Converting solar energy into fuel (without harming the environment) is possible

Converting solar energy into fuel through a zero-emissions closed cycle is a promising approach to reduce energy production from fossil fuels. In nature, photosynthetic organisms exploit solar radiation to produce energy-rich compounds from water and atmospheric CO2 through refined biochemical machinery, but the overall efficiency of the conversion of light into biomass is limited by the fact that the chemical species obtain (the biomass components) are diverse and depend on an infinite number of different production methods with low average efficiency. In other words, although the photosynthetic organelles of living beings are very efficient in collecting light photons and using them to transform carbon dioxide into sugars and ATP, everything that follows downstream, and which serves to obtain the extremely numerous variety of components of the organism, lowers the final yield considerably, so that, if you want to use biomass (and consequently also its derivatives coal and oil) as fuel, you have a relatively low efficiency compared to the solar energy used for their production.

However, the high stability, selectivity and efficiency of photosynthetic first steps offers a design principle for highly efficient man-made photocatalytic systems, i.e. transforming CO2 by light directly into a useful product, without intermediate steps and unwanted products. Chemists have long found engineered photocatalytic systems with very high efficiency, but these, until now, have required pure or highly concentrated CO2, due to their low selectivity, as well as organic solvents to reduce water-induced catalyst degradation.

Even the replication of natural light-gathering structures using appropriate biochemical and biotechnological techniques is difficult to implement and far from cost-effective. What would be needed are self-assembling systems (that is, that do not require expensive and difficult chemical or biochemical synthesis routes), based on cheap and abundant components, without the use of precious metals or rare earths.

Well, in an article just published in Nature Catalysis, a group of scientists managed to achieve this very goal. Looking at the spherical, water-soluble photosynthetic structure of the bacterium Rhodobacter sphaeroideswhich collects light with unrivaled energy transfer efficiency, researchers have built an artificial photosynthetic nanomicelle that self-assembles from compounds similar to the pigment that colors plant leaves or our hemoglobin, called porphyrins. The light-sensitive beads thus obtained have the property of capturing a specially added cobalt-based catalyst from the solvent, ultimately obtaining a complete system that has been found to be stable and recyclable for the highly efficient and selective photocatalytic reduction of CO2 in aqueous solution.

The nanomicelle system can selectively catalyze the conversion of CO2 to methane in water with a selectivity of 89% at ambient temperature and pressure under visible light irradiation, with a demonstrated lifetime of at least 30 days of activity before eventual recycling. In the irradiation conditions tested, the conversion of about 15.1% of water and CO2 into methane was obtained in two hours.

The formation of methane from the CO2 supplied to the nanomicelle system was confirmed by isotopic labeling experiments, which showed that in aqueous solution the reduction of CO2 to carbon monoxide (CO) takes place first, and then the reduction of this to methane. At the atmospheric concentration of carbon dioxide, over time this system reduces 96% of the CO2 to methane, of which 92% is found as methane.

In essence, therefore, the researchers have managed to obtain the self-assembly of a photosynthetic artificial organelle that does not use rare earths or precious metals and that does not require a synthetic procedure for its production, other than that of the precursors. This organelle exploits visible light for the selective and efficient production of methane in water, thus providing an example of a closed cycle in which the combustion of the product, exploitable for energy needs, produces CO2 which in turn can be reconverted in a closed, with few losses in the process.

This is a first example, which evidently illustrates a path for very useful solutions to deal with both the climate crisis and energy needs at the same time; we cannot yet know if this will be precisely the way that will prove useful for products and industrial applications on a sufficient scale, but certainly the solution to the problems illustrated at the beginning of this article bodes well for future possibilities.