Graphene-reinforced metal-organic frameworks derived cobalt sulfide/carbon nanocomposites as efficient multifunctional electrocatalysts

MXene boosted MOF-derived cobalt sulfide/carbon nanocomposites as efficient bifunctional electrocatalysts for OER and HER

The development of effective bifunctional electrocatalysts that can realize water splitting to produce oxygen and hydrogen through oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is still a great challenge to be addressed. Herein, we report a simple and versatile approach to fabricate bifunctional OER and HER electrocatalysts derived from ZIF67/MXene hybrids via sulfurization of the precursors in hydrogen sulfide gas atmosphere at high temperatures. The as-prepared CoS@C/MXene nanocomposites were characterized using a series of technologies including X-ray diffraction, gas sorption, scanning electronic microscopy, transmission electronic microscopy, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy. The synthesized CoS@C/MXene composites are electrocatalytically active in both HER and OER, and the CSMX-800 composite displayed the highest electrocatalytic performance towards OER and HER among all the produced samples. CSMX-800 exhibited overpotentials of 257 mV at 10 mA cm-2 for OER and 270 mV at 10 mA cm-2 for HER. Moreover, it also possesses small Tafel slope values of 93 mV dec-1 and 103 mV dec-1 for OER and HER, respectively. The enhanced electrocatalytic performance of the MXene-containing composites is due to their high surface area, enhanced conductivity, and faster charge transfer. This work demonstrated that CoS@C/MXene based electrocatalyst has great potential in electrochemical water splitting for hydrogen production, thus reducing carbon emissions and protecting the environment.

Theoretical Screening of Highly Efficient Single-Atom Catalysts Based on Covalent Triazine Frameworks for Oxygen Reduction

Covalent triazine frameworks (CTFs) obtained from the trimerization of aromatic nitriles are expected to be the preferred carrier for single-atom catalysts (SACs). Using density functional theory methods, the oxygen reduction reaction (ORR) performance of a series of 3d, 4d, and 5d transition metals supported on the 6N or 9N pore of the CTF system [M-CTF(6N) or M-CTF(9N)] is explored. At first, 32 kinds of M-CTF(6N) and M-CTF(9N) are screened out with high thermodynamic and electrochemical stability. The binding energy of ORR intermediates and the change of Gibbs free energy in each step of the ORR are calculated. The overpotential of Pd-CTF(6N) is the lowest, which is 0.38 V. Considering that the ORR activity of M-CTFs is mainly limited by the strong binding of *OH, M-CTF(6N) and M-CTF(9N) are further modified by the OH ligand, namely, M-OH-CTF(6N) and M-OH-CTF(9N). After being modified by the OH ligand, due to the weakened binding strength of *OH, all these screened M-CTFs exhibit better ORR activity. Among them, the η values of Cu-OH-CTF(6N), Pd-OH-CTF(6N), Rh-OH-CTF(6N), Ir-OH-CTF(6N), Rh-OH-CTF(9N), and Ir-OH-CTF(9N) are 0.39, 0.38, 0.24, 0.30, 0.31, and 0.33 V, respectively, which possess better ORR activity than the Pt(111) surface (η = 0.45 V). This work highlights the great potential of CTFs as an efficient carrier for SACs.

Metal-Organic Framework Composites and Their Derivatives as Efficient Electrodes for Energy Storage Applications: Recent Progress and Future Perspectives

Metal-organic frameworks (MOFs) have been important electrochemical energy storage (EES) materials because of their rich species, large specific surface area, high porosity and rich active sites. Nevertheless, the poor conductivity, low mechanical and electrochemical stability of pristine MOFs have hindered their further applications. Although single component MOF derivatives have higher conductivity, self-aggregation often occurs during preparation. Composite design can overcome the shortcomings of MOFs and derivatives and create synergistic effects, resulting in improved electrochemical properties for EES. In this review, recent applications of MOF composites and derivatives as electrodes in different types of batteries and supercapacitors are critically discussed. The advantages, challenges, and future perspectives of MOF composites and derivatives have been given. This review may guide the development of high-performance MOF composites and derivatives in the field of EES.

Two-Dimensional Metal-Organic Frameworks as Ultrahigh-Performance Electrocatalysts for the Fuel Cell Cathode: A First-Principles Study

Except for metal-organic frameworks (MOFs) with traditional metal-nitrogen sites, MOFs with metal-oxygen sites may also possess good oxygen reduction reaction (ORR) catalytic activity due to their unique electronic structures. Herein, using density functional theory methods, the ORR performances of a series of M₃(HHTT)₂ (where M is a 3d, 4d, or 5d transition metal and HHTT is 2,3,7,8,12,13-hexahydroxytetraazanaphthotetraphene)) catalysts are explored. The binding energy (ΔE_species) results suggest that the binding energy of *OH (ΔE_*OH) shows a good linear relationship with the binding energies of *O and *OOH (ΔE_*O and ΔE_*OOH, respectively), indicating that ΔE_*OH can serve as a descriptor to reflect the catalytic activity of M₃(HHTT)₂. In addition, the volcano plot suggests that M₃(HHTT)₂ catalysts with a moderate binding strength of the intermediate *OH (0.6 eV < ΔE_*OH < 0.9 eV) show relatively high ORR activity. Therefore, four highly active ORR catalysts are screened out, namely, Fe₃(HHTT)₂, Co₃(HHTT)₂, Rh₃(HHTT)₂, and Ir₃(HHTT)₂, which possess very small overpotentials of 0.35, 0.24, 0.31, and 0.29 V, respectively. Their potential-determining step is the reduction of O₂ to the intermediate *OOH. It is encouraging that the theoretically lowest overpotential of this kind of catalyst is 0.21 V, which is superior to that on Pt(111). Moreover, Co₃(HHTT)₂ has excellent poisoning-tolerance ability for impurity gases (CO, NO, and SO₂) as well as fuel molecules (CH₃OH and HCOOH).

Bimetallic Co-Mo sulfide/carbon composites derived from polyoxometalate encapsulated polydopamine-decorated ZIF nanocubes for efficient hydrogen and oxygen evolution

The increased call for carbon neutrality by 2050 makes it compelling to develop emission-free alternative energy sources. Green hydrogen produced from water electrolyzers using renewable electricity is of great importance, and the development of efficient transition-metal-based materials for hydrogen production by electrolysis is highly desirable. In this report, a new approach to produce defect-rich and ultra-fine bimetallic Co-Mo sulfides/carbon composites from polyoxometalates@ZIF-67@polydopamine nanocubes via carbonization/sulfurization, which are highly active for hydrogen and oxygen evolution reactions (HER and OER), have been successfully developed. The coating of polydopamine (PDA) on the surface of the acid-sensitive ZIF-67 cubes can prevent the over-dissociation of ZIF-67 caused by the encapsulated phosphomolybdic acid (PMA) etching through PDA chelating with the PMA molecules. Meanwhile, the partially dissociated Co²⁺ from ZIF-67 can be captured by the coated PDA via chelation, resulting in more evenly dispersed active sites throughout the heterogeneous composite after pyrolysis. The optimized bimetallic composite CoMoS-600 exhibits a prominent improvement in HER (with an overpotential of -0.235 V vs. RHE at a current density of 10 mA cm^-2) and OER performance (with an overpotential of 0.350 V vs. RHE at a current density of 10 mA cm^-2), due to the synergistic effect of ultra-fine defect-rich Co-Mo-S nanoparticle active sites and N,S-codoped porous carbons in the composites. Moreover, this synthesis approach can be readily expanded to other acidic polyoxometalates to produce HER and OER active bimetallic Co-W sulfide/carbon composites by replacing PMA with phosphotungstic acid. This new synthesis strategy to modify acid-sensitive ZIFs with selected compounds offers an alternative approach to develop novel transition metal sulfide/carbon composites for various applications.

Exploring high-efficiency electrocatalysts of metal-doped two-dimensional C4N for oxygen reduction, oxygen evolution, and hydrogen evolution reactions by first-principles screening

The HER/ORR/OER on 3d, 4d, and 5d transition metal doped C4N are studied using DFT methods.

Screening of catalytic oxygen reduction reaction activity of 2, 9-dihalo-1, 10-phenanthroline metal complexes: The role of transition metals and halogen substitution

The sluggish kinetics of oxygen reduction reaction (ORR) restricts the employment of fuel cells, it is urgent to design ORR catalysts with excellent performance. The ORR performances of 2, 9-dihalo-1, 10-phenanthroline metal complexes (named as TM-X, X = Cl, Br, I) are comprehensively studied by the density functional theory methods. From the stability point of view, chlorine is more suitable for substitution. The adsorption free energy reveals that the liner relationship between adsorption free energy of *OOH and *OH is changed positively by the steric hindrance caused by the orthogonal TM-X structures. The Ni-Br stands out with the lowest overpotential of 0.34 V, and many other TM-X also show the promising ORR activity. Combining with the analysis of the Gibbs free energy diagrams and d-band center results, the substitution of halogen can improve the electronic structures of TM-X, thus enhancing their ORR activities and changing the ORR mechanism possibly.