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The abundant resource of sodium (Na) makes the Na-ion batteries a promising alternate to the Li-ion batteries in electric energy storage, especially when iron (Fe)-based oxide electrodes are used. Layer-structured NaxFeyM1-yO2 (M for Mn and Ni, etc.), for example, has drawn much attention and is one type of the most attractive cathode materials for the Na-ion batteries. However, the poor cycling performance hinders their applications and the underneath mechanism for their poor performances remains unclear. In this study, the structural transition of NaFeO2 was characterized and the driving forces for the irreversible Fe migration and O oxidation were explored during Na extraction. The Fe migration from the Fe layer to the Na layer was observed for the first time at the atomic scale. The chemical states and coordination environment of the Fe ions within the Na layer were determined. The density functional theory (DFT) calculations were carried out to understand the Fe migration and O oxidation. These findings provide helpful revelation on constructing the structure and improving the performance of the NaFeO2-structured cathode materials for the Na-ion batteries.

Exploring earth-abundant electrocatalysts to realize efficient oxygen evolution reaction (OER) is highly desired for developing sustainable electrochemical energy storage and conversion technologies. Herein, ultrathin single-crystalline Fe-doped nickel thiophosphate (NiPS3) nanosheets prepared in large scale by an easy solid-state method were demonstrated to be highly efficient OER electrocatalysts. The density functional theory (DFT) calculations reveal that the Fe-doping effectively decreases the energy barrier of OER path by reducing the binding of the oxygen-containing species on the surface of NiPS3. As such, the Fe-doped NiPS3 nanosheets show a low overpotential of 256 mV to reach a current density of 30 mA cm−2 and a small Tafel slope of 46 mV dec−1. To our knowledge, this is one of the best OER electrocatalysts in alkaline medium to date. The in-depth mechanism study demonstrates that the in-situ formed Fe-doped nickel oxides/hydroxides shell, resulting from the surface oxidation during the OER process, not only may serve as favorable electrocatalytic species but also improves the chemical stability of the Fe-doped NiPS3 in alkaline electrolyte. This work provides a new perspective for designing highly efficient OER electrocatalysts based on the ternary two-dimensional layered metal thiophosphates. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore)

NASICON-type (Na super ionic conductor) Na4VMn(PO4)3 (NVMP) cathode material has attracted increasing attention due to its higher sodium de-/intercalation voltage, lower cost and greener resources compared with Na3V2(PO4)3. However, the poor electronic conductivity limits the exploitation of its electrochemical performance. In this work, we achieved an ultrahigh rate capable and long cycle life NVMP/C-rGO composite using the simple and scalable spray-drying technique, in which low-fraction rGO nanosheets (2.14 wt%) highly dispersed and bridged the carbon shelled NVMP nanoparticles to form an effective electron conductive network. Benefiting from this favorable nano-architecture, the elaborately designed NVMP/C-rGO composite exhibited excellent performance as cathode in sodium ion batteries, delivering reversible capacities of 105.5 mA h g−1 at 1 C (1 C = 110 mA h g−1) and 102.6 mA h g−1 at 50 C with capacity retention of 90.6% (1 C) and 80.4% (50 C) over 500 cycles, respectively. This is the highest rate performance for Na4VMn(PO4)3 materials reported so far. This work provides a practicable tactic for fabricating advanced polyanion-type cathode materials which can be commercially scale up for sodium energy storage.

Harvesting solar energy for vapor generation has attracted large amount of attention due to its promise for applications in water purification, desalination, power generation, and so on. Many structures based on the two-layered design or the plasmonic enhanced evaporation have been reported to promote the efficiency of solar vapor generation. Inspired by the transpiration phenomenon in plant, we report that a capillary-driven pump can be used for highly efficient solar vapor generation. The pump is mainly consisted of a porous hydrophilic modified NiO (M-NiO) disc and a one-dimension water supply channel. The M-NiO with a three-layered structured TiAlON-based nanocomposite absorber deposited on its surface can efficiently capture solar radiation (absorptance of 0.97). Driven by the capillary force in the porous M-NiO, the pump can continuously wick water via the one-dimension channel to the solar absorber for evaporation, achieving solar-to-vapor efficiency of 73% at 1 sun and 90% at 4 suns. The high conversion efficiency can be attributed to the high absorption ability of the M-NiO disc and the one-dimension water supply design to limit the thermal loss. This new design with advantage of easy scale-up provides an efficient approach to harvest sunlight for solar vapor generation in low solar concentrations.

Few-layer black phosphorus (BP) is a promising anode material for sodium ion batteries (SIBs) due to its high theoretical capacity and favorable layered structure. However, practical implementation is hindered by sluggish reaction kinetics and large volume change during de/sodiation process. Especially, combining BP with large portion of low-capacity carbonaceous materials is a common strategy to improve the Na storage properties, but leading to reduced specific capacity based on the overall mass of the whole electrode. To address these challenges, nanoscale surface engineering of few-layer BP is herein performed by homogeneously depositing horizontally aligned Poly(3, 4-ethylenedioxythiophene) (PEDOT) nanofibers on specially surface-modified BP nanosheets. Such material design could achieve simultaneously: (1) enhanced charge transfer kinetics and (2) super surface wettability with electrolyte. Benefiting from the unique functionalization, the reaction kinetics are greatly enhanced accordingly for both sodium and lithium storage. Our strategy sheds light on designing advanced electrodes for high-performance rechargeable batteries and other energy storage/conversion devices.

Photocatalytic CO2 reduction has been regarded as an appealing pathway for CO2 conversion to hydrocarbon fuels. To boost the CO2 photoreduction performance, developing suitable cocatalyst on the photocatalysts is an efficient strategy. Herein, Co2N is employed as novel noble-metal-free cocatalyst to promote the CO2 photoreduction performance of BiOBr ultrathin nanosheets. The optimal Co2N/BiOBr delivers a high selectivity CO formation rate of 67.8 µmol g−1 h−1 in pure water without sacrificial reagent or extra photosensitizer, roughly 6 times higher than BiOBr. Co2N can create strong electronic interactions with BiOBr, steering the electron transfer from BiOBr, across the interface to metallic Co2N and finally to the surface. Apart from the charge separation steering, the activation energy barrier can be lowered on Co2N surface via stabilize COOH* intermediates, tuning the rate-limiting step from the formation of COOH* on BiOBr to the formation of CO* on Co2N, jointly optimize the CO2 photoreduction activity. This strategy affords an accessible pathway for designing cocatalysts for efficient CO2 photoreduction.

Tin disulfide is considered as a promising electrode material for sodium-ion batteries because of its two-dimensional layered structural characteristics allowing the intercalation of Na ions. Understanding the underlying reaction mechanisms and the decisive step of the reaction reversibility is critical for its applications. Herein, we investigate the sodiation and desodiation processes of SnS2 by employing in situ transmission electron microscopy (TEM). After the initial intercalation reaction, a rock-salt NaySnS2 phase with disordering Na and Sn cations is observed, followed with a conversion reaction and an alloying reaction. The disordering reaction occurs along direction of pristine SnS2 phase which is correlated with local bonding rearrangements induced by the exchange of Sn and Na cations. In-situ TEM studies and first-principles calculations indicate that the original 2D SnS2 structure could not be recovered during desodiation. Instead, the disordered NaySnS2 phase is finally formed, which indicates that the irreversible disordering transition is the determining step of irreversible cycling. This work probes the structural evolution of sodiation, providing a fundamental understanding of the electrochemical properties of metal sulfides and inspiring rational designs of high performance electrodes for sodium-ion batteries.

The fundamental electrochemical reaction mechanisms and the phase transformation pathways of layer-structured α-MoO 3 nanobelt during the sodiation/desodiation process to date remain largely unknown. To observe the real-time sodiation/desodiaton behaviors of α-MoO 3 during electrochemical cycling, we construct a MoO 3 anode sodium-ion battery inside a transmission electron microscope (TEM). Utilizing in situ TEM and electron diffraction pattern (EDP) observation, α-MoO 3 nanobelts are found to undergo a unique multi-step phase transformation. Upon the first sodiation, α-MoO 3 nanobelts initially form amorphous Na x MoO 3 phase and are subsequently sodiated into intermediate phase of crystalline NaMoO 2 , finally resulting in the crystallized Mo nanograins embedded within the Na 2 O matrix. During the first desodiation process, Mo nanograins are firstly re-oxidized into intermediate phase NaMoO 2 that is further transformed into amorphous Na 2 MoO 3 , resulting in an irreversible phase transformation. Upon subsequent sodiation/desodiation cycles, however, a stable and reversible phase transformation between crystalline Mo and amorphous Na 2 MoO 3 phases has been revealed. Our work provides an in-deepth understanding of the phase transformation pathways of α-MoO 3 nanobelts upon electrochemical sodiation/desodiation processes, with the hope of assistance in designing sodium-ion batteries with enhanced performance.

Large-scale preparation of single-crystalline V 2 O 5 nanobelts is successfully demonstrated with a simple solution treatment process under ambient condition using commercial V 2 O 5 powders as the precursor. Unlike the commonly recognized Ostwald ripening process that involves the dissolution of small crystals and the redeposition of the dissolved species on the more energetically favored large particles, this preparation shows that the reaction mechanism of our method follows a different route, in which the large commercial V 2 O 5 powders (1–4 μm) dissolve in the solution and eventually transform into V 2 O 5 nanobelts with lengths up to several tens of micrometers, widths of 5–50 nm, and thicknesses of only ~5 nm. The density function theory (DFT) calculation indicates that the preferential growth of V 2 O 5 nanobelts along the [010] direction is attributed to the anisotropic bonding of V 2 O 5 layered structure resulting in the fastest nucleation rate at the V 2 O 5 (010) surface. These nanobelts possess a remarkably large surface area, which is about 14 times higher than that of the V 2 O 5 precursor. Binder-free bulky papers can be prepared by the intertwining V 2 O 5 nanobelts with the acid-treated multi-walled carbon nanotubes. When the V 2 O 5 nanobelts are applied as the cathodes in lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs), such robust and flexible electrodes demonstrate superior lithium and sodium storage performances at fast charge/discharge rates, delivering 144 mA h g −1 at 20 C in LIBs and 61 mA h g −1 at 10 C in SIBs respectively.

Transitional metal oxides are promising anode materials for sodium ion batteries (SIBs) due to their high theoretical capacities and material abundance; however, their sodium storage capability is significantly hindered by the sluggish sodiation/desodiation reaction kinetics. Herein, towards achieving fast and durable sodiation/desodiation reaction, Fe 3 O 4 and Co 3 O 4 nanocrystals encapsulated in carbon micro-spheres are synthesized via a biochemistry approach using recombinant elastin-like polypeptides containing hexahistidine tag (ELP16-His) followed by annealing. Fe 3 O 4 and Co 3 O 4 nanocrystals of approximately 5 nm in size, which are uniformly dispersed in a carbon matrix, are obtained. The carbon-encapsulated metal oxides exhibit encouraging sodium storage capacities (657 and 246 mA h g −1 at 0.1 and 2 A g −1 , respectively, for carbon-encapsulated Fe 3 O 4 ; 583 and 183 mA h g −1 at 0.1 and 2 A g −1 , respectively, for carbon-encapsulated Co 3 O 4 ), and have a high capacity retention after 100 cycles at 0.5 A g −1 . The superior electrochemical properties of the carbon-encapsulated metal oxide nanocrystals demonstrate their potential for use as anode materials for high-capacity, high-rate and durable sodium storage.

Quantum-dot sensitized solar cells (QDSCs), as a promising type of new generation photovoltaic devices, require an efficient and inexpensive counterelectrode (CE) for their commercial application. In this work, novel noble-metal-free Cu-doped CoS (Cu–CoS) CEs are designed for QDSCs. The hierarchical three-dimensional (3D) Cu–CoS porous nanosheet films on fluorine doped tin oxide (FTO) glass are prepared by a simple hydrothermal reaction followed by a cation exchange reaction process. As an alternative to Pt electrode, the 3D structure of porous Cu–CoS nanosheets with high reflectivity can provide a great large number of active catalytic sites and easy accessibility toward S n 2 − / S 2 − electrolyte solution, leading to high electrocatalytic activity. Theoretical calculations are further performed to demonstrate the superior catalytic activity for the Cu–CoS CE. As a result, the QDSC with an optimized Cu–CoS CE exhibits a high photovoltaic conversion efficiency of 6.1% under 100 mW cm −2 irradiation, which is much higher than that of the references Pt and bare CoS CEs. Furthermore, this QDSC device shows long-term stability, which makes it as a very promising and low cost effective material for QDSCs.

β-FeOOH has recently been reported to have a very high lithium storage capacity of ~1400 mAh g −1 , 20–40% higher than those widely reported for iron-based anodes, such as Fe 2 O 3 and Fe 3 O 4 . However, many properties of this material that are important for their use in lithium-ion batteries remain unknown. Here, we present a study on the effects of particle size on their lithium storage performance and the kinetics associated with the redox reactions. The study is based on β-FeOOH nanorods prepared by a simple hydrolysis method which is able to control the size of the rods in a wide range, diameter from tens of nanometers down to ~5 nm. Three materials with different sizes, mean diameter 5, 13 and 53 nm, are investigated for lithium storage. They show a very high and comparative capacity at a low current density, but different initial Coulombic efficiencies and rate capabilities. The kinetic study reveals the initial size of the material influences the formation of the SEI layers on the material and has significant effects on the kinetics of the redox reactions that lead to different rate behaviors. This study provides fundamental information and understanding of β-FeOOH anodes for their further development.

Rechargeable lithium–oxygen (Li–O2) batteries are attracting much interest due to their highest energy density among chemical batteries. Recent studies have focused on designing highly efficient catalysts to improve the electrochemical performance of Li–O2 batteries for long-term cycling. In this work, we report controlled synthesis of high efficient spinel catalysts in Li–O2 batteries. The catalysts present core–shell spheres, which are constructed by porous nanoplates. The remarkable feature of the hierarchical porous structure cannot only provide many electrocatalytic sites but also facilitate the flow of oxygen and infiltration of the electrolyte, and eventually improving the capacity and cyclability. As a representative catalyst, the synergistic effects of the inherited superior catalytic activity and the unique porous structure of NiCo2O4 lead to low overpotentials, high rate capacity as well as excellent long-term cyclability in Li–O2 cells. This study affords a new strategy to achieve optimal performance in spinel catalysts, which may be extended to the preparation of other materials for a broad range of energy and environmental applications.