Direct Ammonia Fuel Cell. 03/25/2024

ScienceDirect

March 2024
font size="2">By Ibrahim Dincer, Calin Zamfirescu, in Advanced Power Generation Systems, 2014

Direct Ammonia Fuel Cell
In DAFCs, ammonia is input at the cathode that reacts electrochemically with hydroxyl (OH−) ions to emit electrons and form nitrogen and water molecules.
From: Ammonia Fuel Cells, 2020

Chapters and Articles
Hydrogen and Fuel Cell Systems

4.4.8 Direct Ammonia Fuel Cells
DAFCs are of the SOFC + type with selected catalysts at the anode where gaseous ammonia is fed as a source of hydrogen. The schematic of a DAFC is shown in Figure 4.33. Ammonia fed at the anode decomposes thermo-catalytically and generates protons that diffuse through the porous electrolyte. Water is formed at the cathode where protons encounter the oxygen. The achievable DAFC efficiency is on the order of SOFC + fueled with hydrogen, i.e., over 55%. The system half-reactions are then given as follows:

Ammonia fuel cells

Ibrahim Dincer, Osamah Siddiqui, in Ammonia Fuel Cells, 2020

4.2 Direct ammonia fuel cells entailing oxygen anion conducting solid oxide electrolyte

The DAFC operating with the transfer of negatively charged oxygen anions have been developed and investigated in the recent past. Ammonia is fed at the anodic side of the electrochemical cell that is maintained at temperatures in the range 500–1000°C. At this high temperature, the ammonia molecules are expected to dissociate internally into constituent hydrogen and nitrogen molecules, which can be depicted by the following equation:

(4.10)

N H_{3} \leftrightarrow \frac{3}{2} H_{2} + \frac{1}{2} N_{2}

Furthermore, the cathode feed comprises generally pure oxygen or air. The oxygen molecules that enter the cathodic compartment interact electrochemically in the presence of a suitable electro-catalyst and accept electrons to form negatively charged oxygen anions. This half-cell electrochemical interaction at the cathode that is characteristic of such type of DAFC can be expressed as

(4.11)

\frac{1}{2} O_{2} + 2 e \to O^{2 -}

As the oxygen molecules at the cathode and electrolyte interface keep converting to O^2 − ions, the negatively charged ions keep migrating through the ceramic electrolyte to reach the anode and electrolyte interfaces. Here, the oxidation reaction or the half-cell anodic reaction of the fuel cell takes place. If all of the ammonia molecules are converted to hydrogen molecules owing to the high cell temperature, the anodic reaction can be expressed as

(4.12)

H_{2} + O^{2 -} \to H_{2} O + 2 e

The fuel cell operation for the ammonia-fueled SOFC-O is depicted in Fig. 4.2. As depicted in the figure, the exhaust stream of the anode comprises unreacted ammonia and hydrogen along with the reaction products of nitrogen and water molecules. Also, some studies have suggested that nitrogen oxides may also be formed due to the high temperature and presence of oxygen molecules. Under these conditions, nitrogen molecules can react with oxygen ions to form nitrogen oxides. These are environmentally harmful and their presence in SOFC-O operation raises several concerns about its environmental performance. However, further studies need to be conducted to confirm the presence, ratios, and amounts of nitrogen oxides that are formed during a typical ammonia-fueled SOFC-O. Moreover, the electrons are emitted at the interaction surface of hydrogen atoms and the negatively charged oxygen anions, which travel through an external circuit. This flow of electrons created through an external circuit enables electrons to reach the cathodic side of fuel cell where the oxygen molecules accept electrons and get reduced to negatively charged O^2– ions that migrate through the solid oxide electrolyte to reach the anodic side of the cell. Hence, a continuous flow of electrons and ions is generated as a continuous feed of fuel and oxidant is supplied to the cell.

The electrolytes primarily employed in ammonia-fueled SOFC-O comprise yttria-stabilized zirconia (YSZ) and samarium-doped ceria (SDC) materials. A study conducted on an ammonia-fueled SOFC-O showed that power density of 168.1 mW/cm² can be obtained with the usage of an SDC electrolyte with a thickness of 50 μm. Moreover, the developed cell entailed low-cost nickel material for the anode and a samarium-strontium-cobalt oxide-based cathode. The power density reported was for an operating temperature of 600°C. Nevertheless, the same cell showed better performance when operated with hydrogen fuel providing a maximum power density value that was nearly 23.1 mW/cm² higher. Also, the performances at other temperatures were studied and increasing operating temperatures were reported to provide higher fuel cell performances [47]. Furthermore, another study utilizing a low-cost nickel anode along with an SDC electrolyte was conducted with direct ammonia feed. The cathodic material was composed of barium-strontium-cobalt-iron oxide-based material. In this cell, however, the electrolyte utilized was five times thinner with a thickness of nearly 10 μm. The output power obtained from the cell per unit cell area was reported to be 1190 mW/cm². This was obtained when the cell temperature was maintained at 650°C. Also, the power density was found to increase with the utilization of hydrogen fuel. The increase was found to be significant with a value of nearly 682 mW/cm² [48]. In addition to this, another SDC electrolyte entailing direct ammonia SOFC was tested [49]. They also employed the cost-effective option of using a nickel oxide-based anode with an electrolyte thickness of 24 μm. Also, the cathode utilized in the study was a combination of samarium, strontium, and cobalt oxide materials. The fuel cell performance was not found to be comparatively higher than other cells developed and the power density was found to be nearly 467 mW/cm² when utilizing an operating temperature of 650°C and assessing at the maximum value. Moreover, other type of solid oxide electrolyte entailing the YSZ material was also investigated for direct ammonia solid oxide fuel cells. Nevertheless, the performance of these fuel cells was comparatively lower than the SDC-based cells. For instance, a power density of 202 mW/cm² was found when assessed at the maximum value for a direct ammonia-fueled cell entailing an YSZ electrolyte [50]. The thickness of the electrolyte used was 15 μm. The low-cost nickel material was employed for the anodic fuel cell side and a lanthanum-strontium-manganese oxide-based cathodic material was used. The reported fuel cell performance was for an operating temperature of 800°C. Although the electrolyte entailed a considerably less thickness, the fuel cell performance was observed to be significantly low as compared to SDC electrolyte-based cells. In addition to this, another similar study was conducted with the same type of electrolyte entailing double thickness [51]. Also, the anodic material was composed of nickel composition. Moreover, the cathodic material was composed of lanthanum, strontium, and manganese oxide as in the previously described study. However, the power density values obtained in this study were comparatively higher. For instance, a 299 mW/cm² of power density was reported to be obtained when the cell was operated at 750°C and this value was observed to increase when a higher operating temperature was employed. An increase of nearly 227 mW/cm² was reported when the operating temperature was increased by 100°C. Another direct ammonia-fueled SOFC with considerably higher electrolyte thickness was conducted [52]. The study also considered an YSZ electrolyte, however, significantly higher thickness of 400 μm was utilized. Nevertheless, nonsophisticated electrode materials were employed. The anode was composed of nickel oxide material and the cathode was composed of silver material. The power density when assessed at the maximum value was found to be 60 mW/cm². This was reported to be observed when the cell operating temperature was set at 800°C. The low power density obtained in this case can be attributed to the usage of a high-thickness electrolyte. The higher the electrolyte thickness, the higher the resistance to ionic transfer. This further leads to higher amounts of polarization losses as well as irreversibilities. Also, another thick electrolyte entailing direct ammonia SOFC-O was investigated [53]. The YSZ electrolyte utilized was 200 μm thick. Further, nickel entailing anode was utilized in this study as well and the cathodic material was also composed of lanthanum, strontium, and manganese. This study also found low power densities of nearly 88 mW/cm², although a high fuel cell operating temperature of 900°C was used. Furthermore, when the operating temperature was reduced by 200°C, the power density was found to decrease significantly by nearly 50 mW/cm². Hence, both operating temperatures and electrolyte thicknesses play a vital role in determining the performance of direct ammonia SOFC-O. At high temperatures, the rate of dissociation of ammonia molecules increases. Thus, as the number of ammonia molecules dissociating and forming hydrogen molecules increases, the electrochemical interaction at the anode becomes more favorable owing to higher number of reactive molecules that can participate in the electrochemical reactions providing using power outputs. Moreover, platinum has also been investigated as the anodic material [54]. The study utilized a platinum-based anode along with the BCG electrolyte. Also, the cathodic material was composed of silver entailing material. However, the power density results were not substantially better as compared to the nickel-based anode and lanthanum-strontium-based cathode containing fuel cell. When the fuel cell temperature was set at 800°C, the power output density was observed to be 50 mW/cm² when assessed at the peak value. Furthermore, when the operating temperature was increased by 200°C, the power output density was found to rise by a value of nearly 75 mW/cm².

The different types of direct ammonia-fueled SOFC-O developed and investigated have been summarized in Table 4.1. The type of electrolytes utilized and their corresponding thickness have also been listed as well as the type of anodic and cathodic materials employed. In addition, the performance of the developed fuel cell is described in terms of open-circuit voltage as well as maximum power density.

Table 4.1. Direct ammonia-fueled oxygen anion conducting solid oxide fuel cells.


Peak power density (mW/cm²)	Operating temperature (°C)	OCV (V)	Electrolyte thickness (μm)	Electrolyte	Electrodes	Source
65	500	0.9	50	SDC	Ni-SDC (anode) SSC-SDC (cathode)	[47]
168	600	0.88
250	700	0.83
167	550	0.795	10	SDC	NiO (anode) BSCF (cathode)	[48]
434	600	0.771
1190	650	0.768
60	800	1.22	400	YSZ	NiO-YSZ (anode) Ag (cathode)	[52]
202	800	1.06	15	YSZ	Ni-YSZ (anode) YSZ-LSM (cathode)	[50]
467	650	0.79	24	SDC	NiO-SDC (anode) SSC-SDC (cathode)	[49]
65	800	1.02	200	YSZ	Ni-YSZ (anode) LSM (cathode)	[53]
88	900	0.99

These type of electrolytes as well as electrode materials have also been extensively investigated for usage with hydrogen fuel. However, as discussed earlier, their application with ammonia fuel opens a new range of possibilities to utilize direct ammonia fuel electrochemically to convert its chemical energy into useful electrical energy. These type of fuel cells, however, necessitate high operating temperatures of nearly 500–1000°C. To achieve satisfactory performances, the operating temperatures in the upper range need to be utilized as temperatures in the lower range of about 500°C do not provide sufficient power outputs. However, in case of exothermic reactions, it is generally expected that the reactor in which the electrochemical reactions take place will entail the heat generated and gradually the external source of heat input can be eliminated. Nevertheless, in case of ammonia-fueled SOFC-O, further investigation is needed to analyze the amount of heat generation per unit time and the corresponding temperature increase and control. For instance, initially SOFC-O can be provided external heat to maintain the high temperatures required. However, as the fuel cell operation is proceeded and ammonia fuel is input, the increase in temperatures should be recorded with time. Correspondingly, the external heat input can be reduced in steps to achieve a stable operating temperature as required by the cell.

Integrated ammonia fuel cell systems

Ibrahim Dincer, Osamah Siddiqui, in Ammonia Fuel Cells, 2020

6.7 Closing remarks

In this chapter, renewable energy-based integrated systems with DAFCs, which have been developed in the recent past are presented. These systems include the opportunity to use the electrochemical interactions of ammonia molecules to produce several useful outputs through system integration. The hybrid DAFC and TES system is firstly presented, which includes the usage of a molten alkaline electrolyte to generate clean power through electrochemical interactions of ammonia molecules as well as stored thermal energy for later usage. The system entails discharging of both electrical as well as thermal energy during the discharging phase. Further, solar- and wind-based integrated energy systems are presented that incorporate the usage of AFCs to produce clean electrical, stored thermal energy as well as electrical energy. Solar thermal power plants entailing excess solar energy are integrated with the hybrid system and their dynamic performances are investigated considering the changes in the solar intensities throughout the year. Also, integrated solar and wind-based energy systems are discussed where the excess energy from both plants is employed for synthesizing ammonia that is used to generate electrical power through DAFCs when there are low wind velocities or solar intensities. The performance of each system is assessed through energy and exergy efficiencies where the total useful energetic and exergetic output is determined as a ratio of the total energetic and exergetic input.

NH3 Oxidation on Well-Defined Surfaces and Proxies of the Same

B. Tam, ... D. Guay, in Encyclopedia of Interfacial Chemistry, 2018

Introduction

The electrooxidation of ammonia has come to the forefront of modern electrochemistry. This toxic gas and significant environmental pollutant has also been touted for its potential use in energy storage as a hydrogen sink1–3 and in energy production through direct ammonia fuel cells.4–6 The electrooxidation of ammonia is also of the utmost importance in NH3 sensors7,8 and decontamination of wastewater.9–11 Accordingly, advancing the state of knowledge for ammonia oxidation remains vital. Since ammonia oxidation occurs with much lower activity in acidic media,12 following discussion will exclusively focus on examples in alkaline media. Despite the fact that NH3 dehydrogenation to N2 may seem straightforward, several reactive intermediates exist with many reaction pathways that cause the so-called “Nitrogen Cycle” to be more nuanced.13
The most frequently cited mechanism for NH3 electrooxidation on Pt electrodes was initially suggested by Gerischer and Mauerer in 1970.14 In this mechanism, Pt dehydrogenates adsorbed NH3 to form reactive intermediates NHx (0 ≤ x ≤ 2). For x = 1 or 2, these intermediates may then combine amongst themselves (or with NH3 in the case of x = 1) to form N2Hy (2 ≤ y ≤ 4). This species is then rapidly oxidized by OH− to form adsorbed N2 (especially so in the case of hydrazine oxidation15), which desorbs readily to free up the surface. For x = 0, adsorbed N or Nad acts as a poison on the reaction as it is strongly adsorbed on the Pt surface and its accumulation reduces the number of reaction sites over time. This proposed mechanism has since been reinforced by numerous studies utilizing in situ techniques such as cyclic voltammetry (CV), differential electrochemical mass spectrometry (DEMS),16 surface-enhanced Raman spectroscopy (SERS), and rotating ring-disk electrode (RRDE) studies as noted in thorough reviews by Rosca et al.,13 Bunce and Bejan,17 and Zhong et al.18
Pt metal and Ir, to a lesser extent, are the most promising catalysts for ammonia electrooxidation.16,19 Pt metal is favored as the catalyst because N2 forms at low potentials with high current densities and the main poisoning by-product, Nad, forms at higher oxidative potentials than for other metal catalysts. As a highly surface-sensitive reaction, numerous studies have demonstrated the superior activity of Pt(100) orientated terrace surfaces over the other basal crystal planes Pt(111) and Pt(110).20,21 Pt(100) surfaces were shown to be the most stabilizing towards the NH2 adsorbed species, instead of NHad and Nad, which were found to have low reactivity once formed.21 Herein, we discuss various methods for synthesizing wide Pt(100) terraces for ammonia oxidation, starting with single crystals and then moving on to epitaxial thin films, nanoparticles, and electrodeposited Pt high surface area films. Functionality may also be added to these films, in order to lower the onset potential of the reaction, by addition of adatoms to form bifunctional catalysts with sustainable activity towards ammonia oxidation.

Energy Materials
Ibrahim Dincer, Yusuf Bicer, in Comprehensive Energy Systems, 2018
2.1.1.3 Ammonia Utilization
The ability to use one fuel in all types of combustion engines, gas turbines, burners, and directly in fuel cells is a tremendous advantage. Storage and delivery infrastructure would be significantly reduced if ammonia is employed rather than hydrogen. NH3 is one of a very short list of fuels that can be used in nearly every type of engine and gas burner with only minor modifications. Gas burners can be equipped with in-line partial reformers to split approximately 5% of the NH3 into hydrogen. This mixture produces a robust, unpolluted burning open flame. One pipeline to a home could provide NH3 to furnaces/boilers, fuel cells, stationary generators and even vehicles. Due to the very minor enthalpy of reforming exhibited by NH3, it can easily be reformed to hydrogen for any application that would require hydrogen. Relatively minor modifications allow efficient use of ammonia as a fuel in diesel engines; high compression ratio spark ignition engines can produce astounding efficiencies of over 50% using NH3 fuel; direct ammonia fuel cells promise to be low-cost, robust, and very efficient; NH3 is also a very suitable fuel for use in solid oxide fuel cell (SOFC) and gas turbines. These medium-temperature (approximately 400°C) fuel cells promise to be low-cost, highly efficient, and very robust [39].

The global ammonia demand is forecasted to grow at an average annual rate of approximately 3% over the next 5 years. The historical growth rate was 1%. Therefore, currently, it is 2% above. The global ammonia consumption amounts are shown in Fig. 6. Solid agricultural materials are expected to drive this growth as fertilizer uses account for approximately 80% of global ammonia demand [42]. The global ammonia exports and imports based on the selected years are illustrated in Fig. 7. The United States and Asia have quite close import rates whereas Western Europe’s imports are almost half of Asia. In recent years, the export of ammonia from Africa and Middle East increases gradually. The United States is recognized as the largest ammonia importer and typically accounts for approximately 35–40% of world trade. Europe, a higher-cost producer, accounts for roughly 25% of commerce. The majority of growth in imports is expected in Asian countries, for industrial uses and the production of fertilizer products.