With pressure mounting to transition away from fossil fuels, fuel cell research has grown significantly in the last several years. In the simplest sense, fuel cells are a battery that you refuel slowly, regulating a chemical reaction and harvesting that energy in the form of usable electrical current. Current solutions use exotic materials to regulate the reactions and often require fossil fuels to generate the chemicals, defeating the purpose of the exercise. Today's release from Science includes three articles that detail methods that may help us overcome the problems with current-generation fuel cells.
Cheap catalyst splits water
Widespread use of fuel cells will rely on cheap sources of hydrogen and oxygen. Researchers at MIT have now made an oxygen-producing catalyst that operates on water in a neutral environment (pH 7 at atmospheric pressure) and can be coupled with solar cells; it's essentially a man-made equivalent to photosynthesis.
Platinum has been used as a catalyst for this reaction in the past, but the costs associated with platinum (it closed today at over $1,730 per ounce) have prompted efforts to eliminate its use. The new research describes the formation of a catalyst composed of a combination of cobalt, potassium, and phosphorous—all cheap and easy to obtain. The researchers found that two different inert electrodes would, when placed into a dilute solution containing cobalt and buffered with potassium phosphate, spontaneously form a coating of the catalyst. When provided with relatively low electrical potentials, such as those obtained from a solar cell, the catalyst would liberate oxygen gas by splitting the water that was acting as a solvent.
The key breakthroughs here are the elimination of precious metals from the catalyst, the in situ formation of the catalyst, and the benign operating conditions of the reaction. All of this adds up to big cost savings in splitting water into is component gasses. Platinum's cost is all too apparent to anybody that has ever been to a jewelry store, but less apparent is the costs associated with producing catalyst materials, a process all but eliminated in this research.
Using less metal
Another use of platinum may go to the wayside in favor of an organic alternative, courtesy of Australian researchers. The metal is often used as a cathode that forms the interface between air and an electrolyte, used in both fuel cell and air/metal battery applications. This electrode's job is to reduce oxygen from the air and diffuse it into the electrolyte, so that it can be put to work in further chemical reactions that generate electricity. Here, platinum has issues beyond its exorbitant cost. It suffers from inactivity in the presence of carbon monoxide gas and diffusion of the platinum particles in the carbon substrate to form agglomerates that harm performance.
Electrically conductive polymers have been tested, but the performance simply wasn't sufficient to justify replacing platinum. But developments in gas-phase deposition techniques have now allowed for higher-quality electrically conductive thin-film polymers to be produced, opening the door for fuel cell applications. In this case, the researchers focused on a conductive polymer called poly(3,4-ethylenedioxythiophene), or PEDOT. The need for both a high surface area in contact with the incoming gas and to avoid moisture ingression led scientists to coat the PEDOT on every hiker's best friend: Gortex fabric. To further enhance conductivity, a 40nm gold coating was added.
The PEDOT electrode is homogeneous, eliminating catalyst agglomerations that plagued the long-term reliability of platinum based electrodes. It's also insensitive to carbon monoxide poisoning, another performance-robbing problem with platinum. The optimal PEDOT coating thickness was found to be 400nm, and performance was on par with that of the standard platinum-based electrodes. Researchers ran the electrode for 1,500 hours with no loss in performance. With the cost of the platinum in a fuel cell being equal to the total cost of an equivalent gasoline engine, this breakthrough has huge potential to drive down the cost of fuel cells, although researchers were quick to point out that similar breakthroughs are needed to get rid of platinum at the anode side.
Solid oxides get to chill
Solid oxide fuel cells (SOFCs) represent a completely different approach to the problem. They're one of the leading options because, compared to many other green technologies, they have relatively high efficiency, high energy storage density, and produce only water as a byproduct. While SOFCs have not made substantial in-roads in the consumer space, they are being adopted as emergency power systems for hospitals, 911 dispatch centers, and other critical entities.
The primary limitation of SOFCs is high operating temperature. SOFCs operate by diffusing O2- across a ceramic electrolyte. Current generation systems use Y2O3 doped ZrO2 (YSZ) electrolytes that require operating temperatures above 700°C because the diffusion is a thermally activated process. A variety of alternatives to YSZ have been suggested, but they offer only modest improvements in operating temperature. In this week's Science, researchers from Madrid and Oak Ridge National Lab describe a novel SOFC membrane that operates at room temperature.
In these ceramics, solid state diffusion of the oxygen can be thought of as occurring through a series of atomic jumps, where ions leap from one lattice site to the next provided the next site is vacant. The easiest way to increase ionic conduction is to increase the number of vacancies—raising the temperature is typically the easiest way to do that. This temperature effect gives rise to the high operating temperatures in conventional SOFCs. The materials in this study are unique because they stabilize incredibly high fractions of vacancies at room temperature.
Instead of using monolithic YSZ, the authors used thin-film growth techniques (molecular beam epitaxy) to grow 5-60 nanometer thick, alternating layers of YSZ and SrTiO3 (STO). They found that these two materials form an interface where the anions (O2-) become highly disordered, causing an anomalously high numbers of vacancies. These unique interfaces form a superhighway of O2- conduction.
Electrical measurements showed that the primary conduction pathway in the materials went through the YSZ/STO interface, but the YSZ layers showed some conduction as well. This work conclusively shows that the conductivity is thermally activated and thus is a result of ionic motion, rather than charge migration. This data is incredibly important because previous reports of high ionic conductivity ultimately turned out to be a result of electronic conduction through defective membranes, making the materials useless as fuel cells.
Despite the substantial promise of these materials, it is probably premature to start placing orders for your room-temperature SOFC; drawbacks include processing that is not amenable to mass production, fast conduction in only two dimensions, and a lack of long-term stability information. Despite these concerns, this work is likely to represent a major step in the march towards wider SOFC commercialization.
The same general note of caution applies to all of these developments, as it's possible that some of these techniques won't scale, or will only find a home in some specific applications. Still, they highlight how focused research and development can produce significant improvements in clean energy technology.
Nobel Intent writers Todd Morton and Adam Stevenson produced this report.
Sciencexpress, 2008. DOI: 10.1126/science/1162018
Science, 2008. DOI: 10.1126/science.1159267
Science, 2008 DOI: 10.1126/science.1156393