Highly Efficient Conversion of Ammonia in Electricity by Solid Oxide Fuel Cells

Hydrogen is the fuel for fuel cells with the highest cell voltage. A drawback for the use of hydrogen is the low energy density storage capacity, even at high pressures. Liquid fuels such as gasoline and methanol have a high energy density but lead to the emission of the greenhouse gas $CO2$⁠. Ammonia could be the ideal bridge fuel, having a high energy density at relative low pressure and no (local) $CO2$ emission. Ammonia as a fuel for the solid oxide fuel cell (SOFC) appears to be very attractive, as shown by cell tests with electrolyte supported cells (ESC) as well as anode supported cells (ASC) with an active area of $81cm2$⁠. The cell voltage was measured as function of the electrical current, temperature, gas composition and ammonia (⁠$NH3$⁠) flow. With $NH3$ as fuel, electrical cell efficiencies up to 70% (LHV) can be achieved at $0.35A∕cm2$ and 60% (LHV) at $0.6A∕cm2$⁠. The cell degradation during 3000 h of operation was comparable with $H2$ fueled measurements. Due to the high temperature and the catalytic active $Ni∕YSZ$ anode, $NH3$ cracks at the anode into $H2$ and $N2$ with a conversion of $>99.996%$⁠. The high $NH3$ conversion is partly due to the withdrawal of $H2$ by the electrochemical cell reaction. The remaining $NH3$ will be converted in the afterburner of the system. The $NOx$ outlet concentration of the fuel cell is low, typically $<0.5ppm$ at temperatures below $950°C$ and around $4ppm$ at $1000°C$⁠. A SOFC system fueled with ammonia is relative simple compared with a carbon containing fuel, since no humidification of the fuel is necessary. Moreover, the endothermic ammonia cracking reaction consumes part of the heat produced by the fuel cell, by which less cathode cooling air is required compared with $H2$ fueled systems. Therefore, the system for a $NH3$ fueled SOFC will have relatively low parasitic power losses and relative small heat exchangers for preheating the cathode air flow.