Ongoing research in triboelectric nanogenerators (TENGs) focuses on increasing power generation, but obstacles concerning economical and eco-friendly utilization of TENGs continue to prevail. Being the second most abundant biopolymer on earth, lignin offers a valuable opportunity for low-cost TENG applications in biomedical devices, benefitting from its biodegradability and biocompatibility. Here, we develop for the first time a lignin biopolymer based TENGs for harvesting mechanical energy in the environment, which shows great potential for self-powered biomedical devices among other applications and opens doors to new technologies that utilize otherwise wasted materials for economically feasible and ecologically friendly production of energy devices.
Recently, energy harvesting through sustainable approaches has become of interest not only to address the global energy crises but also to provide power for micro-scale electronics and sensors in emerging applications, such as wearable and implantable devices.1 An assortment of technologies have been developed to transform environmental energy into electrical power via a variety of mechanisms, including electromagnetic, electrostatic, piezoelectric, and, recently, triboelectric processes.2,3 Triboelectric nanogenerators (TENGs) are highly capable of efficiently harvesting ubiquitous mechanical energy, hinged on principles of contact triboelectrification and electrostatic induction, and have received considerable attention in recent years.4–9 Ongoing efforts are primarily focused on augmenting power generation by increasing the triboelectrification surface area, engineering the physical/chemical properties of contacting surfaces, and implementing practical applications. Most of the demonstrated TENGs were built based on synthetic polymers for the ease and cost of manufacturability. However, TENGs utilizing naturally abundant biological materials have received considerably less attention.9,10 Obstacles concerning practical, eco-friendly utilization of TENGs such as the intricate fabrication and expensive machinery continue to prevail.
Lignin, despite being the second most abundant biopolymer on earth, has few practical applications and a small market value starting from around $300/ton.11 Water insoluble Kraft lignin, being the most abundant side product, is produced by the pulp and paper industry at a scale of 50–100 × 106 tons/annum, most of which is nevertheless burned as a cheap fuel, an economically unfeasible solution.11 Current applications of lignin are scarce, using only approximately 2%–5% of all lignin produced, and primarily utilize lignosulphonate, a chemically modified water-soluble lignin.12,13 Lignosulphonate supply is limited, and thus applications of lignin using insoluble Kraft lignin should be conceived.12 Current utilization of lignin includes binders for animal feed, bricks, ceramics, and road dust, in addition to adhesives. This limited employment is mostly due to the undefined, remarkably irregular structure of lignin, which is a highly branched, hydrophobic, three-dimensional biopolymer of p-hydroxyphenyl propanoid units.14,15 Still, the presence of highly active functional groups results in lignin being exceptionally accessible for chemical modification or polymerization to create high-value materials, e.g., carbon fibers and artificial perfumes.16 However, many of these applications for Kraft lignin are low yielding or manufactured at a small scale.12
Being an antioxidant, naturally degradable, biocompatible, and lacking in cytotoxicity, lignin offers a valuable opportunity as potential constituents in biomedical devices. The vast disparity in structure and surface properties make it finely tunable for controlled degradation which is desirable in implanted applications.11 Here, we present for the first time a lignin-derived nano-composite where lignin is integrated with starch to facilely produce a biocompatible film for harvesting mechanical energy via TENG methodology. By adjusting the glycerol and NaOH concentrations in the nano-composite, we can systematically engineer the physical and chemical properties of the nanocomposites for optimized triboelectric power generation, transforming the otherwise wasted biomaterials into a functional device like flexible TENG. This opens doors to new technologies that utilize waste materials for economically feasible and ecologically friendly systems in self-powered biomedical applications.1
As shown in Fig. 1, the structure of a lignin-based nano-composite TENG consists of a lignin-based film and a Kapton film, both of which are backed by copper electrodes. Lignin-based films were developed using natural waste materials and are biodegradable, eco-friendly, and low cost.11,17,18 According to the triboelectric series, Kapton has a stronger ability to acquire electrons while wood, of which lignin is a part, has a stronger tendency to lose electrons.19 When the lignin-based nano-composite film is brought into contact with the Kapton film, the difference from these triboelectric polarities leads to electrons flowing between the two films. The direction of electron flow is dictated by the relative difference in the surface properties, e.g., the surface work function, between the two films. Such surface properties of interest for the triboelectrification process are strongly dependent on the composition and preparation of the nanocomposites. Figure 1(a) shows a typical case where the as-prepared lignin nano-composite induces positive triboelectric charges upon the contact with the Kapton film. Separation of the two films [Fig. 1(a)] results in a potential difference between the two electrodes and causes electrons to flow in the opposite direction. These mechanically induced contact and separation events, therefore, give rise to the output electrical power through the back-and-forth flow of electrons in the external circuit [Fig. 1(a)]. The testing setup for this is shown in Fig. S1 of the supplementary material. A linear motor is applied to apply the controlled mechanical stimuli, with a peak force of 2.08 N bringing the electrode to contact.
We prepared three groups of nanocomposites to develop the lignin-derived TENG with engineered functionalities, e.g., stretchability and transparency, which are appealing for biomedical applications. The first group includes composites of lignin and starch. Since lignin is made of insoluble fibers that could not form a uniform film or gel by itself, we added starch to bind the lignin materials into a stable film. For the untreated films, a total of 1.064 g of Kraft lignin (Sigma-Aldrich) and potato starch (Alfa Aesar) were added to 10 ml water in the weight ratios of 1:9, 3:7, 5:5, and 7:3. The samples were stirred for 20 min at 135 °C until the starch gelatinized, and the solutions were slightly viscous. Each sample was then transferred into a square mold where it was further dried for approximately 24 h. The second group of composites was prepared by mixing the lignin-starch films obtained from the above procedures with glycerol. For the films with glycerol, lignin and starch in the weight ratio of 3:7 were added to 15 ml water and stirred for 20 min at 135 °C, similar to the procedure above. Then, glycerol was added at 3%, 6%, 9%, and 12% of the original solution weight. This mixture was stirred for 5 additional minutes at the same temperature and transferred to the square molds where they were dried for approximately 48 h. The third group was prepared by adding glycerol and sodium hydroxide to the lignin-starch composites. 0.1594 g lignin was dissolved in 10 ml each of 0.25M NaOH, 0.5M NaOH, and 0.75M NaOH and stirred for 5 min to ensure dissolution. For each sample, 1.4345 g starch was added to 5 ml water and stirred for 20 min at 135 °C. Then, the lignin solution and 0.9 g glycerol were added to the starch mixture and stirred for 5 min. This was transferred to plastic molds of dimensions 6.5 cm × 6.5 cm and dried for 48 h.
Fourier Transform Infrared Spectroscopy (FTIR) was used to examine the interactions between the constituents of the composite films [Fig. 1(b)]. Peaks at 1440 cm−1 and 2910 cm−1 indicate C–H bond bending and stretching, respectively. It can be seen that films containing NaOH have an intensified peak at 1440 cm−1, which is likely to be caused by the cleavage of the ether linkage between the p-hydroxyphenyl propanoid units in lignin.15 This is supported by the peak at approximately 1010 cm−1, which is attributed to the C–O stretching in starch and lignin. Meanwhile, the peak at 3305 cm−1 is accredited to the vibration of OH groups in starch, glycerol, and/or lignin stretching, in addition to the formation or lack thereof of intra- and inter-molecular hydrogen bonds. It can be seen that starch has a higher peak than lignin, most likely from the disruption of hydrogen bonding between the starch molecules by the relatively large lignin particles. Also, it can be clearly seen that glycerol has a profound effect here as the peak attributed to the vibration of OH groups is intensified. It is likely to be due to the increased number of hydroxyl groups and the increase in hydrogen bonding between glycerol and starch. These interactions have a profound effect on the TENG outputs of the final films to be shown in the following.
Pure starch and lignin-starch films without any additives tend to be brittle and delicate, as shown in Fig. 2(a). The surface of pure starch film is very smooth. The SEM image from the edge of a pure starch film (Fig. S2 of the supplementary material) was taken in comparison with the pure starch to demonstrate its flat surface. Moreover, the films demonstrate increased surface roughness with the increase in lignin to starch concentration, most likely caused by the large lignin particle size compared to the gelatinized starch [Fig. 2(b)]. Likewise, the hydrophobic lignin particles could disrupt the crosslinking within the starch matrix making the films weaker with the addition of lignin.19 However, the surface roughness caused by the lignin particles expands the area with which the Kapton film makes contact for potential triboelectrification. Such an increase in the surface area is expected to lead to enhanced triboelectrification which, in turn, ensues the increase in the output short-circuit current and open-circuit voltage from the pure starch film to the 3:7 lignin-starch film shown in Figs. 2(c) and 2(d) respectively, and summarized in Fig. 2(e). We further performed a one-way analysis of variance (ANOVA) to reveal the difference between the electrical outputs of these nanocomposites. The results of a multiple-comparison of the group means for triboelectric current with the p-value << 0.05 (Fig. S3 of the supplementary material) show that there is a statistically significant difference between the starch group and the other four groups, suggesting that the lignin significantly influences the TENG performance. The enhanced output with greater surface area and roughness is consistent with many other reports.20,21 The increased output could also be caused by the decrease in free hydroxyl groups. Starch consists of numerous hydroxyl groups and oxygen atoms, which have high electronegativity. However, lignin is a large molecule that could significantly disrupt the hydrogen bonding between starch chains, which is supported by the FTIR data [Fig. 1(b)] as seen by the decrease in peak intensity at 3305 cm−1. This reduction in free hydroxyl groups results in a relative weaker ability to absorb electrons, thus increasing the differences between lignin-starch films and Kapton. Hence, the electrical outputs improved noticeably by coupling the surface engineering and material modification. Interestingly, it can also be seen that the TENG output decreases with increasing the lignin-starch ratio further [Figs. 2(c)–2(e)]. The decreases in output for films with 5:5 and 7:3 lignin-starch ratios are expected to be induced by the aggregated lignin clusters. When the intermolecular reactions between lignin and starch were saturated, the excess lignin molecules aggregated as clusters [Fig. 2(b)]. Those lignin units have relatively high conductivity compared with the uniform films, weakening the ability to trap electrons, which means that the attracted electrons cannot be effectively induced in electrodes. Overall, the electrical outputs were attenuated. When the lignin-starch ratio is 1:9 or 3:7, the lignin addition was not substantial enough to result in a significant decrease in the TENG output, while the increase in the electrical output due to the enhanced surface area and electronegativity dominates. Thus, the largest output observed through these films is the 3:7 lignin-starch film, which could achieve an average short-circuit current of up to 3.96 nA/cm2 and an open-circuit voltage of 1.04 V/cm2. However, produced through a casting process, these films tend to deform with curvatures and break easily, lacking the mechanical and morphological stability necessary for practical applications.
Films with glycerol were attempted to improve both the mechanical and TENG properties of the films. Since the 3:7 lignin-starch film resulted in the highest TENG output, this ratio was selected to further investigate the effects of glycerol. We observed that even with the smallest addition of glycerol (3% w/w), the film drastically improved in strength and flexibility, making it much more useful in practical applications and this pattern continued with the increase in glycerol. Optical images in Fig. 3(a) demonstrate films that are less likely to crack and break with the addition of glycerol. This improvement in stability is probably due to the OH groups in glycerol which interact with H2O from the original casting solution through the hydrogen bond, resulting in more flexible films. However, as glycerol concentration increases, more H2O stays within the film instead of evaporating, resulting in softer and more delicate films.22 Also owing to the OH groups in glycerol, it can be seen from Figs. 3(b)–3(d) that glycerol additionally has a favorable effect on the TENG properties of the film. The ANOVA results of a multiple-comparison of the group means for triboelectric current with the p-value << 0.05 (Fig. S4 of the supplementary material) suggest that the added glycerol significantly influences the TENG performance. The observed polarity reversal of the TENG output from the original films to those with glycerol is, in part, likely to be induced by the dramatic increase in OH groups from the glycerol, as supported by the FTIR data [Fig. 1(b)]. With more free hydroxyl groups, which have high electronegativity, the ability of lignin-based films for attracting electrons are improved, even stronger than that of Kapton, resulting in the reversal of the electrical output direction. The initial decrease in output current with the 3% supplement of glycerol is caused by the relatively small addition of glycerol, offsetting the original electron-losing tendency of lignin-starch films; however, with the increase of glycerol to 6%, the output is significantly enhanced, with the highly electronegative hydroxyl groups giving rise to a larger potential difference between the lignin-starch and the Kapton films. Also, it can be seen that the output decreases in the films with 9% and 12% glycerol due to the tendency of the hydroxyl groups to hydrogen bond to H2O. The excess H2O dominates the interactions between the two films, leading to a lower output.23 Nevertheless, flexible films such as these have great potential in the electronic skin,23,24 wearable devices,25–27 and self-powered sensor systems.1,2,28
Efforts were further made to develop a transparent and more stretchable film by dissolving the large particles of lignin in NaOH. The composite film with 1:9 lignin-starch ratio was selected for this study due to the relatively high TENG output produced by this composite film [Fig. 2(e)] and its improved transparency compared to that of the 3:7 lignin-starch film. Additionally, 6% glycerol was added to the composite as this film had the highest output when the lignin to starch ratio is 3:7 [Fig. 3(d)]. Lignin-based films with NaOH were fabricated by the method described above. The resulting films were more elastic, transparent, and stable than those without NaOH. As shown in Fig. 4(a), the transparency of the composite was drastically improved with the addition of 0.25M NaOH. However, the transparency became noticeably worse as the concentrations of NaOH increased. This is probably due to the increased concentration of smaller molecules that have the ability to settle more densely. The sample with 0.25M NaOH was more translucent than the rest which could be due to the decrease in the size of the particles making it more transparent than the film without NaOH, but not small enough to densely pack the film.
The addition of NaOH also increases the TENG output of the composite films, as shown in Figs. 4(b)–4(d), which is further confirmed by the one-way ANOVA analysis (Fig. S5 of the supplementary material). Such enhancement is likely to be due to the decrease in the conductivity of lignin. Lignin is intrinsically conductive, but the cleavage of ether bonds by NaOH could reduce this, resulting in a higher dielectric constant and enhanced TENG output with more efficient charge transfer.22 Interestingly, it can be seen that there is a decrease in output from the film with 0.5M NaOH to the film with 0.75M NaOH. At this point, we expect that NaOH has reacted with all possible bonds within the lignin particles. Afterward, extra NaOH remaining in films functions as the ionic conductor, partially shadowing the electrons transfer and leading to decreased electrical outputs. A maximum output power density of 173.5 nW/cm2 was achieved for the composite film with 0.50M NaOH. This was calculated from the product of the voltage and current under different loads, which is summarized in Fig. S6 of the supplementary material. Transparent films such as these have many potential applications such as for harvesting energy from water29 or for raindrops on solar cells.30 Also, the gained stretchability might enable applications in the novel flexible electronics. Furthermore, the long-term mechanical durability of the lignin-based TENG using a lignin-composite film with 6% glycerol and 0.25M NaOH was shown in Fig. S7 of the supplementary material. We can see that the open-circuit voltage is stable over 1 h with 0.5 Hz frequency (about 1800 cycles of contact-separation). The good durability indicates the potential of lignin-based TENG for practical applications.
In conclusion, lignin biopolymer composite films were investigated to optimize the triboelectric power generation for practical applications. Properties of films were engineered by chemical and physical modifications. Statistical analysis was performed to guide the materials and device design. Using environmentally friendly and otherwise wasted materials, we developed an economically feasible approach for producing flexible biopolymer based TENG that has a relatively high power density of 173.5 nW/cm2. The lignin-based TENG demonstrated here shows great potential for self-powered biomedical devices and opens doors to new technologies that utilize waste materials for economically feasible and ecologically friendly production of functional devices in energy, electronics, and sensor applications.
See supplementary material for the supplementary data and figures.
W.Z.W. is thankful to the startup fund from the College of Engineering and School of Industrial Engineering at Purdue University.