Text preview for : 2581 Fuel Cell.pdf part of Keithley 2581 Fuel Cell Keithley Appnotes 2581 Fuel Cell.pdf
Back to : 2581 Fuel Cell.pdf | Home
A G R E AT E R M E A S U R E O F C O N F I D E N C E ing the final cathode reactant. Measurements
of these electrical and ionic currents are ex-
tremely useful in understanding a fuel cell's
characteristics and operation.
Cell Energy and Voltage
A key variable in describing cell charac-
teristics is a quantity called the Gibbs free
energy. This describes the amount of free
energy or chemical energy that a material
possesses. Cell voltage and other electrical
characteristics are related to the Gibbs free
energy, which is a function of the electronic
structure of the molecules undergoing reac-
tion. In the case of a hydrogen fuel cell, the
total energy available from its chemical re-
Unraveling
actions is equal to the free energy of the H2
and O2 reactants, minus the free energy of
the reaction product, H2O.
Fuel Cell Electrical
The potential difference(voltage) devel-
oped from the cell's electrical energy can be
found from the following:
Measurements Electrical power, P = E * I (Eq. 1)
where I is the current, moving through a po-
tential difference E, and P is the power de-
livered. Power is energy per unit time, and
James Niemann, Keithley Instruments, Inc. current is charge per unit time, thus:
W/t = E * Q/t (Eq. 2)
A
where W is energy in Joules, t is time in sec-
NYONE involved in the design, ducting, catalyzed anode and cathode elec- onds, and Q is charge in Coulombs.
manufacture, application or trodes. See Figure 1. By eliminating the time variable,
repair of fuel cells, or fuel cell The desired reactions in a hydrogen fuel
W=E*Q (Eqn. 3)
powered devices, needs cell data cell are described by the equations in Figure
that is available only through di- 1. Usually these reactions yield only heat, but W represents the energy required to move a
rect electrical measurements. When properly in a fuel cell the principal result is electrical charge Q through a potential E.
interpreted, this data not only provides an un- energy. The electrons from the anode reac- The Gibbs free energy can be equated to
derstanding of how and why fuel cells work, tion flow through the external circuit and W and the equivalent heat energy released
but also indicates the health of operational back to the surface of the cathode, supply- when hydrogen and oxygen react (undergo
cells, helps predict their life, reveals failure
mechanisms, and provides insight into their
suitability for specific applications. Electron Flow
2H2 = 4H+ + 4e