Advanced Nanostructures for Proton Exchange Membrane Fuel Cells: From Liquid Electrolyte to Membrane Electrode Assembly

摘要

During the last decade, the landscape of Pt-based electrocatalysts for the oxygen reduction reaction, cathodic reaction of the proton exchange membrane fuel cell, dramatically changed. Huge enhancements in specific and mass activity, versus the standard, mono-metallic platinum nanoparticles supported on carbon, have been achieved. This was made possible by following two different approaches, namely (i) the order-based and (ii) the disorder-based approaches. The order-based consists in modifying the platinum lattice parameter and coordination number, thus impacting its electronic structure, in an identical fashion throughout the nanostructure. To undergo this path, the Pt can be alloyed with 3d-transition metals or rare-earth elements, or the nanostructures can be shaped to exhibit a preferential orientation, such as (111) facets, with a specific Pt atoms packing. By opposition, the disorder-based approaches imply that the electrocatalytic activity is carried, not by the entire nanostructure, but by some specific active sites exhibiting an ideal generalized coordination number and an ideal distance with their nearest neighbour. Such approaches resulted in notable nanostructures, such as the PtNi octahedra, the PtNi nanoframes or others that presented notable activity enhancements in liquid electrolyte vs. standard Pt/C. The highest activity improvement reported was for the PtNi nanowires, namely 35-fold higher specific activity than standard Pt/C. However, there is only little reports on their activity in membrane electrode assemblies. From these reports, it mainly appears that the liquid electrolyte activity is often not reproduced, and that the improvements factors in solid electrolyte environment range from 1 to 5 vs. standard Pt/C. However, an extensive comparison of the liquid vs. solid performance for a wide range of electrocatalysts is currently missing. In this work, a group of electrocatalysts was synthesized from using microwave-assisted syntheses, including standard Pt/C, PtM alloys, ordered (nano cubes, octahedra) and disordered nanostructures. These electrocatalysts were characterized in liquid electrolyte and solid electrolyte environment, and the changes in specific surface, mass and specific activity were assessed.