Undesired Bulk Oxidation of LiMn₂O₄ Increases Overpotential of Electrocatalytic Water Oxidation in Lithium Hydroxide Electrolytes

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.

Reversible and irreversible processes during cyclic voltammetry of an electrodeposited manganese oxide as catalyst for the oxygen evolution reaction

“…While many of the more active Mn-based electrocatalysts contain Mn 4+ in addition to Mn 3+ [10,39,40,[81][82][83][84], those with predominately Mn 4+ are inactive [3]. Recently, Baumung et al [18] showed that (bulk) oxidation of LiMn 3.5+ 2 O 4 toward Mn 4+ increases the overpotential of the OER. This can be rationalized qualitatively using the occupancy of the e g orbitals as proposed by Suntivich et al [47]where the lowest overpotential is predicted for an e g occupancy of about one (Mn 3+ ) and both higher or lower e g occupancy (i.e.…”

“…Usually, degradation is associated with changes of the surface and bulk oxide composition, long range order (i.e. crystallinity) and/or microstructure, but it can be also caused by detachment, particle agglomeration, or blocking by oxygen bubbles [17][18][19][20][21]. These material modifications due to activation and degradation can be electrochemically observed as changes in overpotential at fixed current or changes in current at fixed overpotential [17,[22][23][24].…”

Aqueous lithium‐ion batteries

“…LiMn 2 O 4 and its close derivatives, thus, became a mainstay in the early years of aqueous lithium‐ion battery research 31–37 . LiMn 2 O 4 would also become part of work developing supercapacitors due to its stability 38 and high rate capability 39 …”

Characterization of a Modular Flow Cell System for Electrocatalytic Experiments and Comparison to a Commercial RRDE System

“…For flow rates of 0.5 to 8 ml/min, the Reynolds number is calculated as Re DEFC = 2.5-40 ( = 0.01004 cm 2 /s at 20 °C) using Eq. (12). The low Reynolds number (˂˂2000) suggests that the cell has a laminar flow, which allows a controlled mass transport to and from the working electrodes.…”

“…Generator-collector experiments are most commonly performed using rotating ring-disk electrodes (RRDE; Fig. 1a), [11][12][13] which are commercially available from several manufacturers. In this kind of setup, a rotated disk is the generator electrode.…”