Charge Transfer Reactions Research Articles

Study of apparent effective ionization coefficient in CO2 and Ar gas mixtures

Ar:CO2 gas mixtures have recently received research interest due to the possibly beneficial effects of Ar addition to CO2 for CO2 conversion using electrical discharges. For any gas discharge, knowledge of fundamental parameters, such as the effective ionization coefficient, is necessary to optimize the efficiency of the discharge for a particular application. The reduced apparent effective ionization coefficient αea/N is a measure of total ionization. αea/N is influenced by electron impact ionization, electron attachment and also by charge transfer reaction, Penning ionization, and photoionization. This study determined the αea/N of Ar:CO2 gas mixtures in the pressure range of 10–800 Torr and reduced electric field strength E/N range of 40–1200 Td utilizing a steady-state non-self-sustaining Townsend discharge. Experimental results were compared with calculations of Boltzmann equation solver BOLSIG+. Differences between measurements and calculations increased with decreasing CO2 content in the mixture down to 20%, and the differences were highest at low E/N values (below 150 Td). As the simple modification of the model, contribution of ionization of CO2 by Penning transfer from Ar* 3p53d excited states (13.86 eV) was added to the BOLSIG+ calculations, which resulted in good fit of the experimental measurements. Comparison of CO2 addition to Ar with the addition of O2 or N2 revealed that ionization of CO2 or O2 from Ar* 3p53d excited states influences ionization in Ar:CO2 and Ar:O2 mixtures but not in Ar:N2 mixtures, due to the different ionization energies of CO2, O2, and N2.

Enhanced hydrogen evolution performance of Co/CoP in electrolysis of water by interfacial regulation of crystal plane orientation

Modulating the heterojunction interface is an effective way to improve the electrocatalytic efficiency of water electrolysis, and the metal centers with preferred crystal plane orientation offer the potential to enhance the interfacial charge transfer and electrode reaction kinetics. In this study, a Co/CoP modified hydrogen evolution electrode with selective exposure of the metal-centered Co(002) crystal face was prepared using a one-step electrodeposition method by adjusting the P/Co molar ratio. Results demonstrate that the preferential orientation of Co(002) crystal face in Co/CoP leads to an increased electrochemical active surface area, significantly reduced interfacial impedance and Tafel slope. Moreover, it brings the d-band center closer to the Fermi level, which is thermodynamically favorable for hydrogen evolution reaction (HER). Under optimized conditions, Co/CoP exhibits overpotentials of 85 mV and 183 mV at 10 mA cm−2 and 100 mA cm−2, respectively. After long-term and cyclic stability tests, no significant activity decay is observed, demonstrating excellent hydrogen evolution activity and cyclic stability.

The Difference in the Effects of IR-Drop from the Negative Capacitance of Fast Cyclic Voltammograms

Diffusion-controlled cyclic voltammograms at fast scan rates show peak shifts, as well as decreases in the peak currents from predicted diffusion-controlled currents, especially when the currents are large in a low concentration of supporting electrolytes. This has been conventionally recognized as an IR-drop effect due to solution resistance on the peaks, as well as a heterogeneously kinetic effect. It is also brought about by the negatively capacitive currents associated with charge transfer reactions. The reaction product generates dipoles with counterions to yield a capacitance, the current of which flows oppositely to that of the double-layer capacitance. The three effects are specified here in the oxidation of a ferrocenyl derivative using fast scan voltammetry. The expression for voltammograms complicated with IR-drop is derived analytically and yields deformed voltammograms. The peak shift is approximately linear with the IR-voltage, but exhibits a convex variation. The dependence of some parameters on the peaks due to the IR-drop is compared with those due to the negative capacitance. The latter is more conspicuous than the former under conventional conditions. The two effects cannot be distinguished specifically except for variations in the conductance of the solution.

Promotion of borophene/NiO-based electrocatalyst for oxygen evolution reaction

Electrochemical water splitting has great potential for converting electricity into clean energy through electrode reactions. However, the anodic oxygen evolution reaction (OER) exhibits serious drawbacks such as: (i) high overpotential, slow reaction dynamics, limited robustness at high current density, and (ii) high cost of standard catalysts in OER (IrO2 or RuO2). Therefore, in our study, we address those issues and present a new composite based on 2D borophene functionalized with nickel oxide nanoparticles with highly active electrocatalytic properties in OER, with a low overpotential of 191 mV reaching j = 10 mA/cm2 and Tafel slope of 44 mV/dec. Additionally, the structure showed electrochemical and physical stability during long-term measurements, with excellent potential retention of 99.9 % at 50 mA/cm2. The electrochemical surface area (ECSA) revealed an increase in the number of active sites available for the reaction to occur, with respect to individual components of composites. Detailed spectroscopic and microscopic analyses of the electrocatalyst conducted before and after OER allowed us to propose insight into the reaction mechanism indicating the synergy between borophene and NiO nanoparticles (facilitated electron charge transfer and additional reaction intermediate such as NiB12O14(OH)10). The proposed fabrication process of borophene functionalized by NiO is facile, low-cost, and scalable. Moreover, this strategy offers an additional avenue to explore a 2D family of electrocatalysts in OER enriching the current state of the art in the field.

Spectroscopy electrochemical impedance characterization of membranes electrode assemblies for PEM electrolyzers

Abstract Hydrogen is projected as a key energy vector to achieve aspiring global decarbonization goals mainly for those sectors where greenhouse gas emissions are highly challenging to reduce. Therefore, green hydrogen production using water PEM electrolysis has become the aim of numerous recent studies. One of the crucial challenges for improving hydrogen production is reaching a deep knowledge of the role of anodes electrocatalysts. In this investigation we studied and modeled the effect of different anode electrocatalysts and solid electrolyte’s configurations in a single-cell PEM electrolyzer performance, using EIS. We assembled and tested PEM electrolysis MEAs with two different configurations. MEA-1 comprises in catalyst-coated membrane (MEA)-type with a geometric area of 25 cm2 containing Nafion™ 115, 3.0 mg cm−2 IrRuOx as the anode, and 3.0 mg cm−2 PtB for the cathode. MEA-2 involves catalyst-coated membrane (MEA)-type with a geometric area of 25 cm2 containing Nafion™ 117, and 3.0 mg cm−2 PtB both for the anode and cathode. Sintered Ti plates were used as gas diffusion layers. After hydration and activation steps at room temperature, the polarization curve was performed. Hysteresis curves were achieved to provide information on thermal equilibrium during the measurements. The oxygen and hydrogen production rate were measured and compared to theoretical values. Experiments were carried out at 20 and 40 °C. EIS measurements were performed at different points of the polarization curve, sweeping a frequency range between 10 kHz and 10 mHz, with an amplitude of 5 mV and taking 10 points per decade. To avoid electrocatalysts area effects in charge transfer reaction, time constants were calculated. It can be clearly observed the trend of resistances and overpotentials according to different process control. Low onset potential for oxygen evolution reaction depicts for MEA-1 means that there will be less power consumption to achieve the same efficiency or same amount of gas generation rate compared to high onset potential case depicted for MEA-2. Although different membrane thicknesses would result in different electrochemical performances due to the change in the ionic resistance, for the thickness studied we did not observe a change in the ionic resistance. The experimental results show an increase in efficiency with temperature, reflected in higher currents for a given potential and lower time constants resulting in the EIS runs.

Origin of Cycle Performance in the Ag2O/TiO2 Heterostructure Photocatalyst.

In recent years, many studies on photocatalysis focused on improving efficiency. However, the cycle performance is also an important index for industrialization. Here, an Ag2O/TiO2 heterostructure photocatalyst is prepared for continuous photodegradation of methylene blue (MB) under visible light, and the samples after the first and fifth round reactions are recycled to study the microstructure evolution of the photocatalyst. The results show that the performance is obviously improved in the second round and remains stable in the following reaction round. Due to the charge transfer, Ag2O/TiO2 gradually changes to Ag2O@Ag-TiO2-x during the photocatalytic reaction. The resulting localized surface plasmon resonance effect and the change of the interface structure greatly increase the number of carriers and prolong the lifetime of carriers. Such variations of microstructures and photoelectric properties of the samples due to the charge transfer and redox reaction on the surface of the photocatalyst dominate the cycle performance.

Development of Two Novel One-Step and Green Microwell Spectrophotometric Methods for High-Throughput Determination of Ceritinib, a Potent Drug for Treatment of Anaplastic Lymphoma Kinase-Positive Non-Small-Cell Lung Cancer.

Background and Objectives: Ceritinib (CER) is a potent drug of the third-generation tyrosine kinase inhibitor class. CER has been approved for the treatment of patients with non-small-cell lung cancer (NSCLC) harboring the anaplastic lymphoma kinase (ALK) mutation gene. In the literature, there is no green and high-throughput analytical method for the quantitation of CER in its dosage form (Zykadia® capsules). This study describes, for the first time, the development and validation of two novel one-step and green microwell spectrophotometric methods (MW-SPMs) for the high-throughput quantitation of CER in Zykadia® capsules. Materials and Methods: These two methods were based on an in microwell formation of colored derivatives upon the reaction of CER with two different benzoquinone reagents via two different mechanisms. These reagents were ortho-benzoquinone (OBQ) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), and their reactions proceeded via condensation and charge transfer reactions, respectively. The reactions were carried out in 96-well transparent plates, and the absorbances of the colored reaction products were measured with an absorbance microplate reader at 540 and 460 nm for reactions with OBQ and DDQ, respectively. The optimum conditions of reactions were established, their molar ratios were determined, and reaction mechanisms were postulated. Under the refined optimum reaction conditions, procedures of MW-SPMs were established and validated according to the guidelines of the International Council on Harmonization. Results: The limits of quantitation were 6.5 and 10.2 µg/well for methods involving reactions with OBQ and DDQ, respectively. Both methods were applied with great reliability to the determination of CER content in Zykadia® capsules and their drug uniformity. Greenness of the MW-SPMs was evaluated using three different metric tools, and the results proved that the two methods fulfil the requirements of green analytical approaches. In addition, the simultaneous handling of a large number of samples with microvolumes in the proposed methods gave them the advantage of a high-throughput analysis. Conclusions: The two methods are valuable tools for rapid routine application in pharmaceutical quality control units for the quantitation of CER.

The initial stages of Li2O2 formation during oxygen reduction reaction in Li-O2 batteries: The significance of Li2O2 in charge-transfer reactions within devices

Lithium-oxygen batteries are a promising technology because they can greatly surpass the energy density of lithium-ion batteries. However, this theoretical characteristic has not yet been converted into a real device with high cyclability. Problems with air contamination, metallic lithium reactivity, and complex discharge and charge reactions are the main issues for this technology. A fast and reversible oxygen reduction reaction (ORR) is crucial for good performance of secondary batteries’, but the partial knowledge of its mechanisms, especially when devices are concerned, hinders further development. From this perspective, the present work uses operando Raman experiments and electrochemical impedance spectroscopy (EIS) to assess the first stages of the discharge processes in porous carbon electrodes, following their changes cycle by cycle at initial operation. A growth kinetic formation of the discharge product signal (Li2O2) was observed with operando Raman, indicating a first-order reaction and enabling an analysis by a microkinetic model. The solution mechanism in the evaluated system was ascribed for an equivalent circuit with three time constants. While the time constant for the anode interface reveals to remain relatively constant after the first discharge, its surface seemed to be more non-uniform. The model indicated that the reaction occurs at the Li2O2 surface, decreasing the associated resistance during the initial discharge phase. Furthermore, the growth of Li2O2 forms a hetero-phase between Li2O2/electrolyte, while creating a more compact and homogeneous on the Li2O2/cathode surface. The methodology here described thus offers a way of directly probing changes in surface chemistry evolution during cycling from a device through EIS analysis.

DRT analysis and transmission line modeling of ceria based electrodes for solid oxide cells

For the analysis of performance limiting processes in various electrochemical systems like lithium ion batteries, polymer electrolyte membrane fuel cells or solid oxide cells, impedance spectroscopy is a powerful tool. The distribution of relaxation times (DRT) enables the deconvolution of the polarization processes in the spectrum. The simple approach, to correlate each peak in the DRT with a single polarization process fails for multiphase electrodes as the complex coupling of electronic, ionic and transport of different species by spatially distributed charge transfer reactions leads to a number of correlated peaks in the DRT.In this contribution such coupling of transport and reaction is analyzed for ceria based fuel electrodes applied in solid oxide cells. A physicochemically meaningful two-channel transmission line model is developed. Due to the ambiguity of impedance spectra and DRT, the straightforward approach of fitting this model to measured spectra does not allow an unambiguous model parameterization. Additional methods as conductivity measurements and focused ion beam-tomography are indispensable to obtain physicochemically meaningful parameters to be applied in the fitting procedure. With this parameterization approach DRT based modeling becomes feasible and ionic/electronic transport resistances and area specific charge transfer resistance of the ceria surface can be quantified.

Physics-Based Model to Represent Membrane-Electrode Assemblies of Solid-Oxide Fuel Cells Based on Gadolinium-Doped Ceria

This paper reports a physics-based model that predicts membrane-electrode assembly (MEA) performance of solid-oxide fuel cells (SOFCs) with Ce0.9Gd0.1O2−δ (GDC10) electrolyte membranes. The paper derives self-consistent thermodynamic and transport properties for GDC1o mobile charged defects (oxide vacancies and reduced-ceria small polarons) by fitting published measurements of oxygen non-stoichiometry and conductivity over ranges of temperature and O2 partial pressures. The button-cell model is applied to evaluate how mixed ionic-electronic conductivity influences the performance of an SOFC MEA with a GDC10 electrolyte sandwiched between a porous, composite Ni-GDC10 anode and a porous, composite cathode of Sm0.5Sr0.5CoO3−δ (i.e., SSC) and GDC10. SSC properties are also derived by fitting published conductivity and oxygen non-stoichiometry measurements. Mixed conductivity of GDC10 and competing charge transfer reactions at both electrodes reduce open circuit voltages due to leakage current and buildup of defect concentrations at electrode-electrolyte interfaces. To fit polarization data, the button-cell model includes heterogeneous reaction rates for defect incorporation on the GDC10 surface along with Butler–Volmer expressions derived for competing charge transfer reaction rates from rigorous analyses assuming rate-limiting, elementary charge transfer reactions for each electrode. The calibrated MEA model can support rigorous SOFC modeling with GDC10 electrolytes over the range of conditions within a fully operating cell.

N-doped MnO2 with abundant oxygen vacancies achieves high-capacity and stable ammonium ion capture by capacitive deionization

Following the concept of sustainable development and circular economy, MnO2 is used as a capacitive deionization (CDI) electrode to remove ammonium (NH4+), which not only prevents water pollution and eutrophication, but also achieves the purpose of recycling precious resources. However, the structural instability and low electron conductivity of MnO2 are the major limitations to obtain excellent deammonium performance. Here, a vacancy-doping modification strategy was proposed to prepare N-doped MnO2 with abundant oxygen vacancies and high specific surface area (N-MnO2-x) by a simple one-step hydrothermal method. The synergistic effect of high N doping, abundant oxygen vacancies and porous structure in N-MnO2-x can effectively inhibit the Jahn-Teller distortion, improve the conductivity, boost ion diffusion and electron conduction, accelerate the charge-transfer reaction kinetics. Consequently, the N-MnO2-x presents an ultrahigh salt adsorption capacity (SAC) (134.3 mg g−1) and displays a superior cyclic stability over 100 cycles, exceeding that of most NH4+ removal materials. Interestedly, N-MnO2-x delivers a high selectivity for NH4+ in multi-ion mixed solution. The mechanism of NH4+ removal and selective strengthening were revealed by spectroscopic studies and theoretical calculations. The present study proposes a novel approach for the development of an efficient electrode capable of capturing NH4+ and facilitating the recycling of ammonia resources.

Cobalt ferrite/semiconducting single-walled carbon nanotubes based field-effect transistor for determination of carbamate pesticides

Carbaryl and carbofuran are the carbamate pesticides which have been widely used worldwide to control insects in crops and house. If the pesticides entered in to the food products and drinking water, they could cause serious health effects in humans. Therefore, the development of a rapid, simple, sensitive and selective analytical device for on-site detection of carbamates is crucial to evaluate food and environmental samples. Recently, semiconducting single-walled carbon nanotube-based field effect transistors (s-SWCNT/FETs) have shown several advantages such as high carrier mobility, good on/off ratio, quasi ballistic electron transport, label-free detection and real-time response. Herein, cobalt ferrite (CFO) nanoparticles decorated s-SWCNTs have been prepared and used to bridge the source and drain electrodes. As-prepared CFO/s-SWCNT/FET had been used for the non-enzymatic detection of carbaryl and carbofuran. When used as a sensing platform, the CFO/s-SWCNT hybrid film exhibited high sensitivity, and selectivity with a wide linear range of detection from 10 to 100 fMand the lowest limit of detections for carbaryl (0.11 fM) and carbofuran (0.07 fM) were estimated. This sensor was also used to detect carbaryl in tomato and cabbage samples, which confirmed its practical acceptance. Such performance may be attributed to the oxidation of carbamates by potent catalytic activity of CFO, which led to the changes in the charge transfer reaction on the s-SWCNTs/FET conduction channel. This work presents a novel CFO/s-SWCNT based sensing system which could be used to quantify pesticide residues in food samples.

Mechanistic insights into photo-assisted DMF cleavage mediated by charge transfer reaction

Mechanistic insights into the synthetically-valued metal-free sustainable N,N-dimethylamination of perfluoroarenes have been investigated. Density functional theory (DFT) and time-dependent DFT (TD-DFT) have been applied to provide details regarding photo-assisted cleavage of N, N-dimethylformamide (DMF) for the valued N, N-dimethylamination reaction. The radical reaction mediated by the electron transfer process causes DMF cleavage, which is responsible for the amination reaction. It was found that the energy barrier for the ground state reaction is thermally prohibitive. On the other hand, visible-light irradiation activates the electron-donor properties of DMF and acceptor properties of perfluoronaphthalene (PFN), so that the energy barrier for electron transfer reduces to ∼ 14 kcal/mol. The redistribution of the electronic charge between the DMF and PFN generates an imbalance of charge on the overall molecule. These events prompt the relocation of extraneous charges to some antibonding molecular orbitals so that one of the C-F bonds in PFN gets weakened and is easily broken. The reductive cleavage of the C-F bond enables the radical ions to react quickly to afford the amination products, which eventually deactivate to the ground state.

Perovskite SrCo0.5Fe0.25Cu0.2Nb0.05O3-δ cathode for intermediate-temperature solid oxide fuel cells: Improved working stability and CO2 tolerance

Commercial solid oxide fuel cells (SOFCs) primarily operate in the intermediate temperature range (600 °C–800 °C); hence, developing cathode materials with good stability and excellent catalytic activity in this temperature range is necessary. Herein, we are reporting the performance of SrCo0.5Fe0.5O3−δ (SCF) and SrCo0.5Fe0.25Cu0.2Nb0.05O3−δ (SCFCN) perovskite oxide as cathode materials for solid oxide fuel cells (SOFCs). The SCFCN sample maintains a cubic structure under a CO2 atmosphere. The polarization resistance of the SCFCN cathode on La0.9Sr0.1Ga0.83Mg0.17O3−δ (LSGM) reaches 0.067 Ωcm2 at 700 °C. The charge transfer and oxygen reduction reactions of the SCFCN cathode on the LSGM electrolyte are the main rate-limiting steps at 600 °C–700 °C. The SCFCN cathode maintains good electrochemical stability during a 200-h test with a configuration of SCFCN/LSGM/SCFCN symmetrical cell. Compared with SCF, the doping of Cu and Nb effectively reduces the Rp attenuation of SCFCN from 0.00012 to 0.00005 Ωcm2/h. The output power density of the fuel cell with SCFCN as the cathode on LSGM reaches 1124 mWcm−2 at 800 °C. This indicates that SCFCN can be a potential material for fabricating cathode for intermediate-temperature SOFCs (IT-SOFCs).

(Invited) Modeling the Charge Transfer Reactions at Li/SEI/Electrolyte Interfaces in Lithium-Ion Batteries

Two kinds of charge transfer reactions are critical for the performance and life of lithium battery: the desired ion transfer reaction occurring during each charge/discharge cycle, , and the undesired electron transfer reactions leading to the parasitic chemical decomposition of the electrolyte and solid electrolyte interphase (SEI) formation/growth. The heterogeneous multi-component nature of SEI dominates its ionic and electronic transport properties and controls these two charge transfer reactions. Density Functional Theory (DFT)-informed multiscale modeling has been providing valuable insights under the scarcity of quantitative experiments. For example, the LiF/Li2CO3 interface was demonstrated to increase the ionic conductivity of mixed SEI nanocomposite by forming an ionic space charge region near the interface and promoting more Li-ion interstitials in Li2CO3, although LiF itself has low Li-ion conducting carriers and conductivity. To form a LiF-rich SEI layer, the electrolyte compositions need to be designed. Since the SEI formation occurs on the charged surface, the electric double layer (EDL) structure near the charged surfaces needs to be incorporated into the modeling. Here interactive classical molecular dynamics (MD), DFT, and data statistical analysis were combined to illustrate the effect of EDL on SEI formation in two essential electrolytes, the carbonate-based electrolyte for Li-ion batteries and the ether-based electrolyte for batteries with Li-metal anodes. It was found the effectiveness of adding fluoroethylene carbonate (FEC) to form the beneficial F-containing SEI component (e.g., LiF) varies with the electrolyte and temperature, because of the interplay of ion-solvent interactions with the surface charge. These integrated modeling provided quantitative guidance for electrolyte/SEI/Li-metal interface design.

Infiltration of La2NiO4+ᵟ-Type Electrodes with Nano-Oxide Catalyst to Modify Electrochemical Properties and Stability Using Biomolecular Surface Engineering

Solid oxide electrolysis cells (SOECs) have increased in importance in recent years due to their potential to cleanly and efficiently separate hydrogen and oxygen, or to instigate other chemical conversion reactions. SOECs require a combination of both high start performance and low long-term degradation for optimal electrodes. Additionally, Cr-poisoning from metal interconnect components threatens long-term degradation due to the ambient humidity of the electrolysis process. The most promising candidate for the oxygen electrode was (La,Sr)(Co,Fe)O3±δ (LSCF) due to its excellent electrical and catalytic properties, but it is challenged by a SrZrO3 phase, Cr-poisoning, and decreased oxygen ionic conductivity under high current loads due to its reliance on oxygen site vacancy (VÖ ) concentration. This drives interest into the Ruddlesden-Popper (R-P) phase, which has the general K2NiF4-type structure. Of these, La2NiO4+δ (LNO) is interesting, as its ionic conductivity mechanism is interstitial oxygen (Oï ) and it has no apparent reaction under Cr-poisoning. A challenge, however, is LNOs relatively low surface oxygen transport. As such, this project’s focus is on the surface engineering of a heterostructured oxygen electrode with LNO as a backbone and an infiltrated LaCoO3-δ (LCO)-based perovskite layer.In this work, the surface engineering of the LNO surface was completed by utilizing a novel wet biomolecular infiltration technique to ensure homogenous nano-particle coatings. The process involves a primary deposition of a catechol surfactant throughout the LNO electrode, and then a secondary deposition of the nano-catalyst solution. The catechol molecule that is deposited in the first step enables the adhesion of the nano-oxide, helping to chelate the complex oxide and assisting in the formation of the compound. No further deposition steps are needed, increasing the overall efficiency and the controllability of the microstructure, and allowing for the formation of the multicomponent nanomaterial at temperatures <800 °C. This has the additional benefit of minimalizing the densification and/or reaction of the nanomaterial with the electrode backbone (if not desired).The initial objective of the work was to characterize the nano-catalyst deposition process on the electrode surface, and define characteristics such as surface roughness, homogeneity, and particle size as function of deposition parameters and surfactant type/concentration. This work was completed initially on an electrode surrogate, YSZ single-crystal substrates. First, a deposition study was run on single-crystal, polished YSZ substrates, chosen for their flatness which simplifies the microstructural characterization through atomic force microscopy (AFM). AFM images from this deposition were processed to gather data on sample surface roughness, and both particle homogeneity and volumetric data, indicating which variables were most important to adjust. An example of these images can be seen in Figure 1 below, an AFM graph of the topography of deposited nano-particles. After defining the deposition process on flat YSZ substrates, the deposition methods were transferred to LNO symmetrical cells and full SOECs. Half and full cells were screen-printed on a YSZ electrolyte, with a GDC barrier layer, a 50-50 wt % LNO/GDC active layer, and an LNO current collector layer. The electrochemical impedance spectroscopy (EIS) tests were completed on infiltrated and non-infiltrated cells at different loading levels (defined by the impregnation variables) to evaluate the effectiveness of the nano-oxide surface engineering on the desired characteristics of surface oxygen transport and charge transfer reactions. The cells were tested in air and various steam concentrations up to 800 °C. Acknowledgments: This research was funded by the US Department of Energy, Project DE-FE0032116. We would also like to thank the WVU Shared Research Facilities for their support, specifically Dr. Qiang Wang, Dr. Marcela Redigolo, and Mr. Harley Hart for their help. Figure 1

Spin-orbit charge transfer from guanine and 9-methylguanine radical cations to nitric oxide radicals and the induced triplet-to-singlet intersystem crossing.

Nitric oxide (●NO) participates in many biological activities, including enhancing DNA radiosensitivity in ionizing radiation-based radiotherapy. To help understand the radiosensitization of ●NO, we report reaction dynamics between ●NO and the radical cations of guanine (a 9HG●+ conformer) and 9-methylguanine (9MG●+). On the basis of the formation of 9HG●+ and 9MG●+ in the gas phase and the collisions of the radical cations with ●NO in a guided-ion beam mass spectrometer, the charge transfer reactions of 9HG●+ and 9MG●+ with ●NO were examined. For both reactions, the kinetic energy-dependent product ion cross sections revealed a threshold energy that is 0.24 (or 0.37) eV above the 0K product 9HG (or 9MG) + NO+ asymptote. To interrogate this abnormal threshold behavior, the reaction potential energy surface for [9MG + NO]+ was mapped out at closed-shell singlet, open-shell singlet, and triplet states using density functional and coupled cluster theories. The results showed that the charge transfer reaction requires the interaction of a triplet-state surface originating from a reactant-like precursor complex 3[9MG●+(↑)⋅(↑)●NO] with a closed-shell singlet-state surface evolving from a charge-transferred complex 1[9MG⋅NO+]. During the reaction, an electron is transferred from π∗(NO) to perpendicular π∗(9MG), which introduces a change in orbital angular momentum. The latter offsets the change in electron spin angular momentum and facilitates intersystem crossing. The reaction threshold in excess of the 0K thermochemistry and the low charge-transfer efficiency are rationalized by the vibrational excitation in the product ion NO+ and the kinetic shift arising from a long-lived triplet intermediate.

Reaction Environment Impacts Charge Transfer but Not Chemical Reaction Steps in Hydrogen Evolution Catalysis

Heterogeneous electrocatalysis involves elementary chemical and charge transfer reaction steps, with the kinetics of each step contributing to the overpotential requirement at a given reaction rate. Typical experiments report on the aggregate rate-overpotential profile with no information about the relative contributions from charge transfer and chemical steps. For the hydrogen evolution reaction (HER), the applied overpotential can be partitioned into a charge transfer overpotential, the overpotential necessary to drive proton-coupled electron transfer (PCET) to and from the surface, and a chemical overpotential, corresponding to a change in surface H activity. Reaction conditions can affect either or both the charge transfer and chemical components. Herein, we employ a Pd membrane double cell to spatially separate the charge transfer and chemical reactions steps of HER catalysis, enabling quantification of the chemical and charge transfer overpotential. We further analyze how each depend on pH, and the introduction of HER poisons and promoters. We find that for a given rate of H2 release, the chemical overpotential is constant across diverse reaction environments whereas the charge transfer overpotential is strongly sensitive to reaction conditions. These findings suggest that reaction condition dependent-HER efficiencies are driven predominantly by changes to the kinetics of charge transfer rather than the chemical reactivity of surface H.

Modeling the Local Environment inside a GDE Pore for CO2 Electrochemical Reduction

Porous Gas Diffusion Electrodes (GDE) have gained a lot of attention in recent years for their ability to overcome the solubility and mass transfer limitations of a planar electrode assembly in CO2 electrochemical reduction (CO2ER) systems. In this work, we present a continuum scale modeling technique based on a set of size-modified Poisson-Nernst-Planck (SMPNP) equations to accurately account for the local environment including the Electric Double Layer (EDL) inside a catalytic nanopore. Most GDE models for CO2ER ignore the EDL or are limited in their applicability at relevant applied potentials due to numerical instability. The local conditions found inside an EDL heavily influence the charge transfer reactions and thus the productivity of desired CO2ER products. Our SMPNP model is able to solve for the concentration and electric field profiles at practical cathodic potentials. We highlight the variation of the predicted CO2 concentration which is often assumed to remain uniform along the radii of the catalytic pores even at high applied potentials. The predicted pH profile also varies significantly along the pore radius, influencing both the CO3 2- balance and the charge transfer reactions. Geometrical parameters such as the diameter and length of the pore, influence the electric field and can potentially be used as control parameters for increasing the selectivity of desired products. We also discuss the implications of double layer overlap as a consequence of small pore radii and low electrolyte concentration. Overall, using the SMPNP model we gain insights into key performance-controlling parameters inside a catalytic nanopore. Furthermore, the model can be used to inform boundary conditions in larger-scale GDE models.

Enhancement of Ni-YSZ Fuel Electrode Performance Via Pressurization and GDC Infiltration

Solid oxide cell systems are often designed for operation with a pressurized stack. Although the cell performance is expected to improve with pressurization, the details of how pressure affects the performance of various technologically-relevant electrodes are typically not known. Here we investigate the electrochemical characteristics of Ni-YSZ and GDC-infiltrated Ni-YSZ fuel electrodes in Ni-YSZ-supported cells as a function of total pressure P from 1 to 5 atm in H2/H2O fuel mixtures with humidification of 25%, 50%, and 75% and temperatures of 600˚C and 700˚C. Using electrochemical impedance spectroscopy, the two limiting electrode processes are identified: charge transfer reactions and gas diffusion. The charge transfer resistance is significantly reduced for Ni-YSZ:GDC compared to Ni-YSZ for all conditions, with total polarization resistance RP reduced by 30 - 40%. Fitting the data to a power-law dependence, RP ∝ P−n, yields a power law exponent of n = 0.28 for Ni-YSZ and 0.36 for Ni-YSZ:GDC (at 600˚C) and n = 0.32 for Ni-YSZ and 0.39 (at 700˚C). That is, GDC infiltration improved electrode performance more at higher pressure. Increasing the total pressure from 1 to 5 atm results in a 42% and 47% reduction in RP for infiltrated electrodes at 600˚C and 700˚C; these values are averaged for all humidities. Increasing humidity from 25 to 50% at 1 atm resulted in a ~26% reduction in total RP. The Ni-YSZ:GDC electrode at 5 atm had a RP value 63% - 65% lower than that of the Ni-YSZ electrode at 1 atm, a very substantial combined effect. The impact of pressurization on overall cell area-specific resistance is assessed based on the present data combined with prior measurements of oxygen electrode pressurization effects.