Chemical reactions driven by solar energy are not only possible, but necessary for a sustainable future, report a group of European chemists.
The comment comes in a 20-page whitepaper, “Solar-Driven Chemistry: A Vision for Sustainable Chemistry Production,” published in November, following a brainstorming session on solar-driven chemistry held in Berlin. The German Research Foundation and the European Association of Chemical and Molecular Sciences (EuCheMS) organized the event.
Among the experts at the session was Joost Reek, a professor at the Van’t Hoff Institute for Molecular Sciences at the University of Amsterdam (UvA), The Netherlands.
“The whitepaper reflects the findings of our sustainable chemistry research at UvA where we combine the efforts of dozens of researchers in homogeneous and heterogeneous catalysis, molecular photonics and theoretical chemistry. We do make progress on both fundamental and technological levels, but much more effort is needed to establish a really meaningful European solar-driven chemical community in research as well as industry,” he says.
Reek points out that UvA already has achieved such progress in the form of new molecules for solar-driven hydrogen generation and reducing carbon dioxide (CO2) to methanol and other organic compounds. Pharmaceutical giant Merck also is involved in these projects.
The whitepaper identifies much-needed scientific breakthroughs crucial for making solar-driven chemistry a future reality.
In the short term, it suggests electricity generated by photovoltaic (PV) devices and transported through the grid could power commercial electrolyzers that split water into its components to generate molecular hydrogen. This hydrogen then can be used directly as a fuel or as an intermediate for producing carbon-based fuels and chemicals through known thermal routes.
The first demonstrators for semi-industrial scale production of hydrogen from water and solar energy are expected within the next five years. Additionally, electricity produced through PV would progressively substitute current electrical technologies, particularly those based on fossil fuels, as is already happening in several developed countries.
However, the whitepaper also notes this scenario has some significant weaknesses: “The currently best-performing water electrolyzers based on proton-exchange membranes (PEM) utilize noble metal catalysts, which hinders utilization on the large scale eventually required. The alternative, alkaline electrolyzers require steady current-voltage input, which is not compatible with intermittent sources, and would therefore require large increases in grid-based, renewable electricity production with concomitant challenges to balance intermittencies.”
Research challenges include improving light harvesting; understanding interfacial energy/electron transfer processes and finding means to prevent charge recombination; and developing cheaper and more stable electro- and photo-catalysts.
Coupling water electrolysis and electrochemical reduction of CO2 could directly produce simple carbon-based compounds of high energy value such as formic acid, formaldehyde, methanol, dimethyl ether, methane, ethanol and acetic acid — avoiding the intermediate production and handling of hydrogen. Such processes are likely in prototype demonstrators within a decade. However, the whitepaper notes the disadvantages of coupling such an electrochemical device to a PV system remain the same.
Long-term, photo-induced reactions could produce directly hydrogen or carbon-based compounds. Instead of generating electrons through PV and using them in electrochemical cells, the solar energy would be captured and used in an integrated system, such as photoelectrochemical cells or photocatalytic devices developed to function in intermittent sunlight.
“The realization of these devices will require at least 15–20 years. Large-scale direct hydrogen production in water-splitting systems powered by sunlight might be feasible on a shorter time scale than producing carbon-based fuels and commodities because proton reduction to hydrogen gas is far less difficult than carbon dioxide reduction,” the paper notes.
Again, achieving this will require research to overcome a number of challenges, including better understanding of the complex molecular mechanisms and key steps in natural and artificial photosynthesis, for example C-C bond formation and multiple electron/proton transfer; developing cheaper, more selective and more stable photo-catalysts for CO2 reduction; finding cheap solutions to concentrate and use CO2 from the atmosphere; developing electro- and photo-catalysts, as well as electrodes for ammonia production; and developing new catalysts for synthesis of hydrogen peroxide, driven by renewable energy.
“It requires a large integrated and synergic approach to meet these ambitious objectives, not least research that encompasses catalysis, electrochemistry, photochemistry, nanosciences, in concert with semiconductor physics, engineering, biosciences and social sciences,” concludes the report.
Reek hopes the EuCheMS will use the findings to raise awareness and generate future research funds from European Union policymakers. Download the report.
Seán Ottewell is Chemical Processing's Editor at Large. You can email him at firstname.lastname@example.org.