Page 38 - Handbook of Battery Materials

4 1 Thermodynamics and Mechanistics

conversion chemical conversion
electrical -> chemical storage chemical -> electrical

electrolysis fuel cell

primary battery
charging secondary battery discharging

Figure 1.1 Chemical and electrical energy conversion and possibilities of storage.

consumer
e - e -
e - e -

Zn (s) Cu (s)

Zn 2+ Cu 2+
(aq)
ZnSO 4 (aq) CuSO 4 (aq)
Figure 1.2 Daniell element.

2) Secondary cells are rechargeable several times [1]. Only reversible electrochemi-
cal reactions offer such a possibility. After the cell is discharged, an externally
applied electrical energy forces a reversal of the electrochemical process; as a
consequence the reactants are restored to their original form, and the stored
electrochemical energy can be used once again by a consumer. The process
can be reversed hundreds or even thousands of times, so that the lifetime of
the cell can be extended. This is a fundamental advantage, especially as the
cost of a secondary cell is normally much higher than that of a primary cell.
Furthermore, the resulting environmental friendliness should be taken into
account.
3) Fuel cells [2]: In contrast to the cells so far considered, fuel cells operate in
a continuous process. The reactants – often hydrogen and oxygen – are fed
continuously to the cell from outside. Fuel cells are not reversible systems.
Typical fields of application for electrochemical energy storage systems are
in portable systems such as cellular phones, notebooks, cordless power tools,