Particle size and shell thickness determine both the shape and durability of core–shell electrocatalyst nanoparticles. Below approximately 1.5 nm, or fewer than roughly 100 atoms, the smaller nickel core strongly influences the particle’s diameter, curvature, and atomic arrangement. As the particle grows beyond about 2 nm and the platinum shell reaches two or three layers, the nickel influence weakens and the geometry increasingly resembles pure platinum; however, even a single platinum layer can substantially change oxygen adsorption and electrochemical stability.
The central design trade-off is that smaller particles and thinner shells maximize core-induced electronic and geometric effects, but they also have higher surface free energy and greater susceptibility to dissolution or restructuring. Larger particles and thicker shells generally improve geometric and chemical stability, although they can reduce the extent to which the core modifies platinum’s catalytic behavior.
How Particle Size Changes Nanoparticle Geometry
Very small particles are dominated by the core
At very small sizes, the nanoparticle contains relatively few atoms, so the nickel core is not geometrically isolated from the surface. Because nickel has a smaller atomic radius than platinum, it can strongly influence the overall particle diameter, local bond lengths, surface curvature, and atomic packing.
In particles below approximately 1.5 nm, these effects are especially pronounced because a large fraction of the atoms are located at or near the surface. The distinction between “core” and “shell” is therefore less like a bulk object coated with a film and more like a small atomic cluster whose entire structure is core-influenced.
Growth reduces curvature and core control
As particle size increases, the surface-to-volume ratio decreases. The surface becomes less sharply curved, and the relative number of atoms affected directly by the nickel core becomes smaller.
When the particle exceeds roughly 2 nm, particularly with two or three platinum shell layers, its diameter and curvature approach those of a pure platinum nanoparticle. The platinum shell increasingly determines the external geometry, while nickel acts primarily as an internal structural and electronic modifier.
Size controls the available catalytic surface
Reducing particle size exposes more surface atoms relative to the total amount of material. This can increase the number of accessible catalytic sites and improve catalyst utilization.
However, surface atoms in very small particles are also more under-coordinated and structurally flexible. Their altered coordination can change adsorption energetics and may promote reconstruction under electrochemical operating conditions.
How Shell Thickness Changes the Core–Shell Structure
A single platinum layer creates a strong surface effect
A one-layer platinum shell, often described as a Pt-skin structure, places the outermost platinum atoms under strong geometric and electronic influence from the nickel core. The resulting surface is not equivalent to bulk platinum.
This thin shell can significantly alter oxygen adsorption energies and shift oxidation behavior toward higher stability. Such changes are important because oxygen-containing intermediates and surface oxidation often influence electrocatalytic activity and durability.
Additional layers progressively screen the nickel core
Adding platinum layers increases the distance between the nickel core and the reacting surface. With two or three platinum layers, the outer surface becomes more platinum-like in both geometry and chemical behavior.
The nickel core can still affect the platinum lattice through strain and electronic interactions, but these effects are generally weaker at the outermost surface than in a single-layer Pt-skin structure. Shell thickness therefore provides a way to balance core-induced activity with platinum-like surface stability.
Shell thickness affects protection as well as activity
A thicker platinum shell offers greater physical separation between nickel and the electrolyte. This can reduce direct exposure of nickel to the reaction environment and improve resistance to core dissolution.
The protection is not absolute. Defects, incomplete coverage, strain, and particle restructuring can create pathways for electrolyte access to the core, particularly in very small or highly stressed particles.
Why Smaller Particles Are Less Thermodynamically Stable
High surface energy drives instability
According to the Gibbs–Thomson relationship, small nanoparticles have a high surface-to-volume ratio and elevated surface free energy compared with bulk materials. Their atoms are therefore in a less favorable thermodynamic state.
This size dependence can shift redox behavior and make smaller particles more prone to oxidative dissolution at lower applied potentials than larger particles or bulk metals. The exact potential depends on composition, surface structure, electrolyte, and operating conditions, so the trend should not be treated as a universal fixed voltage.
Small particles are vulnerable to dissolution and ripening
In electrochemical environments, unstable small particles may dissolve and redeposit onto larger particles. This process, known as electrochemical Ostwald ripening, gradually broadens the particle-size distribution and reduces the population of the smallest particles.
The result can be loss of active surface area, changes in shell coverage, and deterioration of the intended core–shell structure. A catalyst that performs well initially may therefore change substantially during long-term cycling.
Quantum and coordination effects become important
At approximately the 1–10 nm scale, nanoparticles can exhibit electronic structures that differ from bulk metals. Discrete or size-dependent electronic states can modify surface reactivity, reaction pathways, and selectivity.
These effects may improve catalytic performance, but they also make behavior more sensitive to particle size, shape, defects, and shell thickness. Small changes in synthesis can consequently produce measurable differences in electrochemical response.
How Geometry and Stability Interact
Curvature changes adsorption behavior
Highly curved surfaces contain more low-coordination atoms and different atomic arrangements than flatter surfaces. These geometric differences influence how oxygen and other reaction intermediates bind to the catalyst.
A thin platinum shell on a small nickel core can therefore produce adsorption energies that differ substantially from those of either bulk platinum or a larger, thick-shell particle.
Strain and electronic effects work together
The nickel core can modify the platinum shell through lattice strain and electronic interaction. These effects may shift adsorption energies into a more favorable range and alter oxidation potentials.
As the shell becomes thicker or the particle becomes larger, the outermost platinum layers become progressively less affected by nickel. The catalyst then moves toward the behavior of pure platinum, with reduced core-induced modification.
Stability depends on the outer surface and the buried core
A stable electrocatalyst must resist both surface oxidation or restructuring and core dissolution. Thin shells may provide strong catalytic modification but leave the core more exposed to defects and electrolyte penetration.
Thicker shells generally improve protection, while larger particles reduce the thermodynamic penalty associated with high surface energy. These benefits must be weighed against lower surface-area utilization and weaker core influence.
Understanding the Trade-offs
Smaller particles are more active per unit mass but less stable
Very small particles can expose many active sites and generate strong size-dependent electronic effects. Their high curvature and surface energy, however, increase the risk of dissolution, restructuring, and ripening.
Thin shells maximize core influence but offer less protection
A single platinum layer can produce substantial changes in oxygen adsorption and oxidation behavior. It also provides less physical protection for the nickel core than a multilayer shell and is more sensitive to defects or incomplete coverage.
Thick shells improve stability but can reduce tunability
Two or three platinum layers make the external surface more platinum-like and can better isolate nickel from the electrolyte. At the same time, the core has less influence on the surface, potentially reducing the intended strain and electronic modifications.
Poor size control undermines performance comparisons
A synthesized powder may contain a broad distribution of particle sizes and shell thicknesses. Its measured electrochemical response will then combine multiple geometries and stability regimes, making it difficult to determine which structure is responsible for the observed performance.
Reliable comparison therefore requires controlled synthesis, structural characterization, and standardized electrode preparation. Precision powder processing and compaction are useful only when they preserve comparable loading, morphology, and electrode structure across samples.
Making the Right Choice for Your Goal
The appropriate design depends on whether the priority is maximum surface utilization, core-induced activity, protection against degradation, or reproducible long-term testing.
- If your primary focus is maximum active surface area: Use small particles, but control their size distribution carefully because high surface energy increases dissolution and ripening.
- If your primary focus is strong nickel-induced electronic or strain effects: Favor a very thin platinum shell, particularly a Pt-skin structure, while accounting for its lower protection against core exposure.
- If your primary focus is long-term structural stability: Use larger particles and a multilayer platinum shell, recognizing that the surface will behave increasingly like pure platinum.
- If your primary focus is reliable performance comparison: Control both particle diameter and shell thickness, then combine electrochemical testing with structural characterization and standardized electrode preparation.
The most durable catalyst is not necessarily the smallest or thinnest-shell particle, but the structure whose geometry, surface chemistry, and stability match the intended operating conditions.
Summary Table:
| Parameter | Smaller Particles | Larger Particles |
|---|---|---|
| Surface-to-volume ratio | High | Low |
| Surface free energy | High (less stable) | Lower (more stable) |
| Curvature | High (more low-coordination sites) | Low (flatter surfaces) |
| Core influence | Strong (dominates geometry) | Weaker (geometry resembles pure Pt) |
| Catalytic surface area | High (more active sites per mass) | Lower |
| Stability | Susceptible to dissolution and ripening | More resistant to degradation |
| Shell thickness effect | Thin shell (e.g., Pt-skin) enhances core effects | Thick shell screens core effects |
| Protection of core | Less protection (defects may expose core) | Better protection (thicker shell) |
| Catalytic behavior | Strongly modified vs. bulk Pt | Approaches pure Pt behavior |
| Comparison of samples | Requires tight size control for reliable data | Easier to reproduce but less tunable |
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