Dual sites of CoO nanoparticles and Co-Nx embedded within coal

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Dual sites of CoO nanoparticles and Co-Nx embedded within coal-based support toward advanced triiodide reduction Hongyu Jing, Yantao Shi, Xuedan Song, Suxia Liang, Danyang Wu, Yonglin An, and Ce Hao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00938 • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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ACS Sustainable Chemistry & Engineering

Dual sites of CoO nanoparticles and Co-Nx embedded within coal-based support toward advanced triiodide reduction

Hongyu Jing, Yantao Shi*, Xuedan Song, Suxia Liang, Danyang Wu, Yonglin An, Ce Hao State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology, 2# Linggong Road, Dalian 116024, Liaoning, China

*Y. Shi. Email address: [email protected]

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ABSTRACT Herein, coal-based composites embedded with CoO nanoparticles and Co-Nx dual sites are constructed and used for triiodide reduction in a dye-sensitized solar cell. Concentrated hydrochloric acid is firstly provided to pretreat coal so as to eliminate interference from other metal impurities. The ash-rich coal with addition of cobalt phthalocyanine and pyrolysed at 800 ℃ (denoted as Coal-10 wt% CoPc-800) as counter electrode shows a much enhanced activity for triiodide reduction reaction with the power conversion efficiency of 8.32%, which is better than that of Coal-800 counter electrode (7.83%), and also exceed that of Pt counterpart (8.02%), revealing the generated Co-Nx and CoO nanoparticles sites derived from CoPc can synergistically boost the electrochemical activity. In particular, a significantly enhanced electrochemical performance is achieved after introducing an additional 33 wt% TiO2 binder into the above mentioned Coal-10 wt% CoPc-800 (denoted as Coal-10 wt% CoPc-800-33 wt% TiO2). The binding strength between the obtained counter electrode material and the fluorine doped tin oxide glass substrate is improved and shows the highest power conversion efficiency of 8.82%, the lowest Epp (the energy of peak-to-peak separation) of 0.16 V as well as the lowest Rs (serial resistance) of 6.3 Ω, exceeded those of Pt and other coal-based counter electrodes. This work paves a facile and viable avenue for the tailoring of coal-derived high-efficient yet cost-effective catalysts promising for renewable energy conversion and storage technologies.

KEYWORDS: dye-sensitized solar cell, counter electrode, coal-based support, binding strength, dual sites synergistic catalysis


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INTRODUCTION Dye-sensitized solar cells (DSCs) are widely recognized as a promising photovoltaic technology due to their environmental friendliness, green and low fabricating cost.1 A DSC typically comprises a dye-sensitized TiO2 film on a fluorine doped tin oxide glass substrate (FTO) as a working electrode and a counter electrode (CE), bridged by an redox electrolyte.2 Triiodide reduction reaction (IRR) process occurring on the CE is critical and determines the overall electrochemical performance.3 Traditionally, Pt is recognized as the most preferred CE material for the IRR because of its excellent catalytic activity and electrical conductivity. Nevertheless, the high cost and reserves scarcity together with the susceptibility to the electrolyte significantly hamper its commercialization.4 Accordingly, to develop low cost, high performance, scalable, and environmental friendly electrocatalyst composed of earth-abundant elements is imperative. To date, considerable efforts have been devoted to the alternative CEs for DSCs, for instance, transition metal carbides/nitrides/oxides,5 metal-organic frameworks (MOFs) modified carbon materials,6 graphene-beaded carbon nanofibers with incorporated Ni nanoparticles (G/CNFs-Ni),7 Electrospun carbon nanofibers with surface-attached platinum nanoparticles (ECNFs-PtNPs),8 and carbon-supported single-atom catalysts such as M-N4 (M = Fe, Co, Ni, Cu, or Mn) materials.9 However, the exploration of stable single-atom catalyst remains a significant challenge due to the severe aggregation and migration of the metal atom as well as the low yield and complicated synthesis.10 Among various catalysts explored, the carbon-based materials containing M-Nx coordination sites hold great potential for Pt-based catalysts. Besides, it is well established that heteroatom doping, particularly nitrogen doping can induce charge redistribution, creating more catalytic active sites, thus being favorable for the reduction of triiodide.11,12 For strongly anchoring the metal atom, an effective approach may be to utilize M-Nx macrocycles (such as metal phthalocyanine or porphyrin and its derivatives), where the metal has been stabilized to nitrogen in advance to form definite M-Nx centers, which can offer high electrocatalytic activities



for many electrochemical applications.13,14 Nevertheless, the stability and catalytic activity are still unfavorable owing to their poor electrical conductivity.15 And there is a general agreement that an ideal substitute should exhibit superior electrical conductivity for charge transfer and electrocatalytic ability for the reduction of triiodide.16 With this knowledge, Yan and Guo et al. have functionalized of multi-walled carbon nanotubes or graphene oxide with iron phthalocyanine precursor by a chemical route.17,18 However, harsh conditions in the actual electrochemical reaction process need the definite M-Nx strongly immobilized by carbon support. Thus, a facile high-temperature carbonization strategy to obtain stable M-Nx sites supported on a carbon matrix is desirable.19,20 Besides, the calcined metal macrocycles structures are more active and stable than the pristine ones.21 Coal as an inexpensive, widely available energy resource has recently garnered much attention, it features abundant ash constituents (especially SiO2), which can effectively catalyze the formation of graphitized carbon and enhance electrical conductivity. Besides, the ash of SiO2 can also serve as a candidate for an active species.22 Recently, coal tar- or bituminous coal-derived carbon has been utilized as electrode materials for supercapacitors and lithium-iron batteries.23-25 Additionally, Nagai et al have extensively investigated N-doped porous carbon from different coal as cathode catalyst towards ORR for a fuel cell.26,27 However, as the richest carbon precursor, coal-based materials toward DSC applications are ignored. Inspired by the proofs-of-concept, we tailor ash-rich coal and cobalt phthalocyanine (CoPc) moieties to active site-rich hybrids as efficient electrocatalysts for DSCs via a combination of mechanochemically assisted synthesis and a direct one-pot carbonization procedure. During the high-temperature carbonization, CoPc precursor can transformed into definite Co-Nx coordination configurations together with graphene-encapsulated metallic Co cluster and CoO nanoparticles supported on the coal-based support. Specifically, in this work, raw coal is firstly pretreated with concentrated hydrochloric acid to eliminate interference from other metal impurities, such as Ca, Co, Sn. Noteworthy is that the ash of SiO2 is too stable to be etched out (silica can be removed by harsh post-treatments such as concentrated alkaline reagent at an elevated


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temperature or toxic HF solution, which will seriously hinder the scale-up protocols and inevitably leach out some active species). The ash of SiO2 can serve as a candidate for an active species thus there is no need to be removed as an internal additive. Afterward, CoPc is introduced to form Co-Nx sites as an additional synergistic catalytic active center. Note that Co can exist in the atomic scale, some finely dispersed species can agglomerate into small nanoparticles upon heat treatment. The combined results of XRD, HRTEM, and XPS demonstrate the coexistence of CoO nanoparticles and Co-Nx sites in the catalyst. The prepared Coal-10 wt% CoPc-800 CE shows a PCE of 8.32%, which is higher than that of Coal-800 CE (7.83%), and also comparable to the state-of-the-art Pt counterpart (8.02%), demonstrating that the generated Co-Nx and CoO nanoparticles sites derived from CoPc can synergistically boost the activity of IRR. The effect of TiO2 addition (20-50 wt%) on the electrochemical properties is further discussed and the binding strength between CE catalyst and FTO substrate is greatly improved. Impressively, a significantly enhanced electrochemical performance is achieved when an optimized proportion of TiO2 is added into the above slurry, the DSC based on Coal-10 wt% CoPc-800-33 wt% TiO2 CE delivers a satisfied PCE of 8.82% together with the lowest Epp (the energy of peak-to-peak separation) of 0.16 V and Rs (serial resistance) of 6.3 Ω in comparison with Pt and other coal-based CEs. It is envisioned that our facile and scalable approach could shed light on the design of cost-effective and highly efficient electrocatalysts that can be utilized in versatile electrochemical energy conversion and storage devices.

EXPERIMENTAL SECTIONS Material and chemicals. Hydrochloric acid was available from Tianjin Damao Chemical Reagent Factory, China. Cobalt (II) phthalocyanine (92%) was purchased from J&K Scientific LTD. All reagent and chemicals were used without further purification. Deionized water was available from our lab. One Chinese brown coal from Anshan mine was utilized. Synthesis of coal-derived catalysts.



The schematic diagram of the fabrication process of Coal-CoPc-800 composite is illustrated in Scheme 1. For a typical run, the raw brown coal was first crushed with a coffee machine and ground in a planet-wheel ball mill at a rotation speed of 500 rpm for 30 min, then was sieved to a powder in a diameter of fewer than 80 μm and was dried under vacuum at 120 ℃ for overnight. Thereafter, the amounts of 15.0 g of raw coal powder were added into 50 mL of concentrated hydrochloric acid (12 M) at 60 ℃ for 2 h to remove any incorporated metals. Note that the coal used afterward is acid treated. To explore the influence of CoPc content added into coal on the electrochemical performance, different CoPc contents (0.115 g, 0.5 g, 1.125 g) were mixed with 4.5 g of fine pretreated coal powders in a weight ratio of 2.5 wt%, 10 wt%, 20 wt%, respectively for 5 h via solid-state grinding process in a ball mill under an Ar atmosphere. Subsequently, the composites were subjected to carbonization at 800 ℃ for 2 h in a stream of N2. Then the pyrolysis product was immersed in ethanol for several hours and times (note that coal will release tar during heating, which will block the pores after solidification, so ethanol soaking is used to remove impurities). The precipitate was obtained through filtration and dried overnight at 120 ℃. The product obtained was defined as Coal-mCoPc-800 (m = 2.5 wt%, 10 wt%, 20 wt%). For comparison, Coal-800 was fabricated under similar conditions but without CoPc addition. CoPc-800 was also obtained without coal addition. The content of Co in Coal-10 wt% CoPc-800 was 6.18 wt% as measured by ICP-AES (inductively coupled plasma-atomic emission spectroscopy). The counter electrodes fabrication. The CEs were obtained via a spray coating technique as follows. Typically, an 80 mg Coal-10 wt% CoPc-800 sample was uniformly dispersed into 8 mL of isopropanol and milled with a ball mill (QM-QX04) for 6 h. The obtained slurry was sprayed onto the FTO glass (0.8 cm×10 cm, 14 Ω/□, Japan). Subsequently, the as-prepared CE was dried at 200 ℃ for 2 h. Specifically, 80 mg Coal-10 wt% CoPc-800 composite was first added to 8 mL of isopropanol and then homogeneously mixed with 20 mg, 40 mg, 80 mg of TiO2, respectively, which was denoted as Coal-10 wt% CoPc-800-nTiO2 (n = 20 wt%, 33 wt%, 50 wt%). All the films were cast onto FTO via a spray-coating


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method, and the average thickness of the CEs was about 2.00 μm, which can be controlled by tuning the slurry dispersion amounts, followed by annealing at 500 ℃ for 30 min under the N2 atmosphere, yielding different CEs. For preparing Pt CE, 8 mM H2PtCl6 (dissolved in isopropanol) was deposited onto the FTO substrate and calcinated at 400 ℃ for 0.5 h.

RESULTS AND DISCUSSION Morphology and Structures. Morphological structure of the as-made Coal-10 wt% CoPc-800 is examined by field emission scanning electron microscope (FESEM). As showed in Figure 1a-c, when treated by ball milling and carbonization process, the coal particles are accumulated with carbon blocks in the sub-micron order, and macroporous structures are observed owing to the presence of oxygenated species in coal and produced CO2, H2O, tars and other volatiles when subjected to high annealing temperature, which would act as the pore-forming agents, thus leading to porous structures. Noting that some generated hole defects generated during the thermal decomposition can anchor CoNx active sites.9 Taken together, such macroporous structures could act as ion-buffering reservoirs, guarantee the penetration of the liquid electrolyte by minimizing ion transport channels and contribute to improved electrochemical performance.28 Figure 1d shows the cross-section image of the optimum thickness of the CE on FTO substrate. The thickness of all CEs is similar because of the same concentration of CE slurry, which is determined to be ca. 2.00 μm on average. Element mapping and energy-dispersive X-ray (EDX) spectrum of the pretreated coal with HCl verifies the homogeneous distribution of C, N, O and Si elements (Figure S1). It should be pointed out that concentrated HCl has efficiently washed off the interference metals such as Co, Ca, and Sn. In addition, the elemental compositions of Coal-800 and Coal-10 wt% CoPc-800 were further revealed by element analysis and ICP-AES, the detailed contents of which are showed in Table 1. It is worth noting that the content of nitrogen element in Coal-10 wt% CoPc-800 (2.33 wt%) is higher than that in Coal-800 (0.78 wt%), proving that CoPc can contribute nitrogen to Coal-800, which is expected to cause electron modulation for desired electronic and geometric structure. In addition, there is a trace amount of Co (the content is around 0.1 wt%, which is decreased to negligible) in Coal-800. When



combined with CoPc, most of the Co content is originated from CoPc (up to 6.18 wt%). The introduction of CoPc serves as a key role in generating nitrogen species and promotes the formation of Co-Nx by inducing a strong interaction between metallic Co and nitrogen. Surface chemical compositions and valence states of Coal-10 wt% CoPc-800 are identified by X-ray photoelectron spectroscopy (XPS). The survey XPS displayed in Figure 2a shows five peaks of C, N, O, Si, and Co with atomic compositions of 80.05%, 3.41%, 10.38%, 3.71%, and 2.11%, respectively, indicating the successful incorporation of N and Co elements in Coal-10 wt% CoPc-800. As can be observed in Figure 2b, the high-resolution C 1s spectrum exhibits configurations of C-C / C=C at 284.6 eV, C-O / C-N at 285.6 eV and O=C-OH at 288.4 eV.29 Three peaks observed from the N 1s spectrum (Figure 2c) are assigned to pyridinic N (398.5 eV, which has been demonstrated as an efficient active site in the electrocatalytic process), quaternary N (400.9 eV) and N-oxide (406.0 eV).30 The chemical states of the O atoms are examined as well. The O 1s spectrum (Figure 2d) is deconvoluted into OL (530.4 eV, lattice oxide, probably associated with Si-O and surface CoO), OV (532.0 eV, defective oxide in vacancy), OC (533.7 eV, surface adsorption oxygen), further revealing the existence of cobalt-based oxides. Note that the oxygen-containing groups can also benefit the wettability of the electrode in aqueous electrolytes.31 While for the Si2p spectrum shown in Figure 2e, it can be verified the presence of organic Si (102.0 eV, which is important in both organic animal and plant life) and SiO2 (103.1 eV). Furthermore, the Co 2p spectrum (Figure 2f) displays two peaks corresponding to Co2p3/2 (780.7 eV) and Co2p1/2 (796.3 eV). The energy difference between Co 2p1/2 and Co 2p3/2 is 15.6 eV, demonstrating the presence of Co2+, which might be due to the oxidation of Co (II) species with O-rich coal during pyrolysis.32-34 The Coal, CoPc, Coal-CoPc and Coal-10 wt% CoPc-800 samples are analyzed by X-ray diffraction (XRD). For comparison, the XRD of Coal-800 and CoPc-800 is also recorded. From Figure 3a-c, it can be seen that SiO2 (PDF#65-0466) still exist in the treated coal-based support. From the XRD pattern of Coal-CoPc composite, it can be seen that the crystal structure of CoPc is partially destroyed after ball milling. Before 10°, the composite has some characteristic peaks of CoPc (indicated by a yellow rectangle). As illustrated in Figure 3d, the XRD pattern of CoPc-800 shows a broad peak at 26.0°, which can be assigned to C (002). The peaks at 44.2°, 51.7°, 75.8° can be indexed to the Co (111), Co (200) and Co (220) planes of CoPc-800.


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Notably, the crystallinity improved, which are consistent with the work by Sun et al. (Co JCPDS No. 15-0806).35 Furthermore, it also confirms that the original CoPc crystal structure has been damaged and the residues CoNx species may be incorporated in coal-based support through the ball milling and pyrolysis process. In addition, it is found that there is no difference between the two samples of Coal-800 and Coal-10 wt% CoPc-800, demonstrating that the addition of a low content of CoPc can not change the structure of the Coal-800. With Coal-800 as the standard reference, no Co crystal phase was detected in the Coal-10 wt% CoPc-800, implying that some amorphous or clusters or atomically dispersed Co compound might be embedded into carbon in disordered form and the peaks can be hardly detected by XRD technique. The microscopic morphology of Coal-10 wt% CoPc-800 and CoPc-800 is further determined by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). Several nanoparticles are clearly identified, which have been agglomerated in the coal-based support as showed in Figure 4a. It can be inferred that if the CoPc concentration is low enough, most of the Co atoms will react with nitrogen atoms to generate the Co-Nx species before aggregation. When the CoPc concentration increased, an excessive Co will aggregate into Co nanocrystals, which can catalyze the formation of graphitized carbon during the pyrolysis. In addition, part of the Co nanocrystals can be oxidized by oxygen species contained in coal to produce divalent cobalt oxides. HRTEM analysis (Figure 4d) further reveals that the metal nanoparticles have a lattice spacing of 0.21 nm, suggesting some of the cobalt atoms exists as the amorphous CoO phase.35 Corresponding fast fourier transform (FFT) patterns in Figure 4e indicate that Co is poorly crystallized as a whole. The identification of active sites of Co-Nx sites from the visible particles is elucidated through TEM and HRTEM of pure CoPc-800. From the whole TEM image shown in Figure 5a, no obvious cobalt nanoparticle aggregates are found. The HRTEM image shown in Figure 5b indicates that the CoPc-800 displays graphitized carbon walls with an interlayer distance of 0.34 nm, which is consistent with C (002) plane of the hexagonal graphite structure in XRD result, note that the walls also appeared in Figure 4d. The inset in Figure 5c shows that CoPc-800 also has amorphous features. Compared with the HRTEM image in Figure 5c, CoPc-800 exists monocrystalline cobalt (indicated by parallelogram array) with a fringe distance of 0.20 nm are found to be encapsulated with a few graphene layers (d-spacing = 0.34 nm) as showed in Figure 5d-e. Herein, high-density Co-Nx species or clusters may be formed in



CoPc-800 during the heat treatment process without the formation of visible Co nanoparticles. Furthermore, the lattice spacing of 0.20 nm and 0.21 nm shown in the HRTEM image (Figure 4b-e and Figure 5e) are corresponding to Co (111) and CoO (200), respectively. Note that CoO is not detected in the above XRD patterns, which may arise from the presence of the amorphous species observed from the HRTEM image. All the aforementioned results of XPS, XRD, and HRTEM confirmed the coexistence of the amorphous CoO domain and metallic Co crystallographic array on the coal-based support. Electrocatalytic activities and photovoltaic performance. The effects of the added amount of CoPc (2.5-20 wt%) and TiO2 (20-50 wt%) on the electrochemical properties of the coal-based CEs are studied. To evaluate the charge transfer process of the electrocatalysts, electrochemical impedance spectroscopy (EIS) is initially performed. Both original and fitted curves have been presented in all EIS figures, the dot plot represents the raw curve and the line plot represents the fitted curve. Figure 6a-b shows the Nyquist plots of EIS spectra of Pt and Coal-10 wt% CoPc-800 symmetric cells with a corresponding equivalent circuit. Apparently, the Pt dummy cell exhibits two typical semicircles, the high-frequency intercept represents Rs. The first semicircle in the high-frequency region corresponds to the charge transfer resistance (Rct) at the interface between CEs and electrolyte, which is an important parameter to evaluate the electrocatalytic activity of the CEs toward the I3- reduction. The other semicircle in the low-frequency area is attributed to the Nernst diffusion impedance (Zn) in the electrolyte of the redox couple. However, that is not the case for the Coal-10 wt% CoPc-800 CE, where I3- reduction occurs on the porous surface and Zpore occurs (Zpore: an additional distorted arc at the high frequency indicates the Nernst diffusion impedance resulting from diffusion through the electrode pores36,37), the relevant equivalent circuit of which is also different from that of the Pt electrode. Fitted EIS parameters extracted from the Nyquist plots of the dummy cells at the bias of 0 V are summarized in Table 2. As showed in Figure 6a-d and Table 2, Pt has a smallest Rs, which reflects a good binding strength on the substrate. In order to improve the catalytic performance of the coal-derived carbon, a trace amount of CoPc with a loading amount of 2.5 wt% to 20 wt% is introduced. The Coal-800 without CoPc addition exhibited a Rct of 20.1 Ω, the addition of CoPc to the coal from 2.5 to 10 wt% decreases Rct value from 17.8 Ω to 15.5 Ω. However, when a higher amount


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of 20 wt% of CoPc is added, the Rct value increases to 26.3 Ω. An excess content of CoPc into the precursor would result in the formation of a higher content of Co particles or Co oxides instead of Co-Nx moieties, which may block the real active sites thus resulting in a decrease of IRR activity. In addition, from Figure 6b-c, it can be found that the bulk structure of coal-based support (as indicated by the FESEM image from Figure 1c) has made itself to be in poor contact with the FTO substrate. Therefore, how to ameliorate the poor binding strength between coal-based CEs and the FTO substrate is of great significance. Studies have shown that the combination of TiO2 and FTO is relatively robust.38-40 To overcome the disadvantage of poor binding strength, TiO2 particle is added into the paste of Coal-10 wt% CoPc-800 to improve the binding strength. Therefore, effects of different TiO2 dosage on the catalytic performance of coal-based electrodes are further investigated. Since TiO2 is a semiconductor material, the addition of too many TiO2 results in a decrease of electrode conductivity. From Figure 6d and Table 2, it can be seen that the Rs decreases gradually with increasing TiO2 content from 0 to 33 wt%. When adding 33 wt% TiO2, the Rs reduces from 11.5 Ω to 5.5 Ω, which can be ascribed to an enhanced binding strength between the CE and the FTO substrate thus makes electrons transfer easily. It is noted that the Rs and the Rct of Coal-10 wt% CoPc-800-33 wt% TiO2 is smaller than that of other coal-based CEs, demonstrating that the binding strength outperforms the other samples, revealing a superior conductivity and electrocatalytic activity of the CE towards the I3- reduction due to the synergistic effect of CoPc and TiO2 addition. On the other hand, when added 50 wt% TiO2, the Rs and the Rct increase rapidly probably originating from the inferior conductivity of TiO2. Compared with other coal-based electrodes, Coal-10 wt% CoPc-800-33 wt% TiO2 has the smallest Rs, which demonstrates a better contact with the FTO. The Rct of Coal-10 wt% CoPc-800-33 wt% TiO2 electrode is 8.8 Ω, revealing a fast response towards the regeneration of I3- in the electrode/electrolyte interface. In addition, Zn is the Nernst diffusion impedance and represents the redox ions diffusion coefficient. Generally, larger Zn value is associated with porous CE with smaller-sized specific pores due to which the ionic diffusion could be constrained. From Table 2, it can be found that the Zn value of Coal-800 is 2.8 Ω, which is close to the value of Pt counterpart (2.5 Ω), indicating a good pore structure in Coal-derived sample. After pyrolysis of Coal and CoPc complexes, the Zn value increases with the



addition of CoPc, which is due to the fact that the Co nanoparticles will aggregate during the pyrolysis process and partially block the pores, resulting in a slower diffusion of electrolyte ions. However, When 33 wt% of TiO2 is added to the above composite, the value of Zn is further decreased, indicating the positive variation of pore structure. In addition, we conjecture that proper addition of TiO2 can strengthen the adhesion between CE and FTO and alleviate the undesired aggregation of carbon (favorable to enlarging the effective area), endowing the electrode with a smaller value of Rs and Rct. Moreover, Tafel curves of dummy cells are also recorded to confirm the electrocatalytic activities. The extrapolated intercept of the cathodic branch at the Tafel zone (intermediate potential) determines the exchange current density (J0). The intersection of the cathodic branch with the Y-axis dictates the limiting diffusion current density (Jlim). The indication for both J0 and Jlim are defined on one curve of Figure 7a. Meantime, the values of J0 and Jlim are calculated and listed in Table 2. As illustrated in Figure 7a, a proper amount of CoPc (10 wt%) added to ash-rich coal-derived carbon provides an improved J0. However, an excess amount of CoPc (20 wt%) in the precursor would form several Co and Co oxides particles, which may plug some of the real active sites and decrease the J0. It can be noted that the Coal-10 wt% CoPc-800 achieves a relatively larger J0 compared with other coal-based CEs. Such a larger J0 is responsible for smaller Rct, which is in line with the EIS results above. As showed in Figure 7b, Tafel curves exhibit a much larger Jlim for the Coal-10 wt% CoPc-800-33 wt% TiO2 electrodes, which can catalyze the reduction of I3- to I- effectively. In addition, the Coal-10 wt% CoPc-800-50 wt% TiO2 electrode possesses a little Jlim, indicating it cannot serve as good as other coal-based materials in catalyzing triiodide reduction. The variation trend of Jlim in Tafel plots is also coherent with EIS results. To gain further insight into the electrocatalytic activities of CEs based on these materials, cyclic voltammogram (CV) measurements are assessed. The left pair of redox peaks at a lower potential attributes to the redox reaction in Eq. (1) (the main reaction that responsible for the electrocatalytic performance of DSCs).41 3I2 + 2e- ⇄ 2I3-

(1)

I3- + 2e- ⇄ 3I-

(2)

Epp is a key parameter for judging catalytic activity and reversibility of CEs. Note that coal-based support can anchor a part of CoPc, but when the CoPc content increases,


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the nanoparticles not only present inside the carbon shell but also exposed on the outer surface, and the particles aggregated on the outer surface are easily corroded by the electrolyte thus leading to a decreased electrocatalytic activity (Figure 7c). As presented in Figure 7d, the CE based on Coal-10 wt% CoPc-800-33 wt% TiO2 exhibits an Epp value of 0.16 V, which is lower than that of 0.27 V achieved with a Pt electrode as well as those of other coal-based CEs, indicative of its excellent electrocatalytic activity for the IRR that arising from effective Co addition and better substrate adhesion of TiO2. However, when CoPc or TiO2 are increased to maximum, the Coal-20 wt% CoPc-800 (Epp = 0.47 V) and Coal-10 wt% CoPc-800-50 wt% TiO2 (Epp = 0.38 V) electrode in the CV curve indicate poor redox reversibility, which is in line with the above EIS and Tafel results. Encouraged by the obviously enhanced electrocatalytic activities, the DSCs with different CEs are assembled and tested. The photocurrent density-photovoltage (J-V) curves are displayed in Figure 8 with detailed photovoltaic parameters summarized in Table 3. To make comparative research, Pt-based DSC is also assembled, which exhibits a short-circuit current density (Jsc) of 14.73 mA cm-2, yielding a PCE of 8.02%. When the Coal-10 wt% CoPc-800 is applied as a CE of the DSC, a PCE of 8.32% is delivered with an improved Jsc value of 15.56 mA cm-2, which is higher than that of the DSC based on Coal-800 CE, and even comparable to that of Pt CE. More impressively, further improvement in PCE is achieved for the Coal-10 wt% CoPc-800-33 wt% TiO2 (8.82%) with a Jsc value of 16.06 mA cm-2, being superior to that of DSCs using Pt CE and other coal-based CEs, which is in agreement with the EIS results. It is worth noting that the relatively lower Rs and Rct from the above EIS result will decrease the loss of charge transfer and improve the charge collection efficiency on the interface, thereby being beneficial to deliver higher photovoltaic performance for DSCs.

CONCLUSIONS To summarize, an inexpensive synthesis pathway for the preparation of dual sites of CoO nanoparticles and Co-Nx doped coal-based matrix containing earth-abundant chemical elements (such as Si and O) is developed. With the proper combination of concentrated hydrochloric acid pretreated coal with optimized content of CoPc and TiO2 addition, an excellent electrochemical and photovoltaics properties are obtained.



Consequently, Coal-10 wt% CoPc-800 (Coal/CoPc = 10/1 in mass ratio) CE delivers much better photovoltaic performance (8.32%) than Coal-800 CE (7.83%). Further, the adding amount of TiO2 is optimized to enhance the binding strength between the coal-based CEs and the FTO substrate. The addition of TiO2 further increases the PCE to 8.82% for the Coal-10 wt% CoPc-800-33 wt% TiO2, which is superior to that of coal-based CEs without binder of TiO2 and Pt reference. The outstanding electrocatalytic performance is attributed to the good conductivity, abundant dopant species such as CoO nanoparticles and Co-Nx dual sites derived from CoPc and rich SiO2 in coal-based support merited by a combination of ball milling assisted synthesis and a direct one-pot carbonization procedure. The investigation provides a simple, sustainable, scalable and cost-effective method to produce a high value-added coal-based electrocatalyst, which will pave a promising path for not only low-cost photovoltaic device but also other renewable energy conversion and storage applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Element mapping and EDX spectra, TGA curve, and UV-vis spectra (PDF) AUTHOR INFORMATION Corresponding Author *Y. Shi. Email address: [email protected]

ORCID Yantao Shi: 0000-0002-7318-2963 Xuedan Song: 0000-0001-7531-4344 Ce Hao: 0000-0002-4379-0474


Page 14 of 31

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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors deeply appreciate financial assistance from the National Natural Science Foundation of China (Grant No. 51402036 and 51773025), the International Science & Technology Cooperation Program of China (Grant No. 2013DFA51000) and the Fundamental Research Funds for the Central Universities of China (Grant No. DUT18ZD208).

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Figures and Tables

Coal CoO par ticle

Si

O

Co

N

C

Ball milling

p or e

Coal-CoPc composite Pyrolysis

p or t coa l-b a sed su p

800 ℃ , N2 Scheme 1 Schematic illustration of the approach for the Coal-CoPc-800 composite.



(a)

(b)

(c)

(d)

2.00 μm

Figure 1. (a-c) FESEM images of Coal-10 wt% CoPc-800 at different magnifications, and (d) cross-section image of the optimum thickness of the CE on FTO substrate.


Page 20 of 31

284.60 C1s

Coal-10 wt% CoPc-800

532.0 O1s 398.6 N1s

102.1 Si2p 153.6 Si2s

(b)

C1s

780.4 Co2p

288.4 eV O=C-OH

285.6 eV C-O / C-N

N1s

398.5 eV 400.9 eV Quaternary N Pyridinic N

Raw Fitted

406.0 eV N-oxide

Element C N O Si Co at % 80.05 3.41 10.38 3.71 2.11

400

600

294

800

292

532.0 eV OV

Raw Fitted

530.4 eV OL 533.7 eV OC

(e)

288

286

Si2p

284

408

282

404

400

396

Binding Energy / eV

(f)

102.0 eV Organic Si

Raw Fitted

Intensity / a.u.

O1s

290

Binding Energy / eV

Binding Energy / eV

Co2p Raw Fitted

Intensity / a.u.

200

(d)

(c)

284.6 eV C-C / C=C

Raw Fitted

Intensity / a.u.

Intensity / a.u.

(a)

Intensity / a.u.

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


Intensity / a.u.

Page 21 of 31

103.1 eV SiO2

780.7 eV Co 2p 3/2

796.3 eV Co 2p 1/2

     eV   eV

536

534

532

530

Binding Energy / eV

528

108

106

104

102

100

98

Binding Energy / eV

800

796

792

788

784

780

776

Binding Energy / eV

Figure 2. (a) Survey XPS spectra, (b-f) fine spectra of the region in C1s, N1s, O1s, Si2p and Co2p for Coal-10 wt% CoPc-800, the table inset shows the element contents.



(a)

(b)

CoPc

Intensity / a.u.

Intensity / a.u.

Coal

10

20

30

40

50

60

70

10

80

20

(c)

(d)

Coal-CoPc PDF# 65-0466 Quartz low, syn SiO2

10

20

30

40

50

30

40

2  / deg

2  / deg

60

70

Coal-800 Coal-10 wt% CoPc-800 CoPc-800

Intensity / a.u.

Intensity / a.u.

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

Page 22 of 31

80

C(002)

10

20

Co(111) Co(200)

30

40

50

60

Co(220)

70

80

2  / deg

2  / deg

Figure 3. XRD patterns of (a) Coal, (b) CoPc, (c) Coal-CoPc, (d) Coal-800, Coal-10 wt% CoPc-800 and CoPc-800. Partial characteristic peaks of CoPc are marked with a yellow rectangle.


Page 23 of 31 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


(a)

50 nm

(e)

(d)

(3)

1

0.21 nm

2 (4)

(1)

3

0.21 nm 4

5

0.21 n m (2)

(5)

graphene

nm 0.21

5 nm

0.21 n m

Figure 4. (a) TEM image of Coal-10 wt% CoPc-800, (b) corresponding HRTEM image, (c) corresponding FFT pattern of the crystallite in (b) marked in a red rectangle. (d) HRTEM image at other areas, (e) corresponding FFT pattern of the crystallite in (d) marked in a red rectangle, (1-5) an enlarged image denoted by a yellow rectangle.

5 nm

5 nm



Page 24 of 31

(a) (a)

100 nm

(c)

(d)

(e) d=0.34 nm d=0.20 nm d=0.20 nm

5 nm

5 nm

Figure 5. (a) TEM image of CoPc-800, (b) corresponding HRTEM image, (1-3) an enlarged image denoted by a yellow rectangle, (c) HRTEM image of CoPc-800 at other area, the inset shows corresponding FFT pattern marked in a red rectangle, (d) HRTEM image of CoPc-800 at other areas, (e) corresponding FFT pattern in (d) marked in a red rectangle.


Page 25 of 31

(a) 10

Raw Fitted

Rs

4

Zn

8

10

12

14

Z' /ohm

16

18

Zn

CPE

10 R ct

Z pore

Zn

3 2 1

20

30

12

40

Z' /ohm

14

16

50

18

25

30

35

40

2

4

0 10

20

Coal-10 wt% CoPc-800-20 wt% TiO

40

5

10

15

Z' /ohm

(d) 50

Coal-800 Coal-2.5 wt% CoPc-800 Coal-10 wt% CoPc-800 Coal-20 wt% CoPc-800

20

0 10

20

-Z''/ohm

6

(c) 30

0 10

Z pore

5

2 0

R ct Rs

CPE

R ct


Raw Fitted

15 -Z''/ohm

-Z''/ohm

Zn

R ct

6

(b) 20

Pt

8

-Z''/ohm

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


20


2


2

30 20 10

60

0

0

20

40

60

Z' /ohm

80

100

Figure 6. Nyquist plots for (a and b) Pt and Coal-10 wt% CoPc-800 symmetric cell with corresponding equivalent circuit, (c) Coal-800-based symmetric cells with various amounts of CoPc biased at 0 V, inset shows an expansion of the high-frequency region, (d) Coal-10 wt% CoPc-800-based symmetric cells with different amounts of TiO2 binder at bias of 0 V. Note: the dot plot represents the raw curve and the line plot represents the fitted curve.


2 1

log J 0

0 -1

Coal-800 Coal-2.5 wt% CoPc-800 Coal-10 wt%CoPc-800 Coal-20 wt%CoPc-800

-2

-3 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

Voltage / V

1 0 Coal-800 Coal-2.5 wt% CoPc-800 Coal-10 wt% CoPc-800 Coal-20 wt% CoPc-800

-0.2 0.0

0.2

0.4

0.6

0 Coal-10 wt% CoPc-800-20 wt% TiO

-1

2


2

-2


2

Pt

-3 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

0.8

Potential vs Ag/Ag Cl /V

1.0

Current Density / mA cm -2

2

-1

1

Voltage / V

(d)

(c)

-2

Page 26 of 31

(b) 2

log J lim

log J / log (mA cm -2)

(a)


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

log J / log (mA cm -2)


Epp

2 1


0

2


2

-1


2

-2

Pt

-0.2 0.0

0.2

0.4

0.6

0.8

1.0

Potential vs Ag/Ag Cl /V

Figure 7. (a and b) Tafel polarization curves of the symmetric cells based on Coal-based symmetric cells and Pt counterpart using I3-/I- as redox couple with a spacer, (c and d) cyclic voltammograms of Coal-800-based CEs and Pt CE.


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 27 of 31

0

Coal-800 Coal-10 wt% CoPc-800

-5


2

Pt

-10 -15 0.0

0.2

0.4 0.6 Voltage / V

0.8

Figure 8. J-V curves of the DSCs using different Coal-800-based CEs and a Pt CE.



Table 1. Elemental compositions and ICP-AES results of Coal-800 and Coal-10 wt% CoPc-800. Testing means

Element (wt%)

Coal-800


Element analysis

C

60.37

68.67

N

0.18

2.33

H

0.56

0.57

Co

0.11

6.18

ICP-AES


Page 28 of 31

Page 29 of 31 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 2. Fitted EIS parameters extracted from the Nyquist plots at the bias of 0 V, Tafel parameters of the dummy cells and Epp value of different CEs obtained from cyclic voltammograms. CEs

Rs / Ω

Rct / Ω Zn / Ω

CPE / F

J0 / mA cm-2

Jlim / mA cm-2

Epp / V

Coal-800

12.0

20.1

2.8

6.6×10-4

2.4

32.5

0.31

Coal-2.5 wt% CoPc-800

11.5

17.8

6.6

1.1×10-4

3.3

32.4

0.26


11.6

15.5

9.4

2.4×10-4

4.1

56.4

0.27


12.8

26.3

15.1

2.0×10-4

3.3

46.9

0.47

10.3

9.5

20.6

3.2×10-5

5.9

62.0

0.26

5.5

8.8

9.1

2.1×10-5

6.0

93.8

0.16

35.2

31.5

15.7

5.2×10-5

1.4

20.6

0.38

7.8

5.5

2.5

1.7×10-5

6.8

29.7

0.27

Coal-10 wt% CoPc-80020 wt% TiO2 Coal-10 wt% CoPc-80033 wt% TiO2 Coal-10 wt% CoPc-80050 wt% TiO2 Pt



Page 30 of 31

Table 3. Photovoltaic parameters of the DSCs using different Coal-800-based CEs and a Pt CE. CEs

Voc / V

Jsc / mA cm-2

FF

PCE / %

Coal-800

0.78

14.65

0.69

7.83


0.78

15.56

0.69

8.32

Coal-10 wt% CoPc-80033 wt% TiO2

0.79

16.06

0.70

8.82

Pt

0.78

14.73

0.70

8.02


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GRAPHICAL

ABSTRACT

defect

Ienhanced IRR I 3-

CoO par ticle Co

Si

O

N

C

Schematic diagram of Coal-CoPc-800 hybrids with dual sites of CoO nanoparticles and Co-Nx sites embedded within ash-rich coal-based support used as enhanced IRR electrocatalyst for DSCs.


Dual sites of CoO nanoparticles and Co-Nx embedded within coal

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