NaxCoO2 Research Articles

Activating the Basal Planes and Oxidized Oxygens in Layer-Structured Na0.6 CoO2 for Boosted OER Activity.

With the CoO2 slabs consisting of Co4 O4 cubane structure, layered Nax CoO2 are considered promising candidates for oxygen evolution reaction (OER) in alkaline media given their earth-abundant and structural advantages. However, due to the strong adsorption of intermediates on the large basal planes, Nax CoO2 cannot meet the activity demands. Here, a novel one-pot synthesis strategy is proposed to realize the high solubility of iron in Nax CoO2 in an air atmosphere. The optimist Na0.6 Co0.9 Fe0.1 O2 exhibits enhanced OER activity compared to their pristine and other reported Fe-doped Nax CoO2 counterparts. Such an enhancement is mainly ascribed to the abundant active sites on the activated basal planes and the participation of oxidized oxygen as active sites independently, which breaks the scaling relationship limit in the OER process. This work is expected to contribute to the understanding of the modification mechanism of Fe-doped cobalt-based oxides and the exploitation of layer-structured oxides for energy application.

Supported Ruthenium Single-Atom and Clustered Catalysts Outperform Benchmark Pt for Alkaline Hydrogen Evolution.

Guaranteeing satisfactory catalytic behavior while ensuring high metal utilization has become the problem that needs to be addressed when designing noble metal based catalysts for electrochemical reactions. Here, the well-dispersed ruthenium (Ru) based clusters with adjacent Ru single atoms (SAs) on layered sodium cobalt oxide (Ru/NC) is demonstrated to be a superb electrocatalyst for alkaline HER. The Ru/NC catalyst manifests an activity increase by a factor of two relative to the commercial Pt/C. Operando characterizations in conjunction with density functional theory (DFT) simulations uncover the origin of the superior activity and establish a structure-performance relationship, that is, under HER condition, the real active species are Ru SAs and metallic Ru clusters supported on the NC substrate. The excellent alkaline HER activity of the Ru/NC catalyst can be understood by a spatially decoupled water dissociation and hydrogen desorption mechanism, where the NC substrate accelerates the water dissociation rate, and the generated H intermediates would then migrate to the Ru SAs or clusters and recombine to have H2 evolution. More importantly, comparing the two forms of Ru sites, it is the Ru cluster that dominates the HER activity. This article is protected by copyright. All rights reserved.

Development of titanium and yttrium doped sodium-cobalt oxide cathods and their behaviour in cells with electrolytes based on NaTDI Hückel salt with NaPF6 salt additive

To meet the needs for the increasing demands for portable energy storages, the alternatives for Li-ion batteries must be researched. One of the ideas is sodium-ion technology, where the development of the novel cathodic material and electrolytes is crucial. In this paper, the new types of electrodes based on the sodium-cobalt oxide doped with yttrium or cobalt are presented. For testing them in cells, at first only two electrolytes were introduced (0.75 M NaTDI in EC:DMC and 0.75M NaTDI in EC:DMC+3% FEC). However, during experiments, the performance of some cells was poor, even after initiating the new protocol of cell assembly. That is why new formulations with the best conductivity performance, physicochemical stability, and interfacial contact with metallic sodium electrodes were used for further testing of cathodes. During the SEM, Raman, EDS, and XRPD analysis, the full characterization of the active materials and electrodes was performed and the best composition of electrode and electrolyte have been chosen basing on the electrochemical cycling of the half-cell, which achieved 143.64 mAh·g −1 discharge capacity value. Also, the metallic sodium electrode stability studies were performed to study the influence of the electrolyte on the anodic side of the cell.

Tracking sodium cobaltate formation pathways and its CO2 capture dynamics in real time with synchrotron X-ray diffraction

Time-resolved synthesis and CO2 capture dynamics of sodium cobaltate solid sorbent allowed to unveil chemical reactions occurring under realistic operative conditions.

Development of Titanium and Yttrium Doped Sodium-Cobalt Oxide Cathods and Their Behaviour in Cells with Electrolytes Based on Natdi Hückel Salt with Napf6 Salt Addititive

Development of Titanium and Yttrium Doped Sodium-Cobalt Oxide Cathods and Their Behaviour in Cells with Electrolytes Based on Natdi Hückel Salt with Napf6 Salt Addititive

Na0.78CoO2 as a novel microwave absorbing material

A novel microwave absorbing material, sodium cobaltates (Na0.78CoO2) was synthesized by solid-state reaction, which shows a good electromagnetic attenuate property. Because of preferential growth along the C axis, the Na0.78CoO2 powder reveals a typical lamellar structure. Theoretical calculations suggest that there is a minimum reflection loss (RL) around −51.7 dB at a frequency of 8.2 GHz in the composite with 50 wt% Na0.78CoO2 and 50 wt% paraffin when the thickness of the composite is about 2.41 mm. Moreover, its effective absorbing bandwidths (RL < −10 dB) are in a range from 12.5 to 15.6 GHz in the case of the sample thickness of around 1.5 mm. The electromagnetic attenuation mechanisms of the composite are relevant to conductive loss, multiple reflection loss, and dipole or interfacial polarization loss.

Lattice strain and atomic replacement of CoO6 octahedra in layered sodium cobalt oxide for boosted water oxidation electrocatalysis

Layered alkali metal oxides have been emerged as an alternative group of low-cost and promising electrocatalysts in water oxidation. The distinct layered configuration may offer some interesting possibilities to tune the intrinsic activity by regulating the intralayer edge-shared CoO6 octahedra and the CoO2 interlayer spacing/strain. In this work, electrochemical desodiation tuning method is explored on intralayer Ag, Cu, Ce-doped Na0.7CoO2 for highly active OER catalysts. It is demonstrated that the ΔGOH* value in the volcano plot is optimized by proper desodiation. Meanwhile, the lattice strain introduced along with the desodiated process modulates the ΔGOH*, according to first principle calculations. It shows that ∼0.157 % compressive strain in the CoO2 layers and ∼1% tensile strain between CoO2 layers are introduced in the desodiated Ag doped Na0.7CoO2. Among these catalysts, the desodiated Ag-Na0.7CoO2 sample exhibits an optimal RuO2-beyond water oxidation (OER) activity with the lowest overpotential of 236 mV@10 mA/cm2, the smallest Tafel slope of 48 mV/dec and the highest mass current density of 227.8 A/g. This work provides an interesting avenues to optimize existing layered materials with inter/intralayer modifications for highly efficient water oxidation electrolysis.

Incommensurately modulated crystal structure of α'(O'3)-type sodium cobalt oxide NaxCoO2 (x ∼ 0.78).

A single-phase sample of α' (O'3)-type layered sodium cobalt oxide NaxCoO2 (x ∼ 0.78) was prepared and its incommensurately modulated crystal structure was analyzed using the (3+1)-dimensional superspace approach to the powder neutron diffraction data. The crystal structure of the cobaltate is accurately described based on the superspace group C2/m(α0γ)00, wherein the positions of Na atoms are most significantly modulated in the monoclinic a direction to form an ordered arrangement. Such a displacive modulation causes a quasi-periodic shift of Na atoms from the centers of the NaO6 polyhedra between undulated CoO2 sheets, changing the form of the NaO6 polyhedron from an octahedral coordination (O) to a trigonal prismatic (P) one, via an intermediate capped trigonal prismatic NaO7 coordination (C). At the positions where the Na atoms are most significantly shifted, the neighboring Na atoms are located at almost touching distances. However, the occupation factor of Na atoms becomes zero at such positions, yielding Na-deficient sites VNa, sandwiched either between C and P, or C and C-type polyhedra.

The role of covalent bonding and anionic redox for the performance of sodium cobaltate electrode materials

It is common belief that conventional layered oxide battery cathode materials operate on a cationic redox mechanism, and that oxygen oxidation triggered by charge compensation results in oxygen loss and limited practical capacity. In this work, thin films of sodium cobaltate demonstrate a mixed redox behavior with strong oxygen participation well within the reversible intercalation range. This unexpected behavior is attributed to an increasing degree of covalency of the transition metal-oxygen (TM-O) bonds during deintercalation in combination with a high structural flexibility of the material. These insights are based on electronic structure data obtained using an operando solid state cell, allowing analysis with unpreceded control, and are supported by density functional theory (DFT) calculations. They demonstrate that the spectrum of possible charge compensation mechanisms is broader than commonly believed, and indicate that tuning TM-O covalency is a key factor for increasing the capacity also of conventional layered materials.

Improved environmental stability of thermoelectric ceramics based on intergrowths of Ca3Co4O9–Na0.75CoO2

Ceramics based on calcium and sodium cobaltates are promising high-temperature thermoelectric oxide materials with complementary advantages. Ca3Co4O9 is stable at high temperatures, whereas Na0.75CoO2 can be easily processed as a textured ceramic with excellent thermoelectric properties. Na0.75CoO2, however, lacks long-term stability and degrades in even a relatively mild humid environment. In this work, we present a novel approach to the synthesis of complex composite materials based on intergrowths of sodium and calcium cobaltates that have excellent thermoelectric performance and improved stability. We synthesized samples with the mixed composition (3-x)Ca3Co4O9–4x(Na0.75CoO2) in an over-pressured oxygen atmosphere. Samples with the mixed Ca–Na composition developed textured microstructures composed of intergrowths of both end-members, as revealed by transmission electron microscopy. We also examined the thermoelectric performance of the investigated materials after exposure to high humidity and found that the composition with x = 0.8 (Ca:Na = 2.75) has long-term stability.

Research Progresses of Sodium Cobalt Oxide as Cathode in Sodium Ion Batteries

Research Progresses of Sodium Cobalt Oxide as Cathode in Sodium Ion Batteries

Microstructure and Thermoelectric Characterization of Composite Nanofiber Webs Derived from Polyacrylonitrile and Sodium Cobalt Oxide Precursors

We report the microstructure and thermoelectric properties of composite nanofiber webs, which were fabricated by dual-electrospinning of polyacrylonitrile (PAN) and sodium cobalt oxide (NaCo2O4) precursor solutions with different input compositions and following heat-treatment at 600–900 °C for simultaneous carbonation and calcination. The SEM and EDS mapping images revealed that PAN-derived carbon nanofibers (CNFs) and NaCo2O4-based ceramic nanofibers coexisted in the composite nanofiber webs and that their relative contents could be controlled by the input compositions. The Seebeck coefficient increased from ~26.77 to ~73.28 μV/K and from ~14.83 to ~40.56 μV/K with increasing the relative content of NaCo2O4 nanofibers in the composite nanofiber webs fabricated at 700 and 800 °C, respectively. On the other hand, the electrical conductivity of the composite nanofiber webs increased with the decrement of the relative content of NaCo2O4 nanofibers as well as the increment of the heat-treatment temperature. Owing to the opposite contributions of NaCo2O4 nanofibers and CNFs to the Seebeck coefficient, electrical conductivity and thermal conductivity, a maximum power factor of ~5.79 μW/mK2 and a figure of merit of ~0.01 were attained for CNF/NaCo2O4-based composite nanofiber webs fabricated at 45 wt% input composition of NaCo2O4 and at heat-treatment of 700 °C.

Original Layered OP4-(Li,Na)xCoO2 Phase: Insights on Its Structure, Electronic Structure, and Dynamics from Solid State NMR.

The OP4-(Li/Na)xCoO2 phase is an unusual lamellar oxide with a 1:1 alternation between Li and Na interslab spaces. In order to probe the local structure, electronic structure, and dynamics, 7Li and 23Na magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was performed in complementarity to X-ray diffraction and electronic and magnetic properties measurements. 7Li MAS NMR showed that NMR shifts result from two contributions: the Fermi contact and the Knight shifts due to the presence of both localized and delocalized electrons, which is really unusual. 7Li MAS NMR clearly shows several Li environments, indicating that, moreover, Co ions with different local electronic structures are formed, probably due to the arrangement of the Na+ ions in the next cationic layer. 23Na MAS NMR showed that some Na+ ions are located in the Li layer, which was not previously considered in the structural model. The Rietveld refinement of the synchrotron XRD led to the OP4-[Li0.42Na0.05]Na0.32CoO2 formula for the material. In addition, 7Li and 23Na MAS NMR spectroscopies provide information about the cationic mobility in the material: Whereas no exchange is observed for 7Li up to 450 K, the 23Na spectrum already reveals a single average signal at room temperature due to a much larger ionic mobility.

Hf doping for enhancing the thermoelectric performance in layered Na0.75CoO2

Abstract Doping has been extensively used as the main strategy for improving the thermoelectric performance of sodium cobaltate (NaxCoO2). Here, by examining the formation energy of Hf dopant in Na0.75CoO2, we demonstrate that under Na and O rich environments, Hf replaces a Co while in all other conditions Hf replaces a Na. We then discuss the critical dependence of the thermoelectric performance of Na0.75CoO2:Hf on the Hf location in the host lattice by examining how the spin entropy flow is influenced by Hf doping. Our results indicate that Hf dopant at Co site increase the Seebeck coefficient to 274 μV/K while Hf dopant at Na site increases the Seebeck coefficient to 388 μV/K.

From molecules to nanoparticles to functional materials

Functional nanomaterials have held a steady position at the frontier of materials science and engineering in the 21st century. ?Molecular Designing of Nanoparticles with Controlled Morphological and Physicochemical Characteristics and Functional Materials Based on Them? was the title of the research project funded by the Ministry of Education, Science and Technological Development of the Republic of Serbia and performed between 2011 and 2019 in the interdisciplinary area of nanoscience and nanotechnologies. Research activities within this program were divided into five interrelated topics: 1) from molecules to nanoparticles; 2) advanced ceramics with improved functional properties; 3) electrode materials for lithium?ion batteries; 4) nano-calcium phosphate in preventive and regenerative medicine; 5) biodegradable microand nano-particles for the controlled delivery of medicaments. This report gives an insight into this bibliographically most impactful Serbian national project on nanotechnologies executed within the aforementioned nine-year cycle, 2011?2019, focusing here only on the results achieved in the past three years. The project provided an outstanding and internationally recognized contribution to synthesis, characterization and functional design of a number of materials systems, including pure and lanthanide?doped hydroxyapatite, zinc oxides, sodium cobaltates, lithium iron pyrophosphates, lithium iron silicates and a number of polymeric systems.

Na site doping a pathway for enhanced thermoelectric performance in Na1−xCoO2; the case of Gd and Yb dopants

Doping is considered to be the main method for improving the thermoelectric performance of layered sodium cobaltate (Na1−xCoO2). However, in the vast majority of past reports, the equilibrium location of the dopant in the Na1−xCoO2’ complex layered lattice has not been confidently identified. Consequently, a universal strategy for choosing a suitable dopant for enhancing Na1−xCoO2’s figure of merit is yet to be established. Here, by examining the formation energy of Gd and Yb dopants in Na0.75CoO2 and Na0.50CoO2, we demonstrate that in an oxygen poor environment, Gd and Yb dopants reside in the Na layer while in an oxygen rich environment these dopants replace a Co in CoO2 layer. When at Na layer, Gd and Yb dopants reduce the carrier concentration via electron–hole recombination, simultaneously increasing the Seebeck coefficient (S) and reducing electric conductivity (σ). Na site doping, however, improves the thermoelectric power factor (PF), only in Na0.50CoO2. When replacing a Co, these dopants reduce S and PF. The results demonstrate how thermoelectric performance critically depends on the synthesis environment that must be fine-tuned for achieving any thermoelectric enhancement.

CO2–CO capture and kinetic analyses of sodium cobaltate under various partial pressures

Sodium cobaltate and some Fe-containing samples were evaluated on the CO, CO2 and CO–CO2 sorption at high temperatures and low CO2 partial pressures, in the presence and absence of oxygen. Initially, CO2 chemisorption on these samples was analyzed using different PCO2. Results indicated that all the samples were able to chemisorb CO2 (if PCO2 ≥ 0.2), where Fe-containing NaCoO2 samples clearly showed higher CO2 chemisorption efficiencies than pristine NaCoO2. These results were explained by the partial iron reduction and the consequent oxygen release. When oxygen was added the chemisorption process was improved as a result of an iron reduction–oxidation mechanism. These results were confirmed kinetically by the Jander-Zhang and Eyring models. The temperature for complete CO catalytic conversion was shifted to lower temperatures as a function of iron content. Finally, simultaneous CO2 and CO sorption as well as catalytic experiments were tested (in absence or presence of O2). These results showed that CO was mainly oxidized and chemically captured, over the CO2 direct capture, in oxygen absence and presence. Iron is able to release and capture oxygen by reduction–oxidation effect and facilitates oxygen dissociation for the carbonation process, through the Mars van Krevelen reaction mechanism.

Development of Sodium-Ion Rechargeable Battery Using Sodium Cobalt Phosphate Cathode

Lithium-ion batteries are the most popular kind of rechargeable batteries accommodate in portable electronic devices up to date. As the Lithium deposits are depleting the cost of Lithium-ion batteries is increasing. Sodium- ion batteries can be introduced as an alternative technology which can replace expensive Lithium-ion batteries. Sodium sources are highly abundant and therefore Sodium-ion batteries could be made cheaper than Lithium-ion batteries. A number of cathode materials which were accommodated in Lithium-ion batteries have also been tested as cathode materials for Sodium-ion batteries. This research was based on a Sodium-ion battery which cathode was prepared using Sodium cobalt phosphate. The cathode material was prepared using a simple solid-state reaction between Cobalt (II) oxide and Sodium phosphate. The prepared material was characterized using powder XRD. Chargedischarge cycles, cyclic voltammetry analyzing, impedance curve matching to obtain equivalent circuit was used in order to analyze the performance of the prepared cathode in the battery. The discharge capacity of the cathode was calculated as 9.58 mA h g-1. The cyclic voltammetry curve has shown that an oxidation and reduction processes involved in the battery cycle but the battery cycle was not completely reversible. KEYWORDS: Rechargeable batteries, Sodium cobalt oxide, cyclic voltammetry, charge-discharge cycles.

London Penetration Depth as a Test of Order Parameter Symmetry in Sodium Cobaltate Superconductors

Temperature dependence of the magnetic field penetration depth λ was calculated for water intercalated sodium cobaltate superconductor Na x CoO 2 · y H 2 O. Assuming that the system is in the chiral d+id–wave superconducting state, it was shown that the shifting of the excitation spectrum nodal points off the normal phase Fermi surface due to variation of the sodium content x changes the functional form of the temperature dependence of λ − 2 from exponential to linear in the low temperatures region. It is argued that this change in the functional form of T–dependence of the λ − 2 can serve as a proof for the chiral symmetry of the superconducting order parameter in the sodium cobaltate.

Enthalpy of Formation of NaxCoO2 and (Na, Mg)xCoO2 Systems: A First Principle Calculation Study

Enthalpy of formation of NaxCoO2 (NCO) compounds with 0 < x < 1 and partially substituted Na+ ions with Mg2+ ions, (Na, Mg)xCoO2 (NMCO) has been calculated using full potential-linear augmented plane wave method within density functional theory. The enthalpy of formation of NCO was found to decrease as sodium concentration, x increases. We found that the enthalpy of formation decreases more rapidly in region of 0 < x < 0.55 and slower in 0.55 < x < 1, which is indicating the presence of two different stability region against sodium concentration. The substitution of Mg2+ into Na+ sites decreases the enthalpy of formation of NCO compound and suggests Mg2+ substitution could increase the stability of NCO compound. The Mg2+ substitution also affect the lattice parameters. In the electronic structure, Mg2+ substitution is found to lower the position of unfilled Co-3d band for about 0.2 eV compared to the NCO compound. The computational results also indicate that the Mg2+ substitution will not affect Seebeck coefficient especially at high temperature.