## 2 Materials and Methods
### Preparation of the Modified Electrodes  
The following materials and reagents were used for the electrochemical experiments: spectroscopic graphite bars (\(\omega\)= 6 mm) from \"Kablo Bratislava\" (National Corporation \"Electrocarbon Topolcany\" Factory, Bratislava, Slovakia), black laboratory-synthesized \(\alpha\)-MnO\({}_{2}\), carbon black--Vulcan XC72 (FuelCellStore), and Nafion(r) 117-solution (Sigma-Aldrich, St. Louis, MO, USA), as well as potassium chloride, N,N-dimethylformamide (DMF),  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**MnO\({}_{2}\)-Type**} & **Catalyst** & **Volume and** & **Degradation** & **Source** \\  & **Concentration** (g/L) & **Concentration** & **of Rhodamine B** & **Percent of RhB** & **Source** \\ \hline \multirow{2}{*}{\(\alpha\)-MnO\({}_{2}\)} & 0.2 (\(\alpha\)-MnO\({}_{2}\)/PMS & 100 mL; & 99\% in 90 min & [26] \\  & system; PMS-0.20 g/L) & 20 mg/L & & \\ \hline \multirow{2}{*}{MnO\({}_{2}\) aerogel} & 0.5 & 10 mL; 5 mg/L & \multirow{2}{*}{100\% in 40 min} & [27] \\  & 0.2 & 50 mL; 40 mg/L & \\ \multirow{2}{*}{nanorods} & \multirow{2}{*}{0.2} & 50 mL; 40 mg/L & \multirow{2}{*}{94.4\% in 80 min} & [28] \\  & & (pH = 6.6) & & \\ \hline \multirow{2}{*}{\(\alpha\)-MnO\({}_{2}\)} & 1 & 30 mL; 10 mg/L & \multirow{2}{*}{90\% in 60 min} & [29] \\  & & (pH = 7) & & \\ \hline \multirow{2}{*}{\(\alpha\)-MnO\({}_{2}\)} & \multirow{2}{*}{0.0045} & 100 mL; & 80\% in 70 min & \multirow{2}{*}{[30]} \\  & & 0.25 mg/L & (UV light) & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Degradation of RhB over manganese dioxide using various experimental conditions.  
ethanol, and acetone (purchased from Sigma-Aldrich and Merck, Kenilworth, NJ, USA). The role of the Vulcan conductive carbon was to enhance the transfer of electrons between the electroactive species and the electrode, while the Nafion solution was used to improve the binding between the powder materials and the surface of the graphite support. All aqueous solutions were prepared using double-distilled water.  
The graphite bars were fixed in polyethylene tubes that became tightly attached to the bars following heat treatment at 180 \({}^{\circ}\)C. During the electrochemical experiments, one of the two ends of the bars was connected to the potentiostat, while the other end was modified and subsequently immersed in the electrolyte solution. Before the modification, the surface of each graphite support was polished using silicon carbide paper with different grit sizes (600, 800, and 1200) and felt. The supports were further washed with double-distilled water, ethanol, and acetone. After drying at RT, their surface was modified by drop-casting a volume of 10 \(\upmu\)L from suspensions prepared by ultrasonication, using the compositions specified in Table 2. This stage was followed by solvent evaporation at 40 \({}^{\circ}\)C for 4 h and at RT for 20 h.  
Ethanol and DMF were utilized to obtain the suspensions for the manufacturing of the modified electrodes based on reported studies in which the same reagents had been selected to prepare MnO\({}_{2}\)-containing suspensions for the same purpose [31; 32] and also because they ensure an improved dispersion of the hydrophobic Vulcan particles.