Most life depends on the Sun. Through photosynthesis, plants and other organisms harness the energy of the Sun to convert water and CO­ 2 into sugars, forming the base of the food chain. Scientists and engineers around the world are trying to develop processes that are as sustainable and elegant as photosynthesis.

But it’s really not that easy to make use of natural systems as an energy source. When such organisms are transplanted into bioreactors, the overall efficiency of the photosynthesis achieved is typically quite low, less than five percent. But there have been attempts to improve on this low efficiency.

Recently, a team of scientists developed a hybrid inorganic-biological system capable of driving an artificial photosynthetic process. Their system relies on an "artificial leaf" as well as some bacteria to power carbon fixation into biomass and liquid fuels.

Designing the device

Initially, the scientists worked with a system where a combination of catalysts would split water molecules: cobalt phosphate produced oxygen, while a NiMoZn alloy to produced hydrogen under the presence of an applied voltage. This system produced reactive oxygen species at one of the electrodes, which was detrimental to bacterial growth.

To overcome this biotoxicity, the scientists switched catalysts. The initial cobalt phosphate alloy drives oxygen production at the anode, while a cobalt phosphorous alloy catalyzes hydrogen production at the cathode. This combination of electrodes maintains low concentrations of extraneous cobalt ions. The electrodes also require low applied voltages to split the water.

But the key to their device is wha happens after water-splitting generates hydrogen. Raistonia eutropha is a bacterial species that normally uses hydrogen from its environment to power its metabolism. They react the hydrogen with carbon dioxide, generating complex organic molecules at a high efficiency. These organic molecules can then be isolated for use as a biomass or biofuel.

Validating the device

To evaluate their system, the scientists deposited the cobalt phosphate catalyst on a carbon cloth with a high surface area, which acted as an electrode support. This configuration resulted in high currents and high faraday efficiency—the efficiency with which electrons are transferred into the chemical reaction (96 ± 4 percent). The bacteria were allowed to grow onto the cathode interface. When this system was placed in a batch reactor half-filled with a solution of inorganic salts and trace metals, carbon dioxide reduction occurred under constant voltage.

The hybrid system stored more than half of the input energy in the chemical products of carbon dioxide fixation. They determined that the optimal efficiency for biomass production (54 ± 4 percent) could be achieved using 36 mM phosphate and a voltage of 2.0 V over a six day time period.

The biomass yield they attained would amount to 180g of captured carbon dioxide at a cost of 1 kWh of electricity. If their hybrid device was coupled to existing photovoltaic systems, it would yield a carbon dioxide reduction energy efficiency of 10 percent, which would exceed natural photosynthetic systems.

The scientists also evaluated the scale-up of this system. They enlarged the reactor by an order of magnitude and found that the efficiency was unaffected.

This process results in biomass and liquid fuel efficiencies that are significantly higher than previous integrated bioelectrochemical systems. Thee energy conversion efficiencies achieved through this process are also more than competitive with natural photosynthetic yields.

Science, 2016. DOI 10.1126/science.aaf5039 (About DOIs).