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Electrochemical oxidation of carbon monoxide in reformate hydrogen for PEM fuel cells

ProQuest Dissertations and Theses, 2011
Dissertation
Author: Sivagaminathan Balasubramanian
Abstract:
Carbon monoxide (CO) poisoning of platinum anode significantly decreases the performance of proton exchange membrane fuel cells (PEMFC). Various techniques studied to remove CO from source or mitigate CO poisoning have met with limited success due to fuel loss or increases infrastructure requirements to meet the operating needs. We developed a twin cell electrochemical filter that is similar in construction of the PEMFC and remove CO with minimal fuel loss. In this CO in fuel gas is adsorbed on a Pt electrode and oxidized by applying a pulse potential. The CO kinetics on Pt electrode was characterized by employing cyclic voltammetry and chronoamperometry techniques. A fixed bed adsorber model was used to characterize the adsorption part of the filtering. The model was validated by demonstrating as a filter cell. The performance was compared with other CO removing techniques.

T ABLE OF C ONTENTS

A CKNOWLEDGEMENTS ........................................................................................................ iii A BSTRACT .......................................................................................................................... iv L IST OF F IGURES ................................................................................................................. vi L IST OF S YMBOLS ............................................................................................................... ix L IST OF A BBREVIATIONS .......................................................................................................x C HAPTER 1

I NTRODUCTION ...................................................................................................1 C HAPTER 2

E LECTRO - OXIDATION OF C ARBON -M ONOXIDE ON P T E LECTRODES ...................5 2.1

I NTRODUCTION ............................................................................................................5 2.2

E XPERIMENTAL ...........................................................................................................8 2.3

R ESULTS AND D ISCUSSIONS ......................................................................................13 2.4

C ONCLUSIONS ...........................................................................................................20 C HAPTER 3

A NALYSIS OF O PERATION OF AN E LECTROCHEMICAL F ILTER ..........................21 3.1

I NTRODUCTION ..........................................................................................................21 3.2

T HEORY .....................................................................................................................24 3.3

E XPERIMENTAL .........................................................................................................26 3.4

R ESULTS AND D ISCUSSIONS ......................................................................................28 3.5

C ONCLUSIONS ...........................................................................................................40 C HAPTER 4

C ONCLUSIONS AND F UTURE A SPECTS ..............................................................42 4.1

C ONCLUSIONS ...........................................................................................................42 4.2

F UTURE A SPECTS ......................................................................................................43 B IBLIOGRAPHY ...................................................................................................................45

vi

L IST OF F IGURES

Figure 1.1. Cross section of a PEM fuel cell. ......................................................................2 Figure 2.1. Comparison of cyclic voltammograms of Pt/C electrode with pre-a dsorbed CO(  ) and blank (--). Inset shows the deconvoluted oxidation peaks of CO ad-species.. 9 Figure 2.2. CO oxidation peaks obtained from baseline corrected CO-SCVs on CO saturated Pt/C electrode after applying pulse potential 550 mV for differen t durations at 45°C. ..................................................................................................................................12 Figure 2.3. Comparison of the predicted and experimental fractional CO coverage a t 45 o C and for pulse potential of 550 mV. The lines represent predictions and symbols show experimental results (

-total CO,

- I ad CO ,

II ad CO ). ....................................................13 Figure 2.4. CO oxidation peaks obtained from baseline corrected CO-SCVs on CO saturated Pt/C electrode after applying pulse potential (550, 600, 650, 700) mV for different durations at 45°C. ................................................................................................15 Figure 2.5. Comparison of experimental fractional CO coverage (symbols) and fit f rom equation 8 (lines) at 45 o C for pulse potentials of varying amplitudes between 500 and 700 mV vs. DHE. ......................................................................................................................16 Figure 2.6. CO oxidation peaks obtained from baseline corrected CO-SCVs on CO saturated Pt/C electrode after applying pulse potential of 550 mV for 10 seconds, for

temperatures 25, 45, 60 and 70°C.. ....................................................................................17

vii

Figure 2.7. Dependence of oxidation reaction rate constant (K I ) of I ad CO ad-species on the potential for different temperatures (

-25,

-45,

-60,

-70 o C). ...................................18 Figure 2.8. Dependence of oxidation reaction rate constant (K II ) of II ad CO ad-species on the potential for different temperatures (

-25,

-45,

-60,

-70 o C)............................18 Figure 2.9. Order-of-time dependence of X I and X II of I ad CO (open symbols) and II ad CO

(closed symbols) ad-species, respectively, on the applied potential for diffe rent temperatures (

- 25,

- 45,

- 60,

- 70 o C). ............................................................19 Figure 3.1. Schematic description of a twin cell filter setup describing the ox idation and adsorption steps of each of the filter cells. .........................................................................23 Figure 3.2: Normalized CO break through at the exit of a filter cell plotted aga inst the corresponding normalized time. Symbols represent experimental data and the line

represents the model fit for a single mass transfer coefficient...........................................29 Figure 3.3. CO adsorption break through curves showing the effect of different feed C O concentration in hydrogen for a flow rate of 100 cm 3 /min at 25 o C and 1 atm. pressure on a filter cell with a Pt-black loading of 4 mg/cm 2 . Symbols and lines (

and dotted - 1000 ppm,

and squared - 5000 ppm and

and straight - 10000 ppm) represent experimental data and the model fit of the equation 11, respectively. ..............................30 Figure 3.4. CO adsorption break through curves showing the effect of different flow r ates for a feed CO concentration of 10000 ppm in hydrogen at 25 o C and 1 atm. pressure on a filter cell with a Pt-black loading of 4 mg/cm 2 and an electrode area of 25 cm 2 . Symbols and lines (

and dotted – 50,

and straight - 100 and

and dashed - 150cm 3 /min) represent experimental data and the model fit of the equation 11, respectively. ...............32

viii

Figure 3.5. CO adsorption break through curves showing the effect of different cata lyst loadings for a feed CO concentration of 10000 ppm in hydrogen at 25 o C and 1 atm. pressure. Symbols and lines (

and dotted – 1.5,

and dashed – 4.0 and

and dashed – 8.0 mg/cm 2 ) represent experimental data and the model fit of the equation 11, respectively. .......................................................................................................................33 Figure 3.6. CO adsorption break through curves showing the effect of different temperatures for a feed CO concentration of 10000 ppm in hydrogen at 1 atm. pressure. Symbols and lines (

and straight – 25,

and dashed – 45 and

and dotted – 65 o C) represent experimental data and the model fit of the equation 11, respectively. ...............34 Figure 3.7. CO adsorption break through curves showing the effect of different fill er gases for a feed CO concentration of 1000 ppm at 1 atm. pressure. Symbols and lines (

and straight – CO/N 2 ,

and dashed – CO/He and

and dotted – CO/H 2 ) represent experimental data and the model fit of the equation 11, respectively. ..............................36 Figure 3.8. The exit CO concentration of a filter operating with a feed CO c oncentration of 10,000 ppm CO/H 2 at 25 o C and 1 atm. pressure. ..........................................................37 Figure 3.9. Oxidation response cycle of a filter operating with a switching cycle of 20 seconds and 0.7 V. .............................................................................................................38

ix

L IST OF S YMBOLS

C Concentration of carbon monoxide, mol/cm 3

F Faraday’s constant. H 2

Hydrogen. K Oxidation rate constant Q Adsorbed phase CO concentration, mol/cm 3

T Time, s CO Carbon monoxide Pt Platinum

θ Fractional Coverage of CO. τ Dimensionless time.

C Concentration of carbon monoxide, mol/cm 3

x

L IST OF A BBREVIATIONS

CA ....................................................................................................... Chronoamperometry CV ........................................................................................................ Cyclic Voltammetry ECF ................................................................................................... Electrochemical Filter PEMFC ................................................................. Proton Exchange Membrane Fuel Cells

1

CHAPTER 1 I NTRODUCTION

Fuel cell is an electrochemical device that extract ele ctrical energy directly from chemical reactions. Conventional heat engine cycles, like Carnot engine cycle, convert energy from chemical to thermal, through combustion, to mechanical a nd finally into electrical form. However in a fuel cell, an ion conducting electr olyte separates the net combustion process and thereby directly tap the electron flow assoc iated with fuel oxidation to an external circuit as electrical power. Theoretica lly, the direct utilization of electron flow results in higher efficiency in conversion of chemica l energy into electrical energy [1]. A fuel cell has two porous electrodes separated by an ion conducti ng electrolyte. Depending on the electrolyte used, fuel cells are classified i n to solid oxide fuel cells, molten carbonate fuel cells, alkaline fuel cells, phosphoric acid duel cells and PEMFC (Proton exchange membrane fuel cells). The PEMFC technology is considered to be the next generation power s ource for automotive power applications [2-4]. The main driving force for this technology is it s low operating temperature (60~90 o C) and near zero emissions, while using hydrogen as its fuel [4]. A PEMFC is comprised of a solid polymer electrolyte m embrane that conducts proton from anode to cathode. Platinum supported on carbon (Pt/C) is widely used as the electrode catalyst material. Hydrogen (H 2 ) or methanol is used as the fuel at the anode and oxygen (O 2 ) or oxygen from air is used as oxidant at the cathode.

2

3

Anode contaminants: Hydrogen obtained from reforming fossil fuels is considered to be one of the economically viable sources [3]. Carbon monoxide (CO) formed during r eforming and hydrogen sulfide (H 2 S) that occur in fossil fuels are the predominant contaminants in the

reformate gas. Studies revealed that the catalytic deactivation due to adsorption of CO and H 2 S on the Pt active sites results in decreased cell efficienc y. Springer et al.[8] showed that a CO concentration as low as 10 ppm in reformate H 2 could result in current loss of 40 % compared to pure H 2 performance. The effects of CO present in reformate hydrogen on fuel cell was well documented. After much research, a number of counter measures like pres sure swing adsorption [7, 10-14], catalytic preferential oxidation or air bleed [ 15-31], low temperature water gas shift reaction[32-34, 13, 35-37], bimetallic catalysts[38-42, 5, 28], and catalytic methanation[43-46] have been proposed to overcome the c ontaminants’ impacts. An additional technique is the electrochemical filtering (ECF) to decrease the CO content in reformate gas. ECF technique taps the difference i n energy of adsorption of CO and H 2 on Pt. This technique has the advantage of low volume space compared to other techniques, because of high turnover frequency of catalyst sites at low temperatures [47]. However, this technique was not explored thoroughly, due to insufficie nt data on CO adsorption and oxidation at room temperature and the difficulty in finding the parameters needed to model the technique. I studied the oxidation of CO monolayer and presented the results in chapter 2. The chapter 3 deals with the adsorption dynamics of carbon monoxide on a Pt electrode. A fixed bed adsorber model was used t o extract useful design parameter from CO adsorption data. The extracted para meter was used to

4

predict suitable switching time for a filter. The prediction wa s validated by demonstrating in a filter cell. The final chapter deals with the possible rol e that ECF can play in the larger realm of CO handling techniques already in development for PEMFC applications. In this chapter we also discuss about possible improvement in electro chemical filtering technique to make it viable for commercial applications.

5

CHAPTER 2 E LECTRO - OXIDATION OF C ARBON -M ONOXIDE ON P T E LECTRODES

2.1 Introduction

The need for carbon monoxide (CO) tolerant catalysts for proton-e xchange- membrane fuel cells (PEMFC) has brought considerable attention to understand the kinetics of CO adsorption and oxidation on single and polycrystalline plat inum (Pt) electrodes [48]. Various electrochemical and spectral techniques have been employed to: (i) delineate the mechanism of CO poisoning in PEMFC; (ii) es timate the CO induced performance loss in PEMFC; and (iii) develop techniques to mitiga te the performance loss [49-65]. Electrochemical filtering is one such technique, in which the concentration of CO in the fuel stream is decreased by electrochemically oxidizing it to CO 2 [47, 66- 67]. Developing CO tolerant catalysts and designing electrochemic al filters require a quantitative understanding of CO adsorption and electro-oxidation kinetics. In the literature, the kinetic parameters for the electro-oxidation of CO adsorbed on Pt were either measured from electrode-in-solution experiments [68] or deduc ed from fitting a model to the CO poisoning induced fuel cell performance-loss data [8, 69-71]. However, in situ measurements of electro-oxidation kinetic parameters from ele ctrodes reflecting PEMFC are rare due to the complex overlapping of different oxidation mechanisms.

The generally accepted mechanism for CO electro-oxidation is the Langmuir- Hinshelwood reaction between adsorbed CO (CO ad ) and a neighboring oxygen-

6

containing species (Eqn. 2.2), which we refer as * ad OH , generated from hydrolysis (Eqn. 2.1) [72]. * + - 2 ad H O OH + H + e → [2.1]

I, II * + - ad ad 2 CO OH CO + H + e + → [2.2] The superscripts I and II indicate the two CO ad-species. It ha s been hypothesized that this reaction is limited by the availability of the * ad OH adjoining a CO ad molecule, which causes the left-skewed current response with time under potentiosta tic conditions (see fig. 7 of reference [73]). Different models proposed to explain the acces sibility of the adjacent * ad OH include nucleation and growth (NG) of * ad OH islands that grow to consume the CO ad , surface diffusion of CO ad-species to active sites on which t he CO oxidation reaction occurs [74-88]. However, it was difficult to arrive at a physically justifiable yet experimentally quantifiable general solution fr om the models proposed for the electro-oxidation of CO ad on a Pt/C electrode. For example, Friedrich et al. [89-90] and Koper et al. [74-82] established the importance of the surface mobility of the s pecies, and their single-crystal electrode studies showed higher activit y in kinks rather than terraces supporting the active sites [91]. While Chang et al. ’s observation of CO islands supports the NG mechanism [92-104], NG mechanism could not explain the rol e of crystalline defects in CO electro-oxidation [74, 76-79]. As the experimental evidence agrees with each of the models under a limited set of conditions, a single physical model has not been shown to explain the CO electro-oxidation rates under most of the possible conditions. In addition, CO ad stripping cyclic voltammetry (CO-SCV) on Pt electrodes shows

a dual peak response during the electro-oxidation of CO ad [105, 72]. Gilman attributed

7

this to the two prominent CO adsorption forms (ad-species) - atop and bridged [72], which were first observed by Eischens and Pliskin through infrared t echniques [106].

Various explanations presented in the literature for the two CO ad -species include: particle size variation [83-85, 73, 107-110], terrace vs. edge sites distribution [111], crystallographic orientation [112-114, 74, 76-79, 115-118], difference in nuclea ting sites of oxygen-containing species [83-85] and difference in the mobility of surface species on different crystal facets [119-121]. The widely accepted view is that the differences in the pattern of adsorption on Pt exhibit the two oxidation peaks in a CO- SCV [58-63, 122- 146] . By deconvoluting the dual peak into individual peaks representing the two ad- species, our group has quantified desorption and rearrangement kineti cs of the two CO ad-species over polycrystalline platinum supported on carbon (Pt/C) electrode s [147]. The objective of this work is to develop and test a semi-empirical methodology to quantify the potentiostatic oxidation rates of the two distinct CO ad-species on a polycrystalline Pt/C electrode. This was done by quantifying the total unreacted CO ad

after applying a potentiostatic pulse for certain duration on a Pt/C electrode with pre- adsorbed CO. The unreacted CO ad is quantified by CO-SCV. The dual peaks observed in the CO-SCV were deconvoluted into two Gaussian peaks representi ng the two CO ad- species. This procedure was repeated for different pulse durations under a constant potential to obtain the change in the overall CO coverage with pulse time. The electro- oxidation kinetic parameters of the two ad-species were deduced by fitting an equation derived from NG mechanism with the fractional unreacted CO ad for different pulse durations. The model predictions of the CO ad-species coverage were compared and

8

validated with the individual ad-species coverage estimated from the deconvoluted CO- SCV. This procedure was repeated for different cell potentials and temperature s. 2.2. Experimental 2.2.1. Experimental Setup The membrane electrode assembly (MEA) used has a Nafion 115 mem brane sandwiched between two polycrystalline Pt/C (XC-72R, E-Tek) ele ctrodes with a Pt loading of 0.5 mg/cm 2 coated over a carbon cloth gas with an area of 10cm 2 . The MEA was assembled into a cell with single-channeled serpentine gra phite flow fields. The electrodes were conditioned as reported in our previous work [147]. A ga s concentration of 500 ppm of CO in nitrogen, flowing at the rate of 100 cm 3 /min for 5 minutes was used as the CO source to saturate the working electrode. A gas mi xture of 4% hydrogen in nitrogen was passed (100 cm 3 /min) through the other electrode, which acted as both counter and reference electrode. All the gases used were procured from Air Products, Inc. and were certified for purity. The electrochemical expe riments were conducted using a M263A potentiostat/galvanostat from Princeton Applied Researc h, Inc. and ECHEM software from EG&G. 2.2.2. CO electro-oxidation and SCV To estimate the oxidation rate of CO ad , a potentiostatic pulse was applied on a CO-saturated electrode for certain duration, t d , at a constant temperature, followed by a CO-SCV. The SCV is carried out by scanning the electrode fr om 50 to 1,100 mV, with respect to reference electrode, back and forth for 3 cycles at a rate of 50 mV/s. The CO- SCV gives the quantity of unreacted CO after the pulse. The chan ge in the quantity of the unreacted CO ad after different pulse durations indicates the oxidation rate of C O ad for

9

that particular applied potential. This procedure of CO saturation and potentiostatic oxidation followed by stripping voltammetry is repeated for pulses of different durations, at different applied potentials in the range of 450~700 mV and for temperatures of 25, 45, 60 and 70°C.

Figure 2.1. Comparison of cyclic voltammograms of Pt/C electrode with pre- adsorbed CO(  ) and blank (----). Inset shows the deconvoluted oxidation peaks of CO ad-species. 2.2.3. Analysis of a CO-SCV Figure 2.1 compares the anodic scans of CO-SCV (line) and a blank, i.e. , no CO adsorbed (dashed line). Compared to the blank one, the hydrogen oxidati on region (< 400 mV) is depressed in the CO-SCV. This is due to the occupation of ac tive sites by CO ad . Whereas in the CO oxidation region (400 to 900 mV), the dual peaks i n CO-SCV

10

represent the electro-oxidation of the two CO ad-species. As the blank did not have CO ad , there was no peak in that potential range. The net charge, corr esponding to the area under the dual peak, includes the charge for CO electro-oxidation as i n equation 2.1 and 2.2. To correct for the background current including double layer capacitance and Pt oxidation, the anodic current response of the blank was subtracted from CO-S CV. The area under the background corrected CO oxidation current is shown as the shaded re gion in the potential range from 400 to 900 mV. All of the CO oxidation peaks prese nted henceforth are background corrected CO oxidation peaks obtained from respective CO-SCV.

2.2.4. Deconvolution of the CO oxidation peaks The magnified portion of the background corrected CO oxidation peaks is s hown in the inset of Fig. 2.1. The dual peak was deconvoluted as two individual Gaussian peaks representing the two ad-species, I ad CO and II ad CO . For more information on the deconvolution of peaks, please refer to our previous work [147]. The peak at l ower potential was assigned to I ad CO and the peak at higher potential was assigned to II ad CO . The oxidation charge for each of the ad-species was estimated by integrating the background corrected and deconvoluted oxidation current peaks over time and wa s referred as Q I and Q II , respectively. This charge is proportional to the quantity of the unreacted CO ad-species the Pt/C electrode (Refer eqn. 2.1 and 2.2). T he fractional coverage of unreacted CO ad was estimated by taking the ratio of CO ad coverage after the application of a potential pulse with respect to the saturation CO ad coverage with no pulse applied. The fractional surface coverage of each of the CO ad-sp ecies ( θ I and θ II ) is defined in the equations 2.3 and 2.4.

11

I I I II 0 Q θ = Q +Q t =     [2.3] II II I II 0 Q θ = Q +Q t =     [2.4] The total initial fractional coverage of CO saturated electrode , when no pulse applied, is taken as unity (i.e. T I II t=0 t=0 t=0 θ = θ + θ =1 ). The change in the total quantity of CO ad with pulse time is taken as the fraction change in CO ad from the initial saturation CO ad coverage. 2.2.5. Model Equation The electro-oxidation of CO is a surface reaction between CO ad -species and neighboring * ad OH formed from the hydrolysis of water. To estimate the oxidation rates of the CO ad-species, the bimodal CO ad oxidation current peaks are treated as current response of two independent Gaussian populations that oxidize independently and simultaneously, albeit with differing rates. The desorption and re -arrangement rates, estimated in our previous work [147], are an order of magnitude lesser than the oxidation rates and are assumed to be negligible in relation to the oxidation rates. According to NG mechanism, CO electro-oxidation rate increases with the rate of nucleation and growth of * ad OH islands and then decreases as these growing * ad OH islands overlap resulting in the shrinkage of CO ad islands [148]. Vollhardt and Retter lumped the nucleation, growth and collapse of islands to a simple expression of fr actional coverage as a function of time as [149], X t t=0 θ = θ exp(-Kt ) [2.5] Where K is the lumped rate constant and X is the order of time. X indicates the type of NG mechanism and takes the value 1, 2 or 3 for the limiting cases of exponential decay,

12

instantaneous nucleation and progressive nucleation [51], respectively. E xtending this relation for two independent populations of CO ad-species, the fractiona l coverage of CO can be written as, I II T I I X II II X t t=0 t=0 θ = θ exp(-K t ) + θ exp(-K t ) [2.6] Love and Kapowski observed that part of the Pt crystal facets e xhibit progressive nucleation and other part exhibit instantaneous nucleation, for the same applied potential [150-151]. Therefore, we consider X I,II as a parameter that encompasses the changing nature of oxidation mechanism from progressive nucleation to exponential decay mechanism for different crystal sites.

Figure 2.2. CO oxidation peaks obtained from baseline corrected CO-SCV s on CO saturated Pt/C electrode after applying pulse potential 550 mV for different durations at 45°C.

13

2.3. Results and Discussion 2.3.1. Effect of oxidation time Figure 2.2 shows the effect of pulse duration on the CO oxidation peaks obtained at 45 o C for the potential 550 mV. With the increase in the duration of potent ial pulse, the height of the CO oxidation peaks decreased. The height of the peak 1 decreased rapidly than that of the peak 2. For the pulse duration of 240s, the peak 1 is absent indicating a complete oxidation of the I ad CO . This shows that II ad CO oxidizes at a lower rate, which implies the significant difference in the oxidation rates of the two ad-speci es.

Figure 2.3. Comparison of the predicted and experimental fractional CO coverage at 45 o C and for pulse potential of 550 mV. The lines represent predicti ons and symbols show experimental results (

-total CO,

- I ad CO ,

II ad CO ).

14

The integrated total charge under the CO ad oxidation peaks, obtained at 45 o C for different durations of 550mV potential pulse, were fitted against the e quation 2.6 and parameters K I,II and X I,II were estimated for the two ad-species. The fractional covera ge of each of the CO ad-species was estimated by using the est imated parameters in the equation 2.5. Fig. 2.3 compares the experimentally measured and deconvolute d CO ad

coverage (symbols) with the fitted and predicted CO ad coverage (lines) for different duration of potential pulses. The figure shows fractional coverage of CO ad-species predicted using the oxidation rate parameters, which were estima ted from fitting the kinetic equation 2.6 with the overall coverage, agrees with the coverage calculated from the deconvolution of CO-SCVs. The total coverage declined rapidly during initial period followed by a gradual decline indicates two different oxidation ra tes. The sharp initial drop in the overall coverage agrees with the higher oxidation rate of the I ad CO ad-species. This is consistent with the sharp decrease in the height of peak 1 with time as observed in Fig. 2.2. The delay in oxidation of the II ad CO ad-species was attributed to slow rate of initiation of the nucleation of the * ad OH islands. This is evidenced from the fitted values of X for I ad CO and II ad CO ad-species, which are 1 and 2.3 corresponding to exponential decay and nucleation and growth mechanism. 2.3.2. Effect of applied potential Figure 2.4 shows the base-line corrected CO oxidation peaks obtaine d after applying pulse potentials (550, 600, 650, 700 mV) for 10 seconds at 45°C of the CO saturate d Pt electrode. This figure shows the effect of applied potential on t he oxidation of CO ad from the electrode surface. With the increase in applied potential, the overall area under the peaks decreased at a faster rate. The first peak decreas ed rapidly and its contribution was

15

not distinguishable for the CO peaks obtained for the pulse potentials 650 and 700 mV. In addition, as the applied potential increased, the CO oxidation time de creased. This shows that an increase in the pulse potential promotes the CO oxidation reaction.

Figure 2.4. CO oxidation peaks obtained from baseline corrected CO-S CVs on CO saturated Pt/C electrode after applying pulse potential (550, 600, 650, 700) mV for different durations at 45°C. Figure 2.5 shows the fitted fractional coverage change with tim e for various pulse-potential amplitudes ( i.e. , 500 - 700 mV vs. DHE) at 45 o C. The line represents the fitted results and the symbols represent the fractional CO covera ge measured after applying the pulse. When 500 mV was applied, the CO coverage graduall y dropped from around 1.0 to 0.25 after 800 seconds. However, when 700 mV was applied within 20 seconds the coverage dropped to zero. For intermediate cases of 550, 6 00 and 650 mV pulse, a sharp initial drop followed by a relatively stagnant peri od and a quick drop in the CO coverage is observed. At lower potentials, both of the CO ad-species oxidi ze at a very

16

low rate. However as the potential increased, nucleation of the * ad OH is facilitated in I ad CO ad-species coverage contributing to the increased oxidation of I ad CO , while II ad CO being relatively dormant until the nucleation of * ad OH starts near the II ad CO islands. The procedure of parameter estimation and deconvolution was repeated for the CO oxidation data obtained for temperatures 25, 60 and 70 o C and the parameters of K and X were estimated for both of the CO ad-species.

Figure 2.5. Comparison of experimental fractional CO coverage (sym bols) and fit from equation 8 (lines) at 45 o C for pulse potentials of varying amplitudes between 500 and 700 mV vs. DHE.

2.3.3. Effect of temperature The effect of temperature on CO oxidation is shown in Fig. 2.6. The CO ad

oxidation peaks shifted to lower potential with the increase in temperature from 25 o C to 70 o C. Also, the total peak area decreased with increase in the t emperature, for e.g. when

17

the temperature was increased from 25 o C to 60°C, CO peak area dropped from 688 to 551 mC. The distance between the peak centers decreased with an inc rease in the temperature, i.e. from two distinguishable peaks at 25 o C to the seemingly indistinguishable peak at 60°C. This shows that oxidation rates of the t wo CO ad-species are equivalent, at higher temperatures.

Figure 2.6. CO oxidation peaks obtained from baseline corrected CO-SCV s on CO saturated Pt/C electrode after applying pulse potential of 550 mV for 10 seconds, for temperatures 25, 45, 60 and 70°C.

Full document contains 80 pages
Abstract: Carbon monoxide (CO) poisoning of platinum anode significantly decreases the performance of proton exchange membrane fuel cells (PEMFC). Various techniques studied to remove CO from source or mitigate CO poisoning have met with limited success due to fuel loss or increases infrastructure requirements to meet the operating needs. We developed a twin cell electrochemical filter that is similar in construction of the PEMFC and remove CO with minimal fuel loss. In this CO in fuel gas is adsorbed on a Pt electrode and oxidized by applying a pulse potential. The CO kinetics on Pt electrode was characterized by employing cyclic voltammetry and chronoamperometry techniques. A fixed bed adsorber model was used to characterize the adsorption part of the filtering. The model was validated by demonstrating as a filter cell. The performance was compared with other CO removing techniques.