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Journal of Power Sources 303 (2016) 159e167

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Journal of Power Sources

journal homepage: www.elsevier .com/locate/ jpowsour

Nitrogen-doped carbon onions encapsulating metal alloys as efficientand stable catalysts for dye-sensitized solar cells

Chongyang Zhu a, Feng Xu a, *, Jing Chen b, Huihua Min c, Hui Dong a, Ling Tong d,Khan Qasim b, Shengli Li a, Litao Sun a

a SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of the Ministry of Education, Southeast University, Nanjing 210096, Chinab School of Electronic Science & Engineering, Southeast University, Nanjing 210096, Chinac Electron Microscope Laboratory, Nanjing Forestry University, Nanjing 210037, Chinad Jiangnan Graphene Research Institute, Changzhou 213149, China

h i g h l i g h t s

* Corresponding author.E-mail address: fxu@seu.edu.cn (F. Xu).

http://dx.doi.org/10.1016/j.jpowsour.2015.10.1110378-7753/© 2015 Elsevier B.V. All rights reserved.

g r a p h i c a l a b s t r a c t

� N-doped carbon onions encapsu-lating alloys are developed as counterelectrodes.

� Scalable production of carbon onionsencapsulating alloys can be achievedfacilely.

� Superior electrochemical stability aredemonstrated for counter electrodes.

� Ultrahigh power conversion effi-ciencies can rival that of traditionalcostly Pt.

a r t i c l e i n f o

Article history:Received 1 September 2015Received in revised form27 October 2015Accepted 30 October 2015Available online 11 November 2015

Keywords:Dye-sensitized solar cellsN-doped carbon onionMetal alloyStabilityCounter electrode

a b s t r a c t

Designing a new class of non-noble metal catalysts with triiodide reduction activity and stability com-parable to those of conventional Pt is extremely significant for the application of dye-sensitized solarcells (DSSCs). Here, we demonstrate newly designed counter electrode (CE) materials of onion-like ni-trogen-doped carbon encapsulating metal alloys (ONC@MAs) such as FeNi3 (ONC@FeNi3) or FeCo(ONC@FeCo), by a facile and scalable pyrolysis method. The resulting composite catalysts show superiorcatalytic activities towards the triiodide reduction and exhibit low charge transfer resistance between theelectrode surfaces and electrolytes. As a result, the DSSCs based on ONC@FeCo and ONC@FeNi3 achieveoutstanding power conversion efficiencies (PCEs) of 8.26% and 8.87%, respectively, which can rival the8.28% of Pt-based DSSC. Moreover, the excellent electrochemical stabilities for both the two catalysts alsohave been corroborated by electrochemical impendence spectra and cyclic voltammetry (CV). Noticeably,TEM investigation further reveals that the N-doped graphitic carbon onions exhibit the high structuralstability in iodine-containing medium even subject to hundreds of CV scanning. These results makeONC@MAs the promising candidates to supersede costly Pt as efficient and stable CEs for DSSCs.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Dye-sensitized solar cells (DSSCs), which directly convert solarenergy into electricity, have become one of the most promising

C. Zhu et al. / Journal of Power Sources 303 (2016) 159e167160

photovoltaic devices to address the energy depletion and envi-ronmental pollution due to their low-cost fabrication, environ-mental friendliness, and relatively high power conversionefficiency (PCE) [1e3]. Generally, four key components have beenbelieved to determine the performance of a DSSC device: photo-anode, dye, electrolyte, and counter electrode (CE). Among these,the CE plays an important role in collecting electrons from theexternal circuit and catalyzing reduction of the redox couples inelectrolytes [4,5]. In this respect, the conventional platinum (Pt),which shows high conductivity and efficiently catalyzes thereduction of I3d to I d, is most commonly used as the CE materialfor DSSCs [6,7]. Nevertheless, as a noble metal, the limited avail-ability and high cost of Pt seriously hinder the large-scale produc-tion of DSSCs. Therefore, considerable efforts have been made onthe development of alternatives that are highly catalytic andabundantly available, including carbon materials [8e10], con-ducting polymers [11,12], transition-metal compounds [13e15],and binary-metal alloy nanoparticles [16e18].

Recently, metal alloy nanoparticles are emerging candidates forCE and contribute to the impressive efficiencies of DSSCs. On onehand, Wan et al. [19] and He et al. [20] in their work, focused onreducing the use amount of Pt metal and reported the utilization ofPt3Ni and CoPt0.02 alloys as CEs in DSSCs. The proposed low-Ptbased alloy CEs exhibited much higher catalytic activities thanthe metallic Pt for I3d reduction, and generated promising PCEs of8.78% and 10.23%, respectively. On the other hand, for the appli-cation of Pt-free metal alloy CEs, a series of FeeCo nanofiber alloyCEs were prepared by hydrothermal approach and exhibited thehighest efficiency of ~5.6% for DSSCs [21]. Later on, a new type ofbinary CoeNi alloy nanoparticles also was successfully employed asthe CEs, which produced an efficiency of 8.39% in a device, out-performing that of Pt-based DSSC [22]. Moreover, it has demon-strated that the metal alloy of FeNi nanoparticles can enhance thecatalytic activity of the carbon walls, consequently resulting in anenhanced PCE [23]. However, apart from the preliminary meritsachieved above, it remains a question for the electrochemical sta-bility of the metal alloy catalysts in electrolytes, due to the potentialcorrosion of metal CE in electrolytes by the formation of halides. Infact, previous studies have proved the chemical instability of thevapor deposited Pt CE in corrosive I d/I3d redox system [24e26],which further leads to a significant reduction of catalytic activityand efficiency of the devices. Noticeably, such corrosive phenom-enon should be avoided for DSSCs in long-term commercial ap-plications. Hence, it is required to design an ideal structurepossessing extraordinary stability when exploring high-performance metal alloy CEs in the future.

In this work, inspired by the facts that carbon material ischemically stable for the electrochemical applications, and coree-shell structure hybrid materials can provide enhanced electro-chemical performance in both catalytic activity and electronicconductivity [27e30], we firstly report a new class of composite CEmaterials of N-doped carbon onion encapsulating metal alloynanoparticles including FeNi3 and FeCo, through a simple pyrolysisapproach. Structure characterizations prove that the N-dopedgraphitic carbon onion was successfully synthesized which couldbenefit the charge transfer between catalysts and provide richcatalytic active sites on the surface of carbon shell, as corroboratedby the electrochemical impedance spectra and cyclic voltammetry.For the CEs application of DSSCs, the ONC@FeCo and ONC@FeNi3have achieved comparable and even superior PCEs to Pt electrode.Further electrochemical measurements as well as TEM observa-tions also have demonstrated the highly chemical stability for theas-prepared catalysts. Hence, our work promisingly provides a newpathway for developing high-performance and stable catalysts forapplications in optoelectronic or energy-storage devices.

2. Experimental

2.1. Preparation of ONC@FeNi3 and ONC@FeCo catalysts

The ONC@FeNi3 catalyst was synthesized through direct pyrol-ysis of the precursor. In detail, the precursor Ni2Fe(CN)6 was firstlyprepared by dropwise adding the 200 mL 0.1 M Ni(NO3)2 aqueoussolution into the 200 ml 0.05 M K4Fe(CN)6 aqueous solution undervigorous stirring at room temperature. Light blue homogeneousmixture was obtained after stirring for 15 min. Later, the mixturewas isolated by centrifugation, washed with deionized water forthree times, and then dried in a vacuum oven at 100 �C for 4 h. Afterthat, the resulting precursor was highly grinded into powder beforeannealing at 600�C for 2 h in nitrogen atmosphere. Finally, thesystem was cooled down naturally to room temperature and theresulting ONC@FeNi3 was obtained for characterization and ex-periments. Similarly, ONC@FeCo catalyst was synthesized by thesame procedure except that the Ni(NO3)2 aqueous solution wasreplaced by Co(NO3)2 aqueous solution.

2.2. DSSC fabrication and testing

For counter electrodes preparation, the as-obtained materialswere ultrasonically dispersed in mixture solutions of isopropanoland ethanol (2:1 in volume) for 30 min, and were subsequentlyspin-coated onto ITO as the CEs. For comparison, the standard PtCEs (Dalian HepatChroma SolarTech) were used. TiO2 nanocrystalanode films consisting of a 12 mm-thick transparent layer and a4 mm-thick scattering layer on FTO glass substrate was prepared bythe screen-printing technique. After that, the resulting TiO2 pho-toanodes were sensitized in a 0.3 mM ethanol solution of ruthe-nium dye N719 at 45 �C for 6 h. Finally, the TiO2 photoanodes wereassembled with the Pt CE, ONC@FeCo CE, and ONC@FeNi3 CE intoDSSCs, respectively. The dye-sensitized TiO2 photoanode and the CEwere separated by a hot-melt Surlyn film (60 mm thick) and sealedthrough hot-pressing. The DSSC electrolyte with 0.1 M LiI, 0.05 M I2,0.3 M 1,2-dimethyl-3-propylimidazolium iodine, and 0.5 M tert-butylpyridine in 3-methoxypropionitrile was injected into thegap between the photoanode and CE by capillarity action.

2.3. Characterization

Phase identification and surface morphology of the productswere characterized by powder X-ray diffractometer (XRD, ARLXTRA, Thermo Electron Co., USA) with Cu Ka radiation as the X-raysource and scanning electron microscope (SEM, JSM-7600F, JEOL,Japan). Further structural analyses were carried out by transmissionelectron microscope (FEI Titan 80e300) at 300 kV equipped withan attachment for element mapping. Raman spectroscopy wasrecorded on Renishaw laser Raman spectrometer (inVia Reflex),using a 523 nm laser source; X-ray photoelectron spectroscopy(XPS) measurement was performed on a K-Alpha spectrometer(Thermo Scientific, Al-Ka X-ray source, 1486.6 eV, spot size of400 mm)with power of 200Wand pass energy of 50.0 eV was used.The specific surface area was measured on a QuantachromeAutosorb-1 volumertic analyzer, using nitrogen adsorption and theBrunauereEmmetteTeller method, and the pore size distributionwas calculated from the isotherms using the BJH (Bare-tteJoynereHalenda) procedure. Cyclic voltammetry (CV) was car-ried out on a CHI-660D electrochemical workstation (CHInstruments, Inc., USA) using a three-electrode system with ananhydrous acetonitrile (ACN) solution of 0.1 M LiClO4, 10 mM LiI,and 1mM I2 at a scan rate of 50mV s�1, with a platinum sheet as thecounter electrode, a statured calomel electrode (SCE) as the refer-ence electrode, and the as-prepared CEs as the working electrodes.

C. Zhu et al. / Journal of Power Sources 303 (2016) 159e167 161

Tafel polarization curves of the CEs were obtained using symmet-rical cells at a scan rate of 10 mV s�1. Electrochemical impedancespectra (EIS) were carried out (Autolab, Eco-Chemie, TheNetherlands) at zero bias using symmetrical cells by applying an ACamplitude of 10 mV in a frequency range from 0.05 Hz to 100 kHz.The resultant impedance spectra were analyzed and fitted with Z-view software. The electrolytes used for testing both EIS and Tafelpolarization curves were the same as those used in the DSSCs. Thecurrent-voltage (IeV) characteristics of DSSCs were assessed with aNewport solar simulator (300 W Xe lamp source), and a Keithley2400 source meter under 1 sun illumination (AM 1.5G,100 mW cm�2). A black mask with an aperture area of around0.16 cm2 was applied on the surface of DSSCs to avoid stray lightcompletely.

3. Results and discussion

The morphology and structure of the prepared samples werecharacterized by TEM. Fig. 1a, e show typical TEM images ofspherical ONC@FeNi3 and ONC@FeCo with an onion-like structurecarbon shells encapsulating metal alloys. No free metal particleswere found outside the carbon layers during the TEM observations,indicating a perfect combination between the carbon shells andmetal alloys. The size distributions of ONC@FeNi3 and ONC@FeConanoparticles were calculated in Fig. 1b, f, with an average size ofabout 30 nm and 66 nm in diameter, respectively. HRTEM images(Fig. 1c, g) of individual ONC@FeNi3 and ONC@FeCo reveal that theshell of samples consist of ordered graphitic layers with latticefringes of about 0.34 nm, corresponding to the (002) plane ofgraphite, while the lattice spacing of 0.204 nm and 0.202 nm in theselect zones marked with red box match well with the (111) planeof FeNi3 alloy and the (110) plane of FeCo alloy, respectively. Theelement mapping (Fig. 1d, h) was also performed to confirm thepresence and well distribution of C, N, Fe, and Ni or Co elementsaround the particles, verifying the well formation of the ONC@-FeNi3 and ONC@FeCo.

XRD measurements were conducted to evaluate the crystalphases of the samples. Fig. 2a shows a series of characteristic peaksat 43.5�, 50.7�, and 74.6�, corresponding to the (111), (200), and

Fig. 1. (a, e) TEM and (c, g) HRTEM images of the obtained ONC@FeNi3 and ONC@FeCo cnanoparticles. (b, f) The size distributions of ONC@FeNi3 and ONC@FeCo catalysts. Scanning Tcorresponding electron energy loss spectroscopy elemental maps.

(220) planes of the cubic phase FeNi3 (JCPDS, PDF no. 47-1417).Meanwhile, the typical peaks located at 44.9� and 65.3� can bereadily indexed to the (110) and (200) planes of the cubic phaseFeCo alloy (JCPDS, PDF no. 49-1568), further confirming the resultsof TEM analysis. A Raman spectrumwas used to identify the degreeof the graphitization of as-prepared samples. As displayed inFig. 2b, both the ONC@FeNi3 and ONC@FeCo show the higher in-tensity of G bands than the D bands, and give ID/IG values of 0.80and 0.85, respectively, indicating a high degree of graphitization.This is attributed to the formation of onion-like carbon structureand is in favor of improving the electrical conductivity of the system[31]. To analyze the elemental composition and nitrogen bondingconfigurations in ONC@FeNi3 and ONC@FeCo samples, we furtherperformed XPS measurements. As displayed in Fig. 2c, d, XPSspectra confirm the element presence of C, N, Fe, and Ni or Co intwo samples, and reveal that the N content in ONC@FeCo andONC@FeNi3 is about 6.74% and 10.31% (Table S1), respectively. Thisresult suggests that the high-level nitrogen atoms have been suc-cessfully and uniformly (Fig. 1d, h) incorporated within the onion-like carbon structure by direct pyrolysis method. The high-resolution N1s spectra of ONC@FeCo and ONC@FeNi3 (Fig. 2e, f)show that N1s spectrum can be mainly resolved into two N peaks,pyridinic N in six-membered pyradazole ring (398 ± 0.1 eV) andpyrrolic N in five membered pyrazole ring (399.8 ± 0.1 eV). Apartfrom those, a weak N peak at around 400.9 eV corresponding tographitic N is also observed for ONC@FeCo. Generally, it has shownthat both pyridinic N and graphitic N could promote the electrontransport of carbon materials and provide highly catalytic activesites for iodine reduction reaction due to the shift in redox potentialas well as the lowered absorption energy [32e34]. Obviously, hightotal contents of pyridinic and graphitic N have been demonstratedfor ONC@FeCo (5.71%) and ONC@FeNi3 (8.93%), respectively. Hence,we expect ONC@FeCo and ONC@FeNi3 CEs to yield outstandingperformance in DSSCs.

For DSSC applications, the obtained ONC@FeCo and ONC@FeNi3catalysts were dispersed in solutions of isopropanol and ethanol,and then spin-coated on ITO substrates as the CE films that werefurther characterized by SEMmeasurements. Obviously, the imagesin Fig. 3 suggest a high surface coverage of catalysts on the ITO and

atalysts. Inset in c, g are the corresponding HRTEM images of FeNi3 and FeCo alloyEM images of the individual (d) ONC@FeNi3 and (h) ONC@FeCo nanoparticles and their

Fig. 2. (a) XRD pattern, (b) Raman spectra of the ONC@FeCo and ONC@FeNi3 catalysts. (c, d) X-ray photoelectron survey spectra and (e, f) high-resolution N 1s spectra of ONC@FeCoand ONC@FeNi3 catalysts.

C. Zhu et al. / Journal of Power Sources 303 (2016) 159e167162

reveal the spherical structure of ONC@FeCo and ONC@FeNi3, whichis in agreement with the TEM observations both in size and shape.Notably, a lot of pores were observed with red marks in Fig. 3a, c,indicating the relatively loose structure between the aggregations,which was further confirmed by the surface area and pore struc-tures characterization in Fig. S1 and Table S2. In spite of low specificsurface area, the relative loose structure would provide facilechannels for the transportation of triiodide ions and thus facilitatethe reduction reactions.

To evaluate the catalytic activities of ONC@FeNi3 and ONC@FeCotowards the reduction of I3d in the electrolyte, we performed cyclicvoltammetry (CV) measurements for ONC@FeNi3 and ONC@FeCoCEs, as well as for Pt electrode for comparison. For all three CEs, twopairs of typical oxidation and reduction peaks are well distin-guished in Fig. 4a (Ox-1/Red-1, Ox-2/Red-2). The left pair isattributed to the reaction of Equation (1), whereas the right pair isassigned to the process of Equation (2). Typically, the peak currentdensity and peak-to-peak separation (Epp), which is negativelycorrelated with the standard electrochemical rate constant of a

redox reaction, are two crucial parameters for evaluating catalyticactivities of different CEs [35]. Since the CEs in DSSCs are respon-sible for catalyzing reduction of I3d to I d, the characteristics of leftpair are at the main concern of our analysis. It can be seen that theONC@FeNi3 CE as well as ONC@FeCo CE has a smaller Epp value thanPt electrode (Fig. 4a and Table 1), demonstrating good reversibilityfor as-prepared CEs. Besides, cathodic peak potentials of ONC@-FeNi3 and ONC@FeCo CEs locate at �0.09 V and �0.14 V, respec-tively, which are larger than that of �0.20 V for Pt CE, suggestingthat I3d is able to be reduced more easily at the interface of the as-prepared CEs [36]. On the other hand, the highest peak currentdensity is achieved by the ONC@FeNi3 CE, revealing that theONC@FeNi3 is a remarkable electrochemical catalyst for reductionof I3d, even better than Pt. However, the ONC@FeCo CE shows alower peak current density compared with Pt electrode, which alsodirectly determines the lowest photocurrent density in DSSCs(Fig. 5 and Table 1). Judged from the peak current density and Epp,ONC@FeCo is inferior to Pt and ONC@FeNi3, but still exhibits goodelectrocatalytic activity.

Fig. 3. SEM images of the CE films prepared by (a) ONC@FeCo and (c) ONC@FeNi3 catalysts. (b), (d) are the corresponding high magnification images.

Fig. 4. (a) Cyclic voltammetry curves obtained in ACN solution containing 10.0 mM LiI, 1.0 mM I2, and 0.1 M LiClO4, at a scan rate of 50 mV s�1. (b) Relationship between the peakcurrent density for the iodide/triiodide redox reaction and the square root of scanning rate of the ONC@FeCo, ONC@FeNi3, and the sputtered Pt CEs. (c) Electrochemical impedancespectra and (d) Tafel polarization curves of the three CEs, obtained with two identical electrodes in the same electrolyte as that used in DSSCs. Inset in c gives the equivalent circuitused in DSSCs.

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I3� þ 2e�43I� (1)

3I2 þ 2e�42I3� (2)

We also have conducted the cyclic voltammograms for above

three CEs at different scanning rates so as to investigate the rela-tionship between the peak current densities of Ox-1/Red-1 and thescanning rates. Evidently, an outward extension of the peaks wasobserved in Fig. S2 for all three CEs with the increase of scanningrates, and a fitting linear relationship was found (Fig. 4b) between

Table 1Parameters of electrochemical and photovoltaic performance using different CEs.

CEs Voc (V) Jsc (mA cm�2) FF PCE (%) Rs (U cm�2) Rct (U cm�2) Epp (mV)

Pt 0.75 17.57 0.63 8.28 4.24 1.22 404ONC@FeCo 0.77 16.58 0.65 8.26 4.71 1.23 320ONC@FeNi3 0.76 17.59 0.67 8.87 4.38 1.02 312

Fig. 5. (a) Scheme configuration of a DSSC used (not scaled), with the obtained ONC@FeCo or ONC@FeNi3 as CE catalysts. (b) Photocurrent density�voltage (J-V) curves of DSSCsconstructed using Pt, ONC@FeCo, and ONC@FeNi3 CEs under a simulated solar illumination with a light intensity of 100 mW cm�2 (AM 1.5).

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the peak current densities and the square root of the scanning rates,which implies that the redox reaction are diffusion-controlledprocess for the transportation of iodide species towards thecounter electrode in the cells [37,38].

EIS represents the intrinsic interfacial charge transfer andcharge transport kinetics at the electrode/electrolyte interface [22].It was performed on symmetric cells to further investigate thecharge transfer process between the interface of electrolytes andthree CEs. The obtained Nyquist plots were fitted by the Z-viewsoftwarewith an appropriate equivalent circuit (Inset in Fig. 4c) and

Fig. 6. EIS spectra of stability examination for symmetrical cells fabricated with (a) conventioscanning (from 0 V / 1 V / �1 V / 0 V at a scanning rate of 50 mV s�1), followed by 60100 k Hz with an AC amplitude of 10 mV. This sequential electrochemical test was repeateONC@FeCo, and ONC@FeNi3 CEs.

the corresponding electrochemical parameters were summarizedin Table 1. As shown in Fig. 4c, two semicircles for each curve wereobtained at the high-frequency (left semicircle) and low-frequency(right semicircle) regions, which stand for the charge transferresistance (Rct) at the interface of CE/electrolyte and the ionicdiffusion impedance (ZN) of the I d/I3d redox couple in the elec-trolyte, respectively. The parameter Zpore corresponds to the secondNernst diffusion process in the holes of nanomaterials [32,39]. Inthe case of ONC@FeNi3 and ONC@FeCo, the diffusion in the porescan be neglected because a distorted arc did not appear within the

nal Pt CE, (b) ONC@FeCo CE, and (c) ONC@FeNi3 CE. The cells were first subjected to CVs relaxation at 0 V, and then EIS measurement was performed at 0 V from 0.05 Hz tod for 10 times. (d) Cycles-dependent changes of Rct and Rs values of conventional Pt,

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first semicircle [32]. The high-frequency intercept (Rs) on the realaxis of ONC@FeNi3 (4.38 U cm2) and ONC@FeCo (4.71 U cm2) areslightly larger than that of Pt (4.24 U cm2), which may be caused bythe poor contact between catalysts and ITO glass through spin-coating method. However, it is worth noting that the ONC@FeNi3exhibits the smallest Rct (1.02 U cm2) at the electrode/electrolyteinterface, whereas the ONC@FeCo shows a slightly higher Rct of1.23 U cm2 but comparable to that of Pt (1.22 U cm2). The differencein Rct of ONC@FeNi3 and ONC@FeCo is likely attributed to thedifferent nitrogen amount and states. Typically, larger total contentof pyridinic and graphite N would provide more active sites andaccelerate the electron transport to I3d, thus resulting in lower Rct[33]. Obviously, a lower Rct is favorable for the charge collectionefficiency, thereby facilitating the enhancement of photovoltaicperformance for DSSCs. Finally, Tafel polarization curves were alsorecorded on symmetrical dummy cells used in the EIS experiments.From Fig. 4d, one can find that the slopes of the anodic or cathodicbranches varied in the order of ONC@FeCo < Pt < ONC@FeNi3,implying a higher exchange current density (J0) generated from theONC@FeNi3 CE [22]. According to following Equation (3), the high J0corresponds to low Rct, which is in good agreement with EIS'results.

Fig. 7. Sequential cyclic voltammograms curves of (a) ONC@FeCo CE and (b) ONC@FeNi3 CEcycles scanning at a scan rate of 50 mV s�1. TEM images of (c, d, e) ONC@FeNi3 and (f, g, h) ONI d/I3d system with the same scan rate. (c, f) 200 cycles; (d, g) 300 cycles; (e, h) 600 cycle

J0 ¼ RTnFRct

(3)

Where R is the gas constant, F is Faraday's constant, T is the absolutetemperature, and n is the number of electrons involved with thereduction of I3d.

Based on the electrochemical characteristics above, it is ex-pected for ONC@FeNi3 and ONC@FeCo potentially to use as CEs forDSSCs. As presented in Fig. 5, a typical scheme configuration of aDSSC device is illustrated. The only difference with classic DSSCdevice is the alternation of conventional Pt particles with the as-prepared ONC@FeNi3 and ONC@FeCo nanocatalysts. The photo-current density-voltage (J-V) curves of ONC@FeNi3, ONC@FeCo, andPt as CEs were performed under a light intensity of 100 mW cm�2.The corresponding photovoltaic parameters are summarized inTable 1. The J-V curves of pure ONCs (Fig. S3) as CEs were alsoconducted (Fig. S4, and Table S3) and yield PCEs of 7.8% and 8.2% forONCCo and ONCNi based DSSCs, respectively, indicating promisingpotential of ONCs as CEs in DSSCs. It is interesting to be noted thatcompared with ONC CEs, the ONC@FeNi3 and ONC@FeCo showenhanced PCEs, which may be attributed to the existence of metalnanoparticles within carbon materials that can change the

obtained in ACN solution containing 10.0 mM LiI, 1.0 mM I2, and 0.1 M LiClO4, with 200C@FeCo catalysts after undergoing a certain number of CV corrosion tests in the above

s.

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electronic structure and reduce the surface work function of thecarbon walls, and thus contribute to enhanced catalytic activity[23]. In detail, when ONC@FeCo is used as CE, the DSSC produced aremarkable short-circuit photocurrent density (Jsc), open-circuitvoltage (Voc), fill factor (FF) and PCE of 16.58 mA cm�2, 0.77 V,0.65, and 8.26%, respectively. Obviously, this impressive PCE couldrival the 8.28% PCE of Pt. By replacing ONC@FeCo with ONC@FeNi3,the PCE of the corresponding DSSC increased to 8.87%, superior tothat of Pt-based device. The higher PCE of ONC@FeNi3 thanONC@FeCo is directly ascribed to higher Jsc and FF, which originfrom the larger content of pyridinic and graphite N in ONC@FeNi3and the resultant higher catalytic activities and lower Rct. Overall,the extraordinary performance of ONC@FeNi3 and ONC@FeCo fromconjunct contribution of metal alloy and the N-doped carbon onionhave demonstrated the feasibility of using them to supersede thecostly conventional Pt CE.

In addition to the conversion efficiency, the stability of a CE isanother important parameter for evaluating the performance of aDSSC device, especially for the long-term commercial applications.In this regard, we have conducted experiments on the electro-chemical stability of ONC@FeNi3, ONC@FeCo and Pt CEs by repeatedEIS measurements with a pretreatment of CV scanning before eachEIS measurement. Fig. 6 presents the evolution of impedancespectra for the each dummy cell. For all three CEs, there werealmost no changes in Rs after 10 cycles of scanning, revealing thatthe potential cycling has no influence on series resistance. On theother hand, therewas negligible change in Rct for ONC@FeNi3, whilea slight increase was observed for both Pt and ONC@FeCo, sug-gesting the best electrochemical stability of ONC@FeNi3. This resultwas further corroborated by the sequential CV measurements thatwere performed for all three CEs in the electrolyte containing10.0 mM LiI, 1.0 mM I2, and 0.1 M LiClO4 in acetonitrile. Typically, atotal of 200 consecutive cyclic voltammograms were recorded at ascan rate of 50 mV s�1. As shown in Fig. 7a,b, the cathodic currentdensities and the Epp had no apparent change for ONC@FeCo andONC@FeNi3 electrodes, while for the Pt electrode subjected to thesame scanning, the reduction peak current density decreased andthe Epp increased noticeably (Fig. S5), implying the deactivation ofPt in I d/I3d medium, which has been reported previously [24,25].

To have insight into the origin of the excellent electrochemicalstability of ONC@FeNi3 and ONC@FeCo, we further used TEM toinvestigate the structure or morphology evolution of two catalystsscraped from the substrate after undergoing a certain number of CVcorrosion tests. Fig. 7ceh presents the resulting TEM images, whichreveal that both two catalysts did not undergo a noticeable changein the structure and morphology after 200 or 300 cycles CV scan-ning (Fig. 7ced, and feg), whereas obvious damages to carbon shellwere observed for both ONC@FeNi3 and ONC@FeCowith increasingCV scanning to 600 cycles. Meanwhile, amorphous materials alsowere found for ONC@FeNi3 at the edge of crystal carbon layer.Although the corrosionmechanism to carbon shell in I d/I3d systemis still unclear, however, it demonstrates that as-prepared catalystsare able to maintain the integrity of the structure including shape,size, and crystallinity only if subjected to the extremely severecorrosion conditions. Moreover, it should be noted that such casesseldom occurred in our repeated observations. Accordingly, weassume that the strong tolerance of onion-like N-doped carbonstructure to I d/I3d medium is responsible for the excellent elec-trochemical stability of ONC@FeNi3 and ONC@FeCo catalysts.

4. Conclusions

In conclusion, the onion-like nitrogen-doped carbon shellencapsulating FeNi3 or FeCo alloy have been prepared through asimple and large-production pyrolysis approach. When used as the

CEs in DSSCs for the first time, both ONC@FeCo and ONC@FeNi3exhibit extraordinary catalytic activities towards the reduction ofI3d and thus contribute to the impressive PCE of 8.26% and 8.87%,respectively, which even outperforms the conventional Pt with an8.28% PCE. The significantly lower peak-to-peak separation, lowercharge-transfer resistance at the interface of the CE/electrolyte, andthe higher exchange current density were found responsible for theenhanced performance of the devices. It is worth mentioning thatthe excellent chemical ability has also been demonstrated for bothtwo catalysts, with negligible changes in the charge-transferresistance and the cathodic peak potential, as well as the struc-ture or morphology of catalysts itself. These results may pave theway for further study of environmentally friendly, stable, andhighly efficient counter electrodes for DSSCs.

Acknowledgments

This work was supported by the National Basic Research Pro-gram of China (973 Program, Grant No. 2015CB352106), the Na-tional Natural Science Foundation of China (NSFC, Grant Nos.61574034, 51372039, and 51202028), China Postdoctoral ScienceFoundation Funded Project (Grant Nos. 2014M550259 and2015T80480), and the Jiangsu Province Science and TechnologySupport Program (Grant No. BK20141118).

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2015.10.111.

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