Electrocatalysis and Volcano Plots

Introduction

Many electrode reactions involve adsorbed intermediates. The catalyst must bind reactants enough to activate them, yet release products and free sites for another cycle. This is the Sabatier principle. When measured activity is plotted against an adsorption-energy descriptor across a family of materials, the trend may form a volcano: activity rises toward an intermediate binding strength and falls on either side. The shape is a model-guided pattern, not a law that every dataset must obey.

Core explanation

Consider hydrogen evolution in acid in a simplified surface scheme. A proton and electron can form adsorbed H , where denotes a free surface site. Hydrogen gas is then produced by reaction of adsorbed species with another proton–electron pair or with another H . If H binds too weakly, forming or retaining it is difficult. If H binds too strongly, H₂ formation or site regeneration can be slow because the intermediate is trapped. A near-thermoneutral hydrogen adsorption free energy , ΔG H , often serves as a useful activity descriptor within an appropriately compared metal series.

A volcano plot places a descriptor such as ΔG H on the horizontal axis and a rate measure such as log exchange current density on the vertical axis. The weak-binding branch is limited by insufficient intermediate formation or coverage. The strong-binding branch is limited by difficult removal or blocked sites. The apex represents a balance within the model and the conditions of the dataset. In a published comparison, Nørskov and co-workers related calculated H adsorption energies to measured hydrogen-evolution exchange currents across metals, motivating this style of analysis.

Adsorption free energy includes energetic and entropy contributions under defined conditions. It is not identical to a bare electronic adsorption energy calculated for an isolated slab. Potential, pH, coverage, solvent, electrolyte ions and co-adsorbates can change effective binding and rates. Surface structure matters as well: a terrace, step or nanoparticle edge may have different adsorption from a nominally identical bulk metal. Normalising current to geometric area, electrochemically active area or site number gives different comparisons.

Descriptors help screen candidates, but a reaction rate depends on activation barriers and prefactors, not adsorption thermodynamics alone. Two catalysts with similar ΔG H can differ in electron-transfer kinetics, water dissociation, site density, stability or conductivity. A volcano compiled from different studies can also suffer inconsistent measurement conditions. For a reliable comparison, use the same electrolyte, temperature, reference scale, potential correction and activity normalisation, and check stability over the test period.

Adsorption can also poison a catalyst. CO binding on a surface may block sites needed for another reaction; stronger binding is then harmful. Conversely, weak adsorption of a necessary reactant can mean almost no surface reaction. The optimal binding value is reaction-specific; “zero eV” is not a universal target for every intermediate or potential.

Step-by-step reasoning

Write a plausible sequence of elementary steps and identify an intermediate marked with . Predict what fails at weak binding and at strong binding. Choose a descriptor linked to that intermediate and an activity metric measured under common conditions. Place candidate catalysts on the qualitative volcano, but then check site density, transport and durability before recommending one. Distinguish a thermodynamic descriptor from a directly measured activation barrier.

Visual explanation

Draw activity rising then falling as binding changes from weak through intermediate to strong. Label the weak side “hard to form H ” and the strong side “hard to remove H .” Mark several hypothetical catalysts but do not imply exact ranks without data. Beside the plot draw a two-step surface cycle with empty site , adsorbed H and released H₂, showing why both uptake and release must occur.

Real-world analogy

A worker needs a grip strong enough to pick up an object but loose enough to hand it onward. Too weak a grip fails at pickup; too tight a grip slows release. This captures the Sabatier trade-off. It does not calculate rates because surface reactions also require electrons, solvent motion and transition-state barriers.

Real-world example

A group screens several metals for acidic hydrogen evolution. They measure current near equilibrium, correct for ohmic drop and compare electrochemically active surface areas. Independently, they estimate H adsorption free energies for corresponding surface structures. A volcano trend guides which metals merit closer study. If a candidate dissolves or reconstructs under operating potential, its apparent initial position on the plot is not a durable device ranking.

Why?

Why can an adsorption descriptor correlate with activity? The same metal–intermediate interaction influences the ease of forming and removing a surface species. Why is the relationship often volcano-shaped rather than monotonic? Improving one step by stronger binding can worsen a later release step. Why test real current anyway? Adsorption energetics alone does not encode every rate constant or available active site.

Common misconception

The peak of a volcano plot does not prove a universal best catalyst. It belongs to a selected reaction, descriptor, surface family and set of conditions. Another misconception is that an adsorption free energy near zero means no intermediate exists; it means adsorption and desorption are thermodynamically balanced under the stated standard conditions, while coverage can still be substantial.

Worked example

Question: Catalysts A and B have estimated ΔG H values of +0.30 eV and −0.80 eV, respectively, under one consistent model. Which likely faces a weak-binding problem, and which a strong-binding problem?

Reasoning: Positive adsorption free energy disfavors H formation, so A is on the weak-binding side. A large negative value favours H strongly, so B can have difficulty releasing hydrogen or freeing sites. The numbers do not quantify their currents without a kinetic model and site information.

Answer: A is weak-binding; B is strong-binding. Neither can be ranked precisely from these values alone.

Quick check

1. What does the asterisk in H represent in a surface reaction scheme? Answer: It denotes an adsorbed H intermediate on a surface site; alone denotes a free site.

Exam focus

Show the two limiting branches of a volcano and name the activity metric and descriptor axes. Use adsorption free energy when comparing thermodynamics. State at least one experimental factor, such as area normalisation or stability, that can change an apparent ranking.

Advanced insight

For multistep reactions, adsorption energies of different intermediates may correlate through scaling relations. Such correlations can constrain how independently a catalyst can optimise each step, shifting or flattening a volcano's best achievable region. Breaking a scaling relation is a possible research strategy, but an apparently improved descriptor can be offset by a slower kinetic step or instability. Modern operando studies are valuable because the active surface under current may differ from the pre-reaction structure used in a calculation.

Summary

The Sabatier principle describes the need for an intermediate to bind strongly enough to react but weakly enough to leave. Volcano plots visualise this balance across a material series using an adsorption-energy descriptor and an activity metric. They guide screening while remaining conditional on mechanism, measurement conditions, active structure and durability.

Practice questions

1. Why does too-strong H binding reduce a hydrogen-evolution rate? Answer: It may hinder H₂ formation or release and leave sites occupied for the next cycle.

2. What is one reason geometric current density could misrank two catalyst powders? Answer: They may expose different numbers of active sites per geometric area, so current also reflects surface area or loading.

3. Does ΔG H = 0 directly give a rate constant? Answer: No. A rate also depends on activation barriers, prefactors, electron transfer and site availability.

4. Why should a volcano plot specify the electrolyte and potential scale? Answer: Potential, pH and solution chemistry alter coverage and reaction driving force, affecting measured activity and comparisons.

Sources: Nørskov et al., Trends in the Exchange Current for Hydrogen Evolution; Original author manuscript at DTU.