Preparation And Performances Of Manganese(Molybdenum)-based Water Splitting Electrocatalysts Derived From Prussian Blue Analogs As Templates

Electrochemical water splitting is an efficient,convenient and sustainable technology to produce hydrogen for future generation,conversion and employment of renewable energy.however,However,due to intricate electron-transfer mechanisms and thus sluggish kinetics,it often takes a far high overpotential to achieve rapid reaction.At present,the first-rank OER and HER catalysts are noble metal-based materials with scarce earth abundance and costly price.Recently non-noble metal catalysts with significant catalytic activity have been widely reported,and to save the cost,simplify synthetic process and increase availability of catalysts,it is best to develop overall water splitting electrocatalysts.Most of non-noble metal bifunctional electrocatalysts are focused on Fe,Co and Ni-based nanomaterials,while the studies on manganese or molybdenum-based bifunctional electrocatalysts are relatively few.Among them,manganese oxides and molybdenum selenides can both possess two-dimensional layered structures,with considerable earth abundance,low price and simplicity of preparation.They are thus promising to be candidates for low-cost overall water splitting.In this paper,Prussian blue analogues?PBA?with uniform size and unique hydrothermal reactivity are used as templates to fabricate Mn oxides-based and Mo selenides-based electrocatalysts with elaborate nanostructures,abundant defects,and desired doping and synergistic effects.These catalysts are endowed with significantly improved conductivity and enhanced electrocatalytic activity by the unique and ingenious synthetical methods,which can be applied as effective water splitting electrocatalysts.The main research results in this paper are as follows:?1?Defect-rich Ni-Fe-doped K_0.23MnO₂ cubic nanoflowers(Ni-Fe-K_0.23MnO₂CNFs-300)are fabricated by a controllable KNi[Fe?CN?₆]?Ni-Fe PBA?-sacrificed strategy followed by annealing process.Benefiting from many merits including unique morphology,abundant defects and doping effect,Ni-Fe-K_0.23MnO₂ CNFs-300 shows the best electrocatalytic performances among currently reported Mn oxide-based electrocatalysts.This catalyst affords low overpotentials of 270 mV at 10 mA cm^-2 for OER with a small Tafel slope of 42.3 mV dec^-1,while requiring overpotentials of 116mV to attain 10 mA cm^-2 for HER respectively.Moreover,Ni-Fe-K_0.23MnO₂ CNFs-300applied to overall water splitting exhibits a low cell voltage of 1.62 V at 10 mA cm^-2and excellent durability,even superior to the Pt/C||IrO₂ cell at large current density.Density functional theory calculations further confirms that doping Ni and Fe into the crystal lattice of?-MnO₂ can not only reinforce the conductivity but also reduce the adsorption free-energy barriers on the active sites during OER and HER.?2?Using a hydrothermal method with KNi[Co?CN?₆]?Ni-Co PBA?as hard templates and subsequent low-temperature NH₃ treatment for regulating the phase composition and oxygen vacancies,defect-rich solid core-shell microcages(N-K_0.23MnO_2-x/Ni-Co PBA-200)and hollow core-shell microcages?N-Mn₃O₄/Ni-Co-C-N-300?are obtained respectively.Under alkaline conditions,the N-Mn₃O₄/Ni-Co-C-N-300 has the best OER performance,which requests the overpotentials of 355 mV to attain 10 mA cm^-2 respectively.And the N-K_0.23MnO_2-x/Ni-Co PBA-200 exhibits the best HER catalytic activity,which affords the overpotential of 89 mV at 10 mA cm^-2.Moreover,when applied to overall water splitting,the electrode pair constituted by the above two catalysts shows a cell voltage of 1.71 V at 10 mA cm^-2 and good durability.?3?Using hydrothermal methods with KCo[Co?CN?₆]?Co-Co PBA?and KNi[Co?CN?₆]?Ni-Co PBA?as the sacrificial templates respectively,followed by annealing process in NH₃,N-doped Co-Mo selenides cubic hollow microcages?N-Co-Mo-Se Microcages?and N-doped Co-Ni-Mo selenides cubic microframes?N-Ni-Co-Mo-Se Microframes?were fabricated.Under alkaline conditions,the two catalysts exhibit low OER overpotentials of 310 and 290 mV at 50 mA cm^-2 respectively,significantly superior to IrO₂.Meanwhile,they needs the overpotentials of 136 and 110mV to achieve 10 mA cm^-2.Moreover,when applied to overall water splitting,the two catalysts show far low cell voltages of 1.58 V and 1.54 V at 10 mA cm^-2 and good durability,which are close to the IrO₂?Pt/C?1.53 V?.The excellent electrocatalytic performances of them should be attributed to the unique morphologies and structures,the synergistic effects of?Ni-?Co-Mo selenides and nitrogen doping effects.