Nanomaterials and technologies for low temperature solid oxide fuel cells : Recent advances, challenges and opportunities

Solid-oxide fuel cells are devices for the efficient conversion of chemical energy to electrical energy and heat. Research efforts are currently addressed toward the optimization of cells operating at temperatures in the region of 600°C, known as intermediate-temperature solid-oxide fuel cells, for which materials requirements are very stringent. In addition to the requirements of mechanical and chemical compatibility, the materials must show a high degree of oxide ion mobility and electrochemical activity at this low temperature. Here we mainly examine the criteria for the development of two key components of intermediate-temperature solid-oxide fuel cells: the electrolyte and the cathode. We limit the discussion to novel approaches to materials optimization and focus on the fluorite oxide for electrolytes, principally those based on ceria and zirconia, and on perovskites and perovskite-related families in the case of cathodes.

Materials for Intermediate-Temperature Solid-Oxide Fuel Cells

La0.6Sr0.4CoO3−δ(LSC) thin film cathodes synthesized by pulsed laser deposition at 450°C (LSC_450°C) and 650°C (LSC_650°C) exhibit different electrochemical performance. The origin of the differences in the oxygen reduction activity and stability of these cathodes is investigated on the basis of their surface chemistry and their surface atomic and electronic structures. Angle resolved X-ray photoelectron spectroscopy and nanoprobe Auger electron spectroscopy are used to identify the surface cation content, chemical bonding environment, and the spatial heterogeneities with nanoscale resolution. The higher electrochemical activity of LSC_450°C is attributed to the more stoichiometric cation content on the surface and the more uniform lateral and depth distribution of constituent cations. The poorly crystalline atomic structure of the LSC_450°C was found to prohibit the extensive segregation and phase separation on the surface because of the more favorable elastic and electrostatic interactions of Sr in the ...

https://pubs.acs.org/doi/pdf/10.1021/cm203501u

Chemical Heterogeneities on La0.6Sr0.4CoO3−δ Thin Films—Correlations to Cathode Surface Activity and Stability

Pulsed laser deposited La 0.6 Sr 0.4 CoO 3―δ (LSC) thin film electrodes on yttria stabilized zirconia (YSZ) single crystals were investigated by impedance spectroscopy, time of flight secondary ion mass spectrometry (ToF-SIMS) and inductively coupled plasma optical emission spectrometry (ICP-OES). Effects caused by different film deposition temperatures, thermal annealing and chemical etching were studied. Correlations between changes in electrode polarization resistance of oxygen reduction and surface composition were found. At high deposition temperatures and after thermal annealing an inhomogeneous cation distribution was detected in the surface-near region, most manifest in a significant Sr enrichment at the surface. An activating effect of chemical etching of LSC is described, which can lower the polarization resistance by orders of magnitude. Chemistry behind this activation and thermal degradation was analyzed by ToF-SIMS and ICP-OES measurements of in-situ etched LSC films. The latter allow quantitative depth resolved compositional analysis with nominally sub nm resolution. High resolution scanning electron microscopy images illustrate the accompanying changes in surface morphology. All measurements suggest that stoichiometric LSC surfaces intrinsically exhibit very high activity towards oxygen reduction.

Relationship between Cation Segregation and the Electrochemical Oxygen Reduction Kinetics of La0.6Sr0.4CoO3−δ Thin Film Electrodes

Amorphous metal oxides (AMOs) have aroused great enthusiasm across multiple energy areas over recent years due to their unique properties, such as the intrinsic isotropy, versatility in compositions, absence of grain boundaries, defect distribution, flexible nature, etc. Here, the materials engineering of AMOs is systematically reviewed in different electrochemical applications and recent advances in understanding and developing AMO-based high-performance electrodes are highlighted. Attention is focused on the important roles that AMOs play in various energy storage and conversion technologies, such as active materials in metal-ion batteries and supercapacitors as well as active catalysts in water splitting, metal-air batteries, and fuel cells. The improvements of electrochemical performance in metal-ion batteries and supercapacitors are reviewed regarding the enhancement in active sites, mechanical strength, and defect distribution of amorphous structures. Furthermore, the high electrochemical activities boosted by AMOs in various fundamental reactions are elaborated on and they are related to the electrocatalytic behaviors in water splitting, metal-air batteries, and fuel cells. The applications in electrochromism and high-conducting sensors are also briefly discussed. Finally, perspectives on the existing challenges of AMOs for electrochemical applications are proposed, together with several promising future research directions.

Research Advances of Amorphous Metal Oxides in Electrochemical Energy Storage and Conversion.

La0.6Sr0.4CoO3–δ (LSC) thin-film electrodes are prepared on yttria-stabilized zirconia (YSZ) substrates by pulsed laser deposition at different deposition temperatures. The decrease of the film crystallinity, occurring when the deposition temperature is lowered, is accompanied by a strong increase of the electrochemical oxygen exchange rate of LSC. For more or less X-ray diffraction (XRD)-amorphous electrodes deposited between ca. 340 and 510 °C polarization resistances as low as 0.1 Ω cm2 can be obtained at 600 °C. Such films also exhibit the best stability of the polarization resistance while electrodes deposited at higher temperatures show a strong and fast degradation of the electrochemical kinetics (thermal deactivation). Possible reasons for this behavior and consequences with respect to the preparation of high-performance solid oxide fuel cell (SOFC) cathodes are discussed.

Optimized La0.6Sr0.4CoO3–δ Thin-Film Electrodes with Extremely Fast Oxygen-Reduction Kinetics

The separation of grain and grain boundary impedance in thin yttria stabilized zirconia (YSZ) layers

The invention relates to a photochemical cell (1) and to a method for the solar-driven decomposition of a starting material, in particular water or carbon dioxide, into a product gas bound therein, in particular hydrogen or carbon monoxide, comprising a supply line (7) for the starting material, a discharge line (9) for the obtained product gas, a first electrode (2) made of a photoelectrically active material and exposed to solar radiation (3', 3'') during operation, and a second electrode (5), wherein the electrodes (2, 5) are connected to each other in a closed circuit by means of an electron conductor (4) for transporting electrons excited by the solar radiation (3', 3'') in the first electrode (2) and an ion conductor (6) for transporting ions produced in the decomposition of the starting material, wherein an electrolyte made of a heat-resistant solid material and arranged between the electrodes (2, 5) is provided as the ion conductor (6)

Martin Ahrens

Papers

Optimized La0.6Sr0.4CoO3–δ Thin-Film Electrodes with Extremely Fast Oxygen-Reduction Kinetics

The separation of grain and grain boundary impedance in thin yttria stabilized zirconia (YSZ) layers

Photoelectrochemical cell and method for the solar-driven decomposition of a starting material

Heterogenität der Kompetenzentwicklung in der beruflichen Bildung