Co-free electrodes

Figure 9. Charge capacity, Q, vs. number of charge-discharge cycles for three mischmetal AB5 electrodes. Note the high decay rate in charge capacity for Co-free electrode [42].

Mm )Ni3 55Co 75Mti 4Al 3 (Mm refers to Ce free mischmetal), and two Co-free electrodes, Mm(or Mm )Ni4 3Mn4Al3. Both Co-ffee electrodes rapidly corrode and would not be suitable for battery applications. Obviously alloy composition is responsible for the observed behavior, and this is discussed in the following sections. The results of these experiments are summarized in Table 9.3. 

Figure 9.20 Comparison of charge capacity, Q, vs charge-discharge cycles for LaNi4.g4Sno.32, A vs simulated commercial, and a Co-free electrode, O [53].

The most essential question is why the CO-free sites are secured for H2 adsorption and oxidation. Watanabe and Motoo proposed a so-called bifunctional mechanism originally found at Pt electrodes with various oxygen-adsorbing adatoms (e.g., Ru, Sn, and As), which facilitate the oxidation of adsorbed COad at Pt sites [Watanabe and Motoo, 1975a Watanabe et al., 1985]. This mechanism has been adopted for the explanation of CO-tolerant HOR on Pt-Ru, Pt-Sn, and Pt-Mo alloys [Gasteiger et al., 1994, 1995], and recently confirmed by in sim FTIR spectroscopy [Yajima et al., 2004]. To investigate the role of such surface sites, we examined the details of the alloy surface states by various methods. 

Figure 10.8 Arrhenius plots for the apparent rate constant for the HOR (CO-free) at Pt (O), Pt5iCo49 (A), and Pt54Ru46 ( ) working electrodes at 0.020 V vs. RHE(/). (From Uchida et al. [2006], reproduced by permission of the American Chemical Society.)...

Figure 12.4 A series of SFG spectra in the CO stretch region of chemisorbed CO on polycrystalline Pt in a CO-free 0.1 M H2SO4 electrolyte. The atop spectra were fit to (12.5) (see text) to extract the amplitude, frequency, and width [Lu et al., 2005 Lagutchev et al, 2006] (each displayed data point is the average of three or five spectra). The electrode potential was swept at a rate of 5 mV/s, and SFG spectra were obtained every 200 ms. Spectra were obtained at 1 mV intervals, but, to avoid congestion in the plot, averaged spectra are displayed at 10 mV intervals in the pre-oxidation region (V < 0.43 V) and at 3.3 mV intervals in the oxidation region (V > 0.43 V) [Lu et al., 2005].

The bottom spectrum was obtained by cycling the electrode in CO-free SnCl /HjSO solution to ensure formation of a partial Sn adlayer and then replacing the cell contents with CO-saturated solution. The v(C0) band is still observed, which shows that the Sn adatoms do not saturate the surface even in the absence of competitive CO adsorption. The intensity and frequency of the v(C0) band have both decreased, which confirms that the CO adlayer is only partially complete. There is no evidence for a change in v(C0) beyond that expected for the coverage dependence expected in acid solution. This shows that there is no strong interaction between adsorbed CO molecules and neighboring Sn adatoms, in support of the assumptions used in the adatom oxidation model discussed above. 

During sensor response testing, the flow rate was initially reduced on both sides. Then, the gas at the working electrode was changed to a gas mixture of type 1 and type 2 (CO -free dry air), mixed at a ratio of 1 9 by a gas mixer. Thus, the virtual CO partial pressure decreased by an order of magnitude at the working electrode... 

Figure 16 Stripping voltammetry (first cycle) (0.5 M H2SO4, 100mVs ) of saturated CO layers on clean and ruthenium-modified Pt(l 11) and Pt(533) surfaces. The ruthenium has been deposited by dipping, and following rinsing CO was adsorbed and stripped in CO-free electrolyte (Pt(l 11) and Pt(533) ). The same surfaces were subsequently reduced in a flow of 10% H2 in Ar, CO adsorbed and stripping voltammetry carried out (Pt(lll) and Pt(533) °). The second cycle in each case corresponds to the CO-free surface. (From Ref. 98.) The working electrode area was about 20mm. ...

Figure 15 Potentiodynamic (ImV/s) oxidation current densities for 0.1% CO/H2 on sputter-cleaned Pt and Pt-Ru RDEs at 2500 rpm in 0.5-M sulfuric acid at 62 °C. Prior to electrochemical measurements, the electrode potential was held at 0.05 V at 2500 rpm for about 300 s. (a) Magnification of the low-current density region for the positive going sweeps, (b) Comparison of the potentiodynamic and potentiostatic (1000-s) oxidation current densities, (c) Potentiodynamic (20mV/s) oxidation of pure H2 on CO-poisoned Pt (Pt-CO/ H2) CO was adsorbed at 0.05 V, and then the electrode was cycled between 0.05 and 0.22 V in CO-free solution (Pt-CO/no H2) the voltammetry of the unpoisoned Pt surface at the same conditions is added for reference.

Fig. 11 SEIRA spectra of water on a Pt electrode in 0.1 M H2SO4 recorded at the potentials indicated and referenced to the single-beam spectrum of the CO-covered surface. The upward and downward bands correspond to the species on the CO-free and CO-covered Pt surface, respectively. [Reprinted from ref 44 by permission of the American Chemical Society copyright 2008.]...

In this section, we review the use of MC simulations to understand the complex reaction kinetics of CO electrooxidation near Pt(lll) surfaces in sulfuric acid solutions. We address the role of specific adsorption of anions on electrode surfaces, and its effect on CO electrooxidation. We also review some results of CO oxidation in alloy surfaces [55]. This section Is Arranged as follows. In Section 18. 3.1, we provide a brief background onanion adsorption and CO oxidation. Section 18. 3.2 outlines themodel used. In Section 18. 3.3, we briefly outline the KMC methodology. In Section 18. 3.4 we review the effect of competitive adsorption on base voltammograms (i.e. under CO-free conditions) and CO electrooxidation. We also briefly review CO diffusion effects and CO electrooxidation on PtRu alloys [55]. 

Anodic Reactions in Electrocatalysis -Oxidation of Carbon Monoxide, Fig. 3 Cyclic voltammogram of a rotating polycrystalline Pt disk electrode in CO saturated 0.1 M H2SO4. Potential sweep rate 0.050 V s rotation rate 900 rpm. The inset shows the integrated IR peak of COj,j plotted versus the position on the electrode for the applied current of 0.3, 1.4, 1.5, 1.7, 1.75, and 1.8 mA left to right and top to bottom). The red color indicates a high CO j coverage and the blue a CO-free surface (Reproduced from Ref [28] with permission of the American Chemical Society)... 

We describe in this section an enzyme-free electrode responsive to urea. A sensitizer used was again a synthetic polypeptide, poly(a-L-glutamate) (PLG), which undergoes conformational changes depending on the concentration of urea. PLG was easily immobilized on a Pt wire with the aid of the multiphase polymer material, poly(styrene-co-acrylonitrile)-PLG block copolymer, as described above. Electrochemical response towards urea was obtained based on the permeability change of the redox-active couple ions (the "ion-channel" mechanism). 

This mechanism suggests that the substantial oxidation of adsorbed CO occurs in the oxide-formation potential region. However, cyclic voltammograms in Figs. 9 and 10 show that the CO oxidation occurs at the oxide-free electrodes and is inhibited by the oxide formation. The CO oxidation is also likely to proceed via the following mechanism ... 

FIGURE 4.5 Voltammograms recorded at 60 mV s for gold (a) in CO-saturated, quiescent base as a function of electrode holding, or CO adsorption, time (the values for the latter were 0, 10, 20, 40, 60, 80, 120, 180, 360, and 720 s), and (b) in CO-free solution in this case the surface initially adsorbed CO (for 5 min at -0.25 V) and then He bubbling was carried out for 10 min to remove dissolved CO prior to recording the scans shown here. (Reprinted from Electrochemical oxidation of CO on Au in alkaline solution, H. Kita, H. Nakajima, and K. Hayashi, J. Electroanal. Chem., 1985, 190, 141. With permission.)... 

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