Pressure Dependence of the Equilibrium Cell Voltage

By integration the dependence of the equilibrium cell voltage on the partial pressure of the dissolved gas (with the integration constant K equivalent to A%, [10]) is obtained. 

Electrochemical cells may consist of two electrodes of the same type, but with different concentrations of the electroactive species in the electrolyte. Such cells are known as concentration cells. For example, two platinum electrodes operate in two H+/H2 solutions of different activity, separated by a membrane. The equilibrium cell voltage is defined by Equation (21a). As the standard potential is the same for both electrode reactions, the measurable cell voltage will depend only on the activity ratios, Equation (21b). If in this system both electrolytes were in equilibrium with the same EE pressure, the measured E would respond linearly to the pH difference between the two electrolytes, Equation (21c) (i.e. a pH electrode). 

Introducing Nemst s law for the equilibrium case, the simation when no current (and hence power) is delivered by the cell, the equilibrium ceU voltage under nonstandard conditions for a H2/O2 ceU in dependence of the respective reactant/ product concentration (partial pressures) can be expressed as ... 

Equilibrium of a reaction can be shifted by a change in pressme or temperature the equilibrium constant depends on temperature and pressure. Because of the Nemst relation, such a change results in a voltage change in an electrochemical cell. Example 6.5 illustrates this. 

To predict the local polarisation in a full-scale cell or stack at any point, its dependence on composition, pressure, and temperature of the gas flowing in the gas channel contacting the electrode must be known. In a large cell, these bulk gas properties vary from one point to the next. Electrode polarisation or overpotential - the difference between the local potential of the electrode under load and the potential at open circuit (equilibrium potential) - is also a local quantity because it depends not only on the bulk gas composition but also on the current density. In a large cell the current is usually distributed nonuniformly, as discussed in Sections 11.2-11.5. Similar to Eq. 7, one can express the local cell voltage under load, i.e., when current is passed, as the thermodynamic cell potential minus three loss terms the ohmic loss, the cathode polarisation, and the anode polarisation ... 

Therefore such sensors are called Nernstian sensors. As a reference air with defined humidity is used. In reducing gases that are in chemical equilibrium (e.g., H2, H2Oj CO, C02 water gas) the oxygen partial pressure is determined by the mass law constant Kv and this in turn depends on the temperature. In the case of H2,H20-mixtures the cell voltage is obtained by insertion of a temperature function of log Kp into the Nernst equation... 

PEM fuel cell characteristics are generally described with polarization curves. The thermodynamic equilibrium potential of the hydrogen/oxygen reaction is reduced by various overvoltage terms that depend on mass transport, kinetic, and ohmic phenomena within cell. In other words, the output voltage of a single cell is attributable to different current, temperature, and pressure dependant factors [1]. 

The position of equilibrium of a reaction maybe affected by changes in the concentration of reagents, temperature and pressure of gases. The voltage of an electrochemical cell will also depend on these factors, so we should use standard conditions when comparing electrode potentials. These are ... 

---