An electrode is an electrical conductor that connects an external circuit to an electrochemical system. In electrochemistry, the term can denote either the electron-conducting component itself or a half-cell comprising that component and an electrolyte. Electrodes provide the interfaces through which electrical and chemical processes interact; they can operate with substantial current or under conditions where virtually no current flows. Their classification depends on composition, geometry, and function rather than on a single characteristic material or shape. (goldbook.iupac.org)
Charge transport and electrode reactions
An electrochemical system generally joins two different kinds of charge transport. Electrons move through the conducting electrodes and external circuit, while ions carry charge through the electrolyte. In a battery, coordinated movement of these charge carriers enables chemical energy to be stored during charging and released as electricity during discharge. Electrons do not normally travel through the electrolyte along the same path as ions. (energy.gov)
At a reacting electrode, a oxidation–reduction reaction transfers electrons between the conductor and an electroactive substance. The substance may be dissolved in the electrolyte or incorporated into the electrode material. These interfacial reactions distinguish electrochemical conduction from the passage of current through an ordinary metallic wire. However, an electrode need not continuously undergo a net chemical transformation: electrodes also support potential measurements and interfacial charge storage. (goldbook.iupac.org)
Anodes, cathodes, and polarity
The anode is the electrode at which oxidation occurs; the cathode is the electrode at which reduction occurs. These names identify reaction direction, not an unchanging positive or negative polarity. In a galvanic cell delivering electrical energy, the anode is negative and the cathode positive. In an electrolytic cell driven by an external power supply, the anode is positive and the cathode negative. (doi.org)
For a rechargeable battery, reversing operation reverses the electrode reactions. Strictly, a physical electrode that acts as the anode during discharge acts as the cathode during charging. Battery literature nevertheless commonly retains the discharge-based labels “anode material” and “cathode material” throughout operation. “Positive electrode” and “negative electrode” can therefore be clearer when describing the physical components independently of reaction direction. (doi.org)
Potential and current
An electrode potential is conventionally determined relative to a reference electrode, rather than measured as an isolated absolute quantity. The reference provides a reproducible basis for comparing electrode systems. The standard hydrogen electrode defines the conventional zero of standard electrode potential in protic solvents; practical measurements frequently use other reference systems, including silver–silver chloride electrodes. (iupac.org)
For an equilibrium electrode reaction, the Nernst equation relates potential to standard potential, temperature, and the activities of participating substances. Potential is consequently a property of an electrode–electrolyte system under specified conditions, not simply an intrinsic number belonging to a piece of metal. (media.iupac.org)
When current flows, the electrode potential can depart from its equilibrium value. This departure, associated with sustaining a specified current, is called overpotential, conventionally written (\eta=E-E_{\mathrm{eq}}). It must be distinguished from the ohmic potential drop through the electrolyte and electrical connections. Both affect measured operating voltages, but they describe different contributions. (goldbook.iupac.org)
Materials and construction
Electrodes may be foils, wires, discs, meshes, or more complex structures. Platinum is a widely used metallic working-electrode material, but it is not chemically featureless: it adsorbs hydrogen and can develop oxide films. Its accessible operating range depends on the electrolyte and the presence of substances such as water and oxygen. Thus, a material described as “inert” is inert only within particular experimental conditions. (goldbook.iupac.org)
Battery electrodes often use materials that reversibly accommodate ions. In many lithium-ion batteries, the negative electrode contains graphite, while the positive electrode contains a lithium-bearing transition-metal oxide or phosphate. Charging transfers lithium ions from the positive material into the graphite structure; discharge reverses the process. These electrodes are consequently active storage materials, not merely electrical terminals. (energy.gov)
Electrode architecture also influences performance. Internal pathways must permit both ionic and electronic transport, while accessible surface area determines how much interface is available. Nanostructured and porous electrodes can shorten transport distances, although their behavior depends on the particular material and reaction. In double-layer supercapacitors, porous electrodes store charge through the organization of ions at the electrode–electrolyte interface. (energy.gov)
Experimental and technological functions
In electrochemical analysis, a working electrode provides the response associated with the substance or reaction being investigated. A three-electrode arrangement separates this function from those of the reference electrode and the counter electrode, also called the auxiliary electrode. Current passes between working and counter electrodes, while the working-electrode potential is measured relative to the reference. A potentiostat maintains the selected potential difference by controlling the current circuit. (old.goldbook.iupac.org)
In electrolysis, externally supplied electricity drives electrode reactions. In fuel cells, electrodes instead support reactions that generate electricity from supplied reactants. A proton-exchange-membrane fuel-cell assembly combines catalyst layers, an ion-conducting membrane, and gas-diffusion media. Supported platinum mixed with an ion-conducting polymer illustrates how an electrode can integrate electronic conduction, ionic conduction, reactant transport, and catalysis within one engineered structure. (goldbook.iupac.org)