【作者】 苏华能；

【导师】 廖世军；

【作者基本信息】 华南理工大学 ， 应用化学， 2010， 博士

【摘要】 质子交换膜燃料电池（PEMFC）因其比能量高、无污染、可快速低温启动等优点而受到人们的广泛关注。膜电极（MEA）是质子交换膜燃料电池的核心部件,不仅对燃料电池的性能有很大的影响,而且对降低其生产成本、加快其商业化进程具有很重要的现实意义。低温燃料电池大量使用贵金属铂做为催化剂的活性组分,成为燃料电池成本居高不下的重要因素,严重影响了低温燃料电池的商业化进程,因此,研究和开发具有低铂载量和超低铂载量的膜电极,对于有效降低燃料电池的成本,促进燃料电池的发展和商业化进程具有十分重要的意义。同时,自增湿膜电极的研究是燃料电池领域的具有挑战性的课题,自增湿的实现将大大简化燃料电池系统,降低燃料电池的成本,并且有效解决质子交换膜燃料电池水热管理困难的问题。本文从降低燃料电池成本和提高其性能出发,开展了超低铂载量膜电极、自增湿膜电极的研究和开发工作,同时开展了微型再生式质子交换膜燃料电池系统的研制工作。采用一种新型的光照下直接喷涂膜技术,通过对催化剂浆料的优化研究以及对制备技术的优化,制备出了一种高性能超低铂载量膜电极。该膜电极的阴、阳两极铂载量分别可低至0.12 mg cm^-2和0.04 mg cm^-2,在0.7 V时电流密度仍高达0.7 A cm^-2。考察了催化剂浆料中Nafion含量以及与催化剂的比例、电池温度以及反应气体背压对这种超低铂载量膜电极性能的影响。结果表明:Nafion的最优含量约为25 wt.%左右,低于文献报道的其它方法的相应数值,表明本技术所制备的膜电极的催化层和质子交换膜之间有良好的界面接触,扫描电镜和阻抗测量证明了这种超低铂载量膜电极的催化层与膜之间的紧密接触,膜电极的内阻和质量传输阻力明显小于其它高铂载量膜电极和其它方法制备的膜电极。本文还研究了采用“双催化层”技术制备超低铂载量的膜电极,为了解决普通双催化层结构在保持高铂利用率和促进传质之间的内在矛盾,我们通过使用两种不同铂含量的催化剂制备了一种新型的双催化层结构阴极。将高铂含量催化剂用在内层以集中铂,而低铂含量催化剂用在外层以保持一个合适的催化层厚度。在常压下和氢-空燃料电池中,具有新型双催化层结构的膜电极的性能明显优于具有普通双催化层结构的膜电极和和单催化层结构膜电极的性能。在阴、阳两极铂载量分别低至0.12和0.04 mg cm^-2（超低铂载量）的情况下,新型双催化层膜电极在通常的工作电压0.65 V时的电流密度仍高达0.73 A cm^-2,可完全媲美高铂载量单催化层膜电极。此外,这种新型双催化层超低铂载量膜电极的最大功率密度高达0.66 W cm^-2,比单催化层电极要高出11.9%,证明了新型双催化层结构对传质过程的改善。交流阻抗和循环伏安测试说明这种超低铂载量膜电极具有高效的电化学活性层和高铂利用率。采用新型双催化层技术制备的常规铂载量(阴极:0.2 mg cm^-2,阳极:0.1 mg cm^-2)膜电极的性能比相同铂载量的单催化层膜电极和普通双催化层膜电极可分别提高35%和20%。采用课题组开发的一种新型的含有二氧化硅的Pt/SiO₂/C催化剂做为阳极催化剂,我们得到了一种具有极好自增湿（免增湿）性能的膜电极;在相对湿度为28%的低湿度条件下,采用含有10 wt.%二氧化硅的Pt/SiO₂/C催化剂做为阳极催化剂制备的膜电极可表现出非常好的性能;在电池温度50 oC时,该膜电极展示出优越的低湿度稳定性:在28%相对湿度下经过120小时测试,0.6 V时电流密度维持在0.65 A cm^-2左右,没有明显的衰减。实验发现:自增湿膜电极对于运行温度较为敏感,随着温度升高,膜电极的低湿度运行性能急剧下降。本文还采用XRD、SEM和吸水性测试对该复合催化剂进行了表征。本文尝试制作了一种微型平面6-cell组合再生式燃料电池电源系统。该系统可在电解水模式和燃料电池模式工作,电解模式时,产生的氢气可以储存在储氢合金中,供燃料电池模式工作时使用,所需要的氧可直接从空气中通过自呼吸方式得到。该微型电源系统的开路电压达4.9 V,可在20 mA cm^-2下恒电流放电40多分钟。在工作电压2.9 V时,系统展示出很好的电池性能和可逆性能:最大功率密度达74.8 mW cm^-2,放电电流达34 mA cm^-2,经过10个充放电循环,系统的性能也没有明显的衰减。该微型燃料电池电源系统为燃料电池在便携式电源方面的应用提供了一个很好的发展方向。更多还原

【Abstract】 Proton exchange membrane fuel cells （PEMFC） have attracted much attention due to their advantages, such as high power density, zero or low exhaust and quick startup at low temperature et al. Membrane electrode assembly （MEA） is the key component of PEMFC, which has a great influence on fuel cell performance and is important for cost reduction to a commercially acceptable level. At present, carbon supported platinum is still the widely used electrocatalyst in MEA, which accounts for a large portion of PEMFC cost. In regard to there reasons, the study of membrane electrode assemblies with low or ultra-low platinum loading has always been one of the hot topics in the field of fuel cell. In addition, with the growth of energy demand in different applications, the development of self-humidifying MEA and the study of micro fuel cell have also received considerable attention.MEA with low and ultra-low platinum loadings and self-humidifying of MEA and miniaturization of PEMFC were studied in this thesis for cost reduction and improved cell performance. Firstly, we prepared a high performance MEA with ultra-low platinum loading by using a novel catalyst spraying technique. A cell performance of 0.7 A cm^-2 at 0.7 V was achieved when the platinum loading of the anode and cathode was lowered to 0.04 and 0.12 mg cm^-2 respectively. The effects of Nafion content, cell temperature and back pressures of the reactant gases on the cell performance were investigated. The optimal Nafion content in the catalyst layer was found to be ca. 25 wt.%, which was significantly lower than that for low platinum loading MEAs prepared by other methods, indicating adequate interfacial contact between the catalyst layer and membrane in our home-made MEAs. Scanning electron microscopy （SEM） observation and electrochemical impedance spectroscopy （EIS） measurements revealed that our home-made MEA possessed very thin anode and cathode catalyst layers which is in close contact with the membrane, resulting in low resistance and reduced mass transport limitations.Secondly, a novel double catalyst layer （DCL） cathode was prepared with different amounts of platinum at each electrode to maintain a dedicated balance between improved mass transfer and good platinum utilization: the catalyst with higher platinum loading was used in the inner layer to concentrate the platinum, and the catalystwith less platinum was used in the outer layer to maintain a suitable layer thickness. Polarization characteristics of cathode with this novel DCL, a conventional DCL, and a single catalyst layer （SCL） were obtained at ambient pressure in an H2/air PEMFC. The results showed a significant enhancement of cell performance with the novel DCL cathode. Compared with the SCL cathode, the current density of the novel DCL cathode at 0.6 V was increased by 35.9%, whereas that of the conventional DCL cathode was increased by 8.8% only.Thirdly, an ultra-low platinum loading MEA was prepared using above-mentioned novel DCL technique. Polarization characteristic of the MEAs with novel DCL, general DCL and SCL were evaluated in H2/air single cell system. The results showed that the novel DCL MEA performance was improved significantly, especially at high current densities. When the platinum loading of the anode and cathode was as low as 0.04 and 0.12 mg cm^-2 respectively, the current density of the novel DCL MEA can reach 0.73 A cm^-2 at a proposed working voltage of 0.65 V, which was comparable with that of the SCL MEA. In addition, the maximum power density of the novel DCL MEA reached 0.66 W cm^-2 at 1.3 A cm^-2 and 0.51 V, 11.9% higher than that of the SCL MEA, indicating mass transfer improvement for the novel MEA. EIS and cyclic voltammetry （CV） tests revealed that the novel DCL MEA possessed an efficient electrochemical active layer and good platinum utilization efficiency.Fourthly, we developed a novel self-humidifying MEA with Pt/SiO₂/C anode composite catalyst to improve the performance of PEMFC operating at low humidity conditions. The characteristics of the composite catalysts were investigated by XRD, SEM and water uptake measurement. The optimal performance of the MEA was obtained with 10 wt.% silica in the composite catalyst by single cell tests under both high and low humidity conditions. The low humidity performance of the novel self-humidifying MEA was evaluated in a H2/air PEMFC at ambient pressure under different relative humidity （RH） and cell temperature. The results showed that the MEA performance was almost unchanged when the RHs of both anode and cathode decreased from 100% to 28%. However, the low humidity performance of the MEA was quite susceptible to the cell temperature, which decreased steeply as the cell temperature increased. At a cell temperature of 50 oC, the MEA showed excellent stability for low humidity operating: the current density remained at 0.65 A cm^-2 at a usual work voltage of 0.6 V without any degradation after 120 h operation under 28% RH for both the anode and cathode.Finally, a novel micro planar fuel cell power supplier, in which a six-cell PEM unitized regenerative fuel cell （URFC） stack was used as the power generator, was designed and fabricated. Six membrane electrode assemblies were prepared and integrated on one piece of membrane by spraying catalyst slurry on both sides of the membrane. Each cell was made by sandwiching a MEA between two graphite monopolar plates, and six cell units were mechanically fixed in two organic glass endplates. When the stack was operated in electrolysis mode, hydrogen was generated by water splitting and was stored using a hydrogen storage alloy; conversely, when the stack was operated in fuel cell mode, hydrogen was supplied by the hydrogen storage alloy and oxygen was supplied from air by self-breathing of the cathode. The open-circuit voltage （OCV） of the system reached 4.9 V at room temperature and standard atmospheric pressure; the system could discharge at a constant current density of 20 mA cm^-2 for about 40 min at 2.9 V. The system showed good stability for 10 charge-discharge cycles. It suggests a potential orientation for the application of PEMFC in the field of portable devices.更多还原