Development of nickel alloy hydrogen electrodes for solid oxide electrolysis cell

Abstract

The conventional nickel-yttrium stabilized zirconia (Ni-YSZ) cathode in solid oxide electrolysis cell (SOEC) suffers from oxidation after exposed to steam during electrolysis for hydrogen production. The Ni-chromium (Cr)-iron (Fe) alloys with various compositions are introduced to improve cathode oxidation resistance. This study aims to study the effect of alloy contents and determine suitable fabrication method for SOEC cathode. The alloy containing cathode should satisfy both high electrochemical performance and low degradation rate. The content of Fe and Cr are varied in the range of 0-20%wt when the weight ratio of metal to YSZ is maintained constantly at 60:40. The SOEC having Ni-Cr-Fe alloy cathode are fabricated. The oxidation resistances are studied using thermogravimetric analysis (TGA). The electrochemical performance is carried out using current/potential characteristic curve and electrochemical impedance spectroscopy (EIS). The operating temperature is controlled between 650-900 °C with the feed containing steam to hydrogen ratio of 70:30. Electrolyte-supported 55Ni5Cr-YSZ/YSZ/Pt shows the highest current density of -1.09 A/cm2 at 1.8 V, 800 °C, compared to the other NiCr alloy compositions. For NiFe cathode in the electrolyte-supported cell, the sample cracked due to the thermal expansion after sintering at 1,100 °C for 2 hrs. The sintered cathode supporting layer of various alloys loading were fragile and brittle. The wash-coating and impregnating technique was introduced as an alternative method to fabricate the alloy containing cathode. The Ni alloys impregnated improved the durability of SOEC while lowering the performance. The highest current density of conventional Ni-YSZ/ YSZ/ BSCF and alloy-impregnated Ni-YSZ/ YSZ/ BSCF was -0.50 and -0.26 A/cm2, respectively. For wash-coated electrode, the dense grains were formed which was not suitable for the hydrogen electrode due to lacking of three phase boundary.