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Energy Storage in Nanomaterials − Capacitive, Pseudocapacitive, or Battery-like? discharge rate, v (V/s): i = Cv. The units of the capacitance, C, are Farads. The sum of these two charge-storage components comprises the “non-insertion capacitance” of the material, C. This term is meant to convey that the slow, diffusion-controlled motion of charge compensating ions is not required to access this capacity. The total charge stored by this mechanism, Q = CΔE (in Coulombs), where ΔE is the width of the potential window over which these two charge processes are operating. It is important to remember that the above equations are only applicable to materials with a capacitor-like response that show rectangular cyclic voltammograms (Figure 1a,b) and a linear voltage response (a triangular-shaped curve) during constantcurrent charging/discharging (Figure 1c). Energy storage involving pseudocapacitance occupies a middle ground between electrical double-layer capacitors (EDLCs) that store energy purely in the double-layer on a high surface area conductor and batteries, which rely predominantly on Faradaic electron transfer to metal centers (usually) that is made possible by the intercalation of chargecompensating ions, such as Li+ or Na+. Current versus voltage curves (Figure 1) provide a means for categorizing the mode of charge storage. In response to potential scanning, EDLCs usually show a potential-independent capacitance and, therefore, potential-independent current (Figure 1a,b). Batteries, on the other hand, show prominent and widely separated peaks associated with the reduction and oxidation of the metal centers involved in charge storage (Figure 1g,h). Constant current discharge of a EDLC results in a linear E versus t plot (Figure 1c), whereas E versus t for the discharge of a battery is profoundly nonlinear and characterized by plateaus of nearly constant potential corresponding to the potentials at which the Faradaic reduction or oxidation of the metal centers is occurring (Figure 1i). Behavior intermediate between these two extremes signals the presence of pseudocapacitance (Figure 1d−f). Missing from the description provided by Figure 1 is the time scale involved in these processes and the magnitude of the stored charge. Solid-state and electrolytic capacitors can charge and discharge on the microsecond to millisecond time scales. Applications for capacitors require this speed. They are used, for example, to smooth the output of half-wave rectifiers that convert alternating current at 120 Hz (ripple frequency doubles if the supply frequency is 60 Hz) to direct current. Supercapacitors and pseudocapacitors can typically be charged/discharged in less than a second to a minute. The corresponding time scales for batteries, capable of storing orders of magnitude more charge, are measured in minutes or hours. By exploiting pseudocapacitance, the charge-storage capacity of EDLCs can be enhanced, and the power of batteries can be
n electrical energy storage science, “nano” is big and getting bigger. One indicator of this increasing importance is the rapidly growing number of manuscripts received and papers published by ACS Nano in the general area of energy, a category dominated by electrical energy storage. In 2007, ACS Nano’s first year, articles involving energy and fuels accounted for just 1.6% of the journal’s 64 papers published (we published just one paper!), whereas in 2017, the fraction was over 10% of the 1296 publications (149 papers). Moreover, 6 of the 10 most-cited papers published in ACS Nano between 2013 and 2017 deal with energy-related topics. Among other impacts, “nano” has enabled electrically insulating pseudocapacitive materials, like transition-metal oxides, to be used more efficiently in both batteries and capacitors.1 There are increasing numbers of new electrode materials (e.g., transition−metal oxides, hydroxides, sulfides, carbides, nitrides, conducting polymers, etc.) that display electrochemical characteristics that are neither purely capacitive nor purely Faradaic. The introduction of these new materials has contributed to blurring of the distinctions between these two fundamentally different energy-storage modalities, leading to confusion for both readers and authors. We are not the only ones grappling with this issue. Recent papers quantitatively discuss the differences between true electrochemical capacitors, pseudocapacitors, and batteries.2,3 The purpose of this editorial is to sharpen the distinction using a short list of criteria already outlined in these papers,2,3 so that we are all speaking the same language.
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There are increasing numbers of new electrode materials (e.g., transitionmetal oxides, hydroxides, sulfides, carbides, nitrides, conducting polymers, etc.) that display electrochemical characteristics that are neither purely capacitive nor purely Faradaic. Pseudocapacitive materials such as RuO2 and MnO2 are capable of storing charge two ways: (1) via Faradaic electron transfer, by accessing two or more redox states of the metal centers in these oxides (e.g., Mn(III) and Mn(IV)) and (2) via non-Faradaic charge storage in the electrical double layer present at the surfaces of these materials. The metal centers that contribute to the Faradaic pseudocapacitance are located near the surface of the oxide, at a distance, l ≪ (2Dt)1/2, where D is the diffusion coefficient for charge-compensating ions (cm2/s), and t is time (s); the Faradaic reaction is electrochemically indistinguishable from the non-Faradaic reaction. That is, both of these processes are characterized by a current, i (A), that is directly proportional to the charge/ © 2018 American Chemical Society
Published: March 27, 2018 2081
DOI: 10.1021/acsnano.8b01914 ACS Nano 2018, 12, 2081−2083
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Cite This: ACS Nano 2018, 12, 2081−2083
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Figure 1. (a, b, d, e, g, h) Schematic cyclic voltammograms and (c, f, i) corresponding galvanostatic discharge curves for various kinds of energy-storage materials. A pseudocapacitive material will generally have the electrochemical characteristics of one, or a combination, of the following categories: (b) surface redox materials (e.g., MnO2 in neutral, aqueous media), (d) intercalation-type materials (e.g., lithium insertion in Nb2O5 in organic electrolytes), or (e) intercalation-type materials showing broad but electrochemically reversible redox peaks (e.g., Ti3C2 in acidic, aqueous electrolytes). Electrochemical responses in (g−i) correspond to battery-like materials.
elevated. “Nano” enters the discussion here.1 As the critical dimensions of energy-storage materials are reduced to the nanoscale, diffusion path lengths for ions are reduced, and surface areas available for non-insertion charge storage are dramatically enhanced. These two effects conspire to increase the pseudocapacitive character of virtually any battery materiala phenomenon called “extrinsic” pseudocapacitance.3 Using pseudocapacitance as an excuse to conflate the concepts of capacitors and batteries risks confusion. An EDLC having enhanced capacity as a consequence of the inclusion of pseudocapacitive materials will begin to exhibit a battery-like current−voltage relationship (Figure 1g) and, in many cases, reduced speed and power. It may not be useful as a capacitor or as a battery. Increasing of the power of a battery electrode by increasing its electrochemically active surface area will only widen its separated peaks (current versus voltage behavior in Figure 1g), or its distinctive potential versus time behavior (Figure 1i). It is still distinctively a battery, not a capacitor. Simon et al. suggested that the term “oxide (or nitride, carbide, etc.) supercapacitor” be applied to describe devices exploiting pseudocapacitance for capacitive energy storage.3 This label conveys that Faradaic electron transfer is involved in charge storage, clarifying a discussion of the properties of a particular device. At ACS Nano, we recommend this practice to authors. The first question a researcher should ask when doing electrochemical data analyses on a new nanomaterial is whether the material is battery-like or capacitor-like. Any material with cyclic voltammograms containing intense, clearly separated oxidative and reductive peaks (Figure 1g,h), or constant-current charge/discharge curves with obvious plateaus (Figure 1i), should be categorized as a battery-type electrode. On the other
end of the spectrum, materials with a capacitor-like response will show rectangular voltammograms (Figure 1a) and linear voltage responses during constant-current discharging (Figure 1c). The peak current (i) response of a battery-type electrode will be proportional to the square root of the scanning rate (i ∼ v1/2), whereas a capacitor-type material will show linear current response dependency on the scan rate (i ∼ v). With this delineation in mind, it should be clear why materials with the electrochemical response shown in Figure 1g (typical of nickel, cobalt, and iron compounds in basic electrolytes) should be classified as battery-type materials.
The first question a researcher should ask when doing electrochemical data analyses on a new nanomaterial is whether the material is battery-like or capacitor-like. Once an electrode material has been categorized, researchers should look to quantify and to report on the total charge, Q, stored and delivered by the material. This charge is calculated by Q = ∫ i × t. If the electrode material falls into the supercapacitor/pseudocapacitor category, it is necessary to convert this charge into capacitance (C in Farads), as described earlier. For battery-type materials having a plateau during the charging/discharging (Figure 1i), the definition of capacitance does not apply. Instead, the capacity (Q/3.6, mAh) should be calculated. Regardless of the nature of the material in question, the values reported should be normalized to the mass, area, or volume of the electrode for comparison to literature reports. It 2082
DOI: 10.1021/acsnano.8b01914 ACS Nano 2018, 12, 2081−2083
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is important to stress that calculating the energy density of an electrode or device showing battery-like behavior requires integrating the cell voltage versus capacity curve and normalizing it by the mass or volume of active material. Graphically, it is the area under the voltage versus specific capacity curve. By taking a ratio of the areas under the discharge and charge curves, one can calculate the energy efficiency of the deviceit should be close to 100% for a supercapacitor. Calculating the energy density of battery-like materials (e.g., nickel, cobalt, and iron compounds) by using the equation derived for capacitors leads to greatly (often, by an order of magnitude) overestimated energy density values that can be found on Ragone plots in many publications. In view of the above discussion, the ability to decompose the total current into contributions from Faradaic pseudocapacitance and double-layer capacitance is critically important in terms of providing an understanding of the charge-storage mechanisms that are operating. A recent comparison of three methods for carrying out this decomposition has been discussed by Forghani and Donne.4 Although a detailed review of these methods is beyond the scope of this editorial, the conclusion of their study is that step potential electrochemical spectroscopy, or SPECS, can accurately carry out this deconvolution as a function of the electrochemical potential, providing a measure of the fraction of the measured current associated with pseudocapacitive processes at every potential.4 We hope and expect to see the SPECS method used more frequently in investigations of pseudocapacitive battery and capacitor materials.
Editorial
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] *E-mail:
[email protected] ORCID
Yury Gogotsi: 0000-0001-9423-4032 Reginald M. Penner: 0000-0003-2831-3028 Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.
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ACKNOWLEDGMENTS We are grateful to Ph.D. students and postdocs of Yury GogotsiTyler S. Mathis, Dr. Xuehang Wang, Dr. Narendra Kurra, and Dr. David Pintofor providing the illustration and contributing to the discussion of best practices for data analysis presented in this editorial.
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REFERENCES
(1) Gogotsi, Y. What Nano Can Do for Energy Storage? ACS Nano 2014, 8, 5369−5371. (2) Brousse, T.; Belanger, D.; Long, J. W. To Be or Not To Be Pseudocapacitive. J. Electrochem. Soc. 2015, 162, A5185−A5189. (3) Simon, P.; Gogotsi, Y.; Dunn, B. Where Do Batteries End and Supercapacitors Begin? Science 2014, 343, 1210−1211. (4) Forghani, M.; Donne, S. W. Method Comparison for Deconvoluting Capacitive and Pseudo-Capacitive Contributions to Electrochemical Capacitor Electrode Behavior. J. Electrochem. Soc. 2018, 165, A664−A673.
The ability to decompose the total current into contributions from Faradaic pseudocapacitance and doublelayer capacitance is critically important in terms of providing an understanding of the charge-storage mechanisms that are operating.
Over the course of this editorial, we emphasized the need for the electrochemical energy storage field to be united in its stance with material characterization and the reporting of material performance metrics. This discussion is by no means exhaustive but is meant to guide researchers toward conducting electrochemical analysis based on the energy-storage mechanisms of emerging nanomaterials, which often do not fit simple “battery” or “capacitor” definitions.
Yury Gogotsi,* Associate Editor
Reginald M. Penner,* Associate Editor 2083
DOI: 10.1021/acsnano.8b01914 ACS Nano 2018, 12, 2081−2083
