A Study Of Alkaline Polymer Electrolytes For Fuel Cell Application

Fuel cells are a class of clean, efficient converter of energy, particularly ofhydrogen energy. Featuring in compact structure, high power density, and roomtemperature startup, polymer electrolyte fuel cell (PEFC) has been considered an idealpower source for vehicles and portable devices. Presently, proton exchange membranefuel cell (PEMFC) is the most advanced PEFC, whose electrolyte, proton exchangemembrane (PEM), however, is a strong acid, hence only noble metals can be used asthe catalyst. This has been precluding the PEMFC from widespread applications.Emerging recently as a novel type of PEFC, alkaline polymer electrolyte fuel cell(APEFC) is expected to combine the advantages of PEMFC and traditional alkalinefuel cell (AFC), and to fundamentally get rid of the dependence of noble metals. Thekey for developing APEFC relies on high-performance alkaline polymer electrolyte(APE). However, there has hitherto been no commercial APE product satisfyingcompletely the requirements of fuel cell application.A qualified APE should have both high OH conductivity and low swellingdegree. Unfortunately, these properties are usually not compatible, because highconductivity normally demands high ion exchange capacity (IEC), but the higher theIEC, the more water the APE soaks and the greater the swelling. To resolve thisproblem, we propose in the present work several strategies for APE design, andachieve at several types of highly qualified APEs. The major progress is summarizedas follows:1. Quaternary ammonia polysulfone (QAPS)Polysulfone (PS), an engineering plastic of high chemical stability, is chosen as thebackbone and modified into quaternary ammonia polysulfone (QAPS) by chemicalgrafting. During this process, a new catalyst for the chloromethylated reaction and anew procedure for the quarternized reaction are applied, both help to raise thesynthetic efficiency and to make the resulting IEC controllable. QAPS of IEC1.0mmol/g exhibits optimal performance: the ionic conductivity reaches0.03S/cm at60°C and the swelling degree is less than30%. Using such QAPS as the electrolyte,the first prototype of APEFC completely free from noble metals has been realized.2. Self-crosslinked quaternary ammonia polysulfone (xQAPS)To further restrain the swelling degree of QAPS, crosslinking is effective, but conventional crosslinking methods will make ionomer solution unavailable. Wepropose a self-crosslinking strategy to address this problem. By introducing a tertiaryamine onto the QAPS, the polymer solution can remain stable at room temperature,while the polymer chains will get tightly crosslinked upon solidification, leading to amembrane with extremely high mechanical strength. The resulting self-crosslinkedQAPS (xQAPS) is exceptionally anti-swelling, even with an IEC as high as1.34mmol/g, the swelling degree is only3%at90°C, and the ionic conductivity reaches0.063S/cm. Such performance remains stable during a test period of1000h.3. Self-aggregated quaternary ammonia polysulfone (aQAPS)To boost the ionic conductivity of APE to a level comparable to PEM, efficientionic channels are required so as to enhance the effective mobility of OH. Byintroducing hydrophobic side chains (without ionic functional groups) onto the QAPS,efficient hydrophilic/hydrophobic phase separations can be resulted, which drive theaggregation of OH in the hydrophilic domain, and effectively increase the ionicconductivity. By adjusting the length of the hydrophobic side chain, the phaseseparation morphology of such self-aggregated QAPS (aQAPS) can be tuned. TheC6-aQAPS turns out to be the best APE with the highest OH mobility (in comparisonto all APE reports in the literature), whose conductivity reaches the order of10-1S/cmat60°C to80°C, comparable to that of Nafion.4. Crosslinking-network quaternary ammonia polysulfone (xaQAPS)Combining xQAPS and aQAPS, a crosslinking network QAPS (xaQAPS) can beresulted, which exhibits superior mechanical strength as xQAPS and excellent ionicconductivity as aQAPS, and thus is very suitable for fuel cell application. An APEFCprototype using C6-xaQAPS as the electrolyte is operated stably at75°C.5. Quaternary methylimidazole polysulfone (QMIPS)To further enhance the stability of the cationic functional group in APEs, we haveexplored several types of APEs with methylimidazole functional groups, amongwhich self-crosslinked QMIPS (xQMIPS) is as good as xQAPS in terms of ionicconductivity and anti-swelling property, with its cationic group being more stable inhot alkaline solutions.6. APEs in electrode catalyst layerWe devise methods for the separate evaluation of ionic and electronic conductivities in catalyst layer, and investigate in detail the influence of APE on theionic and electronic conductivity as well as the gas transportation in the electrode. Inan APEFC, the anode needs to be more hydrophobic than the cathode. Wedemonstrate how to optimize the hydrophobic/hydrophilic balance based on the use ofthose QAPS we developed in this work, and provide guidelines for the rational designof APEFC electrode.