Electrocatalytic water splitting presents an exciting opportunity to produce environmentally benign fuel to power human activities and reduce reliance on fossil fuels. Transition metal nanoparticles (NPs) and their alloys are emerging as promising candidates to replace expensive platinum group metal (PGM) catalysts. Herein, we report the synthesis of distinct crystal phases and compositions of Ni1–x Mo x alloy NPs as low-cost, earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) in alkaline medium. Phase-pure cubic and hexagonal Ni and Ni1–x Mo x alloy NPs, with sizes ranging from 18 to 43 nm and varying Mo composition (∼0–11.4%), were produced by a low-temperature colloidal chemistry method. As-synthesized NPs show spherical to polygonal morphologies and a systematic shifting of Bragg reflections to lower 2θ angles with increasing Mo, suggesting the growth of homogeneous alloys. XPS analysis indicates the dominance of metallic Ni(0) and Mo(0) species in the core of the alloy NPs as well as the presence of higher valent Ni n+ and Mo n+ surface species, stabilized by surfactant ligands. The cubic alloys exhibit significantly higher HER activity in comparison to the hexagonal alloys. For a current density of −10 mA/cm2, the cubic alloys demonstrate overpotentials of −62 to −177 mV compared to −162 to −242 mV for the hexagonal alloys. The overpotentials of cubic alloys are comparable to the commercial Pt-based electrocatalysts for which the overpotentials range from −68 to −129 mV at −10 mA/cm2. In general, a decrease in overpotential and an increase in HER activity were observed with increasing concentration of Mo (up to 6.6%) for the cubic alloys. The cubic Ni0.934Mo0.066 alloy NPs exhibit the highest activity as alkaline HER electrocatalysts.

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Electrochemical water splitting represents a sustainable method to produce molecular hydrogen, a foreseeable clean energy alternative to exhaustible fossil fuels. Transition-metal phosphides (TMPs) are emerging as earth-abundant catalysts for water splitting, and their activity can be further improved by incorporation of synergetic metals to produce bimetallic TMP catalysts. Herein, two distinct colloidal chemistry methods were developed to produce discrete nickel molybdenum phosphide (Ni−Mo−P) nanoparticles (NPs) that show varying crystal structures, morphologies, and compositions as alkaline hydrogen evolution reaction (HER) catalysts. The one-pot route produced smaller homogeneous NPs, ranging from 4 to 11 nm, with a nearspherical morphology. The two-pot synthesis resulted in larger heterogeneous NPs, ranging from ∼50 to 80 nm, with a polygonal morphology. Both nanostructures show either a hexagonal Ni 2 P or tetragonal Ni 12 P 5 crystal structure and a shift in X-ray diffraction patterns to lower 2θ angles, consistent with the formation of bimetallic TMPs. The X-ray photoelectron spectra indicate the presence of partially charged core species (Ni δ+ , Mo δ+ , and P δ− ) as well as minor higher valent (Ni n+ , Mo n+ , and PO 4 3− , n ≥ 2) surface species, presumably bound to surfactant ligands and/or oxides. Among heterogeneous and homogeneous NPs investigated, the hexagonal Ni 2−x Mo x P NPs show lower overpotentials (i.e., high HER activity) in comparison to tetragonal Ni 12−x Mo x P 5 NPs. The HER activity of both nanostructures follows a mixed Volmer− Heyrovsky reaction mechanism consistent with Tafel slopes of 49.5−100.6 mV/dec. The homogeneous and heterogeneous Ni 1.87 Mo 0.13 P NPs showed the lowest overpotentials of 101 and 96 mV, respectively, and outperformed both hexagonal Ni 2 P (156 mV) and tetragonal Ni 12−x Mo x P (198 mV) NPs at a current density of −10 mA/cm 2 . This work provides insights into the design and synthesis of high-efficiency TMP nanostructures for alkaline HER studies.

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The rechargeable lithium-sulfur (Li-S) battery is a promising candidate for the next generation of energy storage technology,o wing to the high theoreticalc apacity,h igh specific energy density,a nd low cost of electrode materials. The main drawbacks in the development of long-life Li-S batteries are capacity fading and the sluggish kinetics at the cathode caused by the polysulfides shuttle. These limitations are addressed through the design of novel nanocagesc ontaining cobalt phosphide (CoP) nanoparticles embedded in highly porousnitrogen-doped carbon (CoP-N-GC) by thermal annealing of ZIF-67 in ar eductivea tmosphere followed by a phosphidation step using sodium hypophosphite. The CoP nanoparticles, with large surface area and uniform homogeneous distribution within the N-doped nanocage graphitic carbon,a ct as electrocatalysts to suppress the shuttle of soluble polysulfides through strong chemical interactions and catalyze the sulfur redox. As ar esult,t he S@CoP-N-GC electrode delivers an extremely high specific capacity of 1410 mA hg À1 at 0.1 C(1C= 1675 mA g À1)w ith an excellent coulombic efficiency of 99.7 %. Moreover,c apacity retention from 864 to 678 mA hg À1 is obtained after 460 cycles with a very low decay rate of 0.046 %p er cycle at 0.5 C. Therefore, the combination of the CoP catalyst and polar conductive porousc arbon effectively stabilizes the sulfur cathode, enhancing the electrochemical performance and stabilityo f the battery.