Speciation and Proton Conductivity of Phosphoric Acid Confined in Mesoporous Silica

TREVANI, LILIANA N.; LÉPORI, CRISTIAN M. O.; GARRO LINCK, YAMILA; MONTI, GUSTAVO A.; ABUIN, GRACIELA C.; VACA CHÁVEZ, FABIÁN; CORTI, HORACIO R.

ACS APPLIED MATERIALS & INTERFACES

AMER CHEMICAL SOC

Lugar: Washington; Año: 2022 vol. 14 p. 33248 - 33256

1944-8244

Phosphoric acid (PA) confined in a commercialmesoporous silica (CARIACT G) with porous size in the range of3 to 10 nm was studied in relation to its coordination with thesilanol groups on the silica surface as a function of temperature, upto 180 °C, using 31P and 29Si MAS NMR spectroscopy. As thetemperature increases, the coordination of Si and P in themesopores depends on the pore size, that is, on the area/volumeratio of the silica matrix. In the mesoporous silica with the higherpore size (10 nm), a considerable fraction of PA is nonbonded tothe silanol groups on the surface, and it seems to be responsible forits higher conductivity at temperatures above 120 °C as comparedto the samples with a smaller pore size. The electrical conductivityof the functionalized mesoporous silica was higher than thatreported for other silico-phosphoric composites synthesized by sol−gel methods using soft templates, which require hightemperaturecalcination and high-cost reagents and are close to that of the best PA-doped polybenzimidazole membranes used inhigh-temperature proton exchange membrane fuel cells (HT-PEMFCs). The rate of PA release from the mesoporous silica matrixwhen the system is exposed to water has been measured, and it was found to be strongly dependent on the pore size. The low costand simplicity of the PA-functionalized mesoporous silica preparation method makes this material a promising candidate to be usedas an electrolyte in HT-PEMFCs.