Carbon-based anodes

Figure 7. Section through cell after electrolysis starts (7 V) with operating carbon-based anode showing lenticular bubbles of fluorine on anode surface.

State-of-the-art thin film Li" cells comprise carbon-based anodes (non-graphitic or graphite), solid polymer electrolytes (such as those formed by solvent-free membranes, for example, polyethylene oxide, PEO, and a lithium salt like LiPFe or LiCFsSOs), and metal oxide based cathodes, in particular mixed or doped oxides... 

With regard to electrode material, it can be seen (Tables 4 and 5) that cathodic limits on mercury are displaced by a few tenths of a volt to more negative potentials than on platinum. On the anodic side, the number of practically useful electrode materials is limited to noble metals and different types of carbon one case (anodic limit of pyridine nos. 35 and 36) shows that the anodic limit is lower on graphite than on platinum, and this seems to be a general trend for the comparison of carbon based anode materials, except possibly for vitreous carbon (Table 5) and bright (smooth, polished or shiny) platinum. 

The electrochemical 2-chlorophenol and 2,6-dichlorophenol removal from aqueous solutions using porous carbon felt (Polcaro and Palmas 1997) or a fixed bed of carbon pellets (Polcaro et al. 2000) as three-dimensional electrodes was investigated by Polcaro s group. The group s experimental setup consisted of a two-compartment electrochemical cell separated by an anionic membrane where the carbon felt or pellets could be lodged and the solution was recirculated by peristaltic pumps. Both carbon-based anodes effectively removed the chlorophenols as well as their reaction... 

The irreversible capacity loss in the first cycle of carbon-based anode materials through consumption of a considerable amount of lithium is detrimental to the specific energy of the whole cell and has to be minimized for optimum cell... 

The chloride ion itself is negative and will be repelled by the negatively charged cathode (reinforcing steel). It will move towards the (new external) anode. With the carbon-based anodes it may then combine to form chlorine... 

Reaction (7.5) is the reverse of reaction (7.2), that is, alkalinity is formed at the steel cathode (enhancing the passivity of the steel) and consumed at the anode. These and related reactions can carbonate the area around the anode (especially where carbon-based anodes are used, where the carbon also turns into carbon dioxide) and can lead to etching of the concrete surface and attack on the cement paste and even some aggregates once the alkalinity is consumed. 

On the anode, the surface of carbon electrode is covered with the products of the electrolyte reduction which form a protective solid electrolyte interface (SEl) [12-15]. SEl layer on the anode s surface controls the safety behavior of lithium-ion cells to a great extent. This behef is supported by voluminous research done predominantly on carbon-based anodes, such as graphites and cokes (Chap. 1), cycled with solutions of hthium-ion salts in organic carbonates. With the emerging... 

From the results summarized by chart (Fig. 6.9) it is apparent that carbon-based anodes can contribute as much as two-thirds of the internal cell resistance. This is mostly due to the resistive solid electrolyte interphase (SEI) film formation on the... 

Hayes, S., van WiiUen, L., Eckert, H., Even, W.R., Crocker, R.W., and Zhang, Z. Solid-state NMR strategies for the structural investigation of carbon-base anode materials. Chem. Mater. 1997 9 901-911. 

The electrolyte used by the fuel cell is a solid gas—impermeable zirconia known as zirconium oxide (ZrOj). This ZrOj is doped with calcium oxide (CaO) to supply enough oxide ions to carry the cell current. The oxidant air or oxygen is bubbled through the molten silver cathode, which is held inside the zirconia cup. At the fuel electrode or the carbon-based anode electrode, the oxide ions are combined with carbon monoxide (CO) and give up their electrons to an external circuit. The cell by-products CO and hydrogen, which are formed in the initial fuel decomposition, are burned outside the cell to keep the fuel cell at operating temperature. The hydrogen is not involved in the electrochemical cell reaction. 

There are basically three broad categories of ICCP anodes consumable, semiconsumable, and nonconsumable anodes. For consumable metals and alloys such as scrap steel or cast iron, the primary anodic reaction is the anodic metal dissolution reaction. Metal dissolution is negligible on inert or nonconsumable anodes and the main reactions are the evolution of gases. Oxygen can be evolved in the presence of water, whereas chlorine gas can be formed if chloride ions are present (seawater, deicing salts). On partially inert anodes, both the metal dissolution and gas evolution reactions are important. Graphite and other carbon-based anodes can additionally produce carbon dioxide gas as these anodes are partially consumed. 

Polymeric anodes are a different type of carbon-based anodes consisting of flexible wire anodes with a copper core surrounded by a polymeric material that is impregnated with carbon. The impregnated carbon is gradually consumed in the conversion to carbon dioxide, with ultimate failure by perforation of the copper strand. The anodes... 

Liu, Y Harnisch, F., Schroder, U., Fricke, K., Climent, V., and Feliu, J.M. (2010) The study of electrochemically active mixed culture microbial biofilms on different carbon-based anode materials. Biosensors Bioelectronics, 25, 2167-2171. 

In the meantime, it was demonstrated that lithium can be reversibly inserted into graphite at room temperature in an organic electrolyte in 1983. Lithium-ion battery was first commercialized with this carbon-based anode in 1991. Graphite bas a capacity of 372 mAh/g, corresponding to the intercalation of one lithium atom per six carbon atoms. Though carbon bas ratber lower capacity than lithium metal and lithium alloy anodes, volume change was small and represented longer cycle performance. After this commercialization, many researchers and companies have put their effort on new carbonaceous anodes. 

The nature of the SEI, which is dependent upon the nature of the electrolyte and the nature and composition of the solvent components is a crucial issue governing the performance of both hard and soft carbon anode materials as discussed above. The determination of individual processes which limit the low temperature curabilities of carbon-based anodes is complex, but experiments have been designed to identify, at least qualitatively, individual mechanisms relating to dissolution or growth of the SEI, Li(m) deposition, or to slow electrode kinetics and/or slow solid-state diffusion. In "half-cells" of the type... 

---