Towards a Highly Stable and Performing Cathode Catalyst for PEM Fuel Cells for Automotive Applications

The oxygen reduction reaction (ORR) is the rate-determining step of polymer electrolyte membrane fuel cells; moreover since the oxygen reduction catalyst operates at high potentials, cathode corrosion represents a significant issue. The cathode catalysts thus deserves special attention both with regard to the electrocatalytic activity and resistance to degradation. In the present work, the activity dealing with the cathode catalyst development has regarded the synthesis of Pt-Co/C catalysts with an ordered cubic primitive structure and Pt segregation in the outermost surface layers to achieve superior electrocatalytic activity and stability. The synthesis involved the preparation of an amorphous PtO_x/C precursor by the sulfite complex route, an impregnation with Co(NO₃)₂, a high temperature (800°C) carbothermal reduction and, finally, a leaching procedure. This specific preparation procedure was selected to achieve a good dispersion of metal particles for the catalysts while assuring good stability due to the thermal treatment. The catalyst was physico-chemically characterised by using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectroscopy (LE-ISS). The herein reported method led to the occurrence of a Pt₃Co/C catalyst with a primitive cubic ordered (L1₂) phase and a mean crystallite size of 3.3 nm, as well as a suitable degree of alloying. A pre-leaching procedure at 80 °C in 0.5 M HClO₄ was necessary to remove most of the Co atoms segregated on the outer surface of PtCo catalysts after the thermal reduction. To evaluate the electrochemical behaviour of this cathodic catalyst, different electrodes were prepared by using a standardised spray technique. As anode catalyst, a reference commercial 40%Pt/C Alfa Aesar was used. The catalytic ink was prepared by mixing in an ultrasonic bath the catalyst and the ionomer. The catalytic layer was deposited onto a gas diffusion layer SGL25BC. A Pt loading of 0.2 mg/cm² was used for both electrodes. The MEAs were prepared by hot pressing, assembling the electrodes with a standard NR212 membrane at 125°C. The electrochemical characterization was carried out in terms of polarisation curves at different operative conditions. Moreover, to evaluate the improved stability of the PtCo/C cathodic catalyst respect to a benchmark Pt/C catalyst, an accelerated stress test was carried out on both catalysts. The accelerated stress test procedure is performed in H₂/O₂ at 80°C 100% RH and 1.5 bar, by cycling the cell potential between 0.6 and 0.9 V for about 10⁴ cycles, checking the performance (I-V curve) and the electrochemical surface area (CV) variations at the beginning (BOL) and the end (EOL) of life. No significant change in performance as a consequence of the stress test is highlighted for the PtCo/C catalyst. XRD analyses carried out on the catalysts after the stress test pointed out an increase of the crystallite size from 3.5 nm to 4.5 nm for the benchmark Pt/C, whereas the crystallite size of 3.3 nm was unaltered for the PtCo/C catalyst.

Acknowledgments

The research leading to these results has received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) for Fuel Cell and Hydrogen Joint Technology Initiative under Grant no 303452 (IMPACT).