Demonstration of Multiscale Proton Transport in Hybrid Organic-Inorganic Membrane

Summary: Hybrid membranes containing a continuous functionalized silica network were synthesized by combining electrospinning and the sol-gel chemistry. The in-plane proton conductivity was evaluated at 80°C under 80% relative humidity and values of 100 mS/cm have been measured. We investigated the proton transport in these multiscale membranes through different techniques including electrochemical impedance spectroscopy, FG-NMR and QENS. We demonstrated that hydrated protons interact with silica network, but connected pathways for hydrated proton exist in the all membranes that facilitate in fine the proton transport.

 Introduction

Improvements need to be performed before Polymer Electrolyte Membrane Fuel cells will be commercialized for transport application. One of the challenges is to replace Nafion, the state of the art by membranes that are efficient at high temperature (³ 100°C) and low humidity (50% RH). Up to date, the development on polymer electrolyte membranes capable of working at higher temperatures with dry gases is still under consideration. Among the different approaches, the addition of inorganic materials capable of retaining water to perfluorosulfonated ionomers has been developed [1]. But, the results are often poor mechanical properties of the membrane, inorganic particle leaching out of the membrane when the fuel cell is running. Only few examples point out the benefit of these particles and how their size, distribution, functionalization can result in unexpected activity. [2, 3]

We propose here to develop hybrid organic-inorganic membranes, exempt of Nafion. These membranes consist of two intermingled networks of PVDF-HFP and functionnalized silica network.[3] To avoid the phase separation between these two components, we synthesized the hybrid membrane by combining electrospinning and the sol-gel process. To achieve satisfactory proton conduction, a comprehensive study was performed on the proton transport in these multiscale membranes. To do so, we employed different techniques including electrochemical impedance spectroscopy, PG-NMR and QENS.

 Experimental

Field Gun Emission-Scanning Electron Microscopy (FG-SEM), High Resolution Transmission Electron Microscopy were used to characterize the microstructure of the hybrid membranes while Small Angle Neutron Scattering (SANS) was performed to study the hybrid organic/inorganic interfaces. In-plane proton conductivity was determined as function of temperature and humidity. Electrochemical impedance spectroscopy (EIS), 2H NMR relaxation and 1H pulsed field gradient NMR (PFGNMR), quasielastic neutron scattering (QENS) were used to study the proton transport at various scales.

 Results

Hybrid organic-inorganic membranes were fabricated by electrospinning in controlled atmosphere (Relative Humidity ~ 20% and T = 25°C) on aluminum foil which serves as counter electrode. The electrospun solution consists of dissolved polymer and pre-hydrolyzed silica precursors. After processing and heat treatment at 70°C, the hybrid membrane is homogenous, flexible with a thickness ranging from 10 to 100 mm depending on the volume of the used hybrid solution. Characterization techniques including FG-SEM, SANS and HR-TEM demonstrated that the fibers in the membrane are composed of an alternation of a thin layer of polymer and oblong functionalized silica domains connected to each other to form a continuous network.

To characterize the proton transport in the electrospun hybrid organic-inorganic membranes, their proton conductivities were measured at 80°C under various relative humidity (RH) conditions. We found that this complex architecture gives rise to efficient proton transport as conductivity values superior to 100 mS/cm is achieved at 80°C under 80% relative humidity. Lately, macroscopic diffusion coefficient was measured by EIS and values compare well with Nafion, confirming the fast transport of hydrated proton through the membrane. Unexpectedly, hydrated proton and water molecule interact with the silica network, giving rise to local diffusion coefficient one order of magnitude lower than Nafion (PG-NMR and QENS) but diffuse rapidly throughout the membrane, as the diffusion coefficient of water is comparable to Nafion for the range of 1 to 10 mm.

Conclusion

In contrast to what it has been observed in Nafion, we found that the water diffusion coefficient (m to Å) can be locally slowdown (2 x 10^-6 cm²/s) due to weak interactions with the silica network but diffusion coefficient determined by EIS is high (9.6 x 10^-6 cm²/s) at least comparable to one observed in Nafion, the state of art at the mm scale.

Acknowledgements

The authors would like to thank ANR MéconPrhy for the financial support.

References

[1]     Kreuer, K. et al. ; “Chem. Mater., 8, 610–641 (1996).

[2]    Mauritz, K.A. et al. Chem. Rev., 104, 4535–4585 (2004).

[3]   Laberty-Robert C. et al. , Chem Soc Rev, 40 (2011) 961.