Triboelectric nanogenerator based on human hair - ACS Sustainable

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Triboelectric nanogenerator based on human hair E. Nirmada Jayaweera, K. Rohana Wijewardhana, Thilini K. Ekanayaka, Amir Shahzad, and Jang-Kun Song ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00136 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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ACS Sustainable Chemistry & Engineering

Triboelectric nanogenerator based on human hair

E. N. Jayaweera, K. Rohana Wijewardhana, Thilini K. Ekanayaka, Amir Shahzad, and Jang-Kun Song* School of Electronic & Electrical Engineering, Sungkyunkwan University, Jangan-Gu, Suwon, Gyeonggi-do 440-746, Korea. *Corresponding author: Jang-Kun Song E-mail address: [email protected]; Fax: +82-(0)502-302-0912; Tel:+82-(0)31-299-4599.

ABSTRACT Triboelectric nanogenerators (TENGs) can be incorporated into modern electronic devices requiring sustainable, renewable, and reliable microscale energy sources. We report the first use of human hair, which is known to be a highly triboelectric material, for the fabrication of bio-based TENGs. Ethanolic NaOH was used to dissolve hair, and two simple fabrication techniques, bar- and spin-coating methods, were used to prepare hair-based films on electrode substrates. The dissolved hair paste has somewhat different chemical composition from original hair, but the hair-based film has almost the same level as the untreated human hair in the triboelectric series. The spin-coated film is thinner and has more even surface compared to the bar-coated one, and exhibits better performance in triboelectric generation. The TENG using a spin-coated hair film produced the maximum peak-to-peak voltage of 103 V and the power density of 60 mW m-2 across a 1.2 MΩ resistor; using the TENG device, an array of LEDs was in-situ lighted without an aid of energy collecting capacitors. A bio-waste human hair offers the advantage of easy accessibility and processability into a highly tribopositive material for TENGs,

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and our work thus broadens the choice of positive tribo-materials and offers a novel approach for fabricating cost-effective, high efficient, and bio-based TENGs.

Keywords: Energy harvesting, Hair, Triboelectricity, Spin-coated, Bar-coated

INTRODUCTION The proliferation of electronic devices has made the quest for viable technologies capable of harvesting energy from their surrounding environment a prime concern. A promising approach involves the use of triboelectric nanogenerators (TENG), which effectively convert mechanical energy into electricity and have attracted considerable attention due to their high efficiency, large power output, and low cost.1 When two materials with relatively opposite tribopolarities are brought into contact, charges move between them due to their differing electron affinities. Consequently, repetitive contact and separation of two such materials can produce a significant triboelectric charge on interacting surfaces, thus generating triboelectricity, which is often combined with electrostatic induction.1,2 Four different operating modes are typically used in TENGs: vertical contact-separation,3 in-plane sliding,4 single-electrode,5 and freestanding triboelectric layer modes.6 These different modes have been used to scavenge mechanical energy associated with vibration, human motion, air bubble or water droplets, and wind.7−11 Following the initial development of triboelectric generators, several approaches have been adopted to improve their performance, including using different combinations of materials, and modifying their structures or electrode surfaces.1−4 Electrode surface roughness can have a pronounced effect on TENG performance.1,3 The choice of materials can significantly enhance the performance of a corresponding TENG, since employing two materials positioned at opposite


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ends of the triboelectric series can produce higher output voltages. While a lot of synthetic materials are available for the negative side of triboelectric series such as fluorinated polydimethylsiloxane and fluorinated ethylene propylene, only a few such materials are known for the positive side and further development is required.12 Human skin is known to be positioned at the positive end of the triboelectric series.11 Human hair also shows a profound triboelectrification effect which is often considered a nuisance in daily activities. Hair is a structurally complicated material comprising keratinized proteins, lipids and water.13 Since human skin and hair retain relatively high positive tribopolarity, several studies have reported TENGs based on human skin.14,15 However, human hair has not yet been used to fabricate TENGs, owing to the difficulty of processing hair into a form useful for incorporation into practical devices. However, the use of human hair in preparing electrodes for Li-ion batteries and super capacitors has been investigated.6−18 In this study, we report the use of human hair in the preparation of triboelectric nanogenerators. Generally, although human hairs are abundant, they are often considered a slowly-degrading waste material causing environmental problems. Thus, utilizing human hair enables the fabrication of cost-effective TENGs with a material that would otherwise be disposed of as waste, potentially easing a challenge to urban waste management systems. We dissolve human hair in an alkaline solution and then use it to prepare electrodes for TENGs operating in a contact separation mode. In this context, human hair-based material offers the advantage of easy processability which does not require lengthy and sophisticated fabrication techniques. The dissolved hair material demonstrates hair-like triboelectric properties making it preferable for use in antistatic combs. EXPERIMENTAL SECTION



The human hair used in this study was Asian black hair. Ethanol (Daejung, Korea, 94.5%) and NaOH (Daejung, Korea, 97%) were used without further purification. Hair was thoroughly washed with acetone, dried and chopped into small pieces (Figure 1a). Then, 0.6 g of hair was immersed overnight in a 2 M ethanolic NaOH solution at 60 °C (Figure 1b). The resulting hair pulp was washed several times with ethanol (Figure 1c). Then, indium tin oxide (ITO) glass substrates (20 Ω cm2) were cleaned in an acetone bath via ultrasonication. Two types of dissolved hair-based films were fabricated using the bar and spin-coating methods (Figure 2). For the former, a small amount of dissolved hair paste was applied on the ITO substrate and the hair film was fabricated using a bar coater (OSP 50). For the latter film, a solution was prepared by adding dissolved hair and de-ionized water in a weight ratio of 1:2 (Figure 1d), and the solution was then spin-coated onto the ITO substrates at 2000 rpm for 30 s. The prepared hair films were then dried at 60 °C for 4 h. The active area of the prepared electrodes was 2 cm × 3 cm and a Kapton film (on an ITO substrate) with the same active area was used as the other electrode.

Figure 1 (a) Chopped hair, (b) hair dissolved in ethanolic NaOH, (c) hair pulp after washing with ethanol, and (d) hair pulp dissolved in de-ionized water.


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The open-circuit voltage was measured using a Keithley digital multimeter (DMM 7510) with an input impedance of 10 MΩ at an average tapping frequency of 3.0 Hz. Scanning electron microscopy images (SEM) of hair-based films were obtained using a JEOL JSM 7600F field emission scanning electron microscope. Structural characterization and elemental analysis of the dissolved hair films were conducted with Fourier transform infrared spectroscopy (FTIR) and energy dispersive X-ray spectroscopy (EDX) (JEOL JSM 7600F).

RESULTS AND DISCUSSION Chemical and morphological analyses of the hair films were conducted by EDX and SEM. Figure 2a-i shows hair films prepared using bar-coating, with thickness of ~55 µm as seen in Figure 2a-ii. The surface morphology of the film is shown in Figures 2a-iii and 2a-iv. The thick horizontal lines are the bar-coater pattern, and irregular cracks appeared owing to increased surface tension caused by drying the sample. The SEM image at high magnification in Figure 2aiv shows that the film prepared using dissolved hair were even with no partially dissolved hair fibers present. Figure 2b-i shows a transparent hair film prepared using the spin-coating method at 2000 rpm. Its transparency is due to a reduced thickness of 6.9 µm as shown in Figure 2b-ii. Unlike the bar-coated hair film, the spin-coated film had an even surface morphology without any cracks. The differences in the thickness and surface morphology of the two types of films are mainly caused by the different features of two coating methods. For the bar-coating method, a viscous hair paste was required, and the film was rather thick. In addition, owing to the sticky nature of the hair paste, a thinner bar-coater did not produce a uniform film in actual experiments.



On the other hand, a diluted solution was used in the spin-coating method, and the film was thin and uniform.

Figure 2. (a-i) Human hair-based film on ITO prepared by bar-coating, (a-ii) cross-sectional SEM image, and (a-iii, a-iv) surface morphology at different magnifications. (b-i) Human hairbased film on ITO prepared by spin-coating, (b-ii) cross-sectional image, and (b-iii, b-iv) surface morphology at different magnifications.

Table 1 shows the results of the EDS analysis, including the weight and atomic percentage of each element in the hair film. The Na concentration exceeding 11 wt.% was higher than that of normal hair, which is less than 1 wt.%. Although samples were thoroughly cleaned during preparation, Na ions from the ethanolic NaOH still remained in the dissolved sample. Amide groups present in human hair fibers can be hydrolyzed, yielding carboxylic acid groups


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and amines upon reaction with strong bases such as NaOH. The resulting carboxylic acid groups then produce a Na+ carboxylate salt. Other elements listed are primarily from the hair.

Table 1. EDS analysis of hair film. Element

Weight (%)

Atomic (%)

C

40.10

49.04

N

15.99

16.76

O

26.17

24.03

Na

11.72

7.49

S

5.21

2.39

Ca

0.81

0.30

FTIR analysis offers a reliable approach for qualitatively analyzing organic compounds, and human hair is a structurally complicated biomaterial that has been extensively studied using FTIR.19−21 Here, FTIR spectroscopic studies were performed in order to analyze the functional groups in the hair paste and dried hair films (Figure 3). For high wavenumbers, a broad absorption peak appears at 3290 cm-1 for the hair paste and at 3294 cm-1 for the hair film. The protein amide A band, corresponding to the N–H stretching mode, is also present as a strong absorption peak in the range of 3300–3250 cm-1.21,22 Note that the asymmetric and symmetric stretching modes of H–O–H strongly absorb at 3490 cm-1 and 3280 cm-1, respectively,22 and so the broad absorption peaks of the dissolved hair paste and film appearing at 3290 cm-1 and 3294 cm-1 are possibly due to the stretching of both N–H and the H–O–H group. Furthermore, a slight wavenumber shift of 4 cm-1 and the relative broadening of this peak for hair film compared to



hair paste may be caused by a further degradation of its chemical structure when the sample was heated at 60 °C for 4 h. The absorption bands at 2972–2910 cm-1 for the hair paste and at 2964– 2922 cm-1 for the hair film are the asymmetric stretching modes of CH3 and CH2, respectively.21,22 These bands show comparatively stronger absorption along with a redshift in wavenumber for the hair paste as compared to the hair film, revealing that further structural

3100

877

669

1334

1134 999

1622 1576

2964 2922

3600

671

1398

1637 1574 3294

1442

2972 2910

3290

Hair film Hair paste

1446 1400 1317 1276 1142 1047 1088 1001 880

deformation of the hair proteins occurred when the hair film was heated.

Absorbance (arb. units)

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

2600

2100

1600

1100

600

Wavenumber (cm-1)

Figure 3. FTIR spectra of dissolved hair-based film and paste.

The fingerprint region of FTIR at low wavenumbers is used to positively identify the presence of organic chemical compounds. Owing to the structural complexity of the material, a number of peaks appear in the fingerprint region of the spectra of the hair paste and film, and hence, it is difficult to thoroughly determine which structural deformations occurred. Generally, untreated human hair is comprised of proteins, which give rise to IR-active amide modes. Thus, three main amide bands caused by the peptide bonds in natural hair appear: amide I, at 1700– 1580 cm-1; amide II at 1580–1500 cm-1; and amide III at 1320–1210 cm-1.19−22 However, the two characteristic peaks usually attributed to the amide I and amide II bands shown in the FTIR


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spectrum of natural hair were not observed in the dissolved hair samples. Although a strong absorption peak at 1576/1574 cm-1 and a shoulder peak at 1622/1637 cm-1 were observed in the dissolved hair samples, they are not due to pure amide modes but rather to amides that hydrolyzed into carboxylic acid groups as they reacted with strong bases. The 1622/1637 cm-1 peaks could therefore be due to overlapping N–H bending and C=C stretching modes. The relative shift of the 1637 cm-1 peak of the dissolved hair-based film with respect to that of the paste may have been caused by adsorbed moisture. Strong peaks appearing at 1576/1574 cm-1 and 1400/1398 cm-1 can be assigned to the symmetric and anti-symmetric stretching modes of – COO-, respectively, as the carboxylic acid groups in the hair fibers ionized after reaction with NaOH.20 The shoulder appearing at 1446/1442 cm-1 is due to the scissoring mode of the CH2 group. The peaks observed at 1317/1334 and 1142/1134 cm-1 in the FTIR spectra of the dissolved hair paste/film may be due to the S=O groups produced by the oxidation of cysteine disulfide crosslinks that are present in natural hair fibers.20 The peak at 1276 cm-1 corresponding to the N–H bending mode is only discernible in the FTIR spectra of the dissolved hair paste, and strong absorptions at 1088 cm-1 and 1047 cm-1 are also observed only for the dissolved hair paste. The peak at 1088 cm-1 may be due to the presence of S=O, and the peak at 1047 cm-1 may be due to C–O stretching in the alcohol present in the hair paste as well as due to cysteic acid. Moreover, the absorption peaks at 1001/999 cm-1 and 880/877 cm-1 are related to the deformation of the hydrocarbons in the keratin proteins, and the peaks at 671/669 cm-1 can be assigned to the C–OH out-of-plane bending mode. Thus, the FTIR analysis reveals that the chemical structure of the natural hair was severely deformed after treatment with NaOH. Further, it also signifies that additional changes in the chemical structure of the material were caused by heating the hair paste at 60 °C for 4 h.



After tapping it with a given electrode, the polarity of a material helps determine whether it gains or loses electrons upon contact with another material. This explains why one material may have a higher affinity for gaining negative tribocharges relative to another. To determine the relative position of a human hair-based material in the triboelectric series, human skin, hair, aluminum, paper, and copper were tapped with a hair-based film, and the polarities of the corresponding signals for all materials were studied. Figure 4 depicts the voltages of the respective materials after tapping 20 times with the hair-based film. Paper, Cu, and Al provided negative voltages, and the voltage from skin became positive after 20 cycles of contact and separation. Notably, the human hair demonstrated a negligible voltage. Thus, the dissolved hairbased material lost more electrons than Al and gained more than human skin, and had a similar tribo-polarity to that of human hair. Interestingly, although the FTIR analysis shows evidence for chemical deformation as the hair dissolution occurred, the triboelectric nature of the two materials was quite similar.

Figure 4. (a) Measured voltage of different materials after tapping 20 times with hair-based film, and (b) corresponding order of the materials in the triboelectric series.


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The device structure and the working mechanism of hair-based TENGs is schematically represented in Figure 5. The TENG device is composed of two ITO glass substrates with hair film or Kapton film (2 cm × 3 cm), as illustrated in the right of Figure 5a. Initially, no charges are present on any surface as shown in Figure 5a. Since Kapton has a higher electron affinity than hair, electrons are transferred from the latter to the former when these two materials are brought into contact. As both materials are insulators, opposite tribocharges are confined to the interactive surfaces of the two films as depicted in Figure 5b.1 Upon releasing the two substrates, a potential difference is induced as a consequence of contact-electrification, driving electrons via an external circuit from the hair-based electrode towards that with the Kapton film, thus causing a negative current to flow to the external circuit (Figure 5c). When the two films are completely released (Figure 5d), the current drops to zero and no current reappears until the two electrodes are pressed again (Figure 5e). On subsequent pressing, an instantaneous positive current appears due to flowing electrons until the induced charges are neutralized.1,2 The release and pressing processes can be repeated to harvest triboelectric signals.



Figure 5. Working mechanism of hair-based TENGs. (a) Initial state of the TENG. The Device structure is shown in the right panel. (b) At first contact, no signal appears. (c) On subsequent releasing a negative electric signal is generated. (d) When the two films are completely released, the current drops to zero. (e) On subsequent pressing, a positive electric signal is generated.1 By collecting the generated signals, electric energy can be obtained. The electrode with Kapton is connected to the positive terminal of the multimeter.

The performance of hair-based TENGs was evaluated. The open-circuit voltages and output current for the two TENGs were measured across a 1.2 MΩ resistor. Figures 6a and 6b show the generated voltage and current as a function of time for the TENG using a bar-coated hair film, and Figures 6c and 6d show the same for a TENG prepared using a spin-coated hair film. The bar-coated TENG produces a maximum peak-to-peak voltage of 78 V and the spincoated TENG produced 103 V. The maximum peak-to-peak currents for the two devices were


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5.7 µA and 10.9 µA, respectively, and hence the calculated power densities were 19 mW m-2 and 60 mW m-2 for the TENGs employing bar-coated and spin-coated hair films, respectively, when these were connected to 1.2 MΩ resistor. The differences in the output power may be due to difference in film thicknesses and the surface morphologies of the films. Since the induced charge density inversely depends on the thickness of the triboelectric film, the thinner TENG with the spin-coated hair film will produce higher induced charge densities.1,2 However, the surface roughness of a triboelectric film is known to improve triboelectrification, but that advantage of the bar-coated hair film may be negated by its larger thickness, resulting in comparatively lower power. 45

a

b

35

Current (µA)

Voltage (V)

15 5 -5 -15 -25

0.8 -0.2 -1.2 -2.2

-35 -45

-3.2 0

10

20 Time (s)

30

40

55 45 35 25 15 5 -5 -15 -25 -35 -45 -55

d

0

10

20 Time (s)

30

40

0

10

20 Time (s)

30

40

6.0 4.0

Current (µA)

c

2.8 1.8

25

Voltage (V)

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


2.0 0.0 -2.0 -4.0 -6.0

0

10

20 Time (s)

30

40

Figure 6. (a,b) Generated voltage and current for a TENG using bar-coated hair film, and (c,d) for a TENG using spin-coated hair film.



The TENG using the spin-coated hair film was used to charge a 1 µF capacitor coupled to a rectifying circuit, and the corresponding charging curve is depicted in Figure 7. The tapping frequency was ~3.0 Hz, and only 100 s was needed to fully charge the capacitor. Further, the feasibility of hair-based TENGs as a direct power source to drive electronics was demonstrated by lighting nine serially-connected tri-chip LEDs (SMD 5050, SIRS-E Inc., USA). Figure 8 depicts the real time lighting of LEDs using spin-coated hair-based TENGs without the use of a capacitor. The output power from the large area 9 cm × 10 cm TENG was sufficient to brightly light up the LEDs (see Video S1 in Supporting Information for clearer observation). This area is small enough to be equipped under a shoe to collect energy from walking, as shown in Figure 8c, which shows the in-situ lighting of LEDs by gently tapping on the TENG with foot. The LEDs could also be lit even using a smaller area (2 cm × 3 cm) TENG although the brightness was lower than that with the larger TENG (Figure 8d). 45 40 35 30

Voltage (V)

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

25 20 15 10 5 0 0

50

100

150

200

Time (s)

Figure 7. Charging curve of a 1 µF capacitor obtained using the TENG employing the spincoated hair-based film.


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Figure 8. (a-i) Serially connected LEDs, (a-ii) real-time lighting of LEDs using a 9 cm × 10 cm TENG, (b) schematic diagram of the circuit used to light LEDs, (c) real-time lighting of LEDs by tapping with foot on 9 cm × 10 cm TENG and (d) LED lighting using small area 2 cm × 3 cm TENG.

Stability of the two TENGs based on spin-coated and bar-coated hair films were tested by measuring the output voltage of the respective TENGs after storing them in the ambient environment for 3 months (Figure 9). As shown in Figure 9a, the aged spin-coated hair-based TENG produced comparable output voltages to that of the fresh TENG. Yet, the performance of the TENG based on bar-coated film had slightly reduced after 3 months (Figure 9b). In the barcoated film, the cracks shown in Figure 2a developed further during the storage, and some of films were detached from the electrode, which causes the reduction in efficiency. Therefore, in this aspect, it can be shown that the spin-coated hair-based TENG is superior to that of barcoated hair-based TENG.



a-ii 45

45

25

25

Voltage (V)

Voltage (V)

a-i

5 -15 -35

5 -15 -35

-55

-55 0

10

20

30

40

Time (s)

b-i

0

10

45

45

35

35

25

25

15

15

5 -5 -15

20

30

40

30

40

Time (s)

b-ii

Voltage (V)

Voltage (V)

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

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5 -5 -15

-25

-25

-35

-35 -45

-45 0

10

20

30

40

0

10

20

Time (s)

Time (s)

Figure 9. Output voltage of the TENGs based on (a-i) fresh spin-coated hair film, (b-i) fresh barcoated hair film, and (a-ii), (b-ii) after storing spin-coated and bar-coated hair films in ambient environment for 3 months, respectively

CONCLUSIONS Human hair, dissolved using ethanolic NaOH, was successfully used to fabricate costeffective TENGs. Simple bar- and spin-coating fabrication techniques were used to prepare films from the dissolved hair. During the process, the films’ chemical structure was significantly changed, but the level of the hair films in the triboelectric series were nearly the same as those of untreated hair, indicating similar triboelectric property. The thicknesses of the bar- and spincoated hair films were 55 µm and 6.9 µm, respectively, and the electrical power collected from repeated contact/separation events were 19 mW·m-2 and 60 mW·m-2 across a 1.2 MΩ resistor respectively. The differences between the two devices primarily originated from the difference in their thickness. Hair ranks highest among materials in the positive triboelectric series, and the


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use of hair-based materials can improve the efficiency of any type of TENG device without sophisticated fabrication techniques. Moreover, the use of hair that would be otherwise wasted can remediate other issues affecting waste management systems.

ASSOCIATED CONTENT The Supporting Information is available free for charge on the ACS Publications website at DOI: Video clip for real-time lighting an array of LEDs using the hair-based nanogenerator (Video S1.avi).

AUTHOR INFORMATION Corresponding Author *[email protected]. Fax: +82-(0)502-302-0912; Tel:+82-(0)31-299-4599 ORCID J.K. Song: 0000-0003-1666-8438 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Korea Research Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF2016H1D3A1938043).



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Synopsis Human hair-based film has highly positive tribopolarity, and the bio-based material can be useful for efficient triboelectric nanogenerators.


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Triboelectric nanogenerator based on human hair - ACS Sustainable

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