Membrane Electrode Assembly for Electrocatalytic CO2 Reduction: Principle and Application

Electrocatalytic CO₂ reduction reaction (CO₂RR) in membrane electrode assembly (MEA) systems is a promising technology. Gaseous CO₂ can be directly transported to the cathode catalyst layer, leading to enhanced reaction rate. Meanwhile, there is no liquid electrolyte between the cathode and the anode, which can help to improve the energy efficiency of the whole system. The remarkable progress achieved recently points out the way to realize industrially relevant performance. In this review, we focus on the principles in MEA for CO₂RR, focusing on gas diffusion electrodes and ion exchange membranes. Furthermore, anode processes beyond the oxidation of water are considered. Besides, the voltage distribution is scrutinized to identify the specific losses related to the individual components. We also summarize the progress on the generation of different reduced products together with the corresponding catalysts. Finally, the challenges and opportunities are highlighted for future research.     

低铂膜电极结构优化途径

膜电极是质子交换膜燃料电池的核心组件,长期以来,在衣院士的指导下,我国高度重视膜电极技术的开发. 目前,燃料电池的研发和产业化进入了一个新的时代,对膜电极提出来更高的要求,特别是在降低铂载量方面,提出了0.125 mg·W^-1的挑战性指标. 本文从活化极化、欧姆极化和传质极化三个方面分析了低铂载量情况下电池性能下降的原因,提出应重点关注催化剂在燃料电池工作区间（0.6 V ~ 0.8 V)的催化活性,并讨论了用电荷传输阻抗作为催化剂活性指示符的合理性. 从优化潜力来说,传质极化优化>活化极化优化>欧姆极化优化. 催化层结构优化是实现低铂目标的关键,重点是解决离子聚合物（ionomer）传递质子和阻碍气体的矛盾.     

MEA with double-layered catalyst cathode to mitigate methanol crossover in DMFC

Methanol permeation is one of the key problems for direct methanol fuel cell (DMFC) applications. It is necessary to change the structure of the cathode of membrane electrode assembly (MEA). Therefore, a novel MEA with double-layered catalyst cathode was prepared in this paper. The double-layered catalyst consists of PtRu black as inner catalyst layer and Pt black as outer catalyst layer. The inner catalyst layer is prepared for oxidation of the methanol permeated from anode. The results indicate that this double-layered catalyst reduced the effects of methanol crossover and assimilated mixed potential losses. The performance of MEA with double-layered catalyst cathode was 52.2 mW cm⁻², which was a remarkable improvement compared with the performance of MEA with traditional cathode. The key factor responsible for the improved performance is the optimization of the electrode structure.     

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.     

Polarization curve of partially degraded catalyst layer

A model of a partially degraded catalyst layer (CL) in a fuel cell is reported. The degradation is modeled as a lowering of the exchange current density in a sub-layer at the membrane/CL interface. Degradation of 30 to 40% of the CL thickness may dramatically increase the polarization voltage. At a large current density, the voltage loss in such a two-layer system is close to the loss in a fully degraded CL. Estimates of this effect for a PEMFC cathode and a DMFC anode are performed.     

Operando 3D Visualization of Migration and Degradation of a Platinum Cathode Catalyst in a Polymer Electrolyte Fuel Cell

The three‐dimensional (3D) distribution and oxidation state of a Pt cathode catalyst in a practical membrane electrode assembly (MEA) were visualized in a practical polymer electrolyte fuel cell (PEFC) under fuel‐cell operating conditions. Operando 3D computed‐tomography imaging with X‐ray absorption near edge structure (XANES) spectroscopy (CT‐XANES) clearly revealed the heterogeneous migration and degradation of Pt cathode catalyst in an MEA during accelerated degradation test (ADT) of PEFC. The degradative Pt migration proceeded over the entire cathode catalyst layer and spread to MEA depth direction into the Nafion membrane.     

自呼吸直接甲醇燃料电池膜电极的优化(英文)

针对空气自呼吸式直接甲醇燃料电池甲醇易渗透和阴极易水淹的特点,通过对催化层催化剂载量、阴极微孔层、阳极微孔层和膜等因素进行调控,对膜电极结构和性能的进行了优化.结果表明,使用高载量催化剂能有效降低甲醇渗透,但载量过高会引起传质阻力.当阳极微孔层PTFE含量为30%(bymass)时,可以有效促进CO2的均一析出,从而降低甲醇浓度梯度,减小甲醇透过.综合考虑甲醇渗透和阴极自返水,经优化后所得MEA在室温时自呼吸工作条件下,比功率密度达到33mW·cm-2,最优甲醇工作浓度为4mol·L-1.     

A novel MEA architecture for improving the performance of a DMFC

Direct methanol fuel cell (DMFC) consisting of a double-catalytic layered membrane electrode assembly (MEA) provide higher performance than that with the traditional MEA. This novel structured MEA includes a hydrophilic inner catalyst layer and a traditional electrode with an outer catalyst layer, which was made using both catalyst coated membrane (CCM) and gas diffusion electrode (GDE) methods. The inner catalyst was PtRu black on anode and Pt black on cathode. The outer catalyst was carbon supported Pt–Ru/Pt on anode and cathode, respectively. Thus in the double-catalytic layered electrodes three gradients were formed: catalyst concentration gradient, hydrophilicity gradient and porosity gradient, resulting in good mass transfer, proton and electron conducting and low methanol crossover. The peak density of DMFC with such MEA was 19 mW cm⁻², operated at 2 M CH₃OH, 2 atm oxygen at room temperature, which was much higher than DMFC with traditional MEA.     

碱性聚合物电解质膜的表面锥形阵列结构提升燃料电池性能

燃料电池作为一种清洁高效的能量转换装置,被认为是构建未来社会可再生能源结构的关键一环。不同于质子交换膜燃料电池(PEMFC),碱性聚合物电解质燃料电池(APEFC)的出现使非贵金属催化剂的使用成为可能,因而受到了日益广泛的关注和研究。APEFC的关键结构是膜电极,主要由聚合物电解质膜和阴阳极(含催化层、气体扩散层)组成,膜电极是电化学反应发生的场所,其优劣直接决定着电池性能的好坏。因此,基于现有的碱性聚合物电解质及催化剂体系,如何构筑更加优化的膜电极结构,使APEFC发挥出更高的电池性能是亟待开展的研究。本文首先通过模板法在碱性聚合物电解质膜的表面构建出有序的锥形阵列,再将具有阵列结构的一侧作为阴极来构筑膜电极,同时,作为对比,制备了由无阵列结构的聚合物电解质膜构筑而成的膜电极,最后对基于两种不同膜电极的APEFC的电化学性能进行了对比研究。实验结果表明,锥形阵列结构可以将APEFC的峰值功率密度由1.04 W·cm-2显著提高到1.48 W·cm-2,这主要归因于在APEFC的阴极侧具有锥形阵列结构的聚合物电解质膜的亲水性的提升和催化剂电化学活性面积的增加。本工作为碱性聚合物电解质燃...     

DMFC寿命测试过程中膜电极的表面和结构研究(英文)

设计并组装单电池寿命测试系统,测试直接甲醇燃料电池(DMFC)的运行寿命,获得不同运行时间下单电池的极化和功率曲线.测试结束后,分别对运行过的膜电极(MEA)催化剂(铂黑和铂钌黑)和Nafion117(膜作XRD,HRTEM,FTIR及Raman等表征.考察在长期运行条件下电池寿命性能与膜电极中催化剂的颗粒大小、分布、形态、表面物种以及膜的结构之间的关系.寿命测试结果表明,单电池在不同运行阶段其性能变化也不同.运行前200 h,电池性能衰减较显著;运行200~704 h性能较稳定,运行1 002 h后电池性能恶化.波谱实验发现,单电池长期运行后,其膜电极的阴、阳极催化剂颗粒变大.电池寿命性能的衰退伴随膜电极微结构、表面组成、催化剂/膜界面结构的变化以及Nafion 117(膜的老化.     

Developing a membrane-electrode assembly with reduced Pt content for a hydrogen—air fuel cell based on a PtCoCr catalyst and F-950 membrane

Laboratory methods are developed for forming an active layer (AL) with a synthesized PtCoCr catalyst (20 wt % Pt) on the F-950 perfluorinated membrane. AL composition and the conditions for forming 3- and 5-layer membrane-electrode assemblies (MEA) are optimized. Reproducible, stable, and high-discharge characteristics are obtained for a hydrogen-air fuel cell (HAFC). At a current density of 0.5 A/cm², the voltage of an MEA with cathode based on a PtCoCr catalyst is 0.66–0.68 V, and the maximum power density is 500 mW/cm². Replacing the commercial HiSPEC 4000 catalyst (40 wt % Pt) with PtCoCr (20 wt % Pt) in the AL composition of the cathode makes it possible to reduce Pt consumption by a factor of two without decreasing MEA discharge characteristics. The parameters that characterize the catalytic activity of catalysts under model conditions and in the MEA cathode composition are shown to be correlated.     

硫化氢燃料电池的无机质子传导膜与MEA制备和性能

制备了硫化氢固体氧化物燃料电池的无机质子传导膜和膜-电极-组装(MEA)。用扫描电镜(SEM)和电化学阻抗(EIS)技术表征了无机质子传导膜和MEA的形貌与性能。研究了不同膜厚和掺杂或没有掺杂Li₂WO₄组分的传导膜和MEA的性能。结果表明，与没有掺杂Li₂WO₄组分制备的MEA相比，掺杂了Li₂WO₄组分制备的MEA的电导提高了一个数量级，掺杂了Li₂WO₄制备的MEA硫化氢燃料电池在操作条件下具有更好的化学稳定性和电化学性能。以Mo-Ni-S为主要成分的复合阳极、0.8 mm厚和组成为67wt% Li₂SO₄ + 8wt% Li₂WO₄ + 25wt% Al₂O₃复合材料制备的质子传导膜、NiO为主要组分的复合阴极构成的MEA硫化氢燃料电池，在650、700和750 ℃时，最大输出功率密度分别达到50、85和130 mW·cm^-2，最大电流密度分别为200、350和480 mA·cm^-2。     

PEMFC膜电极组件(MEA)制备方法的评述

膜电极组件(MEA)是质子交换膜燃料电池的核心部件.本文在简述MEA结构的基础上,根据MEA制备过程中催化层支撑体不同,将目前已有的多种MEA制备方法分为两类制备模式:以GDL为支撑体和以PEM为支撑体的制备模式.文中对这些制备方法的特点进行了详细评述,对MEA制备方法的发展趋势进行了展望,认为以PEM为支撑体的制备模式是今后MEA制备的主要发展方向.     

碳纤维基PtSn催化剂直接乙醇燃料电池制备及性能研究

采用自制的碳纤维基PtSn催化剂薄膜作为阳极催化剂,商用Pt/C作为阴极催化剂,Nafion 115膜作为质子交换膜,通过热压制成膜电极,组装平板型直接乙醇燃料单电池,搭建测试系统并进行性能的测试,研究了温度、乙醇浓度、溶液流量、进气流量等参数对DEFC的影响。结果表明,当乙醇溶液浓度为1.0 mol/L、溶液进样流量为1.0 mL/min、溶液温度为80 ℃、氧气进样流量为100 mL/min时结果较优,单电池的最高功率密度达18.2 mW/cm²。     

Platinum dissolution and deposition in the polymer electrolyte membrane of a PEM fuel cell as studied by potential cycling

The behavior of platinum dissolution and deposition in the polymer electrolyte membrane of a membrane-electrode-assembly (MEA) for a proton-exchange membrane fuel cell (PEMFC) was studied using potential cycling experiment and high-resolution transmission electron microscopy (HRTEM). The electrochemically active surface area decreased depending on the cycle number and the upper potential limit. Platinum deposition was observed in the polymer electrolyte membrane near a cathode catalyst layer. Platinum deposition was accelerated by the presence of hydrogen transported through the membrane from an anode compartment. Platinum was transported across the membrane and deposited on the anode layer in the absence of hydrogen in the anode compartment. This deposition was also affected by the presence of oxygen in the cathode compartment.     

Multi-Physics Modeling Assisted Design of Non-Coking Anode for Planar Solid Oxide Fuel Cell Fueled by Low Steam Methane

Internal reformation of low steam methane fuel is highly beneficial for improving the energy efficiency and reducing the system complexity and cost of solid oxide fuel cells (SOFCs). However, anode coking for the Ni-based anode should be prevented before the technology becomes a reality. A multi-physics fully coupled model is employed to simulate the operations of SOFCs fueled by low steam methane. The multi-physics model produces I-V relations that are in excellent agreement with the experimental results. The multi-physics model and the experimental non-coking current density deduced kinetic carbon activity criterion are used to examine the effect of operating parameters and the anode diffusion barrier layer on the propensity of carbon deposition. The interplays among the fuel utilization ratio, current generation, thickness of the barrier layer and the cell operating voltage are revealed. It is demonstrated that a barrier layer of 400 μm thickness is an optimal and safe anode design to achieve high power density and non-coking operations. The anode structure design can be very useful for the development of high efficiency and low cost SOFC technology.     

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     

降低直接甲醇燃料电池浓差极化的含磺化碳管阻水夹层的构建

采用直接接枝法, 将来自对氨基苯磺酸的苯磺酸官能团引入氧化多壁碳纳米管, 制得磺化多壁碳纳米管(SO₃-MCNT). 再以SO₃-MCNT为填料, 以Nafion离聚物为黏结剂, 利用超声喷涂在商业N212质子交换膜一侧构建了新的膜层, 获得了一种复合膜(SO₃-MCNT?N212). 使用SO₃-MCNT?N212制备燃料电池膜电极(MEA), 并用于直接甲醇燃料电池(DMFC)测试. 与使用普通N212膜的膜电极相比, 该膜电极的性能得到明显提升. 进一步分析表明, SO₃-MCNT膜层的引入降低了阳极向阴极的跨膜水迁移作用, 缓解了阴极的水淹, 从而降低了浓差极化, 提升了膜电极的性能.     

Degradation Investigation of Electrocatalyst in Proton Exchange Membrane Fuel Cell at a High Energy Efficiency

The development of high efficient stacks is critical for the wide spread application of proton exchange membrane fuel cells (PEMFCs) in transportation and stationary power plant. Currently, the favorable operation conditions of PEMFCs are with single cell voltage between 0.65 and 0.7 V, corresponding to energy efficiency lower than 57%. For the long term, PEMFCs need to be operated at higher voltage to increase the energy efficiency and thus promote the fuel economy for transportation and stationary applications. Herein, PEMFC single cell was investigated to demonstrate its capability to working with voltage and energy efficiency higher than 0.8 V and 65%, respectively. It was demonstrated that the PEMFC encountered a significant performance degradation after the 64 h operation. The cell voltage declined by more than 13% at the current density of 1000 mA cm⁻², due to the electrode de-activation. The high operation potential of the cathode leads to the corrosion of carbon support and then causes the detachment of Pt nanoparticles, resulting in significant Pt agglomeration. The catalytic surface area of cathode Pt is thus reduced for oxygen reduction and the cell performance decreased. Therefore, electrochemically stable Pt catalyst is highly desirable for efficient PEMFCs operated under cell voltage higher than 0.8 V.     

Recent advances in integrating platinum group metal-free catalysts in proton exchange membrane fuel cells

Platinum group metal (PGM)-free catalysts are promising low-cost materials for the oxygen reduction reaction in proton exchange membrane fuel cells (PEMFCs). A variety of chemical precursors and synthesis methods have been proposed to increase their catalytic activity. In comparison, significantly less attention has been dedicated to the integration of these PGM-free catalysts into operating electrodes by investigating the role of the membrane electrode assembly (MEA) fabrication on the PEMFC performance. We discuss here some remarkable performance improvements recently achieved by tuning catalyst loading, ionomer content, and ink solvent composition, and call for further explorations of the ink processing and MEA fabrication to improve performance.