Fine valence regulation of hydrated vanadium oxide as a novel cathode for stable potassium-ion storage

Layered V10O24·nH2O with a large interlayer spacing of 14 Å is hydrothermally synthesized and used as a cathode for potassium-ion batteries. It exhibits a capacity of 110 mA h g-1 with a capacity retention of 99.2% over 700 cycles. Its storage mechanism is identified as pseudo-capacitive intercalation.

Structural design enabled a hypotoxic Na3.36FeV(PO4)3 cathode with ultra-fast and ultra-long sodium storage

Among various cathode materials for sodium-ion batteries, Na₃V₂(PO₄)₃ has attracted much attention due to its outstanding electrochemical performance. However, the toxicity and expensive price of vanadium limit its practical application. Therefore, the substitution of vanadium with nontoxic and inexpensive transition metal elements is significant. We select the earth-abundant iron element to partially replace the vanadium element, and successfully synthesize Na_3.36FeV(PO₄)₃ with a Na superionic conductor structure. Furthermore, a Na_3.36FeV(PO₄)₃ cathode with an optimal carbon content can deliver an initial capacity of 97.6 mA h g^-1 at 0.5C with a high capacity retention of 96.4% after 200 cycles. In addition, it also delivers an initial capacity of 90.4 mA h g^-1 at 10C, and a capacity retention of 80% can be obtained after 5000 cycles. We also found that the lack of sodium in the material can be compensated by an electrochemical reaction. Furthermore, the in situ X-ray diffraction analysis reveals that the sodium storage process follows a pseudo-solid-solution reaction mechanism and the volume change ratio is less than 3% during charging/discharging. In order to study the practical application capability of Na_3.36FeV(PO₄)₃, we assemble the pre-activated cathode and a hard carbon anode into a full cell, which exhibits high initial discharge capacities of 103 and 91.3 mA h g^-1 at 0.5C and 10C, respectively. This work will provide new insights into the structural engineering of low-toxicity and ultralong-life NASICON-type cathode materials for SIBs.

Ultra-stable potassium storage and hybrid mechanism of perovskite fluoride KFeF3/rGO

Potassium-ion batteries (PIBs) are promising for large-scale energy storage due to the abundant reserves of the element potassium yet few satisfactory cathode materials have been developed due to the limitation of the large ionic radius of the potassium ion. Cubic perovskite fluorides have three-dimensional diffusion channels and a robust structure, which are favorable for ion transfer, but their poor electronic conductivity needs to be compensated. Here, we synthesized cubic KFeF₃ powder by a solvothermal procedure. After the combination with reduced graphene oxide (rGO) and carbon coating, its electronic conductivity is greatly improved. In the optimal sample KFeF₃/rGO-PVA-500, KFeF₃ nano-particles (smaller than 50 nm) distribute on the rGO surface evenly. Owing to the special structure, KFeF₃/rGO-PVA-500 provides an excellent rate performance and cycling stability. In particular, a high capacity retention of 94% is obtained after 1000 cycles at 200 mA g^-1. In addition, a hybrid reaction mechanism combining mainly solid solution and partly conversion processes is revealed by employing in situ and ex situ characterization.

Active-Site-Specific Structural Engineering Enabled Ultrahigh Rate Performance of the NaLi3Fe3(PO4)2(P2O7) Cathode for Lithium-Ion Batteries

Iron-based mixed-polyanionic cathode Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) has advantages of environmental benignity, easy synthesis, high theoretical capacity, and remarkable stability. From NFPP, a novel Li-replaced material NaLi₃Fe₃(PO₄)₂(P₂O₇) (NLFPP) is synthesized through active Na-site structural engineering by an electrochemical ion exchange approach. The NLFPP cathode can show high reversible capacities of 103.2 and 90.3 mA h g^-1 at 0.5 and 5C, respectively. It also displays an impressive discharge capacity of 81.5 mA h g^-1 at an ultrahigh rate of 30C. Density functional theory (DFT) calculation demonstrates that the formation energy of NLFPP is the lowest among NLFPP, NFPP, and NaFe₃(PO₄)₂(P₂O₇), indicating that NLFPP is the easiest to form and the conversion from NFPP to NLFPP is thermodynamically favorable. The Li substitution for Na in the NFPP lattice causes an increase in the unit cell parameter c and decreases in a, b, and V, which are revealed by both DFT calculations and in situ X-ray powder diffraction (XRD) analysis. With hard carbon (HC) as the anode, the NLFPP//HC full cell shows a high reversible capacity of 91.1 mA h g^-1 at 2C and retains 82.4% after 200 cycles. The proposed active-site-specific structural tailoring via electrochemical ion exchange will give new insights into the design of high-performance cathodes for lithium-ion batteries.

Hollow-Sphere-Structured Na4Fe3(PO4)2(P2O7)/C as a Cathode Material for Sodium-Ion Batteries

The mixed polyanionic material Na₄Fe₃(PO₄)₂(P₂O₇) combines the advantages of NaFePO₄ and Na₂FeP₂O₇ in capacity, stability, and cost. Herein, we synthesized carbon-coated hollow-sphere-structured Na₄Fe₃(PO₄)₂(P₂O₇) powders by a scalable spray drying route. The optimal sample can deliver a high discharge capacity of 107.7 mA h g^-1 at 0.2C. It also delivers a capacity of 88 mA h g^-1 at 10C and a capacity of retention of 92% after 1500 cycles. Ex situ X-ray diffraction analysis indicates a slight volume change (less than 3%) in the Na₄Fe₃(PO₄)₂(P₂O₇) lattice cell. Therefore, such a spraying-derived carbon-coated Na₄Fe₃(PO₄)₂(P₂O₇) powder is a very attractive cathode electrode for sodium-ion batteries.

Continuous electricity production from leachate in a novel upflow air-cathode membrane-free microbial fuel cell

In this study, a novel microbial fuel cell, i.e. upflow air-cathode membrane-free microbial fuel cell (UAMMFC) was reported and its performance in electricity generation from original leachate was examined. The experimental results demonstrated that the UAMMFC could continuously generate electricity from leachate (0.3V; REX=150 Omega) for an operational period of time (50 h). The maximum volumetric power reached 12.8 W/m3 at current density of 41 A/m3 (93 Omega). NH4+-N elimination from the leachate was shown to be a consequence of electrochemistry-independent oxidation occurred in the MFC. Increasing organic loading rate from 0.65 to 5.2 kgCOD/m3 d resulted in a decrease of overall Coulombic efficiency (CE) from 14.4% to 1.2%. The low CE obtained here should be attributed to severe oxygen diffusion from the open-to-air cathode.

Assessing optimal fermentation type for bio-hydrogen production in continuous-flow acidogenic reactors

In this study, the optimal fermentation type and the operating conditions of anaerobic process in continuous-flow acidogenic reactors was investigated for the maximization of bio-hydrogen production using mixed cultures. Butyric acid type fermentation occurred at pH>6, propionic acid type fermentation occurred at pH about 5.5 with E(h) (redox potential) >-278mV, and ethanol-type fermentation occurred at pH<4.5. The representative strains of these fermentations were Clostridium sp., Propionibacterium sp. and Bacteriodes sp., respectively. Ethanol fermentation was optimal type by comparing the operating stabilities and hydrogen production capacities between the fermentation types, which remained stable when the organic loading rate (OLR) reached the highest OLR at 86.1kgCOD/m(3)d. The maximum hydrogen production reached up to 14.99L/d.

Biodegradation of pyridine using aerobic granules in the presence of phenol

Aerobic granules cultivated with 500 mg/L phenol medium effectively degraded pyridine at a concentration of 250-2500 mg/L; maximum degradation rate was 73.0 mg pyridineg/VSS/h at 250 mg/L pyridine concentration. Phenol concentrations of 500-2000 mg/L limited pyridine degradation in a competitive inhibition pattern, as interpreted using Michaelis-Menten kinetics with corresponding parameters V(max), K(m) and K(I) of 63.7 mg/Lh(-1), 827.8 and 1388.9 mg/L, respectively. Fluorescent staining and confocal laser scanning microscopy (CLSM) tests suggested that an active biomass accumulated at the granule outer layer. Denaturing gradient gel electrophoresis (DGGE) fingerprint profile demonstrated that dominating microbial strains exist in phenol and pyridine-degrading aerobic granules.

Synthesis of PHAs from waster under various C:N ratios

Polyhydroxyalkanoates (PHAs) production was carried out under various C:N ratios. A ratio of 100 resulted best polymer yield. C-source was an important factor in synthesis. For example, as the ratio of valeric acid (C5) to butyric acid (C4) in N-free medium was increased, the mole fraction of HV in the copolymer increased. When soy waste was used as a C-source a copolymer, a high HV mole fraction (HB:HV, 75:25) was produced while when malt waste was used, a much lower HV mole fraction (HB:HV, 90:10) was generated. It was concluded that activated sludge bacteria could be induced to produce PHAs using food wastes as C-sources and this could be the basis for production of biodegradable plastics.

Evaluation of heavy metal inhibition of activated sludge by TTC and INT-electron transport system activity tests

TTC and INT-electron transport system activity tests were compared for assessing heavy metal inhibition of activated sludge. The median inhibitory concentrations (IC50s) of Cu2+, Zn2+, Cd2+, Hg2+, Ni2+, Pb2+ and Ag+ measured via TTC test were lower than those measured via INT test, which indicates that the INT-electron transport system activity test was less sensitive to heavy metal toxicity than the TTC test. Tested heavy metals brought about similar decrease in TTC-electron transport system activity and COD removal rate, but less decrease in INT-electron transport system activity than COD removal rate, which suggests that the TTC-electron transport system activity is a better parameter for reflecting heavy metal inhibition of activated sludge than INT-electron transport system activity. The ranking of tested heavy metals in order of decreasing toxicity based on TTC test was Hg2+, Cd2+, Cu2+, Ag+, Zn2+, Ni2+ and Pb2+, and the ranking based on INT test was Hg2+, Ag+, Cu2+, Cd2+, Zn2+, Ni2+ and Pb2+.

Start-up of bio-hydrogen production reactor seeded with sewage sludge and its microbial community analysis

Start-up of a continuously stirred tank reactor for bio-hydrogen production under different initial organic loading rate (OLR) of 3, 7 and 10 kgCOD/m3 d, respectively, was carried out with sewage sludge as inoculum. Molasses wastewater was used as substrate and hydraulic retention time was kept at 6 h. This study aimed to assess OLR on the formation of fermentation types and the structure of microbial communities during the start-up period. It was found that at an initial OLR of 7 kgCOD/m3 d and an initial biomass of 6.24 gVSS/L, an equilibrial microbial community of ethanol-type fermentation could be established within 30 days. The observed average specific hydrogen production rate was 276 mLH2/gVSS d, which was 40% higher than that of the one acclimated with 3 kgCOD/m3 d. Based on denaturing gradient gel electrophoresis profiles, significant microbial population shifts took place at the first 15 days, but a longer period up to 30 days was required to establish a microbial community with stable metabolic activity.