Preparation of graded microporous layers for enhanced water management in fuel cells

The newly prepared gas diffusion layer (GDL) was studied in this article. The new GDL mainly includes a carbon paper layer and two microporous layers (MPLs) on the same side of carbon paper. The two MPLs were fabricated by using conductive carbon blacks of two different carbon morphology, Vulcan XC-72, and acetylene black carbon, respectively. From the results of the water contact angle and conductivity, it was found that the GDL prepared by spraying has not only good hydrophobicity but also low resistivity. Polarization curve and electrochemical impedance spectroscopy indicated that the spraying sequence of two different carbon black pastes seriously affects the fuel cell performance, and the mercury intrusion test proved that the change in the fuel cell performance is due to the change in the pore-size distribution of the newly prepared GDL. By comparing with commercial GDL29BC, the GDL first sprayed with a layer of acetylene black carbon and then a layer of Vulcan XC-72 has more pore size of 7–20 μm and 20–100 μm to improve the water and gas management capability of proton exchange membrane fuel cells. When the humidity is 60%, the maximum power density is increased by 25%, and when the humidity is 100%, it is increased by 12%.     

聚合物电解质膜燃料电池气体扩散层上新型双层微孔层的制备和性能

聚合物电解质膜燃料电池（PEMFC）的微孔层对电池性能有重要影响。首次使用多壁碳纳米管（MWCNTs）在碳纸上先制备1层微孔层,再用碳黑（CB）在其上制备第2层微孔层,形成双层微孔层。从不同尺度上观察了微孔层（MPL）的形貌和结构,测量了气体扩散层（GDL）垂直向电阻,并测试了电池性能。结果表明,双层微孔层的平整程度与单独使用碳黑制备的微孔层相似,比单独使用MWCNTs制备的微孔层更加平整;双层微孔层的GDL垂直向的电阻比单层微孔层的GDL更小;使用双层微孔层制备的膜电极比相同碳载量下的单层微孔层制备的膜电极性能更好。     

Two-phase transport and the role of micro-porous layer in polymer electrolyte fuel cells

Two-phase transport of reactants and products constitutes an important limit in performance of polymer electrolyte fuel cells (PEFC). Particularly, at high current densities and/or low gas flow rates, product water condenses in open pores of the cathode gas diffusion layer (GDL) and limits the effective oxygen transport to the active catalyst sites. Furthermore, liquid water covers some of the active catalytic surface, rendering them inactive for electrochemical reaction. Traditionally, these two-phase transport processes in the GDL are modeled using so-called unsaturated flow theory (UFT), in which a uniform gas-phase pressure is assumed across the entire porous layer, thereby ignoring the gas-phase flow counter to capillarity-induced liquid motion. In this work, using multi-phase mixture (M²) formalism, the constant gas pressure assumption is relaxed and the effects of counter gas-flow are studied and found to be a new oxygen transport mechanism. Further, we analyze the multi-layer diffusion media, composed of two or more layers of porous materials having different pore sizes and/or wetting characteristics. Particularly, the effects of porosity, thickness and wettability of a micro-porous layer (MPL) on the two-phase transport in PEFC are elucidated.     

Experimental Investigation of Air Relative Humidity (RH) Cycling Tests on MEA/Cell Aging in PEMFC Part I: Study of High RH Cycling Test With air RH at 62%/100%

The effect of high air relative humidity (RH) cycling (RH_C 62%/100%) on the degradation mechanisms of a single (5 × 5 cm²) proton exchange membrane fuel cells was investigated. The cell performance was compared to a cell operated at constant humidification (RH_C = 62%). Runs were conducted over approximately 1,500 h at 0.3 A cm^–2. The overall loss in cell performance for the high RH cycling test was 12 μV h^–1 whereas it was at 3 μV h^–1 under constant humidification. Impedance spectroscopy reveals that the ohmic and charge transfer resistances were little modified in both runs. H₂ crossover measurement indicated that both high RH cycling and constant RH test did not promote serious effect on gas permeability. The electroactive surface loss for anode and cathode during high air RH cycling was more significant than at constant RH operation. The water uptake determined by ¹H nuclear magnetic resonance within the membrane electrode assembly (MEA) after high RH cycling was reduced by 12% in comparison with a fresh MEA. Transmission electron microscopy showed bubbles and pinholes formation in the membrane, catalyst particles agglomeration (also observed by X‐ray diffraction), catalyst particles migration in the membrane and thickness reduction of the catalytic layers. Scanning electron microscopy was conducted to observe the changes in morphology of gas diffusion layers after the runs.     

PEM fuel cells operated at 0% relative humidity in the temperature range of 23-120 °C

Operation of a proton exchange membrane (PEM) fuel cell without external humidification (or 0% relative humidity, abbreviated as 0% RH) of the reactant gases is highly desirable, because it can eliminate the gas humidification system and thus decrease the complexity of the PEM fuel cell system and increase the system volume power density (W/l) and weight power density (W/kg). In this investigation, a PEM fuel cell was operated in the temperature range of 23-120 °C, in particular in a high temperature PEM fuel cell operation range of 80-120 °C, with dry reactant gases, and the cell performance was examined according to varying operation parameters. An ac impedance method was used to compare the performance at 0% RH with that at 100% RH; the results suggested that the limited proton transfer process to the Pt catalysts, mainly in the inonomer within the membrane electrode assembly (MEA) could be responsible for the performance drop. It was demonstrated that operating a fuel cell using a commercially available membrane (Nafion^® 112) is feasible under certain conditions without external humidification. However, the cell performance at 0% RH decreased with increasing operation temperature and reactant gas flow rate and decreasing operation pressure.     

Effect of wettability of anode microporous layer on performance and operation duration of passive air-breathing direct methanol fuel cells

The influence of the wettability of the anode microporous layer (MPL) on both cell performance and operation duration of air-breathing direct methanol fuel cells (DMFCs) was investigated. The experimental results demonstrated that a passive DMFC with a hydrophilic MPL (DMFC-L) in the anode gas diffusion layer (GDL) showed performance superior to that with a hydrophobic MPL (DMFC-B), when the performance evaluation was conducted by applying a holding time of 45 s at each current density. DMFC-B showed good performance at medium and high current densities when a holding time of 150 s was used. The observation of water accumulation on the cathode of DMFC-L indicated that the decreased performance resulted mainly from blockage of the oxygen supply path. The constant-current discharging test showed that DMFC-B exhibited a lower performance at the beginning of discharge. However, it showed a lower rate of water accumulation on the cathode and thereby relatively stable operation. Operating the passive DMFCs at high water evaporation rate confirmed the important role of the wettability of anode GDLs for cathode oxygen transport.     

EFFECT OF GAS DIFFUSION LAYER CHARACTERISTICS AND ADDITION OF PORE-FORMING AGENTS ON THE PERFORMANCE OF POLYMER ELECTROLYTE MEMBRANE FUEL CELLS

The performance of five layer membrane electrode assemblies having different characteristics of gas diffusion layers was determined at 70°C cell temperature and ambient pressure. The maximum power density at 0.6 V was 0.36 W/cm² for the membrane electrode assembly prepared with the gas diffusion layer having minimum thickness (SGL BC 30 type). On the other hand, the maximum power density at 0.5 V was 0.44 W/cm² for the membrane electrode assembly prepared with SGL BC 34 type gas diffusion layer. It was found that resistance of a membrane electrode assembly is strongly dependent on gas diffusion layer thickness. Moreover, membrane electrode assemblies prepared with carbon paper gas diffusion layers resulted in higher performance than the assembly prepared with carbon cloth gas diffusion layer. Addition of pore-forming agents, which were ammonium carbonate, ammonium bicarbonate, ammonium sulfonate, and ammonium oxalate, to the catalyst ink lowered the performance.     

Effects of membrane electrode assembly preparation on the polymer electrolyte membrane fuel cell performance

This paper presents results of recent investigations to develop an optimized in-house membrane electrode assembly (MEA) preparation technique combining catalyst ink spraying and assembly hot pressing. Only easy steps were chosen in this preparation technique in order to simplify the method, aiming at cost reduction. The influence of MEA fabrication parameters like electrode pressing or annealing on the performance of hydrogen fuel cells was studied by single cell measurements with H₂/O₂ operation. Toray paper and carbon cloth as gas diffusion layer (GDL) materials were compared and the composition of electrode inks was optimized with regard to most favorable fuel cell performance. Commercial E-TEK catalyst was used on the anode and cathode with Pt loadings of 0.4 and 0.6 mg/cm², respectively. The MEA with best performance delivered approximately 0.58 W/cm², at 65 °C cell temperature, 80 °C anode humidification, dry cathode and ambient pressure on both electrodes. The results show, that changing electrode compositions or the use of different materials with same functionality (e.g. different GDLs), have a larger effect on fuel cell performance than changing preparation parameters like hot pressing or spraying conditions, studied in previous work.     

The Effect of Clamping Pressure on Gas Diffusion Layer Performance in Polymer Electrolyte Fuel Cells

Clamping pressure applied in polymer electrolyte fuel cell (PEFC) assembly is known to have a significant effect on performance. Compression applied on the membrane electrode assembly (MEA) deforms the gas diffusion layer (GDL) and causes a decrease in the GDL thickness, porosity and electrical resistance. These changes in the GDL properties have a significant influence on the MEA performance. In this study, three sets of GDL samples were tested in situ under varied clamping pressure levels to demonstrate the difference in the GDL behaviour with compression, and to optimize the MEA clamping pressure. The paper focuses on the change in the performance of MEAs with various types of GDLs, and relates the properties of the GDL to the behaviour of the MEA under compression. The study discusses the change in GDL compression behaviour with the change in the GDL thickness, density and structure.     

In situ grown carbon nanotubes on carbon paper as integrated gas diffusion and catalyst layer for proton exchange membrane fuel cells

In situ grown carbon nanotubes (CNTs) on carbon paper as an integrated gas diffusion layer (GDL) and catalyst layer (CL) were developed for proton exchange membrane fuel cell (PEMFC) applications. The effect of their structure and morphology on cell performance was investigated under real PEMFC conditions. The in situ grown CNT layers on carbon paper showed a tunable structure under different growth processes. Scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) demonstrated that the CNT layers are able to provide extremely high surface area and porosity to serve as both the GDL and the CL simultaneously. This in situ grown CNT support layer can provide enhanced Pt utilization compared with the carbon black and free-standing CNT support layers. An optimum maximum power density of 670 mW cm⁻² was obtained from the CNT layer grown under 20 cm³ min⁻¹ C₂H₄ flow with 0.04 mg cm⁻² Pt sputter-deposited at the cathode. Furthermore, electrochemical impedance spectroscopy (EIS) results confirmed that the in situ grown CNT layer can provide both enhanced charge transfer and mass transport properties for the Pt/CNT-based electrode as an integrated GDL and CL, in comparison with previously reported Pt/CNT-based electrodes with a VXC72R-based GDL and a Pt/CNT-based CL. Therefore, this in situ grown CNT layer shows a great potential for the improvement of electrode structure and configuration for PEMFC applications.     

阳极进气湿度对质子交换膜水含量及电流密度分布影响

针对质子交换膜中水分布不均匀造成燃料电池性能降低的问题,将膜和催化层中水传递方程进行耦合实现水在膜和催化层之间连续传递,建立了质子交换膜燃料电池三维稳态模型。利用有限元分析软件COMSOL进行模拟计算,研究了阳极气体在不同湿度下膜电流密度分布并组装单电池进行了验证,分析实验模拟结果表明:模拟极化曲线与实验极化曲线吻合良好。湿度对电流密度分布影响很大,低湿度条件下,脊背下方电流密度大于气体流道下方;高湿度条件下,电流密度分布比较均匀;采用Nafion117较厚膜时,高电流密度下,即使阳极加湿,阳极侧也有脱水的可能。     

低流速燃料电池重力辅助排水

实验研究了反应气体低流速下质子交换膜燃料电池内液滴自身重力对电池性能的影响。结果显示,自身重力有利于液滴脱离气体扩散层,使液态水有效排出电池堆。电池水平放置阴极向下时,液滴重力与其脱离气体扩散层方向一致,电池性能最佳;电池竖直放置时,液滴重力与气体将其吹扫出电池方向一致,其向外排水能力最强。反应气体流速较低时,电池在不同放置方式下,提高其温度,电池性能上升;电池竖直放置时,气体加湿对电池性能影响不大。电池测试时,应该避免电池阴极水平向上。     

亲疏水交替碳纸的制备及其在气体扩散层中的应用

气体扩散层（GDL）作为控制质子交换膜燃料电池（PEMFC）水气传输的核心部件，对PEMFC的性能具有重要影响。将碳纤维纸（CP）浸渍聚四氟乙烯（?PTFE）乳液后，通过模具夹持进行干燥，得到了亲疏水交替的CP，并通过微孔层（MPL）涂敷工艺制备了基底层亲疏水交替的GDL，以期提高PEMFC性能。通过SEM、EDS、接触角、垂直平面（TP）电阻率、垂直平面（TP）透气率和电化学性能等测试对基底层亲疏水交替GDL的结构与性能进行了分析。结果表明：浸渍PTFE乳液的CP经模具夹持干燥后，形成条纹状亲水区和疏水区。将基底层亲疏水交替的GDL组装成单电池，在2 A/cm2电流密度（简称电密）下的电压为0.47 V，功率密度为948 mW/cm2；相同条件下，采用基底层无差别疏水处理的GDL组装成单电池，其电压为0.44 V，功率密度为884 mW/cm2，与基底层无差别疏水处理的GDL相比，电压及功率密度分别提高了6.82%和7.24%。     

Polymer electrolyte fuel cells employing electrodes with gas-diffusion layers of mesoporous carbon derived from a sol-gel route

Sol-gel derived mesoporous carbon (MC) for the preparation of gas-diffusion layer (GDL) and its ameliorating effect on the performance of polymer electrolyte fuel cells (PEFCs) is reported. MC with a specific surface area of 370 m²/g, pore diameter of 6.7 nm and pore volume of 0.45 cm³/g has been synthesized by co-assembly of a tri-block copolymer, namely pluronic-F127, as a structure directing agent, and a mixture of phloroglucinol and formaldehyde as carbon precursor. X-ray diffraction and transmission electron microscopy have been employed to examine the structural properties of the MC. Surface morphology of the GDL comprising MC has been studied by scanning electron microscopy. A peak power density of 0.53 W/cm² at a load current-density of 1.1 A/cm² is achieved for the PEFC employing electrodes with GDL of MC compared to the peak power density of 0.47 W/cm² at a load current-density of 0.93 A/cm² for the PEFC employing electrodes with GDL of commercial Vulcan XC-72R carbon, while operating at 70 ^oC with H₂ and air feeds at atmospheric pressure.     

Characterization of gas diffusion layers for PEMFC

A carbon-filled gas diffusion layer (CFGDL), which is in the configuration similar to conventional carbon cloth gas diffusion layer (GDL) coated with carbon layer on both faces, was investigated and compared with conventional carbon paper-based single-layer and dual-layer GDLs. Like the carbon cloth GDL, CFGDL has presented superior performances over the single-layer or dual-layer GDL in all three polarization (activation, ohmic and concentration) controlled regions under electrochemical characterizations (steady-state polarization and electrochemical impedance spectra). The results from SEM showed that CFGDL has the same thickness of 0.11 mm as that of single-layer GDL, while dual-layer GDL has a thickness of 0.18 mm. The fully filled carbon paper with carbon/PTFE filler, as seen in the SEM image, displayed good support for the catalyst layer and electrolyte phase, allowing good electrical contact between the GDL/catalyst/membrane and GDL/flow field plate to be achieved. From porosimetry analysis, CFGDL presented a lower porosity of 67% and a much smaller average pore diameter of 4.7 μm compared to the single-layer GDL (porosity of 77% and pore diameter of 35.8 μm) and dual-layer GDL (porosity of 73% and pore diameter of 25.5 μm); however, it also gave the largest limiting current density, which reflects the improvement in mass transportation. This phenomenon is likely attributed to the fast removal of micro-water droplets formed in the CFGDL structure of the electrode.     

Effect of fabrication methods of bifunctional catalyst layers on unitized regenerative fuel cell performance

In order to understand the origins of performance variations in unitized regenerative fuel cells (URFCs), bifunctional catalyst layers (BCLs) fabricated with two different methods, i.e., ink deposition on membrane or GDL, were designed in this paper. The performances of the two different methods were evaluated, and their reaction dynamics were measured by electrochemical impedance spectra. The different BCLs, caused by the different preparation processes, were found to influence the fuel cell performance. The cell potentials of the URFCs using platinum sprayed onto the gas diffusion layer (GDL) are above 0.100 V higher than those with platinum sprayed onto the membrane at 800 mA cm⁻² in fuel cell (FC) mode. The mass transport resistances of the URFCs at different operation modes were also compared. It was proved that the platinum layer formed by applying platinum onto the GDL could prevent the cell from water flooding in FC mode. However, it was found that the cell performance changed slightly in water electrolysis mode with different BCLs. The electron conduction path was also found to be hindered by an IrO₂ agglomerate, which led to a decrease in cell performance. The highest and lowest round-trip efficiencies of the URFC with different BCLs were 42.1% and 22.3%, respectively, at 800 mA cm⁻².     

PEMFC阴极扩散层结构特性对水淹影响的数值分析

建立质子交换膜燃料电池一维两相传递模型,通过达西定律和菲克定律的联立求解得到扩散层中的液体饱和度和氧气浓度分布。考察扩散层特性参数孔隙率、厚度、接触角、渗透率对阴极水淹的影响,结果表明扩散层表面憎水将有助于液态水移出,但当达到憎水条件后,增大接触角对液态水传输和氧气传质的影响逐渐变小。憎水条件下孔隙率和厚度对液态水传输的影响不是很明显,但孔隙率增大和扩散层厚度减小均有利于氧气传质,实际应用中孔隙率增大的同时,厚度也要适当增大,极限电流密度相差不大。模型计算结果与文献中不同PTFE含量条件下实验的Tafel斜率和极限电流密度比较,吻合较好。     

分体式质子交换膜燃料电堆的膜加湿实验

在已开发的分体式质子交换膜燃料电池电堆基础上,对膜（Nafion115）加湿器子系统进行了详细的研究,揭示了质子交换膜燃料电池膜加湿方法的特性.初步分析了膜加湿的原理,并在不同操作条件下,定量地对膜透过水量与反应气体的润湿程度进行了测量,得到了Nafion115膜加湿器对电堆的润湿性能.发现操作温度在50～70℃之间,加湿速率最强;随着反应气体流量的增大,加湿速率呈非线性增加,但润湿程度反而降低;增加质子交换膜面积会增大反应气体的相对湿度,而加湿速率将下降;较高反应气体压力下不利于加湿.在适当的膜加湿器工况下,当电流密度为2.1A•cm^-2时,电堆最大功率密度可超过1.2W•cm^-2.     

Oxygen mass transport limitations at the cathode of polymer electrolyte membrane fuel cells

Oxygen transport across the cathode gas diffusion layer (GDL) in polymer electrolyte membrane (PEM) fuel cells was examined by varying the O₂/N₂ ratio and by varying the area of the GDL extending laterally from the gas flow channel under the bipolar plate (under the land). As the cathode is depleted of oxygen, the current density becomes limited by oxygen transport across the GDL. Oxygen depletion from O₂/N₂ mixtures limits catalyst utilization, especially under the land.The local current density with air fed PEM fuel cells falls to practically zero at lateral distances under the land more than 3 times the GDL thickness; on the other hand, catalyst utilization was not limited when the fuel cell cathode was fed with 100% oxygen. The ratio of GDL thickness to the extent of the land is thus critical to the effective utilization of the catalyst in an air fed PEM fuel cell. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Micro-porous layer with composite carbon black for PEM fuel cells

Effects of carbon black in micro-porous layer (MPL) on the performance of H₂/air proton exchange membrane fuel cells (PEMFCs) were studied and characterized extensively. Physical properties of gas diffusion layers (GDLs) involving surface morphology, gas permeability, hydrophilic/hydrophobic porosity and electron conductivity were examined. To construct an effective bi-functional pore structure, a novel MPL using composite carbon black consisting of Acetylene Black and Black Pearls 2000 carbon was presented for the first time. An optimal cell performance with the maximum power density of 0.91 W cm⁻² was obtained by the MPL containing 10 wt.% Black Pearls 2000 in composite carbon black.