A solid oxide fuel cell (SOFC) plays a significant role in converting chemically stored energy to electrical energy by using clean and renewable fuels, such as H2 and CO. The LSFCr (La0.3Sr0.7Fe0.7Cr0.3O3) perovskite is one of the few materials that is especially efficient and stable as a reversible SOFC (RSOFC) by performing not only the direct fuel cell reaction to generate power but also the CO2 conversion back to CO. Many surface chemical reactions were studied for different perovskites, but the CO2 reduction to CO at gaseous conditions has been reported for only a few materials. Unfortunately, for the LSFCr perovskite the precise atomic structures during mechanism of CO2 electrolysis is unknown. This study identifies among many adsorption modes for CO2 on the LSFCr surface the preferred active site and a suggested mechanism for the reaction using density functional theory (DFT) with nudged elastic band (NEB) tools. Surprisingly, the mechanism involves a stable, linear O–C–O angle during adsorption of CO2 and bending of the angle is achieved only during the transition state. The results demonstrate the importance of oxygen vacancies in the catalytic process, as well as the importance of a Cr dopant in the reduction despite the direct bonding of CO2 to Fe atom. Our results on the necessity of a particular oxygen vacancy concentration for the chemical reaction is supported by our thermogravimetric analysis (TGA) measurement.

We experimentally investigate the mechanism of formation of self-assembled arrays of nanoislands surrounding dopant sources on the (001) surface of yttria-stabilized zirconia. Initially, we used lithographically defined thin-film patches of gadolinia-doped ceria (GDC) as dopant sources. During annealing at approximately one-half the melting temperature of zirconia, surface diffusion of dopants leads to the breakup of the surface around the source, creating arrays of epitaxial nanoislands with a characteristic size (~100 nm) and alignment along elastically compliant directions, <110>. The breakup relieves elastic strain energy at the expense of increasing surface energy. On the basis of understanding the mechanism of island formation, we introduce a simple and versatile powder-based doping process for spontaneous surface patterning. The new process bypasses lithography and conventional vapor-source doping, opening the door to spontaneous surface patterning of functional ceramics and other refractory materials. In addition to using GDC solid-solution powders, we demonstrate the effectiveness of the process in another system based on Eu2O3.

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The growth and phase evolution characteristics of exsolved metal nanoparticles (NPs) in a Ni-doped La0.3Ca0.70Fe0.7Cr0.3O3-δ (LCFCrN) perovskite is investigated in H2-N2 and CO-CO2 environments. Exsolution kinetics are rapid in H2-N2...

A detailed study aimed at understanding and confirming the reported highly promising performance of a La 0.3 Sr 0.7 Fe 0.7 Cr 0.3 O 3−δ (LSFCr) perovskite catalyst in CO 2 /CO mixtures, for use in reversible solid oxide fuel cells (RSOFCs), is reported in this work, with an emphasis on chemical and performance stability. This work includes an X-ray diffraction (XRD), thermogravimetric analysis (TGA), and electrochemical study in a range of pO 2 atmospheres (pure CO 2 , CO alone (balance N 2 ), and a 90−70% CO 2 /10−30% CO containing mixture), related to the different conditions that could be encountered during CO 2 reduction at the cathode. Powdered LSFCr remains structurally stable in 20−100% CO 2 (balance N 2 , pO 2 = 10 −11 −10 −12 atm) without any decomposition. However, in 30% CO (balance N 2 , pO 2 ∼ 10 −26 atm), a Ruddlesden−Popper phase, Fe nanoparticles, and potentially some coke are observed to form at 800 °C. However, this can be reversed and the original perovskite can be recovered by heat treatment in air at 800 °C. While no evidence for coke formation is obtained in 90−70% CO 2 /10−30% CO (pO 2 = 10 −17 −10 −18 atm) mixtures at 800 °C, in 70 CO 2 /30 CO, minor impurities of SrCO 3 and Fe nanoparticles were observed, with the latter potentially beneficial to the electrochemical activity of the perovskite. Consistent with prior work, symmetrical two-electrode full cells (LSFCr used at both electrodes), fed with the various CO 2 /CO gas mixtures at one electrode and air at the other, showed excellent electrochemical performance at 800 °C, both in the SOFC and in SOEC modes. Also, LSFCr exhibits excellent stability during CO 2 electrolysis in medium-term potentiostatic tests in all gas mixtures, indicative of its excellent promise as an electrode material for use in symmetrical solid oxide cells.

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Spontaneous nanoripple formation on (110) surfaces of yttria-stabilized zirconia, YSZ-(110), is achieved by diffusional surface doping with rare-earth oxides. Periodic arrays of parallel nanobars separated by channels (period ∼100 nm) grow out of the dopant sources, covering relatively wide areas of the surface (∼10 μm). The nanobars mound up on the surface by diffusion, exhibiting morphological uniformity and alignment, with their long axis lying parallel to the [11̅0] direction in the YSZ-(110) surface. The process for forming these nanobar arrays can be as simple as sprinkling of rare-earth oxide powder (dopant source) on YSZ-(110) surface and annealing in a high temperature air furnace. However, higher control on dopant dispersion on the surface is demonstrated with other techniques, including photolithography and inkjet printing. The ripple arrays extend anisotropically on the (110) surface, obeying the parabolic growth law, and showing principal values of the rate constant along [11̅0] (maximum) and [001] (minimum), as expected from the symmetry of the (110) surface. The self-patterned ceramic substrates are well-suited for pattern transfer by replica molding, as illustrated by single-step molding with polydimethylsiloxane (PDMS), which is a widely used biomaterial in cell-culture studies.