6 research outputs found
Electrochemical Characterization of Low-Temperature Direct Ethanol Fuel Cells Using Direct and Alternate Current Methods
Here we report for the first time the results of systematic characterization of a low-temperature polymer electrolyte membrane direct ethanol fuel cell using DC and AC electrochemical methods. Model catalysts (carbon supported Pt nanoparticles) painted on carbon paper are used as anode and cathode. Influence of physical parameters, such as cell temperature, current density, ethanol concentration and anode fuel flow rate on overall cell impedance is studied. Analysis of the obtained impedance spectra in connection with DC measurements allows us to comment on cell properties and to separate different contributions to the overall cell polarization. Our results suggest that the cell impedance is dominated by anode faradaic impedance, with negligible contribution from cathode faradaic impedance. The anode impedance depends strongly on current density and cell temperature, but is not significantly influenced by ethanol concentration. Presence of anode mass-transfer impedance, even when ethanol was fed to the cell in high excess, is confirmed. Based on the results we conclude that changes in ethanol electro-oxidation mechanism might manifest themselves on the impedance spectra in the low-frequency inductive loop. Nonetheless, further studies involving equivalent circuit modelling are needed to determine the exact influence of the cell parameters on the anode kinetics
Media Effects on the Mechanism of Antioxidant Action of Silybin and 2,3-Dehydrosilybin: Role of the Enol Group
Silybin
(SIL) and 2,3-dehydrosilybin (DHS) are constituents of milk thistle
extract (silymarin) applied in the treatment of cirrhosis, hepatitis,
and alcohol-induced liver disease. The molecular mechanism of their
action is usually connected with antioxidant action. However, despite
experimental and theoretical evidence for the antioxidant activity
of SIL and DHS, the mechanism of their antiradical action still remains
unclear. We studied the kinetics of SIL/DHS reactions with 2,2-diphenyl-1-picrylhydrazyl
radical in organic solutions of different polarity and with peroxyl
radicals in a micellar system mimicking the amphiphilic environment
of lipid membranes. Kinetic studies together with determination of
acidity and electrochemical measurements allowed us to discuss the
structure–activity relationship in detail. In nonpolar solvents
the reaction with free radicals proceeds via a one-step hydrogen atom
transfer (HAT) mechanism, while significant acceleration of the reaction
rates in methanol and water/methanol solutions suggests the dominating
contribution of a sequential proton-loss electron-transfer (SPLET)
mechanism with participation of the most acidic hydroxyl groups: 7-OH
in SIL and 7-OH and 3-OH in DHS. In a heterogeneous water/lipid system,
both mechanisms operate; however, the reaction kinetics and the antioxidant
efficacy depend on the partition between lipid and water phases
Strong and Long-Lived Free-Radical Oxidizer Based on Silver(II). Mechanism of Ag(I) Electrooxidation in Concentrated H<sub>2</sub>SO<sub>4</sub>
Electrooxidation
of silver(I) was studied in concentrated sulfuric
acid and oleums at fluorine-doped tin oxide electrode. Electrochemical
processes taking place at the electrode/solution interface have been
investigated by using classical transient methods. We report a catalytic
reaction pathway, in which one-electron transfer leading to solvated
Ag(II) species is followed by a homogeneous chemical step with regeneration
of depolarizer. By means of digital simulations this mechanism and
the half-life of the Ag(II) complex was determined, τ<sub>1/2</sub> = 10 s. Activation energies of the electrode process and chemical
reaction were found to be <i>E</i><sub>a,E</sub> = 52 ±
5 kJ·mol<sup>–1</sup> and <i>E</i><sub>a,C</sub> = 60 ± 5 kJ·mol<sup>–1</sup>, respectively. The
chemical step likely corresponds to a free-radical oxidation of HSO<sub>4</sub><sup>–</sup> anions by Ag(II) radical species. Highly
oxidative abilities of long-lived Ag(II) can be used for mediated
electrochemical oxidation or electrochemical combustion
Spontaneous Chemical Ordering in Bimetallic Nanoparticles
In most experimental studies on bimetallic
nanoparticles the homogeneous
alloying is consistently reported or a priori assumed. It is generally
believed that at nanoscale, alloying is promoted even for otherwise
immiscible systems. In this study we present evidence that Pd–Pt
nanoalloys are much more susceptible to segregation than their bulk
counterparts. The spontaneous segregation (chemical ordering) of Pd–Pt
nanoalloys was evidenced in hydrogen absorption studies and additionally
confirmed by using X-ray photoelectron spectroscopy. The results clearly
show that phase segregation in bimetallic nanoparticles do occur despite
the lack of the miscibility gap in the Pd–Pt phase diagram,
negative heat of mixing and small lattice mismatch for both metals.
Phase segregation for other bimetallic systems is also discussed.
Hydrogen solubility in the investigated nanoparticles is enhanced
nearly by 3 orders of magnitude with respect to H solubility in unsegregated
Pd–Pt alloys. This superior solubility points out the importance
of phase segregation, which is frequently overlooked, yet fundamental
for systems designed for heterogeneous catalysis and hydrogen storage