Hydrodynamic electrodes flow cells

Flow-through electrodes -> flow cell, and - hydrodynamic electrodes... 

In the application of tubular electrodes and electrodes in flow cells, there may often be hydrodynamic complications, especially in voltammetry. 

Substrate orientation should be examined to determine if some planes are preferentially etched. If there is preferential etching taking place, what is its dependence on the etching cycle conditions The hardware being used for these studies should also be investigated. Very little has been done to optimize the flow cell. It is anticipated that a hydrodynamic electrode system such as a rotating disk or wall jet should work as well. 

Certain criteria have to be met in the construction of hydrodynamic electrodes, such that the laminar flow pattern, which is used in the derivation of the theoretical equations, is conformed to. Thus edge effects, which are due to the fact that electrode and surrounding mantle are not of infinite size and which are also dependent on cell dimensions, must be minimised. The shape of the electrode and mantle is important the surfaces must be smooth and there must be no discontinuities or electrolyte penetration at the electrode/mantle junction. 

While thin-layer cells have most commonly been used with flow parallel to the electrode surface as described earlier, several detectors have employed a radial-flow geometry and operate with flow entering from a jet perpendicular to and centered on the electrode surface as illustrated in Figure 27.7. This is intended to reduce dead volume and provide more effective mass transport to the electrode surface. The cell illustrated acts as an end fitting for a microbore LC column. Thin-layer cells with a radial-flow (vs. cross-flow) geometry give superior performance at lower flow rates [13]. While conventional LC columns operate at 1 mL/min, it is not uncommon to use microbore columns at 10 pL/ min, a hundredfold lower flow. It is important not to confuse these cells with the wall-jet concept. Here the orifice is very small and close to the working electrode. The cell is very thin and the wall-jet hydrodynamics are blocked since there are two walls. 

The use of hydrodynamic electrodes in these experiments has been very important in that they increase sensitivity because of higher mass transport, ensure good reproducibility, and sometimes gives better resolution in solutions of mixtures. The use of cells such as the wall-jet in flow systems is particularly useful, as response is fast and it is easy to introduce them at any point in the flow system. 

Hydrodynamic boundary layer — is the region of fluid flow at or near a solid surface where the shear stresses are significantly different to those observed in bulk. The interaction between fluid and solid results in a retardation of the fluid flow which gives rise to a boundary layer of slower moving material. As the distance from the surface increases the fluid becomes less affected by these forces and the fluid velocity approaches the freestream velocity. The thickness of the boundary layer is commonly defined as the distance from the surface where the velocity is 99% of the freestream velocity. The hydrodynamic boundary layer is significant in electrochemical measurements whether the convection is forced or natural the effect of the size of the boundary layer has been studied using hydrodynamic measurements such as the rotating disk electrode [i] and - flow-cells [ii]. 

Hydrodynamic electrodes — are electrodes where a forced convection ensures a -> steady state -> mass transport to the electrode surface, and a -> finite diffusion (subentry of -> diffusion) regime applies. The most frequently used hydrodynamic electrodes are the -> rotating disk electrode, -> rotating ring disk electrode, -> wall-jet electrode, wall-tube electrode, channel electrode, etc. See also - flow-cells, -> hydrodynamic voltammetry, -> detectors. 

The characteristics of laminar flow can allow mathematical prediction of the solution velocity and this has led to a range of hydrodynamic devices which use forced convection as a transport component under laminar flow conditions, examples include, the -> rotating disk electrode [i],-> wall jet electrode [ii], and channel flow cell (see -> flow cell). 

We saw above that the concentration gradient at an electrode will be linear with respect to the spatial coordinate perpendicular to the electrode surface if the anode/cathode cell were operated at a constant current density and if the fluid velocity were zero. In actuality, there will always be some bulk liquid electrolyte stirring during current flow, either an imposed forced convection velocity or a natural convection fluid motion due to changes in the reacting species concentration and fluid density near the electrode surface. In electrochemical systems with fluid flow, the mass transfer and hydrodynamic fluid flow equations are coupled and the solution of the relevant differential equations is often a formidable task, involving complex mathematical and/or numerical solution techniques. The concept of a stagnant diffusion layer or Nemst layer parallel and adjacent to the electrode surface is often used to simplify the analysis of convective mass transfer in... 

The hydrodynamics of flow of solution past the electrode, which is essential to the cell design, was rigorously investigated. In the range of flow rates used (10 4to 10-1cm3s-1), the flow was laminar (Reynolds Number, Re < 10) and hence, beyond a lead-in section of length 0.1 Re b, a parabolic velocity profile developed across the narrow channel. Thus, the hydrodynamics of the coaxial cell were equivalent to those of the conventional channel electrode [59]. It was predicted that the diffusion-limited current would obey the Levich equation... 

C. H. P. Bruins, D. A. Doombos, and K. Brunt, The Hydrodynamics of the Amperometric Detector Flow Cell with a Rotating Disk Electrode. AnaL Chim. Acta, 140 (1982) 39. 

Cell Design Waller and Compton [85] effectively mimicked the Allendo-erfer coaxial design vide supra), whilst simultaneously maintaining the mathematically well-defined laminar flow of the channel flow cell. This improved cell for electrochemical EPR [85] allowed an improvement in the channel cell regarding lifetimes of radicals amenable to study, whilst retaining the hydrodynamic flow that is essential for the investigation of electrode reaction mechanisms. 

Xe is the electrode length, D is the diffusion coefficient of the electrochemically active species, and n is the number of electrons transferred per mole reactant during the electrode reaction. The applicability of this equation was tested using the known one-electron reduction of fluorescein in aqueous alkali, to yield the semifluorescein radical anion, and it was found that the well-defined current potential curves observed gave rise to hnear Tafel plots, with the expected room temperature value of 59 mV decade" slope. A linear Levich plot allowed inference of a diffusion coefficient of 3.0 X 10 cm s , in agreement with literature values [87]. Thus, the hydrodynamics of this particular flow cell are such that there is the expected parabolic, laminar flow. 

The rotating disc electrode (RDE) is the classical hydrodynamic electroanalytical technique used to limit the diffusion layer thickness. However, readers should also consider alternative controlled flow methods including the channel flow cell (38), the wall pipe and wall jet configurations (39). Forced convection has several advantages which include (1) the rapid establishment of a high rate of steady-state mass transport and (2) easily and reproducibly controlled convection over a wide range of mass transfer coefficients. There are also drawbacks (1) in many instances, the construction of electrodes and cells is not easy and (2) the theoretical treatment requires the determination of the solution flow velocity profiles (as functions of rotation rate, viscosities and densities) and of the electrochemical problem very few cases yield exact solutions. 

---