How Fuel Cell Works

Energy Studies

How a Fuel Cell Works

The following information has been obtained from:
Center for energy, Economics & Environmental Policy
Edward J. Bloustein School of Planning and Public Policy.
New Jersey: Opportunities and Options in the hydrogen Economy.
July 2004. Chapter 1 Pages 11-16.

Hydrogen Properties
Hydrogen is the simplest of all elements with one electron and one proton. Two hydrogen atoms form one hydrogen gas molecule, or H2, but this gas is rarely found in large quantities in nature. Hydrogen’s chemical properties allow it to combine easily with other elements to form other molecules. The simplest example is hydrogen’s presence in water, or H2O. As water makes up 70 percent of the Earth’s surface, hydrogen is in abundant supply. Moreover, hydrogen can be extracted from fossil fuels through reformation. Similarly, hydrogen can be extracted from organic materials such as bio-waste, solid waste, landfill gases or biomass (agricultural products specially grown for fuel or parts of agricultural products, such as stalks and stems, not used for human or animal consumption). Hydrogen has the highest energy content by weight of any fuel – 52,000 Btu per pound. [3] Hydrogen gas is nontoxic with no color, odor or taste; a pure hydrogen flame is invisible without special glasses. Like gasoline, hydrogen ignites easily. Hydrogen compared to other gases has a high diffusion rate, the process by which the gas molecules spread out and interact as a result of energy and random motion. This requires that hydrogen be stored in ways to ensure the gas has a reasonable density for applications. Hydrogen can be used to increase efficiency in internal combustion engines (ICEs). It is estimated that a direct-injected hydrogen ICE could have 20-25 percent greater efficiency than a similar gasoline ICE. [4] However, most proponents of hydrogen envision its use to generate electricity when powering a fuel cell. In a fuel cell, the theoretical efficiency can reach 83 percent; in practice 60 percent of hydrogen’s energy is converted to electricity with the rest generating heat energy that can be used in combined heat and power (CHP) applications. Comparing gasoline to hydrogen, the energy in one gallon of gasoline is roughly the equivalent to 1 kg of hydrogen. [5] By weight, hydrogen has about three times the amount of energy as gasoline. Most hydrogen today is not used as a fuel source, but rather as a chemical for oil refining and ammonia production. Hydrogen is used in ammonia production for fertilizer. [7] Hydrogen can also be used in fat hydrogenation, methanol production, welding, and the production of hydrochloric acid. To give an idea of the amount of hydrogen in use in today’s economy, the small amount of merchant hydrogen produced in the United States in 2002, according to one estimate, could suffice to support a fleet of 20-30 million fuel cell cars. [8]

Fuel Cells
The technical understanding of fuel cells has existed since the 19th century. Fuel cells were first created in 1839 by Sir William Grove and refined in 1932 by Francis Bacon. The most well known application of fuel cells was aboard NASA space shuttles to provide electricity to various systems. A fuel cell provides electricity in a manner similar to a battery. Like a battery, a fuel cell produces direct current (DC) power, not alternating current (AC) power. However, the fuel cell can continue to provide energy so long as a fuel is present. A battery, in contrast, has a finite storage of energy before it needs to be recharged. A graphical depiction of the electrochemical process of turning hydrogen fuel into energy using a fuel cell is shown in Figure 2. All fuel cells contain an anode, cathode and electrolyte. The hydrogen fuel is broken into electrons and protons by virtue of a catalyst, and combines with oxygen supplied to the fuel cell to create electricity, water and heat. The hydrogen fuel is fed into the anode (a negative electrode that repels electrons) of the fuel cell. Oxygen enters through the cathode (a positive electrode that attracts electrons). Encouraged by a catalyst, such as platinum, the hydrogen atom splits into a proton and an electron. The electrons cannot permeate the electrolyte and therefore are released through an external current to produce electricity. The hydrogen protons filter through the electrolyte to the cathode. The electrons provide an electrical current before returning to the cathode to be reunited with the hydrogen and oxygen (usually coming from ambient air, but sometimes pure oxygen) in a molecule of water.

Types of Fuel Cells
There are different types of fuel cells that can be used to generate energy. The properties of each fuel cell provide the basis for deciding their most suitable application. The top fuel cell designs are Polymer Electrolyte also known as Proton Exchange Membrane (PEM), Phosphoric Acid, Molten Carbonate, and Solid Oxide. Main characteristics of each fuel cell design presented in Table 1 and the text that follows below are adapted from the U.S. Department of Defense’s online Fuel Cell Information Guide. [10] There are a few other types of fuel cells, but these are the models being developed and marketed by manufacturers for commercial applications.

Polymer Electrolyte/Proton Exchange Membrane Fuel Cell (PEM)
The PEM fuel cell uses an advanced plastic electrolyte to move protons from the anode to the cathode. The PEM uses a solid electrolyte and operates at a low temperature. The PEM uses a thin platinum catalyst to split the electrons from the hydrogen protons. PEM fuel cells are best suited for 1kW to 100kW applications.

Phosphoric Acid Fuel Cell (PAFC)
This fuel cell has been commercially available since 1992. The PAFC is suited for small Distributed Generation (DG) units. They are highly reliable, quiet to operate, and highly efficient. The PAFC runs at a medium temperature range and uses impure hydrogen, which makes them more flexible with multiple sources of hydrogen and production methodologies.

Molten Carbonate Fuel Cell (MCFC)
MCFCs use a ceramic electrolyte filled with carbon and salt. MCFCs operate at high temperatures (800°F), which best suits them for large stationary applications. These fuel cells operate at 85 percent efficiency when operated in conjunction with traditional energy grids. MCFCs are currently used in many demonstration projects, and are expected to be market ready in 2004. Large buildings like hospitals, hotels, or other industrial facilities that require electricity and heating (or cooling) around the clock would be likely applications for the MCFC.

Solid Oxide Fuel Cell (SOFC)
These fuel cells are considered utility grade and are well suited for large scale stationary power generators that could provide electricity for factories or towns. SOFCs use a ceramic oxide electrolyte. Like MCFCs, they operate at higher temperatures (about 1,000°F) and work best as generation devices for industrial applications where high temperature steam is required. These should be commercially competitive in the 2005 to 2007 timeframe. SOFCs are also being developed for residential CHP applications.

Hydrogen Applications
Hydrogen’s proponents envision a future where end uses for energy are domestically supplied principally by hydrogen through the generation of heat and electricity. This, they argue, will complete the historical transition from a carbon-based energy economy to one based on carbon-free hydrogen. The transition from solid fuels (like wood and coal) to liquid fuels (like oil) and then to gaseous fuels (like natural gas) has decreased the amount of carbon in each unit of fuel. With hydrogen as a fuel this transformation becomes complete, providing energy to applications in transportation, stationary CHP systems, portable power systems and microelectronics without carbon. Critics charge that if fossil fuels like coal and natural gas are used to produce hydrogen, the dependence on carbon-based energy will only continue. Also, the transition from solid to gaseous fuels has never been a complete one. Even as natural gas usage has increased, coal continues to be a dominant source of electricity generation and related greenhouse emissions. Transportation modes, from automobiles to trucks and specialty vehicles, would be powered hydrogen fuel cells. Stationary power to buildings is provided through cogeneration of electricity from fuel cells and through heat generated from hydrogen electric conversion. Heat is captured to provide additional ambient environmental control. Portable fuel cell systems would replace portable gasoline generators for various power needs. Micro-fuel cells, using a direct methanol version of a miniature PEM fuel cell, would provide power to consumer electronics such as laptops and cell phones. How hydrogen is generated and distributed in this future is the source of heated debate among hydrogen’s proponents; critics argue that alternatives to hydrogen can provide greater short-term benefits. Many hydrogen and fuel cell research, development and demonstration (RD&D) projects are aimed at creating vehicles that can be powered by fuel cells. The primary objective is to replace the current ICE with a fuel cell “stack” (multiple fuel cells bundled together to provide produce more power) to power the various systems of the vehicle. The challenges faced in achieving this goal differ depending on the type and use of the vehicle – personal vehicles or fleet applications. Personal automobiles are consumer owned, residentially stored and range from compact cars to light trucks (such as SUVs). Fleet vehicles are generally government or commercially owned, have defined routes of travel, generally are stored in a central location, and often centrally fueled as well. Examples of fleet vehicles include taxis, buses, trucks, and delivery vehicles. While transportation often is the main focus of the debate between hydrogen proponents and its critics, there are other applications that also employ hydrogen fuel cells and may provide less controversy. Hydrogen fuel cells can be used in distributed generation (DG) systems to power buildings. Distributed generation describes small electricity generating power plants that are located near or at the site of the end user. Not all DG systems use hydrogen fuel cells, but their potential high efficiencies and low environmental impact at the point of deployment have made them attractive to DG proponents. The onsite storage, production and release of hydrogen in DG systems can be in a cogeneration configuration where the heat generated in operating fuel cells can be used to provide climate control, thereby increasing the function, efficiency and value to the system. In addition, portable gasoline generators used in a variety of applications can be replaced with quieter, cleaner, more efficient hydrogen fuel cells. Furthermore, both stationary and portable fuel cells offer the potential for reduced permitting and site rules due to their low to zero emission operation. Consumer electronics are also envisioned to be powered through the use of a fuel cell. The range of power generated from these fuel cells, which are still under development, is between 25 watts and 10 kilowatts of power. In these applications, the goal is to create a fuel cell that would provide a much longer operating life than a conventional battery, in a package of lighter or equal weight per unit of power output. Fuel cells also have an environmental advantage over batteries, since certain kinds of batteries require special disposal. If successfully developed, these “micro” fuel cells could deliver much higher power density, storing more power in a smaller space than current batteries.