Hydrogen Passivation of M-N-C (M = Fe, Co) Catalysts for Storage Stability and ORR Activity Improvements

The N-anchoring effect in NH2-functionalized MOF-derived iron-carbon nanomaterials for oxygen reduction

The incorporation of an iron source into NH2-MOF-5, followed by thermal decomposition, yields a porous metal-carbon catalyst (MOF5A-Fe@NC). This catalyst possesses significant N content, a high degree of graphitization, and abundant Fe-Nx sites, which contribute to enhanced oxygen reduction. Specifically, the obtained MOF5A-Fe@NC demonstrates a positive onset potential (0.972 V), a substantial limiting current density (4.815 mA cm-2), and a small Tafel slope (58.7 mV dec-1), and maintains a high current retention of 96.3% after 10 hours.

Electrodeposition of Ni/Cu Bimetallic Conductive Metal-Organic Frameworks Electrocatalysts with Boosted Oxygen Reduction Activity for Zinc-Air Batteries

Zinc-air batteries employing non-Pt cathodes hold significant promise for advancing cathodic oxygen reduction reaction (ORR). However, poor intrinsic electrical conductivity and aggregation tendency hinder the application of metal-organic frameworks (MOFs) as active ORR cathodes. Conductive MOFs possess various atomically dispersed metal centers and well-aligned inherent topologies, eliminating the additional carbonization processes for achieving high conductivity. Here, a novel room-temperature electrochemical cathodic electrodeposition method is introduced for fabricating uniform and continuous layered 2D bimetallic conductive MOF films cathodes without polymeric binders, employing the organic ligand 2,3,6,7,10,11-hexaiminotriphenylene (HITP) and varying the Ni/Cu ratio. The influence of metal centers on modulating the ORR performance is investigated by density functional theory (DFT), demonstrating the performance of bimetallic conductive MOFs can be effectively tuned by the unpaired 3d electrons and the Jahn-Teller effect in the doped Cu. The resulting bimetallic Ni2.1Cu0.9(HITP)2 exhibits superior ORR performance, boasting a high onset potential of 0.93 V. Moreover, the assembled aqueous zinc-air battery demonstrates high specific capacity of 706.2 mA h g-1, and exceptional long-term charge/discharge stability exceeding 1250 cycles.

Encapsulating CoNi nanoparticles into nitrogen-doped carbon nanotube arrays as bifunctional oxygen electrocatalyst for rechargeable zinc-air batteries

The high theoretical specific energy and environmental friendliness of zinc-air batteries (ZABs) have garnered significant attention. However, the practical application of ZABs requires overcoming the sluggish kinetics associated with oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, 3D self-supported nitrogen-doped carbon nanotubes (N-CNTs) arrays encapsulated by CoNi nanoparticles on carbon fiber cloth (CoNi@N-CNTs/CFC) are synthesized as bifunctional catalysts for OER and ORR. The 3D interconnected N-CNTs arrays not only improve the electrical conductivity, the permeation and gas escape capabilities of the electrode, but also enhance the corrosion resistance of CoNi metals. DFT calculations reveal that the co-existence of Co and Ni synergistically reduces the energy barrier for OOH conversion to OH, thereby optimizing the Gibbs free energy of the catalysts. Additionally, analysis of the change in energy barrier during the rate-determining step suggests that the primary catalytic active center is Ni site for OER. As a result, CoNi@N-CNTs/CFC exhibits superior catalytic activity with an overpotential of 240 mV at 10 mA cm-2 toward OER, and the onset potential of 0.92 V for ORR. Moreover, utilization of CoNi@N-CNTs/CFC in liquid and solid-state ZABs exhibited exceptional stability, manifesting a consistent cycling operation lasting for 100 and 15 h, respectively.

General Design Concept of High-Performance Single-Atom-Site Catalysts for H2O2 Electrosynthesis

Hydrogen peroxide (H2O2) as a green oxidizing agent is widely used in various fields. Electrosynthesis of H2O2 has gradually become a hotspot due to its convenient and environment-friendly features. Single-atom-site catalysts (SASCs) with uniform active sites are the ideal catalysts for the in-depth study of the reaction mechanism and structure-performance relationship. In this review, the outstanding achievements of SASCs in the electrosynthesis of H2O2 through 2e- oxygen reduction reaction (ORR) and 2e- water oxygen reaction (WOR) in recent years, are summarized. First, the elementary steps of the two pathways and the roles of key intermediates (*OOH and *OH) in the reactions are systematically discussed. Next, the influence of the size effect, electronic structure regulation, the support/interfacial effect, the optimization of coordination microenvironments, and the SASCs-derived catalysts applied in 2e- ORR are systematically analyzed. Besides, the developments of SASCs in 2e- WOR are also overviewed. Finally, the research progress of H2O2 electrosynthesis on SASCs is concluded, and an outlook on the rational design of SASCs is presented in conjunction with the design strategies and characterization techniques.

Regulating d-Orbital Hybridization of Subgroup-IVB Single Atoms for Efficient Oxygen Reduction Reaction

Highly active single-atom electrocatalysts for the oxygen reduction reaction are crucial for improving the energy conversion efficiency, but they suffer from a limited choice of metal centers and unsatisfactory stabilities. Here, this work reports that optimization of the binding energies for reaction intermediates by tuning the d-orbital hybridization with axial groups converts inactive subgroup-IVB (Ti, Zr, Hf) moieties (MN4) into active motifs (MN4O), as confirmed with theoretical calculations. The competition between metal-ligand covalency and metal-intermediate covalency affects the d-p orbital hybridization between the metal site and the intermediates, converting the metal centers into active sites. Subsequently, dispersed single-atom M sites coordinated by nitrogen/oxygen groups have been prepared on graphene (s-M-N/O-C) catalysts on a large-scale with high-energy milling and pyrolysis. Impressively, the s-Hf-N/O-C catalyst with 5.08 wt% Hf exhibits a half-wave potential of 0.920 V and encouraging performance in a zinc-air battery with an extraordinary cycling life of over 1600 h and a large peak power-density of 256.9 mW cm-2. This work provides promising single-atom electrocatalysts and principles for preparing other catalysts for the oxygen reduction reaction.

Concave Structural Carbon Co-Doped with Iron Atom Pairs and Nitrogen as Ultra-High Performance Catalyst Toward Oxygen Reduction

It is crucial to rationally design and synthesize atomic-scale transition metal-doped carbon catalysts with high electrocatalytic activity to achieve a high-efficient oxygen reduction reaction (ORR). Herein, an electrocatalyst comprised of Fe-Fe dual atom pairs and N-doped concave carbon are reported (N-CC@Fe DA) that achieves ultrahigh electrocatalytic ORR activity. The catalyst is prepared by a gaseous doping approach, with zeolitic imidazolate framework-8 (ZIF-8) as the carbon framework precursor and cyclopentadienyliron dicarbonyl dimer as the Fe-Fe atom pair precursor. The catalyst exhibits high cathodic ORR catalytic performance in an alkaline Zn/air battery and proton exchange membrane fuel cell (PEMFC), yielding peak power densities of 241 mW cm-2 and 724 mW cm-2, respectively, compared to 127 mW cm-2 and 1.20 W cm-2 with conventional Pt/C catalysts as cathodes. The presence of Fe atom pairs coordinate with N atoms is revealed by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analysis, and Density Functional Theory (DFT) calculation results show that the Fe-Fe pair structure is beneficial for adsorbing oxygen molecules, activating the O─O bond, and desorbing OH* intermediates formed during oxygen reduction, resulting in a more efficient oxygen reaction. The findings may provide a new pathway for preparing ultra-high-performance doped carbon catalysts with Fe-Fe atom pair structures.

Anchoring a Pt-based alloy on oxygen-vacancy-defected MXene nanosheets for efficient hydrogen evolution reaction and oxygen reduction reaction

Rational design and controllable synthesis of Pt-based materials with intimate interfacial contact open up the possibility for boosting the performance of the ORR (oxygen reduction reaction) and HER (hydrogen evolution reaction). However, it is still challenging to prevent the oxidation of Pt during the formation of alloys and to clarify the interfacial synergistic effects on the catalytic performance between Pt alloys and the dispersed substrate. Herein, the wet chemical stripping and intercalation methods were employed to synthesize a two-dimensional (2D) MXene with abundant defect sites, which can anchor Pt3Co/Pt3Ni nanoparticles and prevent the oxidation of Pt during the process of atomic rearrangement at high temperatures. The obtained Pt3Co/MXene and Pt3Ni/MXene displayed different phase compositions and alloying degrees on adjusting the annealing temperature. Electrochemical test results showed that the optimized HER and ORR electrocatalytic activities occurred at 700 °C. Compared with Pt3Ni/MXene-700, Pt3Co/MXene-700 exhibited an HER overpotential of 1.3 mV at a current density of 10 mA cm-2, and a Tafel slope of 27.11 mV dec-1 in 0.1 M HClO4 solution. Furthermore, Pt3Co/MXene-700 exhibited an ORR half-wave potential of 0.897 V, and a mass activity of 241.1 mA mg-1Pt in 0.1 M HClO4 solution. This can be attributed to the formation of intermetallic compounds in Pt3Co/MXene. The electronic structure analysis showed that the enhanced performance could be assigned to the electron-capturing capability of the MXene, less oxidation of Pt and synergistic interactions between the Pt alloy and the MXene substrate. These findings provide a new strategy for the synthesis of highly active HER/ORR catalysts and broaden the way for the design of MXene-based catalysts.

Single-Atom-Based Oxygen Reduction Reaction Catalysts for Proton Exchange Membrane Fuel Cells: Progress and Perspective

Single-atom catalysts (SACs) are regarded as promising non-noble-metal alternatives for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells due to their high atom utilization efficiency and excellent catalytic properties. However, the insufficient long-term stability issues of SACs under the working conditions seriously hinder their practical application. In this perspective, the recent progress of SACs with optimized ORR catalytic activity is first reviewed. Then, the possible degradation mechanisms of SACs in the ORR process and effective strategies for improving their ORR durability are summarized. Finally, some challenges and opportunities are proposed to develop stable single-atom-based ORR electrocatalysts in the future.

Atomically dispersed Fe-O4-C sites as efficient electrocatalysts for electrosynthesis of hydrogen peroxide

The electrochemical reduction of oxygen via the 2e pathway is an environmentally friendly approach to the electrosynthesis of H2O2. Nevertheless, its sluggish kinetics and limited selectivity hinder its practical application. Herein, single Fe atoms anchored on graphene oxide (SA Fe/GO) with Fe-O4-C sites are developed as an efficient electrocatalyst for the electro-synthesis of H2O2. These Fe-O4-C site active centres could efficiently enhance the activity and selectivity towards 2e electrochemical oxygen reduction in an alkaline environment. The newly-developed SA Fe/GO electrocatalyst demonstrates exceptional electrochemical performance, exhibiting impressive activity with an onset potential of 0.90 and H2O2 production of 0.60 mg cm-2 h-1 at 0.4 V. Remarkably, it achieves a remarkable H2O2 selectivity of over 95.5%.

Bifunctional Single Atom Catalysts for Rechargeable Zinc-Air Batteries: From Dynamic Mechanism to Rational Design

Ever-growing demands for rechargeable zinc-air batteries (ZABs) call for efficient bifunctional electrocatalysts. Among various electrocatalysts, single atom catalysts (SACs) have received increasing attention due to the merits of high atom utilization, structural tunability, and remarkable activity. Rational design of bifunctional SACs relies heavily on an in-depth understanding of reaction mechanisms, especially dynamic evolution under electrochemical conditions. This requires a systematic study in dynamic mechanisms to replace current trial and error modes. Herein, fundamental understanding of dynamic oxygen reduction reaction and oxygen evolution reaction mechanisms for SACs is first presented combining in situ and/or operando characterizations and theoretical calculations. By highlighting structure-performance relationships, rational regulation strategies are particularly proposed to facilitate the design of efficient bifunctional SACs. Furthermore, future perspectives and challenges are discussed. This review provides a thorough understanding of dynamic mechanisms and regulation strategies for bifunctional SACs, which are expected to pave the avenue for exploring optimum single atom bifunctional oxygen catalysts and effective ZABs.

Cu-based catalysts with the co-existence of single atoms and nanoparticles for basic electrocatalytic oxygen reduction reaction

Developing efficient and stable oxygen reduction reaction (ORR) catalysts to replace the precious Pt/C is very important for the industrial application of proton-exchange membrane fuel cells. Herein, using bismuth-based metal-organic frameworks as the substrate to disperse copper ions, we prepared a catalyst containing both Cu single atoms and Cu nanoparticles (CuSACuNP/BiCN) by a pyrolysis method. In 0.1 M KOH electrolyte, the electrocatalytic ORR performance of CuSACuNP/BiCN was superior to that of commercial Pt/C. With a hierarchical porous architecture, CuSACuNP/BiCN displayed a half-wave potential of 0.86 V vs. RHE and a diffusion-limiting current density of 5.82 mA cm-2 with a four-electron transfer process. In addition, it was stable during a 12-hour durability test. This study provides guidance for the synthesis of advanced Cu-based nano-single-atom catalytic materials for ORR applications.

Understanding the selection of catalytic pathway on graphene-supported nitrogen coordinated Ru-atom by ab initio molecular dynamics simulation

CONTEXT

In the paper, the ORR/OER on graphene-supported nitrogen coordinated Ru-atom (Ru-N-C) is simulated. We discuss nitrogen coordination influences electronic properties, adsorption energies, and catalytic activity in a single-atom Ru active site. The over potentials on Ru-N-C are 1.12 eV/1.00 eV for ORR/OER. We calculate Gibbs-free energy (ΔG) for every reaction step in ORR/OER process. In order to gain a deeper understanding of the catalytic process on the surface of single atom catalysts, the ab initio molecular dynamics (AIMD) simulations show that Ru-N-C has a structural stability at 300 K and that ORR/OER on Ru-N-C can occur along a typical four-electron process of reaction. AIMD simulations of catalytic processes provide detailed information about atom interactions.

METHODS

In this paper, we use density functional theory (DFT) with PBE functional to study the electronic properties and adsorption properties of graphene-supported nitrogen coordinated Ru-atom (Ru-N-C) Gibbs-free energy and Gibbs-free energy for very reaction step. The structural optimization and all the calculations are carried out by Dmol3 package, adopting the PNT basis set and DFT semicore pseudopotential. Ab initio molecular dynamics simulations (AIMD) were run for 10 ps. The canonical (NVT) ensemble, massive GGM thermostat, and a temperature of 300 K are taken into account. The functional of B3LYP and the DNP basis set are chosen for AIMD.

Nitrogen-doped porous carbon encapsulates multivalent cobalt-nickel as oxygen reduction reaction catalyst for zinc-air battery

In this study, we present a bimetallic ion coexistence encapsulation strategy employing hexadecyl trimethyl ammonium bromide (CTAB) as a mediator to anchor cobalt-nickel (CoNi) bimetals in nitrogen-doped porous carbon cubic nanoboxes (CoNi@NC). The fully encapsulated and uniformly dispersed CoNi nanoparticles with the improved density of active sites help to accelerate the oxygen reduction reaction (ORR) kinetics and provide an efficient charge/mass transport environment. Zinc-air battery (ZAB) equipped CoNi@NC as cathode exhibits an open-circuit voltage of 1.45 V, a specific capacity of 870.0 mAh g^-1, and a power density of 168.8 mW cm^-2. Moreover, the two CoNi@NC-based ZABs in series display a stable discharge specific capacity of 783.0 mAh g^-1, as well as a large peak power density of 387.9 mW cm^-2. This work provides an effective way to tune the dispersion of nanoparticles to boost active sites in nitrogen-doped carbon structure, and enhance the ORR activity of bimetallic catalysts.

Strategies for Sustainable Production of Hydrogen Peroxide via Oxygen Reduction Reaction: From Catalyst Design to Device Setup

An environmentally benign, sustainable, and cost-effective supply of H2O2 as a rapidly expanding consumption raw material is highly desired for chemical industries, medical treatment, and household disinfection. The electrocatalytic production route via electrochemical oxygen reduction reaction (ORR) offers a sustainable avenue for the on-site production of H2O2 from O2 and H2O. The most crucial and innovative part of such technology lies in the availability of suitable electrocatalysts that promote two-electron (2e-) ORR. In recent years, tremendous progress has been achieved in designing efficient, robust, and cost-effective catalyst materials, including noble metals and their alloys, metal-free carbon-based materials, single-atom catalysts, and molecular catalysts. Meanwhile, innovative cell designs have significantly advanced electrochemical applications at the industrial level. This review summarizes fundamental basics and recent advances in H2O2 production via 2e--ORR, including catalyst design, mechanistic explorations, theoretical computations, experimental evaluations, and electrochemical cell designs. Perspectives on addressing remaining challenges are also presented with an emphasis on the large-scale synthesis of H2O2 via the electrochemical route.

Challenges and Perspectives of Single-Atom-Based Catalysts for Electrochemical Reactions

Single-atom catalysts (SACs) are emerging as the most promising catalysts for various electrochemical reactions. The isolated dispersion of metal atoms enables high density of active sites, and the simplified structure makes them ideal model systems to study the structure-performance relationships. However, the activity of SACs is still insufficient, and the stability of SACs is usually inferior but has received little attention, hindering their practical applications in real devices. Moreover, the catalytic mechanism on a single metal site is unclear, leading the development of SACs to rely on trial-and-error experiments. How can one break the current bottleneck of active sites density? How can one further increase the activity/stability of metal sites? In this Perspective, we discuss the underlying reasons for the current challenges and identify precisely controlled synthesis involving designed precursors and innovative heat-treatment techniques as the key for the development of high-performance SACs. In addition, advanced operando characterizations and theoretical simulations are essential for uncovering the true structure and electrocatalytic mechanism of an active site. Finally, future directions that may arise breakthroughs are discussed.

Dual Metal Site Fe Single Atom Catalyst with Improved Stability in Acidic Conditions

Dual atom catalysts (DACs) not only retain uniform active sites and high atomic utilization efficiency as the single atom catalysts, but the two adjacent metal sites also cooperate and play a synergistic role to achieve additional benefits. However, the relationships connecting their dual-site synergistic effects on catalytic performance are not well rationalized due to limited pairs available from experiments. Herein, Fe/M dual sites supported by nitrogen doped carbon (Fe/M-N-C whereby M from 3 d–5 d electron containing transition metals) have been screened as an oxygen reduction reaction (ORR) catalyst. The results show that the absorption strength of ORR intermediates on four nitrogen coordinated metals is weaker than the three coordinated metals, which promotes favourable ORR activities. As a result, we recommended FeIr, FeRh, FeRu and FeOs as promising ORR catalysts. Ab initio molecular dynamic (AIMD) simulations suggest Fe/M-N-C (M = Ir, Rh, Ru and Os) catalysts with encouraging structural stability at room temperature. Furthermore, it is found that the nitrogen atoms in-between metals are vulnerable sites for proton attacking, yet the protonation process demands high energy, even under O2 atmosphere, which underlines good tolerance under acidic conditions. This work provides a broad understanding of Fe based catalyst and a new direction for catalytic design.

Generalized Encapsulations of ZIF-Based Fe-N-C Catalysts with Controllable Nitrogen-Doped Carbon for Significantly-Improved Stability Toward Oxygen Reduction Reaction

The vigorous development of efficient platinum group metal-free catalysts is considerably important to facilitate the universal application of proton exchange membrane fuel cells. Although nitrogen-coordinated atomic iron intercalated in carbon matrix (Fe-N-C) catalysts exhibit promising catalytic activity, the performance in fuel cells, especially the short lifetime, remains an obstacle. Herein, a highly-active Fe-N-C catalyst with a power density of >1 w cm^-2 and prolonged discharge stability with a current density of 357 mA cm^-2 after 40 h of constant voltage discharge at 0.7 V in H₂ -O₂ fuel cells using a controllable and efficient N-C coating strategy is developed. It is clarified that a thicker N-C coating may be more favorable to enhance the stability of Fe-N-C catalysts at the expense of their catalytic activity. The stability enhancement mechanism of the N-C coating strategy is proven to be the synergistic effect of reduced carbon corrosion and iron loss. It is believed that these findings can contribute to the development of Fe-N-C catalysts with high activity and long lifetimes.

Curvature effects on the bifunctional oxygen catalytic performance of single atom metal-N-C

Understanding the fundamental relationship between the structural information of electrocatalysts and their catalytic activities plays a key role in controlling many important electrochemical processes. Recently, single-atom catalysts (SACs) with the so-called MN₄ structure, consisting of a central transition metal quadruply bound to four pyridine nitrogen atoms all situated in an extended carbon-based matrix, have attracted intensive scientific attention owing to their exceptional catalytic performance. In this work, we perform the first-principles density functional theory (DFT) calculations to explore the curvature effects of the carbon matrix surfaces on the catalytic activities for two fundamental electrochemical processes, namely, the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Our DFT results suggest that the curved surface structure can weaken the interaction between the metal atom and the N-doped carbon matrix, modify the electronic structure of the metal atom, and thus increase the adsorption strength of the reaction intermediates, resulting in enhanced OER and ORR catalytic activities of MN₄ catalysts. More importantly, a prediction model is developed to evaluate the bifunctional catalytic activities of such catalysts based on their directly obtained parameters including the surface curvature of the catalysts, the number of d electrons of the metal element, and the electronegativity of the metal atom and its coordination atoms in MN₄ catalysts. This prediction model not only provides some candidates, for example, FeN₄, CoN₄ and OsN₄ for the ORR; CoN₄, NiN₄, RuN₄, RhN₄ and IrN₄ for the OER; and CoN₄, RuN₄, IrN₄ and OsN₄ for the bifunctional ORR and OER, but also reasonably links the structure of catalysts with their catalytic performance, providing new possibilities for the quick design of high-performance catalysts.

Regulating Interlayer Confinement FeOCl for Accelerating Polymerization of Pollutants to Reduce Carbon Emission in Water Purification

The spatial structure regulation of catalysts could optimize the reaction pathway and enhance the mass transfer kinetics, which might realize the efficient and low-consumption removal of pollutants in Fenton-like technology. In this study, N,N-dimethylformamide (DMF) intercalation was used to adjust the interlayer spacing of FeOCl from 7.90 to 11.84 Å by a simple and rapid intercalation method, thereby enhancing the mass transfer kinetics and altering the catalytic pathway. The removal rate of BPA in the DMF-FeOCl/PS system increased by 8.78 times, showing good resistance to complex water environments (such as pH, humic acid, and anions), especially in 5 g/L high-salt wastewater. The direct electron transfer processes between Fe(IV) and pollutants mediated by interlayer Fe sites generate phenoxy radicals, and the polymerization processes occur, achieving efficient removal of pollutants and low CO₂ emissions. This study provides new insight into the efficient and low-carbon treatment of high-salt wastewater.

Ultrastable Fe-N-C Fuel Cell Electrocatalysts by Eliminating Non-Coordinating Nitrogen and Regulating Coordination Structures at High Temperatures

Pyrolyzed Fe-N-C materials have attracted considerable interest as one of the most active noble-metal-free electrocatalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Despite significant progress is made in improving their catalytic activity during past decades, the Fe-N-C catalysts still suffer from fairly poor electrochemical and storage stability, which greatly hurdles their practical application. Here, an effective strategy is developed to greatly improve their catalytic stability in PEMFCs and storage stability by virtue of previously unexplored high-temperature synthetic chemistry between 1100 and 1200 °C. Pyrolysis at this rarely adopted temperature range not only enables the elimination of less active nitrogen-doped carbon sites that generate detrimental peroxide byproduct but also regulates the coordination structure of Fe-N-C from less stable D1 (O-FeN₄ C₁₂ ) to a more stable D2 structure (FeN₄ C₁₀ ). The optimized Fe-N-C catalyst exhibits excellent stability in PEMFCs (>80% performance retention after 30 h under H₂ /O₂ condition) and no activity loss after 35 day storage while maintaining a competitive ORR activity and PEMFC performance.

Single Atom Catalysts with Out-of-Plane Coordination Structure on Conjugated Covalent Organic Frameworks

Adjusting the local coordination environment of single-atom electrocatalysts is a viable way to improve catalytic performance. The diversity of coordination geometric structures is limited to the traditional in-plane configuration, with only a little consideration paid to out-of-plane configurations due to the lack of suitable carriers and fabrication methods. This study reports out-of-plane coordination of Co-based single-atom catalysts mediated by the conjugated bipyridine-rich covalent organic framework (COF). The bipyridine nitrogen on the COF layer backbone of these catalysts serves as the linker center for cobalt sites anchoring, while the complementary moieties are coordinated at the other side of the Co metal and reside beyond the COF backbone plane, thus yielding out-of-plane coordination. The electrochemical experiments and density functional theory calculations reveal that catalysts with multiple out-of-plane coordinations exhibit different electrocatalytic oxygen evolution activities and catalytic pathways. The out-of-plane coordination enabled by COFs provides a strategy for designing single-atom electrocatalysts, expanding the application of COFs in the field of electrocatalysis.

Ultrathin Nanotube Structure for Mass-Efficient and Durable Oxygen Reduction Reaction Catalysts in PEM Fuel Cells

It remains a challenge for platinum-based oxygen reduction reaction catalysts to simultaneously possess high mass activity and high durability in proton-exchange-membrane fuel cells. Herein, we report ultrathin holey nanotube (UHT)-structured Pt-M (M = Ni, Co) alloy catalysts that achieve unprecedented comprehensive performance. The nanotubes have ultrathin walls of 2-3 nm and construct self-supporting network-like catalyst layers with thicknesses of less than 1 μm, which have efficient mass transfer and 100% surface exposure, thus enabling high utilization of Pt atoms. Combined with the high intrinsic activity produced by the alloying effect, the catalysts achieve high mass activity. Moreover, the nanotube structure not only avoids the agglomeration problem of nanoparticles, but the low curvature of the tube wall also gives UHT a low surface energy (less than 1/3 of that of the same size nanoparticle), so UHT is more resistant to the Ostwald ripening and is stable. For the first time, the U.S. DOE mass activity target and dual durability targets for load and start-stop cycles are achieved on one catalyst. This study provides an effective structural strategy for the preparation of electrocatalysts with high atomic efficiency and excellent durability.

B, P-co-doped PdCu nanothorn assemblies for enhanced oxygen reduction electrolysis

Nonmetal doping is a promising strategy to improve electrocatalytic performance of noble metal based catalysts for oxygen reduction reaction (ORR). Herein, we report a facile method to fabricate PdCuBP nanothorn assemblies (PdCuBP NTAs) by co-doping B and P into pre-synthesized PdCu NTAs using NaBH₄and NaH₂PO₂as B source and P source, respectively. The metal-nonmetal structure and multi-branched morphology can optimize oxygen adsorption energy and avoid catalyst migration, agglomeration and Ostwald ripening. As such, the obtained PdCuBP NTAs exhibit efficient activity and excellent long-term stability for ORR. This research offers an excellent strategy for co-doping nonmetal elements into metal nanocrystals with controllable composition and structure to improve electrocatalytic ORR performance.

General Carbon-Supporting Strategy to Boost the Oxygen Reduction Activity of Zeolitic-Imidazolate-Framework-Derived Fe/N/Carbon Catalysts in Proton Exchange Membrane Fuel Cells

The oxygen reduction reaction (ORR) activity of the Fe/N/Carbon catalysts derived from the pyrolysis of zeolitic-imidazolate-framework-8 (ZIF-8) has been still lower than that of commercial Pt-based catalysts utilized in the proton exchange membrane fuel cells (PEMFCs) due to low density of accessible active sites. In this study, an efficient carbon-supporting strategy is developed to enhance the ORR efficiency of the ZIF-derived Fe/N/Carbon catalysts by increasing the accessible active site density. The enhancement lies in (i) improving the accessibility of active sites via converting dodecahedral particles to graphene-like layered materials and (ii) enhancing the density of FeN_x active sites via suppressing the formation of nanoparticles as well as providing extra spaces to host active sites. The optimized and efficient Fe/N/Carbon catalyst shows a half-wave potential (E_1/2) of 0.834 V versus reversible hydrogen electrode in acidic media and produces a peak power density of 0.66 W cm^-2 in an air-fed PEMFC at 2 bar backpressure, outperforming most previously reported Pt-free ORR catalysts. Finally, the general applicability of the carbon-supporting strategy is confirmed using five different commercial carbon blacks. This work provides an effective route to derive Fe/N/Carbon catalysts exhibiting a higher power density in PEMFCs.

Iron atom-cluster interactions increase activity and improve durability in Fe-N-C fuel cells

Simultaneously increasing the activity and stability of the single-atom active sites of M–N–C catalysts is critical but remains a great challenge. Here, we report an Fe–N–C catalyst with nitrogen-coordinated iron clusters and closely surrounding Fe–N₄ active sites for oxygen reduction reaction in acidic fuel cells. A strong electronic interaction is built between iron clusters and satellite Fe–N₄ due to unblocked electron transfer pathways and very short interacting distances. The iron clusters optimize the adsorption strength of oxygen reduction intermediates on Fe–N₄ and also shorten the bond amplitude of Fe–N₄ with incoherent vibrations. As a result, both the activity and stability of Fe–N₄ sites are increased by about 60% in terms of turnover frequency and demetalation resistance. This work shows the great potential of strong electronic interactions between multiphase metal species for improvements of single-atom catalysts.

It is challenging to break the activity–stability trade-off in Fe–N–C fuel cell catalysts. Here, the authors show that interactions between iron atoms and clusters accelerate reaction kinetics and suppress demetalation to improve fuel cell stability.

Identification of Catalytic Active Sites for Durable Proton Exchange Membrane Fuel Cell: Catalytic Degradation and Poisoning Perspectives

Recent progress in synthetic strategies, analysis techniques, and computational modeling assist researchers to develop more active catalysts including metallic clusters to single-atom active sites (SACs). Metal coordinated N-doped carbons (M-N-C) are the most auspicious, with a large number of atomic sites, markedly performing for a series of electrochemical reactions. This perspective sums up the latest innovative and computational comprehension, while giving credit to earlier/pioneering work in carbonaceous assembly materials towards robust electrocatalytic activity for proton exchange membrane fuel cells via inclusive performance assessment of the oxygen reduction reaction (ORR). M-N_x -C_y are exclusively defined active sites for ORR, so there is a unique possibility to intellectually design the relatively new catalysts with much improved activity, selectivity, and durability. Moreover, some SACs structures provide better performance in fuel cells testing with long-term durability. The efforts to understand the connection in SACs based M-N_x -C_y moieties and how these relate to catalytic ORR performance are also conveyed. Owing to comprehensive practical application in the field, this study has covered very encouraging aspects to the current durability status of M-N-C based catalysts for fuel cells followed by degradation mechanisms such as macro-, microdegradation, catalytic poisoning, and future challenges.

Recent Advances in ZIF-Derived Atomic Metal-N-C Electrocatalysts for Oxygen Reduction Reaction: Synthetic Strategies, Active Centers, and Stabilities

Exploring highly active, stable electrocatalysts with earth-abundant metal centers for the oxygen reduction reaction (ORR) is essential for sustainable energy conversion. Due to the high cost and scarcity of platinum, it is a general trend to develop metal-N-C (M-N-C) electrocatalysts, especially those prepared from the zeolite imidazolate framework (ZIF) to replace/minimize usage of noble metals in ORR electrocatalysis for their amazingly high catalytic efficiency, great stability, and readily-tuned electronic structure. In this review, the most pivotal advances in mechanisms leading to declined catalytic performance, synthetic strategies, and design principles in engineering ZIF-derived M-N-C for efficient ORR catalysis, are presented. Notably, this review focuses on how to improve intrinsic ORR activity, such as M-N_x -C_y coordination structures, doping metal-free heteroatoms in M-N-C, dual/multi-metal sites, hydrogen passivation, and edge-hosted M-N_x . Meanwhile, how to increase active sites density, including formation of M-N complex, spatial confinement effects, and porous structure design, are discussed. Thereafter, challenges and future perspectives of M-N-C are also proposed. The authors believe this instructive review will provide experimental and theoretical guidance for designing future, highly active ORR electrocatalysts, and facilitate their applications in diverse ORR-related energy technologies.

N-Doped Graphene Supported Cu Single Atoms: Highly Efficient Recyclable Catalyst for Enhanced C-N Coupling Reactions

Heterogenization of homogeneous catalysis through supported single-atom catalysts (SACs) provided a feasible solution to recycling catalysts while keeping its efficiency in chemical synthesis. In this work, Cu SACs anchored on N-doped graphene (Cu SACs/NG) were prepared and first used for C-N coupling reactions. During the preparation, Cu-N-C structures, including Cu-N₄ moieties, were formed in a one-step pyrolysis method. As-prepared Cu SACs/NG exhibited excellent catalytic activity toward C-N coupling reactions with a broad scope of substrates and showed outstanding performance of recycling. Compared with Cu nanoparticles (Cu NPs/NG), the advantages of single-atom catalysts were validated via experimental and theoretical calculations. The enhanced performances were attributed to increasing the number of active sites and increasing the intrinsic activity of each active site. This work provides an alternative synthetic strategy for fabricating atomically dispersed SACs and represents a significant advance for coupling reactions.

Synthesis of mixed-valence Cu phthalocyanine/graphene/g-C3N4 ultrathin heterojunctions as efficient photocatalysts for CO2 reduction

Mixed-valence Cu phthalocyanine/graphene/g-C3N4 ultrathin heterojunctions for efficient photocatalytic CO2 reduction.

Atomic and nanosized Co species functionalized N-doped porous carbon hybrids for boosting electrocatalytic oxygen reduction

Co@NRPC electrocatalysts with excellent ORR performance were synthesized by pyrolyzing the hybrid precursors. Atomic CoNx and nanosized metallic Co species were active sites. Porous carbon hybrids ensured efficient charge and mass transport.

The Underlying Molecular Mechanism of Fence Engineering to Break the Activity-stability Trade-off of Catalysts

Non-precious-metal (NPM) catalysts often face the formidable challenge of a trade-off between long-term stability and high activity, which has not yet been widely addressed. Here we propose distinct molecule-selective fence as a promising novel concept to solve this activity-stability trade-off. This unique fence has the characteristics of preventing poisonous species from invading catalysts, but allowing catalytic reaction-related species to diffuse freely. We applied this concept to construct CoS2 layer with the function of molecular selectivity on the external surface of highly active Co doped MoS2, achieving a remarkable catalytic stability towards alkaline hydrogen evolution reaction, along with a further optimized activity. In situ spectroscopy technologies uncovered the underlying molecule mechanism of the CoS2 fence for breaking the activity-stability trade-off of the MoS2 catalyst. This work offers valuable guidance for rationally designing efficient and stable NPM catalysts.