Designing Advanced Catalysts for Energy Conversion Based on Urea Oxidation Reaction.

Urea oxidation reaction (UOR) is the underlying reaction that determines the performance of modern urea-based energy conversion technologies. These technologies include electrocatalytic and photoelectrochemical urea splitting for hydrogen production and direct urea fuel cells as power engines. They have demonstrated great potentials as alternatives to current water splitting and hydrogen fuel cell systems with more favorable operating conditions and cost effectiveness. At the moment, UOR performance is mainly limited by the 6-electron transfer process. In this case, various material design and synthesis strategies have recently been reported to produce highly efficient UOR catalysts. The performance of these advanced catalysts is optimized by the modification of their structural and chemical properties, including porosity development, heterostructure construction, defect engineering, surface functionalization, and electronic structure modulation. Considering the rich progress in this field, the recent advances in the design and synthesis of UOR catalysts for urea electrolysis, photoelectrochemical urea splitting, and direct urea fuel cells are reviewed here. Particular attention is paid to those design concepts, which specifically target the characteristics of urea molecules. Moreover, challenges and prospects for the future development of urea-based energy conversion technologies and corresponding catalysts are also discussed.

Fabrication of Hollow CoP/TiOx Heterostructures for Enhanced Oxygen Evolution Reaction.

Transition-metal phosphides have flourished as promising candidates for oxygen evolution reaction (OER) electrocatalysts. Herein, it is demonstrated that the electrocatalytic OER performance of CoP can be greatly improved by constructing a hybrid CoP/TiOx heterostructure. The CoP/TiOx heterostructure is fabricated using metal-organic framework nanocrystals as templates, which leads to unique hollow structures and uniformly distributed CoP nanoparticles on TiOx . The strong interactions between CoP and TiOx in the CoP/TiOx heterostructure and the conductive nature of TiOx with Ti3+ sites endow the CoP-TiOx hybrid material with high OER activity comparable to the state-of-the-art IrO2 or RuO2 OER electrocatalysts. In combination with theoretical calculations, this work reveals that the formation of CoP/TiOx heterostructure can generate a pathway for facile electron transport and optimize the water adsorption energy, thus promoting the OER electrocatalysis.

Antiperovskite Intermetallic Nanoparticles for Enhanced Oxygen Reduction.

Antiperovskite Co3 InC0.7 N0.3 nanomaterials with highly enhanced oxygen reduction reaction (ORR) performance were prepared by tuning nitrogen contents through a metal-organic framework (MOF)-derived strategy. The nanomaterial surpasses all reported noble-metal-free antiperovskites and even most perovskites in terms of onset potential (0.957 V at J=0.1 mA cm-2 ) and half-wave potential (0.854 V). The OER and zinc-air battery performance demonstrate its multifunctional oxygen catalytic activities. DFT calculation was performed and for the first time, a 4 e- dissociative ORR pathway on (200) facets of antiperovskite was revealed. Free energy studies showed that nitrogen substitution could strengthen the OH desorption as well as hydrogenation that accounts for the enhanced ORR performance. This work expands the scope for material design via tailoring the nitrogen contents for optimal reaction free energy and hence performance of the antiperovskite system.

Puffing Up Energetic Metal-Organic Frameworks to Large Carbon Networks with Hierarchical Porosity and Atomically Dispersed Metal Sites.

Large carbon networks featuring hierarchical pores and atomically dispersed metal sites (ADMSs) are ideal materials for energy storage and conversion due to the spatially continuous conductive networks and highly active ADMSs. However, it is a challenge to synthesize such ADMS-decorated carbon networks. Here, an innovative fusion-foaming methodology is presented in which energetic metal-organic framework (EMOF) nanoparticles are puffed up to submillimeter-scaled ADMS-decorated carbon networks via a one-step pyrolysis. Their extraordinary catalytic performance towards oxygen reduction reaction verifies the practicability of this synthetic approach. Moreover, this approach can be readily applicable to a wide range of unexplored EMOFs, expanding scopes for future materials design.

"One-for-All" Strategy in Fast Energy Storage: Production of Pillared MOF Nanorod-Templated Positive/Negative Electrodes for the Application of High-Performance Hybrid Supercapacitor.

Currently, metal-organic frameworks (MOFs) are intensively studied as active materials for electrochemical energy storage applications due to their tunable structure and exceptional porosities. Among them, water stable pillared MOFs with dual ligands have been reported to exhibit high supercapacitor (SC) performance. Herein, the "One-for-All" strategy is applied to synthesize both positive and negative electrodes of a hybrid SC (HSC) from a single pillared MOF. Specifically, Ni-DMOF-TM ([Ni(TMBDC)(DABCO)0.5 ], TMBDC: 2,3,5,6-tetramethyl-1,4-benzenedicarboxylic acid, DABCO: 1,4-diazabicyclo[2.2.2]-octane) nanorods are directly grown on carbon fiber paper (CFP) (denoted as CFP@TM-nanorods) with the help of triethylamine and function as the positive electrode of HSC under alkaline electrolyte. Meanwhile, calcinated N-doped hierarchical porous carbon nanorods (CFP@TM-NPCs) are produced and utilized as the negative counter-electrode from a one-step heat treatment of CFP@TM-nanorods. After assembling these two electrodes together to make a hybrid device, the TM-nanorods//TM-NPCs exhibit a wide voltage window of 1.5 V with a high sloping discharge plateau between 1-1.2 V, indicating its great potential for practical applications. This as-described "One-for-All" strategy is widely applicable and highly reproducible in producing MOF-based electrode materials for HSC applications, which shortens the gap between experimental synthesis and practical application of MOFs in fast energy storage.

Influence of transition metal catalysts on the decomposition product distribution of PCBs and PCDD/Fs.

In this study, reliable and stable polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) generation systems were used to investigate catalyst performance. The distribution characteristics of PCBs and PCDD/Fs after reaction were evaluated under different simulated flue gas conditions using NiO- and MnO2-loaded γ-Al2O3 and ZSM-5 catalysts. The results showed that the active metal of the catalyst affected mainly the decomposition efficiency, the substrate type affected the distribution characteristics of PCBs, and both the active metal and substrate type jointly determined the fingerprint distribution characteristics of PCBs. Moreover, there was an apparent marginal effect of the active metal loading on the same catalyst and the decomposition efficiency of the pollutants. In the temperature range of 100-350 °C, temperature variation had little effect on the removal efficiency of PCBs, but the gas-solid phase distribution characteristics of the pollutants changed significantly, and large amounts of di- and tri-CBs were generated in the products at 200 °C. A small amount of water generated hydrogen via the water-gas shift reaction at medium temperature, which promoted the hydro-dechlorination reaction of the chlorinated organics. However, excess water in the substrate gas competes with the pollutants for adsorption sites and reduces the reaction activity of the catalyst.

Dual-mode optical thermometer based on fluorescence intensity ratio of Eu3+/Mn4+ co-doping zinc titanate phosphors.

Zinc titanate phosphors containing Eu3+/ Mn4+ as active ions were synthesized by using the solid-state method. XRD patterns of the powders confirmed that the samples were a mixture of cubic Zn2TiO4 and hexagonal ZnTiO3 phases. The luminous intensity of ZTO: Eu3+phosphors and ZTO: Mn4+ phosphors both increased with the increase of doping concentration, reaching the maximum at 2 mol% Eu3+ and 0.3 mol% Mn4+, respectively. In the photoluminescence spectra of ZTO: Eu3+(2 mol%) phosphors with different Mn4+ doping amounts excited at 465 nm, the emission spectra revealed the characteristic peaks of Eu3+ with low Mn4+ content, and with the Mn4+ content increasing, the emission spectra contained both Mn4+ and Eu3+ luminescence peaks. In the variable temperature spectra, the relative sensitivity of the samples was improved with the concentration of Mn4+ increasing and achieved the maximum value of 3.2 %/K.

A high sensitivity dual-mode optical thermometry based on charge compensation in ZnTiO3:M (M = Eu3+, Mn4+) hexagonal prisms.

Optical thermometer based on dual-mode fluorescence intensity ratiometric thermometry has been attracted more attention due to its higher sensitivity. In order to obtain optical thermal probe with high sensitivity, ZnTiO3 hexagonal prisms with hexagonal perovskite structure were fabricated by using self-assembly method, and Al3+ ions were introduced into the crystal lattices of ZnTiO3 doped with Eu3+/Mn4+ to improve the optical properties. The emission intensity assigned to Eu3+ was enhanced about twice with the charge compensation of Al3+ between Eu3+ and Ti3+. While the luminescence ratio between the thermal coupled level of Eu3+ revealed poor temperature dependence property. The emission assigned to 2Eg→4A2g (713 nm) transition of Mn4+ revealed an huge thermal quenching. Using the luminescence ratio between 5D0→7F2 (5D0→7F1) transition of Eu3+ to 2Eg→4A2g of Mn4+, the higher relative sensitivity of 2.7 %K-1was obtained. The charge compensation of Al3+ improved the coordination and reduced the relative sensitivity, Sr =1.85 %K-1. The results suggested the potential application in optical temperature probes for ZnTiO3: Mn4+,Eu3+ phosphor.

Mn2+ ions substitution inducing improvement of optical performances in ZnAl2O4: Cr3+ phosphors: Energy transfer and ratiometric optical thermometry.

Zn1-xMnxAl2O4:0.1 mol% Cr3+ (0.04≤x≤0.16) phosphors with single spinel phase were synthesized by using sol-gel method and the structure, optical and temperature sensing performances were reported herein. The results of X-ray photoelectron spectra indicate that the inversion defects related to octahedral Zn are reduced and the crystal field surrounding Al changes with Mn2+ doping in ZnAl2O4 lattices. Mn2+/Cr3+ co-doped ZnAl2O4 nanophosphors reveal a green emission band assigned to Mn2+ and a series of red emission peaks assigned to Cr3+, respectively. With the concentration of Mn2+ increasing, the intensity of zero phonon line (R line) assigned to Cr3+ increases, reaching the maximum at the optimal Mn2+ concentration of x=0.14. The energy transfer from Mn2+ to Cr3+ is confirmed with the energy transfer efficiency of 83%. The separation between 2E(eg) and 2E(tg) of Cr3+ is enlarged due to Mn2+ dopants giving rise to a change of crystal field. The luminous intensity ratio between two separated emission peaks at 685 nm (R3) and 689 nm (R2) reveals an obvious temperature dependence. The relative sensitivity changes from 3.7 %K-1 to 0.25 %K-1 with the temperature increasing from 80 K to 310 K, which is much larger than that of ZnAl2O4:Cr3+ nanophosphors without Mn2+, indicating its good application prospect in optical thermometry.

Characterization and degradation mechanism of bimetallic iron-based/AC activated persulfate for PAHs-contaminated soil remediation.

In this research, a novel iron based bimetallic nanoparticles (Fe-Ni) supported on activated carbon (AC) were synthesized and employed as an activator of persulfate in polycyclic aromatic hydrocarbons (PAHs) polluted sites remediation. AC-supported Fe-Ni activator was prepared according to two-step reduction method: the liquid phase reduction and H2- reduction under high temperature (600 °C), which was defined as Fe-Ni/AC. Characterizations using micropore physisorption analyzer, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HR-TEM) showed that the synthetic material had large specific surface area, nano-size and carbon-encapsulated metal particles, moreover, the lattice fringes of metals were clearly defined. The PAH compound types and their concentrations were determined by gas chromatography mass spectrometry (GC-MS) with SIM mode, the method detection limit (MDL) was estimated to about 0.21 µg/kg for PAHs, and the average recovery of PAHs was 96.3%. Mechanisms of PAH oxidation degradation with the reaction system of Fe-Ni/AC activated persulfate were discussed, the results showed that short-life free radicals, such as SO4-·, OH·, and OOH· were generated simultaneously, which acted as strong oxidizing radicals, resulting in the oxidation and almost complete opening of the PAH rings.

Ultrafast Sodium/Potassium-Ion Intercalation into Hierarchically Porous Thin Carbon Shells.

The large-scale application of sodium/potassium-ion batteries is severely limited by the low and slow charge storage dynamics of electrode materials. The crystalline carbons exhibit poor insertion capability of large Na+ /K+ ions, which limits the storage capability of Na/K batteries. Herein, porous S and N co-doped thin carbon (S/N@C) with shell-like (shell size ≈20-30 nm, shell wall ≈8-10 nm) morphology for enhanced Na+ /K+ storage is presented. Thanks to the hollow structure and thin shell-wall, S/N@C exhibits an excellent Na+ /K+ storage capability with fast mass transport at higher current densities, leading to limited compromise over charge storage at high charge/discharge rates. The S/N@C delivers a high reversible capacity of 448 mAh g-1 for Na battery, at the current density of 100 mA g-1 and maintains a discharge capacity up to 337 mAh g-1 at 1000 mA g-1 . Owing to shortened diffusion pathways, S/N@C delivers an unprecedented discharge capacity of 204 and 169 mAh g-1 at extremely high current densities of 16 000 and 32 000 mA g-1 , respectively, with excellent reversible capacity for 4500 cycles. Moreover, S/N@C exhibits high K+ storage capability (320 mAh g-1 at current density of 50 mA g-1 ) and excellent cyclic life.

Synergistic Effect of Co-Ni Hybrid Phosphide Nanocages for Ultrahigh Capacity Fast Energy Storage.

Rational design of metal compounds in terms of the structure/morphology and chemical composition is essential to achieve desirable electrochemical performances for fast energy storage because of the synergistic effect between different elements and the structure effect. Here, an approach is presented to facilely fabricate mixed-metal compounds including hydroxides, phosphides, sulfides, oxides, and selenides with well-defined hollow nanocage structure using metal-organic framework nanocrystals as sacrificial precursors. Among the as-synthesized samples, the porous nanocage structure, synergistic effect of mixed metals, and unique phosphide composition endow nickel cobalt bimetallic phosphide (NiCo-P) nanocages with outstanding performance as a battery-type Faradaic electrode material for fast energy storage, with ultrahigh specific capacity of 894 C g-1 at 1 A g-1 and excellent rate capability, surpassing most of the reported metal compounds. Control experiments and theoretical calculations based on density functional theory reveal that the synergistic effect between Ni and Co in NiCo-P can greatly increase the OH- adsorption energy, while the hollow porous structure facilitates the fast mass/electron transport. The presented work not only provides a promising electrode material for fast energy storage, but also opens a new route toward structural and compositional design of electrode materials for energy storage and conversion.

Highly Dispersed Co-B/N Codoped Carbon Nanospheres on Graphene for Synergistic Effects as Bifunctional Oxygen Electrocatalysts.

Oxygen reduction and evolution reactions as two important electrochemical energy conversion processes in metal-air battery devices have aroused widespread concern. However, synthesis of low-cost non-noble metal-based bifunctional high-performance electrocatalysts is still a great challenge. In this work, we report on the design and synthesis of a novel Co-B/N codoped carbon with core-shell-structured nanoparticles aligned on graphene nanosheets (denoted as CoTIB-C/G) derived from cobalt tetrakis(1-imidazolyl)borate (CoTIB) and graphene oxide hybrid template. Compared with pristine CoTIB-derived bulk structure (CoTIB-C), CoTIB-C/G particles with an average size of 25 nm are uniformly dispersed on highly conductive graphene sheets in the hybrid material, thus dramatically increasing the utilization efficiency and activity of the active components upon oxygen reduction and evolution. After all, because of the "barrier effect" of graphene sheets toward CoTIB-C/G and the synergistic effect between Co nanoparticles and carbon shells linked to the graphene sheets, as well as heteroatoms' doping effect, the as-obtained bifunctional electrocatalyst exhibits remarkable oxygen reduction and evolution reaction activities in alkaline media, indicating its feasibility and potential in practical applications.