Electron Transfer Step Research Articles

Mechanisms of cadmium mitigation in tomato plants under orthophosphate and polyphosphate fertilization regimes

Cadmium (Cd) is one of the most toxic elements in soil, affecting morphological, physiological, and biochemical processes in plants. Mineral plant nutrition was tested as an effective approach to mitigate Cd stress in several crop species. In this regard, the present study aimed to elucidate how different phosphorus (P) fertilization regimes can improve some bio-physiological processes in tomato plants exposed to Cd stress. In a hydroponic experiment, the impact of two phosphorus fertilizer forms (Polyphosphate (poly-P): condensed P-form with 100% polymerization rate and orthophosphate (ortho-P): from orthophosphoric acid) on the photosynthetic activity, plant growth, and nutrient uptake was assessed under three levels of Cd stress (0, 12, and 25 µM of CdCl2). The obtained results confirmed the negative effects of Cd stress on the chlorophyll content and the efficiency of the photosynthesis machinery. The application of poly-P fertilizer significantly improved the chlorophyll stability index (82%) under medium Cd stress (Cd12), as compared to the ortho-P form (55%). The analysis of the chlorophyll α fluorescence transient curve revealed that the amplitude of Cd effect on the different steps of electron transfer between PSII and PSI was significantly reduced under the poly-P fertilization regime compared to ortho-P, especially under Cd12. The evaluation of the RE0/RC parameter showed that the electron flux reducing end electron acceptors at the PSI acceptor side per reaction center was significantly improved in the poly-P treatment by 42% under Cd12 compared to the ortho-P treatment. Moreover, the use of poly-P fertilizer enhanced iron uptake and its stoichiometric homeostasis in the shoot tissue which maintained an adequate absorption of iron under Cd stress conditions. Findings from this study revealed for the first time that inorganic polyphosphate fertilizers can reduce Cd toxicity in tomato plants by enhancing photosynthesis activity, nutrient uptake, plant growth, and biomass accumulation despite the high level of cadmium accumulation in shoot tissues.

Redirecting surface reconstruction of CoP-Cu heterojunction to promote ammonia synthesis at industrial-level current density

The electrochemical reduction of nitrate (NO3−RR) represents a compelling approach for the treatment of wastewater, serving as both a sustainable substitute to the energy-intensive Haber-Bosch process and a viable alternative to direct electroreduction of N2. However, the process involves multiple electron and proton transfer steps and a complex reaction pathway, leading to low Faraday efficiency and selectivity. Herein, we demonstrated a directional surface reconstruction to generate CoP-Cu/Co(OH)2 heterojunction for synergistic catalysis of NO3−RR. Impressively, a high ammonia generation rate of 9.91 mmol h−1cm−2 and a Faraday efficiency of 99.2 % can be achieved at an industrial-relevant current density of 2 A cm−2. Moreover, the catalyst exhibited exceptional durability, maintaining the activity for 110 h under industrial current density. Such outstanding NO3−RR performance can be ascribed to the synergistic catalytic effect among the active sites of Cu, CoP and Co(OH)2, as well as the excellent stability of self-supported catalyst. Specifically, Cu and CoP sites synergistically promote the conversion of NO3− to NO2− and NO2− to NH3. Meanwhile, the partial reconstructed Co(OH)2 from CoP enhances water dissociation, thereby supplying active hydrogen (*H) essential for NO3−RR. When applied to a membrane electrode assembly (MEA) system, CoP-Cu/Co(OH)2 can deliver satisfactory ammonia production rate with appreciable economic benefits at industrial-level current densities, highlighting its potential for industrial applications.

Secondary Amines Functionalized Organocatalytic Iodine Redox for High-Performance Aqueous Dual-Ion Batteries.

Aqueous dual-ion batteries (ADIBs) based on the cooperative redox of cations and iodine anions at the anode and cathode respectively, are attracting increasing interest because of high capacity and safety. However, the full-cell performance is limited by the sluggish iodine redox kinetics between iodide and polyiodide involving multiple electron transfer steps, and the undesirable shuttling effect of polyiodides. Here, this work reports a versatile conjugated microporous polymer functionalized with secondary amine groups as an organocatalytic cathode for ADIB, which can be positively charged and electrostatically adsorb iodide, and organocatalyze iodine redox reactions through the amine groups. Both theoretical calculations and controlled experiments confirm that the secondary amine groups confine (poly)iodide species via hydrogen bonding, which is essential for accelerating iodine redox kinetics and reducing the polyiodide shuttling effect. The ADIB achieves an ultrahigh capacity of 730 mAh g-1 with an ultrasmall overpotential of 47mV at 1 A g-1 , which also exhibits excellent rate performance and long cycling stability with a capacity retention of 74% after 5000 cycles at a high current density of 5 A g-1 . This work demonstrates the promise of developing organocatalysts for accelerating electrochemical processes, which remains a virtually unexplored area in electrocatalyst design for clean energy applications.

Is the first step in MALDI in-source decay electron transfer or hydrogen atom abstraction?

Matrix-assisted laser desorption/ionization in-source decay (MALDI-ISD) with reducing matrices enables rapid mass spectrometric characterization of intact proteins. Several novel matrices have been developed for improving the fragmentation efficiency of MALDI-ISD. In particular, matrix having aniline group stronger facilitate MALDI-ISD than that having phenol group. Because the aniline N–H bond is stronger than the phenol O–H bond, the efficient fragmentation induced using matrices containing aniline group cannot be explained by the previously proposed mechanism involving the matrix-peptide hydrogen atom transfer. To explain these phenomena, electron transfer from the matrix to the peptide was recently proposed as the initial step in the MALDI-ISD processes. In this study, to estimate whether a peptide acquires hydrogen atoms or electrons in the first step of MALDI-ISD, the transition-state barriers of the reaction between dipeptide and hydrogen atoms produced from the excited-state matrix were calculated, assuming the peptide in the ground state during the MALDI-ISD process. The barrier obtained for the corresponding hydrogen atom attachment reaction did not correlate with the yield of the fragment ions produced by MALDI-ISD. In contrast, a correlation has been reported between the yields of the fragment ions produced by MALDI-ISD and the ionization energy of the matrix solids, indicating that MALDI-ISD occurs efficiently using a matrix that easily emits electrons. These results strongly suggest that MALDI-ISD of peptides using reducing matrices is well explained by the transfer of electrons and subsequent protons from the matrix to the peptide.

Spatial configuration of Fe–Co dual-sites boosting catalytic intermediates coupling toward oxygen evolution reaction

Oxygen evolution reaction (OER) is the pivotal obstacle of water splitting for hydrogen production. Dual-sites catalysts (DSCs) are considered exceeding single-site catalysts due to the preternatural synergetic effects of two metals in OER. However, appointing the specific spatial configuration of dual-sites toward more efficient catalysis still remains a challenge. Herein, we constructed two configurations of Fe-Co dual-sites: stereo Fe-Co sites (stereo-Fe-Co DSC) and planar Fe-Co sites (planar-Fe-Co DSC). Remarkably, the planar-Fe-Co DSC has excellent OER performance superior to stereo-Fe-Co DSC. DFT calculations and experiments including isotope differential electrochemical mass spectrometry, in situ infrared spectroscopy, and in situ Raman reveal the *O intermediates can be directly coupled to form *O-O* rather than *OOH by both the DSCs, which could overcome the limitation of four electron transfer steps in OER. Especially, the proper Fe-Co distance and steric direction of the planar-Fe-Co benefit the cooperation of dual sites to dehydrogenate intermediates into *O-O* than stereo-Fe-Co in the rate-determining step. This work provides valuable insights and support for further research and development of OER dual-site catalysts.

Rationalizing the Regioselectivity of Azolation of Benzylic C–H Bonds under Photoredox Catalysis

AbstractA density functional theory (DFT) study was performed to evaluate the reaction mechanism of the C–N bond formation under an integrated hydrogen atom transfer/radical-polar crossover photoredox catalytic cycle. The regioselective activation of a model substrate, including three reactive positions (3° benzylic C–H bond, 2° benzylic C–H bond, and primary C–Cl bond) was addressed to distinguish among the radical C–H activation mechanism and the standard SN2 reaction. We demonstrated that activation of tertiary benzylic C–H bond is the most favored and forms exclusively the experimentally observed product. In addition, the whole photoredox catalytic cycle, including the outer-sphere electron-transfer steps, was characterized computationally.

Promoted selectivity of photocatalytic CO2 reduction to C2H4 via hybrid CuxCoSy possessing dual unsaturated sites

Ethylene production by CO2 reduction is sluggish because the repulsive dipole-dipole interaction and 12 proton-coupled electron-transfer steps consecutively. Amorphous structured photocatalysts possess few grain boundaries and abundant unsaturated sites, accelerating the reaction efficiency from the angle of dynamics and thermodynamics, which still not yet be used in PCR to C2 products currently. Herein, an amorphous CuxCoSy composed of the minority crystalline CuCo2S4 is fabricated to realize an excellent C2H4 selectivity in terms of Relectron (94.9%). Unsaturated Co and S play the key roles in the improved efficiency of C2H4 generation. C-C coupling is achieved via shortening Co-S bonds distance, and *CO-*CO coupling barrier is decreased by more electrons accumulated on unsaturated S. Water is adsorbed on Co adjacent to S and provide protons for *COCO to form *CH2 = C. This work paves a new way for broadening the efficient of C2H4 photocatalytic evolution using amorphous photocatalyst.

Oxidation of Methanesulfinate by Hexachloroiridate(IV) and the Standard Electrode Potential of the Aqueous Methanesulfonyl Radical.

The aqueous reaction of [IrCl6]2- with CH3SO2- is biphasic and yields a 1:1 mixture of [IrCl6]3- and [IrCl5(H2O)]2- and CH3SO2Cl in the initial rapid phase. The next slow phase corresponds to the hydrolysis of CH3SO2Cl to yield CH3SO3- and Cl-. The initial phase shows kinetic inhibition by [IrCl6]3- that can be minimized by the addition of the radical scavenger propiolic acid. A detailed analysis of the kinetics indicates a mechanism with reversible outer-sphere electron transfer from CH3SO2- to [IrCl6]2- as the first step, followed by the irreversible inner-sphere oxidation of CH3SO2• by [IrCl6]2- to yield [IrCl5(H2O)]2- and CH3SO2Cl. Analysis of the inhibition by [IrCl6]3- and the kinetic effects of propiolic acid enable the determination of the equilibrium constant for the first electron-transfer step. This equilibrium constant then yields E° (CH3SO2•/CH3SO2-) = 1.01 V vs NHE at 25 °C. This is the first report of a standard potential for an alkanesulfonyl radical.

Copper Ions Reduced Toxicity of Sodium Azide and Lipopolysaccharide on Cultured Cerebellar Granule Neurons

Introduction. Copper ions (Cu2+) are structural elements of proteins such as cytochrome с oxidase (Complex IV), an enzyme that catalyzes the final step of electron transfer to oxygen during oxidative phosphorylation in the mitochondria. With Cu2+ homeostasis being of utmost importance, its disturbances in the central nervous system are involved in the mechanisms of many neurodegenerative and other brain disorders.
 This study aimed to assess the effects of non-toxic copper ion levels on death of cerebellar granule neurons associated with lipopolysaccharide (LPS; in vitro inflammation model) or azide sodium (NaN3; cytochrome с oxidase inhibitor).
 Materials and methods. LPS (10 μg/mL) or NaN3 (250 μM) was added on day 7 to 8 to the culture medium with rat cerebellar cells for 24 hours in vitro. Nitrite concentrations were measured in the culture medium by Griess assay; absorbance was recorded with a spectrophotometer at 540 nm, and morphologically intact cells were counted as survived neurons.
 Results. Added to the culture medium, LPS or NaN3 reduced neuron survival to 15 ± 2% or 20 ± 3% vs. control, respectively. Cu2+ (0.5 to 5.0 μM) increased neuron survival in a dose-dependent manner to 78 ± 4% with toxic levels of LPS and to 86 ± 6% with NaN3 with 5 μM Cu2+. The concentration of nitrites in the control culture medium was 2.0 ± 0.2 μM. Added to the cell cultures, LPS increased the concentration of nitrites to 8.5 ± 0.5 μM. Cu2+ 5 μM did not show any significant effects on nitrite accumulation in the culture medium.
 Conclusions. We showed that copper ions can exert protective effects on neurons against LPS-induced or NaN3-induced toxicity. This protection is likely to be associated rather with Cu2+ interaction with Complex IV of the electron transfer chain in the mitochondria than with inhibition of NO production. Effects of Cu2+ on apoptosis pathway proteins also cannot be ruled out.

Unusual redox stability of pentavalent uranium with hetero-bifunctional phosphonocarboxylate: insight into aqueous speciation.

The +5 state is an unusual oxidation state of uranium due to its instability in the aqueous phase. As a result, gaining information about its aqueous speciation is extremely difficult. The present work is an attempt in that direction and it provides insight into the existence of a new pentavalent species in the presence of hetero-bifunctional phosphonocarboxylate (PC) chelators, other than the carbonate ion, in the aqueous medium. The aqueous chemistry of pentavalent uranium species with three environmentally relevant PCs was probed using electrochemical and DFT methods to understand the redox energy and kinetics of conversion of the U(VI)/U(V) couple, stability, structure, stoichiometry, binding modes, etc. Interestingly, pentavalent uranium complexes with PCs are quite persistent over a wide range of pH starting from acidic to alkaline conditions. The PC chelators block the cation-cation interaction (CCI) of U(V) through strong hetero-bidentate chelation and intermolecular hydrogen bonding (IMHB) interactions which stabilize the pentavalent metal ion against disproportionation. For uranyl species in the presence of PCs, acting as chelators, CV plots were obtained at varying pH values from 2 to 8. The obtained results indicate an irreversible single redox peak involving U(VI) to U(V) conversion and association of a coupled chemical reaction with the electron transfer step. ESI-MS studies were performed to understand the speciation effect on the U(VI)/U(V) redox couple with varying pH. Speciation modelling of U(V) with the PC ligands was carried out, which indicated that the U(V) is redox stable in nearly 47% of the pH region in the presence of the PCs as compared to the carboxylate-based chelators. The free energy and reduction potential of the U(V) complexes and the reduction free energy and disproportionation free energy for the U(VI)/U(V) couple were determined by DFT computations in the presence of the PCs. In situ spectroelectrochemical spectra were recorded to provide evidence for the existence of U(V) species with PCs in the aqueous medium and to acquire its absorption spectra. The present study is highly significant for understanding the coordination chemistry of pentavalent uranium species, accurate modelling of uranium, and isolation of U(V).

A first-principles study of electro-catalytic reduction of CO2 on transition metal-doped stanene.

Employing first-principles calculations based on density functional theory, this work examines the activity of 3d transition metal-doped stanene for electro-catalytic CO2 reduction through the first two electron transfer steps to CO. Our results related to CO2 activation, the first and a crucial step of the reduction process revealed that, among the entire 3d transition metal row studied, only Ti- and Fe-doped stanene can bind and significantly activate the CO2 molecule, while the rest of the TM single atoms are inert in activating the molecule. The activation of the CO2 molecule on Ti- and Fe-doped stanene has been observed in the presence of water as well. In addition, the formation of OCHO has been observed to be energetically preferred over COOH formation as a reaction intermediate, indicating the preference for the formate path of the reduction reaction. Furthermore, despite the strong adsorption of H2O on the catalyst surface, the presence of water seems to enhance CO2 adsorption on the catalysts, contrary to what has been observed recently in graphene-based catalysts. Finally, our difference charge density and the Bader charge calculations reveal that the ability of Ti- and Fe-doped stanene in activating the CO2 molecule and their potential catalytic activity for CO2 reduction is to be attributed to the charge transfer between the catalyst and the molecule, providing new insights into the rational design of 2D catalysts beyond graphene.

Shining light on tryptamine-derived isocyanides: access to constrained spirocylic scaffolds.

Dearomatization of indoles through a charge transfer complex constitutes a powerful tool for synthesizing three-dimensional constrained structures. However, the implementation of this strategy for the dearomatization of tryptamine-derived isocyanides to generate spirocyclic scaffolds remains underdeveloped. In this work, we have demonstrated the ability of tryptamine-derived isocyanides to form aggregates at higher concentration, enabling a single electron transfer step to generate carbon-based-radical intermediates. Optical, HRMS and computational studies have elucidated key aspects associated with the photophysical properties of tryptamine-derived isocyanides. The developed protocol is operationally simple, robust and demonstrates a novel approach to generate conformationally constrained spirocyclic scaffolds, compounds with high demand in various fields, including drug discovery.

Heterogeneous Photocatalytic Synthesis of Sulfenamide with Carbon Doped Potassium Poly(heptazine imide) through effective electron delocalization

Sulfur–heteroatom bonds such as S–N are valuable motifs in pharmaceuticals and agrochemicals, but exploring the green synthetic methods to construct these compounds by rationally designing catalysts remains challenging. Hence, we...

Restructuring multi-phase interfaces from Cu-based metal-organic frameworks for selective electroreduction of CO2 to C2H4.

Multi-phase interfaces are promising for surmounting the energy barriers of electrochemical CO2 reduction involving multiple electron transfer steps, but challenges still remain in constructing interfacial micro-structures and unraveling their dynamic changes and working mechanism. Herein, highly active Ag/Cu/Cu2O heterostructures are in situ electrochemically restructured from Ag-incorporating HKUST-1, a Cu-based metal-organic framework (MOF), and accomplish efficient CO2-to-C2H4 conversion with a high faradaic efficiency (57.2% at -1.3 V vs. RHE) and satisfactory stability in flow cells, performing among the best of recently reported MOFs and their derivatives. The combination of in/ex situ characterizations and theoretical calculations reveals that Ag plays a crucial role in stabilizing Cu(i) and increasing the CO surface coverage, while the active Cu/Cu2O interfaces significantly reduce the energy barrier of C-C coupling toward the boosted ethylene production. This work not only proves MOFs as feasible precursors to derive efficient electrocatalysts on site, but also provides in-depth understanding on the working interfaces at an atomic level.

Strategies in photochemical alcohol oxidation on noble-metal free nanomaterials as heterogeneous catalysts.

Using light absorbing semiconductors to harvest solar energy has long been considered as a promising approach for driving chemical reactions. Photochemical oxidation of alcohols catalyzed by noble-metal free nanomaterials is particularly attractive due to the bio-derivable nature of the substrate and the wide availability of such catalysts. Here, a number of strategies that could facilitate the progress are discussed, where it is concluded that the photochemical alcohol oxidation process can benefit from both mechanistic optimization of the electron-transfer steps and structural engineering of the light-absorbing nanomaterials.

Reactivity of radiolytically and photochemically generated tertiary amine radicals towards a CO2 reduction catalyst.

Homogeneous solar fuels photocatalytic systems often require several additives in solution with the catalyst to operate, such as a photosensitizer (PS), Brønsted acid/base, and a sacrificial electron donor (SED). Tertiary amines, in particular triethylamine (TEA) and triethanolamine (TEOA), are ubiquitously deployed in photocatalysis applications as SEDs and are capable of reductively quenching the PS's excited state. Upon oxidation, TEA and TEOA form TEA•+ and TEOA•+ radical cations, respectively, which decay by proton transfer to generate redox non-innocent transient radicals, TEA• and TEOA•, respectively, with redox potentials that allow them to participate in an additional electron transfer step, thus resulting in net one-photon/two-electron donation. However, the properties of the TEA• and TEOA• radicals are not well understood, including their reducing powers and kinetics of electron transfer to catalysts. Herein, we have used both pulse radiolysis and laser flash photolysis to generate TEA• and TEOA• radicals in CH3CN, and combined with UV/Vis transient absorption and time-resolved mid-infrared spectroscopies, we have probed the kinetics of reduction of the well-established CO2 reduction photocatalyst, fac-ReCl(bpy)(CO)3 (bpy = 2,2'-bipyridine), by these radicals [kTEA• = (4.4 ± 0.3) × 109 M-1s-1 and kTEOA• = (9.3 ± 0.6) × 107 M-1s-1]. The ∼50× smaller rate constant for TEOA• indicates, that in contrast to a previous assumption, TEA• is a more potent reductant than TEOA• (by ∼0.2V, as estimated using the Marcus cross relation). This knowledge will aid in the design of photocatalytic systems involving SEDs. We also show that TEA can be a useful radiolytic solvent radical scavenger for pulse radiolysis experiments in CH3CN, effectively converting unwanted oxidizing radicals into useful reducing equivalents in the form of TEA• radicals.

Vanadium-Containing Keggin-Type Polyoxometalates, [VM12O40]3- and [VVM11O40]4- (M = Mo, W): Structural Characterization and Voltammetric, NMR, and EPR Studies Related to Electrochemical Reduction at Framework and Central Vanadium Sites.

Vanadium is accommodated in both the framework (VoutV) and central positions (VinV) in the Keggin-type polyoxometalates (POMs) [VinVVoutVM11O40]4- (M = Mo, W; VinVVoutVM11) and in the central position in [VinVM12O40]3- (VinVM12). The structures of the VinVVoutVM11 class have been determined by X-ray crystallography and compared to those of VinVM12 reported previously. A major feature of interest with POMs is their capacity for very extensive reduction, particularly when protonation accompanies the electron transfer step. With VinVVoutVM11 and VinVM12 POMs, knowledge as to whether reduction occurs at V or M sites and the concomitant dependence on acidity has been obtained. Frozen solution EPR spectra obtained following bulk electrolysis showed that the one-electron reduction of VinVMo12 occurs at the molybdenum framework site to give VinVMoVMo11. In contrast, EPR spectra of one-electron reduced VinVW12 at <30 K are consistent with the electron being accommodated on the central V atom in a tetrahedral environment to give VinIVW12. In the case of VinVVoutVM11, the initial reduction occurs at the framework VoutV site to give VinVVoutIVM11. The second electron is delocalized over the Mo framework in two-electron reduced VinVVoutIVMoVMo10, whereas it is accommodated on the central V site in VinIVVoutIVW11. The distance between VinIV and VoutIV in VinIVVoutIVW11 estimated as 3.5 ± 0.2 Å from analysis of the EPR spectrum is consistent with that obtained in VinVVoutVW11 from crystallographic data. Simulations of the cyclic voltammograms as a function of CF3SO3H acid concentration for the initial reduction processes provide excellent agreement with experimental data obtained in acetonitrile (0.10 M [n-Bu4N][PF6]) and allowed acid association constants to be estimated and compared with the literature values available for [XVoutVM11O40]n- (X = S (n = 3), P and As (n= 4); M = Mo, W). The interpretation of the voltammetric data is supported by 51V NMR measurements on the oxidized VV forms of the POMs.

Activity and Selectivity Modulation of CO2 Electroreductions at Au-Water Interfaces Via Tuning Localcation Concentrations

Electrochemical CO2 reduction reaction (CO2RR) is a promising technique for converting the greenhouse gas CO2 into valuable fuels and chemicals in a clean energy society. To understand the underlying electrocatalytic reaction mechanism from the atomic level, researchers are investigating the role of electrolyte ions, particularly alkali metal cations, in various electrocatalytic reactions including CO2RR. Despite this attention, the impact of alkali metal cations is still a topic of debate, and it remains unclear how cations affect the CO2RR and the competing hydrogen evolution reaction (HER).In this study, explicit cations and water solvents were added to Au-water interfacial models to simulate the pathways of CO2RR and HER. Two cations (K+, Li+, and/or H+) were used in the model system to create similarly charged Au surfaces, and the local concentration of alkali metal cations (AM+) was adjusted by replacing AM+ (K+ and Li+) with H+. The first electron transfer step was considered critical in electrocatalytic reductions, so CO2 activation and water dissociation were evaluated for CO2RR and HER, respectively. Ab initio molecular dynamics (AIMD) simulations with the slow-growth sampling approach (SG-AIMD) were used to simulate the corresponding reaction mechanisms, and kinetic barriers were obtained through thermodynamic integrations. With these Au-water-cations interfacial models, a systematic study was conducted to investigate the mechanism of CO2 activation at Au-water-2AM+, Au-water-1AM+, and Au-water-0AM+ interfaces. These results show that a high concentration of metal cations with 2AM+ promotes CO2 activation through short-range electrostatic interactions between cations and key intermediates.Besides CO2 activation, this study also investigated water dissociation during the competing HER. Contrary to the promotion effect observed in CO2RR, local alkali metal cations were found to suppress water dissociation with a high reaction barrier (Figure 1). This can be attributed to the broken connectivity of the hydrogen bond network at Au-water-2AM+ interfaces. The reaction kinetics can be improved by reducing the metal cation concentration. Notably, K+ was found to have a more pronounced promotion effect than Li+ on CO2 activation, while the opposite suppression effect was observed on HER. By tuning the local alkali metal cation concentration, it is anticipated that the overall performance of CO2RR, including both activity and selectivity, can be engineered. Figure 1

Heterogeneous Electron Transfer with Redox Active Insulating Solid

For more than a century, electron transfer kinetics at electrode/electrolyte interface has been studied for dissolved redox active species at conductors (heterogeneous) or amongst dissolved redox species (homogeneous). However, a third major case is almost unexplored: dissolved redox species exchanging charge with redox active insulators. The electron transfer with redox active insulators such as Lithium peroxide (Li2O2), Lithium sulfide (Li2S), Sulfur (S8), Lithium carbonate (Li2CO3) are salient feature and central difficulty of next generation ‘beyond intercalation’ batteries such as metal-air (O2), metal-CO2 and metal-sulfur (Li-S) batteries.Redox mediation is the key process in electrochemistry with insulators. Mediators shuttle charge between the electron-conducting electrode and the surface of the insulating storage material to drive the discharge or charge reaction by reducing or oxidizing the surface of the redox active insulator. Mediation is now widely accepted to be key to high efficiency, rate capability, and lifetime. It may on the one hand involve external mediators that are added as an electrolyte additive and which have been studied in a wide variety for metal-O2 cells and a few examples for metal-sulfur. On the other hand, there is inherent mediation, where soluble intermediates act as mediators, such as lithium polysulfides in lithium-sulfur batteries. However, surprisingly little attention has been devoted to quantitatively assessing heterogeneous reaction kinetics between mediator and storage material. Work so far clearly shows that the kinetics of this electron transfer step is overall rate limiting and decisive to understand and to control reaction mechanisms towards improving energy efficiency, rate capability, capacity, and cycle life, which underscores the urgency to measure them and to identify governing factors.Here we systematically assess heterogeneous electron transfer at redox active insulators and identify governing factors. Particular systems we present to investigate are twofold. First, mediated kinetics of (su)peroxide oxidation. Our previous work has strikingly discovered decreasing electron transfer rate as the driving force (mediator redox potential) increases beyond a certain value, which represents a first example of Marcus inverted-region behaviour for heterogeneous reactions1. We are leveraging the predictive value of this finding regarding possibilities to impact maximum kinetics and the occurrence of a separate kinetic parabola for singlet oxygen evolution. Second, mediated kinetics of S8 reduction and Li2S oxidation2. Major tools to assess kinetics include following species concentration over time using UV-Vis-NIR spectroscopy and Scanning Electrochemical Microscope (SECM). Doing so requires deconvoluting spectra of mixtures of mediator oxidation/reduction states even if pure substance spectra cannot be obtained. Particularly, obtaining kinetics in S chemistry requires assessing dynamic equilibria of multiple species, for which we develop chemo-metrics tools. The results afford detailed insights into kinetics, thermodynamics, and reaction mechanisms, allowing them to influence in an informed way.References K. Petit, E. Mourad, C. Prehal, C. Leypold, A. Windischbacher, D. Mijailovic, C. Slugovc, S. M Borisov, E. Zojer, S. Brutti, O. Fontaine, S. A. Freunberger, Nature chemistry, 13, (2021)465–471 (https://doi.org/10.1038/s41557-021-00643-z)Cao, X. Shen, A. Wang, F. Yu, Y. Wu, S. Shi, S. A. Freunberger, Y. Chen, Nature Catalysis, 5, (2022) 193–201 (https://doi.org/10.1038/s41929-022-00752-z)

Enhancement in the Rate of CO2 Photoreduction Using Divalent Strontium Cation (Sr2+) Doped TiO2 Nanotube Arrays

The sunlight-assisted conversion of CO2 into portable value-added products has the potential to address both the energy and environmental challenges faced by the world. Therefore, we currently witness intense research activity and substantial progress in the field of CO2 photoreduction. Anatase TiO2 nanotube arrays (TNTAs) formed by electrochemical anodization constitute a well studied semiconductor photocatalyst with excellent optoelectronic properties and photocatalytic behavior [1, 2]. Yet, many aspects related to structure, surface composition, adsorbates, reaction intermediates, rate of reaction, multi-step electron transfer steps, etc. remain poorly understood even for TiO2. This led to our interest in forming surface coatings and bulk nanocomposites of TNTAs with metal oxides of Sr, Mg, Ca, Zn etc, which provide basic sites on the surface of the catalyst for the chemisorption of CO2 molecules. The higher basicity of hydroxyl groups associated with alkaline earth metals generate carbonate and bicarbonate adsorbates which enhance the reactivity of CO2 [3]. Divalent metal cations are also known to influence and modify the crystallographic texture of TiO2 during the annealing treatment step at elevated temperature [4].Here we performed divalent strontium cation (Sr2+) doping of anodically grown TNTAs using two different techniques – an electrochemical cathodic treatment in Sr2+-containing electrolytes and a wet impregnation technique using an aqueoussolution of strontium salts. The X-ray diffraction pattern and Raman spectra indicate the formation of Sr compounds at the TNTA surface. Sr-doping results in modification of the crystallographic texture of anatase phase TNTAs, increased lattice strain, reduced crystal size, and phonon confinement. Subtle changes are observed in the infrared spectra of the CO2 adsorbed on the Sr-doped TNTAs specifying a larger prevalence of monodentate and bidentate carbonate species on the surface. This attributes to the higher alkalinity of surface hydroxyls bound to Sr in comparison to the Ti-bound hydroxyls. In addition to linearly adsorbed CO2, a smaller population of bent CO2 molecules on the surface is observed, which suggests an enhanced stabilization of the carbon dioxide anion radical on the Sr-doped TNTA surfaces. Finally, under AM1.5G one sun illumination, the cathodized TNTAs produce 25 µmolg-1hr-1 whilethe wet impregnated Sr-doped TNTAs produce 17 µmolg-1hr-1 of CO which are nearly 3 and 2 times the amount of CO generated by bare TNTAs (7.7 µmolg-1hr-1) respectively.REFERENCES: Vahidzadeh, Ehsan, et al. "Asymmetric multipole plasmon-mediated catalysis shifts the product selectivity of CO2 photoreduction toward C2+ products." ACS Applied Materials & Interfaces13.6 (2021): 7248-7258.Zeng, Sheng, et al. "Optical control of selectivity of high rate CO2 photoreduction via interband-or hot electron Z-scheme reaction pathways in Au-TiO2 plasmonic photonic crystal photocatalyst." Applied Catalysis B: Environmental267 (2020): 118644.Kwon, Stephanie, et al. "Alkaline-earth metal-oxide overlayers on TiO 2: application toward CO 2 photoreduction." Catalysis Science & Technology6.21 (2016): 7885-7895.Kisslinger, Ryan, et al. "Preferentially oriented TiO2 nanotube arrays on non-native substrates and their improved performance as electron transporting layer in halide perovskite solar cells." Nanotechnology30.20 (2019): 204003.