Hydrogen and Energy: The Fissionary Sun

"Hydrogen and other important chemicals can be produced using solar energy. Two current research projects show how this is possible.

Green hydrogen is considered the key to a low-emission energy economy. The generation of the energy carrier usually takes place via the electrochemical decomposition of water with the help of regeneratively generated electricity. An alternative approach is direct solar water splitting, in which solar energy is used to split water without any major detours.

While previous processes for solar water splitting only achieved a maximum efficiency of three percent, engineers led by Zetian Mi from the University of Michigan have set a new record with an efficiency of nine percent. In a field test, they achieve a yield of at least six percent.

The structure of the experiment, which the authors present in the journal "Nature Chemistry", is simple: a lens focuses the sunlight and directs it onto a chamber filled with water. Inside is the heart of the system: a silicon plate with nanowires made of indium gallium nitride. The semiconductor, which is also responsible for the eponymous blue laser in blue-ray technology, first converts the visible and ultraviolet components of the incident light into electrical energy. Surrounding metal oxide nanoparticles use the latter to split water into its elementary components, hydrogen and oxygen.

The combination of semiconductors and metal oxides acts here as a so-called photocatalyst: it uses light to drive a chemical reaction, in this case water splitting. In this way, the system produces the desired hydrogen emission-free – without any additional energy supply.

To reach the optimum operating temperature of around 70 degrees Celsius, the system uses the infrared components of sunlight. This spectral range does not have enough energy to drive the photocatalyst reaction, but enough to heat the test chamber. The heat supplied accelerates the water splitting and at the same time suppresses the undesired reverse reaction to water. The research group is now working on increasing efficiency and production volume in order to produce cheap, green hydrogen in large quantities in the future.

Another way of using solar energy for chemical reactions is presented by British chemists led by Erwin Reisner from the University of Cambridge in the journal "Nature Synthesis". They have developed a process that uses sunlight to generate valuable chemicals from carbon dioxide (CO2) and plastic PET waste products. The principle is similar to solar water splitting; in this case, however, a lead-based perovskite solar cell converts the light into electrical energy. But instead of splitting water, two separate processes take place. At the negative pole, metal-based catalysts convert CO2 into carbon monoxide (CO) or formic acid, both of which are valuable building blocks for the chemical industry. Which product is obtained can be controlled by the choice of catalyst. At the same time, PET waste is converted to glycolic acid at the positive pole, which is one of the major innovations compared to existing processes. Glycolic acid is a component of many skin creams and scrubs.

Of course, such developments cannot be immediately translated into large-scale processes. Until such a process becomes scalable and profitable, the scientists still have to invest a lot of work and time. However, these developments show that more sustainable processes without fossil fuels can also be implemented.”