Alloying electrodes

Fig. VIII-2. Scanning tunneling microscopy images illustrating the capabilities of the technique (a) a 10-nm-square scan of a silicon(lll) crystal showing defects and terraces from Ref. 21 (b) the surface of an Ag-Au alloy electrode being electrochemically roughened at 0.2 V and 2 and 42 min after reaching 0.70 V (from Ref. 22) (c) an island of CO molecules on a platinum surface formed by sliding the molecules along the surface with the STM tip (from Ref. 41).

Figure 16. Charge-discharge cycle characteristics of various MH alloy electrodes.

Figure 41. Cycling performance of several Li-Al alloy electrodes (discharge end 6% of total Li in Li-Al alloy current density 1.1 mA cm 2 ).

Several metal additives were investigated to improve this nonuniform reaction. Figure 41 shows the cycle performance of several Li-Al alloy electrodes. It was found that Li-Al-Mn and Li-Al-Cr alloys had better rechargeability than Li-Al alloy in the Li-Al-Mn alloy, particularly no de-... 

The general thermodynamic treatment of binary systems which involve the incorporation of an electroactive species into a solid alloy electrode under the assumption of complete equilibrium was presented by Weppner and Huggins [19-21], Under these conditions the Gibbs Phase Rule specifies that the electrochemical potential varies with composition in the single-phase regions of a binary phase diagram, and is composition-independent in two-phase regions if the temperature and total pressure are kept constant. 

The good cycling stability of the tin in TCO is quite unusual, because the electrochemical cycling of Li ASn and also of other Li alloy electrodes is commonly associated with large volume changes in the... 

Cd + Bi alloy electrodes (1 to 99.5% Bi) have been prepared by Shuganova etal. by remelting alloy surfaces in a vacuum chamber (10-6 torr) evacuated many times and thereafter filled with very pure H2. C dispersion in H20 + KF has been reported to be no more than 5 to 7%. C at Emin has been found to be independent of alloy composition and time. The Emin, independent of the Bi content, is close to that ofpc-Cd. Only at a Bi content 95% has a remarkable shift of toward less negative E (i.e., toward o ) been observed. This has been explained by the existence of very large crystallites (10-4 to 10-3 cm) at the alloy surface. Each component has been assumed to have its own electrical double layer (independent electrode model262,263). The behavior of Cd + Bi alloys has been explained by the eutectic nature of this system and by the surface segregation of Cd.826,827... 

Figure 8.36. Scanning electron micrographs of an Ag-25at%Pd alloy electrode deposited on YSZ, (a,c,e) sample 1 (b, d, f) sample 2.39...

Another very interesting result obtained from these FURS measurements is the difference between adsorbed CO obtained from dissolved CO and that from the dissociation of adsorbed methanol. The shift in wave number is more important with dissolved CO. These shifts may also be correlated with the superficial composition of the alloys, and it was observed that the optimized composition for the oxidation of CO (about 50 at.% Ru) is different from that for the oxidation of methanol (about 15 at.% Ru). FTIR spectra also revealed that the amount of adsorbed CO formed from methanol dissociation is considerably higher on R than on Pt-Ru. For a Ptog-Ru-o i alloy, the amount of linearly adsorbed CO is very small (Fig. 8), suggesting a low coverage in the poisoning species. Moreover, by observing the potentials at which the COj IR absorption band appears, it is possible to conclude that the oxidation of both (CHO)ads and (CO)acis species occurs at much lower potentials on a R-Ru alloy electrode than on pure Pt. 

Nishimura K, Kunimatsu K, Enyo M. 1989. Electrocatalysis on Pd + An alloy electrodes Part III. IR spectroscopic studies on the surface species derived from CO and CH3OH in NaOH solution. J Electroanal Chem 260 167. 

We prepared thin film Pt alloy electrodes by Ar-sputtering Pt and the second metal targets simultaneously onto a disk substrate at room temperature (thickness approximately 200 nm). The resulting alloy composition was determined by gravimetry and X-ray fluorescent analysis (EDX). Grazing incidence (i7= 1°) X-ray diffraction patterns of these alloys indicated the formation of a solid solution with a face-centered cubic (fee) crystal stmeture. 

Before the measurement of HOR activity, a pretreatment of the alloy electrode was carried out by potential sweeps (10 V s ) of 10 cycles between 0.05 and 1.20 V in N2-purged 0.1 M HCIO4. The cyclic voltammograms (CVs) at all the alloys resembled that of pure Pt. As described below, these alloy electrodes were electrochemically stabilized by the pretreatment. Hydrodynamic voltammograms for the HOR were then recorded in the potential range from 0 to 0.20 V with a sweep rate of 10 mV s in 0.1 M HCIO4 saturated with pure H2 or 100 ppm CO/H2 at room temperature. The kinetically controlled current 4 for the HOR at 0.02 V was determined from Levich-Koutecky plots [Bard and Faulkner, 1994]. 

This is the first experimental demonstration of changes in the strength of CO adsorption at Pt-based alloy electrodes. Nprskov and co-workers theoretically predicted a similar linear relation between changes in ads(CO) and shifts in the (i-band center [Hammer et al., 1996 Hammer and Nprskov, 2000 Ruban et al., 1997]. Because the Pt4/7/2 CL shift due to alloying can be more easily measured by XPS than the li-band center can, this should be one of the most important parameters to aid in discovering CO-tolerant anode catalysts among Pt-based alloys or composites. 

As described in the previous sections, a stable Pt skin of a few nanometers is formed on the Pt-Fe, Pt-Co, and Pt-Ni alloy surfaces after electrochemical stabilization. Figure 10.12 shows Arrhenius plots of kapp on the alloy electrodes at —0.525 V vs. E° in comparison with that of a pure Pt electrode. In the low temperature region (20-50 °C for Pt54Fe45, 20-60 °C for Pt6gCo32 and Ptg3Ni37), linear relationships between log kapp and 1 / Tare observed at all the electrodes, corresponding to the following Arrhenius equation ... 

Uchida H, Ozuka H, Watanahe M. 2002. Electrochemical quartz crystal microhalance analysis of CO-tolerance at Pt-Fe alloy electrodes. Electrochim Acta 47 3629-3636. 

Wakabayashi N, Takeichi M, Uchida H, Watanahe M. 2005b. Temperature dependence of oxygen reduction activity at Pt-Fe, Pt-Co, and Pt-Ni alloy electrodes. J Phys Chem B 109 5836-5841. 

Watanabe M, Zhu Y, Uchida H. 2000. Oxidation of CO on a Pt-Fe alloy electrode studied by surface enhanced infrared reflection-absorption spectroscopy. J Phys Chem B 104 1762-1768. 

Kabbabi A, Faure R, Durand R, Beden B, Hahn F, Leger JM, Lamy C. 1998. In situ FTIRS study of the electrocatalytic oxidation of carbon monoxide and methanol at platinum-ruthenium bulk alloy electrodes. J Electroanal Chem 444 41-53. 

Beltowska-Brzezinska M, Heitbaum J. 1985. On the anodic oxidation of formaldehyde on Pt, An and Pt/Au-alloy electrodes in alkaline solution. J Electroanal Chem 183 167-181. 

The decrease in free energy (—AG) which provides the driving force in a cell may ensue either from a chemical reaction or from a physical change. In particular, one often studies cells in which the driving force is a change in concentration (almost always a dilution process). These cells are called concentration cells. The alteration in concentration can take place either in the electrolyte or in the electrodes. As examples of alterations in concentration in electrodes, mention may be made of amalgams or alloy electrodes with different concentrations of the solute metal and in gas electrodes with different pressures of the gas. 

Electrode-concentration is based on dilution of the electrode material itself. For the electrode material to be engaged in such a process, it must have a changeable concentration. Amalgams or alloy electrodes with different concentrations of solute metal, and gas elec-... 

Fig. 23. Surface composition x of a Rujr,- alloy electrode after electrochemical treatment at different potentials for 5 min in 0.5 mol L-1 H2S04. Electron emission angle 90°. After [82],...

Fig. 24. Ru34 and Ir4/binding energies for a RuxIrl x alloy electrode after polarizaition at 1.7 V for 5 min in 0.5 molL-1 H2S04 as a function of composition x. Electron emission angle was 90° for Ru34 and 20 for Ir4/...

Figure 5.12 Atomically resolved STM image of a PtsoRusofl 1 1) alloy electrode in 0.01 M NaF after annealing and cooling in H2/Ar. The arrows mark bright spots that are assigned to Ru atoms. (Reproduced with permission from Ref. [43].)...

For the simplest analyses of metals and alloys, electrodes made of the sample material itself may be used, a disc or cylinder being cut, cast or... 

Platinum-group minerals, classes of, 79 603 Platinum halides, 79 657 Platinum-iridium alloy electrodes, medical applications for, 79 629... 

---