Chlorine evolution reaction electrocatalysis on RuO(110) and IrO(110) grown using molecular-beam epitaxy.

We report the electrocatalysis of the chlorine evolution reaction (CER) on well-defined RuO(110) and IrO(110) surfaces. RuO and IrO are known for their capabilities to catalyze the CER. Until now, the CER measurements have only been reported on well-defined RuO surfaces and only at high Cl concentrations. We present the CER measurement and the role of Cl at lower concentration on single-orientation RuO(110) and IrO(110) films. We find that RuO(110) is two orders of magnitude more active than IrO(110). Moreover, we observe the correlation between the CER activity and the O formation potential on RuO and IrO, supporting the prior suggestion that the O 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 species on oxides for the CER electrocatalysis and suggests the electrochemical formation of Cl on O (for example, Cl + O ↔ OCl + e) as the crucial step in the CER electrocatalysis.