Conceptual design of a miniature DMFC system and ab-initio molecular dynamics study of reactant adsorption on cathode platinum catalyst.

摘要

The growing demand for efficient, non-polluting sources of utility electric power require a novel design of miniature direct methanol fuel cell (DMFC) system with high performance, small size, simplicity, low cost, rapid start-up and quick response to changing load characteristics. The work presented in this thesis dedicates from two different aspects to optimize the DMFC design and maximize the system performance.; The first part is the conceptual design of a miniature DMFC based on the systematic analysis of engineering challenges in fuel cell development. Based on the easy-for-manufacture and easy-for-assembly design principle, a silicon-based miniature DMFC designed to replace state-of-art lithium-ion batteries used in cellular phones is presented. An efficient and reliable fluidic system with the application of natural drafts is incorporated to deal with water management, gas management and fuel delivery. Overall, the proposed miniature DMFC system has a simple configuration with a unified fabrication process. It is designed to continuously deliver power to cellular phones with reliable performance, longer service life and reasonable efficiency. With the comparable dimensions to cellular phone lithium-ion batteries, the proposal DMFC has the capability of continuously producing a net power output of 0.65 W, a typical power requirement of cellular phones, for 20 hours. Compared to state-of-the-art lithium-ion batteries, the proposed DMFC will, at least: (1) provide a four-fold operating life span, (2) exhibit a five-fold advantage on a weight basis, and (3) lessen the environmental impact of battery disposal.; Despite the apparent simplicity of the oxygen reduction reaction on DMFCs cathode catalysts, the reaction mechanism remains elusive, and 30--40% of the total performance losses in PEMFCs incurs from it. To design a high performance catalyst, in the second part of this thesis, ab-initio quantum molecular dynamics is used to analyze the sluggish reactants (O, H and OH) adsorption process on low index (111), (110) and (100) Pt surfaces. Adsorption sites, adsorption geometries, chemisorption energies and electronic density distributions are calculated by density functional theory (DFT) based Car Parrinello Molecular Dynamics program (CPMD), with Goedecker-Hartwigsen norm conserving pseudopotentials, using periodic boundary conditions and a slab model of the Pt surface. With the consideration of the influence of the Pt surface reconstruction, favorite adsorption sites for each reactant species on different Pt surfaces are found and discussed. In order to quantitatively evaluate the overall performance of Pt catalysts on the reactant adsorption, the adsorption capability is introduced as the area weighted average of the adsorption energy. Among all three low index Pt catalyst surface, (111) surface possesses the highest adsorption capability, which explains the high surface activity of Pt (111) surfaces in the oxygen reduction reaction.