MOF-mediated Synthesis And Electrocatalysis Of Co-based Composite Electrocatalyst

Electrochemical water splitting into hydrogen and oxygen is a promising means of storing renewable energy in chemical fuels with high energy density.However,the half-water oxidation reaction(OER)has a sluggish kinetics and involves four-electron transfer process,which is the main barrier limiting the practical application of electrochemical water splitting.Although Ru-and Ir-based materialshave been recognized as efficient OER electrocatalysts,the scarcity and high-cost of noble metals severally hinder their large-scale application.Developing high efficiency,stable and inexpensive electrochemical catalysts(e.g,transition metal oxides)is an effective way to solve above problem.In recently developed transition metal oxide catalysts,most of materials are still suffered by lowly exposed active sites and slow charge transfer speed.To solve the above problems,we propose to create OER catalysts by the use of metal-organic frameworks(MOFs)as precursor or conductive species so as to improve their electrochemical performance.The detailed research work is described as follows:First,we developed a face method to obtain self-supported three dimensional Co(OH)2 nanosheets.We found that the introduction of 2-methylimidazole(2-MeIM)during the synthesis can serve as organic linkers to rapidly produce Co-MOF intermediates at the initial stage,and subsequently work as a weak base to transform the Co-MOFs to α-Co(OH)2 nanosheets at the end.Since the-Co2+ ions have been confined by the framework,the formation of a-Co(OH)2 nanosheets can be regulated and thus the packing of individual nanosheets can be effectively avoided.The developed α-Co(OH)2 nanosheets exhibit an obviously better OER activity than the a-Co(OH)2 nanosheets produced with traditional methods.The onset potential of Co(OH)2 nanosheets in 0.1 M KOH electrolyte solution is 1.52 V;and the Tafel slope is 44.6 mV/dec;ECSAs of Co(OH)2 nanosheets is 9.17 cm2.The developed Co(OH)2 nanosheets exhibit obviously large data than the other samples,which are responsible for their better OER activity.Second,we have obtained a series of Fe/Co metal oxides with controllable compositions by loading different amount of Co2+ ions in MIL-53(Fe)precursor and calcinating them under inert atmosphere.The prepared materials(CoFe2O4/Fe3O4,CoFe2O4,Co/CoO/CoFe2O4 and CoO/CoFe2O4)can preserve an uniform morphology as that of the MOF template and become porous as the increase in Co2+ amount.These prepared electrocatalysts exhibit a high surface area and active sites,much benefiting for their electrochemical OER activities.With a suitable Fe/Co ratio,it was found that some metallic Co could be formed in the catalysts along with CoO.Due to the co-exist of Co and CoO species,the three-component electrocatalysts of Co/CoO/CoFe2O4 displayed the best OER activity.At the same time,metallic Co enabled the sample with HER activity.This bifunctional shows good electrocatalytic activity for overall water splitting with a onset voltage of 1.61 V at the current density of 20 mA/cm2.Finally,we have prepared Co(OH)2/GONRs(graphene oxide nanobelts)composites by in situ growing dispersive Co(OH)2 nanosheets on the GONRs.We discovered that the electrochemical OER activity of Co(OH)2 could be greatly enhanced by spreading them on the GONRs substrate.Due to the presence of plenty of C=O groups,the GONRs can bind Co2+ ions and thus produce monodisperse Co(OH)2 nanosheets on their surface with an intimate contact and stable structure.Since GONRs possess one-dimensional structure with high aspect ratio and high electron transportation capability,they can strongly support the charge transfer of Co(OH)2 during OER.The developed Co(OH)2/GONRs composites exhibit an impressive OER activity with a onset potential of 1.46 V,a Tafel slope of 66 mV/dec,and an overpotential of 280 mV(at 10 mA/cm2).There is only 6.2%decrease in current density for a period of 12 h OER reaction.Compared to the composites made by carbon nanotubes(CNTs)and Co(OH)2,the GONRs derived samples show an obviously high performance in all aspects.