The energy transition and a hydrogen-based economy depend on the availability of energy in forms that allow long-term storage and transport from remote areas with abundance of renewable energy (RE) sources, and liquefied forms are the preferred option. Ammonia has potential to meet several requirements and to constitute one of the best energy carrier alternatives, besides being an important feedstock for the fertilizer and chemical industries. While current industrial processes rely on fossil fuels, new green paths with H2 providing from water electrolysis have emerged as best short-term solution. However, this approach faces certain incompatibilities between the two processes and limitations to tolerate RE intermittency. Direct electro-synthesis of ammonia from N2, H2O and RE has the potential to simplify the entire process, to be easily implemented in islanded and remote areas, to endure energy intermittency and curtailment, to accept less pure raw materials and to require less energy deriving in lower costs. However, the initial N2 adsorption and first electron transfer steps have intrinsic high-energy barriers, which might favor competing HER and impact the final NH3 productivity. As alternative, nitrogen oxyanions such as nitrates, mostly present as dissolved anions in polluted water from industrial sources, domestic sewage, sodium nitrate ore, and nitrification of bacteria, constitute a promising nitrogen source to synthesize ammonia via electrochemical route. This route has potential to achieve higher productivity values, as mass transfer limitations and inherent stability of molecular N2 are avoided, while at the same time constitutes an environmental remediation strategy.

Transition metals, such as titanium (Ti) and copper (Cu) along with their respective metal oxides have been widely studied as electrocatalysts for nitrate electrochemical reduction with important outcomes in the fields of denitrification and ammonia generation. In this work, we have synthesized and evaluated a composite electrode that integrates materials with different intrinsic activities (i.e. CuxO for higher activity for nitrate conversion; Ti for higher faradaic efficiency to ammonia) and have found synergistic effects in the ammonia generation, with improved performances at lower overpotentials. The specific results of single-metal and composite electrodes show a strong dependence on pH and nitrate concentration conditions. Faradaic efficiency to ammonia of 92% and productivities of 0.28 mmolNH3·cm-2·h-1 at 0.5 VRHE values are achieved, demonstrating the implicit potential of this approach.