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 H₂SO₄

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_1/2 = 10 s. Activation energies of the electrode process and chemical reaction were found to be <i>E</i>_a,E = 52 ± 5 kJ·mol^–1 and <i>E</i>_a,C = 60 ± 5 kJ·mol^–1, respectively. The chemical step likely corresponds to a free-radical oxidation of HSO₄^– 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