【作者】 秦海英；

【导师】 李洲鹏；

【作者基本信息】 浙江大学 ， 化学工程， 2009， 博士

【摘要】 直接硼氢化钠燃料电池（DBFC）由于其较高的理论能量密度(5.67 Ah g^-1)、较高的理论电动势（1.64V）以及较高的理论能量转换效率（91%）而广受关注。然而,使用昂贵的Pt作为阴极催化剂限制了其产业化发展。因此,开发非铂催化剂成为近年来DBFC技术研究的热点之一。本文旨在开发高催化活性的过渡金属大环化合物——聚吡咯修饰碳载氢氧化钴（Co（OH）₂-PPY-C）催化剂及其在DBFC中的应用。本文以吡咯、碳材料和Co（NO₃）₂·6H₂O为原料,采用化学法合成了Co（OH）₂-PPY-C。采用X射线粉末衍射仪、X射线光电子谱、电子显微镜等材料分析方法以及恒电流放电、循环伏安法、电化学阻抗谱和旋转圆盘电极伏安法等电化学测试技术,对Co（OH）₂-PPY-C的表面成分、微结构、电化学性能以及三者之间的联系进行了较为系统的研究。研究了影响Co（OH）₂-PPY-C催化活性的关键因素以及O₂和NaBH₄在催化剂表面发生还原反应（ORR）和氧化反应（BOR）的催化机制,为进一步开发适用于DBFC的过渡金属大环化合物催化剂提供了重要的理论和实验依据。研究表明,化学法合成的Co（OH）₂-PPY-C中Co（OH）₂以片状物形式分布在碳载体上。以Super P作为碳载体,Na⁺型Nation膜117膜为电解质膜,储氢合金复合催化剂为阳极催化剂、Co（OH）₂-PPY-C作为阴极催化剂的DBFC在室温下获得的最大输出功率密度为65 mW cm^-2,表现出与Pt/C相当的电化学催化活性和稳定性。Co（OH）₂-PPY-C适合作碱性环境下的氧气还原催化剂,其ORR电子数为2.9。片状的Co（OH）₂在DBFC工作时被氧化为针状CoHO₂,而CoHO₂同样具有良好的ORR催化活性。当Co（OH）₂-PPY-C作为DBFC阴极催化剂时,其表面发生ORR的反应途径是:催化剂中Co离子表面发生4电子反应和PPY修饰的碳载体表面发生2电子反应。当Co（OH）₂-PPY-C作为DBFC的阳极催化剂时,催化剂表面Co原子的排列方式是影响NaBH₄在其表面发生BOR的表观库仑数的关键因素。Co（OH）₂-PPY-C作为阳极催化剂时表现出比储氢合金复合催化剂更好的阳极极化性能,以Co（OH）₂-PPY-C作为阳极和阴极催化剂的DBFC在室温下获得的最大功率密度为83 mW cm^-2。Co（OH）₂-PPY-C的BOR表观库仑数是3.0,并且片状的Co（OH）₂使用后也转变为针状CoHO₂。碳载体类型、聚吡咯和Co（OH）₂的担载量是影响Co（OH）₂-PPY-C催化活性的重要因素。使用BP 2000取代Super P作为碳载体合成催化剂（Co（OH）₂-PPY-BP）,ORR和BOR分别为3.7和4电子反应。随着聚吡咯担载量的增加可提高催化剂的催化活性,但不会改变ORR反应电子数。Co（OH）₂担载量对于催化活性的提高存在饱和点,只有当Co:N（原子比）≈1:2时,合成的催化剂获得相对较佳的氧气还原催化活性。电解质膜的类型、膜的厚度、燃料的浓度、燃料的组分和工作温度对于提高DBFC的性能具有明显效果。采用薄Na⁺型Nation膜N211膜为电解质膜,5 wt.%NaBH₄-10 wt.%NaOH溶液为燃料的DBFC在常温下的最大输出功率密度为207 mW cm^-2,60℃下的最大输出功率密度为324 mW cm^-2。当Co（OH）₂-PPY-C在400～800℃热处理后,Co（OH）₂-PPY-C中的Co（OH）₂的衍射峰逐渐消失,Co₃O₄、CoO和Co峰逐渐出现,片状物逐渐长大甚至球化。随着热处理温度的升高,Co（OH）₂-PPY-C的ORR和BOR的催化活性先上升后下降,其中600℃热处理后的Co（OH）₂-PPY-C的催化性能相对最好。作为一种非铂催化剂,Co（OH）₂-PPY-C在催化活性和稳定性上都已经显示出诱人的应用前景,通过进一步优化催化剂的成分与制备工艺,有望推动DBFC技术的产业化进程。更多还原

【Abstract】 The direct borohydride fuel cell（DBFC） has attracted much attention as a potential candidate of high power supplier due to its high capacity of 5.67 Ah g^-1,high electromotive force（EMF） of 1.64 V and high theoretic energy conversion rate（91%）.However,DBFC system for portable and mobile applications suffers from its high cost caused by the use of platinum as cathode catalyst.The development of non-platinum catalysts to replace the conventional platinum-based catalysts is one of the critical issues in the research of DBFC.In the present work,a transition metal macrocycles composite,polypyrrole modified carbon-supported cobalt hydroxide（Co（OH）₂-PPY-C） has been fabricated and its electrochemical performances as cathode and anode catalysts for DBFC are evaluated.Co（OH）₂-PPY-C was prepared by chemical method using Co（NO₃）₂·6H₂O,pyrrole and carbon as the raw materials.The surface composition,microstructure were characterized by typical material identification methods such as powder X-ray diffraction,X-ray photoemission spectroscopy,field emission scanning electron microscopy,transmission electron microscopy.Its electrochemical performances were systematically studied by electrochemical analysis methods including the galvanostatic discharge,cyclic voltammetry, electrochemical impedance spectroscopy and rotation disk electrode cyclic voltammetry etc. Based on the obtained results,some main factors to influence catalyst activity are evaluated and discussed.The catalytic mechanisms of oxygen reduction reaction（ORR） and borohydride oxidation reaction（BOR） on Co（OH）₂-PPY-C were suggested.The prepared Co（OH）₂-PPY-C isβ-Co（OH）₂ with a shape of platelet dispersed on carbon particles.As a cathode catalyst,Co（OH）₂-PPY-C exhibits fairly good electro-catalytic activity and durability comparable to Pt/C for electrochemical oxygen reduction in the DBFC.At ambient condition,a maximal power density of 65 mW cm^-2 has been obtained by the DBFC using hydrogen storage alloy composite as the anode catalyst,Co（OH）₂-PPY-C as the cathode catalyst,Super P as the catalyst support material and Nafion 117 as the electrolyte. Co（OH）₂-PPY-C is an applicable cathode catalyst for the DBFC and the number of electrons involved in the ORR is 2.9.It has been found that the Co（OH）₂ platelet would be transformed into CoHO₂ with a shape of needle during operation.It is considered CoHO₂ is also a good ORR catalyst like Co（OH）₂.The DBFC using Co（OH）₂-PPY-C as the anode catalyst exhibits better cell performances compared to the DBFC using hydrogen storage alloy as anode catalyst.A maximal power density of 83 mW cm^-2 has been obtained at ambient condition. The Co（OH）₂ platelet would be also transformed into CoHO₂ in the shape of needle during Co（OH）₂-PPY-C being used as anode catalyst in DBFC,and the apparent coulombic number of BOR on Co（OH）₂-PPY-C is 3.0.The arrangement pattern of cobalt atoms on the surface of catalyst is a key factor deciding the apparent coulombic number of BOR.When the Co（OH）₂-PPY-C is used as ORR catalyst in alkaline medium,oxygen is electroreduced with a 4-electron-reaction on the cobalt sites and a 2-electron-reaction on the PPY modified carbon substrate.The catalysis of the Co（OH）₂-PPY-C is strongly dependent upon the catalyst support material as well as the loading of PPY and Co（OH）₂.The electron number of ORR and BOR on the catalyst could reach up to 3.7 and 4 respectively when using BP 2000 instead of Super P as the catalyst support material.The catalytic activity of catalyst could be improved with the increase of loading of PPY on the catalyst,but the number of reaction electron keeps unchanged.There is a optimum loading of Co（OH）₂ on the PPY modified carbon.When the atomic proportion of Co to N is close to 1:2,the catalyst demonstrates a better ORR catalytic activity.The influences of fuel composition,operation temperature and applied electrolyte membrane on the performance of the DBFC using Co（OH）₂-PPY-BP as cathode and anode catalyst are investigated.Maximal power density of 207 mW cm^-2 at room temperature and 324 mW cm^-2 at 60℃have been achieved respectively when the DBFC uses N 211 as the electrolyte,5 wt.%NaBH₄-10 wt.%NaOH solution as the fuel.When Co（OH）₂-PPY-C is annealed at temperatures ranging from 400 to 800℃,the diffraction peak of Co（OH）₂ disappears and the diffraction peaks of Co₃O₄,CoO and Co appears.Meanwhile,the platelet grows up and turns to particles.The ORR and BOR catalyst activity of catalyst is improved with increasing the annealing temperature but decreased when annealing temperature reaches up to 800℃.Co（OH）₂-PPY-C annealed at 600℃exhibits a better catalytic activity.From above obtained results,it can be concluded that the Co（OH）₂-PPY-C catalyst has demonstrated a rather promising possibility to substitute commercialized platinum catalyst.It is believed that these achievements in this research have given a certain contribution to the development of DBFC technology.更多还原