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.