Die autokatalytische H₂O₂-Reduktion an Ag-Elektroden

Recently, it was shown that the hydrogen peroxide (H2O2) reduction on silver in perchloric acid (HClO4) proceeds in two parallel paths. The normal reduction is observed at an overpotential of â??1.5 V, whereas a second reduction reaction occurs at a significantly lower overpotential at â??1.0 V. The second reaction involves the unstable intermediate OHad, which also acts as a catalyst. Hence, the second reaction has been proposed to be an autocatalytic one. This autocatalytic reaction is deactivated after a certain time that depends on the rotation speed of the electrode. It can be recovered if the electrode is negatively polarized. In this thesis work, ex-situ XPS measurements on emersed Ag(111) electrodes were conducted. The analysis leads to the conclusion that the deactivation is caused by a small amount of chloride contamination. Electrochemical impedance spectra are presented together with numerical simulations for the faradaic impedance in the autocatalytic region based on previously suggested kinetic rate laws. The experimental data fit well with the results of the theoretical calculations, which strongly supports the autocatalytic mechanism

Faradaic impedance studies of the autocatalytic reduction of H₂O₂ on Ag electrodes in HClO₄

Recently, it was shown that the reduction of hydrogen peroxide on silver in perchloric acid proceeds in two parallel paths. The normal reduction is observed at an overpotential of −1.5 V whereas a second reduction reaction occurs at a significantly lower overpotential at −1.0 V. The second reaction involves the unstable intermediate OHad, which also acts as a catalyst. Hence, the second reaction has been proposed to be an autocatalytic one. In this paper we present electrochemical impedance spectra together with numerical simulations for the faradaic impedance in the autocatalytic region based on previously suggested kinetic rate laws. It will be shown that the experimental data fit well with the results of the theoretical calculations, thus strongly supporting the autocatalytic mechanism

Modeling galvanostatic potential oscillations in the electrocatalytic iodate reduction system

An experimental and theoretical description of the oscillatory electrocatalytic iodate reduction system in alkaline solution is presented. Experimental measurements suggested the crucial role of a negative differential resistance as well as the presence of hydrogen evolution in the oscillatory instability. A simple kinetic model with one chemical species was developed which involved: (i) an iodate reduction current with N-shaped potential dependence due to a Frumkin repulsion effect; and (ii) an additional ‘iodate-independent’ current providing process. The dynamical behavior of the model compares favorably with experiments as far as voltammetric profiles, stirring effects and bifurcations between stable and oscillatory states are concerned. It is further shown that the model can account for oscillatory states in a number of related electrocatalytic systems

Electrochemical oscillations in the methanol oxidation on Pt

Experimental observations of temporal dynamics in the electrocatalytic oxidation of methanol (CH3OH) on a polycrystalline platinum electrode are reported, Hidden negative differential resistance (HNDR) and instabilities of the system were investigated by means of an electrochemical impedance spectrum analysis and potential oscillations under galvanostatic control. This result could be applied for a direct methanol fuel cell (DMFC) with higher energy efficiency

The electrochemical quartz crystal microbalance (EQCM) in the studies of complex electrochemical reactions

Many electrochemical processes contain more than one charge transfer step. If one of these steps leads to a mass change of the working electrode one can use the electrochemical quartz crystal microbalance (EQCM) in situ to determine the rate of this process as a function of the applied potential. Examples for the use of the EQCM for the elucidation of complex electrochemical reaction, which are discussed in this paper are the electrodeposition of alloys with simultaneous hydrogen evolution, the corrosion of alloys, and the autocatalytic reduction of hydrogen peroxide on silver electrodes in acidic solution

Autocatalytic mechanism of H₂O₂ reduction on Ag electrodes in acidic electrolyte: experiments and simulations

It is shown that the cathodic reduction of hydrogen peroxide (H2O2) on silver electrodes in acidic electrolyte can proceed by two parallel mechanisms: first, by the ‘normal’ mechanism that has been discussed in the literature, second, by a novel mechanism proceeding at a significantly more positive potential. It is proposed that the second mechanism involves the activating adsorbate (OH)ad, that forms in the course of the H2O2 reduction reaction as an unstable intermediate. The coverage of the electrode with (OH)ad increases with the rate of H2O2 reduction, i.e., the process is autocatalytic. At more negative potentials the coverage decreases as the rate of adsorbate reduction/desorption rises. This leads to a potential region of negative differential charge-transfer resistance and thus to complex dynamic phenomena, in particular to electrochemical oscillations. Model calculations based on these considerations yield the potential dependent OH-adsorption, the N-shaped current/voltage curves and current oscillations that agree well with the experimental findings

Electrochemical and XRD characterization of platinum-ruthenium blacks for DMFC anodes.

It is generally accepted that Pt-Ru alloy catalysts with an atomic Pt-to-Ru ratio of 1:1 generate the best anode perform'ance in the direct methanol fuel cell (DMFG). However, at near-ambient cell operating temperatures, Gasteiger et al. reported that a catalyst with significantly lower Ru content, {approx} 10 at %, offers the highest activity towards methanol. Recently, Dinh et al. demonstrated that the activity of different Pt-Ru catalysts with the same Pt-to-Ru atomic ratio in the bulk might vary depending on the actual surface composition, which is often significantly different from that in the bulk phase, In this work, we study several experimental Pt-Ru catalysts (Johnson Matthey) with Pt-to-Ru atomic ratio ranging from 9: 1 to 1 :2. Electrocatalytic activity of these catalysts in methanol oxidation reaction is investigated in a regular DMFC 'and probed using voltammetric stripping of surhce CO

Six cell 'single cell' stack diagnostics and membrane electrode assembly evaluation

Polymer electrolyte fuel cells are promising candidates as energy conversion devices in applications from portable power to stationary applications or electric vehicles. In order to achieve practical voltage, power and energy density, stacks are employed for almost all applications. Here, we present a six-cell 'single cell' stack in which individual cells can be isolated from the stack by current carrying leads found within each of the bipolar plates. The current carrying leads allow individual cells to be isolated from the rest of the stack, so that cells can either be tested together or independently. The design of the stack, utility for specific applications, including stack diagnostics and membrane electrode assembly (MEA) testing, and some experimental results, obtained using the stack, are presented. Special focus is given in this paper to the area of direct methanol fuel cell (DMFC) stacks, however the equipment and many of the experimental results presented are appropriate for other fuel cell systems