Around 3.5 billion years ago, plants became able to turn sunlight and carbon dioxide into energy. Researchers in the field of artificial photosynthesis are figuring out how humans could do the same.
Plant photosynthesis consists of three steps. First, sunlight is harvested. Second, this sunlight is converted into electricity. Finally, this electricity is used to break down carbon dioxide into glucose. Over the last few decades, scientists have been reconstructing each stage to piece together a total artificial photosystem. This is the field of artificial photosynthesis, where man-made devices replicate the way plants obtain energy from sunlight, water, and carbon dioxide.
Scaling artificial photosynthesis would revolutionise energy production because AP overcomes a major drawback of existing solar and wind power technologies – the fact that they can only produce electricity.
Anyone who finds themselves regularly caught out on a 1 percent charge knows that electricity is difficult to store. Batteries are simply not cut out for powering long-haul flights or heavy manufacturing. Because sunlight and wind fluctuate in daily and seasonal cycles, the absence of effective storage solutions is a major obstacle to scaling renewables.
Photosynthesis avoids the storage problem that wracks renewables. This is because it goes a step further than simple electricity generation. Just like plants, it converts the solar electricity into stable, high-energy substances – fuels and high-value materials. In short, while solar panels create electric charge, AP creates stuff from that charge.
The products that can be made using AP include hydrogen and ethanol fuels and carbon-based polymers. Because it is capable of splitting water into its components of oxygen and hydrogen, AP offers a sustainable method for producing hydrogen fuel. Achieving a cheap, safe, and reliable ‘artificial leaf’ would give us renewable replacements for many petro-based resources we use today.
To date, the Rheticus II project in Germany is the most significant proof-of-concept for commercialised artificial photosynthesis. It was jointly established by Evonik, a major player in the space, and Siemens. Their process combines artificial photosynthesis with fermentation to create high-value materials.
The Rheticus project began operating in 2020. It is scheduled to finish in 2022 with a pilot plant ready to produce 10 tonnes per year of product. The next step is to scale production to 5000 tonnes per year. Meanwhile, Evonik is also in a research partnership with skincare chemicals company Beiersdorf to develop sustainable raw materials for care products using artificial photosynthesis.
Their system is made up of chemical and biological components. First, electricity from sunlight is used to break down carbon dioxide and water into carbon and hydrogen respectively. At this point, most AP systems would turn these basic elements into useful materials through chemical reactions. Rheticus however rely on fermentation. The carbon and hydrogen are fed into a bioreactor vat filled with selected bacteria that metabolise these substances into butanol and hexanol. These alcohols are useful in creating industrial solvents, fuels, and other materials.
Rheticus was made possible by a grant from the German Federal Ministry of Education and Research. Thomas Haas who manages the project underlined the importance of public funding in emerging industries such as this. “Public funding, in particular, is very crucial because without this funding such projects would not take place. The risks would be too high to be entirely taken by industry, especially considering that within companies there is very high internal competition for research and development resources, which are often allocated to lower-risk projects.”
Twelve, based in the Bay Area, is one of the world’s first artificial photosynthesis startups. It uses renewable electricity to break apart carbon dioxide and water into simpler atoms. These atoms are then recombined to form carbon, a versatile material that Twelve convert into ethylene used in plastic manufacture. In collaboration with Mercedes-Benz, the company made the first-ever auto part from carbon dioxide. Twelve managed to raise
a staggering $57 million in Series A funding in 2021, signalling that investors recognise the game-changing potential of artificial photosynthesis.
Challenges and solutions
AP promises a techno-utopian future of near-frictionless commodity production. Yet cost-effective products are still far off and basic research is ongoing. The various AP systems in existence today remain energy-intensive. There are also major challenges in finding cheap materials that perform as well or better than their biological counterparts.
In the last decade, dye-sensitised cells have attracted immense attention as a method for collecting and converting sunlight into electricity. First invented in 1988 by Brian O’Regan and Michael Grätzel, the pigments in the dye perform the same chemical reactions as chlorophyll molecules do in plants. When photons strike photovoltaic material like dye-sensitised cells at light speed, this displaces the material’s electrons, sparking an electrical charge.
While plants are good at turning sunlight into electricity, they perform poorly in converting this electricity into fuel. Researchers must improve this efficiency for artificial systems. The conversion rates for dye-sensitised cells are still lower than ordinary first and second-generation silicon-based solar cells, which offer 20-30 percent efficiency. So far, the artificial photosynthesis devices that have solved the electricity-to-fuel conversion problem use very rare and expensive materials that cannot be scaled.
Mimicking the first and second steps of natural photosynthesis (light-harvesting and light-to-electricity conversion) is relatively straightforward. These are basically what solar panels do. However, photosynthesis is distinguished by its ability to use that electricity to make fuel and materials. To achieve this, scientists need to develop cheap and effective catalysts – the substances that speed up chemical reactions. Over the last few decades, hundreds of different catalysts for converting electrical charge into chemical energy have been developed.
In 2020, the US Department of Energy awarded $100 million to artificial photosynthesis research. One recipient was the Liquid Sunlight Alliance (Lisa). In 2022, this project announced cuprous oxide (copper oxide) as a candidate material for a photosynthetic catalyst. They used this material to turn energy from sunlight into ethylene and hydrogen. But although Cuprous oxide is highly effective as a catalyst, it is highly unstable. The JCAP is currently experimenting with ways of preventing the material from corroding with use.
In 2021, another breakthrough in AP catalysts came out of Imperial College London. Researchers developed a catalyst made from an organic material known as hyper-crosslinked polymers. The researchers used this to convert carbon dioxide into carbon monoxide – a basic feedstock for many chemicals. Unlike the precious metals used in most traditional catalysts, the polymers used here are low cost and deliver more sustainability, boosting the prospect of commercialisation.
The Joint Center for Artificial Photosynthesis (JCAP) is a US organisation founded in 2011. It is led by principal investigator Nathan Lewis at Caltech, a doyen of artificial photosynthesis since the early days of the field. The group has developed photocells that sandwich a light-absorbing material between layers of catalysts. The device uses solar electricity to split water molecules and thus obtain hydrogen fuel. The team managed to create a device that attained 19 percent solar-to-hydrogen efficiency, a record-breaking figure.
Again, however, corrosion is a problem. With coating adjustments, the team was able to develop a device that kept stable for over a thousand hours of operation. ‘To make it really industrially relevant you need thousands, hundreds of thousands of hours,’ says one researcher who previously worked at the centre. ‘And that’s neither practical, nor achievable, I think, so far. So that’s the challenge.’
Researchers are also trying to work out a technique for obtaining hydrogen fuel from water-splitting that matches the efficiency of natural photosynthesis. In 2019, University of Tokyo scientists Domen developed a system that finally achieved this. Their catalysts for water splitting were made of ‘aluminum-doped strontium-titanium oxide’, a material that acts as a light absorber and catalyst for the reactions that extract hydrogen from water molecules. Despite this advance, however, it remains far cheaper to obtain hydrogen from conventional methods.
Another problem is that while plants can use photons from across the entire spectrum of light, artificial photosynthesis can only work with a fraction of incoming photons. For example, Domen’s photocatalyst only works well on near-UV light. Capturing and using the full range of light is the next step in his research programme.
Artificial photosynthesis systems have a long way to go before they could become widely commercialised. However, a cheap, safe, and reliable system would represent a significant step towards a carbon-neutral economy. Because it draws on carbon dioxide for feedstock, any emissions from fuels created by the process would be offset from the get-go. If photosynthetic reactions became the energetic base for our economy, human activity could become as benign to the planet as plant growth. Scientists are hoping that developing low-cost component materials in combination with oil price shocks will eventually make fuel from these systems competitive.