# Preparation of Magnetically Driven Nickel Phosphide Nanowires and Their Electrochemical Properties
## 1 Introduction  
Transition metal phosphides have beneficial physical, chemical, and mechanical properties owing to their excellent electronic conductivity [1] and stability in acidic and alkaline conditions [2], as well as a high melting point, hardness, and abrasion resistance [3]. In addition, the coexistence of metal-metal, metal-phosphorus, and phosphorus phosphorus bonds make it possible to synthesize transition metal compounds of various stoichiometries, compositions, and crystal structures. Accordingly, there have been many studies to apply these compounds to engineering applications, and nickel phosphides have emerged as notable candidates [4; 5; 6; 7]. In particular, research has been actively conducted on using nickel phosphides as electrodes for different electrochemical devices such as catalysts for hydrogen and oxygen generation [8], electrodes for secondary lithium batteries [9], and electrodes for supercapacitors [10; 11; 12; 13].  
Many studies have been conducted to improve the output and efficiency of electrochemical devices by fabricating electrodes using nanomaterials. Because such electrodes possess a large surface area, the surface reaction resistance is significantly reduced. Accordingly, the reaction overpotential can be reduced, enabling high-rate and high-efficiency operation of the device. Nickel phosphide nanomaterials are synthesized by various methods, such as pyrolysis of organometallic precursors [14; 15], high-temperature reduction [16; 17], solid-state phase transformation [18], and the vapor-liquid-solid (VLS) method [19; 20]. However, because the phosphorus precursors used for pyrolysis are expensive, flammable, and toxic, the synthesis of nanoparticles is still challenging. High-temperature reduction and solid-state phase transformation also incur high production costs due to synthesis at high temperatures. In addition, with the VLS method, it is challenging to remove the liquidcapsule that is formed during the process, and high temperatures are required. Hence, it is of high engineering significance to develop a cost-effective and highly safe method for synthesizing nickel phosphide nanomaterials. In this regard, a recent study on the synthesis of pure nickel nanostructures by liquid phase chemical reduction is noteworthy, where nickel nanoparticles were relatively quickly, inexpensively, and safely synthesized by chemical reduction. In particular, pure nickel nanowires could be synthesized without a catalyst or template due to the ferromagnetic properties of nickel in a magnetic field [21,22].  
In this study, we present our preliminary results on nickel phosphide nanowires with various structures and compositions, prepared by liquid phase synthesis in a magnetic field. The phosphorus content and nanowire shape of the nickel phosphides according to synthesis conditions (i.e., the solution pH, reducing agent concentration, and magnetic field strength) were analyzed. In addition, the pure nickel phosphide electrodes were fabricated without binder and conducting agents, which might possibly distort the inherent properties, to test their intrinsic electrochemical properties. In particular, we focused on the ratio of nickel to phosphorus, the effect of the reaction surface area on the reaction degree, and the effect of the nanowire shape and structure on the reaction rate and reaction-induced material degradation.