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 MD_xCr_2-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, 10^1.5 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.

References:

[1] M. Carmo, D. L. Fritz, J. Mergel, D. Stolten, Int. J. Hydrogen Energy, 38, 4901–4934 (2013).

[2] E. Oakton, D. Lebedev, M. Povia, D. F. Abbott, E. Fabbri, A. Fedorov, M. Nachtegaal, C. Copéret, T. J. Schmidt, ACS Catal.,7, 2346-2352 (2017).

[3] H. S. Oh, H. N. Nong, T. Reier, A. Bergmann, M. Gliech, J. Ferreira De Araújo, E. Willinger, R. Schlögl, D. Teschner, P. Strasser, J. Am. Chem. Soc., 138, 12552–12563 (2016).

[4] E. Stefan, P. Connor, A. K. Azad, J. T. S. Irvine, J. Mater. Chem., A 2, 18106–18114 (2014).

[5] A. La Fontaine, H. W. Yen, P. J. Felfer, S. P. Ringer, J. M. Cairney, Scr. Mater., 99, 1–4 (2015).

[6] C.-W. Chen, P.-A. Chen, C.-J. Wei, H.-L. Huang, C.-J. Jou, Y.-L. Wei, H. P. Wang, MPB, 124, 1106-1110 (2017)