Development of Nanostructured-Carbon-Supported Gold Nanoparticles As Catalysts for Electroreduction of Oxygen and Carbon Dioxide

Abstract

There has been growing interest in the field of oxygen electroreduction, particularly with respect to potential applications in the science and technology of low-temperature fuel cells.. Obviously, many efforts have been made to develop suitable alternative electrocatalysts efficient enough to replace electrocatalysts based on scarce strategic elements such as platinum-group metals. Despite intensive research in the area, there are still a number of fundamental problems to be resolved, and the practical oxygen reduction catalysts still utilize systems based on platinum. Various homogeneous and heterogeneous catalytic systems have been proposed to induce the CO2 reduction and, depending on reaction conditions (applied potential, strength of the electrolyte, local pH, local concentration of CO2 and the catalyst applied), various products can be obtained. Because the CO2 molecule is very stable, its electroreduction would be characterized by large over-potentials. In this respect, behavior of carbon dioxide resembles that of dioxygen. In both cases, the limiting steps involve protonation of the respective species. The present study refers to a novel and unique approach of fabrication and deposition of gold nanoparticles on the surfaces of reduced graphene oxide and multi-walled carbon nanotubes. Among important issues is application of inorganic (rather than organic) capping ligand, heteropolymolybdate, to modify and stabilize (as well as probably also to link with the oxygen or hydroxyl groups on graphene surfaces) gold nanoparticles. During operation in alkaline medium or neutral media, polyoxometallates disappear but catalytically highly active gold remains and it exhibits excellent stability. The resulting material has occurred to show highly potent electrocatalytic properties toward electroreductions of carbon dioxide in neutral medium and oxygen in alkaline solution. What is even more important is that both carbon nanotubes and graphene have occurred to act effectively as carriers for gold nanostructures. Mutual activating interactions are feasible. The conclusions are reached on the basis of diagnostic electrochemical (e.g. rotating ring disk voltammetry), spectroscopic (FTIR) and microscopic (SEM, TEM) experiments. A series of comparative experiments with different carbon carriers and model catalytic materials (e.g. Vulcan-supported platinum) have also been performed. With respect to oxygen reduction, our diagnostic experiments at different concentrations of H2O2, support a view that the effect of the fast following chemical (H2O2-reductive-decomposition) reaction could be the dominating factor in explaining the observed positive potential shift observed during the oxygen reduction. The fact, that the optimum graphene-based catalytic system produced the oxygen reduction peak current comparable to that observed at the model platinum containing catalyst, would imply the efficient four-electron-type reduction mechanism.