633 Practical Limitations

The reversible standard potentials presented in the previous section determine the fundamental limitations on the performance of fuel cell technologies. These fundamental limitations would suggest that voltage levels greater

Reversible Potential (V)

300 400 500 600 700 800 900 1000 1100

Reversible ideal potential for FC electrochemical reactions versus temperature.

Reversible ideal potential for FC electrochemical reactions versus temperature.

than one volt could be achieved with fuel cells. This would correspond to very high efficiencies for the conversion of fuel chemical energy to electricity. However, there are practical limitations to the performance of fuel cells that are due to several physical processes (e.g., transport and chemical reaction) which do not occur without losses.

The physical processes associated with these losses include: (1) transport of reactants to the gas/electrolyte interface, (2) dissolution of reactant in electrolyte, (3) transport of reactant through electrolyte to the electrode surface,

(4) pre-electrochemical homogeneous or heterogeneous chemical reaction,

(5) adsorption of electro-active species onto the electrode, (6) surface migration of adsorbed species, (7) electrochemical reaction involving electrically charged species, (8) post-electrochemical surface migration, (9) desorption of products, (10) post-electrochemical reaction, (11) transport of products away from the electrode surface, (12) evolution of products from electrolyte, and (13) transport of gaseous products from electrolyte/gas.

The losses associated with these chemical and physical processes are generally manifested in three major fuel cell losses. In electrochemical terms, losses are often referred to as overpotentials (i.e., the potential over that observed to reach theoretical potential), polarizations, or overvoltages. The three major losses are:

1. Activation overpotential (polarization)

2. Ohmic overpotential (polarization)

3. Concentration overpotential (polarization)

These losses are irreversible and result in a practical cell voltage that is less than the ideal voltage. Activation polarization is generally a result of losses associated with slow chemical reactions (e.g., overcoming the activation energy of chemical reactions). Ohmic polarization is loss due to the flow of electricity through the fuel cell which resists the flow of electricity, and concentration polarization is caused by transport phenomena which lead to lower concentrations of reactants at the electrochemical surface than in the bulk flow (Appleby and Foulkes, 1989).

Figure 6.5 presents a comparison of the ideal and actual voltage versus current characteristics of a typical fuel cell. Note that activation polarization is dominant at lower current densities where electronic barriers must be overcome before significant current can flow and reactant species can be consumed. Ohmic polarization varies directly with current and increases over the whole range of current densities. Gas transport losses occur throughout the current density range but are most prominent in leading to concentration polarization when current densities are large and reactants are rapidly consumed at the electrode surfaces. A thermodynamic analysis of these losses can be performed to yield the dependence of the major losses on cell operating parameters, which is presented in the following sections.

Theoretical EMF or Ideal Voltage

Theoretical EMF or Ideal Voltage

Operation Voltage, V, Curve

Current Density (mA/cm2)

FIGURE 6.5

Ideal and actual fuel cell current and voltage characteristics.

Operation Voltage, V, Curve

Current Density (mA/cm2)

FIGURE 6.5

Ideal and actual fuel cell current and voltage characteristics.

6.3.3.1 Activation Polarization

Thermodynamic analyses indicate that activation polarization occurs when the rate of an electrochemical reaction on an electrode surface is controlled by slow electrode kinetics. The rate of electrochemical reactions, similar to chemical reactions, involves overcoming an activation barrier before chemical reaction can occur. In the case of electrochemical reactions, this activation energy ranges from 50 to 100 mV and is governed by the Tafel equation (Atkins, 1986) as follows:

where a is the electron transfer coefficient, n is the number of electrons participating in the reaction, R is the universal gas constant, T is the temperature, T is Faraday's constant, i is the current, and io is the exchange current density. The exchange current density is a measure of the maximum current that can be drawn with negligible polarization (i.e., r]act = 0).

6.3.3.2 Ohmic Polarization

Ohmic polarization is caused by resistance to the flow of ions in the electrolyte and resistance to the flow of electrons through the electrodes. The dominant losses are those associated with flow of ions through the electrolyte. These losses can be reduced by decreasing the distance between the electrodes (shortening the ionic flow distance) and/or enhancing the ionic conductivity of the electrolyte material. Typically, the losses due to electrolyte and electrode resistance are lumped together. Ohm's law governs these losses as follows:

Vohm = iRcell where i is the current and Rcell is the overall cell resistance which includes ionic, electrode, and interconnect resistances. The overall cell resistance can be obtained experimentally.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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