C A R B O N 5 7 ( 2 0 1 3 ) 3 1 7 – 3 2 8

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Supercapacitors based on carbons with tuned porosity derivedfrom paper pulp mill sludge biowaste

Huanlei Wang a,b, Zhi Li a,b,*, Jin Kwon Tak c, Chris M.B. Holt a,b, Xuehai Tan a,b,Zhanwei Xu a,b, Babak Shalchi Amirkhiz a,b, Don Harfield c, Anthony Anyia c,Tyler Stephenson a,b, David Mitlin a,b,*

a Chemical and Materials Engineering, University of Alberta, 9107 – 116 Street Edmonton, Alberta, Canada T6G 2V4b National Institute for Nanotechnology (NINT), National Research Council of Canada, 11421 Saskatchewan Drive,

Edmonton, Alberta, Canada T6G 2M9c Bioresource Technologies, Alberta Innovates – Technology Futures, P.O. Bag 4000, Vegreville, Alberta, Canada T9C 1T4

A R T I C L E I N F O

Article history:

Received 21 August 2012

Accepted 28 January 2013

Available online 8 February 2013

0008-6223/$ - see front matter � 2013 Elsevihttp://dx.doi.org/10.1016/j.carbon.2013.01.079

* Corresponding authors.E-mail addresses: lizhicn@gmail.com (Z.

A B S T R A C T

Hydrothermal carbonization followed by chemical activation is utilized to convert paper

pulp mill sludge biowaste into high surface area (up to 2980 m2 g�1) carbons. This synthesis

process employs an otherwise unusable byproduct of paper manufacturing that is gener-

ated in thousands of tons per year. The textural properties of the carbons are tunable by

the activation process, yielding controlled levels of micro and mesoporosity. The electro-

chemical results for the optimized carbon are very promising. An organic electrolyte yields

a maximum capacitance of 166 F g�1, and a Ragone curve with 30 W h kg�1 at 57 W kg�1 and

20 W h kg�1 at 5450 W kg�1. Two ionic liquid electrolytes result in maximum capacitances

of 180–190 F g�1 with up to 62% retention between 2 and 200 mV s�1. The ionic liquids

yielded energy density–power density combinations of 51 W h kg�1 at 375 W kg�1 and 26–

31 W h kg�1 at 6760–7000 W kg�1. After 5000 plus charge–discharge cycles the capacitance

retention is as high at 91%. The scan rate dependence of the surface area normalized

capacitance highlights the rich interplay of the electrolyte ions with pores of various sizes.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Electrochemical capacitors or supercapacitors are currently

targeted as possible auxiliary energy storage devices to be

used along with rechargeable batteries or fuel cells to allow

for short duration high power service. The advantages of sup-

ercapacitors include their ability to operate in a wide temper-

ature range, their long cycle-life, and the ability to deliver high

power densities [1,2]. Porous carbon materials are considered

as excellent electrode materials for electric double-layer

capacitors due to their high surface area, controllable pore

structure, excellent thermal and chemical stability, and low

cost [3]. Multiple factors affect the performance of carbon-

er Ltd. All rights reservedLi), dmitlin@ualberta.ca (

based supercapacitors, and the more important ones are the

surface chemistry and intrinsic pore characteristics [4–6]. In

2006, Gogotsi’s group and Beguin’s group have reported signif-

icantly increased specific capacitance in micropores [4,7],

which challenged the traditional theory, and demonstrated

that desolvated ions can form a monolayer or wire inside

slit-shaped or cylindrical micropores of carbon [8]. In addition

to micropores, mesopores are necessary for rapid ion trans-

port. This is the reason for the high capacitance retention at

high scan rate for ordered mesoporous carbons, 3D hierarchi-

cally porous carbons, carbon onions, CNTand graphene [9–11].

Several methods have been developed for controlling the

pore structure in carbon materials. For example, templated

.D. Mitlin).

318 C A R B O N 5 7 ( 2 0 1 3 ) 3 1 7 – 3 2 8

carbonization was utilized to prepare carbons with an or-

dered microporous structure, exhibiting excellent specific

capacitances and rate capability [12]. Another promising

way to produce carbon materials with controlled pore struc-

ture is the carbide-derived carbon (CDC) technique [4]. Recent

notable performance improvements have been achieved by

tuning the pore size distribution in CDCs [2,4]. However, the

surface areas and micropore volumes of CDCs are lower than

those of zeolite-templated carbons. In the last several years,

the traditional activation process has been revived in order

to tune the pore distributions in carbons [13,14]. Physical

and chemical activation are well-developed industrial meth-

ods that can produce carbons in large quantity and low cost.

Carbon materials can also be synthesized by hydrothermal

carbonization (HTC). The HTC processes are considered low-

energy due to their intrinsically low synthesis temperatures

(150–350 �C). Antonietti’s group has done extensive work on

the HTC process. Widely available precursors can be con-

verted into nanostructured carbon materials in a few environ-

mentally friendly steps [15]. The resulting carbons exhibited

high capacitances and good rate capability [16]. More recently,

carbon materials prepared from biowaste, such as seaweed

and eggshell membrane, have been recently employed for

supercapacitor applications [17,18].

The paper industry generates ‘‘Pulp Sludge’’ as a ubiqui-

tous solid byproduct of the manufacturing process. In 2012

a liquid byproduct of paper manufacturing, termed brown li-

quor, was successfully converted into promising cathode

material [19]. In North America alone thousands of tons of

pulp sludge waste are annually generated from paper pulp

mills. Currently there are no economical uses of this waste

material, though researchers are attempting to utilize it as

an agricultural soil enhancement additive [20]. Pulp sludge

is currently disposed of through incineration or placed into

landfills. The sludge releases CO2 into the atmosphere as

it either burns or decomposes. The scale of its availability

and negative cost make pulp sludge a very attractive feed-

stock for high surface area carbons. In the present work

we employ an environmentally friendly process of hydro-

thermal carbonization followed by chemical activation to

convert this otherwise unusable green house gas emitting

biowaste into a value added pulp sludge derived activated

carbon (PSDAC).

To ascertain the potential of these materials for superca-

pacitor applications, we assembled and tested commercial-

type 2032 battery button cells with opposing electrodes. Three

different electrolytes were employed: a popular organic elec-

trolyte (1.5 M tetraethylammonium tetrafluoroborate solution

in acetonitrile, TEABF4/AN) and two emerging ionic liquids (1-

ethyl-3-methylimidazolium bis(trifluoromethylsulfony)imide

(EMIM TFSI) and 1-butyl-1-methylpyrrolidinium bis(trifluo-

romethylsulfony)imide (BMPY TFSI)). We examine the three-

way interrelation between the pore size distributions, the

dimensions of the electrolyte ions, and the capacitances of

the electrodes, both on a gravimetric and a surface area nor-

malized basis. A rich interplay is demonstrated, with narrow

micropores having a key role in obtaining anomalously high

surface area normalized capacitance at low scan rates, and

pores larger than 1.2 nm (in organic electrolyte) and 2 nm

(in ionic liquid) being essential for achieving high power.

2. Experimental

2.1. Synthesis of hydrothermal carbon

The HTC process was carried out in a pressure reactor (1L,

Parr Instrument Company, Moline, Illinois, USA). Nitrogen

gas was supplied to the pressure reactor from a gas cylinder.

A PID controller was used to regulate the set temperature

within 1 �C accuracy. The pulp sludge (wet basis, 143.1 g,

60.22% water content) was dispersed in 266 mL distilled water

containing H2SO4 (0.6 g), and Fe2O3 (1.2 g). The mixture was

then loaded into the pressure reactor. The reaction parame-

ters were: pressure: 8.2 MPa, temperature: 225 �C, and reac-

tion time: 8 h. At the end of the reaction time, the pressure

reactor was cooled down to room temperature. KOH was

added to the HTC product to adjust the pH to 7 before the sep-

aration of the HTC-solids and HTC-liquids. The HTC-solids

were filtered and dried in an oven at 105 �C for 12 h.

2.2. Activation of HTC carbon

The hydrothermal carbon was chemically activated by heat-

ing a KOH–HTC mixture (weight ratio 1:1 or 3:1) under a nitro-

gen atmosphere in the temperature range of 700–800 �C. The

heating rate used was 3 �C min�1 with a ‘‘soak time’’ of 1 h

at the target temperature. After the activation process, the

mixture was recovered and washed several times with 2 M

HCl to remove any inorganic salts and then washed with suf-

ficient deionized water. The sample was then dried at 100 �Cin ambient overnight, followed by thermal annealing at

800 �C under argon for 1 h.

2.3. Material characterization

Scanning electron microscopy (SEM) analysis was performed

using a Hitachi S-4800 instrument. Transmission electron

microscopy (TEM) analysis was performed using a JEOL JEM-

2010, with an accelerating voltage of 200 kV. X-ray photoelec-

tron spectroscopy (XPS) measurements were performed using

an Axis Ultra spectrometer. The Raman spectra were recorded

with a confocal microprobe Raman system (Thermo Nicolet

Almega XR Raman Microscope). Nitrogen adsorption–

desorption isotherms and the textural properties of the car-

bons were determined with a Quantachrome Autosorb-1 at

77 K. Prior to the gas sorption measurements, the samples

were outgassed at 200 �C for 4 h under vacuum. The pore size

distributions were evaluated by a non-local density functional

theory (DFT) method using nitrogen adsorption data and

assuming slit-pore geometry. The weighted mean micropore

size (Dmean) was calculated from [21]:

Dmean ¼Pn

i¼1dimiPn

i¼1mið1Þ

where d and v are pore width and volume, respectively.

2.4. Electrochemical evaluation

Carbon electrodes were prepared by mixing 80 wt.% activated

carbon material, 10 wt.% carbon black, and 10 wt.%

poly(vinylidenedifluoride), using N-methyl pyrrolidone to

C A R B O N 5 7 ( 2 0 1 3 ) 3 1 7 – 3 2 8 319

form a slurry. The resulting composite was rolled onto Al foil.

After drying at 100 �C overnight, circular discs (�1.5 cm diam-

eter) were then punched out from the films. Stainless-steel

coin cells (2032 type) with two symmetrical carbon electrodes

separated by a porous polymeric separator were assembled

inside an Ar-filled glove box (<0.1 ppm of both oxygen and

H2O). Each electrode had a mass loading of �2.5 mg cm�2

after drying. The electrode thickness is in the range of 60–

100 lm.

Cyclic voltammetry (CV) curves were measured using a

Solartron 1470E Multichannel Potentiostat/Cell Test System

in the voltage range of 0–2.3/3 V and at scan rates from 2 to

200 mV s�1. The gravimetric capacitance, Cc (F g�1) was calcu-

lated according to

Cc ¼2ilm

ð2Þ

where i is the current (A), l is the scan rate (V s�1), and m is

the mass (g) of active material in each electrode.

Galvanostatic charge–discharge tests were also performed

using current densities between 0.1 and 10 A g�1. The gravi-

metric capacitance, Cg (F g�1), was calculated according to

Cg ¼2I

ðdV=dtÞm ð3Þ

where I is the current (A), dV/dt is the slope of the discharge

curve after the ohmic drop (V s�1), and m is the mass (g) of ac-

tive material in each electrode.

Electrochemical impedance spectroscopy measurements

were carried out using a VersaSTAT 3 potentiostat (Princeton

Applied Research, USA) in the frequency range from 1 MHz to

1 mHz at open circuit with a 5 mV ac amplitude. The capaci-

tance C(x) based on impedance measurements was calcu-

lated from

CðxÞ ¼ �ðZ00ðxÞ þ jZ0ðxÞÞxjZðxÞj2

ð4Þ

Z0ðxÞ2 þ Z00ðxÞ2 ¼ jZðxÞj2 ð5Þ

where x is the angular frequency, Z 0(x) and Z00(x) are the real

part and the imaginary part of the impedance, and |Z(x)|2 is

the modulus of the impedance Z 0(x) and Z00(x). Detailed infor-

mation can be found in reference [22]. The capacitance C(x)

can be defined as

CðxÞ ¼ C0ðxÞ � jC00ðxÞ ð6Þ

where C 0(x) is the real part of capacitance, C00(x) is the imagi-

nary part of capacitance. Then, the real part of capacitance,

C 0(x) (F), was calculated according to [22,23]

C0ðxÞ ¼ �Z00ðxÞxjZðxÞj2

ð7Þ

3. Results and discussion

3.1. Physicochemical characterization

After the pulp sludge was hydrothermally carbonized, it was

chemically activated. During chemical activation, the reaction

between KOH and the carbonaceous material is mainly de-

scribed as follows [24]:

6KOHþ 2C$ 2Kþ 3H2 þ 2K2CO3 ð8Þ

When the activation temperature is higher than 700 �C, the

decomposition of K2CO3 will contribute to additional physical

activation through site-specific gasification [25]. As can be

seen from Table 1, the HTC derived carbon utilized in this

study contains significant levels of oxygen (23.24 at.%). The

high oxygen functionalization and low degree of condensa-

tion makes hydrothermally synthesized carbons quite reac-

tive for KOH activation [26]. The pulp sludge derived

activated carbons thus synthesized were labeled as PSDAC-

T-n, where T is the activation temperature and n is the KOH/

HTC weight ratio. For comparison, a commercial activated

carbon (Norit) labeled as CAC was also analyzed.

Figs. 1a, b, c and S1 (in Supplementary material) show the

SEM images of the as-prepared activated carbons. Regardless

of the preparation conditions, the resulting activated carbon

particles were �1–40 lm in dimensions. When the mass ratio

of KOH/HTC is 1, the PSDAC materials consisted of vesicu-

lated particles. However, sample PSDAC-800–3, activated with

a high mass ratio of KOH/HTC, exhibited irregular shaped par-

ticles with sharp corners. High-resolution TEM micrograph of

PSDAC-800–3 revealed a predominantly disordered structure,

with contrast typical of highly micro and mesoporous materi-

als (Fig. 1d). Raman spectra of the activated carbons and of

the CAC are shown in Fig. S2 (in Supplementary material).

The D-band originating from the disordered and defective

portions of the carbon was at �1340 cm�1, while the G-band

originating from the graphitic portions of the carbon was at

�1590 cm�1. The integrated intensities of D and G bands (ID/

IG) are 1.80, 1.60 and 1.90 for PSDAC-700–1, PSDAC-800–1 and

PSDAC-800–3. Such ratios are typical for porous carbon with

high degree of structural disorder [27], confirming the TEM

results.

The surface chemical groups of the activated carbons were

characterized using XPS. As revealed in Table 1, an increase in

the activation temperature and in the KOH/HTC mass ratio

promoted higher surface oxygen content. This increase is as-

cribed to the KOH activation process, which introduces oxy-

gen-containing groups onto the carbon surfaces [28].

Although there are nitrogen functional groups in PSDAC-

700-1 and PSDAC-800-1, the content of nitrogen is miniscule

(<0.6 at.%). There is no detectable nitrogen in sample

PSDAC-800-3. This is due to the different gasification mecha-

nisms of carbon versus that of nitrogen, with the nitrogen

being gasified at higher activation temperatures and with

higher KOH/HTC mass ratios [29].

The nitrogen adsorption–desorption isotherms are shown

in Fig. 2a. All the adsorption curves contain a region of a well-

defined plateau (type I isotherm) indicating significant levels

of microporosity. The broadening of the knee in the low-pres-

sure range is attributable to mesoporosity, with PSDAC-700-1

containing the least and PSDAC-800-3 containing the most

mesopores. The pore size distributions are shown in Fig. 2b.

It is known that the existing DFT approach generates an arti-

ficial valley in the pore size distribution at around 0.9 nm [12].

The plots indicate that the porosity of the PSDAC-700-1 and

PSDAC-800-1 samples mostly consist of narrow micropores

(<1 nm) and wider micropores (1–2 nm). The PSDAC-700-1

specimen contains negligible levels of mesoporosity. The

Fig. 1 – (a) SEM micrograph of PSDAC-700-1 specimen. (b) SEM micrograph of PSDAC-800-1 specimen. (c) SEM micrograph of

PSDAC-800-3 specimen. (d) TEM micrograph with a high resolution insert of PSDAC-800-3.

1 2 3 4 50.00

0.03

0.06

0.09

0.12

0.15

Mesopores

Incr

emen

tal P

ore

Volu

me

(cm

3 g-1)

Pore Size (nm)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3CAC

Micropores

b

0.0 0.2 0.4 0.6 0.8 1.00

200

400

600

800

1000

1200a

Qua

ntity

Ads

orbe

d (c

m3 g

-1)

Relative Pressure (P/Po)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

Fig. 2 – (a) Nitrogen adsorption–desorption isotherms of the activated carbons (solid: adsorption; hollow: desorption). (b)

Associated density functional theory pore size distributions.

Table 1 – Porosity parameters and element distribution obtained from nitrogen sorption isotherms and the XPS analysis.

Samples SBET (m2 g�1)a Vt (cm3 g�1)b Vmicro (cm3 g�1)c Vmicro/Vt Dmean (nm) CXPS (at.%) NXPS (at.%) OXPS (at.%)

HTC – – – – – 76.01 0.75 23.24PSDAC-700-1 1470 0.84 0.57 0.68 0.94 98.37 0.57 1.06PSDAC-800-1 2340 1.30 0.86 0.66 0.99 98.49 0.31 1.20PSDAC-800-3 2980 1.75 0.66 0.38 1.16 97.22 �0 2.78CAC 2050 1.17 0.65 0.55 1.14 95.35 �0 4.65

a Surface area was calculated with Brunauer–Emmett–Teller method, using a pressure range between 0.05 and 0.20.b The total pore volume was determined from the amount of nitrogen adsorbed at a relative pressure of 0.98.c The volume of micropores was obtained by the t-plot analysis.

320 C A R B O N 5 7 ( 2 0 1 3 ) 3 1 7 – 3 2 8

PSDAC-800-1 specimen contains minor levels of porosity in

the range of 2–3 nm. The PSDAC-800-3 specimen consists of

wider pores distributed in two well-defined pore systems:

micropores and small mesopores (2–4 nm).

The porosity characteristics of the PSDAC and CAC sam-

ples are listed in Table 1. It can be seen that the surface area

and pore volume increase with higher activation tempera-

tures and KOH/HTC mass ratios. The trend is from

1470 m2 g�1/0.84 cm3 g�1 for PSDAC-700-1, to 2340 m2 g�1/

1.30 cm3 g�1 for PSDAC-800-1, and 2980 m2 g�1/1.75 cm3 g�1

for PSDAC-800-3. Moreover, the mean micropore size also in-

creases with higher activation temperatures and KOH/HTC

C A R B O N 5 7 ( 2 0 1 3 ) 3 1 7 – 3 2 8 321

mass ratios. The mean diameter slightly increases from

0.94 nm for PSDAC-700-1, to 0.99 nm for PSDAC-800-1, and

1.16 nm for PSDAC-800-3. Importantly the fraction of mesop-

ores in the PSDAC-800-3 specimen is significantly higher than

in the others. Although CAC has a similar pore size distribu-

tion to PSDAC-800-3, its surface area and pore volume are sig-

nificantly lower. The surface area and pore volume of PSDAC-

800-3 is higher than what has been previously reported for

activated hydrothermal carbons and for carbons derived from

biowastes [16,18,26]. In fact the surface area of PSDAC-800-3

exceeds the theoretical surface area of graphene

(2630 m2 g�1). This can be attributed to the combination of

its highly defective structure with a large fraction of the car-

bon being amorphous and hence less densely arranged than

graphene, and the very high content of micro and

mesoporosity.

3.2. Capacitance, energy and power in organic electrolyte

The electrochemical performance of the activated carbon

materials was evaluated at room temperature in TEABF4/AN,

which is widely used in the commercial supercapacitors.

Fig. 3a and b exhibit the CV curves at 2 and 200 mV s�1. In this

voltage range all the samples demonstrate fairly rectangular

CV curves. Fig. 3c and d display the charge–discharge curves

of the activated carbons at a high current density of 1 and

10 A g�1. The curves are nearly symmetric with only a minor

voltage drop during discharge. The specific capacitance and

capacity retention of the activated carbons measured by both

cyclic voltammetry and charge–discharge techniques are

listed in Table 2. The reported capacitance is the typical aver-

aged value of three measurements. PSDAC-800-3, the sample

with highest surface area, demonstrates the highest capaci-

tance of 162.2 F g�1 at 0.1 A g�1. And the highest total and rel-

ative amount of mesoporosity of PSDAC-800-3 is beneficial for

increasing its accessible surface area. This value exceeds the

measured capacitance of commercial CAC by approximately

50%. The capacitance retention for PSDAC-800-3 is also quite

promising with the sample retaining 74% of its capacity over a

100 times increase in the current density (0.1–10 A g�1). The

capacitance values obtained here are compared with other

carbon materials reported in literature (Table S1 in Supple-

mentary material).

The high specific capacitance coupled with excellent

capacity retention of PSDAC-800-3 is comparable or even

superior to that of ordered/templated carbons that would nat-

urally be more expensive to synthesize in large quantities,

including zeolite-templated carbons; templated-carbide de-

rived carbons; 3D ordered macroporous carbon [12,30–32].

PSDAC-800-3 specimen also displays comparable perfor-

mance to metal–organic framework derived carbon, activated

graphene, carbide derived carbons, SWCNT, and chemically

modified graphene [4,14,33–35]. Recently, activated hydrother-

mal carbons prepared using cellulose, starch and wood saw-

dust precursors were reported to exhibit very high

capacitance values (145–235 F g�1) at low scan rates (1 mV s�1)

[23]. Compared with activated hydrothermal carbons, the

lower specific capacitance of PSDAC-800-3 at similarly slow

scan rates (2 mV s�1) may be ascribed to its lower content of

suitably sized micropores (the pore size – ion size interactions

leading to an optimum surface area normalized capacitance

will be discussed in Section 3.5) due to their similar surface

areas. However, PSDAC-800-3 demonstrates better capaci-

tance retention, and hence exhibits a comparable specific

capacitance at the higher scan rate of 200 mV s�1. This is

attributable to the higher total content of mesoporosity in

PSDAC-800-3, which facilitates fast ion diffusion kinetics.

Fig. 3e displays the relationship between the potential

drop and current density for the carbon electrodes. From

the linear fit of IR drop values, we can obtain the equation of

IR ¼ aþ bI ð9Þ

where a represents the difference between the 2.3 V applied

potential and the charged potential of the capacitor, b repre-

sents double the value of the internal resistance Rs, and I is

the discharge current density [34]. The behavior of the CAC,

PSDAC-700-1 and PSDAC-800-1 samples is similar in magni-

tude. However the PSDAC-800-3 specimen demonstrates not

only lower resistive loses at each current density, but weaker

current density dependence. The PSDAC-800-3 (IR

[V] = 0.000101 + 0.01161I) possesses the least internal resis-

tance, with the other specimens having the following behav-

ior: PSDAC-700-1 (IR [V] = 0.0061 + 0.02090I), PSDAC-800-1 (IR

[V] = 0.002 + 0.01835I) and CAC (IR [V] = 0.00198 + 0.01988I).

The maximum power density (Pmax) can be calculated using

the equation [34]

Pmax ¼V2

max

4Rs¼ ð2:3� aÞ2

2bð10Þ

The obtained maximum power density of PSDAC-800-3 is

228 kW kg�1 in organic electrolyte, which is much higher than

that of PSDAC-700-1 (126 kW kg�1), PSDAC-800-1

(144 kW kg�1) and CAC (133 kW kg�1).

The Ragone plot presented in Fig. 3f demonstrates that

PSDAC-800-3 exhibited the highest energy density of the car-

bons, being approximately 30 W h kg�1. It also exhibited the

best energy density retention with increasing current density.

This sample’s energy density–power density curve was the

flattest, with 30 W h kg�1 at 57 W kg�1 and 20 W h kg�1 at

5450 W kg�1. A power density of 5450 W kg�1 corresponds to

the coin cell being tested at a current density of 10 A g�1.

The optimum combination of energy and power density for

PSDAC-800-3 is related to its high capacitance and low IR drop

at very high current.

3.3. Capacitance, energy and power in ionic liquid

To further increase the operation potential window, electro-

chemical performance of carbons was also tested in two typ-

ical ionic liquids (EMIM TFSI and BMPY TFSI) at 60 �C. The two

ionic liquids have the same anion, with the maximum dimen-

sion being 0.79 nm [36]. The maximum dimension of the cat-

ion in EMIM TFSI and BMPY TFSI is 0.76 and 1.1 nm,

respectively [36,37]. The electrochemical performance of the

carbons was evaluated using cyclic voltammetry, at scan rates

ranging from 2 to 200 mV s�1.

Fig. 4 shows the lowest and the highest scan rate data. Ta-

ble 2 provides the associated values of capacitance and capac-

itance retention. Specimen PSDAC-700-1 demonstrates the

worst distortion in what would be ideally rectangular CV

0 5 10 15 20 25 300.0

0.5

1.0

1.5

2.0

2.5

Volta

ge (V

)

Time (s)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

d

0.0 0.5 1.0 1.5 2.0

-200

-100

0

100

200

300

400

Cap

acita

nce

(F g

-1)

Voltage (V)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

a

0.0 0.5 1.0 1.5 2.0-200

-150

-100

-50

0

50

100

150

200

Cap

acita

nce

(F g

-1)

Voltage (V)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

b

0 100 200 3000.0

0.5

1.0

1.5

2.0

2.5

Volta

ge (V

)

Time (s)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

c

0 2 4 6 8 10-0.1

0.0

0.1

0.2

0.3

0.4

0.5

IR D

rop

(V)

Current Density (A g-1)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

e

10 100 1000 10000

1

10

100

Ener

gy D

ensi

ty (W

h kg

-1)

Power Density (W kg-1)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

f

Fig. 3 – Electrochemical performance of carbon electrodes in TEABF4/AN electrolyte, tested in button cell, at room temperature.

(a) CV curves at the scan rate of 2 mV s�1. (b) CV curves at the scan rate of 200 mV s�1. (c) Galvanostatic charge–discharge

curves at high current density of 1 A g�1. (d) Galvanostatic charge–discharge curves at high current density of 10 A g�1. (e)

Potential drop associated with internal resistance vs. discharge current density. (f) The resultant Ragone chart generated from

the galvanostatic data.

322 C A R B O N 5 7 ( 2 0 1 3 ) 3 1 7 – 3 2 8

curves, which can be attributed to its higher micropore vol-

ume ratio. In EMIM TFSI electrolyte, sample PSDAC-700-1

shows a more rectangular CV curve than in BMPY TFSI, and

the improved electrochemical performance can be ascribed

to the smaller size of the EMIM cation. As Table 2 demon-

strated, PSDAC-700-1 also possesses the lowest overall capac-

itance for a given scan rate, and the worst capacitance

retention. In EMIM TFSI, PSDAC-800-1 and PSDAC-800-3 do

not show ideal CV curves at 2 mV s�1. This phenomenon

has been observed by other groups and has been attributed

to pseudocapacitance from impurities in ionic liquid and/or

the reaction of ionic liquid with functional groups on the car-

bon surface [33,38]. The CV curves for PSDAC-800-3 demon-

strate the least distortion. This carbon is able to achieve

both the highest overall capacitance values and highest

capacitance retention. The variation of the total capacitance

and its rate dependence are attributed to the predominant

mechanism of IR loss that in turn depends both on the net le-

vel and on the size distribution of the porosity. The details of

this will be discussed later in the manuscript.

Fig. 5a and b display the galvanostatic charge–discharge

curves at 1 A g�1 in BMPY TFSI and EMIM TFSI electrolytes.

PSDAC-800-3 and CAC can be charged and discharged

smoothly with symmetric and well-defined charge–discharge

lines. However, PSDAC-700-1 and PSDAC-800-1 materials

show non-linear charge–discharge curves. The galvanostatic

results are analogous to the CV results: PSDAC-800-3 carbon

possesses the best combination of specific capacitance and

capacitance retention, while the PSDAC-700-1 carbon pos-

sesses the worst. Fig. 5c and d display the relationship

Table 2 – Specific capacitance and capacity retention of the activated carbons measured in the organic and ionic liquidelectrolytes.

Electrolyte Samples Capacitance, CV (F g�1) Capacitance, charge–discharge (F g�1)

2 mV s�1 200 mV s�1 Retention (%) 0.1 A g�1 10 A g�1 Retention (%)

TEABF4/AN PSDAC-700-1 118.6 46.0 38.8 116.7 42.5 36.4PSDAC-800-1 128.1 64.1 50.0 121.7 64.3 52.8PSDAC-800-3 165.6 107.9 65.2 162.2 120.0 74.0CAC 111.6 66.5 59.6 108.4 69.7 64.3

BMPY TFSI PSDAC-700-1 85.7 18.1 21.1 82.4 8.0 9.7PSDAC-800-1 136.1 38.8 28.5 126.5 32.2 25.5PSDAC-800-3 180.1 111.3 61.8 163.3 121.7 74.5CAC 122.1 69.2 56.7 113.6 75.0 66.0

EMIM TFSI PSDAC-700-1 119.3 29.6 24.8 113.4 19.3 17.0PSDAC-800-1 149.4 49.9 33.4 140.7 43.6 29.3PSDAC-800-3 190.3 113.5 59.6 161.5 95.3 59.0CAC 123.4 75.5 61.2 114.4 85.6 74.8

0.0 0.5 1.0 1.5 2.0 2.5 3.0

-200

-100

0

100

200

300

400

Cap

acita

nce

(F g

-1)

Voltage (V)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

a

0.0 0.5 1.0 1.5 2.0 2.5 3.0-200

-150

-100

-50

0

50

100

150

200

Cap

acita

nce

(F g

-1)

Voltage (V)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

b

0.0 0.5 1.0 1.5 2.0 2.5 3.0

-200

-100

0

100

200

300

400

Cap

acita

nce

(F g

-1)

Voltage (V)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

c

0.0 0.5 1.0 1.5 2.0 2.5 3.0-200

-150

-100

-50

0

50

100

150

200

Cap

acita

nce

(F g

-1)

Voltage (V)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

d

Fig. 4 – CV curves of the carbons in BMPY TFSI electrolyte at a scan rate of (a) 2 mV s�1 and (b) 200 mV s�1. CV curves of the

carbons in EMIM TFSI electrolyte at (c) 2 mV s�1 and (d) 200 mV s�1.

C A R B O N 5 7 ( 2 0 1 3 ) 3 1 7 – 3 2 8 323

between potential drop and current density. The strongest IR

drop dependence on current density is observed for the

PSDAC-700-1 and PSDAC-800-1 carbons.

Fig. 5e shows the Ragone plot for the two ionic liquids, cal-

culated from the galvanostatic charge–discharge curves. The

PSDAC-800-3 sample yielded very promising energy-power

combinations in both ionic liquids: 51 W h kg�1 at 375 W kg�1

and 26–31 W h kg�1 at 6760–7000 W kg�1. Because the carbon

weight accounts for about 30 wt.% of the total mass of the

packaged commercial electrochemical capacitors, a practical

energy density of about 15 W h kg�1 for a packaged device is

expected, which is about three times higher than existing

activated carbon-based capacitors [33].

In both ionic liquids, PSDAC-800-3 exhibited excellent per-

formance in terms of high capacitance and capacitance reten-

tion. In fact this material performed on-par or even superior

to the potentially more expensive and less industrially scal-

able carbons such as mesoporous activated carbon fibers,

mesoporous carbons prepared from barium citrates, sucrose

derived carbons, carbide-derived carbons, templated car-

bide-derived carbons, activated graphene, reduced graphene

oxide, and CNT-activated carbon composites (see Table S1 in

0 100 200 300 400 5000.0

0.5

1.0

1.5

2.0

2.5

3.0

Volta

ge (V

)

Time (s)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

b

0 100 200 300 400 5000.0

0.5

1.0

1.5

2.0

2.5

3.0

Volta

ge (V

)

Time (s)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

a

0 2 4 6 8 10

0.0

0.5

1.0

1.5

2.0

IR D

rop

(V)

Current Density (A g-1)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

c

0 2 4 6 8 10

0.0

0.5

1.0

1.5

2.0

IR D

rop

(V)

Current Density (A g-1)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

d

10 100 1000 100000.1

1

10

100

EMIM TFSIBMPY TFSIEner

gy D

ensi

ty (W

h kg

-1)

Power Density (W kg-1)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

e

Fig. 5 – Galvanostatic charge–discharge curves at 1 A g�1 in (a) BMPY TFSI and (b) EMIM TFSI electrolytes. Potential drop

associated with internal resistance vs. discharge current density in (c) BMPY TFSI and (d) EMIM TFSI electrolytes. (e) Resultant

Ragone plots utilizing BMPY TFSI and EMIM TFSI electrolytes. (f) Photo image of a commercial flashlight powered by

supercapacitor with pulp sludge derived activated carbon electrodes.

324 C A R B O N 5 7 ( 2 0 1 3 ) 3 1 7 – 3 2 8

Supplementary material) [27,33,36,37,39–43]. Recently, poly-

pyrrole derived activated carbon displayed the highest re-

ported capacitance of up to 300 F g�1 at 1 mV s�1 and

180 F g�1 at 100 mV s�1 [38]. The ultra-high surface area

(3432 m2 g�1) of these materials should be responsible for this

extraordinarily high capacitance. Though the surface area of

PSDAC-800-3 is lower, its capacitance at 100 mV s�1 is

140 F g�1, which is not very far off.

The cycling performance of PSDAC-800-3 over 5000 cycles

at 10 A g�1 is shown in Fig. S3 (in Supplementary material).

PSDAC-800-3 retains up to 91% capacity after 5000 cycles,

which indicates PSDAC-800-3 can be used for extended cy-

cling applications.

To show the practical applications of our supercapacitor

based on pulp sludge derived activated carbon, two 2032-type

button cells with 2.5 mg PSDAC-800-3 carbon on each elec-

trode operated in BMPY TFSI electrolyte (around 0.2 F per cell)

were used to power two light-emitting diode bulbs (0.06 W per

bulb) in a commercial mountaineering helmet-mounted

flashlight. Those two bulbs require a current of 20 mA at

6 V, equal to 8 A g�1 for each carbon electrode. As shown in

Fig. 5f, even at this high current density the supercapacitors

can power the bulbs at full intensity. After roughly 10 s the

bulbs begins to show signs of dimming due to the voltage

drop. However even after about 10 min, when the voltage

has dropped to 1.2 V, the bulbs still emit weak light.

3.4. Electrochemical impedance behavior

The electrochemical impedance data for the carbons in the

organic and in the two ionic liquid electrolytes are shown in

Fig. 6a–c. The Nyquist plots of the samples in all the electro-

lytes exhibit the typical features of porous electrodes with a

45� Warburg region at high-medium frequencies, and an al-

most vertical line at low frequencies, where the behavior be-

comes mainly capacitive [41]. The length of the 45� segment is

related to the resistance caused by ion diffusion into the bulk

of the electrode particles [44,45]. In the organic electrolyte,

0 200 400 600 800 1000 12000

300

600

900

1200

0 20 40 600

20

40

60

-Z'' (

ohm

)

Z' (ohm)

-Z''

(ohm

)

Z' (ohm)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

a

0 500 1000 1500 20000

500

1000

1500

2000

0 50 1000

50

100

-Z'' (

ohm

)

Z' (ohm)

-Z''

(ohm

)

Z' (ohm)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

b

10-3 10-2 10-1 100 101 102

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Cap

acita

nce

(C/C

o)

Frequency (Hz)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

d

0 500 1000 1500 20000

500

1000

1500

2000

0 50 1000

50

100

-Z'' (

ohm

)

Z' (ohm)

-Z''

(ohm

)

Z' (ohm)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

c

10-3 10-2 10-1 100 101 102

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Cap

acita

nce

(C/C

o)

Frequency (Hz)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

e

10-3 10-2 10-1 100 101 102

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Cap

acita

nce

(C/C

o)

Frequency (Hz)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

f

Fig. 6 – Nyquist plots of the carbons in (a) TEABF4/AN, (b) BMPY TFSI, and (c) EMIM TFSI electrolytes. Frequency response of the

carbons in (d) TEABF4/AN, (e) BMPY TFSI, and (f) EMIM TFSI electrolytes.

C A R B O N 5 7 ( 2 0 1 3 ) 3 1 7 – 3 2 8 325

PSDAC-700-1 and PSDAC-800-1 possess a Warburg impedance

of similar magnitude, distinct from PSDAC-800-3 and CAC. In

the BMPY TFSI, the PSDAC-700-1 is distinct from the other

three. In EMIM TFSI, PSDAC-700-1 is also by far the most resis-

tive. An increased length of the Warburg segment indicates a

higher resistance faced by the ions during their transport into

the small micropores. Clearly the PSDAC-700-1 specimen with

its pore size distribution biased towards narrow micropores

provides the most tortuous diffusion path for the absorbing

ions. In the high frequency region all samples show a semicir-

cle of larger radius in the ionic liquids than in the organic

electrolyte, indicative of higher charge-transfer resistance in

the former. The very high frequency response of all the car-

bons in the three electrolytes is analogous, demonstrating

low equivalent series resistances (0.4–2.5 Ohms).

Fig. 6d–f shows the frequency response of the supercapac-

itors in organic and ionic liquid electrolytes. Specimen

PSDAC-800-3 shows the highest operating frequency (fre-

quency at which capacitance is 50% of max.) with 0.22 Hz in

organic electrolyte, and 0.032–0.050 Hz in ionic liquids. The

carbons can be ranked based on the operating frequency in

the following order: PSDAC-800-3 > CAC > PSDAC-800-

1 > PSDAC-700-1. Based on this criterion the electrolytes can

also be ranked: TEABF4/AN > EMIM TFSI > BMPY TFSI. We be-

lieve that the fastest response time observed in PSDAC-800-3

is directly related to its combined suitable micro and mesopo-

rosity. The performance in different electrolytes is related to

both the ion size and the viscosity of the electrolyte. Of the

two ionic liquids BMPY TFSI has the larger cation size (0.76

vs. 1.1 nm) and yields the slowest response.

3.5. Surface area normalized capacitance: ion size andpore size effects

Fig. 7a shows the relationship between the surface area nor-

malized capacitances and the scan rates in TEABF4/AN

0 50 100 150 2000

2

4

6

8

10

Nor

mal

ized

Cap

acita

nce

(µF

cm-2)

Scan Rate (mV s-1)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

b

0 50 100 150 2000

2

4

6

8

10

Nor

mal

ized

Cap

acita

nce

( µF

cm-2)

Scan Rate (mV s-1)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

c

0 50 100 150 2000

2

4

6

8

10

Nor

mal

ized

Cap

acita

nce

(µF

cm-2)

Scan Rate (mV s-1)

PSDAC-700-1 PSDAC-800-1 PSDAC-800-3 CAC

a

Fig. 7 – Surface area normalized capacitance of the carbons in (a) TEABF4/AN, (b) BMPY TFSI, and (c) EMIM TFSI electrolytes.

326 C A R B O N 5 7 ( 2 0 1 3 ) 3 1 7 – 3 2 8

electrolyte. At the lower scan rates the surface area normal-

ized capacitance for PSDAC-700-1 was significantly higher

than for the other carbons. The sizes of the solvated cation

and anion in TEABF4/AN electrolyte are 1.30 and 1.16 nm [46].

This is much larger than the mean micropore size of PSDAC-

700-1. In fact according to Fig. 2b the majority of the pores in

this specimen are significantly smaller than the solvated ion

dimensions. The uniquely enhanced capacitance for PSDAC-

700-1 at the low scan rates provides evidence that there must

be partial desolvation of the ions as to allow their penetration

into narrow micropores [46]. Since many more desolvated ions

can fit onto a given surface than could solvated ions, at slow

scan rates (i.e. low diffusional losses) a pore size distribution

biased towards the small micropores and ultramicropores

would store the most charge on a surface area normalized ba-

sis. The bare cation size of TEABF4/AN is 0.67 nm. The pore size

distribution for PSDAC-700-1 has an incremental pore volume

peak centered at 0.8 nm, which is well suited for cations of

TEA+. Although PSDAC-800-1 and PSDAC-700-1 possess similar

Vmicro/Vt ratio, the mean micropore size of PSDAC-700-1

(0.94 nm) is smaller than PSDAC-800-1 (0.99 nm), better corre-

lating to the size of the electrolyte-ions.

Examining the other carbons shows a trend that the rate

dependence of the surface area normalized capacitance be-

comes progressively weaker with increasing levels of meso-

porosity, with PSDAC-800-3 being superior at rates above

100 mV s�1. The optimum performance of PSDAC-800-3 at

high rates is expected since high relative levels of mesoporos-

ity (Table 1) reduce the diffusional losses associated with tor-

tuous narrow micropores [3,9].The surface area normalized

capacitances are relatively close to each other at the high

scan rates. This may be explained by the finding that in an or-

ganic electrolyte, large micropores (>1.2 nm), in addition to

mesopores, would allow for facile ion diffusion [12].

Fig. 7b and c shows the relationship between surface area

normalized capacitances and scan rates in BMPY TFSI and

EMIM TFSI electrolytes. The high surface area normalized

capacitance of PSDAC-700-1 in EMIM TFSI at the low scan

rates may be explained by the close correlation between the

mean pore size and the ion size. Recent experimental and

theoretical investigations highlight an anomalous increase

in capacitance when the pore size approaches the ionic

dimensions [36,47]. In both electrolytes the maximum dimen-

sion of the anion is 0.79 nm. The maximum dimension of the

cation in EMIM TFSI and BMPY TFSI is 0.76 and 1.1 nm,

respectively. The 0.8 nm distribution peak of PSDAC-700-1 is

almost ideally matched to the size of the EMIM+ ion. For the

case of the ionic liquid with the larger cation, BMPY TFSI,

such an enhancement is not observed. The larger ion size of

BMPY+ couldn’t fully utilize micropores smaller than 1 nm.

This scenario agrees with recent simulation results regarding

the role of sub-nanometer pores in improving the electrical

double layer capacitance [48]. In both ionic liquids, PSDAC-

800-3 displays the best surface area normalized capacitances

at the higher scan rates. This agrees with the diffusional loss

scenario and the need for mesopores in ionic liquid

electrolytes.

At high scan rates, PSDAC-700-1 and PSDAC-800-1 exhibit

higher surface area normalized capacitance in organic

electrolyte than in ionic liquid, and this observation further

indicates that pores larger than 1.2 nm contribute to fast

ion-transport in organic electrolyte and mesopores (>2 nm)

play a more important role in fast ion-transport in ionic liquid

electrolyte. Comparing PSDAC-800-3 with CAC, PSDAC-800-3

with higher mesopore proportion shows similar or a little

higher surface area normalized capacitance at high scan

rates, and this can be partially ascribed to the lower capaci-

tance contribution from mesopores and/or the pore connec-

tivity [4].

4. Conclusions

We demonstrate that paper pulp mill sludge, a widely avail-

able industrial biowaste, can be converted into a high-perfor-

mance carbon for electrochemical capacitor applications.

With the activation conditions optimized (PSDAC-800-3), the

material exhibits excellent specific capacitances and rate

capabilities in both organic and ionic liquid electrolytes. This

is a direct result of its high surface area, which actually ex-

ceeds that of graphene, combined with high levels of mesopo-

rosity. Given the large availability of paper pulp mill sludge

and the energy reduced nature of the hydrothermal carbon-

ization process, such carbons can be produced on large scales

at low environmental and economic costs.

Acknowledgement

The authors acknowledge financial support from ALMA/AB

Bio and NINT NRC.

C A R B O N 5 7 ( 2 0 1 3 ) 3 1 7 – 3 2 8 327

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at http://dx.doi.org/10.1016/j.carbon.

2013.01.079.

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