Dealloyed core-shell platinum-copper nanoparticle electrocatalysts.

摘要

The prospect of Polymer Electrolyte Membrane fuel cells (PEMFCs) producing electricity directly from chemical sources with possibly zero emission makes them attractive as the next generation of power generators for automotives. The key challenges for PEMFCs are sluggish electroreduction of oxygen and high cost of platinum for electrocatalysts. Many Pt-rich alloys had been studied for decades but none meet today's activity targets.;This dissertation reports the discovery and structural and functional characterization of a novel class of Pt bimetallic electrocatalysts that meet today's cost and performance target for PEMFC technology. The active Pt-Cu nanoparticle electrocatalyst is prepared from Cu-rich Pt-Cu precursors. The surface Cu atoms were then selectively dissoluted, resulting in Pt-enriched surfaces. Electrocatalytic testing revealed unprecedented activities for electroreduction of oxygen of these Pt-Cu nanoparticles.;Compositional and structural analyses resulted in a core-shell hypothesis suggesting a Pt-rich shell surrounding Pt-Cu core. A lattice strain hypothesis was put forth based on the assumption that Cu in particle core causes shorter Pt-Pt interatomic distances in the Pt-shell as surface Cu atoms were removed. Previous Density Functional Theory calculation revealed that compressive lattice strain caused metal d-bands to downshift, weakening metal-adsorbate bond strength. Since pure Pt binds oxygen too strongly thus poisoning the active surface, weaker adsorption strength enhances oxygen reduction reaction rates.;Compressive lattice strain in the activated Pt-Cu core-shell nanoparticles was experimentally observed with Anomalous X-ray Diffraction (AXRD) and quantified using a core-shell model. Activities of the activated core-shell Pt-Cu were correlated with corresponding lattice strain for various Pt-Cu precursors. This resulted in a volcano-shaped relation in accordance with theoretical Density Functional Theory (DFT) calculation. Higher Cu content in the precursor causes higher compressive strain and catalytic activities. That composition and thickness of Pt-shell in the particles is controllable experimentally offers a unique feasibility for tuning electrocatalytic activities of Pt-Cu electrocatalysts.;Further characterization addressed the effects of compositions and synthesis conditions on catalytic activity. Higher annealing temperatures showed more significant effects on precursor structure and activities than longer annealing durations. Aging studies of Pt-Cu precursors in the form of powder, film and catalyst ink showed good stability of powders and inks.