(a) Schematic representations for the design and preparation of the palladium (Pd)-modified hollow spherical carbon (P-HSC) deposited onto carbon paper (CP) cathode. (b) The field emission scanning electron microscopic (FESEM) image and photograph (inset) of the pristine CP. Scale bars, 50 μm. (c) FESEM image and photograph (inset) of the as-prepared P-HSC deposited onto CP cathode. Scale bars, 50 μm. (d) The enlarged image of c. Scale bars, 500 nm. The red arrows in d point out the hierarchically porous structure of P-HSC deposited onto CP. (e) X-ray diffraction patterns of the as-prepared P-HSC and the standard pattern of Pd nanocrystals. (f) TEM image of the P-HSC. The inner wall surface of the hollow carbon spheres (light area) is covered with well-dispersed Pd NPs with 10–15 nm in diameter (dark areas). Scale bars, 10 nm. (g) Nitrogen adsorption-desorption isotherms and pore-size distribution (inset) of the P-HSC.