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

FeCoNi molybdenum-based oxides for efficient electrocatalytic oxygen evolution reaction

The search for highly efficient and inexpensive electrocatalysts is crucial to the advancement of environmentally friendly and sustainable energy sources. Here, adopting a one-step hydrothermal method, we have effectively fabricated a self-supported multi-metal molybdenum-based oxide (FeCoNi-MoO4) on nickel foam (NF). In addition to changing the catalyst's microstructure, the introducing of Fe and Co, enhanced its active center count, improved its electronic structure, and in turn reduced the difficulty for high-valence Ni and Fe species to form, which accelerates the oxygen evolution reaction (OER) kinetics by promoting the development of the actual active materials, NiOOH and FeOOH. FeCoNi-MoO4 has outstanding OER performance, requiring just 204 mV overpotentials at 10 mA cm-2 and 271 mV at 100 mA cm-2. Its exceptional OER kinetics at both low and high currents are indicated by a Tafel slope of 50.6 mV dec-1, which is attributed to the combined effect of its multi-metal composition and a higher number of active sites. Moreover, the FeCoNi-MoO4 electrode was operated continuously for over 48 h. Furthermore, the density functional theory (DFT) results demonstrated that the introducing of Fe and Co, which quickens the rate of electron transfer during the electrocatalytic process, improves the ability of oxygen intermediate species to adsorb, and ultimately lowers the overpotential, is responsible for the increased electrocatalytic activity of FeCoNi-MoO4. This work offers hope for further developments in the sector by proposing an efficient approach for creating multi-active electrocatalysts that are stable, economical, and efficient.

Fe-induced crystalline-amorphous interface engineering of a NiMo-based heterostructure for enhanced water oxidation

Engineering heterostructures with a unique surface/interface structure is one of the effective strategies to develop highly active noble-metal-free catalysts for the oxygen evolution reaction (OER), because the surface/interface of catalysts is the main site for the OER. Herein, we design a coralloid NiMo(Fe)-20 catalyst with a crystalline-amorphous interface through combining a hydrothermal method and an Fe-induced surface reconfiguration strategy. That is, after Fe3+ impregnation treatment, the Ni(OH)2-NiMoO4 pre-catalyst with a complete crystalline surface is restructured into a trimetallic heterostructure with a crystalline-amorphous interface, which facilitates mass diffusion and charge transfer during the OER. As expected, self-supported NiMo(Fe)-20 exhibits excellent electrocatalytic water oxidation performance (overpotential: η-10 = 220 mV, η-100 = 239 mV) in the alkaline electrolyte, and its electrocatalytic performance hardly changes after maintaining the current density of 50 mA cm-2 for 10 hours. Furthermore, nickel foam (NF) supported commercial Pt/C and self-supported NiMo(Fe)-20 served as the cathode and anode of the Pt/C‖NiMo(Fe)-20 electrolyzer, respectively, which exhibits a lower cell voltage (E-100 = 1.53 V) than that of the Pt/C‖RuO2 electrolyzer (E-100 = 1.58 V) assembled with noble metal-based catalysts. The enhanced electrocatalytic performance of the NiMo(Fe)-20 catalyst is mainly attributed to the synergistic effect between the crystalline-amorphous interface and the coralloid trimetallic heterostructure.

Scalable Synthesis of Ir Cluster Anchored on Porous Hollow Carbon Nanobowls for Enhancing pH-Universal Hydrogen Evolution

Design high-loading with superior activity and high atomic efficiency has consistently been a new frontier of heterogeneous catalysis while challenging in synthetic technology. In this work, a universal solid-state strategy is proposed for large scalable production of high-loading Ir clusters on porous hollow carbon nanobowls (Ir CSs/PHCNBs). The strong electronic interaction between metallic Ir cluster and C on PHCNBs leads to electron redistribution, which significantly improves the electron transfer rate on the interface. The obtained Ir CSs/PHCNBs only require overpotentials of 35, 34, and 37 mV for the hydrogen evolution reaction (HER) with stable outputting of 10 mA cm-2 under acidic, alkaline, and neutral conditions, respectively, which exceeds the state-of-the-art HER electrocatalysts. Meanwhile, the Tafel slopes of Ir CSs/PHCNBs for the HER process are 23.07, 48.76, and 28.95 mV dec-1 , greatly lower than that of PHCNBs (152.73, 227.96, and 140.29 mV dec-1 ) and commercial Pt/C (20%) (36.33, 66.10, and 36.61 mV dec-1 ). These results provide a new strategy for the universal synthesis of clusters catalysts and insight into understanding the interface effects between clusters and carbon substrate, facilitating the industrial application of hydrogen production.

Interface engineering of Ni2P/MoOx decorated NiFeP nanosheets for enhanced alkaline hydrogen evolution reaction at high current densities

The rational design of high-performance non-noble metal electrocatalysts at large current densities is important for the development of sustainable energy conversion devices such as alkaline water electrolyzers. However, improving the intrinsic activity of those non-noble metal electrocatalysts remains a great challenge. Therefore, Ni₂P/MoO_x decorated three-dimensional (3D) NiFeP nanosheets (NiFeP@Ni₂P/MoO_x) with abundant interfaces were synthesized using facile hydrothermal and phosphorization methods. NiFeP@Ni₂P/MoO_x exhibits excellent electrocatalytic activity for hydrogen evolution reaction (HER) at a high current density of -1000 mA cm^-2 with a low overpotential of 390 mV. Surprisingly, it can operate steadily at a large current density of -500 mA cm^-2 for 300 h, indicating its long-term durability under high current densities. The boosted electrocatalytic activity and stability can be ascribed to the as-fabricated heterostructures via interface engineering, leading to modifying the electronic structure, improving the active area, and enhancing the stability. Besides, the 3D nanostructure is also beneficial for exposing abundant accessible active sites. Therefore, this research proposes a considerable route for fabricating non-noble metal electrocatalysts by interface engineering and 3D nanostructure applied in large-scale hydrogen production facilities.

Synthesis of Urchin-like Ni@NP@NCP Composites with Three Solvothermal Systems for Highly Efficient Overall Seawater Splitting

In this work, an urchin-like Ni@Ni₂P@NiCoP (Ni@NP@NCP) composite was prepared on nickel foam by a simple hydrothermal treatment process. Using the prepared NiO nanosheets as templates, the NiCo precursor was prepared in the presence of three solvothermal systems of water/dimethylformamide (DMF)/dimethyl sulfoxide (DMSO) by the hydrothermal process. After mixing and calcining with sodium hypophosphite under a nitrogen atmosphere at a high temperature for phosphating, an urchin-like Ni@NP@NCP(F/SO/H) nanostructured catalyst was obtained with superior hydrogen evolution and oxygen evolution performance. To further explore their efficiency in seawater splitting. Ni@NP@NCP(F/SO/H) composites were used as the cathode and anode of an electrolytic cell, which delivered 1.822 V potential at 300 mA cm^-2 in simulated seawater (1 M KOH and 0.5 M NaCl). This may provide an effective way of developing clean energy.

Tunable d-band center of a NiFeMo alloy with enlarged lattice strain enhancing the intrinsic catalytic activity for overall water-splitting

Developing efficient bifunctional electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) under alkaline conditions is prospective for reducing energy consumption during water electrolysis. In this work, we successfully synthesized nanocluster structure composites composed of NiFeMo alloys with controllable lattice strain by the electrodeposition method at room temperature. The unique structure of NiFeMo/SSM (stainless steel mesh) facilitates the exposure of abundant active sites and promotes mass transfer and gas exportation. The NiFeMo/SSM electrode exhibits a low overpotential of 86 mV at 10 mA cm^-2 for the HER and 318 mV at 50 mA cm^-2 for the OER, and the assembled device reveals a low voltage of 1.764 V at 50 mA cm^-2. Moreover, both the experimental results and theoretical calculations reveal that the dual doping of Mo and Fe can induce the tunable lattice strain of nickel, which in turn changes the d-band center and electronic interaction of the catalytically active site, and finally enhances the HER and OER catalytic activity. This work may provide more options for the design and preparation of bifunctional catalysts based on non-noble metals.

Defect engineering of two-dimensional materials for advanced energy conversion and storage

In the global trend towards carbon neutrality, sustainable energy conversion and storage technologies are of vital significance to tackle the energy crisis and climate change. However, traditional electrode materials gradually reach their property limits. Two-dimensional (2D) materials featuring large aspect ratios and tunable surface properties exhibit tremendous potential for improving the performance of energy conversion and storage devices. To rationally control the physical and chemical properties for specific applications, defect engineering of 2D materials has been investigated extensively, and is becoming a versatile strategy to promote the electrode reaction kinetics. Simultaneously, exploring the in-depth mechanisms underlying defect action in electrode reactions is crucial to provide profound insight into structure tailoring and property optimization. In this review, we highlight the cutting-edge advances in defect engineering in 2D materials as well as their considerable effects in energy-related applications. Moreover, the confronting challenges and promising directions are discussed for the development of advanced energy conversion and storage systems.

Oxygen vacancies refilling and potassium ions intercalation of δ-manganese dioxide with high structural stability toward 2.3 V high voltage asymmetric supercapacitors

Oxygen vacancies occupation and coordination environment modulation of the transition-metal based electrodes are effective strategies to improve the structural stability and electrochemical performance. In this work, the 2-methylimidazole (2-MI) doped manganese dioxide (MnO2) anchored on carbon cloth (CC) is fabricated via a simple method (MI-MnO2-x/CC), where the oxygen defects on/inside the K+ doped δ-MnO2 nanosheets are in-situ created by reductive ethanol/Mn2+ and occupied by 2-MI ligands. With the pre-embedded K+ ions and abundant ligand-refilled defects, the electronic coordination structure, structural stability and electron/ion diffusion efficiency can be effectively enhanced. Therefore, the MI-MnO2-x/CC reveals a remarkable specific capacitance of 721.2 mF cm-2 with excellent cycle durability (capacitance retention of 93.4% after 10,000 cycles) under 1.3 V operation potential window. In addition, an asymmetric supercapacitor assembled by MI-MnO2-x/CC and activated mechanical exfoliated graphene oxide yields a maximum energy density of 57.0 Wh kg-1 and a highest power density of 23.0 kW kg-1 under 2.3 V. This effective oxygen defect stabilization strategy by ligands refilling can be extended to various metal oxide-based electrodes for energy storage and conversion.

Plasma-modified graphitic C3N4@Cobalt hydroxide nanowires as a highly efficient electrocatalyst for oxygen evolution reaction

The key to electrocatalytic water splitting is the discovery of efficient, low-cost electrocatalysts for oxygen evolution reaction (OER). g-C₃N₄@Co(OH)₂ + PA/X nanowire materials were prepared by a combined strategy of thermo-hydraulic and DBD plasma modification. The morphological structure of the plasma modification for 60 s was then characterised by SEM and TEM patterns. In alkaline media, the g-C₃N₄@Co(OH)₂ catalyst subjected to 60-s plasma treatment had excellent durability and exhibited outstanding electrochemical performance, displaying a low overpotential (329 mV). The number of Co³⁺ active sites, high conductivity, and large surface area of the g-C₃N₄@Co(OH)₂ + PA/60s catalyst contribute to the remarkable OER activity. This research offers a novel approach to rationally designing effective electrocatalysts for water splitting.

Bifunctional keel flower-like Ni-Co-V multicomponent oxide catalyst with enhanced electron transport for accelerating overall water splitting

Heteroatom doping is considered a typical method for improving the electrochemical properties of composites. In this work, the multi-component oxide catalyst (Ni(VO₃)₂ and Co₂V₂O₇ on Ni foam, referred to as NiCoVO_x@NF) is formed by hydrothermal doping of V element into NiCo-based precursors followed by co-oxidation. In the catalyst NiCoVO_x@NF, all three components of Ni, Co and V are particularly tightly coordinated, exhibiting an integrated structure of keel flower-like arrays. The catalyst NiCoVO_x@NF's contact surface with water is increased thanks to this unusual structure, exposing a high number of active sites. Furthermore, NiCoVO_x@NF owns efficient electronic pathways, which greatly enhances the electron transport ability. To generate a current density of 10 mA cm^-2 for hydrogen evolution reaction, just a 107 mV overpotential is required. The electrode exhibits a low overpotential of 217 mV to deliver 50 mA cm^-2 for oxygen evolution reaction. In addition, the total water splitting performance of NiCoVO_x@NF is also excellent, which could be achieved by only one 1.5 V AA battery. This study provides a feasible heteroatom doping route to design bifunctional catalysts with improved performances.

Quasi-Parallel NiFe Layered Double Hydroxide Nanosheet Arrays for Large-Current-Density Oxygen Evolution Electrocatalysis

Designing advanced electrocatalysts for oxygen evolution at large current density (>500 mA cm^-2 ) is critical to practical water splitting applications. Herein, a novel quasi-parallel NiFe layered double hydroxide (NiFe LDH) nanosheet arrays with pattern alignment on Ni foam was developed. The initial α-Ni(OH)₂ layer induced effective coprecipitation between Ni²⁺ and Fe³⁺ for the formation of LDH phase, guaranteeing the electronic pulling effect among metal cations and enhancing the interaction between active materials and substrate for excellent adhesion and electrical conductivity. Quasi-parallel NiFe LDH nanoarrays exhibited outstanding oxygen evolution activity with a small Tafel slope of 30.1 mV dec^-1 and overpotentials of 196, 255, and 284 mV at a current density of 10, 500, and 1000 mA cm^-2 in 1.0 m KOH solution, respectively, and high stability over 40 h at 750 mA cm^-2 . This work presents a new strategy towards fabricating electrode materials with exceptional performance.