Design,Synthesis,and Studies Of Copper-based Materials For Electrocatalytic Water Splitting

Electrocatalytic or photocatalytic water splitting to produce hydrogen,is one of the most promising pathways to solve energy crisis and environmental pollution.In the literature,precious metals-based materials(Ru,Ir,and Pt)have been developed as very efficient water oxidation/reduction catalysts.However,owing to the low abundance and high cost of these noble metals,the practical applications of these catalysts are significantly hampered.Therefore,many efforts have been devoted to find efficient,earth-abundant,and low-cost transition metal-based materials to catalyze water splitting.Copper-based materials,as one member of the first-row transition metals,were less been explored for catalytic water splitting reaction.There are only few Cu(?)complexes reported for water oxidation and no copper-based heterogeneous water oxidation catalysts(WOCs)were reported.This present thesis is to study various kinds of copper-based materials for electrocatalytic water oxidation and hydrogen production.Chapter 1 is a review of the field for water oxidation and reduction.It introduces an overview of the background of the energy crisis and environmental pollution.Subsequently,the theory of nature photosynthesis and the structure of oxygen evolution center(OEC)in photosystem ?(PS?)are reviewed.The research progresses of water oxidation catalysts including precious metals,and non-precious metals(especially copper-based materials)are discussed.Chapter 2 includes two parts about copper oxide(CuO)heterogeneous catalysts for water oxidation.The first part studied nanostructured CuO thin films electrodeposited on conductive fluorine doped tin oxide(FTO)plates from copper(?)pyridine complexes for catalytic oxygen evolution reaction(OER).This is the first report of copper-based heterogeneous WOCs.The second part investigated copper(?)ethylenediamine complexes as different precursors to deposit CuO materials which exhibited a much better activity toward OER.A catalytic current density of 1.0 mA/cm2 and 10 mA/cm2 for the copper oxide material requires the overpotentials of only-370 mV and 475 mV in 1.0 M KOH with a high Faradaic efficiency of>95%.Chapter 3 includes three parts about CuO materials synthesized from simple copper salts for OER.The first part is to study the effect of different morphologies(microsphere,nanosheets,nanowires).Among these three morphologies,CuO nanowires exhibited the best performance for OER,with the lowest onset overpotential of-340 mV in 0.1 M KBi(pH 9.2).A catalytic current density of 0.1 mA/cm2 and 1.0 mA/cm2 for the CuO nanowires requires the overpotentials of only-430 mV and 550 mV,respectively.The Tafel slope is 54.4 mV/dec.The second part introduces an annealing approach to synthesize a binder-free,self-supported CuO catalyst on conductive FTO electrode for OER.Under optimal conditions,the CuO-based OER catalyst showed an excellent activity.A catalytic current density of 1.0 mA/cm2 and 10 mA/cm2 for this annealed CuO material requires the overpotentials of only-430 mV and 580 mV,respectively.The Tafel slope is 61.4 mV/dec.The third part examines CuO nanosheets synthesized by a simple molten salt method for OER.A catalytic current density of 10 mA/cm2 for the CuO nanosheets requires the overpotentials of only-420 mV.Furthermore,when loading the CuO nano sheets on multi-walled carbon nanotubes(MWCNTs),the amount of CuO catalyst is as low as 0.14 mg/cm2 and the catalytic performance is enhanced to achieve a high catalytic current density.Chapter 4 presents the studies of Cu20 materials as an excellent electrocatalyst precursor for OER.Cu2O thin films were facilely electrodeposited on FTO substrates from a simple Cu(II)salt solution under a very low applied potential(-0.17 V or-0.23 V vs.Ag/AgCl).Two morphologies(dendritic branching and cluster-like)were obtained under different potentials.Both Cu2O films can be used for OER,and the dendritic branching Cu2O material exhibited a better performance.Under optimal conditions,water oxidation was achieved under an onset potential at 0.92 V(vs.Ag/AgCl)in 0.1 M borate solution at pH 9.2.A catalytic current density of 0.1 mA/cm2 requires a low overpotential of-430 mV.The slope of the Tafel plot is 59.9 mV/dec and the Faradaic efficiency is close to 93%.In Chapter 5,a Cu(0)catalyst electrodeposited from copper(II)ethylenediamine complex is studied for electrocatalytic hydrogen evolution reaction(HER).In neutral water(0.1 M KPi,pH 7.0),an artificial leaf-like Cu(0)-based catalyst was prepared from the complex under a reductive potential.The catalyst showed an onset catalytic potential for the HER at a very low overpotential of 70 mV.To reach 1.0 mA/cm,the catalysis required a low overpotential of 157 mV and the Faradaic efficiency is>97%.Furthermore,this leaf-like Cu(0)-based catalyst also exhibited high and stable activity toward HER in strongly basic(1.0 M KOH,pH 13.6)and acidic(0.5 M H2SO4,pH 0.4)solutions.Chapter 6 studies copper-based bifunctional catalyst composite for both hydrogen production and water oxidation reactions for the first time.The catalyst was prepared from a series of water-soluble copper complexes as catalyst precursors.Under an applied cathodic potential in 0.1 M KBi(pH 9.2),a thin catalyst film(H2-CuCat)was generated on a FTO electrode,which was composed of Cu2O and Cu(OH)2.This H2-CuCat material can catalyze both HER and OER.The H2-CuCat material can be converted to O2-CuCat material for OER under anodic potential,which was composed of Cu2O,Cu(OH)2,and CuO.The H2-CuCat catalyst is robust and active for water reduction to produce hydrogen at a low onset overpotential of-450 mV with a high Faradaic efficiency of-100%.In addition,the O2-CuCat catalyst showed a low onset overpotential of-330 mV with a high Faradaic efficiency of-97%.Chapter 7 summarizes the research of this dissertation,and presents recommondations of copper-based materials toward water splitting for future resesarch.