Preparation Of Photo-assisted Supercapacitors Of Co-based Electrode And Exploration Of Their Mechanism

In order to achieve the“double carbon”goal of“carbon peaking”and“carbon neutrality”,the rational development and utilization of renewable energy have become a hot issue for scientists.Among them,solar energy is an environmentally friendly,inexpensive and abundant natural energy source that plays a pivotal role in energy utilization by converting it into various kinds of needed energy sources to meet global energy demand.Although solar radiation has been widely investigated in many fields such as photovoltaic,photocatalytic and photothermal systems,it cannot be used as a consecutive energy supply owing to the restrictions by essential daily and seasonal variations.Currently,the most effective strategy to solve this problem is to store solar energy efficiently and then explore new technologies for photovoltaic conversion.Photo-assisted supercapacitors are novel devices for storing solar energy and improving energy storage performance.The development of electrodes with good photoresponsive performance and in-depth investigation of the photo-assisted enhancement mechanism are of great significance for the effective utilization of solar energy.In this paper,with the goal of obtaining light-responsive supercapacitors,cobalt-based photosensitive electrodes are designed and prepared for photo-assisted supercapacitors by means of morphology control and in-situ growth.And then,a combination of experimental and theoretical approaches is used to reveal the mechanism of light-induced charge transfer,which can provide a theoretical basis and a new strategy for the rational design of light-assisted supercapacitors.Firstly,we have synthesized spinel structured Co₃O₄nanospheres by hydrothermal strategy and thermal decomposition,and then proposed a new type of light-assisted strategy by using an n-type semiconductor（Ti O₂）as an additional electrode.Combining the experiments and density functional theory（DFT）calculations,the photo-generated charge produced by Co₃O₄with semiconductor characteristics will promote the charging reaction under continuous light.After the charging process,the semiconductor characteristics gradually disappear,and the generated Co²⁺-OH is not sensitive for light response.On the basis of this result,the new type of light-assisted strategy possesses satisfactory results.The specific capacitance is 523 F g^-1at 1 A g^-1under light with Ti O₂,which is increased by 16.2%when compared with that without Ti O₂-assisted under dark(450 F g^-1@1 A g^-1).The Co₃O₄//AC ASC achieved a remarkable energy density of58.72 W h kg^-1,whereas the energy density of Co₃O₄nanospheres with Ti O₂photo-assisted electrode reaches 97.6 W h kg^-1at the same power density.Secondly,Ni O/Fe Co₂O₄heterostructures have been in-situ grown on nickel foam（denoted as Ni O/FCO）via a facile two-step hydrothermal method to explore the photo-responsive heterogeneous interfaces between Ni O and Fe Co₂O₄.Experimental results show that the specific capacitance of Ni O/FCO can reach 7933 F g^-1at 1 A g^-1under light,which is 1.06 times higher than that in the absence of light(3845 F g^-1@1 A g^-1),exhibiting an ultra-high energy storage capacity.And then,Ni O/FCO//AC light-assisted ASC was assembled by using Ni O/FCO as the positive material.The energy density of the device was enhanced from 35.6 Wh kg^-1to61.9 Wh kg^-1at a power density of 1.5 k W kg^-1before and after illumination.We propose the structure-activity relationship of improve the performance of light-assisted energy storage by density functional theory（DFT）calculations.A built-in electric field（E）directed from Ni O to Fe Co₂O₄will be established due to the difference of Fermi level.The photo-generated electrons would transfer from the conduction band of Ni O to the conduction band of Fe Co₂O₄along the electric field direction.Then,there are more electrons transferred from Fe Co₂O₄to the nickel foam substrate.That is,the photo-generated electrons in the Ni O/FCO heterojunction are injected into the external circuit,facilitating the charging process of the supercapacitor.At the same time,photo-generated holes can flow and participate in the charging process,which is because that the valence bands of Fe Co₂O₄and Ni O are very close.Finally,owing to the rational design,a flower-like Zn Co₂O₄（ZCO NF）was successfully designed as the photosensitive positive electrode and Cu Co₂S₄hollow spheres（CCS HS）was prepared as photo-responsive negative electrode.We assembled a novel photo-assisted asymmetric supercapacitor（ZCO NF//CCS HS ASC）based on dual photo-electrodes to explore the advantage of dual photo-electrodes.The specific capacitance of ZCO NF is 563 F g^-1at 1 A g^-1in a three-electrode system under light,which is higher than that without light(456 F g^-1@1 A g^-1).The specific capacitance of CCS HS in the presence of light(305 F g^-1@1 A g^-1)is approximately twice higher than that in the absence of light(174 F g^-1@1 A g^-1).The ZCO NF//CCS HS ASC possesses an energy density of 60.9 Wh kg^-1at 700 W kg^-1under illumination but only46.5 Wh kg^-1under dark.The photo-responsive performance of the ASC is due to the generation of electron-hole pairs,which is from the positive and negative electrodes under light.Combined with density functional theory（DFT）calculations show that the injection of photo-generated electrons generated from the positive electrode into the external circuit,and and most of the photo-generated holes are located on the surface of Co atoms to adsorb OH^-from the electrolyte to promote the oxidation process.The photo-generated electrons from Cu Co₂S₄are easy to accumulated on S atoms to attract the positively charges K⁺ions.Then,the leaving photo-generated holes combine with the electrons from the external circuit,which promote the energy storage performance.