a high performance pseudocapacitive suspension electrode for the electrochemical flow capacitor

Accepted Manuscript

Title: A High Performance Pseudocapacitive SuspensionElectrode for the Electrochemical Flow Capacitor

Author: Kelsey B. Hatzell Majid Beidaghi Jonathan W.Campos Christopher R. Dennison Emin C. Kumbur YuryGogotsi<ce:footnote id="fn1"><ce:note-paraid="npar0005">Tel.: +1 215 895 6446; fax: +1 215 8951934</ce:note-para></ce:footnote>

PII: S0013-4686(13)01625-3DOI: http://dx.doi.org/doi:10.1016/j.electacta.2013.08.095Reference: EA 21120

To appear in: Electrochimica Acta

Received date: 1-5-2013Revised date: 17-7-2013Accepted date: 17-8-2013

Please cite this article as: K.B. Hatzell, M. Beidaghi, J.W. Campos, C.R. Dennison,E.C. Kumbur, Y. Gogotsi, A High Performance Pseudocapacitive SuspensionElectrode for the Electrochemical Flow Capacitor, Electrochimica Acta (2013),http://dx.doi.org/10.1016/j.electacta.2013.08.095

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

http://dx.doi.org/doi:10.1016/j.electacta.2013.08.095

http://dx.doi.org/10.1016/j.electacta.2013.08.095

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A High Performance Pseudocapacitive Suspension Electrode

for the Electrochemical Flow Capacitor

Kelsey B. Hatzella, Majid Beidaghia, Jonathan W. Camposa, Christopher R. Dennisona,b,Emin C. Kumburb,†, and Yury Gogotsia,*

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

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

Corresponding authors:* E-mail: [email protected] Tel.: +1 215 895 6446 Fax: +1 215 895 1934† E-mail: [email protected] Tel.: +1 215 895 5871 Fax: +1 215 895 1478

Abstract

The electrochemical flow capacitor (EFC) is a new technology for grid energy storage that is

based on the fundamental principles of supercapacitors. The EFC benefits from the advantages of

both supercapacitors and flow batteries in that it is capable of rapid charging/discharging, has a

long cycle lifetime, and enables energy storage and power to be decoupled and optimized for the

desired application. The unique aspect of the EFC is that it utilizes a flowable carbon-electrolyte

suspension (slurry) for capacitive energy storage. Similar to traditional supercapacitor electrodes,

this aqueous slurry is limited in terms of energy density, when compared to batteries. To address

this limitation, in this study a pseudocapacitive additive has been explored to increase

capacitance. A carbon-electrolyte slurry prepared with p-phenylenediamine (PPD), a redox

mediator, shows an increased capacitance on the order of 86% when compared with KOH

electrolytes, and a 130% increase when compared to previously reported neutral electrolyte

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

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.

Keywords: Electrochemical Flow Capacitor; Energy Storage; P-phenylenediamine; Pseudocapacitor; Supercapacitor; Redox Mediator.

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1. Introduction

Currently, the primary technologies employed for grid-level energy storage are pumped

hydroelectric and compressed air energy storage [1]. These technologies are geographically

constrained and require large capital investments when compared with electrochemical energy

systems (EES), such as batteries and supercapacitors [2-4]. Thus, EES systems have emerged as

the most opportunistic type of technology for grid-scale applications [1, 5]. In transportation or

portable electronics applications, where energy density is the critical factor, batteries are favored

[2]. However, in large-scale stationary storage applications, the scalability, low power density,

and cost hinder the widespread implementation of battery systems [6, 7]. The critical

performance requirements for grid-level energy storage are cost, scalability, and lifetime [8].

Furthermore, the market for stationary energy storage is vast, requiring capacities that range

from kW to GW systems which most likely cannot be achieved with a single technology [1, 9].

Thus, similar to energy generation systems, there is a need for a portfolio approach of tailored

energy storage systems designed for specific applications such as voltage and frequency

regulation, load leveling, and bulk power management [1, 9]. In order to address the scalability

requirements and broader power/energy range, various flow-assisted electrochemical systems

have been recently proposed. The architecture seen in the redox flow battery [10], semi-solid

lithium battery [11], and electrochemical flow capacitor (EFC) [12, 13] offers tremendous design

flexibility for use in grid-level energy storage.

Recently, a new energy storage concept called the electrochemical flow capacitor (EFC)

has been proposed by our group at Drexel University. The EFC is a rechargeable EES that

utilizes a flow battery architecture and is based on the fundamental working principles of

supercapacitors [3, 12]. The primary difference between traditional flow cells and the EFC is that

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the EFC utilizes a flowable carbon-electrolyte electrode, similar to the ‘slurry electrode’

introduced by Kastening et al. [14], for capacitive energy storage (Fig. 1b). During operation the

slurry is pumped from a storage reservoir through two polarized plates (charging process). Once

fully charged the slurry is pumped out of the cell and stored in external reservoirs until the

process is reversed and the slurry is discharged (Fig. 1a). The charged slurry stores charge

electrostatically at the carbon/electrolyte interface which allows for rapid charging and

discharging (Fig. 1c). Traditional flow batteries suffer from slow response rates due to faradaic

reactions at the electrode-electrolyte surface and have limited cycle lifetimes (<12,000 cycles)

which is a critical factor for grid scale energy storage[8, 15]. The EFC in comparison is capable

of rapid charging/discharging, is based on cheap and abundant materials (e.g., carbon), and

enables energy storage and power to be decoupled and optimized. Furthermore, since the charge

storage mechanism is physical, the EFC has the potential to exhibit long cycle life, a

characteristic commonly seen in supercapacitors. Power is directly related to stack size, and

energy capacity can be scaled according to reservoir volume.

(Insert Figure 1)

Although the EFC has high power density, its energy density needs to be improved in

order to compete with alternative systems for grid-level energy storage applications. The energy

released by a supercapacitor, E, is:

25. CVE (Eq. 1)

where C is the capacitance (in F), and V is the operational voltage window (in V). Therefore, in

order to increase the energy density, one can either increase the voltage window or the active

material’s capacitance. For aqueous electrolytes, the voltage window is thermodynamically

limited to approximately 1.0V because water decomposition occurs above 1.23 V [16].

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Nevertheless, it has been shown that the voltage window of supercapacitors based on neutral

electrolytes may extend beyond 1.23 V because the di-hydrogen overpotential is higher for

neutral electrolytes[16-18]. In a recent study on the EFC, flowable electrodes based on sodium

sulfate demonstrated voltage stability up to ~1.5V[19]. While an increased voltage window will

yield increased energy density the current study seeks to examine methods to enhance the

capacitance of the flowable electrode. Thus, this study restricts its focus to available methods to

enhance the capacitance of the flowable electrode.”

One method for increasing capacitance is through the addition of quick redox reactions at

the electrode-electrolyte interface (pseudocapacitive effects). The primary means for achieving

pseudocapacitance is through the use of transition metal oxides [21], conducting polymers [21,

24] or the addition of surface functionalities such as nitrogen or oxygen to the surface of carbon

materials [25]. Recently a fourth strategy, the use of redox-mediated electrolytes, has emerged

as a new method of achieving pseudocapacitive behavior [26]. The addition of redox-mediators

into electrolytes has been shown to increase ionic conductivity and provide an additional charge

storage mechanism, and thus has been explored in supercapacitor applications as a way to

enhance specific capacitance [27]. Furthermore, there is no need for surface modification

because the redox species react at the carbon|electrolyte interface. Most redox-mediated

electrolytes are based on proton-coupled electron transfer (PCET) mechanisms that are pH-

dependent and prefer basic or acidic environments in order to facilitate proton transfer at the

active material surface [28-30]. In supercapacitors, electrochemically active compounds such as

indigo carmine [31] and hydroquinone [33], have shown significant capacitance increases in

acidic mediums. In addition, it has been recently shown aromatic diamines such as p-

phenylenediamine (PPD) [21, 35-37], and m-phenylenediamine [38] (MPD) are possible redox

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mediators in basic electrolyte systems. A MnO2-based supercapacitor[36] demonstrated a 6-fold

increase in capacitance and a activated carbon supercapacitor[35] showed a 4-fold increase in

capacitance after the addition of PPD into KOH electrolytes[36]. Thus, PPD as a redox mediator

has been shown to dramatically enhance capacitance in supercapacitors system.

Pseudocapacitance, where the source of the redox reaction is in the electrolyte, is favorable for

systems such as the EFC because the major component in the slurry is the electrolyte. Thus,

redox-mediators integrated into a capacitive slurry offer opportunities for increased capacitance

and enhanced electrochemical performance. However, in many cases pseudocapacitors suffer

from poor rate capacities due to irreversible redox reactions and physicochemical changes that

take place during charging and discharging [39].

The objective of this study is to develop a carbon-based pseudocapacitive flowable

electrode for the EFC that utilizes the phenylenediamine/p-phenylenediimine (PPD/PPI) redox

couple for additional storage of energy. Herein, a redox-mediated slurry is prepared by the

addition of PPD to 2 M KOH electrolyte and the performance of the slurry is characterized

electrochemically. This study also examines the effectiveness of PPD in a suspension

environment (flowable electrode) and assesses its performance (rate handling and capacitance)

across a wide range of rates and concentrations.

2. Materials and Methods

2.1 Chemicals. All the reagents used in this study were of analytical grade. PPD anti-fade

reagent supplied by Sigma-Aldrich was employed as a redox mediator, and 2 M potassium

hydroxide (KOH) supplied from Alfa Aesar, was used as the supporting electrolyte. A baseline

experiment was performed in 1 M sodium sulfate supplied by Sigma-Aldrich for comparison

with KOH electrolyte experiments. Six different concentrations of the redox mediators were

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examined: 0.046 M, 0.092 M, 0.139 M, 0.185 M, and 0.277 M PPD in the supporting electrolyte

solution (2 M KOH). Different concentration solutions were explored in order to identify the

optimal conditions for improved system performance (capacitance and rate handling).

Previously, it has been shown high concentrations of redox mediators can lead to aggregation of

free ions and thus diminish the performance [38]. Thus, this study intends to demonstrate the

optimal concentration for a flowable electrode system. . The molar concentrations correspond to

increments of 5 mg /ml solutions. The PPD was sonicated in 2 M KOH electrolyte until

completely dissolved. All solutions were prepared immediately prior to the testing.

2.2 Electrode Active Materials. Carbon beads derived from phenolic resins were obtained from

MAST Carbon (United Kingdom) and used as the active capacitive material in the slurry

electrodes (Fig. 1d). In our previous work we characterized the active material, MAST carbon

beads, extensively in terms of the physical attributes and electrochemical performance [12, 19].

Three different sizes (diameter) of MAST beads were examined ranging from 9 µm to 500 µm,

and it was determined that the mid-size beads (particle size: 125-250 µm) produced the best

electrochemical and rheological performance [19], and thus was selected as the active material in

this study.

N2 gas sorption was carried out in a Quadrasorb gas sorption instrument (Quantachrome,

USA) and the average, volume-weighted pore size was derived from the cumulative pore volume

assuming slit-shaped pores and using the quenched-solid density function theory (QSDFT)

algorithm. The pore size distributions of the carbon beads (average particle size: 161±35µm) had

an average volume-weighted pore size of 4.7 nm. The Brunauer-Emmett-Teller (BET) method

was used to calculate a specific surface area (SSA) of 1576 m2·g-1. This value is in agreement

with the SSA value calculated from QSDFT with a maximum of 10% deviation. Carbon black

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(100% compressed; Alfa Aesar, USA) was employed as the conductive additive between carbon

beads. For calculation purposes, the active mass was considered to be the total mass of the

carbon black and activated carbon in the slurry. A Zeiss Supra 50VP scanning electron

microscope (Carl Zeiss AG, Germany) operating at 3 kV was used for observing any surface

changes to the carbon beads as a result of the redox-mediator.

2.3 Preparation of Slurry Electrodes. The slurry electrodes were prepared by mixing carbon

beads with carbon black (100% compressed; Alfa Aesar, USA) to achieve a 9:1 weight ratio, as

this ratio is defined as the baseline condition in our previous study. The carbon black served as a

conductive additive. The aqueous electrolyte (2 M KOH) was then added to achieve 23 wt%

(solid-to-liquid ratio), as it was shown previously to exhibit favorable capacitive and rheological

properties[12, 19]. In order to achieve a wetted carbon surface to adsorb the carbon black, an

incremental amount of deionized water (<5wt% of slurry) was weighed and added to the slurry

mixture. The mixture (with the excess deionized water) was mildly heated and stirred until a

homogenous slurry was obtained, and the excess deionized water evaporated.

2.4 Electrochemical Performance. The slurry performance was studied in a symmetric static

cell configuration, with equal masses in both electrode supensions. The static cell is comprised of

two stainless steel current collectors, onto which the slurry was uniformly loaded into a channel

region defined by latex gaskets. The channel region (active area) was 1 cm2. The latex gaskets

were 610 µm thick when completely compressed prior to testing. A polyvinylidene fluoride

(PVDF) membrane separator with a mesh width of 100 nm (Durapore®; Merck Millipore,

Germany) was used as the separator between the two slurry electrodes. All electrodes tested had

a carbon content of 32 mg per electrode and a solid fraction of 23 wt%.

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All electrochemical measurements were performed at room temperature with a VMP3 or

VSP potentiostat/galvanostat (BioLogic, France). Cyclic voltammetry (CV) was carried out at 2,

5, 10, 20, 50, and 100 mV·s-1 sweep rates. From CV, the specific gravimetric capacitance (Csp)

was derived using the following equation.

m

idV

ECsp

2 (Eq. 2)

where ΔE is the width of the voltage window, i is the discharge current, V is the voltage, is the

sweep rate, and m is the mass of carbon in one electrode. The factor of two accounts for the two

electrode setup, assuming that the charge is evenly distributed between two capacitors in

series[40]. Three flowable electrodes were tested for each case (concentration), and an average

value is reported in the figures. Error bars are plotted for each average values, and represent the

standard deviation from the average capacitance.

All values for the capacitance were normalized by the weight of the carbon material in

one electrode, not the total slurry mass, to enable a direct comparison with conventional

supercapacitor electrodes (which are also normalized to the content of active material in a single

electrode). Csp was also calculated from galvanostatic cycling (GC) using equation (3):

dt

dVm

iCsp

2 (Eq. 3)

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 voltage is equal to zero. The whole slope was taken to best account

for the redox behavior exhibited in the galvanostatic charge/discharge results. The equivalent

series resistance (ESR) of the cell was determined by dividing the total change in voltage of the

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IR drop in a galvanostatic cycle with the total change in current (between charge and discharge)

using equation (4).

I

V

II

V

I

VESR

eDischeCh 2argarg

(Eq. 4)

Potentiodynamic electrochemical impedance spectroscopy (PEIS) was used to assess the

change in ohmic resistance due to changes in concentrations of PPD in the slurries. The ohmic

resistance of the static cell, RΩ, was determined from the impedance spectrum intersection with

the real axis. This value is representative of the total resistances associated with the current

collectors, active material, electrolyte, and separator [41]. With all characteristics held constant

(except PPD concentration), the ohmic resistance highlights the kinetic changes in the slurry due

to the addition of PPD. Furthermore, PEIS was used to evaluate the changes in ion diffusion at

the surface of the electrode. In order to characterize changes in the diffusion coefficient, a

Randles circuit was employed to analyze the impedance responses. All spectra were run in the

frequency range of 200 kHz to 10 mHz at a sine wave signal amplitude of 10 mV.

3. Results and Discussion

The morphology of the carbon beads with and without PPD was analyzed by a

scanning electron microscope (SEM) as seen in Fig. 2. PPD, as a monomer, has been reported to

polymerize under electrochemical oxidative conditions [42]. Polymerization can result in

electrode “poisoning”, or the coating of a polymeric film on the carbon surface [43]. This can

cause a blocking effect on the carbon surface that can result in limited ion mobility and poor

electronic conductivity. These limitations can be observed electrochemically by a rapid decrease

in the peak current of a CV plot during successive cycles. In order to examine whether

polymerization occurred during cycling, a SEM micrograph of pristine carbon beads, Fig. 2a,

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was compared with a SEM micrograph of cycled slurry (0.139 M PPD in 2 M KOH electrolyte)

after undergoing 1000 cycles at 20 mV s-1 between 0V and 1V. From the SEM micrographs in

Fig. 2b, it is observed that the cycled slurry shows little evidence of polymer films on the carbon

surface. The noticeable particles around the carbon beads in Fig. 2b are aggregates of carbon

black, which is added to the slurry as a conductive additive. Furthermore, it should be noted that

no drastic declines in the peak current were observed during the first 20 CV cycles typically

when polymerization is expected to occur. A gas sorption analysis was taken on a sample of the

cycled slurry with a concentration of 0.139 M PPD. The surface area of the cycled slurry (carbon

beads, carbon black, PPD, and 2 M KOH) exhibited a BET surface area of ~1210 m2/g. Carbon

black is approximately 10 wt% of the slurry, and explains this decrease in surface area when

compared to dry carbon beads (~1576 m2/g). Thus, the fact that a large decrease in surface area

was not observed supports the notion that the PPD is not polymerizing. This is advantageous for

our system because polymerization during oxidation and reduction cycles form polymer chains

which could contribute to irreversible reactions and deterioration of PPD over the cycle life. We

assessed only the flowable electrode concentration with the greatest overall performance in order

to ascertain whether polymerization occurred. It is expected that below 0.139 M, there is no

polymerization. In later sections we assess the effect of the concentration of PPD on the

electrochemical performance of the flowable electrode.”

(Insert Figure 2)

Cyclic voltammetry (CV) was utilized for characterizing the capacitance of the slurries in

a static cell configuration. Fig. 3a shows that for the cases without the redox mediator (i.e., 2 M

KOH and 1 M Na2SO4 electrolyte), the slurries demonstrate a typical quasi-rectangular shape

without redox peaks, implying an ideal electric double layer capacitor [44]. KOH electrolyte

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based slurries exhibit a higher capacitance (118 F g-1 at 2 mV s-1) when compared to sodium

sulfate solution (94 F g-1 at 2 mV s-1). This may be attributed to the fact that protons as mobile

species may exhibit faster ionic diffusion inside the pores and access a larger pore surface

causing a greater capacitance[45]. Furthermore, the conductivity of 1 M Na2SO4 electrolyte is

observed to be about 60 mS cm-1 whereas 2 M KOH electrolyte has a conductivity of

approximately 170 mS cm-1, facilitating transport throughout the slurry medium. The KOH

based flowable electrode exhibits higher capacitance, but also exhibits signs of faradaic

reactions as the voltage approaches 1V in Fig. 3a. The sudden increase in current at 1V is a sign

that electrolysis of water occurs which evolves hydrogen and oxygen as byproducts[46]. In

constrast, neutral electrolytes have a stability window that reaches around 1.6 V because of a

high over-potential for di-hydrogen evolution, whereas KOH electrolytes are limited to about

1V [17]. Fig. 3b shows the rate performance for the slurries without a redox-mediated slurry.

Across all rates tested, the 2 M KOH slurry outperformed the sodium sulfate based slurries as a

working electrolyte for the flowable electrode (Fig. 3b). Above 20 mV s-1 both slurries exhibit a

decline in rate performance (capacitance decay). At 100 mV s-1 the Na2SO4 based slurry saw a

66% decrease from its initial capacitance taken at 2 mV s-1, which is comparable to the 2 M

KOH electrolyte which exhibited 65% capacitance decay. The observed capacitance decay at

high sweep rates for both slurries is a result of ion transport limitations, which is typically seen in

porous activated carbon [47]. Diffusion limitations in slurry arise as a result of the slurry’s

thickness and as a result of the network of particle-particle contacts. In traditional

supercapacitors with film electrodes, it has been shown that increased thickness yields increased

barriers to ion diffusion within the inner region of the electrode [47]. This results in longer

diffusion paths, higher electrode resistivity, and poor power performance at high rates. This

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explanation can be transferred to slurry electrodes which are on average three to five times

thicker than film electrodes, and can explain the decreased power performance at high rates. In

addition to diffusion length challenges, while loading the cell during charging, some losses can

arise when beads are not in direct contact, and transport paths are broken. Nevertheless, the

desired rate handling of the slurry for a grid scale applications will most likely lie below 20 mV

s-1. which is very well in the range of acceptable performance for the EFC.

(Insert Figure 3)

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

of the redox mediator (PPD) in the slurry. In contrast to the slurries without redox mediators, the

redox-active slurries demonstrate an increased capacitance. The area under CV curve is

proportional to the capacitance, thus in Fig. 4a the increase in capacitance can be observed

qualitatively by the expansion of the CV curve with changing concentration of PPD. The anodic

and cathodic peaks centered around 0.4 V can be attributed to a pseudocapacitive behavior

produced by the redox couple p-phenylenediamine and p-phenylenediimine (PPD/PPI), resulting

in a two-electron two-proton transfer reaction (see Fig. 5) [29]. Following the redox peak, the

CVs of each redox-active slurry (across all concentrations of PPD) exhibits a relatively flat

region with a current density of about 0.175 A/g. The similar voltammetric current 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 concentration of

PPD. This behavior can be attributed to increases in 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 storage mechanisms will be discussed in detail later.

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(Insert Figure 4)

(Insert Figure 5)

As shown in Fig. 4b, between the sweep rates of 2-20 mV s-1, capacitances remain

relatively stable for the redox-mediated slurries. The PPD concentrated slurries (0.046-0.277M)

demonstrate a typical decreasing capacitance with increasing sweep rates. Above 20 mV s-1, all

slurries are observed to demonstrate a more rapid drop in capacitance (Fig. 4b). This capacitance

drop is more pronounced in the redox-mediated electrolyte with 0.277 M PPD concentration

which exhibits a 69% capacitance decay between 2 and 100 mV s-1. Table 1 shows a comparison

of the observed capacitance decay between 2 and 100 mV s-1 for each redox-active slurry

studied. The slurry with 0.139M PPD shows a 57% capacitance decay, and the slurry without

PPD has a 65% capacitance decay, indicating that rate handling can be improved by the addition

of PPD into the slurry. However, the deepest capacitance decay was observed in the highest

concentration PPD slurry which underscores the concentration limitations of PPD.

From the CV experiments (Fig. 4), at low sweep rates below 5 mV s-1, the capacitance

increases for all concentrations studied (0.046-0.277M). Fig. 6 shows the effects of varying PPD

concentration on the both rate handling and capacitance of the slurry. At sweep rates greater than

5 mV s-1, the optimal concentration for rate handling and capacitance is 0.139 M PPD in 2 M

KOH solutions. It appears that at high sweep rates, transport properties become an issue for

highly concentrated solutions. In slurries with high concentrations of the PPD, the PPD may

obstruct ion mobility at the pore level, and thus might contribute to the deep capacitance decay

with sweep rate. Thus, in order to optimize for both rate performance and capacitance, 0.139 M

of PPD (2 M KOH ) yields the best results for the rates that an EFC is expected to operate (< 20

mV s-1). At a concentration of 0.139 M, for our two electrode set-up, it is estimated that the

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increased capacitance only amounts to utilizing ~3.26% of the available PPD molecules in the

suspension. This number suggests that with further optimization of the electrode, we can achieve

an even higher capacitance in the flowable electrode using PPD as a redox-mediator.

(Insert Table 1)

(Insert Figure 6)

Utilizing a partition method, commonly applied in examining the electrochemically

active sites at the outer and inner regions of a metal oxides electrode [48-50], it is possible to

distinguish between pseudocapacitive and double layer charge storage mechanisms for the

various redox-mediated slurries (0.046-0.277M). In metal oxides, the inner region is

characterized by slower diffusion dynamics, due to extended diffusion lengths in comparison to

the surface accessibility. Thus, in this analysis the slower diffusion dynamics (i.e. inner region) is

analogous to pseudocapacitive effects that occur at slower timescales than electric double layer

charge storage mechanisms. As demonstrated in Fig. 4b, the capacitance (charge storage)

decreases as the sweep rate (υ) increases. Thus, if we assume semi-infinite linear diffusion, the

charge storage mechanism is expected to be linearly related to υ-1/2 [49].

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

can be extrapolated from the plot of 1/q vs. υ1/2 as υ→0 (Fig. 7b), and the double-layer charge,

qdl can be estimated from the plot of q vs. υ-1/2 as υ→ ∞ (Fig. 7a) [48]. In this analysis, a linear

fit is used for the data at low sweep rates (2, 5, 10, and 20 mV s-1), which demonstrated the linear

dependence between q and υ-1/2 and 1/q vs. υ1/2. At higher rates polarization can occur, and thus

are not included in the in the linear fit [51]. The fitted regions are represented by the shaded

regions on Fig. 7a and Fig. 7b. The total and double layer charge storage values (q) were

obtained from the cathodic portion of the CV curve:

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Idtq (Eq. 5)

Intuitively, as the sweep rate goes to infinity, the total charge will be stored electrostatically in

the double layer, because faradaic reactions are too slow to occur. Furthermore, as the sweep rate

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,

these approximations are valid assumptions for deciphering the different storage mechanisms.

Finally, the difference between the total charge and the double-layer charge is assumed to equal

the pseudocapacitive charge storage contributions. In Fig. 7c, the pseudocapacitive contributions

increase with concentration, while double layer storage appears to remain relatively stable across

all concentrations studied. The stable double layer charge storage mechanism can also be

observed in Fig. 4a as the current density after the redox peak for each concentration is

approximately equal. Furthermore, this is expected because for all slurries studied, 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 demonstrates the greatest area at 2 mV s-1.

However, at increased rates it is expected that the charge storage at 0.277 M would be sub-

optimal to 0.139M because it’s performance degrades above 2 mV s-1 (Fig. 6).

(Insert Figure 7)

Fig. 8 shows typical galvanostatic cycling for the redox mediated slurries at a charge rate

of 200 mA g-1. Each curve shows a similar charge and discharge time which indicates good

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coulombic efficiency across all redox slurries examined. Typical supercapcacitors 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 electrolytes. Furthermore, the slurry with 0.139 M of PPD charges in ~600

sec (~6C), which demonstrates its ability charge and discharge quickly, a characteristic that is

desired in grid energy storage applications. The equivalent series resistance which reduces the

power output of supercapacitors is found to be low, around 1.5 Ω·cm2 (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 was shown that for rates above 2 mV s-1, 0.139 M of PPD surpasses 0.277M

PPD in terms of capacitance. This trend is confirmed with the galvanostatic results shown in Fig.

8.

(Insert Figure 8)

Fig. 9 shows the Nyquist plot for all redox-mediated slurries. The Nyquist plots all exhibit a

small semi-circle in the high frequency region, and a linear region at low frequency. The

imaginary 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 (RΩ) from 0.8 to 0.2 Ω·cm-2. The maximum specific power for a supercapacitor

can be found from:

mR

VP

*4

2max

max

(Eq. 6)

where V is the voltage window, and m is the mass of the active material in both electrodes. When

all the parameters are held constant, except for concentration, the only parameter that changes is

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the ohmic resistance. Thus, we see with increasing concentrations of PPD, RΩ decreases and thus

the maximum specific power increases.

(Insert Figure 9)

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 parallel 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 circuit

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

jj 1)(= Z 2/12/12/1w (Eq. 7)

where j is the imaginary number, ω is the angular frequency, and σ is Warburg coefficient. Thus

the slope of a plot of ω-1/2 versus either the real or imaginary part of the impedance spectra

provides the Warburg coefficient (Randles Plot, Fig. 9b). The Warburg coefficient is defined

as[52]:

*0

2/10

2

1

2 CDAnF

RT (Eq. 8)

where R is the ideal gas constant, T is the temperature, n is the charge transfer number, A is the

area of the electrode surface, F is faraday’s constant, D0 is the diffusion coefficient, and C0 is the

bulk concentration. The effects on the diffusion coefficient with concentration of PPD can be

determined qualitatively by examining how the magnitude of the Warburg coefficient changes

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with concentration of PPD because the diffusion coefficient is inversely related to the Warburg

coefficient.

The slope of the Randles plot is , the Warburg coefficient. From the Randles plot (Fig.

9b) it can be seen that increasing the concentration of PPD decreases the ohmic resistance, and

increases the diffusion coefficient (Fig. 9b). The diffusion coefficient reflects the mobility of the

ionic species across the supercapacitor. Increasing the diffusion coefficient is advanatageous for

obtaining fast reactions. Thus, the addition of PPD increases the performance of the

supercapacitor initially by both decreasing the ohmic resistance and increasing the diffusion

coefficient of the ionic species.

Fig. 10b shows the specific discharge capacitances for a slurry containing 0.139 M PPD

in 2 M KOH during successive cycling. Over 1000 CV cycles (20 mV s-1), an 11% decrease in

capacitance was observed. In typical pseudocapacitor systems cycle life is often an issue

because volume changes during the redox reaction (metal oxides and conducting polymers) leads

to losses of active material [53]. This loss is typically alleviated by integrating pseudocapacitive

material into a carbon support. In our carbon-based redox-mediated slurries the redox peaks

appears to compress toward the inside of the CV curve with cycling, which represents a decrease

in capacity (Fig. 10a). It can be assumed from this transition that the system is losing faradaic

charge storage over the cycle lifetime as a result of possible loss of active PPD. This is evident

because the flat region beyond the redox bump remains relatively stable, indicating that we are

not losing double layer charge storage with cycling. The loss of faradaic charge storage could be

due to degradation of the redox-active PPD during cycling. Furthermore, deterioation might be

related to pH-related changes of the electrolyte which would stifle redox reactions. This test was

done in a static cell, which does not fully mimic the conditions that would be encountered during

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flow cell operations. In an actual flow system, a simple feedback control system could be

implemented in the storage tanks to ensure that pH remained constant, and the active material

weight percentage remains constant, which may contribute to better electrolyte stability.

(Insert Figure 10)

4. Conclusions

The high-power, low cost, and long lifetime characteristics of the capacitive flowable carbon-

based electrodes in the electrochemical flow capacitor (EFC) offer the opportunity to address a

diverse range of applications on the grid level. The limited energy density, common in carbon-

based electrodes that use physical adsorption of ions as their charge storage mechanism, can be

enhanced through the addition of pseudocapacitance. This study introduced a redox-mediated

slurry as a facile method for increasing the energy density of the flowable electrodes for the

EFC. A pseudocapacitive slurry utilizing a PPD/PPI redox-couple in 2 M KOH electrolyte

demonstrated high capacitance for the EFC. The alkaline redox-mediated slurry shows increases

in capacitance on the order of 86% when compared alkaline solutions without redox mediators,

and a 130% increase compared to previously reported neutral slurries[12]. Furthermore, the rate

handling performance of the redox-mediated slurry increased from approximately 64%

capacitance decay to about 57% between rates of 2-100 mV s-1. The redox-mediated slurry is

found to decrease the ohmic resistance, with increasing concentrations of PPD, which can be

described by the increase in the ionic diffusion coefficient. Among tested slurries, a

concentration of 0.139 M of PPD in 2 M KOH yielded the highest capacitances in both CV and

GC experiments over a wide range of charging rates. The high performance is attributed to the

addition of quick redox reactions at the electrolyte/electrode interface as PPD undergoes a two-

proton/two-electron reduction and oxidation during cycling.

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Acknowledgements

Y.G. and M.B. would like to acknowledge support from the Department of Energy (DOE)

Energy Frontier Research Center (EFRC). Partial support from the Ben Franklin Technology

Partners of Southeastern Pennsylvania group (Grant# 001389-002) and the National Science

Foundation (NSF) (Grant# 1242519) is also appreciated. K.B.H. was supported by the NSF

Graduate Research Fellowship (Grant# 1002809). J.C. was supported by the NSF Bridge to the

Doctorate Fellowship (Grant# 1026641). C.R.D. was supported by the NSF IGERT program

(Grant# 0654313). The authors would also like B. Dyatkin for assistance with gas adsorptions

data and helpful conversations. SEM was carried out using instruments in the Centralized

Research Facility (CRF) of the College of Engineering at Drexel University

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List of Tables

Table 1: Observed decreases in capacitances for different PPD concentrated redox-active

slurries after increasing the sweep rate from 2 mV s-1 to 100 mV s-1.

List of Figures

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.

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.

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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-1. (b) Rate dependence of base-case

slurries from 2-100 mV s-1, showing decreasing capacitance with increasing sweep rates.

Fig. 4. (a) Cyclic voltammograms at 2 mV s-1 of redox-active slurries (23 wt% CB2) in 2M

KOH electrolyte with various amounts of PPD added. (b) Rate performance for the various

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

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

phenylenediamine to p-phenylenediimine. (dark grey, dark blue and white correspond to carbon,

nitrogen and hydrogen atoms).

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

rates ranging from 2 mV s-1 to 100 mV s-1.

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

pheneylenediamine (υ = sweep rate), (b) The total charge storage mechanism, derived as the

sweep rate approaches very 0 mV s-1, (c) Partitioned charge storage quantities (total, double

layer, and pseudocapacitive contributions).

Fig. 8. (a) Galvanostatic charging/discharging at a constant current density of 200 mA/g for the

various concentrations of PPD in 2M KOH electrolyte. (b) Capacitance dependence on

concentration of PPD in 2 M KOH electrolyte.

Fig. 9. (a) Nyquist plots for the various redox slurries in 2M 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.

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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-1. (b) Capacitance as a function of cycle

number.

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Table 1. Observed decreases in capacitances for different PPD concentrated redox-active slurries

after increasing the sweep rate from 2 mV s-1 to 100 mV s-1.

Concentration of PPD (M L-1)

0 0.046 0.092 0.139 0.184 0.277

Capacity Decay (%) 64 53 52 57 60 69

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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.

Figure 1

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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.

Figure 2

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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-1

. (b) Rate dependence of base-case

slurries from 2-100 mV s-1

, showing decreasing capacitance with increasing sweep rates.

Figure 3

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Fig. 4. (a) Cyclic voltammograms at 2 mV s-1

of redox-active slurries (23 wt% CB2) in 2M

KOH electrolyte with various amounts of PPD added. (b) Rate performance for the various

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

Figure 4

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Fig. 5. Standard two-proton/two-electron oxidation/reduction reaction of p-phenylenediamine to

p-phenylenediimine. (dark grey, dark blue and white correspond to carbon, nitrogen and

hydrogen atoms).

Figure 5

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

rates ranging from 2 mV s-1

to 100 mV s-1

.

Figure 6

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

pheneylenediamine (υ = sweep rate), (b) The total charge storage mechanism, derived as the

sweep rate approaches very 0 mV s-1

, (c) Partitioned charge storage quantities (total, double

layer, and pseudocapacitive contributions).

Figure 7

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Fig. 8. (a) Galvanostatic charging/discharging at a constant current density of 200 mA/g for the

various concentrations of PPD in 2M KOH electrolyte. (b) Capacitance dependence on

concentration of PPD in 2 M KOH electrolyte.

Figure 8

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Fig. 9. (a) Nyquist plots for the various redox slurries in 2M 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.

Figure 9

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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-1

. (b) Capacitance as a function of cycle

number.

Figure 10

a high performance pseudocapacitive suspension electrode for the electrochemical flow capacitor

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