Rotating Disk Bulk Electrolysis (BE-RDE)

Posted 1/4/2016 in Uncategorized

This article is part of the AfterMath Data Organizer Electrochemistry Guide

Rotating disk bulk electrolysis (BE-RDE) is very similar to bulk electrolysis (BE) in that a constant current or constant potential is applied to a two or three compartment electrochemical cell in order to effect a large change in the oxidation state of an analyte. The amount of charge passed during the electrolysis can be calculated by integrating the current with respect to time. In almost any bulk electrolysis experiment, the solution is stirred in some way. The unique aspect of the method discussed here (BE-RDE) is that a rotating electrode is used both to stir the electrode and to convey the analyte towards the electrode surface.

Detailed Description

Like most of the other electrochemical techniques offered by the AfterMath software, this experiment begins with an induction period. During the induction period, a set of initial conditions is applied to the electrochemical cell and the cell is allowed to equilibrate to these conditions. The default initial condition involves holding the working electrode potential at the Initial Potential for a brief period of time (i.e., 3 seconds). If the potentiostat is being used to control the rotator speed, the rotator is also spun at the desired speed during this time. The WaveNow and WaveNano are capable of outputting a potential proportional to the desired rotator speed. This output can then be coupled to the input on an MSR rotator for fully automated control of the rotation speed.

Bulk electrolysis can be used to impart a large electrochemical change on a system. This can be complete oxidation or reduction or a species, the complete production of a product through electrochemical synthesis or just a partial oxidation or reduction of a compound to change the ratio of oxidized and reduced species.

Bulk electrolysis typically consists of a three chamber electrochemical cell with the chambers separated by glass frits. One chamber has the working electrode, which is the rotating electrode in this case. The second chamber usually has the reference electrode and the third chamber houses the auxiliary electrode.

After the induction period, a potential is applied to the cell for the specified period of time. After electrolysis the conditions specified in the relaxation period are applied to the cell.

Finally, the Post Experiment Conditions are applied to the cell.

The results of an electrolysis are usually presented as a plot of current versus time. Integrating the current versus time data produces a plot of charge versus time.

Parameter Setup

The parameters for this method are arranged on various tabs on the setup panel. The most commonly used parameters are on the Basic tab, and less commonly used parameters are on the Advanced tab. Additional tabs for Ranges and post experiment idle conditions are common to all of the electrochemical techniques supported by the AfterMath software.

Basic Tab

The Basic Tab (see Figure 1) contains parameters related to the Induction period, Electrolysis period, Relaxation period, Electrode range and Sampling control.

Some Pine potentiostats (such as the WaveNow and WaveNano portable USB potentiostats) have current and voltage autoranging capabilities. To take advantage of this feature, set the Electrode range parameter to “Auto”. This allows the potentiostat to choose the current and voltage ranges “on-the-fly” while the electrolysis is conducted. Please see the wiki regarding Electrode Range for a more detailed description and example of how the integrity of the data is affected by the choice of the range.

Typically, the Induction period, Relaxation period, and Sampling control parameters are filled in. You must enter a Potential and Duration for the electrolysis.

The Number of intervals in the Sampling control box is the number of data points taken during the experiment. A larger number of intervals means that the number of data points acquired will be more and data files will therefore be larger. Likewise, a smaller intervals means that fewer points will be acquired and data files will be smaller.

Figure 1: Basic setup for rotating disk controlled potential bulk electrolysis.

The waveform that is applied to the electrode first starts with the potential entered in the Induction period, followed by the potential entered in the Electrolysis period, and finally followed by the potential entered in the Relaxation period (see Figure 2).

Figure 2 : Waveform applied to electrode throughout experiment

The Electrode Range on the Basic tab is used to specify the expected range of current. If the choice of electrode range is too small, actual current may go off scale and be truncated. If the electrode range is too large, the current-time curve may have a noisy, choppy, or quantized appearance. Please see the ugly duckling webpage for an example of what happens when an improper range is selected.

Finally, the Rotation Parameters box contains the Speed parameter. Even if the potentiostat is not being used to control the rotator, you must enter a rotation speed.

Post Experiment Conditions Tab

After the Relaxation Period, the Post Experiment Conditions are applied to the cell. Typically, the cell is disconnected but you may also specify the conditions applied to the cell. Please see the separate discussion on post experiment conditions for more information.

Typical Results

In the typical result discussed below, the starting material, $K_4Fe(CN)_6$ , is partially oxidized to $K_3Fe(CN)_6$ by applying a sufficiently positive potential ( $+500 mV$ ) to drive the oxidation. The end result of the experiment is a mixture of $K_4Fe(CN)_6$ and $K_3Fe(CN)_6$ .

Prior to performing the electrolysis, the observed value of the open circuit potential ( $~50 mV$ ) was consistent with a solution containing primarily the reduced form of the analyte, $K_4Fe(CN)_6$ . In addition, a preliminary cyclic voltammogram (see Figure 3, experimental parameters: $K_4Fe(CN)_6$ in $0.1 \; M \; Na_2SO_4$ , $2 \; mm$ Pt disc working electrode, Pt counter electrode, sweep rate $100 \; mV/s$ ) confirms that a significant oxidation current is observed if the working electrode potential is at or above $500 \; mV$ .

Figure 3: Cyclic voltammogram of a Potassium Ferrocyanide Solution

The principle result from a bulk electrolysis is a plot of current versus time (see Figure 4, experimental parameters: $K_4Fe(CN)_6$ in $0.1 \; M \; Na_2SO_4$ , $5 \; mm$ Pt Disk, $2800 \; rpm$ , Pt mesh counter electrode, Electrolysis Potential = $0.5 V$ ). Integrating the current with respect to time will yield total charge ( $Q$ ) passed during the experiment. One way to accomplish this integration is to use the Area Tool. This tool can be added to a trace with a right-click on the trace followed by choosing Add Tool » Area from the menu. Once the tool is placed on the trace, you can manipulate the control points to choose the limits of the integration (see Figure 5). In this example, the total charge ( $Q$ ) passed during the electrolysis is $26.96 \; mC$ .

Figure 4: Bulk electrolysis of a Potassium Ferrocyanide Solution

Figure 5 : Using the control points to manipulate the area tool

Finally, the open circuit potential measured after the electrolysis period is $0.275 \; V$ . The ratio of oxidized to reduced species can then be calculated using the Nernst equation shown below

$E = E^{0'} + \frac{RT}{nF} ln \left(\frac{[K_3Fe(CN)_6]}{[K_4Fe(CN)_6]}\right)$

where $E$ is the open circuit potential, $E^{0'}$ is the formal potential ( $0.230 \; V$ in this instance), $R$ is the Universal Gas Constant, $T$ is the absolute temperature, $n$ is the number of electrons and $F$ is the Faraday Constant ( $96485 \; C/mol$ ). The ratio of oxidized to reduced species in this example is $5.77:1$ , meaning that the solution contains approximately $85\% \; K_3Fe(CN)_6$ and $15\% \; K_4Fe(CN)_6$ .

For a complete electrolysis example, consult the basic discussion of bulk electrolysis (BE).

Theory

The theory of bulk electrolysis is really quite simple. However, for a more thorough description of the technique and general considerations, you are directed to the literature.¹

Consider a reaction $O + e^- \rightarrow R$ , where $O$ is reduced to $R$ in a one electron reaction. A solution of $m$ moles of $O$ would require $m$ moles of electrons to completely reduce $O$ to $R$ . Upon applying a sufficient reducing potential, a large amount of $O$ is converted to $R$ and subsequently swept away from the electrode by stirring. As more $O$ is converted to $R$ the current falls off exponentially until it reaches background level. It is at this point, the electrolysis can be stopped. The charge ( $Q$ ) passed during the experiment can be obtained by integrating the current with respect to time.

$Q = {\int}i(t) \; dt$

The charge can then be converted to the number of moles of $O$ using the equation

$m = \frac{Q}{Fn}$

where $F$ is Faraday's constant, and $n$ is the number of electrons transferred during the reaction.

Application

The first instance of BE-RDE reduced $Cu(II)$ onto a copper disk electrode. Puglisi and Bard² utilized BE-RDE to remove most of the stirring noise associated with traditional electrolysis. Also, the authors were able to calculate a diffusion coefficient from the coulometric current-time curve that matched well with a value obtained using RDE.

One recent application highlights the usefulness of BE using RDE. Chardon-Noblat et al.³ synthesized a the macromolecule $[Ru(bpy)(MeCN)_2Cl_2]$ in a one-electron reduction of a $Ru(III)$ precursor. Previously, this molecule was only obtained in mixtures with the corresponding tris(acetonitrile) derivative, $[Ru(bpy)(MeCN)_3Cl]$ . The authors also chemically synthesize the $[Ru(bpy)(MeCN)_2Cl_2]$ using a one-electron reducing reagent, sodium diethyldithiocarbamate. Correlating the amount of charge passed during electrolysis with the amount of liberated chloride ion confirmed that the electrochemical synthesis of the desired complex proceeded with no side reactions. This application highlights the usefulness of bulk electrolysis for synthesis.