Descriptive Role of Pt/PtOx Ratio on the Selective Chlorine Evolution Reaction under Polarity Reversal as Studied by Scanning Electrochemical Microscopy

This study investigated competing chlorine evolution reaction (ClER) and oxygen evolution reaction (OER) on Pt electrodes under variable polarity reversal intervals (±16.7 mA cm^-2, 30-600 s) in the context of distinctive roles of Pt(0) and PtO_x on the surface in dilute (0.1 M) NaCl solutions. The substrate generation/tip collection mode of scanning electrochemical microscopy (SECM) quantified the current efficiency (CE) of ClER with a large tip-to-substrate distance (>500 μm) to avoid intervention of bubbles and spatial variations. Surface interrogation SECM using [Ru(NH₃)₆]^2+/3+ coupled with X-ray photoelectron spectroscopy (XPS) identified the Pt⁴⁺-enriched surface of PtO_x with a bilayer structure to give more efficient regeneration of Pt(0) under the shorter reversal interval. The in situ SECM complemented bulk electrolysis and XPS to demonstrate that ClER on Pt(0) and OER on PtO_x primarily determine the CE of ClER, in agreement with a kinetic model. The descriptive role of surface Pt/PtO_x ratio rationalized the enhanced selectivity for ClER upon the polarity switching, being independent on a scaling relationship. The current reversal (not allowed to IrO₂ electrodes) also alleviated calcareous scale deposit in the electrolyte with hardness.