From catalysts to cathodes: a controlled-architecture approach for PEMFC electrodes catalyzed by Earth-abundant metals – CAT2CAT

The hydrogen-air Polymer Electrolyte Membrane Fuel Cell (PEMFC) is an emerging technology with applications in portable electronics and automotive propulsion. However, the current use of cathode catalysts based on platinum is a long-term impediment to their sustainable development. The project CAT2CAT aims at advancing PEMFC cathodes based on abundant chemical elements. Novel synthesis strategies based on composites of metal-organic frameworks and ordered carbon supports will be developed to marry the antagonistic properties of microporosity, macroporosity and graphitization that are needed for combined activity, mass-transport and durability. With this novel strategy, our final aim is to integrate active moieties comprising iron, nitrogen and carbon in three-dimensional self-standing electrode architectures having a controlled porosity and graphitization.
The currently-employed electrode fabrication method, whereby catalytic layers are formed from the uncontrolled deposition and drying of a catalytic ink involving the catalyst and a proton-conducting ionomer, does not allow controlling the electrode structure. This results in narrow pores leading to gas molecule/pore wall collisions, in turn leading to slow gas diffusion. Increased access of the reactants (O2, protons and electrons for a PEMFC cathode) to the active sites in these novel electrode structures will lead to a breakthrough in their power performance under air operation, needed for a practical application of this class of inexpensive catalysts.
While previous efforts for a rational control of PEMFC cathode morphology have mainly focused on 2D arrays of vertically-aligned multi-wall carbon nanotubes (MWCNT) or carbon nanofibres (CNF), these efforts have not lead to major breakthroughs in their power performance. Reaching a high activity for oxygen reduction with Fe-N-C catalysts on MWCNT or CNF substrate is difficult due to the highly-ordered nature of these carbon structures. It is now recognized that the Fe-based active centres necessitate the presence of disordered carbon.
To overcome this caveat, we will design self-standing cathodes whose 3D structures relies on MWCNT and CNF with high mechanical and chemical stability but whose ORR activity relies on localized carbon domains with high disorder and high density of Fe-based active sites. These disordered carbon domains will be obtained from the pyrolysis of nanocrystals or thin-films of Metal-Organic-Frameworks deposited on MWCNT and/or CNF. The 3D electrode architecture will be formed via electrospinning of the catalyst (or catalyst precursor) and a selected polymer, forming mats having a controlled web morphology and numerous macropores. Electrospinning is an upscalable production technique under intensive R&D efforts for various application fields. The combined control on the microporous and macroporous structure achieved with this approach will allow us preparing self-standing electrodes with state-of-art ORR activity and beyond state-of-the-art mass-transport properties. Improved electron conduction due to unfailing path between all catalytic sites, improved proton conduction due to higher ionomer-to-catalyst ratio than presently allowed and improved O2 diffusion and/or convection due to the existence of numerous macropores will lead to a breakthrough in the performance of cathodes based on Fe-N-C catalytic sites for oxygen reduction. Compared to present Fe-N-C electrodes, the power performance under air operation will expectedly be improved by a factor 3 relative to the present status, critical for automotive and portable applications. This novel approach of electrode preparation will also be transferable to other electrochemical energy conversion devices for which the displacement of noble-metal catalysts by less expensive but intrinsically less active catalysts is also currently limited by the poor mass-transport properties of necessarily thicker electrodes.