Phosphine-Functionalized Graphene Oxide, A High Performance Electrocatalyst for Oxygen Reduction Reaction

Recent interests in generating a sustainable energy economy have given much interest to the concept of the hydrogen economy1. Fuel cells have drawn substantial interest as energy production devices, due to their potentials in low pollution output and high theoretical energy densities. Proton exchange membrane fuel cells (PEMFCs) are, arguably, the most commercialized fuel cells, but one of the key practical challenge for PEMFCs is the slow cathodic kinetics of oxygen reduction reaction (ORR) at the cathode that reduces the performance of fuel cells. Another major challenge is the high cost of Platinum based catalysts. Also platinum can suffer from strong adsorption of carbon monoxide (CO) at the platinum surface and blocking the active sites, so dictating the use of highly purified hydrogen sources. Among the various alternatives for Pt-based catalyst, carbon-based materials are of great interest as non-precious metal catalysts for the ORR due to their relatively high activity and superior stability2. Here, a novel approach for the preparation of phosphine-functionalized graphene oxide (GOPPh2) is developed. Using a simple method, diphenylphosphine group was linked to the hydroxyl group of OH-functionalized graphene. The GO-PPh2 catalyst was characterized using some different microscopic and spectroscopic techniques including XPS, XRD, TGA, SEM and FT-IR analysis. The electrocatalytic activities of GO-PPh2, rGO, and Pt/C toward the ORR were studied using linear sweep voltammetry (LSV) in N2 and O2 saturated 0.1 mol L‒1 KOH solution, from +0.20 V to ‒0.60 V vs. Ag/AgCl and at a scan rate of 10 mV s‒1 for GO-PPh2, rGO, and Pt/C catalysts displayed featureless curves in N2-saturated solution. On the other hand, the LSV curves present a well-defined ORR peak in O2-saturated solution for proposed electrocatalysts, which demonstrate the good electrocatalytic activity of GO-PPh2 in comparison with unmodified glassy carbon electrode and rGO modified glassy carbon electrode catalyst toward the ORR. For 228 investigation the number of electron at different potential (from ‒0.3 to ‒0.8 V), the slopes remain nearly constant, which confirms that at the different electrode potentials, numbers of the electron transfer for reduction of oxygen are analogous. Based on the K–L equation, the calculated n was 3.83 for GO-PPh2 (closing to 4e- pathway) and 2.27 for rGO. These results confirm that the ORR activity of GO-PPh2 follows the 4-electron transfer pathway. The electrocatalytic activity of GO-PPh2 in ORR vs. the electrooxidation of methanol was studied. After addition of methanol into the O2-saturated solution for Pt-rGO, a large current from the oxidation of methanol was observed. On the other hand, with GO-PPh2 electrode at the same conditions, no significant difference between the peak currents can be observed, with and without methanol. From these results, it is suggested that GO-PPh2 is free from methanol poisoning. So the modified electrode exhibit a high selectivity for ORR in a nearly 4e transferred per O2 molecule, with a remarkable tolerance to crossover effects compared to platinum nanoparticle decorate on reduce graphene oxide (Pt/rGO) and they have good durability over the several hours. Therefore GO-PPh2 is convenient electrocatalyst for its further applications in fuel cells. Figure. (Left) Survey XPS data for GO-PPh2; (Right) (A) The GO-PPh2 LSV curves of ORR of modified GCE obtained at different rotating rates; (B) K-L plots of current reciprocal (-i-1) versus ω-1/2 at different potential on GO-PPh2 modified GCE.