A review and comparative assessment of direct ammonia fuel cells

Thermal Science and Engineering Progress 5 (2018) 568–578

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Thermal Science and Engineering Progress journal homepage: www.elsevier.com/locate/tsep

A review and comparative assessment of direct ammonia fuel cells ⁎

T

Osamah Siddiqui , Ibrahim Dincer Clean Energy Research Laboratory, Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario L1H 7K4, Canada

A R T I C L E I N F O

A B S T R A C T

Keywords: Ammonia Hydrogen Fuel cells Comparative assessment Efficiency Energy

In this study, we present a comparative assessment of direct ammonia fuel cells and a discussion of these from various perspectives by considering the effective criteria. The effect of electrolyte and electrode materials, electrolyte thicknesses and operating temperatures on the performance of direct ammonia fuel cells are studied and discussed comparatively for evaluation purposes. A comparison of which cell types and configurations provide the optimum performance is conducted by utilizing the experimental results reported in the literature. The results of this study are expedient to provide important inferences about the performances of direct ammonia fuel cells. Ammonia fed oxygen ion conducting Samarium doped ceria electrolyte based solid oxide fuel cells have comparatively the highest peak power density of 1190 mW/cm2 when operated at 650 °C with a 10 µm thick electrolyte. Direct ammonia proton conducting electrolyte based solid oxide fuel cells have lower cell performance than oxygen ion conducting fuel cells due to the dilution of hydrogen at the anode by undecomposed ammonia as well as formed nitrogen gas. The operating temperature of the fuel cell and electrolyte thickness affect the cell performance considerably. A 200 °C increase in operating temperature increases the peak power density by nearly three to four times for ammonia fed solid oxide fuel cells. The molten alkaline electrolyte based fuel cells can be a promising technology. Further research is required for these type of fuel cells to investigate their performance with low electrode separation distance and more conductive alkaline electrolytes.

1. Introduction Energy forms an integral part of any economy. Energy demands are increasing incessantly across the globe. In order to meet the high demands, the environment has been affected adversely. Utilization of fossil fuels has been the primary reason of environmental damage. Combustion of fossil fuels for energy production has caused massive amounts of environmental contaminants and greenhouse gases to be emitted to the atmosphere. In order to overcome the problems associated with the usage of fossil fuels, renewable energy resources are being implemented. However, renewable energy resources, such as solar energy, are intermittent in nature. Hence, storage media are required in order to better utilize these resources. Hydrogen has been considered as a promising storage medium as the end usage of hydrogen does not emit harmful pollutants [1]. However, hydrogen does not have a high volumetric energy density. Thus, this poses a drawback in storage as well as transportation of hydrogen. In addition, hydrogen is also an odorless flammable gas. It is considered dangerous while storage or transportation. In order to overcome these drawbacks, alternative hydrogen carriers are being investigated. Several hydrogen carriers including ammonia, hydrocarbons and alcohols have been

⁎

considered. Ammonia is a promising candidate due to various advantages. It has a high energy density of 4 kWh/kg and is easy to liquefy as it has a boiling point of −33.4 °C at standard atmospheric pressure. Furthermore, it has a comparatively high hydrogen content of 17.7 wt% and has a constricted flammable range of approximately 16–25 vol.% in air [2,3]. Table 1 provides a comparison of volumetric and energy density of hydrogen and ammonia. Hydrogen gas has a low energy density of 0.9 MJ/L at a temperature of 25 °C and a high pressure of 10 MPa. In order to increase the volumetric density, hydrogen can liquefied for storage and transportation. However, as shown in Table 1, liquefying hydrogen requires a very low temperature of −253 °C at atmospheric pressure. Ammonia liquid has significantly higher volumetric density as compared to hydrogen liquid or gas. Furthermore, ammonia also has a higher energy density than liquid hydrogen when stored as liquid at 25 °C and a pressure of 10 atmospheres. Ammonia is free of carbon and is cost effective [4], hence, it provides an alternative fuel source to achieve lower environmentally harmful emissions in the process of energy generation. Moreover, ammonia is produced by various countries in large quantities. Generation of electricity through fuel cells is considered as a clean source of energy production. Currently, hydrogen is the prominent fuel

Corresponding author. E-mail addresses: [email protected] (O. Siddiqui), [email protected] (I. Dincer).

https://doi.org/10.1016/j.tsep.2018.02.011 Received 10 August 2017; Received in revised form 4 October 2017; Accepted 17 February 2018 2451-9049/ © 2018 Published by Elsevier Ltd.

Thermal Science and Engineering Progress 5 (2018) 568–578

O. Siddiqui, I. Dincer

Nomenclature

B0 D E F f G g J N n P p R T y z

eff TB

permeability (m2) diffusion (m2/s) reversible cell potential (V) Faraday’s constant fugacity Gibbs energy (J) specific Gibbs energy (J/mol) current density (A/m2) molar flux (mol/m2s) moles (mol) pressure (kPa) partial pressure (kPa) gas constant (J/mol·K) temperature (K) molar fraction number electrons transferred per 1 mol of fuel

Subscripts act an ca conc elec k sys Ω

δ ε σ τ μ ϕ ΩD

BCG BCGE BCGO BCGP BCNO BSCF BZCY CDN/C CPPO DABC LSCO LSM PVA SDC SSC TB YSZ

coefficient of charge transfer Lennard-Jones parameter or dimensionless temperature (K) thickness of electrolyte (m) porosity of electrode diameter of molecular collision tortuosity of electrode viscosity (N s/m2) over-potential (V) diffusion collision integral

Superscripts 0

activation anode cathode concentration electric Knudsen diffusion system Ohmic

Acronyms

Greek letters

α Γ

effective triple phase boundary

Gadolinium-doped barium cerate Europium doped barium cerate BaCe0.8 Gd 0.2 O2.9 Gadolinium and Praseodymium-doped barium Cerate BaCe0.9 Nd 0.1O3 − δ Ba0.5Sr0.5Co0.8Fe0.2O3-δ BaZr0.1Ce0.7 Y0.2O3 − δ Chromium decorated nickel/carbon Chloroacetyl poly(2,6-dimethyl-1,4-phenylene oxide) Direct ammonia borane fuel cell La 0.5Sr0.5CoO3 − δ La 0.67Sr0.33MnO3 − δ poly vinyl alcohol Samarium doped ceria Sm0.5Sr0.5CoO3-δ Triple phase boundary Yittria stabilized zirconia

standard conditions

and operating temperatures provide optimum performance with ammonia as a fuel. In this study, we present a comprehensive comparative study of the performance of different types of direct ammonia fuel cells. The results of this study are expedient to provide important inferences about their performance under varying conditions. In this paper, the primary objectives of comparatively assessing different types of ammonia fuel cell are listed as follows:

for fuel cell technology. However, in order to overcome the drawbacks of hydrogen, ammonia can be used as an alternative fuel. Incorporating ammonia as a fuel for fuel cells has been considered in various studies. In the utilization of ammonia as a fuel for fuel cells, either it can be decomposed into nitrogen and hydrogen externally or it can be directly fed into the cell. Direct ammonia fuel cells allow feeding of ammonia as a fuel directly without requiring an external decomposition unit. Hence, provide more applicability for various uses. Ammonia was initially investigated as a source of electricity generation from fuel cells as well as a source to produce nitrogen oxide as a useful chemical [5,6]. Several studies have followed which investigated ammonia as a fuel source for different types of fuel cells. Although, studies have been conducted on various types of direct ammonia fuel cells with different electrolytes, electrodes materials and operating temperatures. It is important to analyze, compare and determine which types of electrolyte and electrode materials, thicknesses

• Analyzing the effects of electrolyte and electrode materials on the performance of direct ammonia fuel cell, • Analyzing the effects of electrolyte thicknesses, • Analyzing the effects of operating temperatures, and • Determining which direct ammonia fuel cell technologies, configurations and operating conditions provide the optimum performance.

2. Direct ammonia solid oxide fuel cells Table 1 Comparison of energy and volumetric density of ammonia and hydrogen.

Hydrogen gas Hydrogen liquid Ammonia liquid

Temperature (°C)

Pressure (MPa)

Weight % H2

Energy density (MJ/L)

Density (g/L)

25

10

100

0.9

7.7

−253

0.1

100

8.6

71.3

25

1

17.7

12.9

603

Ammonia fed solid oxide fuel cells (SOFC) can be classified mainly into two types according to the type of electrolyte used: Proton conducting electrolyte based solid oxide fuel cells (SOFC-H) and Oxygen anion conducting electrolyte based solid oxide fuel cells (SOFC-O). 2.1. Oxygen anion conducting electrolyte based solid oxide fuel cells (SOFCO) Solid oxide fuel cells based on oxygen anion conducting electrolytes entail the working principle of the transport of O2− ions through the 569

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circuit voltages of various ammonia fed SOFC-O operated at varying temperatures and electrolyte thicknesses. The reasons for varying performances of ammonia fed SOFC-O and optimum conditions for higher fuel cell performance are discussed is Section 6.

electrolyte. Pure oxygen gas or air is fed at the cathode side of fuel cell. At the interface between the cathode and electrolyte, reduction of oxygen gas to oxygen ions occurs. These ions pass through the electrolyte and an electrochemical reaction takes place with hydrogen at the interface of the anode and electrolyte to produce water vapor. The hydrogen is produced by thermal decomposition of ammonia fuel. The ammonia fuel is fed into the fuel cell at the anode side, where it undergoes a thermal decomposition under the presence of a catalyst.

2.2. Proton conducting electrolyte based solid oxide fuel cells (SOFC-H)

The hydrogen gas produced as a result of this decomposition diffuses to the interface between the anode and electrolyte where it reacts electrochemically with oxygen ions to form water vapor. The produced water vapor, nitrogen gas, unreacted ammonia fuel and hydrogen gas leave the fuel cell. The production of nitrogen during thermal decomposition of ammonia results in a dilution of hydrogen concentration, which reduces the reversible cell potential of the fuel cell. Fig. 1 depicts the working principle of SOFC-O. The cathodic and anodic reactions are given below: Anodic reactions:

In ammonia fed proton conducting electrolyte based solid oxide fuel cells, the ammonia fuel decomposes to form nitrogen and hydrogen. The hydrogen gas produced due to the decomposition of ammonia oxidizes to protons (H+) in the presence of a catalyst. The H+ ions are transferred through a proton-conducting electrolyte to reach the cathode-solid electrolyte interface, where it reacts with oxygen to form water vapor. The produced water vapor and unreacted oxygen leave the fuel cell from the cathode side exit port of the fuel cell. At the anode side exit port of the fuel cell, unreacted ammonia, nitrogen gas and unreacted hydrogen leave the fuel cell. Ammonia fed SOFC-H avoid the formation of nitrogen oxides as in the case of SOFC-O. The operation principle of a SOFC-H is shown in Fig. 2. The anode and cathode electrochemical reactions are given below: Anodic reactions:

H2 + O 2 − → H2 O + 2e

(2)

H2 → 2H+ + 2e

(3)

1 O2 + 2H+ + 2e → H2 O 2

3 1 NH3 ↔ H2 + N2 2 2

(1)

Cathodic reactions:

1 O2 + 2e → O 2 − 2

(4)

Cathodic reactions:

2.1.1. Performance of oxygen anion conducting electrolyte based solid oxide fuel cells (SOFC-O) Direct ammonia fuel cells with oxygen anion conducting Samarium doped ceria (SDC) and Yittria stabilized zirconia (YSZ) electrolytes have shown promising performance. For a 50 µm SDC electrolyte based SOFC-O with nickel anode and Sm0.5Sr0.5Co3-δ (SSC) cathode, a maximum power density of 168.1 mW/cm2 was obtained with ammonia as the fuel at 600 °C. However, a peak power density of 191.8 mW/cm2 was observed with hydrogen as a fuel at the same temperature [7]. In addition, at higher temperatures, greater power densities were obtained. Furthermore, a 10 µm thick SDC electrolyte with nickel based anode and Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) cathode was observed to have a peak power density of 1190 mW/cm2 at a temperature of 650 °C. Whereas, when fed with hydrogen as the fuel, a higher peak power density of 1872 mW/cm2 was observed at the same temperature [8]. In case of a 24 µm thick SDC electrolyte with nickel oxide based anode and SSC cathode, a peak power density of 467 mW/cm2 is observed at a temperature of 650 °C with ammonia as the fuel [9]. Yittria stabilized zirconia (YSZ) is also used as an electrolyte in ammonia fed solid oxide fuel cells. However, lower performance is observed as compared to SDC electrolytes. An ammonia fed fuel cell with 15 µm thick YSZ electrolyte, nickel anode and LSM cathode is observed to have a peak power density of 202 mW/cm2 when operated at a temperature of 800 °C [10]. In addition, an ammonia fed fuel cell comprised of nickel based anode and YSZ-LSM cathode with a 30 µm YSZ electrolyte provides a peak power density of 299 mW/cm2 and 526 mW/cm2 at 750 °C and 850 °C respectively [11]. Moreover, for a 400 µm thick YSZ electrolyte based ammonia fed solid oxide fuel cell with nickel oxide based anode and silver cathode, a peak power density of 60 mW/cm2 was obtained at a temperature of 800 °C [12]. In addition, at a high temperature of 900 °C, the peak power density obtained for a 200 µm thick YSZ electrolyte with nickel based anode and LSM cathode was 88 mW/cm2. However, at a lower temperature of 700 °C, the peak power density reduced to 38 mW/cm2 [13]. With a platinum-based anode and silverbased cathode and YSZ electrolyte, the peak power density obtained at a high temperature of 1000 °C was 125 mW/cm2. When the operating temperature was reduced to 800 °C, the peak power density decreased to 50 mW/cm2 [14]. Table 2 lists the peak power density and open

(5)

2.2.1. Performance of proton conducting electrolyte based solid oxide fuel cells (SOFC-H) Ammonia fed proton conducting electrolyte based solid oxide fuel cells have lower power densities as compared to ammonia fed SOFC-O. Mainly, BCG based electrolytes have been utilized in SOFC-H with platinum or nickel based anodes. Platinum or nickel act as catalysts in the dissociation of ammonia. In the case of a 1300 µm thick BCGP solid electrolyte with ammonia as the fuel, the peak power density obtained with platinum electrodes at 700 °C was 35 mW/cm2 [15]. In addition, for a BCG electrolyte with the same electrode materials, electrolyte thickness and operating temperature fed with ammonia as the fuel, a peak power density of 25 mW/cm2 was obtained [16]. In the case of a BCGP electrolyte operating at a temperature of 600 °C, peak power densities of 28 and 23 mW/cm2 were observed for Ni-BCE and platinum anodes respectively with ammonia as the fuel [17]. Furthermore, a BCGE electrolyte based solid oxide fuel cell with ammonia as the fuel was observed to have a peak power density of 32 mW/cm2. The electrolyte was 1000 µm thick and platinum electrodes were utilized, the operating temperature was 700 °C. In addition to this, a 50 µm thick BCGO electrolyte was investigated [18]. The anode comprised of NiBCGO, and a LCSO-BCGO cathode was utilized. Temperatures were varied from 600 to 750 °C to test the performance of the fuel cell. The

Fig. 1. Working principle of SOFC-O.

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Table 2 Peak power densities and open circuit voltages of various ammonia fed SOFC-O operated at varying temperatures and electrolyte thicknesses. Electrodes

Electrolyte

Electrolyte thickness (µm)

Operating temperatures (°C)

OCV (V)

Peak power density (mW/cm2)

Reference

Ni-SDC (anode) and SSC–SDC (cathode)

SDC

50

SDC

10

Pt-YSZ (anode)Ag (cathode)

YSZ

200

0.9 0.88 0.83 0.795 0.771 0.768 0.9

NiO-YSZ (anode)Ag (cathode) Ni-YSZ (anode) YSZ-LSM (cathode) Ni-YSZ (anode) YSZ-LSM (cathode) NiO-SDC (anode)SSC-SDC (cathode) Ni-YSZ (anode)LSM (cathode)

YSZ

400

1.22

65 168 250 167 434 1190 50 90 125 60

[7]

NiO (anode)-BSCF (cathode)

500 600 700 550 600 650 800 900 1000 800

YSZ

30 15

1.07 1.03 1.06

299 526 202

[11]

YSZ

750 850 800

SDC

24

650

0.79

467

[9]

YSZ

200

700 800 900

1.03 1.02 0.99

38 65 88

[13]

[8]

[14]

[12]

[10]

was 384 mW/cm2. Moreover, a 30 µm BCGO electrolyte with Ni-CGO anode and BSCFO-CGO cathode had a peak power density of 147 and 200 mW/cm2 at temperatures of 600 and 650 °C respectively with ammonia as the fuel [19]. With a 1300 µm thick BCG electrolyte with platinum electrodes, a peak power density of 25 mW/cm2 is obtained at a temperature of 700 °C [20]. In addition, BZCY electrolyte based proton conducting electrolyte was also tested in an ammonia fed solid oxide fuel cell [21]. A Ni-BZCY anode and BSCG cathode was utilized. The peak power densities were obtained in the range 25–390 mW/cm2 with ammonia as the fuel for temperatures varied from 450 to 750 °C. Further, a 20 µm thick proton conducting BCNO electrolyte based solid oxide fuel cell with a LCSO cathode and Nickel oxide based anode had a peak power density of 315 mW/cm2 at a temperature of 700 °C [22]. Table 3 lists the peak power density and open circuit voltages of various ammonia fed SOFC-H operated at varying temperatures and electrolyte thicknesses. The reasons for varying performances of ammonia fed SOFC-H and optimum conditions for higher fuel cell performance are discussed is Section 6.

Fig. 2. Working principle of SOFC-H.

peak power densities were found to increase with increasing temperature. At 600 °C, a peak power density of 96 mW/cm2 was obtained, whereas, at a temperature of 750 °C, the peak power density recorded

Table 3 Peak power densities and open circuit voltages of various ammonia fed SOFC-H operated at varying temperatures and electrolyte thicknesses. Electrodes

Electrolyte

Electrolyte thickness (um)

Operating temperature (°C)

OCV (V)

Peak power density (mW/cm2)

Reference

Pt (Anode and cathode) Pt (Anode and cathode) Pt (Anode and cathode) Ni-BCE (Anode)Pt (Cathode)

BCGP BCG BCG BCGP

1300 1300 1000 1000

0.85 0.85 0.66 0.92

BCGP BCGO

1000 50

Ni-CGO (anode)BSCFO-CGO (cathode) Ni-BZCY (anode)BSCF (cathode)

BCGO

30

BZCY

35

NiO-BCNO (anode)LSCO (cathode) Pt (anode and cathode)

BCNO

20

0.95

35 25 32 15 18 28 23 96 184 355 384 147 200 25 65 130 190 275 325 390 315

[15] [16] [17] [17]

Pt (Anode and cathode) Ni-BCGO (anode)LSCO (cathode)

700 700 700 500 550 600 600 600 650 700 750 600 650 450 500 550 600 650 700 750 700

BCG

1300

700

0.85

25

[20]

1.102 1.095 0.995 0.985 1.12 1.10 0.95

571

[18] [18]

[19] [21]

[22]

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3. Ammonia alkaline membrane and molten or aqueous alkaline electrolyte fuel cells

were chosen for the study. The peak power density obtained at 200 °C was 10.5 mW/cm2. However, at a higher temperature of 220 °C, it increased to 16 mW/cm2. Electrodes used comprised of platinum material. Furthermore, a CPPO-PVA based alkaline membrane electrolyte fuel cell with CDN/C anode and MnO2/C cathode provided a peak power density of 16 mW/cm2 at room temperature operation [26]. Table 4 lists the open circuit voltages and peak power densities of ammonia fed alkaline fuel cells at various operating temperatures.

Alkaline fuel cells were investigated and developed to be utilized in practical applications in the early stages of fuel cell research and development. They were employed in space applications, vehicles, energy storage and decentralized energy supply [23]. Alkaline membrane and molten or aqueous electrolyte ammonia fuel cells employ the exchange of anions. Air is fed into the fuel cell at the cathode side. The oxygen molecules in the air react with water molecules to form hydroxide ions. The hydroxide anions travel through the electrolyte, which can be an anion exchange membrane or an alkaline electrolyte such as aqueous or molten potassium or sodium hydroxide. In ammonia based alkaline fuel cells, ammonia is fed as a fuel into the fuel cell at the anode side. It combines with the hydroxide anions to form nitrogen and water. The reaction of oxygen with water molecules at the cathode side consumes electrons, whereas, the reaction of ammonia with hydroxide ions at the anode side produces electrons. Hence, producing an electric current through an external electrical path. The cathode, anode and overall reactions for alkaline ammonia fuel cells are given below: Anodic reaction:

2NH3 + 6OH− → N2 + 6H2 O + 6e

4. Ammonia borane fuel cells Direct ammonia borane fuel cell was first investigated by Zhang et al. [27]. Ammonia borane (NH3BH3) solution is fed as the fuel into the fuel cell at the anode side, where it reacts with hydroxyl anions. The hydroxyl anions participating in the reaction are either formed by the reduction of oxygen at the cathode side or are present in the medium. The electrolyte comprises of a cation or anion exchange membrane. The cathodic reaction of reduction of oxygen to form hydroxyl ions is similar to other types of fuel cells:

3 O2 + 3H2 O + 6e → 6OH− 2

(6)

The produced hydroxyl anions diffuse through the membrane electrolyte to reach the anode side of the fuel cell, where the oxidation of NH3BH3 occurs. The anodic reaction for a direct ammonia borane fuel cell is:

Cathodic reaction:

3 O2 + 3H2 O + 6e → 6OH− 2

(7)

NH3 BH3 + 6OH− → BO 2 − + NH4+ + 4H2 O + 6e

Overall reaction:

2NH3 +

(10)

3 O2 → N2 + 3H2 O 2

The overall reaction for an ammonia borane fuel cell can be expressed as:

(8)

In alkaline fuel cells, the presence of carbon dioxide gas degrades the performance of the cell. The carbon dioxide gas reacts with hydroxide anions present to form carbonate ions. This reaction reduces the number of hydroxide anions available to react with ammonia. In addition, the ionic conductivity of the alkaline electrolyte is reduced due to the formation of carbonate ions. Precipitates of carbonate compounds are found in alkaline molten or aqueous electrolyte, however, in alkaline membrane electrolytes, the formation of precipitates is avoided.

CO2 + 2OH− → CO32 − + H2 O

(11)

NH3 BH3 +

3 O2 → BO 2 − + NH4+ + H2 O 2

(12)

A sodium hydroxide electrolyte based ammonia borane fuel cell operating at room temperature was observed to have a peak power density of 14 mW/cm2. The working electrode comprised of platinum and gold [27]. Furthermore, an ammonia borane fuel cell with an anion exchange membrane and platinum electrodes provided peak power densities of 40–110 mW/cm2 when the operating temperatures were varied from 25 to 45 °C [28]. Table 5 lists the peak power densities of ammonia borane fuel cells operated at varying temperatures and electrolyte and electrode materials.

(9)

A molten potassium and sodium hydroxide mixture electrolyte based ammonia fed fuel cell at operating temperatures between 200 and 450 °C was observed to have a peak power density of 16 mW/cm2 at a temperature of 200 °C. However, as the temperature was increased to 450 °C, a considerable increase in peak power density was observed. At 450 °C, it increased to 40 mW/cm2 [24]. In addition, another study on ammonia fed alkaline fuel cell with molten potassium and sodium hydroxide electrolyte was conducted [25]. Temperatures of 200–220 °C

5. Thermodynamics of fuel cells In order to evaluate the performance of a fuel cell, it is essential to consider the underlying thermodynamic principles. In alkaline electrolyte based ammonia fed fuel cells, anodic reaction proceeds according to reaction (7), without the decomposition of ammonia to nitrogen and hydrogen. However, in solid oxide fuel cells fed with

Table 4 Peak power densities and open circuit voltages of various ammonia fed alkaline fuel cells operated at varying temperatures and electrolyte and electrode materials. Electrodes

Electrolyte

Operating temperature (°C)

OCV (V)

Peak power density (mW/cm2)

Reference

Nickel (anode and cathode)

Molten Potassium hydroxide–Sodium hydroxide (KOHNaOH)

Molten Potassium hydroxide–Sodium hydroxide

Chromium decorated nickel/carbon (CDN/C) (anode)Manganese oxide/carbon (MnO2/C) (cathode)

Chloroacetyl poly(2,6-dimethyl-1,4-phenylene oxide)Poly vinyl alcohol (CPPO-PVA membrane)

0.82 0.819 0.817 0.816 0.813 0.811 0.76 0.74 0.73 0.85

16 18 21 25 31 40 10.5 12 16 16

[24]

Platinum (anode and cathode)

200 250 300 350 400 450 200 210 220 25

572

[25]

[26]

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V = E −ϕact ,an−ϕact ,ca−ϕΩ−ϕconc,an−ϕconc,ca

Table 5 Peak power densities of ammonia borane fuel cells operated at varying temperatures and electrolyte and electrode materials.

(20)

Electrodes

Electrolyte

Operating temperature (°C)

Peak power density (mW/cm2)

Reference

The loss occurring in a fuel cell due to electrode kinetics irreversibilities, is known as activation over-potential. The relationship between the activation over-potential and the current density of the fuel cell is expressed by the Butler-Volmer expression as:

Au/Pt (working electrode) Pt

NaOH

25

14

[27]

Anion exchange membrane KOH

25 35 45 45

40 70 110 315 (10 cell stack)

[28]

αzFϕact ⎞ (1−α ) zFϕact ⎞ ⎤ −exp ⎛− J = J0 ⎡exp ⎛ ⎢ ⎥ RT ⎝ RT ⎠ ⎠⎦ ⎝ ⎣

–

⎜

ammonia fuel, the decomposition of ammonia occurs to form nitrogen and hydrogen. This reaction is expressed by reaction (1). After the formation of hydrogen, the anodic reactions proceed according to reaction (2) in case of SOFC-O and reaction (5) in case of SOFC-H. The rate of the reactions in either direction is dependent on the temperature and pressure. The Gibbs energy minimization method can be used to obtain the thermodynamic equilibrium of the reactions [26,27]:

ϕact ,i =

(13)

∑ ni gi

(15)

In case of ideal gases, the partial pressure of the species is equal to their activity. In addition, for liquid and solid species (condensed matter), the activity is unity. Hence, the total system Gibbs energy can be expressed as: (16)

The Nernst equation can be utilized to determine the open circuit voltage. The partial pressures of the reactants and products affect the fuel cell equilibrium potential. The open circuit voltage for SOFC-O and SOFC-H can be expressed as [31]:

E=

E0

0.5 R T ⎡ p H2 (pO2 ) ⎤ + ln ⎢ ⎥ 2F p H2 O ⎣ ⎦

Ni Dieff ,k

2

(17)

ΔG zF

+

(23)

yj Ni−yi Nj

∑ j = 1,j ≠ i

Dieff ,k =

Dijeff

=−

1 ⎡ dyi dP ⎛ B0 P ⎞ ⎤ 1+ + yi ⎢P ⎟⎥ RT dx dx ⎜ μmix Dieff ,k ⎠ ⎦ ⎝ ⎣

(24)

2ε 3τ

8R T rp πMi

(25)

The Chapman-Enskog equation can be used to evaluate the effective binary diffusion of species:

(18)

Dijeff = 0.0018583

Where the standard equilibrium cell potential is calculated as:

E0 = −

(22)

denotes the Knudsen diffusion which includes the effect of where the tortuosity as well as the porosity of the electrode. It can be evaluated as [35]:

1.5

R T ⎡ (p NH3 ) (pO2 ) ⎤ ln 6F ⎢ (p H2 O )3 (p N2 ) ⎥ ⎣ ⎦

⎟

Dieff ,k

The open circuit voltage for alkaline electrolyte based ammonia fuel cells can be expressed as:

E = E0 +

⎜

The magnitude of the electrolyte resistance depends on the material utilized. In addition, it is also affected by the operating temperatures. In addition, losses occur due to the resistance to transference of the involved species from the porous electrode gas channel to the triple phase boundary reaction locations. These losses are represented by the concentration over-potential. The phenomenon of mass transport occurs in electrodes due to the concentration and pressure gradients, which cause diffusion and permeation of reactants. Various models for mass transport have been utilized to quantify the effect of concentration over-potential. The dusty gas model has demonstrated a high level of accuracy [34]. In this model, the porous medium is considered as a collection of suspended spherical particles in space. The effect of permeation as well as diffusion mechanisms on the concentration of species participating in the reactions is accounted for in this model. The multi-component one-dimensional mass transport dusty gas model is expressed as

where the specific Gibbs energy of the individual chemical species is the summation of the Gibbs energy of formation and the amount of chemical activity. The chemical activity of a species of a real gas is the ratio of the fugacity in the current system to the fugacity at standard conditions of temperature and pressure:

Gsys = {∑ ni [gf ,i 0 + R T ln(yi P )] } + ={∑ ni gf ,i 0} g cond

RT J ⎞ sinh−1 ⎛ , i = cathode or anode F ⎝ zJ0,i ⎠

ϕΩ = JδRΩ

(14)

f gi = gf ,i 0 + R T ln ⎜⎛ i0 ⎞⎟ f ⎝ i ⎠

(21)

The Ohmic over-potential (ϕΩ ) accounts for the ionic resistance of the electrolyte. Losses due electrolyte ionic resistance are considerably higher than the electronic resistance losses in the fuel cell. The Ohmic over-potential can be evaluated based on the Ohm’s law [33]. An expression in terms of the current density, electrolyte resistance and electrolyte thickness is:

The system Gibbs energy can be obtained from the number of moles of the chemical species and their individual specific Gibbs energies at a given pressure and temperature from the equation:

Gsys =

⎟

where J0 denotes the exchange current density, which quantifies the electron activity at the equilibrium potential. It is dependent on the material and structure of the electrodes used. In addition, it is also affected by the reaction temperature and the triple-phase boundary length [32]. The ratio of forward to backward activation barrier is affected by the electrical potential. This affect is described by the charge transfer coefficient (α) , which lies numerically between zero and one. However, experimentally it is generally found to be around the value of 0.5. The activation over-potential can thus be expressed as:

[38]

[ΔGsys]T ,P = 0

⎜

⎟

(19)

ε 1 1 ⎞ 1 T 3 ⎛⎜ + ⎟ τ Mj ⎠ Pσij2 ΩD,ij ⎝ Mi

(26)

where σij denotes the average characteristic length of the diameter of collision occurring between molecules of species i and j. It can be evaluated as

The reversible cell potential denotes an ideal performance of the cell. However, the irreversibilities within the cell cause energy losses, which are represented by the over-potentials. The cell voltage can hence be denoted as

σij = 573

σi + σj 2

(27)

Thermal Science and Engineering Progress 5 (2018) 568–578

O. Siddiqui, I. Dincer

The collision integral ΩD,ij can be obtained from the Lennard-Jones parameter, which is expressed as

kT εij

Γ=

ϕconc,an =

(28)

ϕconc,ca =

whereεij = εi εj the collision integral can be evaluated from the Lennard-Jones parameter as

ΩD,ij

(29)

ϕconc,ca =

TB R T ⎛ pH2 O ln ⎜ 2F p ⎝ H 2O

η=

4ε 3rp2 72τ (1−ε )2

dP =− dx

∑ i=1

⎛⎜

Ni

dP dx

1 RT

can thus be evaluated as [37]:

⎞

eff ⎟

⎝ Di,k ⎠

( ) + ( ) ∑ ⎛⎝ B0 P μR T

⎜

i=1

yi Dieff ,k

⎞⎟ ⎠

J zF

i

Ni Mi = 0

Welec ṁ NH3 LHVNH3

Ammonia has been tested as a fuel mainly in oxygen anion conducting and proton-conducting electrolyte based solid oxide fuel cells. Also, alkaline membrane or molten electrolyte ammonia fed fuel cells have been investigated in a few studies. In addition, ammonia borane has also been investigated as a fuel. The results this study analyze, compare and determine which types of electrolyte and electrode materials, thicknesses and operating temperatures provide optimum performance for direct ammonia fuel cells. Fig. 3 depicts a comparison of the maximum peak power densities obtained from different types of direct ammonia fuel cells. The peak power density is a good indicator of overall cell performance. Oxygen anion conducting electrolyte based solid oxide fuel cells attained the highest maximum peak power density of 1190 mW/cm2. The fuel cell in this case was comprised of a nickelbased anode, SDC electrolyte and BSCF cathode. The high peak power

(33)

In addition, Graham’s law of diffusion can be utilized to evaluate the molar flux of the produced water vapor:

∑

(38)

6. Comparative assessment

(32)

In an electrochemical reaction, the relationship between molar fluxes of gaseous reactants and the current density can be expressed as

Ni =

pO2 ⎞ pOTB2 ⎟ ⎠

where Welec denotes the electrical work output of the fuel cell, and LHVNH3 represents the lower heating value of ammonia and ṁ NH3 denote the mass flow rate of ammonia. In addition, it is important to consider the fuel utilization factor and oxygen utilization factors, which denote the ratio of the consumed fuel to the fed fuel and the ratio of the consumed oxidant to the oxidant fed into the fuel cell [31].

(31)

The pressure gradient

(37)

The thermodynamic efficiency of ammonia fed fuel cells can be expressed as:

(30)

The Kozeny-Carman relationship [36] can be used to estimate the porous electrode permeability:

B0 =

(36)

R T ⎛ p H2 ⎞ ln ⎜ ⎟ 2F ⎝ PHTB2 ⎠

Mi Mi

R T ⎛ pO2 ⎞ ln 4F ⎜ pOTB2 ⎟ ⎝ ⎠

ϕconc,an =

In order to evaluate the gas mixture viscosity, a semi-empirical equation can be utilized [31]:

∑ yi μi ∑ yi

(35)

In case of solid oxide fuel cells with proton conducting electrolytes, the concentration over-potentials can be expressed as:

1.06036 0.19300 1.03587 1.76474 = 0.15610 + + + Γ exp(0.47635Γ) exp(1.52996Γ) exp(3.89411Γ)

μmix =

TB R T ⎛ pH2 p H2 O ⎞ ln ⎜ 2F P p TB ⎟ ⎝ H2 H2 O ⎠

(34)

In case of sold oxide fuel cells, the concentration over-potentials can thus be related to the partial pressures. For solid oxide fuel cells with oxygen anion conducting electrolytes, the concentration over-potentials can be expressed as:

Fig. 3. Comparison of the maximum peak power densities of direct ammonia fuel cells.

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O. Siddiqui, I. Dincer

ions, that travel through the solid electrolyte and reach the cathode, where they react with the oxygen gas to form water molecules. The dilution of hydrogen at the anode due to unreacted ammonia as well as the formed nitrogen reduce the fuel cell performance, which is not accounted for in the theoretical modelling. However, in SOFC-O, oxygen gas is fed at the anode and oxygen atoms convert to negatively charged oxygen ions that travel through the solid electrolyte. Hence, if pure oxygen is fed at the cathode, no dilution occurs.

density obtained can be attributed to the low thickness of the electrolyte. The solid electrolyte had a thickness of 10 µm. The comparatively high peak power density was obtained at a temperature of 650 °C and a fuel flow rate of 40 ml/min. The open circuit voltage obtained at these conditions was 0.768 V. At lower temperatures of 550 and 600 °C the peak power density decreased considerably to 167 and 434 mW/cm2 respectively. This is attributed to incomplete ammonia decomposition at lower temperatures. In case of incomplete decomposition, the undecomposed ammonia particles adsorb on the surface of the anode. Hence, this degrades the cell performance. In addition, when hydrogen was fed as a fuel, a higher peak power density of 1872 mW/cm2 was obtained at 650 °C. Lower fuel cell performance with ammonia as fuel is attributed to the endothermic ammonia decomposition reaction, which reduces the inside cell temperature. The maximum peak power density reported for proton-conducting electrolyte based solid oxide fuel cells was 390 mW/cm2 at a working temperature of 750 °C. The fuel cell in this case comprised of a NiO-BSCF anode, a 35 µm thick BZCY electrolyte and a BSCF cathode. The open circuit voltage obtained was approximately 0.98 V. The peak power density dropped considerably with decreasing temperature, however, the open circuit voltage was fairly constant. Although, the working temperature of this fuel cell was higher than the SOFC-O with a peak power density of 1190 mW/cm2, a considerably lower power density was obtained. This can be due to the dilution of hydrogen by the unreacted ammonia or the formed nitrogen at the anode side. In SOFC-H, the ammonia decomposes to nitrogen and hydrogen. The hydrogen forms H+ ions, which react with the oxidant at the cathode side to form water vapor. Nitrogen or unreacted ammonia dilute the hydrogen at the anode side, hence, resulting in lower cell performance. However, in case of SOFC-O, two reactions are possible at the anode. The ammonia can be directly oxidized to nitrogen and water vapor, or the ammonia can decompose to form hydrogen, which oxidizes to form water vapor. In addition, the difference in power densities can also be attributed to the effect of catalyst on ammonia decomposition or the higher electrolyte thickness in case of SOFC-H. The maximum power density reported for alkaline ammonia fed fuel cells was 40 mW/cm2 at a temperature of 450 °C. A molten alkaline eutectic mixture of NaOH and KOH was utilized as the electrolyte. The peak power density obtained in this case is significantly lower than SOFC-O or SOFC-H. This is attributed to the electrode separation thickness. In this study, the electrode separation thickness was 2 cm, which is substantially higher than the electrolyte thicknesses of SOFC-O or SOFC-H discussed above. Hence, usage of thinner electrode separation may result in better fuel cell performance. Alkaline fuel cells are cost effective, as they require low cost electrolytes. In addition, they do not require expensive catalyst metals as in the case of solid oxide fuel cells. Furthermore, a peak power density of 110 mW/cm2 was obtained at a low temperature of 45 °C in a direct ammonia borane fuel cell. The electrolyte was an anion exchange membrane. This type of cell does not utilize ammonia as a fuel. Aqueous ammonia borane was used. This fuel cell showed promising performance with a high power density at low temperatures. However, further research is required on this type of fuel cells to investigate the viability of ammonia borane as an effective hydrogen carrier.

6.2. Effect of electrolyte thickness and operating temperature on performance of proton conducting electrolyte based ammonia fed solid oxide fuel cells The effect of electrolyte thickness and operating temperature on the peak power density of SOFC-H is shown in Fig. 4. In the selection of the proton conducting fuel cell electrolyte, BZCY and BCGO electrolytes provide improved performance at high temperatures, hence, if these electrolytes are utilized, higher operating temperatures are preferable. The peak power density and fuel cell performance increases considerably with increasing temperature and decreasing electrolyte thickness. For the case of a 35 µm BZCY electrolyte, the peak power density is increased significantly from 25 mW/cm2 at 450 °C to 390 mW/cm2 at 750 °C. In addition, for a 50 µm BCGO electrolyte, the peak power density increased by 4 times as the temperature was increased from 600 to 750 °C. At a temperature of 600 °C, a peak power density of 96 mW/cm2 was obtained, whereas at a high temperature of 750 °C, a considerably higher peak power density of 384 mW/cm2 was observed. Moreover, when the BCGO electrolyte thickness is decreased to 30 µm, higher power densities were observed at the same temperatures. The peak power density obtained at 650 °C increased from 184 mW/cm2 at 50 µm electrolyte thickness to 200 mW/cm2 at an electrolyte thickness of 30 µm. However, different electrode materials were used, which can also be responsible for higher cell performance. The increase in fuel cell performance with increasing temperature is attributed to the higher ammonia decomposition rates. Higher amount of ammonia decomposition results in more availability of H+ ions in proton conducting ammonia fed solid oxide fuel cells. At lower temperatures, substantially low power densities were obtained for SOFC-H as can be depicted from Fig. 4. Hence, these type of ammonia fed fuel cells might not be feasible when operated at low or intermediate Table 6 Modelling parameters utilized for ammonia fed solid oxide fuel cells. Parameter

Value

Anodic exchange current density (A/ m2) for SOFC-O

70 × 108 (pH2O/p0)(pH2 /p0) exp(

Cathodic exchange current density (A/ m2) for SOFC-O Tortuosity of electrode Porosity of electrode Resistance of electrolyte (Ω m) for SOFC-O Thickness of cathode (µm) Thickness of anode (µm) Thickness of electrolyte (µm) Average electrode pore radius (µm) Resistance of electrolyte (Ω m) for SOFC-H Anodic exchange current density (A/ m2) for SOFC-H Cathodic exchange current density (A/ m2) for SOFC-H Oxidant utilization Fuel utilization Operating temperature

6.1. Polarization curves of SOFC-O and SOFC-H The modelled polarization curves of SOFC-O and SOFC-H are depicted in Figs. 8 and 9. The modelling parameters utilized are listed in Table 6. An open circuit voltage of 1.1 V and a peak power density of nearly 7000 W/m2 is obtained for SOFC-H. In addition, a slightly lower open circuit voltage and a peak power density of nearly 6000 W/m2 is found for SOFC-O. However, in the experimental studies discussed in this paper, SOFC-O are found to have higher peak power densities than SOFC-H. This can be attributed to the actual working principle of SOFCH. In SOFC-H, the fed ammonia is decomposed to nitrogen and hydrogen. The formed hydrogen converts to protons or positive hydrogen

Sources: [22–43].

575

−100 × 103 ) RT

70 × 108 (pO2 /p0)1/4 exp( 4.25 0.4

2.9 × 10−5 exp( 30 500 30 0.5 0.37 2500 1700 5% 5% 800 °C

10350 ) T

−130 × 103 ) RT

Thermal Science and Engineering Progress 5 (2018) 568–578

O. Siddiqui, I. Dincer

Fig. 4. Effect of electrolyte thickness and operating temperature on peak power density for direct ammonia SOFC-H.

Fig. 5. Effect of electrolyte thickness and operating temperature on peak power density for direct ammonia SOFC-O.

Fig. 6. Effect of temperature on peak power density of Ni-BCGO/BCGO/LSCO and NiBZCY/BZCY/BSCF direct ammonia fuel cells.

Fig. 7. Effect of temperature on peak power density of molten alkaline KOH-NaOH electrolyte ammonia fed fuel cell.

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Thermal Science and Engineering Progress 5 (2018) 568–578

O. Siddiqui, I. Dincer 7000

1.2 Power density Voltage

5000

Voltage (V)

0.8 4000 0.6 3000 0.4 2000 0.2

Power Density (W/m 2)

6000

1

0 0

was increased from 500 to 700 °C. Moreover, as can be depicted in Fig. 5, YSZ electrolyte based fuel cells have lower peak power densities than SDC electrolyte based fuel cells. This can be attributed to the conductivity of the electrolytes as well as the effect of electrode materials. 6.4. Effect of operating temperature on the performance of nickel anode based BCGO and BZCY electrolyte fuel cells The nickel anode based BCGO and BZCY electrolyte fuel cells were observed to provide the highest peak power densities as compared to other proton conducting electrolyte ammonia fuel cells. The effect of operating temperature on the peak power density of Nickel anode based BCGO and BZCY electrolyte fuel cells is shown in Fig. 6. The increase in power density with temperature is consistent in both types of configurations. Similar increments in power densities are observed for every 50 °C rise in operating temperature. The effect of temperature on fuel cell performance is dependent on the type of electrolyte and electrode materials utilized. However, Ni-BCGO/BCGO (50 µm)/LSCO and NiBZCY/BZCY (35 µm)/BSCF fuel cells showed similar trends. Ni-BZCY/ BZCY (35 µm)/BSCF fuel cell provided higher power densities than NiBCGO/BCGO (50 µm)/LSCO. This can be attributed to the electrolyte thickness and different catalyst materials utilized at the cathode. Lower electrolyte thickness results in lower Ohmic losses and hence higher power densities are obtained, however, with increasing electrolyte thickness, the Ohmic losses increase resulting in lower voltage and power densities. However, the difference in the peak power densities reduced with increasing temperatures as can be depicted from Fig. 6. This can be attributed to the change in membrane conductivity with changing temperature.

1000

5000

10000

15000

2

0 25000

20000

Current Density (A/m ) Fig. 8. Polarization curve for oxygen anion conducting solid electrolyte based direct ammonia fuel cell (SOFC-O). 8000

1.2 Power Density Voltage

7000

Voltage (V)

6000 0.8 5000 4000

0.6

3000 0.4 2000

Power Density (W/m2)

1

0.2 1000 0 0

5000

10000

15000

20000

Current Density (A/m 2)

25000

6.5. Effect of operating temperature on the performance of molten KOHNaOH electrolyte based ammonia fuel cells

0 30000

Fig. 9. Polarization curve for proton conducting solid electrolyte based direct ammonia fuel cell (SOFC-H).

The effect of temperature on peak power densities of alkaline KOHNaOH molten electrolyte based fuel cell is shown in Fig. 7. For a temperature rise of 250 °C, the peak power density increases by nearly 1.5 times. This can be attributed to the increased ionic conductivity of the molten electrolyte at higher temperatures, which leads to lower Ohmic potential losses. Ohmic potential losses are significant in case of aqueous or molten electrolytes if the ionic travel within the electrolyte is subjected to larger distances. Hence, to improve the performance of molten or alkaline electrolyte based ammonia fuel cells, it is essential to utilize electrolytes with lower Ohmic resistance and efforts are required to fabricate fuel cells with lower electrode separation distances.

temperatures. In addition, fuel cells with thick electrolytes were also observed to have substantially low power densities. For a 1300 µm thick BCGP electrolyte based ammonia fed fuel cell had a considerably low peak power density of 35 mW/cm2 at a high temperature of 700 °C. Therefore, the electrolyte thickness is required to be kept at minimum in order to achieve high fuel cell performance. 6.3. Effect of electrolyte thickness and operating temperature on performance of oxygen anion conducting electrolyte based ammonia fed solid oxide fuel cells

7. Conclusions An overview and comparative study of the performance of direct ammonia fuel cells is conducted. The oxygen anion conducting electrolyte based solid oxide fuel cells with a nickel-based anode, a 10 µm thick SDC electrolyte and BSCF cathode is found to provide comparatively the highest maximum peak power density of 1190 mW/cm2 at an operating temperature of 650 °C, when tested experimentally. The open circuit voltage obtained at these conditions is 0.768 V. Furthermore, experimental studies conducted on proton conducting electrolyte-based ammonia fed solid oxide fuel cells have shown lower cell performance than oxygen ion conducting fuel cells. However, theoretical modelling predicts higher peak power density and open circuit voltage for SOFC-H as compared to SOFC-O. The lower performance of SOFC-H obtained in experimental studies can be attributed to the dilution of hydrogen by undecomposed ammonia as well as formed nitrogen gas at the anode that is not accounted in the theoretical analysis. Moreover, it is deduced that in solid oxide fuel cells, an increase in the peak power density of nearly three to four times is observed, when the operating temperature is increased by 200 °C. Whereas, it can be observed that an increase in

The effect of electrolyte thickness and operating temperature on the peak power density of ammonia fed solid oxide fuel cells based on oxygen ion conducting electrolytes is shown in Fig. 5. The electrolyte thickness contributes significantly to fuel cell performance. For a 10 µm thick SDC electrolyte based fuel cell, a peak power density of 1190 mW/ cm2 was observed, however, in case of a 24 µm thick SDC electrolyte, a lower peak power density of 467 mW/cm2 was observed. Both fuel cells included a nickel-based anode, however, different cathode materials were utilized which might also affect the fuel cell performance. In addition, at high electrolyte thicknesses, low fuel cell performance is observed even at high temperatures. A 200 µm thick YSZ electrolyte based ammonia fed fuel cell gave a low peak power density of 125 mW/ cm2 at a substantially high temperature of 1000 °C. This can be attributed to the decreased ionic conductivity of electrolyte with increasing thickness. Furthermore, the fuel cell performance of ammonia fed SOFC-O improved significantly with increasing temperature. The peak power density increased by nearly 4 times from 65 to 250 mW/ cm2 for a 50 µm SDC electrolyte based fuel cell when the temperature 577

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electrolyte thickness degrades the cell performance significantly. Hence, to obtain better fuel cell performance for ammonia fed solid oxide fuel cells, it is essential to operate at high temperatures and utilize electrolytes with low thickness. In addition, solutions to the problem of hydrogen dilution at the SOFC-H anode need to be investigated to improve their performance. Further research is required for alkaline molten electrolyte based fuel cells to investigate the fuel cell performance with low electrode separation distance and more conductive alkaline electrolytes.

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