Publisher Summary Limiting-current density refers to the maximum rate at 100% current efficiency, at which a particular electrode reaction can proceed in the steady state. This rate is determined by the composition and transport properties of the electrolytic solution and by the hydrodynamic condition at the electrode surface. It has been observed that in many cases mass transfer is not the sole cause of unsteady-state limiting currents, observed when a fast current ramp is imposed on an elongated electrode. Major conditions for valid measurement and interpretation of limiting currents include reaction characterized by slow surface kinetics, progress of the electrode position reaction, use of low concentrations of the reacting ionic species in free convection studies. This chapter outlines the underlying principles, discusses the conditions of validity, and highlights some selected applications and basic features of the limiting-current phenomenon. The chapter also illustrates a synopsis of electrochemical mass-transfer theory, to the extent required for analysis of limiting-current conditions. Various complicating factors—migration effects, the choice of appropriate diffusivity values, unsteady-state conditions, current distribution below and at the limiting current, and the effect of formation of rough metallic deposits—in the interpretation of limiting-current measurements are considered.

Mass-Transfer Measurements by the Limiting-Current Technique

The heterogeneously catalyzed oxidation of glucose is performed in a rotor−stator spinning disk reactor. One side of the rotor is coated with a Pt/C and Nafion catalytic layer, resulting in a liquid−solid interfacial area of 274 mi2 mR−3. At the lowest rotational disk speed, 26 rad s−1, the reaction is liquid−solid mass transfer limited; at the highest rotational disk speed, 180 rad s−1, the intrinsic kinetics are rate determining. The experimental overall reaction rates are fitted with a resistances in series model, with the activation energy, pre-exponential factor, and volumetric liquid−solid mass transfer coefficient as parameters. The volumetric liquid−solid mass transfer coefficient, kLSaLS, increases from 0.02 to 0.22 mL3 mR−3 s−1 for a rotational disk speed of 26 to 157 rad s−1. These values are high in comparison to conventional reactors, like packed beds, in spite of the low liquid−solid interfacial area used in this study. The values of the liquid−solid mass transfer coefficient kLS are 1 order...

Liquid-solid mass transfer and reaction in a rotor-stator spinning disc reactor

Durch Elektrolyse last sich die Reaktivitat eines Substrates andern oder sogar umpolen („Redox-Umpolung”). Damit werden das Kohlenstoffgerust und die funktionellen Gruppen eines Synthesebausteins okonomischer nutzbar, und zugleich lassen sich Reaktionsschritte in mehrstufigen Synthesen einsparen. Als Werkzeug fur die Elektrolyse werden die Zelle, die Stromquelle, die Elektroden und der Elektrolyt benotigt, wobei man Elektroden und Elektrolyt nach Kenntnis des Reduktions- oder Oxidationspotentials des Substrates auswahlt. Eine Reihe elektroanalytischer Methoden ermoglicht Aussagen uber den Mechanismus der Elektrodenreaktion. An der Anode dimerisieren Arene, Phenylether und elektronenreiche Olefine uber Radikalkationen als Zwischenstufe. Carbonsaure-Anionen decarboxylieren bei der Kolbe-Elektrolyse zu Radikalen, die kuppeln oder sich an Olefine addieren; die Kolbe-Elektrolyse eignet sich auch zur Synthese von Pheromonen. An der Kathode lassen sich aktivierte Olefine uber Radikalanionen hydrodimerisieren oder mit entsprechenden Reagentien acylieren, alkylieren und carboxylieren. Pinakole, gekreuzte Hydrodimere, cyclische und arylierte Verbindungen sind uber kathodisch erzeugte Radikale zuganglich, wahrend die Herstellung gespannter Kleinringe oder die reduktive Addition von Halogeniden an Carbonylverbindungen uber intermediare Carbanionen gelingt.

CC-Verknüpfungen an Anode und Kathode

This paper provides a review of a range of process intensification methods which have been applied to electrochemical processes and reactions. The electrochemical reactions include those in electrolysers where gas evolution occurs, in metal deposition reactions and reactions occurring in fuel cells and batteries. The process intensification methods include the use of centrifugal force fields, ultrasonics, rotating electrodes and rotating cells which operate with and without the application of magnetic fields. Such devices can be used in principle to carry out electrolysis without the application of an external potential or current. Other methods considered include the use of light. The area of magnetic field induced fluid flow is also discussed which has applications in lab on a chip technologies. The area of process intensification has largely focused a lot on hydrogen generation as a means of providing a clean, sustainable fuel.

R.E.W. Jansson

Papers

Mass transfer and pressure distributions in electrochemical pump cells

Flow field in the rotating electrolyser

Mass transfer and pressure distribution in capillary gap cells