Nanoscale Trimetallic Metal–Organic Frameworks Enable Efficient Oxygen Evolution Electrocatalysis

Metal–organic frameworks (MOFs) are a class of promising materials for diverse heterogeneous catalysis, but they are usually not directly employed for oxygen evolution electrocatalysis. Most reports focus on using MOFs as templates to in situ create efficient electrocatalysts through annealing. Herein, we prepared a series of Fe/Ni‐based trimetallic MOFs (Fe/Ni/Co(Mn)‐MIL‐53 accordingly to the Material of Institute Lavoisier) by solvothermal synthesis, which can be directly adopted as highly efficient electrocatalysts. The Fe/Ni/Co(Mn)‐MIL‐53 shows a volcano‐type oxygen evolution reaction (OER) activity as a function of compositions. The optimized Fe/Ni_2.4/Co_0.4‐MIL‐53 can reach a current density of 20 mA cm^?2 at low overpotential of 236 mV with a small Tafel slope of 52.2 mV dec^?1. In addition, the OER performance of these MOFs can be further enhanced by directly being grown on nickel foam (NF).     

Electrochemical codeposited polypyrrole–tungsten oxide composite materials with wide potential windows

The three-dimensional morphology has sufficient interface contact and can be in favor of the electronic transport process. In this work, the demand for high-performance electrodes such as energy storage devices has been designed. Polypyrrole and tungsten oxide composite materials (PPy-WO₃) have been synthesized by cyclic voltammetry (CV) technology at −0.6 to 0.9 V versus saturated calomel electrode (SCE) for 20 cycles. The PPy-WO₃^{20 mV/s}, PPy-WO₃^{60 mV/s}, and PPy-WO₃^{120 mV/s} electrodes have been prepared by CV technology at sweep rates of 20, 60, and 120 mV/s. The influences of scan rate on morphologies and charge storage properties of the composites are discussed. Among them, a three-dimensional flake structure for PPy-WO₃^{20 mV/s} with a size of up to several micrometers was synthesized. PPy-WO₃^{20 mV/s} composites as electrode materials exhibit a wide charge storage potential window of 1.4 V (between −0.9 and 0.5 V vs. SCE) and a specific capacitance of 145.13 F/g at 1 mA/cm². Moreover, the long-term stability of PPy-WO₃^{20 mV/s} and PPy has been investigated in 5 M LiCl aqueous electrolyte. The stability of the materials can be improved by inorganic and organic composites.     

Efficient and Robust Hydrogen Evolution: Phosphorus Nitride Imide Nanotubes as Supports for Anchoring Single Ruthenium Sites

Amorphous phosphorus nitride imide nanotubes (HPN) are reported as a novel substrate to stabilize materials containing single‐metal sites. Abundant dangling unsaturated P vacancies play a role in stabilization. Ruthenium single atoms (SAs) are successfully anchored by strong coordination interactions between the d orbitals of Ru and the lone pair electrons of N located in the HPN matrix. The atomic dispersion of Ru atoms can be distinguished by X‐ray absorption fine structure measurements and spherical aberration correction electron microscopy. Importantly, Ru SAs@PN is an excellent electrocatalyst for the hydrogen evolution reaction (HER) in 0.5 m H₂SO₄, delivering a low overpotential of 24 mV at 10 mA cm^?2 and a Tafel slope of 38 mV dec^?1. The catalyst exhibits robust stability in a constant current test at a large current density of 162 mA cm^?2 for more than 24 hours, and is operative for 5000 cycles in a cyclic voltammetry test. Additionally, Ru SAs@PN presents a turnover frequency (TOF) of 1.67 H₂ s^?1 at 25 mV, and 4.29 H₂ s^?1 at 50 mV, in 0.5 m H₂SO₄ solution, outperforming most of the reported hydrogen evolution catalysts. Density functional theory (DFT) calculations further demonstrate that the Gibbs free energy of adsorbed H* over the Ru SAs on PN is much closer to zero compared with the Ru/C and Ru SAs supported on carbon and C₃N₄, thus considerably facilitating the overall HER performance.     

Ozone reduction at Nafion modified Au electrode and the measurement of ozone concentration in highly resistive solutions

The mass transport and charge transfer kinetics of ozone reduction at Nafion coated Au electrodes were studied in 0.5 mol/L H2SO4 and highly resistive solutions such as distilled water and tap water. The diffusion coefficient and partition coefficient of ozone in Nafion coating are 1.78×10-6 cm2·s-1 and 2.75 at 25℃ (based on dry state thickness), respectively. The heterogeneous rate constants and Tafel slopes for ozone reduction at bare Au are 4.1×10-6 cm·s-1, 1.0×10-6 cm·s-1 and 181 mV, 207 mV in 0.5 mol/L H2SO4 and distilled water respectively and the corresponding values for Nafion coated Au are 5.5×10-6 cm·s-1, 1.1×10-6 cm·s-1 and 182 mV, 168 mV respectively. The Au microelectrode with 3 μm Nafion coating shows good linearity over the range 0-10 mmol/L ozone in distilled water with sensitivity 61 μA·ppm-1 ·cm-2, detection limit 10 ppb and 95% response time below 5 s at 25℃. The temperature coefficient in range of 11-30℃ is 1.3%.     

Manganese Oxide/Graphene Aerogel Composites as an Outstanding Supercapacitor Electrode Material

Graphene aerogels (GA), prepared with an organic sol–gel process, possessing a high specific surface area of 793 m² g^?1, a high pore volume of 3 cm³ g^?1, and a large average pore size of 17 nm, were applied as a support for manganese oxide for supercapacitor applications. The manganese oxide was electrochemically deposited into the highly porous GA to form MnO₂/GA composites. The composites, at a high manganese oxide loading of 61 wt. %, exhibited a high specific capacitance of 410 F g^?1 at 2 mV s^?1. More importantly, the high rate specific capacitances measured at 1000 mV s^?1 for these composites were two‐fold higher than those obtained with samples prepared in the absence of the GA support. The specific capacitance retention ratio, based on the specific capacitance obtained at 25 mV s^?1, was maintained high, at 85 %, even at the high scan rate of 1000 mV s^?1, in contrast with the significantly lower value of 67 % for the plain manganese oxide sample. For the cycling stability, the specific capacitance of the composite electrode decayed by only 5 % after 50,000 cycles at 1000 mV s^?1. The success of this MnO₂/GA composite may be attributed to the structural advantages of high specific surface areas, high pore volumes, large pore sizes, and three‐dimensionally well‐connected network of the GA support. These structural advantages made possible the high mass loading of the active material, manganese oxide, large amounts of electroactive surfaces for the superficial redox events, fast mass‐transfer within the porous structure, and well‐connected conductive paths for the involved charge transport.     

The kinetics of complexation of nickel(II) and cobalt(II) with cinchomeronate in aqueous solution

The pressure-jump method has been used to determine the rate constants for the formation and dissociation of nickel(II) and cobalt(II) complexes with cinchomeronate in aqueous solution at zero ionic strength. The forward and reverse rate constants obtained are k_f = 2.27 × 10⁶ M^?1 s^?1 and k_r = 3.81 × 10¹ s^?1 for the nickel(II) complex and k_f = 1.23 × 10⁷ M^?1 s^?1 and k_r = 2.66 × 10² s^?1 for the cobalt(II) complex at 25°C. The activation parameters of the reactions have also been obtained from the temperature variation study. The results indicate that the rate determining step of the reaction is a loss of a water molecule from the inner coordination sphere of the cation for the nickel(II) complex and the chelate ring closure for the cobalt(II) complex. The influence of the pyridine ring nitrogen atom of the cinchomeronate ligand on the complexation of cobalt(II) ion is also discussed.     

A novel vic-dioxime ligand and its Ni(II), Cu(II) and Co(II) complexes containing calix[4]pyrrole moiety: synthesis, characterization and redox properties

A new calix[4]pyrrole functionalized vic-dioxime, 3-(4-methyl-9,9,14,14,19,19-hexaethylcalix[4]pyrrole)benzoaminoglyoxime (LH₂) was synthesized from anti-chloroglyoxime and 3-aminophenyl-calix[4]pyrrole at room temperature. The mononuclear complexes {nickel(II), copper(II) and cobalt(II)} of this vic-dioxime ligand were prepared and their structures were confirmed by elemental analysis, IR and UV–Vis spectrophotometry, magnetic susceptibility; the MS, ¹H and ¹³C NMR spectra of the LH₂ ligand and its Ni(II) complex were also recorded. The experimental results indicated that the ligand:metal ratio was 2:1 in the cases of Ni(II), Cu(II) and Co(II) complexes as is with most vic-dioximes. Electrochemical properties of the ligand, and its complexes were investigated in DMSO solution by cyclic voltammetry at 200?mV?s^?1 scan rate.     

Electroanalytical Performance of (SiPy+Cl−/CuTsPc)5 LbL Film for Detecting Promethazine Hydrochloride

A (SiPy⁺Cl^?/CuTsPc)₅ layer‐by‐layer film was employed for the electroanalytical determination of promethazine hydrochloride in BR buffer pH 5.0 with peaks at 0.48 and 0.79 V. After optimisation of the square‐wave parameters (f=100 s^?1, a=40 mV and ΔE_s=2 mV), the peak at 0.79 V was used for quantification and a detection limit of 8.71×10^?9 mol L^?1 and a quantification limit of 9.31×10^?8 were calculated. The applicability of this procedure was tested on commercial formulations of promethazine hydrochloride by observing the stability, specificity, recovery and precision of the procedure in complex samples, without any preliminary treatment.     

Solid-state electrochemistry of silicotungstic acid immobilized by sol-gel chemistry

Voltammetry of silicotungstic acid (STA), H₄SiW₁₂O₄₀, that was encapsulated in silica was performed in the absence of a contacting liquid phase. Two one-electron reductions that are separated by 200?mV were observed, which is the same behavior as in aqueous solution. At scan rates, v, below 10?mV s^?1 with a 10?μm dia. carbon fiber indicator electrode, plateaus with limiting currents which are independent of v were observed, which is indicative of spherical diffusion from a field that is much larger than the electrode area. At v?>?20?V?s^?1, peaks were observed with currents directly proportional to v ^½. For a gel aged for 2 days, an effective diffusion coefficient, D _eff, of 3?×?10^?7?cm² s^?1 was estimated by voltammetry and chronoamperometry; the concentration of the redox sites thereby determined was about 0.5?M. The D _eff values that were obtained in this study were larger than expected for a solid electrolyte, which suggests an important role of residual water. In support of this model, gels that were aged in a humidistat at 33% humidity at room temperature for 2 and 5 days lost 16% and 13%, respectively, of their mass when dried at 120°.     

Electrocatalytic Hydrogen Production by a Nickel(II) Complex with a Phosphinopyridyl Ligand

A novel nickel(II) complex [Ni(L)₂Cl]Cl with a bidentate phosphinopyridyl ligand 6‐((diphenylphosphino)methyl)pyridin‐2‐amine (L) was synthesized as a metal‐complex catalyst for hydrogen production from protons. The ligand can stabilize a low Ni oxidation state and has an amine base as a proton transfer site. The X‐ray structure analysis revealed a distorted square‐pyramidal Ni^II complex with two bidentate L ligands in a trans arrangement in the equatorial plane and a chloride anion at the apex. Electrochemical measurements with the Ni^II complex in MeCN indicate a higher rate of hydrogen production under weak acid conditions using acetic acid as the proton source. The catalytic current increases with the stepwise addition of protons, and the turnover frequency is 8400 s^?1 in 0.1 m [NBu₄][ClO₄]/MeCN in the presence of acetic acid (290 equiv) at an overpotential of circa 590 mV.     

Singly versus Doubly Reduced Nickel Porphyrins for Proton Reduction: Experimental and Theoretical Evidence for a Homolytic Hydrogen‐Evolution Reaction

A nickel(II) porphyrin Ni‐P (P=porphyrin) bearing four meso‐C₆F₅ groups to improve solubility and activity was used to explore different hydrogen‐evolution‐reaction (HER) mechanisms. Doubly reduced Ni‐P ([ Ni‐P ]^2?) was involved in H₂ production from acetic acid, whereas a singly reduced species ([ Ni‐P ]^?) initiated HER with stronger trifluoroacetic acid (TFA). High activity and stability of Ni‐P were observed in catalysis, with a remarkable i_c/i_p value of 77 with TFA at a scan rate of 100 mV s^?1 and 20 °C. Electrochemical, stopped‐flow, and theoretical studies indicated that a hydride species [H‐ Ni‐P ] is formed by oxidative protonation of [ Ni‐P ]^?. Subsequent rapid bimetallic homolysis to give H₂ and Ni‐P is probably involved in the catalytic cycle. HER cycling through this one‐electron‐reduction and homolysis mechanism has been proposed previously but rarely validated. The present results could thus have broad implications for the design of new exquisite cycles for H₂ generation.     

Arginine‐Containing Ligands Enhance H2 Oxidation Catalyst Performance

Hydrogenase enzymes use Ni and Fe to oxidize H₂ at high turnover frequencies (TOF) (up to 10 000 s^?1) and low overpotentials (<100 mV). In comparison, the fastest reported synthetic electrocatalyst, [Ni^II(P^Cy₂N^tBu₂)₂]²⁺, oxidizes H₂ at 60 s^?1 in MeCN under 1 atm H₂ with an unoptimized overpotential of ca. 500 mV using triethylamine as a base. ¹ Here we show that a structured outer coordination sphere in a Ni electrocatalyst enhances H₂ oxidation activity: [Ni^II(P^Cy₂N^Arg₂)₂]⁸⁺ (Arg=arginine) has a TOF of 210 s^?1 in water with high energy efficiency (180 mV overpotential) under 1 atm H₂, and 144 000 s^?1 (460 mV overpotential) under 133 atm H₂. The complex is active from pH 0–14 and is faster at low pH, the most relevant condition for fuel cells. The arginine substituents increase TOF and may engage in an intramolecular guanidinium interaction that assists in H₂ activation, while the COOH groups facilitate rapid proton movement. These results emphasize the critical role of features beyond the active site in achieving fast, efficient catalysis.     

Electrochemical performance of grown layer of Ni(OH)2 on nickel foam and treatment with phosphide and selenide for efficient water splitting

Active nanocomposites synthesized by the electrochemical approach play a vital role in energy generation, conversion, and storage technologies. Recently, scientists began to explore the use of earth-rich transition metal-based materials to replace precious metal-based catalysts. Transition metals (TMs) based nickel (Ni) and their pnictides compounds such as phosphides and selenides exhibit good activity for hydrogen evaluation reaction (HER) and the entire water electrolysis process. In this study, we first prepared Ni(OH)₂ and grown its layer on Ni foam (NF) and treated it with selenide (Se) and phosphide (P) then nickel-based selenide-phosphide catalyst (Ni–P–Se) was prepared by simultaneous selenization and phosphidation process for the first time. The as-obtained composite was then analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), elemental mapping and transmission electron microscope (TEM) means to study the composition, structure, and micro-morphology of materials. Furthermore, we also observed electrocatalytic water splitting activity using electrochemical cell. The results of electrochemical tests depicted that the selenization and phosphidation treatments significantly enhanced the electrocatalytic HER activity of the starting materials. The overpotentials required for Ni–P–Se to reach 10 ?mA ?cm^?2 and 100 ?mA ?cm^?2 were only 242 ?mV and 282 ?mV. The Tafel slope of Ni–P–Se is 151 ?mV dec^?1, which is lower than that of nickel phosphide, selenide, and hydroxide indicating that selenide-phosphide enhances the HER reaction kinetics of the material, which in turn increases hydrogen output rate as compared with previous studies.     

Bench-scale biosynthesis of isonicotinic acid from 4-cyanopyridine by Pseudomonas putida

Pseudomonas putida CGMCC3830 harboring nitrilase was used in isonicotinic acid production from 4-cyanopyridine. This nitrilase showed optimum activities towards 4-cyanopyridine at pH 7.5 and 45°C. The half-life of P. putida nitrilase was 93.3 h, 33.9 h, and 9.5 h at 30°C, 38°C, and 45°C, respectively. 4-Cyanopyridine (100 mM) was fully converted into isonicotinic acid within 20 min. The bench-scale production of isonicotinic acid was carried out using 3 mg of resting cells per mL in a 1 L system at 30°C and finally, 123 g L^?1 of isonicotinic acid were obtained within 200 min without any by-products. The conversion reaction suffered from the product inhibition effect after the tenth feeding. The volumetric productivity was 36.9 g L^?1 h^?1. P. putida shows significant potential in nitrile hydrolysis for isonicotinic acid production. This paper is the first report on isonicotinic acid biosynthesis using Pseudomonas putida and it represents the highest isonicotinic acid production reported so far.     

31P-NMR study of the organophosphorous complexes of uranium(VI)

The intermolecular ligand exchange in uranyl nitrate complexes with TBP and TOPO is studied by³¹P-NMR. The constant rates at 25°C in CCl₄ are: (8.47±1.86)·10³ s^?1 for U-TBP and (1.3±0.04)·10⁴M^?1·s^?1 for U-TOPO system. The very similar activation parameters values of the ligand exchange suggest the same mechanism for both systems, namely an one-step interchange mechanism. The differences between the systems regarding the rate equations and the extraction properties are discussed.     

Electroconductive Hydrogels: Electrical and Electrochemical Properties of Polypyrrole‐Poly(HEMA) Composites

Composites of inherently conductive polypyrrole (PPy) within highly hydrophilic poly(2‐hydroxyethyl methacrylate)‐based hydrogels (p(HEMA)) have been fabricated and their electrochemical properties investigated. The electrochemical characteristics observed by cyclic voltammetry suggest less facile reduction of PPy within the composite hydrogel compared to electropolymerized PPy, as shown by the shift in the reduction peak potential from ?472 mV for electropolymerized polypyrrole to ?636 mV for the electroconductive composite gel. The network impedance magnitude for the electroconductive hydrogel remains quite low, ca. 100 Ω, even upon approach to DC, over all frequencies and at all offset potentials suggesting retained electronic (bipolaronic) conductivity within the composite. In contrast, sustained application of +0.7 V (vs. Ag/AgCl, 3 M Cl^?) for typically 100 min. (conditioning) to reduce the background amperometric current to <1.0 μA, resulted in complete loss of electroactivity. Nyquist plots suggest that sustained application of such a modest potential to the composite hydrogel results in impedance characteristics that resembles p(HEMA) without evidence of the conducting polymer component. PPy composite gels supported a larger ferrocene monocarboxylate diffusivity (D_appt=7.97×10^?5 cm² s^?1) compared to electropolymerized PPy (D_appt=5.56×10^?5 cm² s^?1), however a marked reduction in diffusivity (D_appt=1.01×10^?5 cm² s^?1) was observed with the conditioned hydrogel composite. Cyclic voltammograms in buffer containing H₂O₂ showed an absence of redox peaks for electrodes coated with PPy‐containing membranes, suggesting possible chemical oxidation of polypyrrole by the oxidant     

Thermodynamic parameters of complexation reactions between nickel(II) ion and L-serine in aqueous solutions

A thermochemical study of coordination equilibria in the nickel(II) ions-L-serine system was carried out. The heats of reaction between solutions of an amino acid and nickel(II) nitrate at 288.15, 298.15, and 308.15 K and ionic strength of 0.25, 0.50, and 0.75 (KNO₃) were measured. The heats of dilution of nickel nitrate solution in background electrolyte solutions were determined under identical conditions to make proper corrections. The results were processed with allowance for stepwise equilibria. The standard thermodynamic parameters of the complexation processes were calculated. The effect of temperature on heats of complexation reactions in the L-serine-nickel(II) ions was studied. The standard enthalpies of formation of NiSer⁺, NiSer₂, and NiSer ₃ ^? complexes in aqueous solutions were determined.     

Study of the Electrochemical Behavior of Disperse Blue 1‐Modified Graphite Electrodes. Application to the Flow Determination of NADH

The preparation and the electrochemical study of Disperse Blue 1‐chemically modified electrodes (DB1‐CME), as well as their efficiency for the electrocatalytic oxidation of NADH is described. The proposed mediator was immobilized by physical adsorption onto graphite electrodes. The electrochemical behavior of DB1‐CME was studied with cyclic voltammetry. The electrochemical redox reaction of DB1 was found to be reversible, revealing two well‐shaped pair of peaks with formal potentials 152 and ?42 mV, respectively, (vs. Ag/AgCl/3M KCl) at pH 6.5. The current I_p has a linear relationship with the scan rate up to 800 mV s^?1, which is indicative for a fast electron transfer kinetics. The dissociation constants of the immobilized DB1 redox couple were calculated pK₁=4 and pK₂=5. The electrochemical rate constants of the immobilized DB1 were calculated k₁°=18 s^?1 and k₂°=23 s^?1 (Γ=2.36 nmol cm^?2). The modified electrodes were mounted in a flow injection manifold, poised at +150 mV (vs. Ag/AgCl/3M KCl) and a catalytic current due to the oxidation of NADH was measured. The reproducibility was 1.4% RSD (n=11 for 30 μM NADH) The behavior of the sensor towards different reducing compounds was investigated. The sensor exhibited good operational and storage stability.     

Syntheses,Structures and Sorption Properties of Three Isoreticular Trinuclear Indium‐Based Amide‐Functionalized Metal–Organic Frameworks

Amide‐functionalized metal–organic frameworks (AFMOFs) as a subclass of MOF materials have received great interest recently because of their intriguing structures and diverse potential applications. In this work, solvothermal reactions between indium nitrate and two mixed‐linkers afforded two new isoreticular 8‐connected trinuclear indium‐based AFMOFs of [(In₃O)(OH)(L2)₂(IN)₂]?(solv)_x ( 2‐In ) and [(In₃O)(OH)(L2)₂(AIN)₂]?(solv)_x ( NH₂‐2‐In ) (H₂L2=4,4′‐(carbonylimino)dibenzoic acid and HIN=isonicotinic acid or HAIN=3‐aminoisonicotinic acid), respectively. Moreover, by means of reticular chemistry, an extended network of [(In₃O)(OH)(L3)₂(PB)₂]?(solv)_x (3‐In) (H₂L3=4,4′‐(terephthaloylbis(azanediyl))dibenzoic acid, HPB=4‐(4‐pyridyl)benzoic acid) was also successfully realized after prolongation of the former dicarboxylate linker and HIN, resulting in a truly 8‐connected isoreticular AFMOF platform. These frameworks were structurally determined by single‐crystal X‐ray diffraction (SCXRD). Sorption studies further demonstrate that 2‐In and NH₂‐2‐In exhibit not only high surface areas and pore volumes but also relatively high carbon capture capabilities (the CO₂ uptakes reach 60.0 and 75.5 cm³ g^?1 at 298 K and 760 torr, respectively) due to the presences of amide and/or amine functional groups. The selectivity of CO₂/N₂ and CO₂/CH₄ calculated by IAST are 10.18 and 12.43, 4.20 and 4.23 for 2‐In and NH₂‐2‐In , respectively, which were additionally evaluated by mixed‐gases dynamic breakthrough experiments. In addition, high‐pressure gas sorption measurements show that both materials could take up moderate amounts of natural gas.     

Improving the Electrocatalytic Activity of a Nickel-Organic Framework toward the Oxygen Evolution Reaction through Vanadium Doping

Metal-organic frameworks (MOFs) have been considered as potential oxygen evolution reaction (OER) electrocatalysts owning to their ultra-thin structure, adjustable composition, high surface area, and high porosity. Here, we designed and fabricated a vanadium-doped nickel organic framework (V_1−x−Ni_xMOF) system by using a facile two-step solvothermal method on nickel foam (NF). The doping of vanadium remarkably elevates the OER activity of V_1−x−Ni_xMOF, thus demonstrating better performance than the corresponding single metallic Ni-MOF, NiV-MOF and RuO₂ catalysts at high current density (>400 mA cm⁻²). V_0.09−Ni_0.91MOF/NF provides a low overpotential of 235 mV and a small Tafel slope of 30.3 mV dec⁻¹ at a current density of 10 mA cm⁻². More importantly, a water-splitting device assembled with Pt/C/NF and V_0.09−Ni_0.91MOF/NF as cathode and anode yielded a cell voltage of 1.96 V@1000 mA cm⁻², thereby outperforming the-state-of-the-art RuO₂⁽⁺⁾||Pt/C⁽⁻⁾. Our work sheds new insight on preparing stable, efficient OER electrocatalysts and a promising method for designing various MOF-based materials.