Preparation Of Transition Metal Ni/Co Phosphides And Research Of Their Electrocatalytic Hydrogen-evolution Properties

Hydrogen has become one of the most attractive energy carriers due to its excellent environmental compatibility,renewability and high energy density.Currently,water splitting is recognized as an efficient method for hydrogen evolution reaction?HER?.Recently,hydrogen-evolution electrocatalysts have attracted the increasing attention because they can effectively reduce hydrogen-evolution overpotential,which results in low hydrogen-evolution cost.Among various HER electrocatalysts,transition metal phosphides are called as quasi-platinum catalysts owing to the similar properties to noble metals,and they are considered as promising catalysts for hydrogen evolution.However,catalytic performances of transition metal phosphides are not ideal owing to their poor conductivity and limited active sites.In this paper,NaOH was used as an activator to activate the precursor to create defects,resulting in exposing more active sites which can contribute to the superior performance.Then,fullerols C₆₀?OH?_16-24 and fullerene C₆₀ were introduced into the precursors to promote the HER performances by improving the electronic conductivity of the composites.Additionally,integrated electrodes were prepared to simplify the preparation procedure of electrode and improve its cycling life.?1?A kind of defect-rich Al-doped Co-P material was synthesized using Co-Al LDH as the precursor which was activated by NaOH as the activator,and its main structure remain unchanged after NaOH activation which only etches its local area.Nevertheless,the surface roughness for its phosphide of P-Na-Co-Al became larger and the structural defects increases after NaOH activation,which are beneficial to expose more active sites,thus resulting in high catalytic activity.Electrochemical tests display that,NaOH-activated P-Na-Co-Al electrode shows the optimal electrocatalytic activity for HER with a current density of 10 mA cm^-2 at a small applied overpotential(?₁₀)of only 86 mV in 0.5 M H₂SO₄.Additionally,it demonstrates excellent cyclic stability with a small increase rate for?₁₀ of only 2.3%.Electrochemical impedance spectrum?EIS?confirms that P-Na-Co-Al possesses the small charge transfer resistance and hydrogen adsorption resistance,indicating the facile electrochemical reaction for hydrogen evolution because of the promoted electron transport rate after NaOH activation.?2?C₆₀?OH?_16-24 was introduced into the precursor,and then C₆₀?OH?_16-24-decorated Co-P was successfully synthesized after phosphorizaition treatment.Herein,we also systematically investigated the electrocatalytic activity of the composites with different C₆₀?OH?_16-24-doped contents.The metal phase of the as-prepared composite is confirmed to be Co₂P,and it is composed of nanosheets attached by nanoparticles.The thickness of the nanosheets is⁵0 nm and the diameter of the nanoparticles is⁸ nm.Electrochemical analyses show the optimum doping content of C₆₀?OH?_16-24 is 21%and the corresponding composite CPC-21 presents the best electrocatalytic hydrogen-evolution performance with a current density of 10 mA cm^-2 at the applied overpotential of 88 and 124 mV in 0.5 M H₂SO₄ and 1.0 M KOH,respectively.Additionally,CPC-21 displays excellent catalytic stability,and the increase rate of?₁₀ is only 8.0%after 2000 cycles in alkalic system.EIS confirms that C₆₀?OH?_16-24-decorated Co-P shows the fast charge-transfer kinetics and hydrogen-adsorption kinetics,indicating that C₆₀?OH?_16-24 can effectively improve the elctrocatalytic activity by enhancing the electronic conductivity.?3?The doping of C₆₀ can effectively improve the electronic conductivity of Ni-Co-P,and affect the crystallinity,morphology and surface state of the product by affecting the mass transfer process during phosphorizaition treatment.The as-prepared C₆₀-decorated Ni-Co-P nanoparticles possess an average diameter of⁴ nm with rich structural defects.Electrochemical tests show that the as-obtained C₆₀-decorated Ni-Co-P catalyst with a C₆₀-content of 3.93 wt%presents the superior HER performance with a current density of 10mA cm^-2 at the applied overpotential of 97 mV in 0.5 M H₂SO₄.Moreover,it exhibits excellent hydrogen-evolution durability with an increase rate for?₁₀ of only 5.1%.Additionally,EIS confirms that the C₆₀-coupled Ni-Co-P electrode possesses the fast charge-transfer kinetics and hydrogen-adsorption kinetics,indicating C₆₀ decoration can improve the hydrogen-evolution activity by enhancing the electronic conductivity.?4?The foamed nickel-based integrated electrode Ni-foam-Ni-Co-P with the structure of nanosheet arrays was successfully prepared using the potentiostatic electrochemical deposition method,and the density and thickness of the nanosheet arrays are effectively regulated by controlling the Ni/Co ratio,which is proved to be also a significant factor to maintain the nanosheet arrays during phosphorizaition treatment.The nanosheet arrays for Ni-foam-Ni-Co-P electrode with the Ni/Co ratio of 2:1 remain unchanged and possess abundant structural defects after phosphorizaition treatment.In HER tests,it shows the optimal hydrogen-evolution performance with a current density of 10 mA cm^-2 at a low applied overpotential of 110 mV in 1.0 M KOH.Moreover,Ni-foam-Ni-Co-P electrode exhibits a long hydrogen-evolution life owing to the intimate contact between active material and current collector,and its catalytic activity is improved due to the activation during the long hydrogen-evolution operation.Electrochemical analyses demonstrate that the synergistic effect among abundant structural defects,3D interconnected nanoarray structure and the intimate contact between active material and current collector significantly contributes to the superior HER performance for Ni-foam-Ni-Co-P electrode.