Issue Date: September 25, 2017
Fuel-cell cars finally drive off the lot
In brief
The idea of powering a car with a fuel cell has been around for decades. In principle, these cars, which run on electricity generated on board by electrochemically combining hydrogen with oxygen from the air, could reduce global dependence on petroleum while emitting just water from their tailpipes. But despite extensive fleet testing, fuel-cell passenger cars have always seemed to be another five years away. No longer. Motorists can now buy or lease their very own fuel-cell cars. The numbers today are low, and the cars are available only in a few geographic regions equipped with public hydrogen-filling stations. But the industry is gearing up to manufacture more of these cars and expand refueling infrastructure. And researchers continue to look for ways to reduce fuel-cell costs and improve durability.
Raymond Lim, a psychology and statistics instructor, describes himself as an "automobile enthusiast who likes to try out new technology." Celso Pierre also has a thing for cool gadgets. He's a mechanical engineer who loves hiking and the great outdoors. Anytime Pierre hears about new technology, he rushes to learn about it. For both men, that excitement has long included electric vehicles and fuel-efficient cars. So Lim and Pierre jumped at the opportunity to join the small but growing number of motorists who zip around California's roadways in their own fuel-cell vehicles. Lim drives a Toyota Mirai and Pierre motors around in a Hyundai Tucson.
These hydrogen-powered, all-electric cars have been in development for decades as alternatives to conventional cars; they do not depend on fossil fuels and do not pollute—they emit just water vapor. During that time of development, numerous prototypes and fleets of fuel-cell demonstration vehicles logged millions of miles, advancing the transportation technology far beyond the laboratory test stage. Yet industry watchers grew disheartened at the seemingly endless delays that kept fuel-cell vehicles from auto dealers' showrooms. And upon hearing projections year after year that these cars would hit the market "five years down the road," technology enthusiasts figured the automobile industry had largely given up on mass-producing fuel-cell cars.
That impression is just plain wrong. The industry continued working away on the technology, and those "five years down the road" projections finally came true in the past couple of years. Although the numbers of fuel-cell cars for sale or for lease today are relatively low and the vehicles are available only in select geographical areas, it is finally possible for a private motorist to drive one off the lot. Meanwhile, industry is expanding the hydrogen-refueling infrastructure in the U.S. and other countries and continuing to find ways to make the vehicles cheaper and more durable.
Rise of the fuel cell
The fuel-cell concept dates back to the 1800s. But it wasn't until the past century that various types of demonstration units proved that these electrochemical devices could reliably produce electric current. They came to be recognized as reliable devices when the U.S. National Aeronautics & Space Administration used these power generators in the 1960s and 1970s in the Gemini and Apollo missions and other space programs.
Similar to their electrochemical cousin the battery, fuel cells contain electrodes that extract usable electricity from chemical reactions. In both batteries and fuel cells, redox reactions occur when a positive electrode is connected to a negative electrode through an external circuit. When oxidation reactions take place at an anode and reductions proceed at a cathode, electrons flow through the circuit, powering the device connected to it—an electric motor, in the case of a fuel-cell car.
But unlike batteries, which store the oxidant and reductant within the electrochemical package, fuel cells draw oxidizers and fuels from the outside. As a result, fuel cells don't get used up or need to be recharged like batteries do. In principle, fuel cells can continue generating electricity as long as fresh reactants continue to flow into the devices.
Numerous types of fuel cells have made their way through research and development stages, and several versions have been commercialized. The devices differ principally in terms of the electrolyte, which is the medium that transports ions between the electrodes; the materials that make up the electrodes and other components; and the intended application.
Fuels also vary from device to device. In a basic fuel cell, hydrogen serves as the fuel and oxygen as the oxidant. But there are also systems that derive hydrogen from alcohols or hydrocarbons, as well as ones that use methanol directly, without first converting it to hydrogen.
Fuel cells in automobiles rely on a polymer electrolyte membrane (PEM). The micrometers-thick film serves two functions: It's a solid electrolyte that conducts hydrogen ions from the anode to the cathode, and it's a gas separator that prevents direct, uncontrolled mixing of hydrogen and oxygen. Such mixing wastes fuel, causes the fuel cell to operate inefficiently, and leads to by-products that can degrade fuel-cell components.
The number of fuel-cell vehicles has been growing steadily since they entered the retail market in mid-2015, when Toyota began selling them in Japan and California. Hyundai and Honda have also moved into the retail market, and so the numbers are starting to climb.
In 2016, Toyota boosted production of its four-seat fuel-cell car, the Mirai, which means "future" in Japanese, from the 2015 level of 700 units to approximately 2,000 cars. This year the carmaker plans to produce about 3,000 of them.
According to Bo Ki Hong, a research fellow at Hyundai's Fuel Cell Research Lab, the South Korean carmaker expects to produce about 1,000 of its Tucson Fuel Cell compact sport-utility vehicles by the end of this year and distribute them to 18 countries. Honda is producing similar numbers of its Clarity, a sporty five-passenger fuel-cell sedan. And all three automakers, which are currently the only companies selling or leasing fuel-cell passenger cars in the U.S., collectively aim to boost production levels to the tens of thousands by the end of the decade.
So what allowed fuel-cell cars to move from perpetually five years away from dealership lots to finally parking in people's garages? To begin with, carmakers have continuously been gaining engineering and manufacturing experience, which has helped lower production costs. They have also steadily improved the efficiency of PEM fuel cells and learned how to significantly reduce the amount of costly platinum needed to make the devices work effectively. Those advances translate to less-expensive, smaller, and more-powerful devices that provide flexibility to design cars in a range of sizes and prices attractive to customers.
How hydrogen powers a car
Room for growth
But whether or not carmakers will reach their production goals will depend in large part on how satisfied owners are with their fuel-cell cars. "Customers expect the same level of performance and overall driving experience they get with gasoline- and diesel-powered vehicles," Hong says.
Lim raves about the handling and performance of his Mirai. "This car is wonderful," he says. "The ride is smooth, quiet, and powerful." And when it comes to refueling, the process is quick—"less than five minutes, and that gets me over 300 miles [about 480 km] of driving," he says.
These similarities to gasoline-powered vehicles stand out as advantages for fuel-cell vehicles over battery-powered, all-electric cars. Many of those kinds of cars, which are also known as plug-in electrics, require from 30 minutes to 12 hours for a full charge, depending on the type of charger. And many of them travel less than 150 miles (about 240 km) per charge.
Those factors seem to make a strong case for fuel-cell vehicles. But fuel-cell cars need hydrogen, and currently there are only 29 retail hydrogen filling stations in the U.S., all in California.
"It's a chicken-and-egg scenario," says Joseph Cargnelli, chief technology officer at Hydrogenics, a Toronto-area fuel-cell manufacturer.
Fuel-cell carmakers hesitate to ramp up production if customers don't have convenient access to hydrogen, he says. And gas suppliers are iffy about building hydrogen filling stations without ample demand for the fuel.
But the number of hydrogen stations is about to grow. California expects to see 36 more stations by 2018, half in the north and half in the south.
Hydrogen filling stations are also coming to the Northeast. According to Jana L. Hartline, a Toyota communications manager, Toyota, in partnership with Air Liquide, is supporting construction of 12 hydrogen fueling stations in New York, New Jersey, Massachusetts, Rhode Island, and Connecticut. The first of those stations should be completed before the end of the year, she says. And in Japan, Air Liquide, Toyota, and nine other Japanese companies agreed to build 160 hydrogen stations and aim to put 40,000 fuel-cell vehicles on Japan's roads by 2020.
Fuel-cell passenger cars massively outnumber other types of vehicles powered by this electrochemical technology, and as a result, they get the most attention. Yet other vehicle types have seen notable success. For example, nonpolluting, fuel-cell-powered transit buses have traversed congested city streets since the early 2000s. According to a U.S. Department of Energy report, worldwide, 370 fuel-cell buses were delivered or were on order in 2015.
Also, although 18-wheelers aren't likely to be propelled down the highway by fuel cells anytime soon, Toyota earlier this year began experimenting with one prototype semitrailer at the Port of Los Angeles.
Fuel-cell forklifts rack up far larger numbers than higher road vehicles. Major warehouse operators in North America, including Amazon, Walmart, and FedEx, use some 15,000 of these indoor vehicles to shuttle products and equipment to and fro. Unlike standard battery-powered versions, these fuel-cell-powered versions don't have to sit idle for 30 minutes or longer to recharge.
Problems to solve
Even as the numbers of commercially available fuel-cell vehicles rise, researchers continue to search for ways to reduce costs and improve durability. One of the best-studied options for lowering the sticker price calls for reducing the amount of platinum used as the fuel-cell electrode catalyst to mediate the electrochemical reactions.
"Platinum has long been the poster child for fuel-cell cost," says Mark F. Mathias, director of fuel-cell R&D at General Motors. In the past 10 years, GM, like other manufacturers of fuel-cell vehicles, has succeeded in reducing the amount of platinum used in a vehicle's fuel-cell stack from roughly 80 g to below 30 g per vehicle. Currently, 10 g per vehicle is the target, a value that is well within reach, Mathias says.
Fuel-cell makers have employed numerous strategies to reduce the mass of precious metal required while ensuring that the fuel cells operate reliably. One key approach has been dispersing the platinum as thoroughly as possible—for example, on a high-surface-area carbon support material, which maximizes the surface area of platinum available for fuel-cell reactions.
Some researchers are exploring the possibility of reducing costs by avoiding platinum and other precious metals altogether. Piotr Zelenay of Los Alamos National Laboratory and colleagues have prepared catalysts consisting of nitrogen, carbon, and an inexpensive transition metal such as iron or cobalt that exhibits high activity for the oxygen-reduction reaction, a key process in hydrogen-driven fuel cells.
Boosting the performance of these promising catalysts requires a detailed understanding of the nature of their active sites. So the Los Alamos group teamed up with researchers at Oak Ridge National Laboratory to analyze Fe-N-C fuel-cell catalysts by using advanced electron microscopy and computational techniques. The study revealed a carbon-embedded nitrogen-coordinated FeN4 unit as the species most likely responsible for the high catalytic activity (Science 2017, DOI: 10.1126/science.aan2255).
Meanwhile, the University of Delaware's Ajay K. Prasad, Dionisios G. Vlachos, and coworkers are working on improving fuel-cell durability. The team devised a simple, low-cost way to improve the durability of the perfluorosulfonic acid membranes commonly used in PEM fuel cells. Leakage of even a small amount of hydrogen and oxygen through the membrane, a common problem, leads to reactions responsible for degrading these membranes and shortening device lifetimes. The leak, or "crossover," results in the formation of hydrogen peroxide and free radicals, including OH• and OOH•, that attack the membrane's C–S bonds and form defects and eventually pinholes. The team showed that treating the membrane with inexpensive tungsten carbide nanoparticles extends its lifetime via a mechanism in which the particles capture the free radicals and inhibit formation of hydrogen peroxide (Nat. Commun. 2017, DOI: 10.1038/s41467-017-00507-6).
As automakers continue to lower vehicle sticker prices and boost production of fuel-cell passenger cars, more and more private motorists will have the opportunity to own one. Some will be motivated by the cars' environmentally friendly zero emissions, while others will be drawn by a love of latest technology or intrigued by hydrogen power.
Still others may catch the bug directly from proud Mirai owner Lim. "I get asked about the car all the time," he says gleefully. "People are always looking in my direction and waving at me." Lim enjoys talking with them about his hydrogen-powered fuel-cell vehicle and makes sure to keep a large stack of car brochures handy. "I've already passed out more than 100 of them," he says.
- Chemical & Engineering News
- ISSN 0009-2347
- Copyright © 2017 American Chemical Society
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