Switching on electrocatalytic activity in solid oxide cells

Solid oxide cells (SOCs) can operate with high efficiency in two ways - as fuel cells, oxidizing a fuel to produce electricity, and as electrolysis cells, electrolysing water to produce hydrogen and oxygen gases. Ideally, SOCs should perform well, be durable and be inexpensive, but there are often competitive tensions, meaning that, for example, performance is achieved at the expense of durability. SOCs consist of porous electrodes - the fuel and air electrodes - separated by a dense electrolyte. In terms of the electrodes, the greatest challenge is to deliver high, long-lasting electrocatalytic activity while ensuring cost- and time-efficient manufacture. This has typically been achieved through lengthy and intricate ex situ procedures. These often require dedicated precursors and equipment; moreover, although the degradation of such electrodes associated with their reversible operation can be mitigated, they are susceptible to many other forms of degradation. An alternative is to grow appropriate electrode nanoarchitectures under operationally relevant conditions, for example, via redox exsolution. Here we describe the growth of a finely dispersed array of anchored metal nanoparticles on an oxide electrode through electrochemical poling of a SOC at 2 volts for a few seconds. These electrode structures perform well as both fuel cells and electrolysis cells (for example, at 900 °C they deliver 2 watts per square centimetre of power in humidified hydrogen gas, and a current of 2.75 amps per square centimetre at 1.3 volts in 50% water/nitrogen gas). The nanostructures and corresponding electrochemical activity do not degrade in 150 hours of testing. These results not only prove that in operando methods can yield emergent nanomaterials, which in turn deliver exceptional performance, but also offer proof of concept that electrolysis and fuel cells can be unified in a single, high-performance, versatile and easily manufactured device. This opens up the possibility of simple, almost instantaneous production of highly active nanostructures for reinvigorating SOCs during operation

Cation-swapped homogeneous nanoparticles in perovskite oxides for high power density

Exsolution has been intensively studied in the fields of energy conversion and storage as a method for the preparation of catalytically active and durable metal nanoparticles. Under typical conditions, however, only a limited number of nanoparticles can be exsolved from the host oxides. Herein, we report the preparation of catalytic nanoparticles by selective exsolution through topotactic ion exchange, where deposited Fe guest cations can be exchanged with Co host cations in PrBaMn1.7Co0.3O5+delta. Interestingly, this phenomenon spontaneously yields the host PrBaMn1.7Fe0.3O5+delta, liberating all the Co cations from the host owing to the favorable incorporation energy of Fe into the lattice of the parent host (Delta E-incorporation = -0.41 eV) and the cation exchange energy (Delta E-exchange = -0.34 eV). Remarkably, the increase in the number of exsolved nanoparticles leads to their improved catalytic activity as a solid oxide fuel cell electrode and in the dry reforming of methane

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This paper compares the effects of Ni and Ru dopants in lanthanum chromite anodes by correlating structural characterization and electrochemical measurements in solid oxide fuel cells ͑SOFCs͒. Transmission electron microscope observations showed that nanoclusters of Ni or Ru metal precipitated onto lanthanum chromite ͑La 0.8 Sr 0.2 Cr 1−y X y O 3−␦ , X = Ni,Ru͒ surfaces, respectively, after exposure to hydrogen at 750-800°C. Ni nanoclusters were typically ϳ10 nm in diameter immediately after reduction and coarsened to ϳ50 nm over ϳ300 h at 800°C. In contrast, Ru cluster size was stable at Յ5 nm, and the cluster density was Ͼ10 times larger. SOFC tests were done with the doped lanthanum chromite anodes on La 0.9 Sr 0.1 Ga 0. 3,4 Clearly, nanoparticles may coarsen at the relatively high firing temperatures ͑ϳ1400°C͒ used to process SOFCs. Thus, nanoparticles must be introduced after high temperature firing steps. A new method for introducing nanoscale metal particles into oxide anodes was recently reported. The oxide material, La 0.8 Sr 0.2 Cr 1−y Ru y O 3−␦ ͑LSCrRu͒, was fired in air at elevated temperature, but when the anode was reduced during the initial SOFC operation, Ru nanoparticles Ͻ5 nm in diameter formed on the oxide surface. Experimental Procedures Powders of LSCrRu and LSCrNi were synthesized by solid-state reaction at 1200°C for 3 h, yielding particle sizes of ϳ1 to 2 m. In the discussion below, the different Ni or Ru contents are given, for example, as LSCrNi31 ͑La 0.8 Sr 0.2 Cr 0.69 Ni 0.31 O 3−␦ ͒. The SOFC anodes consisted of 50 wt % of one of the above chromite powders mixed with 50 wt % Gd-doped ceria ͑GDC͒. All SOFCs utilized La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3−␦ ͑LSGM͒ electrolytes, ϳ400 m thick. The LSGM powders were fabricated via solid-state reaction at 1250°C, followed by uniaxial pressing and sintering for 6 h at 1450°C to form the electrolyte pellets. The cathodes were La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−␦ ͑LSCF͒ mixed with 50 wt % GDC. The anodes and cathodes ͑0.5 cm 2 active area, ϳ25 m thick͒ were screen printed on the LSGM electrolytes and fired for 3 h at 1200 and 1000°C, respectively. Au current collector grids were screen printed over the electrodes and contacted using Ag wires. Single cell tests were performed as described previously 9 using a four-wire setup for current-voltage ͑I-V͒ and electrochemical impedance spectroscopy ͑EIS͒ measurements ͑BAS-Zahner IM-6͒. In life tests, the cells were first stabilized at temperature with Ar at the anode before starting H 2 flow. The H 2 flow to the cells was first humidified by bubbling the gas through H 2 O at room temperature, resulting in Ϸ3% H 2 O in H 2 . Times given in life test results are after the start of humidified H 2 flow. Measurements on various other SOFCs indicated that Ar was almost entirely purged from the anode compartment before the first electrical measurements ͑15 min͒. X-ray diffraction measurements of anode powders were done with a standard diffractometer ͑Rigaku 0.8 kW Dmax͒. Scanning electron microscopy ͑SEM͒ measurements were done along with energy-dispersive spectroscopy ͑EDS͒ ͑Hitachi S3400N-II, S3500, S3800, and S4800-II cFEG͒. High resolution electron microscopy ͑HREM͒ studies were carried out on the powder samples using a JEOL JEM-2100F electron microscope operated at 200 kV. The powders were annealed in either dry H 2 or 3% H 2 O/H 2 . Micrographs were digitally acquired on a 2 ϫ 2 k charge-coupled device camera using a Gatan Imaging Filter system. A small amount of powder was added to acetone, followed by ultrasonic mixing to achieve a particle dispersion. A drop of the resulting suspension was deposited on carbon-coated transmission electron microscope ͑TEM͒ grids ͑Ted Pella͒. Samples were stored in a desiccator overnight before HREM examination. Experimental Results The following sections describe first the structural observations of LSCrNi, with results for LSCrRu included for comparison. In the second section, electrochemical test results for SOFCs with these anodes are discussed. Structural characterization.-Ni-doped anodes.

In situ growth of nanoparticles through control of non-stoichiometry

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