# Sulfur-Doped Carbon Nanohorn Bifunctional Electrocatalyst for Water Splitting
## 3 Results and Discussion
Additional analysis reveals that S-CNHs present a Tafel slope of 82 mV/dec, which is slightly higher than the one of Pt/C (i.e., 35 mV/dec). The latter demonstrates that the electrochemical desorption of adsorbed hydrogen atoms onto the electrode (Heyrovsky desorption) is the rate-limiting step for HER. On the other hand, HER of ox-CNHs and pristine CNHs is rate-limited by the initial adsorption of a proton onto electrode's surface (Volmer adsorption), which is screened by the much higher Tafel slope values of 240 and 320 mV/dec, respectively. The easier kinetics of HER, revealed from the much lower Tafel slope for S-CNHs, signifies more reactive/accessible protons and water molecules that reach the electrochemical active sites [33]. Next, EIS assays were performed to evaluate the HER kinetics. EIS measurements further confirm the superior HER kinetics of S-CNHs (Figure 6c), which show the smallest R\({}_{\rm ct}\) value of 30.4 \(\Omega\), comparable to that of Pt/C (i.e., 6.1 \(\Omega\)). On the other hand, ox-CNHs and pristine CNHs exhibited much higher R\({}_{\rm ct}\) values, 197 and 123.4 \(\Omega\), respectively. This tendency is in line with the results of Tafel slope values, which reveal improved kinetics and higher electrocataly activity of S-CNHs. Additionally, the long-term stability of S-CNHs was assessed on the basis of durability studies. Figure 6a shows the LSV polarization curves for HER of S-CNHs in comparison to that for ox-CNHs, pristine CNHs and Pt/C, after ongoing cycling for 10,000 cycles, where negligible loss of the cathodic current is noted. Table 2 summarizes the HER-derived data for all tested materials, before and after 10,000 cycles. Finally, the ECSA values were also calculated. Cyclic voltamographs for HER of S-CNHs, as compared to that for ox-CNHs, pristine CNHs and Pt/C, in a non-Faradaic region were measured at scan rates of 50, 100, 200, 300, 400 and 500 mV/sec (Figure 7). Particularly, S-CNHs exhibit the higher ECSA value, \(\sim\)205 cm\({}^{2}\), while the equivalent values that are noted for ox-CNHs and pristine CNHs are  
Figure 5: Cyclic voltamographs for OER of S-CNHs (**a**) before and (**b**) after 2000 cycles, ox-CNHs (**c**) before and (**d**) after 2000 cycles and pristine CNHs (**e**) before and (**f**) after 2000 cycles measured in a nitrogen saturated aqueous 1.0 M KOH, at a rotation speed of 1600 rpm and scan rates from 50 to 500 mV/s. Inset: Scan rate dependence of the current densities for the corresponding materials. Figure 6: (**a**) LSVs, before (solid lines) and after 10,000 cycles (dashed lines) for HER, (**b**) Tafel slopes, and (**c**) Nyquist plots of S-CNHs (red), ox-CNHs (black), pristine CNHs (grey) and Pt/C (blue). The LSV polarization curves obtained at 1600 rpm rotation speed and 5 mV/s scan rate. significantly lower, i.e., \(\sim\)130 and \(\sim\)31.25 cm\({}^{2}\), respectively. Similarly, the estimated ECSA values for the same materials after 10,000 cycles are remarkably lower than that recorded for S-CNHs. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Catalyst** & \begin{tabular}{c} **Onset Potential** \\ **(V vs. RHE)** \\ \end{tabular} & \begin{tabular}{c} **Potential (V vs. RHE)** \\ **at -10 mA/cm\({}^{2}\)** \\ \end{tabular} & \
\begin{tabular}{c} **Tafel Slope** \\ **(mV/dec)** \\ \end{tabular} &
\begin{tabular}{c} **ECSA** \\ **(cm\({}^{2}\)) b** \\ \end{tabular} & \(\mathbf{R_{ct}}\) **(\(\Omega\))** \\ \hline S-CNHs & \(-\)0.2 & \(-\)0.29 & 82 & 205 & 30.4 \\ S-CNHs a  & \(-\)0.21 & \(-\)0.33 & 82 & 190 & - \\ ox-CNHs & \(-\)0.25 & \(-\)0.43 & 240 & 130 & 197 \\ ox-CNHs a  & \(-\)0.26 & \(-\)0.48 & 245 & 102 & - \\ pristine CNHs & \(-\)0.34 & \(-\)0.75 & 320 & 47.5 & 123.4 \\ pristine CNHs a  & \(-\)0.35 & \(-\)0.79 & 320 & 31.25 & - \\ Pt/C & 0.034 & \(-\)0.0025 & 35 & 364 & 6.1 \\ Pt/C a  & 0.011 & \(-\)0.02 & 35 & 350 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Electrochemical parameters for HER.