What the Overpotential Does Not Tell You: Diagnosing Non-Comparability Across Hydrogen and Oxygen Evolution...

Overpotential at 10 mA cm^-2 (η₁₀) is the default benchmark for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysts, but the number is rarely comparable across studies. This Perspective identifies six measurement and reporting stages, from synthesis reproducibility to the measurement platform itself, at which undisclosed methodological choices shift reported activity by tens to hundreds of millivolts without changing the catalyst. The benchmark's device-context origin matches neither laboratory practice nor industrial current density, and its geometric normalization is sensitive to catalyst loading. Comparable artefacts recur in Tafel slope extraction, electrochemically active surface area, turnover frequency, and Faradaic efficiency, each traced to a specific undisclosed choice and a minimum reporting requirement. We test the framework on literature values checked against their primary sources. Of 11 NiFe oxyhydroxide reports, five give η₁₀ on a matched basis (1 M KOH, room temperature), and these span 134 to 372 mV. For commercial Pt/C, the field's own reference material, five matched-basis reports span 33 to 93 mV, and loading is stated directly in only two of the seven reports checked. We close with a concise reporting checklist organized by metric.