| All vanadium redox flow battery?VRFB?is a novel green secondary battery,which has such advantages as adjustable capacity,deep charge-discharge,high efficiency,easy operation and maintenance,long life-time and low cost,etc.The membrane is one of three key components of VRFB,which separates the anolyte and catholyte to reduce the self-discharge of battery and transfers protons to complete the circuit.Ideal membranes for VRFB application should have high proton conductivity and chemical/electrochemical stability,low vanadium permeability and cost,and so on.Currently,widely-used membranes for VRFB application are perfluorosulfonate membranes such as Nafion series made by Dupont Co.USA.Nafion membranes have high proton conductivity and chemical/electrochemical stability,but their vanadium permeability and cost are too high.Therefore,it is necessary to develop novel membranes to satisfy their large scale industrial application in VRFBs.Among several newly-developed membranes for VRFB application,non-fluorinated polymer membranes are of great importance.Compared with Nafion membranes,non-fluorinated polymer membranes commonly exhibit low vanadium permeability and cost.However,the stability of such non-fluorinated membranes is low,which limits their long-term application in VRFBs.Thus,it is very urgent to study the degradation mechanism of the non-fluorinated polymer membrane in VRFB to explain its decreased stability.In this thesis,the sulfonated polyimide?SPI?membrane is chosen as a representative non-fluorinated polymer membrane,and its degradation behavior in VRFB is investigated through in-situ and ex-situ measurements.Morphological,mechanical and physico-chemical properties,together with structure information of the SPI membrane or its oligomer before and after degradation were characterized.Both optical photographs and SEM images show more serious destruction of surface morphology of SPI membrane during in-situ degradation than ex-situ one.Decrease in mechanical property and viscosity average molecular weight of SPI membrane after degradation in different immersion solutions verifies that both H+ and V?V?play important roles in accelerating the decomposition.FTIR results demonstrate that –SO3H groups of SPI membrane in VRFB aren't destructed.1H-NMR spectra verify the scission of polymer chain and the hydrolysis of imide ring of SPI membrane after degradation.XPS spectra evidence the oxidation of –NH2 groups of one hydrolysis product of SPI.According to all the experimental results,the 2-step degradation mechanism including hydrolysis and oxidation of SPI membrane in VRFB is proposed.In order to increase the stability of SPI membrane,a series of sulfonated polyimide/chitosan?SPI/CS?composite membranes with the sulfonation degree changed from 30% to 70% were prepared,and their performance in VRFB was also studied.The structure and morphology were characterized by ATR-FTIR and AFM respectively.The physico-chemical properties of membranes were measured respectively.Results show that the as-prepared SPI30/CS membrane has lowest proton conductivity and negligible vanadium ion permeability.The VRFB performance for SPI30/CS membrane is acceptable and stable at 10 mA cm-2,but the charge-discharge process can't be finished at higher current density such as 20 and 30 mA cm-2,etc.on account of its too low proton conductivity.Although SPI70/CS membrane has the highest proton conductivity(4.88 × 10-2 S cm-1),the coulomb efficiency?CE?of the VRFB containing SPI70/CS is the lowest one due to its highest vanadium ion permeability?10.47 × 10-7 cm2 min-1?.Specially,the VRFB assembled with SPI40/CS membrane undergoes 100 charge-discharge cycles at 50 mA cm-2,showing good stability in CE?99.3%?and energy efficiency?EE??70.5%?.Both SPI40/CS and SPI50/CS membranes are potential candidates for the VRFB application considering their high proton selectivity,good chemical stability and excellent VRFB performance. |
PDF Full Text Request