Report On Adsorption/Desorption Studies of CO on PEM Electrodes Using Cyclic Voltammetry Sethuraman, Vijay Anand I. AIM: The aim of this study is to calculate the adsorption and desorption rate constants of CO (Carbon monoxide) on platinum electrode in Polymer Electrolyte / Proton Exchange Membrane Fuel Cells (PEMFC). II. III. MOTIVATION: Oil is used to meet most of our energy demands today. It is widely accepted 1 now that the world will be running out of oil in about 25 to 30 years. So, we have to look for an alterative energy source that will help us to lead a normal life during the inevitable oil crisis. One hope would be Fuel Cells. Fuel Cells are energy conversion devices that utilize hydrogen and oxygen as fuels and produce electrical energy. The efficiency with which chemical energy is being converted to electrical energy in a fuel cell is very high. The problems we face in commercializing fuel cells are many. One important problem is the fuel. Hydrogen storage is a main concern. So, we have to use reformers to produce hydrogen and this reformate contains CO (as high as 1%). CO lowers the performance of the fuel cell by getting adsorbed on the Pt catalyst surfaces thereby blocking the hydrogen oxidation. In order to solve this problem of CO poisoning, we need to understand the mechanisms of CO adsorption and desorption. But there aren t any experimental data describing the CO adsorption and desorption mechanisms on PEMFC electrodes. INTRODUCTION: Proton exchange membrane fuel cells (PEMFC) operating on pure hydrogen show very good polarization characteristics over a wide range of load and temperature conditions. However, in reality, the use of reformate gas (a mixture of nitrogen (40 50%), hydrogen (35 45%), CO 2 (10 20%), CO, water vapor and traces of other gases) causes a drop in Fuel Cell performance due to CO poisoning. In this study, we conduct experiments on a fully assembled fuel cell to estimate the rates of CO adsorption and desorption under different temperature conditions. We use Cyclic Voltammetry as the analytical tool to determine the CO adsorption/desorption rates. Studies were conducted on Pt electrode at 323K and 343K and the preliminary results are presented here.
IV. EXPERIMENT: The experiment consisted of four parts. They were, 1. MEU (Membrane Electrode Unit) Fabrication & Fuel Cell Assembly. 2. Optimal Performance Test. 3. CO Adsorption Experiments. 4. CO Desorption Experiments. 1. MEU Fabrication & Fuel Cell Assembly: Single sided ELAT Electrode of 10 cm 2 in area [Gas Diffusion Layer (GDL)] from E-TEK was coated with Pt ink [Pt on Vulcan XC 72 R] from E-TEK. An optimum loading of about 5 mg/cm 2 was achieved. Two such GDLs were made. Nafion 117 from Alfa- Aesar was treated in boiling (373 K) H 2 SO 4 (0.1 M) to get rid of surface impurities and was used as the membrane. Using the GDLs and the Nafion membrane, a 10-cm2 fuel cell (from Fuel Cell Technologies Inc.) was assembled. The cell was tested for crossover leaks, overboard leaks and throughput. 2. Optimal Performance Test: This assembled cell was tested on the Fuel Cell Test Station (from Fuel Cell Technologies inc.). Initial conditioning of the cell was done for about 5-6 hours. Hydrogen (120 cc/min) and Air (360 cc/min) (both from Air Technologies Inc.) were flowing on the anode side and the cathode side respectively. Open Circuit Potential of about 1.006 V was noted. A V-I test was done and the V-I curve obtained showed high current (0.75 0.3 A/cm 2 ) for a wide range of voltages (0.5 0.9 V). 3. CO Adsorption Experiments: Once the cell was equilibrated, the CO experiments were started. Hydrogen (50 cc/min) was made to flow on the cathode side and Nitrogen (100 cc/min) was made to flow on the anode side. All the gases were humidified at cell s operating temperature before they were let into the cell. The following describes one cycle of the experiment, a. Switch the flow on the anode side from N 2 to CO, flow rate being the same. b. Let CO flow till the electrode surface gets saturated and there is no more significant increase in the number of molecules adsorbed onto the electrode surface (this time vary from cell to cell and has to be determined experimentally). c. Switch back the flow to N 2. d. Wait for time t (Waiting Time) and run the voltammogram. This is repeated for various waiting times.
4. CO Adsorption Experiments: An experiment was previously conducted to determine the time for CO (at the same flow rate) to adsorb to saturation. The following describes one cycle of such an experiment, a. Switch the flow on the anode side from N 2 to CO, flow rate being the same. b. Now wait for time t 1 (Passing Time). c. Switch back the flow to N 2. d. Immediately after switching back to N 2, run the voltammogram. This was repeated for various passing times III. RESULTS & DISCUSSIONS: A typical Cyclic Voltammogram (E (v) vs. I (A) curve) for the oxidation of CO adsorbed on Pt electrode looks like the following. The following can be observed from this plot, 1. The peak obtained during the first cycle was due to the oxidation of adsorbed CO on the surface of Platinum. That was why there wasn t any peak during the second or the third cycle. 2. Except for the peak caused by the CO oxidation, the CV looks like that of plain platinum electrode. 3. The area under the peak gives us the surface coverage of CO on the platinum electrode.
a. Adsorption Experiment: The following were the results obtained for the adsorption experiment, As seen, the CO Coverage area increases initially with the CO passing time but saturates after some time. A plot of time (s) vs. CO Coverage area was made and from the plot, the saturation time for CO adsorption was found to be 400 s. Time (seconds) Peak Area Base Area CO Coverage Area Theta 15 1.263 1.23 0.033 0.040441176 30 1.338 1.222 0.116 0.142156863 60 1.542 1.338 0.204 0.25 75 1.723 1.346 0.377 0.462009804 120 1.954 1.339 0.615 0.753676471 180 2.069 1.259 0.81 0.992647059 240 2.027 1.238 0.789 0.966911765 360 1.917 1.157 0.76 0.931372549 500 1.988 1.192 0.796 0.975490196 750 1.972 1.17 0.802 0.982843137 1450 2.017 1.201 0.816 1 From the CO Saturation Time plot above, the saturation time was calculated to be 400 s.
Upon plotting the CO coverage (θ), vs. time (t), we get a straight line that describes how fast CO is getting adsorbed on the surface of the platinum electrode. The saturation time for CO adsorption was found to be 400 seconds. b. Desorption Experiment: The following were the results obtained for the adsorption experiment, Run order Time Co Peak Back Area Peak Area Theta Ln (theta) Curve fit 6 0 1.785 1.045 0.74 0.934 0.3011 0.6900 2 5 1.843 1.051 0.792 1.000 0.2332 0.6839 11 10 1.753 1.005 0.748 0.944 0.2904 0.6779 12 21 1.768 1.033 0.735 0.928 0.3079 0.6654 10 30 1.744 0.9904 0.7536 0.952 0.2829 0.6557 13 60 1.691 0.9935 0.6975 0.881 0.3603 0.6266 4 100 1.449 0.8364 0.6126 0.773 0.4900 0.5944 5 405 1.748 1.145 0.603 0.761 0.5058 0.4778 9 505 1.687 1.135 0.552 0.697 0.5942 0.4611 1 641 1.644 1.094 0.55 0.694 0.5978 0.4439 3 750 1.643 1.143 0.5 0.631 0.6931 0.4325 8 817 1.625 1.121 0.504 0.636 0.6852 0.4260 7 1523 1.647 1.182 0.465 0.587 0.7657 0.3687
The plot shows the decrease in the peak area due to CO desorption. The slope gives the desorption rate constant of CO from the Pt electrode at this temperature. Since the desorption of CO depends of the amount of CO present on the surface at any given time, we can define two stages of desorption.
Desorption that takes place immediately when the surface is saturated with CO and that that takes place over a longer period of time. We call the former Short desorption rate and the latter the Long desorption rate. The following plot shows the shift in the Starting oxidation potential of CO,
V. CONCLUSION: The short and the long desorption rate constants for the adsorption of CO on Platinum electrode at 323 K was found out to be 2.1 X 10-3 molecules of CO/s and 4 X 10-4 molecules of CO/s respectively. Also, the shift in the oxidation potential was because of the existence of strongly bonded CO and weakly bonded CO on the surface of platinum electrode. The strongly bonded CO molecules stay on the surface of the platinum electrode for a longer period of time and they also require a less potential for oxidation. This paves the way to look into how strongly or weakly does each CO molecule is adsorbed onto the surface of the platinum electrode. Also, what fraction of CO molecule is strongly adsorbed onto the surface of the platinum electrode can be determined.