Silver nanoparticles in microbial fuel cell

Silver-infused bacteria boost efficiency of fuel cells

Image credit: AKang, Sphere Studio. Copyright: Yu Huang and Xiangfeng Duan.

A team of engineers and chemists from the University of California-LA (UCLA) have taken a major step towards the development of microbial fuel cells – which use bacteria to extract electrons from organic matter and generate an electrical current – adding silver nanoparticles to the bacterial film.

Microbial fuel cells directly convert chemical energy stored in organic matter, such as that found in wastewater, into electrical energy. They are of interest for both wastewater treatment and power generation applications.

“Living energy-recovery systems utilising bacteria found in wastewater offer a one-two punch for environmental sustainability efforts,” said Professor Yu Huang, a UCLA engineer. “The natural populations of bacteria can help decontaminate groundwater by breaking down harmful chemical compounds.”

“Now, our research also shows a practical way to harness renewable energy from this process.”

The UCLA researchers focused on the genus of bacteria Shewanella, which have attracted considerable attention due to their energy-generation capabilities and their astonishing hardiness: they can grow and thrive in all types of environments, including soil, wastewater, and seawater, regardless of oxygen levels. Species in this genus naturally break down organic waste into smaller molecules, with electrons as a by-product of the metabolic process. When the bacteria grow as films on electrodes, some of the electrons can be captured, forming an electricity-generating ‘microbial fuel cell’.

Microbial fuel cells powered by Shewanella oneidensis, however, have not captured sufficient currents from the bacteria to make the technology feasible for industrial use. This is because few electrons could move quickly enough to escape the bacterial membrane and enter the electrodes. “The current [microbial fuel cells] typically exhibit unsatisfactorily low power densities that are largely limited by the sluggish transmembrane and extracellular electron-transfer processes,” the authors wrote in their Science paper.

In an effort to work around this issue, the researchers added silver nanoparticles to graphene oxide electrodes. The nanoparticles release silver ions, which bacteria reduce to silver nanoparticles with electrons emitted by their metabolic process, and then incorporate into their cells. Once inside the bacteria, the silver particles act effectively as transmission wires, capturing more metabolic electrons.

Chemist Professor Xiangfeng Duan explained: “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.”

The resulting Shewanella film outputs more than 80 per cent the metabolic electrons to the external circuit, generating power of 0.66mW per square centimetre. This more than doubles the previous record efficiency for microbial fuel cells and suggests it may be possible to harness bacteria to generate electrical currents for some practical applications.

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