PEMFC膜电极电催化层的制备和性能

膜电极催化层的组成和电催化剂的活性对质子交换膜燃料电池的性能有很大影响．采用浸渍还原法制备出了Pt平均粒径为3．1nm的Pt／C催化剂．催化剂中Pt的粒径和在碳黑载体（VulcanXC-72）表面的分散程度采用透射电镜（TEM）进行测试．用Pt／C催化剂、适量的Nation溶液和PrFE乳液制备出质子交换膜燃料电池（PEMFc）膜电极的催化剂层,并研究了该催化剂层中PTFE含量对其性能的影响．实验表明,PTFE强烈的疏水性可以迫使部分水分向阳极反扩散,催化层中加入适量的PTFE可以使膜电极具有一定的水管理能力,在去掉辅助增湿系统的条件下具有良好的极化性能．     

Preparation and fuel cell performance of catalyst layers using sulfonated polyimide ionomers

Sulfonated polyimide (SPI-8) ionomers were used as binders in the catalyst layers, and their fuel cell performance was evaluated. SPI-8 ionomers functioned well in the anode with only minor overpotential even at low humidity (50% relative humidity (RH)). In contrast, the cathode performance was significantly dependent on the content and molecular weight of the ionomers and humidity of the supplied gases. Higher molecular weight of the ionomer caused larger potential drop at high current density at 80 and 100% RH since oxygen supply and/or water discharge became insufficient due to higher water uptake (swelling) of the ionomer. Similar results were obtained at higher ionomer content, because of the increase of thickness in the catalyst layer. The mass transport was improved with decreasing humidity, however, proton conductivity became lower. While the maximum values of j(@0.70?V) for all membrane electrode assemblies (MEAs) were ca. 0.35 A/cm(2), each electrode could have the different appropriate operating conditions. The results suggest that the parameters such as oxygen supply, proton conductivity, and water uptake and discharge need to be carefully optimized in the catalyst layers for achieving reasonable cathode performance with hydrocarbon ionomers.     

环境对自呼吸式MEMS微型H2/空气质子交换膜燃料电池的影响

利用MEMS技术制备了一种自呼吸式微质子交换膜燃料电池(PEMFC),阳极采用点蛇混合结构,阴极采用双层镂空微流场结构,阴极靠近膜电极侧微孔尺寸从5～50m不等。鉴于自呼吸式电池的性能受环境的影响很大,本文着重研究了环境湿度和温度对电池性能的影响。结果表明阴极微孔尺寸为11m和15m的电池孔径适度,在环境20℃、30%-70%RH时两电池的极限电流密度(Jmax)和峰值功率密度(Pmax)均表现出较高值,性能良好;阴极微孔尺寸为11m的电池在空气维持50%RH下,温度由10℃升到40℃时Pmax逐渐增大,增幅达14.9%;若不维持空气湿度而改变温度,则温度由10℃升高到40℃时Pmax先增大后减小,20℃时达最大。     

炭材料的孔隙——调控的重要性

简要说明炭材料中的孔隙分类和特征后,就其孔隙调控方法及调控的重要性作了述评,首先评述了新近所知的常规活化工艺,而后展示了非活化处理的孔结构调控技术,诸如:模板法,PTFE脱氟,炭气凝胶,聚合物共混法,特定前躯体及制备条件选择等.提出了五类应用实例:CO2捕集、水蒸气吸/脱附、汽油蒸气吸附、超级电容器炭电极、重油吸附和回收等,进一步强调了炭材料孔隙结构凋控的重要性.     

A simple method for preparing 3D hierarchical carbide derived carbon by single step molten salts electrolysis

In this work, we have proposed a simple, cost-effective and green method for the production of 3D hierarchical carbide derived carbon by single step molten salts electrolysis at 710°C. During the electrolysis process, the high carbon ferromanganese fed manganese and iron ions into molten salts and then those ions were reduced on cathode. At last, there is only porous left on the anode. By XRD and Raman analysis, it was found that the as-prepared porous carbon is composed of amorphous carbon and ordered graphitic carbon with a high degree of graphitization. SEM, TEM and N₂ sorption measurement indicate that this type of CDC has a multimodal pore system consisting of micro-pores, meso-pores and macro-pores.     

Supported and unsupported platinum catalysts prepared by a one-step dry deposition method and their oxygen reduction reactivity in acidic media

The present study examines the physical and electrochemical properties of platinum particles generated by a combustion method for use in oxygen reduction on the cathode side of a proton exchange fuel cell (PEMFC). This method employs a one-step, open-atmosphere, and dry deposition technique called reactive spray deposition technology (RSDT). The objective of this study is to characterize the intrinsic activity of the platinum produced for incorporation into low-loading cathode electrodes in high performing membrane electrode assemblies (MEA). The process allows for independent real-time control of the carbon, platinum, and ionomer ratios in the final electrode. In this research work we examine the oxygen reduction reaction via a rotating disk three electrode set-up to understand the intrinsic activity of the as-sprayed platinum as well as platinum condensed onto a carbon support. The mass and specific activities were measured in a 0.1 M perchloric acid electrolyte under different deposition conditions and loading was verified by atomic emission spectroscopy inductively coupled plasma (AES-ICP). Microscopy results indicate that the platinum particle sizes are 5 nm (σ = 2.8 nm) in diameter while TEM and XRD show that the platinum generated by the process is pure and crystalline without bulk oxides or precursor material present. The initial rotating disk electrode result shows that the RSDT technique is capable of producing catalysts with an oxygen reduction mass activity at 0.9 V of 200 mA/mg_Pt rotating at 1600 rpm and 30 °C. The electrochemically active surface area approaches 120 m²/g for the platinum, carbon, and ionomer samples and the unsupported sample with only platinum has an active area of 92 m²/g. The rather larger surface area of the unsupported sample exists when the platinum is deposited as a highly porous nanostructured layer that allows for high penetration of reactant.     

造孔剂对气体扩散阳极性能的影响

研究了气体扩散阳极,以活性炭、碳黑、PVDF、PTFE、造孔剂和催化剂为原料,采用热压成型法制备多孔气体扩散阳极.系统探讨了电极组成、造孔剂、活性炭粒径对气体扩散阳极透气性、孔率和电性能的影响.研究结果表明,采用质量分数为20%的氯化钠和80目左右的活性炭制得的阳极,其造孔效果和电化学性能最好.     

聚丙烯亲水性微孔膜的制备及在锂电池中的应用

以聚丙烯接枝马来酸酐(PP-g-MAH)为改性剂与聚丙烯熔融共混,通过单向拉伸工艺制备亲水性聚丙烯微孔膜。使用差示扫描量热仪和红外光谱表征流延基膜的取向片晶结构,使用压汞仪和扫描电镜表征微孔膜的孔结构,使用Gurley值、水接触角和水蒸气透过率表征微孔膜的透过性和亲水性,将微孔膜组装成电池并测定电池的性能,研究PPg-MAH含量对微孔膜孔结构、亲水性、透过率及电池性能的影响。结果表明,PP-g-MAH共混含量为2.5%的微孔膜孔隙率和透气性得到提升,进一步增加PP-g-MAH含量会导致微孔膜孔结构变差,透过性能下降。微孔膜表面亲水性随PP-g-MAH含量增加持续改善,水蒸气透过率在PP-g-MAH含量为5%时达到最大值,但由于微孔膜孔结构在较高PPg-MAH含量下受到破坏,使得高PP-g-MAH含量下微孔膜水蒸气透过率下降。PP-g-MAH含量为2.5%和5%的微孔膜由于具有较好的孔结构和一定的电解液浸润性,使得锂电池的电荷转移电阻较低,首次充放电比容量较高。     

Optimization of nafion ionomer content using synthesized Pt/carbon nanofibers catalyst in polymer electrolyte membrane fuel cell

In this study, carbon nanofiber (CNF) was used as a support in which 47.5 wt% Pt/CNFs catalyst was prepared by a modified polyol method. The platinum particle size and dispersion on the CNFs are approximately 2-4 nm as determined by X-ray diffractometry and transmission electron microscopy. The specific surface area was approximated as 55.90 m2/g by BET analysis. Electrodes were prepared by the spray method and have a size of 5 cm2. A commercial catalyst (TKK, 46 wt% Pt/C) was used as the anode and the cathode was Pt/CNFs. Different amounts of Nafion ionomer (Aldrich, 5 wt% solution, in the range of 0-20 wt%) were coated on a membrane (Dupont, Nafion 212) with 0.4 mg/cm2 of Pt catalyst at the cathode side. The resulting polarization, ohmic and mass transfer resistances changed significantly based on the Nafion ionomer content. Optimum Nafion ionomer content in the 47.5 wt% Pt/CNFs was 5 wt%. The well-dispersed Nafion ionomer was observed on the catalyst surface area using SEM-EDAX analysis. A sufficient triple-phase boundary was formed by a small amount of Nation ionomer due to the BET surface area of the Pt/CNFs.     

Dispersion kinetics of carbon black for the application in lithium-ion batteries

For production of electrodes for lithium-ion batteries, conductive carbon black (CB) has to be dispersed within the anode and cathode slurry. A sufficient dispersing degree has to be reached in order to ensure the formation of an adequate conductive network within the electrodes. As intermediate product, CB is dispersed in binder solution, prior to addition of active materials. As binder system on anode side carboxymethylcellulose in water is used, whereas on cathode side polyvinylidene fluoride in N-Methyl-2-pyrrolidone is applied. The conductive carbon slurry facilitates the characterization of the slurry properties, based on changes of CB. The structure and amount of conductive carbon black influences the slurry properties decisively. Viscosity increases with increasing CB content, which affects the shear stress within the mixing process. Rheological properties and particle size distributions are investigated over time while dispersing CB with various tip speeds in a dissolver. Dispersion kinetics are described on behalf of an existing model for tip speed variations. Based on the investigations of rheological changes due to varying amount of CB, an extended model enclosing the CB concentration as variable was developed. Using the extended model, particle sizes for new process parameters can be predicted.     

等离子喷涂制造固体氧化物燃料电池三合一电极

采用大气等离子喷涂法在阳极支撑上制备了电解质与阴极.利用X射线衍射分析了阴极与阳极喷涂前后的成分和相结构,用扫描电镜观察了SOFC( solid oxide fuel cell)三合一电极的截面形貌以及阳极、电解质与阴极的表面形貌.结果表明:喷涂前后阴极的化学成份未发生改变,阴极为单斜相、钙钛矿型的(La0.8Sr0.2MnO3)LSM;阳极在喷涂前为Ni/YSZ(Y2O3稳定ZrO2),在喷涂后Ni被氧化成NiO.由阳极、电解质与阴极构成的三合一电极界面不明显,其中电解质致密,阳极与阴极有一定孔隙.     

Evaluation of water distribution in a small operating fuel cell using neutron color image intensifier

Neutron radiography is one of the useful tools for visualizing water behavior in operating fuel cells. In order to observe the detailed information about the water distribution in membrane electrode assembly (MEA) and gas diffusion layer (GDL) in fuel cells, a high performance neutron imaging system is required. A neutron color image intensifier (NCII) is a high spatial resolution and high sensitivity neutron image detector. We have developed an imaging system using an NCII for visualizing the behavior of water in fuel cells. The pixel size of the imaging system is around 4.7 μm in the small view field. By using this system, water distribution of a small sized fuel cell was observed continuously every 20 s at the Thermal Neutron Radiography Facility (TNRF). In the results, the water area appears from the GDL and MEA regions, and expanded to the cathode side channel with time. However, the voltage was gradually reduced with time, and steeply dropped. It is considered that the reduction and the drop of voltage were caused by a blockage of gas flow due to accumulation of water in the GDL and the gas flow channel in the cathode side.     

Nano-morphology of a polymer electrolyte fuel cell catalyst layer—imaging, reconstruction and analysis

The oxygen reduction reaction (ORR) in the cathode catalyst layer (CCL) of polymer electrolyte fuel cells (PEFC) is one of the major causes of performance loss during operation. In addition, the CCL is the most expensive component due to the use of a Pt catalyst. Apart from the ORR itself, the species transport to and from the reactive sites determines the performance of the PEFC. The effective transport properties of the species in the CCL depend on its nanostructure. Therefore a three-dimensional reconstruction of the CCL is required. A series of two-dimensional images was obtained from focused ion beam — scanning electron microscope (FIB-SEM) imaging and a segmentation method for the two-dimensional images has been developed. The pore size distribution (PSD) was calculated for the three-dimensional geometry. The influence of the alignment and the anisotropic pixel size on the PSD has been investigated. Pores were found in the range between 5 nm and 205 nm. Evaluation of the Knudsen number showed that gas transport in the CCL is governed by the transition flow regime. The liquid water transport can be described within continuum hydrodynamics by including suitable slip flow boundary conditions.      

Retention of sulfur dioxide as sulfuric acid by activated coal reject char

Sorption of sulfur dioxide by carbonaceous adsorbents in the presence of water vapor and oxygen results in the formation of sulfuric acid. The retention capacity of sulfur dioxide in this manner is much greater than that of physical adsorption. This article reports the experimental results on the sulfuric acid loading in coal reject-derived char. It is shown that both temperature and concentrations of sulfur dioxide and water vapor have significant effect on the sulfuric acid loading rate. Compared with the commercial-activated carbon, the coal reject char has lower sorption capacity but higher initial rate owing to the catalytic effect of oxides present in the char.     

The Swelling & Water Uptake of Tablets III: Moisture Sorption Behavior of Tablet Disintegrants

Water vapor sorption properties and the thermal behavior of four disintegrants including microcrystalline cellulose (Avicel PH102), croscarmellose sodium (Ac-di-sol), corn starch, and sodium starch glycolate (Primojel), were studied. They all exhibited type II-like isotherm. The apparent monolayer sorption for each of disintegrants was found to be significantly greater than the specific surface obtained from nitrogen adsorption. It is proposed that water molecules interact with specific sites on the disintegrant glassy polymer. Water tends to stay as a condensed phase on the polymer, rather than to diffuse into the bulk. Water plasticization caused glass transition temperature (T_g) of all disintegrant polymers to decrease. It facilitated a change from glass to the rubber state. Because the sorption sites were in the glassy state, the change from glass to rubber, which in turn kinetically reduced the available sites, would reflect the sorption capacity of a disintegrant polymer. In addition, the difficulty in freezing a disintegrant's sorbed water was encountered.     

Hydrogen Storage Alloy and Carbon Nanotubes Mixed Catalyst in a Direct Borohydride Fuel Cell

In this study, carbon nanotubes (CNTs) were mixed with AB5-type hydrogen storage alloy (HSA), as catalyst for an anode in a direct borohydride fuel cell (DBFC). As comparision, a series of traditional carbon materials, such as acetylene black, Vulcan XC-72R, and super activated carbon (SAC) were also employed. Electrochemical measurements showed that the electrocatalytic activity of HSA was improved greatly by CNTs. The current density of the DBFC employing the HSA/CNTs catalytic anode could reach 1550 mA·cm-2 (at -0.6 V vs the Hg/HgO electrode) and the maximum power density of 65 mW·cm-2 for this cell could be achieved at room temperature. Furthermore, the life time test lasting for 60 h showed that the cell displayed a good stability.     

微管式固体氧化物燃料电池阳极微结构及其性能

利用相转化纺丝法制备了NiO-YSZ中空纤维, 在其外表面负载YSZ膜1450℃共烧后形成YSZ/NiO-YSZ双层中空纤维。阳极孔结构通过芯液(N-甲基砒咯烷酮(NMP)+乙醇)中溶剂NMP的含量来控制。 当NMP含量从0、30wt%、50wt%、70wt%增加到100wt%时, 阳极的孔结构由指状孔/海绵孔/指状孔三明治结构逐渐成为贯通的指状孔结构, 电解质膜致密性、还原后的双层中空纤维的机械强度、阳极电导率逐渐减小, 而孔隙率则增加。多孔的阴极Ag涂敷于致密的电解质膜外表面构成微管SOFC。H₂/空气微管SOFC的浓差极化随着指状孔长度的增加而减小, 当NMP含量为70wt%时, 输出性能最佳, 最大功率密度为662 mW/cm² (800℃), 此时极化阻抗最小。     

Measurements of water distribution in through-plane direction of PEFC by using neutron radiography

Fuel gas (hydrogen gas) and oxidant gas (air) are supplied to a polymer electrolyte fuel cell (PEFC). Protons pass through the electrolyte membrane, and combine with oxygen to form water in the cathode reaction site. The generated water must be supplied appropriately to the membrane for proton conduction. On the other hand, the generated water may affect the fuel cell performances because of the blocking of oxygen from reaching the cathode reaction site. Therefore, water management in the PEFC is important, and water distribution during the operation in the through-plane direction has been of wide concern. In order to obtain the water distributions in a membrane electrode assembly (MEA) and a gas diffusion layer (GDL), a borescope system was newly employed using neutron radiography. The system could obtain the water distribution in the MEA and the GDL, and pixel size of 6.5 μm was achieved. Furthermore, the system was applied for a tilted converter system. The pixel of 1.0 μm at an angle of 81° was achieved, and improvement of the spatial resolution was confirmed.     

Selective catalysts for the hydrogen oxidation and oxygen reduction reactions by patterning of platinum with calix[4]arene molecules

The design of new catalysts for polymer electrolyte membrane fuel cells must be guided by two equally important fundamental principles: optimization of their catalytic behaviour as well as the long-term stability of the metal catalysts and supports in hostile electrochemical environments. The methods used to improve catalytic activity are diverse, ranging from the alloying and de-alloying of platinum to the synthesis of platinum core-shell catalysts. However, methods to improve the stability of the carbon supports and catalyst nanoparticles are limited, especially during shutdown (when hydrogen is purged from the anode by air) and startup (when air is purged from the anode by hydrogen) conditions when the cathode potential can be pushed up to 1.5 V (ref. 11). Under the latter conditions, stability of the cathode materials is strongly affected (carbon oxidation reaction) by the undesired oxygen reduction reaction (ORR) on the anode side. This emphasizes the importance of designing selective anode catalysts that can efficiently suppress the ORR while fully preserving the Pt-like activity for the hydrogen oxidation reaction. Here, we demonstrate that chemically modified platinum with a self-assembled monolayer of calix[4]arene molecules meets this challenging requirement.     

Chronoamperometric Study of Ammonia Oxidation in a Direct Ammonia Alkaline Fuel Cell under the Influence of Microgravity

This is a study of the chronoamperometric performance of the electrochemical oxidation of ammonia in an alkaline fuel cell for space applications. Under microgravity the performance of a fuel cell is diminished by the absence of buoyancy since nitrogen gas is produced. The following catalysts were studied: platinum nanocubes of ca. 10nm, platinum nanocubes on carbon Vulcan ? and platinum on carbon nanoonion support of ca. 10nm. These nanomaterials were studied in order to search for catalysts that may reduce or counter the loss of ammonia oxidation current densities performance under microgravity conditions. Chronoamperometries at potential values ranging from 0.2 V to 1.2V vs. cathode potential (breathing Air/300ml/min/82737 Pa) in 1.0 M NH₄OH (30ml/min in anode) were done during over 30 parabolas in NASA’s C9 airplane The Weightless Wonder in January 2016 from Ellington Field Houston. The current densities at 15s in the chronoamperometry experiments showed diminishing values under microgravity and in some cases improvements of up to 92%, for Pt-carbon nanoonions, and over 70% for the three catalysts versus ground at potentials ranging from 0.2 to 0.4V after 5 minutes of chronoamperometric conditions. At higher potentials, 1.0V or higher, Pt nanocubes and Pt-carbon nanoonions showed enhancements of up to 32% and 24%, respectively. At these higher potentials we will have a contribution of oxygen evolution. The changes in current behavior are attributed to the sizes of the catalyst materials and the time needed for the N₂ bubbles detachment from the Pt surface under microgravity conditions.