Ion Conduction Mechanisms and Thermal Properties of Hydrated and Anhydrous Phosphoric Acids Studied with ¹H, ²H, and ³¹P NMR

Abstract

To understand the behaviors of phosphoric acids in fuel cells, the ion conduction mechanisms of phosphoric acids in condensed states without free water and in a monomer state with water were studied by measuring the ionic conductivity (sigma) using AC impedance, thermal properties, and self-diffusion coefficients (D) and spin-lattice relaxation times (T1) with multinuclear NMR. The self-diffusion coefficient of the protons (H+ or H3O+), H2O, and H located around the phosphate were always larger than the diffusion coefficients of the phosphates and the disparity increased with increasing phosphate concentration. The diffusion coefficients of the samples containing D2O paralleled those in the protonated samples. Since the 1H NMR T1 values exhibited a minimum with temperature, it was possible to determine the correlation times and they were found to be of nanosecond order for a distance of nanometer order for a flip. The agreement of the ionic conductivities measured directly and those calculated from the diffusion coefficients indicates that the ion conduction obeys the Nernst-Einstein equation in the condensed phosphoric acids. The proton diffusion plays a dominant role in the ion conduction, especially in the condensed phosphoric acids.

Proton Conductivity in Phosphoric Acid: The Role of Quantum Effects

“…Thus this crossover in the temperature dependence of conductivity and τ σ provides another way to estimate Tg of our materials. Comparison of conductivities in deuterated and hydrogenated PAs (Fig.2c) reveals a strong isotope effect at all studied temperatures and compositions R. This result agrees with earlier studies [22] where a decrease of conductivity was reported in partially deuterated PAs, although these measurements were limited to high temperatures only. The observed strong isotope effect (Fig.2c) contradicts the vehicular mechanism for proton conductivity, for which no measurable isotope effect is expected due to very small changes of the mass of molecular units (vehicle) upon D/H substitution.…”

“…NMR studies of PA found that the average proton mobility is more than 4 times higher than phosphorus mobility, clearly indicating a proton transfer mechanism of conductivity [22]. Recent detailed analysis based on ab initio molecular dynamics studies [21] suggested that, indeed, proton transport in PA involves the intermolecular Grotthuss-like proton transfer mechanism.…”

Proton conductivity of Al(H2PO4)3–H3PO4 composites at intermediate temperature

“…(2)). According to Chuang et al [34] a decrease of conductivity of orthophosphoric acid was observed on the formation of pyrophosphoric acid at higher temperatures [35]. The activation energy calculated from the regression lines of samples AP4 and AP5 are listed in Table 4.…”

Limits of Proton Conductivity

“…But even this mechanism leads to a proton conductivity of only approximately 0.1 S cm À1 just above the melting point, which actually increases through the addition of water to a maximum of about 0.3-0.5 S cm À1 . [17] The discussion of the conduction mechanism in the nanoassembly of trimesic acid and melamine by Ji and coworkers is by far not meeting todays understanding of proton-conduction phenomena, which is just adumbrated here. But since we did not want to exclude the possibility of an unexpected discovery, we prepared whiskers in the same way that Ji and co-workers did and measured the conductivity in humidified atmosphere.…”