| 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. |
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