Synthesis And Electrocatalytic Performances Of Cobalt-and Iron-based Phosphides And Their Composites

Developing a sustainable and high-efficient approach to obtain hydrogen is the key to achieving the hydrogen economy,which may be a viable totechnological route to solve the global energy and environmental problems.Electrocatalytic water splitting,driven by intermittent renewable energy such as solar energy and wind power,is an environmentally friendly and economic hydrogen production technology.It employ electric energy as an energy link to realize the cycle of hydrogen,and finally forms a net zero emission sustainable electric-hydrogen energy system to promote energy transformation.Therefore,the exploration of high-efficient,earth-rich elements based electocatalysts is highly urgent for the realization of efficient and low-cost electrocatalytic hydrogen production.To date,transition metal phosphides（TMPs）has been hailed as one of the most burgeoning catalysts towards water splitting,and one of the competive alternatives to precious metal based electrocatalyts.Therefore,we first studied the synthesis and electrocatalytic performance of mono-metal CoP,FeP and their composites hybriding with graphene oxide.Subsequently,one iron and cobalt bimetallic phosphide species was conducted,as a bifuntional electrocatalyt towards overall water spitting.Additonally,the synthesis and perfoemances of its composite decorating with N-dopped graphene were operated.The specific research contents and significant conclutions of this paper are as follows:1.Fe₃O₄ particles were synthesized by hydrothermal method,then pure porous FeP particles were prepared by phosphating,and their electrocatalytic hydrogen evolution reaction（HER）performance in alkaline electrolyte was studied.Then,graphene oxide（GO）was added during the synthesis of FeP to prepare porous particles in-situ grown on reduced graphene oxide（FeP@rGO）.Systematic study shows that the catalytic performance of FeP@rGO is significantly better than that of pure FeP in alkaline:the overpotential at 10 m A cm^-2 is-191 mV and Tafel slope is 67.2 mV dec^-1 for FeP@rGO,which is significantly lower than that of pure FeP(76.4 mV dec^-1).After 16 hours of current-time test,FeP@rGO still has high hydrogen evolution activity.The introduction of rGO can weaken the agglomeration phenomenon of the material during the growth process and also increase the electrical conductivity of the material.In addition,the porous structure can increase the electrochemically active area of the material,so that more catalytic active sites are exposed and the mass transfer process is accelerated.2.Flower-like CoP was synthesized through simple hydrothermal method and low temperature phosphorization process.Then GO was added and treated with hydrazine hydrate.Finally,the in-situ growth of nitrogen dopped graphene with cobalt phosphide nanocomposites was obtained（CoP@NG）.The electrocatalytic evaluation of HER shows that CoP@NG owns a low Tafel slope of 63.8 mV dec^-1 in acidic electrolyte and59.6 mV dec^-1 in alkaline electrolyte.When used as a bifunctional catalyst for oxygen evolution reaction（OER）in alkaline electrolyte,only 354 mV overpotential is needed to reach 10 m A cm^-2,and the Tafel slope is 63.8 mV dec^-1.N-doped graphene nanosheets with high conductivity and flexibility not only promote the electron transfer and mechanical and chemical stability,but also prevent the aggregation of CoP nanosheets and expose more active sites.3.A novel bimetallic phosphides,constructed by transforming dual-ligands co-coordination compound into nanorod-like FeCoP decorated with nitrogen doped graphene（DLC FeCoP@NG）and its pure counterpart（DLC FeCoP）,synthesized via a facile room-temperature co-coordination reaction followed by phosphorization process.Compared with single-ligand coordination derived phosphides/nitrogen doped graphene hybrid counterparts（such as SLC CoP@NG,SLC FeP）and DLC FeCoP,the DLC FeCoP@NG is undoubtedly the best bifunctional catalyst with excellent performance.Tafel slope of DLC FeCoP@NG is 49.2 mV dec^-1 in OER,and 66.7 mV dec^-1 in HER.When a two-electrode electrolytic cell is assembled for overall water splitting,10 m A cm^-2 can be obtained at a voltage of only 1.63V,and continuously processing for 30hours.On the one hand,the graphene oxide induces FeCoP nanorods to grow on its surface and intertwines between FeCoP nanorods,exposing a large number of active sites.On the other hand,bimetallization changes the electronic structure of the material.