Battery Operation

Why does this produce electricity? The flow of current can be understood as the flow of ions from the more reactice metal to the less reactive metal. The ions moving from one electrode to the other creates an electrical charge which is neutralised by the flow of electrons across the wire.

Before considering the reaction of two metals, consider what happens when we place a single metal electrode in an electrolyte. Some of the metal atoms in the electrolyte go into solution as ions while the remaining electrons create a negative charge on the metal. The separation of ions and electrons leads to a separation of charge. However, this build up of charge cannot continue indefinitely because as the negative charge builds up in the metal it becomes increasingly difficult for positive metal ions to go into solution. A similar build up in positive charge in the electrolyte also prevents the build up of charge. This degree of charge build up depends on the metal and represents the work required to separate electrons from the ions. This is known as the electroneutrality principle

Similarly, if a copper strip is placed in an aquaous Copper (II) Sulfate solution the copper will also lose ions. These reactions are often written as Cu | Cu⁺² this is the half-cell reaction.

The tendancy for Zinc to lose ions is greater than that of Copper. When the two cells are joined together (using a copper wire to connect the electrodes and porous barrier that allows the ions to pass known as a salt-bridge connect the elecrolytes, the build up of electrons on the zinc will flow to through the wire onto the copper.

The copper ions in the electrolyte gain electrons and become copper atoms. Thus the reaction can be written,

Zn | Zn²⁺ | | Cu²⁺ | Cu

To continue the reaction, the charge must be removed. This can be acheived by coupling a second reaction which uses the electrons in the metal to convert the ions in the electrolyte into a metal. For a more specific example, consider a zinc electrode in an electrolyte of Copper (II) Sulphate solution.

Figure 3. Danile Cell

The loss of electrons by the Zinc is known as oxidation.

Zn(s) → Zn²⁺ + 2e^-.(1)

A wire connecting the Zinc electrode to a Copper electrode, allows the electrons to flow to the Copper electrode. Copper ions in the copper sulphate solution take up the electrons and become atoms of copper on the copper electrode. The gaining of electrons by ions is known as reduction;

Cu⁺² + 2e^- → Cu(s).(2)

The net reactions is then,

Zn(s) + Cu²⁺ → Cu(s) + Zn²⁺

When the two electrodes are joined by a wire the charge stored can flow and the electrons combine. The simplest kinds of battery have two conductors made of different materials which are partially emersed in a solution which allow the electrons and ions to flow freely known as an electrolyte.

At the copper electrode (cathode), the acid dissolves the copper metal producing hydrogen gas, H⁺. The reaction will continue until the supply of zinc is used up. The electrons, with their negative charge, are attracted to the copper electrode which causes a current to flow. One of the problems with this cell is that the current stops flowing after a short time because the hydrogen bubles block the current.

Cells using aqueous (containing water) electrolytes are limited in voltage to less than 2 Volts because the oxygen and hydrogen in water dissociate in the presence of voltages above this voltage. Lithium batteries (see below) which use non-aqueous electrolytes do not have this problem and are available in voltages between 2.7 and 3.7 Volts. However the use of non-aqueous electrolytes results in those cells having a relatively high internal impedance.