Structural Control And Electrocatalytic Performance Of Doped Carbon Materials

With the rapid development of economy,increasingly prominent environmental issues and growing demand for energy have caused widespread concern.The metal-air batteries with high theoretical energy density,safety and environmental benignity are considered as promising and feasible energy storage/conversion technologies.However,the sluggish kinetics of oxygen reduction and oxygen evolution reaction（ORR and OER）gives rise to undesirable overpotential on air electrodes,which hinders commercial implementation of this energy storage device.At present,the high-active ORR catalysts are Pt-based materials and its derivatives,wheareas the most prominent OER catalysts are Ir/Ru-based materials and their derivatives.Unfortunately,the scarcity of noble metal catalysts severely limits extensive commercialization of rechargeable metal-air batteries.Recently,bimetallic and bi-nonmetallic codoping carbonous materials with abundant active sites have drawn intensive attention owing to its excellent electrocatalytic activity.Hence,it is greatly crucial and urgent to synthesize bimetallic and bi-nonmetallic codoping carbonous materials and understand origin of synergistic contribution by different dopants.In this work,a novel and versatile synthesis is cleverly proposed to derive bimetallic and bi-nonmetallic doped porous carbon nanosheets by an inorganic salt-template method.X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,high angle annular dark-field scanning transmission electron microscopy（HAADF-STEM）,X-ray photoelectron spectroscopy（XPS）,N₂ adsorption-desorption isotherms,Raman spectra and scanning probe microscope measurements were performed to characterize the structural characterization of catalysts.Simultaneously,the rotating ring-disk electrode（RRDE）measurements were carried out to investigate the catalytic activity of ORR and OER.The self-made zinc-air batteries were assembled to evaluate practical application of as-synthesized catalysts.A novel and multifunctional inorganic salt mixing system（FeCl₃ and CoSO₄）was ingeniously designed to synthesize 2D layered Fe/Co-N/S-doped hierarchical porous carbon（Fe/Co-N/S-Cs）.The FeCl₃·6H₂O with 2D layered crystal structure is mainly served as 2D layered inorganic molten salt-template and dopamine hydrochloride（DA）acted as carbon and nitrogen precursor are treated at high temperature.Besides,CoSO₄can offer Co and S heteroatom doping.Fe/Co-N/S-Cs with uniform and ultrathin thickness of about 4.63 nm exhibit ultrahigh specific surface area(SSA:1589 m² g^-1),total pore volume(TPV:0.92 cm³ g^-1).Impressively,the CoSO₄ not only increases SSA and TPV observably,but also improves N content（pyridinic N and graphitic N）and carbon defects owing to doping S atom.Fe/Co-N/S-Cs show high half-wave potential(E_1/2=0.835 V),limiting current density(I_ld=6.77 mA cm^-2)and electron transfer number（n=3.96）for ORR and low overpotential of 285 mV at 10 mA cm^-2 for OER.Na SCN poison experiments disclose that the excellent ORR activity can be mainly attributed to an appropriate amount of carbon defects tailored by doping thiophene sulfur and active Fe-Nx/Co-Nx sites play an important role in enhancing OER activity of carbonous materials.Furthermore,assembled Zn-air batteries employing Fe/Co-N/S-Cs as cathode catalyst exhibit high power density(102 m W cm^-2)and initial round-trip efficiency of 66.67%.After 80 cycles,round-trip efficiency can be maintained at63.68%and the layered structure of Fe/Co-N/S-Cs does not obviously change.Two dimensional CoFe/SN-doping and tailor-made open porous carbon nanosheets were successfully prepared by inorganic eutectic salts（FeCl₃ and NaCl）as layered templates.Impressively,as-synthetized open pore structure of Co Fe/SN-carbon can be precisely tailored by eutectic salt-templates（FeCl₃ and NaCl）with different NaCl content.2D CoFe/SN-doping carbon nanosheets（CoFe/SN-C）exhibit high specific surface area(1004 m² g^-1)and ultrahigh pore volume(1.54 cm³ g^-1).Besides,the 2D CoFe/SN-C with ultrahigh meso/macro-porous pore volume(1.25 cm³ g^-1)exhibits superior catalytic activity for ORR(E_1/2=0.843 V and I_ld=6.96 mA cm^-2)and OER(E_i=10=1.504 V)in alkaline medium,surpassing most of reported electrocatalysts.Simultaneously,the assembled zinc-air batteries with open porous CoFe/SN-C catalysts display high power density(169 mW cm^-2).After galvanostatic discharge-charge experiments(5100 min at 10 mA cm^-2),round-trip efficiency can be maintained at 56.80%（initial round-trip efficiency of 58.05%）.Then,Density functional theory（DFT）calculation was used to predict the catalytic activity of Fe/Co and N/B codoped carbon materials.Theoretical calculation reveals that catalytic activity of carbon materials is strengthened because B and Co dopants can increase the highest occupied molecular orbitals（HOMO）energy and density of states（DOS）of catalysts.Based on DFT calculations,dopamine hydrochloride（DA）mainly serves as nitrogen-containing carbon precursors and layered inorganic molten salt（FeCl₃）as salt-template can induce the formation of porous carbon nanosheets.CoCl₂ and H₃BO₃ offer Co and B dopants.Fe/Co and N/B codoped mesoporous carbon nanosheets were successfully synthesized by heat treatment of mixtures of DA,FeCl₃,CoCl₂ and H₃BO₃.Furthermore,H₃BO₃ as adopant can also improve mesoporosity of carbon nanosheets.As a result,the Fe/Co and N/B codoped mesoporous carbon nanosheets（Fe Co/NB-Cs）display ultrahigh SSA of 1584 m² g^-1 and TPV of 1.21 cm³g^-1.More importantly,the 2D FeCo/NB-Cs with high mesoporous specific surface area(746 m² g^-1)and mesoporous pore volume(0.81 cm³ g^-1)exhibit remarkable catalytic activity for ORR(E_1/2=0.843 V)and OER(E_i=10=1.504 V)and low total overpotential of 0.653V(△E=E_i=10-E_1/2=0.653V)in alkaline medium.Meanwhile,the FeCo/NB-Cs based zinc-air batteries display high power density(218 mW cm^-2)and perfect charge-discharge stability(stable running for 6700 min at 10 mA cm^-2).