Heteroatom (N, O, and S)-Based Biomolecule-Functionalized Graphene Oxide: A Bifunctional Electrocatalyst for Enhancing Hydrazine Oxidation and Oxygen Reduction Reactions

In this work, a new method is developed to synthesize an L-cysteine-based graphene oxide (L-Cy-rGO) electrocatalyst by a chemical synthesis approach. The electrocatalytic studies of L-Cy-rGO for the oxygen reduction reaction (ORR) and hydrazine oxidation reaction (HOR) have been demonstrated, as important fuel-cell oxidation and reduction reactions confirm its bifunctional nature. The electrochemical ORR performance of L-Cy-rGO is significantly improved with an onset potential of 0.77 V vs reversible hydrogen electrode (RHE) and a current density of -2.32 mA/cm(2) in O-2-saturated 0.5 M KOH electrolytes. The electrochemical impedance spectroscopy (EIS) and chronoamperometric (i-t) measurements of the electrocatalyst are also carried out toward determining the feasibility of electron transfer and current/potential stability at the interface. The L-Cy-rGO electrocatalyst shows excellent activity toward ORR in alkaline medium. Furthermore, L-Cy-rGO shows better electrocatalytic activity toward HOR at an onset potential of 1.01 V vs RHE and the maximum current density of 65 mA/cm(2) at a potential of 1.59 V vs RHE at 35 mu M hydrazine hydrate in 0.5 M KOH. The electrochemical studies show that the L-Cy-rGO exhibits the highest electrocatalytic activity toward hydrazine oxidation. Moreover, the L-cysteine-functionalized graphene oxide supporting material plays an excellent role that could be from their synergistic catalytic effect. The L-Cy-rGO electrocatalyst shows excellent electrochemical ORR and HOR performances due to the presence of S- and N-heteroatom-containing surface of GO that enhances the electrocatalytic activity and electron transfer capabilities toward the ORR. Morphological studies based on high-resolution transmission electron microscopy (HRTEM) confirm that the size of L-Cy-rGO is similar to 10 nm. X-ray photoelectron spectroscopy (XPS) analysis confirms the surface functionalization of GO by L-cysteine (L-Cy-rGO) from the binding energies of C-S, C-N, C-O, and C-C signals. Based on these findings, we find that the metal-free amino acid-functionalized carbon-based electrocatalyst shows excellent electrochemical ORR and HOR performances and demonstrate its key role toward enhancement in activities.

Automated summary

The authors developed a new method to synthesize an L-cysteine-based graphene oxide electrocatalyst and demonstrated its bifunctional nature in the oxygen reduction reaction and hydrazine oxidation reaction. The electrocatalyst showed excellent activity in alkaline medium and towards hydrazine oxidation. High-resolution transmission electron microscopy confirmed the size of the electrocatalyst to be around 10 nm, and X-ray photoelectron spectroscopy analysis confirmed the surface functionalization by L-cysteine. The metal-free amino acid-functionalized carbon-based electrocatalyst exhibited outstanding performances in both ORR and HOR.