Template-free synthesis of mesoporous manganese oxides with catalytic activity in the oxygen evolution reaction

Abstract

Porous manganese carbonate was obtained via solvothermal synthesis using ethanol and urea. The manganese carbonate was subsequently used as a precursor to synthesise mesoporous manganese oxides via thermal treatments at three various temperatures. X-ray diffraction and Extended X-ray Absorption Fine Structure (EXAFS) results shows that g-MnO 2 is synthesised at 380 and 450 C while Mn 2 O 3 is produced at the annealing temperature of 575 C. X-ray absorption spectra show that g-MnO 2 converts completely to Mn 2 O 3 after annealing over the 450-575 C range. The oxides obtained at 380 C and 450 C possess extremely high specific surface area, which is of interest for catalytic applications. The oxides were investigated as electrocatalysts for the oxygen evolution reaction; the oxide prepared at the lowest annealing temperature was found to be the optimum catalyst with an overpotential of 427 AE 10 mV at a current density of 10 mA cm À2 , normalised by the geometric area. The improved catalytic activity was related to the presence of defect-rich and highly porous manganese dioxide at the lowest annealing temperature. Fig. 3 TEM images of (a) MnCO 3 , (b) Mn-380 (c) Mn-450 and (d) Mn-575. Scale bar for all images is shown in (d). 784 | Sustainable Energy Fuels, 2017, 1, 780-788 This journal is

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

“…Nakamura and coworkers [44,45] discuss that more Mn 3+ ions lead to higher activity, which is in line with the e g orbital descriptor proposed by Suntivich et al [47]. In their catalytic model, comproportionation of Mn 4+ serves as a secondary supply of the catalytically active Mn 3+ , which may explain that the lowest overpotential in systematic studies of the OER on simple manganese oxides is usually found for mixed Mn 3+/4+ oxides [3,4,[38][39][40]42]. Therefore, it is critical to control the distribution of Mn cations in Mn-based electrocatalysts for the OER to understand their activity and potential degradation.…”

“…Yet, small amounts of Mn 2+ and Mn 4+ are beneficial if Mn comproportionation of these ions to Mn 3+ is possible on the surface of the Mn oxide [44,45]. Thus, the lowest overpotential in systematic studies of the OER on manganese oxides is usually found between Mn 3.5+ and Mn 3.7+ [3,4,[38][39][40]42], rather than Mn 3+ . Nonetheless, materials with mainly Mn 4+ perform significantly worse than those with sizable amount of Mn 3+ , e.g.…”

“…Valuable insight into the mechanism of catalysis, activation and degradation of manganese oxides has been obtained in the last decade from spectroscopy including X-ray absorption spectroscopy (XAS) [35][36][37][38][39][40][41][42][43], UV-Vis spectroscopy [44][45][46] and Raman spectroscopy [46]. We focus on alkaline electrolytes herein.…”

Emerging trends in metal oxide electrocatalysis: Bifunctional oxygen catalysis, synergies and new insights from in situ studies

“…Further discussion of chemical stability toward ageing of many of the oxide materials reported would also be recommended. Most of the highly active oxide materials are disordered, porous and highly defective, therefore chemical ageing effects are likely to be important; in our own work on MnO x for the OER we have noted how structural changes in highly disordered phases can lead to activity loss [47] . A better understanding of structural instability and its undesirable but also desirable effects, as outlined in [48] , would be thus beneficial.…”

Recent Progresses in Electrocatalysts for Water Electrolysis

“…For manganese oxides, the electrochemical and physicochemical properties are highly dependent on morphology and crystallographic nature [173][174][175][176][177][178][179][180]. Therefore, to study the effects of structure on OER activity, Meng et al [177] synthesized four different crystal lattices of MnO 2 , including α, β, σ-MnO 2 and amorphous MnO 2 (AMO), and analysed them. Here, XRD patterns confirmed the amorphous nature of the AMO with three weak peaks and SEM images revealed aggregated particles of less than 50 nm.…”