Replacing conventional carbon supports by metal oxides
Replacing carbon blacks by metal oxides implies that they fulfill at least three criteria, of equal relevance: (i) be electron-conducting and (ii) corrosion-resistant, and (iii) possess an opened and interconnected structure compatible with facile gas–transport and ionomer insertion.
Recently, we met the first and second criteria by doping SnO2 – a metal oxide that is stable at pH close to 0, 0 < E < 1.5 V vs. RHE and T = 80°C – with Sb. In collaboration with Mines ParisTech at Sofia-Antipolis, we synthesized high surface area antimony-doped tin oxide (ATO) aerogels with different Sb contents (5, 10 and 15 at. %) and tunable pore size distribution using a sol-gel route starting with metal alkoxide precursors followed by drying under CO2 in supercritical conditions (Figure 1).
Figure 1. Structural characteristics of the doped and undoped SnO2 aerogels used in this study. (a) SEM, (b, c) TEM images, (e) pore size distribution and (f) particle size distribution of the doped and undoped SnO2 aerogels used in this study. NT is the total number of counted particles (NT = ca 200). (d) X-ray diffractograms of the synthesized Sb-doped and the undoped SnO2 aerogels.
It is possible to decorate an ATO aerogel (10 at.% Sb-doped SnO2) with Pt nanoparticles via a modified polyol route, and to compare its ORR activity and durability with reference materials (e.g. Pt/Vulcan XC72 and Pt/undoped SnO2) synthesized using the same Pt colloidal suspension. A 2-fold enhancement in specific activity for the ORR is measured on Pt/ATO over the reference Pt/Vulcan XC72, suggesting strong metal support interactions between the Pt nanoparticles and the ATO support (Figure 2).
Figure 2. Electrocatalytic properties of the Pt/C and the Pt/ATO samples. a) Electrochemically active surface area (ECSA) and Pt specific surface area (SPt). The ECSA was determined by using the coulometry required to adsorb and desorb Hupd from the Pt nanoparticles. b) Linear sweep voltammograms recorded on the Pt/ATO and Pt/C samples recorded at vb = 5 mV s-1 in O2 saturated electrolyte. c) Specific activity (SA0.90) and mass activity (MA0.90) for the ORR determined at E = 0.90 V vs RHE. d) Tafel plots of Ohmic and mass-transport corrected linear sweep voltammograms displayed in b). T = 330 K – Electrolyte: 0.1 M H2SO4 – LPt = 60 μgPt cmgeo-2 – The error bars are the standard deviations of at least three independent measurements.
Moreover, contrary to what is observed on Vulcan XC72, Pt nanoparticles do not detach from the ATO support during accelerated stress test (AST) performed in extreme PEMFC operating conditions. The limitations of this type of support concern their stability at high potential E > 1.0 V vs. RHE. We provide evidences that a core@shell structure, with a Sb-poor surface covering a core with a Sb content close to the nominal, forms during the AST. This core@shell structure restricts the capacity of the Pt nanoparticles to exchange electrons, as evidenced by the attenuated Pt surface oxide formation/reduction features, and the decreased catalytic activity for the ORR.
This work was performed within the framework of the Centre of Excellence of Multifunctional Architectured Materials "CEMAM" n° AN-10-LABX-44-01 funded by the "Investments for the Future" program. The authors acknowledge financial support from the French National Research Agency through the SURICAT project (grant number ANR-12-PRGE-007) as well as Tenerrdis.