Nafion含量对直接甲醇燃料电池阴极催化剂性能表达的影响（英文）

燃料电池是一种将燃料反应的化学能转化为电能的装置,可分为氢氧质子交换膜燃料电池(PEMFCs)、直接甲醇燃料电池(DMFCs)和直接甲酸燃料电池等.与PEMFCs相比,DMFCs以甲醇为燃料,燃料的储存运输和电池操作运行具有较高的安全性,所以近年来受到人们的广泛关注.膜电极组件(MEA)是DMFCs的核心部分,由气体扩散层(GDL)、催化层(CL)和质子交换膜(PEM)三部分组成.GDL用于提高电池传质能力,并同时作为MEA的集流体.PEM主要用于隔离燃料和氧气,进行质子传导.CL是MEA中的主要组成部分,为电化学反应提供场所.催化层由催化剂,质子传输介质和电子传输介质组成.通常,阳极催化剂采用Pt Ru/C,阴极采用Pt/C,质子传输介质为全氟磺酸树脂,如Nafion.CL的结构对电池性能有直接的影响,因此人们对CL的结构进行了详细的研究,并通过调节CL亲水性能、梯度催化层的结构设计等优化其结构.研究表明,当CL中Nafion含量为33 wt.%,PEMFCs具有最佳的电池性能.DMFCs与PEMFCs对MEA要求不同,其阴极更容易发生水淹现象.本文结合非接触式三维光学轮廓仪、接触角测试系统和电化学测试对阴极不同Nafion含量的膜电极进行了表面形貌、亲水性、循环伏安和DMFC性能测试.本文利用喷涂法制备了GDE,然后与Nafion115热压形成MEA.由三维表面形貌图可以看出,随着催化层中Nafion含量的增加,GDE表面的粗糙度变大,尤其是N35和N45.理论上,表面粗糙有利于Pt的暴露和传质扩散,但是其电池性能并未与粗糙度呈现出正相关的关系,因为Nafion含量高于35 wt.%,Pt被Nafion过度包裹,抑制了O2至催化剂表面的传输,且随着Nafion含量由15 wt.%增加至45 wt.%,其GDE表面的接触角由166.8o减至143.1o,说明CL的亲水性增强,易导致阴极产生的水无法及时排出,从而造成阴极水淹现象.从不同Nafion含量制备MEA的CV图可以看出,随着Nafion含量的增加,Pt的电化学活性面积(ESA)增加.当Nafion含量较少时,Nafion无法对全部Pt纳米粒子(NPs)形成包覆或无法形成连贯的质子传输通道,从而导致大部分的Pt NPs催化活性较低变为无效Pt.而有效Pt NPs要求与连贯的质子传输通道相连接.当Nafion含量高于35 wt.%时,其ESA基本保持不变,因为Pt载量一定,从而限制了ESA,此时达到该载量条件下的极限ESA.但是电池极化曲线表明,30wt.%Nafion含量的MEA具有最佳的电池性能.因为有效Pt NPs不一定是高效的,当他们全部被Nafion包裹后,O2只能依靠溶解在Nafion中才可以到达催化剂表面,从而阻碍传质.只有Pt NPs表面包裹和暴露面积达到一定比例时才变得高效.所以当Nafion含量低于30 wt.%时,主要由质子传输通道导致的有效Pt NPs较少;当Nafion含量高于30 wt.%时,出现Nafion过度包裹Pt NPs,阻碍O2传质.因此,Nafion含量30 wt.%时,Pt的包裹面积和裸露面积达到所研究的最佳状态.     

直接甲醇燃料电池催化活性层的优化

本文考察了直接甲醇燃料电池 (DMFC)不同催化剂载量的膜电极性能 .对催化剂层中Nafion含量进行优化 ,研究了Nafion含量对电池的阻抗的影响 .实验发现 :DMFC适宜的阳极Pt_Ru/C载量为Pt 4mg/cm2 、Nafion质量百分含量为 2 1.4 % ;高电流密度下 ,阴极Pt/C载量为Pt4mg/cm2 、Nafion质量百分含量为 2 1.4 %时 ,有较好的放电性能 ,继续增加Nafion含量 ,阴极的欧姆极化和浓差极化增大 ,电池性能下降     

乙烷质子传导膜燃料电池的极化与反应机理

制备了以乙烷作为燃料电池膜电极组装(MEA)及构建了单电池系统。研究了Nafion材料作为质子传导膜、Pt/C作为电极催化剂构成的燃料电池在105 ℃和0.4 MPa电化学性能。采用交流阻抗分析法、色谱分析法及根据Faraday定律,考察了电池的电极极化过程,确定了电池的反应产物并探讨了电极的电化学反应机理。研究结果表明,乙烷燃料电池内阻引起的欧姆极化很小,电池阴极的极化主要是欧姆极化过程所控制,阳极极化主要为活化和浓差过程控制,阳极极化比阴极极化显著,乙烷燃料电池的极化主要在阳极侧;在实验操作条件下,阴极反应产物为水,阳极反应的主产物为CO₂且含有少量的CO,电池反应产物不含乙烯。     

燃料电池

正燃料电池是一种发电装置,它突破了卡诺循环的限制,有望获得更高的能量利用效率。目前,使用质子交换膜(PEM)作为电解质的质子交换膜燃料电池(PEMFCs)是最有可能商用化的车用燃料电池。日本丰田Mirai车型使用的燃料电池技术代表了目前最先进的PEMFCs技术。随着PEMFCs阴、阳极贵金属催化剂用量的不断减少(Mirai的Pt载量为0.15 mg·cm-2),催化剂的成本占比也逐渐下降。目前的高压储氢罐压力最高可以达到80 MPa,可以满足商品化的需求。此外,包括流场、双极板、     

层级多孔碳载铂催化剂的制备及可达性

层级多孔碳作为氧还原铂基催化剂载体的选择之一, 简单的旋转圆盘电极(RDE)验证此类催化剂具有较高的氧还原活性, 但几乎都缺少膜电极(MEA)性能验证, 实用性无法保证. 本文设计制备了基于聚苯胺的层级多孔碳（NHPC）载铂催化剂（Pt/NHPC850）, 研究了其氧还原活性、 MEA质子传输和氧传输特性. RDE测试研究表明, Pt/NHPC850催化剂在低I/C（离聚物与碳载体质量比）时的面积活性低于实心碳载铂催化剂（Pt/XC-72）, 但当I/C增大到与膜电极中一致时, 由于Nafion树脂对Pt催化剂的毒化作用增强, 其面积活性反而优于 Pt/XC-72. Pt/NHPC850催化剂的高Pt分散性及其优异的抗Nafion毒化性能, 使其在I/C为0.8时的质量活性为Pt/XC-72催化剂的1.34倍. MEA质子传输研究表明, 即使在高加湿条件下, Pt/NHPC850质子电阻率仍高达72.6 mΩ·cm², 为Pt/XC-72的3倍. Pt/NHPC850制备的膜电极极化曲线在500 mA/cm²电流密度下性能迅速下降, Pt/NHPC850的氧增益电压达到144.4 mV, 比Pt/XC-72高56.7 mV. 表明Pt/NHPC850膜电极的质子传输和氧传输性能较差. 对比Pt/NHPC850催化剂的RDE和MEA的测试结果, 说明以层级多孔碳为载体的铂碳催化剂虽然耐Nafion毒化能力提高, 但是质子和氧气的氧传输性较差, 此类层级多孔碳还需进一步优化其结构, 才有可能满足低铂质子交换膜燃料电池（PEMFC）的应用需求.     

直接甲醇燃料电池用磺化聚醚醚酮膜初探

应用电化学方法研究了SPEEK膜的甲醇渗透性能.SPEEK膜具有比Nafion115膜低的甲醇渗透.以其作质子交换膜电解质组装的直接甲醇燃料电池(DMFCs)开路电压高于Nafion115膜组装的DMFC开路电压,但电池的放电性能尚待改进.本研究可为SPEEK应用于直接甲醇燃料电池提供一定的依据.     

Synthesis of Nafion~-stabilized Pt nanoparticles to improve the durability of proton exchange membrane fuel cell

Nafion-stabilized Pt nanoparticle colloidal solution is synthesized through ethylene glycol reduction.Pt/Nafion added with carbon black as electric conduction material(labeled Pt/Nafion-XC72) shows excellent electrochemical property compared with Pt/C.After a 300-cycle discharging durability test,the cell performance of membrane electrode assembly(MEA) with the Pt/Nafion-XC72 and Pt/C catalysts indicates a 29.9% and 92.2% decrease,respectively.The charge transfer resistances of Pt/Nafion-XC72 and Pt/C increase by 27.2% and 101.9%,respectively.The remaining electrochemically active surface area of Pt is about 61.7% in Pt/Nafion-XC72 and about 38.1% in Pt/C after the durability test.The particle size of Pt/C increases from about 5.1 nm to about 10.8 nm but only from 3.6 nm to 5.8 nm in the case of Pt/Nafion-XC72.These data suggest that Pt/Nafion-XC72 as a catalyst can enhance the durability of PEMFCs compared with Pt/C.     

质子交换膜燃料电池膜电极耐久性的提升

质子交换膜燃料电池由于具有能量转换效率高、操作温度低、环境友好等优点而备受人们关注。随着2014年丰田发布燃料电池电动汽车Mirai,带来了新一轮燃料电池及燃料电池汽车的产业化热潮。然而,提升质子交换膜燃料电池的寿命,开发新一代长寿命燃料电池膜电极及燃料电池仍然是本领域的挑战性课题。膜电极(MEA)是质子交换膜燃料电池最核心的部件,其耐久性直接决定着燃料电池的寿命。MEA主要由质子交换膜、催化剂层、气体扩散层三部分组成。本文从质子交换膜、催化剂及载体、气体扩散层三个方面介绍了近年来国内外在提升燃料电池膜电极的寿命(耐久性)方面所做的工作,并对未来的相关研究和发展做了述评及展望。     

PEMFC电极中表面扩散现象的初步研究

制作双催化层结构的PEMFC电极.该双催化层由含有Nafion的内催化层、无Nafion的外催化层组成.循环伏安测试表明,未与Nafion直接接触的外催化层Pt/C催化剂也参与发生在"Pt/Nafion"界面氢原子的吸脱附反应和Pt表面含氧粒子的电化学氧化还原.当电势扫描速率较低时,未与Nafion直接接触的外层Pt/C催化剂,其对氢脱附电流的贡献和直接与Nafion接触的内催化层的Pt/C催化剂大致相当.以双催化层电极作PEMFC阴极,单电池(PEMFC)极化曲线测试表明,其阴极外催化层能明显地提高该单电池在活化极化区的输出性能.进一步证明了PEMFC阴极外催化层不与Nafion直接接触的Pt/C催化剂可通过其表面吸附含氧粒子的表面扩散参与发生在"Pt/Nafion"界面氧的电化学还原反应.上述实验为设计PEMFC电极提供了一定的新思路.     

银在质子交换膜燃料电池空气阴极内促进氧传递研究

利用离子交换及随后的氢还原,将单质银负载在质子交换膜(Nafion)孔道内.TEM、XRD表征载银Nafion膜的结构,电化学极限电流法测定氧在载银Nafion膜内的扩散系数.结果表明,因银晶颗粒大于Nafion孔道直径,致使Nafion孔道有所扩张;氧在载银Nafion膜内的扩散系数是无银Nafion膜的4倍.据此,把银引入质子交换膜燃料电池空气阴极催化剂表面的Nafion薄层,则电池的性能在高电流密度下有明显的提高,显示了银对该电极内氧传递的促进作用.     

Anode Catalytic Dependency Behavior on Ionomer Content in Direct CO Polymer Electrolyte Membrane Fuel Cell

In this work, the effect of Nafion ionomer content on the structure and catalytic performance of direct CO polymer electrolyte membrane fuel cell(CO-PEMFC) by using Rh-N-C single-atom catalyst as the anode catalyst layers was studied. The ionic plaque and roughness of the anode catalyst layers increase with the increase of Nafion ionomer content. Furthermore, the contact angle measurement results show that the hydrophilicity of the anode catalyst layers also increases with the increase of Nafion ionomer content. However, when the Nafion ionomer content is too low, the binding between microporous layers, catalyst layers and membrane cannot meet the requirement for either electric conductivity or mass transfer. While Nafion ionomer content increased above 30%, the content of water in anode is difficult to control. Therefore, it was found that AN 30(30% Nafion ionomer content of anode) is the best level to effectively extend the three-phase boundary and improve CO-PEMFCs performance.     

The effect of Nafion ionomer on electroactivity of palladium–polypyrrole catalysts for oxygen reduction reaction

Present studies concentrated on the preparation, characterization, and electroactivity of palladium–polypyrrole (Pd/PPY) catalysts for oxygen reduction reaction. In particular, the effect of Nafion ionomer on their electroactivity was evaluated. In all catalysts prepared by “water-in-oil” microemulsion method, the Pd nanoparticles of ca. 7 nm in size appeared regardless of the Pd content (ranging from 2 to 20 wt.%). For comparison, carbon black-supported (Vulcan XC-72) catalyst (20 wt.% Pd) was also synthesized. Coating of the Pd/PPY samples with Nafion ionomer reduced their surface area and porosity. Chemical interaction due to Nafion acid functionalities affected the N-state of pyrrole as well as electron state of Pd in the Pd/PPY catalysts. As a result, the contribution of more oxidized palladium (Pd^δ+) increased. These interactions played an essential role in the electroactivity of Pd/PPY for oxygen reduction reaction. The increased amount of Nafion relative to that of PPY reduced limiting current density whereas the half-wave potential shifted to a more positive value and the fraction of hydrogen peroxide remarkably decreased.     

Preparation of Nafion/sulfonated poly(phenylsilsesquioxane) nanocomposite as high temperature proton exchange membranes

Nafion/sulfonated poly(phenylmethyl silsesquioxane) (sPPSQ) composite membranes are fabricated using homogeneous dispersive mixing and a solvent casting method for direct dimethyl ether fuel cell (DDMEFC) applications operated above 100 °C. The inorganic conducting filler, sPPSQ significantly affects the characteristics in the nanocomposite membranes by functionalization with an organic sulfonic acid to PPSQ. Moreover, sPPSQ content plays an important role in membrane properties such as microstructure, proton conductivity, fuel crossover, and single cell performance test. With increasing sPPSQ content in the nanocomposite membrane, the proton conductivity increased and fuel crossover decreased. However, in a higher temperature range above 110 °C, Nafion/sPPSQ 5 wt.% composite membrane has the highest proton conductivity. Also, the DME permeability for the composite membrane with higher sPPSQ content increased sharply. The excessive sPPSQ content caused a large aggregation of inorganic fillers, leading to the deterioration of membrane properties. In this study, the optimal sPPSQ content for maximizing the DDMEFC performance was 5 wt.%. Our nanocomposite membranes demonstrated proton conductivities as high as 1.57 × 10⁻¹ S/cm at 120 °C, which is higher than that of Nafion. The cell performances were compared to Nafion/sPPSQ composite membrane with Nafion 115, and the composite membrane with sPPSQ yielded better cell performance than Nafion 115 at temperatures ranging from 100 to 120 °C and at pressures from 1 to 2 bar.     

Hydrophilic-hydrophobic and sorption properties of the catalyst layers of electrodes in a proton-exchange membrane fuel cell: A stage-by-stage study

In the electrodes of a proton-exchange membrane fuel cell, the hydrophilic-hydrophobic properties of the catalyst layers (CL), which contain a carbon substrate (CS), an ionomer in the form of Nafion resin, and a platinum catalyst, are investigated with standard contact porosimetry. Regularities in the influence of ionomer on the hydrophilic-hydrophobic properties of ten different CS, including Vulcan XC-72 carbon black, are investigated. The following plots are obtained: pore distribution curves with respect to radii, water desorption isotherms, moisture content distribution curves with respect to capillary pressure and to the free energy of binding water to material, and the wetting angle of water for the samples under investigation as a function of pore radius. It is established that both a hydrophobic effect and a hydrophilic effect occur in CL as a result of ionomer application to the CS under investigation. It is concluded that these different effects are determined by the orientation of the sulfonate groups (inside and outside) in the resin particles. This orientation depends on the extent of the binding of sulfonate groups to the CS surface by adsorption. CS surface properties are determined by the type and concentration of surface groups. Thus, the phenomenon of ionogenic-group inversion is established. When platinum is applied to CS, the CL become partially hydrophilic.     

墨水溶剂对低铂含量质子交换膜燃料电池性能的影响

Owing to the scarcity of platinum, it is of high importance to develop electrodes with low platinum metal loading and to thereby improve the utilization of Pt for the commercialization of proton-exchange membrane fuel cells (PEMFCs). In comparison to conventional high-platinum electrodes, the thickness of the catalyst layer (CL) is thinner and the interatomic Pt spacing is larger for the low-Pt loading electrodes. The distribution of electrolyte ionomer and the electrode morphology, which are strongly influenced by the solvents used in the fabrication process, are therefore increasingly important factors for achieving high performance in the membrane electrode assembly (MEA). In this work, different solvents with various dielectric constants and evaporation rates were used to prepare the inks for low-Pt loading cathode (0.1 mg·cm^-2) fabrication. First, the inks were fabricated by dispersing the catalyst and ionomer in different solvents which were then coated onto carbon paper to prepare the gas diffusion electrodes. The anode and cathode electrodes were then hot-pressed together with the Nafion membrane to produce the MEAs. The results showed a mixture of isopropanol-water (4:1) yielded the best-performing MEA during the single-cell tests compared to the other solvents tested. In order to elucidate the relationship between the performance of MEAs and the solvents, the structure and the surface morphology of the CL and the distribution of Nafion ionomer in the CL was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A comparison of the SEM and TEM images of representative samples indicated that the best performing electrode had a much improved homogeneity in the surface morphology as well as the dispersion of catalyst and ionomer. This was because of the moderate evaporation rate and better dispersion, caused by the increased hydrogen bonding and high dielectric constant, respectively. The results from dynamic light scattering (DLS) showed that the size of the catalyst and ionomer aggregates are influenced by the solvents. It is suggested that larger aggregates might help the formation of holes in the CL for gas diffusion and water removal, with the optimum size found to be around 400–800 nm. In conclusion, the MEA fabricated from the isopropanol-water solvent exhibited a significantly increased power density (1.79 W·cm^-2), and the utilization of Pt was increased to approximately 0.047 mg·W^-1, which is among the best-performing fuel cells reported to date.     

The effect of water content on chain dynamics in nafion membranes measured by neutron spin echo and dielectric spectroscopy

Neutron spin‐echo spectroscopy has been used to measure the time‐scale of fluctuations associated with the inter‐chain correlation peak observed in the diffraction pattern of the perfluorosulfonate ionomer, Nafion^®. We have successfully measured the chain dynamics as a function of water content and temperature and have demonstrated that the chain dynamics become faster with increasing temperature. The addition of water also results in shorter relaxation times due to a plasticization of the chain motions. At water contents above a λ ≈ 6, the chain dynamics seem to approach a plateau and the relaxation times of the chains are no longer being plasticized by the presence of additional water molecules. The increased mobility in molecular relaxations induced by the presence of water points to the molecular origins of the temperature‐ and humidity‐dependent softening mechanisms in Nafion and other perfluorinated sulfonic acid membrane materials. Moreover, these results show that there is a strong inter‐dependence of the dynamics of water and the polymer chains in Nafion membranes. © 2014 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. 2014 , 52, 624–632     

Preparation of Nafion/PTFE/Zr(HPO4)2 composite membranes by direct impregnation method

The viscosities of as received 5.1 wt.% Nafion solutions (EW = 1100, Du Pont Co) blended with various concentrations of ZrOCl₂ were studied. We show the solution viscosity decreases as the wt. ratio of [ZrOCl₂]/[Nafion] is increased from 0.0 to 0.03, then the viscosity does not change significantly as the wt. ratio of [ZrOCl₂]/[Nafion] is increased from 0.03 to 0.16, and then the viscosity increases dramatically as the wt. ratio of [ZrOCl₂]/[Nafion] is increased above 0.16. Four Nafion solutions consisting of 5.1 wt.% Nafion and ZrOCl₂ with [ZrOCl₂]/[Nafion] wt. ratios of 0.019–0.24 were used with porous poly(tetrafluoroethylene) (PTFE) film to prepare zirconium hydrogenphosphate (ZrP) hybridized Nafion/PTFE (NF–ZrP) composite membranes by direct impregnating porous PTFE in Nafion/ZrOCl₂ solutions. The influence of [ZrOCl₂]/[Nafion] wt. ratio of Nafion/ZrOCl₂ solution on the membrane morphology of NF–ZrP and polyelectrolyte membrane fuel cell (PEMFC) performance at temperatures of 110–130 °C with relative humidity of 51.7–28.8% RH was investigated.     

The morphology in nafion perfluorinated membrane products,as determined by wide- and small-angle x-ray studies

The morphology of the ionomer resin from which Nafion perfluorinated membrane products are made was studied with wide-angle and small-angle x-ray diffraction. A reflection observed in the small-angle x-ray scan from hydrolyzed polymer is attributed to ionic clustering. The effects of equiv wt, cation form, temperature, water content, and tensile drawing on this reflection were studied and are discussed.     

Mechanical properties of Nafion and titania/Nafion composite membranes for polymer electrolyte membrane fuel cells

Measurements of the mechanical and electrical properties of Nafion and Nafion/titania composite membranes in constrained environments are reported. The elastic and plastic deformation of Nafion‐based materials decreases with both the temperature and water content. Nafion/titania composites have slightly higher elastic moduli. Thecomposite membranes exhibit less strain hardening than Nafion. Composite membranes also show a reduction in the long‐time creep of ～40% in comparison with Nafion. Water uptake is faster in Nafion membranes recast from solution in comparison with extruded Nafion. The addition of 3–20 wt % titania particles has minimal effect on the rate of water uptake. Water sorption by Nafion membranes generates a swelling pressure of ～0.55 MPa in 125‐μm membranes. The resistivity of Nafion increases when the membrane is placed under a load. At 23 °C and 100% relative humidity, the resistivity of Nafion increases by ～15% under an applied stress of 7.5 MPa. There is a substantial hysteresis in the membrane resistivity as a function of the applied stress depending on whether the pressure is increasing or decreasing. The results demonstrate how the dynamics of water uptake and loss from membranes are dependent on physical constraints, and these constraints can impact fuel cell performance. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2327–2345, 2006     

Nafion Ionomer Dispersion in Mixtures of 1-Propanol and Water Based on the Martini Coarse-Grained Model

The self-assembly of Nafion ionomer in a mixture of 1-propanol (NPA) and water was investigated using coarse-grained molecular dynamics simulations. Ionomer formation into cylindrical bundle-like aggregates is observed when the ionomer chain size is sufficiently large. The size of ionomer bundles decreases (the diameter decreased from ~2.5 to ~1.9 nm) with increasing NPA content, which indicates that the ionomers tend to be more dispersed at higher NPA content. The results of simulations are in good quantitative agreement with the stable size of an ionomer bundle estimated from our free energy calculations as well as the available experimental data. Furthermore, in contrast to the polarizable NPA model, the standard nonpolarizable NPA model shows the opposite trend that the ionomer size increases with increasing NPA content because of a higher localization of hydronium ions at the bundle surface, revealing that the polarization effect plays a significant role in determining the aggregation behaviors. The present study provides insight into the control of ionomer self-assembly toward obtaining targeted structures for specific purposes. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 487–499