## 1 Introduction  
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]. In pursuit of fast electrode kinetics, operating PEMFC at medium temperatures (70\(\sim\)100 \({}^{\circ}\)C) has been attempted [3]. Hence, equipping Nafion membrane with a strong moisture-retention capability becomes necessary for maintaining the required proton conductivity through this temperature range. Incorporation of inorganic particles (e.g., silica, zeolites) into Nafion membrane matrix to enhance hygroscopy of membrane is representative of earlier efforts [4; 5; 6; 7; 8; 9; 10]. In the resulting composite membrane, hydrogen bonding between the surface hydroxyl group of inorganic filler and the pendant sulfonic acid group (-SO\({}_{3}\)H) of Nafion assists the dispersion of filler particles in the host matrix [11; 12]. Grafting hydrophilic oligomeric chains to inorganic particles [3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15] has furthered the preparation technique of Nafion nanocomposite membranes. With the aim of achieving the forestated goal, use of hydrophilic hollow polymer spheres (HPS) as filler is apparently more attractive [16; 17; 18; 19; 20; 21] because this particulate structure possesses larger interior space for storing water and the porous hydrophilic polymeric shell prevents quick evaporation of water while the membrane is subjected to dehydration [22; 23]. It has also been considered that the Grothuss mechanism became dominant in the presence of hydrophilic HPS in a PEM matrix [24] because the matrix water strongly associates with pendant hydrophilic functional groups and is therefore immobile. 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. Furthermore, it is imperative to understand how explicitly a particular type of HPS filler in a PEM, for instance Nafion, alters the matrix structure and then properties of the composite membrane, including water uptake, proton conductivity under low humidity condition, and ion-exchange capability. Hence, this work explored the attainable maximum power density of single fuel cell loaded with a HPS-Nafion composite membrane and its structural origins. HPS (100-200 nm) were synthesized through a three-step procedure as illustrated in Scheme 1, which is similar to the literature method [25].  
**Scheme 1.** Schematic illustration for the preparation of s-HPS (hydrophilic hollow polymer spheres), where 3: DVB/Styrene, 4: H\({}_{2}\)SO\({}_{4}\), and 5: HF.  
Two types of hydrophilic HPS, with poly (sulfonated styrene-divinylbenzene) shell (s-HPS) and poly (methacrylic acid-divinylbenzene) shell (c-HPS), respectively, were obtained from this preparation. 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 results revealed that the reduction in proton conductivity, caused by removing moisture from the HPS-Nafion compositemembranes in dry or hot surroundings, could be noticeably held back. Furthermore, the obvious superiority of the composite membranes over the control samples, the cast and commercial Nafion membranes is reflected in the single fuel cell evaluation. The present work validates the role of microreservoirs of water and proton in a PEM matrix, which not only helps to retain matrix water but also promote convection of protons.