Abstract
The global demand for ammonia, primarily used as fertilizer, is crucial for sustaining agricultural productivity to support the growing world population. Traditional ammonia production through the Haber-Bosch process is energy-intensive and significantly contributes to greenhouse gas emissions, raising concerns about its environmental impact. The high temperatures and pressures required for the process necessitate large, centralized production facilities, presenting logistical and environmental challenges. In light of the 2015 international agreement to limit global temperature rise and achieve zero carbon dioxide emissions by 2050, exploring alternative, sustainable methods for ammonia synthesis is imperative.
This thesis investigates the potential of the electrochemical nitrogen reduction reaction (ENRR) as a sustainable alternative for ammonia synthesis. ENRR offers a carbon-neutral pathway, leveraging renewable electricity to drive the reaction at ambient conditions. This could decentralize ammonia production, reducing the need for extensive transportation and storage infrastructure. However, ENRR faces several challenges, including low activity and selectivity of electrocatalysts, and pervasive environmental ammonia contamination that leads to false positive results. To address these issues, a new method to estimate energy efficiency was developed, alongside a reliability indicator to assess the reliability of published ENRR studies and avoid benchmarking false positive results. This was a comprehensive benchmarking study, that compared ENRR's energy efficiency with the Haber-Bosch process. The assessment revealed that many ENRR studies suffer from adequate experimental protocols, leading to probably false positive results. Only a small percentage of the studies were found reliable, with aqueous ENRR showing potential for higher energy efficiencies up to 55%, whereas nonaqueous ENRR, despite lower energy efficiency, demonstrated higher technology readiness levels and greater ammonia production. In response to these findings, a rigorous ENRR experimental protocol was developed and implemented. This protocol was essential for ensuring the accuracy and reproducibility of subsequent experiments. The research then focused on evaluating new catalysts and improving electrochemical setups to enhance ENRR efficiency.
Gallium (Ga) was identified as a potential catalyst for ENRR. Despite theoretical predictions of its suitability, experimental results indicated that Ga did not effectively catalyze ammonia synthesis under the tested conditions. This led to the exploration of manganese (Mn) as an alternative catalyst. Mn electrodeposition was tested in both aqueous and non-aqueous media. Although Mn showed theoretical promise, practical issues such as hydrogen evolution reaction (HER) dominance and passivation layer formation limited its effectiveness. Various strategies, including modifying electrolytes to increase pH, using different solvents in non-aqueous experiments, were tested but did not yield significant ammonia production.
The research then investigated the use of water vapor as a proton source in non-aqueous lithium mediated electrochemical nitrogen reduction reaction (Li-ENRR) systems aiming to improve energy efficiency compared to methods using ethanol or hydrogen gas. Two methodologies were investigated: a two-compartment parallel-plate flow cell and a three-compartment gas diffusion electrode (GDE) flow cell. In the two-compartment system, managing water crossover proved challenging. However, consistent lithium plating was achieved by integrating a Nafion membrane and using a water-in-salt electrolyte (WISE) in the anode compartment. Despite these efforts, ammonia production remained insignificant compared to background levels, indicating that using an aqueous solution as a proton source under these conditions was ineffective for ENRR. The three-compartment GDE flow cell showed better control over water crossover by introducing water vapor into the anode gas compartment. This method resulted in reproducible lithium plating and a Faradaic efficiency of 5.6%, demonstrating improved performance in controlling water crossover and enhancing ENRR efficiency. This indicates that the introduction of water vapor as a proton source in Li-ENRR systems has a promise to improve efficiency and potentially lead to more viable methods for ammonia synthesis in future applications. Further research and optimization are necessary to build upon these findings and develop more efficient ENRR systems.
Overall, this thesis highlights the complexities and challenges in developing a viable ENRR process. Significant progress has been made in understanding the fundamental mechanisms, improving experimental protocols, and testing new catalysts. However, further research is necessary to overcome practical challenges and enhance the efficiency of ENRR. This work contributes to the broader effort of developing sustainable and energy-efficient ammonia production methods, aligning with global goals to combat climate change and reduce greenhouse gas emissions.
This thesis investigates the potential of the electrochemical nitrogen reduction reaction (ENRR) as a sustainable alternative for ammonia synthesis. ENRR offers a carbon-neutral pathway, leveraging renewable electricity to drive the reaction at ambient conditions. This could decentralize ammonia production, reducing the need for extensive transportation and storage infrastructure. However, ENRR faces several challenges, including low activity and selectivity of electrocatalysts, and pervasive environmental ammonia contamination that leads to false positive results. To address these issues, a new method to estimate energy efficiency was developed, alongside a reliability indicator to assess the reliability of published ENRR studies and avoid benchmarking false positive results. This was a comprehensive benchmarking study, that compared ENRR's energy efficiency with the Haber-Bosch process. The assessment revealed that many ENRR studies suffer from adequate experimental protocols, leading to probably false positive results. Only a small percentage of the studies were found reliable, with aqueous ENRR showing potential for higher energy efficiencies up to 55%, whereas nonaqueous ENRR, despite lower energy efficiency, demonstrated higher technology readiness levels and greater ammonia production. In response to these findings, a rigorous ENRR experimental protocol was developed and implemented. This protocol was essential for ensuring the accuracy and reproducibility of subsequent experiments. The research then focused on evaluating new catalysts and improving electrochemical setups to enhance ENRR efficiency.
Gallium (Ga) was identified as a potential catalyst for ENRR. Despite theoretical predictions of its suitability, experimental results indicated that Ga did not effectively catalyze ammonia synthesis under the tested conditions. This led to the exploration of manganese (Mn) as an alternative catalyst. Mn electrodeposition was tested in both aqueous and non-aqueous media. Although Mn showed theoretical promise, practical issues such as hydrogen evolution reaction (HER) dominance and passivation layer formation limited its effectiveness. Various strategies, including modifying electrolytes to increase pH, using different solvents in non-aqueous experiments, were tested but did not yield significant ammonia production.
The research then investigated the use of water vapor as a proton source in non-aqueous lithium mediated electrochemical nitrogen reduction reaction (Li-ENRR) systems aiming to improve energy efficiency compared to methods using ethanol or hydrogen gas. Two methodologies were investigated: a two-compartment parallel-plate flow cell and a three-compartment gas diffusion electrode (GDE) flow cell. In the two-compartment system, managing water crossover proved challenging. However, consistent lithium plating was achieved by integrating a Nafion membrane and using a water-in-salt electrolyte (WISE) in the anode compartment. Despite these efforts, ammonia production remained insignificant compared to background levels, indicating that using an aqueous solution as a proton source under these conditions was ineffective for ENRR. The three-compartment GDE flow cell showed better control over water crossover by introducing water vapor into the anode gas compartment. This method resulted in reproducible lithium plating and a Faradaic efficiency of 5.6%, demonstrating improved performance in controlling water crossover and enhancing ENRR efficiency. This indicates that the introduction of water vapor as a proton source in Li-ENRR systems has a promise to improve efficiency and potentially lead to more viable methods for ammonia synthesis in future applications. Further research and optimization are necessary to build upon these findings and develop more efficient ENRR systems.
Overall, this thesis highlights the complexities and challenges in developing a viable ENRR process. Significant progress has been made in understanding the fundamental mechanisms, improving experimental protocols, and testing new catalysts. However, further research is necessary to overcome practical challenges and enhance the efficiency of ENRR. This work contributes to the broader effort of developing sustainable and energy-efficient ammonia production methods, aligning with global goals to combat climate change and reduce greenhouse gas emissions.
| Original language | English |
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| Place of publication | Aarhus |
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| Publisher | Aarhus University |
| Number of pages | 154 |
| Publication status | Published - Aug 2024 |