The device through which chemical energy is converted into electrical energy is known as galvanic cell or electrochemical cell or voltaic cell. In a galvanic cell, a redox reaction is carried out in an indirect manner and the decrease in free energy process appears as electrical energy. An indirect redox reaction is such that reduction and oxidation processes are carried in separate vessels. In order to understand this phenomenon, let us consider the Zn - CuSO4 reaction as the basis of the cell reaction.
In its simple form, a zinc strip is dipped in the ZnSO4 solution and copper strip is dipped in the CuSO4 solution taken in separate beakers. The two metallic strips which act as electrodes are connected by the stripes conducting wires through a voltmeter. The two solutions are connected through a reversed U-tube known as salt bridge. The U-tube is filled with the solution of some electrolyte such KCl, KNO3 or NH4Cl to which gelatin or agar-agar has been added to convert it into semi-solid paste. A schematic of this cell has been shown. The deflection in voltmeter shows that there is a potential difference between the two electrodes. It has been establish that the conventional current flows from the outer circuit from copper to zinc strip. It means that the electrons flow occurs from zinc to copper strip. Let us now understand the working of the cell. (i) Zinc undergoes oxidization to form zinc ions Zn(s) Zn2+(aq) + 2e- (oxidization) (ii) The electrons liberated during oxidization are pushed through the connecting wires to copper strip. (iii) Copper ions move towards copper strip, pick up the electrons, and get reduced to copper atoms which hare deposited at the copper strip. (iv) Cu2+(aq) + 2e- Cu(s) (reduction) The electrode at which oxidization occurs is anode and that at which reduction occurs is cathode. In the above that, zinc strip is anode and the copper strip is cathode. Due to the oxidization process occurring at the anode it becomes a source of electrons and acquires a negative charge in the cell. Similarly, due to the reduction process occurring at the cathode it acquires positive charge and becomes a receiver of the electrons. Thus the electrochemical cell, anode electrode acts as negative terminal and cathode electrode acts as a positive terminal.