Ammonia is an important chemical raw material and an excellent hydrogen carrier. Its production relies on the energy-, carbon-emission-, and capital-intensive Haber-Bosch process. With the increasing severity of energy and environmental issues, there is an urgent need to develop alternative technologies for green and sustainable ammonia synthesis. Compared with traditional nitrogen reduction to ammonia in aqueous electrolytes, electrocatalytic nitrate reduction to ammonia (eNO3−RR-to-NH3) offers significant thermodynamic and kinetic advantages. Moreover, nitrate is widely present in natural water bodies and industrial/agricultural wastewater, providing a strong foundation for eNO3−RR-to-NH3 technology. In the long term, the eNO3−RR-to-NH3 technology offers a win-win opportunity for decentralized ammonia production and nitrogen pollution control.
In aqueous electrolytes, eNO3−RR suffers from the competition of hydrogen evolution reaction (HER), leading to a low efficiency of ammonia production. Alkaline conditions can inhibit HER and accelerate the reaction kinetics of eNO3−RR. Actually, most reported catalysts' high ammonia production performance is achieved in strongly alkaline electrolytes. However, using strongly alkaline electrolytes presents several practical challenges, such as reacting with carbon dioxide, causing equipment corrosion, and increasing costs. Thus, developing efficient electrocatalysts suitable for neutral/near-neutral electrolyte conditions is highly necessary.
In principle, the eNO3−RR-to-NH3 process is accompanied by the generation of OH− (NO3− + 6H2O + 8e−→NH3 + 9OH−), which can increase the pH around the catalytic sites, thereby inhibiting HER. However, for traditional catalysts, these OH− ions rapidly diffuse into the bulk electrolyte due to disturbance or concentration gradients, making it difficult to significantly increase the local pH. Effectively confining the OH− around the catalytic sites to create a favorable high pH environment is crucial for enhancing the ammonia production performance of the catalysts and is a highly challenging topic.
In recent years, Zheng Hu's group has developed the hierarchical carbon nanocages featuring by 3D porous structure, high conductivity, large specific surface area and convenient doping, which can much facilitate the mass/charge synergetic transfer, thus becoming an advanced platform for energy conversion and storage (Acc. Chem. Res. 50(2017)435; Adv. Mater. 24(2012)347, 24(2012)5593, 27(2015)3541, 29(2017)1604569, 29(2017)1700470, 31(2019)1804799, 32(2020)1904177, 32(2020)2004632, 35(2023)2304551; Sci. China Chem. 63(2020)665; Sci. China Mater. 64(2021)217; Nat. Commun. 10(2019)1657; J. Am. Chem. Soc. 146(2024)9365; Angew. Chem. Int. Ed. 63(2024)e202401304, etc.).
This study leverages the internal cavities and microporous channels to fill the active components into the nanocage cavities (Fig. 1). During the eNO3−RR, the nanocage hinders the outward diffusion of in-situ generated OH−, thereby spontaneously forming a local high pH environment within the nanocage (Fig. 1). This mechanism has been validated by experiments and theoretical simulations (Fig. 2). The confined catalyst effectively inhibits HER and demonstrates high performance under neutral conditions, avoiding the use of strongly alkaline electrolytes. Additionally, the encapsulation protection of the nanocage prevents the aggregation and loss of internal active components, enhancing the stability of the catalyst (Fig. 3). With the advantages of this confined catalyst, a coupled system with plasma-driven nitrogen oxidation and nitrate reduction demonstrates sustainable ammonia synthesis only using air and water under neutral conditions (Fig. 4).
This work provides new ideas for catalyst design and microenvironment engineering for some key reactions. The related paper entitled Self-enhanced localized alkalinity at the encapsulated Cu catalyst for superb electrocatalytic nitrate/nitrite reduction to NH3 in neutral electrolyte has been published on Science Advances on July 10, 2024. Nanjing University is the first affiliation. Associate Professor Lijun Yang, Professor Qiang Wu, and Professor Zheng Hu from Nanjing University are the co-corresponding authors, and Ph.D. student Zhen Shen from Nanjing University is the first author.
This work was funded by the National Key Research and Development Program of China, the National Natural Science Foundation of China, and the Natural Science Foundation of Jiangsu Province, and supported by the High-Performance Computing Center of Nanjing University and Professor Jiong Li from the BL11B beamline station of the Shanghai Synchrotron Radiation Facility.
Article Link: https://www.science.org/doi/10.1126/sciadv.adm9325
Fig. 1 Morphology of the confined catalyst and schematic illustration of spontaneously formed local high-pH environment inside the nanocage during eNO3−RR.
Fig. 2 Evaluation and theoretical simulation of the local pH. (A,B) Photograph of pH monitor (A) and schematic illustration (B) for the generation and diffusion of OH−. (C) Concentration of OH− at electrode surface. (D) eNO3−RR-to-NH3 performances in different intervals. (E,F) Theoretical simulation of the local pH.
Fig. 3 eNO3−RR-to-NH3 performances. (A-C) Evolutions of NH3 Faradaic efficiency (A), NH3 yield rate and partial current density (B), and H2 Faradaic efficiency (C). (D) Linearized pseudo–first-order kinetic profiles. (E) Stability.
Fig. 4 Demonstration of sustainable NH3 synthesis by coupling plasma-driven N2 oxidization with eNOx−RR-to-NH3. (A) Schematic illustration of the coupled reaction system. (B) Plots of NOx− concentration versus plasma operation time. (C) i-t curves. (D) Faradaic efficiency and yield rate of NH3 after 30 min reaction.