Controlled Preparation And Electrocatalytic Performance Of Three-dimensional Self-supported Transition Metal Nanocomposites

Environmental pollution and energy crisis resulted from massive fossil fuel combustion have stimulated global efforts to explore ecofriendly alternatives as energy carriers.H₂ is recognized as a promising clean energy fuel for its high energy density and environment-friendly features.Water electrolysis is certified as a high efficiency and clean pathway to produce H₂ in a large scale with a short time.Water electrolysis involves hydrogen evolution reaction（HER）on cathode and oxygen evolution reaction（OER）on anode to prepared H₂ and O₂,respectively.However,the energy conversion efficiency of electrolytic water system is low due to the high potential,which limits its wide application.In order to improve the energy conversion efficiency of water electrolysis system,it is usually necessary to use effective catalysts to reduce the energy barrier of HER and OER process.Now,Pt and Ir/Ru-based nanomaterials are the state-of-the-art catalysts for HER and OER,respectively.However,the high price and scarcity severely restrict their commercial application.As a result,developing high-efficiency,economic and green electrocatalysts is very essential yet urgent.In this work,we have prepared three three-dimensional self-supported transition metal-based electrocatalysts on nickel foam（NF）,showing high HER or OER activity and stability.（1）Synthesis of three-dimensional self-supported straw-like phosphorus-doped Co₂MnO₄ nanoneedle arrays for hydrogen evolution reactionThe three-dimensional straw-like phosphorus-doped Co₂MnO₄ nanoneedle arrays supported on NF（P-Co₂MnO₄/NF）were prepared by successive hydrothermal treatment,oxidation,and P doping for high-efficiency HER.Benefiting from the unique needle-like arrays and compositions advantages,P-Co₂MnO₄/NF exhibited excellent HER activities with the low overpotentials of 33,43 and 102 mV to afford the current density of 10 mA cm^-2 in 1.0 M KOH,0.5 M H₂SO₄ and 1.0 M PBS,respectively,coupled with the long-term electrochemical stability.（2）Synthesis of three-dimensional self-supported walnut kernel-like iron-cobalt-nickel sulfide nanosheets for oxygen evolution reactionThe simple room-temperature sulfuration method was developed for in situ synthesis of three-dimensional walnut kernel-like iron-cobalt-nickel sulfide nanosheets on NF（FeCoNiS_x/NF）.The unique nanosheets exposed abundant active sites and offered large electrochemically active surface area.The FeCoNiS_x/NF exhibited high OER activities with the small overpotentials of 231 and 268 mV to afford 10 and 50 mA cm^-2 in 1.0 M KOH,respectively,coupled with a small Tafel slope of 55 mV dec^-1.Furthermore,the as-fabricated FeCoNiS_x/NF and Pt/C/NF were used as the anode and cathode for overall water electrolysis in 1.0 M KOH solution,respectively.The FeCoNiS_x/NF||Pt/C/NF only needs 1.52 V cell potential to afford 10mA cm^-2.（3）Synthesis of three-dimensional self-supported ultrathin rhodium iridium nanosheets for hydrogen evolution reactionThe three-dimensional ultrathin rhodium iridium nanosheets（NSs）were successfully prepared on NF（RhIr NSs/NF）by a simple one-pot aqueous synthesis strategy under room temperature.The obtained free-standing RhIr NSs/NF catalyst performed an enhanced HER activity in pH-universal media.Concretely,RhIr NSs/NF only needs the overpotentials as low as 15 and 14 mV to achieve 10 mA cm^-2current density in 1.0 M KOH and 0.5 M H₂SO₄ with excellent stability,respectively,compared favorably with most reported Pt-free catalysts.