Enhanced Electrochemical H2O2 Production via Two-Electron Oxygen Reduction Enabled by Surface-Derived Amorphous Oxygen-Deficient TiO2-x

Understanding Advanced Transition Metal-Based Two Electron Oxygen Reduction Electrocatalysts from the Perspective of Phase Engineering

Non-noble transition metal (TM)-based compounds have recently become a focal point of extensive research interest as electrocatalysts for the two electron oxygen reduction (2e- ORR) process. To efficiently drive this reaction, these TM-based electrocatalysts must bear unique physiochemical properties, which are strongly dependent on their phase structures. Consequently, adopting engineering strategies toward the phase structure has emerged as a cutting-edge scientific pursuit, crucial for achieving high activity, selectivity, and stability in the electrocatalytic process. This comprehensive review addresses the intricate field of phase engineering applied to non-noble TM-based compounds for 2e- ORR. First, the connotation of phase engineering and fundamental concepts related to oxygen reduction kinetics and thermodynamics are succinctly elucidated. Subsequently, the focus shifts to a detailed discussion of various phase engineering approaches, including elemental doping, defect creation, heterostructure construction, coordination tuning, crystalline design, and polymorphic transformation to boost or revive the 2e- ORR performance (selectivity, activity, and stability) of TM-based catalysts, accompanied by an insightful exploration of the phase-performance correlation. Finally, the review proposes fresh perspectives on the current challenges and opportunities in this burgeoning field, together with several critical research directions for the future development of non-noble TM-based electrocatalysts.

Highly Efficient Acidic Electrosynthesis of Hydrogen Peroxide at Industrial-Level Current Densities Promoted by Alkali Metal Cations

Acidic H2O2 synthesis through electrocatalytic 2e- oxygen reduction presents a sustainable alternative to the energy-intensive anthraquinone oxidation technology. Nevertheless, acidic H2O2 electrosynthesis suffers from low H2O2 Faradaic efficiencies primarily due to the competing reactions of 4e- oxygen reduction to H2O and hydrogen evolution in environments with high H+ concentrations. Here, we demonstrate the significant effect of alkali metal cations, acting as competing ions with H+, in promoting acidic H2O2 electrosynthesis at industrial-level currents, resulting in an effective current densities of 50-421 mA cm-2 with 84-100 % Faradaic efficiency and a production rate of 856-7842 μmol cm-2 h-1 that far exceeds the performance observed in pure acidic electrolytes or low-current electrolysis. Finite-element simulations indicate that high interfacial pH near the electrode surface formed at high currents is crucial for activating the promotional effect of K+. In situ attenuated total reflection Fourier transform infrared spectroscopy and ab initio molecular dynamics simulations reveal the central role of alkali metal cations in stabilizing the key *OOH intermediate to suppress 4e- oxygen reduction through interacting with coordinated H2O.

Creating Conjugated C-C Bonds between Commercial Carbon Electrode and Molecular Catalyst for Oxygen Reduction to Hydrogen Peroxide

Immobilizing molecular catalysts on electrodes is vital for electrochemical applications. However, creating robust electrode-catalyst interactions while maintaining good catalytic performance and rapid electron transfer is challenging. Here, without introducing any foreign elements, we show a bottom-up synthetic approach of constructing the conjugated C-C bond between the commercial Vulcan carbon electrode and an organometallic catalyst. Characterization results from FTIR, XPS, aberration-corrected TEM and EPR confirmed the successful and uniform heterogenization of the complex. The synthesized Vulcan-LN4 -Co catalyst is highly active and selective in the oxygen reduction reaction in neutral media, showing an 80 % hydrogen peroxide selectivity and a 0.72 V (vs. RHE) onset potential which significantly outperformed the homogenous counterpart. Based on single-crystal XRD and NMR data, we built a model for density functional theory calculations which showed a nearly optimal binding energy for the *OOH intermediate. Our results show that the direct conjugated C-C bonding is an effective approach for heterogenizing molecular catalysts on carbon, opening new opportunities for employing molecular catalysts in electrochemical applications.

Favoring the Selective H2O2 Generation of a Self-Antibiofouling Dissolved Oxygen Sensor for Real-Time Online Monitoring via Surface-Engineered N-Doped Reduced Graphene Oxide

Dissolved oxygen (DO) is a key parameter in assessing water quality, particularly in aquatic ecosystems. The oxygen reduction reaction (ORR) has notable prevalence in energy conversion and biological processes, including biosensing. Nevertheless, the long-term usage of the submersible DO sensors leads to undesirable biofilm formation on the electrode surface, deteriorating their sensitivity and stability. Recently, the reactive oxygen species (ROS), such as the two-electron pathway ORR byproduct, H2O2, had been known for its biofilm-degradation activity. Herein, for the first time, we reported N-doped reduced graphene oxide (N-rGO) for H2O2 selectivity as the self-antibiofouling DO sensor. Introducing foreign atom doping could reorient the electron network of graphene by the electronegativity gap, which facilitated highly selective and efficient two electron pathway of ORR. Mitigating the N content of N-rGO had enhanced the H2O2 selectivity (57.5%) and electron transfer number (n = 2.84) in neutral medium. Moreover, the N-rGO could be integrated to a wireless DO monitoring device that might realize an applicable device in the aquatic fish farming.

Local Built-In Field at the Sub-nanometric Heterointerface Mediates Cascade Electrochemical Conversion of Lithium-sulfur Batteries

Heterogeneous catalytic mediators have been proposed to play a vital role in enhancing the multiorder reaction and nucleation kinetics in multielectron sulfur electrochemistry. However, the predictive design of heterogeneous catalysts is still challenging, owing to the lack of in-depth understanding of interfacial electronic states and electron transfer on cascade reaction in Li-S batteries. Here, a heterogeneous catalytic mediator based on monodispersed titanium carbide sub-nanoclusters embedded in titanium dioxide nanobelts is reported. The tunable catalytic and anchoring effects of the resulting catalyst are achieved by the redistribution of localized electrons caused by the abundant built-in fields in heterointerfaces. Subsequently, the resulting sulfur cathodes deliver an areal capacity of 5.6 mAh cm-2 and excellent stability at 1 C under sulfur loading of 8.0 mg cm-2 . The catalytic mechanism especially on enhancing the multiorder reaction kinetic of polysulfides is further demonstrated via operando time-resolved Raman spectroscopy during the reduction process in conjunction with theoretical analysis.

Machine learning screening of high-performance single-atom electrocatalysts for two-electron oxygen reduction reaction

Electrocatalysis has emerged as one of the most promising alternatives to conventional anthraquinone for preparing hydrogen peroxide (H₂O₂) with high energy consumption and pollution because of its simplicity, convenience, and environmental friendliness. However, the oxygen reduction reaction (ORR) generating H₂O₂viathe2e^- path is acompetitive path for 4e^-ORR to generate H₂O. Therefore, it is crucial to identify an electrocatalyst with high selectivity and activity of 2e^-ORR. Here, we established five machine learning (ML) models based on the adsorption free energy of O* (△G (O*)) of 149 single-atom catalysts (SACs) collected and the limiting potential (U_L) of 31 SACs calculated using density functional theory (DFT) from the literature. We then obtained descriptors that could accurately describe SACs. Furthermore, 690 unknown SACs' 2e^-ORR catalytic performance was well predicted. Four 2e^-ORR materials with high selectivity and activity were screened: Zn@Pc-N₃C₁, Au@Pd-N₄, Au@Pd-N₁C₃, and Au@Py-N₃C₁. We verified the U_L of these SACs through DFT calculation, which was higher than the standard value, proving the ML model's validity. The ML-based method to predict the material properties with highly selective and active electrocatalysts provides an efficient, rapid, and low-cost method for discovering and designing more valuable SACs catalysts.

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.

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 SAC_Dp) is mainly responsible for the 2e^– ORR, while pyridine-type CoN₄ catalyzes the 4e^– ORR. Indeed, Co–N SAC_Dp 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 SAC_Dp—with weakening O₂/HOO* interaction—boosts the H₂O₂ production.

Composition Engineering of Amorphous Nickel Boride Nanoarchitectures Enabling Highly Efficient Electrosynthesis of Hydrogen Peroxide

Developing advanced electrocatalysts with exceptional two electron (2e^- ) selectivity, activity, and stability is crucial for driving the oxygen reduction reaction (ORR) to produce hydrogen peroxide (H₂ O₂ ). Herein, a composition engineering strategy is proposed to flexibly regulate the intrinsic activity of amorphous nickel boride nanoarchitectures for efficient 2e^- ORR by oriented reduction of Ni²⁺ with different amounts of BH₄ ^- . Among borides, the amorphous NiB₂ delivers the 2e^- selectivity close to 99% at 0.4 V and over 93% in a wide potential range, together with a negligible activity decay under prolonged time. Notably, an ultrahigh H₂ O₂ production rate of 4.753 mol g_cat ^-1 h^-1 is achieved upon assembling NiB₂ in the practical gas diffusion electrode. The combination of X-ray absorption and in situ Raman spectroscopy, as well as transient photovoltage measurements with density functional theory, unequivocally reveal that the atomic ratio between Ni and B induces the local electronic structure diversity, allowing optimization of the adsorption energy of Ni toward *OOH and reducing of the interfacial charge-transfer kinetics to preserve the OO bond.

Synergetic Dual-Ion Centers Boosting Metal Organic Framework Alloy Catalysts toward Efficient Two Electron Oxygen Reduction

Herein, a strategy of synergetic dual-metal-ion centers to boost transition-metal-based metal organic framework (MOF) alloy nanomaterials as active oxygen reduction reaction (ORR) electrocatalysts for efficient hydrogen peroxide (H₂ O₂ ) generation is proposed. Through a facile one-pot wet chemical method, a series of MOF alloys with unique Ni-M (M-Co, Cu, Zn) synergetic centers are synthesized, where the strong metallic ions 3d-3d synergy can effectively inhibit O₂ cleavage on Ni sites toward a favorable two-electron ORR pathway. Impressively, the well-designed NiZn MOF alloy catalysts show an excellent H₂ O₂ selectivity up to 90% during ORR, evidently outperforming that of NiCo MOF (45%), and NiCu MOF (55%). Moreover, it sustains efficient activity and robust stability under a continuous longterm ORR operation. The correlative in situ synchrotron radiation X-ray adsorption fine structure and Fourier transform infrared spectroscopy analyses reveal at the atomic level that, the higher Ni oxidation states species, regulated via adjacent Zn²⁺ ions, are favorable for optimizing the adsorption energetics of key *OOH intermediates toward fast two electron ORR kinetics.

Entropy Enhanced Perovskite Oxide Ceramic for Efficient Electrochemical Reduction of Oxygen to Hydrogen Peroxide

The electrochemical oxygen reduction reaction (ORR) offers a most promising and efficient route to produce hydrogen peroxide (H₂O₂), yet the lack of cost‐effective and high‐performance electrocatalysts have restricted its practical application. Herein, an entropy‐enhancement strategy has been employed to enable the low‐cost perovskite oxide to effectively catalyze the electrosynthesis of H₂O₂. The optimized Pb(NiWMnNbZrTi)_1/6O₃ ceramic is available on a kilogram‐scale and displays commendable ORR activity in alkaline media with high selectivity over 91 % across the wide potential range for H₂O₂ including an outstanding degradation property for organic dyes through the Fenton process. The exceptional performance of this perovskite oxide is attributed to the entropy stabilization‐induced polymorphic transformation assuring the robust structural stability, decreased charge mobility as well as synergistic catalytic effects which we confirm using advanced in situ Raman, transient photovoltage, Rietveld refinement as well as finite elemental analysis.

Tailoring surface carboxyl groups of mesoporous carbon boosts electrochemical H2O2 production

Oxygen-doped porous carbon materials have been shown promising performance for electrochemical two-electron oxygen reduction reaction (2e^- ORR), an efficient approach for the safe and continuous on-site generation of H₂O₂. The regulation and mechanism understanding of active oxygen-containing functional groups (OFGs) remain great challenges. Here, OFGs modified porous carbon were prepared by thermal oxidation (MC-12-Air), HNO₃ oxidation (MC-12-HNO₃) and H₂O₂ solution hydrothermal treatment (MC-12-H₂O₂), respectively. Structural characterization showed that the oxygen doping content of three catalysts reached about 20%, with the almost completely maintained specific surface area (exception of MC-12- HNO₃). Spectroscopic characterization further revealed that hydroxyl groups are mainly introduced into MC-12-Air, while carboxyl groups are mainly introduced into MC-12- HNO₃ and MC-12- H₂O₂. Compared with the pristine catalyst, three oxygen-functionalized catalysts showed enhanced activity and H₂O₂ selectivity in 2e^- ORR. Among them, MC-12-H₂O₂ exhibited the highest catalytic activity and selectivity of 94 %, as well as a considerable HO₂^- accumulation of 46.2 mmol L^-1 and excellent stability in an extended test over 36 h in a H-cell. Electrochemical characterization demonstrated the promotion of OFGs on ORR kinetics and the greater contribution of carboxyl groups to the intrinsically catalytic activity. DFT calculations confirmed that the electrons are transferred from carboxyl groups to adjacent carbon and the enhanced adsorption strength toward *OOH intermediate, leading to a lower energy barrier for forming *OOH on carboxyl terminated carbon atoms.

Electrocatalytic two-electron oxygen reduction over nitrogen doped hollow carbon nanospheres

The two-electron oxygen reduction reaction (2e^- ORR) has become a hopeful alternative for production of hydrogen peroxide (H₂O₂), but its practical feasibility is hindered by the lack of efficient electrocatalysts to achieve high activity and selectivity. Herein, we successfully synthesized outstanding nitrogen doped hollow carbon nanospheres (NHCSs) for electrochemical production of H₂O₂. In 0.1 M KOH, NHCSs exhibit superior and sustained catalytic activity for the 2e^- ORR with an unordinary selectivity of 96.6%. Impressively, such NHCSs manifest an ultrahigh H₂O₂ yield rate of 7.32 mol g_cat.^-1  h^-1 and a high faradaic efficiency of 96.7% at 0.5 V in an H-cell system. Density functional theory calculations were performed to further reveal the catalytic mechanism involved.

A TiO2-x nanobelt array with oxygen vacancies: an efficient electrocatalyst toward nitrite conversion to ammonia

Electrocatalytic nitrite reduction not only holds significant potential in the control of nitrite contamination in the natural environment, but also is an attractive approach for sustainable ammonia synthesis. In this communication, we report that a TiO_2-x nanobelt array with oxygen vacancies on a titanium plate is able to convert nitrite into ammonia with a high faradaic efficiency of 92.7% and a large yield of 7898 μg h^-1 cm^-2 in alkaline solution. This monolithic catalyst also shows high durability with the maintenance of its catalytic activity for 12 h. Theoretical calculations further reveal the critical role of oxygen vacancies in nitrite electroreduction.

Coupling Co-N-C with MXenes Yields Highly Efficient Catalysts for H2O2 Production in Acidic Media

The electrochemical oxygen reduction reaction (ORR) offers a promising method to replace the anthraquinone process for hydrogen peroxide (H₂O₂) production. However, the efficiency of this process suffers from sluggish kinetics, particularly in an acidic environment. Therefore, employing catalysts with high electroactivity is highly desirable for H₂O₂ synthesis. Here, an effective strategy for preparing Co-N-C/Ti₃C₂T_x with high H₂O₂ selectivity and ORR reactivity is proposed. The acquired Co-N-C/Ti₃C₂T_x shows excellent H₂O₂ electrosynthesis performance in acidic media with H₂O₂ productivity of up to 3200 ppm h^-1, superior to state-of-the-art catalysts. Interestingly, a H₂O₂ concentration of 6.0 wt % was obtained after the stability test, and the Co-N-C/Ti₃C₂T_x catalyst was found to effectively catalyze organic dye degradation. Further analysis reveals that the enhanced H₂O₂ electrosynthesis performance originates from the layered structure and the oxygen functional groups of Ti₃C₂T_x. The layered structure can effectively promote increased exposure of active sites, while the oxygen functional groups will fine-tune the electronic structure of Co atoms, allowing a selective ORR pathway to produce H₂O₂. This work provides a strategy to design and fabricate highly efficient catalysts for H₂O₂ production and degradation of organic pollutants.

Photoelectrochemical Antibacterial Platform Based on Rationally Designed Black TiO2-x Nanowires for Efficient Inactivation against Bacteria

A hyphenated strategy for photoelectrochemical (PEC) disinfection is proposed with rationally designed black TiO_2-x as a photoresponsive material. The PEC method, a fusion of electrochemistry and photocatalysis, is an innovative and effective way to improve photocatalytic performance, which imparts certain properties to the photoelectrode and greatly promotes the charge transfer, thus effectively inhibiting the recombination of carriers and greatly promoting the catalytic efficiency. The oxygen vacancy (V_o) engineering on TiO₂ is conducted to obtain black TiO_2-x, which shows a much larger light absorption region in the full solar spectrum than the pristine white TiO₂ and presents excellent PEC sterilization performance. The bactericidal efficiency with the PEC method is 10 times higher than that with the photocatalytic method, inactivating 99% of bacteria within a short time of 30 min. In addition, the black titanium oxide nanowires (B-TiO_2-x NWs) are grown on flexible carbon cloth (CC) to form a sandwich structural self-cleaning face mask. The bacteria adhering to the surface of the mask will be sterilized quickly and efficiently under the illumination of visible light. The face mask with the characteristics of superior antibacterial effect and long service lifetime will be used for epidemic prevention and reduce the second pollution.

Photocatalytic Recovery of Gold from a Non-Cyanide Gold Plating Solution as Au Nanoparticle-Decorated Semiconductors

In this work, a photocatalytic process was carried out to recover gold (Au) from the simulated non-cyanide plating bath solution. Effects of semiconductor types (TiO₂, WO₃, Nb₂O₃, CeO₂, and Bi₂O₃), initial pH of the solution (3-10), and type of complexing agents (Na₂S₂O₃ and Na₂SO₃) and their concentrations (1-4 mM each) on Au recovery were explored. Among all employed semiconductors, TiO₂ exhibited the highest photocatalytic activity to recover Au from the simulated spent plating bath solution both in the absence and presence of complexing agents, in which Au was completely recovered within 15 min at a pH of 6.5. The presence of complexing agents remarkably affected the size of deposited Au on the TiO₂ surface, the localized surface plasmon effect (LSPR) behavior, and the valence band (VB) edge position of the obtained Au/TiO₂, without a significant change in the textural properties or the band gap energy. The photocatalytic activity of the obtained Au/TiO₂ tested via two photocatalytic processes depended on the common reduction mechanism rather than the textural or optical properties. As a result, the Au/TiO₂ NPs obtained from the proposed recovery process are recommended for use as a photocatalyst for the reactions occurring at the conduction band rather than at the valence band. Notably, they exhibited good stability after the fifth photocatalytic cycle for Au recovery from the actual cyanide plating bath solution.

Carbonate-Induced Electrosynthesis of Hydrogen Peroxide via Two-Electron Water Oxidation

Electrochemical synthesis of hydrogen peroxide (H₂ O₂ ), via the two-electron water oxidation reaction (2e^- WOR), is an attractive method for the sustainable production of valuable chemicals in place of oxygen during water electrolysis. While the majority of 2e^- WOR studies have focussed on electrocatalyst design, little research has been carried out on the selection of the supporting electrolyte. In this work, we investigate the impact of potassium carbonate (K₂ CO₃ ) electrolytes, and their key properties, on H₂ O₂ production. We found that at electrolyte pH values (>9.5) where the carbonate anion (CO₃ ^2- ) was prevalent in the mixture, a 26.5 % increase in the Faraday efficiency (%FE) for H₂ O₂ production was achieved, compared to bicarbonate (HCO₃ ^- ) dominant solutions. Utilising boron-doped diamond (BDD) in highly concentrated K₂ CO₃ solutions, current densities of up to 511 mA cm^-2 (in 4 m) and %FEs of 91.5 % (in 5 m) could be attained. The results presented in this work highlight the influence of CO₃ ^2- on electrochemical H₂ O₂ generation via the 2e^- WOR and provide novel pathways to produce desirable commodities at the anode during electrochemical water splitting.

Plasma-etched Ti2O3 with oxygen vacancies for enhanced NH3 electrosynthesis and Zn–N2 batteries

Plasma-etched OV-Ti2O3 behaves as an active and stable catalyst for electrochemical N2 reduction to yield NH3, capable of attaining a large NH3 yield of 37.24 μg h−1 mgcat.−1 and high faradaic efficiency of 19.29%.

Greatly Facilitated Two-Electron Electroreduction of Oxygen into Hydrogen Peroxide over TiO2 by Mn Doping

Ambient electrochemical oxygen reduction into valuable hydrogen peroxide (H₂O₂) via a selective two-electron (2e^-) pathway is regarded as a sustainable alternative to the industrial anthraquinone process, but it requires advanced electrocatalysts with high activity and selectivity. In this study, we report that Mn-doped TiO₂ behaves as an efficient electrocatalyst toward highly selective H₂O₂ synthesis. This catalyst exhibits markedly enhanced 2e^- oxygen reduction reaction performance with a low onset potential of 0.78 V and a high H₂O₂ selectivity of 92.7%, much superior to the pristine TiO₂ (0.64 V, 62.2%). Additionally, it demonstrates a much improved H₂O₂ yield of up to 205 ppm h^-1 with good stability during bulk electrolysis in an H-cell device. The significantly boosted catalytic performance is ascribed to the lattice distortion of Mn-doped TiO₂ with a large amount of oxygen vacancies and Ti³⁺. Density functional theory calculations reveal that Mn dopant improves the electrical conductivity and reduces ΔG_*OOH of pristine TiO₂, thus giving rise to a highly efficient H₂O₂ production process.

Electrocatalytic H2O2 production via two-electron O2 reduction by Mo-doped TiO2 nanocrystallines

Mo acts as an effective dopant to boost the catalytic activity of TiO2 for the 2e− O2 reduction reaction. Such Mo–TiO2 electrocatalyst achieves a high Faradaic efficiency of 92% and a large H2O2 yield of 395.3 mmol gcat−1 h−1 in alkaline medium.