Consider the voltage of an aluminum air battery.
Citing a DIY photo of an aluminum-air charcoal battery from Saito's Science published in 1999.
A stainless steel or corrosion-resistant plated spoon is used as the core.
It is surrounded by activated charcoal of about 3mm diameter.I am curious about the nature and shape of this activated charcoal.
The activated charcoal is wrapped with kitchen paper, aluminum foil, and saran wrap.
Just touching aluminum and stainless steel with a human hand has an electromotive force of 0.5 volts, he said.
We have also been able to measure 0.5 volts of electromotive force when placing aluminum and copper coins (1 yen and 10 yen coins) on the tongue or between them with kitchen paper.
than aluminum and copper.
The aluminum and carbon electrodes have a higher electromotive force and current.
The potential difference in the database is as follows.
1.6 volts between aluminum and hydrogen
2.0 volts between aluminum and copper
0.5 volts between aluminum and copper, so the actual measurement is
1.5 volts less than the theoretical value.This 1.5 volts is presumed to be the voltage loss.
Aluminum and copper will melt on the tongue in extremely small amounts.Copper, in particular, is a precious metal that does not dissolve easily.However, some people can taste them, so they probably dissolve.
The article states that a battery made of activated charcoal and aluminum foil with a spoon as its core measured a voltage of 1.25 volts.Add to this a voltage loss of 1.5 volts and you get 2.75 volts.
This 2.75 is almost identical to the theoretical value of 2.71 volts on the wiki.
The potential at which aluminum dissolves in alkali is
minus 2.31 volts,
The potential for oxygen to dissolve in water is
plus 0.4 volts
The actual measurement for the case where strong alkali is not used is as follows
The actual value without strong alkali is 0.7 volts.
Saito's science measured 0.5 volts for an aluminum-copper battery in tongue,
0.7 volts actual value for aluminum air battery in salt water in wiki,
These are realistic values of actual measurements.
I have also measured about 0.5 volts for an aluminum air battery.
The brine aluminum-air battery has a potential difference of 1.2 volts between the aluminum foil and the carbon air electrode, if the voltage loss is compensated for, which is supposed to be the case.
The 1.25 volts in Saito's aluminum-activated carbon air battery is a very large voltage compared to the 0.7 volts in the wiki.It is also above the upper limit of what is considered to be the original potential difference of an aluminum air battery in salt water.
The photographic evidence shows 1.014 volts.It is being held tightly by hand and the aluminum foil has been deformed to fit the shape of the activated carbon.The temperature may have also increased.
In wiki,
The original theoretical potential difference of an aluminum air battery using salt water is 1.2 volts
and it does not describe the anode and cathode reactions corresponding to the potential difference in the description.
I am not sure of the theory behind the actual measured 1.25 volts with the hand-holding effect compared to the 0.7 volts of a typical salt aluminum-air battery.
https://sai.ooiso.net/r19/990818/000.html
This is Saito's science URL.
Voltage is a mystery.
Intercalation of porous carbon.
The schematic diagrams of lithium-air and zinc-air batteries show schematically how oxygen enters the holes or layers of porous carbon.
The schematic diagram of a zinc electrode also shows the electrode state where metal and oxide coexist.
Between the electrodes are partitions of solid electrolyte and ion exchange membranes.For this partition, Saito's DIY method uses kitchen paper moistened with salt water.
A demonstration by the Chiba Institute of Technology introduces an aluminum air battery at 0.8 volts.
He did not clench the aluminum foil around the activated carbon grains by hand.If carefully made, 0.8 volts can be produced.
Incidentally, Chiba Institute of Technology's tester has two ranges for voltage, millivolts or volts.The current was in three ranges: microamperes, milliamperes, and amperes.
Carbon pores come in various sizes.It seems that not only are there pores that allow oxygen to enter, but also pores that allow lithium and aluminum ions to enter (intercalation).
The difference in potential between the intercalation potential of those ions and the ionization potential of the metal may be the true potential of the aluminum-air battery that is actually measured?I imagine that this is the true nature of the measured potential of the aluminum air battery.
While the theoretical value of the ionization of aluminum is clearly defined at minus 1.6 volts, the potential of intercalation to carbon is not so clear.Comparing the reaction of metal ions precipitating from water and intercalating into the carbon layer, the same is true for adsorption from the electrolyte solution to the electrode side.
This is similar to electrolytic refining, where a small voltage is applied externally to dissolve the metal on one side and precipitate it on the opposite side.
I imagine that the voltage for this intercalation to occur and the voltage loss of the aluminum-air battery may be related.
There may also be a voltage difference between the potential of oxygen entering the carbon layer and the potential of oxygen leaving the battery.
Also, the metal and oxygen would combine inside the carbon layer to form hydroxide.Even if hydroxide precipitation is formed inside the pores, the carbon electrode as a whole is conductive.
Let us imagine that there should be a carbon layer where oxygen can enter and a carbon layer where metal ions can enter, and they should be close to each other.
Role of the porous carbon electrode in an air battery
What is the role of the carbon electrode during discharge?
The role of the carbon electrode during discharge would be to take in oxygen and metal ions as hydroxides, while maintaining conductivity as an electrode.
In the cathode of a lead-acid battery, lead dioxide is used for its conductivity.
In air batteries, the key technology would be a carbon electrode that retains conductivity as an entire electrode even when less conductive oxides or hydroxides are deposited, similar to the lead oxide in lead-acid batteries.
Is the low conductivity oxide or hydroxide intercalating into the carbon electrode aluminum oxide or aluminum hydroxide, or sodium oxide or sodium hydroxide?
Thinking up to this point, we realize that a porous carbon electrode could localize sodium hydroxide as well as aluminum hydroxide.
It is possible that the activated carbon used in Saito's science class could have generated chlorine gas from the salt to make sodium hydroxide, and that the strong alkali would dissolve the aluminum, resulting in a high voltage.
What kind of activated carbon would be used to generate trace amounts of chlorine gas from salt?
Electrolysis of brine generates hydrogen and chlorine.Hydrogen is generated at the cathode and becomes alkaline.The anode generates chlorine, making it acidic.
This may be the reaction that occurs in the electrode of activated carbon particles.It can be assumed that chlorine gas is generated locally in the carbon electrode.
Just as corrosion occurs in a local battery, there could be a local reaction within the carbon electrodes of the battery.
I tried various imaginations to explain the 1.25-volt voltage of the aluminum-air battery.
The conductive porous carbon electrode has a secret.I would like to process this with salt agar with activated carbon to make it easier to use.I plan to make an electrode with more activated carbon and less agar, using the sponge as the agar core.
アルミニウム空気電池の電圧を考える。
1999年公開の斎藤の理科からアルミ空気炭電池のＤＩＹ写真を引用する。
https://sai.ooiso.net/r19/990818/000.html
また亜鉛電極の模式図では金属と酸化物の共存する電極状態も示されている。
電極間には固体電解質やイオン交換膜の仕切りがある。この仕切りを斎藤さんのＤＩＹでは食塩水で湿らせたキッチンペーパーを使っている。
千葉工業大学のデモンストレーションでアルミ空気電池が0.8ボルトであると紹介されている。
活性炭粒の周りのアルミ箔を手で握り締めるようなことはしていない。丁寧に作れば0.8ボルトは出せる。
ちなみに千葉工業大学のテスターは電圧はミリボルトかボルトの２レンジ。電流はマイクロアンペア、ミリアンペア、アンペアの３レンジのあるものを使っていた。
炭素の細孔にはさまざまな大きさがある。酸素が入ると細孔だけでなく、リチウムやアルミのイオンが入り込む（インターカレーション）する細孔もあるようだ。
そのイオンのインターカレーションの電位と金属のイオン化の電位差が実測されるアルミ空気電池の電位の正体ではないか？と想像している。
アルミのイオン化がマイナス1.6ボルトに理論値がはっきりしているのに対して、炭素へのインターカレーションする電位ははっきりしない。金属イオンが水から析出する反応と炭素層にインターカレーションする反応を比べてみると、電解質液から電極側に吸着するという点は同じだ。
これは外部から小さな電圧をかける電解精錬のように一方で金属を溶かし、対極で金属を析出させる時と似ている。
このインターカレーションが起きるための電圧と、アルミ空気電池の電圧損失は関係あるかもしれないと想像している。
炭素層に酸素が入ってくる電位と電池内へ出ていく時の電位でも、電圧の違いがありそうだ。
また炭素層内部で金属と酸素が結合して水酸化物ができるだろう。細孔内部に水酸化物の析出ができても、炭素電極全体では導電性がある。
酸素が入る炭素層と金属イオンが入る炭素層があって互いに近いと良いと、想像してみる。
【空気電池の多孔質炭素電極の役割】
放電時の炭素電極の役割は、
酸素を取り込み金属イオンを水酸化物として取り込みながら、電極としての導電性を保つことだろう。
鉛蓄電池の正極では、二酸化鉛が導電性があることを利用している。
空気電池では導電性の低い酸化物または水酸化物が析出しても電極全体として導電性を保つ炭素電極が、鉛蓄電池の酸化鉛と似たような鍵の技術となる。
炭素電極にインターカレーションする導電性の低い酸化物や水酸化物はアルミ酸化物や水酸化アルミニウムなのか、ナトリウム酸化物や水酸化ナトリウムなのか？
ここまで考えて、多孔質炭素電極は水酸化アルミニウムだけでなく水酸化ナトリウムも局所的に発生する可能性に気がつく。
斎藤の理科で使われた活性炭が、食塩から塩素ガスを発生させて水酸化ナトリウムをできて、その強アルカリでアルミニウムが溶けて高い電圧が得られた可能性がある。
食塩から微量な塩素ガスが発生させるための活性炭はどんなものだろうか？
食塩水を電気分解すると水素と塩素が発生する。陰極で水素発生してアルカリ性になる。陽極で塩素発生して酸性になる。
これが活性炭粒子の電極のなかで起きた反応かもしれない。炭素電極のなかで局所的に塩素ガスが発生したと推測できる。
腐食が局所電池で進むように、電池の炭素電極内の局所反応もあり得るだろう。
アルミニウム空気電池の1.25ボルトの電圧を説明するためにいろんな想像をしてみた。
導電性多孔質炭素電極には秘密がある。これを活性炭入りの食塩寒天で使いやすく加工してみたい。スポンジを寒天の芯にして活性炭を増やして寒天を減らした電極を作る予定だ。