## 2 Experimental Section  
Norit RX 1.5 Extra activated carbon, a product of CABOT, was used as carbonaceous support for transition metal decoration. The metal precursors were: hexachloroplatin acid (H\({}_{2}\)PtCl\({}_{6}\) H\({}_{2}\)O, 99%, WinLab, Pt 39.96% min. ), palladium acetate [Pd(O\({}_{2}\)CCH\({}_{3}\))\({}_{2}\), 99%, Aldrich], nickel acetate tetrahydrate [Ni(O\({}_{2}\)CCH\({}_{3}\))\({}_{2}\) 4H\({}_{2}\)O, 99% Merck], rhodium(III) 2,4-pentanedonate (C\({}_{15}\)H\({}_{21}\)O\({}_{6}\)Rh, 99.99%, Alfa Aesar, Rh 25.2% min. ), iridium(III) 2,4-pentanedonate (C\({}_{15}\)H\({}_{21}\)IrO\({}_{6}\), 99.99%, Alfa Aesar, Ir 37.5% min. ), and ruthenium(III) 2,4-pentanedonate (C\({}_{15}\)H\({}_{21}\)O\({}_{6}\)Ru, 99.99%, Alfa Aesar, Ru 24% min.). The ultrasound-assisted impregnation method was adopted because of its simplicity and large-scale applicability [45; 46; 92; 93; 94], where the NH\({}_{3}\)-treated or untreated activated carbons were dispersed within the dissolved metal ions to form 5.0 wt.% decorated metal/AC. De-ionized water was used as the dissolving medium for Pt and Ni salts, while the precursors of Pd, Ru, Rh and Ir were dissolved by using ultra-high purity benzene as a solvent. The synthesis method, structure, morphology, textural, elemental analysis (CHNS), and experimental details were reported by [PERSON] et al. [90]. Additionally, JEOL (SX-3202M) with built-in energy-dispersive fluorescence X-ray spectrometer (EDS) and high-resolution ultra-thin window (UTW) detector, for C-peak detection, was used for elemental analysis. The Debye-Scherrer formula, L (20) = (K \(\times\) )/(\(\beta\)\(\times\) cos\(\theta\)) [95], was used to estimate the crystallite size of the decorating metals, where: L is the crystallite size, \(\lambda\) is the X-ray wavelength (Cu K\({}_{\alpha}\) radiation = 0.15405 nm), K is constant and equals to 0.94, \(\theta\) is the angle of diffraction, and \(\beta\) is the full width at half maximum (FWHM) at 20 multiplied by \(\pi\)/180 after considering the broadening contributions from instrument and other factors, which was around 29%. If we did not consider such a broadening effect, the uncorrected crystallite size, calculated from uncorrected \(\beta\), would be smaller by a factor of 3.4. We used at least 5 diffraction peaks for calculating the average crystallite sizes, and we compared this average with the average particle sizes, obtained from scanning electron microscope (SEM) images. The hydrogen storage measurements were conducted via the volumetric technique ([PERSON]'s method) by using a high-pressure volumetric analyzer (HPVA-II, Micromerities), which could reach up to 200 bars. The hydrogen uptake was assessed by measuring the sorption isotherms (pressure-composition-isotherms (PCIs) at 77 K and 298 K, and within a pressure range from ambient to 100 bars. A 3-litre, well-insulated Dewar flask was used for performing the measurements at 77 K, where one filling of the flask was sufficient during the experiment. The mass of the adsorbent specimen was not less than 2.0 g for reducing the experimental errors as much as possible. Before any sorption measurement, the adsorbent specimens were degassed for 12 h, at 200 \({}^{\circ}\)C, under vacuum, where the resulting mass loss was determined, and the degassed mass was considered to investigate the hydrogen uptake. The adsorbent specimen volume was corrected on the basis of skeletal density measurements, corresponding to the excess values obtained by helium gas. The specimen holder was dipped in liquid nitrogen at 77 K and was kept for at least 45 min before performing the measurements at cryogenic temperature. Ultra-high-purity hydrogen gas grade (6N) was used in all adsorption experiments. The PCI curves were measured several times to verify the hysteresis and the repeatability. All of the adsorbent specimens exhibited good repeatability.