“The 250 mV Barrier”: A Thermodynamic Ceiling That Every OER/ORR Researcher Must Know!

Abstract

The anodic oxygen evolution reaction (OER) in room-temperature water electrolysis is a key bottleneck in achieving green, economical, and efficient hydrogen production. Thermodynamic constraints dictate that liquid water cannot be oxidized to gaseous O₂ at its reversible potential of 1.23 V, with a minimum overpotential of 0.25 V required, raising the threshold to 1.48 V (the thermoneutral potential at room temperature). Alarmingly, many recent studies claim OER overpotentials below 250 mV, an impossibility under standard conditions. Our analysis reveals that these inaccuracies often stem from a lack of understanding of thermodynamic constraints, misinterpretation of catalyst redox currents as OER activity, use of uncalibrated reference electrodes, and experiments conducted above 298 K without proper corrections. To address these issues, we propose simple countermeasures for accurate OER characterization and analysis. Hence, understanding why an OER electrocatalyst will never be able achieve an overpotential below 250 mV is important to eliminate errors, ensure reliable data, and promote a fair and credible effort to develop advanced electrocatalysts for efficient green hydrogen production, avoiding the premature dismissal of promising materials.