Monday, August 5, 2013

Mg Soleil Project-9

V.   Magnesium Fuel Cell Technology
5.1  Self-Discharging
5.2  Kohama MgFC technology
5.3  Present Status of MgFC
5.4  Resistive to Disaster
5.5  Major Advantages of MgFC

V. Magnesium Fuel Cell Technology
The magnesium fuel cell (MgFC) has a mass energy density of 2.6 kWh/kg. The figure is 10 times larger than that of the lithium battery. MgFC is capable of powering the vehicle 500 km at 20 kg magnesium.  MgFC is now commercially available from MagPower Systems Inc. It is capable of driving the devices, for example, coffee maker. MagPower uses a "hydrogen inhibitor" as its properietry technology. Kohama Lab. et al.* are now actively developing the magnesium fuel cell (MgFC) functioning like a primary battery. The MgFC has been developed to such a level of an emergency-use battery. The MgFC uses a flame-retardant magnesium alloy for its anode (negative electrode).
* Co-developers = Tohoku university (Prof. Kohama), AIST, Furukawa battery, Nihon materials, and other companies

MgFC belongs to the "metal-air battery" category, and uses magnesium (special magnesium alloy to be described later) for its metal. "Magnesium, oxygen and water reactively cooperate to generate a magnesium hydride (Mg (OH)2), so that an electromotive force is generated between the positive and the negative electrodes. When a load, for example, a lamp, is connected between those electrodes, current flows through a circuitry including the load and the battery. The fact clearly shows that the MgFC is a chemical electric generator, which chemically generates electricity. Kohama MgFC operates like a primary battery that stops its electric generation when the magnesium at the negative electrode is used up.
The function of the MgFC may be roughly interpreted as follows.
The smelted magnesium has stored solar energy in the form of free electrons. When the magnesium is applied to the negative electrode of the metal-air battery, the above chemical reaction takes place in the battery.  Electrons flows from the negative electrode to the positive electrode.  The current is reverse to the electrons in flow.  The MgFC extracts the free electrons from the smelted magnesium, and feeds to the load them in the form of current. 

MgFC will continuously generate electricity if it has such a function as to continuous supply magnesium to the negative electrode. Such an improvement of the MgFC will be made in future. The hydrogen fuel cell (H2FC) is also a chemical electric generator, which generates electricity using hydrogen as its fuel. It continues its electricity generation so long as the fuel supply continues. As known, the ordinary battery is supplied with electricity from an external electric source, stores the electricity supplied, and feeds it to exterior. When the electricity stored in the battery is used up, it is necessary to charge the battery. If it is left for a long time, it naturally discharges and loses the electricity stored. Functionally, the ordinary battery is passive.  The MgFC generates electricity. It function is active.
5.1 Self-discharge phenomenon
The self- or natural discharge is known, which is inherent to this type of battery. When the magnesium (Mg) metal of the negative electrode dissolves, electrons are generated and those react with hydrogen ions to generate hydrogen. The hydrogen generation results in no or little flow of electrons to the positive electrode. No electrons-flow hinders increase of the battery capacity. Where the used electrolyte is acid, the self-discharge phenomenon is distinctive.
To avoid this, the alkaline electrolyte is used. In this case, another problem arises. When the alkaline electrolyte is used, a coating of insoluble magnesium hydroxide is formed on the surface of the magnesium of the negative electrode. The film allows neither electrons nor ions to pass through there. The battery reaction stops and the battery loses its function.  (Dr. Kurihara (SAITEC). 

Some solutions to self-discharge problem
1) Magpower systems successfully solved the self-discharge problem by using "hydrogen inhibitors" as its properietry technology. Reference is made to the company's website.
2) Susumu Suzuki succeeded in solving the self-discharge problem by using a polyvalent carboxylate aqueous solution. For details, reference is made to WO/2011/125150 MAGNESIUM BATTERY.
3) Prof. Kohama solved the same problem by using a special flame-retardant magnesium alloy for the negative electrode. The technology is detailed in JPA No. 2012-234799.

South-Korea's MgFC recently developed is newsed. The researchers succeeded in powering the car a distance of 200 km by the 40 kg MgFC set on the car. The positive and the negative electrodes of the MgFC are improved, and the battery structure is modified for improvement. The results are that the chemical reaction efficiency at the negative electrode and the chemical reaction rate are both increased, and that the energy efficiency and the energy density are doubled when compared with those of the conventional one. It is unknown how to overcome the self-discharge problem inherent to the MgFC.

V. マグネシウム燃料電池
マグネシウム燃料電池(MgFC) の理論エネルギー貯蔵量2.6Ah/g。 リチウムイオン電池の10倍以上。車に適用した場合、重量20kgで500kmの走行が可能。
現在、MgFCはMagPower Systems Inc. より入手可能である。コーヒーメーカーを駆動可能。MagPowerは"hydrogen inhibitor"(特許取得済み)を使用している。
Kohama Labo. et al.* もMgFCを開発している。緊急用電源程度までは実用化の域に達している。特殊な難燃性マグネシウム合金をアノード(負極)に使用している。
* Co-developers = Tohoku university (Prof. Kohama), AIST, Furukawa battery, Nihon materials, and other companies

MgFCは金属―空気電池であり、その金属がマグネシウム(特殊マグネシウム合金、後で詳しく述べる)である電池。この電池では、マグネシウム、酸素及び水から水酸化マグネシウム(Mg(OH)2)が生成され、その結果正負電極間に起電力が発生する。この両電極間に負荷を接続すれば、電気が流れる。 つまり、この電池は発電する。化学的に電気を発生する化学発電装置である。Kohama MgFCはマグネシウムを使い切れば発電機能が失われる一次電池のように働く。
一般の電池は電気の供給を受け、それを蓄電し、出力(放電)する。電池に蓄電した電気を使いきれば、再び充電する必要がある。放っておけば自然に放電する。機能的には 受動的だ。MgFCは発電する。機能的には能動的だ。

5.1 自己放電
MgFCの負極のマグネシウム金属が電解液に溶出し、電子が発生する。この電子が水素イオンと反応し水素を発生する。電子の正極への流れが阻害される。これが自己放電。これにより電池容量を大きくできなかった。これがMgFCの実用化を阻んできた。よく知られている。電解液が酸性(水素イオン濃度が高い)であると、この現象の発生の度合いが大きい。 これを避けるためアルカリ電解液を使用する。しかし、この場合、別の問題が発生する。アルカリ電解液の場合、負極のマグネシウムの表面に不溶性の水酸化マグネシウムの被膜が形成される。この皮膜は電気もイオンも通さない。熱も発生する。結果は電池反応が停止し、電池機能が停止する (栗原英紀博士(SAITEC) 。

1) Magpower systemsはhydrogen inhibitorsでこれを解決している。詳しくは同社のhomepageをご参照願いたい。
2) 鈴木進氏が、多価のカルボン酸塩の水溶液を用いることで自己放電の問題を解決している。詳しくは、特許公開2010-182435をご参照願いたい。
3) Prof. Kohama (Tohoku University) は特殊難燃性マグネシウム合金を負極に用いて自己放電問題を解決している。特開2012-234799に詳しく書いてある。

>> To be continued to Mg Soleil Project-10

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