Preparation Of Sheet-Based CoOOH Hollow Nanospheres And Their Electrochemical Properties

The fast-growing consumption of fossil fuels and the environmental problems associated with their combustion have prompted significant research interests in developing the renewable energy storage and conversion technology.Electrochemical water splitting provides an attractive way to obtain clean hydrogen fuels.As an important family of functional materials,hollow nanostructures with well-defined central void space?e.g.,hierarchical hollow nanospheres,tubes,and cages assembled by nanosheets and nanowire?have been widely applied in the development of Li-ion/Na-ion battery,supercapacitor and water splitting due to their large specific surface area,low density,high permeability and large void space.The oxygen evolution reaction?OER?has received great interest in the fields of energy conversion and storage?e.g.,water splitting,rechargeable metal–air batteries and fuel cells?.Due to its sluggish reaction kinetics,the OER requires a high overpotential???to overcome the activation energy barriers of O-H bond breaking and O-O bond formation.Consequently,a variety of OER catalysts have been introduced to reduce the overpotential.Precious metal oxides?e.g.,RuO₂ and IrO₂?are commercially available OER catalysts,but their high cost,scarcity and instability limit their wide application.Therefore,the development of efficient,stable,low cost and earth abundant OER catalysts is highly desirable.Cobalt oxyhydroxide?CoOOH?is one of the important transition metal oxyhydroxides with wide application in electrochemical water oxidation due to its unique electronic characteristics.Especially,the ultrathin nanosheets endow CoOOH with fast electron transfer,intimate contact with the electrode and abundant active sites,effectively reducing the required OER potential.In this study,we report the rational design and synthesis of hierarchical ultrathin nanosheet-based CoOOH hollow nanospheres with rich oxygen vacancies using a room-temperature self-template strategy with the organic-inorganic hybrid Pi-Co glycerate hollow nanospheres as the templates?Pi,namely H₂PO₂^-?.The Pi-Co-glycerate hollow nanospheres can be further converted to the CoOOH hollow nanospheres through an outside-in pathway via the ionization,anion-exchange reaction,and oxidation in an alkaline environment.To maintain the spherical structure,the precursors are coated with the Nafion membrane which can be simply removed by ethanol washing.The resultant unique structure features?i.e.,ultrathin thickness,rich oxygen vacancies and central void space?endow the CoOOH hollow nanospheres with excellent performance in electrocatalytic OER.The obtained CoOOH hollow nanospheres exhibits a Tafel slope of 49 mV/dec and a low overpotential of 275 mV at the current density of10 mA/cm²,superior to the?-CoOOH nanosheets with an overpotential of 304 mV and the bare Ni foam with an overpotential of 362 mV at same current density.Importantly,this CoOOH hollow nanospheres possess the lowest overpotential of 340 mV at the current density of 200mA/cm² among the reported Co-based nanomaterials?370-770 mV?.This study provides a new pathway for the designing of hierarchical hollow metal oxyhydroxide nanomaterials,facilitating the development of efficient OER catalysts.