Ti Mesh Counter Electrode for

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Energy Conversion and Storage; Plasmonics and Optoelectronics

MOF Derived Co,N Codoped Carbon/Ti Mesh Counter Electrode for High Efficiency Quantum Dot Sensitized Solar Cells Yu Lin, Han Song, Huashang Rao, Zhonglin Du, Zhenxiao Pan, and Xinhua Zhong J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b02082 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019

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The Journal of Physical Chemistry Letters

MOF Derived Co,N Codoped Carbon/Ti Mesh Counter Electrode for High Efficiency Quantum Dot Sensitized Solar Cells Yu Lin,† Han Song,† Huashang Rao,† Zhonglin Du,‡ Zhenxiao Pan,*,† and Xinhua Zhong*,† †College

of Materials and Energy, South China Agricultural University, 483 Wushan Road,

Guangzhou 510642, Guangdong China ‡College

of Materials Science and Engineering, the National Base of International Science

and Technology Cooperation on Hybrid Materials, Qingdao University, 308 Ningxia Road, Qingdao 266071, Shandong China

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] (Z. P.) *Email: [email protected] (X. Z.)

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Carbon supported on titanium mesh electrodes have been recognized as the best performing counter electrodes (CEs) in quantum-dot sensitized solar cells (QDSCs). Herein, layered double hydroxides (LDHs) are applied as scaffold template for the growth of cobalt-zeolite-imidazole framework (ZIF-67) crystals, and micrometer sized Co, N co-doped porous carbon materials (Co,N-C) are obtained through a carbonization process. The as-prepared Co,N-C exhibits favorable features for electrocatalytic reduction of polysulfide, including a high surface area of 491.36 m2/g, highly effective active sites, and a hierarchical micro/mesoporous structure. Due to the large particle size, the obtained Co,N-C can couple with Ti mesh substrate for the fabrication of high performance Co,N-C/Ti CEs for QDSCs. As a result, the corresponding QDSCs exhibit an average efficiency of 13.55% (Jsc = 25.93 mA/cm2, Voc = 0.778 V, FF = 0.672), which is a 10.5% enhancement compared to the previous best result from N-doped mesoporous carbon counterpart.

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Quantum dot sensitized solar cells (QDSCs), with reported highest power conversion efficiency (PCE) more than 12%,1-4 have being become a promising representative for new generation solar cells, and therefore attract great attention in recent years.5-7 A typical QDSC device is usually composed of a photoanode with QD light absorbers immobilized on nanocrystal TiO2 mesoporous film, polysulfide/sulfide redox couple electrolyte, and a counter electrode (CE) with electrocatalytic materials supported on conducting substrate (mainly FTO glass, or metal foil/mesh). As an integral part of a QDSC device, CEs are responsible for collecting photogenerated electrons from external circuit and transferring them to the electrolyte through electrocatalytically reducing polysulfide to sulfide ion.8-10 Metal chalcogenides, especially Cu2S, have been widely used as the catalytic material, and the corresponding Cu2S/brass foil electrode is the most common CE in QDSCs due to its easy accessibility and moderate performance.4,10-12 Alternatively, carbon materials have been explored as catalytic materials in CEs of QDSCs owing to their good electrocatalytic activity, electrical conductivity, high durability and low cost.13,14 A series of carbon materials, such as mesoporous carbon, graphene, carbon nanofiber and foam, have been studied and confirmed to be effective for catalytic reduction of polysulfide in QDSCs.15-29 It has been recognized that nonmetal or metal elements (such as N, Co, Cu etc.) doped carbon materials with uniform doping of heteroatoms, suitable mesopore distribution, and high specific surface area exhibit better electrocatalytic activity.24-26 Among them, CEs of mesoporous carbon (MC), or nitrogen doped mesoporous carbon (N-MC) supported on Ti mesh substrate (MC/Ti, or N-MC/Ti) have been recognized as one of the best performance CEs due to the unique electronic and structural features of the carbon catalytic materials as well as the excellent conductivity and confined area from the grid of the Ti mesh substrate to realize the catalytically active layer with thickness of sub-millimeter.23-25 More importantly, the reported carbon supported on Ti mesh CEs can reduce the redox potential of polysulfide/sulfide redox couple, and enhance the photovoltage of the cell device accordingly.2-4,23-25 The pyrolysis of metal-organic frameworks (MOFs) has been demonstrated to be a convenient and effective route to obtain heteroatom doped porous carbon as catalytic materials in electrochemical reduction reactions.26,27,30-32 In a recent work by Li and coworkers, Zn and Co bimetallic zeolite-imidazole framework (ZIF-0.035) derived Co,N-codoped carbon materials were prepared as catalytic materials in CE for QDSCs.26 It was reported that ZIF-0.035 based carbon exhibited much better performance than Zn-zeolite-imidazole framework (ZIF-8), or Co-zeolite-imidazole framework (ZIF-67) based 3 ACS Paragon Plus Environment


ones due to the synergistic effect of high specific area from the ZIF-8 derived carbon, and Co, N dual catalytic active sites from the ZIF-67 derived carbon. Regrettably, FTO glass, but not the distinguished Ti mesh, was used as substrate in the fabrication of these carbon materials based CEs since these carbon materials cannot form a robust film Ti mesh substrate due to the small particle size. Accordingly, the best PCE for the resultant cell devices based on these carbon catalytic materials is only 9.05%. In this work, layered double hydroxide (LDH) is adopted as scaffold template for the growth of ZIF-67, and micrometer sized Co, N co-doped porous carbon materials (denoted as: Co,N-C) are developed through a high temperature carbonization process.33 For comparison, free-Co,N-C and N-doped mesoporous carbon (N-MC) were prepared. Detailed synthetic procedures are supported in the Supporting Information (SI). The as-prepared Co,N-C materials exhibit favorable features for electrocatalytic reduction of polysulfides, including a high surface area of 491.36 m2/g, highly effective active sites, and a hierarchical micro/mesoporous structure with cellular morphology. In contrast to the mismatch between reference free-Co,N-C and Ti mesh substrate, the obtained Co,N-C can form a robust layer on Ti mesh substrate in the fabrication of high performance CEs for Zn-Cu-In-Se QDSCs. As a result, QDSCs based on the Co,N-C/Ti CEs exhibit an average PCE of 13.55%, which is a 10.5% enhancement in comparison with the previously best result from N-MC counterpart,25 and also among the highest performance for liquid-junction QDSCs.1-4 As illustrated in Figure 1a, a sacriﬁcial template approach was adopted for the preparation of Co,N-C materials.33 First, CoAl-LDH nanosheets were synthesized via the hydrolysis reaction of CoCl2 and AlCl3 in urea aqueous solution at refluxing temperature. Then, cobalt coordinated zeolitic imidazolate framework (ZIF-67) crystals were uniformly grown on both sides of the LDH templates to form sandwich-like CoAl-LDH@ZIF-67 composite by mixing Co(NO3)2 and 2-methylimidazole with CoAl-LDH templates in methanol solution at room temperature. After pyrolysis of the formed CoAl-LDH@ZIF-67 composite under Ar atmosphere at 800 oC, followed with an acidization treatment, quasi-2D Co,N−co-doped porous carbonaceous material with continuous honeycomb holes (denoted as: Co,N-C) was obtained. It is noted that pyrolysis temperature affects the catalytic activity of the resultant carbon materials, and affect the photovoltaic performance of the corresponding cell devices accordingly. Before the determination of the selected pyrolysis temperature of 800 oC, a series of pyrolysis temperatures (750, 800, and 850 oC) were tested, and the photovoltaic performance of corresponding QDSCs based on carbon materials obtained from different 4 ACS Paragon Plus Environment

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carbonization temperatures were shown in Table S1 and Figure S1. It can be found that pyrolysis temperature of 800 oC corresponds to the best performance carbon materials for catalyzing reduction in polysulfide in QDSCs, and therefore this temperature was adopted henceforth. In order to demonstrate the comparative advantage of the obtained Co,N-C in serving as catalytic material for CEs in QDSCs, two reference samples (free-Co,N-C and N-doped mesoporous carbon N-MC) were prepared. The free-Co,N-C material was derived from the free ZIF-67, which was synthesized without the presence of LDH templates according to literature procedure.34 N-MC, the ever reported best performance catalytic material for CE in QDSCs, was prepared via a colloidal silica nanocasting approach with use of formaldehyde, melamine, and phenol as carbon precursors, followed with pyrolysis and removal of silica templates.25,35 a)

b)

d)

c)

Figure 1. (a) Schematic demonstration of the preparation of quasi-2D porous Co,N-C, and the corresponding SEM images of: (b) CoAl-LDH, (c) CoAl-LDH@ZIF-67, and (d) Co,N-C. SEM images of the prepared CoAl-LDH templates (Figure 1b) show a well-defined hexagonal morphology with size of ~ 6 µm. The images of CoAl-LDH@ZIF-67 composites (Figure 1c) show that ~200 nm sized ZIF-67 crystals are deposited on both sides of CoAl-LDH templates. After a pyrolysis and acidization treatment, the derived Co,N-C material (shown in Figure 1d) shows a quasi-2D sandwich-like structure with honeycomb holes. It is noted that the derived Co,N-C material remains the approximate size and morphology of the parent CoAl-LDH templates and CoAl-LDH@ZIF-67 precursors, and the diameter of the honeycomb hole in Co,N-C is about 200 nm, close to the sizes of ZIF-67



crystals. The fine structure of the obtained Co,N-C was characterized by transmission electron microscopy (TEM), and HRTEM with corresponding images shown in Figure 2a. From the wide-area TEM images, Co nanoparticles with diameter of ~10 nm are dispersed in the substrate. This is consistent with the XRD result as discussed in the following section. Furthermore, graphite-like structure can be well identified with an interlayer spacing of 0.334 nm in the HRTEM image (inset in Figure 2a), ascribing to the (002) plane of graphitic carbon. This result indicates that the pyrolysis process renders the ZIF-67 shell convert to porous graphitic carbon, while the LDH nanosheet template transform into Co nanoparticles. In contrast, the SEM images of free-Co,N-C (Figure S2) derived from free ZIF-67, grown without the presence of LDH templates, only gives agglomerated carbon-based particles without porous nanostructure. The powder X-ray diffraction (XRD) patterns (Figure 2b) of the obtained CoAl-LDH@ZIF-67, Co,N-C, and free-Co,N-C materials have been recorded to track the composition and the structural evolution during the pyrolysis process. It is found that the pattern for CoAl-LDH@ZIF-67 shows the integration of those from both CoAl-LDH and ZIF-67 (shown in Figure S3). After the pyrolysis and acidization treatment, both the derived Co,N-C, and free-Co,N-C show diffraction signals from graphitic carbon (with diffraction peak at 26o corresponding to (002) crystal planes), and metallic Co (with diffractions at 44o and 51o derived from (111), (200) planes, respectively). It is noted that cobalt and aluminum hydroxides in the original CoAl-LDHs template could be reduced to corresponding metal nanoparticles (i.e. Co, and Al nanoparticles) since the strong reduction ability of formed carbon at high temperature inert atmosphere.33 The formed Co particles can incorporate into the N-doped carbon matrix as confirmed by the XRD pattern, but the formed metallic Al was evaporated and disappeared in the samples due to its relatively low melting point (660 oC for bulky Al, and much lower temperature for corresponding Al nanoparticles).


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a)

C Co

b)

0.334nm C(002)

Co,N-C

Normalized intensity

5 nm

free-Co,N-C

LDH@ZIF-67

200 nm 10

0.6

Co,N-C free-Co,N-C

3

400

c)

20

d)

30 40 o 2 ( )

50

Co,N-C free-Co,N-C

0.4

3

300

Dv(d) (cm /g )

500

Volume (cm /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200

0.2

100 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/P0)

0.0 1

10 100 Pore diameter (nm)

Figure 2. (a) TEM image of Co,N-C (inset: HRTEM image of Co,N-C) , (b) XRD patterns of LDH-@ZIF-67, free Co,N-C and Co,N-C, (c) N2 adsorption−desorption curves, and (d) pore size distributions for free-Co,N-C and Co,N-C samples. N2 adsorption-desorption experiments were conducted with the purpose of analyzing the specific surface area and pore volume distribution of the obtained carbon materials. As shown in Figure 2c, a typical H3-type hysteresis loops can be observed for Co,N-C sample, indicating the presence of mesoporous structure.36 While the curve of the free-Co,N-C sample shows very few mesopores. Meanwhile, Co,N-C materials show a greater BET surface area than that of free-Co,N-C (491.36 vs. 386.76 m2/g). The pore size distributions can be observed in Figure 2d. It is found that the portion of mesopore for Co,N-C is significantly greater than that of free-Co,N-C. The large BET surface area as well as abundant mesopores highlight the advantages of Co,N-C related to free-Co,N-C material in serving as electrocatalytic materials for CEs in QDSCs due to the sufficient contact between the catalytically active sites and the electrolyte.26,37 The full X-ray photoelectron spectroscopy (XPS, Figure 3a) demonstrates that C, N, O, and Co elements exist in the obtained Co,N-C samples. In the fitted high resolution C 1s spectrum (Figure 3b), two main peaks at around 284.8 and 285.8 eV are observed, corresponding to C=C, C−N species, respectively. The high 7 ACS Paragon Plus Environment


resolution Co 2p3/2 spectrum (Figure 3c) shows two kinds of Co species: metallic Co at 778.5 eV with weight coefficient of 34.8%, and Co − Nx at 780.3 eV with weight coefficient of 65.2%. These results confirm the presence of Co−N species. Furthermore, the N 1s spectrum (Figure 3d) can be curve-fitted with two peaks corresponding to pyridinic N (398.3 eV) with a content of 41.4%, and graphitic N (401.0 eV) with a content of 58.6%. Both of them were reported to be active sites in the electrocatalytic reduction reaction.25,26,33,34 b)

Full spectrum

a)

Co2p

1000

800

O1s N1s

600

400

200

Binding energy (eV)

c)

786

Intensity (a.u.)

metal Co

783

780

777

Binding Energy (eV)

291

288

285

774

282

Binding Energy (eV)

d)

Co2p3/2 Co-N

C-N

294

0

C1s

C=C

Intensity (a.u.)

Intensity (a.u.)

C1s

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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N1s pyridinic N

graphitic N

405

402

399

Binding Energy (eV)

396

Figure 3. (a) Full XPS spectrum of Co,N-C sample, and high-resolution XPS spectra of: (b) C 1s, (c) Co 2p3/2, (d) N 1s in Co,N-C sample. In order to evaluate the electrocatalytic performance, the obtained Co,N-C, as well as the reference carbon materials (free-Co,N-C and N-MC), as catalytic materials in the fabrication of CEs were supported on Ti mesh substrate (the corresponding CEs refereed as: Co,N-C/Ti, free-Co,N-C/Ti and N-MC/Ti, respectively) for assembly of QDSCs. First, carbon pastes were prepared according to literature procedure by dispersing corresponding carbon materials in terpineol solution containing ethyl cellulose and titanium isopropoxide.24,25 Then, the CEs were fabricated by screen printing the corresponding carbon pastes onto Ti mesh substrate, and the thickness of the formed carbon film were all kept at the optimum thickness of ~ 290 µm.24 The obtained carbon based CEs were used for the construction of Zn−Cu−In−Se


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(ZCISe) QDSC devices using polysulfide/sulfide as the electrolyte. The adoption of ZCISe QDSCs as a model cell device is based on the fact that the ever reported highest performance QDSCs are all derived from this kind of QD sensitizer.1-4,38 It is noted that in the case of free-Co,N-C/Ti CEs, the carbon materials fall off from the Ti mesh substrate when contacting with the polysulfide/sulfide electrolyte (the corresponding photograph before and after contacting with electrolyte are available in Figure S4), and the corresponding cell devices show

non-repeatable

poor

photovoltaic

performance.

Therefore,

the

photovoltaic

performance for cell devices based on free-Co,N-C/Ti CEs is not supported in this work. This phenomenon has been observed in previous report.26 The lack of mechanical stability for the free-Co,N-C film is mainly due to small particle size (herein 100-200 nm) of the carbon materials.39

25

80

a)

20

IPCE (%)

2

Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N-MC

15

Co,N-C

b)

60 40

N-MC

10

Co,N-C

20

5 0 0.0

0

0.2

0.4 0.6 Potential (V)

400

0.8

600

800

Wavelength (nm)

1000

Figure 4. Photovoltaic performance characterizations of ZCISe QDSCs based on N-MC/Ti, and Co@N-C/Ti CEs. (a) J-V curves under the illumination of 1 full sun intensity; (b) IPCE curves.



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Table 1. Photovoltaic and EIS Parameters of ZCISe QDSCs based on Different CEsa Jsc

CEs

(mA/cm2)

N-MC/Ti

Co,N-C/Ti aAverage

Voc (V)

FF

PCE (%)

Rs (Ω cm2)

25.42±0.13 0.763±0.008 0.632±0.008 12.26±0.12 (25.50)

(0.756)

(0.647)

(12.47)

25.93±0.14 0.778±0.006 0.672±0.006 13.55±0.11 (25.75)

(0.787)

(0.678)

(13.74)

R1

Rct

(Ω cm2) (Ω cm2)

1.283

0.1186

1.077

1.269

0.1821

0.395

photovoltaic values with standard deviation from five cells in parallel. The numbers

in parentheses are the photovoltaic values for the champion cells. The photovoltaic performance (i.e. current density (J)-photovoltage (V) curves) of champion cells based on Co,N-C/Ti, and N-MC/Ti CEs under one full sun illumination are shown in Figure 4a. Average photovoltaic parameters based on five cells were listed in Table 1. The detailed photovoltaic parameters and corresponding J−V curves of individual cells are shown in Table S2 and Figure S5. The QDSCs based on N-MC/Ti CEs in the reference samples showed an average PCE of 12.26% (Jsc = 25.42 mA/cm2, Voc = 0.763 V, FF = 0.632), which is at the same level as that reported in previous literature.25 With the replacement by Co,N-C/Ti CEs, the corresponding PCE was increased to 13.55% (Jsc = 25.93 mA/cm2, Voc = 0.778 V, FF = 0.672), which is a 10.5% enhancement in comparison with that of the reference sample, and also among the highest PCE for liquid-junction QDSCs.1-4 The improvement of photovoltaic performance is mainly derived from the enhancement of FF, while the Jsc and Voc values have also a slightly increase. This should be ascribed to the better electrocatalytic performance of the Co,N-C material. It is noted that the slightly increased Jsc values in the Co,N-C/Ti CE based QDSCs is also reflected by the IPCE (incident photon-to-electron conversion efficiency, Figure 4b) results, wherein the Co,N-C/Ti CE based QDSCs show a slightly higher IPCE value in the whole photocurrent response range in comparison with the reference N-MC/Ti CE based ones. The integrated photocurrents for corresponding QDSCs are 25.70, and 25.22 mA/cm2, respectively, which are in good agreement with the values obtained in the J-V measurement (Table 1).


3

b)

J lim J0

2

2

1.0 N-MC/Ti Co,N-C/Ti

0.5

0.0 1.0

1.5

2.0 2.5 3.0 2 Z' ( cm )

3.5

2 1

N-MC/Ti Co,N-C/Ti

0

-1 -0.6 -0.4 -0.2 0.0 0.2 Voltage (V)

600

2

a) Log J (log mA/cm )

1.5 -Z'' ( cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Current density (mA/cm )

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c)

400 200 0 N-MC/Ti Co,N-C/Ti

-200 -400 -600

0.4

0.6

-1.5

-1.0 -0.5 0.0 Voltage (V vs SCE)

0.5

Figure 5. (a) Nyquist plots, (b) Tafel polarization curves, and (c) Cyclic voltammograms based on different CEs. In order to investigate the mechanism for the improved photovoltaic performance of the Co,N-C/Ti CE based cell devices, electrochemical impedance spectroscopy (EIS) and Tafel polarization measurements were performed on symmetric dummy cells.10,24,25 This cell structure can exclude the interference from other components in a complete cell device.10,40 The Nyquist plots obtained from this test are shown in the Figure 5a, and the corresponding EIS parameters as shown in Table 1 are extracted with use of standard equivalent circuit (shown in Figure S6).41,42 In the Nyquist plots, the intercept of the horizontal axis represents the series resistance (Rs) of the counter electrodes. Two semicircles can be observed in the Nyquist plots of both counter electrodes. The first semicircle at high frequencies is generally considered to be related to the resistance (R1) of the solid-solid contact between the substrate and the carbon material. The second semicircle at the lower frequency corresponds to the charge transfer resistance (Rct) at the solid-liquid phase interface between the electrolyte and the carbon material.43,44 It is found that both types of CEs show a similar low Rs value as the pervious report due to the excellent electrical conductivity of the Ti mesh. The Rct value of Co,N-C/Ti mesh CE is substantially lower than the that of N-MC/Ti CE, corresponding to better catalytic reduction activity and higher FF value in QDSCs.8,10 The lower Rct value for Co,N-C/Ti mesh CE may be ascribed to the increase in the content of catalytic active sites as well as the unique structure in Co,N-C.25,26 The increase of catalytic active sites will improve the reduction ability of Sn2− to S2−, which is beneficial to reduce the reaction barrier of the reduction of the polysulfide electrolyte. Furthermore, the honeycomb structure and the sheet structure formed large pores in Co,N-C facilitate the penetration of the electrolyte in the carbon material which are more favorable for the sufficient contact between the electrolyte 11 ACS Paragon Plus Environment


and the catalytic active sites.26,37,45 Tafel polarization measurement was also employed to further demonstrate the improvement of the catalytic performance of Co,N-C/Ti CEs. Figure 5b shows the Tafel curves of different CEs. The exchange current density (J0) for Co,N-C is significantly greater that for N-MC, agreeing well with the result of Rct in EIS measurement. This result confirms that Co,N-C/Ti CE exhibits the better catalytic activity.10 The Jlim value of Co,N-C/Ti mesh CE is also higher than that of N-MC/Ti CE, implying the higher diffusion of polysulfide redox in the electrolyte. Furthermore, cyclic voltammogram (CV) measurement results (Figure 5c) indicate that highest current density of Co,N-C/Ti CE is higher than that for N-MC/Ti CE. This demonstrate the better catalytic activity for Co,N-C. This result is consistent with the variation of the Rct value obtained in the EIS measurements. In summary, quasi-2D sheet-like Co,N-C materials were prepared through the pyrolysis of CoAl-LDH@ZIF-67 composite, which was obtained via the in situ nucleation and epitaxial growth of ZIF-67 crystals on pre-prepared CoAl-LDH templates. The obtained Co,N-C materials can couple with Ti mesh substrate to form a robust catalytically active layer in the fabrication of high performance CEs for QDSCs. The sheet structure and the honeycomb morphology on the surface of Co,N-C are beneficial to increase the specific surface area and improve the permeability of the polysulfide electrolyte, and therefore facilitate the exposure of the catalytic active sites, and enhance the catalytic activity of the CEs. Furthermore, Co, N co-doping also improves the electrocatalytic activity. As a result, an average PCE of 13.55% was achieved for ZCISe QDSCs based on this carbon CEs, which is a 10.5% enhancement in comparison with the previously best result from N-doped mesoporous carbon counterpart, and also among the highest performance for liquid-junction QDSCs. Electrochemical measurement results confirm that the improved performance of C,N-C/Ti CEs is attributed to the stronger catalytic reduction activity of the Co,N-C materials toward polysulfide electrolyte.


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ASSOCIATED CONTENT Supporting Information Detailed experimental procedures, additional characterization as well as photovoltaic measurement results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Email: [email protected] (Z. P.) *Email: [email protected] (X. Z.) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research is supported by the Natural Science Foundation of China (Nos. 51732004, 21703071, 21805093 and 51802169).



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