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

Javier Villalobos,Lifei Xi,Ronny Golnak,Götz Schuck,Marcel Risch

doi:10.1088/2515-7655/ab9fe2

Abstract

Manganese oxides have received much attention over the years among the wide range of electrocatalysts for the oxygen evolution reaction (OER) due to their low toxicity, high abundance and rich redox chemistry. While many previous studies focused on the activity of these materials, a better understanding of the material transformations relating to activation or degradation is highly desirable, both from a scientific perspective and for applications. We electrodeposited Na-containing MnOx without long-range order from an alkaline solution to investigate these aspects by cyclic voltammetry, scanning electron microscopy and x-ray absorption spectroscopy at the Mn-K and Mn-L edges. The pristine film was assigned to a layered edge-sharing Mn3+/4+ oxide with Mn-O bond lengths of mainly 1.87 Å and some at 2.30 Å as well as Mn-Mn bond lengths of 2.87 Å based on fits to the extended x-ray fine structure. The decrease of the currents at voltages before the onset of the OER followed power laws with three different exponents depending on the number of cycles and the Tafel slope decreases from 186 ± 48 to 114 ± 18 mV dec−1 after 100 cycles, which we interpret in the context of surface coverage with unreacted intermediates. Post-mortem microscopy and bulk spectroscopy at the Mn-K edge showed no change of the microstructure, bulk local structure or bulk Mn valence. Yet, the surface region of MnOx oxidized toward Mn4+, which explains the reduction of the currents in agreement with literature. Surprisingly, we find that MnOx reactivates after 30 min at open-circuit (OC), where the currents and also the Tafel slope increase. Reactivation processes during OC are crucial because OC is unavoidable when coupling the electrocatalysts to intermittent power sources such as solar energy for sustainable energy production.

Highlights

Among the wide range of transition metal oxides used for the oxygen evolution reaction (OER), manganese oxides have received much attention over the years because their low toxicity and their high abundance (10th in Earth crust)
The decrease of the currents at voltages before the onset of the OER followed power laws with three different exponents depending on the number of cycles and the Tafel slope decreases from 186±48 to 114±18 mV dec-1 after 100 cycles, which we interpret in the context of surface coverage with unreacted intermediates
Manganese oxides were deposited on glassy carbon electrodes for electrocatalytic investigations and on graphite paper for X-ray absorption spectroscopy (XAS)

Summary

Introduction

Among the wide range of transition metal oxides used for the oxygen evolution reaction (OER), manganese oxides have received much attention over the years because their low toxicity and their high abundance (10th in Earth crust). 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 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]. The activation is performed for 20 to 50 cycles, but in some cases, the final activated material does not form until the 200th cycle [31]

Methods

Results

Conclusion