Taking large-scale advantage of intermittent renewable energy such as that produced by wind and sunlight will require storage technology - referred to as massive electricity storage (MES) - for integration with the power grid. Candidates under consideration for MES include batteries, supercapacitors, and flywheels, as well as electrochemical systems.
Several research groups are working on electrochemical storage approaches that they call artificial photosynthesis. Two teams - one from the Univ. of Illinois at UrbanaChampaign and the other from the Massachusetts Institute of Technology (MTT) - have separately come up with ways to mimic a plant's foodproducing process to convert sunlight into chemical energy that can be stored and used at a later time.
Photosynthesis consists of two main steps: water Splitting, and carbon dioxide fixation. The water-splitting step takes energy directly from the sun and uses it to decompose water nito oxygen, hydrogen, and electrons, while the CO2 fixation step uses the electrons produced in the first step to convert CO2 hito carbon monoxide.
The engineers from the Univ. of Illinois partnered with startup company Dioxide Materials and focused on the second step - converting CO2 into CO - with solar panels providing the electrons needed to drive the. reaction.
Normally, metal catalysts are used to form CO from CO2. This process has been stymied, however, because it has a high overpotential and thus requires a large amount of energy input To reduce the overpotential, the engineers used an ionic liquid as a cocatalyst. Ionic liquids consist entirely of positively and negatively charged ions, which under normal conditions would exist as salts rather than as liquids.
"To accomplish this electrochemical conversion without a catalyst present, you need on the order of 2.2 volts; we reduced that potential to 1.5 volts," says chemical and biological engineering professor and chair Paul Kenis. "By lowering the potential that you use to drive this process, you dramatically improve the energetic efficiency, that is, how much energy you have to put in to make the process work," Kenis explains.
In the Illinois process, the ionic liquid, in this case l-ethyl-3-methyIimidazolhim tetrafluoroborate (EMIMBF4), stabilizes the CO2 intermediate (CO2*~X which has been previously identified as the culprit of the high overpotential. The intermediate then reacts with H+ on a silver cathode to produce CO.
The CO can be stored for later use or mixed with hydrogen to produce synthesis gas, which can then be converted into highermolecular-weight hydrocarbons such as liquid fuels via Fischer Tropsch.
The next step will be increasing the throughput of the reaction. 'To have a usefiil process of CO2-to-CO conversion, both the energetic efficiency and the current density have to be high," Kenis says. "The current density relates to the number of molecules you convert per unit of time, which is referred to as turnover number. You can increase that through reactor design, which is typically not as big a challenge as catalysis."
The engineers hope to have a preeommercial prototype process ready in the next 18 months to two years, and will be working on the reactor design under a Small Business Technology Transfer (STTR) grant from the U.S. Dept. of Energy (DOE).
At MIT, chemistry professor Daniel Mocera and his team have developed a process that mimics the water-splitting step of photosynthesis. Instead of using electrons produced by a solar cell, this process creates its own electrons by splitting water into hydrogen and oxygen.
"What Mocera tries to do is a much harder process," Kenis says. "He uses semiconductors to catch the energy from the sun and uses those electrons immediately for chemical conversion."
The MIT device, which the team refers to as an artificial leaf, consists of a silicon solar cell sandwiched between catalytic materials. When placed in water and exposed to sunlight, it creates streams of bubbles - hydrogen gas from one side and oxygen bubbles from the other. These streams can be collected and later fed into a fuel cell to produce electricity, or the hydrogen can be used as a fuel.
"Fuel-forming catalysts interfaced with light-harvesting semiconductors afford a pathway to direct solar-tofuels conversion that captures many of the basic functional elements of a leaf," Nocera says.
Devices that use sunlight to split water into hydrogen and oxygen have been realized before. However, practical problems have limited their widespread use, Nocera says. Previous methods with reasonable efficiency use prohibitively expensive light-absorbing materials (e.g., gallium arsenide) and fuel-forming catalysts (e.g. , platinum, ruthenium dioxide, and iridium oxide), as well as strongly acidic or basic media, which are expensive to manage.
The artificial leaf relies on earthabundant materials at near-neutral pH conditions. Amorphous silicon, an earth-abundant material prevalent in the electronics and photovoltaic industries, is used as the solar cell, with a cobalt compound as the oxygen-evolving catalyst and a nickel-molybdenum-zinc alloy to release hydrogen from water.
The MIT team has achieved efficiencies of 4.7% for a wired device and 2.5% for a wireless configuration. This compares to an average efficiency of 10% for today's solar cells.
Before the artificial leaf can be used in real-world applications, technology to collect, store, and use the hydrogen and oxygen gases must be developed. In addition, Nocera plans to drive up the efficiency and reduce the costs of the device.
"It's a step," Nocera says. "It's heading in the right direction."
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The "artificial leaf" can harness sunlight to split water, into hydrogen and oxygen without any external connections. Image courtesy of Dominlck Reuter.
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