Résumé
The electrochemical activation of oxygen is the cornerstone of electrochemical conversion and storage devices, such as fuel cells, metal-air batteries, and electrolysers. It is well established that Pt is the only metal that can catalyse efficiently the oxygen reduction reaction (ORR) in acidic electrolyte, the reaction limiting the performance of low temperature proton-exchange membrane fuel cells (PEMFCs). However, due to the high cost and scarcity of Pt, research efforts recently focused on enhancing simultaneously its intrinsic activity (specific activity i.e . the current produced per cm 2 of Pt) and its mass activity (the current produced per gram of Pt). Studies on Pt or PtNi single crystals have established that the ORR is a structure sensitive reaction, which is best electrocatalyzed on (111) facets in acidic electrolyte. Combining alloying and ensemble effects recently led to 20-30-fold enhancement of the specific activity for the ORR on PtNi/C nanooctahedra relative to Pt/C nanoparticles. However, due to the highly oxidizing conditions of the PEMFC cathode (high electrochemical potential, presence of oxygen, acidic pH), the stability of these “dream” catalysts was found poor in PEMFC cathode operating conditions, thus compromising their utilization in real devices 1 . Strikingly, it also turned out recently that alloyed but structurally-disordered nanocatalysts, such as hollow PtNi/C nanoparticles, porous PtNi/C nanoparticles, PtNi aerogels or PtNi nanosponges also feature highly desirable and sustainable ORR activity (x 10-12 in specific activity relative to pure Pt/C). Even more striking, the ORR kinetics depends on the concentration of structural defects: the higher the structural disorder in a given nanocatalyst, the best is its intrinsic activity for the ORR but also other oxidation reactions 2 . This talk will address our recent insights about the quantification and the role played by structural defects in heterogeneous electrocatalysis from the beaker cell to the fuel cell device. Our proposal is based on Rietveld refinement of wide angle high energy X-rays scattering measurements and high resolution electron microscopy for a broad range of nanocatalysts combined with density functional theory calculations (DFT) 3 . References: C. Cui, L. Gan, M. Heggen, S. Rudi, and P. Strasser, “Compositional segregation in shaped Pt alloy nanoparticles and their structural behaviour during electrocatalysis,” Nat. Mater., vol. 12, no. 8, pp. 765–771, 2013. R. Chattot, T. Asset, P. Bordet, J. Drnec, L. Dubau, and F. Maillard, “Beyond strain and ligand effects: Microstrain-induced enhancement of the oxygen reduction reaction kinetics on various PtNi/C nanostructures,” ACS Catal., vol. 7, pp. 398–408, 2017. R. Chattot et al., “Surface distortion as a unifying concept and descriptor in oxygen reduction reaction electrocatalysis,” Nat. Mater., vol. 17, no. September, pp. 827–833, 2018. Figure: DFT-derived schematic describing different approaches towards enhanced ORR kinetics: the ‘scattergun’ approach (distorted suface) vs. the ‘high-precision gun’ approach (homogeneously-packed surface) for ‘structurally-disordered’ and ‘structurally-ordered’ catalysts, respectively. Figure 1