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Deciphering the Electrocatalytic Activity of Nitrogen-Doped Carbon Embedded with Cobalt Nanoparticles and the Reaction Mechanism of Triiodide Reduction in Dye-Sensitized Solar Cells Sunghee Ahn, Chi Ho Lee, Min Soo Kim, Seul Ah Kim, Byungwuk Kang, Hee-eun Kim, Sang Uck Lee, and Jin Ho Bang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09758 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on December 2, 2017

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The Journal of Physical Chemistry C 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.

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The Journal of Physical Chemistry

Deciphering the Electrocatalytic Activity of Nitrogen-Doped Carbon Embedded with Cobalt Nanoparticles and the Reaction Mechanism of Triiodide Reduction in Dye-Sensitized Solar Cells Sung Hee Ahn,†,# Chi Ho Lee,†,# Min Soo Kim,† Seul Ah Kim,† Byungwuk Kang,† Hee-eun Kim,† Sang Uck Lee,†, ‡,* and Jin Ho Bang†, ‡,* Department of Bionano Technology and Department of Chemical and Molecular Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Republic of Korea AUTHOR INFORMATION Corresponding Authors: Jin Ho Bang, Sang Uck Lee *

Email: [email protected] (J.H.B), [email protected] (S.U.L)

†

Department of Bionano Technology, Hanyang University

‡

Department of Chemical and Molecular Engineering, Hanyang University

#

These authors contributed equally to this work.

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ABSTRACT: The electrocatalytic activity of carbon materials for triiodide (I3-) reduction has spurred the development of low-cost electrocatalysts as an alternative to platinum (Pt) in dye-sensitized solar cells (DSSCs). While many catalytic aspects of nitrogen-doped carbons have been unveiled in recent years, not all underlying factors that dictate their electrocatalytic activity have been fully considered; the current understanding of the electrocatalytic activity of nitrogen-doped carbons is limited. In addition, the synergistic effect of metal nanoparticles embedded in nitrogen-doped carbon, which was recently demonstrated as a facile way to boost the electrocatalytic activity of carbon, remains elusive. This work sheds light on these unknown aspects of carbon’s electrocatalytic activity by carrying out a systematic investigation of nitrogen-doped carbon with incorporated cobalt nanoparticles. Furthermore, the generally accepted mechanism of the I3- reduction reaction (IRR) is re-evaluated in this work with the aid of density functional theory (DFT) calculations and in-depth electrochemical analysis. A new insight into this mechanism, which suggests that there is another possible reaction pathway available for the IRR on carbon, is provided.


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1. INTRODUCTION Platinum has long held a unique position as the best electrocatalyst in a variety of energy conversion systems. Despite its exceptional electrocatalytic activity, the scarcity and high cost of Pt limit its wide use; these limitations have stimulated tremendous efforts to find alternative materials that are earth-abundant and have high electrocatalytic activity.1 For instance, a great number of materials have been explored to replace Pt in fuel cell-related areas.2 However, scientists have not confined their goal merely to reducing the cost. They have also attempted to develop materials that have improved kinetics for the oxygen reduction reaction, in which Pt is known to be sluggish. The goal of obtaining low-cost, highly efficient electrocatalysts has also recently been extended to other energy applications. Efforts to replace Pt in DSSCs are a good example of this. Pt seems to be indispensable due to its excellent electrocatalytic activity for the reduction of I3- that is a vital component of the most common electrolytes. Along with cost-related issues, however, the gradual deterioration in its electrocatalytic activity by corrosion imposes serious limitations on the wide use of Pt.3 To address these issues, a variety of alternative electrocatalysts have been explored, including metal chalcogenides, oxides, nitrides, carbides, conducting polymers, and carbons.4-9 From a practical point of view, carbons are likely the best candidate to replace Pt due to their good chemical stability, high electrical conductivity, low cost for mass production, and feasibility in large-scale fabrication.10-12 Despite their potential, further improvement in the IRR activity of carbons is still desired before they can replace Pt; to accomplish this, it is of great importance to establish a new design principle for highly efficient carbon-based electrocatalysts. To this end, it is necessary to understand the effects of all physical



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parameters that influence carbon’s IRR activity and to elucidate the IRR mechanism on carbon-based electrocatalysts. As demonstrated by several reports that have investigated the origin of carbon’s IRR activity, some of which have been recently discovered by the DSSC community, pristine carbon is known to be catalytically inert; however, incorporating surface defects and/or heteroatoms (e.g., N, S, and B) into the carbon network renders the material catalytically active.13-22 While such insights are beneficial for establishing design principles, one must recognize that various physical parameters of carbon, including the nature and degree of doping, crystallinity, surface area, and microtextural structure, complexly govern its electrocatalytic activity. Therefore, seeking a comprehensive understanding about the influence of all of these parameters is

necessary.

In

addition,

interesting

observations

regarding

carbon-based

electrocatalysts have been made in recent years. For example, embedding a small amount of metal nanoparticles in carbon can boost its electrocatalytic activity.23-27 Unfortunately, the mechanism behind this synergistic effect remains unclear. Given that the doping strategies to render carbon materials IRR-active are now quite mature, developing this metal-incorporation approach can offer a new way to endow higher electrocatalytic activity to carbon. In this work, we prepared N-doped carbons embedded with cobalt nanoparticles (Co/N-Cs) and explored the effects of the structural characteristics of Co/N-Cs on their IRR activity. Our in-depth investigation reveal that all of the physical parameters of Co/N-Cs interact to make them active for the IRR. The underlying cause of the synergistic boost induced by the metal nanoparticles is also unveiled. Another important contribution of this work is that it provides new insight into the mechanism of the IRR on carbon electrocatalysts. To date, little is known about the mechanism on carbon, and even the mechanism of the


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IRR on Pt is disputed.28-33 A computational study in this work explored two feasible mechanisms, evaluated them in light of our electrochemical analysis, and provided a new guide to help understand the implication of these mechanisms.

2. EXPERIMENTAL SECTION 2.1 Synthesis of Co/N-C and N-C. Co/N-Cs were prepared by carbonizing zeolitic imidazolate framework-67 (ZIF-67) at various temperatures. To synthesize ZIF-67, cobalt nitrate hexahydrate (0.719 g) and 2-methylimidazole were dissolved in methanol at a molar ratio of 1:8:700 under stirring. After stirring for 20 min, the solution was left undisturbed for 1 h. The precipitate was then washed with methanol several times and dried in an oven at 70 °C (characterization results shown in Figures S1-2). The obtained violet powder was subsequently heated in a nitrogen atmosphere at a heating rate of 3°/min to each target carbonization temperature (600, 700, 800, and 900 °C), at which the tube furnace was held for 50 min to complete the carbonization. The resulting carbonized products will hereafter be referred to as Co/N-C_600, Co/N-C_700, Co/N-C_800, and Co/N-C_900, respectively. To remove Co nanoparticles from Co/N-C, Co/N-C_600 in a 5 M HNO3 solution was refluxed, rinsed with deionized water several times, dried in a vacuum oven, and heated in N2 at 700 °C for 50 min. This will be referred to as N-C_600-700. 2.2 Characterization. X-ray diffraction (XRD) patterns of ZIF-67 and Co/N-Cs were obtained using an X-ray diffractometer (Rigaku D/Max-2500/PC), and the crystallinities of carbon in Co/N-Cs were analyzed using Raman spectroscopy (Renishaw RM-1000). The morphologies and microstructures of Co/N-Cs were examined by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL2010F equipped with an energy dispersive X-ray spectroscopy (EDS) detector (Oxford INCA 30 mm ATW detector)). N2 adsorption/desorption isotherms were recorded using a surface



area analyzer (BELSORP-mini II, BEL Japan). The microtextural properties (i.e., the total pore volume (Vtotal) and the specific surface area (SBET)) of Co/N-Cs were derived from adsorption branches. The Brunauer–Emmett–Teller (BET) method was used to calculate SBET values, and the Barrett–Joyner–Halenda (BJH) model was employed to analyze the pore size distributions. The physical properties of doped N in Co/N-Cs were examined by X-ray photoelectron spectroscopy (XPS) using a PHI Versa Probe system equipped with a 100 W Al K alpha X-ray source. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed by an SDT-Q600 (TA Instruments) at a heating rate of 5°/min under nitrogen flow. 2.3 Fabrication of Co/N-C and Pt Counter Electrodes (CEs). Each Co/N-C powder was thoroughly blended with a binder (carboxymethyl cellulose) at a weight ratio of 16:1 with the aid of a small amount of deionized water. The resulting homogeneous Co/N-C pastes were applied to clean F-doped SnO2 (FTO) glass via the doctor-blade technique using a micrometer adjustable film applicator. Films were then dried for 12 h in a vacuum oven at 120 °C. All the films were examined under SEM to determine their thickness; they were found to be uniform and almost identical in thickness (~12.5 µm; Figure S3). Because the thickness of the carbon layer affects the electrocatalytic activity,13,34,35 this careful control in the thickness allowed us to exclude its influence on the IRR activity. For comparison, Pt CEs were prepared by applying Platisol S/TP (Solaronix) onto FTO glass in the same way that the Co/N-C CEs were prepared and then by annealing the samples at 450 °C for 15 min. 2.4 Electrochemical Measurements and Evaluation of Solar Cell Performance. Symmetric dummy cells made of two identical CEs with an iodide-based electrolyte (Dyesol, EL-HPE) between them were employed, and the IRR activity of Co/N-C was investigated using electrochemical impedance spectroscopy (EIS) and Tafel polarization measurements. A


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potentiostat (CH Instruments, CHI 660D electrochemical workstation) was used to acquire EIS spectra that were measured in a frequency range from 0.1 Hz to 100 kHz with a perturbation amplitude of 10 mV. Tafel polarization curves were obtained at a scan rate of 50 mV/s. To examine the stability of Co/N-Cs, cyclic voltammetry (CV) was performed at a scan rate of 100 mV/s in an electrolyte containing 50 mM LiI, 10 mM I2, and 500 mM LiClO4 in acetonitrile. The IRR activity of Co/N-Cs was double-checked with full cells. DSSCs equipped with Co/N-C-based CEs were prepared as described in a previous report.36 Photoelectrodes with B2 (N719) dye (Dyesol)-sensitized 12-µm-thick TiO2 films (made by screen-printing Ti-Nanoxide T/SP (Solaronix) and DSL 18NR-AO (Dyesol) pastes onto FTO glass) were assembled into sandwich-type cells with the Co/N-C-based CEs and the EL-HPE electrolyte. The solar cell performance was evaluated under one sun illumination (air mass 1.5G) generated by a solar simulator (HAL-320, Asahi Spectra). A Keithley 2400 source meter was used to record the photocurrent-photovoltage (J-V) curves of each cell. 2.5 Theoretical Calculations. Ab initio calculations were performed by the Vienna Ab initio Simulation Package (VASP 5.4)37-40 using the projector augmented wave (PAW) method41,42 with the generalized gradient approximation (GGA) based on the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional.43,44 The Brillouin zone was sampled with a k-point grid of 2×2×1 according to the Monkhorst-Pack scheme.45 A plane-wave cutoff energy of 500 eV was employed to control the fineness of this mesh. The fully self-consistent nonlocal optB86b-vdW functional46 proposed by Dion et al.47 was used to study the adsorption of atomic and molecular iodine species on the carbon surface while considering van der Waals interactions. In our calculations, we modelled six graphene-based structures (Figure S11)— graphene (G), graphitic and pyridinic nitrogen-doped graphenes (gN-G and pN-G), and cobalt cluster-incorporated graphenes (G_Co12, gN-G_Co12, and pN-G_Co12)—to obtain a fundamental understanding of the synergistic effects of nitrogen doping and the incorporated ACS Paragon Plus Environment


Co nanoparticles on the IRR activity. To prevent the lattice mismatch between the graphene and Co nanoparticles at the interfacial contact, we employed a Co12 cluster model that was constructed from the most stable icosahedral Co13 cluster by removing the apex atom for interfacial connection.48,49 Only the carbon surface was considered for an IRR active site because the IRR activity of Co nanoparticles is substantially less than that of N-C, as previously noted.50 Additionally, most of the Co nanoparticles in our catalysts were embedded inside the carbon, such that the exposure of the Co surface to the electrolyte was very limited. Each model structure had a hexagonal unit cell repeated along the a and b directions with a vacuum region along the c direction up to 20 Å in order to exclude mirror interactions. The lattice vectors and ionic positions were optimized until the maximum atomic forces were less than 0.04 eV/Å. The overpotential (η) and iodine adsorption energy (∆Gads), which served as the measures of IRR activity, were determined from free energy diagrams (FEDs).

3. RESULTS AND DISCUSSION 3.1 Characterization of Co/N-Cs ZIF-67 was selected as a carbon precursor in this work because it possessed a high nitrogen content, allowing for more catalytic active sites and Co2+ in its building block; this Co2+ can be reduced to form Co nanoparticles during carbonization.51-55 ZIF-67 was carbonized at different temperatures to delicately manipulate the physical properties of Co/NCs, and the resulting Co/N-C samples were thoroughly analyzed by various characterization techniques in order to elucidate the assorted factors affecting IRR activity. We noticed that the investigation on the effect of embedded Co particles in the carbons derived from ZIF-67 had recently been reported,27 but this work failed to provide detailed characterization of these catalysts and the influence of all physical parameters on the IRR activity. Therefore, a


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comprehensive understanding about the role of their physical characteristics in the IRR activity remains elusive. Our investigation sheds light on this unexplored aspect of Co/N-Cs. Figure 1 shows the XRD patterns and N2 physisorption isotherms of Co/N-Cs. An obvious feature in the XRD patterns (Figure 1A) was the diffraction peaks from Co metals, which appeared at 44, 51, and 75° (JCPDS 15-0806), along with a broad (002) peak of carbon at ~26°. As the temperature increased, the diffraction peaks from Co became sharper, implying the growth of Co particles with increased temperature. The increase in grain size was calculated using the Scherrer equation (D=0.89λ/(B·cos θ); here, D is the average size of crystallites, λ is the X-ray wavelength, B is the full width at half maximum of the diffraction peak, and θ is the Bragg angle). The grain sizes were calculated to be 8.4, 12.2, 28.6, and 43.2 nm, respectively. The microtextural properties of Co/N-Cs were examined by analyzing the N2 physisorption measurements (Figure 1B). At a quick glance, all of the isotherms featured with the type I/IV appeared similar, suggesting the presence of micro- and mesopores. However, the textual parameters of Co/N-Cs summarized in Table 1 were slightly different. According to the TGA/DSC analysis (Figure S4), carbon loss during the carbonization continued even after a substantial weight loss between 500 and 550 °C. Therefore, controlling the temperature enabled us to delicately manipulate the degree of carbon burn-off, resulting in subtle differences in SBET and Vtotal. Both parameters increased up to 700 °C but started to decrease at higher temperatures. The SBET values (m2/g) of Co/N-Cs were in the order of Co/N-C_700 (243.44), Co/N-C_800 (216.36), Co/N-C_600 (208.05), and Co/N-C_900 (206.76); however, the order of the Vtotal values (cm3/g) was slightly different: Co/N-C_700 (0.483) > Co/N-C_800 (0.443) > Co/N-C_900 (0.379) > Co/N-C_600 (0.337). This subtle change in these orders was associated with pore widening at the expense of surface area that occurred at elevated carbonization temperatures. It is worth noting that mesopores play a much more important role in the IRR activity than micropores, because micropores are too ACS Paragon Plus Environment


small for I3- to diffuse through; thus, they cannot efficiently serve for IRR active sites, as noted previously.13,56 This claim was also supported by our molecular dynamics simulation of a solvated I2 cluster modeled in acetonitrile (Figure S5). Here, the solvated I2 cluster was surrounded by 12 acetonitrile molecules that formed the first solvation shell, and it was ~2 nm in size. In this regard, the surface area and pore volumes of mesopores (Smeso and Vmeso), as opposed to the SBET and Vtotal values, would serve as better metrics in judging the effect of carbon’s microtexture on its IRR activity. As shown in Table 1, the overall trends in Smeso and Vmeso were the same as those in SBET and Vtotal because the mesopores were primarily responsible for the SBET and Vtotal values of Co/N-Cs. A

Co/N-C_600

Co (111)

Intensity (a.u.)

C (002)

Co (200)

Co (220)

Co/N-C_700

Co/N-C_800

Co/N-C_900

Co (JCPDS 15-0806)

20

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2-Theta (Degree) 350 Adsorption (Co/N-C_600) Desorption (Co/N-C_600) Adsorption (Co/N-C_700) Desorption (Co/N-C_700) Adsorption (Co/N-C_800) Desorption (Co/N-C_800) Adsorption (Co/N-C_900) Desorption (Co/N-C_900)

280 210 140 70 0

0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

Figure 1. (A) XRD patterns and (B) nitrogen adsorption/desorption isotherms of Co/N-Cs.


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Table 1. Microtextural Parameters of Co/N-Cs and N-C_600-700.

The fine structural characteristics of carbon can also determine the IRR activity of Co/N-Cs. Raman spectroscopy was carried out to examine the structural variation in the carbon network of Co/N-Cs. The Raman spectra displayed in Figure 2 show two characteristic peaks centered at 1350 and 1580 cm-1, which correspond to the D band (disordered carbon) and the G band (graphitic carbon), respectively. It is evident that the ratio of the peak intensities (ID/IG) decreased with temperature, indicating the improved graphitic nature in Co/N-C at higher temperatures; this resulted from the catalytic effect of the in situgenerated Co particles.57 However, such simple ratios often failed to provide an accurate quantitative picture of the structural variation in the carbon network, especially when the depth of the valley between the D and G bands substantially changes (as is the case here).58 To obtain more insight about the structure, the Raman spectra were deconvoluted into four peaks, as proposed by Shimodaira et al.58 Here, G1 and D1 represent the winding short basal planes with ordered bond angles, and G2 and D2 arise from sp2-cluster-like amorphous carbon with disordered bond angles. According to this report, the intensity ratio of G1 and G2 (IG1/IG2) is related to the relative content of order/disorder in carbons. The ratios of our samples are given in Table 2. Given the change in the ratios, we could draw two conclusions: (i) the structural disorder was most dominant in Co/N-C_600, and (ii) this disorder gradually



resolved with increasing temperature until 800 °C and abruptly vanished when the carbonization temperature increased to 900 °C. This implies that a decent amount of defects, which can potentially serve as IRR active sites, can reside in Co/N-C up to 800 °C; however, only a small portion of these defects can survive in Co/N-C_900 because of the improved structural ordering.

B

1000

1200

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Raw Data Background Fitting Line D1 D2 G1 G2

1400

Raman Shift

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1600

1800

(cm-1)


1000

(cm-1)

1200

1400

Raman Shift

1600

1800

(cm-1)

Figure 2. Raman spectra of (A) Co/N-C_600, (B) Co/N-C_700, (C) Co/N-C_800, and (D) Co/N-C_900.


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Table 2. Order/disorder ratio in carbons (IG1/IG2) determined by Raman analysis and XPS analysis of Co/N-Cs and N-C_600-700.

XPS analysis was performed to elucidate the nature of the N species in the Co/N-Cs. The survey spectra in Figure S6 show that the intensity of the N peak at ~400 eV gradually diminished with temperature, which was attributed to denitrogenation during the carbonization.59 The quantitatively determined N contents in the Co/N-Cs, which were 13.4, 9.1, 4.7, and 2.5 atomic wt%, respectively, were consistent with this observation. Figure 3 displays the XPS spectra of Co/N-Cs in the N 1s region, each of which was deconvoluted into three characteristic peaks: pyridinic (N-6), pyrrolic (N-5), and quaternary (N-Q) N species. All of the Co/N-C samples possessed three N species, but their relative portions in each spectrum were quite different. With increasing temperature, the N-5 peak disappeared rapidly, whereas the relative portions of N-Q and N-6 increased (Table 2). This trend was associated with the relative thermal stability of these N species: N-Q > N-6 >> N-5.60 While the doped N is typically regarded as an IRR active site, it has been reported that not all N species contribute equally to the IRR activity.15-17,61 However, the role of each N species is still disputed in the literature. For instance, a recent report by Qiu and coworkers used DFT



calculations to show that N-Q was the most critical N species for the IRR activity; N-5 and N-6 were less important.16 However, experiments by Zou and colleagues showed that both NQ and N-6 species played an important role in the IRR activity, while N-5 was rather inactive toward the IRR.15 Alternatively, a very recent report by Li and coworkers shared a computational study where the N-5 and N-6 species dictated the IRR activity of N-doped carbon.17 Because of these discrepancies, it was difficult to determine which N species would be primarily responsible for the IRR activity. For the sake of XPS analysis, we decided to rely on Zou’s argument, partly because their claim was experimentally proven by systematically controlling the content and nature of the doped N in the graphene (similar to our current work). Therefore, the amounts of N-Q and N-6 (and not the total N content) were be taken into account for further discussion. Nevertheless, it is important to remember that our assumption does not necessarily invalidate the other two theoretical predictions; therefore, more investigations must be performed to clarify this ambiguity. (Note that our argument goes along with Qiu’s explanation as well, because the order of N-Q content in our results is consistent with the solar cell performance as will be discussed. Li’s argument on the catalytically active nature of N-5 is controversial in that it was determined to be inactive in other two reports. We diagnosed this discrepancy and figured out that it may come from different ways of introducing nitrogen in carbon matrix. Similar to our work, the nitrogen incorporation was performed by a thermal treatment in Zou and Qiu’s works, during which the degree of surface defects was diminished. However, it was done by a nitrogen plasma treatment in Li’s work, which inevitably introduced many defect sites into their carbon. While further investigation is necessary, we speculate that the association of N-5 with these defects could render N-5 the catalytic activity that was not observed in other two reports.) As summarized in Table 2, the order of the amount of N-Q and N-6 in each Co/N-C remained the same as that of the total N content, despite the larger amount of N-5 species found in Co/N-


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C_600.

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Figure 3. XPS N 1s spectra of (A) Co/N-C_600, (B) Co/N-C_700, (C) Co/N-C_800, and (D) Co/N-C_900.

Figure 4 shows the electron micrographs of Co/N-Cs. While the ZIF-67 particles shrank to 200-300 nm in diameter, the original polyhedral shape was preserved, regardless of the carbonization temperature (Figures 4A-D). The TEM images in Figures 4E-H show the growth of Co particles, presumably by merging, which was consistent with the sharpened peaks in the XRD patterns. High-resolution TEM (HRTEM) images (Figures 4I-L) also reveal that the graphitic nature became more visible with temperature, which was in accordance with the Raman analysis. The EDS mapping analysis (Figure S7) shows that the



N and Co were fairly evenly distributed throughout the Co/N-C samples.

Figure 4. (A-D) SEM, (E-H) TEM, and (I-L) HRTEM images of Co/N-Cs (A, E, I: 600 °C; B, F, J: 700 °C; C, G, K: 800 °C; and D, H, L: 900 °C).

3.2 IRR activity of Co/N-Cs Tafel polarization measurements, where the relationship between the current (i) and the overpotential (η) is dictated by the Butler−Volmer equation, are commonly used to judge the IRR activity of electrocatalysts. The exchange current (i0) estimated from the extrapolated intercepts of the anodic and cathodic branches in the Tafel region serves as a good metric for the IRR activity because it is related to the charge transfer resistance (Rct), as expressed by the following equation:62


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=

(1)

where R is the gas constant, T is the absolute temperature, n is the number of electrons involved in the IRR, F is the Faraday constant, and Rct is the charge transfer resistance. Therefore, a higher i0 implies faster charge transfer kinetics for the IRR. The Tafel polarization curves (Figure 5A) revealed that the i0 value was highest for Co/N-C_700. The i0 of Co/N-C_800 was the second largest, followed by the polarization curves of Co/N-C_900 and Co/N-C_600. These were nearly overlapped, but the polarization curve of Co/N-C_900 lied very slightly higher than that of Co/N-C_600. Figure 5A also provides information about the diffusion kinetics at the electrode; the limiting current (Jlim) at the high overpotential region can be associated with the diffusion coefficient (D) of I3- according to the following equation:23,24

=

(2) 2

where l is the spacer thickness, and C is the concentration of I3-. It is apparent in Figure 5A that the Jlim values of Co/N-C_700 and Co/N-C_800 were higher than those of Co/N-C_600 and Co/N-C_900, implying that more effective diffusion could occur in Co/N-C_700 and Co/N-C_800. These observed trends coincided with the results from the EIS analysis (Figure 5B). The Nyquist plots were fitted into an equivalent circuit (shown in the inset in Figure 5B) to extract various EIS parameters, including the equivalent series resistance (Rs), which corresponded to the intercept of the real axis; Rct, which was represented by the following semi-circle; and the Nernst diffusion impedance (ZN), which appeared at the low-frequency region. These parameters are summarized in Table 3. The Rs values (~13 Ω) were nearly similar for all Co/N-Cs, implying that the resistances of the Co/N-C electrodes, including contact resistance with the FTO glass, were almost identical. The Rct values were in the order



of Co/N-C_700 (2.3 Ω) > Co/N-C _800 (4.9 Ω) > Co/N-C_900 (7.3 Ω) > Co/N-C_600 (7.8 Ω), which echoed the trend in the exchange current. The ZN values also reflected the trend in Jlim with the order of Co/N-C_700 (6.5 Ω) > Co/N-C _800 (6.7 Ω) > Co/N-C_900 (10.7 Ω) > Co/N-C_600 (11.7 Ω). The fact that the results from these two different experiments coincided, assured the validity of our evaluation results. When these four electrodes were utilized as the CEs in DSSCs, the DSSC performance exhibited the same trends as were observed in the electrochemical measurements (Figure 5C). The solar cell parameters, including the short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE), are summarized in Table 3. Note that multiple DSSCs were fabricated and tested to ensure the reliability of our results (Table S1). The PCE of the DSSC with Co/N-C_700 was as high as 7.84%, which was superior to the other devices. With Co/NC_800, the DSSC performance decreased to 7.69%, and the PCE became slightly lower when Co/N-C_900 and Co/N-C_600 were used for the CEs. Compared to Pt, Co/N-C_700 exhibited better IRR activity (Figure 5D-E) and yielded a slightly higher PCE than Pt (7.40%) in the full-cell operation (Figure 5F). These results again highlight the potential usefulness of carbon as an alternative to Pt. In addition to the good IRR activity, maintaining the initial IRR activity over time is an important requirement of electrocatalysts; Pt is known to undergo serious catalytic degradation in iodide-based electrolytes.3 The stability test by the CV measurements (Figure S8) revealed that the catalytic deactivation was much less pronounced in Co/N-C_700 than in Pt during continuous potential sweeping, demonstrating the chemical robustness of Co/N-C_700 against the corrosion. This is an important merit of this carbonbased electrocatalyst (in addition to reducing the cost).


Page 18 of 37

0.6

C

15 ZN

2Rct

12

Co/N-C_600 Co/N-C_700 Co/N-C_800 Co/N-C_900

Rs

0 -0.6 Co/N-C_600 Co/N-C_700 Co/N-C_800 Co/N-C_900

-0.1

½ CPE

6 3

0

0.1

0 10

0.2

Potential (V)

20

25

30

35

40

45

Z' (Ω)

E

1.5

15

F

20

15

- Z" (Ω)

Log i (Log mA)

Pt Co/N-C_700

0

-1.5 -0.6

5

Co/N-C_700 Pt

-0.3

10

0

0.3

0

0.6

0

10

20

30

Potential (V)

50

H

1.5

0

-1.5

I

20

10

5

Co/N-C_700 N-C_600-700

-0.3

0

12 9 6

Co/N-C_600 Co/N-C_700 Co/N-C_800 Co/N-C_900

3 0

0

0.2

0.3

0.6

0

Co/N-C_700 N-C_600-700

0

10

20

30

Potential (V)

40

0.4

0.6

0.8

0.6

0.8

0.6

0.8

18 15 12 9 6 Co/N-C_700 Pt

3 0

0

0.2

0.4

Voltage (V)

15

-3.0 -0.6

15

Z' (Ω)

- Z" (Ω)

G

40

18

Voltage (V) Current Density (mA/cm2)

-0.2

9

50

60

70

Current Density (mA/cm2)

-1.2 -1.8

D

B

1.2

- Z" (Ω)

Log i (Log mA)

A

Log i (Log mA)

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/cm2)

Page 19 of 37

18 15 12 9 6 3 0 0

Z' (Ω)

Co/N-C_700 N-C_600-700

0.2

0.4

Voltage (V)

Figure 5. (A) Tafel polarization curves and (B) Nyquist plots (the inset shows the equivalent circuit) of symmetrical dummy cells. (C) J-V curves of solar cells with different CEs made of Co/N-Cs. (D-F) Comparison between Pt and Co/N-C_700. (G-I) Comparison between Co/NC_700 and N-C_600-700.

Table 3. Solar cell performance parameters of DSSCs with different CEs. EIS parameters for the symmetric dummy cells made of Co/N-Cs, Pt, and N-C_600-700. CEs

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

Rs (Ω)

Rct (Ω)

ZN (Ω)

Co/N-C_600

15.62

0.75

0.63

7.38

12.9

7.8

11.7

Co/N-C_700

16.86

0.75

0.62

7.84

12.7

2.3

6.5

Co/N-C_800

15.77

0.74

0.66

7.69

13.5

4.9

6.7



Page 20 of 37

Co/N-C_900

15.40

0.76

0.63

7.37

13.6

7.3

10.7

Pt

15.23

0.74

0.66

7.40

18.2

13.9

6.8

N-C_600-700

15.20

0.71

0.61

6.60

10.8

23.6

8.3

To explore the role of the Co nanoparticles embedded in the Co/N-Cs, we intentionally removed them by refluxing Co/N-C_600 in a hot acidic solution. The resulting powder underwent a subsequent heat treatment at 700 °C to remove the excess surface functional groups that were created during the refluxing. The characterization results of the obtained product (Co/N-C_600-700) are provided in Figures S9-10 and Tables 1-2. It is noteworthy that selectively removing Co nanoparticles while maintaining the original physical properties of Co/N-C was a very challenging task because the acid treatment inevitably altered the porous structure and surface properties. The XRD analysis (Figure S9A) and electron micrographs (Figure S10) confirmed the removal of Co nanoparticles. However, N2 physisorption, Raman, and XPS analyses revealed that, after these treatments, a greater number of larger pores (i.e., higher Smeso and Vmeso) were developed, more defects were introduced, and the amount of N was reduced. Nevertheless, the amount of N in Co/NC_600-700 was comparable to that in Co/N-C_700 (albeit slightly less), and the increased number of mesopores and defects in Co/N-C_600-700 could be beneficial for its IRR activity. This speculation led us to a tentative conclusion that comparing Co/N-C_600-700 and Co/NC_700 could provide some information about the role of Co nanoparticles. The electrochemical investigation in Figures 5G-H shows that the IRR activity of Co/N-C_600700 was notably less than that of Co/N-C_700. This inferior electrocatalytic activity was also reflected in the substantially lower Jsc and Voc values in the full-cell evaluation results (Figure 5I). The lower Voc was attributed to the slower regeneration kinetics of I3- into I- on Co/N-


Page 21 of 37 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


C_600-700 because it was inversely related to the concentration of I3- in the electrolyte, as shown by the following equation:63

=

! ln ( (3) ! " #$%& '

where kB is the Boltzmann constant, T is the temperature, q is the elementary charge, η is the quantum yield of photo-generated electrons, ϕ0 is the incident photon flux, n0 is the electron density of the conduction band of TiO2 in the dark, ket is the rate constant for recombination, and [I3-] is the concentration of I3- in the electrolyte. Provided that the photoelectrodes are identical for both DSSCs, the Voc can be influenced by the IRR activity of the CE materials. Our electrochemical analyses proved that many physical properties of Co/N-Cs interacted with each other to dictate the IRR activity. From the viewpoint of surface defects and N doping (i.e., the number of active sites potentially available), one could expect Co/NC_600 to work the best. However, the limited access of I3- to these active sites, due to the small pores developed in Co/N-C_600, prevented it from fully harnessing its potent electrocatalytic ability; this was supported by the largest ZN and the lowest Jlim. Alternatively, the enhanced electrical conductivity in Co/N-C_900, due to improved carbon ordering, might be beneficial for the IRR activity; however, given the Rs values in Table 3, this turned out to be untrue. We attributed this incorrect prediction to the presence of well-dispersed Co nanoparticles throughout the carbon. These can compensate for the lower electrical conductivity of N-Cs prepared at lower temperatures. Rather, in Co/N-C_900, the low content of defects and N dopants and the relatively small Smeso and Vmeso (obtained at the expense of better crystallinity) led to the lower IRR activity. Therefore, it was evident that a compromise must be made between these physical characteristics because they were delicately controlled in different manners depending on the carbonization temperatures. When considering all of



these parameters comprehensively, one can speculate that Co/N-C_700 and Co/N-C_800 could work better than the other two materials, which was indeed supported by our results. The slightly superior activity of Co/N-C_700 compared to Co/N-C_800 was ascribed primarily to the larger amount of active sites available. The embedded Co nanoparticles might serve as active sites, but their IRR activity was known to be inferior to that of carbon;50 hence their contribution, if any, would be negligible. Given the poor IRR activity of Co itself, its effect on IRR activity was believed to stem from the altered electronic structure of carbon in the vicinity of Co nanoparticles along with improved electrical conductivity. This possibility was indeed affirmed by the following DFT calculation results. 3.3 Mechanism of the IRR on Co/N-C Despite several recent reports, a comprehensive understanding of the IRR mechanism on carbon-based electrocatalysts is still lacking. An important issue with recent articles that have discussed this mechanism is the assumption that the IRR pathways are based on the following elementary reactions:16,17,64,65 I3-(sol) ↔ I2(sol) + I-(sol)

(4)

I2(sol) + 2* → 2I*

(5)

I* + e- → I-(sol)

(6)

where (sol) is the acetonitrile solution, and * represents the catalytically active sites. These pathways were adapted from the IRR mechanism proposed by Hauch and Georg,30 which has generally been considered the most plausible IRR mechanism on Pt. However, given the ongoing disagreement about the mechanism of the I-/I3- redox reaction (I%& + 2e& ↔ 3I& ) on Pt,28-33,66 this assumption might not serve as a solid basis for theoretical discussions of the IRR mechanism on carbon. Additionally, the IRR on carbon might proceed differently than it


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does on Pt, which has never been proven experimentally. In this regard, it is necessary to evaluate the validity of this unproven general consensus by exploring other possible reaction pathways.

Figure 6. (A) Anodic Volmer-Heyrovsky oxidation of I- and the IRR mechanism based on the reverse Volmer-Heyrovsky mechanism and (B) anodic Volmer-Tafel oxidation of I- and the IRR mechanism based on the reverse Volmer-Tafel mechanism. * represents the active site on the catalyst’s surface. Free energy diagrams of (C) the consecutive IRR mechanism and (D) the concerted IRR mechanism (a) at zero potential (U=0 V), (b) at the equilibrium reduction potential of I2 (U=0.54 V), and (c) at overpotential conditions (U=0.54+η V, only for the consecutive mechanism).

Two representative mechanisms have been used for the I-/I3- redox reaction, each of which can be divided into several elementary reactions. As shown in Figures 6A and 6B, in the typical anodic oxidation of I-, the Volmer reaction is followed either by the Heyrovsky reaction (V-H mechanism) or by the Tafel reaction (V-T mechanism). Therefore, reverse



reactions of this anodic oxidation can be considered as IRR pathways; these will be denoted as the consecutive IRR mechanism and the concerted IRR mechanism, respectively. In our DFT calculations, these two mechanisms were examined as possible reaction pathways on our Co/N-C catalysts. It is noteworthy that we focused only on the reduction reaction of I2 in our computational study instead of dealing with the reduction of I3-, because it was widely accepted that I3- and I2 are in equilibrium, as described in reaction (4). As depicted in Figure 6A-B, a single-electron reduction reaction took place in each elementary reaction in the consecutive IRR mechanism. Alternatively, in the concerted IRR mechanism, no electron transfer reaction was involved in the dissociative adsorption step, and a two-electron reduction reaction followed in the subsequent I- desorption step. Given this difference in the electron transfer, one should use different measures to evaluate the IRR activity in the consecutive and concerted IRR mechanisms; these measures were the overpotential (η) and the iodine adsorption energy (∆Gads),64 respectively. The η and ∆Gads values represented the relative stability of the intermediate state (. ∗ )

as compared to the initial state (.0 + ∗ + 01& ) and the final state (0. & + ∗). These descriptors could be calculated by applying standard DFT in combination with the computational standard hydrogen electrode model (Figure 6C-D). Because the equilibrium reduction potential of I2 was 0.54 V, the chemical potential difference between the initial and final states of this two-electron reaction should be 1.08 V (part (a) of Figure 6C-D). At the equilibrium reduction potential (U=0.54 V), the chemical potential of the intermediate state should ideally be the same as those of the initial and the final states in both consecutive and concerted IRR mechanisms (part (b) of Figure 6C-D). However, the real catalytic behavior deviated from the ideal case due to various binding energies associated with the intermediate, which appeared as an energy barrier (Eb). As a result, a catalyst required extra energy to


Page 24 of 37

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overcome the Eb; this could be expressed as η in the consecutive IRR mechanism (Figure 6C, part (b)). To complete the overall downhill reaction, a rigid shifting approach of the free energy states67 was applied depending on the number of electrons participating in the reaction (Figure 6C, part (c)). In the concerted IRR mechanism, however, this rigid shifting approach could not be applied for the overall downhill reaction because the Eb was associated with the ∆Gads of the I2 dissociative adsorption step, which involved no electrochemical reaction (Figure 6D, part (a)). In this way, ∆Gads could simply serve as the measure of IRR activity in the concerted IRR mechanism (Figure 6D, part (b)). Consequently, the catalytic IRR activity can be evaluated by η and ∆Gads, as determined by the free energy diagrams (FEDs). From the thermodynamic viewpoint, low values of η and ∆Gads can be linked to high IRR activity in each mechanism.

Figure 7. Calculated overpotentials (η) on graphene-based catalysts with and without Co clusters in (A) the consecutive IRR mechanism. (B) The adsorption energies of I on graphene-based catalysts with and without Co clusters in the concerted IRR mechanism.

The calculated η and ∆Gads of the graphene-based model catalysts (Figure S11) are depicted in Figure 7 and summarized in Table S1. Detailed calculation results are shown in Figure S12. The computational results for both mechanisms (Figure 7) show that N-doped graphene models (gN-G and pN-G) work better for the IRR than pristine graphene (G), which



is consistent with previous theoretical studies.16,17,19 However, the overall trends in the predicted IRR activity (i.e., η and ∆Gads) of the Co-incorporated graphene models appeared to be different in each mechanism. In the consecutive IRR mechanism, η gradually decreased in the order of G < gN-G < pN-G, and the incorporation of Co clusters further reduced η while maintaining this trend (Figure 7A). Alternatively, in the concerted IRR mechanism, the presence of Co clusters reduced ∆Gads for all of the model catalysts; however, the calculated ∆Gads of G_Co12 was far beyond the optimal range, given the poor IRR activity of pristine graphene (Figure 7B). Therefore, the prediction of the IRR activity based on the consecutive IRR mechanism matched our experimental results better than the prediction of the concerted IRR mechanism. This suggests that the IRR on carbon-based catalysts is more likely to follow the consecutive mechanism, which has never been considered for the IRR pathways of carbon-based electrocatalysts. The improved IRR activities obtained by the N-doping and the presence of embedded Co clusters can be attributed to electronic effects. The band structure analysis shown in Figure S13 indicates that the N-doping resulted in a higher density of states at the Fermi level for the N-doped graphenes (gN-G and pN-G) compared to the pristine graphene (G); hence, doping can facilitate electron transfer to I2. The total charge distribution on the carbon surface obtained by the Bader charge analysis68 (Figure S14) revealed that the incorporated cobalt clusters could provide more electron-enriched active sites on the carbon surface via electron transfer from cobalt clusters to nearby carbon atoms.69,70 On the basis of these theoretical insights, we concluded that both doped N and incorporated cobalt clusters synergistically improved the IRR activity of graphene by creating more electron-enriched active sites on the carbon surface.

4. CONCLUSION Carbon can be used as a low-cost electrocatalyst and has the potential to replace


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noble metal catalysts in various electrochemical reactions due to its good catalytic activity and unique physicochemical properties. In DSSCs, the catalytic activity of carbons for the IRR has been extensively explored in an attempt to find a Pt substitute in counter electrodes. Since pristine carbon is almost inactive for the IRR, a variety of heteroatom-doped carbons have been designed, and the origin of their electrocatalytic activity has been investigated. Despite recent advances, the comprehensive understanding about the underlying factors governing the IRR activity of N-doped carbons is still lacking, and little is known about the IRR mechanism on carbon-based electrocatalysts. In this report, we prepared N-doped carbons with incorporated cobalt nanoparticles and examined all possible factors that can affect the IRR activity in an attempt to address these unresolved issues. Our investigation revealed that the nature and degree of doping, crystallinity, surface area, and microtextural structure of N-doped carbon worked together to govern the IRR activity. A compromise must be made between these characteristics because they were delicately controlled in different manners depending on the synthesis conditions. The synergistic effect of the embedded Co nanoparticles on the IRR activity was attributed to the electronic effect (i.e., electron enrichment via electron transfer from Co to nearby carbon) according to our DFT calculations. In particular, the discrepancies in the role of doped N in previous computational studies and experimental results led us to explore other possibilities for the commonly used IRR mechanism on carbon; these have generally been accepted despite the fact that they have never been verified. Two mechanisms—the consecutive IRR mechanism and the concerted IRR mechanism—were considered in our computational study, and their validity was judged by our experimental results. While both mechanisms can explain the overall IRR activity of N-doped carbon well, the failure of the concerted IRR mechanism in predicting the effect of Co suggests that the consecutive IRR mechanism is more plausible than the concerted IRR mechanism. However, it is worth mentioning that this tentative conclusion does not



Page 28 of 37

necessarily deny the validity of the concerted IRR mechanism. Rather, this work provides a new insight into the longstanding disagreements regarding the IRR mechanism and calls more attention to this unsolved issue to establish a firm design principle for highly efficient carbon-based electrocatalysts. We speculate that spectroscopic analysis combined with electrochemical investigation on well-defined carbon films that are more amendable to theoretical treatment could provide more insight into this challenging problem.

ASSOCIATED CONTENT Supporting

Information.

Additional

characterizations

of

ZIF-67

and

Co/N-Cs,

electrochemical analysis results, characterization results of Co/N-C_600-700, and detailed DFT calculation results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *(J.H.B) E-mail: [email protected] *(S.U.L) E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS. This research was supported by grants from the Basic Science Research Program (NRF-2016R1A1A1A05005038, NRF-2015R1C1A1A02036670, and 2008-0061891) and the Creative Materials Discovery Program on Creative Multilevel Research Center (2015M3D1A1068062) through the National Research Foundation of Korea


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(NRF) funded by the Ministry of Science, ICT, and Future Planning. This work was also supported by the Supercomputing Center/Korea Institute of Science and Technology Information with supercomputing resources, including technical support (KSC-2017-C30032).

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