Redox transformation of (NdCrO4)-O-V involved by insertion/desertion of protons was examined by XRD, XPS, in situ FT-IR and fuel cell tests. It was found that NdCrO4 caused bulk reduction with H-2 gas at around 400 degrees C, changing to amorphous Nd(III)-Cr(III) hydroxide phase. This hydroxide phase, however, could recover to the original zircon type oxide phase by air oxidation at the same temperature region. Proton-conducting ceramic fuel cells having NdCrO4 film as an interlayer between cathode and electrolyte was fabricated and the cell was confirmed to exhibit remarkable power generation giving rise to OCV of more than 1.0 Vat 500 degrees C. It was speculated that the interlayer facilitates proton transfer from electrolyte to cathode because redox cycling of NdCrO4 and the corresponding hydroxide was driven by the high proton chemical potential at the interface with electrolyte and plenty of oxygen in cathode gases. (C) 2015 Elsevier B.V. All rights reserved.