Tuesday, August 21, 2012

Self-Charging Power Cell Converts and Stores Energy

Researchers have developed a self-charging power cell that directly converts mechanical energy to chemical energy, storing the power until it is released as electrical current. By eliminating the need to convert mechanical energy to electrical energy for charging a battery, the new hybrid generator-storage cell utilizes mechanical energy more efficiently than systems using separate generators and batteries.

At the heart of the self-charging power cell is a piezoelectric membrane that drives lithium ions from one side of the cell to the other when the membrane is deformed by mechanical stress. The lithium ions driven through the polarized membrane by the piezoelectric potential are directly stored as chemical energy using an electrochemical process.

By harnessing a compressive force, such as a shoe heel hitting the pavement from a person walking, the power cell generates enough current to power a small calculator. A hybrid power cell the size of a conventional coin battery can power small electronic devices -- and could have military applications for soldiers who might one day recharge battery-powered equipment as they walked.

"People are accustomed to considering electrical generation and storage as two separate operations done in two separate units," said Zhong Lin Wang, a Regents professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. "We have put them together in a single hybrid unit to create a self-charging power cell, demonstrating a new technique for charge conversion and storage in one integrated unit."

The research was reported Aug. 9, 2012 in the journal Nano Letters. The research was supported by the Defense Advanced Research Projects Agency (DARPA), the U.S. Air Force, the U.S. Department of Energy, the National Science Foundation, and the Knowledge Innovation Program of the Chinese Academy of Sciences.

The power cell consists of a cathode made from lithium-cobalt oxide (LiCoO2) and an anode consisting of titanium dioxide (TiO2) nanotubes grown atop a titanium film. The two electrodes are separated by a membrane made from poly(vinylidene fluoride) (PVDF) film, which generates a piezoelectric charge when placed under strain. When the power cell is mechanically compressed, the PVDF film generates a piezoelectric potential that serves as a charge pump to drive the lithium ions from the cathode side to the anode side. The energy is then stored in the anode as lithium-titanium oxide.

Charging occurs in cycles with the compression of the power cell creating a piezopotential that drives the migration of lithium ions until a point at which the chemical equilibrium of the two electrodes are re-established and the distribution of lithium ions can balance the piezoelectric fields in the PVDF film. When the force applied to the power cell is released, the piezoelectric field in the PVDF disappears, and the lithium ions are kept at the anode through a chemical process.

The charging cycle is completed through an electrochemical process that oxidizes a small amount of lithium-cobalt oxide at the cathode to Li1-xCoO2 and reduces a small amount of titanium dioxide to LixTiO2 at the anode. Compressing the power cell again repeats the cycle.

When an electrical load is connected between the anode and cathode, electrons flow to the load, and the lithium ions within the cell flow back from the anode side to the cathode side.

Using a mechanical compressive force with a frequency of 2.3 Hertz, the researchers increased the voltage in the power cell from 327 to 395 millivolts in just four minutes. The device was then discharged back to its original voltage with a current of one milliamp for about two minutes. The researchers estimated the stored electric capacity of the power cell to be approximately 0.036 milliamp-hours.

So far, Wang and his research team -- which included Xinyu Xue, Sihong Wang, Wenxi Guo and Yan Zhang -- have built and tested more than 500 of the power cells. Wang estimates that the generator-storage cell will be as much as five times more efficient at converting mechanical energy to chemical energy for as a two-cell generator-storage system.

Much of the mechanical energy applied to the cells is now consumed in deforming the stainless steel case the researchers are using to house their power cell. Wang believes the power storage could be boosted by using an improved case.

"When we improve the packaging materials, we anticipate improving the overall efficiency," he said. "The amount of energy actually going into the cell is relatively small at this stage because so much of it is consumed by the shell."

Beyond the efficiencies that come from directly converting mechanical energy to chemical energy, the power cell could also reduce weight and space required by separate generators and batteries. The mechanical energy could come from walking, the tires of a vehicle hitting the pavement, or by harnessing ocean waves or mechanical vibrations.

"One day we could have a power package ready to use that takes advantage of this hybrid approach," Wang said. "Almost anything that involves mechanical action could provide the strain needed for charging. People walking could be generating electricity as they move."


Monday, August 6, 2012

Scientists Use Microbes to Make 'Clean' Methane

Microbes that convert electricity into methane gas could become an important source of renewable energy, according to scientists from Stanford and Pennsylvania State universities.

Researchers at both campuses are raising colonies of microorganisms, called methanogens, which have the remarkable ability to turn electrical energy into pure methane -- the key ingredient in natural gas. The scientists' goal is to create large microbial factories that will transform clean electricity from solar, wind or nuclear power into renewable methane fuel and other valuable chemical compounds for industry.

"Most of today's methane is derived from natural gas, a fossil fuel," said Alfred Spormann, a professor of chemical engineering and of civil and environmental engineering at Stanford. "And many important organic molecules used in industry are made from petroleum. Our microbial approach would eliminate the need for using these fossil resources."

While methane itself is a formidable greenhouse gas, 20 times more potent than CO2, the microbial methane would be safely captured and stored, thus minimizing leakage into the atmosphere, Spormann said.

"The whole microbial process is carbon neutral," he explained. "All of the CO2 released during combustion is derived from the atmosphere, and all of the electrical energy comes from renewables or nuclear power, which are also CO2-free."

Methane-producing microbes, he added, could help solve one of the biggest challenges for large-scale renewable energy: What to do with surplus electricity generated by photovoltaic power stations and wind farms.

"Right now there is no good way to store electricity," Spormann said. "However, we know that some methanogens can produce methane directly from an electrical current. In other words, they metabolize electrical energy into chemical energy in the form of methane, which can be stored. Understanding how this metabolic process works is the focus of our research. If we can engineer methanogens to produce methane at scale, it will be a game changer."

'Green' methane

Burning natural gas accelerates global warming by releasing carbon dioxide that's been trapped underground for millennia. The Stanford and Penn State team is taking a "greener" approach to methane production. Instead of drilling rigs and pumps, the scientists envision large bioreactors filled with methanogens -- single-cell organisms that resemble bacteria but belong to a genetically distinct group of microbes called archaea.

By human standards, a methanogen's lifestyle is extreme. It cannot grow in the presence of oxygen. Instead, it regularly dines on atmospheric carbon dioxideand electrons borrowed from hydrogen gas. The byproduct of this microbial meal is pure methane, which methanogens excrete into the atmosphere.

The researchers plan to use this methane to fuel airplanes, ships and vehicles. In the ideal scenario, cultures of methanogens would be fed a constant supply of electrons generated from emissions-free power sources, such as solar cells, wind turbines and nuclear reactors. The microbes would use these clean electrons to metabolize carbon dioxide into methane, which can then be stockpiled and distributed via existing natural gas facilities and pipelines when needed.

When the microbial methane is burnt as fuel, carbon dioxide would be recycled back into the atmosphere where it originated from -- unlike conventional natural gas combustion, which contributes to global warming.

"Microbial methane is much more ecofriendly than ethanol and other biofuels," Spormann said. "Corn ethanol, for example, requires acres of cropland, as well as fertilizers, pesticides, irrigation and fermentation. Methanogens are much more efficient, because they metabolize methane in just a few quick steps."

Microbial communities

For this new technology to become commercially viable, a number of fundamental challenges must be addressed.

"While conceptually simple, there are significant hurdles to overcome before electricity-to-methane technology can be deployed at a large scale," said Bruce Logan, a professor of civil and environmental engineering at Penn State. "That's because the underlying science of how these organisms convert electrons into chemical energy is poorly understood."

In 2009, Logan's lab was the first to demonstrate that a methanogen strain known as Methanobacterium palustre could convert an electrical current directly into methane. For the experiment, Logan and his Penn State colleagues built a reverse battery with positive and negative electrodes placed in a beaker of nutrient-enriched water.

The researchers spread a biofilm mixture of M. palustre and other microbial species onto the cathode. When an electrical current was applied, the M. palustre began churning out methane gas.

"The microbes were about 80 percent efficient in converting electricity to methane," Logan said.

The rate of methane production remained high as long as the mixed microbial community was intact. But when a previously isolated strain of pure M. palustre was placed on the cathode alone, the rate plummeted, suggesting that methanogens separated from other microbial species are less efficient than those living in a natural community.

"Microbial communities are complex," Spormann added. "For example, oxygen-consuming bacteria can help stabilize the community by preventing the build-up of oxygen gas, which methanogens cannot tolerate. Other microbes compete with methanogens for electrons. We want to identify the composition of different communities and see how they evolve together over time."

Microbial zoo

To accomplish that goal, Spormann has been feeding electricity to laboratory cultures consisting of mixed strains of archaea and bacteria. This microbial zoo includes bacterial species that compete with methanogensfor carbon dioxide, which the bacteria use to make acetate -- an important ingredient in vinegar, textiles and a variety of industrial chemicals.

"There might be organisms that are perfect for making acetate or methane but haven't been identified yet," Spormann said. "We need to tap into the unknown, novel organisms that are out there."

At Penn State, Logan's lab is designing and testing advanced cathode technologies that will encourage the growth of methanogens and maximize methane production. The Penn State team is also studying new materials for electrodes, including a carbon-mesh fabric that could eliminate the need for platinum and other precious metal catalysts.

"Many of these materials have only been studied in bacterial systems but not in communities with methanogens or other archaea," Logan said. "Our ultimate goal is to create a cost-effective system that reliably and robustly produces methane from clean electrical energy. It's high-risk, high-reward research, but new approaches are needed for energy storage and for making useful organic molecules without fossil fuels."

The Stanford-Penn State research effort is funded by a three-year grant from the Global Climate and Energy Project at Stanford.