Ind. Eng. Chem. Res. 1996, 35, 1235-1242
1235
Analysis of Reaction Kinetics for Carbon Monoxide and Carbon Dioxide on Polycrystalline Platinum Relative to Fuel Cell Operation Richard J. Bellows,* Elise P. Marucchi-Soos, and D. Terence Buckley Exxon Research and Engineering, Route 22 East, Annandale, New Jersey 08801
Polymer electrolyte fuel cells operate at high efficiencies using pure H2 fuel. H2 produced by reforming hydrocarbons or alcohols contains CO and CO2 (COx) impurities which readily adsorb onto anode Pt electrocatalysts, reducing the efficiency for H2 electrooxidation. A COx inventory model is useful for describing the behavior of adsorbed CO on Pt electrocatalyst surfaces. The model compares three fluxes: (1) direct adsorption of CO, (2) the electroreduction of CO2, and (3) electrooxidation of adsorbed CO. The fluxes of CO2 electroreduction and CO electrooxidation were measured electrochemically on polycrystalline Pt electrodes (surface roughness ) 30100) in 1 N H2SO4 between 23 and 66 °C. This study concludes that, under polymer electrolyte fuel cell conditions, CO tolerance is achieved when the flux of CO electrooxidation balances the combined adsorption fluxes from both CO and CO2. Furthermore, under most conditions, CO adsorption will generally exceed the fluxes of CO2 electroreduction. 1. Introduction Fuel cells are emerging as promising candidates for powering electric vehicles. Fuel cell developers continue to demonstrate improved power density and improved efficiency. The developers believe that future fuel cells will achieve low production costs and emission-free operation. The polymer electrolyte fuel cell (PEFC) has received the most attention for mobile power applications. The operation of fuel cells is discussed in recent reviews (Appleby and Foulkes, 1989; Appleby and Yeager, 1986). The choice of fuels for fuel-cell-powered electric vehicles is an important consideration. Pure H2 is the preferred fuel for low-temperature fuel cells, especially the PEFC. H2, however, has many limitations as a fuel. There is no infrastructure to deliver H2 to the consumer. H2 refueling processes tend to be slow. Most H2 storage systems, such as compressed H2, are heavy and bulky. Various studies have concluded that the practical range of electric vehicles powered by stored H2 will be less than that for typical gasoline-powered vehicles (Bentley et al., 1994). Various groups have proposed generating H2 on board the electric vehicle by reforming low-cost liquid hydrocarbon or alcohol fuels (Loftus et al., 1994; Sutton and Vanderborgh, 1992). The resulting H2 contains CO and CO2 (COx) impurities. These impurities cause large polarization losses during electrooxidation of H2 (Wilson et al., 1993), which greatly reduce the fuel’s cell efficiency and power output. Efficient management of COx presents important design considerations for developing on-board H2 generators. Several strategies have evolved to reduce COx polarization. One approach involves using multiple processes to remove CO from the reformate gas (Sutton and Vanderborgh, 1992). Other approaches use modified electrocatalysts and/or in situ fuel cell processes (Gottesfeld, 1990), which make the electrodes more “COx tolerant”. In the phosphoric acid fuel cell, CO polarization losses were reduced by increasing the fuel cell operating temperature above 150 °C (Vogel et al., 1975). Combinations of these strategies are possible. The motivation for this effort was the need for an engineering model which could relate COx impurity levels to performance losses in PEFCs based on funda0888-5885/96/2635-1235$12.00/0
mental experiments. An inventory model provides a simple yet physically realistic basis for comparing COx reactions on the Pt electrodes using feed stoichiometry and electrochemical half-cell experiments. The model is useful for evaluating various design strategies for COx management of reformed liquid fuels. A major concern was the potential need to remove large amounts of CO2 from the H2, as is necessary for CO. If all CO2 must be removed from feed H2, then the “clean” H2 would be an equally attractive feed for alkaline fuel cells. Alkaline fuel cells have an advantage of not needing Pt electrocatalysts. 1.1. Reaction Network for Reformate Gases on Platinum Electrodes. Reformed fuel mixtures contain H2, CO, CO2, and H2O which undergo well-known reactions on Pt electrodes. The electrooxidation of H2 is a two-step reaction (Stonehart and Ross, 1975). The first step (1), dissociation, is rate limiting and requires two adjacent bare Pt sites. The second step (2), discharge, is relatively rapid on Pt.
H2 + 2Pt T 2Pt-H
(1)
Pt-H T Pt + H+ + e-
(2)
CO can adsorb directly onto either bare Pt sites (3) or Pt-H sites (4) . CO is well-known as a poison for the electrooxidation of H2 on Pt. CO binds strongly to Pt sites, thereby blocking them for the H2 dissociation reaction (Stonehart and Ross, 1975).
CO + Pt f PtdCO
(3)
2CO + 2Pt-H f 2PtdCO + H2
(4)
Acid electrolyte fuel cells were developed with the anticipation that the electrolyte would reject any CO2 in the feed gases. Acid rejection appeared to give acid electrolyte fuel cells a major advantage over alkaline fuel cells where CO2 reacts with the electrolyte. Recently, however, PEFC developers have reported that CO2 can polarize anode performance (Wilson et al., 1993; Swathirajan, 1994). At low potentials, CO2 is electroreduced (eq 5) by Pt hydrides (Giner, 1963). (The exact chemical nature of “reduced CO2” has been the subject of some controversy in the literature.) The PtdCO, © 1996 American Chemical Society
1236
Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996
formed by reducing CO2, polarizes H2 electrooxidation in the same manner as PtdCO formed by direct CO adsorption. Reaction (5) can be considered the surface equivalent of the well-known water gas shift reaction.
CO2 + 2Pt-H f PtdCO + H2O + Pt
(5)
Adsorbed CO from either reaction (3), (4), or (5) can be electrooxidized at higher electrode potentials via the “reactant pair” (eq 6) mechanism (Gilman, 1964). CO
PtdCO + PtdO f CO2 + 2Pt
(6)
electrooxidation can be followed electrochemically from the current given off by the formation of fresh Pt oxides (or hydroxides) (eq 7). More generally, PtdO should be considered PtOxHy.
Pt + H2O f PtdO + 2H+ + 2e-
(7)
“CO tolerance” is usually defined as the ability to electrooxidize H2 in the presence of CO at an acceptable polarization loss. CO tolerance is usually quantified at some current density in terms of the maximum CO concentration (typically ppm) which can be tolerated, as defined by some nominal polarization loss at the anode (typically 20-100 mV). Losses are referenced to performance on pure H2. Various groups have reported a wide range of CO tolerance levels in acid fuel cells, as summarized below. This range can be attributed to differences in electrode structure, operating conditions, electrolytes, electrocatalysts, and the definition of CO tolerance. Details of some reports are incomplete or not available. Studies on low Pt-loaded electrodes typically report that CO must be reduced to only a few (2-5) ppm (Wilson et al., 1993; Swathirajan, 1994; Thompsett and Cooper, 1994; Simpson et al., 1995; Schmidt et al., 1995). Reports on Pt-Ru electrodes claim higher CO tolerances (between 10 and 100 ppm) (Ralph et al., 1994; Schmidt et al., 1995; Iwase and Kawatsu, 1995). Other reports claim achieving CO tolerances as high as 20-1000 ppm on Pt or modified Pt (Cipriano, 1995; Stonehart, 1994). Cipriano reports that Pt is more “CO tolerant” during higher temperature (120 °C) operation. Knowledgeable sources have reported that Lawrence Berkeley Labs has developed Pt-Sn electrodes which can tolerate up to 10 000 ppm CO (Landgrebe, 1995). To date, however, published reports have not claimed specific CO tolerance levels (Gasteiger et al., 1995). Earlier reports published in the 1960s and 1970s suggest that higher tolerances are possible, using tungsten- and molybdenum-based electrocatalysts (Niedrach and Weinstock, 1965; Bohm et al., 1970). 1.2. Effect of COx on Anode Performance. Platinum is a very active electrocatalyst for H2. Modern well-designed gas diffusion electrodes, operating on pure H2, show virtually no polarization losses even at high current densities (∼1 A/cm2). However, even small amounts of CO in the H2 can increase polarization losses to unacceptable levels. Performance curves for PEFC electrodes with low Pt loadings have been reported as a function of added CO or CO2 (Wilson et al., 1993) and are shown in Figures 1 and 2, respectively. Characteristically, both sets of data show little or no polarization at low current densities. Polarization increases somewhat abruptly at higher current densities. These data show that, for operation at 500 mA/cm2, 5 ppm CO polarized performance about
Figure 1. H2/air PEFC polarization curves showing the effects of CO contamination for a 0.14 mg of Pt/cm2 thin film anode at 80 °C (source: Wilson et al., 1993).
Figure 2. H2/air PEFC polarization curves showing the effects of CO2 contamination for a 0.12 mg of Pt/cm2 thin film anode at 80 °C (source: Wilson et al., 1993).
150 mV and 25% CO2 polarized performance about 75 mV. (Polarization from the pure H2 performance is interpreted solely as an impurity poisoning effect at the anode.) A few ppm CO causes more polarization than 25% CO2. The relative poisoning of CO and CO2 is curious because, at equilibrium under PEFC operating conditions, the water gas shift reaction could produce about 100-200 ppm CO with a 75% H2/25% CO2 feed. CO at these equilibrium levels is a severe poison and would make it necessary to remove most of the CO2 from the feed. If most of the CO2 had to be removed from the reformed H2, a PEFC would have few advantages over an alkaline fuel cell. H2 electrooxidation activity in the presence of CO (iH2/CO) is reported to be a function of anode activity on pure H2 (i°H2(V, T, P)) multiplied by a poisoning parameter ([1 - ΘCO]2), where V ) potential, T ) temperature, P ) pressure, and ΘCO ) fraction of Pt surface covered by CO (Ross and Stonehart, 1974; Stonehart and Ross, 1975).
iH2/CO ) i°H2(V, T, P) [1 - ΘCO]2
(8)
Stonehart and Ross’s interpretation is that the limiting reaction mechanism requires two adjacent sites for (1) and that the probability of finding two adjacent sites is reduced by the square of the surface fraction not
Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996 1237
onto the electrode with the flux of CO removal by electrooxidation processes. A similar model is assumed to interpret recent Pt3Sn performance on H2/CO mixtures (Gasteiger, 1995).
dΘCO/dt )
∑(CO adsorption + CO2 reduction -
CO oxidation) (9)
Figure 3. Cyclic voltammetry [0.05-1.5 V at 50 mV/s] of polycrystalline Pt at 23 °C with and without CO in the electrolyte.
poisoned by CO. Equilibrium considerations suggest that CO coverage on Pt should be decreased by using lower CO partial pressures and/or higher temperatures. Recent studies report [1 - ΘCO] on Pt in gas diffusion electrodes as a function of temperature and CO levels (Thompsett and Cooper, 1994; Ralph et al., 1994). These recent studies report that CO approaches complete monolayer coverages, even at levels as low as 1 ppm. It should be noted that these recent studies model the reduced H2 activity as first order in 1 - ΘCO. 1.3. Pt Surface Studies Using Cyclic Voltammetry and Spectroscopy. Cyclic voltammetry of polycrystalline Pt electrodes in the presence and absence of CO has been reported by many groups (Stonehart and Ross, 1975; Parsons and VanderNoot, 1988). Many CO studies report a strong electrooxidation peak near 900 mV (see Figure 3). Strong CO adsorption onto Pt sites prevents the formation of underpotential deposited hydrogen at potentials below 350 mV. At potentials above 800 mV, the initial oxidation of CO correlates with the initial formation of Pt hydroxides/oxides, as observed under N2 atmospheres. Many groups have proposed that CO oxidation proceeds via the reactant pair mechanism with surface Pt oxides or hydroxides (eq 6) at these potentials. A series of papers reported that various surface ad-atoms shift the onset of CO oxidation on Pt from 800-900 mV to lower potentials (Watanabe and Motoo, 1975; Motoo and Watanabe, 1976, 1980; Motoo et al., 1980; Shibata and Motoo, 1985). Effective surface ad-atoms included Sn, Ru, Ge, Bi, S, and Se. IR spectroscopic studies of CO on Pt report that the major species is linearly bonded CO with a lesser amount of bridge-bonded CO. There is controversy concerning the presence of species such as triply-bonded CO, COH, and CHO species (Parsons and VanderNoot, 1988). There has also been controversy over the chemical nature of reduced CO2. IR spectral studies of reduced CO2 report the same IR frequencies as for adsorbed CO on Pt (Beden et al., 1982; Rodes et al., 1993; Taguchi et al., 1994). 2. Theory: COx Inventory Model An inventory model for the accumulation of CO on polycrystalline Pt is useful for comparing CO fluxes. This model combines the three major reactions which add or remove CO to the electrocatalyst surface: (1) direct adsorption of CO, (2) electroreduction of CO2, and (3) electrooxidation of adsorbed CO. The model assumes that all CO on the electrode are equivalent. The model quantitatively compares CO and CO2 adsorption fluxes
In this model, CO adsorption onto Pt can be assumed to be kinetically very fast. Gas-phase adsorption experiments of CO onto Pt show high-sticking coefficients (Yates et al., 1980). Similar kinetic experiments are not possible in the solution phase. The access of gaseous reactants to the electrocatalyst in gas diffusion electrodes will be rapid. Therefore, CO in the anode feed will be strongly adsorbed onto any available Pt sites. The model assumes that CO approaches a complete monolayer coverage, even for CO as low as 1 ppm. In the discussion of the inventory model, the COx fluxes are interpreted as equivalent current densities in a PEFC in order to compare fluxes in electrochemical units. For purposes of comparison, we assumed a PEFC electrode operating at 500 mA/cm2 on H2 feed containing various levels of COx. The PEFC electrode was assumed to have a low Pt loading (∼100 µg of Pt/cm2) and an equivalent surface roughness of about 100 cm2/cm2. Anode feed stoichiometry will be slightly above 1.0, limited by efficiency considerations. Sample calculations describing the conversion of experimental results into equivalent PEFC current densities are available as supporting information. 3. Experimental Procedures Electrochemical techniques were used to measure the rates for CO electrooxidation and CO2 electroreduction on polycrystalline Pt in 1 N H2SO4 at ambient pressure and at temperatures between 23 and 66 °C. The electrolyte was made with purified sulfuric acid (Ultrex Grade from Baker) and triply distilled water. Experiments were conducted using an EG&G PAR potentiostat/galvanostat Model 263A and ECHEM software. Voltages were referenced to the reversible hydrogen electrode (RHE) using a dynamic H2 electrode (Gileadi et al., 1975). The working electrodes were platinized Pt using a technique recommended in Gileadi (1975). Pt surface roughness (R) was typically between 30 and 100 cm2/cm2, as measured by integration of the Pt-H peaks desorption region. All experiments were conducted under gentle gas sparging, which enhanced mass transport to the electrodes. CO2 electroreduction kinetics were measured using two techniques which give complimentary results. CO2 was adsorbed onto a platinized Pt wire having a superficial area of about 1 cm2. The Pt electrode roughness was about 100 cm2/cm2. The electrolyte was first sparged with N2 for 20-60 min to remove dissolved O2 and then was saturated with pure CO2. Prior to each experiment, reduced CO2 was oxidatively removed by holding the electrode at 1550 mV/RHE for 3 min. In the first technique, an electroreduction current was measured directly using chronoamperometry (CA). Following the electrooxidation at 1550 mV/RHE, the electrode potential was stepped into the platinum hydride region, to 150 mV/RHE, where an electroreduction current was measured at constant potential versus time. In the second technique, linear scanning voltammetry (LSV) was used to measure the current required to oxidize the electroreduced CO2 as a function of adsorp-
1238
Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996
Figure 4. Chronoamperometry for CO2 electroreduction transient on polycrystalline Pt at 25 °C. Data corrected for residual O2 current.
tion times. The electrode is first held at 1550 mV/RHE to remove preadsorbed CO2. Then the electrode potential was stepped to 150 mV/RHE and held briefly to electroreduce CO2 and then scanned at 10 mV/s to 1550 mV/RHE to oxidize any reduced CO2. A series of experiments measured electroreduction vs time. Many groups have reported CO2 electroreduction transients using this technique (Brummer and Cahill, 1969; Marcos et al., 1990). CO electrooxidation experiments used Pt electroplated onto the end of a 0.20 cm2 glassy carbon rod. The Pt had roughness factors of 30-60 cm2/cm2. The electrolyte was first exhaustively sparged with N2 for 20-60 min to remove dissolved O2. Then the electrolyte was saturated with pure CO. Before each measurement the electrode was cleaned by electrooxidation at 1550 mV/RHE for 3 min. The electrode potential was then stepped to the adsorption potential for 3-10 min. Typical adsorption potentials were 50, 100, and 450 mV/ RHE. LSV scans were recorded at rates between 0.1 and 10 mV/s and between 25 and 66 °C. Similar experiments are reported by many authors (Stonehart and Ross, 1975). 4. Experimental Results 4.1. CO2 Electroreduction. A transient current is measured during CO2 electroreduction on Pt at 25 °C, as shown in Figure 4. This current falls off rapidly during the first 100 seconds followed by a gradual approach to zero current. This current arises from reactions (5) and (2). Each adsorbed CO2 (reaction (5)) creates one bare Pt site which immediately proceeds to form Pt-H. Pt-H formation is the reverse of reaction (2), giving rise to a measurable reduction current. As the surface gradually becomes covered with reduced CO2, fewer adsorption sites are available. The adsorption rate gradually approaches zero as the surface becomes fully covered with reduced CO2. The presence of residual amounts of dissolved O2 in the electrolyte can cause a finite limiting current. Exhaustive sparging can reduce the O2 limiting current to negligible levels. The data shown in Figure 4 have been corrected by subtracting about 1 µA of O2 limiting current. A kinetic model for CO2 electroreduction is proposed which assumes that each surface Pt site is covered by either a hydride or a reduced CO2. CO2 electroreduction is not first order in Pt-H surface sites, because the data
Figure 5. Integrated second-order model correlating electroreduced CO2 vs adsorption time on polycrystalline Pt at 25 °C in 1 N H2SO4.
Figure 6. Linear scanning voltammetry for CO electrooxidation at 10 mV/s on polycrystalline Pt at 25 °C in 1 N H2SO4. Dependence on adsorption potential.
are nonlinear on the semilog plot shown in Figure 4. Equation 5 suggests that the adsorption rate could be second order in surface Pt-H concentration. Equation 10 proposes a model for CO2 electroreduction where iCO2 Red. ) CO2 reduction current, k ) reaction constant, PCO2 ) partial pressure of CO2, ΘPt-H ) surface fraction of Pt hydrides.
iCO2 Red. ) k(PCO2)(ΘPt-H)2 ) k(PCO2)[1 - ΘCO]2 (10) The transient in Figure 4 fits the second-order model (10) over a current range of 2-1/2 orders of magnitude. The data are consistent with an interpretation of the reaction mechanism which involves two adjacent Pt-H sites. CO2 electroreduction kinetics using the second technique give complimentary results. An integrated form of the second-order model is used in Figure 5 to correlate reduced CO2 vs the electroreduction time. The data fall onto a straight line, which is again consistent with the proposed second-order kinetic model. 4.2. CO Electrooxidation. CO electrooxidation depends on the potential at which CO is adsorbed. Two different LSV experiments at 10 mV/s are compared in Figure 6. Adsorption at higher potentials, for example, at 450 mV, shows a pronounced single oxidation peak at 900 mV with no measurable oxidative current below
Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996 1239
Figure 7. Linear scanning voltammetry for CO electrooxidation at 10 mV/s on polycrystalline Pt in 1 N H2SO4. CO adsorption at 450 mV. Dependence of CO electrooxidation on temperature.
Figure 8. Linear scanning voltammetry for CO electrooxidation at 10 mV/s on polycrystalline Pt in 1 N H2SO4. CO adsorption at 100 mV. Dependence of CO electrooxidation on temperature.
800 mV. Adsorption at lower potentials, for example, at 100 mV, shows a small oxidation peak beginning at about 400 mV and centered near 500 mV, with a second larger oxidation peak centered near 700 mV. Both types of CO oxidation behavior vary consistently as a function of increasing temperature or decreasing scan rate. The area under both curves approaches a complete monolayer of CO, when compared to the area of surface hydrides measured during cyclic voltammetry under N2. At 450 mV, CO adsorbs onto bare Pt. In contrast, at 100 mV, CO adsorbs onto a surface covered with Pt-H. It is not apparent why CO adsorbed onto Pt-H should be more electroactive than CO adsorbed onto bare Pt. Increased temperature shifts both sets of electrooxidation features to progressively lower potentials, as shown in Figures 7 and 8. For example, at 66 °C, CO adsorbed at 450 mV shows that the oxidation peak shifts to about 620 mV, with no significant oxidation before 500 mV. In contrast, when CO is adsorbed at 100 mV and 66 °C, the peak in the oxidation rate shifts to 580 mV and the onset of oxidation begins near 100 mV. This study shows that, at higher temperatures, CO adsorbed at potentials in the Pt-H region remains more easily oxidized than CO adsorbed onto Pt in the double-layer region. The combination of low adsorption potentials and higher temperatures is more effective for oxidizing
Figure 9. Linear scanning voltammetry for CO electrooxidation on polycrystalline Pt in 1 N H2SO4 at 66 °C. CO adsorption at 50 mV. The insert shows a semilog plot of a 0.1 mV/s scan between 150 and 350 mV. Dependence of CO electrooxidation on potential scanning rate.
CO at low potentials on Pt than many of the various ad-atom modifications which were cited above. CO electrooxidation rates approach quasi steady state at low scan rates. The effect of scan rate is shown in Figure 9 for CO absorbed at 100 mV and 66 °C. (The Figure 9 inset shows a semilog plot of the 0.1 mV/s scan between 50 and 350 mV.) Current plateaus at about 0 and 25 µA for 0.1 and 10 mV/s scans, respectively, represent double-layer charging effects. Currents below 100 mV include some H2 evolution. During scans at 0.1 mV/s, the CO oxidation current increases up to about 1 µA between 50 and 200 mV. CO oxidation approaches a diffusion-limited rate at higher potentials (∼45 µA above 450 mV). In the slow scan below 300 mV, oxidized CO can be immediately replaced by freshly diffused CO so that the surface coverage should remain close to a complete CO monolayer. At potentials above 450 mV, the CO oxidation is diffusion rate limited, so that CO coverage would gradually become depleted. Many previous workers have studied CO electrooxidation on Pt electrodes, but relatively few have noted the dependence upon the adsorption potential (Grambow and Bruckenstein, 1977; Kita et al., 1988; Gutierrez and Caram, 1991). The dependence of CO as a function of adsorption potential has been studied using IR and mass spectroscopy (Kunimatsu et al., 1986; Taguchi et al., 1994; Iwasita and Vogel, 1988). The mechanism of CO oxidation following adsorption at low potentials is not well understood. It seems likely that water acts as the oxidant in a reactant pair mechanism. This dependence on CO adsorption potential deserves more experimentation to clarify its mechanism. 5. Discussion Using the COx Inventory Model The COx inventory model (eq 9) is useful for explaining some of the behavior reported in previous studies of COx tolerance. The model also provides a quantitative basis for evaluating both new materials for CO tolerance and various alternatives for COx control. 5.1. Condition for “CO Tolerance”. A CO tolerant electrode must have sufficient active area so that H2 electrooxidation can proceed at low polarization. CO tolerance requires that the fraction of the surface not poisoned by CO, [1 - ΘCO], be as large as 1-10% of the total Pt surface. However, as noted above, studies conducted at PEFC temperatures suggest that CO
1240
Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996
coverage on Pt approaches a full monolayer even with CO levels as low as 1 ppm. Such high equilibrium coverages would preclude any CO tolerance. Nonequilibrium CO coverages are possible whenever the flux of CO removal (via CO electrooxidation) exceeds the flux of CO accumulation (from both CO adsorption and CO2 electroreduction). The inventory model defines a nonequilibrium condition for CO tolerance as when the fluxes:
CO electrooxidation g (CO adsorption + CO2 electroreduction) (11) If the accumulation terms are larger, then CO will continue to build up on the Pt surface, CO coverage will approach a complete monolayer, and polarization losses will become unacceptably large. However, if the electrooxidation term is larger, then CO coverage will gradually decrease, providing additional sites for H2 activation and lowering electrode polarization. Electrode polarization and CO coverage will gradually adjust until the fluxes are in balance. 5.2. CO Adsorption Rate. CO adsorption onto the Pt in gas diffusion electrodes is feed rate limited. CO can be adsorbed very rapidly so that all of the feed CO will be adsorbed if any bare Pt is available. CO adsorption fluxes can be roughly quantified by assuming that all CO in the feed is uniformly adsorbed onto Pt surfaces within the fuel cell electrode. (A note of caution is necessary on this assumption, because CO adsorption will be faster near the gas inlet.) The adsorption rate is estimated by using a mass balance where all CO in the feed is uniformly adsorbed over the superficial electrode area. The assumed CO adsorption rate is thus proportional to CO impurity levels, electrode current density, and H2 feed stoichiometry at the anode. For modeling purposes, we express CO adsorption fluxes in electrochemical units of “equivalent current density”. (CO adsorption is not an electrochemical reaction so that its adsorption does not generate any current. However, eventual electrooxidation of the CO would generate a current.) The model expresses the CO adsorption rate as an equivalent oxidation current by assuming two electrons to oxidize each molecule of CO. Hence, the CO adsorption flux can be expressed as:
2 × 10-12 mol of CO/cm2/s ≈ 1 µA/cm2
(12)
Mass balances predict that the equivalent current density for CO is about 1 µA/cm2 for each 1 ppm of CO in the feed H2 under the assumed PEFC conditions (Pt surface roughness ) 100 cm2/cm2, current density ) 500 mA/cm2). For example, a H2 feed containing 50 ppm CO would adsorb CO at an average equivalent current density of 50 µA/cm2. 5.3. CO2 Electroreduction Rate. Analysis of CO2 electroreduction experiments suggests that CO2 fluxes on PEFC electrodes will be kinetically limited rather than feed rate limited. Furthermore, CO2 electroreduction kinetics decrease rapidly as ΘCO increases, as shown in Figure 4. CO2 electroreduction fluxes become very small, dropping below 1 µA/cm2 after a few hundred seconds (either in the experiment or by implication under the assumed PEFC operation). CO2 reduction fluxes can also be expressed as equivalent current density for oxidizing CO, assuming 2 electrons/molecule to oxidize reduced CO2. 5.4. CO Electrooxidation Rate. Equation (11) predicts CO tolerance by requiring 1 µA/cm2 of CO
oxidation current to balance each 1 µA/cm2 of CO adsorption flux (or in effect each 1 ppm of CO in the feed under the assumed PEFC conditions). CO electrooxidation rates on polycrystalline Pt vary considerably, depending on the oxidation potential, the temperature, and the CO adsorption potential. CO does not show measurable CO oxidation rates on Pt until 500800 mV after the CO has been adsorbed at potentials in the double-layer region, as shown in Figure 7. Using eq 11, the behavior in Figure 7 cannot explain any useful CO tolerance, even for levels as low as 2-3 ppm CO in the feed H2. However, CO oxidation does proceed at measurable rates when the CO is adsorbed at potentials in the Pt-H region, as shown in Figures 8 and 9. Low potential oxidation is necessary to explain the approximate order of magnitude reported by recent CO tolerance studies in PEFCs. For example, Pt electrodes oxidize CO at currents 0.03-1 µA between 50 and 200 mV polarization, as shown in the inset of Figure 9. These rates project to 0.3-10 µA/cm2 under the assumed PEFC operation, suggesting that Pt electrodes could tolerate about 0.3-10 ppm CO in the feed at these polarizations. This level of CO tolerance is in approximate agreement with the polarization curves shown in Figure 1. CO oxidation rates increase at higher anode polarization as shown in Figure 9, implying that higher polarization is necessary to balance higher levels of CO in the feed. This is also qualitatively consistent with the trends in Figure 1. CO electrooxidation rates increase with increasing temperature, as shown in Figure 8. However, CO adsorption flux is feed rate limited, and therefore CO adsorption is effectively independent of temperature. On this basis, eq 11 predicts that higher temperature operation will improve an electrode’s CO tolerance. This conclusion is consistent with Dow’s report of improved CO tolerance at 120 °C (Cipriano, 1995). (However, the higher temperature may have also reduced equilibrium CO coverage.) Equation 11 can be used to set quantitative design targets for improved electrocatalyst materials by relating CO oxidation rates in half-cell experiments to CO tolerance in the fuel cell. Many groups are studying modifications of Pt such as Pt-Ru and Pt-Sn. If these modifications can oxidize CO at lower potentials, then eq 11 suggests that they should tolerate more CO in the PEFC feed. (However, these modifications may also effect CO adsorption strength, which would also improve CO tolerance by reducing equilibrium CO monolayer coverage.) 5.5. CO2 Removal Processes. CO2 is a weak anode poison, because the kinetics of CO2 electroreduction are very slow. In general, CO adsorbs onto Pt much more rapidly than CO2. This conclusion is in approximate agreement with Figures 1 and 2, where 5 ppm CO causes more polarization than 25% CO2. CO adsorption kinetics are several orders of magnitude faster than that for CO2 reduction per unit of impurity concentration. A comparison of the above experimental data shows that, as the adsorption time proceeds, even trace amounts of CO (2 ppm) can adsorb at higher fluxes than much larger concentrations of CO2 (1 atm ) 106 ppm). In effect, eq 11 predicts that it will be difficult to measure any CO2 polarization unless the feed H2 is virtually “CO free” (
