Experimental correlation of Mn3+ cation defects and electrocatalytic activity of α-MnO2 – an X-ray photoelectron spectroscopy study†
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 2p1/2 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 2p3/2 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 (E1/2) 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, ECSAouter 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.