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Electrospinning: a method to elaborate membrane-electrode assemblies for fuel cells

Final Report Summary - SPINAM (Electrospinning: a method to elaborate membrane-electrode assemblies for fuel cells)

The SPINAM project proposed an integrated approach to nanostructuring of components of the functional core of proton exchange membrane fuel cells (PEMFC), the membrane-electrode assembly. Concerning the membrane, the objective was to induce nanoscale phase separation between a support component and the matrix polymer to promote proton transport and to improve mechanical properties. For the electrodes, the project targeted the design and elaboration of novel architectures based on nanofibres (carbon, conductive ceramics) used as substrates for the uniform deposition of thin layers of noble metals by combining spinning processes and different methods of electrocatalyst deposition.
The methodology chosen to induce nanostructuring is electrospinning, which allows one-dimensional nanomaterials to be obtained in a controlled fashion with a great versatility of morphology and composition, and which can be thus applied both in the electrodes and in the ionomer membranes of the fuel cell. These innovative concepts and approaches have led to fuel cell materials with performance and durability exceeding those of the state of the art, and to the development of a radically new methodology for fuel cell materials elaboration.
To develop these novel membrane architectures required multi-scale organisation of nanofibres. The functions of proton conduction and mechanical strength are dissociated between the electrospun web and the matrix. We demonstrated that synergistic effects between them offer additional possibilities for tuning membrane properties, in particular, specific interactions such as ionic cross-linking improve the fibre/ionomer adhesion/interface with overall improvement of dimensional, proton conduction and mechanical properties. Amongst the various composite membrane compositions were those based on a short-side-chain perfluorosulfonic acid ionomer and nanofibres of multifunctional polymers such as triazole-functionalised polysulfone and polybenzimidazole that both have inherent basic functionalities. This basic character leads to ionic cross-linking with partial proton transfer from the ionomer to the nanofibers, which leads to significant increase in the membrane mechanical strength as determined by its Young’s modulus or toughness. These membranes have excellent stability in accelerated ageing testing such as open-circuit voltage hold and wet-dry cycling compared to a benchmark membrane, despite being fabricated from a lower equivalent weight ionomer.
Another important achievement of SPINAM is the development of a novel class of self-standing nanofibrous electrodes comprising an electrodeposited conformal layer of platinum at the surface of an array of electrospun carbon nanofibres. These structured electrocatalyst layers have high electrical conductivity for fast charge transport and interconnected macroporosity for efficient reactant/product mass transport. They can be used directly in a membrane electrode assembly, where their highly porous structure is exploited. To favour electron transfer through the thickness of the nanofibrous electrodes, we developed an approach enabling fuller control of the nanofibre array architecture by growing a secondary structure of whiskers of carbon nanotube on a primary nanofibre structure. Multi-layered hierarchical electrodes were achieved by integrating this nano-whisker/nanofibre layer into a layer by layer deposition of nanofibers of increasing porosity within the electrode from the side in contact with the membrane (more dense) to the side in contact with the gas diffusion layer (more porous).
Beyond carbon nanofibre structures, novel electrochemically stable supports of metal oxide nanofibres were also developed. Doped tin oxide fibre-in-tubes have the potential to replace carbon supports in PEMFC due to their high corrosion resistance, electrical conductivity and strong interaction with the metal catalyst that improves the overall stability and activity of the catalyst layers.
SPINAM has opened up a new area for study since the innovative methodology associating deposition methods along with the new materials offer significant perspectives for application in the development of water electrolysis and energy storage materials.