Nitrogen Doping in Oxygen-Deficient Ca2Fe2O5: A Strategy for Efficient Oxygen Reduction Oxide Catalysts

Electrocatalytic Properties of Oxygen-Deficient Perovskites Ca3Fe3-xMnxO8 (x = 1-2) for the Hydrogen Evolution Reaction

We have demonstrated a systematic trend in the electrocatalytic activity for the hydrogen evolution reaction (HER) and its correlations with transition-metal type, structural order, and electrical conductivity. The materials studied in this work, Ca3FeMn2O8 (CaFe1/3Mn2/3O3-1/3), Ca3Fe1.5Mn1.5O8, and Ca3Fe2MnO8, belong to the family of oxygen-deficient perovskites and show a gradual increase in the ordering of oxygen vacancies. Ca3FeMn2O8 (CaFe1/3Mn2/3O3-1/3) contains randomly distributed oxygen vacancies, which begin to order in Ca3Fe1.5Mn1.5O8, and are fully ordered in Ca3Fe2MnO8. The gradual increase in the structural order is associated with a systematic enhancement of the electrocatalytic activity for HER in acidic conditions, Ca3FeMn2O8 < Ca3Fe1.5Mn1.5O8 < Ca3Fe2MnO8. While the improvement of the HER activity is also associated with an increase in the Fe content, we have shown that the type of structural order plays a more important role. We demonstrated this effect by control experiments on an analogous material where all Mn was substituted by Fe, leading to a different type of structural order and showing an inferior HER activity compared to the above three materials. Furthermore, electrical conductivity studies in a wide range of temperatures, 25-800 °C, indicate that the trend in the electrical conductivity is the same as that of the HER activity. These findings reveal several important structure-property relationships and highlight the importance of synergistic effects in enhancing the electrocatalytic properties.

Enhanced photo-fenton and photoelectrochemical activities in nitrogen doped brownmillerite KBiFe2O5

Visible-light-driven photo-fenton-like catalytic activity and photoelectrochemical (PEC) performance of nitrogen-doped brownmillerite KBiFe₂O₅ (KBFO) are investigated. The effective optical bandgap of KBFO reduces from 1.67 to 1.60 eV post N-doping, enabling both enhancement of visible light absorption and photoactivity. The photo-fenton activity of KBFO and N-doped KBFO samples were analysed by degrading effluents like Methylene Blue (MB), Bisphenol-A (BPA) and antibiotics such as Norfloxacin (NOX) and Doxycycline (DOX). 20 mmol of Nitrogen-doped KBFO (20N-KBFO) exhibits enhanced catalytic activity while degrading MB. 20N-KBFO sample is further tested for degradation of Bisphenol-A and antibiotics in the presence of H₂O₂ and chelating agent L-cysteine. Under optimum conditions, MB, BPA, and NOX, and DOX are degraded by 99.5% (0.042 min^-1), 83% (0.016 min^-1), 72% (0.011 min^-1) and 95% (0.026 min^-1) of its initial concentration respectively. Photocurrent density of 20N-KBFO improves to 8.83 mA/cm² from 4.31 mA/cm² for pure KBFO. Photocatalytic and photoelectrochemical (PEC) properties of N-doped KBFO make it a promising candidate for energy and environmental applications.

Theoretical DFT Study on the Mechanisms of CO/CO2 Conversion in Chemical Looping Catalyzed by Calcium Ferrite

The CO/CO₂ conversion mechanism on the calcium ferrite (CFO) surface in chemical looping was explored by a computational study using the density functional theory approach. The CFO catalytic reaction pathway of 2CO + O₂ → 2CO₂ conversion has been elucidated. Our results show that the Fe center in CFO plays the key role as a catalyst in the CO/CO₂ conversion. Two energetically stable spin states of CFO, quintet and septet, serve as the effective catalysts. The presence of the triplet O₂ molecule caused the conversion of these two spin-state structures into each other along the catalytic reaction pathway. A double release of CO₂ was predicted following this reaction mechanism. The rate-determining step is the formation of the 2CO₂-CFO complex (P4) in the quintet state (19.0 kcal/mol). The predicted energy barriers for all the steps suggest that the proposed pathway is plausible.

Nitrogen Incorporated Photoactive Brownmillerite Ca2Fe2O5 for Energy and Environmental Applications

Ca₂Fe₂O₅ (CFO) is a potentially viable material for alternate energy applications. Incorporation of nitrogen in Ca₂Fe₂O₅ (CFO-N) lattice modifies the optical and electronic properties to its advantage. Here, the electronic band structures of CFO and CFO-N were probed using Ultraviolet photoelectron spectroscopy (UPS) and UV-Visible spectroscopy. The optical bandgap of CFO reduces from 2.21 eV to 2.07 eV on post N incorporation along with a clear shift in the valence band of CFO indicating the occupation of N 2p levels over O 2p in the valence band. Similar effect is also observed in the bandgap of CFO, which is tailored upto 1.43 eV by N⁺ ion implantation. The theoretical bandgaps of CFO and CFO-N were also determined by using the Density functional theory (DFT) calculations. The photoactivity of these CFO and CFO-N was explored by organic effluent degradation under sunlight. The feasibility of utilizing CFO and CFO-N samples for energy storage applications were also investigated through specific capacitance measurements. The specific capacitance of CFO is found to increase to 224.67 Fg⁻¹ upon N incorporation. CFO-N is thus found to exhibit superior optical, catalytic as well as supercapacitor properties over CFO expanding the scope of brownmillerites in energy and environmental applications.

Photoactive Brownmillerite Multiferroic KBiFe2O5 and Its Potential Application in Sunlight-Driven Photocatalysis

KBiFe₂O₅ (KBFO) is an upcoming promising brownmillerite-structured multiferroic photoactive material for next-generation photovoltaic and photocatalytic applications. In the present work, KBFO has been developed using multistep thermal treatment method to reduce the volatility of constituent elements and improve the stability of compound. The band gap of KBFO (found to be ∼1.68 eV) extends to the near-infrared region compared to traditional perovskite-structured multiferroics. The magnetic and dielectric transitions occur in the same temperature range (740 K-800 K), reflecting the existence of magneto-dielectric effect in the as-synthesized sample. It also shows promising photocatalytic activity by degrading organic effluents under natural sunlight compared to regular perovskite BiFeO₃ photocatalyst (operating under visible light). A new application of brownmillerite multiferroic KBFO photocatalyst in environmental and energy applications has been explored by integrating the structural, optical, magnetic, and dielectric properties of the same.

Cobalt-Doped Ba2In2O5 Brownmillerites: An Efficient Electrocatalyst for Oxygen Reduction in Alkaline Medium

A series of compounds with cobalt doping in the indium site of Ba₂In₂O₅ brownmillerites exhibited excellent oxygen reduction activity under alkaline conditions. Doping (25%) retains the brownmillerite structure with disorder in the O3 site in the two-dimensional alternate layer along the ab plane. Further substitution of cobalt in the indium site leads to the loss of a brownmillerite structure, and the compound attains a perovskite structure. Cobalt-doped samples exhibited far better oxygen reduction reaction (ORR) activity when compared to the parent Ba₂In₂O₅ brownmillerite. Among the series of compounds, BaIn_0.25Co_0.75O_3-δ with the highest Co doping and oxygen vacancies randomly distributed in the lattice exhibited the best ORR activity. BaIn_0.25Co_0.75O_3-δ showed a 40 mV positive shift in the onset potential with better limiting current density and a nearly four-electron-transfer reduction pathway when compared to the parent Ba₂In₂O₅ brownmillerite.

Nitrogen-Doped Perovskite as a Bifunctional Cathode Catalyst for Rechargeable Lithium-Oxygen Batteries

In this work, nitrogen-doped LaNiO₃ perovskite was prepared and studied, for the first time, as a bifunctional electrocatalyst for oxygen cathode in a rechargeable lithium-oxygen battery. N doping was found to significantly increase the Ni³⁺ contents and oxygen vacancies on the bulk surface of the perovskite, which helped to promote the oxygen reduction reaction and oxygen evolution reaction of the cathode and, therefore, enabled reversible Li₂O₂ formation and decomposition on the cathode surface. As a result, the oxygen cathodes loaded with N-doped LaNiO₃ catalyst showed an improved electrochemical performance in terms of discharge capacity and cycling stability to promise practical Li-O₂ batteries.

Unraveling the Role of Structural Order in the Transformation of Electrical Conductivity in Ca2FeCoO6-δ, CaSrFeCoO6-δ, and Sr2FeCoO6-δ

The ability to control the electrical conductivity of solid-state oxides using structural parameters has been demonstrated. A correlation has been established between the electrical conductivity and structural order in a series of oxygen-deficient perovskites using X-ray and neutron diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and electrical conductivity studies at a wide temperature range, 25-800 °C. The crystal structure of CaSrFeCoO_6-δ has been determined, and its stark contrast to Ca₂FeCoO_6-δ and Sr₂FeCoO_6-δ has been demonstrated. The Fe/Co distribution over tetrahedral and octahedral sites has been determined using neutron diffraction. There is a systematic increase in the structural order in progression from Sr₂FeCoO_6-δ (δ = 0.5) to CaSrFeCoO_6-δ (δ = 0.8) and Ca₂FeCoO_6-δ (δ = 0.9) . The oxygen contents of these materials were determined using iodometric titration and TGA. At room temperature, there is an inverse correlation between the electrical conductivity and structural order. The ordered Ca₂ and CaSr compounds are semiconductors, while the disordered Sr₂ compund shows metallic behavior. The metallic nature of the Sr₂ material persists up to 1073 K (800 °C), while the Ca₂ and CaSr compounds undergo a semiconductor-to-metal transition above 500 and 300 °C, respectively, highlighting another important impact of the structural order. At high temperature, the CaSr compound has the highest conductivity compared to the Ca₂ and Sr₂ materials. There appears to be an optimum degree of structural order that leads to the highest conductivity at high temperature. Another consequence of the structural order is the observation of mixed ionic-electronic conductivity in CaSr and Ca₂ compounds, as is evident from the hysteresis in the conductivity data obtained during heating and cooling cycles. The average ionic radius required for each structural transition was determined through the synthesis of 21 different materials by systematic variation of the Ca/Sr ratio. In addition, SEM and XPS were employed to gain insight into the crystallite morphology and oxidation states of transition metals, revealing an interesting redox process between Fe and Co.

Oxygen electrode reactions of doped BiFeO3 materials for low and elevated temperature fuel cell applications

Bi_0.6Ca_0.4FeO₃ demonstrates potential as an oxygen electrode material (for oxygen evolution and reduction reactions) for operation at room and elevated temperatures.