An Effective Way to Improve Bifunctional Electrocatalyst Activity of Manganese Oxide via Control of Bond Competition
Graphical abstract
Introduction
Electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) attract intense research interest because of their critical roles in many emerging energy technologies such as fuel cells, metal−air batteries, water electrolyzers, photoelectrochemical cells, and so on [[1], [2], [3], [4]]. Since both the oxidative and reductive transformations between O2− and O2 require the simultaneous transfer of four electrons, it is not easy to explore efficient electrocatalysts for these sluggish reactions [5,6]. Even though noble metals like Pt and Ir show promising performance as electrocatalysts, their high price and limited abundance frustrate the commercialization of these materials [7]. Also, these noble metal-based electrocatalysts are active only for either OER or ORR, although the application as electrode for metal−air batteries demands bifunctional electrocatalyst active for both the reactions [8]. As an alternative electrocatalyst, transition metal oxides receive special attention because of their remarkable cost-effective advantages over noble metal [9,10]. Among diverse metal oxides, α-MnO2-structured manganese oxide boasts remarkable cost-effectiveness and environmental friendliness as well as promising bifunctional electrocatalytic activity for both OER and ORR [10]. Yet, the electrocatalytic activity of α-MnO2 is still far poorer than those of commercial Pt/C and Ir/C [11]. Both the OER and ORR processes induced by manganese oxide involve reversible cycling among Mn2+, Mn3+, and Mn4+ oxidation states as well as surface adsorption/desorption process of oxygen species [12]. Since Jahn-Teller active Mn3+ ion is the most unstable Mn oxidation state [12], it is of crucial importance to stabilize this species for improving the electrocatalyst performance of MnO2 [13,14]. There are several attempts to stabilize this unstable Mn3+ species by increasing the structural deformation of MnO2 lattice [15]. In one instance, the Cu2+ doping for α-MnO2 induces the formation of surface Mn3+ ion due to the increase of surface defect [16]. Although an enhanced distortion of local symmetry around Mn ion is effective in improving the stability of Mn3+ ion [17], the increased structural distortion would have detrimental effect on the electrical conductivity of manganese oxide and thus its electrocatalyst activity. Alternatively, the tuning of bond competition between neighboring (metal−O) bonds can provide another way of stabilizing a specific oxidation state of metal ion; the highly covalent (metal−O) bond leads to the weakening of adjacent (metal’−O) bond and the stabilization of low-valent metal’ ion due to the decrease of the bond covalency [18]. Thus, the partial substitution of electronegative metal ion into the α-MnO2 lattice can stabilize unstable Jahn-Teller active Mn3+ ion via the creation of the tetragonal distortion of MnO6 octahedra with elongated (Mn−O) bond distance. Simultaneously, the elongation of (Mn−O) bond distance can facilitate the surface adsorption of oxygen species on manganese oxide. Judging from the high electronegativity of Ru4+ ion and the excellent electrical conductivity and high OER electrocatalytic activity of RuO2 material [19], Ru4+ ion can be one of the most promising substituents for stabilizing unstable Mn3+ state in α-MnO2 lattice and thus enhancing the electrocatalytic activity of α-MnO2 material. Yet, at the time of this submission, we are unaware of any other report about the optimization of Mn valency and electrocatalyst performance of manganese oxides in terms of the tuning of bond competition.
In the present study, the (Mn−O) bond covalency, Mn valence state, and electrocatalyst activity of α-MnO2 nanowire can be tailored by the partial substitution of Ru4+ ion through the bond competition. The effects of Ru substitution on the crystal structures, morphologies, and chemical bonding nature of α-Mn1−xRuxO2 1D nanowires are systematically investigated. The obtained α-Mn1−xRuxO2 nanowires are applied as bifunctional electrocatalysts for both OER and ORR to probe the influence of Ru substituent on the electrocatalytic activity of manganese oxide.
Section snippets
Sample preparation
The Ru-substituted α-Mn1−xRuxO2 nanowires were synthesized by the one-pot hydrothermal reaction for the ion-adduct of MnO4−−Ru3+ as follows [20]; 0.5 g of KMnO4, 0.2 g of MnSO4∙H2O, and 0.048−0.23 g of RuCl3∙H2O were dissolved in distilled water and then the obtained solution was transferred to hydrothermal vessel for the reaction at 140 °C for 12 h. After the reaction, the obtained powders were separated by centrifugation, washed thoroughly with distilled water, and dried overnight at 50 °C in
Powder XRD Analysis
Fig. 1a presents the powder XRD patterns of Ru-substituted α-Mn1−xRuxO2 materials. Regardless of Ru substitution rate, all the present materials display typical Bragg reflections of α-MnO2 phase without any impurity peaks, indicating the formation of single-phase Ru-substituted α-Mn1−xRuxO2 materials. The complete Ru substitution for α-MnO2 material is further confirmed by Rietveld refinement, see Fig. 1b and Table S1 of Supporting Information. All the XRD peaks of the most heavily
Conclusions
In this work, highly efficient bifunctional MnO2-based electrocatalysts for both OER and ORR can be synthesized by controlling the bond covalency of (Mn−O) and Mn valence state via Ru substitution in terms of bond competition with (Ru−O). The replacement of Mn ion with more electronegative Ru ion weakens adjacent (Mn−O) bonds due to bond competition, enhancing the stability of Jahn-Teller active Mn3+ species, a crucial component for the electrocatalyst activity of manganese oxide. Although the
Acknowledgments
1 These authors contributed equally to this work. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2017R1A2A1A17069463) and by the Korea government (MSIT) (No. NRF-2017R1A5A1015365). The experiments at PAL were supported in part by MOST and POSTECH.
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These authors contributed equally to this work.