Costate Research Articles

Occurrence and enrichment of critical metals in ferromanganese deposits in the western Pacific

Co-rich ferromanganese nodules and crusts are economically valuable deep-sea ferromanganese deposits widely distributed in the western Pacific Ocean and are rich in critical metals such as Co, Ni, Cu and rare earth elements (REEs). However, the lack of fine delineation and systematic comparisons of the distributions of these critical metals in these ferromanganese deposits limits the understanding of the occurrence states of Co, Ni, Cu and REEs and the metallogenesis of ferromanganese deposits. Therefore, the authors selected one nodule and one crust from the western Pacific Ocean, and utilized high-resolution methods such as micro-area X-ray fluorescence spectrometry (μ-XRF), laser-ablation inductively-coupled plasma time-of-flight mass spectrometry (LA-ICP-TOF-MS), and laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) to depict elemental distribution and elemental profile.The results show that the ferromanganese deposits are predominantly composed of Fe-vernadite with low Mn/Fe ratios. Co, Ni and Cu exist primarily in the manganate octahedral layer of Fe-vernadite. REEs mostly exist in feroxyhite of Fe-vernadite, and manganate octahedral layer of Fe-vernadite also have high Ce content. Co, Ni and Cu contents of ferromanganese deposits are closely correlated with Mn/Fe ratios: Co, Ni and Cu contents increase with Mn/Fe ratios until the Mn/Fe ratios rises up to 4, and remain stable when Mn/Fe ratios > 6. The enrichment of Co, Ni and Cu in ferromanganese deposits is controlled by the redox conditions of seawater during accretion, whereas REEs enrichment is related to Mn and Fe fluxes in seawater during accretion. The coupling relationships between elemental distributions help to reveal the elemental occurrence state, and the geochemistry of ferromanganese deposits are analyzed to identify element enrichment mechanism.

Regulating the Spin-State of Cobalt in Three-Dimensional Covalent Organic Frameworks for High-Performance Sodium-Iodine Rechargeable Batteries.

Although the catalytic activity is heavily reliant on the electronic structure of the catalyst, understanding the impact of electron spin regulation on electrocatalytic performance is still rarely investigated. This work presents a novel approach involving the single-atom coordination of cobalt (Co) within metalloporphyrin-based three-dimensional covalent organic frameworks (3D-COFs) to facilitate the catalytic conversion for sodium-iodine batteries. The spin state of Co is modulated by altering the oxidation state of the porphyrin-centered Co, achieving optimal catalysis for iodine reduction. Experimental results demonstrate that CoII and CoIII are incorporated into the 3D-COFs, exhibiting spin ground states of S = 1/2 and S = 0, respectively. The low spin state of CoIII is favorable to hybridize with the sp 3d orbitals of I3-, thus facilitating the conversion of I3- to I-. Density-functional theory (DFT) calculations further reveal that the presence of CoIII enhances iodide adsorption and accelerates the formation of NaI in 3D-COFs-CoIII, thereby promoting its rapid kinetic behaviors. Notably, the I2@3D-COFs-CoIII cathode achieves a high reversible capacity of 227.7 mAh g-1 after 200 cycles at 0.5 C and demonstrates exceptional cyclic stability, exceeding 2000 cycles at 10 C with a minor capacity fading rate of less than one 0.01% per cycle.

Regulating the Spin‐State of Cobalt in Three‐Dimensional Covalent Organic Frameworks for High‐Performance Sodium‐Iodine Rechargeable Batteries

Although the catalytic activity is heavily reliant on the electronic structure of the catalyst, understanding the impact of electron spin regulation on electrocatalytic performance is still rarely investigated. This work presents a novel approach involving the single‐atom coordination of cobalt (Co) within metalloporphyrin‐based three‐dimensional covalent organic frameworks (3D‐COFs) to facilitate the catalytic conversion for sodium‐iodine batteries. The spin state of Co is modulated by altering the oxidation state of the porphyrin‐centered Co, achieving optimal catalysis for iodine reduction. Experimental results demonstrate that CoII and CoIII are incorporated into the 3D‐COFs, exhibiting spin ground states of S = 1/2 and S = 0, respectively. The low spin state of CoIII is favorable to hybridize with the sp 3d orbitals of I3‐, thus facilitating the conversion of I3‐ to I‐. Density‐functional theory (DFT) calculations further reveal that the presence of CoIII enhances iodide adsorption and accelerates the formation of NaI in 3D‐COFs‐CoIII, thereby promoting its rapid kinetic behaviors. Notably, the I2@3D‐COFs‐CoIII cathode achieves a high reversible capacity of 227.7 mAh g‐1 after 200 cycles at 0.5 C and demonstrates exceptional cyclic stability, exceeding 2000 cycles at 10 C with a minor capacity fading rate of less than one 0.01% per cycle.

OPTIMIZING ROYALTY PAYMENTS FOR MAXIMUM ECONOMIC BENEFIT: A CASE STUDY UTILIZING MODIFIED SHOOTING AND DISCRETIZATION METHODS

This research delves into the application of the modified shooting method for the numerical resolution of non-standard optimal control (OC) problems. More precisely, it concentrates on scenarios where the final state value component remains unknown and unconstrained, leading to a non-zero final shadow value or costate variable. Moreover, the objective function involved a piecewise royalty function, which poses a challenge due to its lack of differentiability within a specific time interval. Consequently, the novel modified shooting method was employed to ascertain the elusive final state value. The model’s differentiability is maintained throughout by adopting a continuous hyperbolic tangent (tanh) approximation. In addition, the construction of the Sufahani-Ahmad-Newton-Golden-Royalty Algorithm (SANGRA) and Sufahani-Ahmad-Powell-Golden-Royalty Algorithm (SAPGRA) was accomplished using the C++ programming language to formulate the problem. The outcomes of these algorithms, satisfying the criteria for optimality, were juxtaposed with non-linear programming (NLP) techniques such as Euler and Runge-Kutta, aside from Trapezoidal and Hermite-Simpson approximations. This groundbreaking discovery carries extensive practical implications as it propels the field forward and ensures the application of contemporary problem-solving methodologies. Moreover, the study underscores the significance of fundamental theory in effectively tackling current economic challenges.

Enhanced energy storage efficiency of an innovative three-dimensional nickel cobalt metal organic framework nanocubes with molybdenum disulphide electrode material as a battery-like supercapacitor

Recently, metal-organic frameworks (MOFs) with transition metal sulfides have gained much attention as electrode materials for supercapacitors (SCs). In this work, 3D-NCMOF@MS NCs, incorporating two-dimensional molybdenum disulphide (2D-MS) nanosheets and three-dimensional nickel cobalt-MOF nanocubes (3D-NCMOF NCs) are synthesized by a solvothermal and precipitation process. Owing to their enhanced electrical conductivity and huge surface area, the 3D-NCMOF@MS NCs produce superior electrochemical responses in SCs compared to 2D-MS and 3D-NCMOF NCs. The results demonstrate that the synergistic 3D-NCMOF@MS NCs increased specific capacitance up to 1048 Fg−1 at 1 Ag−1, revealing outstanding cycle stability with a capacitance retention of 99.06 %. Besides, the exposed high-power density of 3D-NCMOF@MS NCs reached 400 W kg−1 as well as an outstanding maximum energy density of 188.64 Wh kg−1. Such synergistic effects of the multivalent states of Ni, Co, and Mo promote electrochemical characteristics, implying that the 3D-NCMOF@MS NCs could potentially be used in energy storage systems, boosting electrochemical performance.

Theoretical investigations of optoelectronic properties, photocatalytic performance as a water splitting photocatalyst and band gap engineering with transition metals (TM = Fe and Co) of K3VO4, Na3VO4 and Zn3V2O8: a first-principles study.

First-principles density functional investigations of the structural, electronic, optical and thermodynamic properties of K3VO4, Na3VO4 and Zn3V2O8 were performed using generalized gradient approximation (GGA) via ultrasoft pseudopotential and density functional theory (DFT). Their electronic structure was analyzed with a focus on the nature of electronic states near band edges. The electronic band structure revealed that between 6% Fe and 6% Co, 6% Co significantly tuned the band gap with the emergence of new states at the gamma point. Notable variations were highlighted in the electronic properties of Na3V(1-x)Fe x O4, Na3V(1-x)Co x O4, K3V(1-x)Fe x O4, K3V(1-x)Co x O4, Zn3(1-x)V2(1-x)Co x O8 and Zn3(1-x)V2(1-x)Fe x O8 (where x = 0.06) due to the different natures of the unoccupied 3d states of Fe and Co. Density of states analysis as well as α (spin up) and β (spin down) magnetic moments showed that cobalt can reduce the band gap by positioning the valence band higher than O 2p orbitals and the conduction band lower than V 3d orbitals. Mulliken charge distribution revealed the presence of the 6s2 lone pair on Zn, greater population and short bond length in V-O bonds. Hence, the hardness and covalent character develops owing to the V-O bond. Elastic properties, including bulk modulus, shear modulus, Pugh ratio and Poisson ratio, were computed and showed Zn3V2O8 to be mechanically more stable than Na3VO4 and K3VO4. Optimal values of optical properties, such as absorption, reflectivity, dielectric function, refractive index and loss functions, demonstrated Zn3V2O8 as an efficient photocatalytic compound. The optimum trend within finite temperature ranges utilizing quasi-harmonic technique is illustrated by calculating thermodynamic parameters. Theoretical investigations presented here will open up a new line of exploration of the photocatalytic characteristics of orthovanadates.

Heat-Equation-Based Smoothing Homotopy Method for Nonlinear Optimal Control Problems

This paper presents a new heat-equation-based smoothing homotopy method for solving nonlinear optimal control problems with the indirect method. The surrogates, derived from the heat equation solution, are first incorporated into the necessary conditions as replacement of the terminal state and costate variables. The homotopy process is then applied to the heat conduction time: a longer time results in a more uniform temperature distribution and greater stability against initial temperature variations, thereby making the corresponding homotopy problem much easier to solve; zero time implies no heat conduction and reverts to the original problem. Furthermore, capitalizing on the heat equation’s boundedness characteristic, an efficient approach for determining the hypersensitive parameters is proposed, thus obviating the necessity of manual tuning. Challenging numerical examples are provided to demonstrate the superior performance of the proposed method, indicating that its convergence and efficiency are significantly enhanced compared to the original smoothing homotopy methods.

Synergistic interface engineering of ZnCo2O4/NiMnCo-layered double hydroxides for boosted reactivity in advanced supercapacitor electrodes

NiMn layered double hydroxide (LDH) exhibits high redox activity as a cathode material for hybrid supercapacitors, but it has the disadvantages of low conductivity and poor stability. In this work, the electrochemical properties of NiMnCo-LDH with different Co contents prepared by electrodeposition were tested. The results showed that the addition of an appropriate amount Co increased the adsorption of OH− on NiMn-LDH and improved the reactivity of the material. ZnCo-MOF grown on carbon cloth was transformed into flake ZnCo2O4 by calcination, and NiMnCo-LDH was electrodeposited on it to prepare ZnCo2O4/NiMnCo-LDH composite electrode. The core-shell structure increases the reaction area between the LDH material and the electrolyte. Physical characterization and density functional theory (DFT) calculations show that the structure and electronic distribution of ZnCo2O4/NiMnCo-LDH at the heterogeneous interface are reconstructed, which increases the valence state of Co and Mn elements, and finally exhibits enhanced electrochemical performance. Due to the existence of this synergistic effect, the specific capacity of the electrode is as high as 231.5 mAh g−1, with excellent stability demonstrated after 5000 cycles compared to the NiMnCo-LDH. The assembled hybrid supercapacitor exhibits a high energy density of 39.4 Wh kg−1, and has high cycle stability (90 % after 5000 cycles).

Comparison between Co(II) and Ni(II) cycling at goethite-water interfaces: Interplay with Fe(II)-catalyzed recrystallization

Cobalt (Co) and nickel (Ni) are critical metals for modern renewable energy technologies as well as essential micronutrients for terrestrial plant health and marine primary production. Both metals are commonly surface-adsorbed onto and/or structurally-incorporated into iron (oxyhydr)oxide minerals, such as goethite (α-FeOOH), that are ubiquitously present in soils and sediments at the Earth’s surface. In sub- and anoxic environments, aqueous ferrous iron (Fe(II)) generated from dissimilatory Fe(III) reduction can induce the recrystallization of goethite and subsequently influences the speciation and mobilization of associated metals, e.g., Co(III) being reduced to Co(II). While it is generally considered that divalent Co and Ni behave similarly at goethite-water interfaces, there lacks a direct comparison of their cycling through goethite in response to Fe(II)-catalyzed recrystallization. Here, under circumneutral anoxic conditions with and without addition of aqueous Fe(II), we performed batch reaction experiments on Co(II)-substituted goethite and Co(II) and Ni(II) sorption experiments on pure goethite. The redox state and coordination environment of Co associated with goethite were determined using high energy resolution fluorescence detected X-ray absorption spectroscopy at the Co K-edge, and the distribution of solid-bound Co and Ni in goethite following sorption was revealed by sequential dissolution. Our results show that substitution of Co(II) for Fe(III) in goethite has less negative feedback on Fe(II)-catalyzed recrystallization than Ni(II), which may be related to their differing structural distortions as evidenced by EXAFS. In the absence of Fe(II), aqueous Co(II) is more favourably adsorbed onto and incorporated deeper into goethite than Ni(II), suggesting that Co(II) is more structurally compatible with goethite. The presence of Fe(II) markedly enhances the structural incorporation of both Co(II) and Ni(II) as a result of goethite recrystallization, which in turn can be inhibited by the accumulation of metals at the mineral surface. Furthermore, structurally-incorporated Co(II) is more difficult to be released back into solution (i.e. sum of aqueous and extract fractions) than Ni(II) during Fe(II)-catalyzed goethite recrystallization. Overall, our work highlights the significant differences between Co(II) and Ni(II) in their cycling at goethite-water interfaces and intricate interplay with Fe(II)-catalyzed recrystallization. This improves our understanding of their mobilization and distribution in anoxic goethite-rich systems and can provide useful insights for their enrichment and extraction from natural and artificial laterites generated from the enhanced weathering of ultramafic mine tailings.

Co(III)–Co(II)–Co(III) edge-sharing trinuclear complexes having the doubly methoxido-bridged core bridged by acetato ligand

Two trinuclear cobalt complexes with the edge-sharing linear-type framework having benzylbis(2-pyridylmethyl)amine (bbpma), [{CoIII(bbpma)(μ-O2CCH3)(μ-OCH3)2}2CoII](X)2 (X=ClO4; [1](ClO4)2, X =PF6; [1](PF6)2), and one corresponding complex having ethylbis(2-pyridylmethyl)amine (ebpma), [{CoIII(ebpma)(μ-O2CCH3)(μ-OCH3)2}2CoII](PF6)2 ([2](PF6)2), were synthesized and the crystallographic and electronic structures were investigated. X-ray crystallography and magnetic measurements suggested that [1](PF6)2 and [2](PF6)2 were in the mixed-valent state and the mixed-spin state of Co(III)–Co(II)–Co(III) (LS-HS-LS), in which the electron spins were much localized on the central Co(II).

Mainardi smoothing homotopy method for solving nonlinear optimal control problems

This paper proposes a new type of smoothing homotopy method for solving general nonlinear optimal control problems via the indirect method, leveraging the Mainardi kernel as the smoothing kernel. The Mainardi kernel is firstly derived from the fundamental solution of the time-fractional diffusion-wave equation, representing a generalized form of the Gaussian kernel. By altering the fractional derivative order, the kernel can seamlessly switch between non-Gaussian and Gaussian forms. Then, parts of the two-point boundary value problems are convolved with the smoothing kernel, and the resulting surrogates are incorporated into the necessary conditions, replacing the terminal state and costate variables. The homotopy process is used for the smoothing parameter: increasing this parameter enhances the smoothing effect, simplifying the homotopy problems, while a parameter value of zero implies no smoothing, reverting to the original problems. Additionally, explicit expressions for the Mainardi kernel at specific derivative orders are derived, avoiding the inefficiencies associated with fractional derivatives. Simulation examples demonstrate that the proposed method offers significant advantages in flexibility and convergence compared to the traditional Gaussian smoothing homotopy method.

First-Principles Nonadiabatic Dynamics of Molecules at Metal Surfaces withVibrationally Coupled Electron Transfer.

Accurate description of nonadiabatic dynamics of molecules at metal surfaces involving electron transfer has been a long-standing challenge for theory. Here, we tackle this problem by first constructing high-dimensional neural network diabatic potentials including state crossings determined by constrained density functional theory, then applying mixed quantum-classical surface hopping simulations to evolve coupled electron-nuclear motion. Our approach accurately describes the nonadiabatic effects in CO scattering from Au(111) without empirical parameters and yields results agreeing well with experiments under various conditions for this benchmark system. We find that both adiabatic and nonadiabatic energy loss channels have important contributions to the vibrational relaxation of highly vibrationally excited CO(v_{i}=17), whereas relaxation of low vibrationally excited states of CO(v_{i}=2) is weak and dominated by nonadiabatic energy loss. The presented approach paves the way for accurate first-principles simulations of electron transfer mediated nonadiabatic dynamics at metal surfaces.

Competition between Cubic and Tetragonal Phase Induced Unusual Vertical Magnetization Shift and Exchange Bias in CoxMn3−xO4 (1 ≤ x ≤ 2) Spinel Nanoparticles

CoxMn3−xO4 (1 ≤ x ≤ 2) spinel nanoparticles synthesized through conventional coprecipitation technique exhibit the coexistence of tetragonal and cubic phases in the range of 1 ≤ x ≤ 1.5, while a pure cubic phase is observed for 1.75 ≤ x ≤ 2 at room temperature. Reduction in tetragonal phase fraction from 92 % for x = 1 to 47% for x = 1.5 is attributed to diminution of Jahn–Teller (J–T) active Mn3+ ions occupying the octahedral site of spinel lattice. X‐ray photoelectron spectroscopy confirms both +2 and +3 oxidation states for Co and Mn. Surprisingly, cubic and tetragonal phases exhibit magnetic transition, Tc1 and Tc2, corresponding to a paramagnetic to a high and low temperature ferrimagnetic state, respectively. Tetragonal phase induces high spontaneous (HSEB) and conventional (HCEB) exchange bias with unusually high vertical magnetization shift (VMS) than that of the pure cubic phase, shows maximum HCEB of 4.062 kOe for x = 1.5 and a VMS of 2.5 emu g−1 for x = 1. Such dependence of VMS and exchange bias on tetragonal to cubic phase ratio in CoxMn3−xO4 is demonstrated for the first time and interpreted based on the interaction between different arrangement of spins in tetrahedral and octahedral sublattice.

The Indirect Bang-Singular Algorithm (IBSA) for singular control problems with state-inequality constraints

In this study, we present the Indirect Bang-Singular Algorithm (IBSA), a straightforward computational framework developed to solve a wide range of Singular Optimal Control Problems (SOCP) with state-inequality constraints. The algorithm is a type of arc-classification technique which reformulates the SOCP as a nonlinear programming problem over the switching times, and possibly some other parameters such as the co-states’ initial values at the entry time to a singular interval. We derive the singular control feedback using the Pontryagin’s maximum principle and analyze the possibility of an interval where multiple controls are simultaneously singular. Furthermore, we incorporate the state-inequality constraints using the direct-adjoining method. Owing to the linear property of the co-state dynamics, the co-state variables and consequently, the singular controls are computed automatically using MATLAB’s symbolic platform. The nonlinear programming is constructed in a manner to circumvent the challenges posed by state-inequality constraints in more intricate scenarios involving singular controls expressed in terms of incomplete state-feedback functions. We also present several theorems that are integral to devising a straightforward computational approach for solving SOCPs. To assess the effectiveness of the proposed algorithm, we solve the following novel problems: (1) time–fuel-optimal commercial aircraft cruise flight in a vertical plane (i.e., with state-inequality constraint, a scalar singular control, and wind shear effects), and (2) the free-routing time–fuel-optimal commercial aircraft flight in a vertical plane (i.e., with state-inequality constraint, a dual-entry singular control, and wind shear effects). Notably, the optimality of the graphed results has been carefully inspected through first and second-order optimality conditions.

Probing quantum discord and coherence dynamics in the discrete-time quantum walk under generalized amplitude damping noise channel

We investigate the behaviour of one-dimensional dissipative Hadamard discrete-time quantum walk (DTQW) in the generalized amplitude damping channel. By manipulating the noise intensity, we uncover intriguing dynamics in the coherence of the coin state such as rebounding and freezing phenomena. The entanglement between the quantum walker’ states diminishes at lower dissipation rates despite the pronounced coherence. The non-classical characters of the dissipative DTQW becomes evident through the manifestation of quantum discord. Our study shows that even under maximum dissipation stemming from both zero and finite-temperature environment, the quantum walker retains rudimentary non-classical behaviours. It is shown that fine-tuning the parameter governing the coin rotation angle optimizes the quantum discord of the walker. These results are useful for optimizing quantum walks in the presence of noisy channels.

Highly sensitive 2D X-ray absorption spectroscopy via physics informed machine learning

Improving the spatial and spectral resolution of 2D X-ray near-edge absorption structure (XANES) has been a decade-long pursuit to probe local chemical reactions at the nanoscale. However, the poor signal-to-noise ratio in the measured images poses significant challenges in quantitative analysis, especially when the element of interest is at a low concentration. In this work, we developed a post-imaging processing method using deep neural network to reliably improve the signal-to-noise ratio in the XANES images. The proposed neural network model could be trained to adapt to new datasets by incorporating the physical features inherent in the latent space of the XANES images and self-supervised to detect new features in the images and achieve self-consistency. Two examples are presented in this work to illustrate the model’s robustness in determining the valence states of Ni and Co in the LiNixMnyCo1-x-yO2 systems with high confidence.

Investigation of the stability and physical properties of the newly synthesized Ta2AC (A=Co and Ni) MAX phases from first principles

Recently, the MAX phase compounds Ta2NiC and Ta2CoC have been successfully synthesized. In order to theoretically explore their properties, we conducted first-principles calculations based on density functional theory to investigate the structural stability, elastic anisotropy, mechanical behavior, electronic, and thermodynamic properties of these MAX phases. Our results reveal that the calculated structural parameters agree well with the experimental data. The MAX phases under investigation have been observed to exhibit thermodynamic, dynamic, and mechanical stability. Their electrical conductivity primarily originates from the d states of Ta, Ni, and Co at the Fermi level. They are ductile, elastically anisotropic, and demonstrate favorable machinability. The thermal parameters obtained from our calculations were compared with the reported values of a well-established material commonly used in thermal barrier coatings (TBCs). It has been concluded that the newly synthesized MAX phases Ta2NiC and Ta2CoC are potential thermal barrier coating materials.

An iterative method to solve the time delay optimal control problem with bounded control variable

This paper presents an iterative approach for solving the optimal control problem (OCP) with time-delayed terms and bounded control variables. The projection function is used to construct the iterative method to approximate the control law. Employing the approximation of control law the approximation of state and the co-state variables are obtained. To the best of our knowledge, this paper is the first attempt to solve bounded and delayed OCPs with a projection function. The convergence theorem for the proposed method is also proven. Several numerical examples are added, showing the developed method's effectiveness to solve the time-delayed OCPs with bounded control variables.

Performance improvement of oxygen electrode in reversible protonic ceramic electrochemical cell by co-doping of La and F

Reversible protonic ceramic electrochemical cell (R-PCEC) has demonstrated high potential as an energy storage and conversion device. Efforts have primarily concentrated on the development of oxygen electrodes being to increase their performance and reduce performance degradation. In this work, three perovskite materials, namely Ba0.9Co0.2Fe0.7Nb0.1O3-δ (BCFN), Ba0.8La0.1Co0.2Fe0.7Nb0.1O3-δ (BLCFN), and Ba0.8La0.1Co0.2Fe0.7Nb0.1F0.1O2.9-δ (BLCFNF) are investigated. Both doping with La and co-doping with La and F lead to a notable reduction in the average valence states of Fe, Co, and Nb elements, further resulting in an increase in oxygen vacancy concentration, thereby enhancing the oxygen migration ability of the material. The H2O partial pressure does not have a significant impact on the polarization resistance (Rp) of the oxygen electrode. Conversely, the rise in Rp is highly correlated with the decrease in oxygen partial pressure, suggesting that the primary factor influencing the reaction rate of the oxygen electrode process is the exchange of oxygen ions on the surface of the electrode. BLCFNF exhibits the best oxygen migration and surface exchange ability, resulting in the lowest Rp. When BLCFNF was used as the oxygen electrode in a R-PCEC with the humidity air (10%H2O) and humidified H2 (3%H2O) as oxidant and fuel, respectively, it exhibits the highest maximum power density of 472 mW cm−2, and a current density of −682.6 mA cm−2 (1.3 V) at 650 °C. In addition, BLCFNF also shows good durability in both discharge and electrolysis process. The results indicate that La and F co-doped BLCFNF material is a promising oxygen electrode material for R-PCECs.

Redox reactivity of LMCT and MLCT excited states of Earth‐abundant metal complexes

AbstractPrecious metal complexes, which act as excellent photoredox catalysts, have now being replaced by earth‐abundant metal complexes. This review focused on redox reactivity of ligand‐to‐metal charge transfer (LMCT) and metal‐to‐ligand charge transfer (MLCT) excited states of earth‐abundant metal complexes. Iron complexes with strongly σ‐donating NHC‐ligands (NHC = N‐heterocyclic carbene) has emerged, featuring long lived LMCT excited states due to a significantly increased barrier for deactivation via metal centered states. A Fe(III)‐NHC complex acts as an effective photoredox catalyst for various photocatalytic redox reactions. Although manganese(IV)‐oxo complexes have no long‐lived excited states (τ < < 1 ns). Once acids such as Sc(OTf)3 and HOTf are bound to the oxo moiety of Mn(IV)‐oxo complexes, the photoexcitation of acid‐bound Mn(IV)‐oxo complexes resulted in formation of the excited states with microseconds lifetimes, which are capable of oxidation of substrates including benzene to phenol. Photoexcited states of Mn(III), Mn(II) and Mn(I) complexes act as photoreductants to reduce substrates including O2. On the other hand, photoexcited states of Co(IV) and Co(III) complexes act as photooxidants, whereas those of Co(II) and Co(I) complexes act as photoreductants. With regard to the excited state lifetime, [Cr(tpe)2]3+ (tpe = 1,1,1‐tris(pyrid‐2‐yl)ethane) exhibited the longest luminescence lifetime (τ = 4500 μs), acting as an effective photoredox catalyst for photocatalytic redox reactions. The LMCT state of a Cr(0) complex acts as a super photoreductant. Thus, LMCT and MLCT excited states of earth‐abundant metal complexes are utilized as strong photooxidants and photoreductants, respectively.