A tetranuclear nickel(II) complex for water oxidation: Meeting new challenges

Ni complexes are promising catalysts for water splitting. Herein, a tetranuclear nickel(II) complex with bis-[(E)-N′-(1-(pyridin-2-yl)ethylidene)]carbohydrazide (HL), was synthesized and characterized by spectroscopic methods and single crystal X-ray analysis. The complex is a tetranuclear complex and the ligands are coordinated to the metal ions in the mono-negative form, (L)^- to form a tetranuclear [NiL]₄⁴⁺ unit. Each Ni(II) ion is six-coordinated by pyridine nitrogen, azomethine nitrogen and oxygen atoms of two perpendicular carbohydrazone ligands in the mer configuration. The complex was studied as a water-oxidizing catalyst. In the next step, the role of the Ni-based compound for water oxidation on the surface of fluorine doped tin oxide as one of the true catalysts was investigated by scanning transmission electron microscopy, scanning transmission electron microscopy, spectroelectrochemistry, and electrochemistry. The electrode after water oxidation by the complex was studied and a relation between the decomposition of the Ni complex and water-oxidation reaction was proposed. The experiments show that under water-oxidation condition in the presence of the complex, a Ni-based compound on the electrode is a candidate as a contributor to the observed catalysis.     

Rethink about electrolyte: Potassium fluoride as a promising additive to an electrolyte for the water oxidation by a nanolayered Mn oxide

Water oxidation is a bottleneck of the hydrogen production through the water-splitting reaction. Herein, the promising role of fluoride on the water-oxidizing activity of a nanolayered Mn oxide under the electrochemical condition is reported. The experiments show the increase of the water-oxidizing activity of the nanolayered Mn oxide under an electro-water oxidation circumstance in the presence of potassium fluoride as a promising additive to an electrolyte. As a result, the required overpotential is decreased and the yield of oxygen evolution raised in the water-oxidation reaction.     

A chromium complex under water oxidation: A conversion mechanism and a comprehensive hypothesis

Water splitting toward hydrogen production is a promising method to store energy, but water oxidation is a bottleneck for water splitting. First-row transition metal complexes have been extensively used for water-oxidation reaction. However, only one chromium complex has been applied for water-oxidation reaction until now. Thus, the reinvestigation of water-oxidation reaction by this chromium complex in detail is interesting. Herein, water-oxidation reaction by the chromium complex with diphenoxy N,N′-ethylenebis(salicylimine) (SALEN) ligand is reinvestigated using scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction studies, X-ray photoelectron spectroscopy, thermal gravimetric analysis, elemental analysis, Fourier-transform infrared spectroscopy, and electrochemical methods. All the experiments showed that the chromium complex is neither a catalyst nor a precatalyst for water-oxidation reaction. During OER, a relationship between first-row transition metal complexes and the related metal oxides has been proposed.     

Mn-containing ZSM-5 type zeolite as a water-oxidizing catalyst: New findings and current controversies

Water-splitting for H₂ and O₂ production is a key reaction toward a clean energy future. However, the water-oxidation is one of the major limiting factors affecting the efficiency of this important reaction. Thus, the development of low-cost and efficient water-oxidizing catalysts is a key challenge in the artificial photosynthesis field. In this work, Mn-containing ZSM-5 type zeolite was synthesized using silicic acid, manganese(II) chloride tetrahydrate, potassium carbonate and tetrapropylammonium bromide (TPAB). Samples were characterized by different methods. The water-oxidizing activity of the catalysts were studied in the presence of cerium(IV) ammonium nitrate (Ce(IV)) and in the electrochemical water-oxidation condition. The effect of K⁺ ions on the structural properties and the catalytic performance of the compounds was also investigated. It was observed that after treating of manganosilicate with a Ce(IV) solution or applying a 2 V constant potential to the sample, in a convective–suspension–collision system, framework located Mn ions completely leaked. These results are important to reconsider the true catalysts in the different catalytic reactions in the presence of manganosilicates.     

Electrochemical induction of Mn(III) in the structure of Mn(IV) oxide: Toward a new approach for water splitting

Oxygen-evolution reaction (OER) through water-oxidation reaction is an essential reaction for water splitting, which provides electrons for hydrogen production. Mn oxides are among the commonly investigated types of metal oxides as OER catalysts. Recent studies showed that Mn(III) ions in the structure of Mn(IV) oxide have an essential role in OER. Previous works have only focused on adding different materials to induce Mn(III) ions in the structure of Mn(IV). Indeed, Mn(III) ions could be induced in the Mn(IV)-oxide structure in the presence of organic compounds, reductants, fluoride, chloride, and gold nanoparticles. However, a challenging issue in the field is using a stable and perdurable force during OER to induce Mn(III) ions in the structure of Mn(IV). Herein, the effects of inducing Mn(III) ions on OER by different potentials on the surface of the prepared Mn(IV) oxide on fluorine-doped tin oxide were investigated in the phosphate buffer at pH 11, 7, and 3. Mn(IV) oxide under OER/Mn(III) induction shows no decrease in the current density, but for the same sample under OER alone, a decrease (15%) is observed in the current density after only 1000 s. At a lower inducing potential, a decrease for OER is observed, which corresponds to Mn(IV)/(III) reduction to Mn(II), and Mn(II) leaking effect. This investigation sheds new light on OER in the presence of Mn oxide and is a promising, low-cost, and environmentally friendly strategy, which results in an increase in the yield of OER.     

Oxygen-evolution reaction by gold and cobalt in iron and nickel free electrolyte

Hydrogen production by electrolysis is a promising method for storing energy. For water electrolysis, water-oxidation reaction is critical to provide electrons for water reduction. However, water-oxidation reaction is sluggish, and thus, remarkable efforts to understand and improve the performance of water-oxidation reaction have been made. The present study investigates the Fe and Ni impurities of electrolytes as an important issue on oxygen-evolution reaction (OER) and electrochemistry of cobalt and gold. For the first time, in this study, OER for cobalt and gold was investigated in Fe and Ni-free KOH. This paper is highlighted the role of impurities in electrolyte for OER. The experiments showed that cobalt oxide is a relatively good catalyst for OER in a pure (Ni and Fe free) electrolyte. However, gold is not a good catalyst for OER in a pure electrolyte.     

Toward Escherichia coli bacteria-machine for water-oxidation reaction at neutral conditions: Using Ruthenium Red

Water splitting into hydrogen and oxygen is a promising method for storing sustainable but intermittent energies. Ruthenium compounds are promising for the water-oxidation reaction. Herein, an easy method is employed to load a water-oxidation catalyst (Ruthenium Red; ([(NH₃)₅RuORu(NH₃)₄ORu(NH₃)₅]Cl₆)) on the surface of the Escherichia coli bacterial template. After the synthetic procedure, the catalyst is characterized by field emission-scanning electron microscopes, high-resolution transmission electron microscopy, visible spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy. Some of the employed methods show that the bacterial template is intact after the synthetic procedure. In the next step, the catalyst is investigated by linear sweep, cyclic, and square wave voltammetry methods. The water-oxidation reaction of the compounds is examined under electrochemical conditions in LiClO₄ (0.25 M) at pH = 6.3. Linear sweep voltammetry shows that the onset overpotential of the water-oxidation reaction of the catalyst is 720 mV (based on extrapolation of the Tafel plot) with a Tafel slope of 226.4 mV per decade. Thus, the presence of Ruthenium Red in the material remarkably increases the activity of the water-oxidation reaction.     

Water oxidation by a nickel complex: New challenges and an alternative mechanism

Water splitting toward hydrogen production is a promising way for energy storage. The catalytic role of the metal hydroxides formed by the decomposition of molecular catalysts toward oxygen-evolution reaction (OER) is a challenge in water-splitting systems. Recently, [Ni(TMC)(CH₃CN)](NO₃)₂ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, complex 1), was reported to be the most efficient nickel-based complex at neutral pH. In this study, the role of the formed Ni hydroxide-like nanosized species in the presence of this complex under oxygen-evolution reaction (OER) is studied by X-ray photoelectron spectroscopy, scanning electron microscopy, energy dispersive spectrometry, and electrochemical method. A new method for detecting trace Ni hydroxide-like nanosized species on the surface of an electrode is also reported, which is promising for the detection of heterogeneous nickel-based catalysts on the surface of an electrode. The experiments suggest that Ni hydroxide-like nanosized species formed during reaction is an alternative catalyst for OER.     

Synthesis of supported Pt nanoparticles by sonication for ORR: Effect of the graphene oxide-carbon composite

Pt supported on graphene oxide (GO), reduced graphene oxide (RGO), carbon Vulcan (C) and GO-C composite were prepared by using sonication as a simple synthesis method in the absence of any special capping agent or thermal treatment. X-ray diffraction (XRD), Raman spectroscopy, Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques were used to characterize the materials synthesized. The electrocatalysts synthesized were evaluated for oxygen reduction reaction (ORR) in acid medium by using cyclic and linear voltammetry tests. The characterization results indicated that the highest dispersion of small Pt particles was observed on the samples supported on GO materials compared to carbon Vulcan due to the oxygen functional surface groups, which promoted a homogeneous distribution of Pt nanoparticles. The electrochemical characterization indicated that Pt/GO-C composite exhibited a 50% more specific and mass activity at 0.85 V for ORR than the conventional Pt/C catalysts, which is associated to Pt-support interaction that modifies the electronic properties of Pt for electrochemical application in fuel cell. GO-C (1:1) can be a promising support for improving the electrochemical activity for ORR.     

The CO removal performances of Cr-free Fe/Ni catalysts for high temperature WGSR under LNG reformate condition without additional steam

The goal of this study was to investigate Cr-free, Fe/Ni, metal oxide catalysts for the high temperature shift (HTS) reaction of a fuel processor using liquefied natural gas (LNG). As hexavalent chromium (Cr⁶⁺) in commercial HTS catalyst is a hazardous material, we selected Ni as a substitute for chromium in the Fe-based HTS catalyst and investigated the HTS activities of these Cr-free, metal oxide catalysts under the LNG reformate condition. Cr-free, Fe/Ni-based catalysts containing Ni instead of Cr were prepared by coprecipitation and their performance was evaluated under a gas mixture condition (56.7% H₂, 10% CO, 26.7% H₂O, and 6.7% CO₂) that simulated the gas composition from a steam methane reformer (SMR, at H₂O/CH₄ ratio = 3 with 100% CH₄ conversion). Under this condition, the Fe/Ni catalysts showed higher CO removal activities than Fe-only and Cr-containing catalysts, but the methanation was promoted when the Ni content in the catalyst exceeded 50 wt%. Brunner-Emmett-Teller (BET), X-ray diffraction (XRD), inductively coupled plasma (ICP) and X-ray photoelectron spectroscopy (XPS) analyses were performed to explain the HTS activity of the Fe/Ni catalysts based on the catalyst structure.     

Efficient hydrogen evolution performance of phase-pure NiS electrocatalysts grown on fluorine-doped tin oxide-coated glass by facile chemical bath deposition

The production of hydrogen, the future fuel, on stable, efficient, and robust electrocatalysts represents an attractive approach for the conversion and storage of carbon-free energy resources. In this study, earth-abundant nickel sulfide (NiS) electrocatalyst were grown on fluorine-doped tin oxide (FTO) substrate by a simple and cost-effective chemical bath deposition for hydrogen evolution reaction (HER). Energy dispersive X-ray analysis and X-ray photoelectron spectra indicated the presence of highly pure NiS. The HER performance of the catalyst was examined in alkaline solution (1.0 M NaOH; pH = 13.5). Notably, NiS film prepared at 100 °C demonstrated superior HER activity with an overpotential of 290 mV to afford a current density of 10 mA/cm² and a Tafel slope of 143.4 mV/dec which are among the promising results obtained for sulfide-based HER electrocatalysts. The catalyst exhibited 100% faradaic efficiency and electrochemical stability which indicate its potential as noble-metal-free HER electrocatalyst.     

Fluoride etched Ni-based electrodes as economic oxygen evolution electrocatalysts

Electrochemical water splitting is a promising technology for eco-friendly energy storage. However, the design principles for highly active, robust, and noble metal-free electrocatalysts for industrial-scale hydrogen production remain controversial. Oxygen-free compounds containing anionic species with a very high oxidation potential, such as fluorides, have emerged as high-performance targets for thermodynamically stable oxygen evolution reaction (OER) catalysts. They can further be designed to fit the key criteria of high electrical conductivity and stability. Herein, we present a facile and scalable etching method for constructing fluoride doped metallic nickel-based anodes from industrial Ni foam sources with high application potential for large-scale hydrogen production setups. The fluoride-etched Ni-catalysts were investigated with a wide range of techniques, such as powder X-ray diffraction (PXRD), extended X-ray absorption fine structure spectroscopy (EXAFS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). Optimized catalysts displayed a promising overpotential of 220 mV for the OER at a current density of 60 mA cm^?2, which is competitive with noble metal-based reference catalysts, such as iridium oxide. Electrochemical impedance spectroscopy (EIS) studies demonstrated that etching of the electrode surface in fluoride medium leads to a drastic decrease of R_ct. The corresponding decreased resistivity towards electrochemical OER on the electrode surface gives rise to the notably enhanced performance, with a minimum of synthetic effort.     

Facile deposition of NiFe-LDH ultrathin film on pyrolytic graphite sheet for oxygen evolution reaction in alkaline electrolyte

NiFe-layered doubly hydroxide (LDH) is one of the most active materials for hydroxyl oxidation in an alkaline electrolyte. In this study, we explored a facile method of depositing NiFe-LDH catalyst on pyrolytic graphite sheets (PGSs) to explore the synergistic effects at the substrate–electrocatalyst interface. The catalyst was electrodeposited on PGSs using chronoamperometry. The homogeneous distribution of the catalyst with nanosheet morphology on the PGS's surface produced an ultrathin film. The NiFe25/PGS sample showed a low overpotential (332 mV) and Tafel slope (33 mV dec^?1). The fitting of the Nyquist plots was performed using an equivalent circuit, and the NiFe25/PGS (1.69 Ω) sample showed the lowest charge transfer resistance among the studied catalysts. In addition, PGS proved a better substrate for the alkaline oxygen evolution reaction compared to glassy carbon and fluorine-doped tin oxide.     

A dinuclear iron complex as a precatalyst for water oxidation under alkaline conditions

Water oxidation is a key reaction for water splitting. The decomposition of Fe-based-molecular structures toward Fe-based oxides is a promising method for oxygen-evolution reaction (OER) through water oxidation. The decomposition of Fe-based-molecular structures method results in a slow decomposition of precatalysts and forms Fe oxide-based catalysts. In this study, the Fe species formed through the decomposition of a dinuclear Fe(III) complex under OER is investigated by X-ray photoelectron spectroscopy, scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, and the electrochemical method. In addition, using Ni(OH)₂, a new approach is reported for detecting trace Fe species on the electrode surface. The resulting Fe oxide-based catalyst shows a catalytic current with an onset of 621 mV overpotential and the Tafel slope of 113.7 mV/decade at pH 11 in a buffer phosphate.     

Nickel phosphate as advanced promising electrochemical catalyst for the electro-oxidation of methanol

Mesoporous nickel phosphate nanotube (Meso NiPO NT) and mesoporous nickel phosphate nanosheet (Meso NiPO NS) are developed as catalysts for electrochemical methanol oxidation. Conventional mesoporous nickel phosphate which is composed of stacked nanocrystals (Meso NiPO), microporous VSB-5 and commercial nickel oxide (NiO) are used as control materials. Notably, both Meso NiPO NT (40.83 mA cm^?2) and Meso NiPO NS (44.97 mA cm^?2) exhibit much higher oxidation current density than VSB-5 (13.41 mA cm^?2), Meso NiPO (19.85 mA cm^?2) and commercial NiO (0.87 mA cm^?2). As for the durability test on these materials modified fluorine-doped tin dioxide transparent conductive glass (FTO) electrodes, Meso NiPO NT displays the most stable performance and still retains 91.3% electrochemical activity, which perhaps benefit from its nanotube structure and large specific surface area (99.6 m²/g). Moreover, Meso NiPO NT has higher activity and more excellent stability than many of the previously reported nickel-based materials, suggesting a potential development for direct methanol fuel cells.     

Coke resistance of Ni-based catalysts enhanced by cold plasma treatment for CH4–CO2 reforming: Review

Carbon dioxide reforming of methane can reduce emissions of greenhouse gases, and has been extensively studied. The conventional Ni-based catalysts easily coke, sinter, and deactivate in the CRM reaction. The studies suggested that the cold plasma treatment can improve the structure of Ni-based catalysts, and so enhance coke resistance of the catalysts. The review summarized the effect of cold plasma treatment on the coke resistance of Ni-based catalysts for the CRM reaction. The main goal of the paper was to illuminate: the structure change of catalysts treated by cold plasma, such as crystal planes of Ni particles, the particles size of Ni, the Ni dispersion, the metal-support interaction, and CO₂ absorption capacity; the correlation between plasma treatment conditions (treatment way and parameters) and coke resistance of the catalyst treated by cold plasma; and the mechanism of plasma treatment to improve the coke resistance of the catalysts.     

Ultra-thin metal layer passivation of the transparent conductive anode in organic solar cells

Efficiency of organic solar cells shows a strong improvement when the transparent conductive anode (indium tin oxide—ITO, aluminium-doped zinc oxide—AZO, fluorine-doped tin oxide—FTO), is covered with an ultra-thin metallic film. It is shown that the best results are achieved with a gold film (0.5 nm). The efficiency of the solar cells using AZO or FTO is improved up to one order of magnitude, while in the case of ITO it is at least 50%. It is shown that if the matching between the work function of the anode and the highest occupied molecular orbital (HOMO) of the organic electron donor is the most important factor limiting the hole transfer efficiency, others factors such as transparent conductive oxide (TCO) surface roughness and adhesion of the organic layer are also key factors.     

Heat-resisting TCO films for PV cells

We developed new heat-resisting transparent conductive oxide (TCO) films with resistivity of 1.4×10⁻⁴ Ω cm, an optical transmittance of above 80% (at 550 nm) and heat-resisting temperature at above 600 °C. The TCO films consists of fluorine-doped tin oxide films coated on indium–tin oxide films. They were prepared by a spray pyrolysis deposition method on glass substrates. The 100×100 mm² dye-sensitized solar cells (DSCs) were prepared with the TCO films. An energy conversion efficiency of the DSC was improved drastically in comparison to the case with conventional TCO films.     

Polyol synthesis of reduced graphene oxide supported platinum electrocatalysts for fuel cells: Effect of Pt precursor,support oxidation level and pH

In this work, a comprehensive study on the polyol synthesis of platinum supported on reduced graphene oxide (Pt/rGO) catalysts, including both ex-situ and in-situ characterizations of the prepared Pt/rGO catalysts, was performed. The polyol synthesis was studied considering the influence of the platinum precursor, oxidation level of graphite oxide and pH of reaction medium. The as-prepared catalysts were analyzed using thermo-gravimetric (TG) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and cyclic voltammetry (CV). The best results in terms of platinum particle size and distribution were obtained when the synthesis was performed in acidic medium, using chloroplatinic acid as precursor and using graphene oxide with high oxidation level. The most promising graphene-supported catalyst was used to prepare a polymer electrolyte membrane fuel cell electrode. The membrane electrode assembly (MEA) prepared with graphene-based electrode was compared with a MEA prepared with catalyst based on commercial platinum supported in carbon black (Pt/C). Single cell characterization included polarization curves and in-situ electrochemical impedance spectroscopy (EIS). The graphene-based electrode presented promising albeit unstable electrochemical performance due to water management issues. Additionally, EIS measurements revealed that the MEA made with Pt/rGO catalyst presented a lower mass transport resistance than the commercial Pt/C.     

A series of Ni–Al2O3 catalysts derived from layered double hydroxides for vapor phase catalytic exchange between water and hydrogen

Fabricating supported metal catalysts from layered double hydroxides (LDHs) is a promising strategy to develop high-efficient and low-cost materials for water detritiation. In this work, a series of NiAl-LDHs with different Ni/Al ratios were prepared and reduced to obtain Ni-based catalysts with Al₂O₃ support (Ni_x-Al₂O₃) and further tested in vapor phase catalytic exchange (VPCE) process. Results revealed that the activity of catalysts varies with Ni/Al molar ratios and is also affected by textural properties. Remarkably, the Ni₂–Al₂O₃ with the Ni/Al molar ratio of 2 exhibited excellent activity, due to the balanced factors of Ni content, specific surface area, Ni particle size and Al³⁺ configuration proportion. This study sheds some light on reaction mechanism of VPCE process and may be applicable for the rational design of highly efficient catalysts.