Theoretical and experimental insights into the effects of oxygen

Subscriber access provided by WESTERN SYDNEY U

Article

Theoretical and experimental insights into the effects of oxygencontaining species within CNTs towards triiodide reduction Jiafu Hong, Chang Yu, Xuedan Song, Xiangtong Meng, Huawei Huang, Changtai Zhao, Xiaotong Han, Zhao Wang, and Jieshan Qiu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05327 • Publication Date (Web): 01 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 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

ACS Sustainable Chemistry & Engineering

Theoretical and experimental insights into the effects of oxygen-containing species within CNTs towards triiodide reduction Jiafu Hong, Chang Yu*, Xuedan Song, Xiangtong Meng, Huawei Huang, Changtai Zhao, Xiaotong Han, Zhao Wang and Jieshan Qiu*

Carbon Research Laboratory, Liaoning Key Lab for Energy Materials and Chemical Engineering, State Key Lab of Fine Chemicals, Dalian University of Technology, Dalian 116024, PR China

*Corresponding author: Prof. Chang Yu, Prof. Jieshan Qiu *Email: [email protected] (C. Yu); [email protected] (J. Qiu)

ABSTRACT Heteroatom-doped micro/nano-structured carbon materials feature unique superiorities for replacement of noble metal Pt counter electrode (CE) in dye-sensitized solar cells. Nevertheless, the effects of oxygen-containing species on/within carbon matrix on its electrocatalytic activity are seldomly considered and concerned, which will be hindered by a trade off between oxygen defects and conductivity. Herein, we present activated carbon nanotubes (P-CNTs) with abundant active edge sites and oxygen species for simultaneous achieving the activation of sidewalls and open ends. Also, the positive effects of oxygen species are decoupled by experimental data together with theoretical analysis. When capitalizing on the P-CNTs as the CE of DSSCs, the device delivers a high power conversion efficiency of 8.35% and an outstanding

ACS Paragon Plus Environment

1

ACS Sustainable Chemistry & Engineering 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

Page 2 of 28

electrochemical stability, outperforming that of Pt reference (8.04%). The density functional theory calculation reveals that compared with the carboxylic groups, the hydroxyl groups and carbonyl groups on the surface of CNTs can greatly reduce the ionization energy of reaction, accelerate the electron transfer from external circuit to triiodide, thus being responsible for an enhanced electrocatalytic performance. This work demonstrates that a certain amount of oxygen atoms within carbon materials is also indispensable for the improvement in the reactivity of the triiodide. KEYWORDS:

Defective

carbon

nanotubes;

Oxygen

species;

Counter

electrodes;

Electrochemical stability; Ionization energy; Triiodide reduction

INTRODUCTION The effective reduction of triiodide (I3-) to iodide (I-) is one of concerned and key reactions for high-efficiency and cost-effective dye-sensitized solar cells (DSSCs).1-5 Therefore, a desirable counter electrode (CE) is of great concern that plays a vital role in collecting electrons derived from the conduction band of the photoanode and in catalyzing the I3- reduction.6-9 The noble metal Pt is well-known to be a highly active CE material of DSSCs due to its superior electrocatalytic activity.10-13 However, its poor electrochemical stability in corrosive electrolytes and limited resources seriously inhibit the large-scale application of DSSCs. Therefore, an inexpensive and high-performance alternative to Pt is highly desired and taken into consideration.4, 8 In response, micro/nano-structured materials have been developed and investigated as the Pt replacements in DSSCs due to their large surface area, abundant active sites, and high


2

Page 3 of 28 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


conductivity, such as the heteroatom (N, P, S, B, etc.)-doped carbon materials.14-18 The incorporation of heteroatoms with different electronegativity and atomic size will result in the charge redistribution of conjugated carbon matrix and distort the lattice structure, thus triggering novel physicochemical properties.19-21 Doping heteroatoms into carbon materials can not only enhance their intrinsic activity but also create more active sites. For example, doping N atoms into the basal or edge planes of carbon networks can prolong the delocalization of the π-electron domains, resulting in a "healing" effect.22 Meanwhile, the introduction of N atom in carbon materials can produce more active sites for the redox reactions of I3-/I- due to the incorporation of lone pair electrons into the large π-conjugated system.14, 23 The embedment of B atom in carbon matrix could transform electron-deficient B atom to electron-donating site by utilizing the abundant p electrons in the carbon conjugated system.16, 24 More importantly, the introduction of B atoms can activate the π electrons of neighbouring carbon atoms to enhance their catalytic activity and conductivity.25-26 With this information in mind, a highly active and conductive 3D N-doped graphene foam was reported previously, as the CE for DSSCs, showing a power conversion efficiency (PCE) up to 7.07%.23 Recently, the N-doped holey graphene was fabricated to achieve a low charge transfer resistance (Rct) and excellent electrocatalytic activity towards the I3-reduction.27 Moreover, the B-doped materials have also been investigated and developed as the CE for DSSCs, and exhibited the enhanced capability for the reduction of I3-.16 Others on the P and S doped materials have also been concerned and studied.17-18, 28 As such, the co-doping with different elements is also an effective strategy to synergistically regulate the electrocatalytic activity through coordinated electron interactions between different doped heteroatoms and neighbouring carbon atoms. The N, P co-doped carbon nanosheets that were prepared by directly pyrolyzing aminotris(methylene phosphonic acid) had contributed to deliver


3


Page 4 of 28

a high PCE of 6.74%, outperforming that of Pt (5.76%).29 Recently, our work presents a B, N codoped graphene by chemically grafting ionic liquid to GO and an enhanced electrochemically catalytic activity was demonstrated.25 Indeed, various efforts have been devoting to focus on the development of novel doped materials and the structure-effect relationship of heteroatom (N, B, S, P)-doping for electrochemical performance. Nevertheless, O-doped carbon materials and the effects of O atoms on electrochemical performance in DSSCs are rarely concerned and often overlooked. One reason may be the negative effect of lowered conductivity caused by oxygen species, which is not favorable for the electrochemical process. Even so, the research of oxygen species doping is still significant for electrochemical catalysis due to the additionally introduced defects, the improved polarity of carbon materials and the redistributed electronic structure. Wang et al. investigated the effects of oxygen-containing functional groups on the activity for oxygen reduction reaction and oxygen evolution reaction through the in-situ exfoliated graphene from carbon fibers, and further confirmed that the content of oxygen species positively correlates with the increased active sites and improved electrochemical activity.30 In this context, understanding the effects of oxygen species on the reduction of I3- will be of great significance for rational construction of high-efficiency electrocatalysts in DSSCs. Herein, we regulated the oxygen defects of carbon nanotubes (CNTs) by a facile method of oxygen-plasma treatment (P-CNTs), and also revealed the positive effects and electrochemical behaviors of oxygen species towards the triiodide reduction in terms of the density functional theory (DFT) combined with experimental results. Also, the strong oxygen etching occurring on the surface of CNTs generates the cleaved P-CNTs with more exposed open ends and edges. Benefiting from the combined superiorities of P-CNTs, a high PCE of 8.35% was delivered when applying the P-CNTs as CE in DSSCs, which obviously precedes the Pt reference (8.04%).


4

Page 5 of 28 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


DFT results uncovered that the oxygen species on the carbon surface, especially the hydroxyl groups and the carbonyl groups, could adjust the electronic properties of adjacent carbon atoms, and decrease the ionization energy of the P-CNTs. This will effectively reduce the charge transfer resistance to accelerate the electron transfer. Moreover, an outstanding electrochemical stability of P-CNTs CE was also achieved, indicative of great potential to supersede Pt. The present work provides a simple yet effective strategy to engineer the high-performance CE by regulating the oxygen species to modulate the electronic structure of CNTs, and it may be suitable for other energy storage and conversion systems.

EXPERIMENTAL SECTION Preparation of P-CNTs The original and commercial CNTs (Shenzhen Nanotech Port Co.,Ltd, Diameter: 40-60 nm, Length: >5 µm) were treated by plasma (commercial 13.56 MHz PCE-6 plasma system) for different irradiation time (0, 1, 3, 5, and 30 min) with a power of 29.6 W and a pressure of 50 Pa in oxygen atmosphere. The treated and etched P-CNTs for 3 min were used for detailed study. In addition, plasma-etched CNTs under Ar conditions (Ar-P-CNTs) were also prepared for comparison. Preparation of CEs The as-made sample (CNTs or P-CNTs) was mixed with a few drops of carboxyethyl cellulose solution through adequate grinding, then, the well-mixed slurry was transferred onto the FTO glasses using the doctor-blade method. After annealing at 500 oC for 30 min in N2 atmosphere, the CEs were yielded.


5


Page 6 of 28

Assembly of DSSCs TiO2 photoanodes (Yingkou OPV Tech New Energy Co. Ltd, China) were sintered at 500 oC for 30 min in a muffle furnace. After cooling down, the photoanodes were immersed in ethanol solution including N719 dye (Solaronix SA, Switzerland) for 24 h to adsorb sufficient dye. The dye-loaded TiO2 photoanodes and the CEs were sandwiched together using a 45 μm Surlyn as spacer and sealed by hot pressing. Then, the redox electrolyte (OPV-AN-I, Yingkou OPV Tech New Energy Co., Ltd., China, Chemical composition: iodine, anhydrous lithium iodide, 1propyl-3-methylimidazolium iodide, guanidine isothiocyanate, tributyl phosphate, acetonitrile) was slowly injected into the DSSCs through the predrilled holes on the CEs edge. The sealed cells with an active area of 0.16 cm2 were used for the photocurrent-voltage test. Materials Characterization X-ray diffraction (XRD) patterns were collected on Rigaku D/Max 2400 diffractometer with Cu Ka radiation (λ = 1.5406 Å). The morphology and structure of the as-obtained materials were investigated by using scanning electron microscopy (SEM, NOVA NanoSEM 450) and transmission electron microscopy (TEM, FEI TF30). The chemical composition of all samples were determined by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250) with Al Kα X-ray radiation, and the measurement of Raman spectroscopy was recorded on DXR Raman microscope (Thermo Scientific, US). The Brunauer-Emmett-Teller (BET) specific surface areas based on the nitrogen physical adsorption were tested with Micromeritics 3Flex 3500. Computational details To further investigate the ionization ability of CNTs and P-CNTs towards the change from I* to I-, of which the ionization energy on carbon surface were calculated using DFT based on the


6

Page 7 of 28 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


B3LYP functional 6-31G(d,p) basis set.31-33 The solvent effect was also considered, in which the acetonitrile was selected as the solvent through our calculations based on the polarizable continuum model using the integral equation formalism variant (IEFPCM).34-35 All electronic calculations were carried out by using the Gaussian 09 program suite.36 Five optimized molecular structures (G, G-OH1, G-OH2, G-COOH and G-CO) as well as two optimized overall molecular structure (G-96, G-96-O) were selected to simplify the computational process. Electrochemical tests Cyclic voltammetry (CV) measurement was performed by a typical three-electrode system in an anhydrous acetonitrile solution which were composed of 0.1 M LiClO4, 10 mM LiI and 1 mM I2. Also, the as-prepared materials were taken as the working electrode, Pt foil as the counter electrode and Ag/Ag+ electrode as the reference electrode, respectively. Electrochemical impedance spectroscopy (EIS) and Tafel polarization plots of symmetrical dummy cells were tested by electrochemical workstation at zero bias potential, where the symmetrical dummy cell consisted of two identical CEs was filled with the redox electrolyte. The photocurrent densityvoltage (J-V) curves of DSSCs were measured with an AAA solar simulator (94032A, Newport, US) under AM 1.5G and 100 mW cm-2. The incident photon-to-electron conversion efficiency (IPCE) spectra of DSSCs were measured by a Hypermono-light (SM-25, Jasco Co., Ltd., Japan).

RESULTS AND DISCUSSION The preparation route of P-CNTs is shown in Scheme 1. With this strategy, the rich active sites and defects are produced on the surface of CNTs. This will finally result in a better affinity towards electrolyte, which is confirmed by the decreased contact angle. The good surface affinity


7


Page 8 of 28

of P-CNTs to the electrolyte acetonitrile will facilitate the accessibility for the CE to the electrolyte and improve the mass transfer ability at CE/electrolyte interface. SEM and TEM were used to investigate the changes in the morphologies of CNTs before and after O2 plasma etching.

Scheme 1. Schematic illustration for the preparation process of P-CNTs and its surface affinity ability towards the acetonitrile solvent. The representative SEM image (Figure S1, Supporting Information) illustrates that the original CNTs were effectively etched to produce graphene-like nanosheets on its surface, without destroying the overall structure of nanotubes. From the TEM images shown in Figure 1a and 1c, it can be clearly seen that CNTs are broken and partially cut into many shorten tubes. The magnified TEM images of CNT and P-CNTs, as show in Figure 1b and 1d, exhibit that the surface of P-CNTs presents a relatively rough feature and clear broken ends in comparison to that of the pristine CNTs. In addition, the high resolution TEM (HR-TEM) image shown in Figure 1e indicates the as-made P-CNT features numerous defects especially on the end face. These combined results demonstrate that the amount of the edge sites are increased after the oxygen engraving, and more defects are produced and exposed. The abundant open ends, edge


8

Page 9 of 28 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


sites, and defects will serve as active sites to boost the catalytic activities, which will be confirmed by the following experimental and theoretical investigations.

Figure 1. TEM images of (a, b) pristine CNTs and (c, d) P-CNTs; (e) HRTEM image of PCNTs. The structural information of as-obtained samples was further investigated by XRD, of which the detailed results are presented in Figure 2a. It can be seen that after etching, the diffraction peak at ~ 26o corresponding to the (002) plane of graphite becomes weak, indicating that O2 plasma-engraving treatment destroys the crystal structure of CNTs to some degree. Raman spectroscopy is an effective method to unravel the defect level of carbon materials. As shown in Figure 2b, it can be noted that two obvious peaks are observed at 1369 and 1596 cm-1, which correspond to the D-band and G-band of graphitized carbon materials, respectively. The intensity ratio of D band to G band (ID/IG) is often employed to evaluate the defective states of carbon materials.14 It can be clearly seen that the ID/IG value of P-CNTs (1.46) is obviously higher than that of pristine CNTs (1.28), suggesting the increase of the defects/disorder level and the activated sidewalls, which agrees with the TEM results. Additionally, it can be seen that a


9


Page 10 of 28

small red shift in the D band and G band positions for the P-CNTs, further indicative of an enhancement of the defective density. Moreover, that the nitrogen adsorption amount slightly increases after the oxygen plasma etching and the corresponding specific surface area increases from 98 m2 g-1 to 112 m2 g-1 (Figure S3), which can be ascribed to the abundant defects and broken structure.

Figure 2. (a) XRD patterns and (b, c) Raman spectra and close-up Raman spectra of pristine CNTs and P-CNTs. XPS was used to analyze the content of oxygen species and chemical states of original CNTs and P-CNTs, of which the detailed results are shown in Figure 3. As expected, the O 1s peak intensity (about 532 eV) in P-CNTs is obviously higher than that in CNTs, and the corresponding atom ratio of C to O for P-CNTs (9.5) is also much lower than that of the original CNTs (33.48) (Figure S2, Supporting Information). The high resolution XPS spectra of C 1s and O 1s were further analyzed to in-depth explore the bonding state of C and O. As shown in Figure 3b and c, the O 1s peak can be deconvoluted into COO-/O=C-O (the carboxyl group in carboxylate and the oxygen double to carbon) (531.6 eV), C-OH/C=O (the hydroxyl and carbonyl) (532.6 eV), O-C-O (the oxygen single bond in ester and carboxylic acids) (533.8 eV), H-O-H (chemisorbed oxygen or water) (535.0 eV).30 The contents of these chemical species are


10

Page 11 of 28 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


listed in the Table 1. It can be clearly noted that the bonded oxygen increases from 2.09% for CNTs to 9.44% for P-CNTs, which can be attributed to the oxygen-related defects and edgedangling bonds induced by the plasma etching. As shown in Figure 3d and 3e, the high resolution C 1s XPS spectra for the CNTs and P-CNTs show the peaks including C=C (284.6 eV), C-C (285.6 eV), C-O (286.9 eV), and C=O (288.8 eV).37 It can be observed that compared with the original CNTs, the intensities of C-O and C=O peak of P-CNTs increase significantly. Further representative FT-IR results also reveal that the density of oxygen-containing functional groups increases indeed after plasma treatment. Specially, an increment of the intensity is found in the region of C-OH groups (1046 cm-1). Additionally, there is also a slight increase in the strength of the C-O/C=O group. The high-content C-O/C=O and -OH species could regulate the electronic structure of the conjugated system and lower the ionization ability of carbon towards the transformation from I* to I-, thus promoting the reduction of I3-, which will be further confirmed by the following theoretical calculations.


11


Page 12 of 28

Figure 3. (a) XPS survey spectra of CNTs and P-CNTs, and high resolution O1s, C1s XPS spectra of CNTs (b,d) and P-CNTs (c,e). (f) FT-IR spectra and locally enlarged spectra of CNTs, P-CNTs. Table 1. The amount of oxygen-containing functional groups for the P-CNTs and CNTs samples derived from O 1s XPS spectra.

Samples CNTs P-CNTs

COOO=C-OH (at %)

C-OH C=O (at %)

O=C-O

H-O-H

(at %)

(at %)

0.40 2.81

0.77 3.51

0.55 2.11

0.37 1.01

The electroactive surface areas (Se) of electrodes were measured by CV with [Fe(CN)6]4/[Fe(CN)6]3- redox couple. And the detailed data are shown in Figure 4a and Table S1 (Supporting Information). It can note that the Se of P-CNTs (1.936 cm2) is higher than that of pristine CNTs (1.672 cm2) and Pt (0.308 cm2), demonstrating that open ends, rich edge sites, and defects produced by plasma-engraving are capable of contributing the positive effects towards the electrochemical process. The high Se of P-CNTs could facilitate the good contact between the electrode and the electrolyte, thus leading to an improved rate of electron transfer.23


12

Page 13 of 28 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


Figure 4. (a) CV curves of Pt, CNTs, and P-CNTs in 5×10-3 M K3Fe(CN)6/0.1 M KCl solution, scan rate: 50 mV s-1. (b) Schematic illustration of electrocatalytic mechanism for the I3- reduction over the P-CNTs CE. J-V curves of the assembled cells with Pt, CNTs, and P-CNTs and Ar-P-CNTs CEs were measured and shown in Figure 5a and Figure S4 (Supporting Information). The detailed electrochemical parameters are summarized in Table 2. It can be seen that the DSSCs based on Pt delivers a PCE of 8.04% with an open circuit voltage (Voc) of 0.76 V, a short-circuit current density (Jsc) of 14.63 mA cm-2, and a fill factor (FF) of 72.0%. By contrast, a high PCE of 8.35% is delivered for P-CNTs and larger than that of the pristine CNTs (7.29%), Ar-P-CNTs (7.52%) and Pt reference, in which the Voc, Jsc and FF are 0.76, 15.92 and 69.2%, respectively. These results clearly indicate that the activated CNTs with abundant edge sites and defects have better electro-catalytic activities. Taking account of the effects of oxygen species, a series of samples are prepared at different plasma etching time. The corresponding electrochemical performance are shown in Figure S5 and Table S2 (Supporting Information). It can be seen that as the etching time increases, the PCE increases firstly and then decreases. When the processing time reaches 30 min, the efficiency decreases to 7.47%, being obviously lower than Pt, which is probably due


13


Page 14 of 28

to the poor electrical conductivity of CNTs caused by the large amount of oxygen species. This is to say, the appropriate amount of oxygen functional groups can improve the photovoltaic performance of CNTs. Table 2. Photovoltaic parameters of various CEs. Samples

Jsc (mA cm-2)

Voc (V)

FF (%)

η (%)

Pt

14.63±0.16

0.76±0.01

72.0±0.8

8.04±0.04

CNTs

14.80±0.23

0.74±0.01

66.9±1.2

7.29±0.06

Ar-P-CNTs

15.72±0.19

0.74±0.01

64.7±0.8

7.52±0.04

P-CNTs

15.92±0.21

0.76±0.02

69.2±0.6

8.35±0.03

To better understand the improved DSSC performance of the P-CNTs, the CV curves of various CEs were measured. The corresponding curves and parameters are shown in Figure 5b, Figure S6 and Table S3 (Supporting Information). A couple of oxidation and reduction peaks ((Ox-1/Red-1(left)) at low potential corresponds to I3- + 2e- ↔ 3I-, while at high potential ((Ox2/Red-2(right)) relates to 3I2 + 2e- ↔ 2I3-, as shown in Figure 5b. Herein, we only focus on the redox pair from I3- to I- on the left (IRR = triiodide reduction reaction, Ox-1 and Red-1). The intensity of cathodic peak Red-1 and separation between Ox-1 and Red-1 peaks (Epp) represent the catalytic activity of CEs.37 It can be seen that P-CNTs have smaller Epp (171 mV) than that of pristine CNTs (355 mV) and Pt (297 mV). Moreover, the cathodic peak current density of PCNTs is obviously higher than that of Pt and original CNTs. These information further confirm that of the CEs adopted, the P-CNTs have the most outstanding electrochemical activity for IRR, which is consistent with the J-V curves.23 The CV curves of P-CNTs and Pt at different scan


14

Page 15 of 28 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


rates were also tested and analyzed. The detailed results are presented in Figure S6. It is notable that the peak potential moves in the opposite direction and the peak current density increases with an increase of the scan rates. The increased electrochemical polarization is responsible for higher overpotential and poor reversibility, while the thin of the diffusion layer contributes to the increase of peak current density.38

Figure 5. IRR performance of CNTs, P-CNTs, Pt electrodes. (a) J-V curves; (b) CV curves at a scanning rate of 50 mV s-1; (c) Nyquist plots of the symmetrical dummy cell (inset: the magnified plots and equivalent circuits), EIS measurement was conducted at 0 V from 0.8 MHz to 0.1 Hz; (d) Tafel polarization curves of symmetrical dummy cells with P-CNTs and Pt CEs in acetonitrile solution of I3−/ I−.


15


Page 16 of 28

EIS is another effective method to study the charge transfer and charge transport kinetics at the electrode/electrolyte interface. In order to further explain the excellent electrochemical activity of P-CNTs, EIS were measured at 0 V by using symmetrical dummy cells fabricated by two identical electrodes. The detailed Nyquist plots of different CEs are shown in Figure 5c and the corresponding EIS parameters are shown in Table S3. Compared with that of Pt, three semicircles are manifested for the as-made P-CNTs, corresponding to a new model proposed by Aksay (Figure 5c, top inset).12 The semicircle in the high frequency region represents the pore diffusion resistance, which derives from the ion diffusion in the pores of the electrode material. The semicircle in the low frequency region represents the Nernst diffusion resistance (ZN) and the semicircle in the middle means the charge-transfer resistance (Rct) related to electrocatalytic activities of CEs. For Pt CE, the Zpore is not visible because of nonporous structure. After fitting the equivalent circuit, the Rs of P-CNT is much smaller than Pt, indicative of a strong bonding force between FTO conductive substrate and P-CNTs. And its Rct value is also smaller than Pt and CNTs, indicating its superior electrochemical activity.39-40 The corresponding Tafel plots of various CEs were tested by using dummy cells and the detailed results are reproduced in Figure 5d. As we known, diffusion-limiting current density (Jlim) is positively proportional to the diffusion coefficient related to the electrode structure. It can be seen that Jlim value of P-CNTs CE is higher than Pt, corresponding to fast diffusion of electrolyte within P-CNT electrode. And the exchange current density (J0) of P-CNTs is also higher than that of Pt, indicating the stronger reduction ability of P-CNTs for I3-.41 DFT Calculation


16

Page 17 of 28 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


To further understand the catalytic mechanisms of multi-defected and oxygen-enriched P-CNTs, the DFT calculation was conducted. Generally, the IRR reactions could be described in a general equation: I3-+2e-↔ 3I-, and the detail processes can be depicted as follows: I3- (sol) ↔ I2 (sol) + I- (sol)

(1)

I2 (sol) + 2* → 2I*

(2)

I* + e- → I- (sol)

(3)

where * indicates the active sites on the CE surface and (sol) represents the acetonitrile solution. It was known that the step (3) is the rate-determining step for the overall triiodide reduction in DSSCs.42 Thereby, the reaction (3) will be focused on in the present system, where the graphene slabs with seven hexagonal carbon rings and different oxygen species and the optimized structure are shown in Figure 6. The detailed ionization energy (Ei) values for different slabs are summarized in Table 3. It is obviously noted that the Ei values vary with O species and follow in an order of graphene slabs with G-OH2 (4.80 eV) < G-CO (5.15 eV) < G-OH1 (5.23 eV) < G (5.45 eV) < G-COOH (5.55 eV). It can be concluded that the introduction of carbonyl and hydroxyl groups greatly reduces the ionization energy of graphene slabs in comparison to carboxyl groups. Interestingly, hydroxyl groups and carbonyl groups are the dominant oxygen species within the P-CNTs matrix, which are responsible for the excellent performance of PCNTs towards the I3- reduction. Moreover, we also extend this system, of which the different oxygen species were integrated on one graphene slab with 96 carbon atoms according to the atomic ratio of C/O as well as hydroxyl, carbonyl and carboxyl groups derived from XPS spectra. The detailed model and Ei values are also shown in Figure 6 (f, g) and Table 3. The oxygen species are capable of decreasing the Ei value from 4.65 eV for G-96 to 4.47 eV for G-


17


Page 18 of 28

96-O indeed. These combined results indicate that the chemical doped oxygen species within CNTs matrix can improve the IRR activity.

Figure 6. The optimized structure of (a) G, (b) G-OH1, (c) G-OH2, (d) G-COOH, (e) G-CO, (f) G-96 and (g) G-96-O at B3LYP/6-31G(d, p) calculation level. The gray, white and red balls stand for C, H and O atoms, respectively.


18

Page 19 of 28 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


Table 3. The ionization energies (Ei) for seven kinds of simulated graphene slabs*. Species

G

Ei (eV)

5.45

*OH1

G-OH1 G-OH2 5.23

4.80

G-COOH

G-CO

G-96

G-96-O

5.55

5.15

4.65

4.47

located at the edge of the slab; OH2 located inside the slab.

The electrochemical stability is another important factor that needs to be considered to evaluate the performance of CEs. For such a study, freshly assembled dummy cells were subjected to the repeated EIS measurement under ambient conditions. The detailed results are shown in Figure 7. Obviously, the Rs and ZN for P-CNTs dummy cells remain almost unchanged after ten cyclic measurements, suggesting the well-stabilized fast charge and mass transport. More importantly, compared with Pt, the Rct reflecting electrocatalytic activities of P-CNTs dummy cells keeps a slight increase and little changes after 10 cycles, indicative of the excellent electrochemical stability of P-CNTs.


19


Page 20 of 28

Figure 7. Electrochemical stability of symmetrical dummy cells with (a) P-CNTs and (b) Pt CEs in acetonitrile solution of I3−/ I−, of which the corresponding magnified Nyquist plots and equivalent circuits are shown in the inset. The cells were first subjected to CV scanning (from 0 V →1 V→-1 V→0 V at a scan rate of 50 mV s-1), and 30 s relaxation at 0 V was followed, then EIS measurement was performed. The test was repeated for ten times. (c) Rct changes of P-CNTs and Pt CEs versus the EIS scan number. CONCLUSIONS In summary, we present an activated CNTs matrix with abundant active edge sites and oxygen species through oxygen plasma-engraving CNTs, thus simultaneously achieving the activation of the sidewalls and the opened ends of CNTs. These increased open ends and edge sites mainly originate from the plasma-treated CNTs, while the activated sidewalls derive from the introduced


20

Page 21 of 28 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


O species with minimal variation in conjugate length. Benefiting from these superiorities, the asmade P-CNTs shows a higher PCE of 8.35%, lower Rct and better electrochemical stability than Pt CE. The DFT calculation further reveals that the introduction of hydroxyl groups and carbonyl groups greatly reduces the ionization energy for determining-rate step of triiodide reduction reaction. This will finally lead to the fast electron transfer from external circuit to the I3-, thus achieving an excellent performance. This work sheds a new light on simultaneous activating the basal and edge planes for carbon materials, as well as positive effects of oxygen species on carbon surface in carbon-related energy and catalysis systems.

ACKNOWLEDGEMENTS This work was partly supported by the NSFC of China (Nos. 21522601, U1508201) and the National Key Research Development Program of China (2016YFB0101201).

SUPPORTING INFORMATION Supporting information associated with this article has been provided. AUTHOR INFORMANTION Corresponding Author *Email: [email protected] (C. Yu); [email protected] (J. Qiu)


21


Page 22 of 28

NOTES The authors declare no competing financial interest. REFERENCES (1) Oregan, B.; Grätzel, M. A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 films. Nature 1991, 353 (6346), 737-740, DOI 10.1038/353737a0. (2) Boschloo, G.; Hagfeldt, A. Characteristics of the iodide/triiodide redox mediator in dyesensitized solar cells. Acc. Chem. Res. 2009, 42 (11), 1819-1826, DOI 10.1021/ar900138m. (3) Hagfeldt, A.; Boschloo, G.; Sun, L. C.; Kloo, L.; Pettersson, H. Dye-sensitized solar cells. Chem. Rev. 2010, 110 (11), 6595-6663, DOI 10.1021/cr900356p. (4) Meng, X.; Yu, C.; Lu, B.; Yang, J.; Qiu, J. Dual integration system endowing twodimensional titanium disulfide with enhanced triiodide reduction performance in dye-sensitized solar cells. Nano Energy 2016, 22, 59-69, DOI 10.1016/j.nanoen.2016.02.010. (5) Lee, W.; Ramasamy, E.; Lee, D.; Song, J. Efficient dye-sensitized solar cells with catalytic multiwall carbon nanotube counter electrodes. ACS Appl. Mater. Interfaces 2009, 1 (6), 11451149, DOI 10.1021/am800249k. (6) Chang, J.; Rhee, J.; Im, S.; Lee, Y.; Kim, H.; Seok, S.; Nazeeruddin, M.; Grätzel, M. Highperformance nanostructured inorganic-organic heterojunction solar cells. Nano Lett. 2010, 10 (7), 2609-2612, DOI 10.1021/nl101322h. (7) Das, S.; Sudhagar, P.; Verma, V.; Song, D.; Ito, E.; Lee, S. Y.; Kang, Y. S.; Choi, W. Amplifying charge-transfer characteristics of graphene for triiodide reduction in dye-sensitized solar cells. Adv. Funct. Mater. 2011, 21 (19), 3729-3736, DOI 10.1002/adfm.201101191.


22

Page 23 of 28 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


(8) Yu, C.; Meng, X.; Song, X.; Liang, S.; Dong, Q.; Wang, G.; Hao, C.; Yang, X.; Ma, T.; Ajayan, P. M.; Qiu, J. Graphene-mediated highly-dispersed MoS2 nanosheets with enhanced triiodide reduction activity for dye-sensitized solar cells. Carbon 2016, 100, 474-483, DOI 10.1016/j.carbon.2016.01.042. (9) Xin, X.; He, M.; Han, W.; Jung, J.; Lin, Z. Low-cost copper zinc tin sulfide counter electrodes for high-efficiency dye-sensitized solar cells. Angew. Chem. Int. Ed. 2011, 50 (49), 11739-11742, DOI 10.1002/anie.201104786. (10) Kim, D.; Koh, J.; Lee, C.; Kim, J. Mesh-shaped nanopatterning of Pt counter electrodes for dye-sensitized solar cells with enhanced light harvesting. Adv. Energy Mater. 2014, 4 (18), 1400414, DOI 10.1002/aenm.201400414. (11) Joshi, P.; Zhang, L.; Chen, Q.; Galipeau, D.; Fong, H.; Qiao, Q. Electrospun carbon nanofibers as low-cost counter electrode for dye-sensitized solar cells. ACS Appl. Mater. Interfaces 2010, 2 (12), 3572-3577, DOI 10.1021/am100742s. (12) Joseph D. Roy-Mayhew, D. J. B., Christian Punckt, and Ilhan A. Aksay. Functionalized graphene as a catalytic counter electrode in dye-sensitized solar cells. ACS Nano 2010, 4 (10), 6203–6211, DOI 10.1021/nn1016428. (13) Wang, X.; Zhi, L. J.; Mullen, K. Transparent, conductive graphene electrodes for dyesensitized solar cells. Nano Lett. 2008, 8 (1), 323-327, DOI 10.1021/nl072838r. (14) Meng, X.; Yu, C.; Song, X.; Liu, Y.; Liang, S.; Liu, Z.; Hao, C.; Qiu, J. Nitrogen-doped graphene nanoribbons with surface enriched active sites and enhanced performance for dyesensitized solar cells. Adv. Energy Mater. 2015, 5 (11), 1500180, DOI 10.1002/aenm.201500180.


23


Page 24 of 28

(15) Wang, G.; Fang, Y.; Lin, Y.; Xing, W.; Zhuo, S. Nitrogen-doped graphene as transparent counter electrode for efficient dye-sensitized solar cells. Mater. Res. Bull. 2012, 47 (12), 42524256, DOI 10.1016/j.materresbull.2012.09.023. (16) Fang, H.; Yu, C.; Ma, T.; Qiu, J. Boron-doped graphene as a high-efficiency counter electrode for dye-sensitized solar cells. Chem. Commun. 2014, 50 (25), 3328-3330, DOI 10.1039/c3cc48258h. (17) Zhang, C.; Mahmood, N.; Yin, H.; Liu, F.; Hou, Y. Synthesis of phosphorus-doped graphene and its multifunctional applications for oxygen reduction reaction and lithium ion batteries. Adv. Mater. 2013, 25 (35), 4932-4937, DOI 10.1002/adma.201301870. (18) Yang, W.; Ma, X.; Xu, X.; Li, Y.; Raj, S. I.; Ning, G.; Wang, A.; Chen, S. Sulfur-doped porous carbon as metal-free counter electrode for high-efficiency dye-sensitized solar cells. J. Power Sources 2015, 282, 228-234, DOI 10.1016/j.jpowsour.2015.02.060. (19) Chang Hyuck Choi, S. H. P., and Seong Ihl Woo. Binary and ternary doping of nitrogen, boron, and phosphorus into carbon for enhancing electrochemical oxygen reduction activity. ACS Nano 2012, 6, 7084-7091, DOI 10.1021/nn3021234. (20) Zhao, Y.; Yang, L.; Chen, S.; Wang, X.; Ma, Y.; Wu, Q.; Jiang, Y.; Qian, W.; Hu, Z. Can boron and nitrogen co-doping improve oxygen reduction reaction activity of carbon nanotubes? J. Am. Chem. Soc. 2013, 135 (4), 1201-1204, DOI 10.1021/ja310566z. (21) Chen, S.; Bi, J.; Zhao, Y.; Yang, L.; Zhang, C.; Ma, Y.; Wu, Q.; Wang, X.; Hu, Z. Nitrogendoped carbon nanocages as efficient metal-free electrocatalysts for oxygen reduction reaction. Adv. Mater. 2012, 24 (41), 5593-5597, DOI 10.1002/adma.201202424.


24

Page 25 of 28 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


(22) Yu, D.; Nagelli, E.; Du, F.; Dai, L. Metal-free carbon nanomaterials become more active than metal catalysts and last longer. J. Phys. Chem. Lett. 2010, 1 (14), 2165-2173, DOI 10.1021/jz100533t. (23) Xue, Y.; Liu, J.; Chen, H.; Wang, R.; Li, D.; Qu, J.; Dai, L. Nitrogen-doped graphene foams as metal-free counter electrodes in high-performance dye-sensitized solar cells. Angew. Chem. Int. Ed. 2012, 51 (48), 12124-12127, DOI 10.1002/anie.201207277. (24) Sreekanth, N.; Nazrulla, M. A.; Vineesh, T. V.; Sailaja, K.; Phani, K. L. Metal-free borondoped graphene for selective electroreduction of carbon dioxide to formic acid/formate. Chem. Commun. 2015, 51 (89), 16061-16064, DOI 10.1039/c5cc06051f. (25) Yu, C.; Fang, H.; Liu, Z.; Hu, H.; Meng, X.; Qiu, J. Chemically grafting graphene oxide to B,N co-doped graphene via ionic liquid and their superior performance for triiodide reduction. Nano Energy 2016, 25, 184-192, DOI 10.1016/j.nanoen.2016.04.039. (26) Zheng, Y.; Jiao, Y.; Ge, L.; Jaroniec, M.; Qiao, S. Z. Two-step boron and nitrogen doping in graphene for enhanced synergistic catalysis. Angew. Chem. Int. Ed. 2013, 52 (11), 3110-3116, DOI 10.1002/anie.201209548. (27) Yang, W.; Xu, X.; Hou, L.; Ma, X.; Yang, F.; Wang, Y.; Li, Y. Insight into the topological defects and dopants in metal-free holey graphene for triiodide reduction in dye-sensitized solar cells. J. Mater. Chem. A 2017, 5 (12), 5952-5960, DOI 10.1039/c7ta00278e. (28) Meng, X.; Yu, C.; Song, X.; Liu, Z.; Lu, B.; Hao, C.; Qiu, J. Rational design and fabrication of sulfur-doped porous graphene with enhanced performance as a counter electrode in dyesensitized solar cells. J. Mater. Chem. A 2017, 5 (5), 2280-2287, DOI 10.1039/c6ta09505d.


25


Page 26 of 28

(29) Chen, M.; Shao, L.-L.; Guo, Y.-X.; Cao, X.-Q. Nitrogen and phosphorus co-doped carbon nanosheets as efficient counter electrodes of dye-sensitized solar cells. Chem. Eng. J. 2016, 304, 303-312, DOI 10.1016/j.cej.2016.06.067. (30) Liu, Z.; Zhao, Z.; Wang, Y.; Dou, S.; Yan, D.; Liu, D.; Xia, Z.; Wang, S. In situ exfoliated, edge-rich, oxygen-functionalized graphene from carbon fibers for oxygen electrocatalysis. Adv. Mater. 2017, 29 (18), 1606207, DOI 10.1002/adma.201606207. (31) Becke, A. D. Density ‐ functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98 (7), 5648-5652, DOI 10.1063/1.464913. (32) Becke, A. D. Density functional calculations of molecular bond energies. J. Chem. Phys. 1986, 84 (8), 4524-4529, DOI 10.1063/1.450025. (33) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B 1988, 37 (2), 785-789, DOI 10.1103/PhysRevB.37.785 (34) Barone, V.; Cossi, M. Quantum calculation of molecular energies and energy gradients in solution by a conductor solvent model. J. Phys.Chem. A 1998, 102 (11), 1995-2001, DOI 10.1021/jp9716997. (35) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum mechanical continuum solvation models. Chem. Rev. 2005, 105 (8), 2999-3094, DOI 10.1021/cr9904009. (36) Yoon, S.-J.; Park, S. Polymorphic and mechanochromic luminescence modulation in the highly emissive dicyanodistyrylbenzene crystal: secondary bonding interaction in molecular stacking assembly. J. Mater. Chem. A 2011, 21 (23), 8338-8346, DOI 10.1039/c0jm03711g.


26

Page 27 of 28 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


(37) Wang, H.; Sun, K.; Tao, F.; Stacchiola, D. J.; Hu, Y. H. 3D honeycomb-like structured graphene and its high efficiency as a counter-electrode catalyst for dye-sensitized solar cells. Angew. Chem. Int. Ed. 2013, 52 (35), 9210-9214, DOI 10.1002/anie.201303497. (38) Wu, J.; Li, Q.; Fan, L.; Lan, Z.; Li, P.; Lin, J.; Hao, S. High-performance polypyrrole nanoparticles counter electrode for dye-sensitized solar cells. J. Power Sources 2008, 181 (1), 172-176, DOI 10.1016/j.jpowsour.2008.03.029. (39) Kavan, L.; Yum, J.-H.; Gräetzel, M. Graphene-based cathodes for liquid-junction dye sensitized solar cells: Electrocatalytic and mass transport effects. Electrochim. Acta 2014, 128, 349-359, DOI 10.1016/j.electacta.2013.08.112. (40) Ju, M. J.; Jeon, I. Y.; Kim, J. C.; Lim, K.; Choi, H. J.; Jung, S. M.; Choi, I. T.; Eom, Y. K.; Kwon, Y. J.; Ko, J.; Lee, J. J.; Kim, H. K.; Baek, J. B. Graphene nanoplatelets doped with N at its edges as metal-free cathodes for organic dye-sensitized solar cells. Adv. Mater. 2014, 26 (19), 3055-3062, DOI 10.1002/adma.201304986. (41) Meng, X.; Yu, C.; Song, X.; Iocozzia, J.; Hong, J.; Rager, M.; Jin, H.; Wang, S.; Huang, L.; Qiu, J.; Lin, Z. Scrutinizing Defects and Defect Density of Selenium-Doped Graphene for HighEfficiency Triiodide Reduction in Dye-Sensitized Solar Cells. Angew. Chem. Int. Ed. 2018, 57(17) 4782-4786, DOI 10.1002/anie.201801337. (42) Hou, Y.; Wang, D.; Yang, X. H.; Fang, W. Q.; Zhang, B.; Wang, H. F.; Lu, G. Z.; Hu, P.; Zhao, H. J.; Yang, H. G. Rational screening low-cost counter electrodes for dye-sensitized solar cells. Nat. Commun. 2013, 4, 1583, DOI 10.1038/ncomms2547.


27


Page 28 of 28

Table of Contents A trade off strategy between oxygen-containing species and conductivity for CNTs is presented by introducing O species with minimal conjugated variation in length. The positive effects of oxygen-containing species on triiodide reduction are decoupled and confirmed in terms of the experimental data together with theoretical analysis. The broken and activated CNTs feature the fully exposed active sites, delivering a high PCE up to 8.35% and low Rct towards triiodide reduction.


28

Theoretical and experimental insights into the effects of oxygen

Recommend Documents