Dissolution Of Au Research Articles

Graphene/chitosan nanoreactors for ultrafast and precise recovery and catalytic conversion of gold from electronic waste

The extraction of gold (Au) from electronic waste (e-waste) has both environmental impact and inherent value. Improper e-waste disposal poses environmental and health risks, entailing substantial remediation and healthcare costs. Large efforts are applied for the recovery of Au from e-waste using complex processes which include the dissolution of Au, its adsorption in an ionic state and succeeding reduction to metallic Au. These processes themselves being complex and utilizing harsh chemicals contribute to the environmental impact of e-waste. Here, we present an approach for the simultaneous recovery and reduction of Au3+ and Au+ ions from e-waste to produce solid Au0 forms, thus skipping several technological steps. We develop a nanoscale cross-dimensional composite material via self-assembly of two-dimensional graphene oxide and one-dimensional chitosan macromolecules, capable of acting simultaneously as a scavenger of gold ions and as a reducing agent. Such multidimensional architecture doesn't require to apply any voltage for Au adsorption and reduction and solely relies on the chemisorption kinetics of Au ions in the heterogeneous GO/CS nanoconfinements and their chemical reduction on multiple binding sites. The cooperative phenomena in ionic absorption are responsible for the extremely high efficiency of gold extraction. The extraction capacity reaches 16.8 g/g for Au3+ and 6.2 g/g for Au+, which is ten times larger than any existing gold adsorbents can propose. The efficiency is above 99.5 wt.% (current limit is 75 wt.%) and extraction ability is down to very low concentrations of 3 ppm.

A Combined Physical-Mechanical and Hydrometallurgical Approach for Recovering Valuable Metals from Spent Lithium-ion Batteries

In this work, the chemical composition of electrode materials from two samples of lithium-ion batteries (LiB) is comprehensively investigated. The material balance of the physical and mechanical processing of the LiBs mixture is determined. The developed dry process scheme made it possible to extract the following components (wt.%): 15.6 plastic (ABS), 1.89 electronic materials (PP), 59.1 black mass (three types), 6.43 plastic (PVC), 2.97 Al, 6.31 Cu and 7.1 magnetic fraction (Fe). The thermodynamics of reductive leaching of LiCoO<sub>2</sub> in the H<sub>2</sub>SO<sub>4</sub>-HCOOH system was studied. It was calculated that the Gibbs energy of the leaching reaction at 363 K is -327.4 kJ/mol, the equilibrium constant is 2.02×10<sup>44</sup>. All these factors showed the potential of using formic acid as a reducing agent instead of the commonly used hydrogen peroxide solution. It is known to be unstable, since when the solution is heated from 20 to 50<sup>0</sup> C, the decomposition of H<sub>2</sub>O<sub>2</sub> increases 20 times, and the presence of copper sulfate, which usually accompanies the decomposition of the black mass in a sulfuric acid medium, leads to the destruction of 76% of hydrogen peroxide. Therefore, from a practical point of view, it was of interest to study the kinetics of cobalt dissolution from the cathode material in the H<sub>2</sub>SO<sub>4</sub>-HCOOH system. It was shown that the Crank-Ginstling-Braunstein equation agrees satisfactorily with the experimental data, which indicates intra-diffusion limitation of dissolution. The activation energy was determined. The optimal composition and conditions of the leaching solution for dissolving cobalt, lithium and associated transition metals from the black mass were determined. The thermodynamics and kinetics of dissolution of Ag, Au and Pd from electrode materials in a bromide-bromine solution were also studied.

Morphology Control of Nanoporous Gold Through Selective Dissolution of Au–Ge Eutectic Microstructures

The synthesis of nanoporous gold (np‐Au) through the conventional method of dealloying a more reactive element from AuTherma alloys has been extensively studied and applied in various fields, particularly catalysis. Herein, a novel approach to creating droplet‐like np‐Au through the selective dissolution of Au–Ge eutectic microstructures is explored. This work reveals that adjusting the undercooling of the eutectic melt during solidification allows for the tuning of pore and ligament sizes. It is demonstrated that this undercooling can be modified, either directly or indirectly, by adjusting either the cooling rate or the heterogeneous nucleation site density of the eutectic melt. The findings, aligned with the model based on classical theory for eutectic solidification, elucidate the connection between the tuning of pore and ligament sizes in the eutectic microstructure and the undercooling of the eutectic melt. Importantly, it is established that this phenomenon is applicable across a variety of compositions, including hypereutectic, eutectic, and hypoeutectic. The ability to regulate pore and ligament size in single crystals of np‐Au droplets offers a novel approach to synthesizing catalytic np‐Au crystals with enhanced mechanical and thermal stability compared to conventional methods.

PH Dependence of Noble Metals Dissolution: Gold

AbstractThe electrochemical applications of gold span the entire pH spectrum. Recently, gold dissolution in acidic and alkaline media has been studied, but less attention has been given to electrolytes at intermediate pH values. To address this gap, this work uses on‐line electrochemical dissolution inductively coupled plasma mass spectrometry (ICP‐MS) to examine gold dissolution across a pH range of 1 to 12.7 using phosphate buffer solutions. All experimental parameters, except pH, are kept constant, enabling a clear investigation of pH effects on anodic (gold oxidation) and cathodic (gold oxide reduction) dissolution processes. Results show that dissolution amounts are lowest at neutral pH values between 3 and 7, varying with the applied potential and exposure time. Anodic and cathodic dissolution dominate in acidic and alkaline electrolytes, respectively. Depending on the highest applied potentials and time exposure, the main dissolution mechanism shifts at pH=5, 7, and 9. The pH dependence of Au dissolution is proposed to be linked to the nature of gold oxides formed, the kinetics of oxide formation/reduction, gold ion redeposition, and the influence of the oxygen evolution reaction (OER) on dissolution. These results provide fundamental insights into gold dissolution under neutral pH conditions.

Atomically dispersed Au single sites and nanoengineered structural defects enable a high electrocatalytic activity and durability for hydrogen evolution reaction and overall urea electrolysis

The development of exceptionally active and stable trifunctional electrocatalysts is essential to facilitate industrial applications of proton exchange membrane electrolyzers. Rare earth catalysts have a wide range of applications in alkaline solution oxidation, but limitations in their stability and activity prevent their further application. In this paper, Au doped CeO2 coated porous Aux/CeO2 (x = 1,2,3) was designed as a three-function electrocatalyst with excellent performance and stability. At 1 M KOH conditions, both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) reactions exhibit excess potentials of 159.6 and 320.8 mV at a current density of 10 mA cm-2. In addition, when incorporated into an Au2/CeO2 (0.05 mmol, HAuCl4·4H2O) microparticles, the catalyst exhibits the potential of 1.42 V(10 mA cm-2) containing 0.33 M urea in 1 M KOH. Remarkably, for hydrogen production driven by urea electrolysis, Au2/CeO2 demonstrated exceptional performance by requiring only a voltage input of 1.63 V. The reducing agent that forms the Au/CeO2 heterojunction modulates the electronic structure of the Au active site and optimizes the adsorption of the intermediate. In addition, the interaction of the Au monolayer with the CeO2 nanosphere effectively prevents excessive oxidation and dissolution of Au, and the porous hollow structure provides additional exposed active sites and facilitates mass transfer.

Recovering palladium and gold by peroxydisulfate-based advanced oxidation process.

Palladium (Pd) and gold (Au) are the most often used precious metals (PMs) in industrial catalysis and electronics. Green recycling of Pd and Au is crucial and difficult. Here, we report a peroxydisulfate (PDS)-based advanced oxidation process (AOPs) for selectively recovering Pd and Au from spent catalysts. The PDS/NaCl photochemical system achieves complete dissolution of Pd and Au. By introducing Fe(II), the PDS/FeCl2·4H2O solution functioned as Fenton-like system, enhancing the leaching efficiency without xenon (Xe) lamp irradiation. Electron paramagnetic resonance (EPR), 18O isotope tracing experiments, and density functional theory calculations revealed that the reactive oxidation species of SO4·-, ·OH, and Fe(IV)═O were responsible for the oxidative dissolution process. Lixiviant leaching and one-step electrodeposition recovered high-purity Pd and Au. Strong acids, poisonous cyanide, and volatile organic solvents were not used during the whole recovery, which enables an efficient and sustainable precious metal recovery approach and encourage AOP technology for secondary resource recycling.

A microscopic view on the electrochemical deposition and dissolution of Au with scanning electrochemical cell microscopy – Part II: Potentiostatic dissolution and correlation with in-situ EC-TEM

The electrochemical oxidation of gold nanoparticles (NPs) has been examined in H2SO4 and acidified NaCl by combining multiple techniques including scanning electrochemical cell microscopy (SECCM) and in-situ electrochemical transmission electron microscopy (EC-TEM). Our findings provide novel insights into the intricate oxidation dynamics of Au NPs, which are determined by the interplay between passivation and dissolution. SECCM chronoamperometric measurements in the Cl− containing electrolyte reveal distinct current peak events during the dissolution process. We attribute these events to delayed one-by-one NP dissolution following the rapid breakdown of the passive layer formed during the initial stages of anodic polarization, and exposure of the metallic gold core to Cl− ions. Statistical analysis of these peak events further uncovers relationships between the applied potential and the distribution of the peak descriptors over time. The analysis of the charge consumed during each event indicates that NPs undergo partial dissolution interrupted by rapid re-passivation. As the potential increases, the peak current rises and the duration of the peak events decreases due to intensified dissolution and faster re-passivation. The onset time of the peak events displays a highly stochastic nature. In-situ EC-TEM measurements support these findings by confirming that Au NPs dissolve one-by-one at different time intervals and stages, revealing a core-shell structure during the dissolution process. The shell, which is more resistant, leads to a delayed, particle-by-particle dissolution once it is broken down. Quantification of the EC-TEM video allows recreating an equivalent current-time transient which presents peak events similar to these measured by SECCM. These findings contribute to our understanding of the complex and stochastic behavior of nanoparticle dissolution, which cannot be fully explained by traditional (macro) electrochemistry alone. The combination of SECCM and EC-TEM offers high potential to understand complex nanomaterial degradation pathways, essential for the design of durable electrocatalysts for electrochemical conversion and storage applications.

Microstructural evaluation of interfacial intermetallic compounds between Sn58Bi and ENEPIG

BackgroundLow carbonization and Restriction of Hazardous Substances (RoHS) regulations have promoted the development of low-temperature lead-free solder. The solder and substrate are critical entities of joint connections. MethodsSn58Bi has been considered a promising low-temperature solder. Electroless nickel/electroless palladium/immersion gold (ENEPIG) is a good surface finish that prevents solder joints from oxidizing and extends the shelf life of the substrates. In this study, the interfacial reactions between the Sn58Bi solder and ENEPIG surface finish were systematically analyzed. Findings(Au,Pd,Ni)(Sn,Bi)4 was observed after reflow at 160 °C. Au and Pd dissolved into solder rapidly in the beginning. NiSn4 and Ni3Sn4 with small amounts of Au and Pd dissolution formed at the interface after aging. The morphology of NiSn4 evolved from block-like to needle-shaped and finally to a layered configuration as the aging temperature and time increase. Ni3Sn4 was observed between NiSn4 and substrate above 100 °C. This study established the growth mechanism of intermetallic compounds (IMCs) in the reaction between Sn58Bi and ENEPIG for low-carbon emitting processes.

Study of gold electrodissolution by scanning electrochemical microscopy in different modes

The electrodissolution of gold has already been widely studied by several methods, but no consensus concerning the reaction mechanism has been reached. In this work, the scanning electrochemical microscopy (SECM) has been used in transient condition for studying the electrodissolution of gold in H2SO4 solutions, with different concentration of chloride added, in order to elucidate the elementary steps of this mechanism. The electroactive species generated during Au dissolution, namely AuCl2−, AuCl3− and AuCl4−, were detected using a Pt microelectrode positioned at various distances from the Au electrode. Additionally, the SECM in transient mode (AC-SECM), which allows several transfer functions to be determined, has been used to characterize the presence and the relaxation time-constants of adsorbed intermediates formed at the Au electrode during its dissolution. The interpretation of the results is presented and discussed according to the different mechanisms reported in the literature, and subsequently, a new reaction mechanism is proposed.

Toward Eco‐Friendly E‐Waste Recycling: New Perspectives on Ozone‐Assisted Gold Leaching

Global demand for more effective methods to reclaim gold from electronic waste (E‐waste) has never been greater. Alternatives to hydrometallurgical methods, such as cyanide, are still limited. This work examines utilizing ozone and chlorides to recycle Au from E‐waste. It is started with a fundamental investigation of Au dissolution processes on the extended surface of Au polycrystalline and Au nanoparticulated electrodes. An online electrochemical scanning flow cell coupled with inductively coupled plasma mass spectrometry quantifies the rates and amounts of Au leaching. Identical‐location scanning electron microscopy (IL‐SEM) further correlates dissolution events with electrode morphological changes. It is demonstrated that ozone in the electrolyte imposes an anodic potential on the electrode, leading to anodic Au dissolution. Passivation disappears when small amounts of chlorides are added to the electrolyte, significantly enhancing the leaching yield. IL‐SEM images of gold nanoparticles (NPs) before and after exposure to ozone reveal heterogeneity in NP size‐dependent dissolution, showing higher dissolution for smaller particles. An effective Au leaching procedure is further demonstrated in a lab‐scale reactor using real E‐waste where almost complete recovery of Au is achieved. This research suggests that with engineering optimization in reactor applications based on ozone‐stimulated gold, dissolution can pave the way for environmentally friendly gold recycling.

Application of Pulse Techniques for the Electropolishing of Titanium and Gold Alloys in Deep Eutectic Solvents

Electropolishing is widely used industrially to create smooth and bright surface finishings. This technique is particularly relevant for components with complex geometries, especially in the onset of additive manufacturing. (1, 2) As E. J. Taylor showed for different types of materials, pulse techniques can be used to control the surface modification and drive the electrochemical process towards a specific desired result. (3)The changes in the surface layers and concentration profiles caused by shifts in polarization lead to results difficult to attain in constant current/potential conditions, especially in metals that passivate and alloys containing metals with dissimilar electrochemical stabilities.Titanium and noble metal-alloys are materials used in a variety of applications. The passivation layer of titanium is very difficult to remove without using strong acids or fluoride-containing compounds. In the case of noble-metal alloys, the problem lies in the non-uniform dissolution of the material under anodic polarization and in the cyanide-containing electrolytes and strong acids which are used for electropolishing.Water-free solvents have been described by Jacquet for this purpose (4). Ionic liquids and deep eutectic solvents, DES, are increasingly being used to work in potential windows otherwise not reachable in aqueous solutions. Reports indicate that a mixing of choline chloride and ethylene glycol can be used for electropolishing of titanium and other metals (5, 6) and that both dissolution and recovery of Au and Pd is feasible in ionic liquids and deep eutectic solvents. (7, 8)Using DES for electropolishing applications offers therefore an opportunity to substitute dangerous substances, such as cyanide and fluoride compounds. This could increase the scope of application of electropolishing in terms of work safety, recycling and disposal.The aim of this work is to show the interplay between pulse parameters (frequency, pulse height and reverse pulses) and the viscosity of the solution. Together with temperature and convection, pulses can be used to selectively control the dissolution of the metal towards the desired surface finish. Using this principle, an average roughness of 20-30 nm was achieved for titanium samples with complex geometry, both under laboratory conditions as well as in a pilot plant. Furthermore, it will be shown how pulses can be used to control the dissolution of electrochemical less noble elements in gold alloys. Acknowledgements: The project is financially supported by the German Federal Ministry for Economic Affairs and Climate Action (ZIM Program). References W. Han and F. Fang, International Journal of Machine Tools and Manufacture, 139, 1–23 (2019).E. J. Taylor, H. McCrabb, H. Garich and T. Hall, A pulse/pulse reverse electrolytic approach to electropolishing and through-mask electroetching PF Products Finishing, Electroplating, www.pfonline.com (published on 26.09.2011).E. J. Taylor, M. Inman, T. D. Hall, S. Snyder, J. Mammosser and F. Furuta, ECS Meet. Abstr., vol. MA2017-01(24), 1197 (2017).P. A. Jacquet, Trans. Electrochem. Soc., 69(1), 629 (1936).W. O. Karim, J. A. Juma, K. M. Omer, Y. M. Salih, K. H. Aziz and S. B. Aziz, Electrochemistry, 88(5), 447–450 (2020).A. P. Abbott, G. Frisch, J. Hartley, W. O. Karim and K. S. Ryder, Progress in Natural Science: Materials International, 25(6), 595–602 (2015).E. Billy, E. Chainet and F. Tedjar, Electrochimica Acta, 56(28), 10340–10346 (2011).P. Giridhar, K. A. Venkatesan, B. P. Reddy, T. G. Srinivasan and P. R. Vasudeva Rao, Radiochimica Acta, 94(3), 131–136 (2006).

Electrodeposition Behavior of Al-Au Alloys in AlCl3-NaCl-KCl-Aucl Molten Salt

Quartz Crystal Microbalance (QCM) has been used as a highly sensitive mass sensor. This study aims to form porous gold thin films on QCM elements and apply them to highly sensitive gas sensors by attaching selective media. Physical vapor deposition and chemical etching are known methods for forming porous Au thin films. In this study, the electrochemical formation of porous gold films in AlCl3-NaCl-KCl molten salts containing AuCl was investigated. There are few reports about electrodeposition and dissolution potentials of Au in the molten salt electrolyte. As a first step in this study, voltammogram measurements and constant potential electrolysis were performed in the electrolyte, and electrodeposition potentials of Au and Al-Au alloys were discussed.An electrolyte in this study is a mixture of 61 mol% AlCl3- 26 mol% NaCl- 13 mol% KCl molten salt. The mixture salt was melted at 443 K and AuCl was added to the molten salt. A glassy carbon plate (GC) and a pure Al wire were used as the working electrode and reference electrode, respectively. The GC plate and Au mesh were used as the counter electrode for voltammogram measurement and constant potential electrolysis, respectively. Voltammogram measurements were carried out at potential range from -0.1 to 2.4 V vs. Al/Al(Ⅲ). Constant potential electrolysis was performed in the range from -0.2 to 0.3 V with an electric charge density of 40 C cm-2. The electrodeposits were observed by scanning electron microscopy (SEM) and determined the composition of Al and Au by energy dispersive X-ray spectroscopy (EDS).Au ions concentration in the electrolyte was 2.5 mmol dm-3. In voltammogram measurements, a small cathodic current was observed at about 0.8 V, then gradually increased from 0.2 V. In the potential lower than 0 V, a drastical increase of cathodic current was observed, which was attributed to Al electrodeposition. When the potential was reversed, large anodic current flowed from 0 V and a small current peak was also observed at 0.2 V. These were considered to correspond to the dissolution of Al and Al-Au alloys, respectively. Furthermore, a relatively large anodic current peak was observed at about 2 V, which was expected to be due to the dissolution of Au. From the results of constant potential electrolysis, the Au concentration in the electrodeposits at -0.1 V and -0.2 V was less than 1 at%. At 0 V, the Au concentration in the electrodeposit increased sharply to 84 at%, and at 0.3 V it increased moderately to 89 at%. From SEM observation, the morphology of the electrodeposits at -0.1 V and -0.2 V was dense, while that at potentials more than 0 V was sponge-like.AcknowledgementsThis work was supported by JST SICORP and JPMJSC2108.

Electric field-assisted dissolution of bimetal-dielectric multilayer systems

The electric field-assisted dissolution (EFAD) of Ag and Au metal island films (MIFs) in multilayer structures is investigated. The samples consist of Au and Ag MIFs separated by SiO2 layers deposited on soda-lime glass substrates. The samples were subjected to two types of process: a) thermal annealing at 200°C, and b) application of 500 V dc voltage at 200°C. Spectroscopic ellipsometry and secondary ion mass spectrometry were used to track sample changes. It is shown that thermal annealing induces ion exchange between Ag and Na ions from the glass, leading to partial dissolution of Ag MIFs and Ag being incorporated in the glass and Au MIFs. EFAD process leads to the partial or complete dissolution of Ag films depending on the process duration, while Au dissolution turns out to be more difficult. The optical properties are consistent with the compositional changes and additionally suggest MIF morphology modifications. Numerical simulations of ion drift and diffusion reveal the importance of ion exchange during thermal annealing to enable an efficient MIF dissolution. Overall, the study helps to understand the EFAD of different metals in complex systems and provides insights for the application of this technique in systems containing several metals.

Resource Recovery Strategy of Rapid Screening Kits for Diseases through Ozonation.

Rapid disease screening kits are widely used due to their efficiency and convenience in disease screening. However, the resulting medical waste poses a significant environmental burden. Recycling this waste in a resourceful manner not only protects the environment from pollution but also maximizes savings in resources and energy consumption. In this study, we developed a scalable, simple, and efficient method for the resourceful recovery of assay kits. The method achieves simultaneous 100% pathogen inactivation in the kits, complete leaching of gold nanoparticles (Au NPs), and nondestructive recovery of plastics and tampons. Additionally, our mechanistic study on Au leaching has demonstrated that water plays a crucial role as the initiator for the Au dissolution reaction, and the hydroxyl radical (•OH) serves as an important reactive oxygen species. This research introduces a novel approach for dissolving the precious metal gold and provides a viable technology for the resource utilization of waste disease screening kits.

Study of the basic laws of dissolution of gold and copper in solutions with an ultra-low concentration of sodium cyanide

The work sets out to study the basic physicochemical dissolution patterns of gold, copper, and natural copper-containing minerals (chalcopyrite, bornite and azurite) in solutions with an ultra-low concentration of sodium cyanide (from 0.102∙10-3 to 4.08∙10-3 mol/L). The influence of various factors on the rate of dissolution of Au and Cu in solutions with ultra-low NaCN concentrations was studied by the rotating disk method; for natural copper minerals, the powder diffraction method was used. The concentration of gold and copper in solutions was determined by atomic absorption analysis. The chemical composition of the studied copper minerals was determined using the X-ray phase method, while the specific surface of the minerals was detected using a laser granulometer. The process of gold dissolution is shown to proceed in both diffusion and kinetic regions. In the diffusion region, the rate constant was 0.334∙10-6 L∙cm-2∙s-1/2∙rad-1/2; in the kinetic region – 0.919∙10-6 L∙cm-2∙s-1/2. The calculated value of the apparent activation energy for the diffusion region was 22.5 kJ/mol; for the kinetic region – 40.1 kJ/mol. The addition of glycine to a solution with an ultra-low concentration of sodium cyanide is shown to increase the specific dissolution rate of gold by 1.2 times: from 0.692∙10-9 to 0.82∙10-9 mol/cm2∙s. The process of copper dissolution is shown to take place in the diffusion r egion. The rate constant was 0.496∙ 10-6 L∙cm-2∙s-1/2∙rad-1/2 at an activation energy of 17.0 kJ/mol. With a fractional supply of sodium cyanide, the dissolution rate of copper minerals is reduced by 10–30% compared to a single load. The calculated apparent activation energy values for chalcopyrite, bornite, and azurite were 22.03, 24.2, and 24.1 kJ/mol, respectively. Thus, the use of ultra-low concentrations of NaCN in the process of cyanidation of gold and copper has a positive effect, which can be used i n the processing of gold-copper raw materials to significantly reduce the consumption of sodium cyanide.

Noble Metals Dissolution Catalyzed by [AlCl4 -]-Based Ionic Liquids.

Imidazolium-based ionic liquid mixtures with [NO3]- and [AlCl4]- anions were used as oxidizing agents for the dissolution of Au, Pd, and Pt metals under mild conditions. The thermodynamic reduction of [NO3]- to [NO] is catalyzed by [AlCl4]- anions and coupled with the oxidation process of noble metals. The developed ionic liquid system for dissolving Au can reactivate the Au0 formed in the deactivation process of the catalyst in vinyl chloride production. This demonstrates the relevance of the here-presented work for technical noble metal recycling.

A microscopic view on the electrochemical deposition and dissolution of Au with scanning electrochemical cell microscopy – Part I

The nucleation and dissolution of gold nanoparticles (NPs) on a glassy carbon substrate have been studied with a local electrochemical approach based on Scanning Electrochemical Cell Microscopy (SECCM). By performing hundreds of locally-resolved voltammograms, a distribution of responses for nucleation was found when scanning multiple regions of the substrate, addressing experimentally the statistical nature of the nucleation and growth process at the micro-scale. Moreover, the confinement of the electrochemical cell to the SECCM meniscus enables us to resolve a diversity of events during the electrochemical dissolution of previously electrodeposited NPs, characterized as current spikes during the voltammetric scans. These current events do not manifest in the macro-scale due to the averaging of the electrochemical response over the large electrode area and reveal previously untapped information from the electrochemical nucleation, growth, and dissolution of supported metal nanoparticles. This approach opens up new opportunities for the rational design (electrodeposition) of functional nanostructured materials and the evaluation of their durability under electrochemical polarization (resistance to electrodissolution). The ability to study these considering the heterogeneous nature of the supports and the differences within nanomaterial ensembles is essential for applications in electrochemical conversion and storage.

A Comparative Study on Gold Dissolution in Broad pH Range from Acidic to Alkaline Media

Gold is often employed in nanotechnology to develop biosensors and new drug delivery and cancer therapy strategies. Since these applications are used in physiological pH, understanding the stability of gold in a broader pH range including neutral values will provide an useful insight into how to expand the use of gold safely.By using buffer solutions with a constant concentration of phosphate species, electrochemical dissolution of gold in pH 1, 3, 5, 7, 9, 11 and 12.7 was investigated. The concentration of phosphate species was kept constant in order to minimize the possible effect of anion concentration on gold dissolution. Oxidation/reduction of the metal surface leads to transient dissolution peaks, which are labelled as A1 and C1 on the dissolution profiles on Figure 1. An additional dissolution peak A2 appears when the oxygen evolution reaction (OER) takes place. Quantitatively, there are significant differences. The amount of gold that is transiently dissolved in an alkaline medium is more than an order of magnitude higher than that in an acidic medium at lower potentials. Oppositely, steady-state gold dissolution in alkaline pH in the potential region of OER is hindered. There is an apparent shift in the mechanisms at pH 5. At higher potentials, i.e 1.9 V RHE, there is an apparent loss of peak A2 and a growth of C1. The loss of dissolution of gold due to a change in the mechanism of OER in more alkaline electrolytes is clear at pH 7. To explain the observed difference in gold dissolution, a difference in the thickness of compact oxide formed in acid and base is suggested. The formation of thicker oxide as a result of an increase in pH becomes noticeable with the observed increase in both peaks A1 and C1 that can be seen when the pH is increased from 7 to 12.7. These new data on Au dissolution in a broad potential and pH range should be used to minimize Au dissolution in various applications. Figure 1

Identical‐location Transmission Electron Microscopy of Pt Nanoparticle Electrocatalysts Using Iridium‐coated Au Grids for Fuel Cell Durability Tests Simulating Start‐up and Shutdown Conditions

AbstractIn an identical‐location TEM (IL‐TEM) analysis of an electrocatalyst, an Au grid is usually selected owing to its chemical stability under potential cycling conditions. A potential cycle between 1.0 and 1.5 V is applied to the catalyst‐cast Au grid for cathode durability testing that simulates the start‐up and shutdown conditions of fuel cells. Because of the redox potential of Au (1.36 V vs. RHE), the grid dissolves and redeposits on the catalyst under the applied potential, making it complicated to evaluate the catalyst nanoparticle shape and size after degradation. We fabricated an Au grid coated with an iridium oxide layer (Ir‐coated Au grid) to suppress the dissolution of Au. The Ir‐coated Au grid with carbon support was compared to the Au grid to confirm the effect of the iridium oxide layer. No Au deposition was observed, even after 3000 cycles at 60 °C for the Ir‐coated Au grid, but Au was deposited on carbon on the Au grid after 1000 cycles. Consequently, the alkylamine‐modified Pt nanoparticle catalyst (unwashed catalyst) was observed along with the Ir‐coated Au grid using IL‐TEM under durability tests simulating start‐up and shutdown conditions. A catalyst with less alkylamine content was prepared by butylamine washing (washed catalyst) and observed using IL‐TEM for comparison. The Pt nanoparticles of the washed catalyst aggregated and changed their morphology after 1000 cycles, while the alkylamine‐modified nanoparticles of the unwashed catalyst almost maintained their original size and shape up to 1000 cycles. The Ir‐coated gold grids allow proper IL‐TEM analysis of catalysts in durability tests without the interference of Au dissolution.

Structural Evolution of Au Electrodes during Cathodic Corrosion: Initial Stages of Octahedral-Nanocrystal Growth

Cathodic corrosion of Au wires in highly concentrated NaOH (5–20 M) is systemically investigated to monitor the structural transformation as a function of applied electrode potential, polarization time, and electrolyte concentration. Current–potential curves for the restructured Au electrodes in 0.1 M H2SO4 demonstrate preferential (111)-orientation of nanofacets and an increase in electrochemically active surface area as a function of the applied parameters. SEM shows that polarizing the Au surfaces at sufficiently negative potentials leads to the growth of nanosized octahedral and triangular crystallites that can be tailored by varying NaOH concentrations. The nanocrystal growth occurs through dissolution and metal redeposition. The formation of etching pits indicates that Au dissolution is dominating during the initial stages of cathodic corrosion. The number of nanocrystals increases at more negative potentials due to the greater nucleation density induced by the higher concentration of dissolved Au species. Polarization times above 5.0 min result in fractal growth indicated by the appearance of multi-armed nanostructures. This work not only advances our understanding of the initial stages of nanocrystal formation but also offers a powerful means for controlling the shape and size of metal nanocrystals simply by the rational design of the cathodic corrosion.