# Sulfur-Doped Carbon Nanohorn Bifunctional Electrocatalyst for Water Splitting
## 1 Introduction  
Electrochemical water splitting is a cost-effective clean method for breaking down water to hydrogen (H\({}_{2}\)) and oxygen (O\({}_{2}\)) and is absolutely important in hydrogen economy. In a water electrolyzer, water electrolysis is separated into two half-reactions: hydrogen evolution reaction (HER) happens at the cathode, i.e., 2H\({}^{+}\) + 2e\({}^{-}\)\(\rightarrow\) H\({}_{2}\) (under acidic conditions), while oxygen evolution reaction (OER) occurs at the anode, i.e., 2H\({}_{2}\)O \(\rightarrow\) O\({}_{2}\) + 4H\({}^{+}\) + 4e\({}^{-}\). Notably, OER is more kinetically sluggish compared to HER, as it is a four-electron transfer reaction, while HER requires only two electrons. To date, both of these half-reactions have relied on noble-metal-based electrocatalysts to reach their best performance, namely Pt for HER, and IrO\({}_{2}\) and RuO\({}_{2}\) for OER [1; 2; 3; 4]. Despite their high activity, noble-metal-based electrocatalysts are expensive, and this limits their employment in real applications. In this context, progress has been made towards the exploration of non-precious-based metal electrocatalysts, while recent trends want transition metal-based and carbonaceous electrocatalysts to reach low overpotentials for HER and OER [5; 6; 7; 8; 9; 10; 11; 12; 13]. Moreover, due to the different mechanisms that govern HER and OER, it is quite often a good electrocatalyst that can perform at lower overpotential for HER to show poor activity for OER and vice versa. Therefore, it still remains quite a challenge to effectively employ highly active, stable, abundant and low-cost bifunctional electrocatalysts for water electrolysis that compete the performance of the precious metals ones [1; 14].  
Carbon nanohorns (CNHs) consist of single-graphene tubules, with highly strained conical ends that aggregate in larger spherical superstructures [15; 16; 17]. Their unique shape, chemical defects and crystal edges make them interesting material as electrocatalyst [15; 16]. Specifically, CNHs have mostly been employed as supports, and have shown encouraging results in crucial electrocatalytic reactions involved in fuel cells, assisting in reaching superior performances as well as improved durability and stability [15]. In addition, doping with heteroatoms has proven to be an efficient method for increasing the catalytic activity of graphene-related materials [18; 19; 20; 21] and specifically CNHs [22; 23; 24; 25]. Nevertheless, effective, easy and inexpensive ways of incorporating heteroatom dopants is still challenging, since doping methods mostly involve tedious and expensive preparation techniques [15; 26; 27]. However, CNHs are still not fully exploited as electrocatalysts, since there are only very few works dealing with CNHs in HER [28; 29] and OER [30] and none as bifunctional electrocatalyst for both.  
Considering all of the aforementioned points, we embarked on the preparation of sulfur-doped CNHs (S-CNHs) and the subsequent assessment of their electrocatalytic ability. Herein, S-CNHs were prepared by employing a simple and inexpensive one-pot solvothermal method, fully characterized using complementary spectroscopic, thermal and microscopy imaging means, and their bifunctional nature to electrocatalyze the HER and OER was screened. Notably, for the first time, we report data showing excellent electrocatalytic activity and durability of S-CNHs for both HER and OER.