Ionic conducting materials are the most important parts in electrochemical devices such as batteries, fuel cells and electrolysers. Different types of ionic conducting materials, which conduct O^2-, H⁺ or OH^- ions, have been widely used in solid oxide fuel cells (SOFCs), solid oxide electrolytic cells (SOECs), proton exchange membrane fuel cells (PEMFCs) and electrolysers (PEM electrolysers), anion exchange membrane fuel cells (AEMFCs) and electrolysers (AEM electrolysers). The typical materials are solid oxides such as doped ZrO₂ or CeO₂ with fluorite structures or doped BaCeO₃ (BCO)/BaZrO₃ (BZO)/LaGaO₃ (LGO) with perovskite structures^1-10. The representative high temperature proton conductors based on doped perovskite oxides BCO/BZO were discovered in 1980 and 1990s respectively^{2, 6}, while the high temperature O^2- ion conductor based on doped LGO was discovered in 1994^{3, 11}. Those materials have been applied in SOFCs and SOECs. The typical operating temperature is above 550 °C while the durability of the solid oxide cells (SOCs) are not ideal due to the high operating temperature. When temperature drops, the O^2- or H⁺ ionic conductivity in oxides is not high enough to provide high power density for the SOCs. Therefore it is desired to develop low temperature O^2-, H⁺ or OH^- ionic conducting materials.

Polymer membranes such as acidic proton-conducting Nafion membrane have been used in PEMFCs and PEM electrolysers but the acidic membrane are very expensive. Expensive noble electrode materials such as Pt, IrO₂ have to be used in PEMFCs or PEM electrolyers leading to high cost, which is the key barrier for large scale applications. Also European proposes to ban all fluorine-based membrane such as Nafion in 2026-27. It is desired to find a replacement membrane for Nafion. The alkaline OH^- ionic conducting membranes based quaternary ammonium groups have been investigated for over a decade but their stability is not ideal, also reacting with CO₂ in air leading to reduced OH^- ionic conductivity. It is urgent to discover new stable OH^- ionic conducting materials for AEMFCs and AEM electrolysers.

Fortunately, in 2020, Tao's group at University of Warwick found that some ceramic materials exhibit high mixed OH^-/H⁺ ionic conduction in water, with ionic conductivity exceeds 0.01 S cm^-1 and ion transfer number above 98% at a temperature below 100 °C. Under certain conditions, the ionic conductivity exceeds 0.1 S cm^-1. Some of ceramic OH^- and H⁺ ionic conductors are very stable, ideal electrolyte materials for low temperature electrochemical devices such as fuel cells, electrolysers and batteries.

For the first time, ceramic low temperature OH^- and H⁺ ionic conducting materials have been discovered. The application in direct ammonia fuel cells has been demonstrated as well.