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Summary 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.

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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 ...

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Knowledge of the relation between electrocatalytic activity and the rate-determining step or the surface coverage of reaction intermediates is crucial to design next-generation electrocatalysts that may contribute to the sustainability of our society. Commonly, microkinetic models making use of the quasi-equilibrium or steady-state assumptions are applied to correlate potential mechanistic descriptions to experimental data, which are usually depicted in the form of a Tafel plot. Yet, there is a discrepancy in the literature on the utility of the quasi-equilibrium and steady-state conditions, both of which are approximations per se. This inconsistency is the starting point of the present work, which compares the quasi-equilibrium and steady-state approaches for the analysis of a Tafel plot by the concept of free-energy diagram for a two-electron process. A correlation between the two frameworks is deduced and important guidelines for the application of the quasi-equilibrium and steady-state assumptions are obtained. While the quasi-equilibrium approach is a suitable approximation for the analysis of a linear Tafel line without change in the Tafel slope, it may fail in the evaluation of a Tafel plot with two linear Tafel regimes. There, the binding energy of the reaction intermediate governs whether the quasi-equilibrium model is justified, or the steady-state approach is needed. This implies that density functional theory calculations are indispensably required as a supplement to analyze a Tafel plot with two different Tafel slopes, a situation that is particularly observed for highly active electrocatalysts. CA Exner

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 Norskov 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.

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.

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.

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 mod...

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.

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.

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 benc...

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...

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.

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.

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 s-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.