Ammonia crossover in hydroxide exchange membrane (HEM) poses a significant challenge to the advancement of low-temperature DAFCs. In this study, we have developed two types of macromolecule crosslinked HEMs (PNH-ene-x and PNH-yne-x) through a straightforward in-situ thermal crosslinking method using alkenes and aromatic crosslinked moieties. The engineered crosslinked networks demonstrate dual functionality: effectively limiting water absorption to suppress ammonia permeation (2.62 × 10 −7 cm 2 s −1 for PNH-ene-2%) while maintaining a well-defined microphase-separated morphology to promote hydroxide ion conduction (45.1 mS cm −1 at 30 °C). PNH-ene-2% achieves a superior membrane selectivity (9.57 × 10 7 mS s cm −3 ) through optimal balance between transport properties and structural stability. Accordingly, the DAFC with PNH-ene-2% exhibits a high cell energy efficiency (26.9%) and a modest peak power density (256.8 mW cm −2 ), representing the best record for low-temperature DAFCs to date. This work suggests that crosslinking is an effective approach to prepare high-performance HEMs for low-temperature DAFCs.
Low-temperature direct ammonia fuel cells (DAFCs) have attracted widespread attention due to carbon-neutral property, high volumetric energy density, and low source-to-tank costs of ammonia. However, high ammonia crossover which leads to poor cell energy efficiency, is a big challenge for DAFCs. Ammonia crossover is dominantly related to hydroxide exchange membranes (HEMs), and the deep understanding of ammonia crossover behavior in HEMs is lacking in this field. Herein, we have elaborately investigated physical characterizations of different poly (aryl piperidinium)-based HEMs and find that the ammonia permeability is exponentially inversely proportional to the reciprocal of water volume fraction for HEMs. We have also evaluated the energy efficiency of DAFCs with different HEMs by combining two factors of ammonia crossover and cell performance. The DAFC based on the TP-100-QC6 with low ammonia crossover and modest ionic conductivity can achieve a high cell energy efficiency of 33 %, which is much higher than DAFC (14 %) with a benchmark HEM of TP-100. This work suggests that pursuing highly conductive HEMs simultaneously with low water volume fraction to lower ammonia crossover is of vital importance for the future development of DAFCs.
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