Zi-Zhang Liu

Coupling Ag-doping and rich oxygen vacancies in mesoporous NiCoO nanorods supported on nickel foam for highly efficient oxygen evolution

A crucial challenge still remains in the development of efficient and stable electrocatalysts for oxygen evolution reaction (OER) with desirable conductivity, a high surface area and rich oxygen vacancies. Herein, a type of Ag-doped mesoporous NiCoO nanorod with rich oxygen vacancies (NiCoO@Ag40/NF-Ar) for OER is prepared via an electrodeposition-hydrothermal reaction and the subsequent annealing treatment process under an Ar atmosphere. The electrodeposited Ag film is found to direct the uniform growth of the nanowire arrays of NiCo hydroxide precursors compared to the nanoparticles of NiCo hydroxide in the absence of the Ag film. Interestingly, the addition of C2H8N2 (EN) during the electrodeposition of the Ag film and the subsequent calcination under an Ar atmosphere collectively contribute to the formation of mesoporous nanorod structures and rich oxygen vacancies. The calcined NiCoO in air mainly have the Co3O4 phase, implying that it has fewer oxygen vacancies and weak activity for OER. The high surface area and one-dimensional feature of mesoporous nanorods are responsible for the increased exposure of active sites and fast charge transport behavior. Moreover, Ag doping can also improve the conductivity of NiCoO nanorods. NiCoO@Ag40/NF-Ar exhibits a highly efficient activity for OER with a current density of 140 mA cm−2 at an overpotential of 370 mV and a remarkable stability. The suitable Ar annealing treatment coupling Ag films and oxygen vacancies into transition metal oxide precursors may be a facile and promising method for constructing mesoporous nanostructures with rich oxygen vacancies for efficient water oxidation.

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.

Probing the active sites of Co3O4 for the acidic oxygen evolution reaction by modulating the Co2+/Co3+ ratio

Exploring active and stable electrocatalysts for the acidic oxygen evolution reaction (OER) is necessary to broaden the practical applications of proton exchange membrane electrolyzers. Unfortunately, the active sites of electrocatalysts for the acidic OER, which are the most powerful tool for designing and optimizing OER electrocatalysts, still remain ambiguous. Herein, we synthesize Ag doped Co3O4 with different atomic ratios of Co2+/Co3+ and investigate the effect of preferential exposure of Co2+ in Co3O4 on the acidic OER through systematic experiments for the first time. X-ray photoelectron spectroscopy is used to probe the atomic ratio of Co2+/Co3+ on the surface of Co3O4. The results demonstrate that Co3O4 richer in Co2+ shows the best acidic OER performance, and affords a current density of 10 mA cm−2 at an overpotential of 470 mV along with having a satisfactory stability in H2SO4 solution. Moreover, low-temperature calcination treatment is found to be an effective method to aid preferential growth of Co2+ on the surface of Co3O4, further making our synthesis process more practical and universal. Therefore, this work provides some insight into designing non-precious electrocatalysts for the acidic OER, by identifying active sites and offering a versatile modulation strategy on the preferential growth of real active sites.

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.

A triple synergistic effect from pitaya-like MoNix–MoCx hybrids encapsulated in N-doped C nanospheres for efficient hydrogen evolution

To enhance the intrinsic activity and the density of active sites of catalysts for the hydrogen evolution reaction (HER), a facile strategy of using an organic–inorganic precursor followed by carbonization is adopted to prepare ternary core–shell nanostructures composed of ultrafine hybrids of MoNi alloys and MoCx nanoparticles encapsulated in N-doped carbon nanospheres (MoNix–MoCx@NC). The well-defined pitaya-like nanostructures of MoNix–MoCx nanoparticles encapsulated by N-doped carbon nanospheres can be obtained with MoNix–MoCx hybrids as the core and ultrathin N-doped C layers as the shell. A triple synergistic effect has been achieved for the HER. The first synergistic effect from homogeneously dispersed MoNix alloys and MoCx nanoparticles can improve the intrinsic activity and conductivity of MoCx. The second synergistic effect from the MoNix–MoCx and NC shell can enhance the density of active sites and conductivity of MoNix–MoCx. The third synergistic effect from N-doped C can accelerate the charge transfer rate and improve close interaction between NC and MoNix–MoCx. The MoNix–MoCx@NC sample at an optimized low temperature of 700 °C exhibits excellent performance and long-term stability in both acidic and alkaline solution. It requires a lower overpotential of only 172 mV and 168 mV at 10 mA cm−2 in acidic and alkaline solution, respectively. This work provides a new approach to design multiple synergistic effects from excellent catalytic interface through an organic–inorganic hybrid method for efficient electrocatalysis.