Computational-experimental study of the onset potentials for CO2 reduction on polycrystalline and oxide-derived copper electrodes

The electrocatalytic reduction of CO2 (CO2RR) is a promising yet intricate process to alleviate the alarming imbalance in the carbon cycle. One of the intricacies of CO2RR is its structural sensitivity, which is illustrated by the varying onset potentials and selectivities of the reaction products depending on the electrode morphology. Here, using electrochemical real-time mass spectrometry (EC-RTMS), we accurately determine the onset potentials for seven CO2RR products including C1, C2, and C3 species on polycrystalline and oxide-derived Cu electrodes. Density functional theory calculations affordably including solvent and cation effects produce onset potentials of C2 species matching those obtained with EC-RTMS. Our analysis leads us to conclude that the elusive active sites at oxide-derived Cu, known to enhance ethanol production, are undercoordinated square ensembles of Cu atoms

Towards an efficient liquid organic hydrogen carrier fuel cell concept

The high temperature required for hydrogen release from Liquid Organic Hydrogen Carrier (LOHC) systems has been considered in the past as the main drawback of this otherwise highly attractive and fully infrastructure-compatible form of chemical hydrogen storage. According to the state-of-the art, the production of electrical energy from LOHC-bound hydrogen, e.g. from perhydro-dibenzyltoluene (H18DBT), requires provision of the dehydrogenation enthalpy (e.g. 65 kJ mol-1(H2) for H18-DBT) at a temperature level of 300 °C followed by purification of the released hydrogen for subsequent fuel cell operation. Here, we demonstrate that a combination of a heterogeneously catalysed transfer hydrogenation from H18-DBT to acetone and fuel cell operation with the resulting 2-propanol as a fuel, allows for an electrification of LOHC-bound hydrogen in high efficiency (> 50 %) and at surprisingly mild conditions (temperatures below 200 °C). Most importantly, our proposed new sequence does not require an external heat input as the transfer hydrogenation from H18-DBT to acetone is almost thermoneutral. In the PEMFC operation with 2-propanol, the endothermal proton release at the anode is compensated by the exothermic formation of water. Ideally the proposed sequence does not form and consume molecular H2 at any point which adds a very appealing safety feature to this way of producing electricity from LOHC-bound hydrogen, e.g. for applications on mobile platforms.Fil: Sievi, Gabriel. Forschungszentrum Jülich; AlemaniaFil: Geburtig, Denise. Universitat Erlangen-Nuremberg; AlemaniaFil: Skeledzic, Tanja. Forschungszentrum Jülich; AlemaniaFil: Bösmann, Andreas. Universitat Erlangen-Nuremberg; AlemaniaFil: Preuster, Patrick. Forschungszentrum Jülich; AlemaniaFil: Brummel, Olaf. Universitat Erlangen-Nuremberg; AlemaniaFil: Waidhas, Fabian. Universitat Erlangen-Nuremberg; AlemaniaFil: Montero, María de Los Angeles. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; ArgentinaFil: Khanipour, Peyman. Forschungszentrum Jülich; AlemaniaFil: Katsounaros, Ioannis. Forschungszentrum Jülich; AlemaniaFil: Libuda, Jörg. Universitat Erlangen-Nuremberg; AlemaniaFil: Mayrhofer, Karl J. J.. Forschungszentrum Jülich; AndorraFil: Wasserscheid, Peter. Universitat Erlangen-Nuremberg; Alemani

Analysing the relationship between the fields of thermo- and electrocatalysis taking hydrogen peroxide as a case study

Catalysis is inherently driven by the interaction of reactants, intermediates and formed products with the catalyst’s surface. In order to reach the desired transition state and to overcome the kinetic barrier, elevated temperatures or electrical potentials are employed to increase the rate of reaction. Despite immense efforts in the last decades, research in thermo- and electrocatalysis has often preceded in isolation, even for similar reactions. Conceptually, any heterogeneous surface process that involves changes in oxidation states, redox processes, adsorption of charged species (even as spectators) or heterolytic cleavage of small molecules should be thought of as having parallels with electrochemical processes occurring at electrified interfaces. Herein, we compare current trends in thermo- and electrocatalysis and elaborate on the commonalities and differences between both research fields, with a specific focus on the production of hydrogen peroxide as case study. We hope that interlinking both fields will be inspiring and thought-provoking, eventually creating synergies and leverage towards more efficient decentralized chemical conversion processes

On the assessment of electrocatalysts for nitrate reduction

The electrochemical reduction of nitrate is attracting attention in the context of producing ammonia, besides the traditional removal to harmless N2. To make progress in this complex reaction and facilitate the search for active and selective catalysts, we need to establish generalized testing protocols which will enable to compare and complement data from different laboratories. The purpose of this article is to raise awareness on the importance of (i) solution processes that involve products of the electrode reaction, (ii) determination of products with appropriate, product-specific quantitative methods, (iii) the strong sensitivity of the reaction on experimental parameters, (iv) the cell design for the separation of anode from cathode processes, and (v) the increase in the interfacial and solution pH that occurs during the electrolysis at high current densities

Electrochemical reduction of nitrate on tin cathode: removal of nitrate from the ion exchange regenerant solution

The electrochemical reduction of nitrate was achieved efficiently with both high rate and high selectivity of nitrogen (~90%) on a tin cathode at potentials more ne­ga­ti­ve than -1.8 V vs. Ag/AgCl. The other products at pH > 4 were nitrous oxide, ammo­nia, nitrite and traces of nitrogen oxide, while at pH 0-4, ammonia and hydro­xy­la­mi­ne were mainly formed. Hyponitrite and nitramide were detected for the first time as intermediates of the reduction, in considerable concentrations. The rate determining step of the reduction between the potential range of -1.6 V and -1.8 V is the slow electron transfer to the nitrate ion, whereas at potentials more negative than -2.0 V the reduction proceeds through electrochemical hydro­ge­nation. The selectivity of nitrogen increases as the negative potential increases from ?1.8 V to ?2.8 V, while that of nitrite decreases. The selectivity of ammonia displays a maximum at ?2.4 V and consequently decreases. The rate of the reduction at ?1.8 V increases significantly as the concentration of NaCl increases. The cation of the supporting electrolyte increases the rate of the reduction along the series Li+ Br? > Cl? > F?. The experimental results were explained by the theory of cationic catalysis. Τhe nitrate ion forms an ion pair with the cation of the supporting electrolyte which facilitates its approach to the likely charged electrode surface, prior to the electron transfer. The reduction of nitrate follows zero-order kinetics at high concentrations of nitrate (>0.25 M) and first-order at lower concentrations. An equation similar to Langmuir kinetics was derived, where the formation of ion pairs was taken into account. Furthermore, the rate of the reduction increases linearly with the concentration of hydroxonium ion in the pH range 0-4, whereas it is not dependent on the pH at higher pH values. This was attributed to the fact that the proton donor in the pH range 0-4 is the hydroxonium ion while at pH > 4 the proton donor is the water molecule. Based on the experimental results, a reaction mechanism was proposed in which nitrite and nitramide are the main intermediates before the formation of the final products. The applicability of the method for the treatment of the brine wastes which result from the regeneration of the ion exchange resins was investigated. The obtained experimental results showed that the removal of nitrate from these wastes can be carried out with high rate and high selectivity of nitrogen. However, the energy consumption was high but it can be significantly reduced by a better cell configuration.Η ηλεκτροχημική αναγωγή των νιτρικών επιτεύχθηκε αποτελεσματικά με ταυτόχρονα υψηλή ταχύτητα και υψηλή εκλεκτικότητα σε άζωτο (~90%) σε κάθοδο κασσίτερου σε δυναμικά πιο αρνητικά από -1,8 V vs. Ag/AgCl. Τα άλλα προϊόντα σε pH > 4 ήταν υποξείδιο του αζώτου, αμμωνία, νιτρώδη και ίχνη μονοξειδίου του αζώ­του, ενώ σε pH 0-4, σχηματίζονταν κυρίως αμμωνία και υδροξυλαμίνη. Τα υπονι­τρώδη και το νιτραμίδιο ανιχνεύθηκαν για πρώτη φορά ως ενδιάμεσα της αναγωγής, σε σημαντικές συγκεντρώσεις. Το καθορίζον την ταχύτητα της αναγωγής στάδιο στην περιοχή δυναμικών με­ταξύ -1,6 V και -1,8 V είναι η αργή μεταφορά ηλεκτρονίου προς το νιτρικό ιόν, ενώ σε δυναμικά πιο αρνητικά από -2,0 V η αναγωγή χωρεί μέσω ηλεκτροχημικής υδρο­γόνωσης. Η εκλεκτικότητα του αζώτου αυξάνει καθώς το αρνητικό δυναμικό αυξάνει από ?1,8 V σε ?2,8 V, ενώ αυτή των νιτρωδών ελαττώνεται. Η εκλεκτικότητα της αμ­μωνίας εμφανίζει ένα μέγιστο στα ?2,4 V και στη συνέχεια ελαττώνεται. Η ταχύτητα της αναγωγής στα ?1,8 V αυξάνει σημαντικά καθώς η συγκέ­ντρωση του NaCl αυξάνει. Το κατιόν του φέροντα ηλεκτρολύτη αυξάνει την ταχύ­τη­τα της αναγωγής κατά τη σειρά Li+ Br? > Cl? > F?. Τα πειραματικά αποτελέσματα εξηγήθηκαν με τη θεωρία της κατιονικής κατάλυσης σύμ­φωνα με την οποία το νιτρικό ιόν σχηματίζει ένα ζεύγος με το κατιόν του φέροντα ηλεκτρολύτη το οποίο βοηθά την προσέγγισή του στην ομοίως φορτισμένη επιφάνεια του ηλεκτροδίου, πριν από την μεταφορά ηλεκτρονίου. Η αναγωγή των νιτρικών ακολουθεί κινητική μηδενικής τάξης σε μεγάλες συγκεντρώσεις νιτρικών (>0,25 M) και πρώτης τάξης σε μικρότερες συγκεντρώσεις. Μία εξίσωση παρόμοια με την κινητική Langmuir εξήχθη, στην οποία ο σχηματισμός των ζευγών ιόντων λαμβάνεται υπόψη. Περαιτέρω, η ταχύτητα της αναγωγής αυξάνει γραμμικά με τη συγκέντρωση του ιόντος του οξωνίου στην περιοχή του pH 0-4, ενώ δεν εξαρτάται από το pH σε μεγαλύτερες τιμές του. Αυτό αποδόθηκε στο γεγονός ότι ο δότης πρωτονίων στην πε­ριοχή pH 0-4 είναι το ιόν του οξωνίου ενώ σε pH > 4 ο δότης πρωτονίων είναι το μό­ριο του νερού. Με βάση τα πειραματικά αποτελέσματα, ένας μηχανισμός προτάθηκε στον ο­ποίο τα νιτρώδη και το νιτραμίδιο είναι τα κύρια ενδιάμεσα πριν από το σχηματισμό των τε­λικών προϊόντων. Η δυνατότητα εφαρμογής της μεθόδου εξετάστηκε για την κατεργασία των αποβλήτων που προκύπτουν από την αναγέννηση των ρητινών ιοντοεναλλαγής. Τα πειραματικά αποτελέσματα που λήφθηκαν έδειξαν ότι η απομάκρυνση των νιτρικών από τα απόβλητα αυτά μπορεί να γίνει με μεγάλη ταχύτητα και μεγάλη εκλε­κτι­κό­τη­τα σε άζωτο. Εντούτοις, η ενεργειακή κατανάλωση ήταν υψηλή αλλά αυτό μπορεί να ελαττωθεί σημαντικά με έναν καλύτερο σχεδιασμό του κελιού

Oxide Reduction Precedes Carbon Dioxide Reduction on Oxide-Derived Copper Electrodes

Herein, we focus on the time and potential dynamics of the carbon dioxide electroreduction reaction and investigate the effects during the first seconds after applying a reductive potential to an oxide-derived copper electrode in a three-electrode cell. Thereby, we aim to investigate a widespread point of contention about the in situ active phase of initially oxidized copper catalysts under reductive reaction conditions. For this purpose, we utilize the electrochemical real-time mass spectrometry approach developed for such investigations requiring high temporal resolution. By applying three different electrochemical protocols, we come to the same conclusion in all three cases: the reduction of the oxide catalyst precedes the formation of products on either a time or potential scale. Intriguingly, after proper compensation for the electrolyte resistance, we show that the evolution of products in a first negative-going scan right after the thermal treatment of the electrode is identical to consecutive scans on the reduced electrode and that the oxide reduction in the first scan takes place at a more positive potential than the onset of product formation

Implementation of an enclosed ionization interface for the analysis of liquid sample streams with direct analysis in real time mass spectrometry

Rationale: The development of an interface to introduce liquid sample streams to directanalysis in real time mass spectrometry (DART-MS) is of great interest for coupling variousanalytical techniques, using non-volatile salts, with MS. Therefore, we devised an enclosedionization interface and a sample introduction system for the versatile analysis of liquidsamples with DART-MS.Methods: The sample introduction system consists of a nebulizer, a spray chamber and atransfer line, while the confined ionization interface is created by implementing a cross-shapedhousing between the ion source and the MS inlet. Methodical studies of the effects of varioussetup parameters on signal intensity and peak shape were conducted, while its diverseapplicability was demonstrated by coupling with high-performance liquid chromatography(HPLC) for the analysis of alcohols, organic acids and furanic compounds.Results: The confinement of the ionization interface results in a robust setup design with awell-defined ionization region for focusing of the sprayed sample mist. Thereby, an increasein analyte signal intensity by three orders of magnitude and improved signal stability andreproducibility were obtained in comparison to a similar open ionization interfaceconfiguration. Additionally, the successful quantification of alcohols could be demonstrated aswell as the compatibility of the setup with HPLC gradient elution.Conclusions: A versatile setup design for the analysis of liquid sample streams with DARTMSwas devised for monitoring reactions or hyphenating analytics with MS. It minimizesThis article is protected by copyright. All rights reserved.interferences from the laboratory surrounding as well as allows for safe handling of hazardousand toxic chemicals, which renders it suitable for a broad range of applications

CO 2 Electroreduction on Silver Foams Modified by Ionic Liquids with Different Cation Side Chain Length

Ionic liquids (ILs) are capable of tuning the kinetics of electroreduction processesby modifying a catalyst interface. In this work, a group of hydrophobic imidazoliumbasedILs was immobilized on Ag foams using a procedure known as "solid catalystwith ionic liquid layer" (SCILL). The derived electrocatalysts demonstrated alteredselectivity and CO production rates for the electrochemical reduction of CO2 comparedto the unmodified Ag foam. The activity change caused by the IL was dependenton the length of the N-alkyl substituent. The rate of CO production is optimized atmoderate chain length and IL loadings. The observed trends are attributed to a localenrichment of CO2-based species in the proximity of the catalyst and a modification ofthe environment of its active sites. On the contrary, high loadings or long IL chainsrender the surface inaccessible and favor the hydrogen evolution reaction