Résumé
Proton exchange membrane fuel cells now present performances that enable their commercialization, as demonstrated by the recent release of fuel-cell powered vehicles (forklifts by Plug Power, FCEV Miraï by Toyota, etc.). The goal of PEMFC manufacturers is now to reduce the materials cost of their system and to enhance their durability/reliability, so that they can meet the market requirements. To that goal, highly-efficient oxygen reduction reaction (ORR) electrocatalysts have been developed recently [1-6], these materials mostly originating from the dealloying of PtM/C electrocatalysts (M = Ni, Cu, etc.) initially rich in M. Unfortunately, to date, the scientific community has failed to properly use these materials in an operating PEFMC: neither are their apparent performances approaching those expected from tailored data obtained in rotating disk electrode setup (Figure 1), nor are the materials presenting a sufficient durability in the operating conditions of a PEMFC (see for example [7]). In this tutorial, some reasons for our present incapacity to operate properly membrane electrode assemblies based on low-Pt-loading of highly-efficient Pt-based electrocatalysts will be discussed. In particular, it will be shown that the degradation mechanisms at stake for present (conventional) PtM/C electrocatalysts (coarsening of the nanoparticles, loss of their shape/texture, selective dissolution of the M element, corrosion of the carbon support, etc.) still operate (at least for some of them) for the advanced electrocatalysts. Besides, the inevitable leaching of the M element raises issues of pollution of the ionomer [8], which is particularly detrimental, not only to the proton conduction, but also to the mass-transport of oxygen to the active sites. In addition, as these electrocatalysts are more active, their optimized operation relies on an emphasized mass-transport of reactants (both H + and O 2 ) to the catalytic sites, which is not granted for present ionomers and MEA structure [9, 10]. References [1] O. Le Bacq, A. Pasturel, R. Chattot, B. Previdello, J. Nelayah, T. Asset, L. Dubau, F. Maillard, Effect of Atomic Vacancies on the Structure and the Electrocatalytic Activity of Pt-rich/C Nanoparticles: A Combined Experimental and Density Functional Theory Study, ChemCatChem, 9 (2017) 2324. [2] R. Chattot, T. Asset, J. Drnec, P. Bordet, J. Nelayah, L. Dubau, F. Maillard, Atomic-Scale Snapshots of the Formation and Growth of Hollow PtNi/C Nanocatalysts, Nano Lett., 17 (2017) 2447. [3] L. Dubau, J. Nelayah, S. Moldovan, O. Ersen, P. Bordet, J. Drnec, T. Asset, R. Chattot, F. Maillard, Defects do Catalysis: CO Monolayer Oxidation and Oxygen Reduction Reaction on Hollow PtNi/C Nanoparticles, ACS Catal., 6 (2016) 4673. [4] C. Chen, Y. Kang, Z. Huo, Z. Zhu, W. Huang, H.L. Xin, J.D. Snyder, D. Li, J.A. Herron, M. Mavrikakis, M. Chi, K.L. More, Y. Li, N.M. Markovic, G.A. Somorjai, P. Yang, V.R. Stamenkovic, Highly crystalline multimetallic nanoframes with three-dimensional electrocatalytic surfaces, Science, 343 (2014) 1339. [5] L. Gan, M. Heggen, R. O'Malley, B. Theobald, P. Strasser, Understanding and Controlling Nanoporosity Formation for Improving the Stability of Bimetallic Fuel Cell Catalysts, Nano Lett., 13 (2013) 1131. [6] C. Cui, L. Gan, M. Heggen, S. Rudi, P. Strasser, Compositional segregation in shaped Pt alloy nanoparticles and their structural behaviour during electrocatalysis, Nat Mater, 12 (2013) 765. [7] V.R. Stamenkovic, N.M. Markovic, Nanosegregated Cathode Alloy Catalysts with Ultra-Low Platinum Loading, DOE Hydrogen and Fuel Cells Program, FY 2015 Annual Progress Report, V.A.2, in, 2015. [8] J. Durst, M. Chatenet, F. Maillard, Impact of metal cations on the electrocatalytic properties of Pt/C nanoparticles at multiple phase interfaces, Phys. Chem. Chem. Phys., 14 (2012) 13000. [9] Y. Fukuyama, T. Shiomi, T. Kotaka, Y. Tabuchi, The Impact of Platinum Reduction on Oxygen Transport in Proton Exchange Membrane Fuel Cells, Electrochim. Acta, 117 (2014) 367. [10] T.A. Greszler, D. Caulk, P. Sinha, The Impact of Platinum Loading on Oxygen Transport Resistance, J. Electrochem. Soc., 159 (2012) F831. Figure 1