Densely Populated Isolated Single Co?N Site for Efficient Oxygen Electrocatalysis

Atomically dispersed transition metals confined with nitrogen on a carbon support has demonstrated great electrocatalytic performance, but an extremely low concentration of metal atoms (usually below 1.5%) is necessary to avoid aggregation through sintering which limits mass activity. Here, a salt‐template method to fabricate densely populated, monodispersed cobalt atoms on a nitrogen‐doped graphene‐like carbon support is reported, and achieving a dramatically higher site fraction of Co atoms (≈15.3%) in the catalyst and demonstrating excellent electrocatalytic activity for both the oxygen reduction reaction and oxygen evolution reaction. The atomic dispersion and high site fraction of Co provide a large electrochemically active surface area of ≈105.6 m² g^?1, leading to very high mass activity for ORR (≈12.164 A mg_Co^?1 at 0.8 V vs reversible hydrogen electrode), almost 10.5 times higher than that of the state‐of‐the‐art benchmark Pt/C catalyst (1.156 A mg_Pt^?1 under similar conditions). It also demonstrates an outstanding mass activity for OER (0.278 A mg_Co^?1). The Zn‐air battery based on this bifunctional catalyst exhibits high energy density of 945 Wh kg_Zn^?1 as well as remarkable stability. In addition, both density functional theory based simulations and experimental measurements suggest that the Co? N₄ sites on the carbon matrix are the most active sites for the bifunctional oxygen electrocatalytic activity.     

用钴卟啉修饰电极催化测定多巴胺等神经递质

将5, 10, 15, 20－四－（3－甲氧基－4－羟基苯基）卟啉钴（CoTMHPP）修饰在玻碳电极表面,制备成对多巴胶等神经递质有高灵敏度响应的CoTMHPP修饰电极．电极具有灵敏度高、响应快、稳定性好等特点．电极响应时间小于10s,儿茶酚类化合物的检测浓度为10^-6mol／L．     

Formation of electrochemically reduced graphene oxide on melamine electrografted layers and its application toward the determination of methylxanthines

The current study describes the electrografting of 2,4-diamino-1,3,5-triazine (AT) groups at the surfaces of glassy carbon electrode (GCE) and indium tin oxide (ITO) through in situ diazotization of melamine. The presence of AT groups at the surface of the electrode was confirmed by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). Furthermore, graphene oxide (GO) was self-assembled on AT grafted GCE. The oxygen functional groups present on the surface of GO were electrochemically reduced to form an electrochemically reduced graphene oxide (ERGO) on AT grafted electrode surface. Raman spectra show the characteristic D and G bands at 1340 and 1605 cm⁻¹, respectively, which confirms the successful attachment of GO on AT grafted surface, and the ratio of D and G bands was increased after the electrochemical reduction of GO. EIS shows that the electron transfer reaction of [Fe(CN)₆]^3−/4− was higher at the ERGO modified electrode than at bare, AT grafted, and GO modified GCEs. The electrocatalytic activity of ERGO was investigated toward the oxidation of methylxanthines. It shows excellent electrocatalytic activity toward these methylxanthines by not only shifting their oxidation potentials toward less positive potentials but also enhancing their oxidation currents.     

Reduced Mesoporous Co3O4 Nanowires as Efficient Water Oxidation Electrocatalysts and Supercapacitor Electrodes

While electrochemical water splitting is one of the most promising methods to store light/electrical energy in chemical bonds, a key challenge remains in the realization of an efficient oxygen evolution reaction catalyst with large surface area, good electrical conductivity, high catalytic properties, and low fabrication cost. Here, a facile solution reduction method is demonstrated for mesoporous Co₃O₄ nanowires treated with NaBH₄. The high‐surface‐area mesopore feature leads to efficient surface reduction in solution at room temperature, which allows for retention of the nanowire morphology and 1D charge transport behavior, while at the same time substantially increasing the oxygen vacancies on the nanowire surface. Compared to pristine Co₃O₄ nanowires, the reduced Co₃O₄ nanowires exhibit a much larger current of 13.1 mA cm^‐2 at 1.65 V vs reversible hydrogen electrode (RHE) and a much lower onset potential of 1.52 V vs RHE. Electrochemical supercapacitors based on the reduced Co₃O₄ nanowires also show a much improved capacitance of 978 F g^‐1 and reduced charge transfer resistance. Density‐functional theory calculations reveal that the existence of oxygen vacancies leads to the formation of new gap states in which the electrons previously associated with the Co‐O bonds tend to be delocalized, resulting in the much higher electrical conductivity and electrocatalytic activity.     

Synthesis, characterization and crystal structure of a novel 2D network structure based on hexacopper(II) substituted tungstoantimonate

The dimeric, hexacopper(II) substituted tungstoantimonate NaK₆[Cu₆Cl(SbW₉O₃₃)₂] · 14H₂O (1a) has been synthesized in good yield using a one-pot procedure by reaction of Cu²⁺ ions with the trilacunary precursor salt Na₉[α-B-SbW₉O₃₃] and characterized by IR spectroscopy, UV electronic spectroscopy, cyclic voltammetry and single-crystal X-ray diffraction. The title polyanion represents the first tungstoantimonate substituted by six copper centers and the polyanion as basic building unit is assembled into a two-dimensional network-like complex by Cu-OW bonds. The electrochemical behavior of 1a was investigated in buffer solution (pH 3.0) by cyclic voltammetry. In the potential range between −0.7 and 0.6 V, the compound exhibits the successive redox processes of the addenda atoms (W) followed by the redox reaction originating from the substituted Cu^II, which revealed the stepwise reduction of Cu²⁺ centers within the polyanion 1 before deposition of Cu⁰ on the glassy carbon electrode surface. The compound 1a is shown to have electrocatalytic properties towards the reductions of nitrate and nitrite.     

Hierarchical Porous Carbonized Co3O4 Inverse Opals via Combined Block Copolymer and Colloid Templating as Bifunctional Electrocatalysts in Li–O2 Battery

Hierarchically organized porous carbonized‐Co₃O₄ inverse opal nanostructures (C‐Co₃O₄ IO) are synthesized via complementary colloid and block copolymer self‐assembly, where the triblock copolymer Pluronic P123 acts as the template and the carbon source. These highly ordered porous inverse opal nanostructures with high surface area display synergistic properties of high energy density and promising bifunctional electrocatalytic activity toward both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). It is found that the as‐made C‐Co₃O₄ IO/Ketjen Black (KB) composite exhibits remarkably enhanced electrochemical performance, such as increased specific capacity (increase from 3591 to 6959 mA h g^?1), lower charge overpotential (by 284.4 mV), lower discharge overpotential (by 19.0 mV), and enhanced cyclability (about nine times higher than KB in charge cyclability) in Li–O₂ battery. An overall agreement is found with both C‐Co₃O₄ IO/KB and Co₃O₄ IO/KB in ORR and OER half‐cell tests using a rotating disk electrode. This enhanced catalytic performance is attributed to the porous structure with highly dispersed carbon moiety intact with the host Co₃O₄ catalyst.     

Qualitative (sunlight) and quantitative (xenon light source) aspects of the photochemistry of [S2W18O62] in the presence of electron donors

The photoelectrochemical properties of the polyoxotungstate anion [S₂W₁₈O₆₂]⁴⁻ dissolved in acetonitrite have been surveyed. Qualitative studies revealed that the rate of oxidation of the electron donor in the presence of sunlight correlated well with the voltammetric peak potential for oxidation of the electron donor at a glassy carbon electrode. Quantitative studies on the photooxidation of tetrahydrofuran, dimethylformamide, triphenylphosphine and ferrocene, as detected at a platinum channel electrode in the presence of a xenon light source, were assigned to a CE mechanism. Simulated fits for the proposed mechanism provide estimates of the reaction rates and data are compared with those obtained previously on the basis of a CECE scheme for the molybdenum analogue.     

Three isomorphous threefold interpenetrated 2D supramolecular frameworks: Synthesis, structure and sorption properties

Strategically designed and synthesized three isomorphous mononuclear complex, M(bpee)₂(6-me-2,3-pyrdcH)₂ [M = Co^II (1), Ni^II (2) and Fe^II (3)] using the mixed ligand system. Structure determination reveals that each mononuclear fragment is engaged in bidirectional H-bonding (O-H···N) interactions forming a 2D supramolecular rectangular grid. Each rectangular grid undergoes threefold interpenetration resulting a 2D interpenetrated supramolecular framework with hydrophobic small pores. CO₂ sorption at 195 K in 1 shows no occlusion in the pore surface, however hysteretic sorption observed with H₂O and MeOH, correlated with the H-bonding interaction of H₂O and MeOH with the pendant carboxylate O-atoms, which are aligned on the 2D surface.     

Electrochemical properties of heme-protein in lauric acid films and its application as a biosensor

In this research, the enhancement of electron-transfer activity of hemoglobin (Hb) in lauric acid film was investigated for the first time. This type of composite film was made on a glassy carbon electrode by a casting method. Cyclic voltammetric result of the modified electrode displays a well-defined redox peak, which was attributed to the direct electrochemical response of Hb. Our results illustrate that Hb exchange electrons directly with electrode and exhibits the characteristics of peroxidase. When we use this modified electrode as a biosensor, it gives excellent performance in the electrocatalytic reduction of hydrogen peroxide (H₂O₂). The parameters such as pH and applied potential of the biosensor influencing in H₂O₂ detection were optimized carefully. Through the optimal conditions, the proposed biosensor shows the linear range for H₂O₂ determination was from 1×10⁻⁵ to 1.25×10⁻⁴ mol L⁻¹ with a detection limit of 1×10⁻⁷ mol L⁻¹. The biosensor retained more than 90% of the initial response after 14 d.     

Silver electrodeposition catalyzed by colloidal gold on carbon paste electrode: application to biotin–streptavidin interaction monitoring

A new electrochemical method to monitor biotin–streptavidin interaction on carbon paste electrode, based on silver electrodeposition catalyzed by colloidal gold, was investigated. Silver reduction potential changed when colloidal gold was attached to an electrode surface through the biotin–streptavidin interaction. Thus, the direct reduction of silver ions on the electrode surface could be avoided and therefore, they were only reduced to metallic silver on the colloidal gold particle surface, forming a shell around these particles. When an anodic scan was performed, this shell of silver was oxidized and an oxidation process at +0.08 V was recorded in NH₃ 1.0 M. Biotinylated albumin was adsorbed on the pretreated electrode surface. This modified electrode was immersed in colloidal gold-streptavidin labeled solutions. The carbon paste electrode was then activated in adequate medium (NaOH 0.1 M and H₂SO₄ 0.1 M) to remove proteins from the electrode surface while colloidal gold particles remained adsorbed on it. Then, a silver electrodeposition at −0.18 V for 2 min and anodic stripping voltammetry were carried out in NH₃ 1.0 M containing 2.0×10⁻⁵ M of silver lactate. An electrode surface preparation was carried out to obtain a good reproducibility of the analytical signal (5.3%), using a new electrode for each experiment. In addition, a sequential competitive assay was carried out to determine streptavidin. A linear relationship between peak current and logarithm of streptavidin concentration from 2.25×10⁻¹⁵ to 2.24×10⁻¹² M and a limit of detection of 2.0×10⁻¹⁵ M were obtained.     

Mediatorless electrocatalytic reduction of hydrogen peroxide at graphite electrodes chemically modified with peroxidases

The electrocatalytic reduction of H₂O₂ was studied for carbonaceous electrodes modified with horse-radish peroxidase (HRP), microperoxidase (MP), and lactoperoxidase (LP). The carbonaceous electrodes were of three different graphites, carbon and glassy carbon. The peroxidase modified electrode was inserted as the working electrode in a flow through amperometric cell of the wall jet type and connected to a flow injection system. The effect of different pretreatments of the electrode surface prior to adsorption of the enzyme was investigated. Heating the electrodes in a muffle furnace at 700°C for 1.5 min was found to yield the highest currents. The electrocatalytic current for HRP-modified electrodes starts at about +600 mV vs. Ag/AgCl (pH 7.0) and reaches a maximum value at about −200 mV. For MP- and LP-modified electrodes the currents start at a lower potential (≈ 300 mV). For the best electrode material for HRP, straight calibration curves were obtained between 1 and 500 μM H₂O₂ at 0 mV. The mechanism for the electron transfer from the electrode to the adsorbed peroxidase is discussed. Deliberate modification of the electrode surface with quinoid type electroactive species was found to mediate the reaction. It is proposed that spontaneously occurring electrochemically active surface groups mediate the electron transfer to the adsorbed enzyme. However, a contribution to the observed current from a direct electron transfer cannot be ruled out.     

Determination of dopamine in the presence of ascorbic acid using poly(3,5-dihydroxy benzoic acid) film modified electrode

A polymerized film of 3,5-dihydroxy benzoic acid (DBA) was prepared on the surface of a glassy carbon electrode (GCE) in neutral solution by cyclic voltammetry (CV). The poly(DBA) film-coated GCE exhibited excellent electrocatalytic activity toward the oxidation of dopamine (DA). A linear range of 1.0 × 10⁻⁷ to 1.0 × 10⁻⁴ M and a detection limit of 6.0 × 10⁻⁸ M were observed in pH 7.4 phosphate buffer solutions. Moreover, the interference of ascorbic acid (AA) was effectively eliminated. This work provides a simple and easy approach to selective detection of DA in the presence of AA.     

A Porphyrin/Graphene Framework: A Highly Efficient and Robust Electrocatalyst for Carbon Dioxide Reduction

Developing immobilized molecular complexes, which demonstrate high product efficiencies at low overpotential in the electrochemical reduction of CO₂ in aqueous media, is essential for the practical production of reduction products. In this work, a simple and facile self‐assembly method is demonstrated by electrostatic interaction and π–π stacking for the fabrication of a porphyrin/graphene framework (FePGF) composed of Fe(III) tetraphenyltrimethylammonium porphyrin and reduced liquid crystalline graphene oxide that can be utilized for the electrocatalytic reduction of CO₂ to CO on a glassy carbon electrode in aqueous electrolyte. The FePGF results in an outstanding robust catalytic performance for the production of CO with 97.0% faradaic efficiency at an overpotential of 480 mV and superior long‐term stability relative to other heterogeneous molecular complexes of over 24 h (cathodic energy efficiency: 58.1%). In addition, a high surface area carbon fiber paper is used as a substrate for FePGF catalyst, resulting in enhanced current density of 1.68 mA cm^?2 with 98.7% CO faradaic efficiency at an overpotential of 430 mV for 10 h, corresponding to a turnover frequency of 2.9 s^?1 and 104 400 turnover number. Furthermore, FePGF/CFP has one of the highest cathodic energy efficiencies (60.9%) reported for immobilized metal complex catalysts.     

A novel hydrogen peroxide sensor based on immobilization of haemoglobin on nano-TiO2/DTAB composite film

Abstract

The direct electron transfer of immobilized haemoglobin (Hb) on nano-TiO₂ and dodecyltrimethylammonium bromide (DTAB) film modified carbon paste electrode (CPE) and its application as a hydrogen peroxide (H₂O₂) biosensor were investigated. On nano-TiO₂/DTAB/Hb/CPE, Hb displayed a rapid electron transfer process with participation of one proton and with an electron transfer rate constant which estimated as 0.29 s^??1. Thus, the proposed biosensor exhibited a high sensitivity and excellent electrocatalytic activity for the reduction of H₂O₂. The catalytic reduction current of H₂O₂ was proportional to H₂O₂ concentration in the range of 0.2–4.0 mM with a detection limit of 0.07 mM. The apparent Michaelis–Menten constant (K_m^app) of the biosensor was calculated to be 0.127 mM, exhibiting a high enzymatic activity and affinity. This sensor for H₂O₂ can potentially be applied in determination of other reactive oxygen species as well.     

Electrochemistry and electrocatalysis of myoglobin immobilized in sulfonated graphene oxide and Nafion films

In this study, sulfonated graphene oxide (SGO) was synthesized and characterized by Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). It was used to make Mb–SGO–Nafion composite films by coating myoglobin (Mb) on the glassy carbon electrodes (GCE). Positions of the Soret absorption bands suggested that Mb retained its native conformation in the films. Mb–SGO–Nafion film modified electrode showed a pair of well-defined and nearly reversible cyclic voltammetry peaks at around −0.39 V versus saturated calomel electrode (SCE) in pH 7.0 buffers, characteristic of heme Fe(III)/Fe(II) redox couples. Electrochemical parameters such as electron transfer rate constant (k_s) and formal potential (E^o′) were estimated by fitting the data of square-wave voltammetry with nonlinear regression analysis. Experimental data demonstrated that the electron transfer between Mb and electrode was greatly facilitated and showed good electrocatalytic properties toward various substrates, such as H₂O₂ and NaNO₂, with significant lowering of reduction overpotential.     

Vitamin B(12) incorporated with multiwalled carbon nanotube composite film for the determination of hydrazine

Electrochemically active composite film containing multiwalled carbon nanotubes (MWCNTs) and vitamin B₁₂ was synthesized on glassy carbon, gold, and indium tin oxide electrodes by the potentiodynamic method. The presence of MWCNTs in the composite film (MWCNT–B₁₂) modified electrode mediates vitamin B₁₂’s redox reaction, whereas vitamin B₁₂’s redox reaction does not occur at bare electrode. The electrochemical impedance spectroscopy studies reveal that MWCNTs present in MWCNT–B₁₂ film enhance electron shuttling between the reactant and electrode surface. The surface morphology of bare electrode, MWCNT film. and MWCNT–B₁₂ composite film was studied using atomic force microscopy, which reveals vitamin B₁₂ incorporated with MWCNTs. The MWCNT–B₁₂ composite film exhibits promising enhanced electrocatalysis toward hydrazine. The electrocatalysis response of hydrazine at MWCNT film and MWCNT–B₁₂ composite film was measured using cyclic voltammetry and amperometric current–time (i–t) curve techniques. The linear concentration range of hydrazine obtained at MWCNT–B₁₂ composite film using the i–t curve technique is 2.0 μM–1.95 mM. Similarly, the sensitivity of MWCNT–B₁₂ composite film for hydrazine determination using the i–t curve technique is 1.32 mA mM⁻¹ cm⁻², and the hydrazine’s limit of detection at MWCNT–B₁₂ composite film is 0.7 μM.     

Distinct Cd and Co thiocyanate coordination complexes with 2,5-bis(pyrazinyl)-1,3,4-oxadiazole: Metal-directed assembly of a 1-D polymeric chain and a 3-D supramolecular network

The reactions of 2,5-bis(pyrazinyl)-1,3,4-oxadiazole (bpzo) with Cd^II or Co^II salt in the presence of thiocyanate afford two distinct complexes, a 1-D coordination array [Cd(bpzo)₂(SCN)₂]_n (1) and a 3-D hydrogen-bonded supramolecular network [Co(bpzo)₂(SCN)₂(H₂O)₂](CH₃CN)₂(H₂O)₂ (2). X-ray single-crystal structural determination reveals that the extended networks of complexes 1 and 2 are manipulated via different directional propagating forces. In 1, the adjacent Cd^II centers are bridged by a pair of μ_1,3-SCN⁻ anions to form a 1-D array, whereas in 2, the monomeric Co^II coordination entities are hydrogen-bonded into a novel 3-D architecture in which the thiocyanate ions take the only N-binding mode. In both cases, bpzo behaves as monodentate terminals. These results indicate that the choice of metal ions does play a critical role in the supramolecular assembly. The structural and binding features of bpzo in all related compounds have also been discussed.     

The impact of spermine competition on the efficacy of DNA-binding Fe(II), Co(II), and Cu(II) complexes of dimeric chromomycin A3

Chromomycin (Chro) forms a 2:1 drug/metal complex through the chelation with Fe(II), Co(II), or Cu(II) ion. The effects of spermine on the interaction of Fe(II), Co(II), and Cu(II) complexes of dimeric Chro with DNA were studied. Circular dichroism (CD) measurements revealed that spermine strongly competed for the Fe(II) and Cu(II) cations in dimeric Chro-DNA complexes, and disrupted the structures of these complexes. However, the DNA-Co^II(Chro)₂ complex showed extreme resistance to spermine-mediated competition for the Co(II) cation. According to surface plasmon resonance (SPR) experiments, a 6 mM concentration of spermine completely abolished the DNA-binding activity of Fe^II(Chro)₂ and Cu^II(Chro)₂ and interfered with the associative binding of Co^II(Chro)₂ complexes to DNA duplexes, but only slightly affected dissociation. In DNA integrity assays, lower concentrations of spermine (1 and 2 mM) promoted DNA strand cleavage by Cu^II(Chro)₂, whereas various concentrations of spermine protected plasmid DNA from damage caused by either Co^II(Chro)₂ or Fe^II(Chro)₂. Additionally, DNA condensation was observed in the reactions of DNA, spermine, and Fe^II(Chro)₂. Despite the fact that Cu^II(Chro)₂ and Fe^II(Chro)₂ demonstrated lower DNA-binding activity than Co^II(Chro)₂ in the absence of spermine, while Cu^II(Chro)₂ and Fe^II(Chro)₂ exhibited greater cytoxicity against HepG2 cells than Co^II(Chro)₂, possibly due to competition of spermine for Fe(II) or Cu(II) in the dimeric Chro complex in the nucleus of the cancer cells. Our results should have significant relevance to future developments in metalloantibiotics for cancer therapy.     

Ultrathin Co3O4 Layers with Large Contact Area on Carbon Fibers as High‐Performance Electrode for Flexible Zinc–Air Battery Integrated with Flexible Display

A facile and binder‐free method is developed for the in situ and horizontal growth of ultrathin mesoporous Co₃O₄ layers on the surface of carbon fibers in the carbon cloth (ultrathin Co₃O₄/CC) as high‐performance air electrode for the flexible Zn–air battery. In particular, the ultrathin Co₃O₄ layers have a maximum contact area on the conductive support, facilitating the rapid electron transport and preventing the aggregation of ultrathin layers. The ultrathin feature of Co₃O₄ layers is characterized by the transmission electron microscopy, Raman spectra, and X‐ray absorption fine structure spectroscopy. Benefiting from the high utilization degree of active materials and rapid charge transport, the mass activity for oxygen reduction and evolution reactions of the ultrathin Co₃O₄/CC electrode is more than 10 times higher than that of the carbon cloth loaded with commercial Co₃O₄ nanoparticles. Compared to the commercial Co₃O₄/CC electrode, the flexible Zn–air battery using ultrathin Co₃O₄/CC electrode exhibits excellent rechargeable performance and high mechanical stability. Furthermore, the flexible Zn–air battery is integrated with a flexible display unit. The whole integrated device can operate without obvious performance degradation under serious deformation and even during the cutting process, which makes it highly promising for wearable and roll‐up optoelectronics.