Zirconia, Sulphated Zirconia and Zirconium Phosphates as additives for membranes in PEM Fuel Cell

Abstract

This research investigates the impact of zirconia nanoparticle in conductivity, water uptake, fuel crossover and fuel efficiency of modified Nafion® membranes. Synthesized water-retaining mesoporous zirconia nanoparticles (ZrO2) were used to modify Nafion® membrane in order to enhance the thermal properties, water uptake, proton conductivity and mechanical strength of composited membrane for fuel cell applications. Recast and impregnation methods were used to prepare a nanocomposite membrane with required weight% of zirconia nanoparticles. The mechanical stability of modified membranes has become a priority for fuel cell applications as the membranes must endure all the fuel cell operations (to prevent crossover of the fuel while still conducting). Their mechanical stress and yielding stress in the recast and impregnation methods compared with the commercial Nafion® membrane were observed under tensile tests. The incorporated membrane with zirconia nanofiller shows an improvement in mechanical strength, due to the hydrophilic phase domains in the nanocomposite membrane. The water contact angle and water uptake of the composited membrane were measured. The modified membranes with zirconia nanoparticles showed a significant improvement in water uptake and contact angle leading to enhanced hydrophilicity when compared to unmodified hydrophobic Nafion® membrane. This shows the potential for use as electrolytes in fuel cell applications. Zirconia nanoparticles were further impregnated with sulfuric acid and phosphoric acid to introduce the additional acid sites for absorption of water. In addition, zirconium phosphates (ZrP) and sulfated zirconia (S-ZrO2) were incorporated into Nafion® 117 membrane by impregnation method to obtain a reduced methanol permeation and improved proton conductivity for fuel cell application. The mechanical properties and water uptake of Nafion® membrane incorporated with zirconium phosphates and sulfonated zirconia nanoparticles were much more improved when compared to the commercial Nafion® 117, due to the presence of acid site within the nanoparticles. Furthermore, the results showed that incorporating ZrP and S-ZrO2 nanoparticles enhanced proton conductivity and IEC of modified Nafion ® membrane as they sustain water affinity and strong acidity. The results show that nanocomposite membranes have low water content angle, improved thermal degradation, higher conductivity and lower methanol permeability than commercial Nafion® 117 membrane, which holds great promise for fuel cell application. The Nafion®/ sulfated zirconia nanocomposite membrane obtained a higher IEC and water uptake due to the presence of SO 2-4 providing extra acid sites for water diffusion. To reduce the agglomeration of ZrO2 nanoparticles and improve the water diffusion, ZrO2 was electrospun with polyacrylonitrile (PAN) solution to obtain a 1D morphology. The recast method was used to synthesize the high thermal and mechanical stability of Nafion® membrane incorporated with polyacrylonitrile (PAN) nanofibers. The modified Nafion® membranes exhibited improved fuel cell efficiency when tested in direct methanol fuel cells with a high proton conductivity due to incorporating PAN/Zr nanofibers that retain water within the membrane. Moreover, nanocomposite membranes achieved a reduced methanol crossover of 4.37 x 10-7 cm2 /s (Nafion®-PAN/ZrP nanofibers), 9.58 x 10-8 cm2 /s (Nafion®- PAN/ZrGO nanofibers) and 5.47 x 10-8 cm2 /s (Nafion®-PAN/Zr nanofibers), which is higher than 9.12 x 10-7 cm2 /s of recast Nafion® membrane at the higher concentration of 5M. All the blended membranes showed increase in power density at a temperature of 25 °C in comparison with pristine recast Nafion® membrane (76 mW·cm−2 , 69 mW·cm−2 , 44 mW·cm−2 , 18 mW·cm−2 ). Finally, incorporating electrospun PAN/ Zr nanofibers into Nafion® membrane has successfully reduced the use of Nafion® solution that will eliminate the cost problems, while improves the protons conductivity and the methanol permeability which influence the fuel cell efficiency and long-term stability.