In Situ Synthesis Of Transition Metal Oxide/Hydroxide Catalysts On Nickel Foam For Water Splitting

Developing highly performence and cheap electrocatalysts with supernor stability for both the oxygen evolution reaction(OER)and hydrogen evolution reaction(HER)is a grand challenge to produce hydrogen by water electrolysis.On the other hand,urea electrolysis offers the prospect of energy-saving H2 production together with urea-rich wastewater purification,but the lack of inexpensive and efficient urea oxidation reaction(UOR)electrocatalysts places constraints on the development of this technique.It has been reported that the surface of the transition metal(nitrogen,phosphorus,sulfur)was oxidized to an oxide or hydroxide under strong oxidizing conditions.Therefore,the actual reactive site of OER should be surface metal oxide/hydroxide.Against these backgrounds,three kinds of transition metal oxide or hydroxide catalysts were developed for hydrogen production under alkaline conditions.The main works were as follows:(1)Charpter 1:The FeOOH/CoO core-shell nano wires were directly grown on three-dimensional(3D)nickel foam by hydrothermal deposition method.The water splitting performance of the catalysts were investigated by electrochemical characterization methods such as linear scanning voltammetry(LSV),cyclic voltammetry(CV),electrochemical impedance(EIS)and the time-current(I-T).The results show that the combination of the 5-FeOOH/CoO/NF catalyst with highly electro-conductive substrate exhibits exceptionally good electrocatalytic activity and long-term durability towards OER and HER in alkaline electrolyte.The optimal 5-FeOOH/CoO/NF catalyst presents low overpotentials of 256 and89 mV to reach a current density of 10 mA cm-2in alkaline electrolyte for OER and HER,respectively.Using the 5-FeOOH/CoO/NF as bifunctional catalysts for overall water splitting,a current density of 10 mA cm-2 was achieved at a voltage of 1.578 V.(2)Charpter 2:The NiCoFe-LDH catalyst was successfully prepared by simple hydrothermal method using Co(NO3)2.6H20,FeCl3·6H2O and urea as raw materials.The morphology and structure of the materials were characterized by X-ray diffraction(XRD),fourier transforming infrared spectrum(FT-IR),scanning electron microscope(SEM),X-ray photoelectron spectroscopy(XPS)and transmission electron microscopy(TEM).Moreover,electrochemical technologies such as LSV,CV,EIS and the potential-time(P-T)were used to study the effect of trace doping of cobalt on the activity of the catalyst.The results demonstrated that the 3D self-supporting catalysts exhibited outstanding OER activity as well as excellent HER performance in an alkaline medium.For the OER,the overpotential and 2,Tafel slopes of the NiCoFe-LDH/NF catalyst at 10 mA cm-2 current density were 208 mV and 48.41 mV-dec-1,respectively.For the HER,the overpotential and Tafel slopes required for the catalyst to achieve a current density of 10 mA cm-2 were 113 mV and 114.5 mV-dec-1,respectively.Using the NiCoFe-LDH/NF as bifunctional catalysts for overall water splitting,a current density of 10 mA cm-2 was achieved at a voltage of 1.570 V.(3)Charpter 3:The NiMoO4 precursors grown on nickel foam were prepared by hydrothermal method using Ni(NO3)2·6H2O and(NH4)6Mo7O24-4H2O as raw materials.A series of NiMoO4-x/NF catalysts were obtained by annealing at different temperatures(200??500?)in N2 atmosphere.The catalytic performance of NiMoO4-x/NF catalyst for HER and OER was tested by electrochemical characterization.The effect of catalyst on urea oxidation(UOR)performance was also studied.For the HER,the overpotential and Tafelslopes required for the NiMoO4-200/NF catalyst to achieve a current density of 10 mA cm-2 were 113 mV and 114.5 mV-dec-1,respectively.For the OER,the overpotential and Tafel slopes of the NiMoO4-300/NF catalyst at 10 mA cm-2 current density were 208 mV and 48.41 mV·dec-1,respectively.Furthermore,a rod-like NiMoO4-300/NF catalyst enabled highly efficient UOR electrocatalysis,which exhibits an ultralow potential of 1.359 V at 10 mA cm-2.Assembling an electrolytic cell using our developed UOR and HER catalysts as the anode and cathode can provide a current density of 10 mA cm-2 at a cell voltage of mere 1.380 v.