Proton Substructure and Conductivity of the Room-Temperature Proton Conductors Zr 1− x Sc x H 5+ x (PO ₄ ) ₃

Solid proton conductors are an important materials class when it comes to the further development of hydrogen-based devices for energy generation and storage. These applications make it essential to get high proton conductivity and stability at around 25 °C without the need for additional humidification. The proton conductor zirconium acid triphosphate (ZrH 5 (PO 4 ) 3 ) has been shown to meet these requirements and is therefore of interest. In this work, we present a systematic study of Sc-substituted Zr 1− x Sc x H 5+ x (PO 4 ) 3 samples in the range of 0 ≤ x ≤ 1. The structure and proton transport properties are investigated by neutron powder and X-ray diffraction data, second-harmonic generation measurements, nuclear magnetic resonance, and impedance spectroscopy. The obtained results provide a better understanding of the H + network in ZrH 5 (PO 4 ) 3, implying a disordered arrangement. This is not observed in ScH 6 (PO 4 ) 3, which, in contrast to ZrH 5 (PO 4 ) 3 with 1.7(4) mS cm −1 at 25 °C, has a significantly lower proton conductivity of 3.1(6) × 10 −4 mS cm −1 . The aim of this work is to give deeper insight into the structure−property relationship of this class of proton conductors to contribute to the further development of materials.