Covalently Immobilized Anthraquinone on Carbon and Its Use As a Proton-Exchange Counter Electrode in Impurity Sensitive Electrochemical Measurements

Abstract

The choice of counter electrode has emerged as an important topic in our search for non-platinum group metal (PGM) materials that can catalyze reactions that are important for fuel cells and electrolyzers such as oxygen reduction reaction (ORR), H2 evolution reaction (HER), and the CO2 reduction reaction. Recent investigations on the stability of Pt materials have revealed that significant dissolution occurs during O2 evolution, making the use of Pt as a counter electrode unsuitable for developing non-PGM materials for ORR and HER, in particular. That has led to increased use of carbon as the counter electrode, which can also oxidize at high voltages and generate partial oxidization compounds such as CO and other small organic molecules, which in turn, are detrimental to metal surfaces. Therefore, developing a stable counter electrode material is key for artifact-free electrocatalytic measurements. In this work, we will discuss the synthesis and application as a counter electrode of a proton-exchange anthraquinone-based reversible redox pair that was covalently grafted onto a high-purity (e.g. 99.99995%) carbon rod. The anthraquinone-grafted counter electrodes were evaluated in aqueous electrolytes using metal single crystals such as Pt(111) and Ir(111) surfaces as the working electrodes, given their extreme sensitivity to impurities in the electrochemical cell. Our results show that the counter electrode voltage is far more stable and centered around the proton-exchange process happening at the anthraquinone centers. This is in contrast to the wide-range voltage changes observed when using the un-grafted carbon rod. There were no differences in the single-crystal surface cyclic voltammetry profile, even after performing HER or OER, which typically causes the biggest swings in the counter electrode voltages. Lastly, there were no detectable amounts of grafted material leaching or exfoliated carbon, confirming that the use of a known redox pair such as from anthraquinone is an effective strategy to prevent artifacts in electrocatalysis research.