Electrocatalysis and the Hydrogen Evolution Reaction

Introduction

The thermodynamic hydrogen potential is fixed by hydrogen and proton activities under the chosen reference, yet different electrode materials require very different overpotentials to make hydrogen at the same current density. The hydrogen evolution reaction, HER, is a case study in electrocatalysis because surface binding and elementary-step barriers strongly affect exchange current.

Core explanation

In acidic notation, the overall cathodic reaction is 2H+ + 2e− → H2. A surface site is written . One possible first step is Volmer adsorption: H+ + e− + → H . The adsorbed H can form hydrogen through a chemical Tafel recombination, 2H → H2 + 2 , or an electrochemical Heyrovsky step, H + H+ + e− → H2 + . These are alternative pathway elements, not three steps that must all occur sequentially in every catalyst.

In alkaline electrolyte, water often supplies hydrogen: a Volmer-type step can be H2O + e− + → H + OH−. Subsequent H2 formation can again proceed by a water-involving electrochemical step or recombination of adsorbed H. The surface may also bind OH or other electrolyte species, influencing kinetics. Changing pH therefore changes more than a symbol in the overall equation.

A useful qualitative catalyst criterion is balanced hydrogen adsorption. If H binds too weakly, producing enough H can be difficult. If it binds too strongly, releasing H2 or clearing sites can be difficult. Real activity depends on additional factors such as water dissociation in alkaline media, surface structure, coverage and electronic conductivity. Platinum is often a high-activity benchmark, but catalyst ranking depends on electrolyte and normalization.

Exchange current density reflects equal forward and reverse partial rates at equilibrium. Different materials can change the barriers of adsorption, electron transfer and H2 release, yielding very different j0 values. At a fixed small current density, a higher j0 generally reduces activation overpotential. At high current, bubble blockage, ion transport and solution resistance can limit performance even for a fast catalyst.

Tafel slopes are often used to investigate HER, but assigning a unique rate-determining elementary step from one slope alone is risky. Coverage, transport, uncompensated iR and parallel pathways can distort the simple textbook values. A primary mechanistic study discusses this ambiguity for platinum-catalyzed hydrogen evolution and oxidation.

Step-by-step reasoning

Write the overall HER equation for acidic or alkaline medium. Identify the adsorption step and possible H2-forming routes. Compare catalysts at the same electrolyte, temperature, geometric and active area basis. Separate thermodynamic Eeq from kinetic overpotential, then check whether measured current is affected by bubbles, transport or resistance before inferring intrinsic activity.

Visual explanation

Draw a catalyst surface with vacant sites , one adsorbed H and two routes to H2: H meeting another H for the Tafel route, or H receiving a proton and electron for the Heyrovsky route. Place the Volmer adsorption arrow first. Beside it draw a volcano-shaped qualitative trend against H-binding strength, labelling overly weak and overly strong binding regions.

Real-world analogy

A useful workbench must grip a part firmly enough to process it but release it when finished. A surface that cannot hold hydrogen has trouble making H , while one that holds it too tightly has trouble clearing sites for another cycle. The actual catalyst is more complex because electron transfer and solvent reactions also matter.

Real-world example

An electrolyzer cathode coated with a suitable catalyst can deliver a target hydrogen-production current at less negative potential than a sluggish bare surface under comparable conditions. The catalyst improves activation kinetics, but total cell energy use also includes oxygen-evolution polarization, electrolyte resistance and other balance-of-plant losses.

Why?

Different surfaces stabilize reactants, H intermediates and transition states by different amounts. These energy differences change elementary-step rates and equilibrium exchange current. The overall 2H+/H2 thermodynamics remain tied to activities, while the catalyst affects how easily the system approaches useful current.

Common misconception

A catalyst does not create extra thermodynamic energy or shift the standard hydrogen potential solely by making reaction faster. Another error is to infer that a high HER current always indicates superior intrinsic catalyst sites; larger surface area or better bubble removal can also raise measured current.

Worked example

Question: Two cathodes in the same acidic electrolyte have equal geometric area. At the same small cathodic current, catalyst A needs 0.05 V activation overpotential and B needs 0.20 V. What can be inferred and what cannot?

Reasoning: Under matched conditions and after correcting iR and transport, A has faster effective HER kinetics at that operating point. The lower activation loss is consistent with a larger exchange-current scale or favorable mechanism. The observation alone does not give exact j0, identify a unique Volmer/Tafel/Heyrovsky rate-determining step, or prove A has more intrinsically active sites without active-area measurement.

Answer: A shows lower activation polarization under the stated comparison; exact mechanism and intrinsic site activity need additional evidence.

Quick check

1. In acid notation, which step forms adsorbed H from H+ and an electron? Answer: The Volmer adsorption step.

Exam focus

Balance the overall two-electron H2 reaction, then distinguish the one-electron Volmer and Heyrovsky steps from chemical Tafel recombination. Compare catalysts using consistent area and corrected potential. Treat Tafel slopes as evidence, not unique mechanistic fingerprints.

Advanced insight

At high current, local pH, gas bubble coverage and electrolyte transport can differ strongly from bulk conditions. A catalyst that excels in a low-current laboratory measurement may not deliver the same advantage in a full electrolyzer without electrode architecture that removes gas and supplies ions efficiently.

Summary

HER can proceed through Volmer adsorption followed by Tafel recombination or Heyrovsky electrochemical release. Electrode materials change intermediate binding and barriers, altering exchange current and activation overpotential. Thermodynamics, active area, transport and bubble management remain separate performance factors.

Practice questions

1. What is the overall acidic HER stoichiometry? Answer: 2H+ + 2e− → H2. 2. Which H2-forming route is a chemical recombination of two adsorbed H atoms? Answer: The Tafel step. 3. Does a faster catalyst automatically remove all cell voltage losses? Answer: No. Ohmic, transport and opposite-electrode losses remain. 4. Why might very strong H binding lower HER turnover? Answer: H can occupy sites and make product release or site regeneration difficult.