Synthesis of Ammonia Directly from Air and Water via a Single-Chamber Reactor Using Lanthanum Chromite-Based Composite as an Electrocatalyst

Carbon-free electrosynthesis of ammonia using water (H2 source) and air (N2 source) is promising technology to reduce the global CO2 emission resulting from the industrial ammonia production process (Haber-Bosch). In this study, electrocatalysis activity of non-noble metal perovskite-based catalyst (La0.75Sr0.25Cr0.5Fe0.5O3-δ-Ce0.8Gd0.18Ca0.02O2-δ, LSCrF-CGDC) for ammonia synthesis directly from air and water was explored. Ammonia was successfully from wet air (3%H2O) synthesized in a single-chamber type reactor. The highest ammonia formation rate and Faradaic efficiency of about 1.94×10-11 mol s-1 cm-2 and 2.01% were achieved at 375 oC and 1.2 V, respectively. The observed ammonia formation rate is higher than reported for an expensive noble metal-based catalyst (Ru/MgO). The obtained results indicated that the direct synthesis of ammonia from air and water is a promising technology for green and sustainable ammonia synthesis.

Perovskite Chromite With In-Situ Assembled Ni-Co Nano-Alloys: A Potential Bifunctional Electrode Catalyst for Solid Oxide Cells

Solid oxide fuel cell (SOFC) is an advanced electricity generation device with attractive fuel flexibility and conversion efficiency. As its reversed process, solid oxide electrolysis cell (SOEC) can efficiently electrolyze notorious CO2 to valuable chemical product such as CO, by utilizing renewable energy. To achieve long-term operation, the development of catalytically active electrode materials in both SOFC/SOEC modes is highly desirable, yet still challenging. In this research, an A-site deficient perovskite oxide (lanthanum chromite) decorated with in-situ exsolved Ni-Co nano-alloy has been fabricated and applied as a potential fuel electrode for both SOFC/SOEC. The influences of A-site non-stoichiometry and B-site dopant concentration on structural properties and in-situ exsolution process have been elaborately studied from various aspects. Diverse characterizations collectively confirm that the existence of A-site deficiency helps the formation of oxygen vacancies and stimulates the exsolution of B-site cations. In addition, the synergistic effect between the dopants of Co and Ni manipulates the reducibility and promotes carbon deposition resistance of the material. The electrolyte-supported SOFC with self-assembled Ni-Co nano-alloy electrode has shown maximum power densities of 329 mW/cm2 (in H2) and 258 mW/cm2 (in syngas, H2 + CO) at 850 °C, which are 50% better than those of the fuel cell with the exsolved Ni nanoparticles only. Also, the nano-alloy decorated electrode catalyst promotes a 30% increase in SOEC performance for CO2 electrolysis with prominently enhanced resistance against carbon deposition, suggesting the versatile functionality of the materials.

A reappraisal of peridotite solidus phase equilibria from 6 to 14 GPa in the system CaO-MgO-Al2O3-SiO2

Experimentally determined isobaric invariant melting phase relations from 6 to 14 GPa in the system CaO-MgO-Al2O3-SiO2 (CMAS), involving the crystalline phases, forsterite + orthopyroxene + clinopyroxene + garnet, and liquid, are reported. Experiments were conducted using a multianvil device with stepped lanthanum chromite heaters in the pressure cells. At a fixed pressure, the five-phase assemblage identified above can exist only at a single temperature. As such, these isobaric invariant points correspond to the solidus of model garnet peridotite in this part of the composition space in the studied system, as is the case at lower pressures in some previous studies. The solidus of model peridotite is univariant in pressure-temperature space, has a positive Clapeyron slope, and the isobaric invariant solidus temperatures, at, 6, 8, 10, 12, and 14 GPa, are, 1965oC, 2090oC, 2200oC, 2280oC, and 2320oC, respectively. Over the investigated pressure range, orthopyroxene is in reaction relation with the liquid, with the fusion reaction taking the form, forsterite + clinopyroxene + garnet = orthopyroxene + liquid. The compositions of liquids in the experiments reported here do not seem to depend on orthopyroxene being present in the experiments. Compositionally, liquids here are quite magnesian and siliceous, and have lower alumina and lime concentrations than at lower pressures with the identical crystalline phase assemblage in the system CMAS. In contrast to some previous studies, in this work, there is evidence neither of maximum and minimum normative forsterite concentration of the isobaric invariant liquid at around 8 GPa and 12 GPa, respectively, nor of a substantial curvature in the track of liquid compositions, when such liquids coexist with the mentioned four-phase crystalline phase assemblage. Instead, here, with increasing pressure from 6 to 14 GPa, liquids at the isobaric invariant points (defining the univariant solidus) become progressively (quasi-linearly) enstatite-normative. This experimental observation on liquid compositions from the present study might be important for future work directed at attempting to investigate chemistry of liquids derived from partial fusion of anhydrous peridotite at pressures, and corresponding depths in Earth, greater than investigated here.

Electrocatalytic Activity of Lanthanum Chromite-Based Composite Cathode for Ammonia Synthesis from Water and Nitrogen

The electrocatalytic ammonia synthesis using water (along with nitrogen) as a hydrogen source is proposed as an alternative green and clean technology to the energy-intensive and CO2-emitting process (Haber-Bosch) for ammonia production. Besides, a selective electrocatalyst for ammonia synthesis versus the competing hydrogen evolution remains elusive. This study aims to investigate the electrocatalytic activity of non-noble metal Co and Fe-free perovskite oxide-based composite cathode (La0.75Sr0.25Cr0.5Mn0.5O3-δ-Ce0.8Gd0.18Ca0.02O2-δ) towards ammonia synthesis from H2O and N2. The electrocatalyst was synthesized via a sol-gel process and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Ammonia was successfully with a maximum formation rate of 2.5 × 10-10 mol s-1 cm-2 and Faradaic efficiency of 0.52% at 400 oC and applied voltage of 1.4 V. The results demonstrated that the proposed non-noble metal-based electrocatalyst is a promising material for the carbon-free ammonia synthesis process.