A Highly Conductive Oxide Anode for Solid Oxide Fuel Cells

Abstract

Ceramic anodes comprising infiltrated SrMoO 4 in porous ytttria-stabilized zirconia were investigated. Upon reduction at 1073 K, the electronically insulating SrMoO 4 phase transformed to SrMoO 3 , which has a bulk electronic conductivity of 10 3 S cm −1 under fuel cell conditions. An anode conductivity of 20 S cm −1 was achieved with a low SrMoO 4 loading of 13 vol % of the total anode. The infiltrated composite is dimensionally stable upon redox cycling, and a Pd catalyst was required to achieve good fuel cell performance. Fuel cell performance with methane was lower than with hydrogen. This lower methane performance could be due to coking.Solid oxide fuel cells ͑SOFCs͒ are a promising fuel flexible technology for clean and efficient conversion of chemical energy to electrical energy. While the fuel flexibility of SOFCs is widely considered a key benefit, it is well known that the conventional nickelyttria-stabilized zirconia ͑Ni-YSZ͒ cermet anode catalyzes the formation of deleterious carbon filaments in the presence of hydrocarbons. 1 This carbon fiber formation results in deactivation of the Ni surface, loss of Ni by metal dusting, 2 and anode fracture induced by carbon fiber growth. 3 Other drawbacks of Ni include catalytic poisoning in the presence of sulfur and low redox stability. 4,5 Replacement of Ni with an electronic conducting ceramic has generated great interest because ceramics tend to be more fuel flexible and sulfur tolerant than Ni. The target anode must offer ionic/ electronic conductivity, gas transport, and catalytic activity for fuel oxidation. Of these requirements, Ni provides both electronic conductivity and catalytic activity for the oxidation reaction. With a single metal oxide material, it is difficult to achieve high electronic conductivity and catalytic activity. Implementation of a single metal oxide to fulfill both roles would be simpler; however, incorporating two separate materials allows for independent optimization of an electronic conductor and oxidation catalyst. 6 In this paper, we focus on the development of a highly conductive ceramic anode into which any catalyst of choice can be incorporated.The target electronic conductivity for anode materials has been outlined in a review by Atkinson et al. 7 These authors have argued that the required anode conductivity under operating conditions can be relaxed to as low as 1 S cm −1 depending on cell design. The highest performing anodes are typically porous composites of the electrolyte ͑YSZ͒ and an electronic conductor. The composite conductivity of the electrode is strongly dependent on the structure. For example, the conductivity of a conventional Ni-YSZ composite is 10 3 S cm −1 , even though Ni has a conductivity of 10 6 S cm −1 . 8 Compared to conventional cosintering methods, composites formed by infiltration have the advantage of providing higher conductivity at lower volume percent ͑vol. %͒ loadings of the electronic conductor. 9 However, even with infiltrated composites, the conductivity of the composite will be at leas...

Conductivity and redox stability of new double perovskite oxide Sr1.6K0.4Fe1+x Mo1−x O6−δ (x = 0.2, 0.4, 0.6)

“…SrMoO 4 is a stable oxide. It has been reported that reduction of SrMoO 4 in 4 %H 2 /Ar began at 750°C and completed after holding at 800°C for about 3 h [24]. The reducing temperature, 700°C in this study was not enough to convert SrMoO 4 into SrMoO 3 .…”

“…2b, of all samples exhibits an exothermic peak at a temperature around 500°C upon heating is probably due to the reduction of perovskite oxides because endothermic peaks were not observed on cooling therefore it is irreversible indicating not related to phase changes. For samples with second-phase SrMoO 4 , the reduction of SrMoO 4 starts at 750°C in 4 %H 2 /Ar which happens at a much higher temperature [24]. There were some exothermic effects around 700°C on heating which could be related to the phase transformation associated to the reduction of perovskite oxides (Fig.…”

Insights into the Design of SOFC Infiltrated Electrodes with Optimized Active TPB Density via Mechanistic Modeling

“…Our results are consistent with previous infiltrated electrode experimental work, which has reported the conductivity of infiltrated electrodes to be approximately 2% to 5% of the bulk conductivity. 5,6,16,22 …”

Effect of Au and/or Mo Doping on the Development of Carbon and Sulfur Tolerant Anodes for SOFCs—A Short Review

“…A vast number of published studies can be summarized with a selection of other anode candidates such as: (a) Ru (Sauvet and Fouletier, 2001;Bebelis et al, 2006;Caillot et al, 2007), (b) Cu (Slater and Irvine, 1999;Park et al, 2000), (c) Fluorite (e.g., Cu-CeO 2 -ScSZ; Ye et al, 2007), (d) Tungsten bronze (e.g., (Ba/Sr/Ca/La) 0.6 M x Nb 1−x O 3−δ , where M=Mg, Ni, Mn, Cr, Fe, In, Ti, Sn;Tao and Irvine, 2004), (e) Pyrochlore (e.g., Gd 2 Ti 2 O 7 ) anode materials (Goodenough and Huang, 2007;Sun and Stimming, 2007;Tsipis and Kharton, 2008), (f) LSCM (e.g., La 0.75 Sr 0.25 Cr 0.5 Mn 0.5 O 3−δ ) (Sfeir et al, 2001;Liu et al, 2002;Tao and Irvine, 2004;Ruiz-Morales et al, 2007), SrMoO 4 (Smith and Gross, 2011), and other complex perovskites (Xiao et al, 2010), such as double perovskites (e.g., Sr 2 Mg 1−x Mn x MoO 6−δ; Huang et al, 2006 or Sr 2 CoMoO 6; Zhang et al, 2011), chromites (Vashook et al, 2003), and titanates (Li X. et al, 2009). Although most of the above materials are characterized by high carbon resistance, nevertheless, in most of the cases their practical use is inhibited by the poor electrochemical or catalytic activity, the relatively low electronic conduction, the low thermal and chemical stability, the use of prohibitively expensive materials and/or the high cost of processing for their commercial use (Tsipis and Kharton, 2008;Niakolas et al, 2010).…”