Discover Discover the energy the energy of hydrogen of hydrogen

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Discover Discover the energy the energy of hydrogen of hydrogen

Grzegorz Karwasz Anna Kamińska

University Nicolaus Copernicus

Didactical material: Secondary School Part III: Volta’s pile

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This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking (JU) under grant agreement No 826246.

The JU receives support from the European Union’s Horizon 2020 research and innovation programme and Italy, Denmark, Poland, Germany, Switzerland.

Galvani and (dead) frogs

https://en.wikipedia.org/wiki/Luigi_Galvani „Volta, Filosofo Naturale” - Quaderni „Scienze”, 1997

Luigi Galvani, working in Bologna, was a physi- cian and surgeon. Performing experiments on (freshly killed) frogs he noticed (we believe by* touching the nerves of frog positioned on a tin plate by a silver knife) that the legs move.

He believed "animal electricity" to be a third form of electricity (not from glass neither sulphur). His findings were revelatory at that time, but Ales- sandro Volta (from Pavia, different state than Bologna) gave a different explanation: metals and not animals are the source of „electricity”.

*Galvani reported that dead frogs moved when a spark ocured from an electrostatic machine in the nearby or during thunder storms. These would be electromagnetic waves or some shortcuts – hardly to believe.

Bringing electrical charge from electrostatic machine makes the muscle move.

Galvani’s instructions how one should prepare the frog for electrical experiments

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Electricity was in XVIIth century

a social play at aristocratic courts

Thales of Miletus some 500 BC

noticed particular properties of amber, electron in Greek.

In XVIIth century experiments with sulphur or glass balls were know: by friction „electricity” was created, up to great sparks if a (light) girl was kept isolated (by silk) from the ground.

Robert Symmer (1707-1763) wrote on his white and black, woollen and silk, stockings which got electrified if worn each on other.

At the same time, Benjamin Franklin studies electricity form thunder storms.

„1799:.. E la corrente fu. Duecento anni dalla pila di Volta”, Universita’ di Pavia

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The JU receives support from the European Union’s Horizon 2020 research and innovation programme and Italy, Denmark, Poland, Germany, Switzerland. http://www.esdsystems.com/whitepapers/wp_tribocharging.html

Alessandro Volta, a school inspector in Como and later professor of physics in Pavia undertook systematic studies of different materials.

He ordered all material from glass to silk, suphur and resin in a kind of series.

He was also the first who measured electrical forces, acting between two flat

metallic plates: this was the way to define the electrical potential, now called also voltage and measured by voltmeter in volts.

Interactive exhibition „Volta’s pile” , Słupsk, 2001

dydaktyka.fizyka.umk.pl/zabawki/files/zrodla/ogniwa.html

Volta, and the current was…

„1799:.. E la corrente fu. Duecento anni dalla pila di Volta”, Universita’ di Pavia

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In 1799 Volta took a pile of coins, Sn and Ag…

The letter of Volta from March 1800 where he describes his „pile” of coins: every second pair is separated by paper immersed in salty water.

Original piles of Volta, from his Mausoleum in Como (photo GK).

The conversion of „chemical energy” into electricity in Volta’s pile is extremely

efficient. This bedside clock works from 25 years with the same Ag mini-battery: little energy is consumed for the liquid-crystal display (and no ring is used).

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Electrochemical reactions

H₂SO₄ → 2H⁺ + SO₄^-2

Cu (Cathode): 2H⁺ + 2e^- → H₂↑ Zn (Anode): Zn → Zn²⁺ + 2e^-

Programme: dr Mauro Brunato: http://www.science.unitn.it/~brunato/Terra/index.html#documenti

This is how the Volta battery works: the oxidation of a zinc atom produces two electrons: Zn → Zn²⁺ + 2e^- (oxidation reaction at the anode – the negative pole).

These electrons react with water molecules in the conductive solution: 2 H₂O + 2e^-→ 2 HO^- + H₂

(reduction reaction at the cathode – positive pole).

In total, there is an oxido-reduction reaction

accompanied by the creation of a potential difference and a current: hydrogen is produced and the zinc disc is gradually consumed.

Due to the reduction of H⁺ , the potential from Volta pile is not 1.2 V but lower.

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Daniell pile (1837)

https://www.methodephysique.fr/piles_daniell/

The problem with Volta’s pile, and with electro-

chemical reactions in general, is that they do not go like we expect.

In the Zn/Cu pair we expect the effective potential 0.4 V (Cu) + 0.8V (Zn) = 1.2 V

But it is more „convenient” in Volta pile that it is not the Cu²⁺ ion to be reduced (Cu²⁺ + e^-→ Cu) but H⁺ + e- → H with the pile potential of only 0.8 V.

Not only the potential is lower but also the H₂gas bubbles on the electrode (cathode).

To avoid this one should stop the flow of H⁺. But the electrical circuit must be closed.

So a „salt bridge” is used, with ions that do not alter the reactions, like K⁺and NO₃^-.

Direction of current, as established by Volta

Zn → Zn²⁺ + 2e^- Cu²⁺ + 2e^- → Cu

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How did Volta measure voltage? In volts?

Volta not only invented the electro-chemical pile but contributed also to electrostatics.

He invented the electrosope: two thin metal pieces inside a glass container, that repel each other when charged.

He invented also the electrophore: a metal plate held via an insulating stick. Placed on an electrified insulator, and touched in a proper sequence, seems to generate the electricity like perpetuum mobile. In practice this is the work by rising the metal plate which delivers the energy (and generates the potential).

Home/ school experiments inspired by Volta’s works (from left):

- Volta’s electroscope: two thin metal pieces inside a glass - Two ends of a silk scarf repel if taken striding from woolen coat- A pile of old (Al and Ni) Polish coins and pieces of paper with salty water; with eurocents the same can be done using pieces of Al foil

Here: a battery made from pencil-sharpener:

Al and inox steel and electroscope in glass

Here above: Volta’s electroscope used to measure the voltage from electrostatic machine

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Trento, 3 Agosto,1802

Stos Volty i kompas

Playing with electricity was an expensive adventure: piles of Ag coins were needed.

A lawyer from Rovereto in North Italy,

Romagnosi, described in 1802 how he took a pile of copper and zinc round blades, put

cloth immerses in ammonia salt in-between every second pair and connected the ends with a silver wire.

He made not only sparks, but noticed also that a magnetic needle changes its directions when placed near the wire with the current.

A similar experiment was reported almost twenty years later by Ørsted.

Romagnosi (1802) or Ørsted (1817)

http://dydaktyka.fizyka.umk.pl/zabawki/files/zrodla/Romagnosi.pdf

S. Stringari, R. R. Wilson

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Volta’s electrochemical series

Li⁺(aq) + e⁻ → Li(s) −3,04 2H_2O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq) −0,828

Zn²⁺(aq) + 2e⁻ → Zn(s) −0,762

PbO(s) + H₂O + 2e⁻ → Pb(s) + 2OH⁻ −0,576 Fe²⁺(aq) + 2e⁻ → Fe(s) −0,41 Sn²⁺(aq) + 2e− → Sn(s) −0,14 Pb²⁺(aq) + 2e⁻ → Pb(s) −0,13 2H⁺(aq) + 2e⁻ → H₂(g) 0,00

Sn²⁺ (aq) + 2 e− → Sn(s) 0.14

Cu²⁺(aq) + 2e⁻ → Cu(s) 0,34

Ag⁺(aq) + e⁻ → Ag(s) 0,80

F₂(g) + 2e⁻ → 2F⁻(aq) 2,87

+1.83 O₂^* Au

The „school” version of Volta series is given, usually, as here in the table below: Li is the most negative, Al, Zn and Fe are negative, hydrogen is the reference point, Cu, Ag and Au are positive.

But a more detailed version (from wikipedia, but based on serious scientific references, is given here to the left: not only the metal determines the electrochemical potential created, but the very type of the ionic reaction – the level of „oxidation”. So reactions with Cu⁺ and Cu²⁺ ions give different potentials

Source: https://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page)

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Grove’s hydrogen fuel cell

Like Zn and Cu, any other metal (and non-metal) can be used, including hydrogen and oxygen.

William Grove invented the „fuel cell” by case: during the electrolysis of water he disconnected the electrodes from the voltage supplier and observed some „reversed” current.

The reactions on electrodes are:

H₂ → 2H⁺ + 2e^- (negative electrode, called anode)

Electrons flow via a wire to the positive electrode, called cathode while H⁺ ions flow via the solvent, and reaction at cathode occurs:

O₂ + 4H⁺ + 4e^- → 2H₂O

The difficulty stays in suplying hydrogen (and oxygen) into water:

„well-proven” catalysts, like colloidal (nanostructured) Pt are used.

William Grove in 1843 published this drawing:

five H/O cells were used to produce the electrical current that made the electrolysis (the rigth cell)

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In polymer-membrane cells (using nafion) H⁺ ions are transported through the separating diaphragm. But also OH^- could be transported in another type, alkaline cells (those used in Apollo flights).

J. R. Varcoe et al.. Energy Environ. Sci. 2014, 7. 3135

Courtesy: Dr Johannes Töpler, Deutscher Wasserstoff-und Brennstoffzellenverband

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We still do not understand even the electrostatics

In spite of centuries of research, the phenomena studies by Volta are still far from being fully understood.

In the paper shown here, authors look with modern microscopes how pieces of one material are transported into the second in the tribo- electrical phenomena.

In next page we show how scientists try to

explain potentials on the interface metal-water.

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First trials to understand Volta’s potentials

„The description of electrode-electrolyte interfaces is based on the concept of the formation of an electric double layer. This concept was derived from

continuum theories extended by introducing point charge distributions. Based on ab initio molecular dynamics simulations, we analyze the electric double layer in an approach beyond the point charge scheme by instead assessing charge polarizations at electrochemical metal-water interfaces from first principles. We show that the atomic structure of water layers at room

temperature leads to an oscillatory behavior of the averaged electrostatic potential. We address the relation between the polarization distribution at the interface and the extent of the electric double layer and subsequently derive the electrode potential from the charge polarization.”

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Experiment: electrolysis

Needed:

- Low voltage (0-10 V), high current (1A) supplier, 2 cables with crocodile clips, - 2 pieces of aluminium foil (1cmx1cm) for electrodes

- a glass (about ½ l) container, possible „rectangular” in shape - distilled water + salt (NaCl)

- voltmeter Procedure:

1. fill the glass with water (3/4), insert aluminium strips into water, keep them with clips -connect the cables to the supplier, in-between insert the voltmeter fixed to measure the current, fix voltage on the supplier to 0 V, switch on the supplier

-rise the voltage by 0.1 V, measure the current, observe electrodes; stop rising voltage at 3 V.

2. Add some (1 tea spoon) salt to water, repeat the las point (i.e. the rampo of voltage) - try to evaluate the amount of gas formed on the

Make graphs of the current in the two cases (with salt and without) vs. the voltage.

Try to make a similar graph for the amount of gas formed.

Draw conclusions.

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Many other funny experiments can be done…

Needed:

- Pieces of Zn (or Fe) and Cu connected with a (Cu) wire - A voltmeter

- Few pupils, with wet hands:

these are in Toruń To be done:

-a closed electrical curcuit („do not touch the wire, but only the metal pieces”)

To be obtained: Fun!

Discover Discover the energy the energy of hydrogen of hydrogen