## 3 Results and Discussion
### Tafel Polarization Plots  
Furthermore, Figures 3 and 4 present potentiostatic Tafel cathodic and anodic polarization plots (correspondingly), recorded on the polycrystalline nickel electrode in the unmodified NaOH solution and in the presence of acetone, at the concentrations of 1.0 \(\times\) 10\({}^{-7}\), 1.0 \(\times\) 10\({}^{-5}\) and 1.0 \(\times\) 10\({}^{-3}\) M (CH\({}_{3}\))\({}_{2}\)C=O. Thus, the recorded cathodic slopes,  
Figure 2: The cyclic voltammograms recorded on the surface of polycrystalline Ni electrode (on the second cycle), presenting the potential range for OER in 0.1 M NaOH solution, in the absence and presence of acetone, at the indicated (CH\({}_{3}\))\({}_{2}\)C=O concentrations.  
\(b_{\rm c}\) (for the overpotential range 150-300 mV in Figure 3) came to 96, 85 and 92 mV dec\({}^{-1}\) for the unmodified NaOH, and in the presence of 1.0 \(\times\) 10\({}^{-5}\) and 1.0 \(\times\) 10\({}^{-3}\) M of acetone, respectively. Simultaneously, the above is aligned with the general facilitation of the exchange current density, \(j_{0}\) parameter. The corresponding Tafel-derived values of the \(j_{0}\) parameter came to 9.4 \(\times\) 10\({}^{-6}\), 4.6 \(\times\) 10\({}^{-6}\) and 1.2 \(\times\) 10\({}^{-5}\) A cm\({}^{-2}\).  
On the other hand, for the OER process, the derived anodic slopes, \(b_{\rm a}\) (for the overpotential range 250-350 mV in Figure 4) came to 43, 45, 42 and 37 mV dec\({}^{-1}\) for the  
Figure 4: Quasi-potentiostatic anodic Tafel polarization plots (recorded at a scan rate of 0.5 mV s\({}^{-1}\)) for polycrystalline Ni wire electrode, recorded in 0.1 M NaOH solution (in the absence and presence of acetone, at the indicated concentrations). Appropriate \(iR\) correction was made based on the solution resistance derived from the impedance measurements.  
Figure 3: Quasi-potentiostatic cathodic Tafel polarization plots (recorded at a scan rate of 0.5 mV s\({}^{-1}\)) for polycrystalline Ni wire electrode, recorded in 0.1 M NaOH solution (in the absence and presence of acetone, at the indicated concentrations). Appropriate \(iR\) correction was made based on the solution resistance derived from the impedance measurements.  
unmodified NaOH, and in the presence of 1.0 \(\times\) 10\({}^{-7}\), 1.0 \(\times\) 10\({}^{-5}\) and 1.0 \(\times\) 10\({}^{-3}\) M of acetone, correspondingly. Again, the latter is aligned with the facilitation of the reaction's exchange current density parameter. The Tafel-based values of the \(j_{0}\) parameter amounted to 1.5 \(\times\) 10\({}^{-11}\), 1.2 \(\times\) 10\({}^{-10}\), 1.6 \(\times\) 10\({}^{-10}\) and 1.2 \(\times\) 10\({}^{-11}\) A cm\({}^{-2}\), respectively (compared with the literature-based values given for the HER and OER processes in [20; 23; 24; 25]). For the highest acetone concentration, an estimated value of the exchange current density became significantly reduced, which is in line with the cyclic voltammetry features observed in Figure 2. The latter is supposedly related to partial Ni surface blockage by acetone molecules or otherwise to increased electrode contamination by acetone-based impurities.  
Interestingly, the initial anodic polarization of the polycrystalline nickel electrode revealed a typical passivation feature (observed in Figure 4 over the potential range ca. 1.43-1.48 V/RHE), which led to a temporary reduction in the current density, due to the formation of a passive oxide layer on the nickel electrode surface (see, e.g., [26] for details).