ORR on Simple Manganese Oxides: Molecular-Level Factors Determining Reaction Mechanisms and Electrocatalytic Activity

Abstract

In this study we combine experimental rotating ring disc electrode data, theory, molecular-level modeling and microkinetic simulations in order to gain a deeper insight into the oxygen reduction reaction (ORR) mechanism on simple manganese oxides in alkaline media. We demonstrate that "thermodynamic" approach based on periodical density functional theory calculations is unable to explain the experimentally observed differences in the ORR kinetics on the most (α-Mn 2 O 3 ) and the least (α-MnOOH) active oxide. We perform quantum mechanical cluster calculations and show that faster kinetics of the hydrogen peroxide reduction on the surface of Mn 2 O 3 oxide and the ensuing lower peroxide yield during the ORR are corroborated by the lower barrier for the dissociation of hydrogen peroxide adsorbed on the surface of Mn 2 O 3 arising from adsorbate-adsorbate interactions. We provide the arguments in favor of the outersphere nature of initial O 2 reduction steps and demonstrate that this hypothesis does not contradict the experimental trends observed for ORR and hydrogen peroxide reduction reactions on Mn 2 O 3 and MnOOH.

Scalable, inexpensive, one-pot, facile synthesis of crystalline two-dimensional birnessite flakes

“…[13][14][15][16][17][18][19][20][21][22] The electrocatalytic activity and reversibility have been related to the Mn 2+/3+ /Mn 4+ redox couples. 14,15,[23][24][25][26][27][28] And while the specific/normalized electrocatalytic activity for MnO x -based materials remains below that of respective PGM materials, the increase in material abundance and decrease in cost more than offsets this activity disparity. This renders MnO x nanomaterials attractive replacements for PGM catalysts when the total electrode area of an electrochemical energy conversion/storage scheme is not limiting, as in stationary power applications, for example.…”

In situ Probing of Mn2O3 Activation toward Oxygen Electroreduction by the Laser-Induced Current Transient Technique

“…Insufficient durability and catalytic poisoning of state-of-the-art Pt/C encourages moving away from an expensive Pt/C catalyst. Toward this end, manganese oxides have been quite popular and are currently known to be one of the best oxide catalysts having wide abundance with substantial stability and affordability. − Among the diverse variants available, both chemically and structurally, bixbyite Mn 2 O 3 has emerged as one of the most catalytically active contenders. , The fundamental reason being the presence of Mn in the +3 oxidation-state whose Mn 3+ /Mn 4+ redox couple lies just below the thermodynamic oxygen reduction potential. , Besides the electrochemical nature, a structural reorganization following redox transition facilitates O 2 electrosorption at potentials closer to the ideal 1.23 V increasing the overall oxygen reduction reaction (ORR) activity . However, these phenomena have not been studied in detail, rendering them as assumptions, which form the basis of the present study.…”

Mn2O3 oxide with bixbyite structure for the electrochemical oxygen reduction reaction in alkaline media: Highly active if properly manipulated

“…According to microkinetic modeling, the differences in the ORR kinetics of Mn oxides do not solely originate from different formal potential values (affecting the coverage of various adsorbates) but also from the structuredependent rate constants of the reaction steps (ii), (iii) (corresponding to consecutive steps of oxygen reduction to hydrogen peroxide), and (v) (bond breaking in hydrogen peroxide ultimately leading to its transformation into water). Quantum chemical calculations of Nikitina et al [26] have demonstrated the influence of surface structure-dependent adsorbateadsorbate interactions on the activation barrier of the O-O bond breaking in the adsorbed hydrogen peroxide (in the considered ORR mechanism this corresponds to step (v), while quantum-chemical calculations suggest that this step involves OHanion rather than H 2 O molecule formation [26]). Insets in Fig.…”

“…This results in a high Mn(IV)/Mn(III) potential due to high ionicity of the Mn-O bond. While quantumchemical calculations performed by Nikitina et al [26] for the -Mn 2 O 3 (111) and -MnOOH (110) plane (the best and the worst catalysts in the experimentally studied series, correspondingly) suggested that steps (ii) to (v) may not be elementary, they have provided further justification for the high activity of -Mn 2 O 3 .…”