Research Progress on Atomically Dispersed Fe-N-C Catalysts for the Oxygen Reduction Reaction.

The efficiency and performance of proton exchange membrane fuel cells (PEMFCs) are primarily influenced by ORR electrocatalysts. In recent years, atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts have gained significant attention due to their high active center density, high atomic utilization, and high activity. These catalysts are now considered the preferred alternative to traditional noble metal electrocatalysts. The unique properties of M-N-C catalysts are anticipated to enhance the energy conversion efficiency and lower the manufacturing cost of the entire system, thereby facilitating the commercialization and widespread application of fuel cell technology. This article initially delves into the origin of performance and degradation mechanisms of Fe-N-C catalysts from both experimental and theoretical perspectives. Building on this foundation, the focus shifts to strategies aimed at enhancing the activity and durability of atomically dispersed Fe-N-C catalysts. These strategies encompass the use of bimetallic atoms, atomic clusters, heteroatoms (B, S, and P), and morphology regulation to optimize catalytic active sites. This article concludes by detailing the current challenges and future prospects of atomically dispersed Fe-N-C catalysts.

Role of amorphous engineering and cerium doping in NiFe oxyhydroxide for electrocatalytic water oxidation.

Amorphous engineering and atomistic doping provide an effective way to improve the catalytic activity in the oxygen evolution reaction (OER) of transition metal layered double hydroxides. Herein, Cerium (Ce) was introduced into NiFe-based oxyhydroxide using a modified aqueous sol-gel procedure. Ce as an electron acceptor promoted the coupling oxidation of Ni2+/3+ in NiFe oxyhydroxide, and the activated oxyhydroxide showed excellent catalytic activity in OER. The amorphous NiFeCe oxyhydroxide electrocatalyst demonstrated great modified OER catalytic activity under alkaline conditions and excellent cyclic stability, with an overpotential of only 284 mV at 50 mA cm-2, which was significantly better than amorphous NiFe oxyhydroxide and crystalline NiFeCe oxyhydroxide. Theoretical investigations further indicated that the overpotential of the rate-determining step (*OOH deprotonation) decreased from 0.66 to 0.41 V after Ce doping and strong electron interaction, effectively reducing the dependence of proton activity in the solution of OER, and optimizing the adsorption/desorption process of related oxygen-containing species in the reaction. This work also provides a good reference for optimizing OER activity by using rare-earth-metal induced electronic regulation strategies.

Accelerating Charge Kinetics in Photocatalytic CO2 Reduction by Modulating the Cobalt Coordination in Heterostructures of Cadmium Sulfide/Metal-Organic Layer.

Artificial photocatalytic CO2 reduction (CO2 R) holds great promise to directly store solar energy into chemical bonds. The slow charge and mass transfer kinetics at the triphasic solid-liquid-gas interface calls for the rational design of heterogeneous photocatalysts concertedly boosting interfacial charge transfer, local CO2 concentration, and exposure of active sites. To meet these requirements, in this study heterostructures of CdS/MOL (MOL = metal-organic layer) furnishing different redox Co sites are fabricated for CO2 R photocatalysts. It is found that the coordination environment of Co is key to photocatalytic activity. The best catalyst ensemble comprising ligand-chelated Co2+ with the bipyridine electron mediator demonstrates a high CO yield rate of 1523 µmol h-1 gcat -1 , selectivity of 95.8% and TON of 1462.4, which are ranked among the best seen in literature. Comprehensive photochemical and electroanalytical characterizations attribute the high CO2 R performance to the improved photocarrier separation and charge kinetics originated from the proper energy band alignment and coordination chemistry. This work highlights the construction of 2D heterostructures and modulation of transition metal coordination to expedite the charge kinetics in photocatalytic CO2 reduction.

WCx-Supported RuNi Single Atoms for Electrocatalytic Oxygen Evolution.

Single-atom catalysts anchored to oxide or carbonaceous substances are typically tightly coordinated by oxygen or heteroatoms, which certainly impact their electronic structure and coordination environment, thereby affecting their catalytic activity. In this study, we prepared a stable oxygen evolution reaction (OER) catalyst on tungsten carbide using a simple pyrolysis method. The unique structure of tungsten carbide allows the atomic RuNi catalytic site to weakly bond to the surface W and C atoms. XRD patterns and HRTEM images of the WCx-RuNi showed the characteristics of phase-pure WC and W2C, and the absence of nanoparticles. Combined with XPS, the atomic dispersion of Ru/Ni in the catalyst was confirmed. The catalyst exhibits excellent catalytic ability, with a low overpotential of 330 mV at 50 mA/cm2 in 1 m KOH solutions, and demonstrates high long-term stability. This high OER activity is ascribed to the synergistic action of metal Ru/Ni atoms with double monomers. The addition of Ni increases the state density of WCx-RuNi near the Fermi level, promoting the adsorption of oxygen-containing intermediates and enhancing electron exchange. The larger proximity of the d band center to the Fermi level suggests a strong interaction between the d electrons and the valence or conduction band, facilitating charge transfer. Our research offers a promising avenue for reasonable utilization of inexpensive and durable WCx carrier-supported metal single-atom catalysts for electrochemical catalysis.

Electrolyte-Impregnated Mesoporous Hollow Microreactor to Supplement an Inner Reaction Pathway for Boosting the Cyclability of Li-CO2 Batteries.

Li-CO2 batteries that integrate energy storage with greenhouse gas fixation have received a great deal of attention in the pursuit of carbon neutrality. However, cyclic accumulation of the insulative and insoluble Li2CO3 on the cathode surface severely restrains the battery cyclability, especially under a high depth of discharge/charge. Herein, we design and fabricate a microreactor-type catalyst by embedding Ru nanoparticles into the shells of mesoporous hollow carbon spheres. We show that both the hollow cavity and mesoporous shell are indispensable for concertedly furnishing a high activity to catalyze reversible Li2CO3 formation/decomposition. This unique structure ensures that the Ru sites masked by exterior Li2CO3 deposits during charging can resume the redox process of discharge by working with the prestored electrolyte to establish an inner reaction path. The thus fabricated Li-CO2 batteries demonstrate remarkable cyclability of 1085 cycles under 0.5 Ah g-1 and 326 cycles under 2 Ah g-1 at 1 A g-1, outshining most of the literature reports. This study highlights a smart catalyst design to boost the reversibility and cyclability of Li-CO2 batteries through an "in & out" strategy.

Iron clusters regulate local charge distribution in Fe-N4 sites to boost oxygen electroreduction.

The atomically-dispersed and nitrogen-coordinated iron (FeNC) on a carbon catalyst is a potential non-noble metal catalyst that can replace precious metal electrocatalysts. However, its activity is often unsatisfactory owing to the symmetric charge distribution around the iron matrix. In this study, atomically- dispersed Fe-N4 and Fe nanoclusters loaded with N-doped porous carbon (FeNCs/FeSAs-NC-Z8@34) were rationally fabricated by introducing homologous metal clusters and increasing the N content of the support. FeNCs/FeSAs-NC-Z8@34 exhibited a half-wave potential of 0.918 V, which exceeded that of the commercial benchmark Pt/C catalyst. Theoretical calculations verified that introducing Fe nanoclusters can break the symmetric electronic structure of Fe-N4, thus inducing charge redistribution. Furthermore, it can optimize a part of Fe 3d occupancy orbitals and accelerate OO fracture in OOH* (rate-determining step), thus significantly improving oxygen reduction reaction activity. This work provides a reasonably advanced pathway to modulate the electronic structure of the single-atom center and optimize the catalytic activity of single-atom catalysts.

Homogenizing Li2CO3 Nucleation and Growth through High-Density Single-Atomic Ru Loading toward Reversible Li-CO2 Reaction.

The high activation barrier and sluggish kinetics of Li2CO3 decomposition impose a severe challenge on the development of a Li-CO2 battery with high Coulombic efficiency. To tackle this issue, herein we devise a novel synthetic tactic by combining electrostatic assembly with in situ polycondensation to obtain a single-atomic Ru catalyst of high density up to ∼5 wt %. When deployed to the CO2 cathode, the catalyst delivered an extraordinary capacity of 44.7 Ah g-1, an ultralow charge/discharge polarization of 0.97 V at 0.1 A g-1 (1.90 V at 2 A g-1), and a long-term cycling stability up to 367 cycles at 1 Ah g-1 (196 cycles at 2 Ah g-1), outshining most of the state-of-the-art CO2 cathode catalysts reported today. Further through extensive in situ and ex situ electroanalytical, spectroscopic, and microscopic characterizations, we attribute the superb battery performance mainly to the highly reversible Li2CO3 formation/decomposition, facilitated by the homogenized and downsized Li2CO3 nucleation and growth on account of the high density single-atomic Ru loading. This work not only offers a facile method to fabricate single-atom catalysts with high mass loading but also sheds light on promoting the reversible Li-CO2 reaction by mediating product morphology.

Geometric Modulation of Local CO Flux in Ag@Cu2 O Nanoreactors for Steering the CO2 RR Pathway toward High-Efficacy Methane Production.

The electroreduction of carbon dioxide (CO2 RR) to CH4 stands as one of the promising paths for resourceful CO2 utilization in meeting the imminent "carbon-neutral" goal of the near future. Yet, limited success has been witnessed in the development of high-efficiency catalysts imparting satisfactory methane selectivity at a commercially viable current density. Herein, a unique category of CO2 RR catalysts is fabricated with the yolk-shell nanocell structure, comprising an Ag core and a Cu2 O shell that resembles the tandem nanoreactor. By fixing the Ag core and tuning the Cu2 O envelope size, the CO flux arriving at the oxide-derived Cu shell can be regulated, which further modulates the *CO coverage and *H adsorption at the Cu surface, consequently steering the CO2 RR pathway. Density functional theory simulations show that lower CO coverage favors methane formation via stabilizing the intermediate *CHO. As a result, the best catalyst in the flow cell shows a high CH4 Faraday efficiency of 74 ± 2% and partial current density of 178 ± 5 mA cm- 2 at -1.2 VRHE , ranking above the state-of-the-art catalysts reported today for methane production. These findings mark the significance of precision synthesis in tailoring the catalyst geometry for achieving desired CO2 RR performance.

Wax-Transferred Hydrophobic CVD Graphene Enables Water-Resistant and Dendrite-Free Lithium Anode toward Long Cycle Li-Air Battery.

One of the key challenges in achieving practical lithium-air battery is the poor moisture tolerance of the lithium metal anode. Herein, guided by theoretical modeling, an effective tactic for realizing water-resistant Li anode by implementing a wax-assisted transfer protocol is reported to passivate the Li surface with an inert high-quality chemical vapor deposition (CVD) graphene layer. This electrically conductive and mechanically robust graphene coating enables serving as an artificial solid/electrolyte interphase (SEI), guiding homogeneous Li plating/stripping, suppressing dendrite and "dead" Li formation, as well as passivating the Li surface from moisture erosion and side reactions. Consequently, lithium-air batteries fabricated with the passivated Li anodes demonstrate a superb cycling performance up to 2300 h (230 cycles at 1000 mAh g-1 , 200 mA g-1 ). More strikingly, the anode recycled thereafter can be recoupled with a fresh cathode to continuously run for 400 extended hours. Comprehensive time-lapse and ex situ microscopic and spectroscopic investigations are further carried out for elucidating the fundamentals behind the extraordinary air and electrochemical stability.

Revisiting the Grain and Valence Effect of Oxide-Derived Copper on Electrocatalytic CO2 Reduction Using Single Crystal Cu(111) Foils.

Oxide-derived Cu (OD-Cu) has been viewed as a highly active form for catalyzing the multielectron transfer of electrochemical CO2 reduction, but the underlying catalytic mechanism is still controversial. In the current study, the crystalline and valency factors that influence the CO2R activities of OD-Cu are revisited by employing single crystal Cu(111) foils that exclude convolutions from initial morphological and crystallographic heterogeneity. We observe that the overall CO2R performance, especially the C2H4 selectivity, correlates well with the initial oxidation level of the Cu(111) foil, of which the surface oxide layer is reduced into small fragments comprising rich grain boundaries and diversely orientated facets. Nonetheless, we find that the polycrystallinity and grain boundaries of OD-Cu, in this circumstance, are not the major causes of the observed activity enhancement. Instead, a transition state between the initial oxide and the finally reduced copper phases, as well as its longevity, dictates the catalytic property of OD-Cu in electrochemical CO2 reduction. Consequently, this work furnishes further evidence and in-depth understanding to help clarify the catalytic mechanism of OD-Cu in CO2R.

Breaking the Linear Scaling Relationship by Compositional and Structural Crafting of Ternary Cu-Au/Ag Nanoframes for Electrocatalytic Ethylene Production.

Electrocatalytic conversion of carbon dioxide into high-value multicarbon (C2+ ) chemical feedstocks offers a promising avenue to liberate the chemical industry from fossil-resource dependence and eventually close the anthropogenic carbon cycle but is severely impeded by the lack of high-performance catalysts. To break the linear scaling relationship of intermediate binding and minimize the kinetic barrier of CO2 reduction reactions, ternary Cu-Au/Ag nanoframes were fabricated to decouple the functions of CO generation and C-C coupling, whereby the former is promoted by the alloyed Ag/Au substrate and the latter is facilitated by the highly strained and positively charged Cu domains. Thus, C2 H4 production in an H-cell and a flow cell occurred with high Faradic efficiencies of 69±5 and 77±2 %, respectively, as well as good electrocatalytic stability and material durability. In situ IR and DFT calculations unveiled two competing pathways for C2 H4 generation, of which direct CO dimerization is energetically favored.

Topotactically Transformed Polygonal Mesopores on Ternary Layered Double Hydroxides Exposing Under-Coordinated Metal Centers for Accelerated Water Dissociation.

Layered double hydroxides (LDHs) have been recognized as potent electrocatalysts for oxygen evolution reaction (OER), but are lacking in hydrogen evolution reaction (HER) activities due to the sluggish kinetics of water dissociation in alkaline medium. Herein, aiming to simultaneously bolster the HER and OER kinetics, a metal-organic framework (MOF) mediated topotactic transformation tactic is deployed to fabricate holey ternary CoFeNi LDHs on nickel foam, exposing polygonal mesopores with atomistic edge steps and lattice defects. The optimized catalyst requires only an external voltage of 1.49 V to afford the water splitting current density of 10 mA cm-2 apart from the superb electrolytic stability, far surpassing the benchmark Pt/C||RuO2 couple. More importantly, mechanistic investigations utilizing advanced spectroscopies in conjunction with density function theory (DFT) understandings unravel while the synergetic effect among under-coordinated metal centers lowers the energy barrier of water dissociation, Fe-doping enables further modulating the d-band density of states (DOS) of Co and Ni in favor of intermediates binding, thereby promoting the intrinsic HER activity. Operando Raman studies reveal negligible structural change of the LDHs during the HER process, whereas for OER the active sites can quickly turn into oxyhydroxides in the presence of lattice defects and under-coordinated metal centers.

Elucidation of Active Sites on S, N Codoped Carbon Cubes Embedding Co-Fe Carbides toward Reversible Oxygen Conversion in High-Performance Zinc-Air Batteries.

The development of high-performance but low-cost catalysts for the electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of central importance for realizing the prevailing application of metal-air batteries. Herein a facile route is devised to synthesize S, N codoped carbon cubes embedding Co-Fe carbides by pyrolyzing the Co-Fe Prussian blue analogues (PBA) coated with methionine. Via the strong metal-sulfur interaction, the methionine coating provides a robust sheath to restrain the cubic morphology of PBA upon pyrolysis, which is proved highly beneficial for promoting the specific surface area and active sites exposure, leading to remarkable bifunctionality of ORR and OER comparable to the benchmarks of Pt/C and RuO2 . Further elaborative investigations on the activity origin and postelectrolytic composition unravel that for ORR the high activity is mainly contributed by the S, N codoped carbon shell with the inactive carbide phase converting into carbonate hydroxides. For OER, the embedded Co-Fe carbides transform in situ into layered (hydr)oxides, serving as the actual active sites for promoting water oxidation. Zn-air batteries employing the developed hollow structure as the air cathode catalyst demonstrate superb rechargeability, energy efficiency, as well as portability.

Self-Phosphorization of MOF-Armored Microbes for Advanced Energy Storage.

Utilization of microbes as the carbon source and structural template to fabricate porous carbon has incentivized great interests owing to their diverse micromorphology and intricate intracellular structure, apart from the obvious benefit of "turning waste into wealth." Challenges remain to preserve the biological structure through the harsh and laborious post-synthetic treatments, and tailor the functionality as desired. Herein, Escherichia coli is directly coated with metal-organic frameworks (MOFs) through in situ assembly to fabricate N, P co-doped porous carbon capsules expressing self-phosphorized metal phosphides. While the MOF coating serves as an armoring layer for facilitating the morphology inheritance from the bio-templates and provides metal sources for generating extra porosity and electrochemically active sites, the P-rich phospholipids and N-rich proteins from the plasma membrane enable carbon matrix doping and further yield metal phosphides. These unique structural and compositional features endow the carbon capsules with great capabilities in suppressing polysulfide shuttling and catalyzing reversible oxygen conversion, ultimately leading to the superb performance of lithium-sulfur batteries and zinc-air batteries. Combining the bio-templating strategy with hierarchical MOF assembly, this work opens a new avenue for the fabrication of highly porous and functional carbon for advanced energy applications.

Nitrogen-doped carbon fibers embedding CoOx nanoframes towards wearable energy storage.

As continuous consumption of the world's lithium reserves is causing concern, alternative energy storage solutions based on earth-abundant elements, such as sodium-ion batteries and zinc-air batteries, have been attracting increasing attention. Herein, nanoframes of CoOx are encapsulated into carbonized microporous fibers by electrospinning zeolitic imidazolate frameworks to impart both a sodium-hosting capability and catalytic activities for reversible oxygen conversion. The ultrahigh rate performance of sodium-ion batteries up to 20 A g-1 and ultrastable cycling over 6000 cycles are attributed to a dual-buffering effect from the framework structure of CoOx and the confinement of carbon fibers that effectively accommodates cyclic volume fluctuation. Both in situ Raman and ex situ microscopic analyses unveil the reversible conversion of CoOx during the sodiation/desodiation process. The excellent ORR activity, superior to that of commercial Pt/C, is mainly ascribed to the abundant Co-N-C species and the full exposure of active sites on the microporous framework structure. Flexible and rechargeable sodium-ion full batteries and zinc-air batteries are further demonstrated with great energy efficiency and cycling stability, as well as mechanical deformability.

Redox-Driven Lithium Perfusion to Fabricate Li@Ni-Foam Composites for High Lithium-Loading 3D Anodes.

As the hostless nature of the conventional Li anodes with planar surfaces inevitably causes volume expansion and parasitic dendrite growth, it is essential to develop a composite electrode structure with improved Li plating/stripping behaviors to mitigate such issues. Herein, a composite Li@NF anode was successfully fabricated through lithium perfusion into the commercial nickel foam (NF) decorated with lithiophilic NiO nanosheets, demonstrating an exceptionally high areal Li loading of 53.2 mg cm-2 with suppressed Li dendrite formation and volume expansion, improved Coulombic efficiency, as well as extended cycling stability in all half, symmetric, and full cell tests. More importantly, density functional theory calculations and control studies with Fe2O3@NF, pristine NF, and Cu2O@CF reveal a linear correlation between the thermodynamics of the surface reactions and the lithiophilicity of the reaction products, attesting to a redox-driven Li perfusion process. Further, through ex situ scanning electron and in situ optical microscopy, the enhanced performance of Li@NF is mainly attributed to the mediation of Li plating/stripping through homogenizing the Li+ flux, decentralizing local charge density, and accommodating multidirectional Li deposition by the conductive 3D scaffolds. Consequently, this study offers important insights into the driving of thermal Li perfusion through appropriate material and surface design for achieving safe and stable lithium metal anodes.

Unpaired 3d Electrons on Atomically Dispersed Cobalt Centres in Coordination Polymers Regulate both Oxygen Reduction Reaction (ORR) Activity and Selectivity for Use in Zinc-Air Batteries.

Reversible oxygen conversion is important for various green energy technologies. Herein we synthesize a series of bimetallic coordination polymers by varying the Ni/Co ratio and using HITP (HITP=2,3,6,7,10,11-hexaiminotriphenylene) as the ligand, to interrogate the role of metal centres in modulating the activity of the oxygen reduction reaction (ORR). Co3 HITP2 and Ni3 HITP2 are compared. Unpaired 3d electrons in Co3 HITP2 result in less coplanarity but more radical character. Thus, despite of a reduced crystallinity and conductivity, the best ORR activity, comparable to 20 % Pt/C, is obtained for Co3 HITP2 , showing the 3d orbital configuration of the metal centre promotes ORR. Experimental and DFT studies show a transition of ORR pathway from four-electron for Co3 HITP2 to two-electron for Ni3 HITP2 . Rechargeable zinc-air batteries using Co3 HITP2 as the air cathode catalyst demonstrate excellent energy efficiency and stability.

Octahedral gold-silver nanoframes with rich crystalline defects for efficient methanol oxidation manifesting a CO-promoting effect.

Three-dimensional bimetallic nanoframes with high spatial diffusivity and surface heterogeneity possess remarkable catalytic activities owing to their highly exposed active surfaces and tunable electronic structure. Here we report a general one-pot strategy to prepare ultrathin octahedral Au3Ag nanoframes, with the formation mechanism explicitly elucidated through well-monitored temporal nanostructure evolution. Rich crystalline defects lead to lowered atomic coordination and varied electronic states of the metal atoms as evidenced by extensive structural characterizations. When used for electrocatalytic methanol oxidation, the Au3Ag nanoframes demonstrate superior performance with a high specific activity of 3.38 mA cm-2, 3.9 times that of the commercial Pt/C. More intriguingly, the kinetics of methanol oxidation on the Au3Ag nanoframes is counter-intuitively promoted by carbon monoxide. The enhancement is ascribed to the altered reaction pathway and enhanced OH- co-adsorption on the defect-rich surfaces, which can be well understood from the d-band model and comprehensive density functional theory simulations.

A Double-Buffering Strategy to Boost the Lithium Storage of Botryoid MnOx /C Anodes.

Transition metal oxides (TMOs) are regarded as promising candidates for anodes of lithium ion batteries, but their applications have been severely hindered by poor material conductivity and lithiated volume expansion. As a potential solution, herein is presented a facile approach, by electrospinning a manganese-based metal organic framework (Mn-MOF), to fabricate yolk-shell MnOx nanostructures within carbon nanofibers in a botryoid morphology. While the yolk-shell structure accomodates the lithiated volume expansion of MnOx , the fiber confinement ensures the structural integrity during charge/discharge, achieving a so-called double-buffering for cyclic volume fluctuation. The formation mechanism of the yolk-shell structure is well elucidated through comprehensive instrumental characterizations and cogitative control experiments, following a combined Oswald ripening and Kirkendall process. Outstanding electrochemical performances are demonstrated with prolonged stability over 1000 cycles, boosted by the double-buffering design, as well as the "breathing" effect of lithiation/delithiation witnessed by ex situ imaging. Both the fabrication methodology and electrochemical understandings gained here for nanostructured MnOx can also be extended to other TMOs toward their ultimate implementation in high-performance lithium ion batteries (LIBs).

Carved nanoframes of cobalt-iron bimetal phosphide as a bifunctional electrocatalyst for efficient overall water splitting.

Water electrolysis for hydrogen production has long been regarded as an ideal tactic for renewable energy conversion and storage, but is impeded by the sluggish kinetics of both the hydrogen and oxygen evolution reactions, which are therefore in urgent need for high-performance but low-cost electrocatalysts. Herein, nanoframes of transition metal phosphides (TMPs) with the 3D framework carved open have been demonstrated as highly potent bifunctional catalysts for overall water splitting, reaching the benchmark performance of the Pt/C‖RuO2 couple, and are much superior to their nanocubic counterparts. This excellent water splitting behavior can be attributed to the enlarged active surface area, less obstructed electrolyte infiltration, promoted charge transfer, and facilitated gas release. Further through in-depth activity analysis and post-electrocatalysis characterization, special attention has been paid to the fate and role of phosphorus in the electrocatalytic process, suggesting that despite the chemical instability of the TMPs (especially under OER conditions), excellent electrocatalytic stability can still be achieved through the amorphous bimetallic hydroxides/oxides formed in situ.