Highly efficient cathode catalyst layer based on nitrogen-doped carbon nanotubes for the alkaline direct methanol fuel cell

The performance of a direct methanol alkaline anion-exchange membrane (Fumatech FAA3) fuel cell with Pt-free nitrogen-doped few-walled carbon nanotubes (N-FWCNT) as the cathode catalyst is compared with a commercial supported Pt catalyst. The ionomer content of the N-FWCNT cathode catalyst layer is therefore optimized and it is shown to be 40wt% of FAA3. Scanning electron microscopy images of the catalyst layer show that the ionomer forms aggregates with N-FWCNTs probably due to their charged nature and that the catalyst layer structure is remarkably open even with high ionomer contents facilitating the mass transfer of reactants and products to the active sites. With oxygen as the oxidant, the maximum power density obtained with our Pt-free N-FWCNTs (0.78mWcm^-2) is slightly higher than with the Pt catalyst (0.72mWcm^-2). However, when more practical air is used as the oxidant, the N-FWCNTs (0.73mWcm^-2) show clearly superior performance compared to the Pt catalyst (0.18mWcm^-2). The lower performance with the Pt catalyst is attributed to the denser electrode layer structure resulting in higher mass transport resistance and to the presence of methanol in the cathode, which poisons the Pt but not the N-FWCNTs.