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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，为了更好地进行气液相管理，需再涂覆上一层微孔层（MPL）。本文以炭纤维（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炭纤维中呈梯度分布，亦利于电子传输，形成的多级孔隙结构，亦利于物料分配，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 of fuel cells (FCs). Its roles include supporting the catalyst layer, collecting current, and transferring and redistributing materials. A conventional GDL consists of a backing layer, typically of commercial carbon paper or carbon cloth, but it suffers from its high cost, narrow pore-size distribution, lack of flexibility and poor conductivity, and a micro-porous layer (MPL) is necessary for better gas/liquid management. A novel flexible gas diffusion layer (GDL) was prepared by vacuum filtration of a suspension of carbon fibers (CFs) and highly-dispersed multi-wall carbon nanotubes (MWCNTs) in a polytetrafluoroethylene (PTFE) binder and water repellent. SEM observations, gas permeability and porosity tests indicate that there is a gradient in the concentration of highly-conductive MWCNTs in the CNT-CF GDL network that facilitates electron transport. A multi-level pore structure is formed, which is beneficial to mass transport. The PTFE is distributed uniformly, which is favorable for the discharge of condensed water from the FCs. When the GDL/CNT-CF is used in the cathode, or in both the cathode and anode in direct methanol FCs, the maximum power densities of single cells are increased by 20% and 35%, respectively, compared with those using a commercial GDL consisting of carbon paper with a MPL due to its excellent mass transfer performance.
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Figure 1. Schematic diagram of the structure design of (a) GDL/CNT-CF and (b) optical photography.
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 3. The measured gas permeability of GDL/CNT-CF with different ratios of MWCNTs to CFs: (a) Linear relationship between gas flow rate and pressure difference on both sides of the samples and (b) comparison of calculated gas diffusion coefficients of the specific GDLs.
Figure 4. GDL/CNT-CF with different ratios of MWCNTs to CFs applied to the cathodes 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 a concentration of 0.5 mol L−1 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 a constant discharge current density of 100 mA cm−2. The test conditions are presented as follows: temperature is 80 °C, and the anode is fed with a methanol solution with a concentration of 0.5 mol L−1 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 mol L−1 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 are different and temperature is 80 °C. The anode is fed with methanol with a concentration of 0.5 mol L−1 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 mol L−1 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 a constant discharge current density of 100 mA cm−2 and (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 mol L−1 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 mol L−1 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 and (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 mol L−1 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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