In situ assembly of nickel-based ultrathin catalyst film for water oxidation

A nickel-based ultrathin catalyst film is assembled in situ from a solution of Ni(OAc)2 and a Schiff-base ligand L. The resulting ultrathin catalyst film shows a low overpotential of 330 mV, a steady current of 7 mA cm-2 for water oxidation over 10 h.

Combustion process of a promising catalyst [Cu(Salen)] for HMX-CMDB propellants

Metal organic complexes are regarded as a series of promising combustion catalysts for solid rocket propellants. Their effects on the combustion performance of propellants are closely related to the reaction mechanism. Here, the metal-organic complex Cu(Salen) was investigated as a candidate material for the combustion catalyst of the HMX-added composite modified double-base propellant (HMX-CMDB). The combustion performance of the propellant was found to be evidently enhanced in the presence of Cu(Salen) compared with the propellant samples containing Benzoic-Cu or without catalyst. The addition of Cu(Salen) can improve the burning rate and combustion efficiency of the propellant - and greatly reduce the burning rate pressure index. Analysis shows that the addition of Cu(Salen) can increase the combustion area, flame brightness and combustion surface uniformity of the propellant to a higher degree. The sample can spray more beams of bright filaments on the flat combustion section, and the amount of gas generated by decomposition also greatly increases. In addition, Cu(Salen) shows amazing advantages in improving the surface of the propellant and the temperature gradient of the combustion flame.

Ni(II)-Salophen-Comprehensive Analysis on Electrochemical and Spectral Characterization and Biological Studies

New aspects of the Ni(II)-salophen complex and salophen ligand precursor were found during deep electrochemical and optical characterization, as well as biological studies for new pharmacological applications. Physicochemical and spectroscopic methods (¹H- and ¹³C-NMR, FT-IR and UV-Vis, electrospray ionization mass spectroscopy, thermogravimetric analysis, and molar conductance measurements) were also used to prove that the salophen ligand acts as a tetradentate and coordinates to the central metal through nitrogen and oxygen atoms. The electrochemical behavior of the free Schiff salophen ligand (H₂L) and its Ni(II) complex (Ni(II)L) was deeply studied in tetrabutylammonium perchlorate solutions in acetonitrile via CV, DPV, and RDE. Blue films on the surfaces of the electrodes as a result of the electropolymerization processes were put in evidence and characterized via CV and DPV. (H₂L) and Ni(II)L complexes were tested for their antimicrobial, antifungal, and antioxidant activity, showing good antimicrobial and antifungal activity against several bacteria and fungi.

Candidate for Catalyst during Water-Oxidation Reaction in the Presence of Manganese Compounds, from Nanosized Particles to Impurities: Sleep with One Eye Open

Water-oxidation reaction (WOR) catalysts are critical for energy conversion. WOR is a four-electron oxidation and sluggish reaction. WOR needs a high thermodynamic driving force; it is also a kinetically slow reaction. Different compounds have been used for WOR; among these compounds, Mn materials have proven to be interesting because Mn is low-cost and also nontoxic, at least compared to many transition metals. Naturally, it has also been used in the biological water-oxidizing complex (WOC). Indeed, WOR has occurred on a huge scale in natural photosynthesis.For a long time, efforts have been made to design and synthesize various ligands and generate Mn compounds toward WOR catalysts. However, the addition or removal of electrons inside Mn compounds during harsh WOR conditions can lead to the formation or the breakage of bonds and result in the conversion of a precatalyst to a catalyst.Here, our findings on the conversion of Mn compounds to catalysts during WOR are presented. Many Mn compounds have been claimed to be catalysts for WOR in the presence of various chemical oxidants or under electrophotochemical conditions. Currently, the advances in characterization techniques and different spectroscopic methods have enabled a better understanding of catalysts. Different conversions such as that of the Mn complex to Mn oxide and Mn salts to Mn oxide during WOR have been explained. Indeed, the morphology and size of the Mn oxide formed depend on several factors such as the origin compounds, pH, ligands, and conditions. Thus, different Mn compounds show different activities toward WOR. The biomimetic models with Mn-Ca clusters are also decomposed during WOR. On the other hand, stable Mn complexes such as Mn phthalocyanines, which are very stable in the absence of potential, are easily decomposed during WOR. It is noted that for many of these Mn compounds, two steps result in the formation of Mn oxide during WOR: (i) Mn(II) or (III) leaching into the electrolyte and (ii) deposition of the leached Mn ions into the solution.Considering these steps, it can be seen that challenges remain in the area of Mn compounds, given the fact that even in the catalytic cycle at low oxidation numbers no Mn(II) or (III) should be leached to the electrolyte.

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.

Two biologically inspired tetranuclear nickel(II) catalysts: effect of the geometry of Ni4 core on electrocatalytic water oxidation

Two biologically inspired tetranuclear nickel complexes [Ni4(L-H)4(CH3COO)3]·Cl (1) and [Ni4(L-H)4(CH3COO)4]·2CH3OH (2) (L = di(pyridin-2-yl)methanediol) have been synthesized and investigated by a combination of X-ray crystallography, PXRD, electrochemistry, in-situ UV-Vis spectroelectrochemistry and DLS. Both of the two complexes feature a core composed of four Ni(II) ions with the same peripheral ligation provided by the anionic di(pyridin-2-yl)methanediol and MeCOO- ligands. Whereas, complex 1 possesses one distorted cubane-like [Ni4(µ3-O)4] core, while 2 has one extended butterfly-like [Ni4(µ3-O)2] core. The homogeneous electrocatalytic reactivity of the two water-soluble complexes for water oxidation have been thoroughly studied, which demonstrates that both of them can efficiently electrocatalyze water oxidation with high stability under alkaline conditions, at relatively low over-potentials (η) of 420-790 mV for 1 and 390-780 mV for 2, both in the pH range of 7.67-12.32, with the high TOF of about 139 s-1 (1) and 69 s-1 (2) at pH = 12.32, respectively. By a series of comparative experiments for complexes 1 and 2, we proposed that their crystal geometries play an important role in their electrocatalytic reactivity for water oxidation. We verified that biomimetic cubane geometry could promote OER catalysis with two very similar compounds for the first time. Compared with 2, the biomimetic cubane topology of 1 could promote OER catalysis by facilitating efficient charge delocalization and electron-transfer.

Dendrimer-Ni-Based Material: Toward an Efficient Ni-Fe Layered Double Hydroxide for Oxygen-Evolution Reaction

Ni/Fe oxides are among the most widely used catalysts for water splitting. This paper outlines a new approach to synthesize Ni-Fe layered double hydroxides (Ni-Fe LDHs) for oxygen-evolution reaction (OER). Herein, we show that a dendrimer with carboxylate surface groups (generation 3.5) could react with Ni(II) ions to form a precatalyst for OER. During electrochemical OER, this precatalyst converted to Ni-Fe LDH, which is an efficient catalyst toward OER in the presence of Fe(III) ions. The catalyst was characterized by a number of methods and applied for OER using fluorine-doped tin oxide (FTO), Au, Pt, Ni foam, and glassy carbon electrodes. The catalyst shows a current density of 100 mA/cm² on the surface of the Ni foam, using only 297 mV overpotential and with the Tafel slope of 60.8 mV/decade. A current density of 50 mA/cm² on the surface of Au or Pt requires 333 and 317 mV overpotentials, respectively. The slopes of the Tafel plots for the catalyst on Au, GC, and Pt are 52.5, 47.1, and 37.4 mV/decade, respectively. The dendrimer resulted in a large dispersibility and an increase in active sites of Ni-Fe LDH, as well as the formation of Ni-Fe LDH.

4,4′-Bipyridine-based Ni(ii)-metallogel for fabricating a photo-responsive Schottky barrier diode device

4,4′-Bipyridine-based Ni(ii)-metallogel has been implemented to execute a light-responsive semiconducting Schottky barrier diode device with advanced functionality.

Potential- and Buffer-Dependent Catalyst Decomposition during Nickel-Based Water Oxidation Catalysis

The production of hydrogen by water electrolysis benefits from the development of water oxidation catalysts. This development process can be aided by the postulation of design rules for catalytic systems. The analysis of the reactivity of molecular complexes can be complicated by their decomposition under catalytic conditions into nanoparticles that may also be active. Such a misinterpretation can lead to incorrect design rules. In this study, the nickel-based water oxidation catalyst [NiII (meso-L)](ClO4 )2 , which was previously thought to operate as a molecular catalyst, is found to decompose to form a NiOx layer in a pH 7.0 phosphate buffer under prolonged catalytic conditions, as indicated by controlled potential electrolysis, electrochemical quartz crystal microbalance, and X-ray photoelectron spectroscopy measurements. Interestingly, the formed NiOx layer desorbs from the surface of the electrode under less anodic potentials. Therefore, no nickel species can be detected on the electrode after electrolysis. Catalyst decomposition is strongly dependent on the pH and buffer, as there is no indication of NiOx layer formation at pH 6.5 in phosphate buffer nor in a pH 7.0 acetate buffer. Under these conditions, the activity stems from a molecular species, but currents are much lower. This study demonstrates the importance of in situ characterization methods for catalyst decomposition and metal oxide layer formation, and previously proposed design elements for nickel-based catalysts need to be revised.

A dinuclear iron complex as an efficient electrocatalyst for homogeneous water oxidation reaction

A novel dinuclear iron complex of a Schiff base ligand has been exploited as a homogeneous water splitting electrocatalyst having possible real life application in renewable energy.

Electrocatalytic water oxidation by a Ni(ii) salophen-type complex

A new mononuclear Ni(ii) complex, NiL (1), was synthesized from the reaction of Ni(OAc)2·4H2O and salophen-type N2O2-donor ligand, H2L (where H2L = 2,2'-((1E,1'E)-((4-chloro-5-methyl-1,2-phenylene)bis(azanylylidene))bis(methanylylidene))diphenol), in ethanol. The obtained complex was characterized by elemental analysis, spectroscopic techniques and single crystal X-ray analysis. The complex was studied as a water oxidizing catalyst and its electrocatalytic activity in the water oxidation reaction was tested in 0.5 M of borate buffer at pH = 3, 7 and 11 in a typical three-electrode setup with a carbon paste electrode modified by complex 1 as a working electrode. The linear sweep voltammetry (LSV) curves indicated that complex 1 has a much superior activity and only needs 21 mV vs. Ag/AgCl overvoltage to reach a geometrical catalytic current density of 2.0 mA cm-2 at pH = 11. The onset potential decreased from 1.15 V to 0.67 V vs. Ag/AgCl with an increase of pH from 3 to 13 under a constant current density of 1.0 mA cm-2. Then, to determine the true catalyst for the water oxidation reaction in the presence of complex 1 at pH = 3, 7 and 11, cyclic voltammetry was also performed. The continuous CVs for complex 1 at neutral and alkaline solutions showed significant progress for the water oxidation reaction. In addition, the amperometry tests exhibited excellent stability and high constant current density for water oxidation by CPE-complex 1 under electrochemical conditions at pH = 11 and 7. Although X-ray powder diffraction analysis did not show a pure and crystalline structure for NiO x , the scanning electron microscopy images showed that nickel oxide at pH = 11 and nickel oxide or other Ni-based compounds at pH = 7 are true water oxidizing catalysts on the surface of a CPE electrode. Moreover at pH = 3, no clear water oxidation or NiO x formation was observed.

A new decomposition mechanism for metal complexes under water-oxidation conditions

Herein, water-oxidation reaction by cobalt(II) phthalocyanine, N,N′-bis (salicylidene) ethylenediamino cobalt(II), nickel(II) Schiff base (N,N′-bis (salicylidene)ethylenediamino nickel(II), nickel(II)) phthalocyanine-tetrasulfonate tetrasodium, manganese(II) phthalocyanine, 5,10,15,20-Tetraphenyl-21H,23H-porphine manganese(III) chloride, manganese(III) 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine chloride tetrakis(methochloride) was investigated using electrochemistry, UV-vis spectroscopy and spectroelectrochemistry. According to our results, a new decomposition pathway for these metal complexes under water-oxidation conditions was proposed. The produced metal oxide obtained by decomposition of metal complex under water -oxidation conditions not only catalyzes water-oxidation reaction but this metal oxide also accelerates decomposition of the corresponding complex to form higher amounts of the metal oxide. We hypothesize that such a mechanism could be investigated for many metal complexes under different oxidation or reduction reactions.

Nanosized (Ni1-x Zn x )Fe2O4 for water oxidation

Performing water splitting for H2 production is an interesting method to store different energies. For water splitting, an efficient and stable water-oxidizing catalyst is important. Ni-Fe (hydr)oxides are among the best catalysts for water oxidation in alkaline electrolytes. An Fe amount higher than 50% in Ni-Fe (hydr)oxides increases the overpotential for water oxidation. Thus, Ni-Fe (hydr)oxides with a high ratio of Fe to Ni have rarely been focused on for water oxidation. Herein, we report water oxidation using nanosized (Ni1-x Zn x )Fe2O4. The catalyst was characterized via some methods and tested at pH values of 3, 7 and 11 in phosphate buffer. Nanosized (Ni1-x Zn x )Fe2O4 is a good catalyst for water oxidation only under alkaline conditions. In the next step, amperometry studies showed current densities of 3.50 mA cm-2 and 11.50 mA cm-2 at 1.25 V in 0.10 M and 1.0 M KOH solution, respectively. The amperometric measurements indicated high catalyst stability in both 0.10 M and 1.0 M KOH. Tafel plots were obtained in KOH solution at concentrations of both 0.10 M and 1.0 M. At pH = 13 in KOH solution (0.10 M), linearity of lg(j) vs. potential was shown, with two slopes relating to both relatively low (170.9 mV per decade) and high overpotentials (484.2 mV per decade). In 1.0 M KOH solution, the Tafel plot showed linearity of lg(j) vs. potential, with two slopes relating to both relatively low (192.5 mV per decade) and high overpotentials (545.7 mV per decade). After water oxidation, no significant change was observed in the catalyst.

Kinetics and mechanisms of catalytic water oxidation

The kinetics and mechanisms of thermal and photochemical oxidation of water with homogeneous and heterogeneous catalysts, including conversion from homogeneous to heterogeneous catalysts in the course of water oxidation, are discussed in this review article. Molecular and homogeneous catalysts have the advantage to clarify the catalytic mechanisms by detecting active intermediates in catalytic water oxidation. On the other hand, heterogeneous nanoparticle catalysts have advantages for practical applications due to high catalytic activity, robustness and easier separation of catalysts by filtration as compared with molecular homogeneous precursors. Ligand oxidation of homogeneous catalysts sometimes results in the dissociation of ligands to form nanoparticles, which act as much more efficient catalysts for water oxidation. Since it is quite difficult to identify active intermediates on the heterogeneous catalyst surface, the mechanism of water oxidation has hardly been clarified under heterogeneous catalytic conditions. This review focuses on the kinetics and mechanisms of catalytic water oxidation with homogeneous catalysts, which may be converted to heterogeneous nanoparticle catalysts depending on various reaction conditions.

A nickel(ii) complex under water-oxidation reaction: what is the true catalyst?

A Ni(ii) complex as a water-oxidizing catalyst under electrochemical conditions was studied and the role of Ni oxide as a true catalyst was investigated.

A manganese(ii) phthalocyanine under water-oxidation reaction: new findings

The decomposition reaction for a manganese complex under water oxidation was investigated.

A mononuclear cobalt complex for water oxidation: new controversies and puzzles

Herein the role of a mononuclear cobalt(iii) complex, [Co^III(DPKOH)₂]ClO₄ (DPK = di(2-pyridyl)ketone), in the water electrooxidation process is reinvestigated.