(414g) Charge-Storage Mechanisms for High Surface Area Carbides and Nitrides

Charge-Storage Mechanisms for High
Surface Area Carbides and Nitrides

Abdoulaye Djire^a, Jason B.
Siegel^b, Lilin He^c, Alice E. S. Sleightholme^a,
Saemin Choi^ad, Paul Rasmussen^ad, and Levi T. Thompson*^abd

^aDepartment
of Chemical Engineering

^bDepartment
of Mechanical Engineering

^cOak Ridge
National Laboratory

^dHydrogen
Energy Technology Laboratory

University of Michigan, Ann Arbor, MI 48109-2136, USA

Tel. (734) 936-2015

E-mail: ltt@umich.edu

Introduction

Early transition-metal carbides and
nitrides are being considered for use as electrode materials in supercapacitors
due to their high accessible surface areas, high electronic conductivities and low
cost. They possess high pseudocapacitances, good capacitance retention during
cycling and wide voltage windows. The
nitrides of vanadium (VN) and molybdenum (γ-Mo₂N) have received
the most attention due to their high pseudocapacitances [1]. Previously, we demonstrated,
using ion isolation experiments, that H⁺ and OH^- were the
active ions that gave rise to the pseudocapacitance [2]. Using X-ray absorption
spectroscopy to track changes in the oxidation state of V and Mo in the VN and γ-Mo₂N
materials, respectively, a pseudocapacitive charge storage mechanism was
proposed for both materials in aqueous electrolytes [3]. The
distribution of H⁺ and OH^-
with applied potential (location and depth of storage), however, remains
poorly understood. This poses significant challenges in the design and full
exploitation of carbide and nitride materials for supercapacitors. Here we
report a detailed investigation of the charge storage mechanisms for early
transition-metal carbides and nitrides in aqueous media. The pseudocapacitive charge
storage mechanism has been investigated using x-ray absorption spectroscopy and
neutron scattering and a combination of electrochemical techniques including
cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The amount
of inserted hydrogen and hydroxide per mole of electron during pseudocapacitive
storage was determined, and pseudocapacitive charge storage mechanisms were
proposed and compared to those previously reported [3]. Additionally, the
location of inserted hydrogen and hydroxide, depth of storage, and distribution
in pores were determined.

Experimental

High-surface-area carbides and nitrides
of Ti, V, Nb, Mo, and W were prepared from TiO₂ (99.9%, Alfa Aesar), V₂O₅
(Alfa Aesar), Nb₂O₅
(99.9985%, Alfa Aesar), (NH₄)₆Mo₇O₂₄.4H₂O
(81-83% as MoO₃, Alfa Aesar) and WO₃ (Alfa Aesar) by
temperature-programmed reaction (TPR) method with 15% CH₄ in H₂
(Cryogenic Gases) or NH₃ (Cryogenic Gases), respectively. Characterization
of the structural properties was performed using nitrogen  physisorption and X-ray diffraction. The
total capacitance and extent of pseudocapacitance was determined based on
results from CV and EIS. For selected materials, details regarding the insertion
and location of H⁺ and OH^- and metal oxidation state
changes during electrochemical cycling were determined using neutron scattering
and x-ray absorption.

Results and Discussion

Figures 1a and 1b show ratios of H⁺/e^-
and OH^-/e^-, for γ-Mo₂N
and VN, respectively, as function of applied potential. There was insertion of
1H⁺ per 2e^- and removal of 2OH^- per 1e^- for
these materials, as they were electrochemically cycled, suggesting reduction of
the Mo and V metals. Reduction of the metals was subsequently confirmed using x-ray
absorption spectroscopy. Combining physical, electrochemical, surface, and bulk
properties, we proposed the following reaction mechanisms for γ-Mo₂N and VN in
aqueous media, which are in good agreement with those previously reported [3].

Mo2αN+ 2H++ 4e- ↔ Mo2α-1N(H)2

VN+ 2OH- ↔ Vα-1N(OH)2 + e-

During
charge storage, application of an electric field induces insertion of H⁺
and extraction of OH^- in/from the pores of γ-Mo₂N and VN materials, respectively (Fig. 1c
and 1d). There was minimal contribution from large pores (mesopores and
macropores) to the pseudocapacitive storage. Conversely, significant amount of
H⁺ and OH^- were located in small pores (~ 2 nm, upper
limit for micropores) indicating that the origin of the high pseudocapacitances
observed for γ-Mo₂N
and VN is from the insertion/extraction of H⁺ and OH^- in
small pores. In other words, small pores (micropores) are the key players in
the pseudocapacitive charge storage mechanisms for early transition-metal
carbides and nitrides.

Figure 1 Hydrogen (a,c) and hydroxide (b,d) insertion and location, respectively,
as function of applied potential.

References

(1)  Simon, P., and Gogotsi, Y., Nature Mat., 2008, 7,
845-854

(2)  Thompson, L., et al., J. Power Sources, 2012, 207, 212-215.

(3)  Thompson, L., et al., J. Power Sources, 2015, in Press.