Activity And Stability of Single- And Di-Atom Catalysts for the O2 Reduction Reaction.

Efficient and inexpensive catalysts for the O2 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 0.8 V vs RHE. Among the best SACs, MnNC shows high stability in both acidic and alkaline media according to our model. For the DACs, we found four possible candidates, MnMn, FeFe, CoCo, and MnNi doped on quad-atom vacancy sites having considerable stability over a wide pH range. The remaining SACs and DACs with high activity are either less stable or show a stability region at an alkaline pH.

High-Voltage-Enabled Stable Cobalt Species Deposition on MnO2 for Water Oxidation in Acid.

The design and maintenance of highly active sites in an acidic environment is vital and challenging for the oxygen evolution reaction (OER). In this work, it is found that the obtained CoO2 under high applied potential can be stable on MnO2 host in acidic environment, which may act as an effective means to solve the instability of cobalt-based electrocatalyst. The significant improvement of acidic OER activity (6.9 times) and stability (46.4 times) of 90-Co-MnO2 (treated by molten salt with more Co deposition sites) demonstrates the advantages of this approach. In situ Raman and the Pourbaix diagram suggest that the enhanced performance derives from the stable presence of CoO2 at the voltage >1.8 V versus reversible hydrogen electrode (RHE). However, when the potential is <1.8 V, the corresponding other cobalt species is too unstable to facilitate the OER. Density functional theorycalculations reveal that the deposited cobalt oxides can act as active sites, thus effectively reducing the reaction energy barrier of the rate-determining step. This work provides a new perspective for enhancing the stability of cobalt-based electrocatalyst. In the future, the dual consideration of applied potential and stable species of active element in the Pourbaix diagram may be a new direction for developing acid-stable electrocatalysts.

Advancing MXene Electrocatalysts for Energy Conversion Reactions: Surface, Stoichiometry, and Stability.

MXenes, due to their tailorable chemistry and favourable physical properties, have great promise in electrocatalytic energy conversion reactions. To exploit fully their enormous potential, further advances specific to electrocatalysis revolving around their performance, stability, compositional discovery and synthesis are required. The most recent advances in these aspects are discussed in detail: surface functional and stoichiometric modifications which can improve performance, Pourbaix stability related to their electrocatalytic operating conditions, density functional theory and advances in machine learning for their discovery, and prospects in large scale synthesis and solution processing techniques to produce membrane electrode assemblies and integrated electrodes. This Review provides a perspective that is complemented by new density functional theory calculations which show how these recent advances in MXene material design are paving the way for effective electrocatalysts required for the transition to integrated renewable energy systems.

Proton Conduction at High Temperature in High-Symmetry Hydrogen-Bonded Molecular Crystals of RuIII Complexes with Six Imidazole-Imidazolate Ligands.

A new H-bonded crystal [RuIII (Him)3 (Im)3 ] with three imidazole (Him) and three imidazolate (Im- ) groups was prepared to obtain a higher-temperature proton conductor than a Nafion membrane with water driving. The crystal is constructed by complementary N-H⋅⋅⋅N H-bonds between the RuIII complexes and has a rare Icy-c* cubic network topology with a twofold interpenetration without crystal anisotropy. The crystals show a proton conductivity of 3.08×10-5  S cm-1 at 450 K and a faster conductivity than those formed by only HIms. The high proton conductivity is attributed to not only molecular rotations and hopping motions of HIm frameworks that are activated at ∼113 K, but also isotropic whole-molecule rotation of [RuIII (Him)3 (Im)3 ] at temperatures greater than 420 K. The latter rotation was confirmed by solid-state 2 H NMR spectroscopy; probable proton conduction routes were predicted and theoretically considered.

Strong Proton-Electron Coupling in π-Planar Metal Complex with Redox-Active Ligands.

Proton-coupled electron transfer (PCET) of metal complexes has been widely studied, especially in biochemistry and catalytic chemistry. Although metal complexes bearing redox-active ligands play a part in these research areas, those with π-planar structure remain entirely unexplored, which are vital for future development of iono-electronics. Here, proton-electron coupling of a π-planar nickel complex bearing redox-active N,S-ligands, Ni(itsq)2 , was investigated by combining experimental and theoretical approaches. Strong proton-electron coupling was manifested in a large potential shift, which is twice greater than that of a typical PCET-type π-planar metal complex with redox-inactive ligands, [Ni(dcpdt)2 ]2- . Theoretical calculations affirmed that the stabilization of frontier orbitals by protonation is greater in Ni(itsq)2 than that in [Ni(dcpdt)2 ]2- . These results indicate that π-planar metal complexes with redox-active ligands are promising for developing novel PCET-type materials.

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─MSb2O6, where M = Mn, Fe, Co, and Ni─as nonprecious metal catalysts with good oxygen binding energetics, conductivity, thermodynamic phase stability, and aqueous stability. Among the considered antimonates, MnSb2O6 shows the highest theoretical ORR activity based on the 4e- ORR kinetic volcano. Experimentally, nanoparticulate transition metal antimonate catalysts are found to have a minimum of a 2.5-fold enhancement in intrinsic mass activity (on transition metal mass basis) relative to the corresponding transition metal oxide at 0.7 V vs RHE in 0.1 M KOH. MnSb2O6 is the most active catalyst under these conditions, with a 3.5-fold enhancement on a per Mn mass activity basis and 25-fold enhancement on a surface area basis over its antimony-free counterpart. Electrocatalytic and material stability are demonstrated over a 5 h chronopotentiometry experiment in the stability window identified by theoretical Pourbaix analysis. This study further highlights the stable and electrically conductive antimonate structure as a framework to tune the activity and selectivity of nonprecious metal oxide active sites for ORR catalysis.

Combining pressure and electrochemistry to synthesize superhydrides.

Recently, superhydrides have been computationally identified and subsequently synthesized with a variety of metals at very high pressures. In this work, we evaluate the possibility of synthesizing superhydrides by uniquely combining electrochemistry and applied pressure. We perform computational searches using density functional theory and particle swarm optimization calculations over a broad range of pressures and electrode potentials. Using a thermodynamic analysis, we construct pressure-potential phase diagrams and provide an alternate synthesis concept, pressure-potential ([Formula: see text]), to access phases having high hydrogen content. Palladium-hydrogen is a widely studied material system with the highest hydride phase being Pd3H4 Most strikingly for this system, at potentials above hydrogen evolution and ∼ 300 MPa pressure, we find the possibility to make palladium superhydrides (e.g., PdH10). We predict the generalizability of this approach for La-H, Y-H, and Mg-H with 10- to 100-fold reduction in required pressure for stabilizing phases. In addition, the [Formula: see text] strategy allows stabilizing additional phases that cannot be done purely by either pressure or potential and is a general approach that is likely to work for synthesizing other hydrides at modest pressures.

On the Origins of Some Spectroscopic Properties of "Purple Iron" (the Tetraoxoferrate(VI) Ion) and Its Pourbaix Safe-Space.

In this work, the authors attempt to interpret the visible, infrared and Raman spectra of ferrate(VI) by means of theoretical physical-inorganic chemistry and historical highlights in this field of interest. In addition, the sacrificial decomposition of ferrate(VI) during water treatment will also be discussed together with a brief mention of how Rayleigh scattering caused by the decomposition of FeVIO42- may render absorbance readings erroneous. This work is not a compendium of all the instrumental methods of analysis which have been deployed to identify ferrate(VI) or to study its plethora of reactions, but mention will be made of the relevant techniques (e.g., Mössbauer Spectroscopy amongst others) which support and advance this overall discourse at appropriate junctures, without undue elaboration on the foundational physics of these techniques.

Investigating trace metal precipitation in highly concentrated cell culture media with Pourbaix diagrams.

Commercial production of therapeutic proteins using mammalian cells requires complex process solutions, and consistency of these process solutions is critical to maintaining product titer and quality between batches. Inconsistencies between process solutions prepared at bench and commercial scale may be due to differences in mixing time, temperature, and pH which can lead to precipitation and subsequent removal via filtration of critical solution components such as trace metals. Pourbaix diagrams provide a useful tool to model the solubility of trace metals and were applied to troubleshoot the scale-up of nutrient feed preparation after inconsistencies in product titer were observed between bench- and manufacturing-scale batches. Pourbaix diagrams modeled the solubility of key metals in solution at various stages of the nutrient feed preparation and identified copper precipitation as the likely root cause of inconsistent medium stability at commercial scale. Copper precipitation increased proportionally with temperature in bench-scale preparations of nutrient feed and temperature was identified as the root cause of copper precipitation at the commercial scale. Additionally, cell culture copper titration studies performed in bench-scale bioreactors linked copper-deficient mammalian cell culture to inconsistent titers at the commercial scale. Pourbaix diagrams can predict when trace metals are at risk of precipitating and can be used to mitigate risk during the scale-up of complex medium preparations.

Structural Evolution and Underlying Mechanism of Single-Atom Centers on Mo2C(100) Support during Oxygen Reduction Reaction.

The single-metal atoms coordinating with the surface atoms of the support constitute the active centers of as-prepared single-atom catalysts (SACs). However, under hash electrochemical conditions, (1) supports' surfaces may experience structural change, which turn to be distinct from those at ambient conditions; (2) during catalysis, the dynamic responses of a single atom to the attack of reaction intermediates likely change the coordination environment of a single atom. These factors could alter the performance of SACs. Herein, we investigate these issues using Mo2C(100)-supported single transition-metal (TM) atoms as model SACs toward catalyzing the oxygen reduction reaction (ORR). It is found that the Mo2C(100) surface is oxidized under ORR turnover conditions, resulting in significantly weakened bonding between single TM atoms and the Mo2C(100) surface (TM@Mo2C(100)_O* term for SAC). While the intermediate in 2 e- ORR does not change the local structures of the active centers in these SACs, the O* intermediate emerging in 4 e- ORR can damage Rh@ and Cu@Mo2C(100)_O*. Furthermore, on the basis of these findings, we propose Pt@Mo2C(100)_O* as a qualified ORR catalyst, which exhibits extraordinary 4 e- ORR activity with an overpotential of only 0.33 V, surpassing the state-of-the-art Pt(111), and thus being identified as a promising alternative to the commercial Pt/C catalyst.

Building Ab Initio Interface Pourbaix diagrams to Investigate Electrolyte Stability in the Electrochemical Double Layer: Application to Magnesium Batteries.

Insights into the electrochemical processes occurring at the electrode-electrolyte interface are a crucial step in most electrochemistry domains and in particular in the optimization of the battery technology. However, studying potential-dependent processes at the interface is one of the biggest challenges, both for theoreticians and experimentalists. The challenge is pushed further when stable species also depend on the concentration of specific ligands in the electrolyte, such as chlorides. Herein, we present a general theoretical ab initio methodology to compute a Pourbaix-like diagram of complex electrolytes as a function of electrode potential and anion's chemical potential, that is, concentration. This approach is developed not only for the bulk properties of the electrolytes but also for electrode-electrolyte interfaces. In the case of chlorinated magnesium complexes in dimethoxyethane, we show that the stability domains of the different species are strongly shifted at the interface compared to the bulk of the electrolyte because of the strong local electric fields and charges occurring in the double layer. Thus, as the interfacial stability domains are strongly modified, this approach is necessary to investigate all interface properties that often govern the reaction kinetics, such as solvent degradation at the electrode. Interface Pourbaix diagram is used to give some insights into the improved stability at the Mg anode induced by the addition of chloride. Because of its far-reaching insights, transferability, and wide applicability, the methodology presented herein should serve as a valuable tool not only for the battery community but also for the wider electrochemical one.

Electrochemical Hydrogen Evolution at the Interface of Monolayer VS2 and Water from First-Principles Calculations.

Density functional theory calculations are carried out to study the hydrogen evolution reaction (HER) at the electrochemical double-layer interface of monolayer 2H phase VS2 and water. Under typical conditions of HER, the catalyst surface is predicted to have a low hydrogen coverage of about 12%, whereas the aqueous solution side features a high hydronium concentration of about 8.3%. As a result, the HER takes place through the Volmer-Heyrovsky route, with an overall reaction barrier of about 1.0 eV, much larger than that of 1T phase VS2. This result demonstrates that 2H phase VS2 is much less reactive than its 1T phase counterpart, and the 1T-to-2H phase transformation induced by thickness reduction may deteriorate the HER activity of VS2.

Iron and redox cycling. Do's and don'ts.

A major form of toxicity arises from the ability of iron to redox cycle, that is, to accept an electron from a reducing compound and to pass it on to H2O2 (the Fenton reaction). In order to do so, iron must be suitably complexed to avoid formation of Fe2O3. The ligands determine the electrode potential; this information should be known before experiments are carried out. Only one-electron transfer reactions are likely to be significant; thus two-electron potentials should not be used to determine whether an iron(III) complex can be reduced or oxidized. Ascorbate is the relevant reducing agent in blood serum, which means that iron toxicity in this compartment arises from the ascorbate-driven Fenton reaction. In the cytosol, an iron(II)-glutathione complex is likely to be the low-molecular weight iron complex involved in toxicity. When physiologically relevant concentrations are used the window of redox opportunity ranges from +0.1 V to +0.9 V. The electrode potential for non-transferrin-bound iron in the form of iron citrate is close to 0 V and the reduction of iron(III) citrate by ascorbate is slow. The clinically utilised chelators desferrioxamine, deferiprone and deferasirox in each case render iron complexes with large negative electrode potentials, thus being effective in preventing iron redox cycling and the associated toxicity resulting from such activity. There is still uncertainty about the product of the Fenton reaction, HO• or FeO2+.

Toxicity assessment and geochemical model of chromium leaching from AOD slag.

AOD (Argon Oxygen Decarburization) slag is a by-product of the stainless steel refining process. The leaching toxicity of chromium from AOD slag cannot be ignored in the recycling process of the AOD slag. To assess the leaching toxicity of the AOD slag, batch leaching tests have been performed. PHREEQC simulations combined with FactSage were carried out based on the detailed mineralogical analysis and petrophysical data. Moreover, Pourbaix diagram of the Cr-H2O system was protracted by HSC 5.0 software to explore the chromium speciation in leachates. It was found that AOD slag leachate is an alkaline and reductive solution. The Pourbaix diagram of the Cr-H2O system indicated that trivalent chromium, such as Cr(OH)4(-), is the major chromium species in the experimental Eh-pH region considered. However, toxic hexavalent chromium was released with maximum concentrations of 30 µg L(-1) and 18 µg L(-1) at L/S 10 and 100, respectively, during the earlier leaching stage. It concluded that the AOD slag possessed a certain leaching toxicity. After 10 d of leaching, trivalent chromium was the dominant species in the leachates, which corresponded to the results of PHREEQC simulation. Leaching toxicity of AOD slag is based on the chromium speciation and its transformation. Great attention should be focused on such factors as aging, crystal form of chromium-enriched minerals, and electrochemical characteristics of the leachates.

In situ Electrochemical Small-Angle Neutron Scattering (eSANS) for Quantitative Structure and Redox Properties of Nanoparticles.

The rapid growth in nanomaterial applications have revealed limitations in available physicochemical characterization methods. An in situ electrochemical small-angle neutron scattering (eSANS) methodology was devised that enables direct measurements of nanomaterial dispersion structure while undergoing reduction-oxidation (redox) reactions at the vitreous carbon electrode. Furthermore, these porous electrodes are amenable to contrast-variant neutron scattering strategies to measure nanoparticle structure and polymer conformation in multicomponent systems. The eSANS method was tested for feasibility by characterizing ZnO nanoparticles in 50 mmol/L NaCl deuterium oxide solution undergoing bulk electrolysis at negative potentials. Irreversible nanoparticle structural changes are observed during the potential cycle. The complete reduction of Zn(2+) to Zn(0) nanoparticles is unlikely, but a peak in the characteristic correlation length occurs during the redox bias with reduced average characteristic size.