Fuel Cell Technology

A fuel cell is an electrochemical device that combines hydrogen fuel with oxygen to produce electricity, heat and water. The fuel cell is similar to a battery in that an electrochemical reaction takes place as long as fuel is available. The hydrogen fuel is stored in a pressurized container and oxygen is taken from the air. Because of the absence of a burning process, there are no harmful emissions, and the only by-product is pure water.

Fundamentally, a fuel cell is electrolysis in reverse, using two electrodes separated by an electrolyte. The anode (negative electrode) receives the hydrogen and the cathode (positive electrode) collects the oxygen. A catalyst at the anode separates hydrogen into positively charged hydrogen ions and electrons; the oxygen is ionized and migrates across the electrolyte to the anodic compartment, where it combines with hydrogen. A single fuel cell produces 0.6–0.8V under load. To obtain higher voltages, several cells are connected in series. Figure 1 illustrates the concept of a fuel cell.

Figure 1: Concept of a fuel cell The anode (negative electrode) receives the hydrogen and the cathode (positive electrode) collects the oxygen. Source: US Department of Energy, Office of Energy Efficiency and Renewable Energy

Fuel cell technology is twice as efficient as combustion in turning carbon fuel to energy. Hydrogen, the simplest chemical element (one proton and one electron), is plentiful and exceptionally clean as a fuel. Hydrogen makes up 90 percent of the universe and is the third most abundant element on the earth’s surface. Such a wealth of fuel would provide an almost unlimited pool of clean energy at relatively low cost. But there is a hitch.

Hydrogen is usually bound to other substances and “unleashing” the gas takes technology and a substantial amount of energy. In terms of net calorific value (NCV), hydrogen is more costly to produce than gasoline. Some say that hydrogen is nearly energy neutral, meaning that it takes as much energy to produce as it delivers at the end destination. Read more about Batteries Against Fossil Fuel.

Storage of hydrogen poses a further disadvantage. Pressurized hydrogen requires heavy steel tanks, and the NCV by volume is about 24-times lower than a liquid petroleum product. In liquid form, which is much denser, hydrogen needs extensive insulation for cold storage.

Hydrogen can also be produced with a reformer by means of extraction from an existing fuel, such as methanol, propane, butane or natural gas. Converting these fossil fuels into pure hydrogen releases some leftover carbon, but this is 90 percent less harmful than what comes from the tailpipe of a car. Carrying a reformer would add weight to the vehicle and increase its cost. Reformers are also known to be sluggish.

The net benefit of hydrogen conversion is in question because it does not solve the energy problem. With the availability of hydrogen through extraction, the fuel cell core (stack) to convert hydrogen and oxygen to electricity is expensive and the stack has a limited life span. Burning fossil fuels in a combustion engine is the simplest and most effective means of harnessing energy, but this contributes to pollution.

Sir William Grove, a Welsh judge and gentleman scientist, developed the fuel cell concept in 1839, but the invention never took off. This was in part due to the rapidly advancing internal combustion engine, which promised better results. It was not until the 1960s that the fuel cell was put to practical use during the Gemini space program. NASA preferred this clean power source to nuclear or solar power. The alkaline fuel cell system generated electricity and produced the drinking water for the astronauts.

High material costs made the fuel cell prohibitive for commercial use at that time. This did not hinder Karl Kordesch, the co-inventor of the alkaline battery, from converting his car’s power source to an alkaline fuel cell in the early 1970s. Kordesch drove his car for many years in Ohio, USA. He placed the hydrogen tank on the roof and utilized the trunk to place the fuel cell as well as backup batteries. According to Kordesch, there was “enough room for four people and a dog.” The 1990s brought renewed interest in the fuel cell; however, this enthusiasm started to diminish again in the 21st century.

Just as there are different battery chemistries, so also are there several fuel cell systems to choose from. Let’s look at the most common types and examine the applications.

Proton Exchange Membrane Fuel Cell(PEMFC)

The proton exchange membrane, also known as PEM, uses a polymer electrolyte. PEM is one of the furthest developed and most commonly used fuel cell systems; it powers cars, serves as a portable power source and provides backup power in lieu of stationary batteries in offices. The PEM system allows compact design and achieves a high energy-to-weight ratio. Another advantage is a relatively quick start-up when applying hydrogen. The stack runs at a moderate temperature of 80°C (176°F) and has an efficiency of 50 percent. (In comparison, the internal combustion engine is only about 25 percent efficient.)

The limitations of the PEM fuel cell are high manufacturing costs and complex water management systems. The stack contains hydrogen, oxygen and water. If dry, water must be added to get the system going; too much water causes flooding. The system requires pure hydrogen; lower fuel grades can cause decomposition and clogging of the membrane. Testing and repairing a stack is difficult, given that a 150V, 50kW stack to power a small car requires 250 cells.

Extreme operating temperatures are a further challenge. Freezing water can damage the stack, and the manufacturer recommends heating elements to prevent ice formation. When the fuel cell is cold, start-up is slow and the performance is poor at first. Excessive heat can also cause damage. Controlling the operating temperatures as well as supplying enough oxygen requires compressors, pumps and other accessories that consume about 30 percent of the energy generated.

If operated in a vehicle, the PEMFC stack has an estimated service life of 2,000-4,000 hours. Start-and-stop conditions induce drying and wetting that contribute to membrane stress. Running continuously, the stationary stack is good for about 40,000 hours. Stack replacement is a major expense.

Alkaline Fuel Cell(AFC)

The alkaline fuel cell has become the preferred technologyfor aerospace, including the space shuttle. Manufacturing and operating costs are low, especially for the stack. While the separator for the PEM costs between $800 and $1,100 per square meter, the same material for the alkaline system is almost negligible. (The separator for a lead acid battery costs $5 per square meter.) Water management is simple and does not need compressors and other peripherals. A negative is that AFC is larger in physical size than the PEM and needs pure oxygen and hydrogen as fuels. Carbon dioxide in a polluted city can poison the stack.

Solid Oxide Fuel Cell (SOFC)

Electric utilities use three types of fuel cells, which are molten carbonate, phosphoric acid and solid oxide fuel cells. Among these choices, the solid oxide (SOFC) is the least developed but has received renewed attention because of breakthroughs in cell material and stack design. Rather than operating at the very high operating temperature of 800–1,000°C (1,472–1,832°F), a new generation of ceramic material has brought the core down to a more manageable 500–600°C (932–1,112°F). This allows the use of conventional stainless steel rather than expensive ceramics for auxiliary parts.

High temperature allows direct extraction of hydrogen from natural gas through a catalytic reforming process. Carbon monoxide, a contaminant for the PEM, is a fuel for the SOFC. Being able to accept carbon-based fuels without a designated reformer and delivering high efficiency pose significant advantages for this type of fuel cell. Cogeneration by running steam generators from the heat by-product raises the SOFC to 60 percent efficiency, one of the highest among fuel cells. As a negative, high stack temperatures require exotic materials for the core that add to manufacturing costs, lower longevity and decrease reliability. With the newer SOFC systems operating at lower temperatures, however, this drawback has been reduced.

Direct Methanol Fuel Cell (DMFC)

During the past years, portable fuel cells, also known as miniature or micro fuel cells, have gained public attention, and the most promising development is the direct methanol fuel cell. This small fuel cell is inexpensive to manufacture, convenient to use and does not require pressurized hydrogen gas. DMFC provides a reasonably good electrochemical performance, and charging occurs by simply replacing the fuel cartridge. This enables continued operation without downtime. Fuel cells with liquid fuels (ethanol or methanol) have a further advantage over hydrogen in the automotive market in that the fuel can be transported, stored and dispensed with known technologies. Hydrogen, on the other hand, exhibits safety risks, storage problems and needs large investments in special pipelines. Read about the miniature fuel cell.

Table 3 describes the applications and summarizes the advantages and limitations of common fuel cells. For completeness of listing, the table also includes the Molten Carbonate (MCFC) and Phosphoric Acid (PAFC), two varieties not described in the text above. These two fuel cell versions have been around for a long time also but have received less publicity than the others.
 

Type of Fuel Cell	Applications	Advantages	Limitations	Status
Proton Exchange Membrane(PEMFC)	Medium to large systems for portable, stationary and automotive	Compact design; long operating life; offers quick start-up, 50% efficient	Expensive to build; needs pure hydrogen; complex heat and water management.	Practical and most widely developed
Alkaline(AFC)	Space (NASA), terrestrial transport, submarines	Low manufacturing, operation costs; no compressor; fast cathode kinetics	Large size; needs pure hydrogen and oxygen	New interest due to low manufacturing, operating costs
Molten Carbonate(MCFC)	Large-scale power generation	Efficient; co-generation utilizes heat to run turbines	Electrolyte instability; limited service life	Well developed; semi-commercial
Phosphoric Acid(PAFC)	Medium to large power generation	Lenient to fuels; for cogeneration	Low efficiency; limited service life; expensive catalyst	In competition with PEMFC
Solid Oxide(SOFC)	Medium to large power generation	Lenient to fuels; uses natural gas; 60% efficient with cogeneration	High temperatures; exotic metals; high manufacturing costs; short life	New material, stack design sets off renewed development
Direct Methanol(DMFC)	Portable, mobile and stationary use	Compact; feeds directly off methanol; no compressor	Complex stack; slow load response; 20% efficient	Liquid fuels are easier to handle than hydrogen

Table 3: Advantages and disadvantages of various fuel cell systems
The development of the fuel cell has not advanced at the same pace as batteries; a direct battery replacement is not yet feasible.

Developments

In spite of environmental benefits, the fuel cell requires extensive development before it can compete in industrial and consumer markets. The existing problems revolve around slow start-up times, low power output, sluggish response on power demand, poor loading capabilities, narrow power bandwidth, short service life and high cost. These negative traits are especially noticeable for the direct methanol fuel cell. Similar to batteries, the performance of all fuel cells degrades with age, and the stack gradually loses efficiency.

The relatively high internal resistance of full cells poses a challenge. Each cell of a stack produces about one volt when in open-circuit condition, and a heavy load causes a notable voltage drop. Figure 4 illustrates the voltage and power bandwidth as a function of load.

Figure 4: Power band of a portable fuel cell High internal resistance causes the cell voltage to drop rapidly with load. The power band is limited to between 300 and 800mA. Courtesy of Cadex

Fuel cells operate best at a 30 percent load factor; higher loads reduce efficiency. A load factor approaching 100 percent, as is common with a battery, is not practical with the fuel cell. In addition, the fuel cell has poor response characteristics and takes a few seconds to react to power demands. Rather than acting as a stand-alone engine, as the developers had hoped, the fuel cell works as a support function by giving the battery the master status. The fuel cell becomes a slave providing the charge duty. This relationship enables both parties to deliver continuous service.

Having failed in the automotive field, fuel cell manufacturers explore new applications. Large 40,000kW fuel cells are being built to generate electricity in remote locations. Fuel cells also replace battery banks and diesel generators in office buildings, as they can be installed in tight storage places and on rooftops with minimal maintenance. Forklifts running non-stop are further candidates for fuel cell technology, which allows continuous, pollution-free operation.

Paradox of the Fuel Cell

The fuel cell enjoyed the height of popularity in the 1990s, when scientists and stock promoters envisioned a world run on a clean and inexhaustible resource — hydrogen. They predicted that cars would run on fuel cells and households would generate electricity from back-yard fuel cells. The stock prices skyrocketed, and it took a few years before marginal performance, high manufacturing costs and short service life brought the hydrogen dream down. Hype and investment funding have since moderated, and it is hoped that a more sensible approach will eventually find the proper use for the fuel cell.

It had been said that the fuel cell would transform the world as the microprocessor did. Experts further claimed that using an inexhaustible source of fuel, hydrogen, would improve the quality of life, and the environmental consequences of burning fossil fuels would be solved forever. From 1999 through 2001, more than 2,000 organizations were actively involved in fuel cell development, and four of the largest public fuel cell companies in North American raised over a billion dollars in public stock offerings. What went wrong?

Hydrogen is not a source of energy per se. We must look at it as a medium to transport and store energy. When envisioning “burning an endless supply of hydrogen,” we need to first produce the resource because hydrogen is not available abundantly in the earth, ready to burn, as oil and natural gas are. To retrieve the hydrogen for fuel, we need energy to convert the resource into a usable product in a similar way as we use electricity to charge a flat battery. If electricity produces hydrogen, then this energy source should come from a renewable resource. This is often not the case; much of our current energy comes from burning coal, oil and natural gas.

Fossil fuel lends itself well to producing hydrogen, but taking this valuable fuel to unleash hydrogen does not make much sense when it costs as much or more for extraction than burning it directly. It is conceivable that the fuel cell will never become the engine that scientists had hoped, just as the attempt to fly airplanes on steam failed in the mid 1800s.

Hydrogen is, however, being used to propel satellites and space vehicles into orbit. Liquid hydrogen has the highest energy-to-mass ratio; but the specific energy by volume reveals a truer picture when considering storage and delivery for terrestrial use. Gasoline has almost 24 times the specific energy of pure hydrogen by volume.

Is there hope for the fuel cell? Many hope it will succeed. Taxpayers may one day need to subsidize this clean energy source similar to subsidizing the electric car. Furthermore, governments may mandate its use for environmental reasons or as an alternative for the dwindling fossil fuels. Wehope that the development of fuel cells will eventually succeed in finding a replacement for the polluting internal combustion engine.