Microwave-Assisted Solvothermal Synthesis of Pd-MnO2 Hybrid at Onion-like Carbons: Electrocatalytic Oxidation of Alcohols in Alkaline Media

The use of less toxic glycerol, (a non-flammable, volatile, and little-valued residue of biodiesel production), ethanol as well as ethylene glycol (EG) as fuel may be an interesting alternatives to methanol for Direct Alcohol Fuel Cell (DAFC) technology. The volumetric energy densities of glycerol, ethanol and EG are 5.80, 6.31 and 5.97 kW h L^-1 compared to 5.81 kW h L^-1for methanol makes them good fuel cell candidates [1-2]. The secret to the efficient electrocatalysis of alcohols is the use of efficient nanocatalyst. Palladium-based catalysts have emerged as the most viable electrocatalysts for alcohol in alkaline electrolytes. To harness the great catalytic potential offered by Pd, and to reduce cost, Pd-based alloys supported on different carbon supports are constantly used in direct alcohol fuel cells (DAFCs).

In this work, palladium-manganese oxide nanoparticles supported on carbon nano onions (Pd-MnO₂/OLC) were synthesized using the microwave-assisted solvothermal technique.  Palladium-manganese oxide nanoparticles supported on Vulcan carbon (Pd@MnO₂/XC–72) were also synthesized using the same technique for comparison. The catalysts were characterized using XRD, XPS, FESEM and HRTEM techniques. Electroxidation of the alcohols was performed in 0.5 M alcohol / 0.5 M KOH via cyclic and linear sweep voltammetry, chronoamperometric and elelctrochemical impedance spectroscopy.

Results obtained show that the catalyst supported on OLC exhibited the highest current density and least charge transfer resistance relative to the catalyst supported on XC–72. Pd-MnO₂/OLC gave the best result amongst the catalyst synthesized with  respect to the current density, and electrochemical stability as well as exhibited the least charge transfer resistance in all the alcohols studied. For instance, the onset oxidation potential and current density of the glycerol oxidation process were -0.5386 V and 5.58 mAcm^-2, respectively. The implications of these results relative to methanol oxidation using the best catalyst system synthesized are discussed.

References

1. L.M. Palma, T.S. Almeida, A. R. de Andrade, ECS Trans. 2013 58(1): 651-661.

2. X. Zhang, P.K. Shen, Int. J. Hydrogen Energy 2013, 38, 2257-2262