Investigation of Charge-Storage Mechanisms of Early-Transition-Metal Nitrides and Carbides as Electrodes for Electrochemical Capacitors.

Abstract

Early-transition-metal nitrides and carbides have been shown to be promising candidates for use in electrochemical capacitors due to their high electronic conductivities, surface areas and electrochemical stabilities. In addition to double-layer (electrostatic) charge storage, these materials store charge via a pseudocapacitive mechanism involving charge-transfer reactions. The goal of my research is to investigate the charge-storage mechanism of these materials as electrochemical capacitor electrodes. Nitrides and carbides of molybdenum, vanadium and tungsten were synthesized using a temperature-programmed reaction method. Electrochemical characterization of these materials was performed in aqueous sulfuric acid and potassium hydroxide. Results from cyclic voltammetry showed that molybdenum nitride in sulfuric acid (voltage window of 0.8V and capacitance of 346F/g) and vanadium nitride in potassium hydroxide (voltage window of 1.1V and capacitance of 210F/g) were the most promising systems. These systems were further investigated to establish their charge-storage mechanisms. Charge-storage reactions were determined by identifying the role of the active electrolyte species, number of electrons transferred, and the changes in the electrode material. The active electrolyte ions contributing to charge storage were identified by pairing each ion in the standard electrolyte with an inactive counter ion. Cyclic voltammetry experiments were carried out in each of these electrolytes. Protons and hydroxyl ions were found to be active for charge storage in molybdenum nitride and vanadium nitride respectively. Chronopotentiometry at varying concentrations of active ion was used to measure the open-circuit potential. The linear Nernstian relationship between the open-circuit potential and active species concentration was then used to estimate the number electrons reacted per active species. It was found that to store an electron in molybdenum nitride two protons reacted and two hydroxyl ions reacted to store an electron in vanadium nitride. In situ X-ray absorption spectroscopy was used to characterize the changes in the electrode material. It was found that approximately one electron is removed or added per molybdenum and vanadium during storage and discharge. Based on these findings charge-storage reactions were proposed. Theoretical capacitances for molybdenum nitride and vanadium nitride were estimated as 2340 F/g and 1350F/g respectively.