Controlled Potential Bulk Electrolysis (BE)

Posted 1/4/2016 in Uncategorized

This article is part of the AfterMath Data Organizer Electrochemistry Guide

Bulk Electrolysis is a technique where either a constant current or constant potential is applied to an two or three comparment electrochemical cell in order to effect a large change in the oxidation state of a species of interest. The amount of charge passed during the electrolysis can be calculated by integrating the current with respect to time.

Synonyms: Controlled Potential Electrolysis, Potentiostatic Coulometry, Controlled Potential Coulometry,

Detailed Description

The basic setup for Controlled Potential Bulk Electrolysis is shown in Figure 1. 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. This chamber should be stirred to provide maximum mass transport to the electrode during electrolysis. The second chamber usually has the reference electrode and the third chamber houses the auxiliary electrode. Note that both the working and auxiliary electrodes should have a large surface area (typically platinum meshes or vitrious carbon).

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 which you specify is applied to the electrochemical cell and the cell is allowed to equilibrate to these conditions.

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.

Current is plotted as a function of time which can be integrated to yield total charge passed during electrolysis.

Parameter Setup

The parameters for this method are arranged on the two tabs of the setup panel. The parameters relating to bulk electrolysis are on the Basic tab and the post experimental parameters are on the Post experiment conditions tab.

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 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

Post Experiment Conditions

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, $K_4Fe(CN)_6$ was oxidized to $K_3Fe(CN)_6$ by the application of a potential of $0.57 \;V$ . The purpose of the experiment was to convert $Fe^{2+}$ in the complex to $Fe^{3+}$ . A cyclic voltammogram of the solution is shown below to show that the potential of $0.57 \; V$ will oxidize completely $K_4Fe(CN)_6$ (see Figure 3, a $2 \; mm$ Pt disc working electrode and Pt counter electrode were used with a sweep rate of $100 \; mV/s$ ).

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 conditions were: $5 \; mL$ of $K_4Fe(CN)_6$ in $0.1 \; M \; KCl$ , Pt mesh working electrode, Pt mesh counter electrode, Electrolysis Potential $= 0.57 V$ ). Integration of current with respect to time will yield total charged ( $Q$ ) passed during the experiment.

Figure 4 : Bulk Electrolysis of a Potassium Ferrocyanide Solution.

Right-click on the trace to bring up a dialog box and choose Add Tool » Area in order to integrate the area under the curve (see Figure 5).

Figure 5 : Add Area Tool to electrolysis results

You can manipulate the control points of the tool in order to obtain a proper area under the curve (see Figure 6).

Figure 6 : Drawing the proper baseline

The total charge ( $Q$ ) passed during electrolysis was $459.0 \; mC$ or $0.459 C$ . This charge can then be used to calculate the number of moles of species in solution through the use of Faraday's Constant ( $F, \; 96485 \; C/mol$ ) using the equation below:

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

where $n$ is the number of electrons for the redox process. $F$ and $Q$ are defined above. The number of moles of
$K_4Fe(CN)_6$ in solution was $4.76$ x $10^{-6}$ . Dividing this by the volume of solution ( $4 \; mL$ in this instance) in the cell, yields a concentration of $1.19$ x $10^{-3} \; M$ .

Theory

The theory of bulk electrolysis is really quite simple. However, for a more thorough description of the technique and general considerations, please see 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

Four examples of the usefulness of bulk electrolysis are shown below.

The first example shows has bulk electrolysis can be used to quantify the amount of a species in solution. Wolfe and coworkers² electrolyzed solutions of gold nanoparticles passivated with a layer of 6-(ferrocenyl) hexanethiol. Typically, quantitation of the number of ligands protecting a nanoparticle is accopmlished through the use of thermogravimetric analysis (TGA). However, since ferrocene sublimes easily, TGA was inconclusive. The researchers were able to quantitate the number of ligands by using solutions of known nanoparticle concentration and oxidizing ferrocene to ferrocenium. By determining the charge passed during the experiment, they were able to calculate the number of ligands per nanoparticle. This first example is useful because it shows that an electrochemical technique such as bulk electrolysis can be used in places whether traditional techniques might fail.

The second example uses bulk electrolysis to deposit a film of cobalt-based water oxidation catalysts. Surendranath and coworkers³ oxidized $Co^{2+}$ to $Co^{3+}$ in the presence of various electrolyte solutions. $Co^{3+}$ reacts with the electrolytes, forming a thin film of particles on an ITO electrode. Complete electrolysis of the solution was not necessary to form the films that were later used as water oxidation catalysts. This second example is useful since it illustrates that bulk electrolysis need not be solely used to completely oxidize or reduce a species in solution. Rather, the partial electrolysis was enough for the researchers to make their thin films.

The third example shows how bulk electrolysis can be used in a chemical synthesis. Blattes and coworkers⁴ used bulk electrolysis to simultaneously generate cycloaddition partners for inverse-electron demand Diels-Alder reactions. Inverse-electron demand Diels-Alder reactions have not been used as extensively as the normal Diels-Alder reaction due to the limited availability of readily accessible simple electron-deficient dienes. Using bulk electrolysis, the researchers were able to access, in situ, two unstable entities for use in cycloaddition reactions. The researchers were even able to show regiospecificity and diastereospecificity in the case of heterocyclic annulations.

Finally, the fourth example shows how bulk electrolysis can be used to teach electrochemisty to first-year undergraduate students. Chyan and Chyan⁵ used screen-printed electrodes to examine the metal-deposition process through bulk electrolysis. Normally, electrochemistry is introduced at a later time in the undergraduate curriculum when students have had more time to develop essential lab skills. Since the electrodes are screen printed the usually labor-intensive step of electrode preparation is removed. Using a variety of deposition solutions, students are able to generate films of different, visually-attractive films in one lab period. Along with the introduction of Faraday's Law, the researchers are also able to introduce electrodeposition efficiency since these electrodes can be weighed after the deposition process. This great example shows a wonderful way to get students hooked on the fascinating world of electrochemistry.