Synthesis and characterization of novel proton-conductive composite membranes derived from the hybridization of metal oxyhydroxide nanoparticles and organic polymers for fuel cell applications

Abstract

<p>A fuel cell is a device that converts chemical energy directly to electricity. Fuel cells have high energy conversion efficiencies with potentially less release of environmental pollutants. Therefore, they are regarded as a promising technique to address future energy needs. The proton exchange membrane (PEM) is one of the fundamental parts in fuel cell system and the synthesis of novel PEMs is an active area of research. In this work, several metal oxyhydroxide (FeOOH and CoOOH) + organic polymers (polyvinyl alcohol and polysulfone) composite proton exchange membranes are synthesized and their proton conductivities, surface properties, spatial structures and mechanical strength are investigated by Electrochemical Impedance Spectroscopy (EIS), Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, scanning electron microscope (SEM) and tensile resistance measurement. </p> <p>Experimental results of EIS reveal that the protonic conductivities of PVA membrane filled with FeOOH treated by sonication were high at moderate and high RHs (6.54×10<super>-3</super> S/cm at 81% RH and 0.0106 S/cm at 97% RH), and are perhaps comparable to those of Nafion measured by the same method (1.87×10<super>-3</super> S/cm at 81% RH and 4.26×10<super>-3</super> S/cm at 97% RH). A PVA membrane filled with FeOOH treated only by acetic acid had lower protonic conductivities (1.33×10<super>-3</super> S/cm at 81% RH and 3.18×10<super>-3</super> S/cm at 97% RH) compared with the PVA membrane filled with sonication-treated FeOOH at moderate and high RHs. All cobalt-filled PVA membranes had lower protonic conductivities than iron-filled PVA membranes. The protonic conductivities of FeOOH-PVA and CoOOH-PVA composite membranes were close to those of corresponding FeOOH and CoOOH nanoparticles at high RHs, but the protonic conductivities of composite membranes dropped considerably as RH values decreased (<60%). FeOOH-polysulfone and CoOOH-polysulfone composite membranes exhibited low protonic conductivities at all RHs (most conductivity values were below 10<super>-5</super> S/cm), suggesting that polysulfone is not an ideal material for support of proton-conductive metal oxyhydroxides. </p> <p>Experimental results of ATR-FTIR suggest that the reaction with acetic acid can lead to a decrease of hydroxyl groups on the surface of FeOOH nanoparticles, as evidenced by the fact that the protonic conductivities of samples treated by acetic acid only were lower than those of samples treated by sonication. The spectrum of FeOOH (sonication) + PVA composite membrane displayed the characteristic absorption band of FeOOH very clearly, which implies that a high percentage of FeOOH is distributed on the surface of the composite membrane. However, there were no significant differences between the spectra of pure PVA and CoOOH (sonication) + PVA, which means that there are few CoOOH nanoparticles distributed on the surface of the composite membrane. Similarly, no significant differences between the spectra of pure polysulfone and polysulfone + metal oxyhydroxide composites were observed. These results suggest that the percentage of metal oxyhydroxide particles that are distributed on the surface of the composite is low, and is consistent with the fact that the polysulfone-based composite membranes had very low protonic conductivities at all RHs.</p> <p>The SEM images of ferroxane (carboxylate FeOOH) + PVA composites revealed that there was significant aggregation among ferroxane nanoparticles in the composite, which results in a decrease of specific surface area of ferroxane and a drop of protonic conductivity of the composite at low RHs.</p> <p>The tensile force resistance of the ferroxane-PVA composite membrane was tested and the stress-strain curve of the membrane was obtained, which showed that the composite membrane had a higher tensile force resistance than the Nafion membrane. A higher content of ferroxane in the ferroxane-PVA composite membrane resulted in a stronger tensile resistance. Though the increase of ferroxane in the composite also resulted in a decrease in elasticity, the elasticity of the composite membrane with highest ferroxane content (&#949;_B = 0.381) was still comparable to that of Nafion membrane (&#949;_B = 0.417). In summary, the ferroxane + PVA composite membrane had better mechanical properties than Nafion.</p> <p>Based on the findings achieved in this paper, it would appear that the FeOOH + PVA composite membranes are like Nafion, limited with respect to low protonic conductivities at low RHs. To overcome this drawback, several possible approaches are proposed, including the exploration of selection methodology for quick pick-up of polymers that can work well at low RHs, prevention of nanoparticles from aggregation during the preparation process, exploration of new metal oxyhydroxide nanoparticles that may possess high protonic conductivity at low RHs and synthesis of composite membrane that can endure high-temperature sintering.</p>