FUEL CELL RESEARCH IN SWITZERLAND 837 CHIMIA 2004, 58, No. 12

Chimia 58 (2004) 837–850 © Schweizerische Chemische Gesellschaft ISSN 0009–4293

Solid Oxide Fuel Cells: Systems and Materials

Ludwig J. Gauckler*, Daniel Beckel, Brandon E. Buergler, Eva Jud, Ulrich P. Muecke, Michel Prestat, Jennifer L.M. Rupp, and Jörg Richter

Abstract: A solid oxide fuel cell (SOFC) is a solid-state energy conversion system that converts chemical energy in- to electrical energy and heat at elevated temperatures. Its bipolar cells are electrochemical devices with an anode, electrolyte, and cathode that can be arranged in a planar or tubular design with separated gas chambers for fuel and oxidant. Single chamber setups have bipolar cells with reaction selective electrodes and no separation be- tween anode and cathode compartments. A nickel/yttria-stabilized-zirconia (YSZ) cermet is the most investigated and currently most widespread anode material for the use with hydrogen as fuel. In recent years, however, doped ceria cermet anodes with nickel or copper and ceria as the ceramic phase have been introduced together with ce- ria as electrolyte material for the use with hydrocarbon fuels. The state-of-the-art electrolyte material is YSZ of high ionic and nearly no electronic conductivity at temperatures between 800–1000 °C. In order to reduce SOFC sys- tem costs, a reduction of operation temperatures to 600–800 °C is desirable and electrolytes with higher ionic con- ductivities than YSZ are aimed for such as bismuth oxide, lanthanum gallate or mixed conducting ceria and the use of thin electrolytes. Proton conducting perovskites are researched as alternatives to conventional oxygen con- ducting electrolyte materials. At the cathode, the reduction of molecular oxygen takes place predominantly on the

surface. Today’s state-of-the-art cathodes are LaxSr1–xMnO3–d for SOFC operating at high temperature i.e. 800–1000 °C, or mixed conducting LaxSr1–xCoyFe1–yO3–d for intermediate temperature operation, i.e. 600-800 °C. Among the variety of alternative materials, SmxSr1–xCoO3–d and BaxSr1–xCoxFe1–xO3–d are perovskites that show very good oxygen reduction properties. This paper reviews the materials that are used in solid oxide fuel cells and their properties as well as novel materials that are potentially applied in the near future. The possible designs of single bipolar cells are also reviewed.

Keywords: Anode · Cathode · Electrolyte · Materials · SOFC

1. Introduction A fuel cell is an electrochemical device electrons are forced to flow through an ex- that can convert chemical energy of a fuel ternal circuit. At the electrodes, the charge Fuel cells are one of the most attractive en- and an oxidant into heat and electric power. carrying species is changed from electrons ergy conversion systems because they offer In solid oxide fuel cells (SOFCs) the elec- from the outer circuit to the charged species high efficiency and low pollution. An ad- trolyte consists of an oxygen ion conduct- the electrolyte can conduct. In the SOFC vantage of fuel cells is the decentralised ing ceramic such as yttria-stabilized zirco- the electrolyte conducts O2–-ions. The driv- generation of electricity and the prospective nia (YSZ). One advantage of SOFCs is their ing force for the migration of O2– is the applications in mobile devices. possibility to directly use natural gas and oxygen chemical potential gradient be- the high reaction rate given by the high op- tween the anode (low) and cathode (high). erating temperature. Thus, no expensive At the cathode, side air is usually used cor- catalysts are needed. However, the ceramic responding to an oxygen partial pressure materials of the SOFC are difficult to (pO2) of 0.21 atm. At the anode, the pO2 is process and not easy to assemble. very low due to the consumption of oxygen A schematic of a SOFC is shown in Fig. ions by the used fuel (in most cases hydro- 1. A single cell consists of three basic ele- gen) to form water. The operating tempera- ments: electrolyte, anode and cathode. The ture of a SOFC is between 500 and 1000 °C cathode and anode electrodes are porous because the conduction of oxygen ions in layered ceramic (cathode) and ceramic- the solid electrolyte is a thermally activated *Correspondence: Prof. Dr. L.J. Gauckler metal (anode) components enabling easy process. In contrast to other fuel cell types, Nonmetallic Inorganic Materials Department Materials gas diffusion to and from the a solid oxide fuel cell can be operated with Swiss Federal Institute of Technology electrode–electrolyte interfaces. They ex- a variety of fuels such as CH4 with steam Wolfgang-Pauli-Str.10 hibit high electronic conduction and prefer- reforming and within a wide temperature CH-8093 Zurich ably also ionic conduction. The reduction range (500–1000 °C). Tel.: +41 1 632 56 46 Fax: +41 1 632 11 32 and oxidation at the cathode and the anode At the cathode, electrochemical reduc- E-Mail: [email protected] respectively are spatially separated and the tion of oxygen occurs and the oxygen ions FUEL CELL RESEARCH IN SWITZERLAND 838 CHIMIA 2004, 58, No. 12

Fig. 2. The tubular design from Siemens-West- inghouse

mainly due to the mismatch of thermal ex- Fig. 1. Schematic of a solid oxide fuel cell (SOFC) element with anode, cathode and electrolyte pansion coefficient between the electrolytes and support structures and the mechanical properties of the sealing materials. In order migrate through the electrolyte via a vacan- reforming of hydrocarbons is possible di- to overcome these problems a tubular con- cy mechanism to the anode. At the anode, rectly on the anode without the need for an figuration (i.e. cylindrical design) was de- hydrogen is electrochemically oxidized to external reformer [1]. veloped by Westinghouse and taken over by water. Each cell delivers a maximum of 1 V One important design criterion for a sol- Siemens and improved over the last 20 and is typically operated at around 0.6 to id oxide fuel cell is the separation of anode years. In this design (Fig. 2) the electrolyte 0.7 V at a power output of typically 250 to and cathode by the gas tight electrolyte. and anode are supported on a thick cathode 450 mW/cm2. In SOFC systems many cells Pinholes or cracks in the electrolyte can tube that is closed at one end. The materials, are stacked in series connected with a cause the hydrogen to leak to the cathode their dimensions, and fabrication processes metallic conducting interconnect. compartment where it reacts directly with are summarized in Table 1. The electrolyte Research and development in the field oxygen. This will decrease the open circuit is deposited onto the cathode support after of SOFC are currently concentrating on voltage (OCV) and might even render the fabrication of the interconnection. In a last lowering the operating temperature in order fuel cell inoperable. The development of a step, the anode is applied. The gas manifold to reduce costs and increase lifetime and to suitable stack sealant still presents a chal- of the Siemens-Westinghouse design is il- increase reliability of the ceramic stack el- lenging task because the requirements for lustrated in Fig. 3. Air is introduced via a ements and interconnects. New manufac- the sealants are stringent due to harsh envi- central Al2O3-tube to the end of the cathode turing technologies are demanded when ronments and the high operating tempera- tube. The oxidant flows back across the thin electrolytes are used reducing the elec- tures. Sealing of SOFCs can be done by us- cathode while the fuel flows in the same di- trical resistance of the cell. Materials as ing bonding seals or pressurized seals. For rection at the exterior of the tube. At the well as systems development aim also for bonding seals materials like high-B2O3 plenum of each cell, the depleted flow of air better fuel utilization and higher electrical glasses [2], earth-alkali silicate glasses such and fuel recombine and the remaining active efficiency. as BaOAl2O3SiO2 [3] or glass ceramics gases react. The generated heat serves to This paper reviews the different possi- are commonly used. Another solution re- preheat the incoming oxidant stream. One of ble designs of SOFC cells including the lies on compressive seals based on mica the most attractive features of this fuel cell possibilities when reaction selective elec- that do not bond chemically to the SOFC design is that it eliminates the need for leak- trodes are used. Commonly used materials materials [4]. free gas manifolding of the fuel and oxidant and some of their properties, as well as nov- streams in the hot zone. The drawback is el materials that could be applied in near fu- 2.1. Tubular Design that the electric current has to flow along the ture are also reviewed as well as a search In the 1960s experimental SOFCs with circumference of the tube in the anode and strategy for those materials. planar geometry were evaluated and it was the cathode. This increases the length of the found that it is very difficult to obtain ade- conducting path and thus the ohmic resist- quate gas sealing at the edges of the cell, ance of the cell as compared to a planar one. 2. Design

The design of a single cell is closely re- Table 1. Materials and fabrication processes for state-of-the-art cathode supported cells of the Siemens-Westinghouse solid oxide fuel cell lated to the design of an entire stack. Be- cause a single cell only delivers 1 V, more than one cell is usually connected in series Component Material Thickness Fabrication Process using interconnects. The open circuit volt- age (OCV) of the SOFC, i.e. the voltage of Cathode Tube Doped LaMnO3 2.2 mm Extrusion-sintering the system when no current is flowing, cor- Electrolyte ZrO2(Y2O3) 40 mm Electrochemical vapour responds to the number of individual cells deposition in the stack. Over the last two decades, Interconnect Doped LaCrO 85 mm Plasma spraying SOFCs based on yttria-stabilised zirconia 3 have been developed for an operating tem- Anode Ni-ZrO2(Y2O3) 100 mm Slurry spraying or electro- perature range of 900–1000 °C. The advan- chemical vapour deposition tage of the high temperature is that internal FUEL CELL RESEARCH IN SWITZERLAND 839 CHIMIA 2004, 58, No. 12

Siemens-Westinghouse has been work- paths, which in turn decrease the ohmic re- Currently the life of a fuel cell is in the or- ing on this problem and has come up with a sistance of each cell and increase the power der of 3000–7000 h and needs to be im- new design that is called the high-power density of cell stacks. The Siemens-West- proved by optimizing the mechanical as density SOFC (HPD-SOFC) [5]. In this de- inghouse power systems are well estab- well as electrochemical stability of the used sign, shown in Fig. 4, a flat cathode tube lished and development has shifted from materials [9]. with ligaments is used instead of a cylindri- basic technology to cost reduction and scale cal one. It allows easier manifolding of air up [6]. inside the tube and higher packing density of cells as compared to the cylindrical con- 2.2. Planar Design figuration. This leads to higher volumetric A planar design of the bipolar plates en- power densities of a complete cell stack. ables the electrical connection of cells in se- Most important is that the bridges within ries to be simplified without long current the cathode tube allow for shorter current paths. Another advantage of the planar de- sign is that low-cost fabrication methods such as screen-printing and tape casting can be used. However, because of thermal Fig. 5. Ring-type solid oxide fuel cell, with metal- stresses, the size of the cells was limited in lic interconnect from Sulzer Hexis the past. Today 10¥10 cm2 planar cells can routinely be produced and operated [7]. Sulzer Hexis aims at building systems for 2.3. Single Chamber Design the cogeneration of electricity and heat for Conventional fuel cells rely on the strict residential applications in the 1 kW power separation of fuel and oxidant by the elec- regime with cells of planar design [8]. A trolyte membrane and seals. By separating single cell with endplate (top) and intercon- the fuel and oxidant, direct parasitic chem- nect (bottom) is shown in Fig. 5. The fuel is ical reactions of fuel and oxidant are avoid- fed into the centre of a cylindrical stack ed. However, it has been shown that it is not consisting of layered circular cells. Each in- mandatory to separate the fuel and the oxi- terconnect serves as gas manifold and en- dant for operating a fuel cell: By using re- sures that the reactant air is preheated. It is action-selective electrodes a fuel cell can be made via powder metallurgy of oxide dis- operated in a single gas chamber, fed by a persion strengthened alloy (95% Cr and 5% mixture of fuel and air. Such a cell is often Fe) with 1% Y2O3. The materials, thick- referred to as Mixed Gas Fuel Cell or Sin- nesses, and fabrication processes of each gle Chamber SOFC (SC-SOFC). component are given in Table 2. A crucial Already in 1965 van Gool proposed a point is the metal/ceramic contact between device using ‘surface migration’ of an inert the electrodes and interconnects, which is substrate with two different electrodes [10]. made at the cathode side by applying a LSC The electronically insulating substrate slurry to the pins of the interconnect and a should permit easy surface transportation of Ni gauze at the anode side. To the exterior, at least one of the reactants in ionic form. the cell is not sealed and the unreacted fuel The electrodes are placed on the same side is burnt with the unreacted oxygen from air. of this substrate and have different catalyt-

Table 2. Materials and fabrication processes of the components for the electrolyte supported Sulzer Hexis solid oxide fuel cell.

Component Material Thickness Fabrication Process

Cathode LaSrMnO3 (LSM) 20–100 mm Screen printing Electrolyte ZrO (Y O ) (TZP/FSZ) 150–250 mm Tape casting Fig. 3. Schematic view of gas flows in the 2 2 3

Siemens-Westinghouse SOFC design Interconnect CrFe5Y2O3 Powder metallurgy

Anode Ni-ZrO2(Y2O3) 20–100 mm Screen printing

Fig. 4. Flat tube design by Siemens-Westing- house Fig. 6. The first single chamber fuel cell proposed by Dyer in 1965 [11] FUEL CELL RESEARCH IN SWITZERLAND 840 CHIMIA 2004, 58, No. 12

state-of-the-art electrolyte for SOFCs. These solid solutions are primarily ionic conductors and show nearly no electronic conductivity. They have to be operated at high temperatures, around 800–1000 °C [19]. The amount of oxygen vacancies and consequently ionic conductivity is in- creased by the introduction of the trivalent yttria dopants into the zirconia lattice [20–22]. This stabilizes the cubic phase at Y2O3 contents of 8 mol%. The tetragonal form (3mol% Y2O3) shows time-depend- ent degradation [21][23–25] because wa- ter is produced at the anode which leads to hydrothermally assisted transformation of the tetragonal to the monoclinic phase [26]. On the cathode side, YSZ is in contact with LaCoO3 or LaMnO3 based cathode materials. At high operating temperatures of 800 to 1000 °C both materials react forming insulating La7Zr2O7 which leads to a gradual increase of cathode overpotential Fig. 7. Possible designs for SC-SOFCs. a) classic sandwich design, b) side by side c) fully porous [27–29]. It has been recognized that for smaller SOFC stacks the operating temper- ature should be lowered without increasing ic properties. One is active for the reduction An advantage of the Single Chamber the internal resistance of the cell [30–33]. In of oxygen and the other for the activation of approach is that completely new designs the following, alternative materials to state- the fuel, i.e. adsorption and dissociation of can be envisaged such as illustrated in Fig. of-the-art YSZ such as scandia-doped zir- hydrogen from a mixture of hydrogen and 7. For research and development the classic conia, doped ceria solid solutions, bismuth- air. Van Gool suggested the use of gold or design (a) appears to be most feasible be- based oxides or lanthanum gallate based silver as the cathode material (stable oxide, cause of simple geometry and easy fabrica- electrolytes are discussed [34][35]. In Fig. unstable hydride) and platinum, palladium tion procedures. The ‘side by side’ design 8 the ionic conductivity of these electrolyte or iridium as the anode (stable hydride, un- shown in (b) allows easy interconnection of materials are plotted as a function of tem- stable oxide). In 1990 Dyer was able to gen- cells located on the same side of an elec- perature [36][37]. It has been well known erate electrical power from a device with trolyte substrate. Very thin layers of active since the 1970s that Sc-stabilized zirconia electrodes made of platinum separated by a components can be used and this reduces (ScSZ) shows the highest ionic conductivi- thin, ion conducting and porous film [11]. the material costs as well as increases the ty of all zirconia solid solutions. The reason Fig. 6 schematically shows the design of the specific power density [14]. The feasibility for this is the smallest tendency for vacan- electrochemical device and the used mate- of the side by side design and the optimum cy cluster formation with increasing dopant rials. A voltage of approximately 1 V was geometry have recently been evaluated for concentration due to the close match of the achieved at room temperature on a mixture the case of mixed reactant direct methanol Sc3+ ionic radius with the Zr4+ host cation of hydrogen and air. The achieved power fuel cells [16]. In the case of SC-SOFCs gas [38–40]. However, Sc-doped zirconia be- density was in the range of 1 to 5 mW/cm2. leaks in the electrolyte are of no concern. comes unstable especially at intermediate Hibino and Iwahara have been working The fully porous design shown in Fig. 7(c) temperatures [41]. Politova and Irvine re- on SC-SOFCs in recent years. The first makes use of the absent constriction of a cently investigated the possibility of ScSZ cells had similar power densities to the cells gas tight electrolyte. The concept of fully stabilization by yttria doping. Small addi- described by Dyer, i.e. in the range of 2–5 porous fuel cells has been proposed for di- tions of yttria considerably stabilized the mW/cm2 [12]. With very similar materials rect methanol fuel cells [17] and can easily cubic phase of ScSZ at the prospective fuel Gödickemeier et al. proved the feasibility be adopted for SOFCs. cell operating temperature. However, it was of connecting individual cells on one elec- not possible to overcome the time-depend- trolyte plate in series without the need for ent degradation of the conductivity during having sealed gas compartments for each 3. Electrolyte long annealing periods [42]. cell [13]. Thus, with one element consisting Ceria (CeO2) based electrolytes offer an of series connected cells it is possible to ob- 3.1. Oxygen Ion Conducting ionic conductivity up to 4–5 times higher tain useful voltages higher than only 1 V. Electrolytes than that of zirconia solid solutions in the Hibino et al. also used alternative elec- Solid oxide fuel cell (SOFC) electrolyte intermediate and low temperature regime materials should have high ionic conductiv- trolyte materials, e.g. La0.9Sr0.1Ga0.8Mg0.2 [43]. Doping of ceria with e.g. Gd2O3, ity and low electronic conductivity. The O3–d (LSGM), which showed better per- Y2O3, CaO or Sm2O3 introduces oxygen formance than YSZ [14]. This was mainly available electrolyte materials differ main- vacancies and induces ionic conductivity due to the higher ionic conductivity of the ly in the nature of their conductivity either [44]. The development of these materials utilized materials. Ceria (CeO2) based SC- having purely ionic or mixed ionic elec- for intermediate temperature SOFCs has SOFCs showed maximum power densities tronic conductivity (MIEC). The ionic con- been extensively reviewed by Steele [45]. 2 2 ductivity of an electrolyte can be enhanced of 644 mW/cm at 550 °C and 269 mW/cm Sm2O3 doped ceria (CSO) and Gd2O3 at 450 °C [15] with a fuel utilization that by introducing acceptor dopants and conse- doped ceria (CGO) exhibit the highest con- quently oxygen vacancies [18]. YSZ is the was estimated to be around 10%. ductivities of all rare earth doped CeO2 sol- FUEL CELL RESEARCH IN SWITZERLAND 841 CHIMIA 2004, 58, No. 12 id solutions [46]. Again, it is assumed that trolytes are superior to YSZ for low tem- ibility of this V-O network on the other this is due to the ionic radii of Sm3+ and perature SOFCs because at low tempera- hand. Moreover, the same material is able, Gd3+ which nearly match the ionic radius of tures CGO behaves as a pure ionic conduc- under imposed polarisation, to self convert Ce4+ [46–49]. Furthermore, these com- tor with much higher ionic conductivity. reversibly and dynamically from elec- pounds show the lowest electronic conduc- Several authors proposed doped ceria elec- trolyte to electrode. All these specific char- tion at low oxygen partial pressures. At 700 trolytes for intermediate and low tempera- acteristics led to a new concept of ceramic °C the conductivity of CGO and CSO (both ture fuel cell operation [19][53][54][58]. oxygen generator based on a unique mate- with 10–25% dopant) come close to the Dikmen et al. investigated the influence rial [64]. However, the main drawback of conductivity of YSZ at 1000 °C [50][51]. A of high ionic conductive bismuth oxide as a BIMEVOX electrolytes is that they slowly monotonic increase of ionic conductivity is dopant in ceria. The authors report higher decompose at SOFC operating tempera- observed with increasing Sm2O3 or Gd2O3 ionic conductivities due to the bismuth ox- tures. Reviews on stability and ionic con- content until a maximum is reached. The ide doping compared to gadolinia doping of ductivity of Bi2O3-based electrolytes are oxygen vacancies then begin to form defect ceria. However, it remains unclear how given by Shuk et al. [37] and Sammes et al. clusters with the doped cations (e.g. chemically stable this electrolyte is to re- [65]. Sm’CeVÖ) which will decrease the mobility ducing atmospheres and phase transitions Doped lanthanum gallates (LaGaO3) of the oxygen vacancies [52]. As ceria be- of bismuth oxide [59]. are currently attracting considerable atten- comes reduced under low oxygen partial The highest ionic conductivities at tion as promising electrolytes for inter- pressures at the anode-electrolyte interface 300–700 °C are found in Bi2O3-based elec- mediate temperature SOFC applications. the material exhibits n-type electronic con- trolytes like BIMEVOX (Fig. 8) [60][61]. When the trivalent lanthanum and gallium ductivity [53] especially at higher operation For temperatures as low as 300 °C are doped with divalent cations like Sr and temperatures. Therefore ceria solid solu- BIMEVOX electrolytes show conductivi- Mg forming La1–xSrxGa1–yMgyO3–x/2–y/2 tions are recommended for operation tem- ties as high as YSZ at 800 °C [62]. (LSGM) the ionic conductivity is signifi- peratures below 800 °C where excellent BIMEVOX are bismuth vanadium oxides cantly higher than that of YSZ but still low- SOFC performance can be obtained [54] or Bi4V2O11 where the vanadium is partially er than that of CGO [66]. The stability in combination with YSZ layers blocking substituted to yield Bi2V1–xMexOy solid so- seems to be higher than that of CGO and electronic conduction. lutions [63]. The BIMEVOX family of ma- thus, it seems attractive to use LSGM elec- In contrast to zirconia-based elec- terials exhibits specific properties as elec- trolytes at temperatures of 600–800 °C trolytes, ceria solid solutions exhibit lower trolytes as well as oxygen electrodes. The [67]. However, it is difficult to produce sin- cathode-electrolyte overpotentials [55–57]. high oxide anion diffusion observed at gle phase LSGM since secondary phases Doshi et al. measured a high power output moderate temperature results from the syn- such as La4Ga2O9 and SrLaGa3O7 prevail at 500 °C of a fuel cell with CGO elec- ergy between the highly polarisable ion pair at grain boundaries reducing the conductiv- trolyte, lanthanum cobalt based cathode of the BiIII cation in the vicinity of the V-O ity [68]. Furthermore Weitkamp and co- and a Ni-CGO anode [54]. CGO elec- diffusion slab on the one hand, and the flex- workers report a limited stability of LSGM under reducing and oxidizing conditions followed by the development of n-type con- ductivity at low and p-type at high oxygen partial pressures [69]. Increased power densities and/or re- T [°C] duced operation temperatures can also be achieved with reducing the thickness of the electrolytes and thereby reducing the ohmic losses. In many concepts of flat bipolar cells, the electrolyte thickness is in the or- der of 100 to 300 mm and serves also as the structural load bearing component. When reducing the thickness of the electrolyte to

] the range of mm or even to several hundred -1 nanometres, the anode or the cathode is cm

-1 used as support structure. Good power den- W

[ sities in SOFCs have been obtained with

s thin YSZ electrolytes prepared by colloidal methods by Will et al. [70]. Electrophoret- log ic deposition of fine YSZ particles dis- persed and stabilized in water was used to produce 20 mm thin electrolytes that result- ed in power densities of more than 200 mW/cm2 at reduced operating temperatures of 700 °C. Other methods have been reported con- cerning the development of thin-film processes for SOFC applications such as electrochemical vapour deposition [71], 1000/T [K] plasma spraying [72], physical vapour dep- osition [73] and pyrolysis of dip coated or sprayed metal salt solutions [74][75]. Although some of these physical and Fig. 8. Ionic conductivities of different electrolyte materials [36][37] chemical methods produce dense layers, FUEL CELL RESEARCH IN SWITZERLAND 842 CHIMIA 2004, 58, No. 12 they are less suitable for mass production except spray deposition. Perednis et al. ob- tained more than 600 mW/cm2 at 700 °C with anode supported cells with bi- and tri- layer electrolytes as thin as 300 nm based on ceria as shown in Fig. 9 [76][77]. Bilayer electrolytes can combine advan- tages of two electrolytes. In case of a ce- ria/zirconia based bilayer, ceria is used at the cathode side being in thermodynamic equilibrium with lanthanum strontium iron perovskite avoiding the La7Zr2O7 forma- tion which degrades the cell when zirconia is combined with these cathodes. On the other side, when using zirconia on the an- ode side, the ceria-based electrolyte is pro- tected against reduction and electronic con- Fig. 9. Thin-film SOFC with bi-layer YSZ/CGO electrolyte and power output at 620 and 720 °C [76][77] ductivity is avoided in the electrolyte [77–81]. The different electrolyte materials suitable for SOFCs have been extensively reviewed elsewhere [1][18][19][31][34] [67][68][82–84]. Zirconates, such as Y-doped BaZrO3, The diffusion of the protonic defects offer high proton conductivity with the nec- across the electrolyte material requires a 3.2. Proton Conducting Electrolytes essary thermodynamic stability for fuel cell counter flux of oxygen vacancies in order to Various ceramic materials exhibit pro- applications [94][96]. By doping BaZrO3 maintain charge neutrality. This counterdif- tonic conductivity at moderate temperatures. with 15–20 mol% of yttrium, proton con- fusion represents one of the main advan- By replacing the oxygen ion conductive ductivities were found to be higher than the tages of proton conductors for fuel cells, the electrolyte in a SOFC with a proton conduc- conductivities of the best oxygen ionic con- ambipolar steam permeation [99]. Since the tor, several improvements regarding the fuel ductors [97]. Even for high dopant levels, incorporation of water vapour according to cell performance can be envisaged. The first the proton mobility is not changed, making Eqn. (1) is reversible and independent of re- studies in the field of protonic conductivity Y-doped BaZrO3 a suitable candidate as actions (2) and (3), proton conduction will and its application to SOFCs were conduct- electrolyte material [92]. Appreciable pro- take place due to any steam concentration ed by Iwahara et al. for SrCeO3-based mate- ton conduction in hydrogen containing at- gradient. Typical values of activation ener- rials [85]. The highest proton conductivities mospheres and p-type conductivity for high gies for proton conduction are around 0.5 have been reported for perovskites (ABO3) oxygen partial pressures have also been in- eV [100]. If an external load is applied, hy- such as BaCeO3-based materials [86–90]. vestigated for divalent doped scandates like drogen will be incorporated into the elec- Proton conductivity is achieved by the partial LaSc1–xMgxO3– [98]. Acceptor-doped Sr- trolyte according to Eqn. (2) and steam will substitution of the B site cation with an ac- d TiO3 also showed protonic conductivity be produced on the cathode side according ceptor dopant ion which is charge compen- combined with a high thermodynamic sta- to Eqn. (3) as shown in Fig. 10. Conse- sated by oxygen vacancies. Trivalent bility although the protonic defect forma- quently, the steam partial pressure will in- dopants have been demonstrated to be more tion is less favoured compared to acceptor- crease on the cathode side, so that some of effective than bivalent ones due to their high- doped BaZrO3 [97]. the steam will react according to Eqn. (1) er protonic defect concentration and mobili- In order to form proton defects, water and return back to the anode. Therefore, the ty [91]. Most BaCeO3-based materials dis- vapour is incorporated into the crystal lat- Faradaic current of the cell is independent play protonic conduction at intermediate tice of the proton conductor according to of the steam permeation and only depends temperatures and become oxygen ion con- Eqn. 1: on the concentration and mobility of the ductors at higher temperatures, see e.g. [87]. protonic defects [99]. If the cell is operated The atmosphere can also influence the con- with hydrocarbons, coking cannot take duction mechanism. Typical conductivities (1) place at the anode side as long as the diffu- are between 0.1 to 0.001 S/cm for tempera- sion of water through the electrolyte keeps tures from 1000 to 600 °C [90]. A compari- The positively charged protonic defect up with the adsorption and decomposition son of the proton conductivities for various forms a covalent bond with oxygen of the of the fuel. Furthermore, water vapour is oxides is given elsewhere [92]. BaCeO3- lattice. If the concentration of protonated produced at the cathode side and thus can- based materials possess the highest molar oxygen atoms is sufficiently high, a proton- not dilute the fuel [99]. volume and the deviation from the ideal cu- ic current flows across the electrolyte. The Typically achieved maximum power bic perovskite structure is small [93]. These protons are then supplied on the anode side: outputs of cerate- as well zirconate-based properties are assumed to be necessary pre- cells are around 20 mW/cm2 [92][99]. Fur- requisites for a material to exhibit high pro- ther research on proton conducting materi- tonic conductivity [92]. However, these ma- (2) als is therefore needed to make proton con- terials usually lack sufficient thermodynam- ductor based cells to serious competitors ic stability. Cerates, for example, form After crossing the electrolyte, the de- for fuel cells based on oxygen conductors. carbonates in air [94] as well as in CO2-con- fects are removed by: taining atmospheres [95]. A number of in- vestigations have therefore been conducted 4. Anode in the last years with the aim to combine high proton conductivity with improved thermo- The main functionality of a SOFC an- dynamic stability. (3) ode is to provide electrochemically active FUEL CELL RESEARCH IN SWITZERLAND 843 CHIMIA 2004, 58, No. 12

Therefore, nickel is combined with a ce- CO [121] and methane [122]. Pure CH4 can ramic compound, such as zirconia or ceria, either be directly electrochemically oxi- forming three interconnected frameworks dized with oxygen ions at the anode or it of metal, ceramic and pores. This cermet can, as well as any other hydrocarbon, be becomes a good metallic conductor for internally or externally steam reformed nickel contents above the percolation with water vapour to yield carbon monox- threshold. In the past, research has been ide and hydrogen [105]. In conventional Ni- mainly focused on yttria-stabilized zirconia YSZ anodes the nickel can be used as steam (YSZ) as ceramic material for electrolytes reforming catalyst to form hydrogen at the and in cermets for anodes for its good struc- anode. Water can either originate from an tural stability, good electrical conductivity external source through the humidification at high temperatures and stability under all of the fuel gas to obtain large steam to car- atmospheric conditions. bon ratios or in parts from water produced In a purely ionic conductor like YSZ the by the fuel oxidation reaction. Methane at oxidation of the fuel gas with oxygen ions high steam to carbon ratios can be reformed coming from the cathode side through the without carbon deposits on nickel contain- electrolyte is believed to occur only in the ing anodes but the excellent steam reform- triple phase boundary (tpb), the connecting ing properties of Ni leads to a total conver- points of metal, ceramic and pore. The ce- sion within the first few millimetres of the ramic network not only provides structural fuel inlet resulting in steep thermal gradi- integrity and hinders the trapped nickel par- ents within the cell due to the endothermic ticles from excessive grain growth but also character of the reaction. provides a pathway for oxygen ions, effec- The major problem associated with the tively extending the triple phase boundary use of dry methane or higher hydrocarbons from the flat electrolyte interface into the for the direct oxidation is the formation of anode structure. carbon deposits in the form of filamentous Nickel-YSZ anodes have been thor- carbon, tar, and soot during operation at oughly investigated for the use with hydro- high temperatures. This is due to the high Fig. 10. Schematic drawing of a fuel cell with gen in terms of manufacturing, raw materi- catalytic activity of metallic nickel towards proton-conducting electrolyte als selection and microstructural properties. carbon formation, rapidly clogging the Anodes based on Ni-YSZ cermets have pores and blocking reaction sites on the been steadily improved through ceramic nickel surface [123][124]. Even at low car- processing, e.g. careful selection of raw ma- bon levels the reaction of Ni with carbon reaction sites for the oxidation of the fuel terials [102], adjustment of particle sizes will finally lead to a disintegration of the gas molecules and to transport electrons [103] and grading of nickel content in the anode by a process called metal dusting from the oxidation reaction to connecting structure [104] in the last few years. Some [125]. Takeguchi et al. [126] added small cell components. Many factors determine of these materials optimizations are report- amounts of precious metals to conventional the materials choice for the anode. Anodes ed in [105–108] and some in a more gener- Ni-YSZ cermets to shift the active sites for provide pathways for the fuel to reach the al context [36][68][84][109–111]. Möbius steam reforming from Ni to the noble met- reaction sites and for the reactants to diffuse recently reviewed the history of solid elec- al and observed less carbon deposits with away from the reaction sites: They also re- trolyte fuel cells and especially the anodes Ru and Pt during steam reforming of quire a high electronic conductivity for cur- herein [112]. methane. rent transport and should be chemically One of the most promising new materi- Another problem at the anode associat- compatible to adjacent cell components als for intermediate temperatures is doped ed with the use of natural gas based fuels is such as the electrolyte, current collector, ceria, a mixed ionic electronic conductor, poisoning by adsorption of traces of H2S and structural elements. Specifically when which has found considerable attention as usually present in any natural fuel on the used in anode supported fuel cells, they also electrolyte [44]. As ceria becomes reduced nickel surface [127]. Dilution of the fuel have to be structurally stable over an ade- at the anode side of the fuel cell and there- gas by steam reforming products and oxi- quate lifetime. by an n-type semiconductor it can be as- dized fuel such as carbon dioxide and water In the early development of SOFC, no- sumed that the triple phase boundary is no vapour can result in performance loss at ble metals such as ruthenium, rhodium, pal- longer defined by single connecting points high fuel utilization [128] or even reoxida- ladium, silver, platinum and gold and from of pore, metal and ceramic but is enlarged tion of metallic nickel to nickel oxide near the transition metal group manganese, iron, to the surface of all ceramic grains in the the fuel outlet. cobalt, nickel and copper were considered microstructure. Ni-CGO anodes have been The search for alternative anodes with [101]. Platinum is a good electrocatalyst al- successfully fabricated and excellent per- lower activity for cracking of hydrocarbons though the high vapour pressure of plat- formances have been reported in hydrogen and better stability than pure Nickel has inum sub-oxides prevents its use in SOFC as fuel at intermediate temperatures proceeded in various directions. The cat- operating between 900 and 1000 °C. Gold [113–115]. Additions of doped ceria can alytic activity of nickel itself can be gradu- shows almost no catalytic activity and poor also be used to increase the performance of ally reduced by alloying the metal with oth- adhesion to oxides. From the transition conventional Ni-YSZ composites er elements, e.g. gold [129] or copper metal oxides, nickel proved to be the best [116][117]. [130–132]. choice in terms of catalytic activity and re- One advantage of SOFCs as compared Copper, similar to gold, exhibits almost dox stability. However, the pure metal has a to PEM or MCFC is their potential to be op- no electrochemical activity and the com- strong tendency towards grain growth at el- erated directly on hydrocarbon or alcohol plete replacement of Ni by Cu to form a cer- evated temperatures and a significantly dif- fuels without complex fuel processing [67]. met with ceria leads to an anode with the ferent thermal expansion coefficient than More exotic fuels include CH3OCH3 [118], copper being a purely electronically con- commonly used electrolyte materials. wood gasification gases [119], H2S [120], ducting current collector and the ceramic FUEL CELL RESEARCH IN SWITZERLAND 844 CHIMIA 2004, 58, No. 12 being the actual electrochemically active pyrochlores and tungsten bronzes have a high electrocatalytic activity towards component [133]. [146][154–158] were investigated. Rutile oxygen reduction and a high chemical sta- Pure and doped ceria are known for structures such as Nb2TiO6 show a high bility in an oxidizing environment without their good performance as oxidation cata- electronic conductivity, especially under re- forming highly resistive reaction products lysts or as catalyst supports. CGO ducing atmospheres but have very low ther- with the electrolyte and current collector (Ce0.9Gd0.1O2–d) was found to have almost mal expansion coefficients compared to [179][180]. The material should exhibit no tendency towards carbon formation standard fuel cell materials [159]. Reich et similar thermomechanical properties as the [134][135] but exhibits a rather low al. [160] related the poor electrochemical electrolyte to avoid stresses developing up- catalytic activity for steam reforming and performance of niobates to the slow ionic on heating and cooling [181] and it should cracking of methane at 1000 °C. The diffusion in the material and proposed to have high electrical conductivity. results of Marina et al. [136][137] for use it as a current collector instead of an an- Most reviews on SOFCs deal with state- increased gadolinia dopant levels in ode. Tungsten bronzes showed either poor of-the-art cathode materials such as La1–x Ce0.6Gd0.4O2–d/gold cells are consistent stability under hydrogen, too large thermal SrxMnO3–d (LSM) and La1–xSrxCo1–y with these findings. Zhao and Gorte. [138] expansion coefficient mismatch to the elec- FeyO3-d (LSCF) [1][34][36][67][68][82] examined the catalytic activity of various trolyte or poor electrochemical perform- [84][110][182–186]. A few of these re- doped cerium oxides for the direct n-butane ance [161–163]. views also include emerging materials oxidation and reported that pure CeO2 al- Amongst the more promising candi- [19][68][84][184]. The following will be ways outperforms doped samples and that dates to replace established anodes are lan- limited to cathode material aspects and ex- increasing dopant levels reduce reaction thanum strontium chromite La1–xSrxCrO3 clude most processing related techniques rates. The catalytic oxidation of methane (LSC) perovskites [164]. This class of ma- which can be found elsewhere [187]. has been recently addressed by Horita et al. terial is already used as interconnect in The oxygen reduction reaction at the [139] using the isotope labelling technique SOFC stacks and shows good stability un- interface between a SOFC cathode and an to identify reaction sites on YSZ and yttria- der operating conditions [165]. Vernoux et O2– conducting electrolyte is: doped ceria (YDC) with gold and nickel al. [166] reported stable electrochemical electrodes. The YDC substrate proved to be behaviour of B-site vanadium-doped LSC. (4) efficient in reducing carbon deposits on Ni Sfeir et al. [167][168] investigated the cat- by increasing the oxygen concentration on alytic activity of various A and B site the Ni surface through proton interaction dopants of LaCrO3 and found Sr and Ni to and is schematically represented in between Ni and YDC. be the most suitable substituents for anode Fig. 11. SOFC cathodes are usually p-type Gorte and co-workers [140][141] as purposes, although it is not clear whether semi-conductors [188][189] that can be ei- well as other groups have fabricated and the exsolution of Ni from the structure led ther an electronic or mixed ionic-electron- tested Cu-pure/doped ceria anodes for the to the good performance. Sauvet et al. ic conductor (MIEC). Reduction of the direct oxidation of methane and higher hy- [169][170] tried to improve reforming ac- electrokinetic losses and optimization of drocarbons. However, their spectacular in- tivity by small ruthenium additions to the electrode performance are two major terpretations of the activity of Cu to process La1-xSrxCrO3. Gonzales-Cuenca et al. goals of research and development. In case propane had to be corrected. The power [171] tested lanthanum-based chromite-ti- of pure electronic conductors, the oxygen output of Cu-pure/doped ceria anodes con- tanate perovskites and found insufficient adsorbs on the surface of the material and taining fuel cells was solely due to H2 as fu- electronic conductivity. Interesting results diffuses over its surface towards the tpb el originating from thermal decomposition have also been obtained with lanthanum where it becomes charged and incorporat- of propane to propene occurring at 700 °C strontium titanates [172][173]. Hui and ed in the electrolyte. The electrode acts as also in absence of Cu as recently shown by Petric [174–176] reported the properties of an electron supplier. Accordingly, improv- Jörger [142]. rare-earth-doped SrTiO3 and propose yttri- ing the cathode performance towards high Copper-containing anodes are also be- um doping for further investigations. Slater current density and low overpotentials is lieved to be more tolerant against sulphur et al. [177] reported conductivity data on closely related to the increase of the tpb- than nickel-based electrodes [140]. The A-site deficient Sr1–3x/2LaxTiO3–d. length. steam reforming capabilities of Cu-CGO Based on the experience with lanthanum If the SOFC cathode is a MIEC, oxygen cermets can be further enhanced by the ad- strontium chromites Tao and Irvine [178] in- can be reduced on the surface and diffuse dition of small amounts of noble metals vestigated complex perovskites of the struc- through the bulk of the electrode. Conse- such as Ru [143][144]. ture (La,Sr)2M1–xCr1+xO6–d with transition quently surface and bulk pathways that co- Irvine and co-workers [105][145][146] metals M on the B-sites. Excellent electro- exist in parallel are in competition and the investigated the mixed ionic electronic con- chemical performance comparable to that of fastest one determines the kinetics of the ductor titania-doped YSZ (YTZ) and YTZ Ni-YSZ and material stability in hydrogen overall reaction. If the surface pathway is with yttrium substituted by scandium [147] and dry methane were achieved with high rate-determining, the electrode exhibits a and compared it to ceria. The thermal, me- levels (x = 0.5) of Mn doping. similar behaviour as for a purely electronic chanical and electrical properties of YTZ in The requirements for an efficient fuel conductor, as described previously. On the a fuel cell environment seem to be electrode are many and some of the new other hand, if the oxygen migrates mainly favourable [148]. The pure form [149] as materials show very promising properties through the bulk of the cathode, the electro- well as Ni [150] and Cu [151] cermets per- for the development of next generation an- chemical reaction is promoted by produc- formed well in hydrogen. YTZ was found odes that will enable the use of available fu- ing dense thin layers, enhancing thereby not to promote methane cracking [152] but els and operate at lower temperatures than oxygen exchange at both the MIEC/gas and was catalytically less active than ceria and existing ones. MIEC/electrolyte interfaces. These materi- showed only limited electronic conductivi- als should have a high oxygen exchange ca- ty. pacity for an easy incorporation of oxygen Efforts have been made to replace the 5. Cathode in their lattice and high oxygen diffusivity traditional cermet anode by a pure ceramic for high transport rates. However, the material [153] for the direct utilization of For proper function as a cathode in a mechanism and kinetics of oxygen reduc- natural gas as fuel. Perovskites, fluorites, solid oxide fuel cell, the material should tion at SOFC cathodes are still under ques- FUEL CELL RESEARCH IN SWITZERLAND 845 CHIMIA 2004, 58, No. 12

creased by using Pr0.6–xSr0.4MnO3 [218][219], but for substitution of Mn with Co in Y0.6Sr0.4Mn1–yCoyO3 (0 £ y £ 0.4) mixtures, increasing y resulted in lower conductivity [220], the same is observed for adding Al to LSM [221]. The La1–xSrxCoO3–d (LSC) based cath- odes [222–224] are typical mixed conduc- tors offering the advantage of higher elec- tronic and, more important, higher ionic conductivity (see Table 3). By providing this second pathway for oxygen ions, activ- ity of the cathode is increased and lower op- erating temperatures are feasible. The dis- advantage is that those materials react with YSZ [202][224], thus either ceria-based electrolytes or protective layers of ceria [224] or LSGM [223][225][226] on YSZ electrolytes should be used. In order to adjust the TEC of LSC-based cathodes to the one of CGO, Fe was introduced to ob- tain lower TEC [227]. Depending on the composition, the conductivities of La1–x SrxCoyFe1–yO3–d can vary about one order of magnitude [181][188][227][228]. One Fig. 11. Schematic representation of oxygen reduction in a mixed ionic-electronic conductor: Sur- strategy to improve performance of LSCF face and bulk reaction pathways are parallel and in competition. On the surface pathway, charge cathodes is the fabrication of composite transfer occurs at the triple phase boundary. electrodes with CGO [54][229], CGO/Ag [55] or SDC [230] or to obtain higher sur- tion. The interaction between oxygen and graded compositions [201]. Besides YSZ, face exchange coefficient k by impregnat- the MIEC and oxygen diffusion have been CGO [208], Sm0.2Ce0.8O2 (SDC) [209] and ing LSCF with Pd [231]. subject to numerous studies [190–192]. Ce0.7Bi0.3O2 [210] are also used for fabri- Cathode performance can also be im- Comprehensive understanding and model- cation of composite cathodes with LSM proved by substituting one or more of the ling of these reaction mechanisms consti- with improved performance. elements in Ln1–xSrxCoyFe1–yO3–d. En- tute an ongoing field of investigations, from As for most perovskite materials, the hanced performance at low temperatures which controversial results were published properties of LSM can be tailored by (~600 °C) is obtained for Ln = Ce, Dy so far [193–197]. partially substituting the A and B sites of [232], whereas TEC is lowered for Ln = Nd the ABO3 perovskite. The thermal expan- [233]. Reaction products with YSZ are less 5.1. La1–xSrxMnO3–x/2 (LSM) and sion coefficient (TEC) can be further pronounced for Ln = Pr, Nd, Gd [57]. On LaxSr1–xCoyFe1–yO3 (LSCF) adjusted to that of the YSZ electrolyte by CGO, no reaction products are found for Ln Cathodes using (La1–xYx)0.7Sr0.3MnO3 [211] or = La, Gd, Sm, Nd [234][235], although no The choice of cathode materials is Sr1–xCexMnO3–d [212]. Compositions distinct reaction products with LSGM are rather limited: Noble metals such as Pt are which are compatible with CGO as regards found, codiffusion into the electrolyte is de- suitable, but exhibit prohibitive costs for TEC and chemical stability are Gd1–xSrx tected [236]. Sr-doped lanthanum ferrites SOFC application at higher temperatures MnO3,Nd1–xSrxMnO3–d [213] and Pr1-x have also been investigated, since they have due to high Pt suboxide vapour pressure. SrxMnO3 [214]. The formation of reaction a lower TEC than LSCF [237], but they al- La1–xSrxMnO3–x/2 (LSM), as the state-of- products between the YSZ electrolyte and so form Sr- or La-zirconates with YSZ the-art electronic conducting material, is the cathode can be suppressed for Ln1–x [238], which can be reduced by adding Al widely used since it fulfills most of the re- SrxMnO3 (Ln = Pr, Nd) [215] and to LaFe1–xAlxO3 systems without Sr doping quirements listed above; its properties are Pr1–xCaxMnO3 [216], whereas for [239] or using Ce0.8Sm0.2O1.9 protection given in Table 3 with the data taken from La1–xCaxMnO3 on a CaO-stabilized ZrO2 layers [238]. The conductivity is compara- references [181][198–200]. Usually LSM electrolyte no stable composition was ble to that of LSCF, and is enhanced by is used for the cathode when YSZ is used as found [217]. The conductivity can be in- adding Ni [240][241], or replacing Sr with the electrolyte, because the thermal expan- sion coefficients match well [201]. Howev- er, the rather high operating temperatures of Table 3. Coefficient of thermal expansion (TEC) (30–1000 °C), electronic (se) and ionic (si) conduc- the SOFC around 900 to 1000 °C promote tivity and bulk diffusion D as well as surface exchange coefficient k at 800 °C for some SOFC cathode degradation of the cathode and the forma- materials tion of undesired resistive reaction prod- Material TEC/10–6K–1 s /[S/cm] s /[S/cm] D/[cm2/s] k/[cm/s] ucts, such as La2Zr2O7, especially during e i manufacturing of LSM on YSZ La Sr MnO 12.3 [181] 102 [181] 1.7·10–4 4·10–14 5·10–8 [180][202–205]. 0.65 0.35 3-d Increased triple phase boundary length, (YSZ: 11.0·[198]) [181] [198] [198] (at 900 °C) (at 900 °C) better adhesion to the electrolyte and lower –3 –8 –6 thermal expansion mismatch is achieved La0.6Sr0.4Co0.2Fe0.8O3 17.5 [181] 302 [181] 8·10 2.5·10 5.6·10 when using a LSM-YSZ composite materi- (CGO: 10.5 [199]) [181] [200] [200] al [204][206][207] or even composites with FUEL CELL RESEARCH IN SWITZERLAND 846 CHIMIA 2004, 58, No. 12

Ni [242], but is decreased by adding Al high conductivity and gives reasonably low parameter ZB/rB is a measure for the [243][244]. overpotential [252]. La2Ni1–xCuxO4+d on Coulomb potential of the outermost d-elec- Another material that is investigated for the other hand shows high diffusion and trons from the centre of the B ion. In this cathodes is Sm1–xSrxCoO3 (SSC) surface exchange coefficients, but rather potential map, we find two well-defined re- [202][245][246], showing lower overpoten- low conductivity, comparable to LSM gions. The region of compounds with local- tial than LSC [246]. Fabricating composites [253]. Composite cathodes of Ag and yttri- ized electrons and that of itinerant elec- with the electrolyte material (Ce0.8 um doped bismuth oxide show comparable trons, both separated by the line in the Sm0.2O1.9), the interfacial resistances are performance to LSCF [54]. For graph [179][256]. The most interesting reduced [247]. SSC is also used for single Y1Ba2Cu3O7 an additional layer of Pt or Ag compounds and corresponding solid solu- chamber SOFC applications [14][248]. is needed to promote oxygen adsorption tions are located with their potentials di- Barium cobaltates Ba1–xLnxCoO3, Ln = [254]. Nd2NiO4+d cathodes show lower po- rectly on, or close to the dividing line be- La, Pr are studied on either BaCeO3 larization resistance than LSM but long- tween these two regions. Along this line, we [245][249] or LSGM [250] based elec- term stability tests have not been performed will find new catalysts as well as materials trolytes and found to have less polarization [255]. with interesting electrical properties such losses than SSC for Ln = Pr [245], but high- In the search for new cathode materials as high mixed electronic/ionic conductivity. er overpotentials than SSC for Ln = La based on perovskites a structural field map [250]. of perovskites, containing transition metal cations may be useful. In Fig. 12, the 6. Summary and Conclusions 5.2. New Cathode Materials Coulomb potentials from the A and B Pyrochlore ruthenates have been inves- cations in perovskites ABO3 are plotted as One of the main problems of SOFCs is tigated with compositions of Bi2Ru2O7.3, ZA/rA and ZB/rB. Thereby ZA and ZB are the the high operating temperature leading to a Pb2Ru2O6.5 and Y2Ru2O7. Only the latter formal valence of the A and B cations and fast degradation rate of cell performance was found to be stable on CGO electrolytes, rA respectively rB are their ionic radii. The and the need for more expensive intercon- but additional doping with SrO is necessary smaller the value of ZA/rA along the ordi- nect and sealing materials. The electrolyte in order to reach reasonable conductivity nate is, the more itinerant the d-electrons of resistance mainly determines the operating [251]. the perovskite become. Similarly the small- temperature of the cell. Two ways are pos- The search for new cathode materials er the value ZB/rB along the abscissa gets, sible to decrease the latter, either by de- for intermediate temperatures led to the dis- the more itinerant the d-electrons of the per- creasing the electrolyte thickness or by us- covery of La1–xSrxCuO2.5–d. This material ovskite are. The physical meaning of the ing alternative electrolyte materials with is a possible cathode candidate because it parameter ZA/rA is a measure for the per- higher ionic conductivity. One of the mate- shows no reaction with YSZ, it exhibits turbation of the covalent B–0 bond and the rials that have been proposed for low tem-

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