Photosynthesis – turning the sun's energy into food for plants – is the biological system that feeds the world. But despite its awesome power, the process is extremely inefficient. We can't really blame plants for this because photosynthesis evolved for ancient earth's conditions.
What if we could improve on this fundamental, ancient process? One could imagine using solar energy and biology to make chemicals or liquid fuels in a far less-polluting way than today's methods. For years, synthetic biologists have made numerous attempts to improve the ability of organisms, such as bacteria, to translate solar energy into chemical production, but efforts have been met with little success.
My research group set out to rethink photosynthesis by assessing how a leaf converts solar energy to growth. We developed a biological process that makes chemicals or liquid fuels directly from solar cells. This proof of concept offers a new way to think about renewable energy.
By breaking photosynthesis down into reactions, it becomes very clear that one reaction is the heart of it: water splitting. Water splitting is the process by which water is broken down into hydrogen gas (H2) and oxygen gas (O2). Luckily for us, Harvard University professorDaniel Nocera has already produced a catalyst, the “artificial leaf”, that is capable of breaking water into its component elements.
While there are many water-splitting catalysts available, the artificial leaf is special for several reasons: it's made of cheap materials, it operates at neutral pH (i.e. tap water), and it can be operated by ordinary solar panels. Therefore, an artificial leaf catalyst is capable of splitting water from one of the most abundant energy sources in the world: sunlight.
The artificial leaf catalyst converts solar energy to H2 and O2 gasses. These are attractive fuels in principle but lack the infrastructure to be widely adopted in the near future. For example, without hydrogen filling stations for cars or hydrogen pipelines into homes, it's difficult to imagine hydrogen rapidly replacing fossil fuels.
We decided to combine a futuristic fuel – hydrogen – with what can be considered old news: bacteria. The soil bacterium Ralstonia eutropha is an organism capable of growing mainly from H2 and carbon dioxide. We figured it would be possible to use the hydrogen generated from the artificial leaf to drive the growth of Ralstonia. And from that bacteria's growth, we could get our desired product: a liquid fuel or chemical.
Dubbed the “bionic leaf”, this system converts sunlight shining on a solar panel to electricity. Electricity travels to a glass vial containing liquid where both Ralstonia and the water-splitting catalyst are immersed. The electricity drives the catalyst to generate O2 and H2, which Ralstonia consumes along with bubbled carbon dioxide to grow. In the lab, we piped in carbon dioxide from a tank; in a commercial situation, we could use carbon dioxide emissions from a polluter, such as a power plant.
Just like plants, the bionic leaf converts sunlight into “biomass” or, biological material. Here, we produced the alcohol isopropanol, a compound which can be used for the production of fuels. Where many terrestrial plants convert sunlight to biomass at an efficiency of about 1%, the bionic leaf does so at an efficiency of up to 3.2%.
The key to this efficiency is the increased light harvest from solar panels to drive water splitting. The solar photovoltaic panels act as a sort of amplifier, increasing the amount of solar energy delivered to the bacteria-growth medium than what a typical plant can harvest.
Author providedIn addition to increased efficiency, this setup bridges the strengths of each technology. Solar panels are great at harvesting sunlight but storing energy is a challenge. Also, panels aren't really designed to produce chemicals. Microbes, in contrast, can produce a wide range of high-value compounds but require constant “food” to grow - in this case, hydrogen, sunlight and CO2. By combining these technologies, solar energy produces the necessary molecules our Ralstonia require to grow and produce chemicals.
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