Pt系单晶电极上氧还原反应超电势的起源研究

The high activation overpotential and high loading of Pt based precious metals at the cathode for oxygen reduction reaction(ORR) are the bottlenecks which limits the large scale commercialization of polymer electrolyte membrane fuel cells. In order to unravel the origin for the high activation overpotential as well as the key factors which limit ORR kinetics, in this project, the thermodynamic and kinetic behavior of ORR and its key elementary steps will be examined systematically at M(S) [n (1 1 1)×(1 0 0)] \M(S) [n (1 1 1)×(1 1 0)]) and (M (S)[n (1 0 0)×(1 1 1)] electrodes，where M=Rh，Pd，Pt. Fast scan voltammetry, potential step method as well as electrochemical in situ vibrational spectroscopy will be used. H-D kinetic isotope effect, temperature effect will be studied in order to gather the kinetic data for O2 dissociation, OHad adsorption/desorption, the thermodynamics and kinetics for H2O2 redox reaction as well as ORR. The effect of electrode structure, electrolyte compositions (cations, solvent activity) on such reactions will be systematically examined. The potential determining step and rate determining step for ORR, as well as the relationship between the thermodynamics, kinetics for the elementary reaction to that of ORR will be deduced based on the data from the above study. Such information will be of great help to guiding the design of cathode catalysts and the choice of reaction conditions for PEMFCs.

氧还原反应超电势高、需要使用较多贵金属催化剂等问题是制约质子交换膜燃料电池的性价比及其大规模商业化的瓶颈。为揭示氧还原反应超电势的起源，找出制约氧还原性能的关键因素，本项目拟系统考察在各种单晶电极(M(S) [n (1 1 1)×(1 0 0)] 或M(S) [n (1 1 1)×(1 1 0)])或(M (S)[n (1 0 0)×(1 1 1)]，其中M=Rh，Pd，Pt)上氧还原及其关键中间反应的热力学和动力学行为。利用快速电势扫描法、电势阶跃法及电化学谱学技术，结合开展H-D同位素效应、温度效应的研究，获得氧的吸附解离、OHad的吸脱附以及过氧化氢的氧化还原反应的热力学与动力学行为与电极的结构以及溶液组成之间的关系。确定氧还原反应的电势决定步骤与决速步骤，揭示基元步骤的热力学与决速步骤的动力学及其与总反应动力学行为之间的关系，为高效氧还电催化剂的理性设计提供科学基础。

降低氧还原反应催化剂成本是燃料电池汽车市场化的必由之路，针对氧还原反应高超电势的起源这一关键科学问题，本项目：a) 研究了单晶模型电极上氧还原反应与电极的晶面取向以及溶液的组成（pH、阳离子、溶剂的活度）等因素之间的关系；b）结合H-D同位素以及温度效应的研究，对氧还原反应及中间物H2O2的氧化还原反应，及O2 的吸附解离、OHad的吸、脱附的热力学与动力学进行了对比分析。.以科学问题为导向，项目研究结果显示：水溶液中的氧还原反应是一个极为复杂的体系。氧的吸附解离、OHad等氧物种的吸脱附、H2O2的氧化还原等众多因素在氧还原超电势起源上都可能扮演重要角色。另外，结合本项目研究结果，我们在指前因子、活化能等一些更为基础的问题上提出了新的认识。方法学上:制备了系列贵金属单晶电极，探索并制定了可靠的单晶电化学清洁与实验标准，设计出了适宜于研究ORR体系的三电极控温电解池，确定了可靠的温度效应、pH效应、同位素效应实验方案及数据分析方法。.同过本项目的实施，加深了目前对氧还原反应中基元步骤以及与氧还原平行发生的其它过程的热力学、动力学与目标反应的动力学之间关系的理解。为优化设计高活性、高选择性的燃料电池电催化剂，选择合适的燃料电池运行条件（如pH, 溶液组成、反应温度）提供了有价值的参考信息。

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