The selected representative SEM images of structural electrolytes with various contents of hemp fiber are presented in Fig.4.As the content of hemp fiber increases,wider inter layer voids are found in the matrix.The processed images of structural electrolyte with various contents of hemp fiber are shown in Fig.5.Area considered here is defined as the number of pixels.The white part represents pore structure,while the black part is matrix.The pore structure here is mainly macro-pores,which have the greatest influence on storage of aqueous electrolyte and ionic migration.Interconnected pore structure is consisting of some isolated macro-pores in the matrix.Thus,the porosity P of structural electrolyte is calculated by P = S1/S,where S1 is the area of all the white parts,and S is the area of the overall area.And the pore connectivity PC of structural electrolyte is measured according to PC = S2/S,where S2 is the area of all the interconnected pore structure.The porosity and pore connectivity of structural electrolytes with various contents of hemp fiber are depicted in Fig.6.It is found that the volume of hemp fiber is between 5 vol% and 15 vol%,a gradual increase from 8.3% to 18.6% in porosity and from 5.6% to 15.5% in pore connectivity of the structural electrolyte.The increase in porosity and pore connectivity attributes to the interlayer voids trapped beneath the fiber during casting.However,as the content of hemp fiber increases to 5 vol%,the porosity of the sample decreases from 10.8% and the pore connectivity decreases from 7.2%.It is consistent with that low volume of fiber is effective in reducing free plastic shrinkage and pore structure development.Moreover,cannabis grow racks as the content of hemp fiber is higher than 15 vol%,the porosity and pore connectivity of the samples decreases due to the disorganized arrangement and massive stacking dispersion of hemp fiber in the matrix.Thus when compared with foaming method by adding air entraining agent,synchronized changes of porosity and pore connectivity of the structural electrolyte by adding hemp fiber show better.

This is mainly due to that the macropores created by air entraining agent are independent bubbles and difficult to connect with each other,but lots of interconnected pore structure can be formed by adding large content of hemp fiber.Fig.7 shows the pore size distribution curve and porosity of structural electrolytes with various contents of fibers tested by MIP.It is clear that most probably aperture diameter is about 115 μm,54.6 μm and 86.5 μm for the structural electrolyte with 0 vol%,15 vol% and20 vol% of hem fiber,respectively.As the content of hemp fiber increases to 15 vol%,the pore number of structural electrolyte dramatically increases.The structural electrolyte has a total porosity of 13.0%,24.2% and 21.1% with 0 vol%,15 vol% and 20 vol% of hemp fiber,respectively.The pore content obtained by MIP is a little higher than by image analysis,which is mainly due to sampling,but the variation trend of porosity is consistent with that of image analysis.Fig.8 displays the CV curves of structural super capacitors with different contents of hemp fiber.All the CV curves are relatively in rectangular shapes and exhibit near mirror-image current response on voltage reversal,meaning ideal capacitive behavior.Based on the area of CV curves,the specific capacitance of the structural super capacitor initially increases with the increasing content of hemp fiber and then decreases,which are resulted from the porosity and pore connectivity of structural electrolytes.Here,as the volume content of hemp fiber increases from 15 vol% to 20 vol%,the area of CV curves decreases,which is mainly due to that the pore structure of structural electrolyte decreases by 33.5%.Thus the pore structure of structural electrolyte significantly affects the specific capacitance of structural super capacitor.From Fig.9a,the typical triangular-shaped galvanostatic charge discharge curves of the structural super capacitors with different contents of hemp fiber are highly linear and symmetrical without obvious iR drop,exhibiting nearly perfect capacitive behavior.In Fig.9b,as the content of hemp fiber increases from 5 vol% to 15 vol%,the specific capacitance of structural super capacitor gradually increases from 35 F·g−1 to 51.4 F·g−1,which ascribes to the increase in porosity and pore connectivity of structural electrolyte.However,as the volume of hemp fiber is lower than 5 vol% or higher than 15 vol%,the specific capacitance of structural super capacitor decreases with the increasing volume of hemp fiber due to the reduction of porosity and pore connectivity of structural electrolyte.It can be deduced that aqueous electrolyte can be stored in the pores of matrix to assemble structural electrolyte,and aqueous electrolyte can pass quickly from the surface to underlying layers of the matrix through the interconnected pore channels during charge-discharge.

Similar findings have been reported by J.M.Xu,W.Y.Ma and N.Shirshova,they all agreed that ionic conductive phase was not only stored in the pore structure of matrix,but also transmitted to the electrode surface through the pore structure during charge-discharge.Yet they didn’t discuss the effect of pore structure on the specific capacitance of the structural super capacitor in depth.The higher porosity in the matrix,the more KOH aqueous stored.And the higher pore connectivity in the matrix,the more passageways provided for ion movement.Thus the specific capacitance of structural super capacitor is mainly affected by the porosity and pore connectivity of structural electrolyte.Furthermore,the specific capacitance  of the structural super capacitor reaches to 41% of solid state super capacitor  under the same test conditions,which is promising to develop structural electrolyte combining magnesium phosphate with KOH aqueous.Fig.9c shows the Nyquist plots of structural super capacitors with various contents of hemp fiber.The nearly vertical line in the low frequency region is a result of ion diffusion in the electrolyte to the electrode surface,signifying an ideal capacitive behavior and the high frequency region represents an arc-like shape.The internal resistance Rs of the structural super capacitor can be obtained from Nyquist curves by the intercept at real part of horizontal axis.The charge transfer resistance Rct at the electrolyte/electrode interface is represented by the arc formed in a high frequency region.Seen from Fig.9d,as the volume fraction of hemp fiber increases from 5 vol% to 20 vol%,the curve trend of Rs and Rct are negatively related with that of porosity and pore connectivity in structural electrolyte.In addition,as the content of hemp fiber increases to 5 vol%,Rs of the structural super capacitor sharply reduces from 40.2 Ω to 17.4 Ω and Rct decreases from 21.2 to 4.4 Ω due to the strong absorption capacity of hemp fiber for aqueous electrolyte.Thus the internal resistance Rs and charge transfer resistance Rct are mainly affected by absorption capacity of hemp fiber to aqueous electrolyte when small content of hemp fiber is added,yet they are mainly affected by the porosity and pore connectivity of matrix when larger content of hemp fiber is added.Low Rct value of the structural super capacitor reflects the improved transfer mechanism at the interface of electrode/electrolyte.

Therefore,structural super capacitors in this work show better electrochemical performance than that of epoxy system.The long-term cycling stability of the structural super capacitor is evaluated by CV method with a scan rate of 100 mV·s−1 for 2000 cycles.From Fig.10,the specific capacitance of the structural super capacitors with 0 vol% and 15 vol% of hemp fiber retain about 92.2% and 88.2% after 2000 cycles,respectively.The cycling stability of the structural super capacitor slightly decreases after adding 15 vol% of hemp fiber,which is due to the increase of pore structure of the structural electrolyte.It can be concluded that the structural super capacitors in this work have good cycling stability during long-term charge discharge processes.Fig.11a depicted the compressive strength of structural super capacitors with various contents of hemp fiber.It is seen that an increase in the content of hemp fiber causes a gradual reduction in compressive strength of structural super capacitor.Although hemp fiber has been used to enhance the mechanical properties of cement pastes,it has been reported that hemp fiber has a negative effect on the compressive strength of cement matrix,even a small content.In general,high porosity of structural electrolyte can reduce the compressive strength of structural super capacitor,but it is not the only factor.Except for porosity,there are still many reasons for the reduction in compressive of structural super capacitor,such as disorganized arrangement of fiber,agglomeration of fiber and surface interactions between matrix and fiber.To analyze multifunctional properties of the structural super capacitor,two performance parameters are employed in this work,specific capacitance and compressive strength.However,as the mechanical performance and ion conductivity are in a trade-off relationship,an ideal structural super capacitor is designed in this work.The ideal structural super capacitor exhibits the highest values of the two parameters among all the samples,with a compressive strength of 24.1 MPa and a specific capacitance of 51.4 F·g−1.Fig.11b presents the multi-functionality of structural super capacitors with various contents of hemp fiber.It is known that the shorter the distance of scatter to the ideal point,the closer to the ideal structural super capacitor.As the content of hemp fiber increases from 5 vol% to 15 vol%,multi-functionality of the structural super capacitor gradually increases.Nevertheless,as the content of hemp fiber is smaller than 5 vol% or N15 vol%,the multi-functionality of structural super capacitor decreases with the increasing content of hemp fiber.It can be inferred that the multifunctional properties of the structural super capacitor is mainly affected by the porosity and pore connectivity of structural electrolyte.Lots of pore structure causing by adding large content of hemp fiber result in low compressive strength of structural super capacitor,yet such pore structure are beneficial to improve electrochemical properties of structural super capacitor.Moreover,when specific capacitance and compressive strength are combined for evaluation of structural super capacitor,cannabis grow system the porosity and pore connectivity are positively correlated with its multi-functionality.

The sample containing 15 vol% of hemp fiber appears for the optimum balance,displaying a specific capacitance of 51.4 F·g−1 and a compressive strength of 13.2 MPa.The comparison of this study with other relevant structural super capacitors is listed in Table 3.It can be seen that the structural super capacitor with 15 vol% of hemp fiber in this work shows good electrochemical and mechanical properties.In fact,perfect performance of structural super capacitor is still difficult to achieve,but this work is highly encouraging to improve the multi-functionality of structural super capacitor by developing pore structure of structure electrolyte with hemp fiber.Hemp   is one of the plants domesticated and used by humans.This important natural bast fiber crop originated from the central Asia and is belonging to the genus Cannabis,in the family of Cannabiaceae.Hemp is extensively used for paper,textiles,food,medicine,synthetic plastics,and fiberglass production.According to its usage,hemp can be divided into three types,seed hemp,industrial hemp,and drug hemp.For seed hemp,the seeds can generate oil rich in omega-3 and omega-6 polyunsaturated fatty acids that are useful in treating atopic dermatitis symptoms.For industrial hemp,the bast fiber of hemp is widely used in garment production as an environmentally friendly raw material because of its excellent features including biodegradable,heat resistance,softness,and fitness.Also,hemp has a great value in the production of cosmetics and building materials.Because of these significance aspects,hemp has received much considerations from plant breeders in the recent years.As one of the largest transcription factor families in plants,WRKY gene family is widely distributed and is an important part of plant transcription regulation and signal transduction.It regulates the expression of related genes by combining cis-elements or interacting with other regulatory factors,thereby playing an important role in plant growth and development,defense against abiotic stresses,and other biological processes.All known WRKY proteins contain at least one DNA binding domain of about 60 amino acids residues,a highly conserved WRKYGQK heptapeptide at the N-terminus,and a C2H2 or C2HC zinc finger motif at the C-terminus.WRKY proteins can be classified into three groups  based on the number of the conserved WRKY domains and the type of zinc finger structure,and group II was further divided into five subgroups.Previous studies have shown that WRKY genes in group I exist not only in higher plants but also in ferns while group III WRKY genes only exists in higher plants and responds to abiotic stress.WRKY proteins usually preferentially recognizing and binding the W-box cis-element of the target gene to regulate gene expression,respond to environmental stresses,and regulate plant growth and development.The first WRKY gene  was cloned in sweet potato and then identified in Arabidopsis,corn,rice,rapeseed,potato,tomato,cotton,cucumber  and other plants.Previous studies have demonstrated that WRKY transcription factors play a vital role in response to biological/abiotic stress and plant development.In Arabidopsis,WRKY8 and WRKY75 regulate salt tolerance,while multiple WRKY genes  participate in drought stress response through different signaling pathways.In rice,over-expression of OsWRKY53 can promote cellulose synthesis in leaves,WRKY47 and WRKY80 participate in drought stress,while OsWRKY11 over-expression can improve the tolerance of high-temperature stress,and OsWRKY31 is also found to be related to lateral root growth.In addition,recent studies showed that the expression patterns of VviWRKY40 and VviGT14 are negatively correlated with that in grapefruits.