Steady-state Polarization Measurements Research Articles

Feasibility of using PtFe alloys as cathodes in direct methanol fuel cells

Carbon-supported platinum–iron catalysts were fabricated and characterised by means of scanning electron microscopy, energy-dispersive X-ray system and X-ray diffraction. The catalysts were tested in electrochemical half cells for oxygen reduction using voltammetry and steady-state polarisation measurements and in direct methanol fuel cells. Use of PtFe/C cathodes, instead of a Pt/C cathode, partially suppressed methanol oxidation and led to higher net oxygen reduction currents in the presence of methanol. Consequently, an increase in power density up to 30% was achieved in direct methanol fuel cells with PtFe/C cathodes, compared to that with Pt/C cathode. The influence of alloy composition and operation conditions on the cell performance has been investigated.

Hydrogen production by advanced proton exchange membrane (PEM) water electrolysers—Reduced energy consumption by improved electrocatalysis

Proton exchange membrane (PEM) water electrolysis systems offers several advantages over traditional technologies including greater energy efficiency, higher production rates, and more compact design. Normally in these systems, the anode has the largest overpotential at typical operating current densities. By development of the electrocatalytic material used for the oxygen evolving electrode, great improvements in efficiency can be made. We find that using cyclic voltammetry and steady state polarisation analysis, enables us to separate the effects of true specific electrocatalytic activity and active surface area. Understanding these two factors is critical in developing better electrocatalytic materials in order to further improve the performance of PEM water electrolysis cells. The high current performance of a PEM water electrolysis cell using these oxides as the anode electrocatalyst has also been examined by steady state polarisation measurements and electrochemical impedance spectroscopy. Overall the best cell voltage obtained is 1.567 V at 1 A cm−2 and 80 °C was achieved when using Nafion 115 as the electrolyte membrane.

Silver-cobalt bimetallic particles for oxygen reduction in alkaline media

The kinetics of the oxygen reduction reaction (ORR) was studied on ultra thin layer electrodes formed by bimetallic Ag-Co nanoparticles dispersed on a carbon powder (Ag-Co/C) in KOH electrolyte. The morphological features of the materials were studied by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Cyclic voltammetry and steady state polarization measurements for the ORR were obtained using the rotating ring/disk electrode (RRDE) technique. TEM measurements for the bimetallic Ag-Co/C catalysts showed a heterogeneous crystallite size distribution and, together with XRD, suggested the presence of a segregated phase of cobalt, mainly as Co 3O 4. The characterization of the materials by XRD also showed a negligible decrease in the Ag face centered cubic (fcc) lattice parameter, which was ascribed to a negligible Co insertion into the Ag structure. Levich plots obtained from rotating disk electrode data have shown a higher number of electrons for the Ag-Co/C materials, compared to Ag/C. The polarization results showed higher activity for the ORR on Ag-Co/C materials in comparison to Ag/C.

Hydrogen electrosorption on TiHg alloy in acidic solution

Hydrogen evolution and electrosorption on/in TiHg alloy have been investigated in acidic solution using cyclic voltammetry, open circuit potential transient, steady-state polarization, chronopotentiometry and electrochemical impedance measurements. Comparison has been made with pure Ti. Cyclic voltammetry and open circuit potential measurements show that hydrogen absorption into the electrode material occurs during proton discharge. The steady-state polarization curves in the course of positive and negative potential scans illustrate that the formation of absorbed hydrogen slightly improves the electrocatalytic properties of the electrode. The hydrogen diffusion coefficient into the bulk of TiHg alloy was estimated as 1.24 × 10 −13 cm 2 s −1 from a chronoamperometry measurement and was compared with other materials. Finally, the electrochemical impedance spectroscopy was used to further characterize this electrode. The results show that this electrode displayes porous behavior and may be well described by a serial combination of two parallel R-CPE circuit.

Pt-Co/C nanoparticles as electrocatalysts for oxygen reduction in H 2SO 4 and H 2SO 4/CH 3OH electrolytes

The oxygen reduction reaction (ORR) was studied on carbon dispersed Pt and Pt-Co alloyed nanocatalysts with high contents of Co in H 2SO 4 and H 2SO 4/CH 3OH solutions. The characterization techniques considered were transmission electron microscopy (TEM), X-ray diffraction (XRD) and in situ X-ray absorption near edge structure (XANES). The electrochemical activity for the ORR was evaluated from steady state polarization measurements, which were carried out in an ultra thin layer rotating disk electrode. The results showed that with the increase of Co content, the nanoparticle size distributions become sharper and the mean particle diameters become smaller. XRD indicated low degree of alloy formation but significant phase segregation of Co was observed only for Pt-Co/C 1:3 and 1:5 (Pt:Co atomic ratios). The electrochemical measurements indicated that the four-electrons mechanism is mainly followed for the ORR on all materials and the electrocatalytic activities per gram of Pt is higher for the catalysts with higher Co contents. This was explained based on the XANES results which evidenced a decrease of the coverage of oxygenated Pt adsorbates due to the presence of Co. In the methanol-containing electrolyte, the Pt-Co/C 1:5 catalyst showed the highest performance. This was attributed to its low activity for the methanol oxidation due to the smaller probability for presenting three Pt neighboring Pt active sites.

The Effects of Simulated Fission Products in the Reduction of Hydrogen Peroxide on Simulated Nuclear Fuel Electrodes

The electrochemical reduction of hydrogen peroxide has been studied on a series of simulated nuclear fuel (SIMFUEL) materials with different amounts of simulated fission products. Steady-state polarization measurements under diffusion-compensated conditions show that the reaction proceeds with a weak dependence on applied potential and fractional reaction orders with respect to . A mechanism for reduction on SIMFUEL materials is proposed to explain these features. It is found that the activity of the material for the reduction reaction increases with the doping level. This increase can be attributed to noble metal particles present in the SIMFUEL. This enhanced reactivity is most apparent at low overpotentials, where reduction proceeds significantly faster on the noble metal particles than on the lattice.

Electrocatalytic oxidation of glucose at a Ni-curcumin modified glassy carbon electrode

The electrocatalytic oxidation of glucose was investigated on a nickel-basedchemically modified electrode (Ni(II)-curcumin) prepared by electropolymerization of Ni-curcumin complex (curcumin=1,7-bis[4-hydroxy-3-methoxyphenyl]-1,6-heptadiene-3,5-dione) in alkaline solution. Reaction kinetic and mechanism were investigated by using cyclic voltammetry (CV) and chronoamperometry (CA) techniques and steady-state polarization measurements. Cyclic voltammetry studies indicated that in the presence of glucose the anodic peak current of surface redox mediator was increased, followed by decrease in the corresponding cathodic current. This indicates that glucose was oxidized at the surface of this modified electrode. The results were explained based on the concept of electrocatalytic reactions that occur in this chemically modified electrode. The diffusion coefficient of glucose and the rate constant of the catalytic oxidation of glucose were found to be 6.7×10−6 cm2 s−1 and 6.5×103 M−1 s−1, respectively. It has shown that by using the Ni-curcumin modified electrode, glucose can be determined with good response and low detection limit.

Kinetic studies of the electrochemical nitrogen reduction and incorporation into yttria stabilized zirconia

The kinetics of the electrochemical reduction of molecular nitrogen at gold micro electrodes on yttria stabilized zirconia (YSZ) solid oxide electrolyte is studied by steady state polarization measurements. From the η / lg i plot for both cathodic and anodic polarization the apparent transfer coefficients α a and α c are evaluated. The sum of α a + α c exceeds unity and thus a multistep electron transfer process is suggested. The concept of the stoichiometric number is applied to the electrode reaction N 2 + 6 e − = 2N 3− supposing that the overall process involves at least two intermediate species. On the basis of the evaluation of the experimental results the reaction N 2 − + e − ⇌ N 2 2− is suggested as the rate determining reaction step for the cathodic nitrogen reduction and nitride formation.

Electrochemical Analysis of Direct Methanol Fuel Cells for Low Temperature Operation

The electrochemical behaviour of Direct Methanol Fuel Cells (DMFCs) was investigated at low temperatures (30°C-60°C). An 85 wt% Pt-Ru (1:1 a/o)/C anode catalyst and a 60 wt% Pt/C cathode catalyst were in-house prepared and characterised. The influence of noble metal loading on the performance of a DMFC based on these catalysts was studied by steady-state polarisation measurements. The DMFC maximum power density increased linearly from 30 to 75 mW cm at 60°C passing from 1 to 5 mg cm Pt loading in both electrodes. By further increasing the Pt loading at 10 mg cm only a slight increase of power density was recorded (81 mW cm).

Electrochemical behaviour of titanium in H 2SO 4–MnSO 4 electrolytes

The corrosion of titanium in H 2SO 4 electrolytes (0.001–1.0 M) at temperatures from ambient to 98 °C has been investigated using steady-state polarization measurements. Four distinct regions of behaviour were identified, namely active corrosion, the active–passive transition, passive region and the dielectric breakdown region. The active corrosion and active–passive transition were characterized by anodic peak current ( i m) and voltage ( E m), which in turn were found to vary with the experimental conditions, i.e., d ( log ⁡ ( i m ) ) / dpH = − 0.8 ± 0.1 and d E m/d pH which was −71 mV at 98 °C, −58 mV at 80 °C and −28 mV at 60 °C. The activation energy for titanium corrosion, determined from temperature studies, was found to be 67.7 kJ mol −1 in 0.1 M H 2SO 4 and 56.7 kJ mol −1 in 1.0 M H 2SO 4. The dielectric breakdown voltage ( E d) of the passive TiO 2 film was found to vary depending on how much TiO 2 was present. The inclusion of Mn 2+ into the H 2SO 4 electrolyte, as is done during the commercial electrodeposition of manganese dioxide, resulted in a decrease in titanium corrosion current, possibly due to Mn 2+ adsorption limiting electrolyte access to the substrate.

Electrochemical characterisation of Ir xSn 1− xO 2 powders as oxygen evolution electrocatalysts

The electrochemical properties and electrocatalytic performance of nanocrystalline oxide powders of the type Ir x Sn 1− x O 2 ( 0.2 ≤ x ≤ 1 ) have been examined. These oxides have been developed primarily as oxygen evolution electrocatalysts for proton exchange membrane (PEM) water electrolysers. The modified polyol method was used to prepare these oxides, by reducing precursors in ethylene glycol followed by thermal oxidation at 500 ° C. The materials were characterised in 0.5 mol dm −3 H 2SO 4 and PEM electrolytes by cyclic voltammetry, electrochemical impedance spectroscopy, and steady state polarisation measurements. Some comparisons were made between the electrochemical properties of the oxides in the different electrolytes.

Physical Characterization and Electrochemical Activity of Bimetallic Platinum-Silver Particles for Oxygen Reduction in Alkaline Electrolyte

Bimetallic Pt-Ag particles dispersed on a carbon powder with 1:1, 1:2, 1:3, and 1:10 (atomic ratio) Pt:Ag were investigated as electrocatalysts for the oxygen reduction reaction (ORR) in alkaline media. These materials were characterized by X-ray diffraction, in situ X-ray absorption near-edge structure, and extended X-ray absorption fine structure. The electrochemical activity for the ORR was evaluated from steady-state polarization measurements in a rotating ring-disk system and in gas diffusion electrodes (EDG). The results showed that the Pt-Ag particles are formed by a core composed by true Pt-Ag alloys, with the surface covered by a Ag-rich layer. An increase of the particle size and of the fcc unit cell lattice parameter with the increase of Ag content was observed. The electrochemical measurements indicated lower activity for the particles compared to , but higher activity compared to . A number of electrons close to 4 was observed for the ORR in the materials. A discussion of the catalytic activity of the Pt-Ag materials was made in terms of the electronic properties of the Pt and Ag atoms and the morphological structure of the particles.

Electrochemical Performance of Oxide Anode Materials and Catalytic Effect of Current Collector

In order to overcome the problems of SOFC anode such as carbon deposition and micro structural change by oxidation, it is desired to develop a reliable substitute for mckel/yttria stabilized zirconia cermet. Several kinds of oxide materials were tested as fuel electrode by means of impedance spectroscopy and steady state polarization measurements. These measurements confirmed that anode performance of Ce 0.9 Gd 0.1 O 1.95 and Ce 0.992 Nb 0.008 O 2 was good in humid hydrogen atmosphere. On the other hand, La 0.9 Ca 0.1 Cr 1-x Al x O 3 (x=0.2, 0.8), SrZr 0.825 Y 0.1 Ru 0.075 O 3 showed much poorer performance. The reaction kinetics of doped ceria was also investigated and it was revealed that doped ceria has almost the same dependence on gaseous partial pressure and temperature. A possible kinetic equation is proposed, though unclear points still remain.

Ru(bpy)2(L)]Cl2: luminescent metal complexes that bind DNA base mismatches.

Here we report the synthesis of luminescent ruthenium complexes that bind DNA base pair mismatches. [Ru(bpy)2(tpqp)]Cl2 (tpqp = 7,8,13,14-tetrahydro-6-phenylquino[8,7-k][1,8]phenanthroline), [Ru(bpy)2(pqp)]Cl2 (pqp = 6-phenylquino[8,7-k][1,8]phenanthroline), and [Ru(bpy)2(tactp)]Cl2 [tactp = 4,5,9,18-tetraazachryseno[9,10-b]triphenylene] have been synthesized, and their spectroscopic properties in the absence and presence of DNA have been examined. While [Ru(bpy)2(pqp)]2+ shows no detectable luminescence, [Ru(bpy)2(tpqp)]2+ is luminescent in the absence and presence of DNA with an excited-state lifetime of 10 ns and a quantum yield of 0.002. Although no increase in emission intensity is associated with binding to mismatch-containing DNA, luminescence quenching experiments and measurements of steady-state fluorescence polarization provide evidence for preferential binding to oligonucleotides containing a CC mismatch. Furthermore, by marking the site of binding through singlet oxygen sensitized damage, the complex has been shown to target a CC mismatch site directly with a specific binding affinity, Kb = 4 x 10(6) M(-1). [Ru(bpy)2(tactp)]2+, an analogue of [Ru(bpy)2(dppz)]2+ containing a bulky intercalating ligand, is luminescent in aqueous solution at micromolar concentrations and exhibits a 12-fold enhancement in luminescence in the presence of DNA. The complex, however, tends to aggregate in aqueous solution; we find a dimerization constant of 9.8 x 10(5) M(-1). Again, by singlet oxygen sensitization it is apparent that [Ru(bpy)2(tactp)]2+ binds preferentially to a CC mismatch; using a DNase I footprinting assay, a binding constant to a CC mismatch of 8 x 10(5) M(-1) is found. Hence results with these novel luminescent complexes support the concept of using a structurally demanding ligand to obtain selectivity in targeting single base mismatches in DNA. The challenge is coupling the differential binding we can obtain to differential luminescence.

Exploration of electrodeposited platinum alloy catalysts for methanol electro-oxidation in 0.5�M H2SO4: Pt-Ni system

In this work, we examine the electrocatalytic activity of electrodeposited Platinum (Pt)-Nickel (Ni) alloy layers on an inert substrate electrode for methanol oxidation reaction. Analyses using energy-dispersive fluorescent X-ray analysis and powder X-ray diffractometry confirm alloying of Pt with Ni in a range of compositions. Steady-state polarisation measurements in 0.5 M methanol+0.5 M H2SO4 solutions clearly show that the onset of electro-oxidation shifts to less anodic potential values (approximately 160 mV), while also exhibiting current enhancements up to ~15 times the currents obtained for the pure Pt electrodeposit. A linear relationship between the cyclic voltammetric peak (oxidation) current and [MeOH] is observed at a scan rate of 50 mVs−1, thus indicating reduced influence of adsorbed CO (COads) surface poison. A critical composition, Pt (92%)/Ni (8%) [denoted Pt-Ni(3) alloy] is found to exhibit maximum electrocatalytic activity, beyond which the activity drops, whereas pure Ni does not catalyse the reaction. While the promotion of electro-oxidation is understood to be largely due to the alloy catalyst, surface redox species of Ni oxide formed during the electro-oxidation process may also contribute to the oxygenation of COads, thereby enhancing the oxidation current. Plausible mechanisms of methanol oxidation on Pt/ transition metal alloy electrocatalysts are discussed in terms of electron transfer (in the alloy) and the role of Ni oxide species.

Electrochemical oxidation of aniline at boron-doped diamond electrodes

The electrochemical oxidation of aniline at boron-doped diamond (BDD) electrodes was investigated by cyclic voltammetry, steady-state polarization measurements and bulk electrolysis under potentiostatic control. It was found that acidic media is suitable for efficient electrochemical oxidation of aniline, because at low pH, the potential required for avoiding electrode fouling is lower than in neutral and alkaline media. The results of the longtime polarization measurements suggested that more anodic potentials ensure slightly higher efficiency for the conversion of aniline to CO2, while the direct oxidation process does not play a prominent part in the overall electrochemical incineration of aniline. The current efficiencies (∼44%) and the efficiency of aniline conversion to CO2 (∼80%) favourably compare with those reported for other electrochemical methods for aniline destruction. The results demonstrate the possibility of using BDD as an electrode material for electrochemical wastewater treatment, mainly when very high anodic potentials are required.

Kinetic and morphological investigation of CoFe alloy electrodeposition in the presence of organic additives

The use of organic additives is widespread in the manufacturing of thin films using electrodeposition. Common additives like saccharin (SAC) are used as leveling and brightening agents. In the present work, a comparative study of the influence of organic additives containing a similar saccharin molecular structure on CoFe alloy electrodeposition has been performed in order to obtain information about the kinetics of the cathodic process and the morphology of the deposits. The study was based on an electrochemical investigation using steady-state polarization measurements, coupled with scanning electron microscopy and X-ray dispersive analysis of the CoFe deposits. The composition, the current efficiency and the partial current densities of the CoFe alloy were determined in order to study the influence of SAC, phthalimide (PHTA) and another additive containing sulfur element: o-toluene sulfonamide ( oTOL) on the CoFe deposition from sulfate and chloride electrolytes at pH 3. Experimental results indicate that saccharin influence strongly the electrode kinetic contrary to oTOL and PHTA. The morphology of the CoFe deposits depends on used organic additive. It was also noted that the presence of saccharin results in smoother, more compact and more leveled deposits as compared to PHTA or oTOL.

Ruthenium-Based Dimensionally Stable Anode in Chlorate Electrolysis : Effects of Electrolyte Composition on the Anode Potential

In this work the anodic reactions taking place on a dimensionally stable anode (DSA) in chlorate electrolyte have been investigated. Rotating disk electrodes were made from commercial -catalyzed DSAs and studied in steady-state polarization measurements, mainly IR-corrected polarization curves. Effects of varying pH and electrolyte concentrations of chloride, chlorate, chromium(VI), hypochlorite as well as mass transport were studied. The kinetics for the chlorine evolution reaction, with a Tafel slope of 40 mV/decade of current, was not dependent on pH in the region 2-8, at potentials lower than 1.2 V vs. Ag/AgCl. The slope of the polarization curves increased at about 1.2 V vs. Ag/AgCl, a pH-dependent bend not due to mass-transport limitations in the electrolyte. At a pH of 6.5, typical for the chlorate process, oxygen evolution is an important side reaction favored by the dichromate buffer and by increased mass transport, both keeping down the pH at the anode. In the chlorine evolution region the potentials increased when adding Cr(VI) to the electrolyte, whereas no major effect was seen from additions of NaClO. © 2002 The Electrochemical Society. All rights reserved.

Cu-based amorphous alloy electrodes for fuel cells

The present work describes the characteristics of Cu–Zr and Cu–Ti amorphous alloys as catalysts for oxidation of methanol and formaldehyde in alkaline solutions. Two Cu-based amorphous alloys, Cu60Zr40 and Cu60Ti40 both prepared by melt spinning were investigated. Two types of electrode were used: as-quenched amorphous alloys and surface-activated amorphous alloys. The surface-activation treatment consisted in the immersion of the ribbons in 1 M HF solution for 30 s. The determination of the oxidation activity of methanol and formaldehyde was carried out by galvanostatic and by steady-state polarization measurements and cyclic voltammetry experiments in 1 M NaOH solutions containing CH3OH or CH2O, deoxygenated by nitrogen bubbling for 30 min at 30 °C. The HF-treated amorphous alloys exhibit catalytic activity only for formaldehyde oxidation; Zr-based alloys show higher current densities than pure crystalline copper and Ti-based alloys.

Anodic Behaviour of Y0.20Ti0.18 Zr0.62O1.90 Towards Hydrogen Electro-Oxidation in a High Temperature Solid Oxide Fuel Cell

The electrochemical oxidation of humid hydrogen was investigated on a Y0.20Ti0.18Zr0.62O1.90 (YTZ) anode in a high temperature planar solid oxide fuel cell. Cells using platinum counter and reference electrodes on a YSZ based electrolyte were fabricated and tested in H2/H2O atmospheres versus air in the temperature range (788–932) °C. Steady state polarisation measurements, cyclic voltammetry and impedance analysis were used to characterise the behaviour of the anode and determine kinetic parameters of the electro-oxidation reaction. The power output of the cell H2,H2O/YTZ//YSZ-Al2O3//Pt/air was stable over an operation period of 500 h. For 97%H2/3%H2O at 932 °C it amounted to 37 mW cm–2 and the maximum current density obtained was 203 mA cm–2. The apparent reaction order with respect to hydrogen was calculated to be in the region of 0.5 at 932 °C. The impedance spectra showed three apparent processes dependent on temperature, fuel concentration and overpotential. Anode polarisation resistance decreased significantly with anodic overpotential. At open and short circuit (Ecell = 0 V) conditions the total area specific polarisation resistance was 12.67 and 0.96 Ω cm2 respectively.