Showing papers by "Jens K. Nørskov published in 2019"
TL;DR: A broad and historical view of different aspects and their complex interplay in CO2R catalysis on Cu is taken, with the purpose of providing new insights, critical evaluations, and guidance to the field with regard to research directions and best practices.
Abstract: To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol, from electrochemical CO2 reduction (CO2R). There are variety of factors that impact CO2R activity and selectivity, including the catalyst surface structure, morphology, composition, the choice of electrolyte ions and pH, and the electrochemical cell design. Many of these factors are often intertwined, which can complicate catalyst discovery and design efforts. Here we take a broad and historical view of these different aspects and their complex interplay in CO2R catalysis on Cu, with the purpose of providing new insights, critical evaluations, and guidance to the field with regard to research directions and best practices. First, we describe the various experimental probes and complementary theoretical methods that have been used to discern the mechanisms by which products are formed, and next we present our current understanding of the complex reaction networks for CO2R on Cu. We then analyze two key methods that have been used in attempts to alter the activity and selectivity of Cu: nanostructuring and the formation of bimetallic electrodes. Finally, we offer some perspectives on the future outlook for electrochemical CO2R.
2,055 citations
TL;DR: A protocol for the electrochemical reduction of nitrogen to ammonia enables isotope-sensitive quantification of the ammonia produced and the identification and removal of contaminants, and should help to prevent false positives from appearing in the literature.
Abstract: The electrochemical synthesis of ammonia from nitrogen under mild conditions using renewable electricity is an attractive alternative1–4 to the energy-intensive Haber–Bosch process, which dominates industrial ammonia production. However, there are considerable scientific and technical challenges5,6 facing the electrochemical alternative, and most experimental studies reported so far have achieved only low selectivities and conversions. The amount of ammonia produced is usually so small that it cannot be firmly attributed to electrochemical nitrogen fixation7–9 rather than contamination from ammonia that is either present in air, human breath or ion-conducting membranes9, or generated from labile nitrogen-containing compounds (for example, nitrates, amines, nitrites and nitrogen oxides) that are typically present in the nitrogen gas stream10, in the atmosphere or even in the catalyst itself. Although these sources of experimental artefacts are beginning to be recognized and managed11,12, concerted efforts to develop effective electrochemical nitrogen reduction processes would benefit from benchmarking protocols for the reaction and from a standardized set of control experiments designed to identify and then eliminate or quantify the sources of contamination. Here we propose a rigorous procedure using 15N2 that enables us to reliably detect and quantify the electrochemical reduction of nitrogen to ammonia. We demonstrate experimentally the importance of various sources of contamination, and show how to remove labile nitrogen-containing compounds from the nitrogen gas as well as how to perform quantitative isotope measurements with cycling of 15N2 gas to reduce both contamination and the cost of isotope measurements. Following this protocol, we find that no ammonia is produced when using the most promising pure-metal catalysts for this reaction in aqueous media, and we successfully confirm and quantify ammonia synthesis using lithium electrodeposition in tetrahydrofuran13. The use of this rigorous protocol should help to prevent false positives from appearing in the literature, thus enabling the field to focus on viable pathways towards the practical electrochemical reduction of nitrogen to ammonia. A protocol for the electrochemical reduction of nitrogen to ammonia enables isotope-sensitive quantification of the ammonia produced and the identification and removal of contaminants.
819 citations
TL;DR: An iron single atom catalyst is reported that can convert oxygen into hydrogen peroxide with a selectivity of above 95% in both alkaline and neutral pH and demonstrated an effective water disinfection as a representative application.
Abstract: Shifting electrochemical oxygen reduction towards 2e- pathway to hydrogen peroxide (H2O2), instead of the traditional 4e- to water, becomes increasingly important as a green method for H2O2 generation. Here, through a flexible control of oxygen reduction pathways on different transition metal single atom coordination in carbon nanotube, we discovered Fe-C-O as an efficient H2O2 catalyst, with an unprecedented onset of 0.822 V versus reversible hydrogen electrode in 0.1 M KOH to deliver 0.1 mA cm-2 H2O2 current, and a high H2O2 selectivity of above 95% in both alkaline and neutral pH. A wide range tuning of 2e-/4e- ORR pathways was achieved via different metal centers or neighboring metalloid coordination. Density functional theory calculations indicate that the Fe-C-O motifs, in a sharp contrast to the well-known Fe-C-N for 4e-, are responsible for the H2O2 pathway. This iron single atom catalyst demonstrated an effective water disinfection as a representative application.
423 citations
TL;DR: A microkinetic model for CO2 and CO reduction on copper is presented, based on ab initio simulations, to elucidate pH’s impact on competitive reaction pathways and elucidate how reaction conditions can lead to significant enhancements in selectivity and activity towards higher value C2 products.
Abstract: We present a microkinetic model for CO(2) reduction (CO(2)R) on Cu(211) towards C2 products, based on energetics estimated from an explicit solvent model. We show that the differences in both Tafel slopes and pH dependence for C1 vs C2 activity arise from differences in their multi-step mechanisms. We find the depletion in C2 products observed at high overpotential and high pH to arise from the 2nd order dependence of C-C coupling on CO coverage, which decreases due to competition from the C1 pathway. We further demonstrate that CO(2) reduction at a fixed pH yield similar activities, due to the facile kinetics for CO2 reduction to CO on Cu, which suggests C2 products to be favored for CO2R under alkaline conditions. The mechanistic insights of this work elucidate how reaction conditions can lead to significant enhancements in selectivity and activity towards higher value C2 products.
316 citations
TL;DR: In this paper, a two-electron water oxidation reaction (2e-WOR) is used for delocalized production of hydrogen peroxide for water cleaning and other applications.
Abstract: Electrochemical synthesis of hydrogen peroxide (H2O2) via two-electron water oxidation reaction (2e-WOR) is an ideal process for delocalized production for water cleaning and other applications. Pr...
149 citations
TL;DR: In this paper, the authors present a model for the description of the interface between an electrolyte and a charged conductor using Polarizable Continuity Models (PCM) and show that the model is accurate.
Abstract: A major challenge in the modeling of electrochemical phenomena is the accurate description of the interface between an electrolyte and a charged conductor. Polarizable continuum models (PCM) have b...
139 citations
TL;DR: In this article, the authors identify a relationship between the atom-projected density of states of surface oxygen and its ability to make and break bonds with the surrounding metal atoms and hydrogen.
135 citations
TL;DR: The active and selective electroreduction of atmospheric nitrogen (N2) to ammonia (NH3) using energy from solar or wind sources at the point of use would enable a sustainable alternative to the Hab... as mentioned in this paper.
Abstract: The active and selective electroreduction of atmospheric nitrogen (N2) to ammonia (NH3) using energy from solar or wind sources at the point of use would enable a sustainable alternative to the Hab...
129 citations
TL;DR: In this article, the authors predict that doping BiVO4 with optimal concentrations of gadolinium (Gd) not only enhances its activity for H2O2 production but also improves its stability.
Abstract: Photoelectrochemical oxidation of water presents a pathway for sustainable production of hydrogen peroxide (H2O2). Two-electron water oxidation toward H2O2, however, competes with the popular four-electron process to form oxygen and one-electron water oxidation to form OH radical. To date, bismuth vanadate (BiVO4) has been shown to exhibit promising selectivity toward H2O2, especially under illumination, but it suffers from high overpotential and notoriously poor stability. Herein, using density functional theory calculations, we predict that doping BiVO4 with optimal concentrations of gadolinium (Gd) not only enhances its activity for H2O2 production but also improves its stability. Experimentally, we demonstrate that intermediate amounts of Gd doping (6–12%) reduce the onset potential of BiVO4 for H2O2 production by ∼110 mV while achieving a Faradaic efficiency of ∼99.5% under illumination and prolonging the catalytic lifetime by more than a factor of 20 at 2.0 V vs RHE under illumination.
128 citations
TL;DR: Density functional theory calculations are potentially useful for both understanding the activity of experimentally tested catalysts and screening for new catalyst materials for electrochemical ox catalysts as discussed by the authors, but they are computationally expensive.
Abstract: Density functional theory calculations are potentially useful for both understanding the activity of experimentally tested catalysts and screening for new catalyst materials. For electrochemical ox...
119 citations
TL;DR: An active thin-film nickel nitride catalyst synthesized through a reactive sputtering method achieves high activity and selectivity to four-electron ORR and exhibited good stability during 10 and 40 h chronoamperometry measurements in acid and alkaline electrolyte, respectively.
Abstract: With promising activity and stability for the oxygen reduction reaction (ORR), transition metal nitrides are an interesting class of non-platinum group catalysts for polymer electrolyte membrane fuel cells Here, we report an active thin-film nickel nitride catalyst synthesized through a reactive sputtering method In rotating disk electrode testing in a 01 M HClO4 electrolyte, the crystalline nickel nitride film achieved high activity and selectivity to four-electron ORR It also exhibited good stability during 10 and 40 h chronoamperometry measurements in acid and alkaline electrolyte, respectively A combined experiment-theory approach, with detailed ex situ materials characterization and density functional theory calculations, provides insight into the structure of the catalyst and its surface during catalysis Design strategies for activity and stability improvement through alloying and nanostructuring are discussed
TL;DR: In this article, a machine learning approach was used to predict the most expensive and most important parameter in a catalyst's affinity for a reaction, i.e., the reaction barrier.
Abstract: In the past few decades, tremendous advances have been made in the understanding of catalysis at solid surfaces. Despite this, most discoveries of materials for improved catalytic performance are made by a slow trial and error process in an experimental laboratory. Computational simulations have begun to provide a way to rationally design materials for optimizing catalytic performance, but due to the high computational expense of calculating transition state energies, simulations cannot adequately screen the phase space of materials. In this work, we attempt to mitigate this expense by using a machine learning approach to predict the most expensive and most important parameter in a catalyst’s affinity for a reaction: the reaction barrier. Previous methods which used the step reaction energy as the only parameter in a linear regression had a mean absolute error (MAE) on the order of 0.4 eV, too high to be used predictively. In our work, we achieve a MAE of about 0.22 eV, a marked improvement towards the goal of computational prediction of catalytic activity.
TL;DR: In this article, the structure of water and its interactions with various carbon dioxide reduction intermediates adsorbed on a Cu(211) surface was investigated using density functional theory, and the results showed that water and carbon dioxide reduced by different types of carbon dioxide can interact with each other.
Abstract: In this work, the structure of water and its interactions with various carbon dioxide reduction intermediates adsorbed on a Cu(211) surface is investigated using density functional theory. We find ...
TL;DR: This work identifies Ir-Cr as a promising new catalyst system that facilitates reduced precious metal loadings for acid-based OER catalysis and suggests that this enhancement is due to Cr active sites that have improved oxygen binding energetics compared to pure Ir-oxide.
Abstract: Multimetallic Ir-based systems offer significant opportunities for enhanced oxygen evolution electrocatalysis by modifying the electronic and geometric properties of the active catalyst. Herein, a systematic investigation of bimetallic Ir-based thin films was performed to identify activity and stability trends across material systems for the oxygen evolution reaction (OER) in acidic media. Electron beam evaporation was used to co-deposit metallic films of Ir, IrSn2, IrCr, IrTi, and IrNi. The electrocatalytic activity of the electrochemically oxidized alloys was found to increase in the following order: IrTi < IrSn2 < Ir ∼ IrNi < IrCr. The IrCr system demonstrates two times the catalytic activity of Ir at 1.65 V versus RHE. Density functional theory calculations suggest that this enhancement is due to Cr active sites that have improved oxygen binding energetics compared to those of pure Ir oxide. This work identifies IrCr as a promising new catalyst system that facilitates reduced precious metal loadings for acid-based OER catalysis.
TL;DR: Using density functional theory-predicted energies, it is suggested that less than 35 materials are stable under the strongly oxidizing operating conditions of oxygen reduction and/or oxygen evolution reactions in acidic media.
Abstract: Using density functional theory-predicted energies, we performed a high-throughput screening of more than 11 000 two-dimensional materials from available material databases. We suggest that less th...
TL;DR: In this paper, the authors proposed an alternative Pourbaix construction method that filters all potential combinations of species in a system to only those present on a compositional convex hull, by including axes representing the quantities of H+ and e- required to form a given phase.
Abstract: Pourbaix diagrams have been used extensively to evaluate stability regions of materials subject to varying potential and pH conditions in aqueous environments. However, both recent advances in high-throughput material exploration and increasing complexity of materials of interest for electrochemical applications pose challenges for performing Pourbaix analysis on multidimensional systems. Specifically, current Pourbaix construction algorithms incur significant computational costs for systems consisting of four or more elemental components. Herein, we propose an alternative Pourbaix construction method that filters all potential combinations of species in a system to only those present on a compositional convex hull. By including axes representing the quantities of H+ and e- required to form a given phase, one can ensure every stable phase mixture is included in the Pourbaix diagram and reduce the computational time required to construct the resultant Pourbaix diagram by several orders of magnitude. This new Pourbaix algorithm has been incorporated into the pymatgen code and the Materials Project website, and it extends the ability to evaluate the Pourbaix stability of complex multicomponent systems.
TL;DR: In this article, the authors investigated the electrochemical conversion of methane to CO2 on platinum electrodes under ambient conditions through a combination of experimentation, density functional theory (DFT) and density functional analysis.
Abstract: Herein, we investigate the electrochemical conversion of methane to CO2 on platinum electrodes under ambient conditions. Through a combination of experimentation, density functional theory (DFT) ca...
TL;DR: In this article, the authors investigated the basal vacancy on transition metal dichalcogenides (TMDs) for CO2 reduction and found that the change of oxophicility and carbophilicity on each group of TMDs follows different trends, which leads to different scaling relations amongst key intermediates.
Abstract: Transition metal dichalcogenides (TMDs) have shown promising electrocatalytic performance for CO2 reduction (CO2R) recently. However, the development of efficient and selective catalysts remains a major challenge. Although recent studies have suggested the importance of activation energies as activity descriptors for CO2R beyond CO, the scaling of intermediate binding energies presents the first step in computational catalyst screening. Here, we investigate the basal vacancy on 2H and 1T/1T′ phase group V, VI, and X TMDs for CO2 reduction. We find that the change of oxophicility and carbophilicity on each group of TMDs follows different trends, which leads to different scaling relations amongst key intermediates. Our thermochemical analysis also suggests group V and VI TMDs to be either selective for hydrogen evolution reaction or prone to OH poisoning. However, the initial analysis suggests group X TMDs to be possible candidates for active and selective CO2 reduction without suffering from OH poisoning, ...
TL;DR: In this article, a descriptor based micro-kinetic model of the trends in selectivity and activity of non-oxidative dehydrogenation of ethane over close-packed metal facets and through varied reaction conditions is presented.
01 Jan 2019
TL;DR: The authors investigate the solvated proton at the electrochemical interface and show that it unexpectedly carries a fractional charge.
Abstract: A detailed atomic-scale description of the electrochemical interface is essential to the understanding of electrochemical energy transformations. In this work, we investigate the charge of solvated protons at the Pt(111) | H2O and Al(111) | H2O interfaces. Using semi-local density-functional theory as well as hybrid functionals and embedded correlated wavefunction methods as higher-level benchmarks, we show that the effective charge of a solvated proton in the electrochemical double layer or outer Helmholtz plane at all levels of theory is fractional, when the solvated proton and solvent band edges are aligned correctly with the Fermi level of the metal (EF). The observed fractional charge in the absence of frontier band misalignment arises from a significant overlap between the proton and the electron density from the metal surface, and results in an energetic difference between protons in bulk solution and those in the outer Helmholtz plane.
TL;DR: In this paper, the effect of oxygen poisoning due to trace water content in the input gas stream has been investigated, and it has been shown that using a weaker-binding catalyst is one way to avoid oxygen poisoning if it is impractical to remove all water from the reactor.
TL;DR: In this article, the authors estimate the rate of electron transfer to CO2 at the Au (211)|water interface during adsorption in an electrochemical environment under reducing potentials.
Abstract: We estimate the rate of electron transfer to CO2 at the Au (211)|water interface during adsorption in an electrochemical environment under reducing potentials. On the basis of density functional th...
24 Oct 2019
TL;DR: In this paper, the Pt-free oxygen reduction reaction (ORR) electrocatalysts have been actively pursued among the current electrocatalyst research community, and the family of transition-metal chalc...
Abstract: Searching for efficient Pt-free oxygen reduction reaction (ORR) electrocatalysts has been actively pursued among the current electrocatalyst research community. The family of transition-metal chalc...
TL;DR: In this article, the authors presented detailed characterization and activity of silica-supported cobalt catalysts modified by atomic layer deposition of ZnO. The prepared catalysts have up to 46% selectivity toward alcohols with 39% of the alcohols corresponding to ethanol and other higher alcohols, albeit with reduced activity.
Abstract: Controlling selectivity is a key goal in the design of a heterogeneous catalyst. Herein, we present detailed characterization and activity of silica‐supported cobalt catalysts modified by atomic layer deposition of ZnO. After reduction, the resulting catalysts exhibit substantial selectivity towards alcohol production during CO hydrogenation compared to catalysts containing only cobalt. The prepared catalysts have up to 46 % selectivity toward alcohols with 39 % of the alcohols corresponding to ethanol and other higher alcohols, albeit with reduced activity. In situ characterization of the catalyst by X‐ray diffraction and X‐ray absorption spectroscopy reveals details on the structural evolution in syngas, CO+H2, and shows that ZnO promotion of Co results in the formation of Co2C under catalytic conditions. A mechanism is proposed, supported by density functional theory calculations, which explains Co2C formation by the blocking of Co step sites by Zn species. The ZnO acts a dual promoter both by facilitating Co2C formation and by modifying the resulting Co2C. The Co2C formed from the ZnO‐promoted Co catalysts displays improved thermal stability and selectivity compared with similar Co2C catalysts without Zn.
TL;DR: It is demonstrated that noble-metal supports lead to bifunctional enhancement of both the stability and the oxygen reduction reaction (ORR) activity of metal (oxy-hydro) oxide nanoislands, hence boosting ORR activity.
Abstract: Developing cost-effective oxygen electrocatalysts with high activity and stability is key to their commercialization. However, economical earth-abundant catalysts based on first-row transition-metal oxides suffer from low electrochemical stability, which is difficult to improve without compromising their activity. Here, using density functional theory calculations, we demonstrate that noble-metal supports lead to bifunctional enhancement of both the stability and the oxygen reduction reaction (ORR) activity of metal (oxy-hydro) oxide nanoislands. We observe a significant stabilization of supported nanoislands beyond the intrinsic stability limits of bulk phases, which originates from a favorable lattice mismatch and reductive charge transfer from oxophilic supports. We discover that interfacial active sites (located between the nanoisland and the support) reinforce the binding strength of reaction intermediates, hence boosting ORR activity. Considering that both stability and activity lead to discovery of CoOOH|Pt, NiOOH|Ag, and FeO2|Ag as viable systems for alkaline ORR, we then use a multivariant linear regression method to identify elementary descriptors for efficient screening of promising cost-effective nanoisland|support catalysts.
TL;DR: In this article, the authors present a method for constructing multi-element Pourbaix diagrams which uses a pre-processing step to circumvent the most egregious computational bottleneck and make many-element pourbaix diagram computationally efficient.
Abstract: Pourbaix diagrams have long been an essential tool for determining the phase stability of solids and their associated ionic species under electrochemical conditions. In recent years, Pourbaix diagrams have been used for applications ranging from corrosion-resistance alloy design to electrocatalysis, and the data from which they are generated has been enhanced by the availability of materials data in various online databases. However, generation of multi-element Pourbaix diagrams has a critical bottleneck which makes 3-element systems difficult to analyze quickly, and 4 and 5 element systems intractable. In this work, we present a method for constructing Pourbaix diagrams which uses a pre-processing step to circumvent the most egregious computational bottleneck and make many-element Pourbaix diagrams computationally efficient.
TL;DR: An Amendment to this paper has been published and can be accessed via a link at the top of the paper.
Abstract: An Amendment to this paper has been published and can be accessed via a link at the top of the paper.