Application Of Lignin-based Carbon On The Counter Electrodes Of Dye-Sensitized Solar Cells

Dye-sensitized solar cells(DSSCs)have garnered considerable attention as a novel solar cell technology due to their advantageous attributes including cost-effectiveness,high photoelectric conversion efficiency,and environmental friendliness.Consequently,notable advancements have been achieved in the realm of photovoltaics.The counter electrode,a pivotal component of DSSCs,assumes the role of receiving electrons from the external circuit to facilitate the reduction of oxidized electrolyte ions and subsequent regeneration of the dye.It bears paramount significance in ensuring the overall cell’s cycling capability,stability,and high efficiency.Carbon materials,renowned for their commendable catalytic activity,resistance to electrolyte corrosion,and cost-effectiveness,have emerged as a prominent focus of research in diverse electrocatalytic domains.Nevertheless,their intrinsic catalytic performance still falls short when compared to traditional platinum electrodes,necessitating strategies such as heterocombination or heteroatom doping for enhancement.Furthermore,new carbon materials,such as graphene and carbon nanotubes,are synthesized under harsh conditions,demanding substantial energy consumption and intricate processes.Consequently,an urgent requirement arises for a low-cost,high-performance synthesis approach for environmentally friendly carbon materials,which can alleviate the existing developmental challenges in the domain of energy materials.This paper presents a novel and environmentally friendly approach to address the aforementioned challenges by converting lignin residue,a by-product of the paper industry,into oxygen-nitrogen-sulfur co-doped porous carbon microspheres(ONS-doped CMSs).This conversion is achieved through a combined strategy involving pre-oxidation and selfactivation.The pre-oxidation process enhances the cross-linking between lignin molecules,resulting in a more disordered carbon structure upon self-activation.The original inorganic impurities and the presence of oxygen,nitrogen,and sulfur components in lignin waste residue play crucial roles in the formation of a hierarchical porous structure and heteroatom doping,respectively.The well-developed pores contribute to a larger specific surface area,providing abundant catalytic active sites for the reversible adsorption and desorption of I3-.The incorporation of oxygen,nitrogen,and sulfur components redistributes the charge density and spin density,facilitating faster charge transfer and improved catalytic activity.Additionally,density functional theory(DFT)calculations are employed to deeply elucidate the mechanism underlying the influence of heteroatom synergistic doping on the catalytic reduction of I3-on lignin carbon.The main findings of this research are as follows:(1)Successful synthesis of ONS-doped CMSs from lignin residues through a simple pre-oxidation and self-activation process.Low-temperature pre-oxidation introduces oxygen-containing groups and facilitates intermolecular cross-linking via ester groups,resulting in a higher degree of disorder in the derived carbon from lignin residue.This effect leads to a rougher carbon surface and a larger specific surface area,enabling enhanced electrolyte ion accessibility and an increased number of catalytic active sites.During the self-activation process,inorganic salts and the oxygen,nitrogen,and sulfur elements in lignin waste residue exhibit excellent self-activation and self-doping properties.Inorganic salts have an "etching" effect on carbon substrates at high temperatures,facilitating pore formation.Oxygen,nitrogen,and sulfur elements remain partially within the carbon structure after pyrolysis,thereby modifying the carbon material.The optimal pore parameters,including BET specific surface area(626.46 m2 g-1),total pore volume(0.51 cm3 g-1),t-plot micropore specific surface area(416.24 m2 g-1),t-plot micropore volume(0.17 cm3 g-1),mesopore specific surface area(210.22 m2 g-1),and mesopore volume(0.34 cm3 g-1),are obtained when the self-activation temperature is 800℃.Deviating from this temperature range,either lower or higher,results in a reduction in pore properties due to incomplete interaction between carbon and each activator at lower temperatures and collapse and merging of some micro/mesopores at higher temperatures.(2)In the cyclic voltammetry test conducted on a three-electrode system,the pyrolytic carbon obtained at a temperature of 800℃ demonstrated the highest peak current density of 3.94 mA cm-2,albeit with a relatively narrow peak potential interval of only 261 mV.Tafel polarization tests were performed on a symmetric dummy cell comprising two identical electrodes,while electrochemical impedance analysis was carried out on the assembled DSSCs device.The outcomes revealed that the carbon electrode derived from pyrolysis at 800℃ exhibited the most significant redox potential.In terms of diffusion coefficient,the limiting diffusion current density reached 21.88 mA cm-2,accompanied by the lowest series resistance of 8.32 Ω and the highest recombination resistance of 254.16 Ω.Regarding the investigation of photovoltaic characteristics,pyrolytic carbon produced at temperatures of 700,800,and 900℃ yielded photoelectric conversion efficiencies of 6.81%,9.22%,and 8.61%,respectively.Comparatively,the conversion efficiency of Pt-based DSSCs with an identical configuration was measured at 9.12%,exhibiting a 1.08%decrement in comparison to the carbon derived from pyrolysis at 800℃.This observation underscores the remarkable catalytic activity of lignin residue-derived carbon and its potential application in the field of DSSCs.Furthermore,the incident monochromatic photon-electron conversion efficiency(IPCE)of the device was assessed,with DSSCs incorporating 800℃ pyrolytic carbon showcasing the highest efficiency at 88.80%.Additionally,the ONS-doped 800-CMSs displayed excellent chemical stability,exhibiting relative stability during 100 cyclic voltammetry cycles and a 7-day performance monitoring period.(3)Subsequently,to elucidate the mechanism by which heteroatom doping influences the electrochemical properties of carbon materials,a supercell model was established for density functional theory(DFT)calculations,considering the C,O,S,and N contents as well as the chemical bonds present in the 800 ℃ pyrolytic carbon.The adsorption energy,serving as an indicator of the carbon material’s capturing capability for I3-,was assessed.Following heteroatom doping,the adsorption energy of ONS-doped 800-CMSs increased by threefold(from-0.31 eV to-1.27 eV)compared to the pre-doping state.This substantial enhancement in adsorption capacity significantly improved the ability of ONS-doped 800-CMSs to capture I3-.Analysis of the charge density difference map revealed a significant increase in electron-rich and electron-deficient regions upon the incorporation of oxygen,nitrogen,and sulfur elements.This finding indicates faster charge transfer between I3-and ONS-doped 800-CMSs.Furthermore,calculations of the work functions before and after doping were conducted.The results demonstrated that the work function of ONS-doped 800-CMSs(4.38 eV)decreased by 17.6%compared to that of pure carbon(5.15 eV).This decrease in work function better aligns with the redox potential of I-/I3-,indicating a lowered counter electrode/electrolyte interface and a reduced electron transfer barrier from ONS-doped 800CMSs to I3-.Moreover,the total platform density,both before and after doping,further confirms that ONS-doped 800-CMSs exhibit stronger adsorption and interaction with I3compared to pure carbon.In conclusion,the integrated approach of pre-oxidation and self-activation offers a successful means to convert lignin residue derived from papermaking black liquor into hierarchical porous carbon co-doped with oxygen,nitrogen,and sulfur.The resultant carbon material exhibits remarkable catalytic activity towards I3-due to its high specific surface area,abundant pores,and heteroatom doping.This strategy demonstrates notable advantages,including a straightforward process,environmentally friendly characteristics,and sustainability,thereby presenting a novel avenue for the recycling of cellulose industrial wastewater and the fabrication of efficient and cost-effective counter electrodes for DSSCs.