Efficient energy storage systems based on lithium-ion batteries represent a critical technology across many sectors including consumer electronics, electrified transportation, and a smart grid accommodating intermittent renewable energy sources. Nanostructured electrode materials present compelling opportunities for high-performance lithium-ion batteries, but inherent problems related to the high surface area to volume ratios at the nanometer-scale have impeded their adoption for commercial applications. Here, we demonstrate a materials and processing platform that realizes high-performance nanostructured lithium manganese oxide (nano-LMO) spinel cathodes with conformal graphene coatings as a conductive additive. The resulting nanostructured composite cathodes concurrently resolve multiple problems that have plagued nanoparticle-based lithium-ion battery electrodes including low packing density, high additive content, and poor cycling stability. Moreover, this strategy enhances the intrinsic advantages of nano-LMO, resulting in extraordinary rate capability and low temperature performance. With 75% capacity retention at a 20C cycling rate at room temperature and nearly full capacity retention at -20 °C, this work advances lithium-ion battery technology into unprecedented regimes of operation.

Epitaxial LiMn 2 O 4 (LMO)/La 0.5 Sr 0.5 CoO 3 (LSCO) bilayer thin films were grown on single crystalline SrTiO 3 (STO) (111) substrates as model lithium ion battery cathodes. The LSCO layer was used as an electrically conducting buffer layer for electrochemical testing. The LMO and LSCO layers were both epitaxial, with sub-nano flat LMO/LSCO interfaces as seen by X-ray diffraction, synchrotron X-ray scattering, and high-resolution transmission electron microscopy (HRTEM), but with a large LMO surface roughness due to the relatively large lattice mismatch with LSCO. Threedimensional islands and depressions were formed on the strain-relaxed LMO layer, and misfit dislocations at the LMO/LSCO interface were discerned through HRTEM imaging, suggesting a Stranski−Krastanov (SK) mode thin-film growth. A crystalline structural change from cubic spinel at the LMO surface and interior to tetragonal oxygen-deficient LMO at the LMO/LSCO interface was examined. Electrochemical tests along with in situ synchrotron X-ray scattering measurements on the epitaxial LMO/LSCO bilayers showed a significant loss of capacity after the first cycle, which was attributed to an electrical conductivity loss of the LSCO buffer layer due to irreversible lattice oxygen loss.

A direct flame fuel cell setup is designed and built based on a multi-element diffusion flame burner (MEDB). Flame temperature measurements are made using a fine-wire S-type thermocouple to characterize the flat flame burner performance. The MEDB is proven to provide uniform, ∼1D conditions above the surface of the burner, with temperature variations of less than ±2% in the transverse direction (parallel to the burner surface). The temperature distribution in flame height direction is also approximately uniform within the length of 45 mm. Direct flame fuel cell experiments were performed using an anode-supported solid oxide fuel cell (SOFC) based on button cell geometry. The cell power density can reach up to 400 W/m2 with fuel equivalent ratio at ϕ = 1.2 and flame temperature at 1085 K, which is around 1/3 of the original performance of the SOFC fueled with 97% hydrogen at 1073 K. Finally, a novel DFFC configuration is proposed to take advantage the flame uniformity in both the vertical and horizontal directions.

A detailed mechanistic model for solid oxide electrolyte direct carbon fuel cell (SO-DCFC) is developed while considering the thermo-chemical and electrochemical elementary reactions in both the carbon bed and the SOFC, as well as the meso-scale transport processes within the carbon bed and the SOFC electrode porous structures. The model is validated using data from a fixed bed carbon gasification experiment and the SO-DCFC performance testing experiments carried out using different carrier gases and * Corresponding author. Tel.: +86-10-62789955; Fax: +86-10-62770209.Email: shyx@tsinghua.edu.cn.2 at various temperatures. The analyses of the experimental and modeling results indicate the strong influence of the carrier gas on the cell performance. The coupling between carbon gasification and electrochemical oxidation on the SO-DCFC performance that results in an unusual transition zone in the cell polarization curve was predicted by the model, and analyzed in detail at the elementary reaction level. We conclude that the carbon bed physical properties such as the bed height, char conversion ratio and fuel utilization, as well as the temperature significantly limit the performance of the SO- DCFC.Key words: solid oxide electrolyte; direct carbon fuel cell; elementary reaction; modeling; heterogeneous chemistry This work is focuses on the solid oxide electrolyte direct carbon fuel cells (SO-DCFC) which are capable of conversing chemical energy in the solid carbon fuel into electricity.These offer a number of advantages over the traditional carbon conversion technologies 3 as well as alternative DCFCs such as: the abundance of the fuel source, high theoretical efficiency, high CO 2 emission reduction potential, relatively higher reaction activity ascribed to its high operating temperatures, and avoidance of liquid electrolyte consumption, leakage and corrosion [3]. Due to these advantages, some researchers have investigated DFFCs for application in large-scale power plants [4,5] considering their potential merits for high efficiency and emission reduction.SO-DCFC performance improvement relies on optimal electrochemical reactions, carbon gasification and mass transport processes. Since experimental studies on SO-DCFC are rather complex, expensive, and time-consuming, comprehensive mathematical models are essential for the technology development. A validated mechanistic model would offer means to gain insight into the complex physical phenomena governing fthe uel cell performance that is not readily accessible experimentally, and it can also be useful tool for cell design and operating condition optimization.Modeling and experimental studies of SO-DCFCs have been reported recently by several researchers [6][7][8][9]. Numerous SOFC models considering the intricate interdependency among ionic and electronic conduction, gas transport phenomena, and electrochemical processes have been reported in the literatures for pure hydrogen, syngas or methane [10][11][12][13][14][15][16][17][18][19][20]. Hecht et al. [21]...

The performance of a solid oxide electrolyte direct carbon fuel cell (SO-DCFC) is limited by the slow carbon gasification kinetics at the typical operating temperatures of cell: 650-850 o C. To overcome such limitation, potassium salt is used as a catalyst to speed up the dry carbon gasification reactions, increasing the power density by five-fold at 700 -850 o C. The cell performance is shown to be sensitive to the bed temperature, emphasizing the role of gasification rates and that of CO production. Given the finite bed size, the cell performance is time-dependent as the amount of CO available changes. A reduced elementary reaction mechanism for potassium-catalyzed carbon gasification was proposed using kinetic data obtained from the experimental measurements. A comprehensive model including the catalytic gasification reactions and CO electrochemistry is used to examine the impact of the catalytic carbon gasification process on the device performance. The power density is maximum around 50% of the OCV, where carbon utilization is also near maximum. Results show that bed height and porosity impact the power density; a thicker bed maintains the power almost constant for longer times while lower porosity delivers higher power density in the early stages.

A direct flame solid oxide fuel cell unit was designed and built in this work for potential combined heat and power generation. The direct combination of a methane flat flame and a solid oxide fuel cell (SOFC) in a simple, "no-chamber" setup was implemented. Direct flame fuel cell experiments were performed using an anode-supported planar SOFC with 50-mm-long square anode and 30-mm-long square cathode. At the SOFC anode, a multi-element diffusion flame burner with an outlet of 55 mm by 55 mm square provided methane/air rich flames for various equivalence ratios and flow rates. The maximum power generated by the direct flame fuel cell unit reached 0.35 W with the power density of 388 W/m 2 at the equivalence ratio of 1.2. The unit was operated for 3 hours at the equivalence ratio of 1.1 at a fixed voltage of 0.4 V with the current deteriorating from 0.29 A to 0.25 A. The system efficiency of the power generation unit was analyzed by considering the combustion efficiency, the fuel utilization efficiency and the SOFC electrical efficiency. The fuel utilization efficiency was found to be the dominant factor of the total efficiency.

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