Industrial electrolysis involves passing a direct electric current through an electrolyte to elicit a range of chemical reactions. Electrolyzers are key pieces of technology for producing many important industrial chemicals. This one-page reference reviews the basic general operation of an electrolyzer and briefly describes several industrial processes for which electrolysis is a critical part.
Electrolyzer operation
Electrolysis refers to the process of using an electric current to decompose a compound into its constituents. All electrolyzers have an electrolyte, which contains the compound that is to be decomposed and separated, as well as a cathode, from which current flows into the electrolyte, and an anode, into which current flows from the electrolyte. When a direct electric current is applied across the electrodes from an external power source, the ions in the electrolyte are attracted to the oppositely charged electrode (cations are attracted to the negatively charged cathode; anions to the positively charged anode) where they undergo oxidation-reduction (redox) reactions. In redox reactions, the cations gain electrons and are reduced, while the anions lose electrons and are oxidized.
Industrial electrolysis applications
Many important industrial chemical reactions use electrolysis. Listed here are several key electrolysis processes.
Chlor-alkali. The chlor-alkali industry depends on the electrolysis of aqueous sodium chloride (NaCl) to produce sodium hydroxide (NaOH), diatomic chlorine gas (Cl2) and hydrogen (H2). Currently, 95% of the estimated 580 chlor-alkali plants around the world use either membrane technology (83%) or diaphragm technology (12.5%) to keep the products separated [1]. Brine flows into a series of electrolytic cells and direct-current (d.c.) electricity flows between the electrodes, which are submerged in the brine. Chlorine gas bubbles up through the brine, where it is collected, while NaOH is collected from the bottom of the cell. H2 is also collected.
Electrolytic purification of copper. For efficient transmission of electricity through copper wires, the copper must be 99.99% pure. An electrolysis process known as electrowinning can be used to carry out this purification. An electrical current passes through an inert anode (positive electrode) and through the copper solution from a prior step in the process, which acts as an electrolyte. Positively charged copper ions come out of solution and are plated onto a cathode as pure copper.
Electrolytic extraction of aluminum from bauxite. Bauxite is a mixture of hydrated aluminum oxides, along with impurities such as iron oxides, titanium oxides and silicon oxide. The last stage of the purification process involves electrolysis of purified aluminum oxide. The aluminum oxide is contained in a solution of molten cryolite, a natural fluoride mineral that is manufactured artificially. Electricity is applied across the electrodes in rows of reduction cells. Inside the cells, high temperatures and a conductive environment, created by the cryolite, breaks down the bonds between aluminum and oxygen [2]. The aluminum settles at the bottom of the cells, while the oxygen combines with carbon to make carbon dioxide.
Hydrogen production
An increasingly important electrolysis application is the production of hydrogen gas and oxygen from water. There are three main types of water electrolyzers for hydrogen production:
Polymer electrolyte membrane (PEM) electrolyzers. In PEM electrolyzers, water molecules are electrochemically split at the anode into oxygen gas, hydrogen ions (protons) and electrons (Figure 1). The electrons flow through an external circuit while the protons selectively move across the PEM to the cathode. At the cathode, hydrogen ions combine with electrons from the external circuit to form H2. The electrolyte membrane is a solid, specialty-plastic material.
FIGURE 1. In a PEM electrolysis cell, such as the one shown here, hydrogen gas is generated at the cathode
Alkaline electrolyzers. Alkaline electrolyzers operate via transport of hydroxide ions (OH –) through the electrolyte from the cathode to the anode with hydrogen being generated on the cathode side. Electrolyzers using a liquid alkaline solution of sodium or potassium hydroxide as the electrolyte have been commercially available for many years. Newer approaches using solid alkaline exchange membranes as the electrolyte are showing promise at the laboratory scale [3].
Solid oxide electrolyzers (SOEs). Solid oxide electrolyzers operate at elevated temperatures. SOEs combine steam and electrons from the external circuit to form hydrogen and negatively charged oxygen ions (O2–) at the cathode [4]. As the electrolyte, a solid ceramic material selectively conducts O2– to the anode, where it reacts to form oxygen gas and generate electrons for the external circuit.
References
1. World Chlorine Council, Chlor-Alkali Manufacturing Processes, informational PDF, www.worldchlorine.org.
2. Thyssenkrupp Materials, Extraction of Aluminum, thyssenkrupp-materials.co.uk/how-is-aluminum-made, accessed Sept. 2024.
3. U.S. Department of Energy Hydrogen and Fuel Cells Technologies Office, Hydrogen Production: Electrolysis, www.energy.gov.
4. Kumar, S. and Himabindu, V., Hydrogen production by PEM water electrolysis, A review, Materials Science for Energy Technologies, vol. 2, pp. 442–454, 2019.