| The third-generation solar cells-dye-sensitized solar cells(DSSCs)have attracted wide attention due to their simple production process,environmental friendliness and high energy conversion efficiency.DSSCs are mainly composed of three parts:counter electrode,photoanode and electrolyte containing I3-/I-.As an important part of DSSCs,the counter electrode materials can receive electrons transmitted by the photoanode and are responsible for catalyzing I3-reduction(IRR),which is an important factor affecting the photoelectronic conversion efficiency of DSSCs.The precious metal platinum(Pt),due to its good IRR catalytic performance,is widely used as the counter electrode of DSSCs.However,Pt reserves are limited and easily corroded during the reaction process,which severely restricts the large-scale application of DSSCs.In view of this,it is particularly important to explore and develop lowcost,highly active and stable counter electrode materials to replace Pt.Carbon materials have many intrinsic advantages,such as high specific surface area,excellent electrical conductivity,stable electrochemical performance,etc.,and are widely used in the field of energy conversion and storage.Graphene owns two-dimensional crystal structure formed by sp2 hybridization of carbon atoms.Its unique physicoche1ical properties make itself widely concerned in DSSCs as the counter electrode materials.Studies have shown that,compared with the active edge carbon sites,although graphene base planes with long-range conducive network are beneficial to charge transfer,they tend to be electrocatalytically inert,which greatly limits the electrochemical performance of graphene materials.In order to make better use of its excellent electrical conductivity and stimulate potential catalytic activity,the strategies of functionalization of the basal plane by carbon quantum dot coupling and nitrogen doping to induce abundant active sites,were applied to construct efficient carbon-based electrode materials.In addition,the relationship between intrinsic/extrinsic defects and IRR performance of carbon materials was deeply explored.The main results are highlighted as follows:(1)Gravity field-assisted coupling strategy was adopted to realize the controllable adjustment of the concentration distribution of the coupled carbon quantum dots(CQDs).Based on this,the controllable regulation of the concentration of the edge site(defect density)is realized,thereby constructing efficient carbon-based catalytic materials(CQDs/CS-x).Applying CQDs/CS-x as the counter electrodes of DSSCs,the results show that following the increased concentration of CQDs,the catalytic performance is significantly enhanced;DSSCs based on CQDs/CS-4 exhibit a power conversion efficiency as high as 8.42%,which is obviously superior than that of Pt counter electrode.In addition,the continuous electrochemical impedance test also shows the excellent electrochemical stability of CQDs/CS-4.Furthermore,density functional theory(DFT)calculations show that,in comparison with basal plane,edge carbon site with higher charge density(especially armchair site)can significantly reduce the adsorption energy of the key intermediate I2 molecule during the IRR reaction,signifying the importance of edge carbon site in adjusting the electronic properties to create required active sites,which will help the rational design of high-efficiency carbon-based electrocatalysts.(2)Glucose and melamine are applied as carbon and nitrogen source,respectively.Firstly,ball milling is used to provide mechanochemical force to realize the pre-assembly of glucose and melamine molecules.XPS combined with DFT calculation confirms the bonding behavior for the formation of C-N-C through dehydration reaction between glucose and melamine.Based on this intermolecular pre-assembly,the graphite carbon nitride formed by the anchored melamine is connected and confined in the carbon framework produced by the polymerization of glucose,enhancing the local interaction between the pyrolytic nitrogen-containing species and carbon edge sites to selectively generate pyridinic nitrogen,thereby enhancing nitrogen doping efficiency.Based on this,a series of carbon materials(NCs-x)with different nitrogen doping amounts were prepared by adjusting the mass ratio of melamine/glucose.Moreover,when the mass ratio is 20:1,an ultra-high nitrogen doping of 12.4 at%is obtained after 1000? carbonization.The NCs-x was applied to the DSSCs counter electrode,and the experimental results show that the optimal nitrogen doping amount was 11.2 at%,and NCs-16 exhibits a power conversion efficiency as high as 8.86%.Further referring to the natural population analysis(NPA)and electrostatic potential(ESP),it can be concluded that graphitic nitrogen doping has little effect on the charge density of adjacent carbon atom,while pyridinic nitrogen doping significantly increases the charge density of adjacent carbon atom,thereby inducing them to be the active site and thus promoting the adsorption of I2 molecule.(3)2-mercaptobenzimidazole and melamine were adapted as the carbon and nitrogen source,respectively.During the pyrolytic process of graphite carbon nitride,the C=N-C site gradually dissociates to form nitrogen vacancy.As such,the C atom adjacent to the nitrogen vacancy with the dwelling of unpaired electrons becomes more active.Meanwhile,because the thiol group on 2-benzimidazolethiol is highly active and easily subjected to hydrogen extraction to form free radicals,the active C atom is covalently bonded with thiol group probably through a mechanism of radical reaction,and form C-S-C bond to trigger the in situ confined pyrolysis of graphite carbon nitride.Benefiting from this,an ultra-high nitrogen doping up to 13.5 at%is realized after carbonization at 1000?,which is the highest value among the reported results.By changing the mass ratio of melamine/2-mercaptobenzimidazole,a series of carbon materials(NCM-x)with nitrogen doping content ranging from 5.4 to 13.5 at%were prepared,and they were adaplted as model materials to explore structure-activity relationship between nitrogen doping species and IRR activity.The experimental results show that pyridine nitrogen is the key contributor for the improvement of IRR performance.When the pyridine nitrogen content is increased from 1.2 to 6.6 at%,the electrochemical efficient surface area is greatly increased from 1.17 to 2.18 cm2.DFT calculations further reveal that,compared with graphitic nitrogen,pyridinic nitrogen doping significantly reduces the band gap value,thereby promoting electron transfer between carbon atoms and I2 to enhance the adsorption of I2 molecules,resulting in high-efficiency catalytic activity.(4)Specifically,we performed a simple yet high-efficiency copolymerization(that is,liquid paraffin and melamine)to achieve the incorporation of C atoms into the frameworks of graphite carbon nitride.Our combined X-ray photoelectron spectroscopy(XPS)and electron paramagnetic resonance(EPR)measurements demonstrate that the active unpaired electron origining from nitrogen vacancy formed at the C-N=C site is redistributed to realize the in situ incorporation of hydrocarbons and reconstruct dual N-doped hexagonal-C ring,which was evidenced by 13C NMR spectrum(CP/MAS 13C NMR).Thermogravimetric analysis coupled with Fourier transform infrared spectroscopy further verify that the N resource in graphite carbon nitride can be effectively fixed after framework reconstruction.As a strong evidence,a superhigh N-doping content up to 33.8 at%is obtained in the prepared graphene nanoribbon after carbonization at 800?.Furthermore,the prepared superhigh nitrogen-doped graphene nanoribbons were physically mixedwith carbon nanotubes(Mixture-l:x)and then applied as the counter electrodes.After optimizing the mass ratio,Mixture-1:6 exhibits an excellent power conversion efficiency of 8.60%and a series of electrochemical testings such as cyclic voltammetry,electrochemical impedance and Tafel polarization curve also demonstrate that Mixture-1:6 owns the best electrocatalytic activity. |
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