Self-Supported Nickel Iron Layered Double Hydroxide-Nickel Selenide Electrocatalyst for Superior Water Splitting Activity

Obtaining materials from local sources: surface modification engineering enabled substrates for water splitting

The preparation of an efficient electrode is the key to achieving efficient overall water-splitting for H2 production. Substrate surface modification engineering (SSME) provides a feasible method for preparing self-supported electrodes with high active site utilization, fast mass transport, and a simple fabrication process. This review summarizes and discusses the recent advances in preparing transition-metal-based HER/OER electrocatalysts via SSME. We first highlight the description and advantages of SSME, followed by the detailed introduction of electrocatalysts prepared via the SSME, such as hydroxides, oxyhydroxides, chalcogenides, phosphides, and borides. Finally, we provide the challenges and perspectives. We hope that this review will provide inspiration for researchers and stimulate the development of water splitting technology.

Synergistic Coupling Effect and Anionic Modulation of CoFe LDH@MXene for Triggered and Sustained Alkaline Water/Seawater Electrolysis

The application of seawater splitting is crucial for hydrogen production; therefore, efficient electrocatalysts are necessary to prevent chlorine evolution and severe corrosion. A synergistic method is employed on CoFe LDH by integrating a conductive Ti3C2Tx MXene layer and subsequently applying anionic modulation. Robust metal-substrate interaction along with subsequent phosphidation facilitates efficient electron transfer and optimises the electronic structure of Co and Fe sites. The CoFe-P-1000@Ti3C2Tx/CC demonstrates commendable electrochemical performance, requiring overpotentials of 106.6 mV and 276 mV for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at 10 mA cm-2 in 1 M KOH electrolyte, while 292 mV is necessary for OER in a simulated seawater electrolyte (1 M KOH+0.5 M NaCl). Furthermore, the CoFe-P-1000@Ti3C2Tx/CC exhibits an encouraging cell voltage of 1.59 V (j=10 mA cm-2) for comprehensive alkaline seawater splitting, maintaining exceptional stability for over 50 hours.

Partial Selenization Strategy for Fabrication of Ni0.85Se@NiCr-LDH Heterostructure as an Efficient Bifunctional Electrocatalyst for Overall Water Splitting

NiCr-LDH and its partial selenization as Ni0.85Se@NiCr-LDH heterostructure is established here as an alkaline water electrolyzer for achieving enhanced overall water splitting efficiency. The hydrothermally synthesized optimized batch of Ni0.85Se@NiCr-LDH is thoroughly characterized to elucidate its structure, morphology, and composition. Compared to pristine NiCr-LDH, the batch of Ni0.85Se@NiCr-LDH exhibits exceptional alkaline OER and HER activity with low overpotentials of 258 and 85 mV at 10 mA cm-2, respectively. Besides, Ni0.85Se@NiCr-LDH also exhibits excellent acidic HER with an overpotential of only 61 mV at 10 mA cm-2, indicating that Ni0.85Se@NiCr-LDH can operate effectively across a wide pH range. The excellent electrochemical stability of Ni0.85Se@NiCr-LDH for 24 h operation is attributed to the formation of a thin layer of SeOx during OER operation. The role of selenization and the effect of Cr in the LDH lattice toward enhanced electrocatalytic water splitting is discussed. The outstanding OER and HER performances of Ni0.85Se@NiCr-LDH are attributed to the higher electrochemical active surface area, favorable conditions for adsorption of HER/OER intermediates, low charge transfer resistance, and improved conductivity. The practical application of Ni0.85Se@NiCr-LDH as a bifunctional electrocatalyst for overall water splitting is reflected from the low cell voltage of 1.548 V at 10 mA cm-2.

Deep eutectic Solvents-Assisted synthesis of NiFe-LDHs/Mo2Ti2C3: A bifunctional electrocatalyst for overall electrochemical water splitting in alkaline media

The development of efficient and stable electrocatalysts is crucial for the advancement of green and clean hydrogen energy technologies. In this work, we synthesized a nanocomposite of nickel-iron layered double hydroxide/molybdenum titanium carbide (NiFe-LDHs/Mo2Ti2C3) using a deep eutectic solvent (DESs) by the solvothermal method. The formation of NiFe-LDHs/Mo2Ti2C3 nanocomposite was confirmed by various electron microscopic and spectroscopic techniques. The synthesized nanocomposite was investigated as a bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) under the alkaline condition. The NiFe-LDHs/Mo2Ti2C3-based electrodes exhibit small overpotentials of 204 and 306 mV for HER and OER at a current density of 10 mA cm-2. The anchor of NiFe-LDHs on the surface of Mo2Ti2C3 induces an interfacial synergistic effect, leading to a significantly improvement in electrochemical performance. Remarkably, the proposed NiFe-LDHs/Mo2Ti2C3 modified electrode demonstrates superior performance compared to many recently reported LDHs and MXenes-based electrocatalysts in an alkaline environment. Furthermore, a symmetrical two-electrode water splitting setup employing the NiFe-LDHs/Mo2Ti2C3 electrocatalyst requires an electrolysis voltage of 1.65 V to achieve a current density of 10 mA cm-2. The findings provide a new perspective on the rational design and synthesis of multifunctional electrocatalysts for electrochemical applications.

Nitrate-coordinated FeNi(OH)2 for hydrazine oxidation assisted seawater splitting at the industrial-level current density

In this study, we developed a nitrate-coordinated iron-nickel hydroxide [NC-FeNi(OH)2] catalyst for hydrazine oxidation-assisted seawater splitting. Replacement of O2 evolution by hydrazine oxidation in a two-electrode setup resulted in a cell voltage of 1.20 V at 100 mA cm-2. This represents a voltage reduction of 470 mV compared to conventional seawater splitting. Additionally, NC-FeNi(OH)2 demonstrated remarkable stability over a period of 60 hours.

Novel mixed heterovalent (Mo/Co)Ox-zerovalent Cu system as bi-functional electrocatalyst for overall water splitting

A novel hybrid ternary metallic electrocatalyst of amorphous Mo/Co oxides and crystallized Cu metal was deposited over Ni foam using a one-pot, simple, and scalable solvothermal technique. The chemical structure of the prepared ternary electrocatalyst was systematically characterized and confirmed via XRD, FTIR, EDS, and XPS analysis techniques. FESEM images of (Mo/Co)Ox-Cu@NF display the formation of 3D hierarchical structure with a particle size range of 3-5 µm. The developed (Mo/Co)Ox-Cu@NF ternary electrocatalyst exhibits the maximum activity with 188 mV and 410 mV overpotentials at 50 mA cm-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Electrochemical impedance spectroscopy (EIS) results for the (Mo/Co)Ox-Cu@NF sample demonstrate the minimum charge transfer resistance (Rct) and maximum constant phase element (CPE) values. A two-electrode cell based on the ternary electrocatalyst just needs a voltage of about 1.86 V at 50 mA cm-2 for overall water splitting (OWS). The electrocatalyst shows satisfactory durability during the OWS for 24 h at 10 mA cm-2 with an increase of only 33 mV in the cell potential.

Zeolitic Imidazolate Framework-Derived Co3S4@NiFe-LDH Core-Shell Heterostructure as Efficient Bifunctional Electrocatalyst for Water Splitting

The development of stable and efficient bifunctional electrocatalysts is of utmost importance for overall water splitting. This study introduces Co3S4@NiFe-LDH core-shell heterostructure prepared via an electrodeposition of ultrathin NiFe-LDH nanosheet on zeolitic imidazolium framework-derived Co3S4 nanosheet arrays. The bifunctional Co3S4@NiFe-LDH/NF exhibits impressive catalytic performance and long-term stability for both the OER and HER with low overpotentials of 100 mA cm-2 at 235 mV and 10 mA cm-2 at 95 mV in 1 M KOH, respectively. The assembled cell with Co3S4@NiFe-LDH/NF as both cathode and anode shows voltages of 1.595 and 1.666 V at current densities of 10 and 20 mA cm-2, respectively, as well as ultralong stability over 500 h. DFT calculations expose a robust electron interaction at the heterogeneous interface of the Co3S4@NiFe-LDH/NF core-shell structure. This interaction promotes electron transfer from NiFe-LDH to Co3S4 and reduces the energy barriers for OER intermediates, thereby enhancing electrocatalytic activity. This research contributes novel insights toward the promising materials for electrochemical water splitting through the construction of heterojunction interfaces.

Ni(OH)2 nanosheets decorated with FeCoPi on NiO heterostructures: tunable intrinsic electronic structures for improved overall water splitting

Herein, we demonstrate the rational design of 3-dimensional nickel double hydroxide nanosheets decorated with iron-cobalt phosphide on nickel oxide (Ni(OH)2@FexCo1-xPi|NiO) heterostructures for achieving improved overall water splitting. The as-optimized Ni(OH)2@FexCo1-xPi|NiO heterostructures exhibited an overpotential (η) of ∼133 mV and ∼173 mV at 10 mA cm-2 for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), respectively, in an alkaline electrolyte through a tunable electronic interaction and stabilization of the active Ni(OH)2 and FeCoPi interface.

Heteroatom Doping Promoting CoP for Driving Water Splitting

CoP nanomaterials have been extensively regarded as one of the most promising electrocatalysts for overall water splitting due to their unique bifunctionality. Although the great promise for future applications, some important issues should also be addressed. Heteroatom doping has been widely acknowledged as a potential strategy for improving the electrocatalytic performance of CoP and narrowing the gap between experimental study and industrial applications. Recent years have witnessed the rapid development of heteroatom-doped CoP electrocatalysts for water splitting. Aiming to provide guidance for the future development of more effective CoP-based electrocatalysts, we herein organize a comprehensive review of this interesting field, with the special focus on the effects of heteroatom doping on the catalytic performance of CoP. Additionally, many heteroatom-doped CoP electrocatalysts for water splitting are also discussed, and the structure-activity relationship is also manifested. Finally, a systematic conclusion and outlook is well organized to provide direction for the future development of this interesting field.

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.

Size-Dependent Graphene Support for Decorating Gold Nanoparticles as a Catalyst for Hydrogen Evolution Reaction with Machine Learning-Assisted Prediction

Size-dependent two-dimensional (2D) materials (e.g., graphene) have been recently used to improve their performance in various applications such as membrane filtration, energy storage, and electrocatalysts. It has also been demonstrated that 2D nanosheets can be one of the promising support materials for decorating nanoparticles (NPs). However, the optimum nanosheet size (lateral length and thickness) for supporting NPs has not yet been explored to enhance their catalytic performance. Herein, we elucidate the mechanism behind size-dependent graphene (GP) as a support due to which gold nanoparticles (AuNPs) are used as an active catalyst for the hydrogen evolution reaction (HER). Surprisingly, the decoration of AuNPs increased with the increasing nanosheet size, counter to what is widely reported in the literature (high surface area for smaller nanosheet size). We found that a large graphene nanosheet (lGP; ∼800 nm) used as the AuNP support (lGP/AuNPs) exhibited superior performance for the HER with long-term stability. The lGP/AuNPs with a suitable content of AuNPs provides a low overpotential and a small Tafel slope, being lower than that of other reported carbon-based HER electrocatalysts. This results from highly exposed active sites of well-dispersed AuNPs on lGP giving high conductivity. The laminar structure of the stacked graphene nanosheets and the high wettability of the lGP/AuNPs electrode surface also play crucial roles in enhancing electrolytes for penetration in the electrode, suggesting a highly electrochemical surface area. Moreover, machine learning (Random Forest) was also used to reveal the essential features of the advanced catalytic material design for catalyst-based applications.

Zeolitic Imidazolate Framework-Derived Zn/Co-S@Ni(OH)2 Nanoarrays with Excellent Energy Storage and Electrocatalytic Performance

The design and development of high-performance electrochemical electrode materials are crucial for energy storage and conversion systems. This work reports a facile preparation of a self-supported Zn/Co-S@Ni(OH)2 array electrode in which a Zn/Co-S nanosheet is derived from a leaf-like zeolitic imidazolate framework (Zn/Co-ZIF-L). The core-shell structure provides multiple benefits such as enhanced electrical conductivity, an abundance of exposed active sites, and strong electronic interactions between Zn/Co-S and ultra-thin Ni(OH)2 nanosheets, facilitating faster charge transfer. Consequently, Zn/Co-S@Ni(OH)2 demonstrates remarkable electrochemical characteristics as an electrode material for supercapacitors with an area capacitance of 12.9 F cm-2 at a current density of 2 mA cm-2 in 2 M KOH. The assembled asymmetric supercapacitor device achieves a high energy density of 0.95 mW h cm-2, while showing excellent longevity with a retention of 90.9% over 5000 cycles. Additionally, the Zn/Co-S@Ni(OH)2 arrays demonstrate significant oxygen evolution reaction activity with an overpotential of 242 mV at 10 mA cm-2 in 1 M KOH and significant stability for more than 100 h. This work provides a valuable approach for synthesizing bifunctional electrode materials for both energy storage and electrocatalysis applications.

Selective Adsorption Behavior Modulation on Nickel Selenide by Heteroatom Implantation and Heterointerface Construction Achieves Efficient Co-production of H2 and Formate

Glycerol-assisted hybrid water electrolysis is a potential strategy to achieve energy-efficient hydrogen production. However, the design of an efficient catalyst for the specific reaction is still a key challenge, which suffers from the barrier of regulating the adsorption characteristics of distinctive intermediates in different reactions. Herein, a novel rationale that achieves selective adsorption behavior modulation for self-supported nickel selenide electrode by heteroatom implantation and heterointerface construction through electrodeposition is developed, which can realize nichetargeting optimization on hydrogen evolution reaction (HER) and glycerol oxidation reaction (GOR), respectively. Specifically, the prepared Mo-doped Ni3 Se2 electrode exhibits superior catalytic activity for HER, while the NiSe-Ni3 Se2 electrode exhibits high Faradaic efficiency (FE) towards formate production for GOR. A two-electrode electrolyzer exhibits superb activity that only needs an ultralow cell voltage of 1.40 V to achieve 40 mA cm-2 with a high FE (97%) for formate production. Theoretical calculation unravels that the introduction of molybdenum contributes to the deviation of the d-band center of Ni3 Se2 from the Fermi level, which is conducive to hydrogen desorption. Meanwhile, the construction of the heterojunction induces the distortion of the surface structure of nickel selenide, which exposes highly active nickel sites for glycerol adsorption, thus contributing to the excellent electrocatalytic performance.

Tuning the Surface Electronic Structure of Amorphous NiWO4 by Doping Fe as an Electrocatalyst for OER

Water electrolysis is considered as one of the alternative potential approaches for producing renewable energy. Due to the sluggish kinetic nature of oxygen evolution reaction (OER), it encounters a significant overpotential to achieve water electrolysis. Hence, the advancement of cost-effective transition metal-based catalysts toward water splitting has gained global attention in recent years. In this work, the doping of Fe over amorphous NiWO₄ increased the OER activity effectively and achieved stable oxygen evolution in the alkaline medium, which show better electrocatalytic activity as compared to crystalline tungstate. As NiWO₄ has poor activity toward OER in the alkaline medium, the doping of Fe³⁺ will tune the electronic structure of Ni in NiWO₄ and boost the OER activity. The as-synthesized Fe-doped amorphous NiWO₄ exhibits a low overpotential of 230 mV to achieve a current density of 10 mA cm^-2 and a lower Tafel slope value of 48 mV dec^-1 toward OER in 1.0 M KOH solution. The catalyst also exhibits long-term static stability of 30 h during chronoamperometric study. The doping of Fe improves the electronic conductivity of Ni-3d states in NiWO₄ which play a dominant role for better catalytic activity via synergistic interaction between Fe and active Ni sites. In future, these results offer an alternative route for precious metal-free catalysts in alkaline medium and can be explicitly used in various tungstate-based materials to increase the synergism between the doped atom and metal ions in tungstate-based materials for further improvement in the electrocatalytic performance.

Facile synthesis of porous transition metal hydroxides from a poly(4-vinyl pyridine) film by controlling pH

Non-noble transition metal hydroxides have been widely used in electrochemical devices because of low cost and multiple redox states. In particular, self-supported porous transition metal hydroxides are used to improve the electrical conductivity, as well as achieving fast electron and mass transfer and a large effective surface area. Herein, we introduce facile synthesis of self-supported porous transition metal hydroxides using a poly(4-vinyl pyridine) (P4VP) film. We used metal cyanide as a transition metal precursor capable of forming metal hydroxide anions in aqueous solution, which is the seed for transition metal hydroxides. To increase the coordination between P4VP and the transition metal cyanide precursors, we dissolved the precursors in buffer solutions with various pH. When the P4VP film was immersed in the precursor solution with lower pH, the metal cyanide precursors were sufficiently coordinated with the protonated nitrogen in P4VP. When reactive ion etching was performed on the precursor-containing P4VP film, the P4VP region without coordination was etched out and became pores. Then, the coordinated precursors were aggregated as metal hydroxide seeds and became the metal hydroxide backbone, resulting in the formation of porous transition metal hydroxide structures. We successfully fabricated various self-supported porous transition metal hydroxides (Ni(OH)₂, Co(OH)₂, and FeOOH). Finally, we prepared a pseudo-capacitor based on self-supported porous Ni(OH)₂, which showed a good specific capacitance (780 F g^-1 at 5 A g^-1).

Sea urchin-like amorphous MgNiCo mixed metal hydroxide nanoarrays for efficient overall water splitting under industrial electrolytic conditions

Exploring amorphous mixed transition metal hydroxide electrocatalysts with high performance and stability for overall water splitting is a difficult challenge under industrial electrolytic conditions. Herein, a sea urchin-like amorphous MgNiCo hydroxide (Mg_xNi_1-xCo-OH, 0 < x < 1), self-assembled from nanowire arrays, is synthesized by the hydrothermal process. The synergistic effect between Mg and Ni/Co adjusts their crystal structure and morphology, which can improve the inherent activity and provide more active sites. Benefiting from the favorable structural features, Mg_0.5Ni_0.5Co-OH exhibits superior electrocatalytic oxygen and hydrogen evolution reaction (OER and HER) activity with a low overpotential of 277 and 110 mV (10 mA cm^-2) in 1 M KOH at 25 °C. Furthermore, overpotentials of 239 and 197 mV are required to achieve a current density of 50 mA cm^-2 for the OER and HER under simulated industrial electrolysis conditions (5 M KOH at 65 °C). Notably, Mg_0.5Ni_0.5Co-OH remarkably accelerates water splitting with a low voltage of 1.938 and 1.699 V for 50 mA cm^-2 in 1 M KOH at 25 °C and 5 M KOH at 65 °C, respectively. This work presents a novel amorphous strategy to design and construct sea urchin-like mixed metal hydroxide bifunctional efficient electrocatalysts for industrial applications.

Synthesis of Hollow Leaf-Shaped Iron-Doped Nickel–Cobalt Layered Double Hydroxides Using Two-Dimensional (2D) Zeolitic Imidazolate Framework Catalyzing Oxygen Evolution Reaction

Layered double hydroxides (LDHs) have been reported as one of the most effective materials for oxygen evolution reaction (OER) catalysts, which are prone to hydrolysis and oxidation under OER conditions. Metal–organic frameworks (MOFs) are porous materials with high crystallinity and internal surface area. The design of LDHs based on MOFs has attracted increasing attention owing to their high surface area, exposed catalysis sites, and fast charge/mass transport kinetics. Herein, we report a novel approach to fabricate a leaf-shaped iron-doped nickel–cobalt LDH (L-Fe-NiCoLDH) derived from a two-dimensional (2D) zeolitic imidazolate framework with a leaf-like morphology (ZIFL). Iron doping played a significant role in enhancing the specific surface area, affecting the OER performance. L-Fe-NiCoLDH showed high OER performance with an overpotential of 243 mV at 10 mA cm−2 and high durability after 20 h. The design of LDHs based on the leaf morphology of MOFs offers tremendous potential for improving OER efficiency.

Adsorption and photodegradation of reactive red 120 with nickel-iron-layered double hydroxide/biochar composites

Layered double hydroxide (LDH) materials were widely applied for adsorption and photodegradation of pollutants for wastewater treatment. New efficient LDH materials with adsorption and photodegradation abilities will be promising candidates for pollutants removal. Hence, a series of NiFe-LDH/biochar (NiFe/BC) were fabricated by the coprecipitation method for synergistic adsorption and photodegradation anionic dyes of reactive red 120 (RR120). The removal experiment showed that the addition of an appropriate amount of biochar into NiFe-LDH enhanced the adsorption capacity and its photocatalytic ability. The optimized NiFe/BC2 composite can remove 88.5 % of RR120 under visible light by adsorption and photocatalysis, which was much better than NiFe-LDH (63.3 %) and biochar (2.6 %). The photodegradation kinetic constant of the NiFe/BC2 composite was 3.1 and 104.8 times that of NiFe-LDH and BC. In addition, active species capture experiments and electron spin resonance (ESR) tests revealed the removal mechanisms of NiFe/BC composites for RR120 removal. This work affords a feasible strategy for preparing LDH-based photocatalyst with excellent adsorption and photocatalytic performance for wastewater treatment.

Doping Mo into NiFe LDH/NiSe Heterostructure to Enhance Oxygen Evolution Activity by Synergistically Facilitating Electronic Modulation and Surface Reconstruction

It is of great significance to design highly efficient electrocatalysts with abundant earth elements instead of precious metals for water splitting. Herein, Mo-doped NiFe-layered double hydroxides/NiSe heterostructure (Mo-NiFe LDH/NiSe) was fabricated by coupling Mo-doped NiFe LDH and NiSe on nickel foam (NF). The heterostructure electrocatalyst showed ultra-low overpotential (250 mV) and remarkable durability for oxygen evolution reaction (OER) at 150 mA cm^-2 . Both theoretical and experimental results confirmed that Mo doping and interfacial synergism induced the interfacial charge redistribution and the lifted d-band center to weaken the energy barrier (EB) of the formation of OOH^* . Mo doping also facilitated the surface reconstruction of NiFe LDH into Ni(Fe)OOH as the active sites under electro-oxidation process. This work provides a facile strategy for electronic modulation and surface reconstruction of OER electrocatalyst by transition metal doping and heterostructure generation.

Sensitive electrochemical sensor based on nickel/PDDA/reduced graphene oxide modified screen-printed carbon electrode for nitrite detection

A simple, rapid method of the determination of nitrite in food samples is reported by using a highly sensitive electrochemical sensor based on nickel, poly(diallyldimethylammonium chloride) (PDDA), reduced graphene oxide (rGO) and a disposable screen-printed carbon electrode (SPCE). The method is based on a modification of the electrode to enhance the sensitivity and selectivity of the disposable and applicable SPCE, which is essential for the present analytical challenge. The nitrite determination was performed by using a cyclic voltammetry (CV) method under optimum conditions. Ni/PDDA/rGO/SPCE showed a linear working range of 6 to 100 μM of nitrite concentration. The limit of detection and limit of quantification were 1.99 μM (S/N = 3) and 6.6 μM (S/N = 10), respectively. The sensitivities were 0.453 μA μM^-1 cm^-2 for the lower concentration range and 0.171 μA μM^-1 cm^-2 for the higher concentration range. The Ni/PDDA/rGO sensor also showed excellent anti-interference ability and good long-term stability. The purposed disposable sensor was successfully applied to determine nitrite in sausages and pickled vegetable samples with satisfactory recovery.

Chemical Transformation Induced Core-Shell Ni2P@Fe2P Heterostructures toward Efficient Electrocatalytic Oxygen Evolution

The oxygen evolution reaction (OER) is a crucial reaction in water splitting, metal-air batteries, and other electrochemical conversion technologies. Rationally designed catalysts with rich active sites and high intrinsic activity have been considered as a hopeful strategy to address the sluggish kinetics for OER. However, constructing such active sites in non-noble catalysts still faces grand challenges. To this end, we fabricate a Ni₂P@Fe₂P core-shell structure with outperforming performance toward OER via chemical transformation of rationally designed Ni-MOF hybrid nanosheets. Specifically, the Ni-MOF nanosheets and their supported Fe-based nanomaterials were in situ transformed into porous Ni₂P@Fe₂P core-shell nanosheets composed of Ni₂P and Fe₂P nanodomains in homogenous dispersion via a phosphorization process. When employed as the OER electrocatalyst, the Ni₂P@Fe₂P core-shell nanosheets exhibits excellent OER performance, with a low overpotential of 238/247 mV to drive 50/100 mA cm^-2, a small Tafel slope of 32.91 mV dec^-1, as well as outstanding durability, which could be mainly ascribed to the strong electronic interaction between Ni₂P and Fe₂P nanodomains stabilizing more Ni and Fe atoms with higher valence. These high-valence metal sites promote the generation of high-active Ni/FeOOH to enhance OER activity.

Homoleptic Ni(II) dithiocarbamate complexes as pre-catalysts for the electrocatalytic oxygen evolution reaction

Four new functionalized Ni(II) dithiocarbamate complexes of the formula [Ni(L_x)₂] (1-4) (L₁ = N-methylthiophene-N-3-pyridylmethyl dithiocarbamate, L₂ = N-methylthiophene-N-4-pyridylmethyl dithiocarbamate, L₃ = N-benzyl-N-3-pyridylmethyl dithiocarbamate, and L₄ = N-benzyl-N-4-pyridylmethyl dithiocarbamate) have been synthesized and characterized by IR, UV-vis, and ¹H and ¹³C{¹H} NMR spectroscopic techniques. The solid-state structure of complex 1 has also been determined by single crystal X-ray crystallography. Single crystal X-ray analysis revealed a monomeric centrosymmetric structure for complex 1 in which two dithiocarbamate ligands are bonded to the Ni(II) metal ion in a S^S chelating mode resulting in a square planar geometry around the nickel center. These complexes are immobilized on activated carbon cloth (CC) and their electrocatalytic performances for the oxygen evolution reaction (OER) have been investigated in aqueous alkaline solution. All the complexes act as pre-catalysts for the OER and undergo electrochemical anodic activation to form Ni(O)OH active catalysts. Spectroscopic and electrochemical characterization revealed the existence of the interface of molecular complex/Ni(O)OH, which acts as the real catalyst for the OER. The active catalyst obtained from complex 2 showed the best OER activity achieving 10 mA cm^-2 current density at an overpotential of 330 mV in 1.0 M aqueous KOH solution.

Integrating Amorphous Molybdenum Sulfide Nanosheets with a Co9S8@Ni3S2 Array as an Efficient Electrocatalyst for Overall Water Splitting

It is highly challenging to design low-cost, efficient electrocatalysts for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Herein, a hierarchical heterostructure was constructed on three-dimensional (3D) Ni foam, which contains Ni₃S₂ nanorods decorated with both Co₉S₈ and amorphous MoS_x nanosheets and Ni₃S₂ nanowires decorated with amorphous MoS_x nanosheets, namely, MoS_x@Co₉S₈@Ni₃S₂/NF. The synergistic effects from the strong interactions of the heterointerface and unique hierarchical heterostructure endow the MoS_x@Co₉S₈@Ni₃S₂/NF with abundant active sites and effective mass and electron transport pathways, resulting in excellent activity toward both HER and OER in 1 M KOH. It only gives a low overpotential of 76.5 mV to achieve 10 mA cm^-2 for HER and a low overpotential of 310 mV to achieve 100 mA cm^-2 for OER. Based on the superior catalytic activity of MoS_x@Co₉S₈@Ni₃S₂/NF for OER and HER, we demonstrated the activity of overall water splitting using MoS_x@Co₉S₈@Ni₃S₂/NF as both the anode and cathode. It shows a higher catalytic activity for overall water splitting with a low cell voltage of 1.52 V at 10 mA cm^-2 than commercial Pt/C/NF||IrO₂/NF (1.61 V) and superior stability. This work provides a platform for the design and preparation of efficient electrocatalysts with various hierarchical heterostructures.

Catalytic cracking of polystyrene pyrolysis oil: Effect of Nb2O5 and NiO/Nb2O5 catalyst on the liquid product composition

The catalytic cracking of polystyrene pyrolysis oil was investigated over a Nb2O5 and a NiO/Nb2O5 catalyst in a fixed bed reactor. First, the pyrolysis of two different polystyrene feedstock (polystyrene foam and polystyrene pellet) was carried out in a semi-batch reactor, and the resulting polystyrene pellets pyrolysis oil was selected for catalytic cracking reaction because of its high liquid yield (85%). Catalytic cracking experiments were then performed at different temperatures (350-500 °C) using Nb2O5 or NiO/Nb2O5 catalyst. Gas chromatography-mass spectrometry analysis of liquid product obtained from the catalytic cracking process showed that the dimers in the pyrolysis oil were converted to monomers during the catalytic cracking process. The catalytic cracking results also showed that the NiO/Nb2O5 catalyst (having slightly higher acidic sites) had slightly higher activity for monomer conversion than the Nb2O5 catalyst (having less acidic sites). X-ray diffraction, transmission electron microscopy, pyridine Fourier transform infrared spectroscopy, NH3 Temperature Programmed Desorption and X-ray photoelectron spectroscopy were used to characterize the catalyst. The highest catalytic cracking activity was observed at 400 °C with the Nb2O5 catalyst with 4% toluene, 6% ethylbenzene, approximately 50% styrene, 13% α-methyl styrene, and only 6% of dimers in the liquid oil. The increase in temperature positively affected the yield of gases during catalytic cracking process.

NiO nanobelts with exposed {110} crystal planes as an efficient electrocatalyst for the oxygen evolution reaction

The electrocatalytic oxygen evolution reaction (OER) is necessary and challenging for converting renewable electricity into clean fuels, because of its complex proton coupled multielectron transfer process. Herein, we investigated the crystal plane effects of NiO on the electrocatalytic OER activity through combining experimental studies and theoretical calculations. The experimental results reveal that NiO nanobelts with exposed {110} crystal planes show much higher OER activity than NiO nanoplates with exposed {111} planes. The efficient OER activity of the {110} crystal planes comes from their intrinsically high catalytic ability and fast charge transfer kinetics. Density functional theory (DFT) shows that the {110} crystal planes possess a lower theoretical overpotential value for the OER, leading to a high electrocatalytic performance. This research broadens our vision to design efficient OER electrocatalysts by the selective exposure of specific crystal planes.

Preparation and Characterization of Multi-Doped Porous Carbon Nanofibers from Carbonization in Different Atmospheres and Their Oxygen Electrocatalytic Properties Research

Recently, electrocatalysts for oxygen reduction reaction (ORR) as well as oxygen evolution reaction (OER) hinged on electrospun nanofiber composites have attracted wide research attention. Transition metal elements and heteroatomic doping are important methods used to enhance their catalytic performances. Lately, the construction of electrocatalysts based on metal-organic framework (MOF) electrospun nanofibers has become a research hotspot. In this work, nickel-cobalt zeolitic imidazolate frameworks with different molar ratios (NixCoy-ZIFs) were synthesized in an aqueous solution, followed by NixCoy-ZIFs/polyacrylonitrile (PAN) electrospun nanofiber precursors, which were prepared by a simple electrospinning method. Bimetal (Ni-Co) porous carbon nanofiber catalysts doped with nitrogen, oxygen, and sulfur elements were obtained at high-temperature carbonization treatment in different atmospheres (argon (Ar), Air, and hydrogen sulfide (H2S)), respectively. The morphological properties, structures, and composition were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Moreover, the specific surface area of materials and their pore size distribution was characterized by Brunauer-Emmett-Teller (BET). Linear sweep voltammetry curves investigated catalyst performances towards oxygen reduction and evolution reactions. Importantly, Ni1Co2-ZIFs/PAN-Ar yielded the best ORR activity, whereas Ni1Co1-ZIFs/PAN-Air exhibited the best OER performance. This work provides significant guidance for the preparation and characterization of multi-doped porous carbon nanofibers carbonized in different atmospheres.

Finding the True Catalyst for Water Oxidation at Low Overpotential in the Presence of a Metal Complex

The design of molecular-based catalysts for oxygen-evolution reaction (OER) requires more investigations for the true catalyst to be found. First-row transition metal complexes are extensively investigated for OER, but the role of these metal complexes as a true catalyst is doubtful. Some doubts have been expressed about the role of first-row transition metal complexes for OER at high overpotentials (η > 450). Generally, the detection of the true catalyst has so far been focused on high overpotentials (η > 450) because at low overpotentials (η < 450), many methods are not sensitive enough to detect small amounts of heterogeneous catalysts on the electrode surface during the first seconds of the reaction. Ni(II) phthalocyanine-tetra sulfonate tetrasodium (1) is in moderate conditions (at 20-50 °C and pH 5-13) in the absence of electrochemical driving forces, which could make it noteworthy for OER. Herein, the results of OER in the presence of 1 at low overpotentials under alkaline conditions are presented. In addition, in the presence of Ni complexes, using an Fe ion is introduced as a new method for detecting Ni (hydr)oxide under OER. Our experiments indicate that in the presence of a homogeneous OER (pre)catalyst, a deep investigation is necessary to rule out the heterogeneous catalysts formed. Our approach is a roadmap in the field of catalysis to understand the OER mechanism in the presence of a molecular Ni-based catalyst design. Our results shown in this study are likely to open up new perspectives and discussion on many molecular catalysts in a considerable part of the chemistry community.

Binder free 3D core-shell NiFe layered double hydroxide (LDH) nanosheets (NSs) supported on Cu foam as a highly efficient non-enzymatic glucose sensor

Rational design with fine-tuning of the electrocatalyst material is vital for achieving the desired sensitivity, selectivity, and stability for an electrochemical sensor. In this study, a three-dimensional (3D) hierarchical core-shell catalyst was employed as a self-standing, binder-free electrode for non-enzymatic glucose sensing. The catalyst was prepared by decorating the shell of NiFe layered double hydroxide (LDH) nanosheets (NSs) on the core of Cu nanowires (NWs) grown on a Cu foam support. The optimized 3D core-shell Cu@NiFe LDH sensor demonstrated higher sensitivity (7.88 mA mM^-1cm^-2), lower limit of detection (0.10 µM) and wider linear range (1 µM to 0.9 mM) in glucose sensing with a low working potential (0.4 V). The applied sensor also showed excellent stability, reproducibility, interference ability as well as practicability in real environment. The detection of real samples further suggests its great feasibility for practical applications. The superior electrocatalytic performance is collectively ascribed to the excellent electro-conductivity of the Cu substrate, the distinct self-standing 3D porous nanostructure, the ultrathin homogenous architecture, and the appropriate loading amount of NiFe LDH NSs. This study then provides a non-enzymatic glucose sensor with 3D Cu@NiFe LDH electrode for ultrahigh sensitivity and stability.

Electrospun one-dimensional electrocatalysts for boosting electrocatalysis

Electrocatalytic reaction plays a crucial role in determining the energy conversion efficiency in advanced technology. However, it is limited by the sluggish reaction kinetics and high energy barrier. These shortcomings...

Rapid mass production of iron nickel oxalate nanorods for efficient oxygen evolution reaction catalysis

Using a coprecipitation method we synthesized an oxalate, which has a good catalytic performance for oxygen evolution in an alkaline electrolyte. This method can efficiently synthesize a large number of electrocatalysts in a short time.

Hydrogen production coupled with water and organic oxidation based on layered double hydroxides

Hydrogen production via electrochemical water splitting is one of the most green and promising ways to produce clean energy and address resource crisis, but still suffers from low efficiency and high cost mainly due to the sluggish oxygen evolution reaction (OER) process. Alternatively, electrochemical hydrogen-evolution coupled with alternative oxidation (EHCO) has been proposed as a considerable strategy to improve hydrogen production efficiency combined with the production of high value-added chemicals. Although with these merits, high-efficient electrocatalysts are always needed in practical operation. Typically, layered double hydroxides (LDHs) have been developed as a large class of advanced electrocatalysts toward both OER and EHCO with high efficiency and stability. In this review, we have summarized the latest progress of hydrogen production from the perspectives of designing efficient LDHs-based electrocatalysts for OER and EHCO. Particularly, the influence of structure design and component regulation on the efficiency of their electrocatalytic process have been discussed in detail. Finally, we look forward to the challenges in the field of hydrogen production via electrochemical water splitting coupled with organic oxidation, such as the mechanism, selected oxidation as well as system design, hoping to provide certain inspiration for the development of low-cost hydrogen production technology.

Ni-Fe phosphide deposited carbon felt as free-standing bifunctional catalyst electrode for urea electrolysis

A free-standing catalyst electrode for the urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) in a urea electrolysis cell was synthesized by electroplating a Ni-Fe alloy onto carbon felt, followed by phosphidation (P-NiFe@CF). The prepared P-NiFe@CF catalyst consisted of Ni5P4, NiP2, and FeP with 3D flower-like P-NiFe architecture on CF. P-NiFe@CF exhibited excellent electrocatalytic activity for the UOR (demanding only 1.39 V (vs. RHE) to achieve 200 mA cm-2), and for the HER with a low overpotential of 0.023 V (vs. RHE) at 10 mA cm-2, indicating its feasibility as a bifunctional catalyst electrode for urea electrolysis. A urea electrolysis cell with P-NiFe@CF as both the free-standing anode and cathode generated a current density of 10 mA cm-2 at a cell potential of 1.37 V (vs. RHE), which is considerably lower than that of water electrolysis, and also lower than previously reported values. The results indicate that the P-NiFe@CF catalyst electrodes can be used as free-standing bifunctional electrodes for urea electrolyzers.

Low-Cost and Efficient Nickel Nitroprusside/Graphene Nanohybrid Electrocatalysts as Counter Electrodes for Dye-Sensitized Solar Cells

Novel nickel nitroprusside (NNP) nanoparticles with incorporated graphene nanoplatelets (NNP/GnP) were used for the first time as a low-cost and effective counter electrode (CE) for dye-sensitized solar cells (DSSCs). NNP was synthesized at a low-temperature (25 °C) solution process with suitable purity and crystallinity with a size range from 5 to 10 nm, as confirmed by different spectroscopic and microscopic analyses. The incorporation of an optimized amount of GnP (0.2 wt%) into the NNP significantly improved the electrocatalytic behavior for the redox reaction of iodide (I-)/tri-iodide (I3-) by decreasing the charge-transfer resistance at the CE/electrolyte interface, lower than the NNP- and GnP-CEs, and comparable to the Pt-CE. The NNP/GnP nanohybrid CE when applied in DSSC exhibited a PCE of 6.13% (under one sun illumination conditions) with the Jsc, Voc, and FF of 14.22 mA/cm2, 0.628 V, and 68.68%, respectively, while the PCE of the reference Pt-CE-based DSSC was 6.37% (Jsc = 14.47 mA/cm2, Voc = 0.635 V, and FF = 69.20%). The low cost of the NNP/GnP hybrid CE with comparable photovoltaic performance to Pt-CE can be potentially exploited as a suitable replacement of Pt-CE in DSSCs.

Advancing the extended roles of 3D transition metal based heterostructures with copious active sites for electrocatalytic water splitting

The replacement of noble metals with alternative electrocatalysts is highly demanded for water splitting. From the exploration of 3D -transition metal based heterostructures, engineering at the nano-level brought more enhancements in active sites with reduced overpotentials for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). However, recent developments in 3D transition metal based heterostructures like direct growth on external substrates (Ni foam, Cu foam) gave highly impressive activities and stabilities. Research needs to be focused on how the active sites can be enhanced further with 3D heterostructures of transition metals by studying them with various counterparts like hydroxides, layered double hydroxides and phosphides for empowering both OER and HER applications. This perspective covers the way to enlarge the utilization of 3D heterostructures successfully in terms of reduced overpotentials, highly exposed active sites, increased electrical conductivity, porosity and high-rate activity. From the various approaches of growth of transition metal based 3D heterostructures, it is easy to fine tune the active sites to have a viable production of hydrogen with less applied energy input. Overall, this perspective outlines a direction to increase the number of active sites on 3D transition metal based heterostructures by growing on 3D foams for enhanced water splitting applications.