How does structural anisotropy in TMD electrocatalysts affect HER performance, and why is controlled powder processing critical?

Structural anisotropy makes the catalyst’s geometry part of its chemistry. In transition-metal dichalcogenide (MX₂) electrocatalysts, the exposed edge planes generally provide the most active sites for the hydrogen evolution reaction (HER), while the basal planes are comparatively inert. This difference is reinforced by highly directional charge transport: in-plane conductivity can be approximately 2,200 times greater than cross-layer conductivity. Controlled powder processing is therefore essential because electrode fabrication determines whether active edges remain exposed, electrically connected, and accessible to the electrolyte.

The key point: TMD HER performance depends not only on chemical composition, but also on which crystal planes are exposed and how effectively those planes are connected to the electrode. Powder compaction and coating must improve contact without collapsing the layered structure or burying the active edges.

Why Structural Anisotropy Controls HER Activity

Basal Planes and Edge Planes Behave Differently

TMD crystals have a layered structure in which strong in-plane bonding contrasts with weaker interlayer interactions. This produces distinct basal and edge surfaces with different electronic structures, surface energies, and catalytic behavior.

The basal plane is typically electrochemically inactive or weakly active, whereas the edge plane has higher surface energy and more favorable sites for hydrogen adsorption and reaction. As a result, HER activity is often governed by the number, accessibility, and electronic quality of exposed edges.

Edge Sites Are the Main HER Centers

The HER requires adsorption and subsequent conversion of hydrogen-containing intermediates at the catalyst surface. TMD edge sites can provide more suitable chemical environments for these steps than the relatively unreactive basal surface.

This makes edge-site engineering a central design strategy. Increasing the edge-to-basal-plane ratio through nanosheet thinning, controlled orientation, or defect engineering can improve the density of catalytically useful sites.

Charge Transport Follows the Crystal Plane

Structural anisotropy also affects how electrons reach the reaction sites. The reported approximately 2,200-fold difference between in-plane and cross-layer conductivity illustrates why the direction of electrical transport matters during electrode fabrication.

If nanosheets are arranged so that electrons must repeatedly cross poorly conducting interlayer pathways, the catalyst may contain active chemistry that is poorly utilized electrochemically. Efficient electrodes must provide short, continuous electrical pathways to the exposed reactive edges.

How Powder Processing Affects Electrode Performance

Alignment Determines Which Surfaces Remain Accessible

Compaction and coating can change the orientation of layered particles. Excessive alignment parallel to the substrate may improve certain in-plane transport pathways, but it can also reduce the number of edges exposed to the electrolyte if the particles are pressed into a dense, face-on arrangement.

The objective is not simply maximum compaction. It is a controlled structure that balances edge exposure, electrolyte access, mechanical integrity, and electrical connectivity.

Pressure Must Preserve the Layered Architecture

Two-dimensional TMD particles can be damaged by aggressive pressing, milling, or excessive shear. Crushing or restacking the layers may reduce edge accessibility, alter the intended particle morphology, and create transport barriers.

Precision compaction helps establish a reproducible layer density while limiting unnecessary mechanical damage. This is particularly important when comparing catalyst formulations, because uncontrolled processing can obscure the intrinsic performance of the material.

Inter-Particle Contact Controls Utilization

A powder electrode contains many particle-to-particle interfaces. Poor contact increases electronic resistance and can leave portions of the catalyst electrically isolated, even when those particles are chemically active.

Uniform coating and controlled pressing improve the consistency of these contacts. The resulting electrode has a more reliable pathway from the current collector through the catalyst network to the electrochemically exposed edges.

Coating Uniformity Improves Measurement Reliability

Nonuniform coatings can produce variations in thickness, catalyst loading, porosity, and local current density. These differences affect apparent overpotential, Tafel behavior, stability, and the measured current response.

Controlled powder dispersion and coating therefore support more than manufacturing quality. They make electrochemical data more reproducible and allow performance differences to be attributed more confidently to catalyst composition or structure.

Connecting Structure to Measured HER Performance

More Active Edges Can Reduce Apparent Overpotential

When more electrically connected edge sites are available, the electrode can support a larger HER current at a given applied potential. This commonly appears as lower apparent overpotential and improved reaction kinetics.

However, the improvement depends on whether the additional edges are both electrochemically accessible and electrically connected. A high edge density alone does not guarantee high electrode-level performance.

Conductivity and Surface Area Must Be Considered Together

A porous, finely divided powder may offer substantial surface area but still perform poorly if cross-layer transport and inter-particle contact are unfavorable. Conversely, a well-connected compact may have good conductivity while exposing too few catalytic edges.

HER evaluation must therefore distinguish intrinsic catalytic activity from electrode architecture. Processing conditions can change the balance between active area, transport resistance, and accessible porosity.

Electrode Fabrication Can Mask Material Quality

If one sample is more densely packed, more uniformly coated, or better connected than another, its measured performance may reflect fabrication rather than chemistry. This is a major concern when comparing different TMD phases, compositions, flake sizes, or defect concentrations.

Consistent powder handling and electrode preparation reduce this source of uncertainty. They provide a stable baseline for interpreting electrochemical results.

Understanding the Trade-offs

Maximum Compaction Is Not Always Beneficial

Higher density can improve mechanical stability and reduce contact resistance, but it can also block electrolyte pathways and bury edge sites. A dense electrode may therefore show better physical integrity while delivering lower effective catalytic activity.

The appropriate compaction level is the one that maintains robust particle contact without eliminating accessible porosity and reactive surface.

Greater Edge Exposure Can Increase Structural Instability

Edge-rich nanosheets often have higher surface energy and may be more susceptible to reconstruction, oxidation, dissolution, or detachment under operating conditions. The most active initial morphology is not automatically the most stable long-term electrode.

Stability testing should assess whether the original edge-rich structure remains functional during extended HER operation.

Aggressive Mixing Can Improve Dispersion but Damage Particles

Powder mixing is needed to distribute the catalyst and any conductive components uniformly. Excessive mixing energy, however, can fracture or restack the delicate layered particles and change the edge population.

Mixing intensity, duration, and compaction pressure should be treated as process variables rather than incidental laboratory details.

Supplementary Processing Lessons Are Material-Specific

The same fabrication principles apply broadly to electrocatalyst development, including oxide systems used for the oxygen evolution reaction: uniform mixing, coating, density, and phase distribution improve the reliability of electrochemical testing.

The underlying catalytic descriptors are not interchangeable, however. OER trends involving oxide transition enthalpy do not directly explain TMD HER activity, which is primarily discussed here in terms of crystal-plane reactivity, electronic anisotropy, and electrode architecture.

How to Apply This to Your Project

Electrode fabrication should be designed around the active-plane structure of the TMD rather than treated as a final handling step.

• If your primary focus is maximizing HER activity: Increase the population of exposed edge sites while preserving sufficient porosity and ensuring those sites remain electrically connected.
• If your primary focus is accurate catalyst comparison: Use identical powder mixing, coating, loading, and compaction procedures so fabrication differences do not dominate the electrochemical results.
• If your primary focus is low electrode resistance: Optimize inter-particle contact and in-plane transport without forcing electrons through excessive cross-layer pathways.
• If your primary focus is long-term stability: Select processing conditions that preserve the layered structure and prevent particle fracture, restacking, detachment, or loss of electrolyte access.
• If your primary focus is reproducible scale-up: Define and control powder dispersion, coating thickness, compaction pressure, mass density, and electrode orientation as measurable process parameters.

For TMD HER electrodes, controlling the powder structure is a direct way to control which catalytic sites exist, how they are reached, and how reliably their performance can be measured.

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