Preparation And Electrocatalytic Performance Of Self-supported NiFe-based Three-dimensional Electrodes

With the accelerated consumption of fossil fuels and environmental pollution,people are committed to finding renewable clean energy sources.Hydrogen energy is considered to be one of the most ideal renewable energy for replacing fossil fuels.Electrochemical water splitting is an effective technique for hydrogen production.The search for efficient overall water splitting electrocatalysts plays an important role for hydrogen production.However,the noble-metal catalysts are not suitable for the large-scale hydrogen production due to their high cost and scarcity.In particular,non-precious metal based electrocatalysts are unable to integrate the bifunctional hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?properties into the same system.Therefore,the overall efficiency of water splitting hydrogen production cannot be improved.This project aims to develop low-cost,efficient and stable self-supported bifunctional electrocatalysts for overall water splitting.The overall water splitting performance can be promoted on the basis of the electron coupling effect between Ni and Fe.Therefore,a series of self-supported Ni/Fe-based three-dimensional bifunctional electrocatalysts have been desined and prepared in this paper.Owing to the excellent bifunctional HER/OER properties of transition-metal phosphides?TMP?,we first synthesize a free-standing carbon cloth electrode consisting of Fe-doped Ni₅P₄ ultrathin porous nanosheet arrays via hydrothermal reaction and subsequent phosphorization.The introduction of trace Fe?2.5 atom%?in Ni₅P₄ is effective to modulate the adsorbates energy for both HER and OER,which is promising for the rational design of bifunctional transition metal phosphides based water splitting electrocatalysts with high performance.The as-prepared Fe-doped Ni₅P₄ delivered the current density of 10 mA cm^-2 at 131 mV for HER,and needed257 mV to reach the current density of 100 mA cm^-2 for OER.Overall water splitting required only 1.50 V to reach the current density of 10 mA cm^-2.Meanwhile,Fe-doped Ni₅P₄ could effectively catalyze the hydrolysis of NaBH₄ with hydrogen generation rate of 0.175 L g^-1 min^-1 and activation energy of 53.41 kJ mol^-1,comparable to most transition metal based electrocatalysts.In order to satisfy the industrial requirements,it is necessay to develop an facile and low-cost strategy for the preparation of electrocatalysts,which could produce H₂with with high current density and low overpotential for long-term durability.Present work reported a three-dimensional amorphous interwoven Ni/Fe?hydroxyl?oxide nanosheet arrays grown on a commercial nickel foams via“top down”soft etching method.The coexistence of Ni³⁺and Fe³⁺can effectively improve both HER and OER properties.The overpotentials for Ni/Fe-based catalyst were 175 mV for HER at the current density of 10 mA cm^-2,and 237 mV for OER at the current density of 20 mA cm^-2.In addition,the voltage of overall water splitting were 1.691 and 1.787 V at the current density of 20 and 50 mA cm^-2,respectively.The sluggish electrocatalytic kinetics of OER process was replaced by the urea oxidation reaction?UOR?,which has lower thermodynamic potential.The voltage of the two-electrode urea electrolyzer was 1.735 V at the current density of 50 mA cm^-2,smaller than that of water electrolyzer.This work proposes two methods of preparing self-supported Ni/Fe transition-metal-based three-dimensional bifunctional electrocatalysts.The preparation process is simple and the cost is low.Simultaneously,the properties of HER and OER are improved as a result of the electron coupling effect between Ni and Fe,thereby increasing the activity of overall water splitting.