Hierarchically structured catalyst layer for the oxygen reduction reaction fabricated by electrodeposition of platinum on carbon nanotube coated carbon fiber

Abstract

The cost-efficient fabrication of cathode catalyst layers with low Pt-loadings and high oxygen reduction reaction (ORR) performances is of prime importance towards the commercialization of low temperature fuel cells. Here, an attempt has been made to fabricate a hierarchically structured cathode catalyst layer consisting of Pt-nanoparticle clusters supported on defective carbon nanotube (CNT) coated carbon fiber (CNTCF). CNTs were grown on carbon fiber (CF) using chemical vapor deposition (CVD), while electrodeposition was employed to deposit Pt-nanoparticle clusters onto the CNTCF. The effect of Pt-loading on the oxygen reduction reaction (ORR) performance of the Pt-coated CNTCF (Pt-CNTCF) electrocatalysts was studied by varying the electrodeposition time (t_ed) between 5 and 30 min, which resulted in a variation of the Pt weight fraction in Pt-CNTCF from almost zero to ∼0.3. Linear sweep voltammetry using a Pt-CNTCF modified glassy carbon (GC) rotating disc electrode (RDE) was employed to study the ORR performance of the samples. The CNTCF support, due to the defective structure of the CNTs, itself exhibits significant ORR activity with an electron transfer number (n) of ∼2.6, which leads to a synergistic enhancement of the overall electrocatalytic performance of Pt-CNTCF. For low Pt-loading on the GC (∼5 μg cm⁻²), the contribution of the CNTCF support dominated, while the Pt-nanoclusters governed the ORR performance at higher Pt-loading (>10 μg cm⁻²), with n > 3.5. Hence, a very low Pt-loading (∼5 μg cm⁻²) may not be suitable for polymer electrolyte membrane fuel cells as the production of substantial amounts of H₂O₂ on the catalyst supports may accelerate membrane degradation.