Sr-Doped LaCrO[sub 3] Anode for Solid Oxide Fuel Cells

Abstract

The most commonly used anode in development of solid oxide fuel cells (SOFCs) is currently a Ni/yttria-stabilized zirconia (YSZ) cermet. 1 Despite excellent properties for operation in hydrogen, the Nibased anode suffers from some disadvantages related to volume instability upon redox cycling and detrimental carbon formation caused by cracking of methane during operation in dry natural gas. For mixtures of CH 4 and H 2 O the catalytic properties of Ni cause total conversion over the first few millimeters of a cell. The endothermic nature of the steam reforming process can thus cause steep thermal gradients potentially capable of mechanically damaging the cell stack. 2 The basic requirements for an SOFC anode are as follows. 1. Electrochemical ability to oxidize methane directly without deposition of carbon, or alternatively adequate catalytic properties for steam reforming combined with the ability to oxidize H 2 and CO.2. Adequate electronic conductivity and preferably also oxide-ion conductivity to enhance electrode efficiency.3. Physical and chemical stability in oxidizing and reducing atmospheres, and chemical compatibility with the electrolyte during production and operation. 4. A thermal expansion coefficient (TEC) value comparable with that of the supporting element of the cell.Lanthanum-chromite (LC)-based materials have previously been investigated primarily as interconnect material, where typical dopants are Ca (LCC) and Sr (LSC). However, this group of materials may also be considered for use as anode material. The aim of this work has been to examine to which extent selected doped LC materials fulfill the listed requirements for anodes, and to which extent the problems faced with Ni/YSZ electrodes can be avoided. Key properties of selected LC materials are reviewed in the following.Electrochemical and catalytic properties.-Experiments reported in literature indicate little or no reforming activity or direct oxidation of CH 4 on lanthanum chromites, 3-7 disregarding experiments where a catalytic active current collector such as Pt has been applied. 8,9 Considering this, addition of a steam reforming catalyst to the LC electrode might be required. 4,7 Dissociation (cracking) of dry CH 4 occurs on LC materials at temperatures of 800 to 900ЊC, 5 but can apparently be avoided by addition of small amounts of water. 7 Preferably the reforming step should take place internally near the anode, where (i) water is evolved by the anode reaction and therefore does not have to be added or recycled, and (ii) heat for the endothermic steam reforming reaction is available from ohmic losses in the fuel cell during operation.Conductivity.-The electronic conductivity of LC is enhanced by the use of divalent doping on either A or B sites in the perovskite lattice. At high pO 2 , the charge balance is maintained by the formation of Cr 4ϩ . However, when the material is exposed to reducing conditions, the lattice looses oxygen, oxygen vacancies are formed, and the charge balance is now attained by reduction of Cr 4ϩ to Cr 3...

Cromitas de Lantano como potencial electrodos simétricos para Pilas de Combustible de Óxido Sólido

“…The main diffraction peaks do not shift significantly and moreover there are no extra peaks, which may indicate that no reactions occur between the electrolyte and the electrode at least up to 1400°C. Although no degradation of LCC was observed in our short-term experiments under reducing further investigations after longterm operation are highly demanded to clarify this issue as reported by Primdahl et al (24) The Nyquist plots of a dense La 0.7 Ca 0.3 CrO 3 pellet only show the presence of inductive effects indicating predominat electronic contribution in this material. Thus the overall resistance, under both air and 5%H 2 conditions, was obtained from the high frequency x-intercept.…”

Potenciales materiales de electrodo para Pilas de Combustible de Óxido Sólido simétricas

“…Regarding compatibility, XRD studies reveal that the main diffraction peaks of LCC-YSZ composites do not shift significantly and moreover there are no extra peaks, which seem to indicate that no reactions occur between the electrolyte and the electrode at least up to 1400°C. Although no degradation of LCC was observed in our short-term experiments under reducing conditions, further investigations after long-term operation are highly demanded to clarify this issue as reported by Primdahl et al (11), and to check potential problems related with the volatilisation of chromium species.…”

Development of novel anode material for intermediate temperature SOFC (IT-SOFC)

“…[5][6][7][8][9] The weaknesses of the nickel cermet based anode under operating conditions prompted large scale research on the development of novel oxide ceramic based anode materials. [10][11][12][13][14][15][16] It may be noted that most of the anode materials developed so far are based on structures containing rare-earth elements. In fact, all the active components of the SOFC unit cell comprise of rare-earth or low abundant elements.…”