Abstract
HighlightsMnO rich in oxygen vacancies has been synthesized.The synthesized MnO demonstrates excellent oxygen reduction reaction performance and high output power in Zn–air battery.The high catalytic activity is attributed to the synergetic catalytic effect between oxygen vacancies and in situ generated Mn3+/Mn4+.Among various earth-abundant and noble metal-free catalysts for oxygen reduction reaction (ORR), manganese-based oxides are promising candidates owing to the rich variety of manganese valence. Herein, an extremely facile method for the synthesis of cubic and orthorhombic phase coexisting Mn(II)O electrocatalyst as an efficient ORR catalyst was explored. The obtained MnO electrocatalyst with oxygen vacancies shows a significantly elevated ORR catalytic activity with a half-wave potential (E1/2) of as high as 0.895 V, in comparison with that of commercial Pt/C (E1/2 = 0.877 V). More impressively, the MnO electrocatalyst exhibits a marked activity enhancement after test under a constant applied potential for 1000 s thanks to the in situ generation and stable presence of high-valence manganese species (Mn3+ and Mn4+) during the electrochemical process, initiating a synergetic catalytic effect with oxygen vacancies, which is proved to largely accelerate the adsorption and reduction of O2 molecules favoring the ORR activity elevation. Such an excellent ORR catalytic performance of this MnO electrocatalyst is applied in Zn–air battery, which shows an extra-high peak power density of 63.2 mW cm−2 in comparison with that (47.4 mW cm−2) of commercial Pt/C under identical test conditions.
Highlights
Advanced energy storage and conversion systems, such as fuel cells and metal–air batteries, are attracting more and more attentions worldwide due to the ever-increasing fossil energy consumption and accompanying severe environmental problems [1, 2]
Suib and coworkers synthesized a series of manganese oxides including α, β, δ-MnO2 and amorphous MnO2 (AMO) via facile methods and demonstrated that the electrocatalytic activities follow the order of α-MnO2 > AMO > β-MnO2 > δ-MnO2 [20]
The electrocatalytic performances of the synthesized catalysts for oxygen reduction reaction (ORR) were evaluated in N2 and O2-saturated 1 M KOH solutions using a rotating disk electrode (RDE) and a rotating ring-disk electrode (RRDE) system
Summary
Advanced energy storage and conversion systems, such as fuel cells and metal–air batteries, are attracting more and more attentions worldwide due to the ever-increasing fossil energy consumption and accompanying severe environmental problems [1, 2]. Various oxidation states of transition metals enable the electrons transfer among the metal ions, contributing to their excellent ORR performances [13]. Among these transition metal oxides, manganese oxides have induced huge interests owing to its abundance, low cost, non-toxic and its various valence states (I–VI) [14, 15]. The electrocatalytic activities of manganese oxides are still not satisfactory because of the poor intrinsic activity and conductivity impeding the electron transfer during the ORR process, which could be enhanced by the introduction of oxygen vacancies and carbon matrix [21]. The production of Mn3+ and Mn4+ is responsible for the obviously enhanced O2 transformation ability and peroxide decomposition, respectively, further confirming that the presence of M n3+/Mn4+ in a ratio of 2:1 in the present case is vital for the high ORR activity of MnO
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