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.