Electrochemical Ammonia Synthesis: The Energy Efficiency Challenge.

We discuss the challenges associated with achieving high energy efficiency in electrochemical ammonia synthesis at near-ambient conditions. The current Li-mediated process has a theoretical maximum energy efficiency of ∼28%, since Li deposition gives rise to a very large effective overpotential. As a starting point toward finding electrocatalysts with lower effective overpotentials, we show that one reason why Li and alkaline earth metals work as N reduction electrocatalysts at ambient conditions is that the thermal elemental processes, N dissociation and NH desorption, are both facile at room temperature for these metals.

Characterization of adsorption sites on IrO temperature programmed O desorption simulations.

This study presents simulations of temperature-programmed desorption (TPD) profiles using desorption energy data from density functional theory (DFT) calculations. We apply this method to investigate the desorption of oxygen (O) from IrO(110) to gain insight into the kinetics of oxygen coupling and desorption, important elementary steps in the oxygen evolution reaction (OER). Initially, we confirm the thermodynamically stable adsorption site for oxygen in the pristine IrO(110) as Ir, even with a high oxygen coverage.

Spin-mediated promotion of Co catalysts for ammonia synthesis.

Over the past two decades, there has been growing interest in developing catalysts to enable Haber-Bosch ammonia synthesis under milder conditions than currently pertain. Rational catalyst design requires theoretical guidance and clear mechanistic understanding. Recently, a spin-mediated promotion mechanism was proposed to activate traditionally unreactive magnetic materials such as cobalt (Co) for ammonia synthesis by introducing hetero metal atoms bound to the active site of the catalyst surface.

Long-term continuous ammonia electrosynthesis.

Ammonia is crucial as a fertilizer and in the chemical industry and is considered to be a carbon-free fuel. Ammonia electrosynthesis from nitrogen under ambient conditions offers an attractive alternative to the Haber-Bosch process, and lithium-mediated nitrogen reduction represents a promising approach to continuous-flow ammonia electrosynthesis, coupling nitrogen reduction with hydrogen oxidation. However, tetrahydrofuran, which is commonly used as a solvent, impedes long-term ammonia production owing to polymerization and volatility problems.

Phenol as proton shuttle and buffer for lithium-mediated ammonia electrosynthesis.

Ammonia is a crucial component in the production of fertilizers and various nitrogen-based compounds. Now, the lithium-mediated nitrogen reduction reaction (Li-NRR) has emerged as a promising approach for ammonia synthesis at ambient conditions. The proton shuttle plays a critical role in the proton transfer process during Li-NRR.

Calcium-mediated nitrogen reduction for electrochemical ammonia synthesis.

Ammonia (NH) is a key commodity chemical for the agricultural, textile and pharmaceutical industries, but its production via the Haber-Bosch process is carbon-intensive and centralized. Alternatively, an electrochemical method could enable decentralized, ambient NH production that can be paired with renewable energy. The first verified electrochemical method for NH synthesis was a process mediated by lithium (Li) in organic electrolytes.

Activity And Stability of Single- And Di-Atom Catalysts for the O Reduction Reaction.

Efficient and inexpensive catalysts for the O reduction reaction (ORR) are needed for the advancement of renewable energy technologies. In this study, we designed a computational catalyst-screening method to identify single and di-atom metal dopants from first-row transition elements supported on defect-containing nitrogenated graphene surfaces for the ORR. Based on formation-energy calculations and micro-kinetic modelling of reaction pathways using intermediate binding free energies, we have identified four potentially interesting single-atom catalysts (SACs) and fifteen di-atom catalysts (DACs) with relatively high estimated catalytic activity at 0.

Acid anion electrolyte effects on platinum for oxygen and hydrogen electrocatalysis.

Platinum is an important material with applications in oxygen and hydrogen electrocatalysis. To better understand how its activity can be modulated through electrolyte effects in the double layer microenvironment, herein we investigate the effects of different acid anions on platinum for the oxygen reduction/evolution reaction (ORR/OER) and hydrogen evolution/oxidation reaction (HER/HOR) in pH 1 electrolytes. Experimentally, we see the ORR activity trend of HClO > HNO > HSO, and the OER activity trend of HClO [Formula: see text] HNO ∼ HSO.

Electrosynthesis of ammonia with high selectivity and high rates via engineering of the solid-electrolyte interphase.

Ammonia is a large-scale commodity essential to fertilizer production, but the Haber-Bosch process leads to massive emissions of carbon dioxide. Electrochemical ammonia synthesis is an attractive alternative pathway, but the process is still limited by low ammonia production rate and faradaic efficiency. Herein, guided by our theoretical model, we present a highly efficient lithium-mediated process enabled by using different lithium salts, leading to the formation of a uniform solid-electrolyte interphase (SEI) layer on a porous copper electrode.

Analysing oxygen reduction electrocatalysis on transition metal doped niobium oxide(110).

A good oxygen reduction reaction (ORR) catalyst should be stable and active under electrochemical reaction conditions. Niobium pentaoxide (NbO) is known to be stable under ORR conditions. However it has a large band gap, which makes conductivity a challenge during the reaction.

Limits to scaling relations between adsorption energies?

Linear scaling relations have led to an understanding of trends in catalytic activity and selectivity of many reactions in heterogeneous and electro-catalysis. However, linear scaling between the chemisorption energies of any two small molecule adsorbates is not guaranteed. A prominent example is the lack of scaling between the chemisorption energies of carbon and oxygen on transition metal surfaces.

First-Row Transition Metal Antimonates for the Oxygen Reduction Reaction.

The development of inexpensive and abundant catalysts with high activity, selectivity, and stability for the oxygen reduction reaction (ORR) is imperative for the widespread implementation of fuel cell devices. Herein, we present a combined theoretical-experimental approach to discover and design first-row transition metal antimonates as excellent electrocatalytic materials for the ORR. Theoretically, we identify first-row transition metal antimonates─MSbO, where M = Mn, Fe, Co, and Ni─as nonprecious metal catalysts with good oxygen binding energetics, conductivity, thermodynamic phase stability, and aqueous stability.

Opportunities and Challenges in Electrolytic Propylene Epoxidation.

Propylene oxide (PO) is an important chemical. So far, its synthesis protocol relies on expensive oxidants. In contrast, direct epoxidation of propylene (DEP) using molecular oxygen is considered ideal for PO synthesis.

Enhancement of lithium-mediated ammonia synthesis by addition of oxygen.

Owing to the worrying increase in carbon dioxide concentrations in the atmosphere, there is a need to electrify fossil-fuel–powered chemical processes such as the Haber-Bosch ammonia synthesis. Lithium-mediated electrochemical nitrogen reduction has shown preliminary promise but still lacks sufficient faradaic efficiency and ammonia formation rate to be industrially relevant. Here, we show that oxygen, previously believed to hinder the reaction, actually greatly improves the faradaic efficiency and stability of the lithium-mediated nitrogen reduction when added to the reaction atmosphere in small amounts.

Electrochemical oxidation of molecular nitrogen to nitric acid - towards a molecular level understanding of the challenges.

Nitric acid is manufactured by oxidizing ammonia where the ammonia comes from an energy demanding and non-eco-friendly, Haber-Bosch process. Electrochemical oxidation of N to nitric acid using renewable electricity could be a promising alternative to bypass the ammonia route. In this work, we discuss the plausible reaction mechanisms of electrochemical N oxidation (NOR) at the molecular level and its competition with the parasitic oxygen evolution reaction (OER).

Generalizable Trends in Electrochemical Protonation Barriers.

Predicting activation energies for reaction steps is essential for modeling catalytic processes, but accurate barrier simulations often require considerable computational expense, especially for electrochemical reactions. Given the challenges of barrier computations and the growing promise of electrochemical routes for various processes, generalizable energetic trends in electrochemistry can significantly aid in analyzing reaction networks and building microkinetic models. Herein, we employ density functional theory and machine learning nudged elastic band models to simulate electrochemical protonation of *C, *N, and *O monatomic adsorbates from hydronium on a series of transition metal surfaces.

Neural Network Sampling of the Free Energy Landscape for Nitrogen Dissociation on Ruthenium.

In heterogeneous catalysis, free energy profiles of reactions govern the mechanisms, rates, and equilibria. Energetics are conventionally computed using the harmonic approximation (HA), which requires determination of critical states Here, we use neural networks to efficiently sample and directly calculate the free energy surface (FES) of a prototypical heterogeneous catalysis reaction-the dissociation of molecular nitrogen on ruthenium-at density-functional-theory-level accuracy. We find that the vibrational entropy of surface atoms, often neglected in HA for transition metal catalysts, contributes significantly to the reaction barrier.

Tuning the electronic structure of Ag-Pd alloys to enhance performance for alkaline oxygen reduction.

Alloying is a powerful tool that can improve the electrocatalytic performance and viability of diverse electrochemical renewable energy technologies. Herein, we enhance the activity of Pd-based electrocatalysts via Ag-Pd alloying while simultaneously lowering precious metal content in a broad-range compositional study focusing on highly comparable Ag-Pd thin films synthesized systematically via electron-beam physical vapor co-deposition. Cyclic voltammetry in 0.

Effects of a conductive support on the bonding of oxygen containing molecules to transition metal oxide surfaces.

Conventional oxygen electrocatalysts are expensive for industrial use. Transition metal oxides (TMOs), as a more economical option, have emerged as an alternative to potentially replace conventional precious metal catalysts. However, many experimental studies have suggested that although a few of the TMOs supported by conductive substrates are stable under electrocatalytic conditions, their performances are far from the industrial level, especially in the acidic oxygen reduction reaction (ORR).

Acetonitrile Transition Metal Interfaces from First Principles.

Acetonitrile is among the most commonly used nonaqueous solvents in catalysis and electrochemistry. We study its interfaces with multiple facets of the metals Ag, Cu, Pt, and Rh using density functional theory calculations; the structures reported shed new light on experimental observations and underscore the importance of solvent-solvent interactions at high coverage. We investigate the relationship of potential of zero charge (PZC) to metal work function, reporting results in agreement with experimental measurements.

Scaling Relationships and Volcano Plots in Homogeneous Catalysis.

Scaling relations and volcano plots are widely used in heterogeneous catalysis. In this Perspective, we discuss the prospects and challenges associated with the application of similar concepts in homogeneous catalysis using examples from the literature that have appeared recently.

Two-Dimensional Conductive Ni-HAB as a Catalyst for the Electrochemical Oxygen Reduction Reaction.

Catalytic systems whose properties can be systematically tuned via changes in synthesis conditions are highly desirable for the next-generation catalyst design and optimization. Herein, we present a two-dimensional (2D) conductive metal-organic framework consisting of M-N units (M = Ni, Cu) and a hexaaminobenzene (HAB) linker as a catalyst for the oxygen reduction reaction. By varying synthetic conditions, we prepared two Ni-HAB catalysts with different crystallinities, resulting in catalytic systems with different electric conductivities, electrochemical activity, and stability.

Micro-kinetic model of electrochemical carbon dioxide reduction over platinum in non-aqueous solvents.

The competition between the hydrogen evolution reaction and the electrochemical reduction of carbon dioxide to multi-carbon products is a well-known challenge. In this study, we present a simple micro-kinetic model of these competing reactions over a platinum catalyst under a strong reducing potential at varying proton concentrations in a non-aqueous solvent. The model provides some insight into the mechanism of reaction and suggests that low proton concentration and a high fraction of stepped sites is likely to improve selectivity to multi-carbon products.