## 3 Results and Discussion
### Electrochemical Experiments
In all cases, the absolute values of the peak current densities increase proportionally with the scan rate. This increase denotes the specific behavior of diffusion-controlled electron transfer processes [46]. The OER kinetics at the interfaces between the four electrodes and the electrolyte solution were also studied. The obtained Tafel plots are shown in Figure 6, and the determined values of the Tafel slopes can be seen in Table 4. To represent the Tafel curve belonging to each sample the current density was normalized to their estimated ECSA value (iECSA). The smallest Tafel slope value is the one calculated for the Gr4P11+CB sample. A smaller value indicates more favorable kinetics and a higher reaction rate [47]. The electrochemical stability of the four electrodes was evaluated during 7 h chronoamperometric tests at the constant potential value corresponding to i = 5 mA/cm2. Gr4P11+CB was revealed to be more stable than the other investigated samples. The i vs. time curves recorded for this electrode and for Gr4P14+CB are presented in Figure 7. In the case of Gr4P11+CB, the i value stabilized at 5.8 mA/cm2. LSVs were obtained on this sample before and after the experiment and the results can be seen in Inset a. At higher i values the two polarization curves overlap almost perfectly, but this is not the case at values < 24 mA/cm \begin{table}
\begin{tabular}{c c c} \hline
**Electrode** & **Tafel Slope (V/dec)** & \(\mathbf{R^{2}}\) \\ \hline Gr4P11 & 0.389 & 0.9996 \\ Gr4P14 & 0.247 & 0.9993 \\ Gr4P11+CB & 0.197 & 0.9994 \\ Gr4P14+CB & 0.312 & 0.9995 \\ \hline \end{tabular}
\end{table} 
Table 4: Tafel slope values were determined for the Gr4P11, Gr4P14, Gr4P11+CB, and Gr4P14+CB electrodes. \begin{table}
\begin{tabular}{c c c} \hline
**Electrode** & **ECSA (cm2)** & **D (cm2/s)** \\ \hline Gr4P11 & 0.54 & \(2.55\times 10^{-5}\) \\ Gr4P14 & 0.74 & \(4.85\times 10^{-5}\) \\ Gr4P11+CB & 1.19 & \(1.21\times 10^{-4}\) \\ Gr4P14+CB & 0.80 & \(5.51\times 10^{-5}\) \\ \hline \end{tabular}
\end{table} 
Table 3: The ECSA and D values were determined in the case of Gr4P11, Gr4P14, Gr4P11+CB, and Gr4P14+CB. where the stability test led to a decrease in the OER activity of the electrode. For example, at i = 10 mA/cm2 \(\eta_{\text{OER}}\) = 0.88 V. This 50 mV increase is probably due to the high electrochemical potential applied throughout the chronoamperometric study. Still, the Gr4P11+CB electrode was more electrochemically stable than the other samples. To exemplify this, the i vs. time curve recorded under the same conditions but for Gr4P14+CB is also included in Figure 7. Apparently, the behavior of the two samples is not very different, but a closer look at the amperogram (Insets b and c) reveals the presence of features attributable to intense alternate processes of O2 bubble accumulation and release that are not observed on the curve traced on Gr4P11+CB [29]. More importantly, once the test ended and the surface of the sample was investigated visually it was observed that the perovskite-containing composition was partially detached from the graphite surface it was deposited on during the manufacturing process.