Improvement in oxygen reduction activity of polypyrrole-coated PtNi alloy catalyst prepared for proton exchange membrane fuel cells

Sluggish oxygen reduction reaction (ORR) kinetics and excessive usage of expensive platinum (Pt) catalyst are two main roadblocks hampering more popularity of proton exchange membrane fuel cells. As an alternative way to alleviate these problems, we develop a new ORR catalyst. The new catalyst (PtNi/PPy-CNT) is obtained by (1) addition of Ni to Pt to form metal alloy (PtNi) and (2) employment of polypyrrole (PPy) to carbon nanotube (CNT) (PPy-CNT). Optimal thickness of PPy is electrochemically determined while PtNi is well alloyed by the use of sodium borohydride (NaBH₄). In experimental evaluations, despite less Pt portion (31 wt %) in PtNi/PPy-CNT than that in Pt/C (40 wt %), its ORR activities (limiting current density, half-wave potential, transferred electron number and H₂O₂%) are compatible with those in Pt/C with better catalytic stability. It turns out that (i) PtNi-N networks cause increase in oxygen adsorption and weakening of O-O bond, followed by promotion of desirable ORR and (ii) C-N networks formed as pyridinic N improve the electron-donating capability and lead to increase in ORR activity.