The Synthesis And Mechanism Investigation Of Nano Catalysts Based On Mo And Co Towards Water Splitting

The rapid development of global economy has led to serious energy and environmental problems.With increasing demand for renewable energy,there are intense efforts to develop various types of energy storage and conversion systems.Hydrogen is considered to be one of the most promising new energy.The efficient electrochemical and photo/photoelectrochemical conversion of water to hydrogen by catalysts offers an attractive way to store energy.Currently,typical catalysts used for the hydrogen evolution and oxygen evolution reaction?HER and OER?in electrolysis cells are still precious metal-based materials?Pt,RuO₂,and IrO₂?.Therefore,replacing noble metal-based electrocatalysts with highly efficient and inexpensive non-noble metal-based hydrogen and oxygen electrocatalysts is critical for the practical applications of these technologies.Herein,we have summarized the catalysts used for the electrocatalytic hydrogen and oxygen evolution reaction,as well as the cocatalysts for the photo/photoelectrocatalytic hydrogen evolution.We have designed the MoS₂ based materials as the catalysts for facilitating the electrocatalytic hydrogen evolution as well as the cocatalyst for improving the semiconductor's photo/photoelectrocatalytic performance.OER has more sluggish kinetics,therefore we studied transition metal oxides,especially cobalt oxides,as catalysts to reduce the OER energy barrier and to further accelerate the overall water splitting process.Additionally,carbon based material have been reported as a bifunctional catalyst to simultaneously generate H₂ and O₂ in alkaline solution.The composite of layered MoS₂ nanosheets supported on a 3D graphene aerogel network?GA-MoS₂?has been synthesized by two-step hydrothermal method.The flexible graphene sheets partially overlap in 3D space to form an interconnected porous microstructure,which greatly prevents serious restacking of graphene,and further provides large surface area for growing MoS₂ nanosheets.GA-MoS₂ maintains an excellent porous structure and assembles with MoS₂ nanosheets around the edge of the pores,providing relatively large amounts of exposed edge sites for hydrogen evolution.The 3D structure of the catalyst can also supply efficient conducting network for rapid electronic transport during the electrocatalytic process,offsetting the poor intrinsic conductivity of MoS₂,facilitating fast electron transfer.Therefore,GA-MoS₂ exhibits high catalytic performance and strong stability for electrocatalytic HER application.We have prepared a p-type semiconductor of Cu₂O decorated with MoS₂ nanosheets as cocatalyst for efficient solar hydrogen production under visible light.The existence of cocatalyst MoS₂ leads to an efficient interfacial photo-generated electron transferring from the surface of Cu₂O to MoS₂.MoS₂ namosheets act as the active sites for the hydrogen evolution,lowering the electrochemical proton reduction overpotential.Results show that Cu₂O decorated with 1.0 wt%MoS₂ exhibits the maximum reduction photocurrent density of 0.17 mA cm^-2 and H₂-production activity of 50?mol h^-1.Moreover,the presence of MoS₂ as cocatalyst at the surface of Cu₂O also greatly prohibits the photocorrosion of MoS₂@Cu₂O catalyst.The as-prepared MoS₂@Cu₂O exhibits remarkable photostability with only 7%loss of its original photocurrent after 9 h of continuous work.Graphene-Co₃O₄ composite with a unique sandwich-architecture is successfully synthesized and applies as an efficient electrocatalyst towards OER.The graphene nanosheets act as a binder to link neighboring Co₃O₄ particles together and Co₃O₄nanocrystals are homogeneously attached on both sides of graphene nanosheets.The existence of graphene highly increases the conductivity of the composite.The obtained composite shows enhanced catalytic activities in both alkaline and neutral electrolytes.The current density of 10 mA cm^-2 has been achieved at the overpotential of 313 mV in 1 M KOH and 498 mV in phosphate buffer solution,respectively.Furthermore,there is no obvious current density decay after the stability test.An active catalyst composed of porous graphene and cobalt oxide?PGE–CoO?has been synthesized,demonstrating high porosity,large specific surface area and fast charge transport kinetics.The catalyst also exhibits excellent electrochemical performance towards OER with a low onset potential?504 mV vs.Ag/AgCl?and high catalytic current density(overpotential of348 mV for 10 mA cm^-2).The enhanced catalytic activity could be ascribed to porous structure,high electroactive surface area and strong chemical coupling between graphene and CoO nanoparticles.Moreover,CoO nanoparticles are wrapped by the porous graphene,inhibiting the corrosion phenomenon,thus this OER catalyst also shows good stability in the alkaline solution.The high performance and strong durability suggest that the porous structured composite is favorable and promising for water splitting.We report the successful synthesis of cobalt nanoparticle embedded porous nitrogen doped carbon nanofibers?Co-PNCNFs?by a facile and scalable electrospinning technology.The electrospun Co-PNCNF composite exhibits a low onset potential of 1.45 V?vs.RHE?along with high current density(overpotential of 285 mV for 10 mA cm^-2)towards OER.The exceptional performance could be ascribed to the bi-functionalized CNFs with nitrogen doping and cobalt encapsulation,which can convert the inert carbon into active sites.Moreover,the porous structure and synergistic effect further provide a highly effective surface area and facilitate a fast electron transfer pathway for the OER process.Interestingly,the Co-PNCNF composite also displays the capability for the hydrogen evolution reaction?HER?in alkaline solution.A water electrolyzer cell fabricated by applying Co-PNCNFs as both anode and cathode electrocatalysts in alkaline solution can achieve a high current density of 10 mA cm^-2 at a voltage of 1.66 V.