Degradation Analysis of Pt/Nb–Ti4O7 as PEFC Cathode Catalysts with Controlled Arc Plasma-deposited Platinum Content

Abstract

For polymer electrolyte fuel cell cathodes, highly durable supports are required to prevent catalyst degradation in supports. In this study, as model Pt catalysts, 2–10 wt% Pt was deposited on Magnéli-phase niobium-doped macroporous Ti4O7 (Nb–Ti4O7) mounted on glassy carbon rods using the coaxial arc plasma deposition method. The morphologies of 2, 5, and 10 wt% Pt catalysts showed the hemisphere fine particles, islands with ca. 1.4 nm diameter and ca. 2.4 nm thickness, and films with ca. 3.3 nm thickness, respectively. During start/stop accelerated durability tests (ADTs) of 5000 cycles following the Fuel Cell Commercialization Conference of Japan protocol, Pt was slightly agglomerated; consequently, the morphologies of the 2, 5, and 10 wt% Pt catalysts were island-like with 3.5 nm thickness, chain bead-like with 4 nm thickness, and film-like with 4 nm thickness, respectively. This slight agglomeration led to good durability during the ADTs. Herein, the oxygen reduction reaction (ORR) mass activity (MA) values at 0.9 V vs. reversible hydrogen electrode (RHE) of the 2, 5, and 10 wt% Pt catalysts were 79, 60, and 36 A gPt-1 after 5000 cycles ADT, respectively, which had declining ratios after 5000 cycles were 32 %, 17 %, and 0 %, respectively. The island-like and film-like Pt/Nb–Ti4O7 presented activity and durability comparable to a Pt/C catalyst, which was 42 A gPt-1 (0.9 V vs. RHE) with a 12 % of declining ratio after the ADTs. The durability of the MA suggested that the different affinity caused by different crystal faces led to the slight agglomeration of 2, 5, and 10 wt%_Pt/Nb–Ti4O7 catalysts. These catalysts showed electrochemical surface areas (ECSAs) of 36, 27, and 29 m2 g−1 after the ADTs, with declining ratios as low as 20 %, 6 %, and 0 %, respectively. All Pt/Nb–Ti4O7 catalysts showed higher durability of the ECSAs than the Pt/C catalyst, which was 68 m2 g−1 with a 30 % declining ratio after the ADT. Different from common Pt nanoparticle catalysts, which agglomerate into large spherical Pt particles, the slight agglomeration was caused by the interconnection of the deposits and supplemented by a limited increase in the diameter or thickness. The island-like morphology of Pt with a limited thickness presented both high durability and activity among the Pt/Oxide catalysts.

Keywords

• carbon free orr catalysts
• polymer electrolyte fuel cells
• carbon free catalysts degradation analysis
• pt supported cathode catalysts