(5). This pseudocapacitance represents a second mechanism for capacitive energy storage. The most widely known pseudocapacitors are RuO2 and MnO2; recently this list has
expanded to other oxides, as well as nitrides
and carbides, as different pseudocapacitance
mechanisms have been identified (6). Pseudocapacitive materials hold the promise of
achieving battery-level energy density combined with the cycle life and power density
of EDLCs. To avoid further confusion with
EDLCs, we propose that these materials be
called oxide supercapacitors (nitride, carbide,
etc.) to recognize that a substantial fraction
of the charge storage arises from redox reactions. The use of this terminology requires
identifying the charge storage mechanism,
rather than basing the claim on the material
A second feature that blurs the distinction
between batteries and supercapacitors is how
their response changes when nanoscale mate-
rials are used. When battery materials are pre-
pared in nanoscale forms, their power density
increases because of the short transport paths
for ions and electrons (7). However, increased
power density does not necessarily transform
nanoscale materials into oxide supercapaci-
tors because their faradaic redox peaks and
galvanostatic profiles remain battery-like
(see the figure, panels F and H). At smaller
dimensions (<10 nm), there are indications
that traditional battery materials exhibit
capacitor-like properties [e.g., LiCoO2 shown
in panel H of the figure (8); V2O5 may behave
in a similar fashion (9)]. “Extrinsic” pseudo-
capacitance can emerge when a battery mate-
rial is engineered at the nanoscale so that a
large fraction of Li+ storage sites are on the
surface or near-surface region.
Pronounced redox peaks in the voltammetry can be an indication of pseudocapacitance, provided the peak voltage differences
are small and remain so with increasing sweep
rate (5). The kinetic information obtained
from sweep voltammetry can also be used.
For a redox reaction limited by semi-infinite
diffusion, the peak current i varies as v1/2; for
a capacitive process, it varies as v. This relation is expressed as i = avb (10), with the value
of b providing insight regarding the charge
storage mechanism. Over a wide range of
sweep rates v, the well-known battery material LiFePO4 has b ≈ 0.5, whereas b ≈ 1.0 for
the pseudocapacitor material Nb2O5 (6, 11).
In addition to diffusion-controlled behavior,
low Coulombic efficiency and sluggish kinetics are indications that the material is not a
supercapacitor. Thus, an electrode material
or a device with well-separated redox peaks
(panel F) and a discharge curve similar to the
upper curve in panel H should not be considered a supercapacitor.
There is nothing inappropriate in using
nanostructured battery materials in symmetric electrochemical cells or combined with
a capacitive electrode (carbon) to make a
hybrid energy storage device. However, it is
misleading to test such a material or device
at a low rate (for a supercapacitor, at least)
and claim that it is a “high–
energy density supercapacitor.” Additionally, the
use of low weight loadings or thin films of nanostructured battery materials leads to devices with
and limited cycle life (12).
If the materials are to be
considered for high-power
devices, they need to be
evaluated at the rates where
supercapacitor devices are
used (e.g., fully recharged
in 1 min, referred to as a
rate of 60C).
The prospect of developing materials with the
energy density of batteries
and the power density and
cycle life of supercapacitors is an exciting direction
that has yet to be realized.
Whether to approach these
goals by increasing the
power density of battery materials or increasing the energy density of supercapacitors is
one of the enticing features of the field. However, there needs to be clarity in the terminology used in combination with appropriate
measurements and analyses. Proper evaluation of new materials and their charge storage
mechanisms will facilitate progress in this
important field of electrical energy storage.
References and Notes
1. J. R. Miller, P. Simon, Science 321, 651 (2008).
2. F. Beguin, E. Frackowiak, Ed., Supercapacitors: Materials,
Systems and Applications (Wiley-VCH, Weinheim, Germany, 2013).
3. P. Simon, Y. Gogotsi, Acc. Chem. Res. 46, 1094 (2013).
4. B. E. Conway, J. Electrochem. Soc. 138, 1539 (1991).
5. B. E. Conway, Electrochemical Supercapacitors: Scientific
Fundamentals and Technological Applications (Springer,
New York, 1999).
6. V. Augustyn et al., Nat. Mater. 12, 518 (2013).
7. A. S. Aricò, P. Bruce, B. Scrosati, J. M. Tarascon, W. van
Schalkwijk, Nat. Mater. 4, 366 (2005).
8. M. Okubo et al., J. Am. Chem. Soc. 129, 7444 (2007).
9. M. Sathiya, A. S. Prakash, K. Ramesha, J.-M. Tarascon, A.
K. Shukla, J. Am. Chem. Soc. 133, 16291 (2011).
10. H. Lindström et al., J. Phys. Chem. B 101, 7717 (1997).
11. J. Come, P. L. Taberna, S. Hamelet, C. Masquelier, P.
Simon, J. Electrochem. Soc. 158, A1090 (2011).
12. Y. Gogotsi, P. Simon, Science 334, 917 (2011).
13. A. Boisset et al., J. Phys. Chem. C 117, 7408 (2013).
14. W. Shimizu, S. Makino, K. Takahashi, N. Imanishi, W.
Sugimoto, J. Power Sources 241, 572 (2013).
Acknowledgments: Supported by the FIRST ( Y. G.) and
MEEM (B. D.) Energy Frontier Research Centers (funded by the
Office of Basic Energy Sciences, U.S. Department of Energy)
and by European Research Council grant ERC-2011-AdG
Li+ Li+ Li+
Li+ Li+ Li+
Comparing batteries and supercapacitors. (A to D) The different mechanisms of capacitive energy storage are illustrated. Double-layer capacitance develops at electrodes comprising (A) carbon particles or (B) porous carbon. The double layer shown here
arises from adsorption of negative ions from the electrolyte on the positively charged electrode. Pseudocapacitive mechanisms
include (C) redox pseudocapacitance, as occurs in hydrous RuO2, and (D) intercalation pseudocapacitance, where Li+ ions are
inserted into the host material. (E to H) Electrochemical characteristics distinguish capacitor and battery materials. Cyclic voltammograms distinguish a capacitor material where the response to a linear change in potential is a constant current (E), as compared
to a battery material, which exhibits faradaic redox peaks (F). Galvanostatic discharge behavior (where Q is charge) for a MnO2 pseudocapacitor is linear for both bulk and nanoscale material (G) (13, 14), but a LiCoO2 nanoscale material exhibits a linear response
while the bulk material shows a voltage plateau (H) (8).