How does surface oxidation forming a Ni-NiO heterostructure influence the alkaline HOR mechanism, and why is uniform electrode pressing important?

Surface oxidation changes both the active sites and the reaction pathway. On metallic Ni, alkaline HOR is primarily associated with a Heyrovsky–Volmer route. Forming a controlled Ni–NiO surface creates an interface where metallic Ni binds hydrogen while neighboring NiO alters water, hydroxyl, and hydrogen adsorption energetics, favoring a Tafel–Volmer pathway and improving HOR activity.

The Ni–NiO benefit comes from interfacial cooperation, not simply from adding more oxide. Ni provides effective hydrogen-adsorption sites, while NiO modifies the local environment for water dissociation and hydroxyl interactions. Uniform electrode pressing is equally important because inconsistent density, loading, or contact resistance can make apparent electrochemical improvements a sample-preparation artifact.

How NiO Changes the Alkaline HOR Mechanism

The baseline behavior of metallic Ni

On predominantly metallic Ni, alkaline HOR commonly proceeds through a Heyrovsky–Volmer mechanism.

The surface chemistry involves hydrogen-containing intermediates and water or hydroxide species. In this pathway, electrochemical hydrogen conversion and surface hydrogen removal are closely coupled rather than relying primarily on the combination of two independently adsorbed hydrogen atoms.

What the Ni–NiO interface adds

Surface oxidation introduces adjacent metallic Ni and NiO sites with different electronic and chemical properties.

Metallic Ni remains an important site for forming or binding adsorbed hydrogen, represented as Ni–H_ads. Neighboring NiO species influence the adsorption of water and hydroxyl-related intermediates, including Ni–OH_ads.

Why adsorption energetics matter

The interface changes the free energies of hydrogen adsorption, hydroxyl adsorption, and water adsorption.

This matters because a catalyst must bind intermediates strongly enough to activate them, but not so strongly that the intermediates become difficult to remove. The Ni–NiO boundary can therefore provide a more favorable balance than either phase alone.

Why the Reaction Pathway Shifts

Metallic Ni favors the Heyrovsky–Volmer route

For pure metallic Ni, the alkaline HOR is primarily described by a Heyrovsky–Volmer sequence.

The exact rate-controlling behavior depends on surface condition and operating environment, but the central point is that hydrogen oxidation is governed by electrochemical steps involving adsorbed hydrogen and water-derived species.

NiO enables a Tafel–Volmer contribution

At a Ni–NiO interface, neighboring sites can support a Tafel–Volmer mechanism.

In this description, adsorbed hydrogen species associated with metallic Ni can recombine through a Tafel-type step, while the Volmer step remains associated with electrochemical water or hydroxide chemistry. NiO helps organize and tune the interfacial environment required for these processes.

The role of water and hydroxyl species

NiO is not merely an inert coating. Its presence changes how water molecules adsorb and dissociate and how hydroxyl species interact with the surface.

This can assist water activation while also altering the strength and availability of hydroxyl adsorption. The result is a cooperative interface: metallic Ni supplies hydrogen chemistry, and NiO helps manage water and hydroxyl chemistry.

Why the Heterostructure Can Be More Active

Two neighboring functions work together

The most useful way to view Ni–NiO is as a dual-site catalyst.

• Metallic Ni: primarily supports hydrogen adsorption.
• NiO or Ni²⁺-containing regions: facilitate water dissociation and modify hydroxyl interactions.
• The interface: couples these functions over a short distance.

This division of labor can be more effective than asking a single uniform surface to optimize hydrogen, water, and hydroxyl binding simultaneously.

Improved hydrogen binding can accelerate kinetics

The Ni–NiO interface can moderate hydrogen binding relative to pure Ni.

The supplementary reference reports hydrogen-binding energies of approximately −0.47 eV for the interface, compared with −0.54 eV for metallic Ni and −1.18 eV for pure NiO. These values illustrate why the interface can be advantageous: pure NiO binds hydrogen too strongly for efficient activity, while the heterostructure provides a more balanced environment.

Activity improvements depend on controlled oxidation

A reported increase of up to 10-fold in HOR activity demonstrates the potential of Ni–NiO interfaces, but it should not be treated as a universal result.

The improvement depends on oxide coverage, interface density, particle structure, electrode fabrication, electrolyte conditions, and measurement quality. The relevant design variable is therefore controlled surface oxidation, not maximum oxidation.

Why Uniform Electrode Pressing Is Essential

Electrochemical data depend on physical structure

Catalyst testing is not determined by chemistry alone. The measured cyclic voltammetry response and apparent kinetic parameters also depend on electrode density, catalyst loading, film structure, and electrical contact.

If these physical variables differ between samples, the experiment may attribute a fabrication difference to the Ni–NiO chemistry.

Pressing controls contact resistance

Precision pressing creates consistent contact between catalyst particles and the current collector or between particles within a test pellet.

More consistent contact reduces variations in electronic resistance and improves the reliability of current measurements. Without this control, a poorly connected electrode may appear less active even when its intrinsic catalyst chemistry is comparable.

Pressing improves mass-loading consistency

Controlled compaction helps produce a reproducible catalyst mass per geometric area or per test specimen.

This is critical when comparing current density, activity, or the overpotential required to reach a target such as 10 mA cm⁻². An electrode with uneven or uncertain loading cannot provide a clean comparison between oxidation states or catalyst compositions.

Pressing promotes structural homogeneity

Uniform pressure helps reduce local differences in density, thickness, and particle packing.

A homogeneous electrode presents a more consistent pathway for electron transport and electrolyte access. It also reduces the likelihood that one region dominates the measured response simply because it contains more catalyst or has better electrical contact.

Understanding the Trade-offs

Too little compaction creates measurement noise

Insufficient pressing can leave loosely connected particles, variable contact resistance, and mechanically unstable catalyst layers.

The resulting voltammograms may show poor reproducibility or distorted kinetic behavior. Such data make it difficult to distinguish intrinsic activity from electrode-construction effects.

Excessive compaction can also be harmful

Pressing is not automatically beneficial at higher pressure. Over-compaction can alter porosity and restrict electrolyte access to active interfaces.

The objective is therefore controlled, repeatable compaction, not maximum densification. The selected pressure and procedure should be kept constant across all comparative samples.

Excessive oxidation can remove metallic functionality

The activity enhancement depends on retaining accessible metallic Ni alongside NiO.

If oxidation becomes too extensive, the electrode may lose the metallic sites needed for effective hydrogen adsorption and electronic transport. Pure NiO is not an equivalent substitute for a well-connected Ni–NiO heterostructure.

Apparent activity can be confused with intrinsic activity

A higher measured current may result from greater active surface area, higher catalyst loading, lower resistance, or a genuinely faster reaction pathway.

Uniform pressing helps control the first three factors, allowing the experiment to more credibly evaluate the fourth: the intrinsic catalytic contribution of the Ni–NiO interface.

Making the Right Choice for Your Goal

Use the chemistry and fabrication process together when designing or evaluating these catalysts.

• If your primary focus is HOR mechanism: Compare metallic Ni with deliberately oxidized Ni–NiO surfaces and interpret changes through Ni–H_ads, Ni–OH_ads, and water-adsorption energetics.
• If your primary focus is catalytic activity: Optimize controlled, partial oxidation rather than maximizing NiO content, because the active synergy requires both metallic Ni and neighboring oxide.
• If your primary focus is reproducible screening: Use the same pressing pressure, time, catalyst loading, substrate preparation, and electrode geometry for every sample.
• If your primary focus is reliable kinetic data: Minimize contact-resistance and density variations before assigning differences in cyclic voltammetry or overpotential to catalyst chemistry.

A controlled Ni–NiO interface can improve alkaline HOR by coupling complementary adsorption functions, while uniform pressing ensures that the measured improvement reflects chemistry rather than electrode inconsistency.

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