Applied Materials Today
Transition metal doped MnO2 nanosheets grown on internal surface of macroporous carbon for supercapacitors and oxygen reduction reaction electrocatalysts
Graphical abstract
Transition metal ions-doped MnO2 nanosheets are grown on internal surface of macroporous carbon, which show excellent electrochemical performance for supercapacitors and electrocatalysts for oxygen reduction reaction due to appropriate doping and full exposure towards the electrolytes.
Introduction
Manganese oxide (MnO2) has attracted wide interest for many years due to its low cost, environmental friendliness, and excellent electrochemical performance for applications of supercapacitors [1], [2], [3] and electrocatalysts for oxygen reduction reaction (ORR) [4]. Many efforts have been devoted to improving the electrochemical property of MnO2 by taking appropriate measures. The electrochemical property of material is first determined by its electronic structure. Doping can generate new energy level and adjust binding energy of electrons, providing an effective means to tune the electronic structure of metal oxides [5]. It is expected that the pseudocapacitive and electrocatalytic properties of MnO2 can be improved by doping with heteroatoms, which have been demonstrated by previous reports. For instance, for the MnOx films deposited by pulsed laser deposition the specific capacitance increases from 47 F g−1 to 99 F g−1 by doping with Co ions [6]. Liu et al. indicated that doping MnO2 nanocrystals with In3+ changes the average oxidation state of Mn, resulting in considerable improvement of the electrocatalytic performance for ORR [7]. Davis et al. reported that Cu-doping of MnO2 nanowires is able to promote stabilization of O2 adsorption on the nanowire surface, leading to great increase of the current density [8]. Wang et al. [9] prepared Cr-doped Mn2O3 and obtained a specific capacitance of 272 F g−1 at 0.5 A g−1, which is about 35% higher than the undoped one. It is noted that the doped MnO2 in previous reports was mainly prepared by hydrothermal and high temperature reaction processes [6], [7], [8], [9], where the doping reaction is driven by thermal energy provided by external heating. The other important and simple method to prepare MnO2 is to reduce KMnO4 in solution at room temperature with reducing agents such as carbon with the equation 3C + 4KMnO4 + H2O = 4MnO2 + K2CO3 + 2KHCO3. However, the doping experiments for this reaction to prepare MnO2 have not been reported yet. As the reducibility of carbon is generally higher than the transition metal ions the possibility of preparing transition metal doped MnO2 following this reaction is not clear. Due to the simplicity and importance of this reaction it is necessary to explore the doping of MnO2 in this reaction.
The charge storage mechanism of MnO2 is based on rapid and reversible surface reactions related to oxidation/reduction of Mn ions while the electrocatalytic ORR occurs on the surface of MnO2 catalysts also. Therefore, increasing the exposing area and thus the utilization efficiency of MnO2 is of great importance for achieving excellent electrochemical performance. A variety of strategies have been used to increase the utilization efficiency of MnO2. The indispensable step is to make MnO2 into nanostructures such as nanorods [10], nanowires [11], and nanosheets [12]. In order to avoid the problem of agglomeration the nanoscale MnO2 materials are generally dispersed and supported on various supports such as carbon nanotubes [13] and mesoporous carbon [14]. However, in most reports the MnO2 nanomaterials are supported on the out surface of the supports. In this case, the MnO2 nanomaterials suffer from the loss of surface area due to the covering by adjacent particles of active materials, binders, and conductive additives. Some efficient measures such as using planar configuration [15] and conductive wrapping [16] have been reported to increase the utilization efficiency and thus the performance of supercapacitors. Recently, we prepared macroporous carbon (MC) from luffa sponge fibers [17], [18]. The pores in the MC are densely packed, straight, parallel, and completely through with the diameters at micrometer scale. The large pore size allows easy access of the electrolyte and the active sites on the internal pore walls can be fully exposed towards the electrolyte without any surface covering, resulting in excellent electrochemical performance for supercapacitors and ORR electrocatalysis [17], [18]. We expect that the MC may be a good substrate for the internal surface growth of MnO2 and the utilization efficiency of MnO2 can be greatly increased by growing on the internal surface of the MC.
In this work, we grew transition metal ions-doped MnO2 nanosheets (M-MONSs) on internal wall surface of the MC for supercapacitor and ORR electrocatalysis application. The MnO2 nanosheets (MONSs) were prepared by redox reaction between the MC and KMnO4 in solution at room temperature. By introducing the corresponding salts into the reaction solutions four representative transition metal (Fe, Co, Ni, and V) ions were in situ doped into the MnO2 nanosheets during the growth process. The electrochemical performance was improved by doping with appropriate elements. The Ni-doped MONSs exhibits the best performance for supercapacitors with a high specific capacitance of 445 F g−1 at 1 A g−1 (based on the total mass of the MONSs and the MC substrates) and the Co-doped MONSs shows the best ORR electrocatalytic perfromance with an onset potential of 0.01 V and near four electron transfer per oxygen molecules. It has been demonstrated that appropriate doping plus growth on internal surface is a good strategy to prepare high performance active materials for electrochemical applications.
Section snippets
Materials synthesis
The preparation procedures of the MC have been described in our previous reports [17], [18]. In brief, the luffa sponge fibers were first carbonized in NH3 atmosphere at 800 °C for 2 h. Subsequently, the carbonized products were activated for 1.5 h with KOH in N2 at 750 °C, obtaining the so-called MC. In the activation process, the mass ratio of carbon to KOH is 1:2. vanadium (V), iron (Fe), cobalt (Co), and nickel (Ni) doped MONSs were prepared by using the corresponding salts VOSO4, Fe(NO3)3,
Results and discussion
Fig. 1 shows the SEM images of the pristine MC, MONSs/MC, Co-MONSs/MC, and Ni-MONSs/MC. It is observed that the MC possesses fibrous morphology and comprises many pores, which is inherited from the natural structure of the luffa sponge fibers (Fig. 1a). These pores are densely packed, straight, parallel, and completely through with the diameters ranging from several to over ten micrometers. On the cross-section images newly grown coatings with porous structure can be observed on the internal
Conclusions
In conclusion, MnO2 nanosheets doped with transition metal ions were grown on the internal surface of MC by a simple solution reaction at room temperature. The electrochemical performance of MnO2 for supercapacitors and electrocatalysts for ORR was considerably enhanced by doping with Co and Ni ions but degraded when doping with V and Fe ions. The Ni-MONSs/MC possesses excellent electrochemical performance for supercapacitors such as high specific capacitance, rate capability, and operation
Acknowledgments
This work is supported by the National Basic Research Program of China (2012CB933003), National Natural Science Foundation of China (No. 51272057), and Shenzhen Basic Research Program (JCYJ20130329150737027).
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