Function Of Electrode Potential Research Articles

Impact of palladium/palladium hydride conversion on electrochemical CO2 reduction via in-situ transmission electron microscopy and diffraction

Electrochemical conversion of CO2 offers a sustainable route for producing fuels and chemicals. Pd-based catalysts are effective for converting CO2 into formate at low overpotentials and CO/H2 at high overpotentials, while undergoing poorly understood morphology and phase structure transformations under reaction conditions that impact performance. Herein, in-situ liquid-phase transmission electron microscopy and select area diffraction measurements are applied to track the morphology and Pd/PdHx phase interconversion under reaction conditions as a function of electrode potential. These studies identify the degradation mechanisms, including poisoning and physical structure changes, occurring in PdHx/Pd electrodes. Constant potential density functional theory calculations are used to probe the reaction mechanisms occurring on the PdHx structures observed under reaction conditions. Microkinetic modeling reveals that the intercalation of *H into Pd is essential for formate production. However, the change in electrochemical CO2 conversion selectivity away from formate and towards CO/H2 at increasing overpotentials is due to electrode potential dependent changes in the reaction energetics and not a consequence of morphology or phase structure changes.

Operando X-Ray Diffraction Studies of Co Oxide Model Catalysts during Alcohol Oxidation and Oxygen Evolution Reaction

Green energy technology involving hydrogen generation requires developing catalyst materials of commercial viability, good stability and high reactivity for the electrochemical water splitting. Transition-metal oxides such as Co3O4 are among the best precious-metal-free electrode materials for the anodic oxygen evolution reaction (OER) in alkaline electrolysis [1]. Moreover, these spinel-type transition metal oxides are an important class of catalysts for the selective oxidation of organic compounds. Previous in situ and operando surface X-ray diffraction studies of epitaxial Co oxide films by our group showed pronounced changes in the near-surface structure of these catalysts. In particular, a sub-nm CoOx(OH)y skin layer starts forming in the pre-OER potential range which is accompanied by a volume change of the Co3O4 lattice [2]. In a more recent work, where we systematically explored the relationship between the effective thickness of this skin layer and the OER activity, we found clear evidence that the entire skin layer serves as a three-dimensional reactive zone for the OER [3]. Since the skin layer formation already starts several 100 mV negative of the onset of the OER, it is not induced by the catalytic reaction itself, but rather reflects the oxides’ electrochemistry, and may thus be expected also for other electrocatalytic processes, especially other electrocatalytic oxidation reactions which occur in a similar potential range. These oxide materials are not only of interest for OER, but also for the electrochemical oxidation of other compounds, such as alcohols [4]. However, data on the oxides’ structure in the presence of organic compounds does not exist at present. Obtaining such data is of great interest for understanding the mutual influence of oxide-electrolyte interface structure and electrocatalytic activity. In addition, the vast majority of publications on Co oxide oxidation processes, in particular all in situ and operando studies, have been performed up to now at room temperature only, and, thus, the catalytic performance of these materials at higher temperatures remains unknown. As electrolyzers and industrial (electro-) catalytic processes typically operate at elevated temperatures, such data are of considerable interest. Here, we present results of first potential-dependent structural studies of epitaxial Co3O4(111) and CoOOH(001) films electrodeposited on Au(111) single crystal substrates during ethylene glycol oxidation (EG Ox) in 0.1 M NaOH solution and compare those with the behavior found on the same samples in EG-free electrolyte. Ethylene glycol oxidation is a highly suitable reaction for such studies, as it occurs at potentials prior to the onset of the OER [4]. The samples were investigated as a function of electrode potential and temperature by operando surface X-ray diffraction at beamline P23 of PETRA III, DESY. The EG Ox was studied by successively increasing the EG concentration from 0 to 0.1, 0.5 and 2 M, which allows direct quantitative comparison of the potential-dependent structural changes. These studies of the oxides’ structure and electrochemical reactivity were performed at different temperatures in the range of 25 - 70°C. In all cases significant skin layer formation and accompanying changes in the oxide lattice strain were observed. The structural data are correlated with the results of cyclic voltammetry and optical reflectivity measurements that were recorded in parallel with the X-ray diffraction measurements. Together, they provide insights into the potential-dependent evolution of the skin layer thickness and strain and into the stability of the oxides in dependence of EG concentration and temperature. First results indicate that at elevated temperatures larger skin layer thicknesses and strain changes occur. Furthermore, the structural properties recover completely upon cooling back down to room temperature, suggesting that these effects are highly reversible. In the presence of EG, the reversible skin layer formation and changes in the Co3O4 bulk lattice were also observed, even though these structural changes appear to be less pronounced under alcohol oxidation conditions, especially at higher EG concentration. These studies will contribute to an in-depth understanding of the near surface structure of these Co oxide thin films during liquid phase oxidation catalysis, which is a prerequisite for the design of new, abundant and superior catalysts based on transition metal oxides for catalytic selective oxidation processes.We acknowledge financial support by the Deutsche Forschungsgemeinschaft via special research area TRR 247, project B10 (project no. 388390466).[1] A. Bergmann et al., Nature Catalysis 2018, 1, 711 - 719[2] F. Reikowski et al., ACS Catal. 2019, 9, 3811 - 3821[3] T. Wiegmann et al., ACS Catal. 2022 [4] S. N. Sun et al, J. Electrochem. Soc. 2016, 163, H99

Potential program invariant representation of diffusion-adsorption related voltammograms and impedance spectra

A theory is presented by which voltammograms, and dynamic electrochemical impedance spectroscopy (dEIS) measurements of diffusion-hindered adsorption processes can be analyzed. By the proposed procedure, from a set of voltammograms, taken at varied scan-rates, parameters of the kinetics and isotherm of the adsorption can be calculated. These are scan-rate independent functions of potential. The theory also comprises the analysis of the high frequency impedance spectra of the same system, which have been measured during dynamic conditions, i.e., during potential scans. The properties of the analysis are illustrated by transformation of simulated cyclic voltammograms. The presented theory is a synthesis of previous ones on diffusion-controlled charge transfer and on adsorption. The theory supports the determination of rate coefficients as a function of electrode potential from fast dEIS measurements.

Approximating constant potential DFT with canonical DFT and electrostatic corrections.

The complexity of electrochemical interfaces has led to the development of several approximate density functional theory (DFT)-based schemes to study reaction thermodynamics and kinetics as a function of electrode potential. While fixed electrode potential conditions can be simulated with grand canonical ensemble DFT (GCE-DFT), various electrostatic corrections on canonical, constant charge DFT are often applied instead. In this work, we present a systematic derivation and analysis of the different electrostatic corrections on canonical DFT to understand their physical validity, implicit assumptions, and scope of applicability. Our work highlights the need to carefully address the suitability of a given model for the problem under study, especially if physical or chemical insight in addition to reaction energetics is sought. In particular, we analytically show that the different corrections cannot differentiate between electrostatic interactions and covalent or charge-transfer interactions. By numerically testing different models for CO2 adsorption on a single-atom catalyst as a function of the electrode potential, we further show that computed capacitances, dipole moments, and the obtained physical insight depend sensitively on the chosen approximation. These features limit the scope, generality, and physical insight of these corrective schemes despite their proven practicality for specific systems and energetics. Finally, we suggest guidelines for choosing different electrostatic corrections and propose the use of conceptual DFT to develop more general approximations for electrochemical interfaces and reactions using canonical DFT.

The Effect of the SEI Layer Mechanical Deformation on the Passivity of a Si Anode in Organic Carbonate Electrolytes

The solid electrolyte interphase (SEI) on a Si negative electrode in carbonate-based organic electrolytes shows intrinsically poor passivating behavior, giving rise to unsatisfactory calendar life of Li-ion batteries. Moreover, mechanical strains induced in the SEI due to large volume changes of Si during charge-discharge cycling could contribute to its mechanical instability and poor passivating behavior. This study elucidates the influence that static mechanical deformation of the SEI has on the rate of unwanted parasitic reactions at the Si/electrolyte interface as a function of electrode potential. The experimental approach involves the utilization of Si thin-film electrodes on substrates with disparate elastic moduli, which either permit or suppress the SEI deformation in response to Si volume changes upon charging-discharging. We find that static mechanical stretching and deformation of the SEI results in an increased parasitic electrolyte reduction current on Si. Furthermore, attenuated total reflection and near-field Fourier-transform infrared nanospectroscopy reveal that the static mechanical stretching and deformation of the SEI fosters a selective transport of linear carbonate solvent through, and nanoconfinement within, the SEI. These, in turn, promote selective solvent reduction and continuous electrolyte decomposition on Si electrodes, reducing the calendar life of Si anode-based Li-ion batteries. Finally, possible correlations between the structure and chemical composition of the SEI layer and its mechanical and chemical resilience under prolonged mechanical deformation are discussed in detail.

Particle Size and Shape Effects in Electrochemical Environments: Pd Particles Supported on Ordered Co3O4(111) and Highly Oriented Pyrolytic Graphite

Particle size and shape effects control the oxidation behavior of nanostructured electrocatalysts. We investigated the oxidation state of Pd nanoparticles supported on Ar+-sputtered highly oriented pyrolytic graphite (HOPG) and well-ordered Co3O4(111) films on Ir(100) as a function of electrode potential by means of synchrotron radiation photoelectron spectroscopy coupled with an ex situ emersion electrochemical (EC) cell. Scanning tunneling microscopy revealed the growth of hemispherical and flat Pd nanoparticles on Ar+-sputtered HOPG and Co3O4(111), respectively. The oxidation state of Pd nanoparticles is controlled by electronic metal support interaction (EMSI) associated with charge transfer at the interface. We found that the Pd nanoparticles are largely metallic on HOPG and partially oxidized on Co3O4(111). Specifically, we detected the formation of partially oxidized Pdδ+ aggregates in combination with atomically dispersed Pd2+ species. The latter species dominate at small Pd coverage and form the metal/oxide interface at high Pd coverage. Immersion into an alkaline electrolyte (pH 10, phosphate buffer) at potentials between 0.5 and 1.1 VRHE has no significant effect for Pd/Co3O4(111) but yields traces of surface Pd oxide at 0.9 and 1.1 VRHE for Pd/HOPG. Formation of PdO was observed at 1.3 and 1.5 VRHE. Quantitative analysis suggests nearly one monolayer and nearly two monolayers of PdO on the surfaces of the Pd nanoparticles supported on HOPG and Co3O4(111) at 1.5 VRHE, respectively. The differences in the oxidation behavior reveal the decisive role of the EMSI in the stability of the metal/oxide interfaces in an EC environment.

Surface Modification of Three-Dimensional Au Coated Polymer Electrodes for Electrochemical Reduction of CO2

Electrochemical reduction of CO2 (CO2R) into chemical and fuels has found its place among scientists in the past decade as a promising field to invest in for moving towards a greener future. However, finding catalysts for CO2R has been a real challenge for researchers. The four main requirements for a good CO2R catalyst performance include activity, selectivity, stability, and low cost. Furthermore, as catalysis is a surface based process, achieving high catalytically active surface areas exposed to CO2 while the conversion process is occurring is important.The type of transition metal catalyst and the exposed surface facets used for CO2 reduction plays a role on the activity and selectivity towards different reaction products. Furthermore, tuning the 3-dimensional structure of CO2R electrodes enables control of the local reaction environment that can impact overall catalytic performance.1 In this work, we control the 3-dimensional structure of an Au catalyst coated on a polymeric-based substrate, along with investigating the impact of surface decoration of metal adatoms.The coating of Au on the polymer substrate was accomplished following a previously developed electroless deposition method.2 CO2R was carried out in a custom designed three electrode cell reported previously3 with a flow of CO2 maintained over a 1 h chronoamperometry electrolysis process. The counter electrode was Pt foil and 0.1 M KHCO3 was used as the electrolyte.The modified surface showed an almost 5 times higher peak in its cyclic voltammetry (CV) in compare to the plain surface. This means that the actual active surface area of the modified surface sample is almost 5 times higher the plain sample (demonstrated in Figure 1a). This significantly affects the CO2R performance of these surfaces. Figure 1b depicts the activity of the modified three-dimensional electrode surface and the two-dimensional plain surface as a function of electrode potential. By decorating these electrodes with other metal species at the surface we observed interesting activity and selectivity trends that arise due to synergistic interactions between the two metal species and modified transport properties due to the three-dimensionality of the polymer supported electrodes. In this presentation we will discuss these reactivity trends and provide insight into the activity and selectivity observed, supported by detailed structural analyses and supplementary measurements. Figure 1. (a) CV of a plain and modified surface of Au in 2.25 M of H2SO4; the counter electrode was Pt foil and the scan rate was 50 mV/s. (b) Activity (current density vs. different electrode potentials) of both the three-dimensional modified and two-dimensional plain surfaces. References Hall, A. S.; Yoon, Y.; Wuttig, A.; Surendranath, Y., Mesostructure-Induced Selectivity in CO2 Reduction Catalysis. Journal of the American Chemical Society 2015, 137 (47), 14834-14837.Gabardo, C. M.; Adams-McGavin, R. C.; Fung, B. C.; Mahoney, E. J.; Fang, Q.; Soleymani, L., Rapid prototyping of all-solution-processed multi-lengthscale electrodes using polymer-induced thin film wrinkling. Scientific Reports 2017, 7 (1), 1-9.Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F., New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energy & Environmental Science 2012, 5 (5), 7050-7059. Figure 1

(Keynote) Deciphering Electrocatalytic Reactions with Theory and Computation: The Case of CO2 Reduction

The interfacial region between metal surface and aqueous electrolyte is of central importance for electrochemical reactions. The need to understand the properties of this electrochemical double layer (EDL) region drives extensive research.1,2 A foremost goal of research in electrocatalysis is the development of a theoretical-computational framework. The fundamental task for such a framework is to unravel the complex interplay of electronic structure effects of the catalyst material, potential-induced variations of the chemisorption state, local reaction conditions on the electrolyte side, and electrochemical kinetics of reactions of interest. A recently developed theoretical approach revealed the non-monotonic charging behaviour of a Pt electrode3 and it was thereafter used to decipher the oxygen reduction reaction.4 In the present work, we adapt this approach to study how the local reaction environment dictates the mechanism and kinetics of reduction to CO at an Ag electrode. Our hierarchical model accounts for the multistep reaction kinetics of surface reactions, local chemical equilibria (involving CO2, HCO3 -, CO3 2-, OH-, H+), the specific surface charging state at a given electrode potential, solvent polarization and ion density variations, and reactant/product transport. It integrates interface and pore-level models to account for this interplay. The combined approach rationalizes the impact of the considered effects on the kinetics of the reaction, manifested experimentally in changes of the effective Tafel slope as a function of electrode potential. Lateral interactions between chemisorbed species are seen to contribute to the decrease of the CO current density at high electrode potentials, in addition to mass transport effects, surface charging effects and pH increase. We will conclude with a discussion of the parameters that allow tuning the local reaction environment and thus the electrocatalytic activity and selectivity of the electrocatalyst. 1O.M. Magnussen and A. Gross, Toward an Atomic-Scale Understanding of Electrochemical Interface Structure and Dynamics, J. Am. Chem. Soc. 141, 4777–4790 (2019). 2M.J. Eslamibidgoli and M.H. Eikerling, Approaching the Self-consistency Challenge of Electrocatalysis with Theory and Computation, Current Opinion in Electrochemistry 9, 189-197 (2018). 3J. Huang, A. Malek, J. Zhang and M.H. Eikerling, Non-monotonic Surface Charging Behavior of Platinum: A Paradigm Change, J. Phys. Chem. C 120, 13587-13595 (2016). 4J. Huang, J. Zhang and M. Eikerling, Unifying Theoretical Framework for Deciphering the Oxygen Reduction Reaction on Platinum, Phys. Chem. Chem. Phys. 20, 11776-11786 (2018).

Evolution of the Raman 2D’ mode in monolayer graphene during electrochemical doping

Effective charge doping and control of the Fermi level shift is a crucial aspect in graphene device fabrication. Raman spectroscopy is extensively used for primary characterization of such samples, however, quantification of carrier concentration using the known G-2D vector analysis can pose problems when more sources of phonon renormalization are present, for example lattice deformation or disorder. In such cases, analysis of another Raman mode can provide the needed missing information. In this study, chemical vapor-deposition-grown single-layer graphene was monitored by in-situ Raman spectroelectrochemistry with the focus on the behavior of the 2D’ mode upon the application of electrochemical potential. We analyzed the changes of the Raman peak intensity, frequency and linewidth of the 2D’ mode as a function of electrode potential in detail and compared the obtained results with well-known behavior of the G and 2D modes during electrochemical doping.

Role of imidazolium cations on the interfacial structure of room‐temperature ionic liquids in contact with Pt(111) electrodes

AbstractRoom‐temperature ionic liquids (ILs) have gained considerable attention as an important addition to conventional electrolytes because they exhibit large electrochemical windows and can reduce existing overpotentials in electrocatalysis. For the interfacial electrochemistry of ILs, a comprehensive understanding of molecular ions and the resulting electric double‐layer structures as a function of electrode potential is mandatory, but the structures are largely different from conventional electrolytes. For that reason, we have studied the interfaces of Pt(111) in contact with ILs using 1‐butyl‐3‐methylimidazolium [BMIM] and 1‐butyl‐2,3‐dimethylimidazolium [BMMIM] cations as well as bis(trifluoromethylsulfonyl)imide [NTf2] anions. We applied vibrational sum‐frequency generation (SFG), where we interrogate vibrational bands from interfacial cations, anions, as well as interfacial water in situ and under potential control. Structuring of [NTf2] anions and H2O with electrode potential show hysteresis while a strong Stark tuning was absent. This indicates that the IL ions are oriented in the vicinity of the interface, without being directly adsorbed to the Pt(111) surface. Using the C‐H stretching band from CH groups at the imidazolium ring, the ring reorientation with electrode potential was qualitatively determined. The imidazolium ring reorients as a function of potential from a more parallel orientation to an upright orientation with respect to the interfacial plane. This leads to the formation of voids in the layered structure of ions at the interface, which can be then filled with H2O as evidenced by an increased SFG intensity from O‐H stretching modes that are attributable to hydrogen‐bonded interfacial water. Comparing the responses of the ILs, particularly of [BMMIM][NTf2], shows a compact structure and a significantly pronounced rearrangement of the imidazolium ring that can also facilitates better incorporation of H2O and significantly affects the reorientation of [NTf2] anions and, thus, causes a pronounced hysteresis with electrode potential.

Optimization of in Situ pH Detection

High selectivity and electrode efficiency are essential for practical CO2 reduction [1-2]. However, the simultaneous hydrogen evolution reaction in aqueous solutions lowers process efficiency. Both reactions change the local pH, which in turn, has been shown to have an impact on CO2 reduction reactions [2-5]. In situ detection of pH using ultramicroelectrodes can help elucidate the pH dependency of these reactions [5-6]. However, the presence of the ultramicroelectrode tip can significantly influence conditions near the surface due to transport restrictions. Using a numerical model that accounts for the presence of the tip, the operando environment near the surface can be elucidated.In this work, a 2D-axisymmetric finite element (FEM) simulation was developed to predict concentration fields of reacting species near the electrode surface and in the presence of the ultramicroeletrode tip. The simulation can be used to identify optimal geometric and spatial parameters for experimental surface characterization. Using the FEM model, the operando pH within 100 µm of the surface electrode varies by up to 6 pH units within a 0.5 - 0.9 V vs. Ag/AgCl potential window. This wide variation is due to a diffusion boundary layer that grows with decreasing potential. The near-surface concentration of buffer species (Figure 1) was found to vary, along with pH, as a function of electrode potential and time.Lee, C. W.; Cho, N. H.; Im, S. W.; Jee, M. S.; Hwang, Y. J.; Min, B. K.; Nam, K. T. New Challenges of Electrokinetic Studies in Investigating the Reaction Mechanism of Electrochemical CO2 Reduction. J. Mater. Chem. A 2018, 6 (29), 14043–14057. https://doi.org/10.1039/C8TA03480J.Goyal, A.; Marcandalli, G.; Mints, V. A.; Koper, M. T. M. Competition between CO2 Reduction and Hydrogen Evolution on a Gold Electrode under Well-Defined Mass Transport Conditions. J Am Chem Soc 2020, 142 (9), 4154–4161. https://doi.org/10/gg84dq.Zhang, B. A.; Ozel, T.; Elias, J. S.; Costentin, C.; Nocera, D. G. Interplay of Homogeneous Reactions, Mass Transport, and Kinetics in Determining Selectivity of the Reduction of CO2 on Gold Electrodes. ACS Cent. Sci. 2019, 5 (6), 1097–1105. https://doi.org/10/ggbvcs.Seifitokaldani, A.; Gabardo, C. M.; Burdyny, T.; Dinh, C.-T.; Edwards, J. P.; Kibria, M. G.; Bushuyev, O. S.; Kelley, S. O.; Sinton, D.; Sargent, E. H. Hydronium-Induced Switching between CO2 Electroreduction Pathways. J. Am. Chem. Soc. 2018, 140 (11), 3833–3837. https://doi.org/10.1021/jacs.7b13542.Monteiro, M. C. O.; Jacobse, L.; Touzalin, T.; Koper, M. T. M. Mediator-Free SECM for Probing the Diffusion Layer PH with Functionalized Gold Ultramicroelectrodes. Analytical Chemistry 2020, 92 (2), 2237–2243. https://doi.org/10/gg77vg.Carneiro-Neto, E. B.; Lopes, M. C.; Pereira, E. C. Simulation of Interfacial PH Changes during Hydrogen Evolution Reaction. Journal of Electroanalytical Chemistry 2016, 765, 92–99. https://doi.org/10/f8pbnv. Figure 1

STM studies of electron transfer through single molecules at electrode-electrolyte interfaces

Electrochemical scanning tunnelling microscopy (EC-STM) is one of the most important tools in modern interfacial electrochemistry. Over several decades it has proven its worth by providing in-situ atomic and molecular scale visualisations of electrode surfaces, but its value goes far beyond topographic imaging. This article discusses the use of EC-STM as a tool for quantitatively studying mechanisms of electron transfer (ET) at electrode-electrolyte interfaces. In combination with theoretical modelling, it has been revealing mechanistic details of charge transfer across molecules at electrochemical interfaces which are of broad interest to a wide range of electrochemists. This article discusses different ways in which the EC-STM can be deployed to monitor charge flow through single molecules and thereby analyse ET mechanisms. Mechanisms covered range from superexchange to 2-step hopping ones such as the Kuznetsov Ulstrup (KU) mechanism. Literature examples of a variety of redox and non-redox molecular targets are used to describe how measurements of single molecule conductance as a function of electrode potential can be used to analyse charge transfer through single molecules. The review finishes by highlighting some of the most recent work and new developments which will ensure that EC-STM will continue to be an important tool for studying ET mechanisms at electrode-electrolyte interfaces.

Building Ab Initio Interface Pourbaix diagrams to Investigate Electrolyte Stability in the Electrochemical Double Layer: Application to Magnesium Batteries.

Insights into the electrochemical processes occurring at the electrode-electrolyte interface are a crucial step in most electrochemistry domains and in particular in the optimization of the battery technology. However, studying potential-dependent processes at the interface is one of the biggest challenges, both for theoreticians and experimentalists. The challenge is pushed further when stable species also depend on the concentration of specific ligands in the electrolyte, such as chlorides. Herein, we present a general theoretical ab initio methodology to compute a Pourbaix-like diagram of complex electrolytes as a function of electrode potential and anion's chemical potential, that is, concentration. This approach is developed not only for the bulk properties of the electrolytes but also for electrode-electrolyte interfaces. In the case of chlorinated magnesium complexes in dimethoxyethane, we show that the stability domains of the different species are strongly shifted at the interface compared to the bulk of the electrolyte because of the strong local electric fields and charges occurring in the double layer. Thus, as the interfacial stability domains are strongly modified, this approach is necessary to investigate all interface properties that often govern the reaction kinetics, such as solvent degradation at the electrode. Interface Pourbaix diagram is used to give some insights into the improved stability at the Mg anode induced by the addition of chloride. Because of its far-reaching insights, transferability, and wide applicability, the methodology presented herein should serve as a valuable tool not only for the battery community but also for the wider electrochemical one.

Electrochemical Control and Understanding of the Electronic and Optical Properties of ZnO Formed in Rechargeable Zn-Alkaline Batteries

Rechargeable alkaline batteries using zinc metal (Zn) electrodes are of significant interest for next-generation energy storage because of their high energy density, low cost, environmental friendliness, and inherent safety. Rechargeable Zn-alkaline batteries, however, have not been widely commercialized in part due to the low cyclability of Zn electrodes. At high depths of discharge, zinc oxide (ZnO) discharge product accumulates in the electrode, passivating the zinc-electrolyte interface and ultimately leading to cell failure. While the ZnO discharge product has been studied for decades, there is no general consensus on its local structure, composition, and electrochemical properties, nor on how these properties are affected by the electrode potential or cycling conditions.In this work, we used in operando analytical techniques including X-Ray diffraction (XRD), confocal Raman spectroscopy, electrochemical impedance spectroscopy (EIS), and ultraviolet-visible (UV-vis) spectroscopy to demonstrate that the discharge product is proton-doped ZnO with oxygen vacancies, and that the electrical conductivity, band gap, and color vary systematically as a function of electrode potential. To investigate local hydrogen and zinc environments within the ZnO discharge product, we performed ex situ solid-state 1H, 2H, and 67Zn magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Solid-state 1H and 2H NMR measurements establish that hydrogen environments change as a function of potential, while solid-state 67Zn NMR spectra indicate that the local zinc environments are distorted compared to ZnO generated via conventional routes. We also performed 2D 1H-1H exchange (EXSY) NMR measurements to investigate proton mobility between defect environments and acquired 1D 1H-1H dipolar-mediated double quantum-filtered MAS NMR spectra to establish through-space molecular-level proximities between proton sites. This work provides insights into the dynamics of zinc electrodes in alkaline environments, as well as how to electrochemically control the defect structures and electronic and optical properties of ZnO in alkaline media, which are expected to aid the development of next-generation Zn alkaline batteries and electronic devices utilizing ZnO.

Molecular electronics at electrode–electrolyte interfaces

Four contemporary examples, all published in recent years, of studies of molecular electronics at electrode–electrolyte interfaces are reviewed in this opinion article. The first illustrative example involves the switching of the redox active molecular wire between redox states, with concomitant changes in molecular conductance. This example illustrates how molecular electronics at electrode–electrolyte interfaces can be used to analyse mechanisms of electron transfer, to distinguish electrolyte effects and to provide details not readily available from ensemble measurements. The second example shows that the fluctuations of molecular conductance of a redox active molecular wire can be followed as a function of electrode potential. This shows how the stochastic kinetics of individual reaction events at electrode–electrolyte interfaces can be followed. The third example demonstrates how electrochemistry can be used to control quantum interference in single molecular wires. The fourth example shows a single-molecule electrochemical transistor concept for well-defined metal cluster containing molecular wires.

Numerical computations of Marcus–Hush–Chidsey electron transfer rate constants

Two theories are available to predict the values of the rate constants of electron transfer between a redox species and an electrode as a function of electrode potential: the Butler-Volmer theory, and the Marcus “Density of States” (MDoS) theory. While the Butler-Volmer is purely empirical, it is widely used, in part because of its ability to represent experimental data, but also because of its simplicity: the rates are just exponential functions of the potential. By contrast, in the case of the MDoS theory, whose justification comes from the well-established theory of electron transfer proposed by Marcus, the rates are expressed in the form of indefinite integrals, which are harder to compute. A number of algorithms have been presented in the literature, with various merits in terms of accuracy and computation time; however, these algorithms have never been compared under the same conditions, which prevent making informed choices. We systematically compared the algorithms, both in terms of accuracy and computation time, and also propose a new algorithm which is very fast and reasonably accurate.

Long range self-organisations of small metallic nanocrystals for SERS detection of electrochemical reactions

Gold electrodes were modified by silver and gold nanocrystals (NCs) that self-organize onto the surface. Their optical properties were explored by measuring electroreflectance spectra as a function of electrode potential. Below their oxidation potential, no shift of the reflectance maximum was observed for Ag NCs. This can be explained by a low interfacial capacitance resulting from the impossibility for the electrolyte to penetrate into the hydrophobic layer created by the NCs dodecanethiol ligands. Conversely, a non-monotonous evolution was observed with the electrode potential for oleylamine capped Au NCs. This behavior is suggesting a less dense hydrophobic layer, allowing significant electrolyte penetration. Next, electroactive compounds were adsorbed on the the Au NCs assemblies and characterized by Raman spectroelectrochemistry. In the first system displaying a single electron transfer with no coupled chemical reaction, only the spectrum intensity changed, because oxidation generated a Raman resonant radical cation. The second study considered 4-nitrothiophenol, for which up to 6 electrons and 6 protons may be transferred. In this case, the nitro band disappeared upon reduction, and the spectrum displayed typical features of the formed 4-aminothiophenol.

Potential-Induced Adsorption and Structuring of Water at the Pt(111) Electrode Surface in Contact with an Ionic Liquid

Water adsorption is important in many fields from surface electrochemistry to electrocatalysis where molecular level information is much needed in order to gain a detailed understand on the role of interfacial water. Here we report on water at Pt(111) surfaces in contact with an [EIMIM][BF4] ionic liquid, which was spectroscopically resolved by using in situ sum-frequency generation (SFG). O-H modes are used to study water adsorption and water structure as a function of electrode potential, while analysis of C-H modes is used to infer orientational changes of [EMIM] cations at the interface. Different from the bulk where free water molecules are found, SFG spectra provide evidence an interfacial layer with an extended network of hydrogen-bonded water molecules exists and grows with increasing absolute potential which is used to identify the potential of zero charge at +0.1 V SHE where a pronounced minimum in O-H intensity is found.

A Mechanistic Understanding of the Electrochemical Activity of Zinc Oxide in Alkaline Batteries

Rechargeable zinc alkaline batteries are of great interest for stationary storage applications because they are inherently safe, energy dense, environmentally friendly, and low-cost. However, these batteries have not been commercialized on a large scale because of their low cycle life at high depths of discharge. One cause is the buildup of ZnO on the electrode. It has been shown recently that ZnO produced in zinc alkaline electrodes inserts protons and electrons and undergoes electrochromic color changes as a function of electrode potential. This dynamic behavior increases hydrogen catalysis, contributes capacity to the electrode, and changes oxide conductivity as a function of electrode potential. In this work we investigate the disorder and nonstoichiometry of ZnO that allow the material to become electrochemically active. We used 1H, 2H, and 67Zn solid-state MAS NMR to better understand structure, defects, order, and the hydrogen insertion mechanism of the ZnO. Understanding the physical properties of the ZnO will enable better design of electrodes and additives for zinc alkaline batteries in the future.

Discussion on Inductive Loop Observed in Complex Plane Plot of Electrochemical Impedance of Mg

Electrochemical impedance spectroscopy (EIS) is able to discriminate several time constants involved in electrode reactions. The time constant regarding adsorption-desorption relaxation may be detected in the plot of impedance spectrum when the adsorption at liquid/solid interface contributes to the kinetics of electrode reaction. For example, the complex plane plots of the electrochemical impedance of Pt electrode, where oxygen reduction occurs, show an apparent inductive loop, indicating the presence of the reaction intermediate adsorbed on the electrode1). The inductive loop is often observed in the cases of metal dissolutions in aqueous solutions2-4), and the origin has been explained by the adsorption of adsorbed intermediate in dissolution reaction. In this presentation, the author derives the theoretical equations of the Faraday impedance involving the contribution of adsorption process during metal dissolution. A current i, which flows across liquid/solid interface, is a function of electrode potential E and surficial coverage θ of adsorbed species as follows. i = f(E, θ) (1)The Faradaic impedance Z F can be represented from the differential calculus of the equation (1), where, a 1 and a 1 are partial differential coefficients.1/Z F = di/dE = a 1 + a 2(dθ/dE) (2)The equation (2) indicates that the Faradaic impedance is a function of (dθ/dE) and describes a semicircle due to an adsorption relaxation on the complex plane. In these cases, the semicircle in the low frequency range is related to the time constant of adsorption and desorption of the reaction intermediate. The conditions that the electrochemical impedance shows the inductive loop in the low frequency range are discussed on the basis of the numerical simulations.References1) M. Itagaki, H. Hasegawa, K. Watanabe, T. Hachiya, J. Electroanal Chem 557 (2003) 59.2 M. Keddam, O. R. Mattos, H. Takenouti, J. Electrochem. Soc, 128 (1981) 257.3) M. Itagaki, M. Tagaki, K. Watanabe, Electrochim.Acta, 41 (1996) 1201.4) G. Baril, G. Galicia, C. Deslouis, N. Pebere, B. Tribollet, V. Vivier, J. Electrochem. Soc, 154 (2007) C108.