Electrochemical Redox Potentials Research Articles

Synergetic Effect of LiNO3 and Fluorine-Containing Additives on Stabilizing Electrode-Electrolyte Interfaces in Lithium Metal Batteries

Lithium metal batteries (LMBs) are regarded as an ideal candidate for next-generation batteries to meet the increasing demand for high energy density batteries. Due to the high theoretical capacity (3860 mAh g-1) and the low electrochemical redox potential (- 3.04 V vs Standard Hydrogen Electrode (SHE)) of lithium metal anodes, lithium metal batteries can achieve a high energy density. However, the poor compatibility of conventional carbonate electrolytes with lithium metal anodes has limited the commercialization of LMBs. The unstable solid-electrolyte interphase (SEI) generated from the charging and discharging process leads to the dendritic growth of lithium and the continuous depletion of the electrolyte, limiting the cycle life of LMBs. Furthermore, the unstable cathode-electrolyte interphase (CEI) limits the charge transfer kinetics of Li+ and decomposes at high voltage operation of LMBs. Therefore, the formation of stable electrode-electrolyte interfaces is crucial for the adoption of LMBs. Numerous studies have been attempted to regulate the electrode-electrolyte interfaces by introducing additives to the electrolyte. Lithium nitrate (LiNO3) is an effective additive for regulating the SEI layers through the formation of a highly lithium ion-conductive SEI components derived from the reduction of NO3 - ions such as Li3N (2~4 x 10-4 S cm-1). However, it is vulnerable to volume expansion, leading to continuous consumption of the additive itself. Fluorine-containing additives such as fluoroethylene carbonate (FEC) and lithium difluoro(oxalate)borate (LiDFOB) can promote the formation of a robust, high mechanical strength (64.7 GPa) LiF-rich SEI layer to enhance the mechanical strength of the SEI layer to mitigate the lithium dendrite growth. Nevertheless, LiF can hinder the lithium ion transport, due to its low ionic conductivity (10-9 ~ 10-14 cm-2). The synergetic effect of LiF and Li3N as SEI components may be able to enable highly reversible LMBs. In addition, the LiDFOB additive have been reported to form a thin and stable CEI via B-O bonds with anchoring transition metal ions in the cathode, preventing the degradation of the cathode. The electrolyte system with FEC and LiDFOB is able to increase the cycle life of LMBs by the formation of robust SEI and CEI layer. Herein, the synergetic effects of LiNO3, FEC, and LiDFOB additive is discussed to further enhance the stability of the electrode-electrolyte interfaces for high energy density LMBs. The LiNO3 additive is sustainably released through the use of a porous film which consists of LiNO3 embedded polyacrylonitrile (PAN) nanofibers (LiNO3/PAN) to overcome the low solubility of LiNO3 in the carbonate electrolyte. Moreover, fluorine containing additives, LiDFOB and FEC are introduced with LiNO3 in order to form a LiF/Li3N-rich SEI to lengthen the life cycle of LMBs. LiDFOB-derived CEI also contributed to the enhanced stability of the cathode. Consequently, through the synergetic effects of LiNO3 and fluorine-containing additives, the capacity retention of ultra-thin Li (20 μm)||nickel-rich NCM811 cells is improved.

Ultra-Long and Rapid Operating Sodium Metal Batteries Enabled By Multifunctional Polarizable Interface Stabilizer

Sodium metal batteries (SMBs) have been considered as one of the most favorable and economic candidates for next generation batteries to overcome low enenrgy density limitation of sodium ion batteries (SIBs) due to high theoretical specific capacity (1166 mAh g-1), low electrochemical redox potential (-2.71 V vs. SHE). However, ‘moss-like’ dendrite growth that occurs during continuous charging and discharging processes not only threatens the safety of batteries, but also poses several problems to be solved, such as electrolyte consumption, hugh volume expansion, dead Na formation, and low coulombic efficiency. The dendrite problem in the SMBs is similr to that of lithium-metal batteries (LMBs), yet the degree of the issue is further severe than LMBs. Surface stabilization of sodium metal anode should be achieved to realize SMB technologies.The introduction of functional electrolyte additives is an effective and economical way to overcome these problems and improve the cyclability of SMBs. Based on the similarity between electroplating and sodium deposition reaction, utilization of surface leveler additive in the electrolyte could allow a substantial impact to stabilize sodium metal surface during repeated deposition and stripping process.In this presentation, we confirmed the surface flattening effect of molecular dipole electrolyte additives at the anode through the Na-Na symmetric cell test. Sodium metal deposition and stripping behavior has been successfully controlled by introducing surface leveler additives for realization of SMBs. Even under rapid charging and discharging situations, additive containing Na-Na symmetric cells showed stable performance for over 2000 hours, which is 5 times longer than the pristine cell without the additive. We confirmed that controlling intrinsic properties of surface leveler additive can effectively further improve cycle performance of SMBs.[1] S.-Y. Jun, K. Shin, C. Y. Son, J. Park, H.-S. Kim, J.-Y. Hwang, W.-H. Ryu, Advanced Energy Materials, 2024, 2304504[2] S.-Y. Jun, K. Shin, Y. Lim, S. Kim, H. Kim, C. Y. Son, W.-H. Ryu, Small Structures, 2024, 5, 2300578[3] S.-Y. Jun, K. Shin, J.-S. Lee, S. Kim, J. Chun, W.-H. Ryu, Advanced Science, 2023, 10, 2301426[4] J.-S. Lee, K. Shin, S.-Y. Jun, S. Kim, W.-H. Ryu, Chemical Engineering Journal, 2023, 458, 141383 Figure 1

Modifying Intermolecular Interactions of Single-Ion Conducting Polymer Electrolytes through Salt Additives for Stable and High Performance Lithium Metal Batteries

Lithium metal batteries (LMBs) are emerging as a crucial breakthrough in energy storage systems due to their lowest electrochemical redox potential at -3.04V (vs. the standard hydrogen electrode) and highest theoretical capacity of 3860 mAh g-1. However, the irregular deposition on the lithium metal anode during the cycles may lead to the formation of lithium dendrites, adversely affecting the safety and performance of LMBs. Furthermore, concentration polarization can cause lithium depletion at the electrode-electrolyte interface, increasing overpotential and thereby accelerating dendrite growth. Single-ion conducting polymer electrolytes (SIPEs), in which the anion is immobilized, possess the ability to mitigate concentration polarization. Herein, we prepared a salt-in-SIPE (SiSIPE) by incorporating a salt additive (LiTFSI) into SIPEs. Spectroscopy analysis (FT-IR and Raman) and density functional theory (DFT) calculations indicated that NMA, a plasticizer that has crystallinity at 25 ℃, can form a solvation structure with LiTFSI. The intermolecular interactions inhibit the SiSIPE from crystallizing even at -80 ℃, ensure high ionic conductivity (7.0×10-4 S cm-1) at 25 ℃, and lead to a high tLi + (0.67) comparable to that of the SIPE (0.71, 2.2×10-4 S cm-1 at 25℃). Additionally, while NMA is easily decomposed above 4.0 V, the interactions ensure that the SiSIPE possesses a wide electrochemical stability window up to 4.4 V. To suppress the decomposition of the electrolyte and achieve stable cycle performance at high voltage, the electrolyte's HOMO level should be lower than that of the cathode. The DFT calculation results showed that the HOMO level of the solvation structure was found to be lower than when NMA forms hydrogen bonds with each other, which implies that it can suppress the decomposition of NMA at high voltages. Additionally, since the solvation structure has a lower LUMO level than NMA, it is preferentially used for forming the solid electrolyte interphase (SEI) layer. As a result, as shown in the X-ray photoelectron spectroscopy results, due to the salt additive, the SiSIPE forms a more stable SEI layer compared to the SIPE. This leads to improved cycle stability in both Li symmetric cell (0.1 mAh cm-2, 700 hours) and LiFePO4 full cell tests (1 C-rate, 400 cycles, 80% retention). The intermolecular design through the salt additive can not only effectively suppress lithium dendrites but also enhance stability at high voltages and improve the cycle performance of LMBs. These strategies could significantly provide a new perspective in achieving high stability and high-performance LMBs. Figure 1

Effects of ion mass transport on electrochemical reaction pathways in aluminum-anthraquinone batteries

Aluminum anodes and quinone-based organic cathodes couple as earth abundant, sustainable, and safe battery electrodes. However, different numbers of galvanostatic discharge plateaus have been observed in aluminum-quinone cells depending on the experimental conditions, a phenomenon that has not yet been explained and furthermore affects cell energy density. Here, in aluminum-anthraquinone batteries, we show that ion mass transport affects the electrochemical reaction pathway and controls the occurrence of either one or two reduction plateaus during galvanostatic discharge. The effects of electrode mass loading, porosity, rest periods, and cycling rates were analyzed on the reduction potential and relative specific capacity of the galvanostatic plateaus. When AlCl2+ cations charge compensate the electrochemically reduced carbonyl groups concurrently, a single reduction plateau is observed. When ion mass transport is limiting, sequential reduction and charge compensation of each carbonyl group results in two distinct reduction plateaus. The second plateau occurs at a potential approximately 0.3 V lower than the first, thus decreasing the cell energy density. The potential of the electrochemical oxidation reaction where AlCl2+ cations dissociate, however, is not affected. Ion mass transport is thus shown to be a critical variable that can control the electrochemical reaction pathway, redox potentials, and practical energy densities of aluminum-quinone batteries.

Synthesis of Alkoxy-TEMPO Aminoxyl Radicals and Electrochemical Characterization in Acetonitrile for Energy Storage Applications

In this paper, we describe the synthesis and characterization of alkoxylated TEMPO, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, radicals with potential application in organic non-aqueous redox flow batteries. The behavior of a series of TEMPO derivatives with varying lengths of alkoxy chain is analyzed in acetonitrile solutions using electrochemical techniques, electron paramagnetic resonance (EPR) spectroscopy, and measurements of permeability through three different membranes. Electrochemical redox potentials are only weakly dependent on the substituent, but, in contrast, exchange current densities derived from the data do depend on the substitution. EPR lends further insight into these properties via the determination of hyperfine splitting constant and rotational correlation time. There is a negligible effect of the substituents on those parameters among the modified TEMPO radicals. Finally, permeation rates of modified TEMPO derivatives through membranes depend significantly on both the membrane and the substitution of TEMPO, providing insights into capacity fade measurements in the literature.

Structural, photophysical, and electrochemical redox properties of meso-tetrakis(pentafluorophenyl)porphyrins

This study delineates the synthesis and comprehensive characterization of metal complexes of meso-tetrakis(pentafluorophenyl)porphyrin (H2TPF[Formula: see text]P, 1) with first-row transition metal ions (M = Mn(III) 2, Fe(III) 3, Co(II) 4, Ni(II) 5, Cu(II) 6, and Zn(II) 7) to elucidate their structural, electronic spectral, and electrochemical redox properties. Co, Ni, Cu and Zn Complexes exhibit planar structure ([Formula: see text]24 = 0.003-0.021 Å and [Formula: see text]C[Formula: see text] = 0.004−0.041 Å) which is also confirmed by the single crystal data of complex ZnTPF[Formula: see text]P, 7 shows the planar geometry, while Fe and Mn complexes show the slightly nonplanar structure ([Formula: see text]24 = 0.101–0.143 Å and [Formula: see text]C[Formula: see text] = 0.080–0.112 Å), which are distorted due to their occupied axial sites by −Cl ligand as revealed by density functional theory (DFT) calculations. Furthermore, electronic spectroscopy revealed a gradual bathochromic shift following the order of NiTPF[Formula: see text]P < CoTPF[Formula: see text]P < CuTPF[Formula: see text]PCl < H2TPF[Formula: see text]P = FeTPF[Formula: see text]PCl < ZnTPF[Formula: see text]P < MnTPF[Formula: see text]PCl by appending different metal ions in the porphyrin core. MnTPF[Formula: see text]PCl exhibited the most significant bathochromic shift ([Formula: see text] = 63 nm) in the Soret band compared to H2TPF[Formula: see text]P. Also, MnTPF[Formula: see text]PCl and FeTPF[Formula: see text]PCl show a unique pattern of split Soret band at around 364 and 345 nm due to the interaction between the metal [Formula: see text]-orbitals and the surrounding ligands. The electrochemical redox potentials of the filled 3d orbital of Zn endow ZnTPF[Formula: see text]P (ring centered) with the lowest oxidation potential.

Electrochemical/computational analysis and the derivation of Pourbaix diagrams of redox-active structural and adsorbed species in uranyl peroxide clusters (U60)

Electrochemical analysis of U60 nanoclusters (Li40K20[UO2(O2)(OH)]60(H2O)214) and their natural analog, the mineral studtite (UO2)O2(H2O)2·(H2O)2, was carried out using cyclic voltammetry with a powder microelectrode setup. Voltammetric analysis was supplemented by ab initio quantum mechanical modeling to understand the mechanism, thermodynamics, and structural changes during redox transition (with a semi-quantitative kinetics evaluation). This research aims to better understand the redox behavior and transport of dissolved and structural (incorporated) uranyl units in the environment. Further analysis is performed to determine if redox switching of actinyl ions in clusters can be performed while leaving the cluster intact or at what redox transition it disintegrates. Quantum-mechanical calculations were applied to approximate electrochemical peak potential shifts and add them to standard reduction potentials to more accurately assign peaks to specific redox transitions between different species. These peak potentials are calculated from oxidation state-dependent binding energies between different uranyl species either within or adsorbed to U60. This theoretical approach incorporates extensive error cancellation and serves to predict electrochemical redox potentials and identify the reaction taking place. Voltammograms of U60 in electrolyte solutions exhibit kinetically-inhibited coupled redox peaks assigned to the U(VI)/U(V) transition of cluster structural uranyl units at −0.34 V (vs. standard hydrogen electrode), indicative of U60 reduction from UO22+ to UO2+. The U(V)/U(IV) transition at −0.71 V is assigned to the subsequent U60 reduction of UO2+ to UO20. A method of observing these transitions in situ using synchrotron x-ray absorption spectroscopy was explored. Voltammetry, modeling, and peak analysis indicate a kinetically-inhibited two-step, one-e- (per step) reduction of U60 from U(VI) to U(V) to U(IV). Voltammetry and x-ray absorption spectroscopy measurements performed before and after exhaustive cycling and the highly stable nature of U60 provide evidence that clusters typically do not break apart upon reduction. Voltammograms of U60 collected in varied uranyl solutions are indicative of UO22+ adsorption to the cluster surface and its subsequent reduction at +0.24 V (U(VI)/U(V)) and −0.05 V (U(V)/U(IV)). For uranyl solutions in electrolyte, the two-step, one-e- (per step) reduction of U60 is superseded by uranyl adsorption from solution to the electrode surface with subsequent reduction via U(V) disproportionation. This study represents the sole electrochemical investigation of U60 in the literature and only the second of any uranyl peroxide nanocluster. It is also one of the first to use quantum–mechanical approaches in combination with existing solution-based standard reduction potentials to calculate Pourbaix (EH-pH) diagrams for adsorbed and structural species.

Stress Relief in Metal Anodes: Mechanisms and Applications

AbstractMetal anodes (lithium/sodium/zinc) are recognized as the most promising choice for rechargeable batteries due to their high theoretical capacity and low electrochemical redox potential. Unfortunately, metal anodes face serious metal dendrite problems, hindering their practical applications. Recent research has shown that metal dendrites can also be caused by high levels of stress generated during the metal deposition process. To address this issue, an alternative strategy based on stress relief is proposed to inhibit the growth of metal dendrites. Herein, this work aims to investigate the mechanism of stress generation and evolution within metal anodes. In addition, this work explores the utilization of stress to induce metal nucleation. This work further discuss the various experimental techniques used to study stress release in metal dendrites and review recent research findings on stress relief in metal dendrites within metal anodes. Specifically, this work examines how microstructure and processing conditions affect stress release and discuss potential strategies for improving the efficiency of stress relief strategies in metal dendrites. As a result, a deeper understanding of stress release in metal dendrites can lead to the development of rechargeable batteries with superior performance, longer cycle life, as well as enhanced safety for practical applications in various fields.

(Best Student Presentation) A Calorimetric and Potentiometric Titration Method to Elucidate Ca2+ Solvation Thermodynamics

Calcium metal batteries are a promising long-term sustainable alternative to lithium metal batteries due to calcium’s high natural abundance and low electrochemical redox potential. Learning how to control Ca2+ coordination chemistry is essential in enabling high performance and stable calcium batteries. While several spectroscopic and computational studies of Ca2+ coordination have been conducted, they are limited by their qualitative nature and/or require experimental validation. This work introduces a novel methodology using a combination of calorimetric and potentiometric titration techniques, to quantitatively probe electrolyte coordination shell composition, as well as previously unattainable experimental coordination enthalpy, Gibbs free energy, and entropy. To establish the effectiveness of the method, here we investigate the coordination shell thermodynamics in simpler, highly dissociating LiPF6 electrolytes. Starting with a weakly solvated electrolyte, we titrate in a strong solvent—which preferentially coordinates to Li+—and measure the associated heat release/absorption and Li+ potential change with each injection. Fitting both sets of data to a mathematical binding model allows for the extraction of the change in enthalpy (ΔHi), Gibbs free energy (ΔGi), and entropy (ΔSi) associated with each Li+-solvent complex formation. Several weak solvent and strong solvents are investigated to build a dataset of ΔHi, ΔGi, and ΔSi. These thermodynamic values are used to calculate the average coordination number of each solvent in the Li+ coordination shell which agree well with literature. Raman and NMR spectroscopy confirm salt dissociation and solvent displacement during titration. This method unlocks a precise description of the chemical environment around Li+ and enables prediction of the Li+ coordination shell in new electrolytes by rationalizing the hierarchy of solvent binding. Given that this method extends to relevant calcium electrolytes, it is a powerful tool to engineer effective electrolytes for use with a calcium metal anode.

High‐Energy‐Heavy‐Ion Engineering Low‐Tortuosity and High‐Porosity 3D Metallic Electrodes for Long‐Life Lithium Anodes

AbstractLithium metal anodes are the most hopeful candidate for new‐generation energy storage batteries because of their high energy‐density and low electrochemical redox potential. However, the sluggish ion diffusion and nonuniform electric field in lithium anodes hinder their high rate properties and long‐life performance with deep capacities. Here, a highly interconnected 3D metallic Cu&CuAux matrix with both low‐tortuosity (1.3) and ultrahigh porosity (81.5%) is fabricated by using a high energy heavy ion‐tracking method. As a consequence, the 3D metallic Cu&CuAux matrix can highly accelerate the transfer of Li+ and reduce the Li nucleation barrier during the Li deposition process due to its low‐tortuosity and ultrahigh porosity. Furthermore, finite element simulation reveals that the unique 3D Cu&CuAux structure can highly homogenize the electric field and Li‐ion flux as well as decrease the lithium‐ion concentration gradient in Li anodes. As a result, the composite 3D Cu&CuAux‐Li anodes exhibit ultrahigh cycle life more than 2000 h and high rate capabilities. Full cells consisting of the 3D Cu&CuAux‐Li anodes and LiFePO4 cathodes also demonstrate a good capability and stable cycle life up to 200 cycles.

Nonaqueous Electrochemistry of Uranium Complexes: A Guide to Structure-Reactivity Tuning.

Uranium complexes can be stabilized in a wide range of oxidation states, ranging from UII to UVI and a very recent example of a UI complex. This review provides a comprehensive summary of electrochemistry data reported on uranium complexes in nonaqueous electrolyte, to serve as a clear point of reference for newly synthesized compounds, and to evaluate how different ligand environments influence experimentally observed electrochemical redox potentials. Data for over 200 uranium compounds are reported, together with a detailed discussion of trends observed across larger series of complexes in response to ligand field variations. In analogy to the traditional Lever parameter, we utilized the data to derive a new uranium-specific set of ligand field parameters UEL(L) that more accurately represent metal-ligand bonding situations than previously existing transition metal derived parameters. Exemplarily, we demonstrate UEL(L) parameters to be useful for the prediction of structure-reactivity correlations in order to activate specific substrate targets.

Waste Heat Driven Microporous-Carbon-Based Thermo-Electrochemical Cells

Solid-state thermoelectrics have focused on thermal to electrical energy conversion for many decades. Nevertheless, the thermoelectric energy conversion efficiency is limited by the intrinsic coupling between material properties such as electrical conductivity and the Seebeck coefficient, which reduces its efficiency despite much progress in this field. Here, we demonstrate a simple, cost-effective platform for harvesting waste heat into electricity via thermo-electrochemical cells (or thermocells) that utilize the effect of temperature on electrochemical redox potentials (i.e., the electrochemical Seebeck effect). In our preliminary experiments, coin cell type thermocells were prepared using microstructured carbon electrodes, aqueous ferricyanide/ferrocyanide redox couple, and commercial polymer separators. The wettability of the electrodes was varied by changing the duration of reactive ion etching (RIE), which improved the power performance. The highest output power (~ 75 nW) was recorded for RIE time of 3 minutes with a load of 1 kΩ. Separately, we also tested thermocells using natural wood-derived cellulose as separator materials. The performance of our thermocells will be presented, and their ability to directly power small electronic devices will be discussed.

High Current‐Density‐Charging Lithium Metal Batteries Enabled by Double‐Layer Protected Lithium Metal Anode

AbstractThe practical application of lithium (Li) metal anode (LMA) is still hindered by non‐uniformity of solid electrolyte interphase (SEI), formation of “dead” Li, and continuous consumption of electrolyte although LMA has an ultrahigh theoretical specific capacity and a very low electrochemical redox potential. Herein, a facile protection strategy is reported for LMA using a double layer (DL) coating that consists of a polyethylene oxide (PEO)‐based bottom layer that is highly stable with LMA and promotes uniform ion flux, and a cross‐linked polymer‐based top layer that prevents solvation of PEO layer in electrolytes. Li deposited on DL‐coated Li (DL@Li) exhibits a smoother surface and much larger size than that deposited on bare Li. The LiF/Li2O enriched SEI layer generated by the salt decomposition on top of DL@Li further suppresses the side reactions between Li and electrolyte. Driven by the abovementioned advantageous features, the DL@Li||LiNi0.6Mn0.2Co0.2O2 cells demonstrate capacity retention of 92.4% after 220 cycles at a current density of 2.1 mA cm–2 (C/2 rate) and stability at a high charging current density of 6.9 mA cm–2 (1.5 C rate). These results indicate that the DL protection is promising to overcome the rate limitation of LMAs and high energy‐density Li metal batteries.

Composite Coatings of Chitosan and Silver Nanoparticles Obtained by Galvanic Deposition for Orthopedic Implants.

In this work, composite coatings of chitosan and silver nanoparticles were presented as an antibacterial coating for orthopedic implants. Coatings were deposited on AISI 304L using the galvanic deposition method. In galvanic deposition, the difference of the electrochemical redox potential between two metals (the substrate and a sacrificial anode) has the pivotal role in the process. In the coupling of these two metals a spontaneous redox reaction occurs and thus no external power supply is necessary. Using this process, a uniform deposition on the exposed area and a good adherence of the composite coating on the metallic substrate were achieved. Physical-chemical characterizations were carried out to evaluate morphology, chemical composition, and the presence of silver nanoparticles. These characterizations have shown the deposition of coatings with homogenous and porous surface structures with silver nanoparticles incorporated and distributed into the polymeric matrix. Corrosion tests were also carried out in a simulated body fluid at 37 °C in order to simulate the same physiological conditions. Corrosion potential and corrosion current density were obtained from the polarization curves by Tafel extrapolation. The results show an improvement in protection against corrosion phenomena compared to bare AISI 304L. Furthermore, the ability of the coating to release the Ag+ was evaluated in the simulated body fluid at 37 °C and it was found that the release mechanism switches from anomalous to diffusion controlled after 3 h.

Singlet Oxygen Generation in Peripherally Modified Platinum and Palladium Porphyrins: Effect of Triplet Excited State Lifetimes and meso-Substituents on 1 O2 Quantum Yields.

A series of meso-substituted with aromatic (=tolyl, pyrenyl, fluorenyl, naphthyl, and triphenylamine) substituents, platinum (Pt), and palladium (Pd) porphyrins have been synthesized and characterized by spectroscopic and single-crystal X-ray diffraction studies to probe structure-reactivity aspects on the electrochemical redox potentials, and phosphorescence quantum yields and lifetimes. In the X-ray structures, the aromatic meso-substituents were rotated to some extent from the planarity of the porphyrin ring to minimize steric hindrance. Both Pt and Pd porphyrins revealed higher electrochemical redox gaps as compared to their free-base porphyrin analogs as a result of the harder oxidation and reduction processes. The ability of both Pt and Pd porphyrins to generate singlet oxygen was probed by monitoring the photoluminescence of 1 O2 at 1270 nm. Higher quantum yields for both triplet sensitizers compared to their free-base analogs were witnessed. Singlet oxygen quantum yields close to unity were possible to achieve in the case of Pt and Pd porphyrins bearing triphenylamine substituents at the meso-position. The present study brings out the importance of different meso-substituents on the triplet porphyrin sensitizers in governing singlet oxygen quantum yields; a key property of photosensitizers needed for photodynamic therapy, chemical synthesis, and other pertinent applications.

Comparing Isoelectronic, Quadruple-Bonded Metalloporphyrin and Metallocorrole Dimers: Scalar-Relativistic DFT Calculations Predict a >1 eV Range for Ionization Potential and Electron Affinity.

A scalar-relativistic DFT study of isoelectronic, quadruple-bonded Group 6 metalloporphyrins (M = Mo, W) and Group 7 metallocorroles (M = Tc, Re) has uncovered dramatic differences in ionization potential (IP) and electron affinity (EA) among the compounds. Thus, both the IPs and EAs of the corrole derivatives are 1 eV or more higher than those of the porphyrin derivatives. These differences largely reflect the much lower orbital energies of the δ- and δ*-orbitals of the corrole dimers relative to those of the porphyrin dimers, which in turn reflect the higher (+III as opposed to +II) oxidation states of the metals in the former compounds. Significant differences have also been determined between Mo and W porphyrin dimers and between Tc and Re corrole dimers. These differences are thought to largely reflect greater relativistic destabilization of the 5d orbitals of W and Re relative to the 4d orbitals of Mo and Tc. The calculated differences in IP and EA should translate to major differences in electrochemical redox potentials-a prediction that in our opinion is well worth confirming.

Tailored Design of Electrochemically Degradable Anthraquinone Functionality toward Organic Cathodes

In efforts to design organic cathode materials for rechargeable batteries, a fundamental understanding of the redox properties of diverse non-carbon-based functionalities incorporated into 9,10-anthraquinone is lacking despite their potential impact. Herein, a preliminary investigation of the potential of anthraquinones with halogenated nitrogen-based functionalities reveals that the Li-triggered structural collapse observed in the early stage of discharging can be ascribed to the preference toward the strong Lewis acid-base interaction of N-Li-X (X = F or Cl) over the repulsive interaction of the electron-rich N-X bond. A further study of three solutions (i.e., substitution of NX2 with (i) BX2, (ii) NH2, and (iii) BH2) to the structural decomposition issue highlights four conclusive remarks. First, the replacement of N and/or X with electron-deficient atom(s), such as B and/or H, relieves the repulsive force on the N-X bond without the assistance of Li, and thus, no structural decomposition occurs. Second, the incorporation of BH2 is verified to be the most beneficial for improving the theoretical performance. Third, all the redox properties are better correlated with electron affinity and solvation energy than the electronegativity of functionality, implying that these key parameters cooperatively contribute to the electrochemical redox potential; additionally, solvation energy plays a crucial role in determining cathodic deactivation. Fourth, the improvement to the Li storage capability of anthraquinone using the third solution can primarily be ascribed to solvation energy remaining at a negative value even after the binding of more Li atoms than the other derivatives.

Highly elastic wrinkled structures for stable and low volume-expansion lithium-metal anodes

Lithium (Li) metal is promising for high energy density batteries due to its low electrochemical redox potential and high specific capacity. However, the formation of dendrites and its tendency for large volume expansion during plating/stripping restrict the application of Li metal in practical scenarios. In this work, we developed reduced graphene oxide-graphitic carbon nitride (rGO-C3N4, GCN) with highly elastic and wrinkled structure as the current collector. Lithiophilic site C3N4 in GCN could reduce the nucleation overpotential. In addition, this material effectively inhibited electrode expansion during cycling. At the same time, due to its high elasticity, GCN could release the stress induced by Li deposition to maintain structural integrity of the electrode. Li-metal anodes with GCN exhibited small volume expansion, high Coulombic efficiency (CE) of 98.6% within 300 cycles and long cycling life of more than 1700 h. This work described and demonstrated a new approach to construct flexible current collectors for stable lithium-metal anodes.

Exogenous electron transfer mediator enhancing gaseous toluene degradation in a microbial fuel cell: Performance and electron transfer mechanism

Effective electron transfer (ET) between microorganisms and electrodes is essential for the toluene degradation and power generation in a microbial fuel cell (MFC). In this work, the neutral red, with excellent electrochemical reversibility and compatible redox potential as NADH/NAD+, was selected as electron mediator to boost the performance of the MFC. Experimental results revealed that, with the 0.5 μM neutral red, the removal efficiency and coulombic efficiency of the gaseous toluene powered MFC was increased by ~19% and ~400%, respectively. However, further increase in neutral red concentration resulted in a decreased in removal efficiency and coulombic efficiency, which was attributed by the toxicity of neutral red to the microbes. The microbial community analysis indicated that, with the dosage of the neutral red, the dominated bacteria shifted from Geobacter to Ignavibacteriales, resulting in a high coulombic efficiency. With the further increase in the neutral red, the amount of Ignavibacteriales gradually decreased and thus the coulombic efficiency declined at a high neutral red concentration. Based on the cyclic voltammetry analysis, an electron transport pathway involving neutral red, cytochromes, and OMCs in neutral red mediated MFC was proposed. Overall, the dosage of neutral not only enhanced the electron transfer but also induced the growth of the exoelectrogens, and thus significantly improve the MFC performance.

Structural, photophysical, electrochemical and spintronic study of first-row metal Tetrakis(meso-triphenylamine)-porphyrin complexes: A combined experimental and theoretical study

Different metal ions in the center of 3d-metalloporphyrins have subtle influences on their structural, photophysical, electrochemical, and spintronic properties. A new series of symmetrically meso-substituted porphyrins with six first row transition metals, namely TPA-MPs (M = 2H, Mn, Fe, Co, Ni, Cu, and Zn) have been synthesized and characterized by various spectroscopic techniques. The electrostatic potential energy surface (ESP) calculated by density functional theory (DFT) exhibits the concentrated positive electrostatic potential on the substituted triphenylamine group, while the negative electrostatic potential localized on the porphyrin ring varies depending on metal ions. All compounds were crystallographically characterized and showed a coplanar porphyrin macrocyclic ring except for TPA-MnP and TPA-ZnP, which are distorted due to their occupied axial sites by Cl− and H2O, respectively. Furthermore, the X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) confirmed the valence states of the metal ions in the TPA-MPs. Electronic spectra revealed a gradual bathochromic shift following the order of TPA-CoP < TPA-CuP < TPA-P = TPA-NiP < TPA-FeP < TPA-ZnP < TPA-MnP by appending different metal ions in the porphyrin skeleton. TPA-MnP exhibited the most significant bathochromic shift (Δλmax = 49 nm) in the Soret band compared to TPA-P. The electrochemical redox potentials of the TPA-MPs decrease with the stepwise filling of d orbitals of the central metal ions from 0.86 to 0.81 V and the completely filled 3d orbital of Zn endows TPA-ZnP with the lowest oxidation onset. Meanwhile, TPA-MnP and TPA-FeP exhibited dramatically reduced HOMO-LUMO gaps of 1.78 eV and 1.58 eV, respectively, compared to other TPA-MPs. The excellent thermal stability, panchromatic absorption in the whole visible range, and suitable energy levels of TPA-MPs indicate huge potential in the application as dyes.