Durable ceramic materials for improvement of Proton Exchange Membrane electrolysis cells

This work was devoted to explore different classes of oxides as potential auxiliary materials for Proton Exchange Membrane electrolysis cells (PEMEC) anodes. After the preparation of the materials, the stability in acid solutions was evaluated by chemical and electrochemical means. A conductivity target was defined as 0.1Scm^-1 at the temperatures of operation of a PEMEC, the typical conductivity of the electrolyte polymeric membrane.
Given the excellent corrosion stability of SnO₂, the formation of a solid solution SnO₂–Bi₂O₃ was studied to increase the p-type conductivity of tin oxide. Several compositions of Bi-doped SnO₂ were prepared with increasing cat.%_Bi (namely 0.5, 1, 1.5, 2,3,4,5). The preparation procedure did not produce a solid solution. It was found that even for small cat.%_Bi the system was composed of a mixture of SnO₂ and Bi₂Sn₂O₇, a pyrochlore-structured secondary phase. Increasing the fraction of secondary phase decreased both the stability of the composites toward corrosion and the measured electrical conductivity, which did not meet the target of this study. An hypothesis was made, corroborated by evaluation of the activation energy for electrical conduction and microscopy analysis, that the nucleation of the pyrochlore phase at the grain boundaries of SnO₂ is at the origin of the decrease in conductivity and corrosion stability of the compounds.
Several compositions of MCr₂O₄ spinels (with M=Zn,Mg,Ni, NiFe, Mn) were prepared and tested. It was found that the materials possess excellent corrosion stability in acid conditions, but the electrical conductivity was below the target of this study. Doping with Cu and Li was conducted on NiCr₂O₄ and MnCr₂O₄ in order to increase the electrical conductivity. In the case of MnCu_0.25Cr_1.75O₄, MnLi_0.25Cr_1.75O₄ and MnLi_0.5Cr_1.5O₄ a pure cubic spinel phase was obtained. Compared with the conductivity of MnCr₂O₄ (~10^-12Scm^-1 at 70◦C), the conductivity was increased to ~10^-10Scm^-1 in the case of MnLi_0.25Cr_1.75O₄ and MnLi_0.5Cr_1.5O₄ and ~10^-4Scm^-1 in the case of MnCu_0.25Cr_1.75O₄ at 70◦C.
Electrochemical characterization was performed on MnCu_0.25Cr_1.75O₄ and NiCr₂O₄. It was found that the material undergo only partial oxidation upon potential cycling up to 2.0V vs SHE. It was demonstrated that HCrO₄^– and MnO₄^– are dissolution products of the materials as a consequence of the oxidation. A MnCu_0.25Cr_1.75O₄-loaded PEM electrolysis cell was prepared and tested. The total cell resistance irreversible increased after incresing potentials steps. It was concluded that, despite the materials show excellent corrosion stability, the conductivity of spinel structured chromites is too low for the application in PEMEC.
Various compositions of spinel-structured MTi₂O₄ (M=Li,Mg,Mn) were prepared by solid state synthesis. The materials showed high electrical conductivity at temperature of interest for a PEMEC (~30-100Scm^-1), but it was found that they are unstable toward corrosion in acidic medium. A degradation mechanism of the materials in water was proposed, which encompass both dissolution and solid-state oxidation. It was concluded that despite the excellent electrical conductivity, MTi₂O₄ spinels are too unstable for the application in PEMEC anodes.