MOF derived multi-metal oxides anchored N, P-doped carbon matrix as efficient and durable electrocatalyst for oxygen evolution reaction

ZIFs-Derived Hollow Nanostructures via a Strong/Weak Coetching Strategy for Long-Life Rechargeable Zn-Air Batteries

Recently, zeolitic imidazolate frameworks (ZIFs) composites have emerged as promising precursors for synthesizing hollow-structured N-doped carbon-based noble-metal materials with diverse structures and compositions. Here, a strong/weak competitive coordination strategy is presented for synthesizing high-performance electrocatalysts with hollow features. During the competitive coordination process, the cubic zeolitic-imidazole framework-8 (Cube-8)@ZIF-67 with core-shell structures are transformed into Cube-8@ZIF-67@PF/POM with yolk-shell nanostructures employing phosphomolybdic acid (POM) and potassium ferricyanide (PF) as the strong chelator and the weak chelator, respectively. After calcination, the hollow Mo/Fe/Co@NC catalyst exhibits superior performance in both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Interestingly, the Mo/Fe/Co@NC catalyst exhibits efficient electrocatalytic performance for Zn-air batteries (ZABs), with a high power density (≈150 mW cm-2 ) and superior cycling life (≈500 h) compared to commercial platinum/carbon (Pt/C) and ruthenium dioxide (RuO2 ) mixture benchmarks catalysts. In addition, the density functional theory further proves that after the introduction of Mo and Fe atoms, the adsorption energy with the adsorption intermediates is weakened by adjusting the d-band center, thus weakening the reaction barrier and promoting the reaction kinetics of OER. Undoubtedly, this study presents novel insights into the fabrication of ZIFs-derived hollow structure bifunctional oxygen electrocatalysts for clean-energy diverse applications.

Sustainable bacterial cellulose derived composites for high-efficiency hydrogen evolution reaction

Incorporating heteroatoms into carbon structure has been demonstrated to be efficient for hydrogen evolution reaction (HER). However, the preparation complexity and poor durability are insufficient for the future hydrogen economy. In this work, the preparation of ZIF-67/BC precursor with BC as the template was done for the in-situ growth of MOFs (ZIF-67) crystals, followed by the carbonization and phosphating of ZIF-67/BC to prepare the CoP-NC/CBC N-doped composite carbon material with CoP as the primary active material. The results show that as an HER catalyst, CoP-NC/CBC can provide a current density of 10 mA cm^-2 at an overpotential of 182 mV in the acidic electrolyte of 0.5 M H₂SO₄ or the same current density at an overpotential of 151 mV in the alkaline electrolyte of 1.0 M KOH. The work validates a design idea for advanced non-precious metal-based HER catalysts with high activity and stability.

In Situ S-Doping Strategy of Promoting Iron Coordinated by Nitrogen-Doped Carbon Nanosheets for Efficient Oxygen Reduction Reaction

Improving transition metal-nitrogen-carbon (M-N-C) as a noble-metal-free catalyst for the oxygen reduction reaction (ORR) is critical to achieve low-cost electrochemical energy conversion. Herein, an in situ S doping strategy of enhancing Fe-N-C activity for ORR was developed by newly designed Fe(II) ion coordinated S-containing bis(imino)-pyridine-based polymers as precursors, which were synthesized through copolymerizing three monomers of 2, 6-diacetylpyridine (DAP), triamterene (TIT), and 2,5-dithiobiurea (DTB) as both N and S sources. All samples derived from various molar ratios of the three monomers possess a self-supporting structure of nanosheets. Additionally, incorporating DTB into the copolymer can not only strongly affect the derived coordinative species of N dopants to Fe atom but also effectively induce the synergistic effect between S dopants and FeNx moieties, resulting a significant improvement for ORR. The S-doped Fe-N-C nansheets with Fe coordinated by 4 pyrrolic N dopants exhibit the highest ORR activity and stability in alkaline media with a higher power output of Zn-air battery than that of the same loading of Pt/C. Theoretical calculation identifies that the thiophenic S dopant adjacent to Fe-pyrrolic N moiety can decrease the d band center of Fe atom, greatly weakening the energy profiles of oxygenated intermediates and thus enhancing ORR. In addition, because of the designability of transition metal coordinated S-containing bis(imino)-pyridine based polymers in the work, therefore, it is believable that this strategy would open a wide space to explore the structural relationship between precursors and MNx active sites with S dopants for the purpose of achieving highly efficient and robust M-N-C catalysts for energy-related electrocatalysis.

Natural Halloysite-Templated Synthesis of Highly Graphitic Boron-Doped Hollow Carbon Nanocapsule Webs

Hollow carbon nanocapsules have been attracting growing interest due to their fascinating characteristics and extensive potential applications. In this work, a novel natural halloysite-templated synthesis approach for highly graphitic boron-doped hollow carbon nanocapsule webs (B-HCNCWs) using glucose as the carbon source and boric acid as the heteroatom dopant was first reported. The formation process and physicochemical properties of B-HCNCWs were revealed by SEM, TEM, XRD, Raman, Brunauer–Emmett–Teller (BET), and XPS characterization techniques. The outcomes showed that the as-obtained B-HCNCWs with hollow nanocapsule network architecture had a specific surface area of 263 m² g⁻¹, a pore volume of 0.8 cm³ g⁻¹, a high degree of graphitization (81.4%), graphite-like interplanar spacing (0.3370 nm), and B-containing functional groups (0.77 at%). The density function theory (DFT) calculation demonstrated that the adsorption energies of Li on B-HCNCWs were much higher than that of HCNCWs, which proved that B-doping in a carbon matrix could increase the lithium intercalation capacity.

Molybdenum oxide-iron, cobalt, copper alloy hybrid as efficient bifunctional catalyst for alkali water electrolysis

Efficient and durable non-precious catalyst for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is pivotal for practical water electrolysis toward clean hydrogen fuel. Herein, a molybdenum oxide-FeCoCu alloy hybrid (MoO_x-FeCoCu) catalyst was designed by polyoxometallate (POM) molecular cluster mediated solvothermal alcoholysis and ammonolysis of metal salts followed by pyrolytic reduction treatment. The HER efficiency is substantially enhanced by the ternary alloy component, which is more close to the benchmark Pt/C catalyst, and the HER catalytic stability is also superior to Pt/C catalyst. Moreover, the MoO_x-FeCoCu demonstrates high catalytic efficiency and rather good durability for OER. Benefitted by the bifunctional catalytic behaviors for HER and OER, the symmetric water electrolyzer based on the MoO_x-FeCoCu electrode requires a low driving voltage of 1.69 V to deliver a response current density of 10 mA cm^-2, which is comparable to that based on the benchmark Pt/C HER cathode and RuO₂ OER anode. The current work offers a feasible way to design efficient bifunctional catalyst for water electrolysis via POM mediated co-assembly and calcination treatment.

Fe-Triazole coordination compound-derived Fe2O3 nanoparticles anchored on Fe-MOF/N-doped carbon nanosheets for efficient electrocatalytic oxygen evolution reaction

Using FeCl₃·6H₂O and 1,2,4-triazole (Htrz) as starting materials, an Fe coordination compound (CC), [FeCl₃(Htrz)₃]·H₂O, was synthesized at room temperature. Fe-CC can be partially transformed into an Fe metal-organic framework (MOF), [FeCl₂(Htrz)], via low-temperature annealing. After sulfurization at 250, 300, and 400 °C, S-doped Fe₂O₃/N-doped carbon (denoted as NC)/Fe-MOF, FeS₂/NC/Fe-MOF, and FeS₂/NC were obtained, respectively. S-doped Fe₂O₃/NC/Fe-MOF shows the best oxygen evolution reaction (OER) catalytic activity in 1 M KOH solution, with overpotentials (η) of 185, 232, and 258 mV required to reach current densities of 10, 30, and 50 mA cm^-2, respectively, outperforming commercial RuO₂ and most transition-metal oxides reported to date; this high performance is associated with the Fe₂O₃ nanoparticles (NPs) anchored on the Fe-MOF/NC nanosheets. The Fe-MOF/NC matrix can act as a support to prevent the agglomeration of Fe₂O₃ NPs. In addition, S-doped Fe₂O₃/NC/Fe-MOF exhibits long-term OER activity at 20 mA cm^-2, which is related to the partial transformation of Fe₂O₃/Fe-MOF into FeOOH. In addition, density functional theory (DFT) calculations show that the rate-determining step of the OER process at the Fe sites of both Fe₂O₃ and FeS₂ is the formation of Fe*-OH, and the Fe₂O₃ sites display a lower Gibbs free energy (ΔG_max) of 1.674 eV and a smaller η value of ∼0.444 V. Bader charge, differential charge density mapping, and density of states (DOS) analysis all reveal more charge accumulation at the Fe sites of FeS₂ than at the Fe sites of Fe₂O₃, which is due to the lower electronegativity of S than of O. As a result, the Fe sites of FeS₂ show weaker affinity for -OH intermediates, giving rise to inferior OER performance compared with Fe₂O₃.

Bioinspired Camellia japonica carbon dots with high near-infrared absorbance for efficient photothermal cancer therapy

Since carbon dots (CDs) exhibit excellent biocompatibility, low cytotoxicity, near-infrared (NIR) absorbance, and superior photostability, many types of CDs are considered as powerful candidates for photothermal therapy (PTT) applications. However, the development of a desirable CD is still difficult due to insufficient photothermal conversion, thus resulting in the use of high laser power densities at a high dose of CDs for the PTT effect. Herein, bioinspired sulfur-doped CDs (S-CDs) with strong NIR absorbance were prepared from Camellia japonica flowers via a facile hydrothermal method for enhancing the photothermal conversion efficiency. The as-prepared S-CDs exhibited various advantages including cost-effective preparation, good water-solubility, high biocompatibility, intense NIR absorption, and excellent photothermal effect with robust photostability. Most importantly, the optimal low dose of S-CDs (45 μg mL^-1) successfully led to efficient PTT performance with a high photothermal conversion efficiency (55.4%) under moderate laser power (808 nm, 1.1 W cm^-2) for safe and effective cancer therapy.

CoFe alloy embedded in N-doped carbon nanotubes derived from triamterene as a highly efficient and durable electrocatalyst beyond commercial Pt/C for oxygen reduction

For development of green and sustainable energy, it is of importance to search highly efficient and low-cost electrocatalysts of oxygen reduction reaction (ORR) in energy conversion devices. Herein, CoFe alloyed nanocrystals embedded in N-doped bamboo-like carbon nanotubes (CoFe@NCNTs) were facilely synthetized by one-step co-pyrolysis with the mixture of triamterene, metal chlorides and graphitic carbon nitride (g-C₃N₄). The resultant CoFe@NCNTs exhibited excellent ORR activity with the positive shifts in the onset potential (E_onset = 0.97 V) and half-wave potential (E_1/2 = 0.88 V), outperforming commercial Pt/C (E_onset = 0.96 V; E_1/2 = 0.84 V). Compared to metal organic frameworks (MOFs)-based strategy for synthesis of low-cost carbon-based ORR catalysts, this method is simple and convenient, coupled by avoiding the complicated synthesis of MOFs and its ligands. This work provides a promising route to fabricate advanced transition-metal-based carbon catalysts in the researches correlated with energy conversion devices.

Insight into the amorphous nickel-iron (oxy)hydroxide catalyst for efficient oxygen evolution reaction

The specific roles of Ni and Fe in nickel-iron (oxy)hydroxide catalyst (NiFeO_x(OH)_y) are extensively discussed during oxygen evolution reaction (OER). However, there still remains controversy about whether Ni or Fe species as the dominate active site. In this work, we reported the NiFeO_x(OH)_y catalysts with varied atomic ratio of nickel and iron for OER to explore the dominate active site during OER processes. From the electrochemical performances, the similar Tafel slopes of catalysts with Fe species can achieve at a level of 40 mV dec^-1, outperforming the Tafel slopes of catalysts without Fe species. Thus, it can be concluded that the present Fe site can serve as the dominant active site in NiFeO_x(OH)_y for OER. Meanwhile, the Ni species is proved as the OH^- adsorption site, which is beneficial to the Fe site to deliver a better OER performance. As a result, the catalyst with an optimal Ni/Fe interface (atomic ratio of 1 : 1.18) displays outstanding OER performances. It only requires a low overpotential of 250 mV to deliver current density of 10 mA cm^-2 and exhibits a small Tafel slope of 39 mV dec^-1. This catalyst also shows remarkable stability with negligible potential decay after 50 h at a current density of 50 mA cm^-2. This work offers a new sight into the specific roles of Ni and Fe in NiFeO_x(OH)_y for OER.