Non-Carbon Catalyst Support for Polymer Electrolyte Fuel Cells

The most commonly used anode and cathode electrocatalysts in polymer electrolyte fuel cells (PEFCs) are nanosized platinum or platinum alloy (Pt-M₁M₂) particles dispersed on high surface area, porous, conductive carbon black (e.g., Vulcan XC-72). Carbon has become the state-of-the-art support due to its good electrical conductivity (e.g., 2 S/cm for Vulcan XC-72) and chemical stability in the fuel cell environment. However, during extended PEFC operation, or load cycling, and start-up/shut-down, the carbon undergoes electrochemical oxidation to form surface oxides and CO₂, accompanied by catalyst (Pt or Pt-M) oxidation/dissolution.^1-3 The presence of Pt or Pt-M nanoparticles also accelerates the carbon corrosion.¹  As a result, Pt particles sinter into larger particles, accompanied by weakening of the interaction between Pt and the carbon support which results in the dislodging of Pt from the carbon support.^3-5  These processes lead to the loss of electrochemically active surface area (ECSA) of the electrocatalysts, resulting in a decline in fuel cell performance. Carbon corrosion is especially problematic in the automotive application, where the frequent shutdowns result in localized potentials close to 1.0 V at the air/fuel boundary in the Pt/C anode of the PEFC and as high as ~1.5 V at the Pt/C cathode.^6,7

         To improve the durability of electrocatalysts and PEFCs, more durable catalyst support materials than carbon black have been investigated or developed. They include more stable carbon forms such as graphitized carbon^8-9 and carbon nanotubes/nanofibers^10-11 and non-carbon supports such as conducting/semiconducting oxides,^12-14 transition metal carbides,¹⁵ nitrides (mainly TiN),¹⁶ and silicides (mainly TiSi₂)^17-19.  In this work, we report on our investigations of alternative transition metal silicides (TMSs) as PEFC cathode catalyst supports. The candidate support materials were identified by studying the chemical and electrochemical stability and oxygen reduction reaction (ORR) catalytic activity of seven TMSs. Various synthetic approaches including colloidal synthesis using different capping agents, strong electrostatic adsorption (SEA), and wet impregnation were explored to make the silicide-supported Pt catalysts (Pt/TMS) with the desired phase, composition, and microstructure. The effect of the support treatment was also studied. The properties of the prepared Pt/TMS catalysts were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), temperature-programmed oxidation/temperature-programmed reduction (TPO/TPR), and rotating disk electrode techniques.

          Select silicides showed the requisite electronic conductivity and promising stability against oxidative degradation.  Further work is needed to improve the dispersion of Pt nanoparticles on the silicide supports to translate these materials into viable fuel cell catalysts.

Acknowledgements

Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC, under contract DE-AC-02-06CH11357.           

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