Electrochemical oxidation of carbon

Gilman S. 1964. The mechanism of electrochemical oxidation of carbon monoxide and methanol on platinum. II. The reactant-pair mechanism for electrochemical oxidation of carbon monoxide and methanol. J Phys Chem 68 70-80. 

In Chapter 2, the electrochemical oxidation of carbon monoxide, which is considered as the key intermediate for methanol oxidation, is investigated using electrochemical and spectroscopic methods for polycrystalline and single crystal platiniim electrodes. In Chapter 3, the electrochemical oxidation of methanol on the same electrodes was treated. In Chapter 4, electrocatalytic activities of platinum modified by adding secondary elements will be disciissed. 

This section addresses the role of chemical surface bonding in the electrochemical oxidation of carbon monoxide, CO, formic acid, and methanol as examples of the electrocatalytic oxidation of small organics into C02 and water. The (electro)oxidation of these small Cl organic molecules, in particular CO, is one of the most thoroughly researched reactions to date. Especially formic acid and methanol [130,131] have attracted much interest due to their usefulness as fuels in Polymer Electrolyte Membrane direct liquid fuel cells [132] where liquid carbonaceous fuels are fed directly to the anode catalyst and are electrocatalytically oxidized in the anodic half-cell reaction to C02 and water according to... 

Electrochemical oxidation of carbon surfaces is also a possibility. It is very convenient when carbon fibers are to be oxidized in a continuous process [32—35], but it can also be applied to activated carbons [36]. 

Jannakoudakis, A.D., Jannakoudakis, P.D., Theodoridou, E., and Besenhard, J.O. (1990). Electrochemical oxidation of carbon fibers in aqueous solutions and analysis of the surface oxides. J. Appl. Electrochem., 20, 619-24. 

Kinoshita, K. and Bett, J. (1973). Electrochemical oxidation of carbon black in concentrated phosphoric acid at 135C. Carbon, 11, 237. 

Electrochemical oxidation of carbon in 1 mol L" H2S04 is claimed to result in hydroquinone- or quinone-like groups in different environments however there is not much support for these findings in later work . The anodically produced surface film differs from that chemically formed . 

XPS studies of surface oxide groups obtained by electrochemical oxidation of carbons showed that carbonyl type and quinone type groups become tlte major oxide components fonned during dectrochemical (Kidation of carbcais [187]. 

The electrochemical oxidation of carbon black in 96% phosphoric acid has been studied at 135 °C. Two anodic processes were observed to occur (a) the formation of a surface oxide, and (6) the evolution of CO2. Both processes decreased with time but at different rates, so that CO2 evolution eventually became the major reaction. 

This opens up possibilities for an indirect electrochemical utilization of huge coal reserves. The possibility of direct electrochemical oxidation of carbon monoxide was soon questioned. It was suggested that hydrogen, rather than the carbon monoxide, is involved in the electrochemical reaction after being formed from CO by the Boudouard reaction ... 

Van Baar, J.F., J.A.R. Van Veen, J.M. Van der Eijk, T.J. Peters, and N. De Wit (1982). Electrochemical oxidation of carbon monoxide with carbon-supported Group VIII metal chelates Mechanistic aspects. Electrochim. Acta 27(9), 1315-1319. 

When the potential is close to equilibrium potential, the electrochemical oxidation of carbon by the reaction (11) is delayed. Judging by initial sections of polarization curves (Fig. 6, curve 1), in CO atmosphere the most probable electrode process is the reaction... 

Under anodic polarization ( 0.3-5-0,5 V) the electrochemical oxidation of carbon by reaction (11) at a considerable rate ( 1 mA/cm ) is possible. In this case desorption of the (CO)x compoimds from the electrode surface is delayed most. Since under these conditions (/ > KIO mA/cm ) main gaseous product is CO2, anodic reaction can be written as a stepwise process ... 

Generally, carbon has been used as a support in fuel cell systems since it allows one to decrease Pt loading from ca. 4 to 0.2 mg cm [31, 32]. However, thermodynamically favorable and kinetically slow electrochemical oxidation of carbon support during start/stop of a simple fuel cell (Eq. 10) or carbon corrosion that could proceed during recharge of the... 

Gallagher KG, Wong DT, Fuller TF (2008) The effect of transient potential expostffe on the electrochemical oxidation of carbon black in low-temptaatiire fuel cells. J Electrochem Soc... 

For other electrochemical reactions, different ideal efficiencies apply. Curiously, for direct electrochemical oxidation of carbon AG is larger than AH, and consequently the ideal efficiency is slightly greater than 100% when using this definition of ideal efficiency. 

Shao Y, Yin G, Zhang J, Gao Y. Comparative investigation of the resistance to electrochemical oxidation of carbon black and carbon nanotubes in aqueous sulfuric acid solution. Electrochim Acta 2006 51 5853-7. 

Figure 16.1. The energy level of carbon monoxide molecules, and the formation of metal-carbon monoxide bonding [449]. (Reprinted from Grgur BN, Markovic NM, Lucas CA, Ross PN. Electrochemical oxidation of carbon monoxide from platinum single crystals to low temperature fuel cells catalysis. Part 1 carbon monoxide oxidation onto low index platinum single crystals. J Serb Chem Soc 2001 66 785-97. With permission from the Serbian Chemical Society.)...

In principle, electrochemical oxidation of carbon in acid occurs by at least two anodic reaction pathways (Atanassova, 2007) ... 

The possibility of a direct electrochemical oxidation of carbon monoxide was soon questioned. It was suggested that hydrogen, rather than carbon monoxide. 

By the electrochemical point of view, another criterion should be taken account when comparing fasf and slow processes. While keeping driving voltage lower than 2 V, the electrochemical reactions will not occur in carbon-based actuators using nonaqueous solvents or ionic liquids (Janes et al. 2004 Torop et al. 2009). However, in aqueous media toe electrochemical oxidation of carbon-based electrodes appear already at +0.6 V (vs. Ag/AgCl reference) and may lead to intense gas evolution (Xi et al. 2009). 

However, while carbonaceous materials are abundant and therefore cheap, they also suffer from an insufficient long-term stability. Among the major issues hindering a commercial market launch of low-temperature polymer electrolyte membrane fuel cells (PEMFC), the poor durability of the carbon-supported catalysts appears to be the most critical [69,70]. In particular in the harsh conditions at the cathode side, severe corrosion of the carbon support takes place. Electrochemical oxidation of carbon in aqueous solution is already thermodynamically... 

Typically, electrochemical carbon-support corrosion (oxidation) proceeds as in (1). The mechanism and kinetics of electrochemical oxidation of carbon as a function of potential (usually on the basis of the reversible hydrogen electrode (RHE) potential) have mainly been investigated using potentiostatic experiments in phosphoric acid (Kinoshita 1988), and are described by... 

The gas-phase oxidation of carbon blacks by oxygen and/or water is strongly catalyzed by the presence of catalytically active metals, such as platinum (Rewick et al. 1974, Stevens and Dahn 2005), whereby several weight percent of platinum on carbon can increase the gas-phase oxidation rate by orders of magnitude. This, however, is not the case for the electrochemical oxidation of carbon blacks, where at potentials of 0.8 V and higher (vs. RHE) the carbon corrosion rate is within a factor of 2 between that for noncatalyzed and platinum-catalyzed carbon blacks (Roen et al. 2004, Passalacqua et al. 1992, Kinoshita 1988). Therefore, gas-phase oxidation tests to screen potential carbon-black supports is not a reliable method for predicting their stability in the electrochemical environment, so it is essential to measure the carbon corrosion rates directly in an electrochemical cell. 

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