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Electrochemistry of Fuel Cells
Published in Xianguo Li, Principles of Fuel Cells, 2005
From this analysis and discussion, it becomes clear that the energy efficient approach to high current density (hence, high output power density) operation for practical fuel cells is to increase the exchange current density, because the generation of entropy represents the degradation of the useful energy. Increasing the cell operation temperature is commonly most effective to increase the exchange current density because of its strong temperature dependence. However, for each type of fuel cells under development limitations are found on the highest temperatures that a cell can be operated or tolerated. Therefore, other measures have been explored to increase the exchange current density. Some common techniques are to find the most effective catalyst or electrolyte or their combinations to facilitate the heterogeneous electrochemical reaction, and these techniques have been met with considerable difficulty for further significant progress. Since exchange current density is also a function of the reactant concentration, increasing the reactant concentration at the reaction sites is also effective to increase the current density without the increase in the generation of entropy; this can be achieved by increasing the operation pressure, by using the purified reactants (pristine hydrogen or oxygen), and by deploring electrolytes with high reactant solubilities, and so on. Due to the various practical limitations, the approach to minimize transport resistances to mass transfer is presently most feasible for each type of fuel cell, that includes the improved design of gas flow fields on the bipolar plates, electrode backing, and the catalyst layer.
Theory of Compound Semiconductor Electrodeposition
Published in R.K. Pandey, S.N. Sahu, S.N. Sahu, S. Chandra, Handrook Of Semiconductor Electrodeposition, 2017
R.K. Pandey, S.N. Sahu, S.N. Sahu, S. Chandra
The exchange current density j0 and the transfer coefficients αc and αa are two important parameters related to the kinetics of the charge transfer reaction. For a given electrode potential, the net current density will be higher for the process with the higher exchange current density. The exchange current density depends on the nature of the reaction, the electrode material, and the bath composition. The transfer coefficient describes the effect of the electric field on the charge transfer step and the symmetry of the cathodic and anodic processes. Its dependence on the electrode material is usually small.
Vanadium Flow Batteries
Published in Huamin Zhang, Huamin Zhang, Xianfeng Li, Jiujun Zhang, Redox Flow Batteries, 2017
In the preceding equation, i0 is called the exchange current density, and it is influenced by the reaction rate constant, the electrode material, and the reactant concentration. Under the equilibrium potential, although there seems to be no change on the electrode, the reduction and oxidation reactions are still continuing. The reaction rate represented by the current density is the exchange current density.
A simplified state-space model for performance analysis of proton exchange membrane fuel cell
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2020
From Figure 2, at light loading condition there is heavy nonlinear potential drop in polarization characteristics of PEMFC, called as activation polarization drop. The activation polarization drop depends on exchange current density and operating temperature of PEMFC. The exchange current density again depends on electrode surface area and catalyst activity. The electrochemical double-layer capacitance (ELDC) formed between electrode and electrolyte also depends on electrode surface area and contact area between electrode and electrolyte. Hence, increase in electrode surface area and contact area between electrode and electrolyte increases numerical value of double layer capacitance. As double layer capacitance and exchange current density both depend on electrode surface area, therefore increase in double layer capacitance increases exchange current density. Increase in exchange current density decreases activation polarization drop, increases terminal voltage and hence increases power density. The effect of double layer capacitance on terminal voltage of PEMFC is shown in Figure 8. For analysis, four different values of capacitances (4 F, 8 F, 42 F, and 48 F) are considered. The decrease in activation polarization drops improves slow dynamics and increase in terminal voltage increases peak power supplying capability of PEMFC.