This article provides a perspective on solid oxide fuel cells operating at low temperature, defined here to be the range from ∼400 °C to 650 °C. These low-temperature solid oxide fuel cells ...(LT-SOFCs) have seen considerable research and development and are widely viewed as the "next generation" technology, following the 650-850 °C SOFCs that are currently undergoing commercialization. LT-SOFCs have potential advantages for conventional SOFC applications such as stationary power generation, and may be viable for new portable and transportation power applications, along with electrolytic fuel production and energy storage. The characteristics of electrolyte and electrode materials are reviewed, with a focus on materials that have demonstrated good properties and cell performance at low temperature. Only oxygen-ion-conducting electrolytes are considered here. Anode materials are discussed, primarily the various Ni-cermet anode compositions that yield good low-temperature performance. Mixed ionically and electronically conducting cathode materials are described in detail, reflecting the extensive research activity that has aimed at providing useful oxygen reduction kinetics at low operating temperature. Cell design, materials compatibility, processing methods, and resulting microstructures are discussed, along with their role in determining cell performance. Results from state of the art LT-SOFCs are presented, and future prospects are discussed.
This article provides a perspective review of low-temperature solid oxide fuel cells research and development.
An Octane-Fueled Solid Oxide Fuel Cell Zhan, Zhongliang; Barnett, Scott A.
Science (American Association for the Advancement of Science),
05/2005, Letnik:
308, Številka:
5723
Journal Article
Recenzirano
There are substantial barriers to the introduction of hydrogen fuel cells for transportation, including the high cost of fuel-cell systems, the current lack of a hydrogen infrastructure, and the ...relatively low fuel efficiency when using hydrogen produced from hydrocarbons. Here, we describe a solid oxide fuel cell that combines a catalyst layer with a conventional anode, allowing internal reforming of iso-octane without coking and yielding stable power densities of 0.3 to 0.6 watts per square centimeter. This approach is potentially the basis of a simple low-cost system that can provide substantially higher fuel efficiency by using excess fuel-cell heat for the endothermic reforming reaction.
Increasing the power density and reducing the operating temperature of solid oxide fuel cells (SOFCs) is important for improving commercial viability. Here we discuss two strategies for achieving ...such improvements in Ni–YSZ supported SOFCs – electrolyte thickness reduction and cathode infiltration. Microstructural and electrochemical results are presented showing the effect of reducing YSZ/GDC electrolyte thickness from 8 to 2.5 μm, and the effect of PrO x infiltration into the LSCF–GDC cathode. Both of these measures are effective, particularly at lower temperatures, leading to an increase in the maximum power density at 650 °C from 0.4 to 0.95 W cm −2 , for example. Electrochemical impedance spectroscopy utilizing subtractive analysis shows that PrO x enhances the cathode charge transfer process. Reducing the electrolyte thickness reduces not only the cell ohmic resistance but also the electrode polarization resistance. The latter effect appears to be an artifact associated with a slight increase in the steam partial pressure at the anode due to minor gas leakage across the thinner electrolyte.
In the context of materials design and high-throughput computational searches for new thermoelectric materials, the need to compute electron and phonon transport properties renders direct assessment ...of the thermoelectric figure of merit (zT) for large numbers of compounds untenable. Here we develop a semi-empirical approach rooted in first-principles calculations that allows relatively simple computational assessment of the intrinsic bulk material properties which govern zT. These include carrier mobility, effective mass, and lattice thermal conductivity, which combine to form a semi-empirical metric (descriptor) termed beta sub(SE). We assess the predictive power of beta sub(SE) against a range of known thermoelectric materials, as well as demonstrate its use in high-throughput screening for promising candidate materials.
Solid oxide electrolysis cells (SOECs) convert renewable electricity to fuels with efficiency substantially higher than other electrolysis technologies. However, questions remain regarding ...degradation mechanisms that limit SOEC long-term stability. One of the key degradation mechanisms is oxygen electrode delamination; although prior studies have improved the understanding of this mechanism, it is still difficult to predict how degradation depends on SOEC materials and operating conditions,
i.e.
, temperature, voltage, and current density. Here we present a study aimed at developing a quantitative understanding of oxygen electrode delamination. Experimentally, a life test study of symmetric and full cells with yttria-stabilized zirconia (YSZ) electrolytes and Gd-doped ceria (GDC) barrier layers was done with three different perovskite oxygen electrode materials. Fracture was observed at the perovskite-GDC interface above a critical current density and below a critical operating temperature. A theory is presented that combines a calculation of the effective oxygen pressure across the electrolyte with an estimation of the pressure required for fracture. Fracture is correctly predicted for a critical oxygen partial pressure of ∼7200 atm and an associated electrode overpotential of ∼0.2 V, occurring at the electrode/GDC interface because of the relatively low perovskite fracture toughness. Damage at the GDC/YSZ interface was also observed in some cases and explained by a peak in the oxygen pressure at this interface.
The fracture at the electrode/electrolyte interface occurs for a critical oxygen pressure and an associated electrode overpotential.
The exsolution of metal cations from oxides under reducing fuel conditions results in the formation of surface metallic nanoparticles, which can reduce Solid Oxide Fuel Cell anode polarization ...resistance. However, the loss of the B-site cations shifts the stoichiometry of the perovskite oxide. Depending on the amount exsolved and the initial stoichiometry, the exsolution can presumably shift the oxide away from its single-phase perovskite region. Herein, the direct comparison of initially stoichiometric composition Sr(Ti0.3Fe0.63Ni0.07)O3-δ (STFN0) with initially A-site deficient Sr0.95(Ti0.3Fe0.63Ni0.07)O3-δ (STFN5) is conducted and reported. X-ray diffraction along with scanning and transmission electron microscopy analysis of the oxides, which are both reduced at 850 °C in H2/H2O/Ar, shows a similar size and density of exsolved Fe–Ni alloy nanoparticles, albeit with slightly different alloy compositions. Whereas the oxide phase in reduced STFN5 shows a well-ordered perovskite structure, the greater B-site deficiency in reduced STFN0 results in a highly disordered and strained structure. The electrochemical performance of STFN0 anodes is inferior to that of STFN5 anodes, and even worse than SrTi0.3Fe0.7O3-δ (Ni-free) anodes. It appears that an initial Sr deficiency is important to avoid a too-high B-site deficiency after exsolution, which distorts the perovskite structure and impairs electrochemical processes.
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•Direct comparison of stoichiometric and A-site deficient exsolved perovskite anodes.•Sr deficiency has minimal effect on exsolved nanoparticles size and distribution.•Sr deficiency has major impact on electrochemical properties.•A-site deficiency limits oxide structural damage caused by exsolution.
This paper addresses the use of Ce0.8Gd0.2O2−δ (GDC) infiltration into the Ni–(Y2O3)0.08(ZrO2)0.92 (YSZ) fuel electrode of solid oxide cells (SOCs) for improving their electrochemical performance in ...fuel cell and electrolysis operation. Although doped ceria infiltration into Ni–YSZ has recently been shown to improve the electrode performance and stability, the mechanisms defining how GDC impacts electrochemical characteristics are not fully delineated. Furthermore, the electrochemical characteristics have not yet been determined over the full range of conditions normally encountered in fuel cell and electrolysis operation. Here we present a study of both symmetric and full cells aimed at understanding the electrochemical mechanisms of GDC-modified Ni–YSZ over a wide range of fuel compositions and temperatures. Single-step GDC infiltration at an appropriate loading substantially reduced the polarization resistance of Ni–YSZ electrodes in electrolyte-supported cells, as measured using electrochemical impedance spectroscopy (EIS) at various temperatures (600–800 °C) in a range of H2O–H2 mixtures (3–90 vol% H2O). Fuel-electrode-supported cells had significant concentration polarization due to the thick Ni–YSZ supports. A distribution of relaxation times approach is used to develop a physically-based electrochemical model; the results show that GDC reduces the reaction resistance associated with three-phase boundaries, but also appears to improve oxygen transport in the electrode. Increasing the H2O fraction in the H2–H2O fuel mixture reduced both the three-phase boundary resistance and the gas diffusion resistance for Ni–YSZ; with GDC infiltration, the electrode resistance showed less variation with fuel composition. GDC infiltration improved the performance of fuel-electrode-supported full cells, which yielded a maximum power density of 2.28 W cm−2 in fuel cell mode and an electrolysis current density at 1.3 V of 2.22 A cm−2, both at 800 °C.
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▶ 3D tomography showed the high temperature evolution of Ni-YSZ anodes. ▶ Ni particle size changed little, constrained by the high vol% YSZ matrix. ▶ Pore percolation decreased and ...interfacial area decreased. ▶ The pore evolution substantially decreased the electrochemically active TPB. ▶ The loss of active TPBs explained observed anode polarization resistance increase.
Temperature induced degradation in Solid Oxide Fuel Cell (SOFC) Ni-YSZ anodes was studied using both impedance spectroscopy and three-dimensional tomography via Focused Ion Beam–Scanning Electron Microscopy. A 100h anneal at 1100°C caused a 90% increase in cell polarization resistance, which correlated with the observed factor of ∼2 reduction in the electrochemically active three-phase boundary (TPB) density. The TPB decrease was caused by a significant decrease in pore percolation, and a reduction in pore interfacial area due to pores becoming larger and more equiaxed. The anneal caused no measurable change in average Ni particle size; Ni coarsening was apparently highly constrained in these anodes due to the relatively large YSZ volume fraction and low pore volume.
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