Synthesis Of One-dimensional Transition Metal Phosphides And Investigation Of Their Catalytic Mechanisms Toward Hydrogen Evolution Reaction

Hydrogen is a clean and renewable energy source,which addresses the global concerns of energy crisis and environmental pollution arisen from over-consumption of fossil fuels.Currently,hydrogen is mainly produced by fossil fuel gasification,which has a high environmental contamination and is not a sustainable strategy.Water electrolysis is a simple and eco-friendly way to generate hydrogen energy in a large scale from abundant water source,and thus is of importance in the sustainable energy solutions.Water electrolysis involves hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?.It is critical to develop of noble-metal free catalysts substituing the high cost noble-based ones in water electrolysis for its practical applications.However,the noble-metal free catalysts have a sluggish HER kinetics leading to the increased electrical energy consumption.Therefore,highly active noble metal-free electrocatalysts toward HER are greatly demanded for water eletrolysis.Proton exchange membrane?PEM?electrolyzers with the merits of portable design and low contamination,and have gained great success.The PEM based electrolyzers require to operate in strong acidic condition,and platinum-based noble metals are currently the most efficient stable electrocatalysts for HER in the strong acidic electrolyte.Recently,transition metal phosphides are reported as a kind of efficient noble-metal free catalysts toward HER in an acidic condition,thus holding great promises for PEM based electrolyzers.One-dimensional nanomaterials have been intensively investigated for applications in various energy devices due to its unique physical structure.However,one-dimensional transition metal phosphides have not been systematically studied.In this thesis,a series of one-dimensional nanostructured transition metal phosphides are synthesized as electrocatalysts for HER,and their electrocatalytic behaviours are studied to expolore not only their scientific catalysis insights,but also the application potentials for acidic PEM electrolyzer applications.The main research contents and results of this thesis are summarized as follows:1.FeP has been studied as HER catalysts,but there is still a large room to further improve its electrocatalytic activity.One-dimensional nanostructured FeP could be fabricated to further improve the HER catalytic activity due to its superior physical structure;nevertheless,its controllable synthesis still faces great challenges.In this thesis work,well-controlled iron phosphide nanorods?FeP NRs?are synthesized via low-temperature phosphidation of Fe salt with a porous anodic aluminum oxide?AAO?template.The FeP NRs exhibit high catalytic activity toward HER in acidic solutions,showing an onset potential of⁴5 mV,a Tafel slope of 55 mV dec^-1 and nearly 100%Faradaic yield with good stability.Both the catalytic activity and stability of FeP NRs are greatly improved when compared to the bulk FeP prepared without AAO template.The catalytic performance enhancement of FeP NRs could be attributed to its unique one-dimensional structure.On one hand,the one-dimensional structured FeP NRs help to expose more active sites to electrolyte and increase the charge transfer rate for HER;on another hand,the voids between the nanorods are favorable to mass transport process as well as the hydrogen eveluion during HER.2.Materials with tubular structure are favorable for energy conversion applications due to its high surface area and unique physical structure.However,the fabrication and design of nanotube structured material is very challenge in material science,especially the synthesis of transition metal phosphides nanotube.In chapter 4,CoP nanotubes?CoP NTs?are prepared as an efficient catalyst for HER via a template-assisting strategy.As an electrocatalyst for HER,both the outer and inner surface of the CoP NTs can be accessed by the electrolyte,the electrochemical surface area is expected to be increased.Therefore,the CoP NTs catalyst is highly active toward HER,showing a low overpotential of 129 mV at a current density of 10 mA cm^-2 and a small Tafel slope of 60 mV dec^-1 with high Faradaic yield.The catalytic activity and stability are aggressively enhanced compare to CoP nanoparticle counterparts.The great electrocatalytic activity of CoP NTs catalyst probably attributed to its one-dimensional tubular structure,which is benefit to mass transport during HER,such as the reactants and produced hydrogen bubbles could transfer directly via the nanotube.Charge separation phenomenon is observed in P and Co of CoP NTs,in which the P and Co have partial negative and partial positive charge,which is similar to the configuration of hydrogenases.We believe that CoP NTs material shares the similar catalytic mechanism of hydrogenases.During the hydrogen evolution,the P site in CoP NTs with partial negative charge could incorporate proton to the active Co center followed been reduction to hydrogen.The Co sites in CoP NTs are the catalytic active sites for HER.3.A few research works on tungsten phosphides as an electrocatalysts toward HER have been reported,but the catalytic behaviors of one-dimensional tungsten phosphide is not well investigated.In chapter 5,porous tungsten diphosphide nanorods?WP₂ NRs?material with good crystallinity is synthesized as an efficient catalyst for HER by phosphidation of?NH₄?_0.25WO₃ NRs precursor.WP₂ NRs catalyst exhibits an onset overpotential of 56 mV toward HER,a Tafel slope of 52 mV dec^-1 and an exchange current density of 0.013 mA cm^-2 in acidic media.Further,the catalyst only needs an overpotential of 148 mV at a current density of 10 mA cm^-2.In particular,the catalyst can maintain its catalytic activity for 90 hours at least,showing a superior long-term operation stability.It also has excellent activity and stability under both neutral and basic conditions.Tungsten monophosphide?WP?catalyst also prepared via tuning the synthesis temperature,and its HER electrocatalytic activity is inferior than that of WP₂ NRs.The proton adsorption and desorption on the catalyst surface is the important process in HER,which dominates catalytic activities of electrocatalyst.P sites in tungsten phosphide are served as proton acceptors,while the high oxidation state W can tune the bond strength of proton to P site.The WP₂ NRs material with suitable proton adsorption/desorption bond strength ensures its good catalytic activity,but the WP catalyst with strong bond strength to proton thus stoping the hydrogen desorption in HER.4.Although the synthesis of various transition metal phosphides are reported,the strategy to fabricate transition metal phosphide nanofibers is still missing.In chapter 6,molybdenum phosphide nanofibers?MoP NFs?are synthesized as a catalyst for HER via the assistant of electrospinning technique.MoP NFs material is highly active toward HER,which is superior than that of bulk MoP.The MoP NFs catalyst exhibits a low charge transfer resistance in HER,which can be attributed to the following two reasons.For one hand,the unique one-dimensional structure of MoP NFs benefit to charge transfer.For another hand,the residual carbon comes from pyrolysis of PVP help to improve the conductivity of MoP NFs.The low charge transfer resistance of MoP NFs ensure the electron could directly transfer from current collector to the interface of catalyst and electrolyte for hydrogen evolution,thus improving the HER catalytic activity.In brief,four types of one-dimensional transition metal phosphides,including FeP NRs,CoP NTs,WP₂ NRs and MoP NFs,are synthesized and further developed as efficient catalysts for HER.FeP NRs and CoP NTs catalysts are very similar in view of the HER catalytic behaviors,while WP₂ NRs catalyst is same to MoP NFs.The catalytic activity of FeP NRs and CoP NTs is higher than that of WP₂ NRs and MoP NFs,but the stabilities are in opposite situation.For example,the CoP material is unstable in alkaline condition,while WP₂ NRs and MoP NFs are highly stable in all pH range.Except for the intrinsic activity of transition metal phosphides,the unique one-dimensional structure of as-synthesized transition metal phosphides is responsible for their superior catalytic performance.The one-dimensional structure can promote the HER via following three aspects:?1?increased surface area for exposure more active site to electrolyte thus improved the catalytic ability;?2?enhanced charge transfer rate for fast electrode kinetics;?3?the voids between the one-dimensional nanostructure are favorable to mass transport,especially benefits for hydrogen bubble penetrate during HER.This thesis project not only improves the widespread application of transition metal phosphides in water electrolysis for hydrogen production,but also offers a valuable operating experience for the synthesis of one-dimensional nanostructured transition metal phosphides.