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Here, we demonstrate a material design strategy to enhance the electrocatalytic activity for oxygen evolution reaction (OER) by repetitive charge/discharge supercapacitor cycling of Co3V2O8. The charge/discharge cycle helps to alter the electronic structure via stabilizing more active components on the surface and increases the electronic conductivity by increasing the oxygen vacancies. X-ray photoelectron spectroscopy and X-Ray Diffraction assists in identifying the chemical states and the structure of different components. The electrochemical techniques suggest that an increase in Co3+ on the surface with the available oxygen vacancies at 2500 charge/discharge cycled sample promotes intermediates like CoOOH, which help in decreasing the overpotential for the OER. We report specific current density with respect to the electrochemical surface area (ECSA), and our best sample shows a specific activity of ~6.2 µA/cm2ECSA with Tafel slope 85.12mV/dec. Further, the reaction mechanism and different interfacial resistances are identified with the help of electrochemical impedance spectroscopy. This study suggests that an optimum number of charge/discharge cycles are needed to balance the competing factors of electrocatalysts activation and decomposition of the active material. © 2020 Elsevier Ltd

World University root node

Department of Chemical Engineering

Electrochimica Acta

Surface-enhanced OER activity in Co3V2O8 using cyclic charge-discharge to balance electrocatalytic active site generation and degradation

Evolution in material centric devices like batteries and electrocatalytic reactors have predominantly been made possible via the exploitation of the thermodynamic ground state of pristine or defective bulk crystal, referred to as the “Native polymorph” (NP) here. A significant increase in the material search space is possible by utilizing “Non-Native polymorphs (NNP),” which are materials that have different translational symmetry with respect to NP. As the NNP have a distinct coordination structure from that of the NP, critical material properties can be anticipated to be different, making NNP a potential substitute material for the aforementioned applications, which are the focus of this review. To obtain a structure–function relationship, systematic approaches to the synthesis of NNP has been demonstrated. Following certain generalities behind NNP, we classify synthesis techniques into few categories with the hope of rationalizing the underlying mechanism of these synthesis and stabilization strategies. We discuss the utility of NNPs in the context of electrochemical water electrocatalytic reactions. Typically, the NNPs have more open volume space enabling lower lithium-ion diffusion barrier, higher lithium-ion binding energies, thereby making NNP efficient in the context of energy storage material. However, NNP have lesser stability than the NP and methods to calibrate and improve the stability of NNP are important. Overall, the discussion of polymorphic materials by demarcating them as NP and NNP provides a systematic approach towards modulating material properties as a trade-off between thermodynamics and kinetics of physicochemical processes. Finally, the challenges and perspectives in this emerging field are discussed. This article is categorized under: Fuel Cells and Hydrogen > Science and Materials Energy Research & Innovation > Science and Materials Energy and Development > Science and Materials. © 2020 Wiley Periodicals LLC

Wiley Interdisciplinary Reviews: Energy and Environment

Stabilization of non-native polymorphs for electrocatalysis and energy storage systems

We explore the effect of sulfur doping in Co3V2O8 on the electrochemical performance of supercapacitors in terms of enhanced lattice spacing, electrochemical surface area (ECSA), electronic conductivity, and density of states. The sulfur-doped Co3V2O8 (S-Co3V2O8) nanosheets were grown in situ as a binder-free synthesis on nickel foam by the hydrothermal method. The electrochemical performance was analyzed in an aqueous alkaline electrolyte where the S-Co3V2O8 electrode exhibited a specific capacity of 410 mAh/g at 2 A/g with enhanced rate capability and capacity retention of 94.2% at 5 A/g specific currents after 4000 cycles, whereas the undoped Co3V2O8 electrode exhibited a specific capacity of 337.8 mAh/g at 2 A/g. The high capacity of S-Co3V2O8 is attributed to the enhanced ECSA (by ∼45%) and improved electrical conductivity (by ∼22%) upon doping with sulfur. Furthermore, these results corroborated with density functional theory results. The calculations suggest an increase in lattice parameters and the introduction of additional density of states near the valence-band edge due to the doping of sulfur, resulting in a decrease in charge-transfer resistance. An asymmetric supercapacitor was fabricated, using S-Co3V2O8 nanosheets as the cathode and activated carbon as the anode, which shows a high specific capacity (or capacitance) value of 485 mAh/g, an energy density of 36.4 W h/kg, and a power density of 740 W/kg at 2 A/g with 98.4% specific capacity retention after 4000 consecutive charge-discharge cycles. Furthermore, the analysis was extended in a nonaqueous medium for Li-ion storage where S-Co3V2O8 exhibits a specific capacity of 994 mA h/g and a specific energy density of 828 W h/kg at 1 A/g making it a promising candidate for future high-energy storage systems. The concept of doping is extended to other chalcogenide (e.g., selenium) doping (Se-Co3V2O8), which also shows improved device performance and makes this a versatile approach for high-performance devices. ©

ACS Applied Energy Materials

Chalcogenide Dopant-Induced Lattice Expansion in Cobalt Vanadium Oxide Nanosheets for Enhanced Supercapacitor Performance

Non-native structures have more open structure with less stable bulk formation energy compared to their native structure and hence, non-native structured cathodes are expected to generate higher discharge potential and better lithium mobility. However, non-native structures are limited in stability, which can be enhanced by scaffolding with native structure. We synthesize highly crystalline non-native Ramsdellite MnO2 (r/NN1–MnO2) and gradually thermal phase transform to native Pyrolusite MnO2 (β/N–MnO2), intimately interfaced (r/NN1–MnO2)/(β/N–MnO2) intergrowth intermediate structures are realized. As the temperature increases, the ratio of r/NN1–MnO2 to β/N–MnO2 in intergrowth structure decreases, so does the discharge potential and lithium mobility. The wider discharge plateau for intergrowth structures is rationalized using density functional simulations followed by statistical averaging to account for interfacial intercalation sites with greater stability. Improved capacity retention for intergrowth structure is due to the higher structural stability and available free volume, where the presence of native structure acts as a stabilizing element. Electrochemical impedance spectroscopy correlates faster transport in the intergrowth structure with higher percentage of non-native phase with wider 2 × 1 channel compared to native phase with narrow 1 × 1 channel. The insights generated from this study have broad implications for interface engineering in multicomponent electrodes such as Li-rich-NMC oxides. © 2019 Elsevier B.V.

Journal of Power Sources

Higher voltage, wider voltage plateau, longer cycle life, and faster kinetics via thermally modulated interfaces between Ramsdellite and Pyrolusite MnO2 for lithium-ion battery cathodes

The rate capability of Li-ion batteries is crucially influenced by Li-ion diffusion within bulk electrodes and electrolyte, and Li-ion transfer across the electrode-electrolyte interface. Relative magnitudes of both bulk and interfacial diffusion energy barriers are essential in order to identify the rate-limiting component in overall Li-ion transport process. Here, we quantitatively estimate migration energy barrier in bulk cathode and at the cathode-electrolyte interface through density functional theory based nudged elastic band calculations. We study a Li-ion transfer pathway wherein a Li-ion solvated by the first coordination shell of the electrolyte changes its coordination structure to that in cathode sub-surface sites, and electrolyte molecules act as ligands for Li-ion. We compute the energy barrier for Li-ion transfer from ethylene carbonate/LiPF6 electrolyte into (010)-surface of LiFePO4 to be 756meV. We explore the effect of partial fluorination of interface due to electrolytic reactions and find a drastic reduction in energy barrier to 410meV. Nevertheless, the energy barrier for interfacial ion-transfer is much higher than that for diffusion through bulk one-dimensional channels of olivine LiFePO4, which is estimated at 290meV. The higher energy barrier for interfacial Li-ion transfer suggests a rate-limiting behavior of interface in overall kinetics of ion transport in Li-ion batteries. © 2019 The Electrochemical Society.

Journal of the Electrochemical Society

Higher energy barrier for interfacial Li-ion transfer from EC/LiPF6 electrolyte into (010) LiFePO4 cathode surface than bulk Li-ion diffusion within both cathode and electrolyte

Manuscript reports on ferrocenium ion entangled in polyelectrolyte matrix as a redox platform for oxidative and reductive sensing of nitric oxide (NO). It was prepared by oxidation of ferrocene in diethyl ether followed by extraction with polystyrene sulfonic acid (PSS) and Nafion® solution. The presence of stable ferrocenium cation was confirmed by FT-IR, UV-visible spectra, 1 HNMR and XRD. It results into a stable cyclic voltammogram in 100 mM Phosphate buffer, pH 7 solution with Ag/AgCl reference and Pt wire counter electrodes. The electrode sensitivity towards NO was confirmed through the increase in oxidative current at +750 mV and reductive current at −800 mV by the addition of 100 μL saturated NO solution. The chronoamperometric response for NO oxidation followed linearity in the concentration range of 200 nM to 3.5 μM. The lower detection limit and sensitivity were found to be 200 nM and be 33.29 μA/μM/cm 2 respectively. The reductive response also followed a linear trend in the concentration range of 100 μM to 1.2 mM with 15.15 μA/μM/cm 2 sensitivity and 100 μM detection limit. It indicates ferrocenium ion entanglement within polyelectrolyte matrix and its viability as a redox platform for oxidative as well as reductive sensing of nitric oxide. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ChemistrySelect

Ferrocenium Ion Confinement in Polyelectrolyte for Electrochemical Nitric Oxide Sensor

Enhanced electrocatalytic activity of the oxygen evolution reaction (OER) can be achieved through modulation of the electronic structure of the electrocatalytic active sites. This modulation can also be achieved through stabilization of metastable or non-native polymorphs of the electrocatalyst. Non-native (NN) crystal structures differ in their discrete translational symmetry from the bulk native (N) crystal. The variable oxygen evolution reactivity in a basic medium of different polymorphs is demonstrated by synthesizing β/N-, γ/N-NN1-, r/NN1-, α/NN2-, and δ/NN3-MnO2 polymorphs of MnO2 which show different active site densities on the surface, XPS-derived oxidation state of Mn, and bulk electronic conductivity. The specific OER activity [activity per electrochemical surface area (ECSA)] of MnO2 is codependent on the oxidation state of Mn and electronic conductivity. A volcano-based relationship is observed for the specific OER activity of MnO2 polymorphs with the universal descriptor ΔGO∗ - ΔGHO∗, computed from density functional theory (DFT). Both δ/NN3-MnO2 and α/NN2-MnO2 lie closer to the volcano peak but on opposite legs of the volcano. δ/NN3-MnO2 shows higher specific activity due to its low oxidation state (+3.5) of Mn, which is confirmed through the calculated average oxidation states (AOS) from XPS and Bader charge from DFT studies. α/NN2-MnO2 shows higher specific activity due to higher electronic conductivity, which is correlated with its low oxygen vacancy formation energy. Further, the electronic origin of high OER activity in two of the most non-native polymorphs (δ/NN3-MnO2 and α/NN2-MnO2) is ascribed to a shift in the valence Mn-d band closer to the Fermi level leading to stronger O adsorption. The present study demonstrates the efficacy of utilizing non-native polymorphs for OER and unfolds the correlation between OER and variable oxidation states and electronic conductivities, thereby providing directions toward the generalization of these effects to other polymorphic compounds. © 2019 American Chemical Society.

Journal of Physical Chemistry C

Modulating Oxygen Evolution Reactivity in MnO2 through Polymorphic Engineering

The activity of electrocatalysts critically depends on the chemical coordination around the active sites. Amorphous materials have short-range atomic ordering while their crystalline counterparts have both short and long-range ordering. Traditional synthesis of amorphous materials, involving quenching from high temperatures is unsuitable as it results in less porosity and surface area. In this context, room-temperature syntheses of high surface area amorphous materials with high activity are desirable. Here, we contrast two electrochemical synthesis procedures for generating high surface area amorphous Co3O4 at room temperature via electrochemical ion intercalation/deintercalation and surface oxidation/reduction cycles and evaluate their performance for electrocatalytic oxygen evolution reaction (OER). In the first approach, Li-ion is used for the intercalation/deintercalation (Li/D−Li) cycles in Co3O4, which leads to expansion and contraction of structure, inducing amorphization of Co3O4 by the pulverization of crystal structure in non-aqueous medium. In the second approach, rapid electrochemical surface oxidation/reduction (Ox/Red) of Co3O4 in the aqueous medium leads to the formation of a metastable amorphous structure. The OER specific activity (activity per unit electrochemical surface area) for Li/D−Li−Co3O4 is ∼3.5 times and Ox/Red- Co3O4 induced amorphization is ∼2.5 times higher than their crystalline Co3O4. The superior OER metrics of both the room-temperature amorphization techniques are rationalized via the increase in the ratio of Co2+/Co3+ obtained from the Co-2p XPS spectra. Further, the decrease in overall polarization resistance per site for the OER reaction for both amorphous samples were analyzed from the Tafel plot and electrochemical impedance spectroscopy (EIS). In Li/D−Li−Co3O4, the Li-ion intercalation in bulk Co3O4 structure generates higher bulk-oxygen vacancies leading to higher conductivity and reduction in overall charge-transport resistance for electrocatalyst. On the other hand, Ox/Red- induced amorphization is restricted to the surface or near-surface only with the formation of a small amount of metallic Co which hampers the OER. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ChemElectroChem

Electrochemical Cycling-Induced Amorphization of Cobalt(II,III) Oxide for Stable High Surface Area Oxygen Evolution Electrocatalysts

Ni3(VO4)2 nanosheet arrays are grown on aminated r-GO sheets through a facile hydrothermal route. The growth of Ni3(VO4)2 nanosheet arrays on aminated r-GO sheets (NiV@r-GO nanocomposites) is confirmed by XRD, FESEM, TEM, XPS, and Raman spectroscopy. The length, height and thickness of the Ni3(VO4)2 nanosheet arrays are ∼100, ∼20 and ∼10 nm, respectively. The density of the Ni3(VO4)2 nanosheet arrays on the r-GO sheets is directly related to the concentration of precursor used. The generation of mesoporosity during synthesis originates from the high surface area in the NiV@r-GO nanocomposites. The high surface area causes the mesoporous NiV@r-GO nanocomposite based electrode to exhibit a specific capacity of 170 C g-1 at 0.5 A g-1, which is approximately three times higher than that of aminated r-GO sheets. The capacity retention of the NiV@r-GO nanocomposite is 97.2% at 0.5 A g-1 current density after 1400 cycles. Other than the faradic redox reaction, the superior electrochemical performance of NiV@r-GO nanocomposites over the aminated r-GO sheet based electrode is related to the three-dimensional (3D) morphology as an effect of the Ni3(VO4)2 nanosheet array, which results in a higher surface area, more active sites and greater electrolyte penetration in the electrode. © 2018 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

New Journal of Chemistry

Free-standing Ni3(VO4)2 nanosheet arrays on aminated r-GO sheets for supercapacitor applications

Transition metal oxides are the primary choice for cathode materials in lithium ion batteries. These oxides occur as different polymorphs that have varied structures, stabilities, and affinities for Li+, and consequently, it is desirable to develop heuristics for the choice of the optimal polymorph. MnO2 is the most extensively used cathode material for lithium ion battery, and it exists in more than ten polymorphic forms. Among them, γ-MnO2, the most electrochemically active polymorph, is a mixture of pyrolusite and ramsdellite polymorphs. Here we have focused on the less explored ramsdellite MnO2 (R-MnO2). Highly crystalline R-MnO2 has been synthesized, and the experimentally obtained discharge features are compared with the calculations based on density functional theory followed by cluster expansion to obtain molecular insights into the intercalation process. R-MnO2 shows voltage fading beyond x = 0.5 (in LixMnO2). Computational results suggest that change in Li environment from tetrahedral to octahedral beyond x = 0.5 (in LixMnO2) causes the voltage fading. This change in Li environment is also correlated with experimentally obtained Mn 2p, O 1s, and valence band XPS spectra. The shift in the d-band center, plotted for valence band spectra at different lithium concentrations, is adduced for the migration of lithium ion from the tetrahedral site to the less favorable octahedral site beyond x = 0.5. Volume expansion of about 20% at full lithiation is accompanied by Jahn-Teller distortion of MnO6. Further, computations reveal the diffusional energy barrier to be 200 meV in the limit of dilute Li concentration (x = 0.125) and 481 meV in the limit of dilute vacancy concentration (x = 0.875). A structural rationale for the energy barrier is developed. The experimental and computational studies provide insight into the lithiation mechanism in R-MnO2 and are relevant to rationalize the high performance of the intergrowth structure of γ-MnO2. We discuss a wider implication of the study by correlating the voltage profile with structural changes in MnO2 polymorphs, which gives general design principles to make better cathodes through polymorphic engineering. © 2018 American Chemical Society.

Correlating Voltage Profile to Molecular Transformations in Ramsdellite MnO2 and Its Implication for Polymorph Engineering of Lithium Ion Battery Cathodes

Binary transition metal oxides have attracted wide attention in Li-ion batteries due to their multivalent electronic states and improved electrical conductivity. In this context, we have, for the first time, successfully synthesized reduced graphene oxide- nickel tungstate (RGO/NiWO4) nanocomposites by one pot hydrothermal method and employed them as anode for Li-ion battery. Scanning and transmission electron microscopic images show complete dispersion of NiWO4 nanoparticles (NPs) of size 20-50 nm on reduced graphene oxide sheets. The synthesized nanocomposites RGO/NiWO4 (1-3) (RGO:NiWO4 wt ratios of 50:50, 25:75 and 15:85) exhibit enhance electrochemical performance and cycling stability as compared to bare NiWO4 NPs. The electrodes made with bare NiWO4, RGO/ NiWO4-1, RGO/ NiWO4-2 and RGO/ NiWO4-3 nanocomposites deliver specific capacities of 67, 578, 647 and 498 mAh g-1 respectively at 100 mA g-1 current density after 100 cycles achieving 98% coulombic efficiency. The improved cycling performance of nanocomposite is attributed to the incorporation of conducting graphene nanosheets, which not only act as flexible confinements but also prevent the detachment, agglomeration and pulverization of NiWO4 NPs. The excellent electrochemical performance of nanocomposite electrode in terms of specific capacity, cycling stability and rate capability makes it an ideal anode material for lithium ion battery. © 2017 The Electrochemical Society. All rights reserved.

Hydrothermally synthesized reduced graphene oxide-NiWO4 nanocomposite for lithium-ion battery anode

The central governing factors that influence the efficiency of photoelectrochemical (PEC) water splitting reaction are photon absorption, effective charge-carrier separation, and surface electrochemistry. Attempts to improve one of the three factors may debilitate other factors and we explore such issues in hydrogenated TiO2, wherein a significant increase in optical absorption has not resulted in a significant increase in PEC performance, which we attribute to the enhanced recombination rate due to the formation of amorphization/disorderness in the bulk during the hydrogenation process. To this end, we report a methodology to increase the charge-carrier separation with enhanced optical absorption of hydrogenated TiO2. Current methodology involves hydrogenation of non-metal (N and S) doped TiO2 which comprises (1) lowering of the band gap through shifting of the valence band via less electronegative non-metal N, S-doping, (2) lowering of the conduction band level and the band gap via formation of the Ti3+ state and oxygen vacancies by hydrogenation, and (3) material processing to obtain a disordered surface structure which favors higher electrocatalytic (EC) activity. This design strategy yields enhanced PEC activity (%ABPE = 0.38) for the N-S co-doped TiO2 sample hydrogenated at 800 °C for 24 h over possible combinations of N-S co-doped TiO2 samples hydrogenated at 500 °C/24 h, 650 °C/24 h and 800 °C/72 h. This suggests that hydrogenation at lower temperatures does not result in much increase in optical absorption and prolonged hydrogenation results in an increase in optical absorption but a decrease in charge carrier separation by forming disorderness/oxygen vacancies in the bulk. Furthermore, the difference in double layer capacitance (Cdl) calculated from electrochemical impedance spectroscopy (EIS) measurements of these samples reflects the change in the electrochemical surface area (ECSA) and facilitates assessing the key role of surface electrochemistry in PEC water splitting reaction. Additionally, we observed a blue-shift of the absorption spectrum and a decrease in both electrochemical (EC) and photoelectrochemical (PEC) activities after the removal of surface layers through focused ion beam (FIB) sputtering suggesting the importance of surface defects and photon absorption. © the Owner Societies 2016.

Physical Chemistry Chemical Physics

Coupled optical absorption, charge carrier separation, and surface electrochemistry in surface disordered/hydrogenated TiO2 for enhanced PEC water splitting reaction