nanomaterials Article

Facile synthesis of Mesoporouscobalt Hexacyanoferrate Nanocubes for High-Performance Supercapacitors Zhiyong Zhang 1 1

2

*

ID

, Jian-Gan Wang 1, *

ID

and Bingqing Wei 1,2, *

ID

State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Lab of Graphene (NPU), Xi’an 710072, China; [email protected] Department of Mechanical Engineering, University of Delaware, Newark DE19716, USA Correspondence: [email protected] (J.-G.W.); [email protected] (B.W.); Tel.: +86-029-8846-0204 (J.-G.W. & B.W.)

Received: 1 August 2017; Accepted: 18 August 2017; Published: 21 August 2017

Abstract: Mesoporous cobalt hexacyanoferrate nanocubes (meso–CoHCF) were prepared for the first time through a facile sacrificial template method. The CoHCF mesostructures possess a high specific surface area of 548.5 m2 ·g−1 and a large amount of mesopores, which enable fast mass transport of electrolyte and abundant energy storage sites. When evaluated as supercapacitor materials, the meso–CoHCF materials exhibit a high specific capacitance of 285 F·g−1 , good rate capability and long cycle life with capacitance retention of 92.9% after 3000 cycles in Na2 SO4 aqueous electrolyte. The excellent electrochemical properties demonstrate the rational preparation of mesoporous prussian blue and its analogues for energy storage applications. Keywords: prussian blue; electrode material; mesostructure; supercapacitor

1. Introduction Supercapacitors, also known as electrochemical capacitors, are considered one of the most promising energy storage devices, owing to their desirable properties of high power density, high energy density and excellent cycling stability [1]. Based on the charge-storage mechanism, supercapacitors can be generally classified into two types: electrical double-layer capacitors (EDLCs) and pseudocapacitors. The former stores charges through electrostatic adsorption/desorption at the electrode-electrolyte interface; while the energy of pseudocapacitors comes from rapid Faradaic reactions occurred at the surface of electrode materials [2]. For EDLCs, carbonaceous materials with large surface area are widely used, such as activated carbon [3], carbon nanotubes [4], carbon xerogel [5] and graphene [6]. The most widely investigated electrode materials of pseudocapacitors include conducting polymers and transition-metal oxides or hydroxides [7,8]. As a representative of the metal organic framework materials, prussian blue (PB) can be generally expressed as Ax Fe[Fe(CN)6 ]y ·mH2 O (A, alkaline metal; 0 < x < 2, y < 1) [9,10]. In the framework of PB, Fe in low-spin and high-spin state bond with six C and six N atoms, respectively, forming face-centered-cube (FCC) crystal structure and resultant open channels [11–13]. Prussian blue analogues (PBAs), with similar crystal structures to PB, can be obtained by replacing part or all of the irons by other transition-metal elements (e.g., Co, Ni and Mn) [14,15]. As electrode materials, the open framework of PB or PBAs could provide large interstitial sites for insertion/extraction of alkali metals ions [16]. Besides, the PBAs possess the advantages of relatively higher specific capacitance, environmental friendliness and low cost, which make it promising in the field of energy storage [17]. However, the poor electrical conductivity and serious agglomeration of PBAs’ nanoparticles limit the Nanomaterials 2017, 7, 228; doi:10.3390/nano7080228

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effective utilization of their electrochemical [18]. To address this issue, twoTo methods nanoparticles limit the effective utilization performance of their electrochemical performance [18]. addresshave this been developed: (i) combining PBAs with conducting materials [19]; and (ii) increasing the reaction issue, two methods have been developed: (i) combining PBAs with conducting materials [19]; and (ii) sites of PBAs reducing particle with the high specific surface area specific [20]. There have been a increasing the by reaction sitesthe of PBAs by size reducing particle size with high surface area [20]. number of studies synthesize PB/PBA and their composites fortheir supercapacitors [21–24]. It is worth There have been a to number of studies to synthesize PB/PBA and composites for supercapacitors noting are important for mass which determines the accessing capabilitythe of [21–24].that It ismesopores worth noting that mesopores aretransport, important for mass transport, which determines electrolyte ions to theofintrinsic micro-channels of PB/PBA [25]. However, synthesis mesoporous accessing capability electrolyte ions to the intrinsic micro-channels ofthe PB/PBA [25].ofHowever, the PB/PBAs has scarcely been reported. synthesis of mesoporous PB/PBAs has scarcely been reported. In this work, we developed a facile and controllable sacrificial template route to prepare mesoporous cobalt hexacyanoferratenanocubes hexacyanoferratenanocubes (meso–CoHCF). (meso–CoHCF). The as-prepared meso–CoHCF meso–CoHCF 2−1·g−1 and a large amount of mesopores centered 2 possesses aa high highspecific specificsurface surfacearea areaofof 548.5 m 548.5 m ·g and a large amount of mesopores centered at 4.6 at 4.6The nm.meso–CoHCF The meso–CoHCF electrodes exhibit a high specific capacitance of F· 285 ·g−a1 scan at a scan nm. electrodes exhibit a high specific capacitance of 285 g−1Fat rate rate of 2 − 1 −1 in of 2 smV ·s neutral in neutral Na42 SO electrolyte, which is much higher than that of the controlled CoHCF mV· Na2SO electrolyte, which is much higher than that of the controlled 4 −11 nanoparticles (CoHCF, ·g− (CoHCF, 215 215 FF· ).). 2. Discussion and and Results Results 2. Discussion 2.1. Characterization of Meso–CoHCF 2.1. Characterization of Meso–CoHCF Figure 1a–c exhibits the morphology of the meso–CoHCF sample. From the low magnification Figure 1(a–c) exhibits the morphology of the meso–CoHCF sample. From the low magnification field-emission scanning electron microscopy (SEM) image (Figure 1a), the meso–CoHCF sample field-emission scanning electron microscopy (SEM) image (Figure 1(a)), the meso–CoHCF sample is is composed of uniform nanocubes with sizes in the range of 300–500 nm. A closer observation composed of uniform nanocubes with sizes in the range of 300–500 nm. A closer observation from from the high-resolution SEM image (Figure 1b) reveals the presence of mesopores on the exterior the high-resolution SEM image (Figure 1(b)) reveals the presence of mesopores on the exterior surface. surface. The porous microstructure is verified by transmission electron microscopy (TEM) imaging, The porous microstructure is verified by transmission electron microscopy (TEM) imaging, which which shows the existence of numerous voids in the interior of the nanocubes. For comparison, shows the existence of numerous voids in the interior of the nanocubes. For comparison, CoHCF CoHCF nanoparticles were synthesized by conventional chemical precipitation method. Figure 1d–f nanoparticles were synthesized by conventional chemical precipitation method. Figure1 (d–f) show show the SEM and TEM images of the CoHCF nanoparticles. It is observed that the irregular CoHCF the SEM and TEM images of the CoHCF nanoparticles. It is observed that the irregular CoHCF nanoparticles are of solid morphology. nanoparticles are of solid morphology.

Figure 1. SEM and TEM images of meso–CoHCF nanocubes (a–c) and CoHCF nanoparticles (d–f). Figure 1. SEM and TEM images of meso–CoHCF nanocubes (a–c) and CoHCF nanoparticles (d–f).

The phase phase structure structure of of the the products products is is investigated investigated by by X-ray X-ray diffraction diffraction (XRD). (XRD). As As shown shown in in The Figure 2(a), the meso–CoHCF and CoHCF samples show identical diffraction pattern, in which the Figure 2a, the meso–CoHCF and CoHCF samples show identical diffraction pattern, in which the diffraction peaks phase of of CoCo 3[Fe(CN)6]2·xH2O [26]. No diffraction peaks can canbe bewell wellindexed indexedtotothe theface-centered face-centeredcubic cubic phase 3 [Fe(CN)6 ]2 ·xH2 O [26]. other diffraction peaks are noted, indicating a high purity of the products. To determine the chemical No other diffraction peaks are noted, indicating a high purity of the products. To determine the structure of CoHCF, Fourier-transform infrared (FTIR) spectra were collected. As shown in Figure 2(b), the strong absorption band at 2104 cm−1 is characteristic of PB structure, which can be assigned

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chemical structure of CoHCF, Fourier-transform infrared (FTIR) spectra were collected. As shown 3 of 9 band at 2104 cm−1 is characteristic of PB structure, which can be3 of 9 −1 assigned to the stretching vibration of Fe–CN–Co bonds. The absorption at 3407 andcm 1620 cmbe −1 can to the vibration of Fe–CN–Co bonds. The absorption peaks atpeaks 3407 and 1620 −1 can be to stretching the stretching vibration of Fe–CN–Co bonds. The absorption peaks at 3407 and 1620 cm can be attributed to the stretching and bending modes of the H −O−H, respectively, indicating the attributed to the and and bending modes of the respectively, indicating the existence attributed to stretching the stretching bending modes of H−O−H, the H−O−H, respectively, indicating the existence existence of the interstitial water in the crystal lattice of the sample [27]. of the water in the lattice of the [27].[27]. of interstitial the interstitial water in crystal the crystal lattice of sample the sample

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Figure 2. patterns and spectraof meso–CoHCF and Figure 2. (a) (a)2.XRD XRD patterns and(b) (b) FTIR FTIR spectraof meso–CoHCF and CoHCF. CoHCF. Figure (a) XRD patterns and (b) FTIR spectraof meso–CoHCF and CoHCF.

The elemental components and their chemical states ofof the meso–CoHCF were examined by XTheThe elemental components andand their chemical states the were examined by elemental components their chemical states of themeso–CoHCF meso–CoHCF were examined by Xray photoelectron spectroscopy (XPS) measurements. As shown in Figure 3(a), the full scan spectrum X-ray photoelectron spectroscopy (XPS) measurements. As shown in Figure 3a, the full scan spectrum ray photoelectron spectroscopy (XPS) measurements. As shown in Figure 3(a), the full scan spectrum shows the of C in Two pairs of are shows the existence existence of Co, Co, Fe, O, O, N and C elements elements in the the sample. TwoTwo pairs of subpeaks subpeaks are present present shows the existence of Fe, Co, Fe,N O,and N and C elements in sample. the sample. pairs of subpeaks are present in high-resolution spectra of Fe2p (Figure 3(b)). The binding energies at 723.5 and 710.0 eV can be be in the the high-resolution spectra of Fe2p (Figure 3b). The binding energies at 723.5 and 710.0 in the high-resolution spectra of Fe2p (Figure 3(b)). The binding energies at 723.5 and 710.0eV eVcan can 3+ 2+ 3+ in 3+, 3+ 2+ in assigned to the Fe [Fe(CN) 6 ] while the peaks at 721.1 and 708.2 eV belong to the Fe be assigned and 708.2 eVeV belong to the in2+ in 6 ]6]3+,, while assigned to tothe theFeFe3+inin[Fe(CN) [Fe(CN) whilethe thepeaks peaksatat721.1 721.1 and 708.2 belong to the Fe 4+4+ [Fe(CN) 66]] [27,28]. The spectrum of N 1s (Figure3(c)) displays a distinct peak located at 397.9 eV, [Fe(CN) [27,28]. The spectrum of N1 s (Figure 3c) displays a distinct peak located at 397.9 eV, 4+ [Fe(CN)6] [27,28]. The spectrum of N 1s (Figure3(c)) displays a distinct peak located at 397.9 eV, corresponding to 6]6groups [29]. The CC 1s1s spectrum (Figure. 3(d)) corresponding to the the C≡C≡N Ninin[Fe(CN) [Fe(CN) ] groups [29]. The spectrum (Figure 3d) can can be be fitted fitted by by corresponding to C≡N the in [Fe(CN) 6] groups [29]. The C 1s spectrum (Figure. 3(d)) can be fitted by three component curves centered at 288.3, eV, representing representing O–C=O, C≡N and C–C three component curves centered 285.6 and and 284.6284.6 eV, O–C=O, C ≡NC≡N andand C–CC–C three component curves centered at 288.3, 285.6 eV, representing O–C=O, bonds, respectively. bonds, respectively. bonds, respectively.

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Figure 3.XPS spectra of meso–CoHCF: (a) full scan; (b) Fe2p; (c) N1s and (d) C1s. Figure 3.XPS spectra of meso–CoHCF: (a) full scan; (b) Fe2p; (c) N1s and (d) C1s. Figure 3. XPS spectra of meso–CoHCF: (a) full scan; (b) Fe2p; (c) N1s and (d) C1s.

The The porous characteristics of the samples were investigated by N adsorption-desorption porous characteristics of the samples investigated by22 adsorption-desorption N2 adsorption-desorption The porous characteristics of the samples werewere investigated by N measurements. Figure 4(a,b) exhibits the resulting isotherms of meso–CoHCF and CoHCF. The measurements. Figure 4(a,b) exhibits the resulting isotherms of meso–CoHCF measurements. Figure 4a,b exhibits the resulting isotherms of meso–CoHCF and andCoHCF. CoHCF.The meso–CoHCF possesses a large hysteresis looploop in the middle P/P0P/P region, indicating the presence of of large hysteresis inthe themiddle middle region, indicating thepresence presence Themeso–CoHCF meso–CoHCF possesses possesses aa large hysteresis loop in P/P00 region, indicating the uniform mesopores. The corresponding Barrett–Joyner–Halenda (BJH) pore-size distribution plots uniform mesopores. The corresponding Barrett–Joyner–Halenda (BJH) pore-size distribution plots are displayed as inserts in Figure 4. It 4. is It noted thatthat the meso–CoHCF sample has has uniform mesopore are displayed as inserts in Figure is noted the meso–CoHCF sample uniform mesopore size size centered at 4.6 In sharp contrast, the controlled CoHCF sample shows irregular largelarge pores centered atnm. 4.6 nm. In sharp contrast, the controlled CoHCF sample shows irregular pores in the range of 10–100 nm, which come from the agglomeration of the nanoparticles. More remarkably, in the range of 10–100 nm, which come from the agglomeration of the nanoparticles. More remarkably,

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of uniform mesopores. The corresponding Barrett–Joyner–Halenda (BJH) pore-size distribution plots are displayed as inserts in Figure 4. It is noted that the meso–CoHCF sample has uniform mesopore size centered at 4.6 nm. In sharp contrast, the controlled CoHCF sample shows irregular large pores in Nanomaterials 2017, 7, 228 4 of 9 the range of 10–100 nm, which come from the agglomeration of the nanoparticles. More remarkably, 2 −1 the Brunauer-Emmett-Teller Brunauer-Emmett-Teller (BET) the (BET)specific specificsurface surfacearea areaofofthe themeso–CoHCF meso–CoHCFisisasaslarge largeasas548.5 548.5mm·2g·g−1,, 2 − 1 which The high high surface surface area area along along with with the which is is much much higher higher than than that that of ofthe theCoHCF CoHCF(70.0 (70.0mm·2g ·g−1).). The the uniform mesoporous structure of the meso–CoHCF provide more accessible electro-active sites for for uniform mesoporous structure of the meso–CoHCF provide more accessible electro-active sites charge storage charge storage and and allow allow for for easy easy transport transport of of electrolyte electrolyte ions. ions.

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Figure 4. 4. N N22adsorption/desorption adsorption/desorption isotherm and pore size distribution of (a) meso–CoHCF Figure isotherm and pore size distribution of (a) meso–CoHCF andand (b) (b) CoHCF. CoHCF.

2.2. Electrochemical Electrochemical Performance Performance 2.2. The electrochemical properties of of the electrode were were evaluated The electrochemical properties the meso–CoHCF meso–CoHCF electrode evaluated by by cyclic cyclic voltammetry (CV) and galvanostatic charge/discharge techniques in 0.5 M Na SO aqueous electrolyte. 2 4 voltammetry (CV) and galvanostatic charge/discharge techniques in 0.5 M Na2SO4 aqueous Figure 5 shows the5CV curves the meso–CoHCF and CoHCFand electrodes various scan rates. Both electrolyte. Figure shows theof CV curves of the meso–CoHCF CoHCFat electrodes at various scan electrodes typical pseudocapacitive behavior, which features a pair of redox peaks associated rates. Bothexhibit electrodes exhibit typical pseudocapacitive behavior, which features a pair of redox peaks 2+ and Fe3+ in CoHCF [30]. The symmetric peak shape indicates good with transition associated with between transitionFe between Fe2+and Fe3+ in CoHCF [30]. The symmetric peak shape indicates reaction reversibility. Compared to the (Figure 5b), the CV5(b)), shapethe of the good reaction reversibility. ComparedCoHCF to theelectrode CoHCF electrode (Figure CVmeso–CoHCF shape of the − 1 electrode is wellelectrode maintained as the scan rate increases to 30 mV ·s , indicating better rate capability of meso–CoHCF is well maintained as the scan rate increases to 30 mV· s−1, indicating better the meso–CoHCF electrode with small polarization. The larger current density of the meso − CoHCF rate capability of the meso–CoHCF electrode with small polarization. The larger current density of electrode at each scan rate manifests a higher specificacapacitance. In capacitance. addition, it is the meso−CoHCF electrode at each scan rate manifests higher specific In interesting addition, it to is note that the high-specific-surface-area meso–CoHCF electrode exhibits an approximately rectangular interesting to note that the high-specific-surface-area meso–CoHCF electrode exhibits an CV shape in the high potential region V, as shown in of Figure 5c,V, which is totally different approximately rectangular CV shape inof the0.8–1.0 high potential region 0.8–1.0 as shown in Figure 5(c), from theisCoHCF that there are some electrochemical which totally electrode, different demonstrating from the CoHCF electrode, demonstrating thatdouble therelayer are (EDL) some capacitance contributing to the overall capacitance. contributing More importantly, theoverall specificcapacitance. capacitance of the electrochemical double layer (EDL) capacitance to the More −1 at 2 mV·s−1 , which is much higher than that of the meso–CoHCF electrode is as high as 285 F · g −1 importantly, the specific capacitance of the meso–CoHCF electrode is as high as 285 F·g at 2 mV·s−1, −1 controlled electrode F·g the meso–CoHCF exhibit better capacitance which is much higher(215 than that).ofMoreover, the controlled electrode (215electrode F·g−1). Moreover, the meso–CoHCF − 1 −1 ) than the reported manganese hexacyanoferrat/manganese performance (e.g., 272 F · g at 5 mV · s −1 −1 electrode exhibit better capacitance performance (e.g., 272 F·g at 5 mV·s ) than the reported dioxide (MnHCF/MnO F·g−1 at 52)mV ·s−1 ) [21],electrode CoHCF nanoparticles 2 ) composites electrode manganese hexacyanoferrat/manganese dioxide(225.6 (MnHCF/MnO composites (225.6 F g−1 at − 1 − 1 − 1 − 1 ) and meso–CuHCF electrode (250 F · g at 5 mV · s ) [24], meso–NiHCF (184 F · g at 5 mV · s −1 −1 −1 5 mV·s ) [21], CoHCF nanoparticles electrode (250 F·g at 5 mV·s ) [24], meso–NiHCF (184 F·g−1 at 1 at 5 mV·s−1 ) [23]. (243 Fs·−1g)−and 5 mV· meso–CuHCF (243 F·g−1 at 5 mV·s−1) [23]. Figure 6a,b shows the galvanostatic charge/discharge curves of the meso–CoHCF and CoHCF (a) electrodes from 0.5 to 10 A·g−1 . (c)Both curves exhibit a long (b) 2 meso-CoHCF 30at mV s different current densities 30 mV s 12 12 CoHCF 20 mV s 20 mV V, s charge/discharge voltage plateau at 0.3–0.6 which is consistent with the Ni foamredox peaks in CV 10 mV s 10 mV s 1 5 mV s 6 5 mV s 6 curves. The symmetric charge/discharge profiles indicate a high Coulombic efficiency. Figure 6c 2 mV s 2 mV s 0 plots the specific capacitance of the meso–CoHCF and CoHCF electrodes0 at various current densities. 0 The specific capacitance of both electrodes decreases with the increase of the current density due to -6 -1 -6 the reduced diffusion time at a high current rate [31,32]. It is noted that the meso–CoHCF electrode -12 -1

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Figure 5. CV curves of (a) meso–CoHCF and (b) CoHCF electrodes and (c) their comparison at 2 mV s−1.

good reaction reversibility. Compared to the CoHCF electrode with (Figure CVinshape of the charge/discharge voltage plateau at 0.3–0.6 V, which is consistent the5(b)), redoxthe peaks CV curves. −1 meso–CoHCF is well maintained as the ascan increasesefficiency. to 30 mV·Figure s , indicating better The symmetricelectrode charge/discharge profiles indicate highrate Coulombic 6(c) plots the rate capability of the meso–CoHCF electrode with small polarization. The larger current density of specific capacitance of the meso–CoHCF and CoHCF electrodes at various current densities. The the meso−CoHCF electrode each scandecreases rate manifests a higher specific capacitance. In addition, is specific capacitance of both at electrodes with the increase of the current density due to it the interesting to note that the high-specific-surface-area meso–CoHCF electrode exhibits an Nanomaterials 2017, 7, 228 5 of reduced diffusion time at a high current rate [31,32]. It is noted that the meso–CoHCF electrode can9 approximately rectangular CV shape the F· high ofsustains 0.8–1.0 V, shown incapacitance Figure 5(c), −1, and deliver a highspecific capacitance ofin273 g−1 potential at 0.5 A·gregion anasexcellent which is oftotally CoHCF the electrode, demonstrating that can thereonly aredeliver some retention 73.6% different even at 10from A·g−1the . However, controlled CoHCF electrode can deliver a highspecific capacitance ofcapacitance 273 F·g−1 atcontributing 0.5 A·g−1 ,−1and sustains an excellent capacitance electrochemical double layer (EDL) to the overall capacitance. More −1 specific capacitance of 212 and−1119 F·g at 0.5 and 10 A·g , respectively. The higher specific retention of 73.6% even atcapacitance 10 A·g . However, the controlled CoHCF is electrode only deliver −1 at 2 specific −1, importantly, the specific the meso–CoHCF electrode as highcan as 285 mV·sto capacitance along with the better rateofcapability of − the meso–CoHCF electrode can F· beg attributed − 1 1 capacitance of 212 and 119 F · g at 0.5 and 10 A · g , respectively. The higher specific capacitance −1 which is much higher than that of higher the controlled electrode (215 F·gprovides ). Moreover, the meso–CoHCF the unique porous structure: (i) the BET specific surface area more accessible electroalong withexhibit the better ratecapacitance capability of the meso–CoHCF electrode can be attributed thereported unique −1 at −1) than to electrode better performance (e.g., 272 F· g 5 mV· s active sites for charge storage; and (ii) the uniform mesopores facilitate fast mass the transport of porous structure: (i) the higher BET specific surface area provides more accessible electro-active sites manganeseions hexacyanoferrat/manganese (MnHCF/MnO 2) composites electrode (225.6 F g−1 at electrolyte throughout the electrode dioxide for better reaction kinetics. charge storage; andnanoparticles (ii) the uniform mesopores facilitate fasts−1mass transport of electrolyte −1 at 5for mV· s−1)further [21], CoHCF electrode (250 F· g−1 at 5 mV· ) [24], meso–NiHCF (184 F·gions To understand the electrochemical behavior of the electrodes, electrochemical throughout the electrode for better reaction kinetics. −1) [23]. 5 mV·s−1) and meso–CuHCF (243 F·g−1 at 5were mV·sperformed impedance spectrum (EIS) measurements and the resulting Nyquist plots are shown in Figure 6(d). The EIS data can be fitted using an equivalent circuit model (insert). It can be observed (a) (c) 2 meso-CoHCF 30 mV s 30 mV s in the high frequency region and a sloped line in the that the curves are composed of(b) a semicircle 12 12 EIS CoHCF 20 mV s 20 mV s Ni foam 10 mV s 10 mVthe s low frequency region. The first x-intercept on real axis (Z’) represents the bulk resistance of the 1 5 mV s 6 5 mV s 6 2 mV s 2 mV s electrode (Rs), and the diameter of the semicircle corresponds to the charge-transfer resistance (Rct) 0 0 [33]. Notably, both Rs (4.9 Ω) and Rct (3.20 Ω) of the meso–CoHCF electrode are much smaller than that of the-6 CoHCF electrode (i.e., 6.1 and -64.3 Ω). The porous structure of -1the meso–CoHCF electrode -12 provides more accessible sites for the-12 charges accumulated at the surface, thereby resulting @ 2 mV sin a -2 0.0 0.2 layer 0.4 0.6 0.8 1.0 0.0 0.2 85 0.4μF0.6 0.8 that 1.0 of the CoHCF higher double capacitance (Cdl) of around than electrode 0.0 0.2 0.4 0.6 (16 0.8μF) 1.0[4]. Potential (V vs. SCE) Potential (V vs. SCE) Potential (V vs. SCE) In addition, the sloped line is associated with the electrolyte diffusion/transport into the porous electrode [32,33]. Clearly, the meso–CoHCF electrode possesses much vertical shape,atindicating Figure 5. of of (a)(a) meso–CoHCF and (b) CoHCF electrodes and more (c) comparison 2 mV Figure 5. CV CVcurves curves meso–CoHCF and (b) CoHCF electrodes andtheir (c) their comparison at lowers2−1diffusion .mV·s−1 . resistance. It is anticipated that the lower internal and diffusion resistance of the meso–CoHCF electrode renders fast reaction kinetics for better power delivery. -1

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Figure (a) Charge/discharge Charge/discharge curves curvesofofmeso–CoHCF meso–CoHCF electrode and CoHCF electrode; (c) Figure 6. 6. (a) electrode and (b)(b) CoHCF electrode; and and (c) their their corresponding specific capacitance and (d) Nyquist plots. corresponding specific capacitance and (d) Nyquist plots.

To further understand the electrochemical behavior of the electrodes, electrochemical impedance spectrum (EIS) measurements were performed and the resulting Nyquist plots are shown in Figure 6d. The EIS data can be fitted using an equivalent circuit model (insert). It can be observed that the EIS curves are composed of a semicircle in the high frequency region and a sloped line in the low frequency region. The first x-intercept on the real axis (Z’) represents the bulk resistance of the electrode (Rs ), and the diameter of the semicircle corresponds to the charge-transfer resistance (Rct ) [33]. Notably, both Rs (4.9 Ω) and Rct (3.2 Ω) of the meso–CoHCF electrode are much smaller than that of the CoHCF

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Capacitance Retention (%)

electrode (i.e., 6.1 and 4.3 Ω). The porous structure of the meso–CoHCF electrode provides more accessible sites for the charges accumulated at the surface, thereby resulting in a higher double layer capacitance (Cdl ) of around 85 µF than that of the CoHCF electrode (16 µF) [4]. In addition, the sloped line is associated with the electrolyte diffusion/transport into the porous electrode [32,33]. Clearly, the meso–CoHCF electrode possesses much more vertical shape, indicating lower diffusion resistance. Nanomaterials 2017, 7,that 228 the lower internal and diffusion resistance of the meso–CoHCF electrode renders 6 of 9 It is anticipated fast reaction kinetics for better power delivery. Good Good cycling cycling stability stability isis an an important important criterion criterion for for supercapacitors supercapacitors in in practical practical applications. applications. Figure 7 shows the cycling performance of the meso–CoHCF and CoHCF electrodes at a large Figure 7 shows the cycling performance of the meso–CoHCF and CoHCF electrodes at current a large −1. The meso–CoHCF electrode retains 92.9% of the initial capacitance after 3000 density of 10 A· g − 1 current density of 10 A·g . The meso–CoHCF electrode retains 92.9% of the initial capacitance cycles, revealing cycling stability. contrast, CoHCF shows a shows seriousa after 3000 cycles, long-term revealing long-term cycling By stability. By the contrast, theelectrode CoHCF electrode capacitance degradation of 26.9% over the 3000-cycle test. The better cycling stability of the serious capacitance degradation of 26.9% over the 3000-cycle test. The better cycling stabilitymeso– of the CoHCF electrode may benefit from from the voidspace in the interior of of the meso–CoHCF electrode may benefit the voidspace in the interior thenanocubes, nanocubes,which which can can accommodate the possible volume expansion and contraction during the long-term operation. To accommodate the possible volume expansion and contraction during the long-term operation. confirm it, the morphology of the meso – CoHCF electrode after cycling test is characterized. As To confirm it, the morphology of the meso–CoHCF electrode after cycling test is characterized. shown in the inset of Figure 7, the nanocubes areare well maintained As shown in the inset of Figure 7, the nanocubes well maintainedafter aftercycling, cycling,indicating indicatingthe the robust structure of the meso – CoHCF. robust structure of the meso–CoHCF. 100 80 pc-CoHCF p-CoHCF

60 40

@ 10 A g-1

20 0 0

500 1000 1500 2000 2500 3000

Cycle Number Figure Figure 7. 7. Cycle Cyclestability stabilityof of meso–CoHCF meso–CoHCFand andCoHCF CoHCF electrodes. electrodes. Inset Insetshows showsthe themorphology morphologyof ofthe the meso–CoHCF electrode after cycling test. meso–CoHCF electrode after cycling test.

3. Materials and Methods 3. Materials and Methods Materials Synthesis: All chemical reagents were analytical grade and purchased from Sinopharm Materials Synthesis: All chemical reagents were analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). In a typical synthesis procedure of the meso–CoHCF, Chemical Reagent Co., Ltd. (Shanghai, China). In a typical synthesis procedure of the meso–CoHCF, 0.4 g of polyvinylpyrrolidone (PVP) and 100 mg of MnSO4H2O were dissolved in 10 mL ethanol and 0.4 g of polyvinylpyrrolidone (PVP) and 100 mg of MnSO4 H2 O were dissolved in 10 mL ethanol and 10 mL H2O, respectively, under stirring for 30 min to form a homogeneous solution. Next, 10 mL of 10 mL H2 O, respectively, under stirring for 30 min to form a homogeneous solution. Next, 10 mL of 0.03 M potassium ferricyanide (K3Fe(CN)6) was added in 3 min and kept under stirring overnight to 0.03 M potassium ferricyanide (K3 Fe(CN)6 ) was added in 3 min and kept under stirring overnight to form MnHCF template. Then, 10 mL of 0.045 M Co(NO3)26H2O was mixed with the above suspension form MnHCF template. Then, 10 mL of 0.045 M Co(NO3 )2 6H2 O was mixed with the above suspension and let stand for 5 h at 50◦ °C. Finally, the meso–CoHCF products were obtained after filtration (the and let stand for 5 h at 50 C. Finally, the meso–CoHCF products were obtained after filtration (the pore pore diameter of filter paper was ~0.45 μm) and washing with deionized water and finally drying at diameter of filter paper was ~0.45 µm) and washing with deionized water and finally drying at 80 °C for 12 h. For comparison, CoHCF nanoparticles were prepared by directly mixing 20 mL of 15 80 ◦ C for 12 h. For comparison, CoHCF nanoparticles were prepared by directly mixing 20 mL of mM Co(NO3)2 with 20 mL 10 mM K3Fe(CN)6 under stirring for 3 h. The precipitates were filtered, 15 mM Co(NO3 )2 with 20 mL 10 mM K3 Fe(CN)6 under stirring for 3 h. The precipitates were filtered, rinsed and dried using a similar process. rinsed and dried using a similar process. Materials Characterization: The crystalline structures of the as-prepared products were characterized Materials Characterization: The crystalline structures of the as-prepared products were by X-ray powder diffraction (XRD, X’Pert Pro MPD, Philips, Almelo, The Netherlands). The Fouriercharacterized by X-ray powder diffraction (XRD, X’Pert Pro MPD, Philips, Almelo, The Netherlands). transform infrared (FT-IR, Nicolet iS50, Thermo Fisher Scientific, Waltham, MA, USA) spectra were The Fourier-transform infrared (FT-IR, Nicolet iS50, Thermo Fisher Scientific, Waltham, MA, USA) collected in the region from 800 to 4000 cm−1 to detect the chemical components of the samples. X-ray spectra were collected in the region from 800 to 4000 cm−1 to detect the chemical components of photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific, Waltham, MA, USA) the samples. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific, Waltham, measurements were applied to investigate the elemental composition and surface chemistry. The morphology of the samples was observed with field emission scanning electron microscopy (FE-SEM, FEI NanoSEM 450, FEI, Portland, OR, USA) and transmission electron microscopy (TEM, FEI Tecnai F30G2, FEI, Portland, OR, USA). The porous characteristics of the products was measured by N2adsorption-desorption isotherms (ASAP 2020, Mike, Norcross, GA, USA). The specific surface area was calculated using the BET method while the pore size distribution was determined by BJH model.

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MA, USA) measurements were applied to investigate the elemental composition and surface chemistry. The morphology of the samples was observed with field emission scanning electron microscopy (FE-SEM, FEI NanoSEM 450, FEI, Portland, OR, USA) and transmission electron microscopy (TEM, FEI Tecnai F30G2, FEI, Portland, OR, USA). The porous characteristics of the products was measured by N2 adsorption-desorption isotherms (ASAP 2020, Mike, Norcross, GA, USA). The specific surface area was calculated using the BET method while the pore size distribution was determined by BJH model. Electrochemical Measurements:The electrochemical tests of meso–CoHCF electrodes were performed on an electrochemical workstation (Solartron 1260 + 1287, Bognor Regis, West Sussex, UK) with a three-electrode configuration system. For the preparation of the working electrode, the active materials, carbon black and polytetrafluoroethylene (PTFE) were mixed with a mass ratio of 7:2:1 and stirred for 2 h. The working electrode was yielded after uniformly painting the above slurry on the nickel foam (1 × 2 cm) and drying at 90 ◦ C for 12 h in an oven. Platinum foil and saturated calomel electrode (SCE) served as counter electrode and reference electrode, respectively, and a 0.5 M Na2 SO4 aqueous solution was employed as electrolyte. The mass loading of active materials on the working electrode was around 2.0 mg·cm−2 . The cyclic voltammetry (CV) and galvanostatic charge-discharge techniques were employed within a potential window ranging from 0.0 to 1.0 V. to evaluate the electrochemical performance of the working electrodes [34]. The electrochemical impedance spectroscopy (EIS) measurements were conducted in the frequency range of 10 kHz to 0.01 Hz. 4. Conclusions A facile chemical-processing method has been developed in our work to prepare the mesoporous CoHCF nanocubes for supercapcitors. The meso–CoHCF with size between 300 and 500 nm show a high specific surface area of 548.5 m2 ·g−1 and sufficient meso–channels, which not only offers abundant electro-active sites but also facilitates the mass transport of electrolyte. The meso–CoHCF electrodes exhibit a high specific capacitance of 285 F·g−1 at a scan rate of 2 mV·s−1 and excellent rate capability in Na2 SO4 aqueous electrolyte. In addition, the capacitance retention is as high as 92.9% after 3000 cycles at a large current density of 10 A·g−1 . The high specific capacitance, good rate capability and long cycle life makethe meso–CoHCFpromising as supercapacitor electrode materials for practical applications. Acknowledgments: The authors acknowledge the financial support for this work from the National Natural Science Foundation of China (51772249, 51402236, 51472204), the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No.: 123-QZ-2015), the Key Laboratory of New Ceramic and Fine Processingand State Key Laboratory of Control and Simulation of Power System and Generation Equipment (Tsinghua University, KF201607, SKLD17KM02), and the Fundamental Research Funds for the Central Universities. Author Contributions: Jian-Gan Wang and Zhiyong Zhang conceived, designed and performed the experiments; Jian-Gan Wang, Zhiyong Zhang and Bingqing Wei analyzed the results and co-wrote this paper. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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