Glossary

Here are some terms and definitions you may encounter when researching fuel cells.

ALKALINE FUEL CELL (AFC):

A type of fuel cell in which the electrolyte is concentrated potassium hydroxide (KOH) (varies between 35 to 85 percentage weight depending on the intended operating temperature) and hydroxide ions (OH-) are transported from the cathode to the anode. Temperature of operation can vary from below 120°C to approximately 250°C depending on electrolyte concentration.

ANAEROBIC DIGESTION:

A biological process that converts biodegradable material into biogas and effluent through the use of microorganisms without the presence of oxygen. An anaerobic digester is the air-tight vessel through which complex organic material such as animal or food waste is fed. The methane-rich biogas from this process can be used to generate electricity, heat, and hot water.

ANODE:

The electrode at which oxidation occurs.

AUXILIARY POWER:

Power from an independent source that functions as required to augment or support various performance criteria established for the prime power source.

AVAILABILITY:

A factor based on the ratio (%) of a power source’s fully operational hours (the number of hours that the power source is operating and producing power) divided by the total planned or expected hours of full operation.

BALANCE-OF-PLANT:

Supporting or auxiliary components based on the power source or site-specific requirements and integrated into a comprehensive power system package centered near the power source.

BASE LOAD:

The constant electrical demand by a system to a power supply.

BIPOLAR PLATES:

Conductive plate in a fuel cell stack that acts as an anode for one cell and a cathode for the adjacent cell. The plate may be made of metal or a conductive polymer (which may be a carbon-filled composite). The plate usually incorporates flow channels for the fluid feeds and may also contain conduits for heat transfer.

BRITISH THERMAL UNIT (BTU):

The mean British Thermal Unit (BTU) is 1/180 of the heat required to raise the temperature of one pound (1lb.) of water from 32°F to 212°F at a constant atmospheric pressure. It is about equal to the quantity of heat required to raise one pound (1 lb.) of water 1°F.

CATALYST:

A chemical substance that increases the rate of a reaction without being consumed; after the reaction it can potentially be recovered from the reaction mixture chemically unchanged. The catalyst lowers the activation energy required, allowing the reaction to proceed more quickly or at a lower temperature.

CATALYST COATED MEMBRANE (CCM):

Term used to describe a membrane (in a PEM fuel cell) whose surfaces are coated with a catalyst layer to form the reaction zone of the electrode. (See also Membrane Electrode Assembly [MEA]).

CATALYST LOADING:

The amount of catalyst incorporated in the fuel cell per unit area. Typical units are mg/cm2.

CATHODE:

The electrode at which reduction occurs.

COGENERATION:

The simultaneous on-site production of electric energy and process steam or heat from the same power source.

COMBUSTION:

The rapid oxidation of fuel accompanied by the production of heat, or heat and light.

COMPONENTS:

The substructures within a cell that support the reaction, provide physical structure, receive and distribute reactants, remove products, dissipate heat, and perform other functions necessary for operation.

DEGRADATION RATE:

The rate at which a cell’s performance deteriorates over time. The degradation rate can be used to measure both recoverable and permanent losses in cell performance. The typical unit of measure is volts (DC) per unit time.

DIFFUSION:

The random movement of ions, or chemicals from areas of high concentrations to areas of low concentration over time.

DIRECT INTERNAL REFORMING:

Production of a desired product (hydrogen) within a fuel cell from a hydrocarbon based fuel (e.g. methanol, gasoline) fed to the fuel cell or stack.

DIRECT METHANOL FUEL CELL (DMFC):

A type of fuel cell that uses methanol (CH3OH), in gaseous or liquid form. The methanol is oxidized directly at the anode with no reformation to hydrogen. The electrolyte is typically a polymer electrolyte membrane (PEM).

DISTRIBUTED GENERATION:

Any small-scale power generation technology that provides electric power at or closer to the load being served.

EFFICIENCY:

A measure (usually a ratio) of the useful energy provided by a dynamic system versus the total energy supplied to it during a specific period of operation.

ELECTRICAL EFFICIENCY:

The ratio of useful electrical real power output to the total electrical power input.

ELECTRODE:

An electric conductor through which an electric current enters or leaves a medium, whether it be an electrolytic solution, solid, molten mass, gas, or vacuum.

ELECTROLYTE:

A non-metallic electrical conductor in which current is carried by the movement of ions.

EXHAUST HEAT:

Waste heat produced by a mechanical, chemical, or electrochemical process.

EXHAUST HEAT RECOVERY.

The use of exhaust heat as a source of energy.

EXTERNAL REFORMING:

The production of hydrogen from a hydrocarbon fuel (methanol, gasoline, natural gas, propane, etc.) prior to entry to the fuel cell or stack.

GAS CLEAN UP:

Removal of contaminants from gaseous feed streams by a mechanical or chemical process.

GAS TURBINE:

A turbine rotated by expanding gases that can produce electricity.

GAS UTILIZATION EQUIPMENT:

Any device which utilizes gas as a fuel or raw material, or both.

GRID CONNECTED:

A power delivery method that utilizes an independent power source which normally operates in parallel with a utility power system.

GRID INDEPENDENT:

A power delivery scheme consisting of an independent power source that serves a dedicated load and is not interconnected with the utility power system.

HEAT EXCHANGER:

A vessel in which heat is transferred from one medium to another.

IMPURITIES:

Undesired foreign material(s) in a substance or mixture.

INDIRECT INTERNAL REFORMING:

The fuel reformer is separated, but adjacent to, the fuel cell anode. This structure takes advantage of the close coupled thermal benefit from the exothermic reaction of the fuel cell to support the endothermic reforming reaction.

INTERCONNECT:

To link power systems in a way that enables them to draw on one another’s reserves in time of need.

IONIC RESISTANCE LOSS (OHMIC POLARIZATION):

Losses created by the resistance to the flow of ions in the electrolyte and resistance to flow of electrons through the electrode and bipolar plate materials. Because both the electrolyte and fuel cell electrodes obey Ohm’s law, the ohmic losses can be expressed by the equation

ήOhm = iR

LOW EMISSION VEHICLE (LEV):

Light duty passenger vehicles that meet emission control standards set by California’s Air Resources Board (ARB) and the U.S. Environmental Protection Agency (EPA). Since 1998, California has set standards for tailpipe emissions and air emissions related to the volatility of fuel used in passenger cars and light trucks. California has established several categories of vehicles, based on the maximum permitted emissions of several pollutants. LEV is the minimum standard for all new cars sold in California as of 2004.

MEMBRANE:

A thin, selective barrier that separates two fluids, allowing some particles or chemicals to pass through. In a fuel cell, the membrane acts as electrolyte (an ion-exchanger) as well as a barrier film separating the gases in the anode and cathode compartments of the fuel cell.

MEMBRANE ELECTRODE ASSEMBLY (MEA):

Structure consisting of a proton-exchange membrane with surfaces coated with catalyst/carbon/binder layers and sandwiched by two microporous conductive layers (which function as the gas diffusion layers and current collectors).

MOLTEN CARBONATE FUEL CELL (MCFC):

A type of fuel cell consisting of a molten electrolyte of lithium carbonate (Li2CO3) and sodium carbonate salts (Na2CO3) in which carbonate ions are transported from the cathode to the anode. Operating temperatures are typically near 650°C.

NATURAL GAS:

A naturally occurring gaseous mixture of simple hydrocarbon components (primarily methane) used as a fuel for the production of electrical power.

PHOSPHORIC ACID FUEL CELL (PAFC):

A type of fuel cell in which the electrolyte consists of concentrated phosphoric acid (H3PO4). Protons are transported from the anode to the cathode. The operating temperature range is generally 160 – 220°C.

PIPELINE GAS:

Gaseous fuel delivered by pipeline and whose major component is natural gas (methane), but whose lesser components (other gaseous hydrocarbons such as propane, butane, etc. and water vapor) may vary significantly depending upon origin, time of year and other factors.

POWER DENSITY:

The amount of power produced per unit of measure. For a single cell, this is typically measured as kW per square centimeter.

POWER PLANT:

A packaged, self-contained, automatically operated assembly of integrated systems for generating useful electrical energy and recoverable thermal energy.

PROTON EXCHANGE MEMBRANE (PEM):

The separating layer in a PEM fuel cell that acts as an electrolyte (which is proton conducting) as well as a barrier film separating the hydrogen-rich feed in the cathode compartment of the cell from the oxygen-rich anode side.

PROTON EXCHANGE MEMBRANE FUEL CELL (PEMFC, PEFC, or PEM):

A type of fuel cell in which the exchange of protons from the anode to the cathode is achieved by a solid, aqueous membrane impregnated with an acid. The electrolyte is also called a proton-exchange membrane (PEM). These fuel cells typically run at low temperatures (<100°C) and pressures (< 5 atm).

REFORMING:

The thermal or catalytic conversion of a hydrocarbon fuel into more volatile products with higher BTU ratings.

RELIABILITY:

The ability of an item to perform a required function under stated conditions for a stated period of time.

REVERSIBLE FUEL CELL:

A type of fuel cell in which the chemical reactants undergo reversible reactions, such that the cell may be recharged with a separate power source if desired. For example, the hydrogen or oxygen fuel cell may be recharged by providing power for water electrolysis with hydrogen storage. Also called a Regenerative Fuel Cell.

SEPARATOR PLATE:

A solid piece of electrically conductive material (usually a metal or graphite) that is inserted between cells in a stack.

SERIES CONNECTION:

The connection of electrical cells in a positive to negative pattern such that individual cell voltages are additive.

SOLID OXIDE FUEL CELL (SOFC):

A type of fuel cell in which the electrolyte is a solid, nonporous metal oxide or ceramic, typically zirconium oxide (ZrO2) doped with yittrium oxide (Y2O3), and O2- is transported from the cathode to the anode. Any carbon monoxide (CO) in the reformate gas is oxidized to carbon dioxide (CO2) at the anode. Temperatures of operation are typically 800 – 1000°C.

SMART GRID:

An electrical delivery system that uses digital technology to improve reliability, resiliency, flexibility, and efficiency. Refers to a class of technology being used to bring utility electricity delivery systems into the 21st century, using computer-based remote control and automation.

STACK LIFE:

The cumulative period of time that a fuel cell stack may operate before its output deteriorates below a useful minimum value.

STACKING:

The process of placing individual fuel cells in a series to increase voltage and form a fuel cell stack.

STANDBY:

A power circuit equipment, device, or component available to be connected into the circuit to perform a function when the preferred component has failed or is inoperative.

STANDBY POWER:

An independent reserve source of electric energy that upon failure or outage of the normal source, allows the user’s facilities to continue in satisfactory operation.

STATIONARY POWER PLANTS:

Source of electricity that remains in one location.

STEAM REFORMING:

The process of reacting a hydrocarbon fuel, such as natural gas, in the presence of steam to form hydrogen as a product. This is the commonly preferred method of bulk hydrogen generation.

THERMAL EFFICIENCY:

Efficiency with which a power source transforms the potential heat of its fuel into work or output, expressed as the ratio of the useful work done by the power source in a given time interval to the total heat energy contained in the fuel burned during the same time interval, both work and heat being expressed in the same units.

THERMAL MANAGEMENT:

The directing of heat entering or exiting a system.

ZERO EMISSION VEHICLE (ZEV):

A vehicle that produces no air emissions from its fueling or operation. California regulations require that by 2025, approximately 15% of the vehicles sold in California by major auto makers be ZEV or ZEV equivalent. California has established a comprehensive program for determining this equivalency through the state Air Resources Board (ARB). More than a dozen U.S. states have adopted California’s ZEV standards. See also LEV.