Electrochemical Cell

An electrochemical cell is a device that couples chemical change to the flow of electric current. Every cell contains two electrodes, an anode where oxidation (loss of electrons) happens and a cathode where reduction (gain of electrons) happens, immersed in an Electrolyte that carries ions between them. Electrons travel through an external wire while ions move through the electrolyte, completing the circuit. The overall chemistry is split into two complementary half-reactions, one at each electrode. Cells come in two flavours. A galvanic cell (also called a voltaic cell) runs a spontaneous redox reaction and releases energy as electricity; here the anode is the negative terminal and the cathode the positive one. An electrolytic cell does the reverse: an external power supply forces a non-spontaneous reaction, with the anode positive and the cathode negative. Electrolytic cells drive water splitting, electroplating, and industrial extraction such as The Hall-Héroult Process: How Electrolysis Made Aluminum Affordable. Throughout, the rule "anode = oxidation, cathode = reduction" holds regardless of cell type; only the terminal polarity flips. The difference in electrode potentials sets the cell voltage (electromotive force), measured against the standard hydrogen electrode. Aqueous cells are limited to roughly 2.5 volts before the water itself decomposes, and voltage sags as reactants are consumed during discharge. To reach useful voltages, many cells are wired in series to form a battery -- see Why Batteries Are Built From Many Small Cells Instead of One Giant One. A single-use cell with irreversible chemistry is a primary cell, while a rechargeable cell with reversible reactions is a secondary cell, as in the Lithium-Ion Battery. The Flow Battery is a variant that stores its reactants in external tanks, decoupling power from capacity.