CO2 has a potentially bright future as a carbon resource because it is very cheap and abundant. The conversion technology of CO2 into useful chemicals therefore has gained growing attention over ...recent years. Despite many attempts, there have not yet been revolutionary successes for commercialization of such technology. One of the main challenges in this field is to catalytically activate the CO2 molecule on the surfaces of catalysts. Although many researchers have studied the catalytic reactions involving CO2 on the surfaces, the activation process of CO2 is still controversial. Here, we performed density functional theory calculations to understand the CO2 activation and dissociation on a wide range of bimetallic alloy surfaces. To begin with, the adsorption process of CO2 on pure metal surfaces was carefully examined with the analyses of adsorption energetics, geometries, vibrational frequencies, charge transfers, and density of states. From the activated CO2 on the surfaces, we could precisely capture the transition state of the dissociation reaction. On the basis of the information, we found that Brønsted–Evans–Polanyi (BEP) relations hold for CO2 dissociation reaction. It was also verified that the sum of adsorption energies of CO and O is linearly scaled with not only adsorption energy of CO2 δ− but also reaction energy for the CO2 dissociation. As a result, the energy barriers of CO2 dissociation on pure metal and bimetallic alloy surfaces could be rapidly screened by combining the BEP relation, scaling relation, and surface mixing rule. Our results will provide useful insight into designing transition metal catalysts for the CO2-involved reactions.
In reducing the high operating temperatures (≥800 °C) of solid-oxide fuel cells, use of protonic ceramics as an alternative electrolyte material is attractive due to their high conductivity and low ...activation energy in a low-temperature regime (≤600 °C). Among many protonic ceramics, yttrium-doped barium zirconate has attracted attention due to its excellent chemical stability, which is the main issue in protonic-ceramic fuel cells. However, poor sinterability of yttrium-doped barium zirconate discourages its fabrication as a thin-film electrolyte and integration on porous anode supports, both of which are essential to achieve high performance. Here we fabricate a protonic-ceramic fuel cell using a thin-film-deposited yttrium-doped barium zirconate electrolyte with no impeding grain boundaries owing to the columnar structure tightly integrated with nanogranular cathode and nanoporous anode supports, which to the best of our knowledge exhibits a record high-power output of up to an order of magnitude higher than those of other reported barium zirconate-based fuel cells.
•Electrolyte with a thickness less than grain size was fabricated by cost-effective method.•Bamboo-structured thin electrolyte effectively reduces ohmic resistance of protonic ceramic fuel ...cell.•Electrode reactions were analyzed by distribution of relaxation time method.•Surface diffusion of an adsorbed oxygen to the triple phase boundaries at cathode is the most probable rate determining step.
High-performance and cost-effective fabrications should be simultaneously achieved for practical applications of fuel cells. Unfortunately, protonic ceramic fuel cells, which are considered next-generation solid oxide fuel cells operating at lower temperatures (≤600 °C), do not satisfy the requirements. While thin electrolyte and rapid reactions at electrode/electrolyte interfaces are crucial for cell performance, the thickness of the electrolyte via cost-effective ceramic processes is still not satisfactory (currently capable of >10 μm) and the electrode reaction(s) are yet to be clarified. Here we demonstrate the fabrication of a columnar-structured thin electrolyte (∼2.5 μm) of BaCe0.55Zr0.3Y0.15O3-δ, in which no perpendicular grain boundaries exist against the current direction, through a low-cost screen printing method. A high open-cell voltage of 1.10 V ensures that the thin electrolyte is sufficiently dense for gas-tightness, thereby achieving an extraordinary maximum power density of 350 mW/cm2 at 500 °C. The electrode reactions are investigated by distribution of relaxation time method based on electrochemical impedance spectroscopy as a function of oxygen partial pressure and hydrogen partial pressure at 500 °C, suggesting that the reaction step corresponding to the surface diffusion of an adsorbed oxygen to the triple phase boundaries at the cathode is most probably the main contributor to the overall polarization resistances.
Understanding the solid electrolyte/cathode nano-interfacial kinetics is critical in designing advanced all-solid-state lithium metal batteries with improved performance and stability. However, the ...correlation between crystallographic features of cathodes and the solid nano-interface behaviour remains controversial due to the difficulty in eliminating the impact of other factors. Here, we systematically investigated the effect of exposed crystal facets of LiNi0.5Mn1.5O4 on the solid nano-interface using the substrate orientation-dependent epitaxial growth of thin films as a model study. (100), (110), and (111)-oriented Pt/MgO substrates were used to make selective high-quality epitaxial LiNi0.5Mn1.5O4 films with different {100}/{111}-exposed facet ratios. The atomic arrangement of the exposed facets was found to affect the electrochemical performance. Loosely packed {100} facets and densely packed {111} facets were beneficial for lithium ion diffusion and cycle stability, respectively. In particular, stable {111} facets effectively suppressed the dissolution and diffusion of transition metals at the solid nano-interface during charge-discharge, enabling a 99.6% retention after 100 cycles. In addition, this model study reveals that an amorphous cathode surface layer and a twin boundary inside the cathode are crystallographic origins that hinder the electrochemical performance of batteries. These findings suggest that crystallographic modifications of cathodes can be a key to improving the solid nano-interface.
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•Precise epitaxial growth creates different exposed crystal facets of LiNi0.5Mn1.5O4.•Loosely packed {100} facets induces high capacity via fast lithium ion diffusion.•{111} facets blocks mutual diffusion across the nano-interface, enhancing stability.•Thin film model reveals crystallographic origins of the solid interface degradation.
Thin-film electrolytes and nanostructured electrodes are essential components for lowering the operation temperature of solid oxide fuel cells (SOFCs); however, reliably implementing thin-film ...electrolytes and nano-structure electrodes over a realistic SOFC platform, such as a porous anode-support, has been extremely difficult. If these components can be created reliably and reproducibly on porous substrates as anode supports, a more precise assessment of their impact on realistic SOFCs would be possible. In this work, structurally sound thin-film and nano-structured SOFC components consisting of a nano-composite NiO-yttria-stabilized zirconia (YSZ) anode interlayer, a thin YSZ and gadolinia-doped ceria (GDC) bi-layer electrolyte, and a nano-structure lanthanum strontium cobaltite (LSC)-base cathode, are sequentially fabricated on a porous NiO-YSZ anode support using thin-film technology. Using an optimized cell testing setup makes possible a more exact investigation of the potential and challenges of thin-film electrolyte and nanostructured electrode-based anode-supported SOFCs. Peak power densities obtained at 500 degree C surpass 500 mW cm-2, which is an unprecedented low-temperature performance for the YSZ-based anode-supported SOFC. It is found that this critical, low-temperature performance for the anode-supported SOFC depends more on the electrode performance than the resistance of the thin-film electrolyte during lower temperature operation.
•A high-fidelity three-dimensional SOFC physical model was developed and validated.•Thermo-fluid reacting environment was elucidated as fuel utilization is raised.•Hydrogen depletion induces a ...gradient of ionic current density in the electrolyte.•Conductive flow through porous electrodes results in large pressure gradients.•The temperature profile, its increments and maximum location were estimated.
The thermo-fluid reacting environment and local thermodynamic state in solid oxide fuel cell (SOFC) stacks were examined by using three-dimensional numerical simulations. Enhancing the performance and durability of the SOFC stacks is essential when a high fuel utilization scheme is implemented to increase the system efficiency and lower system operating costs. In this study, numerical simulations were conducted to elucidate the effect of fuel utilization on heat and mass transfer as the fuel utilization is raised. A high-fidelity three-dimensional physical model was developed incorporating elementary electrochemical reaction kinetics by assuming rate-limiting steps and spatially-resolved conservation equations. The model considers planar anode-supported SOFC stacks and is validated against their electrochemical performance experimentally measured. A parametric study with respect to fuel utilization was conducted by varying a fuel flow rate while maintaining other operating conditions constant. Results show that, when increasing the fuel utilization, a narrow and non-uniform electrochemical reaction zone is observed near the fuel inlet, resulting in substantial depletion of hydrogen in the downstream fuel flow and thus raising the partial pressure of oxygen in the anode. This subsequently lowers the electrochemical potential gradient across the electrolyte and hence induces a large gradient of ionic current density along the cell. Convective flow through porous electrodes also results in pressure gradients in the direction of both cell thickness and length. In addition, the heat balance between conduction through metallic interconnects, convection by gases and the heat generated from charged-species transport and electrochemical reactions determines a temperature gradient along the cell and its maximum location. All of these gradients may induce chemical, mechanical and thermal stresses on SOFC materials and corresponding degradation.
Solid oxide regenerative fuel cells (SORFCs), which perform the dual functions of power generation and energy storage at high temperatures, could offer one of the most efficient and environmentally ...friendly options for future energy management systems. Although the functionality of SORFC electrodes could be significantly improved by reducing the feature size to the nanoscale, the practical use of nanomaterials has been limited in this area due to losses in stability and controllability with increasing temperature. Here, we demonstrate an advanced infiltration technique that allows nanoscale control of highly active and stable catalysts at elevated temperatures. Homogeneous precipitation in chemical solution, which is induced by urea decomposition, promotes crystallization behavior and regulates precursor redistribution, thus allowing the precise tailoring of the phase purity and geometric properties. Controlling the key characteristics of Sm0.5Sr0.5CoO3 (SSC) nanocatalysts yields an electrode that is very close to the ideal electrode structure identified by our modeling study herein. Consequently, outstanding performance and durability are demonstrated in both fuel cell and electrolysis modes. This work highlights a simple, cost-effective and reproducible way to implement thermally stable nanocomponents in SORFCs, and furthermore, it expands opportunities to effectively exploit nanotechnology in a wide range of high-temperature energy devices.
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•Advanced infiltration technique for solid oxide regenerative fuel cells is introduced.•Highly active and thermally stable nanocatalysts are produced via in operando synthesis.•Geometric properties and crystallization behavior are precisely regulated at high temperatures.•Nano-tailoring of catalysts remarkably improves the performance in both fuel cell and electrolysis modes.•Nanocatalysts do not degrade during long-term operation under harsh environments.
Polyaniline (PANI)/epoxy composites with different polyaniline (PANI) contents were successfully developed by in situ polymerization of aniline salt protonated with camphorsulfonic acid within epoxy ...matrices and fully characterized. The influence of PANI loading levels on various properties was also explored. Dielectric and electrical properties of PANI/epoxy composites were studied for samples in parallel plate configuration. A PANI/epoxy composite prepared in this fashion reached a high dielectric constant close to 3000, a dielectric loss tangent less than 0.5 at room temperature and 10
kHz. The hardener type was also found as a critical parameter for the dielectric properties of PANI/epoxy composites. The distribution of the conductive element clusters within the polymer matrix was studied by SEM and correlated to the dielectric behavior of the composite films.
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•Promising Ni-based binary pair candidates were rationally selected through the stability analyses of the alloys.•In M/Ni(111) surface, doping of a larger transition metal weakens the ...adsorption strength of atomic sulfur by changing the electronic structures.•H2S dissociation can be suppressed by replacing Ni with other transition metals, especially Au.
Sulfur compounds in fuels deactivate the surface of anode materials in solid oxide fuel cells (SOFCs), which adversely affect the long-term durability. To solve this issue, it is important to design new SOFC anode materials with high sulfur tolerance. Unfortunately, it is difficult to completely replace the traditional Ni anode owing to its outstanding reactivity with low cost. As an alternative, alloying Ni with transition metals is a practical strategy to enhance the sulfur resistance while taking advantage of Ni metal. Therefore, in this study, we examined the effects of transition metal (Cu, Rh, Pd, Ag, Pt, and Au) doping into a Ni catalyst on not only the adsorption of H2S, HS, S, and H but also H2S decomposition using density functional theory (DFT) calculations. The dopant metals were selected rationally by considering the stability of the Ni-based binary alloys. The interactions between sulfur atoms produced by H2S dissociation and the surface are weakened by the dopant metals at the topmost layer. In addition, the findings show that H2S dissociation can be suppressed by doping transition metals. It turns out that these effects are maximized in the Au-doped Ni catalyst. Our DFT results will provide useful insights into the design of sulfur-tolerant SOFC anode materials.
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