1 HighlightsFe3O4 nanosheet arrays were successfully assembled on one-dimensional Fe wire by a simple one-step oxidization treatment.The Fe@Fe3O4 electrode displays a high specific capacitance of 20.8 mF cm−1 at 10 mV s−1.A wire-shaped supercapacitor (WSSC) based on Fe@Fe3O4 was assembled, and it exhibited a high energy density (9 µWh cm−2 at 532.7 µW cm−2) and good stability.2 IntroductionNowadays, the increasing demand for portable electronic devices in modern industry requires compatible flexible, lightweight and even wearable miniature energy storage system [1, 2, 3]. Therefore, due to the inherent characteristics of roll-up and micrometer size, one-dimensional (1D) wire-shaped and fiber-shaped supercapacitors (SCs) are being identified as one of the most promising miniature energy storage systems for these portable electronic devices [1]. Compared with the typical two-dimensional (2D) sandwich-structured SCs [4, 5, 6, 7], 1D SCs possess many versatile advantages such as smaller size and higher bendability and also can be converted into many other conceivable model or even woven into textile for unique electronic devices in practical applications [8, 9, 10]. Recently, high-performance wire- or fiber-shaped SCs have been extensively explored based on carbon/CNT (carbon nanotube) fibers [9, 11, 12], Cu wire and Ti wire [13, 14]. However, the complicated synthesized procedure and relatively high cost, as well as low energy density values, hamper their wide applications.Note that iron-based materials have received hugely interest and have been widely used as electrode material for SCs [15, 16, 17, 18]. In particular, among the ordinary electrode materials (nickel, cobalt, manganese, iron and molybdenum), iron is of higher abundance and lower price. In addition, iron oxides have received growing attention due to their suitable negative working window for aqueous supercapacitors [15, 19, 20]. Thus, developing efficient iron-based material for SCs should be highly economically desirable. So far, various iron-based materials, including Fe2O3 and Fe3O4, exhibit a charming electrochemical performance for SCs [15, 16, 18, 21, 22, 23, 24, 25, 26]. For instance, the hollow and porous Fe2O3, which was derived from industrial mill scale, delivers a high capacitance value of 346 F g−1 with outstanding cycling property (88% retention after 5000 cycles) [21]. In addition, Yang and co-authors [27], for the first time, synthesized Fe3O4 nanoparticles, which showed good capacitive property, including high specific capacitance (207.7 F g−1), prominent rate capability and superior cycling stability (100% capacitance retention after 2000 cycles). Nevertheless, to the best of our knowledge, a simple and effective strategy for the preparation of iron-based material remains a great challenge.Here, novel Fe3O4 nanosheet arrays directly supported on Fe wire (Fe@Fe3O4) were efficiently synthesized as electrode for SCs. The purpose of designing such Fe@Fe3O4 electrode material can be summarized as follows: (1) Fe wire is earth-abundant, low cost and high conductivity and suitable as a supporting substrate for supercapacitor electrodes; (2) Fe wire, as source and substrate, has intimate contact with Fe3O4 sheet, which will promote electron interactions between the Fe3O4 and Fe substrate and in turn improve the electrochemical property; and (3) by applying Fe wire as substrate, the wire-shaped SCs would be easily fabricated. The electrochemical properties were measured. Simultaneously, a flexible all-solid-state asymmetric wire-shaped SCs were also assembled and its energy density as well as cycling performance was investigated.3 Experimental Section3.1 Preparation of Fe@Fe3O4To prepare Fe3O4 nanosheets on Fe substrates, pure Fe wire (99.5% purity) with a diameter of 0.5 mm and a length of 8 cm was polished with sandpaper (360 grits), rinsed with distilled water and dried. The Fe wire was then immersed into a 0.1 M KCl solution. The solution was adjusted to pH ≈ 3 by adding 0.1 M HCl and heated to around 70 °C by a hotplate. After pure oxygen bubbles introduced to the solution for 30 min at the flow rate of 150 sccm, the Fe wire was taken out and immersed into 50 mL of distilled water for about 1 h and then dried in N2 environment. The obtained production was named as Fe@Fe3O4-30. For comparison, pure oxygen bubbles introduced to the solution with different reaction time (20 and 40 min) also were prepared, named as Fe@Fe3O4-20 and Fe@Fe3O4-40, respectively.3.2 Fabrication of Wire-Shaped Supercapacitor (WSSC)The WSSC was fabricated as illustrated in Scheme 1 with the Fe@Fe3O4 as the negative electrode and the CF@ MnO2 (MnO2 on carbon fiber) as the positive electrode. The detailed synthesis and properties of CF@MnO2 electrode material are shown in Support Information of Figs. S1, S2 and S3. The PVA-LiCl (PVA, polyvinyl alcohol) gel electrolyte was prepared by dissolving 1 g PVA into 20 mL of 5.0 M LiCl solution at 85 °C under stirring until the solution became clear. The Fe@Fe3O4 and CF@MnO2 cathodes were soaked in the hot gel electrolyte (50–60 °C) for 10 min to allow the electrolyte diffuse into their porous structures and then were carefully entangled with each other. The assembled device was further heated at 35 °C for 12 h to remove excess water in the electrolyte. The specific capacitance is about 3.0 cm, which was calculated based on the length of the total device. The calculation process is shown in Support Information in detail.Open image in new windowScheme 1Schematic illustration for the fabrication of WSSCs3.3 Morphology and Structure CharacterizationThe morphology and structure of the samples were characterized using a field-emission scanning electron microscopic (FESEM, Model JSM-7600F), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (JEOL JEM-20100). Powder X-ray diffraction (XRD) patterns of the samples were recorded with a Bruker D8 Advance powder X-ray diffractometer with Cu Kα (λ = 0.15406 nm) radiation. Raman spectra were recorded on a RENISHAW in via instrument with an Ar laser source of 488 nm in a macroscopic configuration. X-ray photoelectron spectroscopic (XPS) measurements were taken using a PHI X-tool instrument (Ulvac-Phi).3.4 Electrochemical MeasurementsThe electrochemical performances were measured on an electrochemical workstation (CHI 660e, CH Instruments Inc., Shanghai) using a three-electrode mode in 3.0 M LiCl aqueous solution. The as-prepared Fe@Fe3O4 or CF@MnO2, a platinum electrode and a saturated calomel electrode (SCE) were used as the working electrode, counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) tests were done between −0.65 and −1.15 V for Fe@Fe3O4 electrode, 0 and 1.0 V for CF@MnO2 (vs. SCE) at different scan rates, respectively. The electrochemical impedance spectroscopy (EIS) measurements were taken in the frequency range from 0.01 Hz to 100 kHz.4 Results and Discussion4.1 Fe@Fe3O4 Negative Electrode MaterialsThe SEM images of the Fe@Fe3O4-30 are shown in Fig. 1. In Fig. 1a, b, one can see that a thin layer of Fe3O4 has been formed and uniformly decorated on the Fe wire surface after oxidizing treatment. The peeling part on Fe wire is due to the artificial sanding process. The connected nanosheet architecture of Fe3O4 can be evidently observed in the high-magnification SEM images displayed in Fig. 1c, d. The as-formed connected nanosheet structure leads to abundant open spaces, which can provide more active surface sites for effective penetration of the electrolyte and accordingly enhance capacitive property. The comparison morphologies of other two samples of Fe@Fe3O4-20 and Fe@Fe3O4-40 are shown in Fig. S4. It was observed that longer oxidation time (40 min) would cause the nanosheets array structure breakup. Besides, the microstructure of the as-prepared Fe3O4 (scratched from Fe@Fe3O4) was further investigated by TEM (see Fig. 2a), which also shows the nanosheet structure. A lattice fringe spacing of 0.253 nm in the HRTEM image (Fig. 2b) is ascribed to the (311) plane of Fe3O4. Simultaneously, Fig. 2c displays the selected area electron diffraction (SAED) pattern of Fe3O4. The corresponding diffraction rings attribute to the lattice planes (220), (311) and (400) of Fe3O4, which is in good agreement with the following XRD pattern.Open image in new windowFig. 1SEM images of Fe@Fe3O4 at different magnificationsOpen image in new windowFig. 2a TEM and b HRTEM images of the Fe3O4. c The SAED pattern of the Fe3O4 (carefully scratched from Fe@Fe3O4)Figure 3a shows the XRD patterns of the Fe and Fe@Fe3O4-30. For the Fe wire substrate, two typical peaks can be clearly seen at 2θ = 44.7° and 65.0°, corresponding to the diffraction patterns of metallic iron (JCPDS No. 06-0696) [28]. After oxidization treatment in acidic solution, except for characteristic peaks of Fe wire, additional peaks appeared at 30.2°, 35.6°, 43.2°, 57.1°, and 62.7° agree well with the (220), (311), (400), (511), and (440) planes of Fe3O4 (JCPDS No. 75-0033), respectively, confirming the formation of Fe3O4 [29, 30, 31]. No additional peaks of other phases have been detected, indicating high purity and good crystallinity of the obtained Fe3O4. In addition, Raman spectra of the Fe3O4 nanosheets are shown in Fig. S5. The fundamental Raman scattering peaks were observed at 540 and 670 cm−1, corresponding to the T2g and A1g vibration modes, respectively [32, 33, 34]. The T2g is attributed to asymmetric stretch of Fe and O, and the A1g is attributed to symmetric stretch of oxygen atoms along Fe–O bonds.Open image in new windowFig. 3a XRD patterns of Fe and Fe@Fe3O4. b XPS fully scanned spectra, high-resolution XPS spectrum of c Fe 2p and d O 1 s of Fe3O4 scratched from Fe@Fe3O4The XPS was further employed to investigate the composition and valence states of the Fe3O4 gently scratched from the Fe wire. The full XPS spectrum of the Fe3O4 reveals the presence of Fe and O elements along with a small quantity of C element (Fig. 3b). Moreover, the Fe spectrum is depicted in Fig. 3c, and two dominant peaks located at 710.5 and 723.8 eV are in good accordance with Fe 2p3/2 and Fe 2p1/2 spin orbit peaks accompanied by their satellite peaks between 717.2 and 731.2 eV, respectively, which are again consistent with the standard Fe3O4 XPS spectrum [22, 35, 36]. Furthermore, the O 1s spectrum could be deconvoluted into two peaks at 530.3 and 531.8 eV, which results from the oxygen bonds of Fe–O and H–O, as shown in Fig. 3d.The electrochemical properties of as-prepared samples were studied by CV in a typical three-electrode system in 3.0 M LiCl electrolyte. The morphologies, XRD and Fe3O4 content of Fe@Fe3O4 oxidized in different time are shown in Fig. S4, S6 and Table S1. One can see that the nanosheet array structure breaks up under longer oxidation time of 40 min (Fig. S4), and the capacitive performances are reduced due to the poor electron transportation. In addition, it is easy to see that with the increase in reaction time from 0 to 30 min, the content of Fe3O4 is increased, whereas the content of Fe3O4 is decreased when the reaction time is over 40 min. The reason may be that the as-formed Fe3O4 is easy to fall out from Fe substrate, as shown in Fig. S7.As expected, the Fe@Fe3O4-30 electrode in Fig. 4a distinctly presents better capacitive property than pure Fe, Fe@Fe3O4-20 and Fe@Fe3O4-40. In the following section, we mainly discuss the electrochemical performance of Fe@Fe3O4-30 electrode material. The CV curves of Fe@Fe3O4-30 electrode at various scan rates of 10–200 mV s−1 are shown in Fig. 4b and quasi-rectangular shape is inherited even at a very high scan rate of 200 mV s−1, indicating excellent fast electron-transfer characteristics. This was further supported by the low resistance value Rct of 1.2 Ω (Fig. S8). The quasi-rectangular CV shape without any redox peaks indicates a double-layer capacitive behavior [24, 27]. Figure 4c summarizes the specific capacitance from CV tests with different scan rates. The high specific capacitance of 20.8 mF cm−1 is obtained at the scan rate of 10 mV s−1. To further evaluate the electrochemical properties of the as-prepared Fe@Fe3O4-30 electrode, galvanostatic charge–discharge (GCD) tests were performed. The GCD curves (Fig. 4d) at different current (0.5–2.4 mA) display a nearly triangular shape, implying a good electrochemical reversibility. The specific capacitance of the Fe@Fe3O4-30 electrode can also be calculated from the GCD curves (Fig. 4e) and is 12, 8.0, 6.6, 5.8, 4.5, and 4.2 mF cm−1 at 0.6, 0.9, 1.2, 1.5, 2.1, and 2.4 mA, respectively. With the increasing current, the specific capacitance decreases which is similar to the foregoing CV results. In addition, prominent long-term stability is a most important characteristic for state-of-the-art electrode material. The cycling property of the Fe@Fe3O4-30 electrode was tested by continuous GCD curves in Fig. 4f. As expected, the Fe@Fe3O4-30 electrode exhibits a very excellent stability with a small loss of capacitance value (only 8.3% loss) after 2500 cycles. The specific capacitance and stability of Fe@Fe3O4-20 and Fe@Fe3O4-40 are also investigated in Fig. S9. Significantly, the Fe@Fe3O4-30 electrode maintains the nanosheet structures after cycle tests (See SEM image in Fig. S10).Open image in new windowFig. 4a CV curves of Fe, Fe@Fe3O4-20, Fe@Fe3O4-30 and Fe@Fe3O4-40 at the scan rate of 50 mV s−1 in 3.0 M LiCl. b CV curves of the Fe@Fe3O4-30 electrode at different scan rates. c Specific capacitances of the Fe@Fe3O4-30 electrode as a function of scan rate. d GCD curves of the Fe@Fe3O4-30 electrode at different current densities. e Specific capacitances of Fe@Fe3O4-30 as a function of current. f Cycling stability of the Fe@Fe3O4-30 electrode at a current of 0.9 mA. Inset is the last 10 charge/discharge profile of Fe@Fe3O4-30The high performance may be attributed to the following factors: (1) Highly conductive Fe wire as a core was advantageous to the quick transfer of electron; (2) Fe3O4 sheets were in situ synthesized on Fe substrate and possessed intimate contact with Fe wire, which can promote electron interactions between the Fe3O4 and Fe substrate to improve the electrochemical property; and (3) Compared with the SEM images shown in Fig. 1 and S4, the connected nanosheet architecture of Fe@Fe3O4-30 was evidently observed. The as-formed connected nanosheet structure leads to abundant open spaces, which can provide more active surface sites for effective penetration of the electrolyte and accordingly enhance capacitive property. Thus, we think that the enhanced property results from good conductivity of Fe wires, intimate contact between Fe wire and Fe3O4, and the unique nanosheet architecture.4.2 Electrochemical Performance of the WSSCThe WSSC was assembled by using the Fe@Fe3O4-30 as negative electrode and CF@MnO2 as positive electrode (Scheme 1). The gel state PVA-LiCl solution was used as the solid electrolyte. Figure S11 shows the SEM images of as-assembled WSSC. The length and diameter of the WSSC are about 3 cm and 0.5 mm, respectively. Figure 5a displays the CV curves of the assembled WSSC collected in different potential windows, indicating that the potential window of the assembled WSSC can reach up to 2.0 V. Moreover, the CV tests at different scan rates were carried out within the potential window of 0–2.0 V, as shown in Fig. 5b. The voltammetric feature of the assembled WSSC remains almost unchanged with the increasing scan rate from 10 to 500 mV s−1, suggesting fast electron-transfer kinetics. Figure 5c gives the GCD curves of the WSSC at different currents. The corresponding specific capacitances calculated according to the GCD curves are summarized in Fig. 5d. One can see that the WSSC exhibits a length specific capacitance of 5 mF cm−1 and an area specific capacitance of 16 mF cm−2 at the current of 0.5 mA. The delivered specific capacitances are also much higher than that of reported WSSC (Table 1).Open image in new windowFig. 5Electrochemical characterization of the (+) CF@MnO2//Fe@Fe3O4-30 (−) wire-shaped all-solid-state asymmetric supercapacitor device. a CV curves collected in different scan voltage windows at the scan rate of 100 mV s−1. b CV curves of the device at different scan rates. c GCD curves of the device at different current. d Specific capacitance of the device as a function of current. e Ragone plots of the device calculated from GCD curves. The inset is a photograph of a red LED turned on by a wire-shaped all-solid-state asymmetric supercapacitor device. f Cycling stability of the device at a current density of 3.0 mA. The inset shows the last 10 charge/discharge profileTable 1Performance summary of recent reports about one-dimensional wire-shaped supercapacitor1D wire-shaped supercapacitorCL (mF cm−1)/CA (mF cm−2)Potential window (V)Emax (µWh cm−2)Pmax (µW cm−2)ReferencesFe@Fe3O4//CF@MnO25/15.92.094736.8This workMWCNT//MWCNT/MnO20.016/3.162.0––[37]NPG wire/MnO2//CNTs/Carbon paper-/121.85.42531[38]MWCNTs/CMF//CNF6.3/86.81.00.7189.4[39]MnO2/CNT/nylon fiber//MnO2/CNT/nylon fiber5.4/40.91.42.6–[40]ZnO nanowire/MnO2//ZnO nanowire/MnO20.2/2.40.80.02714[41]MnO2-CNT-G-Ni wires//MnO2-CNT-G-Ni tubes–/310.82.76–[42]Ti@MnO2//Ti@MnO2–/15.60.81.4580[13]Cu@CuO@CoFe-LDH//Cu@AC–/1.293.7545,720[43]CL: length specific capacitance; CA: area specific capacitance; E and P are the energy and power energyIt is well known that the energy density (E) and power density (P) of a supercapacitor could be calculated according to Eq. S3 and Eq. S4, respectively. Therefore, this WSSC will also deliver a superior energy density and power density which are plotted on the Ragone diagram in Fig. 5e. Impressively, a maximum energy density of 9 μWh cm−2 at power density of 532.7 μW cm−2 is achieved at a working voltage of 2.0 V. Meanwhile, the large energy density of the assembled WSSC is superior to previously reported WSSCs systems such as MWCNT//MWCNT/MnO2, NPG wire/MnO2//CNTs/carbon paper (Table 1). Furthermore, as shown in the inset of Fig. 5e, a single WSSC device could light a commercial red-light-emitting-diode (1.5 V) for 2 min, implying its practical application. More importantly, the WSSC device also reveals a good cycling stability and 100% of capacitance is retained over 2000 cycles (Fig. 5f).5 ConclusionIn summary, Fe3O4-connected nanosheet arrays growing on the surface of the Fe wire substrate have been successfully synthesized by directly oxidization of Fe wire. Benefiting from the connected nanosheet structure and the intimate contact between the Fe3O4 and Fe substrate, the obtained Fe@Fe3O4 exhibits excellent capacitive behavior with a length specific capacitance of 12 mF cm−1 at 0.6 mA. What is more, the as-assembled asymmetrical WSSC device also presents a high energy density (9 µWh cm−2) at power density of 532.7 µW cm−2 and remarkable long-term cycling performance (100% capacitance retention after 2000 cycles), which will possess enormous potential for practical applications in portable electronic devices.

•Using MoO2 nanowires as templates and conductive substrates synthesized Co−N−MoO2.•The deliberate doping with N and Co atoms exhibited high activities for ORR and HER.•The 1D conductive substrate with co-doping is attractive for heterogeneous catalysis.Oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) are traditionally carried out with noble metals (such as Pt) as catalysts, respectively. Herein, Co-N-doped MoO2 nanowires catalysts were synthesized by employing MoO2 nanowires as templates and conductive substrates. The effect of nanowire structure and non-metal/metal doping on ORR and HER performance were scientific discussed. The most active Co-N-MoO2 (Co-N-doped MoO2) exhibited high ORR catalytic activity (an onset potential of +0.87 V vs. RHE, n values of 3.56 and 3.68, excellent electrochemical stability) and outstanding HER performance with a low overpotential (69 mV vs. RHE), high electrochemical area and robust stability in 0.1 M KOH, which are associated with the defined nanowires structure, and homogeneous doping of Co/N into MoO2 with numerous active sites.Download high-res image (271KB)Download full-size image

•Mo2C/CLCN was synthesized by using Cu-MoO2 rods as Mo source and Cu template.•DFT calculation confirmed Mo atoms on C plane were the main catalytic sites of Mo2C.•Cu plays key roles in the formation of porous carbon and the exposure of Mo2C.Hydrogen evolution reaction (HER) from water electrolysis, currently, is fundamental for large-scale solar fuel production by utilizing earth-abundant element and non-noble metal-based catalysts. Herein, Mo2C on hierarchical porous carbon rods composed of cross-linked carbon networks (Mo2C/CLCN) was synthesized by using Cu-MoO2 rods as Mo source and Cu template. The copper plays key roles in protecting Mo2C from excess covering carbon to enhance the HER performance. When evaluated for HER activity, Mo2C/CLCN exhibit a low onset potential of −85 mV with a Tafel slope of 48.2 mV dec−1, an operating overpotential of 145 mV at the cathodic current density of 10 mA cm−2 and outstanding long-term cycling stability in acidic electrolyte, which superior to most of non-doping Mo2C. This work suggests that the in-situ template method by multi-element compounds is an inspiring strategy for synthesizing the efficient water splitting electrocatalysts with high electrochemical active area and more catalytically active sites.Download high-res image (185KB)Download full-size image

Sunlight-driven electrolytic splitting of water is a promising route to hydrogen production, and widespread implementation has called for the development of inexpensive, robust and large-scale electrodes. Here, nickel nanoparticles partially embedded into carbon fiber cloth (NiΦCFC) is prepared on a large scale (e.g., 40 cm × 40 cm) via a metal-mediated pitting process, which is adopted to fabricate hydrogen evolution reaction electrode for the first time. The partially embedded structure is beneficial for regulating the electron density state of carbon, exposing the nickel catalytic sites and improving the catalytic stability. The two kinds of electrochemical area and density functional theory results confirm that the interface effect between nickel and defective carbon leads into a low Gibbs free energy of H* adsorption. The NiΦCFC as flexible and efficient electrodes require a small overpotential of 131.5 mV to achieve −10 mA cm−2. Note that the two-electrode electrolyzer composed of FeNi layered double hydroxide loaded on CFC (NiFe-LDH/CFC) (+)//NiΦCFC (−) possesses a voltage of 1.54 V at −10 mA cm−2, which can also be powered by a solar cell. The facile and large-scale preparation of NiΦCFC as flexible electrodes could be adapted for the industrial hydrogen production powered by solar energy.Download high-res image (187KB)Download full-size image

Electrochemical H2 generation from H2O has been focused on the exploration of non-noble metals as well as earth-rich catalysts. In our practical work, we provide a simple cost-efficient fabrication process to prepare large Mo sheets via the controlled equilibrium between sublimation of MoO3 and reduction of H2. Porous MoP sheets were synthesized from the obtained Mo sheets as the Mo source and template which exhibit notable activity in the hydrogen evolution reaction with a low onset potential of −88 mV vs RHE, small Tafel value of 54.5 mV/dec, and strong catalytic stability. With Mo sheets as the universal Mo source and template, MoS2 and Mo2C sheets were synthesized by a similar process, and the corresponding catalytic activities were calculated by density functional theory.Keywords: catalytic site; derivative MoX sheet; hydrogen evolution reaction; Mo sheet; sublimation;

•A simple strategy was reported to fabricate N-doped MoS2 as HER catalysts.•N-doped MoS2 revealed an enhanced HER performance than pure MoS2.•The DFT confirmed that more active sites were produced by doped N atoms.Highly active and earth-abundant catalysts for hydrogen evolution reaction (HER) play a crucial in the development of efficient water splitting to produce hydrogen fuel. Here, we reported a simple, facile and effective strategy to fabricate N-doped molybdenum sulfide (N-doped MoS2) as noble metal-free catalysts for HER. Compared with pure MoS2, the obtained N-doped MoS2 catalyst revealed enhanced HER performance with low overpotential of −168 mV (−10 mA cm−2), small Tafel slope of 40.5 mV dec−1 and excellent stability. The superior HER activity may originate from both the exposed Mo active sites due to S defects and the optimized electron density state of S atoms by N doping. More importantly, due to its simple synthesis method, earth-abundant catalysts and high catalytic activity, the N-doped MoS2 will become a promising HER catalysts for water splitting.Download high-res image (244KB)Download full-size image

•A simple one-step hydrothermal method is used to synthesize CdS/MoS2/Mo.•Worm-like pores and nanosheet morphology of MoS2 improve the specific area.•Mo sheets with good conductivity facilitate fast photogenerated electrons transfer.•MoS2 co-catalyst provides active sites and inhibits charge carriers recombination.Co-catalysts for H2 production are often made from expensive noble metals, such as the most efficient Pt. The alternative non-noble metal co-catalysts with low cost and high efficiency are therefore highly desirable for economically viable H2 production. Herein, we demonstrated that a CdS/MoS2/Mo sheets system simultaneously containing photocatalysts, co-catalysts, and conductive supports, was prepared via the one-step hydrothermal process by Mo sheets as template and Mo sources. The obtained CdS/MoS2/Mo sheets possess the superior photocatalytic H2 production via water splitting under visible light irradiation, which achieved an extraordinary H2 production of 4540 μmol h−1 g−1, up to 28.6 and 3.6 times greater than that of CdS alone and Pt/CdS. The synergetic effect of MoS2 as co-catalysts and Mo sheets as conductive supports contribute to the dramatically improved photocatalytic H2 evolution activity of CdS photocatalysts, by means of facilitating charge carriers separation and providing active sites for proton reduction. These findings provide a straightforward and practical route to produce cheap and efficient co-catalysts for large-scale water splitting.Download high-res image (143KB)Download full-size image

•A new catalytic way of suspended hydrogen evolution reaction (SHER) is proposed.•Porous molybdenum carbide microspheres are synthesized by ion exchange reaction and subsequent calcining process.•Mo2C microspheres as binder-free electrocatalysts are easy to be recycled and replaced.Generally, the electrocatalysts are immobilized on conductive electrodes or in-situ grown on current-collecting substrates, which causes some disadvantages. For the first time, the obtained porous molybdenum carbide microspheres with diameters of 200–400 μm are employed as binder-free electrocatalysts in the novel model of suspended hydrogen evolution reaction (SHER), which possess the perfect catalytic stability and high practicability. Herein, porous molybdenum carbide microspheres synthesized by ion exchange reaction and subsequent calcining process are employed as electrocatalysts for HER, which possess a low onset potential of −79 mV vs. RHE and a low overpotential of 174 mV achieving a current density of 10 mA/cm2 in 0.5 M H2SO4. This work may provide a new methodology for rational design and fabrication of reaction pattern for the electrolysis of water.

Core–shell nanocomposites based on Au nanoparticle@zinc–iron-embedded porous carbons (Au@Zn–Fe–C) derived from metal–organic frameworks were prepared as bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). A single Au nanoparticle of 50–100 nm in diameter was encapsulated within a porous carbon shell embedded with Zn–Fe compounds. The resulting Au@Zn–Fe–C hybrids exhibited apparent catalytic activity for ORR in 0.1 M KOH (with an onset potential of +0.94 V vs RHE, excellent stability and methanol tolerance) and for HER as well, which was evidenced by a low onset potential of −0.08 V vs RHE and a stable current density of 10 mA cm–2 at only −0.123 V vs RHE in 0.5 M H2SO4. The encapsulated Au nanoparticles played an important role in determining the electrocatalytic activity for ORR and HER by promoting electron transfer to the zinc–iron-embedded porous carbon layer, and the electrocatalytic activity was found to vary with both the loading of the gold nanoparticle cores and the thickness of the metal–carbon shells. The experimental results suggested that metal-embedded porous carbons derived from metal–organic frameworks might be viable alternative catalysts for both ORR and HER.Keywords: core−shell structure; electron transfer; hydrogen evolution reaction; metal−organic frameworks; oxygen reduction reaction

Acidic electrolytes are advantageous for water electrolysis in the production of hydrogen as there is a large supply of H+ ions in the solution. In this study, with the applied overpotential larger than the equilibrium potential of Ni0/Ni2+, Ni foam as HER electrode exhibits excellent and stable HER activity with an onset potential of −84 mV (vs RHE), a high current density of 10 mA cm–2 at −210 mV (vs RHE), and prominent electrochemical durability (longer than 5 days) in acidic electrolyte. The results presented herein may has potential large-scale application in hydrogen energy production.Keywords: acidic electrolyte; equilibrium potential; hydrogen evolution reaction; nickel foam; stable electrocatalyst

•CoSe2 embedded defective carbon nanotube via carbonization-oxidation-selenylation.•The pre-oxidation as the crucial step introduced defects into carbon nanotubes.•CoSe2 partially embedded defective carbon nanotubes can expose more active sites.Development of electrocatalysts for hydrogen evolution reaction (HER) with low overpotential and robust stability remained as one of the most serious challenges for energy conversion. In this work, CoSe2 nanoparticles embedded in defective carbon nanotubes (CoSe2@DC) was synthesized by a carbonization-oxidation-selenylation procedure of Co-based metal-organic frameworks (MOFs). The pre-oxidation treatment was a crucial step in introducing an increasing number of defects into carbon nanotubes, which promoted the reaction between Co@carbon and selenium and led to the enhanced HER performance. The as-prepared CoSe2@DC exhibited excellent HER catalytic reactivity with a low onset potential of −40 mV vs. RHE, a small Tafel slope of 82 mV dec−1 as well as a high current density with robust catalytic stability in 0.5 M H2SO4. This work may provide a generic methodology for rational design and fabrication of the partially encased core-shell structure derived from MOFs as efficient HER electrocatalysts.

•Heteroatoms including N, S, P and B improve HER catalytic activity of carbon.•Metals as core modulate the electronic state density of carbon shell to produce new active sites.•Some challenges for carbon based electrocatalysts are discussed.Development of effective technologies for clean and sustainable hydrogen energy has been attracting great attention. Toward this end, an effective and promising approach is based on the electrolysis of water for hydrogen production. To date, the most effective hydrogen evolution reaction (HER) electrocatalysts are Pt-group metals with a low overpotential to generate large cathodic current densities. However, the high cost and scarcity severely limit their broad utilization. As alternatives to Pt electrocatalysts, transition metal compounds as effective HER catalysts have been prepared in a series of recent studies. However, thus far, it remained a great challenge to develop highly active HER catalysts with a low overpotential based on earth-abundant and cost-effective materials. Recently, the new significant developments about carbon-based electrocatalysts with a low overpotential toward HER have stimulated a great deal of the researchers' interest. In particular, the catalytic activity of carbon based-catalysts can be enhanced by transition metal nanoparticles as core and nonmetal doping into carbon skeleton, which can modulate the electronic state density of carbon to produce new active sites for HER. In this feature article, we review the research progress in the development of carbon-based electrocatalysts toward HER in acid electrolytes throughout the past few years. In addition, some notable matters and challenge in the research of HER are discussed in this review.

•Defect-rich MoS2/carbon hierarchical spheres with more active sites were prepared by simple micro-emulsion method.•The C–MoS2 hierarchical spheres exhibited a low onset potential of −103 mV (vs. RHE) with a Tafel slope of 56.1 mV dec−1.•The “inverted molybdenum reaction” might provide a novel method to regulate the catalytic sites of MoS2.Highly active and stable MoS2/carbon hierarchical spheres with abundant active edge sites were fabricated by a simple micro-emulsion procedure where PVP was used as the carbon source, and carbon disulfide as the sulfur source and oil phase in micro-emulsion to control the morphology of MoS2. Hierarchical spheres of MoS2/carbon with a diameter of ca. 500 nm were obtained and characterized by scanning and transmission electron microscopic measurements. With a high electrochemically accessible surface area and defect-rich MoS2 nanosheets, the MoS2/carbon hierarchical spheres exhibited an excellent electrocatalytic activity for hydrogen evolution reaction with a low onset potential of −103 mV (vs. RHE), small Tafel of 56.1 mV dec−1, as well as extraordinary catalytic stability. The results were accounted for by the “inverted molybdenum reaction” that served as a novel way of regulating Mo catalytic sites of MoS2 electrocatalysts.

Development of non-noble-metal catalysts for hydrogen evolution reaction (HER) with both excellent activity and robust stability has remained a key challenge in the past decades. Herein, for the first time, N-doped carbon-wrapped cobalt nanoparticles supported on N-doped graphene nanosheets were prepared by a facile solvothermal procedure and subsequent calcination at controlled temperatures. The electrocatalytic activity for HER was examined in 0.5 M H2SO4. Electrochemical measurements showed a small overpotential of only −49 mV with a Tafel slope of 79.3 mV/dec. Theoretical calculations based on density functional theory showed that the catalytically active sites were due to carbon atoms promoted by the entrapped cobalt nanoparticles. The results may offer a new methodology for the preparation of effective catalysts for water splitting technology.

Biomass-derived nitrogen self-doped porous carbon was synthesized by a facile procedure based on simple pyrolysis of water hyacinth (eichhornia crassipes) at controlled temperatures (600–800 °C) with ZnCl2 as an activation reagent. The obtained porous carbon exhibited a BET surface area up to 950.6 m2 g−1, and various forms of nitrogen (pyridinic, pyrrolic and graphitic) were found to be incorporated into the carbon molecular skeleton. Electrochemical measurements showed that the nitrogen self-doped carbons possessed a high electrocatalytic activity for ORR in alkaline media that was highly comparable to that of commercial 20% Pt/C catalysts. Experimentally, the best performance was identified with the sample prepared at 700 °C, with the onset potential at ca. +0.98 V vs. RHE, that possessed the highest concentrations of pyridinic and graphitic nitrogens among the series. Moreover, the porous carbon catalysts showed excellent long-term stability and much enhanced methanol tolerance, as compared to commercial Pt/C. The performance was also markedly better than or at least comparable to the leading results in the literature based on biomass-derived carbon catalysts for ORR. The results suggested a promising route based on economical and sustainable biomass towards the development and engineering of value-added carbon materials as effective metal-free cathode catalysts for alkaline fuel cells.

Advanced materials for electrocatalytic water splitting are central to renewable energy research. In this work, MoS2 nanosheets supported on porous metallic MoO2 (MoS2/MoO2) were produced by sulfuration treatments of porous and highly conductive MoO2 for the hydrogen evolution reaction. Porous MoO2 with one-dimensional channel-like structures was prepared by calcination at elevated temperatures using phosphomolybdic acid as the precursor and mesoporous silica (SBA-15) as the template, and the subsequent hydrothermal treatment in the presence of thioacetamide led to the transformation of the top layers to MoS2 forming MoS2/MoO2 composites. Electrochemical studies showed that the obtained composites exhibited excellent electrocatalytic activity for HER with an onset potential of −104 mV (vs. RHE), a large current density (10 mA cm−2 at −0.24 V), a small Tafel slope of 76.1 mV dec−1 and robust electrochemical durability. The performance might be ascribed to the high electrical conductivity and porous structures of MoO2 with one-dimensional channels of 3 to 4 nm in diameter that allowed for fast charge transport and collection.

The design and engineering of low-cost and high-efficiency electrocatalysts for the hydrogen evolution reaction (HER) has attracted increasing interest in renewable energy research. Herein, MoS2 nanosheet-coated CoS2 nanowire arrays supported on carbon cloth (MoS2/CoS2/CC) were prepared by a two-step procedure that entailed the hydrothermal growth of Co(OH)2 nanowire arrays on carbon cloth followed by reaction with (NH4)2MoS4 to grow an overlayer of MoS2 nanosheets. Electrochemical studies showed that the obtained 3D electrode exhibited excellent HER activity with an overpotential of −87 mV at 10 mA cm−2, a small Tafel slope of 73.4 mV dec−1 and prominent electrochemical durability. The results presented herein may offer a new methodology for the design and engineering of effective multilevel structured catalysts for the HER based on earth-abundant components.

The development of electrodes composed of non-noble-metal catalysts with both excellent activity and high stability for the hydrogen evolution reaction (HER) is essential for hydrogen production. In this work, a flexible and robust film electrode based on cobalt nanoparticles embedded into the interlamination of N-doped graphene film (Co@NGF) is fabricated by simple vacuum filtration combined with subsequently controlled calcination. This flexible three-dimensional (3D) nano-architecture film directly used as the electrode shows a low onset potential of only −14 mV (vs. RHE) with a small Tafel slope of 93.9 mV per dec for the HER in 0.5 M H2SO4. Stability tests through long term potential cycles and extended electrolysis confirm the perfect durability of Co@NGFs in acid media. The remarkable HER catalytic activity is derived from the electron penetration effect of cobalt nanoparticles as the core protected by N-doped graphene as the shell. It is worth noting that the Co@NGF electrodes, for the first time, used as both the anode and cathode in a two-electrode system open up new possibilities for exploring overall water splitting catalysts in an acid electrolyte. This development offers an attractive HER film electrode for large-scale water splitting technology.

Design and engineering of low-cost and high-efficiency electrocatalysts for hydrogen evolution reactions (HER) has attracted increasing interest in renewable energy research. Herein, a highly active and stable metal-free electrocatalyst, N and S co-doped porous carbon derived from human hair, was developed for HER for the first time, with an electrocatalytic performance comparable to that of state-of-the-art commercial 20 wt% Pt/C catalysts. SEM, TEM and nitrogen adsorption–desorption measurements showed that the resultant carbon exhibited a porous structure with a high specific surface area (up to 830.0 m2 g−1) and rich porosity. XPS measurements showed that N and S were co-doped into the carbon molecular skeletons. Importantly, electrochemical measurements showed high activity for hydrogen evolution with a low overpotential of only −12 mV, a Tafel slope of 57.4 mV dec−1, a current density of 10 mA cm−2 at −0.1 V vs. RHE, and remarkable durability. The results highlight a unique paradigm for the preparation of highly efficient electrocatalysts for HER based on abundant biowastes.

N-doped carbon-coated cobalt nanorod arrays supported on a Ti mesh were prepared by a two-step procedure involving hydrothermal synthesis of Co3O4 nanorods followed by thermal reduction to metallic cobalt. The nanocomposites exhibited a remarkable catalytic activity that was comparable to that of leading commercial Pt/C catalysts.

Advanced materials for electrocatalytic water splitting are central to renewable energy research. In this study, we describe a two-step reaction for preparing hydrogen evolution reaction (HER) electrodes composed of Pt nanoparticles and MoS2 nanosheets grown on carbon fibers. The morphology and the structures are characterized by a variety of techniques including SEM, TEM, XRD and XPS. Detailed electrochemical characterizations demonstrate that the Pt nanoparticles/MoS2 nanosheets/carbon fibers electrode (2.03 w% Pt) exhibited an excellent catalytic activity for HER in an acidic electrolyte with an overpotential of −5 mV (vs. HER). And the corresponding Tafel slope is estimated to be 53.6 mV/dec. Stability tests through long-term potential cycles and extended electrolysis confirm the exceptional durability of the catalyst.

•MoO3 nanocrystals were assembled on GO nanosheets by oxygen-bonding interactions.•MoO3-GAs possessed the highest specific capacitance and excellent cycling stability.•A solid-state symmetric supercpacitor based on MoO3-GAs was successfully fabricated.Ultrathin MoO3 nanocrystals were assembled on 3D graphene oxide frameworks via a hydrothermal reaction forming a layered structure by oxygen-bonding interactions at the interface. The structure and morphology of the resulting MoO3-GAs hybrids were characterized by a range of experimental tools including atomic force microscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, infrared and Raman spectroscopy. Because of abundant exposed active sites of the ultrathin MoO3 and rapid ion diffusion and electron transport of 3D graphene frameworks, the resulting MoO3-GAs hybrids possess the highest specific capacitances and excellent cycling stability in both aqueous (527 F g−1 at the current density of 1.0 A g−1, 100% retention after 10,000 cycles) and solid electrolytes (373 F g–1 at 1.0 A g−1, 100% retention after 5,000 cycles) among leading literature results of similar systems.

•S and N self-doped carbon is prepared on a large scale from an abundant biowaste.•The non-metal HER catalysts displayed an onset potential of -0.027 V (vs. RHE).•S doping led to marked changes of the electronic energy structure of catalyst.Development of non-metal catalysts for hydrogen evolution reaction (HER) with both excellent activity and robust stability has remained a key challenge in recent decades. Herein, sulfur and nitrogen self-doped carbon nanosheets are prepared as efficient non-metal catalysts for HER by thermal decomposition of peanut root nodules, an abundant biowaste. The obtained S and N-doped carbons exhibit a porous and multilayer structure with a specific surface area of 513.3 m2/g and high electrochemical area of 27.4 mF/cm2. Electrochemical measurements show apparent electrocatalytic activity for HER in 0.5 M H2SO4, with a small overpotential of only −0.027 V, a Tafel slope of 67.8 mV/dec and good catalytic stability. The density functional theory calculations confirmed that both S and N doping significantly change the electronic structure of carbon catalysts. However, after S-doping into carbon skeleton, the surrounding electric density of S atoms and C atoms increases, but after N-doping it only increases that of C atoms. The results presented herein may offer a novel and effective methodology for the design and engineering of efficacious and ecologically friendly catalysts for water splitting technology.

Advanced materials for electrocatalytic water splitting are central to renewable energy research. In this study, MoO2 nanobelts@nitrogen self-doped MoS2 nanosheets are produced by nitridation and sulfuration treatments of MoO3 nanobelts. The material structures are characterized by a variety of techniques including scanning electron microscopy, transmission electron microscopy, Raman scattering, X-ray photoelectron spectroscopy, and X-ray diffraction spectroscopy. It is found that because of nitrogen doping and the abundance of exposed active edges, the heterostructures exhibit high electronic conductivity, and more importantly, enhanced and stable electrocatalytic activity in hydrogen evolution reaction (HER), as manifested in electrochemical studies. The onset potential is found to be only −156 mV (vs. RHE), which is 105 mV more positive than that of pure MoS2 under identical experimental conditions. The corresponding Tafel slope is estimated to be 47.5 mV dec−1, even slightly less than that of commercial 10 wt% Pt/C (49.8 mV dec−1), suggesting that the reaction dynamics is largely determined by the electrochemical desorption of hydrogen. This is accounted for by nitrogen doping that leads to an enhanced electronic conductivity of the heterostructures as well as a high density of spinning electron states around the N and Mo atoms in MoS2 nanosheets that are the active sites for HER, as manifested in density functional theory studies of a N-doped MoS2 monolayer.

Hierarchical porous carbon-based supercapacitors have been attracting intense attention due to their high and stable electrical double-layer capacitance that may be used for advanced technologies. In this study, porous core–shell carbon fibers were produced by a simple and fast acid oxidation treatment of carbon fibers, and the morphological and structural evolution were examined by SEM, TEM and Raman spectroscopic measurements. Detailed electrochemical characterizations showed that the resulting porous core–shell carbon fibers exhibited an excellent performance for charge storage with a specific capacitance of 98 F g−1 at 0.5 A g−1 in a 1 M H2SO4 liquid electrolyte and 20.4 F g−1 at 1 A g−1 in a H2SO4/PVA solid electrolyte, and excellent capacitance retention at ∼98.5% for the former and ∼96% for the latter over 3000 cycles. The results demonstrated that porous core–shell carbon fibers might be used as effective electrode materials for the fabrication of wire-like all-carbon flexible supercapacitors with high physical flexibility and desirable electrochemical properties.

Titania nanostructured materials have been used extensively for the fabrication of electrochemical capacitors. However, the devices typically exhibit relatively low capacitance and poor cycling stability. Herein, we report the synthesis of a core–shell heterostructure based on layered titanate nanowires coated with nickel hydroxide nanosheets on a titanium mesh, referred to as K2Ti4O9@Ni(OH)2/Ti, by a simple nickel ion exchange reaction. The incorporation of nickel into the titanate nanowires is confirmed by X-ray photoelectron spectroscopic measurements and elemental mapping. Scanning electron microscopic and transmission electron microscopic measurements show the formation of a highly porous network of the hybrid nanowires. Electrochemical studies show that the K2Ti4O9@Ni(OH)2/Ti electrodes possess a high specific capacitance of 340 mF/cm2 at 50 mV/s in an aqueous electrolyte of 3 M KOH and 3 mF/cm2 at 0.04 mA/cm2 in the KOH/PVA solid-state electrolyte, with an excellent retention rate of 92.5% after 2000 cycles and 92.7% after 10 000 cycles, respectively. Such a performance is a few tens of times better than that of the unmodified K2Ti4O9/Ti electrode. The enhanced capability of the chemically modified titanate electrodes may open up new opportunities in the development of low-cost, high-performance, and flexible supercapacitors.Keywords: ion exchange; Ni(OH)2 nanosheet; supercapacitor; titanate nanowire;

Nitrogen self-doped porous carbon was prepared by calcination treatment of surplus sludge, a toxic byproduct from microbial wastewater treatments, and exhibited a mesoporous structure, as manifested in scanning and transmission electron microscopic measurements. Nitrogen adsorption/desorption studies showed that the porous carbon featured a BET surface area as high as 310.8 m2/g and a rather broad range of pore size from 5 to 80 nm. X-ray photoelectron spectroscopic studies confirmed the incorporation of nitrogen into the graphitic matrix forming pyridinic and pyrrolic moieties. Interestingly, the obtained porous carbon exhibited apparent electrocatalytic activity in oxygen reduction in alkaline media, with the optimal temperatures identified within the range of 600 to 800 °C, where the number of electron transfers involved in oxygen reduction was estimated to be 3.5 to 3.7 and the performance was rather comparable to leading literature results as a consequence of deliberate engineering of the graphitic matrix by nitrogen doping.Keywords: carbon nanosheet; carbonization; CO poisoning; methanol crossover; nitrogen adsorption/desorption

Advanced materials for electrocatalytic water splitting are central to renewable energy research. In this work, three-dimensional (3D) hierarchical frameworks based on the self-assembly of MoS2 nanosheets on graphene oxide were produced via a simple one-step hydrothermal process. The structures of the resulting 3D frameworks were characterized by using a variety of microscopic and spectroscopic tools, including scanning and transmission electron microscopies, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman scattering. Importantly, the three-dimensional MoS2/graphene frameworks might be used directly as working electrodes which exhibited apparent and stable electrocatalytic activity in hydrogen evolution reaction (HER), as manifested by a large cathodic current density with a small overpotential of −107 mV (−121 mV when loaded on a glassy-carbon electrode) and a Tafel slope of 86.3 mV/dec (46.3 mV/dec when loaded on a glassy-carbon electrode). The remarkable performance might be ascribed to the good mechanical strength and high electrical conductivity of the 3D frameworks for fast charge transport and collection, where graphene oxide provided abundant nucleation sites for MoS2 deposition and oxygen incorporation led to the formation of defect-rich MoS2 nanosheets with active sites for HER.Keywords: electrocatalytic; graphene oxide; hydrogen evolution reaction; hydrothermal; MoS2 nanosheet; three-dimensional framework

Hierarchical porous carbon-based supercapacitors have been attracting intense attention due to their high and stable electrical double-layer capacitance that may be used for advanced technologies. In this study, porous core–shell carbon fibers were produced by a simple and fast acid oxidation treatment of carbon fibers, and the morphological and structural evolution were examined by SEM, TEM and Raman spectroscopic measurements. Detailed electrochemical characterizations showed that the resulting porous core–shell carbon fibers exhibited an excellent performance for charge storage with a specific capacitance of 98 F g−1 at 0.5 A g−1 in a 1 M H2SO4 liquid electrolyte and 20.4 F g−1 at 1 A g−1 in a H2SO4/PVA solid electrolyte, and excellent capacitance retention at ∼98.5% for the former and ∼96% for the latter over 3000 cycles. The results demonstrated that porous core–shell carbon fibers might be used as effective electrode materials for the fabrication of wire-like all-carbon flexible supercapacitors with high physical flexibility and desirable electrochemical properties.

N-doped carbon-coated cobalt nanorod arrays supported on a Ti mesh were prepared by a two-step procedure involving hydrothermal synthesis of Co3O4 nanorods followed by thermal reduction to metallic cobalt. The nanocomposites exhibited a remarkable catalytic activity that was comparable to that of leading commercial Pt/C catalysts.

The development of electrodes composed of non-noble-metal catalysts with both excellent activity and high stability for the hydrogen evolution reaction (HER) is essential for hydrogen production. In this work, a flexible and robust film electrode based on cobalt nanoparticles embedded into the interlamination of N-doped graphene film (Co@NGF) is fabricated by simple vacuum filtration combined with subsequently controlled calcination. This flexible three-dimensional (3D) nano-architecture film directly used as the electrode shows a low onset potential of only −14 mV (vs. RHE) with a small Tafel slope of 93.9 mV per dec for the HER in 0.5 M H2SO4. Stability tests through long term potential cycles and extended electrolysis confirm the perfect durability of Co@NGFs in acid media. The remarkable HER catalytic activity is derived from the electron penetration effect of cobalt nanoparticles as the core protected by N-doped graphene as the shell. It is worth noting that the Co@NGF electrodes, for the first time, used as both the anode and cathode in a two-electrode system open up new possibilities for exploring overall water splitting catalysts in an acid electrolyte. This development offers an attractive HER film electrode for large-scale water splitting technology.

Advanced materials for electrocatalytic water splitting are central to renewable energy research. In this study, MoO2 nanobelts@nitrogen self-doped MoS2 nanosheets are produced by nitridation and sulfuration treatments of MoO3 nanobelts. The material structures are characterized by a variety of techniques including scanning electron microscopy, transmission electron microscopy, Raman scattering, X-ray photoelectron spectroscopy, and X-ray diffraction spectroscopy. It is found that because of nitrogen doping and the abundance of exposed active edges, the heterostructures exhibit high electronic conductivity, and more importantly, enhanced and stable electrocatalytic activity in hydrogen evolution reaction (HER), as manifested in electrochemical studies. The onset potential is found to be only −156 mV (vs. RHE), which is 105 mV more positive than that of pure MoS2 under identical experimental conditions. The corresponding Tafel slope is estimated to be 47.5 mV dec−1, even slightly less than that of commercial 10 wt% Pt/C (49.8 mV dec−1), suggesting that the reaction dynamics is largely determined by the electrochemical desorption of hydrogen. This is accounted for by nitrogen doping that leads to an enhanced electronic conductivity of the heterostructures as well as a high density of spinning electron states around the N and Mo atoms in MoS2 nanosheets that are the active sites for HER, as manifested in density functional theory studies of a N-doped MoS2 monolayer.

The design and engineering of low-cost and high-efficiency electrocatalysts for the hydrogen evolution reaction (HER) has attracted increasing interest in renewable energy research. Herein, MoS2 nanosheet-coated CoS2 nanowire arrays supported on carbon cloth (MoS2/CoS2/CC) were prepared by a two-step procedure that entailed the hydrothermal growth of Co(OH)2 nanowire arrays on carbon cloth followed by reaction with (NH4)2MoS4 to grow an overlayer of MoS2 nanosheets. Electrochemical studies showed that the obtained 3D electrode exhibited excellent HER activity with an overpotential of −87 mV at 10 mA cm−2, a small Tafel slope of 73.4 mV dec−1 and prominent electrochemical durability. The results presented herein may offer a new methodology for the design and engineering of effective multilevel structured catalysts for the HER based on earth-abundant components.

Design and engineering of low-cost and high-efficiency electrocatalysts for hydrogen evolution reactions (HER) has attracted increasing interest in renewable energy research. Herein, a highly active and stable metal-free electrocatalyst, N and S co-doped porous carbon derived from human hair, was developed for HER for the first time, with an electrocatalytic performance comparable to that of state-of-the-art commercial 20 wt% Pt/C catalysts. SEM, TEM and nitrogen adsorption–desorption measurements showed that the resultant carbon exhibited a porous structure with a high specific surface area (up to 830.0 m2 g−1) and rich porosity. XPS measurements showed that N and S were co-doped into the carbon molecular skeletons. Importantly, electrochemical measurements showed high activity for hydrogen evolution with a low overpotential of only −12 mV, a Tafel slope of 57.4 mV dec−1, a current density of 10 mA cm−2 at −0.1 V vs. RHE, and remarkable durability. The results highlight a unique paradigm for the preparation of highly efficient electrocatalysts for HER based on abundant biowastes.