Nanostructured Palladium-based Alloy Catalysts for Fuel Cells

化石燃料的迅速枯竭和日益严重的环境问题使世界范围内对替代清洁能源技术产生了巨大的需求。能源是21世纪人类面临的最大挑战。燃料电池为解决各种各样的能源需求提供了巨大的希望，从便携式到汽车再到固定电源，减少我们对石油的全球依赖，促进未来的能源安全、繁荣和更清洁的环境。然而，燃料电池技术面临着与耐久性、性能和成本相关的众多材料挑战，阻碍了商业化前景。正如2004年国家研究委员会/国家工程院报告和美国物理学会报告所表明的那样，对燃料电池材料的化学和物理过程的深刻基本理解对于实现重大突破至关重要，这将以可承受的成本提高燃料电池的性能。例如，目前使用的铂催化剂的高成本和有限的丰度给燃料电池的商业化前景带来了严重的问题。本提案通过探索新的钯基合金催化剂来解决这一关键问题；钯的成本是铂的五分之一。用于氧还原反应（ORR）的纳米结构钯基合金催化剂是基于一个指导原则设计的，该指导原则涉及将良好的氧键切割金属（如Co）与良好的氧还原金属（如Pd）配对，首先分裂O-O键以形成吸附氧，从而有效地将吸附氧原子还原为氧化物离子。潜在的催化剂组成通过循环伏安（CV）筛选与玻璃碳微电极确定。由钯和Ti、V、Cr、Fe、Co、Ni、Cu、Mo、W、Ru、Au、Pt等金属组成的多金属二元和三元合金组合物采用新颖的低温方法合成，如采用反相微乳液法，采用甲酸钠或硼氢化钠等不同还原剂和多元醇还原方法，然后在中等温度下进行热处理，以达到高度合金化和均匀性。粒径分布小而均匀，催化活性高，化学稳定性好。合金催化剂通过各种物理技术进行表征，包括衍射、显微镜、光谱学和电化学测量（循环伏安法、线性极化法和旋转圆盘电极法）。对单细胞质子交换膜燃料电池和直接甲醇燃料电池的氧还原反应（ORR）和甲醇氧化反应（MOR）的催化活性进行了评价。基于这些结果，对催化机理有了基本的认识。知识价值：该活动的知识价值在于：(1)对燃料电池中用于氧还原反应和甲醇氧化反应的纳米结构钯基合金催化剂的电催化活性的控制因素有了基本的科学认识；(2)利用这些知识设计和开发新的更便宜、更高效的燃料电池钯基催化剂。根据指导原则设计的钯基合金催化剂，通过控制、低温的方法合成，以保持粒径小，最大限度地提高催化活性，通过循环伏安法筛选，并通过各种物理、化学和电化学技术进行表征，以建立所涉及的催化机理。这项研究活动将增强我们对电催化剂结构-性能-性能关系的基本理解，以及燃料电池技术的商业化前景。更广泛的影响：拟议的研究为学生提供了一个更广泛的跨学科培训，在一个独特的，国家重要的能源转换材料领域，包括材料化学和电化学科学与工程。在这一领域建立强大的科学基础有助于设计和开发便携式、汽车和固定应用的新电源材料，这将产生深远的社会影响。拟议的活动还旨在招募和培训少数民族和妇女学生，并向K-12学生和公众宣传清洁能源技术和材料。

The rapid depletion of fossil fuels and growing environmental concerns have created enormous worldwide demand for alternative, clean energy technologies. Energy is the single greatest challenge facing humankind in the 21st century. Fuel cells offer tremendous promise for solving a variety of energy needs ranging from portable to automobile to stationary power, reducing our global dependence on oil and fostering future energy security, prosperity, and a cleaner environment. However, fuel cell technologies are confronted with numerous materials challenges associated with durability, performance, and cost, impeding the commercialization prospects. As evident from the 2004 National Research Council/National Academy of Engineering report and the American Physical Society report, a profound fundamental understanding of the chemical and physical processes in fuel cell materials is vital for enabling significant breakthroughs that will lead to enhanced fuel cell performance at an affordable cost. For example, the high cost and limited abundance of the currently used platinum catalysts pose serious problems for the commercialization prospects of fuel cells. This proposal addresses this critical issue by exploring new palladium-based alloy catalysts; the cost of palladium is one-fifth of the cost of platinum. Nanostructured palladium-based alloy catalysts for oxygen reduction reaction (ORR) are designed based on a guiding principle involving the pairing of a good oxygen-bond cleaving metal such as Co for first splitting the O-O bond to form adsorbed oxygen with a good oxygen-reduction metal such as Pd for efficiently reducing the adsorbed oxygen atoms to oxide ions. Potential catalyst compositions are identified by a cyclic voltammetric (CV) screening with glassy carbon microelectrodes. Multi-metallic binary and ternary alloy compositions consisting of palladium and other metals like Ti, V, Cr, Fe, Co, Ni, Cu, Mo, W, Ru, Au, and Pt are synthesized by novel low temperature approaches such as a reverse microemulsion method employing different reducing agents like sodium formate or sodium borohydride and polyol reduction methods, followed by heat treatment at moderate temperatures to achieve a high degree of alloying and homogeneity, small and uniform distribution of particle size, high catalytic activity, and good chemical stability. The alloy catalysts are characterized by a variety of physical techniques including diffraction, microscopy, spectroscopy, and electrochemical measurements (cyclic voltammetry, linear polarization, and rotating disk electrode methods). The catalytic activity is evaluated for both oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) in single cell proton exchange membrane fuel cells and direct methanol fuel cells with hydrogen and methanol fuels. Based on the results, a fundamental understanding of the catalytic mechanisms is developed.Intellectual Merit: The intellectual merit of the proposed activity is to (i) develop a basic scientific understanding of the factors that control the electrocatalytic activity of nanostructured palladium-based alloy catalysts for oxygen reduction reaction and methanol oxidation reaction in fuel cells, and (ii) utilize the knowledge to design and develop new less expensive, more efficient palladium-based catalysts for fuel cells. Palladium-based alloy catalysts designed with a guiding principle are synthesized by controlled, low temperature methods to keep the particle size small and maximize the catalytic activity, screened with cyclic voltammetry, and characterized by a variety of physical, chemical, and electrochemical techniques to establish the catalytic mechanisms involved. The proposed research activity will enhance our fundamental understanding of the structure-property-performance relationships of electrocatalysts and the commercialization prospects of fuel cell technology.Broader Impact: The proposed research provides a broader interdisciplinary training to students in a unique, nationally important area of materials for energy conversion, encompassing materials chemistry and electrochemical science and engineering. The realization of a strong scientific basis in this area can help to design and develop new materials for power sources for portable, automobile, and stationary applications, which would have a profound societal impact. The proposed activity also aims to recruit and train minority and women students and educate K-12 students and the general public about clean energy technologies and materials.