| Hydrogen,an abundant and sustainable fuel with 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 the efficient 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.Up to now,noble metal Pt and Pt-based compounds have been known as the most active electrocatalysts toward hydrogen evolution reaction(HER),however,their high expense and low earth-abundance significantly hinder their widespread usage.Therefore,it still motivates intensive research efforts to explore and develop highefficiency,stable and non-noble metal HER electrocatalysts.In the past few years,tungsten diphosphide(WP2)have been reported as excellent electrochemical catalysts toward HER.Therefore,this doctoral thesis focuses on the fabrication of the WP2 nanostructures and the study of their electrochemical performance.The main contents in this thesis are as follows:(1)Monoclinic tungsten diphosphide(α-WP2)and orthorhombic tungsten diphosphide(β-WP2)particles were synthesized through a phase-controlled phosphating reaction route via vacuum encapsulation technique,and were evaluated as HER electrocatalysts.However,α-WP2 exhibit better catalytic activity than β-WP2 with a lower overpotential for achieving cathodic current density of 10 m A cm-2 and a smaller Tafel slope.Combining experimental measurements and theoretical calculations based on density functional theory,we conclude that the higher catalytic activity of α-WP2 can be attributed to the higher electron conductivity,the lower kinetic energy barrier of H atom adsorption on catalysts surface for HER and longer H-P bond length for effective desorption of H atom to form H-H bond.(2)Polymorphic tungsten diphosphide(p-WP2)nanoparticles with mixed monoclinic(α-)and orthorhombic(β-)phases are synthesized by phase-controlled phosphidation route via vacuum capsulation and explored as a novel efficient electrocatalyst towards HER.The p-WP2 catalyst delivers superior performance with excellent stability under both acidic and alkaline conditions over its single phases due to the synergistic effect between the α-WP2 and β-WP2.(3)Self-supported tungsten diphosphide nanowire arrays on carbon cloth(WP2 NWs/CC)was successfully fabricated.Such binder-free flexible HER cathode with integrated three-dimensional nanostructures can not only provide a large surface area to expose luxuriant active sites,but also facilitate the electrolyte penetration for electrons and electrolyte ions.The WP2 NWs/CC electrode exhibits superior catalytic performance,it needs overpotential of 109 m V with a small Tafel slop of 56 m V dec-1 to achieve current density of 10 m A cm-2.Furthermore,high stability is also observed for the WP2 NWs/CC.(4)Porous WP2 nanosheet arrays integrated on graphite paper(P-WP2 NSs/GP)as 3D flexible cathode for electrocatalytic hydrogen evolution are prepared by in situ phosphidation via vacuum encapsulation assisted by pulsed laser deposition technique(PLD).Compared to the electrode without the seed layer prepared by PLD,the catalytic activity is improved,because the PLD pre-deposition increases the effective catalyst loading which increases the active site and reinforces the electrical contact between the active materials and the substrate which enhances the charge transport kinetics of the electrode.(5)Self-supported 3D Mo doped WP2 nanowire arrays on carbon fiber paper(MoWP2 NWs/CFP)is prepared as an efficient HER monolithic electrode.The results indicate that the electrocatalytic performance can be significantly improved via doping.The sample with the Mo: W mole ratio of 5% only needs overpotential of 96 m V to afford the current density of 10 m A cm-2 with a small Tafel slop of 48 m V dec-1,which displays the best HER catalytic activity ever reported among the tungsten phosphides.The Mo doping enhances the electronic conductivity,increases the electrochemical active surface area and modifies the binding energies of P in the WP2,which notably improves its HER activities. |
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