Performance Analysis of Ammonia in Solid Oxide Fuel Cells

Abstract

The transition of the marine propulsion system towards alternative fuels is mandatory to offset carbon emission. Among the alternative fuels, ammonia is carbon-free and can be produced in sustainable ways. Ammonia has 17.8% hydrogen (wt %) and is easily liquified at 25°C and 8 bar pressure. The two-stroke internal combustion engines currently used in the marine sector reach efficiencies of about 50% but generate substantial polluting emissions. Solid oxide fuel cells generate electricity with efficiencies greater than 50 % and can use ammonia as fuel.In this work, a single-cell SOFC was characterised using in-situ ammonia decomposition reaction (Int-ADR) and compared with ex-situ ammonia decomposition reaction (Ext - ADR; composition - N2:H2 = 1:3), and pure hydrogen (H2 100%; composition - H2 = 100%), between temperatures of 750 °C to 850°C. The constant load tests performed at 750°C with 84% fuel utilization revealed that the LHV efficiency of the ammonia fed SOFC was 58%. The J-V characteristics showed similar performances for the three compositions at different temperatures and flowrates. The open-circuit voltage (OCV) of Int-ADR was similar to that of Ext- ADR, confirming that usage of ammonia as fuel in the Ni-YSZ anode involved two steps, (i) decomposition of ammonia into nitrogen and hydrogen, followed by (ii) electrochemical conversion of the hydrogen into steam.The electrochemical impedance spectroscopy (EIS) results showed that the polarisation resistance (Rp) decreased by 15% over this temperature range. The total flowrate of 80 Nml min -1 of ammonia for Int-ADR, 160 Nml min -1 for Ext ADR and 120 Nml min -1 for H2 100% were chosen after performing distribution of relaxation time (DRT) analysis on changing total flowrates at 775 °C. The DRT coupled with complex nonlinear least-square (CNLS) analysis for the temperature range of 750-850°C revealed that the charge transfer resistance reduced by 66%, while the gas conversion resistance increased by 12% due to better evacuation of steam. The Arrhenius plot of DRT peak resistances revealed that the polarisation activation energy was the same for the three gas compositions, at 16 kJ mol -1 . Figure 1