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Active sites-enriched carbon matrix enables efficient triiodide reduction indye-sensitized solar cells: An understanding of the active centers

Xiangtong Menga,c, Chang Yua, Xuepeng Zhangb, Longlong Huanga, Matthew Ragerc, Jiafu Honga,Jieshan Qiua,⁎, Zhiqun Linc,⁎

a State Key Lab of Fine Chemicals, School of Chemical Engineering, Liaoning Key Lab for Energy Materials and Chemical Engineering, Dalian University of Technology,Dalian 116024, Chinab School of Chemistry, Peking University Shenzhen Graduate School, Shenzhen 518055, Chinac School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA

A R T I C L E I N F O

Keywords:Dye-sensitized solar cellCounter electrodeNitrogen-doped carbon nanowireElectrocatalytic activityActive site

A B S T R A C T

Understanding the activity origins of electrocatalysts for the triiodide (I3-) reduction is highly desirable in dye-sensitized solar cells (DSSCs). Herein, we report a robust strategy to craft nitrogen-doped carbon nanowires(NCWs) through combining oxidation polymerization from p-phenylenediamine with carbonization process.Owing to the abundant edges of the graphite microcrystals embedded in the NCWs and the incorporated Nspecies, the NCWs synthesized at 700 °C exhibit a superior response to the I3- reduction in DSSCs with a highpower conversion efficiency of 8.90%, outperforming the Pt reference (8.09%), and a high stability is alsomanifested. Theoretical calculations reveal that, of various doped N species within NCWs, the quaternary Nspecies can significantly decrease the ionization energy and modulate the spin density distribution of carbonframeworks, thus promoting the electron transfer from the external circuit to the electrolyte. Natural populationanalysis further reveals that the active centers within the NCWs for the I3- reduction are those positively chargedcarbon atoms adjacent to the quaternary N. As such, this work will pave an avenue for rational design andengineering of inexpensive yet high-efficiency carbon electrocatalysts for the advanced energy applications.

1. Introduction

Searching for cost-effective yet high-efficiency electrocatalysts inmultifold energy applications is one of the most central and prevailingsubjects because of the increasingly grimmer energy crisis. The inter-conversion of triiodide (I3-) and iodide (I-) is the cornerstone in thestate-of-the-art dye-sensitized solar cells (DSSCs) [1–5], which connectsthe photoanode and counter electrode (CE) and governs the internalcarrier transfer [6,7]. Nevertheless, the cathodic reduction of I3- to I- issubstantially dictated by the precious and electrochemically unstable Ptcounter electrode (CE), which significantly hampers the large-scaleapplications of DSSCs [8,9]. Accordingly, it is imperative to engineerinexpensive Pt alternatives with high catalytic activity and robust sta-bility.

Carbonaceous materials have great potential to replace Pt CEs dueto their ubiquity in nature, economic feasibility, and robust durabilityin the electrochemical environments [10]. The intrinsic electrocatalyticactivity of the carbon materials can be improved through incorporatingforeign atoms, evidenced by the emerging Se and S-doped graphene

[11,12]. In the case of the N-doped carbon electrocatalysts, the intri-guing features have been substantiated because of their adjustableelectronic structures and the tunable surface chemistry [13,14]. Gen-erally, the N doping process can be realized through treatment with N-rich precursors (such as ammonia, amines, and urea), of which theprocedures are complex, environmentally unfriendly, and time-con-suming. By contrast, in situ doping and conversion from the precursorsto the desired N-doped target would be one of the convenient andpromising strategies. Moreover, despite those unique attributes of theN-doped carbon materials, their developments still significantly dependon the trial-and-error approaches due to the lack of sufficient under-standing on not only the specific roles of the diverse N doping typeswithin the doped carbon, including quaternary N, pyridinic N, pyrrolicN, and pyridinic N-oxide, in its catalytic activity towards the I3- re-duction, but also the exact active centers in the doped carbon frame-work for the cathodic reduction of DSSCs. Therefore, systematic in-vestigation into these two critical issues will be the key tounderstanding the underlying mechanism of the I3- reduction over CEsas it will provide insights into the rational design and construction of

https://doi.org/10.1016/j.nanoen.2018.09.070Received 2 August 2018; Received in revised form 17 September 2018; Accepted 30 September 2018

⁎ Corresponding authors.E-mail addresses: jqiu@dlut.edu.cn (J. Qiu), zhiqun.lin@mse.gatech.edu (Z. Lin).

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Available online 09 October 20182211-2855/ © 2018 Published by Elsevier Ltd.

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efficient carbon-based DSSCs electrocatalysts.Herein, we report a facile yet robust strategy to synthesize nitrogen-

doped carbon nanowires (NCWs) composed of the high-edge exposureand nitrogen-rich graphite microcrystals through oxidation poly-merization of p-phenylenediamine (pPD) followed by an in-situ dopingprocess. The impact of various N types and the active centers within theNCWs on the highly efficient I3- reduction in DSSCs has been system-atically scrutinized. The NCWs prepared at 500, 600, 700, and 800 °Care denoted as NCWs-500, NCWs-600, NCWs-700, and NCWs-800, re-spectively. When capitalizing on the NCWs as CEs of DSSCs, it is foundthat the NCWs-700 exhibits superior electrocatalytic response towardsthe I3- reduction with an optimal power conversion efficiency (PCE) of8.90%, outperforming the Pt reference (8.04%) and other NCWscounterparts. Though the photovoltaic performance of device with theNCWs −800 CE (8.31%) is inferior to that of NCWs-700, it is largerthan the Pt reference, indicative of great potential to replace Pt.Experimental analysis shows that the catalytic activity of the NCWstowards the I3- reduction depends on the N types and correspondingdensity. Theoretical calculations unravel that the quaternary N con-figuration can noticeably decrease the ionization energy of the NCWscompared to other sorts of N moieties, which facilitates the electrontransfer from the external circuit to the I3- and suppressing the chargeloss at the NCWs/electrolyte interface, thus contributing to the re-markably enhanced photovoltaic performance of DSSCs. Natural po-pulation analysis further unravels that the active centers within theNCWs for the I3- reduction are those positively charged carbon atomsadjacent to the quaternary N.

2. Experimental section

2.1. Synthesis of poly(p-phenylenediamine) (PpPD)

0.9 mL of 0.6 M ammonium persulfate was dropwise added into0.9 mL of 0.2 M pPD aqueous solution under continuous stirring. Then,the black suspension was washed twice with the deionized water andcentrifuged to get the precipitates followed by lyophilization, thusachieving the black powder.

2.2. Preparation of NCWs

As-made PpPD was annealed in flowing N2 atmosphere for 2 h at500 °C, 600 °C, 700 °C, and 800 °C, which are denoted as NCWs-500,NCWs-600, NCWs-700, and NCWs-800, respectively.

2.3. Fabrication of various CEs

As-made NCWs-500, NCWs-600, NCWs-700, and NCWs-800 wererespectively mixed with the carboxyethyl cellulose solution throughgrinding to form four kinds of slurries. These slurries were respectivelytransferred onto the fluorine-doped tin oxide (FTO) glasses using thedoctor-blade method and then sintered at 500 °C for 30min in N2 at-mosphere, thus producing different CEs.

2.4. Assembly of DSSCs

The TiO2 photoanodes (Yingkou OPV Tech New Energy Co., Ltd.,China) were annealed at 500 °C for 30min in air. After cooling to100 °C, the TiO2 films were immersed in a 5×10−4 M anhydrousethanol solution of N719 dye (Solaronix SA, Switzerland) for 20 h. Thephotoanodes and the CEs were then separated by a hot-melt Surlyn film(Surlyn, Yingkou OPV Tech New Energy Co., Ltd., China) with athickness of 45 µm and assembled by the hot-press. The I3-/I- electrolyte(OPV-AN-I, Yingkou OPV Tech New Energy Co., Ltd., China) was in-jected into the cell through the pre-drilled holes on the CEs. The sealedcells were used for the photovoltaic tests with an active area of0.16 cm2.

2.5. Characterization

The morphologies of all samples were examined using transmissionelectron microscopy (TEM, Philips Tecnai G220). The elemental map-pings of the NCWs-700 were recorded through field-emission scanningelectron microscopy (FESEM, Nova NanoSEM 450). X-ray diffractionpatterns were measured by an X-ray diffractometer (D/Max 2400,RIGAKU, Japan) with Cu Kα radiation (λ=1.5406 Å). The Brunauer-Emmett-Teller (BET) specific surface areas and the pore structures ofthe as-made samples were determined by the N2 adsorption(Micromeritics ASAP 2020, US). The chemical composition was de-termined by X-ray photoelectron spectroscopy with Al Kα X-ray ra-diation (XPS, Thermo ESCALAB 250). The photocurrent density-voltage(J-V) curves were recorded by a Keithley 2400 source meter equippedwith an AAA solar simulator (94,032 A, Newport, US) under AM 1.5 Gand 100mW cm−2. Cyclic voltammetry (CV) was conducted using athree-electrode cell setup, in which the as-prepared CE was taken as theworking electrode, an Ag/Ag+ electrode as the reference electrode, anda Pt wire as the CE in an anhydrous acetonitrile solution containing0.1 M LiClO4, 10mM LiI, and 1mM I2. For electrochemical impedancespectroscopy (EIS) and Tafel polarization measurements, the dummycells were assembled using two identical CEs filled with the sameelectrolyte used in the DSSCs. The Tafel plots and EIS spectra werecharacterized by Multichannel Potentiostats (VSP, Bio-Logic, France).

2.6. DFT calculations

All structural optimizations and thermodynamic data acquisitionswere operated using DFT method at B3LYP/LANL2DZ level in liquidphase with the Gaussian 09 program package [15]. The solvent effectwas estimated by utilizing the polarized continuum model (PCM)[16,17] in acetonitrile solution (Eps = 35.688) with the SMD-Coulomb[18] atomic radii. The D3 version of Grimme's dispersion with Becke-Johnson damping (GD3BJ) [19] was also included to refine the ther-modynamic energies. The ionization energy (Ei) is calculated as: Ei= Eion – En, where En and Eion are the energies of neutral molecule andpositive ion at the ground state, respectively. The atomic partial chargeswere obtained via a natural population analysis (NPA) [20].

3. Results and discussion

3.1. Synthesis and structural characterization

The novel synthetic process for the NCWs including two steps isschemed in Fig. 1. Briefly, ammonium persulfate used as oxidant wasadded into the pPD aqueous solution to induce the oxidation poly-merization of the pPD monomers. The generated black PpPD powderwas washed and collected. It is well known that the PpPD are polymersbased on the aromatic diamine and can deliver high carbonaceous re-sidue after carbonization. When the dried PpPD was subjected to dif-ferent annealing temperature (i.e., 500–800 °C) under nitrogen atmo-sphere, corresponding NCWs with edge-enriched graphite microcrystalswere in situ synthesized.

To uncover the properties of as-made NCWs, systematic character-izations were conducted. The structural information of NCWs wasfirstly revealed by X-ray diffraction (XRD) patterns and Raman spectra,of which the results are shown in Fig. S1a and S1b respectively. Ap-parently, the peak indexed to the (002) plane for the NCWs graduallyshifts to the larger diffractive angle as the annealing temperature in-creases. Based on the Bragg equation, 2dsinθ =nλ, where d is the in-terplanar distance, θ represents the diffractive angle, n=1, and λ is thewavelength of the incident wave, the NCWs-800 possesses a smallerinterlayer spacing for (002) plane in comparison with other kinds ofNCWs, indicating its improved graphite crystallinity. The relatively lowintensity and large full width at half maximum of the (002) diffractionpeak reveals that the as-made NCWs is composed of carbon sheets with

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small grain size [21]. The Raman spectra of all samples (Fig. S1b) ex-hibit two peaks at ca. 1350 cm−1 and 1580 cm−1, which are assigned tothe D band and G band. D band is ascribed to the disorder degree or thedefects in carbon materials, being absent in the Raman spectrum ofpure graphite [22]. G band relates to the vibration of the hexagonalcarbon rings [23]. Therefore the intensity ratio for the D and the Gbands (ID/IG) can reflect the crystallinity or amorphicity. It is notablethat the ID/IG of the NCWs-800 (0.97) is smaller than that of the NCWs-500 (1.01), suggesting the decreased defects and the improved degreeof graphitization, which is consistent with the XRD result.

The morphologies of the as-made products were investigated bytransmission electron microscopy (TEM). It can be clearly seen fromFig. S2a that the PpPD sample presents a wire-like structure with dia-meters of 60–100 nm. As shown in Fig. 2a, b and g, h, the nanowire-likemorphologies of NCWs are well maintained after the high temperatureannealing, which are beneficial for transferring electrons from theNCWs to the electrolyte when applied as CEs for DSSCs. The SEMmappings of the NCWs-700 (Fig. 2d–f) demonstrate that the N elementsuniformly distribute within NCWs, indicative of the in situ incorpora-tion of the N species into the carbon skeleton. It is also noted that theNCWs-700 (Fig. 2c) and the NCWs-800 (Fig. 2i) are constituted byample graphite microcrystals with 3–6 layers of carbon sheets with aninterplanar spacing of 0.34 nm, which is mainly ascribed to the easycrystallization characteristics of the rich aromatic chains within thePpPD. The nitrogen adsorption/desorption technique was performed tomeasure the Brunauer–Emmett–Teller (BET) surface areas and studythe pore structures of various NCWs. As shown in Fig. 3a, the quantityadsorbed of N2 for all NCWs materials saturate rapidly at low relativepressure, indicative of the adsorption of abundant micropores. There isa plateau in the isotherms of NCWs-500 at the medium pressure region(0.2–0.8), of which the isotherm is close to type I and the pores mainlyfocus on the micropores (Fig. 3b) [24,25]. In the range of 0.2–1.0 PP0–1, the quantity adsorbed of N2 for the NCWs-600, NCWs-700, andNCWs-800 increases with the relative pressure, indicating that thepresence of the mesopores and corresponding isotherms belong to typeIV with a H3 hysteresis loop [26–28]. The pore size distributions of theNCWs (Fig. 3b) show that the microvoid sizes for all of the NCWssamples are concentrated on 0.5–0.7 nm. The mesopores of the NCWswith diameters of 2.7–3.5 nm emerge when the annealing temperatureis higher than 500 °C, which is consistent with the related isotherms.The BET surface area and pore structural parameters of the NCWs aretabulated in Table S1, demonstrating a positive correlation between thespecific surface areas of the NCWs or the pore volume and the corre-sponding carbonization temperature. The NCWs-700 and the NCWs-800manifest high surface area up to 657 and 684m2 g−1, respectively. Thecomposition analysis (Table S2) shows that the N content of the NCWsdecreases from 13.0 at% to 9.3 at% as the annealing temperature in-creasing from 500 °C to 700 °C. When the carbonization temperature

further grows to 800 °C, the N content dramatically declines to 5.6 at%due to the partial removal of the thermally unstable N moieties. Thevolatilization of the N species within the NCWs could be responsible forthe increased surface area and pore volume [29], which is in agreementwith the above BET results. The high surface area and the multimodalpores of the NCWs-700 and the NCWs-800 can not only facilitate theaccessibility of the electrolyte and reducing the ion-transfer pathway,but also expose abundant active sites.

To analyze the valence states of the NCWs samples, X-ray photo-electron spectroscopy (XPS) was conducted. As can be observed inFig. 4a, the XPS survey spectra of all samples present the C 1s, O 1s, andN 1s peaks at about 284.6 eV, 530.4 eV, and 400.4 eV, respectively,indicative of the coexistence of C, O, and N elements. The C 1s highresolution spectrum of the NCWs-700, shown in Fig. S3, demonstratesthe C˭C (284.6 eV), C-N/C-O (285.8 eV), C˭N/C˭O (286.5 eV), andO˭C–O (288.5 eV) configurations [30], implying the successful in-corporation of the N species into carbon backbone. By deconvolutingthe N 1s spectra of all NCWs materials, four typical peaks are clearlyobserved (Fig. 4b–e), which are assigned to the pyridinic N (398.2 eV),pyrrolic N (399.3 eV), quaternary N (400.4 eV), and oxidized pyridinicN (403.2 eV) [13,31], respectively. With the N 1s XPS spectra, thecontents of these four types of N species in respective NCWs are listed inTable S2, showing the differences in the content distribution of Nconfigurations. It is clear that the contents of the pyridinic N (6.47 at%)and the quaternary N (5.7 at%) species within the NCWs-500 are largerthan that of the pyrrolic N and the oxidized pyridinic N. For the NCWs-700, the pyridinic N content significantly reduces to 3.7 at%, indicatingthe instability of the pyridinic N species at high temperature. It is alsonotable that in contrast with the contents of other N types, the qua-ternary N content of the NCWs-700 is 4.8 at%, which can be ascribed tothe high thermal stability of the quaternary N species at high tem-perature [32]. As the carbonization temperature continually increasesto 800 °C, the content of the unstable pyridinic N dramatically de-creases to 1.8 at%. Though the content of the quaternary N for theNCWs-800 declines to 3.3 at%, it remains dominant. The dominantquaternary N species within the NCWs-700 and NCWs-800 backbonecould contribute lone-pair electrons to the π-system of carbon frame-works, and function as active sites, thus providing potential superiorityto I3- reduction, which will be demonstrated by the following electro-chemical characterizations and theoretical analysis.

3.2. Electrochemical and photovoltaic characterizations

To examine the potential of various NCWs as inexpensive elecro-catalysts to the I3- reduction in DSSCs, the cells with sandwichedstructure were assembled and measured. Fig. 5a displays the photo-current density-voltage (J-V) curves. The derived parameters, includingthe short-circuit photocurrent (Jsc), open-circuit voltage (Voc), fill factor

Fig. 1. Schematic of the synthesis of NCWs.

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(FF), and PCE, are summarized in Table S3. In DSSC, the FF is a keyfactor to the overall performance, which is mainly related to the chargetransfer process at electrolyte/electrode interface [33]. As can beclearly seen from Fig. 5a that the device with the NCWs-500 CE pro-duces a Jsc of 15.14mA cm−2, Voc of 718mV, and FF of 53.7%. Thesmall FF for the cell with the NCWs-500 results in a poor PCE of 5.45%,indicative of the unsatisfactory electrocatalytic activity of the NCWs-500 towards the I3- reduction and a large internal resistance in thedevice. The PCE of the NCWs-600 is 6.82% (Jsc = 15.80mA cm−2, FF= 58.4%), while the Pt reference cell yields a PCE of 8.04%, Jsc of16.67mA cm−2, and FF of 65.3%. This will be rationalized by cyclicvoltammetry and Tafel polarization measurements below. When theNCWs-700 is capitalized on as the CE for DSSC, an impressive perfor-mance with a PCE of 8.90% is delivered (Jsc = 16.08mA cm−2, FF =74.2%), outperforming the Pt CE, which indicates that the NCWs-700possesses superb response to the I- regeneration. The photovoltaicperformance of the NCWs-700 CE-based DSSC precedes that of the

devices with other N-doped carbon CEs in previous reports (Table S4),suggesting the superiorities of the as-made NCWs-700 CE in the presentwork. It is noteworthy that the FF of device with NCWs-700 CE is muchlarger than that of the device with Pt CE. The high FF of the device withthe NCWs-700 is associated with the small charge transfer resistance atthe electrode/electrolyte interface, which will be corroborated by thefollowing electrochemical impedance analysis. It is interesting to notethat when using the NCWs-800 as CE, the cell generates a PCE of 8.31%(Jsc = 15.91mA cm−2, Voc = 713mV, and FF = 73.3%). Evidently,both the Jsc and FF of the cell with the NCWs-800 CE are smallercompared to those of the NCWs-700-based cell, which could be ascribedto the loss of N active sites within the NCWs-800 backbone at higherannealing temperature, the detailed information will be disclosed bythe following experimental and theoretical analysis. Though the per-formance of the DSSC with the NCWs-800 CE is somewhat weaker thanthat of the NCWs-700, it still holds advantages over the Pt reference,demonstrating its promise in Pt replacement.

Fig. 2. (a) SEM and (b, c) TEM images of the NCWs-700. (d) SEM image of the NCWs-700, and (d, e) corresponding element mappings. (g) SEM and (h, i) TEM imagesof the NCWs-800.

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Fig. 3. (a) Nitrogen adsorption-desorption isotherms. (b) The corresponding pore size distributions of the NCWs-500, NCWs-600, NCWs-700, and NCWs-800.

Fig. 4. XPS spectra of various NCWs. (a) Survey spectra. High resolution spectra of N 1s for (b) NCWs-500, (c) NCWs-600, (d) NCWs-700, and (e) NCWs-800.

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To compare and evaluate the electrocatalytic activity of variousNCWs and Pt CEs to the I3- reduction, cyclic voltammetry (CV) wascarried out, of which the curves are reproduced in Fig. 5b. All the CEs,except for the NCWs-500 with poor catalytic activity, display two pairsof redox peaks. The one (Aox/Ared) at low potential correlates with thereversible reactions: I3- + 2e- ↔ 3I-, and the one at high potential (Box/Bred) corresponds to the reaction: 3I2 + 2e- ↔ 2I3-. Here, the Aox/Ared

peaks are taken into account because of the I3- reduction reaction oc-curred on CE. The large peak intensity of Ared (JAred) and the smallseparation between the Aox and the Ared peaks (Epp) are responsible forthe excellent catalytic activity of CE [34]. As shown in Fig. 5c, theNCWs-600 CE possesses the smallest Ared intensity and the biggest Eppvalue than those of the Pt, NCWs-700, and NCWs-800 CEs, demon-strating the weakest catalytic behavior towards the I- regeneration andin turn the lowest PCE of the device (Fig. 5a). The NCWs-700 CE showsa larger Ared intensity (JAred) of 2.32mA cm−2 than that of the NCWs-800 (1.73mA cm−2), signifying a higher I3- reduction rate. On the otherhand, the Epp value of the NCWs-700 (0.24 V) is smaller compared toother CEs, revealing the striking electrocatalytic activity and thus ac-counting for why the photovoltaic performance of the device with theNCWs-700 CE takes the lead among all CEs. Additionally, it is notice-able that the potential of the Ared peak (VAred) of the NCWs-700 is morepositive than that of other counterparts, demonstrating that the I3- re-duction over the NCWs-700 holds a suppressed polarization and asmallest over potential, which can be ascribed to the dominant catalyticperformance of the NCWs-700. In comparison with the Pt CE (JAred =1.35mA cm−2, Epp = 0.42 V), the NCWs-800 demonstrates a larger Ared

intensity and a lower Epp, indicative of the superior catalytic activity forthe NCWs-800 and great potential to supersede Pt. The electrochemicalstability of the NCWs-700 and the NCWs-800 was also investigated. It isclearly shown that, after 15 times of sequential scanning of CV (Fig. 6),

the JAred and Epp of the NCWs-700 and the NCWs-800 do not changenoticeably, revealing their outstanding electrochemical stability. To getan insight into the reaction kinetics of the NCWs-700, NCWs-800, andPt CEs in DSSCs, their related I3- diffusion coefficients (D) were calcu-lated by means of the CV curves shown in Fig. S4 and are listed in TableS5. It is found that, for all samples, the JAred becomes larger with theincrease of the scan rates, and a linear dependence between the peakcurrent density and the square root of the scan rates is clearly mani-fested, revealing that the electrochemical polarization at the CEs be-comes increasingly serious and the cathodic and anodic reactions aredominated by the electrolyte transportation [23,35]. The linear re-lationship between the Ared current and the square root of the scan ratecan be described by the Randles-Sevcik equation [36]: Ipeak = Kn3/2AC(D)1/2V1/2, where, Ipeak is the peak current, K is the constant of2.69×105, n is the electron transfer number, A is the electrode area, Dis the diffusion coefficient of the I3-, V is the scan rate, and C is theconcentration of electrolyte. Therefore, the slope of the line is positivelyassociated with the D of I3-. Apparently, the absolute value of the slopeof the line for the NCWs-700 CE is the largest (Fig. S4d), correspondingto a predominant D of I3- in the electrolyte, which will accelerate the I-

regeneration.In order to further unveil the reason why the photovoltaic perfor-

mance of the NCWs-700 CE takes the lead among the NCWs CEs, Tafelpolarization curves were recorded by symmetrical dummy cells made oftwo identical CEs (CE/electrolyte/CE), as shown in Fig. S5. Typically,the Tafel curve comprises three zones: polarization zone at the lowpotential, Tafel zone at the intermediate potential, and diffusion zone atthe high potential. The exchange current density (J0) can be deliveredvia the extrapolated intercepts of the cathodic branch at the Tafel zone[12]. A large J0 of CE signifies a high catalytic activity. Apparently, theNCWs-700 exhibits a larger J0 compared with other CEs, indicating that

Fig. 5. (a) J-V curves of DSSCs based on NCWs-500, NCWs-600, NCWs-700, NCWs-800 CEs, and Pt CEs. (b) CV curves of the I3-/I- redox couple for various CEs at scanrate of 50mV s−1. (c) Epp and JAred of various NCWs CEs. (d) Nyquist plots of the symmetrical dummy cells with NCWs-700 and NCWs-800 CEs (inset, correspondingequivalent circuit), EIS measurement was conducted at 0.8 V from 0.8MHz to 0.1 Hz.

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the NCWs-700 has outstanding catalytic activity. Additionally, thelimited current density (Jlim) can be deduced from the Tafel curves bythe intersection of the cathodic branch with the y-axis. In principle, theJlim value is positively proportional to the diffusion coefficient (D) of I3-

in DSSCs [34]: Jlim = 2nFCDδ−1, where n is the electron transfernumber in the I3- reduction, F is the Faraday constant, C is the con-centration of I3-, δ is the distance between two electrodes of the dummycell. Thus it will be easy to note that the D follows in an order of NCWs-700 > NCWs-800 > NCWs-600 > NCWs-500, corresponding to theincremental electrocatalytic activity towards the I3- reduction. There-fore, the DSSCs with the NCWs-700 and the NCWs-800 CEs demonstratethe excellent photovoltaic and electrochemical performance.

The charge transfer process at CEs was sequentially analyzedthrough the electrochemical impedance spectra (EIS), the results ofwhich are reproduced in Fig. 5d and Table S6. It is notable that theNyquist plots exhibit two semicircles. Generally, the high-frequencyintercept of Nyquist plot on the real axis determines the serial resistance(Rs) containing the electrode resistance. The left semicircle representsthe charge-transfer resistance (Rct) at the electrode/electrolyte inter-face, related to the electrocatalytic activity of CEs to I3- reduction, andconstant phase element (CPE). The right one can be ascribed to theNernst diffusion impedance (ZN), stemming from the mass transportlimitation due to the diffusion of the I3-/I- redox couple [37]. TheNCWs-700 CE shows a smaller Rct (5.6Ω cm2) in comparison to that ofthe NCWs-800 (18.0Ω cm2), elucidating that the NCWs-700 can rapidlytransfer the electrons to the electrolyte and suppress the accumulationof electrons effectively at the electrode/electrolyte interface, thus de-livering a high FF (Fig. 5a). In principle, the Rct varies inversely with

the J0 of the Tafel plot [38]. The small Rct for the NCWs-700 indicates alarge J0, which is in line with the Tafel results. The NCWs-700 CEpossesses a smaller ZN (9.9Ω cm2) than that of the NCWs-800(35.2Ω cm2) because of its larger diffusion coefficient, indicating thelarge transfer rate of ions in electrolyte.

3.3. Effect of N species within NCWs on catalytic activity

To shed light on the effect of these different N species within theNCWs on the I3- reduction, the relationship between the atomic ratios ofthe four N configurations in respective NCWs determined by the XPSanalysis, and the related photovoltaic performance is unraveled(Fig. 7a). As for the NCWs-500, the highest total N content (13.0 at%)corresponds to a lowest PCE (5.45%). Though the total N content of theNCWs-600 (12.2 at%) decreases, a larger PCE (6.82%) is presentedcompared with the NCWs-500, implying that the PCE of the deviceswith the NCW CEs is not correlated to the total N content of the NCWs.Thereinto, the content of the pyridinic N species for NCWs-600 has adecrease in contrast with that of the NCWs-500, indicating that thecontribution of the pyridinic N in the NCWs to the PCE improvement isnegligible. When the annealing temperature increases to 700 °C, thetotal N content of the NCWs-700 decreases to 9.3 at% and corre-sponding device delivers a highest PCE of 8.90%. Although the contentsof the pyrrolic N and the pyridinic N-oxide of the NCWs-700 do notchange obviously, the pyridinic N content dramatically decreases to3.7 at%. Interestingly, the content of the quaternary N of the NCWs-700becomes the largest compared with the other N types. This is also thecase for the NCWs-800.

To unveil the roles of various N species in the I3- reduction in detail,DFT calculations were further conducted. The overall reduction reac-tion of I3- over CE can be described in a general equation: I3- (sol) + 2e-

↔ 3I- (sol), which includes three elementary reactions [39]:

I3- (sol) ↔ I2 (sol) + I- (sol) (1)

I2 (sol) + 2* → 2I* (2)

I* + e- → I- (sol) + * (3)

wherein sol means the acetonitrile solution, and * represents the activesite of CE material. In terms of the reported results, the reaction (3) is

Fig. 6. Normalized Epp and normalized JAred versus scan numbers of CV mea-surements: (a, b) for the NCWs-700, and (c, d) for the NCWs-800.

Fig. 7. (a) The atomic contents of different N configurations within variousNCWs versus corresponding photovoltaic performance. (b) The calculated Ei fordifferent kinds of graphene slabs (inset, six kinds of slab models, in which grey,yellow, green, purple, blue, and red balls represent carbon, hydrogen, pyridinicN, quaternary N, pyrrolic N, and oxygen atoms, respectively.).

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considered the rate-determining step for the I- regeneration [40–42].Therefore, this reaction has been focused on in the present work. Tosimplify the calculations, the graphite microcrystal structures withvarious N species in NCWs were simulated by one layer of graphite(graphene, G) composed of ten hexagonal carbon rings. The simulatedstructures and related energies are shown in Figs. S6–S9. Evidently, theG with the pyridinic N located at the zig-zag edge (Pyd N3), G with thequaternary N situated on the arm-chair edge (Qua N1), G with thepyridinic N-oxide at the zig-zag edge (Pyd NO3), and G with the pyr-rolic N1 (Pyr N1) are the most stable configurations due to corre-sponding lowest energies. The ionization energy (Ei) of the electro-catalyst can reflect its electron-donating capability. In this regard, a lowEi enables a fast rate for the reaction (3) [12]. The Ei values of various Gslabs are computed and enumerated in Table S7. It is observed that theEi values of G slabs vary with the containing N types. The Ei values ofthe G slabs with pyrrolic N, pyridinic N, and pyridinic N-oxide are closeto that of the pure G slab (Fig. 7b), indicating that the impacts of theseN species on the I3- reduction are analogous and negligible. The Ei valueof the G with quaternary N is ca. 75 kcal mol−1, which is smaller thanthat of G (113.71 kcal mol−1) and other counterparts, meaning that thequaternary N species can render the charges more readily to transferfrom the NCWs to the electrolyte. In contrast with the NCWs-700, theserious loss of the quaternary N species for the NCWs-800 impairs itscatalytic activity, thus resulting in a diminished PCE. Additionally, thegraphite microcrystals within NCWs-700 expose abundant electro-chemically active edge sites, as well as the multimodal pores and highspecific surface area, thus contributing to the accessibility of CEs to theelectrolyte. These integrated characteristics can improve the electro-chemical response of the NCWs-700 towards the I- regeneration to agreat degree. The stable G slab containing the whole N types was alsoset up (Fig. S10). The corresponding Ei is 88.7 kcal mol−1, which issmaller than that of the pure G. This result further corroborates that theincorporated N species can reduce the Ei of the carbon framework.

To further scrutinize the roles of N species within the NCWs back-bone in the reduction of I3-, spin density distributions of G with thesemost stable N configurations were calculated and displayed in Fig. 8a. Itis clear that all the G slabs are fully covered by corresponding spindensity distributions, indicative of the high electron delocalization. It isinteresting to note that the spin density distribution of G with Qua N1has shifted obviously compared to other G slabs, which could result in

the formation of the carbon active sites. In this context, the atomicpartial charges of G with Qua N1 were obtained via a natural popula-tion analysis (Fig. 8b). We can observe that the carbon atom adjacent tothe Qua N1 is positively charged (red dotted area), which could interactwith the long pair electrons of I2 through the electrostatic interactionsto some extent, being favourable for the absorption of I2 in the elec-trolyte. Therefore, this carbon atom can behave as the potential activesite to the I3- reduction reaction.

4. Conclusions

In summary, active site-enriched NCWs have been crafted via theoxidation polymerization of p-phenylenediamine, followed by in situdoping process. Due to the rich edge sites of graphite microcrystalsembedded in the NCWs, abundant N species, and relatively high spe-cific surface area, both the NCWs-700 and NCWs-800 show superbelectrocatalytic activity towards the I3- reduction and the robust elec-trochemical stability when behaving as the CEs for DSSCs, thus ex-hibiting great potential to supersede the Pt references. The differencesin the catalytic activity between the NCWs and the Pt CEs are scruti-nized by various electrochemical measurements. More importantly,DFT calculations uncover that the N species within the carbon back-bone, in particular the quaternary N significantly reduces the ionizationenergy compared with other N species, which imparts the electrontransfer from the external circuit to the electrolyte, thus contributing tothe enhancement of the photovoltaic performance in DSSCs. It is alsofound that the carbon atoms adjacent to the quaternary N are the activesites to the I3- reduction. As such, this work may open an avenue forrational design and engineering of inexpensive yet high-efficiencycarbon electrocatalysts for the advanced energy applications.

Acknowledgements

This work was partly supported by the National Natural ScienceFoundation of China (NSFC) of China (Nos. 21522601, U1508201), theFundamental Research Funds for the Central Universities of China(DUT16ZD217), and the National Key Research Development Programof China (2016YFB0101201). X. Meng also thanks the financial supportfrom China Scholarship Council (201606060061).

Fig. 8. (a) Spin density distributions of G with respective most stable N species. (b) Natural population analysis of G with quaternary N1 species situated on the Garm-chair edge (Qua N1).

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Appendix A. Supplementary material

Supplementary data associated with this article can be found in theonline version at doi:10.1016/j.nanoen.2018.09.070.

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Xiangtong Meng has been a Ph.D. candidate supervised byProf. Jieshan Qiu after his Master study in the School ofChemical Engineering at the Dalian University ofTechnology (DUT) since 2014. He is currently a joint Ph.D.student co-supervised by Prof. Jieshan Qiu at DUT and Prof.Zhiqun Lin at the Georgia Institute of Technology. His re-search focuses on engineering carbon-based electrode ma-terials for high-performance dye-sensitized solar cells andperovskite solar cells.

Chang Yu received her Ph.D. degree from the School ofChemical Engineering at the DUT in 2008. She is currentlya professor at the DUT. Her research interests mainly focuson carbon coupled two-dimensional inorganic layered ma-terials for energy storage and conversion applications.

Xuepeng Zhang received his Ph.D. degree from the Schoolof Chemistry at the Sun Yat-sen University in 2017. Heworks currently as a post doctor fellow at the PekingUniversity Shenzhen Graduate School. His research interestfocuses on elucidating reaction mechanisms in catalysisemploying metal-complexes, especially in the field of C-Hfunctionalization and phosphoester hydrolysis.

Longlong Huang received his B.S. degree from the Schoolof Chemical Engineering of the DUT in 2015. Now he is aM.S. student supervised by Prof. Chang Yu. His researchtopic is construction of carbon based counter electrodes fordye-sensitized solar cells.

Matthew Rager is a Ph.D. student at the Georgia Instituteof Technology under the advisement of Dr. Zhiqun Lin. Heis currently studying perovskite solar cell technology with afocus on nanomaterials and interfacial engineering ap-proaches to improve device efficiency and stability. Hisinterests lie in nanomaterials for renewable energy, energystorage, and environmental applications.

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Jiafu Hong is a M.S. student now supervised by Prof.Chang Yu in School of Chemical Engineering at the DUT.His research topic focuses on construction of carbon basedcounter electrodes for dye-sensitized solar cells.

Jieshan Qiu obtained his Ph.D degree in the school ofchemical engineering of DUT in 1990. He was also a visitingprofessor at Pennsylvania State University (USA), WestVirginia University (USA), and the University of Reading(UK). He was appointed to a Cheung-Kong DistinguishedProfessor in 2009. Now he is a professor of chemical en-gineering and the director of the Carbon ResearchLaboratory of DUT. His current research includes functionalcarbon nanotubes, graphene, carbon nanohybrids, and theirapplications (energy conversion and storage, capacitivedeionization technique, etc.).

Zhiqun Lin is a Professor in the School of Materials Scienceand Engineering at the Georgia Institute of Technology. Hereceived his Ph.D. in Polymer Science and Engineering fromthe University of Massachusetts, Amherst in 2002. His re-search interests include perovskite solar cells, polymer solarcells, dye-sensitized solar cells, photocatalysis, hydrogengeneration, lithium ion batteries, semiconductor organi-c–inorganic nanocomposites, quantum dots (rods), con-jugated polymers, block copolymers, polymer blends,hierarchical structure formation and assembly, surface andinterfacial properties, multifunctional nanocrystals, andJanus nanostructures.

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