Paper-based triboelectric nanogenerator and activities of salt ions

Aluminum foil (Hindalco, 10 microns thick), PTFE film (ELEX, 99.9% pure, width 12 mm, 0.075 mm thick), thin aluminium wires (0.3 mm diameter, 28.58 gauge by AWG no.), cellulose tissue paper (size 20 meter with 2-Ply approx., 10 cm width) were purchased from a local distributor. I-V characteristics were performed using Keithley 2450 and digital Multimeter Mastech model no. M92A.

We cleaned the Al foil using laboratory-grade detergent and DI water successively. Further, in 1 M aqueous HCl solution dipped the Al foil for ∼20 s to produce tiny micropores across the surface. After drying at room temperature, PTFE film was glued using Fevistik Super Glue Stick over one Al foil side. Secondly, cellulose tissue paper layers were pasted on both sides of the PTFE and Al foil to be a sandwiched structure, as shown in figures 1(a) and (b). The TENG structure is schematically illustrated (figure S1 (available online at stacks.iop.org/ERX/4/015002/mmedia)) and described the detailed methodology in the supplementary information. Figure 1 shows a schematic arrangement of the different materials and their internal mechanism of the charge polarized surfaces. The PTFE surface is considered a reference terminal regarding Al surface as a positive terminal, and TENG works in single-electrode mode. The wires connections are fixed at the junctions of PTFE-Paper and Aluminium-Paper. PTFE is an insulator film referenced to negative electrostatic charge, intermediated by aluminum foil and cellulose paper surfaces.

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Negatively polarized electrostatic charge during mechanical impact partially displaced at the paper surface. At that time, the Al layer develops a positive charge partially moved at the cellulose paper surface (figure 1(c)). The PTFE coated paper induces partial positive charge polarization due to Al foil at the cellulose paper. We analyzed that PTFE transfers the negative charge toward paper and the positive electrostatic charge induced at the paper surface. As a result, a partial positive charge transfers toward the PTFE surface. On removing the force, the charge on the aluminum foil reduces that becomes partially polarized. The system approaches equal charge distribution following the equilibrium state between PTFE and Al layers, as shown in figures 1(d) and (e). This process, repeated by several stress-release cycles, results in charge separation at the opposite polarized charge surfaces. After reaching the initial state, TENG again generated opposite charge polarity at triboelectric layers and started forward charge flow (figure 1(f)). At these transitions, TENG develops a particular potential difference termed an energy harvesting unit under compressive cycles that validate the TENG's single electrode operation. At the molecular scale, microfibrils structured PTFE possesses significant dielectric performance. These structures pierced through the aluminum micropores. These interlayer structures rapidly respond to the charge relocation mechanism among aluminum foil, PTFE film, and tissue paper under mechanical interactions. A symbolic charge separation among the interlayers led by triboelectric layers spacing resulted in an enhanced electrostatic charge generation.

The dimensions and internal structures of the triboelectric layers play a distinct role in providing easy charge transfer. The PTFE film stores negative charge transports at the paper layers due to opposite charge polarization across the Al layer. The generated electrical signals of the TENGs correlated directly to the size (viz., area) of different triboelectric layers. Therefore, the induced electrostatic charge polarization increases as rising the electrode area. As shown in the schematic diagram (figure 2(a)), a series of fabricated nanogenerators are characterized with various sizes (1 × 1, 2 × 2, and 5 × 1 mm²), as shown in the digital image in figures 2(b) and (c).

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We observed that each nanogenerator's electrical performances with the variable sizes give irregular output responses. We explored TENG at the smallest size and its electrical parameters. However, current (I_sc) against current density (J, $\tfrac{{\rm{nA}}}{{{\rm{cm}}}^{2}}$) has revealed an excellent revamped charge performance, corresponding to increasing electrode area (figure 2(d)). In accordance, power density also has shown a non-linear reversible phenomenon against increasing output voltage (figure 2(e)). As a result, the TENG's extent areal density has produced higher electrical signals; therefore, 4 mm² TENG generates higher electrical signals than 1 mm². We obtained significant electrical responses by adding ionic NaCl micro-droplets at the TENG's surface.

Figure 3(a) illustrates that micro-droplets commence electrostatic charge ionization and evolve considerable charge accumulation at the triboelectric layers, enhancing the I-V characteristics. Figure 3(b) shows the PTFE fibrous structure, and at higher magnification figure S2, which is effectively stuck with the Al foil's micropores (figure 3(c)), constructed a close-pack system. The average diameter distribution of the pores of the Al layer is ∼4.44 μm. These structures would establish a direct channeling amidst oppositely polarized triboelectric layers. The PTFE microfibrous morphology can electrostatically accumulate improved charge aggregation. The wet agent through the tissue paper significantly dispersed over the Al and PTFE layers generating opposite charge polarization across the surfaces (figures 3(d) and figure S3). Tissue paper has porous morphology (avg. pore size ∼13 μm) and hygroscopic nature that readily adsorbs the ionic species and helps in rapid charge transport. TENG, without any movement, connected with the Keithley 2450 instrument. The ionic droplets are added over the TENG's surface and allowed to rest for about 60 s. After completing the electrochemical interactions, the generated electrical signals are recorded and observed an increased I_sc in the ± 1 V range (figure 3(e)). The nonlinear shape of the I-V responses is associated with the semiconducting trend. The TENG's internal resistance (R_in) according to sizes 1, 2, and 5 mm² are ∼18, ∼9, and ∼8 MΩ, respectively. The power against current characteristics exhibits a nonlinear behavior in the ± 1 V range (figure 3(f)). Tissue paper's porous surface readily adsorb the ionic species that escalate the charge carrier density and represent curves semiconducting trend. The transition time for each TENG is almost the same, providing a rapid response. Thus, an array of reduced size TENGs can also be possible for significant electrical signals. Moreover, fast electrostatic charge transfer consisting of V_oc and I_sc signals is acceptable operating micro-nano electronic devices.

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As discussed in section 2.1, efficient energy harvesting depends on the dimension and morphology of the electrodes. The 5 μL ionic salt was added over TENG, size of 5 cm², and examined concerned I_sc and V_oc, as shown in figure 4(a). We observed an improved I_sc (18.8 μA) and V_oc (0.803 V) when pouring the ionic salt solutions and simultaneously applying mechanical force at the TENG surface. Further, I_sc (∼15.5 μA) and V_oc (∼0.559) are recorded in the absence of ionic solution under the mechanical interactions. The constant output I_sc (∼7.8 μA) and V_oc (0. 44 V) in the absence of mechanical force and the presence of ionic solution generated electrical signals are slightly lower than the TENG under mechanical force (figure (4)). We also have recorded the generated electrical signals without any mechanical interactions and ionic activity for the pristine TENG samples, shown V_oc (∼0.25 V) and I_sc (∼3.5 μA) in table 1. The % increment of V_oc between ionic salt activated TENG and without any mechanical interactions based TENG is 56.81%. The TENG itself can generate this current quantity, adequate to operate micro-nano devices. Tissue paper's microporous structure (figure 3(d)) is excellent for ionic diffusion across the PTFE layer. The addition of NaCl ions rapidly escalates the charge transfer proficiency attributed to the ionization of electrostatically generated charge carriers. The triboelectric effect generated these charge carriers, and interaction through ionic species stimulated the charge density. The opposite charge carriers instantly accumulated at triboelectric layers when adding ionic droplets, generating higher charge activity. This hybrid mechanism can increase charge transfer capability across the TENG surface. We also carried out the TENG electrochemical performance using Autolab PGSTAT302N performing cyclic voltammogram (CV) under ionic activities in the 0–1 V potential window. Figure 4(b) shows that generated signals at various scan rates significantly exhibit a reduction of the Al layer in the range of 0.2–0.4 V. We envision that the triboelectric effect somewhat affects the ionic activity that leads to instability. Following the Randles−Sevcik equation [17], the peak current I_pa is linearly increasing due to the charge transfer phenomenon through ionically active redox species.

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This ionic charge carrying mechanism could be promising in sensing applications, particularly clinical diagnostic devices. For example, paper colorimetric dipstick test devices [18], paper lateral flow devices for analytes detection [19], and paper-based microfluidic devices [20]. Ionic salt droplets cannot significantly respond beyond a specific amount as I_sc and V_oc get saturated. Alternatively, ionic species raise the diffusional current by adding 0.1 M NaCl ions at the TENG surface. The CV profile shows variable I_sc values (table 1) at the various scan rates leads to reduction supposed to negative charge polarization (–7.65 to – 45 μA) across the triboelectric layers. An overall charge transfer mechanism leads to increased electrical conductivity for specific concentrations after adding fix amount of ionic species. This charge movement activity could consider as a self-detection of the ionic molecules. The following chemical reactions (equations (1) and (2)) exhibit the redox reactions providing cathodic responses in the presence of salt ions at absolute molar concentrations [21].

$$\begin{eqnarray}&&2{{\rm{H}}}_{2}{\rm{O}}+2{{\rm{e}}}^{-}\leftrightharpoons 2{{\rm{OH}}}^{-}+{{\rm{H}}}_{2}\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&\displaystyle \frac{1}{2}{{\rm{O}}}_{{\rm{2}}}+{{\rm{H}}}_{{\rm{2}}}{\rm{O}}+{{\rm{2e}}}^{-}\rightleftharpoons 2{{\rm{OH}}}^{-}\end{eqnarray} \tag{ 2 }$$

Similarly, Al layer gives anodic reaction (equation (3)) [22].

$$\begin{eqnarray}&&{{\rm{Al}}}_{\left({\rm{s}}\right)}+3{{\rm{OH}}}^{-}\leftrightharpoons {\rm{Al}}{\left({\rm{OH}}\right)}_{3}+3{{\rm{e}}}^{-}\end{eqnarray} \tag{ 3 }$$

In reactions (equations (4)), and (6) oxide components act as a protective layer against the material's degradation; however, it may limit the charge transfer efficiency [23].

$$\begin{eqnarray}&&{\rm{2Al}}{\left({\rm{OH}}\right)}_{3\,}\leftrightharpoons {{\rm{Al}}}_{2}{{\rm{O}}}_{3}\cdot {{\rm{3H}}}_{2}{\rm{O}}\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&{\rm{Al}}\leftrightharpoons {{\rm{Al}}}^{3+}{+\mathrm{3e}}^{-}\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&{{\rm{Al}}}^{3+}{+\mathrm{4Cl}}^{-}\leftrightharpoons {{\rm{AlCl}}}_{3}\end{eqnarray} \tag{ 6 }$$

The anodic ${\mathrm{Al}}^{3+}$ ions interacted with cathodic OH⁻ ions at the TENG surface, raising the electrostatic charge. The induced charge is proportional to the ionization of the surface charge density and areal density. Pouring with an ionic solution of NaCl at the TENG's surface exhibits an improved constant output once absorption is accomplished. The ionic molecules get saturated, and the tissue paper layer does not accommodate more ionic solutions. Mechanical stability slightly decays due to damped tissue paper. It can regain its initial state once wet molecules evaporate, which is possible from one to two times after that paper starts to degrade. All the measurements, such as I-V characteristics (with and without applying force) and CV profile, are performed using the single TENG device (figure 4). Once the TENG dries, it can be reused in further experiments, validated stability up to a specific paper limit, and the Al layer for about 100 cycles (100 mV s⁻¹) of the CV performance.

We estimated the energy harvesting characteristics of tissue paper-based TENG devices, V_oc, I_sc, and output power density are determined under a series of NaCl molar concentrations. A combined phenomenon of the triboelectric effect and ionic activity of the salt ions generate the electrical signals. At the time of drop-casting ionic solution over the TENG surface, the impact of the drop itself is considered as mechanical interaction. It is helpful to develop the triboelectric effect. Once the electrostatic charge is generated, the polarized electrostatic charge is stimulated by interacting with ionic species that appear in increased electrical signals. The damp region adsorbed ionic components leads to activated charge enhancement. A comparative study of the ionic solutions estimated the maximum output voltage of the TENG. The generated voltage values at 0.05, 0.5, 0.1 and 1 M solutions are 0.68, 0.65, 0.8, and 0.45 V, respectively for the 5 cm² TENG (figures 5(a) to (d)). The zig-zag patterns are supposed to be the triboelectric effect, which ultimately gets an increasing order due to the electrochemical impacts after interaction with ionic droplets, considered equivalent to sensing behavior. We observed that 0.1 M NaCl harvests a higher voltage value than other molar concentrations. The tissue paper's highly porous and good ion adsorbent nature is liable for increased short circuit displacement current (I_sd) across Al and PTFE layers, resulting in quick response at the various molar concentration of NaCl ions. The maximum I_sd (∼18.5 μA) is due to the triboelectric effect and 0.1 M solution (figure 5(e)). Related R_in is ∼43 kΩ observed, which is significantly low. The lower R_in allows easy charge transport supports in ions detection. Increased current density unveils total power density $\left({P}_{D}={I}_{{\rm{sc}}}\times {V}_{{\rm{oc}}}\right)$ as 0.032 W ${m}^{-2}$ at the 0.1 M NaCl (figure 5(f)). At this molarity, generated voltage reached ∼0.45 V after adding 15 μl, which improved the output response. Continuously adding 5 μl salt ions increased the voltage response (figure 6(a)). A linear calibration curve shows increasing voltage against volumetric ions and analyzed that V_oc response attributed to the ionic sensitivity (figure 6(b)). The above measured I_sd and V_oc are equivalent to the sensing parameters as correlation factors are ${{\rm{R}}}^{2}={\rm{0.9564}}\left({\rm{0.1}}\,{\rm{M}}\right)$ and ${R}^{2}={\rm{0.9632}}\left({\rm{0.5}}\,{\rm{M}}\right),$ and related sensitivities $\left(\tfrac{{\rm{V}}}{{{\rm{m}}}^{2}{\rm{M}}}\right)$ are ∼11.2 (0.5 M) and ∼14 (0.1 M).

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In addition, figures 6(c) and (d) exhibit TENG ionic activities at the higher molar concentrations (1 to 5 M) of the NaCl ions. Discharged ionic solution at the TENG's surface rapidly increases the generated electrical signals. The addition of an ionic solution successively limited the generated voltage value and moved toward saturation after a particular time duration. At higher concentrations (4 and 5 M) of the NaCl solution, TENG typically generates a constant value of the V_oc, reaching almost at the saturation level of the molar solution.