Thermodynamics & electrochemical kinetics of fuel cell

FUEL CELL TECHNOLOGY
Presented by,
Kowshigan S V
Thermodynamics & Electrochemical Kinetics of fuel cell

THERMODYNAMICS IN FUEL CELL
• Fuel cell is an electrochemical device which converts the Gibbs free enthalpy of
the combustion reaction of a fuel and an oxidant gas (Air) as far as possible.
• Hydrogen & oxygen are used to illustrate the simplest case.
GIBBS FREE ENERGY:
• It is the maximum amount of non-expansion work that can be extracted from a
thermodynamically closed system at a constant temperature and pressure.

• The effect of temperature and pressure on the cell potential may be analyzed on
the basis of the Gibbs free energy variation with respect to temperature and
pressure in a fuel cell. This may be written as
• where −∆V is the change in volume, ∆S is the entropy change, E is the cell
potential, T the temperature, P the reactant gas pressure, n the number of
electrons transferred, and F Faraday’s constant.

• Since the entropy change for the H2/O2 fuel cell reaction is negative the
reversible potential (the net ion flow through any channel is 0) of the H2/O2 fuel
cell decreases with an increase in temperature, assuming that the reaction
product is liquid water.
• For the above reaction the volume change is negative, hence the reversible
potential increases with an increase in pressure. The influence of temperature on
the fuel cell voltage is shown schematically in Figure 2.4, where the fuel cell
performance data from typical operating cells and the dependence of the
reversible potential of H2/O2 fuel cells on temperature are given.

• The maximum electrical work obtainable in a fuel cell operating at constant
temperature and pressure is given by the change in the Gibbs free energy of the
electrochemical reaction
Where n is the number of electrons participating in the reaction, F is Faraday’s
constant (96,487 coulombs/g-mole electron), and E is the ideal potential of the cell.
If we consider the case of reactants and products being in the standard state, then

• The maximum work available from a fuel source is related to the free energy of
reaction in the case of a fuel cell, whereas the enthalpy of reaction is the
pertinent quantity for a heat engine, i.e.,
where the difference between ∆G and ∆H is proportional to the change in entropy
∆S. This entropy change is manifested in changes in the degrees of freedom for the
chemical system being considered. The maximum amount of electrical energy
available is ∆G as mentioned above, and the total thermal energy available is ∆H.
The amount of heat that is produced by a fuel cell operating reversibly is T∆S.
Reactions in fuel cells that have negative entropy change generate heat, while
those with positive entropy change may extract heat from their surroundings

• Differentiating Equation 2.12 with respect to temperature or
pressure, and substituting it into Equation 2.10 gives

• The thermal efficiency of an energy conversion device is defined as the amount of
useful energy produced relative to the change in stored chemical energy
(commonly referred to as thermal energy) that is released when a fuel is reacted
with an oxidant. Hence the efficiency may be defined as
• Hydrogen (fuel) and oxygen (oxidant) can exist in each other’s presence at room
temperature, but if heated to above 500 °C and at high pressure they explode
violently. The combustion reaction for these gases can be forced to occur below
500 °C in the presence of a flame, such as in a heat engine. In the case of a fuel
cell, a catalyst can increase the rate of reaction of H2 and O2 at temperatures
lower than 500 °C in the ambient of an electrolyte

• In high temperature fuel cells a non combustible reaction can occur at
temperatures over 500°C because of controlled separation of the fuel and
oxidant. The process taking place in a heat engine is thermal, where as the fuel
cell process is electrochemical. The difference in these two processes in energy
conversion is the fact behind efficiency comparison for these two systems. In the
ideal case of an electrochemical energy conversion reaction such as a fuel cell the
change in Gibbs free energy of the reaction is available as useful electric energy
at the output of the device. The ideal efficiency of a fuel cell operating
irreversibly may be stated as

• The most commonly used way of expressing efficiency of a fuel cell is based on
the change in the standard free energy for the cell reaction
• where the product water is in liquid form. At standard conditions of reaction the
chemical energy in the hydrogen/oxygen reaction is 285.8 kJ/mole and the free
energy available for useful work is 237.1 kJ/mole. Thus, the thermal efficiency of
an ideal fuel cell operating reversibly on pure hydrogen and oxygen at standard
conditions would be

• The efficiency of an actual fuel cell can be expressed in terms of the ratio of the
operating cell voltage to the ideal cell voltage. The actual cell voltage is less than
the ideal cell voltage because of the losses associated with cell polarization and
the iR loss, as discussed in the earlier section. The thermal efficiency of the fuel
cell can then be written in terms of the actual cell voltage,
• As mentioned earlier, the ideal voltage of a fuel cell operating reversibly with
pure hydrogen and oxygen in standard conditions is 1.229 V. Thus, the thermal
efficiency of an actual fuel cell operating at a voltage of Vcell, based on the higher
heating value of hydrogen is given by

• A fuel cell can be operated at different current densities; the corresponding cell
voltage then determines the fuel cell efficiency. Decreasing the current density
increases the cell voltage, thereby increasing the fuel cell efficiency. In fact, as the
current density is decreased, the active cell area must be increased to obtain the
desired amount of power

ELECTROCHEMICAL
• The energy balance analysis in the fuel cell should be based on energy
conversion processes like power generation, electrochemical
reactions, heat loss, etc. The energy balance analysis varies for the
different types of fuel cells because the various types of
electrochemical reactions occur according to the fuel cell type. The
enthalpy of the reactants entering the system should match the sum
of the enthalpies of the products leaving the cell, the net heat
generated within the system, the dc power output from the cell, and
the heat loss from the cell to its surroundings. The energy balance
analysis is done by determining the fuel cell temperature at the exit
by having information of the reactant composition, the temperatures,
H2 and O2 utilization, the power produced, and the heat loss.

• The fuel cell reaction (inverse of the electrolysis reaction) is a
chemical process that can be divided into two electrochemical half-
cell reactions. The most simple and common reaction encountered in
fuel cells is

Thermodynamics & electrochemical kinetics of fuel cell

• The electrochemical reactions taking place in a fuel cell determine the ideal performance
of a fuel cell; these are shown in Table 2.1 for different kinds of fuels depending on the
electrochemical reactions that occur with different fuels, where CO is carbon monoxide,
e− is an electron, H2O is water, CO2 is carbon dioxide, H+ is a hydrogen ion, O2 is oxygen,
CO32− is a carbonate ion, H2 is hydrogen, and OH− is a hydroxyl ion. It is very clear that
from one kind of cell to another the reactions vary, and thus so do the types of fuel. The
minimum temperature for optimum operating conditions varies from cell to cell. This
detail will be discussed in subsequent chapters. Low to medium-temperature fuel cells
such as polymer electrolyte fuel cells
• (PEMFC), alkaline fuel cells (AFC), and phosphoric acid fuel cells (PAFC) are limited by the
requirement of noble metal electro-catalysts for optimum reaction rates at the anode
and cathode, and H2 is the most recommended fuel.
• For high temperature fuel cells such as molten carbonate fuel cells (MCFC) and solid
oxide fuel cells (SOFC) the catalyst restrictions are less stringent, and the fuel types can
vary.
• Carbon monoxide can poison a noble metal electro-catalyst such as platinum (Pt) in low-
temperature fuel cells, but it serves as a potential fuel in high temperature fuel cells
where non-noble metal catalysts such as nickel (Ni), or oxides can be employed as
catalysts.

• The ideal performance of a fuel cell can be represented in different ways.
The most commonly used practice is to define it by the Nernst potential
represented as the cell voltage. The fuel cell reactions corresponding to the
anode and cathode reactions and the corresponding Nernst equations are
given in Table 2.2. The Nernst equation is a representation of the
relationship between the ideal standard potential E0 for the fuel cell
reaction and the ideal equilibrium potential E at other temperatures and
pressures of reactants and products. Once the ideal potential at standard
conditions is known, the ideal voltage can be determined at other
temperatures and pressures through the use of these equations. According
to the Nernst equation for hydrogen oxidation, the ideal cell potential at a
given temperature can be increased by operating the cell at higher reactant
pressures. Improvements in fuel cell performance have been observed at
higher pressures and temperatures. The symbol E represents the
equilibrium potential, E0 the standard potential, P the gas pressure, R the
universal gas constant, F Faraday’s constant and T the absolute
temperature.

• In general in a fuel cell the reaction of H2 and O2 produces H2O. When
hydrocarbon fuels are involved in the anode reaction, CO2 is also
produced.
• For molten carbonate fuel cells CO2 is consumed in the cathode reaction to
maintain the invariant carbonate concentration in the electrolyte. Since
CO2 is generated at the anode and consumed at the cathode in MCFCs, and
because the concentrations of the anode and cathode flows are not
necessarily equal, the Nernst equation in Table 2.2 includes the partial
pressures of CO2 for both electrode reactions.
• The ideal standard potential of an H2/O2 fuel cell (E0) is 1.229 V with liquid
water as the product and 1.18 V for water with gaseous product. This value
is normally referred to as the oxidation potential of H2. The potential can
also be expressed as a change in Gibbs free energy for the reaction of
hydrogen and oxygen. The change in Gibbs free energy increases as cell
temperature decreases and the ideal potential of a cell is proportional to
the change in the standard Gibbs free energy.

Thermodynamics & electrochemical kinetics of fuel cell

More Related Content

What's hot

What's hot (20)

Similar to Thermodynamics & electrochemical kinetics of fuel cell

Similar to Thermodynamics & electrochemical kinetics of fuel cell (20)

Recently uploaded

Recently uploaded (20)