Chlorine evolution reaction electrocatalysis on RuO₂(110) and IrO₂(110) grown using molecular-beam epitaxy

We report the electrocatalysis of the chlorine evolution reaction (CER) on well-defined RuO₂(110) and IrO₂(110) surfaces. RuO₂ and IrO₂ are known for their capabilities to catalyze the CER. Until now, the CER measurements have only been reported on well-defined RuO₂ surfaces and only at high Cl^- concentrations. We present the CER measurement and the role of Cl^- at lower concentration on single-orientation RuO₂(110) and IrO₂(110) films. We find that RuO₂(110) is two orders of magnitude more active than IrO₂(110). Moreover, we observe the correlation between the CER activity and the O_ad formation potential on RuO₂ and IrO₂, supporting the prior suggestion that the O_ad is the active site for the CER. We further use the reaction order analysis to support the Volmer-Heyrovsky mechanism of the CER, which was previously suggested from the Tafel slope analysis. Our finding highlights the importance of the surface O_ad species on oxides for the CER electrocatalysis and suggests the electrochemical formation of Cl_ad on O_ad (for example, Cl^- + O_ad ↔ OCl_ad + e^-) as the crucial step in the CER electrocatalysis.