Combined Synchrotron Nano X-Ray Computed Tomography and XANES to Understand PGM-Free Anodes of Alkaline Fuel Cells

Abstract

Combined chemical information and X-ray CT or X-ray radiography is a new paradigm in energy-conversion and -storage characterization. During electrochemical experiment chemical mapping can be used to differentiate elements within fuel cell catalyst layer. Polymer electrolyte fuel cells (PEFCs) and alkaline exchange membrane (AEM) fuel cells are both low temperature, high efficiency technologies that use hydrogen and air (oxygen) to produce electricity. Most of the current efforts have focused on PEFCs, as commercial availability of polymer electrolyte membrane (PEM), such as Nafion enabled wide-spread research. Recently, AEM fuel cells have shown to produce current densities that are compatible to PEFCs 1-2. The promise of AEM fuel cells is to be more cost-effective as they can use Earth-abundant materials as electrocatalysts. In the past decade platinum group metal-free (PGM-free) catalysts have been developed. We reported novel class of NiMo catalysts, supported on carbon blacks, that showed unprecedented power densities for fully PGM-free fuel cells 3. Understanding chemical oxidation states of Ni, Mo and Cu under applied potentials in hydrogen environment within AEM fuel cell anode is critical to advance the knowledge of this domain of electrocatalysts. NiCu and NiMo bi-metallic catalysts are dispersed onto porous, high surface area supports to achieve high electrochemical surface area and good dispersion. The catalysts clusters are micrometer sized domains composed of nano particles. As hydrogen oxidation reaction proceeds in anode of these AEM fuel cells, the surface of NiCu and NiMo are reduced and thus oxidation state can change depending on local conditions. Our micro X-ray CT results indicate significant difference in electrochemical behavior and morphology between NiMo and NiCu electrocatalysts1, 3. The micro-CT technique provides large field-of-view (FOV) but it is lacking in spatial resolution. Designing experiment with combined nano X-ray computed tomography, X-ray absorption near edge structure (XANES) and/or fluorescence spatial mapping of the electrocatalyst under operating conditions will provide unprecedented insight into structure-function relations of these catalyst layers. In this work we use nano-CT imaging at adsorption edges of Ni and Cu to show electrocatalyst distribution and its XANES spectra at 18-ID National Synchrotron Light Source II (NSLS II) (Figure 1). We are the first group to use operando hardware with gas lines and electric inputs at the 18-ID NSLS II. Figure 1 shows our most recent updated operando cell at the beamline. Using our Gen. 3 operando fuel cell hardware, spatial 2D mapping of Ni and Cu was achieved as a function of applied potential and time with XANES probes. References Roy, A.; Talarposhti, M. R.; Normile, S. J.; Zenyuk, I. V.; De Andrade, V.; Artyushkova, K.; Serov, A.; Atanassov, P., Nickel–Copper Supported on a Carbon Black Hydrogen Oxidation Catalyst Integrated into an Anion-Exchange Membrane Fuel Cell. Sustainable Energy & Fuels 2018.Serov, A.; Zenyuk, I. V.; Arges, C. G.; Chatenet, M., Hot Topics in Alkaline Exchange Membrane Fuel Cells. Journal of Power Sources 375, 149-157 2018 Kabir, S.; Lemire, K.; Artyushkova, K.; Roy, A.; Odgaard, M.; Schlueter, D.; Oshchepkov, A.; Bonnefont, A.; Savinova, E.; Sabarirajan, D. C., Platinum Group Metal-Free Nimo Hydrogen Oxidation Catalysts: High Performance and Durability in Alkaline Exchange Membrane Fuel Cells. Journal of Materials Chemistry A 2017, 5, 24433-24443 Figure 1. a) Our Gen. 4 operando fuel cell at nano TXM beamline at 18-ID NSLS II, b) CAD design of modular operando cell and a photograph of our manufactured cell. c) XANES spectra and 2D projections of NiCu/KB catalyst. Figure 1