A so-called blockage geometry consisting of a rod with a fin positioned concentrically within a pipe is used to asses the capabilities of numerical turbulent flow and mass transfer models to predict the turbulent mass transfer coefficients. Measurements of the mass transfer coefficient have been performed for a range of fin diameters and flow rates. The limiting diffusion current measurements were performed using the ferri-ferrocyanide system and nickel electrodes. Different mass transfer turbulence models are used for the calculations and the results are compared with the measurements. The influence of flow rate and fin diameter on the mass transfer rate is examined.

A direct ethanol fuel cell (DEFC) of 5 cm2 membrane-electrode area was studied systematically by varying the catalyst loading, ethanol concentration, temperature and different Pt based electro-catalysts (Pt–Ru/C, Pt-black High Surface Area (HSA) and Pt/C). A combination of 2 M ethanol at the anode, pure oxygen at the cathode, 1 mg cm−2 of Pt–Ru/C (40%:20%) as the anode and 1 mg cm−2 of Pt-black as the cathode gave a maximum open circuit voltage (OCV) of 0.815 V, a short circuit current density of 27.90 mA cm−2 and a power density of 10.3 mW cm−2. The optimum temperatures of the anode and cathode were determined as 90 °C and 60 °C, respectively. The power density increased with increase in ethanol concentration and catalyst loading at the anode and cathode. However, the power density decreased slightly beyond 2 M ethanol concentration and 1 mg cm−2 catalyst loading at the anode and cathode. These results were validated using cyclic voltammetry at single electrodes under similar conditions to those of the DEFC.

The local and averaged forced-convective heat transfer coefficients were estimated from measured local and averaged mass transfer coefficients in a model slagging-gasifier hearth pool using the Chilton-Colburn analogy. A solution of ferri/ferrocyanide and buffer with addition of CMC (carboxymethylcellulose) was used for the electrochemical mass transfer measurements. This solution had similar properties to those of the slag in the real gasifier. The influence of natural convection due to the differences in temperatures in the hearth was also estimated. Values of heat transfer coefficient similar to those estimated by British Gas for the prototype Westfield gasifier were found using the mass transfer modelling method.

This paper is devoted to the numerical optimization of the dimensions of channels and current transfer ribs of bipolar plates as well as the thickness and porosity of gas diffusion layers. A mathematical model of the transfer processes in a PEM fuel cell has been developed for this purpose. The results are compared with experimental data. Recommendations of the values of operating parameters and some design requirements to increase PEM fuel cell efficiency are suggested.

Since the review by Wragg [Journal of Applied Electrochemistry, 21 (1991) 1047] of more than 15 years ago there has been a considerable research output involving the use of combined electrochemical and flow visualisation techniques to investigate complex flows in natural convection situations. This paper reviews recent work involving electrode geometries and orientations such as vertical cylindrical electrodes of varying aspect ratio, down pointing and up pointing pyramidal electrodes, down facing horizontal circular surfaces with different edge conditions, isosceles triangular surfaces of various inclinations, long narrow upward facing linear tracks, open upward facing cavity electrodes, upward and downward facing truncated cones, and inclined disks with either single or both sides active. This constitutes a wide range of experimentation in which new insights into data treatment and correlation have been obtained. We have used approaches taking account of the differing behaviours of the single sides of three dimensional objects and computing interference factors taking account of flow interactions. Some highly successful illustrations of data correlation for extremely complex situations are demonstrated. Flow visualisation for most of the above mentioned situations have been obtained using schlieren photography which has provided good illustrations of flow structure occurring at single faces of objects and in complexly interacting flows. In addition we have combined some flow visualisation sequences with monitoring of the current-time relationships at the onset of convection to illustrate flow development phenomena from the onset of convective instabilities to the achievement of steady state conditions.

The electrochemical limiting diffusion current technique was used to evaluate free convective mass transfer rates at free-standing circular disk electrodes of varying inclination and diameter. The electrochemical system used was copper deposition from acidified cupric sulphate solution. Experiments were carried with the single faces active and also with both faces simultaneously active. A Toepler–Schlieren system was used to photovisualise the ascending convection streams in order to obtain flow pattern information. For the upward-facing surface, the flow separates at a certain distance from the leading edge of the inclined disk, whereas the flow on the downward-facing surface always stays attached. The data correlations for the doubled disks show an exponent in the Sh–Ra correlation ranging from 0.25 (vertical case), indicating laminar flow, to 0.294 (horizontal case), indicating a mixed effect of turbulent separated flow on the upward-facing side and attached flow on the downward-facing surface. A universal correlation Shd=fA(Rad)fB was obtained for disk diameters d from 5 to 80mm and inclination angle from 0° to 90° (from the vertical) where fA and fB are functions of inclination.

Mass transfer rates were investigated in the presence of simultaneous subcooled pool boiling at a vertical cylindrical electrode using the electrochemical limiting diffusion current technique. Various concentrations of equimolar ferri/ferrocyanide (0.01-0.20M) in 0.5M NaOH electrolyte were used. The initial metal temperature was approximate to500degrees C and liquid subcooling was varied between 25 and 80degrees C. Transient temperature and limiting current histories were recorded following sudden immersion of the hot electrode and Schlieren photographs were taken for a range of electrode temperatures. In the nucleate boiling regime, the variation of mass transfer coefficient with temperature difference between electrode and electrolyte showed similar trends to the variation of heat transfer with temperature difference. Film boiling was briefly observed at low subcooling, eg. 27degrees C. (C) 2003 Elsevier Science Ltd.

This paper reviews the most common reaction pathways for CO 2 electroreduction proposed by various workers in recent years. Each pathway involves certain intermediate compounds and certain end products and thus the pathways promoted, within a specific process, can be deduced from which of these have been detected. There are considered to be four principal pathways, each based on one of the following reactions: (1) CO(g) formation via disproportionation, (2) •CO 2 − radical formation, (3) COOH(ad) formation and (4) the formation of reduced CO x y − (ad) species giving a range of reduced CO 2 end products. This paper places particular emphasis on the formation of HCOOH. The effects of electrocatalyst and process conditions on reaction pathways is also reviewed.

Mass-transfer measurements were made in a parallel plate cell equipped with baffles which produced a three-channel serpentine flow pattern and three-dimensional mass-transfer distribution effects. The entry and exit configurations were in the form of slots of rectangular cross-section. Local mass-transfer coefficient values obtained in the different channels, using surface-flush microelectrodes, reflected the complex hydrodynamics associated with phenomena such as the cell inlet and exit effects, the flow reversal effects at the baffles and preferential flow phenomena in the channels. Mass-transfer measurements obtained by averaging point measurements over various zones of the cell compared favourably with those of other workers. A hydrodynamic model of flow in the cell corresponded well to the measured mass-transfer distribution.

Free convective mass transfer at down-pointing truncated cones was experimentally studied using the electrochemical limiting diffusion current technique. The total mass transfer coefficient has the same dependence on height as that of a conical surface; however, the values are higher because of the high mass transfer at the up-facing horizontal surface. The flow development after sudden switch on of the limiting current potential was observed. The flow begins as a ring of fluid rising from the upper cone periphery followed by necking of the plume and a steady-state situation arises in which there is a complex flow structure consisting of a multiplicity of close spaced interacting convection streams. The experimental total mass transfer data were correlated using a method which includes an interference factor taking account of the fact that the conical and up-facing horizontal surfaces of the truncated cones are exposed to fluid which has already been depleted in cupric ions as it flows up from the down-facing horizontal surface.

Global mass transfer measurements in unbaffled and baffled configurations using different baffle lengths and Reynolds numbers have been made in a parallel plate cell of rectangular geometry. The entry jet arrangement and the repeated 180° changes in direction of the flow, followed by the exit, produces extremely complex hydrodynamics in the cell. A plot of mass transfer coefficient against baffle length shows an increase in mass transfer with baffle length. Comparison of data for the present work with those of other workers for similar devices showed higher mass transfer due to the modifications incorporated in the present cell.

This paper describes an investigation of free convection and mass transfer at narrow upfacing tracks 100 mm long, associated with copper deposition from a sulphate/sulphuric acid bath. The electrode width was varied from 0.8 to 7.5 mm, and the copper sulphate concentration from 0.01 to 0.28 M thus allowing a broad range of Rayleigh number to be investigated. The onset of free convection was identified by measurement of the transient current and use of an interferometric (schlieren) technique. The critical time corresponding to the onset of free convection was shown to depend on the technique used. After one or two minutes the current reached a steady value for high Ra and exhibited regular oscillations of appreciable amplitude for Ra below 106. Determination of mass transfer rates from the steady state currents gave a dependence of Sherwood number on Ra0.33 for Ra over 2×05, indicating turbulent natural convection. The low value for the transitional Ra is attributable to the peculiar aspect ratios of the electrodes.

The dependence of current efficiency for ferrate(VI) synthesis on the superimposed electrolyte flow rate was determined using 14 M NaOH solution and a white cast iron electrode. The electrolyte mean velocity in the cell ranged from 0.4 to 2.9 cm s-1 and current density from 1.4 to 35.3 mA cm-2. It was found that current efficiency was influenced by electrolyte velocity only at current densities lower than 7 mA cm-2. This is explained in terms of the removal of intermediate products from the anode surface by the electrolyte convection. This factor becomes negligible at higher current densities in comparison to mass transfer induced by oxygen evolution on the anode surface.

The mass transfer rate in fluidized beds of inert particles (FIB) is shown to be dependent on the electrolyte flow velocity and the intensity of particle collisions with the electrode. The influence of particle size and density on the ratio of the magnitude of these two influences on the mass transfer rate in a FIB was studied. Use of particle materials of varying density in an FIB permits variation of the two effects. The influence of collision currents prevails in FIBs of low density materials, and the influence of interstitial velocity is dominant in beds of high density material. The ratio of these factors also depends on the size of particles of the same density. With smaller particle size the influence of collision currents is greater. Smoothing of mass transfer maxima in beds of particles both of small and high density is explained. The results establish a basis for the selection of FIB materials for electrochemical processes.

The effect of particle-wall collision on the mass transfer rate mechanism in liquid fluidized bed electrochemical cells was studied. Collision frequencies and currents were measured at microelectrodes set in the bed wall. It is postulated that, at each particle–electrode collision, a specific microvolume of bulk concentration electrolyte is introduced into the near-electrode diffusion layer during particle movement towards the electrode causing an enhancement of the limiting diffusion current. Based on measurements made at microelectrodes calculations of the contribution of the particle collision mechanism to total mass transfer to a planar electrode are attempted and are in good agreement with experimental values.

Free convective mass transfer in open upward-facing cylindrical cavities was studied experimentally using the limiting diffusion current technique. The mass transfer rate for the total vertical cavity with all surfaces active is higher (except for the shallowest cavity) than for cavities with walls only active; this is due to the cavity base flow disturbing the more fully developed boundary layers on the longer vertical surfaces. The present correlation for vertical cavities based on characteristic length Lw (surface area/perimeter projected onto a horizontal plane)ShLw=0.559RaLw0.265for RaLw in the range from 2 × 107 to 1.2 × 1010 and Sc ∈ 〈2050, 2300〉 was found to be applicable for cavities with various height/diameter ratios (0.22–2.8). Re-evaluation of previous experimental data of Somerscales and Kassemi also gave excellent agreement with this equation. The flow development after sudden switch on of the limiting current potential was observed. The flow emerging from the mouth of vertical cavities with either active or inactive base was similar, showing a visible pulsing which was also noticed on the current transients. The current transients for deeper vertical cavities showed the slow build up of the steady state diffusion layer, whereas for shallow cavities an undershoot and overshoot, typical of up-facing horizontal surfaces, was observed.

Electrosynthesis of cupric oxide powder was carried out on a laboratory scale in an electrochemical cell under various experimental conditions. The electrolysis was appraised in terms of the particle size of the cupric oxide product and the anodic dissolution of the copper electrode. Using a previously determined pH value of 7.50, the other optimum electrolysis operating conditions established were a current density of 4000Am−2, temperature of 353K, and Na2SO4 concentration of 0.5m. The optimum values of current efficiency, cell voltage and specific energy consumption for the electrochemical synthesis of cupric oxide powder were determined.

In this paper the current distribution along two parallel segmented 3 dimensional solid electrodes is discussed in terms of the influence of the electrolyte flow, electrolyte temperature and electrode to membrane gap. The experimental work was based in a scale model "zero-gap" cell similar to those used in the chlor-alkali industry evolving oxygen at the working electrode and hydrogen at the counter electrode. A significant influence on the current distribution by both the working electrode and the counter electrode to membrane gap was found, the electrolyte temperature and superimposed flow rate had a lesser effect on the current distribution.

The current yield for the anodic oxidation of a pure iron (99.95%) electrode to ferrate(VI) ions in 14 M NaOH between 30 and 60 °C using a sinusoidal alternating current (a.c.) at amplitudes in the range 38–88 mA cm−2 and frequencies in the range 0.5 mHz to 5 kHz superimposed on direct current (d.c.) of 16 mAcm−2 was measured under conditions of bubble induced convection in a batch cell. The current yield for ferrate(VI) synthesis exhibited a complex dependence on temperature and a.c. frequency, but generally a maximum was observed in a frequency range 2–50Hz depending on the a.c. amplitude. A global maximum current yield after 180 min of electrolysis of 33% was reached at the following conditions: a.c. amplitude of 88 mA cm−2, a.c. frequency of 50 Hz and temperature of 40 °C. At the optimum conditions the highest d.c. electrolysis yield was 23%. Thus, operation with the a.c. component leads to an increase in the yield by 43% with respect to d.c. electrolysis alone.