Hybrid Electrocatalysts Composed of PtSn, Ru or PtRu Nanoparticles for Low-Temperature Oxidation of Dimethyl Ether Fuel

Abstract

The science and technology of proton-exchange membrane (PEM) low-temperature fuel cells is a subject of intense during recent years, particularly with use of hydrogen gas due to its molecular simplicity and fast kinetics of its electrooxidation. However, there are still many fundamental problems to be resolved with respect to generation of pure hydrogen and related pollution side effects, the low density and safety of hydrogen storage and distribution. Thus, there are parallel studies involving such simple organic molecules as methanol, formic acid or ethanol as substitute fuels for a wide range of potential applications. Among important obstacles are slow kinetics of electrocatalytic oxidations of fuels and insufficient durability of operation of catalysts that includes poisoning of active sites by strong adsorption of the reaction intermediate products (e.g. CO-type adsorbates), in addition the fuel cross-over through the membrane, which depolarizes the cathode and decreases its activity. While methanol, which has been the most extensively studied, faces the problem of toxicity and challenges mentioned above, the low electrocatalytic efficiency of the formic acid oxidation (only 2-electron-process) is complicated by corrosion phenomena. When it comes to the electrooxidation of ethanol, extra problems concern difficulty of the strong C-C bond breaking and excessive formation of undesirable intermediate products, acetaldehyde and acetic acid. Dimethyl ether (DME), or methoxymethane having formula CH3OCH3, is the simplest ether, and it can be considered as an isomer of another small organic molecule, ethanol, but with a different functional group. DME is a colorless gas that has been demonstrated in a variety of fuel applications, e.g. as an aerosol propellant or substitute for propane in LPG. DME can be viewed as the synthetic second generation biofuel, which can be produced from lignocellulosic biomass biogas or methane from agricultural waste. With respect to potential applications in low-temperature fuel cells, DME could be an attractive simple fuel due to its high energy density, low toxicity, liquefied easy storage, as well as the molecule’s low dipole moment that should diminish crossover through Nafion membrane, when compared to methanol, ethanol and formic acid. The stoichiometric complete electrochemical oxidation of DME requires three H2O-molecules, produces two CO2-molecules, and yields as many as twelve electrons. Obviously the multi-electron and multi-proton electrode process are complex and composed of distinct and competing reaction pathways and, thus, they are subject to kinetic limitations. The main constraints related to application of common Pt-based catalysts (e.g. Pt and bimetallic PtRu) in direct DME fuel cells concern the system’s low practical electrooxidation efficiency.In the present study, electrocatalytic activity of Vulcan-supported PtSn (PtSn/V) nanostructured alloys toward electrooxidation of dimethyl ether (DME) has been significantly enhanced in acid medium (0.5 mol dm-3 H2SO4) by intentional and controlled decoration of PtSn/V with ruthenium black or bimetallic PtRu nanoparticles. The enhancement effect concerns both shifting the onset potential for the DME-oxidation toward less positive values and increase of the DME electrocatalytic current densities recorded under both cyclic voltammetric and chronoamperometric conditions. The activating capabilities of ruthenium nanostructures seem to originate from the existence of reactive ruthenium oxo/hydroxo groups (even below 0.45 V vs. RHE) on their surfaces capable of inducing the oxidative removal of poisoning (CO-type) adsorbates from the neighboring platinum catalytic sites. In that respect, the Ru-oxo species originating from Ru or PtRu additives seem to support activity of Sn co-catalytic sites interacting with Pt within the PtSn heterogenous alloy. The Ru-decorated PtSn/V and, in particular, PtSn/V admixed with PtRu exhibit very high activity toward the oxidation of methanol which is also an important DME-oxidation intermediate. On the whole, the hybrid materials composed of Vulcan-supported PtSn decorated with Ru or PtRu nanoparticles seem to act as multifunctional nanoreactors inducing not only stripping of poisoning adsorbates but also catalyzing oxidation of the DME-reaction intermediates (methanol). Despite possible differences in the DME-electrooxidation mechanisms at the hybrid and single component systems, the dissociation of H3C-O-CH3 molecule, which involves C-O bond breaking, formation of the methanol reaction intermediate and CO-type poisoning adsorbates should be operative.