BU-210: How does the Fuel Cell Work?

Explore the development of the fuel cell and study the different systems. 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 occurs as long as fuel is available. Hydrogen is stored in a pressurized container and oxygen is taken from the air. Because of the absence of combustion, there are no harmful emissions, and the only by-product is pure water. So pure is the water emitted from the proton exchange membrane fuel cell (PEMFC) that visitors to Vancouver’s Ballard Power Systems were served hot tea made from this clean water. Fundamentally, a fuel cell is electrolysis in reverse, using two electrodes separated by an electrolyte. The anode (negative electrode) receives hydrogen and the cathode (positive electrode) collects 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. With most fuels, hydrogen is bonded to other substances and “unleashing” the gas takes 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. (See BU-1007: Net Calorific Value.) 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 fossil fuel 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 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 during the development of the internal combustion engine (ICE) that showed promising 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 that was chosen generated electricity and produced drinking water for the astronauts. High material costs made the fuel cell prohibitive for commercial use. The fuel cell core (stack) is expensive and has a limited life span. Burning fossil fuel in a combustion engine is the simplest and most effective means to harness energy, but it pollutes. High cost did not discourage the late Karl Kordesch, the co-inventor of the alkaline battery, from converting his car to an alkaline fuel cell in the early 1970s. He mounted the hydrogen tank on the roof and placed the fuel cell and backup batteries in the trunk. According to Kordesch, there was enough room for four people and a dog. He drove his car for many years in Ohio, USA, but the only problem, Kordesch told me in person, was that the car did not pass inspections because it had not tail pipe. Here are the most common fuel cell concepts.

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 is 50 percent efficient. (The ICE is 25–30 percent efficient.) On the negative, the PEM fuel cell has high manufacturing costs and a complex water management system. The stack contains hydrogen, oxygen and water, and if dry, water must be added to get the system started; too much water causes flooding. The stack requires chemical grade hydrogen; lower fuel grades can cause decomposition and clogging of the membrane. Testing and repairing a stack is difficult, given that a 150V stack requires 250 cells. Freezing water can damage the stack and heating elements may be added to prevent ice formation. Start-up is slow when cold and the performance is poor at first. Excessive heat can also cause damage. Controlling temperatures and supplying oxygen requires compressors, pumps and other accessories that consume about 30 percent of the energy generated. Operating a PEM fuel cell in a vehicle, the PEMFC stack has an estimated service life of 2,000–4,000 hours. Wetting and drying caused by short distance driving contributes to membrane stress. Running continuously, the stationary stack is good for about 40,000 hours. The stack does not die suddenly but fades similar to a battery. Stack replacement is a major expense.  

Alkaline Fuel Cell (afc)

The alkaline fuel cell has become the preferred technology for 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; efficiency is in the 60 percent range. A negative is that the AFC is larger in physical size than the PEM and needs pure oxygen and hydrogen as fuels. The amount of carbon dioxide present in a polluted city can poison the stack and this limits the AFC to specialized applications.  

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 it 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 poses 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 temperature requires exotic materials for the core that adds to manufacturing costs and reduces longevity.