Nonprecious Metal Catalysts for Oxygen Reduction in Heterogeneous Aqueous Systems

Mesoporous waffle-like N-doped carbon with embedded Co nanoparticles for efficiently electrocatalytic oxygen reduction and evolution

Rational design and facile preparation of high-performance carbon-based eletrocatalysts for both oxygen reduction and evolution reactions (ORR and OER) is crucial for practical applications of rechargeable zinc-air batteries. Inspired by the fact that the metallic Co catalysis on the formation of carbon nanotubes (CNTs), this work develops a facial compression-pyrolysis route to synthesize a mesoporous waffle-like N-doped carbon framework with embedded Co nanoparticles (Co@pNC) using a Co metal-organic framework and melamine as precursors. The unique porous waffle-like carbon framework is built up of interwoven N-doped CNTs and graphene nanosheets, which offers abundant catalytic-active sites and rapid diffusion channels for intermediates and electrolyte. The optimized Co@pNC shows excellent bifunctional ORR/OER electrocatalytic activity in alkaline media with a half-wave potential (E_1/2) of 0.85 V for ORR and a small potential gap of 0.70 V between ORR E_1/2 and OER potential at 10 mA cm^-2. Its assembled battery exhibits a peak power density up to 150.3 mW cm^-2, an energy density of 928 Wh kg_Zn^-1 and superb rate capability. It highlights a facile component and architecture strategy to design high-performance carbon-based eletrocatalysts.

A rational design of functional porous frameworks for electrocatalytic CO2 reduction reaction

The electrocatalytic CO₂ reduction reaction (ECO₂RR) is considered one of the approaches with the most potential to achieve lower carbon emissions in the future, but a huge gap still exists between the current ECO₂RR technology and industrial applications. Therefore, the design and preparation of catalysts with satisfactory activity, selectivity and stability for the ECO₂RR have attracted extensive attention. As a classic type of functional porous framework, crystalline porous materials (e.g., metal organic frameworks (MOFs) and covalent organic frameworks (COFs)) and derived porous materials (e.g., MOF/COF composites and pyrolysates) have been regarded as superior catalysts for the ECO₂RR due to their advantages such as designable porosity, modifiable skeleton, flexible active site structure, regulable charge transfer pathway and controllable morphology. Meanwhile, with the rapid development of nano-characterization and theoretical calculation technologies, the structure-activity relationships of functional porous frameworks have been comprehensively considered, i.e., metallic element type, local coordination environment, and microstructure, corresponding to selectivity, activity and mass transfer efficiency for the ECO₂RR, respectively. In this review, the rational design strategy for functional porous frameworks is briefly but precisely generalized based on three key factors including metallic element type, local coordination environment, and microstructure. Then, details about the structure-activity relationships for functional porous frameworks are illustrated in the order of MOFs, COFs, composites and pyrolysates to analyze the effect of the above-mentioned three factors on their ECO₂RR performance. Finally, the challenges and perspectives of functional porous frameworks for the further development of the ECO₂RR are reasonably proposed, aiming to offer insights for future studies in this intriguing and significant research field.

Amine Groups in the Second Sphere of Iron Porphyrins Allow for Higher and Selective 4e-/4H+ Oxygen Reduction Rates at Lower Overpotentials

Iron porphyrins with one or four tertiary amine groups in their second sphere are used to investigate the electrochemical O₂ reduction reaction (ORR) in organic (homogeneous) and aqueous (heterogeneous) conditions. Both of these complexes show selective 4e^-/4H⁺ reduction of oxygen to water at rates that are 2-3 orders of magnitude higher than those of iron tetraphenylporphyrin lacking these amines in the second sphere. In organic solvents, these amines get protonated, which leads to the lowering of overpotentials, and the rate of the ORR is enhanced almost 75,000 times relative to rates expected from the established scaling relationship for the ORR by iron porphyrins. In the aqueous medium, the same trend of higher ORR rates at a lower overpotential is observed. In situ resonance Raman data under heterogeneous aqueous conditions show that the presence of one amine group in the second sphere leads to a cleavage of the O-O bond in a Fe^III-OOH intermediate as the rate-determining step (rds). The presence of four such amine groups enhances the rate of O-O bond cleavage such that this intermediate is no longer observed during the ORR; rather, the proton-coupled reduction of the Fe^III-O₂^- intermediate with a H/D isotope effect of 10.6 is the rds. These data clearly demonstrate changes in the rds of the electrochemical ORR depending on the nature of second-sphere residues and explain their deviation from linear scaling relationships.

MOF-Derived CoNi Nanoalloy Particles Encapsulated in Nitrogen-Doped Carbon as Superdurable Bifunctional Oxygen Electrocatalyst

Carbon-encapsulated transition metal catalysts have caught the interest of researchers in the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) due to their distinctive architectures and highly tunable electronic structures. In this work, we synthesized N-doped carbon encapsulated with CoNi nanoalloy particles (CoNi@NC) as the electrocatalysts. The metal-organic skeleton ZIF-67 nanocubes were first synthesized, and then Ni²⁺ ions were inserted to generate CoNi-ZIF precursors by a simple ion-exchange route, which was followed by pyrolysis and with urea for the introduction of nitrogen (N) at a low temperature to synthesize CoNi@NC composites. The results reveal that ZIF-67 pyrolysis can dope more N atoms in the carbon skeleton and that the pyrolysis temperature influences the ORR and OER performances. The sample prepared by CoNi@NC pyrolysis at 650 °C has a high N content (9.70%) and a large specific surface area (167 m² g^-1), with a positive ORR onset potential (Eonset) of 0.89 V vs. RHE and half-wave potential (E_1/2) of 0.81 V vs. RHE in 0.1 M KOH, and the overpotential of the OER measured in 1 M KOH was only 286 mV at 10 mA cm^-2. The highly efficient bifunctional ORR/OER electrocatalysts synthesized by this method can offer some insights into the design and synthesis of complex metal-organic frameworks (MOFs) hybrid structures and their derivatives as functional materials in energy storage.

Steering Selectivity in the Four-Electron and Two-Electron Oxygen Reduction Reactions: On the Importance of the Volcano Slope

In the last decade, trends for competing electrocatalytic processes have been largely captured by volcano plots, which can be constructed by the analysis of adsorption free energies as derived from electronic structure theory in the density functional theory approximation. One prototypical example refers to the four-electron and two-electron oxygen reduction reactions (ORRs), resulting in the formation of water and hydrogen peroxide, respectively. The conventional thermodynamic volcano curve illustrates that the four-electron and two-electron ORRs reveal the same slopes at the volcano legs. This finding is related to two facts, namely, that only a single mechanistic description is considered in the model, and electrocatalytic activity is assessed by the concept of the limiting potential, a simple thermodynamic descriptor evaluated at the equilibrium potential. In the present contribution, the selectivity challenge of the four-electron and two-electron ORRs is analyzed, thereby accounting for two major expansions. First, different reaction mechanisms are included into the analysis, and second, G _max(U), a potential-dependent activity measure that factors overpotential and kinetic effects into the evaluation of adsorption free energies, is applied for approximation of electrocatalytic activity. It is illustrated that the slope of the four-electron ORR is not constant at the volcano legs but rather is prone to change as soon as another mechanistic pathway is energetically preferred or another elementary step becomes the limiting one. Due to the varying slope of the four-electron ORR volcano, a trade-off between activity and selectivity for hydrogen peroxide formation is observed. It is demonstrated that the two-electron ORR is energetically preferred at the left and right volcano legs, thus opening a new strategy for the selective formation of H₂O₂ by an environmentally benign route.

Facile Preparation of Cobalt Nanoparticles Encapsulated Nitrogen-Doped Carbon Sponge for Efficient Oxygen Reduction Reaction

The facile preparation of non-noble metal nanoparticle loaded carbon nanomaterials is promising for efficient oxygen reduction reaction (ORR) electrocatalysis. Herein, a facile preparation strategy is proposed to prepare nitrogen-doped carbon sponge loaded with fine cobalt nanoparticles by the direct pyrolysis of the cobalt ions adsorbed polymeric precursor. The polymeric sponge precursor with continuous framework and high porosity is formed by the self-assembly of a poly(amic acid). Taking advantage of the negatively charged surface and porous structure, cobalt ions can be efficiently adsorbed into the polymeric sponge. After pyrolysis, fine cobalt nanoparticles covered by carbon layers are formed, while the sponge-like structure of the precursor is also well-preserved in order to give cobalt nanoparticles loaded nitrogen-doped carbon sponges (Co/CoO@NCS) with a high loading content of 44%. The Co/CoO@NCS exhibits promising catalytic activity toward ORR with a half-wave potential of 0.830 V and a limiting current density of 4.71 mA cm^-2. Overall, we propose a facile polymer self-assembly strategy to encapsulate transition metal nanoparticles with high loading content on a nitrogen-doped carbon sponge for efficient ORR catalysis.

Effect of molybdenum doping on the catalytic activity of VS2/CNT for the oxygen reduction reaction in alkaline media

This study shows that chemical doping is a promising technique to improve the electrocatalytic activity of TMDs. The Mo-VS2-15/CNT/GCE catalyst with significant ORR activity could be an excellent alternative for platinum.

Colloidal synthesis of monodisperse trimetallic Pt-Fe-Ni nanocrystals and their enhanced electrochemical performances

Among the multi-metallic nanocatalysts, Pt-based alloy nanocrystals (NCs) have demonstrated promising performance in fuel cells and water electrolyzers. Herein, we demonstrate a facile colloidal synthesis of monodisperse trimetallic Pt-Fe-Ni alloy NCs through a co-reduction of metal precursors. The as-synthesized ternary NCs exhibit superior mass and specific activities toward oxygen reduction reaction (ORR), which are ∼2.8 and 5.6 times as high as those of the benchmark Pt/C catalyst, respectively. The ORR activity of the carbon-supported Pt-Fe-Ni nanocatalyst is persistently retained after the durability test. Owing to the incorporation of Fe and Ni atoms into the Pt lattice, the as-prepared trimetallic Pt-alloy electrocatalyst also manifestly enhances the electrochemical activity and durability toward the oxygen evolution reaction with a reduced overpotential when compared with that of the benchmark Pt/C (△η= 0.20 V, at 10 mA cm^-2). This synthetic strategy paves the way for improving the reactivity for a broad range of electrocatalytic applications.

Application of a TEMPO-Polypyrrole Polymer for NOx-Mediated Oxygen Electroreduction

The oxygen reduction reaction (ORR) is one of the key processes for electrochemical energy storage, such as the cathode process in fuel cells and metal–air batteries. To date, the efficiency of the ORR half-reaction limits the overall performance of these energy storage devices. Traditional platinum-based materials are expensive and cannot provide the desired ORR efficiency. As an alternative, a new catalytic scheme for an ORR was proposed, which consisted of an electrode modified with a TEMPO-containing conductive polymer and a solution redox mediator system based on nitrogen oxides (NOx). NOx is perfect for oxygen reduction in solution, which, however, cannot be efficiently reduced onto a pristine electrode, while TEMPO is inactive in the ORR itself but catalyzes the electrochemical reduction of NO2 on the electrode surface. Together, these catalysts have a synergistic effect, enabling an efficient ORR in an acidic medium. In the present study, the synthesis of a novel TEMPO-containing conductive polymer and its application in the synergistic ORR system with a NOx mediator is described. The proposed mediator system may increase the performance of proton-exchange fuel cells and metal–air batteries.

Theoretical Study of Oxygen Reduction Reaction Mechanism in Metal-Free Carbon Materials: Defects, Structural Flexibility, and Chemical Reaction

Metal-free carbon materials are attractive Pt-based catalyst alternatives. However, despite efforts, the reaction mechanism remains elusive. Thus, we investigated the role of defects (dopant nitrogen and carbon vacancy) on the catalytic oxygen reduction reaction in a metal-free carbon material focusing on the effect of structural flexibility. Crucially, defects lower the energy barrier for the sp²/sp³ transition of the carbon-centered O₂-adsorption sites by releasing structural strain during the reaction. In particular, low-coordinated pyridinic-N displaces from the carbon plane to release the strain, whereas weak C-C bonds around the carbon vacancy change the bond lengths to release the strain. Defects indirectly promote the adsorption of oxygen by enhancing structural flexibility. Thus, the nonlocal structural environment is as critical as the direct interaction between adsorption sites and adsorbate in the chemical reaction. Molecular dynamics simulations reveal that pyridinic-N doping is a facile route to introduce stable catalytic active sites. Overall, our results provide a deeper understanding of chemical processes on defective carbon materials.

Molecular Catalyst Synthesis Strategies to Prepare Atomically Dispersed Fe-N-C Heterogeneous Catalysts

We report a strategy to integrate atomically dispersed iron within a heterogeneous nitrogen-doped carbon (N-C) support, inspired by routes for metalation of molecular macrocyclic iron complexes. The N-C support, derived from pyrolysis of a ZIF-8 metal-organic framework, is metalated via solution-phase reaction with FeCl₂ and tributyl amine, as a Brønsted base, at 150 °C. Fe active sites are characterized by ⁵⁷Fe Mössbauer spectroscopy and aberration-corrected scanning transmission electron microscopy. The site density can be increased by selective removal of Zn²⁺ ions from the N-C support prior to metalation, resembling the transmetalation strategy commonly employed for the preparation of molecular Fe-macrocycles. The utility of this approach is validated by the higher catalytic rates (per total Fe) of these materials relative to established Fe-N-C catalysts, benchmarked using an aerobic oxidation reaction.

Stabilization of Ni0/NiII Heterojunctions inside Robust Porous Metal Silicate Materials for High-Performance Catalysis

Heterostructural nanomaterials demonstrate great potential to replace noble metal-based catalysts because heterojunctions could induce relocalization of electrons and facilitate the migration of electrons and charge carriers at the heterostructural boundary between electron-rich and electron-deficient metal sites; however, the instability of heterojunctions greatly hinders their practical applications. We report herein an effective strategy for the fabrication and stabilization of Ni⁰/Ni^II heterojunctions inside a porous metal silicate (PMS) material PMS-22 using a nickel coordination complex as the bifunctional template. The synergistic activity between metallic nickel and nickel silicate in PMS-22 highly boosts the catalytic activity in the hydrogenation of phenol, which could activate phenol at a very low temperature of 50 °C. Most importantly, PMS-22 demonstrates robust stability in catalysis, attributed to the strong interaction and charge transfer between metallic Ni and nickel silicate at the heterointerfaces inside the confined pores. Therefore, this work paves a new pathway to improve the stability and activity of heterostructural nanomaterials for catalytic applications.

Rechargeable Batteries for Grid Scale Energy Storage

Ever-increasing global energy consumption has driven the development of renewable energy technologies to reduce greenhouse gas emissions and air pollution. Battery energy storage systems (BESS) with high electrochemical performance are critical for enabling renewable yet intermittent sources of energy such as solar and wind. In recent years, numerous new battery technologies have been achieved and showed great potential for grid scale energy storage (GSES) applications. However, their practical applications have been greatly impeded due to the gap between the breakthroughs achieved in research laboratories and the industrial applications. In addition, various complex applications call for different battery performances. Matching of diverse batteries to various applications is required to promote practical energy storage research achievement. This review provides in-depth discussion and comprehensive consideration in the battery research field for GSES. The overall requirements of battery technologies for practical applications with key parameters are systematically analyzed by generating standards and measures for GSES. We also discuss recent progress and existing challenges for some representative battery technologies with great promise for GSES, including metal-ion batteries, lead-acid batteries, molten-salt batteries, alkaline batteries, redox-flow batteries, metal-air batteries, and hydrogen-gas batteries. Moreover, we emphasize the importance of bringing emerging battery technologies from academia to industry. Our perspectives on the future development of batteries for GSES applications are provided.

Origin of the Rarely Reported High Performance of Mn-doped Carbon-based Oxygen Reduction Catalysts

Recent efforts to develop durable high-performance platinum-group metal (PGM)-free oxygen reduction reaction (ORR) electrocatalysts have focused on Fe- and Co-based molecular and pyrolyzed catalysts. While Mn-based catalysts have advantages of lower toxicity and higher durability, their activity has been generally poor. Nevertheless, several examples of high-performance Mn-based catalysts have been reported. Thus, it is necessary to understand why Mn-based materials much more rarely show high catalytic ORR performance and to determine the factors that can lead to the achievement of such high performance in these rare cases. We have studied the effects of the changes in the macrocycle structure, axial ligand, distance between the active sites, interactions with the dopant N atoms and the presence of an extended carbon network on the ORR catalysis of various Mn-, Fe-, and Co-based systems through the comparison of the adsorption energies of the ORR intermediates. We find that the sensitivity to the local environment changes is the largest for Mn and is the smallest for Co, with Fe between Mn and Co. Our results showed that the strong binding of OH by Mn and the strong sensitivity of the Mn to the modification of its environment necessitate a precise combination of local environment changes to achieve a high onset potential (V_onset ) in Mn-based catalysts. By contrast, the weaker binding of OH by Fe and Co and their weaker sensitivity to local environment changes lead to a wide variety of local environments with favorable catalytic activity (V_onset >0.7 V) for Co- and Fe-based systems. This explains the scarcity of reported Mn-based pyrolyzed catalysts and suggests that precise material synthesis and engineering of the active site can achieve high-performance Mn-based ORR electrocatalysts with high activity and durability.

Co-N heteroatomic interface engineering in peanut Shell-Derived porous carbon for enhanced oxygen reduction reaction

The development of high-efficiency and low-cost oxygen reduction electrocatalysts have become an urgent need to push fuel cells into practical application. Herein, an effective electrocatalyst Co/NC was successfully constructed, which was derived from abundant peanut shells, obtained by doping with cobalt ions and pyrolyzing in NH₃ atmosphere. Due to the abundant Co-N active sites triggered by Co-N heteroatomic interface, the prepared electrocatalysts present excellent oxygen reduction reaction (ORR) performance with more positive half-wave potential (E_1/2 = 0.83 V), incremental limiting current density (J_L = 5.45 mA cm^-2), higher durability and stronger resistance to methanol, which is superior to that of Pt/C (E_1/2 = 0.81 V and J_L = 5.19 mA cm^-2). This work proposes a potential strategy to synthesize efficient ORR electrocatalysts to instead of Pt-based catalysts.

Crucial Roles of a Pendant Imidazole Ligand of a Cobalt Porphyrin Complex in the Stoichiometric and Catalytic Reduction of Dioxygen

A cobalt porphyrin complex with a pendant imidazole base ([(L₁ )Co^II ]) is an efficient catalyst for the homogeneous catalytic two-electron reduction of dioxygen by 1,1'-dimethylferrocene (Me₂ Fc) in the presence of triflic acid (HOTf), as compared with a cobalt porphyrin complex without a pendant imidazole base ([(L₂ )Co^II ]). The pendant imidazole ligand plays a crucial role not only to provide an imidazolinium proton for proton-coupled electron transfer (PCET) from [(L₁ )Co^II ] to O₂ in the presence of HOTf but also to facilitate electron transfer (ET) from [(L₁ )Co^II ] to O₂ in the absence of HOTf. The kinetics analysis and the detection of intermediates in the stoichiometric and catalytic reduction of O₂ have provided clues to clarify the crucial roles of the pendant imidazole ligand of [(L₁ )Co^II ] for the first time.

Core-Shell Carbon-Based Bifunctional Electrocatalysts Derived from COF@MOF Hybrid for Advanced Rechargeable Zn-Air Batteries

The development of highly active carbon-based bifunctional electrocatalysts for both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is highly desired, but still full of challenges in rechargeable Zn-air batteries. Metal organic frameworks (MOFs) and covalent organic frameworks (COFs) have gained great attention for various applications due to their attractive features of structural tunability, high surface area and high porosity. Herein, a core-shell structured carbon-based hybrid electrocatalyst (H-NSC@Co/NSC), which contains high density active sites of MOF-derived shell (Co/NSC) and COF-derived hollow core (H-NSC), is successfully fabricated by direct pyrolysis of covalently-connected COF@ZIF-67 hybrid. The core-shell H-NSC@Co/NSC hybrid manifests excellent catalytic properties toward both OER and ORR with a small potential gap (∆E = 0.75 V). The H-NSC@Co/NSC assembled Zn-air battery exhibits a high power-density of 204.3 mW cm^-2 and stable rechargeability, outperforming that of Pt/C+RuO₂ assembled Zn-air battery. Density functional theory calculations reveal that the electronic structure of the carbon frameworks on the Co/NSC shell can be effectively modulated by the embedded Co nanoparticles (NPs), facilitating the adsorption of oxygen intermediates and leading to enhanced catalytic activity. This work will provide a strategy to design highly-efficient electrocatalysts for application in energy conversion and storage.

Fe-Fe3C Functionalized Few-Layer Graphene Sheet Nanocomposites for an Efficient Electrocatalyst of the Oxygen Reduction Reaction

Preparation of a high-efficiency, low-cost, and environmentally friendly non-precious metal catalyst for the oxygen reduction reaction (ORR) is highly desirable in fuel cells. Herein, a Fe-Fe₃C-functionalized few-layer graphene sheet (Fe/Fe₃C/FLG) nanocomposite was fabricated through the vacuum heat treatment technique using ferric nitrate and glucose as the precursors and exhibited a high-performance ORR electrocatalyst. Multiple characterizations confirm that the nanosized Fe particles with the Fe₃C interface are uniformly distributed in the FLGs. Electrocatalytic kinetics investigation of the nanocomposite indicates that the electron transfer process is a four-electron pathway. The formation of the Fe₃C interface between the Fe nanoparticles and FLGs may promote the electron transfer from the Fe to FLGs. Furthermore, the Fe/Fe₃C/FLG nanocomposite not only exhibits high ORR catalytic activity but also displays desirable stability. Consequently, the obtained Fe/Fe₃C/FLG nanocomposite might be a promising non-precious, cheap, and high-efficiency catalyst for fuel cells.

Co-doped CeO2/N-C nanorods as a bifunctional oxygen electrocatalyst and its application in rechargeable Zn-air batteries

The development of electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with high-activity and atability still remain great challenges for rechargeable Zn-air batteries. Herein, a new type of Co-doped Ce-N-C bifunctional electrocatalyst has been synthesized through a simple two-step method, which realizes the high dispersion of Co₃O₄on the CeO₂carbon frame and stabilizes its specific surface area. Benefiting from the synergistic interaction between Co₃O₄and CeO₂, the conductivity of the electrocatalyst is improved and the oxygen reduction reaction/oxygen storage properties are promoted. The resultant Co₃O₄-CeO₂@N-C catalyst shows remarkable ORR activity with the high initial potential (E₀ = 0.8 V), the large limiting current density (j_L = 6 mA cm^-2), and a low Tafel slope (81 mV dec^-1). In full cell tests, Co₃O₄-CeO₂@NC as the oxygen electrode exhibites superior charge/discharge capacity and excellent cycle stability. The assembled Zn-air battery achieves a maximum power density of 110 mW cm^-2at a current density of 180 mA cm^-2, and a high specific capacity of 780 mAh g^-1at a discharge current density of 10 mA cm^-2.

Potential-Driven Restructuring of Cu Single Atoms to Nanoparticles for Boosting the Electrochemical Reduction of Nitrate to Ammonia

Restructuring is ubiquitous in thermocatalysis and of pivotal importance to identify the real active site, yet it is less explored in electrocatalysis. Herein, by using operando X-ray absorption spectroscopy in conjunction with advanced electron microscopy, we reveal the restructuring of the as-synthesized Cu-N₄ single-atom site to the nanoparticles of ∼5 nm during the electrochemical reduction of nitrate to ammonia, a green ammonia production route upon combined with the plasma-assisted oxidation of nitrogen. The reduction of Cu²⁺ to Cu⁺ and Cu⁰ and the subsequent aggregation of Cu⁰ single atoms is found to occur concurrently with the enhancement of the NH₃ production rate, both of them are driven by the applied potential switching from 0.00 to -1.00 V versus RHE. The maximum production rate of ammonia reaches 4.5 mg cm^-2 h^-1 (12.5 mol_NH3 g_Cu^-1 h^-1) with a Faradaic efficiency of 84.7% at -1.00 V versus RHE, outperforming most of the other Cu catalysts reported previously. After electrolysis, the aggregated Cu nanoparticles are reversibly disintegrated into single atoms and then restored to the Cu-N₄ structure upon being exposed to an ambient atmosphere, which masks the potential-induced restructuring during the reaction. The synchronous changes of the Cu⁰ percentage and the ammonia Faradaic efficiency with the applied potential suggests that the Cu nanoparticles are the genuine active sites for nitrate reduction to ammonia, which is corroborated with both the post-deposited Cu NP catalyst and density functional theory calculations.

Controlled introduction of defects into single-walled carbon nanotubes via a fluorination-defluorination strategy using xenon difluoride and their alkaline oxygen reduction reaction catalytic activity

The defect edges in carbon nanomaterials have attracted attention as catalytic active sites for the oxygen reduction reaction (ORR) of the cathode in electrolyte fuel cells, and the defect control in carbon nanomaterials is becoming increasingly important. This study evaluates a fluorination-defluorination strategy for the controlled introduction of defects into single-walled carbon nanotubes (SWCNTs) involving the fluorination of SWCNTs using xenon difluoride (XeF₂) and their subsequent defluorination through thermal annealing. We synthesized fluorinated SWCNTs with different fluorine contents using gaseous XeF₂ and annealed the fluorinated SWCNTs at 1000 °C for 3 h under nitrogen gas flow. Structural analyses revealed that SWCNTs derived from fluorinated SWCNTs with low fluorine contents primarily had single point defects. In contrast, SWCNTs derived from fluorinated SWCNTs with high fluorine contents had vacancy defects with edges. According to the ORR catalyst evaluation in alkaline aqueous solution, SWCNTs with edge defects, rather than point defects, can improve the efficiency of ORR catalytic activity. The proposed fluorination-defluorination strategy using gaseous XeF₂ is expected to enable the controlled introduction of defects in different types of carbon materials.

Thermal Hydroquinone Oxidation on Co/N-doped Carbon Proceeds by a Band-Mediated Electrochemical Mechanism

Molecular metal complexes catalyze aerobic oxidation reactions via redox cycling at the metal center to effect sequential activation of O₂ and the substrate. Metal surfaces can catalyze the same transformations by coupling independent half-reactions for oxygen reduction and substrate oxidation mediated via the exchange of band-electrons. Metal- and nitrogen-doped carbons (MNCs) are promising catalysts for aerobic oxidation that consist of molecule-like active sites embedded in conductive carbon hosts. Owing to their combined molecular and metallic features, it remains unclear whether they catalyze aerobic oxidation via the sequential redox cycling pathways of molecules or band-mediated pathways of metals. Herein, we simultaneously track the potential of the catalyst and the rate of turnover of aerobic hydroquinone oxidation on a cobalt-based MNC catalyst in contact with a carbon electrode. By comparing operando measurements of rate and potential with the current-voltage behavior of each constituent half-reaction under identical conditions, we show that these molecular materials can display the band-mediated reaction mechanisms of extended metallic solids. We show that the action of these band-mediated mechanisms explains the fractional reaction orders in both oxygen and hydroquinone, the time evolution of catalyst potential and rate, and the dependence of rate on the overall reaction free energy. Selective poisoning experiments suggest that oxygen reduction proceeds at cobalt sites, whereas hydroquinone oxidation proceeds at native carbon-oxide defects on the MNC catalyst. These findings highlight that molecule-like active sites can take advantage of band-mediated mechanisms when coupled to conductive hosts.

Controllable Solid-Phase Fabrication of an Fe2O3/Fe5C2/Fe-N-C Electrocatalyst toward Optimizing the Oxygen Reduction Reaction in Zinc-Air Batteries

Preparing advanced electrocatalysts via solid-phase reactions encounters the challenge of low controllability for multiconstituent hybridization and microstructure modulation. Herein, a hydrothermal-mimicking solid-phase system is established to fabricate novel Fe₂O₃/Fe₅C₂/Fe-N-C composites consisting of Fe₂O₃/Fe₅C₂ nanoparticles and Fe,N-doped carbon species with varying morphologies. The evolution mechanism featuring a competitive growth of different carbon sources in a closed hypoxic space is elucidated for a series of Fe₂O₃/Fe₅C₂/Fe-N-C composites. The size and dispersity of Fe₂O₃/Fe₅C₂ nanoparticles, the graphitization degree of the carbonaceous matrix, and their diverse hybridization states lead to disparate electrocatalytic behaviors for the oxygen reduction reaction (ORR). Among them, microspherical Fe₂O₃/Fe₅C₂/Fe-N-C-3 exhibits an optimal ORR performance and the as-assembled zinc-air battery shows all-round superiority to the Pt/C counterpart. This work presents a mild solid-phase fabrication technique for obtaining a variety of nanocomposites with effective control over composition hybridization and microstructural modulation, which is significantly important for the design and optimization of advanced electrocatalysts.

The Role of Steps on Silver Nanoparticles in Electrocatalytic Oxygen Reduction

Hydrogen fuel cell technology is an essential component of a green economy. However, it is limited in practicality and affordability by the oxygen reduction reaction (ORR). Nanoscale silver particles have been proposed as a cost-effective solution to this problem. However, previous computational studies focused on clean and flat surfaces. High-index surfaces can be used to model active steps presented in nanoparticles. Here, we used the stable stepped Ag(322) surface as a model to understand the ORR performance of steps on Ag nanoparticles. Our density functional theory (DFT) results demonstrate a small dissociation energy barrier for O2 molecules on the Ag(322) surface, which can be ascribed to the existence of low-coordination number surface atoms. Consequently, the adsorption of OOH* led to the associative pathway becoming ineffective. Alternatively, the unusual dissociative mechanism is energetically favored on Ag(322) for ORR. Our findings reveal the importance of the coordination numbers of active sites for catalytic performance, which can further guide electrocatalysts’ design.

Cobalt-Containing Nitrogen-Doped Carbon Materials Derived from Saccharides as Efficient Electrocatalysts for Oxygen Reduction Reaction

The development of non-precious metal electrocatalysts towards oxygen reduction reaction (ORR) is crucial for the commercialisation of polymer electrolyte fuel cells. In this work, cobalt-containing nitrogen-doped porous carbon materials were prepared by a pyrolysis of mixtures of saccharides, cobalt nitrate and dicyandiamide, which acts as a precursor for reactive carbon nitride template and a nitrogen source. The rotating disk electrode (RDE) experiments in 0.1 M KOH solution showed that the glucose-derived material with optimised cobalt content had excellent ORR activity, which was comparable to that of 20 wt % Pt/C catalyst. In addition, the catalyst exhibited high tolerance to methanol, good stability in short-time potential cycling test and low peroxide yield. The materials derived from xylan, xylose and cyclodextrin displayed similar activities, indicating that various saccharides can be used as inexpensive and sustainable precursors to synthesise active catalyst materials for anion exchange membrane fuel cells.

Understanding hydrazine oxidation electrocatalysis on undoped carbon

Carbons are ubiquitous electrocatalytic supports for various energy-related transformations, especially in fuel cells. Doped carbons such as Fe-N-C materials are particularly active towards the oxidation of hydrazine, an alternative fuel and hydrogen carrier. However, there is little discussion of the electrocatalytic role of the most abundant component - the carbon matrix - towards the hydrazine oxidation reaction (HzOR). We present a systematic investigation of undoped graphitic carbons towards the HzOR in alkaline electrolyte. Using highly oriented pyrolytic graphite electrodes, as well as graphite powders enriched in either basal planes or edge defects, we demonstrate that edge defects are the most active catalytic sites during hydrazine oxidation electrocatalysis. Theoretical DFT calculations support and explain the mechanism of HzOR on carbon edges, identifying unsaturated graphene armchair defects as the most likely active sites. Finally, these findings explain the 'double peak' voltammetric feature observed on many doped carbons during the HzOR.