N- & S-co-doped carbon nanofiber network embedded with ultrafine NiCo nanoalloy for efficient oxygen electrocatalysis and Zn-air batteries

Constructing of Ni-Nx Active Sites in Self-Supported Ni Single-Atom Catalysts for Efficient Reduction of CO2 to CO

The electrochemical carbon dioxide reduction reaction (CO2RR) represents a promising approach for achieving CO2 resource utilization. Carbon-based materials featuring single-atom transition metal-nitrogen coordination (M-Nx) have attracted considerable research attention due to their ability to maximize catalytic efficiency while minimizing metal atom usage. However, conventional synthesis methods often encounter challenges with metal particle agglomeration. In this study, we developed a Ni-doped polyvinylidene fluoride (PVDF) fiber membrane via electrospinning, subsequently transformed into a nitrogen-doped three-dimensional self-supporting single-atom Ni catalyst (Ni-N-CF) through controlled carbonization. PVDF was partially defluorinated and crosslinked, and the single carbon chain is changed into a reticulated structure, which ensured that the structure did not collapse during carbonization and effectively solved the problem of runaway M-Nx composite in the high-temperature pyrolysis process. Grounded in X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS), nitrogen coordinates with nickel atoms to form a Ni-N structure, which keeps nickel in a low oxidation state, thereby facilitating CO2RR. When applied to CO2RR, the Ni-N-CF catalyst demonstrated exceptional CO selectivity with a Faradaic efficiency (FE) of 92%. The unique self-supporting architecture effectively addressed traditional electrode instability issues caused by catalyst detachment. These results indicate that by tuning the local coordination structure of atomically dispersed Ni, the original inert reaction sites can be activated into efficient catalytic centers. This work can provide a new strategy for designing high-performance single-atom catalysts and structurally stable electrodes.

OER catalytic performance of a composite catalyst comprising multi-layer thin flake Co3O4 and PPy nanofibers

The oxygen evolution reaction (OER) plays a crucial role in energy conversion and storage processes, highlighting the significance of searching for efficient and stable OER catalysts. In this study, we have developed a composite catalyst, PPy@Co3O4, with outstanding catalytic performance for the OER. The catalyst was constructed by integrating multi-layer thin flake Co3O4 with attached PPy nanofibers, utilizing the rich active sites of Co3O4 and the flexibility and tunability of PPy nanofibers to optimize the catalyst structure. Through comprehensive characterization and performance evaluation, our results demonstrate that the PPy@Co3O4 (0.1 : 1) catalyst exhibits remarkable OER catalytic activity and stability. This research provides new strategies and insights for the development of efficient and stable OER catalysts, holding promising prospects for energy conversion and storage applications in relevant fields.

Molten salt synthesis of NiCo-NiCo2O4@C nanotubes as anode materials for Li-ion batteries

The construction of carbon-encapsulated transition metal nanotube structures is a preferred method that can effectively slow down volume expansion, improve cycling stability and enhance the electrical conductivity of the reactive sites of lithium-ion batteries. In this study, nanotubes of carbon-coated NiCo-NiCo₂O₄ nanoparticles (NC-NCO@C) were prepared by a one-step molten salt method at high temperature using Ni and Co as catalytic centers and sodium acetate as carbon source. We used NC-NCO@C-2 nanotubes as anode materials for lithium-ion batteries(LIBs), which exhibited excellent lithium storage performance and good stability, with a specific capacity of 616.26 mAh g^-1 after 1000 cycles at a high current density of 1 A g^-1. In addition, NC-NCO@C-2 were used as anodes in lithium-ion full cells and LiFePO₄ (LFP) was used as the cathode. The NC-NCO@C-2//LFP full-cell exhibits high capacity and good cycling stability, with a capacity of 100.7 mAh g^-1 after 100 cycles and a capacity retention rate of 92%. The construction of NC, NCO, and carbon ternary complexes was found to activate and promote the reversible conversion of certain inorganic components at the solid electrolyte interfaces (SEI), which effectively reduced the volume change during cycling, increased the electrical conductivity, and improved the cycling stability of the electrode. The proposed one-step molten salt synthesis of Carbon-coated metals complexes with excellent compatibility characteristics, is expected to solve the problem of volume change in transition metals, which is encountered in LIBs applications.

Ether-based electrolytes enable the application of nitrogen and sulfur co-doped 3D graphene frameworks as anodes in high-performance sodium-ion batteries

The development of graphitic carbon materials as anodes of sodium-ion batteries (SIBs) is greatly restricted by their inherent low specific capacity. Herein, nitrogen and sulfur co-doped 3D graphene frameworks (NSGFs) were successfully synthesized via a simple and facile one-step hydrothermal method and exhibited high Na storage capacity in ether-based electrolytes. A systematic comparison was made between NSGFs, undoped graphene frameworks (GFs) and nitrogen-doped graphene frameworks (NGFs). It is demonstrated that the high specific capacity of NSGFs can be attributed to the free diffusion of Na ions within the graphene layer and reversible reaction between -C-S_x-C- covalent chains and Na ions thanks to the large interplanar distance and the dominant -C-S_x-C- covalent chains in NSGFs. NSGF anodes, therefore, exhibit a high initial coulombic efficiency (ICE) (92.8%) and a remarkable specific capacity of 834.0 mA h g^-1 at 0.1 A g^-1. Kinetic analysis verified that the synergetic effect of N/S co-doping not only largely enhanced the Na ion diffusion rate but also reduced the electrochemical impedance of NSGFs. Postmortem techniques, such as SEM, ex situ XPS, HTEM and ex situ Raman spectroscopy, all demonstrated the extremely physicochemically stable structure of the 3D graphene matrix and ultrathin inorganic-rich solid electrolyte interphase (SEI) films formed on the surface of NSGFs. Yet it is worth noting that the Na storage performance and mechanism are exclusive to ether-based electrolytes and would be inhibited in their carbonate ester-based counterparts. In addition, the corrosion of copper foils under the synergetic effect of S atoms and ether-based electrolytes was reported for the first time. Interestingly, by-products derived from this corrosion could provide additional Na storage capacity. This work sheds light on the mechanism of improving the electrochemical performance of carbon-based anodes by heteroatom doping in SIBs and provides a new insight for designing high-performance anodes of SIBs.

Manganese Oxide/Iron Carbide Encapsulated in Nitrogen and Boron Codoped Carbon Nanowire Networks as Accelerated Alkaline Hydrogen Evolution and Oxygen Reduction Bifunctional Electrocatalysts

Along with the widespread applications of various energy storage and conversion devices, the prices of precious metal platinum (Pt) and transition-metal cobalt/nickel keep continuously growing. In the future, designing high-efficiency nonprecious-metal catalysts based on low-cost iron (Fe) and manganese (Mn) metals for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) is fairly critical for commercial applications of hydrogen fuel cells. In this study, for the first time, we design novel three-dimensional (3D) hybrid networks consisting of manganese oxide (MnO)-modified, iron carbide (Fe₃C)-embedded, and boron (B)/nitrogen (N) codoped hierarchically porous carbon nanofibers (denoted FeMn@BNPCFs). After optimizing the pyrolysis temperatures, the optimal FeMn@BNPCFs-900 catalyst displays the best HER and ORR catalytic activities in an alkaline solution. As expected, the HER onset potential (E_onset) and the potential at a current density of -10 mA cm^-2 for FeMn@BNPCFs-900 in 1.0 M KOH are just 36 and 194 mV more negative than the state-of-the-art 20 wt % Pt/C catalyst with more superior stability. In particular, the FeMn@BNPCFs-900 catalyst shows excellent ORR catalytic activity with a more positive E_onset (0.946 V vs RHE), a more positive half-wave potential (E_1/2 = 0.868 V vs RHE), better long-term stability, and higher methanol tolerance surpassing the commercial 20 wt % Pt/C (E_onset = 0.943 V vs RHE, E_1/2 = 0.854 V vs RHE) and most previously reported precious-metal-free catalysts in 0.1 M KOH. The synergistic effects of 3D hierarchically macro-/mesoporous architectures, advanced charge transport capacity, abundant carbon defects/edges, abundant B (2.3 atom %) and N (4.9 atom %) dopants, uniformly dispersed Fe₃C@BNC NPs, and MnO nanocrystallines are responsible for the excellent HER/ORR catalytic activities of the FeMn@BNPCFs-900 catalyst.

Morphological modulation of iron carbide embedded nitrogen-doped hierarchically porous carbon by manganese doping as highly efficient bifunctional electrocatalysts for overall water splitting

In the development of water splitting, the sluggish electrocatalytic kinetics of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have restricted their energy conversion efficiencies. Along with the continuous rise in the prices of noble metals and transition metals (such as cobalt and nickel), constructing high-efficiency HER/OER catalysts based on low cost transition metals, such as iron and manganese, is becoming more meaningful in developing industrialized water splitting devices. In this paper, in the absence of a template or active agent, three-dimensional, hierarchically porous Fe_xMn_y nanoparticles (NPs) were embedded and nitrogen-doped carbon materials (denoted as Fe_xMn_y@NC; x:y, representing the molar ratio of Fe:Mn) were successfully prepared via pyrolysis of corresponding precursors containing different metallic salt components. Various morphological, structural, and chemical characterization analysis demonstrate that at an Fe:Mn molar ratio of 3:1, the optimal Fe₃Mn₁@NC material shows high graphitization degree, rich mesoporous structures, a large surface area, and abundant carbon defects/edges, which promote the uniform dispersion of pyridinic-N (pyridinic-N-metal), graphitic-N, carbon oxygen bonds (CO), manganese oxide (MnO) nanocrystals, and Fe₃C NPs-embedded, N-doped carbon sheet (Fe₃C@NC) active sites. In alkaline conditions, the HER onset potentials (E_onset) and potentials recorded at 10 mA cm^-2 (E₁₀) of the optimal Fe₃Mn₁@NC are just 84.8 and 156 mV more negative than those of 20 wt% platinum carbon (Pt/C). Meanwhile, the OER E_onset and E₁₀ values of the optimal Fe₃Mn₁@NC are just 8 and 18.7 mV more positive than those of RuO₂. Furthermore, optimized Fe₃Mn₁@NC catalysts were assembled into a water splitting cell, where the catalytic current density achieves 10 mA cm^-2 at a low voltage of 1.6287 V (with superior catalytic stability), which is just 24.9 mV higher than that of the (-) 20 wt% Pt/C||RuO₂ (+) benchmark (1.6038 V) under the same conditions. This work describes the regulating efficiency of Mn toward growing mesopores and opens new possibilities for the development of novel carbonaceous catalysts with excellent hydroxide catalytic efficiencies based on low cost Mn/Fe elements.

Sm0.5Sr0.5Co1−xNixO3−δ—A Novel Bifunctional Electrocatalyst for Oxygen Reduction/Evolution Reactions

The development of non-precious metal catalysts with excellent bifunctional activities is significant for air–metal batteries. ABO3-type perovskite oxides can improve their catalytic activity and electronic conductivity by doping transition metal elements at B sites. Here, we develop a novel Sm0.5Sr0.5Co1−xNixO3−δ (SSCN) nanofiber-structured electrocatalyst. In 0.1 M KOH electrolyte solution, Sm0.5Sr0.5Co0.8Ni0.2O3−δ (SSCN82) with the optimal Co: Ni molar ratio exhibits good electrocatalytic activity for OER/ORR, affording a low onset potential of 1.39 V, a slight Tafel slope of 123.8 mV dec−1, and a current density of 6.01 mA cm−2 at 1.8 V, and the ORR reaction process was four-electron reaction pathway. Combining the morphological characteristic of SSCN nanofibers with the synergistic effect of cobalt and nickel with a suitable molar ratio is beneficial to improving the catalytic activity of SSCN perovskite oxides. SSCN82 exhibits good bi-functional catalytic performance and electrochemical double-layer capacitance.

Constructing Conductive Channels between Platinum Nanoparticles and Graphitic Carbon Nitride by Gamma Irradiation for an Enhanced Oxygen Reduction Reaction

Electrocatalytic performance of low-cost graphitic carbon nitride (g-C₃N₄) is greatly limited by its conductivity. In this work, an innovative method, gamma irradiation technology, was used to efficiently synthesize g-C₃N₄/Pt nanoparticle (CN/Pt) nanocomposites, which can construct conductive channels between the nanostructure g-C₃N₄ and supported platinum nanoparticles (PtNPs). Then, the as-prepared CN/Pt nanocomposites were applied in the oxygen reduction reaction (ORR) as an electrocatalyst, which shows a small Tafel slope and the fast four-electron transfer path in the ORR. The oxygen reduction performance over the CN/Pt nanocomposite is much superior to that of the commercial Pt/C and mostly reported in g-C₃N₄-based electrodes. Experimental results have confirmed the fast charge transfer between PtNPs and g-C₃N₄ through a metal-support interaction, and using gamma irradiation technique to disperse PtNPs on g-C₃N₄ proves to be an effective strategy to enhance the catalytic performance of g-C₃N₄ in ORR. Therefore, gamma irradiation may possess great potential for preparing CN/Pt nanocomposites as a highly efficient ORR catalyst.

Tri-(Fe/F/N)-doped porous carbons as electrocatalysts for the oxygen reduction reaction in both alkaline and acidic media

Developing a low cost, sustainable and high-performance precious-metal free catalyst to replace platinum (Pt)-based catalysts for the oxygen reduction reaction (ORR) in fuel cells has recently attracted significant attention. It is crucial to produce more abundant and more uniformly dispersed ORR active sites for improving the ORR performance of the catalyst. Herein, we synthesized tri-(Fe/F/N)-doped porous carbons as high-efficiency electrocatalysts for the ORR by using Fe-zeolitic imidazolate framework-8 (Fe-ZIF-8) and ammonium fluoride as precursors. The results indicate that the as-prepared FeFNC-5 catalysts exhibit superior ORR activity, methanol tolerance, and long-term stability compared to commercial 20 wt% Pt/C in both alkaline and acidic media because of the abundant and dispersed Fe-N_x and pyridinic-N active sites, high specific surface area, and hierarchical porous structure. This work provides a new method and insights into the synthesis of Fe, F, and N triple-doped porous carbons as high-efficiency ORR electrocatalysts.