(192g) Pretreatment Effects On Charge Storage of Early Transition-Metal Carbides and Nitrides

Pretreatment
Effects on Charge Storage of Early Transition-Metal Carbides and
Nitrides

Abdoulaye Djire^a, Priyanka
Pande^a , Aniruddha Deb^b,Alice E. S.
Sleightholme^a, Paul Rasmussen^ac, James Penner-Hahn^b,
Levi T Thompson*^ac

^a Department of Chemical
Engineering

^b Department of Chemistry

^c Hydrogen Energy Technology
Laboratory

University of Michigan, Ann Arbor, MI_- 48109-2130

E-mail: ltt@umich.edu

Early transition-metal carbides and
nitrides are promising candidates for use in supercapacitor electrodes due to
their high electronic conductivities, high surface areas (can exceed 200 m²/g),
good electrochemical stabilities and high capacitance [1,2]. For example, the
capacitance for VN has been reported to be as high as 1340 Fg^-1 in
aqueous KOH [3]. This high capacitance has been attributed to pseudocapacitive
charge storage involving fast near-surface redox reactions. We observed that
pretreatment of the surface to remove the passivation
layer significantly improved the capacitances and electrochemical
stabilities of early transition-metal carbides and nitrides in aqueous
electrolytes. In this paper we present results from electrochemical
characterization including cyclic voltammetry (CV), chronopotentiometry, and
electrochemical impedance spectroscopy (EIS), and surface characterization
using x-ray photoelectron spectroscopy (XPS) to explain changes in surface
chemistry and electrochemical properties caused by pretreatment. 

The early transition-metal carbides and
nitrides of V, W and Mo were synthesized via temperature-programmed-reaction of
their oxide precursors with 15% CH₄/H₂ or anhydrous NH₃
followed by passivation in 1% O₂/He at room temperature to form a
oxygen-rich passivation layer preventing bulk oxidation on exposure to air [1].
Characterization of the structural properties was performed using nitrogen
physisorption (BET surface area) and X-ray diffraction. Prior to the
electrochemical characterization, the electrodes were pretreated in 0.3M NaOH
aqueous solution for 1 min then rinsed with acetone, ethanol, and ultrapure
water [4]. The CV was used to establish the stability windows and capacitances
for these materials. The capacitance was deconvoluted into double-layer
capacitance and pseudocapacitance by means of CV and EIS. Chronopotentiometry
was used for further stability measurements and to determine changes in surface
activities caused by pretreatment.

As shown in Figure 1, the specific
capacitance for VN increased by 57% after pretreatment. This enhancement in
performance suggests significant improvement in material electrochemical
activities. We
believe
that pretreatment altered the charge storage for the carbides and nitrides. We
also observed good electrochemical stability after successive cycling of the
material for several thousand cycles. In general, the treated materials were more
stable than the passivated materials.

Figure 2 shows the open circuit
potential (OCP) or rest potentials for the passivated and treated VN in acidic
medium. The OCP for the treated material is lower than that for the passivated
material. We attributed this to changes in surface activities due to
pretreatment.  Using the Nernst equation, we found that the treated VN had a
higher pseudocapacitance than the passivated VN. This pseudocapacitive
charge-storage mechanism is being investigated using XPS and in-situ
x-ray absorption spectroscopy.

Figure 1: CV of passivated
and treated VN in 0.1M H₂SO₄ at 50 mVs^-1.

Figure 2: Rest potential
of passivated and treated VN in 0.1M H₂SO₄

The
physical and electrochemical properties will be described in terms of a
proposed mechanism for psuedocapacitive charge storage.

References

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

(2)   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. 1998, 496, 643.

(3)   Choi, D.; 
Kumta, P. N.; Electrochem. Solid-State Lett. 2005, 8, 8,
A418.

(4)   Weidman, M.C.;
Elposito, D.V.; Hsu, I. J.; Chen J. G.; J. Electrochem Soc.2010, 157,
12, F179