Redox competition

The transition metal trichalcogenides MX3 (M = Ti, Zr, Hf, Nb, Ta X = S, Se) have been studied in some detail. All the trichalcogenides contain anion-anion bonding and consequently a redox competition exists between electron transfer to the metal center and electron transfer to the anions. The situation is the converse of the deintercalation of the thallium vanadium sulfides, where the competition is between oxidation of T1+ cations and that of the metal sulfur framework. 

Case (i) corresponds to a reduction of a given ion in the host network, e.g., Fe to Fe " in Li,FeOCl intercalatesSuch systems, which are associated with the most ionic host structures (oxides, fluorides, chlorides), are largely governed by redox competition between A/A" and couples. Lithium can be intercalated in FeOCl but not... 

RC-SECM Redox competition mode scanning electrochemical microscopy... 

Okunola AO, Nagaiah TC, Chen X, Eckhard K, Schuhmaim BM (2009) Visualization of local electrocatalytic activity of metalloporphyrins towards oxygen reduction by means of redox competition scanning electrochemical microscopy (RC-SECM). Electrochim Acta 54(22) 4971 978... 

The redox competition mode consists of two subsequent steps (Fig. 6). In the first step, the species of interest is electrochemically generated at the tip increasing the concentration of this species in the gap between tip and sample surface. In the subsequent step, the generated species is collected at the tip. Simultaneously, the generated species is also collected at the sample, which means that both tip and sample compete in the second step for the species generated at the tip in the first step. 

F. 6 Redox competition mode. Left the species of interest is generated at the tip in the first step. Right Both the tip and sample compete to collect the generated species... 

The electrocatalytic properties of an electrode surface can be mapped via the generation/coUection or the redox competition mode of SECM. However, evaluation of the properties of a single nanopaiticle supported on a noncatalytic macroelectrode requires rather high spatial resolution of the SECM, below a few hundreds of nanometers which is highly challenging. Two approaches have shown the potential to resolve the electrocatalytic properties of single nanopaiticles toward ORR, namely SICM-SECM and SECCM. 

Fig. 24 SECM images of local electrocatalytic activity of Pt/C spots with different catalyst loading, 1 droplet left) 2 droplets (right) showing the local O2 consumption obtained using the redox competition mode of SECM. Sample potential = 200 mV versus Ag/AgCl/3 M KCl electrolyte = H2SO4 (200 mM) tip potential = 0 mV (5 s) and -600 mV (0.5 s)...

Eckhard K, Chen X, Turcu F, Schuhmann W (2006) Redox competition mode of scanning electrochemical microscopy (RC-SECM) for visualisation of local catalytic activity. Phys Chem Chem Phys 8(45) 5359-5365... 

Gonzalez-Garcia, Y., Garcia, S.I, Hughes, A.E. and Mol, IM.C. (2011), A combined redox-competition and negative-feedback SECM study of self-healing anticorrosive coatings, Electrochem. Comm., 13,1094-1097. 

SECM. In conventional amperometric SECM an eleetro-ehemical probe, typically a glass-encapsulated microdisk electrode, is brought into close proximity to the interface of interest, and electroehemical reaetions are driven in the small gap between the tip and the surface. Current can be measured at the tip and/or at the substrate and the response is highly sensitive to both the nature of the surface and the tip-surface separation. There are various modes of SECM, such as feedback mode, generation-collection mode and redox competition mode, each of which can... 

Radial coordinate in polar coordinate system Sample radins Redox competition... 

Kamicka, K., Eckhard, K., Guschin, D. A., Stoica, L., Kulesza, P. J., Schuhmann, W. Visualisation of the local bio-electrocatalytic activity in biofuel ceU cathodes by means of redox competition scaiming electrochemical microscopy (RC-SECM). Electrochem Commun 2007, 9,1998-2002. 

Santana, J. J., J. Gonzalez-Guzman, L. Fernandez-Merida, S. Gonzalez, and R. M. Souto, Visualization of local degradation processes in coated metals by means of scanning electrochemical microscopy in the redox competition mode, Electrochimica Acta, 55, 2010, 4488. 

Electrodep., electrodeposition NPs, nanoparticles MD/SC, micropipette delivery—substrate collection RC, redox competition SG/TC, substrate generation/tip collection TG/SC, tip generation/substrate collection. 

When the species to be studied cannot be conveniently generated at an electrode and no product of the electrode reaction can be detected, then SECM shielding (Figure 16.3d) can be used to screen an array. In shielding, the same reaction occurs at the tip and the substrate so that the current at the tip is decreased compared to what would be observed with an insulating substrate. " Thus the tip is shielded by and competes with the substrate for the same reactant A reactant in bulk solution can be studied by noting how well the substrate competes with the tip for consumption of the reactant This approach, also called redox competition (RC-SECM), was used to study the ORR on model Pt and An catalyst spots as an alternative to TG/SC. " ... 

Guadagnini, L., Maljusch, A., Chen, X., Neugebauer, S., Tonelli, D., Schuhmann, W. Visualization of electrocatalytic activity of microstructured metal hexacyanoferrates by means of redox competition mode of scanning electrochemical microscopy (RC-SECM). Electrochim. Acta 2009, 54, 3753-3758. 

A critical discussion of this approach is possible under the light of the general considerations of section I. VS2 and TiS2 are favorable cases fi om the point of view of both the ionicity of bonds and the d-sp redox competition. 

Now, in the case of a ternary sulfide involving an element of the second part of transition metal rows, the d-sp redox competition will interfere. When deintercalating Li2peS2 (39) a new form of FeS2 is obtained in which both iron and (S") anions are oxidized as shown by the formulation Fe + S (S2")o.5- This is reflected in structural changes, for example, a shifting of the Fe" from octahedral to tetrahedral sites. 

The other option in GC is SG/TC mode, which has been used successfully for imaging activity of anode catalysts such as GOx. However, in this mode, the analyte (O2) concentration is significantly altered, and large enzyme sample surface and small UME tips are needed. Moreover, the application of potential pulse to the sample in TG/SC mode is limited by capacitive charging currents, resulting in a poorly defined UME potential. To overcome these drawbacks in GC mode for imaging cathode catalyst activity, anew method called redox competition (RC) mode was developed in... 

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