The rational synthesis and tuning of nanoparticles (NPs) and nanocomposites plays pivotal role in broad applications of nanomaterials in various fields. The organic solution based synthesis provides a robust approach to obtain the well-controlled NPs with good monodispersity, as well as other fine-tuned parameters, like size, composition, structure, morphology and shape. One of the major applications for these fine-controlled NPs and nanocomposites is as electrochemical catalysts in renewable energy conversion devices, like proton exchange membrane fuel cell (PEMFC), electrolysis water-splitting cell (EWSC) and direct formic acid fuel cell (DFAFC). Electrochemical oxidation (of formic acid and OH-) and reduction (of oxygen and H+) reactions are especially important for energy conversion in fuel cells and for fuel production (like H2) from water splitting. Current state-of-art catalysts for these reactions are all noble metal based. For example, Pt is active for catalyzing oxygen reduction reaction (ORR), hydrogen evolution reaction (HER) and formic acid oxidation reaction (FAOR), but it degrades undesirably under the reaction conditions and can be poisoned by CO. Pd is explored as a Pt-alternative catalyst with higher CO tolerance but without the desired chemical stability. Other catalysts based on Ir or Ru also suffer from surface oxidation and catalysis degradation during the electrochemical oxidation reactions. The first catalyst system I developed is transitional metal M (M = Fe, Co, Ni) doped WO2.72 nanorods (NRs) as a noble metal free catalyst for studying oxygen evolution reaction (OER) in alkaline solution (Chapter 3). Among these composite NR systems studied, Ni0.78WO2.72 with Ni (II) intercalation into defective perovskite-type WO2.72 show much better activity and stability for OER than the common Ir catalyst. The studies of oxygen-defective perovskite-type WO2.72 and its effect on catalysis enhancement lead to the preparation of nanocomposite catalysts of Pd NPs coupled with WO2.72 NRs (Chapter 4). The strong interaction causes the Pd lattice expansion and surface electron density decrease, and thus the improvement of FAOR activity and stability. The Pd catalysis can be further enhanced by alloying Pd with Cu and the Cu can be stabilized by coupling CuPd NPs with WO2.72 NRs (Chapter 5). I also developed chemical methods to prepare core/shell NPs and control shell catalysis by tuning core-shell interactions. The first core/shell system is based on Au/Pt with single Pt atoms deposited on Au NPs by electrochemical cyclic voltammetry (CV) cycling, to enhance Pt activity and CO-tolerance for FAOR (Chapter 6). I further prepared core/shell NiAu/Au NPs with the same CV cycling strategy to form porous Au shell showing Pt-like HER activity and better stability (Chapter 7). Using controlled chemical etching and annealing, I synthesized core/shell L10-FePt/Pt NPs with the core being intermetallic L10-FePt and shell being 2-3 atomic layer thick Pt. The core/shell structure is a much superior ORR catalyst and show much enhanced stability in the acidic fuel cell operation condition (Chapter 8). The WO2.72-, Au- and Pt-based NP systems developed in my Ph.D. study make it possible to understand catalyst interactions with nanoscale supports to enhance electrocatalysis for oxidation and reduction reactions. Such studies will be important to develop practical nanocatalysts for renewable energy applications.
"Rational Synthesis and Tuning of Nanoparticles and Nanocomposites for Electrocatalytic Applications"
Chemistry Theses and Dissertations.
Brown Digital Repository. Brown University Library.