Ultrahigh-Performance Pseudocapacitor Electrodes Based on Transition Metal Phosphide Nanosheets Array via Phosphorization: A General and Effective Approach

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 7530 wileyonlinelibrary.com consumer electronics, and backup energy systems. [ 2 ] In recent years, great efforts have been dedicated to the development and engineering of new materials and structures for enhanced capacitance performance. [ 3 ] For instance, pseudocapacitors that store energy through reversible Faradaic reactions on the electrode surface, such as metal oxides [ 4 ] and hydroxides, [ 5 ] usually provide high specifi c capacitance. Nevertheless, these metal oxides or hydroxides are kinetically unfavorable for fast electron transport required for high power density. [ 6 ] As a comparison, transition metal phosphides possess the metalloid characteristics and superior electrical conductivity, [ 7 ] which have been used as anode materials for lithium-ion batteries, [ 8 ] hydrotreating catalysts, [ 9 ] electrocatalysts for the H 2 evolution, [ 10 ] etc. To the best of our knowledge, reports of transition metal phosphides as a cathode material for electrochemical capacitors have been scarce. This is the primary motivation of the present study in which we use a general and effective phosphorization approach to prepare various transition metals phosphide nanostructures and examined the capacitance performance of the hybrid electrodes. Note that Ni foam has been used extensively as a supporting substrate for supercapacitor electrodes, due to the high conductivity and interconnected pores. [ 11 ] Electroactive materials of various morphologies, including nanoparticle, [ 12 ] nanowire, [ 13 ] and nanosheet, [ 14 ] have been deposited onto nickel foam to construct supercapacitor electrodes. In this study, 3D networks of Ni 2 P nanosheets were grown on the surface of a Ni foam (Ni 2 P NS/ NF) by a phosphorization strategy from Ni(OH) 2 nanosheets. A variety of experimental techniques including transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the resulting composite structures. The hybrid structure was then used directly as a capacitor electrode, and electrochemical studies showed an excellent specifi c capacitance of 2141 F g −1 at the potential scan rate of 50 mV s −1 and remained as high as 1109 F g −1 even at the current density of 83.3 A g −1 in a 6.0 M KOH aqueous electrolyte, much higher that of the as-synthesized Ni(OH) 2 nanosheet/ Ni foam electrode. This is the best performance that has ever Ultrahigh-Performance Pseudocapacitor Electrodes Based on Transition Metal Phosphide Nanosheets Array via Phosphorization: A General and Effective Approach