The role of surface charging and potential redistribution on the kinetics of hole injection reactions at n-GaAs

The dissolution rate of GaAs in alkaline solutions in the pH range 11–14 was shown to be limited by OH- diffusion in the solution. The kinetics of the hole injection from an oxidizing agent in solution into the valence band of n-GaAs changes when the hole injection rate is larger than the dissolution rate. In this study Fe(CN)3-6 was used as an oxidizing agent. From the results of impedance measurements it was shown that, due to interface charging effects, a shift of the semiconductor band edges of more than 0.5 V reduces drastically the overlap between the valence band and the distribution function of Fe(CN)3-6; the hole injection reaction becomes kinetically controlled. Photoanodic dissolution of n-GaAs also influences the kinetics of the hole injection from solution. At higher light intensities the rate of reduction of the oxidizing agent can be reduced almost to zero

3D-integrated all-solid-state batteries

This article has no abstract

Evidence for cathodic protection of crystallographic facets from GaAs etching profiles

Etching experiments were carried out on partially masked GaAs single crystals in alkaline K3Fe(CN)6 solutions inwhich the dissolution rate of all crystal planes is diffusion-controlled. Etching could be rate determined in two ways. Inthe first case, mass transport of OH– ions to the GaAs surface determined the rate of the anodic partial reaction and alsothe etch rate. This resulted in rounded profiles at the semiconductor/resist edge as expected for diffusion limited etching.In the second case, mass transport limited reduction of the oxidizing agent determined the dissolution rate. Etching at theresist edge was now, surprisingly, anisotropic and faceted profiles were observed. On the basis of electrochemical measurementswith these etchants it is concluded that a local galvanic element can be formed between crystallographic facets.As a result, certain facets may be cathodically protected and consequently etch more slowly than the corresponding freecrystal plane

Double-phase hydride forming compounds : a new class of highly electrocatalytic materials

A new class of materials is proposed to improve the electrocatalytic activity of hydride forming intermetallic compoundsof the AB5-type without making use of highly electrocatalytic precious metals like Pd or Pt. These materials, denotedas AB5.5, consist of two different crystallographic phases: the bulk phase, still responsible for hydrogen storage, isformed by the corrosion-resistant multicomponent standard alloy based on LaNi5; and a second phase, homogeneouslydecorating the surface of the bulk-phase particles, provides for the extremely fast electrochemical hydrogen reaction. Thecomposition of the second-phase alloy is such that synergism in the electrocatalysis occurs. A simple metallurgicalmethod of producing double-phase materials is described. Various analytical techniques such as EPMA and x-ray diffractionare employed to characterize the solids produced. It is shown that the kinetics of the charge-transfer reaction can becharacterized electrochemically by the overall exchange current. In accordance with the Brewer-Engel theory, MoCo3 precipitatesare found to be highly electrocatalytic, which is reflected in an increase of the overall exchange current from190 mA · g–1 for the single-phase AB5 compound to 588 mA · g–1. As a consequence very high discharge efficiencies are accomplishedwith these MoCo3-based powder electrodes, even under extreme conditions: at 0°C the efficiency is improvedfrom 34 to 90%

Secondary batteries - nickel systems : nickel–metal hydride : metal hydrides:Metal Hydrides

\u3cp\u3eMany scientists and even policy-makers are nowadays openly speculating about the future hydrogen economy for which it is believed that hydrogen-driven fuel cells are going to play a dominant role in transportation, thereby replacing conventional and inefficient internal combustion engines. To enable such beneficial economy it has been argued that safe and efficient storage of hydrogen gas is one of the crucial aspects to be solved. However, it has generally not been realized that our society has already entered the hydrogen economy a few decades ago with the development of nickel-metal hydride (Ni-MH) batteries. Advantageously, this battery system is indeed based on the smallest energy carrying chemical particle shuttling between the two electrodes. Storage of hydrogen in the negative electrode of Ni-MH batteries has been accomplished efficiently, which is safe and at low pressures in the form of metal hydrides (MH). Since its discovery, this battery type has become widely accepted to power our portable electronics, due to their high storage capacity, excellent rate capability, and environmental friendliness. Its popularity is currently further boosted resulting from the large-scale introduction in hybrid electrical vehicles (HEVs) for which Ni-MH batteries are exclusively used and it is to be expected that this expansion is further amplified in the near future into the direction of plug-in (hybrid) electrical vehicles (P(H)EV). This battery system can, therefore, be considered as the first commercial success toward a fully developed hydrogen era. In this article the basic electrochemical principles underlying Ni-MH operation are first outlined and then the materials research, which has enabled this successful battery system, is reviewed. Finally, more recent developments in materials research, which may lead to a new generation of high-energy density Ni-MH batteries in the near future, are described.\u3c/p\u3

Electrochemical Quartz Microbalance characterization of Ni(OH)\u3csub\u3e2\u3c/sub\u3e-based thin film electrodes

\u3cp\u3eThe use of the Electrochemical Quartz Crystal Microbalance (EQCM) to study the proton intercalation performance of thin film Ni(OH)\u3csub\u3e2\u3c/sub\u3e layers, nowadays widely used as cathode electrode material in rechargeable Ni(OH)\u3csub\u3e2\u3c/sub\u3e-based battery systems such as NiMH and NiCd, is reviewed. In addition, the impact of incorporating foreign metals in these layers on the electrochemical performance will be highlighted. Using EQCM much information can be obtained, as both the electrochemical response and accompanying mass changes can be measured simultaneously. EQCM was extensively used to investigate the effect of the conditions on the formation of Ni(OH)\u3csub\u3e2\u3c/sub\u3e thin layers, the α-to-β modification changes and the details of the redox mechanism. The proposed redox mechanisms differ in whether H\u3csup\u3e+\u3c/sup\u3e or OH\u3csup\u3e-\u3c/sup\u3e is transferred, the reactants and/or products are hydrated and cations from the solution take part in the reaction. By incorporation of other metals in the structure, the characteristics of thin Ni(OH)\u3csub\u3e2\u3c/sub\u3e layers can be tuned. This affects the oxidation and reduction potential, the reversibility, the stability of the structure and the oxygen evolution side reaction. Co\u3csup\u3e2+\u3c/sup\u3e and Fe\u3csup\u3e2+\u3c/sup\u3e were shown to replace Ni-sites in the hydrous oxide lattice, thereby forming very dense structures with higher stability. However, structural changes still occur in most cases. Due to this inhomogeneity, the layers are usually a combination of different structures, depending on the distribution of the incorporated metal(s). Suppression of the oxygen evolution reaction is reported for Co, Pb, Pd, Zn and Mn. The effects of Co and Mn are shown to depend on the incorporated amount. Co shifts the standard redox potential for the oxygen evolution reaction towards more cathodic potentials and decreases the oxygen overpotential significantly. Light-weight rare-earth elements also catalyze the oxygen evolution reaction.\u3c/p\u3

Battery Modeling: A Versatile Tool to Design Advanced Battery Management Systems

Fundamental physical and (electro) chemical principles of rechargeable battery operation form the basis of the electronic network models developed for Nickel-based aqueous battery systems, including Nickel Metal Hydride (NiMH), and non-aqueous battery systems, such as the well-known Li-ion. Refined equivalent network circuits for both systems represent the main contribution of this paper. These electronic network models describe the behavior of batteries during normal operation and during over (dis) charging in the case of the aqueous battery systems. This makes it possible to visualize the various reaction pathways, including convention and pulse (dis) charge behavior and for example, the self-discharge performance