Hydrogen -- Fuel Cell Revolution

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The Fuel Cell Revolution Rising to meet the energy challenges of tomorrow

by Arvind Ravi

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t’s not every day you get to meet a world record holder. But sit down with Stanford Mechanical Engineering Professor Fritz Prinz, and you’ll have a chance to do just that. Addressing a key component of the hydrogen economy, Prinz and his team boast the world’s lowest operating temperature for a solid oxide fuel cell. Although at first this might not seem like the type of record to write home about, it actually represents a potentially significant change in the way our world will be powered. With efficiencies twice that of the conventional gasoline combustion engines that power cars today, fuel cells are becoming an increasingly attractive solution to our perennial energy concerns. And because the only waste produced is water, they are a positive step toward an emissions-free energy future - something that your children might appreciate. Building on technologies for the production and storage of hydrogen, Prinz’s findings represent the final step of a conversion process that could not only power your transportation, but even your home or office.

powering electronic devices. Upon reaching the cathode, the ions and electrons recombine to generate waste. The variety of possible fuel cells stems from the diversity of electronically insulating electrolyte materials and the assortment of fuels that may be used. In the case of the hydrogen fuel cell, H2 is split into electrons and positive ions—in this case protons—

Hydrogen Fuel Cell Water Out

Hydrogen In

Oxygen In

Inside a Fuel Cell Despite being a hot topic of current research, fuel cells themselves have actually been around for quite a while, even predating the combustion engine. First created in 1843 by Welsh researcher Sir William Grove, fuel cells rely on the separation of fuel into positively charged ions and electrons. This chemical separation and the ensuing recombination are catalyzed at two terminals known as the anode and cathode, respectively. Once generated, the positive ions diffuse toward the cathode through an electron-impermeable electrolyte layer (i.e., one that allows the transport of ions), while the electrons are forced to move around an external wire that reaches the cathode. The motion of these electrons in the external loop serves as the current for

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Energy Out http://www.eia.doe.gov/kids/energyfacts/sources/images/IntHydrogen2.jpg

The basic design of a hydrogen fuel cell. Hydrogen is decomposed into protons and electrons, which recombine with oxygen to form water. Electrons from the hydrogen molecules provide the electrical power that can be used to do work.

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Cover: Hydrogen

which ultimately recombine with oxygen to form water. The particular appeal of such fuel cells is that they release only water into the environment rather than harmful pollutants (e.g., sulfur dioxide from fossil fuel combustion is a common source of acid rain). If combined

With efficiencies twice that of the conventional gasoline combustion engines that power cars today, fuel cells are becoming an increasingly attractive solution to our growing energy concerns.

with technologies that use the electrolysis of water to produce hydrogen, fuel cells could eliminate greenhouse gas emissions altogether.

Unique Challenges, Novel Approaches With all of these advantages over traditional conversion devices, why aren’t fuel cells commercial? According to Prinz, the problem has been largely one of materials, as they must be extremely durable. “Fuel cells work in a relatively harsh environment,” he explains. The first issue is one of temperature. All fuel cells naturally produce heat, and so the electrolyte layer must have good thermal integrity. In addition, one side of the electrolyte is exposed to a strongly oxidizing environment - one which is indiscriminately eager to gather electrons, not only from the external wire circuit, but also from the electrolyte itself. While some are daunted by the challenges of advancing fuel cell technology, researchers like Prinz are only encouraged. “It’s certainly intriguing from a technical perspective,” remarks Prinz.

“Fuel cells work in a relatively harsh -Professor Fritz Prinz environment.”

In addition to investigating technologies that convert hydrogen to electric power, Mechanical Engineering Professor Fritz Prinz is also looking into deriving electric power from biological systems. Here he is shown standing with research colleague Rainer Fasching (right) in the Rapid Prototyping Laboratory at Stanford University as they examine materials that could be used to generate “bioelectricity.”

Fortunately, advances in materials science and microfabrication - the science of small-scale design - have paved the way for better approaches. “Now we have materials and fabrication technology that we didn’t have 20 or 30 years ago. That’s why I think there is a new opportunity towards realizing fuel cells and making them even more efficient than what they are now.” Capitalizing on the same technology that has given rise to microprocessors and flat panel displays, researchers like Prinz are making headway in an area that could one day revolutionize power utilization.

The Goldilocks Problem In exploiting these advances, researchers have tried to use many different materials to make the electrolyte layers. Two popular materials in current research are polymeric electrolyte membranes (PEMs) and solid oxide fuel cells (SOFCs). PEMs tend to be relatively small and easy to manufacture, and able operate best below 100 °C. Because fuel cell operation releases a large amount of heat, some of the energy generated must be used to cool PEM fuel cells, compromising their energy efficiency and

“As engineers, we not only look at problems which are scientifically interesting, but also problems that have societal relevance.” -Professor Fritz Prinz

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Hydrogen powered cars employing fuel cell technology are expected to be competitive with the price of traditional automobiles by the year 2010. The prototypes shown above can travel approximately 125 miles without a refueling, and reach top speeds of around 80 mph.

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making them less than ideal. In addition, PEMs must remain humidified in order to retain ionic conductivity, and so care must be taken to prevent them from drying out. As a result of these concerns, Prinz’s team turned their attention to SOFCs, which also have their limitations. Because SOFCs have restricted ionic mobility at low temperatures, they require significant heating to achieve usable power conversion. Unfortunately, whereas the temperature requirement of PEMs is too low, that of SOFCs is currently too high, requiring temperatures

of around 1000 °C. This temperature is highly correlated to the thickness of the electrolyte layer, so a natural solution to this issue is decreasing the thickness of the membrane to achieve greater conductivity at lower © Ceramic Fuel Cells Ltd. temperatures. A commercial 150 W fuel cell stack. With Prinz’s team has been advances in lowering their operating able to shrink the average temperature, they may become the fuel cell membrane thickness of choice. approximately 100-fold, corresponding to approximately 100 atomic layers. These layers are so thin that they can function at 300-400 °C, much closer to real world conditions. Even at these lower temperatures, the SOFCs perform relatively well, with a power density of approximately 400 mW/cm2, or within 20% of the power density of a conventional PEM cell. In reducing the membranes to a record-breaking thickness, Prinz acknowledges the unique benefits of collaborating with other leading research departments. “That is particularly an advantage at Stanford since we have such an outstanding fabrication facility here,” he says.

Beyond the Laboratory While advances in operating temperatures for fuel cells are promising, they represent just one factor in the energy equation. There are still questions on many fronts before we can realize a hydrogen economy. “Where does [the hydrogen] come from?” Printz points out. “How can we economically store and retrieve it?” The answers to these and other related issues surrounding a hydrogen economy will require the continued dedication of scientists and engineers.

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Fortunately, as the importance of alternative energy Hydrogen technologies becomes more apparent, researchers in these areas are finding increasing support from corporate interests that share their goal of sustainability. Prinz, for instance, is partly funded by Honda, which already has a commercial hydrogen car, the Honda FCX. Although cars like this one are prohibitively expensive for most (the FCX is priced at a mere $1,000,000), the growing interest and investment in a hydrogen economy are leading to rapid progress on a variety of fronts – progress that within a few years may bring this very technology to a garage or neighborhood near you. S

Educating Students About Fuel Cells In addition to his pioneering research, Prinz has taken an active role in fuel cell education, serving as coauthor of the first student textbook in this field. Cowritten with two former students Ryan O’Hayre and Suk-Won Cha in addition to postdoctoral fellow Whitney Colella, “Fuel Cell Fundamentals” aims to provide advanced undergraduates and early graduate students a comprehensive background in fuel cell technology. With over 100 problems, the textbook covers the relevant scientific background necessary to understand fuel cell function, but it also extends beyond these basics to issues of environmental impact stemming from their use. Introduced this year in Prinz’s Spring Quarter course, “Fuel Cell Science Technology,” the book is already garnering attention from Prinz’s colleagues, some of whom are planning to incorporate it into their curricula as well. “There is a big need to educate students in energy” remarks Prinz. With the introduction of this textbook, he is taking a real step towards doing just that: preparing students to rise to the scientific challenges of the future, and in particular, energy sustainability.

Arvind Ravi is a senior majoring in Chemistry and Mathematics. A long-time member and former Editor-in-Chief for Stanford Scientific, he has enjoyed watching the magazine’s presence grow at Stanford and beyond.

To Learn More: Prinz Group Homepage http://me.stanford.edu/faculty/facultydir/prinz.html Fuel Cell Fundamentals by Ryan O’Hayre, Suk-Won Cha, Whitney Colella, and Fritz Prinz. -Student textbook on fuel cells introduced by Professor Prinz

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Because of their high efficiencies, fuel cells are an attractive means of converting stored chemical energy into power. In the above artist’s conception, a solar array/ fuel cell power system for a NASA lunar observatory is shown. Such systems may provide more reliable long-term power than battery-based alternatives through the fourteen-day lunar night.

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