Transition Metal Solutes Research Articles

(Invited) On-Line Inductively-Coupled Plasma Mass Spectrometry Characterization of Transition Metal Dissolution in Electrochemical Environments

While in-cell studies are the best way to characterize the performance and stability of electrochemical materials, the device environment often presents challenges to the real-time measurement of degradation processes. Several techniques have been developed to mimic the device environment, but to enable the real-time characterization of degradation processes. For example, low temperature fuel cell catalysts are often characterized using an aqueous electrolyte tuned to have pH and adsorption characteristics comparable to those of the solid ionomers used in fuel cells’ membrane-electrode assemblies. The material can then be subjected to various potential waveforms typical of those encountered in the device or intended to accelerate the materials degradation and the electrolyte analyzed, ex situ, for degradation products using a number of techniques. One such technique which is particularly useful is inductively-coupled mass spectrometry (ICP-MS), as it is able to detect transition metals in solution at concentrations as low at parts-per-trillion (ppt). This method, while valuable, provides limited information regarding the mechanisms of materials degradation (e.g., potential and time-dependence of dissolution processes). Recently, techniques have been developed which couple electrochemical flow or rotating disk electrode (RDE) cells directly to an ICP-MS allowing for precise time- and potential-resolved detection of dissolved species. One iteration of this technique is that pioneered by Mayrhofer et al. 1 in which a cell is sealed to a planar substrate on which the electrode material of interest is deposited. A stream of electrolyte is pumped over the material and the effluent electrolyte analyzed by ICP-MS. This cell can be rastered over the substrate/electrode material and thus has been dubbed a “scanning flow cell”. Lopes et al. 2 developed a stationary ICP-MS probe coupled with an RDE which allows measurement of the kinetics of a reaction simultaneously with detection of dissolution products. Another version of the technique uses a commercial flow cell from BASi with the electroactive material deposited on a glassy carbon working electrode and electrolyte drawn through the cell using the peristaltic pump of the ICP-MS.3,4 An overview of the application and utility of this technique in determining the potential-dependent degradation mechanisms of platinum and platinum alloy fuel cell catalysts will be presented. Acknowledgements This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the auspices of the Fuel Cell Performance and Durability Consortium (FC-PAD). Argonne National Laboratory is managed for the U.S. Department of Energy by the University of Chicago Argonne, LLC, also under contract DE-AC-02-06CH11357. References O. Klemm, A. A. Topalov, C. A. Laska, K. J. J. Mayrhofer, Electrochem. Commun., 13, 1533 (2011).P. Lopes, D. Strmcnik, D. Tripkovic, J. G. Connell, V. Stamenkovic, and N.M. Markovic, Acs Catal, 6, 2536 (2016).P. Jovanovic, A. Pavlisic, V.S. Selih, M. Sala, N. Hodnik, M. Bele, S. Hocevar, and M. Gaberscek, Chemcatchem, 6, 449 (2014).R.K. Ahluwalia, D.D. Papadias, N.N. Kariuki, J.-K. Peng, X. Wang, Y. Tsai, D.G. Graczyk, and D.J. Myers, J. Electrochem. Soc., 165(6), F3024 (2018).

Grain-boundary segregation of 3d-transition metal solutes in bcc Fe: ab initio local-energy and d-electron behavior analysis

We present ab initio calculations of grain-boundary (GB) segregation of the series of 3d transition-metal (TM) solutes in bcc Fe, taking advantage of the local-energy analysis. For 11(3 3 2) and 3(1 1 1) symmetrical tilt GBs, the segregation behaviors of 3d-TM solutes can be classified into three groups. The early TMs (Sc, Ti and V) are preferentially segregated to the looser sites of GBs antiferromagnetically, the late TMs (Co, Ni and Cu) are preferentially segregated to the tighter sites of GBs ferromagnetically, and the middle TMs (Cr and Mn) are segregated antiferromagnetically without fixed site preference. TMs at both ends of the 3d series show larger segregation-energy gains, while Mn shows a cusp at the center, which is similar to the ab initio interaction energies between the 3d-TM solutes and a screw-dislocation core in bcc Fe. By the local-energy analysis combined with the local densities of states, the segregation of the early TMs is mainly attributed to the stabilization of surrounding Fe atoms by the TM solute at the looser sites of GBs, and that of the late TMs is mainly attributed to the stabilization of the TM solute itself from bulk Fe to GB sites and the destabilization of Fe atoms around the TM solute in bulk Fe. The cusp at Mn is mainly caused by the destabilization of Fe atoms around the Mn solute in bulk Fe, due to nearly-localized high-spin d states of Mn, in contrast to substantial d-d hybridization for Mn in GBs. For each group of 3d-TM solutes, the effects on the magnetic and mechanical properties of Fe GBs are also analyzed by the d-electron behavior in common with the segregation mechanism.

Simple co-precipitation synthesis of high-voltage spinel cathodes with different Ni/Mn ratios for lithium-ion batteries

The high-voltage spinel is a promising cathode material in next generation of lithium-ion batteries. Samples LiNi0.5 − xMn1.5 + xO4 (x = 0, 0.05, 0.1) are synthesized by a simple co-precipitation method, in which pH value and temperature conditions do not need control. In the simple co-precipitation method, NaHCO3 solution is poured into transition metal solution to produce precursor. Ni and Mn are distributed uniformly in the products. The as-prepared samples are composed of ~ 200 nm primary particles. Samples LiNi0.5 − xMn1.5 + xO4 (x = 0, 0.05, 0.1) are also tested to study the effects of different Ni/Mn ratios. Sample LiNi0.5Mn1.5O4 delivers discharge capacities of 130 mAh g−1 at 0.2 C. The decreasing of Ni/Mn ratio in samples reduces specific capacity. With the decreasing of Ni/Mn ratios in spinel, amount of Mn3+ are increased. Attributed to its high Mn3+ contents, sample LiNi0.4Mn1.6O4 delivers the highest discharge capacity of 106 mAh g−1 at a large current density of 15 C, keeping 84.5% of that at 0.2 C rate. With the increasing of Ni/Mn ratios in spinel, cycling performance is improved. Sample LiNi0.5Mn1.5O4 shows the best cycling stability, keeping 94.4% and 90.4% of the highest discharge capacities after 500 cycles at 1 C and 1000 cycles at 5 C.

Synthesis and electrochemical performance of transition metal-coated carbon nanofibers as anode materials for lithium secondary batteries

In this study, transition metal coated carbon nanofibers (CNFs) were synthesized and applied as anode materials of Li secondary batteries. CNFs/Ni foam was immersed into 0.01M transition metal solutions after growing CNFs on Ni foam via chemical vapor deposition (CVD) method. Transition metal coated CNFs/Ni foam was dried in an oven at 80°C. Morphologies, compositions, and crystal quality of CNFs-transition metal composites were characterized by scanning electron microscopy (SEM), Raman spectroscopy (Raman), and X-ray photoelectron spectroscopy (XPS), respectively. Electrochemical characteristics of CNFs-transition metal composites as anodes of Li secondary batteries were investigated using a three-electrode cell. Transition metal/CNFs/Ni foam was directly employed as a working electrode without any binder. Lithium foil was used as both counter and reference electrodes while 1M LiClO4 was employed as the electrolyte after it was dissolved in a mixture of propylene carbonate:ethylene carbonate (PC:EC) at 1:1 volume ratio. Galvanostatic charge/discharge cycling and cyclic voltammetry measurements were taken at room temperature using a battery tester. In particular, the capacity of the synthesized CNFs-Fe was improved compared to that of CNFs. After 30 cycles, the capacity of CNFs-Fe was increased by 78%. Among four transition metals of Fe, Cu, Co and Ni coated on carbon nanofibers, the retention rate of CNFs-Fe was the highest at 41%. The initial capacity of CNFs-Fe with 670mAh/g was reduced to 275mAh/g after 30 cycles.

General trends between solute segregation tendency and grain boundary character in aluminum - An ab inito study

Quantum mechanical calculations have been performed to establish general trends in propensity for the segregation of solutes across the periodic table at or in the neighborhood of an extended set of commonly observed special grain boundaries in face centered cubic aluminium. To this end, Al has been considered as the matrix and elements from 3d and 4d transition metals as well as those from group II, III and IV have been selected as solute atoms. For transition metal solutes, we find a concave-up parabolic-like dependency of segregation energy as a function of atomic number that is argued to be caused by the competition between chemical bonding and atomic size effects. The analysis is corroborated quantitatively by the computation of crystal orbital Hamiltonian population for solute-Al and Al-Al pairs as well as the Voronoi polyhedral surrounding solutes at a sample GB. The parabolic-like (concave-down trend) dependency of the cohesiveness of grain boundaries is explained by an equivalent trend in the bonding strength of Al-Al pairs at the segregated GBs. We extend this investigation to examine the stability of the solid solution polycrystalline state by comparing the calculated segregation energy against the combined energetic cost of grain boundary and intermetallic precipitate formation. The results may serve as a design tool for tailoring polycrystalline alloys with desired properties.

A comprehensive first-principles study of solute elements in dilute Ni alloys: Diffusion coefficients and their implications to tailor creep rate

Diffusion regulates a vast number of materials properties and phenomena such as creep, the focus of the present work. However, a deep understanding of the effect of how each alloying element in a Ni-base superalloy affects properties such as diffusion and creep is far from complete. Here, we report temperature-dependent dilute solute diffusion coefficients and their implications to tailor the creep rate for 26 transition metal solute elements, X's, in fcc Ni using first-principles calculations. Calculations are performed using the five-frequency model for dilute solute diffusion and the nudged elastic band method within the local density approximation. Thermodynamic properties at finite temperatures for all configurations are calculated using the quasi-harmonic Debye model. In general, the fastest diffusing solute elements in Ni are found at the left side of the periodic table and the slowest diffusing solute elements are found in group VIIB. In particular, the present work indicates that the diffusivity of the dilute solute elements is correlated to the compressibility of each solute element on the respective Ni31X supercell, and not as strongly to the ionic radius of the solute elements, as previously suggested. Finally, results from the diffusivity study are combined with the previous results of elastic constants and stacking fault energies, and hence, a relative creep rate ratio for these 26 solute elements is modeled. It is shown that in most cases, slower diffusing solute elements provide the most creep resistance. This is true even at higher temperatures, due mainly to the solute's strong bonding with Ni atoms in the host lattice.

Ab initio magnesium-solute transport database using exact diffusion theory

A recently developed Green function approach informed by ab initio calculations models vacancy-mediated transport of 61 solutes in a hexagonal close packed magnesium. The 8- and 13-frequency diffusion models approximate vacancy jump rates near a solute, leading to the inaccurate calculation of Onsager coefficients. We identify all the symmetry-unique vacancy jumps in the Mg lattice and use the Green function approach to calculate the Onsager coefficients exactly in the limit of dilute solute concentration. Density functional theory-computed solute-vacancy interactions and vacancy jump rates inform the Green function approach and previous diffusion models. Solutes with positive size misfit diffuse faster compared to the self-diffusion of Mg due to the relaxation of solute towards vacancy while solutes with negative size misfit diffuse slower. Transition metal solutes show drag for attractive solute-vacancy binding as well as for repulsive binding, due to faster reorientation rates of the vacancy around the solute compared to dissociation rates. Solutes from the s-block, p-block and lanthanide series with attractive solute-vacancy binding and slower reorientation rates compared to the dissociation rates show drag due to vacancy motion around the solute through alternate dissociation and association jumps. The prediction of activation energy of diffusion from the 8-frequency model deviates by more than 50 meV for solutes with significant correlations effect. Our GF approach prediction of solute diffusion coefficients agree well with the available experimental measurements.

Analytical Tools for Characterizing Cellulose-Active Lytic Polysaccharide Monooxygenases (LPMOs).

Lytic polysaccharide monooxygenases are copper-dependent enzymes that perform oxidative cleavage of glycosidic bonds in cellulose and various other polysaccharides. LPMOs acting on cellulose use a reactive oxygen species to abstract a hydrogen from the C1 or C4, followed by hydroxylation of the resulting substrate radical. The resulting hydroxylated species is unstable, resulting in glycoside bond scission and formation of an oxidized new chain end. These oxidized chain ends are spontaneously hydrated at neutral pH, leading to formation of an aldonic acid or a gemdiol, respectively. LPMO activity may be characterized using a variety of analytic tools, the most common of which are high-performance anion exchange chromatography system with pulsed amperometric detection (HPAEC-PAD) and MALDI-TOF mass spectrometry (MALDI-MS). NMR may be used to increase the certainty of product identifications, in particular the site of oxidation. Kinetic studies of LPMOs have several pitfalls and to avoid these, it is important to secure copper saturation, avoid the presence of free transition metals in solution, and control the amount of reductant (i.e., electron supply to the LPMO). Further insight into LPMO properties may be obtained by determining the redox potential and by determining the affinity for copper. In some cases, substrate affinity can be assessed using isothermal titration calorimetry. These methods are described in this chapter.

On the temperature dependent strengthening of nickel by transition metal solutes

Fundamental insights are presented into the factors influencing the temperature-dependent strengthening of nickel by transition metal solutes. Strain rate jump tests are used using the alloying elements Ta, W and Re which lie beside one another in the d-block. At 800 °C, the hardening induced by Ta is significantly greater than that by W and Re. At T ≥ 1000 °C, Re becomes more potent than Ta at slow strain rates; at 1200 °C it is the most effective at all the strain rates employed. The results are interpreted using theory for temperature-dependent strengthening that emphasises solute–dislocation interaction. It is confirmed that at low temperatures the solute strengthening is controlled by paraelastic interaction – the solute with the largest size difference with the host Ni is the most potent. At higher temperatures, the evidence indicates that solutes collect on dislocations such that the slowest diffusing solutes confer maximal resistance to dislocation glide and climb; thus Re is particularly potent. The findings elucidate the role of transition metal solutes in strengthening. Moreover, they provide the necessary quantitative data for ongoing alloy design efforts.

Partitioning and diffusion of transition metal solutes in ternary model Ni-based single crystal superalloys

Partitioning and diffusion of transition metal solutes can significantly affect the coarsening rate of γ′ precipitates in Ni-based single crystal superalloys. Using first-principles density-functional calculations and atom probe tomography, we investigated the partitioning coefficient and diffusion rate of 3d (TiNi), 4d (ZrPd), and 5d (HfPt) transition metal solutes. For ternary model Ni-based single crystal superalloys, the 3d solutes (except Ti) partition to the matrix phase, whereas 4d and 5d solutes (except Ru, Rh, and Ir) prefer γ′ precipitates. The existing atom probe tomography results are consistent with the calculation results. Across the periodic table, middle-row elements have a higher (lower) diffusion activation energy (diffusion rate) than early and late row elements. The band-filling effect that is coupled with electronic structure analysis can explain the parabolic behavior of energetics that involves bond breaking. To minimize the coarsening rate, we have screened alloying elements that both have a low diffusion rate in the γ matrix and partition to the γ′ phase. These alloying elements will suppress the coarsening behavior of the precipitates and are expected to contribute to the stability of precipitates at elevated temperature.

Tuning Solution Chemistry for Compositional Control of Multicomponent Lithium-Ion Battery Materials

A common route to synthesize lithium-ion battery active materials in the literature is to precipitate transition metals with a desired stoichiometry and then perform a calcination to produce the final active material. While this approach has produced many high performance materials, depending on the details of the solution conditions used during precipitation the stoichiometry of the precipitate can deviate from the stoichiometry of the feed. This stoichiometry deviation is largely ignored; however, a few groups have attempted to address this discrepancy with solution equilibrium models to rationally approach compositional control in the final active material. In this talk, we will present results that show the extent of the deviation of the precipitate stoichiometry from the feed stoichiometry across a number of solution conditions and compare experimental results to equilibrium calculations for one co-precipitation process. In addition, we will show initial results for the rate of precipitation of different transition metals in solution and the potential impact on the resulting final material.

Compositional control of precipitate precursors for lithium-ion battery active materials: role of solution equilibrium and precipitation rate

Extent of coprecipitation of Ni and Mn cations with oxalate anions in pure and blended solutions of transition metals.

Theoretical study of the substitutional solute effect on the interstitial carbon in nickel-based alloy

The carbon binding in nickel-based alloy with 3d, 4d and 5d transition metal solutes are investigated by using first-principles methods.

Iron Anode-Mediated Activation of Persulfate

Recently, persulfate (PS) has been applied to the oxidation of organic contaminants in wastewater, groundwater, and soil. However, PS requires activation by UV light, heat, transition metal, or pH control to be useful. In particular, transition metals are able to rapidly activate PS to sulfate radical. However, it is difficult to control the concentration of transition metal solution in an environmental setting. In this study, the potential of an electrochemical reaction using an iron anode to activate PS was investigated with phenol as a model contaminant. Based on Faraday’s law, Fe(II) generated by the electrochemical reaction was regularly supplied to the solution to activate PS to sulfate radical. The activation of PS was influenced by current intensity; however, excess Fe(II) decreased the oxidation rate of phenol because anodic oxidation-generated Fe(II) also scavenged sulfate radical. However, the electrochemical reaction using the iron anode could be readily controlled to supply an optimal amount of Fe(II) for activation of PS. Therefore, it is expected that this electrochemical process using an iron anode could be useful for the effective removal of phenol.

Energetics of cobalt alloys and compounds and solute–vacancy binding in fcc cobalt: A first-principles database

Using extensive first-principles density functional calculations, we calculate thermodynamic properties of binary fcc cobalt-based alloys with 25 different solute elements. For each solute element X, we calculate its (a) nearest- and next-nearest-neighbor solute–vacancy binding energy, (b) dilute impurity mixing energy with respect to the equilibrium and hypothetical fcc-based reference states of the solute, (c) enthalpy of formation of cobalt-rich binary ordered compounds in the Co–X system, (d) solubility enthalpy, and other derived quantities. We find that the solute–vacancy binding energies of all the studied solutes in fcc cobalt are positive (indicating favorable binding), in contrast to nickel, aluminum, magnesium, and copper-based alloys, where mid 3d transition metal solutes have unfavorable solute–vacancy binding. We study the physical and chemical effects influencing solute–vacancy binding energy, and find that (a) it correlates broadly with solute size—larger solute atoms possess stronger binding with vacancies—and this is understood in terms of strain relief and secondary next-nearest-neighbor interactions, and (b) it follows a parabolic trend as a function of d-occupancy across the transition metal series—falling to a minimum at the middle of the series—and this is understood in terms of a d-band filling effect. We also find this d-band filling effect in other calculated quantities such as the dilute impurity volumes and dilute impurity mixing energies. To aid in building thermodynamic databases, we tabulate all the calculated thermodynamic quantities, and compare them with experimental phase diagrams data and previous literature, and find good agreement where such data is available.

Selective removal and recovery of samarium from mixed transition metal solution using phosphoric acid

Cobalt–samarium alloys are generally used for producing magnets. Because samarium is a rare earth element, it is precious and expensive; consequently, it is often recovered by recycling processes to reduce costs. In this paper, a novel technique to recover samarium while avoiding the reported difficulties with previous methods is presented. Because rare earth phosphates are the main components of rare earth ores, a novel phosphate process is suggested. Samarium–cobalt magnets contain other transition metals; therefore, in this work, an attempt was made to precipitate samarium phosphate from a mixed transition metal solution. A samarium–cobalt–iron–copper solution was mixed with a phosphoric acid solution and then adjusted to several pH values using sodium hydroxide solution. The precipitates were filtered and dried. The ratio of samarium and other metals in the precipitates and filtered solutions was estimated by inductively coupled plasma atomic emission spectrometry. Ideally, the samarium phosphate was filtered out, so the filtrate contained the cationic transition metal species. To suppress the formation of iron phosphate, ascorbic acid was added to the mixed transition metal solution before it was mixed with phosphoric acid. The phosphoric acid concentration was varied in order to study the precipitation of samarium compounds. More than 99% of the samarium cations were recovered from the mixed transition metal solution using phosphoric acid. This novel process was observed to be useful for the recovery of samarium, and it may find applications in recycling and recovery of other rare earth elements of interest.

Electronic origin of strain effects on solute stabilities in iron

Nonuniform strain fields might induce the segregation of alloying solutes and ultimately lead to the mechanical performance degradation of body-centered-cubic (bcc) Fe based steels serving in extreme environments, which is worthy of investigation. In this paper, two typical volume-conserving strains, shear strain (SS) and normal strain (NS), are proposed to investigate the strain effects on solute stabilities in bcc iron by first-principles calculations. For solutes in each transition metal group, the calculated substitution energy change due to SS exhibits a linear dependence on the valence d radius of the solutes, and the slope decreases in an exponential manner as a function of the absolute difference between the Watson's electronegativity of iron and the averaged value of each transition metal group. This regularity is attributed to the Pauli repulsion between the solutes and the nearest neighboring Fe ions modulated by the hybridization of valence d bands and concluded to be originated from the characteristics of valence d bonding between the transition-metal solutes and Fe ions under SS. For main-group and post transition-metal solutes, the considerable drop of substitution energy change due to NS is concluded to be originated from the low-energy side shift of the widened valence s and p bands of the solutes. Our results indicate that the stabilities of substitutional solutes in iron under volume-conserving strain directly correlate with the intrinsic properties of the alloying elements, such as the valence d radius and occupancy, having or not having valence s and p bands.

Examination of Solubility Models for the Determination of Transition Metals within Liquid Alkali Metals

The experimental solubility of transition metals in liquid alkali metal was compared to the modeled solubility calculated using various equations for solubility. These equations were modeled using the enthalpy calculations of the semi-empirical Miedema model and various entropy calculations. The accuracy of the predicted solubility compared to the experimental data is more dependent on which liquid alkali metal is being examined rather than the transition metal solute examined. For liquid lithium the calculated solubility by the model was generally larger than experimental values, while for liquid cesium the modeling solubility was significantly smaller than the experimental values. For liquid sodium, potassium, and rubidium the experimental solubilities were within the range calculated by this study. Few data approached the predicted temperature dependence of solubility and instead most data exhibited a less pronounced temperature dependence.

Calibration update strategies for an array of potentiometric chemical sensors

One of the obstacles to the practical use of the multisensor systems – electronic tongues is a drift, i.e. gradual change of the sensor characteristics occurring in the process of their exploitation. Sensor drift leads to the deterioration of the performance of the calibration model that was calculated before changes in the sensor responses occurred. Two approaches can be employed to deal with this phenomenon. One consists of regular re-calibration of the sensor array, which is effective but time and labor consuming. Alternatively, multivariate statistical methods can be applied to the calibration model update using small subset of the standard samples. While significant efforts have been directed to the development of the calibration transfer and calibration update techniques, they were mostly applied to the near infrared spectroscopic instruments. Very few works addressed this issue for the potentiometric sensor arrays. In the present study, applicability of calibration update methods including slope and bias correction, direct standardization using PLS2 regression and auto-associative neural network, calibration update by weighting, Tikhonov regularization and Joint Y-PLS regression to the sensor array data was evaluated. Calibration models for copper and lead quantification in mixed solutions of transition metals using an array of 7 potentiometric sensors was used as an example. Of all studied methods, weighting and JYPLS regression were shown to be the most effective permitting to reduce prediction error for new data to the level of cross-validation error for the initial data.

From electronic structure to phase diagrams: A bottom-up approach to understand the stability of titanium–transition metal alloys

We have computed formation energies for all technologically relevant transition metal solutes in the α, β, and ω phases of Ti, employing ab initio simulations. We analyze and explain their periodic-table trends, and from their differences we derive stabilization energies which provide direct insight into phase stabilization effects of the various solutes with respect to α, β, and ω. This allows us to identify strong β stabilizers in the middle of each electronic d shell in consistency with experimental knowledge. Based on an extension of the stabilization energies to free energies we derive a wide range of Ti-transition metal phase diagrams. A detailed comparison to available experimental martensitic transformation temperatures and to measurements performed in this study shows that, despite some quantitative discrepancies, the qualitative trends can be expected to be correct. An important feature that is displayed by a limited range of the computed phase diagrams is a triple point at which the three phases, α, β, and ω, meet. This insight provides a plausible explanation for the complexity observed in gum metals, a class of Ti alloys with very special materials properties.