Electrochemical nitrogen reduction for ammonia synthesis using gas diffusion electrodes

  • Elektrochemische Stickstoffreduktion für die Ammoniaksynthese mit Gasdiffusionselektroden

Wei, Xin; Wessling, Matthias (Thesis advisor); Eichel, Rüdiger-Albert (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

Nowadays, ammonia ($\rm NH_3$) is one of the most important industrial products due to its vital role in the agricultural industry. $\rm NH_3$ is mainly produced by directly reducing nitrogen ($\rm N_2$) with hydrogen ($\rm H_2$) through the Haber-Bosch process. However, this energy-intensive process, operating under harsh conditions, suffers from a high fossil energy input and emits a substantial amount of carbon dioxide. It is highlighted to develop more sustainable and ecological alternative $\rm NH_3$ synthesis processes, which can operate with low carbon emission and renewable energy supply in the future. The ambient electrochemical $\rm N_2$ reduction reaction (eNRR) for $\rm NH_3$ synthesis in aqueous electrolyte attracts intensively increasing attentions due to its carbon-free process and independence of fossil-fuel-based energy demand. Whereas, the inherent limitations still challenge the feasibility of this process. The inertness of $\rm N_2$ and the competition against the hydrogen evolution reaction obstruct the eNRR performance in terms of $\rm NH_3$ yield rate and current efficiency. Until now, the most operations were in $\rm N_2$-saturated aqueous electrolytes, which further suppressed the eNRR results due to extremely low $\rm N_2$ solubility. However, no studies are reported to develop eNRR by optimizing the operation. This study presents the progress of ambient eNRR by optimizing the operations. In this work, catalyst-modified gas diffusion electrodes (GDEs) were applied in an electrochemical membrane reactor to overcome the restriction caused by the low solubility of $\rm N_2$. More specifically, GDEs with plate and microtubular configurations were proposed. For the plate GDE, the influence of the reactor configurations, electrolytes, and GDE preparation on the eNRR performance was studied. Additionally, the microtubular GDEs based on catalyst-modified carbon nanotubes were, for the first time, proposed to synthesize $\rm NH_3$. The microtubular GDEs facilitate a practicable and accessible catalyst-evaluation system with three-phase boundary for eNRR in the laboratory scale. Moreover, the preparation of microtubular GDEs and the influence of wettability were investigated for further improvement on the eNRR performance. As a result, a significant improvement was achieved. The $\rm NH_3$ yield rate raised from $\rm 2.5\times 10^{-11}$ to $\rm 2.1\times 10^{-9}$ $\rm mol/cm^{2}s$ and current efficiency increased from 0.002% to 71.9%. The remarkable achievement with several orders of magnitude increase is contributed to the combination of GDE and eNRR processes under an appropriate reactor configuration and electrolyte choice. The results obtained in the present thesis encourage further research based on GDE for the electrochemical $\rm NH_3$ synthesis. In particular, the combination of the developed electrocatalysts and GDE is worth promoting.

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