An underwater fuel cell that recharges itself by consuming microscopic marine life could keep ocean sensors in the water powered for longer.

Replacing batteries for underwater sensors can be a tricky endeavour, especially if they are located far out in difficult-to-access locations. Michigan Technological University researchers have developed a system that converts dissolved organic matter and microscopic forms of marine biomass into electrical power. 

“There are increasing deployments of all kinds of sensors in the marine environment for observing ecological conditions, organism migrations and acoustics relevant to naval defence,” said researcher Amy Marcarelli. “Almost all run on batteries, which have to be replaced.”

Some sensing units can run on wave energy, but those devices have to be near the ocean’s surface. The microbial fuel cell (MFC) enables long-endurance ocean-deployed sensor systems that self-refuel while fully submerged.

“The basic idea is that microbes move electrons around during their metabolic processes,” said Marcarelli. “In a microbial fuel cell, those processes transfer electrons from an anode to a cathode, creating an electrical current we can harness.”

MFCs are typically used in environments with high nutrient and organic matter concentrations, such as wastewater treatment plants, where microbes have ample material to metabolise. But in the ocean’s water column, where most aquatic sensors must travel, there is far less organic matter and far more oxygen, which hampers the anoxic conditions needed to generate energy.

To overcome that, the team realised they needed to concentrate organic material from the water column and limit oxygen introduction into the MFC. Jennifer Becker, co-principal investigator on the project, said even in a functioning microbial fuel cell, collecting enough energy from the marine water to make the system effective is a challenge.

“There’s just only so much energy you can extract out of a microbial fuel cell,” said Becker. “The microbes need to take a lot of the energy that is generated, or they won’t be able to grow, and the whole thing won’t work. So we can only harvest the energy that is left over by the microbes.”

To allow the fuel cell to generate sufficient energy, the team used granulated activated carbon to passively concentrate organic material that passes through the system, while also giving microbes a place to grow. 

This allows them to form a biofilm on the activated carbon, which means that even if the fuel cell is oxygenated, the microbes will have anoxic conditions where they can carry out the metabolic processes that are related to energy generation. This energy can then be harvested by the fuel cell and converted into electricity.