【作者】 王广进；

【导师】 高云智； 尹鸽平；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2010， 博士

【摘要】 直接硼氢化钠燃料电池(DSBFC)作为一种新型的燃料电池,具有理论电压高(1.64 V)、能量密度大、可采用非贵金属作为催化剂等优点,近几年受到广泛关注。本论文从阳极催化剂的制备、膜电极的制备工艺和电池运行条件等方面做了系统的研究,并提出了一种提高燃料库仑效率的方法。采用浸渍还原法制备了不同摩尔比PtNi/C、AuNi/C、AuPt/C二元合金催化剂。EDAX结果表明各合金催化剂的金属载量和摩尔比均基本接近理论值。循环伏安和稳态极化的测试结果表明:三种二元合金催化剂的摩尔比均为7:3时催化性能最好,PtNi(7:3)/C在三种最优二元催化剂中性能最佳。采用XRD、XPS等手段深入分析了合金催化剂性能提高的机制。XPS结果发现,由于Ni的加入改变了Pt的电子状态,零价Pt的含量提高,这可能是其性能提高的原因之一。对于AuPt二元催化剂,Pt的加入可以使开路电势和峰值电位明显负移,峰值电流也有所提高。总的来看,合金化可以提高二元催化剂的性能,通过适当的热处理实现了这一目的。对膜电极制备工艺的研究发现,阳极催化剂采用PtNi(7:3)/C时电池性能最好;催化剂的载量提高有利于性能的提高,但载量过高会增加电极厚度对燃料传输不利,最优载量为1 mg·cm-2;Nafion的加入在起到粘结固定催化剂的同时会牺牲部分活性位置,10 mass %的Nafion含量较为适宜;阳极扩散层中PTFE的含量不同可得到不同的憎水性,从而调节扩散层中憎水和亲水孔的比例,结果发现PTFE的含量为5 mass %时膜电极性能最好;扩散层碳粉起到整平的作用,为催化剂提供安全的工作场所,但碳粉载量过高反而使扩散层粗糙度增大,有裂缝出现,另外碳粉过厚对物料传输不利,最后得出碳粉最佳载量为1 mg·cm-2;电解质膜仍然以Nafion117膜最为适宜,AEM性能较差;对电解质膜预先进行Na+化处理有利于缩短膜电极的活化时间,同时提高膜电极的放电稳定性。在最佳制备工艺条件下电池最大功率密度分别达到25.2 mW·cm-2(25℃)和54.8 mW·cm-2(60℃)。研究发现运行温度提高则电池性能大幅度增大,但实际应用时被动式燃料电池的工作温度不宜超过60℃;阳极燃料NaBH4的浓度的提高在一定程度上对提高电池性能有利,但浓度过高时,因为燃料的渗透加重了阴极的极化,而且浓度高会导致燃料库仑效率下降,研究发现NaBH4的浓度为1.0 mol·L-1时电池性能最好;支持电解质NaOH既为电池提供电荷载体,又是NaBH4的稳定剂,同时还参与阳极电极反应,但NaOH浓度过高会导致燃料粘度过大反而使电池性能下降。结果表明:NaOH浓度为6.0 mol·L-1时为最佳;阴极的工作条件也会对电池的性能产生较大影响,因为在电池中阳极极化较弱,阴极极化更为严重,采用纯氧的电池性能优于直接用空气,气体的加湿对电池性能有轻微改善;在采用阳离子膜作为电解质膜时,阴极会有NaOH的累积,及时清除阴极产生的NaOH可以改善电池长时间放电性能。为解决DSBFC燃料库仑效率低的问题,我们设计制备了一种新型复合膜电极,其阳极包含上下两个区域,可以使BH4-和H2同时在这两部分区域发生氧化反应,适当调整两部分区域的面积比例,可以在提高电池性能的同时使燃料库仑效率达到100%,目前条件下得到的最佳面积比例为3:1。更多还原

【Abstract】 In recent years, direct sodium borohydride fuel cell (DSBFC) has been receiving increasing attention because of its advantages, such as high voltage (1.64 V), high energy density (9.3 kWh·kg-1), and using non-noble metal as anode catalyst. The preparation of anode catalyst, preparation technology of membrane electrode assembly (MEA) and effect of operating conditions on the DSBFC performance have been investigated in this dissertation.PtNi/C, AuNi/C and AuPt/C catalysts with various molar ratios were prepared by using impregnation method. EDAX results showed that the molar ratio and metal loading of each catalyst were near to the expect values. Cyclic voltammetry (CV) and steady-state polarization measurement results indicated that 7:3 was the optimal molar ratio for the three binary alloy catalysts, and PtNi(7:3)/C showed the best performance. The absence of characteristic peaks of Ni in XRD pattern of PtNi(7:3)/C catalysts, positive shift of 2θfor characteristic peaks of Pt, and the decrease of the d value indicated that Ni had entered into Pt lattice and formed alloy with Pt. The same results were obtained in AuNi(7:3)/C catalyst. Heat treatment can result in the higher alloying between binary metals, and the catalyst activity can be further improved. XPS patterns of PtNi(7:3)/C shows that the electronic state of Pt was changed, i.e. increase of Pt0 content, due to the electron effect of Ni. This may be one of the reasons to the performance improvement.The MEA with PtNi(7:3)/C as the anode catalyst has the best performance. The increase of catalyst loading can improve the cell performance, but exorbitant catalyst loading can make the electrode too thick and inhibit the diffusion of the fuel. The optimized catalyst loading is 1 mg·cm-2. Nafion can bond and fix the catalyst meanwhile it can also decrease the catalyst active sites. Therefore the best content of Nafion is 10 mass%. The proportion of hydrophobic pore to hydrophilic pore in the anode diffusion layer can be regulated by PTFE content. The experimental results indicated that the MEA with 5% PTFE content had the best performance. The carbon powder in the diffusion layer can provide safe working place for the catalysts, but exorbitant carbon powder loading would make the diffusion layer too coarse and generate cracks, on the other hand, the carbon powder with high thickness can inhibit the diffusion of the fuel. The best carbon powder loading is 1 mg·cm-2. The performance of the MEA with Nafion 117 membrane was higher than that of the MEA with AEM membrane. Na+ pre-treatment on the electrolyte membrane can shorten the MEA activation time and improve the discharge stability. Under the optimized preparation technologies, the maximum power density of the MEA was 25.2 mW·cm-2 and 54.8 mW·cm-2 at 25℃and 60℃, respectively.The DSBFC performance was significantly improved with the increase of operating temperature. And, the concentration of NaBH4 also had effects on the performance of DSBFC, where the optimal concentration is 1.0 mol·L-1. The possible reason is that the high concentration of NaBH4 will increase the cathode polarization and reduce the fuel utilization. In addition, the electrolyte, NaOH is another factor, where the optimal concentration of NaOH is 6.0 mol·L-1. And the cathode working conditions also play an important role on the performance of DBFC. It has been found that the pure and humid oxygen used, and the removal of NaOH from the cathode will improve the long-term performance of DSBFC.To overcome the low utilization of fuel in DSBFC, a novel composite membrane electrode assembly (MEA) is developed, in which the anode zone includes two sections. The utilization of fuel can be increased to 100 % when the area ratio of the two sections is 3:1.更多还原