What Are Electrocatalysts?

Electrocatalysts are materials that accelerate electrochemical reactions at electrode surfaces by lowering activation energy barriers without being permanently consumed in the process. In an electrochemical cell, the rate at which reactants transfer electrons and convert to products is governed by the kinetics of interfacial reactions, and electrocatalysts control those kinetics by providing favorable adsorption sites, tuning the electronic structure of the electrode surface, and stabilizing reaction intermediates. The performance of technologies such as fuel cells, water electrolyzers, and metal-air batteries depends directly on the activity, selectivity, and durability of the electrocatalysts employed at their electrodes.

Electrocatalyst research draws on electrochemistry, surface science, computational chemistry, and materials engineering. The field received major impetus from the development of the hydrogen-oxygen fuel cell in the twentieth century, which required electrode materials capable of catalyzing the oxygen reduction reaction (ORR) at practical current densities. Since then, the search for catalysts that can replace or reduce the loading of scarce platinum-group metals has expanded into a broad materials landscape spanning single atoms, nanoparticles, transition metal oxides, sulfides, and molecular complexes.

Transition Metal and Alloy Catalysts

Platinum and other platinum-group metals (PGMs) remain the reference electrocatalysts for the ORR in acidic media and the hydrogen evolution reaction (HER) in both acidic and alkaline conditions. Their high activity is rooted in the d-band position of the surface metal atoms, which sets the binding energy of reaction intermediates such as atomic hydrogen and peroxy species at values close to the optimum predicted by the Sabatier principle. Pt-alloy catalysts, in which platinum is alloyed with nickel, cobalt, or copper, shift the d-band to improve ORR activity while reducing platinum loading. The ACS Catalysis paper on electrocatalysts for polymer electrolyte fuel cells traces the development from pure Pt black to shaped nanoparticle and core-shell architectures that have substantially increased the platinum mass activity.

For the oxygen evolution reaction (OER) in alkaline media, nickel, iron, cobalt, and manganese oxides and hydroxides show high activity at lower cost than PGMs. In acidic conditions, iridium dioxide (IrO2) is the most durable OER electrocatalyst known, though iridium's scarcity limits its deployment at scale.

Mesoporous and Nanostructured Supports

The activity of an electrocatalyst depends on the accessible surface area of the active sites as much as on their intrinsic per-site activity. Mesoporous materials, defined by the IUPAC convention as having pore diameters between 2 and 50 nanometers, provide high surface area and pore networks through which electrolyte can reach interior active sites. Carbon-based mesoporous supports, including ordered mesoporous carbons and carbon nanotubes, are widely used to disperse and anchor platinum nanoparticles in fuel cell electrodes. Metal-organic frameworks (MOFs) and their derived carbons have also emerged as supports that allow precise control of pore geometry and heteroatom doping to tailor both conductivity and catalyst-support interactions. The PMC review of electrocatalysis for sustainable energy addresses the structural design principles that link support morphology to the stability of dispersed active sites under electrochemical cycling.

Single-Atom and Molecular Electrocatalysts

Single-atom catalysts (SACs) anchor individual metal atoms on a support, maximizing the fraction of active-site utilization while minimizing precious metal loading. The isolated atom coordinates with surrounding nitrogen, oxygen, or carbon atoms on the support, producing a well-defined coordination environment that can be characterized by X-ray absorption spectroscopy and modeled computationally. Molecular electrocatalysts, such as metal porphyrins and phthalocyanines, offer a complementary approach in which the active site is defined by synthetic chemistry, enabling systematic tuning through ligand modification. The PNNL Center for Molecular Electrocatalysis investigates molecular designs for hydrogen and oxygen chemistry with direct relevance to electrolysis and fuel cell applications.

Applications

Electrocatalysts are used across a wide range of energy conversion and industrial processes, including:

• Cathode and anode layers in proton exchange membrane fuel cells for transportation and stationary power
• Anodes for alkaline and PEM water electrolyzers producing hydrogen from renewable electricity
• Air electrodes in zinc-air and lithium-air batteries
• CO2 reduction electrodes that convert carbon dioxide to carbon monoxide, formate, or ethylene
• Chlor-alkali electrolysis for industrial production of chlorine and sodium hydroxide