By combining existing technologies, ETE Ph.D. scientist at Wetsus, Sam Molenaar, developed a rechargeable battery, where microorganisms play a key role. The concept combines the principles of the microbial fuel cell with microbial electrosynthesis. Recent experiments have confirmed that the system is working. Molenaar: ‘It may form a stable, environmental friendly and inexpensive alternative to store energy.’
Renewable energy sources like sun or wind don’t deliver a continuous and steady amount of energy that is matched with consumer demands. During peak production, an excess of energy is delivered, while during periods without sun or wind, they provide too little energy. Therefore, to implement these green energy sources on a larger scale, storing energy to match supply and demand is essential. However, current energy storing systems have several issues, such as safety, the presence of toxic substances or the need for scarce and non-renewable substances. According to Molenaar, there is an environmental friendly alternative. ‘Microorganisms may play an important role in storing electrical energy’, says Molenaar. ‘They are able to use electricity to generate certain compounds; these substances can subsequently be transformed back into electricity by other species of microorganisms.’
For the rechargeable bio-battery, Molenaar was inspired by two different technologies: the microbial fuel cell and microbial electrosynthesis. In the microbial fuel cell, microorganisms produce electricity. They grow on the anode while extracting electrons from organic matter present in for example wastewater or urine. These electrons are subsequently transferred to the anode, generating an electron flow: electricity. So, in this system, chemical energy is converted into electrical energy.
The opposite is essentially happening during microbial electrosynthesis. Microorganisms grow at the cathode, while using electricity (electrons) and a carbon source like CO2. These bacteria take up electrons generated at the anode and produce biomass and acetate at the cathode. ‘Acetate is a perfect substrate for microorganisms in the microbial fuel cell to produce energy’, Molenaar explains. ‘So, when you combine these two systems, you have in fact a rechargeable bio-battery.’ He further developed this concept into his so-called Microbial Rechargeable Battery, briefly MRB, where he connected the two different systems. In the MRB microorganisms at the bio-cathode use electricity and CO2 to produce acetate, while at the bio-anode different microbiota use acetate to produce electricity.
Mimicking solar panels
Subsequent experiments provided solid proof that the concept was working. ‘We successfully operated the two systems in turn during a 24 hour period, with acetate production during the night at the cathode and electricity production during the day at the anode’, Molenaar explains. ‘We were in fact mimicking solar panels that also produce energy only during daytime.’ The overall energy efficiency was about 40%, so the scientist is working to increase efficiency of his MRB: ‘Although the battery will always have some losses due to the fact the bacteria have to grow and maintain themselves, this may surely be further improved’.
Also regarding the energy density further improvements are required. It will most likely never be as high as in lithium batteries, but levels comparable to old-fashioned lead-acid batteries may be achieved. The system is therefore not suitable to be used in electric cars, it would simply become too big.’ According to Molenaar, his finding is a promising alternative for the stationary storage of energy. The working concept has shown that MRB could become an inexpensive and environmentally friendly energy storage device in the future.