Fuel Cell Stack
A single fuel cell is only capable of producing about 1 volt, so typical fuel cell designs link together many individual cells to form a "stack" to produce a more useful voltage. A fuel cell stack can be configured with many groups of cells in series and parallel connections to further tailor the voltage, current, and power. The number of individual cells contained within one stack is typically greater than 50 and varies significantly with stack design.
The basic components of the fuel cell stack include the electrodes and electrolyte with additional components required for electrical connections and/or insulation and the flow of fuel and oxidant. These key components include current collectors and separator plates. The current collectors conduct electrons from the anode to the separator plate. The separator plates provide the electrical series connections between cells and physically separate the oxidant flow of one cell from the fuel flow of the adjacent cell. The channels in the current collectors serve as the distribution pathways for the fuel and oxidant. Often, the two current collectors and the separator plate are combined into a single unit called a "bipolar plate."
A non-metallic electrical conductor in which current is carried by the movement of ions.
In the case of a proton exchange membrane fuel cell (PEMFC), the electrolyte is a membrane that acts as the separating layer in a fuel cell (ion-exchanger). It is also a film barrier that separates gases in the anode and cathode compartments of the fuel cell.
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.
- Anode - The electrode at which oxidation occurs. For cells that create potential, it is also the electrode towards which the negative ion flows.
- Cathode - The electrode at which reduction occurs.
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.
Term used to describe the conductive material in a fuel cell that collects electrons (on the anode side) or disburses electrons (on the cathode side). The current collectors are microporous (to allow for fluid flow through them) and lie in between the catalyst / electrolyte surfaces and the bipolar plates.
Membrane Electrode Assembly (MEA)
In the case of a PEMFC, the MEA is the structure consisting of an electrolyte (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).
Bipolar Plates or Separator 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.
A manifold is the conduit of an appliance that supplies gas to the individual burner. The manifolds of a fuel cell stack can be either external or internal.
An external manifold acts as the plumbing of the system which routes the fuel to anode chamber and the oxidant (air) to cathode chamber. Developmental issues include:
- Even distribution of fuel and oxidant
- Exposure to high temperatures and corrosive environments
- Low pressure drop allowable
- Compact space requirements
An internal manifold is a " system with a self-contained reactant delivery system similar to a boxed fuel cell system that would only require connections to the reactant tanks to become operational." Developmental issues include:
- Delivery of fuel and oxidant uniformly to electrode - electrolyte surfaces
- Multiple concepts for integration
- Integrate with separators, bipolar plates
- Integrate with cooling
- Integrate with electrodes
A device used to transfer heat from a fluid (liquid or gas) to another fluid where the two fluids are physically separated.
Heat exchangers are needed to cool fuel cells and maintain a consistent operating temperature. They are used by primarily by the lower-temperature fuel cells (AFC, PEMFC and PAFC). The typical cooling mechanism is water. There are a number of types of heat exchangers:
- Gas - Gas
- Gas - Liquid
Some of the development areas to date are:
- Size and weight reduction (transportation markets)
- Heat transfer and size (portable power markets)