Nanoporous Iron Oxide/Carbon Composites Through In-Situ Deposition of Prussian Blue Nanoparticles on Graphene Oxide Nanosheets A

Article Nanoporous Iron Oxide/Carbon Composites through In-Situ Deposition of Prussian Blue Nanoparticles on Graphene Oxide Nanosheets and Subsequent Thermal Treatment for Supercapacitor Applications

1, 2,3,4, , 1,5, 6, Alowasheeir Azhar †, Yusuke Yamauchi * †, Abeer Enaiet Allah †, Zeid A. Alothman * , Ahmad Yacine Badjah 6, Mu. Naushad 6 , Mohamed Habila 6, Saikh Wabaidur 6, Jie Wang 1,* and Mohamed Barakat Zakaria 1,3,7,*

1 International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan; [email protected] (A.A.); [email protected] (A.E.A.) 2 Key Laboratory of Eco-Chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China 3 School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane, QLD 4072, Australia 4 Department of Plant & Environmental New Resources, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Korea 5 Chemistry Department, Faculty of Science, Beni-Suef University, Beni-Suef 62511, Egypt 6 Advanced Material Research Chair, Chemistry Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia; [email protected] (A.Y.B.); [email protected] (M.N.); [email protected] (M.H.); [email protected] (S.W.) 7 Department of Chemistry, Faculty of Science, Tanta University, Tanta, Gharbeya 31527, Egypt * Correspondence: [email protected] (Y.Y.); [email protected] (Z.A.A.); [email protected] (J.W.); [email protected] (M.B.Z.) These authors equally contributed to this work. † Received: 20 March 2019; Accepted: 8 May 2019; Published: 21 May 2019

Abstract: This work reports the successful preparation of nanoporous iron oxide/carbon composites through the in-situ growth of Prussian blue (PB) nanoparticles on the surface of graphene oxide (GO) nanosheets. The applied thermal treatment allows the conversion of PB nanoparticles into iron oxide (Fe2O3) nanoparticles. The resulting iron oxide/carbon composite exhibits higher speciﬁc capacitance at all scan rates than pure GO and Fe2O3 electrodes due to the synergistic contribution of electric double-layer capacitance from GO and pseudocapacitance from Fe2O3. Notably, even at 1 a high current density of 20 A g− , the iron oxide/carbon composite still shows a high capacitance retention of 51%, indicating that the hybrid structure provides a highly accessible path for diﬀusion of electrolyte ions.

Keywords: nanoporous materials; iron oxide; carbon composites; Prussian blue; supercapacitors

1. Introduction Hybrid materials have been extensively studied for energy storage applications due to the possibility of combining the advantages of two (or more) individual constituents to meet the demand for recent technological applications [1,2]. In recent years, coordination polymers (CPs), especially Prussian Blue (PB) and its analogues (PBAs), have been widely used for energy storage applications because of their unique features, such as their open frameworks, high stability, and redox activity.

Nanomaterials 2019, 9, 776; doi:10.3390/nano9050776 www.mdpi.com/journal/nanomaterials Nanomaterials 2019, 9, x FOR PEER REVIEW 2 of 11 applications because of their unique features, such as their open frameworks, high stability, and Nanomaterials 2019, 9, 776 2 of 11 redox activity. However, the poor conductivity of these materials presents a major obstacle for practical use. Hybrid materials composed of PB and rGO nanosheets have been investigated to some However,extent recently the poor [2–4]. conductivity of these materials presents a major obstacle for practical use. Hybrid materialsIn general, composed carbon-based of PB and materials rGO nanosheets [5], such have as graphene been investigated oxide (GO), to some reduced extent graphene recently [oxide2–4]. (rGO),In and general, carbon carbon-based nanotubes, materialsor conducting [5], such poly asmers, graphene such as oxide polypyrrole (GO), reduced (PPy) and graphene polyaniline oxide (rGO),(PANI), and are carbon typically nanotubes, used to orimprove conducting the conducti polymers,vity such of PB-based as polypyrrole materials (PPy) [6–10]. and polyanilineThe active (PANI),functional are groups typically on usedthe basal to improve planes theand conductivity edges of GO of nanoshee PB-basedts materialsallow for [6the–10 adsorption]. The active of functionalcations which groups act on as the nucleation basal planes centers and edgesfor the of gr GOowth nanosheets of PB nanoparticles, allow for the adsorption thus making of cations them whichattractive act asto nucleationbe hybridized centers with for PB. the In growth addition, of PB the nanoparticles, developed reduced thus making GO (rGO) them attractivenanosheets to beduring hybridized composites with synthesis PB. In addition, possess the high developed electrical reduced conductivity GO (rGO) and nanosheetsimproved structural during composites stability. synthesisOur group possess has reported high electrical the in-situ conductivity growth andof vari improvedous cyano-bridged structural stability. CPs on Our the group surface has of reported GO by thedifferent in-situ methods growth of[11]. various cyano-bridged CPs on the surface of GO by different methods [11]. PB and PBAs have been utilized as precursors for the preparation of metal oxides with good electrochemical performanceperformance for for energy energy storage storage applications applications by by means means of thermalof thermal decomposition decomposition [12]. In[12]. particular, In particular, iron oxides iron (Fe oxidesxOy) have(FexO showny) have some shown promise some as anpromise electrode as materialan electrode for supercapacitors material for owingsupercapacitors to their high owing theoretical to their high specific theoretical capacitance, specific cost-e capacitance,ffectiveness, cost-effectiveness, low toxicity, excellent low toxicity, safety, andexcellent wide safety, abundance and wide [13 ,14abundance]. In this [13,14]. work, In we this demonstrate work, we demonstrate the facile preparation the facile preparation of a PB/GO of compositea PB/GO composite through the through in-situ the deposition in-situ deposition of PB nanoparticles of PB nanoparticles on GO nanosheets on GO nanosheets and the subsequent and the thermalsubsequent treatment thermal to converttreatment the PBto/ GOconver compositet the toPB/GO nanoporous composite Fe2O 3to/carbon nanoporous composite Fe (Scheme2O3/carbon1). Thecomposite resulting (Scheme Fe2O3 /1)carbon. The compositeresulting Fe shows2O3/carbon high specificcomposite capacitance, shows high excellent specific rate capacitance, capability, andexcellent good rate cycling capability, stability and for good supercapacitors. cycling stability for supercapacitors.

Scheme 1. 1. SchematicSchematic illustration illustration for the for preparation the preparation of the ofPrussian the Prussian blue/graphene blue/graphene oxide (PB/GO) oxide

(PBcomposite/GO) composite and the subsequent and the subsequentthermal conversi thermalon to conversion nanoporous to iron nanoporous oxide/carbon iron (Fe oxide2O3/carbon)/carbon (Fecomposite.2O3/carbon) composite.

2. Experimental Section 2. Experimental Section 2.1. Chemicals 2.1. Chemicals Sodium ferrocyanide (II) decahydrate (Na4[Fe(CN)6] 10H2O) was purchased from Sigma-Aldrich, Sodium ferrocyanide (II) decahydrate (Na4[Fe(CN)· 6]·10H2O) was purchased from Co., St. Louis, MO, USA. The sulfuric acid solution was purchased from Nacalai tesque, Inc., Kyoto, Sigma-Aldrich, Co., St. Louis, MO, USA. The sulfuric acid solution was purchased from Nacalai Japan. Potassium hydroxide, sodium nitrate and iron (III) chloride hexahydrate (FeCl 6H O) were tesque, Inc., Kyoto, Japan. Potassium hydroxide, sodium nitrate and iron (III) chloride3 ·hexahydrate2 purchased from FUJIFILM Wako Pure Chemicals, Osaka, Japan. Nanographite platelets (N008-100-N) (FeCl3·6H2O) were purchased from FUJIFILM Wako Pure Chemicals, Osaka, Japan. Nanographite of 100 nm thickness were used as a raw material to prepare graphene oxide (GO) sheets (Angstron platelets (N008-100-N) of 100 nm thickness were used as a raw material to prepare graphene oxide materials, Dayton, OH, USA). Potassium permanganate (KMnO4) and hydrogen peroxide (H2O2) were (GO) sheets (Angstron materials, Dayton, OH, USA). Potassium permanganate (KMnO4) and purchased from Kanto Chemicals Co., Inc., Tokyo, Japan. All chemical reagents were used without hydrogen peroxide (H2O2) were purchased from Kanto Chemicals Co., Inc., Tokyo, Japan. All further puriﬁcation. chemical reagents were used without further purification. 2.2. Synthesis of GO Nanosheets 2.2. Synthesis of GO Nanosheets GO nanosheets were prepared according to the modiﬁed Hummer’s method [1]. In a typical process, graphite powder (N008-100-N, carbon source) and sodium nitrate were mixed together and then a concentrated sulphuric acid solution (7.67 mL) was added into this suspension under constant Nanomaterials 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/nanomaterials stirring for 1 h. Subsequently, KMnO4 (1.0 g) was added into the resulting mixture solution while maintaining the temperature at lower than 20 ◦C. The reaction mixture was then stirred at 35 ◦C for 2 h Nanomaterials 2019, 9, 776 3 of 11 followed by the addition of pure water (83 mL) under vigorous stirring. To ensure the completion of the reaction, the obtained suspension was further treated with an aqueous H2O2 solution (30% w/w, 1.67 mL). The resulting solution was washed several times using the diluted HCl solution and distilled water. Finally, it was sonicated in pure water for several hours for exfoliation. The GO powder was then collected by centrifugation for 30 min at 14,500 rpm. The precipitate was washed again with distilled water for several times and dried naturally in air at room temperature and then, in vacuum at 1 60 ◦C overnight. The resulting GO powder was used to prepare the aqueous GO solution (2 mg mL− ) in the next step.

2.3. In-Situ Growth of PB Nanoparticles on the Surface of GO Nanosheets (PB/GO Composite) and the Subsequent Thermal Conversion to Nanoporous Iron Oxide/Carbon Composite

In a typical procedure, 40 mL of 0.299 mM FeCl3 6H2O solution was added dropwise to the GO 1 · solution (20 mL, 2 mg mL− ) followed by stirring for 30 min. The obtained mixture was gently mixed with a 0.358 mM Na [Fe(CN) ] 10H O solution (40 mL) by stirring for 30 min. The resulting suspension 4 6 · 2 was aged for two days until the reaction was completed. The precipitate of the PB/GO composite was isolated by centrifugation (for 30 min at 14,500 rpm) and washed with water and ethanol several times. The precipitate was left to dry at room temperature for the next step. The nanoporous Fe2O3/carbon composite was obtained by heating the as-synthesized PB/GO powder at 350 ◦C for 1 h in air with 1 a ﬁxed heating rate of 5 ◦C min− . The PB nanoparticles and GO nanosheets were also heated under the same conditions to compare their electrochemical performance with the Fe2O3/carbon composite. For the synthesis of PB nanoparticles, a 40 mL aqueous solution of 0.299 mM FeCl 6H O was mixed 3· 2 with another 40 mL aqueous solution of 0.358 mM Na [Fe(CN) ] 10H O and stirred for 1 h before 4 6 · 2 aging overnight to ensure a complete reaction. The PB nanoparticles were collected by centrifugation (for 30 min at 14,500 rpm) and washed with water and ethanol several times. The precipitate dried naturally at room temperature (Figure S1).

2.4. Characterization Wide-angle powder X-ray diffraction (PXRD) analysis was performed with Rigaku (Tokyo, Japan) RINT 2500X diffractometer using a monochromated Cu Kα (1.5406 Å, 40 kV, 40 mA) radiation. Nitrogen adsorption-desorption isotherms were obtained with a Quantachrome (Boynton Beach, FL, USA) Autosorb automated gas sorption system at 77 K. Before the measurement, all samples were dehydrated completely by heating at 250 ◦C overnight under vacuum. Morphological observations of the samples were performed using a scanning electron microscope (SEM, Hitachi SU8000, Tokyo, Japan) and transmission electron microscope (TEM, JEOL JEM2100, Tokyo, Japan). Fourier-transform infrared (FTIR) spectroscopy was performed using a Thermoscientific Nicolet 4700 (Waltham, MA, USA).

2.5. Electrochemical Measurements All electrochemical measurements were carried out on an electrochemical analyzer (CH Instruments Model 600E, Austin, TX, USA). The electrochemical measurements of the prepared electrodes were investigated by the cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) using a three-electrode system with platinum wire as the counter electrode, Ag/AgCl electrode as the reference electrode, and 3 M KOH solution as the electrolyte. The working electrode was prepared by coating a slurry containing the active material (1 mg, 85 wt.%), carbon black (10 wt.%), polyvinylidene fluoride binder (PVDF) (5 wt.%), and N-methyl-2-pyrrolidone on a graphite paper (thickness: 1 mm) with an area of 1 1 cm2 as the current collector and dried at 60 C. In our experiments, we used the × ◦ flexible graphite paper as the current collector because of its excellent electrical conductivity as well as 1 its negligible capacitive performance (usually below 1 F g− ), which would contribute little to the total Nanomaterials 2019, 9, x FOR PEER REVIEW 4 of 11 Nanomaterials 2019, 9, 776 4 of 11 paper (thickness: 1 mm) with an area of 1 × 1 cm2 as the current collector and dried at 60 °C. In our experiments, we used the flexible graphite paper as the current collector because of its excellent capacitance of the prepared working electrode. The specific gravimetric capacitance of the electrode electrical conductivity as well as its negligible capacitive performance (usually below 1 F g−1), which was calculated from the CV curves by using the following Equation (1): would contribute little to the total capacitance of the prepared working electrode. The specific gravimetric capacitance of the electrode was calculated from the CV curves by using the following ZV f Equation (1): 1 Cg = I(V)dv (1) ms(V f Vi) 1 − Vi 𝐶 = 𝐼𝑉𝑑𝑣 (1) C 𝑚𝑠𝑉𝑓 − 𝑉𝑖1 s 1 V where g is the gravimetric capacitance (F g− ), is the potential scan rate (V s− ), is the potential windowwhere Cg (V), is theI is gravimetric current (A), capacitance and m is the (F mass g−1), of s activeis the materialpotential (g). scan rate (V s−1), V is the potential windowBased (V), on I galvanostaticis current (A), charge-discharge and m is the mass (GCD) of active curves, material the gravimetric (g). specific capacitance (Cg, 1 F g− Based) was calculatedon galvanostatic using the charge-discharge Equation (2): (GCD) curves, the gravimetric specific capacitance (Cg, F g−1) was calculated using the Equation (2): I t 𝐼·𝑡Cg = · (2) 𝐶 = m ∆ V (2) 𝑚·∆𝑉 · where II isis thethe dischargedischarge current (A), t is the discharge time (s), m is the mass of active material (g), and ∆ΔV is the potential change during the discharge processprocess (V).(V).

3. Results and Discussion The functionalfunctional groups, groups, including including hydroxyl, hydroxyl, carboxyl carboxyl and carbonyland carbonyl on the on GO the nanosheets GO nanosheets enables 3+ 3+ theenables adsorption the adsorption of Fe ions of Fe at3+ theions GO at surfacethe GO insurface the ﬁrst in step.the first Then, step. the Then, adsorbed the adsorbed Fe ions Fe interact3+ ions withinteract the with ferrocyanide the ferrocyanide ligand to ligand initiate to the initiate growth the of PBgrowth nanoparticles of PB nanoparticles on the GO surface.on the GO The surface. change 3+ inThe the change surface in charge the surface of the charge GO nanosheet of the GO suspension nanosheet before suspension and after before the additionand after of the Fe additionions was of studiedFe3+ ions by was measuring studied by the measuring zeta potentials. the zeta The potentia zeta potentialls. The zeta measurements potential measurements reveal that thereveal surface that 3+ chargethe surface of the charge GO nanosheets of the GO wasnanosheets changed was from changed negative from to positive negative due to positive to the adsorbed due to the cations adsorbed (Fe ions)cations [15 (Fe,163+]. ions) During [15,16]. the growth During of the PB, growth the color of of PB, the the suspension color of the gradually suspension changed gradually to blue changed due to the to formationblue due to of the PB formation nanoparticles of PB (Figure nanoparticles1a). The morphology(Figure 1a). The of the morphology PB /GO composite of the PB/GO was investigated composite bywas both investigated SEM and by TEM both (Figure SEM and1b,c). TEM The (Figure high-magniﬁcation 1b,c). The high-magnification SEM and TEM imagesSEM and of TEM the PBimages/GO compositeof the PB/GO clearly composite show theclearly wrinkly show structure the wrinkl of GO-wrappedy structure of PBGO-wrapped nanoparticles. PB nanoparticles.

Figure 1. (a) AA digitaldigital photographphotograph of of the the PB PB/GO/GO precipitate precipitate collected collected by by centrifugation; centrifugation; (b )(b SEM) SEM and and (c) TEM(c) TEM images images ofthe of the as-synthesized as-synthesized PB/ PB/GOGO composite. composite.

The crystalcrystal structurestructure andand phase phase purity purity of of the the PB PB/GO/GO composite composite were were checked checked by XRDby XRD (Figure (Figure2a). The2a). The XRD XRD pattern pattern of theof th GOe GO nanosheets nanosheets shows shows two two sharp sharp di ﬀdiffractionraction peaks peaks at at 2 θ2θ= =11 11°◦ andand 2626°◦ indexed to (001) and (002) planes of GO, respectively,respectively, indicatingindicating the highly ordered nature of thethe graphitic layers [[17,18].17,18]. The XRD pattern of the PB na nanoparticlesnoparticles (Figure2 2a)a) isis assignableassignable toto aa typicaltypical face-centered cubic PB crystalcrystal structurestructure (JCPDS(JCPDS No.No. 73-0687) [[19].19]. The The diffraction diﬀraction peaks of PB still remain inin thethe PB PB/GO/GO composite composite even even after after the the hybridization hybridization with with GO, howeverGO, however the characteristic the characteristic peaks ofpeaks the GOof the nanosheets GO nanosheets are not detected are not becausedetected the because anchored the PB anchored nanoparticles PB nanoparticles cover the surface cover of GOthe nanosheets,surface of GO thus nanosheets, preventing thus their preventing restacking. their restacking.

Nanomaterials 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/nanomaterials

Nanomaterials 2019, 9, 776 5 of 11 Nanomaterials 2019, 9, x FOR PEER REVIEW 5 of 11

Figure 2. ((aa)) Wide-angle Wide-angle XRD XRD diffraction diﬀraction patterns patterns and and ( (bb)) FT-IR FT-IR spectra spectra of of graphene oxide oxide (GO) (GO) nanosheets, Prussian blue (PB) nanoparticles, and PBPB/GO/GO composite.composite.

The chemical compositions of the PB nanoparticles, GO nanosheets, and PB/GO PB/GO composite were investigated by the FTIR spectroscopy (Figure 22b).b). TheThe peakspeaks observedobserved inin thethe IRIR spectrumspectrum ofof GOGO nanosheets can be assigned to the oxygen-containing functional functional groups groups on on the the GO GO surface surface [20–22]. [20–22]. After the hybridization with PB nanoparticles, several bands belonging to GO disappear and/or and/or the peak intensities are largely reduced, due to the successful reduction of GO to reduced graphene 1 oxide (rGO). The The peak peak at at 2080 2080 cm cm−1− is isattributed attributed to to–C –C≡N–N– units, units, thus thus confirming confirming the theexistence existence of PB of 1 ≡ nanoparticlesPB nanoparticles [23,24]. [23,24 The]. The peak peak at at1600 1600 cm cm−1 −onon the the PB/GO PB/GO composite composite isis difficult difficult to to identify identify as as it overlaps with the CC=C=C group group of of GO GO nanosheet nanosheetss and the –OH group of PB nanoparticles. As shown in in Scheme Scheme 1,1, the thermal thermal conversion conversion of of the the PB/GO PB /GO composite composite at at350 350 °C◦ inC inair air results results in thein the formation formation of ofthe the iron iron oxide oxide (Fe (Fe2O2O3)/carbon3)/carbon composite composite [25]. [25]. The The cr crystalystal structure structure of of the the PB/GO PB/GO composite afterafter thethe heat heat treatment treatment was was examined examined by theby wide-anglethe wide-angle XRD XRD (Figure (Figure3a). The 3a). original The original peaks peaksof PB vanishedof PB vanished entirely. entirely The new. The and new broad and peaks broad at 2 θpeaks= 35◦ atand 2θ 63 =◦ 35°can and be assigned 63° can tobe impurity-free assigned to impurity-freeγ-Fe2O3 [26], therebyγ-Fe2O3 confirming [26], thereby the confirming successful conversionthe successful of PB conversion to iron oxide. of PB The todi ironffraction oxide. peaks The diffractionof GO nanosheets peaks of are GO not nanosheets observed are in thenot preparedobserved Fein2 Othe3/ carbonprepared composite. Fe2O3/carbon The morphologycomposite. The of morphologythis composite of wasthis composite investigated was using investigated both SEM usin andg both TEM SEM (Figure and3 TEMb–d). (Figure As revealed 3b–d). byAs therevealed SEM byimage, the SEM the size image, of the the resulting size of the iron resulting oxide nanoparticles iron oxide nanoparticles is nearly similar is nearly as that similar of the as startingthat of the PB startingnanoparticles PB nanoparticles (Figure1b). The (Figure high-resolution 1b). The high SEM-resolution image shows SEM that image the iron shows oxide that nanoparticles the iron oxide are nanoparticlesaggregated to are each aggregated other. To further to each investigate other. To further the morphology investigate of the this morphology composite, TEMof this analysis composite, was TEMcarried analysis out. Figure was3 ccarried confirms out. the Figure presence 3c ofconfirms nanoparticle the presence aggregates of andnanoparticle wrinkled GOaggregates nanosheets. and Thewrinkled high-resolution GO nanosheets. TEM The (HRTEM) high-resolution image shows TEM several (HRTEM) lattice image fringes shows belonging several lattice to iron fringes oxide (Figurebelonging3d). to The iron EDX oxide elemental (Figure mapping3d). The EDX shows elem theental uniform mapping distribution shows the of carbonuniform (C), distribution oxygen (O), of carbonand iron (C), (Fe) oxygen atoms (Figure(O), and4). Theiron Fe:C (Fe) ratio atoms of the (Figure resulting 4). ironThe oxideFe:C /ratiocarbon of composite the resulting is around iron oxide/carbon62:38 (wt.%). Thecomposite presence is ofaround Fe and 62:38 C was (wt.%). also confirmed The presence by FTIR of spectroscopy, Fe and C was as shownalso confirmed in Figure by S2. FTIR spectroscopy, as shown in Figure S2.

Nanomaterials 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/nanomaterials

Nanomaterials 2019,, 9,, 776x FOR PEER REVIEW 6 of 11 Nanomaterials 2019, 9, x FOR PEER REVIEW 6 of 11

Figure 3. (a) Wide-angle XRD pattern, and (b) SEM, (c) TEM and (d) high-resolution TEM (HRTEM) Figureimages 3.of (a )the Wide-angle iron oxide/carb XRD pattern, on composite and ( b) SEM, obtained ( c) TEM by and the ( thermald) high-resolution treatment TEM of the (HRTEM) (HRTEM) PB/GO imagescomposite. ofof the the iron iron oxide oxide/carb/carbon compositeon composite obtained obtained by the by thermal the thermal treatment treatment of the PB /ofGO the composite. PB/GO composite.

Figure 4.4.( a()a HAADF-STEM) HAADF-STEM image image of the of ironthe oxideiron /carbonoxide/carbon composite composite and the correspondingand the corresponding elemental Figuremappingelemental 4. formapping(a) (bHAADF-STEM) carbon, for (b (c) )carbon, oxygen, image (c and ) ofoxygen, (thed) ironiron and contents. oxide/(d) ironcarbon Thecontents. weightcomposite The ratio weight and of carbon ratiothe corresponding of and carbon iron wasand elementalmeasurediron was measured frommapping the selectedfromfor (b the) carbon, area selected highlighted (c )area oxygen, highlighted in and the yellow(d) iniron the box. contents. yellow box. The weight ratio of carbon and iron was measured from the selected area highlighted in the yellow box. Figure5 5shows shows thethe N N22 adsorption-desorption isotherms of PB nanoparticles and PBPB/GO/GO compositeFigure beforebefore 5 shows andand afterafterthe thermalNthermal2 adsorption-desorption treatment.treatment. TheThe textural textural isotherms characteristics characteristics of PB nanoparticles are are summarized summarized and in in TablePB/GO Table1. 2 1 Thecomposite1. The Brunauer-Emmett-Teller Brunauer-Emmett-Teller before and after thermal (BET) (BET) treatment. speciﬁc specific surface suTherface textural area area of ofcharacteristics the the PB PB/GO/GO composite composite are summarized (152.6 (152.6 m m in2g gTable−−1) is 2 1 2 1 remarkably1. The Brunauer-Emmett-Teller improvedimproved compared compared to(BET) pureto purespecific GO (34.9GO su m(34.9rfaceg− aream)[21 ]g andof−1) the[1] pure PB/GOand PB (36.1pure composite mPBg −(36.1). (152.6 The m adsorption2 mg−21 ).g −The1) is isothermsremarkablyadsorption are isothermsimproved gradually are compared increased gradually withto increased pure the increaseGO with (34.9 ofth relativeem increase2 g−1) pressure,[1] of andrelative indicatingpure pressure, PB (36.1 that indicating variousm2 g−1). pores thatThe withadsorptionvarious diﬀ poreserent isotherms porewith sizesdifferent are aregradually randomlypore sizes increased distributedare random with overthlye distributedincrease the entire of relative area.over the The pressure, entire higher area. surfaceindicating The areahigher that of thevarious PB/GO pores composite with different can be pore attributed sizes are to the random goodly dispersion distributed of over the PB the nanoparticles entire area. The on thehigher GO Nanomaterials 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/nanomaterials Nanomaterials 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/nanomaterials

Nanomaterials 2019,, 9,, 776x FOR PEER REVIEW 7 of 11

surface area of the PB/GO composite can be attributed to the good dispersion of the PB nanoparticles surface,on the GO thus surface, enabling thus them enabling to act them as spacers to act toas preventspacers theto prevent GO layers the fromGO layers aggregation from aggregation or restacking. or Followingrestacking. the Following heat treatment, the heat the treatment, surface area the of surf theace iron area oxide of/carbon the iron composite oxide/carbon is mostly composite similar tois thatmostly of thesimilar precursor to that PB of/ GOthe composite.precursor PB/GO composite.

Figure 5.5.Nitrogen Nitrogen gas adsorption-desorptiongas adsorption-desorption isotherms isothe of PBrms nanoparticles, of PB nanoparticles, iron oxide nanoparticles, iron oxide PBnanoparticles,/GO composite, PB/GO and composite, iron oxide /andcarbon iron composite. oxide/carbon composite.

Table 1. Textural characteristics of the obtained samples. Table 1. Textural characteristics of the obtained samples. 2 1 3 1 Sample Surface Area/m g− Pore Volume/cm g− Sample Surface Area / m2 g−1 Pore Volume / cm3 g−1 PB 36.1 0.291 PB 36.1 0.291 Iron oxide 60.3 0.343 Iron oxidePB /GO 152.6 60.3 0.453 0.343 PB/GOIron oxide /carbon 145.5152.6 0.487 0.453 Iron oxide/carbon 145.5 0.487 It is well-reported that graphene/metal oxide composites exhibit good electrochemical It is well-reported that graphene/metal oxide composites exhibit good electrochemical performance performance for supercapacitor applications [27,28]. The electrochemical properties of pure GO, for supercapacitor applications [27,28]. The electrochemical properties of pure GO, pure iron oxide, pure iron oxide, and iron oxide/carbon composite electrodes for supercapacitor applications were and iron oxide/carbon composite electrodes for supercapacitor applications were investigated by investigated by using a three-electrode system in a 3 M KOH electrolyte. The cyclic voltammetry using a three-electrode system in a 3 M KOH electrolyte. The cyclic voltammetry (CV) curves of (CV) curves of the pure GO electrode at different scan rates (Figure 6a) reveal rectangle-like curves, the pure GO electrode at different scan rates (Figure6a) reveal rectangle-like curves, indicating its indicating its electrical double layer capacitive properties, while in the case of iron oxide and iron electrical double layer capacitive properties, while in the case of iron oxide and iron oxide/carbon oxide/carbon electrodes (Figure 6b,c) the CV curves exhibit a pair of redox peaks due to electrodes (Figure6b,c) the CV curves exhibit a pair of redox peaks due to pseudocapacitive behavior. pseudocapacitive behavior. The existence of an oxidation peak at ca. 0.3 V and a reduction peak at The existence of an oxidation peak at ca. 0.3 V and a reduction peak at ca. 0.7 V vs. Ag/AgCl, [29] for ca. −0.7 V vs. Ag/AgCl,[29] for the iron oxide/carbon composite electrode− implies reversible redox the iron oxide/carbon composite electrode implies reversible redox reactions of iron ion [30]. reactions of iron ion [30]. Based on CV measurements, the specific capacitance values of GO, iron oxide, and iron oxide/carbon Based on CV measurements, the specific capacitance values of GO, iron oxide, and iron electrodes at a scan rate of 2 mV s 1 are 220.0, 264.0, and 551.5 F g 1, respectively. More importantly, oxide/carbon electrodes at a scan −rate of 2 mV s−1 are 220.0, 264.0, −and 551.5 F g−1, respectively. More the iron oxide/carbon composite electrode exhibits higher specific capacitance values than both GO importantly, the iron oxide/carbon composite electrode exhibits higher specific capacitance values and iron oxide electrodes at all scan rates owing to the synergistic combination of EDLC from GO and than both GO and iron oxide electrodes at all scan rates owing to the synergistic combination of pseudocapacitance from Fe2O3 (Table2). The capacitance retention value of the iron oxide /carbon EDLC from GO and pseudocapacitance from Fe2O3 (Table 2). The capacitance retention value of the composite electrode at 100 mV s 1 is very high (50%), superior to those of graphene oxide (31%) and iron oxide/carbon composite electrode− at 100 mV s−1 is very high (50%), superior to those of graphene iron oxide electrodes (44%). oxide (31%) and iron oxide electrodes (44%).

Nanomaterials 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/nanomaterials

Nanomaterials 2019, 9, 776 8 of 11

Nanomaterials 2019, 9, x FOR PEER REVIEW 8 of 11

FigureFigure 6.6. ((a–ca–c) )CV CV curves curves at at various various scan scan rates rates for for (a) (agraphene) graphene oxide; oxide; (b) (ironb) iron oxide oxide and and(c) iron (c) −1 1 ironoxide/carbon oxide/carbon electrodes; electrodes; (d) cycling (d) cycling performances performances at 200 mV at 200 s for mV the s− ironfor oxide/carbon the iron oxide composite/carbon compositeelectrode. electrode.

Table 2. Specific capacitance values of GO, iron oxide, and iron oxide/carbon composite at different Table 2. Specific capacitance values of GO, iron oxide, and iron oxide/carbon composite at different scan rates. scan rates. Specific Capacitance/F g 1 Scan Rate/mV s 1 Specific Capacitance / F g−−1 Scan Rate / −mV s−GO1 Iron Oxide Iron Oxide/Carbon GO Iron Oxide Iron Oxide/Carbon 2 220.0 264.0 551.5 20 2 205.3220.0 145.5264.0 551.5 450.0 50 20 110.0205.3 91.70145.5 450.0 340.0 100 50 97.00110.0 82.0091.70 340.0 275.0 100 97.00 82.00 275.0 The long-term stability of the iron oxide/carbon composite electrode was investigated by the CV The long-term stability of the iron oxide/carbon composite electrode was investigated by the −1 measurement at 100 mV s (Figure1 6d). Clearly, the capacitance gradually increases until the CV measurement at 100 mV s− (Figure6d). Clearly, the capacitance gradually increases until the capacitance retention reaches 120% after 8000 cycles. The increase in the specific capacitance during capacitance retention reaches 120% after 8000 cycles. The increase in the specific capacitance during cycling is possibly due to the activation of the hybrid electrode material as a result of the improved cycling is possibly due to the activation of the hybrid electrode material as a result of the improved surface wetting of the electrode, leading to improvement in the electrolyte ion diffusion. surface wetting of the electrode, leading to improvement in the electrolyte ion diffusion. Furthermore, Furthermore, the as-prepared iron oxide/carbon composite electrode shows better electrochemical the as-prepared iron oxide/carbon composite electrode shows better electrochemical performance for performance for supercapacitor applications than previously reported iron oxide/GO electrodes in supercapacitor applications than previously reported iron oxide/GO electrodes in terms of specific terms of specific capacitance and capacitance retention (Table S1). capacitance and capacitance retention (Table S1). Galvanostatic charge-discharge (GCD) measurements were carried out to investigate the rate Galvanostatic charge-discharge (GCD) measurements were carried out to investigate the rate performance of the graphene oxide, iron oxide, and iron oxide/carbon electrodes at current densities performance of the graphene oxide, iron oxide, and iron oxide/carbon electrodes at current densities of of 2, 4, 6, 8, 10, and 20 A g−1 (Figure 7). The GCD curves of the iron oxide/carbon composite electrode 2, 4, 6, 8, 10, and 20 A g 1 (Figure7). The GCD curves of the iron oxide /carbon composite electrode depicts high specific capacitance− in comparison with graphene oxide and iron oxide. A non-linear depicts high specific capacitance in comparison with graphene oxide and iron oxide. A non-linear behavior was observed during charging (at a potential of ca. 0.3 V) and discharging (at a potential of behavior was observed during charging (at a potential of ca. 0.3 V) and discharging (at a potential ca. −0.7 V) vs. Ag/AgCl and these observations are in good agreement with the oxidation and of ca. 0.7 V) vs. Ag/AgCl and these observations are in good agreement with the oxidation and reduction− peaks observed in the CV curves. Figure 7d compares the rate capabilities of graphene reduction peaks observed in the CV curves. Figure7d compares the rate capabilities of graphene oxide, oxide, iron oxide, and iron oxide/carbon composite electrodes at different current densities. At a iron oxide, and iron oxide/carbon composite electrodes at different current densities. At a current current density of 2 A g−1, the specific capacitance of the iron oxide/carbon composite electrode is density of 2 A g 1, the specific capacitance of the iron oxide/carbon composite electrode is 415.0 F g 1, −1 − −1 − 415.0 F g , which is much higher than those of graphene oxide1 (210.0 F g ) and iron oxide electrodes1 which is much higher than those of graphene oxide (210.0 F g− ) and iron oxide electrodes (202.5 F g− ). (202.5 F g−1). Notably, with the increase of current density to 20 A g−1, the iron oxide/carbon

Nanomaterials 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/nanomaterials

Nanomaterials 2019, 9, 776 9 of 11 Nanomaterials 2019, 9, x FOR PEER REVIEW 9 of 11

1 Notably,composite with electrode the increase still ofexhibits current a density high capaci to 20 Atance g− ,retention the iron oxide of 51%./carbon On composite the contrary, electrode pure stillgraphene exhibits oxide a high and capacitance iron oxide retention electrodes of 51%. display On the poor contrary, capacitance pure graphene retention oxide of and21% ironand oxide 29%, electrodesrespectively, display indicating poor capacitance that the hybrid retention structur of 21%e andwas 29%, beneficial respectively, for enhancing indicating the that diffusion the hybrid of structureelectrolyte was ions. beneﬁcial for enhancing the diﬀusion of electrolyte ions.

FigureFigure 7.7.Galvanostatic Galvanostatic charge-dischargecharge-discharge (GCD)(GCD) curvescurves ofof ((aa)) graphenegraphene oxide;oxide; ((bb)) ironiron oxideoxide andand ((cc)) ironiron oxide oxide/carbon/carbon electrodes electrodes at diatff erentdifferent current current densities; densities; (d) specific (d) specific capacitance capacitance compared compared to current to densitycurrent plotsdensity for plots all the for samples. all the samples. 4. Conclusions 4. Conclusions In this study, PB nanoparticles were simultaneously deposited on the surface of GO nanosheets In this study, PB nanoparticles were simultaneously deposited on the surface of GO nanosheets through the adsorption of positively charged Fe3+ ions on GO surface firstly followed by addition of through the adsorption of positively charged Fe3+ ions on GO surface firstly followed by addition of Na4[Fe(CN)6] 10H2O. The oxygen-containing functional groups of the GO surface served as excellent Na4[Fe(CN)6]·10H· 2O. The oxygen-containing functional groups of the GO surface served as excellent anchoring agents for the growth of PB nanoparticles. The PB/GO composite precursor was successfully anchoring agents for the growth of PB nanoparticles. The PB/GO composite precursor was converted into the iron oxide/GO composite with a well-retained morphology by a facile thermal successfully converted into the iron oxide/GO composite with a well-retained morphology by a annealing in air at 350 ◦C. The obtained iron oxide/carbon composite shows superior electrochemical facile thermal annealing in air at 350 °C. The obtained iron oxide/carbon composite shows superior performance as an electrode material for supercapacitors compared to both pure iron oxide and electrochemical performance as an electrode material for supercapacitors compared to both pure graphene oxide electrodes in terms of specific capacitance, rate capability, and capacitance retention. iron oxide and graphene oxide electrodes in terms of specific capacitance, rate capability, and The results will provide a useful guidance for constructing high-performance hybrid materials for capacitance retention. The results will provide a useful guidance for constructing high-performance energy storage applications using PB and PBA. hybrid materials for energy storage applications using PB and PBA. Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/9/5/776/s1, FigureSupplementary S1: (a) SEM Materials: and (b) TMEThe following images of are PB available nanoparticles; online Figure at www.mdpi.com/xxx/s1, S2: FTIR spectrum of Figure iron oxide S1: /(a)carbon SEM composite;and (b) TME Table images S1: Comparisonof PB nanoparticles; of our sample Figure performanceS2: FTIR spectrum with the of previouslyiron oxide/carbon reported composite; iron oxide Table/carbon S1: materials composites for supercapacitors. Comparison of our sample performance with the previously reported iron oxide/carbon materials composites Authorfor supercapacitors. Contributions: Conceptualization, Y.Y. and M.B.Z.; methodology, A.A. and A.E.A.; validation, J.W. and M.B.Z.; formal analysis, J.W. and M.B.Z.; investigation, Y.Y., J.W. and M.B.Z.; resources, Y.Y.; data curation, Z.A.A.,Author M.N.,Contributions: M.H. and Conceptualization, M.B.Z.; writing—original Y.Y. and draftM.B.Z; preparation, methodology, A.A., A.A. A.E.A., and A.E.A.; A.Y.B., validation, M.N. and J.W. M.B.Z.; and writing—review and editing, Y.Y., J.W. and M.B.Z.; supervision, Y.Y. and Z.A.A.; project administration, M.H. and M.B.Z.; formal analysis, J.W. and M.B.Z.; investigation, Y.Y., J.W. and M.B.Z.; resources, Y.Y.; data curation, S.W.; funding acquisition, Z.A.A., M.H. and S.W. Z.A.A., M.N., M.H. and M.B.Z.; writing—original draft preparation, A.A., A.E.A., A.Y.B., M.N. and M.B.Z; Funding: This research received no external funding.

Nanomaterials 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/nanomaterials

Nanomaterials 2019, 9, 776 10 of 11

Acknowledgments: This project was funded by the NSTIP strategic technologies program (Project No. ADV 1285-02) in the Kingdom of Saudi Arabia. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Tanaka, S.; Salunkhe, R.R.; Kaneti, Y.V.; Malgras, V.; Alshehri, S.M.; Ahamad, T.; Zakaria, M.B.; Dou, S.X.; Yamauchi, Y.; Hossain MS, A. Prussian blue derived iron oxide nanoparticles wrapped in graphene oxide sheets for electrochemical supercapacitors. RSC Adv. 2017, 7, 33994–33999. [CrossRef] 2. Wang, Z.L.; Sun, K.; Henzie, J.; Hao, X.; Ide, Y.; Takei, T.; Bando, Y.; Yamauchi, Y. Electrochemically in situ controllable assembly of hierarchically-ordered and integrated inorganic-carbon hybrids for eﬃcient hydrogen evolution. Mater. Horiz. 2018, 5, 1194–1203. [CrossRef] 3. Sun, D.; Wang, H.; Deng, B.; Zhang, H.; Wang, L.; Wan, Q.; Yan, X.; Qu, M. A Mn Fe based Prussian blue Analogue@Reduced graphene oxide composite as high capacity and superior rate capability anode for lithium-ion batteries. Carbon 2019, 143, 706–713. [CrossRef] 4. Cao, L.; Liu, Y.; Zhang, B.; Lu, L. In situ controllable growth of prussian blue nanocubes on reduced graphene

oxide: Facile synthesis and their application as enhanced nanoelectrocatalyst for H2O2 reduction. ACS Appl. Mater. Interfaces 2010, 2, 2339–2346. [CrossRef][PubMed] 5. Jiang, H.; Leeb, P.S.; Li, C. 3D carbon based nanostructures for advanced supercapacitors. Energy Environ. Sci. 2013, 6, 41–53. [CrossRef] 6. Wong, M.H.; Zhang, Z.; Yang, X.; Chen, X.; Ying, J.Y. One-pot in situ redox synthesis of hexacyanoferrate/conductive polymer hybrids as lithium-ion battery cathodes. Chem. Commun. 2015, 51, 13674–13677. [CrossRef][PubMed] 7. Wang, X.J.; Krumeich, F.; Nesper, R. Nanocomposite of manganese ferrocyanide and graphene: A promising cathode material for rechargeable lithium ion batteries. Electrochem. Commun. 2013, 34, 246–249. [CrossRef]

8. Daneshvar, F.; Aziz, A.; Abdelkader, A.M.; Zhang, T.; Sue, H.-J.; Welland, M.E. Porous SnO2-CuxO nanocomposite thin ﬁlm on carbon nanotubes as electrodes for high performance supercapacitors. Nanotechnology 2019, 30, 015401. [CrossRef][PubMed]

9. Chou, S.-L.; Wang, J.-Z.; Chew, S.-Y.; Liu, H.-K.; Dou, S.-X. Electrodeposition of MnO2 nanowires on carbon nanotube paper as free-standing, ﬂexible electrode for supercapacitors. Electrochem Commun. 2008, 10, 1724–1727. [CrossRef] 10. Chen, Z.; Augustyn, V.; Wen, J.; Zhang, Y.; Shen, M.; Dunn, B.; Lu, Y. High-Performance Supercapacitors

Based on Intertwined CNT/V2O5 Nanowire Nanocomposites. Adv. Mater. 2011, 23, 791–795. [CrossRef] 11. Zakaria, M.B.; Tan, H.; Kim, J.; Badjah, A.Y.; Naushad, M.; Habila, M.; Wabaidur, S.; Alothman, Z.A.;

Yamauchi, Y.; Lin, J. Structurally controlled layered Ni3C/graphene hybrids using cyano-bridged coordination polymers. Electrochem. Commun. 2019, 100, 74–8075. [CrossRef] 12. Cai, Z.X.; Wang, Z.L.; Kim, J.; Yamauchi, Y. Hollow Functional Materials Derived from Metal-Organic Frameworks: Synthetic Strategies, Conversion Mechanisms, and Electrochemical Applications. Adv. Mater. 2019, 31, 1804903. [CrossRef] 13. Tanaka, S.; Zakaria, M.B.; Kaneti, Y.V.; Jikihara, Y.; Nakayama, T.; Zaman, M.; Bando, Y.; Hossain, M.S.A. Gold-loaded nanoporous iron oxide cubes derived from Prussian blue as carbon monoxide oxidation catalyst at room temperature. ChemistrySelect 2018, 3, 13464–13469. [CrossRef] 14. Liu, H.D.; Zhang, J.L.; Xu, D.D.; Huang, L.H.; Tan, S.Z.; Mai, W.J. Easy one-step hydrothermal synthesis of nitrogen-doped reduced graphene oxide/iron oxide hybrid as eﬃcient supercapacitor material. J. Solid State Electrochem. 2014, 19, 135–144. [CrossRef] 15. Zakaria, M.B.; Li, C.; Ji, Q.; Jiang, B.; Tominaka, S.; Ide, Y.; Hill, J.P.; Ariga, K.; Yamauchi, Y. Self-construction from 2D to 3D: One-Pot layer-by-layer assembly of graphene oxide sheets held together by coordination polymers. Angew. Chem. Int. Ed. 2016, 55, 8426–8430. [CrossRef][PubMed] 16. Azhar, A.; Zakaria, M.B.; Lin, J.; Chikyow, T.; Martin, D.J.; Alghamdi, Y.G.; Alshehri, A.A.; Bando, Y.; Hossain, M.S.A.; Wu, K.C.W.; et al. Graphene-wrapped nanoporous nickel-cobalt oxide ﬂakes for electrochemical supercapacitors. ChemistrySelect 2018, 3, 8505–8510. [CrossRef] Nanomaterials 2019, 9, 776 11 of 11

17. Chen, Y.L.; Hu, Z.A.; Chang, Y.Q.; Wang, H.W.; Zhang, Z.Y.; Yang, Y.Y.; Wu, H.Y. Zinc oxide/reduced graphene oxide composites and electrochemical capacitance enhanced by homogeneous incorporation of reduced graphene oxide sheets in zinc oxide matrix. J. Phys. Chem. C 2011, 115, 2563–2571. [CrossRef] 18. Huang, X.; Zhi, C.; Jiang, P.; Golberg, D.; Bando, Y.; Tanaka, T. Temperature-dependent electrical property transition of graphene oxide paper. Nanotechnology 2012, 23, 455705. [CrossRef] 19. Muthusamy, S.; Charles, J.; Renganathan, B.; Sastikumar, D. In situ growth of Prussian blue nanocubes on polypyrrole nanoparticles: Facile synthesis, characterization and their application as fiber optic gas sensor. J. Mater. Sci. 2018, 53, 15401–15417. [CrossRef] 20. Vermisoglou, E.C.; Devlin, E.; Giannakopoulou, T.; Romanos, G.; Boukos, N.; Psycharis, V.; Lei, C.; Lekakou, C.; Petridis, D.; Trapalis, C. Reduced graphene oxide/iron carbide nanocomposites for magnetic and supercapacitor applications. J. Alloys Compd. 2014, 590, 102–109. [CrossRef] 21. Liu, X.W.; Yao, Z.J.; Wang, Y.F.; Wei, X.W. Graphene oxide sheet-prussian blue nanocomposites: Green synthesis and their extraordinary electrochemical properties. Coll. Surf. B Biointerfaces 2010, 81, 508–512. [CrossRef][PubMed] 22. Jang, S.C.; Haldorai, Y.; Lee, G.W.; Hwang, S.K.; Han, Y.K.; Roh, C.; Huh, Y.S. Porous three-dimensional graphene foam/Prussian blue composite for efficient removal of radioactive137Cs. Sci. Rep. 2015, 5, 17510. [CrossRef][PubMed] 23. Wang, S.C.; Gu, M.; Pan, L.; Xu, J.; Han, L.; Yi, F.Y. The interlocked: In situ fabrication of graphene@prussian blue nanocomposite as high-performance supercapacitor. Dalton Trans. 2018, 47, 13126–13134. [CrossRef] [PubMed] 24. Yang, H.; Li, H.; Zhai, J.; Sun, L.; Zhao, Y.; Yu, H. Magnetic prussian blue/graphene oxide nanocomposites caged in calcium alginate microbeads for elimination of cesium ions from water and soil. Chem. Eng. J. 2014, 246, 10–19. [CrossRef] 25. Hu, M.; Belik, A.A.; Imura, M.; Mibu, K.; Tsujimoto, Y.; Yamauchi, Y. Synthesis of superparamagnetic nanoporous iron oxide particles with hollow interiors by using Prussian blue coordination polymers. Chem. Mater. 2012, 24, 2698–2707. [CrossRef] 26. Zakaria, M.B.; Belik, A.A.; Liu, C.; Hsieh, H.; Liao, Y.; Malgras, V.; Yamauchi, Y.; Wu, K.C. Prussian blue derived nanoporous iron oxides as anticancer drug carriers for magnetic-guided chemotherapy. Chem. Asian J. 2015, 10, 1457–1462. [CrossRef][PubMed] 27. Ma, W.; Chen, S.; Yang, S.; Chen, W.; Weng, W.; Cheng, Y.; Zhu, M. Flexible all-solid-state asymmetric supercapacitor based on transition metal oxide nanorods/reduced graphene oxide hybrid fibers with high energy density. Carbon 2017, 113, 151–158. [CrossRef] 28. Li, M.; Pan, F.; Shi, E.; Choo, G.; Lv, Y.; Chen, Y.; Xue, J. Designed construction of a graphene and iron oxide freestanding electrode with enhanced flexible energy-storage performance. ACS Appl. Mater. Interfaces 2016, 8, 6972–6981. [CrossRef] 29. Mallick, S.; Jana, P.P.; Raj, C.R. Asymmetric supercapacitor based on chemically coupled hybrid material

of Fe2O3-Fe3O4 heterostructure and nitrogen-doped reduced graphene oxide. ChemElectroChem 2018, 5, 2348–2356. [CrossRef]

30. Liu, M.; Sun, J. In situ growth of monodisperse Fe3O4 nanoparticles on graphene as ﬂexible paper for supercapacitor. J. Mater. Chem. A 2014, 2, 12068–12074. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).