Synthesis Of Azobenzene-Graphene Hybrid Materials For Molecular Energy Storage Tuned By Hydrogen Bond

Azobenzene can transfer from the trans-to cis-configuration by irradiation at a specific wavelength light in the wake of storing the energy in chemical bonds.Under external stimulus,the metastable cis-isomer reverts to trans-isomer and meanwhile releases the stored energy as heat in a complete energy storage cycle.Azobenzenes show great potential for high-energy and long-term storage due to the controllability and well cycle stability of photo-isomerization.However,the low theoretical energy capacity and short-lifetime have greatly limitted their utilization in solar heat storage.Aiming at the problem,on the basis of molecular design,five kinds of azobenzenes(AZO)with different substituents(carboxyl,methoxy and sulfonic group)via diazonium salt coupling reaction were synthesized and covalently attached on the surface of reduced graphene oxide(RGO).By regulating the grafting densiy and steric configuration,the AZO can form multiple intramolecular/intermolecular interactions(especially hydrogen bonds)for obtaining high-energy-density and excellent stability azobenzene-graphene hybrid materials(AZO-RGO).The energy densities were also confirmed by the simulations and calculations based on density functional theory(DFT),which can provide theoretical and experimental basis for solar thermal storage.Single-layer graphene oxide(GO)was prepared by modified Hummers method with well-dispersity and wrinkled microstructure.RGO with partial reduced structure and high crystallinity was obtained by sodium borohydride,showing well-dispersed in water due to the residual oxygen-contained groups.Ortho-bismethoxyl substituted AZO-1-RGO was prepared by repeated diazonium salt free radicals reaction.The grafting density was one AZO-1 supported by 19 carbon atoms of RGO(1:19)based on thermogravimetry analysis(TGA)and X-ray photoelectron spectroscopy(XPS).The high grafting density decreases intermolecular distance and forms stacking interaction,result in enhancing intermolecular interaction and steric hindrance that remarkably improve the energy density and half-lifetime.The kinetic constant of the reversion of AZO-1-RGO was four order of magnitude lower than that of AZO-1.The energy density of AZO-1-RGO was 68.14 Wh/kg with an increasing of 62.08% compared to AZO-1(42.04 Wh/kg)based on differential scanning calorimetry(DSC)data.Futhermore,para-carboxyl substituted AZO-2-RGO and meta-biscarboxyl substituted AZO-3-RGO were prepared at a similar grafting density of 1:16~17.High grafting densities lead to the formation of different numbers of intermolecular hydrogen bonds(H-Bonds)between the trans-and cis-isomers,which can significantly improve the energy-level difference of single AZO molecule.Thus,the energy density and thermal stability of the AZO-RGO hybrid materials are remarkably increased.And the energy densities of AZO-2-RGO and AZO-3-RGO are 112.8 Wh/kg and 138.08,respectively,indicating an increasing of 178.38% and 235.63% to AZO-2(40.52 Wh/kg)and AZO-3(41.14 Wh/kg).Half-lifetime of AZO-2-RGO and AZO-3-RGO are 33 days and 52 days,respectively,showing two and four order of magnitudes than that of AZO-2(10 hour)and AZO-3(0.5 h).Meanwhile,the two kinds of AZO-RGO both keep good cycle stability for 50 cycles without any attenuation.Five different AZO-RGO were simulated and calculated based on DFT.Results show that the change of the numbers of H-Bonds between trans-isomer and cisisomer can effectively improve the energy density and half-lifetime.Theoretical models are constructed according to the grafting density of experiment,and the calculated theoretical energy densities of AZO-2-RGO and AZO-3-RGO are respectively 120 Wh/kg and 159.4 Wh/kg with the same order of magnitude to experimental data.The AZO-RGO hybrid with multiple hydrogen bonds shows high energy density and long half-lifetime,and is expected to be an ideal candidate for solar thermal storage.