The Ni-based cermet SOFC anode material exhibits a number of undesirable properties. It is often redox unstable ¹, Ni-particles agglomerate over time ¹, ultimately reducing the catalytic activity of the anode and, in unprocessed natural gas feeds, this material exhibits severe coking intolerance ¹ as well as sulphur poisoning ¹. Therefore, there is a requirement to produce a novel anode material which can offer better durability under the aforementioned conditions. An ideal replacement material comprises a ‘backbone’ microstructure of the A-site deficient perovskite: La_0.20Sr_0.25Ca_0.45TiO₃ (LSCT_A-), impregnated with an oxide ion conducting material and a metallic electrocatalyst. This ‘backbone’ material, impregnated with Ce_0.8Gd_0.2O_1.9 (CGO) and Ni, has previously been employed as a SOFC anode material on an industrially relevant scale at HEXIS AG, with promising results ².

Current research focusses on optimisation of the LSCT_A- ‘backbone’, to improve current distribution through the anode, and development of impregnated catalyst systems that provide enhanced durability and tolerance to natural gas feeds containing sulphur. Thick-film ceramic processing techniques, such as ink formulation, screen printing and control of sintering protocol, have been used as the primary method in controlling the anode microstructure. Rheological analysis of a variety of LSCT_A- inks showed that a formulation with 75 wt. % solids loading possessed ideal (pseudoplastic) properties for screen printing. Extensive investigation of screen printing parameters and screen mesh counts, as well as sintering temperatures and dwell times, allowed determination of the optimal conditions required to produce a LSCT_A- anode ‘backbone’ microstructure with an advantageous combination of porosity and grain connectivity. Screen printing of the 75 wt. % solids loading ink with a 230 mesh count (per inch) screen and sintering at 1350 °C for 2 hours facilitated production of the required anode microstructure, ensuring sufficient lateral electronic conductivity through the anode to prevent generation of localised temperature ‘hotspots’. Four-point DC conductivity analysis of several LSCT_A- ‘backbone’ microstructures showed that ‘effective’ conductivities of up to 21 S cm^-1 could be achieved (in 5% H₂/Ar), with the highest values pertaining to the most advantageous microstructure. Electrolyte-supported fuel cells employing this ‘backbone’ microstructure, impregnated with 12-16 wt. % (of the ‘backbone’) of CGO and 2-5 wt. % of either Ni, Ru, Rh, Pt or Pd, showed very promising performances during short-term electrochemical testing in humidified hydrogen. Fuel cells with anodes containing Rh/CGO and Pd/CGO catalyst systems were particularly promising, achieving Area Specific Resistances (ASR) of 0.41 Ω cm² and 0.39 Ω cm² (figure 1), respectively.

References

1. Sun, C. & Stimming, U. Recent anode advances in solid oxide fuel cells. J. Power Sources 171, 247–260 (2007).

2. Verbraeken, M. C. et al. Short stack and full system test using a ceramic A-site deficient strontium titanate anode, Fuel Cells, 15, 5, 682 – 688 (2015).