What are the chemical reaction mechanisms involved in HER under acidic vs. alkaline conditions? Key Insights for Catalyst Design

The Hydrogen Evolution Reaction (HER) follows different elementary pathways in acidic and alkaline electrolytes. In acidic media, surface hydrogen is formed directly by electrochemical reduction of protons, while in alkaline media, the catalyst must generally reduce water first, producing surface hydrogen and hydroxide ions. In both cases, the adsorbed hydrogen intermediate, written as *H, can form hydrogen through either an electrochemical Heyrovsky step or a chemical Tafel step.

The central catalyst-design issue is not only hydrogen binding strength. A catalyst must also efficiently activate the available proton donor, provide suitable adsorption sites, and release H2 without blocking those sites. This requirement is usually more demanding in alkaline media because water activation and hydroxide formation add an additional kinetic burden.

The Common Elementary-Step Framework

Surface sites and adsorbed hydrogen

The symbol * represents an available catalytic surface site, and *H represents hydrogen adsorbed on that site. HER proceeds by forming *H and then combining or electrochemically reducing these intermediates to produce molecular hydrogen.

The relative rates of these steps depend on the catalyst surface, electrolyte composition, applied potential, water structure, and the binding energy of hydrogen.

The three canonical HER steps

HER is commonly described using three elementary reactions:

1. Volmer step: electrochemical formation of adsorbed hydrogen.
2. Heyrovsky step: electrochemical hydrogen desorption.
3. Tafel step: chemical recombination of two adsorbed hydrogen species.

The Volmer step is always the hydrogen-formation step. The subsequent rate-controlling pathway may involve Heyrovsky desorption, Tafel recombination, or a combination of both.

HER Mechanism in Acidic Media

Direct proton discharge: the Volmer step

In acidic electrolyte, proton reduction can be represented as:

[

• • \mathrm{H^+} + e^- \rightleftharpoons *\mathrm{H} ]

In aqueous systems, H+ is often used as a shorthand for hydrated proton species such as hydronium. The essential process is the transfer of a proton and an electron to the catalyst surface, forming adsorbed hydrogen.

This step tests whether the catalyst can efficiently accept charge and bind the resulting hydrogen intermediate at an appropriate strength.

Electrochemical desorption: the Heyrovsky step

The adsorbed hydrogen can react with another proton and electron:

[ *\mathrm{H} + \mathrm{H^+} + e^- \rightleftharpoons \mathrm{H_2} + * ]

This pathway produces hydrogen through a second electrochemical event and regenerates the surface site.

A catalyst following a Volmer-Heyrovsky pathway must therefore support both proton discharge and electrochemical removal of *H.

Chemical recombination: the Tafel step

Alternatively, two adsorbed hydrogen species can combine chemically:

[ 2*\mathrm{H} \rightleftharpoons \mathrm{H_2} + 2* ]

This is the Volmer-Tafel pathway. It does not require a second electron-transfer step during hydrogen-hydrogen recombination, but it requires two neighboring sites containing appropriately configured *H species.

Overall acidic reaction

Combining the elementary steps gives the overall cathodic reaction:

[ 2\mathrm{H^+} + 2e^- \rightarrow \mathrm{H_2} ]

The electrolyte supplies protons, while the electrode supplies electrons. The catalyst lowers the kinetic barriers associated with forming and removing adsorbed hydrogen.

HER Mechanism in Alkaline Media

Water reduction: the alkaline Volmer step

In alkaline electrolyte, the concentration of free hydronium is low. HER therefore generally begins with water activation:

[

• • \mathrm{H_2O} + e^- \rightleftharpoons *\mathrm{H} + \mathrm{OH^-} ]

This step forms adsorbed hydrogen while releasing hydroxide into the electrolyte.

Compared with direct proton reduction, water reduction introduces an additional requirement: the catalyst must facilitate cleavage or rearrangement of the O-H bond in an interfacial water molecule.

Electrochemical desorption: the alkaline Heyrovsky step

The adsorbed hydrogen can then react with water and receive another electron:

[ *\mathrm{H} + \mathrm{H_2O} + e^- \rightleftharpoons \mathrm{H_2} + * + \mathrm{OH^-} ]

This produces molecular hydrogen, regenerates the catalytic site, and forms a second hydroxide ion.

The alkaline Volmer-Heyrovsky sequence is therefore sensitive to water adsorption, water orientation, proton transfer through the interfacial hydrogen-bond network, and hydroxide removal.

Chemical recombination: the alkaline Tafel step

After water reduction has generated adsorbed hydrogen, the Tafel route remains possible:

[ 2*\mathrm{H} \rightleftharpoons \mathrm{H_2} + 2* ]

The chemical recombination step itself has the same formal expression as in acidic media. The important difference is that alkaline HER must generally first obtain hydrogen from water rather than from abundant solution-phase protons.

Overall alkaline reaction

Adding the alkaline elementary steps and canceling the hydroxide intermediates gives:

[ 2\mathrm{H_2O} + 2e^- \rightarrow \mathrm{H_2} + 2\mathrm{OH^-} ]

The thermodynamic equilibrium potential for HER shifts with pH when referenced to a fixed reference electrode scale, but the fundamental water-to-hydrogen chemistry is equivalent to proton reduction when the electrode potential is properly referenced.

Why Alkaline HER Often Has Different Kinetics

Water activation becomes a catalyst function

In acidic media, the electrolyte provides a relatively direct proton source. In alkaline media, the catalyst and its interface must help extract hydrogen from water.

This can make the Volmer step more difficult, particularly on surfaces that bind water weakly or do not efficiently stabilize the transition state for O-H bond cleavage.

Hydroxide changes the interface

The hydroxide product is not merely a spectator in the elementary mechanism. Its concentration, adsorption behavior, and transport away from the interface can influence water activation and the availability of catalytic sites.

Surface hydroxyl species may either assist proton transfer or compete with hydrogen and water for adsorption sites, depending on the catalyst and operating conditions.

Proton donors can be more diverse than bulk hydronium

The simplified alkaline equations emphasize water as the proton donor. In practice, interfacial water, surface hydroxyl groups, and other proton-transfer configurations can participate in the local reaction environment.

For catalyst comparison, the simplified mechanism is useful, but it should not be interpreted as proof that every surface follows one identical molecular pathway.

Connecting Mechanism to Catalyst Research

Balance hydrogen binding and release

A useful catalyst must bind *H strongly enough to form the intermediate but weakly enough to release H2 and regenerate * sites.

If hydrogen binds too weakly, the Volmer step may be unfavorable. If it binds too strongly, desorption or site recovery may become limiting.

Design for the proton source

In acidic research, catalyst screening often emphasizes proton adsorption and hydrogen binding. In alkaline research, those factors remain important, but water adsorption and activation must also be considered.

This distinction is why a material that performs well in acid may show substantially slower kinetics in alkaline electrolyte.

Consider neighboring active sites

The Tafel step requires two adsorbed hydrogen species and therefore depends on site proximity and surface coverage. A surface may have favorable individual hydrogen-binding sites but still be inefficient for Tafel recombination if the required site arrangement is unavailable.

Preserve accessible active sites

The number and accessibility of free sites directly affect measured activity. Uniform catalyst slurry coating, controlled catalyst loading, and reproducible compression during electrode preparation help ensure that differences in kinetic performance reflect material chemistry rather than inconsistent electrode structure.

Electrode porosity, electrical contact, wetting, and mass transport should be controlled alongside the catalyst composition.

Understanding the Trade-offs

Mechanistic labels are not complete explanations

Volmer, Heyrovsky, and Tafel labels describe useful idealized elementary steps. They do not, by themselves, identify the true rate-determining event, active site, proton-transfer pathway, or surface coverage under operating conditions.

Mechanistic conclusions should therefore be supported with complementary electrochemical, spectroscopic, structural, and computational evidence where possible.

Tafel slopes require careful interpretation

A measured Tafel slope can suggest a dominant kinetic regime, but it is not a unique fingerprint of one elementary step. Double-layer effects, changing surface coverage, uncompensated resistance, mass transport, catalyst reconstruction, and gas bubbles can all distort the apparent slope.

The slope should be interpreted together with exchange current density, impedance, stability, electrolyte composition, and electrode controls.

Acid and alkaline performance cannot be compared casually

Comparisons must use consistent reference-electrode calibration, catalyst loading, geometric and electrochemical surface area definitions, electrolyte purity, temperature, and iR correction.

A lower overpotential in one electrolyte does not automatically demonstrate superior intrinsic catalyst activity because the proton donor, interfacial structure, and transport conditions also differ.

More active sites are not always better

Increasing surface area can improve apparent current, but poorly connected particles, blocked pores, nonuniform films, or excessive compression can reduce reactant access and alter local gas removal.

The electrode-fabrication procedure must therefore be treated as part of the experimental design, not as an incidental assembly detail.

Making the Right Choice for Your Goal

The most useful mechanism for catalyst research is the one that matches both the electrolyte and the measurement objective.

• If your primary focus is acidic HER: Prioritize direct proton discharge, hydrogen adsorption energetics, and the competition between Heyrovsky and Tafel desorption.
• If your primary focus is alkaline HER: Prioritize water activation, interfacial proton transfer, hydroxide management, and the ability of the surface to stabilize *H without blocking water-accessible sites.
• If your primary focus is mechanism identification: Combine polarization data with controls for resistance, loading, surface area, electrolyte composition, and spectroscopic or computational evidence.
• If your primary focus is reproducible material comparison: Standardize catalyst coating, compression, loading, electrical contact, wetting, and gas removal before attributing kinetic differences to intrinsic chemistry.

The strongest HER catalyst is not simply the material with the lowest measured overpotential; it is the material whose surface, interface, and electrode architecture efficiently coordinate proton or water activation with hydrogen formation and release.

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