Nanostructured materials on 3D nickel foam as electrocatalysts for water splitting

Ternary Mo2 NiB2 as a Superior Bifunctional Electrocatalyst for Overall Water Splitting

Transition metal borides are considered as promising electrocatalysts for water splitting due to their metallic conductivity and good durability. However, the currently reported monometallic and noncrystalline multimetallic borides only show generic and monofunctional catalytic activity. In this work, the authors design and successfully synthesize highly crystalline ternary borides, Mo₂ NiB₂ , via a facile solid-state reaction from pure elemental powders. The as-synthesized Mo₂ NiB₂ exhibits very low overpotentials for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), that is, 280 and 160 mV to reach a current density of 10 mA cm^-2 , in alkaline media. These values are much lower from the ones observed over monometallic borides, that is, Ni₂ B and MoB, and the lowest among all nonprecious metal borides. By loading Mo₂ NiB₂ onto Ni foams as both cathode and anode electrode for overall water splitting applications, a low cell voltage of 1.57 V is required to achieve a current density of 10 mA cm^-2 , comparable with the value required from the Pt/C||IrO₂ /C couple (1.56 V). The proposed synthesis strategy can be used for the preparation of cost-effective, multi-metallic crystalline borides, as multifunctional electrocatalysts.

Using Sawdust Derived Biochar as a Novel 3D Particle Electrode for Micropollutants Degradation

This work examined the use of a 3D combined electrochemical process based on particle electrodes from sawdust-derived biochar pyrolized at T = 550–850 °C to remove persistent pollutants. The as-prepared biochar was characterized by scanning electron microscopy with an X-ray energy dispersive spectrometer (SEM/EDS), nitrogen adsorption (BET method) and X-ray diffraction (XRD) techniques. The use of sawdust biochar pyrolized at 650 °C led to a significant increase in efficiency against the sum of conventional 2D electrochemical systems and adsorption, and the synergy index estimated equal to 74.5% at optimum conditions. Sulfamethoxazole (SMX) removal was favored by increasing particle electrode loading. Despite that, the reaction was slightly favored in near-neutral conditions; the system retained most of its activity in the pH range 3–10. The proposed 3D system could degrade different micropollutants, namely SMX, Bisphenol A (BPA), Propylparaben (PP), and Piroxicam (PR). Of particular interest was that no significant reduction in degradation was observed in the case of complex or real water matrices. In addition, the system retained its efficiency regarding SMX removal after five sequential experiments in the 3D combined electrochemical process. However, further investigation is needed to estimate the contribution of the different mechanisms of micropollutant removal in the proposed system.

Nickel-Based Selenides with a Fractal Structure as an Excellent Bifunctional Electrocatalyst for Water Splitting

Nickel-based selenides are believed to be promising non-precious metal electrocatalysts, and have been widely used for both oxygen evolution reactions (OER) and hydrogen evolution reactions (HER). Here, we control the aging time to prepare Ni_xSe_y with different fractal structures as a bifunctional catalyst. An obtained sample with an aging time of 80 min shows outstanding electrocatalytic performance for hydrogen evolution reactions (HER) with an overpotential of 225 mV (η@10 mA/cm²) and for oxygen evolution reactions (OER) with an overpotential of 309 mV (η@50 mA/cm²). Moreover, to further improve catalytic activity, we doped Fe in Ni_xSe_y to obtain the ternary nickel-based selenide, Fe_0.2Ni_0.8Se (FNSs). The HER activity of FNS increased two-fold at 10 mA/cm², and the overpotential of OER decreased to 255 mV at 50 mA/cm². The synthetic strategy and research results of this work have a certain reference value for other low-cost and high-efficiency transition metal catalysts for electrocatalytic water splitting.

Trimetallic metal-organic frameworks (Fe, Co, Ni-MOF) derived as efficient electrochemical determination for ultra-micro imidacloprid in vegetables

The excessive use of imidacloprid in agricultural production leads to a large number of residues that seriously threaten human health. Therefore, the detection of imidacloprid has become very important. But how to quantitatively detect imidacloprid at ultra-low levels is the main challenges. In this work, trimetallic metal-organic frameworks Fe, Co, Ni-MOF (FCN-MOF) isin situprepared on nickel foam (NF) and then used to make an electrochemical sensor in the detection of imidacloprid. FCN-MOF exhibits the characteristics of ultra-micro concentration detection for imidacloprid with high specific surface area and rich active metal centers. The high conductivity and 3D skeleton structure of the NF electrode enhance the contact site with imidacloprid and promote the transmission of electrons efficiently. All results show that the prepared electrochemical sensor has the advantages of ultra-low detection limits (0.1 pM), wide linear detection ranges (1-5 × 10⁷pM) and good sensitivity (132.91μA pM^‒1cm^‒2), as well as good reproducibility, excellent anti-interference ability, and fantastic stability. Meanwhile, the electrochemical sensor is used to determine imidacloprid in lettuce, tomato, and cucumber samples with excellent recovery (90%-102.7%). The novel electrochemical sensor is successfully applied to the ultra-micro detection of imidacloprid in vegetables, which provides a new way for the efficient monitoring of imidacloprid in agriculture.

Shining Light on Anion-Mixed Nanocatalysts for Efficient Water Electrolysis: Fundamentals, Progress, and Perspectives

This review introduces recent advances of various anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, (oxy)hydroxides, and borides) for efficient water electrolysis applications in detail. The challenges and future perspectives are proposed and analyzed for the anion-mixed water dissociation catalysts, including polyanion-mixed and metal-free catalyst, progressive synthesis strategies, advanced in situ characterizations, and atomic level structure-activity relationship. Hydrogen with high energy density and zero carbon emission is widely acknowledged as the most promising candidate toward world's carbon neutrality and future sustainable eco-society. Water-splitting is a constructive technology for unpolluted and high-purity H₂ production, and a series of non-precious electrocatalysts have been developed over the past decade. To further improve the catalytic activities, metal doping is always adopted to modulate the 3d-electronic configuration and electron-donating/accepting (e-DA) properties, while for anion doping, the electronegativity variations among different non-metal elements would also bring some potential in the modulations of e-DA and metal valence for tuning the performances. In this review, we summarize the recent developments of the many different anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, oxyhydroxides, and borides/borates) for efficient water electrolysis applications. First, we have introduced the general information of water-splitting and the description of anion-mixed electrocatalysts and highlighted their complementary functions of mixed anions. Furthermore, some latest advances of anion-mixed compounds are also categorized for hydrogen and oxygen evolution electrocatalysis. The rationales behind their enhanced electrochemical performances are discussed. Last but not least, the challenges and future perspectives are briefly proposed for the anion-mixed water dissociation catalysts.

Ni(OH)2 Coated CoMn-layered double hydroxide nanowires as efficient water oxidation electrocatalysts

Hierarchical heterostructures with a core of CoMn-LDH nanowires and a shell of Ni(OH)2 were successfully synthesized by a facile hydrothermal method followed by electrodeposition.

Design principles of noble metal-free electrocatalysts for hydrogen production in alkaline media: combining theory and experiment

Water electrolysis is a promising solution to convert renewable energy sources to hydrogen as a high-energy-density energy carrier. Although alkaline conditions extend the scope of electrocatalysts beyond precious metal-based materials to earth-abundant materials, the sluggish kinetics of cathodic and anodic reactions (hydrogen and oxygen evolution reactions, respectively) impede the development of practical electrocatalysts that do not use precious metals. This review discusses the rational design of efficient electrocatalysts by exploiting the understanding of alkaline hydrogen evolution reaction and oxygen evolution reaction mechanisms and of the electron structure-activity relationship, as achieved by combining experimental and computational approaches. The enhancement of water splitting not only deals with intrinsic catalytic activity but also includes the aspect of electrical conductivity and stability. Future perspectives to increase the synergy between theory and experiment are also proposed.

Unraveling a Graphene Exfoliation Technique Analogy in the Making of Ultrathin Nickel-Iron Oxyhydroxides@Nickel Foam to Promote the OER

One of the major objectives of using the improved Hummers' method was to exfoliate the graphene layers by oxidizing and thereafter reducing them to obtain highly conductive reduced graphene layers, which can be used in the construction of electronic devices or as a part of catalyst composites in energy conversion reactions. Herein, we have employed a similar idea to exfoliate the layered double hydroxide (LDH), which is proposed as a promising material for the oxygen evolution reaction (OER) electrocatalysis. Usually, the efficiency of these materials is largely restricted due to their sheetlike morphology, which is susceptible to stacking. In this work, NiFe-LDH sheets were fabricated on nickel foam in a one-step co-precipitation technique and their ultrathin nanosheets (∼2 nm) are obtained by in situ oxygen-plasma-controlled exfoliation. In addition, the oxygen vacancies in exfoliated sheets were generated by a chemical reduction method that further improved the electronic conductivity and overall electrocatalytic performance of the catalyst. This approach can address the limitations of NiFe-LDH, such as poor conductivity and low stability, making it more efficient for electrocatalysis. It is also observed that the catalyst having 60 s O-plasma exposure after chemical reduction, i.e., NiFe-OOH_OV, outperformed remaining electrocatalysts and exhibited superior OER activity with a low overpotential of 330 mV to achieve a high current density of 50 mA cm^-2. The catalyst also displayed an ECSA-normalized OER overpotential of 288 mV at a current density of 10 mA cm^-2 and exhibited excellent long-term stability (120 h) in an alkaline electrolyte. Remarkably, ultrathin defect-rich catalyst continuously produced O₂, resulting in a high faradaic efficiency of 98.1% for the OER.

Promoting electrocatalytic overall water splitting by sulfur incorporation into CoFe-(oxy)hydroxide

The design and fabrication of highly cost-effective electrocatalysts with high activity, and stability to enhance the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has been considered to be one of the most promising approaches toward overall water splitting. In this study, sulfur-incorporated cobalt-iron (oxy)hydroxide (S-(Co,Fe)OOH) nanosheets were directly grown on commercial iron foam via galvanic corrosion and hydrothermal methods. The incorporation of sulfur into (Co,Fe)OOH results in superior catalytic performance and high stability in both the HER and OER conducted in 1 M KOH. The incorporation of sulfur enhanced the electrocatalytic activity by modifying the electronic structure and chemical states of (Co,Fe)OOH. An alkaline water electrolyzer for overall water splitting was fabricated using a two-electrode configuration utilizing the S-(Co,Fe)OOH bifunctional electrocatalyst in both the HER and OER. The fabricated electrolyzer outperformed a precious metal-based electrolyzer using Pt/C as the HER electrocatalyst and IrO2 as the OER electrocatalyst, which are the benchmark catalysts. This electrolyzer provides a lower potential of 1.641 V at 10 mA cm-2 and maintains 98.4% of its performance after 50 h of durability testing. In addition, the S-(Co,Fe)OOH-based electrolyzer successfully generated hydrogen under natural illumination upon its combination with a commercial silicon solar cell and exhibited a solar to hydrogen (STH) efficiency of up to 13.0%. This study shows that S-(Co,Fe)OOH is a promising candidate for application in the future renewable energy industry due to its high cost-effectiveness, activity, and stability during overall water splitting. In addition, the combination of a commercial silicon solar cell with an alkaline water electrolyzer has great potential for the production of hydrogen.

Electronic modulation and proton transfer by iron and borate co-doping for synergistically triggering the oxygen evolution reaction on a hollow NiO bipyramidal prism

Designing an Earth-abundant and inexpensive electrocatalyst to drive the oxygen evolution reaction (OER) for high-purity hydrogen production is of great importance. Herein, the cation (iron) and anion (borate) co-doping strategy was proposed to effectively trigger the OER performance on a low-cost NiO material. The optimal hollow Fe/B_i-NiO bipyramidal prism shows superior OER performance, and displays a low overpotential (261 mV) at 10 mA cm^-2, accompanied by a low Tafel slope (46 mV dec^-1), excellent intrinsic activity and robust stability. The overall alkaline water splitting using Fe/B_i-NiO/NF as an anode affords low cell voltages of 1.50 and 1.63 V at 10 and 100 mA cm^-2, and operates steadily at a high current density of 100 mA cm^-2 for 55 h without decay. The excellent electrocatalytic activity could be ascribed to the hollow structure to shorten the mass transfer pathway, the electronic modulation by Fe doping, the increased accessible electroactive sites created by oxygen vacancies through borate doping, and the formation of BO₃^3--OH^- to accelerate the deprotonation of OH_ads.

Progress in research and development of particle electrodes for three-dimensional electrochemical treatment of wastewater: a review

A three-dimensional (3D) electrochemical technology is regarded as a very effective industrial wastewater treatment method as it has high treatment efficiency, high current efficiency, and low energy consumption, and especially can completely mineralize nonbiodegradable organic pollutants. The core of the 3D electrochemical technology is a particle electrode, and the particle electrode plays several important roles for removing pollutants during the electrochemical reaction process. Many types of particle electrodes have been developed and used for different types of wastewater treatment. In this paper, a comprehensive review on the research and development of particle electrodes of the 3D electrochemical reactors for wastewater treatment is conducted. Specifically, the role that the particle electrode plays during the 3D electrochemical treatment of wastewater is thoroughly investigated and systematized. In addition, the different types of particle electrodes used in the 3D electrochemical wastewater treatment are classified into several types according to the presence or absence of a catalyst and the main components of the particle electrode or carrier. Also, focusing on the recent research results, the structural characteristics, performance, advantages and defects, and the role of catalyst components of each particle electrodes are evaluated. Finally, the direction and prospect of future research on the particle electrode is presented.

From NiMoO4 to γ-NiOOH: Detecting the Active Catalyst Phase by Time Resolved in Situ and Operando Raman Spectroscopy

Water electrolysis powered by renewable energies is a promising technology to produce sustainable fossil free fuels. The development and evaluation of effective catalysts are here imperative; however, due to the inclusion of elements with different redox properties and reactivity, these materials undergo dynamical changes and phase transformations during the reaction conditions. NiMoO4 is currently investigated among other metal oxides as a promising noble metal free catalyst for the oxygen evolution reaction. Here we show that at applied bias, NiMoO4·H2O transforms into γ-NiOOH. Time resolved operando Raman spectroscopy is utilized to follow the potential dependent phase transformation and is collaborated with elemental analysis of the electrolyte, confirming that molybdenum leaches out from the as-synthesized NiMoO4·H2O. Molybdenum leaching increases the surface coverage of exposed nickel sites, and this in combination with the formation of γ-NiOOH enlarges the amount of active sites of the catalyst, leading to high current densities. Additionally, we discovered different NiMoO4 nanostructures, nanoflowers, and nanorods, for which the relative ratio can be influenced by the heating ramp during the synthesis. With selective molybdenum etching we were able to assign the varying X-ray diffraction (XRD) pattern as well as Raman vibrations unambiguously to the two nanostructures, which were revealed to exhibit different stabilities in alkaline media by time-resolved in situ and operando Raman spectroscopy. We advocate that a similar approach can beneficially be applied to many other catalysts, unveiling their structural integrity, characterize the dynamic surface reformulation, and resolve any ambiguities in interpretations of the active catalyst phase.

Transition metal-based catalysts for electrochemical water splitting at high current density: current status and perspectives

As a clean energy carrier, hydrogen has priority in decarbonization to build sustainable and carbon-neutral economies due to its high energy density and no pollutant emission upon combustion. Electrochemical water splitting driven by renewable electricity to produce green hydrogen with high-purity has been considered to be a promising technology. Unfortunately, the reaction of water electrolysis always requires a large excess potential, let alone the large-scale application (e.g., >500 mA cm^-2 needs a cell voltage range of 1.8-2.4 V). Thus, developing cost-effective and robust transition metal electrocatalysts working at high current density is imperative and urgent for industrial electrocatalytic water splitting. In this review, the strategies and requirements for the design of self-supported electrocatalysts are summarized and discussed. Subsequently, the fundamental mechanisms of water electrolysis (OER or HER) are analyzed, and the required important evaluation parameters, relevant testing conditions and potential conversion in exploring electrocatalysts working at high current density are also introduced. Specifically, recent progress in the engineering of self-supported transition metal-based electrocatalysts for either HER or OER, as well as overall water splitting (OWS), including oxides, hydroxides, phosphides, sulfides, nitrides and alloys applied in the alkaline electrolyte at large current density condition is highlighted in detail, focusing on current advances in the nanostructure design, controllable fabrication and mechanistic understanding for enhancing the electrocatalytic performance. Finally, remaining challenges and outlooks for constructing self-supported transition metal electrocatalysts working at large current density are proposed. It is expected to give guidance and inspiration to rationally design and prepare these electrocatalysts for practical applications, and thus further promote the practical production of hydrogen via electrochemical water splitting.

Fe, Ni-codoped W18O49 grown on nickel foam as a bifunctional electrocatalyst for boosted water splitting

Designing cost-effective bifunctional catalysts with high-performance and durability is of great significance for renewable energy systems. Herein, typical Fe, Ni-codoped W₁₈O₄₉/NF was prepared via a simple solvothermal method. The incorporation of Fe ions enhanced the electronic interaction and enlarged the electrochemically active surface area. The increased W⁴⁺ leads to a high proportion of unsaturated W[double bond, length as m-dash]O bonds, thus enhancing the adsorption capacity of water. The valence configuration of nickel (Ni) sites in such dual-cation doping is well adjusted, realizing a high proportion of trivalent Ni ions (Ni³⁺). Due to the orbital interactions, the Fe³⁺/Ni³⁺ ions and OER reaction intermediates exhibit strong orbital overlap. The positions of the valence band and conduction band are well modulated. As a result, the Fe, Ni-codoped W₁₈O₄₉/NF shows improved electrocatalytic activity, and achieves a low decomposition voltage of 1.58 V at 10 mA cm^-2 and retains long-time stability.

Ultrasonically Surface-Activated Nickel Foam as a Highly Efficient Monolith Electrode for the Catalytic Oxidation of Methanol to Formate

Most of the current electrocatalysts for the methanol oxidation reaction are precious group metals such as Pt, Pd, and Ru. However, their use is limited due to their high cost, scarcity, and issues with carbon monoxide poisoning. We developed a simple method to prepare a nickel foam (NF)-based monolith electrode with a NiO nanosheet array structure as an efficient electrocatalyst toward the oxidation of methanol to produce formate. By a simple ultrasonic acid treatment and air oxidation at room temperature, an inert NF was converted to NiO/NF as a catalytically active electrode due to the uniform NiO nanosheet array that was rapidly formed on the surface of NiO/NF. In alkaline electrolytes containing methanol, the as-prepared NiO/NF catalysts exhibited a lower methanol oxidation reaction (MOR) potential of +1.53 V vs RHE at 100 mA cm^-2 compared to that of inert NF samples. The difference in potentials between the E_MOR and the E_OER at that current density was found to be 280 mV, indicating that methanol oxidation occurred at lower potentials as compared to the oxygen evolution reaction (OER). We also observed that the NiO/NF could also efficiently catalyze the oxidation of CO without being poisoned by it. NiO/NF retained close to 100% of its initial activity after 20,000 s of methanol oxidation tests at high current densities above 200 mA cm^-2. Because of the simple synthesis method and the enhanced catalytic performance and stability of NiO/NF, this allows methanol to be used as an OER masking agent for the energy-efficient generation of value-added products such as formic acid and hydrogen.

A strategy for preparing high-efficiency and economical catalytic electrodes toward overall water splitting

Electrolyzing water technology to prepare high-purity hydrogen is currently an important field in energy development. However, the preparation of efficient, stable, and inexpensive hydrogen production technology from electrolyzed water is a major problem in hydrogen energy production. The key technology for hydrogen production from water electrolysis is to prepare highly efficient catalytic, stable and durable electrodes, which are used to reduce the overpotential of the hydrogen evolution reaction and the oxygen evolution reaction of electrolyzed water. The main strategies for preparing catalytic electrodes include: (i) choosing cheap, large specific surface area and stable base materials, (ii) modulating the intrinsic activity of the catalytic material through elemental doping and lattice changes, and (iii) adjusting the morphology and structure to increase the catalytic activity. Based on these findings, herein, we review the recent work in the field of hydrogen production by water electrolysis, introduce the preparation of catalytic electrodes based on nickel foam, carbon cloth and new flexible materials, and summarize the catalytic performance of metal oxides, phosphides, sulfides and nitrides in the hydrogen evolution and oxygen evolution reactions. Secondly, parameters such as the overpotential, Tafel slope, active site, turnover frequency, and stability are used as indicators to measure the performance of catalytic electrode materials. Finally, taking the material cost of the catalytic electrode as a reference, the successful preparations are comprehensively compared. The overall aim is to shed some light on the exploration of high-efficiency and economical electrodes in energy chemistry and also demonstrate that there is still room for discovering new combinations of electrodes including base materials, composition lattice changes and morphologies.

Graphene-coated nanoporous nickel towards a metal-catalyzed oxygen evolution reaction

Developing highly active electrocatalysts with low costs and long durability for oxygen evolution reactions (OERs) is crucial towards the practical implementations of electrocatalytic water-splitting and rechargeable metal-air batteries. Anodized nanostructured 3d transition metals and alloys with the formation of OER-active oxides/hydroxides are known to have high catalytic activity towards OERs but suffer from poor electrical conductivity and electrochemical stability in harsh oxidation environments. Here we report that high OER activity can be achieved from the metallic state of Ni which is passivated by atomically thick graphene in a three-dimensional nanoporous architecture. As a free-standing catalytic anode, the non-oxide transition metal catalyst shows a low OER overpotential, high OER current density and long cycling lifetime in alkaline solutions, benefiting from the high electrical conductivity and low impedance resistance for charge transfer and transport. This study may pave a new way to develop high efficiency transition metal OER catalysts for a wide range of applications in renewable energy.

Interfacial sp C-O-Mo Hybridization Originated High-Current Density Hydrogen Evolution

High-current density (≥1 A cm^-2) is a critical factor for large-scale industrial application of water-splitting electrocatalysts, especially seawater-splitting. However, it still remains a great challenge to reach high-current density due to the lack of active and stable intrinsic catalytic active sites in catalysts. Herein, we report an original three-dimensional self-supporting graphdiyne/molybdenum oxide (GDY/MoO₃) material for efficient hydrogen evolution reaction via a rational design of "sp C-O-Mo hybridization" on the interface. The "sp C-O-Mo hybridization" creates new intrinsic catalytic active sites (nonoxygen vacancy sites) and increases the amount of active sites (eight times higher than pure MoO₃). The "sp C-O-Mo hybridization" facilitates charge transfer and boosts the dissociation process of H₂O molecules, leading to outstanding HER activity with high-current density (>1.2 A cm^-2) in alkaline electrolyte and a decent activity and stability in natural seawater. Our results show that high-current density electrocatalysts can be achieved by interfacial chemical bond engineering, three-dimensional structure design, and hydrophilicity optimization.

Decolorization of acid blue 29, disperse red 1 and congo red by different indigenous fungal strains

Azo dyes are toxic and recalcitrant environmental pollutants in wastewater and soil in many industrial sites in Asia and Arabic countries. The aim of this study was to find fungal species useful in wastewater treatment and soil remediation efforts. We assessed the ability of different indigenous Aspergillus strains (i.e. A. flavus, A. fumigatus, A. niger and A. terreus) to degrade the azo dyes Acid Blue 29 (AB29), Disperse Red 1 (DR1) and Congo Red (CR). The optimal conditions for dye decolorization by the above-mentioned strains appeared to be as follows: temperature range 30-35 °C, pH 7, glucose as the carbon source (10 g/L), ammonium sulphate as the nitrogen source (1.5 g/L) and 100 mg/L initial dye concentration. The Aspergillus strains decolorized all azo dyes more than 86%. The HPLC and GC-MS analyses confirmed that aniline (retention time 9.0 min), 3-nitroaniline (retention time 15.92 min), 4-nitroanline (retention time 17.81 min), N,N' diethyl-1,4-phenylendiamine (retention time 18.184 min), and benzidine (retention time 15.07 min) were formed as the intermediate metabolites of dye degradation. All Aspergillus strains decolorized 85% of the dyes in synthetic wastewater.

Electrocatalytic Properties of Co Nanoconical Structured Electrodes Produced by a One-Step or Two-Step Method

One-dimensional (1D) nanostructures, such as nanotubes, nanopores, nanodots and nanocones, are characterized by better catalytic properties than bulk material due to their large active surface area and small geometrical size. These structures can be produced by several methods of synthesis including the one- and two-step methods. In the one-step method, a crystal modifier is added to the solution in order to limit the horizontal direction of structures growing during electrodeposition. In this work, NH4Cl was used as a crystal modifier. Another way of production of 1D nanocones is the electrodeposition of metal in porous anodic alumina oxide (AAO) templates, called the two-step method. In this case, the AAO template was obtained using a two-step anodization process. Nanocones obtained by the two-step method show smaller geometrical size. In this work, cobalt nanoconical structures were obtained from an electrolyte containing CoCl2 and H3BO3. The electrocatalytic properties of materials fabricated by one-step and two-step methods were measured in 1 M NaOH and compared with bulk material electrodeposited from the same electrolyte. There were several microshell structures in the case of Co deposits obtained by the one-step method. To solve this problem, different conditions of synthesis Co cones by the one-step method were applied. The electrocatalytic activity of these samples was checked as well.

Enhancing the Effectiveness of Oxygen Evolution Reaction by Electrodeposition of Transition Metal Nanoparticles on Nickel Foam Material

Electrochemical oxygen evolution reaction (OER) activity was studied on nickel foam-based electrodes. The OER was investigated in 0.1 M NaOH solution at room temperature on as-received and Co- or Mo-modified Ni foam anodes. Corresponding values of charge-transfer resistance, exchange current-density for the OER and other electrochemical parameters for the examined Ni foam composites were recorded. The electrodeposition of Co or Mo on Ni foam base-materials resulted in a significant enhancement of the OER electrocatalytic activity. The quality and extent of Co, and Mo electrodeposition on Ni foam were characterized by means of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis.

Engineering porous Pd-Cu nanocrystals with tailored three-dimensional catalytic facets for highly efficient formic acid oxidation

Rational synthesis of bi- or multi-metallic nanomaterials with both dendritic and porous features is appealing yet challenging. Herein, with the cubic Cu₂O nanoparticles composed of ultrafine Cu₂O nanocrystals as a self-template, a series of Pd-Cu nanocrystals with different morphologies (e.g., aggregates, porous nanodendrites, meshy nanochains and porous nanoboxes) are synthesized through simply regulating the molar ratio of the Pd precursor to the cubic Cu₂O, indicating that the galvanic replacement and Kirkendall effect across the alloying process are well controlled. Among the as-developed various Pd-Cu nanocrystals, the porous nanodendrites with both dendritic and hollow features show superior electrocatalytic activity toward formic acid oxidation. Comprehensive characterizations including three-dimensional simulated reconstruction of a single particle and high-resolution transmission electron microscopy reveal that the surface steps, defects, three-dimensional architecture, and the electronic/strain effects between Cu and Pd are responsible for the outstanding catalytic activity and excellent stability of the Pd-Cu porous nanodendrites.

Metal doped layered MgB2 nanoparticles as novel electrocatalysts for water splitting

Growing environmental problems along with the galloping rate of population growth have raised an unprecedented challenge to look for an ever-lasting alternative source of energy for fossil fuels. The eternal quest for sustainable energy production strategies has culminated in the electrocatalytic water splitting process integrated with renewable energy resources. The successful accomplishment of this process is thoroughly subject to competent, earth-abundant, and low-cost electrocatalysts to drive the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), preferably, in the same electrolyte. The present contribution has been dedicated to studying the synthesis, characterization, and electrochemical properties of newfangled electrocatalysts with the formal composition of Mg1-xTMxB2 (x = 0.025, 0.05, and 0.1; TM (transition metal) = Fe and Co) primarily in HER as well as OER under 1 M KOH medium. The electrochemical tests revealed that among all the metal-doped MgB2 catalysts, Mg0.95Co0.05B2 has the best HER performance showing an overpotential of 470 mV at - 10 mA cm-2 and a Tafel slope of 80 mV dec-1 on account of its high purity and fast electron transport. Further investigation shed some light on the fact that Fe concentration and overpotential for HER have adverse relation meaning that the highest amount of Fe doping (x = 0.1) displayed the lowest overpotential. This contribution introduces not only highly competent electrocatalysts composed of low-cost precursors for the water-splitting process but also a facile scalable method for the assembly of highly porous electrodes paving the way for further stunning developments in the field.

Construction of self-supporting, hierarchically structured caterpillar-like NiCo2S4 arrays as an efficient trifunctional electrocatalyst for water and urea electrolysis

In this study, we have developed intriguing self-supporting caterpillar-like spinel NiCo2S4 arrays with a hierarchical structure of nanowires on a nanosheet skeleton, which can be used as a self-supporting trifunctional electrocatalyst for the oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). The caterpillar-like NiCo precursor arrays are first in situ grown on carbon cloth (NiCo2O4/CC) by a facile hydrothermal reaction, which is followed by an anion exchange process (or sulfuration treatment) with Na2S to form self-supporting spinel NiCo2S4 arrays (NiCo2S4/CC) with a roughened nanostructure. Taking advantage of the bimetallic synergistic effect, the unique hierarchical nanostructure, and the self-supporting nature, the resultant NiCo2S4/CC electrode exhibits high activities toward the OER, HER and UOR, which are highly superior to the monometallic counterparts of NiS nanosheets and Co9S8 nanowires on a carbon cloth substrate. The comparison of the three electrodes also indicates that the hierarchically structured bimetallic electrode combines the morphological and structural characteristics of monometallic Ni-based nanosheets and Co-based nanowires. When assembling a two-electrode electrolytic cell with NiCo2S4/CC as both the anode and cathode, an applied cell voltage of only 1.66 V is required to deliver a current density of 10 mA cm-2 in water electrolysis. By using the same two-electrode setup, the applied voltage for urea electrolysis is further reduced to 1.45 V that produces hydrogen at the cathode with the same current density. This study paves the way for exploring the feasibility of future less energy-intensive and large-scale hydrogen production.

Metal–organic frameworks and their derivatives as electrocatalysts for the oxygen evolution reaction

This review summarizes the recent progress on MOFs and their derivatives used for OER electrocatalysis in terms of their morphology, composition and structure–performance relationship.

Star-shaped colloidal PbS nanocrystals: structural evolution and growth mechanism

Branched nanostructures have attracted considerable interest due to their large surface-to-volume ratio with benefits in photocatalysis and photovoltaic applications. Here we discuss the tailoring of branched structures with a shape of a star based on PbS semiconductor. It exposes the reaction mechanism and the controlling factors that template their morphology. For this purpose, we varied the primary lead precursors, types of surfactant, lead-to-surfactant molar ratio, temperature and duration of the reaction. Furthermore, intermediate products in a growth reaction were thoroughly examined using X-ray diffraction, transmission electron microscopy, Raman scattering, optical absorbance and Fourier transform infrared spectroscopy. The results designated a primary formation of truncated octahedral seeds with terminating {100} and {111} facets, followed by the selective fast growth of pods along the 〈100〉 directions toward the development of a star-like shape. The examined intermediates possess a cubic rock salt structure. The observations indicated that small surfactant molecules (e.g. acetate) evolve the branching process, while long-chain surfactants (e.g. oleate) stabilize the long pods as well as mitigate the aggregation process. This study conveys fundamental knowledge for the design of other branched structures, that are attractive for practical use in catalysis, electrochemistry and light-harvesting.

Tailoring of branched structures in the shape of stars based on PbS semiconductor, revealing the reaction mechanism and controlling factors that dictate their morphology and associated optical properties.

Ni(OH)2–Ag hybrid nanosheet array with ultralow Ag loading as a highly efficient and stable electrocatalyst for hydrogen evolution reaction

0D Ag nanoparticles were successfully loaded onto 2D Ni(OH)₂ nanosheets on 3D nickel foam by a one-step hydrothermal method for enhancing the electrocatalytic ability.

In situ evolved NiMo/NiMoO4 nanorods as a bifunctional catalyst for overall water splitting

Due to their good conductivity and catalytic performance, Ni-Mo-based catalysts are well-established for highly effective water splitting. However, the know-how required to fabricate distinct interfaces and electronic structures for metal oxides is still a challenge, and the synergistic effect between metal and metal oxides that enhances electrocatalytic activity is still ambiguous. As described here, by controlling the lithium-induced conversion reaction of metal oxides, metal/metal-oxide composites with plentiful interfaces and prominent electrical interconnections were fabricated, which can boost active sites and accelerate mass transfer during electrocatalytic reactions. As a consequence, the superior catalytic activity of ECT-NiMo/NiMoO₄ exhibited a low overpotential of 61 mV at a current density of 10 mA cm^-2 for the hydrogen evolution reaction and 331 mV at 100 mA cm^-2 for the oxygen evolution reaction. When integrated into a two-electrode system, the ECT-NiMo/NiMoO₄ revealed a highly stable and efficient performance in overall water splitting. This work provides a promising approach to enhance the metallicity and electron redistribution of catalysts for numerous water-splitting applications and many other possibilities for energy storage devices.

In situ deposition of MOF-74(Cu) nanosheet arrays onto carbon cloth to fabricate a sensitive and selective electrocatalytic biosensor and its application for the determination of glucose in human serum

A new electrocatalytic biosensor (MOF-74(Cu) NS-CC) based on the in situ deposition of MOF-74(Cu) nanosheet on carbon cloth via a bottom-up synthetic approach in a glass tube was developed. The electrocatalytic activity of the deposited MOF-74(Cu) NS was demonstrated in the oxidation of glucose to gluconate under alkaline conditions. The results revealed that the proposed method of in situ formation of MOF-74(Cu) NS onto a carbon cloth surface in a multi-layer solution is capable to generate a stable MOF-74(Cu) NS-CC electrode with excellent sensing performance. When the as-synthesized MOF-74(Cu) NS-CC was applied directly as the working electrode for glucose sensing, it showed much higher conductivity and redox activity than MOF-74(Cu) NS-GCE. With the potential applied at 0.55 V (vs. Ag/AgCl), this new electrocatalytic biosensor exhibits an excellent linear relationship between current density and concentration of glucose. Moreover, a wide linear range of detection (1.0 to 1000 μM) was observed. The limit of detection was found to be 0.41 μM (S/N = 3). The response sensitivity is 3.35 mA mM^-1 cm^-2 when the concentration of glucose is in the range 1-100 μM and 3.81 mA mM^-1 cm^-2 for the 100-1000 μM concentration range. This study provides a low-cost, easy to prepare, and reproducible methodology for the synthesis of highly redox-active nanomaterials based on the in situ formation of two-dimensional MOF-74(Cu) NS for the development of new electrocatalytic biosensors. Graphical abstract.

Concentration profile of dissolved gas during hydrogen gas evolution: an optical approach

We develop an optical image tracking technique for the simultaneous observation of a wide area in proximity to the electrode, and study the growth of bubbles during hydrogen gas evolution in alkaline water electrolysis. Using a diffusion model we can successfully extract the concentration profile of dissolved hydrogen gas as a function of distance from the electrode. The obtained concentrations agree well with the values by the electrochemical method.

A soft molecular 2Fe-2As precursor approach to the synthesis of nanostructured FeAs for efficient electrocatalytic water oxidation

An unprecedented molecular 2Fe-2As precursor complex was synthesized and transformed under soft reaction conditions to produce an active and long-term stable nanocrystalline FeAs material for electrocatalytic water oxidation in alkaline media. The 2Fe2As-centred β-diketiminato complex, having an unusual planar Fe2As2 core structure, results from the salt-metathesis reaction of the corresponding β-diketiminato FeIICl complex and the AsCO- (arsaethynolate) anion as the monoanionic As- source. The as-prepared FeAs phase produced from the precursor has been electrophoretically deposited on conductive electrode substrates and shown to act as a electro(pre)catalyst for the oxygen evolution reaction (OER). The deposited FeAs undergoes corrosion under the severe anodic alkaline conditions which causes extensive dissolution of As into the electrolyte forming finally an active two-line ferrihydrite phase (Fe2O3(H2O) x ). Importantly, the dissolved As in the electrolyte can be fully recaptured (electro-deposited) at the counter electrode making the complete process eco-conscious. The results represent a new and facile entry to unexplored nanostructured transition-metal arsenides and their utilization for high-performance OER electrocatalysis, which are also known to be magnificent high-temperature superconductors.

Simultaneous Sulfite Electrolysis and Hydrogen Production Using Ni Foam-Based Three-Dimensional Electrodes

The electrochemical oxidation of sulfite ions offers encouraging advantages for large-scale hydrogen production, while sulfur dioxide emissions can be effectively used to obtain value-added byproducts. Herein, the performance and stability during sulfite electrolysis under alkaline conditions are evaluated. Nickel foam (NF) substrates were functionalized as the anode and cathode through electrochemical deposition of palladium and chemical oxidation to carry out the sulfite electro-oxidation and hydrogen evolution reactions, respectively. A combined analytical approach in which a robust electrochemical flow cell was coupled to different in situ and ex situ measurements was successfully implemented to monitor the activity and stability during electrolysis. Overall, satisfactory sulfite conversion and hydrogen production efficiencies (>90%) at 10 mA·cm^-2 were mainly attributed to the use of NF in three-dimensional electrodes with a large surface area and enhanced mass transfer. Furthermore, stabilization processes associated with electrochemical dissolution and sulfur crossover through the membrane induced specific changes in the chemical and physical properties of the electrodes after electrolysis. This study demonstrates that NF-based electrocatalysts can be incorporated in an efficient electrochemical flow cell system for sulfite electrolysis and hydrogen production, with potential applications at a large scale.

Laser-Assisted Synthesis and Oxygen Generation of Nickel Nanoparticles

Nowadays, more than ever, environmental awareness is being taken into account when it comes to the design of novel materials. Herein, the pathway to the creation of a colloid of spherical, almost purely metallic nickel nanoparticles (NPs) through pulsed laser ablation in ethanol is presented. A complex description of the colloid is provided through UV-vis spectroscopy and dynamic light scattering analysis, ensuring insight into laser-induced nanoparticle homogenization and size-control of the NPs. The transmission electron spectroscopy revealed spherical nanoparticles with a narrow size distribution, whereas the energy-dispersive X-ray spectroscopy accompanied by the X-ray photoelectron spectroscopy revealed their metallic nature. Furthermore, an example of the application of the colloidal nanoparticles is presented, where a quick, five-min ultrasound modification results in over an order of magnitude higher current densities in the titania-based electrode for the oxygen evolution reaction.

Flower-like S-doped-Ni2P mesoporous nanosheets-derived self-standing electrocatalytic electrode for boosting hydrogen evolution

Developing cost-effective, highly active, and stable electrocatalysts for boosting electrochemical hydrogen evolution reaction (HER) in alkaline media is playing a critical role to meet hydrogen industry in the future. Herein, an efficient HER electrocatalyst based on flowerlike S-doped Ni2P mesoporous nanosheets supported on nickel foam (S-Ni2P NSs/NF) was developed through an effective approach. The obtained S-Ni2P NSs/NF catalyst required low overpotential of only 87.5 mV and 179.1 mV to reach current density of 10 and 50 mA cm-2, respectively. Moreover, a small Tafel slope of 62.1 mV dec-1 for S-Ni2P NSs/NF demonstrated that HER process occurred with very fast kinetics. Besides high HER activity, the synthesized S-Ni2P NSs/NF catalyst exhibited superior stability and long-term durability toward HER, which had ability to operate over 30 h without degradation in catalytic performance. The unique flower-like nanosheets structure with excellent mesoporous characteristics of S-Ni2P NSs/NF resulted in maximizing electrochemical active surface area for providing a large number of electrocatalytic active sites. In addition, S doping effect could modulate electronic structure of Ni species in Ni2P, leading to accelerating rate adsorption of reaction intermediates on the surface of catalysts toward improving HER kinetics. The results not only demonstrate S-Ni2P NSs/NF as active catalyst for HER, but also offer effective strategy for improving catalytic activity of earth-abundant transition metal-based HER catalysts.

Label-Free Bimetallic In Situ-Grown 3D Nickel-Foam-Supported NH2-MIL-88B(Fe2Co)-MOF-based Impedimetric Immunosensor for the Detection of Cardiac Troponin I

In this study, a simple and competent metal-organic framework (MOF)-based nickel foam (NF)-supported three-dimensional (3D) immunosensor (Ab-NH₂-MIL-88B(Fe₂Co)-MOF/NF) was constructed and utilized for the specific recognition of the biomarker cardiac troponin (I) (cTnI). In the present work, biosensor fabrication was progressed through the modification of the NF substrate with the MOF material (NH₂-MIL-88B(Fe₂Co)-MOF) to enable an amine-functionalized electrode. This amine-functionalized NF electrodes (NH₂-MIL-88B(Fe₂Co)-MOF/NF) were then biointerfaced with anti-cTnI antibodies, which ended up as Ab-NH₂-MIL-88B(Fe₂Co)-MOF/NF electrodes. Analytical executions of the constructed bioelectrode were investigated for the quantitative analysis of cTnI in both buffered and serum solutions. Then, the electrochemical studies were carried out using the electrochemical impedance spectroscopy (EIS) method by monitoring changes concerning the charge transfer resistance (R_ct) characteristics. The limit of detection (LOD) of the Ab-NH₂-MIL-88B(Fe₂Co)-MOF/NF immunosensor was achieved to be 13 fg/mL with great specificity. This kind of immunosensor imparts a new platform for the construction and application of MOF-hybrid 3D electrode materials with enhanced electrochemical behavior in cTnI sensing for the first time.