Beyond conventional electrocatalysts: hollow metal nanocrystallites for PEMFC applications – HOLLOW

There is an urgent need to address the environmental and energy challenges that the planet is facing. The main issue at stake is the so-called global warming, which is linked to the combination of two phenomena: an increasing demand for fossil fuels in spite of their limited availability and a concomitant increasing release of CO2 in the atmosphere. In this context, developing eco-friendly electrochemical generators, like proton-exchange membrane fuel cells (PEMFCs), which can efficiently operate beyond fossil energies is vital. Yet, the widespread commercialization of PEMFCs requires overcoming several hurdles: (i) improving the kinetics of the cathode reaction (the oxygen reduction reaction, ORR), (ii) improving the durability of the cathode material and (iii) decreasing the cost of the catalytic layers. Despite the extensive efforts in research and development made during the last 15 years, Pt-based nanoparticles remain the only – but unstable – electrocatalyst able to accelerate efficiently the rate of the ORR in PEMFC operating conditions. Improving the ORR activity (x 2-3) and decreasing the catalyst cost is currently best achieved using homogeneous Pt-M/C nanoalloys (M being a transition metal, M = Co, Ni, Cu or Fe) or core-shell nanoparticles composed of a Pt-enriched shell and a metallic or alloyed core. The loss of M atoms causes (i) a gradual loss in the intrinsic catalytic activity of the cathode electrocatalyst, (ii) the depreciation of the H+/O2 mass-transport properties of the catalytic layer ionomer/proton exchange membrane (PEM) and (iii) the formation of radical species in the PEM. Even nanostructured bimetallic Pt-M thin films (NSTF), which are composed of large crystallites, do not prevent the dissolution of the less noble element. Therefore, the dilemma of achieving cost-competitive, highly active (platinum-group metal mass normalized) and robust electrocatalysts is still unsolved yet. The HOLLOW project aims at synthesizing; characterizing and investigating the electrocatalytic activity and the stability of a novel class of nanomaterials that are able fulfil cost, performance and stability requirements of PEMFCs. Hollow Pt/C nanocrystallites (i.e. containing a cavity/void in their centre) will be synthesized using the galvanic replacement of a transition metal M by Pt, and the nanoscale Kirkendall effect. Preliminary experiments performed at LEPMI proved this concept feasible; however, further improvements are required to decrease the M content in the final electrocatalyst, control the number of Pt monolayers in the shell and the size of the central cavity and of the outer diameter. The ORR specific activity of the synthesized hollow Pt/C nanocrystallites reached a 4-fold enhancement over standard solid Pt/C nanocrystallites of the same size without any optimization. We strive to understand the physical reasons of this activity enhancement. Such objective will require atomic-scale chemical and structural characterisation with advanced electron microscopy and X-ray techniques, such as high-resolution aberration-corrected scanning transmission electron microscopy with a high-angle annular dark field detector, X-ray energy dispersive spectroscopy, electron energy-loss spectroscopy in spectrum-imaging mode, and in situ X-ray absorption spectroscopy. The information derived from this broad set of physicochemical techniques will provide crystallographic guess inputs for density functional theory calculations aimed to understand the mechanisms at work during the formation of the hollow nanoparticles, determine their electronic properties, rationalize their improved ORR activity, and tailor optimized electrocatalysts. Ultimately, the synthesis of the best-performing hollow Pt/C nanocrystallites will be scaled-up to perform PEM fuel cell tests and compare their activity to reference electrocatalysts in real membrane electrode assemblies.