Enough energy from sunlight strikes our planet in one hour to provide all the energy needed for human activity in one year.
Given this astonishing fact, it is not surprising that governments all over the world now consider the harvesting of solar energy to be a priority. Several approaches exist, the most well known being the direct conversion of sunlight into electricity. However, sunlight is not constant and so to ensure a reliable national power supply an energy storage system is required. This cannot just be a daily charge-recharge cycle. For energy security most countries require a storage buffer. At the moment this often takes the form of an oil stockpile. Batteries could provide part of the solution, but current technology does not have the energy capacity or stability for large-scale long-term storage.
Another possibility is using solar energy to generate a fuel, in much the same way as plants use sunlight to convert carbon dioxide and water into energy-rich carbohydrates. Chemical fuels offer a much higher energy density (amount of energy per unit of mass) than batteries and can be stored for use either in stationary power plants or in vehicles. However, ‘copying nature’ is not straightforward. Photosynthesis is actually quite inefficient and so to make artificial photosynthesis a viable industry we can’t just settle with copying nature. We need to go one better.
Photosynthesis in plants involves two main steps, both of which are driven by sunlight. One step splits water into hydrogen and oxygen. This hydrogen is not released as a gas but is transported as a positively charged hydrogen ion to another enzyme. Here, the hydrogen ion is combined with carbon dioxide to generate sugars. For a chemist, copying this exquisite multistep process is extremely difficult! One alternative is just to focus on part of the photosynthesis reaction: the water splitting. If we can generate materials to split water into hydrogen and oxygen, we could generate hydrogen gas, which is an energy-rich and clean fuel. The materials use energy from sunlight to drive the water splitting and are called photocatalysts.
This system has a lot of potential but many challenges need to be overcome. Current photocatalysts have quite low efficiency and many only work using UV light. When water is split, the hydrogen and oxygen gases need to be kept separate to avoid creating an explosive mixture! Furthermore, many existing photocatalysts for water splitting use toxic elements such as cadmium or extremely rare and expensive elements such as platinum. Viable, large-scale hydrogen production from sunlight will require efficient photocatalysts based on cheap materials and simple preparation methods.
It’s at this stage that you can envisage some of the enormous challenges facing scientists. We’ve already mentioned cadmium being toxic – it’s banned from many applications under EU RoHS (Restriction of Hazardous Substances) legislation.[i] But in artificial photosynthesis, there are materials containing cadmium that work really well! Should we continue to use cadmium, arguing that it may end up being the only material that works? Or perhaps we can learn a lot about the science of artificial photosynthesis by studying cadmium? It’s a very difficult problem and certainly not one that is confined to cadmium, or indeed to artificial photosynthesis. There are countless cases of toxic or expensive elements that perform their jobs extremely well. This is why some toxic elements are exempted from EU chemical hazard regulation for certain devices. I would argue that we have a unique opportunity. In terms of implementing the technology, we are in the very early stages with artificial photosynthesis. There is a lot more work to be done to make this very promising chemistry work and it could genuinely revolutionize our world. If we consider sustainability now, then we won’t be faced with a big clean-up operation in 50 or 100 years.
Dr Zoe Schnepp is a Birmingham Fellow in the School of Chemistry at the University of Birmingham.