The term âasymmetricâ supercapacitor encompasses a wider range of electrode combinations because it can be used for super- capacitors using electrodes of ...
Capacitive and Pseudocapacitive Electrodes for Electrochemical Capacitors and Hybrid Devices
1
Thierry Brousse 1,2 , Olivier Crosnier 1, 2 , Daniel Bélanger 3 , Jeffrey W. Long 4 1 Université de Nantes, CNRS, Nantes, France; 2Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS, Amiens, France; 3Université du Québec a Montréal, Montréal, QC, Canada; 4U.S. Naval Research Laboratory, Washington, DC, United States
1.1
Introduction
Electrochemical capacitors (ECs, also sometimes denoted as “supercapacitors” or “ultracapacitors”) are energy storage devices that bridge the performance gap between the high energy density provided by batteries and the high power density (but very limited energy density) derived from dielectric capacitors. Commercially available ECs exhibit gravimetric energy density up to 8.5 Wh kg1 and usable power density up to 9.0 kW kg1 [1]. In the field of ECs, there is often confusion between the electric parameters of a full device and the electrochemical properties of the individual electrodes that comprise the cell. The aim of this chapter is to describe the distinctions between these various devices and their constituents, starting with a comparison of dielectric capacitors and ECs, followed by discussion of other electrochemical energy storage devices with regard to their electric properties. The electrochemical behavior of common electrode materials used in ECs and related devices will be discussed in terms of capacitive, pseudocapacitive, and faradaic charge storage mechanisms, as well as recommended methods with which such electrodes should be characterized. We highlight the distinctions between carbon-based capacitive electrodes that are commonly found in commercial ECs and pseudocapacitive electrodes [2,3], such as RuO2 [4,5], or MnO2 [6,7], that have the electrochemical signature of a capacitive electrode but express different charge storage mechanisms. In the last part of the chapter, we describe the important distinctions between high-power battery-type electrodes and pseudocapacitive electrodes.
1.2 1.2.1
Devices Dielectric Capacitors
A dielectric is an electronically insulating material that can be polarized by an applied electric field where electric charges do not flow through the material as they do in an Metal Oxides in Supercapacitors. http://dx.doi.org/10.1016/B978-0-12-810464-4.00001-2 Copyright © 2017 Elsevier Inc. All rights reserved.
2
Metal Oxides in Supercapacitors
electronic conductor, such as metals, but are only slightly shifted from their equilibrium positions. Positive charges are displaced in the direction of the field and negative charges shift in the opposite direction. This separation of charge creates an internal electric field that reduces the overall field within the dielectric itself [8]. In a dielectric capacitor (e.g., conventional polymer film or ceramic capacitor), the dielectric material is thin and sandwiched between two current collectors that are usually metals. When a voltage is applied to the dielectric capacitor, an electric field is created within the dielectric, and charges (electrons and holes) accumulate in the metallic current collectors at the interface with the dielectric material. Thus, dielectric capacitors store charges through electrostatic interactions but at levels that are much lower than for standard batteries, and they are usually not designed or used where high energy density is required. Capacitance is the ability of a body to store an electric charge. This capacitance is constant over a given voltage window and can be used to calculate the charge stored using Eq. (1.1), DQ ¼ C DU
(1.1)
where DQ is the charge stored (expressed in coulombs, C) and DU is the width of the voltage window (V). In this case the capacitance, C, is the amount of charge stored when a 1-V window is used. For a given voltage window, there is a direct and simple access to the charge stored. The SI unit of capacitance is farad (symbol: F), named after the English physicist, Michael Faraday. For conventional dielectric capacitors, charges can be stored over a wide voltage window, sometimes reaching several hundred volts. Thus, capacitance was introduced to compare the performance of dielectric materials: the higher the capacitance, the more charge stored within a given voltage. In a conventional dielectric capacitor where a thin dielectric layer separates the two metallic current collectors, the capacitance is proportional to the surface S of the metallic plates and inversely proportional to the thickness of the dielectric, e; the thinner it is, the larger the capacitance (Eq. 1.2). C ¼ ðε0 εr SÞ=e
(1.2)
where ε0 is the electric permittivity of vacuum and εr is the relative permittivity of the given dielectric material. However, thinner dielectric are more susceptible to breakdown. Indeed, the concept of capacitance is accompanied in dielectric materials by that of breakdown voltage. This is characteristic of an insulator that defines the maximum voltage difference that can be applied across a given dielectric material before it collapses and conducts charges. In solid dielectric materials, the breakdown is usually due to the creation of a weakened pathway within the material that enables charge transfer from one electrode to the other. Thus, from relation (1.1), high capacitance values and high breakdown voltage lead to high charge storage. Furthermore, the energy stored in the dielectric capacitor is related to capacitance and cell voltage by relation (1.3): DE ¼ 1=2C ðDUÞ2
(1.3)
Capacitive and Pseudocapacitive Electrodes
3
5
Voltage / V
4 3 2 1 0 2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
Time / s
Figure 1.1 Constant-current charge galvanostatic cycling of a 4.7-nF dielectric capacitor (charge current ¼ 100 nA, discharge current ¼ 100 nA).
Thus, a 1-nF 400-V/DC dielectric capacitor available from any electronic component supplier stores an energy of 80 mJ, i.e., 22 n Wh when translated into units familiar to battery users. Similarly, a 100-mF 6.3-V/DC stores 2 mJ, i.e., 0.55 m Wh. If a constant-current chargeedischarge test is performed between 0 V and a positive voltage value, the voltageetime (or chargeetime) response of the dielectric capacitor will be triangular, as shown, for example, in Fig. 1.1. The capacitance is inversely proportional to the slope DU/Dt (during charge or discharge) according to Eqs. (1.4) and (1.5): DQ ¼ I Dt ¼ C DU
(1.4)
C ¼ I=ðDU=DtÞ
(1.5)
Because Fig. 1.1 clearly shows a linear slope, a constant capacitance can be defined no matter the voltage window used. The cell voltage for a dielectric capacitor can be cycled over a much broader range than accessible with an electrochemical device, but the time scale (microseconds or milliseconds) shows that the amount of charge stored is relatively low, and consequently, energy density is also orders of magnitude lower than that for a battery. This is the main reason why dielectric capacitors are not used to store energy, but instead used as passive components in electronic circuits/devices. Although Fig. 1.1 presents valuable information, it does not identify which kind of dielectric material is involved in the capacitor or the nature of the metallic plates sandwiching this dielectric. This sounds like an obvious remark but many researchers become confused with the concept of two-terminal devices. In most cases, dielectric capacitors do not have a polarity, such that any terminal can be used as the positive polarity and the other one as the negative (the exception being electrolytic capacitors). Dielectric capacitors are based on a very robust technology enabling billions of chargeedischarge cycles without significant fade in their electric properties.
4
1.2.2
Metal Oxides in Supercapacitors
Electrochemical Capacitors
ECs [2,9], also called supercapacitors, ultracapacitors, electric double-layer capacitors, or electrochemical double-layer capacitors, are now commercial devices that are manufactured and sold by many companies all around the world. A wide range of devices are available with capacitance ranging from a few farads to thousands of farads. Unlike dielectric capacitors, the maximum operating voltage of individual cells is usually in the range of 0 V to 2.5e3 V. These two main distinctions from dielectric capacitors, in the maximum operating cell voltage and in the order of magnitude of the capacitance values, suggest that the underlying chargeestorage processes are quite different. Indeed, ECs are typically made of two carbon electrodes, each of them coated on an aluminum current collector. Commonly used materials for commercial ECs mainly consist of various activated carbons. These carbons have a very high specific surface area, typically 1000e2000 m2 g1. They are usually mixed with a carbon additive such as carbon black, which improves the electronic conductivity of the electrode (activated carbons are not inherently good electronic conductors). A polymeric binder is also necessary to provide mechanical stability to the electrode, i.e., between the grains of carbons and also between carbon and the current collector. The two electrodes are sandwiched on either side of a polymeric separator (or a paper), which prevents the electrodes from contacting while also supporting an infiltrated electrolyte solution. Most commercial ECs use tetraethylammonium tetrafluoroborate, ðC2 H5 Þ4Nþ BF4 , as the electrolyte salt, acetonitrile (CH3CN) and propylene carbonate (C4H6O3) being the two most commonly used solvents. Commercial ECs are commonly considered as symmetric devices using two identical electrodes. However, commercially available cells possess a negative and a positive terminal. This is due to the charge storage mechanism that occurs upon polarization of each electrode. Ions from the electrolyte are attracted by each charged carbon surface. Briefly, cations preferentially migrate toward the negatively polarized electrode and anions in the opposite direction toward the positive one, thus giving rise to a double-layer capacitance at each electrodeeelectrolyte interface (Fig. 1.2). Although Electrolyte
Separator
Current Collector (Al)
+
-
Porous carbon electrode
Figure 1.2 (Left) Schematic representation of a double-layer capacitor and (right) a model of the electrochemical double layer. Rreproduced with the kind permission of Patrice Simon and Pierre-Louis Taberna, CIRIMAT, Toulouse, France.
Capacitive and Pseudocapacitive Electrodes
5
it is beyond the scope of this chapter to discuss the mechanistic nuances of double-layer capacitance and their implications for performance, it is important to realize that the charge separation occurs across a narrow distance at the electrode surfaces, typically less than 2 nm. Additionally, unlike the metal plates in a conventional dielectric capacitor, ions carrying charges occupy the extensive porosity of activated carbon grains. Thus, the effective surface area of the capacitor is much higher than the geometric footprint of a given electrode, but rather correspond reasonably well with the specific surface area of the carbon electrode. Considering these two parameters in relation to Eq. (1.2), the very low thickness of the double layer and the large surface area of the carbon electrode, together with the dielectric constant of the electrolyte, give rise to a very high capacitance compared to dielectric capacitors, typically several orders of magnitude more for the same volume of device. Another distinction is that the capacitance of an EC, Ccell, is the result of two capacitances in series, one at the positive electrodeeelectrolyte interface (Cþ) and another at the negative electrodeeelectrolyte interface (C), according to Eq. (1.6): 1=Ccell ¼ 1=Cþ þ 1=C
(1.6)
It can be noted that all the capacitances in Eq. (1.6) are expressed in farad, not gravimetric capacitance, areal capacitance, or volumetric capacitance. Thus, Ccell has to be divided by the mass, surface, or volume of the device to obtain a technologically relevant unit of charge storage performance. Eq. (1.6) does not necessarily reflect the complex charge storage mechanisms that occur at the electrodeeelectrolyte interface; for a more thorough understanding one must consider modern theory involving the influence of pore size distribution and partial desolvation of electrolyte ions when entering the pores [10e13]. Because carbon electrodes do not store anions and cations in the same way, the two electrodes do not have to be necessarily identical in terms of mass loading. Thus it is important to keep the right polarity when connecting the EC to another device. Despite the high capacitance that can be obtained with ECs, they exhibit a cell voltage that is one or two orders of magnitude smaller than the operating voltage of most dielectric capacitors. Thus the gain in energy density for ECs, compared to dielectric capacitors, due to their high capacitance is somewhat mitigated by the low cell voltage. For ECs, the cell voltage is dictated by electrochemical considerations such as electrolyte decomposition on carbon electrodes and subsequent gas evolution reactions, as well as carbon electrode oxidation. Organic electrolytes help achieve a 3 V maximum operating voltage for modern ECs, whereas aqueous electrolytes typically yield a much lower operating voltage. Even though ECs are based on entirely different materials and charge storage mechanisms, the typical constant-current chargeedischarge plot of ECs is similar to that of dielectric capacitors (Fig. 1.3). Eqs. (1.4) and (1.5) still apply, and again one can define a constant capacitance irrespective of the cell voltage used. The main differences between Figs. 1.1 and 1.3 are that the cell voltage is lower for ECs than for dielectric capacitors and the timescale is much higher for ECs, on the order of a few seconds or even minutes.
Metal Oxides in Supercapacitors
Voltage / V
6
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
50
100
150 200 Time / s
250
300
350
Figure 1.3 Constant-current galvanostatic cycling of a 22-F Nichicon electrochemical capacitor (charge current ¼ 1 A, discharge current ¼ 1 A).
According to Eq. (1.3), a 1-F EC over a 3-V voltage window can deliver an energy of 4.5 J (or 1.3 m Wh). A 3000-F cell operated under the same voltage reaches 3.75 Wh. These values are much higher than those calculated for dielectric capacitor, and in this case the stored energy can be used to power a device. It must be noted that the maximum energy density can be determined by operating the EC between 0 V and the positive cell voltage limit indicated by the manufacturer. Operating the capacitor between a negative cell voltage and a positive one does not provide any additional energy (but can severely damage the EC). Thus, studies claiming outstanding energy density by operating a cell between 3V and þ3V are wrong and mislead the reader and the potential users. As for dielectric capacitors, the capacitance and the maximum operating cell voltage of an EC are electric parameters that give pertinent information neither on the electrodes and electrolyte inside the cell nor on the specific charge storage mechanisms involved. Furthermore, the safe electrochemical potential window of a single carbon electrode as well as the specific capacitance of this electrode cannot be deduced from electric parameters listed for the full cell and must be separately determined using particular electrochemical techniques. It can be noted that apart from the capacitance and the rated cell voltage, EC manufacturers also provide the equivalent series resistance of the cell (named ESR, in milliohms), which is of crucial importance for estimating the power density. Similar to dielectric capacitors, ECs can exhibit long-term stability and excellent cycle life (many thousands to millions of cycles) when properly operated within their manufacturers’ specifications.
1.2.3
Secondary (Rechargeable) Batteries
Although it is beyond the scope of this book to detail all the available battery technologies, it is important to focus on the electric parameter usually provided for rechargeable batteries. Unlike capacitors, both dielectric capacitors and ECs, the constant-current chargeedischarge plots of batteries do not exhibit a triangular shape
Capacitive and Pseudocapacitive Electrodes
7
5
Voltage / V
4 3 2 1 0 5
10
15
20
25
30
Time / h
Figure 1.4 Constant-current galvanostatic cycling of a 18650 Li-ion cell (ENIX) between 2 and 3.6 V at C/5 rate (charge current ¼ 300 mA, discharge current ¼ 300 mA).
with linear slopes, but show flat or sloping plateaus at a given voltage or range of voltage, respectively, as shown in Fig. 1.4. Such behavior is typical for faradaic reactions occurring at well-defined potentials at each electrode of the battery. Thus rechargeable batteries typically charge and discharge within narrow voltage windows, which can be quite advantageous when powering an external device that required a specific voltage. As with ECs, a battery has negative and positive terminals that are not interchangeable when powering a device or upon charging. Unlike capacitors, Eqs. (1.3)e(1.5) do not apply to batteries and subsequently a constant capacitance cannot be defined. For example, by considering an ideal battery with a flat discharge plateau, Eq. (1.5), the slope DU/Dt is close to zero and thus the capacitance has an infinite value. At the end of the charge or discharge of a battery, the voltage drastically increases or decreases, respectively (Fig. 1.4). In this voltage window, the slope DU/Dt reaches a very high value, almost infinite, and consequently the capacitance is very small. Thus, it makes no sense to report capacitance as an electric parameter for a battery. Hence, the electric parameters usually provided for a single cell are the average operating voltage and the cell capacity, expressed in C or milliampere hours (mAh). The first approximation is that the energy stored in a cell is the product of the capacity and the average discharge voltage. Unlike ECs, the electrochemical reactions in battery-type electrodes often result in stresses that lead to a steady decay in performance and consequently batteries can only sustain a few thousands of chargee discharge cycles (or less) at 100% nominal depth of discharge.
1.2.4
Asymmetric or Hybrid Device
The past 2 decades have seen the rise of new devices called asymmetric or hybrid capacitors. Multiple versions of these emerging energy storage devices are now commercially
8
Metal Oxides in Supercapacitors
available [14] and, similar to secondary batteries, they exploit a large variety of chemistries for the electrode that cannot be described in great detail here. As a representative example, the lithium-ion capacitor (LIC) is presented in the following [15]. Unlike batteries, LICs include a battery-type electrode (as the negative terminal) and a capacitive electrode (as the positive terminal). More precisely, the negative side is usually a graphite electrode (lithium-ion intercalation compound) and the positive one is activated carbon. On first inspection, the chargeedischarge profile of such hybrid cell, shown in Fig. 1.5A, looks similar to that of an EC. The presence of a capacitive electrode (as the positive electrode in LIC) provides a linear slope to the device when coupled with the flat discharge plot of the faradaic negative electrode. As can be seen from Fig. 1.5B, this slope is the same as that of the capacitive electrode. As the capacitance of a device can be determined from Eq. (1.5), and is inversely proportional to the slope of the discharge plot, DU/Dt, the capacitance of the LIC will be almost the same as the carbon electrode. In an EC, the slope of the full device combines the discharge profile of the two carbon electrodes, resulting in a cell capacitance divided by two compared to a single carbon electrode. Thus, for LICs, (1) a capacitance can be determined, (2) the presence of a battery-type electrode provides a higher capacitance to the LIC than the one of an EC device that uses two capacitive carbon electrodes, (3) the faradaic electrode also determines the minimum voltage at which the cell can be operated, and (4) the maximum operating cell voltage is determined by the faradaic electrode and the electrochemical window in which the capacitive carbon electrode is operated. In summary, the electric parameters of an LIC, and more generally speaking of a hybrid capacitor, can be given as the cell capacitance, the minimum and maximum cell operating voltages, and the ESR. In this example, it can be clearly seen that the electric parameters reflect neither the chemistry nor the electrochemistry of the electrodes. The cycling ability of LICs is usually less than a conventional EC but much better than rechargeable batteries.
(B)
Charge
4.0 Hybrid capacitor -1 157 F g
4 3.5 3 2.5 2 1.5 1 0.5 0
Symmetric capacitor 98 F g-1
0.0
0
100
200
300 400 t/s
500
600
700
Discharge 5.0
AC
Potential (V vs Li/Li+)
U/V
4.5
Cell Graphite
0
100 Time (s)
Cell voltage (V)
(A)
0.0 200
Figure 1.5 (A) Comparison of the galvanostatic chargeedischarge profiles for symmetric activated carbon (AC)/AC electrochemical capacitor and hybrid AC/graphite (SLP-30) hybrid capacitors using 2 mol L1 LiTFSI (lithium bis(trifluoromethanesulfonyl) imide) electrolyte [16]. (B) Schematic galvanostatic chargeedischarge behavior of a lithium-ion capacitor: positive electrode, AC; negative electrode, graphite; electrolyte, LiPF6 in ethylene carbonate diethyl carbonate.
Capacitive and Pseudocapacitive Electrodes
9
For all such devices, it is not possible to discern the electrochemical behavior of a single electrode when only the electric parameters of a device are known. Thus, authors should refrain from calling a device “pseudocapacitive,” a term related to the electrochemical behavior of particular electrode materials, which will be described later in this chapter. The same remark applies for authors applying the term pseudocapacitive to battery-type electrodes inside a hybrid capacitor. In the following, we will focus on the electrodes used in ECs and hybrid capacitors as well as on their electrochemical properties. Contrary to what was presented earlier sections, which mainly focused on commercially available devices, more prospective materials will be discussed in the following section. The term “hybrid” supercapacitor should be preferentially used when pairing two electrodes, one capacitive and one faradaic. The term “asymmetric” supercapacitor encompasses a wider range of electrode combinations because it can be used for supercapacitors using electrodes of the same nature but with different mass loading, or two electrodes using different materials [17e19].
1.3 1.3.1
Electrodes for Electrochemical Capacitors and for Hybrid Capacitors Capacitive Electrodes
A wide variety of carbons have been proposed as capacitive electrodes for ECs [20]. Not all of them are pertinent in terms of capacitance, electronic conductivity, density, specific surface area, pore size distribution, and many other characteristics. The investigation of the carbon electrode itself without the target electrolyte in which it might be operated is useless. Because the capacitance of a single carbon electrode is related to its ability to attract as many ions as possible close to its surface when polarized, the surface accessible to ions should be one of the major parameters for selecting a carbon. This accessible surface is strongly related to the pore size distribution of the carbon. Practically, to obtain a high capacitance, most of the surface measured by gas adsorption method should be available to the electrolyte infused into the pore structure when the material is tested as an electrode or in a device. Even if the porosity is accessible to ions but the tortuosity of the carbon and the electrode is unfavorable to fast ion diffusion, the electrode surface will only be accessible at low cycling rates, which will impact the power capability of the electrode. These important properties can only be investigated by preparing an electrode and testing this electrode in a three-electrode cell, with an adequate reference electrode (e.g., Ag/AgCl in saturated KCl solution, Hg/HgO in KOH) and a high-surface-area counterelectrode. With such a cell, cyclic voltammetry can be used to determine (1) the useful electrochemical potential window of a given carbon electrode (not the voltage window, as too often presented in the literature), (2) the capacitance of the carbon electrode (usually the gravimetric capacitance), and (3) the rate capability of the electrode by varying the potential scan rate. Other electroanalytical methods
10
Metal Oxides in Supercapacitors
may also be used to investigate the behavior and the performance of an electrode, but voltammetry is the most commonly used method in the literature, and thus, we will also use such results for comparison in the discussions of this chapter Interested researchers could also use more powerful techniques, such as electrochemical impedance spectroscopy, as presented elsewhere [21]. Because carbon electrodes are mainly capacitive, i.e., storing charges in the electrochemical double layer, they should exhibit a linear dependence on the width of the investigated potential window with the amount of charge stored. This results in quasi-rectangular cyclic voltammograms (CVs), which means that while scanning the potential at a constant sweeping rate, the measured current is constant. When a potential limit, imposed by the user, is reached, the sweeping direction is reversed and the current should immediately shift to the same current value but of opposite sign. As mentioned earlier, the electrode itself must be correctly prepared to determine its important characteristics. If the electrode is too thick, this will introduce some extrinsic tortuosity, which is not related to the intrinsic tortuosity of the carbon, and then the electrode engineering will become predominant in the electrochemical evaluation of the electrode performance. Thus, low mass loading should be preferred (typically a few micrograms to a few milligrams per square centimeter), i.e., the amount of the investigated carbon per footprint area of the electrode. However, most of the authors who are following this advice also tend to extrapolate the gravimetric values they measure to a device-type carbon electrode, which is definitively not correct [22]. Thus, evaluating the intrinsic performance of a carbon in a three-electrode cell and evaluating the performance of the same carbon in an electrode for a full device are two different things that require different experiments. In the latter case, the electrode preparation is very important and likely requires some additional optimization to determine the most suitable conductive additive, binder, current collector, and electrode porosity when preparing a 100-mm-thick electrode designed for evaluating its performance in a full cell. This engineering process will not be further discussed in the following paragraph but one should keep in mind that the electrode preparation should be suitable to determine the desired parameters. To avoid intrinsic tortuosity, carbons with large geometric surface and low porosity are preferred [20]. Indeed carbons such as zero-dimensional (0D) carbons [carbon quantum dots, onion-like carbons (OLCs)], one-dimensional (1D) carbons (carbon nanotubes or nanofibers), and two-dimensional (2D) carbons (graphene) theoretically provide an easily accessible surface to the ions, thus strongly limiting the problem of tortuosity. OLC-based thin-film electrodes, for example, have demonstrated very high rate capability and they still exhibit a quasi-rectangular shape (Fig. 1.6A) [23]. The same behavior can be found for graphene (Fig. 1.6B) [24] but to a lesser degree because graphene sheets tend to restack, thus rebuilding a porous architecture. Carbide-derived carbons (CDCs) [10,25] have been proposed as three-dimensional (3D) carbon electrodes with tailored porosity that can drastically improve the gravimetric capacitance when used with an adequate supply of electrolyte ions. However, their synthesis requires the use of specific techniques that are not exempt of hazards, such as chlorination of transition metal carbide precursors. 3D carbons such as CDCs (Fig. 1.6C) [25], activated carbons (Fig. 1.6D) [26], and templated mesoporous
Capacitive and Pseudocapacitive Electrodes
(A)
C1350-Tair-H2 C1350-2Tair C1350-Tair C1350
60
11
(B)
40 Capacitance (F/g)
40 20
Capacitance / F g–1
20
0
–20
(a)
0 –20 –1
20 mV s
–40
50 mV s–1 150 mV s–1
–60 –40 0.0
0.5
1.0
1.5
250 mV s–1
–80 –1.0
2.0
–0.8
E(V)
(D)
0.0
0.2
80
(C)
60 40
200
20 C am / F g–1
C m;cv / F g–1
–0.6 –0.4 –0.2 Potential vs. (Hg/HgO) / V
ΔV=3.2 V
100
ΔV=3.0 V
0
ΔV=2.7 V
0 –20 –40 –60
–100
–80 –100
–200
0
2
1 ΔV / V
3
–120 –3
–2
–1 0 E / V vs Ag
1
2
Figure 1.6 Cyclic voltammogram (CV) of different carbon electrodes prepared with (A) cyclic voltammetric curves of onion-like carbons modified by different treatments at scan rate 20 mV s1 using Swagelok assembling (two electrodes) in organic electrolyte [1.5 M NEt4BF4/AN (tetraethylammonium tetrafluoroborate/acetonitrile) [23], (B) graphene nanosheets in KOH aqueous solution (30 wt%) as electrolyte at different cycling rates [24], (C) carbide-derived carbon (derived from Mo2C) symmetric cell at potential scan rate of 1 mV s1 at different cell voltages in 1 M (C2H5)3CH3NBF4 electrolyte in AN [25], and (D) activated carbon electrode in PYR14TFSI electrolyte at 60 C, obtained from CV at 20 mV s1 [26]. inset: capacitance vs. scan rate.
carbons [27] mainly exhibit surface area related to their intrinsic porosity. Their high surface area and corresponding high specific capacitance are technologically interesting, but performance at high rate may be compromised, and their low density (0.8 g cm3) limits volume-normalized capacitance. Most of the carbon investigated as electrodes in ECs are not pure carbon materials, especially with regard to their surface. Indeed, many chemical moieties such as quinoic or carboxylic radicals are usually found by X-ray photoelectron spectroscopic (XPS) analysis. These functional groups provide additional charge storage, which comes in addition to the double-layer capacitance of the carbon. Thus, sometimes CVs deviate
12
Metal Oxides in Supercapacitors
(A)
A 3
T1
(a)
4
2
2
1
0 –2
5 V/s 4 V/s 2 V/s 1.5 V/s 0.5 V/s
–4 –6 –0.2
0.0
0.2
0.4
Potential (V vs. Ag/AgCI)
0.6
j / μA.cm–2
Current density (mA/cm2)
6
(B) a) p-SiNWs b) n-SiNWs a’) p-Si b’) n-Si
0 –1 –2 –3 –4 –1,4
–1,2
–1,0 –0,8 E / V vs Ag+/Ag
–0,6
–0,4
Figure 1.7 Cyclic voltammogram of (A) TiN thin film in 0.5 M K2SO4 electrolyte [28] and (B) silicon nanowires (Si-NWs) in 1 M NEt4BF4 in propylene carbonate (PC) (tetraethylammonium tetrafluoroborate/propylene carbonate) electrolyte [33].
from a quasi-rectangular shape by the presence of bumps assigned to the redox activity of the related moieties. Heteroatoms such as nitrogen or sulfur can also give rise to faradaic contributions, but they can also modify other properties such as the improvement of the electronic conductivity of N-doped carbons. Apart from carbon electrodes, a few materials have been proposed as capacitive electrodes suitable for ECs. Titanium nitride (TiN) thin films have been described as capacitive electrodes [28] (Fig. 1.7A). Because of their good electronic conductivity, they can be used as both active materials and current collectors. However, the lack of in situ or operando experimental results makes it difficult to clearly demonstrate a purely capacitive behavior and the role of titanium cations cannot be definitely ruled out [29]. Silicon-based nanomaterials have also attracted substantial attention owing to their capacitive properties [30], large voltage window [31], and very high cyclability [32] (Fig. 1.7B). Doped silicon is preferred for its higher electronic conductivity. Different morphologies have been proposed, such as bottom-up nanowires [silicon nanowires (Si-NWs)] [33e35] and nanotrees [silicon nanotrees (Si-NTrs)] [32], top-down metal-assisted etched Si-NWs [36] or porous silicon [37e39]. Electrodes based on Si-NWs have also been characterized when coated with silicon carbide (SiC) [40] or gold [38]. In all the cases, the capacitance reported by specific surface area is in the range of a few microfarads per square centimeter and the typical rectangular shape of the related CVs strongly suggests a capacitive behavior similar to that of carbon electrodes. The main interest in capacitive electrodes resides in their ability to store charge at their surface and not in the bulk of the material. Of course, this severely limits the accessible capacity but provides a very long durability to the electrode because it does not suffer from chemical or mechanical changes on cycling. Indeed, when implemented in a full cell, millions of chargeedischarge cycles can be achieved without much fade in capacitance, or significant increase in the ESR.
Capacitive and Pseudocapacitive Electrodes
1.3.2
13
Pseudocapacitive Electrodes
As described earlier, capacitive carbon-based electrodes have many advantages when used as the charge-storing material within ECs: (1) compatibility with organic electrolytes that provide wide voltage windows; (2) a large range of material morphologies (nanotubes, graphene sheets, nanoparticles) that can be selected for either power- or energycentric cell designs; (3) good gravimetric capacitance, particularly with activated carbons; and (4) outstanding long-term cycling ability. However, carbon-based electrodes have nearly reached their limit in terms of specific capacitance. Typical double-layer capacitance at a carbon surface is 15 mF cm2, which is a maximum of z400 F g1 when considering that the theoretical specific surface area of graphene is 2630 m2 g1. Such specific capacitance values have never been reported for practical carbon-based electrodes, especially when organic electrolytes are used. Furthermore, the apparent density of such carbon is usually less than 1 g cm3, which also strongly limits the volumetric energy density of corresponding devices. Ultimately, charge storage at carbon electrodes is constrained by their reliance on double-layer capacitance mechanisms. Some metal oxides, such as RuO2 [4,5] or MnO2 [6,7], have demonstrated much higher gravimetric capacitance than carbon electrodes. Because of their higher solid density than carbon, even their apparent density in practical electrodes, the volumetric capacitance is also much higher than carbon. Although double-layer capacitance also occurs at such materials, many of which are prepared in high-surface-area forms, their main advantage is that charge storage proceeds through fast and reversible redox reactions occurring at the surface and subsurface of the material. These redox reactions typically involve changes in the oxidation state of the metal sites within the oxide. The term “pseudocapacitance” [2] has been proposed to describe the electrochemical behavior of such electrode materials (typically RuO2 or MnO2) because they exhibit the electrochemical signature of a capacitive electrode (Fig. 1.8A and B), i.e., a linear dependence of the charge stored on changing potential within the window of interest. However, charge storage does not originate strictly from capacitive phenomena but rather by electron-transfer mechanisms. The origin of the word “pseudocapacitance” is interesting to trace, constructed by the association of the Greek prefix “pseudo” and the term capacitance. Although this prefix can take two different meanings, the one that applies in this case is “almost, approaching, or trying to be,” as noted in the English dictionary [42]. For example, “pseudomyxoma” in medicine is a gelatinous mass resembling a myxoma but composed of epithelial mucus. Similarly, the term “pseudocapacitance” was created to describe the properties of an electrode that shows the electrochemical signature of a capacitive electrode, but whose charge storage mechanism is primarily faradaic [4e7]. In Conway’s influential book, “Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications,” [2] it is stated, “Regular double-layer capacitance arises from the potential-dependence of the surface density of charges stored electrostatically (i.e., non-Faradaically) at the interfaces of the capacitor electrodes. (.) Pseudocapacitance arises at the electrode surfaces where a completely different charge-storage mechanism applies. It is Faradaic in origin, involving the passage of charge across the double-layer, as in battery charging or discharging, but capacitance arises in account of the special
14
Metal Oxides in Supercapacitors
(A)
(B) Specific Capacitance (F g–1 RuO2)
3
I / mA cm2
2 1 0
–1
1500 1000 500 0 –500
–1000
–2 0.4
0.9 E (SCE) / V
1.4
–1500 0.1
0.2
0.3 0.4 0.5 0.6 0.7 Potential (V vs SCE)
0.8 0.9
Figure 1.8 Cyclic voltammogram of (A) RuO2 electrode at 20 mV sl in 1 M solutions of HCIO4 (dashed line) and KOH (plain line) [4]. (B) Cyclic voltammogram of RuO2 electrode in 0.5 M H2SO4 at 2 mV s1 versus saturated calomel electrode (SCE) as a function of calcination temperature: 100 C [blue], 150 C [red], 200 C [green], 250 C (black), and 300 C [purple] [41].
relation that can originate for thermodynamic reasons between the extent of charge acceptance (Dq) and the change of potential (DV), so that the derivative d(Dq)/ d(DV) or dq/dV, which is equivalent to a capacitance, can be formulated and experimentally measured by dc, ac, or transient techniques. (.).” Since the first claim of pseudocapacitive charge storage for RuO2 electrodes, many materials have been proposed as pseudocapacitive electrodes for ECs. Both pseudocapacitance and battery-like charge storage are due to faradaic processes, which is a source of confusion for many authors in the literature. This misunderstanding arises in part from the similarity between the electric behavior of ECs and hybrid capacitors. It must be emphasized once again that the electric behavior of a given two-electrode device does not presuppose anything about the nature or the electrochemical behavior of the individual electrodes inside the cell. In some cases, material compositions that would normally exhibit battery-type charge storage behavior may appear capacitive when expressed particularly in nanoscale forms. Indeed, nanosized or ultrathin films of certain faradaic electrodes show such pseudocapacitive behavior, but as a consequence of their morphology or electrode construction [43] rather than any intrinsic properties of the material itself. Thus we may distinguish between “intrinsic pseudocapacitance” and “extrinsic pseudocapacitance,” although both types of electrodes still exhibit a capacitive-like electrochemical signature. The two types of behaviors are described in more detail in the following sections.
1.3.2.1
Intrinsic Pseudocapacitance
Intrinsic pseudocapacitance is related to the chemical nature of the material itself and does not depend on its microstructure. Returning to the example of RuO2 and MnO2
Capacitive and Pseudocapacitive Electrodes
15
Figure 1.9 Cyclic voltammograms of (A) an MnO2 thin film in 2 M KCl electrolyte at 25 C with a scan rate of 20 mV s1 [44] and (B) a thick powder-based electrode in 0.1 M K2SO4 at 25 C with a scan rate of 2 mV s1 [45].
electrodes, thin films (100 nm thick) as well as bulk powder-composite electrodes (100 mm thick) have demonstrated exactly the same electrochemical signature, even though the gravimetric capacitance that can be calculated from the CV experiments is usually much larger for thin-film electrodes. Fig. 1.9 compares the CVs of an MnO2 thin film to that of a thick powder-based electrode. The pseudocapacitive behavior of RuO2 or MnO2 electrodes has been investigated by various electrochemical and spectroscopic techniques, sometimes in a coupled fashion, to follow the change in the mean oxidation state of the metal oxide upon cycling the electrode. Fig. 1.10 provides an example of X-ray absorption spectroscopy used in fluorescence mode [44]. This in situ experiment clearly demonstrated a change in the mean oxidation state of Mn from þ3.2 when the electrode was polarized at the lower potential limit of the electrochemical window [0.0 V vs. saturated calomel electrode (SCE)] up to þ3.95 when polarized to þ1.0 V versus SCE. Despite some hysteresis, the process is largely reversible. The same conclusions were reached by another group using in situ Mn K-edge X-ray absorption spectroscopic studies of electrodeposited manganese dioxide thin films [46]. Other techniques such as XPS [47] also led to the same conclusions for MnO2 powderdpseudocapacitance is due to fast and reversible faradaic reactions that involve the electrochemical interconversion between Mn4þand Mn3þ in the solid and the concomitant insertion of cations, Hþ or Cþ (Cþ ¼ Naþ, Kþ, .), from the electrolyte [44,46,48]. Similar studies have been performed on RuO2 electrodes. In this case, proton insertion was ascribed as the main cause of pseudocapacitance, concomitant with a change in Ru oxidation state over the potential window [49,50]. The electrochemical behavior during both charge and discharge of MnO2-based ECs has been analyzed and a 1D model adapted from the transmission line model was proposed, taking into account partial cation diffusion in the solid oxide [51]. A linear relationship between the variation of the mean Mn oxidation state and the
16
Metal Oxides in Supercapacitors
Figure 1.10 (A) Schematic diagram of the spectroelectrochemical cell for in situ X-ray absorption spectroscopic study in the fluorescence mode, and (B) dependence of the Mn oxidation state and the E0 on the applied potential during the electrochemical redox cycle [44].
potential on cycling was used to implement the model (Fig. 1.11A), as experimentally established through previous in situ XPS experiments. As depicted in Fig. 1.11B, the CVs simulated from the model fit well with the experimental CV.
1.3.2.2
Extrinsic Pseudocapacitance
The concept of extrinsic pseudocapacitance can be used to describe materials that exhibit capacitor-like behavior, but only when expressed in particular morphologies
Capacitive and Pseudocapacitive Electrodes
(A)
17
1.0
(B)
0.6 OCP
2
0.4 0.2 0.0 3.75
20 mV.s–1
3
I / A g–1
Eeq vs (Ag/AgCI)/V
0.8
1 0 –1 –2
3.80
3.85
3.90
3.95
–3 –0.1
–0.2 Mn oxidation state
0.1
0.3
0.5
0.7
0.9
U/V
Figure 1.11 (A) Relation between the equilibrium potential of the couple Mn3þ/Mn4þ and its oxidation state, and (B) cyclic voltammograms, as measured (solid line) and as computed (dashed line). Potential is reported versus Ag/AgCl electrode Ref. [51]. OCP, open circuit potential.
(ultrathin films) or nanoscale forms, for example, when the electrode material (e.g., powder, thin film) reaches a critical size [52,53] where diffusion occurs through very limited timescales. But in such cases, capacitorlike behavior is only due to electrode design or architecture and not an intrinsic property of the parent material, as observed with LiCoO2 thin films. LiCoO2 is known as a lithium-intercalation compound that is commonly used as positive electrode in lithium-ion batteries and does not show “pseudocapacitive” behavior in such devices. Yet when synthesized in thin-film form (6 nm thick)[53], LiCoO2 yields a capacitorlike response (Fig. 1.12). MnO2
0.0
nanoscale
bulk
0
I Q/Qmax
LiCoO2
4.2
bulk
Potential (V vs Li/Li+)
Potenal (V vs Ag/AgCl)
0.9
1
3.0
nanoscale
0
I Q/Qmax
1
Figure 1.12 Schematic constant-current galvanostatic discharge of an intrinsic pseudocapacitive electrode and an extrinsic one in the case of a bulk electrode and a nanoscale electrode. Q and Qmax are the charge stored and the maximum charge that can be stored, respectively. Adapted from P. Simon, Y. Gogotsi, B. Dunn, Materials science. Where do batteries end and supercapacitors begin? Science 343 (2014) 1210e1211.
18
Metal Oxides in Supercapacitors
Dunn et al. denoted that such behavior is “extrinsic” pseudocapacitance, as opposed to MnO2, which exhibits capacitive behavior whether in thin-film or thick powdercomposite electrodes and is thus intrinsically pseudocapacitive (Fig. 1.12) [53].
1.3.3
High-Power Battery Electrodes
The electric behavior of a given device does not presume the processes that occur at each electrode inside the device, for example, whether the individual electrodes are capacitive, pseudocapacitive, faradaic, or some combination thereof in nature. Thus, some confusion may arise when considering the so-called asymmetric or hybrid devices that have been discussed in the literature and are also commercially available. Such devices are designed with a battery-type negative [e.g., graphite, Li4Ti5O12, TiO2(B), .] [15,16,54e56] or positive electrode [e.g., PbO2, Ni(OH)2, .] [57,58] and a complementary capacitive electrode, typically activated carbon. These devices exhibit interesting energy and power densities compared to standard ECs (Fig. 1.13). The first report of an energy storage device that combined an electric double-layer capacitor negative electrode with a positive nickel-oxide battery electrode was a patent by Varakin et al. in the mid 90s [59]. The chargeedischarge plots of such
Figure 1.13 Ragone plots of hybrid capacitor systems [Li4Ti5O12/carbon nanofiber (CNF) nanocomposite/activated carbon] and conventional electric double-layer capacity or system (activated carbon/activated carbon). The hybrid capacitor systems were assembled using two types of the composites with weight ratio of Li4Ti5O12/CNFs ¼ 50/50 or 70/30. The power and energy densities were calculated on the basis of the electrode volume. inset: schematic illustration for the two-step formation procedure of the Li4Ti5O12/CNFs nanocomposite [55].
Capacitive and Pseudocapacitive Electrodes
19
devices appear capacitive as a combination of a capacitive electrode (triangular shape) and a faradaic electrode (plateau shape). Activated carbon/Ni(OH)2 hybrid devices have been extensively investigated, and calculations of projected energy densities are as high as 50 Wh kg1 based on a 1.65 V working voltage in 6.25 M KOH [57,58], i.e., six to seven times higher than an electric double-layer capacitor operated at 3 V in a standard organic electrolyte. Although the energy density of these hybrid devices is promising, the power capability is strongly dependent on the kinetics of charge storage at the battery-type electrode. Thus, many investigations have focused on improving the electronic conductivity and ionic transport within such electrodes tuning their nano-/micro-/macroarchitecture. Several strategies have been proposed to circumvent power/rate limitations, focusing on (1) new synthesis routes for Ni(OH)2 nanoparticles [60] and (2) fabrication of composite materials comprising Ni(OH)2 particles that are incorporated with a conductive carbonaceous material such as activated carbon [61] nanotubes or graphene sheets [62,63] to improve electronic conductivity within the electrode.
1.3.4
Capacity Versus Capacitance
Despite historic precedence for a working definition of “pseudocapacitance,” many faradaic electrodes based on Ni or cobalt-based oxides, hydroxides, or even sulfides [64e68] or composite materials involving these compounds [69,70] have been presented in the literature as pseudocapacitive materials; however, these materials exhibit well-defined redox peaks when examined by voltammetry (Fig. 1.14). This misunderstanding confuses the readers because the concept of “capacitance” (F) cannot apply to faradaic behavior; the metric of “capacity” [(coulomb, C, or milliampere hours (mAh)] is the most appropriate and meaningful option in such cases. Indeed, a constant capacitance cannot be determined from the CVs presented in Fig. 1.14 because they are not rectangular and correspondingly have no linear dependence of the charge stored on changing potential within the electrochemical window of interest. Misleading
(A)
CoS NiS Co1.5Ni1.5S4
40
(B) 6 Current density (Ag–1)
Current density / A g–1
30 20 10 0 –10
4
Ni(OH)2 nanosheets Ni(OH)2-GS composite Ni(OH)2-GS-CNT composite
2 0 –2
–20 –30
–4 0.0
0.1
0.2 0.3 0.4 Potential / V vs.HgO/Hg
0.5
0.6
–0.1 0.0 0.1 0.2 0.3 0.4 0.5 Potential vs. Hg/HgO (V)
0.6
Figure 1.14 Cyclic voltammograms of (A) CoS, NiS, and Co1.5Ni1.5S4 electrodes at 5 mV s1 in 2 M KOH and (B) Ni(OH)2 [68] and different Ni(OH)2/carbon nanocomposites at 5 mV s1 in 6 M KOH that exhibit pure faradaic behavior [70].
20
Metal Oxides in Supercapacitors
comments when describing such CVs, for example, “A pair of redox peaks can be observed in each CV curves which indicates a pseudocapacitance characteristic,” [64] must not be used in the scientific literature. Furthermore, a representative example of the misuse of capacitance is presented. Park et al. [61] stated that, “In order to enhance energy density, a hybrid type pseudocapacitor/electric double-layer capacitor (EDLC) was considered and its electrochemical properties were investigated. At various current densities, stable charge/discharge behaviors were observed with much higher specific capacitance values of 530 F g1 compared with that of EDLC (230 F g1), by introducing Ni(OH)2 as a cathode material.” As detailed earlier, it is not appropriate to compare these two values of capacitance because the value of 530 F g1 was determined over a limited potential window and corresponds to an “average” value. For example, if a wider or narrower potential window is arbitrarily chosen, the calculated specific capacitance could decrease or increase, respectively. For an electrode material such as Ni(OH)2, only the capacity in coulomb per gram (C g1) or milliampere hours per gram (mAh g1) provides a useful metric to be used for comparison against other materials [3,71]. Furthermore, charge storage properties could be compared with those of carbon electrode by transforming the constant capacitance of the carbon electrode into C g1 using the width of the potential window over which the device is cycled. Thus a direct comparison of the total charge stored at each electrode can be made. Similar treatment can be applied to an MnO2 pseudocapacitive electrode, which also exhibits a rectangular CV. The signature of a faradaic electrode such as Ni(OH)2 is entirely different.
1.4
Conclusions
The electrochemical behavior of faradaic electrodes are fundamentally different from that of pseudocapacitive electrodes. We propose to use the term “pseudocapacitive” only to describe electrode materials, such as MnO2, that display an electrochemical behavior typical of that observed for a capacitive carbon electrode. Using the same term for materials such as Ni(OH)2 or cobalt oxides that exhibit an electrochemical signature of a “battery” electrode should be avoided. Obviously, many EC-relevant materials display more complex behaviors and exhibit both mechanisms with a pseudocapacitive contribution coming from the surface properties and faradaic contribution coming from intercalation mechanisms, for example, as is the case for birnessite-type MnO2 [48,72]. Such materials must be described in such a way that the readers can understand which mechanisms are involved. Additionally, many references to pseudocapacitive materials can be found in Conway’s book [2], and new pseudocapacitive materials, such as FeWO4, are also proposed in the recent literature [73]. These include hydrous oxides such as RuO2 underpotential-deposition reactions, intercalation in TiS2, and conversion of an oxidized species to a reduced species in a redox system in solution. These three examples obviously do not match the definition given later on in the book: “pseudocapacitance arises when the extent of reaction, Q, is some continuous function of potential, V, so that the derivative, dQ/dV, arises that has the properties of a capacitance.” We hope our explanations will shed some light on this topic and help authors to correctly address the description of their electrodes.
Capacitive and Pseudocapacitive Electrodes
21
Acknowledgments The authors would like to acknowledge Anne-Lise Brisse, Dr. Annaïg Le Comte, Dr. Christophe Aucher, and Alban Morel. J.W. Long acknowledges the U.S. Office of Naval Research.
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Metal Oxides in Supercapacitors
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