Chemical And Photo-enhanced Catalysis In DMFC And DSSC

Direct methanol fuel cell (DMFC) and dye-sensitized solar cell (DSSC) are two kinds of device which directly transform chemical energy and solar energy into electric power, respectively, and attracting more and more interest from the world. However, there is a series of problems for the two devices, such as short service life, high cost and low efficiency, blocking them from the practical application.One of problems blocking DMFC from the practical application is the slow kinetics of methanol electro-oxidation due to CO poisoning to the Pt or PtRu anodic catalysts, resulting in a serious decline in the performance of DMFC. How to effectively remove of CO poison was intensively studied in the chapter 3 and 4 of this thesis. The main results obtained are as follows:(1) COads, an intermediate in the methanol electro-oxidation on Pt-based catalysts, has been thought a primary cause slowing down the kinetics of methanol electro-oxidation. On the basis of the understanding, that is, CO(g) can be oxidized by PMo₁₂ under catalysis of Au, the methanol electro-oxidation was investigated on sub-monolayer Au modified Pt electrode (Au/Pt) in participation of PMo₁₂ to improve the catalytic and anti-poison ability of Pt catalyst. Generally, the sub-monolayer metal is obtained by underpotential deposition (UPD). The occurrence of UPD is in that the electron work function of the substrate metal is greater than that of the deposited metal, ie, the more active metal can be electrochemically deposited on the less active substrate metal at the more positive potential in contrast to the equilibrium potential predicted by the Nernst equation for bulk deposition. Thus, UPD of Au on Pt surface can not occurs to form the sub-monolayer Au, but Cu can. The Au/Pt electrode was prepared by chemically displacing underpotentially-deposited Cu on Pt surface in HAuCl4 solution. The results showed that the methanol electro-oxidation on Pt electrode was markedly enhanced in presence of PMo₁₂ and Au. The onset potential of methanol oxidation shifts 400 mV toward the negative direction on Au/Pt electrode with PMo₁₂ in comparison with Pt electrode with PMo₁₂. It is supposed that adsorbed hydrogen and intermediate CO from the methanol dehydrogenation and oxidation were electro-catalytically oxidized by oxidant state of PMo₁₂ with the aid of Au catalysis.(2) In comparison with TiO₂ nanoparticles (TNPs), TiO₂ nanotubes (TNTs) have a larger surface area, a more effective scattering and absorption of light and less detrimental grain boundaries. TNTs can therefore generate more electron-hole pairs and effectively lessen the electron-hole recombining under illumination. Therefore TNTs can more effectively produce strong oxiding oxygen-containing species ?OH from water under illumination. We expected that CO, the poisoning intermediate of methanol electro-oxidation, can be effectively removed by strong oxiding oxygen-containing species ?OH produced on TNTs under illumination. On the basis of this perception, a Pt/TNTs/Ti electrode was prepared by electrochemically depositing Pt using the modulated pulse current method onto TNTs/Ti substrate, and then the methanol electro-oxidation was investigated on such an electrode under illumination. The results show that the performance and anti-poison ability of the Pt/TNTs/Ti electrode for methanol electro-oxidation under illumination is remarkably enhanced. CO poisoning is no longer a problem during methanol electro-oxidation with the Pt/TNTs/Ti electrode under illumination. The main role of a counter electrode in a DSSC, is to catalyze the reduction I3? by electrons through the outside circuit from the anode to I? instead of by electrons directly from the conduct bands of TiO₂ produced by illumination. How to fabricate an effective counter Pt electrode was intensively studied in the chapters 5 and 6 of this thesis. The main results obtained are as follows:(1) A Pt/FTO counter electrode of the dye-sensitized solar cells (DSSC) was prepared by sputtering (Cu)– displacement (Pt) method (SD) on a conductive glass (FTO) substrates (SD-Pt/FTO). In contrast to PY-Pt/FTO counter electrode prepared by pyrolysis (PY) of Pt salts, SD-Pt/FTO counter electrode not only avoids the expensive Pt target with Pt direct sputtering but also overcomes the poor dispersion of Pt particles and the increased electric resistance of FTO substrate caused by pyrolysis. The results show the photoelectric conversion efficiency of DSSC with SD-Pt/FTO counter electrode increases by 16.5 % relative to that with PY-Pt/FTO counter electrode.(2) To lower electric resistance and improve reflecting ability of Ti substrate compared with that of commonly used FTO glass substrate, Ti sheet rather than FTO glass served as the counter electrode substrate was investigated. A Pt/Ti counter electrode of DSSC was prepared by displacing electrodeposited Cu deposits on a Ti sheet in H2PtCl6 solution. The photocurrent density–volt (J–V) curves show that the photoelectric conversion efficiency of DSSC with the Pt/Ti counter electrode reaches 7.61 %, increased by 20.8 % relative to that with the Pt/FTO counter electrode.