# Sulfur-Doped Carbon Nanohorn Bifunctional Electrocatalyst for Water Splitting
## 3 Results and Discussion
However, since it is hard to determine the exact value, the potential at 10 mA/cm\({}^{\text{2}}\) is commonly used as a benchmark. In OER, overpotential is the potential difference between the potential registered at a specific current density and the equilibrium potential, i.e., 1.23 V [1; 2]. Figure 4a screens the polarization curves of S-CNHs, ox-CNHs and pristine CNHs. It is clear from the LSV curves and the recorded values of overpotentials registered at 10 mA/cm\({}^{\text{2}}\) current density (Table 1) that S-CNHs possess the best electrocatalytic activity with the lowest potential at 1.63 V vs. RHE. Such a low overpotential is considered excellent for the OER, and is close to the overpotentials registered for noble metals [1; 2; 12]. On the contrary, the corresponding potential for both ox-CNHs and pristine CNHs was registered at 1.82 vs. RHE, 220 mV higher than that of S-CNHs. At the same time, S-CNHs reached high current density values compared to the other two reference materials, which is a prerequisite for an effective OER electrocatalyst. The superior electrocatalytic activity of S-CNHs is attributed to the presence of sulfur dopants, which induce a non-uniform spin density distribution due to the mismatch of the outermost orbitals of sulfur and carbon atoms, while further altering the charge density at nearby sites. Figure 2: (a) ATR-IR spectra, (b) Raman spectra (514 nm), and (c) thermogravimetric analysis graphs obtained under nitrogen atmosphere of S-CNHs (red), ox-CNHs (black) and pristine CNHs (grey). To define how swiftly the current increases with the applied overpotential, Tafel slopes were extracted from the LSV curves. Specifically, S-CNHs showed a low Tafel slope value of 80 mV/dec, as shown in Figure 4b. Low Tafel slope values are required for ideal electrocatalysts as they result in an improved OER rate under a constant increase of overpotential, which can be beneficial in real applications. The two reference materials, ox-CNHs and pristine CNHs, accordingly showed \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Catalyst**} & \multicolumn{2}{c}{**Potential (V vs. RHE)**} & \multicolumn{2}{c}{**Overpotential (mV vs.**} & \multicolumn{2}{c}{**Tafel Slope**} & \multicolumn{2}{c}{**ECSA**} \\  & **at 10 mA/cm\({}^{2}\)** & **RHE) at 10 mA/cm\({}^{2}\)** & **(mV/dec)** & **(cm\({}^{2}\)) b** & **R\({}_{\text{ct}}\) (\(\Omega\))** \\ \hline S-CNHs & 1.63 & 400 & 80 & 67.5 & 10.8 \\ S-CNHs a  & 1.65 & 420 & 84 & 61.25 & - \\ ox-CNHs & 1.82 & 590 & 202 & 60 & 560.5 \\ ox-CNHs a  & 2.5 & - & 279 & 13.75 & - \\ pristine CNHs & 1.82 & 590 & 333 & 19.4 & 450.5 \\ pristine CNHs a  & 2.5 & - & 310 & 4.7 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Electrochemical parameters for OER. Figure 4: (a) LSVs, before (solid lines) and after 2000 cycles (dashed lines) for OER, (b) Tafel slopes, and (c) Nyquist plots of S-CNHs (red), ox-CNHs (black) and pristine CNHs (grey). Inset (a) LSVs at 10 mA/cm\({}^{2}\); (c) Nyquist plot for S-CNHs. The LSVs obtained at 1600 rpm rotation speed and 5 mV/s scan rate. Figure 3: (a,b) HR-TEM micrographs showing the structure of S-CNHs. (c) XPS survey spectrum recorded on one of these S-CNHs samples, where S\({}_{\text{2p}}\), C\({}_{\text{1s}}\) and O\({}_{\text{1s}}\) are observed. Impurities \(<\)1% due to F are shown with *. (d) High-resolution S\({}_{\text{2p}}\) XPS of S-CNHs. The peaks are fitted to four components centered at 162.2, 164.1, 165.3 and 168.4, corresponding to S\({}_{\text{p3/2}}\), S\({}_{\text{p3/2}}\), S\({}_{\text{p1/2}}\), and S-O, respectively. much higher Tafel slope values of 202 and 333 mV/dec, which correspond to slower OER kinetics. Electrochemical impedance spectroscopy (EIS) assays were performed to evaluate the OER kinetics. S-CNHs showed the smallest frequency semicircle in the Nyquist plot, which corresponds to a small charge transfer resistance (R\({}_{\text{ct}}\)) value of 10.8 \(\Omega\) (Figure 4c).