Analyzing the Influence of H3PO4 as Catalyst Poison in High

Article pubs.acs.org/JPCC

Analyzing the Influence of H3PO4 as Catalyst Poison in High Temperature PEM Fuel Cells Using in-operando X‑ray Absorption Spectroscopy Sebastian Kaserer,† Keegan M. Caldwell,‡ David E. Ramaker,‡ and Christina Roth*,†,§ †

Renewable Energies Group, Institute for Materials Science, Technische Universität Darmstadt, Petersenstr. 23, 64287 Darmstadt, Germany ‡ Department of Chemistry, George Washington University, 725 21st Street Northwest, Washington, DC 20052, United States § Institute of Chemistry and Biochemistry − Physical and Theoretical Chemistry, Freie Universität Berlin, Takustr. 3, 14195 Berlin, Germany ABSTRACT: The effect of H3PO4 as a poison in high temperature polymer electrolyte fuel cells using polybenzimidazole (PBI) membranes was studied as a function of phosphoric acid loading, potential, and temperature. In this work, for the first time, extensive in-operando X-ray absorption spectroscopy investigations were carried out on Pt/C fuel cell cathode catalysts at different temperatures and H3PO4 concentrations at varying fuel cell voltages. Under in-operando conditions, significant H3PO4 anion coverage of the Pt nanoparticles is observed. The Δμ-XANES analysis shows that the O(H)/H adsorption onset potential increases/ decreases with temperature and that this is a result of phosphate anions being driven off the surface at high temperatures (170 °C). With initial coadsorption of H and O(H), the phosphate anions move into registry with the Pt, whereas random adsorption is observed when only phosphate anions are present on the Pt surface. By varying the temperature and the fuel cell potential, the adsorption geometry of the phosphoric acid anion changes with coverage, but in all cases, the anions block Pt sites and reduce the oxygen reduction reaction (ORR) rate.

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INTRODUCTION Phosphoric acid fuel cells, using concentrated phosphoric acid (H3PO4) as electrolyte, have been utilized since the mid1960s.1,2 These early phosphoric acid fuel cells, with a working temperature of about 170 °C, have several advantages over low temperature PEM fuel cells, indicating it would be advantageous to find ways to bring up the temperature of PEM fuel cells. With the adoption of H3PO4 doped polybenzimidazole (PBI) based membranes in 1995,3,4 it is now possible to utilize high temperature PEM fuel cells (HT-PEMFCs), and interest in these has increased significantly in the past few years. This can be seen by the increasing number of publications which deal with this topic.5 The advantages of higher temperature, for instance, easier water management, enhanced cathode kinetics, higher tolerable amount of impurities (CO6−8 and H2S8) in the anode fuel gases, and simpler cooling systems, make them attractive compared with low temperature PEM fuel cells.5,9 Due to these advantages, HT-PEM fuel cells have already been commercialized for stationary applications1,10,11 and also automobile manufacturers are becoming more interested in these HT fuel cells.12 Currently, the state of the art HT-PEMFC is based on phosphoric acid (H3PO4) imbibed PBI or modified PBI membranes3,5,13,14 with carbon supported platinum nano© 2013 American Chemical Society

particles as anode and cathode catalyst. At temperatures above 100 °C, little water assisted proton conductivity is possible in the membrane. Thus, instead of water, an acid containing membrane represents an effective approach to ensure the proton conductivity at elevated temperatures in the fuel cell. H3PO4 offers high ionic conductivity up to 200 °C;15 that is why it is used as a dopant to assist the proton conduction of the PBI-polymer electrolyte. To provide good proton conductivity throughout the cell, a continuous acid distribution within the electrodes and the membrane must be maintained, and in the electrodes, the contact between the phosphoric acid and the platinum nanoparticles also has to be ensured. However, little is known about the influence of the phosphoric acid on the Pt during fuel cell operation. Previous cyclic voltammetry,16,17 Fourier transformed infrared (FT-IR) spectroscopy measurements,17−20 quartz crystal microbalance investigations,21 and X-ray absorption spectroscopy (XAS) experiments22,23 showed that H3PO4 species can adsorb on the platinum surface, and that the coverage and adsorption geometry are potential, temperature, crystal face, and Received: December 4, 2012 Revised: February 26, 2013 Published: March 1, 2013 6210

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requirement because there is a slight energy shift in the beam energy over time depending on the monochromator and the current loss in the storage ring. The detailed data processing and analysis is described in the Data Analysis section. Fuel Cell Test Bench. For the in-operando XAS measurements, an in house designed high temperature polymer electrolyte membrane test bench was manufactured, which fulfills the requirements of temperatures up to 200 °C. The core of the test bench comprises three gas flow controllers (Alicat Scientific, Inc.) for hydrogen, oxygen, and nitrogen to ensure accurate gas flow control. Also included in this module are two temperature controllers (Jumo, itron32) to guarantee the exact fuel cell operation temperature and the serial interface with the electric load. To control all parts of the test bench, it is connected to a laptop with a Labview programed interface. The modular and flexible design of our test bench allows high mobility and allows it to be used at different XAS beamlines, as, e.g., X1 at HASYLAB, Hamburg, and at ANKA in Karlsruhe. Also, new cell hardware, which resists the higher temperatures and the presence of phosphoric acid, was designed and manufactured in house. For the flow fields, which are in immediate contact with the membrane electrode assembly (MEA), chemically stable graphite was used. These were mechanically stabilized by alumina plates holding the heating elements, which also ensure a homogeneous heat distribution over the whole cell. The end plates, which were also used to fix the whole cell to the Teflon base, were made of a high temperature resistive composite material. For measuring XAS in transmission mode, stepped slits (smallest 13 × 3 mm2) were drilled into all parts of the fuel cell and covered with Kapton foil. To ensure no gas leak at temperatures up to 200 °C, high temperature resistive silicon was used as a glue to fix the foil. An expanded drawing of the cell parts is shown in Figure 1a.

concentration dependent. It is also reported that adsorbed phosphate anions negatively affect the kinetics of the ORR on Pt single crystal electrodes investigated by rotating disc electrode measurements22,24as well as using a micro band electrode.25 A negative effect from the adsorbing phosphoric acid on the cathode performance could hence be expected, because in hydrogen/air operation the kinetics of the oxygen reduction reaction (ORR) significantly limits the overall cell performance. However, all these past measurements were realized in electrochemical half cells (i.e., without membrane), using defined platinum surfaces like Pt(111) single crystals and laboratory conditions mostly at room temperature. The influence of the fuel cell working temperature and the highly concentrated phosphoric acid was not always considered properly. A technique that may be able to close this gap between model catalysts and real systems is X-ray absorption spectroscopy.26 With this technique, it is possible to study the fuel cell catalyst in operation, determining the catalyst structure by analyzing the extended X-ray absorption fine structure (EXAFS) and the adsorbates on the nanoparticle catalyst surface by interpreting the X-ray near edge structure (XANES). For this purpose, the Δμ technique was applied, which is explained in detail in the Data Analysis section. Both methods were used before, and their reliability has been verified in different ex situ and in situ fuel cell measurements. Prominent examples of the effectiveness of this approach are the direct observation of the bifunctional mechanism occurring in PtRu catalysts in PEM and direct methanol/ethanol fuel cells.27−29 The direct relationship between nanoparticle structure, adsorbate coverage, and catalytic activity has been established using this technique. In this work, the Δμ-XANES technique is applied to study the adsorption of phosphoric acid at the fuel cell cathode catalyst under realistic conditions. The in-operando experimental setup in combination with the Δμ-XANES technique has enabled us to follow the kind (anion, poison such as CO or SOx, water decomposition product, or ORR intermediate) of molecule or atomic species bound to the catalyst surface, depending on the fuel cell voltage and the working temperature. To the best of our knowledge, this is the first in-operando study on a high temperature polymer electrolyte fuel cell with a PBI membrane describing in detail the poisoning effect of H3PO4 on the ORR for Pt/C electrocatalysts.

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EXPERIMENTAL SECTION X-ray Absorption Spectroscopy Measurement. All XAS experiments were carried out in transmission mode at the X1 beamline at Hasylab Hamburg and at the XAS beamline at ANKA at Karlsruhe Institute of Technology (KIT), Karlsruhe. The energy range was chosen from 11,300 to 12,800 eV to yield a reliable X-ray absorption spectrum at the Pt L3 edge (11,564 eV). The energy was tuned with a Si(111) double crystal monochromator. The intensity was measured with three gas-filled ionization chambers in series with 10 cm length each. By filling the detectors with different gas mixtures, different absorptions (10% first, 50% second, 90% third) could be obtained. The sample was mounted between the first two chambers and between the second and third detectors a thin Pt metal foil was installed, which was used as a reference. A detailed picture of the setup can be found elsewhere.20 The reference foil is very important for the Δμ data analysis because it allows the alignment of all spectra to one reference energy, a

Figure 1. (a) Expanded drawing of a HT-PEM fuel cell. (A) Membrane electrode assembly (MEA) with gas diffusion electrodes and Teflon gaskets, (B) graphite flow field and current collector, (C) aluminum plates with the heating elements, (D) high temperature resistive composite material. All fuel cell parts (B−D) have a drilled slit for the X-ray beam. (b) Cross-sectional drawing of the used MEAs. (1) Teflon gaskets, (2) PBI membrane, (3) gas diffusion layer (GDL), (4) electrode coating on the GDL, (5) indicated region without catalyst on the anode side.

MEA Samples. Since all samples were mounted for measurement in transmission mode with the X-ray beam perpendicular to the MEA, a slight modification to the MEAs was required. To distinguish the absorption of light from the two Pt-containing electrodes with their different environments (anode, H2; cathode, O2/N2), each MEA was modified to expose only the catalyst from one side of the membrane in the 6211

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adsorbates on the surface and a clean substrate is observed. With the phosphoric acid anions in the cell, either a Vref of 300 or 400 mV depending on temperature (see also the Results and Discussion section) was identified to have the “cleanest” surface. Each of the potentials, V, corresponds to a point in the iV curve of the fuel cell. Theoretical XANES spectra were also obtained using the FEFF 8.0 code;38 these were used to identify the different adsorbate signatures. As a first step, a theoretical XAS spectrum for a three-dimensional clean Pt model cluster (Janin Pt6 cluster39) was calculated. In a subsequent step, theoretical calculations were performed in this same manner for each of several different adsorbates at different adsorption sites on the Pt cluster, and the clean surface calculation subtracted (i.e., Δμtheo = μ(ad/Pt6) − μ(Pt6). The results of these calculations for phosphoric acid are shown in Figure 2. The detailed

beam window region. To fulfill this requirement, a small part (about 20 × 5 mm2) was left out while spraying onto one of the two gas diffusion electrodes (GDE), which were hot-pressed to the membrane. With this hot-pressing step, good electrical contact is guaranteed, so that an inhomogeneous current distribution is largely prevented. A cross-sectional drawing of the MEA is shown in Figure 1b. In order to make sure that a realistic view of what happens at the fuel cell catalysts during operation is obtained, commercial MEAs from either Advent or Fumatech were used. Both companies used carbon supported platinum nanoparticles as anode and cathode catalysts. Phosphoric acid imbibed polybenzimidazole membranes were used. To analyze the influence of the amount of phosphoric acid on the catalyst, some samples were sprayed with 6 g of H3PO4/g of Pt on the cathode side instead of the standardly applied 2 g of acid/g of Pt. The anode loading was 2 g of H3PO4/g of Pt in all cases. Measurements. Under the assumption that, in H2/O2 and accordingly H2/air fuel cell operation, the anode potential remains close to 0 V and that H3PO4 adsorption is potential dependent, all XAS measurements were measured at the cathode side. Here, the influence of the acid on the fuel cell performance and the oxygen reduction reaction (ORR), respectively, is expected to be the most significant. The ohmic drop of the fuel cell membrane is not considered in the analysis. However, because its influence only occurs at high current densities (small cell voltages, respectively), we can still draw significant potential-dependent conclusions at high cell voltages where the ORR occurs. The first measurement was carried out in a fuel cell at normal working conditions (180/ 170 °C, H2/syn. air with λH:1.2/λO:2.0). Spectra were recorded at different fuel cell potentials and are therefore linked with different current densities. The in-operando XAS measurements were carried out on the Pt/C cathode catalyst at varying cell voltages (0−870 mV) relative to the Pt L3 edge as a function of temperature and phosphoric acid loading (2 g/g of Pt or 6 g/g of Pt). The recording time for each potential step was about 40 min for conventional XAS spectra. The temperature-dependent measurements were carried out in Quick-EXAFS mode (3−4 min each, with three repetitions) at the different temperatures.

Figure 2. Comparison of FEFF8 calculated Δμ-XANES signatures for differently adsorbed phosphoric acid anions.23

comparison of these theoretical line shapes allows us to accurately distinguish between the various adsorbates on the electrocatalytic surface. Unfortunately, the Δμ technique cannot distinguish the different phosphate anions (i.e., different n in H3−nPO4n−), so they are referred to collectively as PO4 in the following sections. The line shapes shown in Figure 2 and 3 have either positive, negative, or both positive and negative Δμ magnitudes. The

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DATA ANALYSIS XANES Analysis. A careful alignment of the Pt L3 edges for the XAS samples was carried out by using data from the Pt reference foils. The data were then normalized to isolate the XANES part of the XAS spectra, and the energy range of the beam was tuned to look at the Pt L3 edge (11,564 eV). Our XANES analyses were obtained from −20 to 50 eV relative to the Pt L3 edge. All of this data was processed using the IFEFFIT suite (v.1.2.9).30 A detailed description of the alignment, normalization, and Δμ-XANES subtractive process can be found in the literature.31−37 To be brief, the reference foils were calibrated to the Pt L3 edge energy and then carefully aligned. These energy shifts are then applied to all of the samples. Then, Δμ signals were generated using the subtractive method, Δμ = μads − μclean and thus removing the absorption from the bulk or interior of the Pt clusters. This relationship can be described as follows: Δμ(V ) = μ(ad/Pt, V ) − μ(H3 − nPO4 n − /Pt, Vref )

Figure 3. Comparison of FEFF8 calculated signatures for atop H and 3-fold adsorbed H and O.

varying positive and negative contributions to the Δμ and the magnitudes of these contributions allow us to closely follow the line shapes for each adsorbate and to distinguish between different adsorbates on the surface. The |POS|/|NEG| ratios for the O(H) and PO4 3-fold “inverted” adsorption sites are very different, making them much easier to distinguish than the H 3fold and PO4 3-fold “inverted” binding, which are quite similar in Figure 4. The comparison of the experimental and theoretical results using the Δμ-XANES technique then allows us to see the adsorbate on the surface (i.e., whether that adsorbate is O(H), PO4, or H), and how it is bound to the cluster (atop, bridged, 3-fold, etc.). This procedure was carried

(1)

In the absence of phosphoric acid, normally a potential Vref of ca. 540 mV is used to ensure that there are no H or O(H) 6212

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Figure 4. Height of the |POS| (blue-hatched) and |NEG| (redchecked) amplitudes for the indicated adsorbates on Pt as obtained from FEFF8 calculations such as those indicated above. Also, the potential regions (rel. to RHE) where these adsorbates are expected to dominate on Pt are given.

Figure 6. Plot of positive (2−7 eV) and negative (0 eV) Δμ magnitudes vs cell potential. By comparing the measured magnitudes with the calculated ones, it is possible to identify different adsorbates. The three regions are indicated by the arrows.

out for each change in the fuel cell environment. Further comparison was then made by creating a magnitude of Δμ vs potential plots from this data and detailing the potential at which these adsorbates come on and off the surface of the Pt catalyst. In this work, the Δμ-XANES signatures were exceedingly small because of the higher temperatures driving off adsorbates and the small part of the geometric surface which is electrochemically active (ECSA) in the fuel cell reaction (approximately 20% for the applied catalyst). This means very small errors in alignment of the reference and sample spectra introduced relatively significant errors in the Δμ spectra (usually appearing as spikes near 0 eV such as those illustrated in Figure 5). Therefore, the difference δ(Δμsample − Δμfoil) is

follow the different adsorbates with changing cell voltages. The resulting plots of POS and NEG, which are directly related to the adsorbate coverages, are shown in Figure 7. An illustration

Figure 7. Coverage of different adsorbates by analyzing the |POS| and |NEG|Δμ amplitudes over cell voltage from Figure 6. The lines assist to guide the eye. The broken line indicates the expected PO4 coverage according to He et al.23

summarizing the expected potential-dependent processes at the Pt surface is depicted in Figure 8. We can distinguish three regions: At cell potentials lower than 300 mV, hydrogen is adsorbed at the Pt surface coming from H2O activation and the strong acidic environment. Due to the water produced from the ORR and the phosphoric acid electrolyte, this is possible even if the cathode is saturated with oxygen; at voltages above 800 mV,

Figure 5. Visual explanation of the Δμ and δΔμ technique.

taken where Δμ is equal to μ(V) − μ(Vref) for either the sample or the foil spectra, Figure 5. The δΔμ difference eliminates these artifacts or “spikes” due to misalignment, because they will be present in both the sample and foil Δμ and therefore cancel out when taking the δ(Δμsample − Δμfoil) difference.

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RESULTS AND DISCUSSION Phosphoric Acid Adsorption on Pt Nanoparticles during Fuel Cell Operation. The NEG (negative maximum amplitude around 0 eV vs PtL3) and POS (the positive maximum amplitude at 2−7 eV vs PtL3) amplitudes of the Δμ at different cell voltages for the Advent sample are shown in Figure 6. With a careful classification of the positive and negative amplitudes, using their energy position, the ratio between them, and comparing them with the theoretically calculated signatures for H, O(H), and PO4, it is possible to

Figure 8. Cartoon illustrating the situation of adsorbates on the Pt surface at different cell voltages. 6213

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O(H) is coming on the surface, as indicated by a strong increase of the Δμ POS; with both H and O(H) adsorption on the surface at the extrema, adsorbed phosphoric acid is forced to leave. However, the adsorbed PO4 alters the onset of the H/ O(H) adsorption, shifting it to lower/higher potentials. In the potential region immediately above the H and below the O(H) adsorption onset, a significant PO4 coverage is evident; however, it appears to go through a minimum in the intermediate potential region. First, the zero magnitudes at ca. 450 mV are due to the fact that we used the μ at this potential as the reference spectrum as described in the Data Analysis section, so Δμ is indeed zero. However, the coverage of PO4 at Vref may not be actually zero; in fact, the PO4 coverage may be quite large and even near a maximum in this region; nevertheless, Δμ at Vref is smaller than at the other potentials above and below Vref. How do we explain this? We suggest that, at 180 °C, phosphoric acid anions do not adsorb in an ordered arrangement or in registry on the Pt surface but are mobile, making them invisible to the Δμ technique. Once H or O(H) begins to adsorb, the PO4 anions are forced to adsorb in specific ordered sites and then they become visible in the ΔμXANES. As a result of this coadsorption, the specific signature for phosphoric acid increases with minute amounts of H and O(H) coming on the surface until eventually the H/O(H) forces the PO4 off as described above. A similar behavior was observed previously for HSO4− anions at RT in H2SO4 investigated by Teliska et al.40 Comparing these results with previous papers from other authors on the adsorption of phosphoric acid on Pt surfaces at room temperature and lower concentrations, a different behavior was observed. The FT-IR measurements from Zelenay et al.18 and Iwasita et al.20 both showed an increase of phosphoric acid anion adsorption with potential above 0 mV/ 300 mV. The maximum coverage was observed at 800 mV18 or 650 mV, depending on the concentration and the adsorbing anion, respectively.20 The XAS results from He et al.23 showed an analogous behavior but with the adsorption maximum of PO4 at around 400 mV; this is shown in Figure 7 with the dashed line. The cell voltage range where we observe PO4 adsorption is in the same region (300−800 mV). The fact that PO4 is leaving the surface when H and O(H) significantly adsorb is in good agreement with the He et al.23 investigations where H starts below 300 mV and O(H) above 800 mV. The difference between their results and these here, especially compared to the XAS measurements of He et al.,23 is that while PO4 at low temperature is visible with XAS at all potentials, this is not found for our fuel cell tests at elevated temperatures. This finding suggests that with increasing temperature the mobility of the adsorbed anions increases. Unfortunately, from the perspective of ORR activity, they are apparently not leaving the surface at higher temperature but still block Pt atoms, thus hindering the ORR. Phosphoric Acid Concentration Dependence. To investigate the influence of different phosphoric acid concentrations, samples with different loadings of phosphoric acid on the fuel cell cathode were analyzed. The results of two different samples are shown in Figure 9. The magnitude of the Δμ signature (POS and NEG) increases with increasing amount of phosphoric acid per gram of Pt. This is of course due to more PO4 coming onto the Pt surface. With the more pronounced Δμ magnitudes, a feature at ca. 550 mV becomes increasingly visible (indicated by arrows in Figure 9). The POS and NEG Δμ amplitudes for PO4 at this

Figure 9. The effect of different H3PO4 loadings on the Δμ plots.

point are larger than the ones at higher and lower potentials. As this feature is seen in almost every measurement, we suggest that this peak in the PO4 region is due to a change in adsorption geometry of the adsorbed anion, as has already been described in the literature.19 With more oxygen coming on the surface, PO4 is forced to another adsorption geometry, which needs less space or has a smaller footprint on the surface. Nart et al.19 also discussed a change of the adsorption geometry for H2PO4− ions. They reported that at potentials above 900 mV the adsorption site changes from C2V geometry, where the adsorbed anion is bound with two oxygen atoms to the surface, to the single coordinated CS symmetry due to the hindrance caused by the presence of Pt−OH. The fact that this feature appears in our measurements at lower cell voltages compared to Nart et al.19 might be explained by the higher temperatures in the fuel cell tests. Temperature Dependence of Phosphoric Acid Anion Adsorption on Carbon Supported Pt Nanoparticles. To investigate the temperature effect on the adsorption behavior of phosphoric acid on the Pt catalyst, two different experiments were performed. First, we operated our fuel cell at three different temperatures (50, 110, and 170 °C). The potentials in the iV curve, at which XAS measurements took place, are indicated as dots in Figure 10. With decreasing temperature, the PBI membrane conductivity is decreasing, dominating the performance loss in the fuel cell measurements. However, the increase in temperature clearly decreases the phosphoric acid coverage on the Pt catalyst, thus having the effect of increasing the performance, but the conductivity effect overwhelms the

Figure 10. iV curves of one sample at different working temperatures (50, 110, and 170 °C). The main performance loss is due to the membrane conductivity. XAS spectra were taken at each highlighted data point. 6214

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suggests that at high total adsorbate coverage atop bonded PO4 with the smaller footprint might be preferred over the bridged. The same rearrangement behavior can also be seen in Figure 12 in which the Δμ XANES magnitudes at 540 mV are plotted

adsorbate coverage effect. In a follow-up experiment, the fuel cell voltages were held constant at 540 mV and the temperature changed from 60 to 160 °C and at every 10 °C a XAS spectrum was recorded (Figure 12). We discuss Figure 11 first. To keep the plot less congested, only the results for 50 and 170 °C are shown in this figure. The

Figure 12. Temperature-dependent Δμ magnitudes at 540 mV in a temperature range from 60 to 150 °C. POS magnitudes (solid line), NEG amplitudes (dashed line).

vs temperature. At this cell voltage, as suggested by Figure 11 and the discussion above, the effect of very small amounts of O(H) is to force the PO4 into specifically adsorbed sites, but the direct contribution of the adsorbed O(H) to the total Δμ magnitude is still expected to be minimal. Thus, these data should provide direct information on the desorption of PO4 and/or its rearrangement with temperature. The POS magnitude shows a continuous decline consistent with decreasing PO4 coverage with temperature. Below 110 °C (higher coverage), NEG decreases with T consistent with reducing coverage, but above 110 °C, the NEG increases again. This is consistent with mostly atop/bridged adsorption at low temperature (or coadsorption of 3-fold “inverted” and bridge/ atop-bonded adsorbates), and then, rearrangement to mostly 3fold “inverted” PO4 occurs above 110 °C as the PO4 coverage decreases. Notice that the rate of decrease of the POS also increases slightly at 110 °C. Zelenay et al.18 extrapolated their FT-IR adsorption results at low temperatures to temperature values up to 180 °C and reported a coverage decrease in concentrated phosphoric acid from 100% at 70 °C (or lower) to 70% at around 180 °C. With our measurements and the observed reduced Δμ POS amplitude with increasing temperature, we confirm their results in an in situ environment, but in Figure 12, the decrease in coverage appears to be much larger, a factor of 8 between 60 and 170 °C, rather than 1.4 between 70 and 180 °C. Of course, the rate of decrease here in this work is much larger because the concentration of PO4 is relatively dilute compared to that in “concentrated” H3PO4. Summarizing these temperature-dependent investigations, we conclude that the enhanced temperatures feasible with the state-of-the-art PBI membranes utilized in HT PEM fuel cells are still too low. To clean the cathode surface completely from adsorbed PO4 anions, higher working temperatures are needed. However, due to engineering and degradation issues, an increase in temperature does not seem to be wise. Alternatives to solve this problem could be either to choose another electrolyte, such as proton conductive ionic liquids, or to make the catalyst surface less “attractive” for phosphoric acid anion adsorption.41 For this purpose, alloyed catalysts with a different surface atomic arrangement could be a good choice.22,23 The Choice of the Δμ Reference. With the mentioned effects of temperature and phosphoric acid loading on the fuel cell performance, the reference potentials, Vref, needed to calculate Δμ must be changed with the working temperature

Figure 11. POS (solid line) and NEG (dashed line) amplitudes of 50 and 170 °C measurement. The arrows mark the potential range, in which the adsorption of phosphoric acid is expected. Arrows: 170 °C, 150−600 mV; 50 °C, 100−700 mV.

110 °C measurements (not shown) lie between the two others. At the most extreme cell voltages (700 mV), the POS amplitudes differ the most for the two temperatures. The increasing positive magnitudes for the 170 °C measurement in these regions are due to the increasing H below 100 mV and O(H) above 700 mV coverage on the surface. This is because phosphoric acid anions are driven off the surface by the increasing temperature easier and thus more free surface sites are available for H and O(H) adsorption. The increasing temperature therefore narrows down the cell voltage range in which PO4 adsorption can take place. The expected PO4 adsorption region is indicated with arrows in Figure 11. At cell voltages between 400 and 550 mV, a difference in the Δμ NEG magnitudes is observed for the different temperatures. First, the POS is larger at 50 °C than at 170 °C consistent with the expected greater coverage at the lower temperature. The NEG amplitude is either not visible or even goes to positive values around 400 mV in the 50 °C data. This can be explained by the different adsorption geometries of phosphoric acid and the corresponding Δμ signatures shown in Figure 2. The line shape (Figure 4) of 3-fold inverted PO4 shows a strong negative feature along with a positive feature. Atop/bridge-bonded PO4 has only a positive magnitude. This behavior indicates that at lower temperatures (higher coverage) most of the phosphoric acid anions adsorb in an atop or bridge bonded geometry. The higher 170 °C temperature reduces the atop/bridged coverage and enhances adsorption in the 3-fold “inverted” site, increasing the NEG (i.e., making NEG negative). These results suggest that at higher coverage PO4 exists in a bridge-bonded geometry and at low coverage it prefers the 3fold “inverted” geometry. This is in agreement with previous results reported by He et al.,23 which found 3-fold inverted PO4 in very dilute H3PO4 but atop/bridged PO4 in concentrated H3PO4. Apparently, at high coverage, the PO4 reduces its footprint on the surface by moving to lower coordination, the atop/bridged sites. The Δμ signature for atop and bridge are very similar, so we cannot distinguish atop from bridged with the Δμ signature alone, but the footprint reduction argument 6215

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The Journal of Physical Chemistry C and the electrolyte loading. The applied cell voltages chosen for Vref are shown in Table 1. At lower temperatures, the

50 °C

110 °C

170 °C

300 mV 300 mV

400 mV 300 mV

400 mV 400 mV

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REFERENCES

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membrane resistance contributes more strongly to the cell voltage. Because of this fact, XAS measurements at lower cell voltages were used at lower temperatures to obtain comparable cathode potentials for all measurements. Furthermore, the temperature affects the region where the PO4 is pinned on the surface by other adsorbates (coadsorption). The region in which the phosphate anion is mobile on the surface and therefore invisible to the Δμ is also shifted to higher cell voltages with increasing temperature, and we want the reference potential where the PO4 is the least visible. Only for the sample with reduced PO4 loading at 110 °C was the 400 mV reference used, because, with less PO4, H adsorption also starts at higher cell voltages. Therefore, the region where PO4 is not adsorbed in registry is also shifted to slightly higher cell voltages.

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SUMMARY AND CONCLUSIONS This work investigates the behavior and interaction of H3PO4 in the fuel cell under conditions as close to reality as possible. With in-operando XAS measurements and the Δμ XANES technique, it was possible for the first time to follow the phosphoric acid anion adsorption on the cathode side at different fuel cell potentials. With an enhanced Δμ approach, even the analysis of small changes in the signatures became feasible. The most important finding is the different adsorption behavior of phosphoric acid on Pt nanoparticles at elevated temperatures, which differs from the measurements at room temperature from other groups.18,20,23 At low cell voltages, only hydrogen is present on the surface, while with an increase in the voltage phosphoric acid appears and suppresses other adsorbates. At temperatures of about 170 °C and intermediate cell voltages, only phosphoric acid anions cover the Pt surface. Applying higher cell voltages, O(H) comes down on the cathode catalyst surface and forces adsorbed PO4 into registry again. At cell potentials higher than 800 mV, only O(H) is left on the surface. The temperature-dependent measurements showed that at lower temperatures the phosphoric acid anions adsorb over a wider potential window. Increasing the temperature therefore leads to a decrease in the PO4 coverage and an increase in the PO4 mobility, respectively. As a result, more free Pt sites are accessible for other reactions taking place on the catalyst surface.

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ACKNOWLEDGMENTS

The authors thank Dr. Maria Daletou from Advent (Greece) and Dr. Tomás ̌ Klicpera from Fumatech (Germany) for the preparation of the MEAs. The kind support of Dr. Adam Webb and Mathias Hermann, working at X1 Hasylab DESY, and David Batchelor from ANKA is gratefully acknowledged. Financial support for this effort was provided by the EU project DEMMEA, Seventh Framework Programme.

Table 1. Chosen Reference Measurements (μclean) for the Δμ Calculations 2 g of H3PO4/g of Pt 6 g of H3PO4/g of Pt

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +4930-83850-960. Notes

The authors declare no competing financial interest. 6216

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