Transition Metal Phthalocyanines as Redox Mediators in Li-O2 Batteries: A Combined Experimental and Theoretical Study of the Influence of 3d Electrons in Redox Mediation

A novel overtone peak self-referencing fluorescent sensor based on a bipyridine-linked covalent organic framework for highly sensitive copper ion detection

This study reports a novel ratiometric fluorescence sensor based on a tetraphenylethylene-bipyridine covalent organic framework (TPE-Bpy-COF) for the sensitive detection of Cu2+, leveraging the unique coordination properties of the bipyridine moieties. The interaction between Cu2+ and the nitrogen atoms in the bipyridine units induces fluorescence quenching at 500 nm through an efficient host-guest electron transfer mechanism, where excited-state electrons from the COF framework are transferred to the vacant orbitals of Cu2+. Upon excitation at 410 nm, the sensor exhibits a primary emission peak at 500 nm, which is quenched in the presence of Cu2+, while an overtone peak at 820 nm remains stable, serving as an internal reference for ratiometric measurements and significantly enhancing the accuracy and reliability of the sensor. The detection limit for Cu2+ is 0.1 μM, with the dual-emission system and the strong affinity of the bipyridine units for Cu2+, further improving the sensor's sensitivity and selectivity. Additionally, the sensor demonstrated excellent recovery rates in real water samples, confirming its practical applicability in environmental monitoring.

Operando spectroscopy investigations of the redox reactions in heme and heme-proteins

Operando spectroscopic investigations during molecular redox processes provide unique insights into complex molecular structures and their transformations. Herein, a combination of a potentiodynamic method with spectroscopy has been employed to holistically investigate the structural transformations during Fe-redox (Fe3+ ↔ Fe2+) of hemin vis á vis heme-proteins, e.g. myoglobin (Mb), hemoglobin (Hb) and cytochrome-C (Cyt-C). The UV-vis findings reveal the formation of hemozoin (≈heme-dimer), which can be selectively prevented via a high concentration of strongly interacting ligands, e.g. histidine (the fifth coordinating ligand in the heme-based protein). On the other hand, methionine does not prevent the formation of hemozoin. In Mb, Hb, and Cyt-C, as the fifth coordination site is occupied by histidine, hemozoin formation is inhibited. During Fe3+→ Fe2+, operando circular dichroism exhibits a decrease in the initial helical component in Hb from nearly 40% to 28%, which is close to the initial helix component of Mb (≈25%), strongly indicating denaturation of the protein in the redox pathway. The rate of change of the helices versus potential is almost identical for Mb and Hb, but comparatively faster than Cyt-C. In addition, from the Raman bands of M-N dynamics and protein agglomeration, it is concluded that Cyt-C prefers to agglomerate in the 2+ state, whereas Mb/Hb in the 3+ state. In this report, the power of operando spectroscopy is utilized to unearth the dynamics of hemin and heme-based proteins for comprehending the underlying complexities associated with the molecular redox, which have deep implications in electrocatalysis, energy storage, and sensing.

A MOF-Gel Based Separator for Suppressing Redox Mediator Shuttling in Li-O2 Batteries

Redox mediators (RMs) are widely utilized in the electrolytes of Li-O2 batteries to catalyze the formation/decomposition of Li2O2, which significantly enhances the cycling performance and reduces the charge overpotential. However, RMs have a shuttle effect by migrating to the Li anode side and inducing Li metal degradation through a parasitic reaction. Herein, a metal-organic framework gel (MOF-gel) separator is proposed to restrain the shuttling of RMs. Compared to traditional MOF nanoparticles, MOF gels form uniform and dense films on the separators. When using Ru(acac)3 (ruthenium acetylacetonate) as an RM, the MOF-gel separator suppresses the shuttling of Ru(acac)3 toward the Li anode side and significantly enhances the performance of Li-O2 batteries. Specifically, Li-O2 batteries exhibit an ultralong cycling life (410 cycles) at a current density of 0.5 A g-1. Moreover, the batteries using the MOF-gel/celgard separator exhibit significantly improved cycling performance (increase by ≈1.6 times) at a high current density of 1.0 A g-1 and a decreased charge/discharge overpotential. This result is expected to guide future development of battery separators and the exploration of redox mediators.

del Campo J. Excited States by Coupling Piris Natural Orbital Functionals with the Extended Random-Phase Approximation

In this work, we explore the use of Piris natural orbital functionals (PNOFs) to calculate excited-state energies by coupling their reconstructed second-order reduced density matrix with the extended random-phase approximation (ERPA). We have named the general method PNOF-ERPA, and specific approaches are referred to as PNOF-ERPA0, PNOF-ERPA1, and PNOF-ERPA2, according to the way the excitation operator is built. The implementation has been tested in the first excited states of H2, HeH+, LiH, Li2, and N2 showing good results compared to the configuration interaction (CI) method. As expected, an increase in accuracy is observed on going from ERPA0 to ERPA1 and ERPA2. We also studied the effect of electron correlation included by PNOF5, PNOF7, and the recently proposed global NOF (GNOF) on the predicted excited states. PNOF5 appears to be good and may even provide better results in very small systems, but including more electron correlation becomes important as the system size increases, where GNOF achieves better results. Overall, the extension of PNOF to excited states has been successful, making it a promising method for further applications.

Understanding Catalytic Mechanisms and Cathode Interface Kinetics in Nonaqueous Mg-CO2 Batteries

We leverage first-principles density functional theory (DFT) calculations to understand the electrocatalytic processes in Mg-CO2 batteries, considering ruthenium oxide (RuO2) as an archetypical cathode catalyst. Our goal is to establish a mechanistic framework for understanding the charging and discharging reaction pathways and their influence on overpotentials. On the RuO2 (211) surface, we found reaction initiation through thermodynamically favorable adsorption of Mg followed by interactions with CO2. However, we found that the formation of carbonate (CO32-) and oxalate (C2O42-) intermediates via the activation of CO2 at the catalytic site is thermodynamically unfavorable. We predict that MgC2O4 will form as the discharge product due to its lower overpotential compared to MgCO3. However, MgC2O4 is thermodynamically unstable and is expected to decompose into MgCO3, MgO, and C as final discharge products. Through Bader charge analysis, we investigate the covalent interactions between intermediates and catalyst sites. Moreover, we study the electrochemical free energy profiles of the most favorable reaction pathways and determine discharge and charge overpotentials of 1.30 and 1.35 V, respectively. Our results underscore the importance of catalyst design for the cathode material to overcome performance limitations in nonaqueous Mg-CO2 batteries.

Oxygen Radical Anion Substituted Iron Phthalocyanine as an Effective Redox Mediator for Li-O2 Batteries

Transition metal phthalocyanines are potential soluble redox mediators for Li-O₂ batteries. In this work, effective strategies to control the redox potentials and activities of iron phthalocyanine (FePc) based redox mediators are designed by the introduction of electron-withdrawing or electron-donating groups. Substituted electron-donating groups can shift the oxidation potential of FePc to a higher energy level, consequently reducing the charging voltage of Li-O₂ batteries. Especially, oxygen radical anion (-O^-) modified FePc (FePc-O^-) shows the most significant improvement to the oxygen reduction and evolution reactions of Li-O₂ batteries. Electronic analysis indicates that -O^- substitution can break the symmetry of electronic structures of FePc which further tunes the reduction of O₂ and the oxidation of Li₂O₂. Detailed reaction mechanisms of (FePc-O^-)-mediated Li-O₂ batteries are proposed based on first-principles molecular dynamics simulations and thermodynamic free energy calculations.

Binuclear Cu complex catalysis enabling Li-CO2 battery with a high discharge voltage above 3.0 V

Li-CO₂ batteries possess exceptional advantages in using greenhouse gases to provide electrical energy. However, these batteries following Li₂CO₃-product route usually deliver low output voltage (<2.5 V) and energy efficiency. Besides, Li₂CO₃-related parasitic reactions can further degrade battery performance. Herein, we introduce a soluble binuclear copper(I) complex as the liquid catalyst to achieve Li₂C₂O₄ products in Li-CO₂ batteries. The Li-CO₂ battery using the copper(I) complex exhibits a high electromotive voltage up to 3.38 V, an increased output voltage of 3.04 V, and an enlarged discharge capacity of 5846 mAh g^-1. And it shows robust cyclability over 400 cycles with additional help of Ru catalyst. We reveal that the copper(I) complex can easily capture CO₂ to form a bridged Cu(II)-oxalate adduct. Subsequently reduction of the adduct occurs during discharge. This work innovatively increases the output voltage of Li-CO₂ batteries to higher than 3.0 V, paving a promising avenue for the design and regulation of CO₂ conversion reactions.