A novel integrated gas diffusion layer based on one-dimensional carbon materials and its application to direct methanol fuel cells
摘要: 气体扩散层（GDL）是燃料电池（FCs）膜电极组件（MEA）的重要组成部分，起到支撑催化层、收集电流、对物料进行传输及再分配的作用。传统的扩散层一般以商品的炭纸或炭布为基底，但炭纸价格昂贵、缺乏柔韧性、孔结构单一，为了更好的进行气液相管理，需再涂覆上一层微孔层（MPL）。本文以碳纤维（CF）为骨架，复合多壁碳纳米管（MWCNT），并添加一定量的聚四氟乙烯（PTFE）作为粘结剂和憎水剂，通过减压抽滤成型法一步制成新型一体式气体扩散层（GDL/CNT-CF）。扫描电子显微镜(SEM)结合导电性、气体渗透性及孔隙率等表征表明，一体式扩散层中高导电性的碳纳米管在碳纤维中呈梯度分布，利于电子传输，形成的多级孔隙结构，利于物料分配，均匀分布的PTFE有助于水分排除，从而取代了由炭纸和微孔层构成的传统扩散层。将其应用于直接甲醇燃料电池（DMFC）的阴极或同时作为阴阳极扩散层时，具有优异的传质性能，单电池的最大功率密度相比商品扩散层分别提高了20 % 和35 %。Abstract: The gas diffusion layer (GDL) is an important component of the membrane electrode assembly (MEA) of fuel cells (FCs), which plays a key role in supporting the catalyst layer, collecting current, and transferring and redistributing materials. A conventional GDL consists of a backing layer (BL), typically made of commercial carbon paper or carbon cloth, but it still suffers from some challenges in terms of its high cost, narrow pore-size distribution, lack of flexibility, and poor conductivity. In that case, a micro-porous layer (MPL) is necessary for better gas/liquid management. In this study, a novel, flexible, integrated gas diffusion layer (GDL/CNT-CF) is successfully prepared through vacuum filtration by combining carbon fibres (CFs) with highly-dispersed multi-walled carbon nanotubes (MWCNTs) and polytetrafluoroethylene (PTFE) as a binder and water repellent. Scanning electron microscopy (SEM) combined with characterisation of conductivity, gas permeability, and porosity indicates that the highly-conductive MWCNTs in the GDL/CNT-CF are distributed in gradients in the CF networks, and can facilitate electron transport. Furthermore, the formed multi-level pore structure is beneficial to material distribution and the uniform distribution of PTFE contributes to improved water discharge. Therefore, it replaces the conventional diffusion layer consisting of carbon paper and MPL. When the GDL/CNT-CF is applied as the cathode, or both the cathode and anode, of a direct methanol fuel cell (DMFC), the maximum power density of the single cells is separately increased by 20% and 35% compared with that of a commercial GDL due to its excellent mass transfer performance.
Figure 2. The comparison of surface morphologies between GDL/CNT-CF and GDL/Toray-060H: (a) and (b) Frontal morphologies of GDL/CNT-CF; (c) and (d) Reverse and cross-sectional morphologies of GDL/CNT-CF; (e) and (f) Frontal morphologies of GDL/Toray-060H; (g) and (h) Reverse and cross-sectional morphologies of GDL/Toray-060H
Figure 4. GDL/CNT-CF with different ratios of MWCNTs to CFs applied to cathode of the DMFC. (a) Anodic polarisation curves of the single cells. The test conditions are shown as: temperature is 80 °C, and the anode is fed with methanol solution with concentration of 0.5 M at a flow rate of 1 mL min−1, while the cathode is fed with hydrogen at 100 mL min−1 under a gas pressure of 0.1 MPa. The scanning rate is 1 mV s−1 and scanning range is 0-0.6 V. (b) Cathodic polarisation curves of the single cells; (c) Alternating-current (AC) impedance spectra of the different single cells under constant-current discharge of 100 mA cm−2. The test conditions are presented as follows: temperature is 80 °C, and the anode is fed with methanol solution with concentration of 0.5 M at a flow rate of 1 mL min−1, while the cathode is fed with oxygen at 100 mL min−1 under a gas pressure of 0.1 MPa.
Figure 5. Test results of performance of GDL/CNT-CF(3∶1) as the cathode GDL of DMFC under different conditions. (a) The temperatures are different, and the anode is fed with methanol with a concentration of 0.5 M at a flow rate of 1 mL min−1. The cathode is fed with oxygen at a flow rate of 100 mL min−1 under a gas pressure of 0.1 MPa; (b) The gas flow rate is different and temperature is 80 °C. The anode is fed with methanol with a concentration of 0.5 M at a flow rate of 1 mL min−1; (c) Methanol concentrations are different and the gas flow rate is 100 mL min−1. The temperature is 80 °C and the cathode is fed with oxygen at a flow rate of 100 mL min−1 under a pressure of 0.1 MPa
Figure 6. Performance comparison of single cells assembled with GDL/CNT-CF(3∶1) and GDL/Toray-060H as cathode GDLs of the DMFC. The test conditions are as follows: the temperature is 90 °C, and the anode is fed with methanol with a concentration of 1 M at a flow rate of 1 mL min−1, while the cathode is fed with oxygen at 200 mL min−1. (a) AC impendence spectra of the single cells under constant-current discharge of 100 mA cm−2; (b) Polarisation curves.
Figure 7. Test results of performance in air and oxygen environment when GDL/CNT-CF (3∶1) is applied to the cathode and anode of the DMFC. (a) Test results in the oxygen environment under the conditions: the temperature is 80 °C and the anode is fed with methanol with a concentration of 0.5 M at a flow rate of 1 mL min−1, while gas flow rate at the cathode is 100 mL min−1; (b) Oxygen-gain curves; (c) Anodic polarisation curves obtained when the temperature is 80 °C, and the anode is fed with methanol solution with a concentration of 0.5 M at a flow rate of 1 mL min−1, while the cathode is fed with hydrogen at 100 mL min−1 under a gas pressure of 0.1 MPa; (d) Curves of methanol permeation at the cathode tested under following conditions: the temperature is 80 °C and the anode is fed with methanol solution with a concentration of 0.5 M at a flow rate of 1 mL min−1, while the cathode is fed with nitrogen at 100 mL min−1 under a gas pressure of 0.1 MPa. The scanning rate is 1 mV s−1 and the scanning range is 0 to 0.6 V.
Table 1. Structural parameters of the GDLs.
GDL GDL/CNT-CF GDL/Toray-060H Thickness (mm) 0.1－0.12 0.23－0.25 ASR (mΩ cm2) 7 9 Porosity (%) 75 60 Density (g cm−3) 0.36 0.45 Maximum bending (o) 180 < 90 Hydrophilicity and hydrophobicity/contact angle (o) 145 135
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