The reference electrode dilemma in energy conversion electrocatalysis: “right vs. okay vs. wrong”

Abstract

In this study, to show the extent to which the use of any reference electrode (RE) at any pH could affect the electrochemical data, Hg/HgO, 1.0 M KOH (MMO, E_ref = 98 mV), Hg/Hg₂SO₄, sat. K₂SO₄ (MMS, E_ref = 640 mV), Ag/AgCl, 3.0 M NaCl (SSC, E_ref = 210 mV), and Hg/Hg₂Cl₂, sat. KCl (SCE, E_ref = 241 mV) standard REs destined for use in alkaline, acidic, and neutral waters, respectively, are taken to monitor the hydrogen evolution reaction (HER) onset potential of Pt in 0.5 M H₂SO₄, 1.0 M KOH, and 1.0 M PBS (phosphate buffer solution). The standard potential of the reference electrode can be calculated using the Nernst equation since the HER onset of Pt is known against the reversible hydrogen electrode (RHE) (i.e., 0.0 V vs. RHE). Any change that one might see with the HER onset of Pt while using an inappropriate RE at an inappropriate pH could be attributed to the pH gradient between the RE compartment and the cell. Our results revealed that when an inappropriate RE was used at an inappropriate pH, the standard potential of the RE was shifted either negatively or positively depending on the combination of RE and pH. This shift ranged from +78 mV to −74 mV at their maxima and this is highly significant when we consider the overpotential ranges reported with familiar energy conversion reactions such as the HER, oxygen evolution, oxygen reduction, CO₂ reduction, etc. Hence, it is strongly recommended to use an appropriate RE for a given pH considering its pH-compatibility and the harm that it will otherwise do to the analysis and the results thereof.