Anodes for Direct Oxidation of Hydrocarbons

Figiire 6.3 i Current-potential characteristics of copper cermets at 800°C showing effect of hydrocarbon fuel and dectrocatalysis by ceria (O) hydrogen on CiilYSZ cermet (A) hydrogen on cermet catalyzed by 

Refractory electronic-conducting ceramics may also provide candidate materials for the direct oxidation anode. Ceria and its solid solutions with other rare earths, cerates and titanates [46] have been investigated. Given their variation in conductivity with gas environment they may have to be used with metals in composites. The perovskites, of general formula ABO3, where A and B 

Empirical development of the nickel-zirconia anode over several decades has led to solid oxide fuel cells with adequate service life and performance, but fuel reforming is still required to operate with commercially available hydrocarbon fuels. It has become evident that the anode reactions are dominated by the three-phase boundary and that the microstructure of the composite cermet anodes is pivotal. Consequently, the processing methods used for making the anode powders, and the fabrication techniques used for deposition on the electrolyte are critical in making high performance anodes. Anode-supported cells with very thin electrolyte films are becoming interesting for operation at lower temperatures. 

Anode behaviour is evaluated by d.c. methods under steady state and by impedance spectroscopy under transient conditions. The reaction pathways for hydrogen have been elucidated, and mathematical modelling is providing micro- and nanoscale understanding of electrode processes. At higher current loadings, the diffusion processes have been evaluated showing that the electrochemically active anode thickness is around 10 pm. In practice, however. 

Thampi, A. J. McEvoy and J. Van Herle, in Ionic and Mixed Conducting Ceramics II, eds. T. A. Ramanarayanan, W. L. Worrell and H. L. Tuller, The Electrochemical Society Proceedings, Princeton, NJ, PV94-12,1994, p.239. 

---

FIGURE 6.11 Diagram of the processing technique used to prepare Cu-Ce02-YSZ anodes for direct oxidation of hydrocarbon fuels by preparing a porous preform of YSZ and then infiltrating it with cerium nitrates to form ceria and then with copper nitrates to form metallic copper [84]. Reprinted from [84] with permission from Elsevier. 

R. J. Gorte, S. Park, J. M. Vohs, and C. Wang, Anodes for direct oxidation of dry hydrocarbons in a soUd-oxide fuel cell, Adv. Mater. 12,1465-1469 (2000). 

Gorte R J etal, 2000, Anodes for Direct Oxidation of Dry Hydrocarbons in a Solid Oxide Fuel Cell. Advanced Materials, doi.wiley.com. 

Poor activity for direct oxidation of hydrocarbons and propensity for carbon formation when exposed to hydrocarbons. To improve the activity for direct oxidation and reduce the anode s propensity for carbon formation, copper - ceria anodes are being developed. 

The basic concepts of composite or single-phase MIEC electrodes are equally applicable to anodes. Traditionally, however, the typical anode used to date has been a composite mixture of Ni and YSZ. The presence of YSZ not only suppresses the thermally induced coarsening of Ni, but it also introduces MIEC characteristics. Other anodes currently under investigation are based on cermets of copper, which are being explored for direct oxidation of hydrocarbon fuels [39]. These types of anodes are in an early stage of development and thus their polarisation behavior is not discussed here. In so far as single-phase anodes are concerned, some work has been reported in the literature, most notably on La-SrTi03 [40, 41]. Work on this as well as other perovskite-based anodes is in its infancy, and is not elaborated upon further. The discussion in this chapter is confined to Ni + YSZ cermet anodes. 

Work with the purpose of using ceria based materials as an anode for direct oxidation (i.e. without steam reforming) of methane and other hydrocarbons have been performed by several authors. 

Three strategies for direct conversion of hydrocarbon fuels are possible. The first strategy utilizes conventional Ni-cermet anodes but modifies the operating conditions of the fuel cell [50]. For methane at intermediate temperatures, the rate of carbon deposition may be slow enough so that oxygen anions electrochemically driven through the electrolyte and steam generated by oxidation of methane will remove carbon as it is deposited. 

The situation is more difficult when the anodic oxidation of organic fuels is desired. Oxygen containing fuels (for instance CO, CH3OH) can be oxidized on metals of group Ib of the periodic table such as gold and silver, mixed oxides, and spinels. However, the efficiency is the best for platinum and platinum alloys. For the direct oxidation of hydrocarbons, only platinum has proved useful [6] in low-temperature cells so far. 

Gorte RJ, Kim H, and Vohs JM. Novel SOFC anodes for the direct electrochemical oxidation of hydrocarbon. J. Power Sources 2002 106 10-15. 

It has been argued that all steps in the reaction must be electrochemical in nature for the process to be called direct oxidation. According to this definition, any process that involves cracking of the hydrocarbon on the anode material, followed by electrochemical oxidation of the cracking products, should not be considered to be direct oxidation. The primary reason for using this narrow definition for direct oxidation is that the open-circuit voltage (OCV) of the cell will be equal to the theoretical, Nernst potential if there are no other losses and if all steps in the oxidation mechanism are electrochemical. 

One of the main objective of SOFCs in the future is the use of gaseous mixtures of C0-H2-H20 produced by coal gasification plants or by steam reforming a hydrocarbon fuel, especially methane. Very little data is available about the direct oxidation of methane in SOFCs [96, 97], Steele et al. [97] have recently confirmed the poor electrocatalytic activity of Pt electrodes for the anodic oxidation of methane at 800 °C. Although nickel fulfills major requirements for anode materials when H2 and CO are employed as fuels, its use for the direct oxidation of methane encourages carbon deposition. To overcome this problem, alternative anode materials must be... 

A single-chamber solid oxide fuel cell (SC-SOFC), which operates using a mixture of fuel and oxidant gases, provides several advantages over the conventional double-chamber SOFC, such as simplified cell structure with no sealing required and direct use of hydrocarbon fuel [1, 2], The oxygen activity at the electrodes of the SC-SOFC is not fixed and one electrode (anode) has a higher electrocatalytic activity for the oxidation of the fuel than the other (cathode). Oxidation reactions of a hydrocarbon fuel can... 

Recently, sensational papers about direct oxidation of methane and hydrocarbon in solid oxide fuel eells (SOFC) at relative low temperatures about 700°C were published. Even though the conversion of almost dry CH4 on ceramic anodes were demonstrated more than 10 years ago " at 1000°C, the reports about high current densities for methane oxidation at such low temperatures are indeed surprising. 

The earliest intensive research into the direct electro-oxidation of hydrocarbons for fuel cell applications was in the late 1950 s and the 1960 s. Early attempts to anodically oxidize hydrocarbons were disappointing, in that low rates were observed with both the lower molecular weight gases and higher molecular weight hydrocarbons. To achieve acceptable rates of oxidation. 

The primary motivation for these studies is the analysis of the reactivity patterns of organic compounds, when Ce(IV) is used as an oxidant. These patterns are determined for the most part by product analysis of selected series of organic compounds. The results obtained in two studies that bear more directly on the chemical behavior of Ce(IV) as an oxidant for hydrocarbons have been interpreted to indicate different mechanistic behavior of Ce(IV). In a product study of the oxidation of isodurene (1,2,3,5-tetramethyl benzene) by ceric ammonium nitrate compared to anodic oxidation, Eberson and Oberrauch (1979) concluded that the oxidation by Ce(IV) occurs via a H atom transfer from the alkylaromatic compound to Ce(IV). Badocchi et al. (1980) measured the variation of second-order rate constants for the oxidation of a series of alkylaromatic compounds with added Ce(III). These results along with those from the determination of kinetic deuterium isotope effect were dted to support a mechanism involving radical cations. The Ce(IV)/Ce(III) functions as an electron acceptor/donor in such a mechanism. 

The anodic oxidation of alkanes in anhydrous hydrogen fluoride has been studied at various acidity levels from basic medium (KF) to acidic medium (SbFs) to establish optimum conditions for the formation of carbenium ions . The oxidation potential depends on the structure of the hydrocarbon methane is oxidized at 2.0 V, isopentane at 1.25 V vs Ag/Ag. Three cases of oxidation can be distinguished. In basic medium, direct oxidation of the alkane to its radical cation occurs. In a slightly acidic medium, the first-formed radical cation disproportionates to cation, proton and alkane. The oxidation is, however, complicated by simultaneous isomerization and condensation reactions of the alkane. In strongly acidic medium, protonation of the alkane and its dissociation into a carbenium ion and molecular hydrogen occurs. In acidic medium n-pentane behaves like a tertiary alkane, which is attributed to its isomerization to isopentane. The controlled potential electrolysis in basic medium yields polymeric species. 

As discussed in this entry, a number of novel materials and composites have been proposed as potential anodes for direct hydrocarbon solid oxide fuel cells. While many are promising, a commercially viable solution has not yet been found. The discusskm in this entry is deliberately framed arotmd the cxmcepts of ionic and electronic conductivity, electrocatalysis, and stability. It is essential for future researchers to address all of these topics when discussing new materials. The schematic in Fig. 3.4 represents both the complexity of the problem and the simpUcity that could potentially be achieved if a material meeting all of these requirements can be found. 

---