Abstract
Chemical and structural changes preceding electrocatalysis obfuscate the nature of the active state of electrocatalysts for the oxygen evolution reaction (OER), which calls for model systems to gain systematic insight. We investigated the effect of bulk oxidation on the overpotential of ink‐casted LiMn2O4 electrodes by a rotating ring‐disk electrode (RRDE) setup and X‐ray absorption spectroscopy (XAS) at the K shell core level of manganese ions (Mn−K edge). The cyclic voltammogram of the RRDE disk shows pronounced redox peaks in lithium hydroxide electrolytes with pH between 12 and 13.5, which we assign to bulk manganese redox based on XAS. The onset of the OER is pH‐dependent on the scale of the reversible hydrogen electrode (RHE) with a Nernst slope of −40(4) mV/pH at −5 μA monitored at the RRDE ring. To connect this trend to catalyst changes, we develop a simple model for delithiation of LiMn2O4 in LiOH electrolytes, which gives the same Nernst slope of delithiation as our experimental data, i. e., 116(25) mV/pH. From this data, we construct an ERHE‐pH diagram that illustrates robustness of LiMn2O4 against oxidation above pH 13.5 as also verified by XAS. We conclude that manganese oxidation is the origin of the increase of the OER overpotential at pH lower than 14 and also of the pH dependence on the RHE scale. Our work highlights that vulnerability to transition metal redox may lead to increased overpotentials, which is important for the design of stable electrocatalysts.
Highlights
We investigate the oxygen evolution reaction on LiMn2O4 in LiOH electrolytes with pH between 12 and 14 using rotating ring disk electrodes (RRDE) and X-ray absorption spectroscopy (XAS)
We investigated the effect of bulk oxidation on the overpotential of LiMn2O4 as an electrocatalyst for the oxygen evolution reaction (OER) in LiOH
We found pronounced redox peaks in LiOH electrolytes with pH 13.5 that were not observed at pH 14 and in all previous reports where NaOH and KOH electrolytes were used.[5,6,20,21]
Summary
ChemPhysChem 2019, 20, 2981 – 2988 of oxygen evolution is determined using the ring of the RRDE. We construct an ERHE-pH diagram based on the model and our experimental data. Predictions regarding the oxidation stability are verified using additional ex situ XAS measurements
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