You are correct. I did find those tables. The positive electrode corresponds
to the metal with a positive "electric reduction potential" (ERP) I think it
was called. The negative electrode metal corresponds to the negative ERP.
Subtract the 2 and you get the total potential, which is the POTENTIAL
voltage produced by the cell. The ACTUAL voltage is less than that.
Usually in simple homebrew batteries the positive electrode cannot have
much improvement over hydrogen (going downward) in the electromotive
series.
So, for the negative part, lithium is -3.04. I don't think that's practical
because I thought elemental lithium was very toxic to the skin and other
organs.
Also lithium has severe spontaneous rections with water. So do all
other metals in the first column of the periodic table - there is some
fame for at least sodium and potassium to react with water so violently as
to achieve spontaneous combustion. All metals in the first colum of the
periodic table also react with air so rapidly that a shiny bare metal
surface lasts only seconds. Metals in the first column of the periodic
table are normally kept in a petroleum product to protect them from water
and air - but lithium has the extra problem of floating in anything that
is liquid at or near room temperature and any Earth surface atmospheric
pressure.
Next in line are the metals of the second column of the periodic table.
But calcium, strontium, barium and radium go plop-plop-fizz-fizz in water
(to form hydrogen and hydroxide of the metal in question) - that leaves
beryllium and magnesium as usable in that column of the periodic table.
Of these, beryllium has bigtime toxicity and cost problems, leaving
magnesium as the most electropositive common metal.
Next best was Mg (magnesium?) at -2.38. But that's hard to find. Next
best was aluminum at -1.66, and I do have aluminum foil.
Aluminum's main problem is its tendency to form an insulating oxide
layer.
Next best is silver at -.799. I might have some old silver plated US
coins before 1964.
1964 and older US dimes and quarters and 1965 and older Canadian dimes,
quarters and nickels are solid silver alloy that is mostly silver...BUT:
Silver is not more electropositive than hydrogen (useful for negative
terminal) on the electromotive scale but below copper (good for positive
electrode and and not much more useful in homebrew batteries that
typically have plenty of hydrogen ions that limit the potential of the
negative electrode). I would use copper.
In addition, achievement of positive electrode much improving over
hydrogen in a cell having water typically requires at least one of:
1. The cell having a solution of a salt of a metal that is below hydrogen
on the electromotive scale. But a simpler homebrew cell with that tends
to have problems with the dissolved salt reacting spontaneously with
any metal positive electrode material higher in the electromotive series.
2. Oxidizing agents such as some metal oxides - like lead dioxide or
manganese dioxide - which I do not cosider quite to be in the realm of
homebrew "science fair" batteries.
For the positive electrode we have gold at +.799 (I do not have any gold
coins but I might have gold-plated headphone plugs) and copper at +.34.
The positive electrode in a homebrew science fair style cell has little
chance of benefiting much from choice among metals below hydrogen in the
electromotive series.
So if I use aluminum and copper, we have a potential voltage for one cell of:
.34- (-1.66) of 2.0 volts. Looking at studies the actual voltages runs about
60-80% of potential voltage.
The positive electrode will in most homebrew batteries be no better or
not much better than hydrogen. So I would expect only about 1.66 volts.
Before I start linking multiple cells together I want to maximize the voltage
for one cell. Other studies I read yesterday showed the electrolyte with the
lowest pH produced the most voltage. I think vinegar was the best overall.
Slightly more voltage, much more current, but with the disadvantage of
making the negative electrode react spontaneously. Metals well above
hydrogen in the electromotive scale outright go plop-plop-fizz-fizz in
strongly acid solutions. Life of a zinc electrode could be merely a day
in vinegar, a couple days in lemon juice, a week in orange juice, a month
in a tomato, and a few months in a potato - and I might be somewhat
optimistic for at least some of these!
Standard conditions for electromotive scale are solvent being water.
In addition, when hydrogen is involved (most cases of homebrew batteries
with positive electrode being below hydrogen on the electromotive scale):
1. pH of 0
2. Temperature where product of molarities of H+ and OH_ ions is 1E-14
(a bit above 20 degrees C)
3. The atmosphere above the solvent having hydrogen of pressure (or
partial pressure in a gas mixture) 760 mmHg.
Additional requirement whether or not hydrogen is involved:
Concentration of ions of active ingredients is 1-molar (or is it
1-normal?) 1 molar is per liter grams in solution being same as molecular
weight. For 1-normal grams in solution is molecular weight divided by
valence.
But with typical homebrew batteries concentration of compunds of the
negative electrode material likely to be less than 1-molar and even
1-normal, I would expect potential to improve a little from the
electromotive value of the negative electrode metal. Maybe aluminum's
1.66 will improve to 1.75 or 1.8 or so (with pH 0) oe be 1.5-1.6 (if pH
is between 2 qnd 3) - maybe only if you draw no more than a fraction of a
microamp. But don't expect positive elecrodes to gain much over hydrogen,
which normally forms bubbles on the positive electrode when current is
taken from such an electrochemical cell.
- Don Klipstein (
[email protected])
