# Sulfur-Doped Carbon Nanohorn Bifunctional Electrocatalyst for Water Splitting
## 2 Materials and Methods
### General  
Chemicals, reagents, and solvents were purchased from Sigma-Aldrich and used without further purification. Infrared (IR) spectra were obtained on a Fourier Transform IR spectrometer (Equinox 55 from Bruker Optics, Billerica, Massachusetts, US) equipped with a single reflection diamond attenuated total reflection (ATR) accessory (DuraSamp1 IR II by SensIR Technologies, Danbury, CT, USA). Raman measurements were recorded with a Renishaw confocal spectrometer (Wotton-under-Edge, UK) at 514 nm. The data were obtained and analyzed with Renishaw Wire and Origin software (OriginPro 2018, Northampton, MA, USA). Thermogravimetric analysis was performed using a TGA Q500 V20.2 Build 27 instrument by TA (New Castle, DE, USA) in a nitrogen (purity \(>\) 99.999%) inert atmosphere. High-resolution TEM was performed employing an imaging-side aberration-corrected FEI Titan-Cube microscope working at 80 kV, equipped with a Cs corrector (CETCOR from CEOS GmbH, Heidelberg, Germany). XPS were recorded on a Kratos Axis Supra spectrometer, (Manchester, UK) equipped with a monochromated Al K\(\kappa\) X-ray source using an analyzer pass energy of 160 eV for survey spectra and 20 eV for the core level spectra. Spectra were recorded by setting the instrument to the hybrid lens mode and the slot mode providing approximately a \(700\times 300\)\(\upmu\)m\({}^{2}\) analysis area using charge neutralization. Regions were calibrated using the reference value BE(C 1s sp2) = 284.5 eV.  
All XPS spectra were analyzed using CASA XPS software (2.3.23, Teignmouth, Devon, UK). The XPS peaks were fitted to GL(70) Voigt lineshape (a combination of 70% Gaussian and 30% Lorentzian character), after performing a Shirley background subtraction. For the light-assisted oxidation reaction of CNHs the light source used was a 500 W halogen lamp, which was positioned 20 cm away from the reactor. Autoclave synthesis was undertaken in a Parr autoclave reactor with a 300 mL Teflon-lined stainless-steel vessel. All electrochemical measurements were carried out using an Metrohm Autolab PGSTAT128N potentiostat/galvanostat and were carried out at room temperature in a standard three-compartment electrochemical cell by using a platinum wire as a counter-electrode, an RDE with glassy carbon disk (geometric surface area: 0.0196 cm\({}^{2}\)) as a working electrode, and Hg/HgSO\({}_{4}\) (0.5 M K\({}_{2}\)SO\({}_{4}\)) and Hg/HgO (0.1 M KOH) (AMEL S.r.l., Milan, Italy) as reference electrodes for HER and OER, respectively. LSV measurements for HER were carried out at room temperature in N\({}_{2}\)-saturated 0.5 M H\({}_{2}\)SO\({}_{4}\). LSV measurements for OER were carried out at room temperature in O\({}_{2}\)-saturated 1.0 M KOH. The catalyst ink was prepared by dispersing 4.0 mg of the catalytic powder in a 1 mL mixture of water, isopropanol, and 5% Nafion (_v_/_v_/_v_ = 4:1:0.02) and sonicated for 30 min prior to use. Before casting the electrocatalytic ink on the electrode's surface, the working electrode was polished with 6-, 3- and 1-mm diamond pastes, rinsed with deionized water, and sonicated in double-distilled water. Afterwards, 8.5 \(\mu\)L aliquots of the electrocatalyst were casted on the electrode surface and were left to dry at room temperature. Electrochemical Impedance Spectroscopy (EIS) measurements (Metrohm Autolab, Utrecht, The Netherlands)) were conducted from 10\({}^{5}\) to 10\({}^{-1}\) Hz with an AC amplitude of 0.01 V, at a potential where significant HER and OER current was recorded, at -2 mA/cm\({}^{2}\) for HER and at 3 mA/cm\({}^{2}\) for OER. EIS data were fitted to Randles circuit. In EIS fit and simulation Chi square value is <0.001. The data were obtained and analyzed with Nova 2.1.4 software (Metrohm Autolab, Utrecht, The Netherlands).