• Energy Research

The effect of heat treatment of Ti and Ti–0.2 Pd alloys on their anodic oxidation was studied in deaerated 1% NaCl by means of anodic linear sweep voltammetry, SEM, TEM, EDS, optical microscopy and microhardness measurements. The specimens, as fabricated, consisted of α-phase only. The β-phase, intergranular or with a Widmansttaten type growth, was produced by heat treatment of the Ti–0.2 Pd alloys at the temperature range from 750 to 850 ∘C. The β-phase was transformed into the α′-phase during quenching. The current density against voltage curves for pure Ti and Ti–0.2 Pd, as fabricated or heat-treated, presented an initial plateau at about 1.5 V vs Ag/AgCl/KCl (3 M), an anodic peak at about 4.5 V and a current increase due to the pitting attack at about 10 V. The anodic peak was related to an oxide growth together with a solution electrolysis. Current spikes appeared at random from potentials about 8.3 V, which were related to film breakdown and repair events. The passive films of the alloys oxidized up to about 10 V presented oxidation bands parallel to the surface, with different oxygen content and microhardness, together with a structural transformation of the α′-phase under the titanium oxide layer. The similar behaviour of pure Ti and Ti–0.2 Pd alloys in front of pitting corrosion in chloride was due to such a structural transformation.

The behavior of the O 2 |HO 2 redox couple at equilibrium on a commercial uncatalyzed carbon-polytetrafluoroethylene (PTFE) oxygen-diffusion electrode, fed with O 2 partial pressures between 0.21 and 1.0 atm, has been studied by electrochemical impedance spectroscopy. Measurements have been made in the open-circuit potential using aqueous solutions with KOH concentrations in the range 1.0-6.0 mol dm 3 and HO 2 concentrations up 50 mmol dm 3 at 25.0°C. Under these conditions, the system is controlled by activation, the charge-transfer resistance being much higher than ohmic, adsorption, and diffusion resistive elements. The double-layer capacities show that the wetted electroactive areas, much smaller than the total area, depend on the KOH concentration used for activation. True exchange current densities of about I μA cm 2 are obtained, while their apparent values are two orders of magnitude greater, since the sluggish reaction is compensated by the high electroactive area. The cathodic process is a first-order reaction with respect to the O 2 feed, and Independent of HO 2 and OH concentrations. For the anodic one, a zero order for O 2 and a first order for HO 2 and OH are calculated. These results agree with the previously proposed mechanism for the O 2 reduction to HO 2 on the same electrode from voltammetric studies. Indirect evidence on a weak adsorption of HO 2 is found from the impedance diagrams.

The electrodegradation with electrosynthetic possibilities of trichlorofluoromethane (CFC 11) and 1,1,2-trichloro-1,2,2-trifluoroethane (CFC 113) in acetate-containing methanol-water mixtures, has been studied using constant-potential electrolysis and gas chromatography. A closed cell with a Pb cathode and a Pd-based hydrogen diffusion anode, together with small concentrations of CFCs were employed to follow their conversion. The conversion of CFC 11 into the most dechlorinated derivatives increased with time. Partially and completely dechlorinated ethylene derivatives were obtained in the electroreduction of CFC 113, their relative amounts depending on the Pd 2 + content of the electrolyte. In the negative part, some cathode corrosion during the process was evidenced.

Abstract The behaviour of a recently developed O 2 -diffusion electrode for HO 2 − generation has been studied by electrochemical impedance spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy and X-ray photoelectron spectroscopy. It has been submitted to a cathodic reduction of 100 mA cm −2 in 1.0 and 6.0 M KOH to analyse its evolution with electrolysis time. An increase in the double layer capacity and a decrease in the charge transfer resistance with increasing time were found. The electrode wetting is higher in 6.0 M KOH, but only about 6% of its total surface area is electroactive after 50 h. The charge transfer control for O 2 reduction, the low functionalisation of the electrode surface by oxygen-containing groups and its high hydrophobicity have been related to its high durability.

One of the main challenges of electrochemical Fenton-based processes is the treatment of organic pollutants at near-neutral pH. As a potential approach to this problem, this work addresses the use of a low content of soluble chelated metal catalyst, formed between Fe(III) and ethylenediamine-N,N'-disuccinic (EDDS) acid (1:1), to degrade the herbicide triclopyr in 0.050 M Na2SO4 solutions at pH 7.0 by photoelectro-Fenton with UVA light or sunlight (PEF and SPEF, respectively). Comparison with electro-Fenton treatments revealed the crucial role of the photo-Fenton-like reaction, since this promoted the production of soluble Fe(II) that enhanced the pesticide removal. Hydroxyl radicals formed at the anode surface and in the bulk were the main oxidants. A boron-doped diamond (BDD) anode yielded a greater mineralization than an IrO2-based one, at the expense of reduced cost-effectiveness. The effect of catalyst concentration and current density on the performance of PEF with BDD was examined. The PEF trials in 0.25 mM Na2SO4 + 0.35 mM NaCl medium showed a large influence of generated active chlorine as oxidant, being IrO2 more suitable than RuO2 and BDD. In SPEF with BDD, the higher light intensity from solar photons accelerated the removal of the catalyst and triclopyr, with small effect on mineralization. A plausible route for the herbicide degradation by Fe(III)-EDDS-catalyzed PEF and SPEF is finally proposed based on detected byproducts: three heteroaromatic and four linear N-aliphatic compounds, formamide, and tartronic and oxamic acids.

Abstract The kinetics of conventional gold cyanidation in air has been studied using open circuit potential measurements, voltammetry and atomic absorption spectrophotometry. The experimental results show that this is a complex process characterized by the interdependency of the different variables (cyanide concentration, pH, temperature and stirring speed). The measurement of the mixed potentials at which the process takes place gives valuable information to ascertain the influence of each variable. A good correlation between mixed potential and dissolution rate, thus having a potential interest for an industrial application, has been found. The study of the current—potential curves for oxygen reduction on gold surface and anodic dissolution of gold in cyanide solutions gives more insight into the control of the process: depending on the experimental conditions, gold dissolution takes place in the active region (oxygen diffusion control) or in the potential region where dissolution of adsorbed species limits the rate of the process. Depending also on the experimental conditions, two or four electrons are transferred per oxygen molecule.

Barium strontium cobaltite-ferrite (Ba1-xSrxCoyFe1-yO3-δ, BSCF) is a widely studied mixed ionic-electronic conductor material for air electrode in solid oxide cells (SOC). Despite having excellent features, due to fast oxygen surface exchange and oxygen bulk diffusion, it lacks long-term stability. Electrode/electrolyte thermal expansion coefficient (TEC) mismatch and structural instability at temperature lower than 900 °C are responsible for the increase of electrode polarization which becomes a crucial issue for the long-term stability. In this work, SOC stability was studied by adding a thin porous samarium-doped ceria (SDC) backbone on top of the dense SDC electrolyte. The porous SDC backbone was then infiltrated by precursor nitrates to obtain a Ba0.5Sr0.5Co0.8Fe3-δ composition. The SEM investigation showed a nano-sized BSCF-based layer covering the backbone structure. In addition, symmetrical cells were studied in the 400-700 °C temperature range under anodic and cathodic polarization showing unexpected behavior associated to the electrode microstructure. The modified electrode synergistically enhanced ORR and OER by showing no oxygen vacancies clustering which induces a higher polarization resistance. Ageing procedure was performed for over 120 hours at 600 °C under switched current load of ± 0.2 A·cm-2. The prepared system showed high stability coupled with remarkable electrocatalytic performance and good mechanical properties.

AbstractThe high cost and often complex synthesis procedure of new highly selective electrocatalysts (particularly those based on noble metals) for H2O2 production are daunting obstacles to penetration of this technology into the wastewater treatment market. In this work, a simple direct thermal method has been employed to synthesize Sn‐doped carbon electrocatalysts, which showed an electron transfer number of 2.04 and outstanding two‐electron oxygen reduction reaction (ORR) selectivity of up to 98.0 %. Physicochemical characterization revealed that this material contains 1.53 % pyrrolic nitrogen, which is beneficial for the production of H2O2, and ‐C≡N functional group, which is advantageous for H+ transport. Moreover, the high volume ratio of mesopores to micropores is known to favor the quick escape of H2O2 from the electrode surface, thus minimizing its further oxidation. A purpose‐made gas‐diffusion electrode (GDE) was prepared, yielding 20.4 mM H2O2 under optimal electrolysis conditions. The drug diphenhydramine was selected for the first time as model organic pollutant to evaluate the performance of an electrochemical advanced oxidation process. In conventional electro‐Fenton process (pH 3), complete degradation was achieved in only 15 min at 10 mA cm−2, whereas at natural pH 5.9 and 33.3 mA cm−2, almost overall drug removal was reached in 120 min.