Solar energy conversion by mimicking the structure of chloroplast

The earth is heating up due to the emission of greenhouse gases, among which Carbon Dioxide (CO2)has contributed the most to the phenomenon of global warming due to its high rate of emission and high persistence within the atmosphere. It is also responsible for ocean acidification, which has threatened aquatic lives especially coral. What if we can absorb CO2 from the atmosphere, just like what the chloroplast does in the plant? Well, while it sounds like nonsense, it is in fact doable!

Nature is always a great source of inspiration especially when it comes to sustainable development. The photosynthesis processed by the plant, algae and cyanobacteria has indeed provided an alternative solution for climate change other than carbon capture, that is, the solar energy conversion. 

Broadly, solar energy conversion concerns, first, the absorption and conversion of solar energy, and second, the extraction or storage of this new converted form of energy. Solar energy can be captured by photochemical or photoelectric processes in which a gradient of chemical potential is generated using the redox reaction, and photosynthesis is considered as the most efficient system for quantum conversion and storage energy. To understand the reason behind it, let’s have a look at the mechanisms of photosynthesis: surprisingly, the chemical reaction of photosynthesis is not simply “water + carbon dioxide → glucose + oxygen”, it is far more complicated than that! Photosynthesis can be divided into two parts: the light dependent reaction and the light independent. During the light dependent reaction, light energy is absorbed by the accessory pigments and transferred to a reaction centre where it is used to “excite” electrons. And it is the transport of these electrons that would generate energy used for the later light independent reactions: where carbon dioxide fixed by the protein, RUBISCO, is used to synthesise carbohydrates, where the chemical energy is stored within. Thus, with the application of such a brilliant reaction, we can convert inorganic carbon dioxide into organic carbohydrates such as glucose.

So far there are two major approaches for solar energy conversion: engineered organisms, such as algae and cyanobacteria, and artificial systems that mimic the photosynthesis of the chloroplast or cyanobacteria. While “the first branch is more mature as it is indeed simpler to make modifications on existing organisms which have evolved over millions of years to reduce CO2 effectively,” the second branch is what the current research is focusing on as it would have greater potential and be more robust, for one simple reason, plants and cyanobacteria have to process with proteins, which fulfil many tasks but are not the optimal for light harvesting. However, there are still fundamental problems to solve in developing the catalysts that drive CO2 reduction in an artificial system. Peidong Yang, Chair in Energy at the Department of Chemistry, University of California, Berkeley, in the USA, we still have a long way to go on this road, so far in the synthetic version, what has been achieved is to convert CO2 to CO but anything beyond that is very difficult. Yang said. “So about 6 years ago my group started to look back into Nature and see how to utilise bacteria for the chemistry itself”. This eventually yielded a hybrid system that combines an artificial semiconductor for harvesting light with a bacterial cell to perform the catalytic reduction. “Light goes into the semiconductor nanostructure, producing an electron passed onto the bacteria, which then takes in CO2 and produces products like acetate, methane and later on, after an upgrade, butanol”, Yang explained. As the nano‐structured semiconductors have greater light‐capturing potential, Yang is optimistic that such hybrid systems have the potential to generate fuels on a commercial scale. “Now we are using much simpler semi‐conductors to do the photo sensitization, which is necessary to scale up for the future”.