Direct Measurement of iR-Free Individual-Electrode Overpotentials in

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J. Phys. Chem. C 2007, 111, 6512-6523

Direct Measurement of iR-Free Individual-Electrode Overpotentials in Polymer Electrolyte Fuel Cells Piotr Piela,† Thomas E. Springer, John Davey, and Piotr Zelenay* Los Alamos National Laboratory, Materials Physics and Applications DiVision, MS D429, P.O. Box 1663, Los Alamos, New Mexico 87545 ReceiVed: NoVember 17, 2006; In Final Form: February 8, 2007

A method of measuring error-free potentials of individual electrodes directly in the polymer electrolyte fuel cell and under a full range of load is introduced. Successful measurement is accomplished by placing a reference electrode in ionic contact with the active layer of a fuel cell electrode of interest, that is, away from the electrolyte membrane. Half-cell measurements with the proposed reference electrode configuration are compared with measurements in two other fuel-cell reference electrode arrangements to illustrate the conditions necessary for directly achieving correct measurement of potentials (overpotentials). Functioning of the fuel cell reference electrodes is explained through modeling of electrical potential and reactant distribution in the working fuel cell.

Introduction Polymer electrolyte fuel cells (PEFCs) are promising candidates for powering electrical devices ranging from small electronics to electric cars. Since the space exploration programs, where PEFCs debuted over 35 years ago, great effort has been invested in maximizing the performance-to-cost ratio of these energy converters in order for them to become popular consumer products.1 Today, the moment of commercial introduction of fuel cells seems to be close for only a few selected applications, for example, direct methanol fuel cells for portable electronics.2-6 For both stationary power applications and automotive applications, this goal appears to be much further ahead. Fuel cells in all of their varieties are still the domain of research laboratories, rather than industry, with many fundamental performance as well as cell characterization issues still awaiting solutions. One such issue in the area of fuel cell performance characterization is a successful reference electrode design, which currently does not exist in spite of the fact that reliable measurement of individual fuel cell electrode overpotentials under load is of great importance to fuel cell technology. Knowledge of the overpotentials makes it possible to study the fuel cell electrodes separately in situ and ultimately to determine contributions of unit phenomena to the polarization of the fuel cell with fewer assumptions (e.g., with in situ electrochemical impedance spectroscopy of individual fuel cell electrodes7). To successfully measure an electrode overpotential, one needs (i) a reference electrode of fixed potential placed in ionic contact with the same electrolyte as the electrode of interest and (ii) knowledge of the iR (current density times internal resistance) drop between the reference electrode and the electrode of interest. A stable reference electrode potential is achieved by placing the reference electrode in contact with a fast red-ox couple and/or ensuring that the current drawn through it is negligible. In classical electrochemistry, knowledge of the iR * Corresponding author. Phone: (505) 667-0197; fax: (505) 665-4292; e-mail: [email protected]. † Present affiliation: Industrial Chemistry Research Institute, ul. Rydygiera 8, 01-793 Warsaw, Poland.

drop is acquired by reducing R to almost zero with the use of a Luggin capillary, often used in combination with excess supporting electrolyte and low current density. Alternatively, the iR drop can be calculated from known values of local i and R. Because modern fuel cells often operate at high current density, the internal voltage drop is by no means negligible, easily reaching values as high as several hundred millivolts. To determine the iR drop between the reference and fuel cell electrodes with thin-slab geometry, one would need to place a reference electrode precisely within the thin electrolyte in the main fuel cell current path. Assuming a linear potential drop in the electrolyte in the main current path, one could then calculate the iR drop from the known distances of the reference electrode to the fuel cell electrodes, the specific conductivity of electrolyte, and current density. Unfortunately, this approach is impractical first and foremost because of the very low thickness of electrolyte layers used (e.g., Nafion 112 membrane, commonly used in hydrogen-fueled PEFCs, is merely 50 µm thick) and also because the assumption of a linear potential drop in the electrolyte in the main current path, a consequence of invariability of specific conductivity with distance through the electrolyte, is very often incorrect. Moreover, it has been shown theoretically,8-12 that the electrical potential in the electrolyte layer protruding from the electrolyte-electrode assembly is a strong function of the fine details of the electrode edge geometry and the fuel cell current density/electrode overpotentials. Therefore, when a reference electrode is placed in contact with the protruding part of the electrolyte layer, the necessary effective iR drop has to be obtained by at least two-dimensional modeling of electrical potential distribution in the electrolyte layer. To highlight the results of such modeling, very relevant to the reference electrode placement problem, let us briefly consider an electrolyte-electrode assembly in which there is a relative geometrical misalignment of the anode and cathode. With this geometry, there is an edge of the active area, where one electrode is overhanging the other. Because of the imperfections in the electrolyte-electrode assembly fabrication, this kind

10.1021/jp067669y CCC: $37.00 © 2007 American Chemical Society Published on Web 04/07/2007

iR-Free Individual-Electrode Overpotentials of misalignment is always present. He and Nguyen10 and Liu et al.11 found that (i) Beyond a distance approximately one electrolyte thickness away from the edge of the protruding electrode, the electrical potential in the electrolyte remains constant. (ii) When one electrode overhangs the other more than ca. three times the electrolyte thickness, the electrical potential in the electrolyte far from the electrodes is practically equal to the electrical potential in the electrolyte at the edge of the overhanging electrode. (iii) For the configuration described in ii, the electrical potential in the electrolyte at the edge of the protruding electrode is approximately equal to the electrical potential in the electrolyte at the protruding electrode but in the main fuel cell current path, only when the kinetic resistance (overpotential divided by current density) of the fuel cell reaction proceeding on the protruding electrode is negligible compared to the electrolyte resistance. Otherwise, that is, when the kinetic rate is slow, the two potentials differ markedly and the difference is obviously current-density-dependent. It is quite clear that quantitative interpretation of the reference electrode measurements with the reference electrode placed in contact with the electrolyte extending beyond the perimeter of the fuel cell electrode sandwich requires detailed modeling. However, modeling of fuel cells is always based on many assumptions, leading to uncertainty of the results. Consequently, there is a clear need for having the overpotential of fuel cell electrodes measured directly either to avoid uncertainties involved in the modeling or to validate various models developed to date. In this contribution, we evaluate a method for a direct measurement of iR-free, individual-electrode overpotentials in fuel cells with thin-slab geometry. Although we focus on PEFCs operating in either hydrogen-air or direct methanol-air (DMFC) mode, the proposed method, in principle, should be applicable to any type of fuel cell or other electrochemical energyconversion device with such geometry. The overpotential measurement can be accomplished by placing a reference electrode in direct ionic contact with the catalyst layer of the fuel cell electrode of interest, away from the membrane. This helps to eliminate problems associated with a complex electrical potential distribution in the electrolyte layer of a fuel cell under load. In the theoretical portion of the paper, we seek to understand the necessary conditions for the new reference electrode setup to permit extraction of correct information on the polarization of individual electrodes in a fuel cell. This is accomplished by the modeling of the electrical potential distribution in the membrane, catalyst layers, and ionic conduits from the catalyst layers to reference electrodes. Experimental Section Fuel Cell Testing. Fuel cell testing was conducted in singlecell hardware modified to accommodate two reference electrodes, one on each side of the cell (see below). Membraneelectrode assemblies (MEAs) were prepared using a commercial Pt-Ru black HiSPEC 6000 catalyst (Johnson Matthey, UK) for the anode and a commercial Pt black HiSPEC 1000 catalyst (also Johnson Matthey, UK) for the cathode. Catalysts were applied to Nafion 117 membranes directly.13 The active surface area of the MEA was 22 cm2. Although such designed cells were tailored for highly efficient DMFC operation, they could also be used successfully in the hydrogen-air mode.

J. Phys. Chem. C, Vol. 111, No. 17, 2007 6513 The fuel cell temperature was typically in the 75-80 °C range. Unless specified otherwise, the cathode in DMFC testing was operated on dry (not humidified) air, using a low flow-rate and zero backpressure (an absolute pressure of 0.76 atm at the Los Alamos altitude ∼ 2300 m). Methanol at 0.3 M was used to supply the anode. Methanol solutions were prepared by mixing spectral-grade methanol with the Millipore water. In hydrogen-air mode, both gases were operated at high flow-rate, pressurized to 2 atm, and strongly humidified. Steady-state cell polarization plots were recorded using fuel cell test stations (Fuel Cell Technologies, Inc.). During the polarization experiments, the cell voltage was first stepped down from the open circuit value to its lower limit and then back up. In this communication, runs in both directions are shown each time. To record a methanol anode polarization plot, the cell was operated in a driven mode using an external power supply (HP 6033A, Agilent). In this case, humidified hydrogen was fed to the fuel cell cathode, which in the anode polarization mode served as a hydrogen-evolving counter/quasi-reference electrode. The incoming hydrogen stream was prehumidified at 93 °C and back-pressurized to 1.5 atm to ensure reversible electrochemical conditions. The conditions on the methanol anode side were chosen to match those in the methanol-air mode. The same equipment was used to record steady-state hydrogen-hydrogen cell polarization (“hydrogen pump” experiment). In this case, two independent but identically preconditioned streams of hydrogen were directed to the anode and cathode chambers of the cell. Unless specified otherwise, hydrogen preconditioning and temperature of the hydrogen-hydrogen cell corresponded to the conditions used on the hydrogen side in hydrogen-air or methanol anode polarization experiments. Reference Electrode Setup and Measurements. The reference electrode setup used in this work is illustrated schematically in Figure 1. Single-cell hardware commonly used in our laboratory was modified to accommodate two reference electrodes, one on each side of the cell, as shown in the top left of Figure 1. The reference electrodes were platinum black-covered wires coated with Nafion by dipping in a 5 wt % Nafion suspension (Aldrich). These wires were inserted into the cell through openings in the end plates and flow fields and fixed in place by means of plastic compression fittings to ensure cell sealing and electrical isolation of the reference points from the electrical poles of the fuel cell (end plates). The reference electrode openings in the hardware were such that the reference wire could be inserted to touch the MEA and at the same time remain in intimate contact with the reactant stream of the respective fuel cell electrode (cf. Figure 1, top right). In this communication, RA and RC, with corresponding potentials VRA and VRC, denote reference electrodes in contact with the anode (A) and cathode (C) side of the MEA, respectively. Each of the reference electrodes was operated as a dynamic hydrogen electrode (DHE), which means that a small and constant hydrogen-evolution current was passed through the wire. The fuel cell electrode opposite to a reference electrode served as the counter electrode for that DHE. Each of the two DHE electrodes was driven with a dedicated Keithley 2410 source/ meter (Keithley), and the DHE circuits were electrically independent from each other as well as from the main fuel cell circuit. It was verified experimentally that, as long as the apparent DHE current density was kept in the range of 0-50 mA cm-2, this parameter had virtually no influence on the recorded overpotential of the individual electrodes, or on the fuel cell response. The observed sensitivity of potential readings versus the reference electrodes to the DHE current density was

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Figure 1. Reference electrode configurations (see the experimental section). In a given test, both RA and RC had the same configuration, i.e., a, b, or c. All three configurations are locally cylindrically symmetric with the rotational axes of symmetry indicated.

due to the changing ohmic drop in the ionic connection between the reference electrode and the MEA and, in the case of RC, to a changing red-ox environment around the reference electrode (transitions between an “air reference” at low DHE current and a “hydrogen reference” at high DHE current). A total of five voltages, (VA - VRA), (VC - VRA), (VA - VRC), (VC - VRC), and (VC - VA), were collected automatically during a steadystate polarization of the fuel cell using a multichannel stack testing station and software. Three reference electrode configurations were studied experimentally. In the first situation (Figure 1a), later referred to as “configuration a”, the reference electrode was touching the MEA in an area of exposed membrane through a hole in the gas-diffusion layer (backing). The contact point was at least 10 times the membrane thickness away from the edge of the catalyst layer on that side. The catalyst layer on the opposite side of the membrane extended at least 15 times the membrane thickness beyond the contact point of the reference electrode. The second situation (Figure 1b), later referred to as “configuration b”, differed from the first one in that the reference electrode was touching the MEA in an area of exposed catalyst layer that was applied this time over the membrane to completely match the catalyst layer on the opposite side. The diameter of the hole in the gas-diffusion layer, through which the reference electrode was inserted, was at least 20 times the membrane thickness. Finally, in the third situation (Figure 1c), later referred to as “configuration c”, which, as we show below, constitutes the

most successful fuel-cell reference electrode setup, the continuous structure of the MEA-backing sandwich was preserved. For the purpose of contacting the reference electrode with the MEA, the void space of the gas-diffusion layer was partially filled with recast Nafion in a small area around the reference electrode opening. In this way, an ionic “wire” was formed, through which the reference electrode could be contacted with the catalyst layer. The void volume fraction of the Nafion-treated section of the backing was only slightly lower than that of the untreated backing (ca. 50% in-cell, treated vs ca. 70% in-cell, untreated) and the section could still pass reactant gas and electronic current. Thus, current generation in the catalyst layer adjacent to the reference electrode was maintained essentially the same as in the rest of the catalyst layer. In a given test, both RA and RC had the same configuration, that is, a, b, or c. One should note (Figure 1, parts a-c) that all three experimental configurations were locally cylindrically symmetric. This entailed the same symmetry for the modeling space. Modeling In order to understand why and when the reference electrode configuration c proposed in this article provides for iR-free individual-electrode overpotential measurement in a working fuel cell, we have modeled the electrical potential distribution in representative fragments of the MEA for the three cases illustrated in Figure 1 using COMSOL Multiphysics software

iR-Free Individual-Electrode Overpotentials

J. Phys. Chem. C, Vol. 111, No. 17, 2007 6515 TABLE 1: Modeling: Space Regions and Corresponding Active Variables

Figure 2. Spatial geometries for modeling of electrical potential distribution in the vicinity of reference electrodes (see the modeling section). (b) Configuration from Figure 1b; (c) configuration from Figure 1c. Region numbers 1-5 from Table 1 indicated in parentheses. Rotational axes of symmetry correspond to those indicated in Figure 1.

(COMSOL, Inc.), an environment for numerical solution of partial differential equations. Using the DMFC example, we will initially show that configuration a is equivalent to a reference placed on the bare membrane beyond the perimeter of the MEA, whose opposite fuel cell electrode protrudes toward the reference. Then we will compare the modeled performance of configurations b and c. General Model Assumptions. In a real, operating fuel cell the local potential varies along the reactant flow streams as reactant concentrations and humidity shift, as well as lateral to the reactant flow stream due to the presence of flow field lands. The only approximate constant potentials are the electronic anode- and cathode-current-collector potentials, giving a uniform cell voltage. Concentration-current paths redistribute the potentials in all three dimensions of the MEA to yield this uniform cell voltage. Because they are of a size much smaller than the size of the fuel cell, even “perfect” reference electrodes must be recognized to measure only a local potential. For all reference electrode configurations studied here, a circular region of the fuel cell electrode is modified with the understanding that the diameter of the modified region is small enough to expect a negligible current density shift from feed stream concentration variation across it. At the same time, the diameter is very large relative to the thickness of the membrane and a little larger than the size of the flow field features (lands and channels), cf. Figure 1, top right. For the purpose of this modeling, we chose to assume that there is no variation of current density under the reference caused by the presence of lands and channels. Thus, with the uniform local current density, the reference electrodes may be modeled using cylindrical symmetry. Nonetheless, the practical reference electrode must be envisioned as reporting an average potential from under the land and channel. The model built was macrohomogeneous, steady-state, isothermal, and was based on two two-dimensional, cylindrically symmetric spatial geometries depicted in Figure 2. Labels b and c of this figure correspond to configurations b and c, respectively. For simplicity, the modeled side of the cell was the methanol anode side. In the model, two types of conducting phases were distinguished, the ionically conducting polymer

region number

description

active variables

1 2 3 4 5

fuel cell membrane catalyst layer diffusion backing with ionomer ionomer around reference metal diffusion backing without ionomer

Φ Φ, V, C Φ, V, C Φ V, C

electrolyte phase and the electronically conducting carbon or metallic catalyst phase. Correspondingly, the electrical potential of the former, called the ionic potential (Φ), was distinguished from the electrical potential of the latter, called the electronic potential (V). The purpose of the model was to calculate these two functions of the space coordinates along with a third one, methanol concentration (C). Not all of the space regions depicted in Figure 2 required all three functions. For example, we assumed that methanol penetrated only the regions where there was void space. To include the methanol influence on the cathode potential (methanol crossover), we used an empirical formula (see below). Thus, the membrane region did not involve C. Table 1 lists the regions and the functions considered for each region. Governing Equations. The following partial differential equations governed functions Φ, V, and C:

-∇σi∇Φ ) 6FR

(1)

-∇σe∇V ) -6FR

(2)

-∇D∇C ) -R

(3)

Equations 1 and 2 describe the balance of charge in the ionically and electronically conducting phases, respectively (Poisson’s equations when R * 0, Laplace’s equations when R ) 0). σi and σe are effective conductivities of the respective phases. Equation 3 is the methanol transport (balance of methanol species) equation assuming Fickian diffusion with effective diffusion coefficient D. The rate of methanol oxidation reaction, R, was nonzero only in the catalyst layer, where it was described by the following formula:

R ) k a AV

0 C e(V-Φ-Ea )/ba C + Clim

(4)

Equation 4 reflects the Tafel-like dependence of the methanol oxidation kinetic rate on the electrode potential. It also accounts for the fact that sufficiently above a certain methanol concentration, Clim, the reaction is zero-order on methanol, whereas as the concentration is lowered below this value the reaction gradually becomes first-order. The adjustable parameters in eqs 1-3, that is, σi, σe, and D, have been established for the different regions from Table 1 individually based on the known values for the constituent bulk materials and their volume fractions in the regions by simple multiplication of the two. Because for all electronically conducting regions σe was 3-4 orders of magnitude higher than σi, we have used just one high σe value (100 Ω-1cm-1) for all of those regions. In eq 4, the anode true active area per catalyst layer unit volume, Av, has been calculated from catalyst layer structural data and active surface area data (neither presented here). The other adjustable parameters in this equation, namely, ka, Clim, E0a, and ba, were obtained by least-squares fitting of the equation to experimental kinetic current versus anode potential and methanol concentration data.


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Figure 3. Cell (line only) and individual-electrode (line with symbols) polarization plots in H2-air mode. Reference electrode configuration of Figure 1a. See the experimental section for conditions. Solid line only T measured iR-corrected cell voltage; dashed line only T cell voltage calculated from measured individual-electrode potentials; solid line with symbols T individual-electrode potential reading vs RA; dashed line with symbols T individual-electrode potential reading vs RC; line with squares T cathode potential; line with circles T anode potential.

Figure 4. Cell (line only) and individual-electrode (line with symbols) polarization plots in methanol anode polarization mode. Reference electrode configuration of Figure 1a. See the experimental section for conditions. Lines and symbols code as in Figure 3.

Boundary Conditions. For boundaries where a given conducting phase extended from region o to region p, the standard “transmissive” boundary condition was used for that phase (Ohm’s law), for example

σi,o

∂Φo ∂Φp ) σi,p ) -J⊥ and Φo ) Φp ∂x⊥ ∂x⊥

(5)

x⊥ and J⊥ are the coordinate normal to the boundary and the local current density component normal to the boundary, respectively. Region boundaries being simultaneously physical boundaries of a conducting phase had the standard “reflective” or Neumann boundary condition for the given phase, for example

∂Φ )0 ∂x⊥

(6)

Additionally, because of the very high electronic conductivity of the backing, we have put V ) 0 V along the top boundaries of regions 3 and 5 (Table 1, Figure 2). This also means that all of the potentials in the model are referenced to the potential of this anode metal boundary. The bottom boundary of region 1, that is, the fuel cell electrolyte membrane, is where the cathode catalyst layer begins. The boundary condition used there was an empirical relation between the local current density and the cathode potential extracted by least-squares fitting from the (VC - VRC) line in Figure 16a (DMFC data):

η ) E0c + Φ - Vc σi

∂Φ ) -η(5.5733 + η(-45.804 + 144.61η)) ∂x⊥

Figure 5. Cell (line only) and individual-electrode (line with symbols) polarization plots in DMFC mode. Reference electrode configuration of Figure 1a. See the experimental section for conditions. Lines and symbols code as in Figure 3.

(7)

E0c is the cathode open circuit potential, and Vc denotes the cell voltage. It should be noted that, first, this approach was independent of VRC, because, as we show later, VRC for reference electrode configuration c indeed reported the iR-free cathode

potential, and second, eq 7 permitted us to account for methanol crossover without modeling methanol transport through the electrolyte membrane and the cathode side in more detail. Because the existing purpose was reference electrode modeling, this kind of approach was justified. Methanol concentration along the free reactant access boundaries (thick dashed lines in Figure 2) was fixed at C0, the methanol concentration in the anode feed stream. Table 2 includes all of the parameters used in the model together with their values. Results and Discussion Quality Assessment in Fuel Cell Reference Electrode Measurements. Whenever a reference electrode is used to measure half-cell polarization plots in a fuel cell, the accuracy of the results may be questioned. This is because the reference electrode and the ionic connection affixing it to the cell (often in a “strange” fashion), can undergo separate potential changes from random or directional changes in the red-ox environment of the reference electrode and in the conductivity of the ionic connection. Even when stable and repeatable reference electrode readings are obtained, there is further


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Figure 6. Ionic potentials for reference on anode side of membrane: comparison of reference placement at membrane edge with reference configuration of Figure 1a. Top plot is Cartesian geometry with cathode extending beyond anode but not opposite reference. Bottom plot is axially symmetric geometry with cathode intact everywhere and anode removed for r < 0.2 cm. Vc ) 0.60 V, average current density 0.123 A cm-2.

Figure 7. Cell (line only) and individual-electrode (line with symbols) polarization plots in H2-air mode. Reference electrode configuration of Figure 1b. See the experimental section for conditions. Lines and symbols code as in Figure 3.

Figure 8. Cell (line only) and individual-electrode (line with symbols) polarization plots in methanol anode polarization mode. Reference electrode configuration of Figure 1b. See the experimental section for conditions. Lines and symbols code as in Figure 3.

uncertainty associated with the unknown ionic potential that impacts the reference electrode readings. As the introduction pointed out, this is because ionic potential distribution in thin electrolytes is highly nonuniform and strongly depends on the fuel cell current density (voltage). In view of possible complications discussed above, some test criteria need to be developed to assess the validity of potential measurements performed in polymer electrolyte fuel cells using reference electrodes. The proposed criteria are as follows: Criterion 1: Under fixed fuel cell operating conditions, the reference electrode response should be repeatable and free of excessive noise. Criterion 2: Changes in fuel cell operating conditions, such as temperature, current density, and so forth, should affect the reference electrode signal only to the same degree that they affect the operation of the fuel cell. For example, the conductivity of the ionic connection (bridge) between the reference electrode and the MEA should be independent of the fuel cell current density. In case the latter condition is not met, for example, because of the humidity shift by electroosmotic drag, the potential of the reference electrode itself will exhibit a

directional change with fuel cell current density, a source of an often large systematic error in reference electrode readings. Criterion 3: At a given fuel cell current density, the overpotential of a fuel cell electrode in contact with hydrogen (working as either anode or cathode) measured versus the reference electrode, cannot be larger than the voltage of the cell working in “hydrogen pump” mode (H2 oxidation/H2 evolution) at the same current density. This test is useful for detecting problems with the reference electrode sensing an ionic potential that is strongly driven by the potential of a fuel cell electrode with slow electrode kinetics, for example, an air cathode or a methanol anode. Criterion 4: When a reference electrode is designed to directly measure the iR-free potential of a fuel cell electrode, the measured overpotential of the H2-oxidation electrode (or H2evolution electrode) should be equal to one-half of the iRcorrected “hydrogen pump” voltage at every fuel cell current density (e.g., 10-20 mV per 1.0 A cm-2 in case of Pt-based catalysts). This is because activation polarization of H2 oxidation and H2 evolution are equal at virtually all electrode materials.


Figure 9. Cell (line only) and individual-electrode (line with symbols) polarization plots in DMFC mode. Reference electrode configuration of Figure 1b. See the experimental section for conditions. Lines and symbols code as in Figure 3.

Criterion 5: For a system with two reference electrodes in configuration c of Figure 1, the following relation should hold at every fuel cell current density:

(VC - VRC) - (VA - VRA) + (VRC - VRA)i)0 ) (VC - VA) + iR (8) (VRC - VRA)i ) 0 denotes the potential difference between RC and RA at zero fuel cell current density. Essentially, this means that if the voltages (VC - VRC) and (VA - VRA) are correct measures of the iR-free potentials of the fuel cell cathode and anode, respectively, the difference of these voltages should give the iR-free fuel cell polarization curve accurate to the constant (VRC - VRA)i ) 0. Below, we use these criteria to validate the three reference electrode configurations of Figure 1. Sensing at Membrane, Edge Effects (Configuration a). To illustrate the problems with fuel cell reference electrode measurements encountered when trying to place the reference electrode on the protruding part of the ionically conducting membrane, a cell with two reference electrodes in the configuration a was tested in H2-air (Figure 3), methanol anode polarization (Figure 4), and DMFC (Figure 5) modes. Data in Figure 3 provide clear evidence of significant errors in the response of reference electrode RA placed in direct contact with the exposed part of the membrane, see (VA - VRA) and (VC - VRA) lines. This is because the ionic potential sensed by RA is driven by the strongly changing potential of the air cathode opposite to the contact point between RA and the membrane. Interpretation of readings obtained versus RA requires the modeling of the ionic potential distribution in the membrane, which we later briefly present, for consistency, despite the fact that similar calculations have been done before (cf. refs 10-12). Contrary to RA, reference electrode RC, placed opposite to the H2 anode, seems to function quite well. A closer inspection of the (VA - VRC) line in Figure 3 reveals that the anode potential measured versus reference electrode RC is equal to half of the iR-corrected “hydrogen pump” voltage at any current density used (“hydrogen pump” data not shown), which attests to the fact that reference electrode RC meets criterion 4. This is only possible due to the very fast H2 oxidation at the PEFC anode, which directly determines the potential sensed by RC and makes it largely independent of the fuel cell current density.

Piela et al. A similar situation occurs in the methanol anode polarization experiment (Figure 4). This time RA is located opposite to an H2-evolving cathode. Because of negligible kinetic resistance (overpotential) of hydrogen evolution, the ionic potential sensed by RA is steady, which allows for quantitative interpretation of the measurements versus this reference electrode. However, criterion 5 is not met in this reference electrode configuration because of the errors involved in the response of the reference electrode opposite to a fuel cell electrode with slow kinetics, that is, RA in the H2-air and RC in the methanol anode polarization experiments. In the DMFC mode (Figure 5), neither reference electrode can perform reliably because of the slow kinetics of both electrode processes. (Ionic potentials sensed by RA and RC strongly depend on the rate of these processes.) Overall, reference electrode configuration a has proved to be of very limited value in quantitative measurements of potentials in hydrogen-air and direct methanol fuel cells. Figure 6 shows for configuration a that the potential distribution to a circular reference (lower part) is essentially the same as that to a strip reference at the edge of the membrane with the opposite fuel cell electrode extending more than several membrane thicknesses beyond the fuel cell electrode on the reference side (in Cartesian coordinates, upper part). In both cases, the reference is placed at the origin. The conditions are a DMFC at a cell voltage of 0.60 V, yielding a current density of 0.123 A cm-2 in the main cell on the right side. The detailed potential path from right to left may be slightly different between the Cartesian and the axially symmetric case because the current flow is planar in one case and radially converging in the other. However, the starting and ending potentials are identical (Figure 6). At the cathode, near the reference, negligible current flows in both cases. Thus, the reference ionic potential in both cases must be the cell voltage minus the open circuit cathode potential, Vc - E0c , which is a poor representation of the ionic potential in the main current path at either fuel cell electrode. Sensing Directly at Catalyst Layer (Configuration b). The purpose of using this configuration is to determine if ionic contact of a reference electrode directly to the catalyst layer of a fuel cell electrode can ensure correct reading of that electrode’s overpotential under load. The results of the corresponding H2air, anode polarization, and DMFC testing are shown in Figures 7, 8, and 9, respectively. As demonstrated in Figure 7, the slow electrode process at the fuel cell cathode influences VRA. The effect is a little less than that in configuration a, where the reference electrode contacts exposed membrane. (Compare (VA - VRA) lines in Figures 3 and 7.) Also, contrary to the first configuration, readings versus RC are this time obscured by the iR drop. A close look at the voltage (VA - VRC) reveals that RC does not pass the test in criterion 4, even when the fuel cell anode is operated on H2. Obviously, criterion 5 cannot be met either. In the methanol anode polarization experiment (Figure 8), the ionic potential sensed by both reference electrodes is a function of the rate of fuel cell processes, a priori unknown; cf. lines (VC - VRA) and (VC - VRC). Both VRA and VRC seem to be influenced by the potential of their respective fuel cell electrodes (note a drift of ionic potential sensed by RA with anode potential, evidenced by (VC - VRA)) as well as by the opposite fuel cell electrodes (note a high slope of (VC - VRC) that does not meet criterion 4). Expectedly, no reliable quantitative potential data can be obtained in the DMFC mode of fuel cell operation (Figure 9).


J. Phys. Chem. C, Vol. 111, No. 17, 2007 6519

Figure 10. Bottom: modeled steady-state ionic potential and methanol concentration distributions for reference electrode geometry of Figures 1b and 2b. Top: (solid line) electronic potential at catalyst layer midpoint in z direction and (dashed line) ionic potential at catalyst layer/membrane boundary, both as a function of radius. Vc ) 0.60 V, average current density 0.123 A cm-2.

Figure 11. Modeled steady-state ionic potential and methanol concentration distributions for reference electrode geometry of Figures 1c and 2c. Vc ) 0.55 V, average current density 0.183 A cm-2.

A detailed explanation of the reference electrode signals for configuration b required modeling of electrical potential distribution in the ionically conducting parts of the MEA. The corresponding model geometry is depicted in Figure 2b and the parameters’ values are those from Table 2. At the bottom of Figure 10 is a contour plot of the ionic potential and the methanol concentration obtained by the model at a cell voltage of 0.60 V. The electronic and ionic potentials in the catalyst layer (see figure caption for details) as a function of radius are shown above. An electronic potential drop of 13 mV is caused by the lateral electronic flow in the catalyst layer where there is no adjacent backing layer. With decreasing radius, the ionic potential tracks the electronic potential until r < 0.05 cm where methanol flow to the catalyst layer is impeded and the ionic potential drops further toward the cathode side potential. Half-cell measurement errors encountered with reference electrode configuration b are due to the fact that the catalyst layer underneath the reference electrode is only partially involved in the electrode process. First, this is caused by the

Figure 12. Modeled steady-state ionic potential variation through the cell, from the cathode at z ) 0 cm through the anode catalyst layer at z ) 0.018 to 0.0188 cm at two locations: r ) 0 cm under the reference and r ) 0.5 cm in the main fuel cell current path. Both configurations from Figure 1b and c are shown at a cell voltage of 0.6 V (solid lines). Configuration from Figure 1c is also shown at 0.55 V (dashed lines).

absence of the backing from the section taken up by the reference electrode, which otherwise provides an efficient current collection from the catalyst layer. The lateral resistance in the extremely thin catalyst layer (∼8 µm thick) over a distance of a few millimeters is prohibitively high for maintaining


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Figure 13. Cell (line only) and individual-electrode (line with symbols) polarization plots in H2-air mode. Reference electrode configuration of Figure 1c. See the experimental section for conditions. Lines and symbols code as in Figure 3.

Figure 14. Cell (line only) and individual-electrode (line with symbols) polarization plots in methanol anode polarization mode. Reference electrode configuration of Figure 1c. See the experimental section for conditions. Lines and symbols code as in Figure 3.

significant current flow. Second, at the very contact point of the reference electrode and the catalyst layer reactant access is strongly hindered, which further slows the reaction. Diminished current generation underneath the reference leads to an ionic potential different than that in the areas unobstructed by the reference. In an extreme case, the ionic potential shifts to the value characteristic of the electrolyte at the interface with the opposite fuel cell electrode. In an intermediate case, an iR component constituting an unknown and load-dependent fraction of the full iR drop in the membrane adds to the reference electrode readings. Thus, in order to correctly and directly measure the iR-free overpotential of a fuel cell electrode, the reference electrode has to be contacted with the catalyst layer in such a way that catalyst operation remains unaffected by the reference electrode and therefore stays representative of the whole fuel cell electrode. Sensing at Catalyst Layer through “Ionic Bridge” (Configuration c). In an attempt to fulfill the requirement formulated above, we constructed a cell with reference electrodes in configuration c. Special care was necessary during cell setup to ensure the same current generation rate in the catalyst layer under the reference as elsewhere and thus to avoid an unknown

Figure 15. (a) Cell (line only) and individual-electrode (line with symbols) polarization plots in DMFC mode and (b) comparison of (VA - VRA) signals obtained in DMFC mode (solid line) and methanol anode polarization mode (dashed line). Reference electrode configuration of Figure 1c. Airflow in DMFC mode: 100 sccm, not humidified, not pressurized. See the experimental section for other conditions. Lines and symbols code for part a as in Figure 3.

iR component in the reference electrode readings. To meet this condition the reactant, for example, methanol or air, had to be supplied in ample amount to the catalyst layer under the reference electrode. The need for sufficient reactant is demonstrated in the modeling of this particular reference electrode configuration described below. The DMFC has been modeled for spatial configuration c with parameter values from Table 2. The effective diffusion coefficient of methanol in the diffusion backing with ionomer has been set less than the value for the diffusion backing without ionomer, consistently with the lowered porosity due to the presence of recast Nafion. Figure 11 shows ionic potential and methanol concentration distributions at a DMFC cell voltage of 0.55 V. The model has also been run at 0.60 V (results used in Figure 12). As Figure 11 shows, when the catalyst layer under the reference becomes methanol-starved, methanol oxidation reaction slows down in that part of the cell and a difference arises between the ionic potential sensed by the reference electrode and the ionic potential in the “normally working” catalyst layer, the one we want to sense. Figure 12 allows us to compare the ionic potential through the cell (z direction) at the center of the reference (r ) 0 cm) and in the mainstream (r ) 0.5 cm). We compare three cases: configuration b at cell voltage 0.60 V and configuration c at 0.60 and 0.55 V. Little shift in


Figure 16. (a) Cell (line only) and individual-electrode (line with symbols) polarization plots in DMFC mode and (b) comparison of (VA - VRA) signals obtained in DMFC mode (solid line) and methanol anode polarization mode (dashed line). Reference electrode configuration of Figure 1c. Airflow in DMFC mode: 500 sccm, prehumidified at 90 °C, pressurized to 2.8 atm. See the experimental section for other conditions. Lines and symbols code for part a as in Figure 3.

ionic potential under the reference (z ) 0.0188 cm) for configuration c at 0.60 V (ca. 2 mV) becomes noticeably different at 0.55 V (ca. 6 mV). Configuration b yields considerable error even at the lower current density level (ca. 26 mV). It should be noted that although an “ionic bridge” reference electrode placed at a methanol anode is not likely to easily cause the problems discussed above, the risk is much higher for a reference placed on a gas electrode side, particularly the air cathode side. This is because, although methanol oxidation rate is only weakly dependent on methanol concentration (vide eq 4), oxygen reduction is first-order on oxygen and can easily suffer from oxygen starvation in the cathode catalyst layer, for example, caused by the catalyst flooding. It was, however, possible to overcome these difficulties with a careful construction of the reference electrode fixture. The results of H2-air and methanol anode polarization experiments with a cell possessing two reference electrodes attached to respective catalyst layers through “ionic bridges” in diffusion backings are shown in Figures 13 and 14, respectively. On the basis of excellent fulfillment of all quality criteria, particularly criterion 5, it can be concluded that a direct iR-free measurement of individual fuel cell electrode potentials under load is possible with this reference electrode configuration.

J. Phys. Chem. C, Vol. 111, No. 17, 2007 6521 Below we show data for reference electrode configuration c that proves such a reference electrode configuration returns a local rather than an aVerage fuel cell electrode potential. (Depending on one’s needs, this can be viewed either as an advantage or limitation of the proposed setup.) Figure 15 shows the results from DMFC testing of the fuel cell, for which high-quality hydrogen-air test data are shown in Figure 13. DMFC testing has been performed at a low airflow rate of 100 sccm. The polarization data in Figure 15a clearly indicate that at high current densities criterion 5 is no longer fulfilled. The bottom graph (Figure 15b) shows a comparison of (VA - VRA) readings from the DMFC experiment in Figure 15a and the methanol anode polarization experiment in Figure 14. Despite the fact that in both experiments the conditions on the methanol anode side of the cell were identical, anode potentials measured versus RA are strikingly different in those two experiments at almost every current density. To explain this difference, one needs to go back to Figure 1, the topmost box showing fuel cell geometry. In the experiment presented in Figure 15a, fresh, dry air entered the cathode on the RA side of the cell and left the cathode on the RC side of the cell. Consequently, the cathode section near RC was exposed to all of the humidity associated with low airflow operation of the DMFC and was very likely to become partially flooded with water. In this situation, current density distribution along the air flow channel was likely uneven, with high current generated near RA and low current generated near RC. Consequently, RA must have indicated a larger anode polarization than suggested by the aVerage current density shown on the abscissae of Figure 15, simply because the local current density around RA was much higher than the aVerage current density under these operating conditions of the cathode. In the anode polarization experiment, the cathode was operated as a hydrogen-evolving electrode under reversible conditions, preventing any significant variations in the current density along the flow channel. Moreover, taking into account that the minimum stoichiometric ratio of methanol in the anode polarization experiment was ca. 2, one can assume that the current density variation along the methanol flow channel due to methanol depletion was small. This conclusion is supported by segmented DMFC cell experiments conducted at Los Alamos.14 Therefore, the dashed line in Figure 15b can be viewed as reflecting quite well the local anode potential at a local current density. To support this interpretation of Figure 15 data, two DMFC experiments were conducted. In one of them, the cathode air inlet and outlet were reversed while preserving the same low airflow of 100 sccm. The (VA - VRA) response was found to be strongly dependent on the direction of airflow (data not shown). In the second experiment, high airflow over the cathode was used, at which the system met all reference electrode criteria (Figure 16a). Even more importantly, a difference in the (VA VRA) readings between the DMFC and the methanol anode polarization modes could no longer be seen (Figure 16b). Clearly, by removing air supply limitations in the cathode, one can bring the local anode polarization sensed by RA close to the aVerage anode polarization. Finally, to demonstrate the usefulness of the proposed reference electrode setup, in Figure 17 we present the results of a “hydrogen pump” experiment carried under moderate external humidification of the cell. Such conditions are known to cause strong instability of the internal cell resistance because of membrane drying induced by electroosmotic drag. Such instability is evident from (VA - VRC) and (VC - VRA) signals,


Piela et al.

TABLE 2: Variables and Parameters Description with Parameter Values Used for Modeling parameter

value

definition

Av

cm-1

unit

4.63E+06

ba C C0 Clim Db

V mol cm-3 mol cm-3 mol cm-3 cm2 s-1

0.0237 3.00E-04 5.00E-05 6.00E-05

Dnb

cm2 s-1

5.00E-05

Dcl

cm2 s-1

2.00E-05

E0a E0c

V V

0.631 0.92338

F i J⊥ ka x, y r, z R R VA

C mol-1 A cm-2 A cm-2 mol cm-2 s-1 cm cm Ω cm2 mol cm-3 s-1 V

96 485

VRA Vc VC VRC V η Φ σi,m

V V V V V V V Ω-1 cm-1

0.10

σi,cl σi,nb

Ω-1 cm-1 Ω-1 cm-1

0.02 0.05

σe

Ω-1 cm-1

100

true active surface area of anode per catalyst layer unit volume natural Tafel slope for methanol anode methanol concentration methanol concentration in anode feed stream methanol order-switch concentration effective methanol diffusion coefficient in diffusion backing without ionomer (region 5) effective methanol diffusion coefficient in diffusion backing with ionomer (region 3) effective methanol diffusion coefficient in catalyst layer (region 2) anode potential constant cathode open circuit potential (methanol crossover-affected) Faraday constant current density normal current density methanol oxidation rate constant cartesian coordinates cylindrical coordinates electrolyte resistance methanol reaction rate anode metal potential (universal reference point) anode-side reference electrode potential cell voltage cathode metal potential relative to VA cathode-side reference electrode potential electronic potential overpotential ionic potential ionic conductivity of membrane (region 1) and ionomer around reference metal (region 4) ionic conductivity of catalyst layer (region 2) ionic conductivity of diffusion backing with ionomer (region 3) electronic conductivity (regions 2, 3, and 5)

3.24E-04

0 0.60, 0.55

Figure 17. Cell (line only) and individual-electrode (line with symbols) polarization plots in “hydrogen pump” mode. Reference electrode configuration of Figure 1c. Moderately humidified cell: 300 sccm hydrogen on both sides, prehumidified at 85 °C, pressurized to 2 atm. Lines and symbols code as in Figure 3.

each of which includes the entire iR drop across the membrane. However, the direct iR-free measurements, that is, (VA - VRA) and (VC - VRC), unambiguously demonstrate that neither catalyst layer, including the anode catalyst, theoretically most prone to encountering a kinetic hindrance, suffers any kinetic problems up to 1.0 A cm-2.

Conclusions Placement is crucial to good performance of a reference electrode and often determines its usefulness in probing the potential of a fuel cell electrode. Accurate potential measurements require a direct ionic connection between the reference electrode and the catalyst layer. Such a connection helps to avoid problems with poorly controlled edge effects in electrical potential distribution inside the part of the electrolyte layer extending out of the MEA, the usual place for contacting a reference electrode. Furthermore, the rate of fuel cell current generation in the part of the catalyst layer remaining in contact with the “ionic bridge” linking the reference electrode to the catalyst layer must not be affected by the modification. Only then is the ionic potential sensed by the reference electrode representative of a normally operating catalyst layer and virtually no iR component is introduced into the reference electrode response. From the three fuel cell reference electrode configurations studied in this work, the one in Figure 1c has proven to be fully functional in direct measurements of iR-free individual-electrode potentials in hydrogen-air and direct methanol fuel cells under load. It is further understood that the iR-free potential of the fuel cell electrode measured against such a reference electrode has a local character. Consequently, under some circumstances, for example, depending on reference electrode placement relative to the inlets and outlets of reactants and depending on fuel cell operating conditions, this reported local potential can be significantly different from an average potential of a fuel

iR-Free Individual-Electrode Overpotentials cell electrode. Likewise, if there is non-negligible ionic resistance loss in the catalyst layer, then measurements against the reference electrode in configuration c will detect it because the ionic potential sensed is the potential at the catalyst layer/gasdiffusion layer interface (ionic iR drop in the catalyst layer included), not at the membrane interface. In the view of the authors, this “local potential” property should be regarded as an additional advantage of the proposed reference electrode setup. In combination with the experiment, the finite-element modeling of the MEA with reference electrodes provides indepth insight into the reference electrode operation in a PEFC. The results show that a reference electrode placed in the maincell reactant flow path can work reliably as long as the reaction at the adjacent fuel cell electrode remains undisturbed by the reference electrode’s presence. Acknowledgment. Funding from the Department of Energy (DOE) through Office of Hydrogen, Fuel Cells & Infrastructure Technologies and from the Defense Advanced Research Projects Agency (DARPA) through Defense Sciences Office (Palm Power Program) is gratefully acknowledged. Los Alamos National Laboratory assisted in meeting the publication costs of this article.

J. Phys. Chem. C, Vol. 111, No. 17, 2007 6523 References and Notes (1) Carrette, L.; Friedrich, K. A.; Stimming, U. Fuel Cells 2001, 1, 5. (2) Piela, P.; Zelenay, P. The Fuel Cell ReView 2004, 1, 17. (3) Gottesfeld, S. The Fuel Cell ReView 2004, 1, 25. (4) Wasmus, S.; Ku¨ver, A. J. Electroanal. Chem. 1999, 461, 14. (5) Arico`, A.S.; Srinivasan, S.; Antonucci, V. Fuel Cells 2001, 1, 133. (6) Schultz, T.; Zhou, S.; Sundmacher, K. Chem. Eng. Technol. 2001, 24, 1223. (7) Diard, J.-P.; Glandut, N.; Landaud, P.; Le Gorrec, B.; Montella, C. Electrochim. Acta 2003, 48, 555. (8) Adler, S. B.; Henderson, B. T.; Wilson, M. A.; Taylor, D. M.; Richards, R. E. Solid State Ionics 2000, 134, 35. (9) Adler, S. B. J. Electrochem. Soc. 2002, 149, E166. (10) He, W.; Nguyen, T.V. J. Electrochem. Soc. 2004, 151, A185. (11) Liu, Z.; Wainright, J. S.; Huang, W.; Savinell, R. F. Electrochim. Acta 2004, 49, 923. (12) Piela, P.; Springer, T. E.; Wilson, M. S.; Davey, J.; Zelenay, P. In Proton Conducting Membrane Fuel Cells IV; Murthy, M., Ota, K., Van Zee, J. W., Narayanan, S. R., Takeuchi, E.S., Eds.; Proceedings of the 206th Meeting of the Electrochemical Society, Honolulu, HI, October 3, 2004; Electrochemical Society, Pennington, New Jersey; Vol. 2004-2006.21, pp 537-550. (13) Wilson, M. S.; Gottesfeld, S. J. Electrochem. Soc. 1992, 139, 28. (14) Bender, G.; Pivovar, B. Paper 1006 presented at the 204th Meeting of the Electrochemical Society, Orlando, FL, 2003.

Direct Measurement of iR-Free Individual-Electrode Overpotentials in

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