Solar fuel
[7] The world's dependence on the declining reserves of fossil fuels poses not only environmental problems but also geopolitical ones.
Electricity can be produced directly from sunlight through photovoltaics, but this form of energy is rather inefficient to store compared to hydrogen.
To use sunlight in this process, a photoelectrochemical cell can be used, where one photosensitized electrode converts light into an electric current that is then used for water splitting.
Due to band bending, the electrons and holes move to the surface, where these charges are used to split the water molecules.
Several such catalysts have been developed as proof of concept, but not yet scaled up for commercial use; nevertheless, their relative simplicity gives the advantage of potential lower cost and increased energy conversion efficiency.
[7][12] One such proof of concept is the "artificial leaf" developed by Nocera and coworkers: a combination of metal oxide-based catalysts and a semiconductor solar cell produces hydrogen upon illumination, with oxygen as the only byproduct.
If these genes can be applied, it will take some effort to overcome the problems of oxygen inhibition in the production of hydrogen, but it is estimated that this process can potentially yield as much as 10% solar energy capture.
[19] Because hydrogen manufacture requires continuous performance, the solar thermochemical process includes thermal energy storage.
[22] In a November 2021 publication in Nature, Aldo Steinfeld of Swiss technological university ETH Zurich reported an artificial photosynthesis where carbon dioxide and water vapour absorbed from the air are passed over a cerium oxide catalyst heated by concentrated solar power to produce hydrogen and carbon monoxide, transformed through the Fischer-Tropsch process into complex hydrocarbons forming methanol, a liquid fuel.
Scaling could produce the 414 billion L (414 million m3) of aviation fuel used in 2019 with a surface of 45,000 km2 (17,000 sq mi): 0.5% of the Sahara Desert.
[23][24][25] One author, Philipp Furler, leads specialist Synhelion, which in 2022 was building a solar fuel production facility at Jülich, west of Cologne, before another one in Spain.
Using genetic engineering and synthetic biology techniques, parts of or whole biofuel-producing metabolic pathways can be introduced in photosynthetic organisms.
One example is the production of 1-butanol in Synechococcus elongatus using enzymes from Clostridium acetobutylicum, Escherichia coli and Treponema denticola.
Besides the high volumetric density, ammonia and hydrous hydrazine have a low flammability, which makes it superior to hydrogen by lowering the storage and transportation costs.
When a dip in solar power occurs, a direct ammonia fuel cell kicks into action providing the lacking energy.
This recent research (2020) is a clear example of efficient use of energy, which is essentially done by temporary storage and use of ammonia as a fuel.
Some scientists envision a new ammonia economy that is almost the same as the oil industry, but with the enormous advantage of inexhaustible carbon-free power.
[41] However toxic, this fuel has great potential, because safety measures can be increased sufficiently to safely transport and convert hydrazine back into hydrogen and ammonia.
Researchers discovered a way to decompose hydrazine with a photo catalysis system that works over the entire visible-light region.
This system uses visible light irradiation to excite electrons to the n-type semiconductor creating an electric current.
The holes created in the p-type semiconductor are forced in the direction of the so called Nafion part of the device, which oxidizes hydrazine to nitrogen gas and dissolved hydrogen ions.
[43][44] Recent studies provide much better direct hydrazine fuel cells, for example with the use of hydrogen peroxide as an oxidant.
Making the anode basic and the cathode acidic increased the power density a lot, showing high peaks of around 1 W/cm2 at a temperature of 80 degrees Celsius.
[37] However hydrous hydrazine, which is a water-like liquid retains the high hydrogen density and can be stored and transported safely using the existing fuel infrastructure.
This method has a 20 hour stability and 98% Faradaic efficiency, which is comparable with the best reported claims of self-powered hydrogen generating cells.
