Gold aerogel supported on graphitic carbon nitride: an efficient electrocatalyst for oxygen reduction reaction and hydrogen evolution reaction

Abstract

Fabrication of a high surface area interconnected porous network of metallic nanomaterials is important for their applications in various fields such as catalysis, sensors, and electrochemistry. Here we report a facile bottom up synthesis of high surface area and porous gold aerogel supported on carbon nitride sheets (CN_x). The reduction of HAuCl₄ in the presence of carbon nitride nanosheets using sodium borohydride and ultrasonic treatment produces gold aerogel supported on carbon nitride (Au–aerogel–CN_x). When the reduction of HAuCl₄ in the presence of CN_x nanosheets was performed using only ultrasonication, highly dispersed ultrasmall (∼2 nm) gold nanoparticles on CN_x sheets (AuNPs–CN_x) were formed. The Au aerogel supported on CN_x sheets was well characterized by powder X-ray diffraction, tunneling electron microscopy, selected area electron diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy, UV-visible and X-rayphotoelectron spectroscopic methods. The Au–aerogel–CN_x and AuNPs–CN_x composites exhibited superior electrocatalytic activity towards oxygen reduction reaction (ORR) in alkaline and acidic media. The Au–aerogel–CN_x composite showed ORR onset potentials at 0.92 V and 0.43 V (vs. RHE) in 0.5 M KOH and H₂SO₄ solution. The four electron oxygen reduction process occurred at these supported catalysts in both alkaline and acidic media. In alkaline (KOH) medium the onset potential at Au–aerogel–CN_x was more positive (∼30 mV) than that of commercial Pt/C catalyst. The composites displayed excellent methanol tolerance and comparable durability with commercial Pt/C. Furthermore, the Au–aerogel–CN_x composites exhibited high catalytic activity for the hydrogen reduction reaction (HER) with a small onset potential of −30 mV and a Tafel slope of 53 mV dec⁻¹ in acidic medium. At a small Au loading of 0.130 mg cm⁻², this catalyst also exhibits a current density of 10 mA cm⁻² at a low overpotential of −185 mV with excellent stability. The ORR and HER performances on porous Au–aerogel–CN_x composites were better than those of the AuNPs–CN_x catalyst and commercial flat gold electrode. The superior ORR and HER activities at the Au–aerogel–CN_x composite originated from the unique synergistic effects between the porous Au network and carbon nitride (CN_x) support.