# Preparation of Magnetically Driven Nickel Phosphide Nanowires and Their Electrochemical Properties
## 4 Conclusions  
In this study, nickel phosphide nanowires were fabricated using magnetically-assisted liquid phase synthesis for the first time, and specimens with different structures and phosphorus content were tested as electrodes for the electrochemical system. The experimental results are summarized as follows.  
(1) Highly curved nanowires with low aspect ratios were formed without a magnetic field, while as the magnetic field strength increased, linear and well-defined nanowires with high aspect ratios were obtained. Under the same magnetic field strength, particles were agglomerated around the nanowire, and the aspect ratio tended to decrease as the phosphorus content in the nickel phosphide increased. Structural disturbance of the nanowire due to the increase in the phosphorus content might be caused by the decrease in magnetization due to the reduction of the nickel amount in the initial nanoparticles.  
(2) When the phosphorus content in the nickel phosphide was the same, the reaction charge amount showed a general tendency proportional to the reaction surface area of the electrode. When the phosphorus content was increased, an additional increase in the reaction charge was observed. When the surface area effect was eliminated, the increase in reaction charge occurred solely due to the increase in the phosphorus content, indicating that reaction charge depends entirely on reaction surface area and phosphorus content.  
(3) The rate characteristics increased when the surface area was small and the phosphorus content was low. An electrode consisting of curved nanowires with a low aspect ratio possessed large surface areas and, thus, might have low reaction resistance. However, the complex electron path and many contacts between the nanowires restricts the electron movement, and a relatively closed pore structure retards the ion movement. The unique  
Figure 7: Variations of **(a)** the amount of cathodic charge amount and (b) charge amount retention with repetitive redox reactions.  
rate characteristics might appear because the increase in electron/ion migration resistance according to structural disturbance of the nanowire outweighs the decrease in surface reaction resistance.  
(4) The electrode with a low surface area and low phosphorus content demonstrated the highest robustness against repetitive redox reactions, while an electrode with a low surface area and middle phosphorus content showed comparable robustness. In contrast, the robustness of the electrode having a low phosphorus content and large reaction surface area was significantly low. This suggests that reaction surface area is the dominant factor in nickel phosphide nanowire degradation compared to phosphorus content.  
(5) The results of this study confirmed that the nickel phosphide nanowires can be easily controlled in structure and composition through chemical synthesis in a magnetic field and they have different characteristics in terms of reaction charge amount, rate characteristics, and robustness. The factors governing reaction kinetics and electrode degradation might give some insight into how to design nickel phosphide nanowires for specific applications. Various engineering applications can be possibly explored through a more extensive study on additional process factors such as surfactant concentration, including the process variables covered in this study.  
Conceptualization, formal analysis, investigation, data curation, writing--original draft preparation, H.-W.K.; writing--review and editing, supervision, project administration, funding acquisition, H.-C.S. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Research Foundation (NFR-2018R1A5A1025594) of the Ministry of Science and ICT, and was supported by the Materials Technology Institute at Pusan National University.  
The data that support the findings of this study are available on request from the corresponding author.  
The authors declare no conflict of interest.