High Valence State Research Articles

High-Valence Metals Accelerate the Reaction Kinetics for Boosting Water Oxidation.

The transition metal with high valence state in oxyhydroxides can accelerate the reaction kinetics, enabling highly intrinsic OER activity. However, the formation of high-valence transition-metal ions is thermodynamically unfavorable in most cases. Here, a novel strategy is proposed to realize the purpose and reveal the mechanism by constructing amorphous phase and incorporating of elements with the characteristic of Lewis acid or variable charge state. A model catalyst, CeO2-NiFeOxHy, is presented to achieve the modulation of valence state of active site (Ni2+→Ni3+→Ni4+) for improved OER, leading to dominant active sites with high valence state. The CeO2-NiFeOxHy electrode exhibits superior OER performance with overpotential of 214 and 659mV at 10 and 500mAcm-2, respectively (without IR correction), and high stability, which are much better than those of NiOxHy, NiFeOxHy and CeO2-NiOxHy. These findings provide an effective strategy to achieve the active metals with high-valence state for highly efficient OER.

EGCG mildly reduced single atom Co-MXene catalytic membrane: Catalytic and antioxidant properties

As a 2D nanomaterial with high hydrophilic and co-catalytic properties, MXene has been widely studied in the field of membrane separation and advanced oxidation. However, its susceptibility to oxidation in catalytic environments poses a significant challenge to its long-term stability and performance. Therefore, constructing an oxidation-resistant MXene catalytic membrane technology and ensuring its excellent flux and catalytic performance are important for the practical application of MXene membranes. Herein, we prepared a cobalt monoatomic intercalated M/E0.1-Co catalytic separation membrane by mild reduction of epigallocatechin-3-gallate (EGCG). EGCG not only connects the MXene lamellae during filtration through strong hydrogen bonding, but also protects the chemical stability of the MXene substrate through its antioxidant ability. Furthermore, the trace amount of EGCG in the confined domain space also promotes the rate-limiting step of the transition from the high-valent state to the low-valent state in Co single-atom catalysis. The stability of the catalytic separation layer was ensured along with a 100% degradation performance of sulfamethoxazole. This study establishes a bridge between the catalytic oxidation and chemical stability of a wide range of MXene-based materials nowadays and provides a viable proposal for the practical application of long-term utilized MXene materials in catalysis.

Self-protected Chlorella@Mn catalyst with excellent resistance to alkali/alkaline earth metal for NOx reduction by NH3

In the selective catalytic reduction of NOx by NH3 (NH3-SCR), conventional Mn-based denitration catalysts often suffered from susceptibility to poisoning by alkali and alkaline earth metals, this paper presented an innovative self-protected Chlorella@Mn denitration catalyst. Remarkably, in the presence of high concentrations (2 wt%) of alkali and alkaline earth metal oxides, the Chlorella@Mn catalyst sustained a NOx conversion exceeding 96 % at 175 °C. At an even higher concentration (4 wt%), NOx conversion above 90 % at 175 °C, surface analysis revealed that POMn sites acted as sacrificial sites, binding to the alkali and alkaline earth metals, the Chlorella@Mn catalyst surface naturally carried a spectrum of acidic species (such as SO42−, PO3−, SiO32−), proficient in capturing alkali/alkaline earth metal effectively, elements such as S, P, and Si formed bonds with K, Na, Ca, and Mg. The synergistic protection of the active sites and the surface elements avoided the deactivation of the catalyst. The detrimental effects of high concentrations of alkali and alkaline earth metals were primarily due to promoting an excessively high valence state of Mn on the catalyst surface and the reduction or loss of NH3 adsorption and activation at Brønsted acid sites. This research provided valuable insights for advancing the development of low-temperature denitration catalysts with improved resistance to alkali and alkaline earth metal poisoning.

Boosting the electrocatalytic performance of CuCo2S4 via surface-state engineering for ampere current water electrolysis applications

For the efficient production of green H2 on an industrial scale, developing durable, cost-effective electrocatalysts using earth-abundant transition-metals is crucial. Herein, we report a robust, bifunctional sulfurized CuCo2O4 (CCS/Ov) catalyst with engineered oxygen-vacancies, in which the metal catalyst is tunes to high valence state, significantly altering the intrinsic reaction kinetics and generating better catalytic activity for efficient ion transport in various electrolytes (neutral, seawater, alkaline simulated-seawater (ASW), and KOH). The optimized dual-strategy synthesized CCS/Ov catalysts with hierarchical morphology exhibits modest overpotential of 383 and 355 mV at 1000 mA cm−2 for OER and HER, respectively, in 1 M KOH. Impressively, an electrolyzer cell (CCS/Ov||CCS/Ov) demonstrates low cell-voltage of 1.487 V (6 M KOH), with excellent robustness in an ASW (1 and 2 M) environment against corrosive chlorine. The bifunctional CCS/Ov||CCS/Ov catalyst outperforms a state-of-the-art paired Pt/C||RuO2 catalyst and maintains the lowest potential response at up to 2000 mA cm−2, while demonstrating remarkably stable simultaneous O2 and H2 generation over 10 days at various current rates. Additionally, DFT calculations confirmed that CCS/Ov catalysts demonstrate significantly enhanced HER and OER activities due to reduced H2O dissociation energy and strong intermediate binding energy, which is attributed to the anionic vacancy near Co3+. The excellent bifunctional performance of the hierarchical CCS/Ov highlights its potential as non-precious catalyst with facile fabrication approach.

Steering intermediates coverage of W-doped Fe3N for efficient kinetic promotion in water splitting by lattice and electronic configuration engineering

The development of catalysts that promote kinetic processes represents a significant frontier in the field of catalyst design. Here, a facile strategy is employed to fabricate W-Fe3N@NC. The incorporation of W results in effective lattice and electronic configuration engineering, providing a favorable environment for the in-situ formation of high-valent Fe4+ and the lattice contraction. The kinetics of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are fully improved by steering the intermediate coverages and binding energies. W-Fe3N@NC exhibits superior HER performance at universal pH range and a significantly enhanced alkaline OER performance. Density functional theory (DFT) analysis verifies the lattice and electronic engineering and indicates that the higher valence state and weakened adsorption of the intermediates boost the activity of OER and HER. This study offers new insights into effective electrocatalyst design through lattice and electronic engineering, and provides a basis for further exploration of the kinetic promotion mechanism of catalysts.

Trace element abundances in mineral phases and the groundmass of ocean island basalts

Incompatible trace elements are important tracers in igneous petrogenesis for determining processes acting: (i) prior to partial melting on magma sources; (ii) during partial melting, fractional crystallization or contamination of magmas, and (iii) after emplacement of lavas or intrusive rocks close to Earth's surface. Of particular interest are the high field strength elements (HFSE: Ti, Zr, Nb, Hf, Ta) which are incompatible elements with high valence states that are also relatively immobile in aqueous fluids. Abundances of Ti, Ta, and Nb (TITAN) in ocean island basalts (OIB) have been suggested to reflect recycled oceanic crust in mantle source regions, due to retention of these elements in subducted eclogitic rutile. Some of these elements have also been used to determine ‘canonical’ trace element ratios, such as Nb/U and Ta/U, that are considered invariant during petrogenetic processes. In this contribution, we present an extensive set of in situ trace element data for olivine, clinopyroxene, oxide minerals and groundmass for a suite of well-studied ocean island basalt (OIB) lavas from the Canary Islands, Azores, Samoa, Tubuai and La Réunion. The data were obtained using laser-ablation inductively coupled plasma mass spectrometry, and we also report a new open-source reduction routine for abundance determination using this method. Mineral-groundmass partitioning behavior is determined for olivine, clinopyroxene and oxides, showing that these are consistent with published partition coefficients for clinopyroxene and olivine, but are more variable for oxide phases. This variability is due to the wide variety of oxide phases possible in OIB (e.g., Cr-spinel, ilmenite, titanomagnetite) and their formation within the crystallization sequence. The new data shows that Nb and Ta budgets are dominated by fine-grained groundmass, as are most other incompatible trace elements, while the Ti budget is divided between clinopyroxene, oxide and groundmass. Modally reconstructed TITAN anomalies closely reproduce bulk rock compositions in samples with >40 % groundmass. Olivine and clinopyroxene fractional crystallization have negligible effects on TITAN anomalies, while oxide fractionation has more marked effects, with Ti anomalies being most affected. Variability in the HFSE in OIB and their association with other elements (e.g., Nb/U and Ta/U) can reflect source characteristics, although careful screening for petrogenetic effects should be done prior to ascribing significance to TITAN anomalies or canonical trace element ratios.

Isostructural Series of a Cyclometalated Iron Complex in Three Oxidation States.

An isostructural series of FeII, FeIII, and FeIV complexes [Fe(ImP)2]0/+/2+ utilizing the ImP 1,1'-(1,3-phenylene)bis(3-methyl-1-imidazol-2-ylidene) ligand, combining N-heterocyclic carbenes and cyclometalating functions, is presented. The strong donor motif stabilizes the high-valent FeIV oxidation state yet keeps the FeII oxidation state accessible from the parent FeIII compound. Chemical oxidation of [Fe(ImP)2]+ yields stable [FeIV(ImP)2]2+. In contrast, [FeII(ImP)2]0, obtained by reduction, is highly sensitive toward oxygen. Exhaustive ground state characterization by single-crystal X-ray diffraction, 1H NMR, Mössbauer spectroscopy, temperature-dependent magnetic measurements, a combination of X-ray absorption near edge structure and valence-to-core, as well as core-to-core X-ray emission spectroscopy, complemented by detailed density functional theory (DFT) analysis, reveals that the three complexes [Fe(ImP)2]0/+/2+ can be unequivocally attributed to low-spin d6, d5, and d4 complexes. The excited state landscape of the FeII and FeIV complexes is characterized by short-lived 3MLCT and 3LMCT states, with lifetimes of 5.1 and 1.4 ps, respectively. In the FeII-compound, an energetically low-lying MC state leads to fast deactivation of the MLCT state. The distorted square-pyramidal state, where one carbene is dissociated, can not only relax into the ground state, but also into a singlet dissociated state. Its formation was investigated with time-dependent optical spectroscopy, while insights into its structure were gained by NMR spectroscopy.

Regulating Reconstruction-Engineered Active Sites for Accelerated Electrocatalytic Conversion of Urea.

Reconstruction-engineered electrocatalysts with enriched high active Ni species for urea oxidation reaction (UOR) have recently become promising candidates for energy conversion. However, to inhibit the over-oxidation of urea brought by the high valence state of Ni, tremendous efforts are devoted to obtaining low-value products of nitrogen gas to avoid toxic nitrite formation, undesirably causing inefficient utilization of the nitrogen cycle. Herein, we proposed a mediation engineering strategy to significantly boost high-value nitrite formation to help close a loop for the employment of a nitrogen economy. Specifically, platinum-loaded nickel phosphides (Pt-Ni2P) catalysts exhibit a promising nitrite production rate (0.82 mol kWh-1 cm-2), high stability over 66 h of Zn-urea-air battery operation, and 135 h of co-production of nitrite and hydrogen under 200 mA cm-2 in a zero-gap membrane electrode assembly (MEA) system. The in situ spectroscopic characterizations and computational calculations demonstrated that the urea oxidation kinetics is facilitated by enriched dynamic Ni3+ active sites, thus augmenting the "cyanate" UOR pathway. The C-N cleavage was further verified as the rate-determining step for nitrite generation.

From d8 to d1: Iron(0) and Iron(I) Complexes Complete the Series of Eight Fe Oxidation States within the TIMMNMes Ligand Framework.

Reduction of the ferrous precursor [(TIMMNMes)Fe(Cl)]+ (1) (TIMMNMes = tris-[(3-mesitylimidazol-2-ylidene)methyl]amine) to the low-valent iron(0) complex [(TIMMNMes)Fe(CO)3] (2) is presented, where the tris(N-heterocyclic carbene) (NHC) ligand framework remains intact, yet the coordination mode changed from 3-fold to 2-fold coordination of the carbene arms. Further, the corresponding iron(I) complexes [(TIMMNMes)Fe(L)]+ (L = free site, η1-N2, CO, py) (3) are synthesized and fully characterized. Complexes 1-3 demonstrate the notable steric and electronic flexibility of the TIMMNMes ligand framework by variation of the Fe-N anchor and Fe-carbene distances and the variable size of the axial cavity occupation. This is further underpinned by the oxidation of 3-N2 in a reaction with benzophenone to yield the corresponding, charge-separated iron(II) radical complex [(TIMMNMes)Fe(OCPh2)]+ (4). We found rather surprising similarities in the reactivity behavior when going to low- or high-valent oxidation states of the central iron ion. This is demonstrated by the closely related reactivity of 3-N2, where H atom abstraction with TEMPO triggers the formation of the metallacycle [(TIMMNMes*)Fe(py)]+ (5), and the reactivity of the highly unstable Fe(VII) nitride complex [(TIMMNMes)Fe(N)(F)]3+ to give the metallacyclic Fe(V) imido complex [(TIMMNMesN)Fe(NMes)(MeCN)]3+ (6) upon warming. Thus, the employed tris(carbene) chelate is not only capable of stabilizing the superoxidized Fe(VI) and Fe(VII) nitrides but equally supports the iron center in its low oxidation states 0 and +1. Isolation and characterization of these zero- and monovalent iron complexes demonstrate the extraordinary capability of the tris(carbene) chelate TIMMN to support iron in eight different oxidation states within the very same ligand platform.

Vacancy engineering-induced spin polarization in heterogeneous phosphides by simple iron salt infiltration for efficient overall water splitting

Optimizing electronic structure and facilitating electron transfer modulation in electrocatalyst is the key to enhancing overall water splitting (OWS) performance. In this work, the heterogeneous transition metal phosphide Fe-CoPv/Ni2P@NF, consisting of cobalt phosphide and nickel phosphide, is obtained by simple iron salt infiltration and high-temperature phosphatisation. Due to the doping of iron-atom and the formation of phosphorus vacancies, the activity of metal atoms with higher valence states and spins is induced, and then highly active Fe/Co-O-Ni channels are generated in the dynamic water-splitting process, thereby increasing the exposure of reaction center and total electron transfer rate. The catalytic mechanism is attributed to reducing the potential barrier of phase remodeling and optimizing the intermediate binding strength, thus thoroughly stimulating OER and HER activities. The Fe-CoPv/Ni2P@NF shows superior performance, requiring only total voltages of 1.626 V (OWS) to reach 100 mA·cm−2 and maintaining ultra-high stability for 500 h, suggesting promising industrial applications.

Nanostructure-transportation relation to PEMFCs activity and durability degradation

The Proton exchange membrane (PEM) is a crucial component in the membrane electrode assembly (MEA), playing a vital role in providing a channel for water transport and acting as a bridge for proton conduction. However, during the long-term operation of fuel cells, PEM is susceptible to attack from metal ions such as Cr3+ originating from the corrosion of metallic bipolar plates, leading to changes in its transport properties. Here, we report on the impact of Cr3+ on the microstructure and mechanical properties of PEM and then correlate it with changes in water uptake and proton conductivity, studying the effect of Cr3+ contamination on fuel cell performance and durability. The study reveals that Cr3+ forms a dense cross-linking structure with sulfonic acid groups inside the ionomer, resulting in an increase in storage modulus due to the larger ion radius and higher valence state, leading to a decrease in water uptake and proton conductivity. The membrane transport properties are primarily dependent on its water content. It is observed that Cr3+ contamination reduced the fuel cell voltage and maximum power density by 47.6 % and 57.1 %, respectively, with a voltage attenuation rate up to 2.9 mV/h, nearly 14 times that of the uncontaminated condition (0.21 mV/h). These findings provide valuable insights for developing high-transmission PEMs and aid in predicting the degradation and lifespan of fuel cells under actual operating conditions.

Regulating Reconstruction‐Engineered Active Sites for Accelerated Electrocatalytic Conversion of Urea

AbstractReconstruction‐engineered electrocatalysts with enriched high active Ni species for urea oxidation reaction (UOR) have recently become promising candidates for energy conversion. However, to inhibit the over‐oxidation of urea brought by the high valence state of Ni, tremendous efforts are devoted to obtaining low‐value products of nitrogen gas to avoid toxic nitrite formation, undesirably causing inefficient utilization of the nitrogen cycle. Herein, we proposed a mediation engineering strategy to significantly boost high‐value nitrite formation to help close a loop for the employment of a nitrogen economy. Specifically, platinum‐loaded nickel phosphides (Pt‐Ni2P) catalysts exhibit a promising nitrite production rate (0.82 mol kWh−1 cm−2), high stability over 66 h of Zn‐urea‐air battery operation, and 135 h of co‐production of nitrite and hydrogen under 200 mA cm−2 in a zero‐gap membrane electrode assembly (MEA) system. The in situ spectroscopic characterizations and computational calculations demonstrated that the urea oxidation kinetics is facilitated by enriched dynamic Ni3+ active sites, thus augmenting the “cyanate” UOR pathway. The C−N cleavage was further verified as the rate‐determining step for nitrite generation.

Fe,Ce Co-Doped Ni3S2/NiS Polymorphism Nanosheets with Improved Electrocatalytic Activity and Stability for Water Oxidation.

Balancing the relationship between electrocatalytic activity and stability of sulfide catalysts during oxygen evolution reaction (OER) has been attracting extensive research interest. Here, a simple electrodeposition-vulcanization two-step route was designed to successfully construct nickel foam supported sheet-like Fe,Ce-codoped Ni3S2/NiS polymorphism catalyst (labeled as Fe,Ce-Ni3S2/NiS/NF). Electrochemical measurements showed that the as-obtained Fe,Ce-Ni3S2/NiS/NF electrode presented excellent OER electrocatalytic performances. In 1 M KOH solution, merely 173 and 234 mV of overpotentials were required to deliver the current densities of 10 and 100 mA cm-2, respectively. Further investigations revealed that the Fe,Ce co-doping regulated the electron density around Ni, which promoted the conversion of Ni towards the higher valence state and simultaneously, avoided the stability decrease of the catalyst caused by excessive oxidation corrosion. Moreover, the defects generated during vulcanization also contributed to promoting water oxidation. The present work provides a facile and feasible approach to balance the relationship between the stability and the activity of sulfide catalysts for OER.

Amorphous carbon modulated-quantum dots NiO for efficient oxygen evolution in anion exchange membrane water electrolyzer

Developing efficient electrocatalysts of elements that are abundant on earth crust is crucial for green hydrogen generation technologies. In particular, the oxygen evolution reaction (OER) under alkaline plays a key role in anion exchange membrane (AEM) electrolyzer to produce green hydrogen but suffers from low kinetic. Herein, nickel oxide quantum dots with highly uniform size distribution on ultrathin amorphous carbon nanosheets (NiO dots/a-carbon) were successfully prepared by a one-step method. Introducing NiO quantum dots onto amorphous carbon modifies the local coordination environment of Ni promoting it into a higher valence state. Benefitting from the promoted Niδ+ (2<δ<3) and the strong connection between Ni and amorphous carbon though Ni-O-C and Ni-C bonds, NiO dots/a-carbon exhibits excellent activity and stability towards OER in 0.1 M KOH using the rotating disk electrode. Moreover, a challenging current density of 500 mA cm−2 is achieved at 1.7 V with a lab-scale AEM electrolyzer.

Employing lattice compression spin polarization electric field for boosting degradation of mixed antibiotics in wastewater

Localized polarization electric field is a key factor affecting photocatalytic activity. In this study, the localized polarization electric field of lattice strain was introduced into the electronic structure of Bi2WO6 by using the high and low valence state doping method. The impurity energy level induced by lattice strain further affects the gap energy level. As an intermediate state for electron transfer, the gap level further promotes the dynamics of photo generated charges, reducing the band gap by 0.4 eV and improving the photoelectric performance by 60 %. Various tests and theoretical calculations have confirmed that the internal electric field of local polarization significantly improves the generation and transfer rate of photo-generated charges, thereby boosting catalytic activity. The removal ratios of ciprofloxacin (CIP) and tetracycline (TC) in the mixed wastewater reach 91.5 % and 97.1 % in 90 min, respectively. Cyclic tests have shown that the electronic structure control exhibits excellent stability with removal ratios of CIP and TC of 83.9 % and 85.7 %, respectively. Finally, the 8C of CIP and the 11C and 19O of TC are most susceptible to the attack of free radicals generated by lattice compression spin polarization electric fields based on the theoretical calculations of the Fukui function and the analysis of intermediates. This work provides new insights into the design of catalysts for efficient photo generated charge dynamics with lattice strain.

Bimetallic NiCo-MOF engineering on foam nickel for efficient oxygen evolution reaction in wide-pH-value water and seawater

Oxygen evolution reaction (OER) is a half-reaction that transpires at the anode during water electrolysis. It is a controlling step in the process because of slow kinetics. Therefore, developing OER catalysts with low cost, enduring stability, and wide-pH-value adaptability is a significant challenge. In this article, NiXCo2.4-X-MOF (x = 0.4, 0.6, 0.8, 1) catalysts were synthesized via hydrothermal on nickel foam (NF). The proportion and hydrothermal temperature can affect the performances, and the optimal catalyst is obtained with x = 0.6 and hydrothermal temperature is 150℃ (Ni0.6Co1.8-MOF). This catalyst exhibits outstanding electrocatalytic performances. The overpotentials are 1.77, 1.61, and 1.68 V in 1 M PBS (pH = 7), 1 M KOH, and alkaline seawater at 20, 200, and 200 mA cm−2 with excellent stability towards OER, respectively. In-situ SERS results suggest that during the OER process, hydroxyoxides are formed on the catalyst surface, which serves as the actual active substance. Furthermore, XPS analysis of the OER reaction reveals the formation of M−O and high-valence state oxides. DFT calculations confirm that Ni0.6Co1.8-MOF/NiCoOOH acts as the genuine active site for the OER, formed through the reconstruction of Ni0.6Co1.8-MOF, lowering the energy barrier for OOH* formation further accelerates the reaction kinetics of the OER. This study indicates the broad application prospects of MOFs in wide-pH-value and seawater.

Tailoring the oxygen evolution reaction activity of lanthanide-doped NiFe-LDHs through lanthanide contraction

Elemental doping is employed to tune the inherent activity and electronic structure of electrocatalysts for water electrolysis. Here, unique nanosheet arrays of NiFe-LDH doped with lanthanide metals (NiFeSm-LDH, NiFeCe-LDH, and NiFeLa-LDH) were developed as high-performance electrocatalysts for oxygen evolution reaction (OER). Notably, NiFeSm-LDH exhibits the superior performance, with the lowest overpotentials of 203 mV (at 10 mA cm−2) for OER. Structural analysis, in-situ Raman spectra and DFT calculations reveal that the high activity of the catalysts can be attributed to synergistic functionalities. Firstly, compared to Ce in NiFeCe-LDH and La in NiFeLa-LDH, Sm element in NiFeSm-LDH can attract more electrons from the outermost 3d orbitals of Ni due to the effect of lanthanide contraction, resulting in the highest valence state and d-band center of Ni. Secondly, the formation of NiOOH phase on NiFeSm-LDH requires a lower overpotential (∼1.32 V) than those on NiFeCe-LDH (1.32 ∼ 1.37 V) and NiFeLa-LDH (∼1.37 V) based on in-situ Raman spectra, which indicate that NiFeSm-LDH has faster kinetics to achieve the Ni(II)-Ni(III) transformation. Thirdly, First-principles simulations reveal that NiFeSm-LDH reduces the formation energy from OOH* to O2 significantly, which eventually improves the catalytic activity. Furthermore, the NiFeSm-LDH||Pt/C couple exhibits a low voltage of 1.59 V at 100 mA cm−2 for overall water splitting.

Valence-state mixing and reduced magnetic moment in Fe3−δGeTe2 single crystals with varying Fe content probed by x-ray spectroscopy

We present a spectroscopic study of the magnetic properties of Fe3−δGeTe2 single crystals with varying Fe content, achieved by tuning the stoichiometry of the crystals. We carried out x-ray absorption spectroscopy and analyzed the x-ray circular magnetic dichroism spectra using the sum rules, to determine the orbital and spin magnetic moments of the materials. We find a clear reduction of the spin and orbital magnetic moment with increasing Fe deficiency. Magnetic susceptibility measurements show that the reduction in magnetization is accompanied by a reduced Curie temperature. Multiplet calculations reveal that the Fe2+ state increasingly mixes with a higher valence state when the Fe deficiency is increased. This effect is correlated with the weakening of the magnetic moment. As single crystals are the base material for exfoliation processes, our results are relevant for the assembly of 2D magnetic heterostructures.

Molybdenum-Doped Cobalt-Free Cathode Realizing the Electrochemical Stability by Enhanced Covalent Bonding.

The doping strategy effectively enhances the capacity and cycling stability of cobalt-free nickel-rich cathodes. Understanding the intrinsic contributions of dopants is of great importance to optimize the performances of cathodes. This study investigates the correlation between the structure modification and their performances of Mo-doped LiNi0.8Mn0.2O2 (NM82) cathode. The role of doped Mo's valence state has been proved functional in both lattice structural modification and electronic state adjustment. Although the high-valence of Mo at the cathode surface inevitably reduces Ni valence for electronic neutrality and thus causes ion mixing, the original Mo valence will influence its diffusion depth. Structural analyses reveal Mo doping leads to a mixed layer on the surface, where high-valence Mo forms a slender cation mixing layer, enhancing structural stability and Li-ion transport. In addition, it is found that the high-valence dopant of Mo6+ ions partially occupies the unfilled 4d orbitals, which may strengthen the Mo─O bond through increased covalency and therefore reduce the oxygen mobility. This results in an impressive capacity retention (90.0% after 200 cycles) for Mo-NM82 cathodes with a high Mo valence state. These findings underscore the valence effect of doping on layered oxide cathode performance, offering guidance for next-generation cathode development.

Impact of biochar synergized bacteria on Mn2+ and NH4+-N removal and CO2 immobilization in water by enhanced extracellular electron transfer

Mn2+ and NH4+-N contamination in water by Mn slag leakage is urgent to be resolved. Biochar synergistic strains is considered to be a promising approach to remove pollution with high concentration, however, the specific oxidation and electron transfer processes are rarely elucidated in detail. In this study, under different biochar (BC) additions, NH4+-N concentration and pH ranges, removal efficiency of Mn2+, NH4+-N and COD had reached at 99.7 %-100 %, 100 % and 90.3–96.2 % by the BC & strain AL-6 system (co-system), respectively, and the nitrification processes was completed. Meanwhile, the electrochemical analysis showed that BC had strong tendency to donate, accept and transfer electrons, and electrons transfer property was more responsible for the removal process. Further, the electrical conductivity of co-system was enhanced upon reaction with Mn2+ and NH4+-N, suggesting that co-system promoted the electron transfer processes for pollutants removal. Concretely, Raman spectroscopy specified that conductive graphite region of biochar accelerated electron transfer behavior in the system. As well as, 2D-COS and characterization analyses indicated that C-C and CC functional groups, O2•− radicals, and semiquinone radicals could promote the oxidation of manganese. Eventually, manganese presented 2+ and high valence state was adsorbed on the surface of the co-system. In addition, BC facilitated strain AL-6 to convert organic carbon into CO2 and transfer electrons for N metabolism, and over time, CO2 and N2O could also be absorbed by BC. This study further dissected the efficiency on the Mn-contaminated water bodies and mechanism process by co-system and provided potential value in mitigating climate change.