Synthesis And Catalytic Performance Of The Composite Electrocatalyst Based On Transition Metal Phosphide And MXene

The usage of catalysts to reduce reaction energy barriers of cathodic hydrogen evolution reation（HER）and anodic oxygen evolution reation（OER）is crucial in electrolysis process of hydrogen production.At present,noble metal based electrocatalysts have been proved to be outstanding in performance,except the high price which limits the usage in industrial production.Therefore,the development of cost-effective transition metal-based catalysts can effectively promote the development of industrial water electrolysis for hydrogen production.Since most transition metal-based catalysts cannot perform stable OER ability under acidic conditions,it is significant to investigate stable and efficient OER electrocatalyst under alkaline conditions.In addition,compared with HER,OER is kinetically sluggish which makes it the bottleneck of water electrolysis.Compared with sluggish OER process,urea oxidation reaction（UOR）can significantly reduce anode potential and accelerate cathodic hydrogen production.Based on this strategy,this thesis successfully coupled layered double hydroxide（LDH）with two-dimensional Ti₃C₂T_x MXene via two methods,and further optimized the catalyst using vapor phosphorization to obtain the efficient bifunctional electrolytic water as well as the UOR catalyst.Details are as follows:1.Cobalt iron phosphide（denoted as CoFe-P）nanosheets were successfully coupled with Ti₃C₂T_x MXene via refluxing and vapor phosphorization.By modulating the relative ratio of Ti₃C₂T_x MXene to CoFe LDH precursor,the effect of MXene on morphological structure and catalytic performance was investigated.Characterization showed that Ti₃C₂T_xMXene and P as electron acceptors can effectively promote the electron’s transmission from Co and Fe,which is conducive to the formation of high-valent metal ions thus enhance the OER activity of the material.The OER overpotential of phosphatized CoFe-P/Ti₃C₂T_x is270 m V(j=10 mA cm^-2),and only 337 m V is needed to reach the high current density of 50mA cm^-2.The activity of the catalyst is obviously better than that of relative Co,Fe-based catalysts,and also better than that of noble metal catalyst RuO₂.2.Cobalt iron phosphide（CoFeP）nanosheets were grown vertically on Ti₃C₂T_xMXene modified carbon cloth（CC）by means of two-step electrodeposition and gas-phase phosphorization.The materials with different morphologies and structures were obtained by controlling the time of electrodeposition.The HER,OER of the obtained CoFeP-1000@Ti₃C₂T_x/CC needs only 106.6 and 276 m V overpotential to reach the current density of 10 mA cm^-2.At 50 mA cm^-2 current density,the HER and OER overpotential of the catalyst are 173.5 and 369.5 m V,respectively.The material requires a cell voltage of only 1.59 V(j=10 mA cm^-2)for use in overall water splitting electrolyzer.Also,the UOR of this material requires only 1.359 V vs.RHE(j=10 mA cm^-2),which is 147 m V smaller than the OER potential,demonstrating that the anode potential can be effectively reduced by using UOR instead of OER.The material was used in a urea-assisted electrolyzer with a cell voltage of only 1.475 V at a current density of 10 mA cm^-2.The results showed that the CoFeP-1000@Ti₃C₂T_x/CC had excellent overall water splitting and urea electrolysis performance.