(719c) De-Alloyed Platinum-Bismuth Nanoparticles As Highly Active Electrocatalysts for Dimethyl Ether Oxidation and Oxygen Reduction

Dimethyl ether (DME) is a clean-burning alternative to diesel fuel that meets strict emissions standards in combustion engines. DME is currently derived from natural gas but can also be synthesized from biomass on-site and is thus a sustainable and highly distributed energy source. DME has properties similar to propane and can utilize established fueling infrastructure and handling procedures. In addition to its use in internal combustion engines, the convenience of DME generation and transport can be exploited to overcome the high-pressure infrastructure requirements of hydrogen fuel in more efficient electrochemical fuel cells. Fuel cell-driven vehicle efficiencies are among the highest of all vehicle technologies.

Relative to other direct liquid fuels envisioned for use in electrochemical cells, DME is: (1) more energy-dense than hydrogen, (2) non-toxic compared to methanol, (3) a lower dipole moment than methan which limits fuel crossover, (4) the simplest ether which can be more efficiently oxidized than ethanol through electrocatalytic C-O bond cleavage, and (5) unlike ethanol, does not interfere with world food production. A key stumbling block to the commercialization of DME fuel cells has always been identification of a suitable electrocatalyst. The most active electrocatalyst presently available for DME oxidation is highly dispersed commercial precious metal Pt/Ru nanoparticles supported on carbon. A mass activity no greater than 1.3 A/g_Pt is observed at peak potential and is more than an order of magnitude less than that obtained from hydrogen.

We here discuss the synthesis of a novel DME oxidation electrocatalyst that has more than an order of magnitude greater activity per unit mass of Pt than state-of-the art. The electrocatalyst is composed of de-alloyed PtBi₂ nanoparticles (NPs). The NP pre-cursor material formed without the use of toxic organic solvents, difficult-to-synthesize organic precursors, or dangerous hydrazine reducing agent typically needed to produce bismuth metal. Synthesis involves the use of stannous chloride as both reducing and stabilizing agent in aqueous suspension and is unprecedented in that Sn-Bi ligand formation was unknown prior to our work. Electrochemical and chemical de-alloying of these nanoparticles is shown to alter the surface atomic coordination relative to pure Pt and produce unusually high electrochemical areas and activities. DME electrooxidation activities greater than 30 A/g_Pt are observed in sulfuric acid at peak potential while the activity of the corresponding cathodic oxygen reduction reaction (ORR) increases to over 15 A/g_Pt at 0.9V vs. RHE in perchloric acid. ORR activity is comparable to that obtained with recent dealloyed Pt-Cu catalysts but in a one-pot synthesis approach that avoids the need for a high pressure hydrogen reduction step during synthesis. Despite the significant increases in mass-specific activity observed relative to pure Pt, the area-specific activity of the dealloyed electrocatalyst is lower. This observation is discussed in terms of the corresponding changes in surface atom coordination and electrolyte effects.