Particulate electrodes

A number of recent cell designs have used particulate electrodes. The early versions had slurry electrodes (Butler and Fawcett, 1964 Gerischer, 1963) but more recently the electrodes have been packed or... 

An appreciable increase in working area of the electrodes can be attained with porous electrodes (Section 18.4). Such electrodes are widely used in batteries, and in recent years they are also found in electrolyzers. Attempts are made to use particulate electrodes which consist of a rather thick bed of particulate electrode material into which the auxiliary electrode is immersed together with a separator. Other efforts concern fiuidized-bed reactors, where a finely divided electrode material is distributed over the full electrolyte volume by an ascending liquid or gas flow and collides continuously with special current collector electrodes (Section 18.5). 

Fig. 18.10. In situ FT1R difference spectrum collected at +400 mV vs. RHE during an experiment in which the potential of the platinum particulate electrode (dispersed on carbon), which is initially immersed in 1.0 M H2S04, is first fixed at +50 mV vs. RHE. 1.0M CH3OH is then added to the electrolyte, and the potential was increased from +50 mV vs. RHE in successive 50 mV steps to 400 mV. All the spectra (8 cm-1 resolution, 100 coadded averaged scans requiring ca. 60 scans per second) were normalised to the reference spectrum taken at the base potential of 50 mV.

Fig. 18.12. Cyclic voltammogram of a platinum particulate electrode in 0.5 M H2SO4/I.O M...

Scott [11] compared three types of circulating particulate electrodes for copper recovery from dilute solutions (Fig. 1) spouted (circulating) beds, vortex beds, and moving beds. The beds contained 500— 700 pm spherical copper particles positioned on a stainless steel cathode feeder, and a platinized titanium anode. All electrodes performed similarly in terms of copper recovery current efficiencies. Recovery was found to be more efficient at low pH and high metal concentrations. The spouted bed electrode was preferred on the basis of scaleup. 

J. Munk, P.A. Christensen, A. Hamnett, E. Skou, The electrochemical oxidation of methanol on platinum and platinum plus ruthenium particulate electrodes studied by insitu FTIR spectroscopy and electrochemical mass spectrometry. J. Electroanal. Chem. 1996, 401(1-2), 215-222. 

The General Electric SPE (17) consists of two porous particulate electrodes which are bondecTcohesively with polytetrafluoro-ethylene dispersion particles and connected electrically to the outside of the cell hardware by means of metallic current collectors which are pressed against the SPE by mechanical methods. Such an SPE can be prepared via perfluoroionomer solution techniques. One method is to apply a paste consisting of the electrolyte powder and the perfluoroionomer solution to the membrane and evaporate the solvent. Alternately, the paste can be applied to a sacrificial substrate such as aluminum foil, dried, and subsequently pressed into the membrane as a decal. 

In a staged multi-scale approach, the energetics and reaction rates obtained from these calculations can be used to develop coarse-grained models for simulating kinetics and thermodynamics of complex multi-step reactions on electrodes (for example see [25, 26, 27, 28, 29, 30]). Varying levels of complexity can be simulated on electrodes to introduce defects on electrode surfaces, composition of alloy electrodes, distribution of alloy electrode surfaces, particulate electrodes, etc. Monte Carlo methods can also be coupled with continuum transport/reaction models to correctly describe surfaces effects and provide accurate boundary conditions (for e.g. see Ref. [31]). In what follows, we briefly describe density functional theory calculations and kinetic Monte Carlo simulations to understand CO electro oxidation on Pt-based electrodes. 

Finally, in nanoporous networks, the structural units have dimensions in the range 25-1 nm. For instance, the Ti02 particulate electrode forming the basis... 

Although in situ infrared spectroscopy has been applied widely in terms of the systems studied, the reflective electrodes employed have been predominantly polished metal or graphite, and so an important advance has been the study of electrochemical processes at more representative electrodes such as Pt/Ru on carbon [107,122,157], a carbon black/polyethylene composite employed in cathodic protection systems [158] and sol-gel Ti02 electrodes [159]. Recently, Fan and coworkers [160] took this concept one step further, and reported preliminary in situ FTIR data on the electro-oxidation of humidified methanol vapor at a Pt/Ru particulate electrode deposited directly onto the Nafion membrane of a solid polymer electrolyte fuel cell that was mounted within the sample holder of a diffuse reflectance attachment. As well as features attributable to methanol, a number of bands between 2200 and 1700 cm were observed in the spectra, taken under shortoperating conditions, the importance of which has already been clearly demonstrated [107]. 

Fig. 7.6 Current (top) and integrated fluorescence intensity (bottom) variations during three CV sweeps between 3.5 and 5.0 V at 0.05 mV/s of a carbon- and binder-free LiNiojMni 5O4 particulate electrode in a Li metal cell. Reproduced with permission from [164] copyright 2013, Elsevier...

The physical form of the electrodes used for these purposes may not be simple electrodes-plus-smooth polymer films porous flow-through assemblies, particulate electrodes, electrodes with designed binding sites[79], and electrodes with immobilized heterogeneous as well as mediator catalysts contain promising avenues to improved catalytic efficiencies and to dealing with stability requirements. 

There are many electrode geometries. Particulate electrodes are discussed in Chapter 5, but there is also the single electrode that has... 

Particulate electrodes, however, have difficulties (1) there is an uneven potential distribution and, hence, current distribution along the direction of current flow and (2) the bed tends to coalesce during electrowinning, particularly on a long timescale and with concentrated metal ion solution-this destroys the favourable properties of such electrodes. Hence, at the present time, cells with particulate electrodes are considered more likely to have an impact in metal recovery from industrial process liquors and effluents (Chapter 7) than in large-tonnage dectrowinning. 

ZnO is obviously one of the best candidates among semiconductors for DSSC applications as it can be synthesized easily and inexpensively into different shapes and sizes using various methods, and is environment friendly and stable indefinitely. Photovoltage and photocurrent measurements using modulated illumination on dye-sensitized ZnO solar cells have shown tens to hundreds of times faster electron transport in nanorod array electrodes compared to nanocrystalline particulate electrodes assembled from colloidal nanoparticles, with the electron lifetimes being only slightly smaller [237]. 

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