Design of New Catalytic Architectures for Proton Exchange Membrane Fuel Cell
Authors: Dru Delphine , Baranton Stève , Buvat Pierrick , Coutanceau Christophe 
Published: February 2020

The Proton-exchange membrane fuel cell (PEMFC) will become a mature technology, and several car manufacturers in Asia, United State and Europe are communicating on the commercialization of Fuel Cell cars in the near future. The materials which will be used for the first generation of this technology of energy conversion are currently known: the solid electrolyte will consist in a perfluorinated membrane with attached pendant sidechains and sulfonic acid groups (–SO3H) at the sidechain terminals, which Nafion® is the mostly recognized representative and the electrodes will be composed of platinum-based nanoparticles (NPs) in the diameter range 1 - 10 nm dispersed on a high surface area electron conductive carbon black powder impregnated with proton conducting ionomer. Although such electrode formulation led to a significant increase of the catalyst efficiency, and therefore to a drastic decrease of Pt loading in electrodes, more efforts have still to be done to bring the state-of-the-art specific Pt power value (ca. 0.5 gPt kW−1) down to 0.2 gPt kW−1 for a large scale implementation of the PEMFCs for vehicle propulsion. In order to achieve this breakthrough, it is necessary to improve the platinum utilization efficiency. It was indeed estimated by Eikerling et al. that Pt activity in conventional catalytic layers reached at most 10 - 20 % of its full potential [[i]]. The limitation in the effectiveness factor of Pt utilization, determined as the ratio of the current delivered and the maximum theoretical current expected, is due to several reasons, particularly to mass transport and ohmic losses [[ii]]. So that, the performance improvement of the catalytic layers requires optimizing factors allowing maximizing the fraction of Pt surface atoms, and those allowing improving mass and charge transport properties of the layers.

The optimizations of the catalytic support and of the catalytic layer architecture could greatly contribute to the enhancement of the effectiveness of the PEMFC catalytic layers, and thus enable further reduction of the Pt content, but classical architectures involve their impregnation by an ionomer in order to achieve the three-phase boundaries. A three-phase boundary corresponds to a confined spatial site where the reactant, the ionic conducting polymer and the electronic conductive substrate are present on the same platinum particle [[iii]], allowing the electrochemical reaction to occur with higher efficiency. However, Eikerling et al. also estimated that ca. 60% of unutilized Pt was that not located at the active three-phase boundary [1].

The grafting of ionic conducting polymer to the surface of platinum nanoparticles allows the creation of new catalyst layer architectures (Figure 1 and Figure 2) that promote both the ionic conduction to the solid electrolyte. The synthesis of polyfunctional catalyst, whose properties can be adjusted depending on synthesis parameters, allows optimize  the electrical, chemical and mass transfer to electrodes and also reduce the overhead platinum. In this communication, we will report the synthesis of complex architecture of platinum supported on carbon, their physico-chemical characteristic, electrocatalytic properties (Figure 3) and fuel cell tests (Figure 4).

[[i]] M. Eikerling, A. Korhyshev, A. Kulikovsky, The Fuel Cell Review 15 (20004/2005) FCR.IOP.org.

[[ii]]  H. A. Gasteiger, W. Gu, R. Makharia, M. F. Mathias, B. Sompalli, in: W. Vielstich, A. Lamm, H. A. Gasteiger (Eds.), Handbook of Fuel Cells – Fundamentals, Technology and Applications, Wiley, Chichester, Vol. 3, 2003,  p. 593-610.

[[iii]] R. O’Hayre, D. M. Barnett, F. B. Prinz, J. Electrochem. Soc. 152 (2005) A439-A444.

Figure 1