Electrochemical analysis of nanostructured iron oxides using cyclic voltammetry and scanning electrochemical microscopy

Transpassive Metal Dissolution vs. Oxygen Evolution Reaction: Implication for Alloy Stability and Electrocatalysis

Multi-principal element alloys (MPEAs) are gaining interest in corrosion and electrocatalysis research due to their electrochemical stability across a broad pH range and the design flexibility they offer. Using the equimolar CrCoNi alloy, we observe significant metal dissolution in a corrosive electrolyte (0.1 M NaCl, pH 2) concurrently with the oxygen evolution reaction (OER) in the transpassive region, despite the absence of hysteresis in polarization curves or other obvious corrosion indicators. We present a characterization scheme to delineate the contribution of OER and alloy dissolution, using scanning electrochemical microscopy (SECM) for OER-onset detection, and quantitative chemical analysis with inductively coupled-mass spectrometry (ICP-MS) and ultraviolet visible light (UV/Vis) spectrometry to elucidate metal dissolution processes. In situ electrochemical atomic force microscopy (EC-AFM) revealed that the transpassive metal dissolution on CrCoNi is dominated by intergranular corrosion. These results have significant implications for the stability of MPEAs in corrosion systems, emphasizing the necessity of analytically determining metal ions released from MPEA electrodes into the electrolyte when evaluating Faradaic efficiencies of OER catalysts. The release of transition metal ions not only reduces the Faradaic efficiency of electrolyzers but may also cause poisoning and degradation of membranes in electrochemical reactors.

Study on electrochemical reduction mechanisms of iron oxides in pipe scale in drinking water distribution system

Iron release from pipe scale is an important reason for water quality deterioration in drinking water distribution systems (DWDS) globally. Disruption of pipe scale, release and transformation of iron compounds are hot topics in the field of water supply. The aim of this study is to determine whether and how ferric components in pipe scale be reduced under anoxic condition. In this study, new investigation approaches were applied, which include simplifying the complex scale into electrode pairs, developing novel simulating reactors and conducting tailored electrochemical assays. A galvanic cell reactor with anode of metallic iron (Fe⁰) and various cathode made of certain iron oxide (FeO_x) was firstly developed to simulate the complex niche and components of pipe scale. Electrochemical methods were used to study the reduction characteristics of scale. The results proved that reduction of iron oxide scale did occur under anoxic condition. Electromotive forces between various electrodes match the Nernst Equation quite well. As main components in pipe scale, lepidocrocite (γ-FeOOH) was found to be the most reducible iron oxide but at low rate, while goethite (α-FeOOH) has weak reducibility but can be quickly reduced. As a result of electrochemical reactions, goethite in pipe scale was transformed into magnetite (Fe₃O₄). By these means, electrochemical reaction mechanisms of pipe scale disruption were revealed, which is helpful to restrain pipe corrosion and water deterioration in DWDS.

A bifunctional lead-iron oxyfluoride, PbFeO2F, that functions as a visible-light-responsive photoanode and an electrocatalyst for water oxidation

The oxyfluoride PbFeO₂F was investigated as a photoanode material and as an electrocatalyst for water oxidation. PbFeO₂F powder, which was synthesized by a high-pressure method and had an estimated bandgap of 2.1 eV, was deposited onto a fluorine-doped tin oxide (FTO) substrate. Mott–Schottky plot measurements for the PbFeO₂F/FTO electrode showed n-type semiconductivity of PbFeO₂F, with a flat-band potential of +0.53 ± 0.05 V vs. reversible hydrogen electrode (RHE). The PbFeO₂F/FTO electrode, which was modified with a conductive TiO₂ layer and a cobalt phosphate water-oxidation cocatalyst, showed a clear anodic photocurrent in aqueous K₃PO₄ solution under visible-light irradiation (λ < 600 nm). The PbFeO₂F/FTO electrode without any modification functioned as a stable water-oxidation electrocatalyst to form O₂ with a faradaic efficiency of close to unity. This study demonstrates that PbFeO₂F is a bifunctional material, serving as a water-oxidation photoanode under a wide range of visible-light wavelengths and as an electrocatalyst that operates at a relatively low overpotential for water oxidation.

The PbFeO₂F serves as a bifunctional material for a water-oxidation photoanode workable under a wide range of visible light and a water-oxidation electrocatalyst operatable at a relatively low overpotential.

The Effect of Laser Re-Solidification on Microstructure and Photo-Electrochemical Properties of Fe-Decorated TiO2 Nanotubes

Fossil fuels became increasingly unpleasant energy source due to their negative impact on the environment; thus, attractiveness of renewable, and especially solar energy, is growing worldwide. Among others, the research is focused on smart combination of simple compounds towards formation of the photoactive materials. Following that, our work concerns the optimized manipulation of laser light coupled with the iron sputtering to transform titania that is mostly UV-active, as well as exhibiting poor oxygen evolution reaction to the material responding to solar light, and that can be further used in water splitting process. The preparation route of the material was based on anodization providing well organized system of nanotubes, while magnetron sputtering ensures formation of thin iron films. The last step covering pulsed laser treatment of 355 nm wavelength significantly changes the material morphology and structure, inducing partial melting and formation of oxygen vacancies in the elementary cell. Depending on the applied fluence, anatase, rutile, and hematite phases were recognized in the final product. The formation of a re-solidified layer on the surface of the nanotubes, in which thickness depends on the laser fluence, was shown by microstructure studies. Although a drastic decrement of light absorption was recorded especially in UV range, laser-annealed samples have shown activity under visible light even 20 times higher than bare titania. Electrochemical analysis has shown that the improvement of photoresponse originates mainly from over an order of magnitude higher charge carrier density as revealed by Mott-Schottky analysis. The results show that intense laser light can modulate the semiconductor properties significantly and can be considered as a promising tool towards activation of initially inactive material for the visible light harvesting.

Highly Selective Electrochemical Reduction of Dinitrogen to Ammonia at Ambient Temperature and Pressure over Iron Oxide Catalysts

The catalytic conversion of dinitrogen (N₂ ) into ammonia under ambient conditions represents one of the Holy Grails in sustainable chemistry. As a potential alternative to the Haber-Bosch process, the electrochemical reduction of N₂ to NH₃ is attractive owing to its renewability and flexibility, as well as its sustainability for producing and storing value-added chemicals from the abundant feedstock of water and nitrogen on earth. However, owing to the kinetically complex and energetically challenging N₂ reduction reaction (NRR) process, NRR electrocatalysts with high catalytic activity and high selectivity are rare. In this contribution, as a proof-of-concept, we demonstrate that both the NH₃ yield and faradaic efficiency (FE) under ambient conditions can be improved by modification of the hematite nanostructure surface. Introducing more oxygen vacancies to the hematite surface renders an improved performance in NRR, which leads to an average NH₃ production rate of 0.46 μg h^-1  cm^-2 and an NH₃ FE of 6.04 % at -0.9 V vs. Ag/AgCl in 0.10 m KOH electrolyte. The durability of the electrochemical system was also investigated. A surprisingly high average NH₃ production rate of 1.45 μg h^-1  cm^-2 and a NH₃ FE of 8.28 % were achieved after the first 1 h chronoamperometry test. This is among the highest FEs reported so far for non-precious-metal catalysts that use a polymer-electrolyte-membrane cell and is much higher than the FE of precious-metal catalysts (e.g., Ru/C) under comparable reaction conditions. However, the NH₃ yield and the FE dropped to 0.29 μg h^-1  cm^-2 and 2.74 %, respectively, after 16 h of chronoamperometry tests, which indicates poor durability of the system. Our results demonstrate the important role that the surface states of transition-metal oxides have in promoting electrocatalytic NRR under ambient conditions. This work may spur interest towards the rational design of electrocatalysts as well as electrochemical systems for NRR, with emphasis on the issue of stability.