Transition Metal Carbide Complex Architectures for Energy‐Related Applications

Synergistic catalytic enhancement of metal-organic framework derived nanoarchitectures decorated on graphene as a high-efficiency bifunctional electrocatalyst for methanol oxidation and oxygen reduction

Designing highly efficient, long-lasting, and cost-effective cathodic and anodic functional materials as a bifunctional electrocatalyst is essential for overcoming the bottleneck in fuel cell development. Herein, a novel two-step synthesis strategy is developed to synthesize metal-organic framework (MOF) derived nitrogen-doped carbon (NC) with improved spatial isolation and a higher loading amount of cobalt (Co) and nickel carbide (Ni₃C) nanocrystal decorated on graphene (denoted as Co@NC-Ni₃C/G). Benefiting from multiple active sites of high N-doping level, uniform dispersion of Co and Ni₃C nanocrystals, and a large active area of graphene, the Co@NC-Ni₃C/G hybrids exhibit excellent methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) efficiency in an alkaline environment. For MOR, the optimized Co@NC-Ni₃C/G-350 catalyst achieved a current density of 44.8 mA cm^-2 at an applied potential of 1.47 V (V vs. RHE), which is significantly higher than Co@NC-Ni₃C (42.07 mA cm^-2) and Co@NC (24.1 mA cm^-2) in 0.5 M methanol + 1.0 M KOH solutions. In addition, during the CO retention test, the Co@NC-Ni₃C/G-350 catalyst exhibits excellent CO tolerance capacity. Excitingly, the as-prepared Co@NC-Ni₃C/G-350 hybrid exhibits significantly improved ORR catalytic efficiency in terms of positive onset and half-wave potential (E_onset = 0.90 V, E_1/2 = 0.830 V vs. RHE), small Tafel slope (34 mV dec^-1) and excellent durability (only reduced 0.016 V after 5000 s test). This work provides new insights into MOF-derived functional nanomaterials for anode and cathode co-catalysts for methanol fuel cells.

Mo2 C Nanoparticles Embedded in Carbon Nanowires with Surface Pseudocapacitance Enables High-Energy and High-Power Sodium Ion Capacitors

Electrochemical sodium-ion storage technologies have become an indispensable part in the field of large-scale energy storage systems owing to the widespread and low-cost sodium resources. Molybdenum carbides with high electron conductivity are regarded as potential sodium storage anode materials, but the comprehensive sodium storage mechanism has not been studied in depth. Herein, Mo₂ C nanowires (MC-NWs) in which Mo₂ C nanoparticles are embedded in carbon substrate are synthesized. The sodium-ion storage mechanism is further systematically studied by in/ex situ experimental characterizations and diffusion kinetics analysis. Briefly, it is discovered that a faradaic redox reaction occurs in the surface amorphous molybdenum oxides on Mo₂ C nanoparticles, while the inner Mo₂ C is unreactive. Thus, the as-synthesized MC-NWs with surface pseudocapacitance display excellent rate capability (a high specific capacity of 76.5 mAh g^-1 at 20 A g^-1 ) and long cycling stability (a high specific capacity of 331.2 mAh g^-1 at 1 A g^-1 over 1500 cycles). The assembled original sodium ion capacitor displays remarkable power density and energy density. This work provides a comprehensive understanding of the sodium storage mechanism of Mo₂ C materials, and constructing pseudocapacitive materials is an effective way to achieve sodium-ion storage devices with high power and energy density.

Insights into N, P, S multi-doped Mo2C/C composites as highly efficient hydrogen evolution reaction catalysts

Heteroatom doping has been proved to be an effective strategy to optimize the activity of hydrogen evolution reaction (HER) catalysts. Herein, we report N, P, S multi-doped Mo₂C/C composites exhibiting highly efficient HER performance in acidic solution, which are facilely fabricated via annealing of N, P, S-containing MoO _x -polyaniline (MoO _x -PANI) hybrid precursors. The optimized N, P, S multi-doped Mo₂C/C catalyst with a moderate P dopant level (NPS-Mo₂C/C-0.5) exhibits excellent performance with an overpotential of 53 mV to achieve a current density of 20 mA cm^-2, a Tafel slope of 72 mV dec^-1 and good stability in acidic electrolytes. Based on the study of XPS, EPR and ³¹P MAS NMR, the excellent electrocatalytic performance could be attributed to the effective electronic configuration modulation of both Mo₂C nanorods and the carbon matrix, derived from stronger synergistic N, P, S multi-doping coupling effects. This work provides a promising methodology for the design and fabrication of multi-doped transition metal based electrocatalysts via electronic structure engineering.

Transition-Metal Oxides/Carbides@Carbon Nanotube Composites as Multifunctional Electrocatalysts for Challenging Oxidations and Reductions

The rapid development of renewable-energy technologies such as water splitting, rechargeable metal-air batteries, and fuel cells requires highly efficient electrocatalysts capable of the oxygen-reduction reaction (ORR) and the oxygen-evolution reaction (OER). Herein, we report a facile sonication-driven synthesis to deposit the molecular manganese vanadium oxide precursor [Mn₄ V₄ O₁₇ (OAc)₃ ]^3- on multiwalled carbon nanotubes (MWCNTs). Thermal conversion of this composite at 900 °C gives nanostructured manganese vanadium oxides/carbides, which are stably linked to the MWCNTs. The resulting composites show excellent electrochemical reactivity for ORR and OER, and significant reactivity enhancements compared with the precursors and a Pt/C reference are reported. Notably, even under harsh acidic conditions, long-term OER activity at low overpotential is reported. In addition, we report exceptional activity of the composites for the industrially important Cl₂ evolution from an aqueous HCl electrolyte. The new composite material shows how molecular deposition routes leading to highly active and stable multifunctional electrocatalysts can be developed. The facile design could in principle be extended to multiple catalyst classes by tuning of the molecular metal oxide precursor employed.

Carbon impurity-free, novel Mn,N co-doped porous Mo2C nanorods for an efficient and stable hydrogen evolution reaction

Carbon impurity-free, novel Mn,N co-doped porous Mo₂C nanorods reduce the hydrogen adsorption energy, functioning as efficient HER electrocatalysts.