Electricity is a very crucial part in supporting society and its development. Global electricity consumption has continuously grown over the past half a century. In 2021, the world’s electricity consumption reached approximately 25,300 terawatt-hours. Between 1980 to 2021, electricity consumption more than tripled, while the global population increased by roughly 75%. However, the widespread use of traditional energy sources, such as fossil fuels, has contributed to global warming. Therefore, there is an urgent need to increase the use of green and sustainable energy resources to meet the ever- increasing energy demand.
Different technologies are in vogue to convert green energy into electricity. Among these technologies, Hydroelectric Nano Generators (HENG) is of particular interest, because it uses water, which is abundantly available on earth, to produce energy. A HENG converts nanoscale electro-kinetic streaming energy by absorbing water at the interphase between a conductive nanomaterial like graphene, ketjen black and a substrate made of wool cloth, other textile material, biomolecules, polymers, alumina etc., at its anode side.
The absorbed water is driven through micro-channels by capillary forces to form an asymmetrically wetted HENG. The electricity generated originates from the streaming current of moisture, water, electrolytes i.e., seawater, where movements of ions near the surface of a conducting substrate induce a voltage drop associated with the movement of water molecules. The presence of an Electrical Double Layer (EDL) at the interphase between the liquid and porous wall forms the basis of charge movement during water evaporation.
Overview
HENG is a fascinating technology, because it uses water, a renewable and abundant resource on earth. Hydroelectric nanogenerators are small generators that generate electricity by streaming saline water which has considerable amount of alkali metals salts, or other ambient water/moisture by capillary action, through active materials on the surfaces of core. Since water capillary action is the key mechanism for power generation, the efficiency of HENG primarily depends on the properties of the active materials, core substrates etc. Cellulose-based fibers, wool, textiles are appealing materials, because of their excellent water absorption and wetting properties.
Sometimes MXene is integrated into a woollen cloth to fabricate high efficiency wearable HENGs. MXenes are two-dimensional inorganic compounds that consist of atomically thin layers of transition metal carbides, nitrides, or carbonitrides, but these are quite costly material. Oxidized ketjen black nanoparticles added to the Single Layer MXene (SMX) matrix maintains the HENGs’ hydrophilicity. An Electrical Double Layer (EDL) is formed in the nanochannels of the active material in the wet section and can convert atmospheric thermal energy to electricity during the water evaporation process. HENGs convert nanoscale electrokinetic streaming energy into electrical energy by absorbing water at the interface between a conductive nanomaterial e.g., graphene and MXene and a substrate like biomolecules, polymers at its anode side. It utilizes the synergy between conductive nanomaterials and hydrodynamic flow to generate electricity. Capillary force drives the absorbed water to flow in the charged channels and electric current is generated due to potential difference between the dry and wet ends.
Selection of material
HENGs have the potential for generating power on watt-level, which is essential for practical application. However, this requires that HENGs are constructed with nanomaterials with high conductivity and a substrate with a large surface area. Ketjen Black (KB), an active material with high conductivity, is often used in this technology, which is electro conductive carbon black produced in a furnace by cracking of hydrocarbons. It has several attractive features including a low intrinsic resistivity (0.01 ohm.cm), low toxicity, high specific surface area (1400m2g-1), good structural stability and high chemical stability compared to other carbon-based materials.
KB can be encapsulated by coating with a polymer protection layer, if needed. Textile materials are used in the substrate due to their high breathability, large surface area, flexibility, softness, and low cost. But textiles like cotton, silk have poor water transport, and so they are not preferred. But it has been found that wool fibres are better, because of their excellent water diffusivity due to their tiny pores on the epidermal surface. The pores repel absorbed water and transport them to the outer surface of wool fibres allowing for water absorption up to 30% of their weight. Wool fibers can be stretched up to 30% elongation and recover quickly when stress is released. Their properties can be enhanced by plasma treatment, when many micro pits are formed and become super hydrophilic, which is beneficial for water absorption and diffusion.
For anode electrodes zinc, iron, aluminium can be used and fixed to the KB/wool cloth stripes. Zn electrodes produce in seawater quite high potential of 0.975V due to its high activity.
Fabricating HENGs using flexible plasma treated wool impregnated with KB is at present one of the best material of construction. The surface of KB on the HENGs forms an electrical double layer on contact with water and its high affinity for Ca2+, Na+, or H3O+ ions found in sea water can increase energy density and enhance electric potential. The electricity generated by assembly of 16 HENGs, each with 10 stacked wool cloth stripes can continuously harvest energy from seawater and can be stored in a 5F, 5 VDC supercapacitor with a voltage up to 1.6V for 6 hours. A number of such HENGs in series parallel combination can yield higher voltage and power to serve lighting needs and charging underwater devices.
How it works
The exact mechanism of electricity generation in hydrovoltaics remains controversial, because it is not easy to identify it with experimental measurements. However, the mechanism of generation can be classified mainly into two types namely (a) water flow induced streaming potential and electron drag (refer FIG -2) and (b) ion flow induced, causing gradient of protons or ions (refer FIG -5). The water flow mechanism is from the perspective of conventional streaming potential in a nanochannel or a porous medium.
The charged surfaces of a channel interact with the intrinsic dipole of water molecules and align the latter in specific directions. Then either the transport of charged species through a diffusive layer of Electric Double Layer (EDL) or the flow of aligned dipoles produces an electric current. The ion flow mechanism is based on the ion concentration gradient originating from the different diffusivity between anions and cations, particularly protons and hydroxyls in diluted water. This type of concentration gradient can also be induced either by the direct chemical interaction between water and a substrate or the dissolution of water accompanied by charge transfer between water and a surface.
Water molecules from the surrounding moisture are absorbed by hydrophilic/hygroscopic substrate through functional groups like –COOH and –OH present on their surface via physical or chemical interactions. When the protons and hydroxyl ions are spatially separated, they produce an electric field, which leads to the generation of electricity as shown in FIG -5. However, the above explanations to describe the working mechanism are highly speculative due to lack of quantitative models and as the experimental results are challenging.
Phase transitions of water are essential in the HENGs. A water molecule is formed by one oxygen atom and two hydrogen atoms. The hydrogen atoms are positively charged due to electron transfer to the oxygen atoms, resulting in polarized water molecules. The positive hydrogen atoms attract the adjacent water molecules via hydrogen bonds, which act a vital role in the phase transitions of water.
In HENGs the atmospheric thermal energy from sunlight breaks the hydrogen bonds between the water molecules at the air-water interphase resulting in evaporation of water. The capillary force drives ions in the nanochannels leading to electricity generation.
The water-solid interaction at the active material is very significant. Streaming potentials is formed when liquid moves through a porous structure or micro/nanochannels with charged walls, inducing the migration of free charges generated at the water-solid interphase. Most solids develop a surface charge from the ionization or dissociation of surface groups, adsorption of ions from solution, or the substitution of surface ions when in contact with water or other liquids.
An equal but oppositely charged region of counterions are developed in the liquid outside the solid particle to balance this charge, forming a stern layer consisting of immobile ions and an outer diffusion layer containing mobile ions as shown in FIG- 1. When water molecules come in contact with the charged solid surface, the positively charged H ions and negatively charged ions (e.g., OH) of water are separated out. In line with Coulomb’s Law, the solid surface attracts ions of opposite charges, resulting in an ionic cloud. An Electrical Double Layer (EDL) is created by the surface charge of the material and the ionic cloud as shown in FIG -1. EDL consists of the charges bonded to the surface and free charges in the liquid over the surface.
An EDL layer comprises a stern layer and a diffusion layer. Ions in the stern layer are stationary and well bound to the solid surface due to electrostatic attraction. But in the diffusion layer, ions move under the influence of electrostatic and thermodynamic influence in a specific direction, creating a shear plane (marked “s” in FIG – 1) between the stern layer and the diffusion layer. The electrical zeta potential (ζ), depends on ion concentration and pH of the solution at the shear surface and has profound impact on the water-solid interaction. The characteristic dimension of the EDL is the Debye length, which depends on the nature and concentration of ions in solution over the surface. Debye length is the distance between the stern plane and the nearest region of the bulk liquid, and it generally ranges up to several hundred nanometers.
In a porous medium, the critical parameter is the ratio of Debye length to the channel diameter, as high electrical output is only obtained when the EDL on each side of the channel overlap. Under this condition mobile counterions exist in the center of the channel. These mobile charges in the water within the EDL move under the external pressure or capillary forces resulting in an electric current known as streaming current along the channel. Confining water flow in nanochannels is an efficient way to form a diffusion layer leading to improved output electrical performance of HENGs.
The streaming current, Is= (Ak1k2/ηl) x ΔPζ, where A is the total cross section of the pores, k1 is the dielectric constant of water, k2 is the electrical permittivity of vacuum, η is the viscosity of the solution, l is the pore length, ΔP is the pressure differential, and ζ is the zeta potential. The accumulation of counterions at the downstream end of the channel leads to a potential difference Vs, i.e., streaming potential.
Streaming potential VS= (k1k2/ση)/ΔP ζ, where σ is the electrical conductivity of the solution. The energy that causes the flow or diffusion of water molecules can be harvested and transformed into electrical energy. Mechanical energy is needed to transport water in the large channels of bulk material. But nanomaterials used in HENGs can harvest thermal energy from the ambient environment through the natural process of water evaporation. Water molecules spontaneously flow along the charged nanochannels in HENGs during the natural evaporation process and induce electricity output.
When the water evaporation process stops, electricity generation vanishes. Both the streaming potential and streaming current can be optimized by tuning the zeta potential and pressure difference applied along the channel. The streaming current can also be increased by providing smaller pore length and larger contact area. A high voltage output can be obtained with enhanced charge density. Combination of two factors like streaming potential due to wetting asymmetry of HENGs and the redox reaction of the electrode generate electricity in HENGs. Streaming potential is the potential difference across a capillary tube or membrane measured at zero current when a fluid with charged ionic species flows through the charged channel due to a pressure gradient. The atmospheric thermal energy causes water evaporation and the wicking process of the water flow is improved and the transfer of electrons is accelerated.
Evaporation of water at the top of the HENG surface causes water to flow in the active material under capillary forces, accelerates transfer of electrons and asymmetrically wets the material inducing in the external circuit to form an electrical current. When water is restricted in a channel with dimensions close to Debye length of the solid-water interphase, the EDL overlaps with the channel space that contains either anions or cations based on the charge of the solid surface. As water moves in the channel under a pressure gradient, the ions located outside the shear surface will be transported along with the water molecules inside the channel resulting in streaming potential, which is continuous and unidirectional. The streaming potential has the correlation with the capillary force, zeta potential, channel size and surface tension of the material. Fig -2 shows HENG with streaming potential.
To be continued…
Rathindra Nath Biswas is the Dy. General Manager, In-Charge (Retired), MECON, Durgapur.
