ABSTRACTCarbon nanotubes (CNTs) possess remarkable mechanical and physical characteristics due to their unique structure. The high electrical conductivity of CNTs in combination with their structural and chemical properties make this material attractive for improvement of the performance of the electrodes in electrochemical devices, such as polyelectrolyte-membrane fuel cells. The requirements in particular for this application are manifold. Since the electrode is the reaction layer in a fuel cell, the electrolyte, being proton conductive, has to be combined with catalytic activity and electron conductivity, as well as chemical stability against the reactants. Employed as an electrode, CNTs offer feasibilities to enhance the efficiency of fuel cells due to an improved contact between electrolyte-membrane, electrode and catalyst. An other promising capacity is the large specific surface area of CNTs on which catalyst particles can be distributed in small clusters. This provides an increase of the catalyst efficiency on account of a higher reactive surface area which enables a reduction of the amount of catalyst required. In combination with a good electron conductivity, a CNT based electrode is a potential alternative fuel cell electrode. The presented work focuses on a membrane assembly, consisting of CNTs as electrode and sulfonated polyether ether ketone (SPEEK), which is used as polyelectrolyte in direct methanol fuel cells (DMFCs) and provides good proton conductivity. Nanocomposites were produced by a modified drop casting technique, leading to the formation of an asymmetric structure. Due to the process, the compounded membranes provided a single-sided electron conductivity on account of the CNTs. By using different 3D-structured CNT-carpets, varying in thickness, density and setup, the properties of the electrode membrane could be adjusted for its special application. In consideration of the insertion of platinum and ruthenium particles as catalyst in nano-sized clusters on the CNTs, carpets were modified and grown in a CVD-process at the TUHH. The evaluation of the assemblies comprised SEM-pictures in order to analyse the 3D-nano-structure, measurements of the surface conductivity as well as tests in a fuel cell.
Ways to improve production efficiency problems and ways of their solution