S16_ElectrolysisofWater

Efficiency Dependence on Anodic Surface Area for

Electrolysis Performed with Magnetite Anodes

Bryan Anderson and Samantha Kamal

Introduction

Electrolysis is the process by which gases are formed by splitting liquid solutions with the use of an electric potential. It can be a useful tool for producing chemical fuel in the form of hydrogen by splitting water. In this way, electrical energy can be stored for use later. Previous experiments have shown that using magnetite as the anode material can reduce the potential necessary to generate when driving electrolysis. However, the rate of electrolysis using this method is slow. In an effort to increase the rate of hydrogen production, the dependencies of electrolysis efficiency based on anode characteristics are of interest. In this experiment, the dependence of hydrogen production on anodic surface area, specifically the grain size of powder used to make the anodes, is investigated.

Theory

Magnetite Reactions

The fundamental reactions involved in magnetite facilitating hydrogen production from water are the oxidation (occurring at the anode):

and the reduction (occurring at the cathode):

It can be seen that hydrogen production is tied to the generation and movement of charge carriers. As such, monitoring the current between the anode and the cathode serves to monitor the rate of hydrogen production. The movement of charge carriers around the system can be seen in the figure below.

Electrolysis performed with magnetite in Sodium Hydroxide (NaOH) involves the analogous reactions,

at the anode and,

at the cathode. The charge carriers have changed in the alkaline solution, and water now splits at the cathode rather than the anode. However, the same net reaction occurs in the form of magnetite oxidizing and water splitting. Hydrogen production also remains tied to the production of charge carriers. As such, monitoring current continues to allow monitoring of the rate of hydrogen production. Furthermore, the alkaline medium allows for larger currents to be produced, and thus makes data easier to work with.

Electrolysis Current

After the initial magnetite along the surface of the anode is oxidized, the magnetite-driven electrolysis will become dependent on the diffusion of fresh material from within the magnetite grains to their surface. As such, the electrolysis current as a function of time is expected to follow the electrochemical model for a diffusion-driven current:

[1]

Where D is the diffusion constant of iron within the anode (taken to be 9.52*10^-17cm²/s), A is a free parameter accounting for geometry and initial reactant distributions, and E is a free parameter helping fit the model to data. As E defines the asymptote of the model, it will be taken to represent the steady-state current at large time values.

Once the system reaches steady state, the reaction, and thus the current, will depend heavily on steady state diffusion of material within the anode. Assuming this diffusion to follow a linear density gradient, and that the reaction rates are constant across the entire surface, the current density during the steady state period can be expressed as the system of equations:

Which can be solved to find:

[2]

Where J is the electrolysis current density, L is the distance within the magnetite over which diffusion is expected to take place with significant effect (taken to be 10^-5cm), n(x) is the density of unoxidized material as a function of distance into the anode, m(x) is the density of oxidized material as a function of distance into the anode, and K_f and K_b are reaction constants.

Given these assumptions, it is expected that the steady state current will follow a linear dependence upon surface area.

Optimal Reaction Potentials

The potential region of interest is that in which the magnetite reactions act as the dominate source of hydrogen production. This region is best described as the region for which the potential applied between two platinum electrodes cannot split water or drive current through the solution. To determine this potential range, a range of applied voltages are swept while current is monitored. This test, when the voltage sweep takes the form of a symmetric triangle wave, is known as a Cyclic Voltammogram (CV). An example of a test performed with two platinum electrodes, and an Ag/AgCl reference, in NaOH is included below.

Performing CV tests on the magnetite anodes, sweeping within the platinum flat region, reveals the charge carriers (and thus hydrogen) produced by the magnetite reaction at each potential. As the potential approaches the optimal reaction potential, current will increase due to the increasing rate of oxidation. After this optimal potential, current is expected to decrease (until increasing again outside of the platinum flat region) due to a large decrease in unoxidized material.

During the forward sweep of the CV test, along the positive leg of the triangle wave, current is expected to be driven by oxidation at the anode. During the backward sweep, it is expected to be driven by reduction at the anode. Symmetry in the CV plot would indicate a completely reversible reaction.

Held Currents and Efficiency

The optimal potentials determined by the CV tests should be the potentials for which current is maximized for the anode. As such, these potentials should be used to drive the Chronoamperometry tests -- in which potential is held and current is monitored as a function of time. As described above, the electrolysis current is expected to follow the electrochemical model for a diffusion-driven current, and the value found for E will be taken as the steady state current of the anode.

As the current within the platinum flat region is facilitated by charge carriers whose production is tied to hydrogen production, comparing the steady state currents of each anode will serve to compare the efficiency with which they facilitate electrolysis.

Experimental Setup

Electrode Fabrication

Prior to testing, magnetite anodes of differing surface areas were fabricated using magnetite powder of differing grain sizes. To make the different grain sizes, three groups were creating from magnetite powder with a grain size of 1 micrometer. The first group was left alone and the other two were pulverized in a ball mill for either 5 or 10 minutes. Once the magnetite was ground, it was packed into ceramic tubes (of inner radii 1.75mm and 3mm) and compressed using aluminum dowel pins and an arbor press. Pressure was applied until the ceramic tube fractured in the collet holding it. The produced pellet was then adhered to a wire lead with conductive paste and coated in tool grip plastic. The plastic provided strength and prevents the adhesive and lead from reacting in the solution. An example of a finished electrode is shown below.

Potentiostat and Cell

CV and Chronoamperometry tests were performed with the use of a potentiostat, which is a device that can sweep and hold a voltage at the anode, measure the voltage of the anode with respect to an Ag/AgCl reference electrode, and monitor the current between the anode and cathode. A simplified diagram of the cell is included below.

For each test, the magnetite anode is suspended in a solution of Sodium Hydroxide (NaOH) along with a platinum cathode and an Ag/AgCl reference electrode. The reference electrode maintains a reaction in equilibrium which holds a constant potential with which to measure against.

Results

The Cyclic Voltammetry plots indicate current production within the potential region for which no current is produced using two platinum electrodes. As such, oxidation and reduction appear to be producing charge carriers and driving current along with hydrogen production as expected.

As seen in the images below, there is a notable lack of resolvable current peaks within this region, which was observed for every electrode tested. Because of the lack of peaks, a voltage with which to run chronoamperometry tests was chosen along the plateau near the platinum peak

Chronoamperometry data taken was fit to the diffusion current model by varying the free parameters A and E. As E defines the asymptote of the model at large time values, it is taken to be the steady state current. The chronoamperometry data for two electrodes are shown below as well as the fits with the diffusion model for current. Note the residuals of the fits are good for large values of time, meaning we can use the model to determine the steady state current.

The data displacement plot for Electrode 1 omits the first 38 points, the plot for Electrode 8 omits the first 150. Given hardware limitations in terms of recording the predicted large currents at short time scales, fitting was done with prioritization towards trends at large time values.

A summary of the chosen potentials and steady state currents for each anode is included below. Where R designates the radius (in mm) of the mold used to create the electrode in question.

Discussion and Conclusions

Looking only at steady state current values, there appears to be a mostly positive correlation between efficiency and surface area -- assuming grind time and surface area are positively correlated. However, considering the potential with which the currents were obtained, efficiency may respond non-monotonically to surface area increase, with a favored range including that obtained with 5 minutes of grinding.

That said, the lack of resolvable peaks in the CV plots makes the choice of potentials in the chronoamperometry more difficult, and makes comparison of the steady state currents produced with them less certain. This may have been partially caused by finite resistance in the adhesive used to attach wire leads to the electrodes. However, a more influential cause may have been the use of Ag/AgCl electrodes in an alkaline solution. Free roaming chlorine ions may have contributed to current readings and smoothed out any peaks that existed.

Furthermore, the method of electrode fabrication produced imperfect cylinders and fragments, rather than perfect cylinders. As such, the surface area ordering of electrodes is somewhat uncertain. Further investigations will make use of specific surface area analysis, as well as methods attempting to reduce the variety of macroscopic electrode geometries produced.

Acknowledgements

This experiment was performed under the guidance of Professors Elias Puchner and J.W. Halley.

The potentiostat used in this experiment was designed by Professor Jack Summers of Western Carolina University. He and Jon Huber were of great help in setting up the device and performing repairs as necessary.

References

1 - Bard, A. J., & Faulkner, L. R. (1980). Electrochemical methods: Fundamentals and applications. New York: Wiley.

2 - J.W. Halley, A. Schofield and B. Berntson, “Use of Magnetite as anode for electrolysis of water,” Journal of Applied Physics, 111, 124911 (2012)