Hybrid Inorganic/Organic Membranes for Medium Temperature PEM Fuel Cells

Significant efforts have been made on finding clean and efficient energy sources that will be able to fulfil the needs at a low cost and a minimal environmental impact. Polymer Electrolyte Fuel Cells (PEMFCs) are ideal to work as a clean energy source because there are fuel by hydrogen or methanol and oxygen and generates electricity and water as a secondary product. In PEMFCs research one of the biggest challenges is to be able to work at or above 100 °C because the efficiency of the fuel cell increases with temperature while at the same time the water managing becomes easier, the drawback lies in the need of liquid water in the membrane for good proton transfer. The aim of this work was, using a sulfonated Poly (Ether Ether Ketone) (SPEEK) ^(1,2)together with inorganic fillers that contain water i.e. lithium-based ceramic particles, to try to develop a membrane able to fill these new requirements.

This research is based on a previous studied polymer, sPEEK, and the approach taken is to improve the conductivity by increasing the water content inside the membrane through the use of inorganic compound as ceramic particles that are able to create an adsorbed water layer on top, we believe this extra water content is helping to increase the conduction paths on the membrane and also capable of retain water at higher temperatures and, assuming that the Grötthus hopping mechanism for proton conduction is taking place, increasing the proton conductivity of the sPEEK to the level of Nafion with lower cost for materials and manufacture.

Commercial PEEK (Vitrex 450 PF)  with a Mw: 39200 gr/mol was sulfonated in a reactor (95/5 v/wt.) with sulphuric acid (98% sigma-aldrich) at 55°C for 5 hours (sulfonation degree [SD] 60% obtain using back titration). Before the reaction, the PEEK was dissolve in the sulphuric acid completely. The solution was then washed with demineralized water until the pH was neutral. ⁽¹⁾The modified polymer was dried in a vacuum oven for 1 week at 120°C.

The lithium-based ceramic particles were taken from the cathode materials used in Li ion batteries. ⁽³⁾ LiBPO₄ and Li⁺TiO are the inorganic particles used together with the sPEEK matrix.

The membranes were made with sPEEK and different composition in weight of lithium-based ceramic particles. The polymer was dissolve in N,N-Dimethylacetamide (DMAc) at 120°C with a continuous stirring, when the polymer was completely dissolved in the DMAc, the inorganic compound was added and keep under stirring until an homogeneous solution was obtained. The solution was then casted in to petri dishes and left in a fumed hood for three days for evaporation. The membranes obtained was removed with demineralized water and put it in an vacuum oven at 55°C for final removal of traces of the solvent and water. a pre-treatment was done to the membrane in a solution of 1M H₂SO­₄to activate the protons in the membrane for 1 day, after, the membrane was washed with demineralized water and ready for testing.

A material and electrochemical characterization was performed to the hybrid membranes, that includes Differential Scanning Calorimetry (DSC), Thermo Gravimetrical Analysis (TGA), X-Ray Diffraction (XRD), and Electrical Impedance Spectroscopy (EIS). See Figure 1 for partial results.

Changes in the water content of the samples can be observe in the TGA measurements as a consequence of the extra water incorporated by the inorganic filler. Difference in the glass transition temperature of the polymer matrix are observed by DSC, presumably by the interaction of the sulfonic groups of the matrix with the inorganic fillers, this behaviour also changes depending on the composition of the membrane. The XRD result shows that the crystalline structure of the inorganic compounds is maintained inside the membrane even in the presence of a strong acid group as the -SO₃.

Impedance spectroscopy measurements were made to measure the conductivity of the membranes using a commercial cell able to control the temperature and relative humidity in a confined environment. The measurement preform was a 4 point probe in plane conductivity measurement at different temperatures and constant relative humidity. The Activation Energy for the proton conduction was calculated and compare to Nafion and pristine sPEEK in order to try to understand the proton conduction in the new hybrid membranes. The complete images, graph, tables and further results will be presented in the conference.

References

1. Journal of Applied Polymer Science, 82 (2001) 2651-2660.
2. Journal of Membrane Science ,226 (2003) 159–167.
3. Industrial and Engineering Chemistry  Research, 44 (2005) 7617-7626.