An electrochemical cell is an apparatus that is used to generate electricity from a spontaneous oxidation-reduction reaction, or that uses electricity to drive a nonspontaneous reaction. An electrochemical cell is called such because it utilizes the principles of electrochemistry and is the smallest functional unit of an electrochemical system (a cell). Electrochemistry is the study of electricity and how it relates to chemical reactions. In electrochemistry, electricity can be generated by movements of electrons from one element to another in a reaction known as redox or oxidation-reduction reaction. In redox reactions, electrons are transferred from one species to another. If the reaction is spontaneous, energy is released, which can then be used to do useful work. If the reaction is nonspontaneous, energy is consumed and work must be put into the system.
Electrochemical cells are important in society today especially in terms of energy storage. Electrochemical cells such as batteries can be used on a large scale to store energy from intermittent energy sources like solar and wind. They can also be used on a smaller scale to power our phones, laptops and other electrical appliances.
There are two types of electrochemical cells: galvanic cells (also called voltaic cells) and electrolytic cells. Both types contain two electrodes, one anode and one cathode. Oxidation occurs at the anode and reduction occurs at the cathode. The electrodes are connected to each other with an external electrical connection, such as a wire, to allow electrons to flow from the anode to the cathode. The electrodes are also connected by an electrolyte, an ionic substance or solution that allows ions to transfer between the electrodes. This maintains the system’s electrical neutrality.
Galvanic cells, also called voltaic cells, are driven by a spontaneous chemical reaction. This means that electrons will flow spontaneously from one side of the electrochemical cell to the other. This produces an electric current through an outside circuit. The flow of charge is generated by an electrical potential difference between two points in the circuit. The cell potential is created when the two different metals are connected, and measures the energy per unit charge available from the oxidation-reduction reaction. These cells are the basis for devices like batteries and fuel cells. But they are not the only kind of electrochemical cell.
In an electrolytic cell, a nonspontaneous chemical reaction takes place. A nonspontaneous reaction means that electrons don't want to move. Thus, an electrolytic cell requires an external source of electrical energy to force electrons to flow. Driving the redox reaction in a nonspontaneous direction generates a potential difference between the metal electrodes. In the case of rechargable batteries, they act as galvanic cells when they are discharging electrical energy, but act as an electrolytic cell when they are being recharged.
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