Highly packed graphene–CNT films as electrodes for aqueous supercapacitors with high volumetric performance

Highly packed g aphene–CNT films as elec odes
o aqueous supe capaci o s wi h high olume ic
pe o mance†
Noel D´
ıez, *
a
C is ina Bo as,
a
Roman Mysyk,
a
Eide Goikolea,
a
Te´
ofilo Rojo
ab
and Daniel Ca iazo *
ac
The inc easing complexi y o po able elec onics demands he de elopmen o ene gy s o age de ices wi h
highe olume ic ene gy and powe densi ies. In his wo k we epo a simple s a egy o he p epa a ion o
pa ially educed g aphene oxide/ca bon nano ube composi es (p GO–CNT) as highly packed sel -s anding
binde - ee films sui able as elec odes o supe capaci o s. These ca bon-based films a e easily ob ained by
he hyd o he mal ea men o an aqueous suspension o g aphene oxide and CNTs a 210 Cand hen
compac ed unde p essu e. The p GO–CNT films, which had an appa en densi y as high as 1.5 g cm
3
,
we e in es iga ed as binde - ee elec odes o aqueous supe capaci o s using 6 M KOH solu ion as he
elec oly e. The esul s show ha he p esence o me ely 2 w % o CNTs p oduces a significan
enhancemen o he capaci ance e en ion a high cu en densi ies compa ed o he CNT- ee samples,
and his imp o emen is especially ele an in sys ems o med using elec odes wi h high mass loadings.
Volume ic capaci ance alues o 250 F cm
3
a 1 A g
1
wi h ou s anding capaci ance e en ion (200 F cm
3
a 10 A g
1
)we eachie edusing hep GO–CNT elec odes wi h an a eal mass loading abo e 12 mg cm
2
.
In oduc ion
Elec ochemical capaci o s (ECs), gene ally known as supe -
capaci o s, a e ene gy s o age de ices ha exhibi highe ene gy
han con en ional capaci o s and la ge powe han ba e ies.
1,2
ECs can s o e ene gy by wo diffe en mechanisms: (i) in a pu ely
elec os a ic way h ough he o ma ion o an elec ical double
laye (EDL) be ween he ions om he elec oly e and he su ace
o he elec odes o (ii) h ough as and e e sible a adaic
eac ions unde gone by edox species p esen a he su ace o he
elec odes.
3,4
None o hese mechanisms in ol e he inse ion o
ions in o he elec ode la ice, enabling a as cha ge–discha ge
and ensu ing a e y long cycle li e. Po ous ca bons, pa icula ly
ac i a ed ca bons, a e he mos used ma e ials o he ab ica ion
o EC elec odes due o hei la ge specic su ace a ea, high
elec ical conduc i i y, ine ness and mode a e cos . Un o u-
na ely, hese ca bons ha e a e y low densi y (gene ally below
0.7 g cm
3
) due o he p esence o po osi y ha is needed o
sizeable non- a adaic cha ge s o age. The wide po es a e ooded
wi h he elec oly e, which con ibu es o a dispensable weigh
excess o he de ice bu no o he ene gy s o ed. This limi s he
olume ic ene gy and powe densi y o ECs, hinde ing hei use
in po able elec onics.
5
G aphene, an allo ope o ca bon consis ing o a monolaye
o ca bon a oms wi h ou s anding elec onic, he mal and
mechanical p ope ies, has been ecen ly pu o wa d as
a p omising candida e o eplace po ous ca bons in ECs.
6
The
open su ace o g aphene allows as ion adso p ion/deso p-
ion,
7
which ansla es in o de ices wi h highe powe densi ies.
Mo eo e , he use o g aphene in he elec ode o mula ion also
enables i s p ocessing as a exible sel -s anding and binde - ee
lm wi h enhanced mechanical p ope ies ha a e sui able o
exible ene gy s o age sys ems.
8
Elec odes p ocessed in his
way could be di ec ly assembled in o he cell wi h nei he
adding any conduc i e addi i e o binde , no using any
suppo . This is no a i ial issue since binde s gene ally
ep esen up o 10 w % o he whole elec ode mass and do no
con ibu e o capaci ance; in con as , hey can ha e a nega i e
inuence on he conduc i i y o he elec ode o he capaci-
ance, by pa ially blocking he ac i e si es whe e ions a e
adso bed o o m he EDL.
9
O e he las ew yea s, mos effo s ha e been de o ed o
inc easing he g a ime ic ene gy densi y o ECs wi hou
add essing o he impo an ea u es such as olume ic ene gy
and powe densi ies, which, in he pa icula case o po able
elec onics, a e mo e ealis ic pa ame e s.
10
Wi hin his con ex
and wi h he aim o achie ing high olume ic ene gy densi ies,
se e al esea ch g oups ha e ecen ly epo ed he syn hesis o
a
CIC Ene giGUNE, Pa que Tecnol´
ogico de ´
Ala a, Albe Eins ein 48, 01510 Mi˜
nano,
´
Ala a, Spain. E-mail: [email p o ec ed]; dca iazo@cicene gigune.com
b
Depa amen o de Qu´
ımica Ino g´
anica, Uni e sidad Del Pa´
ıs Vasco UPV/EHU, 48080
Bilbao, Spain
c
IKERBASQUE, Basque Founda ion o Science, 48013 Bilbao, Spain
†Elec onic supplemen a y in o ma ion (ESI) a ailable. See DOI:
10.1039/c7 a10210k
Ci e his: J. Ma e . Chem. A,2018,6,
3667
Recei ed 20 h No embe 2017
Accep ed 24 h Janua y 2018
DOI: 10.1039/c7 a10210k
sc.li/ma e ials-a
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high densi y g aphene-based ma e ials o hei use as elec-
odes in ECs. Yang e al. epo ed he assembly o chemically
hyd azine- educed g aphene oxide in o sel -s anding lms wi h
densi ies o 1.49 g cm
3
; a symme ic EC made ou o hese
lms and using an elec ode mass loading o 1 mg cm
2
showed
a capaci ance o ca. 195 F cm
3
a 10 A g
1
when 1 M H
2
SO
4
was
used as he elec oly e.
11
Tao e al. also epo ed he p epa a ion
o high densi y (1.58 g cm
3
) educed g aphene oxide monoli hs
by he hyd o he mal educ ion o g aphene oxide (GO) and
subsequen acuum d ying, which eached a olume ic
capaci ance o ca. 280 F cm
3
a 10 A g
1
using a 6 M KOH
aqueous solu ion as he elec oly e.
12
Se illa e al. de ailed he
syn hesis o sol a ed g aphene lms wi h high a eal mass and
a densi y o 1.1 g cm
3
, which exhibi ed a cell capaci ance o
47 F cm
3
a 0.1 A g
1
as well as a good capaci ance e en ion.
13
Fan and co-wo ke s epo ed he p epa a ion o densely
packed g aphene nanomesh-ca bon nano ube (CNT) lms o
ca. 0.5 mg cm
2
ha exhibi ed a olume ic capaci ance o
331 F cm
3
in a 3-elec ode congu a ion a a scan a e o
5mVs
1
using 6 M KOH as he elec oly e.
14
Also, Fan and co-
wo ke s epo ed olume ic capaci ance alues as high as
400 F cm
3
on ozone- ea ed GO elec odes wi h a mass o ca.
2mgcm
2
es ed in 1 M Na
2
SO
4
aqueous elec oly e.
15
On he
o he hand, Gogo si and co-wo ke s epo ed he p epa a ion o
e y high densi y (3.1 g cm
3
) sel -s anding binde - ee elec-
odes o MXene/g aphene composi es showing a high olu-
me ic capaci ance (1040 F cm
3
) using 3 M H
2
SO
4
solu ion as
he aqueous elec oly e.
16
Composi es o med using ca bon
nano ubes and g aphene ha e shown hei sui abili y as elec-
odes o supe capaci o s.
17,18
Howe e , as in many o he abo e
ci ed s udies, hese da a a e based on expe imen s using elec-
odes wi h a oo low mass loading, which can lead o he
o e es ima ion o hei elec ochemical pe o mance.
17–19
Also,
hese syn he ic me hods should end owa ds mo e simplis ic
app oaches combined wi h an easy p ocessabili y o ma e ials
wi h a iew o hei upscaling.
In his wo k we p esen a no el and simplis ic me hod o he
p epa a ion o pa ially educed g aphene oxide–ca bon nano-
ube (p GO–CNT) sel -s anding binde - ee lms. The compos-
i es a e p epa ed by he hyd o he mal ea men o a mix u e o
GO and ca bon nano ubes (CNTs), yielding a hyd a ed sel -
assembled composi e ha is nally comp essed o ob ain
high densi y hin lms wi h uned a eal ca bon loading. These
p GO–CNT sel -s anding lms we e di ec ly assembled as
binde - ee elec odes in symme ic cells showing a e y high
olume ic capaci ance o 250 F cm
3
a 1 A g
1
. The homoge-
nous inse ion o CNTs in o he con inuous 3D g aphene
ne wo k p o ided ou s anding capaci ance e en ion a high
cu en densi ies e en in elec odes wi h mass loadings abo e
12 mg cm
2
(ca. 200 F cm
3
a 10 A g
1
).
Expe imen al sec ion
Syn hesis o high densi y p GO–CNT composi e lms
Highly packed p GO–CNT lms we e ob ained by he hyd o-
he mal ea men o 85 mL o a 2 mg mL
1
aqueous GO
suspension (G aphenea) con aining mul i-walled ca bon
nano ubes (MWCNTs, Ald ich) a a concen a ion o
0.04 mg mL
1
. This p opo ion was dened assuming, on he
d y basis, a con ibu ion o 98 w % o GO and 2 w % o
MWCNTs. The mix u e was sonica ed o 30 min, sealed in
aTeon-lined au ocla e and hea ed a 210 C o 24 h. Ae
cooling down o oom empe a u e, he sel -s anding ca bon
monoli hs we e emo ed om he au ocla e and washed wi h
abundan dis illed wa e . In o de o ob ain sel -s anding elec-
odes o diffe en masses, he monoli hs we e sliced a
diffe en hicknesses and hen p essed in he sol a ed s a e a 8
ons be ween wo s ainless-s eel pla es. This simplis ic p oce-
du e was p e iously adop ed by Se illa and co-wo ke s o he
p ocessing o g aphene hyd ogels con aining hema i e nano-
pa icles in o lms.
13
The lms we e nally d ied o e nigh a
80 C be ween he wo s ainless-s eel pla es o keep a a shape
o he lms. Fo compa ison pu poses, p GO lms we e ob-
ained in he same way bu wi hou adding he CNTs o he
ini ial GO suspension.
Cha ac e iza ion
X- ay diff ac ion (XRD) pa e ns we e ob ained o he powde ed
samples using a B uke D8 X- ay diff ac ome e ; da a we e
collec ed a 40 kV and 30 mA using CuKa adia ion o e 2q
wi hin he ange om 5 o 90a s eps o 0.02and a esidence
ime o 5 s. Raman spec a we e eco ded wi h a Renishaw
spec ome e (Nanonics Mul i iew 2000) ope a ing a an exci-
a ion wa eleng h o 532 nm. Spec a we e acqui ed wi h a 10 s
exposu e ime o he sample o he lase beam. Scanning elec-
on mic oscope (SEM) images we e acqui ed using a eld
emission Quan a 200 FEG mic oscope om FEI. Ni ogen
adso p ion–deso p ion iso he ms we e ob ained using an
ASAP2020 ins umen om Mic ome i ics. The samples we e
ou gassed a 80 C o 48 h p io o he analysis. Specic su ace
a ea alues we e de e mined using he BET equa ion wi hin he
0.05–0.2 ela i e p essu e ange. Ca bon and oxygen con en s
we e de e mined using a Flash 2000 o ganic elemen al analyze
om The mo Scien ic.
X- ay pho oelec on spec oscopy (XPS) measu emen s we e
ca ied ou using a UHV spec ome e chambe wi h base
p essu e below 10
10
mba . The chambe ea u es a hemi-
sphe ical analyse PHOIBOS 150 wi h a 2D-DLD de ec o
(SPECS) and a monoch oma ed X- ay sou ce FOCUS 500
(SPECS) wi h wo anodes: Al Ka(hn¼1486.74 eV) and Ag La
(hn¼2984.3 eV).
Elec ode assembly and elec ochemical measu emen s
To pe o m he elec ochemical measu emen s, wo elec odes
wi h a diame e o 11 mm and simila mass loadings ( anging
om 5 o 14 mg cm
2
) we e punched om he dense lms and
di ec ly assembled in a Swagelok™- ype cell placing a po ous
glass be (Wha manGFB)memb anein-be weenas hesepa-
a o . The elec odes and he sepa a o we e we ed wi h a couple
o d ops o 6 M KOH, and wo s ainless s eel ods we e used as
cu en collec o s. Cyclic ol amme y (CV) and gal anos a ic
cha ge–discha ge cycling (GC) measu emen s we e conduc ed
using a mul ichannel VMP3 gene a o om Biologic. The specic
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capaci ance exp essed in a ads pe elec ode olume (F cm
3
)
was ob ained om he discha ge cu e o he gal anos a ic plo s
eco ded a diffe en cu en densi ies acco ding o he equa ion
C¼2(I
d
/mV)
e
,whe eIis he elec ical cu en (A), Vis he
usable ol age ange once he ohmic d op is sub ac ed (V),
d
is
he discha ge ime (s), mis he mass o one elec ode (g) and
e
co esponds o he appa en densi y o he elec odes (g cm
3
).
Equi alen se ies esis ance (ESR, in Ohm cm
2
) was calcula ed
om he ol age d op V
d
acco ding o he equa ion V
d
¼2I(ESR/
0.95). The specic ene gy exp essed as W h dm
3
was ob ained
acco ding o he equa ion:
ED¼
0:277 eIð ðUminÞ
ðUmaxÞ
Uð Þd
2me
whe e E
D
is he discha ge ene gy (W h dm
3
),
e
is he densi y o
he elec ode (g cm
3
), Iis he cu en (A), (U
min
) and (U
max
) a e
he ime co esponding o he maximum and minimum ol -
ages in he discha ge po ion o a gal anos a ic cycle (s), U( )is
he ins an ol age (V) and m
e
is he mass o one elec ode in
a symme ic cell (g). The powe densi y (P, in W dm
3
) was
calcula ed acco ding o:
P¼3600ED
½ ðUmaxÞ ðUminÞ
Resul s and discussion
As shown in Scheme 1, he highly packed ca bon lms we e
ob ained by a clean and simple p ocedu e using GO (p GO lm)
o GO and MWCNTs (p GO–CNT lm) as p ecu so s and wa e
as he eac ion medium. Du ing he hyd o he mal ea men ,
supe hea ed wa e p omo ed he emo al o oxygen unc ional
g oups om GO as well as he c oss-linking o moie ies p esen
on neighbo ing shee s, gi ing ise o sel -assembled sol a ed
hyd ogels o p GO. When a small amoun o MWCNTs (2 w %
on he d y basis) was added o he GO suspension and subjec ed
o hyd o he mal condi ions, a clea supe na an oge he wi h
he ca bon monoli h was ob ained. This obse a ion indica es
ha all o he CNTs we e success ully inco po a ed in o he
pa ially educed g aphene oxide monoli h.
The as-ob ained monoli hs p esen ed in e nal oids c ea ed
by he e olu ion o CO
2
du ing he hyd o he mal educ ion o
GO.
20
As shown in Fig. S1,† he size o he in e nal oids
inc eased wi h he empe a u e o hyd o he mal ea men and,
in con as , he size o he monoli hs dec eased. These wo ac s
e eal ha monoli hs ob ained a he highes empe a u e
(210 C) ha e a mo e compac bulk mic os uc u e. I mus be
ema ked ha , due o a ce ain deg ee o plas ici y ha he
sol a ed monoli hs ha e, all he oids gene a ed du ing he
hyd o he mal ea men collapsed upon he subsequen
mechanical p essing a 8 ons. The p GO hyd ogels ob ained a
210 C and p essed a 8 ons had an appa en densi y as high as
ca. 1.5 g cm
3
. The addi ion o 2 w % o MWCNTs o he GO
suspension did no exe a signican inuence on he densi y
o he p GO–CNT lms ob ained a he same empe a u e,
which was also o ca. 1.5 g cm
3
. p GO hyd ogels ob ained a
180 and 150 C and p essed had lowe densi ies o ca. 1.4 g cm
3
and 1.0 g cm
3
, espec i ely.
The oxygen con en o he ini ial GO (41–50 w %, acco ding
o he manu ac u e 's specica ions) dec eased o 24.7 w % and
20.1 w % in he p GO and p GO–CNT hyd ogels, espec i ely, as
a consequence o hei hyd o he mal educ ion in he p esence
o supe hea ed wa e . The lowe oxygen con en in he p GO–
CNT lm ag ees wi h he p esence o 2 w % o CNTs in he GO
suspension, and his p opo ion becomes highe (3–4 w %) in
he pa ially educed monoli h ae he hyd o he mal ea -
men . The chemical composi ion o GO and p GO was u he
in es iga ed by X- ay pho oelec on spec oscopy (XPS)
measu emen s. In eg a ion o he peaks in he gene al spec a
e ealed oxygen con en s o 43 and 21 w % o GO and p GO,
espec i ely, which a e in ag eemen wi h he alues ob ained
om elemen al analysis. The high esolu ion XPS O1s and C1s
spec a a e shown in Fig. S2.†The in ensi y o he O1s peak is
Scheme 1 Schema ic o he p ocedu e ollowed o he p epa a ion o high densi y p GO and p GO–CNT films.
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no iceably lowe in he case o p GO, which e idences he
pa ial emo al o oxygen unc ionali ies du ing he hyd o-
he mal ea men . Decon olu ion o he C1s peak con med
ha oxygen single-bonded o ca bon (hyd oxyl and epoxy
g oups) was p e e en ially emo ed unde hyd o he mal
condi ions.
21
The con ibu ion o he C–O decon olu ion peak
o he o al C1s signal was 54 and 19% o GO and p GO,
espec i ely, whils he p opo ion o oxygen double-bonded o
ca bon did no change signican ly (8 and 7% o GO and p GO,
espec i ely).
In o de o ge insigh in o he mic os uc u e o he p GO
and he p GO–CNT lms, hese samples we e obse ed by
scanning elec on mic oscopy (SEM). Fig. 1 displays some
ep esen a i e SEM images ob ained o bo h sel -s anding
lms a diffe en magnica ions. The low magnica ion
image acqui ed o he p GO lm in Fig. 1a shows ha hese
lms a e o med by g aphene laye s wi h a p e e en ial o ien-
a ion in pa allel s acking due o he p essing. La ge magni-
ca ion images (Fig. 1b) disclose ha all o he g aphene planes
a e e y well in e connec ed o ming a highly compac 3D
s uc u e wi hou la ge mac opo es. As shown in Fig. 1c and d,
he addi ion o 2 w % o MWCNTs did no modi y he
mo phology o he g aphene-based lm, in which he CNTs
appea homogeneously dis ibu ed.
XRD pa e ns o p GO and p GO–CNT lms a e shown in
Fig. 2. Bo h XRD spec a exhibi ed a b oad and low in ensi y
(002) diff ac ion peak a 2q24–which co esponds o an
in e laye spacing o 3.6 
A, sugges ing ha he g aphene
shee s we e pa ially es acked ae hyd o he mal educ ion
and p essing. The in ensi y o his peak was lowe o he
p GO–CNT lm, sugges ing ha he homogenous in e cala-
ion o CNTs wi hin he g aphene-based lm p e en s he
pa ial es acking o he pa ially educed g aphene oxide
lms. These highly packed lms we e u he analyzed by
Raman spec oscopy (Fig. 2b). The Raman spec a showed wo
cha ac e is ic bands a 1593 cm
1
(G band, o igina ing om
he  s -o de sca e ing o E2g phonons by he g aphi ic
planes) and 1352 cm
1
(D band, a ising om a b ea hing
mode o k-poin phonons o Ag1 symme y). These wo bands –
a ibu able o he p esence o o de ed g aphi ic domains and
s uc u al de ec s in he g aphene nanoshee s espec i ely –
o e lap in bo h spec a, which is consis en wi h he e y small
amoun o CNTs (only 2 w %) used in he syn hesis o he
p GO–CNT lms.
The ex u al p ope ies o he high-densi y lms we e e al-
ua ed by ni ogen adso p ion–deso p ion measu emen s a
196 C. The iso he ms o bo h samples, shown in Fig. 3, a e
a combina ion o ype I and IV p oles dened by he IUPAC
classica ion.
22
These iso he ms a e cha ac e is ic o ma e ials
con aining bo h mic o- and mesopo es, in hese cases gi en by
he p esence o small ca i ies be ween wo g aphene laye s,
be ween he CNTs and he g aphene laye s, o due o he
p esence o de ec s o holes wi hin he g aphene shee s. The
main diffe ence be ween he iso he ms ob ained o hese wo
samples co esponds o he amoun o adso bed ni ogen,
which is highe in he case o p GO–CNT compa ed o he CNT-
ee lm. The BET specic su ace a ea o p GO and p GO–CNT
lms was 160 and 240 m
2
g
1
, espec i ely. This inc ease in he
Fig. 1 SEM images o he p GO (a and b) and p GO–CNT (c and d)
films.
Fig. 2 XRD pa e ns (a) and Raman spec a (b) ob ained o he p GO
(black) and p GO–CNT (blue) films.
Fig. 3 Ni ogen adso p ion (filled symbols) and deso p ion (emp y
symbols) iso he ms ob ained a 196 C o he p GO (black) and
p GO–CNT (blue) films.
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amoun o adso bed gas can be asc ibed o he homogeneous
in e cala ion o CNTs in be ween g aphene laye s ha p e-
en ed he es acking o g aphene shee s, hus making he
inne lm su ace mo e accessible.
Sel -s anding binde - ee disc-shaped elec odes we e cu ou
o he p GO and p GO–CNT lms and di ec ly assembled in o
symme ic ECs o hei elec ochemical es ing using 6 M KOH
aqueous solu ion as he elec oly e. In o de o ob ain elec odes
wi hin a b oad ange o a eal masses, he sol a ed monoli hs we e
sliced o diffe en hicknesses be o e p essing. Rep esen a i e GC
cha ge–discha ge cu es o ECs using elec odes o ca.
13 mg cm
2
a e depic ed in Fig. 4a and b. A a cu en densi y o
1Ag
1
, bo h samples exhibi a quasi-linea ol age inc ease/
dec ease a he cha ge and discha ge (Fig. 4a), cha ac e is ic o
he non- a adaic (EDL) and a adaic (pseudocapaci ance) capac-
i i e cha ge s o age mechanisms aking place. The olume ic
capaci ance alues o p GO and p GO–CNT lms a 1 A g
1
we e
as high as 278 and 255 F cm
3
, espec i ely. The capaci ance o
p GO–CNT was sligh ly lowe p esumably due o he low con i-
bu ion o he CNTs o he capaci ance. These high capaci ance
alues a e he esul o a combina ion o high g a ime ic
capaci ance, gi en by a s ong con ibu ion o pseudocapaci ance
and hei accessible su ace a ea, and a e y compac ca bona-
ceous mic os uc u e ee o la ge emp y oids. A highe cu en
densi ies, he p GO–CNT lm showed much be e capaci ance
e en ion (Fig. 4c). A 2.5 A g
1
bo h samples had he same
capaci ance (238 F cm
3
) and, a a high cu en a e o 10 A g
1
,
he capaci ance o he p GO–CNT lm was wice as high as ha o
CNT- ee elec odes (184 F cm
3
and 97 F cm
3
, espec i ely). The
cha ge–discha ge cu es ob ained a a highe cu en a e o
10 A g
1
(Fig. 4b) show, in he case o he p GO lm, a high
ol age d op a he beginning o he discha ge, and his is much
smalle in he case o he composi e. The ESR o he de ice buil
wi h p GO elec odes, calcula ed om he ol age d op in he
discha ge p ole, was 2.01 Ohm cm
2
. The ESR calcula ed o he
SC made o CNT-con aining lms was as low as 0.68 Ohm cm
2
.
Acco ding o hese esul s, he p esence o CNTs clea ly imp o ed
he conduc i i y o he sel -s anding elec odes e en a such a low
concen a ion. This, oge he wi h a be e elec oly e accessibili y
o he ca bon su ace due o he CNT in e cala ion in be ween
g aphene laye s, allowed he as cha ge and discha ge o he
elec ochemical de ice keeping a high capaci ance e en ion a
high cu en densi ies.
As shown in Fig. 4d, he imp o emen p o ided by he
inse ion o CNTs in o he g aphene-based sel -s anding lms is
especially no iceable in elec odes wi h a high mass loading.
This is o ema kable ele ance, as hese esul s can be di ec ly
ex apola ed o comme cial de ices using elec odes wi h
simila ac i e mass loadings. In con as , he elec odes wi h
mass loadings o ca. 5mgcm
2
ha e a simila elec ochemical
pe o mance (Fig. 4d and S3†). This beha io was con med by
CV. Fig. 5a and b show he cyclic ol ammog ams ob ained o
p GO and p GO–CNT lms wi h lowe (5 mg) and highe
(13 mg) mass loadings a a scan a e o 100 mV s
1
. The CV
cu es o he lms wi h a mass loading o ca. 5mgcm
2
p esen
a quasi- ec angula p ole and hey almos o e lap. When using
elec odes wi h la ge masses, he beha io o he samples
diffe s. The p GO–CNT lm s ill main ains a ec angula -
shaped p ole a a sweep a e o 100 mV s
1
, in ag eemen
wi h i s good capaci ance e en ion a high cu en densi ies. In
con as , he p GO lm showed a e y highly dis o ed p ole
ypical o a mo e esis i e ma e ial.
Fig. 4 Gal anos a ic cha ge–discha ge cu es o p GO (black) and p GO–CNT (blue) eco ded a 1 A g
1
(a) and 10 A g
1
(b). The mass loading o
he elec odes was ca. 13 mg cm
2
. Re en ion o olume ic capaci ance in elec odes wi h a mass loading o ca. 13 mg cm
2
as a unc ion o
cu en densi y (c). E olu ion o he olume ic capaci ance as a unc ion o elec ode mass loading a a cu en densi y o 10 A g
1
(d).
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Fig. 6a includes he Ragone plo s compa ing he olume ic
ene gy and powe densi ies o he p GO and p GO–CNT lms
wi h high and low elec ode mass loadings. Volume ic ene gy
densi ies abo e 15 W h dm
3
@50Wdm
3
we e ob ained o
bo h ma e ials i espec i e o he elec ode mass. The elec odes
wi h low mass loading as well as he p GO–CNT elec odes wi h
high mass loading kep high ene gy densi ies a powe densi ies
abo e 4 kW dm
3
. I should be highligh ed ha he p GO–CNT
lm wi h a mass loading o 13 mg cm
2
had a high ene gy densi y
o 8.2 W h dm
3
@4.2kWdm
3
, while i s CNT- ee coun e pa
ailed a powe densi ies abo e 2 kW dm
3
.
F esh cells we e assembled using p GO and p GO–CNT lms
wi h mass loadings o ca. 13 mg cm
2
and hen hey we e cycled
a 10 A g
1
o 5000 cycles. The p GO–CNT lm exhibi ed an
excellen capaci ance e en ion, main aining 97% o i s ini ial
capaci ance ae 5000 cycles. The cycling pe o mance o p GO–
CNT was be e han ha shown by he CNT- ee elec odes,
which suffe ed om a decay o 15% ae a simila numbe o
cycles.
The imp o ed capaci ance e en ion obse ed o he p GO–
CNT sample can be en a i ely asc ibed o he ac i a ion o
some in e nal unc ional g oups ha become p og essi ely
accessible upon cycling due o he CNT pilla ing effec .
Conclusions
He ein we ha e p esen ed a simplis ic p ocedu e o he p ep-
a a ion o high-densi y (ca. 1.5 mg cm
3
), highly packed,
pa ially educed g aphene oxide–CNT (p GO–CNT) composi es.
These exible lms can be easily assembled in o supe capaci o
cells and used as sel -s anding binde - ee elec odes. The
inco po a ion o a minu e amoun o ca bon nano ubes (only
2 w %) b ings abou a signican imp o emen in bo h capac-
i ance e en ion a high cu en densi ies as well as he cycling
s abili y compa ed o he CNT- ee coun e pa . This enhance-
men is especially ele an in he cells buil wi h high mass
loading elec odes. The high olume ic capaci ance and
ou s anding capaci ance e en ion ob ained o p GO–CNT
elec odes wi h masses abo e 12 mg cm
2
(250 and 200 F cm
3
a 1 and 10 A g
1
, espec i ely) make hese lms p omising
elec ode ma e ials o hei implemen a ion in po able ene gy
s o age sys ems.
Conflic s o in e es
The au ho s decla e no conic o in e es in his a icle.
Fig. 5 Cyclic ol amme y cu es o p GO (black) and p GO–CNT (blue) films eco ded a 100 mV s
1
in symme ic ECs wi h elec odes o low (a)
and high (b) mass loadings.
Fig. 6 (a) Ragone plo showing he olume ic ene gy densi y e sus he olume ic powe densi y o p GO wi h an elec ode mass o ca. 5mg
cm
2
(black ci cles), p GO wi h an elec ode mass o ca. 13 mg cm
2
(black s a s), p GO–CNT wi h an elec ode mass o ca. 5mgcm
2
(blue
ci cles) and p GO–CNT wi h an elec ode mass o ca. 13 mg cm
2
(blue s a s). This Ragone plo was cons uc ed aking in o accoun only he
masses o he wo elec odes. (b) Cycling s abili y o p GO and p GO–CNT elec odes wi h high mass loading.
3672 |J. Ma e . Chem. A,2018,6, 3667–3673 This jou nal is © The Royal Socie y o Chemis y 2018
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Acknowledgemen s
This wo k was nancially suppo ed by he Eu opean Union
(G aphene Flagship, Co e 1) and he Spanish Minis y o
Economy and Compe i i eness (MINECO/FEDER) (MAT2015-
64617-C2-2-R). The au ho s also hank GRAPHENEA Company
o supplying he g aphene oxide used in his s udy. We also
hank D Oleksand Bonda chuk o his ui ul help on he XPS
da a acquisi ion.
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