The construction of a fuel cell itself is
very simple, as it consists of only a few parts. The most important are
the electrolyte and the electrodes. In addition, flow-field plates are
needed to distribute the reactants homogeneously over the cell area. The figure below
shows the fuel cell components (for a PEMFC).
A general description of
all components and their properties is given below.
Electrolyte - The
electrolyte has three main functions in a fuel cell; to conduct ions, act
as an electric insulator and
physically separate the anode and cathode reactants. Ions have to pass through the membrane to maintain charge
equilibrium between the anode and cathode. The charged species and its
direction of flow varies with the type of fuel cell (see "Fuel Cell
As described under "Fuel Cell Principle", the basic concept of
the fuel cell is the separation of the reactants and the situation that the anode
and cathode electrochemical reactions take place separate of each
other. Any flow of current or reactants through the electrolyte will
decrease the performance of the cell, so these properties have a
very large influence on the fuel cell operation.
The electrochemical reactions take place on the electrode surface.
Fuel is oxidised at the anode and oxygen is reduced at the cathode. A
combination of membrane and electrodes is called a membrane electrode
assembly (MEA). For low-temperature fuel cells, noble metals are needed to increase the
reaction rate. Platinum is the most widely used catalyst, sometimes in
combination with other metals. At higher temperatures (MCFC and SOFC), this is not
necessary and cheaper metals/materials can be used.
Diffusion Layers (GDL)- GDL's are only used in low-temperature fuel cells. They are responsible for the distribution of the reactants
to and removal of the
products from the electrode surface. An important issue is water
removal from the cathode in e.g. a PEMFC. The GDL's are optimised by
changing their hydrophobic properties. Since they are placed between
the electrodes and the flow-fields (current collectors), they also
have to be electrically conductive.
The flow-field plates ensure the distribution of fuel and oxidant
the whole cell area. Different channel structures, such as serpentine,
parallel and interdigitated, are machined into the
plates where e.g. hydrogen and air can flow. The choice of materials
varies according to the type of fuel cell.
Examples range from graphite, stainless steel and plastic for low-temperature fuel cells to ceramics for higher temperatures. The flow-fields also act as current
Below is an example of a single cell with
serpentine channel structures for the reactants.
The electrolyte membrane with electrodes and diffusion layers lies
between the graphite plates.
To be able to develop
fuel cell systems of higher power, several cells are combined to
form a so-called stack. In these cases, the flow-field plates have channels on both
sides and are called bipolar
plates. A sketch of the principle of a stack is shown below. There are
just as many stack designs as there are fuel cell types and applications.
Depending on the design specifications (power output, heat, size and
geometry) different flow fields, bipolar plates and complete stacks have
One interesting example is the tubular solid-oxide fuel cell from Siemens Westinghouse
(see "Fuel Cell Types/SOFC").
A complete fuel cell system consists of more than
just the stack itself. A fuel tank, pumps, fans and a control unit is the minimum
requirement to operate a fuel cell. Stationary power stations are the most
complicated, but even small applications can present great challenges in
system development. Below is a picture of a direct methanol fuel cell for
a mobile phone battery charger
In contrast to batteries, the fuel is stored
separate from the energy converter. The size of the fuel storage unit limits the operation time. Hydrogen can be stored in pressurised tanks, in
metal hydrides, as liquid hydrogen or chemically bonded. Heavier
hydrocarbons can also be stored, and either fed directly to the cell or
reformed to hydrogen, depending on the fuel cell type and application. In most cases, the oxidant or oxygen source is air from the
surroundings. Pumps and fans
supply the fuel cell with air and fuel from the tank.
For some fuel cells, the water content of the electrolyte is crucial for the proton conductivity, and
thus the overall performance. A humidifier is therefore used to add water to the
gases to control the humidity in the cell
Even though the theoretical losses in
a fuel cell are low, considerable amounts of heat are produced during
operation. If the overall system efficiency is 50%, the same amount of heat
as electricity is
produced! Due to this, a cooling system, based on either water or air, is
needed to prevent overheating.
All the above mentioned components have to be
managed by a control unit, which assures stable operation and a reliable power output of the total fuel cell system.