# Preparation of Magnetically Driven Nickel Phosphide Nanowires and Their Electrochemical Properties
## 3 Results and Discussion
### Electrochemical Tests  
Several studies have reported the redox reaction mechanism of nickel phosphides in an aqueous solution [29; 30; 31; 32; 33]; however, there is still no solid consensus as to the redox reaction mechanism of nickel phosphides in an aqueous solution, except for the active participation of hydroxyl ions in the reaction. It is suggested that the following irreversible (Equation (5)) and reversible (Equation (6)) reactions take place when nickel phosphides are in an alkaline solution. \[\mathrm{Ni_{x}P_{y}+xOH^{-}\to Ni_{x}P_{y}(OH)_{x}+xe^{-}} \tag{5}\]  
\[\mathrm{Ni_{x}P_{y}(OH)_{x}+xOH^{-}\leftrightarrow Ni_{x}P_{y}O_{x}+xH_{2}O+xe ^{-}} \tag{6}\]  
Based on the reaction mechanism, it is evident that the reaction degree is increased when the interfacial reaction area between the nickel phosphide and the electrolyte is increased or when the phosphorus content in the nickel phosphide is increased. First, we attempted to investigate the effect of the reaction area in a situation where the phosphorus content in the nickel phosphide nanowire was fixed. To this end, the characteristics of electrodes constructed with nickel phosphide nanowires (samples 0L, 2L, and 18L; 7-8 at% P (Figure 2)) obtained under various magnetic fields were analyzed. Figure 4a,b show the cyclic voltammograms in the first and the second cycles. Figure 4c presents a charging current vs. scan rate plot and the ECSA values determined based on the plot (the table inserted in the figure). The presence of the irreversible anodic reaction (Equation (5)) was confirmed by the comparison of the first and the second cyclic voltammograms: Anodic current during the first cycle (Figure 4a) was much larger than that during the second cycle (Figure 4b) throughout the entire potential range, while the cathodic current values during the first and the second cycles are almost the same, strongly indicative of the irreversible anodic process in the first cycle. A redox peak due to the reversible reaction between nickel phosphides and hydroxyl ions (Equation (6)) was also observed in the cyclic voltammogram (Figure 4b). Notably, the electrode (sample 0L) constructed with nanowires obtained in the absence of a magnetic field exhibited the highest reaction degree (i.e., the amount of reaction charge). As the magnetic field increased, the reaction degree was decreased in sample 2L, followed by sample 18L. Based on a quantitative analysis through the cyclic voltammograms, the charge amounts transferred during cathodic reaction of samples 0L, 2L, and 18L were 31.7, 22.8, and 19.4 C/g, respectively (that is, the charge amount ratio is 0L:2L:18L = 1:0.72:0.61). On the other hand, based on Figure 4c, the ECSA values were determined to be 8036, 5550, and 4998 cm\({}^{2}\)/g, respectively, through a capacitance method and the ratio was 1:0.69:0.62. This was approximately the same as the charge amount ratio obtained from the cyclic voltammograms. This result strongly indicates that the degree of reaction (i.e., reaction charge amount) is determined according to the reaction surface area when the phosphorus content in the nickel phosphide is the same. In other words, with respect to the reaction degree itself, heavily bent (i.e., poorly defined) nanowires fabricated in the absence of a magnetic field show high value by having a large active surface area. In contrast, well-defined nanowires fabricated under a high magnetic field show a relatively low value by having a relatively low active surface area. Next, the characteristics of electrodes fabricated using three nanowire specimens having different phosphorus contents (samples 18L, 18M, and 18H) were compared to investigate the effect of the phosphorus content in the nickel phosphide nanowires. Figure 5a,b show the cyclic voltammograms in the first and the second cycles. Figure 5c presents a charging current vs. scan rate plot and the ECSA values determined based on the plot (the table inserted in the figure).