Electrochemical Electron Spin Resonance Research Articles

Ti3+ redox dynamics enabling efficient nitrate reduction to ammonia on Ti2O3

The electrochemical nitrate reduction reaction (NO3−RR) demonstrates significant promise as a sustainable pathway for ammonia (NH3) synthesis and regulating the Earth's nitrogen cycle. However, its advancement is currently hindered by low catalytic activity, particularly limited by the sluggish rate-determining step of NO2− formation. Herein, we capitalize on the highly active and versatile redox properties of Ti3+ in Ti2O3, exploiting its spontaneous reduction of NO3− to NO2− to overcome the sluggish rate-determining step. Subsequently, the electrochemical reduction process continuously reduces the generated high valence titanium species back to Ti3+, creating a redox closed loop, and enabling the continuous NH3 synthesis. The catalyst achieves an outstanding NH3 production rate of 19.04 mg h−1 mgcat.–1 with a Faradaic efficiency of 97.8 %. Using in-situ electrochemical electron paramagnetic resonance technology, we explored dynamic changes of Ti3+ in Ti2O3 during the process, while in-situ electrochemical infrared spectroscopy provided insights into intermediates' evolution. Density functional theory calculations confirmed Ti2O3′s highly active NO3−RR.

Unraveling the Structure and Reactivity Patterns of the Indole Radical Cation in Regioselective Electrochemical Oxidative Annulations

Oxidation-induced strategy for inert chemical bond activation through highly active radical cation intermediate has exhibited unique reactivity. Understanding the structure and reactivity patterns of radical cation intermediates is crucial in the mechanistic study and will be beneficial for developing new reactions. In this work, the structure and properties of indole radical cations have been revealed using time-resolved transient absorption spectroscopy, in situ electrochemical UV-vis, and in situ electrochemical electron paramagnetic resonance (EPR) technique. Density functional theory (DFT) calculations were used to explain and predict the regioselectivity of several electrochemical oxidative indole annulations. Based on the understanding of the inherent properties of several indole radical cations, two different regioselective annulations of indoles have been successfully developed under electrochemical oxidation conditions. Varieties of furo[2,3-b]indolines and furo[3,2-b]indolines were synthesized in good yields with high regioselectivities. Our mechanistic insights into indole radical cations will promote the further development of oxidation-induced indole functionalizations.

Resolution of Lithium Deposition versus Intercalation of Graphite Anodes in Lithium Ion Batteries: An In Situ Electron Paramagnetic Resonance Study

AbstractIn situ electrochemical electron paramagnetic resonance (EPR) spectroscopy is used to understand the mixed lithiation/deposition behavior on graphite anodes during the charging process. The conductivity, degree of lithiation, and the deposition process of the graphite are reflected by the EPR spectroscopic quality factor, the spin density, and the EPR spectral change, respectively. Classical over‐charging (normally associated with potentials ≤0 V vs. Li+/Li) are not required for Li metal deposition onto the graphite anode: Li deposition initiates at ca. +0.04 V (vs. Li+/Li) when the scan rate is lowered to 0.04 mV s−1. The inhibition of Li deposition by vinylene carbonate (VC) additive is highlighted by the EPR results during cycling, attributed to a more mechanically flexible and polymeric SEI layer with higher ionic conductivity. A safe cut‐off potential limit of +0.05 V for the anode is suggested for high rate cycling, confirmed by the EPR response over prolonged cycling.

Resolution of Lithium Deposition versus Intercalation of Graphite Anodes in Lithium Ion Batteries: An In Situ Electron Paramagnetic Resonance Study.

In situ electrochemical electron paramagnetic resonance (EPR) spectroscopy is used to understand the mixed lithiation/deposition behavior on graphite anodes during the charging process. The conductivity, degree of lithiation, and the deposition process of the graphite are reflected by the EPR spectroscopic quality factor, the spin density, and the EPR spectral change, respectively. Classical over‐charging (normally associated with potentials ≤0 V vs. Li+/Li) are not required for Li metal deposition onto the graphite anode: Li deposition initiates at ca. +0.04 V (vs. Li+/Li) when the scan rate is lowered to 0.04 mV s−1. The inhibition of Li deposition by vinylene carbonate (VC) additive is highlighted by the EPR results during cycling, attributed to a more mechanically flexible and polymeric SEI layer with higher ionic conductivity. A safe cut‐off potential limit of +0.05 V for the anode is suggested for high rate cycling, confirmed by the EPR response over prolonged cycling.

The Z-scheme NH2-UiO-66/PTCDA composite for enhanced photocatalytic Cr(VI) reduction under low-power LED visible light

Series Z-scheme NH2-UiO-66/PTCDA (NU100PX) composites constructed from NH2-UiO-66 and PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) were obtained by simple ball-milling method. The photocatalytic Cr(VI) reduction activities of the NU100PX composites were conducted upon the irradiation of low power LED visible light. The results revealed that the introduction of a small amount of PTCDA on the surface of NH2-UiO-66 could broaden the light absorption range and boost the separation of photo-induced charge carriers to promote the photocatalysis efficiency. The influence factors toward photocatalytic Cr(VI) cleanup performances of NU100P10 like pH, initial Cr(VI) concentrations, the impacts of small organic acids as hole capture agents along with various co-existing foreign matters were clarified. After 5 runs’ adsorption-photoreduction towards Cr(VI), the NU100P10 still exhibited superior reduction activity and reusability. The Z-scheme mechanism of photocatalytic Cr(VI) removal over NU100P10 was put forward and certificated by electrochemical experiment, ESR (electron spin resonance) test, XPS determination, photo-deposition and DFT (density functional theory) calculation.

In situ Electron paramagnetic resonance spectroelectrochemical study of graphene-based supercapacitors: Comparison between chemically reduced graphene oxide and nitrogen-doped reduced graphene oxide

An in situ electrochemical electron paramagnetic resonance (EPR) spectroscopic study of N-doped reduced graphene oxide (N-rGO) is reported with the aim of understanding the properties of this material when employed as an electrical double-layer capacitor. N-rGO shows a capacitance of 100 F g−1 in 6 M KOH, which is twice that found for reduced graphene oxide (rGO). The temperature dependence of the rGO EPR signal revealed two different components: a narrow component, following the Curie law, was related to defects; and a broad curve with a stronger Pauli law component was attributed to the spin interaction between mobile electrons and localised π electrons trapped at a more extended aromatic structure. The N-rGO sample presented broader EPR signals, indicative of additional contributions to the resonance width. In situ EPR electrochemical spectroscopy was applied to both samples to relate changes in unpaired electron density to the enhanced capacitance. The narrow and broad components increased and diminished reversibly with potential. The potential-dependent narrow feature was related to the generated radical species from corresponding functional groups: e.g. O- and N-centred radicals. Improved capacitance seen for the N-modified basal graphene planes can be accordingly suggested to underlie the enhanced capacitance of N-rGO in basic electrolytes.

The facile fabrication of 2D/3D Z-scheme g-C3N4/UiO-66 heterojunction with enhanced photocatalytic Cr(VI) reduction performance under white light

Z-scheme g-C3N4/UiO-66 (BGxUy) heterojunctions constructed from 3D UiO-66 and 2D g-C3N4 sheets were facilely fabricated using ball-milling method, which were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), N2 adsorption-desorption measurement (BET), thermogravimetric analysis (TGA), UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS), photoluminescence spectroscopy (PL), electrochemical impedance spectroscopy (EIS), electron spin resonance (ESR) and density functional theory (DFT) calculations. The photocatalytic activities of Cr(VI) reduction over the BGxUy heterojunctions were tested upon the irradiation of white light. The influence factors on photocatalytic performances of BG60U40 like different organics, pH and different co-existing foreign ions (ions in lake water, tap water, simulated sea water, and simulated leather tanning water) were clarified. The experiment results demonstrated that the optimal BG60U40 exhibited superior Cr(VI) reduction efficiency than pristine UiO-66 and g-C3N4 due to the declined recombination of photo-induced charge carriers. As well, the heterojunction displayed good reusability and stability in the cyclic experiment. The corresponding Z-scheme mechanism of photocatalytic Cr(VI) reduction was proposed and verified by electrochemical analysis, ESR measurement and DFT calculations.

Protein film electrochemical EPR spectroscopy as a technique to investigate redox reactions in biomolecules.

Redox reactions and paramagnetic intermediates are ubiquitous in biological chemistry. We report a new method, protein film electrochemical electron paramagnetic resonance spectroscopy (PFE-EPR), that enables the direct and accurate potential control of proteins on the electrode surface for both electrochemical and EPR spectroscopic characterisation of their redox centres.

Electrochemically Driven C−H Hydrogen Abstraction Processes with the Tetrachloro‐Phthalimido‐N‐Oxyl (Cl4PINO) Catalyst

AbstractThe radical redox mediator tetrachloro‐phthalimido‐N‐oxyl (Cl4PINO) is generated at a glassy carbon electrode and investigated for the model oxidation of primary and secondary alcohols with particular attention to reaction rates and mechanism. The two‐electron oxidation reactions of a range of primary, secondary, and cyclic alcohols are dissected into an initial step based on C−H hydrogen abstraction (rate constant k1, confirmed by kinetic isotope effect) and a fast radical‐radical coupling of the resulting alcohol radical with Cl4PINO to give a ketal that only slowly releases the aldehyde/ketone and redox mediator precursor back into solution (rate constant k2). In situ electrochemical EPR reveals Cl4PINO sensitivity towards moisture. DFT methods are applied to confirm and predict C−H hydrogen abstraction reactivity.

In situ electrochemical electron paramagnetic resonance spectroscopy as a tool to probe electrical double layer capacitance.

Electron paramagnetic resonance (EPR) spectroscopy is applied in situ to monitor the electrochemical capacitance of activated carbon in aqueous solutions, thereby revealing aspects of the charge storage mechanism. The EPR signal of activated carbon increases during the charging process and returns reversibly when discharged. Simulation of the spectral lineshape and its temperature dependence indicate that two kinds of spins exist: spin at defects giving rise to a narrow signal, and spins associated with surface-bound aromatic moieties causing a broad signal. A potential-dependent response of the narrow feature is seen in each of the electrolyte solutions used, while changes of the broad feature occur only at higher electrolyte concentrations. The results suggest that the observed increase of unpaired electron density on activated carbon is due to the formation of radical species due to reduction of functional groups. The potential dependence of the broad feature at higher electrolyte concentrations may be related to the further adsorption of ions into the deep porous structure of activated carbon.

Quantitative measurements in electrochemical electron paramagnetic resonance

The quantification of absolute number of paramagnetic centers in continuous wave (CW) Electron Paramagnetic Resonance (EPR) is a demanding measurement, where the interaction between the sample and the EPR spectrometer needs to be understood and controlled precisely. When utilizing electrochemical EPR (EC-EPR), additional challenges are introduced due to the dynamic nature of the electrochemical systems, (e.g. diffusion of radical species on the timescale of EPR experiments). Here we report a full characterization of an EC-EPR setup for quantifying the absolute number of paramagnetic species originating from EC generation. The data reported indicate that with EC-EPR it is possible to quantify intermediates or products from electrode reactions within 3% accuracy over a threefold concentration range, even in organic solvents. The set-up has high sensitivity, with the quantification of 3–4μM concentrations shown to be routine. Furthermore, sub-μM concentrations can be accessed through signal averaging. These studies provide a platform for the future use of quantitative EPR and new capability for studying and characterizing electrochemical systems.

In situ electrochemical-electron spin resonance investigations of multi-electron redox reaction for organic radical cathodes

The multi-electron redox reaction of an organic radical based composite cathode comprised of poly(2,2,6,6- tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA)-Ketjenblack is investigated using an in situ electrochemical-electron spin resonance (ESR) methodology. The experiments allow each electrochemical state to be associated with the chemical state (or environment) of the radical species upon the cell cycling. In situ ESR spectra of the composite cathode demonstrate a two-electron redox reaction of PTMA that is from an aminoxy anion (n-type, at 2.5–2.6 V vs. Li/Li+) via a radical (at 3.2–3.5 V vs. Li/Li+) to an oxoammonium cation (p-type, at 3.7–4.0 V vs. Li/Li+). In particular, an adjustable n-type doping process of PTMA is first observed during the discharging process. Moreover, two different local environments of radical species are found in the PTMA-Ketjenblack composite electrode that includes both concentrated and isolated radicals. These two types of radical species, showing similarities during the redox reaction process while behaving quite different in the non-faradic reaction of ion sorption/desorption on the electrode surface, govern the electrochemical behavior of PTMA based composite electrode.

Electron paramagnetic resonance monitoring for on-demand electrochemically-generated radicals

An in-situ electrochemical-electron paramagnetic resonance (EC-EPR) spectroscopy techniques utilizing a new inexpensive and disposable two-electrode system cell was developed. The EC-EPR cell provided maximal sensitivity, minimal dielectric loss along the central axis of the EPR cavity, and was easy handled/mounted without the need for additional adjustments. The developed method was utilized for the analytical determination of ketoconazole (KTZ). The process relied on monitoring the peak-to-peak EPR signal intensities obtained from KTZ radical species that were generated electrochemically at disposable graphite pencil electrode (GPE) surfaces. Optimization of the EC-EPR parameters enabled KTZ radical detection at a concentration and with a volume that were one order of magnitude lower than the corresponding concentrations and volumes tested using chemical oxidation analysis techniques. Moreover, ‘on-demand’ radical formation was achieved by alternating the applied potentials between the ‘ON’ and ‘OFF’ that was used to explore the kinetic of the KTZ-oxidation and its oxidative products.

Electrochemical electron paramagnetic resonance utilizing loop gap resonators and micro-electrochemical cells.

A miniaturised electrochemical cell design for Electron Paramagnetic Resonance (EPR) studies is reported. The cell incorporates a Loop Gap Resonator (LGR) for EPR investigation of electrochemically generated radicals in aqueous (and other large dielectric loss) samples and achieves accurate potential control for electrochemistry by using micro-wires as working electrodes. The electrochemical behaviour of the cell is analysed with COMSOL finite element models and the EPR sensitivity compared to a commercial TE011 cavity resonator using 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL) as a reference. The electrochemical EPR performance is demonstrated using the reduction of methyl viologen as a redox probe in both water and acetonitrile. The data reported herein suggest that sub-micromolar concentrations of radical species can be detected in aqueous samples with accurate potential control, and that subtle solution processes coupled to electron transfer, such as comproportionation reactions, can be studied quantitatively using EPR.

Mechanistic Study of ORR by Cu(II) Based Electrocatalyst Using Simultaneous Electrochemical EPR Spectroscopy

The mechanistic aspects of the ORR catalyzed by a non-pyrolyzed Cu-based catalyst were studied using a previously reported in situ electrochemical-EPR cell. Measurements made at a series of different O2 concentrations include calculation of Tafel slope and reaction order of O2 and Cu species in the kinetic region. The Cu-species reaction orders reveal that Cu(I) is involved in the RDS in a 1:1 complex with O2 and that Cu(II) reduced to Cu(I) is a fast step before the RDS.

In situ study of the magnetoelectrolysis phenomenon during copper electrodeposition using time domain NMR relaxometry.

Although the effect of magnetic field (B) on electrochemical reactions (magnetoelectrolysis phenomenon) has been long known, it has not been considered in electrochemical reactions analyzed in situ by magnetic resonance methods, such as nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), and magnetic resonance imaging (MRI), which are intrinsically performed in the presence of B. In this report, the effect of B on the copper electrodeposition reaction, measured by a low-field (0.23 T) NMR spectrometer, was demonstrated. As expected, an enhancement in the reaction rate in comparison to the ex situ electrodeposition reaction was observed. Such enhancement was not dependent on electrodes/magnetic field orientations. Parallel and perpendicular orientations showed similar electrodeposition rates, which is explained by the cyclotron flows generated by distortions in electric and magnetic field lines near the electrode and the electrode edge. Therefore, NMR spectroscopy is not a passive analytical method, as assumed in preceding in situ spectroelectrochemical studies. Although the magnetoelectrolysis phenomenon demonstrated in this report used a paramagnetic ion, it can also be observed for diamagnetic species, since the magnetoelectrolysis phenomenon is independent of the nature of the species. Consequently, similar convection effects may occur in other electrochemical nuclear magnetic resonance (EC-NMR) experiments, such as the electrochemical reaction of organic molecules, as well as in electrocatalysis/fuel cells, lithium-ion batteries, and experiments that use electrochemical electron paramagnetic resonance (EC-EPR) and electrochemical magnetic resonance imaging (EC-MRI).

Simultaneous Electrochemical EPR in Probing the Oxygen Reduction Mechanism of Non-Precious Electrocatalysts for PEMFC

The ORR mechanism as catalyzed by non precious metal catalysts to replace Pt based catalysts at the cathode of PEMFC has been controversial for decades, though this category of catalysts has recently achieved remarkable ORR performance.1,2 More understanding of the ORR mechanism is essential to the further improvement of the performance of catalysts of this class. Cu(II) based non precious metal catalysts are advantageous for the study of the ORR mechanism due to their biomimetic significance in mimicking copper oxidases and, when not pyrolyzed to their well defined structure.3 To gain deeper understanding of the ORR mechanism, an in situ electrochemical half cell for EPR has been designed and applied to probe the mechanism of the ORR catalyzed by a Cu(II)-3,5-diamino-1,2,4-triazole based (shown here) and other catalysts. Simultaneous observation of electrochemical behavior and the corresponding Cu(II) EPR signal evolution at different potentials was realized as shown in Figure 1. The intensity of the Cu(II) signal decreases as the potential steps negative because more Cu(II) is reduced to Cu(I) at more negative potentials. In Figure 2, the EPR spectroscopic behaviors of Cu(II) during a cyclic voltammogram are different in the presence and absence of O2, with a shift in potential indicating a complex formation between O2and Cu ions during ORR catalysis. A study of the effects of oxygen concentration will be conducted to estimate the amount of active catalytic sites in the catalyst. A quantitative relationship will be investigated to determine the reaction order of Cu species in the ORR catalysis. Finally, this method will be applied to a number of related non-precious metal catalysts. Acknowledgements: The authors gratefully acknowledge the support of theUS DOE Office of Fuel Cell Technology and the office of Naval Research for financial support of this work.

Antioxidant activities and radical scavenging activities of flavonoids studied by the electrochemical methods and ESR technique based on the novel paramagnetic properties of poly(aniline-co-5-aminosalicylic acid)

Four kinds of flavonoid, viz. flavanone naringenin, Flavone apigenin, flavonol kaempferol, and flavanol (+)-catechin, are used to investigate their antioxidant and radical scavenging activitis in the water-methanol solution of pH 6.3, using the electrochemical methods and electron spin resonance (ESR) technique. Poly(aniline-co-5-aminosalicylic acid) (PAASA) is first used as a radical source that was polymerized on a reduced graphene oxide (RGO)/glassy carbon (GC) disk or on the RGO/graphite fiber electrode. The assessment of the antioxidant activities is performed using both cyclic voltammetry and the open circuit potential measurement. On the basis of results from both electrochemical mathods, the order of the antioxidant actitvities of flavonoids is as follows: (+)-catechin>kaempferol>apigenin>naringeninHowever, the difference in the antioxidant activities between naringenin and apigenin is very small. On the basis of the ESR signal intensities of PAASA, the order of the radical scavenging activities of flavonoids is in good agreement with that of the above antioxdant activities.Three oxidation peaks on the cyclic voltammograms of (+)-catechin are first detected, which gives us a deep insight into the oxidation mechanism of (+)-catechin.

Oncocalyxone A: electrochemical, spectroscopic investigation and studies of its interaction with DNA, nucleobases and N-acetylcysteine

The formation of paramagnetic species from oncocalyxone A in aprotic medium was confirmed by performing in situ electrochemical-electron spin resonance (E-ESR) experiments. The high delocalization of the radical generated at the first reduction potential is clearly evidenced by the hyperfine coupling of H-9 with the larger coupling constant, besides the couplings at the H-3 (close to quinone) and H-7 (far from the quinone nucleus) positions. In protic medium, together with pH dependence experiments, oncocalyxone A showed to be DNA-reactive through experiments with DNA sensors. Its reaction with N-acetylcysteine, with structural characterization of the addition products, proved its ability as Michael acceptor. Both aspects are important in terms of biological/pharmacological activities and indicate the present models as important tools in the screening of biologically active compounds.

Electron reduction processes of nitrothiophenes. A systematic approach by DFT computations, cyclic voltammetry and E-ESR spectroscopy

In the framework of the interest in nitrothiophenes as drugs or hits with different pharmacological applications and considering that in several instances nitroreduction is an essential step for their biological activity, we have studied a complete series of mono-, di-, and tri-nitrothiophenes (1-6) and by comparison some mononitro benzo[b]thiophenes and benzo[b]furans (7-10). Their electroreduction behaviour has been investigated by different techniques: DFT calculations, cyclic voltammetry and electrochemical electron spin resonance spectroscopy. Although, the first reduction process for all of the compounds leads to the relevant radical anions, both the computational and experimental results indicate that there are significant differences in the fate of their corresponding forthcoming reductions, for example, formation of secondary radicals (open-shell electronic structures) or dianions. The effect of the relative positions of the nitro groups during the reduction has also been analysed and rationalised.