Dcsign, Fabrication, And Performance Investigation Of Direct Formic Acid Fuel Cell Catalysts

The extensive usage of fossil fuel has caused significant environmental pollution, climate change and energy crisis. Our future sustainable society has to be built on the basis of development and utilization of sustainable energy. As an efficient kind of energy conversion devices, fuel cells play critical role in the energy utilization sector. With compact structure and low operation temperature, polymer electrolyte membrane fuel cells (PEMFCs) have great application potential in automobiles. When powered by portable liquid fuels, PEMFCs could be used as power sources of portable electronic devices. Now, the large scale application of PEMFCs has been greatly hindered by the high cost which was caused by massive usage of precious metals as catalysts. When fueled by pure hydrogen, the obstacle of PEMFCs is the cathode side because the hydrogen oxidation is a very fast process and catalyst loading at anode could be very low. In contact, the oxygen reduction reaction at cathode side is very slow even on the most active metal surface. When powered by liquid fuels such as methanol and formic acid, both anode and cathode catalysts should be improve to decrease the catalyst loadings as the oxidations of liquid fuels are very slow and catalysts could be readily poisoned by carbon monoxide intermediate. With higher theoretical open circuit potential, lower fuel cross over, non-toxic and non-flammable properties compared with methanol, formic acid is a more promising fuel for liquid fuel cells. In this dissertation, we developed high performance electrocatalysts for both PEMFCs cathode and direct formic acid fuel cells (DFAFCs) anode.1) Nanoporous metal supported catalysts, especially nanoporous gold supported monolayer Pt catalysts, exhibit much higher catalytic activity and Pt utilization compared with traditional Pt/C catalysts. However, as pure Pt surface could be easily poisoned by CO intermediate in formic acid electrooxidation reaction, the catalytic activity of this kind of material should be much improved. Extensive electrochemistry and surface science researches have demonstrated that small Pt ensembles could facilitate the direct reaction path in formic acid electrooxidation, while the formation of CO poisoning intermediate need much larger Pt ensembles. We have developed a low Pt loading, high performance and high stability electrocatalyst based on our understanding of formic acid electrooxidation mechanism and experience in nanoporous metal fabrication. Using under potential deposition mediated method, we deposited monolayer Pt and sub-monolayer Au sequentially on nanoporous gold membrane substrate. With only monolayer Pt, the Pt loading reaches2μg cm-2, which is one of the lowest values reported. As the Pt monolayer was divided into small Pt ensembles, formic acid electrooxidation take place via the direct path and the formation of CO was greatly inhibited and results in a140-fold catalytic activity enhancement compared with Pt/C catalyst. The changing of reaction paths was demonstrated by in-situ surface enhanced infrared spectroscopy. In addition, with the protection of outer layer Au clusters, the catalyst exhibits high stability which was caused by the inhibition of Pt oxidation. After2800cycles'excursion, the catalytic activity decline of this catalyst is only17.4%which is much lower than Pt/C catalyst (declined for40%).2) The Pt surfaces made by traditional plating methods (gas-liquid phase chemical plating method and under potential deposition mediated method) are usually continuous which are easily poisoned by CO intermediate during the formic acid electrooxidation. Further depositing sub-monolayer Au could divide the surface into small Pt ensembles; however, large amount of Pt atoms will be covered by Au and wasted. We have developed a molecule self-assembly and electro-reduction method to deposit Pt onto nanoporous gold. We found that PtCl62-ions could form a stable monolayer structure on the ligament surface of nanoporous gold. With electro-reduction, Pt atoms could be deposited onto nanoporous gold. As the Pt coverage is only1/7monolayer and the extensive existed steps on the ligament surface of nanoporous gold, it is hard for Pt atoms to form large ensembles. With perfect Pt utilization and structure, this material shows380-fold enhanced Pt mass specific catalytic activity compared with Pt/C catalyst and approaching the theoretically highest value calculated based on Bi covered Pt catalyst. More importantly, this catalyst show much improved Pt mass specific performance in real fuel compared with Pt/C catalyst and good stability. Repeating the molecule self-assembly and electro-reduction method, the Pt loading and structure could be tuned easily. A series of high performance catalysts could be prepared easily with typically one order of magnitude enhanced activities toward small molecule oxidation such as methanol and ethanol compared with Pt/C catalyst.3) Within the membrane electrode assembly (MEA) preparation using the traditional Pt/C based catalyst, binders such as Nafion ionomers should be used to fix Pt/C particles onto the diffusion layers which would inevitably increase the electron transportation resistance. Moreover, as Pt/C catalysts are easily poisoned by carbon monoxide intermediate, large amount of catalysts should be used to overcome the large over-potential which in turn further increase the electronic resistance and reactants diffusion resistance. This is why there are lots of high activity electrocatalyst but can not show the same high performance in real fuel cells. On the basis of our understanding on formic acid electrooxidation mechanism and the desired anode structure, we developed a high performance anode for direct formic acid fuel cells which consists of a PtBi catalyst supported on nanoporous gold membrane substrate (NPG-Pt-Bi). With high performance and unique structure, the anode Pt loading of NPG-Pt-Bi catalyst could approach micrograms per centimeters with the same maximum power density with Pt/C anode with Pt loading of2.2mg cm-2. Typically, the Pt mass specific performances of NPG-Pt-Bi catalysts are1-2orders of magnitude higher than commercial Pt/C catalyst. The fuel cell with NPG-Pt-Bi catalyst could be fueled with formic acid with some commonly contaminants such as methanol, methyl formate and acetic acid. During the half-year stability test, the fuel cell with NPG-Pt-Bi catalyst show almost no performance decline which means good stability. Then we assembled a fuel cell stack with maximum power of40W. Compared with stacks using methanol as fuel, the NPG-Pt-Bi catalyst based stack show much improved fuel utilization because of the less fuel cross over of formic acid and the energy efficiency is comparable with reported stacks with Pt/C catalyst. 4) With3-D porous structure, high surface area, kinetically controlled ligament size, and good electrical conductivity, nanoporous metals made by dealloying has been demonstrated for a variety of interesting applications such as catalysis, sensing, actuation, and electrocatalysis. Particularly, large area, free standing, and crack free nanoporous gold leaf could be made by dealloying commercially available white gold leaf on the solution surface of nitric acid. This kind of advanced materials has been demonstrated to be highly effective electrode substrate in fuel cells, supercapacitors, and Li ion batteries. However, the commonly used dealloying method involves the use of harsh corrosion regents such as HNO3and sometimes electrochemical method is needed to assist the dealloying process. We found the galvanic replacement reaction between noble metal ions and active component in alloy could result in nanoporous metals. Without the usage of harsh corrosion regent, this method is a green and facile method to make nanoporous metals. We have prepared high surface area nanoporous PtAu alloy leaf using the galvanic replacement reaction between white gold leaf and H2PtCl6solution. With small Pt ensemble on the surface, we found nanoporous PtAu alloy leaf show high active and stability in formic acid electrooxidation reaction. This facile and green method have open a new way to make nanoporous metals and pave the way for the application of this kind of important materials.5) It is of critical importance to design and fabricate highly active and durable oxygen reduction reaction (ORR) catalysts for the application of proton exchange membrane fuel cells (PEMFCs). At present, the state-of-the-art commercial cathode catalysts for PEMFCs are in the form of carbon supported Pt nanoparticles. Despite the high dispersion of Pt particles, the mass specific activity of Pt/C should be increased at least four times in order to meet the application demand. Moreover, as the Pt nanoparticles only have weak interactions with the carbon support, they tend to aggregate to lose surface area and performance during the long-term operation. By a simple two-step dealloying process, the active components in a Pt/Ni/Al ternary alloy were sequentially leached out in a highly controllable manner, generating a novel nanoporous surface alloy structure. Characterized by an open bicontinuous spongy morphology, the resulting nanostructure is interconnected by～3nm diameter ligaments which are comprised of a Pt/Ni alloy core and a nearly pure Pt surface. In the absence of any catalyst support, these nanoporous surface alloys show much enhanced durability and electrocatalytic activity for ORR as compared to the commercial Pt/C catalyst. At a high potential, such as0.9V versus RHE, nanoporous Pt/Ni surface alloys show a remarkable specific activity of1.23mA cm-2. These nanomaterials thus hold great potential as cathode catalysts in PEMFCs in terms of facile preparation, clean catalyst surface, and enhanced ORR activity and durability.