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The reduction of l-cystine hydrochloride at lead using static and rotating disc electrodes
No full textThe reduction of the disulphide, l-cystine hydrochloride to the l-cysteine hydrochloride thiol, in 0.1 mol dm?3 HCl at 298 K, has been studied at pre-treated, circular, 0.50 cm2 lead disc electrodes using steady state linear sweep voltammetry, non-steady state voltammetry and controlled potential coulometry. The diffusion coefficient for l-cystine hydrochloride was approximately 4.8 × 10?10 m2 s?1 from the three techniques. Reduction of the disulphide was irreversible and hydrogen evolution occurred as a competitive reaction at approximately ?1.35 V vs. SCE. Analysis of the mixed control kinetics, using a Koutecky–Levich approach, allowed the relative roles of charge transfer and mass transport to be resolved. Anomalously high Tafel slopes, of typically ?183 mV, were observed due to disulphide adsorption. The charge transfer kinetics are consistent with the first electron gain being rate determining while reaction orders are +1 with respect to both the disulphide and proton concentrations. The mechanism of l-cystine hydrochloride reduction has been critically discussed
The reduction of l-cystine in hydrochloric acid at mercury drop electrodes
No full textThe reduction of L-cystine in 0.1 mol dm3 HCl at 298 K has been studied at mercury electrodes. Dropping mercury electrode (DME), static mercury drop electrode (SMDE) and hanging mercury drop electrode (HMDE) modes were used with normal, sampled d.c. and differential pulse polarographic detection. The charge transfer kinetics for the irreversible reduction of L-cystine were complicated by reactant and product adsorption, by the formation of cysteinate complexes between mercury and the product thiol as well as by formation of Hg2Cl2. High Tafel slopes of 182 mV per decade were observed with a cathodic transfer coefficient of 0.32. The diffusion coefficient of L-cystine was found to be 5.3 · 10-10 m2 s-1 in 0.1 mol dm-3 HCl and 4.2 · 10-10 m2 s-1 in 2.0 mol dm-3 HCl at 298 K. The
mechanism of L-cystine reduction at mercury has been discussed and the effects of pH and reactant concentration have been quantified
Ionic conductivity of an extruded Nafion 1100 EW series of membranes
Get PDFThe proton conductivity of a series of extruded Nafion membranes @of equivalent weight ~EW! of 1100 and nominal dry thickness of 51, 89, 127, and 178 mm# has been studied. Measurements were made in 1 M H2SO4 at 298 K using a four-electrode, dc technique. The membrane area resistance increases with thickness, as expected, from 0.07 to 0.16 V cm2 for Nafion 112 and Nafion 117, respectively. However, in contrast to the published literature, after correcting for the membrane thickness, the conductivity of the membranes decreases with decreasing membrane thickness. For example, values of 0.083 and 0.16 S cm21 were obtained for Nafion 112 and 117 membranes, respectively. In situ current-interrupt measurements in a proton exchange membrane fuel cell confirmed the relatively poor conductivity of the membrane electrode assemblies ~MEAs! based on the thinner membranes. While a high contact resistance to the electrodes may have contributed to the in situ MEA resistance, water balance measurements over the MEA showed that the high resistance was not due to a low water content or to an uneven water distribution in the MEAs. The implications of the findings for the understanding of the membrane properties are discussed
The reduction of l-cystine hydrochloride at stationary and rotating disc mercury electrodes
No full textThe kinetics of l-cystine hydrochloride reduction have been studied at a mercury-plated copper rotating disc electrode (RDE) and at a stationary mercury disc electrode (SMDE) in 0.1 mol dm?3 HCl at 298 K. The reduction of the disulphide is irreversible and hydrogen evolution is the major side reaction. In contrast to steady state electrode kinetic studies at a mercury drop electrode (which shows a well-defined limiting current), the mercury-plated Cu RDE shows overlap between disulphide reduction and hydrogen evolution. These effects are attributable to strong reactant adsorption with a calculated surface coverage close to 100%. A Tafel slope of ?185 mV per decade is found with a cathodic transfer coefficient of 0.32 and a formal rate constant of 6.7 × 10?9 m s?1. The relative merits of steady state voltammetry at a mercury-plated copper RDE and linear sweep voltammetry at the SMDE are discussed, as is the mechanism of l-cysteine hydrochloride formation
Modelling and simulation of the degradation of perfluorinated ion-exchange membranes in PEM fuel cells
Get PDFA polymer-electrolyte membrane fuel cell model that incorporates chemical degradation in perlfluorinated sulfonic acid membranes is developed. The model is based on conservation principles and includes a detailed description of the transport phenomena. A degradation sub-model describes the formation of hydrogen peroxide \/\it via\/ distinct mechanisms in the cathode and anode, together with the subsequent formation of radicals \/\it via\/ Fenton reactions involving metal-ion impurities. The radicals participate in the decomposition of reactive end groups to form carboxylic acid, hydrogen fluoride and CO_2. Degradation proceeds through unzipping of the polymer backbone and cleavage of the side chains. Simulations are presented and the numerical code is shown to be extremely time efficient. Known trends with respect to operating conditions are qualitatively captured and the exhibited behaviour is shown to be robust to changes in the rate constants. The feasibility of a chemical degradation mechanism based on peroxide and radical formation is discussed
Characterisation of a re-cast composite Nafion® 1100 series of proton exchange membranes incorporating inert inorganic oxide particles
No full textA series of cation exchange membranes was produced by impregnating and coating both sides of a quartz web with a Nafion® solution (1100 EW, 10 %wt in water). Inert filler particles (SiO2, ZrO2 or TiO2; 5–20 %wt) were incorporated into the aqueous Nafion® solution to produce robust, composite membranes. Ion-exchange capacity/equivalent weight, water take-up, thickness change on hydration and ionic and electrical conductivity were measured in 1 mol dm-3 sulfuric acid at 298 K. The TiO2 filler significantly impacted on these properties, producing higher water take-up and increased conductivity. Such membranes may be beneficial for proton exchange membrane (PEM) fuel cell operation at low humidification. The PEM fuel cell performance of the composite membranes containing SiO2 fillers was examined in a Ballard Mark 5E unit cell. While the use of composite membranes offers a cost reduction, the unit cell performance was reduced, in practice, due to drying of the ionomer at the cathode
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Recent trends and developments in polymer electrolyte membrane fuel cell modelling
No full textModelling and simulation are well-established tools for investigating the physical processes inside a polymer electrolyte membrane (PEM) fuel cell. The early literature paid great attention to steady-state transport phenomena in the main components, which continues to be a focus of ongoing activities. There is, on the other hand, a growing interest in modelling other aspects of fuel cell operation, such as transient performance and degradation phenomena. There has also been a growth in the number of molecular and pore-level studies of transport phenomena in fuel cell components, enabled by developments in simulation techniques and enhancements in computer hardware. Such approaches are capable of representing important small-scale phenomena more faithfully than traditional macroscopic models. This review summarises recent activity in PEM fuel cell modelling, with a focus on detailed physical models, and considers its potential significance. An industrial perspective is also provided, highlighting the current use of modelling in the test and design cycles, and outlining future requirements.<br/
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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