Weird Science

Electricity Out of Thin Air: The Aluminium-Oxygen Battery

Polish ver­sion is here

The fol­lo­wing article was ori­gi­nally publi­shed in the jour­nal for edu­ca­tors Che­mia w Szkole (eng. Che­mi­stry in School) (5/2016):

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Ples M., Elek­trycz­ność z powie­trza. Ogniwo gli­nowo-tle­nowe (eng. Elec­tri­city Out of Thin Air: The Alu­mi­num-Oxy­gen Bat­tery), Che­mia w Szkole (eng. Che­mi­stry in School), 5 (2016), Agen­cja AS Józef Szew­czyk, pp. 8-11

Alu­mi­num–air bat­tery

The modern era can cer­ta­inly be descri­bed as the age of elec­tri­city. Much of our civi­li­za­tion's tech­no­logy relies pri­ma­rily on this form of energy.

But what exac­tly is elec­tric cur­rent? It is defi­ned as the orderly move­ment of elec­tri­cally char­ged par­tic­les, known as charge car­riers. The flow of cur­rent in a cir­cuit is made pos­si­ble by the pre­sence of an elec­tro­mo­tive force (EMF), which is pro­du­ced by various types of elec­tri­cal energy sour­ces. In elec­tro­che­mi­cal cells, the EMF ari­ses from che­mi­cal reac­tions occur­ring within the cell. Clo­sing the exter­nal cir­cuit cre­a­tes a poten­tial dif­fe­rence (vol­tage, U) that dri­ves the move­ment of charge, which results in the flow of elec­tric cur­rent.

Sour­ces of elec­tric cur­rent must cre­ate a poten­tial dif­fe­rence by conver­ting other forms of energy. Galva­nic cells, which rely on che­mi­cal energy, date back to the late 18th cen­tury and the work of Luigi Galvani, who obse­rved that dis­sec­ted frog musc­les con­trac­ted upon con­tact with metal instru­ments [1]. Inspi­red by these obse­rva­tions, Ales­san­dro Volta demon­stra­ted that two dis­si­mi­lar metals immer­sed in an elec­tro­lyte could gene­rate an elec­tric cur­rent [2].

Today, one of the most com­monly used cells, altho­ugh incre­a­sin­gly repla­ced by newer tech­no­lo­gies, is the type deve­lo­ped in 1866 by French che­mist Geor­ges Lec­lan­ché. A sche­ma­tic of its dry-cell ver­sion (i.e., without a liquid elec­tro­lyte) is shown in Fig. 1.

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Fig. 1 – Struc­ture of a dry cell: a – cathode (gra­phite C), b – man­ga­nese(IV) oxide MnO2 mixed with gra­phite pow­der, c – paper sepa­ra­tor, d – moist ammo­nium chlo­ride NH4Cl paste, e – anode (zinc Zn), f – sea­led insu­la­ting cap

In addi­tion to Lec­lan­ché (zinc-car­bon) cells, many other types of elec­tro­che­mi­cal cells are widely used today, inc­lu­ding alka­line, lithium, and others.

In most cases, con­struc­ting effi­cient galva­nic cells invo­lves the use of hazar­dous mate­rials, inc­lu­ding toxic or cor­ro­sive sub­stan­ces. This makes them less sui­ta­ble for edu­ca­tio­nal envi­ron­ments, such as clas­sro­oms or school labo­ra­to­ries. For this rea­son, I’d like to intro­duce the alu­mi­num-air cell, a sys­tem that does not rely on toxic che­mi­cals. It is also quite uni­que in that one of its elec­tro­des is atmo­sphe­ric oxy­gen. In a way, we’ll be gene­ra­ting elec­tri­city from the air!

Expe­ri­ment

To build the alu­mi­num air cell descri­bed here, you only need a few easily acces­si­ble mate­rials:

Alu­mi­num is a silvery-white metal with rela­ti­vely low den­sity (2.7 g/cm3 or 0.0975 lb/in3), as well as excel­lent mal­le­a­bi­lity and duc­ti­lity. It is also a good con­duc­tor of elec­tri­city [3]. For this expe­ri­ment, ordi­nary hou­se­hold alu­mi­num foil works well (Photo 1a).

Acti­va­ted car­bon con­si­sts mostly of amor­phous ele­men­tal car­bon, along with tra­ces of micro­cry­stal­line gra­phite and ash. Its most nota­ble pro­perty is its extre­mely large sur­face area. Sur­pri­sin­gly, just one gram (0.035 oz) of acti­va­ted car­bon can have a total sur­face area of up to 3,300 m2 (35,520 ft2) [4]. This makes it a highly effec­tive adsor­bent for many che­mi­cal sub­stan­ces.

Acti­va­ted car­bon has a wide range of uses. In medi­cine, it is used to treat poi­so­ning cau­sed by drugs or other che­mi­cal sub­stan­ces. In the che­mi­cal indu­stry, it serves as a cata­lyst and car­rier, and it is also com­monly used in various types of fil­ters. For our expe­ri­ment, acti­va­ted car­bon works best in the form of small gra­ins or pel­lets (Photo 1b).

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Photo 1 – Mate­rials nee­ded to build the cell: a – alu­mi­num foil, b – acti­va­ted car­bon,
c – table salt

Sodium chlo­ride is sim­ply regu­lar table salt, which can be pur­cha­sed at any gro­cery store (Photo 1c).

Buil­ding the cell is stra­i­ght­for­ward. The anode, which func­tions as the nega­tive ter­mi­nal, is made from a sheet of alu­mi­num foil (Photo 2A).

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Photo 2 – Sta­ges of cell con­struc­tion: A – alu­mi­num foil anode, B – fil­ter paper satu­ra­ted with elec­tro­lyte, C – layer of acti­va­ted car­bon with adsor­bed oxy­gene as cathode, D – alu­mi­num foil col­lec­tor

Next, place a sheet of fil­ter paper over the alu­mi­num foil. Paper towels or labo­ra­tory fil­ters work well for this pur­pose. Satu­rate the paper with a con­cen­tra­ted aqu­e­ous solu­tion of sodium chlo­ride (Photo 2B). Then, spread an even layer of acti­va­ted car­bon over the paper, just thick eno­ugh to cover it. Finally, place ano­ther sheet of alu­mi­num foil on top of the car­bon layer. Make sure the top and bot­tom foil lay­ers do not come into direct con­tact.

It's a good idea to place the entire cell on a water­proof, non-absor­bent sur­face, since the elec­tro­lyte may leak thro­ugh the lay­ers.

In this setup, the bot­tom sheet of alu­mi­num foil serves as the nega­tive ter­mi­nal, while the top sheet func­tions as the posi­tive ter­mi­nal.

Now let’s see whe­ther the cell actu­ally gene­ra­tes elec­tric cur­rent. To ensure good con­tact between the car­bon layer and the top alu­mi­num foil, it’s best to press it down with a heavy object, such as a large bea­ker fil­led with water (Photo 3, top).

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Photo 3 – Assem­bled alu­mi­num–air cell;
top: gene­ral view, bot­tom: mea­su­red vol­tage and cur­rent out­put

A vol­tage mea­su­re­ment taken with only the mul­ti­me­ter as the load sho­wed a value of 0.8 V. The short-cir­cuit cur­rent was sli­gh­tly above 22 mA. These aren’t par­ti­cu­larly high values, but they cle­arly demon­strate that the cell is func­tio­ning, conver­ting che­mi­cal energy into usa­ble elec­tri­cal energy.

But can this energy actu­ally be use­ful? Let’s try to put it to work. Photo 4 shows a small DC elec­tric motor, typi­cally used in toys, con­nec­ted to the cell. For demon­stra­tion pur­po­ses, a pro­pel­ler has been moun­ted on its shaft.

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Photo 4 – An elec­tric motor with a pro­pel­ler, con­nec­ted to the cell

When the cell com­po­nents are firmly pres­sed toge­ther, either by hand or with a heavy object, the cell begins to deli­ver elec­tric cur­rent to the motor, cau­sing it to spin (Photo 5). To ensure the expe­ri­ment works, the motor must be desi­gned for low-vol­tage ope­ra­tion.

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Photo 5 – Com­pres­sing the cell makes the pro­pel­ler start spin­ning

Attempts to power minia­ture light bulbs with this cell often fail. Most of these bulbs requ­ire a cur­rent of at least seve­ral hun­dred mil­liamps and a vol­tage between 2 and 6 volts, which exce­eds the out­put of a sin­gle alu­mi­num air cell. Incre­a­sing the sur­face area of the elec­tro­des can improve per­for­mance, but doing so may be imprac­ti­cal in a clas­sroom set­ting.

Still, powe­ring a light source with this type of cell would be very use­ful. Light is a clear and instan­tly visi­ble indi­ca­tor of energy conver­sion, making it ideal for demon­stra­tions, even in front of lar­ger gro­ups.

While loo­king for a sui­ta­ble option, I tur­ned to a now-com­mon light source: the LED, or light-emit­ting diode. LEDs are opto­e­lec­tro­nic semi­con­duc­tor devi­ces that pro­duce light thro­ugh a pro­cess cal­led radia­tive recom­bi­na­tion, which converts elec­tri­cal energy direc­tly into light [5].

LEDs requ­ire much less cur­rent to ope­rate than tra­di­tio­nal light bulbs. Howe­ver, their mini­mum for­ward vol­tage is typi­cally aro­und 2 volts for infra­red and red LEDs, and often exce­eds 3 volts for white and blue ones. Because of this, it's not pos­si­ble to power an LED direc­tly from a sin­gle alu­mi­num-air cell. Of course, you could build mul­ti­ple cells and con­nect them in series, which also has edu­ca­tio­nal value. But there’s ano­ther option: you can build a sim­ple cir­cuit that boo­sts the vol­tage to a level sui­ta­ble for powe­ring an LED.

Figure 2 shows the sche­ma­tic of such a device.

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Fig. 2 – Sche­ma­tic of the vol­tage-boo­sting cir­cuit; both win­dings are wound in the same direc­tion, with dots mar­king the star­ting points

As shown, the cir­cuit uses only a few ine­xpen­sive and widely ava­i­la­ble elec­tro­nic com­po­nents. It’s a very basic step-up conver­ter [6]. Com­pa­red to more com­mon desi­gns, the resi­stor value has been redu­ced to 1kΩ to allow the cir­cuit to ope­rate more effec­ti­vely at the very low vol­tage and cur­rent levels pro­vi­ded by this type of cell.

The trans­for­mer used dese­rves a more deta­i­led expla­na­tion. It’s best to wind the coils bifi­lar (using two wires wound simul­ta­ne­o­u­sly) on a small toro­i­dal core. In the pro­to­type, a core with an outer dia­me­ter of 12 mm (0.47 in), salva­ged from a dama­ged com­pu­ter mother­bo­ard, was used. Each win­ding has 27 turns of 0.3 mm (0.012 in) ena­me­led magnet wire. The win­ding spe­ci­fi­ca­tions are not cri­ti­cal, so feel free to expe­ri­ment. I assem­bled my cir­cuit on a small squ­are piece of uni­ver­sal prin­ted cir­cuit board (Photo 6). The board mea­su­res about 2 cm (0.79 in) on each side, making the conver­ter truly minia­ture. For good visi­bi­lity, a white LED was used.

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Photo 6 – The fini­shed vol­tage boost conver­ter cir­cuit with an LED

This conver­ter ope­ra­tes as a sim­ple induc­ti­vely cou­pled gene­ra­tor, where, thanks to magne­tic induc­tion, a vol­tage higher than the sup­ply vol­tage is applied to the LED. This is easy to verify. A white LED con­nec­ted direc­tly to the cell will obvio­u­sly not light up. Howe­ver, when using this cir­cuit, the LED shi­nes bri­gh­tly (Photo 7). If the conver­ter does not work, try swap­ping the leads of one of the win­dings.

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Photo 7 – White LED powe­red by the cell thro­ugh the vol­tage boo­sting cir­cuit; top left cor­ner shows a magni­fied view of the wor­king cir­cuit

The cir­cuit built by the author works so effi­cien­tly that an attempt was made to cre­ate a highly minia­tu­ri­zed ver­sion of the cell. In this ver­sion, the anode is a strip of alu­mi­num foil about 1 cm (0.39 in) wide and 4 cm (1.57 in) long. Regar­ding acti­va­ted car­bon, a sin­gle grain is eno­ugh! I sug­gest pres­sing it thro­ugh the fil­ter paper onto the foil using a nee­dle held in pliers, which then serves as the cell’s posi­tive ter­mi­nal. A ready setup before moi­ste­ning the paper with elec­tro­lyte is shown in Photo 8.

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Photo 8 – Minia­ture alu­mi­num-air cell before appli­ca­tion of elec­tro­lyte

Of course, the cell only starts wor­king after the fil­ter paper is moi­ste­ned with the elec­tro­lyte (Photo 9).

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Photo 9 – Wor­king minia­ture alu­mi­num-air cell

It is sur­pri­sing that even such a sim­pli­fied cell, when con­nec­ted to the conver­ter, gene­ra­ted a cur­rent of about 1.2 mA at 0.6 V for at least 15 minu­tes.

The ope­ra­ting time of the cell depends on seve­ral fac­tors, pri­ma­rily the amo­unt of alu­mi­num used for the anode, as it is gra­du­ally con­su­med during ope­ra­tion. This can be obse­rved by exa­mi­ning the anode after use; the foil beco­mes noti­ce­a­bly thin­ner and may even­tu­ally deve­lop holes. The fil­ter paper sepa­ra­ting the anode from the acti­va­ted car­bon must also remain moist at all times; if the elec­tro­lyte dries out, the cell will cease to func­tion.

As you can see, this sim­ple galva­nic cell can be used for a variety of inte­re­sting expe­ri­ments.

Expla­na­tion

Figure 3 shows a cross-sec­tion of the alu­mi­num-air cell’s con­struc­tion.

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Fig. 3 – Cell con­struc­tion; a – anode (alu­mi­num foil), b – fil­ter paper soa­ked with elec­tro­lyte, c – cathode (oxy­gen adsor­bed on the sur­face of acti­va­ted car­bon gra­ins), d – col­lec­tor

The cell gene­ra­tes elec­tri­cal energy thro­ugh che­mi­cal reac­tions invo­lving the elec­trode mate­rials: alu­mi­num from the foil and oxy­gen from the air adsor­bed onto the acti­va­ted car­bon gra­nu­les.

In sum­mary, the cell under­goes an ove­rall reac­tion in which alu­mi­num is oxi­di­zed by atmo­sphe­ric oxy­gen in the pre­sence of water:

4Al + 3O2 + 6H2O → 4Al(OH)3

This is, of course, a redox reac­tion. The key to the cell’s ope­ra­tion is the spa­tial sepa­ra­tion of the oxi­da­tion and reduc­tion reac­tions, known as half-reac­tions. At the anode, the oxi­da­tion of alu­mi­num takes place:

Al + 3OH−→ Al(OH)3 + 3e−

At the cathode, oxy­gen from the air is redu­ced to hydro­xide ions:

O2 + 2H2O + 4e− → 4OH−

The elec­trode are, of course, con­su­med during the reac­tion as they are conver­ted into pro­ducts [7].

Because elec­trons are rele­a­sed at the anode and accep­ted at the cathode, a poten­tial dif­fe­rence deve­lops between the elec­tro­des. When the elec­tro­des are con­nec­ted by a con­duc­tor, an elec­tric cur­rent begins to flow.

You will likely agree, my dear rea­der, that oxy­gen seems like an unco­nven­tio­nal cho­ice for a cell’s elec­trode. It is impor­tant to remem­ber that gase­ous oxy­gen is essen­tially a poor con­duc­tor of elec­tri­city. This is where acti­va­ted car­bon plays a vital role. Thanks to its large sur­face area and adsorp­tion capa­bi­li­ties, it acts as an effec­tive gas rese­rvoir. It also con­ducts elec­tri­city well eno­ugh to col­lect char­ges, which are then trans­fer­red to the upper layer of alu­mi­num foil. The foil’s sole pur­pose is to serve as a charge col­lec­tor from the car­bon par­tic­les and does not par­ti­ci­pate in the che­mi­cal reac­tions.

A cell built this way has rela­ti­vely high inter­nal resi­stance, par­tly due to the limi­ted con­tact area between the acti­va­ted car­bon gra­ins and the col­lec­tor. Com­pres­sing the cell impro­ves con­tact and lowers resi­stance, ena­bling higher cur­rent out­put.

Is oxy­gen really con­su­med in the reac­tion? This is easy to obse­rve in the minia­ture cell ver­sion (Photo 8, Photo 9) by coa­ting the acti­va­ted car­bon grain with some­thing like paraf­fin. This blocks access to air so the oxy­gen sup­ply is quic­kly used up and the cell stops wor­king.

The alu­mi­num air cell is not just a labo­ra­tory curio­sity. With modi­fi­ca­tions, it is being con­si­de­red as a poten­tial energy source for appli­ca­tions such as elec­tric vehic­les [8].

Refe­ren­ces:

All pho­to­gra­phs and illu­stra­tions were cre­a­ted by the author.

This text has under­gone sli­ght edi­to­rial modi­fi­ca­tions com­pa­red to the ver­sion publi­shed in the jour­nal to bet­ter suit online pre­sen­ta­tion.

Marek Ples

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