Lower Potential Limit Research Articles

Structure and inhibition effects of anion adlayers during the course of O 2 reduction

Structures of Br adlayers were determined on the Ag(100), Au(100) and Pt(111) electrode surfaces during the course of O 2 reduction by using synchrotron surface X-ray scattering techniques and correlated with the O 2 reduction activity obtained from rotating disk electrode measurements. On Ag(100), the c(2×2) Br adlayer, precludes the O 2 adsorption in ‘bridge’ configuration, and the O 2 reduction current resulting solely from adsorption in the ‘end-on’ configuration through the four-fold symmetry holes in the c(2×2) Br lattice is observed at large overpotentials. On Au(100), O 2 reduction is completely inhibited by the c(√2×2√2)R45° phase. The reaction takes place only at potentials negative of the low potential limit of the existence of that phase. O 2 reduction is also completely blocked by the commensurate (3×3) Br adlayer on Pt(111) at high potentials. Below 0.5 V, the ordered phase vanishes and O 2 reduction takes place. Adsorbed disordered Br adions change the mechanism of O 2 reduction on Pt(111) into a 2e-reaction. O 2 affects the stability of ordered Br adlayer on Pt(111), but not on Ag(100) and Au(100).

Voltammetric investigation of platinum oxides II. Effect of hydration on their reduction behavior

The reduction behavior of α platinum oxides on PtO x /Ti (fabricated by thermal decomposition) and Pt/Ti (reduced from PtO x /Ti) was compared using linear sweep voltammetry. The peak potential of α oxide reduction was found to shift to a less positive value with decreasing its hydration degree, which depended upon the lower potential limits and cycle number of cyclic voltammetry. Factors affecting the stability of various platinum oxides, including α and β types, were systematically discussed in this work.

Reactions at the lower potential limit in aprotic medium at a platinum cathode revisited: their role in indirect electrochemical reductive degradation of polymers

Abstract Formation of hydroxyl ions when reduction of residual water occurs at the lower potential limit at a Pt cathode in a tetrabutylammonium perchlorate solution in dimethylformamide (DMF), results not only in splitting of TBA + cations via the Hofmann reaction, but also in hydrolysis of the solvent through an intermediate charged complex into dimethylamine (DMA) and a formic acid anion. These solvent hydrolysis products can play a significant role in reactions, which may occur in electrochemical organic systems. For example, such products formed by electrochemical reduction of the background solution at a Pt cathode, interact with some electrophilic compounds present in the solution. In the case of polymers, it results both in dehydrohalogenation of halogenated polyolefins (polyvinyl chloride, poly(vinylidene fluoride), etc.) and in degradation of ester bonds in polycarbonates. That is, the solvent hydrolysis products are the active agents of the indirect electrochemical reductive degradation (ECRD) of polymers. The results obtained suggest that an intermediate charged complex of an aprotic solvent (DMF, MeCN) with OH − ions plays the governing role in the indirect ECRD of halogenated polyolefins. Furthermore, this complex and the DMA which appeared due to DMF hydrolysis are the agents of an indirect ECRD of polycarbonates. Undoubtedly, the electrochemically induced hydrolysis reactions can be minimized both in the thoroughly dried solutions, which have by our CV data a lower potential limit at Pt at about −3.0 V versus SCE, in the case of cathode materials with a high hydrogen overpotential.

Complications associated with kinetic studies of hydrous Ir oxide films

The kinetics of the oxidation and reduction of electrochemically formed Ir oxide films have been examined as a function of the potential limits used, oxide film thickness, solution pH and the degree of reversible aging (diminished kinetics) of the oxide. It is shown first that the oxidation kinetics of the principal Ir(III) to Ir(IV) redox reaction are diminished in acidic solutions if the lower potential limit is set so as to include the kinetically slow process which occurs in the anodic cyclic voltammetric pre-peak, A 0. Consistent with this, in alkaline solutions, little evidence for the prepeak is found, reversible aging does not occur and the kinetics do not depend significantly on the lower potential limit. Other results show that, if kinetic measurements are made employing a lower limit more positive than the A 0 pre-peak and an upper potential limit below that required to generate Ir(V) and/or Ir(VI) states, the kinetics of the Ir(III)/Ir(IV) process are independent of film thickness in acidic solutions. If anodic steps are made to 1.25 V or more in acidic solutions, the Ir(V)/(VI) states generated can mediate and, hence, accelerate, the oxidation of Ir(III) to Ir(IV), as seen by the unusual shapes of the j/ t transients.

Multicomponent hydrous oxide films grown on gold in acid solution

Multilayer hydrous oxide growth on gold in aqueous acid solution was carried out using a repetitive potential cycling technique and the extent of oxide formation, plus the reduction behaviour of the film, was monitored by applying a subsequent negative sweep. The effect of a number of variables in the cycling process e.g. upper and lower potential limits, cycling rate, number of oxide growth cycles, as well as the influence of the oxide reduction sweep rate, were investigated. In many cases three cathodic peaks (along with that for reduction of the monolayer oxide film) were observed in the negative sweep and these were attributed to the presence of three distinct hydrous oxide components (labelled as HO1, HO2 and HO3) in the multilayer deposit. Analogies between these results for gold and recent data obtained for platinum and palladium under similar conditions were noted. It was also observed that the potential values for the reduction of these hydrous oxides and the onset potentials for many electrocatalytic reactions (as investigated earlier) for this electrode system were very similar.

Structure of Br adlayers in the course of electrocatalytic reactions O 2 reduction of Pt(111) and Au(100)

Structures of Br adlayers were determined on the Pt(111) and Au(100) electrodes in the absence and in the presence of O 2 reduction in 0.1 M HClO 4 solutions. On Pt(111), the commensurate (3 × 3) Br adlayer was found between 0.3 and 0.8 V. On Au(100), the c(√2 × 2√2)R 45 and c(√2 × p)R45° phases were found above 0.35 and 0.68 V, respectively, in agreement with previous work in neutral solutions. O 2 reduction is completely inhibited by the c(√2 × 2√2)R45 phase on Au(100). It takes place only at potentials negative of the low potential limit of the existence of the c(√2 × 2√2)R45 phase. On Pt(111), O 2 reduction is also completely blocked by the (3 × 3) Br adlayer up to 0.5 V. At this potential, the ordered phase vanishes and O 2 reduction takes place. Adsorbed disordered Br adions change the mechanism of O 2 reduction on Pt(111) from a 4e- into a 2e-reaction.

The electroreduction of carbon dioxide by macrocyclic cobalt complexes chemically modified on a glassy carbon electrode

Abstract Various macrocyclic cobalt complexes were chemically bonded to a glassy carbon (GC) electrode, on which CO 2 , electroreduction was carried out to give CO in aqueous phosphate buffer solution of pH 6.3. The macrocyclic cobalt complexes were naphthalocyanato cobalt(II), phthalocyanato cobalt(II), dibromo(11-hydroxyimino-4,10-dipropyl-5,9-diazatrideca-4,9-diea-3-one oximato) cobalt(III) (denoted as CoDO), two kinds of hydrophobic vitamin B 12 s (heptamethyl cobyrinate perchlorate and heptapropyl cobyrinate perchlorate 5,10,15,20-tetraphenylporphyrinato cobalt(II), and 5,10,15,20-tetrakis (4-methoxyphenyl)porphyrinato cobalt(II). Their redox potentials Co(I)/Co(II) are in the above order from positive to negative potentials in a 0.05 M TBAP DMSO solution, being between -0.23 and -1.0 V (SCE). These complexes were chemically bonded to GC through -CONH-pyridine which locates perpendicularly to a planar semi-planar complex structure, where the N of the pyridine forms a coordinate bond with the Co atom of the above complexes as a fift ligand. The catalytic activity for hydrogen evolution in aqueous solution was observed to be high on Co naphthalocyanine and C phthalocyanine modified GC electrodes; the latter gave H 2 evolution at the most positive potentials among the Co complexes employee. When the lower potential limit in cyclic voltammetry became less than the hydrogen evolution potential, in the reverse positive-going sweep, and anodic lump current was oberved at -0.35 ∼ -0.78 V, which is assigned to a cobalt hydride oxidation process; the hydrid is suggested to form when hydrogen evolution takes place, and the hump disappeared after the introduction of CO 2 into the solution, was observed that the Co complex chemically bonded on GC can give CO from CO 2 only at relatively low overvoltages, except for CoDO which was not able to reduce CO 2 ; phthalocyanato cobalt(II) gave CO at E = − 1.0V (0.26 V as overvoltage) at 20% current efficiency. The highest CO current efficiency was observed in the case of tetraphenyl-porphyrinato cobalt(II) chemically modified GC.

Electrocrystallisation studied in situ by optical scattering

Optical scattering was measured in situ on an electrochromic tin oxide film during coulometric titration in an electrolyte of 1 M LiClO 4 in propylene carbonate. The normalised scattering increased at the critical point where electrocrystallisation started, according to optical and electrochemical results. A lower potential limit of 1.2V vs Li was established for tin oxide in electrochromic applications. Electrocrystallisation takes place below this limit.

Effect of concentration of lithium ions on the voltammetric responses of nitro-substituted aromatic sulfides in dimethylformamide on glassy carbon electrodes

Lithium salts are used as supporting electrolytes and ion-pair forming reagents during voltammetric investigations in aprotic solvents. In the present work, these ions are found to have a significant influence on the voltammetric responses of aromatic sulfides in dimethylformamide given their concentration and the cathodic potential limits applied. At very low concentrations (< 4 mM) and cathodic potential limits (− 1.4 V), lithium ions form ion-pairs with the anion radicals generated by the reduction of nitro groups on the electrode surface. This is the conventional lithium ion-pair formation effect reported in the literature. With increasing lithium ion concentration and increasing cathodic limit, the ion-pairs tend to form an insoluble salt film on the electrode surface. This leads to inhibition of further electron transfer. The inhibiting effect does not seem to correlate with the size of the reactant organic molecule involved. At cathodic potential limits exceeding − 2 V, lithium ions appear to undergo direct reduction and subsequent reaction with trace levels of water in the solvent to produce a passive LiOH layer. This inhibits all further electron transfer.

Oxidation of formic acid in acid solution on Pt single-crystal electrodes

Abstract The electrochemical oxidation of formic acid in an acidic medium was studied systematically on well-defined Pt single-crystal electrodes, particularly the Pt(100) plane. Focusing on the analysis of the voltammetric profiles, the effects of the experimental conditions (formic acid concentration, upper and lower potential limits of the sweep, sweep rate, solution pH etc.) were examined extensively. The product survey was performed using the newly developed on-line differential electrochemical mass spectrometry (DEMS). The following results were obtained. 1. (1) The surface species X 1 (linearly bonded CO) formed mainly in the hydrogen region and retarded the reaction strongly up to ca. 0.8 V in the positive-going scan. When the X 1 coverage is lower, the reaction occurred with a peak P 1 at ca. 0.4 V (process I). 2. (2) After the removal of X 1 at ca. 0.8 V (peak P 2 ), no reaction occurred up to a potential of 1.0 V, suggesting the presence of retarding process. 3. (3) When the potential was reversed at 1.0 V, the retarding process remained beyond 0.8 V in the negative-going scan. The retarding action decreased exponentially with the negative potential shift, and correspondingly the reaction was accelerated. 4. (4) The reaction continued to occur with a peak P 3 at ca. 0.6 V (process II). With further negative polarization, the reaction ceased beyond 0.3 V with a shoulder corresponding to P 1 (process I). 5. (5) Another retarding process was observed in the potential region between P 1 and P 3 . The species X 2 responsible was electrochemically inactive. The behavior of X 1 and X 2 , processes I and II, and the retarding process on Pt(100), together with the DEMS results, were examined in detail.

Voltammetric Studies on the Palladium Oxides in Alkaline Media

The formation and stripping of palladium oxides on a palladium electrode in a 0.1M solution was studied by cyclic voltammetry. Cyclic polarization methods were used to form palladium oxides on the surface of the palladium electrode. Three different types of palladium oxides were found to be formed in alkaline solutions. A higher oxidation state of palladium oxide can be formed (induced) on the surface of the electrode even at low anodic potential limit, 0.6 V (vs. SCE). Strong evidence that can only be formed in a specific potential range is presented. From the voltammograms obtained after a long cyclic polarization time, the peak in the range of −0.47 to −0.60 V could be attributed to the reduction of dehydrated .

Daily transpiration of field soybeans as related to hydraulic conductance, root distribution, soil potential and midday leaf potential

The present study aims at characterizing plant water status under field conditions on a daily basis, in order to improve operational predictions of plant water stress. Ohm's law analog serves as a basis for establishing daily soil-plant relationships, using experimental data from a water-limited soybean crop: 227-1. The daily transpiration flux, T, is estimated from experimental evapotranspiration data and simulated soil evaporation values. The difference, 227-2, named the effective potential gradient, is derived from i) the midday leaf potential of the uppermost expanded leaves and ii) an effective soil potential accounting for soil potential profile and an effectiveness factor of roots competing for water uptake. This factor is experimentally estimated from field observation of roots. G is an apparent hydraulic conductance of water flow from the soil to the leaves. The value of the lower potential limit for water extraction, required to assess the effective soil potential, is calculated with respect to the plant using the predawn leaf potential. It is found to be equal to −1.2 MPa. It appears that over the range of soil and climatic conditions experienced, the daily effective potential gradient remains constant (1.2 MPa), implying that, on a daily basis, transpiration only depends on the hydraulic conductance. The authors explain this behaviour by diurnal variation of osmotic potential, relying on Morgan's theory (1984). Possible generalization of the results to other crop species is suggested, providing a framework for reasoning plant water behaviour at a daily time step.

Observation of bistability in cyclic voltammetric experiments on ethanol, propanol, butanol and formic acid/formate

The effects of varying the lower potential limit were studied in cyclic voltammetric experiments conducted on several electrocatalysed oxidation processes: the oxidation of formic acid/formate, ethanol, propanol and butanol. In an acidic medium, each oxidation process, subjected to a cycling potential, exhibited a bistable response over a range of values in the lower potential limit. Within the region of bistability, two different cyclic voltammograms, one with a relatively large amplitude and one with a small amplitude, were found to coexist. Experiments conducted on the oxidation of butanol revealed that the region of bistability decreases in size with respect to increases in the alkalinity of the solution; a result that is consistent with the description of bistability provided by the theory of catastrophes. The cause of the cyclic voltammetric bistability is explained in terms of a nonlinear feedback mechanism involving a reaction between strongly bound carbon monoxide and oxides. The reaction couples an indirect path of the oxidation processes to the process by which an oxide layer is formed.

Hydrous Oxide Film Growth on Amorphous Ni‐Co Alloys

The electrochemical behavior of a (weight percent) amorphous alloy ribbon has been investigated in alkaline solutions. When the metal is initially subjected to anodic potentials, an enriched Cr (and possibly B) surface layer is dissolved. Following this, a hydrous oxide film can be readily formed on the electrode surface by a continuous potential cycling method, to a thickness of up to ca. one micron. The film, which has an electrochemical signature which is very similar to the Ni(II)/Ni(III) transition, is electrochromic in nature and displays interference colors when still thin. The maximum growth rate of the film per cycle of potential has been found to be 0.15 to 0.20 mC/cm2, achieved by optimization of the magnitude and time spent at the upper and lower potential limits.

Theoretical assessment of an oxygen heat engine: The effect of mass transport limitation

The efficiency of a closed, reversible cycle consisting of an isothermal expansion of a gas at a higher temperature followed by its compression at a lower temperature is Carnot-limited. If the expansion and compression can be achieved electrochemically through solid electrolytes, heat can be directly converted into electrical energy. A device with oxygen ion conducting electrolytes and oxygen gas as the working fluid may be called an oxygen heat engine in analogy with the sodium heat engine. Theoretical analysis of such a device is presented. The power is shown to exhibit a maximum at some critical pressure in the low pressure chamber. The maximum in power occurs due to a compromise between thermodynamic and kinetic factors. At pressures lower than the critical pressure, mass transport in the low pressure chamber limits power. At pressures greater than the critical, low Nernst potential limits power. The mass transport limitation can, in principle, be eliminated by proper design of the device and/or introduction of an oxygen containing compound with low equilibrium oxygen partial pressure in the low pressure chamber. For example, introduction of a mixture of H 2O and H 2 in the low pressure chamber makes high power densities possible. Theoretical assessment of this type of heat engine shows that power densities in excess of 200 mW/cm 2 are possible.

Dinitrogen fixation in a water-stressed Alnus clone is limited by host xerotolerance

The osmotolerance, rather than the halotolerance, of the endosymbiont predicted the xerotolerance of acetylene reduction by Alnus nodulated withFrankia ARgP5AG. Cloned plants ofAlnus glutinosa (L.) Gaertn. AG8022-16 were subjected to water stress under controlled conditions in an environmental growth chamber. Transpiration, stomatal conductance, and leaf water potential had decreased after successive 10 day periods of moderate (75% of water demand) and severe (50% of water demand) water stress. After severe stress had wilted the plants, reducing leaf water potential to −2.10 MPa, nitrogenase activity had fallen to 2.51 μM per plant per hour. The reported rapid turnover of nitrogenase implies thatFrankia mycelium was metabolically active at this low water potential, a water potential at which no Alnus-derivedFrankia has been reported active. Although ARgP5AG was similar to other such strains in halotolerance (lower limitca.−1.25 MPa), the low water potential limit for growth with glucose (a non-assimilated osmoticum) wasca.−2.53 MPa. Nitrogenase activity was apparently more limited by host xerotolerance than by endophyte xerotolerance.

Transgranular stress-corrosion cracking of disordered Cu-25Au in aqueous chloride and sulfate media

Transgranular stress-corrosion cracking (T-SCC) in disordered copper-25 at. pct gold single crystals was studied in two chloride media: 2 pct (0.13 M) FeCl3 and oxygen-free 0.6 M NaCl. A limited number of tests were also performed on polycrystals in the chloride media and also in acidified 1 M Na2SO4. Potentiostatic polarization, constant deflection tests, and high-resolution SEM examination were used to relate the electrochemical and fractographic features of T-SCC in these systems. Exclusive selective electro-dissolution of copper occurs over the potential domain investigated, and the polarization behavior is characterized at low potentials by a “passive≓ domain in which the current is very low (2 to 4 microamps/cm2), increasing only very slowly with increasing potential up to a “transpassive” potential,E c. AboveE c the current increases by a factor of 103 in a very narrow domain of rising potential ( ≈ 50 mv). In both chloride media Ec was found to be +430 mv (sce). At and above Ec a porous gold-rich “sponge” is readily formed on the surface, and in aq. FeCl3 metallographic evidence for sponge formation was also found in the low-current region of potential, well below Ec. In all three media, fracture surfaces display the characteristic cleavage-like features scen in other T-SCC systems. For the constant deflection loading employed in the present investigation, the lower limit of potential for susceptibility to cracking in aq. NaCl occurs at +400 mv (sce), which is slightly below Ec; whereas, for aq. FeCl3 the limiting potential is 0 mv (sce), considerably belowE c. For both chloride media, the upper limit, if it exists, is greater than +600 mv (sce). These results are analyzed in order to distinguish between proposed mechanisms for T-SCC.

Electrochemical faceting of polycrystalline gold in 1 M H 2SO 4

Electrochemical faceting of polycrystalline (pc) gold electrodes in 1 M H 2SO 4 was investigated by employing wire and bead-shaped electrodes. Electrochemical faceting was produced by applying a repetitive symmetrical square wave potential signal in the 2–4 kHz range and upper and lower potential limits ranging between 1.44 and 1.60 V (vs. RHE) and 0.10 and 1.10 V, respectively. The degree of faceting was followed voltammetrically principally through the O-electroadsorption/electrodesorption process. Electrochemical information was complemented with scanning electron microscopic observations. The results are discussed in terms of the equilibrium potential of reactions involving different gold species, and the potential of zero charge, hydrophobicity and anion adsorbability of the (110), (100) and (111) crystallographic faces of gold. The kinetics of electrochemical faceting of gold can be explained through the two-stage mechanism proposed earlier for polycrystalline platinum in acid electrolyte.

LEED spot profile analysis of the structure of electrochemically treated Pt(100) and Pt(111) surfaces

Abstract The structures formed by oxidation-reduction cycling of ordered Pt(100) and Pt(111) surfaces in aqueous electrolytes were studied using LEED spot profile analysis. Surfaces cycled to anodic potential limits above 1.0 V (RHE) and emersed at 0.5 V gave LEED spots which varied in width periodically with beam energy. The varying spot widths indicated a type of randomly stepped surface, and mean terrace widths were estimated by comparison with the previously published scattering calculations of Henzler and Lu and Lagally. LEED spots from surfaces cycled to potentials below 1.0 V (RHE) remained sharp at all beam energies. The restructuring resulted from the anodic formation of an amorphous surface oxide phaes involving place exchange of platinum and oxygen followed by a reduction process in which the platinum atoms do not all return to their original positions in the surface lattice. The observed spot profiles changed both with the the number of cycles and with the upper potential limit. Cycling to 1.28–1.58 V produced characteristically different structures than cycling to limits of 1.08–1.28 V. At the higher potentials, a randomly-stepped surface was formed whose mean terrace width and extent of vertical relief were functions of the total anodic charge. Cycling to the lower potential limits produced a type of structure which is intermediate between the classical two-level island and the multi-level randomly-stepped structure. A three-level structure with a high degree of correlation between the first (island-like) and third (hole-like) levels is proposed for the latter.

LEED spot profile analysis of the structure of electrochemically treated Pt(100) and Pt(111) surfaces

The structures formed by oxidation-reduction cycling of ordered Pt(100) and Pt(111) surfaces in aqueous electrolytes were studied using LEED spot profile analysis. Surfaces cycled to anodic potential limits above 1.0 V (RHE) and emersed at 0.5 V gave LEED spots which varied in width periodically with beam energy. The varying spot widths indicated a type of randomly stepped surface, and mean terrace widths were estimated by comparison with the previously published scattering calculations of Henzler and Lu and Lagally. LEED spots from surfaces cycled to potentials below 1.0 V (RHE) remained sharp at all beam energies. The restructuring resulted from the anodic formation of an amorphous surface oxide phaes involving place exchange of platinum and oxygen followed by a reduction process in which the platinum atoms do not all return to their original positions in the surface lattice. The observed spot profiles changed both with the the number of cycles and with the upper potential limit. Cycling to 1.28–1.58 V produced characteristically different structures than cycling to limits of 1.08–1.28 V. At the higher potentials, a randomly-stepped surface was formed whose mean terrace width and extent of vertical relief were functions of the total anodic charge. Cycling to the lower potential limits produced a type of structure which is intermediate between the classical two-level island and the multi-level randomly-stepped structure. A three-level structure with a high degree of correlation between the first (island-like) and third (hole-like) levels is proposed for the latter.