Activated Graphene Deposited on Porous Cu Mesh for

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Activated Graphene Deposited on Porous Cu Mesh for

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Activated Graphene Deposited on Porous Cu Mesh for Supercapacitors

TaeGyeong Lim 1, TaeYoung Kim 2 and Ji Won Suk 1,3,4,*

1 School of Mechanical Engineering, Sungkyunkwan University, Suwon 16419, Gyeonggi-do, Korea; [email protected]
2 Department of Materials Science and Engineering, Gachon University, Seongnam 13120, Gyeonggi-do, Korea; [email protected]
3 Department of Smart Fab. Technology, Sungkyunkwan University, Suwon 16419, Gyeonggi-do, Korea 4 SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University,
Suwon 16419, Gyeonggi-do, Korea * Correspondence: [email protected]
Abstract: A porous Cu (P-Cu) mesh was used as a current collector and its morphological effect on the supercapacitor performance was investigated. A porous surface was obtained by thermally annealing the Cu mesh using ammonia gas. Hierarchically porous activated graphene (AG) with a high specific surface area (SSA) was deposited on the P-Cu mesh using electrophoretic deposition, aided by graphene oxide (GO). GO was thermally converted to electrically conductive reduced graphene oxide (rGO). The AG/rGO that was deposited on the P-Cu mesh achieved a high specific capacitance of up to 140.0 F/g and a high energy density of up to 3.11 Wh/kg at a current density of 2 A/g in 6 M KOH aqueous electrolyte. The high SSA of AG and the porous surface morphology of the Cu mesh allowed efficient electric double-layer formation and charge transport. This work offers an alternative to improve supercapacitors by combining a porous metallic current collector with porous AG.

Citation: Lim, T.; Kim, T.; Suk, J.W. Activated Graphene Deposited on Porous Cu Mesh for Supercapacitors. Nanomaterials 2021, 11, 893. https://
Academic Editor: Fabrizio Pirri
Received: 21 February 2021 Accepted: 27 March 2021 Published: 31 March 2021
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Copyright: © 2021 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 (https:// 4.0/).

Keywords: copper mesh; ammonia; activated graphene; supercapacitor
1. Introduction
Supercapacitors have attracted great attention in recent years as energy storage devices owing to their high power density, fast charge and discharge rates, long cycle life, and relatively simple structures [1,2]. However, their wide applications have been limited because their energy density is lower than that of conventional secondary batteries [3,4]. To enhance the performance of supercapacitors, most studies have focused on the development of high-performance electrode materials [5,6]. Graphene-based materials have been widely used as electrode materials due to their high specific surface area (SSA), high electrical conductivity, and thermal stability at moderate temperatures [7–9]. In particular, activated graphene (AG), which is synthesized by KOH-based chemical activation of graphene, has recently been reported to reach an extremely high SSA value of up to ~3100 m2/g with hierarchically porous structures [10,11]. Therefore, AG has been extensively investigated as an electrode material for advanced supercapacitors such as flexible and fiber-shaped supercapacitors with high performances [12,13].
In addition to the electrode materials, several studies have been conducted to improve the current collectors in supercapacitors. For example, the interfacial contact between the active electrode material and current collector was improved by incorporating carbonaceous materials [14], graphite inks [15], nanowires [16], and vertical graphene [17] on the current collector. Furthermore, various forms of current collectors, such as metallic wires and mesh, have been developed for energy storage devices with different shapes [13,18]. For instance, metallic-mesh electrodes have been used to develop planar supercapacitors, which demonstrated flexibility and transparency [19,20].

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In this work, to improve the performance of supercapacitors using metal mesh, we utilized a porous Cu (P-Cu) mesh as a current collector and highly porous AG as an active electrode material. The surface of a commercially available Cu mesh was etched by thermal annealing using ammonia (NH3) gas. Hierarchically porous AG and graphene oxide (GO) were deposited on the P-Cu mesh surface using electrophoretic deposition (EPD). Subsequent thermal annealing was performed to convert the GO into electrically conductive reduced graphene oxide (rGO). The AG/rGO, which was deposited on the P-Cu mesh, was electrochemically tested for supercapacitors.
2. Materials and Methods 2.1. Development of P-Cu Mesh Using Ammonia-Gas Etching
The P-Cu mesh was obtained by thermally annealing a Cu mesh (wire diameter = 160 µm, 50 mesh, Nilaco Corporation, Tokyo, Japan) using ammonia gas. The raw Cu mesh was cleaned using 0.1 M ammonium persulfate for 1 min and rinsed with deionized water. It was annealed using hydrogen (50 sccm) and argon (200 sccm) at 1000 ◦C for 30 min. Then, ammonia (20 sccm) without hydrogen was introduced for 20 min to etch the Cu mesh surface.
2.2. AG Synthesis
To synthesize AG, chemical activation by KOH was performed using rGO powders (rGO-V50, Standard Graphene, Ulsan, Korea) [10,13]. The KOH and rGO powders were mixed in water by stirring (rGO/KOH weight ratio = 1/8). The mixture was placed in an alumina boat and dried in an oven at 120 ◦C for 24 h. Subsequently, it was loaded into a tube furnace and annealed at 800 ◦C for 1 h under an argon atmosphere [10]. AG was washed using acetic acid (10% water) and dried in an oven at 100 ◦C for 24 h.
2.3. Deposition of AG/rGO on the Cu Mesh
AG was deposited on the Cu mesh using EPD [13]. GO (GO-P, Grapheneall, Siheung, Korea) and AG were mixed in deionized water at a weight ratio of 1:1. EPD was performed on the AG/GO aqueous suspension using a two-electrode system where a Cu mesh and a Pt plate were used as working and counter electrodes, respectively. A 10 V voltage was applied on the working electrode for 1 min with mild stirring. The Cu mesh, which was coated with AG/GO, was dried for 24 h. The sample was thermally annealed at 600 ◦C for 1 h to obtain AG/rGO, which was deposited on the Cu mesh.
2.4. Electrochemical Testing
To test the electrochemical performance of the synthesized AG material, AG was mixed with 5 wt.% polytetrafluoroethylene (60 wt.% dispersion in water, Sigma Aldrich, St. Louis, MO, USA) binder to assemble the electrode [11]. The mixture was homogenized and rolled to form a 50 µm thick sheet. The electrodes were prepared by punching the sheet into circular disks with 1 cm diameter. The supercapacitor test cells were assembled in a symmetric two-electrode configuration with two current collectors (conductive films, z-flo 2267P, Transcontinental Advanced Coatings, Matthews, NC, USA), two electrodes, a porous separator (3501, Celgard, Charlotte, NC, USA), and a 6 M KOH aqueous electrolyte. The test cell was supported by two stainless steel plates [11].
The AG/rGO-coated Cu meshes were cut into 1 cm × 1 cm size and had a mass of 0.7 mg. The sample was used as a current collector and an electrode material without any binder. The AG/rGO-coated Cu mesh was electrochemically tested using the same apparatus without any additional current collector.
Electrochemical tests of the assembled symmetric supercapacitors were performed by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) using a potentiostat (Autolab PGSTAT204, Metrohm, Herisau, Switzerland).

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The specific capacitance (Cm) of a single electrode was calculated from the GCD curves using the following equation [13,21]:
2I Cm = m dV/dt ,
where I (A) is the discharge current, dV/dt (V/s) is calculated from the slope of the linearly fitted discharge curve, and m is the mass of the single electrode. The current density in GCD tests was obtained by dividing the applied current by the mass of two electrodes.
The specific capacitance (Cm) of a single electrode was calculated from the CV curves using the following equation [13]:
2 I(V)dV Cm = mv∆V ,
where I (A) is the current, V (V) is the applied voltage, v (V/s) is the scan rate, ∆V (V) is the total scanning voltage, and m is the mass of the single electrode.
The gravimetric energy density (Em) and powder density (Pm) of the device were evaluated from the GCD curve using the following equation [13,21]:
Em = 1 CmU2, 8
Pm = Em , ∆tdischarge
where U (V) is the discharge potential and ∆tdischarge (s) is the discharge time.
2.5. Characterization of the Materials The morphology of the Cu mesh and AG/rGO was observed using scanning elec-
tron microscopy (SEM, JSM7000F, JEOL, Tokyo, Japan). The chemical composition and structure of AG/rGO were characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB250, Thermo Fisher Scientific, Waltham, MA, USA) using monochromated Al Kα radiation and Raman spectroscopy (XperRam35V, Nanobase, Seoul, Korea) with a 405-nm excitation laser. In the XPS analysis, peak deconvolution of the C 1s core-level spectrum was performed using the asymmetric Doniach–Sunjic line shape for the sp2-hybridized carbon, Gaussian–Lorentzian functions for the other spectral components, and the Shirley background model [22–24]. The nitrogen adsorption–desorption isotherms of AG were measured at 77 K (BELSORP-mini II, MicrotracBEL, Osaka, Japan).
3. Results and Discussion 3.1. Fabrication and Characterization of the AG/rGO-Coated P-Cu Mesh
The fabrication process of the P-Cu mesh coated with AG/rGO is schematically shown in Figure 1. Ammonia was used as a copper etchant for the integrated-circuit fabrication and synthesis of the nanoclusters [25,26]. In this work, the Cu-mesh surface morphology was modified by the gas-phase ammonia treatment. The P-Cu mesh was obtained by annealing the Cu mesh in ammonia atmosphere at 1000 ◦C. The ammonia-etching process generated rough and porous surfaces on the P-Cu mesh (Figure 2a,d), as compared with the smooth surface of the raw Cu (R-Cu) mesh (Figure 2b,e). In addition, the copper oxide formed on the Cu mesh was chemically removed by immersing the Cu mesh in 0.1 M ammonium persulfate for 1 min. The bare Cu (B-Cu) mesh without a native oxide exhibited slightly rough surfaces due to the chemical etching by ammonium persulfate (Figure 2c,f).

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The CV curves showed a rectangular shape at scan rates from 0.05 to 0.5 V/s (Figure 4b). The GCD curves exhibited triangular shapes with good symmetry at current densities of 1, 2, 4, and 8 A/g (Figure 4c). These results indicate a good electric double-layer (EDL) formation of AG. Therefore, the AG electrode demonstrated high specific capacitances of 133 and 121 F/g at current densities of 1 and 2 A/g, respectively. In addition, the Nyquist plot in the frequency range from 50 kHz to 0.1 Hz featured a vertical line in the lowfrequency region, which indicated an almost ideal capacitive behavior of AG. In this regard, synthesized AG demonstrated high electrochemical performance for supercapacitors.

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Moreover, the equivalent series resistances of the P-, R-, and B-Cu meshes estimated from the IR drop in the discharge curve at a current density of 2 A/g were 1.88, 4.03, and 1.11 Ω, respectively. This result implies that the removal of the Cu oxide layer in the P- and B-Cu meshes reduced the interfacial resistances of the device. Because of the outstanding

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electrochemical performance of the AG/rGO/P-Cu mesh, the device composed of the AG/rGO/P-Cu mesh exhibited the highest energy density of 3.11 Wh/kg and power Nanomaterials 2021, 11, x FOR PEER REdVeInEsWity of 0.83 kW/kg. In contrast, the R- and B-Cu meshes demonstrated relatively7loowf 1e0r energy densities of 2.71 and 2.46 Wh/kg, respectively.

FFiigguurree 55.. ((aa––cc)) CCVV ccuurrvveess ooff tthhee suppeerrccaappaacciittoorrss uussiinngg AAGG//rGrGOOddeeppoosistieteddoonn(a(a))PP--CCuu,,((bb))RR--CCuu,,aanndd ((cc)) BB--CCuu mmeesshh eelleeccttrrooddeessaattssccaannrraatteessooff00.1.1,,00.5.5, ,aanndd11VV/s/.s(.d(–df–) fG) CGDCDcucruvrevseosfotfhethseuspueprcearpcaapciatcoirtsorussuinsginAgGA/GrG/OrGdOepdoespitoesditoend (odn) (Pd-) CPu-C, (ue,)(Re)-CRu-C, aun, dan(df)(Bf)-CBu-Cmuemshesehleecltercotdroeds east acut rcruernret ndtednesnitsieitsieosf o2f, 24,, 4a,nadn8dA8/Ag./g.
FEigISuraena5ldy–sfisswhoaws ps etrhfeorGmCeDd tcoufruvretsheorf itnhveestuigpaetrecatphaeceitfofercthoaftthuesePd-CthuemAeGsh/r.GTOh-e cNoaytqeudiCstupmloetshoefstahtecAurGre/nrtGdOen-csoitaiteesdofC2u, 4m, aenshde8s Aw/egr.eTohbetGaiCnDedcuinrvtheeofrtehqeuAeGnc/yrGrOan/Pg-e Cfruome5s0hkeHxhzibtoite0d.1sHymz m(Feigtruicrean6d). lTinheearsesmhaicpiersc,lewdhiearmeaestetrheinRt-haenhdigBh-C-furemqueeshnecsycroeagtieodn wofiththeAGN/yrqGuOistshpolowtsedcorrerleastpivoenlydeadsytmo mthetrcihcaargned-tnraonns-flienreraersiGstCaDncecu(rRvcet)s.reTlhateedspteocitfhice cealpecatcriotadnecreessoisfttahneceP,-,cRon-,tanctdbBe-tCwueemnetshheeeslceocatrtoedewainthdAcuGr/rreGnOt caotllaectuorrr,eanntddenlescitryoolyf t2e Nanomaterials 2021, 11, x FOR PEER RAEioV/ngIEicwWrerseist1a4n0c.0e,in1s2i2d.e1,thaendpo1r0es0.o6f Fth/ge,erlecstpreocdteiv[e3l8y,3. 9T]h. eRcAt oGf/trhGeOA/GP-/CruGOm/ePs-hCsuhmowes8ehdowf a1an0s iamlmproosvt e0dΩs,pwechiefriceacsatphaocsietaonfcteh,ecRo-mapnadreBd-Cwuimthestheast wofithAGAGt/esrtGedO watetrhee0.s8a1manedc0u.r6r7enΩt, drenspsietcytiovfe2lyA. /Tgh. TishrisessupletciinfidcicaapteascitthaantcethvealPu-eCouf mthesAhGw/ritGhOa/Pp-oCruoumsessuhrfisacheigphreorvtihdaend oirmcpormovpeadracbolnetatoctsthboestewreeepnotrhtedelienctorothdeermwaotrekrisaltshant dusceudrrmenettacolllilcecmtoershsuesrfacsec,uwrrheincht scluoelrdlfeatccoteoe,rfwsfi;chfioiecrnhetxlceahdmatrpogleef,tftrihcaeinespnfrteecrvhwiaoirutghsewrtaroparnikdssfaehnradwveeitxshhceroalwlpenindtsapEneDdcLifeifxcoccreamlpleaantcitiotEanDnocLnesftohorefm6A3aGtFio//gnrGfooOnr toshxueirdAfiazGcee/dsrG[s3iOn8g–s4lue1r-]wf.aacellsed[3c8a–r4b1o].n nanohorn/nanotube composites on Pt mesh [34], 107.8 F/g for carbon ink coated on Ni/Au-deposited stainless steel mesh [35], 152 F/g for mesoporous carbon nanofibers on Ni mesh [36], and 156 F/g for biomass-derived activated carbon on stainless steel mesh [37].
Moreover, the equivalent series resistances of the P-, R-, and B-Cu meshes estimated from the IR drop in the discharge curve at a current density of 2 A/g were 1.88, 4.03, and 1.11 Ω, respectively. This result implies that the removal of the Cu oxide layer in the Pand B-Cu meshes reduced the interfacial resistances of the device. Because of the outstanding electrochemical performance of the AG/rGO/P-Cu mesh, the device composed of the AG/rGO/P-Cu mesh exhibited the highest energy density of 3.11 Wh/kg and power density of 0.83 kW/kg. In contrast, the R- and B-Cu meshes demonstrated relatively lower energy densities of 2.71 and 2.46 Wh/kg, respectively. FFiigguurree 6. NNyyqquuiissttpplolotstsofotfhethseupsEeuIrpScaeparcnaacapiltayocrsisitsourswsinaugssAipnGge/rAfroGGrOm/rGdeedOpotdoseitpfeudorsotihtneed(ra)oiPn-vC(eaus),t(iPbg-)aCRtue-,Ct(hub,e)aenRfd-fCe(ccu)t,Boa-fnCduthm(ece)PsBh-C-eCuleucmtmroeedsshehs.. The eTlehcetrforedqeuse. nTchyerfarenqguesenfrcoymraN5n0gykeqsHufzirsottomp05l.o10tHksHzo.zf ttoh0e.1AHGz/.rGO-coated Cu meshes were obtained in the frequency range from 50 kHz to 0.1 Hz (Figure 6). The semicircle diameter in the high-frequency region of the NMyoqrueiosvt epr,lothtse ccaoprraecsiptiovnedcehdaratoctethriesticchsaorfgteh-etrAanGs/freGr Ore/sPi-sCtaunmcees(h𝑅w)asrceolantfeidrmteodtbhye tehleecitmropdeedraenscisetapnhcaes,ecaonngtalectpbloettwaesesnhothweneilnecFtriogduerea7nad. Tcuhrerepnhtacsoelalencgtoler,waansdneelaercltyro−ly90te˚ aiotnliocwrefsriesqtaunecneciienss,idwehtihche pisoirneds iocfattihvee eolfecctarpodaceit[i3v8e,3b9e]h. a𝑅viorosf [t1h1e].ATGh/erGsuOp/Per-cCaupamcietsohr uwsains galtmheoAstG0/rΩG,Ow/hPe-Creuasmtehsohsaelsoof ethxehiRbi-teadndthBe-sCtaubmleeasnhdeslowngithcyAclGin/grGpOerfworemrea0n.c8e1waintdh a0p.6p7roΩx,imreastpeelyct8iv2e%lyc. aTphaicsitarenscueltreitnednitcioatnesatthaactutrhreenPt -dCeunsmityesohf w4 iAth/gaafpteorro2u0s00sucyrfcaleces

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Figure 6. Nyquist plots of the supercapacitors using AG/rGO deposited on (a) P-Cu, (b) R-Cu, and (c) B-Cu mesh

electrodes. The frequency ranges from 50 kHz to 0.1 Hz.

MMooreroeovveer,r,ththeeccaappaaccititiviveecchhaarraacctteerriissttiiccssoofftthheeAAGG//rGrGOO/P/-PC-Cuummesehshwwasascocnofnifirmrmededbbyy ththeeimimppeeddaanncceepphhaasseeaannggllee plot as sshhoowwnniinnFFigiguurree77aa. .TThheepphahsaeseanagnlgelwe wasansenaerlayrl−y 9−09◦0a˚ t
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FuFnudnidning:gT: hTihsisstsutduydywwasassuspuppoprotretdedbybythteheBaBsaiscicRReseesaeracrhchPProrgorgarmamththroruoughghththeeNNataitoionnalalRReseesaeracrhch FoFuoundndataitoionn ooffKoKroeare(aNR(FN)RgFra)ntgfruanndtedfubyndtheedMbinyistrthyeof SMciienniscteryandoIfCTSc(Nieon.c2e01a9nRd1A2ICCT208(9N78o5. ).
The work was also supported by the Korea Electric Power Corporation (No. R19XO01-16).
Conflicts of Interest: The authors declare no conflict of interest.
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