Electrolysis

Electrolysis is a process in which externally supplied electrical energy drives a chemical reaction that is nonspontaneous under the operating conditions. It involves electron transfer at electrodes and belongs to electrochemistry. Unlike a galvanic cell, which generates electricity from a spontaneous reaction, an electrolytic cell consumes electricity to produce chemical change. Applications include extracting metals, manufacturing chemicals, depositing coatings, and charging rechargeable batteries. (openstax.org)

Cell structure and electrode reactions

An electrolytic apparatus is a type of electrochemical cell containing two electrodes, an electrolyte, and an external power source. The electrolyte provides ionic conduction and may be a molten salt, an aqueous solution, or a solid ion-conducting material. A membrane or diaphragm can separate the electrode compartments while permitting ionic transport. (openstax.org)

The anode is where oxidation occurs: chemical species lose electrons. The cathode is where reduction occurs: species gain electrons. Together, these processes constitute a redox reaction. In a conventional electrolytic cell, the anode connects to the positive terminal and the cathode to the negative terminal of the power supply. Their names describe their reactions, not an invariant electrical polarity across all electrochemical devices. (openstax.org)

Electrons carry current through the external circuit, while ions carry current through the electrolyte. For molten sodium chloride, the half-reactions are:

The overall reaction produces sodium metal and chlorine gas. The products must remain separated to prevent their recombination. (openstax.org)

Thermodynamics and applied voltage

Thermodynamics establishes the minimum electrical work required for electrolysis. The relationship between reaction Gibbs free energy and reversible cell potential is

where $n$ is the number of electrons transferred per reaction as written and $F$ is the Faraday constant. For the nonspontaneous reaction direction, $\Delta G$ is positive and $E_{\mathrm{cell}}$ is negative. An external source must supply an opposing potential sufficient to drive that direction. (openstax.org)

Equilibrium potential depends on chemical composition and temperature, as described by the Nernst equation. Consequently, the voltage required is not a fixed property of a substance independent of operating conditions. Changes in reactant activities or product pressures alter the reversible potential. (openstax.org)

Practical operation requires additional voltage. Overpotential is the displacement of an electrode’s potential from its equilibrium value needed to sustain a specified current. Electrical resistance also produces potential drops through the electrolyte, electrodes, and connections. Thus, the operating voltage includes both the thermodynamic requirement and losses associated with current flow. (goldbook.iupac.org)

Faraday’s laws and product yield

Faraday’s laws of electrolysis relate chemical conversion to the electric charge passed through a cell. The first law states that the mass transformed is proportional to charge. The second states that, for equal charges, transformed masses are proportional to their chemical equivalent molar masses. These relationships are expressed as

where $m$ is product mass, $M$ is molar mass, $Q$ is charge, and $z$ is the number of electrons required per product formula unit. (goldbook.iupac.org)

For constant current $I$ over time $t$, $Q=It$. This connects electrical measurements with stoichiometry: producing one mole of a species requiring two electrons consumes two Faradays of charge in the ideal case. Actual yields can be smaller when competing electrode reactions consume some current. Calculations therefore distinguish total charge passed from charge contributing to the intended product. (goldbook.iupac.org)

Water electrolysis

Electrolysis of water produces hydrogen at the cathode and oxygen at the anode:

The electrode half-reactions depend on the electrolyte. In an acidic or proton-conducting system, water oxidation generates oxygen, protons, and electrons; protons then combine with electrons at the cathode to form hydrogen. Alkaline systems instead transport hydroxide ions between the electrodes. (energy.gov)

Three major configurations are alkaline, polymer electrolyte membrane, and solid oxide electrolyzers. Alkaline systems commonly employ aqueous potassium or sodium hydroxide. Polymer electrolyte membrane systems use a solid proton-conducting membrane. Conventional solid oxide systems use a ceramic oxygen-ion conductor at elevated temperature and electrolyze steam. High-temperature operation allows part of the energy requirement to be supplied as heat rather than electricity. (energy.gov)

Industrial applications

The chlor-alkali process electrolyzes sodium chloride brine to produce chlorine, sodium hydroxide, and hydrogen. Chlorine forms at the anode, while hydrogen forms at the cathode. A membrane or diaphragm separates the products and restricts their mixing. Unlike molten sodium chloride electrolysis, this aqueous process does not produce sodium metal. (eurochlor.org)

Aluminum production uses the Hall–Héroult process, in which aluminum oxide dissolved in molten cryolite undergoes electrolysis. Charles Martin Hall and Paul Héroult independently developed the process in 1886. Its commercialization established an economical route to aluminum and became the foundation of large-scale production. (acs.org)

Electroplating deposits a metal coating onto an electrically conducting object through cathodic reduction. Related electrolytic methods purify metals and manufacture chemical commodities. Charging a rechargeable battery likewise uses an external electrical source to drive reactions opposite to those occurring during discharge. (openstax.org)

Energy and environmental considerations

Electrolysis converts energy rather than creating it. For hydrogen production, electricity cost, equipment cost, conversion efficiency, and operational lifetime influence economics. Environmental performance depends strongly on how the electricity is generated: using renewable energy or nuclear electricity can substantially reduce production-related emissions, whereas carbon-intensive electricity can undermine that benefit. The absence of carbon dioxide in the water-splitting reaction alone does not establish the emissions of the complete production pathway. (energy.gov)