Charge storage mechanisms in electrochemical capacitors: Effects of electrode properties on performance

Journal of Power Sources 326 (2016) 613e623

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Charge storage mechanisms in electrochemical capacitors: Effects of electrode properties on performance Madeleine F. Dupont, Scott W. Donne* Discipline of Chemistry, University of Newcastle, Callaghan, NSW, 2308, Australia

h i g h l i g h t s
Step potential electrochemical spectroscopy is used to evaluate material behaviour.
A range of common electrochemical capacitor materials have been examined.
Double layer and pseudo-capacitance processes are differentiated.
Differences in charge storage processes for various materials are explained.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 February 2016 Received in revised form 17 March 2016 Accepted 20 March 2016 Available online 16 June 2016

The capacitive behaviour of four commonly studied electrochemical capacitor systems has been analyzed using the step potential electrochemical spectroscopy (SPECS) method. Electrode-electrolyte combinations with different charge storage mechanisms were characterized, including activated carbon in aqueous (H2SO4) and organic (TEABF4 in acetonitrile) electrolytes, manganese dioxide (Na2SO4) and anhydrous ruthenium oxide (H2SO4). The SPECS method was used to separate the charge storage contributions from double layer capacitance (CDL) and diffusion-limited pseudo-capacitance (CD) at scan rates ranging from 0.08 to 125 mV/s. The relative contributions from each process are related to the physicochemical properties of the electrode. Additionally, the effects of these electrode properties on the overall performance of each system, in terms of specific power and energy, are identified. © 2016 Published by Elsevier B.V.

Keywords: Electrochemical capacitors Charge storage processes Double layer charge storage Pseudo-capacitance Step potential electrochemical spectroscopy

1. Introduction Electrochemical capacitors (ECs) are a promising energy storage technology for addressing many of the problems associated with the transition from fossil fuel based energy to renewable energy technologies. In particular, they can be used for mitigating the variable energy supply of some renewable energy sources and can be used in energy harvesting or regenerative energy technologies [1,2]. ECs have a number of advantages. They can be charged and discharged relatively quickly making them ideal for a range of energy harvesting applications [3], they are highly efficient (>98% [4]), have good cyclability (>105 cycles [4]) and are relatively nonhazardous [5]. The development of EC devices with performance characteristics tailored to specific applications is necessary to

* Corresponding author. E-mail address: [email protected] (S.W. Donne). http://dx.doi.org/10.1016/j.jpowsour.2016.03.073 0378-7753/© 2016 Published by Elsevier B.V.

ensure the uptake of this technology. However, current progress in EC development is limited by our understanding of how certain performance characteristics are governed by electrode and electrolyte properties. Identifying this relationship between material properties and device performance will lead to the development of devices with improved performance. An EC consists of two solid electrodes separated by an electrolyte. When a potential is applied to the electrodes, ions in solution accumulate at the surface of the charged electrode, forming an electrical double layer (EDL) [6]. Charge stored via this mechanism (known as double layer capacitance) is restricted to the surface of an electrode and is therefore determined predominantly by the surface area of the electrode accessible by the electrolyte. In addition to double layer capacitance, some materials store charge via fast, reversible redox reactions, known as pseudo-capacitance [6,7]. These reactions are not limited to the surface of the electrode so the bulk of the electrode can be accessed, leading to greater charge storage.

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Double layer capacitance occurs in all electrodes; however, only some materials exhibit pseudo-capacitance. The performance of an EC, in terms of total capacitance, power and energy, is determined predominantly by the relative contributions to charge storage from double layer and pseudo-capacitive processes. Double layer capacitance is a fast process because charge is stored at the surface via electrostatic forces, unlike pseudo-capacitance which is limited by the kinetics of charge transfer and diffusional processes. Therefore, double layer capacitors (EDLCs) often have high power, but low energy compared to pseudo-capacitors as they cannot sustain this discharge for extended times [8]. The relative charge storage contributions from each mechanism in an electrode are determined by the physicochemical properties of the electrode material, such as chemical composition, crystal structure, surface area and porosity, among others. It is therefore necessary to understand how such properties can be tailored to optimise EC performance for applications. EC electrodes are commonly made of one of three materials; i.e., activated carbon, ruthenium oxides or manganese dioxide. Each of these materials has a different charge storage mechanism which is determined by the properties of the material. Activated carbon is an EDLC electrode and is the most commonly used material in commercial ECs. The extremely high surface area attainable for activated carbon (up to 3000 m2/g [9]) leads to a relatively high capacitance (~100 F/g (organic electrolyte) and 200 F/g (aqueous)), despite charge storage being limited to the electrode-electrolyte interface. Due to double layer capacitance being the primary charge storage method, activated carbon electrodes generally have high specific power but low energy [6]. Ruthenium oxide is the prototypical pseudo-capacitor in which charge is stored via both double layer and pseudo-capacitance [7]. This pseudo-capacitance arises through the insertion of an electron from the external circuit to reduce Ru(IV) to Ru(III) coupled with the subsequent insertion of a proton from the electrolyte to maintain charge neutrality. This proton insertion process is very facile due to the high conductivity of ruthenium oxide, hence ruthenium oxide-based electrodes have exhibited some of the highest reported capacitance values, up to 900 F/g [10,11]. The high conductivity and high specific capacitance of ruthenium oxide electrodes means that they generally exhibit both high specific energy and power [12]. However, despite the high performance of ruthenium oxide electrodes, their commercial potential is impeded by their high cost and toxicity [12]. Manganese dioxide has been identified as a promising alternative to ruthenium oxide as it is inexpensive, readily available and non-hazardous. Similarly to ruthenium oxide, manganese dioxide stores charge via a pseudo-capacitive cation insertion mechanism. In manganese dioxide, cation insertion can involve either protons or metal cations from the electrolyte, such as Naþ. Due to their smaller size, proton insertion is often a more facile process and accounts for a large proportion of charge storage, however, in large tunnel and layered manganese dioxide phases (such as the birnessite phase), there is likely to be a more significant contribution from metal cation insertion. Despite manganese dioxide having a higher theoretical capacitance than ruthenium oxide, the reported capacitance of manganese dioxide electrodes have not yet surpassed that of ruthenium oxide, with the exception of some extremely thin electrodeposited films [13]. To improve the performance of these EC electrode materials, it is necessary to identify how their different charge storage mechanisms determine specific performance characteristics (such as power and energy), and furthermore, identify the extent to which the charge storage mechanism is governed by the properties of the electrode material. This will lead to the synthesis of electrode materials with material properties optimized for specific

performance requirements. However, it has previously been difficult to experimentally characterize the different charge storage processes occurring at an electrode. This is due to the inability of conventional electrochemical techniques to separate the charge storage contributions from double layer and pseudo-capacitive processes. In this work, the performance of commonly used EC electrode materials is compared and evaluated in terms of their material properties and different charge storage mechanisms. The charge storage mechanisms of activated carbon, manganese dioxide (birnessite) and hydrous ruthenium oxide (RuO2.nH2O) are characterized using the step potential electrochemical spectroscopy (SPECS) method which allows the contributions from double layer and pseudo-capacitive processes to be separated [14]. 2. Experimental 2.1. Material synthesis 2.1.1. Activated carbon The activated carbon used in this work was prepared in a two stage process. The first stage involved the pyrolysis of coconut husks at 500  C under a nitrogen atmosphere for 3 h. After cooling the resultant char was milled in a zirconia mill to produce a material with a mean particle size of ~20 mm. In the second stage, activation of the char was then carried out with the addition of a small volume of concentrated H3PO4, with the resultant suspension again pyrolyzed at 700  C under a nitrogen atmosphere for 1 h. 2.1.2. Manganese dioxide (birnessite) The Birnessite phase of manganese dioxide was produced as a result of the ambient temperature stoichiometric reaction between solutions of 0.1 M KMnO4 and 0.1 M MnSO4 (2:3 vol/mole ratio) in an aqueous media, automatically pH adjusted via a Metrohm 775 Dosimat with pH feedback loop to a pH of 9 by the addition of a dilute 0.01 M KOH solution; i.e., 2þ 2MnO þ 2H2O / 5MnO2 þ 4Hþ 4 þ 3Mn

(1)

The product was filtered and washed with Milli-Q water and then dried in air at 60BC [15]. 2.1.3. Hydrous ruthenium oxide Hydrous ruthenium oxide (RuO2.nH2O, n~ 0.5) was prepared from a commercially available hydrous ruthenium oxide (Johnson Matthey Co.), which was heat treated at 200BC in air for 24 h to reduce the water content to n~ 0.5 [16]. 2.2. Characterization 2.2.1. X-ray diffraction (XRD) Each material was characterized by X-ray diffraction (XRD) using a Philips 1710 diffractometer equipped with a Cu Ka radiation source (l ¼ 1.5418 Å) operating at 40 kV and 30 mA. The scan range was from 10 to 90B2q with a step size of 0.1B and a count time of 2.5 s. 2.2.2. Gas adsorption The surface area of each sample was determined using a Micrometrics ASAP2020 Gas Adsorption and Porosity Analyzer. Samples were pre-treated by degassing under vacuum at 110BC for 2 h to remove surface water. A N2 adsorption isotherm (at 77 K) was then measured on each sample covering the partial pressure (P/PB) range 0.05e0.30. The specific surface area was then determined using the linearized BET isotherm.

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The working electrode was initially pre-treated using cyclic voltammetry. Cycling was conducted within the potential stability window of each system (Table 2). This was carried out to establish reversible cycling behaviour. The following protocol was then used to conduct the step potential electrochemical spectroscopy (SPECS) experiment. Starting at the minimum potential (Table 2) after the cyclic voltammetry experiments, a potential step of þ0.025 V was applied to the working electrode and held for 300 s to equilibrate. During this time the current flow was measured as a function of time. This step process was repeated until the working electrode potential had reached the maximum potential of its electrochemical stability window (Table 2). The potential step process was then reversed by stepping in 0.025 V increments, each with a 300 s rest time, until the minimum potential was reached. In total, the electrode completed one entire charge/discharge cycle using the SPECS methodology.

2.3. Electrode preparation To prepare each electrode the active material was combined with a conductive agent (carbon black or graphite) and a binder (poly(vinylidenedifluoride); PVdF) in the ratio outlined in Table 1. These materials were ground together using a ceramic mortar and pestle to ensure a homogeneous mixture. This powder was made into an ink by adding N-methylpyrrolidone (NMP; 99%) in a weight ratio of ~20:1 solvent:solid. The working electrode was prepared by dropping 50 mL of the ink onto the end of a clean 13 mm diameter rod substrate, either stainless steel or graphite. The substrate had been previously cleaned by polishing with 1200 grit emery paper before being washed thoroughly with Milli-Q water, and wiped dry with a lint free tissue. The counter electrode was prepared in a similar fashion to the working electrode, except that activated carbon was used as the active material when preparing the electrode ink, and the ratio of activated carbon to carbon black to binder was 80:15:5. Additionally, 150 mL of the ink was dropped onto the substrate to ensure a greater mass of electrode material than the working electrode so that any limitations in the system were a result of the working electrode. The electrodes were then dried in air at 60  C for 8 h.

3. Results and discussion 3.1. Material characterization 3.1.1. Surface area Gas adsorption analysis was used to determine the surface area of each sample. The total surface areas were calculated using the linearized BET isotherm and the results shown in Table 3. Activated carbon has a very high surface area, as is typical for this material [17,18]. The reported surface areas for Birnessite vary depending on the synthesis conditions used, with surface areas ranging from 5 to 230 m2/g being reported [19e21]. The Birnessite phase synthesized here has a surface area within the range reported in the literature. Similarly, the hydrous ruthenium oxide has a surface area value in line with those reported in the literature [10,22].

2.4. Electrochemical cell construction The electrochemical cell used for all experiments was based on a Swagelok 13 mm diameter perfluoroalkoxy alkane (PFA) T-junction. Two coated electrode substrates (working and counter) were inserted loosely from opposite ends into the cell, separated by two layers of porous paper as the separator, leaving the third perpendicular port open. The electrodes were then pressed together at 1.7 MPa using a hydraulic press to ensure good cell conductivity, before the electrodes were secured (screwed) into place. The cell was then filled with electrolyte (Table 1), sealed and left to equilibrate for ~12 h.

3.1.2. XRD analysis Fig. 1 shows the XRD patterns for (a) activated carbon, (b) manganese dioxide and (c) hydrous ruthenium oxide. The activated carbon pattern exhibits two broad, low intensity peaks. Activated carbon is expected to be predominantly amorphous with negligible crystallinity; however, the peaks seen in the pattern indicate that there are some crystalline regions in the material, although the width of the peaks suggests that it is poorly crystalline. The peaks can be attributed to the (002) and the (101) reflections from graphite [23]. The high temperatures involved in the synthesis of activated carbon may cause some graphitization to occur in the material, giving rise the graphitic crystal structure. The manganese dioxide pattern (Fig. 1(b)) shows a number of broad peaks, which can be attributed to the Birnessite crystal structure [24]. The Birnessite phase of manganese dioxide consists of sheets of [MnO6] octahedra joined via edge-sharing, giving rise to a layered structure [25]. The Birnessite phase often has relatively poor crystallinity due to turbostratic distortion arising from uneven stacking of the crystal layers [26]. The hydrous ruthenium oxide (Fig. 1(c)) is almost entirely

2.4.1. Aqueous electrolytes After equilibration the reference electrode (saturated calomel electrode e SCE; Radiometer Analytical) was inserted into the perpendicular port of the Swagelok cell and sealed in place with Parafilm. 2.4.2. Non-aqueous electrolytes After equilibration, the reference electrode (Ag/AgNO3 (0.1 M in acetonitrile)) was inserted into the perpendicular port of the Swagelok cell and sealed in place with Parafilm. The cell was placed in a sealed container with a constant flow of dry N2 pumped through the container for the duration of the experiment. 2.5. Electrochemical protocol All electrochemical experiments were conducted using an Iviumstat Multichannel Potentiostat controlled by Iviumstat software.

Table 1 Electrode preparation. Active material

Activated carbon Birnessite RuO2.nH2O

Substrate

Stainless steel Stainless steel Stainless steel Graphite

Conductor

Carbon black Carbon black Graphite Graphite

Working electrode composition (Mass %)

Electrolyte

Active

Conductor

Binder

80 80 15 15

15 15 80 80

0.05 0.05 0.05 0.05

1 M TEABF4 in acetonitrile 0.5 M H2SO4 0.5 M Na2SO4 0.5 M H2SO4

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M.F. Dupont, S.W. Donne / Journal of Power Sources 326 (2016) 613e623 Table 2 Electrochemical potential window for each system. Electrode material

Minimum potential

Maximum potential

Activated carbon (1 M TEABF4 in acetonitrile) Activated carbon (0.5 M H2SO4) Birnessite (0.5 M Na2SO4) RuO2.nH2O (0.5 M H2SO4)

0.0 V vs Ag/Agþ(0.01 M) 0.5 V vs SCE 0.0 V vs SCE 0.1 V vs SCE

0.9 0.3 0.8 0.9

V V V V

vs vs vs vs

Ag/Agþ(0.01 M) SCE SCE SCE

Table 3 BET surface area, conductivity and specific capacitance for each material. Material

BET SA (m2/g)

Conductivity (S/cm)

Electrolyte

Specific capacitance (F/g)

Activated carbon

1888

1.2 [29]

TEABF4 in acetonitrile 0.5 M H2SO4 0.5 M Na2SO4 0.5 M H2SO4

71 118 111 399

Birnessite RuO2.nH2O

87.1 108.1

1.92  10 750 [10]

6

[48]

integrating the current with respect to time, giving a value for charge passed (Q; C/g) for the anodic and cathodic scans. This was converted to a specific capacitance using:

C¼

Q V

(2)

where C is specific capacitance (F/g), Q is charge passed (C/g), V is the potential window (V). The specific capacitance calculated for each system is shown in Table 3. These values are comparable to other values reported in the literature [8]. Pseudo-capacitive electrodes (MnO2 and RuO2.nH2O) often exhibit relatively high capacitance compared to purely double layer capacitors (activated carbon) [4]. However, the Birnessite phase examined here has a capacitance that is slightly lower than values reported in the literature for the same phase [21]. Additionally, the somewhat higher capacitance exhibited by activated carbon may be due to the hydrogen evolution reaction which contributes to the charge passed, but since it is not a reversible reaction, it is not actually capacitive.

3.3. Step potential electrochemical spectroscopy (SPECS) Fig. 1. XRD spectra for (a) activated carbon, (b) manganese dioxide (birnessite) and (c) hydrous ruthenium oxide (RuO2. nH2O).

amorphous. Anhydrous RuO2 is highly crystalline with a rutile-type crystal structure [27]. However, as the water content of RuO2 increases the crystallinity decreases [28], so the low-temperature treated sample examined here has only two broad, low intensity peaks which can be attributed to the (101) and the (211) peak of the rutile-based RuO2 structure [27]. 3.2. Cyclic voltammetry Fig. 2 shows the cyclic voltammetry profiles for all systems examined, at a cycle rate of 25 mV/s. All systems exhibit typical capacitive behaviour, characterized by a rectangular voltammogram without any significant redox peaks. The activated carbon in H2SO4 voltammogram indicates that a cathodic process is occurring as the potential approaches 0.5 V which has previously been attributed to hydrogen ad-atom formation on the electrode surface [29]. The specific capacitance of each system was calculated by

The SPECS experiment involves cycling an electrode by a series of small (±0.025 V) potential steps. Each step is held for 300 s so that all charging processes equilibrate before further charging or discharging occurs. Given that the different charging mechanisms, such as double layer and pseudo-capacitance, each with a different time constant, the current response for each potential step can be fitted to known expressions to model the current flow arising from each individual process. The current response for the SPECS experiment conducted on hydrous ruthenium oxide is shown in Fig. 3. Each potential step causes a spike in the current which decays to reach an equilibrium current approaching 0 A. The current decay as a function of time is dependent on the processes that are occurring at the electrode; i.e., double layer capacitance will exhibit a faster decay than pseudocapacitance, which in most cases is limited by either solid state diffusion (in manganese dioxide) or by aqueous ion diffusion (in activated carbons). The current response for different processes can be described by known equations. The current response for a double layer capacitor in series with a resistor upon the application of a potential step can be described by Ref. [30]:

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Fig. 2. Cyclic voltammetry profiles (250th cycle) for (a) activated carbon (1 M TEABF4 in acetonitrile) (b) activated carbon (0.5 M H2SO4) (c) manganese dioxide (0.5 M Na2SO4) and (d) hydrous ruthenium oxide (0.5 M H2SO4).


E t iC ¼ exp  RS RS CDL

(3)

where RS is the series resistance (Ug) attributable to the resistance of the electrode, electrolyte and material in the electrochemical cell, CDL is the double layer capacitance (F/g), and t is the time after the potential step (s). It has been demonstrated previously for porous materials that the double layer current response can be most accurately described using two separate double layer terms, CDL1 and CDL2 [31]. These terms represent, respectively, the formation of a double layer on the geometric surface of the material and on the surface of macropores where ion transport is not significantly hindered (CDL1), and double layer formation within micro- and meso-pores where transport of ions to the surface is much slower due to the longer pathways through pores (CDL2) [32,33]. Each of these processes has a series resistance associated with it (RS1 and RS2) which correlates to the movement of ions to the surface such that geometric double layer capacitance has a smaller series resistance than porous capacitance. All of the systems reported here store charge via double layer capacitance; however, manganese dioxide and hydrous ruthenium oxide are both known to have significant contributions from pseudo-capacitance [34,35]. Additionally, some redox reactions have been reported to occur in activated carbons in aqueous electrolyte involving redox-active surface functional groups [12]. In all cases, pseudo-capacitance is a diffusion limited process as it relies on the transport of ions either to the electrode surface or through the electrode in the form of solid state diffusion. Solid state proton

diffusion in small particles can be described by the spherical diffusion model [36]. Aqueous diffusion of ions to an electrode surface is often modelled on the Cottrell equation; however, this model assumes semi-infinite planar diffusion [30]. In the case of ion transport to the surface of an activated carbon electrode, of which the majority of the surface is contained within pores <10 Å [29], this model is no longer applicable and instead, the spherical diffusion model can be used. The current response for spherical diffusion is given by Ref. [36]:

iD ¼


 ∞ 2FADDC X n2 p2 Dt exp  a a2 n¼1

(4)

where iD is the current density (A/g), A is the electrochemically active surface area (m2/g), D is the diffusion coefficient (m2/s), DC is the change in ion concentration in the host structure over the potential step (¼ C1 e C0; mol/m3), a is the particle radius (m), and t is the time (s). The derivation of this equation can be found in Ref. [31]. Therefore, for any given electrode, the total current response to a potential step can be described as a combination of double layer and diffusional processes (Eqn (5)). An additional term, iR (residual current) has been included because in some cases, the current does not decay to zero over the equilibration time, instead there is a constant residual current. This has been attributed to slow, ongoing redox reactions which do not become limited within the equilibration time [14]. Therefore,

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Fig. 4. Total capacitance (F/g) as a function of scan rate for activated carbon in (a) 1 M TEABF4 in acetonitrile and (b) 0.5 M H2SO4 and (c) manganese dioxide and (d) hydrous ruthenium oxide.

Fig. 3. Current (A/g) vs applied potential (V) and time (mins) for hydrous ruthenium oxide in 0.5 M H2SO4.

iTotal ¼ iDL1 þ iDL2 þ iD þ iR

(5)

where iDL1 and iDL2 are the currents resulting from charging of the double layer on the geometric and porous surface areas (as defined by Eqn (3)), iD is the diffusion limited current (Eqn (4)), and iR is the residual current. For each potential step, the current response was fitted to Eqn (5) using least squares regression to obtain values for RS1, RS2, CDL1, CDL2, CD and CR. The current response for each ±0.025 V step can be fitted over the entire equilibration time (300 s). This equates to a scan rate of 0.083 mV/s, which is significantly slower than would typically be used to test a capacitor electrode. The scan rate can be effectively increased by decreasing the time over which the current response is analyzed. For example, fitting the same current response over 10 s is equivalent to a scan rate of 2.5 mV/s. Given that the current response was measured at 0.2 s intervals, the current can be measured up to a maximum scan rate of 125 mV/s.

3.4. Effect of scan rate on specific capacitance The rate capabilities of each system can be evaluated by examining the capacitance as a function of scan rate, as calculated from the fitting results. Fig. 4 shows the total capacitance for each system as a function of the effective scan rate. All systems exhibit a similar trend in that the capacitance increases as the scan rate decreases from 125 mV/s e 0.08 mV/s. At high scan rates, there is less time for processes to equilibrate, therefore only a small proportion of the electrode (primarily the surface) can be accessed for charge storage. As the scan rate decreases, the longer equilibration times allow more of the electrode to be utilized for charge storage via slower equilibrating processes, such as bulk charge storage. This increase in capacitance at low scan rates is dependent on the charge storage

mechanism of the system. Activated carbon (Fig. 4(a)) in organic electrolyte is not expected to have any bulk charge storage (i.e., pseudo-capacitance), hence there is not a significant increase in capacitance at low scan rates, suggesting that the electrode is approaching its maximum charge storage capacity. However, manganese dioxide (Fig. 4(c)) is known to store charge via cation insertion into the bulk structure, the mass transport of which is a relatively slow process, hence the capacitance begins to increase significantly as the scan rate approaches 0.08 mV/s, indicating that the material has yet not reached its maximum capacity. The effect of different charge storage processes on the rate behaviour of each system can be identified by separating the total capacitance into its different components. Fig. 5 shows the breakdown of total capacitance in terms of the double layer capacitances (CDL1 and CDL2) and diffusional processes (CD). This residual capacitance has not been included in these calculations because in some cases, particularly in manganese dioxide, the residual current flows in the opposite direction to the applied potential; i.e., an anodic iR for a cathodic potential step. This has been attributed to extremely slow redox reactions occurring the bulk of the electrode which do not reverse quickly upon a change of potential step direction [14]. Therefore, at extremely slow scan rates, the residual capacitance can become negative. Given that the residual current only becomes significant at very low scan rates (<1 mV/s) its practical effects on capacitor behaviour are negligible, therefore it has not been included.

3.4.1. Surface effects For all systems examined, the sharp increase in capacitance as the scan rate is initially decreased from 125 mV/s is predominantly caused by surface charge storage processes (CDL). As the scan rate is decreased, the depth of ion penetration into pores increases and more of the surface is utilized; however, once the surface is saturated, the capacitance plateaus. The geometric surface area (CDL1) is more easily accessible so it reaches saturation more quickly than the porous surface area (CDL2). This effect is more pronounced in

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Fig. 5. Capacitance contributions from CDL1, CDL2, CD and CTotal as a function of scan rate for (a) activated carbon (1 M TEABF4 in acetonitrile) (b) activated carbon (0.5 M H2SO4) (c) MnO2 (0.5 M Na2SO4) and (d) RuO2. nH2O (0.5 M H2SO4).

highly microporous materials. Activated carbon has a large microporous surface area and therefore slower scan rates are required for it to reach surface saturation. Birnessite has been shown to have some microporous character [14] and it also exhibits a capacitance plateau at relatively low scan rates compared to hydrous ruthenium oxide, which exhibits surface saturation at scan rates >10 mV/s. 3.4.2. Electrolyte effects The difference in rate behaviour of activated carbon in organic and aqueous electrolytes indicates that the electrolyte affects the charge storage mechanism. In the organic electrolyte used here, activated carbon exhibits not only a lower total capacitance, but also reaches surface saturation at a lower scan rate than in aqueous electrolyte. This indicates ion accessibility is limited in the organic system, which is likely due to the larger solvated ion size of the  þ TEAþ and BF 4 compared to the aqueous H and SO4 ions. The movement of large ions through small pores will be more restricted and will take longer to equilibrate, hence requiring slower scan rates to fully saturate the surface, and may also be entirely unable to access some small pores, resulting in a lower total capacitance. Similar ion effects have been reported elsewhere in the literature [33,37,38]. 3.4.3. Diffusional processes (CD) In all systems contributions from diffusional processes (CD) are minimal at the highest scan rates because there is not sufficient

equilibration time for slower process to occur to any significant extent. As the scan rate decreases and more diffusional process can occur, the capacitance increases. Unlike double layer capacitance, CD does not plateau in scan rate range examined here due to the much greater capacity of the bulk electrode compared to the surface. The origin of CD is different for each system, hence has a different rate dependency and overall contribution. Activated carbons are generally thought to store charge predominantly via double layer capacitance (CDL) [12]; however, it is seen here that activated carbon in both aqueous and organic electrolytes exhibits significant diffusional behaviour at low scan rates. The diffusional processes in aqueous electrolyte have been observed previously and have been attributed to the decomposition of the electrolyte via hydrogen evolution at cathodic potentials [29]. The origin of CD can be identified from the calculated voltammogram for CD which is determined from the average current flow from iD at each potential step (Fig. 6). In organic electrolyte (Fig. 6(a)), the diffusional current is relatively constant throughout the cycle, with a moderate increase in current at potentials < 0.2 V, which may indicate some electrolyte decomposition. In the aqueous electrolyte (Fig. 6(b)), there is a significant increase in the diffusional current as the potential approaches 0.5 V (cathodic scan), followed by a significant anodic current as the potential is stepped between 0 and 0.3 V (anodic scan). This cathodic peak is the reduction of hydrogen, followed by a smaller anodic peak, which may be the oxidation of surface functional groups. However,

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Fig. 6. Calculated voltammograms for the diffusional current (iD) for activated carbon in (a) TEABF4 and (b) H2SO4, at an effective scan rate of 1 mV/s.

in both organic and aqueous electrolytes, there is a diffusional current present at all potentials, suggesting that it cannot be attributed solely to electrolyte degradation. Activated carbon has a highly microporous surface and it is well documented that ion transport into small pores is a restricted process [39e41], therefore the equilibration time is much longer than for ion diffusion into larger, more accessible pores. Hence, it may be that microporous double layer formation has a current response more similar to a diffusion limited process than a double layer process. This suggests that a proportion of CD observed at low scan rates may be double layer formation on the surface of micro-pores. This can be supported by comparing the CDL contributions for both activated carbon systems (Fig. 5(a) and (b)) with the CDL contributions in manganese dioxide (Fig. 5(c)). The total CDL contributions for activated carbon are 78 F/g (organic) and 95 F/g (aqueous), and the total CDL for manganese dioxide is 71 F/g. Given that activated carbon has a specific surface area ~20 times that of manganese dioxide, it does not seem reasonable that the two materials would exhibit comparable double layer capacitance. Hence, it is likely that some double layer processes in activated carbon appear diffusion limited. For manganese dioxide (Fig. 5(c)), the diffusional capacitance is predominantly attributable to the solid-state diffusion of protons through the electrode. This is a slower process so it cannot contribute significantly at fast scan rates [42,43], but because it utilizes the bulk of the electrode, rather than just the surface, it contributes a much higher capacitance than CDL. Hydrous ruthenium oxide (Fig. 5(c)) has the lowest CD contribution of any system, which is unexpected as ruthenium oxide is known to have a large pseudo-capacitive contribution via a proton insertion process similar to that in manganese dioxide [10]. Additionally, this system exhibits significantly higher CDL than any other, in fact, almost all of the capacitance arises as a result of CDL processes. Given the relatively low surface area of this material (108 m2/g), the high CDL values cannot be attributed solely to aqueous double layer formation on the external surface area. Pure ruthenium dioxide exhibits almost metallic conductivity (~2.5  104 S/cm [44]). Unlike a semiconductor (such as manganese dioxide), a metallic conductor cannot sustain a potential gradient within its bulk, instead, any excess charge will reside at the surface of the electrode material. The superior performance of hydrous ruthenium oxides compared to the anhydrous form has been attributed to the incorporation of water into the structure. It has been proposed that the structure of hydrous ruthenium oxide

consists of nanocrystals of RuO2 separated by grain boundary regions filled with water molecules [22,45,46]. The high electronic conductivity of RuO2 means that any electron inserted from the external circuit will move to the surface of crystal with no diffusion limitations. Despite its high electronic conductivity, RuO2 has very limited proton conductivity [45], however the hydrous grain boundary regions provide proton conducting pathways effectively creating a large internal surface area for double layer formation. These separate regions maximise both electronic and protonic conductivity within the material, giving rise to high capacitance. This suggests that CDL1 and CDL2 may represent the internal and external surface areas rather than representing porous and geometric CDL (as in other materials). The different accessibility the internal and external surface has been previously examined by Ardizzone et al. [47] and they suggested that proton diffusion to the internal surface was limited by the slower rate of diffusion through the narrow grain boundary regions, hence the utilization of this surface area is lower at high scan rates. The results obtained from the SPECS analysis show that CDL2 has a greater rate dependency than CDL1 which may indicate that CDL2 represents the internal surface area. However, the aforementioned study also found that the internal and external surface areas were of similar magnitude, which is contrary to the SPECS results which show that CDL1 has significantly higher capacitance at all scan rates. The differences in the internal and external surface area may be attributable to the synthesis method used. The hydrous ruthenium oxide samples used here were heat treated at 200BC, whereas the samples examined by Ardizzone et al., were treated at higher temperatures (300e500  C). Higher treatment temperatures remove structural water [10] resulting in a smaller proportion of hydrous grain boundary regions, which will reduce the internal surface area. Additionally, a higher structural water content will lead to the formation of larger, more accessible grain boundaries for which proton diffusion would not be as dependent on the scan rate. Therefore, CDL1 most likely represents the combined capacitance for the external surface area and the easily accessible internal surface area, whilst CDL2 represents the less accessible regions of the internal surface area, i.e., smaller grain boundaries where proton diffusion is limited. The additional contribution from CD at low scan rates may arise as a result of proton insertion into the RuO2 regions. This process has been found to have a high energy barrier [45] so it is likely that it would not occur until the double layer has been saturated; i.e., at low scan rates.

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3.5. Fractional capacitance The fraction of capacitance contributed by each process as a function of scan rate is shown in Fig. 7. In all systems, double layer capacitance is the primary charge storage mechanism at high scan rates. Both activated carbon systems and manganese dioxide (Fig. 7 (a), (b) and (c)) exhibit ~80% contribution from CDL at high scan rates. This decreases as the scan rate is decreased and diffusional processes become significant. For activated carbon in organic electrolyte (Fig. 7(a)) the diffusional processes reach a maximum proportion of 45%. Conversely, in aqueous electrolyte (Fig. 7(b)), diffusional processes become the primary charge storage mechanism at a scan rate of 8 mV/s, reaching a maximum of 76%. Similarly, manganese dioxide transitions from behaving predominantly as a double layer capacitor at high scan rates, to a pseudo-capacitor at low scan rates. The unique charge storage mechanism of hydrous ruthenium oxide (Fig. 7(c)) means that it behaves primarily as a double layer capacitor at all scan rates, with CD contributions only reaching ~25% at the lowest scan rate.

3.6. Effects of mechanisms on power and energy performance The fractional contributions of each charge storage process will determine the specific energy and power of each system at a range of scan rates. Fig. 8 compares the specific energy and power of each

Fig. 8. Ragone diagram for (a) activated carbon (TEABF4) (b) activated carbon (H2SO4) manganese dioxide and (d) hydrous ruthenium oxide.

Fig. 7. Fractional contribution from different charge storage processes (double layer and diffusional) as a function of scan rate for (a) activated carbon (TEABF4) (b) activated carbon (H2SO4) (c) manganese dioxide and (d) hydrous ruthenium oxide.

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system over a range of scan rates in the form of a Ragone diagram. It is important to note that Ragone diagrams generally account for total mass of the energy storage device, which includes the masses of conductive agent, binder and the device housing. However, in the diagrams presented here, only the mass of the active material is considered. Given that all samples were analyzed using the same electrochemical cell setup, it is accurate for making comparisons; however, the values for energy and power cannot be compared with a standard Ragone diagram. All systems exhibit a similar trend in energy and power performance as a function of scan rate. At the highest scan rate, the specific power of all systems is at its maximum value and as the scan rate is decreased, the specific power decreases and the specific energy begins to increase, reaching a maximum at the lowest scan rate. The high power at fast scan rates is primarily due to double layer charge storage, which accounts for >75% of capacitance in all systems (Fig. 5). Since this process has a fast response time, a relatively large proportion of the CDL can be released even at high scan rates. As the scan rate decreases, contributions from CD increase. CD processes have lower power because the equilibration time is longer, but this increases the discharge time which, combined with a greater total charge storage, gives rise to a high specific energy. The transition from high power, CDL processes to high energy CD processes varies depending on the system. Activated carbon in organic electrolyte (Fig. 8(a)) stores charge primarily via CDL at high scan rates. However, the specific energy of these processes reaches a plateau as the scan rate decrease (corresponding with the observed capacitance plateaus in Fig. 5). This is evident from the sharp decrease in specific power at low scan rates. Since CD contributions in this system are relatively small, there is only a minimal increase in specific energy at lower scan rates. By comparison, activated carbon in aqueous electrolyte (Fig. 8(b)) exhibits a similar trend for scan rates >1 mV/s, except both its power and energy are shifter to higher values due to its higher overall capacitance. This higher power can be attributed to better electrolyte accessibility into pores, which is known to improve power capabilities [8]. However, at scan rates <1 mV/s, its behaviour deviates from that observed in organic electrolyte and is characterized by a continual increase in specific energy as the scan rate approaches 0.1 mV/s. Approximately 1 mV/s is the scan rate at which activated carbon in aqueous electrolyte changes from predominantly double layer storage to predominantly diffusional storage (Fig. 7(b)). Conversely, activated carbon in organic electrolyte (Fig. 7(a)) has predominantly double layer storage at all scan rates. This illustrates the importance of CD in increasing the specific energy at low scan rates. However, in the case of activated carbon (aqueous electrolyte) a large proportion of this capacitance is a result of electrolyte degradation. Although this process is diffusion limited, is it not a capacitive process therefore if the electrode were cycled repeatedly, it would likely fail. Manganese dioxide exhibits a similar transition from predominantly CDL to CD at a scan rate of ~1 mV/s (Fig. 7(c)) therefore its specific energy and power exhibit similar behaviour to activated carbon (Fig. 8(c)). It has slightly lower specific energy and power due to it lower capacitance. Hydrous ruthenium oxide exhibits significantly higher energy and power at all scan rates than any other system (Fig. 8(d)). The high power can be attributed to its fast charge storage processes (CDL) which, unlike other systems, is not limited to the external surface of the electrode. This means that most of the electrode is accessible for charge storage at fast scan rates giving rise to both high specific power and energy. As the scan rate is decreased, the specific energy increases slightly as more of the electrode can be accessed, but appears to plateau at ~10 mV/s. It is seen in Fig. 5(d)

that the CDL processes reach saturation at this point and decreasing the scan rate beyond this does not significantly increase charge storage in the electrode, therefore the specific energy plateaus. However, as the scan rate is decreased further below 1 mV/s, the energy begins to increase due to the increasing contributions from CD. These processes do not contribute significantly at higher scan rates due to the larger energy barrier associated with proton insertion into RuO2 [45]. 3.7. Effects of material properties on overall performance Overall, hydrous ruthenium oxide exhibits the best performance in terms of total capacitance, specific power and specific energy, particularly at fast scan rates. This superior rate performance arises due to the structure of the material which consists of solid RuO2 nanocrystals surrounded by hydrous grain boundaries, effectively creating a large internal surface area [22,45,46]. This means that the material has the rate performance of a double layer capacitor (fast equilibration time and high power) but can store this charge in the bulk giving it the high capacitance and energy of a pseudocapacitive electrode. In contrast, manganese dioxide can store charge either via double layer capacitance (CDL) which has fast equilibration time but is limited to the electrode surface, or by pseudo-capacitance (CD) which utilizes the bulk, but is significantly slower. Generally, charge is stored via both CDL and CD with the relative proportions of each being dependent on the scan rate. The charge storage mechanism transitions from primarily CDL at high scan rates to primarily CD at low scan rates. Unlike hydrous ruthenium oxide, which maximises conductivity via metallically conducting RuO2 regions interspersed with protonically conductive water, manganese dioxide is a semiconductor with combined electronic and protonic conductivity in the bulk. The electronic conductivity of manganese dioxide is significantly poorer than RuO2 (Table 3) hence, proton and electron diffusion through the bulk of manganese dioxide is a much slower process than in hydrous ruthenium oxide. Therefore, bulk charge storage in manganese dioxide can only occur at slow scan rates, whereas in hydrous ruthenium oxide, the bulk electrode can be accessed even at high scan rates. This difference in charge storage mechanisms results in the relatively low power of manganese dioxide at fast scan rates compared to hydrous ruthenium oxide. The behaviour of the activated carbon electrode varies according to the electrolyte used. Activated carbon in aqueous electrolyte exhibits higher capacitance (both CDL and CD) than in organic electrolyte. The higher CDL can be attributed to the smaller size of aqueous ions, which have better accessibility into micro-pores. Additionally, the ion access into these pores is less limited at higher scan rates compared to in organic media. Both activated carbon systems exhibit contributions from CD. In aqueous electrolyte, a significant proportion of this charge contribution can be attributed to the electrochemical decomposition of the electrolyte at the cathodic potential limit. Although it contributes to the total current, it is not a capacitive process. There are additional CD contributions across the entire potential range which cannot be attributed to electrolyte decomposition. These CD processes have been attributed to the very slow diffusion of ions into micro-pores to form a double layer, which may behave similarly to a diffusion limited process, rather than a double layer capacitor. 4. Conclusions Here it is demonstrated that the step potential electrochemical spectroscopy (SPECS) technique can be used to characterize the different charge storage mechanisms in a range of commonly used electrochemical capacitor systems. The total charge storage in each

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system was separated into its double layer components CDL1 and CDL2 (representing geometric and porous double layer capacitance, respectively) and diffusional (pseudo-capacitive) processes CD. The relative contributions from these processes was examined as a function of scan rate and the effects of this behaviour on the power and energy performance was identified. It was found that hydrous ruthenium oxide (RuO2.nH2O, n~0.5) had the best overall performance in terms of power and energy which can be attributed to its unique structure which allows it to store charge in its bulk (as in a pseudo-capacitor) but with the rate capabilities of a double layer capacitor. This gives rise to a material with both high specific power and energy. It was found that in all systems, charge is stored predominantly via CDL at fast scan rates and materials with a large double layer capacitance (CDL) contribute to higher power and energy at fast scan rates. At slower scan rates diffusional processes become more significant, and it was shown that systems that transitioned from primarily CDL to CD as the scan rate decreased exhibited increased specific energy at low scan rates. In all cases, a greater total capacitance contributed to better overall power and energy performance at all scan rates. However, the relative contributions from CDL and CD processes determined how the power and energy performance varied with scan rate. Acknowledgements MFD would like to acknowledge the University of Newcastle for the provision of a PhD scholarship. The authors would also like to acknowledge Prof. Wataru Sugimoto (Shinshu University, Japan) for providing the hydrous ruthenium oxide which was studied in this work. References [1] J. Miller, P. Simon, Electrochemical capacitors for energy management, Sci. Mag. 321 (2008) 651e652. [2] J.R. Miller, A.F. Burke, Electrochem. Soc. Interface 17 (1) (2008) 53. [3] P.J. Hall, M. Mirzaeian, S.I. Fletcher, F.B. Sillars, A.J. Rennie, G.O. Shitta-Bey, G. Wilson, A. Cruden, R. Carter, Energy & Environ. Sci. 3 (9) (2010) 1238e1251. [4] Y. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, H. Dong, X. Li, L. Zhang, Int. J. Hydrogen Energy 34 (11) (2009) 4889e4899. [5] G. Wang, L. Zhang, J. Zhang, Chem. Soc. Rev. 41 (2) (2012) 797e828. [6] F. Beguin, E. Frackowiak, Supercapacitors Materials, Systems, and Applications, first ed., Wiley-VCH, Weinheim, 2013. [7] B.E. Conway, Electrochemical Supercapacitors, Scientific Fundamentals, Technological Applications, Kluwer Academic/Plenum Publishers,, New York, 1999. [8] L.L. Zhang, X.S. Zhao, Chem. Soc. Rev. 38 (2009) 2520e2531.

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