Favoring the Originally Unfavored Oxygen for Enhancing Nitrogen-to-Nitrate Electroconversion
Authors: LI Xin , Leung Michael K.H. 
Published: December 2024

Nitrate (NO3
−), one of the most crucial forms of reactive nitrogen, is widely used in industry and agriculture. Currently, NO3
− is manufactured predominantly via a two-step procedure, including the Haber–Bosch process for ammonia synthesis and the Ostwald process for ammonia oxidation. However,both techniques require high-pressure and high-temperature reaction conditions (Haber–Bosch reaction at 150-200 bar and 400-500 ℃; Ostwald reaction at 4-10 bar and 800-1000 ℃), which consumes 1-2% of world energy and releases 1-2% of CO2. In addition, due to the complexity of the Ostwald and Haber-Bosch processes, it is only economical at large scales, leading to centralized production, which situation is poorly matched with the distributed nature of HNO3 utilization. Therefore, bypassing the ammonia route and developing a direct and sustainable approach for NO3
− synthesis is highly desirable.

Electrochemical synthesis of chemicals has been regarded as an attractive alternative to traditional thermochemical methods since electric potential can replace high temperature and pressure in electrochemical reactions as the thermodynamic driving force. Therefore, direct electrochemical oxidation of molecular nitrogen appears to be a potential approach for NO3
− synthesis,which could be sustainable, modular and easily integrated with intermittent renewable electricity. However, due to the lack of natural catalysts as a reference, the nitrogen oxidation reaction (NOR) still needs to be explored despite its enormous practical value as a replacement for the fossil fuel-driven two-step nitrate preparation approach. To date, only a few works have reported nitrate's electrosynthesis from nitrogen, and the proposed electrocatalysts can only achieve limited NOR activity and selectively due to the complex reaction networks which involve multi-electron transfers, multi-bond breaking and formation steps. In addition, oxidation evolution reaction (OER) as a competitive reaction further limits the selectivity toward nitrate synthesis.

One major challenge in improving electrochemical NOR is that its reaction pathways and reactivity are highly sensitive to the catalyst-active-site identity and the non-catalyst components at the electrode/electrolyte interface, such as the local reaction environment. Even minor changes at the catalyst-reaction environment interface can significantly impact the overall catalytic performance.However, the previous works generally regarded the catalyst and reaction environment as two independent systems despite their dynamic interaction throughout the electrochemical reaction20. For example, most works improve NOR performance only by designing the morphologies, chemical states, and compositions of the catalysts. To date, a comprehensive design on the interplays between the above catalyst–reaction environment and the NOR performance is absent, which motivates us to employ a synergistic strategy to regulate both catalyst and reaction environment to achieve the overall optimization of the electrocatalytic interface from the microstructure to the macrosystem, thereby achieving the improvement of NOR performance.

Here, we propose that the intrinsic property of the catalyst structure and its reaction environment can be considered as gradient interfaces, ranging from microstructure to macroenvironment, to achieve the effect of one plus one greater than two in terms of NOR performance. Specifically, we synthesized Ru nanoclusters confined within the lattice of TiO2 (RuNC@TiO2) as a microstructural interface and created a macro-interface environment through which we synergistically achieved exceptional NOR performance. The introduced TiO2, an oxophilic species with strong OHad adsorption capability, inhibits OER on Ru active sites by competitively adsorbing OHad between Ru and TiO2. Consequently, the OER on Ru clusters is effectively suppressed due to the preferential adsorption of OHad on TiO2. Importantly, we propose leveraging a macro-interface environment by utilizing the previously considered unfavored oxygen from OER, successfully enhancing the NOR via Le Chatelier's principle. As an experimental demonstration, the as-prepared RuNC@TiO2 delivered a record-high nitrate yield rate of 26.80 μg h-1 cm-2 (20.54 mol h-1 gRu
-1) and a Faradaic efficiency of 35.52 % under simulated conditions with high-concentration oxygen (8 atm air). In addition, an increasing nitrate yield rate and Faradaic efficiency (38.87%) after a continuous 20-hour electrochemical process was found due to the increasing oxygen concentration in the reaction environment caused by the OER.

Figure 1