Dynamic Phase Transformations of Prussian Blue Analogue Crystals in Hydrotherms.

Prussian blue analogue (PBA)/metal-organic frameworks (MOFs) are multifunctional precursors for the synthesis of metal/metal compounds, carbon, and their derived composites (P/MDCs) in chemical, medical, energy, and other applications. P/MDCs combine the advantages of both the high specific surface area of PBA/MOF and the electronic conductivity of metal compound/carbon. Although the calcination under different atmospheres has been extensively studied, the transformation mechanism of PBA/MOF under hydrothermal conditions remains unclear. The qualitative preparation of P/MDCs in hydrothermal conditions remains a challenge. Here, we select PBA to construct a machine-learning model and measure its hydrothermal phase diagram. The architecture-activity relationship of substances among nine parameters was analyzed for the hydrothermal phase transformation of PBA. Excitingly, we established a universal qualitative model to accurately fabricate 31 PBA derivates. Additionally, we performed three-dimensional reconstructed transmission electron microscopy, X-ray absorption fine structure spectroscopy, ultraviolet photoelectron spectroscopy, in situ X-ray powder diffraction, and theoretical calculation to analyze the advantages of hydrothermal derivatives in the oxygen evolution reaction and clarify their reaction mechanisms. We uncover the unified principles of the hydrothermal phase transformation of PBA, and we expect to guide the design for a wide range of composites.

Synergistically Coupling Atomic-Level Defect-Manipulation and Nanoscopic-Level Interfacial Engineering Enables Fast and Durable Sodium Storage.

Through inducing interlayer anionic ligands and functionally modifying conductive carbon-skeleton on the transition metal chalcogenides (TMCs) parent to achieve atomic-level defect-manipulation and nanoscopic-level architecture design is of great significance, which can broaden interlayer distance, optimize electronic structure, and mitigate structural deformation to endow high-efficiency battery performance of TMCs. Herein, an intriguing 3D biconcave hollow-tyre-like anode constituted by carbon-packaged defective-rich SnSSe nanosheet grafting onto Aspergillus niger spores-derived hollow-carbon (ANDC@SnSSe@C) is reported. Systematically experimental investigations and theoretical analyses forcefully demonstrate the existence of anion Se ligand and outer-carbon all-around encapsulation on the ANDC@SnSSe@C can effectively yield abundant structural defects and Na+-reactivity sites, accelerate rapid ion migration, widen interlayer spacing, as well as relieve volume expansion, thus further resolving the critical issues throughout the charge-discharge processes. As anticipated, as-fabricated ANDC@SnSSe@C anode contributes extraordinary reversible capacity, wonderful cyclic lifespan with 83.4% capacity retention over 2000 cycles at 20.0 A g-1, and exceptional rate capability. A series of correlated kinetic investigations and ex situ characterizations deeply reveal the underlying springheads for the ion-transport kinetics, as well as synthetically elucidate phase-transformation mechanism of the ANDC@SnSSe@C. Furthermore, the ANDC@SnSSe@C-based sodium ion full cell and hybrid capacitor offer high-capacity contribution and remarkable energy-density output, indicative of its great practicability.

Arming Amorphous NiMoO4 on Nickel Phosphide Enables Highly Stable Alkaline Seawater Oxidation.

Seawater electrolysis holds tremendous promise for the generation of green hydrogen (H2 ). However, the system of seawater-to-H2 faces significant hurdles, primarily due to the corrosive effects of chlorine compounds, which can cause severe anodic deterioration. Here, a nickel phosphide nanosheet array with amorphous NiMoO4 layer on Ni foam (Ni2 P@NiMoO4 /NF) is reported as a highly efficient and stable electrocatalyst for oxygen evolution reaction (OER) in alkaline seawater. Such Ni2 P@NiMoO4 /NF requires overpotentials of just 343 and 370 mV to achieve industrial-level current densities of 500 and 1000 mA cm-2 , respectively, surpassing that of Ni2 P/NF (470 and 555 mV). Furthermore, it maintains consistent electrolysis for over 500 h, a significant improvement compared to that of Ni2 P/NF (120 h) and Ni(OH)2 /NF (65 h). Electrochemical in situ Raman spectroscopy, stability testing, and chloride extraction analysis reveal that is situ formed MoO4 2- /PO4 3- from Ni2 P@NiMoO4 during the OER test to the electrode surface, thus effectively repelling Cl- and hindering the formation of harmful ClO- .

Rational Construction of 3D Self-Supported MOF-Derived Cobalt Phosphide-Based Hollow Nanowall Arrays for Efficient Overall Water Splitting At large Current Density.

Developing efficient nonprecious bifunctional electrocatalysts for hydrogen and oxygen evolution reactions (HER and OER) in the same electrolyte with a low overpotential and large current density presents an appealing yet challenging goal for large-scale water electrolysis. Herein, a unique 3D self-branched hierarchical nanostructure composed of ultra-small cobalt phosphide (CoP) nanoparticles embedded into N, P-codoped carbon nanotubes knitted hollow nanowall arrays (CoPʘNPCNTs HNWAs) on carbon textiles (CTs) through a carbonization-phosphatization process is presented. Benefiting from the uniform protrusion distributions of CoP nanoparticles, the optimum CoPʘNPCNTs HNWAs composites with high abundant porosity exhibit superior electrocatalytic activity and excellent stability for OER in alkaline conditions, as well as for HER in both acidic and alkaline electrolytes, even under large current densities. Furthermore, the assembled CoPʘNPCNTs/CTs||CoPʘNPCNTs/CTs electrolyzer demonstrates exceptional performance, requiring an ultralow cell voltage of 1.50 V to deliver the current density of 10 mA cm-2 for overall water splitting (OWS) with favorable stability, even achieving a large current density of 200 mA cm-2 at a low cell voltage of 1.78 V. Density functional theory (DFT) calculation further reveals that all the C atoms between N and P atoms in CoPʘNPCNTs/CTs act as the most efficient active sites, significantly enhancing the electrocatalytic properties. This strategy, utilizing 2D MOF arrays as a structural and compositional material to create multifunctional composites/hybrids, opens new avenues for the exploration of highly efficient and robust non-noble-metal catalysts for energy-conversion reactions.

Tungstate Intercalated NiFe Layered Double Hydroxide Enables Long-Term Alkaline Seawater Oxidation.

Renewable electricity-driven seawater splitting presents a green, effective, and promising strategy for building hydrogen (H2)-based energy systems (e.g., storing wind power as H2), especially in many coastal cities. The abundance of Cl- in seawater, however, will cause severe corrosion of anode catalyst during the seawater electrolysis, and thus affect the long-term stability of the catalyst. Herein, seawater oxidation performances of NiFe layered double hydroxides (LDH), a classic oxygen (O2) evolution material, can be boosted by employing tungstate (WO4 2-) as the intercalated guest. Notably, insertion of WO4 2- to LDH layers upgrades the reaction kinetics and selectivity, attaining higher current densities with ≈100% O2 generation efficiency in alkaline seawater. Moreover, after a 350 h test at 1000 mA cm-2, only trace active chlorine can be detected in the electrolyte. Additionally, O2 evolution follows lattice oxygen mechanism on NiFe LDH with intercalated WO4 2-.

Carbon Oxyanion Self-Transformation on NiFe Oxalates Enables Long-Term Ampere-Level Current Density Seawater Oxidation.

Seawater electrolysis is an attractive way of making H2 in coastal areas, and NiFe-based materials are among the top options for alkaline seawater oxidation (ASO). However, ample Cl- in seawater can severely corrode catalytic sites and lead to limited lifespans. Herein, we report that in situ carbon oxyanion self-transformation (COST) from oxalate to carbonate on a monolithic NiFe oxalate micropillar electrode allows safeguard of high-valence metal reaction sites in ASO. In situ/ex situ studies show that spontaneous, timely, and appropriate COST safeguards active sites against Cl- attack during ASO even at an ampere-level current density (j). Our NiFe catalyst shows efficient and stable ASO performance, which requires an overpotential as low as 349 mV to attain a j of 1 A cm-2 . Moreover, the NiFe catalyst with protective surface CO3 2- exhibits a slight activity degradation after 600 h of electrolysis under 1 A cm-2 in alkaline seawater. This work reports effective catalyst surface design concepts at the level of oxyanion self-transformation, acting as a momentous step toward defending active sites in seawater-to-H2 conversion systems.

Constructing Co@TiO2 Nanoarray Heterostructure with Schottky Contact for Selective Electrocatalytic Nitrate Reduction to Ammonia.

Electrochemical nitrate (NO3 - ) reduction reaction (NO3 - RR) is a potential sustainable route for large-scale ambient ammonia (NH3 ) synthesis and regulating the nitrogen cycle. However, as this reaction involves multi-electron transfer steps, it urgently needs efficient electrocatalysts on promoting NH3  selectivity. Herein, a rational design of Co nanoparticles anchored on TiO2  nanobelt array on titanium plate (Co@TiO2 /TP) is presented as a high-efficiency electrocatalyst for NO3 - RR. Density theory calculations demonstrate that the constructed Schottky heterostructures coupling metallic Co with semiconductor TiO2  develop a built-in electric field, which can accelerate the rate determining step and facilitate NO3 - adsorption, ensuring the selective conversion to NH3 . Expectantly, the Co@TiO2 /TP electrocatalyst attains an excellent Faradaic efficiency of 96.7% and a high NH3  yield of 800.0 µmol h-1  cm-2  under neutral solution. More importantly, Co@TiO2 /TP heterostructure catalyst also presents a remarkable stability in 50-h electrolysis test.

Pd-Doped Co3 O4 Nanoarray for Efficient Eight-Electron Nitrate Electrocatalytic Reduction to Ammonia Synthesis.

Ammonia (NH3 ) is an indispensable feedstock for fertilizer production and one of the most ideal green hydrogen rich fuel. Electrochemical nitrate (NO3 - ) reduction reaction (NO3 - RR) is being explored as a promising strategy for green to synthesize industrial-scale NH3 , which has nonetheless involved complex multi-reaction process. This work presents a Pd-doped Co3 O4 nanoarray on titanium mesh (Pd-Co3 O4 /TM) electrode for highly efficient and selective electrocatalytic NO3 - RR to NH3 at low onset potential. The well-designed Pd-Co3 O4 /TM delivers a large NH3 yield of 745.6 µmol h-1 cm-2 and an extremely high Faradaic efficiency (FE) of 98.7% at -0.3 V with strong stability. These calculations further indicate that the doping Co3 O4 with Pd improves the adsorption characteristic of Pd-Co3 O4 and optimizes the free energies for intermediates, thereby facilitating the kinetics of the reaction. Furthermore, assembling this catalyst in a Zn-NO3 - battery realizes a power density of 3.9 mW cm-2 and an excellent FE of 98.8% for NH3 .

Defective TiO2- x for High-Performance Electrocatalytic NO Reduction toward Ambient NH3 Production.

Synthesis of green ammonia (NH3 ) via electrolysis of nitric oxide (NO) is extraordinarily sustainable, but multielectron/proton-involved hydrogenation steps as well as low concentrations of NO can lead to poor activities and selectivities of electrocatalysts. Herein, it is reported that oxygen-defective TiO2 nanoarray supported on Ti plate (TiO2- x /TP) behaves as an efficient catalyst for NO reduction to NH3 . In 0.2 m phosphate-buffered electrolyte, such TiO2- x /TP shows competitive electrocatalytic NH3 synthesis activity with a maximum NH3 yield of 1233.2 µg h-1  cm-2 and Faradaic efficiency of 92.5%. Density functional theory calculations further thermodynamically faster NO deoxygenation and protonation processes on TiO2- x (101) compared to perfect TiO2 (101). And the low energy barrier of 0.7 eV on TiO2- x (101) for the potential-determining step further highlights the greatly improved intrinsic activity. In addition, a Zn-NO battery is fabricated with TiO2- x /TP and Zn plate to obtain an NH3 yield of 241.7 µg h-1  cm-2 while providing a peak power density of 0.84 mW cm-2 .

Rational Construction of Heterostructured Cu3 P@TiO2 Nanoarray for High-Efficiency Electrochemical Nitrite Reduction to Ammonia.

Electroreduction of nitrite (NO2 - ) to valuable ammonia (NH3 ) offers a sustainable and green approach for NH3 synthesis. Here, a Cu3 P@TiO2 heterostructure is rationally constructed as an active catalyst for selective NO2 - -to-NH3 electroreduction, with rich nanosized Cu3 P anchored on a TiO2 nanoribbon array on Ti plate (Cu3 P@TiO2 /TP). When performed in the 0.1 m NaOH with 0.1 m NaNO2 , the Cu3 P@TiO2 /TP electrode obtains a large NH3 yield of 1583.4 µmol h-1  cm-2 and a high Faradaic efficiency of 97.1%. More importantly, Cu3 P@TiO2 /TP also delivers remarkable long-term stability for 50 h electrolysis. Theoretical calculations indicate that intermediate adsorption/conversion processes on Cu3 P@TiO2 interfaces are synergistically optimized, substantially facilitating the conversion of NO2 - -to-NH3 .

Improved Electrochemical Alkaline Seawater Oxidation over Cobalt Carbonate Hydroxide Nanowire Array by Iron Doping.

Constructing efficient and low-cost oxygen evolution reaction (OER) catalysts operating in seawater is essential for green hydrogen production but remains a great challenge. In this study, we report an iron doped cobalt carbonate hydroxide nanowire array on nickel foam (Fe-CoCH/NF) as a high-efficiency OER electrocatalyst. In alkaline seawater, such Fe-CoCH/NF demands an overpotential of 387 mV to drive 500 mA cm-2, superior to that of CoCH/NF (597 mV). Moreover, it achieves excellent electrochemical and structural stability in alkaline seawater.

Ambient Ammonia Synthesis via Nitrate Electroreduction in Neutral Media on Fe3O4 Nanoparticles-decorated TiO2 Nanoribbon Array.

Electrochemical nitrate (NO3-) reduction is a potential approach to produce high-value ammonia (NH3) while removing NO3- pollution, but it requires electrocatalysts with high efficiency and selectivity. Herein, we report the development of Fe3O4 nanoparticles decorated TiO2 nanoribbon array on titanium plate (Fe3O4@TiO2/TP) as an efficient electrocatalyst for NO3--to-NH3 conversion. When operated in 0.1 M phosphate-buffered saline and 0.1 M NO3-, such Fe3O4@TiO2/TP achieves a prominent NH3 yield of 12394.3 µg h-1 cm-2 and a high Faradaic efficiency of 88.4%. In addition, it exhibits excellent stability during long-time electrolysis.

Improved Alkaline Seawater Splitting of NiS Nanosheets by Iron Doping.

Seawater electrolysis driven by renewable electricity is deemed a promising and sustainable strategy for green hydrogen production, but it is still formidably challenging. Here, we report an iron-doped NiS nanosheet array on Ni foam (Fe-NiS/NF) as a high-performance and stable seawater splitting electrocatalyst. Such Fe-NiS/NF catalyst needs overpotentials of only 420 and 270 mV at 1000 mA cm-2 for the oxygen evolution reaction and hydrogen evolution reaction in alkaline seawater, respectively. Furthermore, its two-electrode electrolyzer needs a cell voltage of 1.88 V for 1000 mA cm-2 with 50 h of long-term electrochemical durability in alkaline seawater. Additionally, in situ electrochemical Raman and infrared spectroscopy were employed to detect the reconstitution process of NiOOH and the generation of oxygen intermediates under reaction conditions.

Cobalt-Nanoparticles-Decorated 3D Porous Nitrogen-Doped Carbon Network for Electrocatalytic Nitrite Reduction to Ammonia.

Electrocatalytic nitrite (NO2-) reduction offers the potential to synthesize high-value ammonia (NH3) while simultaneously removing NO2- pollution from aqueous solutions, but it requires high-efficiency catalysts to drive the complex six-electron reaction. Herein, cobalt-nanoparticle-decorated 3D porous nitrogen-doped carbon network (Co@NC) is proven as a high-efficiency catalyst for the selective electroreduction of NO2- to NH3. Such Co@NC attains a large NH3 yield of 922.7 µmol h-1 cm-2 and a high Faradaic efficiency of 95.4% under alkaline conditions. Furthermore, it shows remarkable electrochemical stability during cyclic electrolysis.

Ultrafast and ultrastable FeSe2embedded in nitrogen-doped carbon nanofibers anode for sodium-ion half/full batteries.

Transition metal selenides are considered as promising anode materials for fast-charging sodium-ion batteries due to their high theoretical specific capacity. However, the low intrinsic conductivity, particle aggregation, and large volume expansion problems can severely inhibit the high-rate and long-cycle performance of the electrode. Herein, FeSe2nanoparticles embedded in nitrogen-doped carbon nanofibers (FeSe2@NCF) have been synthesized using the electrospinning and selenization process, which can alleviate the volume expansion and particle aggregation during the sodiation/desodiation and improve the electrical conductivity of the electrode. The FeSe2@NCF electrode delivers the outstanding specific capacity of 222.3 mAh g-1at a fast current density of 50 A g-1and 262.1 mAh g-1at 10 A g-1with the 87.8% capacity retention after 5000 cycles. Furthermore, the Na-ion full cells assembled with pre-sodiated FeSe2@NCF as anode and Na3V2(PO4)3/C as cathode exhibit the reversible specific capacity of 117.6 mAh g-1at 5 A g-1with the 84.3% capacity retention after 1000 cycles. This work provides a promising way for the conversion-based metal selenides for the applications as fast-charging sodium-ion battery anode.

Durable Electrocatalytic Reduction of Nitrate to Ammonia over Defective Pseudobrookite Fe2 TiO5 Nanofibers with Abundant Oxygen Vacancies.

We propose the pseudobrookite Fe2 TiO5 nanofiber with abundant oxygen vacancies as a new electrocatalyst to ambiently reduce nitrate to ammonia. Such catalyst achieves a large NH3 yield of 0.73 mmol h-1 mg-1 cat. and a high Faradaic Efficiency (FE) of 87.6 % in phosphate buffer saline solution with 0.1 M NaNO3 , which is lifted to 1.36 mmol h-1 mg-1 cat. and 96.06 % at -0.9 V vs. RHE for nitrite conversion to ammonia in 0.1 M NaNO2 . It also shows excellent electrochemical durability and structural stability. Theoretical calculation reveals the enhanced conductivity of this catalyst and an extremely low free energy of -0.28 eV for nitrate adsorption at the presence of vacant oxygen.

Crystal Facet Engineering of MXene-Derived TiN Nanoflakes as Efficient Bidirectional Electrocatalyst for Advanced Lithium-Sulfur Batteries.

The design of nanomaterials with grain orientation structure by crystal facet engineering is of great significance for boosting the catalytic ability and electrochemical properties, but the controllable synthesis is still a challenge. Here, TiN nanoflakes with exposed (001) facets are prepared using 2D Ti3 C2 MXene as the initial reactant and applied as a bidirectional electrocatalyst for the reduction and oxidation process in lithium-sulfur batteries (LSBs). The (001) facet-dominated TiN nanoflakes have a strong adsorption capacity for soluble lithium polysulfides (LiPSs). More importantly, theoretical calculations and experiment results confirm the (001) facet-dominated TiN nanoflakes catalyze the conversion of soluble LiPSs to Li2 S2 /Li2 S to induce the Li2 S uniform deposition in the discharge process and decrease the delithiation barrier of Li2 S in the charge process. Therefore, the excellent electrochemical properties of LSBs are achieved, which demonstrates a high discharge capacity of 949 mAh g-1 at 1 C and maintains high capacity reversibility with a decay rate of 0.033% per cycle after 800 cycles.

Enhanced Activities in Alkaline Hydrogen and Oxygen Evolution Reactions on MoS2 Electrocatalysts by In-Plane Sulfur Defects Coupled with Transition Metal Doping.

2D transition metal disulfides (TMDs) are promising and cost-effective alternatives to noble-metal-based catalysts for hydrogen production. Activation of the inert basal plane of TMDs is crucial to improving the catalytic efficiency. Herein, introduction of in-plane sulfur vacancies (Sv ) and 3d transition metal dopants in concert activates the basal planes of MoS2 (M-Sv -MoS2 ) to achieve high activities in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Acetate introducing mild wet chemical etching removes surface S atoms facilitating subsequent cation exchange between the exposed Mo atoms and targeted metal ions in solution. Density-functional theory calculation demonstrates that the exposed 3d transition metal dopants in MoS2 basal planes serve as multifunctional active centers, which not only reduce ΔGH* but also accelerate water oxidation. As a result, the optimal Ni-Sv -MoS2 and Co-Sv -MoS2 electrocatalysts show excellent stability and alkaline HER and OER characteristics such as low overpotentials of 101 and 190 mV at 10 mA cm-2 , respectively. The results reveal a strategy to activate the inert MoS2 basal planes by defect and doping co-engineering and the technique can be extended to other types of TMDs for high-efficiency electrocatalysis beyond water splitting.

Ambient Ammonia Synthesis via Electrochemical Reduction of Nitrate Enabled by NiCo2 O4 Nanowire Array.

NiCo2 O4 nanowire array on carbon cloth (NiCo2 O4 /CC) is proposed as a highly active electrocatalyst for ambient nitrate (NO3 - ) reduction to ammonia (NH3 ). In 0.1 m NaOH solution with 0.1 m NaNO3 , such NiCo2 O4 /CC achieves a high Faradic efficiency of 99.0% and a large NH3 yield up to 973.2 µmol h-1  cm-2 . The superior catalytic activity of NiCo2 O4 comes from its half-metal feature and optimized adsorption energy due to the existence of Ni in the crystal structure. A Zn-NO3 - battery with NiCo2 O4 /CC cathode also shows a record-high battery performance.

Co Nanoparticles Decorated Corncob-Derived Biomass Carbon as an Efficient Electrocatalyst for Nitrate Reduction to Ammonia.

Nitrate (NO3-) is a type of common pollutant in aqueous systems. Electrochemical NO3- reduction is an ecofriendly and sustainable strategy, which can selectively reduce NO3- to highly value-added NH3 and remove NO3- pollutants at the same time. In this work, Co nanoparticles decorated corncob-derived biomass carbon as a highly active electrocatalyst for NO3- to NH3 conversion. Such a catalyst can achieve an amazing Faradaic efficiency of 93.4% and a large NH3 yield of 0.60 mmol h-1 cm-2 in alkaline media. 15N-Labeling experiment proves that the detected NH3 is derived from NO3- electroreduction. In addition, it also displays excellent durability in long-term and cycle-electrolysis tests.