Cathodes for fuel cells using proton-conducting Li2SO4–Al2O3 electrolyte

The performances of three widely different cathode materials (Pt, strontium-doped lanthanum manganite (LSM), and NiO) have been compared for use with proton conducting Li₂SO₄–Al₂O₃ composite electrolyte, using H₂S–air and H₂–air fuel cells operating at 600 °C. Surface analysis and electrochemical techniques were used to characterize fresh and used electrode materials. Pt or LSM cathodes each became covered with Li₂SO₄ and Al₂O₃ and, as a consequence, the fuel cells showed poor performance. In contrast, the NiO cathode catalyst did not become covered with Li₂SO₄ and good fuel cell performance was achieved. Exceptionally good current densities of over 100 mA/cm² and power densities of over 30 mW/cm² were obtained for H₂S–air fuel cells having Mo–Ni–S anode catalysts. Slight agglomeration of NiO particles during fuel cell operation had only a minor effect on performance.     

Thermodynamic studies of anion adsorption at the Pt(111) electrode surface in sulfuric acid solutions

The thermodynamics of the so-called perfectly polarized electrode were employed to analyze the total charge densities for a Pt(111) electrode in solutions of H₂SO₄ with an excess of an inert electrolyte (0.1 M HClO₄). A high quality Pt(111) single crystal electrode with a low defect density was employed in this study. The total charge densities were calculated by integration of cyclic voltammetry curves. A complete thermodynamic analysis using the electrode potential and the charge as independent variables has been performed. The best results were obtained when the charge was the independent variable. These results show that the maximum Gibbs excess of (bi)sulfate attains a value of ～3×10¹⁴ ions cm^?2, which corresponds to the packing density of ～0.2 monolayer. This number agrees well with the results of a radioactive labeling method (Kolics and Wieckowski, J. Phys. Chem. Sect. B 105 (2001) 2588) and with recent STM studies (Itaya, Prog. Surf. Sci. 58 (1998) 121; Funtikov, Linke, Stimming and Vogel, Surf. Sci. 324 (1995) L343). The calculated Esin–Markov coefficient and electrosorption valencies exhibit an important dependence on the electrode potential (or charge).     

Ionic liquid–polymer gel electrolytes from hydrophilic and hydrophobic ionic liquids1

Ionic liquid–polymer gels were prepared by incorporating hydrophilic EMIBF₄ and EMI(triflate) (EMI⁺=1-ethyl-3-methylimidazolium) and hydrophobic BMIPF₆ (BMI⁺=1-(1-butyl)-3-methylimidazolium) room-temperature ionic liquids into a poly(vinylidene fluoride)–hexafluoropropylene copolymer (PVdF(HFP)) matrix. Gel electrolytes prepared with ionic liquid:PVdF(HFP) mass ratios of 2:1 exhibited ionic conductivities of >10^?3 S cm^?1 at room temperature and >10^?2 S cm^?1 at 100°C. The BMIPF₆-PVdF(HFP) gel was incorporated into electrochemical cells, employing graphite intercalation electrodes for both the anode and cathode, to construct single and bipolar cells displaying open-circuit voltages of 3.77 and 7.86 V, respectively. New inexpensive preparative routes for the hydrophilic ionic liquids were developed that utilize the metathesis reaction of EMICl with the appropriate ammonium salt in acetone or acetonitrile to produce high purity products.     

Spectroelectrochemical investigations on the reduction of thin films of hexadecafluorophthalocyaninatozinc (F16PcZn)

Redox potentials were determined for solutions and thin films of hexadecafluorophthalocyaninatozinc (F₁₆PcZn). A value of ?0.6 V versus SCE was determined for the first reduction to the radical anion of F₁₆PcZn^? in N,N′-dimethylformamide (DMF) and ?0.9 V versus SCE for the second reduction to F₁₆PcZn^2?. Both potentials were shifted about 0.4 V towards positive potentials when compared to the unsubstituted phthalocyaninatozinc (PcZn) caused by the stabilization of the π-system by the electron-withdrawing fluorine atoms in the ligand. Vapor-deposited thin films of F₁₆PcZn on indiumtinoxide (ITO) were reduced in contact with aqueous electrolytes of pH 5.5 to avoid H⁺-reduction in acidic regimes. A chemically reversible reduction accompanied by cation intercalation was found. The kinetics of the reaction were studied in detail by cyclic voltammetry under variation of the intercalating ionic species, the film thickness and sweep rate. From the charge uptake as well as from the dependence of peak current densities on the square root of the sweep rate it was found that the reaction rate is limited by the diffusion of intercalating cations. Optical absorption spectra were collected in situ. An irreversible structural change was observed during initial reduction, also in accordance with a strongly irreversible initial CV before the reversible behavior was obtained. The presence of the dianionic F₁₆PcZn^2? was detected even under conditions of an average charge uptake of less than 1 electron/molecule. From the analysis of peak current densities according to the Randles–Sev?ik equation as well as the observed charge flow dependent on film thickness and chronoamperometric characterization of the reaction a diffusion constant D_i for K⁺ in F₁₆PcZn in the range from 1.6×10^?12 to 8.0×10^?12 cm² V^?1 s^?1 was calculated.     

The effect of hexadecyl pyridinium bromide and hexadecyl trimethyl ammonium bromide on the behaviour of iron and copper in acidic solutions

The effect of hexadecyl pyridinium bromide (HPB) and hexadecyl trimethyl ammonium bromide (HTAB) on the corrosion behaviour of iron and copper in hydrochloric and sulphuric acid solutions has been investigated by potentiodynamic polarization and Tafel extrapolation methods. The polarization curves indicate that the two compounds behave as mixed inhibitors, but the cathode is more inhibited. HPB is more effective than HTAB in both acids; this is explained on the basis of the charge located on the nitrogen atom on the two compounds. The inhibition efficiency of the compounds investigated is more effective for iron and copper metals in HCl than in H₂SO₄, which is explained on the basis of the potential of zero charge of the metal surface and the adsorption ability of both Cl^? and SO₄^2? on the metal surface.     

Effect of pH on direct electron transfer between graphite and horseradish peroxidase

The effect of pH on the kinetics of the electroreduction of H₂O₂ catalysed by horseradish peroxidase (HRP) has been studied with LSV in the potential range from 700 to ?50 mV versus SCE (under steady-state conditions and with an RDE system) and at ?50 mV versus Ag/AgCl on HRP-modified graphite electrodes placed in a wall-jet flow-through electrochemical cell. Increasing [H₃O⁺] was shown to enhance significantly the current of the bioelectroreduction of H₂O₂ due to direct electron transfer (ET) between graphite and the enzyme over the potential range involved. It is demonstrated that at high overvoltages (E<0.2 V) H₃O⁺ does not affect the rate of the enzymatic reduction of H₂O₂, but it increases the rate of direct ET between graphite and HRP. The values of the apparent rate constant of heterogeneous ET between HRP and graphite, k_s, changed from a value of 0.54±0.05 s^?1 in phosphate buffer solution (PBS) at pH 7.9, to a value of 11.0±1.7 s^?1 in PBS at pH 6.0. Analysing the pH rate profile and the variation of the k_s with increasing [H₃O⁺] made it possible to consider the reaction mechanism as implying the participation of a proton in the limiting step of charge transfer.     

Nitrite reduction in acidic solution at a GC/Eastman-AQ-Os(bpy)32+-PVP composite modified electrode

The negatively charged polymer polyester sulfonic acid (Eastman-AQ, abbreviated to AQ), positively charged polymer polyvinyl pyridine (PVP) and mediator Os(bpy)₃²⁺ were used to construct composite modified glassy carbon (GC) electrodes (abbreviated to GC/AQ-Os(bpy)₃²⁺-PVP). The reduction of NO₂^? in acidic solution was taken as a model reaction to explore the properties of the modified electrode. On the steady state polarization curve of NO₂^? reduction there were two well-developed waves with much enhanced plateau current densities and positively shifted half-wave potentials compared with bare GC electrodes. In 0.05 mol/l H₂SO₄ + 5 mmol/l NO₂^? the modified electrode exhibited plateau current densities of 723 and 1153 μA cm^?2 and half-wave potential shifts of 0.29 and 0.56 V for the first and second current wave, respectively, showing promising potential for nitrite detection. The catalytic activity for NO₂^? reduction did not decrease appreciably over 4 months. A number of relevant kinetic and thermodynamic parameters are estimated experimentally. NO₂^? reduction at the composite modified electrodes is suggested to follow the SR mechanism (pure kinetic conditions involving mutual compensation between a catalytic reaction and substrate diffusion in the film in addition to diffusion in the solution phase) according to the Savéant–Andrieux theory.     

Adsorption of atomic hydrogen on a polycrystalline Pt electrode surface studied by FT-IRAS: the influence of adsorbed carbon monoxide on the spectral feature

Adsorption of atomic hydrogen on a polycrystalline Pt electrode surface was studied by in situ infrared reflection absorption spectroscopy (IRAS). When the electrode potential was adjusted in a potential range where the underpotential-deposited (upd) hydrogen was formed, an absorption band assignable to the vibration of on-top CO (which would be formed by the reduction of a trace of CO₂) appeared at ca. 2010 cm^?1 even for highly purified 0.1 M (M=mol dm^?3) H₂SO₄ solution. An absorption band due to the on-top H was observed at ca. 2070 cm^?1 for a conventional acidic solution in a potential range as narrow as ca. 0.1 V just before the hydrogen evolution reaction (her) ascribable to the reduction of hydronium ions began. On the other hand, the on-top H band was observed unequivocally for a solution containing 1 mM H₂SO₄ and 99 mM Na₂SO₄ over a wide potential range where molecular hydrogen was formed by the reduction of hydronium ions. Even for a neutral solution such as 0.1 M KCl, the weak band ascribable to the on-top H was detected. The dependence of the spectral feature on the concentration of hydronium ions and the applied electrode potential strongly suggested that the on-top H is the intermediate in the electrochemical reduction of hydronium ions. We demonstrated that the adsorbed CO is readily formed by the reduction of CO₂ in the 0.1 M H₂SO₄ solution.     

Electrochemical generation of H2O2 using immobilized carbon nanotubes on graphite electrode fed with air: Investigation of operational parameters

Three cathode materials (i.e. bare graphite, activated carbon immobilized onto graphite surface (AC/graphite) and carbon nanotubes immobilized onto graphite surface (CNTs/graphite)) were investigated for electrochemical generation of hydrogen peroxide. The amount of electrogenerated H₂O₂ using CNTs/graphite fed with air was nearly three times higher than that of AC/graphite and seven times higher than that of bare graphite. The effect of some operational parameters such as applied current, supporting electrolyte concentration, air flow rate and pH on the generation of H₂O₂ was investigated. Results indicated that the optimal conditions for H₂O₂ generation were applied current of 100 mA (2.5 mA/cm²), air flow rate of 2.5 L/min, and pH = 3. After eight times reuse, electrochemical generated hydrogen peroxide concentration dropped from 118.65 μM to 114.63 μM, indicating a decay of 3.6%. This fact indicates that the present system can be useful for the in situ electrochemical generation of hydrogen peroxide.     

EIS study of solid-state transformations in the passivation process of bismuth in sulfide solution

Anodic oxidation of polycrystalline bismuth in alkaline medium, containing sulfide ions was investigated in situ. Cyclic voltammetry was used to define the potential regions of formation and stability of anodic Bi₂S₃ and Bi₂O₃ semiconductor films that translate bismuth to the passive state. The mechanism of elementary steps of the passivation process has been investigated using electrochemical impedance spectroscopy (EIS). The electric and dielectric properties of anodic films and structural parameters of the interfaces Bi–growing film–electrolyte in a wide region of potentials and frequencies of six decades, were determined. The EIS data have been analyzed and discussed in the frame of the point defect model (PDM) of anodic film formation and growth. The growth of passive surface films on bismuth takes place via transport of anionic vacancies generated at the metal–film interface. The slow step of the process is the layer-limited diffusion of anionic vacancies (D≈ 10^?16 cm² s^?1).Solid-state transformation of sulfide to the oxide film is a consequence of OH^? ion capture into the anionic vacancies of the sulfide film, the generation and transport of cation vacancies from the film–solution interface, their annihilation and formation of a vacancy condensate of a critical size at the metal–film interface.     

Effect of gas-phase transport in molten carbonate fuel cell

Electrode reaction characteristics involving gas-phase transport effect have been investigated with several 100 cm² class molten carbonate fuel cells (MCFCs). Although the MCFCs operate on gas-phase reactants at relatively high temperature, most of studies on the electrode reaction kinetics have been confined within kinetic-control and liquid-phase mass-transfer regions. To evaluate the gas-phase transport effect in the MCFC, an inert gas step addition (ISA) method was devised in this work. The ISA varies reactant flow rate for an electrode by adding an inert gas, which results in an overpotential shift. Since gas-phase mass-transfer resistance should be a function of reactant flow rate, correlation of the overpotential shifts with the reactant flow rates yields valuable information regarding the gas-phase transport effect in the electrode. The ISA was performed at both the anode and cathode with respect to reactant gas flow rates, addition amounts of inert gas, inert gas species, and currents applied to the cell. The overpotential shifts for both the anode and cathode were found to be inversely proportional to the square root of the reactant flow rate, indicating gas-phase mass-transfer control of both these electrodes. Especially, the overpotential shift values for the anode are much larger than those for the cathode, which suggests that the anode is under severe gas-phase mass-transfer control. From the partial pressure dependence of the overpotential shifts in the cathode, the cathode was found to be a combined gas and liquid-phase mass-transfer control system.     

The temperature dependence of the cyclic-voltammetry response for the Pt(110) electrode in aqueous H2SO4 solution

The under-potential deposition of H (upd H) and anion adsorption on Pt(110) in 0.5 M aqueous H₂SO₄ solution is investigated by application of cyclic-voltammetry (CV). The data show that an increase in T results in a shift of the CV peak towards less positive potentials and changes in its morphology; the peak current is not affected by T. The relation between the peak potential, E_p, and T is linear, the slope being ?4.64×10^?4 V K^?1. The variation of E_p with T allows one to determine the entropy of the process, ΔS, that is ?44.8 J mol^?1 K^?1. Comparison of this value with ΔS for H_upd adsorption on Pt(111) suggests that the sharp peak represents H_upd adsorption on the Pt(110) substrate. The temperature modification does not alter the Pt(110) surface which retains the (1×1) structure. The overall adsorption charge density does not vary upon T variation (for 273≤T≤333 K) and its average value is 213 μC cm^?2.     

In-situ resistance measurements of Nafion® 117 membranes in polymer electrolyte fuel cells

The resistance of the Nafion® 117 membrane in $\text{H}{}_2\text{O}{}_2$ and $\text{H}{}_2\text{air}$ polymer electrolyte fuel cells (PEFCs) has been measured in situ using fast current pulses. The dependence of the membrane resistance on current density, temperature, pressure and flow-field design was investigated. It was found that, independent of other variations, the resistance increases with increasing current density. When the current density in the cell is increased from 0.2 to 0.7 A cm^?2, the membrane resistance increases by up to 22%. Even on open circuit the resistance at 60°C is 15%–35% higher than that measured ex situ, indicating that the membrane is not fully hydrated under the fuel cell operating conditions. The resistance on open circuit also depends on the design of the flow field. In a design with forced gas convection the resistance at 60°C is substantially higher (210 mΩ cm²) than in a design without forced convection (186 mΩ cm²).     

The protective effect of peroxidase and thiocyanate against hydrogen peroxide toxicity assessed by the uptake of [3H]-thymidine by human gingival fibroblasts cultured in vitro

The hypothiocyanite ion (OSCN^?) is the principal oxidation product of the salivary peroxidase-thiocyanate (SCN^?)-hydrogen peroxide antimicrobial system. Supplementation of human saliva in vitro and in vivo with low amounts (< 1.0 mM) of hydrogen peroxide increase the concentration of salivary OSCN^? (in vivo up to 0.3 mM). Elevated concentrations of OSCN^? are strongly antimicrobial and may therefore be protective against dental caries. However, as OSCN^? is a highly-reactive oxidizing agent, its possible toxic effect on human cells was studied using gingival flbroblasts as target cells. Concentrations of OSCN^? (up to 300 μM) had no effect on [³H]-thymidine incorporation into the cells. However, fibroblasts were sensitive to peroxide so that 100 μ M of H₂O₂ caused over 80 per cent reduction in [³H]-thymidine incorporation. The toxicity of H₂O₂ could be entirely prevented by adding lacto-peroxidase and SCN^? to the cell culture before the addition of peroxide. Thus, conversion of toxic H₂O₂ to non-toxic OSCN^? in fibroblast culture by lactoperoxidase and SCN^? suggests a dual role for the salivary peroxidase system: protection of human cells from H₂O₂ toxicity and antimicrobial action against oral pathogens. Furthermore, the elevated concentrations of OSCN ^? which produce inhibition of bacterial metabolism did not damage human cells.     

Bromide oxidation and bromine reduction in propylene carbonate

Bromide oxidation and bromine reduction in propylene carbonate (PC) +1 M LiClO₄ was investigated voltammetrically using polycrystalline platinum rotating disc electrode. The corresponding voltammograms were compared to those of iodide oxidation and HCl decomposition. The oxidation 2Br^?→Br₂+2e^? was found to proceed through two steps, which are caused by the formation of a stable intermediate Br₃^? ion. The stability constant of tri-bromine ion was estimated to be 10^5.5 M. The diffusion coefficient of Br^? ion in this solution was determined to be 3.41×10^?6 cm² s^?1. In the case of bromine reduction, a time dependence of the shape of the voltammogram was observed, which showed that one is dealing with a direct bromination of PC.     

In-vivo fluid movement across dentine in the dog

Conical chambers were cemented to dentine and connected via micropipettes to a pressure bottle to permit measurement of the rate of fluid movement across dentine under positive or negative hydrostatic pressures. Connecting the chamber to a pressure transducer permitted measurement of intra-pulpal tissue pressure through intact dentine, which averaged 24 mm Hg. Knowledge of the intra- and extra-pulpal pressures employed permitted calculation of the in-vivo hydraulic conductance of dog dentine. This was 1.35 × 10^?2 and 4.51 × 10^?3μl cm^?2 min^?1 cmH₂O^?1 for molar and canine teeth respectively.     

Oxygen reduction reaction kinetics and mechanism on platinum nanoparticles inside Nafion®

The aim of this work is to study kinetic parameters and the mechanism of the oxygen reduction reaction (orr) on platinum nanoparticles supported on carbon, inside Nafion^® (i.e. in PEMFC cathode conditions). Stationary and electrochemical impedance spectroscopy techniques were used to measure exchange current densities, Tafel slopes, and reaction orders with respect to O₂ pressure and H⁺ activity. The platinum nanoparticle size effect was confirmed. A specific low frequency inductive behaviour of the cathode impedance was observed. The latter demonstrates the presence of (at least) two electrochemical steps in the orr mechanism. Secondly, dc and ac modelling of the reaction in a gas diffusion electrode is proposed in order to simulate current–potential curves and impedance spectra. This paper reveals that an ECE mechanism for oxygen reduction proposed by Damjanovic and coworkers on bulk platinum in acidic medium is also valid for that on Pt nanoparticles.     

Studies of CO tolerance on modified gas diffusion electrodes containing ruthenium dispersed on carbon

Studies of the CO tolerance for the hydrogen oxidation reaction (hor) on a polymer electrolyte fuel cell were conducted on modified gas diffusion electrodes with CO filtering layers formed by ruthenium dispersed onto carbon (Ru/C) and having platinum dispersed on carbon (Pt/C) in the catalyst layer. Different configurations for gas diffusion electrodes containing Ru hydrated-oxide (RuO_xH_y/C) layers were tested in order to understand the CO oxidation mechanism on these catalysts. Single cell polarization results indicated that a direct contact between Ru distributed on the diffusion layer and the catalyst layer is required to achieve CO tolerance. Results also show that an enhanced activity is achieved for the hor in the presence of CO with PtRu/C in the catalyst layer obtained by an impregnation method. Also, an electrode formed by a RuO_xH_y/C filtering layer in contact with a catalyst layer with PtRu/C showed the highest CO-tolerance. For this system, the cell voltage loss was only 65 mV at 1 A cm^?2 with H₂ + 100 ppm CO, with respect to the value obtained with pure hydrogen.     

Vibrational study of adsorbed phosphate ions on rhodium single crystal electrodes

The adsorption of H₂PO₄^? ions was studied on low Miller index rhodium single crystal electrodes by in situ FTIR spectroscopy. It is found that for Rh(1 0 0) and Rh(1 1 0), H₂PO₄^? ions are the major species at low potentials, but at higher potentials, some of the H₂PO₄^? ions undergo a potential induced deprotonation and probably there is a mixture of H₂PO₄^? and HPO₄^2? ions. On Rh(1 1 1) the deprotonation starts at very low potentials and at higher potentials the H₂PO₄^? is fully converted to HPO₄^2?. The behavior of the band center and of the band intensity with the applied potential was also analyzed. It is found that the adsorption increases from 0.08 V vs. a Pd–H₂ electrode up to 0.5 V and then it decreases when the OH starts to be adsorbed.     

A mechanism for dental caries based on chemical processes and diffusion phenomena during in-vitro caries simulation on human tooth enamel.

Artificial carious lesions were produced in a range of lactate buffers containing disodium-methane-hydroxy-diphosphonate and the depths of the lesions were measured. The depth of the body of the lesion depended on the calculated concentration of un-ionized lactic acid (HL), and the innermost boundary depended on HL concn. and pH. Apparent diffusion coefficients of the order 10^?10cm²/s were calculated from the boundary movement. A mechanism is proposed for the caries process. Diffusion, predominantly of the un-ionized chemical species HL, H₃PO₄, CaHPO₄, Ca(H₂PO₄)₂ and CaL₂ occurs through the intercrystalline and interprismatic pores of enamel filled with water and protein. Dissolution takes place throughout the lesion by H⁺ and L^? as HL dissociates. The outer enamel surface is partly protected by reversible adsorption of suitable chemical species.