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Electrochemical experiments and theoretical calculations have shown that Pd-based metal hydrides can perform well for the CO 2 reduction reaction (CO 2 RR). Our previous work on doped-PdH showed that doping Ti and Nb into PdH can improve the CO 2 RR activity, suggesting that the Pd alloy hydrides with better performance are likely to be found in the Pd x Ti 1-x H y and Pd x Nb 1-x H y phase space. However, the vast compositional and structural space with different alloy hydride compositions and surface adsorbates, makes it intractable to screen out the stable and active Pd x M 1-x H y catalysts using density functional theory calculations. Herein, an active learning cluster expansion (ALCE) surrogate model equipped with Monte Carlo simulated annealing (MCSA), a CO* binding energy filter and a kinetic model are used to identify promising Pd x Ti 1-x H y and Pd x Nb 1-x H y catalysts with high stability and superior activity. Using our approach, we identify 24 stable and active candidates of Pd x Ti 1-x H y and 5 active candidates of Pd x Nb 1-x H y . Among these candidates, the Pd 0.23 Ti 0.77 H, Pd 0.19 Ti 0.81 H 0.94 , and Pd 0.17 Nb 0.83 H 0.25 are predicted to display current densities of approximately 5.1, 5.1 and 4.6 μA cm -2 at -0.5 V overpotential, respectively, which are significantly higher than that of PdH at 3.7 μA cm -2 ., (© 2023 Wiley‐VCH GmbH.)

Transition-metal-mediated dinitrogen fixation has been intensively investigated. The employment of main group elements for this vital reaction has recently sparked interest because of new dinitrogen reaction chemistry. We report ammonia synthesis via a chemical looping process mediated by a transition-metal-free barium hydride (BaH 2 ). Experimental and computational studies reveal that the introduction of hydrogen vacancies is essential for creating multiple coordinatively unsaturated Ba sites for N 2 activation. The adjacent lattice hydridic hydrogen (H - ) then undergoes both reductive elimination and reductive protonation to convert N 2 to NH x . The ammonia production rate supports this hydride-vacancy mechanism via a chemical looping route that far exceeds that of a catalytic process. The BaH 2 -mediated chemical looping process has prospects in future technologies for ammonia synthesis using transition-metal-free materials., (© 2022 Wiley-VCH GmbH.)

PdH-based catalysts hold promise for both CO 2 reduction to CO and the hydrogen evolution reaction. Density functional theory is used to systematically screen for stability, activity, and selectivity of transition metal dopants in PdH. The transition metal elements Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ru, Rh, Ag, Cd, Hf, Ta, W, and Re are doped into PdH(111) surface with six different doping configurations: single, dimer, triangle, parallelogram, island, and overlayer. We find that several dopants, such as Ti and Nb, have excellent predicted catalytic activity and CO 2 selectivity compared to the pure PdH hydride. In addition, they display good stability due to their negative doping formation energy. The improved performance can be assigned to reaction intermediates forming two bonds consisting of one C-Metal and one O-Metal bond on the PdH surface, which break the scaling relations of intermediates, and thus have stronger HOCO* binding facilitating CO 2 activation., (© 2022 The Authors. ChemSusChem published by Wiley-VCH GmbH.)

Graphite felt is a widely used electrode material for vanadium redox flow batteries. Electrode activation leads to the functionalization of the graphite surface with epoxy, OH, C=O, and COOH oxygenic groups and changes the carbon surface morphology and electronic structure, thereby improving the electrode's electroactivity relative to the untreated graphite. In this study, density functional theory (DFT) calculations are conducted to evaluate functionalization's contribution towards the positive half-cell reaction of the vanadium redox flow battery. The DFT calculations show that oxygenic groups improve the graphite felt's affinity towards the VO 2+ /VO 2 + redox couple in the following order: C=O>COOH>OH> basal plane. Projected density-of-states (PDOS) calculations show that these groups increase the electrode's sp 3 hybridization in the same order, indicating that the increase in sp 3 hybridization is responsible for the improved electroactivity, whereas the oxygenic groups' presence is responsible for this sp 3 increment. These insights can aid the selection of activation processes and optimization of their parameters., (© 2021 Wiley-VCH GmbH.)

The development of an efficient electrocatalyst for the oxygen reduction reaction (ORR) is essential for the commercialization of fuel-cell technologies. Iron carbide encapsulated in N-doped graphene (NG/Fe 3 C) has been recognized recently as a promising ORR catalyst. In this study, the stability and catalytic activity of N-doped graphene supported on metal-iron carbide (NG/M_Fe 3 C) toward the ORR are investigated by using DFT calculations. The NG/M_Fe 3 C heterostructure is modeled by substituting Fe atoms in the Fe 3 C substrate near the NG/Fe 3 C interface by metal atoms M (M=Cr-Mn, Co-Zn, Nb-Mo, Ta-W). The calculations show that the introduction of the metal atoms M alters the work function of the overlayer N-doped graphene, which is found to correlate with the binding strength of the ORR intermediates. The introduction of Ni or Co atoms at the interface improves the ORR activity of the NG/Fe 3 C and stabilizes the heterostructure. The ORR activity increases as the concentration of Ni or Co atoms near the interface increases, and the stable heterostructure is available in a wide range of substituted concentrations. These results suggest approaches to improve the ORR activity of NG/Fe 3 C catalysts., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Metal and nitrogen codoped carbons (M-N/Cs) have emerged as promising alternatives to platinum-based catalysts for the oxygen reduction reaction (ORR). DFT calculations are used to investigate the adsorption of anions and impurities from the electrolyte on the active site, modeled as an M-N 4 motif embedded in a planar carbon sheet (M=Cr, Mn, Fe, Co). The two-dimensional catalyst structure implies that each metal atom has two potential active sites, one on each side of the sheet. Adsorption of anions or impurities on both sites results in poisoning, but adsorption on one of the sites leads to a modified ORR activity on the remaining site. The calculated adsorption energies show that a number of species adsorb only on one of the two sites under realistic experimental conditions. Furthermore, a few of these adsorbates modify the adsorption energies of the ORR intermediates on the remaining site, in such a way that the limiting potential is improved., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Zinc-air batteries offer the potential of low-cost energy storage with high specific energy, but at present secondary Zn-air batteries suffer from poor cyclability. To develop economically viable secondary Zn-air batteries, several properties need to be improved: choking of the cathode, catalyzing the oxygen evolution and reduction reactions, limiting dendrite formation and suppressing the hydrogen evolution reaction (HER). Understanding and alleviating HER at the negative electrode in a secondary Zn-air battery is a substantial challenge, for which it is necessary to combine computational and experimental research. Here, we combine differential electrochemical mass spectrometry (DEMS) and density functional theory (DFT) calculations to investigate the fundamental role and stability when cycling in the presence of selected beneficial additives, that is, In and Bi, and Ag as a potentially unfavorable additive. We show that both In and Bi have the desired property for a secondary battery, that is, upon recharging they will remain on the surface, thereby retaining the beneficial effects on Zn dissolution and suppression of HER. This is confirmed by DEMS, where it is observed that In reduces HER and Bi affects the discharge potential beneficially compared to a battery without additives. Using a simple procedure based on adsorption energies calculated with DFT, it is found that Ag suppresses OH adsorption, but, unlike In and Bi, it does not hinder HER. Finally, it is shown that mixing In and Bi is beneficial compared to the additives by themselves as it improves the electrochemical performance and cyclic stability of the secondary Zn-air battery., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Minimizing energy and materials costs for driving the oxygen evolution reaction (OER) is paramount for the commercialization of water electrolysis cells and rechargeable metal-air batteries. Structural stability, catalytic activity, and electronic conductivity of pure and doped α-MnO 2 for the OER are studied using density functional theory calculations. As model surfaces, we investigate the (110) and (100) facets, on which three possible active sites are identified: a coordination unsaturated, a bridge, and a bulk site. For pure and Cr-, Fe-, Co-, Ni-, Cu-, Zn-, Cd-, Mg-, Al-, Ga-, In-, Sc-, Ru-, Rh-, Ir-, Pd-, Pt-, Ti-, Zr-, Nb-, and Sn-doped α-MnO 2 , the preferred valence at each site is imposed by adding/subtracting electron donors (hydrogen atoms) and electron acceptors (hydroxy groups). From a subset of stable dopants, Pd-doped α-MnO 2 is identified as the best catalyst and the only material that can outperform pristine α-MnO 2 . Different approaches to increase the bulk electron conductivity of semiconducting α-MnO 2 are discussed., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A detailed understanding of the electrochemical reduction of CO 2 into liquid fuels on rutile metal oxide surfaces is developed by using DFT calculations. We consider oxide overlayer structures on RuO 2 (1 1 0) surfaces as model catalysts to elucidate the trends and limitations in the CO 2 reduction reaction (CO2RR) based on thermodynamic analysis. We aim to specify the requirements for CO2RR catalysts to establish adsorbate scaling relations and use these to derive activity volcanoes. Computational results show that the OH* binding free energy is a good descriptor of the thermodynamic limitations and it defines the left leg of the activity volcano for CO2RR. HCOOH* is a key intermediate for products formed through further reduction, for example, methanediol, methanol, and methane. The surfaces that do not bind HCOOH* are selective towards formic acid (HCOOH) production, but hydrogen evolution limits their suitability. We determine the ideal binding free energy for H* and OH* to facilitate selective CO2RR over H 2 /CO evolution to be ΔG B [H]>0.5 eV and -0.5 eV<ΔG B [OH]<0.1 eV. The Re-containing overlayers considered in this work display excellent promise for selectivity, although they are active at a highly reducing potential., (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)