Simultaneous Mapping of Electrocatalytic Activity and Selectivity via Hybrid Scanning Electrochemical Probe Microscopy

Nanoscale scanning electrochemical probe microscopy started to elucidate the heterogeneity of electrocatalytic activity at electrode surfaces. However, understanding the heterogeneity in product selectivity, another crucial aspect of interfacial reactivity, remains challenging. Herein, we introduce a method combining scanning electrochemical microscopy (SECM) and scanning electrochemical cell microscopy (SECCM) to enable the spatially resolved mapping of both activity and selectivity in electrocatalysis. A dual-channel nanopipette probe was developed: one channel for activity mapping and the other for product detection with a high collection efficiency (>95%) and sensitivity. Simultaneous mapping of activity and selectivity in the oxygen reduction reaction (ORR) is demonstrated. Combined with colocalized crystal orientation mapping, we uncover the local electrocatalytic performance of ORR at different facets on polycrystalline Pt and Au. The high-resolution selectivity mapping enabled by our method with colocalized structural characterization can provide structure-activity-selectivity relationships that are often unavailable in ensemble measurement, holding promise for understanding key structural motifs controlling interfacial reactivity.

N-Doped porous carbons obtained from chitosan and spent coffee as electrocatalysts with tuneable oxygen reduction reaction selectivity for H2O2 generation

Nitrogen-containing porous carbons prepared by the pyrolysis of adequate biopolymer-based precursors have shown potential in several electrochemical energy-related applications. However, it is still of crucial interest to find the optimal precursors and process conditions which would allow the preparation of carbons with adequate porous structure as well as suitable nitrogen content and distribution of functional groups. In the present work we suggested a straightforward approach to prepare N-doped porous carbons by direct pyrolysis under nitrogen of chitosan : coffee blends of different compositions and using KOH for simultaneous surface activation. The synthetized carbon materials were tested for the electrochemical oxygen reduction to hydrogen peroxide (H₂O₂). A higher fraction of chitosan in the precursor led to a decrease in meso- and nano-porosity of the formed porous carbons, while their activity towards H₂O₂ generation increased. The nitrogen species derived from chitosan seem to play a very important role. Out of the synthesized catalysts the one with the largest content of pyridinic nitrogen sites exhibited the highest faradaic efficiency. The faradaic efficiencies and current densities of the synthesized materials were comparable with the ones of other commercially available carbons obtained from less renewable precursors.

Strategies for Sustainable Production of Hydrogen Peroxide via Oxygen Reduction Reaction: From Catalyst Design to Device Setup

An environmentally benign, sustainable, and cost-effective supply of H2O2 as a rapidly expanding consumption raw material is highly desired for chemical industries, medical treatment, and household disinfection. The electrocatalytic production route via electrochemical oxygen reduction reaction (ORR) offers a sustainable avenue for the on-site production of H2O2 from O2 and H2O. The most crucial and innovative part of such technology lies in the availability of suitable electrocatalysts that promote two-electron (2e-) ORR. In recent years, tremendous progress has been achieved in designing efficient, robust, and cost-effective catalyst materials, including noble metals and their alloys, metal-free carbon-based materials, single-atom catalysts, and molecular catalysts. Meanwhile, innovative cell designs have significantly advanced electrochemical applications at the industrial level. This review summarizes fundamental basics and recent advances in H2O2 production via 2e--ORR, including catalyst design, mechanistic explorations, theoretical computations, experimental evaluations, and electrochemical cell designs. Perspectives on addressing remaining challenges are also presented with an emphasis on the large-scale synthesis of H2O2 via the electrochemical route.

Electrochemistry needs electrochemists: “goodbye to rotating discs”

The essential need for expert, fully trained electrochemists in the successful application of the subject is illustrated with several examples including the use of rotating electrodes and impedance spectroscopy where the use of the techniques in “black box” mode non-experts is likely to lead to disappointment or embarrassment.

Directing oxygen reduction reaction selectivity towards hydrogen peroxide via electric double layer engineering

The electrochemical oxygen reduction reaction (ORR) has been recognized as a promising alternative for the sustainable production of H₂O₂. Here, we report a facile and effective strategy to promote ORR selectivity towards the 2e^- product H₂O₂via electric double layer engineering. Specifically, in a model system using immobilized cobalt phthalocyanine as the electrocatalyst, H₂O₂ selectivity has been improved from below 60% to over 93%, and the intrinsic activity for H₂O₂ formation has been enhanced by more than 3 times upon the introduction of a cationic surfactant (i.e., cetyltrimethylammonium bromide, CTAB) into the electrolyte. Based on detailed kinetics analysis, we conclude that the accelerated H₂O₂ formation rate results from the reduced charge transfer resistance in the rate limiting step and the promoted oxygen uptake rate. We propose that the electric field strength across the electric double layer is enhanced via the self-assembled single-tail cationic surfactant layer at the electrode/electrolyte interface, which is the origin of the enhancement of the 2e^- ORR performance.

Novel Bifunctional Nitrogen Doped MoS2/COF-C4N Vertical Heterostructures for Electrocatalytic HER and OER

Highly active and earth-abundant catalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) play vital roles in developing efficient water splitting to produce hydrogen fuels. Here, we reported an effective strategy to fabricate a completely new nitrogen-doped MoS2/COF-C4N vertical heterojunction (N-MoS2/COF-C4N) as precious-metal-free bifunctional electrocatalysts for both HER and OER. Compared with MoS2 and COF-C4N, the obtained vertical N-MoS2/COF-C4N catalyst showed enhanced HER with a low overpotential of 106 mV at 10 mA cm−2, which is six times lower than MoS2. The superior acidic HER activity, molecular mechanism, and charge transfer characteristic of this vertical N-MoS2/COF-C4N were investigated experimentally and theoretically in detail. Its basic OER activity is almost equal to that of COF-C4N with an overpotential of 349 mV at 10 mA cm−2, which showed that the in-situ growing method maintains the exposure of the C active sites to the greatest extent. The preparation and investigation for vertical N-MoS2/COF-C4N provide ideas and a research basis for us to further explore promising overall water-splitting electrocatalysts.

Recent progress in the design of photocatalytic H2O2 synthesis system

Photocatalytic synthesis of hydrogen peroxide under mild reaction conditions is a promising technology. This article will review the recent research progress in the design of photocatalytic H2O2 synthesis systems. A comprehensive discussion of the strategies that could solve two essential issues related to H2O2 synthesis. That is, how to improve the reaction kinetics of H2O2 formation via 2e- oxygen reduction reaction and inhibit the H2O2 decomposition through a variety of surface functionalization methods. The photocatalyst design and the reaction mechanism will be especially stressed in this work which will be concluded with an outlook to show the possible ways for synthesizing high-concentration H2O2 solution in the future.

Identification of the Highly Active Co-N4 Coordination Motif for Selective Oxygen Reduction to Hydrogen Peroxide

Electrosynthesis of hydrogen peroxide (H2O2) through oxygen reduction reaction (ORR) is an environment-friendly and sustainable route for obtaining a fundamental product in the chemical industry. Co-N4 single-atom catalysts (SAC) have sparkled attention for being highly active in both 2e- ORR, leading to H2O2 and 4e- ORR, in which H2O is the main product. However, there is still a lack of fundamental insights into the structure-function relationship between CoN4 and the ORR mechanism over this family of catalysts. Here, by combining theoretical simulation and experiments, we unveil that pyrrole-type CoN4 (Co-N SACDp) is mainly responsible for the 2e- ORR, while pyridine-type CoN4 catalyzes the 4e- ORR. Indeed, Co-N SACDp exhibits a remarkable H2O2 selectivity of 94% and a superb H2O2 yield of 2032 mg for 90 h in a flow cell, outperforming most reported catalysts in acid media. Theoretical analysis and experimental investigations confirm that Co-N SACDp─with weakening O2/HOO* interaction─boosts the H2O2 production.

Noble Metal-Free Inorganic Photocatalyst Consisting of Antimony-Doped Tin Oxide Nanorod and Titanium oxide for Two-Electron Oxygen Reduction Reaction

This study reports a noble metal-free robust inorganic photocatalyst for H 2 O 2 synthesis via two-electron oxygen reduction reaction (ORR). Antimony-doped tin oxide nanorods were heteroepitaxially grown from rutile TiO 2 seed crystals with an orientation of (001)ATO//(001)TiO 2 (ATO-NR//TiO 2 , // denotes heteroepitaxial junction) by a hydrothermal method. UV-light irradiation of ATO-NR//TiO 2 particles stably and continuously produces H 2 O 2 from aerated aqueous solution of ethanol. Electrochemical measurements using rotating electrodes show that Sb-doping into SnO 2 greatly enhances the electrocatalytic activity for two-electron ORR. The striking photocatalytic activity of ATO-NR//TiO 2 stems from the effective charge separation, electrocatalytic activity for two-electron ORR, and low catalytic activity for H 2 O 2 decomposition.

Effects of g-C3N4 Heterogenization into Intrinsically Microporous Polymers on the Photocatalytic Generation of Hydrogen Peroxide

Graphitic carbon nitride (g-C₃N₄) is known to photogenerate hydrogen peroxide in the presence of hole quenchers in aqueous environments. Here, the g-C₃N₄ photocatalyst is embedded into a host polymer of intrinsic microporosity (PIM-1) to provide recoverable heterogenized photocatalysts without loss of activity. Different types of g-C₃N₄ (including Pt@g-C₃N₄, Pd@g-C₃N₄, and Au@g-C₃N₄) and different quenchers are investigated. Exploratory experiments yield data that suggest binding of the quencher either (i) directly by adsorption onto the g-C₃N₄ (as shown for α-glucose) or (ii) indirectly by absorption into the microporous polymer host environment (as shown for Triton X-100) enhances the overall photochemical H₂O₂ production process. The amphiphilic molecule Triton X-100 is shown to interact only weakly with g-C₃N₄ but strongly with PIM-1, resulting in accumulation and enhanced H₂O₂ production due to the microporous polymer host.

Using Palladium and Gold Palladium Nanoparticles Decorated with Molybdenum Oxide for Versatile Hydrogen Peroxide Electroproduction on Graphene Nanoribbons

Electrocatalytic production of H₂O₂ via a two-electron oxygen reduction reaction (ORR-2e^-) is regarded as a highly promising decentralized and environmentally friendly mechanism for the production of this important chemical commodity. However, the underlying challenges related to the development of catalytic materials that contain zero or low content of noble metals and that are relatively more active, selective, and resistant for long-term use have become a huge obstacle for the electroproduction of H₂O₂ on commercial and industrial scales. The present study reports the synthesis and characterization of low metal-loaded (≤6.4 wt %) catalysts and their efficiency in H₂O₂ electroproduction. The catalysts were constructed using gold palladium molybdenum oxide (AuPdMoO_x) and palladium molybdenum oxide (PdMoO_x) nanoparticles supported on graphene nanoribbons. Based on the application of a rotating ring-disk electrode, we conducted a thorough comparative analysis of the electrocatalytic performance of the catalysts in the ORR under acidic and alkaline media. The proposed catalysts exhibited high catalytic activity (ca. 0.08 mA g_noble metal^-1 in an acidic medium and ca. 6.6 mA g_noble metal^-1 in an alkaline medium), good selectivity (over 80%), and improved long-term stability toward ORR-2e^-. The results obtained showed that the enhanced ORR activity presented by the catalysts, which occurred preferentially via the two-electron pathway, was promoted by a combination of factors including geometry, Pd content, interparticle distance, and site-blocking effects, while the electrochemical stability of the catalysts may have been enhanced by the presence of MoO_x.

Recent Progress of Electrochemical Production of Hydrogen Peroxide by Two-Electron Oxygen Reduction Reaction

Shifting electrochemical oxygen reduction reaction (ORR) via two-electron pathway becomes increasingly crucial as an alternative/green method for hydrogen peroxide (H2 O2 ) generation. Here, the development of 2e- ORR catalysts in recent years is reviewed, in aspects of reaction mechanism exploration, types of high-performance catalysts, factors to influence catalytic performance, and potential applications of 2e- ORR. Based on the previous theoretical and experimental studies, the underlying 2e- ORR catalytic mechanism is firstly unveiled, in aspect of reaction pathway, thermodynamic free energy diagram, limiting potential, and volcano plots. Then, various types of efficient catalysts for producing H2 O2 via 2e- ORR pathway are summarized. Additionally, the catalytic active sites and factors to influence catalysts' performance, such as electronic structure, carbon defect, functional groups (O, N, B, S, F etc.), synergistic effect, and others (pH, pore structure, steric hindrance effect, etc.) are discussed. The H2 O2 electrogeneration via 2e- ORR also has various potential applications in wastewater treatment, disinfection, organics degradation, and energy storage. Finally, potential future directions and prospects in 2e- ORR catalysts for electrochemically producing H2 O2 are examined. These insights may help develop highly active/selective 2e- ORR catalysts and shape the potential application of this electrochemical H2 O2 producing method.

Nitrogen, sulfur co-doped carbon coated zinc sulfide for efficient hydrogen peroxide electrosynthesis

Oxygen electroreduction (ORR) via a two-electron pathway is a promising alternative for hydrogen peroxide (H2O2) synthesis in small-scale applications. In this work, nitrogen and sulfur co-doped carbon coated zinc sulfide nanoparticles (ZnS@C) are synthesized using facile high-temperature annealing. In an alkaline electrolyte, the presence of ZnS suppresses the reduction of H2O2 during the ORR and contributes to high H2O2 selectivity (∼90%) over a wide potential range (0.40-0.80 V). Continuous generation of H2O2 is in turn achieved at an outstanding rate of 1.485 mol gcat.-1 h-1 with a faradaic efficiency of 93.7%.

Modeling and Multi-Criteria Optimization of a Process for H2O2 Electrosynthesis

This article introduces a novel laboratory-scale process for the electrochemical synthesis of hydrogen peroxide (H2O2). The process aims at an energy-efficient, decentralized production, and a mathematical optimization of it is presented. A dynamic, zero-dimensional mathematical model of the reactor is set up in Aspen custom modeler®. The proposed model constitutes a reasonable compromise between complexity and convergence. After thoroughly determining the reaction kinetics by adjustment to experimental data, the reactor unit is embedded in an Aspen Plus® flowsheet in order to investigate its interaction with other unit operations. The downstream contains another custom module for membrane distillation. Electricity appears as a resource in the process, and optimization shows that it reaches product purities of up to 3 wt.-%. Both the process optimization and the adjustment of the reaction kinetics are treated as multi-criteria optimization (MCO) problems.

Ultrasensitive Detection of Hydrogen Peroxide Using Bi2Te3 Electrochemical Sensors

Electrochemical sensors, with high accuracy, good selectivity, and linear response, have been widely used for environmental protection, health monitoring, and disease treatment. However, to date, these sensors still have limit sensitivity or otherwise require the use of high-cost materials such as noble metals and enzymes. Here, we report a novel electrochemical sensor using a topological insulator, Bi₂Te₃. Through liquid-phase exfoliation, we prepared nano- and microflakes of Bi₂Te₃ and measured their performance in hydrogen peroxide sensing via electrocatalytic reduction processes. Our devices exhibit a sensitivity of ∼4900 μA mM^-1 cm^-2 and a detection limit of ∼10^-8 molar, both of which are superior to typical noble metal-based electrochemical sensors. Through electrochemical analysis and microkinetic simulations, we extracted the kinetic parameters and gained insights into the reaction mechanism. We attribute the ultrahigh sensitivity to the facile electron transfer at the Bi₂Te₃-aqueous solution interface.

Electrochemical Stability and Degradation Mechanisms of Commercial Carbon-Supported Gold Nanoparticles in Acidic Media

Electrochemical stability of a commercial Au/C catalyst in an acidic electrolyte has been investigated by an accelerated stress test (AST), which consisted of 10,000 voltammetric scans (1 V/s) in the potential range between 0.58 and 1.41 V_RHE. Loss of Au electrochemical surface area (ESA) during the AST pointed out to the degradation of Au/C. Coupling of an electrochemical flow cell with ICP-MS showed that only a minor amount of gold is dissolved despite the substantial loss of gold ESA during the AST (∼35% of initial value remains at the end of the AST). According to the electrochemical mass spectrometry experiments, carbon corrosion occurs during the AST but to a minor extent. By using identical location scanning electron microscopy and identical location transmission electron microscopy, it was possible to discern that the dissolution of small Au particles (<5 nm) within the polydisperse Au/C sample is the main degradation mechanism. The mass of such particles gives only a minor contribution to the overall Au mass of the polydisperse sample while giving a major contribution to the overall ESA, which explains a significant loss of ESA and minor loss of mass during the AST. The addition of low amounts of chloride anions (10^-4 M) substantially promoted the degradation of gold nanoparticles. At an even higher concentration of chlorides (10^-2 M), the dissolution of gold was rather effective, which is useful from the recycling point of view when rapid leaching of gold is desirable.

Heteroatom-doped carbon-based oxygen reduction electrocatalysts with tailored four-electron and two-electron selectivity

Oxygen reduction reaction (ORR) plays a pivotal role in electrochemical energy conversion and commodity chemical production. Oxygen reduction involving a complete four-electron (4e-) transfer is important for the efficient operation of polymer electrolyte fuel cells, whereas the ORR with a partial 2e- transfer can serve as a versatile method for producing industrially important hydrogen peroxide (H2O2). For both the 4e- and 2e- pathway ORR, platinum-group metals (PGMs) have been materials of prevalent choice owing to their high intrinsic activity, but they are costly and scarce. Hence, the development of highly active and selective non-precious metal catalysts is of crucial importance for advancing electrocatalysis of the ORR. Heteroatom-doped carbon-based electrocatalysts have emerged as promising alternatives to PGM catalysts owing to their appreciable activity, tunable selectivity, and facile preparation. This review provides an overview of the design of heteroatom-doped carbon ORR catalysts with tailored 4e- or 2e- selectivities. We highlight catalyst design strategies that promote 4e- or 2e- ORR activity. We also summarise the major active sites and activity descriptors of the respective ORR pathways and describe the catalyst properties controlling the ORR mechanisms. We conclude the review with a summary and suggestions for future research.

A review on recent developments in electrochemical hydrogen peroxide synthesis with a critical assessment of perspectives and strategies

Electrochemical hydrogen peroxide synthesis using two-electron oxygen electrochemistry is an intriguing alternative to currently dominating environmentally unfriendly and potentially hazardous anthraquinone process and noble metals catalysed direct synthesis. Electrocatalytic two-electron oxygen reduction reaction (ORR) and water oxidation reaction (WOR) are the source of electrochemical hydrogen peroxide generation. Various electrocatalysts have been used for the same and were characterized using several electroanalytical, chemical, spectroscopic and chromatographic tools. Though there have been a few reviews summarizing the recent developments in this field, none of them have unified the approaches in catalysts' design, criticized the ambiguities and flaws in the methods of evaluation, and emphasized the role of electrolyte engineering. Hence, we dedicated this review to discuss the recent trends in the catalysts' design, performance optimization, evaluation perspectives and their appropriateness and opportunities with electrolyte engineering. In addition, particularized discussions on fundamental oxygen electrochemistry, additional methods for precise screening, and the role of solution chemistry of synthesized hydrogen peroxide are also presented. Thus, this review discloses the state-of-the-art in an unpresented view highlighting the challenges, opportunities, and alternative perspectives.

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.

Plasmon-Induced Photoreduction System Allows Ultrasensitive Detection of Disease Biomarkers by Silver-Mediated Immunoassay

Current strategies for the detection of disease biomarkers often require enzymatic assays that may have limited sensitivity due to inferior stability and vulnerable catalytic activity of the enzyme. A new enzyme-free amplification method for identifying suitable biomarkers is necessary to lower the limit of detection and improve many critical diagnosis applications. Here, we presented an enzyme-free amplified plasmonic immunoassay that enhanced the detection sensitivity of disease biomarkers by combining a novel plasmon-induced silver photoreduction system with a silver nanoparticle (AgNP)-linked immunoassay. The key step to achieving ultrasensitivity was to use Ag⁺ from dissolved AgNPs that control the growth rate of the silver coating on plasmonic nanosensors under visible light illumination. We demonstrated the outstanding sensitivity and robustness of this assay by detecting the disease biomarker alpha-fetoprotein (AFP) at a low concentration of 3.3 fg mL^-1. The detection of AFP was further confirmed in the sera of hepatocellular carcinoma patients.

Promoting H2O2 production via 2-electron oxygen reduction by coordinating partially oxidized Pd with defect carbon

Electrochemical synthesis of H₂O₂ through a selective two-electron (2e⁻) oxygen reduction reaction (ORR) is an attractive alternative to the industrial anthraquinone oxidation method, as it allows decentralized H₂O₂ production. Herein, we report that the synergistic interaction between partially oxidized palladium (Pd^δ+) and oxygen-functionalized carbon can promote 2e⁻ ORR in acidic electrolytes. An electrocatalyst synthesized by solution deposition of amorphous Pd^δ+ clusters (Pd₃^δ+ and Pd₄^δ+) onto mildly oxidized carbon nanotubes (Pd^δ+-OCNT) shows nearly 100% selectivity toward H₂O₂ and a positive shift of ORR onset potential by ~320 mV compared with the OCNT substrate. A high mass activity (1.946 A mg⁻¹ at 0.45 V) of Pd^δ+-OCNT is achieved. Extended X-ray absorption fine structure characterization and density functional theory calculations suggest that the interaction between Pd clusters and the nearby oxygen-containing functional groups is key for the high selectivity and activity for 2e⁻ ORR.

Electrocatalysts which deliver high selectivity for two-electron oxygen reduction to hydrogen peroxide in acidic media are sought to replace the conventional anthraquinone process. Here, the authors develop partially oxidised palladium clusters on carbon nanotubes with near 100% selectivity.

Tailoring the Electrochemical Production of H2 O2 : Strategies for the Rational Design of High-Performance Electrocatalysts

The production of H₂ O₂ via the electrochemical oxygen reduction reaction (ORR) presents an attractive decentralized alternative to the current industry-dominant anthraquinone process. However, in order to achieve viable commercialization of this process, a state-of-the-art electrocatalyst exhibiting high activity, selectivity, and long-term stability is imperative for industrial applications. Herein, an in-depth discussion on the current frontiers in electrocatalyst design is provided, emphasizing the influences of electronic and geometric effects, surface structure, and the effects of heteroatom functionalization on the catalytic performance of commonly studied materials (metals, alloys, carbons). The limitations on the performance of the current catalyst materials are also discussed, together with alternative strategies to overcome the impediments. Finally, directions of future research efforts for the discovery of next-generation ORR electrocatalysts are highlighted.

Atomic-level tuning of Co-N-C catalyst for high-performance electrochemical H2O2 production

Despite the growing demand for hydrogen peroxide it is almost exclusively manufactured by the energy-intensive anthraquinone process. Alternatively, H₂O₂ can be produced electrochemically via the two-electron oxygen reduction reaction, although the performance of the state-of-the-art electrocatalysts is insufficient to meet the demands for industrialization. Interestingly, guided by first-principles calculations, we found that the catalytic properties of the Co-N₄ moiety can be tailored by fine-tuning its surrounding atomic configuration to resemble the structure-dependent catalytic properties of metalloenzymes. Using this principle, we designed and synthesized a single-atom electrocatalyst that comprises an optimized Co-N₄ moiety incorporated in nitrogen-doped graphene for H₂O₂ production and exhibits a kinetic current density of 2.8 mA cm^-2 (at 0.65 V versus the reversible hydrogen electrode) and a mass activity of 155 A g^-1 (at 0.65 V versus the reversible hydrogen electrode) with negligible activity loss over 110 hours.

Nanoparticles as Emerging Labels in Electrochemical Immunosensors

This review shows recent trends in the use of nanoparticles as labels for electrochemical immunosensing applications. Some general considerations on the principles of both the direct detection based on redox properties and indirect detection through electrocatalytic properties, before focusing on the applications for mainly proteins detection, are given. Emerging use as blocking tags in nanochannels-based immunosensing systems is also covered in this review. Finally, aspects related to the analytical performance of the developed devices together with prospects for future improvements and applications are discussed.

A full understanding of oxygen reduction reaction mechanism on Au(1 1 1) surface

Oxygen reduction and hydrogen peroxide reduction are technologically important reactions in energy-conversion devices. In this work, a full understanding of oxygen reduction reaction (ORR) mechanism on Au(1 1 1) surface is investigated by density functional theory (DFT) calculations, including the reaction mechanisms of O₂ dissociation, OOH dissociation, and H₂O₂ dissociation. Among these ORR mechanisms on Au(1 1 1), the activation energy of [Formula: see text] hydrogenation reaction is much lower than that of [Formula: see text] dissociation, indicating that [Formula: see text] hydrogenation reaction is more appropriate at the first step than [Formula: see text] dissociation. In the following, H₂O₂ can be formed with the lower activation energy compared with the OOH dissociation reaction, and finally H₂O₂ could be generated as a detectable product due to the high activation energy of H₂O₂ dissociation reaction. Furthermore, the potential dependent free energy study suggests that the H₂O₂ formation is thermodynamically favorable up to 0.4 V on Au(1 1 1), reducing the overpotential for 2e ^- ORR process. And the elementary step of first H₂O formation becomes non-spontaneous at 0.4 V, indicating the difficulty of 4e ^- reduction pathway. Our DFT calculations show that H₂O₂ can be generated on Au(1 1 1) and the first electron transfer is the rate determining step. Our results show that gold surface could be used as a good catalyst for small-scale manufacture and on-site production of H₂O₂.

Epitaxial Growth of Au-Pt-Ni Nanorods for Direct High Selectivity H2 O2 Production

Au-Ni core-shell nanorods (NRs) and Au-Pt-Ni core-sandwich-shell NRs are synthesized and exhibit high activity for selective H₂ O₂ production via direct oxygen reduction. The epitaxial growth with coherent lattice fringes allow for the tuning of the oxygen reduction pathway. Moreover, a selectivity of 95% and mass activity of 192.9 A g^-1_{noble metal} are achieved using Au-Pt-Ni NRs at 150 mV overpotential.

The bifurcation point of the oxygen reduction reaction on Au–Pd nanoalloys

The oxygen reduction reaction is of major importance in energy conversion and storage. Controlling electrocatalytic activity and its selectivity remains a challenge of modern electrochemistry. Here, first principles calculations and analysis of experimental data unravel the mechanism of this reaction on Au–Pd nanoalloys in acid media. A mechanistic model is proposed from comparison of the electrocatalysis of oxygen and hydrogen peroxide reduction on different Au–Pd ensembles. A H₂O production channel on contiguous Pd sites proceeding through intermediates different from H₂O₂ and OOH^σ adsorbate is identified as the bifurcation point for the two reaction pathway alternatives to yield either H₂O or H₂O₂. H₂O₂ is a leaving group, albeit reduction of H₂O₂ to H₂O can occur by electrocatalytic HO–OH dissociation that is affected by the presence of adsorbed OOH^σ. Similarities and differences between electrochemical and direct synthesis from H₂ + O₂ reaction on Au–Pd nanoalloys are discussed.

Reduced graphene oxide-Hemin-Au nanohybrids: Facile one-pot synthesis and enhanced electrocatalytic activity towards the reduction of hydrogen peroxide

A facile and effective strategy is demonstrated for the synthesis of ternary reduced graphene oxide-Hemin-Au (rGO-H-Au) nanohybrids. The nanohybrids were synthesized through a one-pot in situ reduction of GO and HAuCl4 under alkaline conditions using GO, Hemin and HAuCl4 as the starting materials. The synthesis process can be finished within 1h in a solution phase, without adding any additional surfactant, stabilizing agent and toxic or harsh chemical reducing agents. The resulting nanohybrids were characterized by UV-vis spectroscopy, Raman spectroscopy, transmission electron microscopy (TEM), and so on. Electrochemical measurements showed that the rGO-H-Au nanohybrids exhibited good electrocatalytic activity for the reduction of hydrogen peroxide (H2O2). Based on this property, a simple and highly sensitive amperometric biosensor for H2O2 had been developed. The linear relationships were obtained from 0.1 µM to 40 µM and the detection limit was estimated to be 30 nM. The simple and sensitive sensing platform showed great promising applications in the pharmaceutical, clinical and industrial detection of H2O2.