Ag2S-Sensitized Thermal Cell - ACS Publications

Subscriber access provided by UNIV OF LOUISIANA

C: Energy Conversion and Storage; Energy and Charge Transport 2

AgS-Sensitized Thermal Cell Yuri Inagawa, Toshihiro Isobe, Akira Nakajima, and Sachiko Matsushita J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01922 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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

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

Page 1 of 36 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

The Journal of Physical Chemistry

Ag2S-Sensitized Thermal Cell Yuri Inagawa, Toshihiro Isobe, Akira Nakajima, Sachiko Matsushita*

Department of Materials Science and Engineering, Tokyo Institute of Technology, 2-121 S7-8, Ookayama, Meguro-ku, Tokyo 152-8552, JAPAN

Tel.: +81-3-5734-2525, e-mail: [email protected]

ACS Paragon Plus Environment

1

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

Page 2 of 36

ABSTRACT. Sensitized thermal cells (STCs) is a new green power generation system operate by heating without a cooling temperature part. Here we suggest Ag2S-sensitized thermal cell working at low temperature (below 200oC) and have a stylish sheet form. TiO2 was used for the electron transport layer, and ferrocene/ferricinium ions dissolved in dimethylsulfoxide was selected for the redox couple ions. The Ag2S STCs generated power under light irradiation and at 90oC in the dark. Electrochemical measurements and ICP-MS spectroscopy indicated thermally excited carriers caused redox reaction and power generation occurred with heating. This result is an example of the first STC where the semiconductor part is less than micro size, not only improving the volume density of STC but also showing the possibility of STC with design awareness including flexibility.


2

Page 3 of 36 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


Introduction

Acquisition of safety electrical energy is an urgent, global challenge. Thermal energy is a strong candidate as a green energy because it exists not only on surface of the earth but also in the ground. One method of utilizing thermal energy is based on the Seebeck effect where the temperature difference between two different conductors or semiconductors yields a potential difference1. However, because the Seebeck effect requires a temperature difference, one side of the generator must be maintained as the cooling section. Thus, in some cases, additional work, such as cooling circulation, must be provided, and the thermoelectric generator becomes a large-scale and complex module2.

In these situations, we recently reported “sensitized thermal cells 　(STCs)” as a new thermal cell concept that does not require temperature difference and has a simple Figure 1. Schematic image of a sensitized thermal cell.


3


Page 4 of 36

module3-4. The structure of STC imitates the dye-sensitized solar cells (DSSCs)5. In DSSCs, electrons in dyes are photoexcited by light irradiation, and the excited electrons are injected into the conduction band of an electron transport material (a semiconductor such as titanium dioxide (TiO2)). The transported electrons reduced oxidant ions in electrolyte, and the reductant ions were oxidized by the photoexcited holes, and we can get photo-excited current. If thermally excited electrons in the semiconductor can generate the ion redox reactions6, a novel heat-to-electron conversion system functioned without a temperature difference (Figure 1), named a “sensitized thermal cell”, can be obtained.

In our first paper3, STCs successfully operated at relatively high temperature, 600ºC. To utilize the living environment or geothermal temperature as green energy, STCs must operate at 200°C or less. The operating temperature of STCs depends mainly on the thermally excited carrier number of the semiconductor. The thermally excited carrier density ni is expressed by the following equation (1).


4

Page 5 of 36 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


Here, Nc and Nv are effective state density of conduction and valence bands, Eg is the bandgap of semiconductor, k is Boltzmann constant, and T is temperature. As the bandgap is narrower, more thermally excited charges are generated. For utilize low temperature heat, selection of narrow band gap semiconductor is desirable.

To achieve the lower temperature operation, we also reported an STC using organic perovskite. The operating temperature was successfully lower 60 ℃ to 90 ℃.4 However, the CH3NH3PbI3 perovskite was chemically unstable and it was impossible to confirm stable power generation unless making CH3NH3PbI3 powder into a compact body. This compactification greatly impairs the volume energy density and design of the STC. In this paper, silver sulfide (Ag2S) is selected for the semiconductor of a sheet-type STC. Silver sulfide is a near-infrared absorbed binary semiconductor with a narrow bandgap of 0.9~1.1 eV, and is environmentally friendly material7. Moreover, it can be formed nanoparticles and the band gap can be adjusted by the size. Its outstanding properties


5


Page 6 of 36

allowed Ag2S to be used photocatalysts8, bio sensing9, and solar cells there are some reports of Ag2S for sensitized solar cells

11-13.

7, 10-11,

and also,

Using equation (1) with

effective masses of electrons and holes of Ag2S of 1.096 m0, 0.268 m0 and a bandgap of 0.9 eV, the thermal excited carrier density of Ag2S at 90°C is 7.7 × 1012 cm-3 14. Since this is about 80 times the thermally excited charge density of Si at the same 90ºC, it is considered that thermally excited carriers from Ag2S sufficient for power generation. Therefore, in this paper, we report the photoexcitation and thermal excitation power generation performance of Ag2S sensitized cell and tried to fabricate a sheet-type STC.

1. Methods

1.1.

Fabrication of TiO2 layer. Since the photocatalyst and the sensitizing solar cell

researches of the combination of TiO2 and Ag2S had reported that the conduction band of Ag2S is about 0.2 eV above TiO2

13,

TiO2 was selected for the electron

transport layer. Titanium dioxide paste was prepared following the process of Graezel15. A mixture of 1.5 g TiO2 nanoparticles (P25, Nippon Aerosil Co., Ltd., Japan), 0.375 g ethyl cellulose (46070-250G-F, 33837-250G,Sigma Aldrich, U.S.A.),


6

Page 7 of 36 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


and 5 g terpineol (Fujifilm Wako Pure Chemical Co., Japan) was grounded in an aluminum mortar with ethanol and stirred in a plastic vessel. The TiO2 paste was squeegee printed on Fluorite doped tin oxide (FTO, 10 ) substrate with dimensions 1.5 x 2.0 cm using 10 µm spacer. The electrodes were sintered at 450oC for 1 h. The temperature increasing rate was 10 ºC/min.

1.2.

Fabrication of Ag electrode. Silver was photodeposited on the TiO2 electrode

under 1 mW Xenon light irradiated for 1.5 hours in 10 mL of 10 mM silver nitrate (Fujifilm Wako Pure Chemical Co., Japan) solution (water : ethanol = 9:1). The light intensity was measured at 355 nm.

1.3.

Fabrication of Ag2S electrode. The

fabricated Ag electrode was impregnated in 15 mL N,N-Dimethylformamide solution that contains 0.01 g sulfur powder (Fujifilm

Figure 2. Digital camera images of the TiO2 layer (a), Ag-photodeposited electrode (b), and sulfurized Agphotodeposited electrode (c), and the XRD patterns of the TiO2 layer (d) and the sulfurized Ag-photodeposited electrode (e).


7


Page 8 of 36

Wako Pure Chemical Corporation, Japan). The container was maintained at room temperature for 8 h, and after drying in vacuum oven at 100ºC for 24 h, metallic black substrate was obtained (Figure 2a-c).

1.4.

Fabrication of Ag2S electrode without TiO2 layer Silver was sputtered on FTO

substrate with 200 nm thickness. After that, the substrate was sulfided by the same method as the Ag2S electrode described above.

1.5.

Characterization. The crystal structures of each electrode were examined by X-

ray diffraction measurement using an X-ray diffractometer (XRD, XRD-6100, SIMADZU, 40 kV–30 mA, Japan) equipped with a graphite monochromator using the Cu Kα line (λ = 1.54 Å). The ultraviolet–visible–near infrared (UV-Vis-NIR) diffuse reflectance spectra of electrodes was measured by UV–Vis-NIR spectrometer (V770, JASCO, Japan) equipped with integrating-sphere photometer unit. Its wavelength resolution was ±1.5 nm (measured at 1312.2 nm). For reference measurement, white standard (BaSO4) was used. For observation of the surface morphology of electrodes, field emission scanning electron microscope (FE-SEM,


8

Page 9 of 36 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


JSM-7500F/JEOL, Japan) was used. The cross section of Ag2S electrode was also observed and its element distribution was confirmed by using FE-SEM (S4700, HITACHI, Japan) equipped with energy dispersive X-ray analyzer (EDS, Genesis/EDAX, USA). The cross section was obtained by applying stress to the substrate and cracking it.

The temperature dependence on surface resistivity of Ag2S electrodes (1.5 ×1.5 cm) was examined by four-terminal method on a hot plate (Digital hot plate ninos ND-1, ASONE) from room temperature to 110oC. Cu wires (1.6 mm) was adhered to the electrode surface with insulating tape. The terminal spacing was 3 mm. Source meter (Keithley 2400, Keithley, USA) was used for the measurement.

To check the flat band potential of Ag2S electrode, electrochemical impedance spectroscopy measurement （ HSV-100, Hokuto Denko, Japan ） was conducted. The Ag2S electrode, Pt wire for the counter electrode and Ag/AgCl electrode for the reference electrode were dipped in 0.1 M sodium sulphate solution. The applied


9


Page 10 of 36

voltage was -0.5 to 0.2 V, the AC amplitude was 10 mV, and the frequency was 1 MHz to 100 Hz.

For determination of valence band position, atmospheric photoelectron yield spectroscopy was carried out using AC-3 (Riken Keiki, Japan). The incident light energy was 4 to 7 eV.

1.6.

Cell assembling. Pt (100 nm thickness) with 5 -nm-thickness Cr intermediate

layer was sputtered on a FTO substrate with a 1-mm-diameter hole, and used as a counter electrode. The counter electrode and the Ag2S electrode (or the Ag electrode) were adhered with an insulating tape (Double-Coated Kapton® Polyimide Film P-223, Nitto, Japan) with a hole of 6 mm in diameter. The gap was filled with an electrolyte and the 1-mm-diameter hole of counter electrode was sealed with an insulating tape. The electrolyte was 0.1 M ferrocene (98.0%, Fujifilm Wako Pure Chemical Co., Japan,) in dimethylsulfoxide (99.0%, Fujifilm Wako Pure Chemical Co., Japan ） solution having a boiling point of 189oC. The cells named as Ag2S STCs or Ag STCs.


10

Page 11 of 36 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


1.7.

Measurement of cell property. Cyclic voltammetry (CV) measurement of the cells

was carried out on a potentiostat / galvanostat system (HSV-100, Hokuto Denko, Japan). The applied voltage was +0.01 V to -0.10 V and scan rate was 1 mV/sec. The potential sweep started from -0.1 V and the third lap was used for the discussion below. The measurements were performed with a solar simulator (AM1.5, Jasco, Japan), at room temperature in the dark, and at 90oC in the dark. CV measurement was carried out also at 80, 100, and 110oC to check the temperature dependence measurement. For the heating, the cell was put in a constant temperature bath (NS111L, Isuzu, Japan) and maintained at each temperature for at least

10

min

before

measurement.

At

90oC,

electrochemical

impedance

spectroscopy (EIS) measurement was performed by using potentiostat / galvanostat system (VSP-300, Toyo Co., Japan). The AC amplitude was 10 mV, and the frequency was 7 MHz to 1 Hz.

1.8.

ICP-OES Spectroscopy. Inductively coupled plasma – optical emission

spectrometry (ICP-OES) was carried out on ICP-Mass (ICP-MS) Spectrometer


11


Page 12 of 36

(Agilent 7700x Agilent, Japan) to check the electrode dissolution into the electrolyte. For the ICP-MS spectroscopy, Ag2S or Ag electrode was dipped into 8 mL dimethyl sulfide (DMSO) and maintained 24 h at 90oC, and the supernatant was analyzed. Since the area of electrodes was different, analysis results standardized by a 6 mm diameter circle which is cell area.

2. Results and Discussion

In this system, power generation by Ag2S sensitized thermal cells was aimed. From the prior researches of photocatalysts and sensitized solar cells, it has been shown that the conduction band of Ag2S is about 0.2 eV above TiO213. Therefore, TiO2 was selected as the electron transport layer in this study. As redox ions, ferrocene having an oxidation-reduction level near the valence band of Ag2S was selected from welldiscussed redox couples in nonaqueous solvents. Details are described below.

2.1.

Characterization of the electrodes


12

Page 13 of 36 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


Characterization. The XRD pattern of the fabricated TiO2 layer shows peaks of SnO2 contained in FTO substrate and anatase TiO2 (Figure 2d). The XRD pattern of the sulfurized Ag-photodeposited electrode reveals that the electrode includes monoclinic Ag2S and metal silver (Figure 2e). It is confirmed that monoclinic Ag2S is stable from room temperature to 175oC by DTA-TG measurement (Figure S1). A phase transition between monoclinic acanthite to body-centered cubic argentite can be seen at 175oC16. Cubic Ag2S is famous as ionic-electronic mixed superionic conductor and its conductivity behave like metal to temperature17. In this research, Ag2S has a role as a semiconductor, the monoclinic Ag2S preferable. The SEM images of surface of the TiO2

Figure 3. SEM images of the top views

of

TiO2

layer

(a),

Ag-

photodeposited electrode (b), and sulfurized electrode,

Ag-photodeposited and

the

elemental

analysis mapping images of the cross section of the sulfurized Agphotodeposited electrode (d-h). The SEM image (a), S (b), Ag (c), Sn (d),

and

Ti

(e).

Temperature

dependence on sheet resistance of the

sulfurized

ACS Paragon Plus Environment electrode (i).

Ag-photodeposited

13


Page 14 of 36

layer, Ag-photodeposited electrode, and sulfurized Ag-photodeposited electrode were shown in Figure 3. The TiO2 layer presented originally porous structure (Figure 3a). After the photodeposition, the TiO2 layer was covered with horny particles (200~300 nm diameter) (Figure 3b), considered as photodeposited Ag particles. As a result of sulfurization, the particle size became larger (Figure 3c). The particle size around 500 nm diameter was small enough to make a sheet-type STC.

A cross section image of sulfurized Ag-photodeposited electrode indicates that there were two layers on the FTO substrate (Figure 3d). The bottom layer was considered as 400 nm thick TiO2 layer and the upper layer was considered as photodeposited Ag layer with 200 nm thickness from EDS analysis (Figure 3e-3h). Because signal of sulfur was detected from Ag layer (Figure 3e), it is understood that Ag layer was successfully sulfurized. Moreover, the detection of Ag and S signal from TiO2 layer indicate that Ag2S was also fabricated in TiO2 layer. These results suggest that photo-exited electron in TiO2 reduced Ag+ ion to Ag on TiO2, as we expected.


14

Page 15 of 36 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


The sheet resistance decreased due to the temperature rise, and the semiconductor property of the present electrode was confirmed (Figure 3i).

Estimation of band structure of Ag2S electrode. A diffusion reflectance spectrum and Tauc plot of Ag2S electrode are shown in Figures 4a and 4b, respectively. The band structure of Ag2S was reported as an indirect bandgap, but the electrical and optical features similar to a direct bandgap material because of the conduction band proximity to zone center18. Therefore, bandgap of Ag2S can be determined from this Tauc plot. The optical bandgap of the fabricated Ag2S electrode was calculated about ~1.0 eV. This value is consistent with the literature value7.


15


Page 16 of 36

In the Mott-Schottky plot (Figure 4c), Ag2S acted like n-type semiconductor. It is considerable that there are Ag+ ions in the crystal lattice of Ag2S19. The flat band

Figure 4.

Diffusion reflectance spectrum (a), Tauc plot (b), Mott-Schottky plot (c),

atmospheric photoelectron yield spectroscopy (circle, d), and the schematic band diagram (e) of the sulfurized Ag-photodeposited electrode. The atmospheric photoelectron yield spectroscopy of the Ag2S powder on FTO (diamond, d) is also shown.


16

Page 17 of 36 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


potential of Ag2S was shown as -0.23 V (vs. normal hydrogen electrode (NHE)). This result matches well as the previous reseach20. From these results, the lower end of the conduction band is considered to be located at a lower energy level in the vicinity of 0.23 V (vs. NHE).

Using Atmospheric photoelectron yield spectroscopy, measurement of working function of Ag2S electrode was conducted. In this measurement, photoelectrons jumped out of the sample are caught by oxygen molecules in the atmosphere, and the photoelectron yield is estimated by counting them. It is known that there is a relationship for photoelectron yield and energy of incident light such as

where Φ is a work

function, and the value of n varies depending on metal or semiconductor

21-22.

In the

case of direct transition semiconductor, n value is 222. From the result of plot described above (Figure 4d), photoelectron emission of Ag2S electrode was suppressed to 1.46 V (vs. NHE) compared to the electrode without TiO2 layer. Considering that the conduction band position of Ag2S is less than -0.23 V and the bandgap value is about 1.0 eV, this value is not the working function of Ag2S electrode itself, but the diffusion


17


Page 18 of 36

potential reflects the junction with TiO2. That is, it turned out that TiO2 operated effectively as an electron transport material.

Consequently, the band diagram might be as Figure 4e. The reported position of Ag2S conduction band was -0.34 V (vs. NHE)13, thus the valence band might be around +0.66 V. Here, the oxidation-reduction potential of ferrocene in dimethylsulfoxide is +0.59 V (vs. NHE)

23-24.

Since it is lower than the valence band of Ag2S, holes can

transfer from Ag2S to electrolytes. Therefore, it is indicated that redox reaction possibly occurred due to thermally excited carriers in Ag2S.

2.2.

Battery characteristic


18

Page 19 of 36 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


The thickness of the completed cell is 2.5 mm and most of its thickness is the thickness of the FTO glass (1.2 mm thickness). If this part is changed to plastic FTO sheet, a thinner cell can be achieved. With

this

in

mind,

we

confirmed

the

possibility of the power generation by the following experiment.

Light irradiation and thermal excitation. The fabricated battery was in sheet form similar to DSSC, and the thickness was almost substrate dependent (Figure 5a). Figure 5b illustrates I-V characteristics of Ag2S STCs

Figure 5. The digital camera image of the fabricated STC (a). I-V curves under light

with light irradiation, at room temperature in

irradiation at room temperature, without light irradiation at room temperature, and

the dark, and with 90oC heating in the dark.

at 90 oC without light irradiation (b). The temperature dependence of I-V curves (c), and the Arrhenius plot of short circuit current (d). The scan rate for I-V curves

ACS Paragon Plus Environment was 1 mV/ sec.

19


Page 20 of 36

An increase in open circuit voltage and short circuit current was confirmed both at light irradiation and heating compared to the room temperature in the dark. Open circuit voltage (Voc) and short circuit current (Jsc) were 45 mV and 0.14 µA/cm2 at 90oC and 139 mV and 0.2 µA/cm2 with light irradiation. The difference of Voc between light and heat is theoretically intrinsic4, and will be described below.

The temperature dependence of I-V characteristics of Ag2S STCs shown in Figure 5c indicates Jsc increased as the operation temperature rises. The measurement temperature was from 80 ° C at which the electrical resistivity of the Ag2S electrode was sufficiently low (Figure 3i) to 110 ° C at which the spacer did not deteriorate. Since the sheet resistance of the Ag2S electrode was almost the same in the measurement temperature range (Figure 3i), this current change is thought to be due to the redox reaction. Arrhenius plot was performed from this temperature dependence (Figure 5d), and the activation energy was estimated to be 0.66 eV. There are roughly two different process in this system, one is generation of thermally excited carriers by semiconductor Ag2S, and another is a process related to redox reaction of Fc/Fc+ in the electrolyte. The


20

Page 21 of 36 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


activation energy from Arrhenius plot was inconsistent to the bandgap energy of Ag2S electrode from Tauc plot (~1.0 eV, Figure 4e). This suggests that a rate-depending step is the process related to the redox reaction of ferrocene.

In contrast, Voc kept about 45 mV regardless of temperature. Consideration will be given to the Voc difference between light irradiation and heating. First of all, the band narrowing of TiO2 and Ag2S due to heating and the change of redox potential of ferrocene can be conceived as causes Voc diminution at 90oC than with light irradiation. It is known that the temperature dependence of band gap is expressed by the following equation (2)25 and the change of the oxidation reduction level is described by the Nernst's equation as shown in the equation (3). If the influence on these change is large, it is contradictory to the poor temperature dependency of Voc during heating. Thus, these are not considered to be a direct factor of the decrease in open circuit voltage.


21


Page 22 of 36

It is considered that the theoretical maximum Voc of the DSSCs is given by the difference between the quasi Fermi level of the semiconductor electrode and the redox potential of the electrolyte. The quasi Fermi level is a concept introduced to express an increase in the amount of excited electrons during light irradiation, and it exists on the conduction band side of the Fermi level. On the other hand, in the case of heating, the thermally excited electrons exist in equilibrium in the conduction band, thus the theoretical maximum Voc is considered to be the difference between the Fermi level of the semiconductor electrode and the redox potential of the electrolyte.4 Since the Fermi level is almost constant regardless of the temperature, it is consistent with the result of absence of temperature dependence on Voc. It is considered that the difference between the quasi Fermi level and the Fermi level is a major factor of the difference in Voc between light irradiation and heating. From these discussions, the theoretical maximum Voc in this system is considered to be about 0.83 V from the flatband potential measurement and the redox potential of ferrocene (Figure 4).


22

Page 23 of 36 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


I-V characteristics comparison at 90oC between Ag2S STCs and Ag2S cell not containing ferrocene is shown in Figure 6a. Same Ag2S electrode was used. Power generation was observed only in Ag2S STCs, and it shows that the charge exchange successfully occurs between the Ag2S electrode and the redox pair ferrocene. I-V characteristics at

Figure 6. V-I curves with and without ferrocene

(a),

and

before

and

after

sulfurization (b).

90oC comparison between Ag2S STCs and Ag electrodes containing ferrocene illustrated in Figure 6b shows only Ag2S STCs generated power. This comparison clearly demonstrate that the charges derived from Ag2S contribute to increase of Voc and Jsc.

Figure 7a or 7b shows comparisons of Nyquist plots between Ag2S STCs, Ag2S cell not containing ferrocene or Ag cell with ferrocene at 90oC. In the case of no


23


Page 24 of 36

ferrocene, the circular arc became larger (Figure 7a). In the case of no sulfurization, the resistance at 1 Hz was increased (Figure 7b). These results are fitted by equivalent circuit illustrated in Figure 7c following cases of DSSCs. Rsol is resistance of electrolyte and wiring in the highest frequency region, R1 and CPE1 is corresponds to the circular arc appearing around 7 MHz frequency region and is considered to be an electric double layer at the metal/electrolyte interface such as Pt and Ag. Here CPE is the abbreviation of constant phase element. Typical tracing of diffusion appears in the low frequency region, it is described as mixing rate-determining step of charge transfer and diffusion using Warburg impedance. From calculated resistance of each interface shown in Figure 7d, Ag2S STCs and Ag cell had smaller R1 than Ag2S cell not containing ferrocene. This is suggested that the exchange of electrons at the electrode interface is smoothly performed due to the presence of ferrocene. For Ag cell containing ferrocene, R2 is much smaller than other cells. It is considered to be caused by good infiltration of electrolyte into inside of electrolyte decreased resistance because of difference of surface morphology of electrode.


24

Page 25 of 36 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


Influence of Ag2S dissolution. Based on the results so far, it was found that the battery characteristics at 90°C is attributable both Ag2S and ferrocene. In this section, whether the electric carriers from Ag2S is thermally excited carriers is considered. In addition to the current due to the thermally excited carriers, a current due to the charge released by elution of the electrode is conceivable. From ICP-MS analysis, very slight dissolution of

Figure 7. Nyquist plots with and without ferrocene (a) and with Ag2S or Ag electrode (b). The calculated equivalent circuit (c) and the obtained interface resistance of each cells (d). ACS Paragon Plus Environment

25


Page 26 of 36

Ag into DMSO was confirmed in both electrode, Ag2S and Ag electrode. Ag concentrations were 72.3 ppb in Ag2S electrode and 13.0 ppb in Ag electrode.

Because DMSO is a solvent sometimes used as a dispersion medium for Ag2S 26,

Ag2S hardly dissolves to DMSO. Since Ag was also present in Ag2S electrode from

XRD result, it is considered that the reaction of the formula (4) occurs not only in Ag electrode but also in the Ag2S electrode. Less Ag elution from the Ag2S electrode also supports this consideration.

If electrons traveling to the titanium oxide side, it flows to the valence band instead of the conduction band (-0.23 V vs NHE) of titanium oxide (Figure 4e)27. Therefore, it does not affect the STCs’ generation current confirmed this time. It suggests that the power generation at 90°C is attributable to redox reaction in electrolyte caused by thermally excited carriers from Ag2S.

3. Conclusion


26

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


To achieve a stylish, low-temperature operation sensitized thermal cell, Ag2S was investigated as the sensitizer. The electrode stable at 170oC joining Ag2S and TiO2 was successfully fabricated, and a sheet-type sensitized thermal cell (STC) was assembled using ferrocene DMSO solution as electrolyte. The power generation using Ag2S was convinced both by light irradiation and heating over 80oC. Voc and Jsc were 45 mV and 0.14 µA/cm2 at 90oC and 139 mV and 0.2 µA/cm2 with light irradiation. The difference in Voc between light irradiation and heating was interpreted from the difference between quasi Fermi level and Fermi level, as discussed in our previous report4. From electrochemical measurements of the cells with and without sulfurization, with and without redox ions, and ICP-MS analysis, we concluded that the redox reaction by thermally excited carriers derived from Ag2S resulted in the power generation. The thickness of the completed cell is 2.5 mm and most of its thickness is the thickness of the FTO glass (1.2 mm thickness). If this part is changed to plastic FTO sheet, a thinner cell can be achieved. This result is an example of the first STC where the semiconductor part is less than micro size, not only improving the volume density of


27


Page 28 of 36

STC but also showing the possibility of STC with design awareness including flexibility; it opens up the various applications such as IoT power supply and STC fibrosis.

ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the ACS publications website at DOI:

Thermal property of Ag2S powder (PDF)

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected], Tel: +81-3-5734-2525

ORCID Sachiko Matsushita: 0000-0001-8699-295X


28

Page 29 of 36 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


Toshihiro Isobe: 0000-0002-2726-6728 Akira Nakajima: 0000-0003-0056-9283

Author Contributions S.M. organized the research project. Y.I. performed the experiments and analyzed the results. Y.I. and S.M. wrote the manuscript. T.I. and A.N. supported the research.

Funding Sources This work was financially supported by Tohnic Co.(Japan), The Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant Number JP16K14054), and The Japanese New Energy and Industrial Technology Development Organization (NEDO) fund

(16101200-0).

Notes The authors declare no competing interests

ACKNOWLEDGMENT


29


Page 30 of 36

This work was financially supported by The Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant Number JP16K14054) and Tohnic corporation, Japan, and technically supported by Ookayama Analysis Division, Tech. Dept., Tokyo Tech.

ABBREVIATIONS

STC, sensitized thermal cell; DSSC, a dye-sensitized solar cell


30

Page 31 of 36 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


REFERENCES (1)

Uchida, K.; Takahashi, S.; Harii, K.; Ieda, J.; Koshibae, W.; Ando, K.; Maekawa,

S.; Saitoh, E., Observation of the spin Seebeck effect. Nature 2008, 455, 778-781. (2)

Kiely, J. H.; Dong-Hi, L., Characteristics of Bi0.5Sb1.5Te3/Be2Te2.4Se0.6 thin-film

thermoelectric devices for power generation. Meas. Sci. Technol. 1997, 8, 661. (3)

Matsushita, S.; Tsuruoka, A.; Kobayashi, E.; Isobe, T.; Nakajima, A., Redox

reactions by thermally excited charge carriers: towards sensitized thermal cells. Mater.

Horiz. 2017, 4, 649–656. (4)

Matsushita, S.; Sugawara, S.; Isobe, T.; Nakajima, A., Temperature Dependence

of a Perovskite-Sensitized Solar Cell: A Sensitized “Thermal” Cell. ACS Appl. Energy

Mater. 2019, 2, 13-18. (5)

Grätzel, M., Dye-sensitized solar cells. J. Photochem. Photobiol. 2003, 4, 145-

153.


31


(6)

Page 32 of 36

Mizuguchi, J.; Shinbara, T., Disposal of used optical disks utilizing thermally-

excited holes in titanium dioxide at high temperatures: A complete decomposition of polycarbonate. J. Appl. Phys. 2004, 96, 3514-3519. (7)

Lei, Y.; Gu, L.; Zheng, L.; Yang, X.; He, W.; Gao, Y.; Zheng, Z., Wavelength-

dependent charge carrier dynamics: the case of Ag2S/organic thin films heterojunction solar cells. Electrochim. Acta 2017, 252, 33-40. (8)

Li, Y.; Li, L.; Gong, Y.; Bai, S.; Ju, H.; Wang, C.; Xu, Q.; Zhu, J.; Jiang, J.; Xiong,

Y., Towards full-spectrum photocatalysis: Achieving a Z-scheme between Ag2S and TiO2 by engineering energy band alignment with interfacial Ag. Nano Res. 2015, 8, 3621-3629. (9)

Javidi, J.; Haeri, A.; Shirazi, F. H.; Kobarfard, F.; Dadashzadeh, S., Synthesis,

Characterization, In Vivo Imaging, Hemolysis, and Toxicity of Hydrophilic Ag2S NearInfrared Quantum Dots. J. Clust. Sci. 2017, 28, 165-178. (10)

Guo, X.; Gao, J.; Zhang, Z.; Xiao, S.; Pan, D.; Zhou, C.; Shen, J.; Hong, J.;

Yang, Y., Highly efficient interfacial layer using SILAR-derived Ag2S quantum dots for solid-state bifacial dye-sensitized solar cells. Materials Today Energy 2017, 5, 320-330.


32

Page 33 of 36 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


(11)

Tubtimtae, A.; Wu, K.-L.; Tung, H.-Y.; Lee, M.-W.; Wang, G. J., Ag2S quantum

dot-sensitized solar cells. Electrochem. Commun. 2010, 12, 1158-1160. (12)

Tubtimtae, A.; Cheng, K.-Y.; Lee, M.-W., Ag2S quantum dot-sensitized WO3

photoelectrodes for solar cells. J. Solid State Electrochem. 2014, 18, 1627-1633. (13)

Shen, H.; Jiao, X.; Oron, D.; Li, J.; Lin, H., Efficient electron injection in non-toxic

silver sulfide (Ag2S) sensitized solar cells. J. Power Sources 2013, 240, 8-13. (14)

Ovchinnikov, O. V.; Smirnov, M. S.; Shapiro, B. I.; Shatskikh, T. S.; Perepelitsa,

A. S.; Korolev, N. V., Optical and structural properties of ensembles of colloidal Ag2S quantum dots in gelatin. Semiconductors 2015, 49, 373-379. (15)

Ito, S.; Chen, P.; Comte, P.; Nazeeruddin, M. K.; Liska, P.; Péchy, P.; Grätzel,

M., Fabrication of screen-printing pastes from TiO2 powders for dye-sensitised solar cells. Prog. Photovoltaics. 2007, 15, 603-612. (16)

Martínez-Castañón, G. A.; Sánchez-Loredo, M. G.; Dorantes, H. J.; Martínez-

Mendoza, J. R.; Ortega-Zarzosa, G.; Ruiz, F., Characterization of silver sulfide nanoparticles synthesized by a simple precipitation method. Mater. Lett. 2005, 59, 529534.


33


(17)

Page 34 of 36

Hebb, M. H., Electrical Conductivity of Silver Sulfide. J. Chem. Phys. 1952, 20,

185-190. (18)

Lin, S.; Feng, Y.; Wen, X.; Zhang, P.; Woo, S.; Shrestha, S.; Conibeer, G.;

Huang, S., Theoretical and Experimental Investigation of the Electronic Structure and Quantum Confinement of Wet-Chemistry Synthesized Ag2S Nanocrystals. J. Phys.

Chem. C 2015, 119, 867-872. (19)

Morales-Masis, M.; van der Molen, S.; Hasegawa, T.; van Ruitenbeek, J., Bulk

and Surface Nucleation Processes in Ag2S Conductance Switches. Phys. Rev. B 2011,

84, 115310. (20)

Liu, T.; Liu, B.; Yang, L.; Ma, X.; Li, H.; Yin, S.; Sato, T.; Sekino, T.; Wang, Y.,

RGO/Ag2S/TiO2 ternary heterojunctions with highly enhanced UV-NIR photocatalytic activity and stability. Appl. Catal. B: Environ. 2017, 204, 593-601. (21)

Fowler, R. H., The Analysis of Photoelectric Sensitivity Curves for Clean Metals

at Various Temperatures. Phys. Rev. 1931, 38, 45-56. (22)

Fan, H. Y., Theory of Photoelectric Emission from Metals. Phys. Rev. 1945, 68,

43-52.


34

Page 35 of 36 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


(23)

Abeed, F. A.; Al-Allaf, T. A. K.; Sulaiman, S. T., Electroanalytical studies of

ferrocene and substituted ferrocene in non-aqueous solvents by rotating disc voltammetry. Analyst 1988, 113, 333-336. (24)

Tsierkezos, N. G., Cyclic voltammetric studies of Ferrocene in Nonaqueous

solvents in the temperature range from 248.1 to 298.15 K. J. Solution. Chem. 2007, 36, 289-302. (25)

Varshni, Y. P., Temperature dependence of the energy gap in semiconductors.

Physica 1967, 34, 149-154. (26)

Karimzadeh, R.; Aleali, H.; Mansour, N., Thermal nonlinear refraction properties

of Ag2S semiconductor nanocrystals with its application as a low power optical limiter.

Opt. Commun. 2011, 284, 2370-2375. (27)

Gritzner, G., Solvent effects on half-wave potentials. J. Phys. Chem. 1986, 90,

5478-5485.


35


Page 36 of 36

For Table of Contents Only


36

Ag2S-Sensitized Thermal Cell - ACS Publications

Recommend Documents