应用电化学Chapter_5

1. The container
Requirements: Resistant to corrosion (stable) The required mechanical strength Cheap and light Simple of sealing For alkaline batteries: steel polypropylene For systems with an acid electrolyte:
Porous electrodes are commonly used to increase the
surface area and the electrolyte. The porous electrode is a mass of particulate reactants with many random and tortuous electrolyte channels between. The sizes of the particles and the pores (i.e. the porosity of the paste) are important in determining the performance of the battery. In practice, porosity should
The weight of the electrolyte is a major contribution to
that of the complete battery and, hence, must be
minimized.
5. Active materials
Requirements:
a) high electromotive force of the cell
(3) mechanical strength, flexibility, wetability
(4) cheap and readily available
Materials: such as plastic pins, nylon or cellulose-based felt, ion exchange membranes etc.
b) more active electrochemical reaction
c) large specific capacity
d) stable chemically to electrolyte e) good electronic conductivity f) cheap and readily available
Form: microporous (pore size 0.01-10 um) or macroporous (30-70um pores) polymer sheets
3.
Current collectors
Function: a) provide a conducting path through the paste and thereby minimize the resistance of the battery b) act as a physical support for the active mass which otherwise would be a very brittle structure. Form: plate , closely spaced grid, thin metal sheet, expanded metal Corrosion of the collector and shedding of the active paste are two major causes of battery failure.
Chapter 5
Chemical Power Sources
5.1 Introduction
A battery is a device which can store chemical energy and, on demand, convert it into electrical energy to drive an external circuit. The electrical energy results from a spontaneous chemical change within the battery. Development of batteries: 1800 1836 Volta cell Daniell cell
2. Separators
Requirements for separators: (1) stable chemically to the electrolyte and to the active materials (2) low resistance to the transport of ions
4.
Electrolyte
The selection of the electrolyte is determined by the
electrode reactions and its concentration is also
important.
Temperature has a great effect on electrolyte properties.
the range of current to be drawn from the cell (i.e. the
electrode reaction should be fast); (3) designing the cell with a low resistance, i.e. with a high conductivity electrolyte, low resistance separator and small interelectrode gap.
(1) the selection of electrode reactions which lead to
an overall cell reaction with a high negative free
energy change;
(2) electrode reactions without large overpotentials in
2.
Cell resistance
V E iRCELL E i( R R f )
Operation voltage can be expressed as:
where RCELL is the total electric resistance of the battery,
The major contribution will normally be from the charge transfer overpotential since mass transport control has a catastrophic effect on the battery voltage and one would not normally design a battery to operate in such conditions. Examples of nucleation and passivation overpotentials do occur. The nucleation overpotential is normally a transitory phenomenon since, once nuclei of the new phase are present in numbers, the overpotential will disappear. The nucleation overpotential therefore occurs as a dip on the discharge curve at the commencement of discharge.
3. Discharge system (放电制度)
Discharge conditions: Stopping voltage (终止或截止电压): the minimum voltage Modes of discharge:
a) Discharge into fixed resistance
VCELL E E C A iRCELL
C e A e
The cell voltage, excluding the iRCELL of a practical
battery, can be estimated from i-E data for the anode and cathode processes.
(2) Secondary cell/ battery Cell reactions are reversible and can be recharged. e.g. Pb∣H2SO4∣PbO2 Cd∣KOH∣NiOOH Zn∣KOH∣Ag2O
(3) Reserve cell/battery
stored dry and activated before use e.g. Mg∣MgCl2∣AgCl Zn∣KOH∣Ag2O
b) Discharge (and charge) at constant currents c) Discharge at constant wattages continuous discharge and intermittent discharge
(4) Fuel cell
Active mass are not stored within the cell. H2∣KOH∣O2 N2H4∣KOH∣O2
5.1.2 Battery components
The performance of the battery will depend on the cell geometry and on the design and composition of all the components of the battery, in addition to the choice of electrode reactions and their kinetics.