Most life depends on the sun. Through photosynthesis, plants and other organisms harness the sun’s energy to convert water and CO2 in sugars, which form the basis of the food chain. Scientists and engineers around the world are trying to develop processes as sustainable and elegant as photosynthesis.
But it’s really not that easy to use natural systems as an energy source. When such organisms are transplanted into bioreactors, the overall efficiency of photosynthesis achieved is typically quite low, less than five percent. But efforts have been made to improve on this low efficiency.
Recently, a team of scientists developed a hybrid inorganic-biological system capable of driving an artificial photosynthesis process. Their system relies on an “artificial leaf” and some bacteria to enable carbon fixation in biomass and liquid fuels.
Designing the device
Initially, the scientists worked with a system in which a combination of catalysts would split water molecules: cobalt phosphate produced oxygen, while a NiMoZn alloy produced hydrogen in 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 phosphorus alloy catalyzes hydrogen production at the cathode. This combination of electrodes maintains low concentrations of foreign cobalt ions. The electrodes also need low applied voltages to split the water.
But the key to their device is what happens after water splitting generates hydrogen. Raistonia eutropha is a species of bacteria that normally uses hydrogen from its environment to power its metabolism. They react the hydrogen with carbon dioxide, generating complex organic molecules with high efficiency. These organic molecules can then be isolated for use as biomass or biofuel.
Validate the device
To evaluate their system, the scientists deposited the cobalt phosphate catalyst on a large-area carbon cloth, which acted as an electrode support. This configuration resulted in high currents and high faraday efficiency – the efficiency at which electrons are transferred to the chemical reaction (96 ± 4 percent). The bacteria were allowed to grow on 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 took place 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 optimum efficiency for biomass production (54 ± 4 percent) could be achieved with 36 mM phosphate and a voltage of 2.0 V over a six-day period.
The biomass yield they achieved would amount to 180 g of captured carbon dioxide at a cost of 1 kWh of electricity. If their hybrid device were linked to existing photovoltaic systems, it would deliver a 10 percent carbon dioxide reduction energy efficiency, which would surpass natural photosynthesis systems.
The scientists also evaluated the upscaling of this system. They enlarged the reactor by an order of magnitude and found that its efficiency remained unaffected.
This process results in biomass and liquid fuel efficiencies significantly higher than previous integrated bioelectrochemical systems. Thee energy conversion efficiencies achieved by this process are also more than competitive with natural photosynthetic yields.
Science2016. DOI 10.1126/science.aaf5039 (About DOIs).