Double Layer and Pseudocapacitive Charge-Storage Mechanisms in Carbides and Nitrides

Introduction

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 m²g^-1), 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 attributed to a combination of electric double-layer formation and faradaic reactions occurring on the nitride or oxynitride (VN_xO_y) surface [3]. Despite efforts to date, the nature of the faradaic redox reactions or pseudocapacitive properties of early transition-metal carbides and nitrides remains ill-defined. This presents a challenge to the full exploitation of these materials. Here we report a detailed investigation of the charge-storage mechanisms in early transition-metal carbides and nitrides in aqueous media. The contributions of both double-layer and pseudocapacitive mechanisms have been deconvoluted using a combination of electrochemical techniques including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and electrochemical quartz crystal microbalance (EQCM), x-ray absorption spectroscopy and neutron scattering.

Experimental

High-surface-area Ti, V, Nb, Mo, and W carbides and nitrides were prepared from their oxide precursors TiO₂ (Alfa Aesar), V₂O₅ (Alfa Aesar), N₂O₅ (Alfa Aesar), (NH₄)₆Mo₇O₂₄.4H₂O (81-83% as MoO₃, Alfa Aesar) and WO₃ (Alfa Aesar) respectively, by temperature-programmed reaction (TPR) synthesis with 15% CH₄ / H₂ (Cryogenic Gases) or NH₃ (Cryogenic Gases), respectively, then passivated using a flowing mixture of 1% O₂/He (Cryogenic Gases). Characterization of the structural properties was performed using nitrogen physisorption (BET surface area) and X-ray diffraction. 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.  EQCM was used to characterize the nature of the adsorbed/desorbed species during charge/discharge. For selected materials details regarding key species and hydrogen adsorption were determined from the x-ray absorption and neutron scattering results.

Results and Discussion

Figure 1 shows the response of VN in the frequency range of 10 kHz to 10 mHz in acidic medium for selected potentials within the stable potential window. We observed plateaus at -0.7 and -0.44V bias potentials. This region is believed to be a signature of double-layer and surface adsorption, respectively. This behavior is expected given that both processes depend strongly on accessible surface area. As shown in Figure 2, the double-layer capacitance determined for VN in acidic medium was approximately 96 μFcm^-2. The charge-storage mechanism was found to be a combination of surface redox reaction, adsorption and double-layer charging. Similar examinations have been applied to the other carbides and nitrides listed above in aqueous media and the results will be discussed. 

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.