## Materials and Methods
### Electrochemical Experiments  
The electrochemical assembly used during the study of the OER and HER electro-catalytic activity of the electrodes constructed with the four procedures consisted of a Voltalab PGZ 402 potentiostat from Radiometer Analytical (Lyon, France), a glass electrolysis cell, an auxiliary plate-type Pt electrode with the geometric surface area of 0.8 cm\({}^{2}\), the Ag/AgCl (sat. KCl) reference electrode and the working electrode. The role of the working electrode was fulfilled by each of the samples obtained with Procedures 1 to 4, but also by the unmodified glassy carbon and graphite electrodes. The electrolyte employed during the water-splitting experiments was the strongly alkaline 1 M KOH solution. The OER activity of the samples was evaluated by recording anodic linear sweep voltammograms (LSVs) while their HER activity was investigated by tracing cathodic LSVs. The iR-corrected polarization curves were obtained in an unstirred solution at the scan rate (\(v\)) of 5 mV/s. Before each HER experiment the electrolyte solution was deaerated by high-purity nitrogen bubbling. Except if stated differently, the electrochemical potential values (E) are expressed in terms of the Reversible Hydrogen Electrode (RHE) via Equation (1) and the current density values (i) refer to the geometric current density. The water electrolysis electrocatalytic properties of the electrodes identified as having the highest activity for this process were subjected to additional electrochemical evaluation. Their electrochemically active surface area (ECSA), the diffusion coefficient of hexacyanoferrate (III) ions (D), the values of their Tafel slopes and their stability were studied as \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{2}{c}{**Electrode**} & \multicolumn{5}{c}{**Suspension Composition**} \\ \hline
**Procedure 1** & **Procedure 3** & **P11 \({}^{*}\) (mg)** & **P14 \({}^{**}\) (mg)** & **Nafion** & **Carbon** & **Ethanol** \\  & & **(mg)** & **(mg)** & **Solution (\(\upmu\)L)** & **Black (mg)** & **(\(\upmu\)L)** \\ \hline GC1 CB & Gr3 CB & - & - & 50 & 5 & 450 \\ GC1P1I & Gr3P1I & 5 & - & 50 & - & 450 \\ GC1P1I+CB & Gr3P1I+CB & 5 & - & 100 & 5 & 900 \\ GC1P1I & Gr3P14 & - & 5 & 50 & - & 450 \\ GC1P1I+CB & Gr3P14+CB & - & 5 & 100 & 5 & 900 \\ \hline \hline \end{tabular} \({}^{*}\)PVP functionalized with 1% Ag-doped LaMnO\({}_{3}\). \({}^{**}\) PVP functionalized with 1% Pd-doped LaMnO\({}_{3}\).  
\end{table} 
Table 1: Electrodes obtained with Procedures 1 and 3 and the suspensions used to construct them. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{2}{c}{**Electrode**} & \multicolumn{5}{c}{**Suspension Composition**} \\ \hline
**Procedure 2** & **Procedure 4** & **P11** & **P14** & **Nafion** & **Carbon** & **Ethanol** \\  & & **(mg)** & **(mg)** & **Solution (\(\upmu\)L)** & **Black (mg)** & **(\(\upmu\)L)** \\ \hline GC2 CB & Gr4 CB & - & - & 50 & 5 & 150 \\ GC2P1I & Gr4P1I & 5 & - & 50 & - & 150 \\ GC2P1I+CB & Gr4P1I+CB & 5 & - & 100 & 5 & 300 \\ GC2P1I & Gr4P1I & - & 5 & 50 & - & 150 \\ GC2P1I+CB & Gr4P1I+CB & - & 5 & 100 & 5 & 300 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Electrodes obtained with Procedures 2 and 4 and the suspensions used to construct them. well. The OER overpotential was calculated with Equation (2), and the HER overpotential with Equation (3). The Tafel slope was determined with Equation (4) and the ECSA and D values were estimated using Equation (5)--the Randles-Sevcik equation--together with experimental data obtained from cyclic voltammetry experiments. The voltammetry curves were recorded in 1 M KNO\({}_{3}\) solution, in the absence and presence of 4 mM potassium hexacyanoferrate (III), at increasing scan rate values (\(v\) = 50, 100, 150, 200, 250, 300 and 350 mV/s) and in the 0-0.8 V potential range vs. Ag/AgCl (sat. KCl).