Composite Proton Exchange Membranes Based On Chitosan And Functionalized Carbon Nanotubes

Proton exchange membrane fuel cell(PEMFC)has attracted numerous attentions due to it's a novel energy conversion device with high effiency.Proton exchange membrane(PEM)is one of the most critical parts of the proton exchange membrane fuel cells,which takes the responsibility of preventing the fuel leakage and conducting protons as well as blocking electrons transfer between the anode and cathode.The most popular membrane is the Nafion^?membrane developed by Du Pont Company.However,the state-of-the-art Nafion^?membranes suffer from two major obstacles including high methanol crossover and declined proton conductivity under high-temperature and low-humidity condition when applying in fuel cells.Therefore,search for new ways to design and fabricate low-cost and high-performance PEMs has the great application value and theoretical significance in accelerating the commercialization of fuel cells(FC).Chitosan(CS)is one of the most promising alternative materials for PEM due to its low cost,environmental friendliness,available for high temperature and low humidity environment,low methanol permeability,and ease of functionalization by reacting with the-OH and-NH₂ on the CS backbone.Nevertheless,the insufficient flexibility,inadequate mechanical strength,low thermal stability,and particularly the poor proton conductivity restrict the CS based membranes when used as the substrate of PEM.In this paper,the novel PEMs were prepared by self-assemblying of the functionalized CNTs with different surface coating materials and CS in dilute solution.Firstly,the functionalized coating on CNTs surface could improve the dispersibility in CS matrix and strengthen the interface interaction,which led to the enhancement in the mechanical property and thermal stability.Secondly,functionalized coating on the CNTs surface could avoid the directly contact among CNTs,which eliminated the risk of short circuit.Lastly,the inert surface of CNTs was changed due to the coating layer,and then helps to bulid continuous channel for proton transfer,thus improved the proton conductivity of CS-based composite PEMs.This paper expands the application of CS in PEMs,and will provide new methodologies and theoretical bases of producing high-performance proton exchange membranes.The main content and result of this thesis were listed as follow:1.TiO₂@CNTs was prepared by a facile sol-gel method,and then it was used as inorganic filler to self-assembly with CS in dilute solution to fabricate CS/TiO₂@CNTs composite PEMs.The interactions between-OH group in TiO₂and-NH₂ group in CS improved the dispersity and the interface interaction.And the TiO₂coating on the surface of CNTs effectively avoided the short circuit risk of CNTs in the CS matrix.The thermal stability,mechanical stability,oxidation stability,proton conductivity,methanol permeability and the single cell performance of the CS/TiO₂@CNTs composite membrane was improved.2.The STi@CNTs was prepared by using the sulfonic acid to modify the TiO₂coating on the CNTs surface.Then,the CS/STi@CNTs composite membrane was obtained by self-assemblying the STi@CNTs and CS in a dilute solution.Owing to the STi coating,the dispersion ability of carbon nanotubes and interfacial bonding were obviously improved,which made the composite membranes exhibit better mechanical properties than that of pure chitosan membrane.Moreover,with the aid of the excellent proton conductivity of the STi coating,the proton conductivity of CS/STi@CNTs composite membranes was remarkably improved.The CS/STi@CNTs-1 composite membranes reached the highest proton conductivity of0.043 S cm^-1 at 80?,which was about 1.9 times higer than that of pure CS membrane.3.Solvent-free carbon nanotube fluids(CNT fluids)with liquid-like behavior are prepared through an ion exchange method to graft organic long chain onto CNTs surface.Then CNT fluids was used to self-assmbly with CS in dilute solution to fabricate CS/CNT fluids composite membranes.The organic long chain on the CNTs surface promote the dispersion of CNT fluids in the CS matrix and improve the interfacial compatibility between CNTs and CS,subsequently CNT fluids produced simultaneously reinforcement and plasticization effects on CS membrane.Moreover,the grafted organic long chain can eliminate the short circuit risk of CNTs in CS matrix,and the-SO₃^-on the organic long chain can provide extra proton conducting sites,thus the CS/CNT fluids-3 composite membrane reached the highest proton conductivity of 0.044 S cm^-1 at 80? and a maximum power density of 48.46 m W cm^-2 at 70? and 2 M methanol concerntration.4.Polydopamine(PDA)functionalized CNTs(PDA@CNTs)was prepared by oxidative self-polymerization of dopamine on CNTs surface under mild condition.Then,the PDA@CNTs was self-assemblying with CS in dilute solution to fabricate CS/PDA@CNTs composite membrane.PDA coating on CNTs could promote the dispersion of CNTs in CS matrix and improve the interface compability between CNTs and CS,CS/PDA@CNTs composite membrane exhibited better thermal stability,mechanical property,oxidation stability and dimension stability.Moreover,with the help of 1 dimension of CNTs and the electrostatic interactions between the-SO₄^2-and the-NH₃⁺in PDA@CNTs and the CS,the proton conductivity of CS/PDA@CNTs-2 composite membrane reached to the highest proton conductivity of 0.028 S cm^-1 at 80? and a maximum power density of 42.6 m W cm^-2 at 70? and 2 M methanol concerntration.5.Super inorganic proton conductor boron phosphate(BPO₄)was utilized to functionalize the CNTs by using a facile PDA-assisted sol-gel method to yield BPO₄@CNTs.Then this new additive was used to self-assembly with CS in dilute solution to fabricate CS/BPO₄@CNTs composite membrane.The incorporation of BPO₄@CNTs could promote the dispersion of CNTs in CS matrix and improve the interface compability between CNTs and CS,thus lead to the improvement in the thermal stability,mechanical property and oxidation stability of the BPO₄@CNTs composite membrane.Moreover,the BPO₄ on the surface of CNTs can provide a new proton conducting pathway,the CS/BPO₄@CNTs composite membrane showed better single cell performance than that of pure CS membrane.The CS/BPO₄@CNTs-2 composite membrane achieved the maximum proton conductivity of 0.040 S cm^-1 at 80?,and a maximum power density of 49.0 m W cm^-2 at 70? and 2 M methanol concerntration.