molecules, but this process requires energy.
One way to get this energy is from the Sun in the form of light. Photocatalysts are materials that can adsorb water and use light energy to split it, but current photocatalysts are too inefficient at doing this to be viable at the scales needed.
Now, a team from the University of Hong Kong have shown how to improve the efficiency of a photocatalyst by an order of magnitude, paving the way for more improvements. Professors Guo Zhengxiao and David Lee Phillips combined the work of their two groups to make the discovery, reported in the journal Energy & Environmental Science.
Tracking electrons
In photocatalysts, the energy received from the Sun excites the material’s electrons. The electrons then jump to a higher band, which researchers call ‘the electron highway’. It is only then, once they are on this highway, that the electrons have enough energy to perform the water-splitting process, releasing hydrogen from water.
Knowledge of how electrons behave is therefore crucial for the design of more efficient photocatalysts. Towards that end, the team employed a technique that uses one ‘pump’ laser pulse to excite the electrons, and a second ‘probe’ laser pulse to measure the results. They measured the quantity of electrons in the highway and how long they stayed there. Many of the electrons dropped back down to their lower band quickly, suggesting that finding a way to keep them there longer would increase the efficiency.
With this information, the team built in a shallow trap, a small dip next to the highway that can accumulate electrons at an energy still high enough to be used for the water-splitting reaction. This optimised photocatalyst showed almost an 800% increase in hydrogen generation and around a five-fold increase in quantum efficiency.
Guo says: “We expect that our discovery will open up a new line of thinking in the future design of photocatalysts for effective solar energy utilisation, allowing researchers to pump up the efficiency.”
Design principles
The results are impressive, but the efficiency gains are still not enough to make the photocatalyst commercially viable. However, these findings do point to design principles which will increase the efficiency further by keeping the electrons at a higher energy for longer.
There are also other avenues the team are pursuing. The catalysts perform reactions when water is adsorbed on their active sites. If these can be made to hold more water in a smaller area, then they will be able to produce more hydrogen.
These materials design principles also need to be coupled with engineering principles. Perhaps individual active sites may not be so efficient, but one question is whether there is a way to increase the number of sites in a given area, improving the efficiency that way. The optimum will be a balance between these considerations.
Combining the aqueous chemistry and catalyst expertise of both groups is also bearing fruit when examining how the water can actually act as part of the photocatalyst. Often, being immersed in water can cause reactions to go awry. In this case, as water is a key part of the reaction, it can instead replenish the catalyst, helping it last for a long time.
A clean energy future
There are several types of photocatalysts, and here the team used the material graphitic carbon nitride, which has the added advantage of being made from resources that are abundant. The limitations of many catalysts involve the need for rarer transition metals like platinum. Instead, relying on abundant materials means this design could be scalable, as it would be cheaper and easier to produce.
Ultimately, this research could also help liberate hydrogen from an incredibly abundant source: seawater. The team used pure water in the lab, but if photocatalysts could be made that could work despite the impurities in seawater, or even use the salts to create useful by-products, they could be key to providing abundant green fuel.
Phillips concludes: “We are driven by the grand challenge of clean energy provision. We believe we can follow the successful path of solar panels, increasing the efficiency of the photocatalysts from our current standard of 6-7% to 10-20%, at which point they begin to be useful on a commercial scale, producing chemical energy from sunlight.”