Synthesis, characterisation and charge storage behaviour of some transition metal nitrides

Abstract

The application of nanostructured materials with ordered morphologies and properties to electrochemical capacitors (ECs) is being studied in order to provide enhanced energy density without comprising their inherent high power density and excellent cyclability. In this study we have synthesised some metal nitrides through various routes using non-oxide precursor sources and studied their structure, composition and morphology. As synthesised materials were made in to electrodes and their capacitances were measured using cyclic voltammetry.
Manganese nitrides were obtained in a solvothermal synthesis by reacting MnCl₂ and LiNH₂. Nanotubes at 350 °C or spherical shape particles were seen at higher temperatures. Capacitances were seen to drop with scan number when run for longer periods at higher scan rates. Molybdenum nitride samples were obtained by solution phase ammonolysis of MoCl₅ or Mo(NMe₂)₄ to obtain the polymers. Annealing the choloroimide polymer yielded hexagonal MoN at 500 °C, rock salt Mo₂N at 700 °C and above and a mix of both the phases at 600 °C. Imide derivatives yielded distorted cubic up to 800 °C and a mix of hexagonal and rock salt at high temperature. Nanotubes were obtained from imide derivatives up to 900 °C, while spherical particles above this temperature and for chloroimide derivatives at any temperature. CVs obtained for chloroimide derivatives show to double layer supercapacitors dominating over redox reaction, while imide derivatives were found to be redox in nature. Chloroimide samples show high capacities up to 247 F g^-1 at 2 mV s^-1 and long cycling lifetime for both the derivatives through 1000 cycles.
Vanadium nitride samples were obtained by solution phase ammonolysis of V(NMe₂)₄. Annealing the polymer at various temperatures under NH₃ produced VN with standard rock salt structure at 500-1000 °C while distorted rock salt in the samples obtained at lower temperature. Particles were found to be spherical in shape. Highest capacitance of 128 F g^-1 was seen for the sample obtained at 600 °C at lower 2 mV s^-1. Capacitance was found to be stable through 1000 scans only when a short potential window was used. TiN samples were produced by obtaining a precursor from solution phase ammonolysis of Ti(NMe₂)₄ and annealing the precursor under ammonia or through sol-gel route using long chain amines as the templating agents. PXD patterns showed rock salt TiN with broad peaks at 500 °C or highly crystalline at 800 °C. TEM images suggested particles to be spherical in shape. A fairly high surface area of 319 m² g^-1 was observed for the sample obtained from annealing the HDA/PrNH₂ polymer at 500 °C. Cyclic voltammograms showed poor charge storage and most of the samples tend to oxidise rapidly in aqueous KOH. VN/C samples were obtained carrying an overnight reaction of V(NMe₂)₄ with NH₃ in a hydrothermal bomb at various temperature. PXD reviled rock salt VN, some distortion was in the structure was found in the sample obtained at lower temperature. TEM images show vanadium nitride nano particles with carbon nanotubes. Specific capacitances were observed to be decreased with scan number through 100 cycles in 1 M KOH.

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ORCID for Andrew Hector: orcid.org/0000-0002-9964-2163

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