Number Of Redox-active Sites Research Articles

Sulfur scrambling assisted in-situ growth of 3D - hierarchical FeNi2S4@Mo-doped Ni3S2/NF nanosheet arrays: A stellar performer towards alkaline water electrolysis

Industrial alkaline water electrolysis is facing major difficulties in developing inexpensive, abundant and active bifunctional non-noble metal electrocatalysts. Herein, a heterogeneous three-dimensional (3D) FeNi2S4@Mo-doped Ni3S2/NF heterostructure electrocatalyst is synthesized by a controlled two-step hydrothermal method. Based on the unique structural advantages, the number of redox active sites and the conductivity of the heterostructure electrode are improved remarkably resulting the shortening of ion diffusion path, effective penetration of the electrolyte and decrease in equivalent series resistance. The electrode achieves 10 mA cm−2 current density in hydrogen evolution reaction and oxygen evolution reaction at ∼121 and 150 mV overpotential, respectively in alkaline solution. The bifunctional electrochemical performance of the heterostructure is confirmed from the low cell voltage of 1.501 V and higher stability in overall water electrolysis. This work provides a useful strategy to tune the electrocatalytic performance by doping engineering and hetero-interface formation that offers an efficient self-supported 3D heterostructure electrode material for overall water electrolysis.

Polyoxometalates as Electrocatalysts for Electrochemical Energy Conversion and Storage

Polyoxometalates (POMs) are polyatomic ions with closed three-dimensional frameworks. Their unique structure contains a large number of redox active sites, making them promising electrocatalysts for electrochemical energy conversion and storage applications. Thus, this paper presents an overview of the use of POMs as electrocatalysts for electrochemical energy conversion and storage devices, such as batteries, supercapacitors, fuel cells, or water electrolyzers. A discussion of the viability of these materials as alternatives to noble metal-based electrocatalysts is made. The current status of these materials to respond to the challenges of converting modern energy systems into more sustainable ones is also envisaged.

Requirements for Beneficial Electrochemical Restructuring: A Model Study on a Cobalt Oxide in Selected Electrolytes

AbstractThe requirements for beneficial materials restructuring into a higher performance oxygen evolution reaction (OER) electrocatalyst are still a largely open question. Here erythrite (Co3(AsO4)2·8H2O) is used as a Co‐based OER electrocatalyst to evaluate its catalytic properties during in situ restructuring into an amorphous Co‐based catalyst in four different electrolytes at pH 7. Using diffraction, microscopy, and spectroscopy, a strong effect in the restructuring behavior is observed depending of the anions in the electrolyte. Only carbonate electrolyte can activate the catalyst material, which is related to its slow restructuring process. While the catalyst turnover frequency (TOF) undesirably reduces by a factor of 28, the number of redox active sites continuously increases to a factor of 56, which results in an overall twofold increase in current of the restructured catalyst after 800 cycles. The activation is attributed to an adequate local order, a high Co oxidation state close to 3+, and a high number of redox‐active Co ions. These three requirements for beneficial restructuring provide new insights into the rational design of high‐performance OER catalysts by electrochemical restructuring.

Influence of Transition Metals (Cu and Co) on the Carbon-Coated Nickel Sulfide Used as Positive Electrode Material in Hybrid Supercapacitor Device

Herein, a facile, environment-friendly and cost-effective approach was followed for the preparation of metal sulfide-based supercapacitor electrodes. The effect of transition metal interrogation on the morphology and electrochemical performance of carbon-coated nickel sulfide composite electrode was investigated. Physicochemical characterization showed that the enhancement in electrical conductivity and electrochemical reaction sites with the introduction of copper (Cu) and cobalt (Co) was due to the variation in morphology. Fast ionic transformation and improvement in the number of redox active sites might improve the supercapacitor performance. The electrochemical experiment showed that the NCoSC electrode exhibited the highest capacitance value of ~760 F g−1 at 2 A g−1 current density as compared to the NCuSC and NSC electrodes. Therefore, a hybrid supercapacitor (HSC) device was fabricated by using NCoSC as the positive electrode and thermally reduced graphene oxide (TRGO) as the negative electrode. The fabricated device demonstrated maximum energy density of ~38.8 Wh Kg−1 and power density of 9.8 kW Kg−1. The HSC device also showed ~89.5% retention in specific capacitance after 10,000 charge–discharge cycles at 12 A g−1 current density. So, the tuning of electronic and physical properties by the introduction of Cu and Co on nickel sulfide improved the supercapacitor performance.

Vanadium-Incorporated Metallic (1-T) Molybdenum Sulfide Nanoroses for High-Energy-Density Asymmetric Supercapacitors.

The metallic phase (1T) of molybdenum sulfide is critical, pertaining to its exceptional interlayer structure and metastability, but forms up with low content. Herein, 1T-phase-prominent vanadium-incorporated MoS2 (MVS) nanoroses were synthesized through a hydrothermal process. A significant increase in 1T content (50 %) occurred with the addition of vanadium, enhancing the prompt diffusion of lithium ions by two orders. More exposed electroactive basal planes increased the number of redox active sites to 84 %, suggesting an excellent charge storage of 451 F g-1 at 1 A g-1 . On assembling MVS with MnO2 to form an asymmetric cell (MnO2 ∥MVS), a high energy density (62.7 Wh kg-1 at 428 W kg-1 ) with a threefold increment from the MVS‖MVS symmetric cell (21.8 Wh kg-1 at 255 Wkg-1 ) was achieved. The asymmetric cell also exhibited a superior cycling stability with 98 % retention of its capacitance after 10 000 cycles.

Hydrothermal synthesis of NiCo2O4–NiO nanorods for high performance supercapacitors

In this work, spinel-metal oxide (NiCo2O4–NiO) nanorods based electrode was fabricated and tested for their efficacy as supercapacitors. The NiCo2O4–NiO was synthesized by hydrothermal method and characterized using thermogravimetric analysis/differential scanning calorimetry (TGA/DSC), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and transmission electron microscope (TEM) respectively. The TEM images reveal the morphology of the synthesized metal oxides in the form of nanorods with length of several micrometers and diameter in the range of 60–70 nm. The XRD pattern confirmed the formation of cubic spinel structure. A three electrode cell set up was employed to estimate the electrochemical performance of the material. It is observed that the NiCo2O4–NiO showed specific capacitance of 490 F/g at 0.5 A/g as compared to pure NiCo2O4 which is around 216 F/g at 0.5 A/g. The enhanced capacitance mainly arises from the incorporation of NiO secondary phases within the spinels which increases the number of redox active sites. The present work proves to be a promising strategy to improve the performance of the electrode material for supercapacitors.

Morphology-dependent electrochemical supercapacitive characteristics of nanostructured manganese dioxide

The dependence on morphology of the supercapacitive characteristics of manganese dioxide nanospheres (NSs) and nanorods (NRs) was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and a series of electrochemical techniques. Because the nanosized pores in MnO2 NSs resulted in high surface area, MnO2 electrodes made of NSs had higher specific capacitance (SC) than those made of NRs at current densities less than 2.0 A g−1. However, at current densities over 2.0 A g−1, the power density of MnO2 electrodes composed of NRs was better than that of NSs. The high surface area and nanosized pores in MnO2 NSs increase the number of redox active sites, which leads to high specific capacitance. On the other hand, the small pore size in MnO2 NSs restricts the rates of charge and discharge, thus limiting their power density.

Structural and Functional Characterization of Redox Mn and Co Sites in AlPO Materials and Their Role in Alkane Oxidation Catalysis

energy occurred concurrently and at similar rates, indicating excellent agreement between these techniques. No H2 Oo r CO 2 were detected during treatment in H2 or CO, respectively, indicating that reduction from Me 3+ to Me 2+ occurred by introduction of protons. These protons are fully removed by treatment in O2 to 773 K, and O2-H2 redox cycles involving reversible proton formation suggest that cations reside within AlPO framework positions, in which protons reside as charge balancing cations at Me 2+ -O-P bridges. H2 consumption rates measured during H2-TPR could be accurately described by Arrhenius-type behavior, and H2/Me ratios showed that only a fraction of all Me cations undergo reversible redox cycles. This fraction was 0.86 for MnAPO-18 (atomic Mn/P ) 0.05); it decreased from 0.68 to 0.40 for MnAPO-5 as Mn/P ratios increased from 0.028 to 0.10. For CoAPO-18 with 0.028 Co/P and CoAPO-5 with 0.40 Co/P, the fractions of redox sites were 0.64 and 0.40, respectively. UV-visible spectra showed no detectable Me 3+ features after thermal treatment in H2. Thus, cations that do not undergo redox cycles remain as permanently divalent cations throughout O2-H2 cycles. The redox-active fraction also decreased during repeated H 2-O2 redox cycles above 773 K, indicating that redox cations convert into permanently divalent sites. This conversion coincides with the evolution of H2O during H2 treatments above 773 K. These findings together with the effects of Me/P ratio on the redox fraction are consistent with a mechanism in which OH groups at divalent framework cation sites recombine to form H2O and an oxygen vacancy. Cyclohexanol and cyclohexanone formation rates during liquid-phase cyclohexane oxidation with O2 on MnAPO-5 samples increased linearly with the number of redox-active sites, suggesting that elementary steps of cyclohexane oxidation involve cycling between Me 2+ and Me 3+ , and require cation sites able to reversibly form charge balancing cationic species.