Surface-Regulated Nano-SnO₂/Pt₃Co/C Cathode Catalysts for Polymer Electrolyte Fuel Cells Fabricated by a Selective Electrochemical Sn Deposition Method

Abstract

We have achieved significant improvements for the oxygen reduction reaction activity and durability with new SnO2-nanoislands/Pt3Co/C catalysts in 0.1 M HClO4, which were regulated by a strategic fabrication using a new selective electrochemical Sn deposition method. The nano-SnO2/Pt3Co/C catalysts with Pt/Sn = 4/1, 9/1, 11/1, and 15/1 were characterized by STEM-EDS, XRD, XRF, XPS, in situ XAFS, and electrochemical measurements to have a Pt3Co core/Pt skeleton-skin structure decorated with SnO2 nanoislands at the compressive Pt surface with the defects and dislocations. The high performances of nano-SnO2/Pt3Co/C originate from efficient electronic modification of the Pt skin surface (site 1) by both the Co of the Pt3Co core and surface nano-SnO2 and more from the unique property of the periphery sites of the SnO2 nanoislands at the compressive Pt skeleton-skin surface (more active site 2), which were much more active than expected from the d-band center values. The white line peak intensity of the nano-SnO2/Pt3Co/C revealed no hysteresis in the potential up-down operations between 0.4 and 1.0 V versus RHE, unlike the cases of Pt/C and Pt3Co/C, resulting in the high ORR performance. Here we report development of a new class of cathode catalysts with two different active sites for next-generation polymer electrolyte fuel cells.

Key Factors for Simultaneous Improvements of Performance and Durability of Core‐Shell Pt3Ni/Carbon Electrocatalysts Toward Superior Polymer Electrolyte Fuel Cell

“…To examine the compressive strucutre of the activated Oh Pt 3 Ni/CMC( ex situ STEM of Figure (k–m)) and the bond strength with oxygenated species (CV of Figure a), we measured operando Pt L 3 ‐edge XAFS spectra under voltage operating conditions in N 2 ‐saturated 0.1 M HClO 4 . The white line peak intensity (metallic valence) in the in‐situ XANES spectra for the Oh Pt 3 Ni/CMC did not change significantly by increasing the voltage from 0.4 V RHE to 1.0 V RHE (Figure ) and the in‐situ Pt L 3 ‐edge EXAFS analysis revealed only the Pt−Pt bond and no Pt−O bonds were observed at 0.4–1.0 V RHE (Figure (b), Figure S6 and Table S2).…”

“…In situ X‐ray absorption fine structure (XAFS ) . The measurements of potential‐dependent in situ XAFS spectra at Pt L III ‐edge for electrocatalysts at rotating disk electrodes (RDE) in N 2 ‐saturated 0.1 M HClO 4 at 1600 rpm were performed in a fluorescence mode by using a Si(111) double‐crystal monochromator and an ion chamber (I 0 : Ar 5 %/N 2 95 %) for incident X‐rays and a 21 Ge‐elements detector for fluorescent X‐rays at BL36XU station in SPring‐8 similarly to the previous reports . X‐ray absorption near‐edge structure (XANES) spectra were normalized by Athena software.…”

“…These results suggest that most surface oxygen species are weakly adsorbed and disordered without making strong Pt−O bonds at a definite distance. It agrees with no hysteresis of the white line peak intensities in the voltage cycling in Figure (b), contrasted to the hysteresis behavior of the Pt/C with strongly bound oxygenates with definite Pt−O bonds at 0.195±0.006 nm above 0.8 V RHE …”

“…However, it is not an easy task for electrocatalysts to meet all these properties simultaneously because the activity, durability and mass transportation often cause interplay and dilemma in the design of optimal catalyst structures. Numerous monometallic Pt and bimetallic Pt−M electrocatalysts with different shapes, exposed planes, sizes, compositions, stresses, d‐band centers, Pt−O strength, etc., which have been demonstrated as key performance issues, have been synthesized ––. Nevertheless, it remains a big challenge for such Pt and Pt−M alloys to sustain their activities in the harsh cathode environment over a long period of time.…”

“…The Oh Pt 3 Ni/CMC also showed remarkable improvements of MA and SA; 13.6–34.1 and 31.3–37.0 times, respectively higher than those for the two benchmark Pt/C (Table ). In the present account we also found an exponential relationship between the ORR SAs and CO stripping peak potentials for the Oh Pt 3 Ni/CMC and our previous electrocatalysts to enable us to predict a maximum SA at the asymptote of the exponential curve in a series of Pt−M/carbon (M: Ni and Co). The advantage of structured Oh PtNi x nanoparticles and CMC support in achieving the high performance and durability is also discussed.…”