ON THE ROAD TO SOLAR FUELS
In a new paper in the journal Nature Materials (in an edition focused on materials for sustainable energy), a team from Stanford University and SLAC National Accelerator Laboratory has reviewed milestones in the progress of solid-state photoelectrocatalytic technologies toward delivering solar fuels and chemistry.
Noting the “important advances” in solar fuels research, the review team also noted that the largest scientific and technical milestones are still ahead. Following their review, they listed some of the scientific challenges they see as the most important for the coming years. As CO2 levels in the atmosphere increase and it becomes clear that this has substantial consequences for our climate, developing alternatives to fossil fuels is more urgent than ever. Fuels are unrivaled for energy storage and we have an enormous infrastructure built to handle fuels for transportation and heating. In addition, the chemical industry and all the products and materials that we rely on in modern society are completely reliant on fossil feedstocks. This motivates the development of sustainable processes to generate fuels and chemical feedstocks from water and CO2 using solar energy; such a process is analogous to photosynthesis in nature and is sometimes referred to as artificial photosynthesis. There are large scientific and technical challenges involved in making even the simplest fuel, H2, and even more so for carbon-based fuels by means of CO2 reduction. Described is some of the recent progress and the obstacles that need to be surmounted to find economically viable solutions. In accordance with the theme of materials for sustainable energy, thee focus on solid state devices for the production of solar fuels. There have also been important developments in molecular catalysis for fuels and we refer the interested reader to a recent review. Various device configuration powered by a renewable energy source perform electrochemical water splitting or CO2 reduction. One such connects a photovoltaic (PV) to a separate electrolyzer with catalysts that drive the necessary conversion reactions (PV/electrolysis). Another possibility is to combine these two pieces into a fully integrated system in which the catalyst is deposited directly on top of the solar absorbers to create a photoelectrochemical (PEC) device. A range of intermediate configurations also exist that allow partial decoupling of the device components and set various bene fi and constraints on the device operation.
Materials discovery remains the biggest challenge for photoabsorber materials. While experimental and computational screening of new materials has begun, there are still many to be simulated and tested. Some insights into structure–property relationships have been derived from the large quantity of available band structure data, but there are likely to be a number of trends that might yet be discovered and leveraged into more effective searches. Stability, particularly in the aqueous environment at the electrode interface, will also be a key concern.
We are far from having solutions to the production of solar fuels and chemicals, and we cannot rely solely on trial-and-error strategies to solve the challenges ahead. However, by improving fundamental understanding of materials properties relevant to each aspect of the photoelectrocatalytic device that connects the incident photon to the desorbing fuel molecule, we may accelerate the process of making artificial photosynthesis a reality.