No abstract

High surface area carbon materials are promising for low-temperature storage of hydrogen by physisorption. To achieve acceptable hydrogen capacities at ambient temperature, chemisorption must come into play. The dispersion of transition metal catalysts to carbon materials can enhance the ambient temperature adsorption capacity of the carbon materials via the hydrogen spillover mechanism. In this study, three different hydrogen dissociation catalysts (Pd, PdAg, and PdCd nanoparticles) were dispersed onto surfaces of activated carbon. The surface composition of these metal-dispersed carbon materials was analyzed using X-ray photoelectron spectroscopy (XPS) and the specific surface areas, and pore sizes were measured using N2 adsorption/desorption. The effect of the dispersed catalysts on the hydrogen adsorption properties of the activated carbon was systemically investigated at 77 K and room temperature (295 K) using a volumetric gas adsorption technique. At 77 K, the catalysts have no effect, and the hydrogen capacity of the materials is strictly related to the specific surface area. At room temperature, hydrogen spillover was observed from the catalysts to the carbon material. The hydrogen capacity is related to the adsorption strength of hydrogen atoms to the catalyst particle surface atoms, which was verified with DFT calculations. In addition, this study reveals that the PdCd nanoparticle possesses much higher hydrogen spillover enhancement (108%) than the pure Pd and PdAg nanoparticles, promising for hydrogen storage.

In view of the inherent limitations of current portable technology energy sources, the implementation of micro fuel cells is becoming increasingly appealing. This has generated great interest in the development of direct formic acid micro fuel cells. In this study, nanoporous Pd and four nanoporous bimetallic Pd-M catalysts with an atomic ratio of 90:10, where M = Cd, Pb, Ir, and Pt, were synthesized via a facile hydrothermal method and examined for the electrochemical oxidation of formic acid. The electrocatalytic activity of these nanoporous electrode materials was studied with the use of linear sweep voltammetry and chronoamperometry. Our chronoamperometric measurements have shown that the initial electrochemical performance of the nanoporous Pd-M electrodes toward formic acid oxidation was almost independent of the alloying materials; however, the second incorporated metal strongly affected the stability of the Pd-based electrocatalysts. The mechanisms of the oxidation of formic acid were further examined with the aid of in situ electrochemical ATR-FTIR spectroscopy. For the nanoporous Pd, PdCd, and PdPb catalysts, oxidation proceeds through the direct mechanism, whereas the indirect mechanism, along with major CO poisoning, was observed in the case of the PdIr and PdPt catalysts. The incorporation of even small amounts of Pt and Ir to Pd was found to inhibit the oxidation of formic acid. On the other hand, the addition of Pb to Pd served to promote the direct mechanism, which in turn makes these Pd-based catalysts both cost and electrocatalytically more effective.

Hydrogen-absorbing materials are crucial for both the purification and storage of hydrogen. Pd and Pd-based alloys have been studied extensively for their use as both hydrogen dissociation catalysts and hydrogen selective membrane materials. It is known that incorporating metal atoms of different sizes into the Pd lattice has a major impact on the hydrogen absorption process. In this paper, hydrogen electrosorption into nanostructured Pd-Cd alloys has been studied for different compositions of Cd that varied from 0 to 15 at. %. The low cost of Cd makes it an attractive material to combine with Pd for hydrogen sorption. A combination of chronoamperometry and cyclic voltammetric experiments was used to determine the ratio of the H/(Pd + Cd) and the kinetics of hydrogen sorption into these Pd-Cd alloys at different potentials. It was found that the maximum H/(Pd + Cd) value was 0.66 for pure Pd, and this decreased with increasing the amount of Cd. Also, the alpha (solid solution) to beta phase (metal hydride) hydrogen transition was determined to be the slowest step in the absorption process and was practically eliminated when an optimum amount of Cd atoms was doped (i.e., Pd-Cd(15%)). With increasing the amount of Cd, more hydrogen was absorbed into the Pd-Cd nanostructures at the higher potentials (the alpha phase region). The faster kinetics, along with the decrease in the phase transition of hydrogen sorption into the Pd-Cd nanostructures when compared to pure Pd, makes the Pd-Cd nanostructures attractive for use as a hydrogen dissociation catalytic capping layer for other metal hydrides or as a hydrogen selective membrane.

Recent studies have shown that the use of hydrogen peroxide as a fuel cell oxidant instead of oxygen significantly increases the power density. In this paper, we report on highly active carbon supported PdPt catalysts for hydrogen peroxide reduction. Four different PdPt electrocatalysts, Pd/C, Pt/C, Pd 0.5 Pt 0.5 /C, and Pd 0.25 Pt 0.75 /C, were prepared and characterized using transmission electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction, and Brunauer-Emmet-Teller surface area analysis. The electrocatalytic activities of these synthesized catalysts were studied using a rotating disk electrode system with a combination of linear sweep voltammetry and chronoamperometry. Our studies show that the Pd 0.25 Pt 0.