Scientists Use Microbes to Make 'Clean' Methane

 

Post-doctoral fellow Svenja Lohner, left, and Professor Alfred Spormann. Their research, along with the work of others, 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. (Credit: Linda A. Cicero / Stanford News Service))                                        Science Daily  — Microbes that convert electricity into methane gas could become an important source of renewable energy, according to scientists from Stanford and Pennsylvania State universities.

"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."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.

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 dioxide and 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.

Photovoltaics from Any Semiconductor: Opens Door to More Widespread Solar Energy Devices

The SFPV technology was tested for two top electrode architectures: (A) the top electrode is shaped into narrow fingers; (B) top electrode is uniformly ultrathin. (Credit: Image courtesy of DOE/Lawrence Berkeley National Laboratory)                                Science Daily — A technology that would enable low-cost, high efficiency solar cells to be made from virtually any semiconductor material has been developed by researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley. This technology opens the door to the use of plentiful, relatively inexpensive semiconductors, such as the promising metal oxides, sulfides and phosphides, that have been considered unsuitable for solar cells because it is so difficult to taylor their properties by chemical means.

"It's time we put bad materials to good use," says physicist Alex Zettl, who led this research along with colleague Feng Wang. "Our technology allows us to sidestep the difficulty in chemically tailoring many earth abundant, non-toxic semiconductors and instead tailor these materials simply by applying an electric field."

Zettl, who holds joint appointments with Berkeley Lab's Materials Sciences Division and UC Berkeley's Physics Department where he directs the Center of Integrated Nanomechanical Systems (COINS), is the corresponding author of a paper describing this work in the journal Nano Letters. The paper is titled "Screening-Engineered Field-Effect Solar Cells." Co-authoring it were William Regan, Steven Byrnes, Will Gannett, Onur Ergen, Oscar Vazquez-Mena and Feng Wang.

Solar cells convert sunlight into electricity using semiconductor materials that exhibit the photovoltaic effect -- meaning they absorb photons and release electrons that can be channeled into an electrical current. Photovoltaics are the ultimate source of clean, green and renewable energy but today's technologies utilize relatively scarce and expensive semiconductors, such as large crystals of silicon, or thin films of cadmium telluride or copper indium gallium selenide, that are tricky or expensive to fabricate into devices.

"Solar technologies today face a cost-to-efficiency trade-off that has slowed widespread implementation," Zettl says. "Our technology reduces the cost and complexity of fabricating solar cells and thereby provides what could be an important cost-effective and environmentally friendly alternative that would accelerate the usage of solar energy."

This new technology is called "screening-engineered field-effect photovoltaics," or SFPV, because it utilizes the electric field effect, a well understood phenomenon by which the concentration of charge-carriers in a semiconductor is altered by the application of an electric field. With the SFPV technology, a carefully designed partially screening top electrode lets the gate electric field sufficiently penetrate the electrode and more uniformly modulate the semiconductor carrier concentration and type to induce a p-n junction. This enables the creation of high quality p-n junctions in semiconductors that are difficult if not impossible to dope by conventional chemical methods.

"Our technology requires only electrode and gate deposition, without the need for high-temperature chemical doping, ion implantation, or other expensive or damaging processes," says lead author William Regan. "The key to our success is the minimal screening of the gate field which is achieved through geometric structuring of the top electrode. This makes it possible for electrical contact to and carrier modulation of the semiconductor to be performed simultaneously."

Under the SFPV system, the architecture of the top electrode is structured so that at least one of the electrode's dimensions is confined. In one configuration, working with copper oxide, the Berkeley researchers shaped the electrode contact into narrow fingers; in another configuration, working with silicon, they made the top contact ultra-thin (single layer graphene) across the surface. With sufficiently narrow fingers, the gate field creates a low electrical resistance inversion layer between the fingers and a potential barrier beneath them. A uniformly thin top contact allows gate fields to penetrate and deplete/invert the underlying semiconductor. The results in both configurations are high quality p-n junctions.

Says co-author Feng Wang, "Our demonstrations show that a stable, electrically contacted p-n junction can be achieved with nearly any semiconductor and any electrode material through the application of a gate field provided that the electrode is appropriately geometrically structured."

The researchers also demonstrated the SFPV effect in a self-gating configuration, in which the gate was powered internally by the electrical activity of the cell itself.

"The self-gating configuration eliminates the need for an external gate power source, which will simplify the practical implementation of SFPV devices," Regan says. "Additionally, the gate can serve a dual role as an antireflection coating, a feature already common and necessary for high efficiency photovoltaics."

This research was supported in part by the DOE Office of Science and in part by the National Science Foundation.

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