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Hydroelectric cell (HEC)
Hydroelectric Cell (HEC) is a primary source of green energy device that generates electricity by dissociating water molecules on its surface.[1] [2] It has been invented very recently by Dr. R. K. Kotnala and Dr. Jyoti Shah https://scholar.google.co.in/citations?user=NsmkzGYAAAAJ&hl=en in 2015 at National Physical Laboratory , New Delhi and it has attracted a considerable attention recently.[3][4] [5][6][7] Dissociation of water molecules at oxygen deficient nonporous ferrite/oxide surface creates hydronium and hydroxide ions collected by Zn and Ag electrodes respectively to generate electricity. Hence combination of nonporous oxygen deficient magnesium ferrite/metal oxide along with Zn and Ag electrodes has been coined as Hydroelectric Cell. [8] [9] '''In fact, at room temperature dissociation of single water molecule require 1.23 V potential, an external energy is required to split water molecule into its constituent ions that in general in conventional way is provided externally in the form of UV/catalysts, photons, thermal, biochemical etc.[10] [11] [12] [13] [14][15][16] On the other hand, Nano porous surface of ferrite/oxide material provides highly reactive surface for dissociation of water molecules in to its constituent ions at room temperature which is generally available in adsorbed state.[17][18][19] The main advantage of Hydroelectric Cell is that it's Nano porous surface provides chemidissociated H+ and OH- ions at initial stage and subsequently followed by copious physidissociation due to high electric field developed inside nanopores resulting into continuous generation of large number of ions. Zn(Zinc) and Ag(Silver) electrodes attached on opposite surfaces of the ferrite/metal oxide pellet separate and collect opposite polarity ions result into flow of electric current in the cell.''' HEC is environment friendly, produces no green house gases and its low cost raw materials are widely available as well as its disposal after its uses is non toxic. A two inch diameter cell requires only few drops of water to develop a maximum voltage 0.98 V & a short circuit current of 120 mA. Results are highly comparable to solar cell and other portable electrical energy sources that too without use of any electrolyte/acid or alkali but requires only water. Hydroelectric cell different versions have been reported to produce green electricity using water droplets only.[20][21][22][23][24] Hydroelectric Cell popularity among masses crossed over one million viewers just in 18 months in YouTube alone besides millions of persons have watched TV News broadcast on HEC. Besides there have been lot of progress globally in just 3 years of Hydroelectric Cell inception to be used in different remarkable applications[25-32]

Working Principle
Hydroelectric cell (HEC) works on the principle of water dissociation on Nano-porous oxygen deficient ferrite/oxide pellet surface. The water molecules are initially chemidissociated at oxygen deficient nonporous surface into H+ and OH- ions followed by continuous physidissociation by electrostatic field developed inside nanopores present on pellet surface. These ions migrate towards Silver (Ag) inert cathode and Zinc (Zn) anode electrodes respectively and collected via these electrodes for current generation in external circuit. Zinc is consumed in this reaction and slowly converted in to Zn(OH)2 and highly pure hydrogen gas is ejected. For any other electrolytic cell to work, generation of ions is mandatory which is generally achieved by chemical reaction between electrolyte and electrode. In case of Hydroelectric cell, no electrolyte is required but only ferrite/metal oxide pellet surface acts as the continuous source of ions with water. Hydroelectric Cell working is unique based on the combination of material property, oxygen deficiency, Nano science and electrochemistry.

Reaction Mechanism
At Cathode (Ag):

2H2O ↔ 2OH- + H3O + H3O+ + 2e- → H2 ↑ + 2H2O (+0.22 V) 

At Anode (Zn):

2OH- + Zn → Zn(OH)2 + 2e- (-0.76 V)

Ecell = 0.22 -( -.76) = 0.98 V

Both by products of HEC are of commercial high value and the H2 gas as well as Zn(OH)2 Nano powder are biodegradable and environment friendly.

Design
Fabrication of HEC is very simple process, the ferrite material is obtained by simple heat treatment by solid state reaction technique. A small (1 inch square) or large (2 inch diameter) pellet of ferrite/metal oxide material is coated with comb patterned silver electrodes on one surface and back surface is covered with Zinc electrode. Water is sprinkled on the surface of cell while current is collected via two electrodes in an external circuit.

Performance
HEC has been reported to produce maximum 0.125 A current at 0.98 V by 2 inch diameter circular pellet.[20][21][22][23][24]. All the raw materials that are being used in cell fabrication are cheap so it is cost effective choice for electricity production as compared to other resources. Hydroelectric Cell turns chemical energy into electrical energy. In fact it is the most efficient way to produce electricity by water.

Advantage
HEC offers many advantages as compared to other available power resources, and the most important one is its green energy generation where the residues are also non toxic and its low cost component raw materials. Others are: It is portable green energy device which is compact and light weight. It requires only few drops of water to produce energy and no hazardous chemicals are used. Unlike solar cell it can work during day or night and can run small scale devices like LED and fans continuously for couple of days. The two byproducts of Hydroelectric Cell, Hydrogen gas and Zn(OH)2 Nano-particles are environment friendly and can be utilized for other applications.[22] It generates 99.9 % pure H2 gas which can be used as a high quality fuel. It is a reusable device. No tedious manufacturing process is involved in making of HEC. It is very clean and safe device is to be used for electricity production and is highly reliable with acceptable performance.

Application
1. It can be used as a source for power generation at far/interior places like rural areas/farms, forests and mountains.

2. Hydroelectric Cell can be utilized for domestic and residential applications in decentralized mode to generate electricity at small scale.

3. After scaling up it can also be used to fulfill energy needs of automotive industry.

4. At a large scale HEC finds its applications in consumer products specially in rural areas.

5. One another unique application of HEC is to produce high purity Hydrogen gas.[23]