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Limitless Carbon-Neutral Hydrogen With A Little Help From Our Bacteria Friends
October 4, 2011
You may remember an experiment from high school. In order to achieve hydrolysis of water, or separating water into its two base elements, hydrogen and oxygen, you ran a current through water into which you had stirred a pinch of salt. This was usually accomplished by tying two pieces of graphite from disemboweled pencils to two pieces of copper wire and then attaching them to a battery. Run the current from the battery through the slightly salty water, and voila: you create bubbles of hydrogen that you can capture under the water with an upside-down cylinder. After being supremely unimpressed by the whole process, you cleaned up and headed for algebra class, forgetting all about the experiment until you were forced to wonder: is it going to be on the mid-term? Of course, producing more than tiny amounts of hydrogen for commercial purposes requires a slightly larger-scale process. Hydrogen, of course, is the foundation of the "hydrogen economy" that has been promised to the world for decades now as the solution for copious amounts of cheap, clean energy. Perhaps the promise is finally in the early stages of being fulfilled: it's only lately that hydrogen fuel cell products have begun entering the commercial marketplace at a rapid clip. Fuel cells are showing up in everything from low-emissions cars (hydrogen has more efficient energy density than the batteries typically used in electric cars, which make fuel cell cars more efficient and cleaner than electric vehicles) to data centers (in the form of Bloom Energy's pricey but marvelous "Bloom Boxes") to commercial phone-charging products such as Horizon Fuel Cell's MiniPAK home fuel cell device. The problems is (let's go back to that high school physics experiment for a moment) that pesky bit of current required to create the hydrogen must come from an external energy source - a battery or the electric grid - which means that currently, hydrogen is merely an energy carrier and not an actual energy source. Even with the cheapest and most efficient hydrogen creation process today - still hydrolysis of water - it requires more energy to make the hydrogen than the hydrogen itself supplies. So how can it be a clean, cheap source of renewable energy if you still need electricity from toxic batteries or dirty fossil-fuel-fired plants to create the hydrogen? (In the same way people view electric cars as being carbon-neutral, forgetting that the car must be charged by plugging it into an electric source that inevitably ties into a grid powered by coal or oil burning.) The answer is: hydrogen can't be a carbon-neutral source of energy with status quo technology. Not yet. Researchers at Penn State University, however, may have taken a few recent steps toward solving the problem by using - of all things - bacteria. (Though entering a new, modern "Bacteria Economy" just doesn't have quite the same sexy ring to it.) Via the new process, scientists have been able to create hydrogen from scratch, in a carbon-neutral way, minus any input from grid electricity or batteries and minus any output of greenhouse gasses. The new process, which uses something called microbial electrolysis cells (MECs), could ultimately produce fuel cells that are essentially self-powered and therefore limitless in their ability to produce clean, carbon-neutral and emissions-free energy, reported the BBC. "There are bacteria that occur naturally in the environment that are able to release electrons outside of the cell, so they can actually produce electricity as they are breaking down organic matter," said the study's co-author, Bruce Logan, a professor of environmental engineering at Penn State who worked with postdoctoral fellow Younggy Kim on the project. "We use those microbes, particularly inside something called a microbial fuel cell (MFC), to generate electrical power. We can also use them in this device, where they need a little extra power to make hydrogen gas. What that means is that they produce this electrical current, which are electrons, they release protons in the water and these combine with electrons." So what's a "microbial fuel cell" you might ask? It's a bio-electrochemical (part nature, part man-made) system that can mimic bacterial activity normally found in nature, converting chemical energy to electrical energy via the catalytic reaction of microorganisms. In this case, the bacteria can be induced to act in a way that creates an electrical current, without input from any man-made energy source needed. (The process was previously accomplished using electrodes that required an external power source. In this case, the bacteria is replacing the electrodes.) Interestingly, the idea for using microbial cells to produce electricity was first proposed a hundred years ago by British botanist M.C. Potter. Though in the interim many others have worked on producing energy via microbial fuel cells, this new process is the first that requires no external power to induce the reaction. Logan and Kim themselves have worked with microbial fuel cells before. Back in 2005, the pair developed a process that turned human waste into hydrogen, generating about four times as much hydrogen as previous processes that fermented biomass. (The press, in some cases, charmingly dubbed it "Poo Power.") At the time, it was Logan himself who underscored the limitations of that process, pointing out that there's just not enough "waste biomass to sustain a global hydrogen economy," reported Engadget. But there certainly is enough fresh water and salt water, at least in many parts of the world, and nobody's running short on bacteria. The mechanism behind the microbial fuel cell process is called "reverse electrodialysis" or RED, which gathers the tiny bit of energy released when two samples of water, different in their salinity (salt content) meet one another through micro-thin positive and negative ion exchange membranes (known as "membrane pairs") with an anode and a cathode at each end. The ionic discrepancy between the salt water molecules and the fresh water molecules causes a reaction that produces a tiny amount of voltage that could be used or stored. Under normal circumstances, however, the interaction between the membrane pairs generates only about .5 volts of electricity. Since it requires about 1.8 volts to hydrolyze water and produce hydrogen, this wasn't enough. It wasn't until Logan and Kim added the exoelectrogenic bacteria to the process that they reached the electrical potential to create hydrolyze the water and "make" hydrogen. In their report, Logan and Kim say they envision an RED system that would use alternating stacks of membranes to harvest the energy. The more membranes you have, of course, the more energy you can create. "If you think about desalinating water, it takes energy. If you have a freshwater and saltwater interface, that can add energy," Professor Logan told the BBC. "We realized that just a little bit of that energy could make this process go on its own." The "just a little bit of energy" would come from the microbial electrolysis cells. During the course of the experiment, Logan and Kim's device - the bacterial hydrolysis cell - proved to be about 58 to 64 percent energy efficient, and it produced somewhere between 0.8 to 1.6 cubic meters of hydrogen for every cubic meter of liquid it used. In addition, only one percent of that energy produced was required to pump the water through the cells, leaving the other 99 percent of energy potentially usable. So where do you get a ready supply of bacteria? Exoelectrogenic bacteria can be found in ponds, river sediment and soil, for starters. (Basically, all the scummy places your mother told you never to go into, probably including biker bars.) But it's waste water treatment plants the researchers have their eye on for a nearly endless supply of non-salty water - it's hard to call it "fresh" - loaded with bacteria. Of particular interest are waste water treatment plants that are located near the sea (for a nearly endless supply of salt water). The secondary benefit of the process is that it's a way to treat waste-water in a carbon-neutral way. "Biodegradable liquids and cellulose waste are abundant and with no energy in and hydrogen out we can get rid of waste water and by-products," Professor Logan told GizMag "This could be an inexhaustible source of energy." Obviously, the "could be" means that this type of energy generation is still years off. The authors of the study, which was published in the Proceedings of the National Academy of Sciences, note that the process is in its infancy and therefore still too expensive for commercial applications, but they hope in the future the process will become feasible for generating a nearly endless supply of clean energy. Professor Logan likened the process to any other alternative energy solution that needed to learn to crawl, sometimes for decades, before it learned to walk. "Right now, it is such a new technology," said Logan. "In a way it is a little like solar power. We know we can convert solar energy into electricity but it has taken many years to lower the cost. This is a similar thing: it is a new technology and it could be used, but right now it is probably a little expensive. So the question is, can we bring down the cost?" Logan and Kim say they plan to try. Their next step will be to develop larger-scale cells, expanding the scope of the experiment. "Then it will easier to evaluate the costs and investment needed to use the technology," noted Logan. That doesn't mean that the process would solve all the energy problems of the world. Hydrogen storage is still a complicated and energy-intensive process. Since a single gram of hydrogen takes up nearly three gallons of storage space at a normal (Earth) atmosphere, it must be highly pressurized to several hundred atmospheres, and then stored in a container that can accommodate that kind of pressure. (So, we're not talking about plastic margarine tubs.) Alternatively, it can be stored in liquid or "slush" form, but that requires keeping it at a chilly -423 degrees Fahrenheit. It also evaporates easily, losing about one percent of its volume each day, and presents a risk of explosion if not handled properly. If you're still wondering why the U.S. has discontinued the space shuttle program, consider that each shuttle launch required about half a million gallons of liquid hydrogen, all of which had to be kept below -423 degrees. (Think of the electric bill!) Another problem is that Logan and Kim used platinum in the cathode as the catalyst for the process which, at about $1,700 an ounce today and historically as high as $2,200, would drive the price far and away out of the hands of commercial or consumer entities looking to make it practical. Though the team did replicate the process using a catalyst of the non-precious metal molybdenum disulfide (MoS2 ), it lost a little bit of efficiency and produced a little less hydrogen - though still an encouraging amount at about 51 percent efficiency as opposed to the 58 to 64 percent using platinum. Finally, discovering places on earth that have a limitless supply of fresh, bacteria-laden water adjacent to a limitless source of sea water isn't as easy as you might think, and that's what would be required for a plant that would be capable of producing vast quantities of hydrogen. While it's a long step from "we know how to do it" to "we know how to do it cheaply and on a large scale," the potential for hydrogen fuel cell technology was, up until now, limited because no one could figure out how to create hydrogen entirely "off the grid." With Logan and Kim's breakthrough, researchers now know that it can be done. Which probably means that it's only a short matter of time until the world's largest oil companies buy the rights to the process so they can shelve it - Raiders of the Lost Ark style - in the back of a deep, dark closet forever.