Toyota Central R&D Labs Inc. (Toyota CRDL) has established basic technology for achieving artificial photosynthesis with practical-sized cells.
Using a proprietary method that incorporates an electrode combining a semiconductor with a molecular catalyst, Toyota CRDL’s system is notable for having a completely stand-alone design that does not require supporting components. It consists of a setup in which an oxidation electrode with a semiconductor, and a reduction electrode with a molecular catalyst and semiconductor, are connected to a solar cell and submerged in an aqueous solution containing CO2. Solar energy is used to let the oxidation electrode extract electrons from the water, generating hydrogen ions and oxygen. The reduction electrode then utilizes the electrons and the hydrogen ions to synthesize formic acid, an organic compound.
Using a system with a 36-centimeter-squared cell – the size of solar cells in practical use – Toyota CRDL has achieved a solar conversion efficiency of 7.2 percent. This puts the technology in a world-leading class with regard to efficiency, and has allowed for the synthesis of 4 milliliters of formic acid per hour.
Toyota CRDL managed to heighten the system’s formic acid synthesis efficiency through various optimizations, including expansion of the reactive surface area of its oxidation electrode, a decrease of the electric resistance of its reduction electrode and a shortening of the distance that the hydrogen ions must travel between electrodes. Now the company is working on cells that measure 1 meter squared – and for demonstration testing here, it is targeting a solar conversion efficiency of 10 percent.
Toyota CRDL is envisioning the practical implementation of artificial photosynthesis in systems that aim to convert and recycle CO2 – captured from factories, vehicles or the atmosphere – into useful organic compounds. The company estimates that installing artificial photosynthesis technology with a solar conversion efficiency of 10 percent at factory sites measuring 7 hectares would enable the treatment of 5,000 tons of CO2 per year, which would be about 100 times as effective as planting cedar trees across the same area. Further, since the synthesized formic acid is a liquid at room temperature, it has high energy density per volume and is easy to transport and store, which could make it useful as a liquid fuel in the form of a hydrogen carrier.
The main hurdles to clear going forward are durability and system costs. On the first of these points, Toyota CRDL is pursuing R&D with the aim of getting catalysts to last years. Then in looking to reduce the cost of catalyst raw materials, the company is tackling development aimed at replacing the iridium used for the oxidation reaction with iron oxide, and replacing the ruthenium used for the reduction reaction with manganese. And with regard to the synthetic compound generated from the technology, the company is exploring a wide range of possibilities, such as by pursuing new catalyst development and looking into syngas that combines carbon monoxide with hydrogen.