Electrochemistry by A.J.Bard - 3rd Ed Ch. 1
Electrode Process
Experimentally it is not possible to separate single interface from system. We can only handle a electrochemical cell comprised of mutiple interfaces.
Notation for Electrochemical Cell
- Slash(/) indicates phase boundary.
- If two or more components exist within single phase, use comma(,) to separate the components.
- Gaseous phases are written on the side of adjacent conductor phase.
Difference of Interfacial Potential and Cell Potential
Cell potential is the capability of electrochemical cell to work and can be calculated by summing up all the interfacial potential exist along the conduction path inside a cell.
Passing phase boundary accompanies abrupt potential change, meaning there is strong electric field there. Hence, there are large difference in energy of electrons, which controls the direction and rate of electron transfer.
Working Electrode and Reference Electrode
Working electrode is the electrode an electrochemist want to analyze. To facilitate the analysis, it is paired with Reference electrode which is designed to have consistent interfacial potential difference regardless of externally applied potential.
Having a working electrode - reference electrode pair, there is a fixed potential difference at the interface in reference electrode side and this difference can be calculated based on Nernst equation. Therefore, if we apply some potential difference from external source, this change can be wholly assigned to working electrode.
If potential of working electrode is increased to (-) side, electrons in working electrode obtain higher energy. Sufficiently energetic electrons can transit to vacant electronic levels in electrolyte to reduce it, yielding reductive current. On the way back, (+) side increase of potential can contribute to oxidative current
Several Reference Electrodes
Standard Hydrogen Electrode (SHE/NHE)
Cell notation for standard(normal) hydrogen electrode looks like:
\[\ce{Pt/H2(a=1)/H+(a=1, aq)}\]Standard Calomel Electrode (SCE)
Cell notation for standard calomel electrode looks like:
\[\ce{Hg/Hg2Cl2/KCl(saturated in water)}\]At 25 deg., cell potential of SCE is +0.244 V compared with NHE.
Ag/AgCl Electrode
Cell notation for Ag/AgCl electrode looks like:
\[\ce{Ag/AgCl/KCl(saturated in water)}\]At 25 deg., cell potential of SCE is +0.197 V compared with NHE.
Background Current vs. Potential Curve
Analysis on background $i$-$E$ curve must precede to the analysis of specific redox reaction from a electrochemical cell. Continuously increasing the potential of working electrode to -(+) side leads to abrupt increase of unwanted reductive(oxidative) current at some point. As such rise of background current obscures reaction current from redox reaction in concern, it must be guaranteed that a redox reaction in concern is located somewhere in the middle of the points where those unwanted currents dominate.
Choice of Working Electrode and Overpotential
Consider below two kinds of electrochemical cells:
\[\ce{Pt/H^+(1 M),Br^-(1 M)/AgBr/Ag}\] \[\ce{Hg/H^+(1 M),Br^-(1 M)/AgBr/Ag}\]In above cases, working electrodes are platinum and mercury, respectively. On the Pt working electrode, hydrogen evolution reaction occurs at 0 V (vs. NHE). However, if mecury is chosen as working electrode, far lower (negatively larger) potential or far larger electronic energy is required to drive the same reaction. This is because of the kinetics: reaction rate for hydrogen evolution reaction is much slower with Hg electrode.
Open Circuit Potential
Open-Circuit Potential or OCP indicates the potential difference between electrodes in a electrochemical cell when the external conduction path is not applied.
Locating OCP of Electrochemical Cell
- Basically, OCP is located between the potential at which the easiest-to-be-oxidized species starts to oxidize and the potential at which the easiest-to-be-reduced species starts to reduce.
- If an electrolyte is comprised only of the oxidized forms, OCP is located between the potential at which the background oxidative current onsets and the potential where the easiest-to-be-reduced species starts to reduce.
- If an electrolyte is comprised only of the reduced forms, OCP is located between the potential at which the background reductive current onsets and the potential where the easiest-to-be-oxidized species starts to oxidize.
Faradaic and Nonfaradaic Process
If an electrode reaction accompanies with charge transfer at interface, the reaction is called Faradaic Process. If not, it is called Non-Faradaic Process
Interface and Currents
Ideally Polarizable and Ideally Nonpolarizable Electrodes
It is called polarization if the potential for an electrode reaction deviates from its expected value from themodynamic estimation. And the extent of such descrepancy is called overpotential. Therefore, if higher polarization level is involved with an electrode, larger overpotential is expected.
In such perspective, it is called Ideally Polarizable Electrode or IPE if an electrode is unlimitedly easy to be polarized. It means that IPE shows large overpotential even with infinitesimally small amount of reactions proceeding. On $i$-$E$ plot, graph for IPE has $y = constant$ form.
On the other hand, Ideally Nonpolarizable Electrode or INE is an electrode which is infinitely difficult to deviate from themodynamically expected reaction potential. Potential of such electrode cannot off from its thermodynamic equilibrium value unless sufficiently huge mass of reactants participate in reaction at once. This makes $i$-$E$ curve of INE have $x = constant$ form.
Electrical Double Layer
Often in an electrode process, the closest vicinity of electrode surface is packed with solvent molecules. These solvent molecules prohibits reacting species or ions from approaching. If we handle solvent molecules as hard sphere with diameter $d$, it seems like there an ion-inhibiting layer with thickness $d$ and this layer is termed Internal Helmholtz Plane (IHP).
Furthermore, as in solution charged ions are not moving freely as themselves but coupled with solvent molecules also, the limit proximity of reacting ions to electrode surface is farther than $d$ but $d$’ $>d$. This defines an extra layer located farther than IHP which we call Outer Helmholtz Plane (OHP).
The force exerted on solvated ions is electrostatic dragging from the electrode surface, which diminishes along the distance from the electrode surface. In addition, Brownian thermal motion of molecules attempts to negate electrostatic attraction. Therefore, interplay between those two source defines the thickness of diffuse layer.
Practical Tips
Study for printine (blank) electrolyte is as important as that for reaction species but frequently overlooked. First of all, background anodic and cathodic current should be first measured to fix the voltage window where our analysis can be meaningful for our target reaction species. Furthermore, one can also find minor traces of impurities unwantedly contributing to background current by preliminary study for blank solution.
- Deaeration: Bubbling nitrogen for a couple of minutes does.
- Counter electrode: Substantially larger one than working electrode is desired.