Ammonia plays an important role in agriculture since more than 50% of food production relies on ammonia-based fertilizers. Large-scale production of ammonia is obtained by Haber-Bosch process which employs H2 and N2 as reagents under high temperature (500 °C) and pressure (10 MPa). Nonetheless, this process is highly energy demanding and it is responsible of 450 million tons of CO2. Therefore, green alternatives for decentralized ammonia production are required specially in medium scale. In the last years, electrochemical fixation of nitrogen has been presented as interesting alternative. However, nitrogen (N2) reduction presents several drawbacks due to low solubility in water and high energy barrier of nitrogen triple bond. Nitrate is an untapped N-source that overcome N2 reduction limitations. Besides, nitrate is considered in the top 10 water violations in the US. Thus, electrochemical nitrate reduction to produce ammonia may address two needs at once converting a pollutant into an added-value product. However, the most ranked materials used to obtain high ammonia selectivity and ammonia yield are based on platinum-group materials (e.g., Pt, Pd) reducing their operation feasibility. Herein, we report the evaluation of ammonia production of a self-supported Ni foam as substrate modified with Cu and Co(OH)x nanocomposites. Comparison between bimetallic and trimetallic nanodomains on composite electrodes demonstrated the synergistic effect of earth-abundant elements for ammonia production. Experiments with Ni-Cu and Ni-Co(OH)x demonstrate co-catalytic mechanisms of nitrate reduction involving direct charge transfer and catalytic hydrogenation conducting towards preferential ammonia production. The treatment of environmental-relevance nitrate concentration (30 mg L-1 NO3--N) with Ni-Cu-Co(OH)x reached 90.3% of conversion under galvanostatic condition (40 mA cm-2) within 120 min. Ammonia yield of 1.22 mmol NH3 gcat-1 h-1 and Faradaic efficiency of 22% for Ni-Cu-Co(OH)x, which corresponds to an order of magnitude higher outcome than bare metals (e.g., 0.13 mmol NH3 gcat-1 h-1 and Faradaic efficiency of 2% for Ni foam). It is important to remark that the trimetallic electrode outperforms Cu-Pt electrodes on the double. Evaluation of electrode activity stability exhibited high activity retention (90%) after 12 h of continuously. These results may motivate the design of multimetallic configurations using only earth-abundant materials for ammonia production.

Trimetallic synergies improve electrocatalytic response of earth-abundant elements: Superior ammonia production from nitrate wastewater

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