Electrochemical CO2 Reduction with Atomic Iron‐Dispersed on Nitrogen‐Doped Graphene

Electrochemical reduction of CO₂ provides an opportunity to reach a carbon‐neutral energy recycling regime, in which CO₂ emissions from fuel use are collected and converted back to fuels. The reduction of CO₂ to CO is the first step toward the synthesis of more complex carbon‐based fuels and chemicals. Therefore, understanding this step is crucial for the development of high‐performance electrocatalyst for CO₂ conversion to higher order products such as hydrocarbons. Here, atomic iron dispersed on nitrogen‐doped graphene (Fe/NG) is synthesized as an efficient electrocatalyst for CO₂ reduction to CO. Fe/NG has a low reduction overpotential with high Faradic efficiency up to 80%. The existence of nitrogen‐confined atomic Fe moieties on the nitrogen‐doped graphene layer is confirmed by aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy and X‐ray absorption fine structure analysis. The Fe/NG catalysts provide an ideal platform for comparative studies of the effect of the catalytic center on the electrocatalytic performance. The CO₂ reduction reaction mechanism on atomic Fe surrounded by four N atoms (Fe–N₄) embedded in nitrogen‐doped graphene is further investigated through density functional theory calculations, revealing a possible promotional effect of nitrogen doping on graphene.     

Activation of Ni Particles into Single Ni–N Atoms for Efficient Electrochemical Reduction of CO2

Electrochemical reduction of carbon dioxide (CO₂) to fuels and value‐added industrial chemicals is a promising strategy for keeping a healthy balance between energy supply and net carbon emissions. Here, the facile transformation of residual Ni particle catalysts in carbon nanotubes into thermally stable single Ni atoms with a possible NiN₃ moiety is reported, surrounded with a porous N‐doped carbon sheath through a one‐step nanoconfined pyrolysis strategy. These structural changes are confirmed by X‐ray absorption fine structure analysis and density functional theory (DFT) calculations. The dispersed Ni single atoms facilitate highly efficient electrocatalytic CO₂ reduction at low overpotentials to yield CO, providing a CO faradaic efficiency exceeding 90%, turnover frequency approaching 12 000 h^?1, and metal mass activity reaching about 10 600 mA mg^?1, outperforming current state‐of‐the‐art single atom catalysts for CO₂ reduction to CO. DFT calculations suggest that the Ni@N₃ (pyrrolic) site favors *COOH formation with lower free energy than Ni@N₄, in addition to exothermic CO desorption, hence enhancing electrocatalytic CO₂ conversion. This finding provides a simple, scalable, and promising route for the preparation of low‐cost, abundant, and highly active single atom catalysts, benefiting future practical CO₂ electrolysis.     

Tunable and Efficient Tin Modified Nitrogen‐Doped Carbon Nanofibers for Electrochemical Reduction of Aqueous Carbon Dioxide

Efficient and selective earth‐abundant catalysts are highly desirable to drive the electrochemical conversion of CO₂ into value‐added chemicals. In this work, a low‐cost Sn modified N‐doped carbon nanofiber hybrid catalyst is developed for switchable CO₂ electroreduction in aqueous medium via a straightforward electrospinning technique coupled with a pyrolysis process. The electrocatalytic performance can be tuned by the structure of Sn species on the N‐doped carbon nanofibers. Sn nanoparticles drive efficient formate formation with a high current density of 11 mA cm^?2 and a faradaic efficiency of 62% at a moderate overpotential of 690 mV. Atomically dispersed Sn species promote conversion of CO₂ to CO with a high faradaic efficiency of 91% at a low overpotential of 490 mV. The interaction between Sn species and pyridinic‐N may play an important role in tuning the catalytic activity and selectivity of these two materials.     

Densely Populated Isolated Single Co?N Site for Efficient Oxygen Electrocatalysis

Atomically dispersed transition metals confined with nitrogen on a carbon support has demonstrated great electrocatalytic performance, but an extremely low concentration of metal atoms (usually below 1.5%) is necessary to avoid aggregation through sintering which limits mass activity. Here, a salt‐template method to fabricate densely populated, monodispersed cobalt atoms on a nitrogen‐doped graphene‐like carbon support is reported, and achieving a dramatically higher site fraction of Co atoms (≈15.3%) in the catalyst and demonstrating excellent electrocatalytic activity for both the oxygen reduction reaction and oxygen evolution reaction. The atomic dispersion and high site fraction of Co provide a large electrochemically active surface area of ≈105.6 m² g^?1, leading to very high mass activity for ORR (≈12.164 A mg_Co^?1 at 0.8 V vs reversible hydrogen electrode), almost 10.5 times higher than that of the state‐of‐the‐art benchmark Pt/C catalyst (1.156 A mg_Pt^?1 under similar conditions). It also demonstrates an outstanding mass activity for OER (0.278 A mg_Co^?1). The Zn‐air battery based on this bifunctional catalyst exhibits high energy density of 945 Wh kg_Zn^?1 as well as remarkable stability. In addition, both density functional theory based simulations and experimental measurements suggest that the Co? N₄ sites on the carbon matrix are the most active sites for the bifunctional oxygen electrocatalytic activity.     

Atomic Modulation of FeCo–Nitrogen–Carbon Bifunctional Oxygen Electrodes for Rechargeable and Flexible All‐Solid‐State Zinc–Air Battery

Rational design and exploration of robust and low‐cost bifunctional oxygen reduction/evolution electrocatalysts are greatly desired for metal–air batteries. Herein, a novel high‐performance oxygen electrode catalyst is developed based on bimetal FeCo nanoparticles encapsulated in in situ grown nitrogen‐doped graphitic carbon nanotubes with bamboo‐like structure. The obtained catalyst exhibits a positive half‐wave potential of 0.92 V (vs the reversible hydrogen electrode, RHE) for oxygen reduction reaction, and a low operating potential of 1.73 V to achieve a 10 mA cm^?2 current density for oxygen evolution reaction. The reversible oxygen electrode index is 0.81 V, surpassing that of most highly active bifunctional catalysts reported to date. By combining experimental and simulation studies, a strong synergetic coupling between FeCo alloy and N‐doped carbon nanotubes is proposed in producing a favorable local coordination environment and electronic structure, which affords the pyridinic N‐rich catalyst surface promoting the reversible oxygen reactions. Impressively, the assembled zinc–air batteries using liquid electrolytes and the all‐solid‐state batteries with the synthesized bifunctional catalyst as the air electrode demonstrate superior charging–discharging performance, long lifetime, and high flexibility, holding great potential in practical implementation of new‐generation powerful rechargeable batteries with portable or even wearable characteristic.     

Metal‐Free Artificial Photosynthesis of Carbon Monoxide Using N‐Doped ZnTe Nanorod Photocathode Decorated with N‐Doped Carbon Electrocatalyst Layer

An artificial photosynthesis system based on N‐doped ZnTe nanorods decorated with an N‐doped carbon electrocatalyst layer is fabricated via an all‐solution process for the selective conversion of CO₂ to CO. Substitutional N‐doping into the ZnTe lattice decreases the bandgap slightly and improves the charge transfer characteristics, leading to enhanced photoelectrochemical activity. Remarkable N‐doping effects are also demonstrated by the N‐doped carbon layer that promotes selective CO₂‐to‐CO conversion instead of undesired water‐to‐H₂ reduction by providing active sites for CO₂ adsorption and activation, even in the absence of metallic redox centers. The photocathode shows promising performance in photocurrent generation (?1.21 mA cm^?2 at ?0.11 V_RHE), CO selectivity (dominant CO production of ≈72%), minor H₂ reduction (≈20%), and stability (corrosion suppression). The metal‐free electrocatalyst/photocatalyst combination prepared via a cost‐effective solution process exhibits high performance due to synergistic effects between them, and thus may find application in practical solar fuel production.     

Architecting Nitrogen Functionalities on Graphene Oxide Photocatalysts for Boosting Hydrogen Production in Water Decomposition Process

This study elucidates how nitrogen functionalities influence the transition and transfer of photogenerated electrons in graphene‐based materials. Graphene oxide dots (GODs) and Nitrogen‐doped GODs (NGODs) are synthesized by thermally treating graphene oxide (GO) sheets in argon and ammonia, respectively, and then ultrasonically exfoliating the sheets in nitric acid. The nitrogen functionalities of NGODs are mainly quaternary/pyridinic/pyrrolic, and the nitrogen atoms in these functionalities are planar to the GO sheets and repair the vacancy defects on the sheets. Hydrothermal treatment of NGODs in ammonia yields ammonia‐treated NGODs (A‐NGODs), with some pyridinic/pyrrolic groups being converted to amino/amide groups. The nitrogen atoms in the amino/amide groups are not planar to the GO sheets and are prone to donate their lone pair electrons to resonantly conjugate with the aromatic π electrons. The promoted conjugation facilitates the relaxation of photogenerated electrons to the triplet states and prolongs the electron lifetime. When deposited with Pt as the co‐catalyst, the samples catalyze H₂ production from an aqueous triethanolamine solution under 420 nm monochromatic irradiation at quantum yields of 7.3% (GODs), 9.7% (NGODs), and 21% (A‐NGODs). The high activity of A‐NGODs demonstrates that architecting nitrogen functionalities effectively mediate charge motion in carbon‐based materials for application to photoenergy conversion.     

A Disquisition on the Active Sites of Heterogeneous Catalysts for Electrochemical Reduction of CO2 to Value‐Added Chemicals and Fuel

Renewable‐electricity‐powered electrocatalytic CO₂ reduction reactions (CO₂RR) have been identified as an emerging technology to address the issue of rising CO₂ emissions in the atmosphere. While the CO₂RR has been demonstrated to be technically feasible, further improvements in catalyst performance through active sites engineering are a prerequisite to accelerate its commercial feasibility for utilization in large CO₂‐emitting industrial sources. Over the years, the improved understanding of the interaction of CO₂ with the active sites has allowed superior catalyst design and subsequent attainment of prominent CO₂RR activity in literature. This review tracks the evolution of the understanding of CO₂RR active sites on different electrocatalysts such as metals, metal‐oxides, single atoms, metal‐carbon, and subsequently metal‐free carbon‐based catalysts. Despite the tremendous research efforts in the field, many scientific questions on the role of various active sites in governing CO₂RR activity, selectivity, stability, and pathways are still unanswered. These gaps in knowledge are highlighted and a discussion is set forth on the merits of utilizing advanced in‐situ and operando characterization techniques and machine learning (ML). Using this technique, the underlying mechanisms can be discerned, and as a result new strategies for designing active sites may be uncovered. Finally, this review advocates an interdisciplinary approach to discover and design CO₂RR active sites (rather than focusing merely on catalyst activity) in a bid to stimulate practical research for industrial application.     

Nitrogen‐Doped CNx/CNTs Heteroelectrocatalysts for Highly Efficient Dye‐Sensitized Solar Cells

The use of polydopamine as a nitrogen containing precursor to generate catalytically active nitrogen‐doped carbon (CN_x) materials on carbon nanotubes (CNTs) is reported. These N‐doped CN_x/CNT materials display excellent electrocatalytic activity toward the reduction of triiodide electrolyte in dye‐sensitized solar cells (DSSCs). Further, the influence of various synthesis parameters on the catalytic performance of CN_x/CNTs is investigated in detail. The best performing device fabricated with the CN_x/CNTs material delivers power conversion efficiency of 7.3%, which is comparable or slightly higher than that of Pt (7.1%) counter electrode‐based DSSC. These CN_x/CNTs materials show great potential to address the issues associated with the Pt electrocatalyst including the high cost and scarcity.     

A Large Scalable and Low‐Cost Sulfur/Nitrogen Dual‐Doped Hard Carbon as the Negative Electrode Material for High‐Performance Potassium‐Ion Batteries

Among the negative electrode materials for potassium ion batteries, carbon is very promising because of its low cost and environmental benignity. However, the relatively low storage capacity and sluggish kinetics still hinder its practical application. Herein, a large scalable sulfur/nitrogen dual‐doped hard carbon is prepared via a facile pyrolysis process with low‐cost sulfur and polyacrylonitrile as precursors. The dual‐doped hard carbon exhibits hierarchical structure, abundant defects, and functional groups. The material delivers a high reversible potassium storage capacity and excellent rate performance. In particular, a high reversible capacity of 213.7 and 144.9 mA h g^?1 can be retained over 500 cycles at 0.1 A g^?1 and 1200 cycles at 3 A g^?1, respectively, demonstrating remarkable cycle stability at both low and high rates, superior to the other carbon materials reported for potassium storage, to the best of the authors' knowledge. Structure and kinetics studies suggest that the dual‐doping enhances the potassium diffusion and storage, profiting from the formation of a hierarchical structure, introduction of defects, and generation of increased graphitic and pyridinic N sites. This study demonstrates that a facile and scalable pyrolysis strategy is effective to realize hierarchical structure design and heteroatom doping of carbon, to achieve excellent potassium storage performance.     

Ultrafine Metal Nanoparticles/N‐Doped Porous Carbon Hybrids Coated on Carbon Fibers as Flexible and Binder‐Free Water Splitting Catalysts

By employing in situ reduction of metal precursor and metal‐assisted carbon etching process, this study achieves a series of ultrafine transition metal‐based nanoparticles (Ni–Fe, Ni–Mo) embedded in N‐doped carbon, which are found efficient catalysts for electrolytic water splitting. The as‐prepared hybrid materials demonstrate outstanding catalytic activities as non‐noble metal electrodes rendered by the synergistic effect of bimetal elements and N‐dopants, the improved electrical conductivity, and hydrophilism. Ni/Mo₂C@N‐doped porous carbon (NiMo‐polyvinylpyrrolidone (PVP)) and NiFe@N‐doped carbon (NiFe‐PVP) produce low overpotentials of 130 and 297 mV at a current density of 10 mA cm^?2 as catalysts for hydrogen evolution reaction and oxygen evolution reaction, respectively. In addition, these binder‐free electrodes show long‐term stability. Overall water splitting is also demonstrated based on the couple of NiMo‐PVP||NiFe‐PVP catalyzer. This represents a simple and effective synthesis method toward a new type of nanometal–carbon hybrid electrodes.     

2D Assembly of Confined Space toward Enhanced CO2 Electroreduction

Rational design of electrocatalysts toward efficient CO₂ electroreduction has the potential to reduce carbon emission and produce value‐added chemicals. In this work, a strategy of constructing 2D confined‐space as molecular reactors for enhanced electrocatalytic CO₂ reduction selectivity is demonstrated. Highly ordered 2D nanosheet lamella assemblies are achieved via weak molecular interaction of atomically thin titania nanosheets, a variety of cationic surfactants, and SnO₂ nanoparticles. The interlayer spacings can be tuned from 0.9 to 3.0 nm by using different surfactant molecules. These 2D assemblies of confined‐space catalysts exhibit a strong size dependence of CO₂ electroreduction selectivity, with a peak Faradaic efficiency of 73% for formate production and excellent electrochemical stability at an optimal interspacing of ≈2.0 nm. This work suggests great potential for constructing new molecular‐size reactors, for highly selective electrocatalytic CO₂ reduction.     

3D Synergistically Active Carbon Nanofibers for Improved Oxygen Evolution

Developing earth‐abundant and active electrocatalysts for the oxygen evolution reaction (OER) as replacements for conventional noble metal catalysts is of scientific and technological importance for achieving cost‐effective and efficient conversion and storage of renewable energy. However, most of the promising candidates thus far are exclusively metal‐based catalysts, which are disadvantaged by relatively restricted electron mobility, corrosion susceptibility, and detrimental environmental influences. Herein, hierarchically porous nitrogen (N) and phosphorus (P) codoped carbon nanofibers directly grown on conductive carbon paper are prepared through an electrochemically induced polymerization process in the presence of aniline monomer and phosphonic acid. The resultant material exhibits robust stability (little activity attenuation after 12 h continuous operation) and high activity with low overpotential (310 mV at 10 mA cm^?2) toward electrocatalytic oxygen production, with performance comparable to that of the precious iridium oxide (IrO₂) benchmark. Experimental measurements reveal that dual doping of N and P can result in an increased active surface area and abundant active sites in comparison with the single doped and pristine carbon counterparts, and density functional theory calculations indicate that N and P dopants can coactivate the adjacent C atoms, inducing synergistically enhanced activity toward OER.     

Electrocatalytic Reduction of Gaseous CO2 to CO on Sn/Cu‐Nanofiber‐Based Gas Diffusion Electrodes

Earth‐abundant Sn/Cu catalysts are highly selective for the electrocatalytic reduction of CO₂ to CO in aqueous electrolytes. However, CO₂ mass transport limitations, resulting from the low solubility of CO₂ in water, so far limit the CO partial current density for Sn/Cu catalysts to about 10 mA cm^?2. Here, a freestanding gas diffusion electrode design based on Sn‐decorated Cu‐coated electrospun polyvinylidene fluoride nanofibers is demonstrated. The use of gaseous CO₂ as a feedstock alleviates mass transport limitations, resulting in high CO partial current densities above 100 mA cm^?2, while maintaining high CO faradaic efficiencies above 80%. These results represent an important step toward an economically viable pathway to CO₂ reduction.     

FeN4 Sites Embedded into Carbon Nanofiber Integrated with Electrochemically Exfoliated Graphene for Oxygen Evolution in Acidic Medium

Development of inexpensive and efficient oxygen evolution reaction (OER) catalysts in acidic environment is very challenging, but it is important for practical proton exchange membrane water electrolyzers. A molecular iron–nitrogen coordinated carbon nanofiber is developed, which is supported on an electrochemically exfoliated graphene (FeN₄/NF/EG) electrocatalyst through carbonizing the precursor composed of iron ions absorbed on polyaniline‐electrodeposited EG. Benefitting from the unique 3D structure, the FeN₄/NF/EG hybrid exhibits a low overpotential of ≈294 mV at 10 mA cm^?2 for the OER in acidic electrolyte, which is much lower than that of commercial Ir/C catalysts (320 mV) as well as all previously reported acid transitional metal‐derived OER electrocatalysts. X‐ray absorption spectroscopy coupled with a designed poisoning experiment reveals that the molecular Fe?N₄ species are identified as active centers for the OER in acid. The first‐principles‐based calculations verify that the Fe?N₄–doped carbon structure is capable of reducing the potential barriers and boosting the electrocatalytic OER activity in acid.     

Nitrogen‐Doping‐Induced Defects of a Carbon Coating Layer Facilitate Na‐Storage in Electrode Materials

A nitrogen‐doped, carbon‐coated Na₃V₂(PO₄)₃ cathode material is synthesized and the formation of doping type of nitrogen‐doped in carbon coating layer is systemically investigated. Three different carbon‐nitrogen species: pyridinic N, pyrrolic N, and quaternary N are identified. The most important finding is that different carbon‐nitrogen species in the carbon layer have different impacts on the improvement of the electrochemical properties of Na₃V₂(PO₄)₃. Pyridinic N and pyrrolic N significantly increase the electronic conductivity and create numerous extrinsic defects and active sites. Quaternary N only increases the electronic conductivity without creating extrinsic defects. Therefore, it is unexpectedly demonstrated that the Na₃V₂(PO₄)₃/C+N, in which with minimize content of quaternary N or exist most extrinsic defects, exhibits the best electrochemical performance, particularly the rate performance and cycling stability. For example, when the discharging rate increased from 0.2 C to 5 C, its capacity of 101.9 mAh g^?1 decays to 84.3 mAh g^?1 and an amazing capacity retention of 83% is achieved. Moreover, even at higher current density of 5 C, an excellent capacity retention of 93% is maintained even after 100 cycles.     

Ni3Fe‐N Doped Carbon Sheets as a Bifunctional Electrocatalyst for Air Cathodes

A promising bifunctional electrocatalyst is reported for air cathodes consisting of Ni₃Fe nanoparticles embedded in porous nitrogen‐doped carbon sheets (Ni₃Fe/N‐C sheets) by a facile and effective pyrolysis‐based route with sodium chloride (NaCl) crystals as a template. The Ni₃Fe/N‐C sheets show excellent catalytic activity, selectivity, and durability toward both the oxygen‐reduction and oxygen‐evolution reactions (ORR and OER). They are shown to provide a superior, low‐cost cathode for a rechargeable Zn‐air battery. At a discharge–charge current density of 10 mA cm^?2, the Ni₃Fe/N‐C sheets enable a Zn–air battery to cycle steadily up to 420 h with only a small increase in the round‐trip overpotential, outperforming the more costly Pt/C + IrO₂ mixture catalyst (160 h). With the simplicity and scalability of the synthetic approach and its remarkable bifunctional electrocatalytic performance, the Ni₃Fe/N‐C sheets offer a promising rechargeable air cathode operating at room temperature in an alkaline electrolyte.     

Fe?N4 Sites Embedded into Carbon Nanofiber Integrated with Electrochemically Exfoliated Graphene for Oxygen Evolution in Acidic Medium

Development of inexpensive and efficient oxygen evolution reaction (OER) catalysts in acidic environment is very challenging, but it is important for practical proton exchange membrane water electrolyzers. A molecular iron–nitrogen coordinated carbon nanofiber is developed, which is supported on an electrochemically exfoliated graphene (FeN₄/NF/EG) electrocatalyst through carbonizing the precursor composed of iron ions absorbed on polyaniline‐electrodeposited EG. Benefitting from the unique 3D structure, the FeN₄/NF/EG hybrid exhibits a low overpotential of ≈294 mV at 10 mA cm^?2 for the OER in acidic electrolyte, which is much lower than that of commercial Ir/C catalysts (320 mV) as well as all previously reported acid transitional metal‐derived OER electrocatalysts. X‐ray absorption spectroscopy coupled with a designed poisoning experiment reveals that the molecular Fe? N₄ species are identified as active centers for the OER in acid. The first‐principles‐based calculations verify that the Fe? N₄–doped carbon structure is capable of reducing the potential barriers and boosting the electrocatalytic OER activity in acid.     

Bimetallic Zeolitic Imidazolite Framework Derived Carbon Nanotubes Embedded with Co Nanoparticles for Efficient Bifunctional Oxygen Electrocatalyst

Bifunctional oxygen catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with high activities and low‐cost are of prime importance and challenging in the development of fuel cells and rechargeable metal–air batteries. This study reports a porous carbon nanomaterial loaded with cobalt nanoparticles (Co@NC‐x/y) derived from pyrolysis of a Co/Zn bimetallic zeolitic imidazolite framework, which exhibits incredibly high activity as bifunctional oxygen catalysts. For instance, the optimal catalyst of Co@NC‐3/1 has the interconnected framework structure between porous carbon and embedded carbon nanotubes, which shows the superb ORR activity with onset potential of ≈1.15 V and half‐wave potential of ≈0.93 V. Moreover, it presents high OER activity that can be further enhanced to over commercial RuO₂ by P‐doped with overpotentials of 1.57 V versus reversible hydrogen electrode at 10 mA cm^?2 and long‐term stability for 2000 circles and a Tafel slope of 85 mV dec^?1. Significantly, the nanomaterial demonstrates better catalytic performance and durability than Pt/C for ORR and commercial RuO₂ and IrO₂ for OER. These findings suggest the importance of a synergistic effect of graphitic carbon, nanotubes, exposed Co–N_x active sites, and interconnected framework structure of various carbons for bifunctional oxygen electrocatalysts.