Effect of Transition Metals on the Structure and Performance of the Doped Carbon Catalysts Derived From Polyaniline and Melamine for ORR Application

Atomically Dispersed Metal-Nitrogen-Carbon Catalysts for Acidic Oxygen Reduction Reaction

Designing efficient and cost-effective electrocatalysts toward oxygen reduction reaction (ORR) under demanding acidic environments plays a critical role in advancing proton exchange membrane fuel cells (PEMFCs). Metal-nitrogen-carbon (M-N-C) catalysts with atomically dispersed metals have gained attention for their affordability, excellent catalytic performance, and distinctive features including consistent active sites and high atomic utilization. Over the past decade, significant achievements have been made in this field. This review offers a comprehensive summary of the latest developments in atomically dispersed M-N-C catalysts for ORR in acidic environments along with their applications in PEMFCs. The ORR mechanisms, PEMFC configuration, and operational principles are presented first, followed by an in-depth discussion of strategies to improve the activity and stability of the PEMFC using atomically dispersed M-N-C catalysts at the cathode. Lastly, this review highlights the unresolved challenges and proposes future research pathways for advancing high-performance atomically dispersed M-N-C catalysts and PEMFCs.

Strain engineering of CoSANC catalyst toward enhancing the oxygen reduction reaction activity

Developing efficient and cost-effective platinum-group metal-free (PGMF) catalysts for the oxygen reduction reaction (ORR) is crucial for energy conversion and storage devices. Among these catalysts, metal-nitrogen-carbon (MNC) materials, particularly cobalt single-atom catalysts (CoSANC), show promise as ORR electrocatalysts. However, their ORR activity is often hindered by strong hydroxyl (OH) adsorption on the Co sites. While the impact of strain engineering on MNC electrocatalysts has been minimally explored, recent studies suggest its potential to enhance catalytic performance and optimize intrinsic activity in traditional bulk catalysts. In this context, we investigate the effect of surface strain on CoSANC for ORR activity and correlate substrate-strain-induced geometric distortions with catalytic activity using experimental and theoretical methods. The findings suggest that the d-band center gap of spin states (Δεd) may be a preferred descriptor for predicting strain-dependent ORR performance in MNC catalysts. Leveraging CoSANC moiety placed on a substrate with an average size of 1.0 μm, we achieve performance comparable to that of commercial Pt/C catalysts when used as a cathode catalyst in zinc-air batteries. This investigation unveils the structure-function relationship of MNC electrocatalysts regarding strain engineering and provides valuable insights for future ORR activity design and enhancement.

FeCu-N6-C Diatomic Sites Catalyst for the Boosted Oxygen Reduction Reactions in Zinc-Air Batteries

Due to the high catalytic activity and stability for oxygen reduction reaction, N-coordinated Fe-Cu dual-metal doped carbon material (FeCu-N-C) is considered to be one of the promising electrode materials for metal-air battery and fuel cells. Herein, FeCu-N-C dual-metal catalysts was synthesized by an adsorption-calcination strategy. The prepared FeCu-N-C exhibited high activity and stability both in alkaline and acidic media. In alkaline/acid medium, the half-wave potential reaches to 0.90/0.80 V, which is better than Fe-N-C catalyst. The power density for FeCu-N-C in zinc-air battery reaches to 220 mW cm-2 and shows high electrochemical stability for more than 600 hours in charge/discharge cycles, much higher than 130 hours for Pt/C (40 %) and 100 hours for Fe-N-C. Density-functional theory calculations showed that the FeCu-N-C dual-metal catalysts got lower overpotential of 0.50 V than Fe-N-C (0.53 V), which improved the ORR activity. The results are helpful for the deep understanding of high-performance diatomic catalysts.

Unravelling Species-Specific Loading Effects on Oxygen Reduction Activity of Heteronuclear Single Atom Catalysts

Toward high-density single atom catalysts (SACs), the interaction between neighboring SACs and the induced non-linear loading effect become crucial for their intrinsic catalytic performance. Despite recent investigations on homonuclear SACs, understanding such effect in heteronuclear SACs remains limited. Using Fe and Co SACs co-supported on the nitrogen-doped graphene as a model system, the loading effect on the site-specific activity of heteronuclear SACs toward oxygen reduction reaction (ORR) is here reported by density functional theory calculations. The Fe site exhibits an oscillatory decrease in activity with the loading. In contrast, the Co site has a volcano-like activity with the optimum performance achieved at ≈16.8 wt.% (average inter-site distance: ≈7 Å). At the ultra-high loading of 38.4 wt.% (inter-site distance: ≈4 Å), the Co site is the only ORR active site, whereas Fe sites turn into spectators. This distinct loading-dependent activity between the Fe and Co sites can be ascribed to their difference in the binding capability with the substrate and the dxz and dyz orbitals' occupation. These findings highlight the importance of the loading effect in heteronuclear SACs, which could be useful for the development of high-performance heteronuclear and high-entropy SACs toward various catalytic reactions in the high-loading regime.

Single Atom Catalysts Based on Earth-Abundant Metals for Energy-Related Applications

Anthropogenic activities related to population growth, economic development, technological advances, and changes in lifestyle and climate patterns result in a continuous increase in energy consumption. At the same time, the rare metal elements frequently deployed as catalysts in energy related processes are not only costly in view of their low natural abundance, but their availability is often further limited due to geopolitical reasons. Thus, electrochemical energy storage and conversion with earth-abundant metals, mainly in the form of single-atom catalysts (SACs), are highly relevant and timely technologies. In this review the application of earth-abundant SACs in electrochemical energy storage and electrocatalytic conversion of chemicals to fuels or products with high energy content is discussed. The oxygen reduction reaction is also appraised, which is primarily harnessed in fuel cell technologies and metal-air batteries. The coordination, active sites, and mechanistic aspects of transition metal SACs are analyzed for two-electron and four-electron reaction pathways. Further, the electrochemical water splitting with SACs toward green hydrogen fuel is discussed in terms of not only hydrogen evolution reaction but also oxygen evolution reaction. Similarly, the production of ammonia as a clean fuel via electrocatalytic nitrogen reduction reaction is portrayed, highlighting the potential of earth-abundant single metal species.

Correlative Effects of Carbon Support Structures and Surface Properties on ORR Catalytic Activities of Loaded Catalysts

As a complex three-phase heterogeneous catalyst, the oxygen reduction reaction (ORR) catalyst activity is determined by the interfacial and surface structures and chemical state of the catalyst support. As a typical biomass carbon-based support, rice husk-based porous carbon (RHPC) has natural unique hierarchical porous structures, which easily regulate the microstructure and surface properties. This study explored the correlative effects of RHPC structure and surface properties on ORR catalytic activity through the typical modification methods, namely, alkali etching, high temperature, oxidation, and ball milling. The various factors for the joint effects are defined as the specific surface area, oxygen-containing functional groups, graphite edge defects, resistivity, and contact angle. The analysis of such joint influences is difficult to quantitatively evaluate due to the large number of experimental factors and small sample sizes. Partial least-squares (PLS) can better deal with such problems. Therefore, a PLS regression model was established to evaluate the relative weight of each factor on the catalytic activity for the RHPC-based support catalysts. The results reveal that the regression coefficients of four factors yield similar magnitude for the effect of the half-wave potential (E1/2). However, graphite edge defects had a more significant impact on the limiting diffusion current density (J) and electron transfer number (n). Furthermore, an optimal support named BM-RHPC-3 was prepared with more defects and oxygen-containing functional groups, which prepared Fe-NS/BM-RHPC-3 presenting the best ORR catalytic activity (E1/2 = 0.880 V, J of 5.15 mA cm-2), superior to Pt/C (E1/2 = 0.844 V, J of 4.99 mA cm-2). The statistical regression model is validated with a relative error of less than 5% between predicted and true values for analyzing RHPC-based ORR catalysts' catalytic performance. It shows the feasibility of experiment-informed learning for data-driven material discovery and design.

From Charge to Spin: An In-Depth Exploration of Electron Transfer in Energy Electrocatalysis

Catalytic materials play crucial roles in various energy-related processes, ranging from large-scale chemical production to advancements in renewable energy technologies. Despite a century of dedicated research, major enduring challenges associated with enhancing catalyst efficiency and durability, particularly in green energy-related electrochemical reactions, remain. Focusing only on either the crystal structure or electronic structure of a catalyst is deemed insufficient to break the linear scaling relationship (LSR), which is the golden rule for the design of advanced catalysts. The discourse in this review intricately outlines the essence of heterogeneous catalysis reactions by highlighting the vital roles played by electron properties. The physical and electrochemical properties of electron charge and spin that govern catalysis efficiencies are analyzed. Emphasis is placed on the pronounced influence of external fields in perturbing the LSR, underscoring the vital role that electron spin plays in advancing high-performance catalyst design. The review culminates by proffering insights into the potential applications of spin catalysis, concluding with a discussion of extant challenges and inherent limitations.

Co-N-C/C Bifunctional Electrocatalyst for Dual Applications in Seawater Electrolysis and Catalyst in Hydrazine Fuel Cells

The convergence of water electrolysis and alkaline fuel cells offers captivating solutions for sustainably harvesting energy. The research explores both hydrazine-assisted seawater electrolysis (hydrazine oxidation reaction (HzOR) and hydrogen production reaction (HER)), as well as alkaline hydrazine fuel cell reactions (HzOR and Oxygen reduction reaction (ORR)) by using a bifunctional cobalt polyaniline derived (Co PANI/C) catalyst. The catalyst shows excellent performance for hydrazine-assisted seawater electrolysis in harsh seawater environments to produce H2 as fuel with nearly 85% Faradaic efficiency and during alkaline HzOR, the bifunctional catalyst generates H2 with 95% Faradaic efficiency by acting as both anode and cathode side catalyst. Also, the same catalyst requires only a potential of 0.34 V versus RHE and 0.906 V versus RHE for HzOR and ORR, respectively, in 1 m KOH, which makes this overall process useful for a Hz/O2 fuel cell.

Magnetic Order Transition of a Two-Dimensional Square-Lattice Electrocatalyst Assembled by Fe-N4 Units: Crucial Role on Oxygen Reduction

Herein, we theoretically investigate the effect of magnetic orders on electrocatalytic oxygen reduction reaction (ORR) properties on the Fe-N4 site-embedded two-dimensional (2D) covalent organic framework (Fe-N4@COF-C3N2) under realistic environments. The Fe-N4@COF-C3N2 shows a 2D square-lattice (sql) topology with three magnetic order states: one ferromagnetic state (FM) and two antiferromagnetic states (AFM1 and AFM2). Specially, the electrocatalyst in the AFM2 state shows a remarkable onset potential of 0.80 V/reversible hydrogen electrode (RHE) at pH 1, superior to the existing most excellent noble-metal catalysts. Thermodynamically, the onset potential for the 4e- ORR is 0.64 V/RHE at pH 1, with a magnetic state transition process of FM → AFM1 → FM → FM → FM, while at pH 13, the onset potential for the 4e- ORR is 0.54 V/RHE, with the magnetic transition process of FM → FM → AFM1 → FM → FM. Generally, this finding will provide new avenues to rationally design the Fe-N4 electrocatalyst.

Ordered mesoporous carbon with binary CoFe atomic species for highly efficient oxygen reduction electrocatalysis

The exploration of powerful, efficient and precious metal-free electrocatalysts for facilitating the sluggish kinetics of the oxygen reduction reaction (ORR) is a crucial endeavor in the development and application of energy conversion and storage devices. Herein, we have rationally designed and synthesized bimetallic CoFe species consisting of CoFe nanoparticles and atomically dispersed dual atoms anchored on an ordered mesoporous carbon matrix (CoFe/NC) as highly efficient ORR electrocatalysts. The pyrolyzation temperature for CoFe/NC plays a vital role in regulating the morphology and composition of both the carbon matrix and CoFe species. The optimized CoFe/NC-750 exhibits a favorable ORR performance in 0.1 M KOH with a high half-wave potential (E1/2) of 0.87 V vs. RHE, excellent tolerance to methanol and remarkable durability (no obvious decrease in E1/2 value after 3000 cycles), all of which are superior to the performance of commercial Pt/C. Experimental measurements and density functional theory (DFT) calculations reveal that the improved ORR performance of CoFe/NC-750 is mainly attributed to the electronic structure of atomically dispersed Fe active sites modulated by the surrounding CoFe alloys and Co single atoms, which accelerates the dissociation and reduction of intermediate OH* species and promotes the ORR process.

2D Metal Porphyrin-Based MOFs and ZIF-8 Composite-Derived Carbon Materials Containing M-Nx Active Sites as Bifunctional Electrocatalysts for Zinc-Air Batteries

The main impediment to the development of zinc-air batteries is the sluggish kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Transition metal N-doped carbon catalysts offer a promising alternative to noble metal catalysts, with metal-organic framework (MOF)-derived carbon material catalysts being particularly noteworthy. Here, we synthesized MxP-Z-C carbon catalysts by combining two-dimensional (2D) metal porphyrin-based MOFs (MxPMFs, x = Fe, Co, Ni, Mn) and three-dimensional zeolitic imidazole framework-8 (ZIF-8) through electrostatic interaction, followed by carbonization. ZIF-8 was inserted between the layers of MxPMFs to prevent its Π-Π stacking, allowing the active sites to become fully exposed. MxP-Z-C demonstrated an impressive catalytic activity for both the ORR and the OER reactions. Among them, FeP-Z-C showed the best catalytic activity. The half-wave potential for ORR was 0.92 V (vs the reversible hydrogen electrode (RHE)), while the overpotential for the OER was 290 mV. In addition, the zinc-air battery assembled by FeP-Z-C exhibited high power density (133.14 mW cm-2) and significant specific capacity (816 mAh gZn-1), indicating considerable potential as a bifunctional catalyst for electronic devices.

Improved H2O2 Electrosynthesis on S-doped Co-N-C through Cooperation of Co-S and Thiophene S

Co-N-C based catalysts have emerged as a prospective alternative for H2O2 electrosynthesis via a selective 2e- oxygen reduction reaction (ORR). However, conventional Co-N-C with Co-N4 configurations usually exhibits low selectivity toward 2e- ORR for H2O2 production. In this study, the S-doped Co-N-C (Co-N-C@S) catalysts were designed and synthesized for enhancing the electrosynthesis of H2O2, and their S doping levels and species were tuned to investigate their relationship with the H2O2 yield. The results showed that the S doping greatly enhanced the activity and selectivity of Co-N-C@S for H2O2 production. The optimal Co-N-C@S(12) displayed a high H2O2 production rate of 395 mmol gcat-1 h-1, H2O2 selectivity of 76.06%, and Faraday efficiency of 91.66% at 0.2 V, which were obviously better than those of Co-N-C (H2O2 production rate of 44 mmol gcat-1 h-1, H2O2 selectivity of 26.63%, and Faraday efficiency of 17.37%). Moreover, the Co-N-C@S(12) based electron-Fenton system displayed effective rhodamine B (RhB) removal, significantly outperforming the Co-N-C-based system. Experimental results combined with density functional theory unveiled that the enhanced performance of Co-N-C@S(12) stemmed from the combined effect of Co-S and thiophene S, which jointly enhanced electron density of the Co center, reduced the desorption energy of the *OOH intermediate, and then promoted the production of H2O2.

Unraveling the pH-Dependent Oxygen Reduction Performance on Single-Atom Catalysts: From Single- to Dual-Sabatier Optima

Metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) have emerged as a potential substitute for the costly platinum-group catalysts in oxygen reduction reaction (ORR). However, several critical aspects of M-N-C SACs in ORR remain poorly understood, including their pH-dependent activity, selectivity for 2- or 4-electron transfer pathways, and the identification of the rate-determining steps. Herein, by analyzing >100 M-N-C structures and >2000 sets of energetics, we unveil a pH-dependent evolution in ORR activity volcanos─from a single peak in alkaline media to a double peak in acids. We found that this pH-dependent behavior in M-N-C catalysts fundamentally stems from their moderate dipole moments and polarizability for O* and HOO* adsorbates, as well as unique scaling relations among ORR adsorbates. To validate our theoretical discovery, we synthesized a series of molecular M-N-C catalysts, each characterized by well-defined atomic coordination environments. Impressively, the experiments matched our theoretical predictions on kinetic current, Tafel slope, and turnover frequency in both acidic and alkaline environments. These new insights also refine the famous Sabatier principle by emphasizing the need to avoid an "acid trap" while designing M-N-C catalysts for ORR or any other pH-dependent electrochemical applications.

Integration of a Cu2O/ZnO heterojunction and Ag@SiO2 into a photoanode for enhanced solar water oxidation

Photoelectrochemical water splitting (PEC-WS) has attracted considerable attention owing to its low energy consumption and sustainable nature. Constructing semiconductor heterojunctions with controllable band structure can effectively facilitate photogenerated carrier separation. In this study, a FTO/ZnO/Cu2O/Ag@SiO2 photoanode with a Cu2O/ZnO p-n heterojunction and Ag@SiO2 nanoparticles is constructed to investigate its PEC-WS performance. Compared with a bare ZnO photoanode, the photocurrent density of the FTO/ZnO/Cu2O/Ag@SiO2 photoanode (0.77 mA cm-2) at 1.23 VRHE exhibits an increment of 88%, and a cathodic shift of 0.1 V for the on-set potential (0.4 VRHE). Detailed photoelectrochemical analyses reveal that the Cu2O/ZnO p-n heterojunction formed between Cu2O and ZnO can effectively promote photogenerated carrier separation. The surface plasmonic effect of the Ag@SiO2 nanoparticles can further promote the photogenerated carrier transfer efficiency, which synergistically improves the PEC-WS performance.

Recent Progress on Durable Metal-N-C Catalysts for Proton Exchange Membrane Fuel Cells

It is essential for the widespread application of proton exchange membrane fuel cells (PEMFCs) to investigate low-cost, extremely active, and long-lasting oxygen reduction catalysts. Initial performance of PGM-free metal-nitrogen-carbon (M-N-C) catalysts for oxygen reduction reaction (ORR) has advanced significantly, particularly for Fe-N-C-based catalysts. However, the insufficient stability of M-N-C catalysts still impedes their use in practical fuel cells. In this review, we focus on the understanding of the structure-stability relationship of M-N-C ORR catalysts and summarize valuable guidance for the rational design of durable M-N-C catalysts. In the first section of this review, we discuss the inherent degrading mechanisms of M-N-C catalysts, such as carbon corrosion, demetallation, H2 O2 attack, etc. As we gain a thorough comprehension of these deterioration mechanisms, we shift our attention to the investigation of strategies that can mitigate catalyst deterioration and increase its stability. These strategies include enhancing the anti-oxidation of carbon, fortifying M-N bonds, and maximizing the effectiveness of free radical scavengers. This review offers a prospective view on the enhancement of the stability of non-noble metal catalysts.

Bifunctional Single Atom Catalysts for Rechargeable Zinc-Air Batteries: From Dynamic Mechanism to Rational Design

Ever-growing demands for rechargeable zinc-air batteries (ZABs) call for efficient bifunctional electrocatalysts. Among various electrocatalysts, single atom catalysts (SACs) have received increasing attention due to the merits of high atom utilization, structural tunability, and remarkable activity. Rational design of bifunctional SACs relies heavily on an in-depth understanding of reaction mechanisms, especially dynamic evolution under electrochemical conditions. This requires a systematic study in dynamic mechanisms to replace current trial and error modes. Herein, fundamental understanding of dynamic oxygen reduction reaction and oxygen evolution reaction mechanisms for SACs is first presented combining in situ and/or operando characterizations and theoretical calculations. By highlighting structure-performance relationships, rational regulation strategies are particularly proposed to facilitate the design of efficient bifunctional SACs. Furthermore, future perspectives and challenges are discussed. This review provides a thorough understanding of dynamic mechanisms and regulation strategies for bifunctional SACs, which are expected to pave the avenue for exploring optimum single atom bifunctional oxygen catalysts and effective ZABs.

Coordination Engineering of Dual Co, Ni Active Sites in N-Doped Carbon Fostering Reversible Oxygen Electrocatalysis

The development of affordable and non-noble-metal-based reversible oxygen electrocatalysts is required for renewable energy conversion and storage systems like metal-air batteries (MABs). However, the nonbifunctionality of most of the catalysts impedes their use in rechargeable MAB applications. Moreover, the loss of active sites also affects the long-term performance of the electrocatalyst toward oxygen electrocatalysis. In this work, we report a simplistic yet controllable chemical approach for the synthesis of dual transitional metals such as cobalt, nickel, and nitrogen-doped carbon (CoNi-NC) as bifunctional electrode materials for rechargeable zinc-air batteries (ZABs). The spatially isolated Ni-N4 and Co-N4 active units were rendered for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), respectively. The individual efficacy of both reversible reactions enables an ΔE value of ∼0.72 V, which outperforms several bifunctional electrocatalysts reported in the literature. The half-wave potential (E1/2) and overpotential were achieved at 0.83 V and 330 mV (vs RHE) for ORR and OER, respectively. The peak power density of ZAB equipped with the CoNi-NC catalyst was calculated to be 194 mW cm-2. The present strategy for the synthesis of bifunctional electrocatalysts with dual active sites offers prospects for developing electrochemical energy storage and conversion systems.

Research progress on MOFs and their derivatives as promising and efficient electrode materials for electrocatalytic hydrogen production from water

Hydrogen energy is considered to be the most potential "ultimate energy source" due to its high combustion calorific value, cleanliness, and pollution-free characteristics. Furthermore, the production of hydrogen via the electrolysis of water has the advantages of simplicity, high efficiency, environmentally safe, and high-purity hydrogen. However, it is also associated with issues such as high-power consumption for the reaction and limited large-scale application of noble metal catalysts. Metal-organic frameworks (MOFs) are porous composite materials composed of metal ions and organic functional groups through orderly coordination with large specific surface areas and large porosity. Herein, we focus on the research status of MOFs and their transition metal derivatives for electrocatalytic water splitting to produce hydrogen and briefly describe the reaction mechanism and evaluation parameters of the electrocatalytic hydrogen evolution and oxygen evolution reactions. Furthermore, the relationship between the catalytic behavior and catalytic activity of different MOF-based catalysts and their morphology, elemental composition, and synthetic strategy is analyzed and discussed. The reasons for the excellent activity and poor stability of the original MOF materials for the electrolysis of water reaction are shown through analysis, and using various means to improve the catalytic activity by changing the electronic structure, active sites, and charge transfer rate, MOF-based catalysts were obtained. Finally, we present perspectives on the future development of MOFs for the electrocatalytic decomposition of water.

Highly Porous Polypyrrole (PPy) Hydrogel Support for the Design of a Co-N-C Electrocatalyst for Oxygen Reduction Reaction

Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts have emerged as one of the most promising platinum-group metal (PGM)-free cathode catalysts for oxygen reduction reaction (ORR). Among the various approaches to enhance the ORR performance of the catalysts, increasing the density of accessible active sites is of paramount importance. Thus, nitrogen-rich support with abundant porosity can be very propitious. Herein, we report a highly porous polypyrrole (PPy) hydrogel as a versatile support for the facile design of a Co-N-C electrocatalyst for ORR. The resulting Co-N-C catalyst with abundant micro- and mesoporous combinations demonstrates a half-wave potential (E1/2) of 0.825 V vs reversible hydrogen electrode (RHE) in O2-saturated 0.1M KOH with just 2.1 wt % Co content. The ORR performance reduces only 11 mV (E1/2) after 5000 cycles of accelerated durability test (ADT), portraying its excellent stability. The catalyst retains ≈83% of its original current during a short-term durability test at 0.8 V vs RHE for 25 h. Furthermore, the catalyst shows electron transfer approaching ≈4 with low H2O2 yield in the potential range 0.5-0.9 V vs RHE. This work provides a simple design strategy to synthesize M-N-C catalysts with increased accessible active site density and enhanced mass transport for ORR and other electrocatalytic applications.

Carbonized Polydopamine-Based Nanocomposites: The Effect of Transition Metals on the Oxygen Electrocatalytic Activity

The electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the most critical processes in renewable energy-related technologies, such as fuel cells, water electrolyzers, and unitized regenerative fuel cells. N-doped carbon composites have been demonstrated to be promising ORR/OER catalyst candidates because of their excellent electrical properties, tunable pore structure, and environmental compatibility. In this study, we prepared porous N-doped carbon nanocomposites (NC) by combining mussel-inspired polydopamine (PDA) chemistry and transition metals using a solvothermal carbonization strategy. The complexation between dopamine catechol groups and transition metal ions (Fe, Ni, Co, Zn, Mn, Cu, and Ti) results in hybrid structures with embedded metal nanoparticles converted to metal-NC composites after the carbonization process. The influence of the transition metals on the structural, morphological, and electrochemical properties was analyzed in detail. Among them, Cu, Co, Mn, and Fe N-doped carbon nanocomposites exhibit efficient catalytic activity and excellent stability toward ORR. This method improves the homogeneous distribution of the catalytically active sites. The metal nanoparticles in reduced (MnO, Fe₃C) or metallic (Cu, Co) oxidation states are protected by the N-doped carbon layers, thus further enhancing the ORR performance of the composites. Still, only Co nanocomposite is also effective toward OER with a potential bifunctional gap (ΔE) of 0.867 V. The formation of Co-N active sites during the carbonization process, and the strong coupling between Co nanoparticles and the N-doped carbon layer could promote the formation of defects and the interfacial electron transfer between the catalyst surface, and the reaction intermediates, increasing the bifunctional ORR/OER performance.

Atomically Dispersed Fe/N4 and Ni/N4 Sites on Separate-Sides of Porous Carbon Nanosheets with Janus Structure for Selective Oxygen Electrocatalysis

Dual single atoms catalysts have promising application in bifunctional electrocatalysis due to their synergistic effect. However, how to balance the competition between rate-limiting steps (RDSs) of reversible oxygen reduction and oxygen evolution reaction (OER) and fully expose the active centers by reasonable structure design remain enormous challenges. Herein, Fe/N₄ and Ni/N₄ sites separated on different sides of the carbon nanosheets with Janus structure (FeNi_jns /NC) is synthesized by layer-by-layer assembly method. Experiments and calculations reveal that the side of Fe/N₄ is beneficial to oxygen reduction reaction (ORR) and the Ni/N₄ side is preferred to OER. Such Janus structure can take full advantage of two separate-sides of carbon nanosheets and balance the competition of RDSs during ORR and OER. FeNi_jns /NC possesses superior ORR and OER activity with ORR half-wave potential of 0.92 V and OER overpotential of 440 mV at J = 10 mA cm^-2 . Benefiting from the excellent bifunctional activities, FeNi_jns /NC assembled aqueous Zn-air battery (ZAB) demonstrates better maximum power density, and long-term stability (140 h) than Pt/C+RuO₂ catalyst. It also reveals superior flexibility and stability in solid-state ZAB. This work brings a novel perspective for rational design and understanding of the catalytic mechanisms of dual single atom catalysts.

Enhanced redox cycle of Fe3+/Fe2+ on Fe@NC by boron: Fast electron transfer and long-term stability for Fenton-like reaction

In this work, Fe@NC/B material is successfully synthesized and in-situ supported on the surface of amorphous boron (B) using a simple pyrolysis method. The interface between Fe species and B is improved by introducing N-doped carbon (NC) layers as intermediate, fast electron transfer from B to Fe@NC can therefore be achieved, thus could promote the fast redox cycle of Fe³⁺/Fe²⁺. The obtained material can therefore activate peroxymonosulfate (PMS) effectively to degrade Bisphenol A (BPA), a fast degradation rate and a very long lifetime in a continous tubular reactor are realized. Moreover, experiments and DFT calculation indicate that Fe²⁺ containing species are the dominated active sites, while the exposed B atoms and structure defect of B can also activate PMS directly to produce SO₄^•- and ¹O₂ species for BPA degradation. In addition, boric acid is the oxidation product of B, which can be dissolved into the aqueous solution and expose fresh B species again for PMS activation. The combination of B with Fe@NC provide novel materials for long term PMS activation, thus could promote the real application of persulfates on an industrial scale.

2,6-Diaminopyridine-Based Polyurea as an ORR Electrocatalyst of an Anion Exchange Membrane Fuel Cell

In order to yield more Co(II), 2,6-diaminopyridine (DAP) was polymerized with 4,4-methylene diphenyl diisocyanates (MDI) in the presence of Co(II) to obtain a Co-complexed polyurea (Co-PUr). The obtained Co-PUr was calcined to become Co, N-doped carbon (Co-N-C) as the cathode catalyst of an anion exchange membrane fuel cell (AEMFC). High-resolution transmission electron microscopy (HR-TEM) of Co-N-C indicated many Co-Nx (Co covalent bonding with several nitrogen) units in the Co-N-C matrix. X-ray diffraction patterns showed that carbon and cobalt crystallized in the Co-N-C catalysts. The Raman spectra showed that the carbon matrix of Co-N-C became ordered with increased calcination temperature. The surface area (dominated by micropores) of Co-N-Cs also increased with the calcination temperature. The non-precious Co-N-C demonstrated comparable electrochemical properties (oxygen reduction reaction: ORR) to commercial precious Pt/C, such as high on-set and half-wave voltages, high limited reduction current density, and lower Tafel slope. The number of electrons transferred in the cathode was close to four, indicating complete ORR. The max. power density (P_max) of the single cell with the Co-N-C cathode catalyst demonstrated a high value of 227.7 mWcm^-2.

Nitrogen doped carbonaceous materials as platinum free cathode electrocatalysts for oxygen reduction reaction (ORR)

Comparison of physicochemical properties and electrocatalytic behavior of different N-doped carbonaceous materials as potential catalysts for oxygen reduction reaction (ORR) was attended. Ball-milling of graphite with melamine and solvothermal treatment of graphite oxide, graphene nanoplatelets (GNP) with ammonia were used as preparation methods. Elemental analysis and N2 physisorption measurements revealed the synthesis of N-doped materials with strongly different morphological parameters. Contact angle measurements proved that all three samples had good wettability properties. According to analysis of XRD data and Raman spectra a higher nitrogen concentration corresponded to a smaller size of crystallites of the N-doped carbonaceous material. Surface total N content determined by XPS and bulk N content assessed by elemental analysis were close, indicating homogenous inclusion of N in all samples. Rotating disc electrode tests showed that these N-doped materials weremuch less active in acidic medium than in an alkaline environment. Although the presence of in-plane N species is regarded to be advantageous for the ORR activity, no particular correlation was found in these systems with any type of N species. According to Koutecky–Levich analysis, both the N-containing carbonaceous materials and the reference Pt/C catalyst displayed a typical one-step, four-electron ORR route. Both ball-milled sample with high N-content but with low SSA and solvothermally synthesized N-GNP with high SSA but low N content showed significant ORR activity. It could be concluded that beside the total N content other parameters such as SSA, pore structure, structural defects, wettability were also essential for achieving high ORR activity.

Cu/CuOx@C Composite as a High-Efficiency Electrocatalyst for Oxygen Reduction Reactions

Among clean energy transformation devices, fuel cells have gained special attention over the past years; however, advancing appropriate non-valuable metal impetuses to halfway supplant the customary Pt/C impetus is still in progress. In this paper, we propose a specific electrocatalyst in the formula of highly-active Cu species, associated with coated carbon (Cu@C-800), for oxygen reduction reaction (ORR) through post-treatment of a self-assembled precursor. The optimized catalyst Cu@C-800 showed excellent ORR performance (i.e., the onset potential was 1.00 V vs. RHE, and half-wave potential of 0.81 V vs. RHE), high stability, resistance to methanol, and high four-electron selectivity. The enhancement is attributed to the synergy between the carbon matrix and a high explicit surface region and rich Cu nano-species.

Pyridinic nitrogen dominated doping on Pd/carbon catalysts for enhanced hydrogenation performance

The hydrogenation of 4-carboxylbenzaldehyde over Pd catalysts is a crucial process during the production of pure terephthalic acid. Herein, ZIF-8 derived carbon materials (NC) with adjustable N types were synthesized and used as the supports of Pd catalysts. Pd supported on NC with 50.5% of pyridinic N exhibited best hydrogenation activity with a TOF value of 4.1 min-1. The microstructures of NC support and electronic structures of Pd in Pd/NC were investigated by techniques such as XRD, N2 physisorption, XPS, H2-O2 titration and TEM. The nitrogen species in CN surface not only can adjust chemical state and dispersion of Pd nanoparticles (NPs), but also promote the adsorption and activation capability of H2 molecular. Besides, the ratio of Pd0/Pd2+ and Pd dispersion were closely correlated with pyridinic nitrogen content. The improvement in hydrogenation activity and stability of Pd/CN catalyst in relative to Pd/C were ascribed to the synergistic effect of pyridinic nitrogen and active site Pd0.

A fourteen-component high-entropy alloy@oxide bifunctional electrocatalyst with a record-low ΔE of 0.61 V for highly reversible Zn-air batteries

Nanostructured high-entropy materials such as alloys, oxides, etc., are attracting extensive attention because of their widely tunable surface electronic structure/catalytic activity through mixing different elements in one system. To further tune the catalytic performance and multifunctionality, the designed fabrication of multicomponent high-entropy nanocomposites such as high-entropy alloy@high-entropy oxides (HEA@HEO) should be very promising. In this work, we design a two-step alloying-dealloying strategy to synthesize ultra-small HEA nanoclusters (∼2 nm) loaded on nanoporous HEO nanowires, and the compositions of both the HEA and HEO can be adjusted separately. To demonstrate this concept, a seven-component HEA (PtPdAuAgCuIrRu) clusters@seven-component HEO (AlNiCoFeCrMoTi)3O4 was prepared, which is highly active for both oxygen evolution and reduction reactions. Our comprehensive experimental results and first-principles density functional theory (DFT) calculations clearly show that better oxygen evolution reaction (OER) performance is obtained by optimizing the composition of the HEO support, and the seven-component HEA nanocluster is much more active for the ORR when compared with pure Pt due to the modified surface electronic structure. Specifically, the high-entropy composite exhibits an OER activity comparable to the best reported value, and the ORR activity exceeded the performance of commercial Pt/C in alkaline solutions with a record-low bifunctional ΔE of 0.61 V in 0.1 M KOH solution. This work shows an important route to prepare complex HEA@HEO nanocomposites with tuned catalytic performance for multifunctional catalysis and energy conversion.

Recent Progress on Fe-Based Single/Dual-Atom Catalysts for Zn-Air Batteries

As one of the most competitive candidates for large-scale energy storage, zinc-air batteries (ZABs) have attracted great attention due to their high theoretical specific energy density, low toxicity, high abundance, and high safety. It is highly desirable but still remains a huge challenge, however, to achieve cheap and efficient electrocatalysts to promote their commercialization. Recently, Fe-based single-atom and dual-atom catalysts (SACs and DACs, respectively) have emerged as powerful candidates for ZABs derived from their maximum utilization of atoms, excellent catalytic performance, and low price. In this review, some fundamental concepts in the field of ZABs are presented and the recent progress on the reported Fe-based SACs and DACs is summarized, mainly focusing on the relationship between structure and performance at the atomic level, with the aim of providing helpful guidelines for future rational designs of efficient electrocatalysts with atomically dispersed active sites. Finally, the great advantages and future challenges in this field of ZABs are also discussed.

Double-Faced Atomic-Level Engineering of Hollow Carbon Nanofibers as Free-Standing Bifunctional Oxygen Electrocatalysts for Flexible Zn-Air Battery

Flexible solid-state zinc-air batteries (ZABs) with low cost, excellent safety, and high energy density has been considered as one of ideal power sources for portable and wearable electronic devices, while their practical applications are still hindered by the kinetically sluggish cathodic oxygen reduction and oxygen evolution reactions (ORR/OER). Herein, a Janus-structured flexible free-standing bifunctional oxygen electrocatalyst, with OER-active O, N co-coordinated Ni single atoms and ORR-active Co₃O₄@Co_1-xS nanosheet arrays being separately integrated at the inner and outer walls of flexible hollow carbon nanofibers (Ni-SAs/HCNFs/Co-NAs), is reported. Benefiting from the sophisticated topological structure and atomic-level-designed chemical compositions, Ni-SAs/HCNFs/Co-NAs exhibits outstanding bifunctional catalytic activity with the ΔE index of 0.65 V, representing the current state-of-the-art flexible free-standing bifunctional ORR/OER electrocatalyst. Impressively, the Ni-SAs/HCNFs/Co-NAs-based liquid ZAB show a high open-circuit potential (1.45 V), high capacity (808 mAh g^-1 Zn), and extremely long life (over 200 h at 10 mA cm^-2), and the assembled flexible all-solid-state ZABs have excellent cycle stability (over 80 h). This work provides an efficient strategy for developing high-performance bifunctional ORR/OER electrocatalysts for commercial applications.

Simply prepared electrocatalyst of CoFe alloy and nitrogen-doped carbon with multi-dimensional structure and high performance for rechargeable zinc-air battery

Simple and green preparation of highly-performed electrocatalysts for reaction both at cathode (oxygen reduction reaction (ORR)) and anode (oxygen evolution reaction (OER)) is crucial for boosting the application of meta-air battery. CoFe alloy and nitrogen doped carbon (CoFe-NC) material was prepared by a one-step carbonization procedure to construct a highly efficient electrocatalysis in this work. CoFe-NC displays a three-dimensional (3D) flower-like morphology composed of ordered stacked 2D nanosheets, which is entangled by 1D carbon nanotubes (CNTs). Its structure and electrocatalytic performance are compared with that of nitrogen doped carbon materials obtained from 2D zeolitic-imidazolate frameworks (ZIF) with no metal or single metal, as well as 3D ZIF with bimetal. Benefiting from the multi-dimensional structure of bimetal nanoparticles, 1D CNTs, 2D nanosheets, and 3D flowers, as well as the abundant active sites of Co/Fe-N_xand pyridine nitrogen, CoFe-NC displays a high half-wave potential of 0.896 V for ORR and low overpotential of 370 mV at 10 mA cm^-2for OER. Furthermore, compared with the primary and rechargeable Zn-air batteries fabricated with commercial Pt/C-RuO₂catalysts, the CoFe-NC catalysts assembled Zn-air batteries show a higher specific capacity (812.2 mAh g^-1), open circuit potential (1.59 V), power density (183.4 mW cm^-2), and stability. Hence, a facile and environmental-friendly strategy is provided for rational design and synthesis of bifunctional electrocatalysts for zinc-air batteries.

Metal-Organic Frameworks (MOFs) Derived Materials Used in Zn-Air Battery

It is necessary to develop new energy technologies because of serious environmental problems. As one of the most promising electrochemical energy conversion and storage devices, the Zn-air battery has attracted extensive research in recent years due to the advantages of abundant resources, low price, high energy density, and high reduction potential. However, the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) of Zn-air battery during discharge and charge have complicated multi-electron transfer processes with slow reaction kinetics. It is important to develop efficient and stable oxygen electrocatalysts. At present, single-function catalysts such as Pt/C, RuO₂, and IrO₂ are regarded as the benchmark catalysts for ORR and OER, respectively. However, the large-scale application of Zn-air battery is limited by the few sources of the precious metal catalysts, as well as their high costs, and poor long-term stability. Therefore, designing bifunctional electrocatalysts with excellent activity and stability using resource-rich non-noble metals is the key to improving ORR/OER reaction kinetics and promoting the commercial application of the Zn-air battery. Metal-organic framework (MOF) is a kind of porous crystal material composed of metal ions/clusters connected by organic ligands, which has the characteristics of adjustable porosity, highly ordered pore structure, low crystal density, and large specific surface area. MOFs and their derivatives show remarkable performance in promoting oxygen reaction, and are a promising candidate material for oxygen electrocatalysts. Herein, this review summarizes the latest progress in advanced MOF-derived materials such as oxygen electrocatalysts in a Zn-air battery. Firstly, the composition and working principle of the Zn-air battery are introduced. Then, the related reaction mechanism of ORR/OER is briefly described. After that, the latest developments in ORR/OER electrocatalysts for Zn-air batteries are introduced in detail from two aspects: (i) non-precious metal catalysts (NPMC) derived from MOF materials, including single transition metals and bimetallic catalysts with Co, Fe, Mn, Cu, etc.; (ii) metal-free catalysts derived from MOF materials, including heteroatom-doped MOF materials and MOF/graphene oxide (GO) composite materials. At the end of the paper, we also put forward the challenges and prospects of designing bifunctional oxygen electrocatalysts with high activity and stability derived from MOF materials for Zn-air battery.

Rational coordination regulation in carbon-based single-metal-atom catalysts for electrocatalytic oxygen reduction reaction

Single-metal-atom catalysts (SMACs) have garnered extensive attention for various electrocatalytic applications, owing to their maximum atom-utilization efficiency, tunable electronic structure, and remarkable catalytic performance. In particular, carbon-based SMACs exhibit optimal electrocatalytic activity for the oxygen reduction reaction (ORR) which is of paramount importance for several sustainable energy conversion and generation technologies, such as fuel cells and metal-air batteries. Despite continuous endeavors in developing various advanced carbon-based SMACs for electrocatalytic ORR, the rational regulation of coordination structure and thus the electronic structure of carbon-based SMACs remains challenging. In this review, we critically examine the role of coordination structure, including local coordination structure (i.e., metal atomic centers and the first coordination shell) and extended local coordination structure (i.e., the second and higher coordination shells), on the rational design of carbon-based SMACs for high-efficiency electrocatalytic ORR. Insights into the relevance between coordination structures and their intrinsic ORR activities are emphatically exemplified and discussed. Finally, we also propose the major challenges and future perspectives in the rational design of advanced carbon-based SMACs for electrocatalytic ORR. This review aims to emphasize the significance of coordination structure and deepen the insightful understanding of structure-performance relationships.

High-Voltage Redox Mediator of an Organic Electrolyte for Supercapacitors by Lewis Base Electrocatalysis

Redox electrolytes for supercapacitors (SCs) have recently sparked widespread interest. Due to the redox reactions within electrolytes, they can achieve high capacitance and long cycle stability. However, the energy density of SCs with redox electrolytes is limited by the narrow applied electrochemical window due to the irreversible side reaction of redox mediators at high potential. To overcome this issue, a redox mediator with a high redox potential, tetrachloridehydroquinone (TCHQ), is added to organic electrolytes to obtain a broad electrochemical window. TCHQ is designed to undergo a dehydrogenation reaction catalyzed by N-doped activated carbon to provide capacitance. The pyrrole N atoms have the highest electrocatalytic activity based on the theoretical calculation of reaction overpotential with predicted reaction pathways due to their Lewis basicity. Benefitting from that, TCHQ shows promising reversibility with a larger electrochemical window (up to 2.7 V). As a result, a higher energy density is obtained when compared to commercial SCs. This study proposes a strategy for designing redox mediators and interfaces of SCs with high energy density and a calculation method of dehydrogenation reaction electrocatalysis.

Utilizing waste duckweed from phytoremediation to synthesize highly efficient FeNxC catalysts for oxygen reduction reaction electrocatalysis

Duckweed is a universal aquatic plant to remove nitrogen source pollutants in the field of phytoremediation. Due to the naturally abundant nitrogen, synthesis of carbon materials from duckweed would be a high-value approach. In oxygen reduction reaction (ORR) of metal-air batteries and fuel cells, non-noble metals and heteroatoms co-doped electrocatalysts with excellent catalytic activity and remarkable stability are promising substitutes for Pt-based catalysts. The first-class ORR performance is determined by appropriate pore structure and active sites, which are strongly associated with the feasible synthesis methods. Herein, a facile one-step synthesis strategy for the transition metals- and nitrogen-codoped carbon (MN_xC) based catalysts with hierarchically porous structure was developed. The MN_xC (M = Fe, Co, Ni, and Mn) active sites were constructed and FeN_xC (D-ZB-Fe) was the best electrocatalyst with excellent ORR performance. Results showed that D-ZB-Fe exhibited an obvious honeycomb porous structure with specific surface area of 1342.91 m²·g^-1 and total pore volume of 1.085 cm³·g^-1. It also possessed considerable active atoms and sites, where the proportion of pyridine N and graphite N was up to 72.9%. The above feature made for a superior ORR electrocatalytic activity. In specific, the onset and half-wave potential were 0.974 V and 0.857 V vs. RHE (Reversible Hydrogen Electrode), respectively. When compared with performances of commercial Pt/C, the four-electron pathway and relatively low peroxide yield, ca. 5%, were almost equivalent. Furthermore, D-ZB-Fe showed an excellent stability and remarkably methanol tolerance by the durability test. In conclusion, this research provides a new synthesis strategy of electrocatalysts with porous structures and active sites.

Insights into the activity of single-atom Fe-N-C catalysts for oxygen reduction reaction

Single-atom Fe-N-C catalysts has attracted widespread attentions in the oxygen reduction reaction (ORR). However, the origin of ORR activity on Fe-N-C catalysts is still unclear, which hinder the further improvement of Fe-N-C catalysts. Herein, we provide a model to understand the ORR activity of Fe-N4 site from the spatial structure and energy level of the frontier orbitals by density functional theory calculations. Taking the regulation of divacancy defects on Fe-N4 site ORR activity as examples, we demonstrate that the hybridization between Fe 3dz2, 3dyz (3dxz) and O2 π* orbitals is the origin of Fe-N4 ORR activity. We found that the Fe-O bond length, the d-band center gap of spin states, the magnetic moment of Fe site and *O2 as descriptors can accurately predict the ORR activity of Fe-N4 site. Furthermore, these descriptors and ORR activity of Fe-N4 site are mainly distributed in two regions with obvious difference, which greatly relate to the height of Fe 3d projected orbital in the Z direction. This work provides a new insight into the ORR activity of single-atom M-N-C catalysts.

Mechanochemical Synthesis of Fluorine-Containing Co-Doped Zeolitic Imidazolate Frameworks for Producing Electrocatalysts

Catalysts derived from pyrolysis of metal organic frameworks (MOFs) are promising candidates to replace expensive and scarce platinum-based electrocatalysts commonly used in polymer electrolyte membrane fuel cells. MOFs contain ordered connections between metal centers and organic ligands. They can be pyrolyzed into metal- and nitrogen-doped carbons, which show electrocatalytic activity toward the oxygen reduction reaction (ORR). Furthermore, metal-free heteroatom-doped carbons, such as N-F-Cs, are known for being active as well. Thus, a carbon material with Co-N-F doping could possibly be even more promising as ORR electrocatalyst. Herein, we report the mechanochemical synthesis of two polymorphs of a zeolitic imidazole framework, Co-doped zinc 2-trifluoromethyl-1H-imidazolate (Zn_0.9Co_0.1(CF₃-Im)₂). Time-resolved in situ X-ray diffraction studies of the mechanochemical formation revealed a direct conversion of starting materials to the products. Both polymorphs of Zn_0.9Co_0.1(CF₃-Im)₂ were pyrolyzed, yielding Co-N-F containing carbons, which are active toward electrochemical ORR.

Electronic Metal-Support Interaction Modulation of Single-Atom Electrocatalysts for Rechargeable Zinc-Air Batteries

High-performance oxygen electrocatalysts play a key role in the widespread application of rechargeable Zn-air batteries (ZABs). Single-atom catalysts (SACs) with maximum atom efficiency and well-defined active sites have been recognized as promising alternatives of the present noble-metal-based catalysts for oxygen reduction reaction and oxygen evolution reaction. To improve their oxygen electrocatalysis activities and reveal the structure-activity relationship, many advanced synthesis and characterization methods have been developed to study the effects of 1) coordination and electronic structure of the metal centers and 2) morphology and stability of the conductive substrates. Herein, a detailed review of the recent advances of SACs with strong electronic metal-support interaction (EMSI) for rechargeable ZABs is provided. Great emphasis was placed on the EMSI forms and design strategies. Moreover, the importance and the impact of the atomic coordinating structure and the substrates on the oxygen electrocatalytic activity and stability are highlighted. Finally, future directions and perspectives on the development of SACs are also presented.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Heteroatom-Doped Metal-Free Carbon Nanomaterials as Potential Electrocatalysts

In recent years, heteroatom-incorporated specially structured metal-free carbon nanomaterials have drawn huge attention among researchers. In comparison to the undoped carbon nanomaterials, heteroatoms such as nitrogen-, sulphur-, boron-, phosphorous-, etc., incorporated nanomaterials have become well-accepted as potential electrocatalysts in water splitting, supercapacitors and dye-sensitized solar cells. This review puts special emphasis on the most popular synthetic strategies of heteroatom-doped and co-doped metal-free carbon nanomaterials, viz., chemical vapor deposition, pyrolysis, solvothermal process, etc., utilized in last two decades. These specially structured nanomaterials' extensive applications as potential electrocatalysts are taken into consideration in this article. Their comparative enhancement of electrocatalytic performance with incorporation of heteroatoms has also been discussed.

Recycling and valorization of LDPE: direct transformation into highly ordered doped-carbon materials and their application as electro-catalysts for the oxygen reduction reaction

A transformation of LDPE in highly ordered doped-carbon materials by a simple one-step pyrolysis in presence of transition metal precursors is proposed. The graphitization, metal dispersion and CNFs presence are key factors for the high ORR performance.

CoAl-LDHs@Fe3O4 decorated with cobalt nanowires and cobalt nanoparticles for a heterogeneous electro-Fenton process to degrade 1-hydroxyethane-1,1-diphosphonic acid and glyphosate

Heterogeneous electro-Fenton is one of the promising technologies to degrade refractory organic phosphonates. In this work, CoNWs@CoAl-LDHs/Fe₃O₄ and CoNPs@CoAl-LDHs/Fe₃O₄ were successfully synthesized by a co-precipitation method and applied to degrade 1-hydroxyethane-1,1-diphosphonic acid (HEDP) and glyphosate (PMG) via an electro-Fenton process. The results indicated that the removal rate of HEDP (100 mg L⁻¹) and PMG (100 mg L⁻¹) by CoNWs@CoAl-LDHs/Fe₃O₄ increased from 62.09% and 95.31% to 82.45% and 100%, respectively. The CoNPs@CoAl-LDHs/Fe₃O₄ electro-Fenton system could remove 70.03% of HEDP and nearly 100% of PMG within 2 hours at a pH of 3. Moreover, we compared the SEM, EDS, XRD and BET results of CoNWs@CoAl-LDHs/Fe₃O₄ with those of CoNPs@CoAl-LDHs/Fe₃O₄. The effects of initial pH, CoNW dosage and reaction time on the degradation of HEDP and PMG were discussed. CoNWs@CoAl-LDHs@Fe₃O₄ could even remove 71.03% of HEDP at a neutral pH. After four cycles of repeated use at a pH of 3, the removal rate of HEDP by CoNWs@CoAl-LDHs/Fe₃O₄ was still higher than 70%. Radical quenching experiments revealed that ˙OH is the dominant active species participating in the heterogeneous electro-Fenton process. Finally, we would talk about the mechanism of the CoNWs@CoAl-LDHs/Fe₃O₄-based electro-Fenton system.

Cobalt nanowires and cobalt particles are introduced into CoAl-LDHs@Fe₃O₄, and the effect of the former is better in the application of electro-Fenton process.

Atomically Dispersed Transition Metal-Nitrogen-Carbon Bifunctional Oxygen Electrocatalysts for Zinc-Air Batteries: Recent Advances and Future Perspectives

Rechargeable zinc-air batteries (ZABs) are currently receiving extensive attention because of their extremely high theoretical specific energy density, low manufacturing costs, and environmental friendliness. Exploring bifunctional catalysts with high activity and stability to overcome sluggish kinetics of oxygen reduction reaction and oxygen evolution reaction is critical for the development of rechargeable ZABs. Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts possessing prominent advantages of high metal atom utilization and electrocatalytic activity are promising candidates to promote oxygen electrocatalysis. In this work, general principles for designing atomically dispersed M-N-C are reviewed. Then, strategies aiming at enhancing the bifunctional catalytic activity and stability are presented. Finally, the challenges and perspectives of M-N-C bifunctional oxygen catalysts for ZABs are outlined. It is expected that this review will provide insights into the targeted optimization of atomically dispersed M-N-C catalysts in rechargeable ZABs.

Active site engineering of single-atom carbonaceous electrocatalysts for the oxygen reduction reaction

The electrocatalytic oxygen reduction reaction (ORR) is the vital process at the cathode of next-generation electrochemical storage and conversion technologies, such as metal-air batteries and fuel cells. Single-metal-atom and nitrogen co-doped carbonaceous electrocatalysts (M-N-C) have emerged as attractive alternatives to noble-metal platinum for catalyzing the kinetically sluggish ORR due to their high electrical conductivity, large surface area, and structural tunability at the atomic level, however, their application is limited by the low intrinsic activity of the metal-nitrogen coordination sites (M-N x ) and inferior site density. In this Perspective, we summarize the recent progress and milestones relating to the active site engineering of single atom carbonous electrocatalysts for enhancing the ORR activity. Particular emphasis is placed on the emerging strategies for regulating the electronic structure of the single metal site and populating the site density. In addition, challenges and perspectives are provided regarding the future development of single atom carbonous electrocatalysts for the ORR and their utilization in practical use.