Basics of Batteries?

[Ewokinarmor]

Hey guys. I am thinking of creating something relating to batteries (sorry, no specifics!), but since I have not yet formally taken chemistry or any electrical classes, this is, understandably, problematic. Don't get me wrong though, I'm not going into this blindly. I have taken AP Bio, so I have a good understanding of the periodic table, the elements, valence electrons, redox reactions, electronegativity, Endergonic vs. Exergonic, etc, etc. I have been trying lately to get a good understanding of how batteries work, but there are still a few things that confuse me.

 

Here's what I know so far: A battery consists of an anode (negative I think, like ANion, but the charts I see are never consistent), a cathode (+, like CATion?), and an electrolyte between them. Electrons are passed from the negative side to the positive side through the electrolyte, and a wire or other conductor is used to get this charge out of the cell to do work. When most or all of the electrons have done this, the battery is depleted.

 

However, what really confused me was the Daniell Cell. If the point here is to send electrons from one side to another, then why does there seem to be a separate path for electrons and a separate path for ions? Don't the ions have to carry the electrons? What role do the ions and the salt bridge play, if not to shuttle electrons from - to +? Thanks, and sorry for the pretty large post. Here's a link to the diagram in question:

 

http://en.m.wikipedia.org/wiki/Daniell_cell#/image/File:Galvanic_cell_labeled.svg

[Nicholas Kang]

Do you know what is a salt bridge? The key to your answer is in the salt bridge.

 

I am nowhere near a chemistry expert. Sorry.

 

Well, In a simple Daniell cell, the copper is the positive terminal-anode, zinc is the negative terminal-cathode

 

Do not confuse with anion and cation. Anode attracts anion. Recall electrolysis of water, you connect anode to the terminal in which in the end oxygen is collected. Oxygen gas is formed from O^2-. 2 O^2- froms one oxygen molecule-O₂. Oxygen is negative-anion, the terminal is positive-anode. The situation is the reverse in hydrogen terminal, whereas hydrogen is positive in ion H⁺-cation and the terminal is negative-cathode.

 

After realising these principles, let`s start to investigate how a Daniell Cell works.

 

Zinc is immersed in zinc sulphate solution, copper is immersed in copper(II) sulphate solution.

 

A salt bridge contains inert ions or salt that does not react with the electrolyte. Examples are

 

NaCl, KCl, KNO₃, NH₄Cl and dilute H₂SO₄

 

A simple salt bridge can be made by immersing a piece of filter paper in sulphuric acid or in a salt solution. Functions of salt bridge:

To allow the flow of ions so that the circuit is completed.

To prevent the 2 aqueous solutions from mixing. This will prevent displacement reaction between a more electropositive metal and the salt solution of the less electropositive metal from taking place.

 

The reaction:

 

Zinc reacts directly with copper(II) sulphate solution in a displacement reaction.

 

Zn + CuSO₄ produce ZnSO₄ + Cu

 

As a result, the zinc metal will be coated by a layer of copper metal. The electric current decreases rapidly.

 

The salt bridge prevents the zinc from reacting directly with the copper(II) sulphate solution.

 

The final reaction:

 

At the negative terminal, cathode: Zn becomes Zn²⁺ + 2e^-

At the positive terminal, anode: Cu²⁺ + 2e^- becomes Cu

 

Overall reaction of cell: Zn + Cu²⁺ becomes Zn²⁺ + Cu

 

The copper will become thicker(mass increase)

the zinc becomes thinner(mass decrease)

The concentrationof the blue copper sulphate solution becomes paler and the concentration of zinc sulphate solution increases.

 

The key is the salt bridge, which is inert(means lazy and less reactive. Have you heard of inert gases? Neon, Argon, Krypton, Xenon and Radon which are octets/duplets achieve stability in terms of electronic configuration, thus stable and less reactive) don`t react with either solutions and metals. The are just transporter/medium of ions to complete the circuit. Hope that this is clear enough.

[studiot]

This site has a good picture and short explanation showing the workings of the eelctrochemistry of the Daniel cell and the salt bridge that Nicholas mentioned.

 

It also explains why the two halfs of the cell are kept separate, can you spot this?

 

http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Voltaic_Cells/Case_Study%3A_Battery_Types/Batteries%3A_Electricity_though_chemical_reactions

 

This site is generally useful about modern theory and applictions of batteries.

 

http://batteryuniversity.com/learn/

[Ten oz]

Good explanation of ionization by Nicholas Kang. Batteries are electrochemical and not simple as electrons diffusing between high and low potentials. I suggest you read up a bit on thermocouples and understand how they differ from batteries.

[John Cuthber]

OK, for a start, we need to clarify what an anode and cathode are.

http://en.wikipedia.org/wiki/Anode

They are not the positive and negative electrodes respectively.

[Enthalpy]

In what profession is the anode defined that way?

 

In electrical engineering, the cathode of a Zener diode is always the same electrode, whatever the current direction. I haven't seen any single exception in 35 years. Same for tunnel diodes, Impatt, Trapatt and the whole and complete zoo. Same for rechargeable batteries in electrical engineering.

[studiot]

I agree that the Wikipedia definition is cumbersome and not user friendly.

 

The anode is the terminal of a device where electrons leave or conventional current enters.

 

Is IMHO much easier to cope with.

 

This is actually equivalent to the Wiki definition, despite the vebiage.

[Enthalpy]

Better definition indeed.

 

But I'm still surprised that the cathode gets an anode when the current direction changes. This is not the vocabulary in use in electrical engineering - definitely not for diodes, nor did I hear it for accumulators.

[studiot]

 

Better definition indeed.

 

But I'm still surprised that the cathode gets an anode when the current direction changes. This is not the vocabulary in use in electrical engineering - definitely not for diodes, nor did I hear it for accumulators.

 

 

If you look carefully at the picture in the first link in my post#3 you will see that this definition also conforms to the (electro)chemical definition, ie they are the same.

 

Electrons are shown leaving the zinc anode.

 

Further down there is a picture of an (i'm sure familiar) accumulator.

 

Unfortunately it is probably (no disrespect meant) beyond auto techs to understand that the accumulator acts as two different devices when it is charging and when it is supplying current and that the terminals are simply the lumps of metal, but the names anode and cathode are defined by their function, not by a particular lump of metal.

So that the terminal names should therefore be reversed when charging and discharging.

So auto techs call one of the lumps of metal (terminals) the anode and (wrongly) stick to it.

 

For a battery being discharged, the terminal marked + is the cathode, while for the same battery being recharged, the terminal marked + is the anode.

 

The Zener diode is an unusual case since it is normally operated in reverse mode.

However it does work as a normal diode in forward mode where the definition works.

So a simple approach is to say that since we are talking about reverse current the reverse definition applies.

 

Alternatively a good discussion is to be has here

 

http://www.av8n.com/physics/anode-cathode.htm

Edited November 6, 2014 by studiot

[John Cuthber]

Better definition indeed.

 

But I'm still surprised that the cathode gets an anode when the current direction changes. This is not the vocabulary in use in electrical engineering - definitely not for diodes, nor did I hear it for accumulators.

In principle, with a diode, the current direction doesn't change. it's "forward" or zero. (Pace Zener diodes and back diodes if you like)

The words were not invented for electronic engineers, but for electro-chemists and so i don't mind much if they use them differently, but this is a discussion about electrochemical cells..

[Enthalpy]

This document confirms my impression that manufacturers of rechargeable batteries also call "cathode" the negative electrode whatever the current direction:

http://www.saftbatteries.com/force_download/Defense_Systems_Brochure_0.pdf

on page 4 of 6 (or search "cathode") about LiNiCoAlO₂.

 

Looks like the claim at Wiki and the common use of several professions differ widely.

[studiot]

^{This document confirms my impression that manufacturers of rechargeable batteries also call "cathode" the negative electrode whatever the current direction:}

^{http://www.saftbatteries.com/force_download/Defense_Systems_Brochure_0.pdf}

^{on page 4 of 6 (or search "cathode") about LiNiCoAlO₂.}

 

^{Looks like the claim at Wiki and the common use of several professions differ widely.}

 

 

I wasn't convinced by this link since they only mention 'cathode' once, as 'cathode materials', and do not indicate a polarity.

 

Farady introduced the terms anode and cathode in the 1830s, along with his two laws of electrochemistry.

 

Here is the modern version that has been in continuous use since that time by the whole electrochemical industry, not just the battery section.

 

Taken from Moody's Comparative Inorganic Chemistry. I have highlighted the definitions.

 

Edited November 8, 2014 by studiot

[mississippichem]

I've always been taught that the anode is where the oxidation half-reaction occurs. End of story. If the reaction is reversed, the cathode is still the electrode where oxidation is occurring.

 

That way it doesn't matter whether you think of an electric circuit in terms of electron flow or "hole flow". This definition doesn't care about which way you draw the current flowing.

[Enthalpy]

I have no opinion about electrochemistry. I'm just pointing out that the very companies that produce rechargeable batteries call "cathode" the negative electrode whatever the current direction, that's why they say "cathode material" for a rechargeable battery.

Sony use "cathode" and "anode" the same way as Saft

http://www.sony.com.cn/products/ed/battery/download.pdf

for instance on pages 9 to 11, on figure 1 and in the text.

 

Panasonic also call "anode" and "cathode" some components of their rechargeable cells

http://datasheet.octopart.com/VL2330-1VC-Panasonic-datasheet-9706278.pdf

on the drawing titled "construction".

 

So does the academic "Journal of power sources"

http://adsabs.harvard.edu/abs/2014JPS...247..412W

[John Cuthber]

I have no opinion about electrochemistry.

...and the thread is about batteries.

 

As for the people writing about cathodes in pages about batteries- well, they clearly haven't read what Mr Faraday wrote.

Edited November 10, 2014 by John Cuthber

[imatfaal]

This is a bit off-topic but we were kind of at an impasse anyway - the etymology of the words cathode and anode suggest a down / up relationship. kata hodos is the down route and ana hodos is the up route in my schoolboy greek. Is this based on an expected charge carrier's movement in the wire? Or is there another good reason for cathode being down and anode being up?

[studiot]

 

the etymology of the words cathode and anode suggest a down / up relationship. kata hodos is the down route and ana hodos is the up route in my schoolboy greek.

 

You would have to ask a botanist for an explanation, but my Chambers Scientific Dictionary has:

 

Anodal or anodic (Bot)

 

In the upward direction on the genetic spiral.

 

There is, however, no impasse.

 

It's just that some engineers seem to have forgotten what we regard as the internal cicuit and what the external circuit and mix them up. It is important to understand which is which because directions are reversed for these two parts of a complete circuit.

 

So they cheerfully take a battery, write + on one terminal and draw conventional current passing out from this terminal round the 'external' circuit and back into the battery at the other one.

 

Yest these same engineers then take a vacuum tube, mark one terminal + and cheerfully show conventional current entering the valve at this terminal passing through the valve to the other terminal.

 

Strangely they want to call both these terminals marked + the anode!