Solid alkaline fuel cells (SAFCs) have attracted much attention as next-generation energy conversion electrochemical devices. Despite numerous advantages of SAFCs including the usages of various type catalysts and liquid fuels with high energy density, the practical applications of these devices are limited because of the drastic decrease in the cell performance during the high temperature operation under alkaline condition. This low performance of the device is mainly attributed to the severe degradation of membrane-electrode assembly (MEA). Therefore, the development of a durable and stable MEA is critical to improve the cell performances and stabilities in this system.

Here we report the development of highly-durable MEA using a new carbon-free cathode catalyst and an aromatic polyelectrolytes without an ether linkage. As shown in Fig. 1A-a, the carbon-free catalyst consists of a nanosized beaded network formed by the connection of Pt–Fe nanoparticles.^[1–3] The beaded metal network is electrically conductive, enabling the removal of carbon supports from catalyst layer. Hence, carbon corrosion problems can be avoided, leading to high durability against the cyclic start-stop operation of the system.

We successfully demonstrated the high durability of this carbon-free catalyst in alkaline electrolyte solution. In addition, the specific activity of a connected Pt–Fe catalyst for oxygen-reduction-reaction (ORR) is about five times higher than that of a commercial Pt-nanoparticle catalyst supported on carbon black.

Fig. 1A-b represents the chemical structure of new spirobifluorene-based aromatic ionomer with no ether-linkage. This anion-conductive ionomer exhibited high alkaline and oxidative durability because the backbone did not contain ether linkages, heteroatoms, and benzylic C–H bonds. It was also demonstrated that the thin-layer coating (< 1 μm) of spirobifluorene-based ionomer on the anion-exchange membrane (Fig. 1A-c) improved alkaline and oxidative stability, and suppressed the permeability of potassium formate (HCOOK) as fuel.

We fabricated the MEA using the in-house made durable catalyst and membrane material, i.e., the carbon-free connected Pt–Fe catalyst, spirobifluorene-based ionomer, and anion conductive pore-filling membrane with durable thin layer. The in-house made MEA, operated at a high temperature of 80 °C, exhibited high power density of 220 mW cm⁻² and high open circuit voltage of about 1.0 V (Fig. 1B), here, a mixture of aqueous solutions of 4M HCOOK and 2 M KOH was used as the fuel. Further, more importantly, the high performance of the MEA retained even after the operation at 80 °C and 0.2 A cm⁻² for 150 h. Therefore, for the first time, we demonstrated a highly durable and stable MEA in alkaline medium for the direct formate SAFCs.

[1] T. Tamaki, H. Kuroki, S. Ogura, T. Fuchigami, Y. Kitamoto, and T. Yamaguchi, Energy Environ. Sci., 8, 3545–3549 (2015).

[2] H. Kuroki, T. Tamaki, and T. Yamaguchi, J. Electrochem. Soc., 163(8), F927–F932 (2016).

[3] H. Kuroki, T. Tamaki, M. Matsumoto, M. Arao, Y. Takahashi, H. Imai, Y. Kitamoto, and T. Yamaguchi, ACS Appl. Energy Mater., 1(2), 324–330 (2018).