Adsorbed Intermediates in Oxygen Reduction on Platinum Nanoparticles Observed by In Situ IR Spectroscopy

Ultrafast Charge Transfer on Ru-Cu Atomic Units for Enhanced Photocatalytic H2O2 Production

Photosensitizer-assisted photocatalytic systems offer a solution to overcome the limitations of inherent light harvesting capabilities in catalysts. However, achieving efficient charge transfer between the dissociative photosensitizer and catalyst poses a significant challenge. Incorporating photosensitive components into reactive centers to establish well-defined charge transfer channels is expected to effectively address this issue. Herein, the electrostatic-driven self-assembly method is utilized to integrate photosensitizers into metal-organic frameworks, constructing atomically Ru-Cu bi-functional units to promote efficient local electron migration. Within this newly constructed system, the [Ru(bpy)2]2+ component and Cu site serve as photosensitive and catalytic active centers for photocarrier generation and H2O2 production, respectively, and their integration significantly reduces the barriers to charge transfer. Ultrafast spectroscopy and in situ characterization unveil accelerated directional charge transfer over Ru-Cu units, presenting orders of magnitude improvement over dissociative photosensitizer systems. As a result, a 37.2-fold enhancement of the H2O2 generation rate (570.9 µmol g-1 h-1) over that of dissociative photosensitizer system (15.3 µmol g-1 h-1) is achieved. This work presents a promising strategy for integrating atomic-scale photosensitive and catalytic active centers to achieve ultrafast photocarrier transfer and enhanced photocatalytic performance.

O-O Radical Coupling in Ultrathin Reconstructed Co6.8Se8 Nanosheets for Effective Oxygen Evolution and Zinc-Air Batteries

Designing ultrathin transition metal electrocatalysts with optimal surface chemistry state is crucial for oxygen evolution reaction (OER). However, the structure-dependent electrochemical performance and the underlying catalytic mechanisms are still not clearly distinguished. Herein, we synthesize ultrathin Co6.8Se8 nanosheets (NSs) with subnanometer thickness by incorporating catalytically inactive selenium (Se) into ultrathin Co(OH)2, thereby switching the OER reaction pathway from adsorbate evolution mechanism (AEM) to oxide path mechanism (OPM). The prepared ultrathin Co6.8Se8 NSs exhibit an overpotential of 253 mV at 10 mA/cm2, outperforming the mostly reported Co-based electrocatalysts. Advanced operando synchrotron spectroscopies and X-ray absorption spectroscopy reveal the ultrathin Co6.8Se8 NSs, whose surface is reconstructed into Se-doped Co(OH)2 during the OER process, could trigger direct O*-O* radical coupling rather than OOH* intermediates within AEM pathway thus lowering the energy input. Density functional theory calculations confirm that Co6.8Se8 NSs with shorter Co-Co bond length and stable Co-Se bond could optimize the rate-determining step barrier via OPM pathway. Besides, rechargeable zinc-air batteries based on Co6.8Se8 NSs exhibit excellent stability for more than 500 h of continuous charge-discharge cycles at 4 mA/cm2. The present study highlights the structural-dependent switch of OER pathways and provides valuable insights for further development of ultrathin OER catalysts.

Activating the Mn Single Atomic Center for an Efficient Actual Active Site of the Oxygen Reduction Reaction by Spin-State Regulation

The ligand engineering for single-atom catalysts (SACs) is considered a cutting-edge strategy to tailor their electrocatalytic activity. However, the fundamental reasons underlying the reaction mechanism and the contemplation for which the actual active site for the catalytic reaction depends on the pyrrolic and pyridinic N ligand structure remain to be fully understood. Herein, we first reveal the relationship between the oxygen reduction reaction (ORR) activity and the N ligand structure for the manganese (Mn) single atomic site by the precisely regulated pyrrolic and pyridinic N4 coordination environment. Experimental and theoretical analyses reveal that the long Mn-N distance in Mn-pyrrolic N4 enables a high spin state of the Mn center, which is beneficial to reduce the adsorption strength of oxygen intermediates by the high filling state in antibond orbitals, thereby activating the Mn single atomic site to achieve a half-wave potential of 0.896 V vs RHE with outstanding stability in acidic media. This work provides a new fundamental insight into understanding the ORR catalytic origin of Mn SACs and the rational design strategy of SACs for various electrocatalytic reactions.

Ultra-Rapid Electrocatalytic H2O2 Fabrication over Mono-Species and High-Density Polypyrrolic-N Sites

Electrocatalytic hydrogen peroxide (H2O2) production via two-electron oxygen reduction reaction (2e--ORR) features energy-saving and eco-friendly characteristics, making it a promising alternative to the anthraquinone oxidation process. However, the common existence of numerous 2e--ORR-inactive sites/species on electrocatalysts tends to catalyze side reactions, especially under low potentials, which compromises energy efficiency and limits H2O2 yield. Addressing this, a high surface density of mono-species pyrrolic nitrogen configurations is formed over a polypyrrole@carbon nanotube composite. Thermodynamic and kinetic calculation and experimental investigation collaboratively confirm that these densely distributed and highly selective active sites effectively promote high-rate 2e--ORR electrocatalysis and inhibit side reactions over a wide potential range. Consequently, an ultra-high and stable H2O2 yield of up to 67.9/51.2 mol g-1 h-1 has been achieved on this material at a current density of 200/120 mA cm-1, corresponding Faradaic efficiency of 72.8/91.5%. A maximum H2O2 concentration of 13.47 g L-1 can be accumulated at a current density of 80 mA cm-1 with satisfactory stability. The strategy of surface active site densification thus provides a promising and universal avenue toward designing highly active and efficient electrocatalysts for 2e--ORR as well as a series of other similar electrochemical processes.

Merging semi-crystallization and multispecies iodine intercalation at photo-redox interfaces for dual high-value synthesis

The artificial photocatalytic synthesis based on graphitic carbon nitride (g-C3N4) for H2O2 production is evolving rapidly. However, the simultaneous production of high-value products at electron and hole sites remains a great challenge. Here, we use transformable potassium iodide to obtain semi-crystalline g-C3N4 integrated with the I-/I3- redox shuttle mediators for efficient generation of H2O2 and benzaldehyde. The system demonstrates a prominent catalytic efficiency, with a benzaldehyde yield of 0.78 mol g-1 h-1 and an H2O2 yield of 62.52 mmol g-1 h-1. Such a constructed system can achieve an impressive 96.25% catalytic selectivity for 2e- oxygen reduction, surpassing previously reported systems. The mechanism study reveals that the strong crystal electric field from iodized salt enhances photo-generated charge carrier separation. The I-/I3- redox mediators significantly boost charge migration and continuous electron and proton supply for dual-channel catalytic synthesis. This groundbreaking work in photocatalytic co-production opens neoteric avenues for high-value synthesis.

Improvement in ORR Durability of Fe Single-Atom Carbon Catalysts Hybridized with CeO2 Nanozyme

FeNC catalysts are considered one of the most promising alternatives to platinum group metals for the oxygen reduction reaction (ORR). Despite the extensive research on improving ORR activity, the undesirable durability of FeNC is still a critical issue for its practical application. Herein, inspired by the antioxidant mechanism of natural enzymes, CeO2 nanozymes featuring catalase-like and superoxide dismutase-like activities were coupled with FeNC to mitigate the attack of reactive oxygen species (ROS) for improving durability. Benefiting from the multienzyme-like activities of CeO2, ROS generated from FeNC is instantaneously eliminated to alleviate the corrosion of carbon and demetallization of metal sites. Consequently, FeNC/CeO2 exhibits better ORR durability with a decay of only 5 mV compared to FeNC (18 mV) in neutral electrolyte after 10k cycles. The FeNC/CeO2-based zinc-air battery also shows minimal voltage decay over 140 h in galvanostatic discharge-charge cycling tests, outperforming FeNC and commercial Pt/C.

Defect Engineering for Enhanced Electrocatalytic Oxygen Reaction on Transition Metal Oxides: The Role of Metal Defects

Metal defect engineering is a highly effective strategy for addressing the prevalent high overpotential issues associated with transition metal oxides functioning as dual-function commercial oxygen reduction reaction/oxygen evolution reaction catalysts for increasing their activity and stability. However, the high formation energy of metal defects poses a challenge to the development of strategies to precisely control the selectivity during metal defect formation. Here, density functional theory calculations are used to demonstrate that altering the pathway of metal defect formation releases metal atoms as metal chlorides, which effectively reduces the formation energy of defects. The metal defects on the monometallic metal oxide surface (Mn, Fe, Co, and Ni) are selectively produced using chlorine plasma. The characterization and density functional theory calculations reveal that catalytic activity is enhanced owing to electronic delocalization induced by metal defects, which reduces the theoretical overpotential. Notably, ab initio molecular dynamics calculations, ex situ XPS, and in situ ATR-SEIRAS suggest that metal defects effectively improve the adsorption of reactive species on active sites and enhance the efficiency of product desorption, thereby boosting catalytic performance.

Role of Transition Metals in Pt Alloy Catalysts for the Oxygen Reduction Reaction

In pursuit of higher activity and stability of electrocatalysts toward the oxygen reduction reaction, it has become standard practice to alloy platinum in various structural configurations. Transition metals have been extensively studied for their ability to tune catalyst functionality through strain, ligand, and ensemble effects. The origin of these effects and potential for synergistic application in practical materials have been the subject of many theoretical and experimental analyses in recent years. Here, a comprehensive overview of these phenomena is provided regarding the impact on reaction mechanisms and kinetics through combined experimental and theoretical approaches. Experimental approaches to electrocatalysis are discussed.

Unveiling the dynamic active site of defective carbon-based electrocatalysts for hydrogen peroxide production

Active sites identification in metal-free carbon materials is crucial for developing practical electrocatalysts, but resolving precise configuration of active site remains a challenge because of the elusive dynamic structural evolution process during reactions. Here, we reveal the dynamic active site identification process of oxygen modified defective graphene. First, the defect density and types of oxygen groups were precisely manipulated on graphene, combined with electrocatalytic performance evaluation, revealing a previously overlooked positive correlation relationship between the defect density and the 2 e- oxygen reduction performance. An electrocatalytic-driven oxygen groups redistribution phenomenon was observed, which narrows the scope of potential configurations of the active site. The dynamic evolution processes are monitored via multiple in-situ technologies and theoretical spectra simulations, resolving the configuration of major active sites (carbonyl on pentagon defect) and key intermediates (*OOH), in-depth understanding the catalytic mechanism and providing a research paradigm for metal-free carbon materials.

Understanding Synergistic Catalysis on Cu-Se Dual Atom Sites via Operando X-ray Absorption Spectroscopy in Oxygen Reduction Reaction

The construction and understanding of synergy in well-defined dual-atom active sites is an available avenue to promote multistep tandem catalytic reactions. Herein, we construct a dual-hetero-atom catalyst that comprises adjacent Cu-N₄ and Se-C₃ active sites for efficient oxygen reduction reaction (ORR) activity. Operando X-ray absorption spectroscopy coupled with theoretical calculations provide in-depth insights into this dual-atom synergy mechanism for ORR under realistic device operation conditions. The heteroatom Se modulator can efficiently polarize the charge distribution around symmetrical Cu-N₄ moieties, and serve as synergistic site to facilitate the second oxygen reduction step simultaneously, in which the key OOH*-(Cu₁ -N₄ ) transforms to O*-(Se₁ -C₂ ) intermediate on the dual-atom sites. Therefore, this designed catalyst achieves satisfied alkaline ORR activity with a half-wave potential of 0.905 V vs. RHE and a maximum power density of 206.5 mW cm^-2 in Zn-air battery.

Molecular Level Modulation of Anthraquinone-containing Resorcinol-formaldehyde Resin Photocatalysts for H2 O2 Production with Exceeding 1.2 % Efficiency

Designing polymeric photocatalysts at the molecular level to modulate the photogenerated charge behavior is a promising and challenging strategy for efficient hydrogen peroxide (H₂ O₂ ) photosynthesis. Here, we introduce electron-deficient 1,4-dihydroxyanthraquinone (DHAQ) into the framework of resorcinol-formaldehyde (RF) resin, which modulates the donor/acceptor ratio from the perspective of molecular design for promoting the charge separation. Interestingly, H₂ O₂ can be produced via oxygen reduction and water oxidation pathways, verified by isotopic labeling and in situ characterization techniques. Density functional theory (DFT) calculations elucidate that DHAQ can reduce the energy barrier for H₂ O₂ production. RF-DHAQ exhibits excellent overall photosynthesis of H₂ O₂ with a solar-to-chemical conversion (SCC) efficiency exceeding 1.2 %. This work opens a new avenue to design polymeric photocatalysts at the molecular level for high-efficiency artificial photosynthesis.

Surface Activation and Ni-S Stabilization in NiO/NiS2 for Efficient Oxygen Evolution Reaction

Manipulating the active species and improving the structural stabilization of sulfur-containing catalysts during the OER process remain a tremendous challenge. Herein, we constructed NiO/NiS₂ and Fe-NiO/NiS₂ as catalyst models to study the effect of Fe doping. As expected, Fe-NiO/NiS₂ exhibits a low overpotential of 270 mV at 10 mA cm^-2 . The accumulation of hydroxyl groups on the surface of materials after Fe doping can promote the formation of highly active NiOOH at a lower OER potential. Moreover, we investigated the level of corrosion of M-S bonds and compared the stability variation of M-S bonds with Fe at different locations. Interestingly, Fe bonded with S in the bulk as the sacrificial agent can alleviate the oxidation corrosion of partial Ni-S bonds and thus endow Fe-NiO/NiS₂ long-term durability. This work could motivate the community to focus more on resolving the corrosion of sulfur-containing materials.

In situ Spectroscopic Insight into the Origin of the Enhanced Performance of Bimetallic Nanocatalysts towards the Oxygen Reduction Reaction (ORR)

It is vital to understand the oxygen reduction reaction (ORR) mechanism at the molecular level for the rational design and synthesis of high activity fuel-cell catalysts. Surface enhanced Raman spectroscopy (SERS) is a powerful technique capable of detecting the bond vibrations of surface species in the low wavenumber range, however, using it to probe practical nanocatalysts remains extremely challenging. Herein, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was used to investigate ORR processes on the surface of bimetallic Pt₃ Co nanocatalyst structures. Direct spectroscopic evidence of *OOH suggests that ORR undergoes an associative mechanism on Pt₃ Co in both acidic and basic environments. Density functional theory (DFT) calculations show that the weak *O adsorption arise from electronic effect on the Pt₃ Co surface accounts for enhanced ORR activity. This work shows SHINERS is a promising technique for the real-time observation of catalytic processes.