A High Performance Pseudocapacitive Suspension Electrode for The

Home , Supercapacitor

Electrochimica Acta 111 (2013) 888–897

Contents lists available at ScienceDirect

Electrochimica Acta

jou rnal homepage: www.elsevier.com/locate/electacta

A high performance pseudocapacitive suspension electrode

for the electrochemical ﬂow capacitor

a a a a,b

Kelsey B. Hatzell , Majid Beidaghi , Jonathan W. Campos , Christopher R. Dennison ,

b,∗ a,∗∗

Emin C. Kumbur , Yury Gogotsi

A.J. Drexel Nanotechnology Institute, Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, USA

Electrochemical Energy Systems Laboratory, Department of Mechanical Engineering and Mechanics, Drexel University, Philadelphia, PA 19104, USA

r t i c l e i n f o a b s t r a c t

Article history: The electrochemical ﬂow capacitor (EFC) is a new technology for grid energy storage that is based on

Received 1 May 2013

the fundamental principles of supercapacitors. The EFC beneﬁts from the advantages of both superca-

Received in revised form 17 July 2013

pacitors and ﬂow batteries in that it is capable of rapid charging/discharging, has a long cycle lifetime,

Accepted 17 August 2013

and enables energy storage and power to be decoupled and optimized for the desired application. The

Available online 30 August 2013

unique aspect of the EFC is that it utilizes a ﬂowable carbon-electrolyte suspension (slurry) for capac-

itive energy storage. Similar to traditional supercapacitor electrodes, this aqueous slurry is limited in

Keywords:

terms of energy density, when compared to batteries. To address this limitation, in this study a pseudo-

Electrochemical ﬂow capacitor

capacitive additive has been explored to increase capacitance. A carbon-electrolyte slurry prepared with

Energy storage

P-phenylenediamine p-phenylenediamine (PPD), a redox mediator, shows an increased capacitance on the order of 86% when

Pseudocapacitor compared with KOH electrolytes, and a 130% increase when compared to previously reported neutral

Supercapacitor electrolyte based slurries. The redox-mediated slurry also appears to beneﬁt from a decrease in ohmic

Redox mediator resistance with increasing concentrations of PPD, most likely a result of an increase in the ionic diffusion

coefﬁcient. Among the tested slurries, a concentration of 0.139 M of PPD in 2 M KOH electrolyte yields the

largest capacitance and rate handling performance in both cyclic voltammetry and galvanostatic cycling

experiments. The improved performance is attributed to the addition of quick faradaic reactions at the

electrolyte-electrode interface as PPD undergoes a two-proton/two-electron reduction and oxidation

reaction during cycling.

1. Introduction battery systems [6,7]. The critical performance requirements for

grid-level energy storage are cost, scalability, and lifetime [8]. Fur-

Currently, the primary technologies employed for grid-level thermore, the market for stationary energy storage is vast, requiring

energy storage are pumped hydroelectric and compressed air capacities that range from kW to GW systems which most likely

energy storage [1]. These technologies are geographically con- cannot be achieved with a single technology [1,9]. Thus, simi-

strained and require large capital investments when compared lar to energy generation systems, there is a need for a portfolio

with electrochemical energy systems (EES), such as batteries and approach of tailored energy storage systems designed for spe-

supercapacitors [2–4]. Thus, EES systems have emerged as the most ciﬁc applications such as voltage and frequency regulation, load

opportunistic type of technology for grid-scale applications [1,5]. In leveling, and bulk power management [1,9]. In order to address

transportation or portable electronics applications, where energy the scalability requirements and broader power/energy range, var-

density is the critical factor, batteries are favored [2]. However, ious ﬂow-assisted electrochemical systems have been recently

in large-scale stationary storage applications, the scalability, low proposed. The architecture seen in the redox ﬂow battery [10],

power density, and cost hinder the widespread implementation of semi-solid lithium battery [11], and electrochemical ﬂow capac-

itor (EFC) [12,13] offers tremendous design ﬂexibility for use in

grid-level energy storage.

∗ Recently, a new energy storage concept called the electrochem-

Corresponding author. Tel.: +1 215 895 5871; fax: +1 215 895 1478.

∗∗ ical ﬂow capacitor (EFC) has been proposed by our group at Drexel

Corresponding author. Tel.: +1 215 895 6446; fax: +1 215 895 1934.

University. The EFC is a rechargeable EES that utilizes a ﬂow battery

E-mail addresses: [email protected] (E.C. Kumbur), [email protected]

(Y. Gogotsi). architecture and is based on the fundamental working principles of

K.B. Hatzell et al. / Electrochimica Acta 111 (2013) 888–897 889

Fig. 1. (a) Operational schematic of the electrochemical ﬂow capacitor. Uncharged slurry ﬂows through polarized plates and charged. At the pore level, electrode neutrality

is maintained at the interface between the electrolyte and active material. This slurry is then pumped into external reservoirs for storage. The process is reversed during

discharge. (b) Schematic of a carbon|electrolyte interface between charged spherical particles and (c) SEM image of carbon beads.

supercapacitors [3,12]. The primary difference between traditional Although the EFC has high power density, its energy density

ﬂow cells and the EFC is that the EFC utilizes a ﬂowable carbon- needs to be improved in order to compete with alternative systems

electrolyte electrode, similar to the ‘slurry electrode’ introduced by for grid-level energy storage applications. The energy released by

Kastening et al. [14], for capacitive energy storage (Fig. 1b). During a supercapacitor, E, is:

operation the slurry is pumped from a storage reservoir through 2

E = .5CV (1)

two polarized plates (charging process). Once fully charged, the

slurry is pumped out of the cell and stored in external reser- where C is the capacitance (in F), and V is the operational voltage

voirs until the process is reversed and the slurry is discharged window (in V). Therefore, in order to increase the energy density,

(Fig. 1a). The charged slurry stores charge electrostatically at the one can either increase the voltage window or the active mate-

carbon/electrolyte interface which allows for rapid charging and rial’s capacitance. For aqueous electrolytes, the voltage window is

discharging (Fig. 1c). Traditional ﬂow batteries suffer from slow thermodynamically limited to approximately 1.0 V because water

response rates due to faradaic reactions at the electrode-electrolyte decomposition occurs above 1.23 V [16]. Nevertheless, it has been

surface and have limited cycle lifetimes (<12,000 cycles) which is a shown that the voltage window of supercapacitors based on neutral

critical factor for grid scale energy storage [8,15]. The EFC in com- electrolytes may extend beyond 1.23 V because the di-hydrogen

parison is capable of rapid charging/discharging, is based on cheap overpotential is higher for neutral electrolytes[16–20]. In a recent

and abundant materials (e.g., carbon), and enables energy storage study on the EFC, ﬂowable electrodes based on sodium sulfate

and power to be decoupled and optimized. Furthermore, since the demonstrated voltage stability up to ∼1.5 V[19]. While an increased

charge storage mechanism is physical, the EFC has the potential to voltage window will yield increased energy density, the current

exhibit long cycle life, a characteristic commonly seen in superca- study seeks to examine methods to enhance the capacitance of the

pacitors. Power is directly related to stack size, and energy capacity ﬂowable electrode. Thus, this study restricts its focus to available

can be scaled according to reservoir volume. methods to enhance the capacitance of the ﬂowable electrode.

890 K.B. Hatzell et al. / Electrochimica Acta 111 (2013) 888–897

One method for increasing capacitance is through the addi- performance [38]. Thus, this study intends to demonstrate the

tion of quick redox reactions at the electrode-electrolyte interface optimal concentration for a ﬂowable electrode system. The molar

(pseudocapacitive effects). The primary means for achieving pseu- concentrations correspond to increments of 5 mg/ml solutions.

docapacitance is through the use of transition metal oxides [21–23], The PPD was sonicated in 2 M KOH electrolyte until completely

conducting polymers [21,24] or the addition of surface func- dissolved. All solutions were prepared immediately prior to the

tionalities such as nitrogen or oxygen to the surface of carbon testing.

materials [25]. Recently a fourth strategy, the use of redox-

mediated electrolytes, has emerged as a new method of achieving 2.2. Electrode active materials

pseudocapacitive behavior [26]. The addition of redox-mediators

into electrolytes has been shown to increase ionic conductivity Carbon beads derived from phenolic resins were obtained from

and provide an additional charge storage mechanism, and thus MAST Carbon (United Kingdom) and used as the active capacitive

has been explored in supercapacitor applications as a way to material in the slurry electrodes (Fig. 1d). In our previous work we

enhance speciﬁc capacitance [27]. Furthermore, there is no need characterized the active material, MAST carbon beads, extensively

for surface modiﬁcation because the redox species react at the in terms of the physical attributes and electrochemical perfor-

carbon electrolyte interface. Most redox-mediated electrolytes are mance [12,19]. Three different sizes (diameter) of MAST beads were

based on proton-coupled electron transfer (PCET) mechanisms examined ranging from 9 ␮m to 500 ␮m, and it was determined

that are pH-dependent and prefer basic or acidic environments that the mid-size beads (particle size: 125–250 ␮m) produced the

in order to facilitate proton transfer at the active material surface best electrochemical and rheological performance [19], and thus

[28–30]. In supercapacitors, electrochemically active compounds was selected as the active material in this study.

such as indigo carmine [31] and hydroquinone [33], have shown N2 gas sorption was carried out in a Quadrasorb gas sorption

signiﬁcant capacitance increases in acidic mediums. In addition, instrument (Quantachrome, USA) and the average, volume-

it has been recently shown that aromatic diamines such as p- weighted pore size was derived from the cumulative pore

phenylenediamine (PPD) [21,34–37], and m-phenylenediamine volume assuming slit-shaped pores and using the quenched-solid

[38] (MPD) are possible redox mediators in basic electrolyte sys- density function theory (QSDFT) algorithm. The pore size distri-

tems. A MnO2-based supercapacitor[36] demonstrated a 6-fold butions of the carbon beads (average particle size: 161 ± 35 ␮m)

increase in capacitance and an activated carbon supercapacitor[35] had an average volume-weighted pore size of 4.7 nm. The

showed a 4-fold increase in capacitance after the addition of PPD Brunauer–Emmett–Teller (BET) method was used to calculate a

into KOH electrolytes[36]. Thus, PPD as a redox mediator has been speciﬁc surface area (SSA) of 1576 m /g. This value is in agreement

shown to dramatically enhance capacitance. Pseudocapacitance, with the SSA value calculated from QSDFT with a maximum of 10%

where the source of the redox reaction is in the electrolyte, is favor- deviation. Carbon black (100% compressed; Alfa Aesar, USA) was

able for systems such as the EFC because the major component employed as the conductive additive between carbon beads. For

in the slurry is the electrolyte. Thus, redox-mediators integrated calculation purposes, the active mass was considered to be the total

into a capacitive slurry offer opportunities for increased capac- mass of the carbon black and activated carbon in the slurry. A Zeiss

itance and enhanced electrochemical performance. However, in Supra 50VP scanning electron microscope (Carl Zeiss AG, Germany)

many cases pseudocapacitors suffer from poor rate capacities due operating at 3 kV was used for observing any surface changes to the

to irreversible redox reactions and physicochemical changes that carbon beads as a result of the redox-mediator.

take place during charging and discharging [39].

The objective of this study is to develop a carbon-based pseu- 2.3. Preparation of slurry electrodes

docapacitive ﬂowable electrode for the EFC that utilizes the

phenylenediamine/p-phenylenediimine (PPD/PPI) redox couple The slurry electrodes were prepared by mixing carbon beads

for additional storage of energy. Herein, a redox-mediated slurry with carbon black (100% compressed; Alfa Aesar, USA) to achieve

is prepared by the addition of PPD to 2 M KOH electrolyte and the a 9:1 weight ratio, as this ratio is deﬁned as the baseline condi-

performance of the slurry is characterized electrochemically. This tion in our previous study. The carbon black served as a conductive

study also examines the effectiveness of PPD in a suspension envi- additive. The aqueous electrolyte (2 M KOH) was then added to

ronment (ﬂowable electrode) and assesses its performance (rate achieve 23 wt% (solid-to-liquid ratio), as it was shown previously

handling and capacitance) across a wide range of rates and con- to exhibit favorable capacitive and rheological properties [12,19].

centrations. In order to achieve a wetted carbon surface to adsorb the carbon

black, an incremental amount of deionized water (<5 wt% of slurry)

was weighed and added to the slurry mixture. The mixture (with

2. Materials and methods the excess deionized water) was mildly heated and stirred until a

homogenous slurry was obtained, and the excess deionized water

2.1. Chemicals evaporated.

All the reagents used in this study were of analytical grade. 2.4. Electrochemical performance

PPD anti-fade reagent supplied by Sigma–Aldrich was employed

as a redox mediator, and 2 M potassium hydroxide (KOH) supplied The slurry performance was studied in a symmetric static cell

from Alfa Aesar, was used as the supporting electrolyte. A base- conﬁguration, with equal masses in both electrode supensions.

line experiment was performed in 1 M sodium sulfate supplied by The static cell is comprised of two stainless steel current collec-

Sigma-Aldrich for comparison with KOH electrolyte experiments. tors, onto which the slurry was uniformly loaded into a channel

Six different concentrations of the redox mediators were exam- region deﬁned by latex gaskets. The channel region (active area)

ined: 0.046 M, 0.092 M, 0.139 M, 0.185 M, and 0.277 M PPD in the was 1 cm . The latex gaskets were 610 ␮m thick when completely

supporting electrolyte solution (2 M KOH). Different concentration compressed prior to testing. A polyvinylidene ﬂuoride (PVDF)

solutions were explored in order to identify the optimal conditions membrane separator with a mesh width of 100 nm (Durapore ;

for improved system performance (capacitance and rate handling). Merck Millipore, Germany) was used as the separator between the

Previously, it has been shown high concentrations of redox medi- two slurry electrodes. All electrodes tested had a carbon content of

ators can lead to aggregation of free ions and thus diminish the 32 mg per electrode and a solid fraction of 23 wt%.

K.B. Hatzell et al. / Electrochimica Acta 111 (2013) 888–897 891

Fig. 2. SEM image of activated carbon beads. (a) As-received beads with pristine surface, (b) Carbon beads from slurry after being cycled 1000 times in 2 M KOH and 0.139 M

PPD. Insets show magniﬁed images of the bead surfaces.

All electrochemical measurements were performed at room the redox behavior exhibited in the galvanostatic charge/discharge

temperature with a VMP3 or VSP potentiostat/galvanostat (Bio- results. The equivalent series resistance (ESR) of the cell was deter-

Logic, France). Cyclic voltammetry (CV) was carried out at 2, 5, 10, mined by dividing the total change in voltage of the IR drop in

−1

20, 50, and 100 mV s sweep rates. From CV, the speciﬁc gravi- a galvanostatic cycle with the total change in current (between

metric capacitance (Csp) was derived using the following equation. charge and discharge) using Eq. (4). V V V = idV ESR = = (4) 2 I I + I 2I C = × (2) charge discharge

sp E m

Potentiodynamic electrochemical impedance spectroscopy

where E is the width of the voltage window, i is the discharge cur-

(PEIS) was used to assess the change in ohmic resistance due to

rent, V is the voltage, is the sweep rate, and m is the mass of carbon

changes in concentrations of PPD in the slurries. The ohmic resis-

in one electrode. The factor of two accounts for the two electrode

tance of the static cell, R, was determined from the impedance

setup, assuming that the charge is evenly distributed between two

spectrum intersection with the real axis. This value is representa-

capacitors in series [40]. Three ﬂowable electrodes were tested for

tive of the total resistances associated with the current collectors,

each case (concentration), and an average value is reported in the

active material, electrolyte, and separator [41]. With all characteris-

ﬁgures. Error bars are plotted for each average value and represent

tics held constant (except PPD concentration), the ohmic resistance

the standard deviation from the average capacitance.

highlights the kinetic changes in the slurry due to the addition of

All values for the capacitance were normalized by the weight

PPD. Furthermore, PEIS was used to evaluate the changes in ion

of the carbon material in one electrode, not the total slurry mass, to

diffusion at the surface of the electrode. In order to characterize

enable a direct comparison with conventional supercapacitor elec-

changes in the diffusion coefﬁcient, a Randles circuit was employed

trodes (which are also normalized to the content of active material

to analyze the impedance responses. All spectra were run in the fre-

in a single electrode). Csp was also calculated from galvanostatic

quency range of 200 kHz to 10 mHz at a sine wave signal amplitude

cycling (GC) using Eq. (3):

of 10 mV. 2i C = (3)

sp m

× dV/dt

3. Results and discussion

where dV/dt is the slope of the entire discharge curve starting from

the bottom of the IR drop and ending at the time when the volt- The morphology of the carbon beads with and without PPD was

age is equal to zero. The whole slope was taken to best account for analyzed by a scanning electron microscope (SEM) as seen in Fig. 2.

−1

Fig. 3. (a) Cyclic voltammograms (CV) for carbon-electrolyte based slurries with 1 M sodium sulfate and 2 M potassium hydroxide electrolytes at 2 mV s . (b) Rate dependence

−1

of base-case slurries from 2–100 mV s , showing decreasing capacitance with increasing sweep rates.

892 K.B. Hatzell et al. / Electrochimica Acta 111 (2013) 888–897

−1

Fig. 4. (a) Cyclic voltammograms at 2 mV s of redox-active slurries (23 wt% CB2) in 2 M KOH electrolyte with various amounts of PPD added. (b) Rate performance for the

various concentrations of p-phenylenediamine in the slurries based on 2 M KOH.

−1

PPD, as a monomer, has been reported to polymerize under elec- approximately 170 mS cm , facilitating transport throughout the

trochemical oxidative conditions [42]. Polymerization can result slurry medium. The KOH based ﬂowable electrode exhibits higher

in electrode “poisoning”, or the coating of a polymeric ﬁlm on the capacitance, but also exhibits signs of faradaic reactions as the volt-

carbon surface [43]. This can cause a blocking effect on the carbon age approaches 1 V in Fig. 3a. The sudden increase in current at 1 V is

surface that can result in limited ion mobility and poor electronic a sign that electrolysis of water occurs which evolves hydrogen and

conductivity. These limitations can be observed electrochemically oxygen as byproducts [46]. In constrast, neutral electrolytes have a

by a rapid decrease in the peak current of a CV plot during succes- stability window that reaches around 1.6 V because of a high over-

sive cycles. In order to examine whether polymerization occurred potential for di-hydrogen evolution, whereas KOH electrolytes are

during cycling, a SEM micrograph of pristine carbon beads, Fig. 2a, limited to about 1 V [17]. Fig. 3b shows the rate performance for the

was compared with a SEM micrograph of cycled slurry (0.139 M PPD slurries without a redox-mediated slurry. Across all rates tested,

−1

in 2 M KOH electrolyte) after undergoing 1000 cycles at 20 mV s the 2 M KOH slurry outperformed the sodium sulfate based slur-

between 0 V and 1 V. From the SEM micrographs in Fig. 2b, it is ries as a working electrolyte for the ﬂowable electrode (Fig. 3b).

−1

observed that the cycled slurry shows little evidence of polymer Above 20 mV s both slurries exhibit a decline in rate performance

−1

ﬁlms on the carbon surface. The noticeable particles around the car- (capacitance decay). At 100 mV s the Na2SO4 based slurry saw a

−1

bon beads in Fig. 2b are aggregates of carbon black, which is added 66% decrease from its initial capacitance taken at 2 mV s , which is

to the slurry as a conductive additive. Furthermore, it should be comparable to the 2 M KOH electrolyte which exhibited 65% capac-

noted that no drastic declines in the peak current were observed itance decay. The observed capacitance decay at high sweep rates

during the ﬁrst 20 CV cycles typically when polymerization is for both slurries is a result of ion transport limitations, which is typ-

expected to occur. A gas sorption analysis was taken on a sample of ically seen in porous activated carbon [47]. Diffusion limitations in

the cycled slurry with a concentration of 0.139 M PPD. The surface slurry arise as a result of the slurry’s thickness and as a result of the

area of the cycled slurry (carbon beads, carbon black, PPD, and 2 M network of particle-particle contacts. In traditional supercapacitors

∼

KOH) exhibited a BET surface area of 1210 m /g. Carbon black with ﬁlm electrodes, it has been shown that increased thickness

is approximately 10 wt% of the slurry, and explains this decrease yields increased barriers to ion diffusion within the inner region

∼

in surface area when compared to dry carbon beads ( 1576 m /g). of the electrode [47]. This results in longer diffusion paths, higher

Thus, the fact that a large decrease in surface area was not observed electrode resistivity, and poor power performance at high rates.

supports the notion that the PPD is not polymerizing. This is advan- This explanation can be transferred to slurry electrodes which are

tageous for our system because polymerization during oxidation on average three to ﬁve times thicker than ﬁlm electrodes, and can

and reduction cycles forms polymer chains which could contribute explain the decreased power performance at high rates. In addi-

to irreversible reactions and deterioration of PPD over the cycle life. tion to diffusion length challenges, while loading the cell during

We assessed only the ﬂowable electrode concentration with the charging, some losses can arise when beads are not in direct con-

greatest overall performance in order to ascertain whether poly- tact, and transport paths are broken. Nevertheless, the desired rate

merization occurred. It is expected that below 0.139 M, there is no handling of the slurry for a grid scale applications will most likely

−1

polymerization. In later sections we assess the effect of the concen- lie below 20 mV s . which is very well in the range of acceptable

tration of PPD on the electrochemical performance of the ﬂowable performance for the EFC.

electrode. Fig. 4 shows the effect on capacitance and rate handling with

Cyclic voltammetry (CV) was utilized for characterizing the increasing concentrations of the redox mediator (PPD) in the slurry.

capacitance of the slurries in a static cell conﬁguration. Fig. 3a In contrast to the slurries without redox mediators, the redox-

shows that for the cases without the redox mediator (i.e., 2 M active slurries demonstrate an increased capacitance. The area

KOH and 1 M Na2SO4 electrolyte), the slurries demonstrate a typ- under CV curve is proportional to the capacitance, thus in Fig. 4a

ical quasi-rectangular shape without redox peaks, implying an the increase in capacitance can be observed qualitatively by the

ideal electric double layer capacitor [44]. KOH electrolyte based expansion of the CV curve with changing concentration of PPD. The

−1 −1

slurries exhibit a higher capacitance (118 F g at 2 mV s ) when anodic and cathodic peaks centered around 0.4 V can be attributed

−1 −1

compared to sodium sulfate solution (94 F g at 2 mV s ). This to a pseudocapacitive behavior produced by the redox couple p-

may be attributed to the fact that protons as mobile species may phenylenediamine and p-phenylenediimine (PPD/PPI), resulting in

exhibit faster ionic diffusion inside the pores and access a larger a two-electron two-proton transfer reaction (see Fig. 5) [29]. Fol-

pore surface causing a greater capacitance [45]. Furthermore, the lowing the redox peak, the CVs of each redox-active slurry (across

conductivity of 1 M Na2SO4 electrolyte is observed to be about all concentrations of PPD) exhibits a relatively ﬂat region with a cur-

−1

60 mS cm whereas 2 M KOH electrolyte has a conductivity of rent density of about 0.175 A/g. The similar voltammetric current

K.B. Hatzell et al. / Electrochimica Acta 111 (2013) 888–897 893

Fig. 5. Standard two-proton/two-electron oxidation and reduction reaction of

p-phenylenediamine to p-phenylenediimine. (dark gray, dark blue and white corre-

spond to carbon, nitrogen and hydrogen atoms). (For interpretation of the references

to color in this ﬁgure legend, the reader is referred to the web version of the article.)

densities implies that for each concentration studied, the slurries

appear to experience similar double layer storage [48]. The CV

curves show that the capacitance increases as a function of con-

Fig. 6. Capacitance as a function of changing concentrations of p-phenylenediamine

centration of PPD. This behavior can be attributed to increases in −1 −1

and sweep rates ranging from 2 mV s to 100 mV s .

pseudocapacitive charge storage by the fully reversible PPD/PPI

redox couple, rather than changes in double layer charge storage.

This can be observed by the varying redox peak heights on the

voltammagram shown in Fig. 4a. The effect of PPD on the charge

achieve an even higher capacitance in the ﬂowable electrode using

storage mechanisms will be discussed in detail later.

−1 PPD as a redox-mediator.

As shown in Fig. 4b, between the sweep rates of 2–20 mV s ,

Utilizing a partition method, commonly applied in examining

capacitances remain relatively stable for the redox-mediated slur-

the electrochemically active sites at the outer and inner regions

ries. The PPD concentrated slurries (0.046–0.277 M) demonstrate a

of a metal oxides electrode [48–50], it is possible to distinguish

typical decreasing capacitance with increasing sweep rates. Above

−1 between pseudocapacitive and double layer charge storage mech-

20 mV s , all slurries are observed to demonstrate a more rapid

anisms for the various redox-mediated slurries (0.046–0.277 M).

drop in capacitance (Fig. 4b). This capacitance drop is more pro-

In metal oxides, the inner region is characterized by slower diffu-

nounced in the redox-mediated electrolyte with 0.277 M PPD

sion dynamics, due to extended diffusion lengths in comparison

concentration which exhibits a 69% capacitance decay between

−1 to the surface accessibility. Thus, in this analysis the slower diffu-

2 and 100 mV s . Table 1 shows a comparison of the observed

−1 sion dynamics (i.e. inner region) is analogous to pseudocapacitive

capacitance decay between 2 and 100 mV s for each redox-active

effects that occur at slower timescales than electric double layer

slurry studied. The slurry with 0.139 M PPD shows a 57% capac-

charge storage mechanisms. As demonstrated in Fig. 4b, the capac-

itance decay, and the slurry without PPD has a 65% capacitance

itance (charge storage) decreases as the sweep rate () increases.

decay, indicating that rate handling can be improved by the addi-

Thus, if we assume semi-inﬁnite linear diffusion, the charge storage

tion of PPD into the slurry. However, the deepest capacitance decay −1/2

mechanism is expected to be linearly related to [49].

was observed in the highest concentration PPD slurry which under-

The results from the partition method are shown in Fig. 7. The

scores the concentration limitations of PPD.

total voltammetric charge can be extrapolated from the plot of 1/q

From the CV experiments (Fig. 4), at low sweep rates below 1/2

→

−1 vs. as 0 (Fig. 7b), and the double-layer charge, qdl can be

5 mV s , the capacitance increases for all concentrations studied −1/2

estimated from the plot of q vs. as → ∞ (Fig. 7a) [48]. In

(0.046–0.277 M). Fig. 6 shows the effects of varying PPD concen-

this analysis, a linear ﬁt is used for the data at low sweep rates (2,

tration on the both rate handling and capacitance of the slurry. At −1

−1 5, 10, and 20 mV s ), which demonstrated the linear dependence

sweep rates greater than 5 mV s , the optimal concentration for −1/2 1/2

between q and and 1/q vs. . At higher rates polarization

rate handling and capacitance is 0.139 M PPD in 2 M KOH solutions.

can occur, and thus are not included in the in the linear ﬁt [51].

It appears that at high sweep rates, transport properties become

The ﬁtted regions are represented by the shaded regions on Fig. 7a

an issue for highly concentrated solutions. In slurries with high

and b. The total and double layer charge storage values (q) were

concentrations of the PPD, the PPD may obstruct ion mobility at

obtained from the cathodic portion of the CV curve:

the pore level, and thus might contribute to the deep capacitance

decay with sweep rate. Thus, in order to optimize for both rate per-

= Idt (5)

formance and capacitance, 0.139 M of PPD (2 M KOH) yields the best

−1

results for the rates that an EFC is expected to operate (<20 mV s ).

At a concentration of 0.139 M, for our two-electrode set-up, it is Intuitively, as the sweep rate goes to inﬁnity, the total charge

estimated that the increased capacitance only amounts to utilizing will be stored electrostatically in the double layer, because faradaic

∼3.26% of the available PPD molecules in the suspension. This num- reactions are too slow to occur. Furthermore, as the sweep rate

ber suggests that with further optimization of the electrode, we can approaches to zero, or a very slow rate, one can estimate the

total charge storage possible because both faradaic and double

layer charge storage mechanism can occur concurrently. Thus,

Table 1

Observed decreases in capacitances for different PPD concentrated redox-active these approximations are valid assumptions for deciphering the

−1 −1

slurries after increasing the sweep rate from 2 mV s to 100 mV s . different storage mechanisms. Finally, the difference between the

total charge and the double-layer charge is assumed to equal

Concentration of 0 0.046 0.092 0.139 0.184 0.277

−1

PPD (M L ) the pseudocapacitive charge storage contributions. In Fig. 7c, the

Capacity decay (%) 64 53 52 57 60 69 pseudocapacitive contributions increase with concentration, while

double layer storage appears to remain relatively stable across

all concentrations studied. The stable double layer charge storage

894 K.B. Hatzell et al. / Electrochimica Acta 111 (2013) 888–897

mechanism can also be observed in Fig. 4a as the current den-

sity after the redox peak for each concentration is approximately

equal. Furthermore, this is expected because for all slurries stud-

ied, the same active material with the similar surface areas were

used. Overall, double layer charge storage appears to contribute

a greater proportion of the charge storage over all concentrations

of PPD tested, except at 0.277 M of PPD, where double layer and

pseudocapacitive charge storage mechanisms are approximately

equal. This analysis demonstrates that the highest charge storage

was observed in the slurry that had 0.277 M of PPD. This observation

is qualitatively conﬁrmed, by the CV plot shown in Fig. 4a as 0.277 M

−1

demonstrates the greatest area at 2 mV s . However, at increased

rates it is expected that the charge storage at 0.277 M would be

sub-optimal to 0.139 M because it is performance degrades above

−1

2 mV s (Fig. 6).

Fig. 8 shows typical galvanostatic cycling for the redox medi-

−1

ated slurries at a charge rate of 200 mA g . Each curve shows a

similar charge and discharge time which indicates good coulombic

efﬁciency across all redox slurries examined. Typical supercapcac-

itors demonstrate triangular shape galvanostatic curves. However,

for these pseudocapacitive slurries, we see reduction and oxidation

bumps in the middle region, which indicates that redox reactions

occur. The fact that charge and discharge curves are symmetric

emphasizes the reversibility of the redox-couple in KOH elec-

trolytes. Furthermore, the slurry with 0.139 M of PPD charges in

∼

600 s (∼6 C), which demonstrates its ability charge and discharge

quickly, a characteristic that is desired in grid energy storage appli-

cations. The equivalent series resistance which reduces the power

output of supercapacitors is found to be low, around 1.5 cm

(Fig. 8a). Moreover, Fig. 8b shows that the capacitance appears to be

optimized at 0.139 M of PPD in 2 M KOH. In the CV experiments it

−1

was shown that for rates above 2 mV s , 0.139 M of PPD surpasses

0.277 M PPD in terms of capacitance. This trend is conﬁrmed with

the galvanostatic results shown in Fig. 8.

Fig. 9 shows the Nyquist plot for all redox-mediated slur-

ries. The Nyquist plots exhibit a small semi-circle in the high

frequency region, and a linear region at low frequency. The imag-

inary impedance is linearly related to the real impedance at low

frequency which indicates a diffusion controlled system. With

increasing concentrations of PPD, we see a decrease in the ohmic

resistance (R) from 0.8 to 0.2 cm . The maximum speciﬁc power

for a supercapacitor can be found from:

V 2 max Pmax = (6)

4R˝m

where V is the voltage window, and m is the mass of the active

material in both electrodes. When all the parameters are held con-

stant, except for concentration, the only parameter that changes

is the ohmic resistance. Thus, we see with increasing concentra-

tions of PPD, R decreases and thus the maximum speciﬁc power

increases.

A typical Randles circuit was employed to distinguish the effects

of PPD concentration on the faradaic impedance (Fig. 9b inset). A

Randles circuit is comprised of a resistor (which represents ohmic

or solution resistance) in series with a parallel RC circuit. The par-

allel RC circuit is characterized by a branch with a capacitor which

represents pure capacitance, and another branch that captures the

faradaic impedance. The faradaic impedance is comprised of charge

transfer resistance and a Warburg impedance which represents

mass transfer losses [52]. For the pseudocapacitive slurry, we are

interested in examining the changes in Warburg impedance with

concentration of PPD. The Warburg impedance, for the equivalent

Fig. 7. (a) Double layer charge storage contribution for each concentration of p-

circuit described above can be calculate from [39,52]:

phenylenediamine ( = sweep rate), (b) the total charge storage mechanism, derived

−1

as the sweep rate approaches very 0 mV s , (c) partitioned charge storage quantities −1/2 −1/2 −1/2

Zw = [ω − j(ω )] = ω [1 − j] (7)

(total, double layer, and pseudocapacitive contributions).

K.B. Hatzell et al. / Electrochimica Acta 111 (2013) 888–897 895

−1

Fig. 8. (a) Galvanostatic charging/discharging at a constant current density of 200 mA g for the various concentrations of PPD in 2 M KOH electrolyte. (b) Capacitance

dependence on concentration of PPD in 2 M KOH electrolyte.

where j is the imaginary number, ω is the angular frequency, and supercapacitor initially by both decreasing the ohmic resistance

−1/2

is Warburg coefﬁcient. Thus the slope of a plot of ω versus either and increasing the diffusion coefﬁcient of the ionic species.

the real or imaginary part of the impedance spectra provides the Fig. 10b shows the speciﬁc discharge capacitances for a slurry

Warburg coefﬁcient (Randles plot, Fig. 9b). The Warburg coefﬁcient containing 0.139 M PPD in 2 M KOH during successive cycling.

−1

is deﬁned as [52]: Over 1000 CV cycles (20 mV s ), an 11% decrease in capacitance

was observed. In typical pseudocapacitor systems cycle life is

1 often an issue because volume changes during the redox reaction = √ (8)

2 1/2 ∗

nF A 2 D C (metal oxides and conducting polymers) leads to losses of active

0 0

material [53]. This loss is typically alleviated by integrating pseu-

where R is the ideal gas constant, T is the temperature, n is the docapacitive material into a carbon support. In our carbon-based

charge transfer number, A is the area of the electrode surface, F is redox-mediated slurries the redox peaks appears to compress

faraday’s constant, D0 is the diffusion coefﬁcient, and C0 is the bulk toward the inside of the CV curve with cycling, which represents a

concentration. The effects on the diffusion coefﬁcient with concen- decrease in capacity (Fig. 10a). It can be assumed from this transi-

tration of PPD can be determined qualitatively by examining how tion that the system is losing faradaic charge storage over the cycle

the magnitude of the Warburg coefﬁcient changes with concentra- lifetime as a result of possible loss of active PPD. This is evident

tion of PPD because the diffusion coefﬁcient is inversely related to because the ﬂat region beyond the redox bump remains relatively

the Warburg coefﬁcient. stable, indicating that we are not losing double layer charge stor-

The slope of the Randles plot is , the Warburg coefﬁcient. From age with cycling. The loss of faradaic charge storage could be due to

the Randles plot (Fig. 9b) it can be seen that increasing the concen- degradation of the redox-active PPD during cycling. Furthermore,

tration of PPD decreases the ohmic resistance, and increases the deterioation might be related to pH-related changes of the elec-

diffusion coefficient (Fig. 9b). The diffusion coefficient reflects the trolyte which would stifle redox reactions. This test was done in a

mobility of the ionic species across the supercapacitor. Increasing static cell, which does not fully mimic the conditions that would

the diffusion coefficient is advanatageous for obtaining fast reac- be encountered during flow cell operations. In an actual flow sys-

tions. Thus, the addition of PPD increases the performance of the tem, a simple feedback control system could be implemented in

Fig. 9. (a) Nyquist plots for the various redox slurries in 2 M KOH electrolyte across the frequency range from 10 mHz to 200 kHz. (b) Randles plot of the redox slurries based

on KOH electrolyte, with an inset of a Randles circuit.

896 K.B. Hatzell et al. / Electrochimica Acta 111 (2013) 888–897

−1

Fig. 10. (a) Cyclic voltammogramof 0.139 M PPD in 2 M KOH electrolyte slurry at various cycles over a lifetime test. Static cell cycled at 20 mV s . (b) Capacitance as a function

of cycle number.

the storage tanks to ensure that pH remained constant, and the SEM was carried out using instruments in the Centralized Research

active material weight percentage remains constant, which may Facility (CRF) of the College of Engineering at Drexel University

contribute to better electrolyte stability.

4. Conclusions

References

The high power, low cost, and long lifetime characteristics of the

[1] B. Dunn, H. Kamath, J.-M. Tarascon, Electrical energy storage for the grid: a

capacitive ﬂowable carbon-based electrodes in the electrochemi- battery of choices, Science 334 (2011) 928–935.

[2] M. Armand, J.-M. Tarascon, Building better batteries, Nature 451 (2008)

cal ﬂow capacitor (EFC) offer the opportunity to address a diverse

652–657.

range of applications on the grid level. The limited energy density,

[3] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7

common in carbon-based electrodes that use physical adsorption of (2008) 845–854.

ions as their charge storage mechanism, can be enhanced through [4] J.-M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium

batteries, Nature 414 (2001) 359–367.

the addition of pseudocapacitance. This study introduced a redox-

[5] Z. Yang, J. Zhang, M.C. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Elec-

mediated slurry as a facile method for increasing the energy density

trochemical energy storage for green grid, Chem. Rev. 111 (2011) 3577.

of the ﬂowable electrodes for the EFC. A pseudocapacitive slurry [6] Department of Energy, Basic Research Needs for Electrical Energy Storage, 2007.

[7] M. Anderman, Status and prospects technology for hybrid electric vehicles,

utilizing a PPD/PPI redox-couple in 2 M KOH electrolyte demon-

including plug-in hybrid vehicles, Brieﬁng to the U.S. Senate Committee on

strated high capacitance for the EFC. The alkaline redox-mediated

Energy and Natural Resources by the President of Advanced Automotive Bat-

slurry shows increases in capacitance on the order of 86% when teries, 2007.

[8] C.J. Barnhart, S.M. Benson, On the importance of reducing the energetic and

compared alkaline solutions without redox mediators, and a 130%

material demands of electrical energy storage, Energy Environ. Sci. 6 (2013)

increase compared to previously reported neutral slurries [12]. Fur- 1083–1092.

thermore, the rate handling performance of the redox-mediated [9] J. Liu, Addressing the grand challenges in energy storage, Adv. Funct. Mater. 23

(2013) 924–928.

slurry increased from approximately 64% capacitance decay to

−1 [10] M. Skyllas-Kazacos, M. Rychcik, R.G. Robins, A. Fane, M. Green, New all-

about 57% between rates of 2–100 mV s . The redox-mediated

vanadium redox ﬂow cell, J. Electrochem. Soc. 133 (1986) 1057–1058.

slurry is found to decrease the ohmic resistance, with increasing [11] M. Duduta, B. Ho, V.C. Wood, P. Limthongkul, V.E. Brunini, W.C. Carter,

Y.M. Chiang, Semi-solid lithium rechargeable ﬂow battery, Adv. Energy Mater.

concentrations of PPD, which can be described by the increase in

1 (2011) 511–516.

the ionic diffusion coefﬁcient. Among tested slurries, a concentra-

[12] V. Presser, C.R. Dennison, J. Campos, K.W. Knehr, E.C. Kumbur, Y. Gogotsi, Elec-

tion of 0.139 M of PPD in 2 M KOH yielded the highest capacitances trochemical ﬂow cells: the electrochemical ﬂow capacitor: a new concept for

in both CV and GC experiments over a wide range of charging rates. rapid energy storage and recovery, Adv. Energy Mater. 2 (2012) 911.

[13] Y. Gogotsi, V. Presser, E. Kumbur, Electrochemical Flow Capacitor. WO Patent

The high performance is attributed to the addition of quick redox

Application 2,012,112,481, 2012.

reactions at the electrolyte/electrode interface as PPD undergoes a

[14] B. Kastening, T. Bointowitz, M. Heins, Design of a slurry electrode reactor sys-

two-proton/two-electron reduction and oxidation during cycling. tem, J. Appl. Electrochem. 27 (1997) 147–152.

[15] M. Skyllas-Kazacos, M. Chakrabarti, S. Hajimolana, F. Mjalli, M. Saleem, Progress

in ﬂow battery research and development, J. Electrochem. Soc. 158 (2011)

Acknowledgements R55–R79.

[16] K. Fic, G. Lota, M. Meller, E. Frackowiak, Novel insight into neutral medium

as electrolyte for high-voltage supercapacitors, Energy Environ. Sci. 5 (2012)

Y.G. and M.B. would like to acknowledge support from the Fluid 5842–5850.

Interface Reactions, Structures and Transport (FIRST) Center, an [17] L. Demarconnay, E. Raymundo-Pinero, F. Béguin, A symmetric carbon/carbon

supercapacitor operating at 1.6 V by using a neutral aqueous solution, Elec-

Energy Frontier Research Center funded by the U.S. Department

trochem. Commun. 12 (2010) 1275–1278.

of Energy, Ofﬁce of Science, Ofﬁce of Basic Energy. Partial support

[18] Q. Gao, L. Demarconnay, E. Raymundo-Pinero,˜ F. Béguin, Exploring the large

from the Ben Franklin Technology Partners of Southeastern Penn- voltage range of carbon/carbon supercapacitors in aqueous lithium sulfate

electrolyte, Energy Environ. Sci. 5 (2012) 9611–9617.

sylvania group (Grant# 001389-002) and the National Science

[19] J. Campos, M. Beidaghi, K.B. Hatzell, C. Dennison, V. Presser, E.C. Kumbur, Y.

Foundation (NSF) (Grant# 1242519) is also appreciated. K.B.H.

Gogotsi, Investigation of carbon materials for use as a ﬂowable electrode in

was supported by the NSF Graduate Research Fellowship (Grant# electrochemical ﬂow capacitors, Electrochim. Acta 98 (2013) 123–130.

[20] X. Lang, A. Hirata, T. Fujita, M. Chen, Nanoporous metal/oxide hybrid electrodes

1002809). J.C. was supported by the NSF Bridge to the Doctorate Fel-

for electrochemical supercapacitors, Nat. Nanotechnol. 6 (2011) 232–236.

lowship (Grant# 1026641). C.R.D. was supported by the NSF IGERT

[21] H. Yu, J. Wu, J. Lin, L. Fan, M. Huang, Y. Lin, Y. Li, F. Yu, Z. Qiu, A reversible

program (Grant# 0654313). The authors would also like B. Dyatkin redox strategy for SWCNT-based supercapacitors using a high-performance

electrolyte, ChemPhysChem 14 (2013) 394–399.

for assistance with gas adsorptions data and helpful conversations.

K.B. Hatzell et al. / Electrochimica Acta 111 (2013) 888–897 897

[22] B. Conway, V. Birss, J. Wojtowicz, The role and utilization of pseudocapac- [37] Z. Liu, H. Zhou, Z. Huang, W. Wang, F. Zeng, Y. Kuang, Graphene covalently

itance for energy storage by supercapacitors, J. Power Sources 66 (1997) functionalized with poly (p-phenylenediamine) as high performance electrode

1–14. material for supercapacitors, J. Mater. Chem. A 1 (10) (2013) 3454–3462.

[23] T. Brezesinski, J. Wang, S.H. Tolbert, B. Dunn, Ordered mesoporous [alpha]- [38] H. Yu, L. Fan, J. Wu, Y. Lin, J. Lin, M. Huang, Z. Lan, Redox-active alkaline elec-

MoO3 with iso-oriented nanocrystalline walls for thin-ﬁlm pseudocapacitors, trolyte for carbon-based supercapacitor with pseudocapacitive performance

Nat. Mater. 9 (2010) 146–151. and excellent cyclability, RSC Adv. 2 (17) (2012) 6736–6740.

[24] V. Khomenko, E. Frackowiak, F. Beguin, Determination of the speciﬁc capaci- [39] B.G. Choi, M.H. Yang, S.C. Jung, K.G. Lee, J.-G. Kim, H. Park, T.J. Park, S.B. Lee,

tance of conducting polymer/nanotubes composite electrodes using different Y.-K. Han, Y.S. Huh, Enhanced pseudocapacitance of ionic liquid/cobalt hydrox-

cell conﬁgurations, Electrochim. Acta 50 (2005) 2499–2506. ide nanohybrids, ACS Nano 7 (2013) 2453–2460.

[25] T. Brousse, P.-L. Taberna, O. Crosnier, R. Dugas, P. Guillemet, Y. Scudeller, Y. [40] M.D. Stoller, R.S. Ruoff, Best practice methods for determining an electrode

Zhou, F. Favier, D. Bélanger, P. Simon, Long-term cycling behavior of asymmet- material’s performance for ultracapacitors, Energy Environ. Sci. 3 (2010)

ric activated carbon/MnO2 aqueous electrochemical supercapacitor, J. Power 1294–1301.

Sources 173 (2007) 633–641. [41] D. Andre, M. Meiler, K. Steiner, C. Wimmer, T. Soczka-Guth, D.U. Sauer, Charac-

[26] S. Roldán, C. Blanco, M. Granda, R. Menéndez, R. Santamaría, Towards a fur- terization of high-power lithium-ion batteries by electrochemical impedance

ther generation of high energy carbon based capacitors by using redox-active spectroscopy. I. Experimental investigation, J. Power Sources 12 (2011) 8.

electrolytes, Angew. Chem. Int. Ed. 50 (2011) 1699–1701. [42] X.-G. Li, M.-R. Huang, W. Duan, Y.-L. Yang, Novel multifunctional polymers

[27] H. Yu, J. Wu, L. Fan, Y. Lin, K. Xu, Z. Tang, C. Cheng, S. Tang, J. Lin, M. Huang, A novel from aromatic diamines by oxidative polymerizations, Chem. Rev. 102 (2002)

redox-mediated gel polymer electrolyte for high-performance supercapacitor, 2925–3030.

J. Power Sources 15 (2011) 402–407. [43] W.R. Heineman, H.J. Wieck, A.M. Yacynych, Polymer ﬁlm chemically modiﬁed

[28] S. Horvath, L.E. Fernandez, A.M. Appel, S. Hammes-Schiffer, pH-Dependent electrode as a potentiometric sensor, Anal. Chem. 52 (1980) 345–346.

reduction potentials and proton-coupled electron transfer mechanisms in [44] B. Conway, Electrochemical Supercapacitors: Scientiﬁc Fundamentals and

hydrogen-producing nickel molecular electrocatalysts, Inorg. Chem. 52 (2013) Technological Applications, Kluwer Academic/Plenum, New York, 1999.

3643–3652. [45] M. Bichat, E. Raymundo-Pinero,˜ F. Béguin, High voltage supercapacitor built

[29] N.J. Ke, S.-S. Lu, S.-H. Cheng, A strategy for the determination of dopamine at with seaweed carbons in neutral aqueous electrolyte, Carbon 48 (2010)

a bare glassy carbon electrode: p-phenylenediamine as a nucleophile, Elec- 4351–4361.

trochem. Commun. 8 (2006) 1514–1520. [46] K. Fic, G. Lota, E. Frackowiak, Effect of surfactants on capacitance properties of

[30] L.A. Clare, L.E. Rojas-Sligh, S.M. Maciejewski, K. Kangas, J.E. Woods, L.J. Deiner, carbon electrodes, Electrochim. Acta 60 (2012) 206–212.

A. Cooksy, D.K. Smith, The effect of H-bonding and proton transfer on the [47] D.N. Futaba, K. Hata, T. Yamada, T. Hiraoka, Y. Hayamizu, Y. Kakudate, O.

voltammetry of 2,3,5,6-tetramethyl-p-phenylenediamine in acetonitrile. An Tanaike, H. Hatori, M. Yumura, S. Iijima, Shape-engineerable and highly

unexpectedly complex mechanism for a simple redox couple, J. Phys. Chem. densely packed single-walled carbon nanotubes and their application as super-

C 114 (2010) 8938–8949. capacitor electrodes, Nat. Mater. 5 (2006) 987–994.

[31] S. Roldán, Z. González, C. Blanco, M. Granda, R. Menéndez, R. Santamaría, Redox- [48] Y.-H. Lee, K.-H. Chang, C.-C. Hu, Differentiate the pseudocapacitance and

active electrolyte for carbon nanotube-based electric double layer capacitors, double-layer capacitance contributions for nitrogen-doped reduced graphene

Electrochim. Acta 56 (2011) 3401–3405. oxide in acidic and alkaline electrolytes, J. Power Sources 227 (2012) 300–308.

[33] S. Roldan, M. Granda, R. Menendez, R. Santamaria, C. Blanco, Mechanisms of [49] S. Ardizzone, G. Fregonara, S. Trasatti, Inner and outer active surface of RuO2

energy storage in carbon-based supercapacitors modiﬁed with a quinoid redox- electrodes, Electrochim. Acta 35 (1990) 263–267.

active electrolyte, J. Phys. Chem. C 115 (2011) 17606–17611. [50] K.-H. Chang, C.-C. Hu, C.-Y. Chou, Textural, Capacitive characteristics of

[34] M. Hébert, D. Rochefort, Electrode passivation by reaction products of the hydrothermally derived RuO2·H2O nanocrystallites: independent control of

electrochemical and enzymatic oxidation of p-phenylenediamine, Electrochim. crystal size and water content, Chem. Mater. 19 (2007) 2112–2119.

Acta 53 (2008) 5272–5279. [51] J.W. Kim, V. Augustyn, B. Dunn, The effect of crystallinity on the rapid pseudo-

[35] H. Yu, J. Wu, L. Fan, G.G. Luo, J. Lin, M. Huang, A simple and high-effective capacitive response of Nb2O5, Adv. Energy Mater. 2 (2012) 141–148.

electrolyte mediated with p-phenylenediamine for supercapacitor, J. Mater. [52] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applica-

Chem. 22 (2012) 19025–19030. tions, Wiley, New York, 1980.

[36] H. Yu, J. Wu, L. Fan, Y. Lin, S. Chen, Y. Chen, J. Wang, M. Huang, J. Lin, Z. Lan, [53] J. Yan, Z. Fan, T. Wei, W. Qian, M. Zhang, F. Wei, Fast and reversible surface redox

Application of a novel redox-active electrolyte in MnO2-based supercapacitors, reaction of graphene–MnO2 composites as supercapacitor electrodes, Carbon

Sci. China Chem. 55 (2012) 1319–1324. 48 (2010) 3825–3833.