Versatile Strategy for Tuning ORR Activity of a Single Fe-N4 Site by Controlling Electron-Withdrawing/Donating Properties of a Carbon Plane

Dimensionality Engineering of Single-Atom Nanozyme for Efficient Peroxidase-Mimicking

In nature, enzymatic reactions occur in well-functioning catalytic pockets, where substrates bind and react by properly arranging the catalytic sites and amino acids in a three-dimensional (3D) space. Single-atom nanozymes (SAzymes) are a new type of nanozymes with active sites similar to those of natural metalloenzymes. However, the catalytic centers in current SAzymes are two-dimensional (2D) architectures and the lack of collaborative substrate-binding features limits their catalytic activity. Herein, we report a dimensionality engineering strategy to convert conventional 2D Fe-N-4 centers into 3D structures by integrating oxidized sulfur functionalities onto the carbon plane. Our results suggest that oxidized sulfur functionalities could serve as binding sites for assisting substrate orientation and facilitating the desorption of H₂O, resulting in an outstanding specific activity of up to 119.77 U mg^-1, which is 6.8 times higher than that of conventional FeN₄C SAzymes. This study paves the way for the rational design of highly active single-atom nanozymes.

Iron/iron carbide coupled with S, N co-doped porous carbon as effective oxygen reduction reaction catalyst for microbial fuel cells

As a novel energy device, microbial fuel cells (MFCs) have attracted much attention for their dual functions of electricity generation and sewage treatment. However, the sluggish oxygen reduction reaction (ORR) kinetic on the cathode have hindered the practical application of MFCs. In this work, metallic organic framework derived carbon framework co-doped by Fe, S, N tri-elements was used as alternative electrocatalyst to the conventional Pt/C cathode catalyst in pH-universal electrolytes. The amount of thiosemicarbazide from 0.3 to 3 g determined the surface chemical property, and therefore the ORR activity of FeSNC catalysts. The sulfur/nitrogen doping and Fe/Fe₃C embedded in carbon shell was characterized by X-ray photoelectron spectroscopy and transmission electron microscopy. The synergy of iron salt and thiosemicarbazide contributed to the improvement of nitrogen and sulfur doping. Sulfur atoms were successfully doped into the carbon matrix and formed a certain amount of thiophene- and oxidized-sulfur. The optimal FeSNC-3 catalyst synthesized with 1.5 g of thiosemicarbazide exhibited the highest ORR activity with a positive half wave potential of 0.866 V in alkaline and 0.691 V (vs. Reversible Hydrogen Electrode) in neutral electrolyte, which both outperformed the commercial Pt/C catalyst. However, as the amount of thiosemicarbazide surpassed 1.5 g, the catalytic performance of FeSNC-4 was lowered, and this could be assigned to the decreased defects and low specific surface area. The excellent ORR performance in neutral medium urged FeSNC-3 as good cathode catalyst in single chambered MFC (SCMFC). It showed the highest maximum power density of 2126 ± 100 mW m^-2, excellent output stability of 8.14% decline in 550 h, chemical oxygen demand removal of 90.7 ± 1.6% and coulombic efficiency of 12.5 ± 1.1%, all superior to those of benchmark SCMFC-Pt/C (1637 ± 35 mW m^-2, 15.4%, 88.9 ± 0.9%, and 10.2 ± 1.1%). These outstanding results were associated to the large specific surface area and synergistic interaction of multiple active sites, like Fe/Fe₃C, Fe-N₄, pyridinic N, graphite N and thiophene-S.

Exploration of spatial confinement and ligand effects for the oxygen reduction reaction on Fe-Nx embedded hole-graphene

In this work, we constructed theoretical models by embedding Fe-TCPP and Fe-(mIM)_n (n = 2,3,4) active sites into hole-graphene, and the structural stability was evaluated using molecular dynamics simulations. Based on the theoretical models, we systematically studied the oxygen reduction reaction (ORR) mechanism and the effect of spatial confinement and ligands with DFT calculations. The analysis of the ORR reaction pathway shows that Fe-TCPP and Fe-(mIM)₄ have good catalytic activity. Subsequently, the confinement effect (5-14 Å) was introduced to investigate its influence on the catalytic activity. The Fe-TCPP and Fe-(mIM)₄ active sites have the lowest overpotential at an axial space of 8 Å and 9 Å, respectively. We select four ligands (bpy, pya, CH₃, and bIm) to explore their effect on the catalytic activity of the Fe-TCPP active site. With the modification of bpy, pya, and bIm_N (Fe-N₄ sites become Fe-N₅ active sites), the overpotential decreases by 26-31%. In the present work, the best catalytic system is Fe-TCPP_pya, which is on the top of the volcano plot.

Electrochemical Oxidation Encapsulated Ru Clusters Enable Robust Durability for Efficient Oxygen Evolution

Electrochemical oxidization and thermodynamic instability agglomeration are a primary challenge in triggering metal-support interactions (MSIs) by immobilizing metal atoms on a carrier to achieve efficient oxygen evolution reactions (OER). Herein, Ru clusters anchored to the VS₂ surface and the VS₂ nanosheets embedded vertically in carbon cloth (Ru-VS₂ @CC) are deliberately designed to realize high reactivity and exceptional durability. In situ Raman spectroscopy reveals that the Ru clusters are preferentially electro-oxidized to form RuO₂ chainmail, both affording sufficient catalytic sites and protecting the internal Ru core with VS₂ substrates for consistent MSIs. Theoretical calculations elucidate that electrons across the Ru/VS₂ interface aggregate toward the electro-oxidized Ru clusters, while the electronic coupling of Ru 3p and O 2p orbitals boosts a positive shift in the Fermi energy level of Ru, optimizing the adsorption capacity of the intermediates and diminishing the migration barriers of the rate-determining steps. Therefore, the Ru-VS₂ @CC catalyst demonstrated ultra-low overpotentials of 245 mV at 50 mA cm^-2 , while the zinc-air battery maintained a narrow gap (0.62 V) after 470 h of reversible operation. This work has transformed the corrupt into the miraculous and paved a new way for the development of efficient electrocatalysts.

Rational Design of Atomically Dispersed Metal Site Electrocatalysts for Oxygen Reduction Reaction

Future renewable energy supply and a cleaner Earth greatly depend on various crucial catalytic reactions for the society. Atomically dispersed metal site electrocatalysts (ADMSEs) have attracted tremendous research interest and are considered as the next-generation promising oxygen reduction reaction (ORR) electrocatalysts due to the maximum atom utilization efficiency, tailorable catalytic sites, and tunable electronic structures. Despite great efforts have been devoted to the development of ADMSEs, the systematic summary for design principles of high-efficiency ADMSEs is not sufficiently highlighted for ORR. In this review, the authors first summarize the fundamental ORR mechanisms for ADMSEs, and further discuss the intrinsic catalytic mechanism from the perspective of theoretical calculation. Then, the advanced characterization techniques to identify the active sites and effective synthesis methods to prepare catalysts for ADMSEs are also showcased. Subsequently, a special emphasis is placed on effective strategies for the rational design of the advanced ADMSEs. Finally, the present challenges to be addressed in practical application and future research directions are also proposed to overcome the relevant obstacles for developing high-efficiency ORR electrocatalysts. This review aims to provide a deeper understanding for catalytic mechanisms and valuable design principles to obtain the advanced ADMSEs for sustainable energy conversion and storage techniques.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Optimizing d-Orbital Electronic Configuration via Metal-Metal Oxide Core-Shell Charge Donation for Boosting Reversible Oxygen Electrocatalysis

Tuning the d-orbital electronic configuration of active sites to achieve well-optimized adsorption strength of oxygen-containing intermediates toward reversible oxygen electrocatalysis is desirable for efficient rechargeable Zn-Air batteries but extremely challenging. Herein, this work proposes to construct a Co@Co₃ O₄ core-shell structure to regulate the d-orbital electronic configuration of Co₃ O₄ for the enhanced bifunctional oxygen electrocatalysis. Theoretical calculations first evidence that electron donation from Co core to Co₃ O₄ shell could downshift the d-band center and simultaneously weak spin state of Co₃ O₄ , result in the well-optimized adsorption strength of oxygen-containing intermediates on Co₃ O₄ , thus contributing a favor way for oxygen reduction/evolution reaction (ORR/OER) bifunctional catalysis. As a proof-of-concept, the Co@Co₃ O₄ embedded in Co, N co-doped porous carbon derived from thickness controlled 2D metal-organic-framework is designed to realize the structure of computational prediction and further improve the performance. The optimized 15Co@Co₃ O₄ /PNC catalyst exhibits the superior bifunctional oxygen electrocatalytic activity with a small potential gap of 0.69 V and a peak power density of 158.5 mW cm^-2 in ZABs. Moreover, DFT calculations shows that the more oxygen vacancies on Co₃ O₄ contribute too strong adsorption of oxygen intermediates which limit the bifunctional electrocatalysis, while electron donation in the core-shell structure can alleviate the negative effect and maintain superior bifunctional overpotential.

Challenges and Opportunities in Engineering the Electronic Structure of Single-Atom Catalysts

Controlling the electronic structure of transition-metal single-atom heterogeneous catalysts (SACs) is crucial to unlocking their full potential. The ability to do this with increasing precision offers a rational strategy to optimize processes associated with the adsorption and activation of reactive intermediates, charge transfer dynamics, and light absorption. While several methods have been proposed to alter the electronic characteristics of SACs, such as the oxidation state, band structure, orbital occupancy, and associated spin, the lack of a systematic approach to their application makes it difficult to control their effects. In this Perspective, we examine how the electronic configuration of SACs can be engineered for thermochemical, electrochemical, and photochemical applications, exploring the relationship with their activity, selectivity, and stability. We discuss synthetic and analytical challenges in controlling and discriminating the electronic structure of SACs and possible directions toward closing the gap between computational and experimental efforts. By bringing this topic to the center, we hope to stimulate research to understand, control, and exploit electronic effects in SACs and ultimately spur technological developments.

Conductivity-enhanced porous N/P co-doped metal-free carbon significantly enhances oxygen reduction kinetics for aqueous/flexible zinc-air batteries

Heteroatom-doped metal-free carbon catalysts for oxygen reduction reactions have gained significant attention because of their unusual activity and economic cost. Here, a novel N/P co-doped porous carbon catalyst (NPPC) with a high surface area for oxygen reduction reaction (ORR) is constructed by a facile high-temperature calcination method employing ZIF-8 as the precursor and red phosphorus as the phosphorus source. In particular, ZIF-8 is firstly calcined to obtain N-doped carbon (NC) followed by further calcination with red phosphorus to obtain NPPC. Ultraviolet photoelectron spectroscopy (UPS) analysis shows that the ultra-low amount of P doping could significantly decrease the work function from 4.32 to 3.86 eV. The resultant catalyst exhibits a promising electrocatalytic activity with a half-wave potential (E_1/2) of 0.87 V and a limiting current density (J_L) of 5.15 mA cm^-2. Besides, it also shows improved catalytic efficiency and excellent durability with a negligible decay of J_L after 2000 CV cycles. Moreover, aqueous and solid-state flexible zinc-air batteries (ZAB) using the catalyst show a promising application potential. This work provides new insight into developing P/N-doped metal-free carbon ORR catalysts.

Cooperative Electronic Structure Modulator of Fe Single-Atom Electrocatalyst for High Energy and Long Cycle Li-S Pouch Cell

High-energy and long cycle lithium-sulfur (Li-S) pouch cells are limited by the insufficient capacities and stabilities of their cathodes under practical electrolyte/sulfur (E/S), electrolyte/capacity (E/C), and negative/positive (N/P) ratios. Herein, an advanced cathode comprising highly active Fe single-atom catalysts (SACs) is reported to form 320.2 W h kg^-1 multistacked Li-S pouch cells with total capacity of ≈1 A h level, satisfying low E/S (3.0), E/C (2.8), and N/P (2.3) ratios and high sulfur loadings (8.4 mg cm^-2 ). The high-activity Fe SAC is designed by manipulating its local environments using electron-exchangeable binding (EEB) sites. Introducing EEB sites comprising two different types of S species, namely, thiophene-like-S (-S) and oxidized-S (-SO₂ ), adjacent to Fe SACs promotes the kinetics of the Li₂ S redox reaction by providing additional binding sites and modulating the Fe d-orbital levels via electron exchange with Fe. The -S donates the electrons to the Fe SACs, whereas -SO₂ withdraws electrons from the Fe SACs. Thus, the Fe d-orbital energy level can be modulated by the different -SO₂ /-S ratios of the EEB site, controlling the electron donating/withdrawing characteristics. This desirable electrocatalysis is maximized by the intimate contact of the Fe SACs with the S species, which are confined together in porous carbon.

Engineering Energy Level of FeN4 Sites via Dual-Atom Site Construction Toward Efficient Oxygen Reduction

Single-atom catalysts based on metal-N₄ moieties and embedded in a graphite matrix (defined as MNC) are promising for oxygen reduction reaction (ORR). However, the performance of MNC catalysts is still far from satisfactory due to their imperfect adsorption energy to oxygen species. Herein, single-atom FeNC is leveraged as a model system and report an adjacent Ru-N₄ moiety modulation effect to optimize the catalyst's electronic configuration and ORR performance. Theoretical simulations and physical characterizations reveal that the incorporation of Ru-N₄ sites as the modulator can alter the d-band electronic energy of Fe center to weaken the FeO binding affinity, thus resulting in the lower adsorption energy of ORR intermediates at Fe sites. Thanks to the synergetic effects of neighboring Fe and Ru single-atom pairs, the FeN₄ /RuN₄ catalyst exhibits a half-wave potential of 0.958 V and negligible activity degradation after 10 000 cycles in 0.1 m KOH. Metal-air batteries using this catalyst in the cathode side exhibit a high power density of 219.5 mW cm^-2 and excellent cycling stability for over 2370 h, outperforming the state-of-the-art catalysts.

Hierarchical meso-micro porous FeNC derived from tripolycyanamide-based microporous polymer as efficient electrocatalyst for oxygen reduction reaction

Designing porous FeNC nanomaterials with highly efficient active sites is an effective strategy for constructing high-performance oxygen reduction reaction (ORR) electrocatalysts. N-containing porous organic polymers (POPs) have emerged as promising candidates for the preparation of porous FeNC catalysts. Here, N-rich tripolycyanamide-based microporous polymer (TCAMP)-coated SiO₂ nanospheres (SiO₂@TACMP) were prepared as the precursors of an Fe-N doped hierarchical meso-micro porous carbon (Fe-N-HMC) electrocatalyst for the ORR. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) characterizations demonstrated that the Fe-N-HMC catalyst possessed a higher content percentage of Fe-N_x active sites and a better distribution of Fe nanoparticles than its Fe-N doped microporous carbon (Fe-N-MC) counterpart. N₂ adsorption-desorption isotherm analysis showed that Fe-N-HMC catalyst exhibited a hierarchical meso-micro porous system, with a Brunauer-Emmett-Teller (BET) surface area (S_BET) of 733 m² g^-1 (∼2 times of Fe-N-MC's S_BET). As a result, Fe-N-HMC catalyst presented a highly efficient ORR performance with a half-wave potential of 0.856 mV, which is similar to the commercial grade 20 wt% Pt/C catalyst and superior to the Fe-N-MC catalyst. Moreover, the as-synthesized Fe-N-HMC catalyst displayed a better durability and methanol tolerance than the commercial Pt/C catalyst. Therefore, Fe-N-HMC shows great promise as an ORR catalyst for fuel batteries and metal-air cells due to its low-cost, high activity, and good stability.

Insight into oxygen reduction activity and pathway on pure titanium using scanning electrochemical microscopy and theoretical calculations

HYPOTHESIS

Unlike noble metals, the oxygen reduction reaction (ORR) behavior on Ti is more complicated due to its spontaneously formed oxide film. This film results in sluggish ORR kinetics and tends to be reduced within ORR potential region, causing the weak and multi-reaction coupled current. Though Ti is being used in chemical and biological fields, its ORR research is still underexplored.

EXPERIMENTS

We innovatively employed the modified reactive tip generation-substrate collection (RTG/SC) mode of scanning electrochemical microscopy (SECM) with high efficiency of 97.2 % to quantitatively study the effects of film characteristics, solution environment (pH, anion, dissolved oxygen), and applied potential on the ORR activity and selectivity of Ti. Then, density functional theory (DFT) and molecular dynamics (MD) analyses were employed to elucidate its ORR behavior.

FINDINGS

On highly reduced Ti, film properties dominate ORR behavior with promoted 4e^- selectivity. Rapid film regeneration in alkaline/O₂-saturated conditions inhibits ORR activity. Besides, ORR is sensitive to anion species in neutral solutions while showing enhanced 4e^- reduction in alkaline media. All the improved 4e^- selectivities originate from the hydrogen bond/electrostatic stabilization effect, while the decayed ORR activity by Cl^- arises from the suppressed O₂ adsorption. This work provides theoretical support and possible guidance for ORR research on oxide-covered metals.

Metallic Metastable Hybrid 1T'/1T Phase Triggered Co,PSnS2 Nanosheets for High Efficiency Trifunctional Electrocatalyst

The development of trifunctional electrocatalyst for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) with deeply understanding the mechanism to enhance the electrochemical performance is still a challenging task. In this work, the distorted metastable hybrid-phase induced 1T'/1T Co,PSnS₂ nanosheets on carbon cloth (1T'/1T Co,PSnS₂ @CC) is prepared and examined. The density functional theoretical (DFT) calculation suggests that the distorted 1T'/1T Co,PSnS₂ can provide excellent conductivity and strong hydrogen adsorption ability. The electronic structure tuning and enhancement mechanism of electrochemical performance are investigated and discussed. The optimal 1T'/1T Co,PSnS₂ @CC catalyst exhibits low overpotential of ≈94 and 219.7 mV at 10 mA cm^-2 for HER and OER, respectively. Remarkably, the catalyst exhibits exceptional ORR activity with small onset potential value (≈0.94 V) and half-wave potential (≈0.87 V). Most significantly, the 1T'/1T Co,PSnS₂ ||Co,PSnS₂ electrolyzer required small cell voltages of ≈1.53, 1.70, and 1.82 V at 10, 100, and 400 mA cm^-2 , respectively, which are better than those of state-of-the-art Pt-C||RuO₂ (≈1.56 and 1.84 V at 10 and 100 mA cm^-2 ). The present study suggests a new approach for the preparation of large-scalable, high performance hierarchical 3D next-generation trifunctional electrocatalysts.

Role of the Support Effects in Single-Atom Catalysts

In recent years, single-atom catalysts (SACs) have received a significant amount of attention due to their high atomic utilization, low cost, high reaction activity, and selectivity for multiple catalytic reactions. Unfortunately, the high surface free energy of single atoms leads them easily migrated and aggregated. Therefore, support materials play an important role in the preparation and catalytic performance of SACs. Aiming at understanding the relationship between support materials and the catalytic performance of SACs, the support effects in SACs are introduced and reviewed herein. Moreover, special emphasis is placed on exploring the influence of the type and structure of supports on SAC catalytic performance through advanced characterization and theoretical research. Future research directions for support materials are also proposed, providing some insight into the design of SACs with high efficiency and high loading.

Integrating anodic sulfate activation with cathodic H2O2 production/activation to generate the sulfate and hydroxyl radicals for the degradation of emerging organic contaminants

Conventional electrocatalytic degradation of pollutants involves either cathodic reduction or anodic oxidation process, which caused the low energy utilization efficiency. In this study, we successfully couple the anodic activation of sulfates with the cathodic H₂O₂ production/activation to boost the generation of sulfate radical (SO₄^·-) and hydroxyl radical (·OH) for the efficient degradation of emerging contaminants. The electrocatalysis reactor is composed of a modified-graphite-felt (GF) cathode, in-situ prepared by the carbonization of polyaniline (PANI) electrodeposited on a GF substrate, and a boron-doped diamond (BDD) anode. In the presence of sulfates, the electrocatalysis system shows superior activities towards the degradation of pharmaceutical and personal care products (PPCPs), with the optimal performance of completely degrading the representative pollutant carbamazepine (CBZ, 0.2 mg L^-1) within 150 s. Radicals quenching experiments indicated that ·OH and SO₄^·- act as the main reactive oxygen species for CBZ decomposition. Results from the electron paramagnetic resonance (EPR) and chronoamperometry studies verified that the sulfate ions were oxidized to SO₄^·-radicals at the anode, while the dissolve oxygen molecules were reduced to H₂O₂ molecules which were further activated to produce ·OH radicals at the cathode. It was also found that during the catalytic reactions SO₄^·-radicals could spontaneously convert into peroxydisulfate (PDS) which were subsequently reduced back to SO₄^·-at the cathodes. The quasi-steady-state concentrations of ·OH and SO₄^·-were estimated to be 0.51×10^-12 M and 0.56×10^-12 M, respectively. This study provides insight into the synergistic generation of ·OH/SO₄^·- from the integrated electrochemical anode oxidation of sulfate and cathode reduction of dissolved oxygen, which indicates a potential practical approach to efficiently degrade the emerging organic water contaminants.

Electronic Perturbation of Copper Single-Atom CO2 Reduction Catalysts in a Molecular Way

Fine-tuning electronic structures of single-atom catalysts (SACs) plays a crucial role in harnessing their catalytic activities, yet challenges remain at a molecular scale in a controlled fashion. By tailoring the structure of graphdiyne (GDY) with electron-withdrawing/-donating groups, we show herein the electronic perturbation of Cu single-atom CO₂ reduction catalysts in a molecular way. The elaborately introduced functional groups (-F, -H and -OMe) can regulate the valance state of Cu^δ+ , which is found to be directly scaled with the selectivity of the electrochemical CO₂ -to-CH₄ conversion. An optimum CH₄ Faradaic efficiency of 72.3 % was achieved over the Cu SAC on the F-substituted GDY. In situ spectroscopic studies and theoretical calculations revealed that the positive Cu^δ+ centers adjusted by the electron-withdrawing group decrease the pK_a of adsorbed H₂ O, promoting the hydrogenation of intermediates toward the CH₄ production. Our strategy paves the way for precise electronic perturbation of SACs toward efficient electrocatalysis.

Activating Single-Atom Ni Site via First-Shell Si Modulation Boosts Oxygen Reduction Reaction

Atomically dispersed nitrogen-coordinated 3d transition-metal site on carbon support (M-NC) are promising alternatives to Pt group metal-based catalysts toward oxygen reduction reaction (ORR). However, despite the excellent activities of most of M-NC catalysts, such as Fe-NC, Co-NC et al., their durability is far from satisfactory due to Fenton reaction. Herein, this work reports a novel Si-doped Ni-NC catalyst (Ni-SiNC) that possesses high activity and excellent stability. X-ray absorption fine structure and aberration-corrected transmission electron microscopy uncover that the single-atom Ni site is coordinated with one Si atom and three N atoms, constructing Ni-Si₁ N₃  moiety. The Ni-SiNC catalyst exhibits a half-wave potential (E_1/2 ) of 0.866 V versus RHE, with a distinguished long-term durability in alkaline media of only 10 mV negative shift in E_1/2  after 35 000 cycles, which is also validated in Zn-air battery. Density functional theory calculations reveal that the Ni-Si₁ N₃  moiety facilitates ORR kinetics through optimizing the adsorption of intermediates.

Decorating Single-Atomic Mn Sites with FeMn Clusters to Boost Oxygen Reduction Reaction

The regulation of electron distribution of single-atomic metal sites by atomic clusters is an effective strategy to boost their intrinsic activity of oxygen reduction reaction (ORR). Herein we report the construction of single-atomic Mn sites decorated with atomic clusters by an innovative combination of post-adsorption and secondary pyrolysis. The X-ray absorption spectroscopy confirms the formation of Mn sites via Mn-N₄ coordination bonding to FeMn atomic clusters (FeMn_ac /Mn-N₄ C), which has been demonstrated theoretically to be conducive to the adsorption of molecular O₂ and the break of O-O bond during the ORR process. Benefiting from the structural features above, the FeMn_ac /Mn-N₄ C catalyst exhibits excellent ORR activity with half-wave potential of 0.79 V in 0.5 M H₂ SO₄ and 0.90 V in 0.1 M KOH as well as preeminent Zn-air battery performance. Such synthetic strategy may open up a route to construct highly active catalysts with tunable atomic structures for diverse applications.

Heterogenization of Molecular Electrocatalytic Active Sites through Reticular Chemistry

The electrochemical conversion of small molecules, such as CO₂ , O₂ , and H₂ O, has received significant attention as a potential engine for sustainable life. Metal-organic frameworks (MOFs) are a promising class of electrocatalytic materials for such processes. An attractive aspect of utilizing this class of materials as electrocatalysts is that well-known molecular active sites can be introduced to well-defined crystalline heterogeneous catalytic systems with high tunability. This review offers strategic insights into recent studies on MOF-based electrocatalysts by discussing the notable active sites that have been utilized in both homogeneous and heterogeneous catalysts, while highlighting instances where such active sites have been introduced into MOFs. In addition, material design principles enabling the integration of electrochemically active components with the MOF platform are outlined. Viewpoints on the viability of MOFs as an alternative to currently used electrocatalysts are also discussed. Finally, the future direction of MOF-based electrocatalysis research is established.

Regulating the FeN4 Moiety by Constructing Fe-Mo Dual-Metal Atom Sites for Efficient Electrochemical Oxygen Reduction

An Fe-N-C catalyst with an FeN₄ active moiety has gained ever-increasing attention for the oxygen reduction reaction (ORR); however, the catalytic performance is sluggish in acidic solutions and the regulation is still a challenge. Herein, Fe-Mo dual-metal sites were constructed to tune the ORR activity of a mononuclear Fe site embedded in porous nitrogen-doped carbon. The cracking of O-O bonds is much more facile on the Fe-Mo atomic pair site due to the preferred bridge-cis adsorption model of oxygen molecules. The downshift of the Fe d band center when an Mo atom is introduced to the FeN_x configuration optimizes the absorption-desorption behavior of ORR intermediates in the FeMoN₆ active moiety, thus boosting the catalytic performance. The construction of dual-metal atom sites to regulate the catalytically active moiety paves the way for boosting the electrocatalytic performance of other similar non-precious-metal catalysts.

Correlation between Electronic Descriptor and Proton-Coupled Electron Transfer Thermodynamics in Doped Graphite-Conjugated Catalysts

Graphite-conjugated catalysts (GCCs) provide a powerful framework for investigating correlations between electronic structure features and chemical reactivity of single-site heterogeneous catalysts. GCC-phenazine undergoes proton-coupled electron transfer (PCET) involving protonation of phenazine at its two nitrogen atoms with the addition of two electrons. Herein, this PCET reaction is investigated in the presence of defects, such as heteroatom dopants, in the graphitic surface. The proton-coupled redox potentials, E_PCET, are computed using a constant potential periodic density functional theory (DFT) strategy. The electronic states directly involved in PCET for GCC-phenazine exhibit the same nitrogen orbital character as those for molecular phenazine. The energy ε_LUS of this phenazine-related lowest unoccupied electronic state in GCC-phenazine is identified as a descriptor for changes in PCET thermodynamics. Importantly, ε_LUS is obtained from only a single DFT calculation but can predict E_PCET, which requires many such calculations. Similar electronic features may be useful descriptors for thermodynamic properties of other single-site catalysts.

Simultaneous Improvement of Oxygen Reduction and Catalyst Anchoring via Multiple Dopants on Mesoporous Carbon Frameworks for Flexible Al-Air Batteries

Mesoporous carbon supported non-noble metals, as promising catalysts for boosting the oxygen reduction reaction (ORR) in metal-air batteries, usually face challenges of low activity and performance degradation caused by the catalyst detachment from carbon substrates. Herein, a one-stone-two-birds strategy is reported to simultaneously improve the ORR activity and anchor nanosized MnS catalysts on a mesoporous carbon framework via nitrogen (N) and sulfur (S) dopants (MnS/NS-C). Synchrotron-based X-ray absorption spectroscopy (XAS) confirms the existence of Mn-N and Mn-S bonds, which firmly anchor active MnS nanoparticles. Density functional theory (DFT) calculations reveal that the N, S codoping lowers the d-band center of Mn and optimizes ORR intermediate adsorption. An excellent ORR performance (the onset and half-wave potential of 1.07 and 0.91 V) and long-term durability are achieved for MnS/NS-C in alkaline media. The flexible Al-air battery, using MnS/NS-C as the cathode catalyst, shows a power density of 134.6 mW cm^-2 in comparison to the Pt/C-based counterpart of 106.2 mW cm^-2. This study constructs a stable interaction with non-noble catalysts and carbon substrates for enhancing catalytic activity and durability in metal-air batteries.

Pinpointing the axial ligand effect on platinum single-atom-catalyst towards efficient alkaline hydrogen evolution reaction

Developing active single-atom-catalyst (SAC) for alkaline hydrogen evolution reaction (HER) is a promising solution to lower the green hydrogen cost. However, the correlations are not clear between the chemical environments around the active-sites and their desired catalytic activity. Here we study a group of SACs prepared by anchoring platinum atoms on NiFe-layered-double-hydroxide. While maintaining the homogeneity of the Pt-SACs, various axial ligands (−F, −Cl, −Br, −I, −OH) are employed via a facile irradiation-impregnation procedure, enabling us to discover definite chemical-environments/performance correlations. Owing to its high first-electron-affinity, chloride chelated Pt-SAC exhibits optimized bindings with hydrogen and hydroxide, which favor the sluggish water dissociation and further promote the alkaline HER. Specifically, it shows high mass-activity of 30.6 A mgPt⁻¹ and turnover frequency of 30.3  H₂ s⁻¹ at 100 mV overpotential, which are significantly higher than those of the state-of-the-art Pt-SACs and commercial Pt/C catalyst. Moreover, high energy efficiency of 80% is obtained for the alkaline water electrolyser assembled using the above catalyst under practical-relevant conditions.

Establishing robust structure/performance correlations is critical for the development of single-atom-catalysts with improved activity. Here, the axial ligand on Pt single-atom-catalyst is precisely adjusted and studied, showing that the ligand’s first electron affinity is crucial for the catalysis.

Tuning Fe Spin Moment in Fe-N-C Catalysts to Climb the Activity Volcano via a Local Geometric Distortion Strategy

As the most promising alternative to platinum-based catalysts for cathodic oxygen reduction reaction (ORR) in proton exchange membrane fuel cells, further performance enhancement of Fe-N-C catalysts is highly expected to promote their wide application. In Fe-N-C catalysts, the single Fe atom forms a square-planar configuration with four adjacent N atoms (D4h symmetry). Breaking the D4h symmetry of the FeN4 active center provides a new route to boost the activity of Fe-N-C catalysts. Herein, for the first time, the deformation of the square-planar coordination of FeN4 moiety achieved by introducing chalcogen oxygen groups (XO2 , X = S, Se, Te) as polar functional groups in the Fe-N-C catalyst is reported. The theoretical and experimental results demonstrate that breaking the D4h symmetry of FeN4 results in the rearrangement of Fe 3d electrons and increases spin moment of Fe centers. The efficient spin state manipulation optimizes the adsorption energetics of ORR intermediates, thereby significantly promoting the intrinsic ORR activity of Fe-N-C catalysts, among which the SeO2 modified catalyst lies around the peak of the ORR volcano plot. This work provides a new strategy to tune the local coordination and thus the electronic structure of single-atom catalysts.

Atomically Dispersed Pentacoordinated-Zirconium Catalyst with Axial Oxygen Ligand for Oxygen Reduction Reaction

Single-atom catalysts (SACs), as promising alternatives to Pt-based catalysts, suffer from the limited choice of center metals and low single-atom loading. Here, we report a pentacoordinated Zr-based SAC with nontrivial axial O ligands (denoted O-Zr-N-C) for oxygen reduction reaction (ORR). The O ligand downshifts the d-band center of Zr and confers Zr sites with stable local structure and proper adsorption capability for intermediates. Consequently, the ORR performance of O-Zr-N-C prominently surpasses that of commercial Pt/C, achieving a half-wave potential of 0.91 V vs. reversible hydrogen electrode and outstanding durability (92 % current retention after 130-hour operation). Moreover, the Zr site shows good resistance towards aggregation, enabling the synthesis of Zr-based SAC with high loading (9.1 wt%). With the high-loading catalyst, the zinc-air battery (ZAB) delivers a record-high power density of 324 mW cm-2 among those of SAC-based ZABs.

An Initial Covalent Organic Polymer with Closed-F Edges Directly for Proton-Exchange-Membrane Fuel Cells

Covalent organic polymers (COPs) are a class of rising electrocatalysts for the oxygen reduction reaction (ORR) due to the atomically metrical control of the organic molecular components along with highly architectural robustness and thermodynamic stability even in acid or alkaline media. However, the direct application of pristine COPs as acidic ORR electrocatalysts, especially in device manner, e.g., in proton-exchange-membrane fuel cells (PEMFCs), remains a big challenge. Currently, the decoration toward electronic structures of active sites is considered a vital pathway to enhancing the acidic ORR activity of carbon-based electrocatalysts. Here, an initial F-decorated fully closed π-conjugated quasi-phthalocyanine COP (denoted as COP_BTC -F) is reported. The introduction of the closed-F edges stepwise drags more electrons from FeN₄ sites in COP_BTC -F into the catalyst margin, which weakens the occupied numbers of bonding orbitals between COP_BTC -F and OH* intermediates at the rate-determining step, exhibiting over five times intrinsic performance beyond the counterpart without F functionalities (termed as COP_BTC ). Significantly, the maximum power density utilizing COP_BTC -F as a cathode catalyst in PEMFCs is remarkably increased by an order of magnitude compared with COP_BTC , which is a stride forward among catalysts based on a pyrolysis-free conjugated-polymer network in device manner to date.

Isolating Single and Few Atoms for Enhanced Catalysis

Atomically dispersed metal catalysts have triggered great interest in the field of catalysis owing to their unique features. Isolated single or few metal atoms can be anchored on substrates via chemical bonding or space confinement to maximize atom utilization efficiency. The key challenge lies in precisely regulating the geometric and electronic structure of the active metal centers, thus significantly influencing the catalytic properties. Although several reviews have been published on the preparation, characterization, and application of single-atom catalysts (SACs), the comprehensive understanding of SACs, dual-atom catalysts (DACs), and atomic clusters has never been systematically summarized. Here, recent advances in the engineering of local environments of state-of-the-art SACs, DACs, and atomic clusters for enhanced catalytic performance are highlighted. Firstly, various synthesis approaches for SACs, DACs, and atomic clusters are presented. Then, special attention is focused on the elucidation of local environments in terms of electronic state and coordination structure. Furthermore, a comprehensive summary of isolated single and few atoms for the applications of thermocatalysis, electrocatalysis, and photocatalysis is provided. Finally, the potential challenges and future opportunities in this emerging field are presented. This review will pave the way to regulate the microenvironment of the active site for boosting catalytic processes.

Metal-Coordinated Phthalocyanines as Platform Molecules for Understanding Isolated Metal Sites in the Electrochemical Reduction of CO2

Single-atom catalysts (SACs) of non-precious transition metals (TMs) often show unique electrochemical performance, including the electrochemical carbon dioxide reduction reaction (CO₂RR). However, the inhomogeneity in their structures makes it difficult to directly compare SACs of different TM for their CO₂RR activity, selectivity, and reaction mechanisms. In this study, the comparison of isolated TMs (Fe, Co, Ni, Cu, and Zn) is systematically investigated using a series of crystalline molecular catalysts, namely TM-coordinated phthalocyanines (TM-Pcs), to directly compare the intrinsic role of the TMs with identical local coordination environments on the CO₂RR performance. The combined experimental measurements, in situ characterization, and density functional theory calculations of TM-Pc catalysts reveal a TM-dependent CO₂RR activity and selectivity, with the free energy difference of ΔG(*HOCO) - ΔG(*CO) being identified as a descriptor for predicting the CO₂RR performance.

Directional Manipulation of Electron Transfer by Energy Level Engineering for Efficient Cathodic Oxygen Reduction

Electron transfer plays an important role in determining the energy conversion efficiency of energy devices. Nitrogen-coordinated single metal sites (M-N₄) materials as electrocatalysts have exhibited great potential in devices. However, there are still great difficulties in how to directionally manipulate electron transfer in M-N₄ catalysts for higher efficiency. Herein, we demonstrated the mechanism of electron transfer being affected by energy level structure based on classical iron phthalocyanine (FePc) molecule/carbon models and proposed an energy level engineering strategy to manipulate electron transfer, preparing high-performance ORR catalysts. Engineering molecular energy level via modulating FePc molecular structure with nitro induces a strong interfacial electronic coupling and efficient charge transfer from carbon to FePc-β-NO₂ molecule. Consequently, the assembled zinc-air battery exhibits ultrahigh performance which is superior to most of M-N₄ catalysts. Energy level engineering provides a universal approach for directionally manipulating electron transfer, bringing a new concept to design efficient and stable M-N₄ electrocatalyst.

Multilevel Computational Studies Reveal the Importance of Axial Ligand for Oxygen Reduction Reaction on Fe-N-C Materials

The systematic improvement of Fe-N-C materials for fuel cell applications has proven challenging, due in part to an incomplete atomistic understanding of the oxygen reduction reaction (ORR) under electrochemical conditions. Herein, a multilevel computational approach, which combines ab initio molecular dynamics simulations and constant potential density functional theory calculations, is used to assess proton-coupled electron transfer (PCET) processes and adsorption thermodynamics of key ORR intermediates. These calculations indicate that the potential-limiting step for ORR on Fe-N-C materials is the formation of the Fe^III-OOH intermediate. They also show that an active site model with a water molecule axially ligated to the iron center throughout the catalytic cycle produces results that are consistent with the experimental measurements. In particular, reliable prediction of the ORR onset potential and the Fe(III/II) redox potential associated with the conversion of Fe^III-OH to Fe^II and desorbed H₂O requires an axial H₂O co-adsorbed to the iron center. The observation of a five-coordinate rather than four-coordinate active site has significant implications for the thermodynamics and mechanism of ORR. These findings highlight the importance of solvent-substrate interactions and surface charge effects for understanding the PCET reaction mechanisms and transition-metal redox couples under realistic electrochemical conditions.

Engineering at Subatomic Scale: Achieving Selective Catalytic Pathways via Tuning of the Oxidation States in Functionalized Single-Atom Quantum Catalysts

Regulating the catalytic pathways of single-atom sites in single atom catalysts (SACs) is an exciting debate at the moment, which has redirected the research towards understanding and modifying the single-atom catalytic sites through various strategies including altering the coordination environment of single atom for desirable outcomes as well as increasing their number. One useful aspect concerning the tunability of the catalytic pathways of SACs, which has been overlooked, is the oxidation state dynamics of the single atoms. In this study, iron single-atoms (FeSA) with variable oxidation states, dependent on the precursors, are harnessed inside a nitrogen-rich functionalized carbon quantum dots (CQDs) matrix via a facile one-step and low-temperature synthesis process. Dynamic electronic properties are imparted to the FeSAs by the simpler carbon dots matrix of CQDs in order to achieve the desired catalytic pathways of reactive oxygen species (ROS) generation in different environments, which are explored experimentally and theoretically for an in-depth understanding of the redox chemistry that drives the alternative catalytic pathways in FeSA@CQDs. These alternative and oxidation state-dependent catalytic pathways are employed for specific as well as cascade-like activities simulating natural enzymes as well as biomarkers for the detection of cancerous cells.

Macro/Micro-Environment Regulating Carbon-Supported Single-Atom Catalysts for Hydrogen/Oxygen Conversion Reactions

Single-atom catalysts (SACs) have attracted tremendous research interest due to their unique atomic structure, maximized atom utilization, and remarkable catalytic performance. Among the SACs, the carbon-supported SACs have been widely investigated due to their easily controlled properties of the carbon substrates, such as the tunable morphologies, ordered porosity, and abundant anchoring sites. The electrochemical performance of carbon-supported SACs is highly related to the morphological structure of carbon substrates (macro-environment) and the local coordination environments of center metals (micro-environment). This review aims to provide a comprehensive summary on the macro/micro-environment regulating carbon-supported SACs for highly efficient hydrogen/oxygen conversion reactions. The authors first summarize the macro-environment engineering strategies of carbon-supported SACs with altered specific surface areas and porous properties of the carbon substrates, facilitating the mass diffusion kinetics and structural stability. Then the micro-environment engineering strategies of carbon-supported SACs are discussed with the regulated atomic structure and electronic structure of metal centers, boosting the catalytic performance. Insights into the correlation between the co-boosted effect from the macro/micro-environments and catalytic activity for hydrogen/oxygen conversion reactions are summarized and discussed. Finally, the challenges and perspectives are addressed in building highly efficient carbon-supported SACs for practical applications.

Enhancing CO2 electroreduction to CH4 over Cu nanoparticles supported on N-doped carbon

The electroreduction of CO2 to CH4 has attracted extensive attention. However, it is still a challenge to achieve high current density and faradaic efficiency (FE) for producing CH4 because the reaction involves eight electrons and four protons. In this work, we designed Cu nanoparticles supported on N-doped carbon (Cu-np/NC). It was found that the catalyst exhibited outstanding performance for the electroreduction of CO2 to CH4. The FE toward CH4 could be as high as 73.4% with a high current density of 320 mA cm-2. In addition, the mass activity could reach up to 6.4 A mgCu -1. Both experimental and theoretical calculations illustrated that the pyrrolic N in NC could accelerate the hydrogenation of *CO to the *CHO intermediate, resulting in high current density and excellent selectivity for CH4. This work conducted the first exploration of the effect of N-doped species in composites on the electrocatalytic performance of CO2 reduction.

Green Crop Yam-Derived Carbons: Off-Plane Active Sites for Oxygen Electroreduction Identified by First-Principles

Plant-derived nonprecious metal catalysts are considered one of the promising candidates of platinum for oxygen reduction reaction (ORR). In this work, the typical microscopic morphology of fresh green crop yam is first detected by cryoscanning electronic microscopy. Using the green and widely sourced yam with spherical starch in nature as a precursor, well-defined spherical carbons are prepared via hypersaline-assisted hydrothermal carbonization and NH₃activation, featuring a high heteroatom doping level and a hierarchical porous structure. Experimental results and density functional theory (DFT) calculations reveal that diverse off-plane Fe-N_x-C_y ensembles on the spherical carbons trigger the high performance that exceeds state-of-art Pt/C and most reported carbon catalysts toward ORR in a KOH solution. The increased charge density and the bond length of Fe coordinated in the sites should be responsible for the significantly improved property. The easily editing of off-plane active sites from the simple carbon morphology may shed light on optimizing nonprecious carbons as next-generation catalysts for ORR.

Spin engineering of single-site metal catalysts

Single-site metal atoms (SMAs) on supports are attracting extensive interest as new catalytic systems because of maximized atom utilization and superior performance. However, rational design of configuration-optimized SMAs with high activity from the perspectives of fundamental electron spin is highly challenging. Herein, N-coordinated Fe single atoms are successfully distributed over axial carbon micropores to form dangling-FeN₄ centers (d-FeN₄). This unique d-FeN₄ demonstrates much higher intrinsic activity toward oxygen reduction reaction (ORR) in HClO₄ than FeN₄ without micropore underneath and commercial Pt/C. Both theoretical calculation and electronic structure characterization imply that d-FeN₄ endows central Fe with medium spin (t_2g⁴ e_g¹), which provides a spin channel for electron transition compared with FeN₄ with low spin. This leads to the facile formation of the singlet state of oxygen-containing species from triplet oxygen during the ORR, thus showing faster kinetics than FeN₄. This work provides an in-depth understanding of spin tuning on SMAs for advanced energy catalysis.

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Single-site FeN₄ species are designed to dangle over axial carbon micropores (d-FeN₄)

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d-FeN₄ shows much superior oxygen reduction reactivity to traditional FeN₄

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d-FeN₄ facilitates the formation of singlet-state oxygen-containing species with optimized spin states by micropore

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This work provides in-depth understanding of spin tuning for advanced catalyst design

Mesopore-Rich Fe-N-C Catalyst with FeN4 -O-NC Single-Atom Sites Delivers Remarkable Oxygen Reduction Reaction Performance in Alkaline Media

Fe-N-C catalysts offer excellent performance for the oxygen reduction reaction (ORR) in alkaline media. With a view toward boosting the intrinsic ORR activity of Fe single-atom sites in Fe-N-C catalysts, fine-tuning the local coordination of the Fe sites to optimize the binding energies of ORR intermediates is imperative. Herein, a porous FeN₄ -O-NCR electrocatalyst rich in catalytically accessible FeN₄ -O sites (wherein the Fe single atoms are coordinated to four in-plane nitrogen atoms and one subsurface axial oxygen atom) supported on N-doped carbon nanorods (NCR) is reported. Fe K-edge X-ray absorption spectroscopy (XAS) verifies the presence of FeN₄ -O active sites in FeN₄ -O-NCR, while density functional theory calculations reveal that the FeN₄ -O coordination offers a lower energy and more selective 4-electron/4-proton ORR pathway compared to traditional FeN₄ sites. Electrochemical tests validate the outstanding intrinsic activity of FeN₄ -O-NCR for alkaline ORR, outperforming Pt/C and almost all other M-N-C catalysts reported to date. A primary zinc-air battery constructed using FeN₄ -O-NCR delivers a peak power density of 214.2 mW cm^-2 at a current density of 334.1 mA cm^-2 , highlighting the benefits of optimizing the local coordination of iron single atoms.