Rotating Ring–Disk Electrode Study of Oxygen Evolution at a Perovskite Surface: Correlating Activity to Manganese Concentration

Abstract

Transition-metal oxides with the perovskite structure are promising catalysts to promote the kinetics of the oxygen evolution reaction (OER). To improve the activity and stability of these catalysts, a deeper understanding about the active site, the underlying reaction mechanism, and possible side reactions is necessary. We chose smooth epitaxial (100)-oriented La 0.6 Sr 0.4 MnO 3 (LSMO) films grown on Nb:SrTiO 3 (STNO) as a model electrode to investigate OER activity and stability using the rotating ring−disk electrode (RRDE) method. Careful electrochemical characterization of various films in the thickness range between 10 and 200 nm yields an OER activity of the epitaxial LSMO surface of 100 μA/cm 2 ox at 1.65 V vs RHE, which is among the highest reported for LSMO and close to (110)-oriented IrO 2 . Detailed post-mortem analysis using XPS, XRD, and AFM revealed the high structural and morphological stability of LSMO after OER. The observed correlation between activity and Mn vacancies on the surface suggested Mn as the active site for the OER in (100)-oriented LSMO, in contrast to similar perovskite manganites, such as Pr 1−x Ca x MnO 3 . The observed Tafel slope of about 60 mV/dec matches the theoretical prediction for a chemical ratelimiting step that follows an electrochemical pre-equilibrium, probably O−O bond formation. Our study established LSMO as an atomically flat oxide with high intrinsic activity and high stability.

Environmental TEM Investigation of Electrochemical Stability of Perovskite and Ruddlesden–Popper Type Manganite Oxygen Evolution Catalysts

“…Ring currents are not superimposed by corrosion or capacitance, and correspond solely to the reduction of evolved oxygen. [32] Therefore, the simultaneous increase, i.e., similar Tafel slopes, of disk and ring currents suggests that the observed onset in fact corresponds to OER and the absence of parasitic processes. [45] To evaluate the electrochemical stability of P73-PCMO and RP-PCMO, we compare the changes of consecutive voltage cycles relative to the 2nd cycle at a potential of 1.65 V versus RHE.…”

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

“…In both normalizations, it is assumedt hat all surfaceionsare active, which likely leads to underestimation of intrinsic activity,p articularly if all bulk ions are considered. The LiMn 2 O 4 -carbon composite electrode reached 100 mAcm À2 ox at 1.714(3) Vversus RHE (0.485(3) Voverpotential), which is similar to LaMnO 3 /C (1.68 Vv ersusR HE) [7] and Ba 6 Mn 4 O 16 /C (1.66 V versus RHE) [68] particles and La 0.6 Sr 0.4 MnO 3 thin films (1.68(2) V versus RHE) [61,69] but clearly less activea sc ompared to, for example,B a 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-d thin films (1.53 Vv ersus RHE). [70] The TOF-b of the LiMn 2 O 4 -carbon composite electrode was 0.0025 (2) )t han the TOF-b.…”

“…Quantitative detection is hindered by trappingo fb ubbles at the disk and the PTFE (polytetrafluoroethylene) spacer. [56,61] Instead of the conventional collection efficiency, the ring currents were scaled to match the diskc urrents between 20 and 100 mA, where we expect low bubble formation. The scaling factors at 1600rpm were 0.062(22) at pH 13 and 0.061 (14)a veraged over all investigated pH values during the 5th cycle.T he deviation to the expected value of 0.424 for the investigated RRDEs etup suggesteds ignificant trapping of oxygen bubbles.…”

“…Yet, Ta fel slopes of about 60 mV dec À1 were also previously reported for diverse manganese oxidess uch as composite electrodes of Mn 3 O 4 and carbon (61 mV dec À1 ), electrodeposited MnO x (60(3) mV dec À1 ) [75] and epitaxial La 0.6 Sr 0.4 MnO 3 disks (65(5) mV dec À1 ). [61] The latter was also studied on aR RDE setup,w here the ring Ta fel slope was 66(6) mV dec À1 (the values are given for setupu sed at the Institute of Materials Physics (IMP) in Ref. [61],w hich is identical to the one used in this study).…”

“…[61] The latter was also studied on aR RDE setup,w here the ring Ta fel slope was 66(6) mV dec À1 (the values are given for setupu sed at the Institute of Materials Physics (IMP) in Ref. [61],w hich is identical to the one used in this study). These literature values further corroborate that the ring Ta fel slope of 62(1)mVdec À1 is representative of oxygen evolution on LiMn 2 O 4 .…”