Research

Highlights:

• Romo-De-La-Cruz, C., Liang, L., Navia, S. A. P., Chen, Y., Prucz, J., Song, X. (2018). Role of oversized dopant potassium on the nanostructure and thermoelectric performance of calcium cobaltite ceramics. Sustainable Energy Fuels, 2, 876-881. doi:10.1039/C7SE00612H
• Song, X., Paredes Navia, S. A., Liang, L., Boyle, C., Romo-De-La-Cruz, C.-O., Jackson, B., Hinerman, A., Wilt, M. Chen, Y. (2018). Grain Boundary Phase Segregation for Dramatic Improvement of the Thermoelectric Performance of Oxide Ceramics. Acs Applied Materials Interfaces, 10(45), 39018-39024. doi:10.1021/acsami.8b12710
• Carvillo, P., Chen, Y., Boyle, C., Barnes, P. N., Song, X. (2015). Thermoelectric Performance Enhancement of Calcium Cobaltite through Barium Grain Boundary Segregation. Inorganic Chemistry, 54(18), 9027-9032. doi:10.1021/acs.inorgchem.5b01296

Thermoelectric (TE) technology could be more efficient in most applications if the high-performance thermoelectric materials were made of non-toxic and earth-abundant elements. The energy conversion efficiency of the TE materials is described by the figure of merit ZT, which is defined as $ZT=S^2{\rho }^{-1}{\kappa }^{-1}$, where $S$, $\rho$, $S^2{\rho }^{-1}$, and $\kappa$ are the Seebeck coefficient, electrical resistivity, electrical power factor (PF) and thermal conductivity, respectively. P-type calcium cobaltite $C{a}_{3}C{o}_4{O}_{9-\delta }$ is a promising candidate for TE applications over conventional TE materials due to its high thermal stability in the air at high temperatures, low cost, lightweight, and non-toxicity. Single crystal $C{a}_{3}C{o}_4{O}_{9-\delta }$ already possess an excellent TE performance, approaching that of the well-developed conventional TE materials. However, TE energy conversion efficiency of polycrystalline $C{a}_{3}C{o}_4{O}_{9-\delta }$ remains low and accounts to ~30-60% of the single crystals. To overcome the low performance of the ZT in $C{a}_{3}C{o}_4{O}_{9-\delta }$ ceramics, dopants were introduced through both the cation stoichiometric substitution and novel non-stoichiometric addition. The work from Dr. Song’s work shows that dopants intragranular non-stoichiometric addition and concurrent intergranular segregation at the grain boundaries dramatically decrease the electrical resistivity, simultaneously increase the Seebeck coefficient, and ultimately result in the polycrystalline ceramics outperforming the single crystals over a wide range of temperatures. A record high thermoelectric figure of merit ZT~0.9 at 1073 K was achieved for $C{a}_{3}C{o}_4{O}_9$ ceramics modified and synthesized by Dr. Song’s group. The newly $C{a}_{3}C{o}_4{O}_9$ ceramics developed by Prof. Song’s group is surpassing the $C{a}_{3}C{o}_4{O}_{9-\delta }$ single crystals. Furthermore, the engineered $C{a}_{3}C{o}_4{O}_{9-\delta }$polycrystalline ceramics also outperformed the best reported p-type SiGe from 373 to 973K, while $C{a}_{3}C{o}_4{O}_{9+\delta }$ ceramics are having just 5-10% of the cost of SiGe and will be performing directly in air . Most of all, the present work from Prof. Song’s group presents a novel approach to engineer the lattice and grain boundary in ceramics to decouple the strongly coupled thermoelectric parameters of $S$, $\rho$, and $\kappa$ to largely increase the electrical power factor of thermometric oxide ceramic s . 

The latest work from Song’s group on thermoelectric ceramics lead to series peer-reviewed journal paper publications, and two new pending patents since 2016.

Learn more about thermoelectric ceramics

In comparison with the emerging protonic ceramic fuel cells, the solid oxide fuel cells (SOFCs) are nowadays commercially available with applications including stationary power supply and advanced hybrid fuel cell and engine systems that have the potential of achieving ultra-high efficiency of greater than 70%. Nevertheless, there is an urgent need for further improvement of the SOFC performance in term of power density and long-term stability to increase their market competences. The typical power density of a yttria-stabilized zirconia (YSZ)-based commercial SOFC is currently reported to be in the range of ~0.2-0.8 $W/c{m}^2$ depending on the cell configuration (either electrolyte supported, or anode supported), cathode materials, and the cell operating conditions. Regardless the intense research effort during the past decade, the high activation energy for oxygen reduction reaction (ORR) in the cathode is still the major cause hindering the power density the state-of-the-art SOFCs. Infiltration processes have thus been developed for deposition of nanoscale catalysts into the well-developed porous lanthanum strontium manganite (LSM) or lanthanum strontium cobalt ferrite (LSCF) based composite cathodes, to enhance the surface electro-catalytic activity and stability and accelerate the ORR.

• Highlight one: Chen, Y., Liang, L., Paredes Navia, S., Hinerman, A., Gerdes, K., & Song, X. (2019). Synergetic Interaction of Additive Dual Nano Catalysts to Accelerate Oxygen Reduction Reaction in Fuel Cells Cathode. Acs Catalysis.

Amongst the various electrocatalysts, precious metal Pt remains to be one of the most efficient oxygen reduction catalysts employed for various fuel cells operated at different temperatures, while the high cost of Pt prevents its large-scale applications. In recent years, chemical vapor based Atomic Layer Deposition (ALD) is demonstrated to be able to create a conformal and uniform surface coating layer with thickness down to the atomic scale. Such an approach could lead to the minimum loading of catalyst into the cathode of as-fabricated cells to further improve the SOFC performance. For example, when the ALD layer is ~ 5 nm in thickness and consisting discrete ~ 3 nm Pt grains, the loading of Pt is estimated to be minute of ~1.5×10-3 $mg/c{m}^2$, which is significantly lower than the target loading of < 0.1 $mg/c{m}^2$ that needs to be achieved for proton exchange membrane fuel cells in automotive applications. However, for SOFC operated at high temperatures of 750 °C or higher, once the ALD mono-layer of unary Pt is applied to the LSM/YSZ cathode of cells, the power density enhancement induced by ALD coating is limited to be ~140 %. Pt in the ALD layer undergoes immediate agglomeration from ~ 3 nm to ~ 70 nm in dimensions and loss of catalytic surface area due to the electrochemical operation. Pinning the Pt catalyst to be nano-sized and with uniform distribution on the ORR active sites is very much desired to further boost the cell performance while minimizing the Pt loading in the ALD layer. Here we report one simple approach of applying an ultra-thin $Co{O}_x$ layer to stabilize the nano-Pt particles on the LSM/YSZ cathode backbone of as-made SOFCs. Through synergetic electrochemical interactions, the $Pt/\left(MnCo\right)O_x$ couplings were established and uniformly distributed on the entire YSZ surface, the original TPBs and at LSM/LSM surface grain boundaries. Such nanoscale $Pt/\left(MnCo\right)O_x$ couplings are stable and dramatically accelerate the ORR. ALD coating has significantly enhanced the cell peak power density by 200 % over electrochemical operation of 504 h at 750 °C.

• Highlight two: Chen, Y., Gerdes, K., & Song, X. (2016). Nanoionics and Nanocatalysts: Conformal Mesoporous Surface Scaffold for Cathode of Solid Oxide Fuel Cells. Scientific Reports, 6, 32997. doi:10.1038/srep32997

Nanoionics has become increasingly important in devices and systems related to energy conversion and storage. Nevertheless, nanoionics and nanostructured electrodes development has been challenging for solid oxide fuel cells (SOFCs) owing to many reasons including poor stability of the nanocrystals during fabrication of SOFCs at elevated temperatures. In this study, a conformal mesoporous $Zr{O}_2$ nanoionic network was formed on the surface of $L{a}_{1-x}S{r}_{x}Mn{O}_3$/yttria-stabilized zirconia (LSM/YSZ) cathode backbone using Atomic Layer Deposition (ALD) and thermal treatment. The surface layer nanoionic network possesses open mesopores for gas penetration, and features a high density of grain boundaries for enhanced ion-transport. The mesoporous nanoionic network is remarkably stable and retains the same morphology after electrochemical operation at high temperatures of 650–800 °C for 400 hours. The stable mesoporous $Zr{O}_2$ nanoionic network is further utilized to anchor catalytic Pt nanocrystals and create a nanocomposite that is stable at elevated temperatures. The power density of the ALD modified and inherently functional commercial cells exhibited enhancement by a factor of 1.5–1.7 operated at 0.8 V at 750 °C. Nanoionics and nanostructured electrode, which are extraordinary important for ion transport in the other electrochemical systems, have been very challenging to develop for SOFCs. To the authors’ knowledge, the present work from Prof. Song's group is the first demonstration of establishment of nanoionic network for applications of high temperature SOFCs . These experiments contribute to validation of the nanoionic mechanisms reported previously by independent researchers and offer justification for tailoring surface structure of the electrode, via nanostructural engineering with ALD and thermo-treatment. 

• Highlight three: Chen, Y., Gerdes, K., Paredes Navia, S. A., Liang, L., Hinerman A., Song, X. (2019). Conformal Electrocatalytic Surface Nanoionics for Accelerating High-Temperature Electrochemical Reactions in Solid Oxide Fuel Cells, Nano Lett.

Additive implantation of electrocatalysts onto the internal surface of porous cathodes holds great promise to accelerate the electrochemical reactions within solid oxide fuel cells (SOFCs). Here we utilize atomic layer deposition (ALD) to apply dual catalysts with $(M{n}_{0.8}C{o}_{0.2}{)}_3{O}_4$ and a minute amount of Pt on the cathode consisting of lanthanum strontium manganite (LSM) and yttria-stabilized zirconia (YSZ). Coating this material with optimum ALD layer thickness resulted in a 53% reduction of polarization resistance and a 350% SOFC peak power density enhancement at 750 °C. During the electrochemical operations, the dual catalysts interact synergistically and evolve into superjacent conformal electrocatalytic $(M{n}_{0.8}C{o}_{0.2}{)}_3{O}_4$ nanoionics with high-density grain boundaries and subjacent discrete nano Pt particles evenly distributed on both the LSM and YSZ. The configuration consequently extends the active electrochemical reaction sites to the entire internal surface of the cathode. For the first time in the field of SOFCs, the present work demonstrates the formation of the electrocatalytic surface nanoionics and its resultant accelerated mass and charge transfer to dramatically boost the cell performance.

The latest work from Song’s group and her collaborators on solid oxide fuel cells lead to series peer-reviewed journal paper publications, and two patents including one granted patent and one pending patent since 2015. <\p>

Learn more about solid oxide fuel cells

• Highlight one:  X. Song , G. Daniels, M. Feldmann, A. Guriech, D. Larbalestier, “Electromagnetic, atomic structure and chemistry changes induced by Ca-doping of low-angle $YB{a}_{2}C{u}_3{O}_{7-\delta }$ grain boundaries,”  Nature Materials, 4 (2005) 470-475.
• Accompanying News & Views article published in Nature Materials: J. Mannhart and D. Muller, “Superconductors-catching dopants in Action,”  Nature Materials, 4 (2005) 431-432. 
• Highlighted in the cover page of Nature Materials, 4 (2005).

“Largely unknown to the public, university groups, national labs and companies are developing wires from high-temperature superconductors that can be used to build energy-efficient motors, generators, transformers, and cables for the power grid. In long-distance wires and cables, high-temperature superconductors are successfully used to transport enormous amounts of electric power with small losses. The performance and costs of these superconducting cables are mainly controlled by the boundaries between the crystallites of the superconductors. One of the few ways to optimize the transport properties of these grain boundaries is to dope them with calcium. However, calcium also degrades the properties of the bulk material. Now two groups, David Larbalestier and colleagues and Robert Klie et al., have shown how calcium changes the structure of the boundaries, giving guidance to the search for dopants with fewer side effects.”

The above is Quoted from “Mannhart, J., & Muller, D. A. (2005). Superconductors: Catching dopants in action. Nature Materials, 4(6), 431.” <\p>

Learn more about superconductors

• Highlight one:  X. Song *, S. Babcock, C. Paulson, T. Kuech, J. Huang, D. Xu, J. Park, L. Mawst, Nanostructure of $GaA{s}_{0.88}S{b}_{0.10}{N}_{0.02}/InP$ quantum wells grown by metal-organic chemical vapor deposition on InP, J. Cryst. Growth, 310 (2008) 2377-2381.

Dilute nitride III–V semiconductors show great promise for application in GaAs-based diode lasers that emit at wavelengths in 1.30–1.55 μm range, and are thus strategic for telecommunications. The effects of thermal annealing on the emission and microstructural characteristics of $GaA{s}_{0.88}S{b}_{0.10}{N}_{0.02}/InP$ multiple quantum well (QW) structures were studied by photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM). The results show that the optimum annealing conditions lead to improved PL intensity accompanied by only a small blue shift, contrasting the behavior of GaAsSbN/GaAs multiple QWs, and improved structural uniformity. <\p>

Learn more about compound semiconductors

• Highlight one: Ö. N. Dog˘an, X. Song, D. Palacio, M. C. Gao, Coherent precipitation in a high-temperature Cr–Ni–Al–Ti Alloy, J Mater. Sci . 49 (2014) 805-810.

Developing more efficient fossil energy conversion technologies with less environmental impact require high-performance structural materials of long-term reliability that can be used in aggressive environments. Next generation gas turbines combusting hydrogen-rich fuels and oxy-fuels are predicted to have inlet temperatures higher than the current gas turbines. The current gas turbines utilize state-of-the-art nickel-based superalloys in the hottest sections. These materials possess excellent creep strength because they contain high volume fraction of γ’ precipitates coherent with the γ matrix and excellent high-temperature oxidation resistance due to ability to form protective oxide scales on the surface. However, these alloys perform at their temperature capability limit, and there is not much room left for improvement since the application temperatures are already very near their melting points. Chromium alloys present a potential for development of new high-temperature materials. Precipitation characteristics of a Cr–5Ni–5Al–0.5Ti (at.%) alloy were investigated utilizing a series of heat treatments. XRD, SEM, and analytical TEM were used to characterize the microstructure. This study has shown that the small spherical B2-NiAl precipitates forming below 1345 °C are highly coherent and have a well-defined orientation relationship with the Cr-matrix. Also, some evidence has been presented for the formation of $L{2}_1-N{i}_{2}AlTi$ phase within the B2-NiAl phase. <\p>

Learn more about intermetallic compounds