electrochemical cell

A redox reaction is just electrons jumping from one substance to another. Usually they jump at the point of contact and the energy escapes as heat. The trick of the electrochemical cell is to physically separate the two halves so the electrons cannot jump directly — they are forced to travel the long way round, through a wire, where we can put them to work or measure their push. A battery is exactly this: a redox reaction whose electrons are made to flow through a circuit.

A cell has two electrodes sitting in electrolyte. At the anode, oxidation happens — electrons are released; at the cathode, reduction happens — electrons are consumed. (A handy memory: an Ox at a Red Cat — oxidation/anode, reduction/cathode.) In the classic Daniell cell, a zinc rod in zinc sulfate is the anode (Zn -> Zn2+ + 2 e-) and a copper rod in copper sulfate is the cathode (Cu2+ + 2 e- -> Cu). Electrons flow through the external wire from zinc to copper; a salt bridge lets ions drift between the two compartments to keep each side electrically neutral, closing the circuit.

There are two flavours, and the distinction matters. A galvanic (or voltaic) cell runs a spontaneous reaction and gives out electrical energy — every battery and fuel cell is one. An electrolytic cell does the reverse: you push electricity in to force a non-spontaneous reaction, as in electroplating, charging a battery, or extracting aluminium from molten ore. One subtle trap: the anode is the oxidation electrode in both, but it is the negative terminal in a galvanic cell and the positive terminal in an electrolytic one, so 'anode = negative' is not a reliable rule.