Cathodic pre-polarization studies on the carbon felt/KOH interface: An efficient metal-free electrocatalyst for hydrogen generation

The recent decades have witnessed a booming research effort on the development of highly active electrode materials in electrocatalysis with the aim to replace the commonly used noble metal-based ones. Although non-noble metal electrodes can fulfill some requirements in terms of performance and price, their stability in alkaline media is generally weak. Additionally, from the environmental view point, the use of metals can cause water contamination. Therefore, the preparation of metal-free electrocatalysts represents an appealing approach to overcome the above-mentioned shortcuts. In this work, we used a simple electrochemical activation process, chronoamperometry measurements conducted in 1 M KOH at various overpotentials (−2, −2.5, −3, and −4 V vs. RHE) and pre-polarization periods (0.5, 2, 4, and 12 h), of commercially available and cheap carbon felt (CF) electrode to achieve good performance for hydrogen evolution reaction (HER) in alkaline media. Under optimized operating conditions (12 h of chronoamperometry measurement acquired at an overpotential of −4 V vs. RHE), the CF electrode exhibited an overpotential of 182 mV at 10 mA/cm2, a value surpassing that recorded for the most efficacious metal-free HER electrocatalysts published in the literature. The two CF samples exposed to 12 h of cathodic activation at −2.5 and −3.0 V vs. RHE recorded respectively Tafel slope (bc) values of 110 and 126 mV dec−1. These bc values are close to the standard bc value of 120 mV dec−1, suggesting the Volmer step as the main step controlling the HER. Increasing the applied cathodic potential to −4.0 V vs. RHE further decreased the bc value to 90 mV dec−1, referring to increased accessible active catalytic sites for the HER. The cathodically activated CF surface's morphology and composition were assessed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) examinations. Raman spectroscopy was also conducted to assess the influence of the cathodic activation process on the structural defects/disorder of the CF surface. The formation of oxygen-containing groups on the CF surface after cathodic activation, as evidenced from XPS analysis, is believed to enhance water adsorption and act as active sites for hydrogen generation. The technique developed herein is simple and straightforward and can be easily implemented for almost any type of electrode.