Steam and Co-Electrolysis Sensitivity Analysis on Ni-YSZ Supported Cells

The present paper investigates the variation in performances in co-electrolysis of steam/CO₂ on SOEC single cells caused by the progressive increase of CO₂ inlet concentration and current density.

Steam electrolysis and co-electrolysis were investigated on circular Ni-YSZ supported cells produced by SOLIDpower (“ASC-700” type cells). The thin film electrolyte is constituted by YSZ and the oxygen electrode by LSCF-GDC. The active area corresponds to 12.56 cm² (2 cm of radius).

The cells were fed with different H₂O/CO₂ mixtures, while keeping 10% H₂ of the inlet molar flow in order to avoid re-oxidation of the Ni-YSZ support.

A sensitivity analysis was performed looking in particular at the effects of inlet gas composition, gas flows and temperature on the performances. I-V characterization and EIS measurements at different current densities were carried out for each sequence.

In steam electrolysis mode, the reference inlet molar flow was equal to 150 Nml/min (90% H₂0, 10% H₂), at 750°C.  The I-V curve showed a voltage of 0.861V at OCV, 1.049V at 0.5 A/cm² and 1.253V at 1 A/cm². A sensitivity analysis was performed varying the temperature (700°C, 750°C and 800°C), the inlet air flow (100 Nml/min, 300 Nml/min and 500 Nml/min) and the total inlet fuel flow (from 8 to 16 Nml/min/cm²). In this case, the steam conversion factor was kept constant equal to 50% (adjusting the current imposed).

EIS analysis exhibits that ohmic and polarization resistances decrease with current densities lower than 0.4 A/cm² and then rise with higher values. As expected, the increase of temperature has a positive impact on the performances, while the change of air and fuel flow slightly affects them.

In co-electrolysis mode the CO₂ and H₂O content was varied, keeping constant the total molar flow and the hydrogen fraction as explained before. As shown in the enclosed image, the area specific resistance (ASR) increases with the CO₂ concentration in the mixture and gas transport limitation occurs at lower current densities.

Also in this case a sensitivity analysis was conducted changing the temperature (from 700°C to 800°C) and the resulting EIS plots demonstrate different ongoing processes favored at high current densities. In particular, the reverse water-gas-shift reaction plays an important role on the overall balance of the gas species, and it directly depends on temperature and inlet fuel composition.