Carbon-conducting membranes

The concept of the reversed fuel cell, as shown schematically, consists of two parts. One is the already discussed direct oxidation fuel cell. The other consists of an electrochemical cell consisting of a membrane electrode assembly where the anode comprises Pt/C (or related) catalysts and the cathode, various metal catalysts on carbon. The membrane used is the new proton-conducting PEM-type membrane we developed, which minimizes crossover. 

The PEMFCs require expensive polymer membrane (e.g., Nation ), and operate at a low temperature (e.g., 80°C). Although low temperature reduced the cost of material, the heat generated at low temperatures is more difficult to remove. Alternate proton conducting membranes (e.g., inorganic polymer composites) that will operate at a high temperature (e.g., 200°C) are required. The expensive platinum catalyst used for electrochemical reactions can be poisoned by even trace amounts of carbon monoxide in the hydrogen fuel stream. Hence, a more tolerant catalyst material needs to be developed. 

The following example illustrates the potential of membrane-separation processes for precombustion carbon capture in an IGCC. This approach avoids using an expensive air-separation unit (asu) or a difficult-to-implement high-temperature mixed-ion conducting membrane process however, it still enables capture of C02 at purities suitable for commercial use or sequestration. 

Ismail AF and David LIB. A review on the latest development of carbon membranes for gas separation. J. Membr. Sci. 2001 193 1-18. Kilner JA, Benson S, Lane J, and Waller D. Ceramic ion conductive membranes for oxygen separation. Chem. Ind. 1997 17 November 907-911. 

FIGURE 10.28 Scanning electron microscopy (SEM) images of (a, b) Umbite and (c, d) ETS-10 supported on carbon Toray paper. (Erom Aused, M.P., Urbiztondo, M.A., MaUada, R., Pina, M.P., and Santamana, J., Synthesis of proton conducting membranes for direct metbanol fuel cells (DMPCs). Books of abstracts of tbe OSSEP Pinal Workshop, Tenerife, 2004, pp. 90-91.)... 

The greatest interest in Nafion (Nf), a perfluorinated polymer with sulfonate groups, derives from its consideration as a proton conducting membrane in fuel cells to many sensor applications [37], The use of Nf and other polyanions to control ion-transport during electrode reaction is a recurring theme [38], The electrodeposition of platinum nanoparticles at the Nafion (Nf) modified glassy carbon (GC) electrode (GC/Nf/Ptnano) by the two-step method... 

There are essentially four different types of membranes, or semipermeable barriers, which have either been commercialized for hydrogen separations or are being proposed for development and commercialization. They are polymeric membranes, porous (ceramic, carbon, metal) membranes, dense metal membranes, and ion-conductive membranes (see Table 8.1). Of these, only the polymeric membranes have seen significant commercialization, although dense metal membranes have been used for commercial applications in selected niche markets. Commercial polymeric membranes may be further classified as either asymmetric (a single polymer composition in which the thin, dense permselective layer covers a porous, but thick, layer) or composite (a thick, porous layer covered by a thin, dense permselective layer composed of a different polymer composition).2... 

Porous membranes, especially ceramic and carbon compositions, are the focus of intense development efforts. Perhaps, the least studied of the group, at least for hydrogen separations, are the ion-conducting membranes (despite the fact that many fuel cells incorporate a proton-conducting membrane as the electrolyte), and this class of membranes will not be discussed further in this chapter. 

In a later work, both the CuCl/KCl molten salt Wacker oxidation system and a [Bu4N][SnCl3] system (melting point 60 °C) was applied to the electrocatalytic generation of acetaldehyde from ethanol by co-generation of electricity in a fuel cell [56]. In the cell set-up, porous carbon electrodes supported with an ionic liquid catalyst electrolyte were separated by a proton conducting membrane (Fig. 5.6-4), and current efficiency and product selectivity up to 87% and 83%, respectively, were reported at 90 °C. 

Methane coupUng chemistry has been reported [112, 113] in mixed conducting membranes of Bij 5Y0 3Smo 2O3. CoupHng selectivities as high as 54% and yields of 35% were obtained at 900 °C. The simultaneous oxidative coupHng of methane and oxidative dehydrogenation of ethane over a basic catalyst system has been reported [114], as well as the use of carbon dioxide as an oxidant in the methane/ ethane system[115]. In the latter case, ethylene content of up to 16% was obtained in the tail gas. 

Microporous expanded PTFE membrane (ePTFE), ion conductive membrane, 229, 271 Grafil fibers Carbon fiber, 165 Grafoil ... 

There are two possibilities for an electrochemical utilization of the chemical energy of coal (i) via prior coal gasification and the subsequent use of hydrogen and/or of carbon monoxide, thus produced, in various fuel cells and (ii) by a direct electrochemical oxidation within the fuel cell. For the first of these possibilities no insurmountable technical or scientific problems arise. Gasification units and proton-conducting membrane fuel cells are well known and very reliable devices, so, their use is connected only with economic and lifetime problems. 

Figure 1.8 shows a scheme of the microstructxu e of the catalyst layer, where the presence of TPRs allows the simultaneous flows of and electrons toward the PEM and GDL, respectively. Note that the thickness of the proton conducting membrane covering the carbon-supported catalyst nanoparticles is of the order of nanometers, depending of the concentration of proton conducting binder (usually Nafion) in the ink used to prepare the MEA. 

Samsung SDI has developed a prototype of DMFC for use in laptops which is quoted to have a durability almost twice as compared to other systems being developed. SAIT has reduced the amount of catalyst required by 50 %, by developing a mesoporous carbon material, which supports highly efficient 3 nm nanocatalyst particles. In addition, SAIT has developed a unique concept of nanocomposite membrane to reduce methanol crossover by more than 90 %. This composite uses a 30-100 pm thick proton-conducting membrane with a proton conductivity of 0.1 S.cm . The DMFC has an energy density of 650 Wh dm , and fed with about 200 cm of liquid methanol can supply power to a laptop for about 15 h. The cell measures 23 cm x 8.2 cm x 5.3 cm, and its weight is less than 1 kg [60]. 

Unlu M, Zhou J, Kohl P (2009) Anion exchange membrane fuel cells experimental comparison of hydroxide and carbonate conductive ions fuel cells and energy conversion. Electrochem Solid State Lett 12(3) B27-B30... 

Li, J., Yoon, H., Wachsman, E. D. (2012). Carbon dioxide reforming of methane in a SrCeo.7Zro.2Euo.1O3a proton conducting membrane reactor. International Journal of Hydrogen Energy, 37(24), 19125-19132. 

Fuel cell electrodes are a particularly interesting catalyst application. The active layers of the proton conducting membrane fuel cells must have the ability to transport both gases and hydrogen ions to the catalyst as well as conducting water away. Mixtures of carbon blacks with special polymers create the desired hydrophilic and hydrophobic pore structures. 

---