Evidence for OH Radical Production during Electrocatalysis of Oxygen Reduction on Pt Surfaces: Consequences and Application

Deciphering platinum dissolution in neural stimulation electrodes: Electrochemistry or biology?

Platinum (Pt) is the metal of choice for electrodes in implantable neural prostheses like the cochlear implants, deep brain stimulating devices, and brain-computer interfacing technologies. However, it is well known since the 1970s that Pt dissolution occurs with electrical stimulation. More recent clinical and in vivo studies have shown signs of corrosion in explanted electrode arrays and the presence of Pt-containing particulates in tissue samples. The process of degradation and release of metallic ions and particles can significantly impact on device performance. Moreover, the effects of Pt dissolution products on tissue health and function are still largely unknown. This is due to the highly complex chemistry underlying the dissolution process and the difficulty in decoupling electrical and chemical effects on biological responses. Understanding the mechanisms and effects of Pt dissolution proves challenging as the dissolution process can be influenced by electrical, chemical, physical, and biological factors, all of them highly variable between experimental settings. By evaluating comprehensive findings on Pt dissolution mechanisms reported in the fuel cell field, this review presents a critical analysis of the possible mechanisms that drive Pt dissolution in neural stimulation in vitro and in vivo. Stimulation parameters, such as aggregate charge, charge density, and electrochemical potential can all impact the levels of dissolved Pt. However, chemical factors such as electrolyte types, dissolved gases, and pH can all influence dissolution, confounding the findings of in vitro studies with multiple variables. Biological factors, such as proteins, have been documented to exhibit a mitigating effect on the dissolution process. Other biological factors like cells and fibro-proliferative responses, such as fibrosis and gliosis, impact on electrode properties and are suspected to impact on Pt dissolution. However, the relationship between electrical properties of stimulating electrodes and Pt dissolution remains contentious. Host responses to Pt degradation products are also controversial due to the unknown chemistry of Pt compounds formed and the lack of understanding of Pt distribution in clinical scenarios. The cytotoxicity of Pt produced via electrical stimulation appears similar to Pt-based compounds, including hexachloroplatinates and chemotherapeutic agents like cisplatin. While the levels of Pt produced under clinical and acute stimulation regimes were typically an order of magnitude lower than toxic concentrations observed in vitro, further research is needed to accurately assess the mass balance and type of Pt produced during long-term stimulation and its impact on tissue response. Finally, approaches to mitigating the dissolution process are reviewed. A wide variety of approaches, including stimulation strategies, coating electrode materials, and surface modification techniques to avoid excess charge during stimulation and minimise tissue response, may ultimately support long-term and safe operation of neural stimulating devices.

Real-time investigation of reactive oxygen species and radicals evolved from operating Fe-N-C electrocatalysts during the ORR: potential dependence, impact on degradation, and structural comparisons

Improving the stability of platinum-group-metal-free (PGM-free) catalysts is a critical roadblock to the development of economically feasible energy storage and conversion technologies. Fe-N-C catalysts, the most promising class of PGM-free catalysts, suffer from rapid degradation. The generation of reactive oxygen species (ROS) during the oxygen reduction reaction (ORR) has been proposed as a central cause of this loss of activity. However, there is insufficient understanding of the generation and dynamics of ROS under catalytic conditions due to the difficulty of detecting and quantifying short-lived ROS such as the hydroxyl radical, OH˙. To accomplish this, we use operando scanning electrochemical microscopy (SECM) to probe the production of radicals by a commercial pyrolyzed Fe-N-C catalyst in real-time using a redox-active spin trap methodology. SECM showed the monotonic production of OH˙ which followed the ORR activity. Our results were thoroughly backed using electron spin resonance confirmation to show that the hydroxyl radical is the dominant radical species produced. Furthermore, OH˙ and H2O2 production followed distinct trends. ROS studied as a function of catalyst degradation also showed a decreased production, suggesting its relation to the catalytic activity of the sample. The structural origins of ROS production were also probed using model systems such as iron phthalocyanine (FePc) and Fe3O4 nanoparticles, both of which showed significant generation of OH˙ during the ORR. These results provide a comprehensive insight into the critical, yet under-studied, aspects of the production and effects of ROS on electrocatalytic systems and open the door for further mechanistic and kinetic investigation using SECM.

Size Control of Carbon Xerogel Spheres as Key Factor Governing the H2O2 Selectivity in Metal-Free Bifunctional Electro-Fenton Catalysts for Tetracycline Degradation

Carbon xerogel spheres co-doped with nitrogen and eco-graphene were synthesized using a typical solvothermal method. The results indicate that the incorporation of eco-graphene enhances the electrochemical properties, such as the current density (JK) and the selectivity for the four transferred electrons (n). Additionally, nitrogen doping has a significant effect on the degradation efficiency, varying with the size of the carbon xerogel spheres, which could be attributed to the type of nitrogenous group doped in the carbon material. The degradation efficiency improved in the nanometric spheres (48.3% to 61.6%) but decreased in the micrometric-scale spheres (58.6% to 53.4%). This effect was attributed to the N-functional groups present in each sample, with N-CNS-5 exhibiting a higher percentage of graphitic nitrogen (35.7%) compared to N-CMS-5 (15.3%). These findings highlight the critical role of sphere size in determining the type of N-functional groups present in the sample. leading to enhanced degradation of pollutants as a result of the electro-Fenton process.

Pb2+-Selective Nanoemulsion-Integrated Single-Entity Electrochemistry for Ultrasensitive Sensing of Blood Lead

Unequivocally, Pb2+ as a harmful substance damaging children's brain and nerve systems, thereby causing behavior and learning disabilities, should be detected much lower than the elevated blood lead for children, 240 nM, endorsed by US CDC considering the unknown neurotoxic effects, yet the ultralow detection limit up to sub-ppb level remains a challenge due to the intrinsically insufficient sensitivity in the current analytical techniques. Here, we present nanoemulsion (NE)-integrated single-entity electrochemistry (NI-SEE) toward ultrasensitive sensing of blood lead using Pb-ion-selective ionophores inside a NE, i.e., Pb2+-selective NE. Through the high thermodynamic selectivity between Pb2+ and Pb-ionophore IV, and the extremely large partition coefficient for the Pb2+-Pb-ionophore complex inside NEs, we modulate the selectivity and sensitivity of NI-SEE for Pb2+ sensing up to an unprecedentedly low detection limit, 20 ppt in aqueous solutions, and lower limit of quantitation, 40 ppb in blood serums. This observation is supported by molecular dynamics simulations, which clearly corroborate intermolecular interactions, e.g., H-bonding and π*-n, between the aromatic rings of Pb-ionophore and lone pair electrons of oxygen in dioctyl sebacate (DOS), plasticizers of NEs, subsequently enhancing the current intensity in NI-SEE. Moreover, the highly sensitive sensing of Pb2+ is enabled by the appropriate suppression of hydroxyl radical formation during NI-SEE under a cathodic potential applied to a Pt electrode. Overall, the experimentally demonstrated NI-SEE approach and the results position our new sensing technology as potential sensors for practical environmental and biomedical applications as well as a platform to interrogate the stoichiometry of target ion-ionophore recognition inside a NE as nanoreactors.

Electrocatalytic Deep Dehalogenation and Mineralization of Florfenicol: Synergy of Atomic Hydrogen Reduction and Hydroxyl Radical Oxidation over Bifunctional Cathode Catalyst

It is difficult to achieve deep dehalogenation or mineralization for halogenated antibiotics using traditional reduction or oxidation processes, posing the risk of microbial activity inhibition and bacterial resistance. Herein, an efficient electrocatalytic process coupling atomic hydrogen (H*) reduction with hydroxyl radical (•OH) oxidation on a bifunctional cathode catalyst is developed for the deep dehalogenation and mineralization of florfenicol (FLO). Atomically dispersed NiFe bimetallic catalyst on nitrogen-doped carbon as a bifunctional cathode catalyst can simultaneously generate H* and •OH through H2O/H+ reduction and O2 reduction, respectively. The H* performs nucleophilic hydro-dehalogenation, and the •OH performs electrophilic oxidization of the carbon skeleton. The experimental results and theoretical calculations indicate that reductive dehalogenation and oxidative mineralization processes can promote each other mutually, showing an effect of 1 + 1 > 2. 100% removal, 100% dechlorination, 70.8% defluorination, and 65.1% total organic carbon removal for FLO are achieved within 20 min (C0 = 20 mg·L-1, -0.5 V vs SCE, pH 7). The relative abundance of the FLO resistance gene can be significantly reduced in the subsequent biodegradation system. This study demonstrates that the synergy of reduction dehalogenation and oxidation degradation can achieve the deep removal of refractory halogenated organic contaminants.

Boron Bifunctional Catalysts for Rapid Degradation of Persistent Organic Pollutants in a Metal-Free Electro-Fenton Process: O2 and H2O2 Activation Process

Metals usually served as the active sites of the heterogeneous bifunctional electro-Fenton reaction, which faced the challenge of poor stability under acidic or even neutral conditions. Exploring a metal-free heterogeneous bifunctional electro-Fenton catalyst can effectively solve the above problems. In this work, a stable metal-free heterogeneous bifunctional boron-modified porous carbon catalyst (BTA-1000) was synthesized. For the BTA-1000 catalyst, the yield of H2O2 (294 mg/L) significantly increased. The degradation rate of phenol by BTA-1000 (0.242 min-1) increased by an order of magnitude, compared with the porous carbon catalyst (0.0105 min-1). The BTA catalyst could rapidly degrade industrial dye wastewater, and its specific energy consumption was 5.52 kW h kg-1 COD-1, lower than that in previous reports (6.38-7.4 kW h kg-1 COD-1). DFT and XPS revealed that C═O and -BC2O groups jointly promoted the generation of H2O2, and the -BCO2 group played dominant roles in the generation of •OH because the oxygen atom near the electron-giving groups (-BCO2 group) facilitated the formation of hydrogen bond and H2O2 adsorption. This work gained deep insights into the reaction mechanism of the boron-modified porous carbon catalyst, which helped to guide the development of metal-free heterogeneous bifunctional electro-Fenton catalysts.

Carbon Gels-Green Graphene Composites as Metal-Free Bifunctional Electro-Fenton Catalysts

The Electro-Fenton (EF) process has emerged as a promising technology for pollutant removal. However, the EF process requires the use of two catalysts: one acting as an electrocatalyst for the reduction of oxygen to H2O2 and another Fenton-type catalyst for the generation of ·OH radicals from H2O2. Thus, the search for materials with bifunctionality for both processes is required for a practical and real application of the EF process. Thus, in this work, bifunctional electrocatalysts were obtained via doping carbon microspheres with Eco-graphene, a form of graphene produced using eco-friendly methods. The incorporation of Eco-graphene offers numerous advantages to the catalysts, including enhanced conductivity, leading to more efficient electron transfer during the Electro-Fenton process. Additionally, the synthesis induced structural defects that serve as active sites, promoting the direct production of hydroxyl radicals via a 3-electron pathway. Furthermore, the spherical morphology of carbon xerogels enhances the accessibility of the reagents to the active sites. This combination of factors results in the effective degradation of Tetracycline (TTC) using metal-free catalysts in the Electro-Fenton process, achieving up to an impressive 83% degradation without requiring any other external or additional catalyst.

Highly graphitic Fe-doped carbon xerogels as dual-functional electro-Fenton catalysts for the degradation of tetracycline in wastewater

Fe-doped carbon xerogels with a highly developed graphitic structure were synthesized by a one-step sol-gel polymerization. These highly graphitic Fe-doped carbons are presented as promising dual-functional electro-Fenton catalysts to perform both the electro-reduction of O₂ to H₂O₂ and H₂O₂ catalytic decomposition (Fenton) for wastewater decontamination. The amount of Fe is key to the development of this electrode material, since affects the textural properties; catalyzes the development of graphitic clusters improving the electrode conductivity; and influences the O₂-catalyst interaction controlling the H₂O₂ selectivity but, at the same time is the catalyst for the decomposition of the electrogenerated H₂O₂ to OH^• radicals for the organic pollutants oxidation. All materials achieve the development of ORR via the 2-electron route. The presence of Fe considerably improves the electro-catalytic activity. However, a mechanism change seems to occur at around -0.5 V in highly Fe-doped samples. At potential lower than -0.5 eV, the present of Fe^δ+ species or even Fe-O-C active sites favour the selectivity to 2e-pathway, however at higher potentials, Fe^δ+ species are reduced favoring a O-O strong interaction enhancing the 4e-pathway. The Electro-Fenton degradation of tetracycline was analyzed. The TTC degradation is almost complete (95.13%) after 7 h of reaction without using any external Fenton-catalysts.

Mo-Based Heterogeneous Interface and Sulfur Vacancy Synergistic Effect Enhances the Fenton-like Catalytic Performance for Organic Pollutant Degradation

Heterogeneous Fenton-like reactions (HFLRs) based on the in situ electrochemical generation of hydrogen peroxide (H₂O₂) are one of the green methods to remediate organic pollutants in wastewater. However, the design of Fenton-like catalysts with specific active sites and high pollutant degradation rate is still challenging. Here, MoS₂-MoC and MoS₂-Mo₂N catalytic cathodes with heterojunctions were successfully prepared, and the mechanism by which hydroxyl radicals and singlet oxygen (¹O₂) were generated cleanly without adding chemical additives other than oxygen was clarified. The composite catalysts contained more sulfur vacancies, and the catalytic cathode achieved a high paracetamol pollutant degradation efficiency with 0.17 kWh g^-1 TOC specific energy consumption. And almost 5 times higher activity was achieved compared to a pure MoS₂ catalytic cathode. Experimental studies confirmed that the production of ¹O₂ was based on the transformation of superoxide radicals by Mo⁶⁺, and ¹O₂ accounted for approximately 66% of the total degradation and enhanced the nonradical behavior in the reaction. This work provides a sustainable strategy for pollutant utilization, which is valuable for solving the difficult problems of HFLRs and developing new environmental remediation technologies.

Enhanced degradation of micropollutants over iron-based electro-Fenton catalyst: Cobalt as an electron modulator in mesochannels and mechanism insight

Heterogeneous electro-Fenton (hetero-EF) process is an emerging alternative for effective oxidation of recalcitrant micropollutants, but it is hampered by limited hydroxyl radical (•OH) generation and low stability on the iron-based cathodes. Herein, we demonstrate an enhanced hetero-EF performance via modulation of iron electronic structure in an ordered mesoporous carbon (OMC). By tuning the cobalt incorporation, the highly-dispersed iron-cobalt (FeCo) nanoalloys in mesochannels (Fe_0.5Co_0.5@OMC) show a 3-fold increase in •OH yield compared with Fe@OMC, achieving degradation efficiency with 92% of sulfamethazine (SMT) and 99% of rhodamine B (RhB), and the corresponding total organic carbon (TOC) removal with 66% of SMT and 85% of RhB within 2 h in neutral pH, respectively. Experimental results and density functional theory (DFT) calculations demonstrate that iron incorporated with cobalt reduces energy barrier for facile generation of H₂O₂ and •OH from O₂ through direct electron transfer, along with decreased overpotential. Meanwhile, cobalt doping promotes H₂O₂ decomposition by accelerated Fe(II)/Fe(III) cycle and Co(II)/Co(III) redox. Furthermore, spatially confined and half-embedded structure endows the nanocatalyst (8 nm) excellent durability within a wide pH value range and good stability in cycle tests. A plausible reaction mechanism and degradation pathway for SMT are proposed. Moreover, the superiority of Fe_0.5Co_0.5@OMC cathode is maintained in simulated wastewater, suggesting an enormous potential in practical wastewater treatment.

Highly stable silver nanohybrid electrocatalysts for the oxygen reduction reaction

Silver nanoparticles (AgNPs) were deliberately functionalized via aryl diazonium chemistry with a monolayer of calix[4]arenes. The resulting nanohybrids show high efficiency and high selectivity toward the ORR in alkaline media along with an exceptional durability and a high methanol tolerance.

Mechanistic studies of reactive oxygen species mediated electrochemical radical reactions of alkyl iodides

Mechanistic studies of a reactive oxygen species mediated electrochemical radical reaction of alkyl iodides are described. Hydroxyl radicals and ozone are identified to be the active species involved in the formation of alkyl radicals under mildly reducing potential (-1.0 V vs. Ag QRE) in buffered acidic conditions (pH 3.6).

Surface-Enhanced Raman Spectroscopy-Scanning Electrochemical Microscopy: Observation of Real-Time Surface pH Perturbations

Understanding and controlling chemical dynamics at electrode interfaces is key to electrochemical applications in sensing, electrocatalysis, and energy storage. Here, we introduce colocalized surface-enhanced Raman scattering-scanning electrochemical microscopy (SERS-SECM) as a multimodal tool able to simultaneously probe and affect electrochemical interfaces in real time. As a model system to demonstrate SERS-SECM, we used a self-assembled monolayer of 4-mercaptopyridine (4MPy), a pH sensitive Raman indicator, anchored to silver nanoparticles as a substrate. We modulated the local pH at the surface with chronoamperometry, inducing the hydrogen evolution reaction (HER) at the SECM tip and observed subsequent Raman peak height changes in the 4MPy. We then performed cyclic voltammetry of HER at the SECM tip while measuring SERS spectra every 200 ms to highlight the technique's real-time capabilities. Our results show the capability to sensitively interrogate and trigger chemical/electrochemical dynamic surface phenomena. We hope SERS-SECM will provide insight on the link between heterogeneous and homogeneous reactivity at electrochemical interfaces.

Selective Electrocatalytic Reduction of Oxygen to Hydroxyl Radicals via 3-Electron Pathway with FeCo Alloy Encapsulated Carbon Aerogel for Fast and Complete Removing Pollutants

We reported the selective electrochemical reduction of oxygen (O₂ ) to hydroxyl radicals (^. OH) via 3-electron pathway with FeCo alloy encapsulated by carbon aerogel (FeCoC). The graphite shell with exposed -COOH is conducive to the 2-electron reduction pathway for H₂ O₂ generation stepped by 1-electron reduction towards to ^. OH. The electrocatalytic activity can be regulated by tuning the local electronic environment of carbon shell with the electrons coming from the inner FeCo alloy. The new strategy of ^. OH generation from electrocatalytic reduction O₂ overcomes the rate-limiting step over electron transfer initiated by reduction-/oxidation-state cycle in Fenton process. Fast and complete removal of ciprofloxacin was achieved within 5 min in this proposed system, the apparent rate constant (k_obs ) was up to 1.44±0.04 min^-1 , which is comparable with the state-of-the-art advanced oxidation processes. The degradation rate almost remains the same after 50 successive runs, suggesting the satisfactory stability for practical applications.

Phase Segregated Pt-SnO2 /C Nanohybrids for Highly Efficient Oxygen Reduction Electrocatalysis

Strengthening the interfacial interaction in heterogeneous catalysts can lead to a dramatic improvement in their performance and allow the use of smaller amounts of active noble metal, thus decreasing the cost without compromising their activity. In this work, a facile phase-segregation method is demonstrated for synthesizing platinum-tin oxide hybrids supported on carbon black (PtSnO₂ /C) in situ by air annealing PtSn alloy nanoparticles on carbon black. Compared with a control sample formed by preloading SnO₂ on carbon support followed by deposition of Pt nanoparticles, the phase-segregation-derived PtSnO₂ /C exhibits a more strongly coupled PtSnO₂ interface with lattice overlap of Pt (111) and SnO₂ (200), along with enhanced electron transfer from SnO₂ to Pt. Furthermore, the PtSnO₂ active sites show a strong ability to degrade reactive oxygen species. As a result, the PtSnO₂ /C nanohybrids exhibit both excellent activity and stability as a catalyst for the oxygen reduction reaction, with an overall performance which is superior to both the control sample and commercial Pt/C catalyst. This phase-segregation method can be expected to be applicable in the preparation of other strongly coupled nanohybrids and offers a new route to high-performance heterogeneous catalysts for low-cost energy conversion devices.

Recent advances in electrochemical 2e oxygen reduction reaction for on-site hydrogen peroxide production and beyond

The electroproduction of H₂O₂ through 2e oxygen reduction reaction (ORR) as an alternative strategy for the conventional anthraquinone process is highly energy-efficient and environment-friendly. Different kinds of electrocatalysts with high selectivity, activity, and stability have been recently reported, and are an essential part of the whole electroproduction process of H₂O₂. In this review, we expound the ORR mechanism and introduce some methods to screen out potential electrocatalysts through theoretical calculations and experimental verifications. In addition, recent advances in reactor design for large-scale on-site production of H₂O₂ and integrated systems for electricity-H₂O₂ co-generation are mentioned. With ideal electrocatalysts and rational reactor design, different concentrations of H₂O₂ can be obtained depending on the practical applications. Utilizing the solar or chemical energy, it can promote energy efficiency and sustainability of the process. Finally, we make a brief conclusion about recent developments in electrocatalysts, device design, as well as integrated systems, and give an outlook for future research challenges, which are meaningful for advancing the electrochemical on-site production of H₂O₂via 2e ORR to the marketplace.

Construction of a Microchannel Electrochemical Reactor with a Monolithic Porous-Carbon Cathode for Adsorption and Degradation of Organic Pollutants in Several Minutes of Retention Time

A monolithic porous-carbon (MPC) electrode was fabricated to simultaneously intensify mass transfer and enhance reaction activity. The MPC involved channel arrays (about 50 μm of diameter for each channel) with mesopores and micropores in channel walls. The abundant surface pores may improve the reaction efficiency of the reduction of O₂ to produce H₂O₂ and •OH. The function of channel arrays was to shorten the mass-transfer distance not only from O₂ to the electrode surface but also from pollutants to the electrode surface and •OH. A microchannel electrochemical reactor was assembled to evaluate the performance of the MPC cathode. For 20 mg/L of phenol, sulfamethoxazole or atrazine, effluent concentration and total organic carbon (TOC) decreased down to 1.5 and 3 mg/L, respectively, in a retention time of only 100-300 s. Phenol removal was dominated by the MPC cathode, and the contribution of cathodic adsorption, cathodic degradation, and anodic reaction was 46, 33, and 8%, respectively. The proper working potential for the MPC cathode was +0.26 to +0.6 V versus reversible hydrogen electrode; in this potential range, no scaling was observed. For the real surface water (the initial TOC was 41.5 mg/L), TOC in effluent (the retention time was 335 s) was stable at 31.0 mg/L.

Investigation of Protective Properties of Organic Layers toward Reactive Oxygen Species

The antioxidant protective properties of polyaromatic organic layers were evaluated toward reactive oxygen species (ROS) using scanning electrochemical microscopy in a foot-printing strategy. The layers were prepared by electrografting of aryldiazonium salts. Where p-(methyl)phenyl films show only weak protective properties toward ROS, p-(ethynyl)phenyl films evidence efficient protection of the covered surfaces. Applied potentials and electrolytes used during oxygen reduction reaction are critical parameters to control, prevent, or reduce the influence of ROS production and hence enhance the device lifetime.

Alkaline Anion-Exchange Membrane Fuel Cells: Challenges in Electrocatalysis and Interfacial Charge Transfer

Alkaline anion-exchange membrane (AAEM) fuel cells have attracted significant interest in the past decade, thanks to the recent developments in hydroxide-anion conductive membranes. In this article, we compare the performance of current state of the art AAEM fuel cells to proton-exchange membrane (PEM) fuel cells and elucidate the sources of various overpotentials. While the continued development of highly conductive and thermally stable anion-exchange membranes is unambiguously a principal requirement, we attempt to put the focus on the challenges in electrocatalysis and interfacial charge transfer at an alkaline electrode/electrolyte interface. Specifically, a critical analysis presented here details the (i) fundamental causes for higher overpotential in hydrogen oxidation reaction, (ii) mechanistic aspects of oxygen reduction reaction, (iii) carbonate anion poisoning, (iv) unique challenges arising from the specific adsorption of alkaline ionomer cation-exchange head groups on electrocatalysts surfaces, and (v) the potential of alternative small molecule fuel oxidation. This review and analysis encompasses both the precious and nonprecious group metal based electrocatalysts from the perspective of various interfacial charge-transfer phenomena and reaction mechanisms. Finally, a research roadmap for further improvement in AAEM fuel cell performance is delineated here within the purview of electrocatalysis and interfacial charge transfer.

Influence of pH Modification on Catalytic Activities of Metal-Doped IrO2 Nanoparticles

The effects of pH variation on the catalytic activity of IrO₂ nanoparticles (NPs) doped with Cr (an early transition metal) or Ni (a late transition metal) depending on the amount of defect structures on the NP surfaces were analyzed. It was found that both Cr@IrO₂ and Ni@IrO₂ NPs, fabricated under basic conditions (pH = 13.5) denoted as Cr@IrO₂-B and Ni@IrO₂-B, respectively, were the best catalysts among the eight tested ones. Moreover, it was confirmed that variation in pH resulted in the changes in the surface area (defect structure), which were considered to be responsible for the changes in the catalytic properties of these NPs. For the oxygen evolution reaction, these NPs exhibited relatively smaller overpotential (η) values than other tested Cr@IrO₂- and Ni@IrO₂-containing NPs. Furthermore, methylene blue degradation analysis and OH radical formation experiments by benzoic acid showed the same trend. Thus, we confirmed that the catalytic activity of transition metals doped IrO₂ NPs fabricated under basic conditions can be improved.

In Situ Electrochemical AFM Imaging of a Pt Electrode in Sulfuric Acid under Potential Cycling Conditions

Understanding the electrochemical behavior of Pt at the solid/liquid interface is of significant importance for the development of efficient electrochemical devices, such as fuel cells and water electrolyzers. In this work, the evolution of the surface morphology of a polycrystalline platinum under potential cycling conditions was investigated by in situ electrochemical atomic force microscopy (EC-AFM). After 50 cycles between 0.05 and 1.8 V in 0.1 M H₂SO₄, the Pt surface is coarsened and nanoparticles of several nanometers appear on the surface. The critical upper and lower potentials for the formation of nanoparticles are found to be 1.8 and 0.8 V, respectively. The in situ AFM observation coupled with Cyclic Voltammerty reveals the periodic disappearance and reappearance of the nanoparticles, based on which the formation of nanoparticles is attributed to the deposition of dissolved Pt from solution, and a model for the nanoparticle formation is proposed. While the formation of a thick oxide layer is a prerequisite, the reduction process is found to have a strong influence on Pt nanoparticle formation as well. This investigation provides a visualization of the Pt electrode surface under electrochemical control in a large potential window, enabling a broader understanding of the Pt electrode roughening mechanisms.

Mechanism and kinetics of the electrocatalytic reaction responsible for the high cost of hydrogen fuel cells

The sluggish oxygen reduction reaction (ORR) is a major impediment to the economic use of hydrogen fuel cells in transportation. In this work, we report the full ORR reaction mechanism for Pt(111) based on Quantum Mechanics (QM) based Reactive metadynamics (RμD) simulations including explicit water to obtain free energy reaction barriers at 298 K. The lowest energy pathway for 4 e^- water formation is: first, *OOH formation; second, *OOH reduction to H₂O and O*; third, O* hydrolysis using surface water to produce two *OH and finally *OH hydration to water. Water formation is the rate-determining step (RDS) for potentials above 0.87 Volt, the normal operating range. Considering the Eley-Rideal (ER) mechanism involving protons from the solvent, we predict the free energy reaction barrier at 298 K for water formation to be 0.25 eV for an external potential below U = 0.87 V and 0.41 eV at U = 1.23 V, in good agreement with experimental values of 0.22 eV and 0.44 eV, respectively. With the mechanism now fully understood, we can use this now validated methodology to examine the changes upon alloying and surface modifications to increase the rate by reducing the barrier for water formation.

Efficient interfacial charge transfer through plasmon sensitized Ag@Bi2O3 hierarchical photoanodes for photoelectrocatalytic degradation of chlorinated phenols

Understanding the interfacial charge transfer behavior of plasmonically active Ag decorated hierarchical Bi₂O₃ photoanodes for the photo-electro-oxidation of chlorinated phenols.

Dissolution of Pt during Oxygen Reduction Reaction Produces Pt Nanoparticles

The loss of Pt during the oxygen reduction reaction (ORR) affects the performance and economic viability of fuel cells and sensors. Our group previously observed the dissolution of Pt nanoelectrodes at moderately negative potentials during the ORR. Here we report a more detailed study of this process and identify its product. The nanoporous Pt surface formed during the ORR was visualized by AFM and high-resolution SEM, which also showed ∼5 nm sized Pt particles on the glass surface surrounding the electrode. The release of these nanoparticles into the solution was confirmed by monitoring their catalytically amplified collisions with a Hg-coated microelectrode used as the tip in the scanning electrochemical microscope (SECM).

Generation of Hydroxyl Radicals in the Reaction of Dihydrogen with AuNbO4+ Cluster Cations

A molecular-level insight into the nature of reactive oxygen species involved in dihydrogen (H₂ ) dissociation is of great importance to understand gold catalysis. In this study, laser ablation generated and mass-selected AuNbO₄⁺ oxide cluster cations could dissociate H₂ in an ion-trap reactor. The reaction has been characterized by time-of-flight mass spectrometric experiments and density functional calculations. The lowest energy isomer of AuNbO₄⁺ contains two lattice oxygen (O^2- ) and one superoxide (O₂^.- ) species. The gold atom anchors the H₂ molecule in the first step and then delivers one hydrogen atom to the O^2- ion in H₂ dissociation. At the same time, O₂^.- is reduced into a peroxide unit that can accept the second hydrogen atom of H₂ with the generation of a hydroxyl radical as the main product. In this study, the important roles of the O₂^.- unit in the dissociation of H₂ have been identified.

Mapping Cd²⁺-induced membrane permeability changes of single live cells by means of scanning electrochemical microscopy

Scanning Electrochemical Microscopy (SECM) is a powerful, non-invasive, analytical methodology that can be used to investigate live cell membrane permeability. Depth scan SECM imaging allowed for the generation of 2D current maps of live cells relative to electrode position in the x-z or y-z plane. Depending on resolution, one depth scan image can contain hundreds of probe approach curves (PACs). Individual PACs were obtained by simply extracting vertical cross-sections from the 2D image. These experimental PACs were overlaid onto theoretically generated PACs simulated at specific geometry conditions. Simulations were carried out using 3D models in COMSOL Multiphysics to determine the cell membrane permeability coefficients at different locations on the surface of the cells. Common in literature, theoretical PACs are generated using a 2D axially symmetric geometry. This saves on both compute time and memory utilization. However, due to symmetry limitations of the model, only one experimental PAC right above the cell can be matched with simulated PAC data. Full 3D models in this article were developed for the SECM system of live cells, allowing all experimental PACs over the entire cell to become usable. Cd(2+)-induced membrane permeability changes of single human bladder (T24) cells were investigated at several positions above the cell, displaced from the central axis. The experimental T24 cells under study were incubated with Cd(2+) in varying concentrations. It is experimentally observed that 50 and 100 μM Cd(2+) caused a decrease in membrane permeability, which was uniform across all locations over the cell regardless of Cd(2+) concentration. The Cd(2+) was found to have detrimental effects on the cell, with cells shrinking in size and volume, and the membrane permeability decreasing. A mapping technique for the analysis of the cell membrane permeability under the Cd(2+) stress is realized by the methodology presented.

A simple way to fine tune the redox potentials of cobalt ions encapsulated in nitrogen doped graphene molecular catalysts for the oxygen evolution reaction

A simple approach to fine-tuning the redox potential of Co²⁺ ions encapsulated in nitrogen doped graphene (NG) has been proposed. We found that the redox potential determines the oxygen evolution reaction activity of the catalyst.

The oxygen reduction reaction (ORR) on reduced metals: evidence for a unique relationship between the coverage of adsorbed oxygen species and adsorption energy

The ORR current follows a volcano-like dependence on the coverage of oxygen species that adsorb upon exposure to dissolved oxygen and on their adsorption energy.