(106b) Investigation of Charge Storage Mechanism of Nanostructured Metal Nitrides and Carbides Electrodes for Electrochemical Capacitors

     Electrochemical capacitors (ECs) offer a
unique combination of high power and energy densities filling the gap between
conventional capacitors and batteries. These devices store energy via double
layer (i.e. electrostatic mechanism) and/or pseudo-capacitance mechanisms that
involve charge transfer processes [1]. Early transition metal nitrides and
carbides are promising candidates for use as electrode materials due to their
high electronic conductivities, surface areas (up to 200 m²/g) and
electrochemical stabilities [2,3]. For example, vanadium nitride has been
reported to have the highest capacitance up to 1340 Fg^-1 [4].
However the charge storage mechanism for these materials is not well
understood. In this paper we will report results from ion isolation experiments,
impedance spectroscopy, charge-discharge and chronotentiometry and suggest charge
storage mechanisms that reconcile the results.

     Nanostructured V, Mo, W nitrides and
carbides were synthesized via temperature-programmed-reaction of their oxide
precursors with anhydrous NH₃ or 15% CH₄/H₂
followed by passivation in 1% O₂/He at room temperature to form a
oxygen-rich passivation layer preventing bulk oxidation on exposure to air
[2,3]. Physical characterization was performed using BET surface area analysis,
scanning electron microscopy and X-ray diffraction.  The stability of these materials in aqueous KOH and H₂SO₄
was determined by cyclic voltammetry. The species contributing to charge
storage were identified by isolation of the electrolyte ions and pairing them
with inactive counter ions. H⁺ and SO₄^2-, were
isolated as H⁺BF₄^- (tetrafluoroboric acid) and
[(C₂H₅)₄N⁺]₂ SO₄^2-(tetraethylammonium
sulfate, (TEA)₂SO₄), while K⁺ and OH^-
ions were isolated as K⁺(CF₃SO₃)^-
(potassium triflate, K-triflate) and (C₂H₅)₄N⁺OH^-
(tetraethylammonium hydroxide, TEA-OH). Cyclic voltammetry experiments were
carried out in each of these electrolytes. In order to establish the charge
transfer reaction chronopotentiometry experiments were carried out in varying
electrolyte concentrations.

     The redox behavior
confirmed that most of the charge storage was via the pseudocapacitance
mechanism. The ion isolation experiments indicated that OH^- and H⁺
were the primary species contributing to charge storage in VN in KOH (figure 1)
and Mo₂N in H₂SO₄ systems, respectively. The
chronopotentiometry results (figure 2) suggest that the charge storage reaction
for VN was:

Results from impedance
spectroscopy and three-electrode charge-discharge experiments will also be
presented

REFERENCES

[1] Simon
P, Burke A, The Electrochemical
Society: Interface, Spring
(2008) 38.

[2] Cladridge J B, York A P E,
Brungs A J, Green Malcolm L H, Chem. Mater.12
(2000) 132.

[3] Wixom
M R, Tarnowski D J, Parker J M, Lee J Q, Chen P -L, Song I, Thompson L T, Mat.
Res. Soc. Symp. Proc. 496 (1998) 643.

[4] Choi D,  Kumta P N, Electrochem. Solid-State
Lett. 8 8 (2005) A418.

Figure 1:
Cyclic voltammogram for VN in various electrolyte solutions at a scan rate of
2mVs^-1.

Figure 2: pOH vs
Voltage for VN in KOH. Slope of 0.096 indicates that there are 2 OH^-
ions reacting per electron transferred.