Synthesis And Performance Of Mo-doped ZIF-67-derived Electrocatalysts For Overall Water Splitting

The development of inexpensive,highly efficient,and stable noble metal-free bifunctional electrocatalysts is crucial for water electrolysis.Metal-organic frameworks（MOFs）consisting of metal nodes linked coordinatively with different organic ligands are interesting porous materials.Owing to the high surface area,large porosity,high concentration of active sites,tunable morphology and functionality,MOFs and MOF-based materials have found applications in CH₄/H₂storage,drug delivery,catalysis,gas separation/absorption energy storage,and conversion.Basic methods for the synthesis of MOFs include the solvo-/hydro-thermal method,electrochemical deposition,ultrasonication,CVD,microwave irradiation,and template-directed synthesis.MOFs’conductivity and catalytic properties could be increased by heteroatom-doping,combing with carbon-based materials,and growth on conductive substrates.Taking this into account,herein,a zeolitic imidazolate frameworks-67（ZIF-67）is selected,and its conductivity and catalytic properties are enhanced by using two approaches.In the first work,ZIF-67 was first synthesized at room temperature,followed by physical mixing with ammonium molybdate tetrahydrate using a solid-state reaction,and the resultant mixture was thermally annealed under an inert atmosphere resulting in a well-developed core-shell structure（Co@β-Mo₂C-NC;Co nanoparticles as core andβ-Mo₂C in N-doped carbon as shell structure）.The doping of Mo lowers the charge transfer resistance and prevents the nanoparticles from aggregation during the electrocatalytic reaction.The suitable ratio between Co and Mo produces high ECSA,suggesting the appropriate number of active sites,while the NC enhances the electron transport in the core-shell structure.As a result,the Co@β-Mo₂C-NC-0.115（quantity of Mo precursor）illustrates an overpotential of 188 and 330 m V for hydrogen and oxygen evolution reactions（HER,OER）,respectively,and a cell voltage of 1.72 V is required for overall water splitting（OWS）to attain a current density of 10 m A cm^-2in alkaline media.Remarkably,the Co@β-Mo₂C-NC-0.115 catalyst exhibits excellent stability for continuous operation of 15 h chronopotentiometry and 25 h chronoamperometry measurements.These research findings speculate a straightforward strategy to fabricate MOF-derived heterostructures with robust catalytic performance and wide applications in electrocatalysis.Moreover,in the second work,a template-directed method was used to fabricate ZIF-67-derived Mo and P dual-doped electrocatalyst（P-NiCo₂O₄/Co Mo O₄/NF,for simplicity G-3）for HER,OER,and OWS.A three-step procedure was employed for the fabrication of the G-3sample.Firstly,a template was grown on the NF’s surface through a hydrothermal route.Subsequently,the as-prepared sample was soaked in an aqueous solution of 2-methylimidazole（2-MIM）,which is followed by phosphorization in an inert atmosphere,and the final product was labeled as P-NiCo₂O₄/Co Mo O₄/NF（G-3）.The dual doping of Mo and P prompts the formation of nanosheet arrays structure and modifies the surface electronic states,which subsequently enhance the active sites,facilitate the charge transfer and accelerate the reaction kinetics.As a result,the G-3 sample requires a low overpotential of 78.7 m V and 248.6 m V to reach a current density of 25 m A cm^-2for HER and OER,respectively.Furthermore,a cell voltage of 1.729 V is required at 100 m A cm^-2,and the catalyst demonstrates long-term stability of 54 h for overall water splitting.This strategy could be utilized to fabricate other MOF-derived self-supported electrocatalysts for OWS and energy conversion.