Charge-Storage Mechanisms in Nanostructured Carbides and Nitrides for Energy Storage
Early transition-metal carbides and nitrides are potential electrode materials for supercapacitor applications due to their high accessible surface areas, high electric conductivities and low cost. They possess high capacitances, good capacitance retention during cycling and wide voltage windows. For example, the capacitance for VN has been reported to be as high as 1340 Fg-1 in aqueous electrolyte . The origin of this high capacitance was attributed to a combination of electric double-layer formation and faradaic redox reactions occurring on the nitride or oxynitride (VNxOy) surface. However, the contribution of the each individual mechanism is ill-defined. Despite efforts to date, the nature of the pseudocapacitive properties of early transition-metal carbides and nitrides remains vague. Full exploitation of the properties of these materials will require an understanding of the pseudocapacitive charge-storage mechanism. 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), electrochemical impedance spectroscopy (EIS).
High-surface-area Ti, V, Mo, and W carbides and nitrides were prepared from their oxide precursors TiO2 (Alfa Aesar), V2O5 (Alfa Aesar), (NH4)6Mo7O24.4H2O (81-83% as MoO3, Alfa Aesar) and WO3 (Alfa Aesar) respectively, by temperature-programmed reactions (TPR) . Characterization of the structural properties was performed using nitrogen physisorption (BET surface area) 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 adsorption of actives species and metal oxidation state changes during electrochemical cycling were determined using neutron scattering and x-ray absorption.
Results and Discussion
Table 1 lists the total capacitances and percent contribution of pseudocapacitance of each material in aqueous electrolytes. The pseudocapacitive charge storage was obtained by subtracting the double-layer capacitance from the total capacitance. The extent of pseudocapacitive charge storage contribution ranged from 61% for TiN to 88% for WC1-x in acid and base electrolytes, respectively. This result indicates that pseudocapacitance is the dominant charge-storage mechanism in carbides and nitrides. This is expected given that these materials are electroactive and can go through several oxidation state changes during electrochemical cycling. Figure 1 shows the oxidation state changes for Mo2N in acidic electrolyte during electrochemical cycling. There was removal of one electron per Mo as the potential was increased. This result suggests reduction of the metal during electrochemical cycling. Concomitant with changes in oxidation state, neutron scattering indicated that hydrogen was inserted into the material. The amount of hydrogen exceeded the amount attributable to adsorption on the surface. These and other results will be discussed during the presentation.
(1) Kumta, P.N. et al., Adv. Mater., 2006, 18, 1178.