(363f) In-Situ Characterization of Transition Metal Nitrides for Supercapacitor Electrodes

In-situ Characterization of
Transition Metal Nitrides for Supercapacitor
Electrodes

Priyanka Pande^a, Alice E. S. Sleightholme^a,
Paul Rasmussen^ac, Aniruddha
Deb^b, 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- 48109-2100.

E-mail: ltt@umich.edu

Early
transition metal nitrides are promising candidates for use in supercapacitors due to their high electronic
conductivities, surface areas (up to 200 m²/g) and electrochemical
stabilities [1,2]. Of these, V and Mo nitrides have been demonstrated to
possess the highest capacitances. 
For example, VN with 1340 Fg^-1 in aqueous KOH [3] and γ-Mo₂N with 380 Fg^-1
in aqueous H₂SO₄ [4] have been reported. Further
development of these materials would benefit from a better understanding of
their charge storage mechanisms. In this paper we identify active species on
the electrode-surface during charge-storage, present results from in-situ x-ray absorption spectroscopy
(XAS), and suggest mechanisms that reconcile these results. Additionally we
will present results from impedance spectroscopy and charge-discharge,
indicating the device level performance for these materials.

The nanostructured V
and Mo nitrides were synthesized via temperature-programmed-reaction of their
oxide precursors with 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]. Physical characterization was performed using
BET surface area analysis and X-ray diffraction. Our previous results using
electrolyte ion-isolation techniques suggested that OH^- and H⁺
were the contributing species in charge storage for VN and γ-Mo₂N [4]. Chronopotentiometry
at varying concentrations of the electrolyte was used to measure the open
circuit potential (OCP). The OCP values were then used to establish the
relationship between the charge-transferred and the active species. In-situ XAS experiments were carried out
in the cell shown in Figure 1. The stability range was determined using cyclic
voltammetry for VN in 0.1M KOH and γ-Mo₂N
in 0.1M H₂SO₄ inside the XAS cell (vs a Pt wire reference
electrode). The stable potential range was divided into potential steps and chronoamperometry was carried out while x-ray absorption
spectra were collected at these voltages from highest to lowest potential and
then in reverse.

The chronopotentiometry results for VN and γ-Mo₂N suggested that 0.5 and 1.7 electrons are
transferred per OH^- and H⁺ reacted, respectively. The in-situ x-ray absorption near-edge
spectra (XANES) for γ-Mo₂N
are shown in Figure 2. Based on the Mo-edge shift collected at the various
potentials, we estimated the change in oxidation state for Mo at these voltages
(Figure 3). These results indicated the following changes in the Mo oxidation
state during charge-storage: Mo^3.6+Mo^3.2+Mo^2.8+Mo^2.4+. The results for forward and reverse scans
revealed minimal change in Mo oxidation state at a given potential, indicating
the reversibility and cyclability of the material. In-situ X-ray absorption fine structure
(XAFS) results will also be presented to show the changes in the coordination
sphere. Parallel experiments with VN required recording spectra in fluorescence
mode instead of the transmission mode as was used for γ-Mo₂N.  The fluorescence mode is more
difficult to analyze, but our preliminary interpretations indicate reversible redox reactions for VN. 

Reference:

[1] Cladridge J B, York A P E, Brungs A J, Green
Malcolm L H, Chem. Mater.12 (2000)
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. 496
(1998) 643.

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

[4] Pande P, Rasmussen P G, Thompson L T, J. Power Sources. 207 (2012) 212.

Figure 1:    Schematic of in-situ x-ray 
absorption spectroscopy cell.

Figure 2: In-situ XANES
spectra of γ-Mo₂N in
0.1M H₂SO₄ at various potential steps in transmission
mode.  Mo₂N As-is is the
dry electrode. Mo-foil and Mo (IV) were used as model compounds.

Figure 3: Changes in oxidation state of γ-Mo₂N in 0.1 M H₂SO₄
at various potentials during forward and reverse scans.