Effect of Saturating the Electrolyte with Oxygen on the Activity for the Oxygen Evolution Reaction.

Abstract

With the increased interest in the electrochemical conversion of renewable electricity, water, and carbon dioxide to fuels, there is an ever-growing number of papers reporting new electrocatalysts for the oxygen evolution reaction (OER).1−5 The OER is the anode reaction in the electrolysis of water and CO2 and a major source of efficiency loss, because of its high overpotential.6 To meaningfully compare the activity of the many new materials that are currently synthesized and tested, it is important that the research community agrees on proper standardization, benchmarking, and best practices7,8 Several papers reporting on the activity of OER state that it is necessary to saturate the electrolyte with oxygen gas before measurement, in order for the electrode “to reach its rest potential” or “to fix the equilibrium potential”,2,9−14 and saturating the solution with oxygen seems to have become an often-employed practice (see, e.g., refs (15−18)). A recent paper claimed that oxygen in the electrolyte may reduce the OER activity on nickel (supported on graphene) and change the Tafel slope via a van der Waals-type interaction of molecular oxygen with the active site, hampering access by hydroxide ions.11 The argument to fix the equilibrium potential is based on the idea that, in the absence of oxygen, the driving force for oxygen evolution should be higher than in its presence. However, at a given electrode potential, the rate of oxygen evolution itself must be independent of whether O2 is present in solution or not. What potentially changes in the presence of oxygen is the rate of the back reaction, i.e., the oxygen reduction reaction (ORR), and therefore the net production rate of oxygen may be dependent on whether O2 is present or not. This could play a role for a reversible reaction near its equilibrium potential, but it should be irrelevant for an irreversible reaction such as OER (ORR rates can safely be neglected above 1.3 V). The rate of an electrode reaction at a given applied potential is more accurately measured in the absence of the product, regardless of whether, under the experimental conditions, the equilibrium potential is well-defined or not. This also implies that the overpotential, when defined as the difference between the applied potential and the equilibrium potential, is not well-defined in the absence of oxygen in solution, but this has no (theoretical) effect on the rate of the OER at a given applied potential. Of course, close to equilibrium, it may still be necessary to correct for any current due to the back reaction if one wants to know the rate of the forward reaction only. The notion that the presence of O2 in solution may have an effect on the state of the surface (and thereby influence its chemistry and “rest potential”) has been studied for platinum electrodes.19,20 It was found that O2 in solution may indeed have an effect on the activity and stability of platinum electrodes, but apparently only under potential cycling conditions, or at relatively negative potentials, suggesting that oxygen reduction may play a role. Kongkanand and Ziegelbauer concluded that the effect of O2 in solution on the oxide coverage on platinum is negligible.19 To clarify the experimental role of oxygen saturation of the electrolyte on OER activity, we decided to study the effect of electrolyte oxygen on the OER on Pt and Ni-oxyhydroxide electrodes. In this Viewpoint, we show that (i) oxygen dissolved in the electrolyte has no significant effect on the OER activity (at least not on Pt-oxide and Ni-oxyhydroxide surfaces) and (ii) in the standardization of best practices for OER studies, care should be taken in employing a proper (placement of the) reference electrode, and in taking measures that small oxygen bubbles are efficiently removed from the electrocatalyst surface (for instance, by rotation).