## 3 Results and Discussion
### Ac. Impedance Spectroscopy  
The _ac._ impedance electrochemical characteristics of the polycrystalline nickel electrode surface in the 0.1 M NaOH electrolyte, in the absence and presence of acetone, at the concentrations of 1.0 \(\times\) 10\({}^{-7}\), 1.0 \(\times\) 10\({}^{-5}\) and 1.0 \(\times\) 10\({}^{-3}\) M over the potential ranges characteristic for the HER and OER processes, is shown in the Nyquist impedance plots of Figures 5 and 6, and Tables 1 and 2 below. Figure 5: (**a**) The Nyquist impedance plots for HER performed on polycrystalline nickel electrode surface at \(-\)100 mV vs. RHE in 0.1 M NaOH, in the absence and presence of acetone, at the specified concentrations; (**b**) equivalent circuit used to fit the data for HER and OER processes, where \(R_{\rm ct}\) is charge transfer resistance and \(C_{\rm dl}\), and \(R_{\rm s}\) correspond to CPE (constant-phase element)-modified double layer capacitance and solution resistance parameters, respectively. \begin{table}
\begin{tabular}{c c c} \hline  & **0.1 M NaOH** \\ \hline
**E/mV** & \(\mathbf{R_{\text{ct}}/\Omega~{}\text{cm}^{2}}\) & \(\mathbf{C_{\text{dl}}/\mu\text{F cm}^{-2}}\) \\ \hline \(-\)50 & 1076.2 \(\pm\) 3.7 & 217.5 \(\pm\) 1.3 \\ \hline \(-\)100 & 567.3 \(\pm\) 1.4 & 179.3 \(\pm\) 1.2 \\ \hline \(-\)150 & 227.8 \(\pm\) 0.9 & 159.1 \(\pm\) 1.9 \\ \hline \(-\)200 & 80.7 \(\pm\) 0.3 & 132.2 \(\pm\) 2.5 \\ \hline \(-\)250 & 36.5 \(\pm\) 0.1 & 120.2 \(\pm\) 2.5 \\ \hline \(-\)300 & 22.6 \(\pm\) 0.1 & 113.8 \(\pm\) 3.0 \\ \hline \(-\)350 & 15.3 \(\pm\) 0.1 & 108.2 \(\pm\) 2.7 \\ \hline \(-\)400 & 11.5 \(\pm\) 0.0 & 89.3 \(\pm\) 2.6 \\ \hline \(-\)450 & 9.3 \(\pm\) 0.0 & 89.9 \(\pm\) 3.3 \\ \hline \(-\)500 & 7.7 \(\pm\) 0.0 & 85.1 \(\pm\) 0.0 \\ \hline \end{tabular}
\end{table} 
Table 1: The resistance and capacitance parameters for the process of HER on polycrystalline nickel electrode surface in 0.1 M NaOH solution, obtained in the absence and presence of acetone at the specified concentrations, acquired by fitting the equivalent circuit (Figure 5b) to the recorded impedance data (values of dimensionless \(\varphi\) parameter for the CPE circuit fluctuated between 0.84 and 0.97; \(\chi^{2}\) = 3 \(\times\) 10\({}^{-5}\)-4 \(\times\) 10\({}^{-3}\)). Figure 6: The Nyquist impedance plots for OER conducted on the surface of polycrystalline nickel electrode at 1600 mV vs. RHE in 0.1 M NaOH, in the absence and presence of acetone, at the denoted concentrations. Hence, for the hydrogen evolution reaction, a single (and slightly depressed) semicircle on the polycrystalline Ni electrode was present in the Nyquist impedance plots (Figure 5). The impedance measurements were performed over the potential range from \(-50\) to \(-500\) mV vs. RHE. For the acetone-free NaOH solution, the recorded charge transfer resistance, \(R_{\rm ct}\), values reduced from 1076.2 \(\Omega\) cm\({}^{2}\) at an overpotential (\(\eta\)) of 50 mV to 7.7 \(\Omega\) cm\({}^{2}\) at \(\eta\) = 500 mV. On the other hand, for the respective potentials, the double layer capacitance, \(C_{\rm dl}\), parameter values reduced from 217.5 to 85.1 \(\mu\)F cm\({}^{-2}\) (Table 1). The latter is most likely the effect of rising Ni surface blockage by vigorously evolving hydrogen microbubbles at increased overpotentials. \begin{table}
\begin{tabular}{c c c} \hline \multicolumn{3}{c}{**0.1 M NaOH**} \\ \hline
**E/mV** & \(\mathbf{R_{ct}/\Omega\ cm^{2}}\) & \(\mathbf{C_{dl}/\mu F\ cm^{-2}}\) \\ \hline  
\end{tabular}
\end{table} 
Table 2: The resistance and capacitance parameters for the process of OER, conducted on polycrystalline Ni electrode surface in 0.1 M NaOH solution, in the absence and presence of acetone at the specified concentrations, attained by fitting the equivalent circuit (Figure 5b) to the recorded impedance data (values of dimensionless \(\varphi\) parameter for the CPE circuit oscillated between 0.84 and 0.95; \(\chi^{2}=2\times 10^{-5}\)–\(2\times 10^{-3}\)).