00:00:00
we will now examine the effect of over
00:00:02
potentials in our electrochemical cells
00:00:04
in order to do this we need a first
00:00:06
quick recap of the overpotential
00:00:08
so remember we said that the over
00:00:10
potential is simply the difference
00:00:11
between the illiberal potential and the
00:00:13
actual potential at an electrode so it's
00:00:15
a fairly simple quantity to measure we
00:00:18
apply this over potential for an
00:00:19
external source so we connect
00:00:21
potentially a stat and drive this
00:00:24
potential through an electrochemical
00:00:25
cell this over potential is what we need
00:00:28
to drive a current through the cell so
00:00:30
if we simply apply the equilibrium
00:00:31
potential we're not likely to drive a
00:00:33
particularly high current we need to
00:00:35
drive a higher potential in order to
00:00:36
overcome the kinetic barriers to the
00:00:39
electrochemistry going on in our cell
00:00:40
remember that we said a lower exchange
00:00:42
current density the slower the rate at
00:00:44
which electrons are exchanged to
00:00:46
equilibrium the higher the other
00:00:47
potential we need to drive a current
00:00:49
through the cell we're going to first
00:00:52
look at galvanic cells so remember a
00:00:54
galvanic cell is one that we simply put
00:00:57
electrodes into the electrolyte and the
00:00:59
cell potential drives electrons through
00:01:01
the external circuit so let's consider
00:01:03
the cell here where we have copper and
00:01:05
silver present in the cell so this cell
00:01:08
potential we can calculate under
00:01:09
standard conditions would be about plus
00:01:12
0.4 six volts under galvanic conditions
00:01:15
we would expect to have a reduction at
00:01:18
silver electrode so we have the silver
00:01:20
ions being reduced to silver metal and
00:01:23
the copper electrode we would expect
00:01:25
have oxidation where we have the copper
00:01:26
metal going into solution as copper ions
00:01:29
but what we're really interested in with
00:01:30
the galvanic cell is as it's doing work
00:01:32
as it's driving the current through the
00:01:34
external circuit you want to know how
00:01:36
does that potential vary as the current
00:01:37
flows through the circuit and what can
00:01:39
we get from this information a key
00:01:42
factor in our galvanic cells is the
00:01:44
performance of the cell with the over
00:01:46
potential present so whenever we apply
00:01:49
an over potential to a cell remember we
00:01:51
get a different cell responds with a
00:01:54
galvanic cell the one that we were
00:01:56
looking at the copper silver cell we
00:01:57
said that we would have reduction at the
00:01:59
silver electrode so we would expect to
00:02:00
observe in this case we would expect to
00:02:02
observe a cathodic over potential so we
00:02:05
have a the cathodic current or the
00:02:07
reduction current running for a given
00:02:09
cell potential while we would have
00:02:11
oxidation at the copper electrode
00:02:13
driving an analytic current and the
00:02:15
overall current we see if you remember
00:02:16
the butler-volmer that we looked at
00:02:18
before the overall current is the sum of
00:02:20
the reduction and the the oxidation
00:02:22
currents the current voltage profile is
00:02:24
shaped fundamentally by the button of
00:02:25
Ulmer equation so the shapes of these
00:02:28
curves are exponential in nature but
00:02:30
remember this is a galvanic cell so the
00:02:32
passing of current is spontaneous it's a
00:02:34
spontaneous process as this cell strives
00:02:37
to reach equilibrium so let's now look
00:02:39
at what happens in that cell as we allow
00:02:41
that cell to pass the current if we now
00:02:44
connect a potentia stamp which allows us
00:02:45
to control the potential on the cell we
00:02:48
can hold the cell at a particular
00:02:49
potential so let's hold the cell at the
00:02:51
copper potential if we do this no
00:02:54
current flows this copper will be at
00:02:56
equilibrium no current would flow to the
00:02:58
cell however if we then release that
00:03:00
potential if we then allow current to
00:03:02
flow freely under the thermodynamics of
00:03:04
the cell the spontaneous change causes
00:03:06
the potential to increase to become more
00:03:08
positive allowing oxidation current to
00:03:11
flow at the anode and we establish an
00:03:12
equilibrium position conversely if we
00:03:15
were to change the potential and hold
00:03:16
the cell instead at the silver
00:03:18
equilibrium potential again no current
00:03:21
flows but if we've then released that
00:03:23
potential and allow the self-drive
00:03:25
current again spontaneous change will
00:03:27
cause the potential overall to decrease
00:03:29
and become less positive again allowing
00:03:31
a reduction current to flow at the
00:03:32
cathode and establishing that
00:03:34
equilibrium when we consider the
00:03:37
galvanic cell delivering energy we think
00:03:39
of it delivering a current but the it's
00:03:41
ability to deliver a current that the
00:03:44
amount of current it delivers
00:03:45
fundamentally affects the cell potential
00:03:48
because of the need to drive this over
00:03:49
potential if we think about our overall
00:03:52
cell potential we have our equilibrium
00:03:54
cell potential for copper and our
00:03:57
equilibrium cell potential for silver
00:03:58
remember these are the standard
00:04:00
electrode potentials that we can look up
00:04:01
in books and we would expect the overall
00:04:03
cell potential to be the difference of
00:04:05
these and we should be able to measure
00:04:07
this
00:04:08
however by allowing the cell to
00:04:11
equilibrate we will actually get a
00:04:12
different potential measurement from
00:04:13
that which our predictions would give us
00:04:15
as the cell runs the concentrations
00:04:17
change so we move away from ideality we
00:04:20
have an irreversible process we we might
00:04:22
lose material which causes the process
00:04:24
to be irreversible and the cell itself
00:04:26
has a fund
00:04:27
for internal resistance so in order to
00:04:29
identify how that cell potential varies
00:04:31
with current we need to find the
00:04:33
potential at two equal and opposite
00:04:34
currents so if we define two currents we
00:04:38
find that in order to drive a particular
00:04:40
positive current we need a particular
00:04:42
analytic potential and in order to drive
00:04:44
a negative current we would need a
00:04:45
particular cathodic potential what this
00:04:49
gives us over all for a given current we
00:04:51
find the difference between these new
00:04:52
potentials driving these currents and we
00:04:54
can extrapolate this to find the actual
00:04:56
measured cell potential for this current
00:04:58
as the current increases the cell
00:05:01
potential drops we only obtained the
00:05:03
thermodynamically predicted cell
00:05:06
potential under zero current conditions
00:05:08
as soon as current starts to flow we
00:05:10
start to deviate from that equilibrium
00:05:12
position what happens then with an
00:05:15
electrolytic cell where we are driving
00:05:17
the current around with this potentia
00:05:18
stat so let's consider the same cell
00:05:20
again we understand how it works under
00:05:22
galvanic conditions let's look at it now
00:05:24
under electrolytic conditions this time
00:05:26
we're going to impose a voltage to drive
00:05:28
the cell in a non spontaneous direction
00:05:30
so we're now going to drive the
00:05:32
reduction of copper instead so we now
00:05:34
bring copper two-plus to copper metal
00:05:36
and we drive oxidation at the silver
00:05:39
electrode so the opposite way around to
00:05:41
what we had with the galvanic conditions
00:05:43
now we want to think of how does the
00:05:45
current vary with the applied potential
00:05:47
and again what can we gain from this new
00:05:49
information once again we want to look
00:05:52
at the cell performance and how that
00:05:53
varies with the other potential again so
00:05:55
as we say it does still vary so let's
00:05:58
look at the current voltage curve again
00:06:01
this is a familiar shape we've already
00:06:03
seen this in terms of the butler-volmer
00:06:04
kinetics we looked at before but now
00:06:07
we're looking at a much greater
00:06:07
potential range the potential under
00:06:10
which we would expect to do real
00:06:11
electrochemistry I remember we said that
00:06:14
we'd have reduction at the copper
00:06:15
electrode so we have reduction here of
00:06:18
copper ions going to copper solid that's
00:06:21
the copper equilibrium potential no
00:06:24
current flows okay so if we're holding
00:06:26
our cell here we wouldn't expect to get
00:06:28
a current so in order to drive the
00:06:30
current through we have to drive a more
00:06:33
negative potential we have to move the
00:06:34
potential to a more negative value
00:06:36
conversely at the silver electrode if we
00:06:38
hold the cell at the silver equilibrium
00:06:40
again no current would flow we have to
00:06:42
apply an over potential to overcome the
00:06:44
electrode kinetics so that a current can
00:06:46
flow again the curves are still derived
00:06:48
from butler-volmer kinetics are still
00:06:50
fundamentally exponential remember we're
00:06:53
driving a cell against spontaneous
00:06:55
change so in this case we're forcing
00:06:58
copper to be deposited rather than
00:07:00
copper being released into solution but
00:07:03
in order to drive a cell at a particular
00:07:04
current we have to drive a different
00:07:06
potential than we predicted again
00:07:08
remember the difference between these
00:07:10
standard potentials would be expected to
00:07:12
give us our cell potential but in order
00:07:15
to get a measurable current we have to
00:07:17
apply a much greater over potential so
00:07:19
we apply the same principles as for the
00:07:20
galvanic cell we identify current
00:07:22
required so we identify our anodic and
00:07:24
cathodic components and the applied
00:07:27
voltage that we need is once again the
00:07:29
same separation from the tie lines as we
00:07:31
found before meaning that we have to
00:07:33
apply a much greater potential in order
00:07:35
to drive a current through that cell
00:07:36
than thermodynamics would otherwise
00:07:38
predict when we compare the galvanic and
00:07:41
electrolytic cells we can always use
00:07:43
thermodynamics to predict the cell
00:07:44
potential however when we actually come
00:07:47
to do measurements we find that the
00:07:48
output potential of a galvanic cell is
00:07:50
considerably lower than that predicted
00:07:52
by thermodynamics while the charging
00:07:55
potential of an electrolytic cell is
00:07:56
considerably greater than that predicted
00:07:58
by thermodynamics no matter which cell
00:08:00
we're looking at the measured potential
00:08:02
varies with current so depending on what
00:08:05
current is being driven through the cell
00:08:06
or the cell is supplying we would expect
00:08:08
to measure a different potential coming
00:08:10
out of that cell so for a galvanic cell
00:08:12
the effect of the over potential is to
00:08:15
reduce the output of the cell from that
00:08:17
predicted by the Nernst equation whereas
00:08:19
with electrolytic cells the effect of
00:08:20
our potentials is to increase the
00:08:22
applied voltage required to put a
00:08:24
current through the cell we always have
00:08:26
a struggle between kinetics and
00:08:28
thermodynamics thermodynamics predicts
00:08:30
overall outcomes for our reactions for
00:08:32
our cell potentials for everything in
00:08:33
chemistry while kinetics says how fast
00:08:36
something happens so if we consider a
00:08:39
particular cell where we've dissolved
00:08:41
ink chloride in a solution at 10 to
00:08:44
minus 2 molar and we maintain the pH at
00:08:48
7 we would expect to see these cell
00:08:50
potentials when we look at this couple
00:08:52
we can see that we have two
00:08:54
possible reductions happening we can
00:08:56
either reduce H+ to hydrogen gas or we
00:08:58
can reduce zinc two plus two zinc methyl
00:09:01
so more than one outcome can come out of
00:09:03
this cell so are we going to reduce zinc
00:09:06
or are we going to reduce hydrogen if we
00:09:08
look at the current voltage curves again
00:09:11
and we think about sweeping our voltage
00:09:14
to negative potential so we start our
00:09:15
voltage at zero and we drive it to
00:09:18
negative voltages we would expect to see
00:09:21
hydrogen evolution happening once we get
00:09:24
to a potential of minus 0.41 4 volts so
00:09:29
as we come into this region here we
00:09:32
start to apply the over potential and we
00:09:33
would expect start seeing hydrogen
00:09:34
evolution while we would expect to see
00:09:36
zinc if we drive it further past the
00:09:39
zinc electrode potential but the result
00:09:42
that we observe depends fundamentally on
00:09:44
the electrode material we're using so
00:09:46
depending on the value of the over
00:09:47
potential required to deliver a
00:09:49
particular current we may see a
00:09:51
different result so let's look at what
00:09:52
happens once we start varying the
00:09:53
electrode potential let's consider
00:09:55
platinum and mercury electrodes if we
00:09:58
look at the exchange current densities
00:09:59
for each one for the hydrogen couple and
00:10:01
for the zinc couple we can see there's a
00:10:03
big difference between the exchange
00:10:04
current potentials for hydrogen whether
00:10:06
we're at the mercury or the Platinum
00:10:08
electrode while for zinc it's largely
00:10:10
unchanged it's still a very high
00:10:11
exchange current density so let's
00:10:13
consider platinum first let's look at
00:10:15
platinum where hydrogen has a rip
00:10:16
moderately high exchange current density
00:10:18
so zinc reaction has fast kinetics it
00:10:21
has the high exchange current density we
00:10:23
expect to see fast kinetics going on so
00:10:25
we would see high reduction currents
00:10:27
near the zinc equilibrium potential the
00:10:29
hydrogen is also fast it's also
00:10:31
relatively fast we might need a slightly
00:10:33
higher over potential required but we
00:10:34
can still drive a current through the
00:10:36
cell and see hydrogen production so as
00:10:38
we sweep our potential from zero down
00:10:40
through the hydrogen potential we start
00:10:44
to see hydrogen production at the
00:10:46
expected voltage this is an expected
00:10:48
result and something that shouldn't
00:10:49
worry us too much
00:10:50
let's however now consider mercury
00:10:53
electrodes the equilibrium values are
00:10:57
unchanged but we've now gone into a
00:10:59
situation where hydrogen has very poor
00:11:01
kinetics it's got a very very small
00:11:03
we've got nine orders of magnitude
00:11:05
difference in the
00:11:06
James current density so very per
00:11:08
kinetics for hydrogen what that means is
00:11:10
we require a very large overpotential to
00:11:13
drive a current and what that means the
00:11:15
over potential we have to apply is
00:11:16
considerably greater than that for the
00:11:18
zinc couple the overall result of this
00:11:22
is that because the zinc still has fast
00:11:23
kinetics and only requires a small
00:11:25
overpotential predominately at the
00:11:27
mercury electrode we would expect to see
00:11:29
the zinc reaction first the effect of
00:11:32
these electrode kinetics can't be
00:11:34
ignored so depending on how we design
00:11:36
our electrodes I would design the
00:11:37
material that we're working with we can
00:11:39
tailor our exchange current density to
00:11:42
get different results of the electrode
00:11:45
an example of this is a chlor-alkali
00:11:47
industry where we electrolyze sodium
00:11:50
chloride solution or salt water I'm not
00:11:53
going to go into too much detail about
00:11:54
it because it's a standard a-level case
00:11:56
study but fundamentally we have two
00:11:58
electrodes our cathode and our anode
00:12:00
each which has a competing process the
00:12:02
cathode processes center around the
00:12:04
reduction of sodium to sodium metal or
00:12:06
the reduction of water to hydrogen and
00:12:08
hydroxide when we look at this we see
00:12:11
that we have a huge thermodynamic
00:12:13
barrier to overcome to reduce sodium
00:12:15
while a barrier is not quite so great
00:12:16
for the reduction of water however if we
00:12:18
look at the anode processes the
00:12:20
electrode potentials are very similar so
00:12:22
we now need to think about the kinetics
00:12:24
of what's going on looking at the third
00:12:26
dynamics we would expect to see a lower
00:12:29
barrier for the oxidation of water to
00:12:31
hydrogen oxygen than we would for the
00:12:34
oxidation of chloride to chlorine gas
00:12:36
but when we look at the exchange current
00:12:39
densities at the electrode we see that
00:12:40
the kinetics for the chloride oxidation
00:12:43
are considerably better we have a much
00:12:45
greater exchange current density than we
00:12:47
have for the water at that electrode so
00:12:51
thermodynamics predicts that we should
00:12:52
get hydrogen gas and oxygen gas as
00:12:54
products from this reaction because we
00:12:57
have a lower electrode potential for
00:12:59
each one so the thermodynamics predicted
00:13:01
however when we look at the exchange
00:13:04
current density as a function of the
00:13:06
applied potential we see that the
00:13:09
exchange current to that part on time
00:13:11
electrode affect the outcome of the
00:13:13
reaction because of the much much
00:13:15
smaller exchange current density we
00:13:16
require a very high over potential for
00:13:19
the oxygen water
00:13:19
couple and we see that we need to apply
00:13:21
very very high over potential to supply
00:13:23
any measurable amount of currents to
00:13:25
actually make that reaction proceed if
00:13:27
we look at chlorine instead we find that
00:13:30
for the same potential we get a much
00:13:32
greater current happening with the
00:13:33
chlorine so the chlorine production
00:13:35
dominates to summarize we need to
00:13:39
remember that electro process is
00:13:40
fundamentally effect thermodynamic
00:13:42
predictions in a galvanic cell we see
00:13:44
that the actual cell potential is
00:13:46
considerably lower than we would
00:13:47
otherwise expect while for an
00:13:49
electrolytic cell the cell potential is
00:13:52
higher than we would expect because of
00:13:54
the over potentials required to drive a
00:13:55
current through the cell the value of
00:13:59
that cell potential fundamentally
00:14:00
depends on the magnitude of the current
00:14:02
so if we have a high current we would
00:14:04
expect to see a lower galvanic cell
00:14:06
potential whereas if we drive a high
00:14:08
current through an electrolytic cell
00:14:10
you'd expect to see a higher selves of
00:14:11
potential required the effect of faster
00:14:14
kinetics also can't be ignored faster
00:14:16
kinetics can favour adverse
00:14:17
thermodynamics if it forms faster we
00:14:20
will get more of a happening through
00:14:22
modification of our electrodes we can
00:14:23
increase the exchange current density
00:14:25
which will favour particular processes
00:14:27
to our advantage
