Modification of Transition-Metal Redox by Interstitial Water in Hexacyanometalate Electrodes for Sodium-Ion Batteries.

A sodium-ion battery (SIB) solution is attractive for grid-scale electrical energy storage. Low-cost hexacyanometalate is a promising electrode material for SIBs because of its easy synthesis and open framework. Most hexacyanometalate-based SIBs work with aqueous electrolyte, and interstitial water in the material has been found to strongly affect the electrochemical profile, but the mechanism remains elusive. Here we provide a comparative study of the transition-metal redox in hexacyanometalate electrodes with and without interstitial water based on soft X-ray absorption spectroscopy and theoretical calculations. We found distinct transition-metal redox sequences in hydrated and anhydrated NaxMnFe(CN)6·zH2O. The Fe and Mn redox in hydrated electrodes are separated and are at different potentials, leading to two voltage plateaus. On the contrary, mixed Fe and Mn redox in the same potential range is found in the anhydrated system. This work reveals for the first time how transition-metal redox in batteries is strongly affected by interstitial molecules that are seemingly spectators. The results suggest a fundamental mechanism based on three competing factors that determine the transition-metal redox potentials. Because most hexacyanometalate electrodes contain water, this work directly reveals the mechanism of how interstitial molecules could define the electrochemical profile, especially for electrodes based on transition-metal redox with well-defined spin states

Long-term water mass and redox potential dynamics in large weighable fen lysimeter

The water mass and redox potentials dynamics were intensively studied in peat soils. However, most of these studies were conducted either in the laboratory or in situ for the short-term period that does not reflect the actual long-term field conditions. Although sustainable management of peat soils needs to understand long-term water and redox potentials dynamics, there is little information on undisturbed peat soils. To address this problem, a large weighable fen lysimeter (LWFL) was installed at Lysimeter Station Falkenberg in 2003. The LWFL was filled with undisturbed 6 m3 (4 m length, 1 m width and 1.5 m depth) fen monolith. The lysimeter vessel had four electronic load cells at the four supporting points which are sensitive to detect and measure mass changes because of rain and evapotranspiration. Platinum redox electrodes were also horizontally installed in 20, 50 and 120 cm soil depth in three replications. The mass changes and redox potentials were automatically measured every hour interval for 13 years. The preliminary evaluation of the 13 years’ data showed that the mass change was strongly and positively associated with the rainfall distribution pattern. The lowest and highest annual masses were 7092 and 9768 kg that was recorded in 2009 and 2016, respectively. Both the average and individual years of monthly and daily mass changes were also the reflection of seasonal changes that affect the rainfall amount and distribution. However, the hourly mass change was not significant within a year, but among the most years. The change in redox potentials showed similar trends to that of the mass change but in the opposite directions. The redox potentials increased as mass decreased and vice versa. The results indicate that the lysimeter can be used to study long-term water and redox potentials dynamics to understand the biogeochemistry of peat soils

The redox potential of DDT: An environmental perspective

Due to their low water solubility, the reduction potentials of DDT and dicofol are difficult to determine experimentally. Through computational methods, we calculated the one and two electron reduction potentials for DDT as 0.51 V and 0.67 V, respectively, and those for dicofol were 0.50 V and 0.74 V. This study is significant because we are able to calculate reliable redox potentials that may be utilized to predict chemical behavior and degradation pathways of environmental pollutants. To assess the accuracy of theoretical chemistry, we compared experimental and computational data and examined whether our calculated redox potentials fit with the known redox behavior of DDT

Nitrate elimination by denitrification in hardwood forest soils of the Upper Rhine floodplain – correlation with redox potential and organic matter

Denitrification in floodplains is a major issue for river- and groundwater quality. In the Upper Rhine valley, floodplain forests are about to be restored to serve as flood retention areas (polders). Besides flood attenuation in downstream areas, improvement of water quality became recently a major goal for polder construction. Redox potential monitoring was suggested as a means to support assessment of nitrogen elimination in future floodplains by denitrification during controlled flooding. To elucidate the relationship between redox potential and denitrification, experiments with floodplain soils and in situ measurements were done. Floodplain soil of two depth profiles from a hardwood forest of the Upper Rhine valley was incubated anaerobically with continuous nitrate supply. Reduction of nitrate was followed and compared with redox potential and organic matter content. The redox potential under denitrifying conditions ranged from 10 to 300 mV. Redox potential values decreased with increasing nitrate reduction rates and increasing organic matter content. Furthermore, a narrow correlation between organicmatter and nitrate reduction was observed. Experiments were intended to help interpreting redox potentials generated under in situ conditions as exemplified by in situ observations for the year 1999. Results obtained by experiments and in situ observations showed that monitoring of redox potential could support management of the flooding regime to optimize nitrogen retention by denitrification in future flood retention areas

Redox potentials of aryl derivatives from hybrid functional based first principles molecular dynamics

Acknowledgements We acknowledge the National Science Foundation of China (No. 41222015, 41273074, 41572027 and 21373166), Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase), the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (No. 201228), Newton International Fellowship Program and the financial support from the State Key Laboratory at Nanjing University. We are grateful to the High Performance Computing Center of Nanjing University for allowing us to use the IBM Blade cluster system. Open access via RSC Gold for GoldPeer reviewedPublisher PD

Water oxidation catalysis – role of redox and structural dynamics in biological photosynthesis and inorganic manganese oxides

Water oxidation is pivotal in biological photosynthesis, where it is catalyzed by a protein-bound metal complex with a Mn4Ca-oxide core; related synthetic catalysts may become key components in non-fossil fuel technologies. Going beyond characterization of the catalyst resting state, we compare redox and structural dynamics of three representative birnessite-type Mn(Ca) oxides (catalytically active versus inactive; with/without calcium) and the biological catalyst. In the synthetic oxides, Mn oxidation was induced by increasingly positive electrode potentials and monitored by electrochemical freeze-quench and novel time-resolved in situ experiments involving detection of X-ray absorption and UV-vis transients, complemented by electrochemical impedance spectroscopy. A minority fraction of Mn(III) ions present at catalytic potentials is found to be functionally crucial; calcium ions are inessential but tune redox properties. Redox-state changes of the water- oxidizing Mn oxide are similarly fast as observed in the biological catalyst (<10 ms), but 10–100 times slower in the catalytically inactive oxide. Surprisingly similar redox dynamics of biological catalyst and water-oxidizing Mn(Ca) oxides suggest that in both catalysts, rather than direct oxidation of bound water species, oxidation equivalents are accumulated before onset of the multi-electron O–O bond formation chemistry in Mn(III)–Mn(IV) oxidation steps coupled to changes in the oxo-bridging between metal ions. Aside from the ability of the bulk oxide to undergo Mn oxidation-state changes, we identify two further, likely interrelated prerequisites for catalytic activity of the synthetic oxides: (i) the presence of Mn(III) ions at catalytic potentials preventing formation of an inert all-Mn(IV) oxide and (ii) fast rates of redox-state changes approaching the millisecond time domain

Redox Potentials in a Cropped Potato Processing Waste Water Disposal Field with a Deep Water Table

Redox potential measurements were made in a field irrigated with potato processing waste water at seven depths of 5 to 150 cm for 14 mo. Irrigation with canal water mixed with waste water in the summer, and with waste water in the winter, decreased redox potentials in the field at some depths for a short time but not enough to cause denitrification. However, as the soil temperature increased in the spring, and decomposition of the accumulated waste organic matter accelerated, redox potentials decreased after each irrigation at all observed depths. During April, redox potentials low enough to promote denitrification (below + 225 mV) at 90-, 120-, and 150-cm depths in the soil persisted for 2 weeks. Irrigation with nondiluted waste water in June and July decreased redox potentials and denitrification occurred for up to 3 days after irrigations. As the soil temperature increased in the spring, nitrification of accumulated organic matter increased son nitrates. Waste water irrigations from April to July promoted denitrification, removing most of the nitrate from the soil, and thereby decreasing the potential for ground water pollution

Water at an electrochemical interface - a simulation study

The results of molecular dynamics simulations of the properties of water in an aqueous ionic solution close to an interface with a model metallic electrode are described. In the simulations the electrode behaves as an ideally polarizable hydrophilic metal, supporting image charge interactions with charged species, and it is maintained at a constant electrical potential with respect to the solution so that the model is a textbook representation of an electrochemical interface through which no current is passing. We show how water is strongly attracted to and ordered at the electrode surface. This ordering is different to the structure that might be imagined from continuum models of electrode interfaces. Further, this ordering significantly affects the probability of ions reaching the surface. We describe the concomitant motion and configurations of the water and ions as functions of the electrode potential, and we analyze the length scales over which ionic atmospheres fluctuate. The statistics of these fluctuations depend upon surface structure and ionic strength. The fluctuations are large, sufficiently so that the mean ionic atmosphere is a poor descriptor of the aqueous environment near a metal surface. The importance of this finding for a description of electrochemical reactions is examined by calculating, directly from the simulation, Marcus free energy profiles for transfer of charge between the electrode and a redox species in the solution and comparing the results with the predictions of continuum theories. Significant departures from the electrochemical textbook descriptions of the phenomenon are found and their physical origins are characterized from the atomistic perspective of the simulations.Comment: 29 pages, 15 figure

Strong Correlations in Actinide Redox Reactions

Reduction-oxidation (redox) reactions of the redox couples An(VI)/An(V), An(V)/An(IV), and An(IV)/An(III), where An is an element in the family of early actinides (U, Np, and Pu), as well as Am(VI)/Am(V) and Am(V)/Am(III), are modeled by combining density functional theory with a generalized Anderson impurity model that accounts for the strong correlations between the 5f electrons. Diagonalization of the Anderson impurity model yields improved estimates for the redox potentials and the propensity of the actinide complexes to disproportionate.Comment: 17 pages, 10 figure, 3 tables. Corrections and clarifications; this version has been accepted for publication in The Journal of Chemical Physic