Enhancing the Selectivity of H2O2 Electrogeneration by Steric Hindrance Effect

Advanced Nanocarbons Toward two-Electron Oxygen Electrode Reactions for H2O2 Production and Integrated Energy Conversion

Hydrogen peroxide (H2O2) plays a pivotal role in advancing sustainable technologies due to its eco-friendly oxidizing capability. The electrochemical two-electron (2e-) oxygen reduction reaction and water oxidation reaction present an environmentally green method for H2O2 production. Over the past three years, significant progress is made in the field of carbon-based metal-free electrochemical catalysts (C-MFECs) for low-cost and efficient production of H2O2 (H2O2EP). This article offers a focused and comprehensive review of designing C-MFECs for H2O2EP, exploring the construction of dual-doping configurations, heteroatom-defect coupling sites, and strategic dopant positioning to enhance H2O2EP efficiency; innovative structural tuning that improves interfacial reactant concentration and promote the timely release of H2O2; modulation of electrolyte and electrode interfaces to support the 2e- pathways; and the application of C-MFECs in reactors and integrated energy systems. Finally, the current challenges and future directions in this burgeoning field are discussed.

Sn Bulk Phase Doping and Surface Modification on Ti4 O7 for Oxygen Reduction to Hydrogen Peroxide

Developing stable and highly selective two-electron oxygen reduction reaction (2e- ORR) electrocatalysts for producing hydrogen peroxide (H2 O2 ) is considered a major challenge to replace the anthraquinone process and achieve a sustainable green economy. Here, we doped Sn into Ti4 O7 (D-Sn-Ti4 O7 ) by simple polymerization post-calcination method as a high-efficiency 2e- ORR electrocatalyst. In addition, we also applied plain calcination after the grinding method to load Sn on Ti4 O7 (L-Sn-Ti4 O7 ) as a comparison. However, the performance of L-Sn-Ti4 O7 is far inferior to that of the D-Sn-Ti4 O7 . D-Sn-Ti4 O7 exhibits a starting potential of 0.769 V (versus the reversible hydrogen electrode, RHE) and a high H2 O2 selectivity of 95.7 %. Excitingly, the catalyst can maintain a stable current density of 2.43 mA ⋅ cm-2 for 3600 s in our self-made H-type cell, and the cumulative H2 O2 production reaches 359.2 mg ⋅ L-1 within 50,000 s at 0.3 V. The performance of D-Sn-Ti4 O7 is better than that of the non-noble metal 2e- ORR catalysts reported so far. The doping of Sn not only improves the conductivity but also leads to the lattice distortion of Ti4 O7 , further forming more oxygen vacancies and Ti3+ , which greatly improves its 2e- ORR performance compared with the original Ti4 O7 . In contrast, since the Sn on the surface of L-Sn-Ti4 O7 displays a synergistic effect with Tin+ (3≤n≤4) of Ti4 O7 , the active center Tin+ dissociates the O=O bond, making it more inclined to 4e- ORR.

Single-Atom Zinc Sites with Synergetic Multiple Coordination Shells for Electrochemical H2 O2 Production

Precise manipulation of the coordination environment of single-atom catalysts (SACs), particularly the simultaneous engineering of multiple coordination shells, is crucial to maximize their catalytic performance but remains challenging. Herein, we present a general two-step strategy to fabricate a series of hollow carbon-based SACs featuring asymmetric Zn-N2 O2 moieties simultaneously modulated with S atoms in higher coordination shells of Zn centers (n≥2; designated as Zn-N2 O2 -S). Systematic analyses demonstrate that the synergetic effects between the N2 O2 species in the first coordination shell and the S atoms in higher coordination shells lead to robust discrete Zn sites with the optimal electronic structure for selective O2 reduction to H2 O2 . Remarkably, the Zn-N2 O2 moiety with S atoms in the second coordination shell possesses a nearly ideal Gibbs free energy for the key OOH* intermediate, which favors the formation and desorption of OOH* on Zn sites for H2 O2 generation. Consequently, the Zn-N2 O2 -S SAC exhibits impressive electrochemical H2 O2 production performance with high selectivity of 96 %. Even at a high current density of 80 mA cm-2 in the flow cell, it shows a high H2 O2 production rate of 6.924 mol gcat -1  h-1 with an average Faradaic efficiency of 93.1 %, and excellent durability over 65 h.

Tailoring the Atomic-Local Environment of Carbon Nanotube Tips for Selective H2 O2 Electrosynthesis at High Current Densities

The atomic-local environment of catalytically active sites plays an important role in tuning the activity of carbon-based metal-free electrocatalysts (C-MFECs). However, the rational regulation of the environment is always impeded by synthetic limitations and insufficient understanding of the formation mechanism of the catalytic sites. Herein, the possible cleavage mechanism of carbon nanotubes (CNTs) through the crossing points during ball-milling is proposed, resulting in abundant CNT tips that are more susceptible to be modified by heteroatoms, achieving precise modulation of the atomic environment at the tips. The obtained CNTs with N,S-rich tips (N,S-TCNTs) exhibit a wide potential window of 0.59 V along with H2 O2 selectivity for over 90.0%. Even using air as the O2 source, the flow cell system with N,S-TCNTs catalyst attains high H2 O2 productivity up to 30.37 mol gcat. -1  h-1 @350 mA cm-2 , superior to most reported C-MFECs. From a practical point of view, a solid electrolyzer based on N,S-TCNTs is further employed to realize the in-situ continuous generation of pure H2 O2 solution with high productivity (up to 4.35 mmol cm-2  h-1 @300 mA cm-2 ; over 300 h). The CNTs with functionalized tips hold great promise for practical applications, even beyond H2 O2 generation.

Facilitating two-electron oxygen reduction with pyrrolic nitrogen sites for electrochemical hydrogen peroxide production

Electrocatalytic hydrogen peroxide (H₂O₂) production via the two-electron oxygen reduction reaction is a promising alternative to the energy-intensive and high-pollution anthraquinone oxidation process. However, developing advanced electrocatalysts with high H₂O₂ yield, selectivity, and durability is still challenging, because of the limited quantity and easy passivation of active sites on typical metal-containing catalysts, especially for the state-of-the-art single-atom ones. To address this, we report a graphene/mesoporous carbon composite for high-rate and high-efficiency 2e^- oxygen reduction catalysis. The coordination of pyrrolic-N sites -modulates the adsorption configuration of the *OOH species to provide a kinetically favorable pathway for H₂O₂ production. Consequently, the H₂O₂ yield approaches 30 mol g^-1 h^-1 with a Faradaic efficiency of 80% and excellent durability, yielding a high H₂O₂ concentration of 7.2 g L^-1. This strategy of manipulating the adsorption configuration of reactants with multiple non-metal active sites provides a strategy to design efficient and durable metal-free electrocatalyst for 2e^- oxygen reduction.

Intrinsic Carbon Defects for the Electrosynthesis of H2O2

Carbon materials have manifested promising potential in electrochemical reduction of O₂ to H₂O₂. The oxygen functional groups have been identified as the catalytic sites. However, the intrinsic carbon defects abundant in carbon materials have often been neglected. Herein, a three-dimensional carbon framework with abundant intrinsic defects and oxygen functional groups (the oxygen content and chemical states of oxygen are comparable to those of commercial carbon black) was introduced and exhibited outstanding catalytic activity and selectivity toward H₂O₂ electrosynthesis. Through a combination of in situ Raman spectroscopy and density functional theory calculations, the intrinsic carbon defects, such as zigzag edge and zigzag pentagon sites with optimal binding energy for OOH, were also determined to be active sites. It was further revealed that intrinsic carbon defects with large negative charge density and asymmetric spin density may have high activity toward H₂O₂ production.

Radical and non-radical cooperative degradation in metal-free electro-Fenton based on nitrogen self-doped biochar

To achieve sustainable metal-free electron-Fenton, N self-doped biochar air-cathode (BCAC) was prepared by pyrolyzing coffee residues. During the pyrolysis process, the endogenous N transformed from edge-doping to graphite-doping. Particularly, N vacancies started to evolve when the peak temperature exceeded 700 °C. A high Tetracycline removal rate of 70.42% was obtained on the BCAC at the current density of 4 mA cm^-2. Quenching tests incorporated with ESR spectroscopy were adopted to identify the specific oxidants produced on the cathode. The results showed that •OH (37.36%), •O₂^- (29.67%) and ¹O₂ (24.17%) played comparable role in the tetracycline removal, suggesting the coexist of radical and non-radical oxidants in our electro-Fenton system. According to the structure characterization and the DFT calculation, graphitic N was suggested as the critical site for H₂O₂ generation, and both graphitic N and pyridinic N were electroactive sites for H₂O₂ activation to •OH. Graphitic N and N vacancies with stronger capabilities in O₂ adsorption and electron-trapping were proposed as the electroactive sites for ¹O₂ and •O₂^- formation. This work predicts a novel electro-Fenton process with cooperative radical and non-radical degradation on N self-doped carbonaceous catalysts at a mild condition, which is extremely meaningful for boosting sustainable electro-Fenton technology.

Tuning Two-Electron Oxygen-Reduction Pathways for H2 O2 Electrosynthesis via Engineering Atomically Dispersed Single Metal Site Catalysts

The hydrogen peroxide (H₂ O₂ ) generation via the electrochemical oxygen reduction reaction (ORR) under ambient conditions is emerging as an alternative and green strategy to the traditional energy-intensive anthraquinone process and unsafe direct synthesis using H₂ and O₂ . It enables on-site and decentralized H₂ O₂ production using air and renewable electricity for various applications. Currently, atomically dispersed single metal site catalysts have emerged as the most promising platinum group metal (PGM)-free electrocatalysts for the ORR. Further tuning their central metal sites, coordination environments, and local structures can be highly active and selective for H₂ O₂ production via the 2e^- ORR. Herein, recent methodologies and achievements on developing single metal site catalysts for selective O₂ to H₂ O₂ reduction are summarized. Combined with theoretical computation and advanced characterization, a structure-property correlation to guide rational catalyst design with a favorable 2e^- ORR process is aimed to provide. Due to the oxidative nature of H₂ O₂ and the derived free radicals, catalyst stability and effective solutions to improve catalyst tolerance to H₂ O₂ are emphasized. Transferring intrinsic catalyst properties to electrode performance for viable applications always remains a grand challenge. The key performance metrics and knowledge during the electrolyzer development are, therefore, highlighted.

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.

Impact of Pore Structure on Two-Electron Oxygen Reduction Reaction in Nitrogen-Doped Carbon Materials: Rotating Ring-Disk Electrode vs. Flow Cell

The impact of pore structure on the two-electron oxygen reduction reaction (ORR) in nitrogen-doped carbon materials is currently under debate, and previous studies are mainly limited to the rotating ring-disk electrode (RRDE) rather than the practical flow cell (FC) system. In this study, assisted by a group of reliable pore models, the impact of two pore structure parameters, that is, Brunauer-Emmett-Teller surface area (S_BET ) and micropore surface fraction (f_micro ), on ORR activity and selectivity are investigated in both RRDE and FC. The ORR mass activity correlates positively to the S_BET in the RRDE and FC because a higher S_BET can host more active sites. The H₂ O₂ selectivity is independent of f_micro in the RRDE but correlates negatively to f_micro in the FC. The inconsistency results from different states of the electrode in the RRDE and the FC. These insights will guide the design of carbon materials for H₂ O₂ synthesis.

Construction of a Microchannel Aeration Cathode for Producing H2O2 via Oxygen Reduction Reaction

Electrochemical oxygen reduction is a promising method for in situ H₂O₂ production. Its important precondition is that dissolved oxygen molecules have to diffuse to and arrive at the cathode surface for reacting with electrons. Obviously, shortening the diffusion distance is beneficial to improve the reaction efficiency. In this study, a novel microchannel aeration mode was proposed to confine the diffusion distance of O₂ to the micrometer level. For this mode, an aeration cathode was fabricated from a carbon block with microchannel arrays. The diameter of each channel was only 10-40 μm. Oxygen will be pumped into every microchannel from the top entry, while an aqueous solution will permeate into microchannels through the bottom entry and pores in the channel wall. This microchannel aeration cathode exhibited a H₂O₂ yield of up to 4.34 mg h^-1 cm^-2, about eight times higher than that of the common bubbling aeration mode. The corresponding energy consumption was only 7.35 kWh kg^-1, which was superior to most reported results. In addition to H₂O₂, this aeration cathode may also produce ^•OH via a one-electron reduction of H₂O₂. In combination with H₂O₂ and ^•OH, phenol, sulfamethoxazole, and atrazine were degraded effectively. We expect that this microchannel aeration cathode may inspire researchers focused on H₂O₂ production, water pollutant control, and other multiphase interfacial reactions.

Carbon-based metal-free electrocatalysts: from oxygen reduction to multifunctional electrocatalysis

Since the discovery of N-doped carbon nanotubes as the first carbon-based metal-free electrocatalyst (C-MFEC) for oxygen reduction reaction (ORR) in 2009, C-MFECs have shown multifunctional electrocatalytic activities for many reactions beyond ORR, such as oxygen evolution reaction (OER), hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO₂RR), nitrogen reduction reaction (NRR), and hydrogen peroxide production reaction (H₂O₂PR). Consequently, C-MFECs have attracted a great deal of interest for various applications, including metal-air batteries, water splitting devices, regenerative fuel cells, solar cells, fuel and chemical production, water purification, to mention a few. By altering the electronic configuration and/or modulating their spin angular momentum, both heteroatom(s) doping and structural defects (e.g., atomic vacancy, edge) have been demonstrated to create catalytic active sites in the skeleton of graphitic carbon materials. Although certain C-MFECs have been made to be comparable to or even better than their counterparts based on noble metals, transition metals and/or their hybrids, further research and development are necessary in order to translate C-MFECs for practical applications. In this article, we present a timely and comprehensive, but critical, review on recent advancements in the field of C-MFECs within the past five years or so by discussing various types of electrocatalytic reactions catalyzed by C-MFECs. An emphasis is given to potential applications of C-MFECs for energy conversion and storage. The structure-property relationship for and mechanistic understanding of C-MFECs will also be discussed, along with the current challenges and future perspectives.

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.

Efficient Electrochemical Production of H2O2 on Hollow N-Doped Carbon Nanospheres with Abundant Micropores

Electrocatalytic two-electron (2e-) oxygen reduction reaction (ORR) has been regarded as an efficient strategy to achieve onsite H2O2 generation under ambient conditions. However, due to the sluggish kinetics and competitive reaction between 2e- and 4e- ORR, exploring more efficient ORR catalysts with dominant 2e- ORR selectivity is of significance. Herein, hollow N-doped carbon spheres (HNCS) with abundant micropores through a template-directed method are presented. Consequently, the selectivity of the HNCS reaches ∼91.9% at 0.7 V (vs RHE), and the output for H2O2 production is up to 618.5 mmol gcatalyst-1 h-1 in 0.1 M KOH solution. The enhanced performance of HNCS for H2O2 electrosynthesis could be attributed to the hollow structure and heteroatom/defect/pore incorporation. The strategy presented here could shed light on the design of efficient carbon-based materials for improved 2e- ORR.

Electrosynthesized CuOx/graphene by a four-electrode electrolysis system for the oxygen reduction reaction to hydrogen peroxide

Here a facile four-electrode electrolysis system is firstly applied to synthesize a CuOx/graphene hybrid. The exfoliation of graphite via high electrolytic voltage and dissolution of copper via low electrolytic voltage are achieved simultaneously. CuOx/G with the highest content of CuOx shows superior electrocatalytic activity for oxygen reduction to hydrogen peroxide.

Nitrogen and oxygen tailoring of a solid carbon active site for two-electron selectivity electrocatalysis

The second nearest C atoms of pyridinic N were predicted to be an active site for 2e⁻ ORR using DFT calculations, and were experimentally demonstrated to possess a tailoring function of a pyridinic N structure and O dopants on carbon materials.

2D Electrocatalysts for Converting Earth-Abundant Simple Molecules into Value-Added Commodity Chemicals: Recent Progress and Perspectives

The electrocatalytic conversion of earth-abundant simple molecules into value-added commodity chemicals can transform current chemical production regimes with enormous socioeconomic and environmental benefits. For these applications, 2D electrocatalysts have emerged as a new class of high-performance electrocatalyst with massive forward-looking potential. Recent advances in 2D electrocatalysts are reviewed for emerging applications that utilize naturally existing H₂ O, N₂ , O₂ , Cl^- (seawater) and CH₄ (natural gas) as reactants for nitrogen reduction (N₂ → NH₃ ), two-electron oxygen reduction (O₂ → H₂ O₂ ), chlorine evolution (Cl^- → Cl₂ ), and methane partial oxidation (CH₄ → CH₃ OH) reactions to generate NH₃ , H₂ O₂ , Cl₂ , and CH₃ OH. The unique 2D features and effective approaches that take advantage of such features to create high-performance 2D electrocatalysts are articulated with emphasis. To benefit the readers and expedite future progress, the challenges facing the future development of 2D electrocatalysts for each of the above reactions and the related perspectives are provided.

Graphene-Supported Single Nickel Atom Catalyst for Highly Selective and Efficient Hydrogen Peroxide Production

Hydrogen peroxide (H₂O₂) production by electrocatalytic two-electron oxygen reduction shows promise as a replacement for energy-intensive anthraquinone oxidation or H₂/O₂ direct synthesis. Here, we report on graphene-supported Ni single-atom (SA) electrocatalysts, which are synthesized by a simple surfactant-free reduction process with enhanced electrocatalytic activity and stability. Unlike conventional Ni nanoparticles or alloy catalysts, the well-dispersed Ni-SA sites lack adjacent Ni atoms. This structure promotes H₂O₂ production by a two-electron oxygen reduction pathway under an alkaline condition (pH = 13). This catalyst exhibited enhanced H₂O₂ selectivity (>94%) with a considerable mass activity (2.11 A mg_Ni^-1 at 0.60 V vs reversible hydrogen electrode), owing to the presence of oxygen functional groups and isolated Ni sites. Density functional theory calculations provide insights into the role of this catalyst in optimizing the two-electron oxygen reduction reaction pathway with high H₂O₂ selectivity. This work suggests a new method for controlling reaction pathways in atomically dispersed non-noble catalysts.

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.

Oxidase-Inspired Selective 2e/4e Reduction of Oxygen on Electron-Deficient Cu

Development of low-cost and efficient (electro)catalysts with tunable 2e/4e oxygen reduction reaction (ORR) selectivity toward energy conversion, biomimetic catalysis, and biosensing has attracted growing interest. Herein, we reported that carbon nanohybrids with O- or N-coordinated Cu (Cu-OC or Cu-NC) showed superior activity for 2e and 4e electrocatalytic ORR with selectivities of 84.0% and 97.2%, respectively. Experimental evidence demonstrated that the strong electron-rich O-doped carbon in Cu-OC donated electrons to Cu²⁺, weakening the binding strength of H₂O₂ at Cu-O centers and facilitating the 2e ORR pathway for selective production of H₂O₂. However, the poor electron-donor ability of the N-doped carbon in Cu-NC made Cu-N sites more electron deficient due to the reduced electron transfer from N-doped carbon to Cu²⁺, promoting 4e ORR by enhancing adsorption of O₂ and the ORR intermediates. The high 4e ORR activity of Cu-NC rendered its potential for application in a Zn-air battery and oxidase-mimicking activity for 3,3',5,5'-tetramethylbenzidine (TMB) and ascorbic acid (AA) oxidation. The maximal velocity (V_max) of TMB and AA oxidation over Cu-NC was higher than some natural oxidases and noble-metal-based artificial enzymes. The lower activation energy for AA oxidation over Cu-NC resulted in a 263-fold higher oxidative rate than TMB, further prompting nonenzymatic sensing of AA by the competitive oxidation strategy.

Two-electron oxygen reduction on fullerene C60-carbon nanotubes covalent hybrid as a metal-free electrocatalyst

Nanocarbon materials are considered to be active for electrochemical oxygen reduction reaction (ORR) for hydrogen peroxide (H₂O₂) synthesis. In the present work, a new type of fullerene 60 (C₆₀)-carbon nanotubes (CNTs) hybrid with covalently attached C₆₀ onto outer surface of CNTs was synthesized. The structure of C₆₀-CNT hybrid was confirmed by physical and chemical characterizations and its conformation is proposed featuring the covalent incorporation of CNTs and C₆₀ derivative. C₆₀-CNT hybrid showed high efficiencies on electro-generating H₂O₂, owing to huge surface area and intermolecular electron-transfer in the hybrid structure. A high H₂O₂ production rate of 4834.57 mg L⁻¹ h⁻¹ (426.58 mmol L⁻¹) was achieved at − 0.2 V vs saturated calomel electrode (SCE).