Study of the hydrogen absorption/diffusion in Pd80Rh20 alloy in acidic solution

The Origin of the Constant Phase Element

The constant phase element (CPE) is usually observed at solid electrodes. It is a source of controversies and misunderstandings. In this Viewpoint, theory, properties, experimental results, and the origin of the CPE are presented and discussed.

Tuning hydrogen sorption properties of Pd by its alloying with Ru, Rh, and Pt: the study of binary alloys in concentrated alkaline media

The hydrogen electrosorption process was examined in 6 M KOH on Pd binary alloys, containing Rh, Ru, and Pt. Pd-alloys were electrochemically deposited on Au substrate. The electrodes were subjected to activation procedure—hydrogen pretreatment procedure (HPP) at first in 0.5 M H2SO4 and then in 6 M KOH. It was noticed that it was possible to achieve comparable reversibility of hydrogen electrosorption process in acid and in concentrated base. The obtained values of the α→β phase transition potential, hysteresis extent, and maximum hydrogen absorption capacity show good agreement with the data from acidic medium. The observed kinetics of hydrogen electrosorption were strongly hindered in concentrated alkaline media, whereas the influence of the electrolyte on the thermodynamic functions of hydrogen absorption is less pronounced.

Effect of Hydrogen Electrosorption on Mechanical and Electronic Properties of Pd80Rh20 Alloy

The interaction of hydrogen with Pt-group metals and alloys is at the center of research in the fields of electrochemistry, electrocatalysis, hydrogen technologies and fuel cells developed under the Hydrogen Economy. In this work, the material under study was Pd₈₀Rh₂₀ alloy (50 μm foil) subjected to hydrogen electrosorption at potentials corresponding to formation of α, α-β and β phase in 0.1 M H₂SO₄ at 25 °C. The total amount of hydrogen adsorbed at the surface and absorbed in octahedral interstitial positions of fcc Pd₈₀Rh₂₀ alloy, was determined from the oxidation charges. The H/(Pd+Rh) was 0.002, 0.4 and 0.8 for α, α-β, and β Pd₈₀Rh₂₀H, respectively. Microindentation hardness testing and nanoindentation showed weakening of mechanical properties of the Pd₈₀Rh₂₀ alloy after hydrogen electrosorption due to internal stresses. Decrease of work function with increasing amount of hydrogen absorbed occurred due to the surface roughness changes and the presence of electropositive hydrogen atoms absorbed in the crystal lattice responsible for the dipole interaction. The detailed mechanism of hydrogen absorption/diffusion in the Pd₈₀Rh₂₀ alloy structure is discussed. The obtained results give a new insight into the relationship between the amount of absorbed hydrogen and mechanical and electronic properties of the Pd₈₀Rh₂₀ alloy at the micro- and nanoscale.

Multiphase Methods in Organic Electrosynthesis

With water providing a highly favored solution environment for industrial processes (and in biological processes), it is interesting to develop water-based electrolysis processes for the synthesis and conversion of organic and biomass-based molecules. Molecules with low solubility in aqueous media can be dispersed/solubilized (i) by physical dispersion tools (e.g., milling, power ultrasound, or high-shear ultraturrax processing), (ii) in some cases by pressurization/supersaturation (e.g., for gases), (iii) by adding cosolvents or "carriers" such as chremophor EL, or (iv) by adding surfactants to generate micelles, microemulsions, and/or stabilized biphasic conditions. This Account examines and compares methodologies to bring the dispersed or multiphase system into contact with an electrode. Both the microscopic process based on individual particle impact and the overall electro-organic transformation are of interest. Distinct mechanistic cases for multiphase redox processes are considered. Most traditional electro-organic transformations are performed in homogeneous solution with reagents, products, electrolyte, and possibly mediators or redox catalysts all in the same (usually organic) solution phase. This may lead to challenges in the product separation step and in the reuse of solvents and electrolytes. When aqueous electrolyte media are used, reagents and products (or even the electrolyte) may be present as microdroplets or nanoparticles. Redox transformations then occur during interfacial "collisions" under multiphase conditions or within a reaction layer when a redox mediator is present. Benefits of this approach can be (i) the use of a highly conducting aqueous electrolyte, (ii) simple separation of products and reuse of the electrolyte, (iii) phase-transfer conditions in redox catalysis, (iv) new reaction pathways, and (v) improved sustainability. In some cases, a surface phase or phase boundary processes can lead to interesting changes in reaction pathways. Controlling the reaction zone within the multiphase redox system poses a challenge, and methods based on microchannel flow reactors have been developed to provide a higher degree of control. However, detrimental effects in microchannel systems are also observed, in particular for limited current densities (which can be very low in microchannel multiphase flow) or in the development of technical solutions for scale-up of multiphase redox transformations. This Account describes physical approaches (and reactor designs) to bring multiphase redox systems into effective contact with the electrode surface as well as cases of important electro-organic multiphase transformations. Mechanistic cases considered are "impacts" by microdroplets or particles at the electrode, effects of dissolved intermediates or redox mediators, and effects of dissolved redox catalysts. These mechanistic cases are discussed for important multiphase transformations for gaseous, liquid, and solid dispersed phases. Processes based on mesoporous membranes and hydrogen-permeable palladium membranes are discussed.