Local Transients of Flooding and Current in Channel and Land Areas

Jun 16, 2010 - Our results demonstrate that oxygen depletion leads to a strong performance loss under the ribs already before the onset of a notable l...
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J. Phys. Chem. C 2010, 114, 11998–12002

Local Transients of Flooding and Current in Channel and Land Areas of a Polymer Electrolyte Fuel Cell Ingo A. Schneider,*,† Steffen von Dahlen,† Michael H. Bayer,† Pierre Boillat,‡ Malte Hildebrandt,§ Eberhard H. Lehmann,⊥ Pierre Oberholzer,‡ Gu¨nther G. Scherer,‡ and Alexander Wokaun# Electrochemistry Laboratory, Fuel Cell Diagnostics ActiVities, Electrochemistry Laboratory, Laboratory for Particle Physics, Spallation Neutron Source DiVision (ASQ), and Department General Energy, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland ReceiVed: March 12, 2010; ReVised Manuscript ReceiVed: May 22, 2010

Submillimeter resolved current density distribution measurements and in-plane neutron radiography were used simultaneously in voltage step experiments for the local investigation of flooding and cell performance in cathode channel and land areas of a polymer electrolyte fuel cell. Our results demonstrate that oxygen depletion leads to a strong performance loss under the ribs already before the onset of a notable liquid water accumulation. Here, the detrimental effect of liquid water accumulation on cell current is more likely associated with a loss of electrode performance rather than the result of a decreasing effective diffusivity for oxygen in the gas diffusion layer. Introduction Nafion-membrane-based polymer electrolyte fuel cells (PEFCs) operate at temperatures of T < 100 °C. Water coexists, upon reaching saturated vapor pressure, in both the liquid and the vapor phases. The porous gas diffusion layer (GDL) and the catalyst layer (CL) allow simultaneous gas and liquid flows. The local buildup of liquid water in the cell may, however, lead to flooding of reactant gas-transport pathways, and as a consequence, the cell is prone to suffer from phenomena such as performance loss,1,2 reactant starvation, and cell degradation.3 Much effort has been spent on the understanding of twophase flow phenomena by both experimental and modeling investigations. In the face of improved physical models4,5 and emerging diagnostic tools, for example, advanced imaging techniques,6 the picture of liquid water transport and its accumulation in the porous cell components is still nebulous. The same holds for flooding-related phenomena. In particular, the demonstration to correlate the local water content to performance loss1,2,7 remains challenging if we consider that local mass-transport limitations are not necessarily governed by the effect of liquid water flooding. In this context, the use of transient techniques provides an avenue to separate flooding-related losses from the systeminherent limitation due to mass transport in the porous layers by means of the time constant. The time constants for the diffusive transport of oxygen are on the order of only fractions of a second. In contrast, water saturation profiles develop within tens of seconds, while the rearrangement of water may take even minutes. These processes are the slowest of all if effects due to changing membrane hydration can be excluded.4,5,8-11 A separa* To whom correspondence should be addressed. E-mail: ingo.schneider@ psi.ch. † Electrochemistry Laboratory, Fuel Cell Diagnostics Activities. ‡ Electrochemistry Laboratory. § Laboratory for Particle Physics. ⊥ Spallation Neutron Source Division (ASQ). # Department General Energy.

tion of flooding-related losses should therefore be possible by means of the time constant. The application of transient techniques for the in situ investigation of two-phase flow phenomena in PEFCs has been reported recently by Ziegler et al.12,13 They proposed the use of a variety of voltage perturbation waveforms as a lean and quick alternative to neutron imaging. In an attempt to quantify the average saturation change in the cathode GDL after a voltage step, they estimated the accumulation of liquid water from the integral current transient as obtained in a quasi two-dimensional cell. The simplicity of their approach is appealing, yet it is an indirect method, and the evaluation is based on several assumptions.12 A conclusion about the spatial distribution of liquid water or on its effect on local performance cannot be drawn from the experiment. Recently, we reported a significant resolution enhancement with respect to both local current measurement and neutron imaging of liquid water in PEFCs.14,15 In this work, submillimeter resolved current density distribution measurements and high-resolution in-plane neutron radiography have been used simultaneously for the first time. We present a voltage step technique for the simultaneous examination of the temporal evolution of local water accumulation and local performance in cathode channel and land areas. Experimental Section The experiments were performed at the imaging with cold neutrons (ICON) beamline at PSI. For neutron beam detection, a microsetup16 with tilted scintillator (Figure 1a)8 and optical adaptor15 was employed. The image was recorded by a charge coupled device (CCD) camera of 2600 × 4000 pixels with a pixel pitch of 9 µm. This results in a field of view of 24 × 36 mm2. In 2 × 2 pixel binning mode, the magnification in the horizontal direction (tilting angle of R ) 10°) provides an effective pixel size of 3.15 × 18 µm2. By using anisotropic resolution enhancement techniques,15 a full width at halfmaximum (fwhm) resolution of 20 µm in the through-plane

10.1021/jp102259q  2010 American Chemical Society Published on Web 06/16/2010

Flooding and Current in Channel and Land Areas

Figure 1. (a) Experimental setup and timing diagram for high time resolution in plane neutron radiography. Exposure time is accumulated by the averaging of multiple series of neutron images as received upon repetitive cell perturbation from the steady state. The maximum cell current is limited to 3.5 A/cm2 by the potentiostat; the first current measurement was taken after 3 ms at icell < 3.5 A/cm2. (b) Characteristic positions of the cathode flow field used for the evaluation of (i, ii) the local water accumulation (the full line is a guide for the eye (fit y(t) ) k*(1 - e-at))) and (iii) the local current density.

direction and of 200 µm in the in-plane direction could be achieved at a low exposure time of 10 s. In transient measurements, a series of neutron images must be taken after cell perturbation. In the voltage step experiments,

J. Phys. Chem. C, Vol. 114, No. 27, 2010 11999 a temporal resolution on the order of 1-2 s is desirable. A temporal resolution better than the total exposure time can be achieved if the exposure time is accumulated by the averaging of multiple series of neutron images as received upon repetitive cell perturbation from the steady state. Here, the reproducibility of the observed current transients during the flooding period after cell perturbation was considered as a prerequisite for the averaging of image series. The approach requires a synchronization of the cell perturbation, the data acquisition, and the image capture sequence. This is illustrated in Figure 1a, which shows the experimental setup and the respective timing diagram. The CCD shutter signal is used for synchronization. This signal is directly connected to the data acquisition system for current and voltage measurement and to a potentiostat via a trigger circuit. The potentiostat is used for cell perturbation. On the first rising edge of the shutter signal, the voltage step from OCV to 0.1 V is triggered after a delay of td ) 1 s. The shutter signal becomes active for te ) 1.96 s during exposure. During an image capture sequence, a number of n ) 32 images is taken at an interval of te + tr ) 2.08 s. The cell is disconnected from the potentiostat on the last falling edge of the shutter signal. After gas purge at OCV for a period of approximately 30 min, the next sequence is triggered. In total, N ) 5 series of neutron images are averaged to achieve a temporal resolution of ∼2 s at a spatial resolution of 20 µm (in through-plane direction). A special segmented differential cell is used in the measurement (A ) 0.6 cm2).14 The cell (Tcell ) 40 °C) is operated on humidified air and hydrogen (Thum_air ) Thum_H2 ) 40 °C) at a high gas flow rate (Vair ) VH2 ) 200 mL/min) and at ambient pressure. The graphite cathode flow field exhibits three parallel 1.2 mm wide and 1 mm deep gas channels (channel/land ratio is 1:1). An untreated Toray TGP-H-090 carbon paper GDL (78% porosity, 280 µm thickness)17 is employed at the cathode. The cathode is sealed by using a 200 µm thick polytetrafluoroethylene (PTFE) gasket. At the anode, a segmented microstructured pin-type flow field (760 pins/cm2) is directly attached to the catalyst-coated membrane (Nafion 112, 0.5 mg Pt/cm2, 70% Pt/ C).14 The anode is operated in cross-flow mode and acts as a quasi-homogeneous counter electrode across the cell area. This anode design allows a direct measurement of the local current in channel and land areas of the cathode flow field with submillimeter resolution (400 µm),14 as shown in Figure 1b. For current measurement, high-precision shunt resistors are used. The voltage drop over these resistors is very low (approximately 0.5 mV at 1 A/cm2). The voltage signals are amplified before acquisition. Evidently, the liquid water distribution at the cathode will depend on the anode diffuser characteristics, which cannot be chosen arbitrarily in our cell design. However, we focus primarily on a correlation of the liquid water distribution at the cathode and the local cell performance. Before evaluation, the local current transients, as obtained after cell perturbation, are averaged within each of four characteristic areas, (i) CC: channel center, (ii) CE: channel edge, (iii) LE: land edge, and (iv) LC: land center (Figure 1b (iii)). Moreover, the accumulation of liquid water after cell perturbation is determined from the neutron image sequence (Figure 1b (i, ii)). Here, integral and local values (in the through-plane direction) were determined and averaged within the four areas, respectively. Values are stated in percent of the total volume, which includes the pore space and the solid space. Results and Discussion Local transients of the liquid water accumulation and the current density are shown in Figure 1b (i-iii) for the four

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Figure 2. (a) Liquid water profiles in the four characteristic areas CC, CE, LE, and LC (Figure 1b (iii)) for distinct points in time. (b) Current density distribution in channel and land areas for distinct points in time. (c) Transients of liquid water accumulation on a logarithmic time scale (the full line is a guide for the eye (fit y(t) ) k*(1 - e-at))). (d-f) Local current transients in the four characteristic areas CC, CE, LE, and LC on a logarithmic time scale.

characteristic channel (CC, CE) and land (LC, LE) areas. Liquid water accumulates in the cathode GDL within some tens of seconds after cell perturbation (Figure 1b (i, ii)). The weaker increase of the total water content in the land center (Figure 1b (ii)) cannot solely be explained by the lower current density (Figure 1b (iii)) under the assumption of a constant water drag coefficient. A higher compression and lower permeability of the GDL under the ribs requires a higher capillary pressure to enter the porous GDL and might therefore favor water permeation through the membrane.18 However, the average saturation change approaches comparable values of around 13% in channel (CC) and land (LC, LE) areas within a time frame of 60 s (Figure 1b (i, ii)), considering 78% porosity of the uncompressed Toray paper GDL and ∼30% compression of this material in the land area. The same characteristic time constant is observed in the local current transients (Figure 1b (iii)). The associated performance loss is most strongly pronounced in the channel area. In the land area, the local current already drops to low values before a notable liquid water accumulation in the porous GDL. This phenomenon is illustrated in more detail in Figure 2. Figure 2a and b shows local liquid water profiles and the current density distribution in channel and land areas for distinct points in time. Figure 2c-f shows transients of the local current and the average liquid water content on a logarithmic time scale. The characteristics of the local current transients are governed by the ohmic resistance, double layer charging, oxygen transport, and liquid water accumulation. Upon cell perturbation, the local cell current is limited initially by the local ohmic resistance. Under the conditions used here (Tcell ) Thum_H2 ) Thum_air ) 40 °C), the resistance distribution will be fairly homogeneous.

However, if we take into account here electronic GDL properties, the current density distribution is expected to pass a maximum under the ribs. This characteristic governs the current distribution profile for the first 5-10 ms after the step (Figure 2b (a-c)). The current decay within this time frame must be attributed to double layer charging and to a decreasing oxygen concentration at the air electrode. The oxygen concentration profiles develop initially within both channel and land areas primarily in the through-plane direction, and therefore, the observed characteristic decay in both areas is similar within this time frame (Figure 2b (a-c)). With the onset of the development of the oxygen concentration profile in the in-plane direction, the current density starts to drop severely under the ribs as a result of oxygen depletion (Figure 2f (c-g)). This effect is a direct consequence of the inherent longer diffusion path length for oxygen under the ribs and becomes most pronounced in the rib center. The performance loss occurs mainly within the first 100 ms after the step, already before notable liquid water accumulation in the GDL (Figure 2c), and leads temporarily to a pronounced maximum in current density in the channel area (Figure 2b (g)). In the channel region, the short diffusion path length and the low compression of the GDL keep the oxygen concentration at the electrode at a high level. However, any reactant gas consumed either in the channel or in the land area must pass through the GDL of the channel region. It is therefore surprising that the strong performance loss, which appears in the channel region within the first 5-10 s of liquid water accumulation (Figure 2d (g-l)), is not observed in the rib center (Figure 2f (g-l)), although the local current is limited here by the rate of oxygen supply through the porous

Flooding and Current in Channel and Land Areas GDL. Evidently, the experimentally obtained cross-sectional water profiles (Figure 2a) show liquid water build up nearby the air electrode, and therefore, it is reasonable that liquid water accumulation primarily affects local cell performance in channel and land areas. Furthermore, the liquid water accumulation shows the lowest values of all in the rib center (Figure 2a and c (LC)). Nevertheless, the GDL must still provide a sufficient number of reactant pathways to sustain a quasi-constant current in the center of the rib within this time frame (Figure 2f (g-l)). It is likely that the performance loss during liquid water accumulation is not primarily the result of a decreasing effective diffusivity for oxygen in the gas diffusion layer but basically the result of an ongoing performance loss of the air electrode. This hypothesis provides a basis to explain the characteristics of the local current transients during the flooding period. An electrode performance loss has the strongest impact in the channel area. Here, the oxygen concentration is kept at a high level. The current shows initially highest values, but the total performance loss is the highest too (Figure 2d (g-l)). The effect is less pronounced at the rib edge at an overall lower oxygen concentration (Figure 2e (g-l)). However, an ongoing loss of electrode performance has no impact at all on the local current density under limiting current conditions, as long as the limiting current, which is governed by the maximum oxygen-transport rate, can be sustained by the air electrode (note that a limiting current in the rib center will also depend on oxygen consumption in the rib edge and channel). This effect might explain the behavior of the rib center (Figure 2f (g-l)). Here, a quasi-constant current is observed, until the gradually increasing liquid water content (Figure 2a and c) degrades electrode performance to a level where even the small limiting current density cannot be maintained, despite the high cell polarization (Ucell ) 0.1 V). As the electrode performance loss is no longer masked by the oxygen-transport limitation, the local current starts to drop after 5-10 s (Figure 2f (g-l)). Finally, the performance in both channel and land areas is governed more or less by the poor electrode performance, and a quasi-homogeneous current distribution is observed (Figure 2b (n)). At this point, we cannot make any statement on a possible liquid water accumulation at the anode since the high boron content of the ceramic anode flow field material results in strong neutron absorption and impedes liquid water imaging at the anode. However, the symmetry of the observed local current transients in the four characteristic channel and land (CC, CE, LC, LE) areas (Figure 1b) justifies our assumption of a quasihomogeneous anode operation across the cell area.14 It is important to note here that the strong performance loss of the cell upon liquid water accumulation is primarily related to the specific experimental conditions used here (Tcell ) 40 °C, non PTFE treated GDL).12 Low temperatures result in small saturation pressures and should also favor water condensation in front of the electrode.19 The detrimental effect of water accumulation on cell performance was also observed under the same operation conditions in a conventional cell, which employs this type of GDL and the parallel three-channel flow field on both electrodes. The cell performance improves significantly at higher temperature (Tcell ) Thum_air ) Thum_H2 ) 70 °C). Under these more practical conditions, the pronounced maximum in current production in the channel region is still observed at a high cell polarization during steady-state operation (Figure 3), as we have also demonstrated in our earlier work.14 A closer investigation on the effect of operating conditions and materials

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Figure 3. Steady-state current density distribution (Ucell ) 0.1 V) in channel and land areas of the 3 × 1.2 mm channel cathode flow field for operation on fully humidified H2/air at higher temperature Tcell ) Thum_H2, air ) 70 °C. (CCM: N212, 0.5 mg Pt/cm2, 70% Pt/C; GDL: Toray TGP-H-090 (10% PTFE), VH2 ) Vair ) 200 mL/min).

upon liquid accumulation and current distribution by using the method demonstrated here is the aim of a forthcoming study. Conclusions The transient technique presented in this work allows the simultaneous measurement of the local current and the liquid water profile in channel and land areas of a PEFC cathode with a high temporal and spatial resolution. Thereby, the systemimmanent limitation of the local performance due to the cell design can, in principle, be separated from losses due to liquid water accumulation by means of the time constant. The high temporal resolution of the measurement allows us to obtain a current density distribution, which is temporarily practically unaffected by effects of liquid water accumulation. This distribution shows a pronounced maximum in the channel area at higher cell polarization and fully humidified conditions, while the performance drops strongly in the land area and shows a minimum in the rib center. This performance loss is primarily the result of the increasing diffusion path length for oxygen and its depletion in diluting nitrogen underneath of the ribs. The effect is enforced by the compression of the GDL. Under practical conditions, a pronounced maximum of current generation was still observed in the channel region at steady state. In light of this result, a missing clear correlation between the integral cell performance and the liquid water content under the ribs is not an unexpected finding.7 At an already small contribution of the land areas to the total cell current at higher cell polarization, the effect of liquid water accumulation in the GDL on total mass-transport-related losses is expected to be small. The discussion has also shown that any effect of water accumulation on the electrode performance in the land area is almost irrelevant under the limiting current conditions, as long as the already low limiting current density can be sustained by the air electrode. The performance of the land areas and their contribution to the total cell current is therefore more likely governed primarily by the cell design, for example, by the flow field structure or the clamping pressure,20 rather than just by the amount of liquid water. References and Notes (1) Mukundan, R.; Borup, R. L. Fuel Cells 2009, 9, 499. (2) Boillat, P.; Kramer, D.; Seyfang, B. C.; Frei, G.; Lehmann, E.; Scherer, G. G.; Wokaun, A.; Ichikawa, Y.; Tasaki, Y.; Shinohara, K. Electrochem. Commun. 2008, 10, 546. (3) Patterson, T. W.; Darling, R. M. Electrochem. Solid State Lett. 2006, 9, A183.

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(4) Weber, A.; Balliet, R.; Gunterman, H. P.; Newmann, J. LBNL Paper 2008, 316E. (5) Wang, C.-Y. Chem. ReV. 2004, 104, 4727. (6) Bazylak, A. Int. J. Hydrogen Energy 2009, 34, 3845. (7) Zhang, J.; Kramer, D.; Shimoi, R.; Ono, Y.; Lehmann, E.; Wokaun, A.; Shinohara, K.; Scherer, G. G. Electrochim. Acta 2006, 51, 2715. (8) Wang, Y.; Wang, C.-Y. J. Electrochem. Soc. 2007, 154, B636. (9) Wu, H.; Berg, P.; Li, X. J. Electrochem. Soc. 2010, 157, B1. (10) Wang, Y.; Wang, C.-Y. Electrochim. Acta 2005, 50, 1307. (11) Hickner, M. A.; Siegel, N. P.; Chen, K. S.; Hussey, D. S.; Jacobson, D. L. J. Electrochem. Soc. 2010, 157, B32. (12) Ziegler, C.; Heilmann, T.; Gerteisen, D. J. Electrochem. Soc. 2008, 155, B349. (13) Ziegler, C.; Gerteisen, D. J. Power Sources 2009, 188, 184.

Schneider et al. (14) Schneider, I. A.; von Dahlen, S.; Wokaun, A.; Scherer, G. G. J. Electrochem. Soc. 2010, 157, B338. (15) Boillat, P.; Frei, G.; Lehmann, E. H.; Scherer, G. G.; Wokaun, A. Electrochem. Solid State Lett. 2010, 13, B25. (16) Lehmann, E. H.; Frei, G.; Ku¨hne, G.; Boillat, P. Nucl. Instrum. Methods Phys. Res., Sect. A 2007, 576, 389. (17) Toray Carbon Fiber Paper “TGP-H” Property Sheet; Toray Industries Inc.: Tokyo. (18) Ahmed, D. H.; Sung, H. J.; Bae, J. Int. J. Hydrogen Energy 2008, 33, 3786. (19) Weber, A. Z.; Newman, J. J. Electrochem. Soc. 2006, 153, A2205. (20) Ihonen, J.; Mikkola, M.; Lindbergh, G. J. Electrochem. Soc. 2004, 151, 1152.

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