Identification of Catalyst Structure during the Hydrogen Oxidation

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Identification of catalyst structure during hydrogen oxidation reaction in an operating PEM fuel cell Armin Siebel, Yelena Gorlin, Julien Durst, Olivier Proux, Frédéric Hasché, Moniek Tromp, and Hubert A. Gasteiger ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02157 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016

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Identification of catalyst structure during hydrogen oxidation reaction in an operating PEM fuel cell Armin Siebel,a Yelena Gorlin,a* Julien Durst,b Olivier Proux,c,d Frédéric Hasché,a Moniek Tromp,e Hubert A. Gasteigera a.

Chair of Technical Electrochemistry, Department of Chemistry and Catalysis Research

Center, Technical University of Munich, Lichtenbergstr. 4, D-85748 Garching, Germany. * [email protected] b.

Laboratory for Catalysis and Sustainable Chemistry, LSK, Paul Scherrer Institute, Swiss

Light Source, CH-5232 Villigen PSI, Switzerland. c.

BM30B/FAME beamline, European Synchrotron Radiation Facility (ESRF), F-38000

Grenoble, France. d.

Observatoire des Sciences de l’Univers de Grenoble, UMS 832, CNRS, Université

Joseph Fourier, F-38000 Grenoble, France. e.

Van't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904,

1098XH Amsterdam, The Netherlands

ACS Paragon Plus Environment

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Abstract

Palladium is among the most active catalysts for the hydrogen oxidation reaction (HOR) and thus a potential candidate for replacing platinum in fuel cell catalysis. At the same time, it is well known for absorbing large amounts of hydrogen, forming a bulk hydride phase. In several electrochemical studies conducted in liquid electrolytes and temperatures between 60 and 20 °C, the hydrogen from the hydride phase was observed to desorb at potentials positive of ≈32 mV to 50 mV vs. the reversible hydrogen electrode (RHE). Here, we present operando spectroscopic studies in a fuel cell configuration. We first validate our experimental setup by comparing the potential dependence of hydrogen absorption isotherms of a Pd/C catalyst under nitrogen determined both electrochemically and by operando X-ray absorption spectroscopy (XAS) at various temperatures between 20 and 100 °C. Subsequently, we investigate the structure of the Pd/C catalyst during the HOR in a fuel cell operating at 80 °C in a H2-pump configuration. Our results unequivocally show that, in contrast to rotating-disk electrode (RDE) data reported in the literature, the hydride phase is maintained during the HOR in a fuel cell anode environment. The discrepancy between our results and previously published data is explained in terms of the vastly different mass-transport limitations in a fuel cell and in a conventional liquid electrolyte based electrochemical cell, and highlights the importance of investigating catalyst structure in a representative reaction environment.

KEYWORDS HOR, operando EXAFS, palladium hydride, PEMFC, RDE


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Introduction In proton exchange membrane fuel cells (PEMFC) the hydrogen oxidation reaction (HOR) constitutes the anode half-cell reaction, in which H2 is electrochemically oxidized according to equation 1: H2 ⇌ 2H+ +2e-

(1)

It is well established that the best currently known electro-catalysts for the HOR as well as for the hydrogen evolution reaction (HER), are platinum-group metals (PGMs).1,2 Among these, platinum itself is the most active monometallic material, based on its high HOR/HER exchange current density (e.g., i0HOR/HER ≈ 2-5·10-1 A/cm²Pt at 80 °C).3 This exchange current density can be understood as a turn-over frequency (TOF) of the electrochemical reaction and corresponds to a real TOF of ≈ 500-1200 molH2/molPt,surface/s (see description in the Supporting Information (SI) for conversion of i0 to TOF). The exchange current densities of other PGMs, including Ir, Rh or Pd, have recently been identified to be 10-100 times lower than that of Pt and were reported to decrease as the polarization of the anode is increased.2,4 As this decrease in catalytic activity for the HOR with increasing potential could be explained by a change in the catalyst’s bulk (e.g., hydride decomposition) and/or surface (e.g., oxide formation) properties,2 it is important both for the fundamental understanding of the HOR/HER and for the development of non-Pt based catalysts to characterize these properties under real reaction conditions. Our study focuses on the characterization of Pd, an HOR catalyst that can exist in metallic and hydride states. Previously reported electrochemical characterization of Pd in acidic liquid electrolytes under both inert gas (N2- or Ar-saturated)5-9 and reactive gas (H2-saturated)2,10-12 conditions in a three-electrode electrochemical cell have been shown to exhibit an electrochemical feature, which corresponded to the desorption of hydrogen from the hydride


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phase, during positive polarization of the electrode. Due to the use of liquid electrolytes, these studies were typically limited to room temperature and observed a transition from a hydride to metallic phase at ≈50 mV vs. the reversible hydrogen electrode (RHE) potential (from here on referred to as “VRHE“). However, recently published HOR data recorded in a fuel cell setup at 80 °C, i.e., at typical operating conditions of a low temperature PEMFC, do not exhibit a clear electrochemical feature indicative of a phase transition between PdHx and Pd.2 To bridge the gap between room temperature electrochemical studies in liquid electrolytes and fuel cell work conducted at 80 °C, we present electrochemical isotherms for the absorption of hydrogen into a Pd catalyst as a function of applied potential, temperature, and reaction atmosphere. We first conduct our experiments with the Pd working electrode under inert gas atmosphere (N2) at temperatures between 20 and 100 °C, which we refer to as cyclic voltammetry (CV) mode. Then, we extend our characterization to fuel cell anode relevant conditions, where the Pd working electrode is polarized in 1 bar H2 at 80 °C; these experiments are referred to as H2-pump configuration.3 In both cases, a Pt counter electrode under 1 bar H2 is used, which also serves as reference electrode. In the CV mode, we obtain hydrogen absorption isotherms using both electrochemical and X-ray absorption spectroscopy (XAS) characterization, a non-invasive technique that can offer element-specific information about the electronic and local geometric structure of catalysts. Changes in the Pd structure can be evaluated using the extended X-ray absorption fine structure (EXAFS) region of a K-edge X-ray absorption spectrum by taking advantage of the fact that hydride formation not only greatly alters the electronic band structure of the metal but also increases the crystal lattice parameter.8,13,14 After establishing the equivalence of electrochemical and spectroscopic methods in the CV mode under N2 and identifying that, at 80 °C, the transition from a hydride to a metallic state occurs at


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≈25 mV, we extend our characterization to PEMFC operating conditions in the H2-pump mode, i.e., under 1 bar of H2. Owing to the high HOR currents even at low anodic overpotentials, the hydride to metal state transformation in H2 cannot be followed by purely electrochemical methods and, consequently, we apply operando EXAFS to determine the exact catalyst state during the HOR in a PEMFC. Our spectroscopic characterization during hydrogen oxidation unequivocally demonstrates that, in contrast to the previously observed transition from a hydride to a metallic state in liquid electrolytes, the hydride phase is maintained under practical operating conditions of a fuel cell anode even at high anodic potentials. Furthermore, using the obtained data, we clarify that the inconsistency between our results and previous studies can be explained quantitatively by the orders of magnitude lower mass-transport rates in liquid electrolyte based electrochemical cells compared to PEMFCs, which results in different reaction environments and thus, affects the chemical state of the Pd catalyst. Our findings highlight the necessity of characterizing the properties of electrocatalysts under realistic operating conditions. Experimental Operando fuel cell design For this study, an improved X-ray absorption spectroscopy (XAS) electrochemical cell was designed in-house, allowing the investigation of PEMFC electrodes during operation. In fuel cell research, the first in situ cells used liquid electrolytes, which had the disadvantages of specific adsorption of electrolyte anions and limited ability to conduct measurements above room temperature.15-18 More recently, spectro-electrochemical cells have been directly adapted from modified fuel cell hardware by locally thinning the graphite flow field to either 4 mm19,20 or 1.5 mm21 thickness to serve as an X-ray window. While this development is a significant


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improvement over liquid cells, the remaining layer of graphite severely reduces the intensity of X-rays at the sample and at the detector. This phenomenon is especially problematic at lower X-ray energies, at which the signal-to-noise ratio deteriorates, resulting in poor quality of the spectra. The 1.5-4 mm thick graphite windows effectively exclude the operando characterization of 3d transition metal catalysts (e.g., Mn, Fe, Co, Ni, etc.) due to their impermeability for X-rays at the K-edge energies (6-8 keV) of these elements (s. below). In the present work, we have improved the flow field design and were able to reduce the thickness of the graphite window down to 500 µm. The AXF 5Q graphite plates were machined by Poco Graphite (Houston, TX) and subsequently pyro-sealed in order to prevent gas permeation through the very thin graphite layer. Fig. 1a shows a photograph of the flow field block (7.6x7.6 cm²) and flow field geometry with an active area of 5 cm²geo (front) and the rectangular window (back). In order to maintain mechanical stability, which is required to be able to apply sufficient force to seal the cell and ensure good contact between the flow fields and the gas-diffusion media, the window area was kept as small as possible (2x15 mm²) by decreasing the window length through the thickness of the flow field. The round-trip transmission of the window (i.e., X-rays passing the window twice) as a function of the incident X-ray energy was calculated from the increased attenuation length in the 45° geometry required for fluorescence measurements (i.e., increase by a factor of 1.4) and can be seen to approach 100 % at the high energy of the Pd K-edge (Fig. 1b, green dotted line). We also calculated the X-ray transmission as a function of incident beam energy for the two flow fields used in the XAS studies discussed above and included them in Fig. 1b (blue and red dotted lines) to enable a direct comparison of the benefit of our improved cell design: our design provides experimentally feasible round-trip transmission values at the K-edges of Mn (≈8%), Fe (≈12%), Co (≈20%), and


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Ni (≈30%), contrary to 25 mV under N2 atmosphere in the CV mode. The stability of the Pd-Hx phase up to high anodic potentials in the H2-pump mode is in stark contrast to the commonly observed phenomenon of


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Pd β-hydride decomposition at lower anodic potentials in many RDE studies.2,10-12 To understand the origin of this discrepancy, we consider the differences in the experimental conditions for the HOR RDE studies versus those in a fuel cell.

Figure 5: a) Potentiostatic polarization of the 80 wt.% Pd/C (0.25 ± 0.05 mgPd/cm²MEA) vs. current density during the HOR at 80 °C in the H2-pump configuration, where both the Pd/C working electrode and the Pt/C counter/reference electrode are flushed with fully-humidified H2 at a partial pressure of 1 barabs, holding each potential for 1 h. The current corresponds to the average value over the total step duration of 1 h, during which 2 X-ray absorption spectra were recorded. Displayed are the measured cell voltage, Ecell (blue circles), the HFR-corrected cell voltage (EHFR-free ≡ Ecell – i·RHFR; orange diamonds), and the true Pd/C working electrode potential, EPd/C, after correction for the proton transport resistance (corrected acc. to Eq. 7; green stars). Note that the overpotential for the HER on the Pt/C counter electrode was assumed to be negligible, i.e., ηHER(Pt/C) ≈ 0. b) Pd nearest neighbour distance obtained by operando XAS (left y-axis) and value of x in Pd-Hx determined via Eq. 3 (right y-axis) vs. the potential of the Pd/C working electrode: the blue line/symbols represent the 80 °C isotherm recorded in N2 atmosphere (CV mode, s. blue line/symbols in Fig. 4a), whereas the green line/symbols correspond to data measured during the H2-pump experiment.


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Although H2 is supplied at atmospheric pressure in both RDE and H2-pump experimental setups, the mass transport rates are drastically different. Specifically, the diffusion limited current density, ilim, in a typical fuel cell configuration is on the order of 10+2 A/cm2MEA,3 while it is on the order of 10-3 A/cm2disk under typical RDE conditions, e.g., ilim = 2.4 mA/cm²disk at 1600 rpm in 0.1 M HClO4 at 20 °C (s. Fig. 6). The poor mass transport in liquid electrolyte based cells results in a significant change of the H2 concentration at the catalyst surface (expressed as H2 partial pressure at the catalyst surface, pH2(surf)) with respect to the equilibrium concentration in the bulk solution (again expressed as H2 partial pressure, pH2(bulk)), which can be related to the ratio of the HOR disk current density, idisk, over the limiting current density: pH

2 ( )

pH

2 ( )

=1-

iHOR ilim

(9)

This dependence of pH2(surf) on the anode potential (from Eq. 9 with ilim = 2.4 mA/cm²disk) during the HOR on Pd/C measured by RDE in 0.1 M HClO4 (data from Durst et al.2) is shown on the left axis in Fig. 6 (green dashed-dotted line), together with its current response plotted on the right y-axis (solid green line). In the acidic electrolyte, in which the HOR kinetics are very fast (i0HOR/HER ≈ 3 mA/cm²Pd), the current density begins to approach ilim at ≈ 49 mVRHE (dash-dotted green line in Fig. 6a), i.e., immediately prior to the potential-dependent onset of hydrogen desorption from the hydride phase, which has been observed to occur at ≈50 mVRHE (52 mVRHE at 20 °C) in N2 atmosphere both in this study and in the previously published literature.5-9 If the apparent HOR kinetics of the Pd electrode are decreased, for example by using alkaline electrolytes where i0HOR/HER ≈ 60 µA/cm²Pd4, the diffusion limited current is reached much later than at 50 mVRHE, as seen in Figure 6b. On closer inspection of the alkaline RDE data, one can see that approximately at the onset of Pd-Hx desorption (≈165 mVRHE, solid magenta line in


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Fig. 6), pH2(surf) decreases below the Pd-Hx/Pd transition pressure of ≈16 mbar (s. dashed-dotted magenta line in Fig. 6), which clearly demonstrates that the higher Pd-Hx desorption potential in RDE experiments in 0.1 M NaOH is simply related to the higher overpotential required to approach the limiting current, owing to the slower HOR kinetics on Pd/C in alkaline electrolytes. In summary, this analysis demonstrates that in the RDE-based HOR experiments performed in H2-saturated electrolytes, the Pd-Hx/Pd transition occurs at the potential at which pH2(surf) decreases below the transition pressure observed in gas phase experiments (16 mbar) unless this potential is negative of the corresponding Nernst potential (52 mVRHE), in which case the transition occurs at the Nernst potential. In H2-pump experiments, however, pH2(surf) will remain close to 1 bar at the experimentally accessible current densities, and therefore, no Pd-Hx/Pd transition occurs (s. green stars in Fig. 5b). We note that under real fuel cell operating conditions of 80 °C, a fixed pressure of pcell = 1-1.5 barabs, and fully humidified (pH

2O

= 0.47 barabs ) pure H2

anode gas feed, Pd-Hx/Pd transition is also not expected to occur. Under these conditions, the H2pressure at any part of the flow field will be pH = pcell - pH 2

2O

= 0.53-1.03 barabs and lead to a

continuous presence of the Pd-Hx phase.


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Figure 6: a) Representation of the RDE polarization curve measured by Durst et al.2 in 0.1 M HClO4 at 20 °C using a scan rate of 20 mV/s and 10 µgPd/cm2geo and electrode rotation rate of 1600 rpm, which shows the feature corresponding to the desorption of hydrogen from the bulk hydride phase (solid green line); the diffusion-limited current is indicated with a dashed green line. On the basis of the RDE curve, the change in local and effective hydrogen concentration at the electrode surface, pH2(surf), was calculated using Eq. 9 (dash-dotted green line; left axis). The orange dash-dotted line visualizes the lack of change in pH2(surf) in a fuel cell in the same potential range (corresponding HOR polarization curve was omitted for clarity). For comparison with the CV experiments presented above (Fig. 4), the effective H2-pressure as a function of electrode potential in N2-environment is included (Eq. 6, blue line). When HOR/HER kinetics are sufficiently high and ilim is reached at a potential that is more cathodic of 52 mV, hydride desorption in the RDE-experiment will be prescribed by the Nernst equation (dotted green line). b) HOR polarization curves of Pd recorded in an RDE experiment at 20 °C using 0.1 M HClO4 (green line, same curve as in panel a)) or 0.1 M NaOH (magenta line, 10 mV/s and 20 µgPd/cm2geo) as a supporting electrolyte. Both curves exhibit a clear feature related to desorption of the bulk hydride phase (dashed grey lines). Additionally, the current-dependent gradient of the surface reactant concentration for the respective curve is displayed as dash-dotted line of the same color. At 20 °C, the transition from bulk hydride to metallic Pd occurs below a hydrogen pressure of ≈16 mbar or applied potential of 52 mVRHE, whichever occurs at a later time point.


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Conclusions In this work, we have first presented the hydrogen absorption isotherms of a Pd/C catalyst at temperatures ranging from room temperature up to 100 °C in N2 atmosphere in a fuel cell setup by varying the applied electrochemical potential and characterizing the extent of hydride formation using both cyclic voltammetry and X-ray absorption spectroscopy. We then established the consistency of our electrochemically generated isotherms with the widely available gas phase data and extended our characterization of the Pd/C catalyst to fuel cell conditions (during HOR in H2-pump mode at 80 °C and H2 pressure of 1 bar), at which the hydride can form from the electrochemically generated H2 at the surface of the catalyst and from the H2 that is supplied in the gas phase. In contrast to the previously published observations of hydrogen desorption from the hydride phase during HOR in liquid electrolytes, we found that the Pd catalyst exhibited the hydride phase up to high positive potentials in a fuel cell environment. We explain the apparent inconsistency between measurements in liquid electrolytes and an operating fuel cell by the different mass transport in the two experimental techniques and thus highlight the importance of carefully choosing the appropriate characterization conditions to identify catalyst structure. We believe that the necessity of using an appropriate mass transfer regime for the development of structure-activity relationships of electrocatalysts will not be limited to the HOR, but will also have implications for all electrocatalytic reactions in which the reactant is supplied in a gaseous form. Supporting Information Details regarding TOF calculation, the operando experimental setup, characterization of the catalyst by XRD and TEM, MEA preparation, the experimental procedure, EXAFS data analysis,


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electrochemical isotherms plotted on the pressure scale derived for each temperature, as well as Tafel plot and Butler-Volmer analysis are provided as supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgments The authors acknowledge the French SOLEIL-CRG committee for provision of synchrotron facilities at BM30B at the European Synchrotron Radiation Facility (ESRF), Grenoble, France and thank Maximilian Bernt, Alin Orfanidi, and Jacinto Sà for assistance during the measurements. Yelena Gorlin gratefully acknowledges the support of the Alexander von Humboldt Postdoctoral Fellowship and Carl Friedrich von Siemens Fellowship Supplement. Many thanks are also due to Thomas Mittermeier for helpful discussions and comments on the manuscript. Abbreviations CV, cyclic voltammetry; EC, electrochemical; ECSA, electrochemically active surface area; EW, equivalent weight; EXAFS, extended X-ray absorption fine structure; fcc, face-centered cubic; FFT, fast Fourier transform; FWHM, full width at half maximum; GDL, gas-diffusion layer; Had, surface-adsorbed hydrogen; HER, hydrogen evolution reaction; HFR, high-frequency resistance; HOR, hydrogen oxidation reaction; H-upd, hydrogen under-potential deposition; MEA, membrane-electrode assembly; OCV, open-circuit voltage; PEIS, potentiostatic impedance spectroscopy; PEM, proton-exchange membrane; FC, fuel cell; PGM, platinum-group metals; PTFE, poly(tetrafluoroethylene); RDE, rotating-disk electrode; RH, relative humidity; RHE, reversible hydrogen electrode; TEM, transmission electron microscopy; TOF, turn-over frequency; VRHE, Volt vs. RHE; XANES, X-ray absorption near-edge structure; XAS, X-ray absorption spectroscopy; XRD, X-ray diffraction


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(27) Tew, M. W.; Miller, J. T.; van Bokhoven, J. A. J. Phys. Chem. C 2009, 113, 1514015147. (28) Bugaev, A. L.; Srabionyan, V. V.; Soldatov, A. V.; Bugaev, L. A.; Bokhoven, J. A. v. J. Phys.: Conf. Ser. 2013, 430, 012028. (29) Feenstra, R.; Griessen, R.; Groot, D. G. d. J. Phys. F: Met. Phys. 1986, 16, 1933. (30) Wang, J.; Wang, Q.; Jiang, X.; Liu, Z.; Yang, W.; Frenkel, A. I. J. Phys. Chem. C 2015, 119, 854-861. (31) Ratchford, R. J.; Castellan, G. W. J. Phys. Chem. 1958, 62, 1123-1127. (32) Vasile, M. J.; Enke, C. G. J. Electrochem. Soc. 1965, 112, 865-870. (33) Flanagan, T. B.; Lewis, F. A. Trans. Faraday Soc. 1959, 55, 1409-1420. (34) Czerwiński, A.; Kiersztyn, I.; Grdeń, M. J. Solid State Electrochem. 2003, 7, 321-326. (35) Bernardini, M.; Comisso, N.; Fabrizio, M.; Mengoli, G.; Randi, A. J. Electroanal. Chem. 1998, 453, 221-230. (36) Gillespie, L. J.; Hall, F. P. J. Am. Chem. Soc. 1926, 48, 1207-1219. (37) Frieske, H.; Wicke, E. Ber. Bunsenges. Phys. Chem 1973, 77, 48-52. (38) Mütschele, T.; Kirchheim, R. Scr. Metall. 1987, 21, 1101-1104. (39) Kuji, T.; Matsumura, Y.; Uchida, H.; Aizawa, T. J. Alloys Compd. 2002, 330–332, 718722. (40) Züttel, A.; Nützenadel, C.; Schmid, G.; Chartouni, D.; Schlapbach, L. J. Alloys Compd. 1999, 293–295, 472-475. (41) Wu, W.; Baker, D. R.; Liu, Y.; Gasteiger, H. A. in Handbook of Fuel Cells, John Wiley & Sons, Ltd, Chichester, UK 2010, 6, 631-657.


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ACS Catalysis

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ACS Catalysis

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TOC 80x44mm (300 x 300 DPI)


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Identification of Catalyst Structure during the Hydrogen Oxidation

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