Vacancy Formation Energy Research Articles

Interstitial Oxygen Acts as Electronic Buffer Stabilizing High-Entropy Alloys for Trifunctional Electrocatalysis.

Understanding the effect of elements' oxygen affinity is essential for comprehending high-entropy alloys' (HEAs) complete properties. However, the origin of HEAs' oxygen-containing structure and stability remains poorly understood, primarily due to their diverse components, hindering synthesis and analysis. Herein, the O-doping HEAs (HEA-O) have demonstrated outstanding performance and stability in electrolyzed water and Zinc-air batteries which can be reassembled after being stable for more than 1600h when the zinc consumption is over. The experiment and DFT simulation demonstrate that Cr with strong oxygen affinity can introduce more oxygen into the system of HEAs. Consequently, interstitial oxygens act as electronic buffers making the binding energy of other metal elements move to a higher level. Additionally, O-doping lowers the d-band center promoting electrochemical activity and increasing vacancy formation energies of metal active sites leading to super stability. The study provides significant insights into the design and comprehension of interstitial oxygen-doped HEAs.

A Study on Oxygen Vacancy Resistance Mechanism of V2O5

Introduction: Due to its magnetic and semiconductor properties, V2O5 has shown tremendous potential in resistive switching memory. Method: Therefore, this paper investigates the resistive mechanism of oxygen vacancies in V2O5. The formation energies of different oxygen vacancies are calculated. Results: The results indicate that oxygen vacancies tend to form single-component conductive filaments. In mixed oxygen vacancies clusters, the charge transfer characteristics and density of states of the V2O5-VO 13 vacancies are the most significant, which is consistent with the analysis of formation energy data. Conclusions: Additionally, the charge transfer of cluster oxygen vacancies was calculated, showing that V atoms directly connected to oxygen vacancies tend to lose electrons, while adjacent oxygen atoms are more likely to gain electrons. In V2O5-VO 12 and V2O5-VO 13, the number of electrons obtained by O2 and O16 exceeds the average by 36.4% and 33.2%. Thus, the formation of oxygen vacancies effectively improves the resistance characteristics of the V2O5.

Effect of impurities on hydrogen defect stability and migration barrier in yttrium dihydride crystal

The impurity or alloying atoms in YH2 can alter the local electronic structure and so the hydrogen defect stability, as well as the H migration barrier energy. Thus, DFT calculations were employed to determine the effect of foreign elements from alkali and alkaline earth metals to transition metals and one critical impurity element, O, on H vacancy stability and retention characteristics in YH2. Results revealed that alloying elements act as hydrogen vacancy sinks by reducing the vacancy formation energy at neighboring sites. The implantation of non-magnetic foreign elements (s1, s2, and d10 valence electrons) in hydrogen energy landscape was calculated to be minor; while the hydrogen vacancy formation energy was reduced from 1.37 eV to 1.00 eV, the migration energy barrier of hydrogen was increased from 0.87 eV to 1.15 eV for non-magnetic foreign elements. The migration energy barrier monotonically decreased with increasing d-shell occupancy, reaching as low as 0.4 eV for Cr, Mo(d4), and Fe (d4). Alloying with late transition metals (d8 and d9) moderately impacted the hydrogen vacancy formation. Finally, it was found to be O addition into the YH2- lattice did not alter the energy landscape of hydrogen vacancies. Since alloyed YH2 has not been studied extensively, this study provides an atomistic understanding how alloying elements and impurities trap vacancies and affects hydrogen mobility YH2. Meanwhile, the main findings of this study may serve as guidelines for introducing alloying elements in ZrH2 as well.

Promoting Surface Reconstruction in Spinel Oxides via Tetrahedral-Octahedral Phase Boundary Construction for Efficient Oxygen Evolution.

Understanding the origin of surface reconstruction is crucial for developing highly efficient lattice oxygen oxidation mechanism (LOM) based spinel oxides. Traditionally, the reconstruction has been achieved through electrochemical procedures, such as cyclic voltammetry (CV), linear sweep voltammetry (LSV). In this work, we found that the surface reconstruction in LOM-based CoFe0.25Al1.75O4 catalyst was an irreversible oxygen redox chemical reaction. And a lower oxygen vacancy formation energy (EO-V) could benefit the combination of the activated lattice oxygen atoms with adsorbed water molecular. Motivated by this finding, a strategy of phase boundary construction from Co tetrahedral to octahedral was employed to decrease EO-V in CoFe0.25Al1.75O4. The results showed that as the Co octahedral occupancy ratio rose to 64 %, a 3.5 nm-thick reconstructed layer formed on the catalyst surface with a 158 mV decrease in overpotential. Further experiments indicated that the coexistence of tetrahedral-octahedral (O-T) phase would result in lattice mismatch, promoting non-bonding oxygen states and lowering EO-V. Then more active lattice oxygen combined with H2O molecules to generate hydroxide ions (OH-), followed by soluble cation leaching, which enhanced the reconstruction process. This work provided new insights into the relationship between the intrinsic structure of pre-catalysts and surface reconstruction in LOM-based spinel electrocatalysts.

Electronic Confinement-Restrained Anti-Site Defects in Sodium-Rich Phosphates Toward Multi-Electron Transfer and High Energy Efficiency.

Sodium (Na) super-ionic conductor structured Na3MnTi(PO4)3 (NMTP) cathodes have garnered interest owing to their cost-effectiveness and high operating voltages. However, the voltage hysteresis phenomenon triggered by anti-site defects ( -ASD), namely, the occupation of Mn2+ in the Na2 vacancies in NMTP, leads to sluggish diffusion kinetics and low energy efficiency. This study employs an innovative electronic confinement-restrained strategy to achieve the regulation of -ASD. Partial replacement of titanium (Ti) with electron-rich vanadium (V) favors strong electronic interactions with Mn2+, restraining Mn2+ migration. The results suggest that this strategy can significantly increase the vacancy formation energy and migration energy barrier of manganese (Mn), thus inhibiting -ASD formation. As proof of this concept, an Na-rich Na3.5MnTi0.5V0.5(PO4)3 (NMTVP) material is designed, wherein the electronic interaction enhanced the redox activity and achieved more Na+ storage under high-voltage. The NMTVP cathode delivered a reversible specific capacity of up to 182.7 mAh g-1 and output an excellent specific energy of 513.8Wh kg-1, corresponding to ≈3.2 electron transfer processes, wherein the energy efficiency increased by 35.5% at 30C. Through the confinement effect of electron interactions, this strategy provides novel perspectives for the exploitation and breakthrough of high-energy-density cathode materials in Na-ion batteries.

Experimental and DFT studies of mercury adsorption and oxidation on CuCo2O4(110) surface

Elemental mercury (Hg0) is a typical toxic pollutant in flue gas from coal-fired power plants. The specific role of oxygen transfer on Hg0 oxidation over CuCo2O4 oxygen carrier and the involved reaction mechanisms were systematically studied by experimental and theoretical methods. In this study, the CuCo2O4 oxygen carrier with spinel-structure was prepared with salt-assisted combustion method. To reduce the grain size and increase the specific surface area of the oxygen carrier, the optimal KCl: CuCo2O4 ratio of 1:1 was obtained. Oxygen uncoupling ability of CuCo2O4 oxygen carrier was very strong, and the oxygen-releasing performance was slightly decreased after ten cyclic processes. Simulation based on density functional theory (DFT) shows that CuCo2O4 has relative low oxygen vacancy formation energy and oxygen transport energy barrier, and the oxygen vacancy is the preferred O2 adsorption site. Hg0 removal experiments show that CuCo2O4 has a strong ability to remove Hg0. The proportion of Hg2+ increases with the temperature, and the proportion of HgP decreases accordingly. Simulation shows that the 3-Co site on the surface of the CuCo2O4 oxygen carrier is the optimum Hg0 adsorption site. CuCo2O4 is found as a strong oxygen-releasing oxygen carrier, which can effectively promote the oxidation of Hg0.

In Situ Unraveling Surface Reconstruction of Ni‐CoP Nanowire for Excellent Alkaline Water Electrolysis

The surface reconstruction behavior of transition metal phosphides precursors is considered as an important method to prepare efficient oxygen evolution catalysts, but there are still significant challenges in guiding catalyst design at the atomic scale. Here, the CoP nanowire with excellent water splitting performance and stability is used as a catalytic model to study the reconstruction process. Obvious double redox signals and valence evolution behavior of the Co site are observed, corresponding to Co2+/Co3+ and Co3+/Co4+ caused by auto‐oxidation process. Importantly, the in situ Raman spectrum exhibits the vibration signal of Co–OH in the non‐Faradaic potential interval for oxygen evolution reaction, which is considered the initial step in reconstruction process. Density functional theory and ab initio molecular dynamics are used to elucidate this process at the atomic scale: First, OH− exhibits a lower adsorption energy barrier and proton desorption energy barrier at the configuration surface, which proposes the formation of a single oxygen (–O) group. Under a higher –O group coverage, the Co–P bond is destroyed along with the POx groups. Subsequently, lower P vacancy formation energy confirm that the Ni‐CoP configuration can fast transform into a highly active phase. Based on the optimized reconstruction behavior and rate‐limiting barrier, the Ni‐CoP nanowire exhibit an excellent overpotential of 1.59 V at 10 mA cm−2 for overall water splitting, which demonstrates low degradation (2.62%) during the 100 mA cm−2 for 100 h. This work provide systematic insights into the atomic‐level reconstruction mechanism of transition metal phosphides, which benefit further design of water splitting catalysts.

Phonon spectroscopy and features of low-temperature heat capacity of solid solutions of electrolytes

The kinetic characteristics of thermal frequency phonons in the region of helium temperatures in ceramic samples of the Ce1–xGdxO2–y electrolyte solid solution have been studied. To explain the temperature dependence of the phonon mean free path, we used the previously performed calculations of the energy of vacancy formation in the anion sublattice of a solid solution of zirconium dioxide stabilized by yttrium ZrO2:Y2O3 (YSZ) with a similar crystal structure. It is shown that in the Ce1–xGdxO2–y system under study, the formation of structural defects associated with the presence of vacancies in the anion sublattice with energy Δ = 8.53 K is possible. It has been established that analysis of the temperature dependences of the YSZ heat capacity allows one to trace the degree of disorder (amorphization) of the solid solution depending on its level of stabilization.

Using Post‐Treatment Additives for Crystal Modulation and Interface Passivation Enables the Fabrication of Efficient and Stable Perovskite Solar Cells in Air

AbstractHigh‐performance perovskite solar cells (PSCs) fabricated in ambient air are considered inevitable for low‐cost commercial manufacturing. However, passivating perovskite film defects and controlling the crystallization process are critical for achieving high performance in PSCs. This study proposes using the novel 2D material MBene in green antisolvent to simultaneously modulate the crystallization and passivation defects of perovskites. MBene facilitates the passivation of uncoordinated Pb2+ ions, thereby enhancing the formation energy of vacancies within the perovskite film and adjusting the energy level structure. Moreover, MBene increases nucleation sites for perovskite, significantly extending crystal growth and improving film crystallinity, thereby reducing non‐radiative recombination. Consequently, champion devices treated with MBene achieve a power conversion efficiency (PCE) of 24.22% when fabricated in air, and exhibit superior humidity and long‐term stability. Furthermore, PSCs with added MBene exhibit significant long‐term stability under various environmental conditions, including humidity and heat. The study results lay a foundation for the development of MBene materials in photovoltaics, revealing their mechanism as a new type of 2D material in perovskites, and providing new insights for industrially producing efficient and stable solar cells.

Mild and rapid construction of Ti electrodes for efficient and corrosion-resistant oxidative catalysis at industrial-grade intensity

The development of cost-effective and corrosion-resistant catalytic electrodes for chlorine/oxygen evolution reaction (CER/OER) in large-scale industrial applications is a significant challenge. Herein, the sol–gel method is employed to achieve a uniform coating of ruthenium (Ru) doping copper (Cu) on titanium sheet (Ru + 20 %Cu@Ti), and the highly efficient industrial grade stable Ti dimensional stable anode can be quickly constructed at 723.15 K for 2 h. Cu doping reduces the vacancy formation energy of surface oxygen, promotes additional lattice oxygen vacancy assisted hydrolysis dissociation pathway, improves the selectivity and specific activity of CER at high concentration doping, and reduces the binding energy of OER intermediates (e.g., *OH, *O, and *OOH) at adjacent Ru active sites. The overpotentials require to reach the current density of 10 mA cm−2 for CER and OER were only 365 mV and 232 mV at the conditions of 5.0 M NaCl (pH = 7.0) and 1.0 M KOH + 0.5 M NaCl. More importantly, Ru + 20 %Cu@Ti demonstrates excellent stability, operates continuously for over 340h at industrial current density in neutral and alkaline electrolytes, and its strengthening life reaches 64 h, with ultra-low performance attenuation. Impressively, the designed applied electrode (8.0 cm ✕ 15.0 cm) achieves long-term CER at 0.2–0.3 A cm−2. Further industrial grade evaluation of CER shows that its chlorine extraction polarizability, enhances life and weight loss meet the requirements of industrial applications.

Atomic-scale mechanistic study of oxygen reduction mechanism for B-site doped Pr(Ba,Sr)Co2O5+δ by density functional theory calculations

Pr(Ba,Sr)Co2O5+δ is a promising cathode material for solid oxide fuel cell (SOFC) due to its high oxygen ion transport capability and oxygen reduction activity. Density functional theory (DFT) calculations were performed to elucidate the oxygen reduction mechanism of B-site doped Pr(Ba,Sr)(Co,M)2O5+δ (M = Fe, Ni, Cu, and Zn) materials. First, we investigated the formation of O vacancies. The results indicate that Cu and Zn doping facilitate O vacancy formation, resulting in lower O vacancy formation energies. Furthermore, the interaction between oxygen and the (0 0 1) surfaces of B-site doped Pr(Ba,Sr)(Co,M)2O5+δ has been comprehensively discussed, involving both perfect and defective surfaces. The results demonstrate that the presence of O vacancies enhances the catalytic activity for oxygen reduction, by reducing the energy required for O2 dissociation. Zn-doped Pr(Ba,Sr)(Co,M)2O5+δ exhibits a low O vacancy formation energy, resulting in a stable adsorption configuration upon oxygen dissociation, indicating its potential as a cathode material for SOFC.

The influence of different vacancies on Zr(0001)/SiC close-packed interface performance: a first-principles study

The first-principles were employed to investigate the structure, adhesion, and tensile properties of the Zr(0001)/SiC close-packed interface with different vacancies. From the perspective of vacancy formation energy, SiC coating is beneficial for enhancing the irradiation resistance of Zr cladding. When vacancies are present, except for the Zr2 and Zr3 vacancies, introducing other vacancies reduces the stability of Zr/SiC interfaces. The C-terminated interface is more stable than the Si-terminated interface. Through electronic structure analysis, vacancies at the interface primarily reduce the bonds between Zr and SiC, decreasing the interface stability. Vacancies on the side of SiC indirectly alter the strength or quantity of covalent (bonding or anti-bonding) and ionic bonds at the interface, thus intricately lowering the interface stability. In tensile tests, the cleavage of all interfaces with vacancies still occurs on the side of Zr. Vacancies on the SiC side partly lead to increased electrons between Zr1-Zr2 or Zr2-Zr3, strengthening the metallic bonds and enhancing the interface's ideal strength and ductility. The present study offers a novel perspective from the standpoint of bonding mechanisms, providing good insights into the effects of different vacancies on the performance of Zr/SiC interfaces.

Hydrogen-Induced Topotactic Phase Transformations of Cobaltite Thin Films.

Manipulating physical properties through ion migration in complex oxide thin films is an emerging research direction to achieve tunable materials for advanced applications. While the reduction of complex oxides has been widely reported, few reports exist on the modulation of physical properties through a direct hydrogenation process. Here, we report an unusual mechanism for hydrogen-induced topotactic phase transitions in perovskite La0.7Sr0.3CoO3 thin films. Hydrogenation is performed upon annealing in a pure hydrogen gas environment, offering a direct understanding of the role that hydrogen plays at the atomic scale in these transitions. Topotactic phase transformations from the perovskite (P) to hydrogenated-brownmillerite (H-BM) phase can be induced at temperatures as low as 220 °C, while at higher hydrogenation temperatures (320-400 °C), the progression toward more reduced phases is hindered. Density functional theory calculations suggest that hydroxyl bonds are formed with the introduction of hydrogen ions, which lower the formation energy of oxygen vacancies of the neighboring oxygen, enabling the transition from the P to H-BM phase at low temperatures. Furthermore, the impact on the magnetic and electronic properties of the hydrogenation temperature is investigated. Our research provides a potential pathway for utilizing hydrogen as a basis for low-temperature modulation of complex oxide thin films, with potential applications in neuromorphic computing.

Designing water resistant high entropy oxide materials

The ubiquitous presence of moisture usually shows adverse effects on industrial catalysis. Herein, a concept of engineering entropy to design water-resistant oxide catalysts is proposed. The C3H6 oxidation by spinel ACr2O4 (A=Ni, Mg, Cu, Zn, Co) catalysts is selected as a model. Through DFT calculation, the adsorption energy of C3H6, the dissociation energy of molecular H2O on the oxide surface, and the formation energy of oxygen vacancy all suggest better performance induced by higher configurational entropy. Indeed, (Ni0.2Mg0.2Cu0.2Zn0.2Co0.2)Cr2O4 experimentally show excellent water resistance (>100 h) in C3H6 oxidation, while in sharp contrast binary oxides (e.g., NiCr2O4, CoCr2O4) are deactivated in 20 h. H2O-TPD, in-situ Raman, and in-situ FTIR all confirm the low H2O adsorption energy and strong hydrothermal stability of high entropy oxide, which is attributed to their lower Gibbs free energy. This work may inspire the rational design of water-resistant catalysts.

Defects-Rich Heterostructures Trigger Strong Polarization Coupling in Sulfides/Carbon Composites with Robust Electromagnetic Wave Absorption

Defects-rich heterointerfaces integrated with adjustable crystalline phases and atom vacancies, as well as veiled dielectric-responsive character, are instrumental in electromagnetic dissipation. Conventional methods, however, constrain their delicate constructions. Herein, an innovative alternative is proposed: carrageenan-assistant cations-regulated (CACR) strategy, which induces a series of sulfides nanoparticles rooted in situ on the surface of carbon matrix. This unique configuration originates from strategic vacancy formation energy of sulfides and strong sulfides-carbon support interaction, benefiting the delicate construction of defects-rich heterostructures in MxSy/carbon composites (M-CAs). Impressively, these generated sulfur vacancies are firstly found to strengthen electron accumulation/consumption ability at heterointerfaces and, simultaneously, induct local asymmetry of electronic structure to evoke large dipole moment, ultimately leading to polarization coupling, i.e., defect-type interfacial polarization. Such “Janus effect” (Janus effect means versatility, as in the Greek two-headed Janus) of interfacial sulfur vacancies is intuitively confirmed by both theoretical and experimental investigations for the first time. Consequently, the sulfur vacancies-rich heterostructured Co/Ni-CAs displays broad absorption bandwidth of 6.76 GHz at only 1.8 mm, compared to sulfur vacancies-free CAs without any dielectric response. Harnessing defects-rich heterostructures, this one-pot CACR strategy may steer the design and development of advanced nanomaterials, boosting functionality across diverse application domains beyond electromagnetic response.

Thermodynamic and Kinetic Properties of the Lithium-Silver System.

A carbon-silver anode has recently been shown to suppress dendrite formation in all-solid-state lithium-ion batteries. The role that silver plays in enabling the reversible deposition and stripping of lithium remains unknown. Furthermore, very little is known about the thermodynamic and kinetic properties of Li x Ag1-x alloys. Here, we report on an in-depth first-principles study of phase stability and diffusion mechanisms in the Li-Ag alloy system. We identify two new intermetallic phases that are predicted to be stable in Li-rich Li x Ag1-x alloys with stoichiometries of Li3Ag and Li11Ag2. Our calculations show that the peculiar and highly anharmonic energy surface of pure Li along the Bain and Burgers paths persists upon the addition of Ag to BCC Li. This has important implications for room-temperature phase stability and mechanical properties. We have also performed a systematic study of diffusion mechanisms in the Li x Ag1-x alloy system as a function of alloy concentration x. Diffusion in alloys and intermetallics is mediated by vacancies. High vacancy formation energies are predicted in the Li x Ag1-x alloy, especially in Ag-rich FCC solid solutions. Complex diffusion mechanisms are identified in the B2 and γ-brass intermetallic phases that include two-atom hops and second-nearest neighbor hops. The migration barriers are found to decrease with increasing Li concentration, with predictions of exceptionally low migration barriers of 0.1 eV in the D03 Li3Ag phase.

Inhibiting the Appearance of Green Emission in Mixed Lead Halide Perovskite Nanocrystals for Pure Red Emission.

Mixed halide perovskites exhibit promising optoelectronic properties for next-generation light-emitting diodes due to their tunable emission wavelength that covers the entire visible light spectrum. However, these materials suffer from severe phase segregation under continuous illumination, making long-term stability for pure red emission a significant challenge. In this study, we present a comprehensive analysis of the role of halide oxidation in unbalanced ion migration (I/Br) within CsPbI2Br nanocrystals and thin films. We also introduce a new approach using cyclic olefin copolymer (COC) to encapsulate CsPbI2Br perovskite nanocrystals (PNCs), effectively suppressing ion migration by increasing the corresponding activation energy. Compared with that of unencapsulated samples, we observe a substantial reduction in phase separation under intense illumination in PNCs with a COC coating. Our findings show that COC enhances phase stability by passivating uncoordinated surface defects (Pb2+ and I-), increasing the formation energy of halide vacancies, improving the charge carrier lifetime, and reducing the nonradiative recombination density.

Transition-Metal-Doped CeO2 for the Reverse Water-Gas Shift Reaction: An Experimental and Theoretical Study on CO2 Adsorption and Surface Vacancy Effects.

Transition metal-doped ceria (M-CeO2) catalysts (M = Fe, Co, Ni and Cu) with multiple loadings were experimentally investigated for reverse water gas shift (RWGS) reaction. Density functional theory (DFT) calculations were performed to benchmark the properties that impact catalytic activity of CO2 reduction. Temperature-programmed desorption (TPD) was conducted to study the CO2 binding strength on doped CeO2 surfaces; the trend of the energy along increasing metal loading agrees with the DFT calculations. Notably, CO2 dissociative adsorption energy and oxygen vacancy (OV) formation energy are key descriptors obtained from both DFT and experiments, which can be used to evaluate catalytic performance. Results show the effectiveness of transition metal doping in enhancing CO2 adsorption and reducibility of the surfaces, with Fe showing particularly promising results, i.e., CO2 conversion higher than 56% at 600 °C and 100% selectivity to CO. Cu exhibits 100% selectivity to CO but low CO2 conversion, while Co and Ni showed notable ability of methanation, particularly at high loadings. This study finds that an effective CeO2 based RWGS catalyst corresponds to OV sites that have low OV formation energies for surface reduction, and moderate CO2 adsorption energies for strong interaction with the surface to promote C-O bond scission.

Effect of Cu doping on morphology and properties of calcium ferrite and its application as oxygen carrier in chemical looping hydrogen production

Chemical looping hydrogen production with inherent CO2 capture has been widely recognized as a clean and efficient approach to high-purity hydrogen production because of the ultra-pure H2 product without any purification facilities. It is crucial to develop oxygen carriers with better performance to improve fuel conversion and hydrogen production efficiency. The potential of Ca2Fe2O5 in steam converting has been confirmed. However, its lower oxygen transfer capacity, that is, it tends to produce lower fuel conversion in fuel reactors, which will limit its practical application. Here, we report the development of Cu-doped Ca2Fe2O5-based oxygen carriers using sol-gel technology. The effects of B-site substitution of Cu element on the morphological properties and redox properties of Ca2Fe2-xCuxO5 (x = 0, 0.1, 0.25, 0.5, 1) oxygen carriers were evaluated based on experiments and density functional theory calculations. The results show that Cu doping not only elevated the surface oxygen content and enhanced the oxygen activity of the oxygen carriers, but also increased the Fe3+ at the B-site, thus enhanced their binding ability with oxygen molecules. Vacancy formation was a rate-determining step in the chemical looping hydrogen production (CLH), and Cu-doped Ca2Fe2O5 reduced the energy of oxygen vacancy formation. In the CLH process, the doping of Cu significantly improved the hydrogen productivity and fuel conversion rate. The fuel conversion rate was positively correlated with the doping amount of Cu. When x = 1, Ca2Fe2-xCuxO5 had the maximum fuel conversion rate, and its average conversion was 54.2% more than that of undoped Ca2Fe2O5. Ca2Fe2-xCuxO5 with x = 0.25 was the most suitable for CLH with the highest H2 yield, which was 20.3% more than that of Ca2Fe2O5. Moreover, its properties remained stable over multiple redox cycles with high activity and stability for CO-CLH.

Oxygen vacancy modulation for enhanced hydrogen production via chemical looping water-gas shift

Chemical looping water gas shift (CL-WGS)is prospective to generate high-purity hydrogen with integrated CO2 capture. However, this technology is impeded by the lack of active oxygen carriers at mid-temperatures. Here, we synthesized several Ni-doped CoFe2O4-δ to modulate oxygen vacancies and investigate its effect on promoting hydrogen production reaction via chemical looping water gas shift at 650 °C. The findings delineate that doping Ni considerably lowers the energy barriers associated with the oxygen vacancies formation, thereby augmenting their concentration. The underlying mechanism elucidates that within the CL-WGS process, the transfer of lattice oxygen acts as the rate-limiting step. NixCo1-xFe2O4 lowers the formation energy of oxygen vacancies and facilitates the bulk lattice oxygen diffusion through the bulk. Hence, Ni0.5Co0.5Fe2O4 demonstrates the most reduction depth and reversibility via redox reactions, resulting in an elevated hydrogen yield (∼15.5 mmol g−1) at 650 °C, which surpasses the yield from undoped CoFe2O4 by 1.4 times. This performance remains consistently high with only a minimal decline over 100 cycles. The findings introduce a promising approach to promote the reactivity of oxygen carriers, particularly for mid-temperature applications.