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In the last decade, trends for competing electrocatalytic processes have been largely captured by volcano plots, which can be constructed by the analysis of adsorption free energies as derived from electronic structure theory in the density functional theory approximation. One prototypical example refers to the four-electron and two-electron oxygen reduction reactions (ORRs), resulting in the formation of water and hydrogen peroxide, respectively. The conventional thermodynamic volcano curve illustrates that the four-electron and two-electron ORRs reveal the same slopes at the volcano legs. This finding is related to two facts, namely, that only a single mechanistic description is considered in the model, and electrocatalytic activity is assessed by the concept of the limiting potential, a simple thermodynamic descriptor evaluated at the equilibrium potential. In the present contribution, the selectivity challenge of the four-electron and two-electron ORRs is analyzed, thereby accounting for two major expansions. First, different reaction mechanisms are included into the analysis, and second, Gmₐₓ(U), a potential-dependent activity measure that factors overpotential and kinetic effects into the evaluation of adsorption free energies, is applied for approximation of electrocatalytic activity. It is illustrated that the slope of the four-electron ORR is not constant at the volcano legs but rather is prone to change as soon as another mechanistic pathway is energetically preferred or another elementary step becomes the limiting one. Due to the varying slope of the four-electron ORR volcano, a trade-off between activity and selectivity for hydrogen peroxide formation is observed. It is demonstrated that the two-electron ORR is energetically preferred at the left and right volcano legs, thus opening a new strategy for the selective formation of H₂O₂ by an environmentally benign route. CA Exner

The apparent activation energy, Eₐpp, is a common measure in thermal catalysis to discuss the activity and limiting steps of catalytic processes on solid-state materials. Recently, the electrocatalysis community adopted the concept of Eₐpp and combined it with the Butler-Volmer theory. Certain observations though, such as potential-dependent fluctuations of Eₐpp, are yet surprising because they conflict with the proposed linear decrease in Eₐpp with increasing overpotential. The most common explanation for this finding refers to coverage changes upon alterations in the temperature or the applied electrode potential. In the present contribution, it is demonstrated that the modulation of surface coverages cannot entirely explain potential-dependent oscillations of Eₐpp, and rather the impact of entropic contributions of the transition states has been overlooked so far. In the case of a nearly constant surface coverage, these entropic contributions can be extracted by a dedicated combination of Tafel plots and temperature-dependent experiments.

The anodic four-electron oxygen evolution reaction (OER) corresponds to the limiting process in acidic or alkaline electrolyzers to produce gaseous hydrogen at the cathode of the device. In the last decade, tremendous efforts have been dedicated to the identification of active OER materials by electronic structure calculations in the density functional theory approximation. Most of these works rely on the assumption that the mononuclear mechanism, comprising the *OH, *O, and *OOH intermediates, is operative under OER conditions, and that a single elementary reaction step (most likely *OOH formation) governs the kinetics. In the present manuscript, six different OER mechanisms are analyzed, and potential-dependent volcano curves are constructed to comprehend the electrocatalytic activity of these pathways in the approximation of the descriptor Gmₐₓ(U), a potential-dependent activity measure based on the notion of the free-energy span model. While the mononuclear description mainly describes the legs of the volcano plot, corresponding to electrocatalysts with low intrinsic activity, it is demonstrated that the preferred pathway at the volcano apex is a strong function of the applied electrode potential. The observed mechanistic complexity including a switch of the favored pathway with increasing overpotential sets previous investigations aiming at the identification of reaction mechanisms and limiting steps into question since the entire breadth of OER pathways was not accounted for. A prerequisite for future atomic-scale studies on highly active OER catalysts refers to the evaluation of several mechanistic pathways so that neither important mechanistic features are overlooked nor limiting steps are incorrectly determined. in press

While metal-air batteries (MABs) are considered to outperform lithium-ion batteries for energy-storage applications, the sluggish bifunctional oxygen electrocatalysis at the cathode of MABs still represents a major bottleneck that severely limits efficiency. Recently, it was motivated by means of electronic structure calculations to replace the oxygen redox chemistry at the cathode of MABs by the redox chemistry of peroxide, considering that the latter is governed by kinetically facile two-electron processes. Herein, two different concepts of rechargeable metal-hydrogen peroxide batteries are investigated, consisting of either the peroxide reduction (PRR) and peroxide formation (PFR) reactions or the two-electron oxygen reduction (ORR) and two-electron oxygen evolution (OER) reactions at the cathode. Applying a dedicated thermodynamic framework in the spirit of the descriptor Gmₐₓ(U), a potential-dependent activity measure that factors overpotential and kinetic effects into the evaluation of adsorption free energies, generalized volcano plots for the PRR, PFR, two-electron ORR, and two-electron OER as well as their competing side reactions are derived. It is illustrated that for the PFR/PRR, selectivity can be steered toward the desired product without loss in activity whereas for the two-electron ORR/OER, a trade-off between activity and selectivity is encountered. The derived volcano models in this contribution may aid the search for potential material motifs for the PFR/PRR and the two-electron ORR/OER by calculations in the framework of electronic structure theory. CA Exner

Oxygen reduction reaction (ORR) corresponds to the limiting process in fuel cells to convert gaseous hydrogen and oxygen from the air into electricity. Most commonly, electronic structure calculations in the density functional theory (DFT) approximation are applied to comprehend the elementary reaction steps on the atomic scale, including the identification of limiting reaction steps and approximation of electrocatalytic activity. A major challenge in the modeling of the ORR refers to the complexity of the mechanistic pathways given that a plethora of different descriptions, ranging from the mononuclear to OOH dissociation mechanisms, dissociative and oxide pathways, have been reported in the literature. In the present work, the adsorption free energy of the *OH intermediate, ΔG₁, is introduced as an effective descriptor for rapid screening of ORR mechanisms. By applying a rigorous thermodynamic analysis in conjunction with a descriptor-based approach, it is demonstrated how the calculation of a single binding energy in terms of ΔG₁ can be used to identify the energetically favored mechanistic description for any ORR catalyst by analyzing the location of the material in the volcano plot. The introduced procedure may support future DFT-based studies in that the preferred pathway is not missed when modeling the four-electron ORR by a binding-energy approach.

MXenes, a rather new family of 2D carbides and nitrides, have shown to be promising materials in many technological applications, particularly in electrocatalysis. The as-synthesized MXenes exhibit a variety of surface terminations involving mixtures of O, OH, H, or F surface groups. These terminations play a crucial role in the electrocatalytic performance of these materials as these may change depending on the reaction conditions. The Pourbaix diagrams have long being used to provide the thermodynamically stable surface under certain conditions of pH and potential, U. However, experimental determination of Pourbaix diagrams may be quite challenging while first-principles studies, considering the most likely terminations, allow deriving reliable insights. Here, Pourbaix diagrams for a series of representative MXenes are provided; the Ti2C, Ti3C2, V2C, and Mo2C MXenes, with the novelty of considering single and several double mixed terminations. The possible implications of the obtained results are discussed, especially for a proper choice of models in theoretical electrocatalysis studies, including the water splitting related hydrogen evolution reaction (HER), or the oxygen reduction reaction (ORR), but also serving as a guide to any further computational studies and also to electrocatalytic experiments.

Electrochemical nitrogen reduction reaction (NRR) is mainly hampered by two facts. On the one hand, the hydrogen evolution reaction competes with the NRR under cathodic reaction conditions. On the other hand, the sluggish reaction kinetics of the NRR, due to the transfer of six proton-electron pairs in the reaction mechanism, causes large overpotentials, and thus results in humble intrinsic activity for ammonia formation. Optimization strategies for NRR electrocatalysts are ultimately called for to overcome these issues. While breaking scaling relation is considered as a universal remedy to enhance turnover for any electrocatalytic process, recently, the concept of catalytic resonance theory has been put forth as a second option to obtain reaction rates beyond the limiting Sabatier volcano. Yet, a comparison of these two concepts is missing but urgently needed to comprehend design principles for electrocatalyst optimization on the atomic scale. In the present work, breaking scaling relation and catalytic resonance by programmable catalysis are compared on the example of the NRR over transition-metal oxides. It is demonstrated that a fine tuning rather than a full breaking of nitrogen-containing scaling relations is required to enhance electrocatalytic activity of electrocatalysts at the volcano apex. The success of catalytic resonance theory is based on the occurrence of negative scaling correlations whereas in the case of positive scaling correlations, the impact of this approach on the optimization of electrocatalysts strongly depends on the location of the material in the volcano plot. The obtained insight may spur the development of NRR electrocatalysts with enhanced intrinsic activity by following the outlined design principles. in press

Efficient transport of pharmaceuticals to malignant cells in the human body often requires the application of drug-delivery systems (DDSs) consisting of several building blocks, each of them bearing a specific function. While nanoparticles are promising as potential carrier moieties, biomolecules may add to the efficient delivery by binding several drug molecules simultaneously. In this contribution, we apply a combination of atomistic molecular dynamics simulations and density functional theory calculations to characterize a multi-component DDS for the transport of the anthracycline antibiotic doxorubicin (DOX), comprising a gold nanoparticle (NP) and a drug-binding peptide (DBP) grafted on the NP surface. We have shown previously that the DDS can stabilize one DOX per DBP. However, by increasing the drug load to a 2 : 1 DOX : DBP ratio the two drug molecules compete for the available adsorption sites, which may cause spontaneous dissociation of one DOX molecule. We identify the chain length of the DBP as a limiting factor for the drug-loading capacity and provide important guidelines for further optimization of multi-component functionalized DDSs.

Identification of active electrocatalysts for the oxygen evolution reaction (OER), corresponding to the bottleneck in electrolyzers to produce gaseous hydrogen as energy vector, by electronic structure calculations relies on the assumption of the mononuclear mechanism, comprising the *OH, *O, and *OOH intermediates. This mechanistic description is thermodynamically hampered by a scaling relation between the *OH and *OOH adsorbates, which may serve as an explanation why OER catalysts commonly require large overpotentials to reach sufficient current densities. Recently, an alternate OER pathway was proposed that, in contrast to the mononuclear description, consists of the formation of two adjacent *OO adsorbates, and gaseous oxygen is produced by chemical recombination of the neighboring *OO intermediates. In the present manuscript, a data-driven model based on a dedicated assessment of the elementary reaction steps is deduced, which enables evaluating the mononuclear and *OO pathways by the same set of parameters. Potential-dependent volcano plots are constructed to comprehend the energetics of the competing mechanisms. It is demonstrated that the alternate OER pathway consisting of the *OO∙∙*OO recombination step may excel the mononuclear description at overpotentials corresponding to typical OER conditions. Consequently, it is suggested that future studies, aiming at the identification of OER materials, may not omit the *OO∙∙*OO recombination mechanism when using concepts of materials screening in a heuristic fashion or multiscale modeling.

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.