Cr- and Ti-Based Spinels As Materials for Anodic Catalyst Support in PEM Electrolysis Cells: Assessing Corrosion Stability and Support Role in Catalyst Activity of Corrosion Stable Ceramics
Authors: Fenini Filippo , Hansen Kent Kammer , Savaniu Cristian , Irvine John T. S. , Mogensen Mogens Bjerg 
Published: February 2020

PEM electrolysis is an efficient and robust technology for conversion of power-to-H2 for energy storage [1]. However, the cost of the components is still too high for large scale applications. Corrosion resistant catalyst supports with high conductivity have continuously been researched in order to provide a better catalyst utilization and current collection eliminating the need of Pt coated Ti components [2,3]. Spinel-structured chromites are stable toward corrosion. They are the passivating layer on the surface of stainless steels as a product of the oxidation of the surface [4,5]. On the other hand, Ti-based spinels are conductive at room temperature and they are reported to reversibly exchange H+ on Li+ sites [6], which might enable proton conductivity throughout the support.

MCr2O4 (M = Mg, Zn, Mn, Ni, NiFe) compounds were synthesized by solid state route to form a single phase of the cubic spinel compound. In order to increase the overall conductivity of the best performing ceramic support, nitrate combustion synthesis was used to prepare MDxCr2-xO4 (where M=Ni, Mn, and D=Cu, Li) with x=0.25, 0.5.While NiCr2O4 showed limited ability to be doped, single phase spinels were obtained in the case of doped MnCr2O4.

MTi2O4 (with M=Li, Mg, Mn) were also prepared by solid state synthesis. LiTi2O4 pellets were prepared by wrapping cylindrical pellets of the raw materials with Cu foil and firing them in 5% H2/Ar. MnTi2O4, and MgTi2O4 pellets were wrapped in Mo foil, and fired in vacuum in quartz tubes. For all the materials, tiny amount of secondary phases were detected after the synthesis.

The conductivity of the materials was measured on sintered rectangular bars using a 4-probe DC measurement, revealing insulating characteristics of the materials. The conductivity of doped MnCr2O4 showed an increase of several orders of magnitude, reaching 10-4 S/cm at 70°C. In the case of Ti-based spinels, the conductivity was measured on disks using a Van der Pauw set up. The measured conductivities were of the order of 102, 101.5 and 10 S/cm for LiTi2O4, MnTi2O4 and MgTi2O4 respectively.

All the materials were tested chemically for 24 h at 85°C in a 1:1 H2SO4:HNO3 1 M mixture under continuous agitation and electrochemically by repeated cycling of an ink-drop casted layer between 0.5 and 2.0 V vs RHE in 1 M H2SO4 in a RDE setup. The activity test was conducted on physical mixtures with IrO2 and support by CV between 0.5 and 2.0 V vs RHE in 0.5 M H2SO4 in a RDE setup.

The chemical testing of undoped Cr-based spinels showed negligible corrosion. The test did not show any negative effect of doping to the corrosion stability of the compound. Ti-based spinels showed to undergo sensible mass loss after chemical corrosion testing, together with loss of the spinel crystal structure.

Electrochemical characterization of the best performing materials, Cu-MnCr2O4 and LiTi2O4, revealed that the materials are redox active in the potential window under analysis. Both the materials present distinctive oxidation peaks, coupled to reduction peaks in the case of Cu-MnCr2O4. To establish the effect of doping, also the undoped MnCr2O4 was tested. The integrated areas of the voltammetry curves are consistent with the complete oxidation of the deposited material, in the case of MnCr2O4 and LiTi2O4. Interestingly, Cu-MnCr2O4 does not show the same electrochemical activity in the potential window under study. Activity test were conducted on pure IrO2 and on the physical mixtures IrO2/Cu-MnCr2O4 and IrO2/LiTi2O4. The absolute value of current density at 2.0 V showed an increase of 10% in the case of IrO2/Cu-MnCr2O4 compared with pure IrO2.

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