Earth-Abundant Nanomaterials for Oxygen Reduction

One-dimensional metal-organic frameworks for electrochemical applications

Metal-organic frameworks (MOFs) are as a category of crystalline porous materials. Extensive interest has been devoted to energy storage and energy conversion applications owing to their unique advantages of periodic architecture, high specific surface area, high adsorption, high conductivity, high specific capacitance, and high porosity. One-dimensional (1D) nanostructures have unique surface effects, easily regulated size, good agglutination of the substrate, and other distinct properties amenable to the field of energy storage and conversion. Therefore, 1D nanostructures could further improve the characteristic properties of MOFs, and it is of great importance for practical applications to control the size and morphological characteristics of MOFs. The electrochemical application of 1D MOFs is mainly discussed in this review, including energy storage applications in supercapacitors and batteries and energy conversion applications in catalysis. In addition, various synthesis strategies for 1D MOFs and their architectures are presented.

Halide-Doping Effect of Strontium Cobalt Oxide Electrocatalyst and the Induced Activity for Oxygen Evolution in an Alkaline Solution

Perovskites of strontium cobalt oxyhalides having the chemical formulae Sr2CoO4-xHx (H = F, Cl, and Br; x = 0 and 1) were prepared using a solid-phase synthesis approach and comparatively evaluated as electrocatalysts for oxygen evolution in an alkaline solution. The perovskite electrocatalyst crystal phase, surface morphology, and composition were examined by X-ray diffraction, a scanning electron microscope, and energy-dispersive X-ray (EDX) mapping. The electrochemical investigations of the oxyhalides catalysts showed that the doping of F, Cl, or Br into the Sr2CoO4 parent oxide enhances the electrocatalytic activity for the oxygen evolution reaction (OER) with the onset potential as well as the potential required to achieve a current density of 10 mA/cm2 shifting to lower potential values in the order of Sr2CoO4 (1.64, 1.73) > Sr2CoO3Br (1.61, 1.65) > Sr2CoO3Cl (1.53, 1.60) > Sr2CoO3F (1.50, 1.56) V vs. HRE which indicates that Sr2CoO3F is the most active electrode among the studied catalysts under static and steady-state conditions. Moreover, Sr2CoO3F demonstrates long-term stability and remarkably less charge transfer resistance (Rct = 36.8 ohm) than the other oxyhalide counterparts during the OER. The doping of the perovskites with halide ions particularly the fluoride-ion enhances the surface oxygen vacancy density due to electron withdrawal away from the Co-atom which improves the ionic and electronic conductivity as well as the electrochemical activity of the oxygen evolution in alkaline solution.

Fe, P, N- and FeP, N-doped carbon hollow nanospheres: A comparison study toward oxygen reduction reaction electrocatalysts

In recent years, carbon materials co-doped with transition metals and heteroatoms have been widely used in the oxygen reduction reaction (ORR) as an alternative to platinum/carbon catalysts because of their high efficiency, low price, and appropriate sustainability. Herein, we report the synthesis of FeP, N-doped carbon (FeP, N-Carbon) hollow nanospheres (HNSs) and Fe, P, N-doped carbon (Fe, P, N-Carbon) HNSs. The FeP, N-Carbon was obtained via the pyrolysis of poly(o-phenylenediamine) (PoPD) HNSs in the presence of Fe(NO₃)₃ and phytic acid (PA), whereas Fe, P, N-Carbon was obtained by first pyrolyzing PoPD HNSs with Fe(NO₃)₃, followed by another cycle of pyrolysis with PA. Fe, P, N-Carbon exhibited a better ORR performance than FeP, N-Carbon, with an onset potential of 1.03 V, a half-wave potential of 0.89 V and a limiting current density of 5.75 mA cm^-2. The findings will provide insights into the controlled synthesis of transition-metal-heteroatom-codoped carbon nanomaterials for the development of advanced ORR electrocatalysts.

Conductive One-Dimensional Coordination Polymers with Tunable Selectivity for the Oxygen Reduction Reaction

Conductive materials involving nonprecious metal coordination complexes as electrocatalysts for the oxygen reduction reaction (ORR) have received increasing attention in recent years. Herein, we reported efficient ORR electrocatalysts containing M-S2N2 sites with tunable selectivity based on simple one-dimensional (1D) coordination polymers (CPs). The 1D CPs were synthesized from M(OAc)2 and 2,5-diamino-1,4-benzenedithiol (DABDT) by a solvent thermal method. Due to their good electrical conductivities (10-6-10-2 S cm-1), the 1D CPs could be used as ORR catalysts in low catalytic amounts without the addition of carbon materials. Cobalt-based CPs showed a well-organized structure of nanosheets with Co-S2N2 sites exposed and exhibited remarkable electrocatalytic ORR activity (Eonset = 0.93 V vs reversible hydrogen electrode (RHE), E1/2 = 0.82 V, n = 3.85, JL = 5.22 mA cm-2, Tafel slope of 63 mV dec-1) in alkaline media. However, nickel-based CPs favored a 2e- ORR process with ∼87% H2O2 selectivity and an Eonset of 0.78 V. This work provides new opportunities for the construction of ORR catalysts based on conductive nonprecious metal CPs.

In situ encapsulation engineering boosts the electrochemical performance of highly graphitized N-doped porous carbon-based copper-cobalt selenides for bifunctional oxygen electrocatalysis

Transition-metal selenides are gaining prominence as promising electrode materials for energy storage applications owing to their low electronegativity and environment-friendliness compared with metal sulfides/oxides. Herein, a CuCoSe@NC nanocomposite with copper-cobalt selenides embedded in highly graphitized N-doped porous carbon was synthesized by an in situ encapsulation strategy with metal-organic framework crystals (CuCo-BDC) as templates followed by selenization, and used as a bifunctional electrocatalyst for Zn-air batteries in lye. The result shows that the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic activity of the optimal CuCoSe@NC-2 was enhanced, and the assembled Zn-air batteries exhibited a remarkable electrochemical performance with the use of the CuCoSe@NC-2 electrode, including a high power density (137.1 mW cm^-2) and excellent charge-discharge cycling stability, which were better than those of the Pt/C + RuO₂ electrocatalyst. Such improvement is attributed not only to the higher porosity and larger specific surface area (342 m² g^-1) of the carbon matrix, which increased the contact area with oxygen-containing species, but also the encapsulation effect of the highly graphitized N-doped carbon layer and the high content of pyridine-N species also further improved the conductivity of selenide composites. This work has introduced N-doped bimetallic selenides as an ideal candidate for bifunctional electrocatalysts.

Heterostructural Interface in Fe3C-TiN Quantum Dots Boosts Oxygen Reduction Reaction for Al-Air Batteries

Oxygen reduction electrocatalysts play important roles in metal-air batteries. Herein, Fe₃C-TiN heterostructural quantum dots loaded on carbon nanotubes (FCTN@CNTs) are prepared as electrocatalysts for the oxygen reduction reaction (ORR) through a one-pot pyrolysis. The Fe₃C-TiN quantum dots with a diameter of 2-5 nm show the unique characteristic of heterostructural interface. The as-prepared FCTN@CNTs display Pt/C comparable ORR performance (E_onset 1.06 and E_1/2 0.95 V) in alkaline medium, which is ascribed to the heterostructural interface between TiN and Fe₃C. Furthermore, the Al-air batteries with the FCTN@CNT catalyst display superior discharge performance, demonstrating good feasibility for practical application. This work provides an effective new method to synthesize affordable and efficient oxygen reduction reaction catalysts.

Progress in Development of Nanostructured Manganese Oxide as Catalyst for Oxygen Reduction and Evolution Reaction

The rise in energy consumption is largely driven by the growth of population. The supply of energy to meet that demand can be fulfilled by slowly introducing energy from renewable resources. The fluctuating nature of the renewable energy production (i.e., affected by weather such as wind, sun light, etc.), necessitates the increasing demand in developing electricity storage systems. Reliable energy storage system will also play immense roles to support activities related to the internet of things. In the past decades, metal-air batteries have attracted great attention and interest for their high theoretical capacity, environmental friendliness, and their low cost. However, one of the main challenges faced in metal-air batteries is the slow rate of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) that affects the charging and the discharging performance. Various types of nanostructure manganese oxide with high specific surface area and excellent catalytic properties have been synthesized and studied. This review provides a discussion of the recent developments of the nanostructure manganese oxide and their performance in oxygen reduction and oxygen evolution reactions in alkaline media. It includes the experimental work in the nanostructure of manganese oxide, but also the fundamental understanding of ORR and OER. A brief discussion on electrocatalyst kinetics including the measurement and criteria for the ORR and the OER is also included. Finally, recently reported nanostructure manganese oxide catalysts are also discussed.

The Role of Surface Curvature in Electrocatalysts

Excessive consumption of fossil fuels has caused unavoidable environmental problems. The development of renewable and clean alternatives is essential for the sustainable and green development of human society. Electrocatalysts are most important parts in these energy-related devices. Recently, scientists found that the surface curvature of electrocatalysts could play an important role for the improvement of catalytic performance and the optimization of intrinsic catalytic activity during electrocatalytic process. The role of surface curvature in electrocatalysts is still under investigating. In this minireview, we summarized the latest progress of electrocatalysts with different surface curvatures and their applications in energy-related applications. This review mainly involves the strategies for preparation of electrocatalysts with different surface curvatures, three typical electrocatalysts with different surface curvatures (curled surface, onion-like structure, and spiral structure), and the potential mechanisms that surface curvature in electrocatalysts affects activities.

Nanoporous manganese ferrite films by anodising electroplated Fe-Mn alloys for bifunctional oxygen electrodes

An electroplating-anodising method based on a facile and scalable electrochemical process was used to fabricate manganese ferrite porous oxide films for use as precious-metal-free oxygen reduction/evolution reaction (ORR/OER) electrodes. Porous oxide films of spinel manganese ferrites (Mn_xFe_3-xO₄) were formed on electroplated Fe-Mn films. The Mn_xFe_3-xO₄ porous oxide formed on microcracks in the Fe-Mn films constituted a nanoporous/microcrack hierarchical structure (NP/MC), which provided a large electrode surface area for ORR/OER. The electrochemically active surface area of the NP/MC on Fe-36 at% Mn was 33.3 cm², which is nine times that of the nanoporous structure on Fe (3.67 cm²). The onset potential of the NP/MC on Fe-15 at% Mn and Fe-36 at% Mn was 0.88 V vs. RHE (overpotential, ∼350 mV) for the ORR at -0.1 mA cm^-2. The OER onset potentials at 10 mA cm^-2 were 1.79 V on Fe-15 at% Mn (∼560 mV) and 1.74 V on Fe-36 at% Mn (∼510 mV). The OER and ORR activities of the Mn_xFe_3-xO₄ porous oxides are better than those of spinel iron oxide (∼510 and ∼640 mV for the ORR and OER, respectively) because of the good intrinsic activity of Mn_xFe_3-xO₄ and greater surface area of the NP/MC. The ORR activities of the Mn_xFe_3-xO₄ porous oxides decreased to about 30% during ORR durability testing for 7.5 h, and the same level of activity was retained after 24 h of use. The Mn_xFe_3-xO₄ porous oxides retained a high level of activity during OER durability testing for 8 h.

Decarboxylation-Induced Defects in MOF-Derived Single Cobalt Atom@Carbon Electrocatalysts for Efficient Oxygen Reduction

Developing transition metal single-atom catalysts (SACs) for oxygen reduction reaction (ORR) is of great importance. Zeolitic imidazolate frameworks (ZIFs) as a subgroup of metal-organic frameworks (MOFs) are distinguished as SAC precursors, due to their large porosity and N content. However, the activity of the formed metal sites is limited. Herein, we report a decarboxylation-induced defects strategy to improve their intrinsic activity via increasing the defect density. Carboxylate/amide mixed-linker MOF (DMOF) was chosen to produce defective Co SACs (Co@DMOF) by gas-transport of Co species to DMOF upon heating. Comparing with ZIF-8 derived SAC (Co@ZIF-8-900), Co@DMOF-900 with more defects yet one fifth Co content and similar specific double-layer capacitance show better ORR activity and eight times higher turnover frequency (2.015 e s^-1  site^-1 ). Quantum calculation confirms the defects can weaken the adsorption free energy of OOH on Co sites and further boost the ORR process.

Noble-Metal-Free Multicomponent Nanointegration for Sustainable Energy Conversion

Global energy and environmental crises are among the most pressing challenges facing humankind. To overcome these challenges, recent years have seen an upsurge of interest in the development and production of renewable chemical fuels as alternatives to the nonrenewable and high-polluting fossil fuels. Photocatalysis, photoelectrocatalysis, and electrocatalysis provide promising avenues for sustainable energy conversion. Single- and dual-component catalytic systems based on nanomaterials have been intensively studied for decades, but their intrinsic weaknesses hamper their practical applications. Multicomponent nanomaterial-based systems, consisting of three or more components with at least one component in the nanoscale, have recently emerged. The multiple components are integrated together to create synergistic effects and hence overcome the limitation for outperformance. Such higher-efficiency systems based on nanomaterials will potentially bring an additional benefit in balance-of-system costs if they exclude the use of noble metals, considering the expense and sustainability. It is therefore timely to review the research in this field, providing guidance in the development of noble-metal-free multicomponent nanointegration for sustainable energy conversion. In this work, we first recall the fundamentals of catalysis by nanomaterials, multicomponent nanointegration, and reactor configuration for water splitting, CO₂ reduction, and N₂ reduction. We then systematically review and discuss recent advances in multicomponent-based photocatalytic, photoelectrochemical, and electrochemical systems based on nanomaterials. On the basis of these systems, we further laterally evaluate different multicomponent integration strategies and highlight their impacts on catalytic activity, performance stability, and product selectivity. Finally, we provide conclusions and future prospects for multicomponent nanointegration. This work offers comprehensive insights into the development of cost-competitive multicomponent nanomaterial-based systems for sustainable energy-conversion technologies and assists researchers working toward addressing the global challenges in energy and the environment.

Simultaneous sulfamethoxazole degradation with electricity generation by microbial fuel cells using Ni-MOF-74 as cathode catalysts and quantification of antibiotic resistance genes

Antibiotic wastewater presents serious challenges in water treatment. Metal-organic frameworks (MOFs) have received significant attention as promising precursors and sacrificial templates in the preparation of porous carbon-supported catalysts. Herein, we investigated the sulfamethoxazole (SMX) degradation and electrochemical performance of microbial fuel cells (MFCs) that applied as-prepared Ni-MOF-74 and Ni-N-C (Ni-MOF-74 underwent pyrolysis treatment at different temperatures) as air-cathode catalyst. Firstly, the electrocatalytic activity towards oxygen reduction reaction (ORR) of the catalyst was investigated by rotating disk electrode. The results showed that electron transfer number for Ni-MOF-74 was 2.12, while that of 800Ni-N-C was 3.44, which was close to four-electron reduction. Applying Ni-MOF-74 in MFCs, a maximum power density of 446 mW/m² was obtained, which was close to that of 800Ni-N-C. Besides, using Ni-MOF-74 as cathode catalyst, a chemical oxygen demand removal rate of about 84% was obtained, and the degradation rate of 10 mg/L SMX was 61%. The degradation rate decreased with increasing antibiotic concentration, but the average degradation efficiency increased stepwise. Additionally, the relative abundance of resistant gene sul1 in the reactors of the new catalytic material was about 62% lower than that of sul1 in the control (Pt/C) reactors, and the relative abundance of sul2 was about 73% lower. Moreover, cost assessments related to the catalyst performance are presented. The findings of this study demonstrated that Ni-MOF-74 could be considered as a two-electron transfer ORR catalyst, and offers a promising technique for preparation of Ni-N-C for use as four-electron transfer ORR catalysts. In comparison, Ni-MOF-74 could be a promising ORR catalyst of MFCs for antibiotic degradation.

Highly Curved Nanostructure-Coated Co, N-Doped Carbon Materials for Oxygen Electrocatalysis

Nitrogen-doped graphene could catalyze the electrochemical reduction and evolution of oxygen, but unfortunately suffers from sluggish catalytic kinetics. Herein, for the first time, we report an onion-like carbon coated Co, N-doped carbon (OLC/Co-N-C) material, which possesses multilayers of highly curved nanostructures that form mesoporous architectures. These unique nanospheres are produced when surfactant micelles are introduced to synthesis precursors. Owing to the combined electronic effect and nanostructuring effect, our OLC/Co-N-C materials exhibit high bifunctional oxygen reduction/evolution reaction (ORR/OER) activity, showing a promising application in rechargeable Zn-air batteries. Experimental results are rationalized by theoretical calculations, showing that the curvature of graphitic carbon plays a vital role in promoting activities of meta-carbon atoms near graphitic N and ortho/meta carbon atoms close to pyridinic N.

Axial Ligand Coordination Tuning of the Electrocatalytic Activity of Iron Porphyrin Electrografted onto Carbon Nanotubes for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is essential in many life processes and energy conversion systems. It is desirable to design transition metal molecular catalysts inspired by enzymatic oxygen activation/reduction processes as an alternative to noble-metal-Pt-based ORR electrocatalysts, especially in view point of fuel cell commercialization. We have fabricated bio-inspired molecular catalysts electrografted onto multiwalled carbon nanotubes (MWCNTs) in which 5,10,15,20-tetra(pentafluorophenyl) iron porphyrin (iron porphyrin FeF₂₀ TPP) is coordinated with covalently electrografted axial ligands varying from thiophene to imidazole on the MWCNTs' surface. The catalysts' electrocatalytic activity varied with the axial coordination environment (i. e., S-thiophene, N-imidazole, and O-carboxylate); the imidazole-coordinated catalyst MWCNTs-Im-FeF₂₀ TPP exhibited the highest ORR activity among the prepared catalysts. When MWCNT-Im-FeF₂₀ TPP was loaded onto the cathode of a zinc-air battery, an open-cell voltage (OCV) of 1.35 V and a maximum power density (P_max ) of 110 mW cm^-2 were achieved; this was higher than those of MWCNTs-Thi-FeF₂₀ TPP (OCV=1.30 V, P_max =100 mW cm^-2 ) and MWCNTs-Ox-FeF₂₀ TPP (OCV=1.28 V, P_max =86 mW cm^-2 ) and comparable with a commercial Pt/C catalyst (OCV=1.45 V, P_max =120 mW cm^-2 ) under similar experimental conditions. This study provides a time-saving method to prepare covalently immobilized molecular electrocatalysts on carbon-based materials with structure-performance correlation that is also applicable to the design of other electrografted catalysts for energy conversion.

Co/CoOx heterojunctions encapsulated N-doped carbon sheets via a dual-template-guided strategy as efficient electrocatalysts for rechargeable Zn-air battery

Developing highly efficient oxygen electrocatalysts is of vital importance for rechargeable Zn-air batteries (ZABs). Herein, Co/CoO_x nano-heterojunctions encapsulated into nitrogen-doped carbon sheets (NCS@Co/CoO_x) are fabricated via a dual-template-guided approach by using zeolitic imidazolate frameworks (ZIFs) as templates. The synergistic integration of structural and compositional advantages endows such catalyst with superior catalytic properties to benchmark noble-metal catalysts. To be specific, the hierarchical micro/mesopores affords massive mass transport channels and maximizes the exposure of accessible active sites, whereas the NCS matrix accelerates electron transfer and prevents the self-aggregation of active species during the electrocatalytic reaction. Moreover, abundant and synergistic Co-based active sites (CoO, Co₃O₄, Co-N_x) greatly promote the catalytic activity. As the cathode of both liquid and flexible solid-state ZABs, excellent device properties are achieved, outperforming those assembled with commercial Pt/C+RuO₂ catalyst. This work presents a feasible and cost-effective strategy for developing oxygen electrocatalysts derived from ZIFs templates.

Zeolitic imidazole framework derived N-doped porous carbon/metal cobalt nanoparticles hybrid for oxygen electrocatalysis and rechargeable Zn-air batteries

Bifunctional electrocatalysts with high catalytic property for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are vital for high-performance zinc-air batteries (ZnABs). In this study, an efficient bifunctional electrocatalyst with hollow structure (C-N/Co (1/2)) has been successfully prepared through carbonization of ZIF-8@ZIF-67 and evaporation of Zn ions at high temperature. With Co nanoparticles encapsulated by an N-doped porous carbon matrix, the catalyst exhibits excellent stability in aqueous alkaline solution over an extended period and good tolerance to the methanol crossover effect. The integration of an N-doped graphitic carbon outer shell and Co nanoparticles enables high ORR and OER activity, as evidenced by ZnAB using the catalyst C-N/Co (1/2) in an air cathode.

Control of Molecular Catalysts for Oxygen Reduction by Variation of pH and Functional Groups

In the search for replacement of the platinum-based catalysts for fuel cells, MN₄ molecular catalysts based on abundant transition metals play a crucial role in modeling and investigation of the influence of the environment near the active site in platinum-group metal-free (PGM-free) oxygen reduction reaction (ORR) catalysts. To understand how the ORR activity of molecular catalysts can be controlled by the active site structure through modification by the pH and substituent functional groups, the change of the ORR onset potential and the electron number in a broad pH range was examined for three different metallocorroles. Experiments revealed a switch between two different ORR mechanisms and a change from 2e^- to 4e^- pathway in the pH range of 3.5-6. This phenomenon was shown by density functional theory (DFT) calculations to be related to the protonation of the nitrogen atoms and carboxylic acid groups on the corroles indicated by the pK_a values of the protonation sites in the vicinity of the ORR active sites. Control of the electron-withdrawing nature of these groups characterized by the pK_a values could switch the ORR from the H⁺ to e^- rate-determining step mechanisms and from 2e^- to 4e^- ORR pathways and also controlled the durability of the corrole catalysts. The results suggest that protonation of the nitrogen atoms plays a vital role in both the ORR activity and durability for these materials and that pK_a of the N atoms at the active sites can be used as a descriptor for the design of high-performance, durable PGM-free catalysts.

Design Strategies of Non-Noble Metal-Based Electrocatalysts for Two-Electron Oxygen Reduction to Hydrogen Peroxide

Hydrogen peroxide (H₂ O₂ ) is a highly value-added and environmentally friendly chemical with various applications. The production of H₂ O₂ by electrocatalytic 2e^- oxygen reduction reaction (ORR) has drawn considerable research attention, with a view to replacing the currently established anthraquinone process. Electrocatalysts with low cost, high activity, high selectivity, and superior stability are in high demand to realize precise control over electrochemical H₂ O₂ synthesis by 2e^- ORR and the feasible commercialization of this system. This Review introduces a comprehensive overview of non-noble metal-based catalysts for electrochemical oxygen reduction to afford H₂ O₂ , providing an insight into catalyst design and corresponding reaction mechanisms. It starts with an in-depth discussion on the origins of 2e^- /4e^- selectivity towards ORR for catalysts. Recent advances in design strategies for non-noble metal-based catalysts, including carbon nanomaterials and transition metal-based materials, for electrochemical oxygen reduction to H₂ O₂ are then discussed, with an emphasis on the effects of electronic structure, nanostructure, and surface properties on catalytic performance. Finally, future challenges and opportunities are proposed for the further development of H₂ O₂ electrogeneration through 2e^- ORR, from the standpoints of mechanistic studies and practical application.

O,N-Codoped 3D graphene hollow sphere derived from metal-organic frameworks as oxygen reduction reaction electrocatalysts for Zn-air batteries

Although Pt-based oxygen reduction reaction (ORR) catalysts have excellent performance, they are expensive and suffer from poor durability. It is necessary to explore carbon-based ORR electrocatalysts with low cost, high specific surface area, large porosity, and strong chemical stability. Herein, we have synthesized a zinc-based metal-organic framework precursor (Zn-BTC) using a simple solvothermal method. Then, carbonization and N doping have been carried out by means of high-temperature pyrolysis, ultimately affording metal-free 3D hollow spherical O and N dual-doped graphene framework composites (O,N-graphene) with an average diameter of about 4 μm and specific surface area as high as 1801.4 m² g^-1. O,N-Graphene has superior ORR electrocatalytic activity with an onset potential E_onset = 1.01 V vs. RHE and a half-wave potential E_1/2 = 0.842 V vs. RHE, which are comparable with commercial 20 wt% Pt/C with a 4-electron reduction process. The O,N-graphene catalyst shows better durability and methanol tolerance at a lower cost than commercial 20 wt% Pt/C. The peak power density of O,N-graphene as the cathode of a traditional Zn-air battery is 152.8 mW cm^-2, which is higher than that of a commercial 20 wt% Pt/C battery (119.8 mW cm^-2). Our findings indicate that synergy among the 3D hollow structure, large specific surface area, highly conductive graphene framework, and pyridine N and graphite N defects left in O,N-graphene accelerated O₂ diffusion and increased catalytically active sites, thereby affording superior ORR and improved Zn-air battery performance under alkaline conditions.

Tuning the Covering on Gold Surfaces by Grafting Amino-Aryl Films Functionalized with Fe(II) Phthalocyanine: Performance on the Electrocatalysis of Oxygen Reduction

Current selective modification methods, coupled with functionalization through organic or inorganic molecules, are crucial for designing and constructing custom-made molecular materials that act as electroactive interfaces. A versatile method for derivatizing surfaces is through an aryl diazonium salt reduction reaction (DSRR). A prominent feature of this strategy is that it can be carried out on various materials. Using the DSRR, we modified gold surface electrodes with 4-aminebenzene from 4-nitrobenzenediazonium tetrafluoroborate (NBTF), regulating the deposited mass of the aryl film to achieve covering control on the electrode surface. We got different degrees of covering: monolayer, intermediate, and multilayer. Afterwards, the ArNO₂ end groups were electrochemically reduced to ArNH₂ and functionalized with Fe(II)-Phthalocyanine to study the catalytic performance for the oxygen reduction reaction (ORR). The thickness of the electrode covering determines its response in front of ORR. Interestingly, the experimental results showed that an intermediate covering film presents a better electrocatalytic response for ORR, driving the reaction by a four-electron pathway.

Plasma-assisted defect engineering of N-doped NiCo2O4 for efficient oxygen reduction

Defect control is a promising way to enhance the electrocatalysis performance of metal oxides. Oxygen vacancy enriched NiCo₂O₄ was successfully prepared using cold plasma. Oxygen as a plasma-forming gas introduces oxygen vacancies via electron etching. The concentration of oxygen vacancies can be controlled by different plasma-forming gas. CoO, which formed on the plasma samples, is beneficial for quick charge transfer and electrocatalytic performance. A high amount of nitrogen atoms of up to 10.1% was doped on NiCo₂O₄ because of the enriched oxygen vacancies and improved the stability of the oxygen defects and the conductivity of the catalyst. Electrocatalytic studies showed that the plasma-induced N-doped NiCo₂O₄ shows enhanced electrocatalytic performance for the oxygen reduction reaction (ORR). It shows a typical four-electron process that considerably improves the current density and onset potential. The HO₂^- % was as low as 0.59% and current density was 4.9 mA cm^-2 at 0.2 V (Vs. RHE) on the plasma-treated NiCo₂O₄. Calculations based on density functional theory reveal the mechanism for the promotion of the catalytic ORR activity via plasma treatment. This increases the electron density near the Fermi level, reducing the work function, and changing the position of the d-band center.

Hydrothermal-Induced Formation of Well-Defined Hollow Carbons with Curvature-Activated N-C Sites for Zn-Air Batteries

Metal-free carbons have been regarded as one of the promising materials alternatives to precious-metal catalysts for oxygen reduction reaction (ORR) due to their high activity and stability. In this paper, well-defined N-doped hollow carbons (NHCs) are firstly synthesized by using an ammonia-based hydrothermal synthesis that is environmentally friendly and suitable for mass production in industry and a commercial black carbon as raw material. Moreover, the shell thickness of the NHCs can be easily tuned by this hydrothermal strategy. Zn-air battery test results reveal shell thickness-dependent activity and durability for ORR over the NHCs, which exceeds that obtained by commercial Pt/C (20 wt %). The enhanced battery performance can be attributed to the curvature-activated N-C moieties on the hollow carbon surface, which served as the main active sites for ORR as evidenced by DFT calculations. The proposed approach may open a way for designing curved hollow carbons with high graphitization degree and dopant nitrogen level for metal-air batteries or fuel cells.

Reconstructing 1D Fe Single-atom Catalytic Structure on 2D Graphene Film for High-Efficiency Oxygen Reduction Reaction

The ordinary intrinsic activity and disordered distribution of metal sites in zero/one-dimensional (0D/1D) single-atom catalysts (SACs) lead to inferior catalytic efficiency and short-term endurance in the oxygen reduction reaction (ORR), which restricts the large-scale application of hydrogen-oxygen fuel cells and metal-air batteries. To improve the activity of SACs, a mild synthesis method was chosen to conjugate 1D Fe SACs with 2D graphene film (Fe SAC@G) that realized a composite structure with well-ordered atomic-Fe coordination configuration. The product exhibits outstanding ORR electrocatalytic efficiency and stability in 0.1 M KOH aqueous solution. DFT-D computational results manifest the intrinsic ORR activity of Fe SAC@G originated from the newly-formed FeN₄ -O-FeN₄ bridge structure with moderate adsorption ability towards ORR intermediates. These findings provide new ways for designing SACs with high activity and long-term stability.

MOFs-Derived Fe-N Codoped Carbon Nanoparticles as O2-Evolving Reactor and ROS Generator for CDT/PDT/PTT Synergistic Treatment of Tumors

Metal-organic frameworks (MOFs) derivatives had been widely explored in electronic and environmental fields, but rarely evaluated in the biomedical applications. Herein, Fe-N codoped carbon (FeNC) nanoparticles were synthesized and characterized via facile pyrolysis of precursor ZIF-8 (Fe/Zn) nanoparticles, and their potential applications in tumor therapy were assessed in this investigation both in vitro and in vivo. After PAA (sodium polyacrylate) modification, the FeNC@PAA nanoparticles were able to initiate a Fe-based Fenton-like reaction to generate ·OH and O₂ for chemodynamic therapy (CDT) and O₂ evolution. Meanwhile, the porphyrin-like metal center in the FeNC@PAA nanoparticles could be used as a photosensitizer for photodynamic therapy (PDT) of tumors, which could be enhanced by O₂ generated in CDT. Furthermore, the FeNC@PAA nanoparticles were also found to be effective in photothermal therapy (PTT) with a photothermal conversion efficiency of 29.15%, owing to a high absorbance in the near-infrared region (NIR). In conclusion, the synthesized FeNC@PAA nanoparticles exhibited promising applications in O₂ evolution and CDT/PDT/PTT synergistic treatment of tumors.

Direct Pyrolysis of a Manganese-Triazolate Metal-Organic Framework into Air-Stable Manganese Nitride Nanoparticles

Although metal-organic frameworks (MOFs) are being widely used to derive functional nanomaterials through pyrolysis, the actual mechanisms involved remain unclear. In the limited studies to date, elemental metallic species are found to be the initial products, which limits the variety of MOF-derived nanomaterials. Here, the pyrolysis of a manganese triazolate MOF is examined carefully in terms of phase transformation, reaction pathways, and morphology evolution in different conditions. Surprisingly, the formation of metal is not detected when manganese triazolate is pyrolyzed in an oxygen-free environment. Instead, a direct transformation into nanoparticles of manganese nitride, Mn2N x embedded in N-doped graphitic carbon took place. The electrically conductive Mn2N x nanoparticles show much better air stability than bulk samples and exhibit promising electrocatalytic performance for the oxygen reduction reaction. The findings on pyrolysis mechanisms expand the potential of MOF as a precursor to derive more functional nanomaterials.

Transition Metal and Nitrogen Co-Doped Carbon-based Electrocatalysts for the Oxygen Reduction Reaction: From Active Site Insights to the Rational Design of Precursors and Structures

Considering the urgent requirement for clean and sustainable energy, fuel cells and metal-air batteries have emerged as promising energy storage and conversion devices to alleviate the worldwide energy challenges. The key step in accelerating the sluggish oxygen reduction reaction (ORR) kinetics at the cathode is to develop cost-effective and high-efficiency non-precious metal catalysts, which can be used to replace expensive Pt-based catalysts. Recently, the transition metal and nitrogen co-doped carbon (M-N_x /C) materials with tailored morphology, tunable composition, and confined structure show great potential in both acidic and alkaline media. Herein, the mechanism of ORR is provided, followed by recent efforts to clarify the actual structures of active sites. Furthermore, the progress of optimizing the catalytic performance of M-N_x /C catalysts by modulating nitrogen-rich precursors and porous structure engineering is highlighted. The remaining challenges and development prospects of M-N_x /C catalysts are also outlined and evaluated.

Design Strategies of Transition-Metal Phosphate and Phosphonate Electrocatalysts for Energy-Related Reactions

The key challenge to developing renewable energy conversion and storage devices lies in the exploration and rational engineering of cost-effective and highly efficient electrocatalysts for various energy-related electrochemical reactions. Transition-metal phosphates and phosphonates have shown remarkable performances for these reactions based on their unique physicochemical properties. Compared with transition-metal oxides, phosphate groups in transition-metal phosphates and phosphonates show flexible coordination with diverse orientations, making them an ideal platform for designing active electrocatalysts. Although numerous efforts have been spent on the development of transition-metal phosphate and phosphonate electrocatalysts, some urgent issues, such as low intrinsic catalytic efficiency and low electronic conductivity, have to be resolved in accordance with their applications. In this Review, we focus on the design strategies of highly efficient transition-metal phosphate and phosphonate electrocatalysts, with special emphasis on the tuning of transition-metal-center coordination environment, optimization of electronic structures, increase of catalytically active site densities, and construction of heterostructures. Guided by these strategies, recently developed transition-metal phosphate and phosphonate materials have exhibited excellent activity, selectivity, and stability for various energy-related electrocatalytic reactions, showing great potential for replacing noble-metal-based catalysts in next-generation advanced energy techniques. The existing challenges and prospects regarding these materials are also presented.

Heteroatom-doped carbon-based oxygen reduction electrocatalysts with tailored four-electron and two-electron selectivity

Oxygen reduction reaction (ORR) plays a pivotal role in electrochemical energy conversion and commodity chemical production. Oxygen reduction involving a complete four-electron (4e-) transfer is important for the efficient operation of polymer electrolyte fuel cells, whereas the ORR with a partial 2e- transfer can serve as a versatile method for producing industrially important hydrogen peroxide (H2O2). For both the 4e- and 2e- pathway ORR, platinum-group metals (PGMs) have been materials of prevalent choice owing to their high intrinsic activity, but they are costly and scarce. Hence, the development of highly active and selective non-precious metal catalysts is of crucial importance for advancing electrocatalysis of the ORR. Heteroatom-doped carbon-based electrocatalysts have emerged as promising alternatives to PGM catalysts owing to their appreciable activity, tunable selectivity, and facile preparation. This review provides an overview of the design of heteroatom-doped carbon ORR catalysts with tailored 4e- or 2e- selectivities. We highlight catalyst design strategies that promote 4e- or 2e- ORR activity. We also summarise the major active sites and activity descriptors of the respective ORR pathways and describe the catalyst properties controlling the ORR mechanisms. We conclude the review with a summary and suggestions for future research.

One-pot synthesis of mesoporous palladium/C nanodendrites as high-performance oxygen reduction eletrocatalysts through a facile dual surface protecting agent-assisted strategy

Palladium (Pd) is regarded as a potential non-platinum electrocatalyst to drive oxygen reduction in fuel cells. The development of Pd-based electrocatalysts with high performances through structural engineering is still highly desirable. Herein, a facile one-pot synthesis strategy with the assistance of dual surface protecting agents was developed to fabricate carbon-supported Pd (Pd/C) nanodendrites with high mesoporosity. The mesoporous spherical Pd/C nanodendrites are built with connected nanoparticles with a small size of several nanometers and coated by simultaneously formed carbon layers. The used dual protecting agents, glycine and oleylamine, exhibit synergistic effects to engineer Pd growth to form the unique mesoporous dendritic structure. Benefiting from the mesoporous feature, small size, defect-rich surface and carbon coating, the obtained mesoporous Pd/C nanodendrites exhibit great electrocatalytic performance toward the oxygen reduction reaction (ORR).

Effect of the valence state of Mn in MnOx/Ti4O7 composites on the catalytic performance for oxygen reduction reaction and oxygen evolution reaction

Manganese oxide composites with mixed valence states were prepared through the hydrothermal method by compositing with Ti₄O₇ and calcining at different temperatures, and their ORR and OER catalytic performance were investigated. The prepared catalysts were characterized by XRD, SEM-EDS, HRTEM-EDS, and XPS methods to analyse their phase constitution, morphology feature, and surface composition. The major phase of manganese oxides was Mn₃O₄, which is a one-dimensional structure, and its growth was induced by Ti₄O₇. The ORR and OER catalytic activity can be enhanced due to the preferred orientation of manganese oxides. Electrochemical measurements, namely CV, LSV and EIS, were utilized for determining the ORR and OER catalytic activity, whereas CA and ADT were used for studying the durability and stability. A Li–O₂ battery was assembled to test the electrochemical behavior and properties in practical application. MnO_x/Ti₄O₇ calcined at 300 °C exhibited the best catalytic activity of 0.72 V vs. RHE half-wave potential for ORR and 0.67 V vs. RHE overpotential for OER. The proportion of Mn³⁺ was also highest in all the MnO_x/Ti₄O₇ composites. The assembled Li–O₂ battery shows high performance with a voltage gap of only 0.85 V. Therefore, it can be affirmed that the inducement of Ti₄O₇ could strengthen the preferred orientation in manganese oxide growth and Mn³⁺ in MnO_x/Ti₄O₇ plays a vital role in catalyzing ORR and OER, with both improving the ORR and OER bifunctional catalytic performance of manganese oxides.

Manganese oxide composites with mixed valence states were prepared by compositing with Ti₄O₇ and calcination temperature could influence their ORR and OER catalytic performance observably.

Porphyrin-based frameworks for oxygen electrocatalysis and catalytic reduction of carbon dioxide

The recent progress made on porphyrin-based frameworks and their applications in energy-related conversion technologies (e.g., ORR, OER and CO₂RR) and storage technologies (e.g., Zn–air batteries).

The role of metal–organic porous frameworks in dual catalysis

Metal–organic frameworks (MOFs) are a valuable group of porous crystalline solids with inorganic and organic parts that can be used in dual catalysis.