# Electrochemical Characteristics of Amorphous Ni-P Electroplated Thin Film
## 2 Experimental Procedure  
### Preparation of Ni-P Thin Films Using Electroplating  
Ni-P is known to be electroplated through the below indirect mechanism [47].  
\[\mathrm{H_{2}PO_{2}}^{-}+5\mathrm{H^{+}}+4\mathrm{e^{-}}\to\mathrm{PH_{3}}+2 \mathrm{H_{2}O} \tag{1}\] \[2\mathrm{PH_{3}}+3\mathrm{Ni^{3+}}\to 3\mathrm{Ni}+2\mathrm{P}+6 \mathrm{H^{+}}\] (2) \[\mathrm{Ni^{2+}}+2\mathrm{e^{-}}\to\mathrm{Ni}\] (3) \[2\mathrm{H^{+}}+2\mathrm{e^{-}}\to\mathrm{H_{2}} \tag{4}\]  
That is, Ni-P (Equations (1) and (2)) is concurrently coated with pure Ni (Equation (3)), and at the same time, \(\mathrm{H_{2}}\) evolution occurs (Equation (4)). In this study, Ni-P thin films were electrolytically plated using a three-electrode electrochemical cell. A copper foil (99.8%; Alfaaesar, Haverhill, MA, USA) with a thickness of 20 \(\upmu\)m was selected as a substrate and masked so that its exposed area was 1 cm\({}^{2}\), and this was used as a working electrode. To remove the native oxide layer from the surface of the copper substrate, it was pretreated in 2.5 M H\({}_{2}\)SO\({}_{4}\) (98%; Daejeong, Sileung-si, Gyeonggi-do, Korea) for 10 s and then washed with distilled water. A 6 cm\({}^{2}\) platinum metal plate was used as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. To form the plating solution, 0.6 M NiSO\({}_{4}\)\(\cdot\)6H\({}_{2}\)O (Cryst./Certified ACS; Fisher chemical), 0.2 M NiCl\({}_{2}\cdot\)6H\({}_{2}\)O (Cryst./Certified; Fisher chemical), 0.1-1.8 M H\({}_{3}\)PO\({}_{3}\) (98+%; Alfa aesar, Haverhill, MA, USA), and 0.6 M H\({}_{3}\)PO\({}_{4}\) (85%; Junsei chemical, Tokyo, Japan) were used. Then, 3 M NaOH (97%; Junsei chemical, Tokyo, Japan) or ammonia water (35%; Alfa aesar, Haverhill, MA, USA) was added to adjust the acidity of the plating solution. All the electrolytic plating was performed at 60 \({}^{\circ}\)C. With stirring, the hydrogen gas generated during plating was removed from the electrode surface. The reduction current density was 40-100 mA cm\({}^{-2}\), and the thickness of all the plating layers was adjusted to be approximately 100 nm by controlling the plating time. In particular, the phosphorus content of the Ni-P thin film was determined by the H\({}_{3}\)PO\({}_{3}\) concentration, reduction current density, and plating solution acidity [48]. Table 1 lists the plating conditions used in this study, along with the designations of the specimens.  
This work tried to indirectly confirm the adhesive strength by observing whether the film on the substrate maintained its structural integrity when the thin film was bent. The bending test was carried out by manually wrapping a structural body of copper with a 80 \(\upmu\)m diameter cylindrical edge with the Ni-P thin film sample. After bending and straightening, the morphology of the part of the thin film that was bent was observed with an electron microscope. Bending speed was about one cycle per second.  
The electroplating of the Ni-P thin films was conducted using an Iviumstat (Ivium technologies, The Netherland). The morphology of the electroplated Ni-P thin films was analyzed using field-emission scanning electron microscopy (FE-SEM; MIRA3; TESCAN, Brno, Czech Republic), and the composition was verified using an energy-dispersive X-ray spectrometer (EDS; 51-XMZ1004; Oxford instruments, Abingdon, UK). The crystal structure of the thin film was identified using an X-ray diffractometer (XRD; Ultima-IV, Rigaku, Tokyo, Japan).