In-situ probing of electrochemical dissolution and surface properties of chalcopyrite with implications for the dissolution kinetics and passivation mechanism

Effect of Redox Mediators on Corrosion Behavior and Scanning Electrochemical Microscopy Response

Scanning electrochemical microscopy (SECM) is widely used to measure local electrochemical reactivity of corroding surfaces. A major criticism of using SECM in feedback mode for corrosion studies is the requirement of an external redox mediator (RM) as it could react with the metal and affect the Nernst potential at the metal-solution interface. Consequently, it becomes challenging to differentiate the interference caused by the RM from the local reactivity of the metal. Herein, a multiscale electrochemical approach is presented to investigate the effect of RM choice on the corroding substrate. Two common RMs, ferrocenemethanol and hexaammineruthenium(III) chloride, were used to perform SECM over copper and aluminum. It was found during macroscale electrochemical measurements that Ru(NH)63+ acted as an oxidant and promoted corrosion. The SECM feedback behavior varied for copper depending on the RM used, suggesting that the corrosion reactions controlled the negative feedback mechanism, not the formation of an insulating passive film. The passivated aluminum surface consistently exhibited negative feedback, regardless of the RM used. SECM approach curves also displayed a distortion in the steady state current, which was caused by the deposition of substrate-generated species on the microelectrode. These deviations in feedback response were accounted for during analysis through incorporation into a finite element model to accurately extract the RM kinetic rate constants. The importance of understanding these processes is highlighted to avoid misinterpretation of passive behavior and advances toward a more quantitative use of SECM for corrosion studies.

Insight into oxygen reduction activity and pathway on pure titanium using scanning electrochemical microscopy and theoretical calculations

HYPOTHESIS

Unlike noble metals, the oxygen reduction reaction (ORR) behavior on Ti is more complicated due to its spontaneously formed oxide film. This film results in sluggish ORR kinetics and tends to be reduced within ORR potential region, causing the weak and multi-reaction coupled current. Though Ti is being used in chemical and biological fields, its ORR research is still underexplored.

EXPERIMENTS

We innovatively employed the modified reactive tip generation-substrate collection (RTG/SC) mode of scanning electrochemical microscopy (SECM) with high efficiency of 97.2 % to quantitatively study the effects of film characteristics, solution environment (pH, anion, dissolved oxygen), and applied potential on the ORR activity and selectivity of Ti. Then, density functional theory (DFT) and molecular dynamics (MD) analyses were employed to elucidate its ORR behavior.

FINDINGS

On highly reduced Ti, film properties dominate ORR behavior with promoted 4e^- selectivity. Rapid film regeneration in alkaline/O₂-saturated conditions inhibits ORR activity. Besides, ORR is sensitive to anion species in neutral solutions while showing enhanced 4e^- reduction in alkaline media. All the improved 4e^- selectivities originate from the hydrogen bond/electrostatic stabilization effect, while the decayed ORR activity by Cl^- arises from the suppressed O₂ adsorption. This work provides theoretical support and possible guidance for ORR research on oxide-covered metals.

Surface interaction mechanisms in mineral flotation: Fundamentals, measurements, and perspectives

As non-renewable natural resources, minerals are essential in a broad range of biological and technological applications. The surface interactions of mineral particles with other objects (e.g., solids, bubbles, reagents) in aqueous suspensions play a critical role in mediating many interfacial phenomena involved in mineral flotation. In this work, we have reviewed the fundamentals of surface forces and quantitative surface property-force relationship of minerals, and the advances in the quantitative measurements of interaction forces of mineral-mineral, bubble-mineral and mineral-reagent using nanomechanical tools such as surface forces apparatus (SFA) and atomic force microscope (AFM). The quantitative correlation between surface properties of minerals at the solid/water interface and their surface interaction mechanisms with other objects in complex aqueous media at the nanoscale has been established. The existing challenges in mineral flotation such as characterization of anisotropic crystal plane or heterogeneous surface, low recovery of fine particle flotation, and in-situ electrochemical characterization of collectorless flotation as well as the future work to resolve the challenges based on the understanding and modulation of surface forces of minerals have also been discussed. This review provides useful insights into the fundamental understanding of the intermolecular and surface interaction mechanisms involved in mineral processing, with implications for precisely modulating related interfacial interactions towards the development of highly efficient industrial processes and chemical additives.

Recent advances in electrochemical techniques for characterizing surface properties of minerals

Electrochemical techniques are very useful tools for characterizing the surface properties of natural minerals involved in electrochemical reactions. This work reviews the recent advances in electrochemical characterizations of minerals by employing various electrochemical techniques, i.e., open circuit potential, chronoamperometry, potential sweep voltammetry, electrochemical impedance spectroscopy, and electrochemical scanning probe techniques. The fundamental working principles of these electrochemical techniques and their applications for mineral surface characterizations in various research areas, including mineral flotation, mineral leaching, electrocatalysis, energy storage materials and environmental issues, are highlighted. Valuable information such as the redox condition of substrate surface, the current response of substrate with time under polarization, the identification of redox reaction and its kinetics on substrate surface, the structure of substrate/electrolyte interface, and the local electrochemical response on substrate surface at micro-/nano-scale can be obtained by open circuit potential, chronoamperometry, potential sweep voltammetry, electrochemical impedance spectroscopy, and scanning electrochemical microscopy, respectively. Some remaining challenges and future perspectives are discussed. These recent advances in electrochemical techniques can be readily applied to characterize the surface properties and interfacial interactions of a wide variety of material systems and in different engineering processes.