This article discusses the use of fuel cell-powered vehicles that aim to change the face of transportation. These fuel cell-powered vehicles are expected to have a significant impact on reducing both the emissions implicated in global climate change and those that cause local smog. Fuel cells electrochemically oxidize a fuel without burning, thereby avoiding the inefficiencies and pollution associated with the traditional combustion technologies. The U.S. Department of Energy is working with researchers at the University of Waterloo in Ontario and elsewhere to develop non-precious materials to replace the platinum catalysts in fuel cells. European scientists have developed a material for converting hydrogen and oxygen to water that uses only 10% of the amount of platinum that is normally required. The researchers discovered that the efficiency of the nanometer-sized catalyst particles is greatly influenced by their geometric shape and atomic structure. Mechanical engineers play a crucial role in the development of both fuel cell and hydrogen production technologies.
Green is always in the eye of the beholder, but hydrogen-powered fuel cells are regarded by many experts as the cleanest alternative to the internal combustion engine for powering automobiles. Fuel cells, which electrochemically convert chemical energy to electricity, don’t produce the by-products of combustion such as nitrogen oxides and ozone. And when hydrogen is the fuel, the only exhaust from the tailpipe is water. The mass production of fuel cell-powered vehicles would change the face of transportation, and would have a huge impact on reducing both the emissions implicated in global climate change and those that cause local smog.
That's the promise of fuel cell technology. But that promise has long been at odds with reality.
Fuel cells have been used for some time in niche, stationary applications. But in spite of decades of research, fuel cell vehicles have never been able to compete with hybrid vehicles or even battery electrics, let alone standard cars. The fuel cells have been too expensive for the mass market and have not been durable enough for ordinary drivers.
Those drawbacks, however, may not apply much longer. An explosion of research is leading scientists to innovative fuels, materials, and cell designs that are vastly improving the efficiency and reducing the cost of fuel-cell systems. Within a few years, fuel cells may finally come into their own and live up to their potential as a game-changing technology.
Handling the Heat
Fuel cells come in a variety of types, with the electrolyte—the charge-carrying medium—determining the most effective temperature of operation.
“The optimum temperatures for fuel cell types cover a wide range, from about 60 °C for the polymer electrolyte membrane fuel cell up to 1,000 °C for the solid oxide fuel cell,” said Shailesh D. Vora, technology manager for fuel cells for the U.S. Department of Energy's National Energy Technology Laboratory in Pittsburgh. “Cell operating temperature influences fuel cell and system design in areas such as material selection, heat recovery from the fuel cell exhaust, and the potential for applying the fuel cell in a combined cycle, which can have a significant effect on the power system electric efficiency.”
Low-temperature cells such as polymer electrolyte membrane systems have the highest power density, require high-purity hydrogen, allow for quick start-up, and handle dynamics efficiently, making them ideal for transportation applications and as back-up power generators.
“These fuel cells commonly utilize a noble metal as a catalyst, and their performance remains susceptible to both air and fuel impurities,” said Prabhakar Singh, director for the Center for Clean Energy Engineering at the University of Connecticut in Storrs.
High-temperature fuel cells such as solid oxide or molten carbonate systems, on the other hand, can actually reform hydrocarbon fuels internally. Even though their power density is much lower than PEMFCs, they can achieve quite high efficiencies from hydrocarbon fuels, making them best suited for stationary applications. The catalyst is typically a non-strategic metal. High-temperature fuel cells are also more tolerant of fuel and air impurities than low-temperature systems are.
“Both MCFCs and SOFCs operate at higher temperatures, ranging from 650 °C for MCFCs to 650-1,000 °C for SOFCs,” said Etim Ubong, director of the Kettering University Fuel Cell Program in Flint, Mich. “MCFCs can be used as a stationary power plant for distributed power generation, with outputs up to 1.4 MW per unit. The Hilton Hotel Marina in San Diego, for example, is entirely powered by MCFCs. Both MCFCs and SOFCs can use multi-fuels for their operations, including pure hydrogen, reformed fuel, natural gas, and carbon monoxide.”
While PEMFCs and SOFCs have seen the most deployment so far, there is increased interest being shown toward direct methanol fuel cells, a type of PEMFC that is fueled by methanol rather than hydrogen. Methanol as a fuel has some attractive features in terms of stability, energy density, and ease and safety of transport. Because the efficiency of DMFCs is low compared to other cells, direct methanol fuel cells are best suited for portable situations, where energy and power density are more important than efficiency.
“Direct methanol fuel cells are being developed and explored as a replacement for batteries in cell phones, laptops, and tablets,” said Scott Samuelsen, professor of mechanical engineering and director of the National Fuel Cell Research Center at University of California-Irvine. “Potentially, this could mean refueling your laptop with a few milliliters of methanol in a matter of seconds followed by eight hours of operation, rather than having to plug in your laptop for an hour or more to charge a battery. That would be a game-changer for the industry.”
On the Move
The big R&D driver for fuel cells is still the automotive industry. Hydrogen fuel cell vehicles are expected to play an important role in reducing greenhouse gas emissions and meeting tough new emission standards in the U.S. In preparation for a 2020 deadline, California plans to spend about $18 million to expand its network of hydrogen fueling stations across the state, with the goal of supporting one million zero-emission vehicles by 2020. In July 2013 General Motors and Honda announced a joint plan to deliver cheaper power-making fuel cells and hydrogen tanks by 2020. A key goal of the partnership is designing a common hydrogen powertrain that will make low-polluting vehicles more affordable.
Fuel cells could also find their way aboard ships. Sandia National Laboratories is exploring the possibility of using hydrogen fuel cells to power ships at berth, replacing incumbent diesel generators, which release significant greenhouse gas pollution. Researchers have determined that a barge-mounted hydrogen fuel cell system could provide around 1.4 MW to a ship over 48 hours. Vessels with smaller requirements, such as tugboats, could be powered by a single container housing both the fuel cell and hydrogen.
To get there, some materials bottlenecks have to be overcome. Large amounts of platinum are still required for PEMFC electrodes to achieve necessary conversion rates.
“Using expensive platinum as a catalyst for the oxygen reduction reaction results in high fuel-cell cost,” said Partha P. Mukherjee, assistant professor of mechanical engineering at Texas A&M University. “Therefore it is imperative to develop high-performance catalyst layers with reduced platinum-loading or non-precious metal catalyst electrodes with improved kinetics.”
The U.S. Department of Energy is working with researchers at University of Waterloo in Ontario and elsewhere to develop non-precious materials to replace the platinum catalysts in fuel cells. “Platinum is so expensive [about $4,000 per fuel-cell car], and such a limited resource, that we must find a way to replace it if fuel-cell cars are going to succeed,” said Zhongwei Chen, professor of engineering at Waterloo and a researcher on the project.
One way to reduce platinum content is by developing alloys that are just as effective as platinum, but contain much smaller concentrations of the metal.
“Very low-platinum electrodes are being fine-tuned by some suppliers to ensure both performance and durability,” said Scott Blanchet, director of technology development for Nuvera Fuel Cells, a developer of fuel cell technology.
“For example, Nuvera successfully completed a DOE-funded program in 2012 that demonstrated 12.5 W per milligram of platinum in a full-format fuel-cell stack,” Blanchet said. “Today's ultra-low-emission gasoline passenger vehicles contain about 10 grams of platinum-group metals in their catalytic converters. So, at 12.5 W/mg, a 125 kW fuel cell would contain the same amount of platinum as a typical gasoline car on the road today, but produce no carbon emissions.”
European scientists have developed a material for converting hydrogen and oxygen to water that uses only 10 percent of the amount of platinum that is normally required. The researchers discovered the efficiency of the nanometersized catalyst particles is greatly influenced by their geometric shape and atomic structure. Instead of being round, the platinum-nickel alloy nanoparticles are octahedral, which accelerates the chemical reaction between hydrogen and oxygen.
Another new catalyst was recently announced at the University of Wisconsin. Researchers there deposited nanostructures of molybdenum disulfide on a disk of graphite, which is then subjected to a lithium treatment to create a unique structure with improved catalytic properties.
In June, fuel cell developer ACAL, based in Runcorn, England, ran a fuel cell using a non-platinum liquid catalyst for 10,000 hours without any significant signs of degradation.
The Next Steps
Catalysts are not the only point of research. Engineers at the Georgia Institute of Technology in Atlanta are exploring better ways to manufacture polymer electrolyte membranes, the thin film at the heart of PEMFCs and DMFCs. The membrane material is most often cast from solution onto a non-porous, sacrificial backing material and rolled until ready for use. A research team has developed a technique to directly coat the membrane onto another key component, the porous gas diffusion layer. “By directly coating the membrane onto the GDL, subsequent processing steps including cutting and hot pressing can be eliminated,” said the team leader, Tequila Harris, an assistant professor of mechanical engineering at Georgia Tech.
In order for this method to be successful, fluid penetration into the GDL pores must be controlled and the coated film must be defect-free. Harris and her research group have developed an empirical model that predicts the onset of defects, such as pinholes or air bubbles, which can lead to catastrophic failure of a fuel cell or costly waste during production. In addition, “this model allows for maximum coating speeds to be determined a priori versus by trial and error,” Harris said.
How they Work
Fuel cells electrochemically oxidize a fuel without burning, thereby avoiding the inefficiencies and pollution associated with traditional combustion technologies. The end result is clean electrical power, high-quality heat, and no pollutants.
A fuel cell has many of the same components as a battery: an anode, a cathode, and layer of electrolyte in between that serves as the ion conductor. Hydrogen fuel enters the anode, where it's divided into hydrogen ions and electrons. The ions move through the electrolyte to the cathode, where they combine with oxygen to produce water. Electrons cannot pass through the membrane, so they are routed through a circuit to the cathode, producing electricity. Pollution emissions are essentially zero.
Although hydrogen is the optimal fuel for all types of fuel cells, carbon monoxide, methanol, and dilute light hydrocarbons like methane are also used.
Fuel cells are distinguished by the material that is used for the electrolyte layer. There are dozens of types of fuel cells, but five main types are in various stages of development and commercialization:
NETL is also working to improve the reliability and robustness of fuel cell systems. Although single-cell modules have achieved approximately four years of run time, large stacks under real-world operating conditions degrade at approximately 1.5 to 2 percent per 1,000 hours. This limits stacks to about one year before they require replacement.
“Multiple studies by NETL have shown that stack life must be at least four years, and degradation reduced to approximately 0.2 percent per 1,000 hours, to ensure competitiveness with other technologies,” said Vora of NETL. Vora added that the lab's research goal was to reach those benchmarks in a low-cost solid oxide fuel cell.
Mechanical engineers play a crucial role in the development of both fuel cell and hydrogen production technologies. Of the more than 100 degreed engineers at Nuvera, for example, more than 40 percent are mechanical engineers.
“Developing fuel cell stacks and systems requires the use of structural mechanics, shock and vibration, thermodynamics, heat transfer, fluid mechanics, electrical engineering, reacting systems, phase change, material science (polymers, metals, and ceramics), and controls, not to mention operational expertise in design and manufacturing,” Blanchet said.
A common misconception by many engineers is that, because fuel cells have no moving parts, they must be boring. “Having worked as an ME for 20 years, I can say that nothing could be further from the truth,” Blanchet said. “Fuel cells involve multiple disciplines, highly coupled where every component serves several functions. These challenges are particularly well-suited to graduates from mechanical engineering programs. Although most people associate fuel cells and hydrogen production with chemistry and material science, in the end we need cost-effective, functioning machines to do work for society. Mechanical engineers make that happen.”
Advances in fuel cell technology, combined with the increasing availability of natural gas, have created an opportunity for fuel cells to fulfill their longheld promise. Are fuel cells finally going to be the next big thing?
“The biggest difference in five years,” Samuelsen at UC Irvine said, “will be a robust market deployment of fuel cell product domestically and internationally, with the recognition that fuel cells are the product of choice for distributed generation due to the efficiency and environmental attributes, with the ability to produce clean renewable power 24/7.”
If they can meet that potential, fuel cells will upend the energy landscape.