Pourbaix Research Articles

Geochemical and reactive transport modelling in R with the RedModRphree package

Abstract. Advances in computing and experimental capabilities in the research of water-rock-interactions require geoscientists to routinely combine laboratory data and models to produce new knowledge. Data science is hence a more and more pervasive instrument for geochemists, which in turn demands flexible and easy to learn software adaptable to their specific needs. The GNU R language and programming environment has established itself as de facto standard language for statistics and machine learning, enjoying increasing diffusion in many applied scientific fields such as bioinformatics, chemometrics and ecological modelling. The availability of excellent third party extensions as well as its advanced graphical and numerical capabilities make R an ideal platform for comprehensive geochemical data analysis, experiment evaluation and modelling. We introduce the open source RedModRphree extension package, which leverages the R interface to the established PHREEQC geochemical simulator. The aim of RedModRphree is to provide the user with an easy-to-use, high-level interface to program algorithms involving geochemical models: parameter calibration, error and sensitivity analysis, thermodynamical database manipulation, up to CPU-intensive parallel coupled reactive transport models. Among the out-of-the-box features included in RedModRphree, we highlight the computation and visualization of Pourbaix (Eh-pH) diagrams using full speciation as computed by PHREEQC and the implementation of 1D advective reactive transport supporting the use of surrogate models replacing expensive equation-based calculations.

Corrosion behavior in aluminum/galvanized steel resistance spot welds and self-piercing riveting joints in salt spray environment

This paper underlined the corrosion behavior of aluminum alloy/galvanized steel joints fabricated by resistance spot welding (RSW) and self-piercing riveting (SPR) exposed to different salt-spray cycles. It was found that the crevice generated by different joining methods has an important impact on corrosion behavior. Compared with the RSW joint, less corrosion happened on the coupled regions in SPR joints because of the higher local pH value resulted from the smaller crevice. The crevice, however, has less impact on the types of corrosion products. Galvanic corrosion happened in the coupled region for both RSW and SPR joints. The Pourbaix diagram explained the stable corrosion products of Al and ZnFe alloy, which are Al2O3, ZnO, and Fe3O4. Additional corrosion products observed from the experiment included α-Al(OH)3, Ca4Al2(CO3)(OH)12(H2O)5, Zn5(OH)8Cl2(H2O), Zn5(CO3)2(OH)6, and Fe2(OH)2(CO3), which can be understood by considering the metastable corrosion products, Pourbaix diagram, and possible chemical reactions.

Deprotonation and cation adsorption on the NiOOH/water interface: A grand-canonical first-principles investigation

Nickel-based oxides are highly active, cost-effective materials for the oxygen evolution reaction in alkaline conditions. Recent experimental studies have revealed the importance of surface deprotonation and alkali metal cation adsorption on the activity of Ni oxide surfaces, in contact with aqueous alkaline electrolyte. As a first step to elucidate the role of the alkali adsorption for the activity, we performed first-principles electronic structure calculations to address the stable surface structures of β-NiOOH(0001) as a function of the operating conditions in an electrochemical environment. We present a grand-canonical approach to compute the surface Pourbaix diagram of the β-NiOOH/water interface for the processes of deprotonation and alkali metal cation adsorption. The results of this study emphasize the importance of double-layer effects, including the adsorbate-induced change of surface dipole moments and the rearrangement of water molecules due to their strong interaction with the adsorbed species, for the most stable interface structure.

The effect of caustic soda treatment to recover rare earth elements from secondary feedstocks with low concentrations

Caustic soda (NaOH) treatment has enhanced rare earth element (REE) extraction from monazite. This study examined the effectiveness of caustic soda for RE-phosphates with lower concentrations for extraction from coal overburden, an alternative feedstock derived from mining waste. Pourbaix (Eh-pH) diagrams were constructed for La, Ce, Nd, Eu, and Y-PO4-H2O systems to understand the thermodynamics of NaOH-facilitated acid leaching for low-concentration RE-phosphates and oxides and to elucidate the difference in recoveries among individual REEs. The Eh-pH diagrams indicated that low crystallinity is beneficial for extraction. Nd was thermodynamically most favorable for the extraction process among the five elements examined, and Y was the least promising. Experiments were conducted using three Pennsylvania coal overburden samples to validate the effect of caustic soda on rare earth leaching. The experimental results showed that for NaOH (6 M), a leaching time of 2 h at 125 °C was sufficient to obtain twice the recovery values compared to those without the NaOH treatment. After implementing the caustic soda treatment, a high total REE recovery (>60%), especially for Nd and Pr, could be achieved with diluted HCl and more environmentally friendlier citric acid. The results were similar for both coarse and fine particles. This novel approach is likely to improve the feasibility of REE extraction from coal-associated feedstock in a more environmentally benign manner.

Low pH-induced lone-pair activity in the hybrid (C6H10N2)[SnCl3]Cl: Chemical study and physical characterizations

By changing the synthesis method and through the pourbaix diagram, a novel 0D organic−inorganic hybrid like perovskite system was crystallized by slow evaporation in an aqueous solution of 2-(aminomethyl) pyridine (2-amp) and tin chloride. Compound with the formula (C6H10N2)[SnCl3]Cl was characterized by single-crystal X-ray diffraction, Infrared spectroscopy (IR), and UV-Visible (UV-Vis) as well as photoluminescence (PL) technique and thermal analysis. The crystals of (C6H10N2)[SnCl3]Cl belong to the orthorhombic system with the Pbca space group. The crystalline stability is ensured by a three-dimensional network of N-H…Cl hydrogen bonds. Theoretical calculations were performed using density functional theory with the B3LYP/LanL2DZ level for studying the molecular structure and vibrational spectra of the title compound. A satisfactory agreement has been found between the calculated and the experimental vibrational frequencies. UV-Vis reveals that (C6H10N2)[SnCl3]Cl undergoes two absorption bands with an energy gap estimated to 3.27 eV. The material exhibits an important broad-band white light emission with a correlated color temperature of 4989 K with chromaticity coordinates (CIE) of (0.34, 0.33) and CRI of 92. According to others s2 Tin-complexes, this emission is mainly resulting from the self-trapped exciton transitions within the inorganic framework. Our contribution of luminescent 0D organic-inorganic hybrid materials as lead-free lighting systems opens up a new paradigm in functional materials design.

Isolation and Identification of Pseudo Seven-Coordinate Ru(III) Intermediate Completing the Catalytic Cycle of Ru-bda Type of Water Oxidation Catalysts

Isolation and Identification of Pseudo Seven-Coordinate Ru(III) Intermediate Completing the Catalytic Cycle of Ru-bda Type of Water Oxidation Catalysts

온도에 따른 코발트 함유 폐수의 침전 처리 특성 기초 연구

The influences of temperature and hydrogen ion concentration on wastewater containing cobalt ions were investigated by constructing Pourbaix diagrams for the Co⁰-H₂O system at temperatures of 288, 298, 308, and 318 K with varying pH levels. The majority of cobalt in water has been observed to exist in the form of Co²⁺ ions. Co⁰ and Co²⁺ are mutually interchangeable in acidic conditions depending on the reduction potential. Here, the conversion of Co²⁺ to Co(OH)₂ and to Co(OH)₃ was observed when pH was increased to around pH 5.5 to 6.5; the pH at which the change occurred decreased as the temperature increased. The changes in reactivity between the chemical species Co⁰, Co²⁺, Co(OH)₂, and Co(OH)₃ were quantified by measuring the area of stable conditions in the Pourbaix diagrams. The ratios of stable regions between Co(OH)₃/Co²⁺, Co(OH)₂/Co²⁺, Co⁰/Co²⁺, Co(OH)₂/Co⁰, and Co(OH)₂/Co(OH)₃ were observed to increase as the temperature increases. Therefore, the reactivity between these species was considered to in crease with temperature. These results indicate that it is necessary to consider the effect of temperature for the effective treatment of wastewater that contains cobaltions.

Ammoniacal leaching process for the selective recovery of value metals from waste lithium-ion batteries

ABSTRACT The recovery of valuable metals in spent lithium-ion batteries (LIBs) is not only environmentally sustainable but also economically advantageous. In this research, ammoniacal leaching process was adopted to selectively extract valuable metals from the mixed types of cathode active materials. By employing the ammonium sulphite as the reductant, while the ammonia and ammonium carbonate are used to provide NH3 to complex metal elements. The Eh–pH diagrams were used to study the extraction behaviour of Co, Li, Ni and Mn during ammoniacal leaching, and the Co, Ni can be leached out as the complexes ([Ni(NH3) n ]2+, [Co(NH3) m ]2+), while the Li was leached out as metallic ion. Manganese is first leached out as [Mn(NH3)β]2+ and then precipitated as (NH4)2Mn(SO3)2·H2O and (NH4)2Mn(SO4)3 when an excess of ammonium sulphites added to the solution. A detailed understanding of the ammoniacal selective leaching process is carried out by investigating the effects of factors such as reaction temperature (50–90 ℃), leaching time (2–6 h), agitation intensity (400–900 rpm) and solid-to-liquid ratio (10–50 g/L). The experiment of response surface methodology shows that significant interactions exist between ammonium sulphite and ammonium carbonate, ammonium sulphite and ammonia according to the leaching efficiency of Co. Under the optimum conditions, Co, Li, Ni, Mn and Al can be leached out with the efficiencies of 84.56%, 90.31%, 64.13%, 4.53% and 1.72%, respectively. The results proved the possibility of selective leaching. Summarily, this research is potentially beneficial for the design of a selective recycling technique.

Total and Leached Arsenic, Mercury and Antimony in the Mining Waste Dumping Area of Abbadia San Salvatore (Mt. Amiata, Central Italy)

Total and leached Arsenic, Mercury and Antimony were determined in the topsoils developed on the mining waste dumping area of Le Lame (Mt. Amiata, central Italy) where the post-processing Hg-rich ore deposits from the mining area of Abbadia San Salvatore were stored. The concentrations of As, Hg and Sb were up to 610, 1910 and 1610 mg kg−1, respectively, while those in the leachates (carried out with CO2-saturated MilliQ water to simulate the meteoric water conditions) were up to 102, 7 and 661 μg·L−1, respectively. Most aqueous solutions were characterized by Hg content <0.1 μg·L−1. This is likely suggesting that the mine wastes (locally named “rosticci”) were possibly resulting from an efficient roasting process that favored either the removal or inertization of Hg operated by the Gould furnaces and located in the southern sector of Le Lame. The highest values of total and leachate mercury were indeed mostly found in the northern portion where the “rosticci”, derived by the less efficient and older Spirek-Cermak furnaces, was accumulated. The saturation index was positive for the great majority of leachate samples in Fe-oxy-hydroxides, e.g., ferrihydrite, hematite, magnetite, goethite, and Al-hydroxides (boehmite and gibbsite). On the other hand, As- and Hg-compounds were shown to be systematically undersaturated, whereas oversaturation in tripuhyte (FeSbO4) and romeite (Ca2Sb2O7) was evidenced. The Eh-pH diagrams for the three chalcophile elements were also constructed and computed and updated according to the recent literature data.

Impact of sulfate on the solubility of Tc(IV) in acidic to hyperalkaline aqueous reducing systems

Abstract The solubility of 99Tc(IV) was investigated from undersaturation conditions in NaCl–Na2SO4 (0.3 M ≤ I ≤ 5.0 M), MgCl2–MgSO4 (I = 13.5 M) and CaCl2–CaSO4 (I = 13.5 M) systems with 0.001 M ≤ [SO4 2−]tot ≤ 1.0 M and 1 ≤ pH m ≤ 12 (with pH m = −log[H+], in molal units). Reducing conditions were set by either Sn(II) or Fe(0). Special efforts were dedicated to accurately characterize the correction factors A m required for the determination of pH m from the experimentally measured pH values in the mixed salt systems investigated, with pH m = pHexp + A m . The combination of (pe + pH m ) measurements with Pourbaix diagrams of Tc suggests that technetium is present in its +IV redox state. This hypothesis is confirmed by XANES, which unambiguously shows the predominance of Tc(IV) both in the aqueous and solid phases of selected solubility samples. XRD and SEM–EDS support the amorphous character of the solid phase controlling the solubility of Tc(IV). EXAFS data confirm the predominance of TcO2(am, hyd) at pH m > 1.5, whereas the formation of a Tc(IV)–O–Cl solid phase is hinted at lower pHm values in concentrated NaCl–Na2SO4 systems with ≈5 M NaCl. Solubility data collected in sulfate-containing systems are generally in good agreement with previous solubility studies conducted in sulfate-free NaCl, MgCl2 and CaCl2 solutions of analogous ionic strength. Although the complexation of Tc(IV) with sulfate cannot be completely ruled out, these results strongly support that, if occurring, complexation must be weak and has no significant impact on the solubility of Tc(IV) in dilute up to highly saline media. Solubility upper-limits determined in this work can be used for source term estimations including the effect of sulfate in a variety of geochemical conditions relevant in the context of nuclear waste disposal.

Advances in the Chemistry of Astatine and Implications for the Development of Radiopharmaceuticals.

ConspectusAstatine (At) is the rarest on Earth of all naturally occurring elements, situated below iodine in the periodic table. While only short-lived isotopes (t1/2 ≤ 8.1 h) are known, 211At is the object of growing attention due to its emission of high-energy alpha particles. Such radiation is highly efficient to eradicate disseminated tumors, provided that the radionuclide is attached to a cancer-targeting molecule. The interest in applications of 211At in nuclear medicine translates into the increasing number of cyclotrons able to produce it. Yet, many challenges related to the minute amounts of available astatine are to be overcome in order to characterize its physical and chemical properties. This point is of paramount importance to develop synthetic strategies and solve the labeling instability in current approaches that limits the use of 211At-labeled radiopharmaceuticals. Despite its discovery in the 1940s, only the past decade has seen a significant rise in the understanding of astatine's basic chemical and radiochemical properties, thanks to the development of new analytical and computational tools.In this Account, we give a concise summary of recent advances in the determination of the physicochemical properties of astatine, putting in perspective the duality of this element which exhibits the characteristics both of a halogen and of a metal. Striking features were evidenced in the recent determination of its Pourbaix diagram such as the identification of stable cationic species, At+ and AtO+, contrasting with other halogens. Like metals, these species were shown to form complexes with anionic ligands and to exhibit a particular affinity for organic species bearing soft donor atoms. On the other hand, astatine shares many characteristics with other halogen elements. For instance, the At- species exists in water, but with the least range of EH-pH stability in the halogen series. Astatine can form molecular interactions through halogen bonding, and it was only recently identified as the strongest halogen-bond donor. This ability is nonetheless affected by relativistic effects, which translate to other peculiarities for this heavy element. For instance, the spin-orbit coupling boosts astatine's propensity to form charge-shift bonds, catching up with the behavior of the lightest halogens (fluorine, chlorine).All these new data have an impact on the development of radiolabeling strategies to turn 211At into radiopharmaceuticals. Inspired by the chemistry of iodine, the chemical approaches have sparsely evolved over the past decades and have long been limited to electrophilic halodemetalation reactions to form astatoaryl compounds. Conversely, recent developments have favored the use of the more stable At- species including the aromatic nucleophilic substitution with diaryliodonium salts or the copper-catalyzed halodeboronation of arylboron precursors. However, it is clear that new bonding modalities are necessary to improve the in vivo stability of 211At-labeled aryl compounds. The tools and data gathered over the past decade will contribute to instigate original strategies for overcoming the challenges offered by this enigmatic element. Alternatives to the C-At bond such as the B-At and the metal-At bonds are typical examples of exciting new axes of research.

Passivity of Holmium Studied Theoretically by Potential-pH Diagrams for Selection of Electrolytes and Experimental Proof of the Formation of Ultra-Thin Anodic Films

We present an efficient strategy to identify anodizing electrolytes by coupling conventional Eh–pH diagrams with first-principles density functional theory calculations. Herein, the growth of ultra-thin films having a thickness of 10–20 nm is successfully demonstrated on thermally evaporated holmium. Considering only thermodynamic basis, simulated Eh-pH diagrams, and solubility analysis, electrolytes having different compositions and pH values are suggested for the efficient growth of anodic films. The Eh–pH diagrams are modified by incorporation of oxide–forming species in such a way to appreciably extend the stability domain. The predicted diagrams showed a strong agreement with the experimental observations and provide a better understanding of Ho–H2O based aqueous systems and can serve as a guide in other rare-earth elements anodizing. Grown film properties are investigated by using electrochemical impedance studies which disclose a linear increase in inverse capacitance with formation voltage indicating a uniform growth of anodic films irrespective of electrolyte selection. Formation factor, k, of anodic films grown up to 10 V varies from 1.16 to 1.95 nm V −1 in selected electrolytes. The k values greater than unity may contribute to the uniform film growth at different pH values.

Enhancing the Cycling Stability of Transition-Metal-Oxide-Based Electrochemical Electrode via Pourbaix Diagram Engineering

Transition-metal oxides have shown renewed interest as promising electrode materials for high-performance electrochemical energy storage devices. However, applications of these devices are hindered by the instability of the electrode materials, which is originated from the electrochemical redox reactions induced phase transition between materials with different chemical stoichiometries. Herein, the Pourbaix diagram was constructed to predict ways of inhibiting this undesirable phase transition by using iron oxides as a prototypical model. Appropriate metal dopant (e.g., Ni) doping in the iron oxide was theoretically and experimentally found to present single phase under an extra-wide range of applied potential in alkaline aqueous solutions. Typically, the designed material (Ni x Fe 3- x O 4 ) presents high cycling stability with capacitance retention of 94.3% over 5,000 cycles at a current density of 20 A/g when working as a supercapacitor anode. A configured asymmetric supercapacitor with the designed anode exhibits an almost 100% performance retention after 10,000 charging-discharging cycles. This success proposed a new perspective to enhance the stability of electrochemical electrodes for energy storage devices.

Thermodynamic properties and the solubility of the oxide lanthanum compounds in fused 0.36NaCl–0.64CsCl eutectic

The interaction between the oxygen and lanthanum ions in fused 0.36NaCl-0.64CsCl eutectic in the temperature range (823–1023) K has been studied by the method of potentiometric titration using YZME electrodes. The mechanism of interaction between La3+ and O2− ions was established versus the temperature and the basicity of medium. The solubility constants of lanthanum oxychloride and oxide compounds at various temperatures were determined. Specific thermodynamic properties of LaOCl and La2O3 were calculated. The Pourbaix (potential–pO2−) diagram was drawn and the stable lanthanum species in the melt were summarized.

Influence of addition of calcium salts on extraction of potassium from phosphorus- potassium associated mineral

In this article, calcium salts were added into the hydrofluoric acid-sulfuric acid (HF-H2SO4) leaching system, and their effects on the extraction of potassium from phosphorus-potassium associated mineral (mineral deposits containing Ca5(PO4)3F (fluorapatite) and KAlSi3O8 (potassium feldspar)) were investigated. KAlSi3O8 was converted into water-soluble potassium fertilizers, and important factors affecting the extraction, including dosages of H2SO4, types and dosages of calcium salts were studied. The results showed that the maximum extraction of 75.3 wt.% was obtained with H2SO4 (70 wt.%) dosage of 5 mL. When the reaction was added with 0.15 g CaSO4, the extraction rate of K of 95 wt.% was achieved, which was an increase by 19.7 wt.% compared with that without calcium salts. Moreover, the Eh-pH diagrams for K-Si-(P)-(F)-(Ca)-H2O systems were plotted and applied to analyze the mechanism of potassium extraction in the HF-H2SO4 reaction system. The analysis demonstrated that with the presence of F-, the formation of fluoro-complexes enlarged the stable regions of soluble phosphorus (KPF6) and silicon (K2SiF6). However, when calcium salts were added, such region decreased with the increase of [Ca2 +]; and such trends suggested that [K+] could be enhanced by adding [Ca2 +] during the leaching process.

Value extraction from semiconductor industry tantalum scrap through understanding the thermodynamics and chemistry

The electrode in the waste capacitor and capacitor producing semiconductor industry produces a significant amount of tantalum-bearing e-wastes. The lack of proficient technology poses a significant challenge for valorization and the circular economy of tantalum-bearing waste. Currently, we are developing a process for the purification and recovery of metal values from such waste through a combination of hydrometallurgy and pyrometallurgy techniques. In our present investigation, the initiative for the study includes an understanding of rudimentary thermochemistry and thermal characterization of semiconductor industry tantalum scrap for possible pretreatment. Also includes identification of suitable pretreatment conditions for feasible oxidation of semiconductor industry tantalum scrap to generate leachable tantalum oxide. Followed by the possibility for leaching has been investigated through the Pourbaix diagram and selected case studies. Our leaching investigation reasonably indicated that tantalum nitride in the semiconductor industry tantalum scrap can efficiently be oxidized using Na2CO3 (1:1 weight ratio) at 500 °C and roasting time of 3 h. Then Ta leaching using concentrated HF and H2SO4 at 50:50 wt% ratio as a lixiviant can be achieved efficiently. As the roasting and leaching process is feasible and scalable, Ta can be successfully extracted from the semiconductor industry tantalum scrap on an industrial scale.

Cathodic Kinetics on Platinum and Stainless Steel in NaOH Environments

During typical atmospheric conditions, cathodic reduction reactions produce hydroxyl ions increasing the pH in the cathodic region. Therefore, cathodic reduction reactions are investigated on platinum and stainless steel 304 L (SS304L) in NaOH solutions ranging in pH from 13.6 to 16.5. It was found that in solution pHs less than 16.5 the cathodic reduction reaction on Pt and SS304L was ORR with an electron transfer number less than two due to superoxide formation as an intermediate. Increasing pH decreased the number of electrons transferred. At a pH of 16.5, the cathodic reduction reaction on SS304L is no longer ORR and the cathodic current on the surface of the alloy is due to oxide reduction occurring on the surface as indicated by the creation of multi-component Pourbaix diagrams. The results of this study have important implications for predicting corrosion in atmospheric environments.

Redox chemistry, solubility and hydrolysis of Re in reducing aquatic systems. Thermodynamic description and comparison with Tc

The aqueous solution chemistry of rhenium was investigated in acidic to hyperalkaline reducing systems buffered by Sn(II) and Na2S2O4. Oversaturation experiments with [NaReO4]0 = 10−3 M were used to evaluate the redox chemistry of Re(VII)/Re(IV), whereas the solubility and hydrolysis of Re(IV) were investigated in a series of undersaturation experiments with ReO2(s).The predominance of Re(IV) is predicted under the very reducing conditions defined by Sn(II) and Na2S2O4 (at pe + pHm ≈ 1–5) (with pHm = –log [H+] and pe = – log ae–), according to thermodynamic calculations represented as Pourbaix diagrams. In contrast to Tc(VII), Re(VII) shows recalcitrance to the reduction in Sn(II) systems except at pHm ≈ 1 and ≈12.8. As previously reported in the literature, the reduction of Re(VII) observed at pHm ≈ 1 is expectedly triggered by the co-precipitation of Re(IV) with the SnO2(s) solid phase forming through the in-situ oxidation of Sn(II). The faster reduction kinetics reported for Tc(VII) can be partly explained by the more positive standard potential (E°) of the couple TcO4−/TcO2(s) compared to that of ReO4−/ReO2(s), which results in a greater driving force for the reduction of TcO4− under the same conditions.The solubility of ReO2(s) remains low (≈10−6 M) and pH-independent within 1 ≤ pHm ≤ 10, but increases with a slope of +1 (log [Re] vs. pHm) above pHm ≈ 10. A combination of solid phase characterization, solubility data determined in this work and reported in the literature are considered to derive chemical and thermodynamic models including the solubility reactions ReO2(s) + H2O(l) ⇔ ReO(OH)2(aq) and ReO2(s) + 2H2O(l) ⇔ ReO(OH)3– + H+. Thermodynamic data determined in this work for the hydrolysis of Re(IV) show a good agreement with thermodynamic data reported for Tc(IV), and represent a significant improvement with respect to previous studies available on the Re(IV) system.Experimental evidence obtained in this work support the chemical analogy of Re and Tc in the +IV oxidation state, which is rationalized by the very similar ionic radii of both metal ions. Notwithstanding, these elements show a remarkably different redox chemistry, both in terms of kinetics and thermodynamic stability. The use of Re as an inactive analogue of Tc in systems involving redox transformations must therefore be considered with precaution.

A Computational Protocol Combining DFT and Cheminformatics for Prediction of pH-Dependent Redox Potentials.

Discovering new materials for energy storage requires reliable and efficient protocols for predicting key properties of unknown compounds. In the context of the search for new organic electrolytes for redox flow batteries, we present and validate a robust procedure to calculate the redox potentials of organic molecules at any pH value, using widely available quantum chemistry and cheminformatics methods. Using a consistent experimental data set for validation, we explore and compare a few different methods for calculating reaction free energies, the treatment of solvation, and the effect of pH on redox potentials. We find that the B3LYP hybrid functional with the COSMO solvation method, in conjunction with thermal contributions evaluated from BLYP gas-phase harmonic frequencies, yields a good prediction of pH = 0 redox potentials at a moderate computational cost. To predict how the potentials are affected by pH, we propose an improved version of the Alberty-Legendre transform that allows the construction of a more realistic Pourbaix diagram by taking into account how the protonation state changes with pH.

Progress of Nonprecious-Metal-Based Electrocatalysts for Oxygen Evolution in Acidic Media.

Water oxidation, or the oxygen evolution reaction (OER), which combines two oxygen atoms from two water molecules and releases one oxygen molecule, plays the key role by providing protons and electrons needed for the hydrogen generation, electrochemical carbon dioxide reduction, and nitrogen fixation. The multielectron transfer OER process involves multiple reaction intermediates, and a high overpotential is needed to overcome the sluggish kinetics. Among the different water splitting devices, proton exchange membrane (PEM) water electrolyzer offers greater advantages. However, current anode OER electrocatalysts in PEM electrolyzers are limited to precious iridium and ruthenium oxides. Developing highly active, stable, and precious-metal-free electrocatalysts for water oxidation in acidic media is attractive for the large-scale application of PEM electrolyzers. In recent years, various types of precious-metal-free catalysts such as carbon-based materials, earth-abundant transition metal oxides, and multiple metal oxide mixtures have been investigated and some of them show promising activity and stability for acidic OER. In this review, the thermodynamics of water oxidation, Pourbaix diagram of metal elements in aqueous solution, and theoretical screening and prediction of precious-metal-free electrocatalysts for acidic OER are first elaborated. The catalytic performance, reaction kinetics, and mechanisms together with future research directions regarding acidic OER are summarized and discussed.