Fuel Cells: A Primer on the Coming Hydrogen Economy

Understanding Fuel Cells

Fuel cells are not magic. What a fuel cell does and how it does it are well known. Nor are fuel cells very new. They were invented by Sir William Grove, a Welsh judge and gentleman scientist, in 1839. Fuel cells found their first practical use in the U.S. space program in the 1960s, and they have been a key component of power production in space ever since.
Fuel cells are somewhat like batteries. Every battery has two electrodes (an anode and a cathode) separated by an electrolyte. Chemical reactions take place in or near the electrodes, producing free electrons at the anode, which flow out of the battery, through the load (e.g., lamp, motor or other device requiring an electrical current) and then back into the cathode of the battery. Over time, the reactive chemicals are used up and the battery goes dead.

In a fuel cell, there are also two electrodes and an electrolyte. But instead of relying on chemical energy that was stored in the battery, the chemical energy is supplied from external sources. Plus, instead of the chemical energy sources being complicated inorganic compounds (some of which are toxic) such as manganese, zinc, and lead, the chemical energy inputs are simply hydrogen and oxygen. Thus, the only significant emissions from a basic fuel cell are water (H₂O) and heat, and the cell continues to run as long as hydrogen and oxygen are supplied.

Very simply, the fuel cell does the reverse of the common high school chemistry experiment in which electric current is used to split water (H₂O) into hydrogen (H₂) and oxygen (O₂)—a process known as electrolysis. In a fuel cell, the H₂ and O₂ are combined to form H₂O plus free electrons (electrical current).
In the proton-exchange-membrane (PEM) fuel cell (see schematic)—the type most actively being pursued today—hydrogen is fed into the anode side of the fuel cell, and oxygen (usually just air, which contains 21% oxygen) is fed into the cathode side. Special catalysts are located on the two electrodes, and this is where the action happens. The catalyst (usually made out of platinum) splits the hydrogen molecule into hydrogen ions (protons) and free electrons. The electrolyte (proton exchange membrane) allows the hydrogen ion (proton) to pass through but blocks the larger H₂ molecules that have not yet been split apart. The free electrons, meanwhile, return to the positively charged cathode through an external conductor (wire). It is this electron flow, or current, that we can tap as usable electricity. Then at the cathode, the positively charged hydrogen ions and free electrons react with the oxygen molecules (O₂)—the process of oxidation—forming water and releasing heat. As on the anode, a catalyst is used to speed up this reaction.
Individual fuel cells (just like common flashlight batteries) produce relatively low electrical potential (voltage), but by connecting a number of them in series (forming a fuel cell stack), any desired voltage can be obtained. These stacks comprise the core of a functioning fuel-cell power plant. As in a battery, the electricity produced by a fuel cell is DC (direct current); a power inverter is needed to convert this to AC wherever alternating current is required.

Why Fuel Cells Are Attractive Environmentally

Fuel cells have several important environmental features. First, there is no high-temperature combustion involved; the electricity generation occurs through a chemical reaction, which in a PEM fuel cell takes place at a temperature less than the boiling point of water. As a result there are no combustion by-products, such as nitrogen oxides (NO_X) , sulfur oxide (SO_X), or particulates. Second, if hydrogen is used as the fuel (more on this below), the only by-products are water vapor and heat. Third, fuel cells are efficient; they produce more electricity per unit of fuel than most conventional power plants. Fourth, there is significant potential for waste heat utilization in combined heat and power or cogeneration units. This can significantly boost overall conversion efficiency for stationary power plants, and it solves one of the more challenging problems with electric vehicles: providing heat during cold weather.
Pollution emissions from various power generation options are shown in Table 1. Although comparative data for carbon monoxide and particulates are not included, the benefits are just as dramatic for these pollutants as for NO_X and SO_X. Reductions in carbon dioxide (CO₂) emissions, although welcome, are less dramatic. This issue is discussed below.
Another important benefit of fuel cells stems from their suitability for on-site power generation. Since they do not contribute to smog and because they operate very quietly, fuel cells are uniquely suited to the world of distributed generation, in which electricity is produced by relatively small power plants at or near the end-users. Distributed generation avoids the habitat disruption, aesthetic impact, and electromagnetic fields produced by high-voltage transmission lines. If the electricity is produced immediately on-site, local distribution lines are also reduced.

Where Does the Hydrogen Come From?

When fuel cells use pure hydrogen as the fuel, they are a wonderfully clean power generation option, whether for stationary power production or for powering electric vehicles. The only emission product is water vapor. The problem is that hydrogen is not yet a readily available fuel.

Today, most fuel cells are supplied with hydrogen from another device called a fuel processor or reformer. The fuel processor, in turn, is supplied with either a gaseous or liquid hydrogen-rich fuel, such as methane, natural gas, methanol, LPG, or gasoline. These fuels consist of organic molecules, each of which contains hydrogen and carbon atoms, and sometimes other atoms as well. Natural gas (which is mostly methane, CH₄) and methanol (CH₃OH, also called methyl alcohol) have high ratios of hydrogen to other atoms, making them particularly attractive as fuels, even though we cannot yet buy them at the local service station. Gasoline, although widely available and convenient to use, is not a pure chemical, but rather is a varying mixture of hundreds of different molecules; no two refineries make precisely the same mixture. Fuel processors use heat (often as steam) and catalysts to separate the hydrogen. The chemistry is complex and all of the engineering requirements of safety, reliability, cold-weather operation, compact size, low weight, and low cost have not yet been solved.
When hydrocarbons are reformed to obtain hydrogen, the carbon is released as carbon dioxide (CO₂). In steam reforming of methane (CH₄), a catalyst is first used to break the methane into a mixture of hydrogen gas (H₂) and carbon monoxide (CO). Water molecules from the steam then react with the CO to create carbon dioxide (CO₂) and more hydrogen gas. The net result of these reactions is that one molecule of methane is converted into one molecule of CO₂ and four molecules of H₂, with the aid of two water molecules. In practice, these reactions aren’t all 100% complete; a little less hydrogen is produced, a little of the unreacted carbon monoxide and methane are released as emissions. If some of the hydrogen is burned to generate heat for producing steam, the net hydrogen production is reduced.
It surprises many people to learn that hydrogen is produced on a huge scale daily in oil refineries and chemical plants around the world; it is a proven, safe, and economic process. The Praxair Corporation, for example, one of the major industrial gas production companies, supplies more than 50 refineries and petrochemical companies from its 22 methane reforming plants, and distributes more than 500 million cubic feet (15 million cubic meters) of hydrogen each day through a network of 280 miles (450 km) of hydrogen pipelines in the Gulf Coast states.
A cubic foot of natural gas powering a fuel cell releases just as much global-warming-inducing CO₂ (during the reforming stage) as a unit of natural gas burned to operate a gas turbine. And a gallon of gasoline releases just as much CO₂ whether used in a fuel processor/fuel cell, or burned in an internal combustion engine. However, because the overall efficiency of power conversion is higher with a fuel cell typically than with direct combustion, less CO₂ is emitted per kilowatt-hour of electricity or produced per mile traveled.

Hydrogen from water

Due to the high cost of renewable electricity generation, though, there is active research being done on alternative ways of producing hydrogen sustainably. Researchers in Japan and Spain have independently succeeded in splitting water into hydrogen and oxygen at room temperature using special catalysts: cuprous oxide at the Tokyo Institute of Technology and molybdenum at the University of Valencia. Others feel that biological sources of hydrogen may be more attractive. Certain enzymatic reactions produce hydrogen as a by-product. In principle, the proper organisms could be farmed in large ponds or lagoons, and the hydrogen emissions collected for use as fuel. Scientists at Utah State University, for example, have discovered an enzyme in the soil microorganism Clostridium pasteurianum that produces hydrogen. According to the January 1999 Technology Update, published by Fuel Cells 2000, rather than splitting water into its constituent elements, this enzyme uses iron atoms to combine protons and electrons into hydrogen.

Hydrogen from landfills and sewage treatment plants

There is another source of hydrogen that makes fuel cells not merely clean, but actually beneficial to the environment. Landfills and wastewater treatment plants both release methane from the decomposition of organic matter under anaerobic conditions. As a greenhouse gas, methane (CH₄) is 21 times more potent than CO₂ (over a time horizon of 100 years). For this reason methane from landfills is sometimes collected and burned—either with or without recovering the energy. Burning the methane converts it to CO₂ and water, reducing its global-warming impact dramatically. But reforming that methane in a fuel processor and using the hydrogen in a fuel cell is even better, because it generates higher-value electricity without producing any of the smog that comes from burning.

Hydrogen from fertilizer?

Fuel cell expert Doug Holmes, of minergy associates in Lexington, Massachusetts, points out that there is another very common liquid that could be used as a hydrogen source, but it almost never comes up in discussions: ammonia (NH₃). Ammonia is one of our most common fertilizers; millions of gallons are produced and used annually on our crops. Ammonia has no carbons at all. If you dissociate ammonia, you get molecular nitrogen (N₂), which accounts for roughly 79% of our atmosphere, and hydrogen (H₂). Liquid ammonia has a higher content of H₂ (in grams per liter) than even liquid hydrogen! But because of the smell and the toxic gas produced in a spill, the U.S. Department of Energy (DOE) rarely, if ever, mentions ammonia as a potential hydrogen source for fuel cells. (Also, conventional production of ammonia—from atmospheric nitrogen, N₂—is fairly energy-intensive.)

Storing and Moving Hydrogen

There is a lot of talk about the transport of hydrogen for fuel cells, but most of this discussion relates to fuel cell vehicles. When we are using fuel cells in stationary power-generation systems, such as in a building, transporting hydrogen is not usually a concern—at least in the near-term. Instead of moving hydrogen around, we will transport hydrogen-rich fuels (natural gas, methanol, gasoline, etc.) using pipelines and compression technologies that are already widely in use throughout the country. Then we reform those fuels on-site in our fuel processor to generate hydrogen, which is used in our fuel cell.
Once we begin to generate hydrogen directly from water using PV power, catalytic reactions, or biological activities, however, we will have to store and move that hydrogen. Developers of fuel cells for vehicles are already working on ways to store and transport hydrogen, because using pure hydrogen in a vehicle means that the fuel processor can be eliminated, saving space and weight. By the time we need to deliver pure hydrogen to homes and businesses around the country, it is likely that the technological challenges of doing so will have long-since been solved. (As described above, pure hydrogen is already widely transported in various industries.)

Types of Fuel Cells

There are a number of different types of fuel cells, each using different materials and systems but operating with the same basic principles. While the various technologies are often presented as competing with each other, each has its own characteristics and potential applications, so they will doubtless each find distinct markets over time. Five of the more established technologies are described here.

Proton exchange membrane (PEM) fuel cells are the type being researched most widely today. These are also known as solid-polymer fuel cells because the electrolyte is a thin polymer film. PEM fuel cells are considered by DOE to be the leading contender for light-duty vehicles, buildings, and very small applications, such as video camcorder operation. A number of PEM fuel cell prototypes are operating today, including several from Ballard Power Systems that are operating city buses in Vancouver, Canada, and a 7 kW residential system developed and operated by Plug Power in Latham, New York. A handful of other companies are testing PEM fuel cells for residential and light commercial applications. Most are working with units sized to supply about 3 kW continuously, with batteries to provide short term peaks up to 10 kW. PEM fuel cells generate electricity at 35% to 40% efficiency, and operate at 140°F to 212°F (60°C to 100°C).
Phosphoric acid fuel cells are the only type in commercial production for building applications today. The ONSI subsidiary of United Technologies, Inc. has sold more than 120 200-kW fuel cells over the past few years. They currently cost about $800,000 each, produce electricity at roughly 40% efficiency, and are up to 85% efficient if the heat is used for cogeneration. Operating temperatures are about 400°F (200°C).
Molten carbonate fuel cells operate at high temperatures—about 1,200°F (650°C)—and look promising for large commercial buildings and utility-scale power generation. A nickel catalyst is used, and the electrolyte is a mixture of molten lithium and potassium carbonate. Because of the high temperature and the resistance of the catalyst to poisoning, carbon-based fuels can be internally reformed, obviating the need for a separate fuel processor. The high operating temperatures also provide for the possibility that waste heat can be used for many industrial processes, or to run efficient absorption chillers in commercial buildings.
Solid oxide fuel cells use a hard ceramic material instead of a liquid electrolyte. These fuel cells operate at very high temperature—up to 1,800°F (1,000°C)—and are generally considered most appropriate for utility-scale power production. Power generation efficiencies can be as high as 65%. Like molten carbonate fuel cells, the high operating temperature means that readily available fuels can be used directly. This is the least well developed of the five most common fuel cells, but it is considered to offer a lot of potential because of the high efficiency and useful waste heat.
Alkali fuel cells satisfy the very small niche of power supply systems for space vehicles. They operate at very high efficiency—up to 70%—and the pure water produced is a benefit for manned space flight. The electrolyte is potassium hydroxide, and a large amount of platinum is required as the catalyst, keeping the cost high. Also, the catalysts used in alkali fuel cells are degraded or poisoned by carbon dioxide and other contaminants, so high-purity hydrogen is needed, which increases operating costs.

Commercializing Fuel Cells

As noted above, there is currently only one company that is producing fuel cells on what can truly be considered a commercial basis. A handful of other companies, however, have produced working prototypes of fuel cells, and these are operating around the world. The table below lists some of the companies that have announced near-term availability of fuel cells for building applications.
According to an August 1998 report from Business Communications Company entitled Fuel Cells: On the Verge (report #RE069N, cost $3,250), the market for all five types of fuel cells will expand dramatically in the next four years. The report projects the total fuel cell market growing from $355 million in 1998 to $1.3 billion in 2003, for an average annual growth rate of 29.5%. The market for PEM fuel cells is expected to grow the most, going from $80 million in 1998 to $450 million in 2003.
Companies seeking to produce fuel cells for residential and commercial buildings are struggling to bring the cost of their devices into a competitive range, which is generally considered to be between $1,000 and $1,500 per kilowatt. The actual life-cycle cost depends on the relative price of electricity and natural gas (or whatever fuel is used), the value of waste heat generated, maintenance costs, and the anticipated life of the fuel cell. While the fuel cell as a whole should last a long time, the stacks tend to gradually lose efficiency. Stacks in today’s phosphoric acid fuel cells are expected to need replacement after five to ten years, according to ONSI spokesman Michael London, but there is little in the way of a track record for the fuel cell industry, and some experts believe that early fuel cells will require major servicing every two to three years—this might involve swapping out the stack for factory refurbishing.
The price of the ONSI PC25 works out to $4,000 per kilowatt, while others are much higher since they are not yet in full production. At these prices the fuel cells are economical only when there is a need for some of their other benefits. The Electric Power Research Institute and utility companies suggest that the prices will have to get much lower before residential units will sell in any significant numbers—probably below $1,000 per kW.
Fortunately for fuel cell producers today, some users are willing to pay a premium price for high-quality, dependable power. Fuel cells provide very clean power free of the spikes and voltage drops that affect the utility grid. According to a report from the Natural Resources Defense Council entitled Choosing Clean Power: Bringing the Promise of Fuel Cells to New York, such high quality power has a value of $1,000 to $4,000 per kilowatt to facilities with sensitive computers or other electronics. And some facilities, such as hospitals, must have back-up power available to protect the health of patients. Fuel cells can be a cost-effective source of back-up electricity, and once installed, they can provide cheap, clean power as a base-load that is supplemented by power from the grid.
Another niche market for fuel cells is off-the-grid applications, where they might compete with solar and wind generation. For a New York City Police Department Precinct in Central Park the existing electrical service has been inadequate. “It would have been very expensive to trench through the park to the building,” says New York Power Authority spokesman Brian Warner. That precinct is now getting its own ONSI PC25 200 kW power plant, which will supply all the facility’s electrical needs for the near future while also providing charging stations for a small fleet of electric vehicles.

Summary: Fuel Cells and Green Buildings

Fuel cells are on their way. Our grandchildren will almost certainly enjoy the electricity and heat they produce—and their skies will be the better for it. The emergence of fuel cells—and the hydrogen economy—won’t happen overnight, however. Far from it. We are now seeing a gradual process of technological refinement, prototype testing, marketing of systems for very specialized needs where the user is willing to pay a premium, and cost reductions—all of which will lead, finally, to widespread adoption.
It is propitious that fuel cells can evolve in two stages—and that the first stage of evolution can be based on infrastructure we have in place today. Fuel cells (with fuel processors) can operate on a wide range of hydrocarbons already in everyday use. Today’s fuel cells based on reformed hydrogen from hydrocarbons (and perhaps even ammonia) will give us a chance to refine the technology—and get used to it. Then, the transition to the second developmental stage of fuel cells—that based on a true hydrogen economy—will be a relatively easy jump. When we can generate, store, and easily transport hydrogen for fuel cells, releasing no CO₂ in the process, we may truly have a sustainable energy source.
Given the promise that hydrogen-powered fuel cells will play an important role in the power system of the future, it makes sense for green designers and builders to follow the emergence of this technology. It may even make sense for those involved with large commercial building design to begin gaining some experience with methane-, methanol- or natural gas-powered fuel cells—at least for projects where high-quality, uninterruptable power is required. For most buildings, especially small commercial and residential applications, we will have to wait another five to ten years for fuel cells to reach a level of economic competitiveness. Companies that invest in the learning curve with fuel cells over the next several years may be in a better position to effectively integrate them into building projects as costs come down and this power system becomes more practical.

– Alex Wilson and Nadav Malin; Technical assistance by Douglas B. Holmes, minergy associates, Lexington, Mass.

For more information:
 

The American Hydrogen Association
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aha@getneet.com
www.clean-air.org (Web site)
Publishes Hydrogen Today
The Hydrogen & Fuel Cell Letter
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Rhinecliff, NY 12574
914/876-5988, 914/876-7599 (fax)
hfclettr@idsi.net (e-mail)
www.mhv.net/~hfcletter (Web site)
Fuel Cell Technology News
Business Communications Co., Inc.
25 Van Zant Street
Norwalk, CT 06855
203/853-4266, 203/853-0348 (fax)
parrish3@gte.net (e-mail)
The Fuel Cell Institute
P.O. Box 65481
Washington, DC 20035
301/681-3532, 301/681-4896 (fax)
Publishes Fuel Cell News
Fuel Cells 2000
1625 K Street NW, Suite 790
Washington, DC 20006
202/785-9620, 202/785-9629 (fax)
www.fuelcells.org (Web site)
Publishes Fuel Cell Quarterly, Fuel Cell Directory, and a monthly Fuel Cell Technology Update (electronic or fax distribution)
U.S. Fuel Cell Council
1625 K Street NW, Suite 790
Washington, DC 20006
202/293-5500, 202/785-9629 (fax)
www.usfcc.com (Web site)
Natural Resources Defense Council’s Choosing Clean Power report available at
www.nrdc.org/nrdcpro/ccp/chap1.html