Demonstration of hydrogen production in a hybrid lignite-assisted solid oxide electrolysis cell

Abstract

A Ni-YSZ/YSZ/Ag–Pt cell was used to demonstrate the concept of high temperature lignite-assisted electrolysis in hybrid (combined solid oxide and molten carbonate) operation. To assess the performance of the hybrid concept, the same cell was also used for lignite-assisted electrolysis in absence of an anodic carbonate load, as well as for fuel cell measurements using H2 and lignite as fuels, the latter both with and free of molten carbonates. In fuel cell operation, the hybrid direct lignite fuel cell obtained 45% higher maximum power density than the H2 fed SOFC and 160% higher power density than the lignite fuel cell without carbonates. For high temperature electrolysis, the hybrid concept of admixed lignite and carbonates at the anode led to a 350% higher current density (up to 508 mA∙cm−2, at 1.95 V), compared to the lignite-assisted operation in absence of carbonates (145 mA∙cm−2, at 1.95 V). Thus, the anodic addition of carbonates within the same cell, increased H2 production 3.5 times. This was accompanied by an equivalent increase of the anodic fuel consumption, and the cell's efficiency was essentially unaffected. Nonetheless, significant anodic and cathodic resistances at low overpotentials restricted electrolysis performance and efficiency, in either the absence or the presence of carbonates. These resistances, most likely due to both the cathodic steam activation and the anodic “shuttle” of the CO intermediate, were drastically alleviated at higher overpotentials. The presence of carbonates caused an earlier and more rapid decrease of the anodic area specific resistance, to much lower values at high overpotentials, resulting in the considerably higher performance of the hybrid mode.