Niels Janniksen Bjerrum

Limiting Current of Oxygen Reduction on Gas‐Diffusion Electrodes for Phosphoric Acid Fuel Cells

Various models have been devoted to the operation mechanism of porous diffusion electrodes. They are, however, suffering from the lack of accuracy concerning the acid-film thickness on which they are based. In the present paper the limiting current density has been measured for oxygen reduction on polytetrafluorine-ethyl bonded gas-diffusion electrodes in phosphoric acid with and without fluorinated additives. This provides an alternative to estimate the film thickness by combining it with the acid-adsorption measurements and the porosity analysis of the catalyst layer. It was noticed that the limiting current density can be accomplished either by gas-phase diffusion or liquid-phase diffusion, and it is the latter that can be used in the film-thickness estimation. It is also important to mention that at such a limiting condition, both the thin-film model and the filmed agglomerate model reach the same expression for the limiting current density. The acid-film thickness estimated this way was found to be of 0.1 [mu]m order of magnitude for the two types of electrodes used in phosphoric acid with and without fluorinated additives at 150 C.

Influence of Substrates on the Electrochemical Deposition and Dissolution of Aluminum in NaAlCl4 Melts

Le depot et la dissolution electrolytiques de laluminium dans le sel de tetrachloroaluminate de sodium sature en chlorure de sodium sont examines par voltammetrie et potentiometrie pour differents materiaux delectrodes (carbone vitreux, tungstene, cuivre, nickel, aluminium) a 175 o C

Electrochemical Deposition of Aluminum from NaCl ‐ AlCl3 Melts

Electrochemical deposition of aluminum from melts saturated with onto a glassy carbon electrode at 175°C has been studied by voltammetry, chronoamperometry, and constant current deposition. The deposition of aluminum was found to proceed via a nucleation/growth mechanism, and the nucleation process was found to be progressive. The morphology of aluminum deposits was examined with photomicroscopy. It was shown that depending on the current densities (c.d.) applied, three types of aluminum deposits could be obtained, namely, spongy deposits formed at lower c.d. (below 0.7 mA/cm2), smooth layers deposited at intermediate c.d. (between 2 and 10 mA/cm2), and dendritic or porous deposits obtained at high c.d. (above 15 mA/cm2). However, the smooth aluminum deposits were about five times more voluminous than the theoretical value. The spongy deposits were formed due to difficulties in electronucleation and could be inhibited by application of pulsed currents and/or addition of manganese chloride into the melt.

Electrochemical Deposition and Dissolution of Aluminum in NaAlCl4 Melts Influence of and Sulfide Addition

Effects of the additives MnCl 2 , sulfide, and their combined influence on aluminum deposition and dissolution in NaAlCl 4 saturated with NaCl have been studied by polarization measurements, galvanostatic deposition, and current reversal chronopotentiometry

Electroless Growth of Aluminum Dendrites in NaCl ‐ AlCl3 Melts

The spontaneous growth of aluminum dendrites after deposition was observed and examined in sodium chloride‐aluminum chloride melts. The concentration gradient of in the vicinity of the cathode surface resulting from electrolysis constitutes a type of concentration cell with aluminum dendrites as electrodes. The short‐circuit discharge of the cell is found to be the driving force for the growth of aluminum dendrites. Such a concentration gradient is proposed to be one of the causes for dendrite formation in the case of metal deposition.

Electrochemical Behavior of Molten V 2 O 5 ‐ K 2 S 2 O 7 ‐ KHSO 4 Systems

The electrochemical behavior of K 2 S 2 0 7 -KHSO 4 -V 2 O 5 , K 2 S 2 0 7 -V 2 O 4 , and K 2 S 2 0 7 -KHS0 4 -V 2 0 4 melts was studied in argon and SO 2 /air atmospheres using a gold electrode. In order to identify the voltammetric waves due to KHSO 4 , molten KHSO 4 and mixtures of K 2 S 2 O 7 -KHSO 4 were investigated by voltammetry performed with Au and Pt electrodes in an argon atmosphere. It was shown that H + reduction took place at 0.26 V vs. an Ag + /Ag reference electrode, i.e., at a potential in between the V(V) → V(IV) and V(IV) → V(III) reduction stages. The presence of KHSO 4 caused an increased concentration of V(III) species in the V 2 0 5 containing molten electrolytes. This effect may be caused either by protonic promotion of the V(IV) → V(III) reduction (V0 2+ + 2H + + e - → V 3+ + H 2 O) or by chemical reduction of V(IV) complexes with hydrogen, formed from H + as the product of the electrochemical reduction. Both the V(V) → V(IV) reduction and the V(IV) → V(V) oxidation remained one-electron electrochemical reactions after the addition of KHSO 4 (or water) to the H 2 S 2 0 7 -V 2 0 5 melt. Water had no noticeable effect on the V(V) → V(IV) reduction but the V(IV) → V(V) oxidation proceeded at higher polarizations in the water-containing melts in both argon and SO 2 /air atmospheres. This effect may be explained by participation of the water molecules in the V(IV) active complexes.

Electrochemical Investigation of the Catalytical Processes in Sulfuric Acid Production

The electrochemical behavior of molten K 2 S 2 O 7 and its mixtures with V 2 O 5 [2-20 mole percent (m/o) V 2 O 5 ] was studied at 440°C in argon, by using cyclic voltammetry on a gold electrode. The effect of the addition of sulfate and lithium ions on the electrochemical processes in the molten potassium pyrosulfate was also investigated. The potential window for pure K 2 S 2 O 7 was estimated as 2.1 V, being limited by the S 2 O 7 2- oxidation and reduction. The oxidation of SO 4 2- to oxygen is reversible in the basic melt. It is found that V(V) electroreduction proceeds in two steps. The first reduction stage [V(V)→ V(IV)], starting at 0.7-0.8 V vs. Ag + /Ag, is reversible for V 2 O 5 concentrations lower than 5 m/o and at potential scan rates less than 200 mV/s. For all studied compositions, the first reduction stage is a one-electron reaction. The second reduction stage [V(IV)→V(III)], starting at 0.1-0.2 V, is irreversible and under ohmic control at all studied V 2 O 5 concentrations. The presence of Li 2 SO 4 causes a noticeable depolarization effect on the V(V) reduction and the V(IV) oxidation.

Electrochemical behavior of molten V{sub 2}O{sub 5}-K{sub 2}S{sub 2}O{sub 7}-KHSO{sub 4} systems

The electrochemical behavior of K 2 S 2 0 7 -KHSO 4 -V 2 O 5 , K 2 S 2 0 7 -V 2 O 4 , and K 2 S 2 0 7 -KHS0 4 -V 2 0 4 melts was studied in argon and SO 2 /air atmospheres using a gold electrode. In order to identify the voltammetric waves due to KHSO 4 , molten KHSO 4 and mixtures of K 2 S 2 O 7 -KHSO 4 were investigated by voltammetry performed with Au and Pt electrodes in an argon atmosphere. It was shown that H + reduction took place at 0.26 V vs. an Ag + /Ag reference electrode, i.e., at a potential in between the V(V) → V(IV) and V(IV) → V(III) reduction stages. The presence of KHSO 4 caused an increased concentration of V(III) species in the V 2 0 5 containing molten electrolytes. This effect may be caused either by protonic promotion of the V(IV) → V(III) reduction (V0 2+ + 2H + + e - → V 3+ + H 2 O) or by chemical reduction of V(IV) complexes with hydrogen, formed from H + as the product of the electrochemical reduction. Both the V(V) → V(IV) reduction and the V(IV) → V(V) oxidation remained one-electron electrochemical reactions after the addition of KHSO 4 (or water) to the H 2 S 2 0 7 -V 2 0 5 melt. Water had no noticeable effect on the V(V) → V(IV) reduction but the V(IV) → V(V) oxidation proceeded at higher polarizations in the water-containing melts in both argon and SO 2 /air atmospheres. This effect may be explained by participation of the water molecules in the V(IV) active complexes.