What Are Electrochemical Processes?

Electrochemical processes are chemical transformations driven by the controlled exchange of electrons between electrode surfaces and dissolved or solid-phase species in an electrolyte. They operate either galvanically, generating electrical current from spontaneous chemical reactions as in batteries and fuel cells, or electrolytically, consuming electrical energy to drive non-spontaneous reactions as in electrolysis and electroplating. In either mode, the key events occur at the electrode-electrolyte interface: oxidation reactions at the anode release electrons into the external circuit, while reduction reactions at the cathode consume those electrons to convert reactant species into products. The rate and selectivity of these reactions depend on electrode potential, electrolyte composition, temperature, mass transport, and the nature of the electrode material.

Electrochemical processes have been central to industrial chemistry since the mid-nineteenth century, when the development of the dynamo made large-scale electrolytic current available. Their appeal lies in the ability to drive reactions at room temperature using electrical energy, which can increasingly be sourced from renewable generation, and in the high selectivity achievable by adjusting electrode potential to favor one reaction pathway over competing alternatives.

Industrial Electrolysis

The chlor-alkali process is one of the largest electrochemical operations in the world by current consumption, producing chlorine gas at the anode and sodium hydroxide and hydrogen at the cathode from aqueous sodium chloride solutions. Aluminum production by the Hall-Heroult process, in which alumina dissolved in molten cryolite is electrolyzed at temperatures near 960 degrees Celsius, accounts for the majority of global primary aluminum supply. Copper electrorefining purifies blister copper by anodically dissolving impure copper into sulfate solution and selectively depositing pure copper onto cathode blanks. Water electrolysis, either through alkaline electrolyzers using potassium hydroxide solutions or proton exchange membrane (PEM) electrolyzers, splits water into hydrogen and oxygen using electrical energy, and has become a central technology for producing green hydrogen from renewable electricity. The PMC article on emerging electrochemical processes to decarbonize the chemical industry reviews the current state and near-term prospects of electrolytic routes to chemicals produced by energy-intensive thermochemical processes.

Electrolytes and Cell Design

The electrolyte in an electrochemical process serves two functions: it provides the ionic conductivity necessary to complete the current path between electrodes, and it participates in the electrode reactions directly or indirectly as a source of reactants and a carrier of products. Aqueous electrolytes dominate industrial practice, but molten salt electrolytes are required for metals with highly negative reduction potentials, such as aluminum, sodium, and magnesium, because water would be reduced before these metals. Solid polymer electrolytes based on sulfonated fluoropolymers, such as Nafion, are used in PEM electrolyzers and fuel cells because they combine high proton conductivity with low gas crossover and mechanical durability. Ionic liquids, low-melting salts that are liquid at room temperature, offer wide electrochemical windows useful for depositing or dissolving metals that cannot be processed in aqueous media. Electrochemical impedance spectroscopy is routinely used to characterize electrolyte resistance, interfacial impedance, and ion transport in these diverse electrolyte systems. The Electrochemical Society Interface article on electrochemical manufacturing in the chemical industry surveys the cell engineering principles that govern industrial electrochemical cell performance.

Electrochemical Synthesis

Beyond bulk commodity production, electrochemical processes are increasingly applied to the selective synthesis of fine chemicals and pharmaceuticals. Electrosynthesis offers the ability to substitute a stoichiometric chemical oxidant or reductant with an electrode reaction, reducing the generation of salt byproducts and improving atom economy. The Monsanto adiponitrile synthesis, a hydrodimerization of acrylonitrile at a cadmium cathode, was one of the first large-scale electrosynthetic processes and demonstrated that electrochemical routes could compete economically with conventional organic chemistry. CO2 reduction to carbon monoxide, formate, ethylene, and other products is an active research area aimed at using renewable electricity to convert a greenhouse gas into chemical feedstocks. The JACS Au paper on emerging electrochemical processes for chemical industry decarbonization analyzes the economic and technical requirements for these electrosynthetic routes at industrial scale.

Applications

Electrochemical processes are used across a broad range of industrial and energy sectors, including:

• Production of chlorine, sodium hydroxide, and hydrogen via the chlor-alkali process
• Primary aluminum and magnesium smelting through high-temperature electrolysis
• Electrorefining and electrowinning of copper, zinc, and precious metals
• Green hydrogen generation from water electrolysis powered by renewable electricity
• Electrochemical treatment of industrial wastewater to remove heavy metals and organic contaminants