## 3 Results and Discussion
### Effect of coolant Electrical Conductivity on HER  
In this study, coolants were progressively diluted in MEG, and the amount of Cu corrosion inhibitor in each solution was adjusted to match that of the undiluted coolants. The final pH and electrical conductivity of each solution are presented in Table 4. As the  
Figure 4: Linear sweep voltammetry curves of BEV coolants containing different Cu corrosion inhibitors. electrical conductivity of a solution depends on its ionic concentration and mobility, diluted solutions showed reduced electrical conductivity [33]. The LSV cathodic scan of the polarized copper electrodes in progressive dilutions of the BEV and ICE coolants is presented in Figure 5a,b, respectively. The onset potential for hydrogen evolution, i.e., the overpotential at which the HER starts slightly increased with the progressive dilution of the coolants. In Figure 5b, it can be observed that ICE solutions containing less than 25% of coolant (electrical conductivity bellow 712 \(\mu\)S/cm) did not present fluctuations in the current evolution at higher overpotentials due to the reduced hydrogen evolution at the electrode surface. \begin{table}
\begin{tabular}{c c c c c} \hline
**Coolant** & \begin{tabular}{c} **Coolant** \\ **Concentrate (w\%)** \\ \end{tabular} & \
\begin{tabular}{c} **Monoethylene** \\ **Glycol (w\%)** \\ \end{tabular} & **pH** &
\begin{tabular}{c} **eConductivity at 25 \({}^{\circ}\)C** \\ **(\(\mu\)S/cm)** \\ \end{tabular} \\ \hline \multirow{4}{*}{BEV} & 100 & 0 & 8.11 & 86.43 \\  & 75 & 25 & 8.05 & 71.05 \\  & 50 & 50 & 8.04 & 50.44 \\  & 25 & 75 & 7.95 & 28.70 \\  & 10 & 90 & 7.80 & 13.21 \\ \hline \multirow{4}{*}{ICE} & 100 & 0 & 7.54 & 2576.80 \\  & 75 & 25 & 7.51 & 2040.70 \\  & 50 & 50 & 7.47 & 1430.60 \\  & 25 & 75 & 7.43 & 712.20 \\  & 10 & 90 & 7.31 & 299.10 \\  & 2.75 & 97.25 & 7.38 & 86.40 \\ \hline \end{tabular}
\end{table} 
Table 4: Summary of coolant dilutions and their corresponding pH and electrical conductivity at 25 \({}^{\circ}\)C.  
Figure 5: Linear sweep voltammetry curves of different dilutions of (**a**) BEV and (**b**) ICE coolants. All solutions contain a constant amount of TTZ. The absolute current density recorded at \(-5\) V (vs Ag/AgCl) was plotted versus the solution's electrical conductivity, and it is presented in Figure 6. Progressive dilutions from both BEV and ICE coolants resulted in reduced current densities. The relation between the current density at \(-5\) V (vs Ag/AgCl) and the electrical conductivity of the solutions follows a linear correlation for lower conductivities (BEV coolants) and transitions to an exponential relation at higher conductivities (ICE coolants). These results can be explained by the fact that when the copper electrode is polarized in the coolants, the kinetics of HER can be limited by three factors: the activation energy, the concentration polarization, and the ohmic loss due to the low electrical conductivity of the electrolyte [24]. Activation energy, which is the energy necessary for the reactions to initiate, depends on the electrode material and electrolyte nature. For HER in alkaline electrolytes, it involves two electrochemical steps: the adsorption of H\({}_{\text{ads}}\) and OH\({}_{\text{ads}}\) (Volmer step) and the recombination of adsorbed H\({}_{\text{ads}}\) to produce H\({}_{2}\) (Tafel or Heyrovsky step) [34]. The slower step determines the rate of reaction when under activation polarization. The kinetics of HER can also be limited by concentration polarization, where the rate of HER is limited by the availability of reagents in the vicinity of the electrode/electrolyte interface, resulting in an HER rate controlled by mass transport. In our experiment, this is the case for the oxygen reduction reaction but not for HER, as seen in Figure 5. Finally, in electrolytes with low electrical conductivity (i.e., high electrical resistivity), ions that transport the current encounter high resistance, thereby reducing the rate of the reactions.