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.