Electrochemical carbon oxidation melts

Studies of electrochemical carbon oxidation in carbonate melts at 700°C were performed by Weaver et al. (1981) at the Stanford Research Institute, Menlo Park, California. They used rods of different carbon materials as electrodes. The electrode potentials were measured relative to a gold reference electrode in an atmosphere of carbon dioxide mixed with oxygen at the same temperature. The electrodes proved to be more active the lower the degree of crystallinity of the initial powder (from which the rods were pressed). The electrodes had open circuit potentials around 1.1 V. At a current density of lOOmA/cm the potential of the most active sample was 0.8 V (and 0.9 V when the temperature was raised to 900°C). 

At the Hanbat National University in Korea, Lee et al. (2011) investigated the oxidation mechanism of activated carbon made from bamboo in carbonate melts at 850°C. They concluded that carbon oxidation was initiated by chemical oxidation to CO, which was then electrochemically oxidized to CO2. 

In electrochemistry an electrode is an electronic conductor in contact with an ionic conductor. The electronic conductor can be a metal, or a semiconductor, or a mixed electronic and ionic conductor. The ionic conductor is usually an electrolyte solution however, solid electrolytes and ionic melts can be used as well. The term electrode is also used in a technical sense, meaning the electronic conductor only. If not specified otherwise, this meaning of the term electrode is the subject of the present chapter. In the simplest case the electrode is a metallic conductor immersed in an electrolyte solution. At the surface of the electrode, dissolved electroactive ions change their charges by exchanging one or more electrons with the conductor. In this electrochemical reaction both the reduced and oxidized ions remain in solution, while the conductor is chemically inert and serves only as a source and sink of electrons. The technical term electrode usually also includes all mechanical parts supporting the conductor (e.g., a rotating disk electrode or a static mercury drop electrode). Furthermore, it includes all chemical and physical modifications of the conductor, or its surface (e.g., a mercury film electrode, an enzyme electrode, and a carbon paste electrode). However, this term does not cover the electrolyte solution and the ionic part of a double layer at the electrode/solution interface. Ion-selective electrodes, which are used in potentiometry, will not be considered in this chapter. Theoretical and practical aspects of electrodes are covered in various books and reviews [1-9]. 

A number of unique difficulties pertain to oxidation states of metal ions encountered in molten salt solutions. For example, for first-row transition metals, the highest oxidation state prevailing is often +3, as in the case of Fe and Cr. Frequently, for chlorides in particular, the +3 state compounds are volatile at suitable operating temperatures and, hence, their solutions are thermally unstable.Other problems encountered include rapidly dispropor-tionating states, the formation of oxyhalides, and precipitation of complexes by reaction with the melt. While redox reactions per se involve very fast charge transfer steps, these may occur at the extremes of the range of electrochemical stability, thus leading to concomitant solvent melt decomposition. Nevertheless, suitable processes such as Fe /Fe on vitreous carbon in chloride melts can be employed to determine the effective electrochemical areas of electrodes where diffusion coefficients are accurately known. ... 

The MCFC is an electrochemical reaction system where the anode oxidizes Ha to HaO and the cathode reduces Oa to CO as shown in Eqs. 8.1a and 8.1b. Thus carbonate materials serve as the electrolyte, which is generally a mixture of various alkali metal carbonates of LiaCOs, NaaCOa, and KaCOa. Table 8.1 shows the melting points (m.p.), surface tension (y), density (p), electric conductivity (k), and Henry s Law constant of Oa dissolution (/toz) for various eutectic carbonates. 

Using a lithium carbonate electrolyte, containing some lithium oxide, Lubomirsky and his coworkers were able to convert carbon dioxide into carbon monoxide and oxygen electrochemically using a graphite anode and a titanium cathode [5]. A schematic of the cell is shown in Figure 1.2.1. The mechanism is thought to be carbon dioxide reacts with the 0 in the melt ... 

Lithium carbonate is the cheapest precursor for the production of lithium, which is predicted to play a major part in energy storage, but the only production route for lithium is by the electrolysis of lithium chloride. It would be an advantage if lithium carbonate could be electrolysed directly but electrochemical reaction of the carbonate ion is always more favourable than the deposition of lithium, assuming the melt is saturated with lithium oxide ... 

Mohamedi, M., Bprresen, B., Haarberg, G.M. and Tunold, R. (1996) Study of the Anode Process on Carbon Electrodes in the Pure Magnesium Chloride Melt with Dissolved Magnesium Oxide at 1023K, Electrochem. Soc. Proc., 12th Int. Symp. on Molten Salts, 96-41, 417-427. 

In situ spectroscopy measurements were performed in NaCl-KCl-based melts during electrochemical reduction and oxidation of niobium species. Glassy carbon rod, tungsten wire or niobium plate were used as working electrodes in the spectroelectrochemical experiments. A molybdenum wire dipped into a NaCl-CsCl-PbCl2 melt served as a counter electrode and a silver wire in a NaCl-CsCl-AgCl (1 mol %) melt acted as the reference electrode. 

Shapoval, V.L, Malyshev, V.V., Tishchenko, A.A., and Kushkhov, K.B. (2000) Electrochemical behaviour of oxide tungstate-molybdate-carbonate melts and high-temperature electrochemical synthesis of disperse powders of tungsten carbide [in Russian]. Zh. Prikl. Khim. (St. Petersburg), 73, 567-572. 

Linear esters, as mentioned above, are not widely used in Li-ion batteries due to their inferior anodic stability compared to linear carbonates. Although numerous patents for electrolyte formulations claim various ester stmctures as the co-solvents, literature reports on the esters are considerably fewer. Although esters oxidize much more easily than carbonates, at least one reference reported an electrolyte formulated from a mixmre of sulfone and ethyl acetate showing better electrochemical properties than carbonate-based electrolyte in LNMO/Li half cells [108]. Due to their low viscosity and melting point, linear esters are considered to be candidates for low-temperature Li-ion batteries. Both non-lluorinated [94] and fluorinated esters [95] have been examined by scientists at the Jet Propulsion Laboratory as Li-ion battery electrolyte co-solvents for space missions. 

The physical properties of methanol, its melting point, boiling point and solubility in water, approach the ideal for soluble fuels it is also resistant to reduction at the cathode. However, it is inflammable and, although it can be electrochemically oxidized to carbon dioxide and water, its exchange current density is small and suitable inexpensive catalysts to yield high current densities are not yet known. This latter drawback is the one holding back the commercial development of the methanol fuel cells. 

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