微型直接甲酸燃料电池电源的设计、制备及其催化剂研究
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摘要
直接甲酸燃料电池（Direct Formic Acid Fuel Cells，DFAFCs）是一种直接利用甲酸溶液作为燃料的燃料电池。与直接醇类燃料电池（Direct Alcohol Fuel Cells，DAFCs）相比较，DFAFC具有膜渗透率低、更易氧化、可燃性低、毒性小、理论开路电压高等诸多优点，被认为是一类最有可能率先实现商业化的燃料电池。有关DFAFC的研究已成为燃料电池领域的热点研究课题。本文尝试使用印刷电路板（Printed Circuit Board，PCB）技术，设计和制作了一类结构简单、性能可靠的微型空气自呼吸DFAFC电源，系统地考察了这种微型DFAFC电源的性能。同时，还在DFAFC的高度稳定可靠的催化剂研究方面开展了一些探索性的研究工作。
首次采用印刷电路板（作为集电板与端板的统一体）设计和制作了微型空气自呼吸DFAFC，有效简化了电池的结构。考察了甲酸浓度、催化层Nafion载量、集电板类型、阳极催化剂种类等因素对电池放电性能的影响。当使用5 M甲酸为燃料、催化层Nafion载量为20%、使用小圆形集电板时电池性能最佳，在阴、阳极分别采用1 mg/cm~2 Pt/C和Pd/C催化剂时，电池最大功率密度为19.6 mW/cm~2。同时发现Pt/C对甲酸催化具有较好的耐久性能，供应3.5 ml 5 M甲酸，电池可连续恒定电流20 mA放电10 h，且维持电压在0.45 V。
在单电池研究基础上，分别设计了储液腔共通的双电池组和储液腔不共通的四电池组微电源系统。电池组中各单电池性能具有良好的一致性。双电池组使用5 M甲酸为燃料时，电池性能最佳，当在阴、阳极均采用2 mg/cm~2 Pt/C催化剂时，电池最大功率密度为44.5 mW/cm~2。供应3.5 ml 5 M甲酸，双电池组能够恒定电流20 mA工作近5 h，并且电压基本稳定在1.1 V，且重复4次几乎无衰减。四电池组中不连通的设计能有效地防止水解的发生。
通过扫描电镜（SEM）、交流阻抗分析（EIS）和测定阳极电势等方式初步考察了电池长时间放电性能衰减的原因，认为造成电池电压下降的原因包括：膜出现皱褶、电路板腐蚀、阳极电势增加、阳极催化剂活性下降等。
本文还采用有机溶胶法制备了不同粒径的Pd/C催化剂以及不同配比的PdPt合金类催化剂，并考查了其对甲酸催化性能。发现随着络合剂柠檬酸钠与金属前驱体PdCl2比例的增大，催化剂Pd/C颗粒度越小，对甲酸催化性能越好；Pd中添加少量Pt能显著改善Pd对甲酸的催化性能。与Pd/C相比，Pd20Pt/C催化剂的电化学活性比表面积增加，甲酸氧化峰电位负移0.1 V、峰电流密度增加67%。
Direct formic acid fuel cell (DFAFC) is a subcategory of proton exchange membrane fuel cell (PEMFC) where liquid formic acid is fed directly to the cell. Compared with direct alcohol fuel cells (DAFCs), DFAFC offers a broad range of advantages including lower fuel crossover through Nafion membrane, faster anodic electro-oxidition, lower ignitability, lower toxicity and higher theoretic open circuit voltage. DFAFC is therefore recognized as a kind of fuel cell which most likely commercializes in a near future. Nowdays, fundamental research dedicated to the practical application of DFAFC has became a hot topic in fuel cell community. In this thesis, we attempted to apply printed circuit board (PCB) technology in DFAFC, designed and fabricated a series of miniature air breathing DFAFC. Systematical evaluations have found that our DFAFC powers possessed not only simple structure but also reliable performance. Meanwhile, we carried out some exploratory research work on the effects of anodic catalysts for DFAFC.
Firstly, a miniature air breathing DFAFC was designed and fabricated by using a gold covered PCB as the unity of current collector and back board. The merit of technology is to simplify the cell structure effectively. Effects of formic acid concentration, Nafion loading in catalysts layer, types of current collector, anode catalysts and catalyst loading on the cell performance were intensively investigated. A maximum power density of 19.6 mW/cm~2 was achieved at room temperature where the operating parameters are 5 M formic acid, 20% Nafion loading, 1 mg/cm~2 catalyst at both electrodes and with small circular current collector. The home-made DFAFC also displayed good long term discharging stability at constant current density. When Pt/C was used as anode catalysts, the cell discharged for about 10 h at 0.45 V and 20 mA.
Secondly, a twin-cell stack with one shared fuel reservoir and a 4-cell stack with independent fuel reservoir was designed and investigated. The constitutive single cells showed almost identical discharging performance, which can be accounted for the tailorly-made stack structure to provide symmetry for each cells. The twin-cell stack showed the best performance as 5 M formic acid solution was used. A maximum power density of 44.5 mW/cm~2 was obtained with 2 mg/cm~2 Pt/C catalysts in electrodes. The twin-cell stack yields high stability and reproducibility when discharging at a constant current of 20 mA. The output voltage can be maintained at 1.14 V for about 5 h by feeding 3.5 ml 5 M formic acid and the performance can almost be reproduced for 4 times when fresh fuel was injected. The outstanding merit of independent fuel reservoir design in 4-cell stack was the avoidance of water hydrolysis between electrodes.
Thirdly, through scanning electron microscope (SEM) observations, electrochemical impedance spectroscopy (EIS) study and anodic potential test, we discussed the possible reasons for the performance declining during the long-term discharging process. It was found that the possible reasons were a combination of membrane crumple formation, PCB corrosion, the increase of anodic potential and the decrease of anode catalysts activity.
Finally, Pd/C with different sizes and PdPt/C with different PdPt atomic ratios were prepared by an organic colloid method and the catalytic activity for formic acid oxidation was evaluated. It is found that the size of Pd/C decreased with the increase of molar ratio between complexing agent (sodium citrate) and metal precursor (PdCl2). The decreased Pd/C size led to increased catalytic activity. Addition of a small amount Pt to Pd can greatly improve the catalytic activity. Pd20Pt/C showed increased electrochemically active surface area, negatively shifted peak potential (0.1 V) and enhanced peak current density (67%) relative to Pd/C.
首次采用印刷电路板（作为集电板与端板的统一体）设计和制作了微型空气自呼吸DFAFC，有效简化了电池的结构。考察了甲酸浓度、催化层Nafion载量、集电板类型、阳极催化剂种类等因素对电池放电性能的影响。当使用5 M甲酸为燃料、催化层Nafion载量为20%、使用小圆形集电板时电池性能最佳，在阴、阳极分别采用1 mg/cm~2 Pt/C和Pd/C催化剂时，电池最大功率密度为19.6 mW/cm~2。同时发现Pt/C对甲酸催化具有较好的耐久性能，供应3.5 ml 5 M甲酸，电池可连续恒定电流20 mA放电10 h，且维持电压在0.45 V。
在单电池研究基础上，分别设计了储液腔共通的双电池组和储液腔不共通的四电池组微电源系统。电池组中各单电池性能具有良好的一致性。双电池组使用5 M甲酸为燃料时，电池性能最佳，当在阴、阳极均采用2 mg/cm~2 Pt/C催化剂时，电池最大功率密度为44.5 mW/cm~2。供应3.5 ml 5 M甲酸，双电池组能够恒定电流20 mA工作近5 h，并且电压基本稳定在1.1 V，且重复4次几乎无衰减。四电池组中不连通的设计能有效地防止水解的发生。
通过扫描电镜（SEM）、交流阻抗分析（EIS）和测定阳极电势等方式初步考察了电池长时间放电性能衰减的原因，认为造成电池电压下降的原因包括：膜出现皱褶、电路板腐蚀、阳极电势增加、阳极催化剂活性下降等。
本文还采用有机溶胶法制备了不同粒径的Pd/C催化剂以及不同配比的PdPt合金类催化剂，并考查了其对甲酸催化性能。发现随着络合剂柠檬酸钠与金属前驱体PdCl2比例的增大，催化剂Pd/C颗粒度越小，对甲酸催化性能越好；Pd中添加少量Pt能显著改善Pd对甲酸的催化性能。与Pd/C相比，Pd20Pt/C催化剂的电化学活性比表面积增加，甲酸氧化峰电位负移0.1 V、峰电流密度增加67%。
Direct formic acid fuel cell (DFAFC) is a subcategory of proton exchange membrane fuel cell (PEMFC) where liquid formic acid is fed directly to the cell. Compared with direct alcohol fuel cells (DAFCs), DFAFC offers a broad range of advantages including lower fuel crossover through Nafion membrane, faster anodic electro-oxidition, lower ignitability, lower toxicity and higher theoretic open circuit voltage. DFAFC is therefore recognized as a kind of fuel cell which most likely commercializes in a near future. Nowdays, fundamental research dedicated to the practical application of DFAFC has became a hot topic in fuel cell community. In this thesis, we attempted to apply printed circuit board (PCB) technology in DFAFC, designed and fabricated a series of miniature air breathing DFAFC. Systematical evaluations have found that our DFAFC powers possessed not only simple structure but also reliable performance. Meanwhile, we carried out some exploratory research work on the effects of anodic catalysts for DFAFC.
Firstly, a miniature air breathing DFAFC was designed and fabricated by using a gold covered PCB as the unity of current collector and back board. The merit of technology is to simplify the cell structure effectively. Effects of formic acid concentration, Nafion loading in catalysts layer, types of current collector, anode catalysts and catalyst loading on the cell performance were intensively investigated. A maximum power density of 19.6 mW/cm~2 was achieved at room temperature where the operating parameters are 5 M formic acid, 20% Nafion loading, 1 mg/cm~2 catalyst at both electrodes and with small circular current collector. The home-made DFAFC also displayed good long term discharging stability at constant current density. When Pt/C was used as anode catalysts, the cell discharged for about 10 h at 0.45 V and 20 mA.
Secondly, a twin-cell stack with one shared fuel reservoir and a 4-cell stack with independent fuel reservoir was designed and investigated. The constitutive single cells showed almost identical discharging performance, which can be accounted for the tailorly-made stack structure to provide symmetry for each cells. The twin-cell stack showed the best performance as 5 M formic acid solution was used. A maximum power density of 44.5 mW/cm~2 was obtained with 2 mg/cm~2 Pt/C catalysts in electrodes. The twin-cell stack yields high stability and reproducibility when discharging at a constant current of 20 mA. The output voltage can be maintained at 1.14 V for about 5 h by feeding 3.5 ml 5 M formic acid and the performance can almost be reproduced for 4 times when fresh fuel was injected. The outstanding merit of independent fuel reservoir design in 4-cell stack was the avoidance of water hydrolysis between electrodes.
Thirdly, through scanning electron microscope (SEM) observations, electrochemical impedance spectroscopy (EIS) study and anodic potential test, we discussed the possible reasons for the performance declining during the long-term discharging process. It was found that the possible reasons were a combination of membrane crumple formation, PCB corrosion, the increase of anodic potential and the decrease of anode catalysts activity.
Finally, Pd/C with different sizes and PdPt/C with different PdPt atomic ratios were prepared by an organic colloid method and the catalytic activity for formic acid oxidation was evaluated. It is found that the size of Pd/C decreased with the increase of molar ratio between complexing agent (sodium citrate) and metal precursor (PdCl2). The decreased Pd/C size led to increased catalytic activity. Addition of a small amount Pt to Pd can greatly improve the catalytic activity. Pd20Pt/C showed increased electrochemically active surface area, negatively shifted peak potential (0.1 V) and enhanced peak current density (67%) relative to Pd/C.
引文
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