Résumé
Despite the fact that iridium (Ir) is among the rarest elements on Earth [1], it remains the state-of-the-art anode electrocatalyst in Proton Exchange Membrane Water Electrolyzers (PEMWE). Indeed, Ir dioxide (IrO 2 ) presents the best compromise between activity toward the Oxygen Evolution Reaction (OER) and stability under the harsh operating conditions of the anode: E > 1.5 V vs . the reversible hydrogen electrode (RHE), 60 < T < 80 °C, presence of oxygen and highly acidic environment [2]. In this context, research efforts have been dedicated to minimize Ir loading without compromising OER activity and stability. One possible approach is to stabilize the metal nanoparticles (NPs) on a high surface area support. The classical carbon black supports, widely used in fuel cells, are easily oxidized in OER conditions: carbon corrosion leads to particle aggregation and eventually detachment from the support surface [3]. As an alternative to carbon, metal oxides such as tin dioxide (SnO 2 ) [4] or titanium dioxide (TiO 2 ) [5] feature a good corrosion resistance in PEMWE operating conditions and may strongly interact with the catalyst (strong metal–support interaction (SMSI) effect [6]). In this presentation, we shine a spotlight on iridium oxide (IrO x ) NPs supported on SnO 2 aerogels doped with, niobium (Nb), tantalum (Ta) or antimony (Sb). The aerogels, synthesized via a sol-gel process, feature a rutile structure after calcination, a specific surface area about 80 m 2 g -1 and an electronic conductivity spanning from 5 mS cm -1 to 1 S cm -1 depending on the dopant [7-8]. A highly reproducible colloidal polyol route was used to deposit the IrO x NPs onto the aerogels, ensuring a straightforward comparison of their catalytic performance towards the OER and their resistance to corrosion. We used a home-made flow cell coupled to an inductively-coupled plasma mass spectrometer (ICP-MS) to assess metal dissolution under various potential conditions. As expected, carbon support proved to be unstable in OER conditions (Figure 1.a). In contrast, SnO 2 aerogels clearly improved catalyst stability at high potential (Figures 1.b and 1.c). Especially, Ta-doped SnO 2 (TaTO) aerogels showed an outstanding corrosion resistance with a negligible loss of dopant when compared to Sb-doped SnO 2 (ATO). Based on our experimental observations, we demonstrate that a judicious doping of SnO 2 aerogels could help developing highly active and robust OER electrocatalysts in acidic media. Figure 1. Metal concentration under staircase potential profiles from 0.9 V vs . RHE to 1.6 V vs . RHE in 100 mV steps with 300 seconds hold time at each potential for IrO x NPs deposed onto (a) Vulcan© XC-72, (b) ATO (10 at.% of Sb) and (c) TaTO (5 at.% of Ta) . The experiments were performed at room temperature by coupling a home-made flow cell to a PerkinElmer Nexion2000 ICP-MS instrument. A 0.05 M H 2 SO 4 solution deaerated with argon was used as electrolyte. The catalyst ink was deposited on the surface of the glassy carbon electrode (4 mm in diameter) resulting in a loading of 20 μg Ir cm geo -2 . References [1] Haxel, G. Rare Earth Elements: Critical Resources for High Technology ; US Department of the Interior, US Geological Survey, 2002; Vol. 87. [2] Cherevko et al., Oxygen and Hydrogen Evolution Reactions on Ru, RuO 2 , Ir, and IrO 2 Thin Film Electrodes in Acidic and Alkaline Electrolytes: A Comparative Study on Activity and Stability. Catal. Today 2016 , 262 , 170–180. [3] Claudel et al., Degradation Mechanisms of Oxygen Evolution Reaction Electrocatalysts: A Combined Identical-Location Transmission Electron Microscopy and X-Ray Photoelectron Spectroscopy Study. ACS Catal. 2019 , 9 (5), 4688–4698. [4] Oh et al., Preparation of Mesoporous Sb-, F-, and In-Doped SnO 2 Bulk Powder with High Surface Area for Use as Catalyst Supports in Electrolytic Cells. Adv. Funct. Mater. 2015 , 25 (7), 1074–1081. [5] Antolini and Gonzalez, Ceramic Materials as Supports for Low-Temperature Fuel Cell Catalysts. Solid State Ion. 2009 , 180 (9), 746–763. [6] Kamiuchi et al., Nanoscopic Observation of Strong Chemical Interaction between Pt and Tin Oxide. J. Phys. Chem. C 2007 , 111 (44), 16470–16476. [7] Solà-Hernández et al. Doped Tin Oxide Aerogels as Oxygen Evolution Reaction Catalyst Supports. Int. J. Hydrog. Energy 2019 , 44 (45), 24331–24341. [8] Ozouf et al., Niobium- and antimony-doped tin dioxide aerogels as new catalyst supports for PEM fuel cells, J. Mater. Sci., 51(11) ( 2016 ) 5305-5320 Acknowledgments This work was performed within the framework of the Centre of Excellence of Multifunctional Architectured Materials “CEMAM” ANR-10-LABX-44-01. The French National Research Agency (MOISE project, Grant ANR-17-CE05- 0033) financially supported this research. F.C. acknowledges the Region Auvergne Rhône-Alpes for funding his Ph.D. thesis in the frame of the ARC Energies program (ARC 2016 04 ADR). Figure 1