Introduction to solid oxide fuel cells (SOFCs)

Global energy demands have dramatically increased in the last few decades, especially in emerging markets: since 1990, energy consumption in the Middle East has grown by 170%, in China by 146%, in India by 91%, and in Africa and Latin America by 70%.1 The vast majority of this energy continues to be derived from conventional, non-renewable, carbon- based fuels. Despite the promise of renewables such as wind power, hydropower, and solar energy, incompatibilities between these technologies and existing infrastructure have hampered widespread implementation.2,3 Moreover, current fuel-based systems, because they primarily rely on internal combustion engines (ICEs), cannot exceed rather poor theoretical fuel efficiencies of 30–40%. Burning fuels also produces environmental pollutants and greenhouse gases that drive global warming, such as CO, CO2, SOx, and NOx species.4,5

combustion-driven power stations, hinders access to electricity for those in rural and remote areas of the planet.6

Recent developments in fuel cell technology are of great interest in addressing these problems. The past few decades have witnessed significant improvements in polymer electrolyte membrane fuel cells (PEMFCs),7,8 molten carbonate fuel cells (MCFCs),9–12 and solid oxide fuel cells (SOFCs).13–15 SOFCs stand out for their excellent fuel versatility and energy efficiency, but unfortunately require operation at high temperatures (800–1000 °C).16 Like all fuel cells, SOFCs produce electrical energy directly from chemical energy, i.e., without intermediate conversion to thermal energy via combustion, and are therefore not limited by theoretical Carnot cycle efficiencies. Proven fuel sources for SOFCs include pure H2, hydrocarbons, renewable biofuels, and syn-gas (mixtures of H2 and CO).17,18In the mid-

range (100 kW to 10 MW) power class, SOFCs are among the most efficient fuel-powered devices known, and demonstrate up to 70% fuel efficiency in combined heating and power applications (Figure 1.1). Stationary SOFC stacks developed by Siemens Westinghouse have undergone continuous operation for specialized on-site power generation with lifetimes of over 35,000 hours (~4 years).19

Figure 1.1: Comparison of conversion efficiencies for various fuel cell technologies and combustion engines, as a function of typical generated power; figure reproduced from Hayashi et al.20

A typical SOFC comprises two electrodes (the anode and cathode) on either side of the electrolyte (Figure 1.2). Interconnects (typically either metallic or based on rare earth chromites) are used between cells to build larger SOFC stacks. Various geometries are possible, e.g., anode- vs. electrolyte-supported, or planar vs. tubular, with the depicted electrolyte and electrode layers commonly between 10 and 500 μm in thickness, so that each cell is usually no more than 1 to 2 mm thick.21 The constituent materials are ceramics or mixed ceramic–metallic composites (cermets) which allow ionic and/or electronic conductivity at elevated temperatures, and possess mutually compatible thermal expansion coefficients.

Figure 1.2: (a) Simplified cross-sectional schematic of an operational SOFC, highlighting (b) the performance- limiting oxygen reduction reaction at the triple phase boundary (TPB). While the TPB is shown schematically at the edge, for a porous electrode the actual TPB will comprise all regions of the cathode–electrolyte interface with access to air.

During operation at elevated temperatures, O2 from the ambient air is catalytically reduced to

oxide (O2-)anions at the porous cathode; this is typically considered to occur at the triple- phase boundary (TPB) regions where cathode, electrolyte, and gas meet. These then propagate through the dense oxide-ion conducting electrolyte to the interface with the porous anode, reacting with incoming fuel (e.g., H2 or hydrocarbons) to produce gaseous waste

(H2O, CO2) and electrons to drive the external electric circuit. The interconnect material

connects single cells in series or in parallel to form high-voltage, high-power SOFC stacks, and plays no role in electrochemical conversion. The electrolyte material is required to be an excellent oxide-ion conductor but also an electrical insulator, as transport of electrons exclusively via the external circuit is required to minimize polarization losses. Similarly, the electrodes must be optimized for high electrical conductivity and catalytic activity. Finally, all the materials must be stable at high operational temperatures in either highly oxidizing or reducing atmospheres at the cathode and anode, respectively, and exhibit mechanical and chemical compatibility.22

These extreme operating conditions constrain the choices of materials for the anode, cathode, and electrolyte. Operation at 800–1000 °C is particularly problematic but is necessitated by (1) poor oxide-ion conductivity in prospective electrolytes and (2) significantly reduced oxygen reduction reaction (ORR) activity at the cathode at lower temperatures. Typical selections for the electrolyte, anode, and cathode are 8 mol% yttria-stabilized zirconia (YSZ, i.e., (Y2O3)0.08(ZrO2)0.92), a Ni-YSZ cermet, and an electrocatalytic perovskite such as

La1-xSrxMnO3 (lanthanum strontium manganite, LSM), respectively.23 While high-temperature

operation does have benefits, such as facile reforming of diverse fuels and recovery of waste heat via combined heat and power (CHP) systems,19 these are more than negated by long start-up times, accelerated degradation, and delamination of the electrodes on repeated thermal cycling. The majority of recent research has therefore focused on improving performance at lower temperatures (500–800 °C),22,24,25 with devices operating in this range referred to as intermediate-temperature SOFCs (IT-SOFCs). Notably, at intermediate temperatures, less expensive ferritic steel interconnects can also be used, lowering overall manufacturing costs.26

Approaches to reduce the operational temperature of SOFCs have been advanced from both the fields of materials chemistry and engineering, but here only the former is considered. Novel electrolyte materials with impressive oxide-ion conductivity at lower temperatures have been discovered and optimized. Ceria-based electrolytes, just like YSZ, adopt the fluorite-type structure, and doped ceria and YSZ have similar maximum ionic conductivities (~0.1 S cm-1) but at different temperatures: Gd-doped ceria or Sm-doped ceria at ~800 °C,

and YSZ at ~1000 °C.27 A related class of purely oxide-ion conducting materials is the doped δ-Bi2O3 phases which adopt a pseudo-cubic, long-range ordered fluorite-type structure, and

are described in further detail in Section 1.3. The discovery of oxide-ion conductors of other structure types, with aliovalent doping commonly employed to enhance conductivity, has also led to significant progress in the field. Ishihara et al., concurrently with Goodenough, made the critical discovery of electrolytes based on LaGaO3 with a perovskite rather than fluorite

structure.28,29 As perovskites (ABO3) tolerate a wide variety of dopants on both the A and B

sites, a broad compositional space is available for optimization of these materials.

Even with the use of optimized electrolyte materials, as the SOFC operating temperature is reduced, the cathode performance will suffer due to poor reaction kinetics and reduced gas transport, severely limiting the overall performance. The ORR at the cathode (Figure 1.2b) depends on gas transport and electronic and ionic conductivities, and is therefore confined to the gas–cathode–electrolyte TPB. Cathode and thus overall SOFC performance has been shown to directly correlate with the TPB length.30–33 However, by using a compatible mixed ionic–electronic conductor (MIEC) as the cathode, the reaction kinetics are enhanced by increasing the dimensionality of the reaction boundary. In addition to providing electronic conductivity, MIEC cathodes allow transport of oxide ions so that oxygen reduction can occur along the entire cathode surface, and the larger area of the MIEC–gas two-phase boundary determines SOFC performance. Moreover, conventional cathode materials such as LSM, which are very poor oxide-ion conductors, must remain porous at elevated temperature to ensure an active TPB.26 A highly stable porous cathode morphology is less critical for MIEC cathodes, simplifying processing constraints.

Huang and Goodenough showed that the use of MIEC cathodes in SOFCs operating at 800 °C yields impressive current densities at modest overpotentials, especially compared to the non-MIEC material LSM.26 In that study, the MIECs chosen were transition metal perovskite oxides (e.g., La0.6Sr0.4CoO3-δ and SrCo0.8Fe0.2O3-δ) with high concentrations of oxide

vacancies to promote high oxide-ion conductivity. Aspects of the oxygen reduction reaction kinetics and the oxide-ion conductivity in perovskite-based MIECs remain controversial, in particular the effect of cation segregation at the MIEC surface, and the mechanisms of oxygen vacancy formation, trapping, and movement.34–38 As detailed in the next section, a

thorough understanding of the fundamental electronic and ionic conduction mechanisms in MIECs is important to design the next generation of SOFC cathode materials.