Increasing electrochemical chlorine selectivity over oxygen selectivity through the optimal weakening of oxygen bonds in transition metal-doped RuO2†

Abstract

The electrochemical chlorine evolution reaction (CER) is accompanied with the parasitic oxygen evolution reaction (OER) during acidic brine electrolysis, thereby reducing the efficiency of chlorine production. The guiding principles of enhancing the selectivity of the CER are investigated experimentally and computationally in RuO₂ doped with first-row-transition elements. Computational studies suggest that low-valent dopants (e.g., Cu, Zn, Ni, Co, and Fe) tend to bulk segregate in adsorbate-rich conditions. Further, doping elements with higher d-electrons than Ru (e.g., Cu, Zn, Ni, and Co) in RuO₂ tends to lower the binding strength of OER intermediates (e.g., HO–, O–, and HOO–), thereby increasing OER overpotential and providing more active sites for the CER. Doping has less effect on the binding strength of CER intermediates (ClO⁻) than bivalent OER intermediates (O–), resulting in higher CER selectivity. Computational studies suggest that Cu (d⁹)-doped RuO₂ shows maximum CER selectivity, as corroborated by experiments with electrodeposited Cu-doped RuO₂. Electrodeposited Cu-doped RuO₂ (2% dopant concentration) shows a maximum CER selectivity of 95% in an acidic medium. However, doping a low valency aliovalent dopant and d-enriched metals also lowers the bridge-oxygen vacancy formation energy, thereby activating lattice-oxygen vacancy-aided water dissociation pathway in doped RuO₂ and increasing the selectivity of the OER. This results in an optimum doping concentration for maximum CER selectivity, wherein the weakening of surface OER intermediates is achieved without affecting lattice oxygen bond strength. The present work offers insight into catalyst design considering CER and OER selectivity during electrochemical Cl₂ production.