Rutile (β-)MnO2 Surfaces and Vacancy Formation for High Electrochemical and Catalytic Performance

Rational Design of Nanosheet Array-Like Layered-Double-Hydroxide-Derived NiCo2O4 In Situ Grown on Reduced-Graphene-Oxide-Coated Nickel Foam for High-Performance Solid-State Supercapacitors

The investigation of high-performance supercapacitors is essential for accelerating the development of energy storage devices. In this work, a 3D hierarchical nanosheet array-like nickel cobaltite/reduced graphene oxide/nickel foam composite (NiCo2O4/rGO/NF) was assembled via an aqueous coprecipitation-hydrothermal strategy assisted by citric acid. Benefiting from a NiCo layered-double-hydroxide precursor with an atomic-level lattice confinement effect of metal ions and effective hybridization with rGO, the NiCo2O4/rGO/NF composite is featured as thin NiCo2O4 nanosheets (∼113.6 nm × 11.2 nm) composed of NiCo2O4 nanoparticles (∼10.9 nm) vertically staggered on the surface of a rGO-modified NF skeleton, leading to high surface area, abundant mesoporous structure, and active site exposure. The as-obtained NiCo2O4/rGO/NF was directly used as a binder-free integrated electrode for supercapacitors, achieving an excellent specific capacitance of 2863.4 F g-1 (1503.3 C g-1) at 1 A g-1, a superior rate performance of 2335.2 F g-1 at 20 A g-1, and a stability retention of 91.7% after 5000 cycles. More impressively, a solid-state asymmetric supercapacitor assembled by the present NiCo2O4/rGO/NF integrated electrode as the positive electrode and commercial activated carbon as the negative electrode achieved a high energy density of 69.2 Wh kg-1 at a power density of 800 W kg-1, and the energy density at a peak power density of 20004 W kg-1 still remained at 48.9 Wh kg-1, also showing a good cycling stability of 87.2% retention over 10000 cycles. The present facile synthesis strategy of the as-obtained NiCo2O4/rGO/NF nanosheet array composite can be used for the design and construction of many other transition-metal oxide/graphene/NF composite materials with excellent structural stability and performance in energy storage and other related areas.

Zn-ion Batteries: Charge Storing Mechanism and Development Challenges

Improving the energy share of renewable energy technologies is the only solution to reduce greenhouse gas emissions and air pollution. The high-performing green battery energy storage technologies are critical for storing energy to address the intermittent nature of renewable energy resources. In recent years, aqueous batteries, particularly Zn-ion batteries (ZIBs), have achieved and shown great potential for stationary energy storage systems owing to their low cost and safer operation. However, the practical applications of the ZIBs have significantly been impeded due to the gap between the breakthroughs achieved in academic research and industrial developments. The present review discusses the ZIB's advantages, possibilities, and shortcomings for stationary energy storage systems. The Review begins with a brief introduction to the ZIBs and their charge storage mechanisms based on the structural properties of cathode materials. The scientific and technical challenges that obstruct the commercialization of the ZIBs are discussed in detail concerning their impact on accelerating the utilization of the ZIBs for real-life applications. The final section highlights the outlook on research in this flourishing field.

Amorphous Heterostructure Derived from Divalent Manganese Borate for Ultrastable and Ultrafast Aqueous Zinc Ion Storage

Aqueous zinc-manganese (Zn-Mn) batteries have promising potential in large-scale energy storage applications since they are highly safe, environment-friendly, and low-cost. However, the practicality of Mn-based materials is plagued by their structural collapse and uncertain energy storage mechanism upon cycling. Herein, this work designs an amorphous manganese borate (a-MnBO_x ) material via disordered coordination to alleviate the above issues and improve the electrochemical performance of Zn-Mn batteries. The unique physicochemical characteristic of a-MnBO_x enables the inner a-MnBO_x to serve as a robust framework in the initial energy storage process. Additionally, the amorphous manganese dioxide, amorphous Zn_x MnO(OH)₂ , and Zn₄ SO₄ (OH)₆ ·4H₂ O active components form on the surface of a-MnBO_x during the charge/discharge process. The detailed in situ/ex situ characterization demonstrates that the heterostructure of the inner a-MnBO_x and surface multicomponent phases endows two energy storage modes (Zn²⁺ /H⁺ intercalation/deintercalation process and reversible conversion mechanism between the Zn_x MnO(OH)₂ and Zn₄ SO₄ (OH)₆ ·4H₂ O) phases). Therefore, the obtained Zn//a-MnBO_x battery exhibits a high specific capacity of 360.4 mAh g^-1 , a high energy density of 484.2 Wh kg^-1 , and impressive cycling stability (97.0% capacity retention after 10 000 cycles). This finding on a-MnBO_x with a dual-energy storage mechanism provides new opportunities for developing high-performance aqueous Zn-Mn batteries.

Effects of synthesis temperature on ε-MnO2 microstructures and performance: Selective adsorption of heavy metals and the mechanism onto (100) facet compared with (001)

The heavy-metal adsorbent ε-MnO₂ was produced through a simple, one-step oxidation-reduction reaction at three different synthesis temperatures (25 °C, 50 °C and 75 °C) and their morphology and chemical-physical properties were compared. Of the three materials, MnO₂-25 had the largest specific surface area and the highest surface hydroxyl concentration. Its optimal performance was demonstrated by batch adsorption experiments with Pb²⁺, Cd²⁺ and Cu²⁺. Of the three metals, Pb²⁺ was adsorbed best (339.15 mg/g), followed by Cd²⁺ (107.50 mg/g) and Cu²⁺ (86.30 mg/g). When all three metals were present, Pb²⁺ was still absorbed best but now more Cu²⁺ was adsorbed than Cd²⁺. In order to explore the mechanism for the inconsistent adsorption order of Cd²⁺ and Cu²⁺ in single and competitive adsorption, we combined experimental data with density functional theory (DFT) calculations to elucidate the distinct adsorption nature of MnO₂-25 towards these three metals. This revealed that the adsorption affinity of the (100) facet was superior to (001), and since the surface complexes were also more stable on (100), this facet was most likely determining the adsorption order for the single metals. When the metals were present in combination, Pb²⁺ preferentially occupied the active adsorption sites of (100), forcing Cu²⁺ to be adsorbed on the (001) facet where Cd²⁺ was only poorly bound. Thus, the adsorption behavior was affected by MnO₂-25 surface chemistry at a molecular scale. This study provides an in-depth understanding of the adsorption mechanisms of the heavy metals on this adsorbent and offers theoretical guidance for production of adsorbent with improved removal efficiency.

Regulation of Dendrite-Free Li Plating via Lithiophilic Sites on Lithium-Alloy Surface

Lithium (Li) deposition behavior plays an important role in dendrite formation and the subsequent performance of lithium metal batteries. This work reveals the impact of the lithiophilic sites of lithium-alloy on the Li plating process via the first-principles calculations. We find that the Li deposition mechanisms on the Li metal and Li22Sn5 surface are different due to the lithiophilic sites. We first propose that Li plating on the Li metal surface goes through the "adsorption-reduction-desorption-heterogeneous nucleation-cluster drop" process, while it undergoes the "adsorption-reduction-growth" process on the Li22Sn5 surface. The lower adsorption energy contributes to the easy adsorption of Li on the lithiophilic sites of the Li22Sn5 surface. The lower Li reduction energy on the Li metal surface indicates that it is easy for Li to be reduced on the Li metal surface, attributed to its higher Fermi energy level. Furthermore, the faster Li diffusion on the Li22Sn5 surface results in smooth Li deposition, which is based on a "two-Li synergy diffusion" mechanism. However, Li diffuses more slowly on the Li metal surface than on the Li22Sn5 surface due to the "single Li diffusion" mechanism. This work provides a fundamental understanding on the impact of lithiophilic sites of Li alloy on the Li plating process and points out that the future design of 3D Li-alloy substrates decorated with multilithiophilic sites can prevent dendrite formation on the lithium-alloy substrate by guiding uniform Li deposition.

Graphene oxide coupled high-index facets CdZnS with rich sulfur vacancies for synergistic boosting visible-light-catalytic hydrogen evolution in natural seawater: Experimental and DFT study

Constructing photocatalysts with high activity and anti-photocorrosion is a key to harvesting hydrogen energy from seawater efficiently. Herein, graphene oxide closely coupled high-index facets CdZnS with rich sulfur vacancies (Vs-CZS@GO) has been successfully synthesized via one-pot sulfidation accompanied pyrolysis. DFT calculation confirmed the delicate surface/interface/defect engineering endowed high-index facets Vs-CZS@GO with a lower ΔG_H* value and significant charge transfer behavior for efficient H₂-generation. The synergistic effect of sulfur vacancy, high-index facets, and tightly coupling interface not only enhanced intrinsic active sites and carrier separation efficiencies, but also greatly promoted H₂ evolution rate and stability. Consequently, Vs-CZS@GO displayed a significantly high H₂-generation rate of 23.2 mmol∙g^-1∙h^-1 in natural seawater under visible-light irradiation, which is up to 82% of that in pure water. This work provides deeply insight into the synergistic regulation of electronic structure for exposed high-index facets photocatalysts via defect engineering and interface engineering for synergistic boosting visible-light-to-H₂ evolution.

ab initio study of Mn-based systems for oxidative degradation

Organic contaminants can be removed from water/wastewater by oxidative degradation using oxidants such as manganese oxides and/or aqueous manganese ions. The Mn species show a wide range of activity, which is related to the oxidation state of Mn. Here, we use ab initio molecular dynamics simulations to address Mn oxidation states in these systems. We first develop a correlation between Mn partial atomic charge and the oxidation state based on results of 31 simulations on known Mn aqueous complexes. The results collapse to a master curve; the dependence of partial atomic charge on oxidation state weakens with increasing oxidation state, which concurs with a previously proposed feedback effect. This correlation is then used to address oxidation states in Mn systems used as oxidants. Simulations of MnO₂ polymorphs immersed in water give average oxidation states (AOS) in excellent agreement with experimental results, in that β-MnO₂ has the highest AOS, α-MnO₂ has an intermediate AOS, and δ-MnO₂ has the lowest AOS. Furthermore, the oxidation state varies substantially with the atom's environment, and these structures include Mn(III) and Mn(V) species that are expected to be active. In regard to the MnO₄^-/HSO₃^-/O₂ system that has been shown to be a highly effective oxidant, we propose a novel Mn complex that could give rise to the oxidative activity, where Mn(III) is stabilized by sulfite and dissolved O₂ ligands. Our simulations also show that the O₂ would be activated to O₂^2- in this complex under acidic conditions, and could lead to the formation of OH radicals that serve as oxidants.

Nanometer effect promoting arsenic removal on α-MnO2 nano-surface in aqueous solution: DFT+U research

The nanometer effect in the process of arsenic ions removal on α-MnO₂ nano-surface is studied by the first-principle method through microfacet models. Several parameters, such as adhesion energy, electrostatic potential, and Mulliken population were calculated to illuminate the internal mechanism. The results show that the adsorption energies of As(OH)₃ molecules on MnO₂[(100×110)] nanostructure are smaller than that on the bulk surface with the same concentration, which means the nanometer effect is beneficial to enhance the adsorption ability of MnO₂ nano-surface. In an aqueous solution, there exist two possible removal ways of As ions. One is the direct reaction of As(OH)₃→As(OH)₆^-, which occurs both in bulk surface and nano-surface. However, to nanomaterials, there exists another removal way of As(OH)₃→As(OH)₄→As(OH)₆^- through an intermediate As(OH)₄ molecule produced by nanometer effect. Furthermore the smaller electrostatic potential of As ions on [(100×110)] nano-surface is beneficial to enhance the removal capability of As ions. Then the reason why MnO₂ nanomaterials have better catalytic activity than the bulk materials is originated from its much less adhesion energy, much more removal ways, and much smaller electrostatic potential. So this research provides a detailed understanding of the removal capability of toxic ions influenced by a nanometer effect.

High-Performance Aqueous Zn Battery Based on MoS2-Loaded MnO2-x@Carbon Aerogel

The MnO₂-based aqueous Zn cell can meet the requirements of safety, flexibility, and low cost for portable/wearable electronics; however, its low intrinsic conductivity, weak kinetics, and poor high-loading capacity restrict its practical performance. In this study, the synergistic architecture of MoS₂-loaded, oxygen-defect-rich MnO_2-x nanocrystals with a carbon coating (M-PM_2-x-H₂ aerogel) was prepared. As corevealed by various characterizations, this synergistic design not only improves the electronic/ionic conductivity but also motivates the conversion kinetics of the surficial electrochemical reaction. As a result, the M-PM_2-x-H₂ cathode delivers a much improved capacity of 567 mA h·g^-1 at 0.1 A·g^-1 and shows a high capacity retention of 176% after 150 cycles at 0.5 A·g^-1. More impressively, the high areal loading (3.97 mg·cm^-1) of the M-PM_2-x-H₂ electrode also displays a high capacity of 367 mA h·g^-1 at 0.1 A·g^-1. In addition, the derived all-solid-state cell exhibits excellent flexibility and safety under the conditions of weight loading, cutting, and bending.

Dual-site electrocatalytic nitrate reduction to ammonia on oxygen vacancy-enriched and Pd-decorated MnO2 nanosheets

Electrocatalytic nitrate reduction (NRR) represents one promising alternative to the Haber-Bosch process for NH₃ production due to the lower reaction energy barrier compared to N₂ reduction and the potential recycling of nitrogen source from nitrate wastewater. The metal oxides with oxygen vacancy (O_v) display high NH₃ selectivities in NRR (NO₂^-/N₂ as side products), but the complexity in O_v enrichment and the inferior hydrogen adsorption on oxides make NRR an inefficient process. Herein, one superior dual-site NRR electrocatalyst that is composed of O_v-enriched MnO₂ nanosheets (MnO₂-O_v) and Pd nanoparticles (deposited on MnO₂) is constructed over the three-dimensional porous nickel foam (Pd-MnO₂-O_v/Ni foam). In a continuous-flow reaction cell, this electrode delivers a NO₃^--N conversion rate of 642 mg N m^-2_electrode h^-1 and a NH₃ selectivity of 87.64% at -0.85 V vs. Ag/AgCl when feeding 22.5 mg L^-1 of NO₃^--N (0.875 mL min^-1), outperforming the Pd/Ni foam (369 mg N m^-2_electrode h^-1, 85.02%) and MnO₂-O_v/Ni foam (118 mg N m^-2_electrode h^-1, 32.25%). Increasing the feeding NO₃^--N concentration and flow rate to 180.0 mg L^-1 and 2.81 mL min^-1 can further lift the conversion rate to 1933 and 1171 mg N m^-2_electrode h^-1, respectively. The combination of experimental characterizations and theoretical calculations reveal that the MnO₂-O_v adsorbs, immobilizes, and activates the NO₃^- and N-intermediates, while the Pd supplies the O_v sites with sufficient adsorbed hydrogen (H*) for both the NRR and O_v refreshment. Our work presents a good example of utilizing dual-site catalysis in the highly selective conversion of NO₃^- to NH₃ that is important for nitrate pollution abatement, nitrogen resource recycling, as well as sustainable NH₃ production.

Engineering of crystal phase over porous MnO2 with 3D morphology for highly efficient elimination of H2S

In the present work, we report a facile oxalate-derived hydrothermal method to fabricate α-, β- and δ-MnO₂ catalysts with hierarchically porous structure and study the phase-dependent behavior for selective oxidation of H₂S over MnO₂ catalysts. It was disclosed that the oxygen vacancy, reducibility and acid property of MnO₂ are essentially determined by the crystalline phase. Systematic experiments demonstrate that δ-MnO₂ is superior in active oxygen species, activation energy and H₂S adsorption capacity among the prepared catalysts. As a consequence, δ-MnO₂ nanosphere with a hierarchically porous structure shows high activity and stability with almost 100% H₂S conversion and sulfur selectivity at 210 °C, better than majority of reported Mn-based materials. Meanwhile, hierarchically porous structure of δ-MnO₂ nanosphere alleviates the generation of by-product SO₂ and sulfate, promoting the adoptability of Mn-based catalysts in industrial applications.

Efficient catalytic activity and bromate minimization over lattice oxygen-rich MnOOH nanorods in catalytic ozonation of bromide-containing organic pollutants: Lattice oxygen-directed redox cycle and bromate reduction

The inhibition of bromate formation is a challenge for the application of ozonation in water treatment due to the carcinogenicity and nephrotoxicity of bromate. In this study, the high-mobility lattice oxygen-rich MnOOH nanorods were synthesized successfully and applied for the bromate inhibition during catalytic ozonation in bromide and organic pollutants-containing wastewater treatment. The catalytic ozonation system using lattice oxygen-rich MnOOH nanorods exhibited an excellent performance in bromate control with an inhibition efficiency of 54.1% compared with the sole ozonation process. Furthermore, with the coexistence of 4-nitrophenol, the catalytic ozonation process using lattice oxygen-rich MnOOH nanorods could inhibit the bromate formation and boost the degradation of 4-nitrophenol simultaneously. Based on the experiments of ozone decomposition, surface manganese inactivation and reactive oxygen species detection, the inhibition of bromate could be attributed to the effective decomposition of ozone with generating more ·O₂^- and the reduction of bromate into bromide by lattice oxygen-rich MnOOH. The existed surface Mn(IV) on lattice oxygen-rich MnOOH can accept electrons from lattice oxygen and ·O₂^- to generate surface transient Mn(II)/Mn(III), in which Mn(II)/Mn(III) can promote the reduction of bromate into bromide during catalytic ozonation. This study provides a promising strategy for the development of bromate-controlling technologies in water treatment.

Studies of Nickel-Rich LiNi0.85Co0.10Mn0.05O2 Cathode Materials Doped with Molybdenum Ions for Lithium-Ion Batteries

In this work, we continued our systematic investigations on synthesis, structural studies, and electrochemical behavior of Ni-rich materials Li[NixCoyMnz]O2 (x + y + z = 1; x ≥ 0.8) for advanced lithium-ion batteries (LIBs). We focused, herein, on LiNi0.85Co0.10Mn0.05O2 (NCM85) and demonstrated that doping this material with high-charge cation Mo6+ (1 at. %, by a minor nickel substitution) results in substantially stable cycling performance, increased rate capability, lowering of the voltage hysteresis, and impedance in Li-cells with EC-EMC/LiPF6 solutions. Incorporation of Mo-dopant into the NCM85 structure was carried out by in-situ approach, upon the synthesis using ammonium molybdate as the precursor. From X-ray diffraction studies and based on our previous investigation of Mo-doped NCM523 and Ni-rich NCM811 materials, it was revealed that Mo6+ preferably substitutes Ni residing either in 3a or 3b sites. We correlated the improved behavior of the doped NCM85 electrode materials in Li-cells with a partial Mo segregation at the surface and at the grain boundaries, a tendency established previously in our lab for the other members of the Li[NixCoyMnz]O2 family.

Potassium-incorporated manganese oxide enhances the activity and durability of platinum catalysts for low-temperature CO oxidation

Potassium-incorporated manganese oxide is demonstrated as an efficient support for fabricating highly active and stable Pt catalysts for low-temperature CO oxidation.

Contribution of Nitrogen Vacancies to Ammonia Synthesis over Metal Nitride Catalysts

Ammonia is one of the most important feedstocks for the production of fertilizer and as a potential energy carrier. Nitride compounds such as LaN have recently attracted considerable attention due to their nitrogen vacancy sites that can activate N₂ for ammonia synthesis. Here, we propose a general rule for the design of nitride-based catalysts for ammonia synthesis, in which the nitrogen vacancy formation energy (E_NV) dominates the catalytic performance. The relatively low E_NV (ca. 1.3 eV) of CeN means it can serve as an efficient and stable catalyst upon Ni loading. The catalytic activity of Ni/CeN reached 6.5 mmol·g^-1·h^-1 with an effluent NH₃ concentration (E_NH3) of 0.45 vol %, reaching the thermodynamic equilibrium (E_NH3 = 0.45 vol %) at 400 °C and 0.1 MPa, thereby circumventing the bottleneck for N₂ activation on Ni metal with an extremely weak nitrogen binding energy. The activity far exceeds those for other Co- and Ni-based catalysts, and is even comparable to those for Ru-based catalysts. It was determined that CeN itself can produce ammonia without Ni-loading at almost the same activation energy. Kinetic analysis and isotope experiments combined with density functional theory (DFT) calculations indicate that the nitrogen vacancies in CeN can activate both N₂ and H₂ during the reaction, which accounts for the much higher catalytic performance than other reported nonloaded catalysts for ammonia synthesis.

Trends in C-O and N-O bond scission on rutile oxides described using oxygen vacancy formation energies

Reactivity trends on transition metals can generally be understood through the d-band model, but no analogous theory exists for transition metal oxides. This limits the generality of analyses in oxide-based catalysis and surface chemistry and has motivated the appearance of numerous descriptors. Here we show that oxygen vacancy formation energy (ΔE _Vac) is an inexpensive yet accurate and general descriptor for trends in transition-state energies, which are usually difficult to assess. For rutile-type oxides (MO₂ with M = 3d metals from Ti to Ni), we show that ΔE _Vac captures the trends in C-O and N-O bond scission of CO₂, CH₃OH, N₂O, and NH₂OH at oxygen vacancies. The proportionality between ΔE _Vac and transition-state energies is rationalized by analyzing the oxygen-metal bonds, which change from ionic to covalent from TiO₂ to NiO₂. ΔE _Vac may be used to design oxide catalysts, in particular those where lattice oxygen and/or oxygen vacancies participate in the catalytic cycles.

Electrochemical Polarization Dependence of the Elastic and Electrostatic Driving Forces to Aliovalent Dopant Segregation on LaMnO3

Segregation of aliovalent dopant cations is a common degradation pathway on perovskite oxide surfaces in energy conversion and catalysis applications. Here we focus on resolving quantitatively how dopant segregation is affected by oxygen chemical potential, which varies over a wide range in electrochemical and thermochemical energy conversion reactions. We employ electrochemical polarization to tune the oxygen chemical potential over many orders of magnitude. Altering the effective oxygen chemical potential causes the oxygen nonstoichiometry to change in the electrode. This then influences the mechanisms underlying the segregation of aliovalent dopants. These mechanisms are (i) the formation of oxygen vacancies that couples to the electrostatic energy of the dopant in the perovskite lattice and (ii) the elastic energy of the dopant due to cation size mismatch, which also promotes the reaction of the dopant with O₂ from the gas phase. The present study resolves these two contributions over a wide range of effective oxygen pressures. Ca-, Sr-, and Ba-doped LaMnO₃ are selected as model systems, where the dopants have the same charge but different ionic sizes. We found that there is a transition between the electrostatically and elastically dominated segregation regimes, and the transition shifted to a lower oxygen pressure with increasing cation size. This behavior is consistent with the results of our ab initio thermodynamics calculations. The present study provides quantitative insights into how the elastic energy and the electrostatic energy determine the extent of segregation for a given overpotential and atmosphere relevant to the operating conditions of perovskite oxides in energy conversion applications.

Microstructural Engineering of Cathode Materials for Advanced Zinc-Ion Aqueous Batteries

Zinc-ion batteries (ZIBs) have attracted intensive attention due to the low cost, high safety, and abundant resources. However, up to date, challenges still exist in searching for cathode materials with high working potential, excellent electrochemical activity, and good structural stability. To address these challenges, microstructure engineering has been widely investigated to modulate the physical properties of cathode materials, and thus boosts the electrochemical performances of ZIBs. Here, the recent research efforts on the microstructural engineering of various ZIB cathode materials are mainly focused upon, including composition and crystal structure selection, crystal defect engineering, interlayer engineering, and morphology design. The dependency of cathode performance on aqueous electrolyte for ZIB is further discussed. Finally, future perspectives and challenges on microstructure engineering of cathode materials for ZIBs are provided. It is aimed to provide a deep understanding of the microstructure engineering effect on Zn2+ storage performance.

Recent advances in transition metal based compound catalysts for water splitting from the perspective of crystal engineering

A review of the recent progress on the transition metal based catalysts for water splitting with emphasis on crystal engineering.

Hierarchical Z-scheme 1D/2D architecture with TiO2 nanowires decorated by MnO2 nanosheets for efficient adsorption and full spectrum photocatalytic degradation of organic pollutants

A novel hierarchical 1D/2D TiO₂/MnO₂ composite shows superior adsorption, full spectrum photocatalytic activity and excellent stability for organic pollutant removal.

Dispersing transition metal vacancies in layered double hydroxides by ionic reductive complexation extraction for efficient water oxidation

Creating atomic defects in nanomaterials is an effective approach to promote the catalytic performance of a catalyst, but the defective catalysts are often prone to mechanical collapse if not properly synthesized. The uncontrollably formed defects also make it difficult to systematically investigate their effects on the catalytic performance. Herein, we report an efficient method of ionic reductive complexation extraction (IRCE) to fabricate atomic vacancies in a transition metal based nanomaterial without damaging its nanostructure, turning the otherwise catalytically inactive material to an advanced catalyst towards water oxidation in alkaline electrolyte. Here nickel based layered double hydroxide mixed with Cu(ii) is used to demonstrate the concept. With a tunable content and uniform dispersion of Cu(ii) on the brucite layer of the LDH, a suitable complexing agent could specifically combine with and remove the target Cu(ii), thereby creating the desired vacancies. The resulting vacancy rich TM LDH is found to be an excellent OER electrocatalyst with a low overpotential and small Tafel slope, due to the purposely modulated geometric and electronic structures of the active sites, and the greatly decreased charge transfer resistance.

Recent Advancements Towards Closing the Gap between Electrocatalysis and Battery Science Communities: The Computational Lithium Electrode and Activity-Stability Volcano Plots

Despite of the fact that the underlying processes are of electrochemical nature, electrocatalysis and battery research are commonly perceived as two disjointed research fields. Herein, recent advancements towards closing this apparent community gap by discussing the concepts of the constrained ab initio thermodynamics approach and the volcano relationship, which were originally introduced for studying heterogeneously catalyzed reactions by first-principles methods at the beginning of the 21st century, are summarized. The translation of the computational hydrogen electrode (CHE) approach or activity-based volcano plots to a computational lithium electrode (CLiE) or activity-stability volcano plots, respectively, for the investigation of electrode surfaces in batteries may refine theoretical modeling with the aim that enhancements of the underlying concepts are transferred between the research communities. The presented strategy of developing novel approaches by interdisciplinary research activities may trigger further progress of improved theoretical concepts in the near future.

Fe-Doped α-MnO2 nanorods for the catalytic removal of NOx and chlorobenzene: the relationship between lattice distortion and catalytic redox properties

Controllably tuning redox performance is one of the key targets in catalysis.

Enhanced Charge Transport and Increased Active Sites on α-Fe2O3 (110) Nanorod Surface Containing Oxygen Vacancies for Improved Solar Water Oxidation Performance

The effect of oxygen vacancies (V_O) on α-Fe₂O₃ (110) facet on the performance of photoelectrochemical (PEC) water splitting is researched by both experiments and density functional theory (DFT) calculations. The experimental results manifest that the enhancement in photocurrent density by the presence of V_O is related with increased charge separation and charge-transfer efficiencies. The electrochemical analysis reveals that the sample with V_O demonstrates an enhanced carrier density and reduced charge-transfer resistance. The results of DFT calculation indicate that the better charge separation is also contributed by the decrease of potential on the V_O surface, which improves the hole transport from the bulk to the surface. The reduced charge-transfer resistance is owing to the greatly increased number of active sites. The current study provides important insight into the roles of V_O on α-Fe₂O₃ photoanode, especially on its surface catalysis. The generated lesson is also helpful for the improvement of other PEC photoanode materials.

Controllable dynamics of oxygen vacancies through extrinsic doping for superior catalytic activities

Due to its strong redox ability, high stability, cost effectiveness and non-toxicity, cerium oxide (CeO₂) has been extensively researched as an active photocatalyst material. The underlying photocatalytic reactions are mostly associated with the transportation of oxygen ions through vacancies, but the actual transport phenomenon had not been clearly understood. In this work, gadolinium (Gd) is sequentially doped into CeO₂ to investigate how extrinsic doping can modulate oxygen vacancies in CeO₂ and influence photocatalytic activities. From our investigations, it was found that the Gd doping may induce structural symmetry breaking leading to a pure CeO₂ fluorite structure that transforms mobile oxygen vacancies into clustered or immobile vacancies. When the vacancies were set as "mobile" (for Gd doping levels ≤15 at%), maximum photocatalytic activities were obtained. In contrast, suppressed photocatalytic efficiencies were noted for higher Gd doping levels (20 at% or more). The results reported in this research may provide an extra degree of freedom in the form of extrinsic doping to configure the oxygen vacancy defects and their mobility to achieve better catalytic efficiencies.

Synthesis and Progress of New Oxygen-Vacant Electrode Materials for High-Energy Rechargeable Battery Applications

During the last few years, a great amount of oxygen-vacant materials have been synthetized and applied as electrodes for electrochemical storage. The presence of oxygen vacancies leads to an increase in the conductivity and the diffusion coefficient; consequently, the controllable synthesis of oxygen vacancy plays an important role in improving the electrochemical performance, including achieving high specific capacitance, high power density, high energy density, and good cycling stability of the electrode materials for batteries. This review mainly focuses on research progress in the preparation of oxygen-vacant nanostructures and the application of materials with oxygen vacancies in various batteries (such as lithium-ion, lithium-oxygen, and sodium-ion batteries). Challenges related to and opportunities for oxygen-vacant materials are also provided.

Tuning the K+ Concentration in the Tunnels of α-MnO2 To Increase the Content of Oxygen Vacancy for Ozone Elimination

α-MnO₂ is a promising material for ozone catalytic decomposition and the oxygen vacancy is often regarded as the active site for ozone adsorption and decomposition. Here, α-MnO₂ nanowire with tunable K⁺ concentration was prepared through a hydrothermal process in KOH solution. High concentration K⁺ in the tunnel can expand crystal cell and break the charge balance, leading to a lower average oxidation state (AOS) of Mn, which means abundant oxygen vacancy. DFT calculation has also proven that the samples with higher K⁺ concentration exhibit lower formation energy for oxygen vacancy. Due to the enormous active oxygen vacancies existing in the α-MnO₂ nanowire, the lifetime of the catalyst (corresponding to 100% ozone removal rate, 25 °C) is increased from 3 to 15 h. The FT-IR results confirmed that the accumulation of intermediate oxygen species on the catalyst surface is the main reason why it is deactivated after long time reaction. In this work, the performance of the catalyst has been improved because the abundant active oxygen vacancies are fabricated by the electrostatic interaction between oxygen atoms inside the tunnels and the introduced K⁺, which offers us a new perspective to design a high efficiency catalyst and may promote manganese oxide for practical ozone elimination.

Oxygen-Vacancy Abundant Ultrafine Co3O4/Graphene Composites for High-Rate Supercapacitor Electrodes

The metal oxides/graphene composites are one of the most promising supercapacitors (SCs) electrode materials. However, rational synthesis of such electrode materials with controllable conductivity and electrochemical activity is the topical challenge for high-performance SCs. Here, the Co₃O₄/graphene composite is taken as a typical example and develops a novel/universal one-step laser irradiation method that overcomes all these challenges and obtains the oxygen-vacancy abundant ultrafine Co₃O₄ nanoparticles/graphene (UCNG) composites with high SCs performance. First-principles calculations show that the surface oxygen vacancies can facilitate the electrochemical charge transfer by creating midgap electronic states. The specific capacitance of the UCNG electrode reaches 978.1 F g^-1 (135.8 mA h g^-1) at the current densities of 1 A g^-1 and retains a high capacitance retention of 916.5 F g^-1 (127.3 mA h g^-1) even at current density up to 10 A g^-1, showing remarkable rate capability (more than 93.7% capacitance retention). Additionally, 99.3% of the initial capacitance is maintained after consecutive 20 000 cycles, demonstrating enhanced cycling stability. Moreover, this proposed laser-assisted growth strategy is demonstrated to be universal for other metal oxide/graphene composites with tuned electrical conductivity and electrochemical activity.

Element strategy of oxygen evolution electrocatalysis based on in situ spectroelectrochemistry

Oxygen evolution electrocatalysis has received extensive attention due to its significance in biology, chemistry, and technology. However, it is still unclear how the abundant 3d-elements can be used to drive the four-electron oxidation of water as efficiently as in Nature. In this Feature Article, we will propose a design strategy concerning the optimization of the charge accumulation process based on our ongoing spectroelectrochemical study on Mn, Fe, and Ir oxygen evolution catalysts. Spectroscopic identification of the reaction intermediates showed that the activity of MnO₂ and Fe₂O₃ was dictated by the generation of Mn³⁺ and Fe⁴⁺, whereas in the case of IrO_x, the activity did not correlate with the valence change of Ir. The efficiency of charge accumulation through valence change is closely linked with the spin configuration of the metal center, because charge disproportionation, which was found to inhibit charge accumulation in the high-spin 3d metals, requires an electron in the e_g orbital. In addition to directly increasing the overpotential through the generation of an unstable intermediate, charge disproportionation inhibits charge accumulation by dissipating the total oxidative energy of the system. A favorable charge accumulation process may also be beneficial for electrode kinetics due to the enhanced coupling between reaction rates and electrochemical driving force. The model proposed in this study may help explain why low-spin 4d/5d rare metals are often more active than the abundant high-spin 3d materials for multi-electron transfer reactions in general, and provides new insight into how active 3d-metal catalysts can be synthesized by optimizing the energetics of both bond formation and charge accumulation.

Atomic-Scale Influence of Grain Boundaries on Li-Ion Conduction in Solid Electrolytes for All-Solid-State Batteries

Solid electrolytes are generating considerable interest for all-solid-state Li-ion batteries to address safety and performance issues. Grain boundaries have a significant influence on solid electrolytes and are key hurdles that must be overcome for their successful application. However, grain boundary effects on ionic transport are not fully understood, especially at the atomic scale. The Li-rich anti-perovskite Li₃OCl is a promising solid electrolyte, although there is debate concerning the precise Li-ion migration barriers and conductivity. Using Li₃OCl as a model polycrystalline electrolyte, we apply large-scale molecular dynamics simulations to analyze the ionic transport at stable grain boundaries. Our results predict high concentrations of grain boundaries and clearly show that Li-ion conductivity is severely hindered through the grain boundaries. The activation energies for Li-ion conduction traversing the grain boundaries are consistently higher than that of the bulk crystal, confirming the high grain boundary resistance in this material. Using our results, we propose a polycrystalline model to quantify the impact of grain boundaries on conductivity as a function of grain size. Such insights provide valuable fundamental understanding of the role of grain boundaries and how tailoring the microstructure can lead to the optimization of new high-performance solid electrolytes.

Rechargeable aqueous zinc-manganese dioxide batteries with high energy and power densities

Although alkaline zinc-manganese dioxide batteries have dominated the primary battery applications, it is challenging to make them rechargeable. Here we report a high-performance rechargeable zinc-manganese dioxide system with an aqueous mild-acidic zinc triflate electrolyte. We demonstrate that the tunnel structured manganese dioxide polymorphs undergo a phase transition to layered zinc-buserite on first discharging, thus allowing subsequent intercalation of zinc cations in the latter structure. Based on this electrode mechanism, we formulate an aqueous zinc/manganese triflate electrolyte that enables the formation of a protective porous manganese oxide layer. The cathode exhibits a high reversible capacity of 225 mAh g^-1 and long-term cyclability with 94% capacity retention over 2000 cycles. Remarkably, the pouch zinc-manganese dioxide battery delivers a total energy density of 75.2 Wh kg^-1. As a result of the superior battery performance, the high safety of aqueous electrolyte, the facile cell assembly and the cost benefit of the source materials, this zinc-manganese dioxide system is believed to be promising for large-scale energy storage applications.The development of rechargeable aqueous zinc batteries are challenging but promising for energy storage applications. With a mild-acidic triflate electrolyte, here the authors show a high-performance Zn-MnO₂ battery in which the MnO₂ cathode undergoes Zn²⁺ (de)intercalation.

Promoted Electrochemical Performance of β-MnO2 through Surface Engineering

Different crystal facets with different surface atomic configurations and physical/chemical properties will have distinct electrochemical performances during their surface/near-surface redox reactions, and it is important to realize the controllable synthesis of high active surfaces for electrode materials. Herein, using first-principles calculations, the electrochemical performances of different surfaces of β-MnO₂ were investigated. Higher surface adsorption pseudocapacitance and lower ion diffusion barrier from the surface to the near surface make the {001} surface of β-MnO₂ superior to other surfaces when acting as an electrode material. Moreover, β-MnO₂ with a large percentage of the {001} surface was predicted to be obtained through surface F-termination. F-termination decreases the surface energy of the {001} surface while suppressing the growth of {110} surface, which demonstrated as the surface with a much lower electrochemical performance. This work might provide a feasible strategy to synthesize anticipated surfaces with a high electrochemical performance for transition metal oxides.

The effect of acid/alkali treatment on the catalytic combustion activity of manganese oxide octahedral molecular sieves

The acid and alkali treated OMS-2 catalysts were prepared and evaluated as the catalysts for DME and toluene combustion.