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CeO2-based catalysts incorporated with cooperative dopants show excellent potential for promoting H2 production via the water-splitting reaction (WSR) in solid oxide electrolysis cells (SOECs). Here, we report that CeO2 co-doped with Gd+Sb, Pr+Sb, and Bi+Sb trivalent cations have superior performance for WSR with the reaction rate 2-5 orders of magnitude larger than that of individual trivalent dopants including Ga, Sb, Lu, Gd, Sm, Pr, Bi, and La, owing to improved stability of hydridic H-species and accordingly decreased barrier for their decompositions into H2 by co-doping. The energy of hydridic H-species and the turn-over frequency (TOF) toward H2 production is shown to correlate well with the average ionic radius of trivalent cations, where the TOF increases with decreasing ionic radius and Gd+Sb, Pr+Sb, and Bi+Sb with smaller ionic radius show better WSR activity than other dopants. Our studies suggest incorporating ceria with proper co-dopants with an average radius close to Gd+Sb, Pr+Sb, or Bi+Sb pairs as an effective strategy for enhancing WSR performance on CeO2 in SOECs.

Pd hydride has shown much better electrochemical CO2 reduction reaction (CO2RR) performance compared to metal Pd implying that H in the PdHx surface plays a vital role in affecting the performance. Some experiments show phase transformation from metal Pd to PdHx via in-situ X-ray diffraction and electrochemical method. However, studying the stable atomic structures and reaction mechanisms at different H concentrations is experimentally very difficult. Most previous experimental and com- putational studies only focus on the comparison of CO2RR performance of pure metal Pd (0% H concentration) and Pure PdH (100% H concentration). However, there is also no systematical theoretical study about the H concentration effect of PdHx (from 0% to 100%) on CO2RR and the hydrogen evolution reaction (HER), and the atomic structure of the active substoichiometric PdHx catalyst have not been identi- fied. Based on density functional theory (DFT) calculations, active learning cluster expansion equipped with Monte Carlo simulated annealing is carried out to search for stable PdHx(111) surface configurations in this work. We identify 12 stable PdHx(111) configurations from the converged DFT convex hull and theoretically investigate the evolution of the H concentration of the stable structures as a function of applied potential. After automatically finding unique adsorption sites and calculating the binding energies of adsorbates, we analyze their free energy diagram, activity volcano and selectivity toward CO and H2. Finally, we identify the atomic structure of the PdHx phase most likely to produce syngas. It’s activity can be attributed to the fact that H segregation breaks the linear relation between COOH and CO adsorbates.

Single-atom catalysts (SACs) have been considered as a potential candidate for fuel cell application due to the fact that they exhibited good oxygen reduction reaction (ORR) activity. In this study, the ORR catalytic activities of Co-N4 /C, Co-N3 S/C, Co-N2 S2 /C, Co-NS3/C, and Co-S4/C catalysts are studied using density functional theory (DFT) calculations based on BEEF-vdW functional. The reduction of *OH into H2O is found to be the potential determining step of ORR on Co-N4/C catalysts. This implies that the activity of the Co-N4/C system could be improved by weakening the binding energy of the *OH intermediate. The DFT results revealed that the adsorption energy of the *OH intermediate bound on Co-N3S/C, Co-N2S2/C, Co-NS3/C, and Co-S4/C is weaker than that of the Co-N4/C catalyst. The results show that the overpotentials of Co-N4/C, Co-N3S/C, Co-N2S2/C, Co-NS3/C, and Co-S4/C catalysts are 0.57, 0.37, 0.46, 0.40, and 0.42 V, respectively. Thus, the Co-N3S/C catalyst revealed the lower overpotential pathway. Slightly less amount of charges are transferred from Co atom in Co-N3S/C to ORR intermediates as compared to Co-N4/C and the d-band center of Co atom changes from -0.71 eV (Co- N4/C) to -0.91 eV (Co-N3S/C). This can explain the weaker adsorption energy of *OH on Co-N3S/C catalyst. Therefore, Co-N3S/C is a promising non-precious single-atom catalyst for an efficient ORR activity in acidic solution in fuel cells. Keywords: Oxygen reduction reaction, Single-atom catalysts, adsorption energy, catalytic activity, Density functional theory.

Ammonia is the feedstock for nitrogen fertilizers and a potential carbon-free energy carrier; however, its production is highly energy intensive. Conventional heterogeneous catalysts based on metallic iron or ruthenium mediate dinitrogen dissociation and hydrogenation through a relatively energy-expensive pathway. Here we report the ternary ruthenium complex hydrides Li4RuH6 and Ba2RuH6 as an alternative class of catalysts, composed of electron- and hydrogen-rich [RuH6] anionic centres, for non-dissociative dinitrogen reduction, where hydridic hydrogen transports electrons and protons between the centres, and the Li/Ba cations stabilize NxHy (x = 0–2, y = 0–3) intermediates. The dynamic and synergistic involvement of all the components of the ternary complex hydrides facilitates an associative reaction mechanism with a narrow energy span and perfectly balanced kinetic barriers for the multistep process, leading to ammonia production from N2 + H2 with superior kinetics under mild conditions. The mechanism of ammonia synthesis on traditional iron or ruthenium catalysts features a high energetic span. Here, the authors introduce ternary ruthenium complex hydrides of lithium and barium that can activate dinitrogen via a lower-energy path, resulting in the highly efficient production of ammonia under milder conditions.

The application of ab-initio molecular dynamics (AIMD) for the explicit modeling of reactions at the solid- liquid interfaces can provide new understandings towards the reaction mechanisms. However, prohibitive computational cost severely restricts the time- and length-scale of AIMD. Equiv- ariant graph neural network (GNN) based accurate surrogate potentials can accelerate the speed of performing molecular dynamics after learning on representative structures in a data efficient manner. In this study, we combined uncertainty- aware GNN potentials and enhanced sampling to investigate the reactive process of the oxygen reduction reaction (ORR) at Au(100)-water interface. By using a well-established active learning framework based on CUR matrix decomposition, we can evenly sample equilibrium structures from MD simulations and non-equilibrium reaction intermediates that are rarely vis- ited during the reaction. The trained GNNs have shown excep- tional performance in terms of force prediction accuracy, the ability to reproduce structural properties, and the low uncer- tainties when performing MD and metadynamics simulations. Furthermore, the collective variables employed in this work en- ables an automatic search of reaction pathways and provide a detailed understanding towards the ORR reaction mechanism on Au(100). Our simulations identify an associative reaction mechanism where adsorbed O2 reacts with water to form hy- droxyls through an *OOH transition state. The reaction pro- ceeds without formation of *O with a low reaction barrier of 0.3 eV. The low barrier agrees with the fast reaction kinetics observed experimentally. The methodology employed in this study can pave the way for modeling complex chemical reac- tions at electrochemical interfaces with an explicit solvent at ambient conditions.

Many global optimization tasks have partially or completely shared search spaces. This is particularly common in computational heterogeneous catalysis, where catalysts are optimized to their lowest-energy configurations under various idealized chemical conditions to construct phase diagrams. If each task represents the global optimization under a unique reaction condition, all tasks essentially shares the same configurational search space as well as the same black-box function that takes as input the configuration and returns the relaxed energy obtained by an expensive electronic structure calculation. Bayesian optimization represents an efficient paradigm for solving a single global optimization task of an expensive black-box function. Evolutionary multitasking is a newly emerging paradigm for solving multiple global optimization tasks simultaneously. Here we present a Bayesian evolutionary multitasking (BEM) framework combining the best of the two paradigms. As a case study, we aim to derive the phase diagram of Pt-doped Ni nanoparticles covering a wide variety of steam methane reforming conditions. By integrating knowledge such as graph theory and lattice symmetry, the BEM framework as a whole can significantly accelerate the mapping of phase diagrams for very complex heterogeneous systems. The efficient calculation of surface free energies of low index facets in the reactive catalyst environment provides insight into catalyst deactivation through coking and sulfur poisoning.

A single metal site incorporated in N-doped carbon (M/N/C) is a promising electrocatalyst. Here, we perform a computation investigation of the effect of electrolyte anion adsorption on the activity and stability of single-atom catalysts (MN4) with M as transition metal and p-block metal. The MN4 site on two different graphene structures (bulk graphene and graphene edge) is studied under electrochemical conditions for the oxygen reduction reaction (ORR) and the CO2 reduction reaction (CO2RR). Because of the two-dimensional nature of the catalyst, reaction intermediates and electrolyte ions can interact with both sides of the single-atom catalyst. As a result, the electrolyte anions compete with water and adsorbate on the single metal site, in some cases either poisoning or modifying the catalyst activity and thermodynamic stability. We find most electrolyte anions adsorbs on the single metal site under ORR conditions but not at the lower potentials for the CO2RR. Still, the adsorption of water and gas molecules can occur under CO2RR conditions. For example, under ORR conditions, the thermodynamic driving force of the *SO4-FeN4 site in the 0.1 M H2SO4 solution is about 0.47–0.56 eV lower than the *O-FeN4 site in water, depending on the local carbon structure. Additionally, the stabilization by electrolyte anions depends on the nature of the metal atom. Our study demonstrates the important role of electrolytes and the coordination environment for the activity and stability of the M/N/C catalyst.

Solid-state electrolytes (SSEs) are highly desirable for next-generation energy storage and conversion devices. Herein, composite SSEs based on layered double hydroxides (LDHs)/metal-borohydrides (M-BH4 with M = Li, Na, Mg), with a remarkable increase in ionic conductivities, are proposed. The two-dimensional (2D) host layers of LDH, with rich surface hydroxyl groups, maximize the interface interacting area with M-BH4. The consequent defects of the interfacial M-BH4 enable a lower activation energy for boosting ion transfer. As a result, the surfaces of the LDHs serve as an advanced track for improving the ionic mobility of M-BH4. The resultant MgAl-LDH/LiBH4 exhibited a five order of magnitude increase in the room temperature ionic conductivity, reaching values of up to 0.31 mS cm−1, which is superior to most of the previously reported LiBH4-based SSEs. This work provides a new perspective for understanding how the surface effect promotes ionic conductivity of composite SSEs and an avenue towards the realization of low-temperature advanced SSEs.

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

PdH-based catalysts hold promise for both CO

PdH-based catalysts hold promise for both CO2 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 CO2 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 CO2 activation.

Mild-condition ammonia synthesis from N2 and H2 is a long-thought-after scientific goal and a practical need, especially for the intensively pursued "green ammonia" production using renewable H2. Barium-containing materials have recently attracted significant attention as promising catalysts, catalyst supports, or mediators for effective ammonia synthesis under mild conditions. Here, we report that the ternary barium-ruthenium complex hydride, Ba2RuH6, displays outstanding catalytic activity, which is nearly an order of magnitude higher than that of the active BaO-promoted Ru metal (BaO-Ru/MgO) catalyst at temperatures below 573 K. Different from the Ru metal catalyst, the kinetic parameters of Ba2RuH6 catalyst exhibit interesting temperature dependence. The catalytic center, function mechanism, and kinetic behaviors of Ba2RuH6 catalyst are investigated with a combined experimental and computational approach. We find that the N2 reduction reaction (NRR) is preferentially carried out on a defected Ba2RuH6 (110) surface with Ba and H vacancies, in which a complex active center consisting of three Ba atoms and one Ru atom plus the coordinating hydridic hydrogens catalyze nondissociative hydrogenolysis of N2 through the dynamic and synergistic engagement of all of the components of Ba2RuH6 in mediating electron and proton transfers. Specifically, barium plays a unique and vital role in the whole process by directly donating electrons and bonding with reacting NxHy species. Based on the proposed reaction pathway, the catalytic and kinetic performances of the Ba2RuH6 catalyst are analyzed with the energetic span model, and the calculated turnover frequencies are comparable to the experimental results under the ammonia synthesis conditions applied in this study.

The electrochemical reduction of CO

Ammonia is a central vector in sustainable global growth, but the usage of fossil feedstocks and centralized Haber-Bosch synthesis conditions causes >1.4% of the global anthropogenic CO2 emissions. While nitrogenase enzymes convert atmospheric N2 to ammonia at ambient conditions, even the most active manmade inorganic catalysts fail due to low activity and parasitic hydrogen evolution at low temperatures. Here, we show the [RuH6] catalytic center in ternary ruthenium complex hydrides (Li4RuH6 and Ba2RuH6) activate N2 preferentially and avoid hydrogen over-saturation at low temperatures and near ambient pressure by delicately balancing H2 chemisorption and N2 activation. The active [RuH6] catalytic center is capable of achieving an unprecedented yield at low temperatures via a shift in the rate-determining reaction intermediates and transition states, where the reaction orders in hydrogen and ammonia change dramatically. Temperature-dependent atomic-scale understanding of this unique mechanism is obtained with synchronized experimental and density functional theory investigations.

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

Metal and nitrogen co-doped carbons (M-N/Cs) have emerged as promising alternatives to platinum-based catalysts for the oxygen reduction reaction (ORR). Here, density functional theory calculations are used to investigate the adsorption of anions and impurities from the electrolyte on the active site, modelled as an M-N4 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 on one of the two sites only under realistic experimental conditions. A few of these adsorbates furthermore modify the adsorption energies of the ORR intermediates on the remaining site in such a way that the limiting potential is improved.

Ceria is a promising cathode material in solid oxide electrolysis cells (SOECs) because ceria can become a mixed electronic and ionic conductor through doping, which enables a high surface area for electrocatalysis. Here, we systemically investigate the effect of strain on the electrocatalytic water splitting reaction (WSR) for renewable hydrogen production on CeO2(111) by using density functional theory corrected for on-site Coulomb interactions (DFT+U). We find that tensile strain stabilizes the reduced states of ceria such as oxygen vacancies and surface hydroxyls, while compressive strain destabilizes the reduced states. These trends are explained by a downshift of the Ce 4f orbital energy under tensile strain and agree with the larger size of the Ce3+ ion in comparison to the Ce4+ ion. Our results show that hydroxyl decomposition into H2 has the highest activation energy along the WSR pathway (Ea) and that the free energy of hydroxyl formation (ΔGH) prior to hydroxyl decomposition can act as a thermodynamic barrier to the WSR. Compressive strain (2(111) because it reduces the total barrier (ΔGH + Ea). Strain also effectively engineers the reaction pathway of the WSR at T > 1000 K. By a comparison of the total reaction barrier on different hydroxylated surfaces, the WSR is found to proceed readily via a Ce–H intermediate on excessively hydroxylated CeO2(111) under tensile strain because of the lower barrier, while the WSR proceeds preferentially and readily on the partially or fully hydroxylated CeO2(111) under compressive strain. In addition, a direct mapping between the most efficient WSR pathway and strain at different operating temperatures provides a better understanding of the efficient WSR on the CeO2(111) facet by strain engineering, which sheds light on electrocatalysis on oxide catalysts.

Ceria-based materials have attracted considerable interest due to their potential applications in the electrocatalytic splitting of water for hydrogen production. In this work, the reaction mechanism of water splitting on gadolinium-doped ceria (GDC) is investigated using DFT + U calculations. We found that H2 evolution preferentially proceeds on the overly hydroxylated surfaces of pure ceria and GDC via the formation of Ce–H and Gd–H intermediates, respectively, which is much more efficient than the H2 evolution over their partially hydroxylated surfaces. Notably, GDC is more easily and strongly reduced through formation of oxygen vacancies and hydroxyls compared to pure ceria. Thus, the facile formation of the overly hydroxylated states by the incorporation of Gd leads to the improved electrocatalytic activity of water splitting on ceria. These results indicate that a dopant that stabilizes the reduced ceria and favors the formation of metal-H moiety is capable of effectively improving the reaction activity of ceria for water splitting.

The electrochemical carbon dioxide reduction reaction (CO2RR) is a promising technology to establish a sustainable carbon cycle by minimizing the carbon footprint and producing useful chemicals.1-2 Gold is a highly active and selective electrocatalyst towards CO2RR to produce CO, being as one critical syngas component. Previous theoretical simulations are mainly focused on the key reaction intermediates (*COOH and *CO) using the computational hydrogen electrode (CHE) model, where the CO2 activation is usually neglected which is not accessed in the employed vacuum condition. Constructing the Au-water interface with explicit water layers is beneficial to evaluate the CO2 activation, which might be the rate-determining step (RDS) during CO2RR on Au surfaces.3 In this work, the multiple explicit water layers are introduced to simulate the Au(110)-water interface, and the role of applied potential is considered on Au surfaces with different surface charge densities where the K cations are artificially included into water solvents. Combining the standard ab initio molecular dynamics (AIMD) and state-of-the-art constrained AIMD (cAIMD) simulations, the CO2 activation is studied from both kinetic and thermodynamic aspects. Using the slow-growth sampling approach with thermodynamic integrations, we found that CO2 adsorption could be activated at Au-water interface with 2K+, and the free energy barrier is estimated to be 0.61 eV (Figure 1a, top). Additionally, 0.81 electrons are transferred to *CO2 during adsorption based on Bader charge analysis (Figure 1a, bottom). The key intermediates during CO2RR are shown in Figure 1b (including initial state, transition state, and final state), and the reversed CO2 desorption shows very similar free energy diagram, which validates both our sampling approach and interfacial models.4 Besides the CO2 activation, the full reaction pathway including the subsequent proton transfers during CO2RR will be explored in the further study, and the complete free-energy landscape will be constructed using this Au-water interfacial model. Such an AIMD simulation study from the atomic level can largely contribute to the complete understandings of catalytic reaction pathways on Au surfaces and the interfacial behaviors at solid-liquid interfaces. References (1) Nitopi, S.; Bertheussen, E.; Scott, S. B.; Liu, X.; Engstfeld, A. K.; Horch, S.; Seger, B.; Stephens, I. E. L.; Chan, K.; Hahn, C.; Nørskov, J. K.; Jaramillo, T. F.; Chorkendorff, I., Chem. Rev. 2019, 119, 7610-7672. (2) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F., Science 2017, 355, eaad4998. (3) Wuttig, A.; Yaguchi, M.; Motobayashi, K.; Osawa, M.; Surendranath, Y., Proc. Natl. Acad. Sci. 2016, 113, E4585. (4) Qin, X.; Vegge, T.; Hansen, H. A., J. Chem. Phys. 2021, 155, 134703. Figure 1. (a) Free energy profile (top) and Bader charge analysis (bottom) for CO2 adsorption along SG-AIMD in Au(110)-42H2O-2K+. The dashed red lines represent the key intermediates with the CV of 2.74 (transition state, TS), and 2.12 Å (final state), respectively. (b) The key structures during CO2 adsorption, including the initial state (Initial), TS, and final state (Final). Color code: Au, golden; K, purple; C, blue; O, red; H, white. Figure 1

Ammonia is a central vector in sustainable global growth, but the usage of fossil feedstocks and centralized Haber-Bosch synthesis conditions causes >1.4% of global anthropogenic CO2 emissions. While nitrogenase enzymes convert atmospheric N2 to ammonia at ambient conditions, even the most active manmade inorganic catalysts fail due to low activity and parasitic hydrogen evolution at low temperatures. Here, we show that the [RuH6] catalytic center in ternary ruthenium complex hydrides (Li4RuH6) activates N2 preferentially and avoids hydrogen over-saturation at low temperatures and near ambient pressure by delicately balancing H2 chemisorption and N2 activation. The active [RuH6] catalytic center is capable of achieving high yield at low temperatures via a shift in the rate-determining reaction intermediates and transition states, where the reaction orders in hydrogen and ammonia change dramatically. Temperature-dependent atomic-scale understanding of this unique mechanism is obtained with synchronized experimental and density functional theory investigations.

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; thus, improving the electrode’s electroactivity relative to the untreated graphite. In this study, we conduct density functional theory (DFT) calculations to evaluate functionalization’s role 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 VO2+/VO2+ 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 sp3 hybridization in the same order. We conclude that the increase in the sp3 hybridization is responsible for the improved electroactivity, while the oxygenic groups’ presence is responsible for this sp3 increment. These insights can help in the better selection of activation processes and optimization of their parameters.

In this study, we use density functional theory to investigate the catalytic activity of graphene (G), single vacancy defective graphene (GSV), quaternary N-doped graphene (NGQ), and pyridinic N-doped graphene (NGpy, 3NGpy, and 4NGpy) on Co(0001) substrate for an oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). The results show pyridinic N-doped graphene on a Co support exhibited better performance than the NGQ on a Co support and free-standing systems. According to the results, ORR intermediates (*OOH, *O, and *OH) become more stable due to the presence of a Co substrate. The single pyridinic (3NGpy) layer placed on Co(0001) is the most active site. The overpotential for Co/3NGpy is rather higher compared to pure Pt(111) catalyst (0.65 V). Therefore, pyridinic N-doped graphene with a cobalt support could be a promising strategy to enhance the ORR activity of N-doped graphene in PEMFCs.

Discovering low-cost, durable and highly active electrocatalysts with reduced use of precious platinum group metals (PGM) as catalysts for the hydrogen evolution reaction (HER), the oxygen reduction reaction (ORR), and the oxygen evolution reaction (OER) is a key step for large-scale adaptation of fuel cells, electrolyzers, and metal-air batteries. Here we explore the stability and reaction mechanisms of synthesized one-dimensional transition metal dithiolene wire (TM-DWs, TM = Cr – Cu, Rh, Ir, Pt, Pd) for the ORR and the OER in acid solution by density functional theory (DFT) calculations. Our calculations reveal that Co-DW intrinsically exhibits high catalytic activity for bi-functional ORR/OER with low limiting overpotentials (η) of 0.46/0.45 V via four-electron reactions. These low limiting overpotentials arise from modified scaling relations by strengthening the binding free energy of OOH* compared to OH* on TM-DWs, yielding universal minimum ORR/OER overpotentials of η = 0.28/0.22 V, remarkably decreased compared to both metal and oxide surfaces (ηideal = 0.37 V). By applying uni-axial strain, the adsorption strength of reaction intermediates on TM reactive sites can be optimized due to shifts in d-band centers. Our findings provide valuable insight into rational design of non-precious metals based electrocatalysts, and demonstrate a new strategy of tuning adsorptions via uni-axial strain to develop efficient bifunctional electrocatalysts of ORR/OER under optimal conditions.

Metal and nitrogen co-doped carbons (M-N/Cs) are promising alternatives to platinum-based catalysts for the oxygen reduction reaction (ORR). Here, density functional theory calculations are used to compare the ORR activity of Co single and double sites embedded in N-doped carbon. Two different models of a Co double site are investigated, one in which two single sites are stacked on top of each other and one which has two Co atoms next to each other in a single graphene sheet. For both it is found that the ORR can proceed via a dissociative mechanism that splits the ∗OOH intermediate into ∗O and ∗OH, but only for the double site in the graphene sheet does it result in a significant deviation from the scaling relations. The adsorption energies of the ORR intermediates are investigated using different implicit and explicit solvent models, showing some variation in the results. In particular, the addition of explicit water on the same side of the catalyst as the ORR intermediate can result in stabilisation due to hydrogen bonding, while an explicit water molecule adsorbed on the opposite side of the Co atom can have an effect due to the change in coordination which affects the splitting of the Co d orbitals.

Ceria is a promising material for cathodes in high-temperature CO2 electrolysis cells because ceria can become a mixed electronic and ionic conductor through doping, which enables a high surface area for electrocatalysis. Here, we systemically investigate the effect of strain to enhance the activity for electrocatalytic CO2RR on CeO2(111) using density functional theory corrected for on-site Coulomb interactions (DFT + U). We find that tensile strain decreases the oxygen vacancy formation energy due to a downshift of the Ce 4f orbital energy, in agreement with the larger size of the Ce3+ ion than the Ce4+ ion. The corresponding upshift in the Ce f-band center with compressive strain destabilizes the formation energy of the critical surface oxygen vacancies and reduces the energetic span of the reduction reaction, leading to a 4 orders of magnitude higher turnover frequency at 800 K for 4% compressive strain. These findings shed new light on possible pathways to enhance the catalytic activity for CO2RR on CeO2(111) and related catalytic systems by strain engineering.

CeO2-based catalysts as cathodes in solid oxide electrolysis cells (SOECs) have great potential for improving the storage of renewable electricity in the form of H2 via the water-splitting reaction (WSR). A key challenge to promote the WSR on CeO2 is facilitating the decomposition of stable surface hydroxides to form H2. Here, we use density functional theory to investigate the effect of Bi- and Pr- doping for the WSR on CeO2(111). We find that dopants influence the stability of hydridic H, which can be formed during the decomposition of hydroxyl to H2. By stabilizing hydridic H on Bi during the H2 formation step, Bi and Pr co-doped into ceria lower the barrier to H2 formation, enhancing the rate of WSR by 2-4 orders of magnitude compared to individually Pr-, Bi- or Gd-doped CeO2. We suggest co-doping as an effective strategy for improving the WSR in SOECs.

Membrane-based polybenzimidazole (mPBI) emerges as an exciting electrolyte membrane for alkaline fuel cells and water electrolyzers due to its useful ion conductivity range after being doped with aqueous KOH. However, the polymer degradation at highly alkaline concentrations limits its practical use. Herein, the density functional theory (DFT) calculations are used to study the degradation mechanism of mPBI molecule in an alkaline solution. The pristine mPBI molecule deprotonates to form an ionized molecule in an alkaline solution, with the ionized form being predominant at high pH. The nucleophilic hydroxide at the C2 position initiates the degradation, whereas the formation of the fully deprotonated ionic form suppresses the hydroxide ion attack. The degradation reaction then proceeds by ring-opening and chain scission reactions. The ring-opening reaction is preferred with an ancillary hydroxide ion or water molecule during the proton transfer process. The rate-determining state is the transition state involving the amide cleavage during the chain scission. Combining implicit-explicit solvation models is found to stabilize intermediate and transition states, lowering the energy barrier. With one or two explicit water molecules, the free energy barrier agrees well with experimental polymer lifetimes. An increase in KOH concentration increases the degradation rate, agreeing with experiments.

Ammonia is the feedstock for nitrogen fertilizers and a potential carbon-free energy carrier, but the current production emits more CO2 than any other chemical producing reaction in the world. The demand for decarbonizing the ammonia industry by using renewable energy has renewed research interests into catalyst development for effective N2 reduction under mild conditions, a grand scientific challenge. Conventional heterogeneous catalysts based on metallic Fe or Ru mediate dinitrogen dissociation and hydrogenation through a relatively energy-costing pathway. The ternary ruthenium complex hydrides Li4RuH6 and Ba2RuH6 reported in this work, on the other hand, represent an entirely new class of compound catalysts, which are composed of the electron- and H-rich [RuH6] anionic centers for non-dissociative dinitrogen reduction, where hydridic H transports electron and proton between the centers, and the Li(Ba) cations for stabilizing NxHy (x: 0 to 2, y: 0 to 3) intermediates. The dynamic and synergistic involvement of all the components of the ternary complex hydrides facilitates a novel reaction mechanism with a narrow energy span and perfectly balanced kinetic barriers for the multi-step process, leading to ammonia production from N2+H2 with superior kinetics under mild conditions.

The production of hydrogen at a large scale by the environmentally-friendly electrolysis process is currently hampered by the slow kinetics of the oxygen evolution reaction (OER). We report a solid electrocatalyst α-Li2IrO3 which upon oxidation/delithiation chemically reacts with water to form a hydrated birnessite phase, the OER activity of which is five times greater than its non-reacted counterpart. This reaction enlists a bulk redox process during which hydrated potassium ions from the alkaline electrolyte are inserted into the structure while water is oxidized and oxygen evolved. This singular charge balance process for which the electrocatalyst is solid but the reaction is homogeneous in nature allows stabilizing the surface of the catalyst while ensuring stable OER performances, thus breaking the activity/stability tradeoff normally encountered for OER catalysts., Renewable hydrogen production from water will require understanding and improving the oxygen evolution reaction (OER) on catalyst surfaces. Here, authors report α-Li2IrO3 to transform into a hydrated birnessite phase under OER conditions that exhibits enhanced OER performances and durabilities.

Electrochemical reduction of nitrogen to ammonia can potentially replace the existing centralized fossil fuel-based Haber-Bosch process with small, decentralized units relying on electrical energy from renewable sources, thus supporting a sustainable food and energy infrastructure. Recent activities in the development of transition metal nitride electrocatalysts for this reaction have shown promise, but oxynitrides remain unexplored. We have performed a rigorous computational study of the highly promising vanadium oxynitride (VON) to establish for the first time the nitrogen reduction pathway in oxynitrides and the role of the mixed anions that can lead to improved stability of the active surface-states, activity, and selectivity over hydrogen evolution. The electrocatalytic properties are best enhanced at low oxygen content (12.5%) due to optimal balance between consecutive protonation preference at N-sites over V-sites, low onset potential (0.4 V-RHE), and facile N2 adsorption at N-vacancy sites, while a higher oxygen containing VON (31.25%) shows the lowest N2 adsorption/dissociation barrier (∼0.3 eV) on the anion vacancy and can also be a potential N2RR catalyst with a higher NH3 turn over frequency, albeit with a lower stability and higher overpotential (0.6 V-RHE) compared to x = 12.5%. The critical N-vacancy active sites are protected from self-annihilation by the mixed-valency anions, large kinetic barriers, and site blocking by O∗/OH∗/H∗ due to highly favorable N2 absorption.

Electrochemical reaction rates are sensitive to interactions between electrolyte cations and adsorbed reaction intermediates, e.g., cation-*OH interactions in the oxygen reduction reaction on platinum. Here, we calculate the free energy interaction between adsorbed *OH and K+/Li+ situated at the liquid water-Pt(111) interface using ab initio molecular dynamics (AIMD) and metadynamics. Li+ stabilizes *OH by 0.1 ± 0.1 eV and K+ destabilizes *OH by 0.1 ± 0.1 eV, in qualitative agreement with experimental cyclic voltammogram (CV) measurements. In contrast, the internal energy of *OH is stabilized by 0.3 eV and 0.4 eV for Li+ and K+, respectively. This demonstrates that entropy significantly destabilizes cation-*OH interactions and is vital in order to understand even the relative influence of cations at interfaces.

Ceria has attracted considerable interest for the high-temperature electrocatalytic water splitting reaction (WSR) in solid oxide electrolysis cells (SOECs) due to the large active surface area enabled by its accessibility to ions, electrons and gas molecules. The mechanism of the WSR on the (1 1 1) facet of ceria has been studied extensively and hydroxyl decomposition is widely reported as the rate-determining step. However, WSR on other most-often exposed low-index surfaces like CeO2(1 1 0) and CeO2(1 0 0) remains unclear, especially the effect of high temperature on the formation of hydroxyls with different hydrogen coverage (ϴH) and reaction pathways. After identifying stable electron localization on cerium by using density functional theory corrected for on-site Coulomb interactions (DFT + U), we found that the formation of reaction intermediates such as oxygen vacancies, hydroxyls and vacancy-hydroxyl mixed phases on CeO2(1 0 0) is much more stable than that on CeO2(1 1 0) and CeO2(1 1 1). The higher stability of hydroxyls on CeO2(1 0 0) inhibits hydroxyl decomposition into H2, as compared to the reaction on the (1 1 0) and (1 1 1) facets, leading to the WSR on CeO2(1 1 0) and CeO2(1 1 1) 10 ~ 100 times faster than the reaction on CeO2(1 0 0) at temperature (T) < 950 K. In addition, we investigated the WSR via the formation of hydroxylated ceria with different coverage ϴH and found that the most efficient reaction pathway is also strongly facet-dependent, with the WSR on CeO2(1 0 0) and CeO2(1 1 1) preferentially proceeding on their overly hydroxylated surfaces (ϴH > 1, beyond one monolayer), while ϴH of the most active hydroxylated CeO2(1 1 0) decreased with the increase of temperature because of the unstable formation of highly hydroxylated CeO2(1 1 0) at high temperature. Our studies shed more light on the facet sensitivity of electrocatalysis on ceria.

Iron carbide encapsulated in graphitic layers has recently been recognized as an active oxygen reduction reaction (ORR) catalyst made of earth-abundant elements. Here, the ORR activity of graphene ...

Noble metal free catalysts, such as N-doped graphene, have drawn a lot of attention as a promising replacement for platinum in low temperature fuel cells. Computational prediction of catalytic activity requires accurate description of the oxygen reduction reaction (ORR) intermediates adsorption energies. Two stabilizing effects, immanently present in experimental ORR setups with basal plane N-doped graphene catalyst, are studied systematically by means of density functional theory. Distant nitrogen with no adsorbates on neighboring carbon atoms selectively stabilizes *O and *O2 adsorbates. Water solvation stabilizes all ORR intermediates, having a greater impact on *O and *O2, than on *OH and *OOH, in contrast to metal and oxide catalysts. Synergistic stabilization of *O caused by both effects reaches remarkably a high value of 1.5 eV for nitrogen concentrations above 4.2% N. Such a strong effect is explained by a high reactivity of *O and *O2, which possess empty O(sp) states. At 6.25% N, the reaction environment is found to comprise *O and free nitrogen spectators. Finally, strong *O solvation is found to be present in a broader class of systems, comprising all materials where the ORR occurs on a 2nd row element. Including at least a single explicit water layer is paramount to achieve the correct description of the ORR intermediates adsorption energies on these materials.

Non-precious-metal catalysts (NPMC) are promising alternatives to platinum-based catalysts for the oxygen reduction reaction (ORR), which is the cathode reaction in fuel cells. In this paper, we focus on an iron−nitrogen−carbon (Fe/N/C) catalyst, in comparison to platinum, and investigate how these different types of catalysts behave toward selective anion poisoning. The catalysts are studied with respect to their ORR activity, using the rotating disk electrode (RDE) technique in aqueous HClO4, H2SO4, H3PO4, and HCl electrolytes, and the results are supported by density functional theory (DFT) calculations. We find that the ORR on the Fe/N/C catalyst is less affected by anion poisoning than platinum. Surprisingly, it is seen that phosphoric acid not only does not poison the Fe/N/C catalyst, but instead promotes the ORR; this finding is in sharp contrast to the poisoning effect observed on platinum. This is a highly important finding, as modern high-temperature proton exchange fuel cells (HT-PEMFCs) employ membranes consisting of phosphoric acid that is immobilized into a polybenzimidazole (PBI) matrix.

The CO oxidation reaction on single 3d-transition metal catalytic sites in experimentally realized tetracyanoquinodimethane (TM-TCNQ) monolayers (TM = Sc-Zn) is systematically investigated by means of first-principles calculations. Considering the stabilities, adsorption characteristics and thermodynamics of all the ten candidates (Sc-Zn), Sc-TCNQ is found to display the lowest activation energies and yield the highest catalytic activity for room temperature CO oxidation. Exploring the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms, we find that the rate-limiting step of CO oxidation catalyzed by Sc-TCNQ (CO + O2* → OOCO*) can follow the LH mechanism with free energy barriers as low as 0.73 eV at 300 K. The second step of CO + O* → CO2 can occur with rather small energy barriers via either LH or ER mechanisms. The high activity of Sc-TCNQ can be attributed to its unique structural and electronic features by possessing high stability, optimum adsorption energies with adsorbates, and fast reaction kinetics. These results have significant implications for the synthesis of two-dimensional single atom catalysis for CO oxidation with low-cost and high activity at low temperature.

High overpotentials and low faradic efficiencies plague metal catalysts for direct conversion of CO2 to methanol and other liquid fuels. RuO2-based electrocatalysts have been observed to evolve methanol at low overpotentials, which has been attributed to an alternative reaction mechanism with oxygen-coordinated intermediates that can circumvent the limitations imposed by the scaling relations on metal catalysts. Here, we introduce an innovative concept of ligand effects in oxide catalysts. Both IrO2 and RuO2 binds OH* and other intermediates from the electrochemical reduction of CO2 (CO2RR) strongly, but the stable and miscible system IrxRu(1-x)O2 exhibits anomalous weaker binding energy in the presence of CO* spectators, because of Ru–Ir ligand effects. The weakened adsorbate binding leads to a very low CO2RR onset potential (methanol evolution at −0.2 V RHE). An Ir atom at the bridge site with Ru neighbors binds intermediates such as OH* and OCHO* much weaker, because of synergistic ligand effects and adsorbate–adsorbate interactions. Consequently, a RuO2 surface doped with Ir move close to the top of the predicted CO2RR volcano for oxides, which offers a significant improvement over state-of-the-art electrocatalysts for conversion of CO2 into methanol. Analysis of electronic structure parameters with adsorbate binding energies indicates the ligand effect depletes electrons from the Ir atom and shifts the t2g orbitals. The lack of electron donation from CO* spectators to Ir at the active site cause favorable adsorbate binding.

RuO2-based electrocatalysts are found to be active at low overpotential toward direct electrochemical reduction of CO2 to formic acid and methanol. RuO2 can circumvent the thermodynamic bottleneck resulting from the scaling relations observed on metallic electrocatalyst, by utilizing an alternate pathway through oxygen-coordinated intermediates. Employing density functional theory based computational electrocatalysis models we show adsorbate–adsorbate interaction effects for adsorbates and reaction intermediates on the RuO2(110) surface are large and impactful to the reaction thermodynamics. We studied binding energy amendment due to adsorbate interaction (steric and electronic) with varying coverage of CO* spectators on the catalyst surface. Implications on the reaction pathways help us rationalize differences in experimentally observed carbonaceous product mix and suppression of the hydrogen evolution reaction (HER). We show that a moderate CO* coverage (∼50%) is necessary for obtaining methanol as a product and that higher CO* coverages leads to very low overpotential for formic acid evolution. Our analysis also clarifies the importance of the reaction condition for CO2 reduction to liquid fuels utilizing RuO2-based electrocatalysts.

Carbon black, a state-of-the-art cathode material for proton exchange membrane fuel cells (PEMFCs), suffers from severe corrosion in practical applications. Niobium-doped tin dioxide (NTO) is a promising alternative to support the Pt catalysts at the cathodes. Here, through a combined density functional theory and nonequilibrium Green’s function study, we investigate the Nb segregation at Pt/NTO interfaces under operational electrochemical conditions, and reveal the resulting effects on the electronic transport, as well as the catalytic properties. We find that the Nb dopants tend to aggregate in the subsurface layers of the NTO substrate, whereas their transport across the Pt/NTO interface is hindered by a high thermodynamic barrier under the operating condition of PEMFCs. The interfacial transport of Sn is, however, more facile, indicating possible formations of Sn–Pt alloys and tin oxides. The electronic conductivities of the Pt/NTO systems are not particularly sensitive to the distance of the Nb dopan...

The development of 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 recently recognized 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 ORR are investigated using density functional theory (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. We find that introducing Ni or Co atoms at the interface improves the ORR activity of the NG/Fe 3 C as well as stabilize 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.

The identification of catalytically active and stable bifunctional electrocatalysts for the oxygen reduction and evolution reaction (ORR/OER) would revolutionize rechargeable metal-air batteries and regenerative fuel cell technologies. In this study, we use density functional theory (DFT) calculations to systemically investigate 2D transition metal based tetracyanoquinodimethane (TM–TCNQ, TM = Cr-Cu, Ru-Ag, Pt, Ir) monolayers with single TM atoms catalysts (SACs) distributed periodically with high density. We calculate a lower limiting overpotential of ηlim = 0.33 V for Fe-TCNQ yielding a higher expected activity than that of Pt metal (0.48 V) for the ORR under acidic conditions. The catalytic performance of Fe-TCNQ for OER is, however, surpassed by Ni-TCNQs (ηlim = 0.46 V), which is identified as the highest among the candidates. By applying grafting axial ligands and external strain, the adsorption strength of reaction intermediates on TM reactive sites can be further optimized and enhances the activities of Mn-, Fe- and Ni-TCNQs for the ORR or the OER. Here, Fe-TCNQ-Cl (ηlim = 0.27/0.55 V), followed by Fe-TCNQ-CO (ηlim = 0.67/0.43 V), are predicted as novel, high-performance bifunctional ORR/OER catalysts with comparable limiting overpotentials to those of the best commercially used electrocatalysts, Pt (ORR: ηlim = 0.48 V) and RuO2 (OER: ηlim = 0.42 V). In addition, the recently synthesized Fe-TCNQ possess excellent conductivity with fast electron transfer during the catalytic reactions. These results show 2D TM-TCNQ monolayers are durable, low-cost, and efficient catalysts for the ORR and the OER in metal-air batteries and fuel cells.

Catalytic oxidation of CO can remove toxic pollutants from gas emissions, whereas the reduction of CO2 can provide feedstock for fossil free products. Using density functional theory with a Hubbard U correction (DFT + U), the reduction of CO2 into CO is investigated on a reconstructed CeO2−x(110) facet at operatingsolid oxide electrolysis conditions. A reaction pathway is identified with nudged elastic band (NEB) through adsorption of CO2 as a monodentate carbonate, an intermediate CO2− and the transition state to the final adsorbed CO. The reaction barrier for CO2 reduction does not depend on the temperature, whereas the back reaction for CO oxidation decrease with temperatures. Different 4d and 5d transition metals and other suitable metals are screened as dopants to increase the activity for CO2 reduction. After the screening iridium is the most promising candidate. The same temperature dependencies are present for the Ir-doped surface as the undoped, whereas the back reaction barrier is decreased to half the undoped value. There is no evidence of destructive carbon deposition on the reconstructed CeO2−x(110) facet.