Facile Redox Reaction Research Articles

Electrocatalytic properties of transition metal vanadates (T = Ti, Cr, Mn, & Fe) for overall water splitting

In this work, first row transition metal vanadates (TMVs), including titanium vanadate (TiV), chromium vanadate (CrV), manganese vanadate (MnV), and iron vanadate (FeV), were synthesized and investigated their electrocatalytic performance for the alkaline water splitting. The prepared TMVs showed easy electron transfer characteristics and tunable band structures, and among these TMVs, the synthesized FeV demonstrated the best electrocatalytic bifunctional activities for both HER (η100 = 375 mV) and OER (η100 = 260 mV) with a significant electronic and morphological stability over 100 h operation. The examination of electronic structure revealed that the balanced catalyst-intermediate interaction through optimum d-p orbital hybridization in FeV, adhering to the Sabater principle, vouched for such facile redox reaction. The structural stability might be attributed to the synergism between iron and vanadate as the multivalent Fe adjusted the shift in vanadium coordination when the oxidation state of Vn+ (n = 3–5) changed. Moreover, the two-electrode system of FeV || FeV in a single electrolyzer demonstrated superior cell voltage100 of 1.86 V in comparison to commercial Pt/C || IrO2 electrodes (1.91 V). This work offers facile perspective to produce TMVs as the efficient electrocatalysts for green synthesis of fuel gases.

Enhanced and Sustainable Removal of Indoor Formaldehyde by Naturally Porous Bamboo Activated Carbon Supported with MnOx: Synergistic Effect of Adsorption and Oxidation

Novel bamboo activated carbon (BAC) catalysts decorated with manganese oxides (MnOx) were prepared with varying MnOx contents through a facile one-step redox reaction. Due to the physical anchoring effect of the natural macropore structure for catalyst active components, homogeneous MnOx nanoparticles (NPs), and high specific surface area over catalyst surface, the BAC@MnOx-N (N = 1, 2, 3, 4, 5) catalyst shows encouraging adsorption and catalytic oxidation for indoor formaldehyde (HCHO) removal at room temperature. Dynamic adsorption and catalytic activity experiments were conducted. The higher Smicro (733 m2/g) and Vmicro/Vt (82.6%) of the BAC@MnOx-4 catalyst could facilitate its excellent saturated and breakthrough adsorption capacity (5.24 ± 0.42 mg/g, 2.43 ± 0.22 mg/g). The best performer against 2 ppm HCHO is BAC@MnOx-4 catalyst, exhibiting a maximum HCHO removal efficiency of 97% for 17 h without any deactivation as RH = 0, which is higher than those of other MnOx-based catalysts. The average oxidation state and in situ DRIFTS analysis reveal that abundant oxygen vacancies on the BAC@MnOx-4 catalyst could be identified as surface-active sites of decomposing HCHO into the intermediate species (dioxymethylene and formate). This study provides a potential approach to deposit MnOx nanoparticles onto the BAC surface, and this hybrid BAC@MnOx material is promising for indoor HCHO removal at room temperature.

Phase Engineering of Nonstoichiometric Cu2-xSe as Anode for Aqueous Zn-Ion Batteries.

Aqueous zinc-ion batteries (AZIBs) are receiving widespread attention due to their abundant resources, low material cost, and high safety. However, the susceptibility of Zn metal anodes to corrosion and hydrogen evolution limits their further practical applications. Replacing Zn metal with intercalation-type anode material and constructing rocking-chair-type batteries could be an effective way to significantly prolong the cycle life of AZIBs. Herein, we present copper selenide with different crystal phase structures through a facile redox reaction as an anode for AZIBs. By comparing and analyzing different copper selenide phases, it is found that the cubic Cu2-xSe shows superior structural stability and highly reversible Zn2+ storage. Theoretical calculation results further demonstrate that the cubic Cu2-xSe possesses an increased electrical conductivity, higher Zn2+ adsorption energy, and reduced diffusion barrier, thereby promoting the storage reversibility and (de)intercalation kinetics of the Zn2+ ion. Thus, the Cu2-xSe anode delivers a long-term service life of over 15 000 cycles and impressive cumulative capacity. Furthermore, the full-cells assembled with the MnO2/CNT cathode operate stably for over 1500 cycles at 6 mA cm-2 at a negative/positive (N/P) capacity ratio of ∼1.53. This work provides a more ideal Zn-metal-free anode, which helps to push the practical applications of AZIBs.

Engineered Collaborative Size Regulation and Shape Engineering of Tremella-Like Au-MnOx for Highly Sensitive Bimodal-Type Lateral Flow Immunoassays.

Engineered collaborative size regulation and shape engineering of multi-functional nanomaterials (NPs) offer extraordinary opportunities for improving the analysis performance. It is anticipated to address the difficulty in distinguishing color changes caused by subtle variations in target concentrations, thereby facilitating the highly sensitive analysis of lateral flow immunoassays (LFIAs). Herein, tremella-like gold-manganese oxide (Au-MnOx ) nanoparticles with precise MnCl2 regulation are synthesized as immuno signal tracers via a facile one-step redox reaction in alkaline condition at ambient temperature. Avail of the tunable elemental composition and anisotropy in morphology, black-colored tremella-like Au-MnOx exhibits superb colorimetric signal brightness, enhanced antibody coupling efficiency, marvelous photothermal performance, and unrestricted immunological recognition affinity, all of which facilitate highly sensitive multi-signal transduction patterns. In conjunction with the handheld thermal reader device, a bimodal-type LFIA that combines size-regulation- and shape-engineering-mediated colorimetric-photothermal dual-response assay (coined as the SSCPD assay) with a limit of detection of 0.012ngmL-1 for ractopamine (RAC) monitoring is achieved by integrating Au-MnOx with the competitive-type immunoreaction. This work illustrates the effectiveness of this strategy for establishing high-performance sensing, and the SSCPD assay may be extended to a wide spectrum of future point-of-care (POC) diagnostic applications.

Layered MnO2 nanodots as high-rate and stable cathode materials for aqueous zinc-ion storage

MnO2 has recently received great concern as a cathode material for zinc-based energy storage owing to its many advantages. Unfortunately, the low rate capability and poor cyclability hinder its practical application. Herein, novel layered MnO2 nanodots (δ-MnO2 NDs) are synthesized by a facile redox reaction, and utilized as the cathode for aqueous zinc-ion batteries/hybrid capacitors (ZIBs/ZICs) for the first time. Benefiting from the layered structure and nanoscale size, the δ-MnO2 NDs//Zn ZIBs display a considerable specific capacity of 335 mAh g–1 at 0.1 A g–1, an impressive rate capacity of 125 mAh g–1 at 2.0 A g–1, a large specific energy (466.7 Wh kg–1 at 139 W kg–1), and a superior durability with 86.2% capacity retention after 1000 cycles at 1.0 A g–1. Further, the H+/Zn2+ co-insertion energy storage mechanism of the δ-MnO2 NDs cathode is verified by electrochemical kinetics analyses and ex-situ characterizations. Simultaneously, the novel ZICs based on δ-MnO2 NDs cathode exhibit a high specific energy of 68.7 Wh kg–1 and a satisfactory cycle life with 86.9% capacity retention after 5000 cycles at 1.0 A g–1. The quantization design strategy opens a new gateway for the design and exploitation of advanced cathodes for aqueous ZIBs and ZICs.

Graphene electrode functionalization for high performance hybrid energy storage with vanadyl sulfate redox electrolytes

Hybrid energy storage devices face the challenges of pairing suitable redox chemistries with stable electrodes to simultaneously realize the energy storage mechanisms of batteries and supercapacitors. Functionalization of electrodes enables improved catalytic activity and long-term cycling stability by introducing desired functional groups. Herein, we demonstrate novel hybrids using reversible vanadium (V) redox couples along with functionalized graphene electrodes. Redox couples of V(III)/V(IV) and V(IV)/V(V) are formed upon initial charging of the V(IV) electrolyte present in both half-cells, leading to a Galvani potential difference during operation. Moreover, functionalized electrodes with high surface area facilitate both facile redox reactions and electric double layer formation. The unique chemistry offers a straightforward device regeneration step which significantly enhances the cycling stability. Optimized hybrids exhibit a battery-like specific capacity of 700 mAh g−1 at 0.5 A g−1, a supercapacitor-like capacitance of 850 F g−1 over 10,000 cycles at 10 A g−1, and a very low self-discharge rate. This corresponds to power densities of 0.3 kW kg−1 and 9.2 kW kg−1 and energy densities of 363.8 Wh kg−1 and 24.9 Wh kg−1, respectively, outperforming similar aqueous redox hybrid systems.

Unrevealing the Mechanism of Aminoxyl Catalyzed Electrochemical Oxidation of Alcohols By Unpolished Glassy Carbon Electrode

2,2,6,6-tetramethyl-1-piperidine N-oxyl (TEMPO, 1) and related aminoxyl derivatives are important catalysts for alcohol oxidation. They are featured in the synthesis of pharmaceuticals and recently found considerable attention for the conversion of hydroxyl bearing biomass-derived feedstocks to value-added products. Aminoxyl compounds undergo facile redox reactions at electrode surfaces, enabling them to mediate electrochemical oxidation of alcohols.The well-known electron transfer reaction of these persistent radicals involves a reversible one electron oxidation to the corresponding oxoammonium species (TEMPO+ , 3), which are relatively strong oxidants. Aminoxyl radicals can also undergo a proton coupled reduction to hydroxylamines (2), which is quasireversible and less known. However, for many of their electrosynthetic reactions, the catalytic cycle involves not only aminoxyl/oxoammonium redox couple, but also formation of hydroxylamines and regeneration of the oxoammonium from the hydroxylamine. This two steps oxidation has been proposed to occur via two distinct pathways: the comproportionation reaction between the hydroxylamine and a second oxoammonium cation to form aminoxyloxyl radical (two) followed by oxidation of aminoxyl radicals to oxoammonium (pathway b), or the two-electron oxidation of the hydroxylamine to the oxoammonium (pathway a). The electron transfer of the hydroxylamine, at the electrode surface, is a quasireversible ill-defined reaction that strongly depends on the electrode material and conditionings, and its detailed understanding is still lacking. Herein, we investigated the electron transfer reaction of TEMPO and its hydroxylamine, is formed during catalytic alcohol oxidation, using the glassy carbon electrodes that treated under different conditions. Well-treated electrode facilitates the electron transfer of hydroxylamine and enables two-electrons oxidation to oxoammonium, whereas on the untreated electrode, two separated one-electron oxidations are observed that enables measuring the rate of generation and consumption of hydroxylamine. Understanding of how each regeneration pathway affects the catalytic behavior made it possible to comprehensively understand all the features of the current profiles obtained under various alcohol oxidation conditions. Figure 1. Possible mechanism for regeneration of oxoammonium during aminoxyl catalyzed alcohol oxidation. Figure 1

Facile one-step redox synthesis of Bi2O2CO3/Bi2O3/Bi ternary photocatalyst without additional carbon source

Construction of heterojunction and decoration of cocatalyst are two vital strategies to accelerate migration of charge carriers. However, the fabrication routs of multi-composites are usually complex and expensive. In this work, Bi2O2CO3/Bi2O3/Bi ternary composite was fabricated via a facile one-step redox reaction. Ethylene glycol was selected as the solvent during the whole reaction process. Moreover, ethylene glycol as an excellent reductant can reduce Bi3+ into metallic Bi0 and itself is oxidized to CO32−, which would react with Bi2O3 to generate Bi2O2CO3 without additional carbon source. Component proportions in ternary composites were optimized by the control of the ratios of raw materials. Under simulate solar light, Bi-based ternary composites exhibited enhanced photodegradation efficiencies for multifarious pollutants in comparison with single and binary samples. The enhanced photocatalytic activities were ascribed to accelerated migration rate of charge carriers owing to the construction of heterojunction and decoration of cocatalyst.

In situ loading MnO2 onto 3D Aramid nanofiber aerogel as High-Performance lead adsorbent

Fabricating a high-performance adsorbent as a desirable candidate for removing Pb2+ from aqueous water remains a challenge. Aramid nanofibers (ANFs) are promising building blocks that have realized multifunctional applications due to their intrinsic mechanical and chemical stability. Herein, an in situ loading strategy for preparing nanofiber composite aerogel was proposed by assembling ANFs into a 3D aerogel and applying it as host media for the in situ polymerization of pyrrole followed by facile redox reaction between the polypyrrole (PPy) and MnO4–1 to load manganese dioxide (MnO2). The idea was to fully exploit the structural advantages of ultra-low bulk density, large specific surface area, and high porosity of ANFs, and the possible chemical adsorption characteristics of MnO2 on the basis of ion exchange reaction. The adsorption capacity of 3D ANF/MnO2 composite aerogel was as large as 554.36 mg/g for Pb2+. The adsorption mechanism based on an exchange reaction between Pb2+ and protons on the surface of MnO2 was also investigated. The desorption results showed that the adsorption performance could remain up to 90% after five times of usage. In conclusion, this research provides promising insights into the preparation of high-performance lead adsorbent for water treatment.

SiOx/C-Ag nanosheets derived from Zintl phase CaSi2 via a facile redox reaction for high performance lithium storage

SiOx/C-Ag nanosheets derived from Zintl phase CaSi2 via a facile redox reaction for high performance lithium storage

PH and Redox Dual-Response Disulfide Bond-Functionalized Red-Emitting Gold Nanoclusters for Monitoring the Contamination of Organophosphorus Pesticides in Foods.

Most of the fluorescence sensors require choline oxidase or quenchers to detect organophosphorus pesticides (OPs) based on a single hydrolysate and suffer from high cost, complex procedures, weak stability, and low sensitivity. Here, we proposed a brand-new fluorescence strategy for highly sensitive detection of OPs based on both hydrolysate-response disulfide bond-functionalized gold nanoclusters (S-S-AuNCs) without additional substances. S-S-AuNCs were synthesized via a facile one-step redox reaction and emitted bright red light with ultrasmall size and high water dispersion. Interestingly, S-S-AuNCs displayed a unique response to thiol compounds and low pH values and were thus pioneered as a high-efficiency sensor for OPs based on acetylcholinesterase (AChE)-catalyzed hydrolysis of acetylthiocholine into thiocholine and CH3COOH and OP inhibition of AChE activity. Further, S-S-AuNCs were employed to monitor the residue, distribution, and metabolization of methidathion in pakchoi with acceptable results. We believe that this work supplies a simpler and more highly sensitive approach for OP assay than the known ones and opens a new avenue to development of multistimulus-responsive and high-performance fluorescence substances.

Folin-Ciocalteu Assay Inspired Polyoxometalate Nanoclusters as a Renal Clearable Agent for Non-Inflammatory Photothermal Cancer Therapy.

Similar to translated thermal ablative techniques in clinic, the occurrence of cellular necrosis during tumor photothermal therapy (PTT) would induce inflammatory responses that are detrimental to therapeutic outcomes. Inspired by the well-known colorimetric Folin-Ciocalteu assay, monodispersed and renal-clearable tungsten (W)-based polyoxometalate nanoclusters (W-POM NCs, average diameter of around 2.0 nm) were successfully obtained here through a facile redox reaction with natural gallic acid in alkaline aqueous solution. Apart from excellent stability in the form of freeze-dried powder, the as-prepared W-POM NCs occupied considerable biocompatibility toward normal cells/tissues both in vitro and in vivo, since no obvious toxicities were observed by treating female Balb/c mice with concentrated W-POM NCs during the 30 day post-treatment period. More importantly, W-POM NCs exhibited not only considerable near-infrared (NIR) light absorption (coloration effect originated from the existence of electron-trapped W5+) for efficient PTT but also impressive anti-inflammatory ability (eliminating inflammation-related reactive oxygen species by the oxidation of W5+ into W6+ state) to achieve better therapeutic outcomes. Thus, our study pioneers the application of POMs for non-inflammatory PTT with expected safety and efficiency.

Transition-metal doped, magnesium oxide-supported terbium oxides as catalysts for the oxidative coupling of methane

The effects of transition metal dopants on the catalytic properties of magnesia-supported terbium oxide catalysts in the oxidative coupling of methane were investigated. A few transition metals were included based on their oxidation states relative to Tb in TbO1.81 (where Tb is between Tb3+ and Tb4+). Ni and Fe were selected as low valence dopants (LVDs) and Zr and V were considered high valence dopants (HVDs) in the study. The effect of dopant concentration was also investigated, between 0.2 and 1.0 % by weight (wt%), since high loadings could result in segregation of the transition metal (TM) dopants, while low loadings may not yield observable effects. At the higher loading (1.0 wt%) it appears that the oxidation state relative to that of Tb is of minor importance, and the redox properties of the dopant have a larger impact. At this loading, the best performing catalysts were the Ni- and Zr-doped TbOx/n-MgO, while the selectivities of the Fe- and V-doped catalysts toward OCM products were very low, particularly at the lower temperatures (<600 °C). The facile redox reactions that allow Fe and V to easily change oxidation state are likely detrimental to the OCM reaction, particularly at higher concentrations where segregation of the transition metal oxide may occur at the surface. In contrast, at the lower loading (0.2 wt%) all doped catalysts performed better than the undoped TbOx/n-MgO. At this loading, it appears that the LVDs resulted in better performing OCM catalysts, as the Ni- and Fe-doped TbOx/n-MgO both had higher activities and selectivities compared with the Zr- and V-doped catalysts.The characterization data revealed that even at the low concentration of 0.2 wt%, the transition metals markedly alters the properties of the doped TbOx/n-MgO catalysts. For example, the number of basic sites was significantly increased by adding 0.2 wt% of Fe or Zr to the TbOx/n-MgO catalyst and this led to an improvement in the performance of the catalysts in the OCM reaction. The dopants also affected the oxidation state of Tb in the TbOx lattice as well as the dispersion of TbOx on the surface of the MgO. Both XPS and XRD data indicated that the TbO1.81, which was present on the surface of the fresh catalysts, in most cases was reduced to Tb2O3 during reaction. However, there were no evident trends to indicate that LVDs promote, while HVDs suppress, surface reduction of the host oxide, i.e. TbOx. The lack of trends may in part be due to the presence of the MgO support, which results in a complex catalyst system. Nevertheless, the data revealed that even low loadings of a transition metal dopant can positively affect the activity and selectivity of a TbOx/n-MgO catalyst.

The catalytic activity of manganese dioxide supported on graphene promoting the electrochemical performance of lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries have become a research hotspot due to their high energy density, environment friendliness and low-cost. But commercial applications are hampered by rapid capacity fading and low sulfur content. Herein, the composite of integrating manganese oxide on graphene was prepared by a facile redox reaction between potassium permanganate (KMnO4) and graphene in acidic aqueous solution to solve the problems of the Li-S batteries. X-ray Diffraction (XRD), Scanning Electron Microscope (SEM) and transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis show that sulfur was uniformity coated on G/MnO2 composite. TG indicated that the G/MnO2/S composites contained 76.56wt% active sulfur. The prepared cathode of G/MnO2/S composites exhibited excellent rate performance and cycle stability. At a high rate of 1C, its maximum discharge capacity reached 908mAhg−1 and its average capacity decreasing rate was only 0.105%/cycle when running over 500cycles. The interaction mechanism of MnO2 with polysulfides was investigated by adsorption tests and XPS analysis. The excellent electrochemical performance might be due to a combination of adsorption or catalytic properties of MnO2, and the conductivity network of graphene.

A High‐Power Na3V2(PO4)3‐Bi Sodium‐Ion Full Battery in a Wide Temperature Range

AbstractSodium‐ion batteries (SIBs) that operate in a wide temperature range are in high demand for practical large‐scale electric energy storage. Herein, a novel full SIB is composed of a bulk Bi anode, a Na3V2(PO4)3/carbon nanotubes composite (NVP‐CNTs) cathode and a NaPF6‐diglyme electrolyte. The Bi anode gradually evolves into a porous network in the ether‐based electrolyte during initial cycles, and in the NVP‐CNTs cathode the NVP is cross linked by CNTs to show large exchange current density. These unique features merit the full SIB of Bi//NVP‐CNTs with high Na+ diffusion coefficient and low reaction activation energy, and hence fast Na+ transport and facile redox reaction kinetics. The resultant full SIB presents high power density of 2354.6 W kg−1 and energy density of 150 Wh kg−1 and superior cycling stability in a wide temperature range from −15 to 45 °C. This will shed light on battery design, and promote their development for practical applications in various weather conditions.

Oxygen vacancies modulation in graphene/MnOx composite for high performance supercapacitors

An efficient and scalable method to improve the capacitive properties of MnO2 is reported by inducing graphene (rGO) and oxygen vacancies via a thermal treatment in air, in which graphene oxide (GO) is in situ reduced and graphene/MnOx (rGO/MnOx) composite without oxygen vacancies-induced phase transformation is formed. RGO/MnOx composite exhibits a large specific capacitance (274 F g−1 at 0.5 A g−1), excellent cycling stability (98.8% after 5000 cycles at 1 A g−1) and rate performance (171 F g−1 at 10 A g−1). The performance improvement of rGO/MnOx sample is mainly attributed to the moderate concentration of oxygen vacancies, rGO and special 3D porous architecture, which facilitate the rapid ionic and electronic transport and promote a facile redox reaction and surface double-layer capacitance. The asymmetric supercapacitor composed of rGO/MnOx//Kochen Black (KB) also shows an outstanding cyclic stability (89% after 5000 cycles) in all-solid-state asymmetric supercapacitor. The design and synthesis method of rGO/MnOx sample offers a promising approach to broaden MnO2-based electrode materials for a new-generation energy storage device.

Tryptophan-Stabilized Au-FexOy Nanocomposites as Electrocatalysts for Oxygen Evolution Reaction.

Au–FexOy nanocomposites with a variable gold-to-iron ratio were stabilized with l-tryptophan. The synthetic methodology is based on the facile redox reaction between Au(III) and Fe(0) in the presence of gold nanoparticle as a seed at room temperature in an aqueous medium. The synthesis results in the deposition of Au nanoparticles on the surface of iron oxide layers. Composition variation in the nanocomposites was obtained by controlling the seed amount and reducing agent. These nanocomposites are used as electrocatalysts for the thermodynamically unfavorable oxygen evolution reaction (OER) from water. Among the nanocomposites, the most efficient OER activity was observed from the nanocomposite 12. The content of iron with respect to gold is at the maximum in the nanocomposite, which was obtained from the reaction with a minimum seed concentration and maximum reducing agent.

Toward Low-Cost and Sustainable Supercapacitor Electrode Processing: Simultaneous Carbon Grafting and Coating of Mixed-Valence Metal Oxides by Fast Annealing.

There is a rapid market growth for supercapacitors and batteries based on new materials and production strategies that minimize their cost, end-of-life environmental impact, and waste management. Herein, mixed-valence iron oxide (FeOx) and manganese oxide (Mn3O4) and FeOx-carbon black (FeOx-CB) electrodes with excellent pseudocapacitive behavior in 1 M Na2SO4 are produced by a one-step thermal annealing. Due to the in situ grafted carbon black, the FeOx-CB shows a high pseudocapacitance of 408 mF cm−2 (or 128 F g−1), and Mn3O4 after activation shows high pseudocapacitance of 480 mF cm−2 (192 F g−1). The asymmetric supercapacitor based on FeOx-CB and activated-Mn3O4 shows a capacitance of 260 mF cm−2 at 100 mHz and a cycling stability of 97.4% over 800 cycles. Furthermore, due to its facile redox reactions, the supercapacitor can be voltammetrically cycled up to a high rate of 2,000 mV s−1 without a significant distortion of the voltammograms. Overall, our data indicate the feasibility of developing high-performance supercapacitors based on mixed-valence iron and manganese oxide electrodes in a single step.

Bifunctional hexagonal Ni/NiO nanostructures: influence of the core-shell phase on magnetism, electrochemical sensing of serotonin, and catalytic reduction of 4-nitrophenol.

Ni0/NiO (nickel/nickel oxide) core–shell nanostructures were synthesized through a facile combustible redox reaction. Remarkably, the hetero-phase boundary with different crystalline orientations offered dual properties, which helped in bifunctional catalysis. Presence of a metallic Ni phase changed physicochemical properties and some emerging applications (magnetic properties, optical conductivity, electrochemical sensitivity, catalytic behaviour) could be foreseen. Moreover, formation of a NiO layer on metal surface prevented magnetism-induced aggregation, arrested further oxidation by hindering oxygen diffusion, and acted as a good sorbent to enhance the surface adsorption of the analyte. Hexagonal Ni/NiO nanostructures manifested well-defined ferromagnetic behavior and the catalyst could be collected easily at the end of the catalytic reduction. Ni/NiO core–shell catalysts at the nanoscale had outstanding catalytic performance (reduction of 4-nitrophenol to 4-aminophenol) compared with pure NiO catalysts beyond a reaction time of ∼9 min. The estimated sensitivity, limit of detection and limit of quantification towards the electrochemical sensing of serotonin were 0.185, 0.43 and 1.47 μM μA−1, respectively. These results suggest that a bifunctional Ni/NiO nanostructure could be a suitable catalyst for electrochemical detection of serotonin and reduction of 4-nitrophenol.

Cobalt-doped MnO2 ultrathin nanosheets with abundant oxygen vacancies supported on functionalized carbon nanofibers for efficient oxygen evolution

Developing low-cost and efficient catalysts for oxygen evolution reactions (OER) with both excellent activity and robust stability remains a great challenge. Herein, we report a facile spontaneous redox reaction to grow cobalt-doped MnO2 ultrathin nanosheets in situ with abundant oxygen vacancies vertically aligned on cobalt/nitrogen co-functionalized carbon nanofibers (Co-MnO2|OV) as an efficient OER catalyst. It is confirmed that metallic cobalt plays a critical role in the formation of long and ultrathin MnO2 nanosheets during the redox reaction. Furthermore, the cobalt ions doped into MnO2 significantly enhance the catalytic activity of MnO2 nanosheets. Benefiting from the collaborative advantages of doping strategy, fast charge transfer kinetics and strong synergistic coupling effects, Co-MnO2|OV composites exhibit an excellent catalytic activity and a good durability for electrochemical water oxidation, reaching 10 mA cm−2 at an overpotential of 279 mV. According to the density functional theory (DFT) calculations, the enhanced catalytic activity mainly originates from a better conductivity and the decreased adsorption energy barrier of OH- on the O sites neighboring the doped Co and oxygen vacancies. Our findings suggest that the control over the structure and composition of the materials can achieve highly efficient oxygen evolution electrocatalysts.