Epitaxially Grown Heterostructured SrMn3 O6-x -SrMnO3 with High-Valence Mn3+/4+ for Improved Oxygen Reduction Catalysis

Corner-Sharing Tetrahedrally Coordinated W-V Dual Active Sites on Cu2 V2 O7 for Photoelectrochemical Water Oxidation

The sluggish four-electron oxygen evolving reaction is one of the key limitations of photoelectrochemical water decomposition. Optimizing the binding of active sites to oxygen in water and promoting the conversion of *O to *OOH are the key to enhancing oxygen evolution reaction. In this work, W-doped Cu2 V2 O7 (CVO) constructs corner-sharing tetrahedrally coordinated W-V dual active sites to induce the generation of electron deficiency active centers, promote the adsorption of ─OH, and accelerate the transformation of *O to *OOH for water splitting. The photocurrent obtained by the W-modified CVO photoanode is 0.97 mA cm-2 at 1.23 V versus RHE, which is much superior to that of the reported CVO. Experimental and theoretical results show that the excellent catalytic performance may be attributed to the formation of synergistic dual active sites between W and V atoms, and the introduction of W ions reduces the charge migration distance and prolongs the lifetime of photogenerated carriers. Meanwhile, the electronic structure in the center of the d-band is modulated, which leads to the redistribution of the electron density in CVO and lowers the energy barrier for the conversion of the rate-limiting step *O to *OOH.

Strategically Designed Pd-Induced Changes in Alkaline Hydrogen Evolution Reaction and Oxygen Evolution Reaction Performances of Electrochemical Water Oxidation by the Galvanically Synthesized MoO2/MoO3 Composite Thin Film

Electrochemical water oxidation is believed to be an effective pathway to produce clean, carbon-free, and environmentally sustainable green energy. In this work, we report a simple, easy-to-construct, facile, low-cost, and single-step galvanic technique to synthesize a Pd-supported temperature-assisted MoOx thin film nanocomposite for effective water oxidation. The most suitable nanocomposite exhibits very low overpotential at 10 mA/cm2 with smaller Tafel slope values for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) processes in an alkaline medium. The formation of a metal oxide-metal junction accelerates the growth of more active sites, promoting induced electronic synergism at the MoOx-Pd interface. This endows higher electrical conductivity and faster electron transfer kinetics, thus accelerating the faster water dissociation reaction following the Tafel-Volmer mechanism to boost the HER process in an alkaline medium. The excellent electrochemical HER and OER performances of our electrocatalyst even supersede the accomplishments of the benchmark catalysts Pt/C and RuO2. Moreover, neither of these two catalysts demonstrates both catalytic reactions, i.e., HER and OER at the same time, which have been observed for our synthesized catalyst. Our findings illustrate the potential of a thin-film MoOx-Pd nanocomposite to be an exceedingly effective electrocatalyst developed by interface engineering strategies. This also provides insight into designing several other semiconductor composite catalysts using simple synthesis techniques for highly efficient HER/OER processes that could be alternatives to benchmark electrocatalysts for water electrolysis.

Significance of Engineering the MnO6 Octahedral Units to Promote the Oxygen Reduction Reaction of Perovskite Oxides

The electronic structure and geometric configuration of catalysts play a crucial role to design novel perovskite-type catalysts for oxygen reduction reaction (ORR). Nowadays, many studies are more concerned with the influence of electronic structure and ignore the geometric effect, which plays a nonnegligible role in enhancing catalytic performances. Herein, this work regulates the MnO6 octahedral tilting degree of LaMnO3 by modulating the concentration of Y3+ , excluding the electronic effect from the valence state of manganese. Plotting the MnO6 octahedral tilting degree as a function of concentration of Y3+ produces a volcano-shaped plot. The octahedral tilting can reduce the Mn-O covalency, generating more highly active Mn3+ and oxygen vacancies during ORR process. The specific activity has a positive correlation with octahedral tilting degree. Meanwhile, the octahedral tilting stabilizes Mn-O interactions during ORR process and promote stability. Based on experimental results and DFT calculations, octahedral tilting alters the rate-determining step (RDS) and decrease the energy barrier. Subsequent extended experiment confirms that octahedral tilting is the key factor to affect the catalytic performances.

Hetero-Anionic Structure Activated CoS Bonds Promote Oxygen Electrocatalytic Activity for High-Efficiency Zinc-Air Batteries

The electronic structure of transition metal complexes can be modulated by replacing partial ion of complexes to obtain tuned intrinsic oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) electrocatalytic activity. However, the anion-modulated transition metal complexes ORR activity of is still unsatisfactory, and the construction of hetero-anionic structure remains challenging. Herein, an atomic doping strategy is presented to prepare the CuCo2 O4-x Sx /NC-2 (CCSO/NC-2) as electrocatalysts, the structrual characterization results favorably demonstrate the partial substitution of S atoms for O in CCSO/NC-2, which shows excellent catalytic performance and durability for OER and ORR in 0.1 m KOH. In addition, the catalyst assembled Zinc-air battery with an open circuit potential of 1.43 V maintains performance after 300 h of cyclic stability. Theoretical calculations and differential charges illustrate that S doping optimizes the reaction kinetics and promotes electron redistribution. The superior performance of CCSO/NC-2 catalysis is mainly due to its unique S modulation of the electronic structure of the main body. The introduction of S promotes CoO covalency and constructs a fast electron transport channel, thus optimizing the adsorption degree of active site Co to the reaction intermediates.

Axial Oxygen Ligands Regulating Electronic and Geometric Structure of Zn-N-C Sites to Boost Oxygen Reduction Reaction

Zn-N-C possesses the intrinsic inertia for Fenton-like reaction and can retain robust durability in harsh circumstance, but it is often neglected in oxygen reduction reaction (ORR) because of its poor catalytic activity. Zn is of fully filled 3d10 4s2 configuration and is prone to evaporation, making it difficult to regulate the electronic and geometric structure of Zn center. Here, guided by theoretical calculations, five-fold coordinated single-atom Zn sites with four in-plane N ligands is constructed and one axial O ligand (Zn-N4 -O) by ionic liquid-assisted molten salt template method. Additional axial O not only triggers a geometry transformation from the planar structure of Zn-N4 to the non-planar structure of Zn-N4 -O, but also induces the electron transfer from Zn center to neighboring atoms and lower the d-band center of Zn atom, which weakens the adsorption strength of *OH and decreases the energy barrier of rate determining step of ORR. Consequently, the Zn-N4 -O sites exhibit improved ORR activity and excellent methanol tolerance with long-term durability. The Zn-air battery assembled by Zn-N4 -O presents a maximum power density of 182 mW cm-2 and can operate continuously for over 160 h. This work provides new insights into the design of Zn-based single atom catalysts through axial coordination engineering.

Highly Selective Activation of C-H Bond and Inhibition of C-C Bond Cleavage by Tuning Strong Oxidative Pd Sites

Improving the product selectivity meanwhile restraining deep oxidation still remains a great challenge over the supported Pd-based catalysts. Herein, we demonstrate a universal strategy where the surface strong oxidative Pd sites are partially covered by the transition metal (e. g., Cu, Co, Ni, and Mn) oxide through thermal treatment of alloys. It could effectively inhibit the deep oxidation of isopropanol and achieve the ultrahigh selectivity (>98%) to the target product acetone in a wide temperature range of 50-200 °C, even at 150-200 °C with almost 100% isopropanol conversion over PdCu_1.2/Al₂O₃, while an obvious decline in acetone selectivity is observed from 150 °C over Pd/Al₂O₃. Furthermore, it greatly improves the low-temperature catalytic activity (acetone formation rate at 110 °C over PdCu_1.2/Al₂O₃, 34.1 times higher than that over Pd/Al₂O₃). The decrease of surface Pd site exposure weakens the cleavage for the C-C bond, while the introduction of proper CuO shifts the d-band center (ε_d) of Pd upward and strengthens the adsorption and activation of reactants, providing more reactive oxygen species, especially the key super oxygen species (O₂^-) for selective oxidation, and significantly reducing the barrier of O-H and β-C-H bond scission. The molecular-level understanding of the C-H and C-C bond scission mechanism will guide the regulation of strong oxidative noble metal sites with relatively inert metal oxide for the other selective catalytic oxidation reactions.

In-situ gas foaming synthesis of N, S-rich co-doped hierarchically ordered porous carbon as an efficient oxygen reduction reaction catalyst

The design and manufacture of cost-effective and efficient oxygen reduction reaction (ORR) catalysts is critical to the widespread application of multiple energy conversion devices. Herein, a combination of in-situ gas foaming and the hard template method is proposed to construct the N, S-rich co-doped hierarchically ordered porous carbon (NSHOPC) as an effective metal-free electrocatalyst for ORR via carbonizing a mixture of polyallyl thiourea (PATU) and thiourea in silica colloidal crystal template (SiO₂-CCT) voids. Benefiting from the hierarchically ordered porous (HOP) architectures and the mass doping of N and S, NSHOPC displays excellent ORR activities (the half-wave potential of 0.889 V in 0.1 M KOH and 0.786 V in 0.5 M H₂SO₄) and long-term stability, which are all better than those of Pt/C. As the air cathode in a Zn-air battery (ZAB), NSHOPC exhibits a high peak power density of 174.6 mW·cm^-2 and long-term discharge stability. The remarkable performance of the as-synthesized NSHOPC signifies broad prospects for actual applications in energy conversion devices.

Regulating Reversible Oxygen Electrocatalysis by Built-in Electric Field of Heterojunction Electrocatalyst with Modified d-Band

Developing bifunctional catalysts for oxygen electrochemical reactions is essential for high-performance electrochemical energy devices. Here, a Mott-Schottky heterojunction composed of porous cobalt-nitrogen-carbon (Co-N-C) polyhedra containing abundant metal-phosphides for reversible oxygen electrocatalysis is reported. As a demonstration, this catalyst shows excellent activity in the oxygen electrocatalysis and thus delivers outstanding performance in rechargeable zinc-air batteries (ZABs). The built-in electric field in the Mott-Schottky heterojunction can promote electron transfer in oxygen electrocatalysis. More importantly, an appropriate d-band center of the heterojunction catalyst also endows oxygen intermediates with a balanced adsorption/desorption capability, thus enhancing oxygen electrocatalysis and consequently improving the performance of ZABs. The work demonstrates an important design principle for preparing efficient multifunctional catalysts in energy conversion technologies.

Recent advances in epitaxial heterostructures for electrochemical applications

Construction of epitaxial heterostructures is crucial for boosting the electrochemical properties of various materials, however a review dedicated to this attractive topic is still lacking. In this Minireview, a timely summary on the achievements of epitaxial heterostructure design for electrochemical applications is provided. We first introduce the synthesis strategies to provide fundamental understanding on how to create epitaxial interfaces between different components. Secondly, the superiorities of epitaxial heterostructures in electrocatalysis, supercapacitors and batteries are highlighted with the underlying structure-property relationship elucidated. Finally, a discussion on the challenges and future prospects of this field is presented.

Activating C-H Bonds by Tuning Fe Sites and an Interfacial Effect for Enhanced Methanol Oxidation

The interaction mechanism between the reacting species and the active site of α-Fe₂ O₃ -based photoanodes in photoelectrochemical methanol conversion reaction is still ambiguous. Herein, a simple two-step strategy is demonstrated to fabricate a porous α-Fe₂ O₃ /CoFe₂ O₄ heterojunction for the methanol conversion reaction. The influence of the electronic structure of active site and interfacial effect on the reaction are investigated by constructing two different FeO₆ octahedral configurations and heterogeneous structures. The optimal sample ZnFeCo-2 affords high photocurrent density of 1.17 mA cm^-2 at 0.5 V vs Ag/AgCl, which is 3.2 times than that of ZnFe (0.37 mA cm^-2 ). Meanwhile, the ZnFeCo-2 also exhibits 97.8% Faraday efficiency of CH₃ OH to HCHO, and long-term stability over 40 h. Furthermore, density functional theory calculations reveal that the heterostructured α-Fe₂ O₃ /CoFe₂ O₄ with favorable electron transfer effectively lowers methanol adsorption, C-H bond activation, and HCHO desorption energy relative to the pristine α-Fe₂ O₃ , resulting in excellent methanol conversion efficiency.

Synergy in Au-CuO Janus Structure for Catalytic Isopropanol Oxidative Dehydrogenation to Acetone

The controlled oxidation of alcohols to the corresponding ketones or aldehydes via selective cleavage of the β-C-H bond of alcohols under mild conditions still remains a significant challenge. Although the metal/oxide interface is highly active and selective, the interfacial sites fall far behind the demand, due to the large and thick support. Herein, we successfully develop a unique Au-CuO Janus structure (average particle size=3.8 nm) with an ultrathin CuO layer (0.5 nm thickness) via a bimetal in situ activation and separation strategy. The resulting Au-CuO interfacial sites prominently enhance isopropanol adsorption and decrease the energy barrier of β-C-H bond scission from 1.44 to 0.01 eV due to the strong affinity between the O atom of CuO and the H atom of isopropanol, compared with Au sites alone, thereby achieving ultrahigh acetone selectivity (99.3 %) over 1.1 wt % AuCu_0.75 /Al₂ O₃ at 100 °C and atmospheric pressure with 97.5 % isopropanol conversion. Furthermore, Au-CuO Janus structures supported on SiO₂ , TiO₂ or CeO₂ exhibit remarkable catalytic performance, and great promotion in activity and acetone selectivity is achieved as well for other reducible oxides derived from Fe, Co, Ni and Mn. This study should help to develop strategies for maximized interfacial site construction and structure optimization for efficient β-C-H bond activation.

Metalloid-Cluster Ligands Enabling Stable and Active FeN4 -Ten Motifs for the Oxygen Reduction Reaction

In nature, the oxygen reduction reaction (ORR) is catalyzed by cytochrome P450 (CYP) enzymes containing heme iron centers with an axial thiolate ligand (FeN₄ -S), which are among the most finely developed catalysts by natural selection. However, the exceptional ORR activity and selectivity of CYP enzymes originate from their non-rigid and self-adaptive coordination network with molecular ligands, which sacrifices the stability of the active motifs under electrochemical reaction conditions. Here, a design strategy to circumvent this dilemma by incorporating Fe-N₄ motifs into carbon matrices instead of the protein scaffold and replacing the axial molecular thiolate ligand with a stable tellurium cluster (Te_n ) is demonstrated. Theoretical calculations indicate a moderate interaction between Fe 3d and Te 5p orbitals once n > 2, allowing the FeTe bond to dynamically change its strength to adaptively facilitate the intermediate steps during the ORR process, which renders FeN₄ -Te_n active sites with superior ORR activity. This adaptive behavior mimics the conformational dynamics of an enzyme during the reaction, but retains the stability nature as a heterogeneous catalyst. The experiments validate that the as-designed catalyst with a characterized FeN₄ -Te_n structure outperforms the commercial Pt/C catalyst both on activity and stability.

Nanoemulsion-Coated Ni-Fe Hydroxide Self-Supported Electrode as an Air-Breathing Cathode for High-Performance Zinc-Air Batteries

To improve the energy conversion efficiency and durability of zinc-air batteries (ZABs) for large-scale implementations, here we propose an "air-breathing" strategy to significantly enlarge triple-interfaces with intensified mass transfer. By dip-coating the aerophilic perfluorochemical compounds (PFC) and amphiphilic ionomers into the self-supported electrodes, (1) the high solubility of O₂ in the PFC nanoemulsions greatly increases triple-phase boundaries and facilitates the efficient supply/removal of O₂ from the electrolyte; (2) the ionomers with hydrophobic and hydrophilic functionalities enable fast gas, water, and ion transport to the triple-phase boundaries; and (3) the self-supported electrode without binder ensures fast electron transfer while the firm integration prevents catalyst shedding. By applying this strategy, the ZABs show a high power density of 115 mW cm^-2 and a narrow discharge/charge gap of 0.64 V at 10 mA cm^-2 and a long-cycling durability (over 1000 h). This work provides a universal approach to promote gas-evolving reactions for electrochemical applications.

Nanoscale Double-Heterojunctional Electrocatalyst for Hydrogen Evolution

The active sites and charge/mass transfer properties in electrocatalysts play vital roles in kinetics and thermodynamics of electrocatalysis, and impose direct impacts on electrocatalytic performance, which cannot be achieved by a simplex structure. As a prototype, the authors propose a double-heterojunctional nanostructure of NiS₂ /Ni₃ C@C containing NiS₂ /Ni₃ C and Ni₃ C/C heterojunctions as a general model to optimize the above issues and boost electrocatalytic performance. During the thermal reorganization, the in situ reaction between NiS₂ nanoparticles and carbon induces the formation of Ni₃ C between them and constructs tightly contacted two kinds of interfaces among the three components. The TEM and XPS reveal the intimately contacted three components and the as-constructed interacted dual interfaces, further confirming the formation of a porous double-heterojunctional nanostructure. Theoretical calculations uncover that the electron density redistribution occurs at Ni₃ C/C interface by spontaneous electron transfer from defected carbon to Ni₃ C and lower ΔG_H* achieves at NiS₂ /Ni₃ C interface by the concentrated interfacial charge density, which favors the simultaneous realization of high catalytic activity and rapid charge/mass transfer. When applied for hydrogen evolution reaction (HER), the porous double-heterojunctional NiS₂ /Ni₃ C@C exhibits excellent HER activity and durability among all pH values. Profoundly, this special double-heterojunctional structure can provide a new model for high-performance electrocatalysts and beyond.

Cylindrical C96 Fullertubes: A Highly Active Metal-Free O2 -Reduction Electrocatalyst

A new isolation protocol was recently reported for highly purified metallic Fullertubes D_5h -C₉₀ , D_3d -C₉₆ , and D_5d -C_100, which exhibit unique electronic features. Here, we report the oxygen reduction electrocatalytic behavior of C₆₀ , C₇₀ (spheroidal fullerenes), and C₉₀ , C₉₆ , and C₁₀₀ (tubular fullerenes) using a combination of experimental and theoretical approaches. C₉₆ (a metal-free catalyst) displayed remarkable oxygen reduction reaction (ORR) activity, with an onset potential of 0.85 V and a halfway potential of 0.75 V, which are close to the state-of-the-art Pt/C benchmark catalyst values. We achieved an excellent power density of 0.75 W cm^-2 using C₉₆ as a modified cathode in a proton-exchange membrane fuel cell, comparable to other recently reported efficient metal-free catalysts. Combined band structure (experimentally calculated) and free-energy (DFT) investigations show that both favorable energy-level alignment active catalytic sites on the carbon cage are responsible for the superior activity of C₉₆ .

Anion-exchange membrane water electrolyzers and fuel cells

The key components, working management, and operating techniques of anion-exchange membrane water electrolyzers and fuel cells are reviewed for the first time.

Cation-Tuning Induced d-Band Center Modulation on Co-based Spinel Oxide for Rechargeable Zn-Air Batteries

Atomic substitutions at the tetrahedral site (A Td ) could theoretically achieve an efficient optimization of the charge at the octahedral site (B Oh ) through the A Td -O-B Oh interactions in the spinel oxides (AB2O4). However, the precise control and adjustment of the spinel oxides are still challenging owing to the complexity of their crystal structure. In this work, we demonstrate a simple solvent method to tailor the structures of spinel oxides and further use the spinel oxide composites (ACo2O4/NCNTs, A = Mn, Co, Ni, Cu, Zn) for oxygen electrocatalysis. And the optimized MnCo2O4/NCNTs exhibit high activity and excellent durability for oxygen reduction/evolution reactions. Remarkably, the rechargeable liquid Zn-air battery equipped the MnCo2O4/NCNTs cathode affords a specific capacity of 827 mAh gZn-1 with high power density of 74.63 mW cm-2 and no voltage degradation after 300 cycles at a high charging-discharging rate (5 mA cm-2). The density functional theory (DFT) calculations reveal that the substitution could regulate the ratio of Co3+/Co2+ and thereby lead to the electronic structure modulated accompanied with the movement of d-band center. The tetrahedral and octahedral sites interact through the Mn-O-Co, the Co3+ Oh of MnCo2O4 with the optimal charge structure allows more suitable binding interaction between the active center and the oxygenated species, resulting in superior oxygen electrocatalytic performance. This work not only proves the influence of the charge modulation mechanism on the oxygen catalysis process but also provides novel strategies for the subsequent design of other oxygen catalysis materials.

Reconstruction of spinel Co3O4 by inert Zn2+ towards enhanced oxygen catalytic activity

Zn_xCo_3-xO₄ (0 ≤ x≤ 1) coupled with nitrogen-doped hollow porous carbon spheres exhibits a superior oxygen catalytic activity. A Zn-air battery using Zn_0.6Co_2.4O₄/NHCS as a cathodic catalyst affords a high-power density (130 mW cm^-2) and excellent stability. The effect of reconstruction of catalytically active Co ions induced by Zn is well-investigated.