Surface construction of loose Co(OH)2 shell derived from ZIF-67 nanocube for efficient oxygen evolution.

The rational design of nanostructure is very important for improving the number and effective utilization of active sites of the electrocatalysts. Here, a core-shell nanostructure composed of ZIF-67 core and Co(OH)2 shell (ZIF-67@Co(OH)2) has been obtained by subjecting ZIF-67 nanocube to the optimal high temperature etching process. After refluxing and etching in ethanol/water mixed solution, the loose Co(OH)2 shell can be constructed based on the surface of etched ZIF nanocube, which provides the obviously abundant active cobalt sites and better contact for oxygen evolution reaction (OER). Compared to the whole hollow Co(OH)2 nanocube, the solid ZIF-67 core in ZIF-67@Co(OH)2 can be favorable for charge transfer and provide the stable structure. The synergistic effect between Co(OH)2 shell and ZIF-67 core under suitable etching regulation can realize the optimized electrocatalysis for OER. The performance measurements show that Co(OH)2-1 after refluxing 1 h demonstrates the excellent activity requiring 354 mV overpotential at the current density of 10 mA cm-2 and the good stability. The enhanced mechanism may be due to the formation of loose Co(OH)2 as shell with fully exposed active sites, as well as the synergistic effect between ZIF-67 core and Co(OH)2 shell. Therefore, the surface construction of active composites based on ZIF precursor may be a new strategy for efficient electrocatalysis for water splitting.

N-Doped Sandwich-Structured Mo2C@C@Pt Interface with Ultralow Pt Loading for pH-Universal Hydrogen Evolution Reaction.

Designing a unique electrochemical interface to exhibit Pt-like activity and good stability is indispensable for the efficient hydrogen evolution reaction (HER). Herein, we synthesize well-defined Mo2C@NC@Pt nanospheres with a sandwich-structured interface through a facile organic-inorganic pyrolysis and following reduction process. The obtained Mo2C@NC@Pt heterostructures with ultralow Pt loading are composed of well-dispersed Mo2C nanoparticles (NPs) inner layer, N-doped carbon layer, and ultrafine Pt NPs outer layer. Electrochemical measurements demonstrate that Mo2C@NC@Pt heterostructures not only exhibit superior HER activities than commercial Pt/C with small overpotentials of only 27, 47, and 25 mV to achieve a current density of 10 mA cm-2 in acidic, alkaline, and neutral media, respectively, but also possess favorable long-term stability in pH-universal solution. The improved reaction kinetics of Mo2C@NC@Pt heterostructures are mainly attributed to the unique sandwich-structured interface with well-defined Mo2C NPs encapsulated by carbon layers and Pt NPs well-dispersed on the carbon support, synergistic effects among Mo2C NPs, NC, and Pt NPs, high specific surface area, and N-doping into the catalysts. This facile approach not only provides a new pathway for preparing well-defined carbides but also gives insight into the development of low-Pt catalysts for the efficient HER.

Pt-C Interfaces Based on Electronegativity-Functionalized Hollow Carbon Spheres for Highly Efficient Hydrogen Evolution.

The hydrogen evolution reaction activity of carbon-supported Pt catalyst is highly dependent on Pt-C interfaces. Herein, we focus on the relationships between Pt activity and N/O-functionalized hollow carbon sphere (HCS) substrate in acidic media. The electrochemical dissolution of Pt counter electrode is performed to prepare Pt nanoparticles in low loading. The N groups are beneficial for homogeneously sized Pt nanoparticles, whereas the O groups lead to aggregated nanoparticles. Moreover, the proper electronegativity of the N groups may enable capturing of protons to create proton-rich Pt-C interfaces and transfer them onto the Pt sites. The O groups may also capture protons by hydrogen bonding, but the subsequent release of protons is more difficult due to a stronger electronegativity and result in an inferior Pt activity. Consequently, the N-doped HCS with a low Pt loading (1.7 µg cm-2 and 0.05 wt %) possesses a higher intrinsic activity compared with Pt on O-doped HCS. Moreover, it outperforms the commercial 20% Pt/C with a stable operation for 12 h. This work may provide suggestions for constructing the advantageous Pt-C interfaces by proper functional groups for high catalytic efficiencies.

Electrochemical Corrosion Engineering for Ni-Fe Oxides with Superior Activity toward Water Oxidation.

The traditional synthesis for bimetallic-based electrocatalysts is challengeable for fine composition and elemental distribution because of the uncontrollable growth speed of nanostructures utilizing metal salt precursors. Herein, a unique electrochemical corrosion engineering strategy is developed via electrochemically transforming metal solid substrates (iron foil and nickel foam) into a highly active Ni-Fe oxide film for oxygen evolution, rather than directly utilizing metal ion precursors. This synthesis involves electrochemical corrosion of a Fe foil in an aqueous electrolyte along with electrochemical passivation of Ni foam (NF). The released trace Fe ions gradually incorporate into passivated NF surfaces to construct Ni-Fe oxide film and crucially improve composition distribution in the catalyst film. As a result, the resulted film with an ultralow mass loading (0.22 mg cm-2) delivers large current densities of 500 mA cm-2 at overpotential of only 270 mV in 6.0 M KOH at 60 °C, outperforming many reported NiFe catalysts requiring much higher mass loadings. More interestingly, the as-prepared catalyst almost reaches the standard (500 mA cm-2 within the overpotential of 300 mV) in commercial water electrolysis with long-term stability for at least 10 h. This work may provide a unique synthesis strategy for nonprecious transition-metal catalysts for desirable water splitting and can be expanded to many other electrocatalysis systems.

Tuning the morphology and Fe/Ni ratio of a bimetallic Fe-Ni-S film supported on nickel foam for optimized electrolytic water splitting.

The surface composite and morphology of binary metal sulfides are the key for efficient overall water splitting. However, tuning the morphology and surface composition of binary metal sulfides in a facile way is still a challenge. Herein, binary Fe-Ni sulfides supported on nickel foam (FeNi-S/NF) with different morphology and composition ratio of Fe/Ni have been synthesized through a facile one-step electrodeposition assisted by liquidcrystaltemplate (LCT). The binary FeNi-S has improved activity and conductivity compared to single metal sulfides. LCT-assisted porous FeNi-S film composed of uniform nanospheres is obviously different from planar film electrodeposited in water solution. LCT-assisted FeNi-S nanospheres are covered by many interwoven nanosheets, implying more exposed active sites for water splitting. Furthermore, the different Fe/Ni ratios of FeNi-S/NF samples have been systematically studied to explore the influence of Fe-incorporation on intrinsic activity of FeNi-S/NF. And the sample with Fe/Ni ratio (3/1) demonstrates the best activity and excellent stability for overall water electrolysis. Therefore, our work provides a facile and controllable access to binary metal sulfides with excellent performances for overall water splitting.

Hydrogen Evolution Activity of Ruthenium Phosphides Encapsulated in Nitrogen- and Phosphorous-Codoped Hollow Carbon Nanospheres.

RuPx nanoparticles (NPs) encapsulated in uniform N,P-codoped hollow carbon nanospheres (RuPx @NPC) have been synthesized through a facile route in which aniline-pyrrole copolymer nanospheres are used to disperse Ru ions followed by a gas phosphorization process. The as-prepared RuPx @NPC exhibits a uniform core-shell hollow nanospherical structure with RuPx NPs as the core and N,P-codoped carbon (NPC) as the shell. This strategy integrates many advantages of hollow nanostructures, which provide a conductive substrate and the doping of a nonmetal element. At high temperatures, the obtained thin NPC shell can not only protect the highly active phase of RuPx NPs from aggregation and corrosion in the electrolyte but also allows variation in the electronic structures to improve the charge-transfer rate greatly by N,P codoping. The optimized RuPx @NPC sample at 900 °C exhibits a Pt-like performance for the hydrogen evolution reaction (HER) and long-term durability in acidic, alkaline, and neutral solutions. The reaction requires a small overpotential of only 51, 74, and 110 mV at 10 mA cm-2 in 0.5 m H2 SO4 , 1.0 m KOH, and 1.0 m phosphate-buffered saline, respectively. This work provides a new way to design unique phosphide-doped carbon heterostructures through an inorganic-organic hybrid method as excellent electrocatalysts for HER.

Nitrogen, phosphorus dual-doped molybdenum-carbide/molybdenum-phosphide-@-carbon nanospheres for efficient hydrogen evolution over the whole pH range.

MoO42-@aniline-pyrrole (MoO42-@polymer) spheres as precursors have been used to synthesize unique core-shell nanostructure consisting of molybdenum carbide and molybdenum phosphide composites encapsulated into uniformly dual N, P-doped carbon shells (Mo2C/MoP@NPC) through a facile two-step strategy. Firstly, porous core-shell N-doped Mo2C@C (Mo2C@NC) nanospheres have been synthesized with ultrafine Mo2C nanoparticles as core and ultrathin NC as shell by a annealing route. Secondly, Mo2C/MoP@NPC has been obtained maintaining intact spherical-like morphology through a phosphidation reaction in high temperature. The synergistic effect of Mo2C and MoP may reduce the strong MoH bonding energy of pure Mo2C and provide a fast hydrogen release process. In addition, the dual N, P-doped carbon matrix as shell can not only improve the electroconductivity of catalysts but also prevent the corrosion of Mo2C/MoP nanoparticles during the electrocatalytic process. When used as HER cathode in acids, the resulting Mo2C/MoP@NPC shows excellent catalytic activity and durability, which only needs an overpotential of 160 mV to drive 10 mA cm-2. Moreover, it also exhibits better HER performance in basic and neutral media with the need for overpotentials of only 169 and 228 mV to achieve 10 mA cm-2, respectively. This inorganic-organic combination of Mo-based catalysts may open up a new way for water-splitting to produce large-scale hydrogen.