Page 44 - Lindens Handbook of Batteries
P. 44

CHAPTER 2

                                ELECTROCHEMICAL PRINCIPLES
                                AND REACTIONS





                                Mark Salomon













                    2.1  INTRODUCTION

                                Batteries  and  fuel  cells  are  electrochemical  devices  that  convert  chemical  energy  into  electrical
                                energy by electrochemical oxidation and reduction reactions, which occur at the electrodes. A cell
                                consists of an anode where oxidation takes place during discharge, a cathode where reduction takes
                                place, and an electrolyte which conducts the current (via ions) within the cell.
                                   The maximum electric energy that can be delivered by the chemicals that are stored within or
                                supplied to the electrodes in the cell depends on the change in Gibbs energy ∆G of the electrochemical
                                couple, as shown in Eq. (2.5) and discussed in Sec. 2.2.
                                   It would be desirable if during the discharge all of this energy could be converted to useful
                                electric energy. However, losses due to polarization occur when a load current i passes through the
                                electrodes, accompanying the electrochemical reactions. These losses include: (1) activation polar-
                                ization, which drives the electrochemical reaction at the electrode surface, and (2) concentration
                                polarization, which arises from the concentration differences of the reactants and products at the
                                electrode surface and in the bulk as a result of mass transfer.
                                   These polarization effects consume part of the energy, which is given off as waste heat, and thus not all
                                of the theoretically available energy stored in electrodes is fully converted into useful electrical energy.
                                   In  principle,  activation  polarization  and  concentration  polarization  can  be  calculated  from
                                several theoretical equations, as described in later sections of this chapter, if some electro chemical
                                parameters and the mass-transfer condition are available. However, in practice it is difficult to deter-
                                mine the values for both because of the complicated physical structure of the electrodes. As covered
                                in Sec. 2.5, most battery and fuel cells electrodes are composite bodies made of active material,
                                binder, performance enhancing additives, and conductive filler. They usually have a porous structure
                                of finite thickness. It requires complex mathe matical modeling with computer calculations to esti-
                                mate the polarization components.
                                   There is another important factor that strongly affects the performance or rate capability of a cell,
                                the internal impedance of the cell. It causes a voltage drop during operation, which also consumes
                                part of the useful energy as waste heat. The voltage drop due to internal impedance is usually referred
                                to as “ohmic polarization” or IR drop and is proportional to the current drawn from the system.
                                The total internal impedance of a cell is the sum of the ionic resistance of the electrolyte (within
                                the separator and the porous electrodes), the electronic resistances of the active mass, the current
                                collectors and electrical tabs of both electrodes, and the contact resistance between the active mass
                                and the current collector. These resistances are ohmic in nature, and follow Ohm’s law, with a linear
                                relationship between current and voltage drop.


                                                                                                      2.1
   39   40   41   42   43   44   45   46   47   48   49