Performance and Durability of an Alkaline Direct Methanol Fuel Cell Employing a Nitrogen Post-Doped PtRu/C Anode

 

 

         Alkaline direct methanol fuel cells (ADMFCs) are attractive due to the enhanced electro-kinetics and lower methanol crossover compared to acid-based DMFCs¹. However, performance and long-term stability results to-date are insufficient for commercial application. Recently, we have demonstrated that acid based DMFC anodes employing nitrogen pre and post-doped PtRu/C catalytic materials provide markedly higher performance and stability when compared to standard (un-doped) state-of-the-art commercial PtRu/C catalysts^2,3,4. In this work, a commercial PtRu/C catalyst that has been post-doped with nitrogen (PtRu/C-N) is evaluated for the anode of a single-cell ADMFC. In the post-doped approach, nitrogen is incorporated onto the commercial PtRu/C (HS-10000) catalyst by ion implantation technique.

           XPS analysis for the PtRu/C doped with nitrogen using high dosage ion implantation shows that 1-2 at.% nitrogen was incorporated into the material. The formation of lower-binding energy nitrogen species was particularly favored. Furthermore, the XPS and TEM results revealed that a fraction of PtRu particles reorganize to form small well-dispersed nanoparticles on the surface of the carbon. Fig.1 shows the methanol stripping voltammetry curves used to measure the electrochemical surface area (ECSA) for the PtRu/C-N and un-doped PtRu/C catalysts at 25¢ªC (scan rate 5mV sec^-1). The PtRu/C-N exhibits higher ECSA (51.5 m² g^-1) as compared with the un-doped counterpart (35.4 m² g^-1).

              Fig. 2 compares the ADMFC (using a Tokuyama A-201 membrane) performances for anodes with PtRu/C-N (3mg cm^-2) vs. un-doped PtRu/C (3mg cm^-2) in 2M NaOH + 2M CH₃OH at 80¢ªC. In both cases, 40wt.% Pt/C (2mg cm^-2) catalyst was used in the cathode. The anode with PtRu/C-N exhibits improved performance, particularly, in the ohmic and mass transfer regions as compared to the un-doped PtRu/C anode. The improved performance of nitrogen post-doped catalyst can be at least partially attributed to the observed higher ECSA as noted above. The PtRu/C-N delivers maximum power densities of 140 and 110 mW cm^-2 whereas the un-doped PtRu/C delivers maximum power densities of 117 and 100 mW cm^-2using oxygen and air, respectively. The performance of PtRu/C-N based anode among the best ever reported for an ADMFC under similar catalyst loading and measurement conditions. A long-term durability test for the ADMFC with the anodes of PtRu/C-N and un-doped PtRu/C is underway to see the effect of nitrogen on the stability of PtRu.

References

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Acknowledgements

This work was supported by the Army Research Office under grant #W911NF-09-1-0528 and the U.S. Department of Energy EERE, Fuel Cell Technologies Program, under Contract No. DE-AC36-08-GO28308 with the National Renewable Energy Laboratory. The authors also acknowledge surface analysis and fuel cell testing facilities provided at NREL.