•Novel hollow SnO2 nanospheres coated by Ag (Ag/SnO2) have been synthesized.•The coating of Ag homogeneously covers on the surface of SnO2 hollow nanospheres.•Ag/SnO2 has excellent electrocatalytic activity for hydrazine electro-oxidation.Novel Ag/SnO2 has been synthesized by a facile two-step method using hollow SnO2 nanospheres as support. Firstly, monodispersed SnO2 hollow nanospheres have been obtained by solvothermal (ethanol/water) method. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that hollow SnO2 nanospheres exhibit good crystalline and monodispersion with the diameter ranging from 200 to 400 nm. Secondly, Ag/SnO2 nanospheres have been prepared by a facile reduction in silver-ammonia solution. XRD and selected area electron diffraction (SAED) confirm the formation of Ag in Ag/SnO2 nanospheres. SEM and TEM show the homogeneous coating of Ag nanoparticles on hollow SnO2 nanospheres. SEM mapping images confirm the uniform distribution of Ag element on SnO2 nanospheres. The electrocatalytic measurements for hydrazine electro-oxidation have been conducted. The results show that Ag/SnO2 nanospheres have excellent electrocatalytic activity with −0.12 V (vs. SCE) at the scan rate of 20 mV s−1. The hollow structure of SnO2 nanospheres can provide shorter distance of electron transport and better dispersion of Ag nanoparticles, which can be responsible for the enhanced conductivity and efficient utilization of active sites.

In situ cathodic activation (ISCA) of V-incorporated NixSy nanowires supported on nickel foam (VS/NixSy/NF) can be realized in an alkaline hydrogen evolution reaction (HER) process, which provides not only clearly enhanced activity but also ultrahigh stability for HER. The ISCA process is continuous linear sweep voltammetry (LSV) on VS/NixSy/NF as a cathodic electrode with gradually enhanced HER activity. The activated VS/NixSy/NF (A-VS/NixSy/NF) demonstrates enhanced HER activity with an overpotential of 125 mV to drive 10 mA cm−2, which is much lower than that of other samples. It may be predicted that the ISCA-derived amorphous VOOH film covering on A-VS/NixSy/NF accelerates the HER process, and NiOOH may protect active sites from decaying, leading to excellent activity and structural stability. However, for single metal sulfides, the ISCA process of nickel or vanadium sulfides is not available, implying that the synergistic effect between Ni and V of VS/NixSy/NF may be the key to drive ISCA in alkaline HER. In addition, its ultra-high stability confirms that the stable active sites and nanostructures of A-VS/NixSy/NF are derived from ISCA. Therefore, the ISCA of V-incorporated transition metal sulfides in the alkaline HER process may be a facile and promising method to obtain efficient electrocatalysts.

•Ternary Fe0.5Ni0.5Co2O4 nanowires were synthesized by facile hydrothermal process.•FexNi1-xCo2O4 samples with different Fe/Ni ratio have been systematically studied.•Fe0.5Ni0.5Co2O4 exhibits the better OER performances compared to other samples.•The improved OER activity may be due to synergistic effect from ternary nanowires.Designing mixed metal oxides with unique nanostructures as efficient electrocatalysts for water electrolysis has been an attractive approach for the storage of renewable energies. The ternary mixed metal spinel oxides FexNi1-xCo2O4 (x = 0, 0.1, 0.25, 0.5, 0.75, 0.9, 1) have been synthesized by a facile hydrothermal approach and calcination treatment using nickel foam as substrate. Fe/Ni ratios have been proved to affect the nanostructures of FexNi1-xCo2O, which imply different intrinsic activity for oxygen evolution reaction (OER). SEM images show that Fe0.5Ni0.5Co2O4 has the uniform nanowires morphology with about 30 nm of the diameter and 200–300 nm of the length. The OER measurements show that Fe0.5Ni0.5Co2O4 exhibits the better electrocatalytic performances with lower overpotential of 350 mV at J = 10 mA cm−2. In addition, the smaller Tafel slope of 27 mV dec−1 than other samples with different Fe/Ni ratios for Fe0.5Ni0.5Co2O4 is obtained. The improved OER activity of Fe0.5Ni0.5Co2O4 may be attributed to the synergistic effects from ternary mixed metals especially Fe-doping and the uniform nanowires supported on NF. Therefore, synthesizing Fe-doped multi-metal oxides with novel nanostructures may be a promising strategy for excellent OER electrocatalysts and it also provides a facile way for the fabrication of high-activity ternary mixed metal oxides electrocatalysts.Download high-res image (138KB)Download full-size image

•Oriented stacking along vertical (002) plane of MoS2 may form nanocolumn structure.•MoS2 nanocolumns (O-MoS2) may expose the more active sites for HER.•O-MoS2 have about 25 times enhancement of electrochemically active surface area.•Assembling along vertical inert plane may be a novel way to enhance HER activity.The layered structures of MoS2 tend to stack and aggregate and result in decreasing of rims and edges of MoS2 nanosheets, which greatly limited the improvement of activity of MoS2 as electrocatalysts for hydrogen evolution reaction (HER). Herein, owing to the inert nature of (002) basal plane of MoS2 for HER, we propose that oriented stacking along vertical (002) plane can form nanocolumn structure, which may expose an increased number of the active sites per mass and accelerate charge transfer rate for HER. The electrochemical measurements confirm that the nanocolumns of oriented stacking of MoS2 (O-MoS2) have much better HER activity than bulk MoS2 and randomly stacking of MoS2 with about 25 times enhancement of electrochemically active surface area (ECSA). Therefore, assembling style along vertical inert plane may be a novel way to enhance HER performances of layered transition metal sulfides.Download high-res image (218KB)Download full-size image

•NaBH4 as reductant has been to fabricate the reduced CoFe2O4 nanosheets (NS).•Rich oxygen vacancies on CoFe2O4 NS lead to more active sites and better conductivity.•Reduced CoFe2O4 NS show better OER activity than the reduced bulk and nanoparticle.•CoFe2O4 hollow nanospheres exhibit most obvious improvement for OER activity.The efficiency of electrochemical water splitting is greatly hindered by the thermodynamic uphill reaction of oxygen evolution reaction (OER). Thus, it is important to synthesize an active OER electrocatalysts with abundant active sites, favorable conductivity and good durability. Herein, a facile reduction method using NaBH4 as readily available reductant has been developed to fabricate the reduced CoFe2O4 nanosheets (NS). The obtained reduced CoFe2O4 NS are rich in oxygen deficient sites, leading to more active sites as well as the enhanced conductivity than the pristine CoFe2O4 hollow nanosphere, which reaches the current density of 10 mA cm−2 at the overpotential of 320 mV in 1 M KOH. Meanwhile, CoFe2O4 samples with three different morphology nanostructures including hollow nanospheres, bulk and nanoparticles have been provided to study the effect of different morphology on NaBH4 reduction efficiency. As expected, after NaBH4 reduction, CoFe2O4 hollow nanosphere with relatively higher surface area exhibits most obvious improvement for OER activity and also its corresponding reduced CoFe2O4 NS showed best OER performance than the reduced CoFe2O4 bulk as well as the reduced CoFe2O4 nanoparticles, implying the hollow nanospheres feature more accessible surface area than bulk and nanoparticles samples, thus greatly facilitate efficiency of NaBH4 reduction treatment.Download high-res image (202KB)Download full-size image

•Rich N-doped Mo2C nanowires were synthesized by calcination-solvothermal process.•Rich N-doped Mo2C nanowires possess the enhanced activity and long-term stability.•The improved HER activity may be due to 1 D nanostructure and optimized N-doping.Rich N-doped molybdenum carbide (Mo2C) nanowires for hydrogen evolution reaction (HER) have been prepared by a facile calcination-solvothermal treatment utilizing inorganic-organic hybrid MoOx/aniline as precursor. Firstly, MoOx/aniline nanowires provide one-dimensional nanostructure and N element for the synthesis of N-doped Mo2C through calcination process. XRD and SEM show that uniform N-doped Mo2C nanowires with ß-Mo2C phase can be obtained after calcination of 750 °C. The electrocatalytic measurements show N-doped Mo2C nanowires of 750 °C exhibit the better activity compared with other samples at 650 °C, 850 °C and 950 °C. Furtherly, ammonium-hydrazine solvothermal process has been used to optimize the N-doping degree of N-doped Mo2C of 750 °C. XRD shows that rich N-doped Mo2C nanowires contain the crystalline phase structure, which can be identified by HRTEM. XPS, EDX and elemental mapping data reveal the good distribution and valence of Mo, C and N. The electrocatalytic measurements confirm that rich N-doped Mo2C possesses the lower overpotential, smaller Tafel slope, larger double-layer capacitances and excellent long-term stability after 5000 cycles than other samples, which may be attributed to ammonium-hydrazine solvothermal process. Therefore, Mo-based inorganic-organic hybrid nanostructure may be a promising precursor for excellent HER electrocatalysts through a facile calcination-solvothermal treatment.Download high-res image (225KB)Download full-size image

•Vertical MnO2/NiCo2O4 nanoflakes supported on nickel foam (NF) were prepared.•Ternary MnO2/NiCo2O4 possesses enhanced intrinsic activity and stability for OER.•NF as support can provide excellent conductivity and three dimensional skeleton.•The synergistic effect between ternary metal oxides is responsible for enhanced OER.Ternary metal oxides MnO2/NiCo2O4 supported on nickel foam (NF) as electrocatalysts for oxygen evolution reaction (OER) have been fabricated via a two-step hydrothermal process. NF as support can provide excellent conductivity and 3 dimensional skeleton, which facilitate the utilization of active sites and mass transfer of electrolyte. The ternary MnO2/NiCo2O4/NF also possess the advantage of synergistic effect between MnO2 and NiCo2O4, implying the enhanced intrinsic activity for OER. XRD, EDX and XPS confirm the existence and distribution of each element of MnO2/NiCo2O4/NF. SEM shows the hierarchical structure of MnO2/NiCo2O4/NF with vertically aligned MnO2/NiCo2O4 nanoflakes covering on the surface of NF, which is in favor of the more exposed active sites and the fast dissipation of O2 bubbles. Additionally, to clearly testify the significance of the vertically aligned MnO2/NiCo2O4 nanoflakes, non-vertical MnO2/NiCo2O4/NF sample (MnO2/NiCo2O4/NF-1) is prepared for comparing OER performances in the same condition. OER measurements in alkaline solution show that ternary MnO2/NiCo2O4/NF displays better electrocatalytic activity with a low overpotential of 340 mV at a current density of 10 mA cm−2 than other samples. Therefore, ternary metal oxides with hierarchical structure on NF may be a promising choice for excellent electrocatalysts for OER.Download high-res image (204KB)Download full-size image

•Controllable electrodeposition of Fe hydroxides film on NiVS/NF has been utilized.•V-doping of NiVS/NF nanowire structure may introduce more active sites for OER.•Uniform Fe film of eFe/NiVS/NF possess the superior OER activity and stability.•Controllable electrodeposition is an efficient and simple strategy for efficient OER.Developing cost-effective electrocatalysts with both high activity and stability remains challenging for oxygen evolution reaction (OER) in water electrolysis. Herein, based on V-doped nickel sulfide nanowire on nickel foam (NiVS/NF), we further conduct controllable electrodeposition of Fe hydroxides film on NiVS/NF (eFe/NiVS/NF) to further improve OER performance and stability. For comparison, ultrafast chemical deposition of Fe hydroxides on NiVS/NF (uFe/NiVS/NF) is also utilized. V-doping of NiVS/NF may introduce more active sites for OER, and nanowire structure can expose abundant active sites and facilitate mass transport. Both of the two depositions generate amorphous Fe hydroxides film covering on the surface of nanowires and lead to enhanced OER activities. Furthermore, electrodeposition strategy realizes uniform Fe hydroxides film on eFe/NiVS/NF confirmed by superior OER activity of eFe/NiVS/NF than uFe/NiVS/NF with relatively enhanced stability. The OER activity of eFe/NiVS/NF depends on various electrodepositon time, and the optimal time (15 s) is obtained with maximum OER activity. Therefore, the controllable electrodeposition of Fe may provide an efficient and simple strategy to enhance the OER properties of electrocatalysts.Download high-res image (389KB)Download full-size image

•A facile two-step method was used to synthesize NiFeCoSex on carbon fiber cloth.•Trimetallic NiFeCo hydroxides have been electrodeposited on CFC as precursor.•DMF solvothermal selenization was used to convert NiFeCo/CFC into NiFeCoSex/CFC.•The enhanced activity of NiFeCoSex/CFC may be attributed to NiFeCo and metal-CFC.Trimetallic NiFeCo selenides (NiFeCoSex) anchored on carbon fiber cloth (CFC) as efficient electrocatalyst for oxygen evolution reaction (OER) in alkaline medium have been synthesized via a facile two-step method. Firstly, trimetallic NiFeCo (oxy) hydroxides have been electrodeposited on CFC support (NiFeCo/CFC). Secondly, a solvothermal selenization process has been used to convert NiFeCo/CFC into NiFeCoSex/CFC using N, N-dimethylformamide (DMF) as solvent. The composition and homogeneous distribution of NiFeCoSex/CFC nanoparticles are determined by XRD, XPS, SEM elemental mapping and EDX images. Furthermore, SEM images reveal that NiFeCoSex/CFC has volcano-shaped morphology with rough surface and homogenously distributed on the surface of CFC, which may provide more active sites for OER. The electrochemical measurements show that trimetallic NiFeCoSex/CFC possesses the better electrocatalytic activity with the lower overpotential (150 mV at 10 mA cm−2), Tafel slope (85 mV dec−1), larger double-layer capacitance (200 mF cm−2) and long-term stability than unary or binary metal selenides. The enhanced activity of NiFeCoSex/CFC may be attributed to the trimetallic NiFeCo selenides and selenides-CFC synergistic interaction. It may offer a promising way to design transition multimetallic selenides supported on conductive support as electrocatalysts for OER.Download high-res image (253KB)Download full-size image

•N and O dual-doped carbon fiber (CF-O-N) has been hydrothermally synthesized.•The rough surface of CF-O-N is favorable for exposing abundant active sites.•N and O dual-doping can provide dual-active-site and synergistic effect for OER.•The dual-doped CF-O-N show better activity than single doped CF.Highly efficient and metal-free electrocatalysts hold promising in industrial water electrolysis with low cost, environmental friendliness and structural stability in wide pH electrolyte. Herein, N and O dual-doped carbon fiber (CF-O-N) has been hydrothermally synthesized as a three-dimensional (3D) electrocatalyst for oxygen evolution reaction (OER). The co-doping of N and O atoms acts as dual-active-site to improve OER properties, while the rough surface and large pores in CF-O-N electrode may expose abundant active sites and sufficient contacting in catalyst/electrolyte interfaces. CF-O-N electrode behaves excellently for OER with only 130 and 590 mV to generate 10 mA cm−2 and 100 mA cm−2, respectively. The superior OER property of CF-O-N electrode than single N- or O-doped CF implies the synergistic effect between N and O dopants that may further enhance OER activities. More importantly, dual-doped CF-O-N electrode displays roust stability in 10,000 cycles of cyclic voltammogram (CV) and 12 h chronoamperometry test with unchangeable structures proved by post physical characterizations. It may provide a facile and rational design of self-supported and metal-free materials with excellent activities for water oxidation and holds promising for further industrialization.Download high-res image (217KB)Download full-size image

•Binary metal Fe0.5Co0.5Se2 spheres based on carbon fiber cloth (CFC) were prepared.•Binary metal Fe0.5Co0.5Se2/CFC spheres possess enhanced activity for OER.•The synergistic effect between binary metal and CFC is responsible for enhanced OER.Novel binary metal Fe0.5Co0.5Se2 spheres supported on carbon fiber cloth (CFC) as efficient electrocatalyst for oxygen evolution reaction (OER) have been successfully synthesized by a facile solvothermal selenization using N, N-dimethylformamide (DMF) as solvent. Firstly, Fe0.5Co0.5 nanosheets have been electrodeposited on the surface of CFC. Then, Fe0.5Co0.5/CFC as precursor has been converted to uniform Fe0.5Co0.5Se2 spheres on CFC (Fe0.5Co0.5Se2/CFC) by DMF selenization process. XRD and EDX confirm the typical crystalline structure and homogeneous elemental distribution of Fe0.5Co0.5Se2 on the surface of CFC. XPS shows the existence and valence of Fe, Co and Se. The uniform size and good dispersion of Fe0.5Co0.5Se2 spheres with the diameter of about 100 nm have been revealed by SEM and elemental mapping, which may provide more active sites for OER. The electrochemical results demonstrate that the obtained Fe0.5Co0.5Se2/CFC possesses the better OER activity with smaller overpotential, lower Tafel slope and charge transfer resistance than other samples, which may be ascribed to the uniform spherical morphology and the better intrinsic activity from binary metal Fe0.5Co0.5Se2. Moreover, the faster electron transfer rate derived from CFC support and the synergistic effect between Fe0.5Co0.5Se2 and CFC may be responsible for the enhancements of OER performances.

•A facile two-step method was used to synthesize binary metal NiCoS nanorods on NF.•NiCoS nanorods composed of many vertical nanosheets can expose more active sites.•Binary NiCoS/NF has better OER activity than NiCo/NF and CoSx/NF.A facile two-step method has been applied to synthesize novel binary metal NiCoS nanorods supported on nickel foam (NF) as electrocatalysts for oxygen evolution reaction (OER). Firstly, electrodeposition process is conducted to fabricate binary Ni-Co hydroxides on NF (NiCo/NF). Then, a hydrothermal sulfuration of NiCo/NF has been adopted to prepare NiCoS nanorods arrays uniformly grown on the surface of NF (NiCoS/NF). XRD indicates that NiCoS/NF has mixed crystal phases of Ni3S2, CoS and Co9S8. SEM images display the uniform NiCoS nanorods composed of many vertical nanosheets on the surface, implying more exposed active sites. OER measurements demonstrate that NiCoS/NF has better activity with an overpotential of 370 mV to reach 100 mA cm−2 than NiCo/NF and CoSx/NF. Electrochemical impedance spectroscopy (EIS) tests confirm the faster charge-transfer rate of NiCoS/NF and smaller Tafel slope derived from binary NiCoS, implying the excellent electrocatalytic performances of binary metal sulfides.

•Ternary Ni-Co-Fe sulfides based on nickel foam were prepared by two-step method.•Electrodeposition-solvothermal process is suitable for Ni-Co-Fe sulfides film.•Ternary NiCoFeS/NF exhibits excellent OER activity for water splitting in alkaline.•The stability of NiCoFeS/NF can be improved by a secondary electrodeposition of Fe.Ternary mixed metal Ni-Co-Fe sulfides based on three dimensional (3D) nickel foam (NiCoFeS/NF) have been synthesized via facile electrodeposition-solvothermal process. Firstly, a uniform film of Co-Fe oxides has been electrodeposited on the surface of 3D skeleton of NF (CoFe/NF). Secondly, an ethanol solvothermal sulfurization has been adopted to convert CoFe/NF to ternary mixed metal sulfides (NiCoFeS/NF). XRD confirms that ternary NiCoFeS is composed of mixed phases including NiS, Ni3S2 and Co3S4 phases but no Fe sulfides phase, implying amorphous state of Fe sulfides. SEM images of ternary NiCoFeS films shows uniform film covering on the surface of NF, which is composed of many hill-shaped bulges structures. The OER measurements confirm the excellent performance of ternary NiCoFeS/NF in 1.0 M KOH, providing low overpotential of 40 and 160 mV to drive 10 and 100 mA cm−2 in 1.0 M KOH, respectively. A further enhancement of the stability of NiCoFeS/NF have been conducted via a secondary electrodeposition of Fe on NiCoFeS/NF to obtain NiCoFeS-Fe/NF, with slightly lower overpotential of 230 mV to drive 100 mA cm−2 than that (310 mV) of NiCoFeS/NF after stability test. The role of Fe on OER ability and stability of NiCoFeS/NF have been discussed.Download high-res image (142KB)Download full-size image

•Ag-Fe-Co2P nanosphere is prepared as bifunctional electrocatalyst for HER and OER.•The binary Ag-Fe-doping may improve of electronic structure of active site of Co2P.•Ag-Fe-Co2P has the obvious enhancement of activity for HER and OER in 1 M KOH.•Ag-Fe doping may provide the improved conductivity and electron-donating ability.The binary Ag-Fe-doped Co2P (Ag-Fe-Co2P) nanospheres have been prepared as bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) using Ag-Fe-Co hydroxides nanospheres as precursors. XRD and XPS confirm the formation of Ag-Fe-Co2P. SEM and TEM demonstrate that Ag-Fe-Co2P has uniform nanospherical morphology composed of vertical nanosheets, which imply the more exposed active sites for water splitting. For HER measurements in 1 M KOH, Ag-Fe-Co2P with the overpotential of only 41 mV to drive current density of 10 mA cm−2. Remarkably, to achieve current density of 100 mA cm−2 for OER, Ag-Fe-Co2P requires overpotential of 220 mV, which is obviously less than that of Fe-Co2P. Furtherly, when Ag-Fe-Co2P was used as the electrocatalyst for both cathode and anodic, a cell voltage of 1.53 V was able to reach 100 mA cm−2 in 1.0 M KOH, outperforming most reported nonprecious electrocatalysts. The obvious enhancement in activity of Ag-Fe-Co2P for HER and OER derived from its high conductivity, electron-donating ability and augmented accessibility of active sites. Therefore, designing multiple metal doped transition metal phosphides with unique nanostructure may be a promising way for efficient overall water splitting.Download high-res image (112KB)Download full-size image

•Ternary Ni-Fe-V sulfides one-dimensional nanostructures have been fabricated.•Ternary NiFeVS/NF has better activity than binary NiVS, NiFeS and single Ni3S2/NF.•The synergistic effect of mixed metals and 1 D structure is the key for enhanced activity.•Designing mixed metal sulfides may be a new way for excellent electrocatalysts.Designing multiple transition metal sulfides with unique nanostructure may hold promising in highly efficient water electrolysis. Herein, ternary Ni-Fe-V sulfides nanobundles supported on nickel foam (NiFeVS/NF) with one-dimensional structure have been fabricated as efficient electrocatalysts for hydrogen evolution reaction (HER) in alkaline solution. Firstly, a facile electrodeposition realizes homogeneous dispersion of Fe on NF (Fe/NF). Secondly, hydrothermal sulfurization with introduction of V source can obtain ternary NiFeVS/NF. SEM and TEM show that one-dimensional NiFeVS nanobundles are observed to vertically cover on NF, providing abundant active sites with rapid charge transfer. The ternary NiFeVS/NF has superior activity with overpotential to 161 mV at 10 mA cm−2 than binary NiVS/NF, NiFeS/NF and single Ni3S2/NF. The possible synergistic effect of ternary Ni-Fe-V sulfides may lead to enhanced intrinsic activity confirmed by electrochemical active surface area (ECSA) and electrochemical impedance spectroscopy (EIS) data. Further, ternary NiFeVS/NF has robust stability in 10 h water electrolysis with unchanged bundle-like structure. Therefore, the synergistic effect derived from multiple transition metals and unique nanostructure may be the key for excellent electrocatalytic properties.Download high-res image (77KB)Download full-size image

•Three level hierarchical nanostructures of Ni3(VO4)2@NiCo2O4/NF were synthesized.•Ni3(VO4)2@NiCo2O4/NF provide more active sites and faster mass and charge transfer.•Ni3(VO4)2@NiCo2O4/NF demonstrate excellent activity and stability for HER.•Designing hierarchical nanostructures of oxides may be a promising way for HER.Designing mixed transition metal oxides with advantageous nanostructure is efficient access to highly-efficient electrocatalysts for hydrogen evolution reaction (HER) in water electrolysis due to more exposed active sites and synergistic effects in mixed metals. A hierarchically three-level nanostructure combining two types of mixed transition metal oxides on nickel foam (Ni3(VO4)2@NiCo2O4/NF) has been synthesized through a two-step hydrothermal process. The hierarchically three level nanostructure comprises porous nickel foam as robust and conductive substrate in the bottom, vertical and uniform NiCo2O4 nanowires arrays in the intermediate, and top-coated Ni3(VO4)2 nanoparticles in 10 nm. The triple hierarchical nanostructure of Ni3(VO4)2@NiCo2O4/NF is favorable for enlarging surface areas, exposing more active sites, promoting mass and charge transfer and accelerating HER process. Ni3(VO4)2@NiCo2O4/NF electrode can afford a current density of 10 mA cm−2 at a moderate overpotential of 113 mV with robust stability for at least 12 h. It may provide a new strategy to design triple hierarchical nanostructures based on earth-abundant transition metals to achieve large-scale production of hydrogen through water electrolysis in alkaline.Download high-res image (84KB)Download full-size image

Mo2C@NC@MoSx porous nanospheres with sandwich shell have been synthesized for efficient hydrogen evolution in both acidic and alkaline media. Firstly, porous MoO2-Mo2C@NC nanospheres have been obtained with ultrafine Mo2C nanocrystallines as core and ultrathin N-doped carbon (NC) as shell through the carbonization of MoO42--/aniline-pyrrole (MoO42--Polymer) as precursor. Secondly, Mo2C@NC@MoSx porous nanospheres with sandwich shell have been synthesized by hydrothermal sulfurization, which can be confirmed by XPS and HRTEM. The sandwich shell is composed of ultrathin MoSx, NC layer and Mo2C nanocrystallines, which may reduce the strong MoH bonding energy of pure Mo2C and lead to the suitable ΔGH* for HER. In addition, the ultrathin NC can prevent the aggregation of active sites and improve charge transfer rate due to rich N-doping. Mo2C@NC@MoSx exhibits enhanced performance and long-time durability in both acidic and alkaline solution. It requires a low onsetpotential of 119 mV, Tafel slope of 56 mV dec−1 in acidic solution and onsetpotential of 86 mV, Tafel slope of 90 mV dec−1 in alkaline solution. Therefore, designing sandwich nanostructure with better conductivity and optimized MoH bonding energy may be a promising strategy for excellent Mo-based HER electrocatalysts.Download high-res image (383KB)Download full-size image

•Ternary CoS2/MoS2/RGO with CoMoS phase as electrocatalyst for HER was prepared.•CoMoS phase have the metallic nature and highly intrinsic activity for HER.•RGO support ensures good distribution of CoMoS phase and enhances the conductivity.•The introduction of CoMoS and RGO may be a novel strategy for efficient HER of MoS2.CoMoS phase with metallic character plays crucial role on enhancing the activity of MoS2 electrocatalysts for hydrogen evolution reaction (HER). However, only Co atoms located in the edges of MoS2 can create CoMoS phase, so it is a challenge to obtain CoMoS phase with homogeneous distribution limited by the layered MoS2 and doping method of Co. Herein, we reported a simple one-pot hydrothermal method to prepare novel ternary CoS2/MoS2/RGO with CoMoS phase for HER using reduced graphene oxide (RGO) as support. XPS proves the formation of CoMoS phase, implying the enhanced activity for HER. RGO support ensures the well distribution of CoMoS phase and enhances the conductivity of CoS2/MoS2/RGO. Compared to CoS2/RGO, MoS2/RGO and CoS2/MoS2, the obtained CoS2/MoS2/RGO shows superior activity for HER with an onset overpotential of −80 mV (vs. RHE), small Tafel slope of 56 mV dec−1, high exchange current density of 11.4 μA cm−2 and rigid electrochemical durability. The enhanced performances for HER may be ascribed to the formation of CoMoS phase with high activity and the existence of RGO support with good electrical conductivitys in ternary CoS2/MoS2/RGO. Therefore, the introduction of CoMoS phase and RGO into MoS2 could effectively enhance electrocatalytic properties for HER.Download high-res image (133KB)Download full-size image

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.

Many strategies, such as doping metal, designing low-dimensional nanostructures, and enhancing the utilization of active sites based on a conductive support, have been intensively pursued to improve the intrinsic activity of transition metal chalcogenides for the hydrogen evolution reaction (HER). However, integrating all the above-mentioned merits into one electrocatalyst is still a significant challenge. Herein, we have successfully prepared uniform CoMoS/CoMoO4 (CMS) shell–core nanorods, with a diameter of 60 nm and a length of 800 nm, supported on N-doped reduced graphene oxide (NRGO). The obtained CMS/NRGO can combine many advantages, including transition metal doping, one-dimensional nanorods, and the superior conductivity of NRGO, resulting in very promising HER properties and excellent stability. The optimum sulfurization temperature for unsupported CMS nanorods has been explored using uniform CoMoO4 nanorods as a precursor. Although CMS-3 prepared with a sulfurization temperature of 300 °C has been found to possess the optimum activity for the HER, when adopting NRGO as a support, CMS-3/NRGO exhibits an impressive enhancement in HER performances with a low overpotential of 80 mV, a small Tafel slope of 58 mV dec−1, and a large exchange current density of 428 μA cm−2. In addition, the electrocatalytic activity of CMS-3/NRGO shows a negligible delay after 1000 cycles, indicating its robust electrochemical stability in acid electrolyte solution. Therefore, adopting low-temperature sulfurization of one-dimensional metal oxide precursors supported on NRGO may be a promising strategy for obtaining excellent electrocatalysts for the HER.

•WS2/WO3 heterostructure has been synthesized as efficient electrocatalysts for HER.•WS2/WO3 heterostructure improves the intrinsic activity and conductivity of WO3.•Designing heterostructure may be a novel way for efficient HER electrocatalysts.A facile sulfurization process has been utilized to synthesize WS2/WO3 heterostructure using WO3 square nanosheets as support. The thin WO3 nanosheets synthesized by hydrothermal method may provide large surface area with homogeneous dispersion, which is favorable for exposing active sites and fast charge transfer for hydrogen evolution reaction (HER). SEM and TEM images show that the thin square nanosheets of as-prepared WS2/WO3 heterostructure are well kept after sulfurization process. The formation of WS2 on the surface of heterostructured nanosheets may greatly enhance the intrinsic HER activity and conductivity. The electrochemical measurements demonstrate the obviously enhanced HER activity of WS2/WO3 heterostructured nanosheets compared with pure WO3 or bulk WS2, illustrating that the synergistic effect derived from heterostructured WS2/WO3 may be a key to enhance HER performances of WO3. Therefore, designing heterostructure of transition metal sulfides/oxides may be a promising strategy to prepare efficient electrocatalysts for HER.Download high-res image (158KB)Download full-size image

NiSe@NiOOH core–shell hyacinth-like nanostructures supported on nickel foam (NF) have been successfully synthesized by a facile solvothermal selenization and subsequent in situ electrochemical oxidation (ISEO). First, the unique NiSe/NF nanopillar arrays were prepared in N,N-dimethylformamide (DMF) as a precursor template that can provide a large surface area, excellent conductivity, and robust support. Next, amorphous NiOOH covering the surface of NiSe nanopillars was fabricated by ISEO, as confirmed by XPS andEDX spectroscopy. SEM images revealed the hyacinth-like morphology of NiSe@NiOOH/NF with NiOOH as the shell and NiSe as the core. The electrochemical performance of NiSe@NiOOH/NF for the oxygen evolution reaction (OER) was investigated. NiSe@NiOOH/NF demonstrates an obviously enhanced OER activity with much lower overpotential of 332 mV at 50 mA cm–2 compared to other Ni-based electrocatalysts. The low charge-transfer resistance (Rct), large electrochemical double-layer capacitance (Cdl) of electrochemically active surface areas (ECSAs), and excellent long-term stability of NiSe@NiOOH/NF confirm the enhancement of its electrochemical performance for the OER, which can be ascribed to the large amount of active sites derived from the amorphous NiOOH shell and the good conductivity and stability derived from the NiSe core. In addition, the synergistic effect between the NiSe core and NiOOH shell could serve for a highly efficient OER electrocatalyst.Keywords: core−shell structure; NiOOH; NiSe; oxidation; oxygen evolution reaction

•Novel NiSe2 pyramids on oxidized nickel foam were in situ synthesized.•The growth mechanisms of NiSe2 pyramids have been discussed.•NiSe2 pyramids as electrocatalysts for OER showed better activity than NiSe.•The enhanced properties of NiSe2 pyramids for OER were explained in details.In situ grown pyramid structures of nickel diselenides (NiSe2) have been synthesized using oxidized nickel foam (NF (Ox)) as substrate by a facile solvothermal selenization. XRD results show that NiSe phase on NF and NiSe2 phase on NF (Ox) have been obtained after the identical selenization process, respectively. The nanorods morphology of NiSe on NF and pyramid structure of NiSe2 on NF (Ox) have been revealed by SEM images. The different structure and morphology of NiSe/NF compared with NiSe2/NF (Ox) can be ascribed to the oxidation pretreatment of NF which affiliates the formation of ultrathin β-Ni(OH)2 nanosheets on NF. The electrochemical measurements for oxygen evolution reaction (OER) exhibit an enhanced electrocatalytic activity of NiSe2/NF (Ox) with onset potential of 1.54 V (vs. RHE) and small Tafel slope of 96 mV dec−1. Moreover, NiSe2/NF (Ox) possesses lower charge-transfer resistance (Rct) indicating a faster electron transfer rate than NiSe/NF. The excellent stability further confirms the improved elctrocatalytic performance of NiSe2/NF (Ox). We speculate that the high Ni2+ proportion and octahedral structure of NiSe2 may be the keys for excellent electrocatalytic properties for OER.

•Novel MoS2/C-doped MoO2 nanobelts for HER have been successfully synthesized.•MoO2 nanobelts may reduce stacking of MoS2 nanosheets and improve conductivity.•Sulfidizing MoOx-based nanostructures may be an efficient way for electrocatalysts.As efficient electrocatalysts for hydrogen evolution reaction (HER), novel MoS2/carbon-doped molybdenum dioxide (MoS2/C-doped MoO2) nanobelts have been successfully synthesized via sulfuration treatment of the C-doped MoO2 nanobelts. Mo3O10(C6H8N)2•2H2O with one-dimensional structure, high conductivity and large surface area has been used as template to produce C-doped MoO2 nanobelts. After the sulfuration treatment, MoS2 nanosheets vertically grow along the axis direction of conducting C-doped MoO2 nanobelts leading to less stacking of MoS2 nanosheets and improved conductivity. The as-prepared MoS2/C-doped MoO2 nanobelts have been examined by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS). The growth mechanism of MoS2/C-doped MoO2 nanobelts has been discussed. The MoS2/C-doped MoO2 nanobelts exhibit excellent HER activity with the low onset overpotential of 120 mV (vs. RHE) and a small Tafel slope of 55 mV dec−1. The sulfuration treatment of C-doped MoO2 nanobelts could improve the conductivity and decrease the stacking of MoS2 nanosheets, which could provide more exposed active sites and faster rate of electron transfer, thus their electrocatalytic performances for HER have been enhanced drastically. The sulfuration treatment of unique MoOx-based nanostructures may be a potential and efficient strategy to manufacture efficient MoS2-based electrocatalysts for HER.

•The effect of pH on the catalytic activity of MoS2 has been investigated.•Low pH can restrain the growth of (002) plane of MoS2.•Appropriate pH can be used to improve electrocatalytic activity of MoS2.MoS2 with different (002) intensity have been synthesized for hydrogen evolution reaction (HER) through a facile hydrothermal method at different pH value. MoS2 obtained at pH = 2–3 exhibits the smallest intensity ratio between (002) and (100) peaks (I (002)/(100)) and the largest BET surface area, which implying the restrained growth of (002) plane and the decreasing of stacking layers of MoS2 at low pH. Because (002) plane is inert plane for HER, low pH (pH = 2–3) is more suitable for increasing the active sites of MoS2 than high pH. The results of the electrocatalytic HER confirm that MoS2 synthesized at pH = 2–3 has the smaller Tafel slope and the lower resistance than pH = 7–8 or pH = 12–13. The corresponding capacitive properties show similar results that MoS2 obtained at pH = 2–3 has the highest specific capacitance among the different pH, which identified the best dispersion and largest BET surface area of MoS2 at low pH. Therefore, pH has made important impact on the controlling growth of (002) plane and the improved active sites of MoS2.

•Effect of sulfurization degree on crystal phase of NixSy has been investigated.•Altering crystal phase of NixSy can be achieved by changing sulfurization degree.•NiS phase outperforms better activity for OER than other NixSy phases.Relationship between crystal structure of NixSy and sulfurization degree has been investigated via in situ growth NixSy on nickel foam (NixSy/NF) by solvothermal process with thiourea as sulfur source. NixSy with different phases are controlled by mass of thiourea from 1 g, 1.5 g, 2 g, 2.5 g–3 g. XRD shows the presence of NiS2, NiS and Ni3S2 in NixSy. Pure Ni3S2 synthesized by 2.5 g thiourea implies easy access to pristine NixSy phase. Notably, only one sample (synthesized by 2 g of thiourea) contains unique NiS phase with special flake accumulation morphology, owning superior performance for oxygen evolution reaction (OER) by electrochemical measurements. It suggests that NiS phase outperforms other NixSy, probably the best phase for OER in NixSy phases. All data confirm the achievement of altering crystal structure of NixSy by tiny changes of sulfurization degree, which may provide a new strategy to prepare enhanced electrocatalysts for OER.

•Reduced Co3O4 (RCo3O4) was synthesized by a facile NaBH4 reduction treatment.•RCo3O4 has the enhanced OER activity owing the increasing of oxygen vacancies.•Increasing oxygen vacancy provides a new way to prepare efficient electrocatalyst.Novel reduced Co3O4 nanoparticles as electrocatalyst have been synthesized by a facile NaBH4 reduction treatment. Microwave-assisted Co3(CH3COO)5(OH) has been used as precursor to increase the surface area of Co3O4. The as-prepared samples were characterized by X-ray diffraction (XRD), Transmission electron microscope (TEM) and scanning electron microscope (SEM). The original Co3O4 (OCo3O4) nanoparticles have been prepared by annealing of Co3(CH3COO)5(OH). After treatment by NaBH4, OCo3O4 transforms into the reduced Co3O4 (RCo3O4). Electrochemical measurements for oxygen evolution reaction (OER) show that RCo3O4 has more excellent electrocatalytic activity with the lower onset potential of 1.5 V (vs. RHE) and larger current density by over 6.4 times than that of OCo3O4, which may be ascribed to the more oxygen vacancies derived from reduction treatment of NaBH4. Therefore, increasing the number of oxygen vacancies of metal oxides by facile reduction treatment may be a promising way to prepare the efficient electrocatalysts for OER.

Novel three dimensional (3D) electrodeposited Co–P nanosphere arrays on FTO (Co–P/FTO) have been successfully prepared as efficient bifunctional electrocatalysts for overall water splitting in alkaline media. The morphologies and properties of the 3D Co–P nanosphere arrays can be controlled by the electrolyte concentration. At the middle concentration, Co–P nanospheres have a more homogeneous size and array distribution and a rough surface, implying a larger surface area and an increased number of active sites for water splitting. The electrochemical measurements confirm the best electrocatalytic performances of Co–P/FTO at the middle concentration. They show excellent activity, with an overpotential of 125 mV for HER, 420 mV for OER and Tafel slopes of 54 mV dec−1 and 83 mV dec−1, respectively. The fabricated bifunctional systems of Co–P/Co–P can efficiently catalyse HER and OER at the same time, solving the incompatible problem of different media between HER and OER. Therefore, controlling the synthesis of 3D Co–P/FTO nanosphere arrays through electrodeposition can provide a facile way for the bifunctional electrocatalysis of both HER and OER.

•DMF–H2O mixture solution has been used to electrodeposit MoSx thin film on FTO.•The effect of different ratio of DMF to H2O on MoSx film has been investigated.•The appropriate DMF–H2O mixture may increase the active sites of MoSx for HER.•The enhanced activity may be attributed to the polarity difference of DMF–H2O.Nanostructured plate-like island molybdenum sulfides (MoSx) film has been synthesized successfully through a facile one-step electrodeposition method from the mixture solution of N, N-dimethyl-formamide (DMF) and H2O. The effect of different ratio of DMF to H2O on the MoSx film has been investigated. Scanning electron microscopy (SEM) shows that when the ratio of DMF to H2O is 1:20, the as-prepared MoSx film exhibits maximum island structures, exposing most edges and rims. The electrochemical measurements confirm that MoSx film deposited in the mixed solution of VDMF: VH2O = 1:20 has the best hydrogen evolution reaction (HER) activity, with an overpotential of 80 mV. The improved HER activity may be attributed to the polarity difference of DMF-H2O and morphology directing of DMF. Therefore, using DMF–H2O mixture solution with the appropriate ratio may be a facile and effective way to prepare the excellent electrocatalysts for HER.Download high-res image (206KB)Download full-size image

•Novel NiSe-NixSy nanocubes supported on nickel foam have been prepared.•NiSe nanopillar arrays are helpful for the synthesis of NiSe-NixSy nanocubes.•NiSe-NixSy nanocubes have superior catalytic activity for hydrazine oxidation.Novel NiSe-NixSy nanocubes supported on nickel foam (NF) have been prepared by a facile two-step solvothermal process. Firstly, uniform NiSe nanopillar arrays on NF (NiSe/NF) have been achieved using N, N-dimethyl-formamide (DMF) as solvent. Secondly, a sulfuration process has been conducted to synthesize NiSe-NixSy nanocubes on NF (NiSe-NixSy/NF) using NiSe/NF nanopillars as substrate, which may be helpful for the formation and distribution of NiSe-NixSy. XRD indicates the typical crystalline structure of NiSe/NF and NiSe-NixSy/NF without other impurities. The uniformly distributed NiSe nanopillar arrays on NF have been revealed in SEM images. And the unique nanocubes morphology of NiSe-NixSy/NF with the diameter of about 300 nm has been also exhibited. For hydrazine electroxidation, the performances of the as-prepared NiSe/NF and NiSe-NixSy/NF have been investigated as working electrodes, respectively. The electrochemical measurements show a more negative oxidation potential in NiSe-NixSy/NF than the one in NiSe/NF, indicating the excellent activity of novel NiSe-NixSy nanocubes. The corresponding electroxidation mechanisms have been discussed.

•Ag@C/SnO2 hollow spheres for hydrazine electro-oxidation have been synthesized.•C/SnO2 as support provides the synergistic effect of C, SnO2 and hollow structure.•Ag nanoparticles have uniform size and homogeneous dispersion on hollow C/SnO2.•Ternary Ag@C/SnO2 has excellent activity for hydrazine electro-oxidation.Highly dispersed ternary Ag/C/SnO2 hollow spheres have been synthesized for hydrazine electro-oxidation. The sharp XRD peak of Ag shows that Ag nanoparticles of Ag/C/SnO2 have good crystallinity. SEM shows that Ag/C/SnO2 maintained a good spherical morphology with the diameter of about 300 nm. TEM shows that Ag nanoparticles with the average diameter of about 10 nm disperse well on C/SnO2. The advantages derived from C/SnO2 support including carbon, SnO2 and hollow structures may facilitate the dispersion of Ag and the close interaction of Ag with support. The results of hydrazine electro-oxidation show that C/SnO2 has no activity for hydrazine electro-oxidation while ternary Ag/C/SnO2 exhibits excellent electrocatalytic activity with the negative oxidation peak at −0.25 V (vs. SCE) and the high peak current of 473 μA at the scan rate of 20 mV s−1. The enhancement of activity may be attributed to the small size, homogeneous dispersion of Ag and the synergistic effect derived from Ag/C/SnO2 hollow spheres.

•Ultrathin MoS2-coated carbon nanospheres were prepared by a solvothermal method.•The acid-treated CNS improved the uniform coating and dispersion of MoS2.•MoS2/ATCNS have better electrocatalytic activity for hydrogen evolution reaction.The ultrathin MoS2-coated carbon nanospheres have been prepared by a facile solvothermal method using acid-treated carbon nanospheres (ATCNS) as support. TEM images show that ATCNS are homogeneously coated by MoS2 layers. The thickness of MoS2 coating on ATCNS is about 5 nm. XRD data confirm that the few stacking layers and low crystalline of MoS2 nanosheets on ATCNS, which could provide more active sites for hydrogen evolution reaction (HER). The electrocatalytic activity and stability of MoS2/ATCNS for HER have been investigated. The results show that MoS2/ATCNS has better electrocatalytic activity for HER than pure MoS2. It can be speculated that ATCNS play crucial role in increasing conductivity and electrocatalytic activity for HER of MoS2/ATCNS. The nanostructure of ultrathin MoS2 coating on ATCNS is a promising electrocatalyts for HER.

•The highly dispersed Ag/TiO2 spheres have been synthesized by a one-pot facile method.•Ag nanoparticles with uniform diameter about 20 nm homogeneously disperse and strongly anchor on TiO2 spheres.•The nanoporous structure via in situ hydrolysis of TiO2 glycolate precursor is the key for the dispersion, size controlling and junction of Ag nanoparticles.Highly dispersed Ag/TiO2 spheres have been synthesized via in situ hydrolysis of TiO2 glycolate spheres as precursors during a one-pot facile reduction. The structure and morphology of Ag/TiO2 spheres have been characterized by XRD, TEM and SEM. XRD show that Ag nanoparticles on TiO2 spheres have good crystalline and high purity. HRTEM prove that Ag nanoparticles with the uniform diameter of about 20 nm are well dispersed and strongly attached onto TiO2 spheres. The elemental mapping images of Ag/TiO2 show good distribution of Ag on the surface of TiO2 spheres. The enhanced electrocatalytic activity of Ag/TiO2 for hydrazine has been observed, indicating that the small size and good dispersion of Ag nanoparticles may be the key to improve the electrocatalytic activity of Ag/TiO2. The formation mechanisms of Ag/TiO2 spheres using TiO2 glycolate spheres as precursors have also been discussed.

•MoS2/RGO was prepared by one-pot hydrothermal route in different solvents.•The good structure of MoS2/RGO can be obtained in pure DMF.•The obtained MoS2/RGO in DMF exhibits the better catalytic activity for HER.•DMF are helpful on dispersing RGO and MoS42-.MoS2/RGO nanocomposites as highly effective electrocatalysts for hydrogen evolution reaction (HER) have been synthesized by one-pot solvothermal route using dimethyl-formamide (DMF) as solvent. For comparison, MoS2/RGO has also been prepared in pure H2O and H2O–DMF mixture. The good dispersion of MoS2 nanoparticles on the surface of MoS2/RGO obtained in DMF can be seen from TEM and SEM images, which implying the more active sites for HER. The electrocatalytic measurements confirm that the obtained MoS2/RGO in DMF exhibits the smaller onset overpotential of 130 mV and lower tafel slop of 42 mV dec−1, which indicating the better electrocatalytic activity than MoS2/RGO in H2O or H2O–DMF mixture. It may be attributed to the better role of DMF on dispersing RGO and controlling the growth of MoS2. Therefore, DMF may be the more suitable solvent for preparing excellent electrocatalysts for HER than H2O.

•A facile exfoliation process in DMF has been used to prepare E-WS2 for HER.•E-WS2 shows the better electrocatalytic activity than bulk WS2.•DMF provides a promising alternative for enhancing exfoliation of 2D materials.WS2 nanosheets (WS2 NSs) as electrocatalysts for hydrogen evolution reaction (HER) have been prepared based on liquid exfoliation in dimethyl-formamide (DMF) via a direct dispersion and ultrasonication method. X-ray diffraction (XRD) shows the decreasing crystalline of the exfoliated WS2 (E-WS2). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that the as prepared E-WS2 consists of a few two-dimensional nanosheets, with large wrinkles on the surface. Electrochemical measurements show an excellent activity and stability of the E-WS2, with a low overpotential of 80 mV and high current density (10 mA cm−2, at η = 205 mV), which indicates that through the process of exfoliation in DMF, both the dispersion and the amount of active sites have been improved greatly. Therefore, DMF is a promising alternative for exfoliating two-dimensional nanomaterials for highly efficient HER electrocatalysts.

Novel hollow β-MnO2 polyhedral nanorods have been successfully synthesized by a simple hydrothermal process. The morphology and structure of the samples obtained at different reaction time have been characterized by XRD, TEM and SEM. The results indicate that β-MnO2 nanorods prepared at reaction time 12 h have hollow, polyhedral structure and uniform size with the length of about 2–3 μm. At longer reaction time, the shells of the hollow β-MnO2 polyhedral nanorods were broken. After analyzing the above results, it's obvious that the reaction time has a great effect on the structure and morphology of the products. The morphology evolution of MnO2 products demonstrates that the growth mechanism of hollow β-MnO2 polyhedral nanorods could be assigned to the cooperation of oriented attachment and Ostwald ripening mechanism.Graphical abstractHighlights► Hollow polyhedra of β-MnO2 were hydrothermally synthesized. ► The formation process of hollow β-MnO2 polyhedra has been investigated. ► The growth mechanism of hollow β-MnO2 polyhedra was discussed for the first time. ► Hollow β-MnO2 polyhedra form by oriented attachment and Ostwald ripening mechanism.

•The vertical WS2 nanosheets arrays on oxidized carbon fiber have been prepared.•The oxygen functional groups on oCF may be the key for vertical growth of WS2.•WS2/oCF may expose the more active sites and shorten charge transfer distance.•WS2/oCF has the enhanced catalytic activity and stability for HER.Oxidized carbon fiber (oCF) as support successfully realizes the vertical growth of uniform WS2 nanosheets arrays for efficient hydrogen evolution reaction (HER) via a facile hydrothermal process. Thanks to oxygen functional groups on oCF, vertical WS2 nanosheets structures have grown more easily on oCF, which can provide better dispersion, short charge transfer distance and more exposed active sites for HER in comparison with bulk WS2 and WS2 nanosheets on bare carbon fiber (CF) fabricated at the same condition. The electrochemical measurements confirmed that WS2/oCF possesses better HER activity than bulk WS2 and WS2/CF. Especially, the 10-h stability with unchanged vertical WS2 nanosheets morphology further demonstrate the positive effect of oxygen functional groups on the enhanced vertical structure and close combination between WS2 and oCF. It may offer a facile way to realize more exposed active sites from stable electrocatalyst hybrids of transition metal sulfides by surface oxidization of carbon supports.

•(001) plane of MoS2/CNs has been produced using mixture solvent of DMF and H2O.•Solvothermal temperature can affect the growth of (001) plane of MoS2/CNs.•MoS2/CNs with the largest (001) distance has the best HER activity.•Tuning (001) plane of MoS2 can be realized by changing solvent and temperature.MoS2 with (001) plane supported on carbon nanospheres (CNs) has been successfully synthesized by a facile solvothermal process. The effect of solvent and temperature on the growth of MoS2/CNs has been investigated, respectively. XRD results show that (001) plane of MoS2 was synthesized when using mixture solvent of N, N-dimethylformamide (DMF) and H2O. The electrochemical measurements show that MoS2/CNs samples with (001) plane have the enhanced hydrogen evolution reaction (HER) activity than MoS2/CNs without (001) plane. The effect of solvothermal temperature on (001) plane of MoS2 has been investigated. XRD results demonstrate the different interlayer spacing of 9.7 Å, 9.8 Å and 9.4 Å of (001) plane corresponding to 160, 200 and 240 °C, respectively, which may affect the electronic structure of MoS2 and bring different HER activity. TEM reveals the good interface adhesion between MoS2 and CNs at 200 °C, which may provide better conductivity and faster electron transfer rate. MoS2/CNs with the largest (001) distance obtained at 200 °C has the better HER activity than other samples synthesized at other temperatures. Therefore, tuning (001) plane of MoS2 under optimized conditions including solvent and temperature may be a facile way to prepare excellent HER electrocatalysts.

•S22− in MoS2 has been proved to be key factor for improving HER activity.•γ-Al2O3 template has been used to prepare MoS2 with S22− for enhanced HER.•The calcination temperature has important effect on the amount of S22− in MoS2.Owing to the real active sites for hydrogen evolution reaction (HER) located on the rims and edges, it is critical to expose maximum active sites by limiting the stacking of MoS2. On the other hand, the existence of disulfide ligand (S22−) in MoS2 has been proved to be another key factor determining the rate of HER. In this work, a facile γ-Al2O3 template-assisted synthesis has been conducted to prepare highly dispersed MoS2 with appropriate amount of terminal disulfide ligand (S22−) as efficient electrocatalysts for HER. γ-Al2O3 template can facilitate the highly-dispersed growth of MoS2 with small size, which can provide vast active sites for HER. The effect of the different calcination temperature at 300, 350, 400, 450 and 500 °C on the crystallinity and amount of terminal S22− of MoS2 samples has been investigated. After removing γ-Al2O3, MoS2-400 electrocatalyst has the best HER electrocatalystic performance with a low onset overpotential of 120 mV and small Tafel slope of 84 mV dec−1, maybe due to the moderate crystallinity of MoS2 and amount of terminal S22−. Therefore, γ-Al2O3 template synthesis at the optimum calcination temperature may be a promising method for transition metal sulfides as efficient electrocatalysts for HER.

•Liquid crystal template (LCT) assisted electrodeposition is used to prepare MoSx.•MoSx (LCT) has uniform disk bulges and high ratio of S/Mo compared to MoSx (H2O).•MoSx (LCT) exhibits a very small overpotential nearly close to 0 V of Pt/C.•LCT assisted electrodeposition may be a novel way to activate MoSx for HER.MoSx as a promising electrocatalyst for hydrogen evolution reaction (HER) has the active sites of rims and edges and inert sites of basal plane. A strategy to activate MoSx is to produce more unsaturated S or S vacancies on the inert plane by designing unique nanostructure. Herein, we report a facile electrodeposition assisted by liquid crystal template (LCT) to prepare MoSx film on ITO substrate (MoSx (LCT)). XPS confirms the state of Mo and S of the obtained MoSx film. SEM shows that there are many uniform disk bulges with the diameter of about 100 nm on the surface of MoSx (LCT), which may provide more rims and edges of MoSx. While the electrodeposited MoSx film in water (MoSx (H2O)) is a smooth plane structure. The electrochemical measurements show MoSx (LCT) exhibits an ultrahigh activity for HER with the small onsetpotential very close to that of Pt/C, which may be due to not only the uniform disk bulges but also the more unsaturated S atoms of MoSx (LCT). The electrodeposition assisted by LCT may be a novel strategy to prepare novel nanostructure and activate MoSx-based electrocatalysts for HER.

•Ni–P/MoSx was electrodeposited as electrocatalysts for HER in alkaline solution.•The hybrid Ni–P/MoSx can expose more edges and rims as more active sites for HER.•The improved activity may be due to the synergistic effect between Ni–P and MoSx.The hybrid Ni–P/MoSx film as efficient electrocatalyst for hydrogen evolution reaction (HER) in alkaline media has been successfully synthesized through a facile two-step electrodeposition process. The obtained hybrid Ni–P/MoSx can possess the improved activity and conductivity owing to the incorporation of Ni–P. Scanning electron microscopy (SEM) shows that the edge sites of MoSx have not been completely covered by Ni–P, exposing more active sites for HER. The better dispersion of Ni–P on the surface of MoSx has been obtained, implying the enhancement of the conductivity of Ni–P/MoSx film. The electrochemical measurements confirm that Ni–P/MoSx film has the better activity for HER than pure Ni–P and MoSx, with an overpotential of 140 mV (at j = 10 mA cm−2) and a low Tafel slope of 64 mV dec−1. The improved HER activity may be attributed to the excellent conductivity of Ni–P and the synergistic effect derived from hybrid Ni–P/MoSx film.

•Different CoxSy/WS2/CC samples have been fabricated via a facile hydrothermal process.•The molar ratio of CoxSy/WS2 has been investigated in detail.•The molar ratio of W/Co = 1/3 has been proved to have unique spherical nanostructure.•CoxSy/WS2/CC-3 exhibits the best HER activity and excellent stability.To maximum the activity of transition metal sulfides for hydrogen evolution reaction (HER), two strategies usually have been adopted including designing unique nanostructures and integrating other metal element. Herein, CoxSy/WS2 nanosheets supported on carbon cloth (CoxSy/WS2/CC) have been fabricated via a facile hydrothermal process. The cross-linked structures composed of CoxSy/WS2 nanosheets uniformly cover on the surface of CC, which may expose abundant active sites for HER and accelerate charge transfer rate. The molar ratio of CoxSy-incorporating has been investigated in detail. The molar ratio of W/Co = 1/3 (noted as CoxSy/WS2/CC-3) has been proved to have unique spherical CoxSy/WS2 nanostructure, which may further expose more active sites for HER. Electrochemical measurements demonstrate that CoxSy-incorporating can enhance HER activity and conductivity compared with WS2/CC. In addition, CoxSy/WS2/CC-3 exhibits the best HER activity, smallest charge transfer resistance and excellent stability than the counterparts, implying that the degree of CoxSy-incorporating may impact the HER activity of WS2. The mechanisms of CoxSy-incorporating on enhancing HER activity of WS2 have been discussed. It may offer a promising way to design transition metal sulfides-based electrocatalysts for HER by non-precious metal incorporating.Figure optionsDownload full-size imageDownload high-quality image (222 K)Download as PowerPoint slide

•Binary pyrite-type Ni0.5Fe0.5Se2/CFC was prepared by a facile two-step process.•The effect of Ni/Fe (NixFe1-xSe2 x = 0, 0.2, 0.5, 0.8, 1) on OER was investigated.•Ni0.5Fe0.5Se2/CFC (x = 0.5) possesses the better electrocatalytic activity for OER.•The enhanced activity may be attributed to binary Ni0.5Fe0.5Se2 and CFC support.Pyrite-type binary nickel iron diselenides (Ni0.5Fe0.5Se2) supported on carbon fiber cloth (CFC) as electrocatalysts for oxygen evolution reaction (OER) have been prepared by a facile two-step process. Firstly, binary Ni0.5Fe0.5 hydroxide nanosheets have been electrodeposited on CFC. Secondly, a solvothermal selenization process has been used to convert Ni0.5Fe0.5/CFC into Ni0.5Fe0.5Se2/CFC. XRD shows that Ni0.5Fe0.5Se2 on CFC has the typically octahedral crystalline. XPS proves the existence and valence of Ni, Fe and Se. SEM images show that Ni0.5Fe0.5Se2 has novel pyrite-type octahedral morphology with uniform size and good dispersion on the surface of CFC. SEM elemental mapping images confirm the good distribution of Ni, Fe, Se element on CFC. TEM and SAED provide the clear diffraction rings of octahedral Ni0.5Fe0.5Se2, which is consistent with the results of XRD. Furtherly, the effect of different ratio of Ni/Fe (NixFe1-xSe2 x = 0, 0.2, 0.5, 0.8, 1) on OER performances has been systematically investigated. The electrochemical measurements results show that Ni0.5Fe0.5Se2/CFC (x = 0.5) possesses the better electrocatalytic activity with the lower overpotential, Tafel slope and long-term stability than other samples. The enhanced activity of Ni0.5Fe0.5Se2/CFC may be attributed to the intrinsic activity of binary Ni0.5Fe0.5Se2 and faster electron transfer rate derived from CFC support.Figure optionsDownload full-size imageDownload high-quality image (121 K)Download as PowerPoint slide

Many strategies, such as doping metal, designing low-dimensional nanostructures, and enhancing the utilization of active sites based on a conductive support, have been intensively pursued to improve the intrinsic activity of transition metal chalcogenides for the hydrogen evolution reaction (HER). However, integrating all the above-mentioned merits into one electrocatalyst is still a significant challenge. Herein, we have successfully prepared uniform CoMoS/CoMoO4 (CMS) shell–core nanorods, with a diameter of 60 nm and a length of 800 nm, supported on N-doped reduced graphene oxide (NRGO). The obtained CMS/NRGO can combine many advantages, including transition metal doping, one-dimensional nanorods, and the superior conductivity of NRGO, resulting in very promising HER properties and excellent stability. The optimum sulfurization temperature for unsupported CMS nanorods has been explored using uniform CoMoO4 nanorods as a precursor. Although CMS-3 prepared with a sulfurization temperature of 300 °C has been found to possess the optimum activity for the HER, when adopting NRGO as a support, CMS-3/NRGO exhibits an impressive enhancement in HER performances with a low overpotential of 80 mV, a small Tafel slope of 58 mV dec−1, and a large exchange current density of 428 μA cm−2. In addition, the electrocatalytic activity of CMS-3/NRGO shows a negligible delay after 1000 cycles, indicating its robust electrochemical stability in acid electrolyte solution. Therefore, adopting low-temperature sulfurization of one-dimensional metal oxide precursors supported on NRGO may be a promising strategy for obtaining excellent electrocatalysts for the HER.