Multi-Strategy Design of Non-Noble Metal Electrocatalysts in Nitrate Reduction for Ammonia Synthesis

Abstract

Electrochemical nitrate reduction to ammonia (eNO₃RR) is a green and appealing alternative method for ammonia synthesis, serving as an environmentally friendly substitute for the conventional Haber-Bosch process. Non-noble metal electrocatalysts have attracted intense attention in eNO₃RR because of their great advantages in low cost, high activity, and large-scale application potential. However, since the eNO₃RR involves eight-electron transfer processes and is competitive with the undesired hydrogen evolution reaction (HER), most of the previous strategies in designing electrocatalysts met a dilemma for simultaneously promoting activity and selectivity. It is thus urgent to explore promising strategies for developing state-of-the-art electrocatalysts with high selectivity and Faradaic efficiency (FE) toward NH₃/NH₄⁺ for its large-scale deployment.

In general, the eNO₃RR process involves three stages: (i) the adsorption of NO₃^- on the active sites, (ii) the continuous proton-coupled electron transfer (PCET) steps, including nine protons and eight electrons, followed by (iii) the formation and desorption of NH₃ molecules. According to the process, the strategies to design non-noble electrocatalysts with improved eNO₃RR activity could start by regulating the activity of the active site and their local electronic environment. Defect engineering would enhance their electrochemical performance because defects, such as oxygen vacancies (O_v), are generally considered active sites. We investigate the synergistic effect between asymmetric Cu-O_v-W sites and adjacent Mo clusters in copper tungstate (CuWO₄) hollow nanospheres for tuning the local electronic environment around active sites to enhance nitrate reduction substantially. The dynamic balance between the adsorption and desorption of O in NO₃^- influenced by asymmetric O_v and the promoted protonation process facilitated by Mo clusters are responsible for boosting the entire process, leading to the high NH₃ Faradaic efficiency and yield rate of 94.60% and 5.84 mg h⁻¹ mg_cat.⁻¹ at −0.7 V vs. RHE.

Besides, the binding strength of multi-adsorbates on the active sites is decisive for the eNO₃RR activity and selectivity. This strength determines the energy barriers during chemical reactions. Due to the advent of nanotechnology, two-dimensional (2D) materials have unveiled various excellent properties, including large specific areas; therefore, 2D materials are considered appropriate candidates for multiple catalytic reactions. It is widely reported that the interlayer manipulation of 2D materials affects the chemical affinity of adsorbates during electrocatalysis. Therefore, through the active diatomic Pt-Ce pairs, we report the synthesis of 2D SnS nanosheets with tailored interlayer spacing, including both expansion and compression. Taking together the experimental results, in situ Raman spectra, and DFT calculation, we found the compressed interlayer spacing could tune the electron density of localized p-orbital in Sn into its delocalized states, thus enhancing the chemical affinity towards NO₃^- and NO₂^- but inhibiting hydrogen generation simultaneously. This phenomenon significantly facilitates the rate-determining step (^*NO₃→^*NO₂) in eNO₃RR and realizes an excellent Faradaic efficiency (94.12%) and yield rate (0.3056 mmol cm⁻² h⁻¹) for NH₃ at −0.5 V vs. RHE.

Step (ii) of the eNO3RR process involves multi-proton-coupled electron transfer steps, including nine protons and eight electrons. The complex PCET steps plus competitive HER significantly impede the performance of electrocatalysts. To this end, keeping an appropriate equilibrium of electron and proton accessibility kinetics around the active sites of catalysts emerges as a crucial principle to enhance the activity and selectivity of eNO₃RR. Herein, we engineered a remarkable electrocatalyst comprising Co₃O₄ nanoparticles modified with doped rare-earth La atoms and carboxylic (COO^-)-based organic ligands. The COO^--groups efficiently reduce the water activity around the active sites by forming hydrogen bonds, ensuring controlled protons-accessibility, thus effectively regulating the hydrogenation steps and suppressing competing HER. The electrocatalyst exhibits superior activity and selectivity with an ammonia Faradaic efficiency of up to 99.41% and a yield rate of 5.62 mg h⁻¹ mg_cat.⁻¹ at −0.3 V vs. RHE. Notably, the catalyst maintains over 90% Faradaic efficiency for NH3 production across a broad potential range of 400 mV, ranking among reported eNO₃RR electrocatalysts.

In conclusion, we display multi-strategy designs for non-noble electrocatalysts in eNO₃RR. The in-depth mechanisms behind the improved activity and selectivity due to the strategies are investigated using electrochemical in-situ measurement, DFT calculations, etc. The results in this thesis are believed to provide various rational strategies for designing electrocatalysts with high ammonia production rates in eNO₃RR. They are expected to contribute to subsequent catalyst optimization and scaling up in other electrochemical techniques, such as nitrogen reduction.

Date of Award	11 Jul 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Johnny Chung Yin HO (Supervisor)