State Key Laboratory of Fine Chemicals, Liaoning Key Laboratory for Energy Materials and Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China

Keywords:Hydrothermal recrystallization Bamboo-like carbon nanotubes Counter electrodes Electrocatalysis Triiodide reduction

ABSTRACT Carbon nanotubes (CNTs) have been far and wide employed as the counter electrodes (CEs) in dyesensitized solar cells because of their individual physical and chemical properties.However, the techniques available now,such as chemical vapor deposition,arc discharge and laser ablation for synthesizing CNTs,commonly suffer from rigorous operations and complicated steps,which make the process difficult to be controlled.Herein, we present a simple and facile glutamic acid-assisted hydrothermal recrystallization strategy to construct bamboo-like CNTs (GHP-BC-x).Generally, the conventional organic dye 3,4,9,10-perylene tetracarboxylic dianhydride(PTCDA)is used as a precursor and glutamic acid efficiently promotes the recrystallization of the perylene cores’ planar π-conjugated system in PTCDA under hydrothermal conditions and then self-assembles into one-dimensional nanorods with improved crystallization degree, finally resulting in the morphology of bamboo-like CNTs after carbonization.When applied as the counter electrodes, the GHP-BC-3 displays a remarkable power conversion efficiency of 8.25%, benefiting from the superb electrical conductivity and mass transfer dynamics, superior to that of Pt CE (7.62%).

1.Introduction

Dye-sensitized solar cells (DSSCs), as the new generation of photovoltaics with the superior features including competitive cost-effectiveness,simple process engineering and efficient photovoltaic effect, have aroused wide concern to supersede the thinfilm and silicon-based photovoltaic devices [1-6].As an essential part of DSSCs, the counter electrode(CE) plays the dominant roles in the catalytic reduction ofand completing the regeneration of I-in the electrolyte[7].In this case,the electrical conductivity and electrocatalytic activity of the counter electrode electrocatalysts are highly concerned and need to be well constructed [8].

To achieve the synergistic enhancement of the abovementioned characteristics for the counter electrode electrocatalysts, the carbon materials with long-range ordered sp2hybridization are intensively investigated as alternatives for Pt CE on account of their excellent electrical conductivity, economic feasibility and superb corrosion resistance.Thereinto, the onedimensional carbon nanotubes (CNTs) are considered promising because of their extraordinary physical and chemical properties which may result from their unique hollow and linear structures as well as high conductivity [9-11].Noteworthy, the materials with the hollow structure and porosity have improved reaction kinetics and catalytic activity due to the adequate contact between the electrode and the electrolyte.Furthermore, the unique structure is capable of helping to improve their cycling stability[12,13].Inspired by the previously mentioned analysis, rational method to design and synthesis of advanced carbonaceous materials with specific structure as the CEs is extremely pivotal for the practical requirements of DSSCs.

At present, the most common methods to synthesize CNTs are chemical vapor deposition (CVD), plasma-enhanced CVD, arc discharge, and laser ablation.Whereas, these traditional synthesis methods have strict requirements on raw materials, catalyst and the operating parameters including reaction temperature, pressure,gas flow rate,and reaction time,etc.[14].And simultaneously,the demand for special equipment such as high power laser and raw materials such as high purity graphite rods increases their production cost.In this way, it is meaningful to conceive a novel synthetic methodology,which could address these issues of traditional synthetic methods in product purity,energy consumption and process complexity.

In this research, we present a glutamic acid-assisted hydrothermal strategy for synthesizing bamboo-like CNTs with the porous channels by utilizing the conventional organic dye 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) as a precursor.The glutamic acid can efficiently promote the recrystallization of the perylene cores’ planar π-conjugated system in PTCDA and further self-assembly into one-dimensional nanorods with improved crystallization degree under hydrothermal conditions.The microstructure characteristics of the corresponding carbon derivatives are optimized and tuned by adjusting the additive amounts of glutamic acid and the as-synthesized GHP-BC-3 exhibits best activity in catalyzing thereduction reaction because of the excellent reaction kinetics and improved mass transfer dynamics.When employed as CE material, the DSSCs exhibit the power conversion efficiency (PCE) of 8.25%, superior to that of Pt CE (7.62%).

2.Materials and Methods

2.1.Recrystallization of PTCDA

The recrystallization of PTCDA was realized via the conventional hydrothermal synthesis method.To be specific, glutamic acid(0.004, 0.02, 0.04, 0.2, 0.4 mmol) was dissolved into 75 ml deionized water, yielding a series of glutamic acid solution.Then,the PTCDA (2 mmol) was dispersed into the aqueous solution of glutamic acid continuously stirring for 20 min.Subsequently, the red suspensions were transferred into 100 ml hydrothermal synthesis reactor, underwent hydrothermal and recrystallization reactions at 200 °C for 24 h.The red powders were centrifuged,washed by deionized water and lyophilized.The purified samples,recrystallized from the glutamic acid-assisted hydrothermal treatment of PTCDA,were labeled as GHP-x,where the values of x were 1-5,referring to the additive amounts of 0.004,0.02,0.04,0.2,and 0.4 mmol of glutamic acid in water,respectively.As a contrast,the PTCDA was directly dispersed into deionized water and treated under the same hydrothermal conditions, and the as-prepared sample was named as HP.

2.2.Synthesis of bamboo-like CNTs

The powders obtained above (GHP-x and HP) were annealed at 800°C for 2 h in an inert atmosphere(N2)with a rate of heat addition of 2 °C·min-1,which were expressed as GHP-BC-x and HP-BC,respectively.As a contrast, P-C was prepared from the untreated PTCDA under the similar experimental conditions.

2.3.Fabrication of CEs and DSSCs

Briefly, to fabricate CEs, the carboxyethyl cellulose was dissolved into terpilenol and the mixture was used as binder,which was mixed with samples above and ground continuously to form the slurries.The FTO glasses were sintered for 30 min in N2atmosphere, followed by the slurries coated on them with the aid of doctor-blade method, thus obtaining the various CEs.

The TiO2photocathodes were sintered at 500 °C in an ambient atmosphere for 30 min,then the cooled TiO2photocathodes about 100 °C were immersed and kept for 20 h in a N719 of anhydrous ethanol solution(0.5 mmol·L-1).The sandwich configuration made of the as-prepared CEs, a Surlyn film, and photocathodes was assembled based on hot-press approach.Through the pre-drilled holes on CEs, the electrolyte with/I-redox shuttle was injected into cells.

2.4.Materials characterization

The micromorphological characterization of all materials were using the scanning electron microscope (SEM, HITACHI SU8220)and the transmission electron microscopy (TEM, JEOL 2100F).The materials were also examined by the Raman spectroscopy(Thermo DXR Microscope, 532 nm laser),X-ray photoelectron spectroscopy(XPS, Thermo ESCALAB 250), the nitrogen adsorption/desorption techniques (Micromeritics 3Flex 3500), Fourier transform infrared spectroscopy (FT-IR, Bruker EQUINOX 55), and X-ray diffraction(XRD, MiniFlex600).The thermogravimetric analyzer (NETZSCH STA 449 F3) was operated in N2atmosphere to analyze the pyrolysis process of the precursors.

2.5.Electrochemical testing

The photocurrent density-voltage (J-V) curves and cyclic voltammetry (CV) was detected following our previous work[15].The Tafel plots and electrochemical impedance spectroscopy(EIS) were tracked by Multichannel Potentiostats (VSP, Bio-Logic)with the assembled dummy cells.The used light source was an AAA solar simulator (94,032 A, Newport).

3.Results and Discussion

The fabrication of GHP-BC with a structure of bamboo-like CNTs is schematically depicted in Fig.1(a) via a combined technique of glutamic acid-assisted hydrothermal recrystallization and carbonization process.Firstly, the PTCDA went through the recrystallization and self-assembly with the help of glutamic acid by hydrothermal treatment to obtain the organic GHP precursor.Then, the GHP precursor was heat-treated at high temperature to produce the target sample named as GHP-BC.It was believed that such a hydrothermal process is capable of improving the crystallization of organic materials,where the corresponding transformation from amorphous to crystalline structure could not only increase thermal and chemical stability of materials but further change their physical and chemical properties [16].For PTCDA before and after hydrothermal treatment, the evolution of morphology and crystalline structure was revealed from the SEM images shown in Fig.1(b)-(c).Fig.1(b)shows the original morphology of PTCDA powders, indicative of 1 D nanorods with the diameters of about 50 nm.This would be ascribed to the perylene cores’planar π-conjugated system and the PTCDA molecules are apt to ππ stacking[17,18].After the hydrothermal treatment(Fig.1(c)),the morphology of nanorods has been improved significantly.The rough,random surface texture is transformed into smooth and regular structure accompanied by an increase in length and a narrower diameter distribution concentrating upon 100 nm.Here,we have to mention the concept of high-temperature water(HTW),in organic chemistry,liquid water above 200°C and supercritical water is defined as HTW [19].Considering the particularly physicochemical characteristics of HTW compared with water at room temperature, its polarity has a drastic decline arising from the fracture of hydrogen bonds among HTW molecules [16,20],leading to the increased ionic product of HTW.This will also help to facilitate the dissolution and recrystallization of PTCDA.With the assistance of glutamic acid under hydrothermal conditions(Fig.1(d)), the more smooth and regular structure in morphology is observed.It is imagined that the glutamic acid synergistically with hot water work to further improve the crystallinity of PTCDA.

Fig.1.(a)Schematic illustration of preparation procedures for GHP-BC.(b-d)SEM images of PTCDA,HP,and GHP-3 precursors,respectively.(e)XRD patterns,(f)DTG curves,and (g) FT-IR spectra of PTCDA, HP, and GHP-3.

Obviously, the XRD patterns of PTCDA, HP, and GHP-3 precursors (Fig.1(e)) all display the strong diffraction peaks at 9.2°,12.4°, 24.8° and 27.6°, which are indexed to the (0 1 1), (0 2 1),(0 4 2)and(1 0 2)crystal planes of PTCDA,respectively.The above results indicate that after hydrothermal treatment, the HP and GHP-3 still can inherit the structure of pristine PTCDA, displaying a β-form crystal structure that belongs to the monoclinic P21/c space group [21,22].Simultaneously, the peak intensity of GHP-3 is the highest, followed by HP and PTCDA, revealing that the HTW and the glutamic acid-assisted hydrothermal process could significantly increase the crystallinity of PTCDA powders whilst preserve the inherent crystal structure.Combined SEM with XRD results,it is confirmed that the impressive recrystallization of glutamic acid-assisted hydrothermal conditions indeed happens in the present system.

The impact of recrystallization on the physical properties of PTCDA was verified by the thermogravimetric analysis (Fig.1(f)).For the HP and GHP-3 precursors, the maximum weight loss temperatures correspond to 605°C and 617°C shifting 36°C and 48°C towards the high temperature region in contrast to that of PTCAD(569 °C), respectively.The improved thermal stability demonstrates that the HP and GHP-3 have recrystallized after the hydrothermal and the glutamic acid-assisted hydrothermal process, which supplements the results of SEM and XRD.

Moreover,the FT-IR was applied to confirm the chemical bonds and characteristic functional groups of above samples.The stretching vibrations of carbonyl in the anhydride functional group(asymmetric and symmetrical stretching vibration at ～1774 cm-1and 1745 cm-1) and perylene body (～1594 cm-1) [21,23]are detected (Fig.1(g)), which indicate the HP and GHP-3 keep the original chemical structures of the raw PTCDA molecules albeit the rough treatment.Besides, the peaks at ～1693 cm-1and 1654 cm-1arising from imide modes [23]are fully absent for GHP-3, suggesting the amidation of anhydride groups for PTCDA is absent and no chemical reaction takes place with glutamic acid.

After the pyrolysis treatment at 800°C,the structural evolution of PTCDA, HP and GHP-3 precursors is further decoupled by SEM(Fig.2(a)-(c)).The P-C sample (Fig.2(a)) exhibits the interconnected network constituted by atactic nanorod morphologies with the diameter of 50-100 nm.The HP-BC sample (Fig.2(b)) inherits the microstructure of the hydrothermal treatment of PTCDA.Its intermittent pores located on the CNTs walls can be clearly observed, with hollow bamboo-like morphology and its diameter changing in a range of 100-150 nm.The as-synthesized GHP-BC-3 material, as shown in Fig.2(c), also displays bamboo-like morphology the same with the HP-BC sample, with the diameter of about 150 nm.This unconventional carbonaceous network in the HP-BC and GHP-BC-3 that is completely different from common bamboo-like CNTs with perfect tube walls could be attributed to the synergy of the self-assembly of PTCDA molecules and hydrothermal process combined with the presence of glutamic acid.The open intermittent pores could provide the open electrolyte transfer channels, improve the mass transfer kinetics and further increase the rate of triiodide reduction.Fig.2(d)-(f) presents typical TEM images of P-C, HP-BC and GHP-BC-3.It is observed that the P-C sample exhibits irregular and hollow nanorod-like structure,meanwhile,the HP-BC and GHP-BC-3 samples clearly present distinct hollow tubular structure with periodic bamboo joints.Further, the element mapping, as shown in Fig.1(g)-(j), indicates that the C and O species are uniformly distributed.And the mass fractions of corresponding elements are revealed by element analysis (Table S1).

Fig.2.(a-c) SEM images of P-C, HP-BC, and GHP-BC-3, respectively.(d-f) TEM images of P-C, HP-BC, and GHP-BC-3, respectively.(g) SEM image of GHP-BC-3 and corresponding element mapping images of (h) C, (i) O, and (j) EDX spectrum.

Fig.3.(a)XRD patterns of P-C,HP-BC and GHP-BC-3.(b)Nitrogen adsorption-desorption isotherms of P-C,HP-BC and GHP-BC-3.(c)The corresponding pore size distribution curves.Raman spectra of(d)P-C and(e)GHP-BC-3.(f)The calculated percentage contents of four O species and the ratio of sp2 to sp3 carbon deriving from the corresponding high- resolution XPS spectra of P-C, HP-BC and GHP-BC-3.

The crystalline structure information for P-C, HP-BC and GHPBC-3 samples was subsequently uncovered by the XRD technique.The XRD patterns (Fig.3(a)) present the strong (0 0 2) diffraction peaks for three samples at around 25° [24], with the interlayer spacing of about 0.35 nm.Interestingly,the peak of the HP-BC sample assigned to the(0 0 2)plane slightly shifts to a high diffraction angle in comparison to that of P-C.Furthermore,for the GHP-BC-3 sample treated by glutamic acid-assisted process, the diffraction peak mentioned above shifts to a high diffraction angle.The results reveal that the graphite degree/crystallinity of the P-C, HP-BC and GHP-BC-3 samples gradually increases, and suggest that the organic precursors with higher crystallinity can generate the products with higher graphitization degree after carbonization.This also implies that the samples inherit the physical properties of the precursors during carbonization process.The nitrogen adsorption-desorption isotherms (Fig.3(b)) for P-C, HP-BC and GHP-BC-3 are the type-V isotherms with a type-H3 hysteresis loop correlated with flaky grains stacked loosely, corresponding to the presence of mesopores within the carbon networks.The Brunauer-Emmet-Teller(BET)surface areas of above three samples are 114, 102, and 76 m2·g-1, respectively, prone to gradually decline,which can be determined by the altered micromorphology caused by the improved crystallinity and reduced surface defects.The pore size distribution curves (Fig.3(c)) of all three samples suggest that the pores changing from 40 to 60 nm are dominated,meanwhile, the mesoporous ratio is up to 50%.The high mesoporous ratio is conducive to the electrolyte transport, and further improving the reaction kinetics and catalytic activity for triiodide reduction.On account of its high sensitivity to disordered structure or short-range ordered structure,Raman spectroscopy are adopted to uncover the structural characterization of all samples (Fig.S1).For P-C, HP-BC and GHP-BC-3 samples, the intensity ratio of D(1350 cm-1) to G (1580 cm-1) band, correlated to the defects and crystallinity of the graphitized carbon [25,26], is estimated to 0.99,0.96 and 0.92,respectively.The value of GHP-BC-3 sample is the smallest,indicating that the defects decrease and the degree of crystallinity increases.The above results correspond with the XRD patterns.Subsequently, the D bands assigned to the firstorder spectral region were best deconvoluted into four Lorentzian-shaped bands (Fig.2(d)-(e)) constituted by D1 (the vibration of graphene layer edges at 1350 cm-1),D2(the vibration of surface graphene layer at 1620 cm-1),D3(the vibration of amorphous carbon at 1500 cm-1), and D4 (the vibration of polyenes/ionic impurities at 1200 cm-1) bands [27,28].It’s noted that after the glutamic acid-assisted hydrothermal process, the ID3/IGratio decreases to 0.41 in comparison to that of P-C (0.47), indicative of a certain improvement of the ordered degree for GHP-BC-3 sample.

To investigate the conversion of chemical composition and valence states caused by hydrothermal treatment or the glutamic acid-assisted hydrothermal recrystallization, the XPS measurement was conducted over the P-C, HP-BC and GHP-BC-3 samples.The spectrum analysis demonstrates that the three samples are mainly composed of C and O species (Fig.S2) corresponding to the results of element analysis above mentioned(Tab.S1),of which the peaks of C1s and O 1s are located at about 283.5 eV and 532 eV.Fig.S3 shows the high-resolution C 1s XPS spectra which exhibit four typical peaks by the deconvolution method.It can be seen that the four peaks are concentrated around 284.6, 285.6, 286.9 and 288.8 eV, which can be assigned to C-C double and single bonds configurations and C-O single and double bonds configurations,respectively.Calculating the corresponding bond content determined by the peak areas, the ratios of C=C to C-C for the P-C,HP-BC and GHP-BC-3 samples are 6.35, 6.94, and 7.38 (Fig.3(f)),respectively, and correspond to an upward trend, which demonstrate that the C=C is predominated on the surface of materials.It is well known that carbon hybridization is sp3hybridization in C-C while in C=C that is sp2hybridization [29].That is to say,under hydrothermal conditions, especially with the addition of glutamic acid, the proportion of sp2hybridization carbon can be well modulated accompanied by the binding state transformation from C-C to C=C.The increased long-range order structure due to the recrystallization of precursors is in favor of furthering the electrical conductivity and electrochemical stability of the final samples.The high-resolution O 1s XPS spectra(Fig.S3) are deconvoluted into four corresponding bonds.The binding energy of the above bonds is 531.6, 532.6, 533.8, and 535.0 eV, corresponding to COO-in carboxylate and/or O=C-O(the carbon-oxygen double bond), the hydroxyl and/or the carbonyl, O=C-O (the carbonoxygen single bond in carboxylic acids and esters), and chemisorbed oxygen and/or H-O-H (water)[30].In particular, the content of the bonded oxygen decreases from 1.58% to 1.33% as the content of O=C-OH going down (Fig.3f and Tab S1).The C-O/C=O and -OH species could regulate the electronic structure of the conjugated system and lower the ionization ability of carbon toward the transformation from I*to I-,thus promoting the reduction of[31].

To have an insight into the effects of glutamic acid as additives on recrystallization process and subsequent tailoring crystal structure of the GHP-BC,the precursors with different glutamic acid are synthesized and denoted as GHP-x.And after undergoing carbonization treatment, the corresponding samples are named as GHP-BC-1,GHP-BC-2,GHP-BC-3,GHP-BC-4,and GHP-BC-5,respectively.The SEM images of the precursors (Fig.S4(a)-(e)) show the evolution process of precursors’ morphology as the concentration of glutamic acid increases.The general morphology of the precursors has not been changed with/without glutamic acid, presenting nanorod structures with a more uniform size distribution as before.As the concentration continues to increase, the bamboolike carbonization products increase in length, revealing that appropriate glutamic acid addition is conducive to induce recrystallization of the precursors, and adjust the conductive network by altering the macroscopic size of the samples.The XRD patterns of GHP-x precursors (Fig.S5) also suggest the changes in the crystallinity of the reaction precursors, of which the strongest diffraction peak intensity for the GHP-3 is presented, indicating it’s highly ordered.Meanwhile, no new infrared vibration signals are detected besides the peaks at the location of ～1774, 1745 and 1594 cm-1caused by the stretching vibrations of carbonyl in the anhydride functional group and perylene body(Fig.S6),suggesting that glutamic acid could induce the recrystallization process during the robust hydrothermal treatment.Consistent with the XRD results, the DTG analysis (Fig.4(a)) confirms that the maximum weight loss temperatures of five precursor samples first increase and then decrease in which the GHP-3 sample occupies the highest point and it’s proved that the thermal stability of the GHP-3 sample is the highest.After high temperature pyrolysis, the XRD patterns (Fig.S7) reveal the pronounced diffraction peaks identified with the (0 0 2) plane of sp2carbon and the distinctive graphite phase uncovers that the carbonization products inherit the good crystallinity characteristics of the corresponding precursors.Regarding the Raman spectra of GHP-BC-1 to GHP-BC-5 samples(Fig.4(b)), the ratio values of IDto IGshow a trend of decreasing first and then rising,suggesting the variation tendency of relevant amorphicity or defect density in the graphite carbon matrix.Obviously, the GHP-BC-3 sample possesses the minimum ID/IGvalue,determining that it is dominated by the hexagonal sp2carbon network.The XPS detection results of high-resolution C 1s and O 1s spectra can be observed in Fig.4(c) and relevant statistical results are exhibited in Fig.4(d).It is notable that the fraction of graphite carbon and diamond carbon displays transformation laws of volcano plot with glutamic acid concentration, of which the sp2/sp3hybridization ratio of the GHP-BC-3 sample is the highest, consistent with the Raman results.The N 1s peak is absent in the XPS survey spectrum for all the samples above (Fig.S8) confirming no N species introduced into the carbon matrix, which corresponds to the element analysis results(Table S3).Taking the above characterization results into consideration, based on their excellent superiority, the bamboo-like CNTs with appropriate sp2hybrid carbon and open electrolyte transfer channels could be adopted as potential candidates of Pt CEs to investigate the reducing activity of.

Fig.4.(a)DTG curves of GHP-x.(b)Raman spectra of GHP-BC-1,GHP-BC-2,GHP-BC-3,GHP-BC-4,and GHP-BC-5.(c)High-resolution C 1s and O 1s XPS spectra of GHP-BC-1,GHP-BC-2, GHP-BC-3, GHP-BC-4, and GHP-BC-5.(d) The calculated percentage contents of four O species and the ratio of sp2 to sp3 carbon, which are extracted from the corresponding high-resolution XPS spectra of GHP-BC-1, GHP-BC-2, GHP-BC-3, GHP-BC-4, and GHP-BC-5.

To evaluate the electrocatalytic activity of the bamboo-like CNTs on forreduction, the CV technique was conducted on the P-C,HP-BC,GHP-BC-3 samples.Fig.5(a)shows the CV curves associated with electroactive surface areas (Se) at a scan rate of 50 mV·s-1by the[Fe(CN)6]4-/[Fe(CN)6]3-redox couple implementation, The Se of samples subjected to hydrothermal treatment with/without glutamic acid assistance have the striking enhancements compared with that of the traditional noble metal Pt(0.17 cm2), of which the value for the GHP-BC-3 sample reaches to 0.28 cm2.The favorable electrochemical capability is ascribed to the abundant active sites,open channel structures and excellent electrical conductivity,which could improve the contact efficiency between electrolyte and electrode materials and increase electron transfer rate.Two pairs of redox peaks (Aox/Aredcorrespond to the reversible reactions ofcorrespond to the reversible reactions of) can be observed in Fig.5(b),which are located at the low potential and the high potential,respectively.The IRR that occurs on CEs is the imminently concerned points.Consequently, the focus of the electrocatalytic activity study is on the Aoxand Aredpeaks and the correlated derived data are displayed in Fig.5(c).In brief,the catalytic performance of CEs materials could be reflected by the reduction peak(Ared) current density and the difference of potential between the two peaks of Aoxand Ared,which are labeled as JAredand Epp,respectively.Obviously, the JAredvalues of the P-C, HP-BC and GHP-BC-3 samples increase to some degree, superior to that of Pt CEs, of which the GHP-BC-3 possesses the maximum JAredand a smaller Eppvalue, confirming a faster I-regeneration rate and excellent catalytic activity.Moreover, the DSSCs with sandwich configurations utilizing various GHP-BC materials as CEs electrocatalysts were assembled to evaluate their potentialreduction capability.Fig.5(d) displays the J-V curves and Table S3 gives the relevant photovoltaic performance in details.The DSSCs utilizing the GHP-BC-3 as CEs deliver a superior IRR activity with a promising PCE of 8.25%, compared with that of Pt CE-based DSSCs(PCE = 7.62%).Meanwhile, the short-circuit photocurrent (Jsc),open-circuit voltage(Voc)and fill factor(FF)reach 16.43 mA·cm-2,0.70 V and 71.34%, respectively.It is apparent that the PCE of device with GHP-BC-3 CEs is remarkably higher than that for DSSCs assembled by other GHP-BC samples with different additive amounts of glutamic acid.This indicates that the GHP-BC-3 has a significantly improved response toreduction.Obviously, compared with other CEs materials, the GHP-BC-3 is prominent superiorities in both PCE and Jsc, and the improved activity is due to the superb electrical conductivity of sp2hybrid hexagonal lattice carbon skeleton and the smaller charge transfer resistance between electrolyte and electrode interface, which would be validated by the EIS analysis below.Further, the CV curves (Fig.5(e))uncover that the GHP-BC-3 displays a small Eppvalue,a large JAred,and a more positive potential of the Aredpeak (VAred), compared with other GHP-BC samples, which demonstrate that the IRR on the GHP-BC-3 possesses a fasteduction rate, small electrode polarization and low overpotential.To gain a better insight into the reaction kinetics,a series of JAredof GHP-BC-3 and Pt were collected from CV curves with different scan rates (Figs.S9-10).And the linear regression analysis results are related to square root of the scan rate and current density are exhibited in Fig.5(f).Specifically,the absolute value of the slope based on linear equation can reflect the diffusion coefficient of, of which the value for GHPBC-3 is significantly higher than that of Pt,indicating the enhanced IRR kinetics and highreduction catalytic performance.

Fig.5.(a) CV curves tested in K3Fe(CN)6 (0.005 mol·L-1) and KCl (0.1 mol·L-1) aqueous solution, (b) CV curves measured in I2 (1 mmol·L-1), LiI (10 mmol·L-1) and LiClO4(100 mmol·L-1)acetonitrile solution,and(c)JAred and Epp for the CEs of P-C,HP-BC,GHP-BC-3,and Pt.(d)J-V curves on the basis of GHP-BC-1,GHP-BC-2,GHP-BC-3,GHP-BC-4,GHP-BC-5,and Pt CEs.(e)CV curves for GHP-BC-1,GHP-BC-2,GHP-BC-3,GHP-BC-4,and GHP-BC-5 CEs.(f)JAred for the reduction of versus the square root of scan rates of GHP-BC-3 and Pt CEs.(g)Tafel polarization curves,and(h)Rs and Rct for various CEs of GHP-BC-1,GHP-BC-2,GHP-BC-3,GHP-BC-4,GHP-BC-5,and Pt.(i)Nyquist plots of the dummy cells retesting for six times with GHP-BC-3 CEs (inset, corresponding Pt CEs comparison).

Tafel polarization curves derived from symmetrical dummy cells were adopted to elucidate the reason for the prominent photovoltaic performance and the details were depicted in Fig.5(g).The exchange current density(J0)is deduced from the extrapolated interception between the zero potential line and the cathodic branch, whose value is in direct proportion to the rate coefficient of IRR on the electrode,and is correlative with the catalytic activity of the CEs further.The limited current density(Jlim)is the intersection where the cathode branch of Tafel curve in the high potential diffusion zone intersects the Y-axis,theoretically positively related to the diffusion coefficient offor CEs [33].Apparently, the dummy cell employing GHP-BC-3 reveals the maximum J0value and a higher Jlimoutperforming Pt, indicative of its satisfying catalytic activity and mass transfer rate.

To further decouple the charge transfer dynamics of various CEs, the EIS were conducted by dummy cells and the correlative results are illustrated in Figs.S11 and 5h.Generally speaking, the Nyquist plots are constituted by two semicircles, one in the middle-frequency region and the other in the low-frequency region.The above two semicircles determine the charge-transfer resistance (Rct) closely related to the activity of I-regeneration[34], and the Nernst diffusion impedance (ZN), originating from the mass transfer resistance ofin the electrolyte,respectively.The point where the Nyquist plots intersect the real axis represents the serial resistance(Rs)of the whole equivalent circuit.Compared with other CEs materials, the GHP-BC-3 displays the smallest Rsvalue, manifesting the prominent bonding ability between the sample and FTO substrate.Meanwhile, the comparatively low Rctand ZNvalues are conducive to accelerating electron transport at the electrode/electrolyte interface, reducing electron loss and improving mass transfer dynamics, suggesting that GHP-BC-3 has great advantages in catalyzing IRR.Subsequently,the electrochemical stability is recorded by EIS measurement for six times, the Nyquist plots of which are reproduced in Fig.5(i).In sharp contrast with Pt CE, both the Rsand Rctof the GHP-BC-3CE-based dummy cell hardly change after cycling test,demonstrating superior stability than that of Pt.

4.Conclusions

In summary,the bamboo-like CNTs with open channel structure and abundant active sites have been successfully exploited via a novel and facile glutamic acid-assisted hydrothermal recrystallization and pyrolysis process.The hydrothermal process and the addition of glutamic acid co-synergetically promote the significant improvement of PTCDA in crystallinity.Further,the improved crystallinity will help to improve the graphitization degree of subsequent target carbon products.As a result, the superb electrical conductivity and mass transfer dynamics arising from the improved graphitization degree and unique hollow porous structure are beneficial to realize the efficient IRR on DSSCs.Furthermore, the effect of concentration of glutamic acid on recrystallization process and crystal structure has also been investigated in details.The DSSCs fabricated with GHP-BC-3 CE exhibit the PCE of 8.25%,superior to that of the Pt CEs(7.62%).It’s demonstrated that the GHP-BC-3 can be used as a substitute for Pt so as to achieve high-efficiency energy storage conversion.Our strategy may provide a promising method for synthesizing bamboo-like CNTs by utilizing the characters of recrystallization and selfassembly of organic precursors under hydrothermal conditions.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was partly supported by the National Natural Science Foundation of China(51872035 and 22078052),Talent Program of Rejuvenation of the Liaoning (XLYC1807002) and Innovation Program of Dalian City (2019RJ03).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.05.029.