*3.3. Electrocatalytic Properties of the Electrode Materials*

Characterization of the electrodes' surfaces during the optimization of the final electrode modification was performed using CV and EIS. CV was employed to determine the interfacial properties of electrode materials and examine electron transfer kinetics between the electrode surface and the electrolyte. Firstly, the unmodified carbon paste electrode was compared to La(OH)3-, MWCNT-, and La(OH)3@MWCNT-modified CP electrodes to define the optimal nanomaterial composition to use for electrode modification. CVs were performed in PB at pH 6 in the 5 mM redox probe K3[Fe(CN)6]/K4[Fe(CN)6] and 0.1 M KCl support electrolyte, at the scan rate of 50 mV/s (Figure S1). The oxidation and reduction peaks that originate from the Fe2+/3+ redox pair are visible on every voltammogram obtained using unmodified and modified CP electrodes (Figure S1A). The oxidation peak currents, for 2% (weight percent) of individual modifiers in the paste amount 16.73 μA, 12.39 μA, 16.87 μA and 18.21 μA for CP, La(OH)3/CP, MWCNTs/CP and La(OH)3@MWCNTs/CP, respectively. Consequently, La(OH)3@MWCNTs/CP was determined to have the best electrocatalytic properties and was chosen for further examination. The next step in the electrode's surface optimization was varying the modifier percentage in CP. The resulting voltammograms (Figure S1A) show a steady rise in oxidation peak current with the increase in the share of the modifier in CP. The peak current values equaled 18.13 μA for 2%, 23.47 μA for 5%, and 30.81 μA for 10% of La(OH)3@MWCNTs in the CP. Furthermore, with the increase in the modifier content in CP from 2% to 10%, the oxidation peak potential shifted from 0.46 V to 0.32 V, with peak-to-peak separations (ΔE) = 0.69 V, 0.47 V and 0.36 V and oxidation/reduction current ratios (Ia/Ic) = 1.02, 1.01 and 1.04 for 2%, 5% and 10% La(OH)3@MWCNTs/CP, respectively. Higher values for the peak-to-peak separation in this system, in comparison with the theoretical value of 59 mV, is a common phenomenon and it is the product of the heterogeneity of the hand-made carbon paste electrodes. The reported decrease in the ΔE value, as a result of the increase in the amount of the modifier, indicate excellent properties of the selected composite regarding the diffusion properties of the electrode surface. Since oxidation occurs on lower potentials and the other parameters point out that the electrode reaction is reversible when

using 10% La(OH)3@MWCNTs/CP, this electrode material is assumed to have the best electrocatalytic properties.

To further support the abovementioned assumptions, effective surface areas were estimated for each electrode. Calculations were performed using the Randles–Sevcik Equation and the specific surface areas of the electrodes were 2.02 mm2, 1.50 mm2, 2.04 mm2, 2.20 mm2, 2.83 mm2 and 3.72 mm2 for CP, La(OH)3/CP, MWCNT/CP, 2% La(OH)3@MWCNT/CP, 5% La(OH)3@MWCNT/CP and 10% La(OH)3@MWCNT/CP, respectively. This proves, once again, that composite material formation is a crucial part of the modification process. Not only does it enhance the material's electrocatalytic properties, but also leads to an increase in the effective surface area.

In addition to estimating the electron transfer resistance of the chosen electrode and comparing its values to those obtained using different materials, EIS gives information on other conductivity/resistance-related properties of the electrode system, such as doublelayer capacitance or diffusion rate. EIS spectra consist of a semicircle (high frequency) and a linear (low frequency) region. The semicircle radius is electron transport resistancedependent and is defined by its Rct value, while the linear part is diffusion-dependent. The measurements were conducted in PB pH 6, in the 5 mM redox probe containing Fe2+/3+ redox pair from cyano complexes and 0.1 M KCl support electrolyte. Rct values of CP, La(OH)3/CP, MWCNTs/CP and La(OH)3@MWCNTs/CP (Figure S2A) were 40,480 Ω, 29,956 Ω, 48,345 Ω and 35,160 Ω, respectively.

Contrary to our beliefs, these values indicate that MWCNTs have the poorest electrocatalytic features. However, La(OH)3 encourages the electron shuttle and thus has the lowest Rct value, while the composite material exhibits improved properties when compared to the unmodified paste and MWCNTs alone. Moreover, as previously discussed, this material shows the best current response in the cyclic voltammetry measurements, which confirms that La(OH)3@MWCNTs exhibits the optimal electrocatalytic activity. Furthermore, the increase in the La(OH)3@MWCNTs amount in the paste leads to the reduction in Rct and thus the improvement of the overall electrochemical performance, which is evident from the experimental data (Figure S2B). The obtained Rct values are 35,015 Ω, 29,889 Ω, and 19,559 Ω for the 2%, 5%, and 10% La(OH)3@MWCNTs/CP, respectively. In short, better CV response and the decrease in the Rct values with the increase in the modifier content prove the positive impact of the composite formation and the synergetic effect of its components on the electron transfer kinetics enhancement.
