Iron Series Metal Compounds: Regulation Of Morphology & Electronic States For Electrocatalytic Water Splitting

Electrochemical water splitting is a sustainable and efficient approach to obtain hydrogen energy,which is promising to alleviate the rapid consumption of fossil fuels in the future.Theoretically,it only requires 1.23 V to drive the overall water splitting reaction,but practical application requires a larger voltage to overcome the activation barrier of the two half reactions:hydrogen evolution reaction at cathode(HER)and oxygen evolution reaction(OER)at anode.Accordingly,high-efficient catalyst is required to speed up reaction kinetics.To date,noble metal-based materials(Pt,Ir O₂and Ru O₂)are still recognized as benchmark catalysts for water electrolysis.However,these noble metal-based catalysts are severely limited by scarcity and unaffordablity.Iron series metal compounds with high elements reserve(Fe,Co and Ni),among which iron series metal sulfides and phosphides display similar coordination structure to that of hydrogenase,and layered double hydroxides exhibit high OER catalytic activity,is regarded as very promising electrolytic water catalysts.The limited catalytic activity of iron series metal compounds is ascribed to the following aspects:(1)poor conductivity;(2)limited active sites;(3)high water dissociation energy and weak adsorption and desorption capacity towards reaction intermediates(H,OH,OOH and O).In view of the above problems,we synthesized a series of iron series metal compound electrocatalysts by means of morphology engineering and electronic structure modulating,and combined experiments with theoretical calculation to reveal the mechanism underlying the improvement of catalytic performance.The main research contents and results are listed as follows.1.Among of iron series metal compounds,iron sulfide has attracted numerous researches as HER electrocatalysts because it exhibits certain resemblances to the active center of hydrogenase.However,iron sulfide generally shows poor catalytic activity towards the HER.The unsatisfactory activity is mainly attributed to the difficulty in formation of adsorbed hydrogen on its surface,which hinders the hydrogen evolution reaction.In addition,the poor intrinsic conductivity and fewer active sites of iron sulfide also limit its application as HER catalyst.By virtue of morphology engineering and atomic doping,we fabricated a self-supporting electrode by encapsulating Co doped iron sulfide(Fe S₂)nanoparticles in ordered mesoporous carbon(Co_0.25Fe_0.75S₂@OMC/CC)for HER.In the above structure,the ordered mesoporous carbon serves as nanoreactor to restrict the growth of sulfide particles and prevent them from agglomeration.The confinement growth generates sulfide nanoparticles with ultra-small size(?5 nm)and high dispersity,which enables maximum exposure of active sites and prevents aggregation and detachment of active phase during catalytic process,thus effectively improving the catalytic activity.Meanwhile,the Co atom doping improves the intrinsic activity by enhancing the adsorption of the catalyst towards hydrogen.In addition,the self-supported structure also ensures the fast transfer of electron,further accelerating HER.The as-prepared electrode exhibited outstanding catalytic activity for the HER in acid electrolyte,manifesting a current density of 10 m A cm^-2at an overpotential of 92 m V.2.In many iron series metal OER catalysts,nickel–iron layered double hydroxide(Ni Fe-LDH)is regarded as a promising OER catalyst owing to low cost,high abundance of component elements(Ni and Fe),and flexible electronic structure.Nevertheless,the limited edge active sites and intrinsic low conductivity seriously hinder its catalytic activity.Based on the confinement effect of mesoporous carbon can effectively reduce the size of the material,we constructed a highly active self-supporting electrode by entrapping Ni Fe-LDH nanoparticles in ordered mesoporous carbon(Ni Fe-LDH@OMC/CC)through morphology engineering and regulation of electronic structure for OER.The mesopores of OMC as nanoreactors can spatially restrict the growth of Ni Fe-LDH,enabling the formation of ultra-small(?5 nm)nanoparticles.Compared with two-dimensional nanosheets,the Ni Fe-LDH nanoparticles with less shape anisotropy expose more edge atoms as catalytic active sites.In addition,the Ni Fe-LDH particles in-situ grown in the pores form intimate contact with the carbon,which results in the transfer of electrons from Ni Fe-LDH to OMC and forms strong electron interaction,further accelerating OER.The electrode exhibited outstanding catalytic performance towards the OER,delivering 10 and 100m A cm^-2 current density at 223 and 296 m V overpotential,respectively.Moreover,it displays almost nine times higher mass activity than that of Ni Fe-LDH nanosheets at300 m V overpotential.3.Despite the HER catalytic activity of Fe S₂ in acidic media has been improved by our work,it is still insufficient for industrial application in alkaline electrolyte.The unsatisfactory activity of Fe S₂ is mainly attributed to the following aspects.(1)The poor conductivity and difficulty in formation of adsorbed hydrogen on its surface leads to sluggish catalytic kinetics;(2)The repulsion between the S 3p orbital and O2p orbital results in unfavorable coordination of-OH with metal atoms;(3)high energy of the formation of intermediates(O,OOH and OH).Due to high degree of localization and overspreading of the d orbital,intermediate products(such as O,OOH and OOH)would be difficult to be desorbed and dissociated.Herein,we constructed a hierarchical nanostructure with Ni Fe-LDH nanosheets as shell and Co doped Fe S₂ nanowires as core on carbon cloth substrate(Co_0.33Fe_0.67S₂@Ni Fe-LDH/CC)through hydrothermal reaction combined with low-temperature sulfurization and electrodeposition for HER and OER under alkaline conditions.Benefiting from the strong electronic interaction between the Ni Fe-LDH and sulfides,the outer Ni Fe-LDH nanosheets with low electron density as adsorption sites for H₂O and OH^-can effectively accelerate reaction.The inner Co doped Fe S₂ nanowire array facilitates the adsorption of H and exhibits metal-like conductivity to ensure the rapid transfer of electrons.The overall hierarchical structure can promote the penetration of electrolyte and expose a large number of active sites.Under alkaline conditions(1.0 M KOH),the electrode exhibited excellent catalytic activity,which only required 1.58 V overpential to drive 10 m A cm^-2.4.In addition to iron series metal sulfides,iron series phosphides,such as cobalt phosphide(Co P)with low cost and excellent conductivity is regarded as a promising candidate for the HER.However,its catalytic activity is limited by a low density of active sites,weak adsorption towards hydrogen at Co site and a high water dissociation energy in alkaline media.Herein,we constructed a mesoporous iron-doped cobalt phosphide self-supporting electrode(meso-Fe_0.3Co_0.7P/CC)for HER by electronic structure modulating and morphology engineering.The mesoporous structure with high specific surface area(147.5 m² g^-1)in-situ grown on the conductive carbon cloth substrate enables a fully exposure of active sites and a rapid transfer of electrons,and promotes the penetration of electrolyte.The doping of Fe enhances the adsorption of H atoms on by upshifting the d-band center of Co.Meanwhile,the introduction of Fe lowers the energy barrier for water dissociation,which accelerates HER in alkaline electrolyte.Benefiting from the synergic effect,the meso-Fe_0.3Co_0.7P/C electrode exhibited excellent catalytic activity and fast kinetics towards the HER in acidic and alkaline electrolytes,requiring small overpotentials of61 m V and 67 m V to drive 10 m A cm^-2,respectively.