The Preparation Of Integrated Electrode For Direct Alcohol Fuel Cell By Plasma Technique

Fuel cells (FCs) are expected to be promising power sources ranging from automotive to portable electronic device applications due to their high power density and low environmental pollution. Compared to hydrogen-fed fuel cells, direct alcohol fuel cells (DAFCs) have a simpler fuel cell system for usage of liquid methanol fuel, which is easily stored and transported. However, one of the main barriers for the commercial is the high cost and low utilization of the platinum (Pt) based catalyst, which is usually used as the catalysts both in the anode and cathode. In this respect, the main research works of DAFCs catalyst mainly focus on improving the performance and utilization of Pt and Pt based catalysts.An innovation approach which used to prepare integrated direct alcohol fuel cell (DAFC) electrodes has been developed by combining plasma enhanced chemical vapor deposition (PECVD) and plasma magnetron sputtering (PMS) techniques. Pt/carbon nanomaterials (vertically aligned carbon nanofibers and carbon nanowalls) were fabricated by growing carbon nanomaterials on carbon papers with inductively coupled PECVD, followed by sputtering deposition of Pt nanoparticles using a radio-frequency magnetron sputtering system. The integrated electrodes prepared by plasma techniques could not only solve the low utilization and poor electrocatalysis activity of DAFC catalysts, but also decrease the loading of noble catalyst leading to a low cost and high perfomance of DAFC. The plasma discharge parameters and substrate bias are studied which influence the characteristics of carbon nanofibers to control the length and density of carbon nanofiber matrix. The influences of PMS parameters on Pt nanoparticles microstructure are studied and optimized. The microstructure and electrocatalysis properties of the integrated electrode will be characterized to reveal their internal relationship which could give theoretical instructions of the preparation of DAFC electrodes with low Pt loading and high utilization efficiency. The research result will not only provide a new method for the synthesis of the direct alcohol fuel cell electrode, but also show an important scientific value in accelerating the development of the low temperature plasma techniques in the fields of materials and energy science.The main achievements are shown as follow:(1) The vertically aligned carbon nanofibers (VACNFs) matrix directly grown on the carbon paper were prepared by a homemade inductively coupled plasma enhanced chemical vapor deposition (ICP-PECVD) system. The kind and the thickness of the catalyst layer are studied. Meanwhile, the influence of substrate bias and radio-frequency output power on VACNFs growth are revealed. The results show that the VACNFs prepared by Fe catalyst have a better alignment and uniformity than Ni and304stainless steel. The density of VACNFs is increased firstly then decrease as the increasing catalyst thickness. The VACNFs are successfully grown on carbon paper using2nm and5nm Fe catalyst layer and VACNFs couldn’t be prepared under the10nm thickness Fe film. The larger negative substrate bias could lead to a better alignment and faster growing speed of VACNFs. The input radio power indicates a crucial influence of VACNFs. A linear relationship is founded under the allowable parameters of the ICP-PECVD system.(2) The Pt/VACNFs integrated electrodes are developed by directly sputtering Pt nanoparticles onto VACNFs surface. The integrated electrode shows better electro activity, Pt utilization efficiency, methanol oxidation reaction, poisoning-resistance ability and durability than commercial JM catalysts suggesting a potential application in DAFCs. Pt loading as an important factor influencing the electrodes performance has been studied. The Pt nanoparticle diameter becomes larger by increasing Pt loading. Moreover, the electroactivity, Pt utilization efficiency, methanol oxidation activity, anti-poisoning ability are decreases as the Pt loading increase when the VACNFs is fixed.(3) Surface modify effect is studied by treating VACNFs with different kind of plasma. H2, N2, O2, NH3plasma are used to activate VACNFs in-situ. According the particle size distribution, NH3and O2plasma could decrease the particle size of Pt. H2plasma don’t show little influence of Pt nanoparticle. Although the particle dispersion on VACNFs is become better after N2plasma treatment, but the particle size has enlarged obviously. All of the plasma treated Pt/VACNFs electrodes show good electroactivity. The Pt utilization efficiency and methanol oxidation activity of the electrodes have the following order:Pt/VACNFs-25-O2> Pt/VACNFs-25-NH3> Pt/VACNFs-25-N2> Pt/VACNFs-25-H2>Pt/VACNFs-25.(4) Influence of VACNFs surface characteristics on electrocatalytic properties of integrated electrodes are studied. The results indicate that the Pt nanoparticle size becomes smaller and electrochemical properties of electrode become better by increasing density and length of VACNFs. Magnetron sputtering is subject to the ’cosine effect’, which means that more catalyst is deposited on planes facing the sputtering flux, than on inclined planes. In fact, due to this effect larger catalyst particles would like to be formed on planes which are perpendicular to the sputtering flux than on the sidewalls. The area of inclined planes increased with increasing length and density of VACNFs, which implies that the size of Pt nanoparticles will be smaller and the electrocatalytic properties will be better.(5) Pt/carbon nanowalls (Pt/CNWs) and Pt/vertically aligned carbon nanofibers (Pt/VACNFs) electrodes were fabricated by growing CNWs and VACNFs on carbon papers with inductively coupled plasma enhanced chemical vapor deposition, followed by sputter deposition of Pt nanoparticles using a radio-frequency magnetron sputtering system. The CNWs and VACNFs are grown under virtually identical circumstances except that the negative bias that added to the substrate is different. Scanning electron microscopy and transmission electron microscopy results show that the Pt nanoparticles are homogeneously dispersed on the surface of CNWs and VACNFs. The histograms of Pt nanoparticle diameter for both electrodes reveal that the Pt/CNW electrode shows a broader particle size distribution. Cyclic voltammetric measurements show that the Pt/CNWs electrode has a better electrochemical activity and methanol oxidation property than Pt/VACNFs electrode. The unique structure of CNWs ensures that Pt/CNWs electrode has a faster electron transport rate and shorter electron transport path, which lead to an obvious improvement of electro-catalysis activity compared to the Pt/VACNFs electrode and show a further potential application in direct alcohol fuel cells.