Advancing Cr(VI) Electroreduction: A Redox Mediator to Catalyze the Electrochemical Reduction of Cr(VI) in Water While Preventing Fouling of Carbon Electrodes.

Hexavalent chromium is a contaminant of concern and is found in drinking water supplies. Electrochemical methods are well-suited to accomplish the reduction of toxic Cr(VI) to Cr(III). However, high overpotentials and plating of Cr(III) products on electrodes have stymied the development of efficacious purification methods. The Cr(VI) reduction reaction necessitates the transfer of multiple protons and electrons, which is accompanied by a high kinetic barrier. Following recent advances in the electrocatalytic energy storage community, we report that the use of [Fe(CN)6]3- as a small molecular electrocatalyst not only diminishes the overpotential for Cr(VI) reduction on carbon electrodes by 0.575 V, but also prevents electrode fouling by mediating solution-phase homogeneous electron transfers.

Investigation of Iron(III) Tetraphenylporphyrin as a Redox Flow Battery Anolyte: Unexpected Side Reactivity with the Electrolyte.

Redox flow batteries (RFBs) present an opportunity to bridge the gap between the intermittent availability of green energy sources and the need for on-demand grid level energy storage. While aqueous vanadium-based redox flow batteries have been commercialized, they are limited by the constraints of using water as an electrochemical solvent. Nonaqueous redox flow battery systems can be used to produce high voltage batteries due to the larger electrochemical window in nonaqueous solvents and the ability to tune the redox properties of active materials through functionalization. Iron porphyrins, a class of organometallic macrocycles, have been the subject of many studies for their photocatalytic and electrocatalytic properties in nonaqueous solvents. Often, iron porphyrins can undergo multiple redox events making them interesting candidates for use as anolytes in asymmetrical redox flow batteries or as both catholyte and anolyte in symmetrical redox flow battery systems. Here the electrochemical properties of Fe(III)TPP species relevant to redox flow battery electrolytes are investigated including solubility, electrochemical properties, and charge/discharge cycling. Commonly used support electrolyte salts can have reactivities that are often overlooked beyond their conductivity properties in nonaqueous solvents. Parasitic reactions with the cations of common support electrolytes are highlighted herein, which underscore the careful balance required to fully assess the potential of novel RFB electrolytes.

Impact of the choice of buffer on the electrochemical reduction of Cr(vi) in water on carbon electrodes.

Hexavalent chromium is a contaminant of concern in water. Electrochemical methods are being developed to reduce toxic Cr(vi) to benign Cr(iii) at the point of generation or point of use. The effectiveness of glassy carbon electrodes to detect and reduce Cr(vi) in cyclic voltammetry was recently demonstrated. Herein, we report that the nature of the buffer system used, at a fixed pH, has unexpected impacts on the electrochemical reduction of Cr(vi) in water. At low concentrations of Cr(vi), the buffer influences the PCET step gating Cr(vi) reduction on the timescale of cyclic voltammetry experiments. At higher concentrations of Cr(vi), the effect is more complex. Data suggests impacts on both the chemical steps of Cr(vi) reduction and the nature of the products formed, hypothesized to be due to chelation effects. In particular, evidence of adsorption on the electrode surface is seen through cyclic voltammetry studies in certain buffers. Chronoamperometry studies confirm the adsorption of chromium containing species on the electrode surface during Cr(vi) electroreduction. XPS confirms Cr(iii) as the product. The activity of the electrode is regained after an acid wash step, without the need for re-polishing. This work provides a framework to understand the impact of the presence of small organic acids on Cr(vi) reduction for water purification.

A Comparison of the Photophysical, Electrochemical and Cytotoxic Properties of meso-(2-, 3- and 4-Pyridyl)-BODIPYs and Their Derivatives.

Boron dipyrromethene (BODIPY) dyes bearing a pyridyl moiety have been used as metal ion sensors, pH sensors, fluorescence probes, and as sensitizers for phototherapy. A comparative study of the properties of the three structural isomers of meso-pyridyl-BODIPYs, their 2,6-dichloro derivatives, and their corresponding methylated cationic pyridinium-BODIPYs was conducted using spectroscopic and electrochemical methods, X-ray analyses, and TD-DFT calculations. Among the neutral derivatives, the 3Py and 4Py isomers showed the highest relative fluorescence quantum yields in organic solvents, which were further enhanced 2-4-fold via the introduction of two chlorines at the 2,6-positions. Among the cationic derivatives, the 2catPy showed the highest relative fluorescence quantum yield in organic solvents, which was further enhanced by the use of a bulky counter anion (PF6-). In water, the quantum yields were greatly reduced for all three isomers but were shown to be enhanced upon introduction of 2,6-dichloro groups. Our results indicate that 2,6-dichloro-meso-(2- and 3-pyridinium)-BODIPYs are the most promising for sensing applications. Furthermore, all pyridinium BODIPYs are highly water-soluble and display low cytotoxicity towards human HEp2 cells.

Tale of a "Non-interacting" Additive in a Lithium-Ion Electrolyte: Effect on Ionic Speciation and Electrochemical Properties.

New lithium electrolytes compatible with high energy density cells are critical for lithium metal battery applications, but dendrite formation associated with the use of dilute organic electrolytes complicates their realization. High-concentration electrolytes mitigate some of the issues of the electrolytes but introduce additional problems, such as low conductivity and high cost. Hence, pseudo-concentrated electrolytes, wherein a co-solvent is added to a dilute electrolyte, have been presented as a possible alternative to both dilute and concentrated electrolytes. However, the effect that the co-solvent has on the electrolyte properties at both macroscopic and microscopic levels is unknown. Here, a study of the structure and electrochemical properties of two electrolytes as a function of co-solvent concentration is presented using an array of spectroscopies (FTIR, ATR-FTIR, and nuclear magnetic resonance) and computational methods (density functional theory calculations). The chosen electrolytes comprised two different lithium salts (LiPF6 and LiTFSI) in a mixture of dimethyl carbonate (DMC) with 1,1,1,3,3-pentafluorobutane (PFB) as the co-solvent. Our results show that in the case of the LiPF6/DMC electrolyte, the addition of a co-solvent (PFB) with a larger dielectric constant results in the strengthening of the lithium-anion interaction and the formation of aggregate species since PFB does not interact with the anion. Conversely, in the LiTFSI/DMC electrolyte, the co-solvent appears to interact with the anion via hydrogen bonds, which leads to the dissociation of contact ion pairs. The change in ionic speciation of the electrolytes upon addition of PFB provides a reasonable framework to explain the different trends in both the bulk and interfacial macroscopic properties, such as conductivity, viscosity, and electrochemical stability. Overall, our findings demonstrate that the interactions between the anion and the co-solvent must be taken into consideration when adding a co-solvent because they play a major role in determining the final electrolyte properties.

Redox-Induced Structural Reorganization Dictates Kinetics of Cobalt(III) Hydride Formation via Proton-Coupled Electron Transfer.

Two-electron, one-proton reactions of a family of [CoCp(dxpe)(NCCH3)]2+ complexes (Cp = cyclopentadienyl, dxpe = 1,2-bis(di(aryl/alkyl)phosphino)ethane) form the corresponding hydride species [HCoCp(dxpe)]+ (dxpe = dppe (1,2-bis(diphenylphosphino)ethane), depe (1,2-bis(diethylphosphino)ethane), and dcpe (1,2-bis(dicyclohexylphosphino)ethane)) through a stepwise proton-coupled electron transfer process. For three [CoCp(dxpe)(NCCH3)]2+ complexes, peak shift analysis was employed to quantify apparent proton transfer rate constants from cyclic voltammograms recorded with acids ranging 22 pKa units. The apparent proton transfer rate constants correlate with the strength of the proton source for weak acids, but these apparent proton transfer rate constants curiously plateau (kpl) as the reaction becomes increasingly exergonic. The absolute apparent proton transfer rate constants across both these regions correlate with the steric bulk of the chelating diphosphine ligand, with bulkier ligands leading to slower kinetics (kplateau,depe = 3.5 × 107 M-1 s-1, kplateau,dppe = 1.7 × 107 M-1 s-1, kplateau,dcpe = 7.1 × 104 M-1 s-1). Mechanistic studies were conducted to identify the cause of the aberrant kPTapp-ΔpKa trends. When deuterated acids are employed, deuterium incorporation in the Cp ring is observed, indicating protonation of the CoCp(dxpe) species to form the corresponding hydride proceeds via initial ligand protonation. Digital simulations of cyclic voltammograms show ligand loss accompanying initial reduction gates subsequent PCET activity at higher driving forces. Together, these experiments reveal the details of the reaction mechanism: reduction of the Co(III) species is followed by dissociation of the bound acetonitrile ligand, subsequent reduction of the unligated Co(II) species to form a Co(I) species is followed by protonation, which occurs at the Cp ring, followed by tautomerization to generate the stable Co(III)-hydride product [HCoCp(dxpe)]+. Analysis as a function of chelating disphosphine ligand, solvent, and acid strength reveals that the ligand dissociation equilibrium is directly influenced by the steric bulk of the phosphine ligands and gates protonation, giving rise to the plateau of the apparent proton transfer rate constant with strong acids. The complexity of the reaction mechanism underpinning hydride formation, encompassing dynamic behavior of the entire ligand set, highlights the critical need to understand elementary reaction steps in proton-coupled electron transfer reactions.

Electrochemical reduction of Cr(VI) in water: lessons learned from fundamental studies and applications.

Converting toxic Cr(vi) to benign Cr(iii) would offer a solution to decontaminate drinking water. Electrochemical methods are ideally suited to carry out this reduction without added external reductants. Achieving this transformation at low overpotentials requires mediating the transfer of protons and electrons to Cr(vi). In this review thermodynamic parameters will be discussed to understand Cr(vi) speciation in water and identify reduction pathways. The electrochemical reduction of Cr(vi) at bare electrodes is reviewed and mechanistic considerations are discussed. Works on modified electrodes are compared to identify key parameters influencing the reduction. An overview of current applications to Cr(vi) reduction is briefly discussed to link fundamental studies to applications.

Fighting Deactivation: Classical and Emerging Strategies for Efficient Stabilization of Molecular Electrocatalysts.

Development of highly active molecular electrocatalysts for fuel-forming reactions has relied heavily on understanding mechanistic aspects of the electrochemical transformations. Careful fine-tuning of the ligand environment oriented mechanistic pathways towards higher activity and optimal product distribution for several catalysts. Unfortunately, many catalysts deactivate in bulk electrolysis conditions, diminishing the impact of the plethora of highly tuned molecular electrocatalytic systems. This Minireview covers classical and emerging methods developed to circumvent catalyst deactivation and degradation, with an emphasis on successes with molecular electrocatalysts.

Molecular polypyridine-based metal complexes as catalysts for the reduction of CO2.

Polypyridyl transition metal complexes represent one of the more thoroughly studied classes of molecular catalysts towards CO2 reduction to date. Initial reports in the 1980s began with an emphasis on 2nd and 3rd row late transition metals, but more recently the focus has shifted towards earlier metals and base metals. Polypyridyl platforms have proven quite versatile and amenable to studying various parameters that govern product distribution for CO2 reduction. However, open questions remain regarding the key mechanistic steps that govern product selectivity and efficiency. Polypyridyl complexes have also been immobilized through a variety of methods to afford active catalytic materials for CO2 reductions. While still an emerging field, materials incorporating molecular catalysts represent a promising strategy for electrochemical and photoelectrochemical devices capable of CO2 reduction. In general, this class of compounds remains the most promising for the continued development of molecular systems for CO2 reduction and an inspiration for the design of related non-polypyridyl catalysts.

Reaction Parameters Influencing Cobalt Hydride Formation Kinetics: Implications for Benchmarking H(2)-Evolution Catalysts.

The need for benchmarking hydrogen evolution catalysts has increasingly been recognized. The influence of acid choice on activity is often reduced to the overpotential for catalysis. Through the study of a stable cobalt hydride complex, we demonstrate the influence of acid choice, beyond pKa, on the kinetics of hydride formation. A linear free energy relationship between acid pKa and second-order rate constants is observed for weaker acids. For stronger acids, however, further increases in pKa do not correlate to increases in rate constants. Further, steric bulk around the acidic proton is shown to influence rate constants dramatically. Together, these observations reveal the complex factors dictating catalyst performance.

Photocatalytic carbon dioxide reduction with rhodium-based catalysts in solution and heterogenized within metal-organic frameworks.

The first photosensitization of a rhodium-based catalytic system for CO2 reduction is reported, with formate as the sole carbon-containing product. Formate has wide industrial applications and is seen as valuable within fuel cell technologies as well as an interesting H2 -storage compound. Heterogenization of molecular rhodium catalysts is accomplished via the synthesis, post-synthetic linker exchange, and characterization of a new metal-organic framework (MOF) Cp*Rh@UiO-67. While the catalytic activities of the homogeneous and heterogeneous systems are found to be comparable, the MOF-based system is more stable and selective. Furthermore it can be recycled without loss of activity. For formate production, an optimal catalyst loading of ∼10 % molar Rh incorporation is determined. Increased incorporation of rhodium catalyst favors thermal decomposition of formate into H2 . There is no precedent for a MOF catalyzing the latter reaction so far.

Versatile functionalization of carbon electrodes with a polypyridine ligand: metallation and electrocatalytic H(+) and CO2 reduction.

A strategy is proposed for immobilization of homogeneous catalysts whereby a glassy carbon electrode is functionalized by electro-grafting of a ligand, terpyridine. The modified electrode can easily be metallated with cobalt and shows activity towards catalytic proton and CO2 reduction. The metal can be removed and the electrode re-metallated at will.

Turning it off! Disfavouring hydrogen evolution to enhance selectivity for CO production during homogeneous CO2 reduction by cobalt-terpyridine complexes.

Understanding the activity and selectivity of molecular catalysts for CO2 reduction to fuels is an important scientific endeavour in addressing the growing global energy demand. Cobalt-terpyridine compounds have been shown to be catalysts for CO2 reduction to CO while simultaneously producing H2 from the requisite proton source. To investigate the parameters governing the competition for H+ reduction versus CO2 reduction, the cobalt bisterpyridine class of compounds is first evaluated as H+ reduction catalysts. We report that electronic tuning of the ancillary ligand sphere can result in a wide range of second-order rate constants for H+ reduction. When this class of compounds is next submitted to CO2 reduction conditions, a trend is found in which the less active catalysts for H+ reduction are the more selective towards CO2 reduction to CO. This represents the first report of the selectivity of a molecular system for CO2 reduction being controlled through turning off one of the competing reactions. The activities of the series of catalysts are evaluated through foot-of-the-wave analysis and a catalytic Tafel plot is provided.

Terpyridine complexes of first row transition metals and electrochemical reduction of CO2 to CO.

Homoleptic terpyridine complexes of first row transition metals are evaluated as catalysts for the electrocatalytic reduction of CO2. Ni and Co-based catalytic systems are shown to reduce CO2 to CO under the conditions tested. The Ni complex was found to exhibit selectivity for CO2 over proton reduction while the Co-based system generates mixtures of CO and H2 with CO : H2 ratios being tuneable through variation of the applied potential.

Halogen Photoelimination from Dirhodium Phosphazane Complexes via Chloride-Bridged Intermediates.

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.

Stability-enhanced hydrogen-evolving dirhodium photocatalysts through ligand modification.

The two-electron mixed-valent complex Rh(2)(0,II)(tfepma)(2)(CN(t)Bu)(2)Cl(2) (tfepma = CH(3)N[P(OCH(2)CF(3))(2)](2)) photocatalytically splits HCl to generate H(2). Whereas this catalyst degrades rapidly, with H(2) production ceasing after about 36 hours (3 turnovers), a modified complex, Rh(2)(0,II)(tfepma)(2)(CNAd)(2)Cl(2) (CNAd = 1-adamantylisocyanide) displays enhanced stability with sustained H(2) production continuing for >144 h (7 turnovers).

Subcomponent self-assembly and guest-binding properties of face-capped Fe4L4(8+) capsules.

A general method for preparing Fe(4)L(4) face-capped tetrahedral cages through subcomponent self-assembly was developed and has been demonstrated using four different C(3)-symmetric triamines, 2-formylpyridine, and iron(II). Three of the triamines were shown also to form Fe(2)L(3) helicates when the appropriate stoichiometry of subcomponents was used. Two of the cages were observed to have nearly identical Fe-Fe distances in the solid state, which enabled their ligands to be coincorporated into a collection of mixed cages. Only one of the cages combined a sufficiently large cavity with the sufficiently small pores required for guest binding, taking up a wide variety of guest species in size- and shape-selective fashion.