(28e) Highly Durable Direct Formate Solid Alkaline Fuel Cells Using New Aromatic Anion Exchange Polymer and Carbon Free Electro-Catalysts

Solid
alkaline fuel cells (SAFCs) have attracted attention as next-generation energy
conversion devices. Despite numerous advantages including the usages of
non-noble metal catalysts and liquid fuels with high energy density, the
practical applications of these devices are limited by drastically decreasing
cell performance during the operating at high temperature and alkaline
condition due to the severe degradation of membrane-electrode assemblies
(MEAs). Therefore, the development of durable MEAs is critical to improve cell
performances and stabilities in this system.

In this
study, we developed a new spirobifluorene-based aromatic
ionomer with no ether-linkage. The 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 improved alkaline and oxidative stability and suppressed the
permeability of potassium formate (HCOOK) as fuel.

A
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 layers. Carbon-free catalyst layers can eliminate carbon
corrosion problems, leading to high durability against start-stop operations.
We 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 5 times higher than that
of a commercial Pt-nanoparticle catalyst supported on carbon black.

We
prepared the MEA using these durable materials of the spirobifluorene-based
ionomer, and anion conductive pore-filling membrane with the durable thin
layers and the carbon-free connected Pt-Fe catalyst. The MEA operated at high
temperature of 80 áµ’C, and 4M HCOOK and 2 M KOH aqueous solution was used as
liquid fuel. As shown in Fig.1, the MEA exhibited high power density of 220 mW cm^−2 and high open circuit voltage of about
1.0 V. Importantly, the high performance retained even after the operation at
80 °C and 0.2 A cm^−2 for
150 h.

This
study demonstrated, for the first time, long-lasting MEA at high temperature and
alkaline condition in direct formate SAFCs.

References

[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).