Mn2O3 Hollow Nanotube Arrays on Ni Foam as Efficient

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Mn2O3 Hollow Nanotube Arrays on Ni Foam as Efficient Supercapacitors and Electrocatalysts for Oxygen Evolution Reaction Ping-Ping Liu, Yue-Qing Zheng, Hong-Lin Zhu, and Ting-Ting Li ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01918 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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ACS Applied Nano Materials

Mn2O3 Hollow Nanotube Arrays on Ni Foam as Efficient Supercapacitors and Electrocatalysts for Oxygen Evolution Reaction

Ping-Ping Liu, Yue-Qing Zheng *, Hong-Lin Zhu, Ting-Ting Li

Research Center of Applied Solid State Chemistry, Chemistry Institute for Synthesis and Green Application, Ningbo University, 818 Fenghua Road, Ningbo, Zhejiang, 315211, P. R. China.

Corresponding Author: Fax: (+86) 574-87600792; Tel: (+86) 574-87600792. *Email address: [email protected] (Y.-Q. Zheng).

ORCID: Ping-Ping Liu: Yue-Qing Zheng: 0000-0002-8216-1072 Hong-Lin Zhu: 0000-0002-0989-7016 Ting-Ting Li:

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ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract It is imperative to study the earth abundant non-precious metal oxides with high electrocatalytic activity and excellent energy storage performance. Herein, Mn2O3 nanotube arrays (Mn2O3 NAs) have been synthesized by controllably annealing the MnMOFs precursors. The hollow Mn2O3 NAs exhibit high electrocatalytic activity for oxygen evolution reaction (OER), accompanied with a low overpotential of 270 mV to achieve 10 mA cm−2 in 1.0 M KOH solution. The corresponding Tafel slope is only 85 mV dec−1 and the Faradaic efficiency is nearly 100 %. Besides the excellent OER performance, Mn2O3 NAs also exhibit enhanced specific capacitance of 677 Fg−1 at 1 mA cm–2, which is preferable to most Mn2O3 materials. The electrocatalytic results strongly demonstrate that the three-dimensional skeleton structure and micro-structure of hollow nanotube arrays in Mn2O3 NAs are the intrinsical factors for the enhanced OER performance and high specific capacitance. Key words: Metal-Organic Frameworks; Mn2O3; Nanotube Arrays; Oxygen Evolution Reaction; Supercapacitors.


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Introduction Influenced by the excessive use of traditional fossil fuels and the caused growing environmental pollution, it is a very urgent task to develop the renewable energy system. As an efficient energy conversion device, the electrochemical device could effectively realize the conversion of clean energy with high efficiency in lithium ion batteries, supercapacitors, water splitting system and so on. Therefore, it is very important to prepare the highly efficient electrochemical devices. As an important component in the electrochemical device, the electrode can greatly determine the performance of the materials. However, the catalytic activity and stability of the electrode are subject to catalyst itself and the preparation method of the electrode. Compared to precious metal oxides, transition metal oxides containing Mn, Fe, Co or Ni element not only have abundant reserves and low prices, but also show a huge potential in the field of energy storage and energy conversion because of its reversible redox reaction on catalyst surface,1,2 which have been widely utilized as electrocatalysts in the field of water splitting and supercapacitors.3-7 Although transition metal oxides present many merits, there also have some defects, such as poorly conductivity and low durability. In fact, all of these shortcomings can be solved during the preparation of the electrodes. Firstly, the conventional electrode preparation method of drop-casting cannot ensure the integrity of the catalyst morphology, which would affect the activity and stability of the electrodes. Secondly, the additional polymer binder (Nafion film) would greatly increase the interfacial resistance and hinder the electron transfer rate between electrolyte and electrode surface. Finally, the active materials on electrode are easily peeled off during the catalytic process, which also affects the durability. Thus, it is very important to prepare highly efficient electrode. Recently, MOFs, as the promising candidates, have been received growing interest in various applications, especially in the field of gas adsorption,8,9 sensor technology,1012

energy storage13-17 and catalysis18. In addition, due to their periodic network

structures, MOF nano-arrays could be prepared by flexible choosing metal salts and organic ligands, which can be treated as the templates to synthesize transition metal



based derivates with same morphology via heat treatment under different atmosphere. It is recognized that there are many advantages for transition metal oxide nano-arrays with in-situ grown on the substrate. 1) Transition metal oxide nano-arrays in-situ growth on the current collector generally have relative low interfacial resistance, and the integrated electrode exhibits a high specific surface area and an ordered electron transfer channel. 2) The resulting three dimensional (3D) electrode skeleton is beneficial to expose more catalytically active sites, which would enable a larger area of interactions between the catalyst and the electrolyte. 3) The nano-arrays in-situ grown on the substrate lead to a better durability. 4) A variety of MOFs precursors recommend a greater diversification of candidates, which can be used as the templates to obtain efficient electrocatalysts with ideal morphology. To date, the manganese-based MOFs derivative as anode materials have been reported.13,14,19-22 Almost all of those references are concentrated on the energy storage field, but the research for OER and supercapacitors is almost in infancy. In fact, due to the effect of low-cost, manganese oxide catalyst is of great candidates for high-efficient OER electrocatalysis. S. Fiechter and coworkers reported that Mn2O3 in electrodeposited on conductive glass worked as the electrocatalyst toward OER, which only requires an overpotential of 360 mV corresponding to 10 mA cm−2.23 It is obviously that the overpotential for Mn2O3 is far higher than other transition metal oxides values, and the reason is the preparation methods. Due to the special d electronic structure, it is very difficult to synthesize Mn2O3 with pure phase. Thus, it is imperative to synthesize Mn2O3 pure materials with efficient electrocatalytic properties for OER. In this work, we have successful synthesized manganese-based MOFs with growing on Ni foam, and which was via one-step heat treatment method to form the pure Mn2O3 NAs. Because of their three-dimensional skeleton and hollow structure, Mn2O3 NAs demonstrate a high electrocatalytic activity for OER, accompanied with a low overpotential of 270 mV to achieve 10 mA cm−2 in 1.0 M KOH solution. In addition, the specific capacitance of Mn2O3 NAs was determined to be 677 F g−1 at a current


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density of 1 mA cm−2 when used as the supercapacitors, which is obviously larger than other Mn2O3 materials in references. Results and discussion Scheme 1 displays the sythesis route of hollow Mn2O3 NAs derived from MOFs insitu grown on Ni foam. First of all, Mn-MOF-74 were successfully in-situ grown on Ni foam substrate via hydrothermal method, and the XRD patterns and morphology of MnMOF-74 were shown in Figure S1 and Figure S2. The SEM image indicates that the morphology of Mn-MOF-74 is nanorod arrays with the average diameter of 200 nm. Thermogravimetric analysis (TGA) curve (Figure S3) of Mn-MOF-74 indicates its decomposition in air atmosphere starts from 270 ˚C after the volatilization of residual solvents in the MOF channels and reaches a stable state after 330 ˚C. In order to prepare manganese oxides, Mn-MOF-74 precursors were calcined at 350 ˚C in air atmosphere. Three hours later, the obtained sample was characterized by XRD, and the XRD plot in Figure 1a exhibits that the PXRD diffraction peaks can be attributed to Mn2O3 with cubic phase (JCPDS Card No. 41-1442),24 and there is no signs for other impurity peak. SEM images of Mn2O3 materials with high-resolution magnifications are showed in Figure 1b, what we can see is that the self-supported Mn-MOF-74 nanorod arrays were converted to Mn2O3 nanotube arrays (Mn2O3 NAs). It is notorious that Mn-MOF nanorod consists of organic ligands and metal ions. After calcination, the ligands are removed and Mn2O3 nanoparticles are formed, which may lead to the collapse of internal organic skeleton and aggregation of Mn2O3 nanoparticles to form hollow nanotube structure using MOF as the template. Transmission electron microscope (TEM) image (Figure 1c) further confirmed that the hollow nanotubes structure for each Mn2O3 unit are actually assembled from polyhedrons. In fact, the coordinated assembly of metal centers and ligands within metal-organic frameworks is the intrinsic cause to this phenomenon.20 From the high-resolution TEM (HR-TEM) image in Figure 1d, it can be seen that the lattice diffraction fringe spacings of 0.39 nm and 0.33 nm are assigned well to the plane of (211) and (220) for cubic



Mn2O3, respectively. In the selected-area electron diffraction (SAED) pattern (Figure 1d), the observed definite spots confirm the formation of Mn2O3 polycrystals with welldefined feature planes. The X-ray photoelectron spectroscopy (XPS) method was used to determine the chemical bonding states and contents of Mn2O3 NAs. Combined with Figure 2 and Figure S5, the signals of Mn 2p, Mn 3s, Mn 2s and O1s were detected, indicating the presence of Mn and O elements in this sample. The extra C peak is attributed to the conductive paste used in the XPS technology. As shown in Figure 2a, the chemical state of manganese was defined via the positions of multiplet splitting for Mn 2p, and the peaks of Mn 2p3/2 and Mn 2p1/2 located at 641.63 eV and 653.5 eV, respectively, indicating that only MnIII ions is present in the sample.23,25,26 From the XPS spectrum of the deconvoluted O 1s peak, one peak at 529.40 eV was observed，suggesting the presence of O2− ions in Mn-O-Mn bonds.27,28 The isotherm curve for Mn2O3 NAs was shown in Figure S6, which presents type IV characteristic curves, indicating the presence of mesoporous structure.29 The BET specific surface area of Mn2O3 NAs is 14.1 m2 g−1. Besides, the pore size is range from 22 to 140 nm, and the average pore size is about 50 nm. The higher specific surface area, smaller pore diameter and hollow structure of Mn2O3 NAs would be useful in the enhanced electrochemical catalytic activity and energy storage performance. Electrochemical catalytic activity of the Mn2O3 NAs In recent years, Mn2O3 materials have been studied as efficient candidates for OER, but there is no report regarding to Mn2O3 NAs templated by manganese based MOFs. To evaluate the OER performance of Mn2O3 NAs, a three-electrode system was used and 1 M KOH solution (pH = 13.7) was used as the electrolyte. Mn2O3 NAs grown on Ni foam act as the working electrode, and the surface loading is about 1.05 mg cm−2. The electrochemical OER performance of blank Ni foam, commercial IrO2 and Mn2O3 nanoparticles were also measured for comparison. The linear sweep voltammetry (LSV) curves of all electrocatalysts toward OER are shown in Figure 3a. All the


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electrocatalysts exhibited OER activity. Among them, Mn2O3 NAs showed lower overpotentials of 270 mV and 320 mV with 10 mA cm−2 and 20 mA cm−2, respectively. It is worth noting that the OER performance of Mn2O3 NAs is comparable to that of commercial IrO2 nanoparticles, demonstrating that hollow Mn2O3 NAs in our work is a promising noble metal oxide electrocatalyst alternative. As comparison, the electrode prepared by drop-casting Mn2O3 nanoparticles on Ni foam (Mn2O3/NF) showed a higher overpotential of 460 mV to deliver the current density of 10 mA cm−2, indicating that Mn2O3 NAs in-situ grown on the Ni foam could greatly improve the conductivity and expose more catalytically active sites. As we all know that that electrical conductivity and electron transfer resistance are the main factors to affect the performance of electrocatalysts toward OER. Therefore, the electrochemical impedance spectroscopy (EIS) was conducted and the measured impedance data was fitted, and the equivalent circuit consists three components: electrolyte solution resistance (Rs), charge transfer resistance (Rct), and constant phase composition (Figure 3c). Mn2O3 NAs exhibited a smaller semicircular diameter (0.87  cm−2) than that of Mn2O3/NF and IrO2/NF, which means the favorable OER kinetics at the electrode/electrolyte interface of Mn2O3 NAs. Undoubtedly, the electrocatalyst in-situ grown on conductive substrate may be a promising way to decreased charge transfer resistance and facilitate both charge transportation and transfer. Figure 3b shows that the Tafel slope of Mn2O3 NAs is 85 mV dec−1, which is smaller than that of Mn2O3/NF (159 mV dec−1) and IrO2/NF (139 mV dec−1), indicating the enhanced catalytic reaction kinetics of Mn2O3 NAs. Moreover, the electrochemical active surface area (ECSA) was also evaluated as a criterion of high catalytic activity. In Figure S7, Cdl of Mn2O3 NAs is 12 times greater than that of Mn2O3/NF. Besides, the open spaces between the nanotubes and the hollow tubes accelerate the diffusion of electrolytes and effusion of oxygen gas, leading to the efficient OER performance. The

long-term

durability

during

electrolysis

was

measured

by

using

chronopotentiometric method in 1 M KOH solution. As shown in Figure 3d. the current



density remains almost constant at a constant voltage of 1.6 V vs reversible hydrogen electrode (RHE) after 24 h, implying the excellent durability of Mn2O3 NAs toward OER. The SEM images captured after bulk electrolysis show that no significant morphology change occurred for Mn2O3 NAs, and the structure of nanotubes are well reserved. The Faradaic efficiency of Mn2O3 NAs was tested by using a fluorescencebased O2 sensor with the same applied potential during the initial one hour in N2saturated sealed electrochemical cell. The Faradaic efficiency of Mn2O3 NAs was estimated to be nearly 100 % (Figure S8), which further demonstrates that no other side reactions occurred during electrocatalytic OER. Table S1 summarizes all of reported literatures of Mn2O3 electrocatalysts toward electrocatalytic OER. Compared to Mn2O3 materials with other morphologies, Mn2O3 NAs exhibit the lowest catalytic onset potential of 1.50 vs RHE and achieve the current density of 10 mA/cm2 with overpotential of 270 mV, indicating that Mn2O3 NAs possess a larger catalytically active surface area, and the value of electrochemical active surface area also clearly confirms the conclusion. As for the preparation methods of the electrode, the polymer binder-assisted drop-casting technique generally hinders the electrode/electrolyte interfacial contacting. In fact, the electrochemical activity is also largely determined by the substrate of electrode, which could facilitate the penetration of electrolyte with a smaller electrical resistance during the catalytic process. Undoubtedly, the Ni foam substrate with 3D skeleton could dramatically improve the electrocatalysis performance in contrast with some other substrates. Thus, Mn2O3 NAs are found to have an excellent electrocatalytic performance, and the reasons could be assigned to the following two factors: (1) Mn2O3 NAs with one dimensional nanotube arrays could greatly accelerate the rates of ions and electrons diffusion. (2) Mn2O3 NAs directly grown on Ni foam could significantly improve the electron transport efficiency between nanotube arrays and substrates. In total, the 3D electrode with Mn2O3 NAs with three dimensional configuration and in-situ grown on Ni foam are beneficial to enhance the mass/charge transport rates. In order to check if there is any change of composition and morphology of Mn2O3 NAs after bulk electrolysis toward OER, XRD and SEM characterizations were carried out. XRD analysis (Figure S9) reveals Mn2O3


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is still the major form on the surface of Ni foam. SEM images with different magnification in Figure S10 indicate that the morphology of Mn2O3 NAs is well maintained after extensive electrocatalysis. Supercapacitors performance of Mn2O3 NAs The cyclic voltammograms (CVs) of Mn2O3 NAs electrode was collected in 1.0 M Na2SO4 solution with different scan rates of 1 mV s−1, 2 mV s−1, 5 mV s−1, 10 mV s−1 and 20 mV s−1, respectively. From the shape of the CV as shown in Figure 4a, it is obviously that the curves with rectangular shapes remain unchanged, suggesting the excellent reversibility between electrochemical charging and discharging. More importantly, as illustrated in Figure 4b, with the increasement in current densities from 1 mA cm−2 to 8 mA cm−2, the curves of charge-discharge manifest a good symmetry, suggesting a significantly capacitance property. The specific capacitance is calculated by equation 1. C = It/ΔVs

equation 1

Where I is the current intensity, t represents the discharge time, ΔV is the range of charge-discharge potential and s refers to the area of the electrode. The highest specific capacitance is up to 677 mF cm−2 (equal to 677 F g−1) with current density of 1 mA cm−2. With the current densities increase to 2 mA cm−2, 4 mA cm−2, 6 mA cm−2 and 8 mA cm−2, and the responding specific capacitances were calculated to be 478, 259, 174 and 120 mF cm−2, respectively (Figure 4c). The main reason is that the electrolytic ions could not reach the active sites of surface areas at higher current densities. As the important criterions for the supercapacitor, the long-term cycle stability is one absolutely necessary measurement. As shown in Figure 4d, the cycling stability of Mn2O3 NAs was measured by using the galvanostatic charge-discharge at current densities of 2 mA cm−2 for 2000 repeated charging-discharging cycles. After 2000 cycles, the specific capacitance of Mn2O3 NAs shows excellent cycling stability and retains about 90 % of its initial capacitance.



Compared to the reported Mn2O3 materials for supercapacitors in Table S2, the capacitance of 677 F g−1 for Mn2O3 NAs is larger than that of majority, highlights the excellent capability and good cycling stability. The one dimensional hollow nanotube arrays can provide more active site, in favor of the penetration of electrolyte to improve the pseudocapacitive reactions. In addition, Mn2O3 NAs directly grown on nickel foam substrate could greatly strengthen the conductivity between electrocatalysts and current collector. Thus, this method of in situ formed self-supported electrode derived from MOF precursors is feasible and greatly improves the electrochemical performance of materials. Conclusion In summary, Mn2O3 NAs derived from manganese MOFs were prepared by a simple route and it acts as the efficient bifunctional electrocatalysts for both OER and supercapacitor. The performance of Mn2O3 NAs exhibits excellent OER properties compared with other reported Mn2O3 electrodes, and the specific capacitance is also better than most of the reported Mn2O3 materials, which are ascribed to the one dimensional nanotube arrays structure and self-supported configuration on nickel foam. We believe that this work not only provides a new method for preparation of selfsupported 3D electrode derived from MOFs, but also greatly broaden the applications of Mn2O3 electrodes, such as in electrochemical energy storage, water-splitting devices, Li-ions batteries and other renewable energy systems. ASSOCIATED CONTENT Supporting Information Supporting Information contains XRD patterns, SEM images, TGA analysis, additional XPS spectra, N2 adsorption-desorption isotherm and CV curves. Acknowledgements This work was financially supported by Public Projects of Zhejiang Province (Grant No. 2017C33008), K. C. Wong Magna Fund in Ningbo University and K. C. Wong Education Foundation, Hong Kong.


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Scheme 1. The sythesis route of Mn2O3 nanotube arrays in-situ grown on Ni foam.


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Figure 1. (a) XRD patterns, (b) SEM, (c) TEM and (d) HR-TEM images of Mn2O3 nanotube arrays, (inset) the corresponding SAED image.



Figure 2. XPS spectra of the Mn2O3 nanotube arrays for (a) Mn 2p region and (b) O 1s region.

Figure 3. (a) LSV curves of Mn2O3 NAs (red line), commercial IrO2 nanoparticles (black line), Mn2O3/NF (blue line) and blank Ni foam (green line) without iR compensation, the scan rate is 5 mV s−1. (b) The Tafel plots of corresponding materials. (c) EIS spectra of Mn2O3 NAs (red line), commercial IrO2 nanoparticles (black line), Mn2O3/NF (blue line) recorded at 1.7 V vs RHE with the frequency range from 0.1 Hz to 100 kHz. (d) Chronopotentionmetric curves of Mn2O3 NAs in 1 M KOH solution at a fixed potential of 1.6 V vs RHE.


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Figure 4. (a) Cyclic voltammograms curves of Mn2O3 NAs under various scan rates in 1 M Na2SO4 solution. (b) Galvanostatic charge-discharge curves for the Mn2O3 NAs under different current densities. (c) Specific capacitance as a function of the current densities of the Mn2O3 NAs. (d) Cycling performance of the Mn2O3 NAs during 1000 scan cycles at a current density of 2 mA cm−2.



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Scheme 1. The sythesis route of Mn2O3 nanotube arrays in-situ grown on Ni foam. 192x134mm (150 x 150 DPI)



Figure 1. (a) XRD patterns, (b) SEM, (c) TEM and (d) HR-TEM images of Mn2O3 nanotube arrays, (inset) the corresponding SAED image. 460x318mm (96 x 96 DPI)


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Figure 2. XPS spectra of the Mn2O3 nanotube arrays for (a) Mn 2p region and (b) O 1s region. 421x179mm (96 x 96 DPI)



Figure 3. (a) LSV curves of Mn2O3 NAs (red line), commercial IrO2 nanoparticles (black line), Mn2O3/NF (blue line) and blank Ni foam (green line) without iR compensation, the scan rate is 5 mV s−1. (b) The Tafel plots of corresponding materials. (c) EIS spectra of Mn2O3 NAs (red line), commercial IrO2 nanoparticles (black line), Mn2O3/NF (blue line) recorded at 1.7 V vs RHE with the frequency range from 0.1 Hz to 100 kHz. (d) Chronopotentionmetric curves of Mn2O3 NAs in 1 M KOH solution at a fixed potential of 1.6 V vs RHE. 170x127mm (143 x 143 DPI)


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Figure 4. (a) Cyclic voltammograms curves of Mn2O3 NAs under various scan rates in 1 M Na2SO4 solution. (b) Galvanostatic charge-discharge curves for the Mn2O3 NAs under different current densities. (c) Specific capacitance as a function of the current densities of the Mn2O3 NAs. (d) Cycling performance of the Mn2O3 NAs during 1000 scan cycles at a current density of 2 mA cm−2.



287x198mm (150 x 150 DPI)


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Mn2O3 Hollow Nanotube Arrays on Ni Foam as Efficient

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