Preparation of Sulfonated derivatives of Zirconium Phosphates and Their Applications in Composite Proton Exchange Membranes
Date Issued
2011
Date
2011
Author(s)
Pai, Cheng-Wei
Abstract
Most of the inorganic additives used in organic/inorganic composite proton exchange membranes (PEMs) generally exhibited low intrinsic proton conductivity and difficulty in forming homogeneous dispersion of nanoparticles within the membrane. Therefore, the development of inorganic nanoparticles having good proton conductivity and capability of homogeneous dispersion is critical for the further improvement of the composite PEMs. In this study, two different strategies are employed to achieve the above mentioned targets. The first approach is to develop a new methodology of the preparation of the composite membranes using highly proton conductiveα-zirconium phosphate sulfophenylenphonates (α-ZrSPP). The second approach is to synthesize zirconium phosphate-poly(styrene sodium sulfonate) (ZrP-PSStNa) composite nanoparticles with high proton conductivity and good solubility as the additives for the composite membranes..
In the first part, the membranes are prepared by solvent casting from the blends of the reaction mixture of as-synthesized α-ZrSPP and Nafion® solution. The effect of composition, particle size and loading amount of α-ZrSPP on the distributions of α-ZrSPP within the membranes and the transport properties are systematically investigated. SEM cross-sectional images suggest that smaller particles with lower zirconium sulfophenylphosphonate (SPPA) content are more feasible to form homogeneous blends within the composite membrane. With increasing loading ofα-ZrSPP, water uptakes and ion exchange capacities of the composite membranes increase.α-ZrSPP having lower SPPA contents generally lead to higher IEC. Proton conductivity exhibits complicated trends as affected by both the properties and the distribution ofα-ZrSPP as well as the morphologies of the composite membrane. Generally, proton conductivity increased first and then decreased with increasing loading ofα-ZrSPP, indicating the existence of an optimal loading and overloading of ZrSPP would damage the proton conduction pathways. Large ZrSPP particles with lower SPPA contents lead to the highest proton conductivity of the composite membrane. In addition, the incorporation of small α-ZrSPP with high SPPA contents resulted in significantly suppressed methanol permeability and dramatic improved selectivity, at most a 5 fold increase comparing with the pristine Nafion recast . The composite membranes also show improved proton conductivity at low relative humility and elevated temperature, probably owing to the water retention and the proton conduction originated from ZrSPP.
In the second part, two series of ZrP-PSStNa composite particles with different lengths of PSStNa are synthesized by changing the solvent (methanol for M-particles or NMP for N-particles) for polymerization, and the morphologies and the sizes of these particles are significantly affected by the solvent used. The membranes are prepared by solvent casting from the blends of the solution of ZrP-PSStNa and the Nafion solution. The effect of the molecular weight of PSStNa, the particle size/morphology and the loading amount of ZrP-PSStNa on the dispersions of the composite particles within the membranes and the transport properties are also systematically investigated. With increase loading of ZrC-PSStNa, water uptakes and IEC of the composite membranes increase and ZrC-PSStNa having large molecule weight of PSStNa generally lead to higher IEC. Proton conductivity behaves complicatedly, affected by the intrinsic properties and the dispersion of ZrC-PSStNa as well as the morphologies of the composite membrane. Generally, with increasing loading of ZrC-PSStNa, proton conductivity of the composite membranes containing M-particles increased then decreased, but the membranes containing N-particles increased consistently. The SEM cross-sectional images show that N-particles with PSStNa of larger molecule weight can be blended more homogeneously within the composite membranes. The composite membranes containing low loading of N-particles with large molecule weight PSStNa show better selectivity owing to suppressed methanol crossover. Furthermore, the proton conductivity of the composite membranes at low relative humility and high temperature was enhanced significantly, up to a 2.5 fold increase comparing with the pristine Nafion recast.
Subjects
Zirconium phosphate
α-zirconium sulfophenylphosphonate
proton exchange membrane
compositemembrane
composite nanoparticle
poly styrene sodium sulfonate
Type
thesis
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