Rutile (β-)MnO₂ Surfaces and Vacancy Formation for High Electrochemical and Catalytic Performance

Abstract

MnO 2 is a technologically important material for energy storage and catalysis. Recent investigations have demonstrated the success of nanostructuring for improving the performance of rutile MnO 2 in Li-ion batteries and supercapacitors and as a catalyst. Motivated by this we have investigated the stability and electronic structure of rutile (β-)MnO 2 surfaces using density functional theory. A Wulff construction from relaxed surface energies indicates a rod-like equilibrium morphology that is elongated along the c-axis, and is consistent with the large number of nanowire-type structures that are obtainable experimentally. The (110) surface dominates the crystallite surface area. Moreover, higher index surfaces than considered in previous work, for instance the (211) and (311) surfaces, are also expressed to cap the rod-like morphology. Broken coordinations at the surface result in enhanced magnetic moments at Mn sites that may play a role in catalytic activity. The calculated formation energies of oxygen vacancy defects and Mn reduction at key surfaces indicate facile formation at surfaces expressed in the equilibrium morphology. The formation energies are considerably lower than for comparable structures such as rutile TiO 2 and are likely to be important to the high catalytic activity of rutile MnO 2 . ■ INTRODUCTIONEnergy storage for hybrid electric vehicles and renewable energy sources is a pressing technological challenge for which Li-ion batteries and supercapacitors are key candidate systems. The conventional Li-ion intercalation cathode, LiCoO 2 , faces challenges of toxicity and high cost. Current supercapacitors based on carbon cathodes are limited by their storage capacity. The demand for higher capacity and higher power energy storage has led to a surge in interest in nanostructured electrodes. 1 Nanostructuring has been shown to improve the energy storage properties of rutile MnO 2 for both Li-ion batteries 2,3 and supercapacitors. 4−6 However, the mechanisms for this improvement are not fully understood on the atomic-scale. In addition to its role as an electrode material, nanostructured rutile MnO 2 has been shown to give good performance as a catalyst, 7 with much recent interest in its potential role in the Li−O 2 battery. 8 Knowledge of the surface features of rutile MnO 2 at the atomic and electronic level would provide valuable information regarding its energy storage and catalytic mechanisms. With the clear impetus to better understand the performance of rutile MnO 2 in three technologies, Li-ion batteries, supercapacitors and catalysis, in this work we apply first-principles simulations to understand surface phenomena in this promising material.Rutile MnO 2 is the subject of extensive research for its applications in Li-ion batteries, but early work indicated that bulk samples did not allow significant Li-ion intercalation. 2,9,10

Epitaxial single-crystal thin films of Mn Ti1−O2− grown on (rutile)TiO2 substrates with pulsed laser deposition: Experiment and theory

“…4 and 7), it is likely that any oxygen loss required for charge balance is associated with Mn octahedra. This too is consistent with theory that predicts larger oxygen-vacancy formation energies for TiO 2 (rutile) surfaces compared to β-MnO 2 [41][42][43]. Therefore, it is plausible that the surface phase and inclusions, discussed above (Fig.…”

“…The average oxidation state of Mn decreases with increasing Mn concentration and angle resolved XPS also indicates that Mn is more reduced nearer the surface; suggesting that Mn might be more oxidized in the bulk of the film than XPS indicates. If so, this is consistent with ab initio calculations of β-MnO 2 indicating that surface Mn is more easily reduced than bulk Mn [41]. Because Mn(III) has a larger ionic radius than Mn(IV), reduction of Mn(IV) in the film might be a mechanism for relieving accumulated coherent strain to the larger TiO 2 (rutile) lattice; however, it is not clear what role oxygen vacancies play.…”

A facile synthesis of reduced Co3O4 nanoparticles with enhanced Electrocatalytic activity for oxygen evolution

“…More importantly, tuning nanostructures or doping of Co 3 O 4 may provide more surface oxygen vacancies, which has great effective on improving the activity for OER [18,25,26]. Recently, two kinds of method for improving oxygen vacancies of the surface oxides have been investigated including the annealing in H 2 atmosphere [27] and direct reduction treatment by NaBH 4 [19,28]. Owing to the weak CoeO bond and low hopping barriers [29,30], Co 3 O 4 has been regarded as an ideal catalyst by NaBH 4 reduction to realize the increasing of oxygen vacancies.…”

Surfaces of Rutile MnO2 Are Electronically Conducting, Whereas the Bulk Material Is Insulating

“…In recent work the present authors and others have considered the surface energies up to high Miller index and predicted an elongated morphology that is consistent with the large number of nanostructures observed experimentally. 37 That work also demonstrated low formation energies for oxygen vacancies at rutile MnO 2 surfaces, which are likely to be important to its good catalytic performance. This study extends the analysis of important surfaces to consider the electronic conductivity.…”