Abstract
Proton exchange membrane (PEM) capable of working over a broad operating condition window is critical for successful adoption of PEM-based electrochemical devices. In this work, phosphoric acid (PA)-doped biphenyl-backbone ion-pair coordinated PEMs were prepared by quaternization of BPBr-100, a precursor polymer, with three different tertiary amines including trimethylamine, 1-methylpiperidine, and 1,2-dimethylimidazole followed by membrane casting, ion exchange reaction to hydroxide ion, and doping with PA. The resulting PA-doped ion-pair PEMs were characterized in terms of PA doping level, proton conductivity, relative humidity (RH) tolerance, thermal stability, and mechanical properties. PA doping levels were between six and eight according to acid-base titration. The size and structure of the cation group of ion-pair polymers were found to affect the PA doping level and water uptake. Proton conductivity was studied as a function of RH over a wide range of 5% to 95% RH. Stable conductivity at 80 °C was observed up to 70% RH for 10 h. Mechanical property characterization indicates that the PA doping process resulted in more ductile membranes with significantly increased elongation at break due to the plasticization effect of PA. A combination of high proton conductivity at low RH conditions, and good humidity tolerance makes this new class of PEMs great potential candidates for use in electrochemical devices such as proton exchange membrane fuel cells and electrochemical hydrogen compressors.
Highlights
Due to the growing concerns over our society’s heavy energy dependence on fossil fuels, ever-increasing energy demand and CO2 emissions, and accelerated global warming, the identification and development of clean energy technologies have gained significant attention recently [1,2,3,4,5,6,7,8,9,10,11,12]
A combination of high proton conductivity at low relative humidity (RH) conditions, and good humidity tolerance makes this new class of Proton exchange membrane (PEM) great potential candidates for use in electrochemical devices such as proton exchange membrane fuel cells and electrochemical hydrogen compressors
BPBr-100 with a weight-average molecular weight (Mw ) of 90 kg/mol was chosen as the base material platform because of (i) excellent intrinsic chemical stability; (ii) good mechanical strength; (iii) good solubility in common organic solvents at room temperature; and (iv) the potential to form well-connected ionic channels after quaternization
Summary
Due to the growing concerns over our society’s heavy energy dependence on fossil fuels, ever-increasing energy demand and CO2 emissions, and accelerated global warming, the identification and development of clean energy technologies have gained significant attention recently [1,2,3,4,5,6,7,8,9,10,11,12]. Hydrogen (H2 ) has been recognized as an attractive clean fuel because of its high gravimetric energy density (33.3 kW·h/kg) and zero CO2 emission capability [13]. Hydrogen is currently produced mostly from fossil fuel sources, water electrolysis using electricity generated from renewable energy sources is the most desirable approach of hydrogen production. When renewable energy generation from intermittent solar or wind power is greater than energy consumption, the excess of electricity can be used for hydrogen production via water electrolysis and stored as the chemical bond of hydrogen molecules. When electricity demand is greater than renewable energy generation, the stored hydrogen can be used to produce electricity (i.e., electrification) using fuel cells, such as the proton exchange membrane fuel cells (PEMFCs).
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