Study On Synthesis And Performances Of Membrane Materials Used In Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) which convert chemical energy efficiently into electrical energy via oxidation and reduction reactions are receiving considerable attention as transport, stationary, and portable future power sources because of their low emissions and high energy conversion efficiency. The proton exchange membrane (PEM), which provides ionic pathways to transfer protons in PEMFCs as well as a separator between the electrodes for the reactant gases, protons, fuel and electrons, is the key component of the PEMFCs. Nafion, one of the perfluorosulfonic acid membranes, is the current state-of-the art PEM material, due to its good mechanical, thermal and chemical stability, in addition to high proton conductivity at moderate temperatures (< 90 oC) with relatively low ion exchange capacity (IEC). However, several drawbacks of Nafion, such as high cost, high methanol permeability, low humidity and a major reduction in conductivity at high temperatures, have led researchers to investigate promising alternatives. Presently, numerous research efforts are focusing on the sulfonated aromatic polymers because of their low cost, high proton conductivity, and their high thermal, chemical and mechanical stabilities.Poly(aryl ether ketone)s, the important engineering resins, are well known for their excellent high thermal stability, mechanical properties, and oxidation resistance. Sulfonated reaction could be accomplished in two ways: post-sulfonation of polymers and copolymerization using sulfonated monomers with non-sulfonated monomers. The latter affords some advantages over postsulfonation, because it can easily control the position and the content of sulfonated groups, thus further control ion-exchange capacity (IEC) and proton conductivity. Also this method avoids cross-linking and other side reactions. This method makes the sulfonic acid content of copolymers rather easy and facilitates detailed studies of the relationship beteween structure and property. Compared to post-sulfonatin, degree of polymerization of direct copolymerization is lower than that of post-sulfonation because of difference in reactive activeness. Several groups, including our own, use the approach of direct copolymerization of sulfonated monomers, which enables better control of morphology and the number of sulfonic acid functions, so as to synthesize a series of sulfonated copoly(aryl ether ketone)s and sulfonated copoly(aryl ether sulfone)s which differ from each other by the structure of the bisphenol monomers.In this thesis, we used following way to solve these problems and improved the PEM properties:(1) Synthesized a novel series of SPEEKKs with 3,3',5,5'-tetramethyl-4,4'-biphenol (TMBP)(2) Synthesized a novel series of SPEEKKs with bisphenol A(3) Composite membranes for improving the methanol properties(4) Composite membranes for improving the mechanical properties(5) The Pt/Ag nanotubes catalyst were synthesized for fuel cell as an attempt.In chapter 2, a novel series of SPEEKKs with different degrees of sulfonation (Ds) were synthesized from 1,3-bis(3-sodium sulfonate-4-fluorobenzoyl)benzene (1,3-SFBB-Na), 1,3-bis(4-fluorobenzoyl)benzene (1,3-FBB) and 3,3',5,5'-tetramethyl-4,4'-biphenol (TMBP) by aromatic nucleophilic polycondensation. FT-IR spectroscopy was used to confirm the chemical structures of the SPEEKKs. The Ds values were measured by both titration and 1H-NMR data and the results agree with the calculated ones, which clearly suggest that Ds can be readily manipulated by controlling the amount of sulfonated monomer added. The high intrinsic viscosities of the polymers mean that the polymers have high molecular weights. The thermal stabilities of the SPEEKKs in acid and sodium forms were characterized by thermogravimetric analysis (TGA). All the polymers in the sodium form have better thermal stability than those of the acid form. The second thermal degradation in both the sodium and the acid forms around 450 oC are assigned to the degradation of the polymer main chain, indicating that the incorporation of 3,3',5,5'-tetramethyl-4,4'-biphenol did not decrease the thermal stability due to the strong–CH3 bonds, the SPEEKK polymers in acid form have high thermal stability. SPEEKK membranes were easily prepared with solution casting method. The water uptake continues to increase with Ds and temperature, and is especially high for SPEEKK-80, which was about 300 %. The swelling data of the SPEEKK-80 was not obtained at 80 oC because the membrane was unable to maintain a smooth morphology after several hours in water at 80 oC, although the water uptake data could still be obtained at this moment. The proton conductivity of membranes increased when Ds (or IEC) and temperature increased. SPEEKK-60, -70 and -80 membranes showed higher proton conductivities than 10-2 S cm-1 at 80 oC and 100% relative humidity, which is the lowest value of practical interest for use as PEM in fuel cells. The SPEEKK membranes exhibit increased methanol diffusion coefficients at 25 oC depending on the Ds. These membranes show low methanol diffusion in the range of 8.32×10-9 to 1.14×10-7 cm2s-1, which is much lower than that of Nafion (2×10-6 cm2s-1) at the same temperature. The mechanical properties data indicate that the SPEEKK membranes are strong and tough enough for use in PEMFC. Compared to Nafion, SPEEKK membranes show relatively better mechanical stability at ambient condition. And the results show the SPEEKK membranes are promising alternatives to Nafion membranes in DMFCs.In chapter 3, a novel series of SPEEKKs were prepared using BPA as the biphenol. Also we used 1,3-bis(3-sodium sulfonate-4-fluorobenzoyl)benzene (1,3-SFBB-Na), 1,3-bis(4-fluorobenzoyl)benzene (1,3-FBB). The SPEEKK with different sulfonated degrees were prepared by direct synthesis of sulfonated monomer method. The structures of SPEEKKs were characterized by FT-IR and 1H-NMR. 1H-NMR was used to identify the molecular structure of the sulfonated polymers and to evaluate the degree of sulfonation (Ds). The Ds calculated from 1H-NMR was well consistent with values From the TGA curves, we found that the temperatures of 10% weight loss (Td10%) for the sulfonated polymers were all greater than 340oC. This showed that the polymers had high thermal stability. The solubility properties resulted polymers show excellent solubility in aprotic dipolar solvents such as DMF, DMSO and NMP. The water uptake continued to increase with Ds and temperature and particularly sharply increased for SPEEKK-5. Water uptake of the polymers was lower than that of Nation 117. This may suggest the influence of the position of sulfonate groups in polymer on the microstructure of SPEEKK membranes. The water diffusion coefficient of SPEEKK membranes increases with the increment of Ds. The water diffusion of SPEEK-1 to SPEEKK-5 membranes calculated from the slope of the line is 2.987×10-9, 8.870×10-9, 1.764×10-8, 4.685×10-8, and 9.334×10-8m2s-1, respectively. From the results, the velocity of water loss enhance with increasing Ds. The proton conductivity of membranes increase when Ds (or IEC) and temperature increase. Except for SPEEKK-1 and SPEEKK-2, the other three SPEEKK membranes show higher proton conductivity than 10-2 S/cm at room temperature, which is the lowest value of practical interest for use as PEMs in fuel cells. SPEEKK membranes showed methanol diffusion at the range of 4.29×10-8 - 5.03×10-7 cm2/s, which is much lower than Nafion (2×10-6cm2/s) at the same temperature. SPEEKK membranes show relatively better mechanical stability compared with Nafion.In chapter 4, we prepared SPEEK/ETS-10 composite membranes introducing the inorganic ion exchange materials during the membranes synthesis. The performances of the SPEEK/ETS-10 composite membranes were showed in this chapter. The methanol permeability are 4.7004×10-7 cm~2/s 3.0526×10-7 cm~2/s 1.0335×10-7 cm~2/s, from SPEEK/ETS-10-5, -10 to -15, respectively, compared to 7.07×10-7 cm~2/s , the value of bare SPEEK membrane. The Young's Modulus of the membranes was 1.07GPa, 1.04GPa and1.01GPa, while the Tensile Strength was 44MPa, 47MPa and42MPa, and the Maximum elongation were 23%,11%and11% ,respectively, indicated that the mechanical properties were good enough for use in DMFCs applications. The proton conductivities were 0.100S cm-1 for SPEEK/ETS-10-5 at 80oC, showed that the membranes had the potential using in the DMFC. All the results show the ETS-10 can be used in the composite membranes and the membranes are promising alternatives to Nafion in DMFCs. And the 5% percent is the better one in these membranes which need further researches. In chapter 5, we prepared SPEEK/CNTs composite membranes by cast solution method. The carbon nanotubes included four different organic group functional carbon nanotubes, bare carbon nanotubes, -COOH functionalized, -SO3H functionalized, -NH2.functionalized carbon nanotubes. The purpose of this chapter is to see the role of CNTs played in composite membranes. FTIR was used to show the successful preparation of the composite membranes. TGA results showed that the thermal properties of the composite membranes were lower than pure SPEEK membranes. The SPEEK/CNTs composite membranes have better mechanical properties than SPEEK membrane, although the value didn't change too much along with the content of CNTs. The composite membranes got by different CNTs were good enough for use in DMFCs applications. The water uptakes of the composite membranes were changed by mixing the CNTs with SPEEK. The water uptake of SPEEK/CS05 and SPEEK/CS10 were larger than other membranes because of the–SO3H group.The methanol permeabilities were lower than the value of Nafion membrane. The proton conductivities of the membranes didn't change too much with different CNTs with a rule of SPEEK/C-SO3H>SPEEK/C-COOH>SPEEK/C-NH2. All the results show the composite membranes can be used in PEMFC. And the SPEEK/C-SO3H membrane is used for the future development.In Chapter 6, we successfully synthesized Pt nanotubes using two novel Pt precursors. SEM and TEM were used to get the pictures of Ag nanowires and Pt nanotubes. CV curves showed the Pt nanotubes have good electricity properties for the following research in fuel cell.