Influence of hydrophilicity in micro-porous layer for polymer electrolyte membrane fuel cells

Water management is one of the most important factors for improving the performance in polymer electrolyte membrane fuel cells (PEMFCs). The micro-porous layers (MPLs) in the membrane-electrode assembly provide proper pores and paths for mass transport, thereby allowing for the control of the water balance. In this study, a copolymer containing hydrophilic functional groups is introduced into the binder materials of the MPL instead of a highly hydrophobic binder. When 10 wt.% of the binder is incorporated in the MPL on the cathode side, the best performance is exhibited and the ohmic resistance is decreased. Although the charge transfer resistance at low potential is higher than that of the hydrophobic treated MPL, due to the flooding effects, the charge transfer resistance at high potential becomes smaller. This indicates that excess liquid absorption from the catalyst layer to the hydrophilic MPL occurs more strongly than in the case of the hydrophobic MPL. This may bring about an increase in the accessibility of oxygen to the active sites, because the excess liquid near the catalyst agglomerates is expelled as fast as possible. Consequently, the hydrophilicity control in the MPL has a positive effect on the water management in PEMFCs.     

Influence of the Teflon loading in the gas diffusion layer of PBI-based PEM fuel cells

The influence of the PTFE content in commercial Toray graphite paper gas diffusion layer (GDL) on the performance of a PBI-based polymer electrolyte membrane fuel cell (PEMFC) has been studied. These materials have been characterised by evaluating the porosity, pore size distribution, SEM micrographs, hydrophobicity, air permeability and electrical resistance. Fuel cell results show that the lower the Teflon content, the better the cell performance and the lower the losses when oxygen was replaced by air. These results led to non-Teflonized carbon paper to be postulated as the most suitable candidate, provided that its mechanical integrity can be maintained throughout the whole process of preparation and testing of the MEA. However, some practical problems with this type of commercial non-Teflonized carbon paper were experienced in this work and led to damage of the support. The detrimental effects are described and discussed. As conclusion, the use of a minimally PTFE-loaded (10%) carbon paper is suggested because the inclusion of this level of Teflon improved properly the mechanical properties of the carbon support and only caused a very small drop in the performance.     

Gas diffusion layer with titanium carbide for a unitized regenerative fuel cell

In this work, a gas diffusion layer (GDL) prepared with metallic ceramics TiC for a unitized regenerative fuel cell (URFC) was first investigated. By the measurements of morphological characteristic, electrical conductivity, absolute through-plane permeability and electrochemical stability, the characteristics of the novel GDLs and the conventional one were compared. A high corrosion-resistive and low-cost GDL with 80 wt.% TiC and 20 wt.% IrTiO_x was expected to enhance the cycle performance of URFC. And the total loading of Ir in the novel URFC was only 1.3 mg cm⁻². The URFC with the novel GDL exhibited the similar initial performance under both fuel cell and electrolysis modes as that using the conventional GDL. However, the life cycle testing over 60 h showed that the URFC with the novel GDL was more stable than the URFC with the traditional GDL, indicating that the GDL with TiC and IrTiO_x was beneficial to improve the cycle life of the URFC.     

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.     

Gas diffusion electrodes for polymer electrolyte fuel cells using novel organic/inorganic hybrid electrolytes: effect of carbon black addition in the catalyst layer

We have prepared novel gas diffusion electrodes for polymer electrolyte fuel cells (PEFC) using new organic/inorganic hybrid electrolytes. The catalyst layers were prepared by mixing 3-(trihydroxysilyl)-1-propanesulfonic acid [(THS)Pro-SO₃H], 1,8-bis(triethoxysilyl) octane (TES-Oct), Pt loaded carbon black (Pt-CB) and water, followed by a sol-gel reaction. It was found that addition of uncatalyzed carbon black (u-CB) into the cathode catalyst layer enhanced the performance at high current density region, due to an increase in the gas diffusion rate. The optimum volume ratio of u-CB/Pt-CB was found to be 0.1, at which the gas diffusivity and the catalyst utilization are well balanced.     

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.     

Effect of a GDL based on carbon paper or carbon cloth on PEM fuel cell performance

A commercially available GDL based on carbon paper or carbon cloth as a macroporous substrate was characterized by various physical and electrochemical measurements: mercury porosimetry, surface morphology analysis, contact angle measurement, water permeation measurement, polarization techniques, and ac-impedance spectroscopy. SGL 10BB based on carbon paper demonstrated dual pore size distribution and high water flow resistance owing to less permeable macroporous substrate, and more hydrophobic and compact microporous layer, as compared to ELAT-LT-1400 W based on carbon cloth. The membrane-electrode-assembly fabricated using SGL 10BB showed an improved fuel cell performance when air was used as an oxidant. The ac-impedance response indicated that a microporous layer which has high volume of micropores and more hydrophobic property allows oxygen to readily diffuse towards the catalyst layer due to effective water removal from the catalyst layer to the gas flow channel.     

Development of gas diffusion layer using water based carbon slurry for proton exchange membrane fuel cells

The micro-porous layer of gas diffusion layers (GDLs) was fabricated with the carbon slurry dispersed in water containing sodium dodecyl sulfate (SDS), by wire rod coating process. The aqueous carbon slurry with micelle-encapsulation was highly consistent and stable without losing any homogeneity even after adding polytetrafluoroethylene (PTFE) binder for hundreds of hours. The surface morphology, contact angle and pore size distribution of the GDLs were examined using SEM, Goniometer and Hg Porosimeter, respectively. GDLs fabricated with various SDS concentrations were assembled into MEAs and evaluated in a single cell PEMFC under diverse operating relative humidity (RH) conditions using H₂/O₂ and H₂/air as reactants. The peak power density of the single cell using the GDLs with optimum SDS concentration was 1400 and 500 mW cm⁻² with H₂/O₂ and H₂/air at 90% RH, respectively. GDLs were also fabricated with isopropyl alcohol (IPA) based carbon slurry for fuel cell performance comparison. It was found that the composition of the carbon slurry, specifically SDS concentration played a critical role in controlling the pore diameter as well as the corresponding pore volumes of the GDLs.     

Effect of cathode GDL characteristics on mass transport in PEM fuel cells

The effect of the content of the hydrophobic agent in the cathode gas diffusion layer (GDL) on the mass transport in the proton exchange membrane fuel cells (PEMFCs) was studied using mercury porosimetry, scanning electron microscopy, and electrochemical polarization techniques. The mercury intrusion data and SEM micrograph indicated that the hydrophobic agent alters the surface and bulk structure of the GDL, thereby controlling gas-phase void volume and liquid water transport. The electrochemical polarization curves were measured and quantitatively analyzed to determine the oxygen transport limitation both in the catalyst layer and the GDL. Evaluation of the parameter ζ, which represents the cathode GDL characteristics for liquid water transport, indicated that the optimized content of the hydrophobic agent and effective water management results from a trade-off between the hydrophobicity and the absolute permeability for faster water drainage.     

Modification and durability of carbon paper gas diffusion layer in proton exchange membrane fuel cell

To solve the durability problem caused by carbon corrosion in the porous carbon paper gas diffusion layer (GDL) of proton exchange membrane fuel cell (PEMFC), a Cr₇C₃ ceramic coating on porous carbon paper by molten salt method was prepared. The obtained Cr₇C₃ coating is dense and shows good adherent with the carbon paper. Based on the corrosion resistance testes, it is concluded that the Cr₇C₃ coated carbon paper can keep high chemical stability in the long-time acid immersion accelerated corrosion tests and simulated cathodic proton exchange membrane fuel cells environment. The carbon paper with Cr₇C₃ coating shows a small current density with a value of about 1.1 × 10^?5 A cm^?2 in the potentiostatic polarization test under 1 M H₂SO₄ + 2 ppm HF at 70 °C and1.4 V_SCE for 72 h. Meanwhile, the resistivity of carbon paper with Cr₇C₃ coating prepared is 30% lower than that of pure carbon paper, and the conductivity is 1.5 times that of pure carbon paper.     

质子交换膜燃料电池微孔层浆液微观结构与流变性

微孔层（MPL）是质子交换膜燃料电池（PEMFC）水管理的重要部件。本文系统性选择了4种典型炭黑[ACET、XC-72R、BP2000、天然气裂解炭黑（NG）],借助原子力显微镜（AFM）、场发射透射电镜（TEM）和动态光散射粒度分析仪（DLS PSD）对浆液微观结构的研究,给出团聚性这一浆液评价标准,并结合流变性揭示了定量标准。研究结果表明,不同炭黑的团聚程度大小依次为ACET>XC-72R>BP2000>NG。剪切稀化指数n越大,MPL浆液团聚程度越低,颗粒分散越均匀;n越小,团聚程度越高。具体而言,固含量α=6%时,添加NG、BP2000、XC-72R、ACET炭黑的MPL浆液的剪切稀化指数n分别为0.554、0.320、0.118、0.039;测得的ACET、XC-72R、BP2000、NG的团聚体粒径分别为1.358μm、1.149μm、0.732μm、0.406μm,所以浆液的剪切稀化指数与其团聚程度成反比。总之,NG-MPL剪切稀化程度最低,NG更加适合作为MPL的碳材料,也为今后MPL的制备和优化提供指导。     

质子交换膜燃料电池用催化剂及其电极的研究进展

膜电极(MEA)是质子交换膜燃料电池(PEMFC)的核心技术。膜电极包含的催化剂层、材料和结构等对PEMFC的性能影响很大。催化剂面层上供三相(质子、电子、气体)用的通道对于电池使用时的催化作用是必不可少的。介绍了近几年催化剂的研究进展,看重对三相通道进行了详细叙述。也回顾了一些用于改善催化剂活性的其他方法,如阴极催化、合金催化剂,根据这些进展,对今后的研究方向提出了建议。     

质子交换膜燃料电池用高性能碳纤维纸的研究简报

本文介绍了质子交换膜燃料电池（PEMFC）的工作原理，并通过分析气体扩散层材料的性能要求，设计了一条制备高性能碳纤维纸的技术路线。对制得的碳纸样品进行了显微结构和基本性能的表征，同时还将它应用于PEMFC的单电池中进行综合性能的测试，测试结果表明，按照这种技术路线制得的碳纤维纸在性能上基本能满足PEMFC的使用要求。     

Platinum electroless deposition on nafion membrane for PEM fuel cells

The aim of this research was to study the deposition of platinum on Nafion117 membrane by using an electroless technique. The investigated parameters were electroless time (30 to 90 min) and Pt : N₂H₄ ratio (from 1 : 1 to 1 : 3.85). The results indicated that the platinum deposition on the 1st side of membrane was lower than that on the 2nd side because of the effect of N₂H₄ on the property of membrane. Platinum deposition increased when the Pt : N₂H₄ ratio decreased. The optimum condition was found at electroless time of 60 min and Pt : N₂H₄ ratio of 1 : 1.95. At this condition, more than 89% of platinum was deposited on both surfaces. The coated membrane was used to make a membrane electrode assembly (MEA) of a proton exchange membrane fuel cell (PEMFC) which was tested at 60 ‡C in a saturated H₂/O₂ system. The maximum current density was 80 mA/cm² at 0.3 V.     

Investigation of mass transport in gas diffusion layer at the air cathode of a PEMFC

In a polymer electrolyte membrane fuel cell (PEMFC), slow diffusion in the gas diffusion electrode may induce oxygen depletion when using air at the cathode. This work focuses on the behavior of a single PEMFC built with a Nafion^® based MEA and an E-TEK gas diffusion layer and fed at the cathode with nitrogen containing 5, 10 and 20% of oxygen and working at different cell temperatures and relative humidities. The purpose is to apply the experimental impedance technique to cells wherein transport limitations at the cathode are significant. In parallel, a model is proposed to interpret the polarization curves and the impedance diagrams of a single PEMFC. The model accounts for mass transport through the gas diffusion electrode. It allows us to qualitatively analyze the experimental polarization curves and the corresponding impedance spectra and highlights the intra-electrode processes and the influence of the gas diffusion layer.     

Electrical conductivity of carbon black/single-wall carbon nanotube/low-density polyethylene ternary composite foam

In order to obtain high electrical conductive low-density polyethylene (LDPE) foam, carbon black (CB), single-wall carbon nanotube (SWCNT), and LDPE (CB/SWCNT/LDPE) ternary composite foams were successfully fabricated by chemical compression molding method. The electrical conductivity, mechanical properties, microstructure, density, and crystallinity of the foam were studied in detail. It can be found that CB and SWCNT have synergistic effect. For the CB/SWCNT/LDPE composite foam which containing 19 wt % CB and 0.05 wt % SWCNT, its density is only 0.082 g cm⁻¹ and the electrical conductivity can reach at 2.88 × 10⁻⁵ S cm⁻³, which is far more than 15 orders of magnitudes of pure polyethylene and 4 orders of magnitudes times higher than sample which CB content is 19 wt %. It is noteworthy that ultralow concentration of SWCNT could drastically improve the electrical conductivity and reduce the density of LDPE foams. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48382.     

Sub-freezing endurance of PEM fuel cells with different catalyst-coated membranes

The sub-freezing endurance of proton exchange membrane (PEM) fuel cells with hydrophobic and hydrophilic catalyst-coated membranes (CCMs) was investigated. The polarization curves, electrochemical characteristics and physical structures of the CCMs were measured. The cells were frozen at −20 °C with saturated residual water after operating at 60 °C. After eight freeze/thaw cycles, no evident negative effect on the performance of the cell with a hydrophobic CCM was observed, while the cell with a hydrophilic CCM degraded severely. By analyzing the polarization curves, it was concluded that the mass transport limitation was the main reason for the performance loss of the hydrophilic cell. The electrochemical active surface area (ECA) results suggest that the hydrophobicity of the catalyst layer (CL) has an apparent impact on the residual water distribution of the membrane electrode assembly (MEA). A larger water content in the hydrophilic CL has a negative effect on the subzero endurance. From the polarization resistance obtained from electrochemical impedance spectroscopy (EIS) the origin of degradation was further clarified. Mercury intrusion porosimetry showed that the pore size of the hydrophilic catalyst layer changed significantly after freezing; the mean pore size increased from 5.68 to 6.71 nm. However, with a water removal method, namely, gas purging, it was possible to prevent degradation effectively.     

Numerical optimization of bipolar plates and gas diffusion layers for PEM fuel cells

This paper is devoted to the numerical optimization of the dimensions of channels and current transfer ribs of bipolar plates as well as the thickness and porosity of gas diffusion layers. A mathematical model of the transfer processes in a PEM fuel cell has been developed for this purpose. The results are compared with experimental data. Recommendations of the values of operating parameters and some design requirements to increase PEM fuel cell efficiency are suggested.This paper was originally Presented at the CHISA Congress, Prague, August 2004.An erratum to this article can be found at     

多孔纳米碳纤维作为质子交换膜燃料电池微孔层的性能

质子交换膜燃料电池膜电极中的微孔层结构对改善体系的水管理能力,提升膜电极的整体性能发挥重要作用。本文通过静电纺丝和后续热处理的方法制备了多孔纳米碳纤维（PCNF）,并以此构建膜电极的微孔层。与炭黑颗粒作为微孔层呈现出紧密堆积结构不同,由PCNF搭建的微孔层结构疏松呈现三维贯通状。膜电极的发电测试表明,以多孔纳米碳纤维作为微孔层（MPL-PCNF）的膜电极其最大功率密度达70.0mW/cm²,远高于炭黑颗粒为微孔层（MPL-CB）的膜电极（58.1mW/cm²）,而没有微孔层（Ref）结构的膜电极最大功率密度仅为27.7mW/cm²,显示出PCNF作为微孔层材料的明显优势。     

Studies of oxygen reduction reaction active sites and stability of nitrogen-modified carbon composite catalysts for PEM fuel cells

A non-precious nitrogen-modified carbon composite (NMCC) catalyst is synthesized by the pyrolysis of cobalt, iron-ethylenediamine-chelate complexes on silica followed by chemical and pyrolysis treatments. Pyrolysis temperature and time have a remarkable impact on the content and the type of the nitrogen-containing functional groups in the NMCC catalysts, which affect their catalytic activity and stability. Based on the analysis of the nitrogen functional groups before and after the stability tests, the ORR active sites of the NMCC catalysts are proposed to be pyridinic-N and quaternary-N functional groups. However the pyridinic-N group is not stable in the acidic environment due to the protonation reaction.