Hydrogen fuel cells for cars and buses

The use of hydrogen fuel cells for cars is strongly promoted by the governments of many countries and by international organizations like the European Community. The electrochem. behavior of the most promising fuel cell (polymer electrolyte membrane fuel cell, PEMFC) is critically discussed, based on results presented in the literature. Moreover, when some non- electrochem. aspects are taken into consideration, it is concluded that the prospects are not too bright. Moreover, the hydrogen-rich gas generated from small orgs. contains CO2 and a small amt. of CO, its use easily leads to poisoning of the platinum gas diffusion anode in the fuel cell. Also the hydrogen storage problems are still not solved. [on SciFinder (R)

The mechanism of the chlorine evolution on different types of graphite anodes during the electrolysis of an acidic NaCl solution

The behavior of the electrode and the formation of Cl were studied on porous graphite used in the industrial electrolysis of aq. NaCl. This was done for Acheson graphite electrodes and for highly orientated pyrolytic graphite electrodes of which the edge plane or the basal plane served as electrode surface. Voltammograms were detd. by continuous scanning with different rates of the potential between a min. and max. value. From the exptl. results it follows that 2 different surface compds. the lp O and the hp O are formed on all the graphite electrodes investigated. In the stationary state lp O is present at low potential and the hp O at high potentials. The formation and the removal of each compd. depends on the electrolytic conditions, especially on the max. potential of the sweep. The conditions of the sweep determine whether these oxides are really present on the electrode surface during the anodic or the cathodic sweep. Cl is formed according to the Volmer-Heyrovsky mechanism on all the three types of electrodes. The c.d., the transfer coeff. and the activation energy for the edge plane electrode and the Acheson graphite electrode are of the same order of magnitude, while for the basal plane electrode they deviate strongly. [on SciFinder (R)

Behaviour of and mass transfer at gas-evolving electrodes

A completes set of models for the mass transfer of indicator ions to gas-evolving electrodes with different behaviour of bubbles is described theoretically. Sliding bubbles, rising detached single bubbles, jumping detached coalescence bubbles and ensembles of these types of bubbles are taken into consideration as well as the behaviour of various types of bubbles, in particular, the detached coalescence bubble is discussed. The set of model is checked using literature data of mass-transfer coefficients for ferricyanide ions and ferrocyanide ions to, respectively, a hydrogen-evolving and an oxygen-evolving electrode in alkaline solutions under conditions of forced convection of solution. It has been found that the mass transfer of indicator ions to various types of gas-evolving electrodes can be described well by the set of models proposed

Reduction of chromate in dilute solution using hydrogen in a GBC-cell

Redn. of metal ions in dil. solns. is of a great interest for purifn. of waste waters and process liqs. Hydrogen gas is a very attractive reductant, since its use gives no addnl. pollution. The redn. of chromate in a sulfuric acid medium was studied. A new electrochem. cell, a GBC-cell, which is a combination of a gas-diffusion electrode to direct contact with a packed bed of carbon particles, is introduced. Hydrogen gas flows along the hydrophobic side of the gas-diffusion electrode and a chromate soln. is pumped upwards through the bed. Expts. were carried out with H2SO4 solns. initially contg. 70 mol m-3 chromate at various temps., soln. flow rates, H2SO4 solns. initially contg. 70 mol m-3 chromate at various temps., soln. flow rates, H2SO4 concns. and bed thicknesses. Exptl. results for the chromate redn. are described by an empirical relation. The redn. of chromate is a 1st-order reaction in chromate and the apparent rate const. for the chromate redn. increases with decreasing chromate concn. and increasing temp., H2SO4 concn. and bed thickness and is practically independent of the flow rate of the soln. It is concluded that the new GBC-cell is very attractive for the redn. of chromate in dil. solns. and for industrial application on a large scal

Behaviour of a tall vertical gas-evolving cell. Part II: Distribution of current

Vertical electrolysers with a narrow electrode gap are used to produce gases, for example, chlorine, hydrogen and oxygen. The gas voidage in the solution increases with increasing height in the electrolyser and consequently the current density is expected to decrease with increasing height. Current distribution experiments were carried out in an undivided cell with two electrodes each consisting of 20 equal segments or with a segmented electrode and a one-plate electrode. It was found that for a bubbly flow the current density decreases linearly with increasing height in the cell. The current distribution factor increases with increasing average current density, decreasing volumetric flow rate of liquid and decreasing distance between the anode and the cathode. Moreover, it is concluded that the change in the electrode surface area remaining free of bubbles with increasing height has practically no effect on the current distribution factor

Mass transfer to and copper deposition on a round bar in a new type of electrolytic cell: the helix cell

High-rate electrodeposition of copper from CuSO4-H2SO4 baths can be achieved by using crossflow of solution. To obtain copper layers of uniform thickness and quality, a new type of electrolytic cell, the helix cell, has been proposed. An experimental dimensionless relation has been given to describe the mass transfer to a round bar, in crossflow, in a helix cell. Moreover, the current efficiency of copper deposition has been obtained as a function of current density, flow rate of solution, temperature and weight per cent CuSO4 in the CuSO4-H2SO4 solution

Behaviour of a tall vertical gas-evolving cell. Part I: Distribution of void fraction and of ohmic resistance

Electrolysis of a 22 wt % NaOH solution has been carried out in a vertical tall rectangular cell with two segmented electrodes. The ohmic resistance of the solution between a segment pair has been determined as a function of a number of parameters, such as, current density and volumetric rate of liquid flow. It has been found that the ohmic resistance of the solution during the electrolysis increases almost linearly with increasing height in the cell. Moreover, a relation has been presented describing the voidage in the solution as a function of the distance from the electrodes and the height in the cell

Electrochemical oxidation of hypochlorite at platinum anodes

Oxidation of hypochlorite has been studied by cyclic voltammetry with a rotating-ring-disc assembly. Experiments were carried out with sodium chloride or sodium sulphate solutions with hypochlorite present or absent. Hypochlorite is oxidized at the platinum disc anode and the species formed by this oxidation are detected with the platinum ring cathode. From the shape of the current—potential curves at the ring electrode and from the relation between the ring and the disc currents it has been concluded that the first step of the hypochlorite oxidation is the formation of the chloroxyl radical (ClO)