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.
Tip of the volcano: Electrode materials are classically assessed by their location on a volcano curve, using the binding strength of a reaction intermediate (RI) as a descriptor. For a two‐electron process, the apex of the volcano corresponds to thermoneutral binding of the reaction intermediate at zero overpotential. This Viewpoint addresses the definition of an optimum catalyst when kinetic effects and applied overpotential are factored in, illustrating a right shift of the volcano's top with increasing driving force.
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, ΔG ...(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 (ΔG = 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 η, |ΔG(η)|, as thermodynamic descriptor for the kinetics of two-electron processes, which may aid the construction of overpotential-dependent Volcano plots for improved material screening.
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.
Material screening is a powerful tool to assess the activity of electrocatalysts in the oxygen evolution reaction (OER). This progress report summarizes material‐screening methodologies for the OER beyond the assessment of binding energies, such as that encountered with volcano analyses. The recent advancements in this field may contribute to the development of improved OER materials for electrolyzers in the future.
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, Δ
E
O
, 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, Δ
G
2
, rather than Δ
E
O
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.
The intermediate matters: the concept of ESSI-Δ
G
2
activity maps illustrates that the formation of the Cl adsorbate rather than the OCl intermediate is desirable to enhance chlorine selectivity in the competing chlorine and oxygen evolution reactions.
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.
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.
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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 (ΔG2) 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.
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.
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•Apex of the hydrogen-evolution reaction (HER) volcano is modeled by microkinetics.•Apex of the volcano is located at weak bonding of the reaction intermediate (RI).•Microkinetics and scaling relations are combined to construct a volcano plot.•The volcano curve reproduces the high activity of Pt in the HER.•Weak bonding of the RI corresponds to a paradigm change in hydrogen electrocatalysis.
Multielectron 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 G rds #, 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 G max(RI) rather than the highest free energy difference between consecutive reaction intermediates, ΔG loss, as a descriptor for the kinetics.
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