Fuel cells have
attracted interest due to their
low degree of pollution and high (theoretical) efficiency. Since the chemical energy of the fuel is directly converted to
fuel cells are not influenced by the Carnot efficiency limit like combustion
and heat engines. When chemical energy is converted to electricity via heat (by fuel
combustion), the maximum theoretical efficiency is limited by
the operating temperatures:
For fuel cells the theoretical
efficiency can be expressed, among many alternatives, as the relationship between
chemical energy available (free energy change of the fuel cell reaction, DG) and the higher heating value of the fuel cell reaction
(enthalpy change of the fuel cell reaction, DH).
Over a large temperature and
power range fuel cells have a potentially higher efficiency than Carnot
technology, see the figure below. Combustion/heat engines are well suited
for operation under constant conditions, as they have a limitied maximum power range. Outside this
range, the efficiency drops considerably. Fuel cells, however, have high
efficiency over a large power and temperature range and are thus perfect for dynamic load
cycles. The high efficiency at very low loads is particularly valuable.
To understand the
difference between the two types of technology one can consider the case of driving a car.
During highway cruising, the combustion engine operates in the high-efficiency range, the right-hand
section of the red curve. In city traffic however,
the car often starts and stops, which corresponds to going up and down the left-hand
section of the red curve. This corresponds to the well-known fact that city
traffic is more polluting due to the low efficiency. Following the same example
with a fuel cell (green curve), one sees that the fuel cell has its highest efficiency
already at low loads. A fuel cell would operate much more favourably in city
traffic but is also highly efficient under constant load.
Fuel cells are
not always more efficient for all applications than heat engines/turbines. For large stationary applications, where high-temperature fuel cells are used (SOFC and
MCFC), the two types of technology complement each other perfectly and are
combined. The excess heat from the fuel cell is fed into a heat
turbine which is able to extract more electric power.
The losses in fuel cells
are related to the power output. As more current is drawn from a cell,
the losses increase. This can be seen as the cell voltage drops from
the open circuit voltage with increasing current. A typical
current-voltage diagram for a hydrogen PEMFC is shown below.
If there were no losses in the fuel
cell at all, the cell voltage would maintain the theoretical voltage,
independent of cell current. This ideal cell voltage (Erev) of a fuel
cell can be
calculated from the available free energy, DG.
the free Gibbs energy, n is number of electrons transferred in the
electrochemical reaction and F is the Faraday constant (94685 C/mole). The
also dependent on operating conditions like temperature, pressure and
concentration of reactants. The difference between the ideal cell voltage
operating voltage is called overvoltage, and represents the losses in a
fuel cell. The relationship between the theoretical Erev and the actual operating voltage
(Ecell) is a more correct way to express the
efficiency of a fuel cell.
The losses in a fuel cell can be divided into fuel
crossover and internal currents, activation losses, ohmic losses and mass
transport losses. Fuel crossover and internal
current losses result from the flow of fuel and electric current in the
electrolyte. The electrolyte should only transport ions, however a certain
fuel and electron flow will always occur. Although the fuel loss and internal
currents are small, they are the main reason for the real open
circuit voltage (OCV) being lower than the theoretical one (Erev).
Activation losses are caused by the slowness of the
reactions taking place on the electrode surface. The voltage decreases
somewhat due to the electrochemical reaction kinetics.
This can be seen in the left-hand section of the current-voltage curve above.
The ohmic losses
result from resistance to
the flow of ions in the electrolyte and electrons through the cell
hardware and various interconnections. The corresponding voltage drop is
essentially proportional to current density, hence the term "ohmic
Mass transport losses
result from the decrease in reactant concentration at the surface of the electrodes as
fuel is used. At maximum (limiting) current, the concentration at the
catalyst surface is practically zero, as the reactants are consumed as
soon as they are supplied to the surface.