In this work, the utilisation of aqueous ammonium carbonate, (NH₄)₂CO₃, has been investigated as a fuel for solid oxide cell technology for the first time. In its pure form, ammonium carbonate is a solid white power that can easily be stored. It is easily dissolved in water to create an ammonium-rich aqueous solution and can therefore potentially be utilised as a fuel for solid oxide cell technology ¹. Aqueous ammonium carbonate is produced as a waste stream from the coke production processes required for steelmaking. Coke is needed as a fuel for blast furnaces and is typically produced via dry distillation of coal in the absence of oxygen, which yields a waste gas stream known as coke oven gas (COG). Cleaning and treatment of COG yields an aqueous waste stream containing ammonia (28 wt%), water (61 wt%), phenol-based tars (5 wt%), carbon dioxide (3.9 wt%), hydrogen sulfide (0.6 wt%) and trace levels of hydrogen cyanide and sodium hydroxide. Due to the presence of carbon dioxide, a significant fraction of the ammonia is present in the form of ammonium carbonate. Significant amounts of COG are produced due to steelmaking, with 44,000 Nm³ produced every hour at the Tata Steel Port Talbot (UK) site alone, corresponding to 264 kg of ammonia every hour ². Presently, ammonia waste is disposed via incineration, wasting energy and a valuable resource, and contributing to air pollution and greenhouse gas emissions.

Ammonia is a well-established chemical commodity used mainly for the manufacture of mineral fertiliser ³. Recently, ammonia has gained more interest in the energy sector since it is a carbon-free hydrogen carrier and can be used directly and efficiently in fuel cells and gas turbines without emitting CO₂ ⁴. It also has a high energy density (12.7 MJ L^-1) and its storage as a liquid requires less energy-intensive conditions (-33 ˚C) in comparison with hydrogen (-253 ˚C) ⁵. The high volumetric hydrogen density of ammonia (17.8 wt%) and ease of liquefication make it ideal for capturing, storing, and transporting hydrogen.

This work investigates the utilisation of aqueous ammonium carbonate solutions in a solid oxide cell. The electrical performance and gaseous outputs of an anode-supported solid oxide cell operating in fuel cell and electrolysis mode at 750 °C have been established. The cell performance was characterised by current-voltage (I-V) curves and the dried gaseous outputs were characterised by online quadrupole mass spectrometry (QMS).

QMS measurements showed that upon delivery to the anode, ammonium carbonate decomposed to form gaseous hydrogen, carbon dioxide and nitrogen, with no NO_x emissions detected. No carbon monoxide was detected due to the presence of the water-gas shift reaction. In fuel cell mode, QMS measurements showed the cell utilised hydrogen to produce electrical power. The I-V curves (see Figure) showed the electrical power output was lower than pure hydrogen, due to greater open circuit voltage and concentration losses caused by dilution of hydrogen in the steam, carbon dioxide and nitrogen present. The activation losses observed under aqueous ammonium carbonate were very low.

In electrolysis mode, the cell utilised steam to produce hydrogen, which was diluted in carbon dioxide and nitrogen. At 1.4 V, QMS measurements showed the output gases were composed of 60 vol% hydrogen, 29 vol% nitrogen and 11 vol% carbon dioxide. No co-electrolysis of carbon dioxide was observed, and no carbon monoxide was produced. The main losses under ammonium carbonate were ohmic overpotentials (see Figure), with no activation or concentration losses observed. Whilst this work has focussed on the short-term performance and outputs of the cell, no potential causes of long-term deactivation were identified.

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Figure. I-V and power curves of an anode-supported solid oxide cell operating on mixtures of hydrogen and aqueous ammonium carbonate at 750 °C. The mixtures indicated are balanced in pure hydrogen.