Redox Processes of Manganese Oxide in Catalyzing Oxygen Evolution and Reduction: An in Situ Soft X-ray Absorption Spectroscopy Study

Abstract

Manganese oxides with rich redox chemistry have been widely used in (electro)­catalysis in applications of energy and environmental consequence. While they are ubiquitous in catalyzing the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), redox processes occurring on the surface of manganese oxides are poorly understood. We report valence changes at OER- and ORR-relevant voltages of a layered manganese oxide film prepared by electrodeposition. X-ray absorption spectra were collected in situ in O2-saturated 0.1 M KOH using inverse partial fluorescence yield (IPFY) at the Mn L3,2-edges and partial fluorescence yield (PFY) at the O K-edge. Overall, we found reversible yet hysteretic Mn redox and qualitatively reproducible spectral changes by Mn L3,2 IPFY XAS. Oxidation to a mixed Mn3+/4+ valence preceded the oxygen evolution at 1.65 V vs RHE, while manganese reduced below Mn3+ and contained tetrahedral Mn2+ during oxygen reduction at 0.5 V vs RHE. Analysis of the pre-edge in O K-edge XAS provided the Mn–O hybridization, which was highest for Mn3+ (eg 1). Our study demonstrates that combined in situ experiments at the metal L- and oxygen K-edges are indispensable to identify both the active valence during catalysis and the hybridization with oxygen adsorbates, critical to the rational design of active catalysts for oxygen electrocatalysis.

Cover Feature: Mechanistic Parameters of Electrocatalytic Water Oxidation on LiMn2O4 in Comparison to Natural Photosynthesis (ChemSusChem 22/2017)

“…[50] The spectra showed as harp peak near6 42.0 eV (denoted a in Figure 2c)a nd ab roader peak with higher intensity near 644.5 eV (denoted b in Figure 1c) ,i nw hicht he TEY spectra were less shiftedt han the PFY spectra.I ts hould be noted that the PFY spectrum is broader,w hichm ight affect the apparent peak positions. Peak g can be assigned to spin-up holes in the e g orbitals of the Mn 3 + (one e g hole) and Mn 4 + ions (two e g holes), [51,52] which furthero verlap with the t 2g spin-down holes of the Mn 4 + ion.…”

“…[53] The hybridization between these frontier orbitals of Mn and Oc an be experimentally determined by integration of the spectrali ntensity under peaks g and d normalized by the contribution of the holes to spectrali ntensity( e g holes + 1 = 4 t 2g holes). [54] We assume that the integral under thep eaks is ag ood estimationf or the covalence of the MnÀOb ond, which was highest for LiMn 2 [ 50] High covalence wasa lso supported by the MnÀOÀMn angle of 96.78 obtainedb yX RD analysis. It could lead to oxygen radicals, [55] which are also discussed in the mechanism of natural photosynthesis ( Figure 1, S 4 ).…”

“…The buffer capacity of the used alkaline electrolytes is at least four-fold highert han that of the commonly used 0.1 m phosphate buffer at pH 7 [50] so that electrolyte additives could be omitted to avoid possible side reactions. Voltammetry showedd ecreasingc urrents with cycling where the current at the scan apex halved within 10 cycles (Figure 3a).…”

“…Subsequently,asecond-order polynomial was fitted after the Mn L 2 edge or OK edge and divided over the whole range of data to normalize the post-edges to unity. The energy axis was calibrated with respect to the pre-edge in the spectrum of molecular oxygen at 530.8 eV, [50,100] which was acquired using as ample cell filled with ambient air.T he absence of radiation damage was confirmed using KMnO 4 powder as ab enchmark. No Mn reduction was observed during subsequent scans.…”

“…[96] Thus, an early limiting step in the mechanism of LiMn 2 O 4 means that further oxidation steps are required to return the electrocatalyst to the initial state. These steps are likely fast at the highly oxidizing voltages of the OER because voltages above approximately 0.8 Vv ersus RHE were found sufficient to oxidizeM n 2 + /3 + oxide, [50] whereas the mechanistic parameters were evaluated above 1.58 Vv ersus RHE (i.e., > 0.78 Voverpotential with respect to Mn 2 + /3 + oxidation). Changes in the RLS of electrocatalytic oxidesh aver ecently been demonstrated by using electrolyte additives [97] or surfacef unctionalization.…”