Melamine Modification of Pt Surface Increases the ORR Activity and Durability of Pt/C
Authors: Yamazaki Shin-ichi , Asahi Masafumi , Taguchi Noboru , Ioroi Tsutomu 
Published: December 2024

We have investigated surface modification of Pt to enhance the performance of Pt/C. The surface of Pt is partly covered with Pt-O species such as Pt-OH and Pt-O, which are inhibitors for the ORR activity. We try to mitigate the coverage of these species by modification of the Pt surface with external organic compounds, and to enhance the ORR activity. Although most of compounds we tested showed inhibitory effects on the ORR activity, we have found that melamine (and teteraazaporphyrin) exceptionally enhance the ORR activity (Figure 1) [1-5]. This enhancement of the ORR would be reasonably ascribed to the destabilization of Pt-OH species by melamine. Melamine also mitigates the degradation of Pt/C after an accelerated degradation test [5,6]. This enhancement of durability would be related with the destabilization of Pt-OH.

The next step is application of this modification technology to MEA. We made an MEA using melamine-adsorbed Pt/Vulcan as a cathode catalyst. When the amount of melamine is high, the IV performance is lower than MEA using unmodified Pt/Vulcan. This indicates that melamine has negative effect at higher adsorption amount almost due to the over-adsorption of melamine. To investigate this over-adsorption problem, we examined the concentration dependence of the enhancement of the ORR activity of Pt/C (measured with RDE) with regard to triazines including melamine. The results are shown in Figure 1. Melamine can enhance ORR activity at very low concentration but might inhibit ORR at high concentrations (blue line in Figure 1). Cyanuric acid, which is adsorbed on Pt more weakly than melamine, exhibits enhancement of ORR of Pt/C at high concentrations, but no enhancement at low concentrations (green line in Figure 1) [7].

The results of Figure 1 indicate that effect of triazines strongly depends on the number of amino groups, and that melamine, which has three amino groups, exhibit peaky concentration dependence. This dependence is explained by the strongness of adsorption of these triazines on Pt surface. It is shown in an adsorption mechanism presented by DFT calculations that the number of amino groups is closely correlated with the binding energy [8]. This coincides with the concentration profile in Figure 1.

Considering the peaky concentration dependence of melamine in Figure 1, we tested lower amount of melamine to counteract the over-adsorption of melamine. This lower melamine-adsorbed Pt/Vulcan exhibits better MEA performance than unmodified one in low current density (LCD). In this talk, we will discuss the RDE measurement and MEA performance of melamine-modified Pt catalysts.

This paper is based on the results obtained from projects, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

References

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Figure 1