Strategic Improvement of Efficiency for Ambient Electrochemical Ammonia Synthesis


Student thesis: Doctoral Thesis

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Awarding Institution
Award date17 Aug 2021


The ambient electrocatalytic NH3 synthesis (NRR) by utilization of H2O and N2 feedbacks (NRR) has emerged as a promising alternative for the NH3 production to the traditional energy-intensive Haber-Bosch process. However, its large-scale application is hindered by the competing H2 evolution reaction (HER) and the high dissociation barrier of the N≡N bond, which cause the efficiency of NH3 production unsatisfactory, i.e., low NH3 selectivity (Faradaic efficiency, FE) and yield rate. Therefore, developing effective strategies to lower the high dissociation barrier of the N≡N bond and suppress the parasitic HER is highly desirable for improving the efficiency of the NH3 electrosynthesis.

Theoretical investigations have profiled MXenes as promising catalysts for the NRR, especially MXenes containing Ti atoms in favor of weakening the N≡N bond with high adsorption energy and thereby facilitating nitrogen cleavage. However, such Ti-containing MXenes practically exhibit a rather low ammonia yield and unsatisfactory FE (<10%) as the basal plane of MXenes in solution unavoidably suffers from the functionalization of inactive functional F* and OH*groups, which mask the active metal sites for binding N2. To address this issue, we developed a surface termination modified strategy to enhance surface catalytic reactivity of MXene (Ti3C2Tx) nanosheets by ironing out inactive F* and OH* terminals to expose more active sites and introducing Fe to reduce the surface work function. Consequently, the well-optimized MXene/TiFeOx-700 catalyst achieved excellent NH3 FE of 25.44% and NH3 yield of 2.19 µg cm-2 h-1 (21.9 µg mg-1 h-1), outperforming all reported MXene-based NRR catalysts. This surface termination modification methodology provides a feasible strategy for rationally improving the surface reactivity of MXene-based catalysts for efficiency improvement of electrochemical ammonia production.

Precious metal Pd has the intrinsic superiority in adsorbing N2 molecule and wrecking the high cleavage barrier of the N≡N bond; however, its over-strong adsorption ability is unfavorable to the desorption of the produced NH3 during the NRR, which weighs heavily against the NH3 productivity. We presented a hydrogen substituted graphdiyne (HsGDY) tuned strategy for regulating the ability of the active Pd sites toward the nitrogen adsorption and NH3 desorption by in-situ growing Pd clusters onto HsGDY. With the electron-rich acetylene linkages of HsGDY, the d band center of active Pd atoms downward shifts from the Fermi level that reduces the adsorption energy of N2 and benefits the desorption of produced NH3 from the surface of Pd/HsGDY to recover active sites, eventually resulting in a selectively facilitated NH3 production. As a result, the Pd/HsGDY catalyst attained 44.45% of NH3 FE and 11.59 µg cm-2 h-1 (115.93 µg mg-1 h-1) of NH3 yield, far superior to other reported Pd-based catalysts. This strategy provides a novel route by massaging different performance-enhancing factors of an NRR catalyst to promote the production rate and selectivity of NH3 synthesis.

Considering the hydrogenation reactions essential to the aqueous NRR, acidic aqueous electrolytes would be an optimum strategy for the aqueous NRR as long as the proton content and the HER kinetics can be well balanced. To suppress the HER kinetics in favor of the NRR, we developed a HER-suppressed electrolyte for promoting the NRR by adopting hydrophilic poly(ethylene glycol) (PEG) as the aqueous electrolyte additive by virtue of its molecular crowding effect, which promotes the NRR by retarding the HER kinetics. On a TiO2 nanoarray electrode, a significantly improved NRR activity with an NH3 FE of 32.13 % and an NH3 yield of 18.19 µg·cm-2·h-1 (1.07 µmol·cm-2·h-1) was achieved in the PEG-containing acidic electrolyte, 9.4 times and 3.5 times higher than those delivered in the pure acidic electrolytes, respectively. Similar enhancements are achieved with Pd/C and Ru/C as catalysts, as well as in an alkaline electrolyte, demonstrating a universally positive effect of molecular crowding to the NRR.

In summary, three types of performance-enhancing strategies including exposing more active sites, regulating intrinsic activity of the active sites, and suppressing the parasitic HER of the aqueous electrolyte, were rationally proposed and studied in pursuit of high NH3 FEs and yields. It is believed that the strategies and results in this thesis can cast new lights on the efficiency improvement of the artificial NH3 production via NRR and is expected to apply to other electrocatalytic systems, such as electrochemical CO2 reduction, electrochemical nitrate reduction, and so on.