Power/Performance Bits: April 4

Self-sustaining microbial fuel cell; safe thermoelectric material; recycling electronic waste.


Self-sustaining microbial fuel cell

Researchers at Binghamton University developed the first micro-scale self-sustaining microbial fuel cell, which generates power through the symbiotic interactions of two types of bacteria.

A mixed culture of phototrophic and heterotrophic bacteria were placed in a 90-microliter cell chamber, or about one-fifth the size of a teaspoon. Phototrophic bacteria uses sunlight, carbon dioxide, and water to make its own energy, while heterotrophic bacteria feeds on provided organic matter or phototrophic bacteria to survive.

While the cell was exposed to sunlight, an initial dose of food was added to the chamber to stimulate growth of the heterotrophic bacteria. Through cellular respiration, the heterotrophic bacteria produced carbon dioxide waste, which was used by the phototrophic bacteria to kickstart the symbiotic cycle.

After that cycle was established, researchers stopped adding additional food sources for the heterotrophic bacteria, and there were enough phototrophic bacteria to sustain the metabolic processes of the heterotrophic bacteria. Those metabolic processes generated an electrical current of 8 microamps per square centimeter of cell for 13 straight days. The power was about 70 times greater than current produced by phototrophic bacteria alone.

“Heterotrophic bacteria-based fuel cells generate higher power, while photosynthetic microbial fuel cells provide self-sustainability. This is the best of both worlds, thus far,” said Seokheun Choi, an assistant professor of electrical and computer science at Binghamton.

Research into microbial fuel cells, particularly those involving two bacteria, is still in the early stages.

“There are some challenges of using this technique,” Choi said. “Balancing both microorganisms’ growth to maximize the device performance and the need to make sure that this closed system will permanently generate power without additional maintenance are two we have found. Long-term experiments are needed.”

Bacterial fuel cells, with their low power output, are considered a potential way to provide power in remote or dangerous locations for low-power items like health monitors and infrastructure diagnostic sensors.

Safe thermoelectric material

Engineers from the University of Utah developed an inexpensive and bio-friendly material that can generate electricity through a thermoelectric process involving heat and cold air.

The material, a combination of calcium, cobalt and terbium, generates an electrical voltage when one end of the material is hot and the other end is cold, as charge carriers from the hot end move through the material to the cold end. The material needs less than a one-degree difference in temperature to produce a detectable voltage.

While the concept is not new, many of the materials with this property are toxic to humans. This material, however, is non-toxic, as well as being inexpensive to produce yet still efficient at generating electricity.

thermoelectric power generation UUtah apr4

The heat from a hot stove, coupled with the cooler water or food in a cooking pot, could generate enough electricity to charge a cellphone. (Source: Ashutosh Tiwari/University of Utah)

The team envisions a range of applications, from jewelry that uses body heat to power implantable medical devices, to charging mobile devices with the heat of cooking pans, or in cars where it draws from the heat of the engine. Airplanes could generate extra power by using heat from within the cabin versus the cold air outside. Power plants also could use the material to produce more electricity from the escaped heat the plant generates.

“In power plants, about 60 percent of energy is wasted,” according to postdoctoral researcher Shrikant Saini. “With this, you could reuse some of that 60 percent.”

One particularly promising application could be in developing countries where electricity is scarce and the only source of energy is the fire in stoves.

A U.S. patent has been filed for the material, and the team will initially develop it for use in cars and for biosensors.

Recycling electronic waste

Researchers at Rice University and the Indian Institute of Science proposed a method to simplify electronic waste recycling by crushing PCBs into sortable nanoparticles.

E-waste will grow by 33% over the next four years, the researchers estimate, and by 2030 will weigh more than a billion tons. Nearly 80% to 85% of often-toxic e-waste ends up in an incinerator or a landfill, they said, and is the fastest-growing waste stream in the United States, according to the Environmental Protection Agency.

The new recycling process uses a low-temperature cryo-mill to pulverize electronic waste — primarily the chips, other electronic components and polymers that make up PCBs — into particles so small that they do not contaminate each other. The particles can then be sorted and reused.

Cold materials are more brittle and easier to pulverize, said Chandra Sekhar Tiwary, a postdoctoral researcher at Rice and a researcher at the Indian Institute of Science in Bangalore. “We take advantage of the physics. When you heat things, they are more likely to combine: You can put metals into polymer, oxides into polymers. That’s what high-temperature processing is for, and it makes mixing really easy.

“But in low temperatures, they don’t like to mix. The materials’ basic properties – their elastic modulus, thermal conductivity and coefficient of thermal expansion – all change. They allow everything to separate really well,” he said.

Mouse PCBs were used to test the cryo-mill, which contained argon gas and a single tool-grade steel ball. A steady stream of liquid nitrogen kept the container at 154 kelvins (minus 182 degrees Fahrenheit).

When shaken, the ball smashes the polymer first, then the metals and then the oxides just long enough to separate the materials into a powder, with particles between 20 and 100 nanometers wide. That can take up to three hours, after which the particles are bathed in water to separate them.

The process is suggested as a more environmentally friendly method of recycling than burning or treating electronics with chemicals to recover metals and alloys. “In every case, the cycle is one way, and burning or using chemicals takes a lot of energy while still leaving waste,” said Tiwary. “We propose a system that breaks all of the components – metals, oxides and polymers – into homogenous powders and makes them easy to reuse.”