The road to artificial photosynthesis
Ryan Clancy | August 29, 2021
Scientists and engineers have their focus on creating technologies that will aid in the journey toward a carbon-neutral Earth and the fight against climate change. They all share one similar goal: finding/creating/generating renewable energy that will be able to replace fossil fuels and the damage they do to the environment. A method that artificially duplicates photosynthesis (where plants use sunlight in order to synthesize foods) driven by photocatalysts has become a promising new avenue of exploration for these teams. While the technology that drives this process has not yet matured, materials like strontium titanate (SrTiO3) that act as the photocatalysts in these solar devices are a good place to start.
Photosynthesis is a complicated bundle of processes where plants convert the sun’s rays and molecules of water into energy. The process relies on a pigment — the well-known chlorophyll — along with enzymes, metals and proteins.
Photovoltaic technology, in which the sun’s energy is converted into electricity using a solar cell, represents the closest approach to date for achieving artificial photosynthesis. This method is extremely inefficient, utilizing just 20% of the sun’s energy that it collects. Photosynthesis in plants, however, is far more efficient, converting about 60% of the sun’s energy to chemical energy. Existing photovoltaic systems are so inefficient as they are limited to the ability of the semiconductor within the cell to absorb the sun’s light energy and convert it to power. This limit could definitely be overcome using artificial photosynthesis.
In addition to its photocatalytic qualities, SrTiO3 could find use in the construction of fuel cells and resistive switches. This versatility has prompted scientists to investigate properties even further, but for this to happen a deeper understanding of SrTiO3 is required.
Doping is not just for athletes and horses
SrTiO3 and other photocatalytic materials are typically “doped” with other chemicals such as niobium (Nb), to upgrade their electrical features. Charge recombination may occur in these materials however, and greatly reduce their efficiency as mobile charge carriers within the material, like holes and electrons, start to destroy each other when subjected to sunlight. Previous studies have found that crystal defects can affect charge recombination.
Researchers from Nagoya Institute of Technology, Japan, wanted to discover what exactly Nb doping would do to SrTiO3. They tested the impact of low-concentration doping, and no doping at all, on SrTiO3 crystal surface recombination. Finding out the impact of these tests was critical to ensuring that the most effective photocatalysts possible are created, with artificial photosynthesis in mind. They initially examined the decay or surface recombination patterns of SrTiO3 tested with different amounts of Nb, along with samples with no doping whatsoever. Such patterns were identified in terms of microwave photoconductivity decay, and time-resolved photoluminescence methods were applied to examine bulk carrier recombination properties of undoped and doped samples, as well as the energy levels generated by doping with Nb.
The nature of carrier recombination was not observed to be directly proportional to doping concentration, but was indicative of Shockley-Read-Hall and surface processes (both of which are unaffected by exiting carrier concentration). Furthermore, the sample with doping displayed a much faster decay curve, which is most likely because of the inclusion of a recombination center due to the Nb doping. High levels of doping displayed negative results from carrier doping, and photocatalyst size and shape had no impact as was previously thought.
This particular study found that SrTiO3 that was moderately doped (relatively speaking) with Nb may actually be a more effective and efficient photocatalyst than just pure SrTiO3, specifically at higher-than-normal operating temperatures. This is a valuable discovery, as it can greatly help scientists with SrTiO3 photocatalyst design, resulting in increased energy conversion and reduced surface recombination, which will produce a sustainable, renewable source of energy. These finds are a huge step on the road to artificial photosynthesis and a greener, healthier planet Earth.
Future of artificial photosynthesis
There are many different avenues of research with the ultimate goal of creating artificial photosynthesis. An alternative to the above discussed method is a synthetic leaf system, created by a chemist at the University of California Berkeley. This first Photosynthetic Biohybrid System (PBS) utilizes live bacteria and semiconductors to perform the photosynthetic work that actual plants do. This includes absorbing light energy and generating a chemical product using carbon dioxide and water, and releasing oxygen while generating a liquid fuel.
[See also: Artificial leaf designed to consume CO2 from air]
This research has also been advancing at a rapid rate, but has a long way to go before it can be considered a commercial product. PBS is not cost-effective or durable enough at the moment to be sold or even used on a consistent basis. Improvements to this system will be made to swap out the live bacteria for an artificial substitute. Bacteria have proven to be the most efficient catalyst thus far, but due to the nature of live bacteria, they die. Therefore, they are not as durable or long-lasting as an inanimate substance that can last for a longer amount of time.
Either of these technologies could prove to be the next revolutionary advance that changes our energy use and impact on the planet. Our journey toward a future where biology and technology are combined keeps chugging along, with hopes that we may one day grow and use fuels as we need them, with greater reliance on natural products that can be recycled back into the Earth.
Do you think this new study will help to speed up the timeline of artificial photosynthesis? Will artificial photosynthesis be as beneficial as these scientists think? Engineering360 would love to hear your thoughts in the comments below.
I'm not convinced efficiency matters. Area does, and they should know energy produced per square meter. Catalyst cost will depend on what catalyst, of course, but also how long it can keep working. Bacteria die, as noted, but they also reproduce.
Finally and critically, we need to reduce demand as well as change supply source, and know how much difference it would make if we (USA) use it only, or do we need the world on board?