Artificial photosynthesis

*Updated version of the 2023 article

Definition

Processes that use sunlight to split water into hydrogen and oxygen or to produce hydrocarbons are referred to as “artificial photosynthesis”. This mimics natural photosynthesis, whereby plants use light energy to create energy-rich sugar compounds from water and carbon dioxide (CO₂). Similarly, artificial photosynthesis processes convert electrical energy generated in solar cells into products such as hydrogen, methane, methanol, ammonia or even more complex substances. Photoelectrochemical cells and photocatalytic processes are used for this purpose, while metallic or molecular catalysts are used to convert the energy into the desired target products. 

A distinction can be made between three main approaches to this. In multi-stage processes, light absorption, primary charge separation and material conversion are spatially separated. In single-stage approaches, these individual steps are integrated, for example in “artificial leaves” where specific photocatalysts required for value creation are applied directly to solar cells. A third approach is the biotechnological route, whereby all photosynthetic sub-steps take place in living microorganisms, such as genetically modified algae or cyanobacteria.  

Current applications and opportunities

Multi-stage systems that combine photovoltaic cells with electrolysers for hydrogen generation are already operational. However, hydrogen produced in this way is still more expensive than hydrogen produced with conventional processes based on fossil raw materials. So the technology has not yet become widely established. 

Integrated processes have been implemented primarily at laboratory scale so far. In 2023, the EPFL start-up SoHHytec, based in Lausanne, scaled up a pilot project to a demonstration plant, where solar energy is concentrated in a parabolic mirror and used directly with integrated photoelectrochemical cells for hydrogen production. Oxygen and heat are generated as by-products. While oxygen is used in medical applications, heat can be used for space heating purposes. In 2024, the company entered into a collaborative relationship with Indian plant manufacturer TKIL Industries to promote green hydrogen production in India. 

Hydrogen produced using solar energy can make key contributions to sustainable energy supply, whether as fuel in mobility and heating networks or as long-term energy storage systems.  

Besides hydrogen, artificial photosynthesis can also be used to produce other carbon-containing chemicals and fuels such as synthesis gas, formic acid, methanol, methane, formaldehyde, ethene and ethanol. The technology thus covers a wide range of possibilities for the chemical industry. 

All processes require only solar energy, plus water and CO₂ as raw materials. So, unlike other biofuel production processes, they do not conflict with food production or other uses of biomass. 

The use of hydrogen is being driven forwards by private initiatives such as the H2 Mobility Switzerland Association. There is room for improvement in scaling up pilot projects from fundamental research to applications. Innosuisse could provide a vehicle for such projects. The University of Zurich’s university research priority programme Artificial Photosynthesis (duration: 2013–2024) has created an interdisciplinary network of stakeholders pursuing development of a market-ready reactor that takes into consideration all relevant ecological and economic aspects.  

Challenges

A major challenge is solar-to-product efficiency – the ratio of chemical energy stored in the product to solar energy input. Natural photosynthesis stores only about 1 percent of absorbed solar energy as chemical energy in resulting sugar compounds. Depending on the process and product, artificial photosynthesis can achieve solar-to-product efficiencies of 15 to 20 percent. Further increases are expected thanks to the advances made in developing the materials and processes used. SoHHytec’s demonstration plant in Lausanne reportedly achieves 25 percent solar-to-hydrogen efficiency according to its own data. 

A key challenge is developing stable and efficient photochemical catalysts whose structural, optical and electronic properties are optimally adapted to respective products. 

Besides the solar-to-product efficiency of the overall process, the space required for solar cells particularly limits the economic viability of such installations. Since the hours of sunshine in Switzerland are low and space requirements are correspondingly higher, energy companies are increasingly focusing on large-scale installations in the Earth’s sun belt, where firstly larger areas are available without usage conflicts and secondly the solar radiation intensity is higher.  

Focus on industry

The benefits for Swiss companies are currently limited, since the technology is still in its infancy. According to a technology assessment published by the German Federal Ministry of Research, Technology and Space (BMFTR) in 2022, these processes have only reached a technology maturity level of 3 to 4 on a scale of 1 to 10. This means that the methods have only been validated in laboratories as proof-of-concept or in extended demonstration facilities.  

If research and development challenges in artificial photosynthesis are overcome, start-ups and SMEs in renewable energy technologies can assume pioneering roles and gain competitive edges in the cleantech sector.  

International perspective

Research groups active in this field in Switzerland have strong networks both nationally and internationally and conduct high-quality research. However, the lion’s share of research and development – measured by publication numbers – takes place in the US, Japan, China and South Korea. The largest number of innovations – measured by patent numbers – also comes from the US and Asia. In Europe, research and development of prototypes is supported by Horizon Europe, including through the Sunergy and Solar2Chem consortia. 

Future applications

Hydrogen produced through artificial photosynthesis can be stored or used in fuel cells or gas turbines to generate electricity or power hydrogen-fuelled vehicles (see Hydrogen). Hydrogen can also be used as a chemical raw material for industrial production of basic chemicals such as ammonia. Established technologies can be used to produce carbon-neutral fuels from hydrogen and CO₂ from the atmosphere or from industrial exhaust gases. So artificial photosynthesis can contribute longer-term to establishing hydrogen and other synthetic energy sources as important foundations for a sustainable energy economy and decarbonising various applications in transport and industry. 

However, to realise this potential through the competitive application of artificial photosynthesis, significant improvements have to be made both to the processes themselves and to the political and economic framework conditions. If this is successful, artificial photosynthesis will be an alternative for making significant contributions to the energy and raw materials transitions.