The Comprehensive Understanding of 10 mA cm −2_geo as an Evaluation Parameter for Electrochemical Water Splitting

Comparing the overpotential required to reach the current density of −2 10 mA cm_geo (per geometric area of an electrode) (η@10 mA cm⁻2_geo) is a popular method of ranking electrocatalysts for water-splitting reactions, i.e., the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). However, such methodology is, in essence, not reasonable for studying the intrinsic chemistry difference of electrocatalysts. To have a rational understanding of η@ 10 mA cm⁻2_geo, here, its historical origin and its limitations on reflecting the intrinsic electrocatalytic activity are discussed. The η@ 10 mA cm −2_geo is a valid practical parameter to assess water-splitting devices, but it cannot determine whether a particular electrocatalyst is intrinsically active or not.