Pourbaix Research Articles

Thermodynamic Study of Leaching Conditions of Galena with Citrate Ions and Hydrogen Peroxide as Oxidizing Agent.

Galena is the most important mineral for lead production, as it is the main source of lead in the world. Currently, the concentrates of this mineral are mainly treated using pyrometallurgical methods, creating several environmental problems, such as the generation of toxic and greenhouse gases. In addition, these processes involve high energy consumption, which limits their applicability. Hydrometallurgical routes are proposed as alternative processes for obtaining some metals such as silver, copper, gold, etc. The drawback of these processes is that the minerals tend to be passive in aqueous media. To mitigate this issue, researchers have used extreme conditions of pressure and temperature (6 atm. and 155 °C) or the use of very corrosive conditions. In this sense, the use of complexing agents that dissolve the metals of interest has been proposed. Citrate ion is one of the most promising complexing agents for galena leaching, obtaining high percentages of dissolution in relatively short times. Unfortunately, there has not been enough investigation about the concentration optimization of the complexing in the pH range from 5 to 9. In this sense, thermodynamic diagrams, such as the Pourbaix diagrams, are very useful for this purpose. Therefore, in this work, the effects of pH and temperature on the leaching of galena in citrate ion solutions are studied thermodynamically and experimentally. The experimental work was carried out with pure galena samples with a particle size of +149 - 74 µm (-100 + 200 mesh). The results show that higher recoveries were obtained working at a pH of 8 and at temperatures of 30 and 40 °C. The thermodynamic and experimental data demonstrated that the existence of an optimal concentration of citrate ion, due the extraction of lead from galena, has a greater reaction rate at a relatively low initial concentration of 0.3 M. This is due the formation of the complex lead citrate 1 (Pb(cit)-).

Selective Copper Extraction from Spent Lithium-Ion Batteries

This paper is dedicated to the investigation of selective copper extraction from the black mass of lithium-ion batteries (LIBs) using ammonium sulfate. Thermodynamic analysis performed with the use of Pourbaix diagrams allowed to compare and predict the area of stable copper and cobalt ammine complexes, determine operating pH range (8.7-9.7) for solutions. Concentrations of sodium hydroxide and ammonium sulfate were defined experimentally allowing to achieve high copper extraction (82-87 %) at low cobalt recovery into solutions (0.25-0.52 %). By variation of main leaching parameters optimal process conditions were determined: (NH4)2SO4 45 g/dm3, NaOH 9 g/dm3, 25 С, l : s = 5 : 1, 120 minutes.

(Corrosion Division Rusty Award for Mid-Career Excellence) Rust At a Glance: Electrochemical Thermodynamics and Kinetics of Porous Corroding Interfaces

The development of a rust layer is governed by physico-chemical processes at the electrode/electrolyte porous interface, including electrochemical thermodynamics and kinetics, as well as transport and reaction processes. While rust can be protective under passive and inert regions found on the Pourbaix diagram, it becomes interesting to see whether its stability is compromised under harsh conditions that lead to active dissolution of this porous interface.Thermodynamics calculations can predict phase transformation, which nucleation and growth is governed by the chemical activity of the porous interfaces as well as by the saturation of the electrolyte solution. Understanding the electrochemical interface is a major challenge to unravel the corrosion mechanism of porous systems. The heterogeneous nature of the oxide structure as well as the development of amorphous gel regions influences the formation of active sites and point defects with a high lability for reacting with gas phases dissolved, entrapped, or occluded within interfacial substructures.The stabilization and corrosion protection of the rust interface requires to understand the adsorption of corrosion inhibitors on the surface of the active site on the electrode ‒ creating a stable protective passive film. Enthalpy and entropy of the dissolution process can explain the spontaneous formation of a rust layer as well as the displacement of water molecules, which ultimately will result in the effective physico-chemical adsorption of the inhibitor molecules.In summary, this communication is aimed to present the role of rust at a glance, and its implications on the electrochemical corrosion mechanisms on porous interfaces. The application of electrochemical fundamentals and first principles combined with state of the art electrochemical, chemical and surface characterization techniques provides a comprehensive vision and mission on current trends in corrosion and protection of steel in porous media.

Quantification of Hydrogen Evolution on a Zinc Rotating Disk Electrode in Traditional Alkaline Electrolytes and Acetate-Based Water-in-Salt “Wise” Electrolytes

Rechargeable batteries as energy storage devices for the electric grid are advantageous to bridge the intermittency in the daily energy demand and production by renewable energy sources such as solar microgrids. Another important consideration to satiate this energy landscape is the need for inexpensive, safe, and non-toxic rechargeable battery technology and chemistry. Zinc aqueous (alkaline) batteries are one of the prime candidates that satisfies the aforementioned criterion and has thus warranted a lot of scientific and technological research for commercial applicability. There has been considerable work conducted by the researchers in this field to bolster the cycle life and active material utilization of zinc in alkaline systems. However, zinc is thermodynamically susceptible to hydrogen gas evolution in alkaline solutions as witnessed by its Pourbaix diagram. On a device level, this affects the columbic efficiency due to the deleterious side reaction, which in turn negatively impacts the energy density of the battery. Water-in-salt electrolytes (WiSE) are a recently developed class of electrolytes that directly address this shortcoming of dilute electrolytes due to the modified solvation structure of water which lowers the activity of free water molecules in solution.In this work, we show a type of non-toxic and cheap acetate-based WiSE that has lower hydrogen gassing rates when compared to traditional alkaline potassium hydroxide-based electrolytes. Specifically, we quantify hydrogen gas evolution at the zinc electrode-electrolyte interface using the rotating disk electrode technique at different overpotentials to show the remarked improvement in gassing observed. At overpotentials of 100, 300 and 500 mV, 27 m (molal) potassium acetate is observed to have gassing rates of up to 16 times less than that of 25 weight percent potassium hydroxide. Using the Koutecky-Levich equation, mass-transfer and kinetic parameters for the hydrogen evolution reaction were also determined. The results establish the applicability of this class of WiSE in reducing gassing on zinc and thus, improving the overall efficiency of zinc-based aqueous batteries.

Plasma-Chemical Synthesis and Properties of Oxide Compounds of Cobalt

To date, the possibilities of using metal oxide particles in various fields have been developed [1]. At the same time, their characteristics largely depend on the method of preparation, which usually determines their structure, size, and physicochemical properties [2].Among the methods for obtaining nanosized particles, one can single out methods of green synthesis [3], electrochemical [4], electropulse [5], and plasmachemical [6] synthesis based on the reduction or oxidation of metal ions in solutions under favorable conditions, followed by aggregation of nanoparticles.The purpose of this work is to analyze the technology for the synthesis of cobalt oxide compounds using contact nonequilibrium low-temperature plasma and to determine the directions for its further improvement.The object of the study was the process of obtaining particles of cobalt oxide using contact non-equilibrium low-pressure plasma during the processing of a liquid medium.The study was carried out using laboratory equipment and software modules of the HSC Chemistry 5.11 package. To find the preliminary conditions for the formation of interphase boundaries, diagrams were used in the coordinates of the E-pH potential of the aquatic environment (Pourbaix diagrams). It has been established that, depending on the initial state of Co(OH)2 in solution, the mass fraction of oxygen in the precipitate can change. In this case, the addition of hydrogen peroxide leads to a decrease in the proportion of hydroxide compounds in sediments. X-ray phase analysis showed the presence of CoO, CoOOH, Co3O4, β-Co(OH)2, Co in dry sediments. It has been established that the particle sizes lie in the range of 10 – 110 nm. The micrographs confirm the agreement between the calculated values and the experimental data.The phase composition of the deposit is influenced by the following factors: current density and anion concentration, process temperature. Increasing the temperature increases the diffusion coefficient but requires a higher current density to produce a powder. It has been established that a decrease in the liquid layer contributes to an increase in the yield of oxide compounds. The results of the analysis allow us to conclude that when using the technologies of plasma-chemical processing of liquid media to obtain compounds of the ultra- and nanoscale range, it becomes necessary to choose the parameters that are optimal for this process. The results of the analysis allow us to conclude that this technology can be optimized by controlling the initial pH of the solution; use of the spent solution in a container with the initial solution for its acidification; refinement of the reactor block for treating the solution in a film mode or close to it; choice of rational parameters for sludge drying. Laurent S., Boutry S., Muller R. N. Metal oxide particles and their prospects for applications //Iron oxide nanoparticles for biomedical applications. – Elsevier, 2018. –P. 3-42.Wang L. et al. Rational design, synthesis, adsorption principles and applications of metal oxide adsorbents: a review //Nanoscale. – 2020. – Vol. 12. – №. 8. – P. 4790-4815.El Shafey A. M. Green synthesis of metal and metal oxide nanoparticles from plant leaf extracts and their applications: A review //Green Processing and Synthesis. – 2020. – Vol. 9. – №. 1. – P. 304-339.Lawrence M. J., Kolodziej A., Rodriguez P. Controllable synthesis of nanostructured metal oxide and oxyhydroxide materials via electrochemical methods //Current Opinion in Electrochemistry. – 2018. – Vol. 10. – P. 7-15.Gan Z. et al. A laser and electric pulse modulated nonvolatile photoelectric response in nanoscale copper dusted metal‐oxide‐semiconductor structures //Advanced Electronic Materials. – 2018. – Vol. 4. – №. 11. – P. 1800234.Shamanin I. et al. Plasmachemical synthesis and evaluation of the thermal conductivity of metal-oxide compounds for prospective nuclear fuel //Journal of Physics: Conference Series. – IOP Publishing, 2019. – Vol. 1145. – №. 1. – P. 012057.

The Electrochemical Deposition of Pd and Cucl Films on Graphene

Surface limited redox replacement (SLRR) is an electrochemical technique for depositing metal films monolayers in thickness, typically on other metals. As interest in graphene as a highly conductive, chemically resistant support for metal nanoparticles has risen, some have sought to apply this deposition technique to graphene as an electrochemical alternative to other chemical or vacuum-based deposition techniques. To better understand the nature of the interactions between the metal and graphene, our group has used this technique to deposit Pt on graphene to observe the structure of Pt and its epitaxy with the substrate. We have previously found that Pt prefers to bind to the bridge sites of graphene, preferring to form a simple-cubic-like structure and producing a slight lattice compression. We sought to expand on this study by observing the interactions of Pd with graphene. Pd was deposited on graphene using Cu-mediated SLRR, however, most Cu-mediated SLRR is performed with CuSO4 or Cu(ClO4)2 dissolved in H2SO4 or HClO4. This does not necessarily produce a strong Cu underpotential deposition feature on graphene, so HCl was used as the electrolyte instead. This produced a substantially more pronounced underpotential deposition feature, but cyclic voltammograms in this solution do not appear to agree with typical Cu-S-Cl Pourbaix diagram. This also resulted in two different types of structures, primarily Cu-containing films and primarily Pd-containing nanoparticles. The films were always found in the presence of the particles, but the particles did not always have accompanying films. A structural analysis of the particles determined that they preferred to interface with graphene along the (110) planes of Pd and have a slight elongation along the [001] direction of about 1.6% when oriented such that their columns were visible in the scanning transmission electron microscope. Based on an analysis of their orientation, this alignment and elongation appear to be the result of Pd minimizing its interfacial strain with graphene. A structural and compositional analysis of the films determined them to be CuCl, and they prefer to interface with graphene along their (111) plane, although the reason for this particular orientation is unclear as there is weaker evidence for a correlation between the alignment of the graphene and the films.

(Digital Presentation) Investigation of the Influence of Parameters of Contact Nonequilibrium Plasma on the Structure of Copper Oxides

Recently, the interest of researchers in nanodispersed oxides of monovalent and bivalent copper, ferrum, cobalt, nickel, and chromium has significantly increased. At the same time, interest in their micro- and nanosized particles is due, first of all, to a wide range of possibilities for their practical application. Today it is possible to use Cu2O, CuO in the creation of new catalysts for various industrial processes, as a filler for varnishes and paints, antiseptics. Cu2O is used in the production of galvanic cells, other fields of technology, etc. [1]. At the same time, the characteristics and variants of their practical application largely depend on the method of preparation, which usually determines their structure, dimensions, physical and chemical properties, etc. [2].Among the methods for obtaining micro- and nanoparticles, large groups form the methods of chemical, electrochemical, electropulse, and plasma-chemical synthesis, based on the processes of reduction or oxidation of metal ions in solutions, depending on the conditions.Each of these groups has its advantages and disadvantages, but the use of the plasma-electrochemical method (using the treatment of aqueous media with contact low-temperature non-equilibrium plasma (CNP) of reduced pressure) makes it possible to use the advantages of each of the groups [3-5]. However, in this case, it becomes necessary to establish the relationship between the synthesis parameters and the phase composition and morphology of the resulting particles.At the first stage, Pourbaix diagrams were constructed corresponding to the conditions of synthesis in a plasma reactor, and voltammograms were obtained, which made it possible to evaluate the effect of pH synthesis on the composition of products. The morphology of the particles was also studied depending on the current strength of the process at the same initial concentration of Сu2+ in an aqueous solution. The use of X-ray phase analysis and electron microscopy made it possible to evaluate the composition and dimensional characteristics of dry sediments. Thus, at the process current I=200-250 mA, particles were obtained, consisting of intergrowths of crystals, the size of which did not exceed 30 μm, at I=140-200 mA crystals up to 15 μm were obtained, at I=80-140 mA the average crystal size did not exceed 5 µm.The structure of the particles was also established: the largest particles were represented mainly by intergrowths of smaller crystals. It is obvious that the formation of numerous crystallization centers with the simultaneous intense evolution of hydrogen contributes to the fact that the rate of crystal growth at the cathode decreases. At higher current strength parameters, the growth of individual crystals is observed (mainly in the form of octahedrons and cuboctahedrons, the size of which reaches 10-15 μm. A decrease in the current strength leads to a decrease in the particle size - both individual crystals and their intergrowths, which in this case are predominantly octahedral structures, the largest of which are 1–2 µm, and the smaller ones are 100–200 nm.The results of X-ray diffraction analysis show that under the given conditions for the formation of a deposit, the surface of the formed particles consists mainly of Cu2O.On the basis of the conducted studies, it can be noted that the size of the precipitate particles obtained can vary within a fairly wide range. In this case, a change in the process parameters, especially a decrease in the process current, leads to a decrease in the size of the formed particles and an increase in the deposit uniformity, which is confirmed by the results of electron microscopic studies of the synthesized cathode deposits. This makes it possible to control the particle synthesis process by adjusting the parameters of the plasma-chemical processing process, which, due to its simple design, does not cause difficulties and, along with the possibility of using highly diluted solutions, is an advantage of this method.

(Invited) Near-Surface pH during the Electrodeposition of Iron Group Metals

The all-iron flow battery initially proposed in 1981 by Hruska and Savinell represents a cost effective and environmentally benign system for large scale energy storage1. The all-iron chemistry consists of the Fe2+/Fe3+ redox reaction at the positive electrode, and iron plating and stripping at the negative electrode. Because the standard reduction potential for iron plating is more cathodic than that of hydrogen evolution, electrolyte pH heavily influences coulombic plating efficiency as well as deposit morphology and microstructure2-5. Increasing the electrolyte pH decreases the diffusion limited current for hydrogen evolution and shifts the equilibrium potential more negative6. However, iron is not stable at high pH as shown by the Pourbaix diagram, where it will precipitate as hydroxide species. Boric acid is a common component in iron group plating baths where it is believed to function as a buffering agent helping to stabilize pH near the electrode7-9. However, it is not understood how boric acid with a pKa value of 9.27 can buffer at low pH10. The aim of this work is to further understand the effects of pH near the electrode surface and the role of boric acid on the electrodeposition of iron group metals.Electrodeposition of selected iron group metals will be discussed with a focus on the effect of boric acid. We present an in-situ technique for monitoring near surface pH as a function of plating current density. This measurement is accomplished using a tungsten microelectrode which acts as an indicator for hydrogen ion activity11-13. Our results demonstrate that boric acid is not effective in buffering the near surface pH in the acidic baths tested. Even at high current densities in a pH 2.5 electrolyte, the near surface pH did not exceed 6. However, the addition of boric acid improved deposit morphology and current distribution; coupons plated from electrolytes containing boric acid, and those without, are compared and discussed. At high current density and high pH, boric acid incorporates into the deposits, which is studied by use of inductively coupled plasma optical emission spectrometry (ICPOES). Electrochemical impedance spectroscopy, cyclic voltammetry, and coulombic efficiency data aid in understanding additive adsorption as well as the influence of boric acid on the deposition mechanism. In addition to surface specific information, the effects of boric acid on exhaustive plating and long-term bath stability will be presented to account for buffering effects in the bulk electrolyte when pH exceeds 6.References W. Hruska and R. F. Savinell, Journal of The Electrochemical Society, 128, (1) 18-25 (1981)R. Gabe, Journal of Applied Electrochemistry, 27, 908 (1997)Nakahara and S. Mahajan, Journal of the Electrochemical Society, 127 (2) (1980)Cohen-Hyams, Wayne D. Kaplan, Joseph Yahalom, Electrochemical and Solid-State Letters, 5 (8) C75-C78 (2002)Hawthorne et al., Journal of The Electrochemical Society, 162 (1) A108-A113 (2015)Hilbert, Y.Miyoshi, G. Eichkorn, and W. J. Lorenz, Journal of the Electrochemical Society, 118, 1927 (1971)Zech and D. Landolt, Electrochimica Acta, 45 (21) 3461-3471 (2000)P. Hoare, Journal of The Electrochemical Society, 133 (12) 2491-2494 (1986).Santos, R. Matos, F. Trivinho-Strixino and E. C. Pereira, “Effect of temperature on Co electrodeposition in the presence of boric acid” Electrochimica Acta, vol. 53, pp. 644-649 (2007)R. Lide, CRC Handbook of Chemistry and Physics, p. 1274, CRC Press, Boca Raton, FL (2005)E. S. El Wakkad, H. A. Rizk and I. G. Ebaid, The Journal of Physical Chemistry, 59, (10) 1004-1008 (1955)O. Park, C.-H. Paik and H. C. Alkire, Journal of The Electrochemical Society, 8, (143) p. 174 (1996)Webb and R. Alkire, Journal of The Electrochemical Society, 146, (6) p. B280 (2002)

(Digital Presentation) Environmentally Oriented Electrochemistry in the Teaching Laboratory

Electrochemistry has much to offer for a better understanding of natural electron exchange phenomena and multiple environmental remediation schemes. We have designed and tested several laboratory experiments to explore and demonstrate these functions in a teaching lab setting. These include: indirect electrolytic destruction of organic wastes, indirect H2S removal, electrocoagulation of oily wastewater, color removal of simulated wastewater by electrocoagulation-electroflotation, electrokinetic processing of soils, recovery of a soil metal pollutant and its extractant, production of disinfectants (ferrate and ozone), experimental transitions in a potential vs. pH (Pourbaix) diagram, electrolytically-generated activation of a metal surface for CO2 conversion, the indirect generation of colloidal sulfur to demonstrate Rayleigh scattering, a mini-solar cell, and the construction of a solar fuel cell. We have also generated experiments on non-electrodic electron transfer phenomena like simultaneous photocatalysis, removal of metal ions by cementation, destruction of pollutants by Fenton´s reagent, photoreduction of metal ions, production of disinfectants (ClO2, chloramines), water acidification by oxidation of mineral sulfides (acid mine drainage), and the oxidation of Fe(II) ions by molecular dioxygen.

Durability Assessment of Transition Metal Oxygen Electroreduction Catalysts with on-Line Flow Cell Trace Elemental Analysis

Proton exchange membrane hydrogen fuel cell (PEMFC) cathodes for commercial applications are typically based on platinum group materials that exhibit remarkable oxygen reduction reaction (ORR) activity and stability. Despite this performance, the high cost of such platinum electrodes prevents a more rapid transition to a more sustainable hydrogen economy involving PEMFCs. Platinum-free transition metal ORR catalysts have been identified and tested as high activity, low-cost alternatives with well-established protocols, though understanding the durability of these materials would require more experimental efforts. Techniques for measuring catalyst degradation typically involve activity loss measurements, surface-sensitive microscopy and spectroscopy, and liquid aliquot collection for metal loss quantification that are all performed post-catalysis for extended periods of time. Experiments using this approach provide valuable information about overall catalyst material degradation but usually cannot elucidate mechanistic or potential-dependent phenomena that may occur on shorter time scales. More rigorous experiments with synchrotron radiation have been utilized to investigate the degradation mechanisms of platinum-based fuel cell catalysts but studies on other transition metal materials are less widespread due to the high time requirements and limited accessibility of synchrotron beamlines for such experiments. In our work, we utilize an on-line electrochemical flow cell coupled with an in-house inductively coupled plasma-mass spectrometer (ICP-MS) for evaluation of fuel cell catalyst degradation in acidic electrolyte environments. Oxygen and nitrogen gas saturated electrolytes with a constant pH of 1 but differing chemical identity are flowed through the cell and across a working electrode composed of non-platinum transition metals. Dissolved metal ions are convectively transported to the ICP-MS where they are measured with up to part per trillion resolution with a time delay of ~4 seconds, enabling millisecond resolution of transient catalyst surface degradation. We evaluate the oxygen mass transport capabilities of our custom-machined flow cell against a commercial flow cell and compare to the much more established rotating disk electrode (RDE) technique for distinguishing ORR activity. Cyclic voltammetry measurements in oxygen-saturated electrolyte on various metal foils indicate potential dependent corrosion in agreement with what is expected from a Pourbaix (potential-pH) diagram of stability. In contrast, potential cycling these materials in the ORR-relevant range reveals degradation during ORR electrocatalysis that increases with ORR current to differing amounts as a function of the metal in a way that has not been previously characterized. Notably, degradation appears to be suppressed in the ORR potential range when saturating the electrolyte with nitrogen compared to oxygen, suggesting that surface-bound oxygen and ORR intermediates have a potential-dependent role in enabling degradation. Probing degradation during ORR conditions further with varied cyclic voltammetry scan rates and directions reveals more fundamental information about the timescales of catalytic corrosion processes and may inform the development of activation protocols that enhance the lifetime of ORR catalyst materials during operation. Through electrochemical flow cell testing in various pH 1 electrolytes, we make suggestions for electrolyte compositions and catalyst operation potential ranges that maintain activity and stability to advise fuel cell catalyst researchers in developing platinum-free PEMFC cathode materials. We are also developing additional flow cell configurations to allow testing of other non-precious ORR catalyst morphologies such as nanoparticle catalysts on carbon supports. The on-line ICP-MS flow cell architecture accelerates materials stability and degradation experiments for rapid assessment of platinum-free ORR electrocatalysts through in-house experiments and direct comparability to established electrochemical techniques.

Computational Screening of New Dopants for Nife-Based Layered Double Hydroxide Catalysts for Seawater Splitting

Water electrolysis is a key technology to convert and store sustainable energy into high-energy-dense hydrogen. However, slow kinetics of oxygen evolution reaction (OER) at the anode hinders overall reaction rate of water splitting and delays realization of hydrogen energy society. OER consists of four electron steps and an ideal catalyst has 1.23 V potential barrier for each step, but a real catalyst has a step which requires higher potential than 1.23 V, i.e., overpotential. Therefore, exploring OER catalyst with minimized OER overpotential is an important challenge in increasing water electrolysis efficiency. On the other side, the direct use of abundant seawater as a splitting reactant has advantage in terms of resources and cost. It can also help commercialization of water electrolysis. Since seawater contains chloride which causes undesirable side reactions and accelerated corrosion of the anode material, appropriate OER catalyst is necessary to avoid these problems.Conventional trial-and-error method in catalyst design usually requires many years of R&D and high cost. Computational screening can preemptively narrow down the candidate group of high-efficient catalyst to reduce time and economic cost. NiFe-based layered double hydroxide (NiFe-LDH) is a promising OER catalyst due to the comparable activity to commercial IrO2catalyst and can provide easily tunable metal composition during synthesis process. Its unique layered structure and reversible oxidation state change in redox condition also have arisen interest of many researchers. In this study, starting from the mechanism study of OER, chloride evolution reaction (ClER), and chloride-induced corrosion on this material, we performed DFT calculations to predict which transition metal dopants can enhance the OER activity of NiFe-LDH without accelerating selectivity and corrosion problems in seawater.NiFe-LDH is known to experience its transition to NiFeOOH phase under OER condition. [001] and [110] facets were set as edge and terrace sides of NiFeOOH, respectively, after their surface energies were investigated. Surface Pourbaix diagram showed that the [001] facet has clean termination without any adsorbate while [110] facet has dissociated-H2O covered termination under alkaline OER condition. OER energetic profile showed that the Fe site on the [110] surface is an OER active site with a theoretical overpotential of 280 mV, and seawater conditions do not significantly affect the OER activity itself. ClER mechanism study revealed that it occurs via *ClOH intermediates on [110] metal sites or *Cl intermediates on deprotonated [001] oxygen sites. Energy calculation of surface chlorination and metal dissolution steps showed that gradual chlorination accelerates metal dissolution and [110] facet is more vulnerable to chloride-induced corrosion than [001] facet.As the third metal candidate of NiFe-LDH, 3d to 5d transition metals excluding heavy metals were screened. Gibbs free energy correction terms such as vibrational entropy, zero-point energy and solvation parameter were calculated only for NiFeOOH case, and applied them to other candidates to simplify the screening process. NiFeOOH [001] surface could not be better than NiFeOOH [110] surface due to the inherent low activity of the [001] facet, but interestingly seven dopants could reduce the overpotential of NiFeOOH [110] surface by affecting the Fe active site or being active sites themselves. Since these dopants increased the oxidation ability of the active site, all of them also lowered chloride oxidation potential. However, fortunately, none of their ClER operating potential was lower than their OER operating condition, which means that there still exist potential windows where 100% OER selectivity can be achieved. In the last screening step, dissolution energies of fully chlorinated metal sites were calculated for the seven candidates, and two cases of them showed negative dissolution energy, which indicates very unstable doping state. As a result, five candidates passed through all three screening criteria: OER activity, OER vs. ClER selectivity, and durability against corrosion. Further, the experimental validation has performed on three promising candidates considering the raw material price, and it revealed that their activity is higher activity than NiFe-LDH. In addition, for the two abandoned cases in the third screening step (about metal dissolution), the experimental data showed that they were not seem to be actually doped into NiFe-LDH, indicating that the computational screening of seawater compatibility really worked. This study supports the utility of theoretical screening in electrochemical catalyst design and presents computational approach method to consider seawater compatibility.

PH Modulation in the Membrane Capacitive Deionization (MCDI) Cell Using Bipolar Membrane (BPM)

This work investigates bipolar membranes (BPMs) for pH modulation of process streams in a membrane capacitive deionization (MCDI) device. Exploiting Pourbaix diagram knowledge, it was posited that control of electrode potential and process stream pH can enable selective ion removal and recovery from process streams that have ionic species mixtures. For example, making a process stream pH can etch silica particles to silicate or convert boron to borate and make these species susceptible to removal via ion-exchange. A BPM can make an organic acid process stream alkaline for organic acid anion removal via ion-exchange. It can acidify streams to make metals conducive for plating on electrode surfaces. The use of BPMs in Electrodialysis cells (BPM-ED) has been an established technique to attain a particular pH in the cell. However, the ionic separation in the ED cell occurs through the ion-exchange membranes which makes it difficult to differentiate between competing ions in mixtures containing target ions at very low concentrations (e.g., ppm level or less). Prior to using BPM-MCDI for selective ionic separations, this work discusses how BPM properties (onset potential, polarization behavior, etc.) and MCDI cell parameters (operating voltage, feed concentration, residence time, etc.) govern process stream pH.

New insights into the mechanism of localised corrosion induced by TiN-containing inclusions in high strength low alloy steel

This work investigated the chemical and electrochemical mechanisms of localised corrosion triggered by CaS·xMgO·yAl2O3·TiN complex inclusions in high strength low alloy steel (HSLAS) under a simulated marine environment. Special focus was given to the role of the TiN portion of the inclusion on the initiation and growth of the corrosion pits. The thermodynamic process of pitting initiation was investigated by Gibbs free energy, Pourbaix diagram and first principle calculation. Localised corrosion is mainly induced by inclusions and triggered by dissolution of adjacent distorted matrix. Chemical dissolution of CaS portion in CaS·xMgO·yAl2O3·TiN complex inclusion creates an acidic aggressive environment that accelerates the further dissolution of inclusion and matrix. Galvanic coupling effect between TiN inclusion and matrix is directly verified. TiN covered with a TiO2 film acts as the cathodic phase in galvanic corrosion, although it has a lower Volta potential than the matrix. This is an unusual correlation with the scanning Kelvin probe force microscopy result, which has been explained for this special system.

A novel process of immobilizing sodium arsenate crystals as scorodite using Fe(OH)3 as an iron source

Sodium arsenate crystals are a hazardous waste produced from the utilization of arsenic–alkali residue in antimony metallurgy that pose a serious threat to the environment. In this article, to eradicate the damage caused by sodium arsenate and enhance the sustainable development of nonferrous metallurgy, a novel process was proposed to immobilize arsenic as flaky scorodite using Fe(OH)3 as an iron source. Various methods, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and inductively coupled plasma atomic emission spectrometry (ICPAES), were applied to characterize these synthesized products. The whole process is divided into three parts. In the thermodynamic analysis, the stable area of scorodite in the Pourbaix diagram significantly expands at high ionic activity and temperature, which favors the decrease in △rG for scorodite synthesis, and Fe–As coprecipitation is feasible in solution at 0.18 ≤pH≤ 5.30. In the scorodite synthesis, 99.88% of the arsenic was converted to a flaky scorodite with a size of 2–5 µm by optimizing the parameters. In the removal of arsenic, residual arsenic and iron were completely coprecipitated into an iron salt by dropping NaOH solution to a final pH of 5.0. The final products include scorodite and sodium sulfate in this process. Overall, this new approach successfully eliminates the potential arsenic pollution from the utilization of arsenic–alkali residue in antimony metallurgy and can potentially be applied to dispose of other high arsenic-bearing wastes.

Chemical and Redox Speciation of Uranyl with Three Environmentally Relevant Bifunctional Chelates: Multi-Technique Approach Combined with Theoretical Estimations.

Carbon and phosphorous are two primary elements common to the bio-geosphere and are omnipresent in both biotic and abiotic arenas. Phosphonate and carboxylate are considered as building blocks of glyphosate and humic substances and constituents of the cellular wall of bacteria and are the driving functionalities for most of the chemical interactions involving these two elements. Phosphonocarboxylates, a combination of both the functionalities in one moiety, are ideal models to dig deep into for understanding the chemical interactions of the two functional groups with metal ions. Phosphorous and carbon majorly exist as inorganic/organic phosphate and carboxylate, respectively, in the bio-geosphere. Aquatic contamination is a major concern for uranium, and the presence of complexing agents would alter the uranium concentrations in aquifers. Determination of solution thermodynamic parameters, speciation plots, redox patterns, Eh-pH diagrams, coordination structures, and molecular-level understanding by density functional theory calculations was carried out to interpret the uranyl (UO22+) interaction with three environmentally relevant phosphonocarboxylates, namely, phosphono-formic acid (PFA), phosphono-acetic acid (PAA), and phosphono-propanoic acid (PPA). UO22+ forms 1:1 complexes with the three phosphonocarboxylates in the monoprotonated form, having nearly the same stability, and the complexes [UO2(PFAH)], [UO2(PAAH)], and [UO2(PPAH)] involve chelate formation of five, six, and seven membered rings, respectively, through the participation of an oxygen each from the carboxylate and phosphonate, strengthened by an intra-molecular hydrogen bonding through the proton of the phosphonate moiety with uranyl oxygen. The complex formations are favored both enthalpically and entropically, with the latter being more contributive to the overall free energy of formation. The redox speciation showed an aqueous soluble complex formation over a wide pH range of 1-8. Electrospray ionization mass spectrometry and extended X-ray absorption fine structure established the coordination modes, which are further corroborated by density functional calculations. The knowledge gained from the present studies provide potential inputs in framing the cleanup, sequestering, microbial, and bio-remediation strategies for uranyl from aquatic environments.

Could one non-noble metal surface with non-noble substrate be a good hydrogen evolution catalyst: Performance of transition metal A monolayer on B substrate in theory frame

Could materials only including non-noble metals be good HER catalysts? To deal with this puzzle, computational screening of 132 different non-noble transition metal A/B surfaces for HER (A monolayer on B), are carried out by first-principles calculations systematically. The formation energies and dissolution potentials are calculated to access stabilities of A/B in vacuum and in solution, respectively. The realistic catalytic surfaces with oxygen or hydroxyl (co)adsorption are confirmed by analyzing Surface Pourbaix diagrams. Finally, three A/B surfaces (Cu/Mo, Mo/W, and Ti/Nb) with high stabilities and high HER exchange current densities (8.51, 3.39, 2.83 mA/cm2) are screened out. Further electronic properties analysis reveals that the different interactions between H 1s and d orbital, s orbital of A atoms will determine the feasible application of d and s band centers to uncover water effect and substrate effect on hydrogen adsorption ability, and tune the HER activity of A/B in the end.

우라늄 함유 폐수의 침전 처리에 대한 열역학적 기초 연구

Uranium is one of the most frequently used radioactive element but is also known for its toxicity towards variety of species. To study the thermodynamic features and to identify the optimal condition of the precipitation treatment of wastewater that contains uranium, Pourbaix diagram was constructed based on the temperature of 288K, 298K, 308K, and 318K. Between the temperature of 288K to 318K, uranium can exist in the form of U⁴⁺, UO2²⁺, U₃O₈, UO₃, and UO₂ in wastewater. Among these five compounds, UO₂ was the most dominant compound, followed by UO2²⁺, U₃O₈, UO₃, and U⁴⁺. Also, UO2²⁺ and U⁴⁺ existed as ion, while UO₃, UO₂, and U₃O₈ existed as precipitates. At the temperature of 288K, 298K, 308K, and 318K, UO₂ started the precipitation reaction from pH 0.9435, 0.9451, 0.9467, and 0.9481, while U₃O₈ and UO₃ started the precipitation reaction at pH 5.2727, 5.2712, 4.9277, and 4.7716, while UO₃ was from 7.3587, 7.3610, 7.1942, 6.8121 respectively. Between pH 0 to pH 14, the ratio of compound to ion was approximately 2 to 1, and the ratio increased as the temperature increased.

Development of a Model to Estimate the Thermodynamic Stability of Organic Substances in Leaching Processes

The leaching processes for metals using organic substances represent a sustainable approach to recover precious minerals from solid matrices. However, the generation of organometallic species and the lack of thermodynamic diagrams make it difficult to advance the understanding of their behavior and optimize the process. In this work, a thermodynamically and stoichiometrically consistent mathematical model was developed to estimate the thermodynamic stability of organic substances during the leaching process, and iron leaching with oxalic acid was used as a case study. The Pourbaix and the global thermodynamic stability diagrams for the system were developed in this study. Using a Gaussian®, it was estimated that the Gibbs free energy formation for Fe(C2O4)22−, Fe(C2O4)21−, and Fe(C2O4)33− was −1407.51, −2308.38, and −3068.89 kcal/mol. A set of eleven independent reactions was formulated for the sixteen species involved in the leaching process, and its stability functions in terms of Eh and pH were calculated to generate a 3D global thermodynamic stability diagram. According to the Eh-pH diagrams for the leaching process, ferrioxalate was identified as the most stable and predominant species in the leaching process at pH above 6.6 under reductive conditions. The mathematical model developed in this work resulted in a thermodynamic tool for predicting leaching processes.

Crystallization and pyrolysis of nitric acid pressure leach liquor of limonitic laterite and separation of valuable metals

Based on the advantages of nitric acid pressure process (NAPL) for laterite nickel ores, this study innovatively proposes nitric acid pressure leach liquor crystallization pyrolysis-mixed metal oxides separation process. Firstly, the feasibility and the possible reactions of mixed nitrate pyrolysis were confirmed by thermodynamic analysis. Subsequently, the optimal pyrolysis temperature was experimentally investigated. Then, the pyrolysis process of combined nitrate was thermodynamically analyzed, and the phase composition of mixed metal oxides was determined by X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and Scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDS). The Pourbaix diagram shows that using NaOH as leaching agent can selectively separate aluminum from mixed metal oxides, and the variables influencing aluminum's leaching rate are investigated. The experimental results are as follows: the optimal pyrolysis temperature of mixed nitrate crystals is 600 °C; the pyrolysis products are mainly oxides with the formation of NiAl2O4 and MgAl2O4 spinel phases; the aluminum leaching rate is 81.9% under the optimal alkali leaching conditions. The presence of spinel phase is not favorable for aluminum leaching. After leaching, an enriched residue of 15.8% Ni, 1.9% Co and 0.19% Sc was obtained.

Pourbaix Diagram of Astatine Revisited: Experimental Investigations.

The Pourbaix diagram of an element displays its stable chemical forms with respect to the redox potential and pH of the solution, whose knowledge is fundamental for understanding and anticipating the chemistry of the element in a specified solution. Unlike most halogens, the Pourbaix diagram in the aqueous phase for astatine (At, Z = 85) is still under construction. In particular, the predominant domains of two astatine species assumed to exist under alkaline conditions, At- and AtO(OH)2-, need to be refined. Through high-performance ion-exchange chromatography, electromobility measurements, and competition experiments, the existence of At- and AtO(OH)2- has been confirmed and the associated standard potential has been determined for the first time (0.86 ± 0.05 V vs the standard hydrogen electrode). On the basis of these results, a revised version of astatine's Pourbaix diagram is proposed, covering the three oxidation states of astatine that exist in the thermodynamic stability range of water: At(-I), At(I), and At(III) (as At-, At+, AtO+, AtO(OH), and AtO(OH)2-).