Enhancing the Capacitance of Battery-Type Hybrid Capacitors by

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Enhancing capacitance of battery-type hybrid capacitor by encapsulating MgO nanoparticles in porous carbon as reservoirs for OH- ions from electrolyte Junzheng Wang, Changlai Wang, Shipeng Gong, and Qianwang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05275 • Publication Date (Web): 29 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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ACS Applied Materials & Interfaces

Enhancing capacitance of battery-type hybrid capacitor by encapsulating MgO nanoparticles in porous carbon as reservoirs for OH- ions from electrolyte Junzheng Wanga, Changlai Wanga, Shipeng Gonga and Qianwang Chena, b* aDepartment

of Materials Science & Engineering, Hefei National Laboratory for Physical Science at Microscale, and Collaborative Innovation Centre of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei 230026, China bThe Anhui Key Laboratory of Condensed Mater Physics at Extreme Conditions, High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China * Correspondence to: [email protected]

Abstract A novel design of no-loading and bifunctional positive electrode, serving as active materials and current collector simultaneously, has been constructed by grass-like nickel foam which shows a battery-type performance and excellent areal specific capacity at 0.540 mAh∙cm-2 (over 4500 mF∙cm-2). To obtain a high-performance hybrid capacitor, layered porous carbonaceous composites C/MgO negative electrodes were fabricated, in which MgO nanoparticles serve as “reservoirs” for OH- ions from electrolyte. Compared with other carbon materials, such as carbon fibres, hollow nanospheres and nanotube, this three-dimensional (3D) hierarchical hetero-structures C/MgO electrode exhibits higher storage performance 424.1 mF∙cm-2. Assembled by these two working electrodes, hybrid capacitor with uncommon GCD cycling curve has been well investigated in alkaline aqueous electrolyte system. This as-coupled hybrid capacitor exhibits an engaging activation process during multiple cycling tests and leads to a drastically improved energy density by 60 % (from 80.4 to 128.8 μWh∙cm-2), which can be attributed to a “match behavior” between its positive and negative electrodes.

Keywords:

hybrid capacitor, Ni(OH)2, Nickel foam, no-loading and bifunctional design,

reservoirs, match behavior

1. Introduction Electrochemical capacitors (i.e. supercapacitors) have engaged increasing concerns over the past few decades due to their high power density (P), rapid recharge ability and long-cycling life. However, compared with lithium ion batteries (LIBs), supercapacitors exhibit lower energy density (E), because their electricity storages are based on the surface reactions at the electrode/electrolyte interfaces

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without ions diffusion into the bulk of active materials. Therefore, well electrochemical contact and efficient communication at the electrode/electrolyte interfaces is essential to obtain high-performance supercapacitors. Generally, supercapacitors can be mainly divided electrochemical double layer capacitors (EDLCs) and pseudocapacitors1-6. Based on their storage mechanisms, the popular electrode materials for EDLCs are carbon-based, while active materials that exhibit multiple reversible valences are the priority for pseudocapacitors7-8. Recently, a novel charge storage mechanism “intercalation pseudocapacitive behaviour” was proposed, in which its kinetics exhibit pseudocapacitive characteristics (i.e. a linearly proportional voltammetric response), while its electrode has a battery-type electrode behavior (i.e. redox reaction accompanied by the intercalation of cations in the crystalline framework of electrode materials)1. Nickel hydroxides (Ni(OH)2), a battery-type material, are wide-regarded as promising supercapacitor materials due to their low cost, high theoretical capacity, high surface-to-volume ratio, convenient transport channels for electrolyte ions and outstanding stability in alkaline electrolyte1,9-11. Thus, a great number of methods have been reported to fabricate Ni(OH)2 electrode, among which two approaches are adopted widely10. To be specific, one strategy is mixing Ni(OH)2 active materials with the binder agent (mostly poly vinyldifluoride) and conducting agent (mostly acetylene black and conductive carbon black) in organic solvent to form a homogeneous slurry which is then pressed onto a current collector (mostly nickel foam and carbon cloth). This type of Ni(OH)2 electrode fabrication process is complicated and hard to duplicate the exact ratio between active materials and assistant agents. Apart from that, a major defect of this method lies in the addition of the inactive binder which not only impedes the interaction between active materials and electrolyte, but also increases the ineffective loading amount. The other strategy is attaching ink of Ni(OH)2 active materials, diffused in organic solvent, onto a current collector. This method limits the mass loading and often results in an inhomogeneous distribution of active materials. In addition, mechanically depositing often leads to weak electrochemical connection between Ni(OH)2 active materials and its current collector, and finally accounts for the loss of electrochemistry performance. Based on above mentioned problems, a facile fabrication route to prepare Ni(OH)2 electrode with excellent performance is desired. Electrical energy density (E) accumulated in supercapacitors is proportional to specific capacitance and operating voltage window of a cell system. Combining two different behaviour electrode in an exact cell device, so-called asymmetric supercapacitors or hybrid capacitor, is an efficient strategy to enhance both the operating potential and specific capacitance12. Recently, to fabricate EDLCs behaviour electrodes, metal-organic frameworks (MOFs) have become appealing precursors to synthesize porous functional carbon materials, because calcination samples can well inherit the shape from parent MOFs by flexibly choosing the annealing conditions7, and many investigations have been reported in which tuning specific surface area (SSA) and controlling pore size and distribution in carbon materials are two prevalent headlines1. Indeed, specific capacitance and the energy density of these carbon systems are still lower than the expected level. For example, carbon-based microporous nanoplates13 with a ultrahigh SSA of 2557 m2∙g-1 gives a specific capacitance of 264 F∙g-1 at a quite low current density of 0.1 A∙g-1, and ordered mesoporous carbons14 with a pore diameter of 3.9 nm and SSA of 900 m2∙g-1 shows only 90 F∙g-1. Thus, feasible surface structure modification for carbon materials is necessary to increase their energy capacitance.


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In the present article, a novel no-loading and bifunctional strategy was explored by integrating the function of active materials and current collector. For the first time, battery-type material Ni(OH)2 with a grass-like microplates morphology grown on the surface of nickel foams (NFs) were prepared through a one-step ultrasonic treatment to commercial NFs (CNFs) in 3 M HCl aqueous solution without any presence of nickel salt (Scheme 1a-c). The acidized NFs (ANFs) loading no exotic substances, served as active materials by itself and current collector simultaneously without the necessity of either binder or conducting agents and were directly used as positive electrodes in hybrid capacitor in which carbon-based electrodes were designed as negative electrodes. In contrast to high SSA and ordered pore structure, layered porous carbonaceous composites C/MgO were prepared to make EDLC behaviour negative electrodes, in which MgO served as a functional component due to its low coordinated surface and sufficient active sites for generating a great adsorption capacitance on the surface15. To be specific, 3D hierarchical hetero-structures Mg-1,4-BDC MOF precursor was selected and grown on ANF-0.5 (acidized for 0.5 h) through a facile hydrothermal process and then annealed in nitrogen atmosphere directly with a slow heating rate (Scheme 1d-f).

Scheme 1. Schematic illustration for the fabrication of ANF positive electrode (a-c) and C/MgO negative electrode (d-f). (a) CNFs were immersed in 3 M HCl with ultrasonic treatment. (b) After acidification and dried, nickel foam samples were covered by yellow substance. (c) ANF-0.5 positive electrode was obtained after eliminating the yellow substance. (d,e) Mg-1,4-BDC was prepared on the surface of ANF-0.5. (f) C/MgO negative electrode was fabricated after annealing.

2. Experimental Section 2.1 Fabrication for Electrodes Acidized nickel foams (ANFs) electrodes Acidized nickel foams (ANFs) was fabricated through a simple acidification process. In particular, commercial nickel foams (CNFs) was prepared in a size of 22 cm2 (Fig. S1a). Before acidized with 3 M HCl through ultrasonic treatment for different time (including 0.5, 1.0, 1.5, 2.0 and 2.5 h), CNFs should be cleaned with acetone and ethanol for 10 min subsequently. Then, the ANFs were directly dried at 60 °C overnight. Finally, deionized water and ethanol was used to eliminate the yellow substance covering on the surface of nickel foam (Fig. S1b), and the obtained ANFs were dried



under ambient environment overnight. These samples were named as ANF-0.5 (Fig. S1c)/-1.0/-1.5/-2.0/-2.5, respectively, and were used as positive electrodes directly. The yellow substance transformed to a green one after washed with deionized water and ethanol and dried in 60 °C overnight (Fig. S1f). C/MgO-500A/700A/900A electrodes All the chemicals were analytical grade and commercially available from Shanghai Chemical Reagent Co. Ltd and used as received without further purification. C/MgO was fabricated through in situ hydrothermal followed with calcination. First, Mg-1,4-BDC MOF grown on the surface of ANF-0.5 was prepared. To be specific, 0.33 g (2.0 mmol) of benzene-1,4-dicarboxylic acid (BDC) (99.0 %) was dissolved in 20 mL of dimethylformamide (DMF) and 2.5 g polyvinyl pyrrolidone (PVP, K-30) was added to the BDC solution. Another solution was prepared by dissolving 0.6 g (4.0 mmol) of Mg(NO3)2∙6 H2O (99.0 %) in a 20 mL of DMF. Then, these two solutions were mixed dropwise under continuous stirring and kept for 5 min. The resulting mixture was transferred into a 50mL Teflon autoclave containing four pieces of ANF-0.5 in advance and heated at 160 °C for 48 h. After cooling to room temperature, the nickel foam was picked out and washed repeatedly with DMF to remove the excess PVP. After dried at 60 °C overnight, the prepared ANF-0.5 covered with Mg-1,4-BDC MOF (Fig. S1d) was carbonized under different temperatures (including 500, 700 and 900 °C) for 2 h in N2 atmosphere, at a slow heated rate of 3 °C∙min-1. The obtained samples were named as C/MgO-500A, C/MgO-700A and C/MgO-900A (Fig. S1e), respectively. C/MgO-900C electrode To fabricate C/MgO-900C, the same procedure of C/MgO-900A was followed except for using the CNFs in place of ANFs.

2.2 Material Characterization The powder X-ray diffraction (XRD) patterns of the samples were collected from a Japan Rigaku D/MAX-γA X-ray diffractometer equipped with Cu Kα radiation. All the XRD patterns, except for ANF-0.5, were determined using the powder samples collected from corresponding nickel foams. Field emission scanning electron microscopy (FESEM) images were recorded on a JEOLJSM-6700M scanning electron microscope. All the SEM images were determined using corresponding nickel foams rather than the powder samples. Energy dispersive X-ray spectra (EDX) as well as elemental mapping images were obtained on a Mira3 field-emission scanning electron microscope (JEOL JSM-7800F) equipped with an Oxford energy dispersive spectrometer. Transmission electron microscopy (TEM) images were recorded with a Hitachi H-7650transmission electron microscope using an accelerating voltage of 200 kV, and a high resolution transmission electron microscope (HRTEM) (JEOL-2011) was operated at an acceleration voltage of 200 kV. All the TEM and HRTEM images were recorded using powder sample homogeneously diffused in ethanol. Raman spectra were obtained using a LabRAM HR Raman spectrometer. X-ray photoelectron spectroscopy (XPS) was performed on a VGESCALAB MKII X-ray photoelectron spectrometer using an Al Kα excitation source. All the XPS spectra, except for ANF-0.5, were determined using powder samples collected from corresponding nickel foams. The specific surface area was evaluated at 77 K (Micromeritics ASAP 2020) using the Brunauer-Emmett-Teller(BET) method, while the pore volume and pore size


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were calculated according to the Barrett-Joyner-Halenda (BJH) formula applied to the adsorption branch.

2.3 Electrochemical Measurements Electrochemical measurements were carried out in three-electrode and two-electrode system containing an electrolyte of aqueous 0.5 M KOH at room temperature. ANF-0.5 and C/MgO were pressed under a pressure of 10MPa to directly serve as working electrodes, with a piece of nickel belt as the lead wire connected to electrochemical workstation CHI660D (Shanghai, China). In view of the phenomenon of “electro-activation”, all the working electrodes were activated in advance through multi-CV (cyclic voltammogram) tests until exhibiting stable CV curves (Fig. S10). For the three-electrode system, Ag/AgCl reference electrode assembly and platinum wire were used as the reference and counter electrodes, respectively. In this manuscript, all the potentials or voltages were versus Ag/AgCl. The CV test was measured at different scan rates, and the galvanostatic charge/discharge (GCD) tests were performed at different current densities, within a potential range from 0 to 0.43 V for ANF-0.5 and -1.0 to 0 V for C/MgO, respectively. The electrochemical impedance spectroscopy (EIS) measurements were carried out in a frequency range from 0.01 Hz to 100 kHz at an open circuit potential with an AC perturbation of 10mV. In the two-electrode measurements, CV and GCD tests of a hybrid capacitor, assembled by a positive electrode of ANF-0.5 connected to the working electrode clamp and a negative electrode of C/MgO connected with the counter and reference electrode clamps, were operated in a voltage range from 0 to 1.5 V. The cyclic performance was evaluated by GCD measurements of the ANF-0.5// C/MgO-900A and ANF-0.5//C/MgO-900C device at a current density of 2 mA∙cm-2 for 10000 cycles, which was conducted on a Neware CT-3008-5V10mA-164 type battery charger (Shenzhen, China).

3. Results and Discussion 3.1 Composition, Morphology and Microstructures Characterization The ANFs positive electrodes were fabricated through a facile acidized treatment toward CNFs in 3 M HCl. The digital photographs (Fig. S1a-c) show the obvious changes in NFs appearances after the acidification process. Compared with CNFs (Fig. S1a), the 3D crisscrossed architectures were maintained well in ANFs; however, the colour of the nickel foam sample transformed from pristine silver to slight brown after the acidification treatment (Fig. S1c), indicating new moieties may come into being on the CNFs. Nevertheless, from the Xray diffraction (XRD) pattern of as-prepared ANF-0.5 electrode (Fig. S2), no new diffraction peak can be found except for five characteristic peaks assigned to (111), (200), (220), (311) and (222) of Ni metal (JCPDS card no. 65-0380) with face-centred cubic (fcc) structure. In view of the deeper phase determination of XRD and massive Ni metal in ANF-0.5 bulk phase, more-detailed chemical composition of as-prepared ANF-0.5 electrode surface was detected using X-ray photoelectron spectroscopy (XPS), and corresponding results are presented in Fig. 1a-b. The Ni 2p high-resolution spectrum shows two major peaks at 856.4 eV and 874.1 eV (with a spin energy separation of 17.7 eV) corresponding to Ni 2p3/2 and Ni 2p1/2, respectively, along with the typical satellite peaks at 862.0 eV and 880.1 eV, which are characteristics of Ni(OH)2 and match well with previous reports16. The O 1s XPS spectrum (Fig. 1b) can be divided



into two peaks at around 531 and 532 eV, corresponding to the oxygen of hydroxide groups and the physically adsorbed water molecules, respectively17-18. Hence, the ANFs surface phase is confirmed as Ni(OH)2 after facile acidized treatment to CNFs. Additionally, from XPS survey spectrum (Fig. S3a)10, excess O 1s (At %, 53.98) compared with Ni 2p (At %, 18.6), C 1s (At %, 23.25) and a small amount of Cl 2p (At %, 4.17) was determined, which can be ascribed to contamination from air when storage and remained Cl- from HCl, respectively. Moreover, HRTEM image of ANF-0.5 was shown in Fig. S3b to further prove the generation of Ni(OH)2 after acidification treatment towards CNFs, in which lattice fringes of 4.62 and 2.72 Å can be assigned to (001) and (100) planes of Ni(OH)2 (JCPDS card no. 14-0117), respectively. From the mentioned characterization results, the fabrication process of Ni(OH)2 on the surface of ANF-0.5 electrode can be estimated as following forming reaction: Ni (s) + HCl (aq)

Ni2+ (aq)

H2O

Ni(OH)2 (s)

CNFs here served as the Ni element source for the growth of Ni(OH)2. The formation of Ni(OH)2 after the acidification treatment to CNFs in 3 M HCl aqueous solution was mainly due to the strong corrosion from HCl under high-frequency ultrasonic along with adequate attachment to deionized water when obtained ANF-0.5 electrode was washed repeatedly. The carbon-based C/MgO negative electrodes were prepared through hydrothermal reaction followed by calcination (hydrothermal-calcination strategy). To be specific, Mg-1,4-BDC MOF was directly grown on the surface of ANF-0.5 using hydrothermal reaction and white nickel foam samples were obtained (Fig. S1d). Then, these as-prepared nickel foam samples were annealed in nitrogen atmosphere under 500, 700 and 900°C with a slow heated rate of 3 °C∙min-1 to prepare corresponding C/MgO-500A/700A/900A electrodes. The XRD pattern of Mg-1,4-BDC MOF exhibits sharp diffraction peaks, indicating a well-defined crystal structure (Fig. S4), and the major peak positions match well with the previously reported data19, which demonstrates the successful synthesis of Mg-1,4-BDC MOF. Fig. 1c shows the XRD patterns for carbon materials derived from MOF precursors under different annealing temperature from 500 to 900 °C in nitrogen atmosphere, named as C/MgO-500A/700A/900A, respectively. A broad hump at 2θ from 20 to 30° can be found in all of the patterns, indicating the presence of carbon materials. Additionally, five obvious peaks at 2θ of 36.9, 42.9, 62.2, 74.6 and 78.6° can be indexed to the cubic phase of MgO (JCPDS card no. 45-0946). It is clear that the oxygen atom came from the carboxylic group of organic ligands. Because carbon materials were obtained through synthesis of Mg-1,4-BDC MOF on the surface of ANF-0.5, followed by high temperature calcination, typical (111), (200) and (220) lattice planes of Ni metal were detected in C/MgO-700A/900A XRD patterns. No other impure phase was determined, which demonstrates the successful synthesis of C/MgO composites. According to the XPS survey spectrum of C/MgO-900A (Fig. S5a), C 1s, along with Mg 2p, O 1s and a slight amount of Ni 2p were determined, which is in well line with the results of XRD patterns. In high-resolution spectra, the binding energy of Mg 1s signal at 1304.0 eV and Mg 2p signal at 49.9 eV are in good agreement with the tabulated values for MgO (Fig. S5b). The oxygen O 1s peak can be resolved into three main components (Fig. S5c). Signal at 530.4 eV is in accordance with the tabulated values for MgO. The other major component at 532.6 eV can be identified as hydroxide species due to the inevitable exposures to atmosphere when storage20-22. A minor peak at 530.9 eV is assigned to Ni(OH)2, which


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matches well with the Ni 2p high-resolution spectrum in C/MgO-900A (Fig. S5e). Deconvolution of C 1s XPS spectrum indicates the presence of four major surface functional groups, sp3 C-C bond, sp2 C=C bond, CH defect and π-π* bond (Fig. S5d)23. Atomic percentage of Mg, O, C and Ni element determined from XPS are estimated to be 8.55, 15.52, 75.23 and 0.69 %, respectively (Fig. S5a), and these elements proportions are similar to the results from Energy dispersive X-ray spectra (EDX) (Fig. S5f). Element mapping images reveal the uniform distribution of Mg, O, C and Ni elements in C/MgO-900A (Fig. S6). Compared with Mg and O, C element exhibited the heaviest distribution, while Ni element showed the thinnest dispersion, which is in good line with the results of XPS and EDX. Raman spectroscopy is a specific and non-destructive method to observe the ordered and disordered crystal structures of carbon materials. A characteristic property of graphitic layers is G band, which results from the tangential vibration of the carbon atoms, whereas the disordered carbon or defective graphitic structures can be assigned to D band24. As shown in Fig. 1d, C/MgO-900A exhibit a higher ratio of D band to G band intensity (ID/IG), 0.964, while C/MgO-500A/700A show a noticeably lower ratio of 0.817 and 0.939, revealing more structural defects/disorder in C/MgO-900A. To obtain the detailed surface structure information of these carbon materials, Fourier transforms infrared (FTIR) spectra and Brunauer−Emmett−Teller (BET) analysis were performed. Fig. 1e displays the surface groups variations with the calcination temperatures elevated. Apparently, compared with Mg-1,4-BDC precursor, surface groups change drastically under 700 and 900°C calcination, whereas C/MgO-500A still maintains the same surface groups. BET SSA of C/MgO-500A/700A/900A are 18.8, 287.1 and 314.4 m2·g−1, respectively (Fig. S7 and Fig.1f), indicating larger porous structure comes into being under 900 °C calcination and therefore more structural defects/disorder, which matches well with the result of Raman spectra. The corresponding pore-size distribution analysis of C/MgO-900A (inset, Fig.1f) clearly reveals that a great number of nanopores existed in the carbon materials. Most of the pores lie in the range from 2 to 6 nm in diameter and large enough for free diffusion of hydroxyl ion (OH-) and potassium ion (K+), possessing diameters of 0.220 and 0.276 nm, respectively.



Figure 1. XPS spectra for Ni 2p (a) and O 1s (b) in ANF-0.5. XRD patterns (c), Raman spectroscopy (d) and FTIR spectra (e) of C/MgO-500A/700A/900A. (f) Nitrogen adsorption and desorption isotherms and pore size distribution curves for C/MgO-900A.

Field emission scanning electron microscopy (FESEM) and transmission electron microscopic (TEM) images of as-prepared ANF-0.5 and C/MgO-500A/700A/900A electrodes was listed in Fig.2 and Fig. S8. SEM image of ANF-0.5 (Fig. 2a) at low magnification displays an absolutely different morphology in comparison with CNFs (Fig. S8a,b), and the higher magnification image (Fig. 2b)


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clearly demonstrates the homogeneous grass-like microplates morphology resulting from acidification treatment with 3 M HCl solution. SEM images in Fig. S8c,d show the skeletons of ANF-0.5 totally covered by Mg-1,4-BDC precursor and C/MgO-900A, respectively, revealing the successful and feasible hydrothermal-calcination strategy. Fig. 2c,d displays the 3D hierarchical hetero-structures of Mg-1,4-BDC MOF. Noticeably, the layered structure of precursor was well inherited by C/MgO-500A/700A/900A (Fig. S8e,f and Fig. 2e,f), which can be ascribed to the slow heated rate7. Numerous little nanoparticles embedded on the surface, edge and interval of the layered carbon materials were MgO, which can also be observed from TEM image of C/MgO-900A in Fig. 2g. In addition, layered carbon sheets are clearly displayed in TEM image. Fig. 2h shows the high-resolution transmission electron microscopic (HRTEM) image of C/MgO-900A. The distance between adjacent lattice fringes is 2.11 Å, which matches well with the major (200) planes of MgO phase in XRD patterns. In addition, from HRTEM images, massive carbon materials in grey region surrounding the MgO lattice can be discovered clearly (Fig. S8g), further suggesting the pure phase of C/MgO electrode has been successfully prepared through hydrothermal reaction followed by a high temperature calcination treatment to ANF-0.5. Polycrystalline or amorphous feature of C/MgO-900A was further determined by selected area electron diffraction (SAED) ring-patterns (Fig. S8h), which is consistent with the result from Raman studies.



Figure 2. SEM images of (a,b) ANF-0.5, (c,d) Mg-1,4-BDC MOF and (e,f) C/MgO-900A. Inset in (b) clearly displays the well-connected interface between grass microplates and NFs. TEM (g) and HRTEM (h) images of C/MgO-900A.


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3.2 Electrochemical Performance Investigation The electrochemical performances of the as-prepared ANF-0.5 and C/MgO-900A as hybrid capacitor electrodes were measured using three-electrode configurations in 0.5 M KOH aqueous solution. Considering the phenomenon of “electro-activation”25, all the working electrodes were activated in advance through multi-CV (cyclic voltammogram) tests until stable CV curves were obtained (Fig. S9). Fig. 3a shows the CV curves of CNF without any treatment, cleaned CNF using acetone and ethanol and ANF-0.5 at a scan rate of 20 mV∙s-1 in a wide potential range from -0.5 to 1.0 V (vs. Ag/AgCl). CNF and cleaned CNF show small and similar CV curves, whereas the ANF-0.5 electrode demonstrates a dramatically improved integral area and a pair of clear redox peaks of CV curve. On top of that, the pair of redox peaks of ANF-0.5 has a large shift compared to those of CNF and cleaned CNF, which suggests electrical active materials were obtained after an acidification treatment. All these results imply excellent battery-type storage behaviour of ANF-0.5 electrode due to following reversible electrochemical reaction between Ni(II) and Ni(III)10: Ni(OH)2 + OH-

NiO(OH) + H2O + e-

Fig. S10a shows the CV curves of ANF-0.5 at various scan rates from 3 to 20 mV∙s-1. Noticeably, upon improving the scan rate of CV, the current increase, and the positions of the anodic peaks shift to higher potentials, whereas the cathodic peaks oppositely shift to lower potentials, which can be simply ascribed to the expected increase of the internal diffusion resistance within the active material. However, no significant distortion of the CV curves at high scan rates can be found compared to the shapes at low scan rates, indicating rapid, excellent charge transport and small equivalent series resistance within the ANF-0.5 electrode26. Galvanostatic charge/discharge (GCD) tests were operated to value the specific capacity of ANF-0.5 electrode. Fig. 3b shows the GCD curves for CNF without any treatment, cleaned CNF using acetone and ethanol and ANF-0.5 at a current density of 1 mA∙cm-2 in potential range from 0 to 0.43 V (vs. Ag/AgCl). The obvious charge/discharge platforms demonstrate the representative battery-type characteristics of ANF-0.5 electrode either. Corresponded with the CV curves, ANF-0.5 exhibits an excellent areal specific capacity of 0.540 mAh∙cm-2 (equal to a specific capacitance of 4540.0 mF∙cm-2), while the CNF and cleaned CNF hit only 0.005 mAh∙cm-2 (42.3 mF∙cm-2) and 0.007 mAh∙cm-2 (57.4 mF∙cm-2), respectively. Thus, the acidification treatment toward CNFs gives rise to the excellent energy storage performance of this Ni(OH)2 no-loading electrode. Compared with previously reported Ni(OH)2 electrodes prepared through depositing active materials onto current collectors, such as Ni(OH)2/CNT/NF (0.7 F∙cm-2)27, Ni(OH)2/Graphene (0.9 F∙cm-2 )28, Ni(OH)2 grains/NF (1.6 F∙cm-2)29 and Ni(OH)2 film/NF (4.2 F∙cm-2)30, our bifunctional ANF-0.5 electrode shows higher specific capacity, indicating the advantage of the no-loading and bifunctional electrode fabrication method. The excellent specific capacity can be ascribed to the inherent 3D porous structures of NFs and the numerous emerging grass-like Ni(OH)2 microplates, providing this no-loading electrode with huge active surface area, which therefore enlarges the electrode/electrolyte contact area, shortens the diffusion path of electrolyte ions, and consequently boosts the charge storage activity of the ANF-0.5 electrode10. Fig. S10b shows the GCD curves of ANF-0.5 at various current densities of 2, 4, 6, 8 and 10 mV∙cm-2, and the specific capacities are 0.420, 0.331, 0.272, 0.212 and 0.165 mAh∙cm-2 (Fig. S10d), respectively. Confirming the foregoing CV results, the specific capacity decreases while improving current density, suggesting the excellent



electrochemical performance of as-prepared ANF-0.5 electrode. Slight iR drop (~ 7.6 mV) in GCD curves suggests the rapid I-V response, excellent electrochemical reversibility and low electrochemical impedance of ANF-0.5 electrode31. Nyquist ploys were obtained by electrochemical impedance spectroscopy (EIS) measurements in a three-electrode system. The intercept of Nyquist plots at the real axis (Z’) stands for the value of equivalent series resistance (Rs), which is the sum of the internal resistance of grass-like Ni(OH)2 and ionic resistance of the KOH electrolyte. The semicircle loop at high frequency stands for Ni(OH)2/KOH interfaces charge transfer resistance (Rct), and the straight line in the low-frequency range stands for the diffusion impediment of KOH electrolyte ions toward ANF-0.5 electrode32. Fig. S10c shows the quite small Rs (~ 0.68 Ω) and Rct (~ 0.70 Ω), indicating the no-loading and bifunctional design provides the no-loading electrode with low electrochemical resistance. To investigate the contribution of acidification time, CNFs were acidized for different times, including 0.5, 1.0, 1.5, 2.0, and 2.5 h, the capabilities were calculated from GCD curves and listed in Fig. S11, which suggests that all the obtained ANF electrodes have no obvious improvement in specific capacity and rate capability. Thus, prolonging the acidification time cannot introduce enhanced electrochemical performance into ANF electrodes, therefore ANF-0.5 electrode was chosen for further experiments.

Figure 3. CV at 20 mV∙s-1 (a) and GCD at 1 mA∙cm-2 (b) of CNF, Cleaned CNF and ANF-0.5 in three electrodes system. Compared CV at 100 mV∙s-1 (c) and GCD at 1 mA∙cm-2 (d) among C/MgO-900A/700A/500A/900C in three electrodes system.

Different negative electrodes C/MgO-500A/700A/900A were obtained by annealing Mg-1,4-BDC MOF precursor grown on the surface of ANF-0.5 under 500, 700 and 900 °C,


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respectively. Fig. 3c,d compare the CV (100 mV∙s-1) and GCD (1 mA∙cm-2) curves for these C/MgO electrodes in a negative potential range from -1.0 to 0 V (vs. Ag/AgCl). The rectangular CV curves and triangle type GCD curves suggest the representative EDLCs storage behavior of C/MgO electrodes, which suggests the ANF-0.5 electrodes were successfully covered by carbon materials. Noticeably, CV curve of C/MgO-900A electrode shows a higher integral area in comparison with C/MgO-500A and C/MgO-700A, which indicates the more outstanding electrochemical performance obtained from higher temperature carbonization. Corresponded to CV results, longer charge/discharge time and higher specific capacitance (424.1 mF∙cm-2) of C/MgO-900A electrode can be found compared with C/MgO-500A/700A, which are 82.5 and 350.3 mF∙cm-2, respectively. Table S1 compares the reported electrochemical performance and morphology of carbon-based electrodes, and C/MgO-900A with 3D hierarchical hetero-structures shows outstanding specific capacitance which can be ascribed to its high surface/body ratios, large surface areas, better permeability and more surface active sites33. FTIR spectrophotometer was employed to further diagnose the origin of high specific capacitance from C/MgO-900A, and the signals recorded in Fig. 4a are the surface functional groups of C/MgO-900A in different stages, including pristine carbon materials and those after charge and discharge process. In the pristine one, the wide absorption band at 3430 cm-1 is attributed to the stretching vibration of water or the presence of -OH groups on the carbon materials, and the absorption peak observed around 1500 cm-1 are associated with the C=C stretching and C-H bending34. In addition, absorption band at 440 and 593 cm-1 can be assigned to the characteristic stretching vibration of Mg-O35. After electrochemical charge/discharge test, a new peak assigned to -OH anti-symmetric stretching vibration can be observed at 3691 cm-1. According to previous literatures: (1) -OH anti-symmetric stretching vibration was not recorded in the FTIR spectra of MgO@C, whereas it could be found in Mg(OH)2@C35; (2) the surface of MgO is enriched with low coordinated ions, acidic Mg2+ ions, basic oxygen anion, cationic magnesium vacancy and anionic oxygen vacancies, which are active sites for adsorption15, we assume that the newly emerging -OH anti-symmetric stretching vibration peaks come from the OH- electrolyte ions enclosed to MgO nanoparticles. Apart from the new -OH anti-symmetric stretching vibration after electrochemical charge/discharge test, enhanced Mg-O peaks can also be found in the FTIR spectra, which might result from the foregoing assumption. Specifically, after MgO nanoparticles adsorb OH- electrolyte ions, the high electronegativity of O atom in OH- might enforce the dipole moment variation of Mg-O bond, which ultimately causes the sharper signals. Based on the FTIR analysis, a possible energy store model in which MgO nanoparticles serve as “reservoirs” for OH- was suggested to explain the excellent specific capacitance of C/MgO-900A. Once C/MgO-900A electrode is immersed into KOH electrolyte, both carbon and MgO nanoparticles can adsorb OH- ions (Fig. 4b(i)). During charge process, besides KOH electrolyte, OH- ions on the surface of MgO serve as another OH- source and can flow to porous carbon because of EDLC behaviour (Fig. 4b(ii)). In the following discharge process, adsorbed OH- ions on porous carbon surface can migrate to adjacent MgO nanoparticles and be stored there temporarily for next charge process (Fig. 4b(iii)). According to this assumption, carbon materials with large BET SSA and more structural defects/disorder would lead to the fast and easy transportation of OH- ions both from electrolyte and MgO nanoparticles. In consistant with our experiment results of Raman (Fig. 1d) and BET analysis (Fig. 1f and Fig. S6), C/MgO-900A with



higher SSA and ID/IG ratio gives better specific capacitance. To further confirm this model, C-900 powder was prepared from C/MgO-900 powder (not C/MgO-900A electrode), in which MgO nanoparticles were eliminated completely using 3 M HCl, revealed by SEM and XRD (Fig. S12a-d) analysis. The reason why C/MgO-900 powder was chosen rather than C/MgO-900A electrode was to expel the intervention of ANF-0.5 and to specially justify the function of MgO “reservoirs”. Then, both C/MgO-900 and C-900 ethanol ink were decorated onto CNFs to fabricate electrodes with the same mass loading. It’s apparent that C/MgO-900 shows better electrochemical performance than that of C-900 (Fig. S12e,f), which further confirms the function of MgO in C/MgO-900A electrode. Thus, in addition to the classical EDLC behaviour from porous carbon, numerous MgO nanoparticles embedded in carbon materials can provide the latter with “reservoirs” for OH- ions from electrolyte during charge/discharge process, which eventually results in an excellent electrochemical performance. Fig. S13a,b show the CV tests at different scan rates, GCD curves at various areal current densities for C/MgO-900A electrode. No obvious distortion in CV shapes at high scan rates and almost symmetric GCD curves suggest the high-rate and well reversible adsorption and accumulation of KOH electrolyte ions toward electrode surface. Apart from that, good rate capability and high coulombic efficiency of C/MgO-900A electrode can be found in Fig. S13c. Specifically, specific capacitance remains up to 279.6 mF∙cm-2 (65.2 %) even at a large areal current density of 10 mA∙cm-2 and coulombic efficiencies at all current densities hit over 90 %. Moreover, quite low Rs (0.73 Ω), Rct (0.14 Ω) and KOH electrolyte ions diffusion resistance of C/MgO-900A electrode are demonstrated in Fig. S13d. Hydrothermal-calcination strategy, 3D hierarchical hetero-structures and MgO nanoparticles “reservoirs” embedded in carbon materials, which established the strong electrochemical communication among active carbon materials, current collector and electrolyte, account for the excellent specific capacitance, low electrochemical impedance, good rate capability and high coulombic efficiency of C/MgO-900A electrode.

Figure 4. FTIR spectra of pristine, charged and discharged C/MgO-900A (a), and assumed MgO “reservoirs” energy storage model of C/MgO-900A (b).

To clarify the contribution of ANF-0.5 current collector, C/MgO-900C electrode was prepared using the same fabrication strategy except that ANF-0.5 was replaced by CNFs. C/MgO-900C electrode shows similar but smaller CV curve and therefore lower specific capacitance of 365.7


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mF∙cm-2 (Fig. 3c,d), indicating ANF-0.5 current collector accounts for roughly 13.8 % contribution to C/MgO-900A electrode, which could result from the wide working potential of ANF-0.5, leading to an overlapped region from -0.5 to 0 V (vs. Ag/AgCl) in CV curves between carbon materials and ANF-0.5 (Fig. S14a). In other words, ANF-0.5 current collector may also involve into energy storage in virtue of its remarkable integral area between -0.5 and 0 V (vs. Ag/AgCl) in CV curve. On the contrary, the overlapped region cannot be found in CV curves of C/MgO-900C and CNFs current collector (Fig. S14b). ANF-0.5//C/MgO-900A hybrid capacitor was assembled in 0.5 M KOH aqueous electrolyte using ANF-0.5 as positive electrode and C/MgO-900A as negative electrode to further measure the practical application of as-prepared electrodes. Fig. S15a records CV curves of ANF-0.5//C/MgO-900A at different operating voltage windows ranging from 1.0 to 1.5 V at a scan rate of 100 mV∙s-1, and only a little oxygen evolution was observed during the CV measurements, suggesting the hybrid capacitor device can function at a wide voltage range from 0 to 1.5 V26. Fig. S15b shows the CV curves of the hybrid capacitor with various scan rates from 20 to 100 mV∙s-1. Clearly, all the shapes of the CV curves are well preserved, implying the excellent rate capability of the device. However, the CV curves show a small integral area below 0.5 V and a huge one beyond 0.5 V, suggesting an obvious “under-matching” phenomenon exist between ANF-0.5 and C/MgO-900A electrodes, which can also be justified by the huge gap of their storage property (over 4000 mF∙cm-2 at 1 mA∙cm-2) in three electrodes system. GCD curves are demonstrated in Fig. S15c at a series of current densities of 1, 2, 4, 6, 8 and 10 mA∙cm-2, and the calculated specific capacitance of the devices are 257.4, 213.2, 175.6, 153.5, 139.2 and 130.1 mF∙cm-2, respectively (Fig. S15d). Noticeably, all these GCD curves show to be almost vertical below 0.5 V, suggesting the potential window from 0 to 0.5 V has little contribute to the energy storage of ANF-0.5//C/MgO-900A, which matches well with the outcomes of foregoing CV tests. This no-function region is common in such device20, and absolutely hampers the well collaboration between battery-type positive electrodes and EDLCs behaviour negative electrodes. Fortunately, an uncommon activation process of as-assembled ANF-0.5//C/MgO-900A was detected during 10000 GCD cycling tests at a current density of 2 mA∙cm-2. As shown in Fig. 5a, in the initial around 3000 cycles, a significant improvement of specific capacitance can be observed from 138 up to 323 mF∙cm-2. In previous literature, Pan et al. attributed this tendency to the activation of the electrode materials through a slow infiltration of the electrolyte into gaps among Ni(OH)2 platelets during the cycling process5. However, all our working electrodes were activated in advance through multi-CV tests, so there must be other reason accounting for the improved specific capacitance. In the following cycles, specific capacitance of the hybrid capacitor shows a slight drop and finally arrives at 280 mF∙cm-2 which is over twice of the initial value, suggesting an activation process is triggered during the multi-cycling tests. Meanwhile, ultrahigh coulombic efficiencies (almost 100 %) were detected during the whole cycling tests. Well-developed CV and GCD curves of device were recorded after 10000 GCD cycling tests (Fig. 5b,c and Fig. S16), and no functional region below 0.5 V completely disappears, which therefore brings about an increased specific capacitance of the hybrid capacitor from initial 257.4 to 412.3 mF∙cm-2 at 1 mA∙cm-2. Apart from that, widened operating voltage window can be clearly found in GCD curves of the initial and last 10 cycles (insets in Fig. 5a). All these transformations of CV and GCD curves



demonstrate that a “match behaviour” took place between positive and negative electrodes during the multi-cycling tests to compensate for the initial “under-matching” phenomenon, and finally leads to the activation process of the whole device. Moreover, the improvement of energy density from 80.4 μWh∙cm-2 to 128.8 μWh∙cm-2 can also be observed in Rogan plots (Fig. 5d). Deserved to point out, with the improvement of energy density, no cost of power density (still up to 7.5 mW∙cm-2) can be found in the hybrid capacitor. Moreover, no significantly increased electrochemical impediments were determined after 10000 GCD cycling tests, and low and almost similar EIS curves are displayed in Fig. 5e, in which the Rs and Rct are 1.5 Ω and 0.75 Ω, respectively. Understandably, the small electrochemical impediment of ANF-0.5//C/MgO-900A can be ascribed to the low Rs and Rct of both positive and negative working electrodes, which further illustrates the advantage of no-loading and bifunctional design and hydrothermal-calcination strategy. Electrochemical data of supercapacitor devices related with carbon materials in previous reports is listed in the Table S2, in which our ANF-0.5//C/MgO-900A hybrid capacitor shows comparable operating voltage, distinct specific capacitance and excellent energy density.


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Figure 5. (a) Cycling performance of the ANF-0.5//C/MgO-900A hybrid capacitor collected at an areal current density of 2 mA∙cm-2 for 10000 cycles. (Inset: GCD curves of initial and last 10 cycles). CV at 100 mV∙s-1 (b), GCD at 1 mA∙cm-2 (c), Rogan plots (d) and EIS spectra (e) of ANF-0.5//C/MgO-900A hybrid capacitor before and after 10000 GCD cycles.

There are two possible reasons resulting in the “match behavior” between ANF-0.5 positive electrode and C/MgO-900A negative electrode: (1) ANF-0.5 current collector in negative electrode because of its foregoing contribution in energy storage; (2) some transformation takes place in electrode materials. To clarify the exact origin, ANF-0.5//C/MgO-900C was assembled, in which the C/MgO-900C served as negative electrode. Corresponding to the results in three electrodes



configuration (Fig. 3c,d), ANF-0.5//C/MgO-900C hybrid capacitor shows smaller CV curve and lower specific capacitance (140.2 mF∙cm-2) than ANF-0.5//C/MgO-900A (Fig. S17a,b), which further confirms the contribution of ANF-0.5 current collector. After 10000 GCD cycling tests, just same as ANF-0.5//C/MgO-900A, well-developed CV and GCD curves (Fig. S17c,d), improved specific capacitance (Fig. S18) and apparently widened operating potential window in GCD curves (insets in Fig. S18) can also be observed in ANF-0.5//C/MgO-900C, suggesting the ANF-0.5 current collector in negative electrode is not the reason giving rise to the “match behavior”. Compared with the morphologies of working electrodes before 10000 GCD cycling tests, the surface of ANF-0.5 transfers into well-combined 3D sheet networks from grass-like microplates and the C/MgO-900A still maintains the initial layered structures (Fig. S19). Thus, we assume the transformation of ANF-0.5 positive electrode leads to the “match behavior”. To justify this assumption, two specific hybrid capacitors were fabricated: (1) assembled by a newly prepared ANF-0.5 positive electrode and a C/MgO-900A negative electrode going through 10000 GCD cycling test, named as NANF-0.5//C/MgO-900A; (2) assembled by a newly prepared C/MgO-900A negative electrode and a ANF-0.5 positive electrode going through 10000 GCD cycling test, named as ANF-0.5//NC/MgO-900A. CV curves of these two hybrid capacitors are recorded (Fig. S20), and the former device displays a “under-matching” phenomenon for its no functional region, whereas the latter one shows a “match behavior” for its well-developed CV curve. All the experimental outcomes prove that it is the newly emerging well-combined 3D sheet networks of ANF-0.5 positive electrode during multiple cycling tests that leads to the “match behavior” between two working electrodes and consequently the activation process of the whole device. In addition, multiple GCD tests in three electrodes system were conducted to further verify this conclusion, in which C/MgO-900A shows typical receding curve, while ANF-0.5’s curve exhibits an upturn before downturn (Fig. S21). XPS was chosen to clarify the transformation of ANF-0.5. After cycling tests, the sample was collected and washed with deionized water several times and then dried naturally before XPS analysis. As shown in Fig. S3a and Fig. S22a, oxygen atom shows a considerable increase by 11.36 (At %) whereas other elements decrease slightly, indicating oxygen atoms from KOH electrolyte have transferred into ANF-0.5. It cannot be ascribed to air contaminant because a huge jump occurred. On the other hand, according to the electrochemical redox reaction during charge/discharge process, there is no additional oxygen atom introduced during the transformation between Ni(OH)2 and NiO(OH). Based on XPS results for the ANF-0.5 electrode, active material Ni(OH)2 may be produced from nickel foam during cycling, which accounts for the morphology change and results in the capacitance increase in cycling test.

4. Conclusion In summary, no-loading Ni(OH)2 positive electrode was prepared through facile acidification treatment toward commercial nickel foams. The bifunctional design provides as-prepared positive electrode ANF-0.5 with excellent specific capacity of 0.540 mAh∙cm-2, which can be attributed to intrinsic 3D porous structures of CNFs and the numerous emerging grass-like Ni(OH)2 microplates leading to huge active surface area and therefore enlarging the electrode/electrolyte contact area. In


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addition, 3D hierarchical hetero-structures carbon-based negative electrode C/MgO-900A was obtained through hydrothermal-calcination strategy and shows distinct specific capacitance of 424.1 mF∙cm-2. A possible energy storage model in which MgO nanoparticles serve as “reservoirs” for OHfrom electrolyte is proposed. The as-coupled hybrid capacitor exhibits common “under-matching”, however, thanks to the newly emerging well-combined 3D sheet networks in ANF-0.5 electrode during the multiple GCD cycling tests, a “match behaviour” between two working electrodes takes place, and finally contributes to an uncommon activation process of ANF-0.5//C/MgO-900A, which gives rise to an excellent energy density up to 128.8 μWh∙cm-2.

Acknowledgements This study was supported by the National Natural Science Foundation (51772283, 21271163), the Hefei Science Center CAS (No. 2016HSC-IU011), the National Key R&D Program of China (Grant No. 2016YFA0401801) and Fundamental Research Funds for the Central Universities (WK2060140021).

Supporting Information Electrochemical calculation; Digital photographs and additional determinations of nickel foams at different stage; phase analysis of Mg-1,4-BDC MOF and derived carbon materials, supported with SEM, TEM, XRD, XPS, EDX and BET; electrochemical investigation of working electrodes and hybrid capacitor device before and after multiple GCD.

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Enhancing the Capacitance of Battery-Type Hybrid Capacitors by

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