Molten Carbonate Fuel Cells (MCFCs) are a relatively recent development in fuel cell technology, with applications ranging from small to large scale power generation systems. The interconnect is the part of the MCFC to which the anode and cathode are attached and through which the electrical current generated by the cell is conducted. Therefore, the interconnect must not only be mechanically strong and resistant to oxygen and hydrogen, but also maintain high electrical conductivity and corrosion resistance (including on the surface of the interconnect) at high temperatures (550...650°C) for long periods.

Interconnections (0.3-0.5 mm thick) made of stainless steel (which contains 16-18% Cr and has a high density γ ~ 8 g/cm3) lose surface electrical conductivity due to oxidation. MAX phases Ti₂AlC₃ and Ti₂AlC have two times lower density (γ ~ 4.1 - 4.3 g/cm³) than stainless steel, are stable in oxygen and hydrogen atmosphere at high temperature, and have high electrical conductivity. Therefore, the MAX phase based coatings are potentially promising for this application. OT4-1 titanium alloy substrates with protective coatings are being developed for use as interconnects for MCFCs to replace 316L stainless steel.

Ti-Al-C, (Ti,Mo)-Al-C and (Ti,Cr)-Al-C coatings were deposited on OT4-1 alloy substrates by hybrid magnetron sputtering and cathodic arc evaporation. For magnetron sputtering, a MAX phase (Ti₂AlC - 63 wt.% and Ti₃AlC₂ - 37 wt.%) target prepared by hot pressing of TiC, Al and TiH₂ under 20 МPа, at 1350 °С for 10 min was used. Simultaneously with magnetron sputtering of the MAX phase target, chromium or molybdenum was deposited using a cathodic arc plasma source. Three types of coatings were deposited: Ti-Al-C magnetron-only and hybrid (Ti,Mo)-Al-C and (Ti,Cr)-Al-C. The thickness of the deposited coatings was 5-11 µm.

X-ray diffraction analysis showed that all deposited coatings are close to amorphous state. The SEM-EDX study indicated that the average composition of the coatings obtained from the MAX phase based target was Ti₂Al_1.0-1.1C_1.1-1.3 (close to 211), for the coating with additions of Mo: Ti₂ Mo_2.1Al_0.9C_2.8 (close to 413) and Cr: Ti₂Cr_2.6Al_0.8C_1.5. The nanohardness of the coatings varied from 11 to 15 GPa and the Young's modulus from 188 to 240 GPa.

The (Ti,Cr)-Al-C coating showed the highest stability against electrochemical corrosion in 3.5 wt.% NaCl aqueous solution at 20 °C: corrosion potential E_corr = 0.044 V, corrosion current density i_corr = 2.48×10^-9 A/cm², anodic current density i_anodic (at 0.25 V vs. SCE) = 5.18×10^-9 A/cm². This coating also showed the highest long-term oxidation resistance and after heating in air at 600 °C, 1000 h its electrical conductivity s= 9.84×10⁶ S/m was slightly higher than before heating s= 4.35×10⁵ S/m, the nanohardness and Young's modulus are in the range of 15 GPa and 240 GPa, respectively. The increase in electrical conductivity after long-term heating at 600 °C can be explained by the observed crystallization of the amorphous phase in the structure of the coating.

Thus, the hybrid deposited (Ti,Cr)-Al-C coatings exhibit high corrosion and oxidation resistance while maintaining electrical conductivity and can be used to protect titanium alloy interconnects in lightweight MCF cells.

Acknowledgments The work was supported by the III-7-22 (0785) Project of the National Academy of Sciences of Ukraine "Development of wear-resistant electrically conductive composite materials and coatings based on MAX phases for the needs of electrical engineering, aviation, and hydrogen energy"; by the NATO project SPS G6292 “Direct liquid fuelled molten carbonate fuel cell for energy security (DIFFERENT)”, and by the MES Ukraine project №0122U001258 “Development of nanotechnological methods to prevent corrosion of structural materials in thermal and nuclear power plants”.

In the electrodynamic properties study of new AlN- 5wt.%C(diamond powder)-5wt.%Mo composite materials the values ​​and   behavior of the real and imaginary parts of the microwave permittivity were determined and the optimal technological process for materials synthesis was established. The materials with density 3.16 g/cm³ and 3.30 g/cm³ were obtained by the pressureless sintering (PS) at the temperature of 1850 °C and by the hot pressing (HP) at the temperature of 1820 °C (pressure of 15 MPa), respectively.            

The microstructure and phase composition of the composite materials were studied using a scanning electron microscope and the X-ray diffractometer Ultima IV (Rigaku, Japan)  ​​with the Rietveld method for data analysis. The imaginary and real parts of the permittivity were measured by the microwave vector network analyzer Keysight PNA N5227A in the frequency range of 26 - 40 GHz.

The results of structural studies showed that sintering of composites by different methods results in  almost unchanged their phase composition, and the main phases are AlN, Al₃(O,N)₄, Mo₂C, Y₃Al₅O₁₂, and C (graphite) - as in [1].

It was determined that the real part of the permittivity (ε') for both composites obtained by the PS and HP methods is practically frequency independent between 26 to 36 GHz with the value of about 8 and 27 , respectively. At  the same time, the imaginary part of the permittivity (ε″) increases from 0.29 to 1 and from 3.46 to 5.56 for materials made by the PS and HP methods, respectively. When the test frequency is increased from 36 to 40 GHz, significant fluctuations in the permittivity values ​​ are observed, which is possibly related to the greater intensity of electromagnetic wave internal reflections due to the peculiarities of the formation of the materials structure.

As a result of the research, it was established that composite materials obtained by the hot pressing method are characterized by higher density and higher values ​​of the real part of the permittivity in the frequency range 26 - 36 GHz.