Two-phase flow visualization in direct ammonia fuel cells

In this work, the two-phase flow occurring in a direct ammonia fuel cell is visualized to understand the interactions between the complex physico-chemical processes involving in fuel cells. Experimental observations indicate that the appearance, growth, coalescence, detachment and sweeping processes of nitrogen bubbles occur periodically. Besides, the effect of operating conditions on the behavior of two-phase flow as well as cell performance is explored. Experimental results demonstrate that numerous nitrogen gas slugs are generated in the anode flow channel when the fuel flow rate is lower than 1.0 mL min⁻¹, which seriously blocks the transport of NH₃ and OH⁻ to the anode catalyst layer. At an operating temperature of 95 °C, the flow channels are almost completely occupied by gases (a mixture of ammonia, nitrogen, and water), even under open-circuit conditions (a mixture of ammonia and water). In addition, the effects of using parallel and serpentine flow fields on the behaviour and flow patterns of two-phase flows are explored. Experimental results reveal that the parallel flow field is prone to clogging local flow channels by slugs. Moreover, the removal frequency of bubbles in the parallel design is significantly slower than that in the serpentine one.