Design of Single-Atom Catalysts for E lectrocatalytic Nitrogen Fixation.

The Electrochemical nitrogen reduction reaction (ENRR) can be used to solve environmental problems as well as energy shortage. However, ENRR still faces the problems of low NH3 yield and low selectivity. The NH3 yield and selectivity in ENRR are affected by multiple factors such as electrolytic cells, electrolytes, and catalysts, etc. Among these catalysts are at the core of ENRR research. Single-atom catalysts (SACs) with intrinsic activity have become an emerging technology for numerous energy regeneration, including ENRR. In particular, regulating the microenvironment of SACs (hydrogen evolution reaction inhibition, carrier engineering, metal-carrier interaction, etc.) can break through the limitation of intrinsic activity of SACs. Therefore, this Review first introduces the basic principles of NRR and outlines the key factors affecting ENRR. Then a comprehensive summary is given of the progress of SACs (precious metals, non-precious metals, non-metallic) and diatomic catalysts (DACs) in ENRR. The impact of SACs microenvironmental regulation on ENRR is highlighted. Finally, further research directions for SACs in ENRR are discussed.

Crystalline-Amorphous Ni3S2-NiMoO4 Heterostructure for Durable Urea Electrolysis-Assisted Hydrogen Production at High Current Density.

Developing bifunctional catalysts with good performance at a high current density for the urea oxidation reaction (UOR) and the hydrogen evolution reaction (HER) can effectively relieve the severe environmental and energy pressures. Herein, amorphous NiMoO4 decorated Ni3S2 grown on nickel foam (Ni3S2-NiMoO4/NF) is prepared to accelerate UOR and HER. The crystalline-amorphous heterostructure could regulate the interfacial electron structure to reduce the electron density near Ni3S2 for optimizing UOR and HER. The decoration of NiMoO4 enhances its anti-poisoning ability for CO-intermediate species to show good stability at high current densities. Meanwhile, the nano-/microstructure with high hydrophilicity improves mass transfer and the accessibility of electrolyte. Driving high current densities of ±1000 mA cm-2, it merely needs 1.38 V (UOR) and -263 mV (HER). For urea electrolysis, it can deliver 1000 mA cm-2 at 1.73 V and stably operate at 500 mA cm-2 for 120 h. Therefore, this study provides new ideas for durable urea electrolysis-assisted H2 production.

Tuning d-Band Center of Pt by PtCo-PtSn Heterostructure for Enhanced Oxygen Reduction Reaction Performance.

The development of efficient and stable Pt-based catalysts is significant but challenging for fuel cells. Herein, Sn and Co elements are introduced into Pt to form PtCo-PtSn/C heterostructure for enhancing the oxygen reduction reaction (ORR). Electrochemical results indicate that it has remarkable ORR intrinsic activity with a high mass activity (1,158 mA mg-1 Pt) at 0.9 V in HClO4 solution, which is 2.18-, 6.81-, and 9.98-fold higher than that of PtCo/C, PtSn/C, and Pt/C. More importantly, the catalytic activity attenuation for PtCo-PtSn/C is only 27.4% after 30 000 potential cycles, showing high stability. Furthermore, theoretical calculations reveal that the enhancement is attributed to charge transfer and the unique structure of PtCo-PtSn/C heterostructure, which regulate the d-band center of Pt and prevent non-noble metals from further dissolution. This work thus opens a way to design and prepare highly efficient Pt-based alloy catalysts for proton exchange membrane fuel cells.

Surface unsaturated WOx activating PtNi alloy nanowires for oxygen reduction reaction.

PtNi alloy nanoparticles display promising catalytic activity for oxygen reduction reaction (ORR), while the Ostwald ripening of particles and the dissolution/migration of surface atoms greatly affect its stability thus restricting the application. Herein, the WOx-surface modified PtNi alloy nanowires (WOx-PtNi NWs) exhibiting enhanced ORR catalytic property is reported, which has high aspect ratio with the diameter of only 2 âˆ¼ 3 nm. It is found that the WOx-PtNi NWs shows a volcano relationship between the ORR activity and the content of WOx. The WOx-(0.25)-PtNi NWs has the best performance among all the synthesized catalysts. Its mass activity (0.85 A mg-1Pt) is reduced by only 23.89% after 30k cycles durability test, which is much more stable than that of PtNi NWs (0.33 A mg-1Pt, 45.94%) and Pt/C (0.14 A mg-1Pt, 57.79%). Hence this work achieves an effective regulation of the ORR activity for PtNi alloy NWs by the synergistic effect of WOx on Pt.

Three-Phase Heterojunction NiMo-Based Nano-Needle for Water Splitting at Industrial Alkaline Condition.

Constructing heterojunction is an effective strategy to develop high-performance non-precious-metal-based catalysts for electrochemical water splitting (WS). Herein, we design and prepare an N-doped-carbon-encapsulated Ni/MoO2 nano-needle with three-phase heterojunction (Ni/MoO2@CN) for accelerating the WS under industrial alkaline condition. Density functional theory calculations reveal that the electrons are redistributed at the three-phase heterojunction interface, which optimizes the adsorption energy of H- and O-containing intermediates to obtain the best ΔGH* for hydrogen evolution reaction (HER) and decrease the ΔG value of rate-determining step for oxygen evolution reaction (OER), thus enhancing the HER/OER catalytic activity. Electrochemical results confirm that Ni/MoO2@CN exhibits good activity for HER (ƞ-10 = 33 mV, ƞ-1000 = 267 mV) and OER (ƞ10 = 250 mV, ƞ1000 = 420 mV). It shows a low potential of 1.86 V at 1000 mA cm-2 for WS in 6.0 M KOH solution at 60 °C and can steadily operate for 330 h. This good HER/OER performance can be attributed to the three-phase heterojunction with high intrinsic activity and the self-supporting nano-needle with more active sites, faster mass diffusion, and bubbles release. This work provides a unique idea for designing high efficiency catalytic materials for WS.

Ni3S2/Ni Heterostructure Nanobelt Arrays as Bifunctional Catalysts for Urea-Rich Wastewater Degradation.

Urea electrolysis is a cost-effective method for urea-rich wastewater degradation to achieve a pollution-free environment. In this work, the Ni3S2/Ni heterostructure nanobelt arrays supported on nickel foam (Ni3S2/Ni/NF) are synthesized for accelerating the urea oxidation reaction (UOR) and hydrogen evolution reaction (HER). It only needs ultralow potentials of 1.30 V and -54 mV to achieve the current density of ±10 mA cm-2 for UOR and HER, respectively. Meanwhile, the overall urea oxidation driven by Ni3S2/Ni/NF only needs 1.36 V to achieve 10 mA cm-2, and it can remain at 100 mA cm-2 for 60 h without obvious activity attenuation. The superior performance could be attributed to the heterostructure between Ni3S2 and Ni, which can promote electron transfer and form electron-poor Ni species to optimize urea decomposition and hydrogen production. Moreover, the nanobelt self-supported structure could expose abundant active sites. This work thus provides a feasible and cost-effective strategy for urea-rich wastewater degradation and hydrogen production.

N-Doped Graphene-Decorated NiCo Alloy Coupled with Mesoporous NiCoMoO Nano-sheet Heterojunction for Enhanced Water Electrolysis Activity at High Current Density.

Developing highly effective and stable non-noble metal-based bifunctional catalyst working at high current density is an urgent issue for water electrolysis (WE). Herein, we prepare the N-doped graphene-decorated NiCo alloy coupled with mesoporous NiCoMoO nano-sheet grown on 3D nickel foam (NiCo@C-NiCoMoO/NF) for water splitting. NiCo@C-NiCoMoO/NF exhibits outstanding activity with low overpotentials for hydrogen and oxygen evolution reaction (HER: 39/266 mV; OER: 260/390 mV) at ± 10 and ± 1000 mA cm-2. More importantly, in 6.0 M KOH solution at 60 °C for WE, it only requires 1.90 V to reach 1000 mA cm-2 and shows excellent stability for 43 h, exhibiting the potential for actual application. The good performance can be assigned to N-doped graphene-decorated NiCo alloy and mesoporous NiCoMoO nano-sheet, which not only increase the intrinsic activity and expose abundant catalytic activity sites, but also enhance its chemical and mechanical stability. This work thus could provide a promising material for industrial hydrogen production.

Coupling Interface Constructions of FeNi3-MoO2 Heterostructures for Efficient Urea Oxidation and Hydrogen Evolution Reaction.

Urea electrolysis has prospects for urea-containing wastewater purification and hydrogen (H2) production, but the shortage of cost-effective catalysts restricts its development. In this work, the tomentum-like FeNi3-MoO2 heterojunction nanosheets array self-supported on nickel foam (NF) as bifunctional catalyst is prepared by facile hydrothermal and annealing method. Only 1.29 V and -50.8 mV is required to obtain ±10 mA cm-2 for urea oxidation and hydrogen evolution reaction (UOR and HER), respectively, showing great bifunctional catalytic activity. For overall urea electrolysis, it only needs 1.37 V to reach 10 mA cm-2 and can last at 100 mA cm-2 for 70 h without obvious activity attenuation, showing outstanding durability. Coupling interface constructions of FeNi3-MoO2 heterostructures, novel morphology with a mesoporous and self-supporting structure could be the reason for this good performance. This work thus proposes a promising catalyst for boosting UOR and HER to realize efficient overall urea electrolysis.

Novel Bifunctional V2O3 Nanosheets Coupled with N-Doped-Carbon Encapsulated Ni Heterostructure for Enhanced Electrocatalytic Oxidation of Urea-Rich Wastewater.

Developing high performance bifunctional transition metal catalysts would be significantly beneficial for electrocatalytic oxidation of urea-rich wastewater. Herein, we synthesize a V2O3 nanosheet anchored N-doped-carbon encapsulated Ni heterostructure (Ni@C-V2O3/NF) for the reactions of urea oxidation (UOR) and hydrogen evolution (HER). Electrochemical results indicate that it exhibits small potentials of 1.32, 1.39, and 1.43 V for UOR and low overpotentials of 36, 254, and 355 mV for HER at ±10, ± 500 and ±1000 mA cm-2, respectively. It can work at 100 mA cm-2 for over 72 h as cathode and anode electrode without obvious attenuation, suggesting an outstanding durability. The reason for this behavior could be ascribed to the N-doped-carbon coating structure, the synergetic effects between Ni and V2O3, and the nano/micro nanosheets architecture self-supported on nickel foam. This work could provide a promising, inexpensive, and green method for the degradation of urea-rich wastewater and hydrogen production.

Efficient bifunctional catalysts for overall water splitting: porous Fe-Mo oxide hybrid nanorods.

Efficient and inexpensive bifunctional catalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are essential for water splitting. Herein, we successfully prepare porous Fe-Mo oxide hybrid nanorods through a hydrothermal method followed by annealing at high temperature. They exhibit excellent catalytic activity for OER and HER in alkaline media, and produce a current density of 10 mA cm-2 at overpotentials of 200 and 66 mV. Besides, they work as bifunctional electrode materials for overall water splitting, achieving a current density of 10 mA cm-2 at a voltage of 1.52 V, and maintaining a current density of 60 mA cm-2 for 60 h. The unique morphology with self-supported structure can expose more active sites and facilitate charge transfer, and is not easy to peel off, thus it improves the catalytic activity and stability. This work therefore provides a valuable route for designing and fabricating inexpensive and high-performance catalytic materials for overall water splitting.

Adsorption of Cu(ii), Zn(ii), and Pb(ii) from aqueous single and binary metal solutions by regenerated cellulose and sodium alginate chemically modified with polyethyleneimine.

In this paper, crosslinked cellulose/sodium alginate (SA) was modified with polyethyleneimine (PEI) as an adsorbent (PEI-RCSA) for comparative and competitive adsorption of Cu(ii), Zn(ii), and Pb(ii) in single and binary aqueous solutions. FTIR, SEM, TGA and specific surface area analysis were used to characterize the structural characteristics of PEI-RCSA. The effects of initial pH of solutions, contact time and initial concentration of heavy metal ions on the adsorption capacity of PEI-RCSA were investigated. The experimental results revealed that the removal of metal ions on the PEI-RCSA was a pH-dependent process with the maximum adsorption capacity at the initial solution pH of 5-6. The adsorption kinetics were followed by a pseudo-second-order kinetics model, and the diffusion properties played a significant role in the control of the adsorption kinetics. Meanwhile, adsorption isotherms were successfully described by the Langmuir model in a single aqueous solution system. The maximum adsorption capacities of PEI-RCSA for Cu(ii), Zn(ii), and Pb(ii) in a single system were 177.1, 110.2 and 234.2 mg g-1, respectively. The binary-component system was better described with the Langmuir competitive isotherm model. The removal efficiencies didn't change significantly when three adsorption-desorption experimental cycles were conducted. All the above results indicated that PEI-RCSA has promising applications in the treatment of toxic metal pollution.