Design,Preparation And Performance Of Cobalt-based Nanocomposites For Water Electrolysis In Alkaline Media

Proton Water splitting has long been considered as an effective and clean method to produce pure hydrogen,which is a renewable energy for future energy infrastructure and has been intensively researched as an alternative to ever-decreasing fossil fuel energy sources.The adjustable energy consumption of industrial water splitting is mainly the overpotential derived from the activation polarization of catalytic materials for the oxygen evolution reaction（OER）and hydrogen evolution reaction（HER）,especially OER,which contains complex four-electron（4e）transfer process leading to its slow kinetics,and requires relatively high overpotential.Thus,OER and HER are both important to the efficiency of water splitting.The catalysts for HER and OER are the key to reduce overpotential.Most of HER catalysts possess high activity in acidic media,but poor activity in alkaline media of industrial environment,which severely limits the industrial promotion.Therefore,it has become a research hotspot to develop cheap and efficient non-precious metal catalysts that can accelerate the kinetics and increase the activity of catalyst electrolysis in alkaline media.In this thesis,we take full advantage of the structure-activity relationship to design catalysts,to improve the conductivity of the material and the resistance in the alkaline medium preferentially,and enhancing the conductivity of the catalyst by compounding with carbon material or introducing Se element.The design enables high catalytic OER and HER activities.We choose lamellar structures or three-dimensional hollow structures to increase the active sites exposure,which not only improves the activity of the catalyst,but also improves the stability of the catalyst.The prominent structural effects of the catalysts have been extensively discussed to shed light on the eletrocatalytic water splitting catalysts design.The main research contents are as follows:（1）Ni-Co alloys and NiCoO₂（NiCo-NiCoO₂）heterostructures encapsulated in N-doped carbon（NC）were synthesized utilizing the synergies between metal and metal oxides and the advantages of carbon.Bimetallic thin film NiCo-NiCoO₂@NC exhibits high catalytic activity of HER,which is close to that of commercial Pt/C,and its catalytic activity in alkaline medium is better than that of IrO₂.The catalyst shows excellent water electrolysis activity in 1M KOH solution,achieving remarkably small onset potentials of 0 and 200 mV,and overpotentials of 94 and 318 mV at 10 mA·cm^-2for HER and OER respectively,and the TOFs for HER is 2.5 times of Pt/C,2 times of IrO₂ for OER when the overpotential is 500 mV.A water electrolyzer is fabricated by utilizing non-precious NiCo-NiCoO₂@NC as anode and cathode catalysts simultaneously which achieves 20 mA·cm^-2 at a voltage of 1.44 V.The carbon coat and synergy of Ni-Co alloy and NiCoO₂ enables NiCo-NiCoO₂@NC to maintain 20mA·cm^-2 for at least 50 h in alkaline media without trivial activity losses,the current density retention rate is approximately 95.4%.（2）Through the modulation of different solvents,we designed four structures of CoSe₂ using the metal organic framework ZIF-67 as the precursor.The structure of the catalysts-CoSe₂-B（the solvent is benzyl alcohol）,CoSe₂-C（the solvent is cyclohexane）,CoSe₂-D（the solvent is N,N-Dimethylformamide）and CoSe₂-E（the solvent is ethanol）are hollow structure,sheet structure,spherical particles and core-shell structure respectively.The hollow structure can provide open active sites and high specific surface area to improve the activity and durability of the catalyst.The hollow structure of CoSe₂-B also exhibits the highest catalytic activity in 1 M KOH solution,achieving remarkably small overpotential of 268 mV at 10 mA·cm^-2 for OER.At the overpotential of 260 mV,CoSe₂-B exhibits TOF of 0.039 s^-1 per Co atom,which is approximately 8.86 times that of IrO2,at the overpotential of 400 mV,CoSe₂-B exhibits TOF of 0.3 s^-1 per Co atom,and CoSe₂-B has a 67-fold increase in TOF,demonstrating its excellent intrinsic activity.After 2000 cycles of cyclic voltammogram scanning,the activity was almost unchanged.The reaction is continued for 20000 s at the overpotential of 250 mV,the current density retention rate is 94%,demonstrating good stability of CoSe₂-B.