【作者】 段东红；

【导师】 孙彦平；

【作者基本信息】 太原理工大学 ， 化学工程， 2010， 博士

【摘要】 以碱金属硼氢化物为燃料的直接硼氢化物燃料电池(DBFC),具有理论比能量高和燃料效率高、电极反应速度快、燃料易于储存和运输等特点而引起研究者的广泛关注,在小型便携式电源等领域有着潜在的应用前景。DBFC阳极过程是一个复杂的H--H0?-H+之间价态转化的电极反应体系,涉及BH4-的电氧化反应、BH4-的水解释氢反应与氢的电氧化反应,该过程所要求的阳极催化材料与传统的金属或金属氧化物迥然不同。研发能有效抑制释氢的廉价阳极催化材料,是DBFC实用化的关键。本文探索了廉价金属Cu代替贵金属作为硼氢化物燃料电池阳极的可能性,对硼氢化物在Cu电极上的电化学氧化行为与反应动力学进行了系统研究,并对不同金属掺杂的铜基合金电极的性能进行了探讨。论文采用循环伏安法与交流阻抗技术研究了BH4-在Cu电极上的电化学氧化性能,并以Cu为阳极组装成DBFC单元槽,测量了Cu阳极上BH4-氧化的稳态极化数据,提出Cu电极上BH4-的电催化氧化反应的多步骤机理,详细分析了不同超电势区域中电流与反应释放氢气速率的特点,发现:Cu阳极BH4-氧化机理包含电化学和非电化学两种连串的基元步骤。电化学反应步骤在低超电势下为电极反应的速率控制步骤,电极表观反应级数近似为零;非电化学步骤发生于连串的电化学基元步骤之前,在较高超电势下成为速率控制步骤,表观反应级数近似为1,在中等超电势的过渡区,表观反应级数应在0～1之间;极化曲线上限制电流的出现与反应机理中的速率控制步骤有关,而与传质无关;电极表面释放的表观电子数n值,可在0至7之间变化;在中等超电势下,n值在3～5之间。n值与[BH4-]/[OH-]浓度比、温度及超电势有关。根据提出的反应机理,导出了包含表面吸附态BH4? ,ad和OH a?d覆盖率θB和θOH的电极反应动力学方程,归纳出各基元反应步骤的动力学参数,计算出理论极化曲线并与极化实验数据进行了比较,二者吻合良好。同时采用电流分解法将电流定量分解成两类不同的电极反应,确定了Cu电极上不同反应对总电流的贡献,分析了BH4-在铜电极表面上电化学氧化反应的表观电子数的变化规律。在此基础上,通过掺杂“催化”材料Pt与“非催化”材料Ag,进一步研究了碳载铜基合金电极的性能,发现掺杂元素可改进合金电极的性能。Cu-Ag/C和Cu-Pt/C合金电极的性能比Cu/C都有较大改善,其中掺杂Pt的Cu/C电极显示出较高的催化氧化活性。这为研制抑制释氢的新颖催化剂和优化催化剂组成提供了新的思路。更多还原

【Abstract】 Direct borohydride fuel cell (DBFC) using alkali metal borohydride as fuel has attracted much attention in recent years. It has a high energy conversion efficiency and theoretical specific energy, fast electrode reaction rate and easy storage and transport of fuel. For these reasons, DBFC is a potential power supplier in portable applications. The anodic oxidation reactions of BH4- is involved with a novel and complex reaction system of three hydrogen valence state transformation among protide (H-)-protium (H0?)-proton (H+), which related to borohydride direct electrochemical oxidation and hydrolysis reaction with hydrogen evolution followed hydrogen electrochemical oxidation reaction. The anode catalytic materials are very different from traditional metal or metal oxide. The key for DBFC commercial application is to depress hydrogen evolution and reduce costs of anode catalysts. In this paper,we want to explore the possibility using low-cost Cu metal as the anode catalysts for DBFC, study the oxidation behavior and reaction kinetics of borohydride on Cu, and the performance of different metal-doped copper-based alloy electrode.In present dissertation,the electrochemical behavior of NaBH4 alkaline solution on copper was tested by CV, EIS and polarization curve. A multi-step mechanism for the oxidation of BH4- on Cu anode in NaOH solution was presented. In addition, polarization data of BH4- oxidation on Cu anode in alkaline solution were measured at steady state by a self-made experimental cell, and then the relation between hydrogen evolution rate and the anode current at different overpotential regions was analyzed in detail. It was found that the BH4- oxidation mechanism is a multi-step consecutive reaction including electrochemical and prepositive non-electrochemical elementary reaction steps. The result shows that the rate determining step will be the electro- oxidation of adsorbed sodium borohydride at low overpotential regions and it will change to sodium borohydride or hydroxyl activation adsorption at high overpotential. It is a transition area without rate determine step at middle overpotential. The apparent reaction order with respect to BH4- is approximate to zero at low overpotential and it will change to 1 at high overpotential, and it is 0-1 reaction order at middle overpotential. The limiting current density is caused by a limiting elemental step rather than by external mass transport. The apparent number of released electron n by the reaction was calculated by using the measured amounts of hydrogen and coulomb. Higher value of n was obtained at lower [BH4-]/ [OH-] ratio, higher temperature as well as higher over potential. Under the experimental conditions, the value of n varies from 0 to 7, while in the transitional region it is just in the range of 3 to 5. Based on the proposed reaction mechanism, the kinetic expression containing both coverage ratiosθB andθOH of BH4? ,ad and OHa?d adsorbed on the surface of Cu anode was deduced by the steady state approximation theory. Furthermore the kinetic parameters of elementary reaction steps were regressed from the experimental polarization data, and the theoretical polarization curves are calculated then compared with the experimental data. The theoretical calculation is in good agreement with the experimental results. At the same time, the reactions at Cu electrodes in alkaline borohydride solutions are successfully decomposed into two component reactions, i.e., borohydride oxidation and hydrogen electrode reaction by the current decomposition method. The relative contribution of these two reactions to the total current was determined, and the variation of the apparent number of electrons in borohydride oxidation was analyzed.On this basis,the electrochemical oxidation of BH4- in 2 M NaOH on carbon supported copper doped with Pt or Ag (catalytic and non-catalytic materials, respectively, for BH4- hydrolysis accompanied by H2 evolution) has been studied. It was found that that the performance of copper-based alloy electrodes can be improved by doping elements Ag or Pt. The DBFC with Cu-Pt/C as the anode catalyst showed the higher catalytic oxidation activity than that with Cu/C or Cu-Ag/C.更多还原