Charge Transfer Reactions Research Articles

Excited-state structural dynamics of nickel complexes probed by optical and X-ray transient absorption spectroscopies: insights and implications.

Excited states of nickel complexes undergo a variety of photochemical processes, such as charge transfer, ligation/deligation, and redox reactions, relevant to solar energy conversion and photocatalysis. The efficiencies of the aforementioned processes are closely coupled to the molecular structures in the ground and excited states. The conventional optical transient absorption spectroscopy has revealed important excited-state pathways and kinetics, but information regarding the metal center, in particular transient structural and electronic properties, remains limited. These deficiencies are addressed by X-ray transient absorption (XTA) spectroscopy, a detailed probe of 3d orbital occupancy, oxidation state and coordination geometry. The examples of excited-state structural dynamics of nickel porphyrin and nickel phthalocyanine have been described from our previous studies with highlights on the unique structural information obtained by XTA spectroscopy. We close by surveying prospective applications of XTA spectroscopy to active areas of Ni-based photocatalysis based on the knowledge gained from our previous studies.

Investigation of active species in low-pressure capacitively coupled N2/Ar plasmas

In this paper, a self-consistent fluid model is developed focusing on the plasma parameters in capacitively coupled 20% N2–80% Ar discharges. Measurements of ion density are performed with the help of a floating double probe, and the emission intensities from Ar(4p) and N2(B) transitions are detected by an optical emission spectroscopy to estimate their relative densities. The consistency between the numerical and experimental results confirms the reliability of the simulation. Then the plasma characteristics, specifically the reaction mechanisms of active species, are analyzed under various voltages. The increasing voltage leads to a monotonous increase in species density, whereas a less homogeneous radial distribution is observed at a higher voltage. Due to the high concentration of Ar gas, Ar+ becomes the main ion, followed by the N2+ ion. Besides the electron impact ionization of neutrals, the charge transfer processes of Ar+/N2 and N2+/Ar are found to have an impact on the ionic species. The results indicate that adopting the lower charge transfer reaction rate coefficients weakens the Ar+ ion density and yields a higher N2+ ion density. However, the effect on the species spatial distributions and other species densities is limited. As for the excited-state species, the electron impact excitation of background gases remains overwhelming in the formation of Ar(4p), N2(B), and N2(a′), whereas the N2(A) molecules are mainly formed by the decay of N2(B). In addition, the dissociation of N2 collided by excited-state Ar atoms dominates the N generation, which are mostly depleted to produce N+ ions.

Why halides enhance heterogeneous metal ion charge transfer reactions.

The reaction kinetics of many metal redox couples on electrode surfaces are enhanced in the presence of halides (i.e., Cl−, Br−, I−). Using first-principles metadynamics simulations, we show a correlation between calculated desorption barriers of V3+–anion complexes bound to graphite via an inner-sphere anion bridge and experimental V2+/V3+ kinetic measurements on edge plane pyrolytic graphite in H2SO4, HCl, and HI. We extend this analysis to V2+/V3+, Cr2+/Cr3+, and Cd0/Cd2+ reactions on a mercury electrode and demonstrate that reported kinetics in acidic electrolytes for these redox couples also correlate with the predicted desorption barriers of metal–anion complexes. Therefore, the desorption barrier of the metal–anion surface intermediate is a descriptor of kinetics for many metal redox couple/electrode combinations in the presence of halides. Knowledge of the metal–anion surface intermediates can guide the design of electrolytes and electrocatalysts with faster kinetics for redox reactions of relevance to energy and environmental applications.

A comparative study of hydrazine alternatives in simulated steam generator conditions—Oxygen reaction kinetics and interaction with carbon steel

The efficiency of a range of hydrazine alternatives (carbohydrazide, diethyl-hydroxylamine, erythorbic acid and 2-butanone oxime) as oxygen scavengers and their interaction with carbon steel in simulated PWR steam generator inlet conditions are quantitatively compared. Kinetic parameters of oxygen reaction are estimated for the first time in such conditions using both an oxygen and a redox sensor. Electrochemical impedance measurements are performed to investigate the effect of studied alternatives on carbon steel corrosion. Using a quantitative interpretation of the impedance data by the Mixed-Conduction Model, the influence of hydrazine and its alternatives on charge transfer reactions at the oxide/solution interface and film growth/dissolution processes are discriminated. Conclusions are drawn on the comparative reaction rates of hydrazine alternatives with respect to oxygen consumption and steel passivation.

Cation deficiency tuned LaCoO3−δ perovskite for peroxymonosulfate activation towards bisphenol A degradation

Perovskite-based materials have been demonstrated to be effective for peroxymonosulfate (PMS) activation towards wastewater treatment. However, the effect of cation deficiency in perovskite oxides has not been well illustrated. In this study, cation deficient LaCoO3−δ perovskite oxides have been prepared to tune the catalytic activity. B-site cation deficiency on the LaCoO3−δ perovskite oxides promotes the catalytic performance while A-site cation deficiency is detrimental to the catalysis for PMS activation. The first-order kinetic rate of B-site cation deficient LaCoO3−δ perovskites prepared via sol-gel method is 0.591 min−1, higher than that of LaCoO3−δ perovskites with A-site cation deficiency (0.118 min−1) and no cation deficiency (0.243 min−1). About 90% of organics could be mineralized completely through the catalysis of B-site cation deficient LaCoO3−δ perovskites in 60 min. The sulfate radicals and singlet oxygen are responsible for the catalytic degradation of bisphenol A. The surface Co(II) site, surface hydroxyl group and oxygen vacancies on the LaCoO3−δ perovskite oxides are the active sites for catalysis. The excellent catalytic activity induced by B-site cation deficiency can be attributed to the high charge transfer and surface reaction rate, abundant Co(II) site, surface hydroxyl group and oxygen vacancies.

Insight into the Transition‐Metal Hydroxide Cover Layer for Enhancing Photoelectrochemical Water Oxidation

AbstractDepositing a transition‐metal hydroxide (TMH) layer on a photoanode has been demonstrated to enhance photoelectrochemical (PEC) water oxidation. However, the controversial understanding for the improvement origin remains a key challenge to unlock the PEC performance. Herein, by taking BiVO4/iron‐nickel hydroxide (BVO/FxN4−x‐H) as a prototype, we decoupled the PEC process into two processes including charge transfer and surface catalytic reaction. The kinetic information at the BVO/FxN4−x‐H and FxN4−x‐H/electrolyte interfaces was systematically evaluated by employing scanning photoelectrochemical microscopy (SPECM), intensity modulated photocurrent spectroscopy (IMPS) and oxygen evolution reaction (OER) model. It was found that FxN4−x‐H acts as a charge transporter rather than a sole electrocatalyst. PEC performance improvement is mainly ascribed to the efficient suppression of charge recombination by fast hole transfer kinetics at BVO/FxN4−x‐H interface.

Insight into the Transition-Metal Hydroxide Cover Layer for Enhancing Photoelectrochemical Water Oxidation.

Depositing a transition-metal hydroxide (TMH) layer on a photoanode has been demonstrated to enhance photoelectrochemical (PEC) water oxidation. However, the controversial understanding for the improvement origin remains a key challenge to unlock the PEC performance. Herein, by taking BiVO4 /iron-nickel hydroxide (BVO/Fx N4-x -H) as a prototype, we decoupled the PEC process into two processes including charge transfer and surface catalytic reaction. The kinetic information at the BVO/Fx N4-x -H and Fx N4-x -H/electrolyte interfaces was systematically evaluated by employing scanning photoelectrochemical microscopy (SPECM), intensity modulated photocurrent spectroscopy (IMPS) and oxygen evolution reaction (OER) model. It was found that Fx N4-x -H acts as a charge transporter rather than a sole electrocatalyst. PEC performance improvement is mainly ascribed to the efficient suppression of charge recombination by fast hole transfer kinetics at BVO/Fx N4-x -H interface.

Performance degradation of solid oxide fuel cells analyzed by evolution of electrode processes under polarization

The electrochemical impedance spectroscopy (EIS) is one of the most promising methods to unfold the complex electrode processes in solid oxide fuel cells (SOFCs). In this study, evolution of different electrode processes during galvanostatic operation of a single cell is analyzed based on deconvolution of recorded EIS data. The overall operation can be roughly divided into two stages, i.e. a rapid and nonlinear degradation stage, and a slow and quasi-linear degradation stage. Based on the analysis of EIS measured under open circuit voltage (OCV), it is found that the Rp decreases in the first stage while gradually increases in the second stage, which is mainly due to changes in anodic gas-phase diffusion and cathodic O2 surface exchange and bulk diffusion processes. The effect of DC electrical current (polarization) on different electrode processes is emphasized and analyzed in detail. Accordingly, we speculate that the dominant degradation mechanism is closely related to the operating current, i.e. performance degradation in small current region is dominated by the deterioration of O2 surface exchange kinetics at the cathode, while performance degradation in moderate current range is dominated by the weakening of charge transfer reactions at the anode.

Bilateral growth of monoclinic WO3 and 2D Ti3C2Tx on 3D free-standing hollow graphene foam for all-solid-state supercapacitor

• Hollow graphene foam was successfully prepared by TA-CVD method. • m -WO 3 /Ti 3 C 2 T X /HGF-7 electrode was fabricated by UPED and drop-casting methods. • The electrode exhibited a specific capacitance of 573F g −1 with 93.3% cycling stability. • An ASC device showed 27.2 Wh kg −1 energy density at power density of 752 W kg −1 . • The ASC device endured superior cycling stability of 93% over 10,000 GCD cycles. The free-standing three dimensional (3D) hollow graphene foam (HGF) decorated nanostructured pseudocapacitive materials could offer a large active surface area and a extreme conductive porous 3D network for fast reversible charge transfer reactions in the supercapacitor. Herein, the 3D HGF was prepared by a template assisted-chemical vapor deposition (TA-CVD) method and the monoclinic WO 3 ( m- WO 3 ) interconnected nanoparticles as well as 2D Ti 3 C 2 T x sheets were bilaterally loaded on the inner and outer sides of HGF by a simple unipolar electrodeposition (UPED) method and a drop casting method, respectively. The obtained 3D free-standing m- WO 3 /Ti 3 C 2 T X /HGF electrode exhibited an excellent specific capacitance of 573F g −1 at 5 mV s −1 with outstanding rate performance and 93.3% cycling stability over 5000 cycles, which can be attributed to the metal-like conductivity and reversible redox reactions of hydrophilic 2D Ti 3 C 2 T x . The asymmetric solid-state supercapacitor (ASC) illustrated a 2-fold wider potential of 1.4 V in an acidic electrolyte when compared with the MXene-based symmetric supercapacitor. It displayed an excellent specific capacitance of 145.2F g −1 (111.3 mF cm −2 ) at 5 mV s −1 , an ultra-high energy density of 27.2 Wh kg −1 (20.83 μWh cm −2 ) at a power density of 752 W kg −1 along with 93% cycling stability over 10,000 cycles. Therefore, such a m- WO 3 /Ti 3 C 2 T x /HGF electrode should be promising for the fabrication of advanced supercapacitors.

Insight into Electron Transfer from a Redox Polymer to a Photoactive Protein.

Biohybrid photoelectrochemical systems in photovoltaic or biosensor applications have gained considerable attention in recent years. While the photoactive proteins engaged in such systems usually maintain an internal charge separation quantum yield of nearly 100%, the subsequent steps of electron and hole transfer beyond the protein often limit the overall system efficiency and their kinetics remain largely uncharacterized. To reveal the dynamics of one of such charge-transfer reactions, we report on the reduction of Rhodobacter sphaeroides reaction centers (RCs) by Os-complex-modified redox polymers (P-Os) characterized using transient absorption spectroscopy. RCs and P-Os were mixed in buffered solution in different molar ratios in the presence of a water-soluble quinone as an electron acceptor. Electron transfer from P-Os to the photoexcited RCs could be described by a three-exponential function, the fastest lifetime of which was on the order of a few microseconds, which is a few orders of magnitude faster than the internal charge recombination of RCs with fully separated charge. This was similar to the lifetime for the reduction of RCs by their natural electron donor, cytochrome c2. The rate of electron donation increased with increasing ratio of polymer to protein concentrations. It is proposed that P-Os and RCs engage in electrostatic interactions to form complexes, the sizes of which depend on the polymer-to-protein ratio. Our findings throw light on the processes within hydrogel-based biophotovoltaic devices and will inform the future design of materials optimally suited for this application.

Electrochemical Behavior of Dysprosium Complex in Potassium Bis(trifluoromethylsulfonyl)Amide Melts

1. Introduction From the standpoint of energy conservation, the development of a recovery process for rare earth metals with reduced energy consumption is desired. In previous investigations, we demonstrated the recovery of Nd metal using low-temperature molten salts (LTMSs) [1,2], because an LTMS has many useful physicochemical properties [3] such as a wide electrochemical window, low liquid-phase temperature, and high ionic conductivity. In our findings, the reduction process of Dy(III) proceeded in two steps by way of Dy(II), [Dy(III)+e-→Dy(II), Dy(II)+2e-→Dy(0)] in [NTf2 -] based ionic liquids (ILs), because [Dy(III)(NTf2)5]2- and [Dy(II)(NTf2)4]2- in ILs were confirmed by Raman spectroscopy and DFT calculations [4]. However, there were no information about the detailed solvation conformations of Dy(II) and Dy(III) in K[NTf2] melts. Therefore, we have investigated the electrochemical behavior of Dy(II) and Dy(III) in K[NTf2] melts. 2. Experimental The K[NTf2] melts including Dy(III) (molar fraction: x Dy=0.1) were applied as the electrolytic for the electrochemical measurement and this solution was dried at 373 K in a vacuum chamber (<0.1 MPa) for 24 h. Cyclic voltammetry (CV) at 493±1.0 K was carried out using a cylindrical cell constituted from three-electrode system under Ar flow. The Pt electrode was used as a working electrode that was mirror polished using alumina paste. Platinum wires (φ=0.7 mm) were employed as a counter electrode and a quasi-reference electrode (Q.R.E), because the potential obtained using a Pt Q.R.E was stable and exhibited a good reproducibility at medium temperature. All the potentials were compensated for the redox reaction of K/K+ couple. As for the electrodeposition process, a prismatic Dy rod and a Cu substrate were employed as an anode and a cathode, respectively. The anode was surrounded by a soda lime tube with a Vycor glass filter at the bottom to prevent the diffusion of dissolution components from the anode into electrolyte. The potentiostatic electrolysis was carried out while stirring the electrolyte at 500 rpm in order to increase the current density of the electrodeposition process. An in-depth analysis of the electrodeposits was conducted with Al-Kα radiation by XPS. The sputtering rate was 27.2 nm min-1 estimated from the sputtering rate of the Si standard. 3. Results and discussion From the voltammetric analysis of Dy(III) in K[NTf2] melts, there were no apparent anodic peaks corresponding to an oxidation of Dy(0) in voltammogram. This result indicated that the reduction of Dy(III) was an irreversible process. It was also consistent with the reference [5], which was reported that no oxidation peak of Dy(0) in DMF and DMPT. There were two reduction peaks around +1.0 V and +2.3 V vs. K/K+ in voltammogram. The reduction peak around +2.3 V would be based on Dy(III)/Dy(II) charge transfer reaction. The reduction peak around +1.0 V would be corresponded to Dy(II)/Dy(0) deposition process. Regarding the charge transfer reaction, it was confirmed that the plot of the cathodic peak of the current density vs. the square root of the scan rate showed a good linear relation. It indicated that the charge transfer reaction was controlled by the diffusion process.In the electrodeposition process, the blackish-brown electrodeposits had a relatively strong adhesion on the Cu substrate. The Dy3d 5/2 peak of the electrodeposits was shown in Fig. 1. The Dy3d 5/2 peaks for Dy metal and Dy2O3 were theoretically positioned at 1295.8 eV and 1289.0 eV, respectively [6]. The Dy3d 5/2 spectra of the top surface and the middle layer showed a relatively good agreement with the theoretical value. This study enabled us to conclude that Dy metal was able to electrodeposit in a metallic state from the K[NTf2] melts. Acknowledgement This research was partially supported by the Grant-in-Aid for Scientific Research (No. 18H03404) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

A General Theory of Electrochemistry

The electrochemical charge transfer reaction at an electrode/electrolyte interface occurs within close distance from the electrode surface. There is a delicate balance of forces between electrons in the electrode and electrode ions within the electrolyte, and forces between the ions themselves, in this region. Since ions have an electrical charge, a corresponding electrical field is present, which in turn gives rise to the electrode potential. An external current applied to the electrode will affect the distribution of ions, the electric field, and, ultimately, the electrode potential.This report presents a new electrochemical theory that provides a detailed and accurate description of the electric field and charge distributions associated with the charge transfer reaction, together with their relation to time, electrode voltage, and electrode current. With this foundation, it is possible to establish and formulate the basic electrochemical properties of the electrochemical cell.The core of the theory is a partial differential equation that models the electric field in close vicinity of the electrode surface based on fundamental natural constants and specific boundary conditions. The underlying basic equation was initially derived in an attempt to explain and describe the phenomenon of Induced Polarization, which technique is effectively employed by mining companies in the large-scale search for mineral deposits.To demonstrate the general validity of the theory the well-known Butler/Volmer and Nernst’s equations are derived.The theory also permits the accurate modeling of dynamic, or non-equilibrium properties, like the voltage variation with time from the moment an electrode is dipped into an inert electrolyte. To assess the accuracy of the modeling, experiments were carried out where a zinc electrode was dipped into a sodium sulfate electrolyte. The electrode voltage was measured over a 5 to 640 sec time span and would typically be within 5 mV of the predicted value.The electrode voltage was also derived from initial contact with an electrolyte containing the electrode ions until equilibrium was fully established - ending up in agreement with the predictions of Nernst’s equation.The theory also permitted the derivation of the complex impedance of the electrode/electrolyte interface reaction. A comparison with experimental data was carried out, whereby the complex impedance of a copper electrode in contact with nine different concentrations of copper sulfate/sulfuric acid electrolytes was measured. The fit with the theory was typically within 2% over the entire 0.07 to 2400 Hz measurement range. The best fit with the experimental data allowed in turn the exchange current and the electric field distribution thickness to be determined as functions of concentration.(Preprint full report: https://ecsarxiv.org/8njbr/ )

Dual-Cation Electrolyte System Using Quaternary Ammonium Salts and Ionic Liquid for High Power and High Energy Li4Ti5O12//AC Hybrid Capacitor System

[Introduction] The Use of thick electrodes (> 100 µm) is a simple approach to increase the energy density of electrochemical energy storage devices. However, the high ionic resistance in thick electrodes limits the high-power applications of such devices. “Dual-cation” electrolytes for thick-electrode hybrid capacitor systems, typically composed of Li4Ti5O12 (LTO, 200 µm) and activated carbon (AC, 400 µm) electrodes, have been developed[1]. A dual-cation electrolyte is composed of lithium tetrafluoroborate (LiBF4) and spiro-(1,1')-bipyrrolidinium tetrafluoroborate (SBPBF4) dissolved in propylene carbonate (PC). An SBP-based dual-cation electrolyte, comprising 1 M LiBF4 + 2 M SBPBF4 / PC, exhibits a higher total ionic conductivity (7.67 S cm-1) than the single-cation electrolyte (3.40 S cm-1), consisting of 1 M LiBF4 / PC, while Li+ conductivity decreases with an increase of SBPBF4 concentration. Our interesting finding was that the presence of SBP+, despite the low Li+ transport in the electrolyte bulk, reduces overvoltage associated with migration limitation in the thick LTO electrode, and thus enhances power density in LTO//AC hybrid capacitor systems.In this study, we propose another dual-cation electrolyte using a 1-ethyl-3-methylimidazolium (EMI) cation in place of the SBP cation, which further enhances the power performance of LTO//AC hybrid capacitor systems. It has been shown that an EMI-based electrolyte exhibits a higher total ionic conductivity (10.9 S cm-1) than the SBP-based electrolyte. Herein we provide a detailed account of factors that improve the discharge properties of the EMI-based dual-cation electrolyte by using electrochemical and spectroscopic methods.[Experimental] The dual-cation electrolytes were prepared by dissolving 1 M LiBF4 and x M SBPBF4 (0 < x < 3) or x' M EMIBF4 (0 < x' < 4), in PC. The ionic conductivity of the prepared electrolytes was measured using a conductivity meter. The LTO(200 μm)//AC(400 μm) hybrid capacitor system assumed a coin type cell assembly; charge-discharge measurements were conducted at 1 mA cm-2 (at the charge) and 1 to 200 mA cm-2 (at the discharge). Pulsed Gradient Spin Echo (PGSE) NMR analysis was conducted to determine the self-diffusion coefficients of the Li(7Li), SBP (1H), and EMI (1H) cations and the BF4 (19F) anion in the electrolytes.. Electrochemical impedance spectroscopy (EIS) was performed to evaluate the resistance of the LTO electrode using LTO//LTO symmetric cells designed at both the delithiated (SOC of LTO = 0%) and lithiated (SOC of LTO = 25%) states.[Results and discussion]The results of the ionic conductivity for all electrolytes are shown in Figure 1. The EMI-based dual-cation electrolyte exhibited higher ionic conductivity than the SBP-based electrolyte. The results of the charge-discharge test (Figure 2) showed that the EMI-based dual-cation electrolyte exhibits optimal discharge properties among the electrolytes. Moreover, the PGSE NMR analysis results indicated that the self-diffusion coefficient for all ions including Li+ decreased with an increase in EMIBF4 concentration. In contrast, the ionic resistance (R ion) of the LTO negative electrode calculated from the EIS results at the delithiated state (SOC of LTO = 0%) yielded lower values for the EMI-based dual-cation electrolyte (11.1 Ω cm2) than that for the SBP-based (14.0 Ω cm2) and single-cation (27.1 Ω cm2) electrolytes. We also evaluated the charge-transfer resistance (R ct) from the EIS results using LTO//LTO symmetric cells at the lithiated state (SOC of LTO = 25%). The R ct was found to be lower in the EMI-based dual-cation electrolyte (1.71 Ω cm2) than that in the SBP-based (2.28 Ω cm2) and single-cation (3.57 Ω cm2) electrolytes. These results showed that charge-transfer reactions at the LTO electrodes occur more readily in the dual-cation electrolyte.The results indicated that a reduction in R ct occurs because ion potential suppression occurs owing to the accelerated ion transport in the electrode, as well as in our previous study[1]. The suppression of the ionic potential increased the Li+ concentration and reaction current at the collector side, which decreased the R ct. This is because the advantage of an increase in total ionic conductivity is effective even when the transport ofLi+ is reduced in a thick-electrode system. This study confirms that the use of the EMI-based electrolyte, which has a higher total ionic conductivity than the SBP-based electrolyte, exhibits a remarkable suppression of R ion and R ct. The use of thick electrodes, therefore, facilitates an increase in total ionic conductivity, which has a more desirable effect than that of high Li+ conductivity, especially in the EMI-based dual-cation electrolyte, wherein the power density of the thick-electrode LTO//AC hybrid capacitor systems was significantly improved.[1] Y. Chikaoka et al., J. Phys. Chem. C, in press(doi.org/10.1021/acs.jpcc.0c01916). Figure 1

Analysis of Pristine and Tcnq-Loaded MOF FILMS By Contactless Microwave Dielectric Spectroscopy Technique

Metal-organic frameworks (MOFs) are porous and crystalline Coordination Network Compounds (CNCs), also referred to as porous coordination polymers (PCPs), which consist of inorganic metal (or metal/oxo) clusters connected by organic linkers. Surface Anchored Metal-organic-framework (SURMOF) films consist of quasi-liquid phase epitaxially grown highly ordered crystalline MOF films. For this study we used MOF Cu3(BTC)2 (BTC: benzene tricarboxylate) films, also known as HKUST-1 (Hong Kong University of Science and Technology-1), which were infiltrated with tetracyano-quinodimethane (TCNQ) guest molecules to modulate the charge transport characteristics. HKUST-1 MOF thin films have been grown using a liquid phase epitaxy (LPE) spray method in which the metal-containing solution [1 mmol Cu(OAc)2] and the linker solution [0.1 mmol BTC] are sprayed alternately on modified silicon substrates. Subsequently, the samples were immersed into an ethanoic TCNQ solution.We report on localized charge transfer processes at microwave frequencies of porous metal-organic framework (MOF) HKUST-1 Cu2(BTC)3 samples. As part of this study we will compare measurements done in the HKUST-1 pristine form of the material with open pores and HKUST-1, where the pores have been infiltrated with tetracyano-quinodimethane (TCNQ), resulting in a hybrid structure with enhanced electrical conductivity and a significant measurable effect in the microwave propagation characteristics [1]. We present data and analysis from an evaluation of pristine and loaded HKUST-1 SURMOF films using the contactless broadband microwave dielectric spectroscopy (BDS). The technique enables the accurate operando evaluation of the electrical and magnetic properties of materials without compromising the kinetic conditions of the experiment. The BDS method is uniquely suitable for investigating charge transfer reactions, regardless of the nature of the charge carriers, i.e., electrons and holes [2]. As such, these material state-sensitive parameters could be used to monitor charge transfer changes in the SURMOF and SURMOF-TCNQ hybrid analytes.[1] G. M. Espallargas and E. Coronado, “Magnetic functionalities in MOFs: from the framework to the pore”, Chem. Soc. Rev., 2018,47, 533, DOI: 10.1039/c7cs00653e[2] A. M. Wernbacher et al, Operando Electrical Conductivity and Complex Permittivity Study on Vanadia Oxidation Catalysts, J. Phys. Chem. C 2019, 123, 8005−8017, DOI: 10.1021/acs.jpcc.8b07417

Electrodeposition Behavior of Extracted Platinum Complex in Ionic Liquids Evaluated By Electrochemical Quartz Crystal Microbalance

1. Introduction From the perspective of process simplification and secondary waste reduction, the solvent extraction-electrodeposition method is effective in which the ionic liquid (IL) phase containing the extracted complexes is used as an electrodeposition bath and the extracted complexes are electrodeposition to recover the metals efficiently. We have applied the solvent extraction-electrodeposition method to Ru [1], In [2] and Nd [3] in the IL system based on bis(trifluoromethyl)sulfonyl amide (NTf2 -). In addition, the complexation states between [NTf2 -] and the metal have been clarified from a computational chemical point of view [4] based on Raman spectroscopy and density functional theory. The electrochemical quartz crystal microbalance method (EQCM) was used to evaluate the electrodeposition behavior of the extracted platinum complex in the IL phase after the extraction by solvent extraction. The purpose of this study is to analyze the electrodeposition behavior of extracted platinum complex, the weight change of the electrode interface, and the alternation of viscosity or density of the IL at the electrode interface. 2. Experimental The resonance frequency and resistance of an AT-cut Pt-coated quartz oscillator [9 MHz, φ=5.0 mm, Seiko EG&G, QA-A9M-PT(P)] were determined using an electrochemical quartz crystal microbalance (EQCM) system (Seiko EG&G, QCA922) with a well-type cell. The EQCM system was heated with a heating mantle controlled by a thermostat with a PID controller. The applicability of the EQCM technique at elevated temperature was demonstrated in our previous study [5]. Voltammetric measurements were carried out using an electrochemical analyzer (ALS-440A, BAS Inc.) with a Pt-coated quartz oscillator in the EQCM system as a working electrode. The surface area of the QCM crystal was 1.9635×10-5 m2. Two Pt wires with inner diameters of 0.5 mm were used as the counter and quasi-reference electrodes (QRE). The counter electrode was surrounded by a Vycor glass filter at the bottom to prevent diffusion of the decomposition components from the anode into the electrolyte. The Pt QRE showed high stability and good reproducibility of the potential at elevated temperature. The potential was compensated against a Fc/Fc+ redox couple. The electrochemical behavior of the extracted platinum complex was investigated at 323 K with a sweep rate of 2.0 mV s-1. 3. Results and discussion The result of the CV/EQCM analysis of [R3NH]2[PtCl6]IL in Alamine336/[P2225][NTf2] was shown in Fig. 1. Preliminary, in the case of Alamine336/[P2225][NTf2] system, which did not contain the Pt(IV) extraction complex and consisted of the extractant and the diluent, there were no main peaks other than the solvent decomposition during this potential region. It indicated that Alamine336 extractant did not undergo the electrochemical decomposition. On the other hand, a slight reduction peak was observed at -0.53 V for the extracted Pt(IV) complex, which showed little change with respect to Δm, suggesting that it was corresponded to the charge transfer reaction of Pt(IV)/Pt(II). The main reduction peak was observed at -1.65 V, and simultaneously an increase in Δm and a decrease in Δηρ were also observed in this study. The molecular weight of the electrodeposited species: M app was corresponded to be 193.7 calculated from Δm, suggesting that the main reduction reaction was the electrodeposition of Pt(II)/Pt(0); the decrease in Δηρ represented a localized viscous change in the IL phase near the electrode interface, indicating the consumption of Pt(II) by the electrodeposition process. The change in Δηρ during the electrodeposition process was consistent with the EQCM analysis of Ag(I)/Ag(0) [6].

Bilateral Growth of Monoclinic Tungsten Oxide and 2D Ti3C2Tx-MXene on 3D Free-Standing Multilayer Hollow Graphene Foam for Flexible All-Solid-State Pseudocapacitor

To overcome the subordinate electrochemical performance of the pseudocapacitor, it is essential to design two-dimensional (2D) electroactive nanostructured material with enormous electroactive sites on the highly conductive current collector for enriching the rates of reversible charge transfer reactions. Herein, a scalable strategy was employed to prepare the electrodes by depositing pseudocapacitive materials on a three-dimensional (3D) free-standing highly conductive hollow multilayered graphene foam (HGF) synthesized through a template assisted-chemical vapor deposition (TA-CVD) method for the first time. The stable nanostructured monoclinic tungsten oxide (m-WO3) nanoparticles and 2D transition metal carbide (Ti3C2Tx-Mxene) sheets were bilaterally loaded on inner and outer sides of 3D HGF by a unipolar electrodeposition (UPED) method and a drop casting method, respectively. This free-standing electrode exhibited exceptional specific capacitance (Cs) of 487 F/g (439 mF/cm2) at 2 mA/cm2, outstanding rate performance and extraordinary cycling stability of 92.9 % over 5000 cycles owing to its significant metallic conductivity and reversible redox reactions of hydrophilic 2D Ti3C2Tx-Mxene. To assemble asymmetric pseudocapacitor, a MWCNT/RuO2 electrode with tiny RuO2 was also synthesized through a simple and inexpensive layer-by-layer (LBL) method. The obtained flexible asymmetric solid-state pseudocapacitor (FASC) illustrated 2-fold wider potential of 1.4 V in an acidic electrolyte when compared with Mxene based symmetric supercapacitor. This FASC displayed excellent Cs of 152.2 F/g at 5 mV/s, high energy density of 18 Wh/kg at 497 W/kg and an outstanding capacity retention of 93.3 % over 10000 GCD cycles with superb coulombic efficiencies of 99 ~ 100 %. In addition, the extraordinary demonstration performance of this FASC device by driving light emitting diodes (LEDs) and digital stopwatch signified its real-world applicability in electronic gadgets.

Combined Electrokinetic-Electrochemical Probe to Understand Electric Double Layer at Metal-Electrolyte Interface: Application to Polycrystalline Gold and Platinum

We developed a novel technique combining electrochemistry and electrokinetics to understand the structure of electric double layers (EDLs) at metal-electrolyte interface, and we applied it to study polycrystalline Au (poly-Au)-electrolyte and polycrystalline Pt (poly-Pt)-electrolyte interfaces. Traditional electrochemical methods are used to study Faradaic charge transfer reactions and EDLs at a polarized metal-electrolyte interface. On the other hand, electrokinetic methods, such as streaming current/potential are used to study ion distribution and surface conductance of ions in the diffuse layer at the interface of unpolarized solid material and electrolyte. In traditional electrokinetic experiments, the solid material to be studied is mounted inside a microchannel. An electrolyte of known chemical composition (pH, concentration, specifically adsorbing ions etc.) is flown through the microchannel at various pressures. The diffuse layer formed at the interface contains mobile ions with a net positive or negative charge. These ions, flowing with the electrolyte constitute streaming current. Zeta potential is calculated from the streaming current vs pressure data.We combined a 3-electrode electrochemical setup and a pressure regulated microfluidic setup to apply potential on a planar electrode mounted inside a microchannel and flow the electrolyte at various pressures at the same time. The equivalent circuit of our method is shown in Figure 1(a). As charge on the electrode surface strongly depends on the applied potential, different values of streaming currents are obtained for different values of applied potentials. Zeta potential at different applied potentials is calculated from the streaming current vs pressure data. These experimentally calculated zeta potential values are substituted into the exact analytical solution of Poisson-Boltzmann distribution in the diffuse layer to explicitly calculate ion density and ionic conductivity in this layer. The charging mechanism at the interface is studied using this data. We used the simplified Gouy-Chapman interfacial model in the capacitive region of the applied potential to compare the observations and gain insights about the charging behavior.The method was applied to electrochemically clean poly- Au and Pt surfaces. Electrolyte of high concentration (thin diffuse layer) was used to minimize the iR drop. For poly Au-electrolyte interface, we observed an approximately homogeneous charging at the interface, in the applied potential window where Au is known to behave as an ideal capacitor, and zeta potential was found to linearly increase with the iR corrected applied potential (Figure 1(b). Applied potential at which the diffuse layer is completely neutral (0 zeta potential) is found to lie within the experimental range of potential of zero charge (PZC) of Au, implying low adsorption of charges at inner Helmhotz plane (IHP). At high applied potentials ( 750 mV vs Ag/AgCl), an onset of chloride anion adsorption in a chloride rich solution was observed on Au surface. As a consequence of that, we observed a drop in zeta potential from its linearly increasing trend.For Pt, our initial experiments indicate a similar trend: zeta potential is found to increase in the capacitive region (Figure 1(c)). We observed an anomalous charging behavior in the H+ adsorption region (Vapp < PZC), where zeta potential decreased slightly. This drop is presumably a result of the chemisorbed H+ ions. For Pt EDLs, we observed the applied potential at which the diffuse layer is neutral is generally greater than the known PZC of 260 mV vs RHE. This is probably due to the pseudocapacitance which is caused by mild adsorption of ions at Pt IHP.The method developed by us is general and applicable to a broad electrochemistry and electrokinetics community. Ion distribution in the diffuse layer at a polarized metal-electrolyte interface, and its dependence on applied potential and electrolyte chemical composition can be studied efficiently using this method. At present the technical limitation of this method is a high uncompensated resistance, preventing the application of high potentials to observe the effects of surface oxides on the double layer. In future, the knowledge of ion distribution and ionic conductivity calculated from this method will be applied to understand the ion-conduction mechanism inside the micropores of PEM fuel cells, where ionomers cannot penetrate and enhanced double layer conductivity can help the transport of H+ ions to Pt particles. Figure 1

(Invited) Ion Tunable Electronic Materials Systems for Neuromorphic Computing

Tuning electronic conductance through solid state electrochemical ion insertion has emerged as a promising technology to enable next-generation, ultralow energy computing architectures1-3. Unlike two-terminal non-volatile memory elements, the three-terminal redox transistor decouples the ‘write’ and ‘read’ operations using a ‘gate’ electrode to tune the conductance state through charge transfer reactions involving ion injection into the channel electrode through a solid-state electrolyte. The insertion of ions into the bulk of the channel acts to dope the material through a gradual composition modulation that leads up to thousands of finely spaced conductance levels (synaptic weights) with near-ideal analog behavior. These properties enable low-energy operation without compromising analog performance and non-volatility. However, the strong coupling of ionic and electronic processes sharply challenges our current understanding of solid-state electrochemical systems, particularly at decreasing dimensions and timescales relevant to computing technology. In my talk I will discuss the rich portfolio of challenging, exciting fundamental science questions about ion tunable electronic materials systems and how we can harness these to realize a new paradigm for low power neuromorphic computing. Fuller, E. J.; El Gabaly, F.; Leonard, F.; Agarwal, S.; Plimpton, S. J.; Jacobs-Gedrim, R. B.; James, C. D.; Marinella, M. J.; Talin, A. A., Li-Ion Synaptic Transistor for Low Power Analog Computing. Advanced Materials 2017, 29.Fuller, E. J.; Keene, S. T.; Melianas, A.; Wang, Z. R.; Agarwal, S.; Li, Y. Y.; Tuchman, Y.; James, C. D.; Marinella, M. J.; Yang, J. J.; Salleo, A.; Talin, A. A., Parallel programming of an ionic floating-gate memory array for scalable neuromorphic computing. Science 2019, 364 (6440), 570.Li, Y. Y.; Fuller, E. J.; Asapu, S.; Agarwal, S.; Kurita, T.; Yang, J. J.; Talin, A. A., Low-Voltage, CMOS-Free Synaptic Memory Based on LixTiO2 Redox Transistors. Acs Applied Materials & Interfaces 2019, 11 , 38982.

Broadband Dielectric Spectroscopy Detection of Volatile Organic Compounds with ZnO Nanorod Gas Sensors

Historically, metal-oxide semiconductor-based gas sensors, such as SnO2 and ZnO, have been used to detect gases based on chemiresistivity [1]. In this approach, the electrical changes in response to analytes are measured with electrical circuits, that necessarily involve making electrical contacts to the sensing materials. These contacts are imperfect and introduce errors into the measurements. In this paper, we will demonstrate the feasibility of BDS-based metrology in addressing this gap in metrology to avoid these tool-measurand interactions / parasitic errors. Specifically, we will discuss how radio frequency (RF) propagation characteristics can be applied to probe and study charge transfer chemical changes in these metal-oxide VOC sensing materials.For this study, the solid-state gas sensor device under test consisted of hydrothermally synthesized ZnO nanorods grown on a fine grain seed layer of atomic layer deposition (ALD) ZnO on a silicon substrate. The sample was placed on a waveguide situated in a controlled environment to study the redox reactions of the ZnO sensor detecting ethanol target gas under thermal heating. In this configuration, the ZnO gas sensing element couples to the microwave through an evanescent mode. The temperature of the assembly was varied from room temperature to approximately 180 °C, in a variety of gaseous environments, while continuously monitoring the microwave signal going through the waveguide.Analysis of the BDS data (at specific frequencies) gave us electrical information that can be correlated to charge-transfer reactions involving the defects in the ZnO material and the molecules in the gaseous environment. For example, according to the S21 data, the sample became more resistive as temperature increased. This can be attributed to the redox reactions occurring on the ZnO nanorod gas sensor surface. Illustratively, Figure 1 shows time-lapse evolution on microwave signal loss in the experimental setup attributable to the discrete events occurring in the ZnO in response to various environmental conditions.The resistivity (as indicated by the S21) of the ZnO is stable below 120°C. At 120C, the system becomes increasing resistive with time, possibly as adsorbed water molecules on the ZnO rods desorb and to the healing of the ZnO surface defects, in a distinct two-step process [1]. Finally, the system became less lossy (i.e., electrical resistivity decreased) when ethanol target gas (technical grade C2H5OH, EtOH) was injected into the reactor. However, this decrease in resistivity might have been influenced by the decrease in temperature once the EtOH was introduced. From the S-parameters, we calculate a microwave attenuation constant (i.e. the real part of the microwave propagation constant) which quantified energy dissipation into the ZnO material. The attenuation constant data suggest the redox reaction induced changes in the semiconductor rods are reversible within the temperature range in this experiment.To the best of our knowledge, such detailed description of the discrete events that occur in the metal-oxide sensing material cannot be obtained with the traditional electrical DC resistivity measurements. In the future, we may be able to use this technique to discern differences, and distinguish between ZnO surface- and bulk reactions, as well as identify / characterize point defects. References H-J. Kim, and J.-H. Lee, “Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview”, Sensors and Actuators B 192 (2014) 607– 627Heine, F. Girgsdies, A. Trunschke, R. Schlögl, and M. Eichelbaum, “The model oxidation catalyst α-V2O5: insights from contactless in situ microwave permittivity and conductivity measurements”, Applied Physics A, vol. 112, pp. 289–296, 2013 DOI 10.1007/s00339-013-7800-6 Figure 1

In operando x-ray photoelectron spectroscopy studies of H2 oxidation and H2O electrolysis on gadolinia-doped ceria electrodes

In operando, ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) has been used to evaluate surface states of gadolinia-doped ceria (GDC) thin-film electrodes during H2 oxidation and H2O electrolysis, on yttria-stabilized zirconia (YSZ)-supported solid oxide electrochemical cells (SOCs). Porous nickel (Ni) and gold (Au) overlayers deposited on separate GDC thin films served as current collectors and potential electrocatalysts to facilitate heterogeneous chemistry for H2 oxidation or H2O electrolysis. Electrochemical characterization of the GDC thin-film electrodes complemented in operando XPS measurements of the O 1s spectra to correlate electrochemical overpotentials with surface chemistry near the Ni/GDC and Au/GDC interfaces. Shifts in O 1s binding energies across the metal/GDC/YSZ interfaces signified changes of local surface potential and provided a means of estimating kinetic parameters associated with charge transfer reactions. Effective oxygen partial pressure and surface potential impacted oxide vacancy and ceria polaron concentrations in the GDC, resulting in different reactivities of the GDC under the tested conditions. Both the Ni/GDC and Au/GDC demonstrated much higher currents for H2O electrolysis vs. H2 oxidation for comparable metal/GDC overpotentials due to increased electronic conductivity of the GDC under positive potentials and associated spreading of the electrochemically active region away from the triple-phase boundary. Higher electrochemical activity of the Ni/GDC electrode is attributed to the increased H2 activation on Ni in promoting charge transfer reactions (particularly for H2 oxidation). These results provide a basis for developing more informed reaction mechanisms for both H2 oxidation and H2O electrolysis of GDC-based composite electrodes in SOCs.