Solid state 31P MAS NMR spectroscopy and conductivity measurements on NbOPO4 and H3PO4 composite materials

•Composites have been made from NbOPO4 and H3PO4.•The composites composition has been investigated with solid state NMR.•The composites have shown clear signs of acid dehydration upon heating.•The conductivity of the composites increases for increasing acid content.•NbOPO4 has no significant conductivity of its own.

A systematic study of composite powders of niobium oxide phosphate (NbOPO4) and phosphoric acid (H3PO4) has been performed in order to characterize the material׳s ability to perform as an electrolyte material in medium temperature fuel cells and electrolyzers. Powders of H3PO4 contents between 13.1 and 74.2 M% were produced and characterized with powder X-ray diffraction, 31P MAS NMR and impedance spectroscopy. NMR revealed that a significant degree of dehydration and vaporization of H3PO4 takes place above 200 °C, and increases with temperature. At 500 °C the NbOPO4 and H3PO4 has reacted to form niobium pyrophosphate (Nb2P4O15). Impedance spectroscopy showed an increase in conductivity with increasing acid concentration, whereas the conductivity decreased slightly with increasing temperature. The highest conductivity measured was 2.5·10−3 S/cm for a sample containing 74.2 M% of H3PO4. Lastly, it was shown that NbOPO4 has no significant conductivity of its own.

Graphical abstractConductivity of NbOPO4/H3PO4 composites as a function of equivalent P2O5 content. The conductivity is insignificant for pure NbOPO4.Figure optionsDownload full-size imageDownload as PowerPoint slide