Fabrication And Electrocatalytic Hydrogen Production Of WP₂ Nanosheets Assisted By Pulsed Laser Deposition

The increasing global energy demand on fossil fuel has made our economy susceptible to price spikes,which accompanying hostile climate changes as well as air pollution.The urgent need of sustainable and environmentally friendly energy drives the exploration of alternative sources to replace exhaustible fossil fuels.Hydrogen,an abundant and sustainable fuel with a high energy density and zero environmental impact,is a promising candidate to replace fossil fuels in the future.Among the technologies to produce hydrogen,electrochemical water splitting with effcient catalysts,which can achieve large cathodic current densities at low overpotentials,can offer a simple and promising route for large scale evolution of highly pure hydrogen.In the past few years,tungsten based compounds have been reported as electrochemical catalysts toward HER,including carbide,nitride,sulphide and especially oxide compounds.Transition metal phosphides?TMPs?,which are constructed by alloying transition metals?M?and phosphorus?P?,have metalloid characteristics and superior electrocatalytic activities,such as molybdenum,tungsten,iron,cobalt,nickel and copper based phosphides.The excellent electrocatalytic activity makes phosphides promising catalyst for large scale highly pure hydrogen evolution by electrochemical water splitting.This dissertation comprehensively reviews the development history of electrocatalytic water splitting,introduces the basic principles of electrocatalytic water splitting,analyzes the methods to improve the performance of electrocatalytic water splitting and summarizes the recent progress of water splitting.The thesis research mainly focuses on the controllable synthesis of WP₂ electrode and the performance of water splitting,which is rich phosphorus type transition metal phosphide.The enhancement mechanism is investigated for scientific insights.The details are summarized briefly as follows:?1?Preparation of WP₂/GP and its HER water splitting behaviorWO₃ thin film was deposited on graphite paper by PLD?WO₃/GP?,the thickness of WO₃ can be controlled by adjusting deposition time.WP₂ thin film were synthesized by phosphating WO₃/GP through a solid-state reaction method.The structure,composition and electrochemical hydrogen evolution performance of the WP₂/GP films was investigated.The overpotentials at 10 mA cm^-2 of current density for WP₂/GP films with thickness of 54,140,219 and 244 nm were 225,195,208 and 236 mV,respectively.?2?Preparation of three-dimensional porous structure of rich phosphorus WP₂ nanosheets array and its HER water splitting behavior.WO₃ seed layer was deposited on graphite paper by PLD,then WO₃ nanosheet was prepared uniformly on WO₃ seed layer by hydrothermal method.Three-dimensional porous structure of rich phosphorus WP₂ nanosheet array was synthesized through a solid-state reaction method.In addition,we also directly synthesized WO₃ nanosheet film on graphite paper by hydrothermal method and WP₂ nanosheet film was synthesized in same method.WP₂ nanosheet array was more compact when seed layer was deposited on graphite paper by PLD,which can get higher HER performance.WP₂ nanosheet array electrode has much lower overpotential of 156 mv to afford current densities of 10 mA cm^-2,which is superior to the WP₂ nanosheet.The Tafel slope for WP₂ nanosheet array(65 mV dec^-1)is much lower than that of the WP₂ nanosheet(80 mV dec^-1),which implies that WP₂ nanosheet array have a superior activity towards HER than WP₂ nanosheet catalyst.The HER performance of the WP₂ nanosheet array in 1.0 M KOH?pH 14?was also investigated,which needs overpotentials of 207 mV to deliver current densities of 10 mA cm^-2.WP₂ nanoparticles bursting larger H2 bubbles that usually pin at the film surface,among which the WP₂ nanosheets can wick the formed hydrogen bubbles and maintaining the catalyst-electrolyte interface.?3?Influnce of phosphating temperature on HER performanceWe prepared a series of samples with different phosphating temperatures of 700,800 and 900 ?,respectively.The samples show best HER performance when phosphating temperature was 800?.?4?DFT analysisTo understand the superior performance of WP₂,we further investigate the kinetic energy barrier of hydrogen atom adsorption on the catalyst surface by DFT calculations.The energy barrier for the transition state of hydrogen atom adsorption on the WP₂ slab was found to be 0.75 eV and H-P key bond length is 1.43 A,which make for the rapid migration of proton/electronic and H desorption and sorption on the active site.In summary,the works performed in the thesis reveal that seed layer,tempreture and nanostructure can be used to delicately improve the electrocatalytic performance,which can significantly improve the kinetic and thermodynamic energies for efficient HER.we further investigate catalytic mechanism by DFT calculations.