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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.

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

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OA Förderung 2021 Binding energies of reaction intermediates are largely used to comprehend activity trends in a class of electrode materials. For a two-electron process, such as the hydrogen evolution reaction (HER), it is a well-established paradigm that the optimum electrocatalyst binds adsorbed hydrogen thermoneutrally at zero overpotential. While this picture was challenged recently by means of density functional theory (DFT) calculations and microkinetic considerations, reporting a shift of the optimum binding energy to strong or weak bonding with increasing overpotential, now experiments show further evidence for this theory. This perspective article juxtaposes the different views of the optimum binding energy for the HER by means of the Sabatier principle, microkinetic considerations, DFT calculations, and experiments, and provides an outlook of potential future investigations by the combination of experiments with DFT aiming at sustainable materials development for the hydrogen electrocatalysis.

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

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.

Five and six electrons delocalized in the N,N,N,N plane are characteristic of the highly persistent 4N/5e radical anions (deep green) and σ-bis-homoaromatic 4N/6e dianions (red), respectively, which were generated by one- and two-electron reduction of rigid, proximate bis-diazenes (see reaction below). The longest wavelength UV/Vis absorptions of the radical anion and dianion are strongly dependent on the counterion.

In a specifically designed proximate and almost perfectly synperiplanar diazene–diazene oxide substrate, metathesis is the exclusive photoreaction and occurs with retention of optical purity, providing support for the [π2 + π2]photocycloaddition pathway (tetrazetidine oxide intermediate).