At low humidity (<30 RH%), water molecules begin to fill the intermolecular spaces in the membrane. At high humidity (80 RH%) the membrane swells; the free volume size increases.

Results from Pt model catalyst surfaces have demonstrated that surface defects, in particular surface concavities, can improve the oxygen reduction reaction (ORR) kinetics. It is, however, a challenging task to synthesize nanostructured catalysts with such defective surfaces. Hence, we present a one-step and up-scalable top-down approach to produce Pt/C catalyst (with ~3 nm Pt nanoparticle diameter). Using high-resolution transmission electron microscopy and tomography, electrochemical techniques, high energy X-ray measurements and positron annihilation spectroscopy, we provide evidence of a high density of surface defects (including surface concavities). The ORR activity of the developed catalyst exceeds that of commercial Pt/C catalyst, at least 2.7-times in terms of specific (~1.62 mA/cm 2 Pt at 0.9 V vs the reversible hydrogen electrode) and at least 1.7-times in terms of mass activity (~712 mA/mg Pt), which can be correlated to the enhanced amount of surface defects. Besides, the technique used here reduces the Page 2 of 51 ACS Paragon Plus Environment ACS Catalysis 3 complexity of the synthesis (and therefore production costs) compared to state-of-the-art bottom-up techniques. This information is available free of charge on the ACS Publications website. Coincident Doppler broadening spectrum, WAXS measurements, Rietveld refinement data, Reference ORR polarization curve, Accelerated durability test, TEM analysis AUTHOR INFORMATION

Vacancy-type defects in Mg-implanted GaN with and without hydrogen (H) implantation are probed by using monoenergetic positron beams. Mg þ and H þ ions are implanted into GaN(000 1) to obtain 0.1 and 0.7-mm-deep box profiles with Mg and H concentrations of 1 Â 10 19 and 2 Â 10 20 cm À3 , respectively. For the as-implanted samples, the major defect species is determined to be Gavacancy (V Ga ) related defects such as V Ga , divacancy (V Ga V N ), and their complexes with impurities. For Mg-implanted samples, an agglomeration of vacancies starts at 800 C annealing, leading to the formation of vacancy clusters such as (V Ga V N ) 3 . For the samples annealed above 1000 C, the trapping rate of positrons by vacancies is increased by illumination of a He-Cd laser. This is attributed to the capture of photon-excited electrons by the defects and their charge transition. For Mg-and H-implanted samples, the hydrogenation of vacancy-type defects starts after 800 C annealing. Comparing with the annealing behavior of defects for the samples without H-implantation, the clustering of vacancy-type defects is suppressed, which can be attributed to the interaction between Mg, H, and vacancies.