Fuel Cell Basics
Fuel cells are similar to batteries in that both systems have two electrodes separated by an electrolyte and electrical energy can be withdrawn from the cell reaction. However, unlike batteries, the reactants in a fuel cell are supplied from an external source and it operates as long as it is supplied with fuel and oxidant. The fuel is usually hydrogen and the oxidant is usually oxygen (typically from the air). In a hydrogen-oxygen fuel cell, hydrogen molecules are split at the anode by the catalyst into protons and electrons. The protons then travel through the electrolyte to the cathode while the electrons are conducted to the cathode through the external circuit and the load or power application. On the cathode, oxygen, protons, and electrons combine to form water. Depending on the input fuel and the electrolyte, different chemical reactions will occur.
Fuel cells have several highly attractive characteristics. Carnot’s law does not govern electrochemical processes in fuel cells, and therefore high operating temperature is not necessary for achieving high efficiency. The efficiency of a fuel cell can be higher than in conventional energy conversion processes. Low operating temperature guarantees that no nitrous oxide is produced. The only waste product is oxidized fuel. Normally, the fuel is hydrogen and consequently, the product is water. Carbon dioxide may be present as well, if a hydrocarbon fuel is used. Apart from being efficient and nonpolluting, fuel cells are silent, modular in design and respond rapidly to load changes. Fuel reformulation in fuel preprocessors adds the benefit of fuel flexibility.
Alternative Energy Institute, Inc. www.altenergy.org
Information about hydrogen, fuel cells, and other alternative energy sources
California Air Resources Board (CARB) www.arb.ca.gov
Information about fuel cells, zero emission vehicles, health and air pollution
Consumer Energy Center www.consumerenergycenter.org
Information on alternative energy choices, rebates, tax credits
California Energy Commission (CEC) www.energy.ca.gov
Information on energy efficiency, energy statistics, power plants, renewable energy, fuel cells
California Hydrogen Business Council www.californiahydrogen.org
Includes news, information on conferences and state & federal programs, links to related organizations
California Stationary Fuel Cell Collaborative www.casfcc.org
Information on this joint public-private entity, its activities and related efforts
Energy Co-Opportunity (ECO)
ECO is a cooperative which provides its electric co-op members with new energy solutions, including access to distributed energy technologies.
Fuel Cells 2000 www.fuelcells.org
Images, answers to FAQ’s and references to other sites
Link to hydrogen generation, storage, fuel
Hydrogen and Fuel Cell Investor www.h2fc.com
Includes news, and an overview of fuel cell technologies and companies
Hydrogen Fuel Cell Institute www.h2fuelcells.org
Includes links to various fuel cell groups
National Hydrogen Association www.hydrogenus.com
Information on auto companies, fuel cell developers,
US Fuel Cell Council www.usfcc.com
Industry trade association fostering the commercialization of fuel cells in the US
How a Fuel Cell Works
Fuel cells produce electricity though an electrochemical process using hydrogen as fuel and oxygen from the air. The by-products of this reaction are heat and water vapor. There are several types of fuel cells. Altergy’s products utilize the Proton Exchange Membrane or PEM fuel cell.
In the simplest terms, here is the process by which a PEM fuel cell produces electricity:
1. Hydrogen gas enters a fuel cell at the anode (negative cathode) where it attaches to a catalyst layer.
2. The catalyst facilitates the disassociation of the hydrogen gas into electrons and protons (hydrogen ions).
3. The protons are able to pass freely through the membrane—hence Proton Exchange Membrane…
4. …while the electrons pass through an external circuit creating usable electricity.
5. After passing through the membrane the hydrogen protons with the help of a catalyst…
6. …recombine with electrons and oxygen resulting in water vapor.
Hydrogen Energizes the Fuel Cell
Hydrogen, which derives its name from the water generated by its combustion, is currently used in oil refineries, chemical plants, food processing facilities, silicon wafer processing facilities in the hydrogenation of organic materials, as a reducing atmosphere, in oxyhydrogen torches, as rocket fuels, and as a fuel for fuel cells. Use of hydrogen-fueled fuel cells for transportation and electricity production would reduce pollution and increase the efficiency with which natural resources are used, leading us towards a sustainable energy future.
A colorless, tasteless, odorless gaseous element.
The most abundant element in the universe, as it is an ingredient of water and many other substances.
The lightest known substance, being fourteen and a half times lighter than air, and over eleven thousand times lighter than water.
The bulk of hydrogen currently produced is from steam reformation of natural gas. Other processes — such as electrolysis of water, ammonia dissociation, and hydrocarbon oxidation — are used to produce hydrogen as well. Hydrogen is also produced by the action of acids (such as sulphuric) on metals such as zinc and iron.
Types of Fuel Cells
Fuel cells are customarily classified according to the electrolyte employed. The five most common technologies are:
Proton Exchange Membrane (PEMFC)
Alkaline Fuel Cells (AFC)
Phosphoric Acid Fuel Cells (PAFC)
Molten Carbonate Fuel Cells (MCFC)
Solid Oxide Fuel Cells (SOFC)
PEMFCs employ an ion-conducting polymer membrane as an electrolyte and operate in temperatures where water is in liquid form. Low temperature, however, requires the use of expensive platinum catalysts. PEMFCs are intended for use in electric utilities, portable power applications, and transportation.
The electrolyte in an AFC is an aqueous solution on potassium hydroxide. AFCs performance is high due to the fast cathodic reactions in alkaline electrolyte. The electrolyte is very sensitive to carbon dioxide and requires expensive removal of CO2 from fuel and air streams. AFCs are utilized in military and space applications, where cost is not an issue, but high performance is required.
The electrolyte of PAFCs is phosphoric acid. Operation temperature of about 200°C allows very high efficiency electricity and heat co-generation, so a PAFC is able to use impure hydrogen as fuel. PAFCs need platinum catalysts and their low current and power density limit the range of applications. PAFCs have been used in electric utilities and in some transportation applications.
MCFCs and SOFCs share many characteristics. Their operation temperature between 600C and 1000C allows many advantages: high efficiency through electricity and heat co-generation, fuel flexibility, and the use of inexpensive catalysts as the reactions occur much faster as the temperature is increased. MCFCs evolved from the work aimed at producing a fuel cell that would operate directly on coal. MCFC uses a molten carbonate mixture, usually lithium, sodium, and/or potassium carbonates, soaked in a matrix as the electrolyte. SOFC is based on the use of solid ceramic electrolyte-zirconium oxide stabilized with small amounts of yttrium. Unlike MCFCs, SOFCs are safe from corrosion due to the liquid electrolyte. MC and SO fuel cells are intended for use in electric utilities, but are still in the early field test phase.
Another type of fuel cell is the Direct Methanol Fuel Cell (DMFC). As the name implies, DMFC uses methanol as fuel. Cathode reaction is similar to PEMFC cathode reaction but anode reaction is different: methanol molecules are broken apart when methanol-water solution is introduced to the negatively charged electrode. Carbon atoms combine with oxygen atoms from methanol and water to form carbon dioxide. Hydrogen is oxidized on the anode, and protons pass through the electrolyte to the cathode. As reaction byproducts, water is produced on the cathode and carbon dioxide on the anode.
A single fuel cell produces a limited voltage, usually less than 1 volt. In order to produce a useful voltage, a number of fuel cells are connected in series. Series-connected fuel cells form a fuel cell stack. The number of unit cells in a stack depends on the desired voltage.
Aside from military and space flight use, less specialized fuel cell applications can be categorized in three groups: stationary power generation, portable power generation, and transportation.
Stationary power generation applications include both large-scale utility plants and smaller scale systems for distributed electricity and heat generation in buildings and individual homes. Fuel cells are already an alternative for power generation in areas where there is no existing power grid or the power supply is often unreliable.
New applications are emerging in the field of portable power generation, where fuel cell systems are expected to replace primary and rechargeable batteries in portable electronics. Major drawbacks of batteries are limited capacity and slow recharging. With a suitable hydrogen storage method, a fuel cell system can achieve higher power and energy capacity. In addition, battery performance deteriorates when the charge level drops, whereas a fuel cell operates on constant level as long as fuel is supplied.
In the area of transportation, striving for zero-emission vehicles catalyzes the use of fuel cell engines. Typical systems use high pressure fuel storage. This will lower pollution emissions due to higher energy conversion efficiency. Direct hydrogen engines produce no pollutant emissions.