Gel-type shell contributing to the high proton conductivity of pyrophosphates

Metal pyrophosphates have attracted considerable interests due to their high proton conductivity and potentially wide applications in the temperature range of 100-400 °C. However, great difference in conductivity was reported by different groups on the same pyrophosphates. The reason for the huge difference is still in debate up to now, and there is no coherent standpoint in literatures on the proton conduction mechanism. In this study, we chose Fe0.4Nb0.5P2O7, which was reported showing high proton conductivity recently, as an example to disclose the reason inducing the divergence in proton conductivity and conduction mechanism. We found that the as-prepared pyrophosphate grains have three layers, i.e. crystalline pyrophosphate core, amorphous phosphate shell in the middle and gel-type shell composed of amorphous phosphorus species as the outermost layer. The content of amorphous phosphorus species decreases with the increase of the calcination temperature of pyrophosphates, and the calcination temperature-dependent residual soluble phosphorus curve extremely coincides with the conductivity curve. Thus, the proton conduction of pyrophosphates is realized via a gel-type shell formed by residual amorphous phosphorus species on surfaces of pyrophosphate grains. We suggested that the phosphorus content is the key factor to explain the great difference in conductivity of pyrophosphates prepared by different groups.