The lead-acid battery is a water electrochemical system with a positive electrode and limited liquid mass transfer. This system will generate gas (hydrogen evolution, oxygen evolution) during operation, resulting in water loss. Therefore, maintenance of the water rehydration solution is required.
Maintenance-free (meaning no need to add water and rehydration) is the most simple instinct requirement of people. In the process of achieving maintenance-free lead-acid battery, it has gone through a long and tortuous road, including the use of catalytic hydrogen elimination and auxiliary electrodes. .
In China, the development of hydrogen-reducing batteries began in the 1960s [1]. At the time, both internal hydrogen evolution and hydrogen evolution were thoroughly studied. Internal hydrogen depletion is mainly the addition of hydrogen absorbing compounds to the battery electro-hydraulic. The most typical internal hydrogen absorbing compound is anisaldehyde. The anisaldehyde, anisole and anisic acid are used for oxidation-reduction in the battery. It is hoped that the anisaldehyde can be redoxed repeatedly to achieve the purpose of hydrogen elimination and oxygen elimination. However, in fact, due to many problems such as the amount of addition and reversibility change, the hydrogen-suppressing and oxygen-absorbing performance is not satisfactory, and it is not industrialized. The external hydrogen is mainly placed in the catalytic plug by using the catalyst palladium (made as a palladium bead) and mounted on the battery cover. The catalytic plug has a complicated structure, and the catalyst (palladium beads) is placed in the molecular sieve bag. The bag and the catalyst are placed together in a porous (corundum) cap, and a metal cover and a metal cover are arranged outside the porous cap to facilitate water vapor diffusion and water condensation. Accelerating the return of water to the battery does not cause the palladium surface to be coated with water film (wet) and fail. This structure was officially put into production and was put into small-scale hydropower stations for small batches. After 10a, the product was investigated and it was found that the work of hydrogen suppression was satisfactory.
For catalytic hydrogenation of rich liquid batteries, to achieve battery sealing, the key is to establish a thermal balance in the catalytic plug. I once wanted to put the catalyst palladium beads (with small holes in them) on a small tube (glass tube), and put the melted naphthalene in the tube. When the catalyst reacts (H2 has O2), the naphthalene (solid) in the tube melts as a catalyst. When unreacted, the naphthalene (liquid) in the tube is turned into a solid state, and the palladium beads are exothermic, and the phase transition of naphthalene (solid)→naphthalene (liquid) in the tube is used to maintain the heat balance in the catalytic plug.
This kind of idea has not been put into real design and manufacture for various reasons. It has only been a catalytic device. Catalytic hydrogenation is very difficult, and the use of a catalytic device to seal the battery is only a tortuous process that has been on the road to sealing the battery.
In the 1970s, valve-regulated sealed batteries were introduced. In the 1980s, China successfully used it in electric power, post and telecommunications, and UPS, replacing traditional rich liquid batteries. For many years, the reputation of valve-controlled batteries has always been closely linked to the reported failure of failure (pre-capacity PCL) and the unexplained decline. Indeed, a valve-controlled battery (15-20 a) that claims to have a long life seems to be a problem, and most of the cases are only a short-lived (about 5-6 a) design. There are many reasons for this, mainly due to the constraints of electro-hydraulic and negative electrodes. There is little known electrochemical imbalance on the internal anode of the valve-controlled battery; there is a dual role of polarization and depolarization (oxygen compounding). There are many balances inside the valve-controlled battery, with electrochemical equilibrium or hydrogen balance. These balances are extremely important and are key to the stability of the valve-controlled battery and the basic purpose of the battery design. This imbalance has long been discovered [2, 3]. But the information has not been well translated into battery design, and most battery manufacturers do not fully understand the importance of this phenomenon. In this paper, an attempt is made to make a rough analysis from the internal balance of the battery. The purpose is to provide theoretical basis and theoretical support for the majority of valve-controlled battery manufacturers in designing catalytic devices.
What is hydrogen balance? The simple answer is the electrochemical characteristics of a particular battery design [4]. For a valve-controlled battery, the specific hydrogen balance means that two distinct independent reaction rates must be close to or equal to equilibrium. These two reaction rates refer to the self-discharge rate of the negative electrode and the grid corrosion rate of the positive electrode.
When the battery is left open, the self-discharge reaction is always performed on the negative electrode, and the rate can be measured by the hydrogen evolved by the reaction. In fact, self-discharge is also related to many factors: for example, the temperature rises, the impurity content is more, and the self-discharge increases; the organic additive used in the lead paste reduces the self-discharge rate. It is not practical to hope that there is no self-discharge at all, because the autolysis reaction in the lead-acid battery is always present, but the reaction proceeds very slowly.
There is also a tendency for the negative electrode to leak hydrogen at a certain rate. To balance the hydrogen, it is necessary to pump hydrogen (in ion form and electronic form) at the same rate as the leak. In this way, the complete concept of negative charge charging should be to force hydrogen ions and electrons into the negative active material, in other words, to charge hydrogen (ionic form and electrons) into the negative active material.
The source of charged hydrogen is generally overcharged and/or electrolyzed. The charged hydrogen is not hydrogen, but an ionic form and an electronic form.
After the valve-controlled battery is fully charged, water is decomposed at the anode and is divided into three parts:
Part I: Oxygen diffused into the atmosphere (O2) Part II: Hydrogen ions (H+) diffused into the battery electrohydraulic Part III: Electrons flowing on the circuit For liquid-rich batteries, oxygen ( O2) escapes from the battery. It is because of the escape of oxygen that the charged hydrogen (ion form and electron) enters the negative electrode freely. As a result, hydrogen is combined on the negative electrode and the negative electrode is charged. At this time, the negative electrode is only polarized. Little or no depolarization.
For a valve-controlled battery, the situation is different. Oxygen does not escape from the battery. Instead, oxygen, hydrogen ions, and electrons are combined in the negative electrode to form water. At this time, the negative electrode has both polarization and depolarization (oxygen complex). ). At this time, the negative electrode only lie as a charged hydrogen source.
When the oxygen recombination efficiency reaches 100% inside the valve-controlled battery, the charged hydrogen (ion form and electron) from the electro-hydraulic tends to be exhausted, and then what is the reason to keep the negative charge charged? Answering this question is not difficult, this It is because there is another source of charged hydrogen, which is the corrosion of the anode grid. Anode grid corrosion absorbs oxygen from the water and releases a corresponding amount of charged hydrogen (ion form and electrons), which migrates to the negative electrode, helping to charge the negative electrode.
In this mature valve-controlled battery, the negative electrode is truly a useful source of charged hydrogen. However, this source of hydrogen is mainly dependent on the corrosion rate of the anode grid.
The electrons on the external circuit are not shown, but it is clear that the form of the hydrogen ion flow is always opposite to the electron flow and equal in quantity. From the above descriptions, the concept of a balanced battery is that the negative electrode is neither polarized nor discharged, which is an idealized valve-controlled battery. A mature valve-controlled battery has an internal gas reaction efficiency of 100% and does not affect the hydrogen balance of the battery. It is a form of reversible electrolysis, only positive charge (polarization), and the negative electrode is depolarized. Oxygen cycle is the key to sealing, but the depolarization (chemical discharge) of oxygen to the negative electrode will greatly change the hydrogen evolution potential of the negative electrode, the corrosion of the positive electrode grid is greatly accelerated, the water loss of the battery is serious, and the electro-hydraulic dry hydrogen evolution and the positive grid corrosion are achieved. Balance, this is the degree to balance the battery.
The valve-controlled battery has a catalytic device: H2 generated by the partial reaction of the negative electrode and O2 precipitated by the corrosion of the positive electrode grid, and the synthesized water is returned to the battery in the catalytic device. The direct catalysis of H2 becomes water, which can greatly reduce the water consumption, and the O2 from the positive electrode can directly catalyze the formation of water without recombination through the negative electrode, so that the depolarization of the negative electrode can be alleviated, and the positive electrode potential can be lowered. Reduce positive grid corrosion and oxygen evolution.
The valve-controlled battery with the catalytic device is theoretically a long-life design. This is due to the water circulation of both the cathode oxygen composite and the direct hydrogenation, so that the water consumption is greatly reduced and the battery is difficult to occur. Dryness. If a special corrosion-resistant alloy is used in combination with a low self-discharge rate formulation for the negative electrode, a truly long-life valve-controlled battery can be realized.
Catalytic devices are used to correct the imbalance inside the valve-controlled battery. Hydrogen and oxygen can be directly catalyzed into water, and oxygen can be circulated from the oxygen cycle. Therefore, the uncharged charged hydrogen (in the form of electrons and ions) reaches the polarization. negative electrode. It is estimated that about 5% of the oxygen from the oxygen cycle is consumed by the catalyst. The better the battery, the less oxygen is emitted from the oxygen cycle.
The catalytic device can remove some excess oxygen. Repair the battery. Make it completely balanced and reduce the negative chemical discharge (oxygen compounding). The catalytic device for a valve-controlled battery produces much less heat than the catalytic plug of a flooded battery. Generally, the flooded battery is generally 50W/only, which will damage the catalyst in the catalytic plug; the catalytic device for the valve-controlled battery has a heat generation of only a fraction of W/only, and the heat does not damage the catalytic device. The inner space of the valve-controlled battery is dryer than the rich liquid battery, which is advantageous for the long-term stability of the catalyst in the catalytic device.
The valve-controlled battery catalytic device is known as the balancer, which enables the valve-controlled battery to have a balanced design, which can truly treat the disease-causing valve-controlled battery and realize long-life design.
The use of catalytic devices becomes very attractive if a long-life, stable, balanced valve-controlled battery has not been realized before. In particular, it is required to realize the long life of the valve-controlled battery in a high-temperature environment, and the application of the catalytic device is particularly important. The next step is how to design the catalytic device for the use of valve-controlled batteries. Due to the space, the structural design of the catalytic device will be introduced in the next section, and it will be taught by peer experts.
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