Abstract
Assimilating hydrophilic hollow polymer spheres (HPS) into Nafion matrix by a loading of 0.5 wt % led to a restructured hydrophilic channel, composed of the pendant sulfonic acid groups (–SO3H) and the imbedded hydrophilic hollow spheres. The tiny hydrophilic hollow chamber was critical to retaining moisture and facilitating proton transfer in the composite membranes. To obtain such a tiny cavity structure, the synthesis included selective generation of a hydrophilic polymer shell on silica microsphere template and the subsequent removal of the template by etching. The hydrophilic HPS (100–200 nm) possessed two different spherical shells, the styrenic network with pendant sulfonic acid groups and with methacrylic acid groups, respectively. By behaving as microreservoirs of water, the hydrophilic HPS promoted the Grotthus mechanism and, hence, enhanced proton transport efficiency through the inter-sphere path. In addition, the HPS with the –SO3H borne shell played a more effective role than those with the –CO2H borne shell in augmenting proton transport, in particular under low humidity or at medium temperatures. Single H2-PEMFC test at 70 °C using dry H2/O2 further verified the impactful role of hydrophilic HPS in sustaining higher proton flux as compared to pristine Nafion membrane.
Highlights
Proton exchange membrane fuel cell (PEMFC) has attracted much attention because of its high power density and efficiency with nil green-house gas emission [1,2]
The results revealed that the reduction in proton conductivity, caused by removing moisture from the hollow polymer spheres (HPS)-Nafion composite membranes in dry or hot surroundings, could be noticeably held back
Tetraethyl orthosilicate (TEOS, 98%), 3-(trimethoxysilyl) propylmethacrylate (MPS, 98%), styrene (St) (>99%, Aldrich), divinylbenzene (DVB) (80%, Aldrich), 2,2'-azobisisobutyronitrile (AIBN) (98%, Aldrich), methacrylic acid (MAA) (>98%, Aldrich), hydrofluoric acid (HF) (48% in water, Aldrich), doubly distilled water (DDW), ammonia (25% in water, Aldrich) and analytical grade acetonitrile and ethanol were used for the experimental processes
Summary
Proton exchange membrane fuel cell (PEMFC) has attracted much attention because of its high power density and efficiency with nil green-house gas emission [1,2]. Despite recognition of the role of hydrophilic HPS in promoting proton conductivity in PEMs, it is essential to examine how the moisture-retention property of hydrophilic HPS in a PEM affects the fuel cell performance. It is imperative to understand how explicitly a particular type of HPS filler in a PEM, for instance Nafion, alters the matrix structure and properties of the composite membrane, including water uptake, proton conductivity under low humidity condition, and ion-exchange capability. This work explored the attainable maximum power density of single fuel cell loaded with a HPS-Nafion composite membrane and its structural origins. Variations of the water-related membrane properties due to the presence of these two types of HPS in Nafion matrix by 0.5 wt % loading were studied. The present work validates the role of microreservoirs of water and proton in a PEM matrix, which helps to retain matrix water and promote convection of protons
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