Oxygen Reduction Reaction on Au Revisited at Different pH Values using in situ Surface-Enhanced Raman Spectroscopy

Modulating the structure of Cu in Cu2X/CNTs hollow tetrakaidecahedron to enhance high-efficiency H2O2 production

Regulation of active sites of electrocatalysts is critical in adjusting electronic structure and catalytic selectivity towards oxygen reduction reaction (ORR) to hydrogen peroxide (H2O2). Herein, the Cu2X/CNTs (X = Se, SSe, S) hollow tetrakaidecahedron catalysts were synthesized to facilitate the electrocatalytic reduction of O2 to H2O2. The introduction of S resulted in a shift from four-electron pathway on Cu2Se/CNTs to two-electron process on Cu2S/CNTs, ultimately leading to an enhancement in H2O2 productivity. Importantly, the addition of extra S species can modulate the chemical environment of active sites, and electrochemical tests demonstrate that the Cu2S/CNTs catalyst exhibits an enhanced selectivity (over 91 %), production rate (360 mmol gcat-1 h-1), and durability for H2O2 undergoing a two-electron process by an H-type electrolytic cell. The in-situ Raman spectroscopy result confirms that the structural stability of Cu2S/CNTs during the reaction, and the accumulation of H2O2 increased with the extension of reaction time. Various experimental results and density functional theory (DFT) reveal that the S atoms can optimize the adsorption strength of the active sites to reaction intermediates, thereby creating an appropriate energy barrier for the formation of the determinant intermediate OOH* in H2O2 production, while maintain a high energy barrier for OO bond breaking of OOH* towards H2O formation. This study proves insights into strategies for controlling H2O2 production and guiding the optimization of catalysts for H2O2 electrosynthesis.

A High-Rate and Ultrastable Ammonium Ion-Air Battery Enabled by the Synergy of ORR and NH4 + Storage

Ammonium ion batteries (AIBs) offer cost-effectiveness, nontoxicity, and eco-friendly attributes in energy storage technology. However, the constrained capacity and poor stability of conventional cathode materials have impeded their widespread adoption. Herein, a synergistic approach is introduced to overcome these challenges, by enhancing the air cathode with NH4 + and simultaneously leveraging atmospheric oxygen as a reservoir for NH4 + storage. Notably, NH4 + significantly enhances the oxygen reduction reaction (ORR) performance in neutral environments. Through in situ Raman spectroscopy and quantum density functional theory calculations, it is elucidated how NH4 + can act as a proton donor, replacing H2O in neutral media and reducing energy barriers in the protonation of *O2 - and *O, thereby accelerating ORR kinetics. The resulting ammonium ion-air battery, comprising an air cathode and a polymer (PNP) anode, showcases impressive metrics: high energy density of 78 Wh kg-1 and power density of 9369 W kg-1 at 1 A g-1, an initial capacity of 94.3 mAh g-1 and exceptional cycling stability (70.4% capacity retention after 12 500 cycles) at 10 A g-1. This pioneering research highlights the synergistic relationship between ORR and NH4 + storage and opens up new avenues for the design and advancement of innovative, sustainable, and environment-friendly AIBs.

Surface-Enhanced Raman Spectroscopy for the Detection of Reactive Oxygen Species

Reactive oxygen species (ROS) play fundamental roles in various biological and chemical processes in nature and industries, including cell signaling, disease development and aging, immune defenses, environmental remediation, pharmaceutical syntheses, metal corrosion, energy production, etc. As such, their detection is of paramount importance, but their accurate identification and quantification are technically challenging due to their transient nature with short lifetimes and low steady-state concentrations. As a highly sensitive and selective analytical technique, surface-enhanced Raman spectroscopy (SERS) is promising for detecting ROS in real-time, enabling in situ monitoring of ROS-involved electrochemical and biochemical events with exceptional resolution. This review provides a comprehensive analysis of the state-of-the-art in the SERS-based detection of ROS. Herein, the principles and ROS sensing mechanisms of SERS have been critically evaluated, highlighting their emerging applications in direct and indirect ROS monitoring in electrochemical and biological systems. The developments and reaction schemes of selective SERS probes for superoxide (•O2-), hydroxyl radicals (•OH), nitric oxide (•NO), peroxynitrite (ONOO-), and hypochlorite (OCl-) are presented. Finally, technical challenges and future research directions are discussed to guide the design of SERS for ROS analysis.

Pseudo-Jahn-Teller Effect Breaks the pH Dependence in Two-Electron Oxygen Electroreduction

The hydrogenation of small molecules (like O2 and CO2) often exhibits strong activity dependence on pHs because of discrepant proton donor environments. However, some catalysts can show seldom dependence on two-electron oxygen electroreduction, a sustainable route of O2 hydrogenation to hydrogen peroxide (H2O2). In this work, a pH-resistant oxygen electroreduction system arising from the pseudo-Jahn-Teller effect is demonstrated. Thorough operando Raman spectra, local environment analyses and density function theory simulations, the lattice distortion of TiOxFy that introduces the pseudo-Jahn-Teller effect contributing to regulating local pHs at electrode-electrolyte interfaces and the absorption/desorption of key *OOH intermediate is revealed. Consequently, as comparison to 78.6% activity attenuation for common catalyst, the TiOxFy displays minor activity decay (3.2%) in the pH range of 1-13 with remarkable Faradaic efficiencies (93.4-96.4%) and H2O2 yield rates (595-614 mg cm-2 h-1) in the current densities of 100-1000 mA cm-2. Further techno-economics analyses display the H2O2 production cost dependent on pHs, giving the lowest H2O2 price of $0.37 kg-1. The present finding is expected to provide an additional dimension to pseudo-Jahn-Teller effect that leverages systems beyond traditional conception.

Tuning N-heterocyclic carbene wingtips to form electrochemically stable adlayers on metals

Self-assembled monolayers (SAMs) are employed in electrochemical biosensors to passivate and functionalize electrode surfaces. These monolayers prevent the occurrence of undesired electrochemical reactions and act as scaffolds for coupling bioaffinity reagents. Thiols are the most common adlayer used for this application; however, the thiol-gold bond is susceptible to competitive displacement by naturally occurring solvated thiols in biological fluids, as well as to desorption under continuous voltage interrogation. To overcome these issues, N-heterocyclic carbene (NHC) monolayers have been proposed as an alternative for electrochemical biosensor applications due to the strong carbon-gold bond. To maximize the effectiveness of NHCs for SAMs, a thorough understanding of both the steric effects of wingtip substituents and NHC precursor type to the passivation of electrode surfaces is required. In this study, five different NHC wingtips as well as two kinds of NHC precursors were evaluated. The best performing NHC adlayers can be cycled continuously for four days (over 30 000 voltammetric cycles) without appreciably desorbing from the electrode surface. Benchmark thiol monolayers, in contrast, rapidly desorb after only twelve hours. Investigations also show NHC adlayer formation on other biosensor-relevant electrodes such as platinum and palladium.

From 2e- to 4e- pathway in the alkaline oxygen reduction reaction on Au(100): Kinetic circumvention of the volcano curve

We report the free energy barriers for the elementary reactions in the 2e- and 4e- oxygen reduction reaction (ORR) steps on Au(100) in an alkaline solution. Due to the weak adsorption energy of O2 on Au(100), the barrier for the association channel is very low, and the 2e- pathway is clearly favored, while the barrier for the O-O dissociation channel is significantly higher at 0.5 eV. Above 0.7 V reversible hydrogen electrode (RHE), the association channel becomes thermodynamically unfavorable, which opens up the O-O dissociation channel, leading to the 4e- pathway. The low adsorption energy of oxygenated species on Au is now an advantage, and residue ORR current can be observed up to the 1.0-1.2 V region (RHE). In contrast, the O-O dissociation barrier on Au(111) is significantly higher, at close to 0.9 eV, due to coupling with surface reorganization, which explains the lower ORR activity on Au(111) than that on Au(100). In combination with the previously suggested outer sphere electron transfer to O2 for its initial adsorption, these results provide a consistent explanation for the features in the experimentally measured polarization curve for the alkaline ORR on Au(100) and demonstrate an ORR mechanism distinct from that on Pt(111). It also highlights the importance to consider the spin state of O2 in ORR and to understand the activation barriers, in addition to the adsorption energies, to account for the features observed in electrochemical measurements.

Beyond the Gold-Thiol Paradigm: Exploring Alternative Interfaces for Electrochemical Nucleic Acid-Based Sensing

Nucleic acid-based electrochemical sensors (NBEs) use oligonucleotides as affinity reagents for the detection of a variety of targets, ranging from small-molecule therapeutics to whole viruses. Because of their versatility in molecular sensing, NBEs are being developed broadly for diagnostic and biomedical research applications. Benchmark NBEs are fabricated via self-assembly of thiol-based monolayers on gold. Although robust for rapid prototyping, thiol monolayers suffer from limitations in terms of stability under voltage modulation and in the face of competitive ligands such as thiolated molecules naturally occurring in biofluids. Additionally, gold cannot be deployed as an NBE substrate for all biomedical applications, such as in cases where molecular measurements coupled to real-time, under-the-sensor tissue imaging is needed. Seeking to overcome these limitations, the field of NBEs is pursuing alternative ligands and electrode surfaces. In this perspective, I discuss new interface fabrication strategies that have successfully achieved NBE sensing, or that have the potential to allow NBE sensing on conductive surfaces other than gold. I hope this perspective will provide the reader with a fresh view of how future NBE interfaces could be constructed and will serve as inspiration for the pursuit of collaborative developments in the field of NBEs.

Explaining the Decay of Nucleic Acid-Based Sensors under Continuous Voltammetric Interrogation

Nucleic acid-based electrochemical sensors (NBEs) can support continuous and highly selective molecular monitoring in biological fluids, both in vitro and in vivo, via affinity-based interactions. Such interactions afford a sensing versatility that is not supported by strategies that depend on target-specific reactivity. Thus, NBEs have significantly expanded the scope of molecules that can be monitored continuously in biological systems. However, the technology is limited by the lability of the thiol-based monolayers employed for sensor fabrication. Seeking to understand the main drivers of monolayer degradation, we studied four possible mechanisms of NBE decay: (i) passive desorption of monolayer elements in undisturbed sensors, (ii) voltage-induced desorption under continuous voltammetric interrogation, (iii) competitive displacement by thiolated molecules naturally present in biofluids like serum, and (iv) protein binding. Our results indicate that voltage-induced desorption of monolayer elements is the main mechanism by which NBEs decay in phosphate-buffered saline. This degradation can be overcome by using a voltage window contained between -0.2 and 0.2 V vs Ag|AgCl, reported for the first time in this work, where electrochemical oxygen reduction and surface gold oxidation cannot occur. This result underscores the need for chemically stable redox reporters with more positive reduction potentials than the benchmark methylene blue and the ability to cycle thousands of times between redox states to support continuous sensing for long periods. Additionally, in biofluids, the rate of sensor decay is further accelerated by the presence of thiolated small molecules like cysteine and glutathione, which can competitively displace monolayer elements even in the absence of voltage-induced damage. We hope that this work will serve as a framework to inspire future development of novel sensor interfaces aiming to eliminate the mechanisms of signal decay in NBEs.

Elucidating the Correlation between ORR Polarization Curves and Kinetics at Metal-Electrolyte Interfaces

The metal-vacuum models used to analyze the thermodynamics of the oxygen reduction reaction (ORR) completely overlook the role of electrolytes in the electrochemical process and thus cannot reflect the actual kinetic process occurring at the metal-electrolyte interface. Therefore, based on the real experimental process, the current work elucidates the chemical interactions between the electrolyte and the chemical species for the ORR via a novel metal-electrolyte model for the first time by effectively elucidating the correlation between ORR kinetics and polarization curves. Our simulation model analysis comprises the study of all possible ORR mechanisms on different Pt surfaces (Pt(111), Pt(110), and Pt(100)) and PtNi alloys with different compositions (Pt₃Ni(111), Pt₂Ni₂(111), and PtNi₃(111)). The obtained results demonstrate that the hydrogenation of adsorbed oxygen to form adsorbed hydroxyl (R8), whose immense control weight is reflected by a coverage of adsorbed oxygen (θ_O*) of about 1, is the rate-determining step (RDS) in the four-electron-dominated ORR process. A direct correlation has been established by the great fitting of polarization curves from theoretical ORR kinetics obtained via both the metal-electrolyte model and experimental measurement. This study reveals that among the different Pt surfaces and PtNi alloys, Pt₃Ni(111) exhibits the highest ORR activity with the lowest free energy barrier of E_a (0.74 eV), the smallest value of |ΔG_O* - 2.46| (0.80 eV), the highest reaction rate r (9.98 × 10⁵ s^-1 per site), and a more positive half-wave potential U_1/2 (0.93 V). In contrast to previous model studies, this work provides a more accurate theoretical system for catalyst screening, which will help researchers to better understand the experimental phenomena and will be a guiding piece of work for catalyst design and development.

Carbon nitride/positive carbon black anchoring PtNPs assembled byγ-rays as ORR catalyst with excellent stability

Electrocatalytic performance of low-cost graphitic carbon nitride (CN) is greatly limited by its limited conductivity and small specific surface area. Herein, a simple and cost-effective idea to produce novel nanocomposite is constructed by the CN and cetyl trimethyl ammonium bromide functionalized carbon black (CB) anchored platinum nanoparticles as highly efficient oxygen reduction catalysts based on gamma irradiation. The assembled carbon nitride/positive carbon black anchoring PtNPs (Pt/CN₂-CB⁺₁) catalyst exhibits significantly improved specific surface area, high graphitization, and uniformly dispersed ultra-small platinum nanoparticles. For the oxygen reduction reaction (ORR) performance, the catalyst shows more positive onset-potential (0.93 V versus RHE) and larger diffusion limiting current density (5.65 mA cm^-2) compared with benchmark Pt/C catalysts in alkaline medium. Moreover, the Pt/CN₂-CB⁺₁catalyst exhibits a small Tafel slope (92 mV dec^-1). Besides, the catalyst was demonstrated the remarkable methanol tolerance and good long-term stability under working conditions. This work provides a new and effectiveγ-rays irradiation for synthesizing the carbon nitride catalysts for energy conversion and storage applications.

Advances and challenges for experiment and theory for multi-electron multi-proton transfer at electrified solid–liquid interfaces

Understanding microscopic mechanism of multi-electron multi-proton transfer reactions at complexed systems is important for advancing electrochemistry-oriented science in the 21st century.