Intermediate Temperature Alkaline Fuel Cells Enabled by Using Inorganic Layered Double Hydroxides as Solid Electrolyte

Layered double hydroxides (LDHs) are a class of two-dimensional material whose nanosheets are comprised of metal cations octahedrally coordinated to hydroxides. LDHs typically exist as micron wide hexagonal crystallites, and within each crystal, tens of nanosheets are vertically stacked together, forming inter-sheet regions that contain water, hydroxides, and other anions to balance charge. Unique among inorganic layered materials, LDH nanosheets have a positive charge which facilitates impressive hydroxide conductivity on the order of 10^-2 – 10^-1S/cm, competitive with state-of-the-art polymer ion-exchange membranes that are employed as solid electrolytes in alkaline fuel cells and electrolyzers. LDHs also excitingly preserve water content in their inter-sheet galleries well beyond the boiling point of water, up to 200 °C in some cases, a temperature range in which polymer electrolytes experience critical loss of humidity and thermal degradation. Therefore, LDHs are a promising candidate for solid electrolyte in alkaline fuel cells and electrolyzers as they could enable higher operating temperatures (100-200 °C) leading to faster kinetics of desired redox reactions and higher energy conversion efficiency.<br/><br/>In my presentation, I will describe my work in implementing LDHs as inorganic solid electrolyte in alkaline fuel cells to access to an elevated range of operating temperatures at and above 100 °C. First, I will discuss the fundamental relationship between hydroxide conductivity and LDH interlayer anion identity under ambient pressure conditions and over a temperature range up to 140 °C that has not yet been explored for this two-dimensional material. I will then share fabrication strategies to transform crystalline LDH powders into gas impermeable, mechanically robust, and chemically stable membrane architectures. Lastly, fuel cell polarization behavior of LDH alkaline fuel cells will be presented, and<i> in operando </i>three electrode characterization of the anode and cathode will be used to understand kinetics of the hydrogen oxidation and oxygen reduction reactions in this new class of inorganic fuel cell.