| Hydrogen energy is regarded as the ideal energy in the future because of its cleanliness and high energy density.The preparation of hydrogen by electrocatalytic water splitting is a promising method.The exploitation of efficient and low-cost electrocatalysts for hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）is the key to the industrialization of this technology because of the slow kinetics in the process of water electrolysis.Molybdenum and nickel-based nanomaterials are easily synthesized in large quantities and exhibit potential catalytic activity.However,the activity still requires improvement due to the limited conductivity.Its composites with carbon materials exhibit improved catalytic activity due to the enhanced electrical conductivity.Low-cost and renewable biomass with micro-channel structure is a natural material for the preparation of carbon-based composites.In this paper,carbon microtube based molybdenum/nickel composites were prepared based on the Metaplexis japonica and Crepis tectorum fluff biomass through heteroatom doping,heterointerface construction,alloying and other strategies,and they were used to catalyze HER and OER reactions.The overall water splitting systems were also assembled to further evaluate the catalytic performances of the prepared catalysts.Detailed descriptions are as follows:Ni-doped Mo S2nanosheets（Ni-Mo S2/a CMT）were prepared by a hydrothermal method based on the Metaplexis japonica fluff-derived carbon microtube.The open porous structures formed by vertically arranged nanosheets can not only expose more edge active sites,but also promote the diffusion of electrolyte and the release of generated hydrogen.5%-Ni-Mo S2/a CMT catalyst shows good HER catalytic activity.In 1.0 mol·L-1KOH,to deliver the current density of 10 mA·cm-2,it requires overpotentials of 140 m V,and Tafel slope is 96 m V·dec-1.In 0.5 mol·L-1H2SO4,5%-Ni-Mo S2/a CMT exhibits an overpotential of88 m V(@10 mA·cm-2)with a Tafel slope of 61 m V·dec-1.The overall water splitting system assembled with 5%-Ni-Mo S2/a CMT as the cathode and commercial Ru O2as the anode only requires the cell voltages of 1.57 V to generate the current density of 10 mA·cm-2in 1.0mol·L-1KOH,and the system can work stably for 30 h at a current density of 50 mA·cm-2.In order to further improve HER activity of catalysts in alkaline conditions,Ru nanoclusters decorated Mo Se2nanosheets supported on the Crepis tectorum fluff-derived carbon microtube（Ru-Mo Se2/CMT）were prepared by hydrothermal reaction,impregnation and calcination.The biomass carbon substrate improves the electronic conductivity.Benefiting from the strong water dissociation ability of Ru,Ru-Mo Se2/CMT exhibits a low overpotential of 70 m V(@10 mA·cm-2)with a Tafel slope of 39 m V·dec-1in 1.0 mol·L-1KOH.Moreover,Ru-Mo Se2/CMT displays considerable stability in both alkaline and acidic media.The cell voltage required for the assembled Ru-Mo Se2/CMT|Ru O2system at 10mA·cm-2is only 1.54 V in 1.0 mol·L-1KOH.Ru-Mo Se2/CMT|Ru O2system shows a stability retention rate of 96%in the 30 h CP test.Ni1Fe2@Fe2O3@C composites embedded in Metaplexis japonica fluff-derived carbon microtube matrix were prepared by nitrate soaking and subsequent high-temperature calcination,which were used to catalyze the anodic oxygen evolution reaction.The existence of carbon matrix effectively inhibits the agglomeration of metal particles and provides the efficient electron transfer path for OER.The synergy between the carbon matrix and the Ni1Fe2core and Fe2O3shell together enhanced the OER catalytic activity.Ni1Fe2@Fe2O3@C exhibits considerable catalytic activity with the overpotential of 271 m V at 10 mA·cm-2and a Tafel slope of 78 m V·dec-1.Ni1Fe2@Fe2O3@C also shows good stability due to the protection of metal particles by biomass carbon matrix.The overall water splitting system assembled with Pt/C as the cathode and Ni1Fe2@Fe2O3@C catalyst as the anode exhibits water splitting activity close to that of Pt/C|Ru O2system in alkaline medium,and the stability retention rate of the system is 93%.Nanowire-like Ni Co precursors were grown on Metaplexis japonica fluff biomass substrates by hydrothermal method,which were then converted into carbon microtube supported Ni Co P-Ni S2/CMT composites by low-temperature phosphorus and sulfidation treatment.Characterization revealed the thin amorphous layer on the periphery of the nanowires and the distinct heterointerface inside.Electrochemical tests show that Ni Co P-Ni S2/CMT has bifunctional catalytic activity,and its HER and OER performances are better than those of single-component samples.Ni Co P-Ni S2/CMT exhibites good electrocatalytic activity in 1.0 mol·L-1KOH,and the needed overpotentials for HER and OER are 134 and 308 m V at 10 mA·cm-2.The assembled symmetric water splitting system shows a cell voltage of 1.62 V at 10 mA·cm-2in alkaline solution,and it exhibits good stability during the 24 h test.Based on the accumulation of previous work,Ru-Ni Co ternary alloy particles embedded in the Metaplexis japonica fluff-derived carbon microtube matrix（Ru-Ni Co@CMT）were prepared through a simple impregnation followed by reduction.The porous structure of the carbon matrix facilitates the penetration of the electrolyte and electron transfer in the reaction process.The optimized electronic structure resulting from the strong electronic interaction between Ru and Ni Co enables Ru-Ni Co@CMT to exhibit considerable HER and OER catalytic activity in alkaline media.For the HER process,Ru-Ni Co@CMT shows a low overpotential of 78 m V(@10 mA·cm-2)with a Tafel slope of 77 m V·dec-1.For the OER process,Ru-Ni Co@CMT requires an overpotential of 299 m V to deliver the current density of 10 mA·cm-2.The assembled Ru-Ni Co@CMT|Ru-Ni Co@CMT overall water splitting system shows a cell voltage of 1.58 V at 10 mA·cm-2in alkaline medium,and it exhibits the faradaic efficiency higher than 98%.The system can work stably for 24 h under the step-change current densities. |
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