【作者】 褚有群；

【导师】 马淳安；

【作者基本信息】 浙江工业大学 ， 工业催化， 2004， 博士

【摘要】 氧气的电化学还原反应是最重要的电催化反应之一。聚四氟乙烯（PTFE）粘结的碳基氧电极广泛地应用在燃料电池、金属／空气电池、有机电合成及电化学废水处理等领域中。本文在全面综述国内外燃料电池和氧电极研究进展的基础上，采用SEM、XRD、TPD、TG、N₂吸附等材料分析测试手段和线性极化、循环伏安、电化学阻抗谱等电化学实验方法对碱性溶液中活性碳（AC）和多壁纳米碳管（MWNTs）氧电极的电化学性能和氧在MWNTs表面的电化学还原行为进行了研究。 本文在第三章中首先从基础工艺入手，研究了催化层和防水层组成、导电骨架等制备条件对氧气扩散电极性能的影响，为后续工作打好基础。研究发现，碳基氧气扩散电极的性能与其制备条件密切相关，采用下述条件制成的气体扩散电极具有较好的综合性能：催化层中PTFE与活性碳的质量比为0.15:0.85；防水层中PTFE、乙炔黑与硫酸钠的质量比为1:1:1；以80目镍网为导电骨架；电极成型压力为10MPa，冷压时间为1min。 在第四章中采N₂吸附和TPD法详细研究了硝酸氧化和空气氧化处理对活性碳孔结构和表面化学性质的影响，考察了活性碳性质与气体扩散电极性能之间的关系。研究发现，氧气扩散电极的性能随催化层中活性碳表面含氧基团的增多而下降；当活性碳孔隙增大时下降趋势更为显著。 在第五章中，采用循环伏安法研究了碱性溶液中溶解氧在纳米碳管和石墨表面的电化学还原行为。结果表明，多壁纳米碳管具有比石墨更好的催化活性，纳米碳管表面上O₂还原为HO₂^-的反应为准可逆反应；石墨电极上O₂还原为HO₂^-的反应和HO₂^-进一步被还原为OH^-的反应均受反应物的扩散过程控制，而在纳米碳管电极上这两个反应均为吸附控制。反应控制步骤的不同可能与碳管表面存在大量拓扑学缺陷有关。 在实验过程中还发现，空气氧化和浓硝酸浸泡处理后的纳米碳管出现了2～4nm的孔隙，BET比表面显著增大，表明处理后MWNTs的端口被打开。但开口率的增加并未引起电极性能的显著改善，空气氧化法处理的MWNTs电极上氧还原反应电流反而下降，而且开口处理后碳管表面氧还原反应转变为扩散控制过程。其原因可能与开口处理时破坏了碳管表面具有较高反应活性的拓扑学缺陷有关。浙江工业大学博士学位论文 第六章中采用线性极化和电化学阻抗谱法研究了纳米碳管氧气扩散电极的极化行为。研究发现，纳米碳管氧电极的Tafel曲线存在两个线性区。低过电位时的Tafe!斜率在一0.055一0.061 v.dec一’之间，其反应对氧分压为一级反应。催化层碳材料的性质对氧气扩散电极的电化学阻抗谱具有显著影响。纳米碳管氧气扩散电极的Nyqulst图在高频部分呈近似45“的直线，而在低频部分形成一个压扁的圆弧。高频部分的线性区对应氧在电极催化层薄液膜中的有限长度扩散。低频部分圆弧的直径随电极极化电位的增加而减小，表明该阻抗对应于氧的电化学还原过程。而活性碳氧气扩散电极在高频部分还出现了一个大小与极化电位无关的圆弧，对应于多孔催化层内的欧姆极化过程。催化层中活性碳与纳米碳管的质量比为卜1时电极具有最好的性能。 在第七章中采用碳还原KMnO;的方法制成碳载锰氧化物（Mnox/C），并对其电化学活性和氧还原催化活性进行了研究。实验结果表明，制成的MnO、具有类似于Bimessites矿的层状结构:Mnox/M WNTs具有比Mnox/AC更好的电化学活性。Mno扩C的加入并未引起MWNTs电极上氧两电子还原反应峰电位的改变，这表明该反应是在MWNTs表面进行的。Mnox/C对Ho万具有良好的催化分解作用，使氧在两电子还原反应电位区被完全还原，从而表现出类似于四电子直接还原的反应电流。Mnox/M WNTs、Mnox/AC和EMD（电解二氧化锰）催化分解H202的活性依次递减。催化分解反应的速率常数k分别为2.384x10一S一’、1.863、1丫s-l、1 .ol6xlo礴s‘1。更多还原

【Abstract】 Electrochemical reduction of oxygen is one of the most important electrocatalytic reations. Polytetrafluoroethylene (PTFE) bonded carbon-based oxygen electrode are widely used in many important electrochemical technologies, such as fuel cells, matal/air batteries, organic electrosynthetic processes and electrochemical devices for decontamination of wastewaters. Based on the review of research and development of fuel cells and oxygen electrodes, electrochemical performances of oxygen electrodes made from activated carbon (AC) and multi-walled carbon nanotubes (MWNTs) as well as electrochemical reduction behavior of oxygen at the surface of MWNTs were investigated by different material characterization methods and electrochemical techniques, such as SEM, XRD, TPD, TG, N2 adsorption, linear polarization, cyclic voltammetry and electrochemical impedance spectroscopy.In the third chapter of this dissertation, the effects of the preparation conditions, such as the compositions of catalyst layer and hydrophobic layer, and different electric conductors on the performance of gas diffusion electrode were studied by using linear polariztion technique. The experimental results showed that the performance of gas diffusion electrode is greatly depended on its preparation conditions, and the electrodes with better performance can be obtained with the following prepartion conditions: mass ratio of PTFE to activated carbon in catalyst layer is 0.15:0.85; mass ratio of PTFE, acetylene black and Na2SO4 in hydrophobic layer is 1:1:1; 80 mesh nickel net is used as electric conductor; each part of the electrode is pressed together at 10 MPa for 1 min.In the fourth chapter, the techniques of N2 adsorption and TPD were used to examine the texture and surface chemical properties of activated carbon before and after treatment of nitric acid and air oxidation, and effects on the performance of gas diffusion electrode were also studied. It is found that the performance of oxygen diffusion electrode declines with the increasing of oxygen functional groups at the carbon surface.In the fifth chapter, electrochemical reduction behavior of dissolved oxygen in alkaline solution at the surface of MWNTs and graphite were investigated by using cyclic voltammetric method. Compared with other carbon materials, MWNTs showhigher electrocatalytic activity for oxygen reduction, and the reduction of O2 to HO2-is a quasi-reversible process at MWNTs electrode. The reduction of O2 to HO2- andthe further reduction of HO2- to OH- at graphite electrode are controlled by thediffusion of reative species in the solution, while at MWNTs electrode, both reactions are controlled by adsorptive process. This can be ascribed to the great difference of the surface texture between MWNTs and graphite powders.Furthermore, it has been found that the tops of MWNTs can be opened by air oxidation and concentric nitric acid treatment, which resuts in the appearance of pores with diameter of 2-4 nm and the great increasement of BET surface of MWNTs. However, the electrode performance of treated MWNTs does not improve remarkably, on the contrary, the decline of the electrode performance of air oxidized MWNTs is observed. These can be partly attributed to the destruction of topological defects on the MWNTs surface after air oxidation.In the sixth chapter, the polarization behavior of MWNTs oxygen diffusion electrode was investigated by linear polarization and electrochemical impedance spectroscopy (EIS). It is found that two Tafel linear parts exist in the polarization curves of MWNTs oxygen diffusion electrode. Tafel slope at low overpotential is -0.055--0.061 V dec-1, and the corresponding reaction is first order for oxygen partial pressure. EIS of gas diffusion electrode depends on the carbonaceous materials of catalyst layer. The Nyquist plots of MWNTs gas diffusion electrode show a linear region with a phase angle of 45 in the high frequency range and a depressed semicircle at lower frequencies. The linear behavior at higher frequency implies a更多还原