In situ mass change and gas analysis of 3D manganese oxide/graphene aerogel for supercapacitors

Abstract

Manganese oxide nanoparticles decorated on 3D reduced graphene oxide aerogels (3D MnO_x/rGO_ae) for neutral electrochemical capacitors were successfully produced by a rapid microwave reduction process within 20 s. The symmetric electrochemical capacitor of 3D MnO_x/rGO_ae (Mn 3.0 at%) storing charges via both electric double layer capacitance (EDLC) and pseudocapacitance mechanisms exhibits a specific capacitance of 240 F g⁻¹ as compared with 190 F g⁻¹ of that using the bare 3D rGO_ae at 0.5 A g⁻¹ in 1 M Na₂SO₄ (aq.) electrolyte. It retains 90% of the initial capacitance after 10 000 cycles, demonstrating high cycling stability. In addition, the charge storage mechanism of 3D MnO_x/rGO_ae was investigated using an electrochemical quartz crystal microbalance. In situ gas analysis using differential electrochemical mass spectrometry (DEMS) shows the CO₂ evolution at a cell potential over 1 V indicating that the positive electrode is possibly the voltage limiting electrode in the full cell. This finding may be useful for further development of practical high power and energy supercapacitors.

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Introduction

Electrochemical capacitors or supercapacitors (SCs) have attracted interest because of their high specific power, high charging–discharging rate, and long cycling stability.^1–3 SCs can be classified by their charge storage mechanisms such as electrical double layer capacitor (EDLC) and pseudocapacitors. Generally, carbon-based materials are widely used as active materials in EDLCs storing charges by the electrosorption of ions on their surfaces. In contrast, pseudocapacitors store charge via redox reaction at the solid–liquid interface or the intercalation of ions at the surface.⁴ The conducting polymers and metal oxides are widely used as pseudocapacitive materials. Among them, manganese oxide is one of the best electrode materials for supercapacitors due to its good stability, low cost, natural abundance, and environmentally friendly nature.^5,6 However, it has poor electrical conductivity (10⁻⁵ to 10⁻⁶ S cm⁻¹).⁷ Therefore, it is usually combined with highly conductive carbon-based material such as carbon nanotube (CNT), carbon fibre, and graphene or reduced graphene oxide (rGO) to improve its electrochemical performance.^2,8–10

Graphene is widely used as supercapacitor electrode material due to its high electrical conductivity and high specific surface area.^11,12 There are many reports relating to reduced graphene oxide/manganese oxide (rGO/MnO_x) composites as supercapacitor electrodes. For example, the rGO/Mn₃O₄ composite produced by a hydrothermal process exhibits a specific capacitance of 121 F g⁻¹ at 0.5 A g⁻¹ in 1 M Na₂SO₄.¹³ Nitrogen doped graphene/Mn₃O₄ produced by the hydrothermal process shows a specific capacitance of 205 F g⁻¹ at 1.0 A g⁻¹.¹⁴ The rGO/Mn₃O₄ prepared by the N₂ dielectric barrier discharge plasma exhibits a specific capacitance of 193 F g⁻¹, while the specific capacitance of pure Mn₃O₄ is only 95 F g⁻¹.¹⁵ Recently, 3D graphene has attracted interest because the 3D interconnected structure of graphene sheets can increase ion mobility and electron transport.¹⁶ For example, the graphene hydrogel shows a specific capacitance of 190 F g⁻¹ at 0.5 A g⁻¹ in 6 M KOH.¹⁷ 3D rGO produced by the chemical reduction method shows a specific capacitance of 204 F g⁻¹ at 0.5 A g⁻¹ in 6 M KOH.¹⁸ 3D graphene network shows a specific capacitance of 190 F g⁻¹ at 0.5 A g⁻¹ in 6 M KOH.¹⁹ The specific capacitance of 3D graphene aerogel is 245 F g⁻¹ at 1.0 A g⁻¹ in PVA/H₃PO₄ solid electrolyte.²⁰

In this work, we further developed the supercapacitor electrode using the 3D manganese oxide/reduced graphene oxide aerogel (MnO_x/rGO_ae) composite synthesized by a microwave reduction method. This technique is facile, effective, rapid, and environmentally friendly.^21–25 The 3D MnO_x/rGO_ae can store charges via the EDLC of 3D rGO_ae and the surface redox of MnO_x leading to high power and energy. The charge storage mechanism of ion sorption was investigated by the electrochemical quartz crystal microbalance (EQCM) and in situ gas analysis using the differential electrochemical mass spectroscopy (DEMS). The DEMS can also provide the working potential limit without the gas evolution, which is a root of the poor stability of supercapacitors. The electrochemical performance of the symmetrical neutral supercapacitors using four composites with different MnO_x loading contents. The symmetric electrochemical capacitor of 3D MnO_x/rGO_ae (Mn 3.0 at%) exhibits a specific capacitance of 240 F g⁻¹ as compared with 190 F g⁻¹ of that using the bare 3D rGO_ae at 0.5 A g⁻¹ in 1 M Na₂SO₄ (aq.) electrolyte.

Experimental

Chemicals and materials

Graphite powder (<20 μm, Aldrich), sodium nitrate (NaNO₃, 99% Ajax Finechem), potassium permanganate (KMnO₄, 99%, UNIVAR), sulphuric acid (H₂SO₄, 98%, QRec), nitric acid (HNO₃ 65%, QRec), hydrogen peroxide (H₂O₂, 30%, Chem Supply), hydrochloric acid (HCl, 37%, QRec), ethanol (99.9%, QRec), polyvinylidene fluoride (PVDF, M_w ∼ 534 000, Sigma-Aldrich), carbon black (99+%, Super P), (N-methyl-2-pyrrolidone) (NMP, 99.5% QRec), and sodium sulfate (Na₂SO₄ 99%, CARLO ERBA) were analytical reagent grade and used without further purification. Carbon fibre paper (CFP) was from SGL CARBON SE, Germany. Deionized water from Millipore system (DI water, 15 MΩ cm) was used in all experiments.

Synthesis of graphene oxide aerogel

Graphene oxide (GO) was synthesised using a modified Hummer's method. Briefly, 5.0 g graphite powder and 7.5 g NaNO₃ were mixed in 500 mL conc. H₂SO₄ in an ice-bath keeping the temperature below 20 °C. 40 g KMnO₄ was slowly added into the mixture and stirred for 24 h at an ambient temperature. Then, 500 mL DI water and 150 mL H₂O₂ were subsequently added into the mixture and kept stirring for 24 h. The mixture was washed with 5% HCl and then centrifuged with DI water for several times until neutral.²⁶ Finally, the obtained product was then washed with DI water for several times and freeze-dried for 72 h.

Synthesis of MnO_x/rGO_ae composite

0.42 g GO aerogel was dispersed in 30 mL KMnO₄ (0.088 M) solution with varying weight of GO : KMnO₄ (1 : 1, 2 : 1, 4 : 1, and 8 : 1 by weight) becoming the suspension. Then, 0.25 mL ethanol was added into the suspension under constant magnetic stirring and then further stirred for 4 h at room temperature.²⁷ Then, the GO/MnO_x was filtered, washed with deionized water and ethanol, and dried at 60 °C for 24 h. The MnO_x/GO was heated by a microwave reduction method at a power of 200 W for 20 s to obtain MnO_x/rGO_ae. Finally, the obtained product was then dried at 80 °C for 24 h. The rGO_ae was synthesised by the microwave method at 200 W using GO aerogel precursor.

Fabrication of MnO_x/rGO_ae composite electrodes

Active material powder (8 mg), carbon black (CB) using as a conductive additive (1 mg), and PVDF using as a binder (1 mg) were dispersed and sonicated in N-methyl-2-pyrrolidone (NMP, 1 mL) solvent. The as-prepared carbon slurry was then drop-coated on the functionalized-carbon fibre paper (f-CFP) with a diameter of 1.58 cm. The mass loading was about 1 mg cm⁻². The electrode was dried at 80 °C for 24 h. The hydrolysed polyethylene (PE) film and 1 M Na₂SO₄ (aq.) were used as a separator and an electrolyte, respectively. Prior to the fabrication of supercapacitor, the separator was soaked in the electrolyte for 10 min. The symmetric supercapacitors were assembled in 2016-type coin cells using a hydraulic compression machine at 800 psi for the evaluation of electrochemical performance. Whilst, 2032-type coin cells with the hole were used for in situ gas analysis using DEMS. The CV was carried out at 10 mV s⁻¹ up to the gas evolution with the predefined voltage intervals. For EQCM measurement, the carbon slurry was drop-coated on Au support (0.67 cm in diameter) about 0.22 mg cm⁻². Ag/AgCl (3 M KCl) with gel electrolyte and Au wire were used as reference electrode and counter electrode, respectively.

Characterization

The morphologies of as-synthesised materials were characterized by FE-SEM/EDS (FE-SEM, JEOL JSM-7610F), and TEM (TEM, Hitachi HT7700). The crystalline structure was characterized by XRD (Bruker D8 ADVANCE using Cu-Kα radiation (λ = 1.5418 Å)). The chemical structure was characterized by FTIR (PerkinElmer) and Raman spectroscopy (Bruker, an excitation wavelength of 532 nm). The specific surface area was measured by nitrogen adsorption/desorption with the BET method (BELSORP-mini, MicrotracBEL). The surface composition was analysed by X-ray photoelectron spectroscopy (XPS, JEOL, JSP-9010MC). In situ gas detection was carried out by DEMS (Hiden analytical, HPR-40). Cyclic voltammetry (CV) and galvanostatic charge–discharge measurement (GCD) were performed using a Metrohm AUTOLAB potentionstat (PGSTAT 302N) and battery tester (Neware, Gelon, HongKong), respectively.

Results and discussion

The morphologies of rGO_ae and MnO_x/rGO_ae composites were characterized by FE-SEM as shown in Fig. 1. The rGO_ae shows a 3D sponge like structure with the interconnected graphene sheets and macroporous voids (inset image in Fig. 1a). Spherical MnO_x nanoparticles decorated on the graphene nano-sheet are observed for MnO_x/rGO_ae composite (8 : 1 GO : KMnO₄ ratio) (Fig. 1a). The number of nanoparticles decorated on the graphene nanosheet increase with increasing the concentration of Mn source for the synthesis of MnO_x/rGO_ae composite. For example, the dense nanoparticle coverage on the graphene sheet is observed for 1 : 1 GO : KMnO₄ ratio. It may decrease the total surface area and pore volume of the composite. Also, the MnO_x particle size increases with increasing Mn source. The size of the nanoparticles is in the range of ∼10 to 50 nm.

Fig. 1 FE-SEM images of MnO_x/rGO_ae with different GO : KMnO₄ precursor weight ratios including (a) 8 : 1, (b) 4 : 1, (c) 2 : 1, and (d) 1 : 1.

The elemental composition of MnO_x/rGO_ae can be confirmed using the EDS technique. The EDS spectra (see Fig. S1†) show the C, O, Mn elements. The at.% of Mn increases with the increasing amount of Mn precursor. For instance, 2.5% and 6.1% of Mn were quantified for 8 : 1 and 1 : 1 GO : KMnO₄, respectively. Also, the residual K element can be observed in the composite. In fact, it will not affect the electrochemical performance of the supercapacitor as it can be considered as a kind of K-intercalated MnO_x.

To further investigate the morphology of samples, TEM images are presented in Fig. 2. The number of MnO_x nanoparticles decorated on the rGO sheet increases with increasing Mn precursor. The MnO_x nanoparticles are in the range of 10–30 nm for low content of KMnO₄ precursor. At higher KMnO₄ precursor content, the particle sizes of MnO_x NPs are 10–50 nm. It corresponds with FE-SEM measurement.

Fig. 2 TEM images of MnO_x/rGO_ae with different GO : KMnO₄ precursor ratios including (a) 8 : 1, (b) 4 : 1, (c) 2 : 1, and (d) 1 : 1 wt%.

The XRD patterns of the GO_ae, rGO_ae, and MnO_x/rGO_ae composites are shown in Fig. 3a. The XRD pattern for GO_ae shows a characteristic peak at around 10°, corresponding to the (001) plane of graphene oxide. After the reduction of GO_ae by the microwave method, rGO_ae shows the broad peak at around 25.8° which is attributed to an (002) plane of graphitic carbon materials. It indicates the effective reduction of the oxygen-functional group on the GO_ae structure by microwave method. For MnO_x/rGO_ae composites, the mixed phase of MnO and Mn₃O₄ and residual GO are observed.⁵

Fig. 3 (a) XRD patterns, (b) FTIR spectra, (c) Raman spectra, (d) nitrogen adsorption/desorption isotherm, (e) XPS C 1s spectrum, and (f) XPS Mn 2p spectrum of MnO_x/rGO_ae composite at the Mn content of 3.7 at%.

The FTIR spectroscopy was used to identify functional groups of the rGO_ae and MnO_x/rGO_ae composite. The FTIR spectra (Fig. 3b) show three peaks at 1610, 1216, and 1132 cm⁻¹ which can be referred to the remaining C=C, C–O, and C–OH, respectively.²⁸ Additionally, MnO_x/rGO_ae composites exhibit two characteristic peaks at 492 and 614 cm⁻¹ which can be attributed to Mn–O stretching.¹⁰

Fig. 3c shows the Raman spectra of the as-synthesized materials showing D and G bands at 1360 and 1598 cm⁻¹, respectively. D band corresponds to the disordered structure or defect (sp³ carbon), while the G band relates to the ordered structure (sp²) of carbon material. The intensity ratio of D to G band (I_D/I_G) can be used to evaluate the sp² domain size and degree of the disorder of the graphene-based materials. The I_D/I_G ratio of rGO_ae is 0.84, while the MnO_x/rGO_ae composites exhibit lower ratios in the range of 0.6–0.8 due to MnO_x formation reaction decreases the surface defects.²⁹ In addition, MnO_x/rGO_ae composites peaks at 577 and 649 cm⁻¹ are attributed to MnO.⁶

The nitrogen adsorption/desorption isotherms of rGO_ae and MnO_x/rGO_ae composites are shown in Fig. 3d. Type-IV isotherms were obtained for all samples.³⁰ The specific surface areas of rGO_ae and MnO_x/rGO_ae with different Mn contents of 2.5, 3.0, 3.7, and 6.1 at% are 1279, 352, 200, 70, and 34 m² g⁻¹, respectively. The total pore volume of MnO_x/rGO_ae decreases with increasing Mn content, which is 1.69, 1.02, 0.34, and 0.17 cm³ g⁻¹, respectively. Whilst, the total pore volume of rGO_ae is 5.80 cm³ g⁻¹. The average pore diameters of rGO_ae and MnO_x/rGO_ae with Mn content of 2.5, 3.0, 3.7, and 6.1 at% are 21.2, 19.1, 20.5, 19.2, 20.6 nm, respectively.

XPS was carried to confirm the incorporation of MnO_x on the graphene sheet. The XPS results of MnO_x/rGO_ae composite with 3.7 at% Mn is shown in Fig. 3e and f. The C 1s spectrum can be deconvoluted into four peaks at 284.7, 286.1, 287.3, and 288.6 eV corresponding to the typical signals of C–C/C=C, C–O (hydroxyl and epoxy groups), C=O (carbonyl groups), and O–C=O (carboxyl groups),²⁹ respectively. Fig. 3f shows that the Mn 2p_3/2 and Mn 2p_1/2 of the MnO_x in the composite exhibit the binding energies at 642.1 eV and 653.6 eV, respectively. The energy separation is 11.5 eV which is in good agreement for Mn₃O₄.³¹

CV and GCD were used to measure the electrochemical performance of symmetrical supercapacitors with different MnO_x/rGO_ae composites. The CV (Fig. 4a) and GCD (Fig. 4b and c) of supercapacitors were measured at a cell potential of 1.2 V in 1 M Na₂SO₄ electrolyte. Fig. 4a shows the CV curves of symmetrical rGO_ae and MnO_x/rGO_ae supercapacitors at a scan rate of 10 mV s⁻¹ which show nearly rectangular shape. The specific capacitance (Fig. 4d) was calculated from the GCD as the following eqn (1):³²

where I is the discharging current (A g⁻¹), Δt is the discharging time (s), ΔV is the potential (V), and m is the total mass of active materials in positive and negative electrodes of the full cell. The plot of the specific capacitance vs. the specific current for rGO_ae and MnO_x/rGO_ae composites is shown in Fig. 4d. The symmetric MnO_x/rGO_ae devices show higher specific capacitances than bare rGO_ae. Among the composites, the higher specific capacitance of 240 F g⁻¹ at a specific current of 0.5 A g⁻¹ was measured for MnO_x/rGO_ae with a Mn content of 3.0 at% as compared to that of other samples. Also, the measured specific capacitance is higher than the reported systems in the literature (Table 1). The maximum cell potential of 0.9 V with an excellent capacitance retention of 90% was estimated after 10 000 cycles at a specific current of 2.0 A g⁻¹ (Fig. 4e). Note, all electrochemical evaluation was operated at ca. 100% coulombic and energy efficiencies (see Fig. 4e).

Fig. 4 (a) CV curves at a scan rate of 10 mV s⁻¹, (b) GCD curves at a specific current of 0.5 A g⁻¹ of symmetric supercapacitors based on rGO_ae and MnO_x/rGO_ae, (c) GCD curves at specific currents from 0.5 to 2.0 A g⁻¹, (d) the specific capacitances of rGO_ae and MnO_x/rGO_ae, (e) cycling performance of symmetric supercapacitor based on MnO_x/rGO_ae at a Mn content of 3.0 at% at a specific current of 2.0 A g⁻¹, and (f) Ragone plot as compared to other previous work.

Table 1 The comparison of the specific capacitance of 3D MnO_x/rGO_ae (Mn = 3.0%) electrode with the reported graphene/MnO_x composites using the 3-electrode configuration

Material	Specific capacitance	Electrolyte	Ref.
Graphene/Mn3O4	121 F g^−1 at 0.5 A g^−1	1 M Na2SO4	13
Graphene/Mn3O4	225 F g^−1 at 5 mV s^−1	1 M Na2SO4	10
Graphene sheet/Mn3O4	181F g^−1 at 0.2 A g^−1	1 M Na2SO4	39
Graphene/Mn3O4	193 F g^−1 at 0.05 A g^−1	Na2SO4 (sat.)	15
Graphene/Mn3O4	192 F g^−1 at 0.5 A g^−1	K2SO4 (sat.)	40
rGO/Mn3O4 nanorods	176 F g^−1 at 1.0 A g^−1	1 M Na2SO4	41
rGO/Mn3O4 nanocomposite	160 F g^−1 at 0.5 A g^−1	1 M Na2SO4	6
Graphene oxide double-shell hollow spheres/Mn3O4	225 F g^−1 at 5 mV s^−1	0.5 M Na2SO4	29
rGO/Mn3O4	186 F g^−1 at 0.25 A g^−1	1 M Na2SO4	42
rGO/Mn3O4	194 F g^−1 at 0.5 A g^−1	1 M Na2SO4	43
Graphene/Mn3O4 nanocrystalline	260 F g^−1 at 0.25 A g^−1	1 M Na2SO4	44
rGO/Mn3O4	195 F g^−1 at 1.0 A g^−1	1 M Na2SO4	45
Graphene–Mn3O4	247 F g^−1 at 5 mV s^−1	K2SO4 (sat.)	46
Porous graphene/Mn3O4	208 F g^−1 at 0.5 A g^−1	1 M Na2SO4	47
3D MnOx/rGO aerogel	238 F g^−1 at 0.5 A g^−1	1 M Na2SO4 (2-electrode system)	This work
	266 F g^−1 at 0.5 A g^−1	1 M Na2SO4

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The specific energy and specific power can be calculated from the following equation:

where E is the specific energy, C_cell is the capacitance of full cell, ΔV is the potential excluding the IR drop, P is the specific power, and t is the discharging time. The Ragone plot of symmetric supercapacitor with rGO_ae and its composites is shown in Fig. 4f. The MnO_x/rGO_ae with a Mn content of 3.0 at% shows the highest specific energy of 10.9 W h kg⁻¹ and a specific power of 286 W kg⁻¹. The measured values are higher than those of the similar reported symmetric supercapacitors in the literature.^33–36

The CV and EQCM results of the MnO_x/rGO_ae at a Mn content of 3.0 at% in 1.0 M Na₂SO₄ (aq.) at a scan rate of 10 mV s⁻¹ are shown in Fig. 5. The quartz resonance frequency (Δf) related to mass change of electrode (Δm) by Sauerbrey's equation:³⁷

Δm = −C_fΔf (4)

where C_f is a calibration constant (0.0815 Hz ng⁻¹ cm²).³⁸ Note that, the negative frequency response (−Δf) increases with increasing the electrode mass (Δm).

Fig. 5 CVs and EQCM frequency response of MnO_x/rGO_ae (3.0 at% Mn) at 10 mV s⁻¹ in 1.0 M Na₂SO₄ (aq.) electrolyte in three electrode systems at (a) negative and (b) positive potential scans. Note that dashed lines present the frequency responses and solid lines present the CV response. Red and black curves represent charge and discharge of ions, respectively.

To study the anion and cation contribution, the data were collected at two different potential ranges; positive and negative potential ranges. In Fig. 5a, the CV is started at the potential from 0.10 V to −1.00 V vs. Ag/AgCl. The opposite frequency response (−Δf) simultaneously increases implying that the electrode mass (Δm) increases owing to the adsorption of solvated Na⁺ ions. During the reverse scan from −1.00 to 0.07 V vs. Ag/AgCl, the mass change of the electrode decreases due to the desorption of solvated Na⁺. Between 0.05 and 0.25 V vs. Ag/AgCl in the positive sweep, the mass change of the electrode is almost the same because of the desorption of the solvated cations and adsorption of the solvated anion in the electrode surface. It is due to the double layer consisting solvated cations and anions at a zero charge potential.⁴⁸ Further increment of potential up to 0.6 V results the steep increase of Δm due to the adsorption of solvated SO₄²⁻ ions.⁴⁹ Thereafter, a steady increment of Δm is noted up to the 0.9 V and the reduction of Δm is observed from 0.9 to 1.0 V. These changes cannot be explained based on the pure ion sorption process as it only increases the mass of the electrode. In this potential region, possibly the redox reaction of MnO_x occurs⁴⁹ along with the ion adsorption process. The former decreases the mass, while the latter increases the mass of the electrode. The mass change as a result of anion adsorption is the dominant process over the redox reaction of MnO_x in the potential range of 0.6 to 0.9 V, which results in a steady increment of the electrode mass. While the dissolution process becomes a major role, thereafter, leading to the reduction of mass up to 1.0 V. On the reverse scan, the mass decreases steadily up to 0.05 V due to the desorption of the anion. A small increment of mass is noted at 0.65 V is possibly due to the redox reaction of MnO_x.⁴⁹ The mass changes in positive and negative potential ranges are 18 and 11 ng cm⁻². The anion is adsorbed on the MnO_x/rGO_ae surface with ca. 1.7 times higher than the cation adsorption.

DEMS was used for the in situ gas evolution analysis of symmetrical supercapacitors using the MnO_x/rGO_ae composite during the CV measurement. Fig. 6 shows the gas evolution profile. The CO₂ evolution starts at a cell potential of 1.0 V and above, possibly due to the carbon corrosion and the subsequent oxidation of corrosion product as a result of the oxygen evolution at the positive electrode (eqn(5)–(8)).⁵⁰ It agrees well with the maximum cell potential identified by the long-term stability measurement. Initially, the oxygen/hydroxyl radicals generated at the carbon electrodes react with carbon leading to CO and CO₂. The detection of CO₂ alone indicates the further oxidation of CO into CO₂.⁵⁰

H₂O − ne⁻ → OH˙ + H⁺ (n = 1), O˙ + 2H⁺ (n = 2) (5)

C + O˙/OH˙ → C=O + C–OH_ads → CO(+H⁺)↑ (6)

C + 2H₂O − 4e⁻ → CO/CO₂↑ + 4H⁺ (7)

CO + O/O₂ − CO₂↑ (8)

Fig. 6 (a) CVs of the symmetrical supercapacitor of MnO_x/rGO_ae (3.0 at% Mn) at 10 mV s⁻¹ with different cell potential intervals and (b) CO₂ evolution profile during CV measurement.

Conclusion

The symmetric neutral electrochemical capacitor of MnO_x/rGO_ae composites synthesised by a chemical and microwave synthesis exhibits a specific capacitance of 240 F g⁻¹ at a specific current of 0.5 A g⁻¹, a maximum specific energy of 10.9 W h kg⁻¹ and a specific power of 286 W kg⁻¹ in 1.0 M Na₂SO₄ (aq.). The cycling stability of 90% was measured after 10 000 cycles at a cell potential of 0.9 V. In addition, the charge storage mechanism was investigated by electrochemical quartz crystal microbalance (EQCM), demonstrating the ion adsorption/desorption process and the dissolution of MnO_x from MnO_x/rGO_ae. DEMS measurement shows CO₂ evolution at 1.0 V, which validates the identified maximum cell potential by the long-term stability test. Also, the detection of CO₂ hints that the positive electrode possibly limits the cell potential. This finding will be useful for further development of practical high power and energy storage devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Vidyasirimedhi Institute of Science and Technology and Thailand Research Fund (RSA6180031 and RTA6080005) as well as Energy Policy and Planning Office (EPPO), Ministry of Energy, Thailand. Support from the Frontier Research Centre at VISTEC is also acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: EDS spectra, electrochemical performance and comparison table of rGO/MnO_x composites. See DOI: 10.1039/c9ra05444h

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