Electrifying the chemical industry: Towards sustainable ammonia production from dinitrogen and nitrogen-containing wastes
Date
2022-12
Authors
Chen, Yifu
Major Professor
Advisor
Li, Wenzhen
Tessonnier, Jean-Philippe
Roling, Luke T
Brown, Robert C
Huang, Wenyu
Committee Member
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Abstract
Ammonia (NH3) plays an indispensable role in global agro-economics and chemical industries as a basic ingredient for fertilizer production that feeds billions of people in the world with the essential N-nutrients. It is also a promising carbon-neutral energy carrier due to its high energy storage density, ease of liquefaction under mild conditions, and its ability to release various forms of energy via direct oxidation or catalytic cracking. To meet the growing demand for NH3 in those diverse roles, the synthesis of NH3 in the industry has relied on the Haber-Bosch process in centralized facilities, which is commercially viable but energy-intensive; the Haber-Bosch process is also associated with considerable release of CO2, since it requires fossil-derived hydrogen (H2) as a reactant.
Motivated by the increasing demand for flexible and sustainable routes of NH3 production, the electrochemical nitrogen (N2) and nitrate (NO3−) reduction reaction (NRR and NO3RR) have attracted intense research interest in recent years, as N2 and NO3− are highly abundant in our ecosystems; in particular, cumulative NO3− in natural water bodies, as a consequence of the imbalance of N-cycle due to human activities, has led to severe environmental issues that adversely impact human health and welfare. Both NRR and NO3RR allow for distributed and on-site generation of NH3 with water as the proton source, thereby reducing the transportation and storage costs of NH3, and facilitating the decoupling of N-centric chemical industry with the consumption of fossil fuels.
This dissertation presents the author’s Ph. D. work in search of sustainable pathways for NH3 production by NRR and NO3RR. Two strategies for electrochemically converting N2 have been explored, including the indirect lithium (Li)-mediated pathway and the direct reduction in the molten alkali (NaOH/KOH) system. For Li-mediated NH3 synthesis, a membraneless biphasic system based on the immiscibility of aqueous/organic electrolytes was adopted for the electro-deposition of lithium, with the interface stability largely improved upon the addition of PMMA [poly(methyl methacrylate)] to limit the transport of water. Subsequent nitridation and hydrolysis lead to a high faradaic efficiency (FE) of 57.2% and a production rate of 1.21 × 10−9 mol cm−2 s−1 towards NH3.
The quest for direct N2 electro-reduction was more tortuous because of the elusive status of the research field, due partly to the ominous presence of reactive N-containing species (Nr) and the low NH3 production rates easily interfered with by such Nr. To aid the research community in rebuilding the rigorous protocol for NRR and avoiding pitfalls in the early stage, a systematic investigation of Nr as impurities in commercial metal-based catalyst materials was carried out. Unexpectedly, we revealed the presence of NOx−-N or nitrides-N at substantial levels in many metal oxides and metallic iron samples from mainstream chemical vendors. As an inevitable consequence, the previously reported NaOH/KOH system was proven ineffective for NRR, signifying the difficulty of replicating efficient N2 fixation under similarly mild conditions as in nature. In addition, we suggested two-step cleansing procedures as a reliable protocol for screening Nr inside catalyst materials, and an improved gas circulation system so that the quantitative assessment of NH3 origins can be significantly economized and facilitated with limited use of 15N2.
While Nr species are an interfering factor in NRR research, their widespread feature offers new pathways that can fulfill the requirements of distributed NH3 production, such as NO3RR. Despite being ineffective for NRR, the NaOH/KOH/H2O system was employed to develop a simple membrane-free alkaline electrolyzer (MFAEL) system for Nr-to-NH3 processes. Taking advantage of the high conductivity, high NH3 selectivity, and strong capability of breaking both N–O and C–N bonds, the MFAEL system enables rapid and efficient NH3 production from various forms of Nr, accompanied by the concurrent separation and collection of the NH3 in the form of pure NH3-based chemicals. A record-high partial current density for NH3 production (4.22 A cm−2) from NO3RR was achieved with a FE of 84.5% on a commercial Ni foam electrode. By combining NO3− reduction in MFAEL with a low-energy cost electrodialysis process for efficient NO3− concentrating, a sustainable and economically competitive pathway for upcycling waste NO3−-N into NH3 product was successfully demonstrated. In addition, various forms of organic N exhibited ~100% of NH3-N recovery upon full conversion, expanding the repository of available Nr for NH3 production to include organic N. Such a finding enables the “one-pot” convergent transformation of N–O and C–N bonds into NH3 as the sole N-containing product. Overall, the electricity-driven waste upcycling process offers an economically viable solution to the growing trend of Nr buildup, and could potentially fulfill the increasing NH3 demand with reduced carbon footprints.
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