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Electrochemical Supercapacitors for Energy Storage and Delivery
The behavior of this redox reaction is similar to the electrochemical reactions in rechargeable batteries. The redox reaction in an electrode layer must
be electrochemically reversible or semi-reversible to ensure efficient charge
and discharge. If the ES electrode layer is entirely composed of a reversible
electrochemical active material, the electrode will behave like a rechargeable
battery. However, in a pseudocapacitor electrode layer containing both an
electrochemically inert material such as active carbon and an electrochemically active substance such as a redox material, the charge or discharge will
involve two processes (1) double-layer charging or discharging as described
in Chapter 2 and (2) the electrochemical redox process.
For electrochemical redox, each reactant molecule in the bulk phase contributes one or more charges toward the stored energy, unlike the case in a double-layer charging or discharging process where only charges can physically
accumulate on the material particle’s surface. Therefore, pseudocapacitance is
much higher for a double-layer mechanism than the possible capacitance.
The electrochemical reversibility of the employed redox material in a pseudocapacitor normally means that the redox process follows Nerstian behavior [2]. These redox materials include: (1) electrochemically active materials
that can be adsorbed strongly on an electrically conductive substrate surface
such as a carbon particle and (2) solid-state redox materials that can combine with or intercalate into an electrode substrate to form a hybrid electrode layer. For example, adsorption on an electrode substrate surface is
commonly observed as underpotential deposition of protons on the surface
of a crystalline metal electrode (Pt, Rh, Pd, Ir, or Ru). In the case of Ru, the
protons can pass through the surface into the metal lattice by an absorption
process, similar to the transitional behavior seen in lithium battery intercalation electrodes.
Another example is a largely conjugated organic molecule that has a redox
group. The molecules can irreversibly adsorb on the carbon particle surface,
providing a pseduocapacitance source. The most common pseudocapacitance is derived from redox reactions on metal oxide materials and conducting polymers that exhibit a combination of protonation reactions and
absorption into the polymer matrix. Several examples of high pseudocapacitance shown in Table 3.1 will be discussed later in further detail.
Besides the need for reversibility of ES pseudocapacitive materials discussed above, their redox reaction pathway(s) must occur over a practical
potential range. Both reversibility and reaction potential range, along with
costs of the materials, are common limitations to the usefulness of pseudocapacitive behavior.
Electrochemical reversibility means that the electroreduction of a pseudomaterial’s oxidation state can occur at almost the same electrode potential
as that of electrooxidation of its reduction state. However, in practice, most
redox reactions are not totally reversible due to their limited reaction kinetics,
particularly if the reactions are driven at high rates. For a non-ideal reversible redox reaction to maintain a desired reaction rate, an over-electrode
Electrochemical Supercapacitors for Energy Storage and Delivery
The behavior of this redox reaction is similar to the electrochemical reactions in rechargeable batteries. The redox reaction in an electrode layer must
be electrochemically reversible or semi-reversible to ensure efficient charge
and discharge. If the ES electrode layer is entirely composed of a reversible
electrochemical active material, the electrode will behave like a rechargeable
battery. However, in a pseudocapacitor electrode layer containing both an
electrochemically inert material such as active carbon and an electrochemically active substance such as a redox material, the charge or discharge will
involve two processes (1) double-layer charging or discharging as described
in Chapter 2 and (2) the electrochemical redox process.
For electrochemical redox, each reactant molecule in the bulk phase contributes one or more charges toward the stored energy, unlike the case in a double-layer charging or discharging process where only charges can physically
accumulate on the material particle’s surface. Therefore, pseudocapacitance is
much higher for a double-layer mechanism than the possible capacitance.
The electrochemical reversibility of the employed redox material in a pseudocapacitor normally means that the redox process follows Nerstian behavior [2]. These redox materials include: (1) electrochemically active materials
that can be adsorbed strongly on an electrically conductive substrate surface
such as a carbon particle and (2) solid-state redox materials that can combine with or intercalate into an electrode substrate to form a hybrid electrode layer. For example, adsorption on an electrode substrate surface is
commonly observed as underpotential deposition of protons on the surface
of a crystalline metal electrode (Pt, Rh, Pd, Ir, or Ru). In the case of Ru, the
protons can pass through the surface into the metal lattice by an absorption
process, similar to the transitional behavior seen in lithium battery intercalation electrodes.
Another example is a largely conjugated organic molecule that has a redox
group. The molecules can irreversibly adsorb on the carbon particle surface,
providing a pseduocapacitance source. The most common pseudocapacitance is derived from redox reactions on metal oxide materials and conducting polymers that exhibit a combination of protonation reactions and
absorption into the polymer matrix. Several examples of high pseudocapacitance shown in Table 3.1 will be discussed later in further detail.
Besides the need for reversibility of ES pseudocapacitive materials discussed above, their redox reaction pathway(s) must occur over a practical
potential range. Both reversibility and reaction potential range, along with
costs of the materials, are common limitations to the usefulness of pseudocapacitive behavior.
Electrochemical reversibility means that the electroreduction of a pseudomaterial’s oxidation state can occur at almost the same electrode potential
as that of electrooxidation of its reduction state. However, in practice, most
redox reactions are not totally reversible due to their limited reaction kinetics,
particularly if the reactions are driven at high rates. For a non-ideal reversible redox reaction to maintain a desired reaction rate, an over-electrode