MOF-derived manganese oxide/carbon nanocomposites with raised capacitance for stable asymmetric supercapacitor

Abstract

Owing to immense application potentials in electrochemical energy storages, metal organic framework (MOF)-derived metal oxide/carbon nanocomposites have attracted extensive interest of research. Although thermolysis has been widely employed to convert MOFs into various active materials, a large set of in situ changes in chemical composition, phase(s) and morphology requires delicate control over heating parameters. Through an innovative two-stage process, Mn-MIL-100 is first transformed into MnO@C by annealing at 700 °C under N₂ flow, which is then transformed into Mn₃O₄@C at 200 °C in air, while retaining a high surface area. The appropriate retention of carbon content for Mn₃O₄@C can also be easily obtained with the control of heating time. In contrast, thermolysis of MnO@C at higher temperatures gives rise to manganese oxides with negligible carbon content and a greatly reduced surface area. The optimized Mn₃O₄@C-2 h, derived from MnO@C at 200 °C for 2 hours, showed the highest capacitance, far exceeding that of MnO@C and other derivatives. When combined with graphene oxide (GO) nanosheets to form a flexible Mn₃O₄@C/rGO paper electrode, it demonstrated a capacitance of 328.4 F cm⁻³. The Mn₃O₄@C/rGO-based asymmetric supercapacitor thus assembled also shows favorable performance. The present work demonstrates the excellent controllability afforded by the innovative two-stage thermolysis in optimizing the electrochemical performance of MOF-derived active materials as electrode materials in supercapacitors.

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1. Introduction

Metal–Organic Frameworks (MOFs) are a class of crystalline materials formed through coordination bonds among metal ions and organic ligands.¹ They have gained substantial popularity in recent years as a group of novel materials, mainly due to their diverse structures, high levels of porosity and a high degree of tunability which expands their applications from gas absorption, separation, catalysis, drug delivery to energy storages.^1–5 Besides being directly utilized as functional materials, MOFs may also be precursors for the synthesis of various porous active structures, such as, nanoporous carbon, metal oxides, metal monoxides and metal oxides/monoxides/carbon nanocomposites.^6–11 Among various MOF-derivatives, the metal oxide/carbon nanocomposites are a family of potentially attractive supercapacitor electrode materials, due to the integration of carbon and active materials within a nanoporous structure.^12–14

Transition metal oxides (TMOs), such as, Co₃O₄, NiO, and MnO₂, have long been regarded as prominent pseudocapacitance materials; however, they are also dragged in this regard by low electrical conductivities, poor stability and generally a small surface area for TMOs derived from normal hydrothermal processes.^15–22 Thanks to the highly tunable nature of MOFs, the MOF-derived metal oxide/carbon nanocomposites can be made to possess a very high surface area, which would increase the available redox-active sites and overall performance.²³ Moreover, the intrinsically poor electrical conductivity of metal oxides can be mitigated by integration with carbon content at the nanoscale.²³ Therefore, MOF-derived metal oxide/carbon nanocomposites are emerging as high-performance electrode materials for supercapacitors and other energy storage applications.^24–27

Manganese oxides are an important family of TMO-based pseudocapacitance materials that exhibit several advantages, including, high theoretical capacitance, abundance, and environment friendliness. However, like other TMO materials, their performance is affected by poor electrical conductivity and low surface area, when derived from the conventional solution processing routes.^28–31 To improve the capacitance of manganese oxides, especially upon high loading mass, an apparent strategy is to develop MOF-derived manganese oxide/carbon nanocomposites, as mentioned above.²⁹ According to Das and et al., it is feasible to transform Mn-MOFs into manganese oxide/carbon nanocomposites by calcination in an inert atmosphere.³² Nevertheless, MnO_x is one of the rather complicated oxides with variable valences, phases and structures. Therefore, to derive a manganese oxide/carbon nanocomposite with optimized performance from MOFs, several factors need to be controlled, such as, the chemical composition of manganese oxide, specific surface area, amount and scale of carbon content, and their combinations, all strongly affected by thermolysis conditions.^8,33,34 While MOF-derived manganese oxide/carbon nanocomposites are traditionally produced through a facile, one-step calcination in N₂, they can be further modified by post-annealing thermolysis, adding a new dimension to the controllability.³⁵ Thus, it would be of interest to study how these modifications can further vary the MOF-derived manganese oxide/carbon nanocomposite, and the impact such variations could have on the overall electrochemical performance.

Herein, MOF-derived MnO@C was subjected to thermolysis in air at various temperatures for different periods to investigate how the chemical, phase and morphological properties of the MnO@C would change. Interestingly, proper thermolysis in air at a moderate temperature of 200 °C would transform MnO@C into Mn₃O₄@C, which would retain the high surface area and part of the carbon content from the original MnO@C. On the contrary, upon thermolysis in air at 300 and 400 °C, the MnO@C would transform into Mn₃O₄-300 and Mn₂O₃-400, respectively, with negligible carbon content and a much-reduced surface area as compared to that of the MnO@C. Electrochemical measurements revealed that the Mn₃O₄@C sample being thermolyzed for 2 h (Mn₃O₄@C-2 h) shows the highest capacitance, far exceeding that of both Mn₃O₄-300 and Mn₂O₃-400, as well as the original MnO@C, indicating the importance of controlling the thermolysis conditions. To further demonstrate the potential of the MOF-derived manganese oxide/carbon nanocomposite, the Mn₃O₄@C-2 h with optimum performance is combined with aqueous graphene oxide (GO) nanosheets to form a flexible Mn₃O₄@C paper by vacuum filtration, which is then electrochemically reduced (denoted as Mn₃O₄@C/rGO) to improve the electrical conductivity. The Mn₃O₄@C/rGO thus made exhibited volumetric capacitance as high as 328.4 F cm⁻³ at 0.5 A cm⁻³. An asymmetric aqueous supercapacitor was then fabricated with the Mn₃O₄@C/rGO as the positive and the carbon nanotube (CNT)/rGO as the negative electrode, respectively. The asymmetric supercapacitor duly demonstrates the viability of the MOF-derived manganese oxide/carbon nanocomposites obtained through the two-stage thermolysis as a class of promising supercapacitor materials. The experimental procedures in this work are briefly summarized in Scheme 1.

Scheme 1 Schematic illustration of the experimental procedure in this work.

2. Results and discussion

2.1 Materials synthesis and characterization

To synthesize a MOF-derived manganese oxide/carbon nanocomposite, it is essential to select an appropriate MOF as the precursor, since the products of MOF pyrolysis, to some extent, inherit the morphology of the parent MOF. In this work, Mn-MIL-100 MOF was selected for its large pores (25 Å and 29 Å, respectively) and a high surface area of 1540 m² g⁻¹, which is one of the best among the Mn-MOFs.³⁶ The Mn-MIL-100 (Fig. S1†) was synthesized by a modified solvothermal method (the details can be found in the Experimental section).³⁶ The Mn-MIL-100 MOF was then calcined at 700 °C for 2 hours in a tube furnace under nitrogen airflow to obtain the MnO@C.^37–39 After cooling to room temperature, the black powdered product was recovered and characterized. As shown in Fig. 1a, the powder X-ray diffraction (XRD) patterns indicate the formation of MnO nanoparticles within the product. The widened diffraction peaks also suggest a small degree of crystallinity according to the Scherrer equation.⁴⁰ From the scanning electron microscopy (SEM) results, the polyhedron shape of the parent MOF was well preserved (Fig. S2† and 1b), and the corresponding EDS mapping confirmed the presence of Mn, O and C elements (Fig. 1b). It should be noted that since there is no peak corresponding to the crystalline carbon in the XRD pattern, the product should be a nanocomposite of MnO and amorphous carbon.⁴¹ The morphology was further investigated by transmission electron microscopy (TEM), where MnO nanoparticles with an average size of around 10 nm were embedded within a carbon matrix, suggesting a MnO@C core/shell nanostructure (Fig. 1c and d), with the lattice fringes of the MnO particles being visible (Fig. 1d). The N₂ adsorption/desorption isotherm of MnO@C corresponds to a type IV isotherm with a type H4 hysteresis loop, which is often found on micro-mesoporous carbon (Fig. 1e).⁴² The DFT pore distribution of the MnO@C suggests the presence of both micro (1–2 nm) and mesopores (2–10 nm) (Fig. 1e inset), and the BET surface area for MnO@C was calculated to be 253.56 m² g⁻¹. TGA was also done to study the thermal stability and decomposition temperature of MnO@C (Fig. 1f). The sharp weight loss at around 250 °C can well be attributed to the carbon loss, from which a carbon content of more than 45 wt% was concluded. The product of TGA testing was found to be Mn₃O₄ by XRD (Fig. S3†). Therefore, a MnO@C nanoporous composite with a relatively high surface area of 253.56 m² g⁻¹ and abundant carbon content of more than 45 wt% was successfully synthesized.^9,39

Fig. 1 Comparison of the (a) EIS, and (b) CV results between Mn₃O₄@C/GO and Mn₃O₄@C/rGO. (c) Cross-section SEM of Mn₃O₄@C/rGO. (d) TEM and SAED of Mn₃O₄@C/rGO. (e) CV curves, (f) GCD curves, (g) volumetric capacitance, and (h) cyclic test results of Mn₃O₄@C/rGO.

To modify the key parameters, e.g., chemical composition, phase and morphology, further thermolysis in air was carried out on the MOF-derived MnO@C. Based on the TGA study for MnO@C, we first investigated the thermolysis products above 250 °C, where a sudden carbon loss occurred. The heating of MnO@C at 300 °C in air for 2 hours transformed the MnO phase into Mn₃O₄ (Fig. 1a). The EDS mapping of the sample (denoted as Mn₃O₄-300) showed no bright contrast for carbon, indicating negligible carbon content (Fig. S4a†). The TEM image of sample Mn₃O₄-300 further indicated the missing carbon matrix as observed in sample MnO@C (Fig. 1g). The DFT pore distribution calculated from the N₂ isotherm (Fig. S5a†) revealed that the large micropores and the small mesopores of the original MnO@C, which contributed to the bulk of the surface area, were significantly reduced and enlarged, respectively, with a much smaller surface area of 91.26 m² g⁻¹. When a higher thermal treatment temperature of 400 °C was used, Mn₂O₃ particles formed after 2 hours, with some peaks belonging to Mn₃O₄ still being visible from XRD patterns (Fig. 1a). Since most manganese oxides had been readily transformed into Mn₂O₃, indicated by the relative peak intensity, the 400 °C sample was named Mn₂O₃-400. TEM studies of sample Mn₂O₃-400 showed particles of much larger grain size than those of sample Mn₃O₄-300 (Fig. 1h). As is the case for Mn₃O₄-300, negligible carbon remained in the Mn₂O₃-400 sample (Fig. S4b†), and the BET surface area was further reduced to 62.53 m² g⁻¹ with larger but fewer pores (Fig. S5b†).

Compared to the original MnO@C, samples treated at both 300 °C and 400 °C have only negligible carbon content, and a much-decreased surface area, which would not be favorable for supercapacitor application. Therefore, we looked at more moderate heating parameters. Referring to the TGA results of the MnO@C (Fig. 1f), the sharp mass loss in association with carbon might be avoided at below 250 °C. Thus, thermolysis was carried out at 200 °C. At this temperature, the carbon loss proceeds at such a low rate that by variation of the heating period, the carbon content can be effectively controlled (Fig. 2a). Therefore, MnO@C samples were heated for 1.0 hour, 2.0 hours, and 5.0 hours, respectively. Upon heating for 1.0 hour, the MnO particles transformed into Hausmannite and became Mn₃O₄, as indicated by the XRD phase analysis shown in Fig. 2b (thus, the samples were denoted as Mn₃O₄@C-1 h, Mn₃O₄@C-2 h and Mn₃O₄@C-5 h, respectively). This transition from the rock salt MnO to spinel Mn₃O₄ Hausmannite at such a low temperature could well be related to the small size of the MnO particles.⁴³ The TEM studies (Fig. 2d–f) show that with increased heating time, the sample profile became rougher and the dark shades within the sample also became sharper. These changes suggest a slowing-down process of carbon depletion, which is also reflected in the XPS results (Fig. 2c), where the intensity of the peak corresponding to the C 1s electrons decreases with heating time. To verify how much carbon is still left, Mn₃O₄@C-5 h was subjected to another TGA study (Fig. S7†), where the characteristic slope at 200 °C suggests that around 15 wt% of carbon remained. The presence of significant carbon was further confirmed by the EDS mapping of Mn₃O₄@C-5 h (Fig. S6†), where the carbon element is visible. By analyzing N₂ isotherms (Fig. 1a, 2g–i), it was observed that the BET surface area of MnO@C increased to 336.07 m² g⁻¹ when it became Mn₃O₄@C-1 h, before finally dropping to 236.46 m² g⁻¹ after 5 hours of heating with some loss in porosity. All Mn₃O₄@C samples exhibited a BET surface area comparable to that of the original MnO@C, thanks to the moderate thermolysis temperature of 200 °C. Fig. S9† summarizes the thermolysis processes involved in this work and their respective products. It is worth noting that the MOF-derived Mn₃O₄@C exhibited a surface area comparable to those of some amorphous manganese oxides materials, which have attracted considerable attention due to the high surface area and excellent chemical stability.⁴¹

Fig. 2 (a) Heating profile (red line), and TGA analysis result (navy blue line) of the MnO@C held at 200 °C in air. (b) XRD traces of Mn₃O₄@C-1 h, Mn₃O₄@C-2 h and Mn₃O₄@C-5 h. (c) XRD results of various samples. TEM images of (d) Mn₃O₄@C-1 h, (e) Mn₃O₄@C-2 h and (f) Mn₃O₄@C-5 h. N₂ isotherms and DFT pore distributions of (g) Mn₃O₄@C-1 h, (h) Mn₃O₄@C-2 h and (i) Mn₃O₄@C-5 h.

2.2 Electrochemical behavior of MOF-derived MnO_x@C

The chemical composition, phase and morphology of the metal oxide/carbon nanocomposites have a significant impact on the electrochemical performance. To study the effects of these factors and aim for the optimum sample with the highest capacitance, MnO@C and its various derivatives were subjected to electrochemical tests in a three-electrode system with 0.5 M Na₂SO₄ aqueous solution as the electrolyte. Contrary to the alkaline electrolyte (e.g., KOH), in neutral electrolyte such as Na₂SO₄, in spite of the generally lower capacitance, Mn₃O₄ showed near-rectangular CV patterns and has demonstrated better cyclic stability, which are favorable for supercapacitor applications.^44,45 This difference can well originate from the different mechanisms for the pseudocapacitance of Mn₃O₄ when used in different electrolytes. Moreover, a neutral electrolyte is also safer and more facile to process. The capacitances of various samples are shown in Fig. 3. Surprisingly, the original MnO@C showed the worst gravimetric capacitance (only 2.1 F g⁻¹), despite a large surface area and a high carbon content. This lack of performance could be a result of the intrinsic capacitance of MnO and the excessive carbon preventing MnO from accessing the cations (Na⁺) of the electrolyte.⁴⁶ The Mn₃O₄-300 and Mn₂O₃-400 were compared as shown in Fig. S10.† Although both exhibited much higher capacitance than that of the MnO@C, the latter was even better with a capacitance of 78.9 F g⁻¹ at 0.2 A g⁻¹. Considering that negligible carbon content remained in both samples and that the Mn₂O₃-400 had the lower surface area, the results suggest the intrinsically better pseudocapacitance of Mn₂O₃, when compared to Mn₃O₄ in Na₂SO₄ aqueous electrolyte.

Fig. 3 Comparison of the gravimetric capacitances of various samples.

The Mn₃O₄@C samples thermolyzed at 200 °C were tested as shown in Fig. S11.† Though the Mn₃O₄@C-1 h demonstrated a huge improvement over the MnO@C (31.8 F g⁻¹ at 0.2 A g⁻¹), it struggled to outperform the Mn₃O₄-300 with a much lower surface area. This may again indicate the negative effect of excessive carbon content. With the extension of heating time by another hour, the Mn₃O₄@C-2 h was much improved and showed the best capacitance values (162.6 F g⁻¹ at 0.2 A g⁻¹) of all the samples. When the thermolysis at 200 °C in air was further extended to 5.0 hours, the capacitance decreased slightly to 129.1 F g⁻¹ at 0.2 A g⁻¹.

The various Mn₃O₄@C nanocomposites demonstrated near-rectangular CV patterns (Fig. S11†) in accordance with the previously reported Mn₃O₄ pseudocapacitive materials in neutral electrolyte. The contribution of the double-layer capacitance relative to the overall capacitance of the best performing Mn₃O₄@C-2 h was estimated by Trassatti's method to be in the region of ∼10% (Fig. S12†); the rest were diffusion-controlled. Although the mechanism for the pseudocapacitance of spinel Mn₃O₄ in a neutral electrolyte (e.g., KCl, Na₂SO₄) has yet to be fully established, previous studies have suggested the reversible Mn(II)/Mn(III) redox reaction and intercalation/deintercalation of cation ions, which can be described as follows:^47,48

MnO_x(OM)_y + zM⁺ + ze⁻ = MnO_x−z(OM)_y+z

where M⁺ represents the cations.⁴⁹

Therefore, the improved capacitance of Mn₃O₄@C-2 h can be attributed to the better exposure of Mn₃O₄ to the electrolyte, whose pseudocapacitance makes up the majority of the capacitive performance. The slight decrease in the capacitance of Mn₃O₄@C-5 h was attributed to a lower surface area and degraded electrical conductivity, due to further changes in the carbon content. Table 1 lists the chemical state, phases, BET surface area and carbon content of various samples along with their respective capacitances at 0.2 A g⁻¹.

Table 1 A summary of various samples studied in this work

Sample	Chemical composition	BET surface area (m^2 g^−1)	Est. carbon (wt%)	Capacitance (F g^−1)
MnO@C	MnO	253.56	45	2.1
Mn3O4-300	Mn3O4	91.26	Negligible	35.6
Mn2O3-400	Mn2O3/Mn3O4	62.53	Negligible	78.9
Mn3O4@C-1 h	Mn3O4	336.07	35	31.8
Mn3O4@C-2 h	Mn3O4	275.23	25	162.6
Mn3O4@C-5 h	Mn3O4	236.46	15	129.1

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Based on the above discussions, the importance of appropriate carbon content and a high surface area is evident from the better performance of Mn₃O₄@C-2 h; it not only outperforms the Mn₃O₄-300, but it is also significantly better than Mn₂O₃-400. However, excessive carbon content is not desirable since it may hinder the interactions between manganese oxide and the electrolyte, as indicated by the lower capacitance of Mn₃O₄@C-1 h as compared to that of Mn₃O₄@C-2 h and the extremely low capacitance of MnO@C. It is worth noting that such a delicate control over the specific surface area and the carbon content is enabled by the two-stage thermolysis of Mn-MOF.

2.3 Electrochemical performance of Mn₃O₄@C/rGO

For a powdered material, e.g., the Mn₃O₄@C in this work, it must be combined with a substrate playing the roles of both structural support and current collector. While the Ni foam-based electrode is sufficient for comparison purposes, it is far from ideal as even the best-performing Mn₃O₄@C-2 h electrode has a relatively low capacitance value, likely due to the severe agglomeration of Mn₃O₄@C particles. To solve this problem, we opted for a two-step strategy: firstly, combining the Mn₃O₄ powder with graphene oxide (GO) sheets by vacuum filtration to form a flexible nanohybrid-type paper; secondly, reducing the GO into conductive rGO (Fig. S13†). The as-prepared Mn₃O₄@C/GO paper exhibited a 3D-layered structure in which the Mn₃O₄@C polyhedrons are sandwiched by GO sheets (Fig. S14†). The layered structure facilitates better electrical conductivity as almost all Mn₃O₄@Cs are in contact with GO. Moreover, the paper demonstrated excellent mechanical flexibility (Fig. S15†). To preserve the flexible nature of the vacuum-filtered GO paper, a safe and facile method of electrochemical reduction was selected for GO reduction.^50,51

The reduction of Mn₃O₄@C/GO paper was conducted in a three-electrode system with 0.5 M Na₂SO₄ aqueous electrolyte, and the process took 30 min at −1.5 V against the SCE reference electrode. To verify the effectiveness of the reduction, the EIS and CV of Mn₃O₄@C/GO before and after reduction were compared, as shown in Fig. 4a and b. The Z′ value of the reduced sample was greatly decreased, suggesting a lower series resistance.⁵² The reduced sample exhibited a greatly increased current when subjected to CV tests at 10 mV s⁻¹ (Fig. 4d). Moreover, Raman spectroscopy of Mn₃O₄@C/GO and Mn₃O₄@C/rGO showed the D and G bands (at ca. 1350 and 1595 cm⁻¹, respectively), which are characteristic of graphite (Fig. S16a†). Upon reduction, the relative intensity of the D band increases due to the introduction of the sp³ defects of carbon, indicating the partial reduction of GO.⁵³ The XPS results also suggest the successful reduction of GO (Fig. S16b†), as the peaks representing the C=O and the C–O bonds were greatly reduced relative to that of the C–C bond. Both the Mn₃O₄@C/rGO and Mn₃O₄@C/GO electrodes were subjected to TGA test in air (Fig. S17†). Due to the electrochemical reduction, which depletes oxygen from GO, there is less residue for the Mn₃O₄@C/rGO sample.⁵⁴ The mass loading of Mn₃O₄@C/rGO was estimated to be 0.85 g cm⁻³ (or 3.40 mg cm⁻², equivalently) from the TGA results, given an areal density of 7.60 mg cm⁻². More significantly, based on the SEM image (Fig. 4c), the layered structure was largely unaffected by the electrochemical reduction, with a thickness measured to be 40 μm. Due to the relatively weak diffraction of Mn₃O₄@C nanostructures dispersed within rGO sheets, XRD was not able to detect Mn₃O₄ crystals. Therefore, SAED was used to confirm that the Mn₃O₄ was intact after the electrochemical reduction (Fig. 4d-inset). From the TEM images, the Mn₃O₄@C sitting within the rGO sheet is visible (Fig. 4d, S18†). Hence, based on the collective results, the Mn₃O₄@C/rGO paper electrode, with uniform distribution, significantly improved electrical conductivity, and high mass loading, has been successfully prepared.

Fig. 4 Comparison of (a) EIS, and (b) CV results between Mn₃O₄@C/GO and Mn₃O₄@C/rGO. (c) Cross-section SEM of Mn₃O₄@C/rGO. (d) TEM and SAED of Mn₃O₄@C/rGO. (e) CV curves, (f) GCD curves, (g) volumetric capacitance, and (h) cyclic test results of Mn₃O₄@C/rGO.

The electrochemical test on the Mn₃O₄@C/rGO was carried out in the same three-electrode system as in the case of electrochemical reduction. Before the test began, a comparative experiment was conducted between the Mn₃O₄@C/rGO and a pure rGO electrode of the same amount of rGO to verify that the capacitance directly generated from rGO only makes up a minor part and the Mn₃O₄@C is largely responsible for the capacitance (Fig. S19†). The volumetric capacitances of Mn₃O₄@C/rGO paper are illustrated in Fig. 4g. At a current density of 0.5 A cm⁻³, the capacitance is as high as 328.4 F cm⁻³ (382.1 F g⁻¹ for Mn₃O₄@C, estimated from mass loading), far exceeding that of the Mn₃O₄@C-2 h loaded on nickel foam. Even under a high current of 8.0 A cm⁻³, a capacitance of 55.2 F cm⁻³ can still be retained. Such improvement is attributable to the fast electron transfer between rGO sheets and the Mn₃O₄@C particles enabled by the layered structure of Mn₃O₄@C.⁵⁴ Noted that an extremely high capacitance (exceeding 1000 F g⁻¹) can only be obtained at very low mass loading, while 100–200 F g⁻¹ is usually achieved for MnO_x-based electrode when the mass loading is up to several milligrams per square centimeter.^23,26 Thus, the gravimetric capacitance established for Mn₃O₄@C/rGO is still a high value given its high mass loading. The electrode also exhibits a good endurance as capacitance retention of 85% is achieved after 10 000 of GCD cycles at 4 A cm⁻³ (Fig. 4h). Compared to previously reported Mn₃O₄/rGO hybrids of a similar design based on pure Mn₃O₄ nanoparticles, the Mn₃O₄@C/rGO exhibits a significant improvement in volumetric capacitance. For example, a Mn₃O₄/rGO paper (MGP) prepared achieved a capacitance of 109 F cm⁻³ at 0.2 A cm⁻³.⁴³ In another work, an electrochemically reduced Mn₃O₄/rGO paper electrode, where the mass loading is estimated to be 0.71 g m⁻³, exhibits a capacitance of 86.0 F cm⁻³ at 10 mV s⁻¹, still inferior to the Mn₃O₄@C/rGO paper in this work.⁵⁵ These results illustrate the advantage of the MOF-derived Mn₃O₄@C nanocomposite, with high surface area and improved electrical conductivity, as a supercapacitor electrode material when compared to pristine Mn₃O₄.

Upon the successful electrochemical reduction, the Mn₃O₄@C/rGO paper is formed into an aqueous asymmetric supercapacitor (AAS) with CNT/rGO as the negative electrode. The CNT/rGO is manufactured through a similar process to that of the Mn₃O₄@C/rGO, and the cross-section SEM is shown in Fig. S20.† The voltage window for the AAS is expected to be 0–1.8 V (Fig. 5a), as the CNT/rGO electrode has an effective working potential between −1.0 and 0.0 V (Fig. S21a†). The areal ratio between the two electrodes (positive/negative = 1/1.3) is determined by their respective CV profiles at 10 mV s⁻¹ (Fig. S21b†). The full device is tested in 0.5 M Na₂SO₄ electrolyte, and the volumes of both the negative and positive electrodes are taken into consideration of the volumetric capacitance, which is shown in Fig. 5c. The volumetric capacitance of the device reaches 31.13 F cm⁻³ at 0.25 A cm⁻³. The assembled device could achieve an energy density of 10.6 mW h cm⁻³ with a power density of 0.17 W cm⁻³ (Fig. 5d). Even at a high-power output of 1.83 W cm⁻³, an energy density of 0.9 mW h cm⁻³ can still be retained. A comparison with devices of reported works is made in Fig. 5d, where the results duly demonstrate the potential of Mn₃O₄@C/rGO as a supercapacitor electrode material. Furthermore, the stability test indicates that the AAS could withstand at least 5000 cycles of charge/discharge cycles at 4.0 A cm⁻³, with an impressive capacitance retention of nearly 100% (Fig. 5e).^20,56–60

Fig. 5 (a) CV patterns, (b) GCD curves, (c) volumetric capacitance, (d) Ragone plot and (e) cyclic stability of the asymmetric aqueous supercapacitor.

3. Conclusion

Starting from Mn-MIL-100, several types of porous manganese oxides and manganese oxide/carbon nanocomposites were derived by a two-stage, successive thermolysis in nitrogen and air, where a series of changes in chemical composition, phase, morphology, level of porosity and carbon content can be controlled. Thermolysis in air at a moderate 200 °C transformed MOF-derived MnO@C into Mn₃O₄@C, while retaining a significant carbon content and high surface area of the original MnO@C, depending on heating time. On the contrary, MnO@C treated at 300 and 400 °C formed the Mn₃O₄ or Mn₂O₃ phase but with much-reduced porosity and negligible carbon content. When electrochemically tested in aqueous electrolyte, the Mn₃O₄@C-2 h sample showed the highest capacitance of all, due to an optimum balance of several factors, namely, the amount of carbon retained, high surface area and the desired phases of manganese oxides. A flexible nanocomposite-type Mn₃O₄@C/rGO paper incorporating Mn₃O₄@C-2 h was then prepared by vacuum filtration and electrochemical reduction, which enabled a volumetric capacitance of 328.4 F cm⁻³ at 0.5 A cm⁻³. An asymmetric aqueous supercapacitor duly fabricated with the Mn₃O₄@C/rGO paper gave rise to an energy density of 10.6 mW h cm⁻³ at the power density of 0.17 W cm⁻³.

4. Experimental section

4.1 Material synthesis

Mn-MIL-100 was synthesized by dissolving 500 mg of manganese(II) nitrate tetrahydrate (Mn(NO₃)₂·4H₂O) and 900 mg of trimesic acid in to 20 ml of methanol, before transferring the solution into a 45 ml flask for solvothermal operation. The oven temperature was set to 125 °C for 2 hours before the sample was cooled to room temperature.

To prepare the nickel foam-based electrodes, of MnO@C powder or its derivatives were mixed with carbon black and polyvinylidene fluoride (PVDF) at a weight ratio of 8 : 1 : 1, followed by ultrasonication for 30 min. The well-dispersed mixture was then applied to clean nickel foam performing as the current collector. The as-prepared electrode was then dried in an oven at 60 °C overnight.

The Mn₃O₄@C/GO paper was prepared as follows: firstly, 30 mg of Mn₃O₄@C powder was mixed into 8 ml of 5 wt% GO aqueous dispersion with the addition of 4 ml deionized water (DI). The mixture was then extensively stirred by a magneton at a rate of 500 rpm for 24 h, followed by sonication for 1.0 h. After that, the well-dispersed Mn₃O₄@C/GO suspension was transferred into a Büchner funnel for vacuum filtration; a filter paper was placed in the bottom of the funnel before the filtration. After 6.0 h, the black Mn₃O₄@C/GO layer was formed on the filter paper. Finally, by immersing the paper into 95 wt% ethanol for 1.0 h, the Mn₃O₄@C/GO was separated from the filter paper due to the different extents of deformation upon contact with ethanol.

The CNT/rGO paper was prepared through a method similar to that of the Mn₃O₄@C/GO paper: a mixture of 30 mg of CNT fibers and 8 ml of 5 wt% GO aqueous dispersion was pressed into a paper by vacuum filtration before the same electrochemical reduction.

4.2 Materials characterizations

XRD patterns were collected on a BRUKER D8 X-ray diffractometer with a Cu Kα radiation source (λ = 0.154 nm). XPS was conducted with a Kratos Axis Ultra DLD photoelectron spectrometer. The morphology study was carried out with the aid of scanning electron microscopy (SUPRA 40 ZEISS, Germany) and transmission electron microscopy (JEOL 100CX 2010F, Japan). Raman scattering spectra were recorded on a LABRAM-HR Raman spectrometer with a 514.5 nm Ar⁺ laser source. TA Instruments TGA Q500 was used for thermal analysis.

4.3 Electrochemical reduction and measurement

Electrochemical reduction of the Mn₃O₄@C/GO hybrid paper as the working electrode was performed in 0.5 M Na₂SO₄ aqueous solution at a potential of −1.5 V for 30 min. A Pt plate and a saturated calomel electrode were used as the counter electrode and the reference electrode, respectively. The electrochemical measurement of nickel foam-based electrode and the Mn₃O₄@C/rGO paper was carried out in the same setting. The voltage window was set at 0 to 0.8 V. For the electrical measurement of the asymmetric aqueous supercapacitor, Mn₃O₄@C/rGO paper and CNT/rGO paper were used as the cathode material and anode material, respectively. After matching their capacitance by adjusting their relative size, the two papers were assembled into a Swagelok cell for capacitance tests in 0.5 M Na₂SO₄ aqueous electrolyte. The voltage window for the full device was controlled at 0 to 1.8 V. CV and galvanostatic charge/discharge (GCD) measurements, and electrical impedance spectroscopy were conducted using the Solartron Systems 1470E and 1400A, respectively.

The working electrode was prepared by mixing MnO@C powder or its derivatives with carbon black and polyvinylidene fluoride (PVDF) as the conducting additive and binder, respectively, by a weight ratio of 8 : 1 : 1, followed by ultrasonication for 30 min. The well-dispersed mixture was then applied onto clean nickel foam, performing as the current collector. The as-prepared electrode was then dried in an oven at 60 °C overnight. All the gravimetric performance parameters (e.g., specific capacitance) in this work were calculated based on the mass of the active material; the weights of carbon black, binder and nickel foam were not included.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank support of MOE, Singapore (Tier 1, R284-000-193-114) for funding of research, conducted at the National University of Singapore.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra05494a

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