Rapid Preparation of All-In-One Compressible Capacitors with Flame Carbonized Melamine Foam for AC Line Filtering

Melamine foam (MF) is a good candidate for electrochemical filter capacitors due to its 3D porous structure, flexibility, and nitrogen-containing property. However, traditional thermal carbonization would cause a severe loss of nitrogen and need several hours to perform the transition from melamine to carbon. Moreover, to construct all-in-one electrochemical capacitors, the nonconductive MF must be deposited a conductive layer on the surface of MFs. Herein, we developed a flame carbonization method to carbonize the MFs to an all-in-one structure in less than one minute. The carbonized MF exhibits 2.6 times higher nitrogen content than that of the traditional carbonized. MF-based all-in-one compressible electrochemical capacitors deliver excellent alternating current (AC) line filtering performance, such as a low phase angle of −83.1° at 120 Hz, a low resistance capacitance time constant of 157 μs, and a short relaxation time constant of 46 μs. The AC signal of 120 Hz, even a higher frequency of 2000 Hz, can be successfully converted into a stable direct current (DC) signal. Besides, the influence of electrolyte, compressive strain, and the thickness/carbonization time of MFs on the electrochemical performance has been studied. This work provides an ingenious design and effective preparation strategy for MFs-based all-in-one electrochemical capacitors.

With the rapid development of the intelligent electronic equipment, the demand for portable and flexible electronic products is growing. 1,2 Alternating current (AC) line filtering is mainly used to convert the AC into the direct current (DC), which plays a crucial role in electronic products. 3 At present, aluminum electrolytic capacitors (AECs), which are the most widely used capacitors in AC line filtering applications, have a large volume and low capacitance, which is inconsistent with the development trend of miniaturization of devices. [4][5][6][7] Although electrochemical capacitors possess high energy density, the frequency response of common electrochemical capacitors (such as activated carbon-based capacitors) cannot satisfy the requirements of AC line filtering. 8 The poor frequency response of common electrochemical capacitors is attributed to the large series resistance of electrode materials and complex microporous structures. 5 Melamine foams (MFs) are low-cost, eco-friendly, and compressible plastic with an open-cell ratio (the ratio of the open-cell volume and the whole foam volume) of over 90%, which are broadly used as flame retardation and sound absorbing materials. Moreover, the excellent mechanical properties (e.g., flexibility) and the 3D porous structure can be maintained even after the thermal carbonization at a high temperature of 800°C. 9 The carbonized MFs have been demonstrated as a candidate material for AC line filtering. 10 With an organic electrolyte of 1 M TEMABF 4 in acetonitrile, a phase angle of −80.1°was obtained at 120 Hz. However, the carbonization process needs several hours and will cause a severe loss of nitrogen in melamine. 11 Therefore, there is a challenge to perform the melamine carbonization at the minute level and with high nitrogen content.
On the other hand, the all-in-one flexible capacitors fabricated by integrating active electrodes on two sides of a flexible substrate are more resistant to deformation and lower interface resistance than the traditional flexible capacitors, which has a configuration with two flexible electrodes sandwiched by the separator and/or gel electrolyte. 12 MFs have been used to fabricate the all-in-one flexible electrochemical capacitors, in which MF is not only a substrate but also a separator. 13 The active material of polypyrrole was deposited on the gold-coated MFs by the electrochemical polymerization of pyrrole. However, the conductive property of MFs must be achieved on each side through the gold sputtering, which needs expensive equipment and a long time.
Considering the excellent mechanical and flame retardation properties of MFs, we developed a flame carbonization method to prepare the all-in-one compressible electrochemical capacitors rapidly (in less than one minute). The nitrogen content of carbonized MFs can also be well maintained, benefiting the high conductivity of electrodes. Moreover, the porous structure and mechanical properties of the carbonized MFs can be inherited from MFs. The all-inone compressible electrochemical capacitors with flame-carbonized MFs was further applied for AC line filtering. With an aqueous electrolyte of 3 M KOH, a phase angle of −83.1°has been obtained at 120 Hz. Moreover, the resistance capacitance (RC) time constant and relaxation time constant is as short as 157 and 46 μs, respectively. The AC signal with the frequencies of 120 and 2000 Hz can be successfully filtered into a stable DC signal in the AC filtering line by the fabricated electrochemical capacitors. In addition, the influence of electrolyte, and compressive strain on the electrochemical performance has been investigated comprehensively. An ingenious design and a facile/effective preparation strategy have been developed in this work for the fabrication of MFs-based all-in-one electrochemical capacitors without other active compounds modification.

Experimental
Preparation of FCMF and CMF.-Melamine foams (MFs) were cut into sheets with appropriate size (e.g., 7 cm × 5 cm × 1 cm). The two sides of MF sheets were flame carbonized rapidly (e. g., 12 s) with isobutane combustion flame sequentially (see the Movie in Supporting Information). The flame carbonization process was controlled through the flame distance (or the outer flame) and the time used. Then, a sandwich structure of carbon/MF/carbon was prepared for a half minute. This structure was named as FCMFs. The preparation procedure of carbonized MFs (CMFs) via thermal carbonization was as follows: MFs were carbonized at 800°C for 2 h with a ramping rate of 5°C min −1 in a nitrogen atmosphere in a tubular furnace.
Assembly of FCMF electrochemical capacitor.-Rapidly flamecarbonized MF sheets were cut into a circle shape with a diameter of z E-mail: pingpingyang@swpu.edu.cn; jialexie@swpu.edu.cn = Equal contribution. prepared all-in-one FCMF sheets were immersed in an electrolyte (e.g., 3 M KOH) to fully absorb. Then, the devices were assembled with the coin cells of CR-2016, which was made of stainless steel. The original thickness of the active electrode was ∼0.5 mm. The original thickness of the "separator" in the all-in-one FCMF structure was ∼5 mm. The electrolyte used was 3 M KOH. When the electrolyte was optimized, the 2 M H 2 SO 4 , 1 M Na 2 SO 4 , and 3 M KOH aqueous electrolytes were used. Moreover, the organic electrolyte of 1 M TEMABF 4 in acetonitrile (TEMABF 4 /AN) was also selected as a control.
Material characterizations.-The morphology of materials was characterized by scanning electron microscopy (SEM, Thermo Scientific Apreo 2 C). Raman spectroscopy was performed by using a Thermo Scientific Dxr2xi Laser confocal Raman spectrometer with laser excitation at 532 nm. The X-ray photoelectron spectroscopy (XPS) measurements were carried out on a ThermoFisher ESCALAB Xi+ X-ray photoelectron spectrometer. X-ray diffraction (XRD) was performed on a Rigaku Ultima IV Xray diffractometer with Cu K α radiation. The nitrogen adsorption −desorption was conducted at 77 K with Micromeritics ASAP 2460. The electrical conductivities of CMF and FCMF were measured by recording the IV curves in the voltage range of −40-40 mV. CMF and FCMF with almost the same geometric area were fixed between two tin foils to get a good electrical contact. From the IV curves, the electrical conductivity can be calculated from the following equation.
where S is the geometric area contacting the tin foils, R is the measured electric resistance, and h is the height of the sample.
Electrochemical measurements.-Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were measured using an electrochemical workstation (CHI660E, Chenhua, China). The two-electrode system was used during the electrochemical measurements. During the process of EIS measurement, the frequency range was from 0.01 Hz to 100 kHz, the voltage used was 0.4 V, and the potential amplitude was 10 mV.
The specific areal capacitance (C A ) of FCMF device was calculated from CV: The specific areal capacitance (C A ) of FCMF device against the frequency was obtained from EIS analysis: The RC time constant (τ RC ) of FCMF devices was calculated using: The imaginary specific areal capacitance (C″) of FCMF devices was calculated by: The relaxation time constant (τ 0 ) was from the frequency at the maximum C″: where "I" represents current (A), "V" is the voltage window (V), "s" is the scan rate (V s −1 ), "A" is the area of a single electrode (cm 2 ), "f" is the frequency (Hz), Z , ′ Z″ and |Z| are the real part of the impedance (Ω), the imaginary part of the impedance (Ω) and magnitude of impedance (Ω), respectively. AC line filtering measurements.-The element of a full bridge rectifier used in this work is KBP206. The AC input sine signal with a frequency of 60 Hz or 1000 Hz was generated by a function generator (MFG3000, Maiwei, China). The DC output signals were measured using an oscilloscope (GDS-300, Guwei, China).

Results and Discussion
The morphology of the carbonized MFs via thermal carbonization in an inert atmosphere is shown in Fig. 1a. The 3D network structure with the interconnected carbon fibers and tens of micrometers pores can be clearly observed, which facilitates fast mass transport. In this work, the MFs are the precursor of electrode materials, and the separator that prevents short circuit. Figure 1b schematically shows the preparation process of all-in-one compressible electrochemical capacitors through a flame carbonization approach. First, each side of MF sheets was flame carbonized rapidly with isobutane combustion flame (∼1000°C). From Fig. S1 and Movie S1 in Supporting Information, it can be easily seen that the MF surface rapidly converted from white to black during the flame treatment process. Thanks to the flame retardation of MFs, the carbonization process can be stopped immediately when the flame is quenched artificially. Then, a typical sandwich structure was prepared, where the uncarbonized MF was used for separating the two carbon layers to prevent short circuit. Figures 1c and 1d display the morphology of FCMFs. FCMFs have similar 3D porous interconnection structures, except that some bubbles intersperse among the carbonized fibers. 14 Some cracked bubbles illustrate they are cavities. These bubbles can be attributed to the rapid carbonization and decomposition of melamine accompanied by the gas generation, such as CO 2 and N 2 . 15 After the flame carbonization, the FCMFs can be compressed and recovered without damaging the structure (Fig. S2). The XRD patterns of MF, CMF and FCMF are shown in Fig. S3. The characteristic peak of (002) from carbon can  be observed at around 26°. The broad diffraction peak suggests the amorphous features of the synthesized carbon. The XRD pattern of the pristine MF shows a peak located at 21.7°, while no this XRD peak can be observed on the patterns of CMF and FCMF. This suggests the CMF and FCMF were totally carbonized. Raman spectra were used to reveal the graphitization degree of carbon frameworks in the CMF and FCMF. In Fig. 2a, the Raman peaks of CMF and FCMF are located at 1328 cm −1 and 1334 cm −1 respectively, corresponding to the D-band due to disorder or defects of carbon frameworks. The G-band of CMF and FCMF are at 1542 cm −1 and 1554 cm −1 respectively, representing to sp 2 vibration of graphite. The Raman shift was caused by the difference in crystallinity and electronic structure in materials. 16 It is also found from the Raman spectra that FCMF has a lower intensity ratio of D to G band (I D /I G ) than CMF, indicating that its graphitization degree is higher, so its conductivity may be better. 10 As shown in Fig. S4,    S5) reveals that the CMF contains C (64.71 at%), N (11.47 at%), and O (23.82 at%) elements, while FCMF contains C (52.55 at%), N (41.84 at%), and O (5.61 at%). This clearly demonstrates FCMFs have a 2.6 times higher nitrogen content than that of the traditional carbonized CMFs. FCMF not only has a significantly higher nitrogen content than CMF, but also has a lower oxygen content than CMF. Therefore, the flame carbonization method is a good candidate strategy to avoid the loss of nitrogen during thermal carbonization. The high nitrogen content of FCMFs is consistent with the high conductivity of FCMFs. The peak at 497 eV should be attributed to the Auger peak of Na element, which is from the emulsifier (e.g., sodium dodecyl sulphate) and the foaming agent (e.g., NaHCO 3 ). As shown in Fig. 2b, the C1s XPS spectrum of FCMFs can be deconvoluted into four peaks located at 284.8, 286.1, 287.8, and 289.8 eV, corresponding to C-C, C-O, C-N, and C=O, respectively. 17 The C1s XPS spectrum of CMFs can be fitted to six peaks as in Fig. 2b. The additional two peaks are at 292.9, and 295.8 eV, corresponding to O-C=O and π-π, respectively. 18,19 For N1s XPS spectra, the peaks can be well fitted with four types of nitrogen-containing species, such as pyridinictype N (N-6, 398.5 eV), pyrrolictype N (N-5, 399.6 eV), quaternarytype N (N-Q, 401.1 eV) and pyridine-N-oxide (N-X, 403.9 eV). 9 For O1s XPS spectra, four oxygen-containing species are used to fitting the C1s peaks in Fig. 2d Table I. The flame carbonization approach can retain a high ratio (56.69%) of active species (such as N-5 and O-II) for charge storage.
To evaluate the specific surface area and pore size distribution of CMF and FCMF, we conducted the nitrogen adsorption/desorption measurements at 77 K (Fig. S6). The Brunauer-Emmett-Teller (BET) specific surface areas of CMF and FCMF are 2.96 and 2.11 m 2 g −1 , respectively. These values are great lower than the usually used active carbons (e.g., 450-1800 m 2 g −1 ), which would induce the low capacitance achieved (see below). The plots of pore size distribution calculated by multiple Barrett-Joyner-Halenda (BJH) method are shown in Fig. S5b. FCMF has more mesopores than that of CMF in the pore diameter range of 2-10 nm. The average pore diameters are 11.71 and 8.79 nm for CMF and FCMF, respectively.
As we all known, the electrolyte is a very important factor for electrochemical capacitors. In this work, we used different electrolytes (such as KOH, Na 2 SO 4 , H 2 SO 4 , and TEMABF 4 /AN electrolytes) to prepare the devices for electrochemical testing. The CV curves of FCMF electrochemical capacitors with different electrolytes at the scan rate of 10 V s −1 are shown in Fig. 3a. When the electrolyte is acid, its integrated area of the CV curve is obviously larger than others. When the electrolyte is KOH, Na 2 SO 4 , H 2 SO 4 , and TEMABF 4 /AN, the voltage windows are 0.8, 1.0, 0.8, and 1.3 V respectively, and the CV curve shape is close to a rectangle, indicating a dominant electric double-layer capacitor (EDLC) charge storage mechanism. The frequency response of FCMF cells was first evaluated by the Nyquist plots of EIS. As shown in Fig. 3b, no semicircular region was observed in all the devices in the high-frequency region, indicating there was probably no passive layer between the FCMF electrodes and the current collector. In addition, the equivalent series resistance (ESR) of all the devices is less than 2 Ω, and the minimum ESR of cells with KOH electrolyte is 0.64 Ω. The above result shows that FCMF has a good conductivity. 20 Bode plot is one of the main electrochemical tests used to characterize the filtering of AC lines. In particular, whether the phase angle value approaches -90°at a frequency of 120 Hz can indirectly show the response ability of the capacitor in filtering applications. 8 The Bode plots of all the devices are shown in Fig. 3c. The phase angle of FCMF devices at 120 Hz with different electrolytes is −83.1°, −80.4°, −69.9°, and −79.2°, corresponding to KOH, Na 2 SO 4 , H 2 SO 4 , and TEMABF 4 /AN electrolytes, respectively. At the frequency of 120 Hz, the RC time constant (τ RC ) can be obtained as Eq. 4. This time constant indicates the time constant of the transition reaction of the capacitor within one filtering period, and its value should be less than one filtering period of 8.3 ms (1/120 Hz). 21 The τ RC of cells with KOH, Na 2 SO 4, H 2 SO 4 , and TEMABF 4 /AN electrolytes are 157, 221, 477, and 248 μs respectively, which are all much less than 8.3 ms. Figure 3d shows the specific areal capacitance varied with frequency. The specific areal capacitances (C A ) of the samples at 120 Hz are 10.1, 10.2, 24.6, to 3.9 μF cm −2 , corresponding to KOH, Na 2 SO 4 , H 2 SO 4 , and TEMABF 4 /AN electrolytes, respectively. These values are all more than 100 times lower than the values reported with the active carbon. The low C A values achieved in this work are mainly due to the low specific surface area and density of FCMF. 10 Figure 3e shows the imaginary part of the specific areal capacitance (C″) changing with the frequency, which is calculated by Eq. 5 from EIS. Its change trend is like that of C A with frequency. Besides, the relaxation times (τ 0 ) derived from the frequency at a maximum of C″ is also an important performance indicator for capcitors to AC line filtering, which represents the shortest time required to release all energy from the device at an efficiency greater than 50% of its maximum. 22 A smaller τ 0 indicates a better frequency response. The τ 0 of cells with KOH, Na 2 SO 4 , H 2 SO 4 , and TEMABF 4 /AN electrolytes are 46, 100, 178, and 46 μs, repectively. As shown in Fig. 3f, the ratio of C´/C A was used to reveal the ideality of the capacitor. Apart from the device with H 2 SO 4 electrolyte, the capacitors with other aquoeous and organic electrolytes deliver a C´/C A ratio value of ∼1 in the frequency range of 1-10 3 Hz. Moreover, dissipation factor (the ratio of the real and reactive power components, DF) is also plotted in Fig. 3f. Apart from the device with H 2 SO 4 electrolyte, other devices exhibte small DF in the low frequency range of 1-10 3 Hz, indicating the small loss characteristic. Considering the above performance, the KOH electrolyte will be used for future investigation. The AC filtering performance has been compared with the related supercapacitors, such as MFs-based supercapacitors (Table II), typical all-in-one electrochemical capacitors (Table SI), and typical electrochemical filter capacitors (Table III). The comparison result illustrates that the fabricated all-in-one devices possess a comparable/superior AC filtering performance. In addtion, the concentration effect study of KOH electrolyte indicates the phase angle at 120 Hz can't be affected significantly by varying the concentration from 0.1 M to 3 M (Fig. S7).
The thickness and carbonization time of MFs were optimized next. As shown in Figs. 4a-4c, the change in foam thickness from 0.5 cm to 1.75 cm has little effect on the electrochemical performance of the device. Only ESR has some visible changes, the FCMFs with a 1 cm thickness deliver the smallest ESR value. The optimization of FC time is shown in Figs. 4d-4f. The phase angle and C A at 120 Hz are highest when the carbonization time is 12 s. The FCMFs with 12 s flame carbonization exhibit a comparative ESR value. Therefore, we chose 12 s as the final condition for carbonization. As shown in Fig. S8, the CV tests at various scan rates were used to evaluate the electrode kinetics. The CV curves of FCMF devices with a thickness of 1 cm and KOH electrolyte from 1 V s −1 to 1000 V s −1 display an approximately symmetrical rectangle. The discharge current increases linearly with the scanning rate, indicating a surface-controlled electrode process. These further prove that FCMF devices have a good frequency response. After the stability test at a scan rate of 10 V s −1 , the capacitance retention of the device is 110% of the initial value after 100000 cycles, which reveals that the device has a reasonable stability (Fig. S9). In addition, the control device with MF as separator was assembled and measured the electrochemical performance. As shown in Fig. S10, the control device shows a lower C A , a higher phase angle at 120 Hz, and a larger ESR. The above results suggest the all-in-one device is more favorable for AC line filtering than the traditional laminated device.
To test the compression performance of the all-in-one devices, we used a syringe to connect six cells in series to evaluate their electrochemical performance. Photographs of this electrochemical capacitor with different compression ratios are shown in Fig. S11. Figure 5 shows the effect of compressibility on the electrochemical performance of FCMF devices. With the increase of compressive strain, the ESR value of the device first decreases and then increases. The minimum value of ESR is 1.3 Ω, when the compressive strain is 20% (Fig. 5a). It can be seen from the Bode plots that the frequency response of the device is less affected by the compression strain (Fig. 5b). With the increase of compressive strain, the C A of the samples at 120 Hz and at scan rate of 10 V s −1 first increases and then decreases (Figs. 5c and 5d). When the compressive strain is 20%, it reaches the maximum value, which is 45.22 μF cm −2 at 120 Hz and 89.1 μF cm −2 at scan rate of 10 V s −1 . The difference between these two values can be attributed to the different chargedischarge rates (8.3 ms vs 160 ms). The compressible cycling performance of FCMF devices was also studied. As shown in Fig.  S12, the results illustrate the good resilience ability of FCMFs during multiple measurements. Figure 6a shows the electrical circuit for AC line filtering and the conversion of AC voltage into DC voltage. Firstly, the AC sinusoidal signal is generated with a function generator, and then the AC sinusoidal signal is converted into the pulse signal with a double frequency by the rectifier, and finally, the pulse signal is converted into a smooth DC signal by the capacitor and output. Figure 6b shows the Bode plots of commercial AEC and six seriesconnected FCMF electrochemical capacitors. The phase angle of FCMF (−80.6°) at 120 Hz is lower than that of AEC (−76.1°). More importantly, the FCMF device shows an excellent frequency response in the high frequency range (e.g., 1000 Hz). Figure 6c shows the waveform diagram of converting AC signal (60 Hz) into DC signal through FCMF devices and AEC, which proves that the fabricated FCMF devices have excellent filtering performance. The filtering frequency range of the filter capacitor is also very important in practical applications. 23 Figure 6d shows the filtering performance of the FCMF device at 2000 Hz, indicating its filtering effect in a wide frequency range. Besides, the CV curves and charge-discharge profiles of three FCMF devices in both series and parallel configurations are shown in Fig. S13. The series and parallel devices obtained the rectangular CV curves and triangular charge-discharge plots. Crucially, the voltage and current variations in the series and parallel configurations correspond exactly to the single-cell characteristics. This demonstrates the repeatability for preparing FCMF all-in-one electrochemical capacitors is pretty good.

Conclusions
In conclusion, flame carbonization was developed for the preparation of MFs-based all-in-one compressible capacitors. The all-in-one compressible electrochemical capacitors were applied for AC line filtering. With an aqueous electrolyte of 3 M KOH, a low phase angle of −83.1°at 120 Hz, a low RC time constant of 157 μs, and a short relaxation time constant of 46 μs have been obtained. In practical AC line filtering measurements, this type supercapacitor can successfully convert the AC signal of 120 Hz and 2000 Hz into a stable DC signal. The strong alkaline electrolyte is favorable for FCMF all-in-one capacitors for AC line filtering. The compressive strain cannot significantly influence the performance of AC line filtering (e.g., the phase angle at 120 Hz). This work provides an ingenious design and a rapid and effective preparation strategy for MFs-based all-in-one supercapacitors, which also has a compressible advantage.