Ammonia as an energy carrier for renewable energy conversion and storage

Abstract

In response to the escalating energy crisis as well as related pollution and climate change problems, renewable energy has attracted widespread attention in recent years. However, the intermittent nature of renewable energy, such as solar and wind, makes it difficult to directly integrate renewable power into the grid. Recently, hydrogen has been considered as a clean energy carrier for renewable energy conversion and storage via an electricity-fuel-electricity approach. First, the renewable power is used to produce hydrogen via the electrolysis of water; and, then hydrogen is fed into a fuel cell to generate electricity when needed. However, the key issue of adopting hydrogen as energy carrier is the difficulty of hydrogen storage (liquefied at -252.87 ℃ at 1 bar or 690 bar at 25.0 ℃) and transportation, which seriously limits its application at a large scale. Ammonia (NH3), as an emerging energy carrier, has attracted ever-increasing attention due to its high hydrogen content (17.65 wt.%) and its ease of storage (liquefied at -33.34 ℃ at 1 bar or 10 bar at 25.0 ℃), transportation and handling. Therefore, the objective of this thesis is concerned with creation and demonstration of using ammonia as an energy carrier for renewable energy conversion and storage via an electricity-fuel-electricity approach. Firstly, to convert nitrogen to ammonia (electricity to ammonia), a self-supporting carbon mat with FeNi-doped Co catalysts fabricated via electrospinning is prepared and employed as an electrode (FeNi-Co@CM) for ammonia synthesis. The electrochemical performance of the FeNi-Co@CM electrode is evaluated in a three-electrode system with FeNi-Co@CM as working electrode, a platinum mesh as counter electrode, and an Ag/AgCl electrode as reference electrode. It results in an ammonia yield rate of 27.9 μg h−1 mg−1 and a Faradaic efficiency of 1.52 % in 1.0 M KOH, which is higher than most of the reported data achieved by using iron-group-based electrocatalysts. The verification resulting from multiple sets of control experiments confirms that the ammonia detected comes from the nitrogen reduction reaction rather than the contaminants in the surrounding environment. In addition, the stability test shows that the FeNi-Co@CM electrode can maintain more than ten cycles. Secondly, to convert ammonia into nitrogen (ammonia to electricity), a fuel cell, which consists of a PtRu/C-based anode, an anion exchange membrane, and a Pd/C-based cathode, respectively, is designed and fabricated. Experimentally, it is demonstrated that this ammonia fuel cell exhibits a peak power density of 20.7 mW cm–2 and an open-circuit voltage of 0.67 V at 95 °C when fed with 3.0 M ammonia and 3.0 M KOH. Besides, the durability test reveals that the developed fuel cell can maintain a stable operation for more than 25 h. In addition, the effects of operating parameters, such as the fuel solution composition, and the flow rates of fuel and oxidant, and the operating temperature on the cell performance, are examined. Thirdly, to gain insights into the complex physical and chemical processes involved in ammonia fuel cells, two-phase flow behaviors are visualized and investigated. It is observed that the process of bubble (nitrogen) emergence, growth, coalescence, detachment and sweeping occurred periodically. Besides, a series of parametric studies are conducted to investigate the effects of the flow rate and operating temperature on two-phase flow behaviors. It is shown that a low flow rate (< 1.0 mL min-1) will cause numerous nitrogen gas slugs in flow channels, seriously blocking the delivery of reactants to the electrode. It is also found that with increasing the operating temperature, the ammonia solution in flow channels start to undergo a gradual phase change of ammonia from liquid to gas. At an operating temperature of 95 °C, the flow channels are almost completely occupied by gases (a mixture of ammonia, nitrogen, water), even under open-circuit condition (a mixture of ammonia and water). Additionally, the effect of the flow-field design on the two-phase flow behavior and pattern is investigated. It can be observed that the parallel flow field are prone to clogging local flow channels by slugs. Moreover, the removal frequency of bubbles in the parallel design is significantly slower than that in the serpentine one. Fourthly, an ammonia fuel cell prototype for scaling-up demonstration is designed, fabricated, and evaluated. It is demonstrated that the developed cell can provide a power of 127.6 mW with a maximum current of 660 mA. Besides, the effects of various design and operating parameters, such as the catalyst loading, the electrode wettability, the operating temperature and the fuel composition on the cell performance, are examined and discussed. Finally, a techno-economic analysis of using ammonia as an energy carrier for converting and storing renewable energy via electricity-ammonia-electricity approach has been carried out. Five different energy conversion and storage routes have been proposed and analyzed. Besides, the effects of the system scale as well as the maturity of the technology have been investigated.