Interface Size-Dependent Changes in Activity of Water Splitting Electrocatalysts.

The traditional method of reporting electrocatalytic activity by normalizing current to the geometric area (mA cm^-2) often results in exaggerated activity for electrodes smaller than 1 cm². Using stainless steel 304 (SS-304) and Ni foam electrodes of varying sizes (0.3-2.1 cm²), along with two modified electrodes with faster kinetics (NiFe LDH for the OER in 1.0 M KOH and Pt/C for the HER in 0.5 M H₂SO₄), we investigated the oxygen and hydrogen evolution reactions. Our results reveal a 2-3-fold exaggeration in activity with electrodes smaller than 1 cm², primarily due to smaller double layers, lower iR drop, and a high edge-to-size ratio, leading to stronger edge effects. These findings suggest that electrodes smaller than 1 cm² may not be ideal for accurately depicting electrocatalytic activity that is always and almost represented in current densities (in mA cm^-2), as geometric area normalization leads to misleading overstatements of performance. In contrast, smaller electrodes may still be useful for studies where specific and accurate sensing of certain analytes (often masked by large iR drop and mass-transport limitations) are necessary (e.g., electrochemical sensing of various analytes), as they offer advantages like lower iR drop and less affected by mass-transport limitations.