Heterogeneous Electron Transfer Research Articles

Elucidation of Design Criteria for V-based Redox Mediators: Structure-Function Relationships that Dictate Rates of Heterogeneous Electron Transfer.

Redox mediators are attractive solutions for addressing the stringent kinetic stipulations required for efficient energy conversion processes. In this work, we compare the electrochemical properties of four vanadium complexes, namely [V(acac)3], [V6O7(OMe)12], [nBu4N]3[V6O13(TRISNO2)2], and [nBu4N]5[V18O46(NO3)] in non-aqueous solutions on glassy carbon electrodes. The goal of this study is to investigate the electron transfer kinetics and diffusivity of these compounds under identical experimental conditions to develop an understanding of structure-function relationships that dictate the physicochemical properties of vanadium oxide assemblies. Complex selection was dictated by two criteria - (1) nuclearity of the transition metal complexes (2) distribution of electron density in the native electronic configuration. Our analyses establish that electronic communication between metal centers significantly impacts charge transfer kinetics of these vanadium-based compounds.

Benchmarking Trisaminocyclopropeniums as Mediators for Anodic Oxidation Reactions.

This report benchmarks a tris(amino)cyclopropenium (TAC) salt as an electron-transfer mediator for anodic oxidation reactions in comparison to two known mediators: a triarylamine and a triarylimidazole derivative. The three mediators have redox potentials, diffusion coefficients, and heterogeneous electron transfer rates similar to those of glassy carbon electrodes in acetonitrile/KPF6. However, they differ significantly in their performance in two electro-organic reactions: anodic fluorination of a dithiane and anodic oxidation of 4-methoxybenzyl alcohol. These differences are rationalized based on variable stability in the presence of reaction components (e.g., NEt3·3HF, lutidine, and Cs2CO3) as well as very different rates of electron transfer with the organic substrate. Overall, this work highlights the advantages and disadvantages of each mediator and provides a foundation for expanding the applications of TACs in electro-organic synthesis moving forward.

Novel Ce(OH)CO3/carbon nanotubes hybrid material towards a highly sensitive electrochemical dopamine determination in real urine samples

Cerium hydroxycarbonate and multiwalled carbon nanotubes (Ce(OH)CO3/CNT), drop-casted onto a glassy carbon (GC) electrode and electrochemically treated (tCe(OH)CO3/CNT) has been obtained as a new hybrid material. tCe(OH)CO3/CNT was characterized through SEM, FT-IR, UV–Vis, and electrochemical techniques such as cyclic voltammetry (CV) with different redox couples as Fe2+/3+, Fe(CN)63-/4− or Ru(NH3)62+/3+and electrochemical impedance spectroscopy (EIS) measurements with Fe(CN)63-/4−. The charge transfer resistance (RCT) value of tCe(OH)CO3/CNT was up to two orders of magnitude lower than the RCT of bare GC electrodes and these results were confirmed by the effective heterogeneous electron transfer rate constant values (k°eff, 2.4 10−3 vs 7.0 10−4, respectively). The calibration curve of DA exhibits a linear response within the range of 0.1 and 70 μmol L−1 with a correlation coefficient (R2) of 0.987 (n = 3) and a limit of detection of 0.003 μmol L−1. Inorganic and organic interferents do not alter significatively the selectivity. The electrochemical treatment of Ce(OH)CO3 and CNT generates a modified electrode which successfully combines the selectivity of Ce towards catechol with the high conductivity of CNT presenting an ideal performance for the electrochemical DA determination in 5 real urine samples (1.94 μmol L−1 in a 1:20 dilution) using differential pulse voltammetry (DPV) with an RSD < 6.1 %.

Efficient Electrocatalytic H2 Production by Immobilized Co(III)‐myoglobin

AbstractThe thermodynamics and kinetics of heterogeneous electron transfer (ET) for Co‐substituted horse myoglobin (Co−Mb) and its derivatives with ammonia and imidazole as heme axial ligands were studied with cyclic voltammetry on a pyrolytic graphite electrode along with their ability to mediate the electro‐catalytic production of H2. All the proteins experience a non‐diffusive electrochemical regime as electrode‐bound species. The adsorbed Co−Mb construct carries out the electro‐catalytic reduction of water protons to H2 with a good efficiency under anaerobic conditions thus yielding a simple and tunable system for H2 production. Replacement of H2O as Co axial ligand by ammonia and imidazole significantly lowers the catalytic currents for H3O+/H2O reduction to H2. The E°’ values of the Co(III)/Co(II) redox couple for all species are mainly determined by the enthalpic contribution. Differences were found in the kinetics of ET for the different protein adducts due to changes in the activation enthalpies. However, all species share the same distance of about 14 Å from the electrode surface to the Co(III)/Co(II) center determined using the Marcus model, consistent with a non‐denaturing adsorption of the protein.

Fe Single-Atom Carbon Dots Nanozyme Collaborated with Nucleic Acid Exonuclease III-Driven DNA Walker Cascade Amplification Strategy for Circulating Tumor DNA Detection.

Circulating tumor DNA (ctDNA), as a next-generation tumor marker, enables early screening and monitoring of cancer through noninvasive testing. Exploring the development of new methods for ctDNA detection is an intriguing study. In this work, a unique electrochemical biosensor for the ctDNA detector was constructed in the first utilizing Fe single-atom nanozymes-carbon dots (SA Fe-CDs) as a signaling carrier in collaboration with a DNA walker cascade amplification strategy triggered by nucleic acid exonuclease III (Exo III). The electrochemical active surface area of AuNPs/rGO modified onto a glassy carbon electrode (AuNPs/rGO/GCE) was about 1.43 times that of a bare electrode (bare GCE), with good electrical conductivity alongside a high heterogeneous electron transfer rate (5.81 × 10-3 cm s-1), that is, as well as the ability to load more molecules. Sequentially, the DNA walker cascade amplification strategy driven by Exo III effectively converted the target ctDNA into an amplified biosignal, ensuring the sensitivity and specificity of ctDNA. Ultimately, the electrochemical signal was further amplified by introducing SA Fe-CDs nanozymes, which could serve as catalysts for 3,3',5,5'-tetramethylbenzidine (TMB) oxidation with facile responding (Vmax = 0.854 × 10-6 M s-1) and robust annexation (Km = 0.0069 mM). The integration of the triple signal amplification approach achieved detection limits as low as 1.26 aM (S/N = 3) for a linearity spanning from 5 aM to 50 nM. In this regard, our proposal for a biosensor with exceptional assay properties in complicated serum environments had great potential for early and timely diagnosis of cancer.

Microbiologically influenced corrosion can cause a dental implant rejection

In the present study, we apply electrochemical and molecular biology approaches to establish whether microbiologically influenced corrosion (MIC) may be the reason for dental implant rejection. Titanium-based implants have been incubated in vitro in Ringer/glucose solution in the presence and absence of a salivary microbiota and the change in their corrosion potential and current density was monitored over time by potentiodynamic polarization. The charge transfers processes occurring on the Ti-based implant surface during the initial bacterial adhesion and after biofilm formation were analyzed by electrochemical impedance spectroscopy. The components of the equivalent electrical circuit as well as the heterogeneous electron transfer rate constants were estimated. In parallel, bacterial species from the salivary microbiota have been identified by sequencing of the 16S ribosomal amplicon method, and their contribution to the corrosion processes of dental implants was verified and compared to that of native saliva in vitro. The scanning electron microscopy observation and energy dispersive spectroscopy analysis of the implant surface after long-term exposure to a bacterial-containing environment revealed more detailed features of the occurring corrosion processes.

Efficacy of films of uncapped gold nanoparticle as electrodes for direct electron transfer to redox proteins

Electrochemical manipulation of enzymes is expected to enable the rapid detection of marker molecules and the efficient production of valuable materials. We have fabricated an Au nanoparticle (AuNP) thin film electrode in our previous study, which enables more enhanced heterogeneous electron transfer reactions than a conventional planner Au electrode. In this study, electrochemical evaluation of cytochrome c was performed using the AuNP thin film electrodes with and without self-assembled monolayer (SAM) modification to assess its practicality for protein electrochemistry. The 4-pyridinethiol- and 7-carboxy-1-heptanethiol-modified AuNP thin film electrodes showed much larger faradaic current values attributed to redox reactions compared to those of the Au electrodes. The results indicate that the nanostructural effects of the AuNP thin film electrode are beneficial. Importantly, even when the non-modified (bare) AuNP thin film electrode was applied, it exhibited the clear electrochemical response of cyt c, while the bare Au electrode showed no such a response. This study provides evidence that the AuNP thin film electrode functions as a potent bioelectrocatalysts due to the nanostructure-specific properties.

Electrogenerated Chemiluminescence Imaging of Single‐Atom Nanocatalysts

AbstractSingle‐atom catalysts (SACs), distinguished by their maximum atom efficiency and precise control over the coordination and electronic properties of individual atoms, show great promise in electrocatalysis. Gaining a comprehensive understanding of the electrochemical performance of SACs requires the screening of electron transfer process at micro/nano scale. This research pioneers the use of electrogenerated chemiluminescence microscopy (ECLM) to observe the electrocatalytic reactions at individual SACs. It boasts sensitivity at the single photon level and temporal resolution down to 100 ms, enabling real‐time capture of the electrochemical behavior of individual SACs during potential sweeping. Leveraging the direct correlation between ECL emission and heterogeneous electron transfer processes, we introduced photon flux density for quantitative analysis, unveiling the electrocatalytic efficiency of individual SACs. This approach systematically reveals the relationship between SACs based on different metal atoms and their peroxidase (POD)‐like activity. The outcomes contribute to a fundamental understanding of SACs and pave the way for designing SACs with diverse technological and industrial applications.

Electrogenerated Chemiluminescence Imaging of Single-Atom Nanocatalysts.

Single-atom catalysts (SACs), distinguished by their maximum atom efficiency and precise control over the coordination and electronic properties of individual atoms, show great promise in electrocatalysis. Gaining a comprehensive understanding of the electrochemical performance of SACs requires the screening of electron transfer process at micro/nano scale. This research pioneers the use of electrogenerated chemiluminescence microscopy (ECLM) to observe the electrocatalytic reactions at individual SACs. It boasts sensitivity at the single photon level and temporal resolution down to 100 ms, enabling real-time capture of the electrochemical behavior of individual SACs during potential sweeping. Leveraging the direct correlation between ECL emission and heterogeneous electron transfer processes, we introduced photon flux density for quantitative analysis, unveiling the electrocatalytic efficiency of individual SACs. This approach systematically reveals the relationship between SACs based on different metal atoms and their peroxidase (POD)-like activity. The outcomes contribute to a fundamental understanding of SACs and pave the way for designing SACs with diverse technological and industrial applications.

Reviving the absorbent chemistry of electrochemically mediated amine regeneration for improved point source carbon capture

Electrochemically mediated amine regeneration (EMAR) is emerging as a promising electrochemical approach for carbon capture from point sources. In this study, for the first time, the absorbent chemistry of the EMAR process was revived by considering criteria beyond metal-amine complexation. The systematic investigation focused on using second amines, which were previously overlooked in EMAR due to limited metal-amine interaction, despite their proven effectiveness in conventional carbon capture processes. The integration of second amines— monoethanolamine (MEA), aminomethyl propanol (AMP), and methyldiethanolamine (MDEA)—with the benchmark ethylenediamine (EDA) was thoroughly studied, evaluating their impact on absorption and desorption kinetics and capacities, and electrochemical performance. Among the blends, EDA + MDEA emerged as the top performer, particularly excelling in electrochemical aspects. The charge transfer resistance (RCT) of the deposition reaction, as the rate-limiting step, for the EDA + MDEA blend was reduced by 40 % compared to pure EDA, and the heterogeneous electron transfer rate constant (k0) improved threefold. In the bench-scale assessments, the EDA + MDEA blend showed a notable decrease in desorption energetics to 37 kJ/molCO2, marking a 34 % reduction compared to the 56 kJ/molCO2 required for pure EDA. This marks the lowest reported energetics for an EMAR process targeting point source carbon capture. This research paves the way for future systematic investigations of a wider range of amine blends for EMAR and eventually the process implementation at larger scales.

Comparison of Different Electrochemical Methodologies for Electrode Reactions: A Case Study of Paracetamol

Understanding electrochemical reactions at the surface of electrodes requires the accurate calculation of key parameters—the transfer coefficient (α), diffusion coefficient (D0), and heterogeneous electron transfer rate constant (k0). The choice of method to calculate these parameters requires careful consideration based on the nature of the electrochemical reaction. In this study, we conducted the cyclic voltammetry of paracetamol to calculate the values of these parameters using different methods and present a comparative analysis. Our results demonstrate that the Ep − Ep/2 equation for α and the modified Randles–Ševčík equation for D0 is particularly effective for the calculations of these two parameters. The Kochi and Gileadi methods are reliable alternatives for the calculation of k0. Nicholson and Shain’s method using the equation k0 = Ψ(πnD0Fν/RT)1/2 gives the overestimated values of k0. However, the value of k0 calculated using the plot of ν−1/2 versus Ψ (from the Nicholson and Shain equation, where ν is scan rate) agrees well with the values calculated from the Kochi and Gilaedi methods. This study not only identifies optimal methodologies for quasi-reversible reactions but also contributes to a deeper understanding of electrochemical reactions involving complex electron transfer and coupled chemical reactions, which can be broadly applicable in various electrochemical studies.

Efficient Electrochemical Lead Detection by a Histidine-Grafted Metal-Organic Framework MOF-808 Electrode Material.

As the excessive presence of heavy metals in the environment significantly affects human health, it becomes necessary to develop efficient, selective, and sensitive methods for their detection. In this study, a novel electrochemical sensor for the detection of Pb2+ ions is described. The proposed sensor is based on a glassy carbon electrode (GCE) modified by a thin film of histidine-grafted metal-organic framework (MOF-808-His). The MOF-808 was obtained solvothermally, and then postsynthetically modified by substituting the coordinated acetate with histidinate. By electrochemistry, the MOF-808-His-modified GCE demonstrated high charge selectivity, while electrochemical impedance spectroscopy (EIS) and kinetic studies gave a lower charge transfer resistance (4196 Ω) and a better standard heterogeneous electron transfer rate constant (1.80 × 10-5 cm s-1) on MOF-808-modified GCE. These results indicated a swift and direct electron transfer rate from [Fe(CN)6]3-/4- to the electrode surface. Using square wave anodic stripping voltammetry (SWASV), the rapid and highly sensitive determination of Pb2+ was achieved on MOF-808-His-modified GCE. By optimizing the accumulation-detection parameters including pH of the detection medium, deposition time and potential, and concentration, a remarkable limit of detection (LoD, based on a signal-to-noise ratio of 3) of (1.12 × 10-10 ± 0.10 × 10-10) mol L-1 was obtained, with a sensitivity of (9.6 ± 0.1) μA L μmol-1. After interference and stability studies, the MOF-808-His-modified GCE was applied to the detection of Pb2+ in a tap water sample with a concentration of 10 μmol L-1 Pb2+.

Electrochemistry at 2D and 3D nanoelectrodes: The interplay between interface kinetics and surface density of states

Heterogenous Electron Transfer (HET) at electrode-electrolyte interfaces depends strongly on the morphological features or geometry of the nanostructure used for modifying the electrode surface. A swift HET results in faster interface kinetics, which has significant impact on the development/calibration of electrochemical devices like biomolecular sensors, supercapacitors, batteries and electrochromic platforms. The interface electrochemistry depends strongly on the electronic Density of States (DOS) of electrode materials. Over the past years, the 2D electron gas nanomaterials - primarily graphene, Graphene Oxide (GO) and Reduced Graphene Oxide (RGO), have garnered significant interest in electrochemical applications due to promising DOS features. However, the electroanalytical dependency on DOS of 3D nanostructures such as Zinc Oxide Tetrapods (ZnOT) is yet unexplored. The current work focusses on a comparative electrochemical analysis of interface kinetics at RGO (2D) and ZnOT (3D) coated screen printed electrodes, with the intention of selecting the suitable material geometry. The analyses were performed using Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). While the dependence of HET on DOS of RGO and ZnOT nanomaterials were studied using both DFT analysis and impedance-derived capacitance spectroscopy – the latter giving insights on quantum capacitance. It was observed that, the 2D RGO nanostructures exhibit higher surface DOS near the Fermi level, along with a high quantum capacitance (∼345 nF) as compared to 3D ZnOT (∼276 nF). This results in enhanced HET at the former, thereby indicating its suitability in developing futuristic electrochemical devices for various applications as desired.

Electrocatalytic Oxidation of Nitrite Using a Composite of Reduced Graphene Oxide and Cobalt‐4‐Pyridylporphyrin Coordination Polymer

AbstractThe present work describes the electrocatalytic oxidation of nitrite in the presence of a composite of coordination polymer of cobalt‐4‐pyridylporphyrin and electrochemically reduced graphene oxide. The electrode was modified by drop‐casting a suspension of graphene oxide and polymeric cobalt‐4‐pyridylporphyrin followed by reduction through cyclic voltammetry in the potential range 0.0 V to −1.5 V. The electrocatalytic investigations were carried out by voltammetric (cyclic, linear sweep, differential pulse and hydrodynamic) and by amperometric methods. The LOD and the working range obtained through different techniques are compared. The rate constant for this heterogeneous electron transfer reaction is 2.45×10−2 cms−1.

Regulation of heterogeneous electron transfer reactivity by defect engineering through electrochemically induced brominating addition.

Enhancing the electrochemical activity of graphene holds great significance for expanding its applications in various electrochemistry fields. In this study, we have demonstrated a facile and quantitative approach for modulating the defect density of single-layer graphene (SLG) via an electrochemically induced bromination process facilitated by cyclic voltammetry. This controlled defect engineering directly impacts the heterogeneous electron transfer (HET) rate of SLG. By utilizing Raman spectroscopy and scanning electrochemical microscopy (SECM), we have established a correlation between the HET kinetics and both the defect density (nD) and mean distance between defects (LD) of SLG. The variation of the HET rate (k0) with the defect density manifested a distinctive three-stage behavior. Initially, k0 increased slightly with the increasing nD, and then it experienced a rapid increase as nD further increased. However, once the defect density surpassed a critical value of about 1.8 × 1012 cm-2 (LD < 4.2 nm), k0 decreased rapidly. Notably, the results revealed a remarkable 35-fold enhancement of k0 under the optimal defect density conditions compared to pristine SLG. This research paves the way for controllable defect engineering as a powerful strategy to enhance the electrochemical activity of graphene, opening up new possibilities for its utilization in a wide range of electrochemical applications.

Electronic structure modification of metal phthalocyanines by a carbon nanotube support for efficient oxygen reduction to hydrogen peroxide.

An active and selective two-electron oxygen reduction reaction (2e- ORR) is required for efficient electrosynthesis of H2O2. This reaction can be promoted by metal phthalocyanines (MPcs), which serve as model catalysts with well-defined structures. MPc molecules have mostly been evaluated on conductive carbon-based substrates, including glassy carbon (GC) and carbon nanotubes (CNTs), yet their influence on the electrocatalytic properties is not well understood. This study demonstrated that the ORR activity per surface area was improved by up to 4-fold with MPc molecules supported on CNTs (MPc/CNTs, M = Co, Mn, and Fe) compared to MPc loaded directly on GC. Ultraviolet photoelectron spectroscopy and density functional theory calculations revealed that the CNTs modified the electronic structure of the MPc molecules to optimize the *OOH binding energy and boost the heterogeneous electron transfer rates. Detailed kinetic analysis enabled multiple reaction pathways to be decoupled to extract the metal-dependent intrinsic 2e-/4e- ORR activities. Finally, MPc/CNT catalysts were employed in an H2O2 electrosynthesis flow cell, which delivered an industrial-scale current density of -200 mA cm-2 and an H2O2 faradaic efficiency of 88.7 ± 0.6% with the CoPc/CNT catalyst in a neutral electrolyte.

The effect of gold nanostructure morphology on label-free electrochemical immunosensor design

In this research, various electrodeposition techniques were used to form gold nanostructures (AuNSs) on the surface of graphite rod electrode (GE). Three distinct AuNS morphologies on GE have been achieved based on the composition of electrodeposition solution. The use of H2SO4 as a supporting electrolyte resulted in the formation of smaller but more numerous AuNSI with a modified electrode’s electroactive surface area (EASA) of 0.213 cm2. Exchanging the supporting electrolyte to KNO3 and increasing HAuCl4 concentration facilitated the formation of bigger AuNSII particles with electrode EASA of 0.116 cm2. Finally, a partial coverage of GE by branched gold nanostructures (AuNSIII) was achieved with an estimated EASA of 0.110 cm2, when the HAuCl4 and KNO3 concentrations were increased. Estimated values of heterogeneous electron transfer rate constant did not depend on AuNS morphology. Electrode modified with AuNSI exhibited the highest bovine serum albumin (BSA) immobilization efficiency and the highest relative response for the detection of specific polyclonal antibodies against BSA (p-anti-BSA) compared to other modified electrodes. The limit of p-anti-BSA detection in PBS buffer was calculated as 0.63 nM, while in blood serum it was 0.71 nM. Linear ranges were from 1 to 7 nM and from 1 to 5 nM, respectively.

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)

Electrochemical Evaluation of Manganese Electrolytes for Redox Flow Batteries

Redox flow batteries (RFBs) are promising candidates for long-duration energy storage mainly due to the lower cost of improving the energy density compared to lithium-ion batteries. However, the full sustainability of RFBs has not been achieved yet, and there is a lot of room for further improvement. One of the main parameters showing a strong influence on the sustainability of RFBs is redox active materials. In this work, manganese was selected for electrochemical evaluation as it fulfills the sustainability requirements. Manganese is a non-toxic, low-cost and abundant material with sufficient aqueous solubility[1]. Moreover, the standard potential of manganese (1.51 V) is higher than that of vanadium (1.26 V).For electrochemical parameters, we have, through cyclic voltammetry (CVs) and polarization curves, estimated the diffusion coefficient (D0 ) and heterogeneous standard electron transfer rate (k 0) of manganese in an acidic electrolyte in the presence of titanium as an additive. In order to investigate the performance of an RFB system using a manganese electrolyte, a hybrid system was designed that uses hydrogen gas on the negative side and an acidic manganese electrolyte the positive side. We have studied the resistance behavior of the hybrid RFB through electrochemical impedance spectroscopy at different states of charge. From EIS data, cell ohmic resistance, and a general idea of the different processes taking place at different state of charge were obtained. Moreover, we studied the performance of the hybrid RFB during 70 cycles, obtaining an average coulombic efficiency of 98.1%, voltage efficiency of 77.8% and capacity retention of 97.7%. Carney, J.R., B.R. Dillon, and S.P. Thomas, Recent Advances of Manganese Catalysis for Organic Synthesis. European Journal of Organic Chemistry, 2016. 2016(23): p. 3912-3929.

Selective Newman-Kwart Rearrangement Under Mild Conditions Enabled by Heterogeneous Photocatalysis and Electrochemical Catalysis

The Newman-Kwart Rearrangement (NKR), the key-step in the three-stage conversion of phenols 1 into thiophenols 4 (see figure below), holds numerous applications in various fields [1]. Usually, the NKR proceeds via thermally induced intramolecular nucleophilic ipso substitution. High temperatures up to 300 °C are required to facilitate the rearrangement, especially for electron-rich substrates, causing side reactions and high energy consumption [1].Over the past decade, several improved protocols have appeared that enable selective conversion under milder conditions. These methods are based either on homogeneous photocatalysis [2], thermochemical Pd catalysis [3], or on the use of reagents [4]. In this context, a new approach was developed in our group, in which heterogeneous electron transfer enables a radical cation pathway and thereby catalyzes the rearrangement (“hole catalysis”, see figure) [5].The radical cation mechanism is driven by catalytic amounts of charge, either induced by anodic oxidation (“electrochemical catalysis”, e-NKR) [5,6] or in a photochemical reactor using suspended titanium dioxide nanoparticles as photocatalyst (p-NKR) [7].The resulting radical cation 2∙+ then undergoes intramolecular electrophilic aromatic substitution (ArSE) to form the product radical cation 3∙+ , which reacts either in a chain process with another substrate molecule 2 (see figure, case A) or undergoes backward electron exchange with the electrode/titanium dioxide particle (case B) to form product 4 [6,7].For both the e-NKR and the p-NKR, highly selective transformation proceeds at room temperature under aerobic conditions. A variety of different substrates can be rearranged, whereby the scope is complementary to the thermal NKR with respect to electron-donating and -withdrawing aryl substituents. For the e-NKR, an electrochemical micro-flow reactor can overcome the need for supporting electrolyte due to a small distance between the electrodes [6,7].Both for the p- and the e-NKR, detailed mechanistic studies were carried out using cyclic voltammetry, operando spectroscopy, computational analysis, and control experiments. In the present contribution, both practical and mechanistic aspects will be discussed with particular focus on the photocatalytic approach [6,7], along with recent applications to biomass-derived compounds[8].