Synthesis Of Transition Metal-based Nanosized Electrocatalysts For Electrochemical Water Splitting

The promise of hydrogen as a sustainable,secure,and clean alternative energy carrier that can satisfy the growing global energy demand will be realized only through its efficient,low-cost,and environmentally friendly production.Water electrolysis to give hydrogen fuel-one of the ideal hydrogen fuel producing methods-is limited by the oxygen evolution reaction?OER?.This reaction is kinetically sluggish and demands an efficient electrocatalysts which can accelerate the reaction rate,lower the overpotential and possess a long-term stability for OER.Currently,noble metal-based catalysts such as IrO₂ and RuO₂ presents good OER performance,but their large-scale application is restricted by the scarcity and high costs.Accordingly,extensive research efforts have been devoted to the development of high-performance earth-abundant OER electrocatalysts based on transition-metal elements that are inexpensive,sufficiently active,and stable upon prolonged exposure under oxidizing conditions.The purpose of this dissertation is to design highly-efficient,cheap,and stable transition-metal-based electrocatalyst for oxygen evolution reaction.We designed and synthesized the monometallic chalcogenide nanocube,bimetallic NiCo oxides and NiFe sulfides,and trimetallic NiFe Cr layered double hydroxides and studied their OER activity and stability.The main contents and conclusions of this dissertataion are listed as follows:?1?The monometallic NiS₂ as an OER catalyst precursor was parepred and studied the OER performance under alkaline conditions.The monometallic NiS₂ nanocubes was synthesized by a simple hydrothermal method,and then this precursor was in situ electrochemical conversed to Ni?OH?₂?NiS₂-DH?at oxidation conditions.The OER activity was compared between NiS₂-DH and directly synthesized Ni?OH?₂ in basic solution.We aim to study the mechanism of the difference of OER performance about this two electrocatalysts.The experimental results show that NiS₂-DH presents a better OER activity with good electrical conductivity and larger electrochemical active surface area than the directly prepared Ni?OH?₂.This work suggests that the chalcognide is not stable under alkaline condition when the chaocognide as an OER electrocatalyst.The real active materials are hydroxide that was formed from the surface of OER catalyst precursor.We also explain the reasons why the OER performane of NiS₂-DH better than Ni?OH?₂.?2?The bimetallic NiCo O₂ nanowires and FeNi₂S₄ nanoplates were designed and synthesized based on the synergistic effect of valance electron in transition-based-metal bimetallic catalysts.The electrochemical results suggest that the overpotential of NiCo O₂ is 303 mV at the current density of 10 mA cm^-2 in 0.1 M KOH solution,which this value is lower than the monometallic NiO and Co O and comparable with noble-metal catalyst RuO₂.Furthermore,the OER catalyst precursor of bimetallic FeNi₂S₄nanoplates was in situ electrochemical conversed to hydroxides?FeNi₂S₄-DH?.The overpotential of FeNi₂S₄-DH and NiFe LDH are 320 mV and 355 mV at the current density of 50 mA cm^-2 in 1 M KOH solution.This work suggests that the OER performance of bimetallic oxides are better than the monometallic oxides and doping is an effective method to improve the OER performance of monometallic electrocatlysts.?3?Cr was doped into the bimetallic NiFe LDH that is a benchmark among non-noble metal electrocatalsts based on the unique electronic structure(t³_2ge_g⁰)of Cr and synthesized the trimetallic NiFeCr LDH by a one-step hydrothermal method.The electrochemical results show that the overpotential of NiFeCr is 280 mV at the current density of 10 mA cm^-2 in 0.1 M KOH solution,which suggest the introduction of Cr really improve the intrinsic activity of bimetallic NiFe LDH.This activity is due to enhancing electron interactions within the metal hydroxide merix typically has a positive effect on OER activity,which possible synergistic roles between redox-acitve cations?Ni,Fe,and Cr?and Lewis-acid cations?Fe and Cr?.Meanwhile,metal cations in the layers are bound to each other with bridging hydroxo linkages.To further improve the OER performance of trimetallic NiFe Cr LDH,we directly grow trimetallic NiFeCr LDH on carbon paper to fabricate 3D electrode and test the OER performance.The overpotential of integrated 3D electrode is 225 mV at the current density of 25 mA cm^-2 in 1 M KOH solution.Furthermore,X-ray photoelectron spectroscopy,electron paramagenetic resonance spectroscopy,and elemental analysis confirmed the durable stability of trimetallc NiFeCr LDH before and after OER measurement.This work synthesized the trimetallic NiFeCr LDH with higher intrinsic activity and good durability and provided general strategies to design more complex earth-abundant LDH catalyst in the future.