Table I is a summary of the various battery types, geometries, and tests undertaken. Each permutation will be discussed below.
Table I Summary of wax-encapsulated batteries tested in this study.
| Battery | Gel electrolyte | Cathode material | Corrosion inhibitors | Discharge load | Testing environment |
| Large cell* | Alkaline | Carbon paper | / | Continuous 30 µA | In the air |
| Large cell | Alkaline | Carbon paper | / | Continuous 30 µA | 5 cm under soil |
| Corn cell** | Alkaline | Carbon paper | PEG 600 +Tween 20 | Continuous 30 µA | In the air |
| Corn cell | Alkaline | Carbon paper | 0.3 g/L MLD | Continuous 30 µA | In the air |
| Corn cell | Alkaline | Carbon paper | 0.3 g/L MLD | 5% duty cycle | In the air |
| Corn cell | Alkaline | Carbon paper | 0.3 g/L MLD | 5% duty cycle | 5 cm under soil |
| Large cell | Neutral | Pt-loaded Carbon paper | / | 5% duty cycle | In the air |
| Large cell | Neutral | Pt-loaded Carbon paper | / | 5% duty cycle | 5 cm under soil |
| * Large cell refers to a battery of size 2x2x0.7 cm3 after encapsulating in wax, with a Zn anode of 1 cm diameter |
| ** Corn cell refers to a battery of size 10x8x5 mm3 after encapsulating in corn-sized wax package |
Prior to implementing the testing of Table I, two studies on battery subcomponents were performed; an alkaline gel study to optimize the gel parameters, and a cathode study to investigate the effect of the presence or absence of Pt in the cathode on the power performance of the battery.
Alkaline gel study
An alkaline gel study was undertaken using the large cell format, alkaline gels of various thicknesses, and Pt-loaded carbon cathode. Our previous work showed that the total output energy of the battery will increase with the gel mass when it operates in the gel-limited condition, and that batteries with minimal exposure to the working environment have the longest lifetime 8,20,21. Wax-encapsulated batteries with increasing amounts of gel electrolyte were assembled and discharged under a constant 30 µA both in the air and soil. Figure 3 (a) shows that an increase in the gel mass from 36 mg to approximately 250 mg resulted in an up to 3.6 times increase in battery lifetime. By increasing the gel electrolyte beyond 300 mg, the battery lifetime did not exhibit significant improvement.
Two potential mechanisms behind the observed variation of battery lifetime include carbonation and slow decomposition of an intermediate reaction product 20. The OH− ions in the hydrogel provide the ionic conductivity for the battery. While theoretically no OH− will be consumed in the overall reaction, CO2 from external sources can diffuse together with O2 into the battery and react with the OH− ions in the alkaline electrolyte to form CO32− or HCO3−, which have much higher ionic resistivity than OH− 22. Zn is oxidized and combines with hydroxide ions in the electrolyte, forming soluble zincate ions (Zn(OH)42−). This process continues until the zincate ions in the electrolyte reach saturation, at which point zincate begins to decompose into zinc oxide and release hydroxide ions 23. A relatively slow zincate decomposition at early stages of discharge may result in slow hydroxide ion regeneration 24–27. When the amount of gel electrolyte is relatively small – e.g., the tested 38 mg gel, the OH− could be consumed gradually over time (as well as the electrolyte pH falling over time) as the battery discharges. When the concentration of OH− falls too low, the reduced ionic conductivity can induce a large overpotential, leading to the end of discharge.
As the battery lifetime increased to over two weeks, cracks in the wax package along the edges were observed as shown in Fig. 3 (b). This cracking resulted from the volume expansion of the Zn anode during the electrochemical reaction. This emerging defect during discharge in the package could expose the internal battery components to the external environment, and the contact between the battery active component layers might loosen due to the lack of stack pressure from the package. Figure 3 (c) shows an additional fabrication step implemented after manually sealing the edges of the wax encapsulation. The entire battery was dip-coated in melted wax as described in the Supplemental Material to form an additional conformal layer, thereby improving the mechanical strength of the wax package and preventing cracking.
The batteries after discharge were disassembled to inspect the condition of the active components. As shown in Fig. 3 (d), it was observed that in some cases, the Zn anode handle had turned into ZnO and broken before the main part of the Zn anode (the part facing the cathode) was fully consumed. This could be attributed to the consumption of the handle if exposed to the electrolyte (e.g., if any liquid electrolyte is squeezed out from the hydrogel and wets the handle) and oxygen 28. To address this issue, a new design of Zn anode with a handle roughly three times as wide was implemented as seen in Fig. 3 (e). The wider handle was wound with Cu wire and coated with silver paste at the connection region. Subsequently, the interconnect was dip-coated in melted wax to form an additional hydrophobic protective layer, thus preventing reaction at the handle and maintaining the electrical connection over the long-term test. Due to the success of this anode passivation technique, it was utilized in all subsequent large cell tests.
Carbon cathode study
An investigation into substitute cathode materials was undertaken to reduce the cost and further increase the biodegradability of the battery. The air cathode typically consists of a gas diffusion layer for oxygen transfer (comprising a macroporous layer) and an active catalyst layer (typically microporous), where the oxygen reduction reaction (ORR) occurs 29. The Pt-loaded cathode used in the above alkaline gel study contains Pt particles in the catalyst layer, with carbon paper serving as the gas diffusion layer. Additionally, hydrophobic material such as poly-tetrafluoroethylene (PTFE) is optionally used to treat the cathode. However, carbon without a metal catalyst has been found to provide sufficient catalytic activity for the oxygen reactions in nonaqueous electrolytes for Li-air batteries, especially at low current density 6. In this case, carbon functions not only as the catalyst support but also as a good ORR catalyst. To explore the feasibility of Pt-free cathodes, four types of carbon-based materials were tested with the alkaline hydrogel and Zn anode (Table 2). Figure 4 (a) and (b) show the microporous layer structure of the Sigracet 22 BB and Freudenberg H24C5.
Table 2
Carbon-based materials properties.
| Materials | Thickness/µm | PTFE treatment | Microporous layer | Macroporous layer |
| Toray 060 | 190 | Yes | No | Yes |
| Sigracet 29 AA | 180 | No | No | Yes |
| Sigracet 22 BB | 215 | Yes | Yes | Yes |
| Freudenberg H24C5 | 270 | No | Yes | Yes |
Batteries were initially assembled with these carbon-based materials as the cathode (with the micro-porous layer, if present, facing the hydrogel) in the large cell format, using approximately 100 mg gel alkaline electrolyte, and clamp boards to characterize the peak power. Figure 6 (c) shows a comparison of the power curves. Batteries with Sigracet 22 BB and Freudenberg H24C5 had a significantly higher peak power compared to those with Toray 060 and Sigracet 22 AA, potentially attributable to the high surface area of the micro-porous layer. Sigracet 22 BB delivered a higher peak power than Freudenberg H24C5, due to its highly porous structure in contrast to the flake structure of the Freudenberg H24C5, as shown in Fig. 4 (a) and (b). The battery with Sigracet 22 BB carbon cathode was also discharged under higher current loads of 600 µA – 8 mA, showing that the carbon cathode alone has sufficient catalytic activity for sensor applications even at higher discharge rates (Figure S4).
For all subsequent testing involving non-Pt cathodes (Table I), Sigracet 22 BB was selected due to its superior power performance. Though the microporous layer of Sigracet 22 BB has been treated with 5 wt% PTFE to make it hydrophobic, the total amount of the PTFE contained in one battery would be less than 0.27 mg (calculated as the PTFE in one cathode \(\:70\:\frac{g}{{m}^{2}}\times\:0.785{cm}^{2}\times\:5\:wt\%\)). Alternatively, biodegradable materials could potentially replace PTFE, such as modified hydrophobic nano-scale cellulose fibers or crystals 30,31.
Wax-encapsulated batteries with carbon cathodes (Table I, Rows 1–2)
The above gel and cathode studies were conducted using clamp boards, which have minimal oxygen mass transport limitations. In actual application, both the wax package and soil could significantly limit oxygen transport. To examine this effect, batteries with wax encapsulation were assembled and tested. The peak power of the battery with the wax package in the soil is roughly 1/3 that of the battery with the clamp boards as shown in Fig. 4 (d). Although the peak power using this carbon cathode was measured to be 10 mW cm− 2, lower than that of the Pt-loaded cathode as shown in Fig. 4 (e), it is still sufficient to fulfill many typical IoT sensor operation requirements of tens of µW.
Furthermore, the battery with the carbon paper cathode discharged for 50 days at the sensor-relevant 30 µA in soil, and a 70 day lifetime was achieved in the air with an increased amount of the gel electrolyte as shown in Fig. 4 (f). The relatively shorter lifetime in the soil that is observed may be attributed to the lesser amount of gel electrolyte in the soil battery, and the higher humidity and CO2 levels in soil 32. The lifetime of these batteries was also found to be longer than the Pt-loaded cathode batteries of the alkaline gel study. These results indicate the utility of carbon cathodes for low-power sensor applications, as well as the potential of Zn-air batteries as biodegradable power sources to sustain long-term operation in subsurface conditions.
Miniaturized batteries (Table I, Rows 3–6)
Battery miniaturization has the potential advantage of utilizing existing agricultural equipment for deployment. For example, batteries of the size of seeds could exploit conventional planters for undersoil positioning. To investigate these potential benefits, the battery was miniaturized to the size of a corn kernel, referred to as a “corn cell”. Corn cells with alkaline gels and Pt-free carbon cathodes were then fabricated as described above. Figure 1 (i) shows the OCV of the corn cell after assembly was approximately 1.46 V, similar to that of the large cell, demonstrating that the wax encapsulation technology developed can be applied to corn cells.
Power performance study
As the output power of batteries is expected to scale with the battery size, the power performance of corn cells was characterized. Figure 5 (a) and (b) show that batteries of footprint 0.785 cm2 (large battery format) and 0.42 cm2 (corn cell format) were assembled with clamp boards. The power density and current density were calculated by dividing the measured power and current by the footprint area. As shown in Fig. 5 (c), the corn cell had a similar power density curve to the large battery, implying that the output peak power of the battery would scale down at the scale of the footprint area when no air diffusion barrier of the package is present. Figure 5 (d) demonstrates the effect of the package on the peak power of the corn cell. The peak power density of the corn cell dropped by approximately half when encapsulated within wax pads used for large batteries as shown in Fig. 5 (e). As the wax package scaled further down to the size of the corn kernel shown in Fig. 5 (e), the peak power density of the corn cell dropped to approximately 7 mW cm− 2, potentially due to the reduced amount of the air encapsulated, and the slow oxygen replenishment rate. Nonetheless, even this reduced peak power of the corn cell is still sufficient for many sensor node applications.
Lifetime improvement with corrosion inhibitors
With a constrained amount of anode material, the lifetime of a corn cell could be limited by self-corrosion, especially under a relatively low operational current load. Therefore, biodegradable corrosion inhibitors to lengthen the lifetime of the corn cell were investigated. Liang et al. introduced a mixture of biodegradable polymer polyethylene glycol 600 (PEG 600) and polysorbate 20 (Tween 20) as composite corrosion inhibitors for Zn/MnO2 button batteries to suppress the self-discharge of the Zn anode at relatively high efficiency and improve the discharge capacity of the battery 33. Maltodextrin (MLD) has also been reported to be a biodegradable, inexpensive, and extremely water-soluble inhibitor for Zn corrosion in alkaline and acidic electrolytes 34,35. These corrosion inhibitors added into the electrolyte would adhere to the Zn surface through physical adsorption, forming a protective layer that avoids the direct contact of the Zn with the electrolyte, therefore reducing the anode self-corrosion rate.
Potentiodynamic polarization tests were performed to quantify the corrosion current and corrosion potential of the Zn anode, using a three-electrode setup shown in Figure S3. Figure 6 (a) shows the collected polarization curves for batteries without any corrosion inhibitors, with PEG 600 and Tween 20 on Zn anode, and with hydrogel containing 0.3 g/L MLD. The corresponding Icorr and Ecorr values are extracted from the polarization curve through Tafel approximation (described in the Supplementary Material with Figure S5), shown in Table 3. The bare Zn anode has the highest corrosion current of 169 µA, which dropped to 21.6 µA after painting the anode with a thin layer of PEG 600 and Tween 20. The bare Zn anode with a gel containing MLD has a corrosion current of 80.2 µA, higher than the Zn paint coated with PEG 600 and Tween 20, but half that of the battery without inhibitors.
Table 3
Corrosion current and corrosion potential of corn cells without or with corrosion inhibitors.
| Materials | Icorr/µA | Ecorr/V |
| Bare Zn | 169 | -1.43 |
| Zn coated with PEG 600 + Tween 20 | 21.6 | -1.44 |
| Zn with MLD gel | 80.2 | -1.46 |
Based on these electrochemical results, corn cells with corn-sized packages were assembled with Pt-free carbon cathode and alkaline electrolyte, with or without inhibitors to characterize their electrochemical performances. A corn cell with MLD gel has a similar peak power density to the cell with normal alkaline gel, approximately 7.2 mW cm− 2 as shown in Fig. 6 (b). The corn cell with PEG 600 and Tween 20 coated Zn anode has a relatively lower peak power density of approximately 6.4 mW cm− 2, potentially due to the polymer film formed at the interface between the anode and the gel. Corn cells were then discharged at 30 µA in the air. Using corrosion inhibitors PEG 600 and Tween 20, together with increased gel electrolyte mass, the lifetime of the corn cell increased from 7.5 days to 17.2 days in the air. In contrast, by introducing MLD into the gel alone, the lifetime of the corn cell was further extended to 21.8 days as shown in Fig. 6 (c). The corn cells were also tested under a 5% duty cycle to mimic a typical real-case application scenario. Figure 6 (d) and (e) show that the corn cells with MLD discharged for over 82 days in the air and 65 days in soil, longer than corn cells with PEG 600 and Tween 20 under both conditions. Since the PEG 600 and Tween 20 were initially applied at the anode-gel interface, it is possible that their interfacial concentration could reduce with time, whereas the MLD is stored in the reservoir of gel electrolyte and could replenish the interface and be effective for a longer duration.
Neutral gel batteries (Table I, Rows 7–8)
In some applications, the expected operational timeframes for batteries in biodegradable IoT systems can extend over multiple months. For example, in agricultural fields, sensor nodes are expected to be distributed throughout the field during the initial planting process and could be required to operate for the entire growing season to capture complex environmental variables 36. Therefore, approaches to further extend the battery operational lifetimes are of interest.
It is observed that the alkaline hydrogel of the battery after long-term discharge darkened or disintegrated as shown in the Supplemental Material (Figure S6). A color change of the alkaline gel from white-yellow to brown to dark brown was noted with time; further, embrittlement of films when stored in high-pH aqueous environments was observed. This discoloration may be attributed to a deterioration of the chemical structure of the PVA, such as forming polyene fractions, generally caused by oxygen or hydroxyl attack 37,38, or the formation of a more porous PVA structure due to long-term exposure to a high-pH solution 39,40. This degradation of the hydrogel may become more severe when the battery is discharged 41,42, especially over longer durations.
To resolve the degradation issue of the alkaline gel and further extend the lifetime of the battery, a neutral gel electrolyte was investigated. The neutral gel containing NH4Cl as the ionic species has stable electrochemical properties and high water retention capability 43,44. Additionally, Zn has a lower self-corrosion rate in the neutral environment 45,46.
As one potential drawback of batteries with neutral gel electrolytes is the lower power performance compared with alkaline gel electrolytes, a characterization of neutral gel batteries was performed using Pt-loaded cathodes. Power curves of neutral gel batteries were collected in an air environment and were compared to those of the alkaline gel batteries. As shown in Fig. 7 (a), a lower peak power of neutral gel batteries was observed, potentially due to the lower catalytic activity of the Pt and the higher overpotential of the redox reaction in neutral electrolytes 44.
Figure 7 (b) shows the discharge curves of neutral gel batteries under continuous 30 µA discharge and under a 5% duty cycle discharge in an air environment. Figure 7 (c) and (d) are zoomed-in views of the 5% duty cycle discharge curve, showing the duty cycle period of 1 hour and the individual 3-minute discharge curve within 1 hour. It was observed that the battery with 330 mg neutral gel has a higher and more stable working voltage under 5% duty cycle discharge. A small degradation in working voltage with time was observed, which may be due to the lower ionic conductivity of the gel as well as the gradual passivation of the Zn anode, since Zn ions have a lower solubility in the neutral environment 43. Nonetheless, the duty-cycled neutral gel batteries discharged over 340 days in the air environment, and 260 days in soil, as shown in Fig. 7 (e). These results demonstrate the potential of a biodegradable neutral gel battery supplying growing-season-long power to an IoT sensor node.
Comparison to the State-of-the-Art
Figure 8 (a) and (b) summarize the performance of the various wax-encapsulated batteries studied here. For corn cells, peak power densities range from 6-7.5 mW/cm2, and lifetimes range from 8 days to 82 days. For large format cells, peak power densities range from 10 to 50 mW/cm2, and lifetimes range from 15 days to 340 days.
Figure 8 (c) compares the lifetime and the corresponding operational voltage of batteries discussed in this work to state-of-the-art long-term biodegradable primary batteries reported in the literature 47–50. The wax-encapsulated Zn-air batteries provided stable operational voltage, with lifetimes exceeding the literature-cited batteries by several orders of magnitude. In addition, the Zn-air chemistry compares favorably to literature-cited batteries under high output as shown in Fig. 8 (d). These results demonstrate that biodegradable Zn-air batteries may be promising as long-term power sources for environmentally friendly IoT sensor nodes.