# Sulfur-Doped Carbon Nanohorn Bifunctional Electrocatalyst for Water Splitting
## 3 Results and Discussion
On the contrary, the very high R\({}_{\text{ct}}\) values of the ox-CNHs and pristine CNHs, 560.5 and 450.5 \(\Omega\), respectively, reveal high charge transfer resistance. The higher R\({}_{\text{ct}}\) value of ox-CNHs compared to pristine CNHs is justified by the disruption of the graphitic network due to the introduction of carboxylic species. Conversely, the low charge transfer resistance that was reported for S-CNHs arises from the increased conductivity due to the improvement of the graphitic degree resulting from the sulfur doping. Stability is a crucial factor for the evaluation of the overall performance of an electrocataly. Figure 4a shows the LSV polarization curves for OER of S-CNHs as compared to that for ox-CNHs and pristine CNHs, after continuous cycling for 2000 cycles. Notably, negligible loss of the anodic current is noted for S-CNHs. As a striking difference, the two reference materials exhibit extremely huge losses of the anodic current that are unable to reach current density values at 10 mA/cm\({}^{2}\). Electrochemical OER data for S-CNHs, ox-CNHs and pristine CNHs before and after 2000 cycles are summarized in Table 1.  
To get a better understanding of the charge transport flow in S-CNHs in OER, the electrochemically active surface area (ECSA) was estimated. In more detail, ECSA values were calculated based on the following equation [20; 33; 34; 35; 36]: ECSA = \(C_{\text{{\it dl}}}\)/\(C_{s}\), where \(C_{\text{{\it dl}}}\) stands for the electrochemical double-layer capacitance, while \(C_{s}\) is the specific capacitance of a flat surface with 1 cm\({}^{2}\) of real surface area with a value assumed to be 40 \(\mu\)F/cm\({}^{2}\) for the flat electrode. For the estimation of the ECSA, cyclic voltamographs were recorded in a non-Faradaic region at scan rates of 50, 100, 200, 300, 400 and 500 mV/sec for all tested materials before and after 2000 cycles (Figure 5). ECSA values were obtained from \(C_{\text{{\it dl}}}\) by plotting the \(\Delta j=(j_{a}-j_{c})\) at a given potential versus the scan rate as stated in the equation \(C_{\text{{\it dl}}}=d(\Delta j)/2 dV_{b}\). Specifically, S-CNHs showed higher ECSA value of \(\sim\)67.5 cm\({}^{2}\). Consistent with the LSV polarization curves for OER, ox-CNHs and pristine CNHs present significantly lower ECSA, ca. \(\sim\)60 and \(\sim\)19.4 cm\({}^{2}\), respectively. These values are in accordance with the electrocatalytic results towards OER, since the efficient accessibility of the active sites is related to higher ECSA values. Moreover, after cycling continuously for 2000 cycles, a slight decrease of the ECSA was noted for S-CNHs, \(\sim\)61.25 cm\({}^{2}\), while ox-CNHs and pristine CNHs exhibited huge drops at \(\sim\)13.75 and \(\sim\)4.7 cm\({}^{2}\), respectively. Next, the electrochemical properties of S-CNHs against HER were evaluated by performing LSV measurements in N\({}_{2}\)-saturated 0.5 M aqueous H\({}_{2}\)SO\({}_{4}\). The polarization curves of S-CNHs, along with ox-CNHs and pristine CNHs as well as commercially available 20 wt% Pt on carbon black (Pt/C) as reference materials, can be seen in Figure 6a. The lowest onset potential is registered at \(-\)0.2 V vs. RHE for S-CNHs, which is more positive by 50 and 120 mV in comparison with the values noted for ox-CNHs (i.e., at \(-\)0.25 V) and pristine CNHs (i.e., at \(-\)0.32 V), respectively. In addition, potential values for HER are often compared at the functional current density for electrochemical water splitting of \(-\)10 mA/cm\({}^{2}\), where sufficient hydrogen production is achieved, while faulty interpretations from intrinsic electroactivity are prevented. It is clear that S-CNHs show higher electrocatalytic activity at \(-\)0.31 V vs. RHE, shifted by 120 and 440 mV to more positive potentials compared to the value recorded for ox-CNHs (i.e., at \(-\)0.43 V) and pristine CNHs (i.e., at \(-\)0.75 V), respectively. Furthermore, to define the rate-limiting step of HER, Tafel slopes were extracted from the LSV curves (Figure 6b).