Carbon-Based Metal-Free Catalysts for Electrocatalysis beyond the ORR

Fabrication of a biocathode for formic acid production upon the immobilization of formate dehydrogenase from Candida boidinii on a nanoporous carbon

The immobilization of the non-metallic enzyme formate dehydrogenase from Candida boidinii (CbFDH) into a nanoporous carbon with appropriate pore structure was explored for the bioelectrochemical conversion of CO₂ to formic acid (FA). Higher FA production rates were obtained upon immobilization of CbFDH compared to the performance of the enzyme in solution, despite the lower nominal CbFDH to NADH (β-nicotinamide adenine dinucleotide reduced) cofactor ratio and the lower amount of enzyme immobilized. The co-immobilization of the enzyme and a rhodium complex as mediator in the nanoporous carbon allowed the electrochemical regeneration of the cofactor. Preparative electrosynthesis of FA carried out on biocathodes of relatively large dimensions (ca. 3 cm × 2 cm) confirmed the higher production rate of FA for the immobilized enzyme. Furthermore, the incorporation of a Nafion binder in the biocathodes did not modify the immobilization extent of the CbFDH in the carbon support. Coulombic efficiencies close to 46% were obtained for the electrosynthesis carried out at -0.8 V for the biocathodes prepared using the lowest Nafion binder content and the co-immobilized enzyme and rhodium redox mediator. Although these values may yet be improved, they confirm the feasibility of these biocathodes in larger scales (6 cm²) beyond most common electrode dimensions reported in the literature (ca. a few mm²).

In Situ/Operando Insights into the Stability and Degradation Mechanisms of Heterogeneous Electrocatalysts

The further commercialization of renewable energy conversion and storage technologies requires heterogeneous electrocatalysts that meet the exacting durability target. Studies of the stability and degradation mechanisms of electrocatalysts are expected to provide important breakthroughs in stability issues. Accessible in situ/operando techniques performed under realistic reaction conditions are therefore urgently needed to reveal the nature of active center structures and establish links between the structural motifs in a catalyst and its stability properties. This review highlights recent research advances regarding in situ/operando techniques and improves the understanding of the stabilities of advanced heterogeneous electrocatalysts used in a diverse range of electrochemical reactions; it also proposes some degradation mechanisms. The review concludes by offering suggestions for future research.

Bifunctional rare metal-free electrocatalysts synthesized entirely from biomass resources

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are important processes for various energy devices, including polymer electrolyte fuel cells, rechargeable metal-air batteries, and water electrolyzers. We herein report the preparation of a rare metal-free and highly efficient ORR/OER electrocatalyst by calcination of a mixture of blood meal and ascidian-derived cellulose nanofibers. The obtained carbon alloys showed high ORR/OER performances and proved to be promising electrocatalysts. The carbon alloys synthesized entirely from biomass resources not only lead to a new electrocatalyst fabrication process but also contribute to CO2 reduction and the realization of a good life-cycle assessment value in fabrication of a sustainable energy device.

N-doped carbon nanotube encapsulated cobalt for efficient oxidative esterification of 5‑hydroxymethylfurfural

The cobalt nanoparticles embedded into graphitic nitrogen-rich carbon nanotube (Co/GCN) was prepared with a facile method and employed as an efficient catalyst for oxidative esterification of 5-hydroxymethylfurfural (HMF). The introduction...

Emerging Electrocatalysts for Water Oxidation under Near-Neutral CO2 Reduction Conditions

Electrocatalytic CO₂ reduction reaction (CO₂ RR), which produces valuable fuels and chemicals under near-neutral conditions, offers a renewable approach to alleviate the global energy crisis as well as the increasing concerns on climate change. However, to implement this strategy, one of the major challenges, the sluggish kinetics of the paired oxygen evolution reaction (OER) at anode, needs to be surmounted. It is therefore highly desirable to explore high-performance and cost-effective OER electrocatalysts suitable for CO₂ RR conditions, which is very different from those widely investigated under acidic or alkaline conditions. In this review, the ongoing development of OER electrocatalysts under near pH-neutral CO₂ -saturated (bi)carbonate solutions are highlighted and the future opportunities are discussed. It is started with a brief introduction on OER paired with CO₂ RR, the relevant theoretical tools such as density functional theory (DFT) and particularly machine learning (ML), and the operando characterization techniques. Then, there are some detailed discussions of recent progress on the rational design of OER electrocatalysts under CO₂ RR conditions ranging from noble-metal oxides to nonprecious metal phosphides, carbonates, (hydro)oxides, and so on. Finally, a perspective for developing OER electrocatalysts integrated with CO₂ electroreduction is proposed.

Nanosized porous artificial enzyme as a pH-sensitive doxorubicin delivery system for joint enzymatic and chemotherapy towards tumor treatment

A porous spherical artificial nanozyme (HF-900) prepared via pyrolysis of a porous organic polymer was used as drug carrier for efficient loading and highly selective pH-responsive delivery of doxorubicin (DOX) for the tumor joint nanotherapy.

Two-dimensional FeCo2O4 nanosheets with oxygen vacancies enable boosted oxygen evolution

Two dimensional FeCo2O4 nanosheets with abundant oxygen vacancies have been fabricated, which exhibit a superior OER performance compared to pristine FeCo2O4.

Two-Dimensional Biphenylene: A Graphene Allotrope with Superior Activity toward Electrochemical Oxygen Reduction Reaction

Developing efficient and inexpensive catalysts for the oxygen reduction reaction (ORR) is a key for sustainable development of fuel cell technologies. Herein, by means of density functional theory calculations and microkinetic modeling, we demonstrate that two-dimensional (2D) biphenylene, a recently synthesized allotrope of graphene composed of tetragonal, hexagonal, and octagonal rings, is a metal-free candidate for facilitating the electrochemical ORR. Different from semimetallic graphene, 2D biphenylene is metallic, and carbon atoms of its tetragonal rings are substantially positively charged, resulting in good ORR activity due to the enhanced binding strength with reaction intermediates. In particular, the ORR activity of 2D biphenylene is pH-dependent, and it can be significantly boosted under alkaline conditions. Moreover, 2D biphenylene possesses rather good electrochemical stability, rendering it attractive for alkaline fuel cells.

A Facile Route to Synthesis of Hierarchically Porous Carbon via Micelle System for Bifunctional Electrochemical Application

Well-ordered hierarchically porous carbon (HPC) nanomaterials have been successfully synthesized by a facile, efficient, and fast heated-evaporation induced self-assembly (HISA) method. A micelle system was employed as the template by using the HISA method for the first time, which possessed great potential in the large-scale production of HPC materials. Various surfactants, including triblock copolymer Pluronic F127, P123, F108, and cationic CTAB, were used in the polymerization process as templates to reveal the relationship between the structure of surfactants and architecture of the as-prepared HPCs. Transmission electron microscopy (TEM), X-ray diffraction (XRD), Nitrogen adsorption, and Fourier transform infrared (FTIR) measurements were conducted to investigate the morphology, structure, and components of HPCs, which further confirmed the well-ordered and uniform mesoporous structure. The as-prepared HPC sample with F127 possessed the largest specific surface area, suitable pore size, and well-ordered mesoporous structure, resulting in better electrochemical performance as electrodes in the fields of energy storage and conversion system. Doped with the metallic oxide MnO₂, the MnO₂/HPC composites presented the outstanding electrochemical activity in supercapacitor with a high specific capacitance of 531.2 F g^-1 at 1 A g^-1 and excellent cycling performance with little capacity fading, even after 5,000 cycles. Moreover, the obtained sample could also be applied in the fields of oxygen reduction reaction (ORR) for its abundant active sites and regulate architecture. This versatile approach makes the mass industrial production of HPC materials possible in electrochemical applications through a facile and fast route.

Carbon-Based Electrocatalysts for Efficient Hydrogen Peroxide Production

Hydrogen peroxide (H₂ O₂ ) is an environment-friendly and efficient oxidant with a wide range of applications in different industries. Recently, the production of hydrogen peroxide through direct electrosynthesis has attracted widespread research attention, and has emerged as the most promising method to replace the traditional energy-intensive multi-step anthraquinone process. In ongoing efforts to achieve highly efficient large-scale electrosynthesis of H₂ O₂ , carbon-based materials have been developed as 2e^- oxygen reduction reaction catalysts, with the benefits of low cost, abundant availability, and optimal performance. This review comprehensively introduces the strategies for optimizing carbon-based materials toward H₂ O₂ production, and the latest advances in carbon-based hybrid catalysts. The active sites of the carbon-based materials and the influence of coordination heteroatom doping on the selectivity of H₂ O₂ are extensively analyzed. In particular, the appropriate design of functional groups and understanding the effect of the electrolyte pH are expected to further improve the selective efficiency of producing H₂ O₂ via the oxygen reduction reaction. Methods for improving catalytic activity by interface engineering and reaction kinetics are summarized. Finally, the challenges carbon-based catalysts face before they can be employed for commercial-scale H₂ O₂ production are identified, and prospects for designing novel electrochemical reactors are proposed.

Alkaline Metal Oxide Assisting the Ionothermal Method for Efficient Fe-NX/C Catalyst Preparation

Owing to low cost and high efficiency, nonprecious metal catalysts have been widely used in various types of fuel cells. To obtain a high-activity electrocatalyst, a simple method for the synthesis of iron-modified covalent triazine frameworks by the direct heating of a mixture of FeCl3, ZnCl2, ZnO, and m-phthalodinitrile is reported. The role and a possible evolution pathway of the oxygen of metallic oxides are well discussed. To further verify our assumption, the Fe3O4 microspherical nanomaterials were synthesized and the relative Fe-based catalyst (Fe-NX/C) was successfully obtained by the ionothermal polymerization method. Fe-NX/C exhibits an extraordinary oxygen reduction reaction (ORR) performance in acidic solution, with a half-wave potential of only 25 mV negative shifts compared with Pt/C, while the power density is approximately 56% of that of Pt/C catalysts under the proton exchange membrane fuel cell testing condition. This work represents a new strategy to synthesize high-performance Fe-based catalysts toward ORR.

Assessment of the Accuracy of Density Functionals for Calculating Oxygen Reduction Reaction on Nitrogen-Doped Graphene

Experimental studies of the oxygen reduction reaction (ORR) at nitrogen-doped graphene electrodes have reported a remarkably low overpotential, on the order of 0.5 V, similar to Pt-based electrodes. Theoretical calculations using density functional theory have lent support to this claim. However, other measurements have indicated that transition metal impurities are actually responsible for the ORR activity, thereby raising questions about the reliability of both the experiments and the calculations. To assess the accuracy of the theoretical calculations, various generalized gradient approximation (GGA), meta-GGA, and hybrid functionals are employed here and calibrated against high-level wave-function-based coupled-cluster calculations (CCSD(T)) of the overpotential as well as self-interaction corrected density functional calculations and published quantum Monte Carlo calculations of O adatom binding to graphene. The PBE0 and HSE06 hybrid functionals are found to give more accurate results than the GGA and meta-GGA functionals, as would be expected, and for a low dopant concentration, 3.1%, the overpotential is calculated to be 1.0 V. The GGA and meta-GGA functionals give a lower estimate by as much as 0.4 V. When the dopant concentration is doubled, the overpotential calculated with hybrid functionals decreases, while it increases in GGA functional calculations. The opposite trends result from different potential-determining steps, the *OOH species being of central importance in the hybrid functional calculations, while the reduction of *O determines the overpotential obtained in GGA and meta-GGA calculations. The results presented here are mainly based on calculations of periodic representations of the system, but a comparison is also made with molecular flake models that are found to give erratic results due to finite size effects and geometric distortions during energy minimization. The presence of the electrolyte has not been taken into account explicitly in the calculations presented here but is estimated to be important for definitive calculations of the overpotential.

Recent progress of electrocatalysts for oxygen reduction in fuel cells

Oxygen reduction reaction (ORR) has gradually been in the limelight in recent years because of its great application potential for fuel cells and rechargeable metal-air batteries. Therefore, significant issues are increasingly focused on developing effective and economical ORR electrocatalysts. This review begins with the reaction mechanisms and theoretical calculations of ORR in acidic and alkaline media. The latest reports and challenges in ORR electrocatalysis are traced. Most importantly, the latest advances in the development of ORR electrocatalysts are presented in detail, including platinum group metal (PGM), transition metal, and carbon-based electrocatalysts with various nanostructures. Furthermore, the development prospects and challenges of ORR electrocatalysts are speculated and discussed. These insights would help to formulate the design guidelines for highly-active ORR electrocatalysts and affect future research to obtain new knowledge for ORR mechanisms.

Nitrogen Functionalization of CVD Grown Three-Dimensional Graphene Foam for Hydrogen Evolution Reactions in Alkaline Media

Three-dimensional graphene foam (3D-GrFoam) is a highly porous structure and sustained lattice formed by graphene layers with sp2 and sp3 hybridized carbon. In this work, chemical vapor deposition (CVD)-grown 3D-GrFoam was nitrogen-doped and platinum functionalized using hydrothermal treatment with different reducing agents (i.e., urea, hydrazine, ammonia, and dihydrogen hexachloroplatinate (IV) hydrate, respectively). X-ray photoelectron spectroscopy (XPS) survey showed that the most electrochemically active nitrogen-doped sample (GrFoam3N) contained 1.8 at % of N, and it exhibited a 172 mV dec-1 Tafel plot associated with the Volmer-Heyrovsky hydrogen evolution (HER) mechanism in 0.1 M KOH. By the hydrothermal process, 0.2 at % of platinum was anchored to the graphene foam surface, and the resultant sample of GrFoamPt yielded a value of 80 mV dec-1 Tafel associated with the Volmer-Tafel HER mechanism. Furthermore, Raman and infrared spectroscopy analysis, as well as scanning electron microscopy (SEM) were carried out to understand the structure of the samples.

Wood-Derived Bimetallic and Heteroatomic Hierarchically Porous Carbon Aerogel for Rechargeable Flow Zn-Air Batteries

It is necessary to correctly research and synthesize efficient and inexpensive catalysts to achieve reversible oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which is also a prerequisite for zinc-air batteries (ZABs). However, it is still a huge challenge to manufacture electrocatalysts with durable and high electrocatalytic performance from biomass. Here, a convenient method of delignification was used to transform natural balsa wood into a layered porous carbon material, FeCo alloy supported on a N, S-doped wood-based carbon aerogel (FeCo@NS-CA) as the cathode in rechargeable flow ZAB. The obtained FeCo@NS-CA with the porous lamellar architecture exhibits superior bifunctional electrocatalysis, including excellent electrochemical activities and superior stabilities. For ORR, relative to the reversible hydrogen electrode, the onset potential of FeCo@NS-CA is 0.97 V, and the half-wave potential is 0.85 V, which is consistent with the potential of commercial Pt/C. For OER, FeCo@NS-CA obtained an overpotential of 450 mV, which is very similar to the overpotential of the benchmark RuO₂. The superior performance could be owing to the alloy carrier interaction between the FeCo alloy and the wood-based carbon aerogel co-doped with N and S. Moreover, the bifunctional air cathode in a flow ZAB assembled with the FeCo@NS-CA catalyst at a current density of 10 mA cm^-2; the power density is 140 mW cm^-2, and the specific capacitance is 760 mA h g_Zn^-1, with a remarkable long-term stability of 400 h better than ZAB of benchmark Pt/C + RuO₂. This research lays the foundation for transforming abundant biomass resources into high environmental protection materials for energy-related applications.

Pyrolysis-Free Synthesized Catalyst towards Acidic Oxygen Reduction by Deprotonation

Acidic oxygen reduction is vital for renewable energy devices such as fuel cells. However, many aspects of the catalytic process are still uncertain-especially the large difference in activity in acidic and alkaline media. Thus, the design and synthesis of model catalysts to determine the active centers and the inactivation mechanism are urgently needed. We report a pyrolysis-free synthesis route to fabricate a catalyst (CPF-Fe@NG) for oxygen reduction in acidic conditions. By introducing a deprotonation process, we extended the oxygen reduction reaction (ORR) activity from alkaline to acidic conditions. CPF-Fe@NG demonstrated outstanding performance with a half-wave potential of 853 mV (vs. RHE) and good stability after 10000 cycles in 1 M HClO₄ . The pyrolysis-free route could also be used to assemble fuel cells, with a maximum power density of 126 mW cm^-2 . Our findings offer new insights into the ORR process to optimize catalysts for both mechanistic studies and practical applications.

Comprehensive Understanding of the Thriving Ambient Electrochemical Nitrogen Reduction Reaction

The electrochemical method of combining N₂ and H₂ O to produce ammonia (i.e., the electrochemical nitrogen reduction reaction [E-NRR]) continues to draw attention as it is both environmentally friendly and well suited for a progressively distributed farm economy. Despite the multitude of recent works on the E-NRR, further progress in this field faces a bottleneck. On the one hand, despite the extensive exploration and trial-and-error evaluation of E-NRR catalysts, no study has stood out to become the stage protagonist. On the other hand, the current level of ammonia production (microgram-scale) is an almost insurmountable obstacle for its qualitative and quantitative determination, hindering the discrimination between true activity and contamination. Herein i) the popular theory and mechanism of the NRR are introduced; ii) a comprehensive summary of the recent progress in the field of the E-NRR and related catalysts is provided; iii) the operational procedures of the E-NRR are addressed, including the acquisition of key metrics, the challenges faced, and the most suitable solutions; iv) the guiding principles and standardized recommendations for the E-NRR are emphasized and future research directions and prospects are provided.

A Gas-Steamed MOF Route to P-Doped Open Carbon Cages with Enhanced Zn-Ion Energy Storage Capability and Ultrastability

Carbon micro/nanocages have received great attention, especially in electrochemical energy-storage systems. Herein, as a proof-of-concept, a solid-state gas-steamed metal-organic-framework approach is designed to fabricate carbon cages with controlled openings on walls, and N, P dopants. Taking advantage of the fabricated carbon cages with large openings on their walls for enhanced kinetics of mass transport and N, P dopants within the carbon matrix for favoring chemical adsorption of Zn ions, when used as carbon cathodes for advanced aqueous Zn-ion hybrid supercapacitors (ZHSCs), such open carbon cages (OCCs) display a wide operation voltage of 2.0 V and an enhanced capacity of 225 mAh g^-1 at 0.1 A g^-1 . Also, they exhibit an ultralong cycling lifespan of up to 300 000 cycles with 96.5% capacity retention. Particularly, such OCCs as electrode materials lead to a soft-pack ZHSC device, delivering a high energy density of 97 Wh kg^-1 and a superb power density of 6.5 kW kg^-1 . Further, the device can operate in a wide temperature range from -25 to + 40 °C, covering the temperatures for practical applications in daily life.

Cobalt nanoparticles/ nitrogen, sulfur-codoped ultrathin carbon nanotubes derived from metal organic frameworks as high-efficiency electrocatalyst for robust rechargeable zinc-air battery

It remains a challenge for efficient and facile synthesis of promising non-noble metal electrocatalysts with outstanding properties. This work reported a simple pyrolysis method to prepare cobalt nanoparticles/nitrogen, sulfur-codoped ultrathin carbon nanotubes (Co NPs/N,S-CNTs) with metal organic frameworks (cobalt 2-methylimidazole, ZIF-67), melamine, polyvinylpyrrolidone (PVP) and thiourea. The prepared catalyst exhibited superior catalytic activity towards oxygen reduction reaction (ORR) such as the more positive onset potential of 0.96 V, half-wave potential of 0.86 V and smaller Tafel slope of 67.9 mV dec^-1, outperforming those of commercial Pt/C. Furthermore, the Co NPs/N,S-CNTs based Zn-air battery not only showed good cycling performance, but also displayed a notable peak power density (153.8 mW cm^-2) and large open-circuit voltage (1.433 V). This study provides some valuable guidelines for synthesizing advanced electrocatalysts in renewable energy techniques.

Tailoring the Co4+/Co3+ active sites in a single perovskite as a bifunctional catalyst for the oxygen electrode reactions

Developing a non-precious metal electrocatalyst for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is desirable for low-cost energy conversion devices. Herein, we designed and developed a new class of layered cation ordered single perovskite oxides (Pr0.9Ca0.1Co0.8Fe0.2O3-δ) with an optimum ratio of the Co4+/Co3+ oxidation state and oxygen vacancy for oxygen electrode reactions. Catalytic activities are investigated as a function of electronic structure and surface composition. A moderate amount of Ca and Fe dopants keeps the B-site Co cations at a higher oxidation state (Co4+) and generates a vast amount of an oxygen defect rich structure. The improved performance in the ORR and OER is explained by the increase in the sites of Co4+ cations, a state responsible for enhanced catalytic activity. A hypothesis for how doped Ca fraction affects the adsorbed oxygen species and contributes to catalytic activity is discussed. This work sheds light on the influence of crystal structure on the catalytic property and reports that ORR and OER activities are affected not only by oxygen vacancy concentration but also by the oxidation state of the transition metal in the perovskite oxide.

Recent progress in pristine MOF-based catalysts for electrochemical hydrogen evolution, oxygen evolution and oxygen reduction

Among various kinds of materials that have been investigated as electrocatalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), metal-organic frameworks (MOFs) has emerged as a promising material for electrocatalyzing these vital processes owing to their structural merits that integrate advantages of both homogeneous and heterogeneous catalysts; however there is still big room for their improvement in terms of inferior activity and poor conductivity, as well as the ambiguity of real active sites. In this review, advanced strategies with the aim of solving the activity and conductivity problems are summarized as microstructure engineering and conductivity improvement, respectively. The structural evolution of some MOFs and their real active species has also been discussed. Finally, perspectives on the development of MOF materials for HER, OER and ORR electrocatalysis are provided.

Nanostructured Iron Sulfide/N, S Dual-Doped Carbon Nanotube-Graphene Composites as Efficient Electrocatalysts for Oxygen Reduction Reaction

Nanostructured FeS dispersed onto N, S dual-doped carbon nanotube-graphene composite support (FeS/N,S:CNT-GR) was prepared by a simple synthetic method. Annealing an ethanol slurry of Fe precursor, thiourea, carbon nanotube, and graphene oxide at 973 K under N2 atmosphere and subsequent acid treatment produced FeS nanoparticles distributed onto the N, S-doped carbon nanotube-graphene support. The synthesized FeS/N,S:CNT-GR catalyst exhibited significantly enhanced electrochemical performance in the oxygen reduction reaction (ORR) compared with bare FeS, FeS/N,S:GR, and FeS/N,S:CNT with a small half-wave potential (0.827 V) in an alkaline electrolyte. The improved ORR performance, comparable to that of commercial Pt/C, could be attributed to synergy between the small FeS nanoparticles with a high activity and the N, S-doped carbon nanotube-graphene composite support providing high electrical conductivity, large surface area, and additional active sites.

Electrospun cobalt Prussian blue analogue-derived nanofibers for oxygen reduction reaction and lithium-ion batteries

Electrospinning is an effective technique to fabricate one-dimensional materials. In this study, cobalt-embedded carbon nanofibers (Co@CNFs) are obtained via carbonization of electrospun cobalt Prussian blue analogue (Co-Co PBA) under nitrogen atmosphere. The Co@CNFs have metallic cobalt surrounded by graphitic carbon shells and possess high specific surface area, rich porosity, high graphitic degree, and rational nitrogen doping. The structure merits endow them with excellent electrocatalytic performances for oxygen reduction reaction (ORR): an onset potential of 0.867 V vs. RHE and 0.784 V vs. RHE at j =  - 3 mA cm^-2 with a four-electron transfer process. Through a further mild oxidation process, we obtain Co₃O₄ nanoparticles-embedded nitrogen-doped carbon (Co₃O₄@CNFs) with spindle-like morphology. When working as the anode materials for lithium-ion batteries (LIBs), Co₃O₄@CNFs show high specific capacity, good stability, and excellent rate capability. The Co₃O₄@CNFs anode delivers a discharge specific capacity of 1404 mA h g^-1 after 100 cycles at a current density of 100 mA g^-1 and about 500 mA h g^-1 after 500 cycles at 2000 mA g^-1. The diffusion- and capacitive-controlled processes both contribute to the charge storage of the Co₃O₄@CNFs electrode. This study provides a new strategy to fabricate the excellent electrocatalysts for ORR and anode materials for LIBs via facile electrospinning.

Synergistic Effect of N-Doped sp2 Carbon and Porous Structure in Graphene Gels toward Selective Oxidation of C-H Bond

N-doped carbon materials represent a type of metal-free catalyst for diverse organic synthetic reactions. However, single N-doped carbon materials perform insufficiently in the selective oxidation reaction of C-H bond compared with metal catalysts or multielement co-doped materials. There are a few reports on the application of three-dimensional (3D) carbon materials in such a reaction. Besides, the relationship between the well-developed porous structures, heteroatom doping, and their catalytic performance is unclear. In this study, 3D porous N-doped graphene aerogel catalysts with high activity and selectivity for the C-H bond oxidation under mild reaction conditions have been synthesized through a two-step method. Systematic studies on the dosage of N sources, pyrolysis temperature, and their influences on the catalytic performances have been evolved. Moreover, solid evidence of the synergistic effect of sp² C atoms adjacent to the N atoms and porous structure promoting the performance has been provided in this work.

A three-dimensional flower-like NiCo-layered double hydroxide grown on nickel foam with an MXene coating for enhanced oxygen evolution reaction electrocatalysis

Electrolysis of water is currently one of the cleanest and most efficient ways to produce high-purity hydrogen. The oxygen evolution reaction (OER) at the anode of electrolysis is the key factor affecting the reaction efficiency, which involves the transfer of four electrons and can slow down the overall reaction process. In this work, using nickel foam coated with MXene (Ti3C2T x ) as the carrier, a three-dimensional flower-shaped layered double hydroxide (NiCo-LDH) is grown on Ti3C2T x by a hydrothermal method to fabricate a NiCo-LDH/Ti3C2T x /NF hybrid electrocatalyst for enhanced OER performance. The results reveal that the hybrid electrocatalyst has excellent OER activity in alkaline solution, in which a low overpotential of 223 mV and a small Tafel slope of 47.2 mV dec-1 can be achieved at a current density of 100 mA cm-2. The interface interaction and charge transfer between Ti3C2T x and NiCo-LDH can accelerate the electron transfer rate during the redox process and improve the catalytic activity of the overall reaction. This NiCo-LDH/Ti3C2T x /NF hybrid electrocatalyst may have important research significance and great application potential in catalytic electrolysis of water.

Defect and Doping Co-Engineered Non-Metal Nanocarbon ORR Electrocatalyst

Exploring low-cost and earth-abundant oxygen reduction reaction (ORR) electrocatalyst is essential for fuel cells and metal-air batteries. Among them, non-metal nanocarbon with multiple advantages of low cost, abundance, high conductivity, good durability, and competitive activity has attracted intense interest in recent years. The enhanced ORR activities of the nanocarbons are normally thought to originate from heteroatom (e.g., N, B, P, or S) doping or various induced defects. However, in practice, carbon-based materials usually contain both dopants and defects. In this regard, in terms of the co-engineering of heteroatom doping and defect inducing, we present an overview of recent advances in developing non-metal carbon-based electrocatalysts for the ORR. The characteristics, ORR performance, and the related mechanism of these functionalized nanocarbons by heteroatom doping, defect inducing, and in particular their synergistic promotion effect are emphatically analyzed and discussed. Finally, the current issues and perspectives in developing carbon-based electrocatalysts from both of heteroatom doping and defect engineering are proposed. This review will be beneficial for the rational design and manufacturing of highly efficient carbon-based materials for electrocatalysis.

Direct Magnetic Reinforcement of Electrocatalytic ORR/OER with Electromagnetic Induction of Magnetic Catalysts

Designing stable and efficient electrocatalysts for both oxygen reduction and evolution reactions (ORR/OER) at low-cost is challenging. Here, a carbon-based bifunctional catalyst of magnetic catalytic nanocages that can direct enhance the oxygen catalytic activity by simply applying a moderate (350 mT) magnetic field is reported. The catalysts, with high porosity of 90% and conductivity of 905 S m^-1 , are created by in situ doping metallic cobalt nanodots (≈10 nm) into macroporous carbon nanofibers with a facile electrospinning method. An external magnetic field makes the cobalt magnetized into nanomagnets with high spin polarization, which promote the adsorption of oxygen-intermediates and electron transfer, significantly improving the catalytic efficiency. Impressively, the half wave-potential is increased by 20 mV for ORR, and the overpotential at 10 mA cm^-2 is decreased by 15 mV for OER. Compared with the commercial Pt/C+IrO₂ catalysts, the magnetic catalyzed Zn-air batteries deliver 2.5-fold of capacities and exhibit much longer durability over 155 h. The findings point out a very promising strategy of using electromagnetic induction to boost oxygen catalytic activity.