Preparation And Properties Of Polynorbornenes Based On Ring-opening Metathesis Polymerization For Proton Exchange Membranes

Fuel cells(FCs) are attractive as promising power source for portable applications due to their several advantages: high energy conversion efficiency, low emission and wide fuel source, etc. Proton exchange membrane fuel cells(PEMFCs), compared with other FCs, are recognized as the preferred power battery for electric vehicles, stationary power stations and spacecraft because of their low temperature and pressure operation, high power density, simple structure. As the core components for PEMFCs, proton exchange membranes(PEMs) serve as an electrolyte for transporting protons from anode to cathode without allowing fuel crossover. Currently, perfluorosulfonic acid membranes such as Nafion, are the most commonly used PEMs and have served as the benchmarks for assessing membrane performance because of their excellent chemical and electrochemical stability and outstanding proton conductivity. However, Nafion has some shortcomings, such as high cost and high fuel permeability. So to overcome these deficiencies in Nafion, researchers have focused on modification of Nafion membranes and on the development of novel PEM materials. At the same time, polynorbornenes(PNB), prepared via ring-opening metathesis polymerization(ROMP), were widely used for functional polymer materials because of simple polymerization method, raw materials, controllable size, good acid and alkali resistance. In this domain, we will introduce our work to investigate whether polynorbornenes prepared via ROMP can be used for PEM applications.First of all, because the function feature size of the PEM was forced on the level of atoms and moleculars, it is important to investigate mechanism and forecast performance on microscopic level while design structure and improve properities. So molecular dynamics(MD) simulation technique was employed to investigate the effect of tempreture on structural and dynamical characteristics of sulfonated PNB membranes. We used the mean square displacement(MSD) and radial distribution function(RDF) from the trace file to calculate the diffusion coefficient, proton conductivity(PC) and the distance between atoms. The computing PC is 56 m S/cm at room temperature and 105 m S/cm at working temperature, which means that the membranes can be used for PEMFCs whose PC of PEM is more than 50 m S/cm at temperature. As a result, the structure of the polymer meets performance requirements of the PEMs and the experimental program can be going on.Secondly, we design the structure of the monomer and polymer based on the properities of PEMs. The Diels–Alder reaction was used to synthesize a norbornene derivative, namely 4-(Bicyclo[2.2.1]hept-5-en-2-yl)-benzene-1-sulfonylchloride(NBSC), which served as sulfonated monomer and helped avoid the degradation and crosslinking reactions that arise during post-synthesis sulfonation using standard monomers. Subsequently, ROMP was used to synthesize a series of copolymers from NBSC, norbornene(NBE) and dicyclopentadiene(DCPD), where NBE contributed to the flexibility of the polymer chain and DCPD served as the crosslinking agent. While, NMR and FTIR spectrums were used to analysis and prove the structure of the monomer and polymer.And then, a series of membranes with different fractions of DCPD units and equal theoretical IECs were prepared and their properties were measured. The results showed that the water uptake and swelling ratio of the membranes were lowered as the DCPD fraction was increased, so too did the conductivity. This trend was seen at all temperatures and was attributed to the narrowing of the proton transport channels due to increasing crosslink density. Among the membranes prepared and evaluated, the high proton conductivity(94.42±2.67 m S/cm) and low methanol permeability(2.07±0.22′10-6 cm2/s) of PNB-0.4 than that of Nafion117.Finally, membrane electrode assemblies(MEA) are prepared with the method of gas diffusion electrode(GDE). We used the PNB as the binder to discuss the influence of the ratio of binder and catalyst on the single fuel cell performance. The maximum power density of the MEA with the PNB-0.4 is 94.5 m W/cm2 under the catalyst loading 1mg/cm2 at 60°C.