Particle size dependence of proton conduction in a cationic lanthanum phosphonate MOF

摘要

A lanthanum(iii) metal-organic framework, PCMOF21-AcO [La-2(H2L)(1.5)(AcO)(3)(H2O)(5.59)], with a 3-D network linked by dicationic bis(dimethylphosphonato)bipiperidinium units and both coordinated and free acetate counter anions is reported. PCMOF21-AcO was water stable and showed very good proton conductivity >10(-3) S cm(-1) at 85 degrees C and 95% relative humidity. PCMOF21-AcO also showed a bimodal particle size distribution and so proton conductivity was further examined as a function of particle size. Large (>= 220 mu m), intermediate (125 <= x < 180 mu m) and small (<38 mu m) particles were sieved and proton conductivity compared. The larger particle samples showed better proton conduction, an observation that supports grain boundaries being a hurdle to proton conduction rather than an enabler (e.g. by degradation routes enabling ion mobility). Proton conductivity as a function of pelletization pressure was also studied and affirmed that, for this system, the single semicircular feature observed in impedance analysis accounted for bulk and grain boundary contributions.