Self-Sustaining Generation of Electricity from Ambient Water

When wool fabrics impregnated with ketjen black is used for fabrication of HENGs, it enhances streaming potential output through ion gradient diffusion mechanism. HENGs have two electrodes for charge extraction and transport. These electrodes are encased in the KB woollen cloth substrates, which are very hydrophilic. Water molecules from the sea or other ambient source are absorbed through functional groups like –COOH and –OH present on the surface of the substrates via physical or chemical interactions. The ionic liberations form various free charge carriers like H⁺ ions, which diffuse across the hydrophilic surface and to the other side of the HENGs.

Capillary force drives water molecules to flow in the charged channels and transfer them to the top of the substrate, where water molecules are evaporated by sunlight leading to asymmetrically wetted HENGs. There is a potential difference between dry and wet ends, which generates electric current to flow. However, short circuits should be avoided by balancing water supply and evaporation. This can be achieved by using a specific number of yarn stripes. Experimental results show that a HENG device (10 pieces) with 4 yarn stripes can generate a voltage of about V_OC of 0.13V without short circuit in seawater achieving well balanced transport and evaporation of water. If ordinary water is used instead of seawater, then voltage generated would be reduced to about 25% of that achieved by seawater.

When seawater is used, the gradient distribution of salt in seawater results in an asymmetric wetting of the two ends of HENG, and accelerates redox reactions at the zinc anode of the device. Due to the anode surface potential of the Zn electrode, an EDL is generated at the Zn-water interface. When the external circuit is closed, the diffusion of ions and charge transfer occurs at the KB particles and Zn electrodes. The redox reactions of the oxygen and moisture adsorbed at the cathode side of the HENG receive electrons to generate OH-, and Zn releases electrons and Zn²⁺ at the anode, where Zn²⁺ combines with OH- migrated from the cathode to generate zinc hydroxide (ZnOH₂) which is porous and loose. It is therefore necessary to clean the electrode surface routinely. As an alternative Fe or Al electrodes can be used which are cheaper. The reactions at the electrodes are given below: –

Anode:  2Zn – 4e ® 2Zn²⁺

Cathode:  O₂ + 2H₂O +4e^– ® 4OH^–

Overall cell reaction:  2Zn + O₂ + 2H₂O = 2Zn(OH)₂

Self-operating HENG producing electricity from seawater

Fabrication of self-operating HENG

For fabrication of HENG, hydrophilic wool cloth (175.5 g.m^-2) with water contact angle (WCA) of 115o stripes are plasma treated for 2 minutes enabling to change its physical characteristics from hydrophobic to hydrophilic due to formation of micropits and hydrophilic functional group on the surface of wool. Its WCA is also reduced to 500. Then it is immersed in KB ink and dried in an oven under vacuum at 40⁰C for 3 hours. This process is repeated 6 times in order to have consistent and long-term stability of impregnation.

WCA of 115 degree indicates that the pristine wool cloth is hydrophobic. Zn electrodes are attached to the cloth stripes. KB ink is prepared by mixing 0.6 mm diameter KB powder and Hexadecyl Trimethyl Ammonium Bromide (CTAB) in deionised water for 4 hours under sonification, using ultrasonic sound for intimate mixing of the constituents. Mass ratio of KB to CTAB is kept around 1:2. CTAB is a very useful surfactant, which provides improved dispersion, enhanced adhesion and stabilizes the suspension of KB in the ink. CTAB helps in dispersing KB uniformly on the wool fibres ensuring that the coating is even and consistent, which is crucial for maintaining the desired electro conductive properties. It helps KB particles to rigidly adhere to the wool fibres providing increased life. It also stabilizes the suspension of KB in the ink, preventing the agglomeration of KB particles forming lumps, which helps in uniform coating.

SEM images show that KB particles sticking to the cloth after coating are of diameter around 40 nm, and the treated clothes remain super hydrophilic without any appreciable change in WCA. FIG – 3 shows the schematic diagram explaining the working principle and fabrication process of wool-based HENG. FIG – 4 shows the SEM image of KB coated woollen cloth with changing WCA during various stages of treatment process.

Advantages and disadvantages

Advantages – HENGs can utilise sea water, which is abundantly available. Cost of its fabrication is low. No external mechanical force or temperature gradient is required for its operation. The device is highly flexible enabling it to fit to any contour. HENGs are very simple and so its fabrication process is also simple.

Disadvantages – Its output power density is low. Voltage generated is low. Adverse environmental condition affects its performance.

Strategies to improve performance

HENG technology has the potential to generate electricity from previously untapped resources such as seawater, moisture in the air, waste water, body sweat etc. leading to the self-powered devices. But the low power density is the main weakness that possibly can be addressed by improving the design, structure and surface modification, as well as reduction of the internal resistance of the functional materials.

High internal resistance reduces the output current and reduces the overall efficiency of the device. Reducing the internal resistance of an active material is beneficial to electrical output. This can be done by ion doping. For example, when pyrrole is immersed in LiClO₄ during synthesising of polypyrrole, a functional material, a 3D network of polypyrrole is formed with low resistance. The amount of doped functional groups (ClO-4) on the active material can be accurately controlled. It has been found that a composite material based on reduced Graphene Oxide (rGO), silicon oxide nanofibers, and SA (sodium alginate) can produce an excellent output energy density of 120mW.cm^-2, open circuit voltage of about 0.5 V, and short circuit current of about 0.1mA at 100% RH. The high performance can be attributed to the following factors: –

The composite material can dissociate the mobile sodium ions due to the surface functional group like –COONa. 2. rGO is capable of building a 3D conductive carbon skeleton, the resistance of which can be tuned. 3. Due to the presence of SiO₂ nanofibers in the matrix a porous structure is formed, which accelerates the transport of ions and water molecules without reducing the structural stability.

Devices with different designs have been fabricated with diverse functional materials. Optimizing various aspects of a device design may provide HENGs with improved functions. The functional layer and the electrode can form either Scottky junctions or non-rectified ohmic contacts. In one of the laboratory experiments, scientists created a Scottky barrier at the interphase between the electrode and active material that resulted in increased output voltage from 0.035 V to 0.7 V.

The power output can be enhanced, because the recombination of generated ions and injected charge carriers from the electrode to active material is reduced. Typical HENG consists of a pair of electrodes, a composite material impregnated with active materials like KB/reduced graphene oxide (rGO), which enhances conductivity allowing electrons to transfer. But if an additional feed layer of graphene oxide is provided as shown in FIG – 6b, then heterogeneous graphene oxide layers (h-GO) are formed and electric output is enhanced. Feed layer with additional graphene oxide supplies the depletion of ions in gradient rGO, and in the electrode /h-GO interphase area, a space charge zone is formed, which acts as gate to block free electrons through hGO and regulates its flow direction enhancing the electric output.

Electrodes with large surface area and porous structure can enhance the output electrical performance of HENGs. Apart from metal electrodes scientists are trying to use fabrics as electrodes. SiNWs based porous electrodes can enhance water evaporation, charge transport and collection.

It has been discovered that substantial voltage generation is possible across an alumina, soaked in water. Although porous alumina is an insulating material, but it can be converted into a surface conductor when in contact with water and yields a sufficient amount of electricity associated with surface evaporation.  The chemical composition of alumina is 47.3% Al, 45% oxygen and rest are alkali metal, iron, silicon etc. It has porosity of about 35%.

Compared to carbon-based devices, microporous alumina driven by water evaporation is an efficient power generating substrate to achieve high voltage. It is chemically stable in aqueous environment and at high temperature. Robust porous alumina without any pre-treatment and coating can be used for electricity generation through the process of water evapotransportation. Its porosity drives the capillary force and transportation of water molecules through the substrate generating power. A 3.0 x 3.0 sq. cm, 0.3 cm thick block partly soaked in water, is capable of producing an open circuit voltage of about 0.27 V with stable power generation performance for more than one year.

Output power can be adjusted as per demand to suit the load by changing the wetting conditions of the porous medium, water temperature, and salt concentration of water, or by alternating the series/parallel combination of the modules. Abundant interconnected pores in the alumina substrate provide a large polarized surface that provides sufficient charge enabling effective carrier diffusion at the water-alumina interphase.

This system does not require any supply of light or heat and can work anywhere and at any time. The zeta potential of the alumina is about -98.19 mV that indicates that there is negative charge on the alumina surface and this negative charge is associated with surface –OH groups on alumina surface and make the interior of the porous alumina super hydrophilic. Therefore, water molecules are attracted to the hydroxylated alumina surface, wetting it and then moving upward through the porous channels via capillary action, until they eventually evaporate.

Protons (H⁺), or hydronium ions (H₃O⁺) are attracted by the negative surface charge and crowd along the alumina surface forming a polarized surface layer i.e., electric double layer, EDL. The thickness of the EDL is quite large to the tune of several hundreds of nanometres due to the large Debye screening length of water. In this EDL region, a substantial number of protons is attracted to the water-alumina interphase. Then the deficiency of protons in the channel centres need to be compensated by additional water self-ionization, which subsequently boosts the conductivity of the entire system. These ions in the pore-confined water freely flow upward along the channel as water evaporates from the alumina. Interfacial charges, both positive and negative migrate uphill together along with water in the same direction under a capillary force as water evaporates from the alumina. Since their distribution near the surface region differs, a positive net current is generated due to the different diffusion speeds of +ve and –ve ions. Then the two electrodes connected to the upper and lower sides conduct the power. Schematic diagram of the process is shown in FIG – 7.

Conclusion

Ambient moisture and natural water sources have attracted huge research interest in the field of energy harvesting due to easy access and good sustainability and ubiquity of water. A new green technology has been developed to harvest electricity from terrestrial water-based devices on movement of water by capillary action of nanopores present in functional materials like textiles, alumina, wood etc.

This technology has now been identified as viable alternatives to conventional power generation technologies. One such device configuration called Hydroelectric Nanogenerators (HENGs) has demonstrated the ability to generate voltage up to a few volts, which can be utilised for operating electronics.

The working mechanism of HENGs is based on the phase transitions of water. The functional materials and device design of HENGs can be optimized to enhance the electrical performance and lifespan for practical applications. KB-wool-based HENG holds great potential for practical application. Due to high affinity of KB for Ca ions, Na ions, hydronium ions, KB coated wool cloth can be used to increase the energy density.

With 10 HENGs each containing 10 stacked KB/wool stripes can achieve a Voc of 2.31 V continuously using seawater, and the process is self-sustaining. Despite this promise, many challenges are to be addressed in order to develop a robust, high output device capable of generating power for years. These problems are being addressed by exploiting recent advances in nanotechnology, material science, improvement in design and engineering which will lead to commercialization of this device in the near future.

Concluded

Rathindra Nath Biswas is the Dy. General Manager, In-Charge (Retired), MECON, Durgapur.