Stabilization Strategies for PtCo/C Catalysts for Htpem Fuel Cells

Because conventional platinum based electrocatalysts in HTPEM fuel cells do not catalyse the oxygen reduction reaction (ORR) sufficiently fast, large amounts of the scarce and expansive noble metal is used. Due to the impact of platinum on the overall system costs, the loading has to be reduced on the roadway towards commercialization of fuel cell systems. Corrosive operating conditions of HTPEM fuel cells require high stability of all compounds that are used within the electrodes. Because not many materials fulfil the stability requirements, increasing the activity of platinum by alloying with transition metals is probably the best approach [1,2].

Due to an easy up-scalability we are using a modified impregnation method to synthesize PtM/C catalysts (M = first row transition metal). Stability and activity increasing post-preparation treatments are conducted. The as-prepared electrodes are leached in acidic solutions in order to remove inactive material from the surface resulting in platinum skeleton type nanoparticles. Additionally, heat treatments are conducted to decompose any  undesired organic residues and to rearrange platinum and the alloy metal at the surface [1–4].

Fig.1: Comparison of a standard Pt/C catalyst (a) to in-house prepared and stabilized PtCo/C catalysts (b) [1].

                All catalysts are testes preliminary ex-situ by means of cyclic voltammetry employing a rotating disk electrode (RDE) and accelerated stress test (AST) protocols. Promising formulations were tested in-situ in HTPEM single cell tests at 160 °C. In Fig. 1 typical cyclic voltammograms are shown of a commercial Pt/C catalyst and of a PtCo/C catalyst before and after AST procedures. After evaluating the most promising composition and post-preparation treatment, in-situ tests were performed.

The polarization curves of MEAs with commercial and in-house prepared cathode catalysts combined with a standard anode are shown in Fig. 2. Both electrodes show similar performance although the commercial Pt/C based electrode has a 20% higher platinum loading than the PtCo/C based electrode. While the Pt/C MEA performance decreases slightly the PtCo/C MEA shows slight increase within the first 600 hours of operation.

Fig.2: a) Polarization curves of the PtCo/C based MEA and the commercial Pt/C based MEA in HTPEM fuel cell operation; b) cell voltages during 600 h long-term operation. The insert shows the voltage gain during the first hours of operation [1].

Acknowledgements

Support by NAWI Graz and financial support by the Austrian Federal Ministry of Transport, Innovation and Technology (BMVIT) and The Austrian Research Promotion Agency (FFG) through the program a3plus and the IEA research cooperation is gratefully acknowledged.

References

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