Nanoconfinement effects in energy storage materials

Nanoconfinement effects have been studied to understand and to modify thermodynamic and kinetic properties of energy storage materials and to improve their cyclic behaviour. The paper addresses various aspects in the research and development of hydrogen storage materials and batteries. Fundamental relationships and the state-of-the art in the prediction of properties and experimental observations will be outlined and structure-property-relationships will be discussed for some hydrogen storage materials. Similar nanoconfinement effects in lithium battery anode materials will be addressed.     

Near-surface ion distribution and buffer effects during electrochemical reactions

The near-surface ion distribution at the solid-liquid interface during the Hydrogen Oxidation Reaction (HOR)/Hydrogen Evolution Reaction (HER) on a rotating platinum disc electrode is demonstrated in this work. The relation between reaction rate, mass transport and the resulting surface pH-value is used to theoretically predict cyclic voltammetry behaviour using only thermodynamic and diffusion data obtained from the literature, which were confirmed by experimental measurements. The effect of buffer addition on the current signal, the surface pH and the ion distribution is quantitatively described by analytical solutions and the fragility of the surface pH during reactions that form or consume H(+) in near-neutral unbuffered solutions or poorly buffered media is highlighted. While the ideal conditions utilized in this fundamental study cannot be directly applied to real scenarios, they do provide a basic understanding of the surface pH concept for more complex heterogeneous reactions.     

Revealing structural effects: electrochemical reactions of butanols on platinum

Spectroelectrochemical studies on the reactivity of butanol isomers on Pt electrodes in perchloric acid medium led to the observation of structural effects that result from the different arrangements of atoms in the organic molecules. The use of differential electrochemical mass spectrometry (DEMS) to detect volatile products showed that all four isomers react on the electrode, though different product yields were observed for each compound. In spite of the differences in the electrochemical behaviour of the butanol isomers, a series of general processes accounts for the results obtained. The formation of strongly adsorbed residues by a dehydration process leading to the formation of a C=C bond was proposed for all isomers. Electroreduction of the adsorbates produces C(4) and C(3) alkanes, and the latter reveal the existence of a fragmentation process. The C(4) hydrocarbons can be formed by hydrogenation of these residues and by hydrogenolysis of alcohol molecules in the bulk solution which react at the electrode with adsorbed hydrogen. On the other hand, CO(2) is formed during electrooxidation of the adsorbed species. Partial-oxidation products containing a carbonyl group were detected from 0.2 M solutions of 1-butanol, isobutyl alcohol and sec-butyl alcohol. The tertiary alcohol tert-butyl alcohol only reacts in its adsorbed state.     

Interionic Interactions in Conducting Nanoconfinement

Interionic interactions in conducting nanopores determine how counterions may be packed in the pores subject to the applied voltage. In ideal metals, interactions are exponentially screened by metallic electrons. However, modern nanoporous electrodes are predominantly made of carbon materials. To what extent is this screening affected by a different mode of dielectric response in such materials? To answer this question we study Coulomb interaction of charges in cylindrical and slit pores that allow finite electric field penetration into the pore walls, as well as the Coulomb interaction in a nanogap between two thin walls of graphene modeled by a non‐local dielectric function. In all cases studied the screening was found to be subtly different than in metallic nanopores, but still strong enough to support realization of the so called superionic state in such pores.     

Nanoconfinement effects on the reversibility of hydrogen storage in ammonia borane: a first-principles study

We investigate atomistic mechanisms governing hydrogen release and uptake processes in ammonia borane (AB) within the framework of the density functional theory. In order to determine the most favorable pathways for the thermal inter-conversion between AB and polyaminoborane plus H(2), we calculate potential energy surfaces for the corresponding reactions. We explore the possibility of enclosing AB in narrow carbon nanotubes to limit the formation of undesirable side-products such as the cyclic compound borazine, which hinder subsequent rehydrogenation of the system. We also explore the effects of nanoconfinement on the possible rehydrogenation pathways of AB and suggest the use of photoexcitation as a means to achieve dehydrogenation of AB at low temperatures.     

Transition metal-catalyzed organic reactions in undivided electrochemical cells

Transition metal-catalyzed organic electrochemistry is a rapidly growing research area owing in part to the ability of metal catalysts to alter the selectivity of a given transformation. This conversion mainly focuses on transition metal-catalyzed anodic oxidation and cathodic reduction and great progress has been achieved in both areas. Typically, only one of the half-cell reactions is involved in the organic reaction while a sacrificial reaction occurs at the counter electrode, which is inherently wasteful since one electrode is not being used productively. Recently, transition metal-catalyzed paired electrolysis that makes use of both anodic oxidation and cathodic reduction has attracted much attention. This perspective highlights the recent progress of each type of electrochemical reaction and relatively focuses on the transition metal-catalyzed paired electrolysis, showcasing that electrochemical reactions involving transition metal catalysis have advantages over conventional reactions in terms of controlling the reaction activity and selectivity and figuring out that transition metal-catalyzed paired electrolysis is an important direction of organic electrochemistry in the future and offers numerous opportunities for new and improved organic reaction methods.

Transition metal-catalyzed organic electrochemistry is a rapidly growing research area owing in part to the ability of metal catalysts to alter the selectivity of a given transformation.     

Nanoconfinement Effects: Glucose Oxidase Reaction Kinetics in Nanofluidics

Size‐tunable nanofluidic devices coupled to an electrochemical detector have been designed and then used to study glucose oxidase (GOx) reaction kinetics confined in nanospaces. The devices are fabricated via a photochemical decomposition reaction, which forms nanochannels covered with carboxyl groups. The generated carboxyl groups enable us to chemically pattern biological molecules on the polymer surfaces via covalent bonding. With this approach, the activity of the immobilized biological molecules confined in nanospaces with different sizes has been investigated. GOx species are chemically immobilized on the surface of the nanochannels, catalyzing the oxidation of substrate glucose as it flows through the channels. The enzyme reaction product, hydrogen peroxide, passing through the nanochannels, reaches an electrochemical detector, giving rise to an increase in anodic current. This steady‐state electrochemical current, which responds to various glucose concentrations, can be used to evaluate the GOx activity under confinement conditions. The results show significant nanoconfinement effects that are dependent on the channel size where the reaction occurs, demonstrating the importance of spatial confinement on the GOx reaction kinetics. The present approach provides an effective method for the study of enzyme activity and other bioassay systems, such as cell assays, drug discovery, and clinical diagnosis.     

On the dynamics of electrochemical ion-transfer reactions

A model electrochemical ion-transfer reaction is investigated by molecular dynamics simulations. Non-equilibrium solvation effects can lead to barrier recrossings when the ion passes the transition state. The resulting transmission coefficient, which measures the deviation of the rate from the predictions of the transition state theory, is in good agreement with the Grote–Hynes theory. By contrast, Kramers theory predicts a much lower rate constant. The reaction occurs in the polarization caging regime of Grote–Hynes theory, in which the solvent motion controls the advance of the reaction. Furthermore, the molecular friction depends strongly on the distance from the electrode.     

Role of magnetic forces in electrochemical reactions at microstructures

The effect of an external magnetic field (up to 0.8 T) on the anodic dissolution of microstructures has been investigated systematically. Copper and silver wires (100 microm in diameter) were embedded in epoxy resin and dissolved potentiostatically while a magnetic field was periodically switched on and off. A special feature of the thus prepared structures is that they show a smooth transition from an inlaid disk to a recessed disk electrode. An increase or a decrease of the limiting current density in the presence of B was found depending on the orientation of the magnetic field and the hydrodynamic conditions in the cell (natural or forced convection). The magnetic forces which are responsible for this are the Lorentz force and the gradient force. We propose a model which discusses the interaction of these forces with the natural and the forced convection to explain the experimental results.     

Solid state electrochemical reactions in systems with miscibility gaps

Cyclic voltammograms of electroactive solid compounds with partial immiscibility between the oxidized and reduced phases can exhibit a splitting of the peaks. If the free energy of transformation between the oxidized and reduced phases is small, the formal potentials of the redox pair will be almost the same in both solid phases. This results in an inert potential range in which no appreciable electrochemical activity is possible. The kinetic implications of this situation have been analysed in relation to the width of the miscibility gap. The diffusion of ions in the particle, which is hindered by the immiscibility, can proceed when a transition zone between the two phases exists in which the crystal structure is changed. If there is no such transition zone the voltammogram will display several spikes, which are caused by the collapse of concentration barriers at the sharp interfaces between the two phases in the mixed crystals. Received: 14 October 1999 / Accepted: 4 November 1999     

Natural convection effects in electrochemical systems

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Scanning electrochemical mapping of spatially localized electrochemical reactions induced by surface potential gradients

The influence of a surface potential gradient on the location and extent of electrochemical reactions was examined using a scanning electrochemical microscope. A linear potential gradient was imposed on the surface of a platinum-coated indium tin oxide electrode by applying two different potential values at the edges of the electrode. The applied potentials were used to control the location and extent of several electrochemical reactions, including the oxidation of Ru(NH3)6(2+), the oxidation of H2, and the oxidation of H2 in the presence of adsorbed CO. Scanning electrochemical mapping of these reactions was achieved by probing the feedback current associated with the oxidation products. The oxidation of Ru(NH3)6(2+) occurred at locations where the applied potential was positive of the formal potential of the Ru(NH3)6(2+/3+) redox couple. The position of this reaction on the surface could be spatially translated by manipulating the terminal potentials. The rate of hydrogen oxidation on the platinum-coated electrode varied spatially in the presence of a potential gradient and correlated with the nature of the electrode surface. High oxidation rates occurred at low potentials, with decreasing rates observed as the potential increased to values where platinum oxides formed. The extent of oxide formation versus position was confirmed with in-situ ellipsometry mapping. In the presence of adsorbed carbon monoxide, a potential gradient created a localized region of high activity for hydrogen oxidation at potentials between where carbon monoxide was adsorbed and platinum oxides formed. The position of this localized region of activity could be readily translated along the surface by changing the terminal potential values. The ability to manipulate electrochemical reactions spatially on a surface has potential application in microscale analytical devices as well as in the discovery and analysis of electrocatalytic systems.     

Nanoconfinement of lithium borohydride in Cu-MOFs towards low temperature dehydrogenation

Successful synthesis and investigation of a new material that uses copper-metal-organic frameworks (Cu-MOFs) as the template for loading LiBH(4) are reported. The nanoconfinement of LiBH(4) in the pores of Cu-MOFs results in an interaction between LiBH(4) and Cu(2+) ions, enabling the LiBH(4)@Cu-MOFs system to achieve a much lower dehydrogenation temperature than pristine LiBH(4).     

Metallophthalocyanine-based molecular materials as catalysts for electrochemical reactions

Metallophthalocyanines confined on the surface of electrodes are active catalysts for a large variety of electrochemical reactions and electrode surfaces modified by these complexes can be obtained by simple adsorption on graphite and carbon. However, more stable electrodes can be achieved by coating their surfaces with electropolymerized layers of the complexes, that show similar activity than their monomer counterparts. In all cases, fundamental studies carried out with adsorbed layers of these complexes have shown that the redox potential is a very good reactivity index for predicting the catalytic activity of the complexes. Volcano-shaped correlations have been found between the electrocatalytic activity (as log I at constant E) versus the Co(II)/(I) formal potential (E°′) of Co-macrocyclics for the oxidation of several thiols, hydrazine and glucose. For the electroreduction of O₂ only linear correlations between the electrocatalytic activity versus the M(III)/M(II) formal potential have been found using Cr, Mn, Fe and Co phthalocyanines but it is likely that these correlations are “incomplete volcano” correlations. The volcano correlations strongly suggest that E°′, the formal potential of the complex needs to be in a rather narrow potential window for achieving maximum activity, probably corresponding to surface coverages of an M-molecule adduct equal to 0.5 and to standard free energies of adsorption of the reacting molecule on the complex active site equal to zero. These results indicate that the catalytic activity of metallophthalocyanines for the oxidation of several molecules can be “tuned” by manipulating the E°′ formal potential, using proper groups on the macrocyclic ligand. This review emphasizes once more that metallophthalocyanines are extremely versatile materials with many applications in electrocatalysis, electroanalysis, just to mention a few, and they provide very good models for testing their catalytic activity for several reactions. Even though the earlier applications of these complexes were focused on providing active materials for electroreduction of O₂, for making active cathodes for fuel cells, the main trend in the literature nowadays is to use these complexes for making active electrodes for electrochemical sensors.     

Probing complex eletrocatalytic reactions using electrochemical infrared spectroscopy

Electrochemical in situ Fourier transform infrared spectroscopy (EC-FTIRS) is one of the most powerful and widely used techniques in studying electrode–electrolyte interfaces and electrode processes thereon. Despite its great developments and wide applications, challenges that may lead to the misinterpretation of the reaction mechanisms are usually overlooked. In this minireview, after a brief overview of the methodology of EC-FTIRS, we will consider the oxidation of small organic molecule, especially formic acid oxidation to introduce the strategies on (i) how to estimate kinetic parameters of specific reaction pathways of complex reactions on the basis of infrared spectroscopic data combined with other techniques and (ii) how to avoid the pitfalls of data interpretation. Our perspective for future development of EC-FTIRS is also provided.     

Catalysis of electrochemical and photoelectrochemical reactions at semiconductorelectrodes

In electrochemistry, multiple electron transfer redox reactions usually need catalysis by strong adsorption of intermediates on the electrode surface. It is shown that chemical modification of the surface of semiconductors is necessary in order to catalyze such reactions to a sufficient extent. Photocatalysis makes use of the photon energy for overcoming activation barriers. As an example, the photooxidation of organic molecules at n-type semiconductors is discussed.     

Iodine(III) mediators in electrochemical batch and flow reactions

The anodic oxidation of aryl iodides is a powerful method for synthesis of hypervalent iodine compounds, which have matured to frequently used reagents in organic synthesis. The electrochemical route eliminates the use of expensive or hazardous oxidants for their synthesis. Hypervalent iodine reagents generated at the anode are successfully used as either in-cell or ex-cell mediators for many valuable chemical transformations including fluorinations and oxidative cyclisations. The recent advances in the area of flow electrochemistry are providing additional benefits and allow new synthetic applications. Mechanistic insights and novel technologies enable the development of new concepts for sustainable chemistry.