Measurement of competition between oxygen evolution and chlorine evolution using rotating ring-disk electrode voltammetry

J.G Vos,M.T.M Koper

doi:10.1016/j.jelechem.2017.10.058

J.G Vos, M.T.M Koper

Open Access

https://doi.org/10.1016/j.jelechem.2017.10.058

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Abstract

Selectivity between chlorine evolution and oxygen evolution in aqueous media is a phenomenon of central importance in the chlor-alkali process, water treatment, and saline water splitting, which is an emerging technology for sustainable energy conversion. An apparent scaling between oxygen vs. chlorine evolution has been established, making it challenging to carry the two reactions out individually with 100% faradaic efficiency. To aid selectivity determination, we developed a new method to quickly measure chlorine evolution rates using a conventional RRDE setup. We showed that a Pt ring fixed at 0.95V vs. RHE in pH0.88 can selectively reduce the Cl2 formed on the disk and this allows precise and flexible data acquisition. Using this method, we demonstrate that oxygen evolution and chlorine evolution on a glassy carbon supported IrOx catalyst proceed independently, and that the selectivity towards chlorine evolution (εCER) rapidly approaches 100% as [Cl−] increases from 0 to 100mM. Our results suggest that on IrOx, oxygen evolution is not suppressed or influenced by the presence of Cl− or by the chlorine evolution reaction taking place simultaneously on the surface.

Highlights

The electrochemical oxidation of chloride ions is a reaction of great importance to the chemical industry
There are some important considerations for the apparent selectivity of oxygen evolution reaction (OER) vs. chlorine evolution reaction (CER), which are directly relevant to our ringdisk electrode (RRDE) method for CER detection
On the RHE scale, the OER and CER in acidic media are given by the following redox reactions with their corresponding standard equilibrium potentials: 2H2O → O2 + 4H+ + 4 e−

Summary

Introduction

The electrochemical oxidation of chloride ions is a reaction of great importance to the chemical industry. All known OER catalysts catalyze formation of chlorine [16,17,18], indicating that the OER and CER are intimately coupled This interdependence makes it challenging to carry them out individually with 100% faradaic efficiency. CER activity and Tafel slopes in such studies were derived from raw electrode current densities, with the assumption that all observed current could be ascribed to CER and that OER plays only a negligible role This assumption is reasonable for high Cl− concentrations, a complete picture of the competition between OER and CER behavior in chloride-containing media cannot be drawn in this way. We explore the CER vs. OER behavior of IrOx nanoparticles, as this material constitutes a stable and active acidic OER and CER catalyst

Experimental

Aspects of selectivity between OER and CER

Conclusions