Fast and Reversible Surface Redox Reaction in Nanocrystalline Vanadium Nitride Supercapacitors

Supercapacitors have been known for over thirty years but of late are emerging as attractive electrochemical energy-storage and conversion devices for future electrical vehicle application with complementary electrochemical characteristics to rechargeable batteries and fuel cells. Amongst the numerous materials studied to date, various forms of ruthenium oxides are clearly noteworthy, exhibiting superior electrochemical response. Unfortunately, the expensive nature of ruthenium has limited its technological viability. A new class of supercapacitors based on nanocrystalline vanadium nitride is reported here, which can deliver an impressive specific capacitance of 1340 F g when tested at a scan rate of 2 mV s. An even more impressive capacitance of 554 F g is noted at a higher scan rate of 100 mV s. Such a high capacitance, which exceeds that of RuO2·nH2O, is believed to be caused by a series of reversible redox reactions through hydroxy bonding confined to a few atomic layers of vanadium oxide on the surface of the underlying nitride nanocrystals, which exhibit a metallic electronic conductivity (rbulk = 1.67 × 10 6 X m). Such a modification of the nanocrystal surface chemistry may lead to the development of supercapacitors that exhibit very high and stable power densities. Supercapacitors are generally classified into electrical double layer capacitors (EDLCs), which build up electrical charge at the electrode/electrolyte interface as described by the Gouy–Chapman–Stern–Grahame model, and pseudocapacitors, which utilize a redox reaction at the interface at certain potentials. Both rely on the physicochemical changes that occur at the electrode/electrolyte interface. Hence, understanding the surface properties is crucial for achieving high power and energy densities. High-surface-area carbon-based materials are widely studied for EDLCs. On the other hand, crystalline RuO2 [1–3,7] and amorphous RuO2·nH2O [1,4–6] are well-known pseudocapacitors that exhibit a specific capacitance as high as 350 and 720 F g, respectively, due to the redox activity via proton adsorption in an acidic electrolyte. Despite the expensive nature of ruthenium oxide, it has been the focus of intense research since Ru offers a variety of oxidation states (II–IV) while exhibiting good electronic conductivity (rbulk = 2.8 × 10 6 X m). Vanadium also exhibits numerous oxidation states (II–V) similar to that of ruthenium in V2O5·nH2O, but its poor electronic conductivity (rbulk ≈ 1 ∼ 10 X m) renders the oxide unsuitable for use in high-rate electrochemical devices. However, exploiting the good electronic conductivity of the vanadium nitrides combined with the variety of oxidation states exhibited by V in vanadium oxides could lay the foundation for a new class of high-performance supercapacitors. The synthesis of these nanocrystalline vanadium nitrides with controlled surface oxidation (as in the present study) results in a unique class of supercapacitors without much loss in the overall electrical conductivity. Furthermore, the low cost, high molar density (≈6 g cm), and good chemical resistance of the transition metal nitrides render them excellent candidates for the next generation of supercapacitors. Although no detailed studies have been conducted, in the past Thompson and co-workers have explored transition metal nitrides and carbides for supercapacitors with moderate specific capacitances (< 226 F g). Amorphous V2O5·nH2O has also been tested for its supercapacitor response, but despite mixing a large amount of carbon (25 wt.-%) to improve its poor electronic conductivity (see above), the highest specific capacitance reported to date is 350 F g at a scan rate of 5 mV s. In this study, a low-temperature route based on a two-step ammonolysis reaction of VCl4 in anhydrous chloroform is used to synthesize nanocrystalline VN (see Experimental). The nanometer-sized crystals increase the susceptibility for surface oxidation, while the high surface area of the nitrides provides more redox-reaction sites. Such a VCl4/NH3 reaction, although known, tends to be largely influenced by the type of solvent used. X-ray diffraction (XRD) and highresolution transmission electron microscopy (HRTEM) are used to characterize the as-prepared and heat-treated VN nanocrystals. The as-prepared precursor consists of amorphous V(NH2)3Cl and crystalline NH4Cl, which transforms into the rock salt (Fm3m)-structured VN at 400 °C, which is C O M M U N IC A TI O N S