Bacteria are already being harnessed in fuel cells to generate electricity from waste organic matter. Scientists in California now report on a microbial fuel cell (MFC) they say is world beating in power density, by taking advantage of the heavy-metal resistance of Shewanella oneidensis (Science, 2021, 373, 1336).
In a typical Shewanella fuel cell, there are anode and cathode chambers (glass or plastic) separated by a proton exchange membrane. In the anodic chamber solution, Shewanella turns the organic matter to CO2 and free electrons, which move through an outer circuit to the cathode, where they combine with electron acceptors like oxygen.
Early versions produced too few electrons to make the technology practical for industrial use, but chemists and engineers at the University of California, Los Angeles (UCLA), US, have boosted power by employing a reduced graphene oxide electrode and silver.
‘This material releases positively charged silver ions, which the bacterial cells convert into [silver] nanoparticles, after discharging respiratory electrons,’ comments Gemma Reguera, an environmental microbiologist at Michigan State University, US, who was not involved in the research. ‘They literally zap the metal cations to mineralise them.’
The result is that the outer surface of the bacteria is doped with conductive nanoparticles, and they are able to discharge more electrons to neighbouring cells and the underlying anode. ‘This allows the cells to form denser biofilms and harvest more electricity per electrode area,’ Reguera says. ‘The silver nanoparticles accumulate on the outer membrane of the cell, making them more conductive. It is like Iron Man wearing his super suit.’
Inside the bacteria, the silver particles act as tiny transmission wires, capturing more electrons produced by the bacteria. ‘Adding the silver nanoparticles into the bacteria is like creating a dedicated express lane for electrons, which enabled us to extract more electrons and at faster speeds,’ says study leader Xiangfeng Duan at UCLA. The silver-infused Shewanella film outputs more than 80% of the metabolic electrons to an external circuit, generating 0.66mWcm-2. The group says this more than doubles the previous best for MFCs.
Bruce Logan, an environmental engineer at Penn State University, US, describes the power densities achieved as remarkable, but notes two reports the authors overlooked, with his lab achieving 0.57 and 0.72mWcm-2, in published research in 2020 and 2021, respectively. ‘While the results using silver are interesting, they are not practical,’ he says, with other groups having spent years ‘trying to optimise MFCs to produce high power but without the use of any precious metals.’
Reguera agrees the prototypes are not scalable. ‘Cost will be prohibitive,’ she explains, with silver used for the anodes and platinum for the cathodes. There could also be regulatory issues for an electrode that releases silver, a toxic metal, she adds, although Huang says it will be possible to remove the metal from the wastewater and re-use it.
‘When dealing with the wastewater, it still remains to be discussed and solved how to keep the output electricity at a more stable level,’ adds Huang. ‘And this may rely on further research on the Shewanella growth period and biofilm formation.’
Meanwhile, a handful of companies are developing early MFCs. Aquacycl in California, for example, has commercialised a bioelectrochemical fuel cell that treats wastewater and sludge and generates some electricity.