Nickel-iron-based Integrated Electrode: Electrochemical Preparation And The Application Of Water Splitting

Exploiting efficient energy conversion,storage and transportation technologies is the key to realize the sustainable energy development in the future.Hydrogen with high-quality energy density is considered to be the ideal clean energy carrier.Electrochemical decomposition of water to produce hydrogen is a very promising strategy for energy conversion and hydrogen production.The electrolysis of water is composed of two half reactions,the hydrogen evolution reaction（HER）at the cathode and the oxygen evolution reaction（OER）at the anode.Although only 1.23 V of voltage can be used to achieve electrochemical water splitting in theory,a higher voltage is usually required to drive the reaction during practical applications due to the slow HER and OER kinetics.Electrocatalysts are indispensable for lowering the reaction energy barrier and improving the energy conversion efficiency of electrolyzed water.Among various electrocatalysts for water splitting,the noble metal catalysts such as Pt/C and Ru O₂/Ir O₂ show excellent catalytic activity in HER and OER,respectively,but the low abundance and high price of noble metals limit their commercial application.In order to overcome the above problems,it is of great significance to develop high-efficiency non-precious metal catalysts with abundant reserves and low prices.In this thesis,therefore,several Fe-Ni-based hydroxides-based catalysts with improved electrocatalytic performances were successfully synthesized through the electrochemical deposition technology at room temperature.The main contents are listed as follows:1.The porous FeNi₃@FeNi LDH multilayer nanosheets on nickel foam（NF）were successfully prepared by one-step fast galvanostatic deposition route.In 1.0 M KOH,the as-prepared electrocatalyst presented excellent catalytic activity for OER and HER simultaneously.The as-constructed FeNi₃@FeNi LDH/NF electrode required only the overpotentials of 199 m V for OER or 106 m V for HER to reach a current density of 10 or-10 m A cm^-2.Also,employing the FeNi₃@FeNi LDH/NF as both the anode and the cathode in a two-electrode setup,the process of overall water splitting could be easily driven by 1.5V of commercial AA battery cell or commercial polycrystalline silicon solar cell.The above-mentioned attractive electrocatalytic activity benefits from the special morphology and structure that can expose more active sites,which is more conducive to the mass transfer and charge transfer in the electrocatalytic process.In addition,the I₃^-anion is generated as an intercalator during the synthesis process and the I₃^-anions act as the intercalation agent,which expands the distances between layers of the catalyst,further improving the electrocatalytic performances of the catalyst.2.The doping of foreign atoms into the electrocatalyst can effectively adjust the electronic structure,and further improve the electrocatalytic activity.The 3D integrated electrode of Ce-doped FeNi LDH（LDH=layered double hydroxide）with amorphous structure on a Ni foam supported open-framework nickel phosphate nanorods array（donated as CFN@VSB-5/NF）via a hydrothermal-electrodeposition two-step process.The as-constructed CFN@VSB-5/NF electrocatalyst merely required overpotential of 224 m V to generate the current density of 100 m A cm^-2 with the Tafel slope of 43.1 m V dec^-1,which was remarkably lower than FN@VSB-5/NF(₁₀₀=273 m V)without the addition of Ce.Also,the CFN@VSB-5/NF could continuously catalyze for 50 h under the current density of 500 m A cm^-2,presenting appealing durability at the large current density.Further investigations revealed that the superior OER electrocatalytic performance should be attributed to the high intrinsic activity,optimal surface and electronic structure of the as-constructed CFN@VSB-5/NF catalyst.Compared with the traditional FeNi based catalysts,moreover,the present CFN@VSB-5/NF integrated electrocatalyst also displayed the stronger OER electrocatalytic activity and the excellent stability at large current densities.3.The S-doped FeNi-LDH@VSB-5/NF（labeled as FNS@VSB-5/NF）integrated electrode was successfully prepared by the hydrothermal-electrodeposition method.The as-prepared FNS@VSB-5/NF integrated electrode has excellent OER electrocatalytic activity.In 1.0 M KOH solution,only overpotential of 208 m V is required at a current density of 100m A cm^-2,and the Tafel slope is as low as 35.1 m V dec^-1.Through physical characterization and electrochemical analysis,it was found that the excellent electrocatalytic performance of the electrode should be attributed to the following reasons:First,the nanorod precursor array produced in situ had a high degree of stability;Then,the introduction of S^2-ions could improve the conductivity of the FeNi LDH host and promote the rapid charge transfer during the catalysis process;Finally,this special layered porous structure facilitated the rapid gas release and the contact between the catalyst and the electrolyte.