Abstract
Ammonia is an attractive carbon-free chemical for electrochemical energy conversion and storage. However, the sluggish kinetic rates of the ammonia electrooxidation reaction, and high cost and poisoning of Pt-based catalysts still remain challenges. This also limits the development of direct ammonia fuel cells. In this work, we directly grew hierarchical mixed NiCu layered hydroxides (LHs) nanowires on carbon fibre cloth electrodes by a facile one-step hydrothermal synthesis method for efficient electro-oxidation of ammonia. This catalyst achieves a current density of 35mAcm−2 at 0.55V vs. Ag/AgCl, which is much higher than that of bare Ni(OH)2 catalyst (5mAcm−2). This is due to abundant active sites and a synergistic effect between Ni and Cu, possibly due to the formation of Ni1−xCuxOOH on the surface of the catalysts through the electrochemical activation of the mixture of Cu(OH)2 and α-Ni(OH)2. In the investigated first row transition elements, it is found that Cu is the sole first-row transition metal to effectively improve activity of Ni(OH)2 for ammonia electrooxidation. This mixed NiCu LHs nano-wire catalyst outperforms commercial Pt/C catalyst in the aspects of ammonia oxidation current and stability, demonstrating it to be a promising low-cost and stable catalyst for efficient ammonia electrooxidation in alkaline condition, which is a potential electrode for ammonia fuel cells for power generation or electrolysis of ammonia for ammonia-containing wastewater treatment.
Highlights
Ammonia is regarded as a carbon-free hydrogen-storage material with a hydrogen weight percent of 17.6 wt% [1,2,3]
Nickel and copper ions were precipitated under basic conditions from progressive hydrolysis of urea to produce bulk NiCu layered hydroxides (LHs) sediment; some Ni1−xCux LHs (x was copper percent of total metals in the precursor solution of hydrothermal reaction) were attached onto the surface of carbon fibre cloth (CFC) to form nanowires
Hierarchical NiCu LHs nanowires have been successfully grown directly on carbon fibre cloth by a hydrothermal reaction followed by electrochemical activation
Summary
Ammonia is regarded as a carbon-free hydrogen-storage material with a hydrogen weight percent of 17.6 wt% [1,2,3]. It has been reported that pure Ni(OH) has good catalytic activity to electrochemical oxidation of ammonia [18], to the best of our knowledge, reports on Ni-based LHs for ammonia electrooxidation are scarce. Introduction of another element in layered hydroxides will significantly change the catalytic properties as described above. Previous work shows bare Cu is almost inactive toward ammonia electrooxidation as Cu binds N atoms too weakly leading to a very high overpotential [33], NiCu LHs are expected to perform well due to a synergistic effect between Cu and Ni, since Ni binds N atoms strongly [32]. The synthesised nickel-copper hydroxide exhibit hierarchical microstructure which can maximise the surface area to further increase the catalytic activity [34,35]
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