Prof. Dr. Kai Exner

Theoretical Inorganic Chemistry
University of Duisburg-Essen

Contact

Hub
  • Circumventing the OCl versus OOH scaling relation in the chlorine evolution reaction: Beyond dimensionally stable anodes
    Exner, K.S. and Lim, T. and Joo, S.H.
    Current Opinion in Electrochemistry 34 (2022)
    The development of selective electrocatalysts for the chlorine evolution reaction (CER) is majorly restrained by a scaling relation between the OCl and OOH adsorbates, rendering that active CER catalysts are also reasonably active in the competing oxygen evolution reaction (OER). While theory predicts that the OCl versus OOH scaling relation can be circumvented as soon as the elementary reaction steps in the CER comprise the Cl rather than the OCl adsorbate, it was demonstrated recently that PtN4 sites embedded in a carbon nanotube follow this theoretical prediction. Advanced experimental analyses illustrate that the PtN4 sites also reveal a different reaction kinetics compared to the industrial benchmark of dimensionally stable anodes (DSA). A reverse Volmer–Heyrovsky mechanism was identified, in which the rate-determining Volmer step for small overpotentials is followed by the kinetically limiting Heyrovsky step for larger overpotentials. Since the PtN4 sites excel DSA in terms of activity and chlorine selectivity, we suggest the Cl intermediate as well as the reverse Volmer–Heyrovsky mechanism as the design criteria for the development of next-generation electrode materials beyond DSA. © 2022 The Author(s)
    view abstract10.1016/j.coelec.2022.100979
  • Method to Determine the Bifunctional Index for the Oxygen Electrocatalysis from Theory
    Razzaq, S. and Exner, K.S.
    ChemElectroChem 9 (2022)
    Metal-air batteries are encountered as a promising solution for energy storage due to their high energy density, cost effectiveness, and environmental benefits. Yet, the application of metal-air batteries in practice is still not mature, which is also related to the bifunctional oxygen electrocatalysis at the cathode, comprising the oxygen reduction (ORR) and oxygen evolution (OER) reactions during discharge and charge of the battery, respectively. Experimentally, the performance of electrocatalysts in the OER and ORR is described by bifunctional index (BI), but, so far, there is no direct approach to capture the BI on the atomic scale. Herein, we present a method to ascertain the BI from ab initio theory, thereby combining a data-driven methodology with thermodynamic considerations and microkinetic modeling as a function of the applied overpotential. Our approach allows deriving the BI from simple adsorption free energies, which are easily accessible to electronic structure theory in the density functional theory (DFT) approximation. We outline how our methodology may steer the design of efficient bifunctional catalysts on the atomic scale. © 2022 The Authors. ChemElectroChem published by Wiley-VCH GmbH
    view abstract10.1002/celc.202101603
  • Statistical analysis of breaking scaling relation in the oxygen evolution reaction
    Razzaq, S. and Exner, K.S.
    Electrochimica Acta 412 (2022)
    The application of proton exchange membrane electrolyzers in practice to produce gaseous hydrogen as energy vector is majorly hampered by the sluggish oxygen evolution reaction (OER) at the anode. On the atomic scale, a scaling relation between the OH and OOH intermediates has been recognized as main limitation for the development of highly active OER catalysts. Breaking scaling relation is considered as a universal remedy to obtain OER materials with enhanced electrocatalytic activity. While it is a well-accepted paradigm that the optimum OER catalyst reveals a symmetric thermodynamic free-energy landscape, recently, it was suggested that the thermodynamic ideal may correspond to a free-energy landscape with asymmetric shape, allowing thermoneutral stabilization of the key intermediate at the target overpotential. In the present manuscript, we analyze breaking scaling relation in the OER to the symmetric and asymmetric thermodynamic free-energy landscapes by statistical methods at different applied overpotentials. Our analysis reveals that breaking scaling relation to the asymmetric rather than to the symmetric picture is statistically more significant as soon as an overpotential is applied, calling for a change in mindset when thermodynamic considerations are used for catalyst optimization. © 2022
    view abstract10.1016/j.electacta.2022.140125
  • A Universal Approach to Quantify Overpotential-Dependent Selectivity Trends for the Competing Oxygen Evolution and Peroxide Formation Reactions: A Case Study on Graphene Model Electrodes
    Ivanova, A. and Chesnokov, A. and Bocharov, D. and Exner, K.S.
    Journal of Physical Chemistry C 125 (2021)
    In this article, we study the competing oxygen evolution and hydrogen peroxide (H2O2) formation reactions for periodic models of graphene with different active-site concentrations by means of density functional theory (DFT) calculations. Linking the DFT calculations to ab-initio thermodynamic considerations in conjunction with microkinetic modeling enables gaining deep insights into the activity and selectivity trends of graphene-based electrodes as a function of applied bias. We illustrate that both the coverage of intermediates on the electrode surface and the applied electrode potential have a significant effect on the Faradaic efficiency for the electrocatalytic production of H2O2. The presented approach to study overpotential-dependent selectivity trends allows deriving design criteria for peroxide formation, which may serve as a guideline for further studies to realize selective formation of H2O2 using carbon-based materials. © 2021 American Chemical Society.
    view abstract10.1021/acs.jpcc.1c03323
  • Boosting the Stability of RuO2 in the Acidic Oxygen Evolution Reaction by Tuning Oxygen-Vacancy Formation Energies: A Viable Approach Beyond Noble-Metal Catalysts?
    Exner, K.S.
    ChemElectroChem 8 (2021)
    RuO2 belongs to the most active electrode materials for the anodic oxygen evolution reaction (OER) within the electrochemical water splitting, such as those encountered in acidic proton-exchange membrane (PEM) electrolyzers. Despite its large activity, RuO2 faces severe stability issues under the harsh anodic operation conditions. Now, a new strategy has been reported to overcome this bottleneck by tuning the free-formation energy of oxygen vacancies, which can be achieved by the co-doping of W and Er into the RuO2 lattice. The resulting W0.2Er0.1Ru0.7O2-δ electrocatalyst is stable long term in acid and, additionally, reveals remarkable OER activity, about 30 times higher than that of commercial RuO2. The notion of tuning the oxygen-vacancy formation energy could be a valuable starting point for the development of non-noble electrocatalysts for the acidic OER with applications in PEM electrolyzers. © 2020 Wiley-VCH GmbH
    view abstract10.1002/celc.202001465
  • Characterization of Ti electrodes electrophoretically coated with IrO2-Ta2O5 films with different Ir:Ta molar ratios
    Herrada, R.A. and Rodil, S.E. and Sepúlveda-Guzmán, S. and Manríquez, J. and Exner, K.S. and Bustos, E.
    Journal of Alloys and Compounds 862 (2021)
    Titanium electrodes coated with transition metal oxides are used in many applications such as the electrogeneration of hydroxyl radicals (•OH) via catalytic water oxidation. Here, we present electrochemical measurements of IrO2-Ta2O5|Ti electrodes with four different Ir:Ta ratios in the coating (100:0, 70:30, 30:70, 0:100) to better understand their electrochemical behavior. From the results, an Ir:Ta content of 70:30 reveals a homogeneous morphology, an outstanding mechanical stability, and the best generation of •OH radicals due to a cooperative enhancement of the electrocatalytic and proton (H+) transfer properties of IrO2 and Ta2O5, which are complemented by a small ohmic drop due to a junction established during the electrophoretic deposition of IrO2 and Ta2O5. The electrochemical data in this work in conjunction with density functional theory calculations provide in-depth insights into the outstanding electrocatalytic properties of the as-prepared IrO2-Ta2O5|Ti electrodes, which may have applications in environmental processes. © 2020 Elsevier B.V.
    view abstract10.1016/j.jallcom.2020.158015
  • Editorial: Material and Composition Screening Approaches in Electrocatalysis and Battery Research
    Kadyk, T. and Xiao, J. and Ooka, H. and Huang, J. and Exner, K.S.
    Frontiers in Energy Research 9 (2021)
    view abstract10.3389/fenrg.2021.699376
  • General Efficacy of Atomically Dispersed Pt Catalysts for the Chlorine Evolution Reaction: Potential-Dependent Switching of the Kinetics and Mechanism
    Lim, T. and Kim, J.H. and Kim, J. and Baek, D.S. and Shin, T.J. and Jeong, H.Y. and Lee, K.-S. and Exner, K.S. and Joo, S.H.
    ACS Catalysis 11 (2021)
    The electrochemical chlorine evolution reaction (CER) is a key anodic reaction in the chlor-alkali process for Cl2production, on-site generation of ClO-, and Cl2-mediated electrosynthesis. Although Ru-based mixed metal oxides have long been used as CER catalysts, they suffer from a selectivity problem due to the competing oxygen evolution reaction. To overcome this shortcoming, we have developed a new CER catalyst composed of atomically dispersed Pt-N4sites on carbon nanotubes (Pt1/CNT). In this study, we demonstrate that the catalytically active Pt-N4sites can be constructed from H2PtCl6·6H2O and an ionic liquid via a bottom-up approach and a Pt-porphyrin-driven top-down method. Both catalysts exhibit excellent CER activity and remarkable selectivity, demonstrating the general efficacy of Pt1/CNT for the CER. The electrochemical and in situ X-ray absorption spectroscopy analyses reveal that Pt1/CNT catalysts show a reaction order of ∼1.8 in the low overpotential regime, where the Volmer step is reconciled with the rate-determining step (RDS). Interestingly, in the high overpotential region, the CER over Pt1/CNT proceeds with a lower reaction order and the RDS switches to the Heyrovský step. These unprecedented kinetic insights are clearly distinguished from the oxide-based CER catalysts with the opposite sequence of the RDS. © 2021 American Chemical Society
    view abstract10.1021/acscatal.1c03893
  • Hydrogen electrocatalysis revisited: Weak bonding of adsorbed hydrogen as the design principle for active electrode materials
    Exner, K.S.
    Current Opinion in Electrochemistry 26 (2021)
    Hydrogen electrocatalysis has been spurred by theoretical predictions, using simple ab initio thermodynamic considerations, in that the free-binding energy of adsorbed hydrogen has been applied in a heuristic fashion to search for sustainable electrocatalysts as a replacement for scarce platinum in electrolyzers and fuel cells. The original volcano model of Nørskov et al. is given in [14] purports that the optimum hydrogen-evolution catalyst binds adsorbed hydrogen thermoneutrally at zero overpotential, a paradigm based on pure thermodynamic considerations. Recently, the Sabatier principle was revisited by factoring the applied overpotential and kinetics into the analysis. The extended Sabatier principle suggests that the optimum hydrogen-evolution catalyst binds adsorbed hydrogen weakly rather than thermoneutrally. This notion is corroborated by the fact that the most active hydrogen-evolution catalysts, Pt, MoS2, or Mo2C, indeed bind hydrogen weakly by about (100–200) meV rather than thermoneutrally at zero overpotential. © 2020 Elsevier B.V.
    view abstract10.1016/j.coelec.2020.100673
  • Method to Construct Volcano Relations by Multiscale Modeling: Building Bridges between the Catalysis and Biosimulation Communities
    Exner, K.S. and Ivanova, A.
    Journal of Physical Chemistry B 125 (2021)
    Understanding the complex interactions of different building blocks within a sophisticated drug-delivery system (DDS), aimed at targeted transport of the drug to malignant cells, requires modeling techniques on different time and length scales. On the example of the anthracycline antibiotic doxorubicin (DOX), we investigate a potential DDS component, consisting of a gold nanoparticle and a short peptide sequence as carriers of DOX. The combination of atomistic molecular dynamics simulations and density functional theory calculations facilitates compiling a volcano plot, which allows deriving general conclusions on DDS constituents for chemotherapeutic agents within the class of anthracycline antibiotics: the nanoparticle and peptide carrier moieties need to be chosen in such a way that the anthracycline body of the drug is able to intercalate between both entities or between two (π-stacking) residues of the peptide. Using the popular volcano framework as a guideline, the present article connects the catalysis and biosimulation communities, thereby identifying a strategy to overcome the limiting volcano relation by tuning the coordination number of the drug in the DDS component. ©
    view abstract10.1021/acs.jpcb.1c00836
  • On the Lattice Oxygen Evolution Mechanism: Avoiding Pitfalls
    Exner, K.S.
    ChemCatChem 13 (2021)
    The oxygen evolution reaction (OER) is often designated as the enigma in water electrolysis because the development of active and stable OER catalysts is a challenging and formidable task. While ab initio theory in the density functional theory approximation initially focused on the mechanistic description via the OH, O, and OOH adsorbates, in recent years the lattice oxygen evolution reaction (LOER) mechanism attracted increasing attention, given that the LOER is seen as the main reason for catalyst instability under anodic potential conditions. The present concept article critically analyzes the LOER and indicates pitfalls in the interpretation of this mechanistic pathway. A method to assess the energetics of the LOER in relation to conventional OER mechanisms by the compilation of free-energy diagrams is introduced, which may contribute to enhance our understanding of the competing LOER and OER on the atomic scale. Further works are urgently needed to comprehend the interrelationship for the evolution of gaseous oxygen from the electrolyte or the crystal lattice. © 2021 The Authors. ChemCatChem published by Wiley-VCH GmbH
    view abstract10.1002/cctc.202101049
  • The electrochemical-step asymmetry index
    Exner, K.S.
    MethodsX 8 (2021)
    The development of oxygen-evolution reaction (OER) electrocatalysts has been spurred by thermodynamic considerations on the free-energy landscape. Most commonly, electrocatalytic activity is approximated by the analysis of the free-energy changes among the mechanistic description, thereby taking only reaction steps with weak-binding adsorbates into account. Herein, a new method, denoted as the electrochemical-step asymmetry index (ESAI), is presented, which approximates electrocatalytic activity by penalizing both too strong as well as too weak bonding of intermediate states in order to mimic the well-known Sabatier principle. • The electrochemical-step asymmetry index (ESAI) is a descriptor to approximate electrocatalytic activity based on the analysis of the free-energy changes for a given mechanistic description, exemplified by the oxygen evolution reaction (OER). • The concept of the ESAI is based on the assumption that the optimum free-energy landscape has an asymmetric shape because this may factor overpotential and kinetic effects in the analysis, and the ESAI penalizes both too strong as well as too weak bonding of intermediate states to render a thorough representation of the Sabatier principle feasible. • The ESAI is a conceptual development of the earlier proposed electrochemical-step symmetry index (ESSI), which relies on a symmetric distribution of the free-energy changes as thermodynamic optimum and which takes only weak-binding adsorbates into account. © 2021
    view abstract10.1016/j.mex.2021.101590
  • The Sabatier Principle in Electrocatalysis: Basics, Limitations, and Extensions
    Ooka, H. and Huang, J. and Exner, K.S.
    Frontiers in Energy Research 9 (2021)
    The Sabatier principle, which states that the binding energy between the catalyst and the reactant should be neither too strong nor too weak, has been widely used as the key criterion in designing and screening electrocatalytic materials necessary to promote the sustainability of our society. The widespread success of density functional theory (DFT) has made binding energy calculations a routine practice, turning the Sabatier principle from an empirical principle into a quantitative predictive tool. Given its importance in electrocatalysis, we have attempted to introduce the reader to the fundamental concepts of the Sabatier principle with a highlight on the limitations and challenges in its current thermodynamic context. The Sabatier principle is situated at the heart of catalyst development, and moving beyond its current thermodynamic framework is expected to promote the identification of next-generation electrocatalysts. © Copyright © 2021 Ooka, Huang and Exner.
    view abstract10.3389/fenrg.2021.654460
  • Why approximating electrocatalytic activity by a single free-energy change is insufficient
    Exner, K.S.
    Electrochimica Acta 375 (2021)
    Progress in the area of electrocatalysis has been spurred by theoretical predictions, using the free energies of reaction intermediates within the electrocatalytic cycle as a measure to assess electrocatalytic activity. Most commonly, the framework of the thermodynamic overpotential, ηTD, is applied to study activity trends of electrodes in a class of materials. The concept of ηTD, however, relies on the evaluation of a single free-energy change at the equilibrium potential of the reaction, which may explain that the notion of ηTD does not always capture activity trends correctly. To compensate this shortcoming, the electrochemical-step symmetry index (ESSI) was introduced, which accounts for all free-energy changes at the equilibrium potential among the mechanistic description. Yet, both ηTD and the ESSI do not consider overpotential and kinetic effects in the analysis, motivating the introduction of an overpotential-dependent activity descriptor for multiple-electron processes, Gmax(η). In this manuscript, these three descriptors to approximate electrocatalytic activity in a heuristic fashion are compared, elaborating that the assessment of activity by a single free-energy change is too simplistic. © 2021
    view abstract10.1016/j.electacta.2021.137975
  • Why the breaking of the OOH versus OH scaling relation might cause decreased electrocatalytic activity
    Exner, K.S.
    Chem Catalysis 1 (2021)
    In the last decade, tremendous efforts have been dedicated to the breaking of the OOH versus OH scaling relation, which is recognized as the bottleneck for electrocatalysts in the oxygen evolution reaction (OER), the anodic process in water electrolyzers. Breaking the OER scaling relation is seen as a universal remedy to enhance electrocatalytic activity, yet no major progress has so far been achieved in the design of improved OER materials according to this strategy. Introducing kinetics into the thermodynamics-based concept of scaling relations illustrates that the breaking of the OER scaling relation could be accompanied by decreased electrocatalytic activity. As a consequence, it appears imperative to progress the theoretical description of the OER in different directions other than the breaking of this scaling relation. This could include the investigation of competing mechanistic pathways, concerted and decoupled proton-electron transfer steps, or microkinetic considerations in conjunction with machine-learning approaches. © 2021 Elsevier Inc.
    view abstract10.1016/j.checat.2021.06.011
  • Why the optimum thermodynamic free-energy landscape of the oxygen evolution reaction reveals an asymmetric shape
    Exner, K.S.
    Materials Today Energy 21 (2021)
    The development of oxygen evolution reaction (OER) electrocatalysts has been spurred by thermodynamic considerations on the free-energy landscape. It is a common paradigm that the optimum thermodynamic free-energy landscape reveals a symmetric shape in that all reaction intermediates are stabilized at the equilibrium potential of the reaction. However, so far, no OER electrocatalyst has been reported that corresponds to the thermodynamic ideal because of the presence of a linear scaling relationship. Therefore, the common approach builds on the breaking of the scaling relations to establish a catalytic material that is close to the symmetric picture, yet, with minor successes. Relating to the simple two-electron hydrogen evolution reaction (HER), it was recently reported that the optimum thermodynamic free-energy landscape reveals an asymmetric shape rather than a symmetric form as soon as overpotential and kinetic effects are factored in the analysis. This finding motivates scrutinizing whether the symmetric free-energy landscape as the thermodynamic ideal in the OER is justified. Transferring the knowledge from the HER to the OER results in the introduction of the electrochemical-step asymmetry index (ESAI), representing the concept of the asymmetric thermodynamic free-energy diagram. By comparing the ESAI to the symmetric picture in terms of the electrochemical-step symmetry index (ESSI), it is demonstrated herein that the asymmetric rather than the symmetric free-energy landscape corresponds to the thermodynamic ideal. This outcome suggests changing the mindset when applying the concept of free-energy diagrams for the discovery of OER materials by heuristic material-screening techniques. © 2021 The Author(s)
    view abstract10.1016/j.mtener.2021.100831
  • A universal descriptor for the screening of electrode materials for multiple-electron processes: Beyond the thermodynamic overpotential
    Exner, K.S.
    ACS Catalysis 10 (2020)
    view abstract10.1021/acscatal.0c03865
  • Beyond Dimensionally Stable Anodes: Single-Atom Catalysts with Superior Chlorine Selectivity
    Exner, K.S.
    ChemElectroChem 7 (2020)
    Dimensionally stable anodes (DSAs) have been established as anode material in the industrially important chlor-alkali process, owing to their long-term stability, high activity, and reasonable chlorine selectivity under operation conditions. Now, a new material, based on the idea of single-atom catalysis, has been reported, which reveals higher activity and chlorine selectivity than DSAs. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.202000224
  • Beyond thermodynamic-based material-screening concepts: Kinetic scaling relations exemplified by the chlorine evolution reaction over transition-metal oxides
    Exner, K.S.
    Electrochimica Acta 334 (2020)
    State-of-the-art material screening in the field of electrocatalysis mainly uses the concept of linear scaling relationships in order to express the (free) adsorption energies of different reaction intermediates, adsorbed on the surface of a solid-state electrocatalyst, as function of a descriptor. This thermodynamic analysis, based on the application of the computational hydrogen electrode approach (CHE), ultimately results in the construction of a Volcano plot, which facilitates identifying promising catalysts within a class of materials. The conventional ab initio Volcano concept, however, lacks of two critical aspects: on the one hand the applied overpotential, which constitutes the driving force of an electrocatalytic reaction, is not included in the underlying approach, since the thermodynamic analysis refers to the standard equilibrium potential of the electrocatalytic process; on the other hand, the kinetics is not accounted for. Herein, an alternate material-screening concept is presented, which promotes a discussion of the catalytic performance within a class of materials by explicitly including both the kinetic description and applied overpotential: kinetic scaling relations enable resolving the rate-determining reaction step in a homologous series of single-crystalline electrocatalysts in the overpotential regime of interest for practical applications. The proposed methodology is exemplified by the chlorine evolution reaction over transition-metal oxides, which corresponds to the anode reaction in the industrially relevant chlor-alkali process for the production of gaseous chlorine as basic chemical. © 2019 Elsevier Ltd
    view abstract10.1016/j.electacta.2019.135555
  • Comparison of the Conventional Volcano Analysis with a Unifying Approach: Material Screening Based on a Combination of Experiment and Theory
    Exner, K.S.
    Journal of Physical Chemistry C 124 (2020)
    Material screening in electrocatalysis has been spurred by the concept of volcano plots, which rely on analyzing binding energies at zero overpotential. Recently, a unifying material-screening approach was introduced by the author: the combination of first-principles calculations with a single experimental input parameter, that is, the threshold overpotential, at which the experimental Tafel slope exceeds 59 mV/dec in the respective class of materials, results in an ESSI-descriptor activity map. Therein, besides the analysis of binding energies, also the applied overpotential, decisive reaction intermediate, rate-determining reaction step, and electrochemical-step symmetry index (ESSI) are accounted for. On the example of the oxygen evolution reaction over RuO2(110) surfaces, the two different frameworks are compared: while the conventional volcano method relies on a rough screening of the parameter space, as possibly promising surface configurations could be overlooked, the concept of ESSI-descriptor activity maps accounts for a more thorough sorting of electrode materials. Copyright © 2019 American Chemical Society
    view abstract10.1021/acs.jpcc.9b10860
  • Design criteria for the competing chlorine and oxygen evolution reactions: avoid the OCl adsorbate to enhance chlorine selectivity
    Exner, K.S.
    Physical Chemistry Chemical Physics 22 (2020)
    The formation of gaseous chlorine within chlor-alkali electrolysis is accompanied by a selectivity problem, as the evolution of gaseous oxygen constitutes a detrimental side reaction in the same potential range. As such, the development of electrode materials with high selectivity toward the chlorine evolution reaction is of particular importance to the chemical industry. Insight into the elementary reaction steps is ultimately required to comprehend chlorine selectivity on a molecular level. Commonly, linear scaling relationships are analyzed by the construction of a volcano plot, using the binding energy of oxygen, ΔEO, as a descriptor in the analysis. The present article reinvestigates the selectivity problem of the competing chlorine and oxygen evolution reactions by applying a different strategy compared to previous literature studies. On the one hand, a unifying material-screening framework that, besides binding energies, also includes the applied overpotential, kinetics, and the electrochemical-step symmetry index is used to comprehend trends in this selectivity issue for transition-metal oxide-based electrodes. On the other hand, the free-energy difference between the adsorbed oxygen and adsorbed hydroxide, ΔG2, rather than ΔEO is used as a descriptor in the analysis. It is demonstrated that the formation of the OCl adsorbate within the chlorine evolution reaction inherently limits chlorine selectivity, whereas, in the optimum case, the formation of the Cl intermediate can result in significantly higher chlorine selectivity. This finding is used to derive the design criteria for highly selective chlorine evolution electrocatalysts, which can be used in the future to search for potential electrode compositions by material-screening techniques. This journal is © 2020 the Owner Societies.
    view abstract10.1039/d0cp03667f
  • Does a Thermoneutral Electrocatalyst Correspond to the Apex of a Volcano Plot for a Simple Two-Electron Process?
    Exner, K.S.
    Angewandte Chemie - International Edition 59 (2020)
    Volcano analyses have been established as a standard tool in the field of electrocatalysis for assessing the performance of electrodes in a class of materials. The apex of the volcano curve, where the most active electrocatalysts are situated, is commonly defined by a hypothetical ideal material that binds its reaction intermediates thermoneutrally at zero overpotential, in accordance with Sabatier's principle. However, recent studies report a right shift of the apex in a volcano curve, in which the most active electrocatalysts bind their reaction intermediates endergonically rather than thermoneutrally at zero overpotential. Focusing on two-electron process, this Viewpoint addresses the question of how the definition of an optimum catalyst needs to be modified with respect to the requirements of Sabatier's principle when kinetic effects and the applied overpotential are included in the analysis. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.202003688
  • Electrolyte Engineering as a Key Strategy Towards a Sustainable Energy Scenario?
    Exner, K.S.
    ChemElectroChem 7 (2020)
    Major challenges still need to be resolved on the way towards a sustainable energy scenario. A future vision comprises the development of electrocatalysts, refraining from using scarce noble metals, for electrocatalytic key processes, such as hydrogen and oxygen evolution and reduction reactions or CO2 reduction. Hitherto, the focus was set on the investigation of electrode materials, whereas only little emphasis was put on the electrolyte solution. Recently, it was reported that, under non-acidic pH conditions, the composition of the electrolyte solution has a non-negligible effect on the activity of electrocatalytic processes: this outcome puts forth the idea of electrolyte engineering, a promising strategy that, besides the study of enhanced electrode materials, should be put in the focus of future research investigations in electrocatalysis. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201902009
  • Identifying a gold nanoparticle as a proactive carrier for transport of a doxorubicin-peptide complex
    Exner, K.S. and Ivanova, A.
    Colloids and Surfaces B: Biointerfaces 194 (2020)
    Efficient drug delivery to malignant cells in the human organism requires the application of drug-delivery systems (DDS) that consist of several building blocks, such as a biomolecule to bind the drug as well as a carrier for transport. In the present study, we investigate a potential DDS component for the cytostatic doxorubicin (DOX) that consists of a gold nanoparticle (Au-NP) and a short drug-binding peptide sequence. Combining molecular dynamics simulations with density functional theory calculations allows resolving the adsorption configurations of DOX at simulated physiological conditions as well as the interaction energies between the building blocks of the DDS. Interestingly, it turns out that the task of the Au-NP is not limited to being a passive carrier. The nanoparticle is directly involved in the stabilization of the drug by intercalating DOX together with a tryptophan residue from the peptide. Another favored adsorption configuration corresponds to an intercalation complex of DOX with two tryptophan residues, reminiscent of the intercalation of DOX between DNA bases. The insights gained in the present study allow deriving general conclusions about the surface chemistry of DOX: its tendency to intercalate seems not to depend on its π-stacking partners (organic or inorganic), as long as they can be properly arranged around the drug. Hence, DOX may be stabilized sufficiently during its delivery if intercalation within the carrier moieties is possible. This finding may assist the construction of a more complex DDS for DOX in the future, for which the investigated drug-peptide-nanoparticle conjugate may serve as a prototype. © 2020 Elsevier B.V.
    view abstract10.1016/j.colsurfb.2020.111155
  • Overpotential-Dependent Volcano Plots to Assess Activity Trends in the Competing Chlorine and Oxygen Evolution Reactions
    Exner, K.S.
    ChemElectroChem 7 (2020)
    The selectivity problem of the competing chlorine evolution (CER) and oxygen evolution (OER) reactions at the anode in chlor−alkali electrolysis is a major challenge in the chemical industry. The development of electrode materials with enhanced stability and CER selectivity could result in a significant reduction of the overall process costs. In order to gain an atomic-scale understanding of the CER versus OER selectivity, commonly, density functional theory (DFT) calculations are employed that are analyzed by the construction of a volcano plot to comprehend trends. Herein, the binding energy of oxygen, ΔEO, has been established as a descriptor in such analyses. In the present article, it is demonstrated that ΔEO is not suitable to assess activity trends in the OER over transition-metal oxides, such as RuO2(110) and IrO2(110). Quite in contrast, the free-formation energy of oxygen with respect to hydroxide, ΔGO−OH, reproduces activity trends of RuO2(110) and IrO2(110) in the CER and OER correctly. Consequently, re-investigation of the CER versus OER selectivity issue, using ΔGO−OH as a descriptor, is strongly suggested. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.202000120
  • Paradigm change in hydrogen electrocatalysis: The volcano's apex is located at weak bonding of the reaction intermediate
    Exner, K.S.
    International Journal of Hydrogen Energy 45 (2020)
    Volcano plots are a powerful tool to screen electrode materials in the catalysis and battery science communities. Commonly, simple binding energies are analyzed by the concept of linear scaling relationships to describe activity trends in a homologous series of materials, putting forward the picture that an optimum electrode material in the hydrogen evolution reaction (HER) binds the reaction intermediate (RI) thermoneutrally at zero overpotential. This approach, however, consists of various oversimplifications since the applied overpotential and kinetics are not accounted for in the evaluation. In the present article, the apex of the HER volcano is modeled by microkinetics. It is demonstrated that the volcano's top shifts to weak bonding of the RI with increasing driving force as soon as kinetic effects are factored in the analysis. This paradigm change is corroborated by the fact that the constructed volcano plots, using microkinetics and scaling relations for the apex and legs of the volcano respectively, reproduce the high activities of Pt in the HER and RuO2 in the chlorine evolution reaction. © 2020 Hydrogen Energy Publications LLC
    view abstract10.1016/j.ijhydene.2020.07.088
  • Recent Progress in the Development of Screening Methods to Identify Electrode Materials for the Oxygen Evolution Reaction
    Exner, K.S.
    Advanced Functional Materials 30 (2020)
    The oxygen evolution reaction (OER) limits the performance of proton-exchange membrane electrolyzers since substantial overpotentials of several hundred millivolts are required for the formation of gaseous oxygen at the anode to reach satisfying current densities. Theoreticians trace this to the occurrence of a linear scaling relationship between the OH and OOH adsorbates within the electrocatalytic OER cycle, which thermodynamically restrains this four-electron process. While commonly the breaking of this particular scaling relation is pursued as a promising strategy to enhance catalytic turnover, the present progress report summarizes recent trends in the screening of electrode materials for the OER aside this notion. This contains an extension of thermodynamic-based screening methods by including the kinetics, applied overpotential, and the electrochemical-step symmetry index into the analysis, enabling material screening within a unifying methodology, or material screening by molecular orbital principles and band theories. The combination of activity-based screening methods with a proper assessment of catalyst stability may aid the further search of electrode materials for the OER in the future. © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adfm.202005060
  • Universality in Oxygen Evolution Electrocatalysis: High-Throughput Screening and a Priori Determination of the Rate-Determining Reaction Step
    Exner, K.S.
    ChemCatChem 12 (2020)
    Material screening is commonly based on the assessment of linear scaling relations that translate to a Volcano curve in order to identify promising electrode materials, situated at the apex of the Volcano. Recently, an advanced material-screening approach, entitled ESSI-descriptor activity maps, has been introduced by the author. This methodology goes beyond the thermodynamic framework of Volcano plots, as the concept of ESSI-descriptor activity maps evaluates, besides binding energies, the kinetics, applied overpotential, and catalytic symmetry in terms of the electrochemical-step symmetry index (ESSI). Herein, the concept of ESSI-descriptor activity maps is exerted to the oxygen evolution reaction (OER) to derive universal relationships. In contrast to the common procedure in the literature, a suitable range of values for the linear scaling relation's offset is a priori included in the presented model, which enables high-throughput screening of OER catalysts and determination of the rate-determining reaction step (rds) at overpotentials corresponding to typical reaction conditions (ηOER=0.40 V). © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cctc.201902363
  • Activity-Stability Volcano Plots for Material Optimization in Electrocatalysis
    Exner, K.S.
    ChemCatChem 11 (2019)
    In the last two decades, research in electrocatalysis has been spurred by theoretical calculations and predictions based on the concept of ab initio thermodynamics, which has become a valuable tool for computational researchers in material screening. These investigations most commonly result into the construction of activity-based volcano plots in order to predict potential electrocatalysts for the application in practice. However, the prototypical activity-based volcano concept neither captures the influence of the applied overpotential on the activity nor the stability of electrode surfaces. Herein, the well-established volcano approach is expanded by constructing activity-stability volcano plots, which, beside the activity, also enclose the stability and the applied overpotential into the analysis. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cctc.201900500
  • Beyond the Rate-Determining Step in the Oxygen Evolution Reaction over a Single-Crystalline IrO2(110) Model Electrode: Kinetic Scaling Relations
    Exner, K.S. and Over, H.
    ACS Catalysis 9 (2019)
    Electrochemical water splitting is a key technology for moving toward a promising energy scenario based on renewable (regenerative) energy resources in that wind and solar energy can be stored and buffered in chemical bonds, such as in H2. The efficiency of water electrolysis is, however, limited by the sluggish oxygen evolution reaction (OER) at the anode, for which IrO2-based electrodes are considered to be the best compromise of a stable and reasonably active OER electrocatalyst in acidic medium. To improve existing OER electrocatalysts and to advance a rational search of promising alternative electrode materials, it is imperative to identify the rate-determining step (rds). We apply here the concept of the free energy diagram along the reaction coordinate to identify the rate-determining step (rds) in the oxygen evolution reaction (OER) over an IrO2(110) model anode in both acidic and basic media. The free energy diagram as a function of the applied electrode potential is constructed from experimental Tafel plots and ab initio Pourbaix diagrams. Quite in contrast to common perception, the rds for the OER over IrO2(110) at high overpotentials is identified with the decomposition of the OOH adsorbate via a decoupled electron-proton transfer to form gaseous O2. Combining linear scaling relationships with the free energy diagram approach leads to the introduction of kinetic scaling relations, which allow us to predict the rate-determining step (rds) of the OER over general transition metal oxide electrocatalysts in the high-overpotential regime by a single descriptor, namely, the free formation energy of oxygen with respect to the OH adsorbate (Î"G2) on the anode surface. On the basis of kinetic scaling relations we suggest that further improvement of the catalytic OER performance may require a decoupling of the electron-proton transfer in the rds. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acscatal.9b01564
  • Beyond the Traditional Volcano Concept: Overpotential-Dependent Volcano Plots Exemplified by the Chlorine Evolution Reaction over Transition-Metal Oxides
    Exner, K.S.
    Journal of Physical Chemistry C 123 (2019)
    The chlorine evolution reaction (CER) over a single-crystalline RuO2(110) model electrode is one of the best understood model systems in the field of electrocatalysis, which is taken here as a benchmark system to advance the concept of activity-based Volcano plots. Volcano curves can be derived from linear scaling relationships, in which thermodynamic considerations based on Sabatier's principle and the Brønsted-Evans-Polanyi relation at zero overpotential are assumed to describe activity trends of electrocatalysts within a homologous series of materials. However, the underlying approach does not capture the influence of the applied overpotential on the activity, which is given by the Tafel slope. This may explain, why in certain cases the traditional Volcano analysis at zero overpotential does not reproduce activity trends of highly active catalytic materials with an overpotential-dependent Tafel slope correctly. Herein, a novel approach of overpotential-dependent Volcano plots is presented, which connects thermodynamics with kinetics at the respective target overpotential and includes the experimental Tafel slope into the analysis to describe the activity. This methodology is applied to the CER over transition-metal oxide electrodes, such as RuO2(110) and IrO2(110): while the traditional Volcano analysis at zero overpotential ascertains IrO2(110) to be more active in the CER, the overpotential-dependent Volcano plot reproduces the experimentally observed higher CER activity of RuO2(110) compared to IrO2(110) qualitatively as well as quantitatively. This result puts additional emphasis on the fact that the applied overpotential needs to be accounted for in material screening trend studies. © 2019 American Chemical Society.
    view abstract10.1021/acs.jpcc.9b05364
  • Controlling Stability and Selectivity in the Competing Chlorine and Oxygen Evolution Reaction over Transition Metal Oxide Electrodes
    Exner, K.S.
    ChemElectroChem 6 (2019)
    Chlor-alkali electrolysis is a large-scale industrial process, where the evolution of gaseous chlorine (chlorine evolution reaction, CER) at the anode is accompanied by a selectivity problem as the evolution of gaseous oxygen (oxygen evolution reaction, OER) constitutes an undesirable side reaction, which diminishes chlorine selectivity. The active component in the anode material is RuO2 with the (110) facet as most stable surface termination, for which the elementary reaction steps on an atomic scale of the competing CER and OER are already resolved. Here, the selectivity issue and the stability range of a RuO2(110) electrode are explored under CER/OER conditions by combining the kinetic description with surface Pourbaix diagrams and linear scaling relationships. These investigations directly merge into a advanced material screening approach, indicating that the free energy difference between the limiting OOH (OER) and OCl (CER) adsorbate is reconciled as a measure for stability and CER selectivity. This finding supports computational researchers within their search of improved electrode materials based on transition metal oxides for electrocatalytic chlorine formation. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201900834
  • Design criteria for oxygen evolution electrocatalysts from first principles: Introduction of a unifying material-screening approach
    Exner, K.S.
    ACS Applied Energy Materials 2 (2019)
    The oxygen evolution reaction (OER) is the bottleneck in proton-exchange membrane (PEM) electrolyzers as substantial overpotentials are required for the formation of gaseous oxygen at anode side to reach a satisfying current density. In the past years, substantial research investigations were dedicated to search for electrode materials with an ameliorated OER activity. Therein, different frameworks are proposed in the literature. The conventional method based on the computational hydrogen electrode approach relies on an assessment of simple binding energies by deriving linear scaling relationships that translate to a volcano plot at zero overpotential. Recently, the traditional volcano concept was extended, in that the applied overpotential and kinetics were accounted for by deducing overpotential-dependent volcano curves or kinetic scaling relations, respectively. An alternative framework corresponds to the electrochemical-step symmetry index (ESSI), which was suggested as an improved measure within the search of potential OER electrocatalysts. Hitherto, there is no connection between these diverse methods, and it remains elusive which of these approaches is most suitable for material-screening purposes. On the example of the OER over transition-metal oxides, porphyrins, perovskites, metal oxides, and functionalized graphitic materials, a powerful combination of linear scaling relationships, kinetic scaling relations, overpotential-dependent Volcano plots, and ESSI is presented. While the computational costs for this unifying approach are the same as for the traditional volcano analysis, the inclusion of a single experimental input parameter in the underlying framework enables gaining unprecedented insights into catalyst design, thereby considering various aspects, such as binding energies, applied overpotential, decisive reaction intermediate, rate-determining reaction step, and catalytic symmetry. It is suggested to employ the concept of activity maps, as introduced in this contribution, for material-screening studies of multielectron processes in energy and environmental science. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acsaem.9b01480
  • Is Thermodynamics a Good Descriptor for the Activity? Re-Investigation of Sabatier's Principle by the Free Energy Diagram in Electrocatalysis
    Exner, K.S.
    ACS Catalysis (2019)
    The computational hydrogen electrode (CHE) approach has spurred ab initio investigations in the field of electrocatalysis, since the underlying concept enables to quantify free energy changes, ?G (thermodynamics), for the formation of reaction intermediates on an electrocatalyst surface. The connection between thermodynamics and kinetics (activity) is achieved by Sabatier's principle: the optimum situation to realize an active electrocatalyst is ascribed to reaction intermediates that are thermoneutrally bound (?G = 0 eV) at zero overpotential. In order to validate the linkage between thermodynamics and kinetics at zero overpotential for two-electron processes, free energy diagrams as a function of the applied electrode potential are compiled. Herein, the chlorine evolution reaction (CER) over RuO2(110), one of the best understood model systems in electrocatalysis, is used as a starting point for this investigation. It turns out that the connection between thermodynamics and kinetics at zero overpotential does not reproduce activity trends correctly if the Tafel slope is overpotential dependent. Therefore, it appears expedient to include the applied overpotential into the thermodynamic framework: for electrocatalysts with a change in the Tafel slope, it is suggested to employ the absolute free energy change for the formation of a reaction intermediate at respective overpotential ?, |?G(?)|, as thermodynamic descriptor for the kinetics of two-electron processes, which may aid the construction of overpotential-dependent Volcano plots for improved material screening. © 2019 American Chemical Society.
    view abstract10.1021/acscatal.9b00732
  • Recent Advancements Towards Closing the Gap between Electrocatalysis and Battery Science Communities: The Computational Lithium Electrode and Activity–Stability Volcano Plots
    Exner, K.S.
    ChemSusChem 12 (2019)
    Despite of the fact that the underlying processes are of electrochemical nature, electrocatalysis and battery research are commonly perceived as two disjointed research fields. Herein, recent advancements towards closing this apparent community gap by discussing the concepts of the constrained ab initio thermodynamics approach and the volcano relationship, which were originally introduced for studying heterogeneously catalyzed reactions by first-principles methods at the beginning of the 21st century, are summarized. The translation of the computational hydrogen electrode (CHE) approach or activity-based volcano plots to a computational lithium electrode (CLiE) or activity–stability volcano plots, respectively, for the investigation of electrode surfaces in batteries may refine theoretical modeling with the aim that enhancements of the underlying concepts are transferred between the research communities. The presented strategy of developing novel approaches by interdisciplinary research activities may trigger further progress of improved theoretical concepts in the near future. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201900298
  • A short perspective of modeling electrode materials in lithium-ion batteries by the ab initio atomistic thermodynamics approach
    Exner, K.S.
    Journal of Solid State Electrochemistry 22 (2018)
    Atomic-scale insights into the performance of electrode materials in lithium-ion batteries require thermodynamic considerations as first step in order to determine potential surface structures that are relevant for subsequent kinetic studies. Within the last 20 years, research in heterogeneous catalysis as well as in electrocatalysis has been spurred by the ab initio atomistic thermodynamics approach, whose application for electrode materials in lithium-ion batteries is eyed and discussed in this perspective article. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s10008-018-4017-9
  • A Universal Approach to Determine the Free Energy Diagram of an Electrocatalytic Reaction
    Exner, K.S. and Sohrabnejad-Eskan, I. and Over, H.
    ACS Catalysis 8 (2018)
    Extended Tafel plots at various temperatures for an electrocatalyzed reaction and (possibly) its reversed reaction on single-crystalline model electrodes allow for constructing the (essential part of the) free energy surface, in particular the free energies of the transition states (TS). Free energies of the reaction intermediates (RIs) including the chemical nature of active surface sites (S) are hardly accessible to experiment and need therefore to be taken from constrained ab initio thermodynamics calculations. The compact compilation of experimental kinetic data in the form of a free energy diagram enables a critical assessment and validation of theoretical free energy landscapes based on first-principles kinetics. For three prototypical electrocatalyzed reactions, namely the chlorine evolution reaction (CER) and oxygen evolution reaction (OER) over RuO2(110) as well as hydrogen evolution reaction (HER) on Pt(111), we exemplify this universal approach and discuss potential benefits for theoretical modeling of electrocatalyzed reaction. © 2018 American Chemical Society.
    view abstract10.1021/acscatal.7b03142
  • Activity – Stability Volcano Plots for the Investigation of Nano-Sized Electrode Materials in Lithium-Ion Batteries
    Exner, K.S.
    ChemElectroChem 5 (2018)
    In the last two decades, materials design in lithium-ion batteries (LIBs) based on first-principles methods has been spurred mainly by computationally demanding investigations of diffusion pathways, redox mechanisms or activation barriers for lithium-ion migration. However, hitherto an expeditious tool with conceptual simplicity that enables a priori computational screening based on thermodynamic considerations in order to propose potential candidates for the usage as electrode materials in LIBs is missing. Here, a novel method based on the application of Volcano plots from catalysis is introduced which allows assessing lithium intercalation in nano-sized electrode materials of LIBs by means of both activity and stability. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201800838
  • Kinetic study of gold nanoparticles synthesized in the presence of chitosan and citric acid
    Simeonova, S. and Georgiev, P. and Exner, K.S. and Mihaylov, L. and Nihtianova, D. and Koynov, K. and Balashev, K.
    Colloids and Surfaces A: Physicochemical and Engineering Aspects 557 (2018)
    In this work colloidal gold nanoparticles (GNPs) are prepared using a citrate-reduction route, in which citric acid serves as reductive agent for the gold precursor HAuCl4. We demonstrate that a temperature variation on the one hand enables to tune the reaction rate of GNP formation and on the other hand allows modifying the morphology of the resulting metal nanoparticles. The use of chitosan, a biocompatible and biodegradable polymer with a multitude of functional amino and hydroxyl groups, facilitates the simultaneous synthesis and surface modification of GNPs in one pot. The resulting GNPs, which are stabilized by a network of chitosan and ß-ketoglutaric acid units, are characterized by UV–vis spectroscopy, atomic force microscopy (AFM), transmission electron microscopy (TEM) as well as fluorescence correlation spectroscopy (FCS) and reveal an average diameter of about 10 nm at the end of the synthesis. The kinetics of GNP formation is studied by calculating activation parameters based on UV–vis and AFM data such as the apparent activation energy, entropy and free energy applying the concept of the Finke-Watzky model and harmonic transition state theory. © 2018 Elsevier B.V.
    view abstract10.1016/j.colsurfa.2018.02.045
  • Constrained Ab Initio Thermodynamics: Transferring the Concept of Surface Pourbaix Diagrams in Electrocatalysis to Electrode Materials in Lithium-Ion Batteries
    Exner, K.S.
    ChemElectroChem 4 (2017)
    DFT-based ab initio Pourbaix diagrams represent a powerful tool to resolve the stable surface structure of an electrocatalyst under different environmental parameters such as the applied electrode potential and pH. Herein, a general approach for anode and cathode materials in lithium-ion batteries (LIBs) is presented that enables to transfer the concept of surface Pourbaix diagrams from electrocatalysis to electrode materials employed in LIBs. This novel approach is exemplified at the example of the (111) facet for a single-crystalline spinel lithium titanate (LTO) model electrode by combining constrained thermodynamics and density functional theory calculations. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201700754
  • Full Free Energy Diagram of an Electrocatalytic Reaction over a Single-Crystalline Model Electrode
    Exner, K.S. and Sohrabnejad-Eskan, I. and Anton, J. and Jacob, T. and Over, H.
    ChemElectroChem 4 (2017)
    A long-term aim of chemical kinetics is to gain detailed information on the full free energy diagram along the reaction coordinate of electrocatalytic processes such as those encountered in fuel cells, batteries, and industrial electrolysis. We present here a universal approach to construct the free energy landscape of an electrocatalyzed reaction over a single-crystalline model electrode without relying on full kinetics from first principles, a highly computer-resource-demanding approach. The free energies of the transition states are determined by a dedicated evaluation scheme of experimental Tafel plots, whereas ab initio thermodynamics calculations provide the free energies of the reaction intermediates. We exemplified this approach with the chlorine and oxygen evolution reactions over a well-defined RuO2(110) model electrode, both reactions constitute large-scale industrial processes. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201700687
  • Kinetics of Electrocatalytic Reactions from First-Principles: A Critical Comparison with the Ab Initio Thermodynamics Approach
    Exner, K.S. and Over, H.
    Accounts of Chemical Research 50 (2017)
    ConspectusMultielectron processes in electrochemistry require the stabilization of reaction intermediates (RI) at the electrode surface after every elementary reaction step. Accordingly, the bond strengths of these intermediates are important for assessing the catalytic performance of an electrode material. Current understanding of microscopic processes in modern electrocatalysis research is largely driven by theory, mostly based on ab initio thermodynamics considerations, where stable reaction intermediates at the electrode surface are identified, while the actual free energy barriers (or activation barriers) are ignored. This simple approach is popular in electrochemistry in that the researcher has a simple tool at hand in successfully searching for promising electrode materials. The ab initio TD approach allows for a rough but fast screening of the parameter space with low computational cost. However, ab initio thermodynamics is also frequently employed (often, even based on a single binding energy only) to comprehend on the activity and on the mechanism of an electrochemical reaction. The basic idea is that the activation barrier of an endergonic reaction step consists of a thermodynamic part and an additional kinetically determined barrier. Assuming that the activation barrier scales with thermodynamics (so-called Brønsted-Polanyi-Evans (BEP) relation) and the kinetic part of the barrier is small, ab initio thermodynamics may provide molecular insights into the electrochemical reaction kinetics. However, for many electrocatalytic reactions, these tacit assumptions are violated so that ab initio thermodynamics will lead to contradictions with both experimental data and ab initio kinetics.In this Account, we will discuss several electrochemical key reactions, including chlorine evolution (CER), oxygen evolution reaction (OER), and oxygen reduction (ORR), where ab initio kinetics data are available in order to critically compare the results with those derived from a simple ab initio thermodynamics treatment. We show that ab initio thermodynamics leads to erroneous conclusions about kinetic and mechanistic aspects for the CER over RuO2(110), while the kinetics of the OER over RuO2(110) and ORR over Pt(111) are reasonably well described. Microkinetics of an electrocatalyzed reaction is largely simplified by the quasi-equilibria of the RI preceding the rate-determining step (rds) with the reactants. Therefore, in ab initio kinetics the rate of an electrocatalyzed reaction is governed by the transition state (TS) with the highest free energy Grds#, defining also the rate-determining step (rds). Ab initio thermodynamics may be even more powerful, when using the highest free energy of an reaction intermediate Gmax(RI) rather than the highest free energy difference between consecutive reaction intermediates, ΔGloss, as a descriptor for the kinetics. © 2017 American Chemical Society.
    view abstract10.1021/acs.accounts.7b00077
  • Temperature-Dependent Kinetic Studies of the Chlorine Evolution Reaction over RuO2(110) Model Electrodes
    Sohrabnejad-Eskan, I. and Goryachev, A. and Exner, K.S. and Kibler, L.A. and Hensen, E.J.M. and Hofmann, J.P. and Over, H.
    ACS Catalysis 7 (2017)
    Ultrathin single-crystalline RuO2(110) films supported on Ru(0001) are employed as model electrodes to extract kinetic information about the industrially important chlorine evolution reaction (CER) in a 5M concentrated NaCl solution under well-defined electrochemical conditions and variable temperatures. A combination of chronoamperometry (CA) and online electrochemical mass spectrometry (OLEMS) experiments provides insight into the selectivity issue: At pH = 0.9, the CER dominates over oxygen evolution, whereas at pH = 3.5, oxygen evolution and other parasitic side reactions contribute mostly to the total current density. From temperature-dependent CA data for pH = 0.9, we determine the apparent free activation energy of the CER over RuO2(110) to be 0.91 eV, which compares reasonably well with the theoretical value of 0.79 eV derived from first-principles microkinetics. The experimentally determined apparent free activation energy of 0.91 eV is considered as a benchmark for assessing future improved theoretical modeling from first principles. © 2017 American Chemical Society.
    view abstract10.1021/acscatal.6b03415
  • Full Kinetics from First Principles of the Chlorine Evolution Reaction over a RuO2(110) Model Electrode
    Exner, K.S. and Anton, J. and Jacob, T. and Over, H.
    Angewandte Chemie - International Edition 55 (2016)
    Current progress in modern electrocatalysis research is spurred by theory, frequently based on ab initio thermodynamics, where the stable reaction intermediates at the electrode surface are identified, while the actual energy barriers are ignored. This approach is popular in that a simple tool is available for searching for promising electrode materials. However, thermodynamics alone may be misleading to assess the catalytic activity of an electrochemical reaction as we exemplify with the chlorine evolution reaction (CER) over a RuO2(110) model electrode. The full procedure is introduced, starting from the stable reaction intermediates, computing the energy barriers, and finally performing microkinetic simulations, all performed under the influence of the solvent and the electrode potential. Full kinetics from first-principles allows the rate-determining step in the CER to be identified and the experimentally observed change in the Tafel slope to be explained. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201511804
  • Combined experiment and theory approach in surface chemistry: Stairway to heaven?
    Exner, K.S. and Heß, F. and Over, H. and Seitsonen, A.P.
    Surface Science 640 (2015)
    In this perspective we discuss how an intimate interaction of experiments with theory is able to deepen our insight into the catalytic reaction system on the molecular level. This strategy is illustrated by discussing various examples from our own research of surface chemistry and model catalysis. The particular examples were carefully chosen to balance the specific strength of both approaches - theory and experiment - and emphasize the benefit of this combined approach. We start with the determination of complex surface structures, where diffraction techniques in combination with theory are clear-cut. The promoter action of alkali metals in heterogeneous catalysis is rationalized with theory and experiment for the case of CO coadsorption. Predictive power of theory is limited as demonstrated with the apparent activity of chlorinated TiO2(110) in the oxidation of HCl: Even if we know all elementary reaction steps of a catalytic reaction mechanism, the overall kinetics may remain elusive and require the application kinetic Monte Carlo simulations. Catalysts are not always stable under reaction conditions and may chemically transform as discussed for the CO oxidation reaction over ruthenium. Under oxidizing reaction conditions ruthenium transforms into RuO2, a process which is hardly understood on the molecular level. Lastly we focus on electrochemical reactions. Here theory is clearly ahead since spectroscopic methods are not available to resolve the processes at the electrode surface. © 2015 Elsevier B.V.
    view abstract10.1016/j.susc.2015.01.006
  • Ligand Effects and Their Impact on Electrocatalytic Processes Exemplified with the Oxygen Evolution Reaction (OER) on RuO2(110)
    Exner, K.S. and Anton, J. and Jacob, T. and Over, H.
    ChemElectroChem 2 (2015)
    By using the abinitio atomistic thermodynamics approach guided by a DFT-derived volcano curve, we demonstrate that the thermodynamic part of the reaction barrier to the oxygen evolution reaction (OER) over RuO2(110) can be significantly reduced when moderately lowering the free adsorption energy of oxygen to the catalytically active Ru center (Rucus). With the selective replacement of metal sites in the second coordination shell of Rucus, the free oxygen adsorption energy is reduced by about 0.8 and 1.0eV for Cr and Ir, respectively. The weakening of Rucus and oxygen-on-top (RucusOot) bonding results in a substantial decrease in the thermodynamic part of the reaction barrier (Gibbs free-energy loss) by 180meV for Cr and 150meV for Ir. The presented strategy is motivated by homogeneous metal catalysis where dedicated modifications of the ligands are able to tune the catalytic performance of the active metal center. Pick′n′Mix: Based on homogeneous metal catalysis, in which dedicated modifications of the ligands are able to tune the catalytic performance of the active metal center, DFT calculations are used to demonstrate that the thermodynamic part of the reaction barrier (Gibbs free-energy loss) to the oxygen evolution reaction over RuO2(110) can be significantly reduced by selectively substituting the metal sites in the second coordination shell of the active center. Copyright © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201402430
  • Microscopic Insights into the Chlorine Evolution Reaction on RuO2(110): a Mechanistic Ab Initio Atomistic Thermodynamics Study
    Exner, K.S. and Anton, J. and Jacob, T. and Over, H.
    Electrocatalysis 6 (2015)
    The frequently discussed mechanisms for the chlorine evolution reaction (CER)—Volmer–Tafel, Volmer–Heyrovsky, and Krishtalik—are assessed for the case of RuO2 within a mechanistic ab initio thermodynamics approach, employing the concept of Gibbs energy loss. The CER over the fully O-covered RuO2(110) surface, the stable surface configuration under CER conditions, is shown to proceed via the Volmer–Heyrovsky mechanism, i.e., the adsorption and discharge of the chloride ion are followed by the direct recombination of this surface species with a chloride ion from the electrolyte solution. The weak adsorption of the chloride ion on the fully O-covered RuO2(110) surface constitutes the elementary reaction step with highest Gibbs energy loss which has its origin in a too strong ruthenium–oxygen bond. Therefore, the activity of the model catalyst RuO2(110) can be enhanced by weakening the surface metal–oxygen bond such as realized with a monolayer of PtO2 coated on RuO2(110). © 2014, Springer Science+Business Media New York.
    view abstract10.1007/s12678-014-0220-3
  • Chlorine Evolution Reaction on RuO2(110): Ab initio Atomistic Thermodynamics Study - Pourbaix Diagrams
    Exner, K.S. and Anton, J. and Jacob, T. and Over, H.
    Electrochimica Acta 120 (2014)
    Constrained ab initio thermodynamics in the form of a Pourbaix diagram can greatly assist kinetic modeling of a particular electrochemical reaction such as the chlorine evolution reaction (CER) over RuO2(110). Pourbaix diagrams reveal stable surface structures, as a function of pH and the potential. The present DFT study indicates that the Pourbaix diagram in the CER potential region above 1.36 V and pH values around zero is dominated by a stable surface structure in which all coordinatively undercoordinated Ru sites (Rucus) are capped by on-top oxygen (Oot). This oxygen saturated RuO2(110) surface is considered to serve as the catalytically active phase in the CER, quite in contrast to the heterogeneously catalyzed HCl oxidation (Deacon process), for which the active RuO 2(110) surface is mainly covered by on-top chlorine. The active sites in the CER are suggested to be RucusOot surface complexes, while in the Deacon process both undercoordinated surface Ru and oxygen sites must be available for the activation of HCl molecules.©2013 Published by Elsevier Ltd.
    view abstract10.1016/j.electacta.2013.11.027
  • Controlling selectivity in the chlorine evolution reaction over RuO2-based catalysts
    Exner, K.S. and Anton, J. and Jacob, T. and Over, H.
    Angewandte Chemie - International Edition 53 (2014)
    In the industrially important Chlor-Alkali process the chlorine evolution reaction (CER) over a ruthenium dioxide (RuO2) catalyst competes with the oxygen evolution reaction (OER). This selectivity issue is elucidated on the microscopic level with the single-crystalline model electrode RuO2(110) by employing density functional theory (DFT) calculations in combination with the concept of volcano plots. We demonstrate that one monolayer of TiO2(110) supported on RuO2(110) enhances the selectivity towards the CER by several orders of magnitudes while preserving the high activity for the CER. This win-win situation is attributed to the different slopes of the volcano curves for the CER and OER. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA Weinheim.
    view abstract10.1002/anie.201406112
  • density functional theory

  • electrocatalysis

  • modelling and simulation

  • transition metals

« back