Changrong Xia

Reduced-Temperature Solid Oxide Fuel Cells Fabricated by Screen Printing

Electrolyte films of samaria-doped ceria ~SDC, Sm0.2Ce0.8O1.9) are fabricated onto porous NiO-SDC substrates by a screen printing technique. A cathode layer, consisting of Sm0.5Sr0.5CoO3 and 10 wt % SDC, is subsequently screen printed on the electrolyte to form a single cell, which is tested at temperatures from 400 to 600°C. When humidified ~3% H2O) hydrogen or methane is used as fuel and stationary air as oxidant, the maximum power densities are 188 ~or 78! and 397 ~or 304! mW/cm at 500 and 600°C, respectively. Impedance analysis indicates that the performances of the solid oxide fuel cells ~SOFCs! below 550°C are determined primarily by the interfacial resistance, implying that the development of catalytically active electrode materials is critical to the successful development of high-performance SOFCs to be operated at temperatures below 600°C.

Enhancement in Three-Phase Boundary of SOFC Electrodes by an Ion Impregnation Method: A Modeling Comparison

Solid oxide fuel cells SOFCs are promising to be the nextgeneration energy-conversion devices due to their high efficiency and ultralow pollution emission. 1 Many efforts have been made recently to lower their operating temperatures from conventional 1000°C to 600–800°C, in order to significantly reduce the manufacturing cost and improve the stability of the SOFC system. At a reduced operating temperature, the electrode performance becomes the most important determinant of the overall cell output, especially when a thin-film electrolyte i.e., 5–20 m is applied. The electrode performance is believed to be determined by the sum of various polarizations typically associated with the length of the socalled three-phase boundary TPB where the electronic conductor, ionic conductor, and gases are in contact with each other so that the electrochemical reaction can take place. Therefore, a large TPB length is generally essential for high electrode performance. Several experimental studies demonstrated the inverse proportionality of activation overpotential with respect to the TPB length in standard composite electrodes such as Ni–yttria-stabilized zirconia YSZ anodes 2,3 and La,SrMnO3 LSM–YSZ cathodes. 4,5 A composite electrode is usually composed of an electronic conducting phase, an oxygen-ion conducting phase, and pores for gas transportation. Typical examples of the electronic phases are Ni and LSM, which also serve as the electrocatalyst in the anode and cathode, respectively. The ionic phase is generally an electrolyte material such as YSZ and doped ceria DCO. The TPB length of such a composite electrode is dominantly affected by its microstructure characteristics including particle size, porosity, and distribution state of the electronic and ionic conducting phases. Various electrode models have been established to predict and improve the performance of the composite electrode with regards to its microstructure parameters.

Sr2Fe1.5Mo0.5O6 as Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells with La0.8Sr0.2Ga0.87Mg0.13O3 Electrolyte

The performance of Sr 2 Fe 15 Mo 0.5 O 6 (SFMO) as a cathode material has been investigated in this study. The oxygen ionic conductivity of SFMO reaches 0.13 S cm -1 at 800°C in air. The chemical diffusion coefficient (D chem ) and surface exchange constant (k ex ) of SFMO at 750°C are 5.0 x 10 -6 cm 2 s -1 and 2.8 x 10 -5 cm s -1 , respectively, suggesting that SFMO may have good electrochemical activity for oxygen reduction. SFMO shows a thermal expansion coefficient (TEC) of 14.5 x 10 -6 K -1 the temperature range of 200-760°C in air. The polarization resistance of the SFMO cathode is 0.076 Ω cm 2 at 800°C in air under open-circuit conditions measured on symmetrical cells with La 0.8 Sr 0.2 Ga 0.87 Mg 0.13 O 3 (LSGM) electrolytes. Dependence of SFMO cathode polarization resistance on the oxygen partial pressure and the cathode overpotentials at different temperatures are also studied. SFMO shows an exchange current density of 0.186 A cm -2 at 800°C in air. Single cells with the configuration of Ni-La 0.4 Ce 0.6 O 2 (LCO)ILCOILSGMISFMO show peak power densities of 349, 468, and 613 mW cm -2 at 750, 800, and 850°C, respectively using H 2 as the fuel and ambient air as the oxidant. These results indicate that SFMO is a promising cathode candidate for intermediate-temperature solid oxide fuel cells with LSGM electrolyte.

Direct synthesis of methane from CO2–H2O co-electrolysis in tubular solid oxide electrolysis cells

Directly converting CO2 to hydrocarbons offers a potential route for carbon-neutral energy technologies. Here we report a novel design, integrating the high-temperature CO2–H2O co-electrolysis and low-temperature Fischer–Tropsch synthesis in a single tubular unit, for the direct synthesis of methane from CO2 with a substantial yield of 11.84%.

An Oxide Ion and Proton Co-Ion Conducting Sn0.9In0.1P2O7 Electrolyte for Intermediate-Temperature Fuel Cells

The ionic conductivity of Sn 009 In 0.1 P 2 O 7 ceramic was investigated under various atmospheres within the temperature range of 130-230°C. Similar to mixed-conductive perovskite oxides at high temperatures (such as SrCe 0.95 Yb 0.05 O 3-α, La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3-α at 600-1000°C), Sn 0.9 In 0.1 P 2 O 7 can conduct both protons and oxide ions at low temperatures (130-230°C). The conductivity of Sn 0.9 In 0.1 P 2 O 7 reaches 0.019 S/cm at 200°C in wet nitrogen. Its transport numbers determined by steam concentration cells are around 0.76 for a proton and 0.12 for an oxide ion. The performance of direct methanol fuel cells at 170°C using mixed-ion conductive Sn 0.9 In 0.1 P 2 O 7 electrolyte is higher than that at 235°C using pure proton conductive CsH 2 PO 4 electrolyte. This is attributed to direct oxidation of CO at the anode by the oxide ions generated at the cathode and moved through the Sn 0.9 In0.1P 2 O 7 electrolyte.

Perovskite Sr1–xCexCoO3−δ (0.05 ≤ x ≤ 0.15) as Superior Cathodes for Intermediate Temperature Solid Oxide Fuel Cells

Perovskite Sr(1-x)Ce(x)CoO(3-δ) (0.05 ≤ x ≤ 0.15) have been prepared by a sol-gel method and studied as cathodes for intermediate temperature solid oxide fuel cells. As SOFC cathodes, Sr(1-x)Ce(x)CoO(3-δ) materials have sufficiently high electronic conductivities and excellent chemical compatibility with SDC electrolyte. The peak power density of cells with Sr(0.95)Ce(0.05)CoO(3-δ) is 0.625 W cm(-2) at 700 °C. By forming a composite cathode with an oxygen ion conductor SDC, the peak power density of the cell with Sr(0.95)Ce(0.05)CoO(3-δ)-30 wt %SDC composite cathode, reaches 1.01 W cm(-2) at 700 °C, better than that of Sm(0.5)Sr(0.5)CoO(3)-based cathode. All these results demonstrates that Sr(1-x)Ce(x)CoO(3-δ) (0.05 ≤ x ≤ 0.15)-based materials are promising cathodes for an IT-SOFC.

Transparent ion-conducting ceria-zirconia films made by sol-gel technology

Abstract Films of Ce–Zr oxide were made by sol–gel deposition according to three different routes. Cyclic voltammetry showed that Li+ intercalation/deintercalation took place if the Ce content was sufficient. The films had a high transmittance for visible light. They are of interest as counter electrodes in electrochromic smart windows.

Bismuth doped lanthanum ferrite perovskites as novel cathodes for intermediate-temperature solid oxide fuel cells.

Bismuth is doped to lanthanum strontium ferrite to produce ferrite-based perovskites with a composition of La(0.8-x)Bi(x)Sr0.2FeO(3-δ) (0 ≤ x ≤ 0.8) as novel cathode material for intermediate-temperature solid oxide fuel cells. The perovskite properties including oxygen nonstoichiometry coefficient (δ), average valence of Fe, sinterability, thermal expansion coefficient, electrical conductivity (σ), oxygen chemical surface exchange coefficient (K(chem)), and chemical diffusion coefficient (D(chem)) are explored as a function of bismuth content. While σ decreases with x due to the reduced Fe(4+) content, D(chem) and K(chem) increase since the oxygen vacancy concentration is increased by Bi doping. Consequently, the electrochemical performance is substantially improved and the interfacial polarization resistance is reduced from 1.0 to 0.10 Ω cm(2) at 700 °C with Bi doping. The perovskite with x = 0.4 is suggested as the most promising composition as solid oxide fuel cell cathode material since it has demonstrated high electrical conductivity and low interfacial polarization resistance.

A nanostructured ceramic fuel electrode for efficient CO2/H2O electrolysis without safe gas

There is increasing interest in converting CO2/H2O to syngas via solid oxide electrolysis cells (SOECs) driven by renewable and nuclear energies. The electrolysis reaction is usually conducted through Ni–YSZ (yttria stabilized zirconia) cermets, state-of-the-art fuel electrodes for SOECs. However, one obvious problem for practical applications is the usage of CO/H2 safe gas, which must be supplied to maintain the electrode performance. This work reports a safe gas free ceramic electrode for efficient CO2/H2O electrolysis. The electrode has a heterogeneously porous structure with Sr2Fe1.5Mo0.5O6−δ (SFM) electrocatalyst nanoparticles deposited onto the inner surface of the YSZ scaffold fabricated by a modified phase-inversion tape-casting method. The nanostructured SFM–YSZ electrodes have demonstrated excellent performance for CO2–H2O electrolysis. For example, the electrode polarization resistance is 0.25 Ω cm2 under open circuit conditions while the current density is 1.1 A cm−2 at 1.5 V for dry CO2 electrolysis at 800 °C. The performance is comparable with those reported for the Ni–YSZ fuel electrodes, where safe gas must be supplied. In addition, the performance is up to one order of magnitude better than those reported for other ceramic electrodes such as La0.75Sr0.25Cr0.5Mn0.5O3−δ and La0.2Sr0.8TiO3+δ. Furthermore, the electrode exhibits good stability in the short-term test at 1.3 V for CO2-20 vol% H2O co-electrolysis, which produces a syngas with a H2/CO ratio close to 2. The reduced interfacial polarization resistance, high current density, and good stability show that the nanostructured SFM–YSZ fuel electrode is highly effective for CO2/H2O electrolysis without using the safe gas, which is critical for practical applications.

A high performance cathode for proton conducting solid oxide fuel cells

Intermediate temperature solid-oxide fuel cells (IT-SOFCs) ), as one of the energy conversion devices, have attracted worldwide interest for their great fuel efficiency, low air pollution, much reduced cost and excellent longtime stability. In the intermediate temperature range (500–700 °C), SOFCs based on proton conducting electrolytes (PSOFCs) display unique advantages over those based on oxygen ion conducting electrolytes. A key obstacle to the practical operation of past P-SOFCs is the poor stability of the traditionally used composite cathode materials in the steam-containing atmosphere and their low contribution to proton conduction. Here we report the identification of a new Ruddlesden–Popper-type oxide Sr3Fe2O7−δ that meets the requirements for much improved long-term stability and shows a superior single-cell performance. With a Sr3Fe2O7−δ-5 wt% BaZr0.3Ce0.5Y0.2O3−δ cathode, the P-SOFC exhibits high power densities (683 and 583 mW cm−2 at 700 °C and 650 °C, respectively) when operated with humidified hydrogen as the fuel and air as the cathode gas. More importantly, no decay in discharging was observed within a 100 hour test.