Synthesis and Modifications of Palladium Icosahedral Nanocrystals for Electrocatalytic Reactions
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Zhou, Siyu
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Abstract
Effective electrocatalysts are critical in fuel cells (e.g., direct formic acid fuel cells, DFAFCs) for accelarating the electrode reactions and thus the conversion of chemical energy into electricity. However, most of current catalysts are made of platinum group metals (PGMs), which are notoriously expensive and have extremely low abundances in the Earth’s crust. To achieve cost-effective use of PGMs (i.e., high mass activity), it is imperative to enlarge the atom utilization efficiency and the electrochemically surface area (ECSA) without sacrificing the specific activity (SA). For oxygen reduction reaction (ORR, the cathode reaction of DFAFCs), current electrocatalysts, including 3-nm Pt nanocrystals and hollow Pt nanocrystals (i.e., nanoframes), have large ECSAs but low SAs due to the poorly-defined surface structure. In this work, I used Pd icosahedra as the template to synthesize Pt nanocrystals with both large ECSAs and high SAs. I first synthesized Pd icosahedra with uniform sizes tunable in the range of 7–20 nm by combining Ostwald ripening and seeded growth. Using the uniform Pd icosahedra as a template, I developed synthetic strategies to obtain small Pt cones and Pt icosahedral nanoframes enclosed by compressively-strained {111} facets. The twin defect of vertices and edges of a Pd icosahedron makes them preferential sites for the nucleation and deposition of newly-formed Pt atoms. The small Pt cones and Pt icosahedral nanoframes were synthesized by confining the Pt adatoms to the vertices and edges, respectively. The conformally-deposited Pt replicated the surface structure of the vertices or edges, generating Pt nanocrystals covered by compressively-strained {111} facets. After removing the Pd template, both the small cones and nanoframes were found to exhibit greatly-enhanced SA and MA toward ORR, compared to the commercial Pt/C catalyst. In addition, I also demonstrated a synthetic method to obtain PdHx icosahedra (x = 0–0.7) that showed superior catalytic performance toward formic acid oxidation (FAO, the anode reaction of DFAFCs). Firstly, the phase transformation from Pd to PdHx was significantly facilitated by achieving a “single-phase pathway”. Then, PdHx icosahedra (x = 0–0.7) were easily obtained by leveraging the “single-phase pathway” and showed enhanced SA due to the insertion of hydrogen atoms. The tensile strain in Pd icosahedra greatly decreased the chemical potential for hydrogen atoms, making it hard for hydrogen to escape from PdHx (x=0–0.7) icosahedra at elevated temperatures or during FAO. Taken together, this dissertation focuses on the rational synthesis of Pd icosahedra-based nanocrystals with well-controlled shapes, strain distributions, and compositions, which were found to exhibit remarkable catalytic performances toward ORR or FAO, the essential electrode reactions involved in DFAFCs.
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2024-05-17
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Dissertation