## 3 Results and Discussion
### Broadening Hydrophilic Channel of Nafion by Hydrophilic HPS  
The cryofractured cross-sections of the four membranes of interest exhibited a clear transition from a dense and homogeneous matrix of the pristine Nafion membrane to a micro-rupturing matrix of the composite membranes, _i.e._, N/s-HPS and N/c-HPS. This change in morphology is typically due to interaction between Nafion matrix and the filler used. The composite membrane N/n-HPS, containing neutral HPS, nevertheless still held a rather homogeneous matrix, analogous to its pristine counterpart (Figure 3a). Assimilating either s-HPS or c-HPS into Nafion matrix was to trigger hydrophilic interaction between -SO\({}_{3}\)H groups of Nafion molecules with s-HPS or with c-HPS [33]. Compared to the microstructure of the pristine Nafion (Figure 4), consisting of perfluoro-polymer domains and a hydrophilic and continuous boundary phase, hence the hydrophilic HPS associated with the boundary composed of pendant -SO\({}_{3}\)H groups to form a particle-domain packing morphology since both have roughly comparable submicron particle sizes (Scheme 2). As a result, the association of the Nafion domains became insignificant or was weakened due to spatially distancing effect. After drying, such composite membrane is mechanically weaker than the denser pristine Nafion membrane matrix. Hence, numerous micro-ruptures could be observed across the cross-section (Figure 3c,d). Owing to considerably low specific density, 0.5 wt % loading of either c-HPS or s-HPS would have taken up a large volume fraction of the hydrophilic phase of Nafion. However, the smaller volume fraction of  
Figure 2: TEM images: (**a**) SiO\({}_{2}\)-P(St-DVB); (**b**) s-HPS; (**c**) SiO\({}_{2}\)-P(methacrylic acid- divinylbenzene) (SiO\({}_{2}\)-P(MAA-DVB)); and (**d**) c-HPS sphere.  
hydrophilic phase, relative to the hydrophobic perfluoropolymer phase, in Nafion could not separate the filler particles extensively and thereby the structure of composite can be illustrated by Scheme 2, which was actually verified by many loosely bound hydrophilic HPS and the broadened gap between the Nafion domains in Figure 3c,d.  
Figure 4: The cross-sectional FESEM of the cast Nafion membrane.  
Figure 3: **(a)** Field emission scanning electron microscopy (FESEM) images of the cross-section of Nafion; **(b)** N/n-HPS; **(c)** N/s-HPS; and **(d)** N/c-HPS composite membranes. A 0.5% loading of HPS filler exists in each composite membrane.  
**Scheme 2.** The formation of the Nafion/hollow sphere composite membrane structure.  
These hydrophilic HPS aggregates functioned as strong moisture reservoir with prominent effect on proton transport, which will be elaborated in the following section. Despite the positive effect of hydrophilic HPS, a loading exceeding 0.5 wt % would further weaken the cohesive strength of membrane and hence be irrelevant. On the contrary, being hydrophobic in nature, n-HPS particles were likely to be entangled by perfluoro-chains upon drying. As the hydrophobic perfluorocarbon chains take up the major volume fraction and this hydrophobic combination does not or only slightly perturb the ionic clustering of -SO3H throughout the membrane, a dense morphology could still remain after being cryofractured. In summary, the hydrophilic HPS filler acted to broaden proton conducting channel in the composite Nafion membrane.