The effectiveness of sputtered PtCo catalysts on Ti O₂ nanotube arrays for the oxygen reduction reaction

Pt and PtCo binary catalyst layers were prepared on self-ordered Ti O ₂ nanotube (TONT) arrays with the length of 1 μm and diameter of 120 nm employing a dual-gun sputtering method. The sputtered Pt induced a slight increase of the wall thickness of TONT. With increasing Co content in the alloy catalyst layer, a slight reduction of pore diameter on the TONT was also found. Well-dispersed mixed nanophases of Pt and Co were observed along the inner wall of TONT, and crystalline properties from the crystalline plane of Pt, Co, and Ti O₂ were identified by selective area electron diffraction. Among several compositions of additional Co on Pt, 30 at % Co catalyst on Pt showed the best oxygen reduction reaction (ORR) activity with the positive shift (200 mV) of onset potential. This is attributed to Pt electronic modification by an alloying effect and the shortening of the Pt-Pt interatomic distance. However, the coverage from the physically deposited sputtering method is incomplete; thus, the uniformity of the catalyst layer could be improved along the inner wall of TONT. Moreover, the influence of the pore size for diffusion of oxygen molecules on the ORR was investigated by increasing the sputtering time for Pt70 Co30 TONT. The Pt70 Co30 TONT sample with the pore diameter of 70 nm (sputtering time of 180 s) showed the maximum ORR activity (maximum upshift of half-wave potential), while above that, the ORR activity started to be degraded, resulting from restricted diffusion of oxygen molecules and the reduced surface area of catalyst layer. In order to examine the effect on large surface area of TONT and pore diameter, a compact Ti O2 film prepared on a Ti substrate was compared for the Pt70 Co30 catalyst layer. This compact Ti O2 support revealed significantly degraded ORR activity.