Polyoxometalate electrodes

PossibiKties of electrocatalysis of reactions at electrodes are among the powerful incentives for the electrochemical study of POMs. Interesting results were obtained both in electrocatalytic reductions and oxidations, provided the appropriate form of the POM is used. Two recent reviews devoted to the electrochemical properties of polyoxometalates as electrocatalysts are available [8, 9]. The second one focuses more specifically on electrocatalysis on modified electrodes. In the present text, attention will be drawn specially to the basic principles that could be considered to govern most of solution processes. The principles will be illustrated by several recent experimental results, even though earlier achievements will also be described briefly. 

Schwenz and Moore introduced cyclic voltammetry as a modem approach to electrochemistry experiments. Three new experiments exploit this technique. One uses the technique as a probe or electrode surface area (82). A second uses the method to study adsorption of polyoxometalates on graphite electrodes (83). A third studies the effect of micelles on the diffusion and redox potentials of the well-studied ferrocene system (84). 

A Simple Student Experiment for Teaching Surface Electrochemistry Adsorption of Polyoxometalate on Graphite Electrodes 83... 

Various polyoxometalates can be reduced electrochemically and reversibly by several electrons at modest potentials (Section VILA), and these properties are exploited in photocatalysis and eiectrocatalysis. In both cases, redox properties of heteropolyanions (Fig. 49) and the organic reactants (Table XXXV) are the principal properties that control the catalytic performance. The selection of the electrode is also important in eiectrocatalysis. Photocatalysis by hereopoly-anions has been reported extensively, but there are only a few reports of eiectrocatalysis by these compounds. 

Solid-state reference electrodes for potentiometric sensors are currently under research. The main problem to be faced in developing this type of electrode lies in connecting the ionic conducting (usually aqueous) solution with an electronic conductor. Since the reference electrode has to maintain a defined potential, the electrochemical reaction with components of the electrolyte has to be avoided. Oxides, mixed oxides, and polyoxometalate salts of transition elements can be proposed for preparing solid-state reference electrodes. Tested compounds include tungsten and molybdenum oxides (Guth et al., 2009). 

Ingersoll, D., Kulesza, P.J., and Faulkner, L.R. 1994. Polyoxometallate-based layered composite films on electrodes. Preparation through alternate immersions in modification solutions. Journal of the Electrochemical Society 141, 140-147. 

Films of oxides can be produced by anodization of metal electrodes. For example, AI2O3 forms on an aluminum anode immersed in a solution of H3PO4. The thickness of the film can be controlled by the applied potential and the time of anodization. Such a film can be used as a support for other materials, such as poly(vinylpyridine) (PVP). Oxide films of other metals, such as Ti, W, and Ta, can be produced in a similar way. Oxide films can also be produced by CVD, vacuum evaporation and sputtering, and deposition from colloidal solution. Related inorganic films are those of polyoxometallates (iso- and heteropolyacids and their salts) (20). For example, the heteropolyanion P2W17M0O62K6 shows a number of reduction waves at a glassy carbon electrode. A wide variety of metallic polyanionic species (e.g., of W, Mo, V) exist and have a rich chemistry. Films of such materials are interesting for their electrocatalytic possibilities. 

Recently, the question whether C04PW9 is a true WOC, or just a precursor, arose. Electrochemical studies on differently aged samples of C04PW9 in phosphate buffer pH 8 evidenced the decomposition of the polyoxometalate structures and the concomitant deposition of an amorphous CoOx film onto the working electrode. ... 

Oxygenic polyoxometalates can be also supported onto electrodes. A proof-of-principle of such an electrode was recently reported, by using a conductive bed of multi-wall carbon nanotubes (MWCNTs) as the support material for the Ru4SiWio OEC (Fig. 8). ... 

Fig. 2 Cyclic voltammograms showing the electrocatalytic reduction of nitrite at PANI/polyoxometalate coated electrodes. Electrolyte 0.2 M Na2S04 + H2SO4 (pH =2). Scan rate = 5 mV s . Solid line cyclic voltammetry, restricted to the stability domain of PANI, of the PANI/SiMoi2 assembly, in the supporting electrolyte Dotted line effect of the addition of 10 M NaN02 to the electrolyte.

Polyelectrolyte multilayers are able to very effectively immobilize various molecules, like enzymes, for example. These multilayers have been shown to exhibit useful biocatalytic (i.e., enzymatic) [219,220] activities hybrid multilayers containing polyoxometalate are promising for catalytic applications [164], ESA multilayers have also been used for molecular recognition [198,341], or, more specifically, as biosensors [75,166,250-252,342,343]. Electrocatalytic [195,253,256] and electrosensing [254] capabilities of multilayers deposited on electrodes have... 

Cheng, L., Niu, L., Gong, J., et al. (1999). Electrochemical Growth and Characterization of Polyoxometalate-Containing Monolayers and Multilayers on Alkanethiol Monolayers Self-Assembled on Gold Electrodes, Chem. Mater., 11, pp. 1465-1475. 

Cox I.A., Hohnstrom S.D., Tess M.E. Oxidation at a sol-gel composite electrode doped with a dirhodium-substituted polyoxometalate for amperometric detection of peptides separated by HPLC. Talanta 2002 52(6) 1081-1086... 

Cheng, L., et al.. Electrochemical growth and characterization of polyoxometalate-containing monolayers and multilayers on alkanethiol monolayers self-assembled on gold electrodes. Chem Mater, 1999. 11(6) p. 1465-1475. 

Han Z, Zhao Y, Peng J, Liu Q, Wang E (2005) Inorganic-organic hybrid polyoxometalate containing supramolecular helical chains preparation, characterization and application in chemically bulk-modified electrode. Electrochim Acta 51 218-224... 

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