Pressure open-circuit voltage dependence

Cell open-circuit voltage depends on temperature. To rationalize this dependence let us now consider the reaction under constant pressure. Setting dP = 0 in (1.7) we arrive at... 

The value of E - y is called the open-circuit voltage of the cell, which is related to the composition of the product. Note that the steam conversion ratio, X, depends on the open-circuit voltage, and is not affected by the pressure or flow rate of the reactant. Also, the open-circuit voltage decreases with increasing temperature because of the endothermic nature of the reaction. However, due to the temperature dependence of the logarithmic term in Equation 4.5, this effect decreases with the value of X. 

The open circuit voltage of a fuel cell varies with the partial pressure of the supplied reactants. The equUibrium condition is expressed by the Nernst equation which relates open circuit (OC) ceU voltages and molar Gibbs energy to reactant pressure and activity (concentration) [2]. The dependence on reaction temperature is specified further below. 

The thermodynamic electromotive force of a polymer electrolyte membrane fuel cell at a temperature of 25°C is given by e = 1.229 V. The open-circuit voltage (OCV) of a hydrogen-oxygen polymer electrolyte membrane fuel cell has values between 0.95 and 1.02 V, depending on the temperature and gas pressures. 

The fuel cell voltage at an open circuit condition is called the open circuit voltage (OCV). This OCV is defined as the difference between the standard potentials of the cathode (.E ) fhe anode (E ), and its value is 1.229 V at standard conditions. Note that this OCV is strongly dependent on the temperature and pressure. If the fuel cell is loaded with an electrical current, the cell voltage will be lower than the OCV due to the electrode polarizations (or electrode overpotentials). 

The macroscopic modeling described for the PEMFCs can also be apphed to the case of an SOFC fed with hydrogen if we have tests which enable us to identify the different parameters. The activation overvoltage term is far smaller in the case of an SOFC. In fact, the open circuit voltage of a cell is nearest to the reversible potential of the cell. Because of the high temperature, all the species are in gaseous form, and their activity is equal to their partial pressure. The Ohmic resistance of the MEA (the electrodes and the membrane) is highly dependent on the temperature. 

With the activity expressed as p =p/po, (partial pressure/standard pressure) the pressure dependence of the open circuit cell voltage can be written as ... 

El = 1.229 V (vs. NHE)), and the Pt/PtO anode reaction potential (Pt + H2O PtO + 2YC + 2e", E°p p,o = 0.88 V (vs. NHE)). The local electrochemical reaction on the Pt surface could create a PtO surface coverage of 30%, with 70% of the surface remaining as pure Pt. At steady-state mixed potential, a complete layer of Pt-0 can never be achieved in order to keep the reaction of Pt to PtO continuous, due to the diffusion of Pt-0 into the bulk metal. The reported mixed cathode potential is around 1.06 V (vs. NHE) at standard conditions [20, 21] with an O2 partial pressure dependence of 15 mV-atm Furthermore, the H2 that has crossed over through the membrane from anode to cathode can form a local half-cell electrochemical reaction on the cathode, such as H2 2H + 2e, resulting in a mixed cathode potential similar to that of the half-cell reaction (Pt -1- H2O PtO -1-2H + 2e ). The mixed potentials are the dominant sources of voltage losses at open circuits [13]. 

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 ... 

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