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Ni-incorporated carbon materials derived from humic acid as efficient low-cost electrocatalysts for dye-sensitized solar cells
Journal Pre-proof Ni-incorporated carbon materials derived from humic acid as efficient low-cost electrocatalysts for dye-sensitized solar cells Yi Di, Suping Jia, Ning Li, Caihong Hao, Huinian Zhang, Shengliang Hu PII:
S1566-1199(19)30414-8
DOI:
https://doi.org/10.1016/j.orgel.2019.105395
Reference:
ORGELE 105395
To appear in:
Organic Electronics
Received Date: 12 June 2019 Revised Date:
19 July 2019
Accepted Date: 2 August 2019
Please cite this article as: Y. Di, S. Jia, N. Li, C. Hao, H. Zhang, S. Hu, Ni-incorporated carbon materials derived from humic acid as efficient low-cost electrocatalysts for dye-sensitized solar cells, Organic Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.105395. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Ni-incorporated carbon materials derived from humic acid as efficient low-cost electrocatalysts for dye-sensitized solar cells Yi Di1*, Suping Jia1, Ning Li1, Caihong Hao1, Huinian Zhang1, and Shengliang Hu1* 1.
School of Energy and Power Engineering, North University of China, Taiyuan
030051, PR China E-mail: [email protected]; [email protected]
Abstract The scale-up application of dye-sensitized solar cells (DSSCs) will desire the efficient and low-cost electrocatalytic counter electrodes. Carbon materials derived from the natural materials have been extensively used in the new energy fields owing to numerously distinct advantages. In this work, a new-style carbon matrix incorporated with Ni species is manufactured through pyrolyzing the humic acid-Ni complex compounds under an inert atmosphere. Systematic electrochemical measurements reveal that the Ni-incorporated carbon holds an elevated triiodide reducing properties in contrast to the single carbonized humic acid. The improvement of electrocatalytic ability may be ascribed to Ni incorporated in carbon matrix that offer more electroactive sites. As a result, when the prepared Ni-incorporated material serves as electrocatalyst in iodide-mediated DSSC, the corresponding device yields a PCE of 7.01%, which increases by 14% in contrast to the single carbonized humic acid (6.14%) and is close to that (7.1%) of as-reference Pt electrode. This study indicates that the proposed carbonaceous material may be in favour of large-scale
production of low cost and high efficiency DSSCs.
Keywords: carbon material;
electrocatalyst;
DSSC;
CH-Ni
1. Introduction As a representative device of transforming sunlight into electricity, dye-sensitized solar cells (DSSCs) possess some merits of easy fabrication process, environmental friendliness and good photovoltaic performance[1,2]. The typical sandwich structure of DSSC includes the photoanode with adsorbed dye molecules, redox electrolyte and counter electrode[3,4]. Counter electrode (CE) occupies an indispensable role for transporting electrons and catalyzing the iodide regeneration from the triiodide species, which strongly affect the cell performance because its sluggish kinetics may result in serious charge recombination at the photoanode and then lead to a low power conversion efficiency (PCE)[5,6]. Due to the favorable conductivity and outstanding electrochemical activity, noble metal platinum (Pt) has been widely employed as the preferred electrocatalyst for CE. However, high cost, scarce resources and relatively weak corrosion-resistant property hinder the large-scale preparation of DSSCs using Pt electrodes[7,8]. Therefore, abundant efforts have been dedicated to exploit the low-cost alternatives to Pt CE materials. Up to now, a variety of non-noble metal electrode materials have been extensively developed, including carbonaceous materials[9,10],conductive polymers[11,12], and transition metal compounds[13-15]. And DSSCs based on abovementioned electrocatalysts show the outstanding
photovoltaic performances. Among various Pt-free functional materials, carbon materials are the most promising catalysts because of their distinct advantages including low cost, high electric conductivity, good chemical and thermal stability. However, the existing carbon-based CEs are still unsatisfactory and generally present the relatively inferior electrocatalytic activity compared to Pt electrode, which require further material design and innovation to boost the intrinsic electrocatalytic activity of carbonaceous materials. Several methods have been utilized to effectively improve the carbon materials in catalyzing the iodide/triiodide redox couple[16,17]. One of which is introducing transition metals into carbon-based materials that can generate abundant catalytic sites for achieving high-performance DSSCs. Li et al. synthesized two kinds of metal-incorporated carbon materials by pyrolyzing a cobalt (II) imidazolate polymer followed by ion exchange, which satisfactorily catalyzed the triiodide reducing reaction[18]. Wu et al. prepared the carbon materials embedded with cobalt nanoparticles through thermal pyrolysis of the metal-organic framework material and displayed a superior electrocatalytic performance[19]. Tsai et al. proposed graphene oxide/macrocyclic cobalt complex nanocomposites and the corresponding cells achieved the decent PCE values[20]. Above reports demonstrate that carbon-based materials incorporated with transition metals is indeed a promising strategy for enhancing catalytic ability. Due to the environmentally friendly and reproducible properties, many biomass materials serve as the precursors of carbon-based materials. Humic acid (HA) as the
natural materials has the abundant reserve and holds various useful functional groups such as carboxylic, phenolic, and hydroxyl groups, thus HA presents the significant effect on the sorption of metal ions[21,22]. At present, carbonaceous materials derived from HA have been adopted as the electrode materials in several new energy devices and showed the satisfied performances[23,24]. In this work, Ni-incorporated carbon materials are prepared through the simple thermal pyrolysis of HA-Ni complex compounds. Carbon matrix incorporated with Ni species can elevate the electrocatalytic ability of the CE by increasing active sites and expediting the transport of electrons and redox electrolytes. Consequently, the as-prepared carbon material as CE for DSSCs presents a promising performance with high electrocatalytic activity as well as excellent electrochemical stability. DSSC using the modified carbon material delivers a PCE of 7.01%, which is close to that (7.1%) of the expensive Pt electrode. Since the type of carbon material is enough efficient and cheaper than Pt, it may hold a promising future as a replacement for Pt electrode.
2. Experiment 2.1. Preparation of materials Humic acid chemically pure as carbon source was provided by Tianjin guangfu chemical Co., Ltd and used as received without further purification. The Ni-incorporated carbon material was prepared through two steps. Firstly, 3g humic acid particles were added into the 10mmol/ml NiCl2 aqueous solution, and pH value of mixed solution was adjusted to 6~7. Then the mixtures in sealed
conical flask were placed in thermostatic oscillator and kept vibrating at 35℃ with the speed 100r/min for 14h. The precipitates were filtrated and washed with deionized water, then dried under vacuum at 60℃ overnight. Secondly, the carbonization of HA-Ni complex compounds was executed in a tube furnace at 900℃ in Ar atmosphere. The heating rate was 2℃/min and holding time was 2h. The achieved products were named as CH-Ni. As a comparision, the undoped carbon material was yielded according to alike process and labelled as CH. The schematic diagram of total preparation processes for the carbon-based electrodes is shown in Scheme 1. 2.2. Fabrication of electrodes and DSSCs The fabrication processes of carbon-based electrodes followed the widely reported methods[25]. As-prepared carbonaceous materials (CH-Ni, CH) were blended with ethyl cellulose, terpineol, and ethanol in a mass ratio of 1:8:9, then the mixture was ball-milled until to form the homogeneous pastes. Clean FTO glasses substrates were covered with the pastes through the doctor-blade method. Subsequently, substrates were placed on a hot plate at 80℃ for 10 min to remove the solvent. Then the substrates were annealed at 450℃ for 30 min in Ar atmosphere to obtain the targeted electrodes. As-reference Pt electrode was yielded by magnetron sputtering. DSSCs were assembled in a typical sandwich-type structure[26,27]. Dye sensitization was conducted through immersing the TiO2 films (0.25 cm2 ) into 0.5 mM N719 dye ethanol solution for 12 h at 60℃. Then the sensitized films
were washed by absolute ethanol and dried at 80℃ for 2 h. The obtained photoanodes were coupled with the prepared electrodes as CEs, they were assembled together by applying a 30 µm Surlyn film as the spacer. The acetonitrile electrolyte composed with 0.6 M 1-butyl-3-methylimidazolium iodide, 0.05 M LiI, 0.03 M I2, 0.5 M 4-tert-butylpyridine, and 0.1 M guanidinium thiocyanate was injected into the inter-electrode gap of the sandwiched cells through the hole at CE prior to sealing with Surlyn film. The symmetric dummy cells were assembled according to the identical method with the conventional DSSC. 2.3. Characterizations and measurements The infrared spectra (IR) of single HA and HA-Ni complex compound were measured on a monocrystalline silicon piece. The crystallographic phases of as-prepared carbon materials were identified using X-ray powder diffraction (XRD, D8 Advance, Germany) , and the chemical compositions in products were detected by X-ray photoelectron spectrometer (XPS, Thermo Fisher Scientific, UK). The morphologies of carbon materials were characterized using field-emission scanning electron microscopy (SEM, Zeiss Ultra Plus, Germany) and
transmission
electronic
microscopy
(TEM,
JEOL-JEM-2012).
Photocurrent-voltage (J-V) curves of all obtained DSSCs were measured under the irradiation of a solar simulator providing solar AM1.5 light of 100 mWcm-2 (SolarIV, Zolix, China). The incident monochromatic photon-to-current conversion efficiency (IPCE) spectra of the DSSCs with different CEs were
obtained under a monochromatic light illumination in the 300~800 nm wavelength range (Enli Technology Co. Ltd. China). Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Tafel polarization analysis were totally conducted on the CHI660C electrochemical workstation. Three-electrode system constituted of the prepared electrodes as working electrode, Pt wire as CE, Ag wire as reference electrode was utilized to carry out the CV tests. The used electrolyte in above system was acetonitrile solution including 0.1 M LiClO4, 10 mM LiI and 1 mM I2. Symmetric dummy cells composed with two identical electrodes were employed to execute the EIS and Tafel measurements.
3. Results and discussion 3.1.Characterization of materials Individual HA and HA-Ni complex compound are investigated by IR and corresponding results are presented in Fig.1A. The broad absorption band at 3430 cm-1 in both samples can be attributed to the stretching vibrations of OH groups (carboxyl, phenol, water), and large width of this band demonstrates the intense hydrogen bonds that involve in these groups. The intensity of peak at 1580 cm-1 should be related to structural vibrations of aromatic moieties, asymmetric vibrations of COO groups, and the deformation vibrations of adsorbed water molecules[21,28,29]. The bands at 1365 cm-1 and 1020 cm-1 originate from C-O stretching of phenolic groups and C-O stretching of
polysaccharide-like components[30]. In Fig.1A, normalized characteristic peaks intensities of complex HA-Ni are obviously different with that of pure HA. Active carboxylic and hydroxyl functional groups from the HA possibly chelate with the metal ions Ni2+ and then form the complex, which gives rise to the decline of characteristic peaks intensities in HA-Ni complex compared to that of original HA[31,32]. XRD patterns of the carbonized products are displayed in Fig.1B. It can be found in Fig.1B that both materials present two broad peaks around 25。assigned to (002) plane of graphite and a recognizable broadband around 43。referred to (100) plane of graphite, which indicate the successful carbonization of raw materials[33-35]. Besides, no characteristic peaks involving Ni are detected in XRD, the possible reasons can be ascribed to the rare content of Ni species. Fig.1C presents the Raman spectra of CH and CH-Ni products. The peak near 1360 cm-1 coincides with the D band of graphite structure, representing disordered structures and defects. The G band at 1590 cm-1 is assigned to the symmetric vibration mode of graphitic carbon atoms. After Ni species incorporated into carbonized HA, the peak positions and intensities of D band and G band remain unchanged[20,36]. The chemical elements in CH and CH-Ni products are also identified using the XPS. As shown in Fig.1D, XPS spectrum from CH-Ni exhibits a distinct characteristic peak of Ni and the Ni atom concentration is 1.19 at.%, which clearly demonstrate that the Ni species exists on the carbon matrix. Fig.1E depicts the Ni2p3/2 XPS results from CH-Ni, the peaks located at 856.7 eV and 853.5 eV are
assigned to the oxidized Ni2+ and metal Ni0, respectively[37,38]. In addition, the high-resolution spectrum of C1s from CH-Ni is displayed in Fig.1F, which contains two absorption peaks with 284.6 eV for C=C/C-C and 286.3 eV for C-O[20]. Furthermore, Fig.2 exhibits the SEM and TEM images of CH and CH-Ni. In Fig.2A and Fig.2B, the prepared materials are observed to present the irregular thin sheets or bulks with a varying size distributions, and the introduction of Ni element do not trigger the obvious change in morphology. TEM results for CH and CH-Ni materials are shown in Fig.2C and Fig.2D, respectively. For the CH-Ni, black nanocrystalline particles of the Ni species with a size range of 10-20 nm are almost homogeneously anchored and well attached to the CH carbon nanosheets, which are formed from the pyrolysis of the HA-Ni complex. The carbon matrix can promote charge transfer because of its facile electron transfer and the Ni species deposited onto the carbon matrix enrich the extra active sites for redox electrolyte, thus leading to an potential improvement for the catalytic activity of the electrode[39,40]. 3.2. Electrochemical analysis Electrochemical impedance spectroscopy (EIS) measurement is conducted on the symmetric dummy cell assembled with two identical electrodes to further study the catalytic characteristics of various electrodes. Fig.3 displays the Nyquist plots of EIS from CH, CH-Ni, Pt electrodes and relevant equivalent circuit model for dummy cells. As shown in Nyquist plots, the intercept of the semicircle on the real axis is assigned to the series resistance (Rs), which
describes the series resistance (Rs) arising from the resistance between CE materials and FTO substrates[41,42]. The left semicircle in the high frequency region originates from the charge transfer resistance (Rct) at the electrode and electrolyte interface, and Rct value is inversely correlative with the electrocatalytic activity of electrode on the reduction of triiodide, i.e. less Rct implys the strong electrocatalytic ability for triiodide reducing. The low frequency region semicircle represents the Nernst diffusion impedance (W) of the triiodide/iodide redox couple in the electrolyte[43,44]. The extracted parameters involving Rs and Rct from Nyquist spectra are listed in Table 1. Total Rs values from three different electrodes nearly appear around 9Ωcm2, manifesting similar conductivity among abovementioned electrodes. While Rct value from the CH-Ni electrode (7.4Ωcm2) is clearly smaller than that of CH electrode (11.5Ωcm2), declaring CH-Ni electrocatalyst holding the character of high-effect charge transfer. This effect exists due to the increasing sites created through incorporating the transition metal in carbon materials, which is responsible for the enhancement of electrocatalytic ability. Meanwhile the Rct value from Pt electrode is also provided to deeper comparing the functional properties of the prepared carbonaceous electrodes. The Rct value of CH-Ni is closer to 5.6Ωcm2 of Pt electrode than that of CH. The result indicates that the CH-Ni carbon-based electrode possesses the noticeable electrocatalytic feature. Fig.4 shows the Bode spectra of EIS from symmetric dummy cells with three kinds of electrodes above. In Bode plot, electrons lifetime (τ) participating in
the I3- reduction process can be calculated by quation: τ=1/(2πfmax) (here fmax is the peak frequency in the Bode plot ). Table 1 summarizes the τ values from various electrodes. CE serves to collect electrons from the external circuit and catalyze the reduction of redox couple. τ describes the lifetime of the electrons at the electrode/electrolyte interface, therefore the shorter τ means the better electrocatalytic activity[45,46]. As seen in Table 1, τ values follow the order of Pt < CH-Ni < CH, the result is in good agreement with Rct data. On the other hand, Tafel-polarization analysis is carried out to reveal the electrocatalytic reaction kinetic about triiodide reducing at the different electrode surfaces, the recorded curves are presented in Fig.5. In a typical Tafel spectrum, exchange current density J0 and limiting diffusion current density Jlim can be achieved from the corresponding Tafel zone and diffusion zone, respectively. J0 is the intersection point of anodic (or cathodic ) ’s tangent and a perpendicular at 0 V, and a large J0 corresponds to a high electrocatalytic activity[47,48]. Compared to the single CH electrode, the J0 value from CH-Ni (1.1 mAcm−2) obviously exceeds
that
from
CH
(0.6 mAcm−2), again confirming
the
boosted
electrocatalytic activity. And J0 value of CH-Ni is comparable to that from Pt (1.5 mAcm−2) , implying the CH-Ni can catalyze the triiodide reduction as effectively as Pt. In addition, J0 can be used to deduce the Tafel charge transfer resistance (Rct-Tafel) through equation: J0=RT/nFRct-Tafel, Where R is the gas constant, F is the Faraday constant, T is the absolute temperature, and n is the number of electrons being transferred during the iodide/triiodide redox
process[49]. Since the great J0 represents the less Rct-Tafel, therefore the Rct-Tafel performances from three electrodes is in line with the EIS analysis. In addition, the Jlim is proportional to the diffusion coefficient (Dn) of the redox couple, as expressed in equation: Dn = lJlim/2nFC (l is the distance between the electrodes, F is the Faraday constant, n is the number of electrons exchanged in the reaction, C is the triiodide concentration )[50]. As shown in Fig.5, the CH-Ni exhibits the similar Jlim with Pt, indicating that CH-Ni owns a fast electrolyte ions diffusion which can facilitate the triiodide reduction reaction[51]. The catalytic behaviors of the carbonaceous electrodes and Pt electrode are further studied by cyclic voltammetry (CV). Fig.6A presents CV curves from three electrodes. In the voltammograms, the left peak in the relatively low potential can be assigned to I3-/I-. The reduction rate from I3- to I- is the dominating factor for photocurrent density of cell, therefore the cathodic reduction peak current density as a paramount parameter is generally used to evaluate the electrocatalytic activities of various electrodes[52,53]. Obviously, CH-Ni and CH exhibit the analogous CV shapes in Fig.6A, but the convincing distinctions exist in the two electrodes. The enlarged reduction current density observed for the CH-Ni can be attributed to the improving catalytic activity which originates from the Ni species introduced to the carbon supports. Moreover, although lacking the well-defined peaks like Pt, the CH-Ni electrode still behaves the similar reduction peak current density with Pt electrode, manifesting the favourable electrocatalytic characteristic of CH-Ni. In addition,
to judge the dissolution-resistant ability of the CH-Ni electrode in iodine-based electrolyte, 50 sequential CV scans for the electrode have been measured and the recorded curves are displayed in Fig.6B. No distortion is observed and the curves show consistent pattern at all cycles, suggesting good electrochemical durability. 3.3. Photovoltaic performances According to above analysis, we can determine that CH-Ni electrode owns an excellent electrocatalytic activity. The actual electrocatalytic performances of CH and CH-Ni carbonaceous electrodes are assessed by assembling the intact cells, meanwhile a Pt-based DSSC is also fabricated for further comparison. The current density-voltage curves (J-V) of DSSCs based on CH, CH-Ni and Pt electrodes are shown in Fig.7, and corresponding photovoltaic performance
parameters
involving
short-circuit
current
density
(Jsc),
open-circuit voltage (Voc), fill factor (FF) and PCE are summarized in Table 2. DSSC fabricated with single CH electrode gives a medium PCE of 6.14% along with the relatively low Jsc of 12.55 mAcm-2. Jsc performance for DSSCs is generally
associated with
electron injection efficiency
of
photo-induced
electron as well as electron transfer mechanism of DSSCs. When holding the identical dye-sensitized photoanodes, the low Jsc mainly stems from the sluggish reception for electrons transferred to the CE and fewer electron transfer occurring on electrode/electrolyte interface, which can be ascribed to the insufficient catalytic ability of electrode materials[54,55]. One can see that the photovoltaic performance parameters of DSSC assembled with the CH-Ni electrode are obviously improved. Cell with the CH-Ni electrode exhibits a higher PCE of 7.01% and an elevated Jsc of 13.51 mAcm-2. The moderately
improved photovoltaic performance for CH-Ni electrode should be ascribed to the enhanced catalytic ability derived from the introduction of Ni species, which has been confirmed by above electrochemical discussion. In addition, the Pt-based cell generates a PCE of 7.1% on the identical test condition. The results demonstrate the Ni-incorporated carbonaceous electrocatalyst holding the outstanding catalytic property like noble Pt. Furthermore, the IPCE spectra of DSSCs with CH, CH-Ni and Pt electrodes are depicted in Fig.S1. Because of all DSSCs assembled with the same N719 dye, the influence of different CE on the DSSC’s photoelectric response is mainly reflected by the varied IPCE peak values. In Fig.S1, the IPCE results present the same trend with the J-V measurements, reconfirming the J-V performance. On the other hand, high reproducibility is a vital aspect of CE research[56]. 3 parallel cells were fabricated for CH-Ni electrode and Pt electrode to investigate their repeatability. Fig.8A exhibits the crucial photovoltaic parameters Jsc and PCE from these cells, data from Voc and FF are summarized in Fig.S2A. As can be seen from Fig.8A, Jsc and PCE for 3 independent devices with CH-Ni electrode are similarity without large variations, which affirm the good reproducibility for the prepared electrode. In addition, stability for the electrode in intact DSSC is also studied by exposing the CH-Ni cell and Pt cell to the air without special protection in a week. Fig.8B displays the parameters Jsc and PCE extracted from each test (Voc and FF in Fig.S2B). Obviously, CH-Ni cell presents the alike tendency with Pt-based cell, indicative of CH-Ni electrode owning the desirable reliability. These measurements highlight the great potential of the present carbon-based electrode as the alternative to Pt.
4. Conclusions In summary, inexpensive biomass humic acid is adopted as the carbon source, following the Ni-incorporated carbon materials are constructed via the pyrolysis of humic acid-Ni complex compounds. Benefiting from the dopant species of Ni, the modified carbon material can offer extra catalytic active sites compared to the directly carbonized humic acid. When the CH-Ni material is assembled into DSSC as the counter electrode catalyst, it exhibits a decent electrocatalytic performance toward the reduction of the triiodide electrolyte. The kind of non-precious carbonaceous catalyst
can be a good candidate to
solve the issues referred to the expensive Pt and avails developing highly efficient and low-cost DSSCs. In addition, due to humic acid holding abundant functional groups, desired structures can be rationally tailored through the skillful material design. The used method in this work may be in favour of exploiting more effective electrode materials for DSSCs and other energy conversion devices.
Acknowledge
This work was financially supported by the Scientific Research Start-up Funds of North University of China (11012505).
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Captions of Figures in article Scheme 1. The whole preparation process of the carbonaceous electrodes. Fig.1. (A) Normalized IR spectra of the pristine HA and HA-Ni complex; (B) XRD patterns of as-prepared carbon materials; (C) Raman spectra of the two carbon materials; (D) XPS spectra of the CH and CH-Ni products; (E) High-resolution XPS spectrum of Ni2p3/2 peak from CH-Ni; (F) High-resolution XPS spectrum of C1s peak from CH-Ni . Fig.2. SEM images of (A) CH, (B) CH-Ni; TEM images of (C) CH, (D) CH-Ni. Fig.3. Nyquist plots of EIS for symmetrical cells with the different electrodes, the inset shows the equivalent circuit. Fig.4. Bode plots of the symmetrical cells based on different electrodes. Fig.5. Tafel polarization curves from various CEs. Fig.6. (A) CV curves of the different CEs; (B) 50 consecutive CVs of the CH-Ni electrode. Fig.7. Photocurrent density-voltage curves of DSSCs with CH, CH-Ni, Pt electrodes. Fig.8. (A) Jsc and PCE collected from 3 independent DSSCs; (B) Jsc and PCE from the stability test of sealed cells with CH-Ni and Pt.
Captions of Figures in Supporting Information Fig.S1. IPCE curves of the DSSCs with various electrodes. Fig.S2. (A) Voc and FF collected from 3 independent DSSCs; (B) Voc and FF from the stability test of sealed cells with CH-Ni and Pt.
Table in article
Table 1
Table 2
Electrochemical parameters from EIS and Tafel
CE
Rs / Ω cm2
Rct / Ω cm2
τ/ µs
J0 / mA cm−2
CH
10.1
11.5
22.8
0.6
CH-Ni
9.1
7.4
13.9
1.1
Pt
8.5
5.6
9.4
1.5
Photovoltaic parameters of DSSCs based on the various CEs.
CE
Jsc /mAcm-2
Voc /V
FF
PCE/%
CH
12.55
0.76
0.65
6.14
CH-Ni
13.51
0.76
0.68
7.01
Pt
13.85
0.75
0.68
7.1
Highlights
The inexpensive biomass humic acid is adopted as the carbon source of electrode materials for DSSCs.
The Ni-incorporated carbon materials were facilely yielded through the pyrolysis of humic acid-Ni complex.
The low-cost modified carbonaceous electrocatalyst presents the Pt-like electrocatalytic performance.
S1566-1199(19)30414-8
DOI:
https://doi.org/10.1016/j.orgel.2019.105395
Reference:
ORGELE 105395
To appear in:
Organic Electronics
Received Date: 12 June 2019 Revised Date:
19 July 2019
Accepted Date: 2 August 2019
Please cite this article as: Y. Di, S. Jia, N. Li, C. Hao, H. Zhang, S. Hu, Ni-incorporated carbon materials derived from humic acid as efficient low-cost electrocatalysts for dye-sensitized solar cells, Organic Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.105395. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Ni-incorporated carbon materials derived from humic acid as efficient low-cost electrocatalysts for dye-sensitized solar cells Yi Di1*, Suping Jia1, Ning Li1, Caihong Hao1, Huinian Zhang1, and Shengliang Hu1* 1.
School of Energy and Power Engineering, North University of China, Taiyuan
030051, PR China E-mail: [email protected]; [email protected]
Abstract The scale-up application of dye-sensitized solar cells (DSSCs) will desire the efficient and low-cost electrocatalytic counter electrodes. Carbon materials derived from the natural materials have been extensively used in the new energy fields owing to numerously distinct advantages. In this work, a new-style carbon matrix incorporated with Ni species is manufactured through pyrolyzing the humic acid-Ni complex compounds under an inert atmosphere. Systematic electrochemical measurements reveal that the Ni-incorporated carbon holds an elevated triiodide reducing properties in contrast to the single carbonized humic acid. The improvement of electrocatalytic ability may be ascribed to Ni incorporated in carbon matrix that offer more electroactive sites. As a result, when the prepared Ni-incorporated material serves as electrocatalyst in iodide-mediated DSSC, the corresponding device yields a PCE of 7.01%, which increases by 14% in contrast to the single carbonized humic acid (6.14%) and is close to that (7.1%) of as-reference Pt electrode. This study indicates that the proposed carbonaceous material may be in favour of large-scale
production of low cost and high efficiency DSSCs.
Keywords: carbon material;
electrocatalyst;
DSSC;
CH-Ni
1. Introduction As a representative device of transforming sunlight into electricity, dye-sensitized solar cells (DSSCs) possess some merits of easy fabrication process, environmental friendliness and good photovoltaic performance[1,2]. The typical sandwich structure of DSSC includes the photoanode with adsorbed dye molecules, redox electrolyte and counter electrode[3,4]. Counter electrode (CE) occupies an indispensable role for transporting electrons and catalyzing the iodide regeneration from the triiodide species, which strongly affect the cell performance because its sluggish kinetics may result in serious charge recombination at the photoanode and then lead to a low power conversion efficiency (PCE)[5,6]. Due to the favorable conductivity and outstanding electrochemical activity, noble metal platinum (Pt) has been widely employed as the preferred electrocatalyst for CE. However, high cost, scarce resources and relatively weak corrosion-resistant property hinder the large-scale preparation of DSSCs using Pt electrodes[7,8]. Therefore, abundant efforts have been dedicated to exploit the low-cost alternatives to Pt CE materials. Up to now, a variety of non-noble metal electrode materials have been extensively developed, including carbonaceous materials[9,10],conductive polymers[11,12], and transition metal compounds[13-15]. And DSSCs based on abovementioned electrocatalysts show the outstanding
photovoltaic performances. Among various Pt-free functional materials, carbon materials are the most promising catalysts because of their distinct advantages including low cost, high electric conductivity, good chemical and thermal stability. However, the existing carbon-based CEs are still unsatisfactory and generally present the relatively inferior electrocatalytic activity compared to Pt electrode, which require further material design and innovation to boost the intrinsic electrocatalytic activity of carbonaceous materials. Several methods have been utilized to effectively improve the carbon materials in catalyzing the iodide/triiodide redox couple[16,17]. One of which is introducing transition metals into carbon-based materials that can generate abundant catalytic sites for achieving high-performance DSSCs. Li et al. synthesized two kinds of metal-incorporated carbon materials by pyrolyzing a cobalt (II) imidazolate polymer followed by ion exchange, which satisfactorily catalyzed the triiodide reducing reaction[18]. Wu et al. prepared the carbon materials embedded with cobalt nanoparticles through thermal pyrolysis of the metal-organic framework material and displayed a superior electrocatalytic performance[19]. Tsai et al. proposed graphene oxide/macrocyclic cobalt complex nanocomposites and the corresponding cells achieved the decent PCE values[20]. Above reports demonstrate that carbon-based materials incorporated with transition metals is indeed a promising strategy for enhancing catalytic ability. Due to the environmentally friendly and reproducible properties, many biomass materials serve as the precursors of carbon-based materials. Humic acid (HA) as the
natural materials has the abundant reserve and holds various useful functional groups such as carboxylic, phenolic, and hydroxyl groups, thus HA presents the significant effect on the sorption of metal ions[21,22]. At present, carbonaceous materials derived from HA have been adopted as the electrode materials in several new energy devices and showed the satisfied performances[23,24]. In this work, Ni-incorporated carbon materials are prepared through the simple thermal pyrolysis of HA-Ni complex compounds. Carbon matrix incorporated with Ni species can elevate the electrocatalytic ability of the CE by increasing active sites and expediting the transport of electrons and redox electrolytes. Consequently, the as-prepared carbon material as CE for DSSCs presents a promising performance with high electrocatalytic activity as well as excellent electrochemical stability. DSSC using the modified carbon material delivers a PCE of 7.01%, which is close to that (7.1%) of the expensive Pt electrode. Since the type of carbon material is enough efficient and cheaper than Pt, it may hold a promising future as a replacement for Pt electrode.
2. Experiment 2.1. Preparation of materials Humic acid chemically pure as carbon source was provided by Tianjin guangfu chemical Co., Ltd and used as received without further purification. The Ni-incorporated carbon material was prepared through two steps. Firstly, 3g humic acid particles were added into the 10mmol/ml NiCl2 aqueous solution, and pH value of mixed solution was adjusted to 6~7. Then the mixtures in sealed
conical flask were placed in thermostatic oscillator and kept vibrating at 35℃ with the speed 100r/min for 14h. The precipitates were filtrated and washed with deionized water, then dried under vacuum at 60℃ overnight. Secondly, the carbonization of HA-Ni complex compounds was executed in a tube furnace at 900℃ in Ar atmosphere. The heating rate was 2℃/min and holding time was 2h. The achieved products were named as CH-Ni. As a comparision, the undoped carbon material was yielded according to alike process and labelled as CH. The schematic diagram of total preparation processes for the carbon-based electrodes is shown in Scheme 1. 2.2. Fabrication of electrodes and DSSCs The fabrication processes of carbon-based electrodes followed the widely reported methods[25]. As-prepared carbonaceous materials (CH-Ni, CH) were blended with ethyl cellulose, terpineol, and ethanol in a mass ratio of 1:8:9, then the mixture was ball-milled until to form the homogeneous pastes. Clean FTO glasses substrates were covered with the pastes through the doctor-blade method. Subsequently, substrates were placed on a hot plate at 80℃ for 10 min to remove the solvent. Then the substrates were annealed at 450℃ for 30 min in Ar atmosphere to obtain the targeted electrodes. As-reference Pt electrode was yielded by magnetron sputtering. DSSCs were assembled in a typical sandwich-type structure[26,27]. Dye sensitization was conducted through immersing the TiO2 films (0.25 cm2 ) into 0.5 mM N719 dye ethanol solution for 12 h at 60℃. Then the sensitized films
were washed by absolute ethanol and dried at 80℃ for 2 h. The obtained photoanodes were coupled with the prepared electrodes as CEs, they were assembled together by applying a 30 µm Surlyn film as the spacer. The acetonitrile electrolyte composed with 0.6 M 1-butyl-3-methylimidazolium iodide, 0.05 M LiI, 0.03 M I2, 0.5 M 4-tert-butylpyridine, and 0.1 M guanidinium thiocyanate was injected into the inter-electrode gap of the sandwiched cells through the hole at CE prior to sealing with Surlyn film. The symmetric dummy cells were assembled according to the identical method with the conventional DSSC. 2.3. Characterizations and measurements The infrared spectra (IR) of single HA and HA-Ni complex compound were measured on a monocrystalline silicon piece. The crystallographic phases of as-prepared carbon materials were identified using X-ray powder diffraction (XRD, D8 Advance, Germany) , and the chemical compositions in products were detected by X-ray photoelectron spectrometer (XPS, Thermo Fisher Scientific, UK). The morphologies of carbon materials were characterized using field-emission scanning electron microscopy (SEM, Zeiss Ultra Plus, Germany) and
transmission
electronic
microscopy
(TEM,
JEOL-JEM-2012).
Photocurrent-voltage (J-V) curves of all obtained DSSCs were measured under the irradiation of a solar simulator providing solar AM1.5 light of 100 mWcm-2 (SolarIV, Zolix, China). The incident monochromatic photon-to-current conversion efficiency (IPCE) spectra of the DSSCs with different CEs were
obtained under a monochromatic light illumination in the 300~800 nm wavelength range (Enli Technology Co. Ltd. China). Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Tafel polarization analysis were totally conducted on the CHI660C electrochemical workstation. Three-electrode system constituted of the prepared electrodes as working electrode, Pt wire as CE, Ag wire as reference electrode was utilized to carry out the CV tests. The used electrolyte in above system was acetonitrile solution including 0.1 M LiClO4, 10 mM LiI and 1 mM I2. Symmetric dummy cells composed with two identical electrodes were employed to execute the EIS and Tafel measurements.
3. Results and discussion 3.1.Characterization of materials Individual HA and HA-Ni complex compound are investigated by IR and corresponding results are presented in Fig.1A. The broad absorption band at 3430 cm-1 in both samples can be attributed to the stretching vibrations of OH groups (carboxyl, phenol, water), and large width of this band demonstrates the intense hydrogen bonds that involve in these groups. The intensity of peak at 1580 cm-1 should be related to structural vibrations of aromatic moieties, asymmetric vibrations of COO groups, and the deformation vibrations of adsorbed water molecules[21,28,29]. The bands at 1365 cm-1 and 1020 cm-1 originate from C-O stretching of phenolic groups and C-O stretching of
polysaccharide-like components[30]. In Fig.1A, normalized characteristic peaks intensities of complex HA-Ni are obviously different with that of pure HA. Active carboxylic and hydroxyl functional groups from the HA possibly chelate with the metal ions Ni2+ and then form the complex, which gives rise to the decline of characteristic peaks intensities in HA-Ni complex compared to that of original HA[31,32]. XRD patterns of the carbonized products are displayed in Fig.1B. It can be found in Fig.1B that both materials present two broad peaks around 25。assigned to (002) plane of graphite and a recognizable broadband around 43。referred to (100) plane of graphite, which indicate the successful carbonization of raw materials[33-35]. Besides, no characteristic peaks involving Ni are detected in XRD, the possible reasons can be ascribed to the rare content of Ni species. Fig.1C presents the Raman spectra of CH and CH-Ni products. The peak near 1360 cm-1 coincides with the D band of graphite structure, representing disordered structures and defects. The G band at 1590 cm-1 is assigned to the symmetric vibration mode of graphitic carbon atoms. After Ni species incorporated into carbonized HA, the peak positions and intensities of D band and G band remain unchanged[20,36]. The chemical elements in CH and CH-Ni products are also identified using the XPS. As shown in Fig.1D, XPS spectrum from CH-Ni exhibits a distinct characteristic peak of Ni and the Ni atom concentration is 1.19 at.%, which clearly demonstrate that the Ni species exists on the carbon matrix. Fig.1E depicts the Ni2p3/2 XPS results from CH-Ni, the peaks located at 856.7 eV and 853.5 eV are
assigned to the oxidized Ni2+ and metal Ni0, respectively[37,38]. In addition, the high-resolution spectrum of C1s from CH-Ni is displayed in Fig.1F, which contains two absorption peaks with 284.6 eV for C=C/C-C and 286.3 eV for C-O[20]. Furthermore, Fig.2 exhibits the SEM and TEM images of CH and CH-Ni. In Fig.2A and Fig.2B, the prepared materials are observed to present the irregular thin sheets or bulks with a varying size distributions, and the introduction of Ni element do not trigger the obvious change in morphology. TEM results for CH and CH-Ni materials are shown in Fig.2C and Fig.2D, respectively. For the CH-Ni, black nanocrystalline particles of the Ni species with a size range of 10-20 nm are almost homogeneously anchored and well attached to the CH carbon nanosheets, which are formed from the pyrolysis of the HA-Ni complex. The carbon matrix can promote charge transfer because of its facile electron transfer and the Ni species deposited onto the carbon matrix enrich the extra active sites for redox electrolyte, thus leading to an potential improvement for the catalytic activity of the electrode[39,40]. 3.2. Electrochemical analysis Electrochemical impedance spectroscopy (EIS) measurement is conducted on the symmetric dummy cell assembled with two identical electrodes to further study the catalytic characteristics of various electrodes. Fig.3 displays the Nyquist plots of EIS from CH, CH-Ni, Pt electrodes and relevant equivalent circuit model for dummy cells. As shown in Nyquist plots, the intercept of the semicircle on the real axis is assigned to the series resistance (Rs), which
describes the series resistance (Rs) arising from the resistance between CE materials and FTO substrates[41,42]. The left semicircle in the high frequency region originates from the charge transfer resistance (Rct) at the electrode and electrolyte interface, and Rct value is inversely correlative with the electrocatalytic activity of electrode on the reduction of triiodide, i.e. less Rct implys the strong electrocatalytic ability for triiodide reducing. The low frequency region semicircle represents the Nernst diffusion impedance (W) of the triiodide/iodide redox couple in the electrolyte[43,44]. The extracted parameters involving Rs and Rct from Nyquist spectra are listed in Table 1. Total Rs values from three different electrodes nearly appear around 9Ωcm2, manifesting similar conductivity among abovementioned electrodes. While Rct value from the CH-Ni electrode (7.4Ωcm2) is clearly smaller than that of CH electrode (11.5Ωcm2), declaring CH-Ni electrocatalyst holding the character of high-effect charge transfer. This effect exists due to the increasing sites created through incorporating the transition metal in carbon materials, which is responsible for the enhancement of electrocatalytic ability. Meanwhile the Rct value from Pt electrode is also provided to deeper comparing the functional properties of the prepared carbonaceous electrodes. The Rct value of CH-Ni is closer to 5.6Ωcm2 of Pt electrode than that of CH. The result indicates that the CH-Ni carbon-based electrode possesses the noticeable electrocatalytic feature. Fig.4 shows the Bode spectra of EIS from symmetric dummy cells with three kinds of electrodes above. In Bode plot, electrons lifetime (τ) participating in
the I3- reduction process can be calculated by quation: τ=1/(2πfmax) (here fmax is the peak frequency in the Bode plot ). Table 1 summarizes the τ values from various electrodes. CE serves to collect electrons from the external circuit and catalyze the reduction of redox couple. τ describes the lifetime of the electrons at the electrode/electrolyte interface, therefore the shorter τ means the better electrocatalytic activity[45,46]. As seen in Table 1, τ values follow the order of Pt < CH-Ni < CH, the result is in good agreement with Rct data. On the other hand, Tafel-polarization analysis is carried out to reveal the electrocatalytic reaction kinetic about triiodide reducing at the different electrode surfaces, the recorded curves are presented in Fig.5. In a typical Tafel spectrum, exchange current density J0 and limiting diffusion current density Jlim can be achieved from the corresponding Tafel zone and diffusion zone, respectively. J0 is the intersection point of anodic (or cathodic ) ’s tangent and a perpendicular at 0 V, and a large J0 corresponds to a high electrocatalytic activity[47,48]. Compared to the single CH electrode, the J0 value from CH-Ni (1.1 mAcm−2) obviously exceeds
that
from
CH
(0.6 mAcm−2), again confirming
the
boosted
electrocatalytic activity. And J0 value of CH-Ni is comparable to that from Pt (1.5 mAcm−2) , implying the CH-Ni can catalyze the triiodide reduction as effectively as Pt. In addition, J0 can be used to deduce the Tafel charge transfer resistance (Rct-Tafel) through equation: J0=RT/nFRct-Tafel, Where R is the gas constant, F is the Faraday constant, T is the absolute temperature, and n is the number of electrons being transferred during the iodide/triiodide redox
process[49]. Since the great J0 represents the less Rct-Tafel, therefore the Rct-Tafel performances from three electrodes is in line with the EIS analysis. In addition, the Jlim is proportional to the diffusion coefficient (Dn) of the redox couple, as expressed in equation: Dn = lJlim/2nFC (l is the distance between the electrodes, F is the Faraday constant, n is the number of electrons exchanged in the reaction, C is the triiodide concentration )[50]. As shown in Fig.5, the CH-Ni exhibits the similar Jlim with Pt, indicating that CH-Ni owns a fast electrolyte ions diffusion which can facilitate the triiodide reduction reaction[51]. The catalytic behaviors of the carbonaceous electrodes and Pt electrode are further studied by cyclic voltammetry (CV). Fig.6A presents CV curves from three electrodes. In the voltammograms, the left peak in the relatively low potential can be assigned to I3-/I-. The reduction rate from I3- to I- is the dominating factor for photocurrent density of cell, therefore the cathodic reduction peak current density as a paramount parameter is generally used to evaluate the electrocatalytic activities of various electrodes[52,53]. Obviously, CH-Ni and CH exhibit the analogous CV shapes in Fig.6A, but the convincing distinctions exist in the two electrodes. The enlarged reduction current density observed for the CH-Ni can be attributed to the improving catalytic activity which originates from the Ni species introduced to the carbon supports. Moreover, although lacking the well-defined peaks like Pt, the CH-Ni electrode still behaves the similar reduction peak current density with Pt electrode, manifesting the favourable electrocatalytic characteristic of CH-Ni. In addition,
to judge the dissolution-resistant ability of the CH-Ni electrode in iodine-based electrolyte, 50 sequential CV scans for the electrode have been measured and the recorded curves are displayed in Fig.6B. No distortion is observed and the curves show consistent pattern at all cycles, suggesting good electrochemical durability. 3.3. Photovoltaic performances According to above analysis, we can determine that CH-Ni electrode owns an excellent electrocatalytic activity. The actual electrocatalytic performances of CH and CH-Ni carbonaceous electrodes are assessed by assembling the intact cells, meanwhile a Pt-based DSSC is also fabricated for further comparison. The current density-voltage curves (J-V) of DSSCs based on CH, CH-Ni and Pt electrodes are shown in Fig.7, and corresponding photovoltaic performance
parameters
involving
short-circuit
current
density
(Jsc),
open-circuit voltage (Voc), fill factor (FF) and PCE are summarized in Table 2. DSSC fabricated with single CH electrode gives a medium PCE of 6.14% along with the relatively low Jsc of 12.55 mAcm-2. Jsc performance for DSSCs is generally
associated with
electron injection efficiency
of
photo-induced
electron as well as electron transfer mechanism of DSSCs. When holding the identical dye-sensitized photoanodes, the low Jsc mainly stems from the sluggish reception for electrons transferred to the CE and fewer electron transfer occurring on electrode/electrolyte interface, which can be ascribed to the insufficient catalytic ability of electrode materials[54,55]. One can see that the photovoltaic performance parameters of DSSC assembled with the CH-Ni electrode are obviously improved. Cell with the CH-Ni electrode exhibits a higher PCE of 7.01% and an elevated Jsc of 13.51 mAcm-2. The moderately
improved photovoltaic performance for CH-Ni electrode should be ascribed to the enhanced catalytic ability derived from the introduction of Ni species, which has been confirmed by above electrochemical discussion. In addition, the Pt-based cell generates a PCE of 7.1% on the identical test condition. The results demonstrate the Ni-incorporated carbonaceous electrocatalyst holding the outstanding catalytic property like noble Pt. Furthermore, the IPCE spectra of DSSCs with CH, CH-Ni and Pt electrodes are depicted in Fig.S1. Because of all DSSCs assembled with the same N719 dye, the influence of different CE on the DSSC’s photoelectric response is mainly reflected by the varied IPCE peak values. In Fig.S1, the IPCE results present the same trend with the J-V measurements, reconfirming the J-V performance. On the other hand, high reproducibility is a vital aspect of CE research[56]. 3 parallel cells were fabricated for CH-Ni electrode and Pt electrode to investigate their repeatability. Fig.8A exhibits the crucial photovoltaic parameters Jsc and PCE from these cells, data from Voc and FF are summarized in Fig.S2A. As can be seen from Fig.8A, Jsc and PCE for 3 independent devices with CH-Ni electrode are similarity without large variations, which affirm the good reproducibility for the prepared electrode. In addition, stability for the electrode in intact DSSC is also studied by exposing the CH-Ni cell and Pt cell to the air without special protection in a week. Fig.8B displays the parameters Jsc and PCE extracted from each test (Voc and FF in Fig.S2B). Obviously, CH-Ni cell presents the alike tendency with Pt-based cell, indicative of CH-Ni electrode owning the desirable reliability. These measurements highlight the great potential of the present carbon-based electrode as the alternative to Pt.
4. Conclusions In summary, inexpensive biomass humic acid is adopted as the carbon source, following the Ni-incorporated carbon materials are constructed via the pyrolysis of humic acid-Ni complex compounds. Benefiting from the dopant species of Ni, the modified carbon material can offer extra catalytic active sites compared to the directly carbonized humic acid. When the CH-Ni material is assembled into DSSC as the counter electrode catalyst, it exhibits a decent electrocatalytic performance toward the reduction of the triiodide electrolyte. The kind of non-precious carbonaceous catalyst
can be a good candidate to
solve the issues referred to the expensive Pt and avails developing highly efficient and low-cost DSSCs. In addition, due to humic acid holding abundant functional groups, desired structures can be rationally tailored through the skillful material design. The used method in this work may be in favour of exploiting more effective electrode materials for DSSCs and other energy conversion devices.
Acknowledge
This work was financially supported by the Scientific Research Start-up Funds of North University of China (11012505).
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Captions of Figures in article Scheme 1. The whole preparation process of the carbonaceous electrodes. Fig.1. (A) Normalized IR spectra of the pristine HA and HA-Ni complex; (B) XRD patterns of as-prepared carbon materials; (C) Raman spectra of the two carbon materials; (D) XPS spectra of the CH and CH-Ni products; (E) High-resolution XPS spectrum of Ni2p3/2 peak from CH-Ni; (F) High-resolution XPS spectrum of C1s peak from CH-Ni . Fig.2. SEM images of (A) CH, (B) CH-Ni; TEM images of (C) CH, (D) CH-Ni. Fig.3. Nyquist plots of EIS for symmetrical cells with the different electrodes, the inset shows the equivalent circuit. Fig.4. Bode plots of the symmetrical cells based on different electrodes. Fig.5. Tafel polarization curves from various CEs. Fig.6. (A) CV curves of the different CEs; (B) 50 consecutive CVs of the CH-Ni electrode. Fig.7. Photocurrent density-voltage curves of DSSCs with CH, CH-Ni, Pt electrodes. Fig.8. (A) Jsc and PCE collected from 3 independent DSSCs; (B) Jsc and PCE from the stability test of sealed cells with CH-Ni and Pt.
Captions of Figures in Supporting Information Fig.S1. IPCE curves of the DSSCs with various electrodes. Fig.S2. (A) Voc and FF collected from 3 independent DSSCs; (B) Voc and FF from the stability test of sealed cells with CH-Ni and Pt.
Table in article
Table 1
Table 2
Electrochemical parameters from EIS and Tafel
CE
Rs / Ω cm2
Rct / Ω cm2
τ/ µs
J0 / mA cm−2
CH
10.1
11.5
22.8
0.6
CH-Ni
9.1
7.4
13.9
1.1
Pt
8.5
5.6
9.4
1.5
Photovoltaic parameters of DSSCs based on the various CEs.
CE
Jsc /mAcm-2
Voc /V
FF
PCE/%
CH
12.55
0.76
0.65
6.14
CH-Ni
13.51
0.76
0.68
7.01
Pt
13.85
0.75
0.68
7.1
Highlights
The inexpensive biomass humic acid is adopted as the carbon source of electrode materials for DSSCs.
The Ni-incorporated carbon materials were facilely yielded through the pyrolysis of humic acid-Ni complex.
The low-cost modified carbonaceous electrocatalyst presents the Pt-like electrocatalytic performance.