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The electric eel has a unique discharge ability, which can generate enough current to knock people unconscious, which is known as “high-voltage line in water”. The overall reaction process of the concentration difference battery is the concentration gradient of the substance in the battery system, and the electrical energy output is achieved through the concentration difference diffusion of the substance.
One is an aquatic organism that can discharge, and the other is a chemical battery that can store energy. What can they be related to? The Ji Xiaobo team, a professor at the School of Chemistry and Chemical Engineering of Central South University, found that the electric eel is the most typical representative of the perfect use of the ion concentration gradient discharge. With the help of this principle, they use two hydrogels to stack to form a trapezoidal “power layer” to create an electric eel-type double-ion gradient battery, and prepare a foldable by the inspiration of origami art. 3D battery.
Recently, the results were published in ACS Applied Materials & Interfaces. Xiao Xiangting, a graduate student of Central South University, is the first author of the paper, and Ji Xiaobo is the corresponding author.
The unpopular direction in the popular research field
Batteries are an important support for new energy vehicles, energy storage, consumer electronics and other fields. With the rapid development of China’s new energy vehicle market and other fields, the battery market demand continues to grow.
Among them, secondary energy storage batteries represented by lithium-ion batteries currently occupy a dominant position in the market and play a key role. However, potential problems such as limited metal resources and potential safety hazards limit their long-term application. Therefore, the design and manufacture of new energy storage devices have gradually attracted the attention of industry personnel and set off a research boom. Solid-state batteries, nano-batteries and other battery technologies have made frequent breakthroughs, and advanced products have emerged one after another.
In this hot research field, the research on thicker batteries seems to be a little “scanned”.
“At home and abroad, there are not many teams specializing in concentrated batteries, and there are few major achievements.” Hou Hongshuai, a professor at Central South University and a member of the Ji Xiaobo scientific research team, said that although the thick difference battery was mentioned by scientists a long time ago, such batteries did not have specific devices, low voltage, and no good application landing cases, so they have not received enough attention.
What is a thick battery? In terms of composition, the concentration difference battery is only composed of positive electrode, negative electrode and electrolyte. It is divided into electrode difference battery and electrolyte concentration difference battery. The former is caused by the difference in the concentration of active substances of the electrode itself, and the latter is the difference in electrode potential caused by the difference in the concentration of electrolyte in the battery, so the concentration difference in the electrode. The size of the potential of the cell electrode is related to the concentration of the electrolyte solution.
The overall reaction process of the concentration difference battery is only the process of transferring a substance such as a single substance or ion from a high concentration state to a low concentration state. In real life, seawater salt capacity power generation is the most typical application representative. It is reported that the energy resources of seawater salt difference in the world can reach 3 billion kilowatts.
In order to fully develop and utilize this energy, scientists use the principle of concentration difference battery to load seawater and river water into two containers separated by the ion exchange membrane and insert electrodes respectively, so that a simple electrolyte concentration battery can be built. High-concentration sodium or chloride ions in seawater can be freely diffused to low-concentration rivers. In water, as long as the salt concentration of seawater and river is different, the electric potential of the two will always exist, and the electricity can continue to generate electricity.
At present, many enterprises have been engaged in the research of salt capacity power generation. For example, Norway’s Stat-Kraft took the lead in completing the demonstration device of 10 kilowatt salt capacity as early as 2009.
The difference in ion concentration is not enough to “tick a ladder” to make up
One of the keys to the concentration difference battery is the construction of the concentration gradient. The greater the ion gradient, the greater the voltage generated. In fact, concentration gradient is a common phenomenon in natural organisms. Only by maintaining the difference in the concentration of specific ions and maintaining a fixed membrane potential inside and outside the cell can we ensure the normal progress of life activities. The disorder of concentration gradient often leads to the end of life.
Among many animals and plants, the electric eel is undoubtedly the most typical representative of the perfect use of ion concentration gradient discharge. There are 6,000 to 10,000 muscle sheets in its body. The thin sheets are separated by connective tissue, and there are many nerves straight to the central nervous system. Each muscle sheet is a power generation cell, that is, a Micro-density battery.
“In short, when the power generation cell is stimulated by nerve signals, the sodium ion channel on the anterior membrane of the cell is opened, and the high concentration of sodium ions outside the cell flows into the low concentration area of the cell, and this diffusion process will generate 65 millivolts. At the same time, the potassium ion channel on the posterior membrane of the cell is opened, and the high concentration of potassium ions in the cell flows out of finely. Cell, and is generated with 85 mV voltage. Therefore, a power generation cell has a voltage of 0.15 volts. Xiao Xiangting said.
It is worth noting that there are thousands of such micro-batteries in the electric eel, and all these batteries are connected in series and in parallel, so the electric eel can accumulate up to 800 volts of voltage and a large enough current between the head and tail of the electric eel. Scientists have studied the discharge capacity of electric eel and found that it can freely control the discharge time and intensity. The electric eel mainly uses this discharge ability to prey on prey and perceive the surrounding environment to defend against the enemy.
In recent years, researchers have gradually used the characteristics of electric eel to design new energy storage and conversion equipment, such as humidity generators, ion selective membranes and flexible supercapacitors, but they are all in the emerging stage of development.
“We found that the discharge principle of the electric eel is just similar to that of the thick battery, and it can make up for the defect of the thick battery.” Ji Xiaobo said that the solubility of the electrolyte is limited, which means that its concentration cannot be infinite, so the ion concentration gradient cannot be as large as expected, and the voltage threshold that can be generated is also low. The power generation principle of electric eel can solve this problem. The number of concentrated batteries designed is large enough that the overall voltage value can rise all the time.
As early as 2017, researchers designed a tetramer gel battery by imitating electric eel, that is, four hydrogel membranes are selected to correspond to the extracellular solution, intracellular solution, selective precellular membrane and selective posterior membrane of electric eel. A battery can generate an average voltage of about 0.18 volts. . However, the preparation process of this gel battery is cumbersome, and the battery composition is complex, which limits the subsequent battery expansion.
On this basis, the scientific research team of the period effect wave combined the principle of electric eel discharge and the basic theory of traditional concentrated difference battery, designed a new type of simple, flexible, safe and easy-to-scale integration of concentrated difference battery. The voltage value of a concentrated difference battery is close to 4 times the power generation capacity of the generating cell.
“The first problem faced by the team at that time was how to construct the ion gradient, which involved the carrier materials that store ions and the types of electrolytes that can release free ions.” Ji Xiaobo said that after three months of literature research and experimental program adjustment, the team finally decided to choose polyvinyl alcohol as the hydrogel base, build a hydrophilic network, and create an adequate water environment to store ions.
In order to ensure the rapid gluing of hydrogels and the presence of free ions in the liquid environment, the researchers did not choose the commonly used and long-consuming freeze-thawing method and the toxic chemical crosslinking method, but chose glycerin and water as binary solvents. It is very easy to form hydrogen bonds through rich oxygen-containing functional groups between the three, thus accelerating the gluing rate of hydrogels. It greatly saves raw materials and time costs.
Then, the researchers tried to find various electrolyte materials. It needs to meet two conditions. One is that the macromolecular skeleton can be bonded to the hydroxyl group on polyvinyl alcohol and glycerol to limit the diffusion of the main body, and the other is that it can produce as many free cations or anions as possible. Xiao Xiangting said that the team tested more than 10 potential electrolyte materials and selected two materials with the best performance – sodium phytate and chitosan quaternium salt.
“We have identified hydrogels rich in sodium ions and hydrogels rich in chloride ions. After stacking these two hydrogels to form a ‘power generation layer’, two concentration gradients are formed, and then the power layer is combined with the electrode, and a double-concentration gradient concentration difference battery is formed.” Xiao Xiangting said. In the end, the team optimized the structure to make the open circuit voltage of the concentrated battery reach 0.54 volts and remain stable for about 2 hours.
Create a foldable 3D battery inspired by origami
Although the concentrated battery developed by the scientific research team has far exceeded the discharge capacity of electric eel eel generating cells, it still faces the second problem – the scale integration of the battery, which is the key to the application of the concentrated battery.
To this end, the team imitated the series structure of electric eel electric cells and realized the series design of concentrated batteries through horizontal stacking method. The voltage value can increase steadily with the increase of the number of series connections, and 126 battery monomer connections can produce a voltage of up to 60 volts.
More interestingly, inspired by the art of origami, the team integrated 56 battery monomers on a piece of paper through a special Miura-ori strategy to form a foldable 3D battery, which can instantly generate about 22 volts of voltage. The integrated concentrated battery has also been proved to be able to supply power to actual electronic equipment, which proves that it has practical application potential.
“This research is not only an innovation of the traditional concept of concentrated battery, but also another example of the application of bionics.” Ji Xiaobo said that the production cost of the double-ion gradient concentration battery designed by the team is low, the structure is simple, safe, flexible and degradable. The electrical performance can change with the actual needs, which can meet the needs of wearable and implantable equipment in the future.
Looking forward to the future, Jixiaobo said that on the basis of this achievement, it will continue to look for “power generation” materials with stronger ionization ability, deeply analyze the real-time diffusion mechanism of ions under changing concentration, optimize the battery integration program, and improve the structure and performance stability of the concentration battery in different application scenarios.
The reviewer of the paper believes that this result reports a unique new type of power supply – electric eel-type double-ion gradient battery; polycationic hydrogel and polyanionic hydrogel generate electrical properties in contact with the electrode, reaching a unit voltage of 0.54 volts; the researchers have achieved through several simulated stacking methods of electric eels. The battery is connected in series.
Related paper information: https://doi.org/10.1021/acsami.3c13008