Electrochemical carbons

Park S, Tong YT, Wieckowski A, Weaver MJ. 2002a. Infrared spectral comparison of electrochemical carbon monoxide adlayers formed by direct chemisorption and methanol dissociation on carbon-supported platinum nanoparticles. Langmuir 18 3233-3240. 

It should be emphasized that the electrochemical carbonization proceeds, in contrast to all other common carbonization reactions (pyrolysis), already at the room temperature. This fact elucidates various surprising physicochemical properties of electrochemical carbon, such as extreme chemical reactivity and adsorption capacity, time-dependent electronic conductivity and optical spectra, as well as its very peculiar structure which actually matches the structure of the starting fluorocarbon chain. The electrochemical carbon is, therefore, obtained primarily in the form of linear polymeric carbon chains (polycumulene, polyyne), generally termed carbyne. This can be schematically depicted by the reaction ... 

The product of the reaction (5.5.29) is, however, unstable against subsequent interchain crosslinking and insertion of supporting electrolyte cations. The electrochemical carbonization of fluoropolymers was recently reviewed by L. Kavan. 

Electrochemical corrosion processes also include a number of processes in organic chemistry, involving the reduction of various compounds by metals or metal amalgams. A typical example is the electrochemical carbonization of fluoropolymers mentioned on p. 316. These processes, that are often described as purely chemical reductions, can be explained relatively easily on the basis of diagrams of the anodic and cathodic polarization curves of the type shown in Fig. 5.54. 

The electroreductive dehalogenation of a-haloacetic acids has been achieved with cobalamin [387]. The hydrophobic vitamin B12 Co complex immobilized on a glassy carbon electrode (252) may catalyze the electrochemical carbon-skeleton rearrangements of... 

Pt particle agglomeration is due to carbon support corrosion. Electrochemical carbon corrosion is known to occur above 0.9 V. It has been suggested that loss of carbon causes Pt particle agglomeration and electrical isolation, leading to loss in activity. 

Cobalamin catalysed electrochemical reduction of the 2-chloroethanol ester 68 at negative potentials, without photochemical assistance, leads to a 1,2-elimination process (see p. 115) [228]. This contrasts with the lack of 1,2-elimination during reaction of 66 and 67, Thus in the purely electrochemical carbon-cobalt bond... 

Related to the fixation of C02, electrochemical carbon , which is prepared by converting carbon halides (e.g. polytetrafluoroethylene) to carbon, is recently attracting attention for its technological applicabilities. Non-aqueous electrolyte solutions are often used in the electrochemical carbonization processes [13]. The use of non-aqueous electrolyte solutions is also popular in electrochemical organic syntheses, as is apparent in Ref. [14], although not dealt with in this book. 

Kavan, L. Electrochemical carbonization of fluoropolymers. In Chemistry and Physics of Carbon, Vol. 23, New York Marcel Dekker, 1991 69. 

Instead of nucleophiles such as H2O, MeOH, RNH2 and halide ions, both the aryl group and olefinic double bond will react with an electrogenerated phenoxonium ion to give carbon-carbon coupled products. In particular, electrooxidative coupling reactions of a,ft)-diarylalkanes leading to cyclic diaryl ethers have been known to take place in a radical or cationic manner depending on the oxidation potential, the nature and location of substituents, the solvent systems and other factors, as cited in many books Electrochemical carbon-carbon bond formations will be described here. 

The electrochemical synthesis of carbon is still considered an academic curiosity. The reaction usually does not require high temperatures and/or pressures, as the traditional pyrolytic syntheses of elemental carbon. Exclusion of air oxygen and humidity is, however, needed, since the electrochemical potentials of carbonization are outside the electrochemical window of H2O [3], The electrochemical carbonization benefits from three specific features compared to traditional high-temperature syntheses ... 

The generic process for electrochemical synthesis of sp-carbon chains was electrochemical reductive carbonization (corrosion) of poly(tetrafluoro-ethylene) (PTFE) by alkali metal amalgams, pioneered by Jansta and dousek [6 9] (for review see Reference 3). The reaction occurs at the interface of a dry contact between PTFE and alkali metal amalgams, hence, it does not seem to recall an electrochemical synthesis in its classical sense. The purely electrochemical carbonization of PTFE on a Pt electrode in aprotic electrolyte solution is also possible [3], but the amalgam-driven process is superior, presenting a clean and well-defined alternative to classical (wet) electrochemistry. 

The theoretical treatment presented (Eqs 4.1-4.5) is applicable also for direct wet electrochemistry on Pt cathode in aprotic electrolyte solution [12,13] (Table 4.1) and for some other chemical reductants, Rj, viz. benzoin dianion [14] and sodium dihydronaphthylide [15] (Table 4.1). Apparently, the decision between chemical and electrochemical carbonization may not be straightforward. The latter scenario requires a compact solid electrolyte with mixed electron/ion conductivity to be present at the interface. This occurs almost ideally in the reactions of solid fluoropolymers with diluted alkali metal amalgams [3]. If the interfacial layer is mechanically cracked, both electrochemical and chemical carbonization may take place, and the actual kinetics deviates from that predicted by Eq. 4.4 [10]. There is, however, another mechanism, leading to the perturbations of the Jansta and Dousek s electrochemical model (Eq. 4.4). This situation typically occurs if gaseous perfluorinated precursors react with Li-amalgam [4,5], and it will be theoretically treated in the next section. 

However, the actual product contained also sp carbon and impurities (H, N, I and Ni) [42,43]. Along with direct electrochemical carbonization processes (Eqs 4.14 4.16) also indirect reactions were explored, in which the active species is electrochemically generated and consumed for the chemical carbonization in a regenerative loop. For instance, dehydrochlorination of 1,1,2-trichloroethylene proceeds with /-buthanolate (BuO ), which is generated from Z-butylalcohol (BuOl 1) on a Ni cathode in dimethylsulfoxide electrolyte solution [44]. 

The electrochemical carbonization of PTFE is anisotropic, propagating rapidly along the oriented macromolecule chains [12,54]. Although the aim of early studies [12,13,50-54] was just surface modification of PTFE for improving of its adhesion, the authors have intuitively suggested the formation of polyyne (cf. Eq. 4.18) [52]. This prediction was later confirmed by IR spectroscopy [56,60,61]. The reactivity of ex-PTFE carbon was recently used for its subsequent functionalization with diazonium salts [62] and metallization [63]. 

Hlavaty, J. Kavan, L. Modification of electrochemical carbon by in-situ generated carbenes. Carbon 1997, 35, 127-131. 

Kavan, L. Micka, K. Kastner, J. UV-vis absorption of thin electrochemical carbon layers on poly(tetrafluoroethylene-co-hexafluoropropene). Synth. Metals 1994, 63, 147-152. 

Kavan [28] and Kijima et al. [29] have used the electrochemical method to synthesize carbyne. This technique may be realized by classical electrochemistry whereby the charge transfer reaction occurs at interface of a metal electrode and liquid electrolyte solution. Electrons in reaction were supplied either through redox active molecules or through an electrode, which contacts an ionically conducting solid or liquid phase and the precursor. In general, the structure and properties of electrochemical carbon may differ considerably from those of usual pyrolytic carbons. The advantage of this technique is the synthesis of carbyne at low (room) temperature. It was shown that the best product was prepared by cathodic defluorination of poly(tetrafluoroethylene) and some other perhalo-//-alkanes. The carbyne... 

According to Kavan (1997), electrochemical carbons are synthetic solids consisting mainly of atoms of elemental carbon, which can be prepared electrochemically from suitable precursors. Electrochemical carbonization is characterized by three specific features (Kavan et al., 2004) (1) ability to obtain relatively unstable carbon chains (2) easy templating of carbon nanostructured materials by the precursors and (3) defined kinetics of certain reactions, allowing for the control of film thickness. 

A. Yasuda, K. Doi, N. Yamaga and S. Kusanagi, Electrochemical characteristics of the planar electrochemical carbon monoxide sensor with a perfluorocarbon ionomer film, Solid State Ionics, 1990, 40/41, 476 A. Yasuda, K. Doi, N. Yamaga, T. Fujioka and... 

S. Kusanagi, Mechanism of the sensitivity of the planar CO sensor and its dependency on humidity, J. Electrochem. Soc., 1992, 139, 3224-3229 S.B. Lee, A. Cocco, D. Keyvani and G.J. Maclay, Humidity dependence of carbon monoxide rate in a Nation-based electrochemical cell, J. Electrochem. Soc., 1995, 142, 157-160 R.J. Mortimer and A. Beech, AC impedance characteristics of solid-state planar electrochemical carbon monoxide sensors with Nation as solid polymer electrolyte, Electrochim. Acta, 2002, 47, 3383-3387. 

Park, S., Y.T. Tong, A. Wieckowski, and M.J. Weaver, Infrared spectral comparison of electrochemical carbon monoxide adlayers formed by direct chemisorption and methanol dissociation on carbon-supported platinum nanoparticles. Langmuir, 2002.18(8) pp. 3233-3240 Park, S., Y. Tong, A. Wieckowski, and M.J. Weaver, Infrared reflection-absorption properties of platinum nanoparticle films on metal electrode substrates control of anomalous opticalejfects. Electrochemistry Communications, 2001. 3(9) pp. 509-513 Park, S., P.K. Babu, A. Wieckowski, and M.J. Weaver, Electrochemical infrared characterization of CO domains on ruthenium decorated platinum nanoparticles. Abstracts of Papers of the American Chemical Society, 2003. 225 pp. U619-U619... 

These data consist with measurements of carbon concentrations and chemical activities with different types of diffusion or electrochemical carbon meters and with the data on the carbon exchange between sodium and stainless steel. 

Fig. 11.7.2. HPLC separation of catecholamines and serotonin. Chromatographic conditions column, Vydac-CX (35 fim) (500 x 2 mm I.D.) mobile phase, 27.4 mM citrate, 50 mM sodium acetate, 18.4 mM acetic acid, 60 mM so um hydroxide, adjusted to pH 5.3 flow rate, 0.6 ml/min temperature, ambient detection, electrochemical, carbon paste working electrode, electrode potential -1-0.55 V vs. an Ag-AgCl reference electrode. Peaks NA, noradrenaline (80 pg) A, adrenaline (100 pg) DA, dopamine (4(X) pg) a-MDA, a-methyidopamine (internal standard) 5-HT, serotonin (200-250 pg). Reproduced from Patthy and Oyenge (1984), with permission.

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