【作者】 张学伟；

【导师】 史鹏飞；

【作者基本信息】 哈尔滨工业大学 ， 应用化学， 2007， 博士

【摘要】 质子交换膜燃料电池采用贵金属铂作催化剂,高昂的成本限制了它的产业化应用。本文以质子交换膜燃料电池的核心组件——膜电极作为研究对象,研究了燃料电池单体制备、操作过程等影响因素。通过对膜电极的三个主要组成部分:质子交换膜、气体扩散层和催化层的深入研究,提出了双催化层结构的电极,降低了贵金属铂的用量,同时提高了燃料电池的性能。膜电极的电化学性能采用极化曲线进行了表征,并采用非线性最小二乘法对部分数据进行拟合及分析;操作条件及材料对膜电极性能的影响采用交流阻抗谱表征,并对实验数据进行拟合及分析,实验研究了采用PTFE和Nafion作为粘结剂的催化层性能,并用扫描电镜观察膜电极(包括支撑层、微多孔层和催化层)的微观形貌并进行分析。对膜电极的制备及操作过程条件进行了研究。研究表明,在130℃和8MPa的条件下,热压120s后得到的膜电极内阻降低到0.4??cm-2。单体运行的装配力矩在3N?m为宜。在1atm下和运行温度低于65℃时,反应气体利用率随流量的增加而降低,随温度升高而升高;运行温度超过65℃时,氧气的利用率降低。考虑温度与电池内阻的关系,电池操作温度选择65℃为宜。膜电极在常压下的活化时,温度为50℃时采用变电流活化,活化10h后的性能稳定。对膜的组件的三个组成部分进行研究表明:质子交换膜厚度越薄,反应过程中产生质子经过膜的传递阻力越小。在实验温度范围内,不同厚度膜制备的膜电极质子传递能力,都随温度升高而提高。气体扩散层包括支撑层和微多孔层,支撑层厚度降低有利于气体传质。微多孔层采用碳材料XC-72和20wt.%的PTFE的混合物,其载量为2mg·cm-2时,膜电极性能达到0.35W·cm-2。当微多孔层的碳材料采用炭黑XC-72时,气体传质阻力小,在电化学极化区性能较好;采用炭黑BP2000时,液体传质阻力小,在浓差极化区性能较好;两种炭黑混合制备气体扩散层,由于大孔和小孔数目的减少,膜电极性能下降。催化层的制备过程中,喷涂法制备催化层的性能好于丝网印刷。当憎水电极的催化层PTFE含量为25wt.%,Nafion溶液的载量为1.5mg·cm-2时,制备膜电极的性能最好。当亲水型电极的催化层Nafion含量为30wt.%、催化剂载量为0.2mg·cm-2的膜电极性能达到0.5W·cm-2。催化层只采用PTFE或者Nafion粘合剂,催化层内存在反应活性层和非反应活性层,电化学反应由活性层向非活性层梯度递减。提出一种双催化层电极结构,包含内外两个催化层。内催化层是亲水结构,采用Nafion作为粘结剂;外催化层是憎水结构,采用PTFE作为催化剂。内外两催化层的交接处在热压过程中形成一个过渡层,反应集中在过渡层内发生。外催化层中PTFE和内催化层Nafion形成相反的梯度分布,有利于反应进行。铂载量均为0.3mg·cm-2时,电流密度1A·cm-2时,双催化层电极性能比憎水电极和亲水电极分别提高了37.2%和20.4%。采用两组铂载量为0.2mg·cm-2双催化层电极,考察Nafion分布的对膜电极性能影响。提高内催化层的Nafion含量,质子传递阻力降低,膜电极在浓差扩散阶段的性能提高;将内外催化层中间的再铸膜含量降低后,膜电极性能由0.64W·cm-2提高到0.77W·cm-2。在双催化层内外层催化剂载总量相同,改变内外催化层中催化剂的分布比例,得到阴极反应中质子传质阻力要大于气体传质,是氧还原反应的控制步骤。本研究将氢氧燃料电池的催化剂铂载量下降到0.2mg·cm-2,膜电极的峰值功率提高到0.7W·cm-2,在降低膜电极制造成本的同时提高了膜电极性能。更多还原

【Abstract】 The proton exchanged membrane fuel cell (PEMFC) adopts platinum as catalyst, which restricts its commertialization due to high price. In this paper, membrane electrode assembly (MEA), a key component of fuel cell, was investigated on fabrication of electrode and operation. Farther research has been done on MEA, including membrane, gas diffusion layer and catalyst layer. Based on work above, a noval structure of dual-bonded catalyst layer electrode was introduced to enhance the cell performance with low Pt loading.Polarization curve was introduced to characterize the cell performance and data was used to fit by non-linear least square. AC impedance was also used to study the effect of opertation conditions on performance of cell. Scanning electron microscope graphs were adapted to investigate MEA, including substrate, gas diffusion layer and catalyst layer.Preparation and operation condition of MEA were studied. Resistance of MEA reduced to 40??cm-2 when it was hot-pressed at 130℃under 8MPa for 120 seconds. Torque of fixture was matained 3N?m for cell assemblage. Utilization of reactant gas was increased with temperature and decreased with increasing flow rate under 1atm when temperature was less than 65℃. Utilization of oxygen was reduced when temperature was more than 65℃.Operation temperature of cell should be chosed as 65℃with lowest resistance. Investigating MEA’s activation at ambient pressure, cell showed stable performance when actived with varied current at 50℃after 10h.Three part of MEA were investigated and found that high protonic conductivity was conrresponding to thinner membrane. Protonic conductivity of different membrane was increased with high temperature. Gas diffusion layer was comprised of substrate and micro-pouose layer. Thinner substrate was favorable for gas transport. Micro-porous layer of electrode with 2mg·cm-2 cabon black XC-72 containing 20wt.% PTFE provided 0.35W·cm-2. Micro-porous laye formed by XC-72 was favorable for gas transport and showed a better performance in active rigion, whilst it formed by BP2000 was advantage in water drainage and showed a better performance in concertration polarization. Cell performance was poor when two types of carbon mixed to make micro-porous laye because incrased of mass transport resistance. Spray was a better choice to make catalyst layer than print. Hydrophobic catalyst layer containing 35wt.% PTFE and 1.5mg·cm-2 Nafion showed high performance. Hydrophilic catalyst layer containing 30wt.% Nafion and 0.2mg·cm-2 Pt loading showed performance of 0.5W·cm-2. There exsit active layer and inactive layer when PTFE or Nafion was chosed as bonded material for catalyst layer. Electrochemical reaction was decreased from active layer to inactive layer.The dual-bonded catalyst layer with inner and outer catalsty layer was designed. Inner catalyst layer was hydrophilic and bonded with Nafion. Outer catalyst layer was hydrophilic and bonded with PTFE. A transition layer formed when two catalyst layers was hot-pressed that main reaction occurs. PTFE of out layer and Nafion of inner layer were distributing in grads for reaction. When electrode with 0.3mg·cm-2 Pt loadings at 1A·cm-2, it was calculated that performance of cathode with dual-bonded catalyst layer offers 37.2% higher than that of hydrophobic cathode and 20.4% higher than that of hydrophilic catalyst layer. Two groups of dual-bonded electrode with 0.3mg·cm-2 Pt loadings were fabricated to investigate Nafion effect on cell performance. Peak power density of electrodes increased from 0.64W·cm-2 to 0.77W·cm-2 at ambient pressure as recast film decrase. The prtonic conductivity affected more on reaction than that of mass transfer when adjust ratio of catalyst layer.Catalyst layer was decreased to below 0.2mg·cm-2 Pt loading and the cell performance was increased to above 0.7W·cm-2 when operated in hydrogen and oxygen. Cell performance was increased whilst cost of MEA was reduced.更多还原