Cell Voltages, Polarisations and Performances

The fuel cell works under defined conditions such as gas variables, temperature, etc. The operating conditions determine the efficiency of fuel cells and hence require rigorous understanding. Behind the operation of every fuel cell, there are series of thermodynamic and chemical equations that should be well understood. In this chapter, we determine the performance, efficiency and then losses of the fuel cell. The open-circuit voltage (OCV) of a fuel cell is also taken into consideration along with various factors including pressure, temperature, fuel constituents and concentration, etc. 

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The current density and impurities also affect the fuel cell performance. In the initial stages, activation polarisation decreases the cell voltage. When current densities increase, then the concentration losses are predominant and a sharp decrease in cell performance is observed. During normal fuel cell application, ohmic losses are observed due to internal resistances of the fuel cell. This section deals with the effect of different variables on fuel cell performance and efficiency. The different fuel cells and their performances are discussed one by one. 

At higher pressure and current density, the diffusion polarisation at the cathode decreases and reversible cell potential increases. At the cathode, increased oxygen and water pressures decrease activation polarisation. As shown in Fig. 2.5, an increase of 44 mV is observed when pressure and temperature are increased by 2.9 atm and 15°C, respectively. An increase in temperature exhibits a beneficial effect on cell performance because the polarisation losses diminish with increased temperature. The relationship between voltage gain and temperature change is given by ... 

For MFC, ferricyanides and manganese oxides have been used as alternatives for oxygen. The main application of MFC is to treat the wastewater. The ohmic resistance plays dominant role in the polarisation curves. The polarisation and power curves for MFC are shown in Fig. 2.12 for starch substrate. The polarisation curves for MFC are usually linear in nature and hence the internal resistance value can be easily obtained from their slope. The performance of MFC depends upon the electrochemical reactions occurring between the electron acceptor with high potential and organic substrate with low potential. In MFC, it becomes uncertain to have an idea of the cell voltage because the electron transfer takes place via the organic substrate and complex chain which varies from microbe to microbe. 

Electrochemical performance Full open circuit voltage Lott polarisation loss Insignificant gas leakage or cross-leakage (no or minimal sealing) No electrical short Uniform gas distribution between cells and across cell Easy gas access to reaction sites... 

The difference E — E(i) is a measure of the change in gas phase compositions just outside of the electrodes. This difference must be accounted for in the overall description of cell performance. The voltage loss term (i) is known as the polarisation or overpotential, and is a function of current density it consists of a number of terms, with their origins related to various phenomena occurring in the cell, under a finite current. The different polarisations are termed (a) ohmic... 

At the effective property or continuum level, the simulation of electrode and cell performance basically requires only a parameterised electrochemical model. Such an electrochemical model is usually described as a current-voltage relation, or I-V curve, for a single cell, in terms of parameters that are effective cell properties and operational parameters. The I-V relation describes the voltage (potential) loss at a specified current with respect to the ideal thermodynamic performance, which is called overpotential or polarisation (q). This cell I-V curve is specific for the materials, structural characteristics, and operational parameters (gas compositions, pressure, temperature) of a given PEN element. 

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