Experimental correlation of Mn3+ cation defects and electrocatalytic activity of α-MnO2 – an X-ray photoelectron spectroscopy study

Abstract

Identification and exploration of catalytically active sites is key to designing and engineering low-cost and earth-abundant highly active electrocatalyst materials as alternatives to Pt/C in a variety of electrochemical energy storage and conversion applications. Herein, the relationship between Mn³⁺ cation defects, which are regarded as the catalytically active sites of α-MnO₂ based catalysts, and the electrocatalytic activity towards the oxygen reduction reaction (ORR) is demonstrated. A set of nine α-MnO₂ catalysts were synthesized via a low-temperature homogeneous one-step solution-based catalytic route yielding mesoporous micrometer-sized 3D hierarchical spherical and core–shell superstructures, i.e., α-MnO₂ microspheres and “sea-urchins”. By varying the synthesis conditions, not only were different physicochemical and structural properties achieved, but in particular the surface Mn³⁺ cation defect fraction was affected. X-ray photoelectron spectroscopy (XPS) was carried out and various spectral features including the Mn 2p_1/2 satellite structure, composition of the O 1s core-level photoemission spectra and Mn 3s multiplet splitting were assessed to elucidate the surface oxidation state of manganese oxides. More importantly, Mn³⁺ cation defects of the as-synthesized α-MnO₂ as well as various reference Mn oxides, including α-MnO₂ short fibers, commercial MnO₂ activated, Mn₃O₄ and Mn₂O₃, were quantified by a mathematical deconvolution of Mn 2p_3/2 core-level photoemission spectra. Thus, significant amounts of Mn³⁺ varying from ∼34 to ∼63 mol% could be revealed. Meanwhile, by thin-film rotating disk electrode (TF-RDE) measurements in 0.1 M KOH aqueous electrolyte half-wave potentials (E_1/2) ranging from −0.19 V to −0.43 V (vs. Ag/AgCl (sat. KCl)) could be demonstrated. To assess the correlation with Mn³⁺ cation defects, intrinsic activities were determined by normalizing to the electrochemically active surface areas (ECSAs) of individual α-MnO₂ catalysts. In the course of this, based on the accessibility of different surface domains within the mesoporous particles an inner and outer ECSA could be evidenced. Ultimately, ECSA_outer normalized intrinsic activities unveiled a volcano-like correlation towards the surface Mn³⁺ cation defect population of the as-synthesized α-MnO₂ indicating ∼50 mol% Mn³⁺ to be the optimum content for the highest ORR electrocatalytic activity.