Hydrogen Evolution Catalyst Comparison

Introduction

Hydrogen evolution is often used to compare electrocatalysts because its overall product, H₂, is simple. The surface mechanism is still conditional: hydrogen must be supplied, adsorbed and released, while water activation can matter in neutral or alkaline media. A reported “low overpotential” is meaningful only with the current, electrolyte, active area, resistance correction and stability protocol stated.

Core explanation

In acid, the Volmer step can be written H⁺ + e⁻ + → H , where is a surface site. H₂ can then form by the Tafel step, 2H → H₂ + 2 , or by the Heyrovsky step, H + H⁺ + e⁻ → H₂ + . In alkaline solution, water is often the proton source: H₂O + e⁻ + → H + OH⁻ in a Volmer-type step. Water cleavage and OH-related interactions can add kinetic requirements beyond H binding. An ACS analysis of HER Tafel interpretation discusses these pathways and warns against oversimplifying measured slopes.

The Sabatier view suggests H should be stable enough to form but not so stable that H₂ release is hindered. ΔG(H ) near a favourable range can screen related surfaces. It does not fully describe water activation, electrolyte cations, oxide formation or surface area. A JACS Au review of alkaline HER catalysts notes that water dissociation and OH-related adsorption can be important alongside hydrogen binding. A material with excellent calculated H energy can underperform if its working surface is different or water delivery is slow.

Overpotential is the extra potential relative to the thermodynamic H₂ equilibrium potential needed to drive a specified current. Comparing η at 10 mA/cm², for example, requires the same area basis, gas, electrolyte, temperature, scan or steady-state method and correction for solution resistance. Geometric-area current is useful for device output, while site-normalised activity helps mechanistic comparison. Gas bubbles can block area; catalyst layers can have different mass transport. A short voltage sweep does not establish long-term performance.

Product verification matters even for HER. Current may include corrosion, support reactions or other reduction processes. Measure H₂ and Faradaic efficiency where possible. A JACS benchmark study used common protocols for activity, surface area and short-term stability across multiple HER catalysts, illustrating the value of controlled comparison.

Step-by-step reasoning

1. Specify acid, neutral or alkaline electrolyte and the potential reference. 2. Map Volmer and possible Tafel or Heyrovsky pathways. 3. Compare H binding with water activation and surface-state evidence. 4. Measure kinetic current, overpotential, H₂ output and resistance correction. 5. Check area normalization, bubble effects and stability over relevant time.

Visual explanation

Draw H at the centre of a fork. One input arrow is H⁺ reduction in acid; another is water reduction in base. Two output arrows are Tafel recombination and Heyrovsky electrochemical desorption. Beside it draw a polarisation curve with a marked current density and corresponding η, explicitly labelled with reference and electrolyte.

Real-world analogy

A factory can have a perfectly tuned final packaging line yet remain slow if incoming components are hard to prepare. H binding resembles holding a component at the workstation, while alkaline water activation can resemble preparing the component. The final output depends on both supply and release, though electrochemical potentials add a control absent from the analogy.

Real-world example

Catalyst A and B have similar calculated H adsorption free energies. In acidic electrolyte their measured site-normalised rates are close, but in alkaline electrolyte B performs worse. A plausible hypothesis is that B is poorer at water activation or accumulates inhibiting OH-related species. The team tests water-splitting kinetics, local pH, site structure and H₂ Faradaic efficiency instead of concluding that the H descriptor is universally wrong.

Why?

Why is η at a chosen current not an intrinsic material constant? It includes catalyst site density, electrolyte conductivity, layer thickness, bubbles and transport, as well as elementary kinetics. Changing electrode fabrication can change η even when the active chemical motif is the same.

Common misconception

“Near-zero ΔG(H ) proves best HER catalyst” ignores water activation and stability. “A Tafel slope uniquely identifies one step” can fail when coverage, resistance or transport affect the curve. “Low η from a rapid scan guarantees durable operation” is false. “All current is hydrogen” requires product confirmation.

Worked example

Two electrodes are measured at 10 mA/cm² geometric current in the same acid electrolyte. A reaches the current at an overpotential of 0.12 V and B at 0.18 V after comparable resistance correction. A appears better for geometric-area device output at that point. But A contains 2.0 mg catalyst/cm² while B contains 0.5 mg/cm². At the same 10 mA/cm², mass-normalised currents are 5 and 20 mA/mg, respectively. These are different comparison questions; neither alone determines intrinsic active-site turnover. If B loses half its current after 24 h while A remains stable, process choice needs a time-dependent test.

Quick check

1. What extra chemical step may be important for alkaline HER compared with an idealised acid H⁺ reduction? Answer: Water activation or cleavage to supply H can be important.

Exam focus

Write Volmer, Tafel and Heyrovsky steps with appropriate medium. Explain the role and limitation of H binding. Compare overpotential only at a stated current and protocol, and distinguish geometric, mass and site normalisations.

Advanced insight

Interfacial water structure and specific cations can influence proton transfer and adsorbate stabilisation. Two catalysts with similar vacuum H energies may therefore behave differently in aqueous electrolyte. Explicit solvent and potential-dependent measurements can refine a one-descriptor screen, but should be tested against independent product and kinetic data.

Summary

HER performance depends on hydrogen formation and release, with water activation and interfacial conditions often important. A fair catalyst comparison joins descriptor chemistry with controlled overpotential, product and stability measurements.

Practice questions

1. Which HER step combines two adsorbed hydrogens without another electron transfer? Answer: The Tafel step. 2. What does the Heyrovsky step do? Answer: It forms H₂ through an electrochemical reaction involving H and a proton donor. 3. Why report electrolyte when giving a HER overpotential? Answer: Proton source, transport, resistance and interfacial chemistry depend on the electrolyte. 4. Can an electrode with worse η at equal geometric current have higher rate per gram of catalyst? Answer: Yes, if it contains much less catalyst or exposes sites differently.