Synthesis Of Cobalt-based Compounds As Electrocatalyts For Water Spiltting

At present,it is still a challenge to find and develop efficient and inexpensive transition metal compounds to replace noble metals as bifunctional electrocatalytic materials for hydrogen production from electrolytic water.Among them,cobalt-based phosphides and oxides have been widely reported in catalytic hydrogen evolution（HER）and oxygen evolution（OER）reactions due to their large reserves,stable chemical properties and Gibbs free energy of hydrogen/oxygen adsorption close to zero.Increasing the number of active sites and conductivity have become the most effective way to further improve the catalytic performance of materials.One dimensional materials have attracted much attention due to their large specific surface area and high electrical conductivity.Therefore,the synthesis of highly efficient and stable one-dimensional cobalt-based phosphides and oxides is expected to be a high performance electrocatalyst for electrolytic water plants.To this end,we have done two parts,respectively.（1）The mechanical mixture of cobalt trioxide（Co₃O₄）nanoparticles and sodium hypophosphite（NaH₂PO₂）was phosphated by solid phase method.By changing the reaction parameters,we realized the controllable synthesis of Co₂P nanorods,and found that the reaction temperature and holding time played a crucial role in the growth process.The electrochemical tests show that Co₂P nanorods are excellent bifunctional electrocatalytic materials for hydrogen evolution（HER）and oxygen evolution（OER）.In order to obtain more than 10 mA cm^-2,HER overpotential is 87mV and the OER overpotential is 310 mV.When applied to all water splitting devices,its current density reached 10 mA cm^-2 at the potential of 1.65 V,and the voltage remained stable after 24 hours of continuous electrolysis.By analyzing the morphology and phase of Co₂P nanorods,it was found that the excellent performance of Co₂P nanorods was mainly attributed to rod-like nanostructures rather than phase composition,which could expose more active sites,increase the contact interface between electrolyte and catalyst,and accelerate the charge transfer.It is more noteworthy that the yield of Co₂P nanorods obtained by this method has certain advantages compared with other literatures and similar results can be obtained by using commercial Co₃O₄,which show that this method has loose production conditions for industrial applications.（2）Using sulfur powder as sulfur source we demonstrate the synthesis of S_x-CoO microrods with adjustable oxygen vacancies（O_v）by a simple sulfur doping strategy.It is found that a trade-off between O_v and conductivity and active sites plays an crucial role in improving OER electrocatalytic activity.With the increase of S content,OER catalytic performances of S_x-CoO show a gradual upward trend,and all of them are superior to CoO.For S_6.64-CoO with the largest S content,its overpotential is 290mV at 10 mA cm^-2,which is about 120 mV smaller than that of CoO.The excellent electrocatalytic activity is mainly attributed to the fact that O_v adds the electrochemical active siteas of the material and improves its conductivity.This provides a simple and effective strategy to control the concentration of Ov in enhancing the OER electrocatalytic preformances of cobalt-based oxides.