# Preparation of Magnetically Driven Nickel Phosphide Nanowires and Their Electrochemical Properties
## 3 Results and Discussion
### Electrochemical Tests
The amounts of reaction charge were increased in the increasing order of the phosphorus content in the nickel phosphide in the cyclic voltammograms (Figure 5a,b). Based on the quantitative analysis of the reversible cyclic voltammograms (Figure 5b), the charge amounts transferred during cathodic reaction of samples 18L, 18M, and 18H were determined to be 19.4, 52.6, and 95.9 C/g, respectively(i.e., the charge amount ratio is 18L:18M:18H = 1:2.47:4.94). In addition, the ECSA values determined based on Figure 5c were 4998, 8540, and 12,123 cm\({}^{2}\)/g, respectively, and the ratio was 1:1.71:2.43, which is significantly different from the charge amount ratio obtained from the cyclic voltammograms. This implies that the charge amount is affected by other factors, apart from the surface area effect. Remarkably, the ratio after simple mathematical correction of the area effect is calculated to be 1:1.44:2.03, which is almost similar to the phosphorus content ratio of the specimen (7.7:10.6:14.3 = 1:1.37:1.85 in Figure 3c). These results suggest that the phosphorus content and the surface area are two main governing factors affecting the reaction charge amount. Figure 6 shows the rate characteristics of the cathodic reaction for the five specimens described above. First, in the case of the samples 0L, 2L, and 18L, which have the same phosphorus content, the larger the reaction surface area, the higher the cathodic charge amount value under a low rate (0.5 A/g) as described above. However, the specimen with a large surface area undergoes a relatively rapid decrease in the charge amount as the reaction rate increases (Figure 6a). For example, when sample 0L is compared with sample 18L, the charge amount, which showed a large difference at a low rate, becomes almost the same at 3 A/g. When the rate is further increased, sample 18L with a small  
Figure 4: (**a**) The first and (**b**) the second cyclic voltammograms and (**c**) a charging current vs. scan rate plot (inset table is the estimated ECSA values), obtained from the nickel phosphide nanowires prepared at different values of magnetic field strength (Samples 0L, 2L, and 18L). surface area shows a significantly higher charge amount. This tendency to achieve better rate characteristics with a small surface area can be confirmed more clearly through the charge amount retention plot (Figure 6b). This unusual rate characteristic can be understood based on the nanowire shapes and pore structures of the electrodes (Figure 1). With regard to the nanowire shape, sample 0L is highly curved and, thus, has a high surface area; however, the electronic resistance might be large due to the complicated electron migration path and the severe contact between the nanowires. In contrast, sample 18L has a well-defined one-dimensional nanowire shape and has a very large aspect ratio, which results in relatively low electronic resistance. In addition, considering pore structure of the electrode constructed with nanowires, the apparent porosities of samples 0L, 2L, and 18L increased in order. Therefore, the open  
Figure 5: (**a**) The first and (**b**) the second cyclic voltammograms and (**c**) a charging current vs. scan rate plot (inset table is the estimated ECSA values), obtained from the nickel phosphide nanowires prepared at different combinations of pH and reducing agent content (Samples 18L, 18M, and 18H). pore structures lead to a decrease in migration resistance of active ions. Therefore, ohmic overpotential due to electronic and ionic resistance is expected to decrease in the order of samples 0L, 2L, and 18L. In other words, as the one-dimensional nickel phosphide nanowire is developed, a competitive situation arises, wherein the ohmic overpotential is decreased (due to the reduced electronic/ionic resistance) and the activation overpotential is increased (due to the reduced reaction surface area and increased reaction resistance).