A pressure driven electric energy generator exploiting a micro- to nano-scale glass porous filter with ion flow originating from water | Scientific Reports

Investigation of sintering temperature conditions for glass filter fabrication

We first established the fabrication process of porous glass filter and then investigated the structure for various fabrication conditions. The fabrication details are described in Fig. 2a and the Methods section. We employed powder sintering by packing a powder of borosilicate glass particles in a carbon mold and thermally fusing this under pressure applied from a weight as shown in Fig. 2b–d. Although laser fabrication30,31,32 is commonly used to make glass filters, sintering is simpler and provides robust filters with large numbers of channels. A borosilicate glass filter plate (diameter of 2 cm) was fabricated (Fig. 2e). Normally for tight bonding by glass thermal fusion, a temperature of 750 °C is used33,34. However, at this temperature, the borosilicate glass particles completely melted, and became gray colored and no water could pass through it. Based on our previous experiences35,36 glass and glass can be bonded to each other if pressure is applied even at a lower temperature and we used the sintering temperatures of 680–720 °C here. Under these conditions, we obtained good glass filters with no discoloration due to degradation. However, observation with a microscope showed that the edge of the glass particles was slightly melted at 710 and 720 °C (Fig. 2f). Also, the filter was rather fragile when it was sintered at 680 and 690 °C.

Figure 2

Fabrication and structure investigation of porous glass filters. (a) Fabrication procedure of glass filters. (b) Carbon mold to fabricate glass filters. (c) A hole in the mold is filled with glass powder. (d) Setup in a furnace with an alumina weight. (e) Fabricated glass filter. (f) SEM images of porous glass filter surfaces after sintering at the temperature indicated at the top of each image. (g) Results of mercury porosimetry of the porous glass filters sintered at various temperatures. Black and red lines indicate pore volume per mass (V) and pore radius (R) distribution (derivation of V by R), respectively. (h) SEM images of the porous glass filter surfaces sintered at 700 °C using ground glass powder and the milling time indicated at the top of each image. (i) Results of mercury porosimetry of the porous glass filters sintered at 700 °C using ground glass powder and the milling time indicated at the top of each image. Scale bars are shown in SEM images of (f) and (h).

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The pore size distribution was measured by the mercury porosimetry for filters prepared at various sintering temperatures and the results are plotted in Fig. 2g. The peak in the pore size distribution was not so different at different temperatures. At all temperatures, the peak was at 20 μm (average pore radius). But the peak height decreased when the temperature was higher. This meant that the pores were filled with melted glass when the sintering temperature was increased. Considering the findings and to avoid fragility, we concluded that the optimum sintering temperature was 700 °C. We used this condition to investigate the effect of particle size in the glass filter fabrication next.

Investigation of glass particle milling condition for glass filter fabrication

To control the glass particle size, particles were milled. The scanning electron microscope (SEM) images and diameter histograms of milled particles with various milling times are shown in Fig. S1. The average Feret diameter, defined as the distance between two parallel planes restricting the object perpendicular to that direction, could be controlled between 4 and 150 μm.

The milled particles were used to fabricate glass filters with sintering at 700 °C. From the SEM images (Fig. 2h), we saw that most of the particles maintained their shapes, although small particles were observed to be partially melted especially at longer milling times. This was consistent with the pore size distribution results (Fig. 2i). At 0, 5, 10, 20 and 30 min milling times, there were peaks at 20, 12, 8, 5 and 1 μm (average pore radius), respectively. At the 40 min milling time, it was difficult to find a peak. This indicated that the pore size became small when milling time was increased, but the pore volume (peak height) decreased due to the melting of the small particles. Especially at milling times of 30 and 40 min, the pore sizes were nanometer scale and the peak height was very small. These properties are deeply related with the electric power generation performance which we investigated next.

Demonstration of the pressure driven electric power generator

Using the fabricated porous glass filters, we demonstrated the electric power generator and investigated the pore size effect using a constant speed water delivery system (Figs. 3a–c). The power generator was constructed by setting a glass filter in a modified commercial filter holder and inserting the electrodes above and below the filter (Fig. S2). All experiments using the water delivery system were carried out at room temperature. Deionized pure water produced using a Milli-Q system was introduced into the generator, and a voltage was generated (Fig. 3d). Repetitive voltage generation was also confirmed. A slight voltage decrement was observed, but this was probably because we recycled the water. The mesh number and the distance between the glass filter and the mesh electrode were optimized and these results are summarized in Fig. S3.

Figure 3

Characterization of power generating performance of fabricated glass filters. (a) Setup for characterization of the glass filters. Water is introduced into the generator by the water delivery system and it is circulated using check valves. Measured voltage is recorded by a PC through an I/O board. (b) Photo of the generator equipped with the glass filter and electrodes. (c) Photo of the water delivery system. (d) Voltage time-course of the repetitive power generation using the filter sintered at 700 °C and 5 min glass powder milling time. The water delivery system speed was 20 mm/s. (e) Voltage time-courses using the porous glass filters sintered at the temperature indicated at the top of each graph. Each graph shows voltage during 3 pressing cycles at water delivery system speeds of 4, 6, 8, 10, 20, 30, 40 and 50 mm/s. (f) Voltage, current and estimated power of the generators versus sintering temperature of the glass filters at the water delivery system speed of 50 mm/s. Voltage and current plots represent average ± S.D. (n = 3). (g) Voltage time-courses using the porous glass filters sintered at 700 °C with average pore radius indicated at the top of each graph. Water delivery conditions were the same as in (e). (h) Voltage, current and estimated power of the generators versus average pore radius. Voltage and current plots represent average ± S.D. (n = 3).

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As show in Fig. 3e and f, the voltage was in proportion to the speed of the water flow. At the same speed, voltage generation performance was almost equal using the filters sintered at 680, 690 and 700 °C, but it decreased for filters sintered at 710 and 720 °C. This meant that the small pores disappeared at higher sintering temperatures. The current was measured from the slope of the voltage saved in a capacitor during the water flowing (Fig. S4). This generated current had the similar tendency as the voltage, although the peak performance was seen for the filter sintered at 700 °C. This was simply because the number of small pore size increased with temperature increment up to 700 °C, but at higher temperatures the pores were almost completely closed. The voltage, current and power calculated by the product of voltage and current had peaks (11 V, 74 μA and 0.80 mW, respectively) for the filter sintered at 700 °C. From this result, we judged the sintering temperature of 700 °C gave the optimum filter for the power generator.

Then, the filters made from milled particles sintered at 700 °C were used for the power generation experiment. As show in Fig. 3g and h, the filter made using the 5 min milling time (12 µm average pore radius) provided the peak voltage, current and power values of 27 V, 0.14 mA and 3.8 mW, respectively.

Evaluation of the pressure driven electric power generator

We also analyzed the power generator performance in detail using the force measurement results to evaluate the data validity of the water pressure based generation method. The force and pressure applied to the syringe by the water delivery system was measured by a force transducer and the results are shown in Fig. S5. The force was generally in proportion to the speed and filter fineness.

From these results, it was reasonable that the generated voltage increased for filters fabricated with no milling (20 μm average pore radius) to 5 min milling time (12 μm average pore radius) in accordance with the increment of pressure according to Eqs. (1) and (2), but the generated voltage decreased for filters fabricated with longer milling times due to closing of the pores. Especially for the milling times of 30 min or longer (5 μm or smaller average pore radius), the filters could not withstand the pressure and they broke during the experiment. So, we had no data for these conditions. Another reason may be that the electric properties of water in sub-microscale channels are different from those in a bulk space37,38. Considering an applied force of 350 N to the water and the syringe pushing speed (50 mm/s), the peak power efficiency using a filter fabricated with 5 min milling time (12 μm average pore radius) was calculated as 0.021%.

Verification experiment using a fused silica filter

Although the power generator was verified in the previous section, we were concerned that the current was generated by impurities included in the borosilicate glass. To confirm that the generation was caused only by the interaction between the glass surface and water, we used fused silica particles which included almost no impurities for the filter. The result is summarized in Fig. S6. The fabricated fused silica filter is shown in Fig. S6b. Although this filter was more fragile than the borosilicate glass filters, it could be used for a power generation experiment at low water flow speed of less than 20 mm/s. The repetitive voltage generation was confirmed, and water flow was smooth as shown by the force measurement data (Fig. S6d). From these results, we confirmed that the electric power generation was not caused by impurities in the borosilicate glass.

Demonstration of power generation device with a foot press unit

Following the fundamental investigations of the previous sections, we fabricated a prototype power generation device with a foot press unit as shown in Fig. 4a–c. The power and the continuing duration of the generation were measured. The foot press unit (Fig. 4b) included a syringe and its holder, a holder for the power generator and a cover; and the unit was designed to easily and effectively transfer the applied external force to the water in the syringe. We introduced 50 mL of deionized pure water (Milli-Q water) into the syringe that was set in the unit. A weight of about 60 kg (588 N, corresponding to 830 kPa pressure) was applied when an experimenter pressed on this unit with the foot (Fig. 4c). The electrical measurement and recording method was the same as that for the size effect experiment.

Figure 4

Demonstration and application of the electric power generator equipped with a foot press unit. (a) A circuit for capacitor storage and device driving applications. A generator was connected to ports 0 and 1. For energy saving applications, the generator was used without connecting ports 2 and 3. (b) Schematic drawings showing the working motion of the foot press unit. (c) Photo of the foot press unit. (d) Voltage time-courses using porous glass filters sintered at 700 °C with ground glass powder for the average pore radius indicated at the top of each graph. Each graph shows the voltage during 1 pressing cycle using a 60 kg weight (foot pressing by an experimenter) starting at about 2 s. (e) Voltage, current, power generating duration, estimated power and energy of the generator versus average pore radius. Voltage and current plots represent average ± S.D. (n = 3). (f) Application of direct LED lighting by connecting the LED to the generator (without a switch and capacitor). The upper photo shows the overall setup and the lower 2 photos are before and after foot pressing. (g) Application to driving a rotator by saving energy in the capacitor. The upper photo shows the overall setup and the lower 2 photos show the rotator (fan) and the multimeter display screen which gives the accumulated voltage in the capacitor before and after turning the switch on to release the energy in the capacitor. (h) Application to a wireless communication tool busing energy stored in the capacitor. After energy accumulation, the communication tool automatically sends a signal to the PC. The top right photo shows the overall setup and the top left photo is an enlarged image of the capacitor and sender. The middle and bottom photos are screen grabs generated by the software to confirm signal receiving and sending.

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The foot press unit was smoothly driven by the experimenter’s foot pressing motions, and it could be continuously used for at least 100 times during an experiment. The 50 mL of water was repeatedly used by recovering it each time after finishing the pressing motion for up to 50 times in an experiment. Figure 4d and e show the power generation experimental results. The voltage increased immediately after applying the pressure (at t = 2–3 s) and dropped to zero when all the water in the syringe was emptied. The generated voltage and current peaked using the filter fabricated with the milling time of 5 min (12 μm average pore radius). This was the same result as obtained for the experiment using the constant speed water delivery system. However, the duration of power generation increased when the milling time increased, because this experiment used a constant pressure for generation (the experimenter’s foot pressing). Conversely, the flow rate decreased when the pore size became smaller (Fig. S7a). So, considering the duration, the graph of harvested energy vs. milling time had a right shifted peak compared to the power graph (Fig. 4e). The voltage, current and power had peaks of 18 V, 0.26 mA and 4.8 mW, respectively, for the filter made with the 10 min milling time (8 μm average pore radius). Since the energy was generated for a duration of 1.7 s for this filter, the harvested energy was 6.8 mJ. Considering the applied force (588 N) and syringe pushing displacement (70 mm), we calculated the peak power efficiency for the filter made with the 10 min milling time (8 μm average pore radius) was 0.017%.

Application of the pressure driven electric power generator

To demonstrate that the obtained power was able to drive circuits and electronic devices, we carried out a light-emitting diode (LED) lighting test, a fan rotation test and a wireless commination tool test. First, a LED light was illuminated via a direct connection without using a capacitor. The experimental setup is shown in Fig. 4f and Supplementary Movie 1. The pressing time for squeezing out all the stored water from the syringe through the glass filter was roughly 2 s. The LED was lit during this pressing time. This application is easy to understand and could be used for lighting when a person is walking in a dark place.

A mini fan was assembled from a mini motor and other 3D printed parts and in a similar way to the LED lighting was driven as shown in Fig. 4g. But unlike for the LED lighting, more electricity was needed for the fan rotation. Therefore, we used a capacitator with a large capacitance (4700 µF) and the capacitator was charged by 50 foot presses. The stored voltage in the capacitor was measured as 5.2 V. The glass filter power generator produced sufficient electricity to rotate the mini fan. Related experimental results are given in Supplementary Movie 2. We demonstrated that it was feasible to drive something like the mini fan, which could be used for cooling when a person is walking.

Finally, a wireless communication tool was driven as shown in Fig. 4h; such tools are widely used for smart monitoring by constantly sending a signal about the immediate surroundings that includes temperature, light, and motion changes39. The signal generator was attached to a capacitor (2200 µF) and it automatically sent the signal after the energy (about 0.2 V) was stored in it. When the foot press unit was stepped on twice, the wireless communication tool transmitted signals to the signal receiver which were monitored by the PC. The actual signal catching motion is shown in Supplementary Movie 3. When a wireless kit composed of a signal generator and a capacitor was powered by the glass filter power generation device, it simultaneously triggered signals and sent them to the receiver. Once the signals were received, their receipt was reflected on the monitor until all the voltage in the capacitor had been consumed. We sent signals over a 3 m distance in this experiment. This application would be practical for personal health monitoring.

Additional validation

According to the principle of electric generation, hydrogen and oxygen gases are generated. Here, we estimated how much volume was generated. The amount of substance of hydrogen (nh) can be calculated by Eq. (3):

$${\text{n}}_{{\text{h}}} = It/{2}F$$

(3)

where I is the current, t is the duration of power generation, and F is the Faraday constant (9.6 × 104 C/mol). In the foot press experiment, the largest current (I) was 0.26 mA and the corresponding duration (t) was 1.7 s. In this condition, nh was calculated as 24 nmol (48 ng). The amount of substance of oxygen (no) was half of nh which was calculated as 12 nmol (380 ng). Since the water volume in this experiment was 50 mL, the concentrations of hydrogen and oxygen were estimated as 0.96 ppb and 7.7 ppb, respectively. Such low concentrations are difficult to be measured even using commercially available gas monitoring devices with high sensitivity. To measure the gases, the current must be increased significantly.

However, it is important to confirm that the current generation was not caused by other reasons (e.g. vibration or noise). Therefore, we have obtained the negative control data. The generated voltage by applying 830 kPa pressure using the press unit without a glass filter in the generator is shown in Fig. S7b. It was 0.12 ± 0.04 V (n = 3, ± S.D.) and significantly smaller than the data with glass filters. From this result, the principle of the generation was confirmed.

In addition, we investigated the influence of flow to the resistance of a glass filter. If the resistance changed significantly by flow, the current might not be measured correctly due to the leak current. The simulation was added in Fig. S8. It shows that the resistance was almost the same regardless of the presence or absence of flow. In addition, the electrical resistance was actually measured using the press unit and the filter with 8 μm average pore radius. The resistance without and with flow (35 mL/s) were 1.50 ± 0.14 MΩ (n = 3, ± SD) and 1.41 ± 0.10 MΩ (n = 3, ± SD). No significant difference was observed. Moreover, compared to the resistance of external circuit calculated from measured voltage and current in Fig. 4e (68 kΩ), the resistance of the filter was large enough to prevent the leak current. From these results, we concluded that the flow does not influence the electrical resistance.