Electrocatalysts are critical for facilitating numerous important electrochemical reactions such as hydrogen evolution reaction (HER) in hydrogen production, oxygen reduction reaction (ORR) in fuel cells,¹ and emerging electrochemical production of hydrogen peroxide (H₂O₂) via a two-electron ORR.² To date, high-performance electrocatalysts have relied heavily on the use of scarce and expensive precious metals. To lower the cost, researchers have focused on developing platinum group metal-free (PGM-free) electrocatalysts based on earth-abundant elements, such as carbon, nitrogen, and transition metals.³ However, these alternative catalysts often suffer from insufficient stability caused by chemical leaching of transition metal components in harsh acidic environments, which restrict their applications in devices requiring acidic electrolytes, including perfluorosulfonic acids such as Nafion^®.

In contrast, organic molecules have the advantages of low cost, well-defined active sites and good chemical compatibility with polymer electrolytes. Previous studies have shown the capability of organic molecules to catalyze HER^{4, 5} and two-electron ORR.^{6, 7} However, in most cases, these demonstrations were carried out in the electrochemical cell filled with non-aqueous electrolytes and the activities were still very low compared to metal-containing materials. Much work is needed to further improve the activity of organic molecular catalysts, especially in practical polymer electrolyte systems.

In this presentation, we will summarize our study of a bifunctional electrocatalyst for HER and two-electron ORR based on a small organic molecule. In rotating disk electrode (RDE) test, the catalyst showed a low onset potential of ca. -0.05 V vs. RHE for HER with an overpotential of ca. -0.35 V at 10 mA/cm² in 0.5 M H₂SO₄ at 80 °C. In the MEA-based electrochemical H₂ pump test, it reached 10 mA/cm² at -0.12 V vs. RHE at 80 °C and showed no performance loss in a 60-hour durability test. In addition to the high HER activity, the molecular catalyst also showed high activity and selectivity for two-electron ORR to yield H₂O₂. The onset potential was ca. 0.6 V vs. RHE with over 80% H₂O₂ yield in a rotating ring disk electrode (RRDE) test in O₂-saturated 0.5 M H₂SO₄. Density functional theory (DFT) calculations were performed to gain preliminary insight into the reaction mechanisms of HER and two-electron ORR on the organic molecule. The results revealed thermodynamically favorable reaction pathways and structure-activity relationship in this system, which will be presented, too.

References

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