1   Graphene  foam  functionalized  with  electrodeposited  nickel  hydroxide  for  energy   applications   Sandra  Ruiz-­‐Gómeza,b,  Alberto  Boscáb,c,  Lucas  Péreza,b,*,  Jorge  Pedrósb,  Javier   Martínezb,  Antonio  Páezd,  and  Fernando  Calleb,c   a  Dept.  Física  de  Materiales.  Universidad  Complutense  de  Madrid.  28040  Madrid   (Spain)   b  Instituto  de  Sistemas  Optoelectrónicos  y  Microtecnología.  Universidad  Politécnica   de  Madrid.  28040  Madrid  (Spain)   c  Dpto.  de  Ingeniería  Electrónica,  Universidad  Politécnica  de  Madrid.  28040   Madrid  (Spain)   d  Repsol,  Centro  Tecnológico,  Móstoles.  28931  Madrid  (Spain)   *  Corresponding  author     Abstract   The  need  of  new  systems   for   the  storage  and  conversion  of   renewable  energy  sources   is   fueling  the  research  in  supercapacitors.  In  this  work,  we  propose  a  low  temperature  route   for  the  synthesis  of  electrodes  for  these  supercapacitors:  electrodeposition  of  a  transition   metal   hydroxide   -­‐   Ni(OH)2   on   a   graphene   foam.   This   electrode   combines   the   superior   mechanical   and   electrical   properties   of   graphene,   the   large   specific   surface   area   of   the   foam  and  the   large  pseudocapacitance  of  Ni(OH)2.  We  report  a  specific  capacitance  up  to   900   F/g   as   well   as   specific   power   and   energy   comparable   to   active   carbon   electrodes.   These  electrodes  are  potential  candidates  for  their  use  in  energy  applications.             2   Graphical  abstract           Highlights     Ni(OH)2  has  been  incorporated  in  3D  graphene  electrodes  by  a  low  temperature  route   The  obtained  electrodes  show  excellent  supercapacitors  characteristics   No  obvious  capacitance  degradation  is  observed  after  3000  cycles  charge/discharge     0 5 1 0 1 5 2 0 0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 Ni(OH)2/GF 1h Ni(OH)2/GF 2h Ni(OH)2/GF 3h Ni(OH)2/GF 4h Ni(OH)2/GF 6h GF Ispe (F/g) C sp e (F /g ) 3   1. INTRODUCTION   The  environmental  problems  arisen   from  the  use  of   fossil   fuels  have  strengthened  the   search   for   the   storage   and   conversion   of   renewable   energy   sources.   Electrochemical   capacitors,   also   know   as   supercapacitors,   have   been   proposed   as   novel   energy   storage   devices  because   they  have  not  only   shorter   charging   time   than  batteries,  but  also  higher   energy   density   than   conventional   dielectric   capacitors1–5.   Supercapacitors   are   promising   energy  storage  devices  as  well  as  power  supplies  for  systems  requiring  high  power  density   and   long   cycle   life   as   electric   vehicles   and   portable   electronic   devices6.   Considering   the   mechanism   involved   in   the   energy   storage   process,   supercapacitors   can   be   divided   into   two   families:   electrical   double   layer   supercapacitors,   based   on   quick   adsorption/desorption  process   on   the   surface   of   the   electrode;   and  pseudocapacitors,   in   which  the  capacitance  is  originated  on  a  fast  faradaic  charge  transfer  reaction  7,8.   Transition-­‐metal  oxides  as  well  as  conductive  polymers  can  be  used  as  active  materials   in  asymmetrical  supercapacitors  9,10.  Among  the  used  materials  to  date,  ruthenium  oxides   have  been  extensively  studied  as  active  electrode  materials  as  they  can  show  a  capacitance   as   large  as  863  F/g   in  acidic  electrolytes11.  However,  ruthenium  is  an  expensive  material   and,   although   its   electrochemical  properties  are  nearly   ideal,  has   limited  possibilities   for   commercialization.   Hence,   much   effort   has   been   aimed   at   searching   for   alternative   inexpensive   electrode  materials   with   good   pseudocapacitance   behavior   as   cobalt,   nickel   and  manganese   oxides   and   hydroxides   because   of   their   reversible   redox   transition   they   show  at  the  electrode  surface12–14.  In  this  sense,  Ni(OH)2  has  attracted  increasing  attention   over   the  past  decades  as  potential  material   for   supercapacitors  due   to   the  extraordinary   high  value  of  the  capacitance  that  can  theoretically  be  achieved  (2082  F/g)15–17.  Moreover,   it  is  a  non-­‐expensive  and  environment-­‐friendly  material.   However,   there   is   still   a   great  number  of   challenges   to  overcome   in  order   to   get  high   capacity  Ni(OH)2  electrodes.  It  is  well  accepted  that  the  overall  performance  of  an  electrode   material   depends   not   only   on   the   microstructure   but   also   on   the   conductivity   of   the   electrode18–20,  and  NiOH2  has  a  poor  electrical  conductivity  (1017  S/cm)21,  which  is  possibly   the   major   drawback   of   this   material   for   being   used   as   active   material.   Fortunately,   the   conductivity  of  the  electrode  can  be  improved  by  introducing  carbon-­‐based  materials6,22,23.   Among   them,   graphene   has   attracted   tremendous   attention   in   the   last   years   due   to   its   4   unique   properties   over   other   carbon   nanomaterials   in   view   of   its   application   in   supercapacitors.   These   unique   properties   include   a   large   electrical   conductivity,   high   specific  surface  area,  high  mechanical  flexibility  and  chemical  stability24,25.  Recently,  three-­‐ dimensional  (3D)  graphene  has  been  proposed  as  a  candidate  for  energy  applications.  This   3D   graphene,   in   the   form  of   graphene   foam,   provides   a   free-­‐standing   3D   interconnected   network   of   graphene   which   keeps   the   main   properties   of   graphene.   These   foams   are   perfect  candidates  to  serve  as  a  robust  scaffold  to  be  functionalized  with  a  metal  oxide  to   get   a   large   pseudocapacitance26–28.   This   combination   of   graphene   and   a   transition   oxide   metal  merges  the  previously  mentioned  advantages  of  both  components29–31.   In  this  work  we  propose  a  low  temperature  route  to  functionalize  graphene  foams  to  be   used  as  an  electrode  in  electrochemical  supercapacitors.  The  functionalization  is  based  on   electrochemical  synthesis  of  NiOH2  which  is  incorporated  to  a  free  standing  graphene  foam,   previously  fabricated  using  Chemical  Vapor  Deposition  (CVD)  of  graphene  on  a  Ni  foam.   2. MATERIAL  AND  METHODS   The  3D  graphene  foam  is  grown  by  using  a  Ni  foam  as  the  catalytic  substrate  in  a  CVD   process.  After  growth,  the  Ni  foam  is  removed  to  obtain  a  free-­‐standing  3D  graphene  foam.   All   the   measured   foams   are   squared   samples   10   x   15   mm2   in   size   approximately.   The   electrodeposition  of   the  Ni(OH)2  coatings  on   the  graphene   foam  was  performed  using  an   Ecochemie  Autolab  PGSTAT  302N  potentiostat/galvanostat  in  a  three-­‐electrode  pyrex  cell   using   a   Pt   gauze   as   counter   electrode,   a   Ag/AgCl   electrode   (BASi)   as   reference   and   the   graphene  foam  as  working  electrode.  For  that,  an  ohmic  contact  was  done  in  the  graphene   foam  using  conductive  silver  paint.  The  contact  was  electrically  isolated  from  the  solution   using   a   chemically   resistant   epoxy   (Loctite   3423AB).   The   electrolyte   contains   0.2   M   Ni(NO)3   ·   6H2O   and   0.2   M   hexamethylenetramine   (C6H12N4).   All   the   chemicals   were   of   analytical  grade,  used  without  further  purification  and  were  mixed  in  deionized  water.   The   mass   of   the   functionalized   electrode   (graphene   foam   +   Ni(OH)2   layer)   was   measured  after  drying  the  sample  in  air  at  room  temperature,  being  their  average  mass  of   the   electrodes   3.5   mg   approximately.   The   crystalline   structure   of   the   Ni(OH)2   was   examined  by  X-­‐ray  diffraction  (XRD)  in  a  Bragg-­‐Brentano  configuration  using  a  PANalytical   5   X’Pert   MPD   diffractometer   using   Cu-­‐Kα   radiation.   The   surface   morphology   and   microstructure   were   examined   by   Scanning   Electron   Microscopy   (SEM).   All   the   electrochemical   measurements   were   performed   in   a   conventional   three-­‐electrode   configuration   in  a  3  M  KOH  aqueous  solution.  Cyclic  voltammetry   (CV)  and  galvanostatic   charge/discharge   tests   were   performed   using   the   Ecochemie   Autolab   PGSTAT   potentiostat/galvanostat.   All   the   reported   values   are   normalized   by   the  mass   of   the   full   electrode  (graphene  foam  +  functionalization).     3. RESULTS  AND  DISCUSSION   Figure   1.a   shows   graphene   foam   before   functionalization.   Due   to   the   excellent   conductivity  and  mechanical  properties  of  graphene,  this  foam  is  a  perfect  scaffold  for  the   electrode:   it   can   be   easily   manipulated   and   provides   an   excellent   charge-­‐extraction   performance.   In   addition,   the   ripples   and   wrinkles   of   graphene   provide   a   large   surface   area,  which  increase  the  capacitance  of  the  electrode.   The  foams  have  been  functionalized  via  potentiostatic  electrodeposition  with  a  cathodic   potential   of   −0.5   V   (vs   Ag/AgCl)   at   70o  C.   To   remove   the   air   trapped   inside   the   foam,   ensuring  that  the  electrolyte  penetrates   in  the  pores  of  the  foam,  the  graphene  foam  was   immersed   in   the   electrolyte   and   kept   under   vacuum   for   one   hour   just   before   electrode   positing.  We  grew  a  series  of  samples  changing  the  deposition  time  from  1  to  6  hours.  The   thickness  of  the  coating,  measured  by  SEM  in  cross-­‐section  view,  increases  from  less  than  1   micron  for  1  hour  to  more  than  3  microns  for  6  hours.  Figures  1.b  to  1.d  show  SEM  images   of  the  3D  graphene  foam  coated  with  a  thin  nickel  hydroxide   layer.  The  functionalization   clearly  changes  the  surface  morphology  of  the  foam.  There  are  nickel  hydroxide  nanoflakes   covering  the  graphene  and  forming  a  compact  but  porous  film,  with  a  petal-­‐like  structure.   In  addition  to  the  pseudocapacitance  of  the  hydroxide,  this  porous  structure  increases  the   surface   area,   which   is   expected   to   increase   the   capacitance.   We   have   not   found   any   qualitative   difference   in   the   morphology   for   all   the   samples   grown   in   this   work.   The   procedure   for   the   synthesis   of   the   electrodes   is   similar   to   the   one   described   by   Cao   et   colaborators31.   However,   it   is   important   to   note   that,   in   our   case,   no   further   thermal   treatment   is   given   to   the   electrodes   after   the   functionalization.   Therefore,   the   foam   and   their  contacts  are  kept  at  low  temperature  during  the  whole  functionalization  process.   6   A   typical   XRD   pattern   of   a   functionalized   electrode   is   shown   in   figure   Figure   2.   The   spectrum   shows   two   series   of   peaks:   one,   marked   with   red   crosses,   can   be   indexed   as   graphite   (cod:  01-­‐089-­‐8487)  and  the  other,  marked  with  orange  stars,  can  be   indexed  as   β−Ni(OH)2   (cod:   00-­‐014-­‐0017).   The   broadening   of   the   peaks   in   XRD   pattern   is   closely   related   to   the  small   crystalline   size  of   the  Ni(OH)2  nanoflakes   that   cover   the   foam.  These   hydroxide   nanostructures   have   already   shown   a   high   electrochemical   activity   on   multiwalled  carbon  nanotubes32.   Cyclic   voltammograms   (CV)   and   galvanostatic   charge/discharge   measurements   have   been  used  to  evaluate  the  electrochemical  behavior  of  the  electrodes.  Figure  3.a  shows  the   CV  for  a  functionalized  electrode  (4  hours)  measured  at  different  scan  rates  from  5  to  20   mV  s−1  in  the  potential  range  of  0  −  0.5  V  (vs.  Ag/AgCl).  All  the  curves  have  a  pair  of  redox   peaks  suggesting  that  the  specific  capacitance  of  Ni(OH)2/graphene  foam  is  primarily  due   to  pseudocapacitance.  For  a  Ni(OH)2  electrode,  it  is  well-­‐accepted  that  the  surface  redox   reaction  is  the  following22:   Ni(OH)2  +  OH−  →  NiOOH  +  H2O  +  e−   According  to  this  reaction,  the  anodic  peak  (positive  current  density)  at  around  0.45  V   (vs.  Ag/AgCl)   is  attributed   to   the  oxidation  of  Ni(OH)2  to  NiOOH,  while   the  cathodic  peak   (negative   current   density)   at   around   0.15   V   (vs.   Ag/AgCl)   is   related   to   the   reverse   reduction   process.   From   Figure   3.a   it   is   clear   that,   when   the   scan   rate   is   increased,   the   current   increases   and   the   reduction   peak   shifts   towards   more   negative   values   (shift   marked   with   an   arrow   in   the   figure).   This   shift   is   due   to   the   increase   of   the   internal   diffusion   resistance  within   the  pseudocapacitive  material   that  normally  happens  with  an   increase  in  scan  rate33,34.   Figure  3.b  compares  the  galvanostatic  charge/discharge  curves  measured  in  a  potential   window  from  0  V  to  0.4  V  at  specific  currents  (from  1  A/g  to  10  A/g)  in  3  M  KOH  electrolyte   for  the  graphene  foam  functionalized  with  Ni(OH)2  for  4  hours.  The  curves  show  a  pair  of   small   plateaus   starting   and   finishing   at   potential   values   close   to   the   peaks   seen   in   the   voltammograms  (Figure  3.a).   The   charging   time   and,   therefore,   the   capacitance   decreases   while   increasing   the   current  as  expected.  Considering  that  the  largest  measured  value  (t∼  1250  s)  corresponds   7   to  a  current  of  1  A/g,  we  have  chosen  this  value  of  the  specific  current  to  compare  between   the   different   functionalized   electrodes.   Figure   3.c   compares   the   electrochemical   performance  of  the  different  Ni(OH)2/graphene  composites.  From  the  figure  it  is  clear  that   the   discharging   time   and,   therefore   the   specific   capacitance,   increases   with   the   functionalization  time  up  to  4  hours.  Afterwards,   the  capacitance  and  the  performance  of   the   electrode   degrades   with   larger   functionalization   time.   This   is   probably   due   to   a   blocking  of  the  electrodeposition  process.  After  4  hours,  the  thickness  of  the  coating  starts   to   be   too   wide   and   the   ohmic   drop   too   large   for   the   electrodeposition   process   and,   therefore,  the  electrodeposition  stops  and  the  electrode  starts  to  degrade.   The  specific  capacitance  can  be  calculated  from  the  galvanostatic  discharge  curves  shown   in  Figure  3.c,  using  the  following  equation35:   𝐶!"# = 𝐼∆𝑡 𝑚∆𝑉   where   I   is   the   current   used   in   the   discharge   process,   ∆t   is   the   time   needed   for   a   full   discharge   process,   m   is   the   mass   of   the   electrode   and   ∆V   is   the   voltage   drop   after   a   discharge.   Figure  4.a  collects  the  capacitance  for  all  the  synthesized  electrodes,  calculated  from  the   curves  shown  in  Figure  3.c.  As  mentioned  before,  the  electrode  functionalized  for  4  hours   shows   the  highest  value  of  capacitance,  900  F/g  at   low  specific  current.  Considering   that   the   main   contribution   to   the   specific   capacitance   in   these   electrodes   is   the   pseudocapacitance,  this  parameter  drops  as  expected  when  the  specific  current  increases.   The  data  of   two  other  electrodes  have  also  been   included   in   the   figure   for  comparison:  a   graphene   foam   without   functionalization   (green   curve)   and   an   electrode   made   with   commercial   activated   carbon,   Maxsorb   (black   curve).   The   specific   capacitance   of   the   functionalized  electrodes  is  clearly  larger  than  the  one  of  the  graphene  foam,  showing  the   importance  of  the  functionalization  in  the  performance  of  the  device.   The   Ragone   diagrams   —   specific   power   as   a   function   of   specific   energy   —   for   all   electrodes  are  shown  in  Figure  4.b.  The  functionalized  graphene  foam  is  clearly  better  than   the   one   without   functionalization   in   terms   of   both   power   and   energy.   In   particular,   a   maximum   power   of   8.5   kW/kg   and   energy   of   11   Wh/g   are   reached.   Compared   with   Maxsorb,  the  functionalized  electrodes  are  better  in  terms  of  power  and  their  behavior  is   8   similar   in   terms   of   energy.   Part   of   this   behavior   is   due   to   the   large   specific   capacitance,   shown  in  previous  figures,  and  partly  due  to  the  low  equivalent  series  resistance  (ESR)  that   these   electrodes   show   (see   Figure   4.c).   The   contact  made   in   the   graphene   foam   for   the   electrodeposition   process   is   not   degraded   during   functionalization   due   to   the   low   temperature  process  used  and,  therefore,  can  be  also  used  as  contact  electrode,  keeping  the   ESR   low   and   improving   the   performance   of   the   device.   Finally,   to   test   the   long-­‐term   performance   of   the   electrodes,   we   have   determined   the   capacitance   retention   for   best   electrode  after  cycling.    The  capacitance  of  capacitance  is  close  to  98%  after  3000  cycles  at   a  current  density  of  2  A/g.     4. CONCLUSIONS   In  this  work  we  have  shown  a   low-­‐temperature  electrochemical  route  to   functionalize   graphene   foams   with   Ni(OH)2   to   produce   electrodes   for   supercapacitors.   This   method   combines   in   a   single   electrode   the   superior   mechanical   and   electrical   properties   of   graphene  and  the  large  specific  area  of  the  foam  with  the  large  pseudocapacitance  of  nickel   hydroxide.   We   have   shown   that   no   further   thermal   annealing   is   needed   after   electrodeposition  to  produce  electrodes  with  large  specific  capacitance.  A  maximum  value   of   900   F/g   has   been   obtained,   two   orders   of  magnitude   higher   than   the   graphene   foam   without   functionalization.   The   functionalized   Ni(OH)2/graphene   foam   electrodes   show   electrochemical   characteristics   that   make   them   potential   candidates   to   be   used   as   electrodes  in  supercapacitors.     ACKNOWLEDGMENTS   We   thank   J.   Velázquez,   from   the   C.A.I.   de   Difracción   de   Rayos   X   de   la   Universidad   Complutense  de  Madrid,  for  XRD  measurements  and  J.M.  Rojo  y  V.  Barranco  from  Instituto   de  Ciencia  de  Materiales  de  Madrid  (CSIC)  for  the  measurements  on  Maxsorb  electrodes.   This  research  was  supported  by  Repsol  under  the  Inspire  program.   9   REFERENCES   1J.  Yan,  Z.  Fan,  W.  Sun,  G.  Ning,  T.  Wei,  Q.  Zhang,  R.  Zhang,  L.  Zhi,  and  F.  Wei,   “Advanced   asymmetric   supercapacitors   based   on   Ni(OH)2/graphene   and   porous   graphene   electrodes  with  high  energy  density,”  Adv.  Funct.  Mater.  22,  2632–2641  (2012).   2G.   R.   Fu,   Z.   Hu,   L.   Xie,   X.   Jin,   Y.   Xie,   Y.   Wang,   Z.   Zhang,   Y.Yang,   and   H.   Wu,   “Electrodeposition   of   nickel   hydroxide   films   on   nickel   foil   and   its   electrochemical   performances  for  supercapacitor,”  Int.  J. 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 can  be  indexed  as  β-­‐Ni(OH)2  (cod:  00-­‐014-­‐0017)       Ϯɽ��Ğ β−Ni(OH) 2 GF 10 20 30 40 * * * × *× � ď͘ � Ŷŝ ƚƐ 50 × 14     Figure   3.   a)   Cyclic   voltammograms   measured   at   different   scan   rates   in   an   electrode   functionalized   for  4  hours.  b)  Charge/discharge  curves  measured  with  different   currents   for   the   same   electrode.   c)   Discharge   curves   measured   at   1   A/g   for   the   different   synthesization-­‐time  electrodes.     Current 1 A/g 2 A/g 2.5 A/g 5 A/g 7.5 0 500 1000 1500 2000 2500 0.1 0.2 0.3 0.4 A/g a) b) c) 0 500 1000 0.2 0.4 Current: 1 A/g 1h 2h 4h 6h Po te nt ia l ( V) V s Ag /A gC l ( V) Time (s) Po te nt ia l ( V) V s Ag /A gC l ( V) Time (s) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.00 Scan Rate 5 mV/s 10 mV/s 20 mV/s 30 mV/s 40 mV/s 50 mV/s 60 mV/s C ur re nt d en si ty (A /g ) 10 5 -5 Potential (V) Vs Ag/AgCl (V) 15     Figure   4.   (a)   Specific   capacitance   retention  with   increasing   specific   current,   (b)   Ragone   diagram  and  (c)  equivalent  series  resistance  for  the  different  electrodes  synthesized  in  this   work.   A   graphene   foam   without   functionalization   (GF)   and   a   electrode   made   with   commercial  active  carbon  (Maxsorb)  are  also  included  for  comparison.   a) c) 0 5 10 15 20 0 200 400 600 800 1000 Ni(OH)2/GF 1h Ni(OH)2/GF 2h Ni(OH)2/GF 3h Ni(OH)2/GF 4h Ni(OH)2/GF 6h GF C sp e ( F/ g) Ispe (A/g) Maxsorb a) b) 0.01 0.1 1 10 100 1000 Ni(OH)2/GF 1h Ni(OH)2/GF 2h Ni(OH)2/GF 3h Ni(OH)2/GF 4h Ni(OH)2/GF 6h GF P sp e ( W /k g) Wspe (Wh/kg)