A one-pot "shielding-to-etching" strategy to synthesize amorphous MoS2 modified CoS/Co0.85Se heterostructured nanotube arrays for boosted energy-saving H2 generation

Research Advances in Amorphous-Crystalline Heterostructures Toward Efficient Electrochemical Applications

Interface engineering of heterostructures has proven a promising strategy to effectively modulate their physicochemical properties and further improve the electrochemical performance for various applications. In this context related research of the newly proposed amorphous-crystalline heterostructures have lately surged since they combine the superior advantages of amorphous- and crystalline-phase structures, showing unusual atomic arrangements in heterointerfaces. Nonetheless, there has been much less efforts in systematic analysis and summary of the amorphous-crystalline heterostructures to examine their complicated interfacial interactions and elusory active sites. The critical structure-activity correlation and electrocatalytic mechanism remain rather elusive. In this review, the recent advances of amorphous-crystalline heterostructures in electrochemical energy conversion and storage fields are amply discussed and presented, along with remarks on the challenges and perspectives. Initially, the fundamental characteristics of amorphous-crystalline heterostructures are introduced to provide scientific viewpoints for structural understanding. Subsequently, the superiorities and current achievements of amorphous-crystalline heterostructures as highly efficient electrocatalysts/electrodes for hydrogen evolution reaction, oxygen evolution reaction, supercapacitor, lithium-ion battery, and lithium-sulfur battery applications are elaborated. At the end of this review, future outlooks and opportunities on amorphous-crystalline heterostructures are also put forward to promote their further development and application in the field of clean energy.

Construction of Fe-doped NiS-NiS2 Heterostructured Microspheres Via Etching Prussian Blue Analogues for Efficient Water-Urea Splitting

Developing efficient and robust non-precious-metal-based catalysts to accelerate electrocatalytic reaction kinetics is crucial for electrochemical water-urea splitting. Herein, Fe-doped NiS-NiS₂ heterostructured microspheres, an electrocatalyst, are synthesized via etching Prussian blue analogues following a controlled annealing treatment. The resulting microspheres are constructed by mesoporous nanoplates, granting the virtues of large surface areas, high structural void porosity, and accessible inner surface. These advantages not only provide more redox reaction centers but also strengthen structural robustness and effectively facilitate the mass diffusion and charge transport. Density functional theory simulations validate that the Fe-doping improves the conductivity of nickel sulfides, whereas the NiS-NiS₂ heterojunctions induce interface charge rearrangement for optimizing the adsorption free energy of intermediates, resulting in a low overpotential and high electrocatalytic activity. Specifically, an ultralow overpotential of 270 mV at 50 mA cm^-2 for the oxygen evolution reaction (OER) is achieved. After adding 0.33 M urea into 1 M KOH, Fe-doped NiS-NiS₂ obtains a strikingly reduced urea oxidation reaction potential of 1.36 V to reach 50 mA cm^-2 , around 140 mV less than OER. This work provides insights into the synergistic modulation of electrocatalytic activity of non-noble catalysts for applications in energy conversion systems.

Advanced Nickel-Based Catalysts for Urea Oxidation Reaction: Challenges and Developments

The electrochemical urea oxidation reaction (UOR) is crucial for determining industrial and commercial applications of urea-based energy conversion devices. However, the performance of UOR is limited by the dynamic complex of the six-electron transfer process. To this end, it is essential to develop efficient UOR catalysts. Nickel-based materials have been extensively investigated owing to their high activity, easy modification, stable properties, and cheap and abundant reserves. Various material designs and strategies have been investigated in producing highly efficient UOR catalysts including alloying, doping, heterostructure construction, defect engineering, micro functionalization, conductivity modulation, etc. It is essential to promptly review the progress in this field to significantly inspire subsequent studies. In this review, we summarized a comprehensive investigation of the mechanisms of oxidation or poisoning and UOR processes on nickel-based catalysts as well as different approaches to prepare highly active catalysts. Moreover, challenges and prospects for future developments associated with issues of UOR in urea-based energy conversion applications were also discussed.

Plasma-Induced Defect Engineering and Cation Refilling of NiMoO4 Parallel Arrays for Overall Water Splitting

Developing highly active water splitting electrocatalysts with ordered micro/nanostructures and uniformly distributed active sites can meet the increasing requirement for sustainable energy storage/utilization technologies. However, the stability of complicated structures and active sites during a long-term process is also a challenge. Herein, we fabricate a novel approach to create sufficient atomic defects via N₂ plasma treatment onto parallel aligned NiMoO₄ nanosheets, followed by refilling of these defects via heterocation dopants and stabilizing them by annealing. The parallel aligned nanosheet arrays with an open structure and quasi-two-dimensional long-range diffusion channels can accelerate the mass transfer at the electrolyte/gas interface, while the incorporation of Fe/Pt atoms into defect sites can modulate the local electronic environment and facilitate the adsorption/reaction kinetics. The optimized Pt-NP-NMC/CC-5 and Fe-NP-NMC/CC-10 electrodes exhibit low overpotentials of 71 and 241 mV at 10 mA cm^-2 for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively, and the assembled device reveals a low voltage of 1.55 V for overall water splitting. This plasma-induced high-efficiency defect engineering and coupled active site stabilization strategy can be extended to large-scale fabrication of high-end electrocatalysts.

Facet-controlled palladium nanocrystalline for enhanced nitrate reduction towards ammonia

Electrochemical nitrate reduction reaction (NO₃^-RR) is considered an appealing way for producing ammonia (NH₃) under ambient conditions and solving environmental problems caused by nitrate, whereas the lack of adequate catalysts hampers the development of NO₃^-RR. Here, we firstly demonstrate that the Pd nanocrystalline with a well-desired facet can act as a highly efficient NO₃^-RR electrocatalyst for ambient ammonia synthesis. Pd (1 1 1) exhibits excellent activity and selectivity in reducing NO₃^- to NH₄⁺ with a Faradaic efficiency of 79.91% and an NH₄⁺ production of 0.5485 mmol h^-1 cm^-2 (2.74 mmol h^-1 mg^-1) in 0.1 M Na₂SO₄ (containing 0.1 M NO₃^-), which is 1.4 times higher than Pd (1 0 0) and 1.9 times higher than Pd (1 1 0), respectively. Density functional theory (DFT) calculation reveals that the superior NO₃^-RR activity of Pd (1 1 1) originates from its optimized activity of NO₃^- adsorption, smaller free energy change of the rate-limiting step (*NH₃ to NH₃), and poorer hydrogen evolution reaction activity (HER, competitive reaction). This work not only highlights the potentials of Pd-based nanocatalysts for NO₃^-RR but also provides new insight for the applications in NO₃^-RR of other facet-orientation nanomaterials.

Fabrication of an Au25 -Cys-Mo Electrocatalyst for Efficient Nitrogen Reduction to Ammonia under Ambient Conditions

Electrocatalysts for efficient production of ammonia from nitrogen reduction reaction (NRR) under ambient conditions are attracted growing interest in recent years, which demonstrate a great potential to replace the Haber-Bosch method which suffers the problems of the huge energy consumption and massive CO₂ production. In this work, a novel electrocatalyst of Au₂₅ -Cys-M is fabricated for NRR under ambient conditions, with transition metal ions (e.g., Mo⁶⁺ , Fe³⁺ , Co²⁺ , Ni²⁺ ) atomically decorated on Au₂₅ nanoclusters via thiol bridging. The Au₂₅ -Cys-Mo catalyst exhibits the highest Faradaic efficiency (26.5%) and NH₃ yield (34.5 µg h^-1  mg_cat ^-1 ) in 0.1 m HCl solution. X-ray photoelectron spectroscopy analysis and high angle annular dark field image-scanning transmission electron microscopy characterization reveal that the electronic structure of Mo is optimized by forming the structure of Au-S-Mo and Mo acts as active sites for activating the nitrogen to promote the electrochemical production of ammonia. This work provides a new insight into the precise fabrication of efficient NRR electrocatalysts.

Hydrogen and Potassium Acetate Co-Production from Electrochemical Reforming of Ethanol at Ultrathin Cobalt Sulfide Nanosheets on Nickel Foam

The sluggish reaction kinetics of the anodic oxygen evolution reaction increases the energy consumption of the overall water electrolysis for high-purity hydrogen generation. In this work, ultrathin cobalt sulfide nanosheets (Co₃S₄-NSs) on nickel foam (Ni-F) nanohybrids (termed as Co₃S₄-NSs/Ni-F) are synthesized using cyanogel hydrolysis and a sulfurization two-step approach. Physical characterizations reveal that Co₃S₄-NSs with a 1.7 nm thickness have abundant holes, implying the big surface area, abundant active edge atoms, and sufficient active sites. Electrochemical measurements show that as-synthesized Co₃S₄-NSs/Ni-F have excellent electrocatalytic activity and selectivity for ethanol oxidation reaction and hydrogen evolution reaction. Due to their bifunctional property of Co₃S₄-NSs/Ni-F nanohybrids, a symmetric Co₃S₄-NSs/Ni-F∥Co₃S₄-NSs/Ni-F ethanol electrolyzer can be effectively constructed, which only requires a 1.48 V electrolysis voltage to reach a current density of 10 mA cm^-2 for high-purity hydrogen generation at the cathode as well as value-added potassium acetate generation at the anode, much lower than the electrolysis voltage of traditional electrochemical water splitting (1.64 V).

Boosting hydrogen generation by anodic oxidation of iodide over Ni-Co(OH)2 nanosheet arrays

For overall water electrolysis, the hydrogen evolution reaction (HER) is severely limited by the sluggish kinetics of the anodic oxygen evolution reaction (OER). Therefore, replacing the OER with a more favorable anodic oxidation reaction with remarkable kinetics is of paramount significance, especially the one that can produce value-added chemicals. Moreover, time-saving and cost-effective strategies for the fabrication of electrodes are helpful for the wide application of electrolysis. Herein, thermodynamically more favorable iodide electrooxidation over Ni doped Co(OH)2 nanosheet arrays (NSAs) in alkaline solution is presented as the alternative to the OER to boost the HER. And the active species are determined to be the reverse redox of the Co(iv)/Co(iii) couple. Remarkably, a negative shift of voltage of 320 mV is observed at a current density of 10 mA cm-2 after using iodide electrolysis to replace ordinary water splitting. The synthetic strategy and iodide oxidation in this work expand the application of Co-based materials in the field of energy-saving hydrogen production.

Oxygen Vacancy-rich Ni/NiO@NC Nanosheets with Schottky Heterointerface for Efficient Urea Oxidation Reaction

H₂ production via electrocatalytic water splitting is greatly hindered by the sluggish oxygen evolution reaction (OER). The urea oxidation reaction (UOR) draws specific attention not only because of its lower theoretical voltage of 0.37 V compared with OER (1.23 V), but also for treating sewage water. Herein, Ni/NiO nanosheets with an ultrathin N-doped C layer containing a Schottky Ni and NiO heterointerface is constructed. Because of the self-driven charge redistribution at the heterointerface, janus charge domains are successfully created to drive the cleavage of urea molecules. Meanwhile, the synergistic effect between N-doped C and Ni/NiO restrains the deactivation of active sites in alkaline solution. The catalyst displays 1.35 V for UOR at 10 mA/cm² , 0.27 V lower than that of OER. The final potential increase is only 2 mV after long-term stability test of 12 h for UOR, much smaller than the uncoated sample (38 mV). The present work shows that C-coated transition metal nanomaterials with oxygen vacancies and a Schottky heterointerface are promising candidates for simultaneously boosting UOR with both high activity and long-term stability.