Study On Pd - Based Anode Catalyst For Direct Liquid Fuel Cell

Because of their potential to reduce the environmental impact of the use of fossil fuels, fuel cells have emerged as tantalizing alternatives to combustion engines. Like a combustion engine, a fuel cell uses some sort of chemical fuel as its energy source; but like a battery, the chemical energy is directly converted to electrical energy, without an often messy and relatively inefficient combustion step. Among them, direct fuel cells using small organic compounds as liquid fuels are promising power sources for portable electronic devices because these fuels are convenient to store, transport and refill, suitable for simple system design and cell operation. However, for practical applications one of the key issues is the design of effective and low-priced anode catalysts.Among the various electrocatalysts, Pt-based catalysts were most widely studied because of their good activity towards both formic acid oxidation and methanol oxidation. However, Pt is susceptible to poisoning by the reaction intermediates such as adsorbed CO. In addition, the high cost and scarcity of Pt also hindered its large-scale application. Therefore much efforts has been made to develop low Pt loading or Pt-free catalysts, among them, Pd is considered as a good alternative due to its lower cost, better resistance to CO poisoning and high activity in alkaline media for methanol oxidation. Nevertheless, the activity and stability of pure Pd is not satisfactory and needs further improvement for commercialization. In this dissertation, different approaches such as using oxides promoters, alloying and microstructure design to improve the catalytic activity and stability of Pd-based catalysts were studied systematically. The main contents are as follow:(1) Electrodeposited Pd-MoOx catalysts with enhanced catalytic activity for formic acid electrooxidationPd-MoOx catalysts supported on glassy carbon electrode were co-deposited using cyclic voltammetry. The influence of two key deposition parameters (i.e. scanning potential range and concentration of sodium molybdate in the electrolyte) on the catalysts was investigated by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), element analysis mapping (EM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and typical electrochemical measurements, respectively. It was found both of the parameters had great effect on the morphology, chemical states, composition and electrochemical performance of the catalysts. By tuning the electrodeposition parameters, we found the optimal condition to prepare the catalysts. The as-prepared catalysts showed much improved catalytic activity and stability for formic acid electrooxidation. The enhanced performance can be attributed to the fine porous structure composed of small particles, hydrogen spillover effect and unique element distribution (different composition between surface and bulk).(2) Preparation of carbon supported Pd-Pb hollow nanospheres and their electrocatalytic activities for formic acid oxidationPd-Pb hollow nanaospheres dispersed on carbon black were developed by a galvanic replacement reaction between sacrificial cobalt nanoparticles and Pd2+, Pb2+ions. The as-prepared catalysts were characterized by TEM, energy EDX, and XRD. The electrochemical measurements show that the as-prepared catalysts have excellent catalytic activity for formic acid electrooxidation, which is attributed to the large surface area caused by the hollow structure and the lead doping effect which might modify the electronic structure of the catalysts.(3) Ultrasonic-assisted synthesis of Pd-Ni alloy catalysts supported on multi-walled carbon nanotubes for formic acid electrooxidationPd-Ni alloys with different compositions (i.e. Pd2Ni, PdNi, PdNi2) dispersed on multi-walled carbon nanotubes (MWCNTs) are prepared by ultrasonic-assisted chemical reduction. The XRD patterns indicate that all Pd and Pd-Ni nanoparticles exist as Pd face-centered cubic structure, while Ni alloys with Pd. The TEM images show the addition of nickel decreases the particle size and improves the dispersion. The electrochemical measurements reveal that the PdNi catalysts have better catalytic activity and stability for formic acid electrooxidation, among them PdNi/MWCNTs is the best. The performance enhancement is ascribed to the increase of electroactive surface area (EASA) and nickel doping effect which might modify the electronic structure.(4) Rapid synthesis of porous Pd and PdNi catalysts using hydrogen bubble dynamic template and their enhanced catalytic performance for methanol electrooxidationPd and Pd-Ni catalysts with three-dimensional hierarchical pores consisting of interconnected dendrite walls are successfully fabricated by cathodic deposition using hydrogen dynamic bubble template. The as-prepared catalysts are characterized by SEM, XRD, TEM, EDX, EM, XPS and typical electrochemical measurements. It is found that the porous structure enlarges the electro-active surface area; the Ni doping modifies the electronic structure of the catalysts and improves their stability. Thus the porous catalysts exhibit excellent catalytic activity and stability towards methanol oxidation.