## 3 Results and Discussion  
### Preparation and Characterization of ERGO-NiNP Film-Modified Electrode  
The modified electrodes were prepared using a simple electrochemical reduction consisting of 10 successive CV scans in the graphene oxide solution without or with NiCl\({}_{2}\) salt. The CV of GO solution exhibits three characteristic signals, two reduction peaks at \(-0.5\) and \(-1.35\) V/ECS and one oxidation peak at 0.15 V/ECS (Figure 1A). The repetitive reduction process led to the electrodeposition of electrochemically reduced graphene oxide films on the GCE surface (denoted as GCE/ERGO) following the previously described procedure [55]. The CV of the GO solution containing NiCl\({}_{2}\) salt displays significant changes (Figure 1B). The first CV scan shows the characteristic signals for GO reduction-oxidation processes along with two additional signals: one oxidation peak at \(-0.2\) V/SCE and one reduction peak at \(-0.8\) V/SCE. Both additional signals could be assigned to the reduction/oxidation of nickel and leading to the deposition of Ni metal nanoparticles (see Equation (1)) along with reduced graphene oxide (see Electronic Supporting Information Figure S1). The second CV scan displays the characteristic signal of nickel reduction at \(-0.8\) V/SCE and a very high current in the cathodic region overlapping the peak of GO reduction (current value increases by 5-fold). This dramatic increase in the current is proposed to be due to a hydrogen evolution reaction catalysed by electrodeposited nickel metal after the first CV scan. Thus, the generated hydroxyl group reacts with Ni\({}^{2+}\) ions to Ni(OH)\({}_{2}\) precipitates that could probably be decomposed into NiO compounds following Equations (2) and (3) [52; 53]. In fact, [PERSON] et al. [56] demonstrated that electrodeposited nickel nanoparticles exhibited high catalytic activity towards hydrogen evolution. It could be proposed that this process led to the deposition of reduced graphene oxide and nickel nanocomposite materials (including Ni and NiO nanoparticles) onto electrode surfaces (denoted ERGO-NiNP nanocomposite). No more significant changes for the following CVs scans (from 3-10 th) are noted, suggesting that the ERGO-NiNP nanocomposite is continuously formed and deposited for the following successive scans. These observations indicate that a simple cycling potential method could allow both synthesis and deposition of ERGO-NiNP nanocomposites onto GCE surfaces. \[\text{Ni}^{2+}+2\text{e}-\rightarrow\text{Ni nanoparticle} \tag{1}\]  
\[\text{Ni}^{2+}+2\text{H}_{2}\text{O}\rightarrow\text{Ni(OH)}_{2}+\text{H}_{2} \tag{2}\]  
\[\text{Ni(OH)}_{2}\rightarrow\text{NiO}+\text{H}_{2}\text{O} \tag{3}\]  
Figure 1: Ten repetitive CVs at the scan rate v = 50 mV s\({}^{-1}\) at GCE electrode in 0.1 M LiClO\({}_{4}\) electrolyte solution containing (**A**) GO and (**B**) GO + 0.5 mM NiCl\({}_{2}\). FE-SEM characterization confirmed the deposition of ERGO and ERGO-NiNP films. In fact, ERGO films exhibit a crumpled lamellar structure of graphene nanosheets (Figure 2A) while ERGO-NiNP films present homogenous dispersion of nickel nanoparticles of size 20-30 nm into the graphene matrix and present a structure of alternating layers of metal nanoparticles and reduced graphene oxide layers (Figure 2B). From the EDX spectrum, the peak of C and Ni originating from ERGO-NiNP nanocomposite onto the ITO substrate could be seen (see Electronic Supporting Information Figure S2). Further characterisation of ERGO-NiNPs onto the ITO plate was performed by the XRD method. However, we observed that the main signals correspond to the ITO phase. It was not possible to clearly identify the phase of electrodeposited metallic nickels, probably because deposited nickel particle layers are too thin with respect to the ITO substrate (see Electronic Supporting Information Figure S3). Increasing the nickel phase by increasing the number of CV scans to 2 folds (20 scans) do not even allow the identification of the exact state of the deposited nickel materials.