Abstract
Polymer electrolyte membrane fuel cell (PEMFC) is an eco-friendly energy conversion device that can convert chemical energy into electrical energy without emission of harmful oxidants such as nitrogen oxides (NOx) and/or sulfur oxides (SOx) during operation. Nafion®, a representative perfluorinated sulfonic acid (PFSA) ionomer-based membrane, is generally incorporated in fuel cell systems as a polymer electrolyte membrane (PEM). Since the PFSA ionomers are composed of flexible hydrophobic main backbones and hydrophilic side chains with proton-conducting groups, the resulting membranes are found to have high proton conductivity due to the distinct phase-separated structure between hydrophilic and hydrophobic domains. However, PFSA ionomer-based membranes have some drawbacks, including high cost, low glass transition temperatures and emission of environmental pollutants (e.g., HF) during degradation. Hydrocarbon-based PEMs composed of aromatic backbones with proton-conducting hydrophilic groups have been actively studied as substitutes. However, the main problem with the hydrocarbon-based PEMs is the relatively low proton-conducting behavior compared to the PFSA ionomer-based membranes due to the difficulties associated with the formation of well-defined phase-separated structures between the hydrophilic and hydrophobic domains. This study focused on the structural engineering of sulfonated hydrocarbon polymers to develop hydrocarbon-based PEMs that exhibit outstanding proton conductivity for practical fuel cell applications.
Highlights
Interest in eco-friendly alternative energy has increased due to the depletion of fossil fuels and environmental pollution, many countries are making great efforts to develop renewable energy technologies that can replace fossil fuels [1]
The development of sulfonated hydrocarbon polymer (SHP)-based polymer electrolyte membrane (PEM) has been actively pursued in order to overcome the inherent drawbacks of the perfluorinated sulfonic acid (PFSA) ionomerbased PEMs
SHP-based PEMs composed of well-known aromatic random copolymers have shown relatively low proton-conducting behavior and physicochemical stability compared to PFSA ionomer-based PEMs due to the difficulty in forming the well-defined hydrophilic/hydrophobic phase-separated structures
Summary
Interest in eco-friendly alternative energy has increased due to the depletion of fossil fuels and environmental pollution, many countries are making great efforts to develop renewable energy technologies that can replace fossil fuels [1]. Some studies have reported that comb-shaped copolymer-based membranes show better PEM properties including proton conductivity and lower water absorption behavior, resulting in smaller dimensional change than block copolymer-based PEMs with similar compositions [61,72]. The authors demonstrated that the rational design of comb-shaped architectures possibly forming additional interactions between the side chains can improve both the physicochemical stabilities and the electrochemical properties of the SHP-based PEMs. Chulsung Bae et al elucidated synthetic strategies for graft copolymers taking into consideration the effect of acidity of the sulfonic acid groups in the side chains and the optimal lengths for the geometric separation between the main and side chains [82].
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