Metallic Ni3S2 Films Grown by Atomic Layer Deposition as an Efficient and Stable Electrocatalyst for Overall Water Splitting

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Active-site-enriched dendritic crystal Co/Fe-doped Ni3S2 electrocatalysts for efficient oxygen evolution reaction

The electrochemical decomposition of water plays a critical role in green and sustainable energy. However, the development of efficient and low-cost non-noble metal catalysts to overcome the high potential of the anodic oxygen evolution reaction (OER) is still challenging. In this work, electrocatalysts with high OER activity were obtained by doping Co/Fe bimetals into Ni₃S₂ (CF-NS) via a simple single-step hydrothermal method by adjusting the doping ratio of bimetals. A series of characterization studies revealed that the introduction of a Co/Fe co-dopant increased the number of active sites and improved the electroconductibility, while optimizing the electronic structure of Ni₃S₂. Meanwhile, Fe-induced high valence Ni contributed to the production of an OER active phase NiOOH. The unique dendritic crystal morphology facilitated the disclosure of the active sites and the expansion of mass transfer channels. The optimized sample required a low overpotential of 146 mV to obtain a current density of 10 mA cm^-2 in 1.0 M KOH solution. The optimized sample also operated stably for at least 86 h. In sum, the proposed method looks very promising for designing efficient, stable, and low-cost non-precious metal catalysts with high conductivity and multiple active sites, useful for future synthesis of transition metal sulfide catalysts.

Integrating Electrochemical CO2 Reduction on α-NiS with the Water or Organic Oxidations by Its Electro-Oxidized NiO(OH) Counterpart to an Artificial Photosynthetic Scheme

Efficient hydrogen production, biomass up-conversion, and CO₂-to-fuel generation are the key challenges of the present decade. Electrocatalysis in aqueous electrolytes by choosing suitable transition-metal-based electrode materials remains the green approach for the trio of sustainable developments. Given that, finding electrode materials with multifunctional capability would be beneficial. Herein, the nanocrystalline α-NiS, synthesized solvothermally, has been chosen as an electrode material. As the first step to construct an electrolyzer, α-NiS deposited on conducting nickel foam (NF) has been used as an anode, and under the anodic potential, the α-NiS particles have lost sulfides to the electrolyte and transform to amorphous electro-derived NiO(OH) (NiO(OH)_ED), confirmed by different spectroscopic and microscopic studies. In situ transformation of α-NiS to amorphous NiO(OH)_ED results in an enhancement of the electrochemical surface area and not only becomes active toward oxygen evolution reaction (OER) at a moderate overpotential of 264 mV (at 20 mA cm^-2) but also can convert a series of biomass-derived organic compounds, namely, 2-hydroxymethylfurfural (HMF), 2-furfural (FF), ethylene glycol (EG), and glycerol (Gly), to industrially relevant feedstocks with a high (∼96%) Faradaic efficiency. During these organic oxidations, NiO(OH)_ED/NF participate in the multiple-electron oxidation process (up to 8e^-) including C-C bond cleavages of EG and Gly. During the cathodic performance of the α-NiS/NF, the structural integrity has been retained and the unaltered α-NiS/NF electrode remains more effective cathode for alkaline hydrogen evolution reaction (HER) and CO₂ reduction (CO₂R) compared to its in situ-derived NiO(OH)_ED/NF. α-NiS/NF can reduce the CO₂ predominantly to CO even at a higher potential like -0.8 V (vs RHE). The fabricated cell with α-NiS and its electro-oxidized NiO(OH)_ED counterpart, α-NiS/NF(-)/(+)NiO(OH)_ED/NF, is able to show an artificial photosynthetic scheme in which the NiO(OH)_ED/NF anode oxidizes water to O₂ and the α-NiS cathode reduces CO₂ majorly to CO in a moderate cell potential. In this study, α-NiS has been utilized as a single electrode material to perform multiple sustainable transformations.

Techniques of Preparation of Thin Films: Catalytic Combustion

Over the past several decades, an increasing amount of attention has been given to catalytic combustion as an environmentally friendly process. However, major impediments to large-scale application still arise on the materials side. Here, we review catalytic combustion on thin film catalysts in view of highlighting some interesting features. Catalytic films open the way for new designs of structured catalysts and the construction of catalysts for catalytic combustion. A special place is occupied by materials in the form of very thin films that reveal catalytic activity for various chemical reactions. In this review, we demonstrate the high catalytic activity of thin film catalysts in these oxidation reactions.

Hierarchical microstructure constructed with graphitic carbon-coated Ni3S2 nanoparticles anchored on N-doped mesoporous carbon nanoflakes for optimized sodium storage

A hierarchical microstructure constructed with graphitic-carbon-coated Ni₃S₂ nanoparticles anchored on N-doped mesoporous carbon nanoflakes was fabricated using a nickel-based micro-nano structure as a precursor and polydopamine as a carbon source. By optimizing the microstructure, the obtained Ni₃S₂/carbon composite compounded with the thickest carbon nanoflakes delivers ultrafast and stable Na-ion storage performance, and can maintain a reversible charge capacity of 372 mA h g^-1 at a current density of 5 A g^-1 over 250 cycles, and 316 mA h g^-1 even at a current density of 20 A g^-1 for 2000 cycles. These remarkable electrochemical properties can be attributed to its hierarchical microstructure of graphitic-carbon-coated Ni₃S₂ particles and N-doped mesoporous carbon nanoflakes, which provide easy accessibility to the electrolyte, fast electron transport and Na⁺ diffusion, and even relieve the strain caused by the volume expansion upon cycling.

Atomic-Layer-Deposition-Based 2D Transition Metal Chalcogenides: Synthesis, Modulation, and Applications

Transition metal chalcogenides (TMCs) are a large family of 2D materials with different properties, and are promising candidates for a wide range of applications such as nanoelectronics, sensors, energy conversion, and energy storage. In the research of new materials, the development and investigation of industry-compatible synthesis techniques is of key importance. In this respect, it is important to study 2D TMC materials synthesized by the atomic layer deposition (ALD) technique, which is widely applied in industries. In addition to the synthesis of 2D TMCs, ALD is used to modulate the characteristic of 2D TMCs such as their carrier density and morphology. So far, the improvement of thin film uniformity without oxidation and the synthesis of low-dimensional nanomaterials on 2D TMCs have been the research focus. Herein, the synthesis and modulation of 2D TMCs by ALD is described, and the characteristics of ALD-based TMCs used in nanoelectronics, sensors, and energy applications are discussed.

Applications of Atomic Layer Deposition in Design of Systems for Energy Conversion

There is a huge demand for clean energy conversion in all industries. The clean energy production processes include electrocatalytic and photocatalytic conversion of water to hydrogen, carbon dioxide reduction, nitrogen conversion to ammonia, and oxygen reduction reaction and require novel cheap and efficient photo- and electrocatalysts and their scalable methods of fabrication. Atomic layer deposition is a thin film deposition method that allows to deposit thin layers of catalysts on virtually any surface of any shape, size, and porosity in an even and easy to control manner. Here the state of the art in applications of atomic layer deposition in the clean energy production and the opportunities it represents for the whole field of the photo- and electrocatalysis for a sustainable future are reviewed.

"Carbon quantum dots-glue" enabled high-capacitance and highly stable nickel sulphide nanosheet electrode for supercapacitors

A facile "carbon quantum dots glue" strategy for the fabrication of honeycomb-like carbon quantum dots/nickel sulphide network arrays on Ni foam surface is successfully demonstrated. This design realizes the immobilization of nanosheet arrays and maintains a strong adhesion to the collector, forming a three-dimensional (3D) honeycomb-like architecture. Thanks to the unique structural advantages, the resulting bind-free electrode with high active mass loading of 6.16 mg cm^-2 still exhibits a superior specific capacitance of 1130F g^-1 at 2 A g^-1, and maintains 80% of the initial capacitance even at 10 A g^-1 after 3000 cycles. Furthermore, the assembled asymmetrical supercapacitor delivers an energy density of 18.8 Wh kg^-1 at a power density of 134 W kg^-1, and outstanding cycling stability (100% of initial capacitance retention after 5000 cycles at 5 mA cm^-2). These impressive results indicate a new perspective to design various binder-free electrodes for electrochemical energy storage devices.

Atomic layer deposited nickel sulfide for bifunctional oxygen evolution/reduction electrocatalysis and zinc-air batteries

Rechargeable Zn-air batteries are a promising type of metal-air batteries for high-density energy storage. However, their practical use is limited by the use of costly noble-metal electrocatalysts for the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occurred at the air electrode of the Zn-air batteries. This work reports a new non-precious bifunctional OER/ORR electrocatalyst of NiS_x/carbon nanotubes (CNTs), which is made by atomic layer deposition (ALD) of nickel sulfide (NiS_x) on CNTs, for the applications for the air electrode of the Zn-air batteries. The NiS_x/CNT electrocatalyst on a carbon cloth electrode exhibits a low OER overpotential of 288 mV to reach 10 mA cm^-2in current density, and the electrocatalyst on a rotating disk electrode exhibits a half-wave ORR potential of 0.81 V in alkaline electrolyte. With the use of the NiS_x/CNT electrocatalyst for the air electrode, the fabricated aqueous rechargeable Zn-air batteries show a fairly good maximum output power density of 110 mW cm^-2, which highlights the great promise of the ALD NiS_x/CNT electrocatalyst for Zn-air batteries.

A highly stable CoMo2S4/Ni3S2 heterojunction electrocatalyst for efficient hydrogen evolution

A CoMo₂S₄/Ni₃S₂ heterojunction is prepared with an overpotential of only 51 mV to drive a current density of 10 mA cm⁻² in 1 M KOH solution and ∼100% of the potential remains in the ∼50 h chronopotentiometric curve at 10 mA cm⁻².

Biomimetic Nanomembranes: An Overview

Nanomembranes are the principal building block of basically all living organisms, and without them life as we know it would not be possible. Yet in spite of their ubiquity, for a long time their artificial counterparts have mostly been overlooked in mainstream microsystem and nanosystem technologies, being a niche topic at best, instead of holding their rightful position as one of the basic structures in such systems. Synthetic biomimetic nanomembranes are essential in a vast number of seemingly disparate fields, including separation science and technology, sensing technology, environmental protection, renewable energy, process industry, life sciences and biomedicine. In this study, we review the possibilities for the synthesis of inorganic, organic and hybrid nanomembranes mimicking and in some way surpassing living structures, consider their main properties of interest, give a short overview of possible pathways for their enhancement through multifunctionalization, and summarize some of their numerous applications reported to date, with a focus on recent findings. It is our aim to stress the role of functionalized synthetic biomimetic nanomembranes within the context of modern nanoscience and nanotechnologies. We hope to highlight the importance of the topic, as well as to stress its great applicability potentials in many facets of human life.

Controlled Synthesis of Cr-Co0.85 Se Nanoarrays for Water Splitting at an Ultralow Cell Voltage of 1.43 V

Water splitting has attracted more and more attention as a promising strategy for the production of clean hydrogen fuel. In this work, a new synthesis strategy was proposed, and Co_0.85 Se was synthesized on nickel foam as the main matrix. The doping of appropriate Cr amount into the target of Co_0.85 Se and the Cr-Co_0.85 Se resulted in an excellent electrochemical performance. The doping of Cr introduces Cr³⁺ ions which substitute Co²⁺ and Co³⁺ ions in Co_0.85 Se, so that the lattice parameters of the main matrix were changed. It is worth noting that the Cr0.15-Co_0.85 Se/NF material exhibits an excellent performance in the oxygen evolution reaction (OER) test. When the current density reaches 50 mA cm^-2 for OER, the overpotential is only 240 mV. For the hydrogen evolution reaction (HER) tests, the overpotential is only 117 mV to drive 10 mA cm^-2 of current density. Moreover, when the Cr0.15-Co_0.85 Se/NF material is used as a two-electrode device for whole water splitting, the required cell voltage is only 1.43 V to reach a current density of 10 mA cm^-2 , which is among the lowest values of the published catalysts up to now. In addition, the Cr0.15-Co_0.85 Se/NF catalyst also exhibits excellent stability during a long period of water splitting. The experimental result demonstrates that the change of the lattice structure has an obvious influence on the electrocatalytic activity of the material. When an external electric field is applied, it facilitates the rapid electron transfer rate and enhances the electrocatalytic performance and stability of the material.

Generation of Ni3S2 nanorod arrays with high-density bridging S22- by introducing a small amount of Na3VO4·12H2O for superior hydrogen evolution reaction

Bridging S22- moieties have been demonstrated to be highly active sites existing in metal polysulfides for the hydrogen evolution reaction (HER), thus the incorporation of high-density bridging S22- into a Ni3S2 material to improve its electrocatalytic HER performance is highly desirable and challenging. Herein, we report a novel Ni3S2 nanorod array decorated with (020)-oriented VS4 nanocrystals grown on nickel foam (Shig-NS-rod/NF) via a simple and facile solvothermal method. Results show that the in situ incorporation of VS4 not only triggers the formation of such a nanorod array structure, but also contributes to the uniform grafting of high-density and high catalytically active bridging S22- sites on the interface between Ni3S2 and VS4 for enhanced HER activity, and also promotes the absorption ability of OH- radicals and thus accelerates the HER Volmer step in alkaline media. As expected, the resultant Shig-NS-rod/NF material exhibits impressive catalytic performance toward the HER, with a much lower overpotential of 137 mV at 10 mA cm-2 and a long-term durability for at least 22 h, and is superior to Ni3S2 nanorod arrays with low-density bridging S22- (Slow-NS-rod/NF) and NS-film/NF counterparts (without VS4), even outperforming the NF-supported 20% Pt/C at a large current density of over 120 mA cm-2. Our findings put forward fresh insight into the rational design of highly efficient electrocatalysts toward the HER for green hydrogen fuel production.

Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond

This review describes an in-depth overview and knowledge on the variety of synthetic strategies for forming metal sulfides and their potential use to achieve effective hydrogen generation and beyond.

Sn-Ni3S2 Ultrathin Nanosheets as Efficient Bifunctional Water-Splitting Catalysts with a Large Current Density and Low Overpotential

Ni₃S₂ nanosheets doped with tin (Sn) grown on nickel foam (Sn-Ni₃S₂/NF) through a facile hydrothermal process were found to be superior water-splitting electrocatalysts. As for overall water splitting (OWS), when the current density is 10 mA cm^-2, the required voltage is only 1.46 V. Meanwhile, it exhibits a large current density property and long-time stability (>60 h current-time tests) for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). In order to reach the current densities of 100 and 1000 mA cm^-2, Sn-Ni₃S₂/NF needs overpotentials of 0.17 and 0.57 V for HER, and 0.27 and 0.58 V for OER, respectively. The water-splitting property of Sn-Ni₃S₂/NF is much better than that of pure Ni₃S₂/NF or even 20 wt % Pt/C/NF and RuO₂/NF. Furthermore, Sn-Ni₃S₂/NF showed a higher turnover frequency at different potentials, with ∼100% Faraday efficiency for both O₂ and H₂. The improved activity of Sn-Ni₃S₂/NF activity for water-splitting is attributed to the doping of Sn, which enhanced the intrinsic activity of Sn-Ni₃S₂/NF for OWS. This article not only provides a new efficient and stable catalyst for OWS, but also proposes an interface design principle for NF-based high-performance water-splitting materials.

NiPS3 Nanosheet-Graphene Composites as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction

Developing new electrocatalysts is essentially important for efficient water splitting to produce hydrogen. Two-dimensional (2D) materials provide great potential for high-performance electrocatalysts because of their high specific surface area, abundant active edges, and tunable electronic structure. Here, we report few-layer NiPS₃ nanosheet-graphene composites for high-performance electrocatalysts for oxygen evolution reaction (OER). The pure NiPS₃ nanosheets show an overpotential of 343 mV for a current density of 10 mA cm^-2, which is comparable to that for IrO₂ and RuO₂ catalysts. More importantly, the NiPS₃ nanosheet-graphene composites show significantly improved OER activity due to the synergistic effect. The optimized composite shows a very low overpotential of 294 mV for a current density of 10 mA cm^-2, 351 mV for a current density of 100 mA cm^-2, a small Tafel slope of 42.6 mV dec^-1, and excellent stability. These overall performances are far better than those of the reported 2D materials and even better than those of many traditional materials even at a much lower mass loading of NiPS₃.