High-temperature polymer electrolyte membrane fuel cell (HT-PEMFC) is operate at over 100 °C, at which the water in the fuel cell cannot exist in the liquid phase. That is why the PBI-based membrane with phosphoric acid is used in HT-PEMFC and phosphoric acid as an electrolyte to carry hydrogen ions in the form of phosphate ions. However, phosphate ions are attached to the surface of the platinum catalyst and inhibit the catalytic activity [1]. In particular, phosphate ions attached to the cathode prevent oxygen from reacting to platinum and become a major resistance factor [2,3]

In this study, melamine was introduced to the Pt/C under 7 different concentrations in a water/ethanol-based solution at ambient temperature and pressure conditions. In the prepared catalyst, the melamine layer formed on the Pt nanoparticles protects phosphate ions while allowing oxygen to pass through.

When the catalyst surface was analyzed using XPS, a clear N 1s peak was confirmed in common in samples containing 10 mM or more melamine. In addition, when the surface element ratio was analyzed, samples of 20 mM or more showed a ratio of harmful nitrogen atoms from melamine of 4 wt% or more in common.

Elemental analysis using EDS mapping and shape change through transmission electron microscopy (TEM) as a result showed no significant difference depending on the melamine concentration, but showed a nitrogen content of about 7 wt% in common.

In a 0.1 M HClO₄ electrolyte, the binding of melamine can be confirmed through the shape change of the hydrogen oxidation and oxygen reduction peaks on cyclic voltammetry. Also, the 5 mM melamine sample showed the greatest half-wave potential in the ORR polarization curve at 1600 rpm. When the CV was measured while increasing the phosphoric acid concentration, the commercial platinum under the high phosphoric acid concentration had a very reduced ECSA, but the melamine sample showed an increased ECSA. In addition, the electrochemical performance of commercial Pt/C through the ORR polarization curve decreased significantly as the phosphoric acid concentration increased. On the other hand, it was confirmed that the reduction amount was greatly reduced in the sample in which 10 mM or more of melamine was synthesized. As a result, the performance under 1 M phosphoric acid solution was the best in the 20 mM melamine sample, in which the voltage at 0.2 mA/cm² is 0.817 V at melamine 10 mM versus 0.715 V at Pt/C.

References

[1] Bevilacqua, N.; George, M. G.; Galbiati, S.; Bazylak, A.; Zeis, R. Phosphoric Acid Invasion in High Temperature PEM Fuel Cell Gas Diffusion Layers. Electrochimica Acta, 2017, 257, 89–98.

[2] He, Q.; Shyam, B.; Nishijima, M.; Ramaker, D.; Mukerjee, S. Mitigating Phosphate Anion Poisoning of Cathodic Pt/C Catalysts in Phosphoric Acid Fuel Cells. The Journal of Physical Chemistry C, 2013, 117, 4877–4887.

[3] Kaserer, S.; Caldwell, K. M.; Ramaker, D. E.; Roth, C. Analyzing the Influence of H3PO4 as Catalyst Poison in High Temperature PEM Fuel Cells Using in-operando X-ray Absorption Spectroscopy. The Journal of Physical Chemistry C, 2013, 117, 6210–6217.