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CoNi alloy incorporated, N doped porous carbon as efficient counter electrode for dye-sensitized solar cell
Journal of Power Sources 348 (2017) 158e167
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
CoNi alloy incorporated, N doped porous carbon as efficient counter electrode for dye-sensitized solar cell Zhiyong Gao a, Lan Wang a, Jiuli Chang a, Chen Chen a, Dapeng Wu a, **, Fang Xu a, Kai Jiang b, * a
School of Chemistry and Chemical Engineering, Henan Key Laboratory of Boron Chemistry and Advanced Energy Materials, Henan Normal University, Henan, Xinxiang 453007, PR China School of Environment, Henan Normal University, Henan, Xinxiang 453007, PR China
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
CoNi alloy incorporated N doped porous carbon was synthesized by facile method. CoNi alloy and N codoping synergistically improved the catalytic activity. High photovoltaic efficiency, good reliability and stability of cell was yielded.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2016 Received in revised form 14 February 2017 Accepted 1 March 2017
The design of efficient non-Pt counter electrode (CE) materials is highly desired in field of dye sensitized solar cell (DSC). Herein, by combining the catalytic features of N doped carbon (NC) and CoNi alloy, CoNi alloy incorporated porous N doped carbon hybrid (CoNi-NC) is synthesized for application as catalytic CE of DSC. Benefiting from the proper meso-/macroporosity with high electroactive surface area, the CoNiNC electrode demonstrates apparently higher electrocatalytic activity for iodine reduction reaction (IRR) over pyrolyzed Pt electrode. As a consequence, the DSC based on CoNi-NC CE yields a power conversion efficiency (PCE) of 7.6%, which is superior over that of Pt CE based cell (7.2%), highlighting the bright potential of CoNi-NC in efficient and economical CE of DSC. © 2017 Elsevier B.V. All rights reserved.
Keywords: Nitrogen doped porous carbon Cobalt nickel alloy Dye sensitized solar cell Counter electrode
1. Introduction Dye-sensitized solar cell (DSC) has long been recognized as an appealing photovoltaic form featured by low cost, eco-friendliness and high theoretical power conversion efficiency (PCE) [1,2]. Although perovskite solar cells have experienced great increment
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D. Wu), [email protected] (K. Jiang). http://dx.doi.org/10.1016/j.jpowsour.2017.03.009 0378-7753/© 2017 Elsevier B.V. All rights reserved.
in PCE and therefore attracted extensive research interests in field of photovoltaic device in recent years [3,4], DSC still maintains considerable performance to cost ratio in electric energy generation. To strengthen the competitiveness of DSC in photovoltaic field, it is imperative to further improve the PCE and reduce the device cost. DSC is mainly composed of a Ru (II) complex or organic dye sensitized porous TiO2 photoanode and a catalytic counter electrode (CE) mediated by a I 3 /I pair containing electrolyte. CE plays important roles in collecting the external circuit electrons and catalyzing I 3 reduction reaction (IRR) to afford I responsible for the regeneration of oxidized dyes, thus sustains
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the continued photoexcitation of dyes, electrons injection to TiO2 photoanode and therefore the power output. High electrons/ions mobility and robust catalytic activity for IRR are essential prerequisite for efficient CE. Traditional Pt CE holds high electrocatalytic activity for IRR while takes up a high fraction of cost in DSC. Hence, efficient non-Pt alternatives are highly desired in designing of cost-effective DSCs. To date, carbonaceous materials [5,6], polymers [7] and inorganic metal compounds [8e11] have proved to be prospective non-Pt CE materials. Of them, carbonaceous materials are attractive by virtue of the large surface area, intrinsic corrosion resistances and the defects derived active sites. However, the catalytic activities of carbonaceous materials for IRR are commonly restrained by the seesaw effect between surface defects correlated active sites and crystallinity related conductivity. This inherent issue can be resolved by N doping, for the pyridinic- and pyrrolic- N can alter the local charge distribution and increase the surface polarity by electro-negativity difference between N (c ¼ 3.04) and C (c ¼ 2.55) [12,13], therefore improves the catalytic activity and ions affinity. Additionally, the graphitic- N can increase the conductivity of carbon motif, hence, NCs are widely employed in catalytic electrode in oxygen reductions (ORRs) [14,15] or heterogeneous catalysts for certain chemical reactions [16,17]. Similar to ORR, IRR also necessitate efficient catalytic electrode, N doped graphenes [13,18,19] and N doped porous carbons (NCs) [20,21] were reported as CE materials of DSCs and credible PCE were achieved. Moreover, NCs can serve as supporters for anchoring of metallic catalysts to afford hybrids [22e24], which could substantially enhance the catalytic activities and reliabilities via the synergy between different components. As indicated in recent studies, metallic alloys have performed competitive catalytic activities for IRR [25e27]. The decreased work function of the host metal upon alloying with guest metal is beneficial for higher catalytic activity relative to single metals [26,27]. Tang et al. have investigated the Pt alloys with various metals [25], their theoretic calculation testified the increased adsorption energy for I 3 and the decreased work function are responsible for the enhanced catalytic activity, therefore led to improved PCEs. However, the alloys inevitably suffer from electrolyte corrosion [25,26], the incorporation of alloys into NCs is a promising way to reinforce the catalytic activity and durability by virtue of the synergy between both components and the corrosion resistance of NCs. Deng et al. [28] have synthesized FeNi alloy embedded carbon nanotubes for catalyses of IRR, theoretical calculation confirmed that the enclosed alloy can dramatically enhance the electrocatalytic activity and conductivity of carbon framework by lowering the work function of carbon motif [23]. Choi and coworkers [29] used graphene as mats for the deposition of Pt alloy via plasma reduction, the advanced electrocatalytic activity and low Pt usage in the composite highlighted the potential of this type of hybrids as efficient and economical CE materials in DSCs. As the neighboring elements to Pt in periodic table with similar valence electron configurations, Co and Ni are deemed as low cost analogs of Pt. Considering the high conductivity and catalytic activity of NCs, as well as the prospective of binary alloys in electrocatalyses, the Co, Ni alloys incorporated NCs are anticipated to be promising catalytic CE materials for IRR. Herein, porous NCs with incorporation of Co, Ni metals (alloys) were synthesized by pyrolysis of proper precursors, and the electrocatalytic activities for IRR were evaluated as CE of DSCs. Under equal atomic ratio, the CoNi alloy incorporated NC (CoNi-NC) demonstrated the highest catalytic activity for IRR and therefore offered prominent PCE in pertinent DSC, highlighting the prospect in non-precious CE.
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2. Experimental 2.1. Synthesis of Co, Ni metals (alloys) incorporated NCs 2.5 g soluble starch and 2 g gelatin serving as N, C sources were initially dispersed in 60 mL deionized water under agitation at 60 C to form a homogeneous suspension. Co(NO3)2$6H2O and (or) Ni(NO3)2$6H2O with total amount of 2.4 g while molar ratio of 1:0, 2:1, 1:1, 1:2 or 0:1 were dissolved into the above suspension, the homogeneous mixture was then dried at 80 C to yield xerogel. The xerogel precursor was placed in a quartz boat and pyrolyzed at 900 C for 4 h under N2 atmosphere to afford black products. The afforded samples were denoted as Co-NC, CoNi0.5-NC, CoNi-NC, Co0.5Ni-NC and Ni-NC, respectively according to the feeding ratios of Co(NO3)2$6H2O to Ni(NO3)2$6H2O. In controlled experiments, neat NC and CoNi alloys were also synthesized under the same procedure whereas in absence of metal precursor(s) and carbon source, respectively. 2.2. Characterizations Morphological and structural characterizations were conducted by scanning electron microscopy (SEM, JEOL JSM-6390) coupled with energy dispersive spectroscopy (EDS) and high resolution transmission electronic microscopy (HRTEM, JEOL JEM-2100). The phases and chemical compositions were characterized by X-ray powder diffraction (XRD, Bruker D diffractometer with Cu Ka radiation), Raman spectroscopy (JOBIN YVON HR800 Confocal Raman spectrometer with 632.8 nm laser excitation) and X-ray photoelectron spectroscopy (XPS, Kratos Amicus X-ray photoelectron spectrometer with Mg Ka radiation under 2 106 Pa). The pore features were analyzed by cryogenic N2 sorption on Micromeritics ASAP 2380 surface analyzer at 77 K. The specific surface areas were calculated by multiple points Brunauer-Emmett-Teller (BET) method, and the pore size distributions were calculated by nonlocal density function theory (DFT) model on assumption of slit-like pore shape. 2.3. Electrodes fabrication and DSCs assembly The CEs were fabricated by grinding the as-prepared active materials and polytetrafluoroethylene binder at a mass ratio of 95:5 to form viscous pastes, which were coated onto clean FTO conductive glass substrates framed by adhesive tape with film thickness of ca. 15 mm via doctor-blade method. Traditional Pt CE via pyrolysis of dip-coated H2PtCl6 was also fabricated for comparison. Bilayered TiO2 photoanode comprising a TiO2 nanocrystallite active layer and a scattering layer was fabricated as reported previously [21]. Typically, FTO substrate, with the non-conductive side masked by tape, was bathed in 0.2 M fresh TiCl4 aqueous solution at 70 C for 30 min and then calcinated at 450 C for 30 min to form a compact layer. TiO2 nanocrystallite (20 nm) was then spread onto the compact layer via doctor-blade method with controlled thickness of ca. 7 mm. After calcination at 500 C for 30 min, large TiO2 crystallite (300 nm) acting as scattering layer was coated over the active layer with controllable thickness of approximately 5 mm and followed by calcination again at 500 C for 30 min. Thereafter, the bilayered photoanode was treated by 0.04 M TiCl4 solution at 70 C for 30 min and calcinated another time at 450 C for 30 min to passivate the surface defects. The resultant TiO2 photoanode was immersed in 0.3 mM N719 dye for 24 h to adsorb dyes. DSC was fabricated by sandwiching electrolyte containing 0.6 M 1-propyl-3-methylimidazolium iodide (PMII), 0.05 M LiI, 0.05 M I2 and 0.5 M 4-tert-butylpyridine in 3-methoxypropionitrile between dyed TiO2 photoanode and CE using polyimide tape (60 m in thickness) as frame and spacer.
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2.4. Electrochemical measurements and photovoltaic tests Cyclic voltammograms (CVs), electrochemical impedance spectroscopy (EIS) and Tafel polarization curves were measured on CHI 660D electrochemical workstation (Chenhua, China). The three-electrode cell for CV measurement includes a CE, a Pt wire auxiliary electrode and a Ag/AgCl reference electrode immersed in acetonitrile electrolyte containing 10 mM LiI, 1 mM I2 and 0.1 M LiClO4 at 50 mV s1. Electrochemical impedance spectroscopy (EIS) and Tafel polarization curves were measured for symmetric cells composing of two identical electrodes. In EISs, the impedances were recorded at zero bias potential and 10 mV amplitude within 0.01~105 Hz. In Tafel polarization tests, the polarization currents were recorded between ±0.6 V. Photocurrent-voltage (J-V) curves of the DSCs were measured on a Keithley 2400 sourcemeter illuminated by solar simulator (Newport) with incident light power of 100 mW cm2 (AM 1.5). A black mask with a pore area of 0.25 cm2 was placed onto the DSCs to avoid stray light. 3. Results and discussion 3.1. Morphologies and structures The dissolution of starch, Co and/or Ni salt, as well as the dispersion of gelatin form homogeneous fluidic sol (Fig. S1a).
Undergoes continued curing, the gelatinization and partial dehydrate of starch toward dextrin, as well as the swelling and emulsification of gelatin lead to interconnected network. Simultaneously, Co2þ, Ni2þ cations are immobilized onto the polymer chains by coordination or electrostatic interactions, thus affords mixture gels. The evaporation of water leads to homogeneous xerogels (Fig. S1b). Given the enriched N in protein chains of gelatin and the presence of metal ions, the xerogels are feasible precursors for preparation of metals (alloys) incorporated NCs. Given the fact that N dopant in NCs can serve as catalytically active sites toward certain redox reactions [30,31], and iron series metals (alloys) are also common catalysts for certain reactions [32e34], the metals (alloys) incorporated NCs hybrids are reasonable catalysts benefiting from the catalytic activities of both components. Furthermore, the electrons transfer from metals (alloys) to NC motif can decrease the work function and therefore improves the reduction capability of CN motif [22]. Hence, significantly enhanced catalytic activities can be achieved. Fig. 1 presents the morphologies of the metals (alloys) incorporated NCs, all of them exhibit porous texture for the escaping of thermally decomposed gaseous species during the condensation of carbon framework by pyrolysis treatment. From Fig. 1a, Co-NC demonstrates porous sheet with the coating and embedding of granules. Detailed observation reveals that the size of granules include two grades, the larger granules coated onto the sheet are
Fig. 1. SEM of (a) Co-NC, (b) CoNi0.5-NC, (c) CoNi-NC, (d) Co0.5Ni-NC, (e) Ni-NC and (f) NC.
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mainly stemmed from the aggregation and fusion of the formed metallic Co, whereas the tiny grains (light spots) densely embedded within the sheet are mainly derived from the anchored Co crystallites by the doped N in carbon framework. Due to the more sufficient interaction with NC framework, it is presumably that the tiny Co crystallites provide the major contribution to the catalytic activity. Similarly, CoNi0.5-NC (Fig. 1b) displays macroporous network with coating of large granules and tiny crystallites of CoNi0.5 alloy onto the porous walls. In contrast, CoNi-NC (Fig. 1c) demonstrates large sheets with rich pores embedded in the walls (less than 100 nm in thickness), the decreased density of large granules means the homogeneous distribution of CoNi alloy within the NC framework with efficient interaction, which is beneficial for higher catalytic activity. Further increase the content of Ni causes drastic change in morphology, from Fig. 1d, Co0.5Ni-NC exhibits cross-linked particles with ill-defined morphology, which suggests the severe collapse of NC sheets under higher Ni content. As for NiNC (Fig. 1e), larger particles and broken layers are simultaneously formed, further signifies that Ni is not favorable for formation of sheets. In these hybrid materials, the hollow texture with embedded pores in walls can provide efficient channel and large accessible surface areas for rapid electrolyte ions sorption, which are essential premises for efficient catalytic CE in DSC. In contrast, the NC only exhibit bulk blocks with rough texture (Fig. 1e), the absence of metals (alloys) causes the disappearance of macropores within the carbonaceous motif. Fig. 2a shows the panoramic image of CoNi-NC at lower magnification, the sample exhibits interconnected network assembled by primary sheets, which can be manifested from Fig. 1c. The interconnected structure with rich macropores allows the facile diffusion of electrolyte, hence rapid interface reaction can be ensured. The translucent appearance of the primary sheet from TEM image (Fig. 2b) further evidences the low density with embedding of rich pores, which ensures a highly accessible surface area. Additionally, alloy particles with two grades in size were also observed, evidencing the embedding of CoNi alloy in the NC
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framework, the electrons transfer from alloy to NC alters the electronic structure and decreases the surface work function of the NC framework, thereby improves the catalytic activity for IRR [28]. HRTEM in Fig. 2c reveals the individual alloy particle is wrapped by graphitic shells with thickness of 2e2.3 nm, which accounts for of 6e8 graphitic layers. It is recognized that iron series metals are common carbonization catalysts, the adsorption, dissolution of thermo-decomposed carbon atoms in catalyst particles, as well as the followed nucleus and growth of evolved carbon yield graphitic layers around the alloy particle [34], which can not only prevent the further aggregation of alloy, but enhance the overall conductivity by virtue of the possibly high graphitization degree. The discrete lattice fringes (Inset) of the carbon layers implies the presence of N dopant derived defects, which facilitates a higher catalytic activity. In selected area electron diffraction (SAED) pattern (Fig. 2d), the sample exhibits two blurry rings indexed to (002) and (101) planes of graphite superimposed by scattered diffraction spots, showing the incorporation of polycrystalline CoNi alloy in NC framework. Elemental mapping of CoNi-NC (Fig. S2) reveals the homogeneous N doping and the uniformly scattering of CoNi alloy within carbon framework. The 3.3 at% N fraction evidences the efficient doping of N in carbon motif. The Co:Ni mass ratios of CoNi0.5-NC, CoNi-NC and Co0.5Ni-NC are 2.1:1, 1.1:1 and 1:1.8, respectively, which basically coincide with the feeding ratios, and the overall metal fractions of all samples are approaching to 9.2 at%, indicating the considerable alloys contents in the hybrids. Fig. 3a shows the XRD patterns of Co, Ni metals (alloys) incorporated NCs and neat NC. NC displays two broad diffractions at 26 and 43 indexing to (002) and (101) planes of graphite, suggesting the formation of partially graphitized phase. As for Co, Ni metals (alloys) incorporated NCs, the diffractions belonging to graphitic carbon disappear, instead, three peaks appear at 44.3, 51.5, 76.1, which are ascribable to (111), (200) and (220) planes of facecentered cubic phased Co or Ni with Fm3m(225) space group [24,35]. All these diffractions evidence the formation of Co, Ni metals (alloys) by thermo-reduction. The much higher peak
Fig. 2. (a) SEM, (b) TEM, (c) HRTEM and (d) SAED of CoNi-NC.
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Fig. 3. (a, b) XRD, (c) Raman, (d) N2 sorption isotherms and DFT pore size distributions (inset) of Co-NC, CoNi0.5-NC, CoNi-NC, Co0.5Ni-NC, Ni-NC and NC.
intensities of metals (alloys) depress the diffractions of graphitic carbon, thus causes the missing of diffractions for graphitic carbon. No diffractions correlate to nitrides or carbides are discernable, suggesting the Co, Ni precursors are effectively reduced to metals (alloys) by pyrolysis treatment. To be noted, the neighboring Co and Ni elements possess similar valence electron configurations (3d74s2 and 3d84s2, respectively), cell parameters (3.544 Å for Co and 3.523 Å for Ni) [35] and therefore similar physical properties, hence metallic CoNi solid solution can be formed by co-reduction. As expected, zoom-in (111) diffraction peaks of alloys incorporated NCs (Fig. 3b) right shift gradually from CoNi0.5-NC to Co0.5Ni-NC, showing the successful formation of binary alloys with narrowed lattice spacing for the slightly smaller atom diameter of Ni. The lattice strain due to the difference in Co, Ni atom diameter endows the increased surface energy and therefore higher catalytic activity toward certain reactions [36]. Additionally, the doped N in the carbon motif can not only serve as catalytic active sites for IRR, but increases the electrons mobility [13]. Given the combined contribution to conductivities and catalytic activities by both components, the Co, Ni metals (alloys) incorporated NCs herein are viable catalytic electrodes. Raman spectra (Fig. 3c) of all samples comprise two peaks located at 1330 (D band) and 1587 cm1 (G band). The D band is aroused by A1g vibration of edge and heteroatoms related defects within carbon framework, whereas G band represents the E2g vibration of graphitic carbon skeleton [37], the D band to G band intensity ratio (ID/IG) is an indicator of the extent of defects within the pseudographitic framework [37]. The ID/IG values of Co-NC, CoNi0.5-NC, CoNi-NC, Co0.5Ni-NC, Ni-NC, and NC are estimated to be 1.13, 1.06, 1.04, 1.07, 1.11 and 1.18, respectively. The slightly decreased ID/IG values of the hybrids relative to NC imply the enlarged graphitization domain sizes with less defects catalyzed by
Co, Ni catalysts undergo pyrolysis treatment, which coincide with the narrower peaks in metals (alloys) incorporated NCs. The enlarged graphitization domain size is beneficial for high conductivity, besides, the electrons transfer from metals (alloys) to NC framework also offer contribution to the conductivity, which is also indispensible for catalytic electrode. N2 sorption isotherms of all samples roughly exhibit a combined I/IV isotherms including a micropore filling at low P/P0 and a hysteresis loop at high P/P0 (Fig. 3d), which manifest the micro-/ mesoporous texture for all samples. In NC, the much pronounced N2 uptake at low P/P0 (below 0.1) indicates the overwhelming micropores, and the type IV isotherm with H2 type hysteresis loop at intermediate P/P0 range validates the coexistence of interconnected mesoporous, all these features suggest that the micro-/ mesoporous texture is formed by pyrolysis of starch and gelatin. As for metals (alloys) incorporated NCs, the much lower micropore filling suggests the obviously lower SBETs, the H3 type hysteresis loop within P/P0 range of 0.4e1 and the unsaturation at P/P0 of 1 indicate the presence of wedge shaped mesopores and macropores. The calculated BET surface area (SBETs) decreases from 631 m2g-1 for NC to 225, 198, 236, 188 and 248 m2g-1, respectively for Co-NC, CoNi0.5-NC, CoNi-NC, Co0.5Ni-NC and Ni-NC, showing the more prominent condensation of CN framework catalyzed by metals (alloys). Meanwhile, the micropore area ratio decreases accordingly from 78.2% for NC to 50.2, 60.6, 60.3, 61.5 and 63.0%, respectively for Co-NC, CoNi0.5-NC, CoNi-NC, Co0.5Ni-NC and Ni-NC. The apparently lower SBETs and micropore areas to some extent verifies the catalytic effect of metals (alloys) on the condensation and the graphitization of NC motif. DFT pore size distributions (inset) for all samples comprise micropores with sizes centered at 0.4 nm and a wide range of mesopores up to 10 nm, further evidences the coexistence of micropores and mesopores in all samples. Relative to
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NC, the probability of micropores in metals (alloys) incorporated NCs decrease apparently, whereas the probability of mesopores elevate slightly, indicating the conversion of partial micropores into meso-/macropores owing to the collapse of micropores during the catalytic graphitization. Given the 0.3e0.6 nm size of I 3 [38], the higher fraction of meso-/macropores allows the facile diffusion of electrolyte ions to access the catalytic sites and subject to rapid interface redox reactions with negligible steric hindrances and high surface utilization ratios. Additionally, the N dopant and co-doped metals (alloys) can not only offer abundant catalytic sites, but significantly improve the electrical conductivity. All of these virtues are essential for application as catalytic CE of DSC. XPS survey spectrum signifies the containing of N, C and Co, Ni elements in the representative CoNi-NC hybrid (Fig. 4a). The doped N not only alters the surface polarity of carbon motif and maintains a high wettability (Fig. S3) [22], but imparts the carbon motif with Lewis basicity by the electron donating feature of pyridinic- N [15], both of these features facilitate the efficient electrons transfer to the adsorbed I 3 ions and therefore enhance the catalytic activity for IRR [39]. To more in-depth identify the bond information, all of the four elemental peaks were deconvoluted. The core-level C1s spectrum (left inset) includes C]C (284.6 eV) and C]N/C]O (286.0 eV) functionalities [40], signifying the graphitization of carbon framework and the doping of N during the carbonization of starch and gelatin precursor catalyzed by the simultaneously formed CoNi alloy. N 1s spectrum (right inset) can be resolved into pyridinic- (398.1 eV) and graphitic-N (400.6 eV), showing the doping of N in form of pyridinic- and graphitic- N [13,41]. The pyridinic- N, commonly serve as Lewis base sites, or induce the Lewis base feature of the adjacent C atoms to motivate the adsorption of I 3 , weakening and dissociation of the IeI bond in I3 ions toward I [29]. Besides, the graphitic- N by replacing skeletal C atom renders higher conductivity by virtue of the rich electron feature, hence, the NC is a possible catalytic CE in DSC. Moreover, the pyridinic- N can serve as active site immobilizing the metal nanoparticles through the coordination between the lone pair electrons of N and the unoccupied d-orbits of metals [42]. The core level spectrum of Co2p orbit (Fig. 4b) includes Co2p3/2 (781.1 eV), Co2p1/2 (796.7 eV) orbits attributable to Co (II) and two satellite peaks, showing the partial oxidation of surface atoms [43]. Similarly, Ni2p spectrum (Inset) also exhibits Ni (II) orbits, including Ni 2p3/2 (855.6 eV) and Ni 2p1/2 (873.1 eV) orbits and other satellite peaks [43]. Both Co2p and Ni2p spectra reveal the inevitable oxidation of partial surface atoms in CoNi alloy for the moderate metallicity, the trace surface oxides are incapable of arousing diffraction in XRD pattern (Fig. 3b). Similar to Pt based alloys
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[25,44], the higher electro-negativity of Ni (1.92) over Co (1.88) [25] leads to slight electrons deviation from Co to Ni, the polar metallic bond of CoNi alloy facilitates higher wettability. Additionally, the decreased work function of CoNi alloy facilitates electrons transfer to I 3 ions in electrolyte and reinforces the catalytic activity for IRR [25]. Furthermore, in CoNi-NC hybrid, the electrons transfer from CoNi alloy to NC decreases the work function of the NC host, thus can enhance the catalytic activity of NC [45]. As a result of the combined catalytic activity of alloy and NC components, as well as the reinforced catalytic feature stemming from the interactions between them, robust catalytic capabilities of alloys incorporated NCs in CE are reasonable. 3.2. Electrochemical measurements Electroactive surface area (ESA) is an indicator reflecting the electrocatalytic activity of electrode [46]. From Fig. S4, the metals (alloys) incorporated NCs electrodes all demonstrate apparently higher ESAs over NC and Pt electrodes. Specifically, the ESAs of alloys incorporated NCs are higher over neat metals incorporated NCs, this trend is mainly attributable to two factors: Firstly, the deviation of electrons from Co to Ni and the discrepancy in lattice constant activate the surface catalytic sites as a result the charge redistribution of surface Ni and the increased lattice strain [36]. Secondly, the higher electrode/electrolyte compatibility as a result of the higher surface polarity by alloying effect increases the adsorbability to electrolyte ions. Under 1:1 automatic ratio, the maximized active sites and polarity render the highest ESA and further the possibly highest catalytic activity of CoNi-NC electrode. To evaluate the electrocatalytic activities for IRR, CVs of CoNi NC, NC and Pt electrodes in I 3 /I pair electrolyte were measured (Fig. 5a), two pairs of redox peaks are observable for all electrodes, which are indexed to the redox reactions of I 3 /I (left pair) and I2/I3 (right) couples [47]. Since IRR actually occurs at CE during photovoltaic process, the cathodic peak current density and the peak-to peak separation (Epp) of I 3 /I pair are major parameters reflecting catalytic activity of CE [22,27]. The higher cathodic peak current density means the more efficient reduction of I 3 , and the narrower Epp suggests lower overpotential viz higher electrochemical rate constant of I 3 /I3 pair [48,49]. The pyrolyzed Pt electrode arouses a well-resolved cathodic peak at 0 V, validates the good catalytic activity for IRR. NC electrode only evokes weak cathodic current and much wider Epp, suggesting the weaker catalytic activity relative to metallic Pt. The cathodic peak current density of CoNi-NCs electrode is higher over NC and Pt electrodes, showing the superior catalytic activity for IRR. Relative to Pt electrode, the wider Epp
Fig. 4. (a) XPS survey spectrum and (b) Co2p spectrum of CoNi-NC, (Inset: panel a: C1s and N1s spectra, panel b, Ni2p spectra).
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Fig. 5. (a) CV of CoNi-NC, NC and Pt electrodes in acetonitrile electrolyte containing 10 mM LiI, 1 mM I2, and 0.1 M LiClO4 at 50 mV s1, (b) CVs of CoNi-NC electrode at various scan 1 rates, (c) cathodic and anodic peak current densities of I 3 /I pair as a function of square root of scan rates, (d) 100 consecutive cycles of CVs of CoNi-NC electrode at 50 mV s .
is presumably ascribed to the less effective electrons mobility within the porous NC framework and the utilization of insulating binder. Albeit this, the porous texture and the rich active sites stemming from N dopant and the co-doped CoNi alloy are beneficial for sufficient redox reaction at electrode/electrolyte interface, hence, an excellent catalytic activity for IRR is achievable [18]. The cathodic current density and Epp of CoNi-NC electrode are also superior to other metals (alloys) incorporated NCs (Fig. S5a), implies the CoNi alloy is the optimal co-catalyst for IRR. CVs at different scan rates were further measured to reveal the redox reaction kinetics of I 3 /I3 pair at CoNi-NC electrode (Fig. 5b). As seen, the anodic and cathodic current densities of I 3 /I pair elevate accordingly with the scan rate, and the overpotential increases accordingly as a result of the electrochemical polarization and the thinner diffusion layer thickness [50]. The cathodic and anodic current densities are proportional to the square root of scan rates (Fig. 5c), suggesting that the I 3 ions diffusion is the ratelimiting step while the IRR at electrolyte/electrolyte interface is much faster [51]. Consider the 0.3e0.6 nm size of I 3 [38], the considerable fraction of meso-/macropores in CoNi-NC allows the facile infiltration of I 3 to the surface active sites and subject to reduction toward I, which are responsible for dye regeneration and therefore facilitates high photocurrent, filling factor (FF) and further high PCE. The electrocatalytic durability of CoNi-NC electrode was further estimated by 100 cycles of sequential CVs (Fig. 5d), the cathodic peak ascribable to IRR only show feeble variations in both current density and Epp, suggesting the good catalytic durability of CoNi-NC. The excellent catalytic activity and durability highlight the potential of CoNi-NC in high-performance and long-term DSC. To further study the electrochemical behavior at electrode surface, EISs of symmetrical cells based on different electrodes were measured. From Fig. 6a, the Nyquist plots of all devices comprise
two arcs in the intermediate and low-frequency regions. The series resistance (Rs), including the intrinsic resistance of electrode material and the contact resistance between electrode material and FTO substrate, can be estimated by the high frequency intercept at real axis [52]. Rs of CoNi-NC electrode (36.6 U) is higher than NC and Pt electrodes (31.9 and 27.6 U, respectively), which is mainly due to the less effective conductive channels within the loosely connected primary sheets (Fig. 2a). The Rs value is similar to other metals (alloys) incorporated NCs (36.4e37.6 U), while higher than neat CoNi alloy electrode (23.2 U) (Fig. S5b). The semicircle at high- and intermediate frequency originates from the charge transfer resistance (Rct) at electrode/electrolyte interface [53,54], whereas the low frequency arc represents the Warburg diffusion resistance (Zw) of electrolyte ions [53e55]. Rct of CoNi-NC electrode is estimated to be 6.4 U, which is slightly lower than that of Pt (8.4 U) and dramatically lower than NC electrode (224 U), suggesting the highest catalytic activity for the redox of I 3 /I pairs, as identified by the highest cathodic current density in CV test (Fig. 5a). The obviously higher ESA is responsible for the high catalytic activity for IRR for the exposure of more surface reactive sites to iodide mediator. Rct of CoNi-NC is also lower than other metals (alloys) incorporated NCs (11.6, 9.2, 9.0, and 9.8, respectively for Co-NC, CoNi0.5-NC, Co0.5Ni-NC, Ni-NC) and neat CoNi (17.0 U) (Fig. S5b). This trend indicates the optimal Co:Ni atomic ratio and N doping in carbon framework converged in providing the most abundant catalytic sites for IRR, and the resultant high concentration of I ions in electrolyte further ensures the sufficient regeneration of oxidized dyes, hence a higher PCE is reasonable. At low frequency region, the much larger Zw (67 U) over Rct for CoNi-NC indicates the electrolyte diffusion offer the highest contribution to the overall inner resistance, which further verifies the CV result that the ions diffusion is rate-limiting step in kinetics of IRR (Fig. 5c). In contrast, the Zw of NC (1888 U) is much higher than CoNi-NC as a consequence of the
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Fig. 6. (a) EISs and (b) Tafel polar curves of symmetrical cells based on CoNi-NC, NC and Pt electrodes. Inset in panel a: the whole EIS of NC.
higher steric hindrance for the limited micropore size. Additionally, the strong electrostatic force with I 3 ions because of the narrow pore size is also adverse for the exchange between I 3 and I ions, resulting in the higher Rct. Pt and neat CoNi electrodes only cause ill-defined arcs, which are mainly due to the heterogeneous electrode surface, which can not evoke well developed electrolyte diffusion resistances. Zw value of CoNi-NC electrode is also lower than other metals (alloys) incorporated NCs electrodes (within 75.6e92.4 U), evidencing the considerable electrolyte diffusion kinetics benefited by the proper fraction of meso-/macropore channels and the high surface wettability. In all, given the balanced Rs, Rct and Zw, CoNi-NC herein offers high electrons/ions mobility and catalytic activity, viz high electrocatalytic kinetics for IRR, the low inner resistance will finally result in high FF for the resultant DSC. Meanwhile, the high concentration of the generated I sustains the sufficient regeneration of dyes and further the efficient photoexcitation and electrons injection, hence enhanced photocurrent is also reasonable. Tafel polarization curves of the symmetric cells were crosschecked to identify the interface catalytic activities and electrolyte diffusion rates of different electrodes (Fig. 6b and Fig. S5c). At Tafel zone within 0.12e0.3 V, the anodic and cathodic profiles of CoNi-NC cell display higher slop than NC and Pt cells, and the ex change current density (J0) for I 3 /I pairs from the interception of tangent line at zero potential are also higher [29,56]. The highest J0 of CoNi-NC cell means the highest catalytic activity for IRR. According to Eq. (1) [57,58]:
J0 ¼ ðRTÞ=ðnFRct Þ
(1)
(where R is the gas constant, T is the thermodynamic temperature, F is the Faraday constant, n is the number of electrons involved in the reduction of I 3 to I and Rct is the charge transfer resistance), J0 varies inversely with Rct, hence the highest J0 of CoNi-NC cell means the lowest Rct whereas the highest Rct of NC, this trend again evidences the high catalytic activity of the former (Fig. 6a and Fig. S5b). Additionally, CoNi-NC cell also demonstrate the highest intersection at ordinate axis, viz the highest limiting diffusion current density (Jlim) at high potential end, which is determined by the diffusion velocity of I 3 across the two electrodes and the catalytic activity of the electrode. Jlim value is directly correlated with the diffusion coefficient (D) of I 3 ions across the porous electrode according to Eq. (2) [57,58]:
D ¼ ðlJlim Þ=2nFC
(2)
(where l represents the spacing distance between the two electrodes, C is the concentration of I 3 ions.) Following this linear
relationship, the highest Jlim of CoNi-NC cell implies the highest D, which is attributable to the porous texture with high fraction of meso-/macropores with high wettability, thus allows the rapid diffusion of I 3 ions to access the surface active sites and subject to electrocatalytic reduction. In all, the Tafel results again manifest the efficient redox exchange of I 3 /I pair and the high ionic diffusion rate within CoNi-NC electrode, which will finally translate into high PCE in the pertinent DSC. 3.3. Photovoltaic performances Given the high catalytic activity and stability for IRR, as well as the rapid electrolyte diffusion rate, CoNi-NC herein is envisaged to be credible CE in DSC. Fig. 7a, Fig. S6a and Table 1 summarize the J-V characteristics of DSCs based on various CEs. The CoNi-NC CE based DSC demonstrates the highest PCE of 7.6% with a photocurrent density (Jsc) of 18.5 mA cm2, a FF of 0.58 and an open circuit voltage (Voc) of 0.70 V. As a reference, a PCE of 7.15% (Jsc ¼ 17.7 mA cm2, FF ¼ 0.56, Voc ¼ 0.716 V) is harvested in Pt CE based cell. The higher Jsc and FF of CoNi-NC CE based DSC are in line with the electrochemical measurements. Specifically, the rich catalytically active sites and efficient ionic diffusion rate as a result of the N and alloy co-doping, as well as high fraction of meso-/ macropores result in efficient ions exchange to afford more I responsible for dye regeneration, electrons injection and therefore higher photocurrent density. Additionally, the low energy loss owing to the low inner resistance also facilitates a higher FF. The PCEs of other metals (alloys) incorporated NCs and neat CoNi based DSCs are relatively lower owing to the lower catalytic activities and higher Zws. The PCE of NC CE based DSC (2.2%) is obviously inferior for the much lower FF (0.20). All these results validate that CoNi-NC is the most suitable Pt free CE material. N doped carbon by acid leaching of CoNi from CoNi-NC was also used as CE of DSC, the PCE (6.1%) is also lower than CoNi-NC based cell as a consequence of the lower electrochemical activity (Fig. S7). Considering the lower PCEs of DSCs based on leached NC and neat CoNi alloy, the high PCE of CoNi-NC based DSC is essentially attributable to the synergistically catalytic effect between N dopant and CoNi alloy. The operation reliability in fast startup is an important indicator for practical application of DSCs [59,60], Fig. 7b compares the photocurrent-time (IT) curves of CoNi-NC, NC and Pt CE based DSCs at 0 V subjected to consecutive on-off switching of the incident light, the photocurrents of all DSCs eliminate promptly upon switching off the illumination while recover promptly upon turning on the illumination, the rapid photo-response reflects the rapid IRR kinetics at CE surface, the photocurrent densities of CoNi-NC CE based DSC remain over 90% of its initial value after 8 cycles of on-off switching, which is comparable to that of NC and Pt CEs based DSCs,
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Fig. 7. (a) J-V plots, (b) IT curves, (c) Photocurrent decay curves of DSCs based on CoNi-NC, NC and Pt based CEs under irradiation power of 100 mW cm2 (AM 1.5), (d) Photograph of a LED lamp powered by 4 in series connected DSCs based on CoNi-NC CE under illumination of AM 1.5.
Table 1 The main photovoltaic parameters of DSCs based on different CE materials. CEs
Jsc (mA cm2)
Voc (V)
FF
PEC (%)
Co-NC CoNi0.5-NC CoNi-NC Co0.5Ni-NC Ni-NC CoNi NC Pt
17.70 17.77 18.53 18.45 17.71 16.84 16.17 17.68
0.704 0.702 0.700 0.710 0.710 0.66 0.673 0.716
0.55 0.58 0.58 0.56 0.57 0.51 0.20 0.56
6.87 7.20 7.58 7.34 7.23 5.67 2.21 7.15
high-performance DSC. To more intuitively demonstrate the output property of CoNi-NC CE based DSC, 4 DSCs were connected in series to form a cell group. The cell group is capable of powering a LED with positive work voltage of 2.4 V under illumination (Fig. 7d), showing the feasibility of CoNi-NC in practical DSC. Photovoltaic characteristics of CoNi alloy incorporated NCs with different NC contents were also compared (Fig. S8), by altering the NC precursor dose, the FF and PCE both deteriorate, showing the CoNi-NC hybrid herein is an optimal CE material.
4. Conclusions showing the operation reliability of all DSCs as quick start devices. The photocurrent maintaining ratio of other metals (alloys) incorporated NCs increases with Ni content (Fig. S6b), showing higher Ni content is beneficial for higher operation reliability in CE of DSCs. In short, as a consequence of the high IRR kinetics at electrode/electrolyte interface, CoNi-NC herein is a reliable material in nonprecious CE of DSC. Photovoltaic durability is another indispensible parameter for practical application of DSC, Fig. 7c compares the photocurrent decay curves of CoNi-NC, NC and Pt CE based DSCs under continued irradiation, the photocurrent density of all cells are well maintained within 1000 s, showing the potential of CoNi-NC in long-term DSC. The photocurrent stabilities for metals (alloys) incorporated NCs also increase with Ni content (Fig. S6c), again signifies that the higher Ni fraction is indeed beneficial for higher photovoltaic stability for the less dissolution as a consequence of the weaker reactivity (reduction potential: 0.28 V for Co2þ/Co pair vs 0.23 V for Ni2þ/Ni pair relative to normal hydrogen electrode). Although the duration of 1000 s is far from sufficient in reflecting the durability of DSC, the high PCE, high operation reliability and durability preliminary highlight the potential of CoNi-NCs in
Co, Ni metals (alloys) incorporated NCs were prepared by facile pyrolysis of xerogels composing of starch, gelatin and Co, Ni salts. The as-prepared metals (alloys) incorporated NCs demonstrated high electrocatalytic activity for IRR benefitting from the meso-/ macroporous texture, N doping and the incorporation of metals (alloys). The DSC based on CoNi-NC CE offers superior PCE to Pt based counterpart, high photovoltaic reliability and durability, showing the great premise as efficient and economical CE materials in DSCs.
Acknowledgments This work was supported by NSFCs (Nos. 21671059, U1304505), Program for Changjiang Scholars and Innovative Research Team in University, Innovation Scientists and Technicians Troop Construction Projects of Henan Province (154200510009), Program for Innovative Research Team and Individuals (in Science and Technology) in University of Henan Province (Nos. 13IRTSTHN026, 15HASTIT006).
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Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.03.009. References [1] A. Yella, H.W. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M. Nazeeruddin, E.W. Diau, €tzel, Science 334 (2011) 629e634. C.Y. Yeh, S.M. Zakeeruddin, M. Gra [2] L.Y. Han, A. Islam, H. Chen, C. Malapaka, B. Chiranjeevi, S.F. Zhang, X.D. Yang, M. Yanagida, Energy Environ. Sci. 5 (2012) 6057e6060. [3] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Science 348 (2015) 1234e1237. [4] A.Y. Mei, X. Li, L.F. Liu, Z.L. Ku, T.F. Liu, Y.G. Rong, M. Xu, M. Hu, J.Z. Chen, €tzel, H.W. Han, Science 345 (2014) 295e298. Y. Yang, M. Gra [5] M.X. Wu, X. Lin, T.H. Wang, J.S. Qiu, T.L. Ma, Energy Environ. Sci. 4 (2011) 2308e2315. [6] H. Wang, Y.H. Hu, Energy Environ. Sci. 5 (2012) 8182e8188. [7] K. Saranya, Md. Rameez, A. Subramania, Eur. Polym. J. 66 (2015) 207e227. [8] J. Briscoe, S. Dunn, Adv. Mater 28 (2016) 3802e3813. [9] M.X. Wu, H.Y. Guo, Y.N. Lin, K.Z. Wu, T.L. Ma, A. Hagfeldt, J. Phys. Chem. C 118 (2014) 12625e12631. [10] B. Lei, G.R. Li, X.P. Gao, J. Mater. Chem. A 2 (2014) 3919e3925. [11] H.K. Zheng, H.Y. Guo, D. An, K.Z. Wu, C.J. Gao, Q.J. Han, Y.J. Zhu, Y.N. Lin, M.X. Wu, J. Mater. Chem. C 4 (2016) 6533e6538. [12] M.J. Ju, I.Y. Jeon, J.C. Kim, K. Lim, H.J. Choi, S.M. Jung, I.T. Choi, Y.K. Eom, Y.J. Kwon, J. Ko, J.J. Lee, H.K. Kim, J.B. Baek, Adv. Mater 26 (2014) 3055e3062. [13] S.C. Hou, X. Cai, H.W. Wu, X. Yu, M. Peng, K. Yan, D.C. Zou, Energy Environ. Sci. 6 (2013) 3356e3362. [14] S.Y. Gao, K.R. Geng, H.Y. Liu, X.J. Wei, M. Zhang, P. Wang, J.J. Wang, Energy Environ. Sci. 8 (2015) 221e229. [15] D.H. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Science 351 (2016) 361e365. [16] X.K. Kong, Z.Y. Sun, M. Chen, C.L. Chen, Q.W. Chen, Energy Environ. Sci. 6 (2013) 3260e3266. [17] Y.J. Gao, G. Hu, J. Zhong, Z.J. Shi, Y.S. Zhu, D.S. Su, J.G. Wang, X.H. Bao, D. Ma, Angew. Chem. Int. Ed. 52 (2013) 2109e2113. [18] P. Zhai, T.C. Wei, Y.H. Chang, Y.T. Huang, W.T. Yeh, H.J. Su, S.P. Feng, Small 10 (2014) 3347e3353. [19] M. Yu, J.D. Zhang, S.M. Li, Y.B. Meng, J.H. Liu, J. Power Sources 308 (2016) 44e51. [20] M. Chen, L.L. Shao, Y.P. Liu, T.Z. Ren, Z.Y. Yuan, J. Power Sources 283 (2015) 305e313. [21] Z.Y. Gao, L. Wang, J.L. Chang, X. Liu, D.P. Wu, F. Xu, Y.M. Guo, K. Jiang, Electrochim. Acta 188 (2016) 441e449. [22] Y.D. Xing, X.J. Zheng, Y.H. Wu, M.R. Li, W.H. Zhang, C. Li, Chem. Commun. 51 (2015) 8146e8149. [23] J. Deng, P.J. Ren, D.H. Deng, L. Yu, F. Yang, X.H. Bao, Energy Environ. Sci. 7 (2014) 1919e1923. [24] Y. Hou, S.M. Cui, Z.H. Wen, X.R. Guo, X.L. Feng, J.H. Chen, Small 11 (2015) 5940e5948. [25] Q.W. Tang, H.H. Zhang, Y.Y. Meng, B.L. He, L.M. Yu, Angew. Chem. Int. Ed. 54 (2015) 11448e11452. [26] P.Z. Yang, C.Q. Ma, Q.W. Tang, Electrochim. Acta 184 (2015) 226e232. [27] Y.J. Li, Q.W. Tang, L.M. Yu, X.F. Yan, L. Dong, J. Power Sources 305 (2016) 217e224. [28] X.J. Zheng, J. Deng, N. Wang, D.H. Deng, W.H. Zhang, X.H. Bao, C. Li, Angew. Chem. Int. Ed. 53 (2014) 7023e7027.
167
[29] S.W. Yoon, V.D. Dao, L.L. Larina, J.K. Lee, H.S. Choi, Carbon 96 (2016) 229e236. [30] X.G. Duan, H.Q. Sun, Y.X. Wang, J. Kang, S.B. Wang, ACS Catal. 5 (2015) 553e559. [31] T.Y. Yang, J. Liu, R.F. Zhou, Z.G. Chen, H.Y. Xu, S.Z. Qiao, M.J. Monteiro, J. Mater. Chem. A 2 (2014) 18139e18146. [32] S. Bai, X.P. Shen, G.X. Zhu, M.Z. Li, H.T. Xi, K.M. Chen, ACS Appl. Mater. Interfaces 4 (2012) 2378e2386. [33] H. Li, J.Y. Liao, Y.C. Du, T. You, W.W. Liao, L.L. Wen, Chem. Commun. 49 (2013) 1768e1770. [34] O.Y. Podyacheva, Z.R. Ismagilov, Catal. Today 249 (2015) 12e22. [35] M. Motlak, N.A.M. Barakat, M.S. Akhtar, A.M. Hamza, B.S. Kim, C.S. Kim, K.A. Khalil, A.A. Almajid, Electrochim. Acta 160 (2015) 1e6. [36] Q.W. Tang, J. Liu, H.H. Zhang, B.L. He, L.M. Yu, J. Power Sources 297 (2015) 1e8. [37] S.B. Yoon, G.S. Chai, S.K. Kang, J.S. Yu, K.P. Gierszal, M. Jaroniec, J. Am. Chem. Soc. 127 (2005) 4188e4189. [38] W.J. Lee, E. Ramasamy, D.Y. Lee, J.S. Song, ACS Appl. Mater. interfaces 1 (2009) 1145e1149. [39] W.J. Lee, U.N. Maiti, J.M. Lee, J. Lim, T.H. Han, S.O. Kim, Chem. Commun. 50 (2014) 6818e6830. [40] L. Qie, W.M. Chen, H.H. Xu, X.Q. Xiong, Y. Jiang, F. Zou, X.L. Hu, Y. Xin, Z.L. Zhang, Y.H. Huang, Energy Environ. Sci. 6 (2013) 2497e2504. [41] T. Sharifi, G. Hu, X. Jia, T. Wagberg, ACS Nano 6 (2012) 8904e8912. [42] H. Feng, J. Ma, Z. Hu, J. Mater. Chem. 20 (2010) 1702e1708. [43] S.W. Lee, J.S. Kang, K.T. Leung, S.K. Kim, Y.K. Sohn, Appl. Surf. Sci. 386 (2016) 393e404. [44] C.X. Xu, J.G. Hou, X.H. Pang, X.J. Li, M.L. Zhu, B.Y. Tang, Int. J. Hydrogen Energy 37 (2012) 10489e10498. [45] D.H. Deng, L. Yu, X.Q. Chen, G.X. Wang, L. Jin, X.L. Pan, J. Deng, G.Q. Sun, X.H. Bao, Angew. Chem. Int. Ed. 52 (2013) 371e375. [46] Y.H. Xue, J. Liu, H. Chen, R.G. Wang, D.Q. Li, J. Qu, L.M. Dai, Angew. Chem. Int. Ed. 51 (2012) 12124e12127. [47] X.L. Duan, Z.Y. Gao, J.L. Chang, D.P. Wu, P.F. Ma, J.J. He, F. Xu, S.Y. Gao, K. Jiang, Electrochim. Acta 114 (2013) 173e179. [48] J.D. Roy-Mayhew, D.J. Bozym, C. Punckt, I.A. Aksay, ACS Nano 4 (2010) 6203e6211. [49] M.X. Wu, X. Lin, Y.D. Wang, L. Wang, W. Guo, D.D. Qu, X.J. Peng, A. Hagfeldt, €tzel, T.L. Ma, J. Am. Chem. Soc. 134 (2012) 3419e3428. M. Gra [50] M.X. Wu, Y.D. Wang, X. Lin, N. Yu, L. Wang, L.L. Wang, A. Hagfeldt, T.L. Ma, Phys. Chem. Chem. Phys. 13 (2011) 19298e19301. [51] P. Wang, S.M. Zakeeruddin, P. Comte, R. Charvet, R. Humphry-Baker, €tzel, J. Phys. Chem. B 107 (2003) 14336e14341. M. Gra [52] M. Chen, L.L. Shao, X. Qian, T.Z. Ren, Z.Y. Yuan, J. Mater. Chem. C 2 (2014) 10312e10321. [53] S.P. Lim, A. Pandikumar, Y.S. Lim, N.M. Huang, H.N. Lim, Sci. Rep. 4 (2014) 5305. [54] V.D. Dao, N.T.Q. Hoa, L.L. Larina, J.K. Lee, H.S. Choi, Nanoscale 5 (2013) 12237e12244. [55] M. Adachi, M. Sakamoto, J.T. Jiu, Y. Ogata, S. Isoda, J. Phys. Chem. B 110 (2006) 13872e13880. [56] M. Zheng, J.H. Huo, B.G. Chen, Y.G. Tu, J.H. Wu, L.H. Hu, S.Y. Dai, Sol. Energy 122 (2015) 727e736. [57] M.X. Wu, Q.Y. Zhang, J.Q. Xiao, C.Y. Ma, X. Lin, C.Y. Miao, Y.J. He, Y.R. Gao, A. Hagfeldt, T.L. Ma, J. Mater. Chem. 21 (2011) 10761e10766. [58] M.X. Wu, X. Lin, A. Hagfeldt, T.L. Ma, Chem. Commun. 47 (2011) 4535e4537. [59] X.X. Chen, Q.W. Tang, B.L. He, L. Lin, L.M. Yu, Angew. Chem. Int. Ed. 53 (2014) 10799e10803. [60] Y.Y. Duan, Q.W. Tang, J. Liu, B.L. He, L.M. Yu, Angew. Chem. Int. Ed. 53 (2014) 14569e14574.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
CoNi alloy incorporated, N doped porous carbon as efficient counter electrode for dye-sensitized solar cell Zhiyong Gao a, Lan Wang a, Jiuli Chang a, Chen Chen a, Dapeng Wu a, **, Fang Xu a, Kai Jiang b, * a
School of Chemistry and Chemical Engineering, Henan Key Laboratory of Boron Chemistry and Advanced Energy Materials, Henan Normal University, Henan, Xinxiang 453007, PR China School of Environment, Henan Normal University, Henan, Xinxiang 453007, PR China
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
CoNi alloy incorporated N doped porous carbon was synthesized by facile method. CoNi alloy and N codoping synergistically improved the catalytic activity. High photovoltaic efficiency, good reliability and stability of cell was yielded.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2016 Received in revised form 14 February 2017 Accepted 1 March 2017
The design of efficient non-Pt counter electrode (CE) materials is highly desired in field of dye sensitized solar cell (DSC). Herein, by combining the catalytic features of N doped carbon (NC) and CoNi alloy, CoNi alloy incorporated porous N doped carbon hybrid (CoNi-NC) is synthesized for application as catalytic CE of DSC. Benefiting from the proper meso-/macroporosity with high electroactive surface area, the CoNiNC electrode demonstrates apparently higher electrocatalytic activity for iodine reduction reaction (IRR) over pyrolyzed Pt electrode. As a consequence, the DSC based on CoNi-NC CE yields a power conversion efficiency (PCE) of 7.6%, which is superior over that of Pt CE based cell (7.2%), highlighting the bright potential of CoNi-NC in efficient and economical CE of DSC. © 2017 Elsevier B.V. All rights reserved.
Keywords: Nitrogen doped porous carbon Cobalt nickel alloy Dye sensitized solar cell Counter electrode
1. Introduction Dye-sensitized solar cell (DSC) has long been recognized as an appealing photovoltaic form featured by low cost, eco-friendliness and high theoretical power conversion efficiency (PCE) [1,2]. Although perovskite solar cells have experienced great increment
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D. Wu), [email protected] (K. Jiang). http://dx.doi.org/10.1016/j.jpowsour.2017.03.009 0378-7753/© 2017 Elsevier B.V. All rights reserved.
in PCE and therefore attracted extensive research interests in field of photovoltaic device in recent years [3,4], DSC still maintains considerable performance to cost ratio in electric energy generation. To strengthen the competitiveness of DSC in photovoltaic field, it is imperative to further improve the PCE and reduce the device cost. DSC is mainly composed of a Ru (II) complex or organic dye sensitized porous TiO2 photoanode and a catalytic counter electrode (CE) mediated by a I 3 /I pair containing electrolyte. CE plays important roles in collecting the external circuit electrons and catalyzing I 3 reduction reaction (IRR) to afford I responsible for the regeneration of oxidized dyes, thus sustains
Z. Gao et al. / Journal of Power Sources 348 (2017) 158e167
the continued photoexcitation of dyes, electrons injection to TiO2 photoanode and therefore the power output. High electrons/ions mobility and robust catalytic activity for IRR are essential prerequisite for efficient CE. Traditional Pt CE holds high electrocatalytic activity for IRR while takes up a high fraction of cost in DSC. Hence, efficient non-Pt alternatives are highly desired in designing of cost-effective DSCs. To date, carbonaceous materials [5,6], polymers [7] and inorganic metal compounds [8e11] have proved to be prospective non-Pt CE materials. Of them, carbonaceous materials are attractive by virtue of the large surface area, intrinsic corrosion resistances and the defects derived active sites. However, the catalytic activities of carbonaceous materials for IRR are commonly restrained by the seesaw effect between surface defects correlated active sites and crystallinity related conductivity. This inherent issue can be resolved by N doping, for the pyridinic- and pyrrolic- N can alter the local charge distribution and increase the surface polarity by electro-negativity difference between N (c ¼ 3.04) and C (c ¼ 2.55) [12,13], therefore improves the catalytic activity and ions affinity. Additionally, the graphitic- N can increase the conductivity of carbon motif, hence, NCs are widely employed in catalytic electrode in oxygen reductions (ORRs) [14,15] or heterogeneous catalysts for certain chemical reactions [16,17]. Similar to ORR, IRR also necessitate efficient catalytic electrode, N doped graphenes [13,18,19] and N doped porous carbons (NCs) [20,21] were reported as CE materials of DSCs and credible PCE were achieved. Moreover, NCs can serve as supporters for anchoring of metallic catalysts to afford hybrids [22e24], which could substantially enhance the catalytic activities and reliabilities via the synergy between different components. As indicated in recent studies, metallic alloys have performed competitive catalytic activities for IRR [25e27]. The decreased work function of the host metal upon alloying with guest metal is beneficial for higher catalytic activity relative to single metals [26,27]. Tang et al. have investigated the Pt alloys with various metals [25], their theoretic calculation testified the increased adsorption energy for I 3 and the decreased work function are responsible for the enhanced catalytic activity, therefore led to improved PCEs. However, the alloys inevitably suffer from electrolyte corrosion [25,26], the incorporation of alloys into NCs is a promising way to reinforce the catalytic activity and durability by virtue of the synergy between both components and the corrosion resistance of NCs. Deng et al. [28] have synthesized FeNi alloy embedded carbon nanotubes for catalyses of IRR, theoretical calculation confirmed that the enclosed alloy can dramatically enhance the electrocatalytic activity and conductivity of carbon framework by lowering the work function of carbon motif [23]. Choi and coworkers [29] used graphene as mats for the deposition of Pt alloy via plasma reduction, the advanced electrocatalytic activity and low Pt usage in the composite highlighted the potential of this type of hybrids as efficient and economical CE materials in DSCs. As the neighboring elements to Pt in periodic table with similar valence electron configurations, Co and Ni are deemed as low cost analogs of Pt. Considering the high conductivity and catalytic activity of NCs, as well as the prospective of binary alloys in electrocatalyses, the Co, Ni alloys incorporated NCs are anticipated to be promising catalytic CE materials for IRR. Herein, porous NCs with incorporation of Co, Ni metals (alloys) were synthesized by pyrolysis of proper precursors, and the electrocatalytic activities for IRR were evaluated as CE of DSCs. Under equal atomic ratio, the CoNi alloy incorporated NC (CoNi-NC) demonstrated the highest catalytic activity for IRR and therefore offered prominent PCE in pertinent DSC, highlighting the prospect in non-precious CE.
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2. Experimental 2.1. Synthesis of Co, Ni metals (alloys) incorporated NCs 2.5 g soluble starch and 2 g gelatin serving as N, C sources were initially dispersed in 60 mL deionized water under agitation at 60 C to form a homogeneous suspension. Co(NO3)2$6H2O and (or) Ni(NO3)2$6H2O with total amount of 2.4 g while molar ratio of 1:0, 2:1, 1:1, 1:2 or 0:1 were dissolved into the above suspension, the homogeneous mixture was then dried at 80 C to yield xerogel. The xerogel precursor was placed in a quartz boat and pyrolyzed at 900 C for 4 h under N2 atmosphere to afford black products. The afforded samples were denoted as Co-NC, CoNi0.5-NC, CoNi-NC, Co0.5Ni-NC and Ni-NC, respectively according to the feeding ratios of Co(NO3)2$6H2O to Ni(NO3)2$6H2O. In controlled experiments, neat NC and CoNi alloys were also synthesized under the same procedure whereas in absence of metal precursor(s) and carbon source, respectively. 2.2. Characterizations Morphological and structural characterizations were conducted by scanning electron microscopy (SEM, JEOL JSM-6390) coupled with energy dispersive spectroscopy (EDS) and high resolution transmission electronic microscopy (HRTEM, JEOL JEM-2100). The phases and chemical compositions were characterized by X-ray powder diffraction (XRD, Bruker D diffractometer with Cu Ka radiation), Raman spectroscopy (JOBIN YVON HR800 Confocal Raman spectrometer with 632.8 nm laser excitation) and X-ray photoelectron spectroscopy (XPS, Kratos Amicus X-ray photoelectron spectrometer with Mg Ka radiation under 2 106 Pa). The pore features were analyzed by cryogenic N2 sorption on Micromeritics ASAP 2380 surface analyzer at 77 K. The specific surface areas were calculated by multiple points Brunauer-Emmett-Teller (BET) method, and the pore size distributions were calculated by nonlocal density function theory (DFT) model on assumption of slit-like pore shape. 2.3. Electrodes fabrication and DSCs assembly The CEs were fabricated by grinding the as-prepared active materials and polytetrafluoroethylene binder at a mass ratio of 95:5 to form viscous pastes, which were coated onto clean FTO conductive glass substrates framed by adhesive tape with film thickness of ca. 15 mm via doctor-blade method. Traditional Pt CE via pyrolysis of dip-coated H2PtCl6 was also fabricated for comparison. Bilayered TiO2 photoanode comprising a TiO2 nanocrystallite active layer and a scattering layer was fabricated as reported previously [21]. Typically, FTO substrate, with the non-conductive side masked by tape, was bathed in 0.2 M fresh TiCl4 aqueous solution at 70 C for 30 min and then calcinated at 450 C for 30 min to form a compact layer. TiO2 nanocrystallite (20 nm) was then spread onto the compact layer via doctor-blade method with controlled thickness of ca. 7 mm. After calcination at 500 C for 30 min, large TiO2 crystallite (300 nm) acting as scattering layer was coated over the active layer with controllable thickness of approximately 5 mm and followed by calcination again at 500 C for 30 min. Thereafter, the bilayered photoanode was treated by 0.04 M TiCl4 solution at 70 C for 30 min and calcinated another time at 450 C for 30 min to passivate the surface defects. The resultant TiO2 photoanode was immersed in 0.3 mM N719 dye for 24 h to adsorb dyes. DSC was fabricated by sandwiching electrolyte containing 0.6 M 1-propyl-3-methylimidazolium iodide (PMII), 0.05 M LiI, 0.05 M I2 and 0.5 M 4-tert-butylpyridine in 3-methoxypropionitrile between dyed TiO2 photoanode and CE using polyimide tape (60 m in thickness) as frame and spacer.
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2.4. Electrochemical measurements and photovoltaic tests Cyclic voltammograms (CVs), electrochemical impedance spectroscopy (EIS) and Tafel polarization curves were measured on CHI 660D electrochemical workstation (Chenhua, China). The three-electrode cell for CV measurement includes a CE, a Pt wire auxiliary electrode and a Ag/AgCl reference electrode immersed in acetonitrile electrolyte containing 10 mM LiI, 1 mM I2 and 0.1 M LiClO4 at 50 mV s1. Electrochemical impedance spectroscopy (EIS) and Tafel polarization curves were measured for symmetric cells composing of two identical electrodes. In EISs, the impedances were recorded at zero bias potential and 10 mV amplitude within 0.01~105 Hz. In Tafel polarization tests, the polarization currents were recorded between ±0.6 V. Photocurrent-voltage (J-V) curves of the DSCs were measured on a Keithley 2400 sourcemeter illuminated by solar simulator (Newport) with incident light power of 100 mW cm2 (AM 1.5). A black mask with a pore area of 0.25 cm2 was placed onto the DSCs to avoid stray light. 3. Results and discussion 3.1. Morphologies and structures The dissolution of starch, Co and/or Ni salt, as well as the dispersion of gelatin form homogeneous fluidic sol (Fig. S1a).
Undergoes continued curing, the gelatinization and partial dehydrate of starch toward dextrin, as well as the swelling and emulsification of gelatin lead to interconnected network. Simultaneously, Co2þ, Ni2þ cations are immobilized onto the polymer chains by coordination or electrostatic interactions, thus affords mixture gels. The evaporation of water leads to homogeneous xerogels (Fig. S1b). Given the enriched N in protein chains of gelatin and the presence of metal ions, the xerogels are feasible precursors for preparation of metals (alloys) incorporated NCs. Given the fact that N dopant in NCs can serve as catalytically active sites toward certain redox reactions [30,31], and iron series metals (alloys) are also common catalysts for certain reactions [32e34], the metals (alloys) incorporated NCs hybrids are reasonable catalysts benefiting from the catalytic activities of both components. Furthermore, the electrons transfer from metals (alloys) to NC motif can decrease the work function and therefore improves the reduction capability of CN motif [22]. Hence, significantly enhanced catalytic activities can be achieved. Fig. 1 presents the morphologies of the metals (alloys) incorporated NCs, all of them exhibit porous texture for the escaping of thermally decomposed gaseous species during the condensation of carbon framework by pyrolysis treatment. From Fig. 1a, Co-NC demonstrates porous sheet with the coating and embedding of granules. Detailed observation reveals that the size of granules include two grades, the larger granules coated onto the sheet are
Fig. 1. SEM of (a) Co-NC, (b) CoNi0.5-NC, (c) CoNi-NC, (d) Co0.5Ni-NC, (e) Ni-NC and (f) NC.
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mainly stemmed from the aggregation and fusion of the formed metallic Co, whereas the tiny grains (light spots) densely embedded within the sheet are mainly derived from the anchored Co crystallites by the doped N in carbon framework. Due to the more sufficient interaction with NC framework, it is presumably that the tiny Co crystallites provide the major contribution to the catalytic activity. Similarly, CoNi0.5-NC (Fig. 1b) displays macroporous network with coating of large granules and tiny crystallites of CoNi0.5 alloy onto the porous walls. In contrast, CoNi-NC (Fig. 1c) demonstrates large sheets with rich pores embedded in the walls (less than 100 nm in thickness), the decreased density of large granules means the homogeneous distribution of CoNi alloy within the NC framework with efficient interaction, which is beneficial for higher catalytic activity. Further increase the content of Ni causes drastic change in morphology, from Fig. 1d, Co0.5Ni-NC exhibits cross-linked particles with ill-defined morphology, which suggests the severe collapse of NC sheets under higher Ni content. As for NiNC (Fig. 1e), larger particles and broken layers are simultaneously formed, further signifies that Ni is not favorable for formation of sheets. In these hybrid materials, the hollow texture with embedded pores in walls can provide efficient channel and large accessible surface areas for rapid electrolyte ions sorption, which are essential premises for efficient catalytic CE in DSC. In contrast, the NC only exhibit bulk blocks with rough texture (Fig. 1e), the absence of metals (alloys) causes the disappearance of macropores within the carbonaceous motif. Fig. 2a shows the panoramic image of CoNi-NC at lower magnification, the sample exhibits interconnected network assembled by primary sheets, which can be manifested from Fig. 1c. The interconnected structure with rich macropores allows the facile diffusion of electrolyte, hence rapid interface reaction can be ensured. The translucent appearance of the primary sheet from TEM image (Fig. 2b) further evidences the low density with embedding of rich pores, which ensures a highly accessible surface area. Additionally, alloy particles with two grades in size were also observed, evidencing the embedding of CoNi alloy in the NC
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framework, the electrons transfer from alloy to NC alters the electronic structure and decreases the surface work function of the NC framework, thereby improves the catalytic activity for IRR [28]. HRTEM in Fig. 2c reveals the individual alloy particle is wrapped by graphitic shells with thickness of 2e2.3 nm, which accounts for of 6e8 graphitic layers. It is recognized that iron series metals are common carbonization catalysts, the adsorption, dissolution of thermo-decomposed carbon atoms in catalyst particles, as well as the followed nucleus and growth of evolved carbon yield graphitic layers around the alloy particle [34], which can not only prevent the further aggregation of alloy, but enhance the overall conductivity by virtue of the possibly high graphitization degree. The discrete lattice fringes (Inset) of the carbon layers implies the presence of N dopant derived defects, which facilitates a higher catalytic activity. In selected area electron diffraction (SAED) pattern (Fig. 2d), the sample exhibits two blurry rings indexed to (002) and (101) planes of graphite superimposed by scattered diffraction spots, showing the incorporation of polycrystalline CoNi alloy in NC framework. Elemental mapping of CoNi-NC (Fig. S2) reveals the homogeneous N doping and the uniformly scattering of CoNi alloy within carbon framework. The 3.3 at% N fraction evidences the efficient doping of N in carbon motif. The Co:Ni mass ratios of CoNi0.5-NC, CoNi-NC and Co0.5Ni-NC are 2.1:1, 1.1:1 and 1:1.8, respectively, which basically coincide with the feeding ratios, and the overall metal fractions of all samples are approaching to 9.2 at%, indicating the considerable alloys contents in the hybrids. Fig. 3a shows the XRD patterns of Co, Ni metals (alloys) incorporated NCs and neat NC. NC displays two broad diffractions at 26 and 43 indexing to (002) and (101) planes of graphite, suggesting the formation of partially graphitized phase. As for Co, Ni metals (alloys) incorporated NCs, the diffractions belonging to graphitic carbon disappear, instead, three peaks appear at 44.3, 51.5, 76.1, which are ascribable to (111), (200) and (220) planes of facecentered cubic phased Co or Ni with Fm3m(225) space group [24,35]. All these diffractions evidence the formation of Co, Ni metals (alloys) by thermo-reduction. The much higher peak
Fig. 2. (a) SEM, (b) TEM, (c) HRTEM and (d) SAED of CoNi-NC.
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Fig. 3. (a, b) XRD, (c) Raman, (d) N2 sorption isotherms and DFT pore size distributions (inset) of Co-NC, CoNi0.5-NC, CoNi-NC, Co0.5Ni-NC, Ni-NC and NC.
intensities of metals (alloys) depress the diffractions of graphitic carbon, thus causes the missing of diffractions for graphitic carbon. No diffractions correlate to nitrides or carbides are discernable, suggesting the Co, Ni precursors are effectively reduced to metals (alloys) by pyrolysis treatment. To be noted, the neighboring Co and Ni elements possess similar valence electron configurations (3d74s2 and 3d84s2, respectively), cell parameters (3.544 Å for Co and 3.523 Å for Ni) [35] and therefore similar physical properties, hence metallic CoNi solid solution can be formed by co-reduction. As expected, zoom-in (111) diffraction peaks of alloys incorporated NCs (Fig. 3b) right shift gradually from CoNi0.5-NC to Co0.5Ni-NC, showing the successful formation of binary alloys with narrowed lattice spacing for the slightly smaller atom diameter of Ni. The lattice strain due to the difference in Co, Ni atom diameter endows the increased surface energy and therefore higher catalytic activity toward certain reactions [36]. Additionally, the doped N in the carbon motif can not only serve as catalytic active sites for IRR, but increases the electrons mobility [13]. Given the combined contribution to conductivities and catalytic activities by both components, the Co, Ni metals (alloys) incorporated NCs herein are viable catalytic electrodes. Raman spectra (Fig. 3c) of all samples comprise two peaks located at 1330 (D band) and 1587 cm1 (G band). The D band is aroused by A1g vibration of edge and heteroatoms related defects within carbon framework, whereas G band represents the E2g vibration of graphitic carbon skeleton [37], the D band to G band intensity ratio (ID/IG) is an indicator of the extent of defects within the pseudographitic framework [37]. The ID/IG values of Co-NC, CoNi0.5-NC, CoNi-NC, Co0.5Ni-NC, Ni-NC, and NC are estimated to be 1.13, 1.06, 1.04, 1.07, 1.11 and 1.18, respectively. The slightly decreased ID/IG values of the hybrids relative to NC imply the enlarged graphitization domain sizes with less defects catalyzed by
Co, Ni catalysts undergo pyrolysis treatment, which coincide with the narrower peaks in metals (alloys) incorporated NCs. The enlarged graphitization domain size is beneficial for high conductivity, besides, the electrons transfer from metals (alloys) to NC framework also offer contribution to the conductivity, which is also indispensible for catalytic electrode. N2 sorption isotherms of all samples roughly exhibit a combined I/IV isotherms including a micropore filling at low P/P0 and a hysteresis loop at high P/P0 (Fig. 3d), which manifest the micro-/ mesoporous texture for all samples. In NC, the much pronounced N2 uptake at low P/P0 (below 0.1) indicates the overwhelming micropores, and the type IV isotherm with H2 type hysteresis loop at intermediate P/P0 range validates the coexistence of interconnected mesoporous, all these features suggest that the micro-/ mesoporous texture is formed by pyrolysis of starch and gelatin. As for metals (alloys) incorporated NCs, the much lower micropore filling suggests the obviously lower SBETs, the H3 type hysteresis loop within P/P0 range of 0.4e1 and the unsaturation at P/P0 of 1 indicate the presence of wedge shaped mesopores and macropores. The calculated BET surface area (SBETs) decreases from 631 m2g-1 for NC to 225, 198, 236, 188 and 248 m2g-1, respectively for Co-NC, CoNi0.5-NC, CoNi-NC, Co0.5Ni-NC and Ni-NC, showing the more prominent condensation of CN framework catalyzed by metals (alloys). Meanwhile, the micropore area ratio decreases accordingly from 78.2% for NC to 50.2, 60.6, 60.3, 61.5 and 63.0%, respectively for Co-NC, CoNi0.5-NC, CoNi-NC, Co0.5Ni-NC and Ni-NC. The apparently lower SBETs and micropore areas to some extent verifies the catalytic effect of metals (alloys) on the condensation and the graphitization of NC motif. DFT pore size distributions (inset) for all samples comprise micropores with sizes centered at 0.4 nm and a wide range of mesopores up to 10 nm, further evidences the coexistence of micropores and mesopores in all samples. Relative to
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NC, the probability of micropores in metals (alloys) incorporated NCs decrease apparently, whereas the probability of mesopores elevate slightly, indicating the conversion of partial micropores into meso-/macropores owing to the collapse of micropores during the catalytic graphitization. Given the 0.3e0.6 nm size of I 3 [38], the higher fraction of meso-/macropores allows the facile diffusion of electrolyte ions to access the catalytic sites and subject to rapid interface redox reactions with negligible steric hindrances and high surface utilization ratios. Additionally, the N dopant and co-doped metals (alloys) can not only offer abundant catalytic sites, but significantly improve the electrical conductivity. All of these virtues are essential for application as catalytic CE of DSC. XPS survey spectrum signifies the containing of N, C and Co, Ni elements in the representative CoNi-NC hybrid (Fig. 4a). The doped N not only alters the surface polarity of carbon motif and maintains a high wettability (Fig. S3) [22], but imparts the carbon motif with Lewis basicity by the electron donating feature of pyridinic- N [15], both of these features facilitate the efficient electrons transfer to the adsorbed I 3 ions and therefore enhance the catalytic activity for IRR [39]. To more in-depth identify the bond information, all of the four elemental peaks were deconvoluted. The core-level C1s spectrum (left inset) includes C]C (284.6 eV) and C]N/C]O (286.0 eV) functionalities [40], signifying the graphitization of carbon framework and the doping of N during the carbonization of starch and gelatin precursor catalyzed by the simultaneously formed CoNi alloy. N 1s spectrum (right inset) can be resolved into pyridinic- (398.1 eV) and graphitic-N (400.6 eV), showing the doping of N in form of pyridinic- and graphitic- N [13,41]. The pyridinic- N, commonly serve as Lewis base sites, or induce the Lewis base feature of the adjacent C atoms to motivate the adsorption of I 3 , weakening and dissociation of the IeI bond in I3 ions toward I [29]. Besides, the graphitic- N by replacing skeletal C atom renders higher conductivity by virtue of the rich electron feature, hence, the NC is a possible catalytic CE in DSC. Moreover, the pyridinic- N can serve as active site immobilizing the metal nanoparticles through the coordination between the lone pair electrons of N and the unoccupied d-orbits of metals [42]. The core level spectrum of Co2p orbit (Fig. 4b) includes Co2p3/2 (781.1 eV), Co2p1/2 (796.7 eV) orbits attributable to Co (II) and two satellite peaks, showing the partial oxidation of surface atoms [43]. Similarly, Ni2p spectrum (Inset) also exhibits Ni (II) orbits, including Ni 2p3/2 (855.6 eV) and Ni 2p1/2 (873.1 eV) orbits and other satellite peaks [43]. Both Co2p and Ni2p spectra reveal the inevitable oxidation of partial surface atoms in CoNi alloy for the moderate metallicity, the trace surface oxides are incapable of arousing diffraction in XRD pattern (Fig. 3b). Similar to Pt based alloys
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[25,44], the higher electro-negativity of Ni (1.92) over Co (1.88) [25] leads to slight electrons deviation from Co to Ni, the polar metallic bond of CoNi alloy facilitates higher wettability. Additionally, the decreased work function of CoNi alloy facilitates electrons transfer to I 3 ions in electrolyte and reinforces the catalytic activity for IRR [25]. Furthermore, in CoNi-NC hybrid, the electrons transfer from CoNi alloy to NC decreases the work function of the NC host, thus can enhance the catalytic activity of NC [45]. As a result of the combined catalytic activity of alloy and NC components, as well as the reinforced catalytic feature stemming from the interactions between them, robust catalytic capabilities of alloys incorporated NCs in CE are reasonable. 3.2. Electrochemical measurements Electroactive surface area (ESA) is an indicator reflecting the electrocatalytic activity of electrode [46]. From Fig. S4, the metals (alloys) incorporated NCs electrodes all demonstrate apparently higher ESAs over NC and Pt electrodes. Specifically, the ESAs of alloys incorporated NCs are higher over neat metals incorporated NCs, this trend is mainly attributable to two factors: Firstly, the deviation of electrons from Co to Ni and the discrepancy in lattice constant activate the surface catalytic sites as a result the charge redistribution of surface Ni and the increased lattice strain [36]. Secondly, the higher electrode/electrolyte compatibility as a result of the higher surface polarity by alloying effect increases the adsorbability to electrolyte ions. Under 1:1 automatic ratio, the maximized active sites and polarity render the highest ESA and further the possibly highest catalytic activity of CoNi-NC electrode. To evaluate the electrocatalytic activities for IRR, CVs of CoNi NC, NC and Pt electrodes in I 3 /I pair electrolyte were measured (Fig. 5a), two pairs of redox peaks are observable for all electrodes, which are indexed to the redox reactions of I 3 /I (left pair) and I2/I3 (right) couples [47]. Since IRR actually occurs at CE during photovoltaic process, the cathodic peak current density and the peak-to peak separation (Epp) of I 3 /I pair are major parameters reflecting catalytic activity of CE [22,27]. The higher cathodic peak current density means the more efficient reduction of I 3 , and the narrower Epp suggests lower overpotential viz higher electrochemical rate constant of I 3 /I3 pair [48,49]. The pyrolyzed Pt electrode arouses a well-resolved cathodic peak at 0 V, validates the good catalytic activity for IRR. NC electrode only evokes weak cathodic current and much wider Epp, suggesting the weaker catalytic activity relative to metallic Pt. The cathodic peak current density of CoNi-NCs electrode is higher over NC and Pt electrodes, showing the superior catalytic activity for IRR. Relative to Pt electrode, the wider Epp
Fig. 4. (a) XPS survey spectrum and (b) Co2p spectrum of CoNi-NC, (Inset: panel a: C1s and N1s spectra, panel b, Ni2p spectra).
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Fig. 5. (a) CV of CoNi-NC, NC and Pt electrodes in acetonitrile electrolyte containing 10 mM LiI, 1 mM I2, and 0.1 M LiClO4 at 50 mV s1, (b) CVs of CoNi-NC electrode at various scan 1 rates, (c) cathodic and anodic peak current densities of I 3 /I pair as a function of square root of scan rates, (d) 100 consecutive cycles of CVs of CoNi-NC electrode at 50 mV s .
is presumably ascribed to the less effective electrons mobility within the porous NC framework and the utilization of insulating binder. Albeit this, the porous texture and the rich active sites stemming from N dopant and the co-doped CoNi alloy are beneficial for sufficient redox reaction at electrode/electrolyte interface, hence, an excellent catalytic activity for IRR is achievable [18]. The cathodic current density and Epp of CoNi-NC electrode are also superior to other metals (alloys) incorporated NCs (Fig. S5a), implies the CoNi alloy is the optimal co-catalyst for IRR. CVs at different scan rates were further measured to reveal the redox reaction kinetics of I 3 /I3 pair at CoNi-NC electrode (Fig. 5b). As seen, the anodic and cathodic current densities of I 3 /I pair elevate accordingly with the scan rate, and the overpotential increases accordingly as a result of the electrochemical polarization and the thinner diffusion layer thickness [50]. The cathodic and anodic current densities are proportional to the square root of scan rates (Fig. 5c), suggesting that the I 3 ions diffusion is the ratelimiting step while the IRR at electrolyte/electrolyte interface is much faster [51]. Consider the 0.3e0.6 nm size of I 3 [38], the considerable fraction of meso-/macropores in CoNi-NC allows the facile infiltration of I 3 to the surface active sites and subject to reduction toward I, which are responsible for dye regeneration and therefore facilitates high photocurrent, filling factor (FF) and further high PCE. The electrocatalytic durability of CoNi-NC electrode was further estimated by 100 cycles of sequential CVs (Fig. 5d), the cathodic peak ascribable to IRR only show feeble variations in both current density and Epp, suggesting the good catalytic durability of CoNi-NC. The excellent catalytic activity and durability highlight the potential of CoNi-NC in high-performance and long-term DSC. To further study the electrochemical behavior at electrode surface, EISs of symmetrical cells based on different electrodes were measured. From Fig. 6a, the Nyquist plots of all devices comprise
two arcs in the intermediate and low-frequency regions. The series resistance (Rs), including the intrinsic resistance of electrode material and the contact resistance between electrode material and FTO substrate, can be estimated by the high frequency intercept at real axis [52]. Rs of CoNi-NC electrode (36.6 U) is higher than NC and Pt electrodes (31.9 and 27.6 U, respectively), which is mainly due to the less effective conductive channels within the loosely connected primary sheets (Fig. 2a). The Rs value is similar to other metals (alloys) incorporated NCs (36.4e37.6 U), while higher than neat CoNi alloy electrode (23.2 U) (Fig. S5b). The semicircle at high- and intermediate frequency originates from the charge transfer resistance (Rct) at electrode/electrolyte interface [53,54], whereas the low frequency arc represents the Warburg diffusion resistance (Zw) of electrolyte ions [53e55]. Rct of CoNi-NC electrode is estimated to be 6.4 U, which is slightly lower than that of Pt (8.4 U) and dramatically lower than NC electrode (224 U), suggesting the highest catalytic activity for the redox of I 3 /I pairs, as identified by the highest cathodic current density in CV test (Fig. 5a). The obviously higher ESA is responsible for the high catalytic activity for IRR for the exposure of more surface reactive sites to iodide mediator. Rct of CoNi-NC is also lower than other metals (alloys) incorporated NCs (11.6, 9.2, 9.0, and 9.8, respectively for Co-NC, CoNi0.5-NC, Co0.5Ni-NC, Ni-NC) and neat CoNi (17.0 U) (Fig. S5b). This trend indicates the optimal Co:Ni atomic ratio and N doping in carbon framework converged in providing the most abundant catalytic sites for IRR, and the resultant high concentration of I ions in electrolyte further ensures the sufficient regeneration of oxidized dyes, hence a higher PCE is reasonable. At low frequency region, the much larger Zw (67 U) over Rct for CoNi-NC indicates the electrolyte diffusion offer the highest contribution to the overall inner resistance, which further verifies the CV result that the ions diffusion is rate-limiting step in kinetics of IRR (Fig. 5c). In contrast, the Zw of NC (1888 U) is much higher than CoNi-NC as a consequence of the
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Fig. 6. (a) EISs and (b) Tafel polar curves of symmetrical cells based on CoNi-NC, NC and Pt electrodes. Inset in panel a: the whole EIS of NC.
higher steric hindrance for the limited micropore size. Additionally, the strong electrostatic force with I 3 ions because of the narrow pore size is also adverse for the exchange between I 3 and I ions, resulting in the higher Rct. Pt and neat CoNi electrodes only cause ill-defined arcs, which are mainly due to the heterogeneous electrode surface, which can not evoke well developed electrolyte diffusion resistances. Zw value of CoNi-NC electrode is also lower than other metals (alloys) incorporated NCs electrodes (within 75.6e92.4 U), evidencing the considerable electrolyte diffusion kinetics benefited by the proper fraction of meso-/macropore channels and the high surface wettability. In all, given the balanced Rs, Rct and Zw, CoNi-NC herein offers high electrons/ions mobility and catalytic activity, viz high electrocatalytic kinetics for IRR, the low inner resistance will finally result in high FF for the resultant DSC. Meanwhile, the high concentration of the generated I sustains the sufficient regeneration of dyes and further the efficient photoexcitation and electrons injection, hence enhanced photocurrent is also reasonable. Tafel polarization curves of the symmetric cells were crosschecked to identify the interface catalytic activities and electrolyte diffusion rates of different electrodes (Fig. 6b and Fig. S5c). At Tafel zone within 0.12e0.3 V, the anodic and cathodic profiles of CoNi-NC cell display higher slop than NC and Pt cells, and the ex change current density (J0) for I 3 /I pairs from the interception of tangent line at zero potential are also higher [29,56]. The highest J0 of CoNi-NC cell means the highest catalytic activity for IRR. According to Eq. (1) [57,58]:
J0 ¼ ðRTÞ=ðnFRct Þ
(1)
(where R is the gas constant, T is the thermodynamic temperature, F is the Faraday constant, n is the number of electrons involved in the reduction of I 3 to I and Rct is the charge transfer resistance), J0 varies inversely with Rct, hence the highest J0 of CoNi-NC cell means the lowest Rct whereas the highest Rct of NC, this trend again evidences the high catalytic activity of the former (Fig. 6a and Fig. S5b). Additionally, CoNi-NC cell also demonstrate the highest intersection at ordinate axis, viz the highest limiting diffusion current density (Jlim) at high potential end, which is determined by the diffusion velocity of I 3 across the two electrodes and the catalytic activity of the electrode. Jlim value is directly correlated with the diffusion coefficient (D) of I 3 ions across the porous electrode according to Eq. (2) [57,58]:
D ¼ ðlJlim Þ=2nFC
(2)
(where l represents the spacing distance between the two electrodes, C is the concentration of I 3 ions.) Following this linear
relationship, the highest Jlim of CoNi-NC cell implies the highest D, which is attributable to the porous texture with high fraction of meso-/macropores with high wettability, thus allows the rapid diffusion of I 3 ions to access the surface active sites and subject to electrocatalytic reduction. In all, the Tafel results again manifest the efficient redox exchange of I 3 /I pair and the high ionic diffusion rate within CoNi-NC electrode, which will finally translate into high PCE in the pertinent DSC. 3.3. Photovoltaic performances Given the high catalytic activity and stability for IRR, as well as the rapid electrolyte diffusion rate, CoNi-NC herein is envisaged to be credible CE in DSC. Fig. 7a, Fig. S6a and Table 1 summarize the J-V characteristics of DSCs based on various CEs. The CoNi-NC CE based DSC demonstrates the highest PCE of 7.6% with a photocurrent density (Jsc) of 18.5 mA cm2, a FF of 0.58 and an open circuit voltage (Voc) of 0.70 V. As a reference, a PCE of 7.15% (Jsc ¼ 17.7 mA cm2, FF ¼ 0.56, Voc ¼ 0.716 V) is harvested in Pt CE based cell. The higher Jsc and FF of CoNi-NC CE based DSC are in line with the electrochemical measurements. Specifically, the rich catalytically active sites and efficient ionic diffusion rate as a result of the N and alloy co-doping, as well as high fraction of meso-/ macropores result in efficient ions exchange to afford more I responsible for dye regeneration, electrons injection and therefore higher photocurrent density. Additionally, the low energy loss owing to the low inner resistance also facilitates a higher FF. The PCEs of other metals (alloys) incorporated NCs and neat CoNi based DSCs are relatively lower owing to the lower catalytic activities and higher Zws. The PCE of NC CE based DSC (2.2%) is obviously inferior for the much lower FF (0.20). All these results validate that CoNi-NC is the most suitable Pt free CE material. N doped carbon by acid leaching of CoNi from CoNi-NC was also used as CE of DSC, the PCE (6.1%) is also lower than CoNi-NC based cell as a consequence of the lower electrochemical activity (Fig. S7). Considering the lower PCEs of DSCs based on leached NC and neat CoNi alloy, the high PCE of CoNi-NC based DSC is essentially attributable to the synergistically catalytic effect between N dopant and CoNi alloy. The operation reliability in fast startup is an important indicator for practical application of DSCs [59,60], Fig. 7b compares the photocurrent-time (IT) curves of CoNi-NC, NC and Pt CE based DSCs at 0 V subjected to consecutive on-off switching of the incident light, the photocurrents of all DSCs eliminate promptly upon switching off the illumination while recover promptly upon turning on the illumination, the rapid photo-response reflects the rapid IRR kinetics at CE surface, the photocurrent densities of CoNi-NC CE based DSC remain over 90% of its initial value after 8 cycles of on-off switching, which is comparable to that of NC and Pt CEs based DSCs,
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Fig. 7. (a) J-V plots, (b) IT curves, (c) Photocurrent decay curves of DSCs based on CoNi-NC, NC and Pt based CEs under irradiation power of 100 mW cm2 (AM 1.5), (d) Photograph of a LED lamp powered by 4 in series connected DSCs based on CoNi-NC CE under illumination of AM 1.5.
Table 1 The main photovoltaic parameters of DSCs based on different CE materials. CEs
Jsc (mA cm2)
Voc (V)
FF
PEC (%)
Co-NC CoNi0.5-NC CoNi-NC Co0.5Ni-NC Ni-NC CoNi NC Pt
17.70 17.77 18.53 18.45 17.71 16.84 16.17 17.68
0.704 0.702 0.700 0.710 0.710 0.66 0.673 0.716
0.55 0.58 0.58 0.56 0.57 0.51 0.20 0.56
6.87 7.20 7.58 7.34 7.23 5.67 2.21 7.15
high-performance DSC. To more intuitively demonstrate the output property of CoNi-NC CE based DSC, 4 DSCs were connected in series to form a cell group. The cell group is capable of powering a LED with positive work voltage of 2.4 V under illumination (Fig. 7d), showing the feasibility of CoNi-NC in practical DSC. Photovoltaic characteristics of CoNi alloy incorporated NCs with different NC contents were also compared (Fig. S8), by altering the NC precursor dose, the FF and PCE both deteriorate, showing the CoNi-NC hybrid herein is an optimal CE material.
4. Conclusions showing the operation reliability of all DSCs as quick start devices. The photocurrent maintaining ratio of other metals (alloys) incorporated NCs increases with Ni content (Fig. S6b), showing higher Ni content is beneficial for higher operation reliability in CE of DSCs. In short, as a consequence of the high IRR kinetics at electrode/electrolyte interface, CoNi-NC herein is a reliable material in nonprecious CE of DSC. Photovoltaic durability is another indispensible parameter for practical application of DSC, Fig. 7c compares the photocurrent decay curves of CoNi-NC, NC and Pt CE based DSCs under continued irradiation, the photocurrent density of all cells are well maintained within 1000 s, showing the potential of CoNi-NC in long-term DSC. The photocurrent stabilities for metals (alloys) incorporated NCs also increase with Ni content (Fig. S6c), again signifies that the higher Ni fraction is indeed beneficial for higher photovoltaic stability for the less dissolution as a consequence of the weaker reactivity (reduction potential: 0.28 V for Co2þ/Co pair vs 0.23 V for Ni2þ/Ni pair relative to normal hydrogen electrode). Although the duration of 1000 s is far from sufficient in reflecting the durability of DSC, the high PCE, high operation reliability and durability preliminary highlight the potential of CoNi-NCs in
Co, Ni metals (alloys) incorporated NCs were prepared by facile pyrolysis of xerogels composing of starch, gelatin and Co, Ni salts. The as-prepared metals (alloys) incorporated NCs demonstrated high electrocatalytic activity for IRR benefitting from the meso-/ macroporous texture, N doping and the incorporation of metals (alloys). The DSC based on CoNi-NC CE offers superior PCE to Pt based counterpart, high photovoltaic reliability and durability, showing the great premise as efficient and economical CE materials in DSCs.
Acknowledgments This work was supported by NSFCs (Nos. 21671059, U1304505), Program for Changjiang Scholars and Innovative Research Team in University, Innovation Scientists and Technicians Troop Construction Projects of Henan Province (154200510009), Program for Innovative Research Team and Individuals (in Science and Technology) in University of Henan Province (Nos. 13IRTSTHN026, 15HASTIT006).
Z. Gao et al. / Journal of Power Sources 348 (2017) 158e167
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.03.009. References [1] A. Yella, H.W. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M. Nazeeruddin, E.W. Diau, €tzel, Science 334 (2011) 629e634. C.Y. Yeh, S.M. Zakeeruddin, M. Gra [2] L.Y. Han, A. Islam, H. Chen, C. Malapaka, B. Chiranjeevi, S.F. Zhang, X.D. Yang, M. Yanagida, Energy Environ. Sci. 5 (2012) 6057e6060. [3] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Science 348 (2015) 1234e1237. [4] A.Y. Mei, X. Li, L.F. Liu, Z.L. Ku, T.F. Liu, Y.G. Rong, M. Xu, M. Hu, J.Z. Chen, €tzel, H.W. Han, Science 345 (2014) 295e298. Y. Yang, M. Gra [5] M.X. Wu, X. Lin, T.H. Wang, J.S. Qiu, T.L. Ma, Energy Environ. Sci. 4 (2011) 2308e2315. [6] H. Wang, Y.H. Hu, Energy Environ. Sci. 5 (2012) 8182e8188. [7] K. Saranya, Md. Rameez, A. Subramania, Eur. Polym. J. 66 (2015) 207e227. [8] J. Briscoe, S. Dunn, Adv. Mater 28 (2016) 3802e3813. [9] M.X. Wu, H.Y. Guo, Y.N. Lin, K.Z. Wu, T.L. Ma, A. Hagfeldt, J. Phys. Chem. C 118 (2014) 12625e12631. [10] B. Lei, G.R. Li, X.P. Gao, J. Mater. Chem. A 2 (2014) 3919e3925. [11] H.K. Zheng, H.Y. Guo, D. An, K.Z. Wu, C.J. Gao, Q.J. Han, Y.J. Zhu, Y.N. Lin, M.X. Wu, J. Mater. Chem. C 4 (2016) 6533e6538. [12] M.J. Ju, I.Y. Jeon, J.C. Kim, K. Lim, H.J. Choi, S.M. Jung, I.T. Choi, Y.K. Eom, Y.J. Kwon, J. Ko, J.J. Lee, H.K. Kim, J.B. Baek, Adv. Mater 26 (2014) 3055e3062. [13] S.C. Hou, X. Cai, H.W. Wu, X. Yu, M. Peng, K. Yan, D.C. Zou, Energy Environ. Sci. 6 (2013) 3356e3362. [14] S.Y. Gao, K.R. Geng, H.Y. Liu, X.J. Wei, M. Zhang, P. Wang, J.J. Wang, Energy Environ. Sci. 8 (2015) 221e229. [15] D.H. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Science 351 (2016) 361e365. [16] X.K. Kong, Z.Y. Sun, M. Chen, C.L. Chen, Q.W. Chen, Energy Environ. Sci. 6 (2013) 3260e3266. [17] Y.J. Gao, G. Hu, J. Zhong, Z.J. Shi, Y.S. Zhu, D.S. Su, J.G. Wang, X.H. Bao, D. Ma, Angew. Chem. Int. Ed. 52 (2013) 2109e2113. [18] P. Zhai, T.C. Wei, Y.H. Chang, Y.T. Huang, W.T. Yeh, H.J. Su, S.P. Feng, Small 10 (2014) 3347e3353. [19] M. Yu, J.D. Zhang, S.M. Li, Y.B. Meng, J.H. Liu, J. Power Sources 308 (2016) 44e51. [20] M. Chen, L.L. Shao, Y.P. Liu, T.Z. Ren, Z.Y. Yuan, J. Power Sources 283 (2015) 305e313. [21] Z.Y. Gao, L. Wang, J.L. Chang, X. Liu, D.P. Wu, F. Xu, Y.M. Guo, K. Jiang, Electrochim. Acta 188 (2016) 441e449. [22] Y.D. Xing, X.J. Zheng, Y.H. Wu, M.R. Li, W.H. Zhang, C. Li, Chem. Commun. 51 (2015) 8146e8149. [23] J. Deng, P.J. Ren, D.H. Deng, L. Yu, F. Yang, X.H. Bao, Energy Environ. Sci. 7 (2014) 1919e1923. [24] Y. Hou, S.M. Cui, Z.H. Wen, X.R. Guo, X.L. Feng, J.H. Chen, Small 11 (2015) 5940e5948. [25] Q.W. Tang, H.H. Zhang, Y.Y. Meng, B.L. He, L.M. Yu, Angew. Chem. Int. Ed. 54 (2015) 11448e11452. [26] P.Z. Yang, C.Q. Ma, Q.W. Tang, Electrochim. Acta 184 (2015) 226e232. [27] Y.J. Li, Q.W. Tang, L.M. Yu, X.F. Yan, L. Dong, J. Power Sources 305 (2016) 217e224. [28] X.J. Zheng, J. Deng, N. Wang, D.H. Deng, W.H. Zhang, X.H. Bao, C. Li, Angew. Chem. Int. Ed. 53 (2014) 7023e7027.
167
[29] S.W. Yoon, V.D. Dao, L.L. Larina, J.K. Lee, H.S. Choi, Carbon 96 (2016) 229e236. [30] X.G. Duan, H.Q. Sun, Y.X. Wang, J. Kang, S.B. Wang, ACS Catal. 5 (2015) 553e559. [31] T.Y. Yang, J. Liu, R.F. Zhou, Z.G. Chen, H.Y. Xu, S.Z. Qiao, M.J. Monteiro, J. Mater. Chem. A 2 (2014) 18139e18146. [32] S. Bai, X.P. Shen, G.X. Zhu, M.Z. Li, H.T. Xi, K.M. Chen, ACS Appl. Mater. Interfaces 4 (2012) 2378e2386. [33] H. Li, J.Y. Liao, Y.C. Du, T. You, W.W. Liao, L.L. Wen, Chem. Commun. 49 (2013) 1768e1770. [34] O.Y. Podyacheva, Z.R. Ismagilov, Catal. Today 249 (2015) 12e22. [35] M. Motlak, N.A.M. Barakat, M.S. Akhtar, A.M. Hamza, B.S. Kim, C.S. Kim, K.A. Khalil, A.A. Almajid, Electrochim. Acta 160 (2015) 1e6. [36] Q.W. Tang, J. Liu, H.H. Zhang, B.L. He, L.M. Yu, J. Power Sources 297 (2015) 1e8. [37] S.B. Yoon, G.S. Chai, S.K. Kang, J.S. Yu, K.P. Gierszal, M. Jaroniec, J. Am. Chem. Soc. 127 (2005) 4188e4189. [38] W.J. Lee, E. Ramasamy, D.Y. Lee, J.S. Song, ACS Appl. Mater. interfaces 1 (2009) 1145e1149. [39] W.J. Lee, U.N. Maiti, J.M. Lee, J. Lim, T.H. Han, S.O. Kim, Chem. Commun. 50 (2014) 6818e6830. [40] L. Qie, W.M. Chen, H.H. Xu, X.Q. Xiong, Y. Jiang, F. Zou, X.L. Hu, Y. Xin, Z.L. Zhang, Y.H. Huang, Energy Environ. Sci. 6 (2013) 2497e2504. [41] T. Sharifi, G. Hu, X. Jia, T. Wagberg, ACS Nano 6 (2012) 8904e8912. [42] H. Feng, J. Ma, Z. Hu, J. Mater. Chem. 20 (2010) 1702e1708. [43] S.W. Lee, J.S. Kang, K.T. Leung, S.K. Kim, Y.K. Sohn, Appl. Surf. Sci. 386 (2016) 393e404. [44] C.X. Xu, J.G. Hou, X.H. Pang, X.J. Li, M.L. Zhu, B.Y. Tang, Int. J. Hydrogen Energy 37 (2012) 10489e10498. [45] D.H. Deng, L. Yu, X.Q. Chen, G.X. Wang, L. Jin, X.L. Pan, J. Deng, G.Q. Sun, X.H. Bao, Angew. Chem. Int. Ed. 52 (2013) 371e375. [46] Y.H. Xue, J. Liu, H. Chen, R.G. Wang, D.Q. Li, J. Qu, L.M. Dai, Angew. Chem. Int. Ed. 51 (2012) 12124e12127. [47] X.L. Duan, Z.Y. Gao, J.L. Chang, D.P. Wu, P.F. Ma, J.J. He, F. Xu, S.Y. Gao, K. Jiang, Electrochim. Acta 114 (2013) 173e179. [48] J.D. Roy-Mayhew, D.J. Bozym, C. Punckt, I.A. Aksay, ACS Nano 4 (2010) 6203e6211. [49] M.X. Wu, X. Lin, Y.D. Wang, L. Wang, W. Guo, D.D. Qu, X.J. Peng, A. Hagfeldt, €tzel, T.L. Ma, J. Am. Chem. Soc. 134 (2012) 3419e3428. M. Gra [50] M.X. Wu, Y.D. Wang, X. Lin, N. Yu, L. Wang, L.L. Wang, A. Hagfeldt, T.L. Ma, Phys. Chem. Chem. Phys. 13 (2011) 19298e19301. [51] P. Wang, S.M. Zakeeruddin, P. Comte, R. Charvet, R. Humphry-Baker, €tzel, J. Phys. Chem. B 107 (2003) 14336e14341. M. Gra [52] M. Chen, L.L. Shao, X. Qian, T.Z. Ren, Z.Y. Yuan, J. Mater. Chem. C 2 (2014) 10312e10321. [53] S.P. Lim, A. Pandikumar, Y.S. Lim, N.M. Huang, H.N. Lim, Sci. Rep. 4 (2014) 5305. [54] V.D. Dao, N.T.Q. Hoa, L.L. Larina, J.K. Lee, H.S. Choi, Nanoscale 5 (2013) 12237e12244. [55] M. Adachi, M. Sakamoto, J.T. Jiu, Y. Ogata, S. Isoda, J. Phys. Chem. B 110 (2006) 13872e13880. [56] M. Zheng, J.H. Huo, B.G. Chen, Y.G. Tu, J.H. Wu, L.H. Hu, S.Y. Dai, Sol. Energy 122 (2015) 727e736. [57] M.X. Wu, Q.Y. Zhang, J.Q. Xiao, C.Y. Ma, X. Lin, C.Y. Miao, Y.J. He, Y.R. Gao, A. Hagfeldt, T.L. Ma, J. Mater. Chem. 21 (2011) 10761e10766. [58] M.X. Wu, X. Lin, A. Hagfeldt, T.L. Ma, Chem. Commun. 47 (2011) 4535e4537. [59] X.X. Chen, Q.W. Tang, B.L. He, L. Lin, L.M. Yu, Angew. Chem. Int. Ed. 53 (2014) 10799e10803. [60] Y.Y. Duan, Q.W. Tang, J. Liu, B.L. He, L.M. Yu, Angew. Chem. Int. Ed. 53 (2014) 14569e14574.