Electrochimica Acta 56 (2011) 8827–8832 Contents lists available at ScienceDirect Electrochimica Acta jou rn al hom epa ge: www.elsevier.com/locate/electacta Operando X-ray absorption and infrared fuel cell spectroscopy Emily A. Lewis a , Ian Kendrick a , Qingying Jia b , Corey Grice c , Carlo U. Segre b , Eugene S. Smotkin a,∗ a Department of Chemistry and Chemical Biology, Northeastern University, Boston, MA 02115, United States b Physics Division, Department of Biological, Chemical and Physical Sciences, Illinois Institute of Technology, Chicago, IL 60616, United States c NuVant Systems Inc., Crown Point, IN 46307, United States a r t i c l e i n f o Article history: Received 20 July 2011 Received in revised form 23 July 2011 Accepted 23 July 2011 Available online 29 July 2011 Keywords: Fuel cell Membrane electrode assembly Operando spectroscopy X-ray absorption spectroscopy Infrared spectroscopy Stark tuning Platinum Nickel XANES a b s t r a c t A polymer electrolyte fuel cell enables operando X-ray absorption and infrared spectroscopy of the membrane electrode assembly catalytic layer with flowing fuel and air streams at controlled temper- ature. Time-dependent X-ray absorption near edge structure spectra of the Pt and Ni edge of Pt based catalysts of an air-breathing cathode show that catalyst restructuring, after a potential step, has time constants from minutes to hours. The infrared Stark tuning plots of CO adsorbed on Pt at 100, 200, 300 and 400 mV vs. hydrogen reference electrode were obtained. The Stark tuning plots of CO adsorbed at 400 mV exhibit a precipitous drop in frequency coincident with the adsorption potential. The turn-down potential decreases relative to the adsorption potential and is approximately constant after 300 mV. These Stark tuning characteristics are attributed to potential dependent adsorption site selection by CO and competitive adsorption processes. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Because the active state of a catalyst exists only during catal- ysis [1], the characterization of catalysts should be done under standard device operating conditions: catalysts, incorporated into membrane electrode assemblies (MEAs), are exposed to flowing reactant streams within flow fields operating from 60 to 80 ◦ C. This work focuses on platinum-based catalysts of polymer electrolyte reactors. Such reactors have applications in organic synthesis [2–9], environmental remediation [10,11] and energy conversion [12]. The MEA, a polymer electrolyte membrane sandwiched between catalytic layers contacting porous carbon paper or cloth gas dif- fusion layers, is optimized for reactant transport and electronic conduction. MEA fabrication methods have been reviewed [13]. Briefly, inks are prepared by dispersing catalysts in solubilized ionomer. The inks are either deposited onto the carbon gas diffu- sion layers followed by hot pressing to the membrane (i.e., 5-layer MEA), or directly deposited onto a heated membrane surface (i.e., 3-layer MEA). The catalyst layers (carbon supported or metal blacks [14]) are a blend of ionomer, catalyst particles and, in some cases, teflon dispersion. In an operating MEA, the catalyst particles are wetted with a sub-m layer of ionomer that conducts protons and ∗ Corresponding author. Tel.: +1 617 373 7526. E-mail address: e.smotkin@neu.edu (E.S. Smotkin). enhances catalysis [15]. Operando characterization of MEA catalytic layers requires an absence of supplemental electrolytes: aqueous electrolytes (e.g., H 2 SO 4 or HClO 4 ) contribute mobile anion adsor- bates, and preclude fuel cell operation at the high end of relevant temperatures (e.g., 70–90 ◦ C). Proper cell design is the key challenge to operando spectroscopy. In addition to a “real world” catalyst environment, standard electro- chemical cell design principles must be adhered to, including equal resistance between any points of the working electrode surface to an auxiliary electrode surface, and a high impedance reference electrode [16]. Stainless steel should be avoided. It includes iron, nickel and chromium, which fluoresce at energies similar to the edge energies of Pt based catalysts. Viswanathan et al. [17] and Stoupin et al. [18] introduced operando X-ray absorption spectroscopy (XAS) of hydrogen and liquid feed direct methanol fuel cells respectively using the cell in Fig. 1. This cell design was used by Principi et al. [19] in low Pt load- ing XAS studies. Palladium at the cathode mitigates interference when studying Pt based catalyst edge energies. A recently reported operando X-ray absorption cell describes a design that mitigates this problem [20]. The resistance between the anode and cathode catalytic lay- ers of a membrane electrode assembly is governed by a uniform polymer electrolyte membrane thickness (ca. 7 mil for Nafion 117) [21]. The counter electrode to the working electrode of interest serves as both the auxiliary and the reference electrode 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.07.091