Surface Active Sites and Electrocatalytic Activity

1. Beyond the Ideal Surface: The Concept of Active Sites

An idealized catalyst surface is often depicted as a perfect, flat crystal plane. In reality, catalyst surfaces, especially those of nanoparticles and complex materials, are rich with heterogeneity. They possess a variety of different atomic environments. An active site is a specific location on a catalyst's surface—an atom or an ensemble of atoms—that exhibits a particularly high rate of reaction. This heightened activity arises because the site has a geometric or electronic structure that provides a near-optimal binding energy for reaction intermediates, placing it closer to the peak of the Sabatier volcano plot.

More than a century ago, H.S. Taylor first proposed that reactions do not occur uniformly over a surface, but are concentrated at "active centers," which he associated with atoms in low-coordination environments like corners and edges. This foundational idea remains central to modern catalysis.

The image below illustrates this concept on a stepped palladium surface.

Atoms on the flat terrace are fully coordinated by their neighbors, whereas atoms at the step edge have fewer coordinating neighbors. This difference in coordination number alters their electronic properties and, consequently, how strongly they bind to reaction intermediates. For many reactions, these low-coordination step and edge sites are significantly more active than the high-coordination terrace sites.

2. A Classic Example: Edge Site Activity in MoS₂ for HER

A powerful demonstration of the active site concept comes from the study of Molybdenum Disulfide (MoS₂) as a catalyst for the Hydrogen Evolution Reaction (HER). MoS₂ has a layered structure, similar to graphite. Each layer consists of a "basal plane" and "edge sites."

Professor Tom Jaramillo, in his "Electrocatalysis 101" lecture, provides a clear explanation of how theory and experiment came together to identify the active sites on this material.

As the video compellingly shows, the hydrogen binding energy on the basal plane of MoS₂ is too weak for it to be an effective catalyst. However, the edge sites exhibit a near-optimal binding energy, placing them close to the peak of the HER volcano. The linear relationship between measured current and edge site density is conclusive proof that, for MoS₂, the edges are the active sites. This example beautifully illustrates how identifying the active site and understanding its properties provides a clear path for catalyst design: to make a better MoS₂ catalyst, you must maximize the density of its edge sites.

3. A Typology of Active Sites

The MoS₂ edge site is a classic example of a structural active site. In general, active sites in electrocatalysis can be created by several features that disrupt the perfect periodicity of a crystal lattice.

Structural and Electronic Origins of Activity

Let's explore the most common ways active sites are formed, using a highly relevant review paper for your field.

As this reading highlights, active sites are fundamentally regions of unique electronic structure. This uniqueness can be engineered through several strategies:

• Structural Defects: As mentioned, sites like steps, kinks, and grain boundaries possess atoms with lower coordination, making them more reactive.
• Vacancy Defects: The absence of an atom from its lattice position creates an "unsaturated" site with altered local electron density. These vacancies can serve as new, highly active adsorption sites and are known to be crucial in materials like oxides and MXenes.
• Heteroatom Doping: Intentionally replacing some atoms in the crystal lattice with atoms of a different element (e.g., doping Fe into a NiOOH lattice) perturbs the electronic structure of the host material. The Fe atom itself and its neighboring Ni atoms all become modified, potentially creating new active sites with more favorable binding energies. This is the core principle behind the high activity of the Ni-Fe systems you work with.
• Strain: Compressing or stretching the crystal lattice alters the distances between atoms, which in turn modifies their electronic orbitals and their binding properties.
• Interfaces: The interface between a catalyst and its support material (e.g., your Ni-Fe catalyst on an MXene support) can create unique active sites that do not exist in either material alone. Electronic interactions and charge transfer across the interface can generate highly active centers.

4. The Dynamic Nature of Active Sites: Operando Reconstruction

A critical concept in modern electrocatalysis, particularly for OER catalysts like the ones you study, is that the material you synthesize (the precatalyst) is often not the material that performs the reaction. Under the harsh oxidizing potentials of OER, many materials undergo in-situ transformation, or reconstruction, to form the true, active catalytic phase.

The following resources introduce this dynamic nature of active sites.

This evolution means that the active sites are not static but are generated dynamically. For many transition metal oxides, sulfides, and phosphides used for OER, the true active phase is a metal (oxy)hydroxide layer that forms on the surface. Your Ni-Fe catalysts are a prime example of this phenomenon.

Understanding and controlling this reconstruction process is a key strategy in catalyst design. The goal is to create a precatalyst that efficiently transforms into a stable, highly active (oxy)hydroxide phase with a high density of optimal active sites.

5. Quantifying Intrinsic Activity: Turnover Frequency (TOF)

Since different catalysts have different numbers of active sites, simply comparing their current density at a given potential can be misleading. A material might show high current simply because it has a huge surface area with many mediocre sites, not because the sites themselves are intrinsically superior.

To make a true apples-to-apples comparison of the intrinsic activity of the sites themselves, we use the metric of Turnover Frequency (TOF).

TOF is defined as the number of product molecules formed per active site per unit of time.

For an electrochemical reaction, it can be calculated from the measured current ($i$):

where:

• $i$ is the measured current (in Amperes, or Coulombs/sec).
• $z$ is the number of electrons transferred per molecule of product (e.g., 4 for O₂).
• $F$ is the Faraday constant ($\approx 96,485$ C/mol).
• $N_{\text{sites}}$ is the total number of active sites on the electrode (in moles).

The greatest challenge in determining TOF is accurately estimating $N_{\text{sites}}$. This is a major area of research, employing a combination of electrochemical methods (which we will touch upon in a later module), spectroscopy, and microscopy.

The webinar from the School of Convergence Science provides a sophisticated example of how this can be done experimentally for doped nickel oxide OER catalysts.

This approach allows researchers to decouple the number of sites from their intrinsic activity, providing much deeper insight. It revealed that iron doping doesn't just create more sites; it makes each site fundamentally better at catalyzing the reaction.

Conclusion

This lesson has connected the macroscopic property of catalytic activity to its microscopic origins at specific active sites. Understanding what these sites are, how they are formed, and how they function is the essence of rational catalyst design.

Key Takeaways:

• Active Sites: Electrocatalytic reactions are concentrated at specific active sites, which are atoms or ensembles of atoms with unique geometric and electronic properties.
• Structural and Electronic Origins: Active sites arise from disruptions in a perfect crystal lattice, such as steps, edges, vacancies, heteroatom dopants, and interfaces. These features tune the local binding energy of reaction intermediates toward the Sabatier optimum.
• Operando Reconstruction: Many electrocatalysts are precatalysts that transform under reaction conditions into the true active phase (e.g., a metal oxyhydroxide for OER). The active sites exist on this reconstructed surface.
• Turnover Frequency (TOF): TOF is the ultimate metric of intrinsic activity, defined as the reaction rate per active site. It allows for a fundamental comparison of catalysts by normalizing for the number of active sites.

Preview of the Next Lesson:

We've established that good active sites lower the energy barriers for a reaction. However, even with the best catalyst, driving a reaction at a meaningful rate requires an extra energy input. In our next lesson, we will begin our study of electrode kinetics by defining overpotential and identifying its three main components: activation, ohmic, and concentration losses. This will provide the framework for understanding the energy efficiency of electrochemical cells.