Preparation And Electrocatalytic Hydrogen Evolution Performance Of Tungsten-based Phosphides

Transition metal phosphides（TMPs）have been standing in the spotlight of hydrogen evolution（HER）catalysts owing to its excellent mechanical strength,high corrosion resistance,and easily tunable electronic structures.The hydrogen temperature-programmed reduction（H₂-TPR）method has attracted the attention of researchers as a relatively green and safe way for the synthesis of TMPs.However,the unavoidable high-temperature reduction process leads to the agglomeration of catalyst particles and the decrease of electrical conductivity,which will eventually impede the HER activity.Meanwhile,the shortage of fresh water resources has attracted much attention for seawater electrolysis,but it also poses great challenges to the activity and stability of catalysts.To solve these two bottleneck problems,a citric acid derived two-step aging strategy is proposed to synthesize carbon-supported TMPs,which effectively improves the negative effects caused by high temperature.Further,assisted by this method,the W₃P/WP/W catalyst containing tungsten-rich phosphide,W₃P,was synthesized for the first time by in situ H₂-TPR.HER performance was evaluated in p H-universal-freshwater electrolytes,simulated seawater,natural seawater,1 M KOH+seawater（for simulating commercial alkaline electrolysis,ALK）,and ion exchange resins purified seawater（for simulating proton exchange membrane electrolysis,PEM）.The main research contents are as follows:（1）A variety of bulk TMPs(Ni₃P,Ni₁₂P₅,Ni₂P,Fe₂P,Fe P,Co₂P,Co P,Mo₃P,Mo P,WP)were prepared by the H₂-TPR,and their HER performances were investigated in acid electrolyte.The results showed that WP had the highest HER activity.With WP as the main active phase,the effects of metal dopant（Cu,Fe,Ni,Co）and dopant ratio（M/W,M is the metal dopant）,phosphorus source ratio（P/W）and citric acid ratio（CA/W）were further investigated.Results unveiled that when the active phase is WP,the dopant is Co and Co/W=0.1（molar ratio,the same below）,P/W=2,CA/W=2,the synthesized Co WP2-CA2 has the highest HER activity,η₁₀=174 m V.（2）Since the traditional one-step aging method cannot effectively promote the complexation of the metal dopant,we proposed a CA-derived two-step aging strategy,which can effectively improve the negative effects caused by high-temperature synthesis.Co WP-CA/KB（two-step aging）prepared by this method has small-size particles,high dispersion,strong electronic metal-support interactions（EMSI）between WP and the carbon support,which leads to high conductivity.More importantly,this strategy can promote the dopant Co to intrude the lattice of WP to cause lattice distortion and improve the intrinsic activity.Co WP-CA/KB（two-step aging）exhibited an excellent HER performance in 0.5 M H₂SO₄:η₁₀=111 m V,the Tafel slope was 58 m V dec^-1,and a benign stability over 60 hours.A universal verification of CA-derived two-step aging strategy was conducted by changing seven carbon supports（BP2000,C,MC,DB,VXC 72,VXC 72R）to prepare supported Co WP-CA.This strategy can significantly improve the HER performance of all the catalysts,proving itself a universal synthesis approach.（3）Ternary-phase crystal catalysts（W₃P/WP/W,nominated as TPMs）with tungsten-rich phosphide,W₃P,were prepared by in situ H₂-TPR method for the first time.The defect vacancies of were further regulated by tuned the final reduction temperature.The characterization results demonstrate that the enhanced conductivity caused by the introduction of homologous metal W,the strong synergy between W₃P and WP,along with the defect sites highly enhanced HER activity.TPM-3（reduced at 850°C）showed excellent HER performance in p H-universal freshwater electrolytes.Theη₁₀ of TPM-3 are 94 m V and 146 m V in acidic and alkaline freshwater electrolytes,respectively.In seawater electrolytes,The HER performance of TPM-3 at a low current density is comparable to that of commercial Pt/C,and exceeds commercial Pt/C in stability and anti-toxicity at industrial grade high current densities.