31 January 2023

Could innovation in artificial photosynthesis help mitigate global climate catastrophe?

On 24 January 2023, the Science and Security Board (SASB) of the Bulletin of the Atomic Scientists announced that the hands of the Doomsday Clock have been moved forward. The clock, which is a universally recognised indicator of the world’s vulnerability to global catastrophe caused by manmade technologies, now stands at 90 seconds to midnight. The primary reason for moving the Doomsday Clock forward was said to be the war in Ukraine, notably the increased risk of nuclear war.

However, the SASB also consider the war to undermine global efforts to combat climate change. In particular, the SASB state that “countries dependent on Russian oil and gas have sought to diversify their supplies and suppliers, leading to expanded investment in natural gas exactly when such investment should have been shrinking.” The announcement is thus another reminder of the importance of investing in research and development of green energy if we are to mitigate a global climate catastrophe. 

One such form of green energy is solar energy, however, sunlight is an intermittent energy source meaning that it is not always reliable. Therefore, we need an efficient means of converting solar energy into another form of energy. This is where artificial photosynthesis may be useful. Plants can survive for periods without sunlight because during the day sunlight, water, and carbon dioxide are converted into chemical energy in the form of sugar.

The ultimate goal of artificial photosynthesis is to mimic this reaction, but on a larger scale and in a more controlled environment. It is hoped that the products produced by artificial photosynthesis could be used in the production of fuel (e.g. for cars), but also for supporting the growth of crops (e.g. using the acetate produced during artificial photosynthesis) in areas of extreme temperatures, drought, floods, and perhaps, even in space.  

Historically, artificial photosynthesis involves a photoelectrochemical (PEC) cell comprising a system that activates a photosensitive substance, such as a semiconductor, submerged in a liquid solution to trigger the chemical reaction. More recently, chemical engineers at École Polytechnique Fédérale de Lausanne in Switzerland developed a prototype of an ‘artificial leaf’ which enables water to be harvested directly from humid air using a novel gas diffusion electrode.

The artificial leaf comprises a small transparent wafer of glass fibres that is made by blending and compressing glass wool fibres into a wafer. The wafer is then coated with a thin porous transparent film of a conductor (fluorine-doped tin oxide) which functions as a gas-electrode. A second coating comprising a sunlight absorbing semiconductor material (i.e. copper(i) thiocyanate) is then applied.

Importantly, the novel gas electrodes have two key characteristics. Firstly, unlike previous gas diffusion electrodes, which are typically made from opaque carbon-based materials and are used in fuel cells which do not require sunlight (e.g. zinc-air batteries and nickel-metal hydride batteries) the electrodes used in the artificial leaf are transparent to maximise sunlight exposure of the semiconductor coating. Secondly, the electrodes are porous to maximise the contact with the water in the air. Specifically, unlike PEC cells which comprise a flat surface onto which the semiconductor material is applied, the artificial leaf comprises a three-dimensional structure that vastly increases the surface area to maximise contact with water in the air. The resulting artificial leaf is able to absorb light and convert gas-phase water into hydrogen.

Although, as with other forms of artificial photosynthesis, the solar to hydrogen conversion efficiency of the artificial leaf is relatively low, the prototype offers an important proof of principle that PEC cells can be adapted to use gas-phase water. It is hoped that following optimisation of, for example, the pore size, thickness of the coatings, and the semiconductor and catalyst used, the artificial leaf may be used in solar cells in both arid and humid environments.

Some sources suggest that the artificial photosynthesis market size will grow from 62 million USD in 2022 to 185 million USD in 2030. Currently, the market appears to be largely driven by government funding and grants for research and development (R&D), however, private companies are also increasingly investing in their own R&D. It is perhaps unsurprising that there has been an upwards trend in the number of patent filings relating to this field. Whilst there is still a way to go before the technology is ready for mass consumption, there are encouraging signs that we are moving ever closer to a truly reliable green energy source.