17 Jul 2015
Hydrogen fuel cells promise clean cars that emit only water. Several major car manufacturers have recently announced their investment to increase the availability of fueling stations, while others are rolling out new models and prototypes. However, challenges remain, including the chemistry to produce and use hydrogen and oxygen gas efficiently. Today, in ACS Central Science, two research teams report advances on chemical reactions essential to fuel-cell technology in separate papers.
Hydrogen (H2) fuel cells react H2 and oxygen (O2) gases to produce energy. For that to happen, several related chemical reactions are needed, two of which require catalysts. The first step is to produce the two gases separately. The most common way to do that is to break down, or “split,” water with an electric current in a process called electrolysis. Next, the fuel cell must promote the oxidation of H2. That requires reduction of O2, which yields water. The catalysts currently available for these reactions, though, are either too expensive and demand too much energy for practical use, or they produce undesirable side products. So, Yi Cui’s team at Stanford University and James Gerken and Shannon Stahl at the University of Wisconsin, Madison, independently sought new materials for these reactions.
Cui’s group worked on the first reaction, developing a new cadre of porous materials for water splitting. They notably used earth abundant metal oxides, which are inexpensive. The oxides also are very stable, undergoing the reaction in water for 100 hours, significantly better than what researchers have reported for other non-precious metal materials. On the side of oxygen reduction, Gerken and Stahl show how a catalyst system commonly used for aerobic oxidation of organic molecules could be co-opted for electrochemical O2 reduction. Despite the complementary aims, the two studies diverge in their approaches, with the Stanford team showcasing rugged oxide materials, while the UW-Madison researchers exploited the advantages of inexpensive metal-free molecular catalysts. Together these findings demonstrate the power and breadth of chemistry in moving fuel-cell technology forward.
More information: The two papers will be freely available July 15, 2015, at these links:
“In Situ Electrochemical Oxidation Tuning of Transition Metal Disulfides to Oxides for Enhanced Water Oxidation” pubs.acs.org/doi/full/10.1021/acscentsci.5b00163
“High-Potential Electrocatalytic O2 Reduction with Nitroxyl/NOx Mediators: Implications for Fuel Cells and Aerobic Oxidation Catalysis” pubs.acs.org/doi/full/10.1021/acscentsci.5b00227
23 Jun 2015
|Stanford University scientists have invented a low-cost water splitter that uses a single catalyst to produce both hydrogen and oxygen gas 24 hours a day, seven days a weekThe device, described in a study published June 23 in Nature Communications (“Bifunctional non-noble metal oxide nanoparticle electrocatalysts through lithium-induced conversion for overall water splitting”), could provide a renewable source of clean-burning hydrogen fuel for transportation and industry.|
|Stanford scientists have invented a device that produces clean-burning hydrogen from water 24 hours a day, seven days a week. Unlike conventional water splitters, the Stanford device uses a single low-cost catalyst to generate hydrogen bubbles on one electrode and oxygen bubbles on the other. (Image: L.A. Cicero/Stanford University)|
|‘We have developed a low-voltage, single-catalyst water splitter that continuously generates hydrogen and oxygen for more than 200 hours, an exciting world-record performance,’ said study co-author Yi Cui, an associate professor of materials science and engineering at Stanford and of photon science at the SLAC National Accelerator Laboratory.|
|In an engineering first, Cui and his colleagues used lithium-ion battery technology to create one low-cost catalyst that is capable of driving the entire water-splitting reaction.|
|‘Our group has pioneered the idea of using lithium-ion batteries to search for catalysts,’ Cui said. ‘Our hope is that this technique will lead to the discovery of new catalysts for other reactions beyond water splitting.’|
|Hydrogen has long been promoted as an emissions-free alternative to gasoline. Despite its sustainable reputation, most commercial-grade hydrogen is made from natural gas, a fossil fuel that contributes to global warming. As an alternative, scientists have been trying to develop a cheap and efficient way to extract pure hydrogen from water.|
|A conventional water-splitting device consists of two electrodes submerged in a water-based electrolyte. A low-voltage current applied to the electrodes drives a catalytic reaction that separates molecules of H2O, releasing bubbles of hydrogen on one electrode and oxygen on the other.|
|Each electrode is embedded with a different catalyst, typically platinum and iridium, two rare and costly metals. But in 2014, Stanford chemist Hongjie Dai developed a water splitter made of inexpensive nickel and iron that runs on an ordinary 1.5-volt battery.|
|In the new study, Cui and his colleagues advanced that technology further.|
|‘Our water splitter is unique, because we only use one catalyst, nickel-iron oxide, for both electrodes,’ said graduate student Haotian Wang, lead author of the study. ‘This bifunctional catalyst can split water continuously for more than a week with a steady input of just 1.5 volts of electricity. That’s an unprecedented water-splitting efficiency of 82 percent at room temperature.’|
|In conventional water splitters, the hydrogen and oxygen catalysts often require different electrolytes with different pH — one acidic, one alkaline — to remain stable and active. ‘For practical water splitting, an expensive barrier is needed to separate the two electrolytes, adding to the cost of the device,’ Wang said. ‘But our single-catalyst water splitter operates efficiently in one electrolyte with a uniform pH.’|
|Wang and his colleagues discovered that nickel-iron oxide, which is cheap and easy to produce, is actually more stable than some commercial catalysts made of precious metals.|
|‘We built a conventional water splitter with two benchmark catalysts, one platinum and one iridium,’ Wang said. ‘At first the device only needed 1.56 volts of electricity to split water, but within 30 hours we had to increase the voltage nearly 40 percent. That’s a significant loss of efficiency.’|
|Marriage of batteries and catalysis|
|To find catalytic material suitable for both electrodes, the Stanford team borrowed a technique used in battery research called lithium-induced electrochemical tuning. The idea is to use lithium ions to chemically break the metal oxide catalyst into smaller and smaller pieces.|
|‘Breaking down metal oxide into tiny particles increases its surface area and exposes lots of ultra-small, interconnected grain boundaries that become active sites for the water-splitting catalytic reaction,’ Cui said. ‘This process creates tiny particles that are strongly connected, so the catalyst has very good electrical conductivity and stability.’|
|Wang used electrochemical tuning — putting lithium in, taking lithium out — to test the catalytic potential of several metal oxides.|
|‘Haotian eventually discovered that nickel-iron oxide is a world-record performing material that can catalyze both the hydrogen and the oxygen reaction,’ Cui said. ‘No other catalyst can do this with such great performance.’|
|Using one catalyst made of nickel and iron has significant implications in terms of cost, he added.|
|‘Not only are the materials cheaper, but having a single catalyst also reduces two sets of capital investment to one,’ Cui said. ‘We believe that electrochemical tuning can be used to find new catalysts for other chemical fuels beyond hydrogen. The technique has been used in battery research for many years, but it’s a new approach for catalysis. The marriage of these two fields is very powerful.’|
|Source: By Mark Shwartz, Stanford University|
Colonies of microbes produce methane gas and other compounds in the lab
New findings by Stanford engineering Professor Alfred Spormann and colleagues could pave the way for microbial “factories” that produce renewable biofuels and chemicals.
Stanford University scientists have solved a long-standing mystery about methanogens, unique microorganisms that transform electricity and carbon dioxide into methane.
In a new study, the Stanford team demonstrates for the first time how methanogens obtain electrons from solid surfaces. The discovery could help scientists design electrodes for microbial “factories” that produce methane gas and other compounds sustainably.
“There are several hypotheses to explain how electrons get from an electrode into a methanogen cell,” said Stanford postdoctoral scholar Jörg Deutzmann, lead author of the study. “We are the first group to identify the actual mechanism.”
The study is published in the current issue of the journal mBio.
“The overall goal is to create large bioreactors where microbes convert atmospheric carbon dioxide and clean electricity from solar, wind or nuclear power into renewable fuels and other valuable chemicals,” said study co-author Alfred Spormann, a professor of chemical engineering and of civil and environmental engineering at Stanford. “Now that we understand how methanogens take up electricity, we can re-engineer conventional electrodes to deliver more electrons to more microbes at a faster rate.”
The study also provided new insights on microbially influenced corrosion, a biological process that threatens the long-term stability of structures made of iron and steel.
“Biocorrosion is a significant global problem,” Spormann said. “The yearly economic loss caused by this process is estimated to be in the $1 billion range.”
Methane from microbes
Methane is an important fuel for heating, transportation, cooking and generating electricity. Most methane comes from natural gas, an abundant fossil fuel extracted from wells. However, burning natural gas emits carbon dioxide, which accelerates global warming.
Methanogens offer a promising alternative. These single-celled organisms resemble bacteria but belong to a genetically distinct domain called Archaea.
Commonly found in sediments and sewage treatment plants, methanogens thrive on carbon dioxide gas and electrons. The byproduct of this primordial meal is pure methane gas, which the microbes excrete into the air.
Researchers are trying to develop large bioreactors where billions of methanogens crank out methane around the clock. These microbial colonies would be fed carbon dioxide from the atmosphere and clean electricity from electrodes.
The entire process would be carbon neutral, Spormann explained. “When microbial methane is burnt as fuel, carbon dioxide gets recycled back into the atmosphere where it originated,” he said. “Natural gas combustion, on the other hand, frees carbon that has been trapped underground for millions of years.”
Producing microbial methane on an industrial scale will require major improvements in efficiency, Deutzmann said.
“Right now the main bottleneck in this process is figuring out how to get more electrons from the electrode into the microbial cell,” he said. “To do that, you first have to know how electron uptake works in methanogens. Then you can engineer and enhance the electron-transfer rate and increase methane production.”
In nature, methanogens acquire electrons from hydrogen and other molecules that form during the breakdown of organic material or bacterial fermentation.
“These small molecules are food for the microbes,” Deutzmann said. “They provide methanogens with electrons to metabolize carbon dioxide and produce methane.”
In the Spormann lab, methanogens don’t have to worry about food. Electrons are continuously supplied by a low-voltage current via an electrode. How those electrons get into the methanogen cell has been the subject of scientific debate.
“The leading hypothesis is that many microbes, including methanogens, take up electrons directly from the electrode,” Deutzmann said. “But in a previous study, we found evidence that microbial enzymes and other molecules could also play a role. From an engineering perspective, it makes a difference if you have to design an electrode to accommodate large microbial cells versus enzymes. You can attach a lot more enzymes to the electrode, because enzymes are a lot smaller.”
Experiments with enzymes
For the experiment, the Stanford team used a species of methanogen called Methanococcus maripaludis. Cultures of M. maripaludis were grown in flasks equipped with a graphite electrode, which provided a steady supply of electrons. The microbes were also fed carbon dioxide gas.
As expected, methane gas formed inside the flasks, a clear indication that the methanogens were taking up electrons and metabolizing carbon dioxide. But researchers also detected a build-up of hydrogen gas. Were these molecules of hydrogen shuttling electrons to the methanogens, as occurs in nature?
To find out, the Stanford team repeated the experiment using a genetically engineered strain of M. maripaludis. These mutant methanogens had six genes deleted from their DNA so they could no longer produce the enzyme hydrogenase, which microbes need to make hydrogen. Although the mutants were grown in the same conditions as normal methanogens, their methane output was significantly lower.
“When hydrogenase was absent from the culture, methane production plummeted 10-fold,” Spormann said. “This was a strong indication that hydrogen-producing enzymes are significantly involved in electron uptake.”
Further tests without methanogen cells confirmed that hydrogenase and other enzymes take up electrons directly from the electrode surface. The microbial cell itself is not involved in the transfer, as was widely assumed.
“It turns out that all kinds of enzymes are just floating around in the culture medium,” Deutzmann said. “These enzymes can attach to the electrode surface and produce small molecules, like hydrogen, which then feed the electrons to the microbes.”
Normal methanogen cells produce a variety of enzymes. Stirring, starvation and other biological factors can cause the cells to break open, releasing enzymes into the culture medium, Deutzmann said.
“Now that we know that certain enzymes take up electrons, we can engineer them to work better and search for other enzymes that do it even faster,” he added. “Another benefit is that we no longer have to design large, porous electrodes to accommodate the entire methanogen cell.”
The Stanford team also discovered that methanogen enzymes play a similar role in biocorrosion. The researchers found that granules of iron transfer electrons directly to hydrogenase. The enzyme uses these electrons to make hydrogen molecules, which, in turn, are consumed by methanogens. Eliminating hydrogenase from the environment could slow down the rate of corrosion, according to the scientists.
“At first we were surprised by these results, because enzymes were thought to degrade very quickly once they were outside the cell,” Spormann said. “But our study showed that free enzymes attached to an electrode surface can remain active for a month or two. Understanding why they are stable for so long could lead to new insights on reducing corrosion and on scaling up the production of microbial methane and other sustainable chemicals.”
The mBio paper was also co-authored by Stanford researcher Merve Sahin. The study was supported by the Global Climate and Energy Project at Stanford.