Fundamental Molecular Electrochemistry - Potential Sweep Voltammetry

Fundamental molecular electrochemistry - potential sweep voltammetry • Potential (aka voltammetric) sweep methods are the most common electrochemical methods in use by chemists today • They provide an efficient and straightforward assessment of the redox behavior of molecular systems. • In all potential sweep methods, the potential (V, volts) of the working electrode is varied continuously with time according to a predetermined potential waveform (aka excitation function), while the current (I, amps) is concurrently measured as a function of the potential. • The applied potential at the working electrode is measured against a reference electrode of choice, while a counter (aka auxiliary) electrode is required to balance the I-V applied. • Thus, three electrodes are required: •
working electrode
reference electrode
counter electrode An electrolyte salt must also be dissolved in solution to maintain sufficient conductivity in the bulk solution and maintain diffusion controlled mass transfer at the electrode interface. (diffusion – migration – convection) Common electrodes Working electrode
Glassy carbon
Platinum
Silver
Gold Counter electrode
Pt wire
Glassy carbon rod Reference electrode (physically but not electrochemically isolated with a porous vycor frit)
Ag/AgCl 3 M aq. KCl
Normal Hydrogen Electrode (NHE)
Saturated Calomel Electrode
Ag/AgNO3 (0.01 M in 0.1 M Bu4NPF6 acetonitrile) Linear sweep voltammetry • Linear sweep voltammetry represents the most basic potential sweep method. • In LSV the potential of the working electrode is varied linearly with time between two values i.e., the initial (Ei) and final (Ef) potentials. • As the electrode potential is constantly rising (or decreasing) throughout the experiment, a level of capacitive (aka Ohmic) current flows continuously. These currents are due to the capacitive charging of the working electrode's double layer. • Faradaic current will also flow when the potential reaches a value at which the species in solution can undergo electrochemical conversions. Excitation function (potential waveform) for a LSV experiment Linear sweep voltammetry of ferrocene • The FeIII/II couple of ferrocene, (η5-C5H5)2Fe, is an example of an electrochemically reversible system in acetonitrile solution and is often used as an internal or pseudo reference when reporting formal potentials of inorganic and organometallic complexes. • For example, taking a 1 mM solution of ferrocene under the following conditions:
Working electrode = 2 mm diameter platinum disc
Counter electrode = Pt wire
Reference electrode = Ag/AgCl 3 M aq. KCl
Electrolyte = 0.1 M Bu4NPF6 in acetonitrile • The voltammetric scan is started at a potential at which no electrochemical reactions may take place, that is, V < < E ° (at least 177 mV). • Scanning the potential linearly in the positive direction gives rise to a Faradaic current upon oxidation of the Fe(II) center to Fe(III). • Importantly, the solution is kept quiescent, i.e. not stirred, so that diffusion is the only mass transport mechanism possible. • If the redox couple is electrochemically reversible a characteristic anodic wave is observed with a maximum current value given by the Randles-Sevcik equation. Linear sweep voltammetry of ferrocene Randles-Sevcik equation (at 25 °C): ip = peak current; n = # electrons; A = electrode area; C = concentration; v = scan rate 0 a Current (µA) b -5 ipa f e c -10 anodic d Excitation function for a LSV experiment 0.8 0.7 0.6 0.5 Epa 0.4 0.3 0.2 0.1 Potential (V) vs. Ag/AgCl 1 mM ferrocene (2 mm Pt disc; 0.1 M Bu4NPF6 acetonitrile; Pt wire; Ag/AgCl 3M aq. KCl) Linear sweep voltammetry of ferrocene a. The initial potential at 0.10 V shows no current and therefore no electrolysis when the electrode is switched on. b. The electrode is scanned toward a more positive potential. c. As the potential is made more positive the electrode is now a sufficiently strong oxidant to oxidize the ferrocene to ferrocenium. d. The concentration of ferrocene decreases rapidly at the electrode surface as the anodic current increases. At point d the concentration of ferrocene is substantially diminished causing the current to peak. f. The current now decays as the ferrocene concentration becomes more depleted and ferrocenium surrounds the electrode. Complete oxidation scanning to 0.75 V. is ensured by a b Current (µA) e. 0 -5 ipa f e c -10 anodic d 0.8 0.7 0.6 0.5 Epa 0.4 0.3 Potential (V) vs. Ag/AgCl 0.2 0.1 Important notes on LSW • Note: Epa does not equal the standard reduction potential of the corresponding redox couple. • For reversible one-electron redox couples the anodic peak Epa occurs at ca. 30 mV more positive than E° and its position is independent of the scan rate, whereas the cathodic peak Epc occurs ca. 30 mV more negative of E°. • For a revesible redox couple, the position of both Epa and Epc are independent of the scan rate but i ∝ v1/2 . • If the voltammogram exhibits an irreversible peak, the corresponding peak potential will shift anodically for Epa as the scan rate increases. Cyclic voltammetry • Cyclic voltammetry (CV) is based on the same principles as linear sweep voltammetry, however, in CV the potential of the working electrode is scanned with a reverse bias after reaching a chosen value, the so-called switching potential (Esp). • Thus, in CV the potential of the working electrode is varied linearly with time between three values i.e., Ei → Esp → Ef . • In fact, modern instruments allow the user to choose multiple switching potentials for carrying out tailored CV scans over a chosen potential window. Excitation function (potential waveform) for a CV experiment • One method to assess the reversibility of a redox couple is the evaluation of the potential difference between the peak potentials (∆Ep) of the anodic and cathodic peaks associated with the couple. • Based on numerical solutions of the current-potential response in CV experiments, a value of 59/n mV (at 25°C, first cycle voltammogram) is expected for a reversible redox couple. • It is extremely important to realize that this value will only be obtained if the switching potential is at least 177 mV beyond the peak potential observed in the forward scan. • The proximity of the switching potential to the voltammetric peaks leads to increased ∆Ep values. • Furthermore, the presence of uncompensated cell resistance also leads to increased ∆Ep values., e.g. poor electrolyte conductivity, non-aqueous /aqueous junction potentials etc. • The formal potential of a reversible redox couple E°′ is typically within a few mV of the formal potential E° for the couple according to the Nernst equation where the ratio of the activity coefficients αOx and αRed is usually very close to unity. The easy determination of half-wave potentials and estimation of formal potentials is an extremely attractive feature of CV. Cyclic voltammetry of ferrocene 1 mM ferrocene 0.1 M Bu4NPF6 acetonitrile 2 mm diameter Pt disc Pt wire Ag/AgCl 3M aq. KCl i 10 cathodic j 5 Current (µA) ipc h g 0 a b f ipa -5 c e d -10 0.8 k 0.7 0.6 0.5 Epa anodic Epc 0.4 0.3 Potential (V) vs. Ag/AgCl 0.2 0.1 Cyclic voltammetry of ferrocene a. The initial potential at 0.10 V shows no current and therefore no electrolysis when the electrode is switched on. b. The electrode is scanned toward a more positive potential. c. As the potential is made more positive the electrode is now a sufficiently strong oxidant to oxidize the ferrocene to ferrocenium. d. The concentration of ferrocene decreases rapidly at the electrode surface as the anodic current increases. At point d the concentration of ferrocene is substantially diminished causing the current to peak. e. The current now decays as the ferrocene concentration becomes more depleted and ferrocenium surrounds the electrode. f. The scan is then reversed at the swithcing potential Esp of 0.75 V. Between points f and g the anodic current continues because the potential is still sufficiently positive to oxidize ferrocene. g. At point h the electrode is a sufficiently strong reductant to reduce ferrocenium, which has accumulated adjacent to the electrode surface and the reaction:[(η5-C5H5)2FeIII]+ + e– → (η5C5H5)2FeII occurs at the electrode surface. h. Thus, the cathodic current rapidly increases to point i until the surface concentration of [(η5C5H5)2FeIII]+ is diminished causing the current to peak. From point i to j the cathodic current decays as solution surrounding the electrode is depleted of [(η5-C5H5)2FeIII]+. CV vs LSV • The key advantage of CV over simple LSV results from the reverse scan. • Reversing the scan after the electrochemical generation of a species is a direct and straight forward way to probe its stability. • A stable electrogenerated species will remain in the vicinity of the electrode surface and yield a current wave of opposite polarity to that observed in the forward scan. • An unstable species will react as it is formed and no current wave will be detected in the reverse scan. • If the electrochemical process is relatively fast within the time scale of the experiment and the electrogenerated species, e.g. ferrocenium, is perfectly stable in the electrolytic solution. Under these conditions, and assuming that the solution is kept quiescent during the experiment, the ratio of the cathodic and anodic peak currents (the peak currents measured in the forward and reverse scans, respectively) should be equal to one. • Deviations from unity reveal the presence of chemical reactions involving either redox partner (Ox or Red) or both partners. • The average of the two peak potentials affords the half-wave potential for the corresponding couple, i.e. • Relative to the bulk solution, redox chemistry only occurs in the electrical double layer. • • IHP – inner Helmholtz plane. OHP – outer Helmholtz plane • Relative to bulk solution redox chemistry only occurs in the electrical double layer. 100 % 0% Reference electrodes vs. Ag/AgCl + vs. NHE (H2/2H ) 10 + vs. SCE (Hg2/2Hg ) + vs. Fc/Fc Current (µA) 5 Ref 0 Epa Epc ∆ Ep E1/2 Ag/AgCl +0.43 +0.37 60 +0.40 NHE +0.63 +0.57 60 +0.60 SCE +0.39 +0.33 60 +0.36 Fc/Fc+ +0.30 -0.30 60 +0.00 -5 -10 1.0 0.8 0.6 0.4 0.2 Potential (V) 0.0 -0.2 -0.4