For solar energy to be able to provide abundant power, scientists have to solve two key issues: improving the cost effectiveness of the technology and storing the energy so that it can be used at all hours, mainly at night. An interdisciplinary team at Stanford has recently made substantial progress toward solving the storage issue. They have demonstrated how electricity captured from sunlight can be stored in the form of chemical bonds. To date, this is the most efficient means of storing electricity. The team say that if they are able to find a way of lowering the cost of their technology, it would be a massive step toward making solar power a viable alternative to current energy sources.
The electricity generated by high efficiency solar cells was used to turn water into a chemical able to store 30 percent of the energy over long periods.
The science used by the team is well understood – they used the electricity captured from sunlight to split water molecules into oxygen and hydrogen gas. The energy stored in this way can later be recovered in two different ways. The first works similarly to using petroleum products and consists of burning the hydrogen gas in an internal combustion engine, while the second recombines the oxygen and hydrogen into water to release electricity.
The team was led by James Harris, a professor of electrical engineering and Thomas Jaramillo, an associate professor of chemical engineering and of photon science, and the challenge facing them has been to turn this well understood science into an efficient industrial process. In work published in Nature Communications, the team described that they have been able to make a significant improvement. With the target being the previous record of storing 24.4 percent of the energy captured from sunlight into stored hydrogen, they have managed to beat this by capturing and storing 30 percent.
Jaramillo notes that this achievement has brought water splitting as a storage technology much closer to a practical and sustainable process. He added that improved efficiencies has an amazing effect on lowering costs. The team will continue to work on finding other ways to lower the costs so that their technology can compete with conventional fuels.
Ironically, the team started their experiments by using a solar cell that is much more expensive than the typical rooftop solar arrays. The solar cell was pioneered by Harris’ lab and it uses three semiconductor materials that are less common that the silicon used in typical arrays. Each material is tuned to capture red, blue or green light respectively. The cells are therefore called triple junction solar cells. As a result of this precision, triple junction solar cells convert 39 percent of incoming solar energy into electricity, as opposed to the roughly 20 percent achieved by silicon based, single junction solar cells.
The team was however not really interested in how much energy they captured, but rather focused on how much energy was stored through water splitting.
Jaramillo and his collaborators have conducted previous research on how to improve the performance of catalysts. Although catalysts speed up chemical reactions, they are not consumed during the process. Using this previous research as a basis, the team looked specifically at water splitting catalysts that break apart the stable water molecules by using electrons flowing through the catalytic materials.
The Stanford team achieved their record energy capture by building their catalytic process based on their previous advances in the area. They did however focus on one particularly important aspect. While a single electrolysis device is used for most photovoltaic powered water splitting reactions, this team made use of their higher efficiency solar cells and managed to combine two identical electrolysis devices in such a way that it produces double the amount of hydrogen.
Harris notes that for the process to work, it was critical that the entire system had to be balanced perfectly. This was achieved by tuning all the elements, the chemistry and the electronics. The experiment’s results showed that 30 percent of the energy originally collected by the triple junction solar cells had been stored in the form of hydrogen gas.
Once the Stanford team had demonstrated this record efficiency in the use of water splitting to store sun power, their focus shifted to costs. Although the triple junction solar cells and catalysts that were used are suitable for ‘proof of concept’ experiments, they include platinum and are therefore too expensive to use for an industrial process. Jaramillo explains that now that they have proven that a systems approach can improve storage efficiency immensely, they now have to find ways to achieve similar results with cheaper devices and materials.
Harris and Jaramillo agree that that one big reason for the success of this research is the collaboration among teams of scientists and engineers. Eleven researchers were brought in by the team. These included experts in electronics, chemistry and process engineering, and collaborators from the SLAC National Accelerator Laboratory.
The researchers set out to achieve two goals. The first was to extract the utmost in power from sunlight, while the second was to use water splitting chemistry to store as much of the power as possible. Harris firmly believes that this result is not a single fix, but that the key to their success lies in how everything links together.