Presence Of Alkali Metal Cations Research Articles

Effect of Electrolyte Composition over CO2 Reduction

The rapid utilization of fossil fuels escorted by an excess of CO2 emissions has led to global energy and environmental crisis. Therefore, the necessity for a clean, renewable and sustainable source of energy is a growing concern for the present and future society. In this regard, economically viable CO2 reduction would be a critical turnover in research, as it has the potential to fulfil a substantial need for clean energy. However, even though many efforts have been done in this field, there is still room for improvements concerning efficiency, material stability, and catalytic enhancement in regard of kinetics and selectivity. Herein, we provide the experimental proof for enhancement of the CO2 reduction efficiency and selectivity from the SEI (semiconductor-electrolyte interface) side through the use of carbonates, borates, sulphates and alkali cations as the electrolyte as well as an overview of the latest developments on Cu2O based PEC CO2 reduction for solar fuel production. Cu2O is a low-cost semiconductor and one of the most promising candidates for PEC CO2RR. However, its stability and performance is still unsatisfactory, thus the CO2 reduction products vary from one investigated system to another, such as: CH3OH, CO, HCOOH, CH3COOH, and CH3CH2OH. Moreover, the instability of Cu2O causes it, to be rarely used for the routine CO2 reduction reaction. In this paper, we use a very facile electrodeposition method, which offers a high level of reproducibility and the possibility of using a new electrode in each experiment, in order to focus our efforts on following the phenomena occurring in the double layer during the photocatalytic run. In this way, we were able to correlate the final CO2RR performance with a reorganization of the cations and anions near the photocatalyst surface. It is shown in the literature that factors such as: carbonate concentration, local pH, the presence of alkali metal cations, the geometry of the anionic group, CO2 solubility, conductivity, as well as pH changes along with the number of H+ in the electrolyte, play a significant role in regulating the partial CO2RR current. In this paper, we would like to shed new light on the influence of the electrolyte composition, cation-anion interaction and local reaction environment around the catalyst on the performance of CO2RR. We found out that the specific interaction between the alkali cation and the anionic group of particular geometry contributes to the formation of a kind of a “rigid layer” within the double diffusion, close to the photocatalyst surface layer, which accounts more for an apparent CO2RR current than the decrease in the finite Warburg element, which is a central key parameter for the diffusion coefficient values. The effectiveness of the strength of the cation-anion interaction in the formation of the “rigid layer” around the photocatalyst surface is found to increase the PEC CO2RR performance. Elucidating this mechanism provides useful information for creating further experimental design and the new pathways for addressing highly efficient PEC CO2RR systems. The authors have noticed a significant knowledge gap in the existing literature, as no prior publications have delved into the concurrent and combined impact of both cations and anions on PEC CO2RR. In this pioneering publication, we present the inaugural investigation of this this intricate phenomenon.

Electrolyte design for the manipulation of gas bubble detachment during hydrogen evolution reaction

During electrochemical gas evolution reactions, the continuous and vigorous formation of gas bubbles hugely impacts the efficiency of the underlying electrochemical processes. In particular, enhancing the detachment of gas bubbles from the electrode surface has emerged as an effective strategy to improve reaction efficiency. In this study, we demonstrate that the detachment of H2 gas bubbles can be controlled by the electrolyte composition, which can be optimized. We employ a well-defined Pt microelectrode and utilize electrochemical oscillation analysis to elucidate the features of H2 gas bubble detachment. Our investigation explores how the behaviour of H2 gas bubbles responds to variations in electrolyte composition and concentration. The coalescence efficiency of electrochemically generated microbubbles, a critical factor determining the mode of H2 gas bubble detachment (random detachment vs. single H2 gas bubble detachment), is profoundly influenced by the electrolyte composition. Specifically, coalescence efficiency follows the Hofmeister series concerning anions and coalescence is consistently inhibited in the presence of alkali metal cations. Furthermore, we establish a comprehensive model that accounts for both thermal and solutal Marangoni effects, allowing us to rationalize the trend of detachment size and period of single H2 gas bubbles under various conditions.

Cation Effects on the Acidic Oxygen Reduction Reaction at Carbon Surfaces.

Hydrogen peroxide (H2O2) is a widely used green oxidant. Until now, research has focused on the development of efficient catalysts for the two-electron oxygen reduction reaction (2e- ORR). However, electrolyte effects on the 2e- ORR have remained little understood. We report a significant effect of alkali metal cations (AMCs) on carbons in acidic environments. The presence of AMCs at a glassy carbon electrode shifts the half wave potential from -0.48 to -0.22 VRHE. This cation-induced enhancement effect exhibits a uniquely sensitive on/off switching behavior depending on the voltammetric protocol. Voltammetric and in situ X-ray photoemission spectroscopic evidence is presented, supporting a controlling role of the potential of zero charge of the catalytic enhancement. Density functional theory calculations associate the enhancement with stabilization of the *OOH key intermediate as a result of locally induced field effects from the AMCs. Finally, we developed a refined reaction mechanism for the H2O2 production in the presence of AMCs.

Immobilized Tetraalkylammonium Cations Enable Metal-free CO2 Electroreduction in Acid and Pure Water.

Carbon dioxide reduction reaction (CO2 RR) provides an efficient pathway to convert CO2 into desirable products, yet its commercialization is greatly hindered by the huge energy cost due to CO2 loss and regeneration. Performing CO2 RR under acidic conditions containing alkali cations can potentially address the issue, but still causes (bi)carbonate deposition at high current densities, compromising product Faradaic efficiencies (FEs) in present-day acid-fed membrane electrode assemblies. Herein, we present a strategy using a positively charged polyelectrolyte-poly(diallyldimethylammonium) immobilized on graphene oxide via electrostatic interactions to displace alkali cations. This enables a FE of 85 %, a carbon efficiency of 93 %, and an energy efficiency (EE) of 35 % for CO at 100 mA cm-2 on modified Ag catalysts in acid. In a pure-water-fed reactor, we obtained a 78 % CO FE with a 30 % EE at 100 mA cm-2 at 40 °C. All the performance metrics are comparable to or even exceed those attained in the presence of alkali metal cations.

Immobilized Tetraalkylammonium Cations Enable Metal‐free CO2 Electroreduction in Acid and Pure Water

AbstractCarbon dioxide reduction reaction (CO2RR) provides an efficient pathway to convert CO2 into desirable products, yet its commercialization is greatly hindered by the huge energy cost due to CO2 loss and regeneration. Performing CO2RR under acidic conditions containing alkali cations can potentially address the issue, but still causes (bi)carbonate deposition at high current densities, compromising product Faradaic efficiencies (FEs) in present‐day acid‐fed membrane electrode assemblies. Herein, we present a strategy using a positively charged polyelectrolyte—poly(diallyldimethylammonium) immobilized on graphene oxide via electrostatic interactions to displace alkali cations. This enables a FE of 85 %, a carbon efficiency of 93 %, and an energy efficiency (EE) of 35 % for CO at 100 mA cm−2 on modified Ag catalysts in acid. In a pure‐water‐fed reactor, we obtained a 78 % CO FE with a 30 % EE at 100 mA cm−2 at 40 °C. All the performance metrics are comparable to or even exceed those attained in the presence of alkali metal cations.

What Is Needed for Industrial Carbon-Dioxide Electroreduction: From Electrode Development to Process Optimization

Electrochemical reduction of CO2 is a promising method for converting a greenhouse gas into value-added products, utilizing renewable energy. Novel catalysts, electrode assemblies, and cell configurations are all necessary to achieve economically appealing performance. In this talk, I am going to present a zero gap electrolyzer cell, which converts gas phase CO2 to products without the need for any liquid catholyte. This is the first report of a CO2 electrolyzer cell, where multiple cells are connected into a stack, thus scaling up the electrolysis process. I will show how proper cell components and operational conditions result in unprecedentedly high partial current densities in zero-gap cells. I will demonstrate the role of each component of the membrane electrode assembly, such as the catalysts, the porous transport layers, the membrane, and the ionomers.In the second part of my talk, I will show that while precipitate formation in the cathode gas diffusion electrode is detrimental for the long-term stability, the presence of alkali metal cations at the cathode improves performance. To overcome this contradiction, I will present different approaches which help to extend the lifetime of the electrolyzer cell. As an outlook, I will present a complex machine learning based approach, through which we aim to find the optimal operating conditions of such CO2 electrolyzer cells.

Causes and Effects of Unintended Cation Crossover in CO2 Reduction Cells

Electrochemically converting carbon dioxide into useful products is a promising approach to combat climate change by reducing our reliance on fossil fuels and promoting a sustainable carbon cycle.The operation of membrane electrode assemblies within gas diffusion cells facilitates the efficient reduction of CO2 at rates relevant to industrial applications. However, their long-term stability is often limited by formation of solid precipitates in the cathode pores. This is a consequence of a combination of 1) local alkalization due to the electrochemical reaction, 2) generation of (bi)carbonate by chemical reaction of CO2 with the alkaline electrolyte, and 3) the presence of alkali metal cations. In catholyte-free, zero-gap cells using anion exchange membranes, the presence of electrolyte cations at the cathode is the result of unintended crossover from the anolyte, and a detailed understanding of the factors enabling this crossover is lacking. Here we show that the anolyte concentration governs the flux of cation migration through the membrane, and this substantially influences the behaviours of copper catalysts in catholyte-free CO2 electrolyzers.Our findings highlight the substantial impact of cation effects, including unintended crossover, even in catholyte-free cells, on reaction pathways. This aspect should be considered in the future development of catalysts and devices. As an outlook a more in-depth knowledge with the help of operando measurements could help to understand and manage cation crossover for optimizing the performance, selectivity, and durability of these electrochemical systems.

Unintended Cation Crossover in CO2 Conversion MEA Cells: Causes and Effects

Membrane electrode assemblies in gas diffusion cells enable CO2 reduction at industrially relevant rates, yet their long-term operational stability is often limited by the formation of solid precipitates (e.g. K2CO3) in the cathode pores. This is a consequence of a combination of 1) local alkalization due to the electrochemical reaction, 2) generation of (bi)carbonate by chemical reaction of CO2 with the alkaline electrolyte, and 3) the presence of alkali metal cations. In catholyte-free, zero-gap cells using anion exchange membranes, the presence of electrolyte cations at the cathode is the result of unintended crossover from the anolyte, and a detailed understanding of the factors enabling this crossover is lacking. Here we show that the anolyte concentration governs the flux of cation migration through the membrane, and this substantially influences the behaviors of copper catalysts in catholyte-free CO2 electrolysers. Systematic variation of the anolyte ionic strength (using aqueous KOH or KHCO3) correlated with drastic changes in the observed product selectivity – most notably, at low ionic strength, Cu catalysts produced predominantly CO, in contrast to the mixture of C2+ products typically observed on Cu. In this talk, we examine the factors influencing ion crossover and the resulting effects on catalyst structure and activity, under conditions of both CO2 and CO reduction. Operando X-ray absorption spectroscopy and quasi in situ X-ray photoelectron spectroscopy were used to study how the catalyst is affected by operation conditions. Our results show that even in catholyte-free cells, cation effects (including unintended ones) can significantly influence reaction pathways, and this must be considered in future development of catalysts and devices.El-Nagar, G. A., Haun, F., Gupta, S., Stojkovikj, S. & Mayer, M. T. Unintended cation crossover influences CO2 reduction activity in Cu-based zero-gap electrolysers. Nature Communications14, 2062 (2023) doi: doi.org/10.1038/s41467-023-37520-x Figure 1

(Invited) Probing the Specific Adsorption of Alkali Metal Cations on Au Electrodes Under CO2 Reduction

Electrolyte engineering has recently been recognized as an integral component in the design of electrode/electrolyte interfaces for electrocatalysis. In particular, alkali metal cations of the supporting electrolyte are known to influence electrocatalytic processes. A better understanding of these cation effects requires methods capable of probing the distribution of alkali metal cations in the electrochemical double layer under reaction conditions. However, the scarcity of experimental technique capable of probing alkali metal cations greatly hinders further advances in leveraging cation effects in electrolyte engineering. In this talk, we will discuss our novel spectroscopic approach for quantifying the surface coverage of specifically adsorbed alkali metal cations, that is, the cations that are in direct contact with the electrode and therefore most likely to influence electrocatalysis. The technique is based on the use of an organic cation, tetramethylammonium (methyl4N+), as a vibrational probe of the electrode/electrolyte interface in the presence of alkali metal cations. We will discuss how we use this approach to characterize the adsorption of alkali metal cation during CO2-to-CO conversion.

CsxCo/Na-MOR Coating on Ceramic Monoliths for Co-Adsorption of Hydrocarbons Mixture and Selective Catalytic Reduction of NOx

In this work, ceramic monoliths were coated with powders based on exchanged Cs and/or Co cations in Na-mordenite (MOR) zeolite. SEM images showed that zeolite particles fill the macropores of cordierite walls and form a continuous layer of approximately 40 µm with good adherence. XPS analysis revealed that Co and Cs are present on the film surface solely as Co2+ and Cs+ at exchange positions in zeolite. The monolithic structures were evaluated for the butane-toluene co-adsorption and SCR of NOx with hydrocarbon mixture as the reducing agent. The presence of alkali metal cations in the zeolitic lattice favored the adsorption capacity of both hydrocarbons, while cobalt cations provoked a decrease in the adsorbed amounts due to its weak interaction with the HCs. Breakthrough curves of butane adsorption showed a roll-up phenomenon, associated with a competitive adsorption effect generated from toluene presence. In the desorption process, it was observed that adsorbed toluene hindered the butane diffusion through mordenite channels, which released at higher temperatures (above 250 °C). Cs2CoM and Cs7CoM monoliths were more active than the CoM monolith for NO-SCR. The presence of Cs cations close to Co cations increased the hydrocarbons concentration around active sites at high temperatures, according to TPD results, promoting the reduction activity of NO.

The role of alkali metal cations and platinum-surface hydroxyl in the alkaline hydrogen evolution reaction

The platinum-catalysed hydrogen evolution reaction (HER) generally shows poorer kinetics in alkaline electrolyte and represents a key challenge for alkaline water electrolysis. In the presence of alkali metal cations and hydroxyl anions, the electrode–electrolyte (platinum–water) interface in an alkaline electrolyte is far more complex than that in an acidic electrolyte. Here we combine electrochemical impedance spectroscopy and an electrical transport spectroscopy approach to probe and understand the fundamental role of different cations (Li+, Na+ and K+) in HER kinetics. Our integrated studies suggest that the alkali metal cations play an indirect role in modifying the HER kinetics, with the smaller cations being less destabilizing to the hydroxyl adsorbate (OHad) species in the HER potential window, which favours a higher coverage of OHad on the platinum surface. The surface OHad species are highly polar and act as both electronically favoured proton acceptors and geometrically favoured proton donors to promote water dissociation in alkaline media, thus boosting the Volmer-step kinetics and the HER activity. Platinum is the most active catalyst for the hydrogen evolution reaction, but the specific mechanism and the influence of the alkali metal cations remain elusive in alkaline media. Now, electrical transport spectroscopy, electrochemical impedance spectroscopy and ab initio molecular dynamics simulations are combined to elucidate the role of alkali metal cations for this reaction in alkaline electrolyte.

Carbodiimide‐Induced Formation of Transient Polyether Cages**

AbstractThe use of “fuel” compounds to drive chemical systems out of equilibrium is currently of interest because of the potential for temporally controlled, responsive behavior. We have recently shown that transiently formed crown ethers exhibit counterintuitive templation effects when generated in the presence of alkali metal cations: “matched” cations, such as K+with an 18‐crown‐6 analogue, suppress the formation of the macrocycles (negative templation). In this work, we describe two macrocyclic diacids that, on treatment with carbodiimides, give transient macrobicyclic cages analogous to polyether cages. Negative templation effects are observed for the smaller cage when generated in the presence of K+and Na+, but there is a weak, but reproducible, positive templation effect in the presence of Li+. The larger cage behaves similarly in the presence of Li+, K+, Rb+, and Cs+, but differently with Na+, which appears to bind to both the cage and the initial macrocycle.

(Invited) Effects of the Local Chemical Environment on the Anode and Cathode Processes of CO2 Electrolyzers

Continuous-flow electrolyzers allow CO2 reduction at industrially relevant rates, but long-term operation is still challenging. In this talk, I am going to present some interesting findings on the role of different ions, crossing the anion exchange membrane in zero-gap electrolyzer cells, contributing to unstable operation.In the first part of my talk, I will show that while precipitate formation in the cathode gas diffusion electrode is detrimental for the long-term stability, the presence of alkali metal cations at the cathode improves performance. To overcome this contradiction, we develop an operando activation and regeneration process, where the cathode of a zero-gap electrolyzer cell is periodically infused with alkali cation-containing solutions. [1] This enables deionized water-fed electrolyzers to operate at a CO2 reduction rate matching that of those using alkaline electrolytes (CO partial current density of 420 ± 50 mA cm−2 for over 200 hours). We deconvolute the complex effects of activation and validate the concept with five different electrolytes and three different commercial membranes. Finally, we demonstrate the scalability of this approach on a multi-cell electrolyzer stack, with a 100 cm2 / cell active area.In the second part of the presentation, I will discuss the role of anode catalyst in CO2R cells. The urge to substitute Ir is driven by its high- and steeply rising market price as well as its limited stability in alkaline media. Although Ni is a ~ten thousand times cheaper, active, and stable oxygen evolution reaction (OER) catalyst in alkaline media, I will demonstrate that there are factors, which hinder its application in CO2 electrolysis. While Ni is a suitable OER catalyst in short experiments, the cell voltage increases, and the measured total Faradaic efficiency decreases continuously during prolonged electrolysis. This is caused by the local acidic pH at the anode surface, the crossing CO3 2- ions and by the gradual change in the anolyte composition, leading to Ni dissolution. The catalyst loss is only a minor part of the problem; the dissolving metal ions also penetrate into the anion exchange membrane, where precipitate forms due to the high local carbonate ion concentration, inducing cell failure.Finally, I will present a complex machine learning based approach, through which we aim to find the optimal operating conditions of such CO2 electrolyzer cells.

Spotlight on the Effect of Electrolyte Composition on the Potential of Maximum Entropy: Supporting Electrolytes Are Not Always Inert.

The influence of electrolyte pH, the presence of alkali metal cations (Na+, K+), and the presence of O2 on the interfacial water structure of polycrystalline gold electrodes has been experimentally studied in detail. The potential of maximum entropy (PME) was determined by the laser‐induced current transient (LICT) technique. Our results demonstrate that increasing the electrolyte pH and introducing O2 shift the PME to more positive potentials. Interestingly, the PME exhibits a higher sensitivity to the pH change in the presence of K+ than Na+. Altering the pH of the K2SO4 solution from 4 to 6 can cause a drastic shift in the PME. These findings reveal that, for example, K2SO4 and Na2SO4 cannot be considered as equal supporting electrolytes: it is not a viable assumption. This can likely be extrapolated to other common “inert” supporting electrolytes. Beyond this, knowledge about the near‐ideal electrolyte composition can be used to optimize electrochemical devices such as electrolyzers, fuel cells, batteries, and supercapacitors.

Operando cathode activation with alkali metal cations for high current density operation of water-fed zero-gap carbon dioxide electrolyzers.

Continuous-flow electrolyzers allow CO2 reduction at industrially relevant rates, but long-term operation is still challenging. One reason for this is the formation of precipitates in the porous cathode from the alkaline electrolyte and the CO2 feed. Here we show that while precipitate formation is detrimental for the long-term stability, the presence of alkali metal cations at the cathode improves performance. To overcome this contradiction, we develop an operando activation and regeneration process, where the cathode of a zero-gap electrolyzer cell is periodically infused with alkali cation-containing solutions. This enables deionized water-fed electrolyzers to operate at a CO2 reduction rate matching that of those using alkaline electrolytes (CO partial current density of 420 ± 50 mA cm−2 for over 200 hours). We deconvolute the complex effects of activation and validate the concept with five different electrolytes and three different commercial membranes. Finally, we demonstrate the scalability of this approach on a multi-cell electrolyzer stack, with a 100 cm2 / cell active area.

Key activity descriptors of nickel-iron oxygen evolution electrocatalysts in the presence of alkali metal cations

Efficient oxygen evolution reaction (OER) electrocatalysts are pivotal for sustainable fuel production, where the Ni-Fe oxyhydroxide (OOH) is among the most active catalysts for alkaline OER. Electrolyte alkali metal cations have been shown to modify the activity and reaction intermediates, however, the exact mechanism is at question due to unexplained deviations from the cation size trend. Our X-ray absorption spectroelectrochemical results show that bigger cations shift the Ni2+/(3+δ)+ redox peak and OER activity to lower potentials (however, with typical discrepancies), following the order CsOH > NaOH ≈ KOH > RbOH > LiOH. Here, we find that the OER activity follows the variations in electrolyte pH rather than a specific cation, which accounts for differences both in basicity of the alkali hydroxides and other contributing anomalies. Our density functional theory-derived reactivity descriptors confirm that cations impose negligible effect on the Lewis acidity of Ni, Fe, and O lattice sites, thus strengthening the conclusions of an indirect pH effect.

Theoretical insight into effect of cation–anion pairs on CO2 reduction on bismuth electrocatalysts

This study presents theoretical insight into the mechanism of the CO2 reduction reaction (CO2RR) to formic acid (HCOOH) on Bi (012) surfaces in the presence of alkali metal cations (M+: Cs+, K+, and Li+) and/or halide anions (X−: Cl−, Br−, and I−) using density functional theory (DFT). The adsorption energy (Eads) and work function (Wf) of the anions increases with decreasing anion size (i.e., Cl− > Br− > I−). On the other hand, the larger the cation size is, the higher is the Eads (i.e., Li+ < K+ < Cs+) but the lower is the Wf (i.e., Cs+ < K+ < Li+). In the presence of the cation–anion pairs (M+/X−), Eads of the pairs on hydrated Bi (Bi-2H) becomes more negative than that in the cases of anions or cations alone, particularly when the ionic radius of the paired cation and anion do not differ significantly. Such a synergistic effect of the mixed ions is also observed for the work function values. In the case of anions alone, CO2 molecules prefer to coordinate directly with hydrated Bi atoms via the oxygen bidentate mode; in the case of cations alone, CO2 molecules directly bind to the cations via the oxygen monodentate mode, rather than the hydrated Bi atoms. Between two possible CO2RR pathways involving *OCHO and *COOH intermediates on Bi-2H pre-adsorbed with M+/X−, the former pathway requires less energy for all M+/X− pairs. In addition, cascaded reaction profiles from CO2* to HCOOH are obtained with Cs+/Cl− and K+/Cl− pairs in the former. This indicates that once CO2 is adsorbed, the following reactions proceed spontaneously on Bi-2H with Cs+/Cl− or K+/Cl− pairs. This study thus shed light on the positive effects of supporting electrolytes (e.g., CsCl and KCl) on catalytic CO2RR.

An experimental study of electroreduction of CO2 to HCOOH on SnO2/C in presence of alkali metal cations (Li+, Na+, K+, Rb+ and Cs+) and anions (HCO3−, Cl−, Br− and I−)

It is well-known that the electrolytes can influence the electrochemical reduction of carbon dioxide (ERCO2) in aqueous media. In this work, we explore the effects of alkali metal cations and anions (Li+, Na+, K+, Rb+, Cs+, HCO3−, Cl−, Br−, I−) on the current density and product selectivity for the ERCO2 into formic acid (HCOOH) on the SnO2/carbon paper (SnO2/C) electrode. Results of the ERCO2 experiments show that for the cations, the promotion effects on current density and faradaic efficiencies (FEs) are in the order of Li+ < Na+ < K+ < Cs+ < Rb+. For the anions, the current density values are in the order of NaHCO3 < NaCl < NaBr < NaI and KHCO3 < KCl ≈ KI < KBr, respectively, and that on the FEs for the formation of the HCOOH (FEHCOOH) is HCO3− < Cl− < Br− < I−. Based on this result, the effects of alkali metal cations and anions on ERCO2 are discussed.

Faceting and Nanostructuring of Gold Surfaces through Cathodic Corrosion: Role of Cationic Electrolyte Species and Water

Gold is one of the most frequently employed electrode materials in electrochemistry due to its exceptional properties such as chemical inertness and high resistance to oxidation and dissolution [1-3]. According to Pourbaix diagrams, gold is regarded stable against degradation under cathodic polarization [4]. In contrast, applying relatively high negative potentials in the presence of alkali metal cations induce notable changes on Au electrode surfaces [5].In this study, cathodic corrosion of polycrystalline gold in different electrolytes of alkali metal hydroxides has been systemically investigated. In comparison to the recent work [5,6], this study provides new findings as a function of the applied parameters for the nanostructuring of gold surfaces. Cell design and the distance between the electrodes have carefully been controlled and optimized. The progression of surface restructuring and corrosion features was investigated using cyclic voltammetry (CV) and scanning electron microscopy (SEM). The results show different corrosion features depending on the nature and the concentration of alkali metal hydroxides, on the applied voltage, and on time. Current-potential curves for the modified electrodes in 0.1 M H2SO4 show characteristic features in the double-layer and oxidation regions. Analyzing the cyclic voltamogramms of the corroded electrodes provides evidence about the preferential orientation of facets and the electrochemical active surface area as a function of the applied parameters. The SEM micrographs show that polarizing the gold surfaces at relatively high negative potentials generates different size and orientation of octahedral nanocrystals using NaOH, spherical nanoparticles and triangular pits using KOH and triangular pits using LiOH. Besides the alkali metal cations, the electrolyte concentration and the applied voltage are important parameters. Furthermore, the role of water in cathodic corrosion of gold has been investigated using mixtures of different mole fraction of water and non-aqueous electrolytes. The absence of corrosion features using water-free non-aqueous electrolytes reveal that water plays a key role in the cathodic corrosion process.In addition, various aqueous mixed electrolytes have been systemically investigated. The results show combined corrosion features according to the mole fraction of different cations. For example, a 1:1 NaOH + LiOH mixture leads to the formation of rectangular pits which are attributed to (100)-facets compared to a 1:1 KOH + LiOH mixture which leads to triangular pits revealing the formation of (111)-facets.By applying alternating current (AC) to Au electrodes in contact with the same electrolytes, colloidal gold nanoparticles could successfully be synthesized. These nanoparticles have been used for the functionalization of water-soluble carbon nitride to enhance its activity for specific photocatalytic reactions such as alcohol oxidation.

Convergent Ditopic Receptors Enhance Anion Binding upon Alkali Metal Complexation for Catalyzing the Ritter Reaction.

A supramolecular approach to catalyzing the Ritter reaction by utilizing enhanced anion-binding affinity in the presence of alkali metal cations was developed with ditopic hydrogen-bonded amide macrocycles. With prebound cations in the macrocycle, particularly Li+ ion, their metal complexes exhibit greatly enhanced catalytic activities. The catalysis is switchable by removal or addition of the bound cation. The method described in this work may be generalized for use in other anion-triggered organic reactions involving heteroditopic receptors capable of ion pairing.