Further Physical Chemistry: Electrochemistry session 10

00:13:32
https://www.youtube.com/watch?v=LiRqWcgJkb4

الملخص

TLDRThe video provides an in-depth explanation of voltammetry, focusing on the principles and practical aspects of this electrochemical technique. Voltammetry examines the effects of varying cell potential on current. The discussion includes how small deviations from equilibrium potential allow for kinetic measurements using Butler-Volmer relationships and the impact of higher overpotentials on concentration changes at the electrode surface. This leads to concentration polarization, where particular species build up at the electrode, affecting the current. The video explains how linear voltammetry involves a constant rate of potential sweep and current increase due to oxidation at the working electrode, while cyclic voltammetry involves sweeping the potential to a maximum before reversing it, presenting different current profiles. It covers how various factors, like ion mobility, affect reaction rates and emphasizes the importance of potential magnitude in determining current. The video describes how voltammetry is used to infer cell potential, reversibility, and kinetic information by analyzing current curves. Cyclic voltammetry allows for deeper analysis with peaks indicating oxidation and reduction processes, providing insights into reversible and irreversible reactions. Additionally, it discusses the effects of asymmetric processes, multiple oxidation peaks, and different reactant behaviors in the voltammetric analysis.

الوجبات الجاهزة

  • 🔋 Voltammetry analyzes current by varying cell potential.
  • 📉 Current peaks indicate limited reactant supply to electrodes.
  • ⚡ High overpotentials alter concentration at electrode surfaces.
  • 🔄 Reversible processes show distinct oxidation and reduction peaks.
  • ⏱️ Sweep rate crucially affects voltammetric results.
  • ⚙️ Reaction kinetics influence curve shapes and current profiles.
  • 🔍 Analyzing voltammetry provides insights into cell potential and reactions.
  • 🔁 Cyclic voltammetry highlights reversible and irreversible processes.
  • 🧪 Concentration polarization impacts electrodic reactions.
  • 🔬 Electrode surface saturation limits further current increases.

الجدول الزمني

  • 00:00:00 - 00:05:00

    The video discusses the fundamentals of voltammetry, a method of exploring electrochemical reactions by measuring current in response to varying cell potential. Initially, small deviations from equilibrium potential allow for kinetics analysis via Butler-Volmer relations, assuming concentrations remain constant. As overpotentials increase, concentration at the electrode surface changes due to diffusion effects, causing concentration polarization which alters current and provides insights into reaction kinetics. In linear voltammetry, potential is increased at a constant rate, boosting oxidation and current at the working electrode until limited by the reactant supply, showing a current peak proportional to reactant concentration.

  • 00:05:00 - 00:13:32

    Moving to cyclic voltammetry, the potential is cycled up and down at a constant rate, creating distinctive current curves that reveal cell potentials. At low potentials, anion migration limits reaction rates; at high potentials, electrode surface saturation sets a peak current. As potential reverses, the reduction current counteracts oxidation, leading to unique cyclic voltammogram features. These curves help identify standard electrode potentials and provide kinetic insights. The video emphasizes voltammetry's power in analyzing electrochemical processes at electrodes and understanding electrode potentials and reaction kinetics through controlled potential changes and current measurement.

الخريطة الذهنية

فيديو أسئلة وأجوبة

  • What is voltammetry?

    Voltammetry is a measurement technique that studies the current response of a chemical substance as the potential is varied.

  • How does concentration polarization affect voltammetry?

    Concentration polarization causes a buildup of species at the electrode, altering current and kinetics during the voltammetric process.

  • What is the role of overpotentials in voltammetry?

    Overpotentials affect the concentrations at the electrode surface, influencing the reaction dynamics and the current measured.

  • Why does the current peak in voltammetry?

    The current peaks due to the limited supply of reactants reaching the electrode surface as the potential increases.

  • How do linear and cyclic voltammetry differ?

    Linear voltammetry involves sweeping the potential in one direction while cyclic voltammetry involves sweeping up to a maximum potential and then reversing back.

  • What insights can voltammetry provide?

    Voltammetry can provide insights into the kinetics of electrode reactions, reaction reversibility, and electrode potential values.

  • How do increased potentials affect ions in voltammetry?

    Increased potentials augment the electric field, accelerating ion migration towards the electrode and increasing current.

  • What happens at the peak current in cyclic voltammetry?

    At peak current, the electrode surface is saturated, limiting further increases in current due to diffusion limitations.

  • What is the significance of reversibility in voltammetry?

    Reversibility indicates whether the reactions at the electrode can go in both directions, providing information about the electrode kinetics.

  • How do reaction kinetics influence voltammetry results?

    Reaction kinetics determine the shape and position of current peaks, influencing the interpretation of voltammetric data.

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الترجمات
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التمرير التلقائي:
  • 00:00:00
    having covered much the theory behind
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    electrochemistry we now turn our
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    attention to looking at the more
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    practical aspects in this case we'll be
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    exploring the principles behind
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    voltammetry voltammetry looks at the
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    effect of currents as we vary the cell
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    potential so we apply a cell potential
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    and we measure the current which comes
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    through so far we've really only
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    considered small deviations from an
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    equilibrium cell potential so are very
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    small over potentials in the region
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    where we can do measurements of the
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    kinetics using butter Vollmer
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    relationships these small deviations are
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    requirement for butler-volmer kinetics
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    to be analyzed and a key assumption made
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    is that the concentrations don't vary
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    significantly however as we go to higher
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    over potentials these can change the
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    concentration of the electrode surface
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    so looking at our cations and our
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    reduced species we get a fundamental
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    change in the concentration and
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    diffusion effects start to change the
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    behavior of what's going on at that
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    electrode these processes cause a change
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    in concentration at the electrode
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    surface in a process called
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    concentration polarization we start to
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    get a buildup of particular species at
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    electrodes which changes the current
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    that we can draw from them and from this
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    we can infer a great deal about the
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    kinetics of the electrode but also
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    insight into the processes of the
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    reactions going on the principles of
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    voltammetry are fairly simple potential
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    is swept through a predefined range so
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    we set a range for our potential and
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    we'd simply ramp the potential through
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    those values we maintain a constant rate
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    of sweep so if we look at the variation
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    of our electrode potential with time we
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    have a constant rate with linear
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    voltammetry we increase the potential at
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    a constant rate so we allow it to
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    equilibrate for a time period and then
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    we ramp up the potential at a constant
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    rate and look at what happens to the
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    current
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    in this case increasing the potential
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    increases oxidation at the working
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    electrode by increasing the oxidation we
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    increase the current of that electrode
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    so let's now look at what happens to the
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    current as we vary the electrode
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    potential the current we would expect to
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    increase rapidly past the reduction
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    potential once we get past that
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    potential we would expect the current to
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    ramp up severely because we now start
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    oxidizing the
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    the species at that electorate what we
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    see is it reaches a maximum value and
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    this is a key observation in voltammetry
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    experiments the reason for this rise is
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    because we find ourselves limited by the
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    supply of reactant to the electrode as
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    material reaches the electrode it's
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    immediately oxidized or reduced and
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    starts to hinder the process of material
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    getting to that electrode thereafter the
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    peak of this maximum is proportional to
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    the concentration of reactant so as we
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    increase the concentration we would
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    expect to see a greater maximum current
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    let's explore this process in a bit more
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    detail so let's look at a cathode
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    process so where we have a reduction of
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    a cation so we have a process which we
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    detail cation picks up an electron at
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    the cathode and is reduced to an
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    uncharged species if you remember from
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    what we did earlier in other sessions
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    the current is related to the rate of
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    reaction and if we have a higher
  • 00:03:07
    concentration that means we have a
  • 00:03:08
    faster rate so basic kinetics tells us
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    that the rate of change of our cation is
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    proportional to its concentration if we
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    have a faster rate there are more
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    electrons being supplied or removed from
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    the electrode which gives us a higher
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    current through the external cell what's
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    the effect of increasing the potential
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    then if we increase the potential we
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    increase the electric field by
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    increasing the electric field that
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    increases the electro-motive force on
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    ions in solution so if we remember our
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    drift speed our ion drift speed which is
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    proportional to the electric field
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    strength if you remember that's the you
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    term here is the ion mobility which
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    considers things like solvent viscosity
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    ionic charge hydrodynamic radius and so
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    on by increasing the drift speeds we
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    increase the rate at which ions arrive
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    at the electrode that allows us to more
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    rapidly deliver more currents so it
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    seems no surprise that increasing the
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    field should increase the current so if
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    we look at our voltammogram we see as we
  • 00:04:05
    increase the potential we see a faster
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    and faster rate of reaction which is
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    manifest as a greater and greater
  • 00:04:10
    increase in the current provided but we
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    see it start to level off and it's worth
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    considering what's happening as that
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    levels off well let's look at what's
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    happening at the electrode surface as we
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    increase the magnitude of the potential
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    we get more and more ions attract
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    surface which are then immediately
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    reduced to this uncharged species but
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    what happens when we increase the
  • 00:04:30
    magnitude of the potential still further
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    well as we increase it still further we
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    see that the cations are immediately
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    reduced at the electrode and these
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    uncharged species start to collect at
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    the electrode surface and they block
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    active sites to new cations so it
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    inhibits the ability for a cation to get
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    to the surface the diffusion force is
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    simply a driving force against a
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    concentration gradient so this diffusion
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    force is not affected at all by the
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    potential so no matter how much of this
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    uncharged species we have they will
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    still be diffusing at the same rate
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    related to their concentration are these
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    high magnitudes of potential these
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    uncharged species will hinder the
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    reductive process at this electrode and
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    this causes the current to tail off from
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    its maximum value it could it reduces
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    the rate at which the reaction can take
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    place and this is a key feature in
  • 00:05:18
    voltammetry linear voltammetry however
  • 00:05:21
    is just one aspect of voltammetry that
  • 00:05:22
    we do and more often we tend to look at
  • 00:05:24
    cyclic voltammetry cyclic voltammetry is
  • 00:05:27
    a situation where potential is swept up
  • 00:05:29
    to a maximum value and then back down
  • 00:05:31
    again we maintain a constant rate of
  • 00:05:33
    sweep up and down which has a profile
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    that looks a little bit like this we
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    have a constant rate increase and then
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    we suddenly turn and ramp it back down
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    again at the same inverse rate and
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    notice again we allow the cell to
  • 00:05:46
    equilibria at beginning and the end with
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    us fixed potential okay let's now look
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    at the current curve what happens to the
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    current as we change that electrode
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    potential well we expect a similar
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    current profile initially to linear
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    volts Hammond tree but we then see a
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    rapid change in current when we reverse
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    it so as we increase the potential the
  • 00:06:04
    potatoes but then as we come back we see
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    a considerably different shape from
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    these curves we can identify cell
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    potentials and the cell potential is
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    identified as the point equidistant
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    between the oxidation peak and the
  • 00:06:17
    reduction peak here so to understand
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    what's going on let's look at the anode
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    solution interphase at five points
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    remember the anode is where we have
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    oxidation taking place the points we're
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    going to look at is point a where we
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    have a low potential on the oxidation
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    sweep then we're going to look at the
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    peak at high potential of the oxidation
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    sweep and then we're going to look at
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    Point C which is the turning point
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    then we're going to go back down at high
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    potential in the reduction sweep and
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    then we're going to look at a low
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    potential on the reduction sweep so
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    let's look at each step in turn
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    considering the anode process so we're
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    looking at the low potential on the
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    oxidation sweep we have a relatively low
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    potential on our anode and as we
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    increase the potential we see the
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    current rising as the anions are
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    oxidized this rate is limited by the
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    migration of anions so the rate at which
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    these anions can move through the
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    solution carried on the electric field
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    affects the rate of our reaction that is
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    what's limiting the rate of reaction the
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    diffusion of the oxidized species these
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    uncharged species here the diffusion of
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    these through solution is considerably
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    faster than iron migration so is not
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    providing a significant inhibitor so we
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    see an increase in the current with
  • 00:07:28
    increased potential when we get to point
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    B where we have a high potential on the
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    oxidation sweep we see that our
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    electrode surface is starting to become
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    crowded so you have an exchange between
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    the migration of the anions and a
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    diffusion of the oxidized species at
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    this point the oxidation current is in
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    the absolute maximum there's no more
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    increase that can be obtained from
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    oxidation and as we say the migration
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    rate of anions is equal to the diffusion
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    rate of the oxidized species so if we
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    look at this equilibrium here where we
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    have this analytic potential in the
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    reductive potential we have established
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    an equilibrium let's now look at the
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    turning point at a very high potential
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    at this point our surface is completely
  • 00:08:07
    saturated with the oxidized species and
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    this hinders the ability for an anion to
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    get to the surface and causes a
  • 00:08:14
    reduction in the current at this point
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    the rate is limited by the diffusion of
  • 00:08:19
    the oxidized species from the electrode
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    as a turning point we start to lower the
  • 00:08:24
    potential and see what happens as we go
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    backwards so on this reduction sweep
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    remember that the surface is still
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    saturated with oxidized species so
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    there's a rapidly reduced back to the
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    anion and the current starts to go in
  • 00:08:35
    the opposite direction so the current
  • 00:08:38
    starts to flow the reduction current
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    starts to become significant and starts
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    to counteract the oxidation current so
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    the cathodic reduction current starts to
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    contribute in a significant way to
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    counteracting the oxidation current
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    remember as we say this is a constant
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    process happening at any one electrode
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    and it's just whether the applied
  • 00:08:57
    potential drives it forward or backwards
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    once we get down to a particular value
  • 00:09:02
    at a low potential on the reduction
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    sweep the cathodic current is at an
  • 00:09:06
    absolute maximum and it dominates the
  • 00:09:08
    current profile meaning that the overall
  • 00:09:10
    current is at a minimum at this point
  • 00:09:13
    the diffusion of the oxidized species is
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    considerably faster than ionic migration
  • 00:09:17
    so these oxidized species are leaving
  • 00:09:20
    the electrode faster than the anions can
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    get to the surface so this summarizes
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    what's going on at each stage in a
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    cyclic voltammogram so let's now explore
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    the features of this voltammogram the
  • 00:09:31
    peaks of the current light either side
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    of this standard electrode potential for
  • 00:09:35
    the cell and this allows us to work out
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    what the standard electrode potential
  • 00:09:38
    might be for a particular species the
  • 00:09:42
    electrode potential can be identified
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    quite simply by being equidistant
  • 00:09:45
    between the reduction peak at the bottom
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    and the oxidation peak at the top we can
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    also tell something about the kinetics
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    of the process by analyzing the shape of
  • 00:09:52
    the curve so depending on what shape the
  • 00:09:54
    curve might be we always expect to see
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    this sort of shape for a reversible
  • 00:09:57
    process so for a fully reversible
  • 00:09:59
    process we see an oxidation as we ramp
  • 00:10:01
    the potential up and a reduction as we
  • 00:10:03
    ramp it back down again so this would be
  • 00:10:05
    for a fully reversible process and
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    nothing really surprising there but the
  • 00:10:09
    timescale of the sweep also affects the
  • 00:10:11
    results if we sweep our potential too
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    quickly
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    we might miss the subtleties of reaction
  • 00:10:17
    processes diffusion or migration
  • 00:10:19
    kinetics so they may not occur on the
  • 00:10:21
    timescale the sweep so we can infer
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    extra information by either slowing the
  • 00:10:25
    speed down or speeding it up let's not
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    consider asymmetric processes so we
  • 00:10:30
    define our standard cell potential
  • 00:10:33
    reaiiy identify particular process where
  • 00:10:35
    we've identified a cell potential so we
  • 00:10:37
    now do our sweep we ramp the potential
  • 00:10:39
    up we see our oxidation as expected but
  • 00:10:42
    then on the way back down we end up with
  • 00:10:44
    an asymmetric curve we don't see a
  • 00:10:46
    reduction peak so this tells us
  • 00:10:48
    something about the process that we're
  • 00:10:49
    seeing are we getting an irreversible
  • 00:10:52
    oxidation serve for example in the
  • 00:10:54
    electrolysis of sodium chloride we see
  • 00:10:57
    the oxidation of chloride two chlorine
  • 00:10:59
    gas which then escapes from solution so
  • 00:11:01
    there is no
  • 00:11:03
    there's no way that that can come
  • 00:11:04
    backwards so we don't see a reduction
  • 00:11:06
    peak happening another possibility is
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    that there are slower reduction kinetics
  • 00:11:11
    so or uncharged oxidants which are not
  • 00:11:13
    carried to the electrode we may not be
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    driving a sufficiently high
  • 00:11:16
    overpotential it may be that our
  • 00:11:18
    reduction peak actually appears at
  • 00:11:19
    considerably lower potentials and we
  • 00:11:21
    need to drive past to try and find it so
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    all of this tells us a little bit about
  • 00:11:26
    what's going on if we have a process
  • 00:11:28
    with a very low exchange current density
  • 00:11:29
    we'd expect to have a high over
  • 00:11:31
    potential required and that might be
  • 00:11:33
    causing us to miss the reduction another
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    form of asymmetric process is where we
  • 00:11:37
    see two oxidation Peaks for example so
  • 00:11:41
    we might see a shape that looks a bit
  • 00:11:42
    like this so in this case we're seeing
  • 00:11:45
    two oxidations present with different
  • 00:11:47
    equilibrium cell potentials so an it for
  • 00:11:50
    an example that we might use here if we
  • 00:11:52
    have chloride being oxidized to chlorine
  • 00:11:55
    and bromine being oxidized to bromine in
  • 00:11:57
    the same solution we would expect to see
  • 00:11:59
    two different oxidation Peaks but we
  • 00:12:03
    don't see a reduction peak in the
  • 00:12:04
    reverse trace for cell two we see a
  • 00:12:07
    reduction peak for cell one but not for
  • 00:12:08
    sale - so is this causing an
  • 00:12:12
    irreversible oxidation so are reforming
  • 00:12:14
    gas could it be flow reduction kinetics
  • 00:12:17
    again we go through the same processes
  • 00:12:18
    whether they require a very high over
  • 00:12:20
    potential but we do see the reduction
  • 00:12:23
    peak for e1 which allows you to
  • 00:12:25
    determine a cell potential for e1 and we
  • 00:12:27
    can identify what's happening in that
  • 00:12:29
    cell so this is once again a reversible
  • 00:12:32
    process so to summarize our discussion
  • 00:12:34
    on voltammetric processes we can
  • 00:12:36
    determine information on cells from
  • 00:12:38
    voltammetry from electrode potentials to
  • 00:12:41
    reaction kinetics we can determine a
  • 00:12:43
    great deal about it we can identify
  • 00:12:44
    reversible processes versus irreversible
  • 00:12:47
    processes and we can determine kinetic
  • 00:12:49
    information on our cells the signals
  • 00:12:53
    that we get are affected by the
  • 00:12:54
    concentration of our reactants and the
  • 00:12:56
    kinetics of those cell processes so if
  • 00:12:57
    you're careful control of concentrations
  • 00:12:59
    we can unravel information on the
  • 00:13:01
    kinetics of cell processes fundamentally
  • 00:13:03
    in voltammetry we are looking at
  • 00:13:04
    processes at a single electrode we're
  • 00:13:07
    ramping the potential of that electrode
  • 00:13:08
    up and down and monitoring the current
  • 00:13:10
    that comes out of it we're looking at a
  • 00:13:12
    working electrode compared to a standard
  • 00:13:14
    reference electrode
  • 00:13:15
    and depending on the potential that
  • 00:13:17
    we're offering it behaves is either an
  • 00:13:18
    anode or cathode depending on the
  • 00:13:20
    relative potential and which way through
  • 00:13:22
    this equilibrium we're working and we
  • 00:13:24
    use counter electrode to complete the
  • 00:13:26
    circuit in our voltammetric measurements
الوسوم
  • voltammetry
  • electrochemistry
  • linear voltammetry
  • cyclic voltammetry
  • kinetics
  • cell potential
  • concentration polarization
  • electrode reactions
  • ion mobility
  • reversibility