Toward Reliable Reference Electrode Calibration In Alkaline Solution

Abstract

A lkaline electrolysis holds great promise in addressing future energy challenges. 1 A wide range of electrocatalytic reactions, including the hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), CO 2 reduction reaction (CO2RR), and organic oxidation reactions (OOR), are conducted in alkaline conditions. 2 Generally, electrocatalysts employed in these processes require an overpotential in addition to the thermodynamic potential needed for the oxidation or reduction.This overpotential is a key metric for evaluating catalyst performance.To assess the performance of individual electrocatalysts (at the cathode or anode), a reference electrode is introduced during electrochemical measurements.The observed redox potential varies depending on the reference electrode type and the electrolyte solution.To standardize catalyst performance, all measured potentials relative to the reference electrode are converted to the reversible hydrogen electrode (RHE) scale.This conversion can be achieved either through a modified Nernst equation or by experimentally calibrating the reference electrode against the RHE.Despite their differences, both methods rely on the same underlying principle.Specifically, the equilibrium potential of the reference electrode is measured against the H 2 O/H 2 equilibrium (in an alkaline solution,) on a platinum electrode in the measuring solution.An analysis of the 200 most-cited publications on alkaline electrolysis from 2014-2023 reveals that experimental calibration methods were initially prevalent, but equation-based methods have been primarily used in recent years (Figure 1a, Tables S1-S3).Moreover, a significant number of publications fail to report either the final calibration value or the method used to obtain it.Although both approaches seem straightforward, they are prone to errors caused by various factors.In equation-based method, errors can arise from incorrect assumptions about ionic activity, pH value, reference electrode potential, and the omission of liquid junction potentials (LJP).Experimentalbased calibration, on the other hand, can be affected by impurities in hydroxide salts, CO 2 contamination from air, and improper RHE electrode preparation.While some of these errors have been previously discussed in the literature, existing reports are scattered and do not comprehensively address calibration values after error correction. [3][4]4][5][6][7][8][9] As a result, there is a great disparity between the calibration values quoted in the literature (Figure 1b).For instance, the calibration potential of

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