Development and performance evaluation of a direct ammonia fuel cell stack

Chemical Engineering Science 200 (2019) 285–293

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Development and performance evaluation of a direct ammonia fuel cell stack O. Siddiqui ⇑, I. 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

h i g h l i g h t s
Development and investigation of a new direct ammonia fuel cell stack.
Open circuit voltage is found to be 1249 ± 37.5 mV.
Peak power density is observed to be 13.4 ± 0.4 W m

2

.


Energy and exergy efficiencies are found to be 52.4 ± 1.6% and 49.3 ± 1.6% respectively.

a r t i c l e

i n f o

Article history: Received 29 July 2018 Received in revised form 6 January 2019 Accepted 26 January 2019 Available online 23 February 2019 Keywords: Direct ammonia Fuel cell stack Peak power density Open circuit voltage Efficiencies

a b s t r a c t In the present study, the experimental investigation and performance evaluation of a newly developed direct type ammonia fuel cell stack are performed. A solid anion exchange membrane electrolyte is also utilized. The performances of a single-cell and a 5-cell stack are investigated through thermodynamic efficiencies. The open circuit voltages for a single cell and 5-cell stack are obtained as 280 ± 8 mV and 1249 ± 37.5 mV respectively. Furthermore, the peak power densities are found as 6.4 ± 0.2 W m2 and 13.4 ± 0.4 W m2 for a single-cell and a 5-cell stack respectively. Moreover, the effects of varying humidifier temperatures on their efficiencies are studied, and hence increasing humidifier temperatures are found to provide higher efficiencies. Both energetic and exergetic efficiencies at the peak power density are determined to be 52.4 ± 1.6% and 49.3 ± 1.6% respectively under the ambient conditions considered. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Increasing energy demands have become a central concern globally. The global primary energy requirements have been forecasted to rise by 50% by the year 2030 (U.S. Energy Information Administration, Monthly Energy Review, 2016). Colossal utilization of carbon-based fossil fuels for massive energy production has adversely effected the environment. Nearly 80% of the energy production across the globe comprises of carbon-based fossil fuels (Worldbank, 2014). Several environmental and human health detriments have been attributed to emissions arising from carbon based fuels. Significant efforts are being conducted globally to reduce the dependence on carbon containing fossil fuels. Furthermore, hydrogen is one such non-carbon based fuel that is considered as a promising candidate, which can be utilized as an alternative to fossil fuels in the near future. Combustion of hydro⇑ Corresponding author. E-mail addresses: [email protected] (O. Siddiqui), ibrahim.dincer@uoit. ca (I. Dincer). https://doi.org/10.1016/j.ces.2019.01.059 0009-2509/Ó 2019 Elsevier Ltd. All rights reserved.

gen does not emit any harmful substances when utilized to produce energy. Furthermore, hydrogen is also considered as a possible energy storage medium, owing to its promising energy carrier properties. However, several challenges that hinder the utilization of hydrogen need to be addressed. Having a low volumetric energy density, hydrogen is difficult to store as well as transport. In addition, having a high flammability range and being odorless, hydrogen is associated with several safety perils if transported or stored. Storing hydrogen through chemical means is being considered as possible solution to address these challenges. Several chemical carriers of hydrogen such as hydrocarbons, ammonia as well as alcohols have been investigated. Ammonia is considered as a viable option due to its various chemical and physical properties that make it a favorable hydrogen storage medium. At atmospheric pressure, it has a boiling point of 33.4 °C that makes it comparatively easy to liquefy. Further, having a gravimetric energy density of 4 kWh/kg, ammonia is considered to be an appropriate energy carrier. Also, it entails a 17.7% hydrogen by weight, that is considered a high hydrogen content. In addition, with small range of flammable limits in air, nearly 16–25 vol% (Okanishi et al., 2017;

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Nomenclature A J LHV n_ T V _ W

area (m2) current density (A m2) molar lower heating value (J mol1) molar flow rate (mol s1) temperature (oC) voltage (V) power density (W m2)

Greek letters g energy efficiency w exergy efficiency Acronyms BCNO BaCe0:9 Nd0:1 O3d BCGP Gadolinium and Praseodymium-doped barium Cerate BCGO BaCe0:8 Gd0:2 O2:9

Schüth et al., 2012), it is less flammable than hydrogen. In addition, it is a carbon free fuel that is also comparatively cost-effective. Therefore, ammonia is considered as a promising fuel that can be utilized for environmentally benign energy generation. Utilizing fuel cells for obtaining clean energy is a viable option that is being further investigated by various researchers. Presently, most widely utilized fuel hydrogen for fuel cells. Nevertheless, to tackle the problems hindering the usage of hydrogen discussed earlier, ammonia can be used as an alternative fuel. When utilizing ammonia as a fuel in fuel cell technologies, external dissociated into hydrogen and nitrogen can be performed. However, in direct ammonia fuel cell type, the ammonia fuel can be fed to the fuel cell without external dissociation. Hence, making them attuned with a varied range of applications and uses. Various solid oxide entailing electrolyte utilizing fuel cells have been investigated with direct ammonia as fuel. Meng et al. (2007) developed and investigated an ammonia fuel cell with SDC electrolyte at temperatures varying from 550 to 650 °C. A voltage at 550 °C was recorded to be 0.79 V in open circuit conditions and a decreasing trend was observed as the operating temperatures were elevated. They reported 167 mW cm2 as the power density at 550 °C at the peak value. Also, higher power density values were found at higher temperatures. At a temperature of 650 °C, they found 1190 mW cm2 as the power density at the peak value. Also, Ma et al. (2006a) used a nickel anode and SDC electrolyte. The SDC electrolyte comprised of a thickness of 50 mm. The performance of the fuel cell was studied with hydrogen as well as ammonia as fuels. With the fuel input as ammonia, 168.1 mW cm2 was found to be the maximum power density. This value was reported when a 600 °C temperature was used. However, hydrogen was found to provide 191.8 mW cm2 as the maximum power density of at the same conditions. Furthermore, the study found an improvement in the fuel cell performance with rising operating temperatures. Fuerte et al. (2009) tested a 200 mm thick electrolyte made from YSZ material. Nearly 0.99 V were reported at a temperature of 900 °C in open circuit conditions with ammonia. In addition, 88 mW cm2 was reported as the maximum power density. They used an LSM cathode and a nickel-YSZ anode. Liu et al. (2008) fabricated and investigated a nickel oxide anode based solid oxide electrolyte fuel cell with a SSC cathode and a 24 mm thick SDC electrolyte. The reported power density was 467 mW cm2, at 650 °C temperature. Moreover, Fournier et al. (2006) also tested an ammonia fuel cell with thick YSZ electrolyte. The electrolyte thickness was reported to be 400 mm. The anode was comprised of nickel oxide and the

BCG BCGE BZCY BSCF CDN/C CPPO DABC LSCO LSM MnO2 Pt PVA Ru SDC SSC YSZ

gadolinium-doped barium cerate europium doped barium cerate BaZr0:1 Ce0:7 Y0:2 O3d Ba0.5 Sr0.5 Co0.8 Fe0.2O3d chromium decorated nickel/carbon chloroacetyl poly(2,6-dimethyl-1,4-phenylene oxide) direct ammonia borane fuel cell La0:5 Sr0:5 CoO3d La0:67 Sr0:33 MnO3d manganese di oxide platinum polyvinylalcohol ruthenium samarium doped ceria Sm0.5 Sr0.5 CoO3d Yittria stabilized zirconia

cathode comprised of silver. A 60 mW cm2 of power density was at 800 °C temperature. However, the power densities were observed to decrease considerably when the lower temperatures were used. For instance, a drop to 38 mW cm2 maximum power density was found when the temperature was decreased to 700 °C from 800 °C Further, Ma et al. (2007) also investigated an ammonia fuel cell with YSZ electrolyte. The electrolyte thickness utilized was 30 mm. A 299 mW cm2 of maximum power density was found when an operating temperature of 750 °C was used. Greater power densities were reported at higher operating temperatures. Numerous studies investigating direct ammonia fuel cells with solid oxide materials providing conduction of protons (SOFC-H) have also been reported. Furthermore, Maffei et al. (2005) investigated an ammonia fed direct type fuel cell consisting of BCG electrolyte. They found the maximum power density as 25 mW cm2. Pelletier et al. (2005) investigated a SOFC-H fabricated from electrodes comprised of platinum material and an electrolyte fabricated with BCGP. The electrolyte thickness was reported to be 1300 mm. They found the power density at the peak value as 35 mW cm2. This was reported when they used 700 °C operating temperature. Moreover, Ma et al. (2006b) tested BCGE electrolyte and platinum electrodes with a 1000 mm thick electrolyte. A 32 mW cm2 of maximum power density was reported when they used a temperature of 700 °C. Maffei et al. (2008) investigated a platinum anode and BCGP electrolyte entailing fuel cell that was fed with direct ammonia as the fuel. The power density observed at the peak value was 23 mW cm2. This was reported for a cell temperature was 600 °C. The electrodes utilized comprised of platinum. McFarlan et al. (2004) examined an ammonia fed SOFC-H. A BCG electrolyte was utilized and platinum-based electrodes were used in the study. The electrolyte thickness was reported to be 1300 mm. The maximum power density found was 25 mW cm2. This power density was found when they used a temperature of 700 °C. Zhang and Yang (2008) examined a solid oxide electrolyte based fuel cell that was fed with ammonia. They used a cathode consisting of BSCFO-CGO and the anode comprising of Ni-CGO. The utilized BCGO electrolyte entailed 30microns thickness. When the operating temperature of 600 °C was utilized, the power density value was found as 147 mW cm2. However, they found a higher power density of 200 mW cm2 when they utilized a 650 °C operation. Furthermore, Xie et al. (2007) investigated a SOFC-H fed with direct ammonia fuel. A BCNO electrolyte was used in the study. The anode comprised of nickel oxide and a LCSO cath-

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ode were utilized. The electrolyte thickness was reported to be 20 mm. With a setup temperature of 700 °C, the power density was observed as 315 mW cm2. Lin et al. (2010) studied a BZCY electrolyte entailing SOFC-H fed with ammonia. The study utilized a Ni-BZCY anode and a BSCG cathode. A 390 mW cm2 of peak power density was found when they used a cell temperature of 750 °C. Fuel cells utilizing solid oxide electrolytes require the cell temperatures to be in the higher ranges (500–800 °C). These high temperature requirements hinder their widespread usage. Hence, ammonia fuel cells that can operate at lower temperatures are required. Marginal number of studies investigated fuel cells with direct ammonia fuel input entailing lower temperatures. Yang et al. (2014) studied an alkaline electrolyte comprising of molten potassium and sodium hydroxide in a direct ammonia fuel cell. They utilized cell temperatures in the range 200–220 °C. With a cell temperature of 200 °C, the power density for the fuel cell was found to be 10.5 mW cm2 at peak values. Further, they found that a 16 mW cm2 of peak power density is achievable with a cell temperature of 220 °C is used. Ganley (2008) also tested potassium and sodium hydroxide eutectic mixture electrolyte. They utilized cell temperatures in the range of 200–450 °C. At a cell temperature of 200 °C, they found 16 mW cm2 as the maximum power density. Further, when they used a higher temperature of 450 °C, an increased maximum power density of 40 mW cm2 was found. Lee et al. (2014) investigated ammonium carbonate as well as ammonia as fuels in an anion exchange membrane-based fuel cell. The open circuit voltages were reported to be 0.36 V and 0.32 V respectively for ammonium carbonate and ammonia. Moreover, they found power densities of 0.11 mW cm2 for ammonium carbonate fuel and 0.22 mW cm2 for the case and ammonia. Further, Suzuki et al. (2012) also studied a membrane entailing ammonia fuel cell. The study investigated various types of electrocatalysts for the fuel cell. The Pt/C electrocatalyst was observed as more favorable when compared to the Pt-Ru/C catalyst. When operating at a temperature of 50 °C, they found an open circuit voltage of 0.42 V. Lan and Tao (2010) tested anion exchange membranes developed from CPPO-PVA materials. The cathode material comprised of MnO2/C and the anode material was composed of CDN/ C. The study found that at an ambient temperature, the maximum power density is 16 mW cm2 with ammonia. Furthermore, Li and Zhao (2016) investigated a direct type fuel cell with the input of liquid fuel of ethanol. The anion exchange membrane was used for the electrolyte. They reported the maximum power density of 38 mW cm2 with a two cell stack. Furthermore, they also operated a miniscule car with the developed fuel cell systems. The car was found to be capable of operating at 0.52 m s1 for a period of one hour. Li and He (2014) proposed a new method to develop the electrodes suitable for utilization in anion exchange membrane fuel cells. The method entailed a new layer reduction methodology that allowed the development of a foam electrode that is threedimensional. The proposed method was found to enhance both the kinetics of the electrochemical reactions as well as specie transport. The obtained power density was reported to be 1.03 times better as compared to the normal fabrication methodology. Moreover, other studies on direct formate fuel cells (DFFC) have also been reported. Li et al. (2017) developed a DFFC that entails the transfer of sodium ions through the anion exchange electrolyte. They found a 33 mW cm2 of power density at the maximum point. Moreover, the developed fuel cell was also reported to produce sodium hydroxide along with electricity. Hence, having two useful outputs, the new fuel cell entailed promising application prospects. Also, Li et al. (2018) developed a direct liquid fuel cell entailing matched phase boundary based sodium ion utilizing technology. The performance of the fuel cell in terms of power density was reported as 45 mW cm2 at the maximum value. In addi-

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tion, this fuel cell also incorporated the formation of sodium hydroxide at the cathode which provided better and higher usage of catalysts. Hence, developing a new way of improving performance of anion exchange membrane fuel cells. Although, previous studies entailed direct ammonia fuel cells, the central area where efforts were directed was solid oxide electrolyte based cells. However, they necessitate the usage of operating temperatures in the high range (500–800 °C). This impedes their usage in widespread applications. However, anionconducting electrolytes open a range of opportunities to utilize ammonia in low temperature fuel cells. Hence, studies in this area are essential for their future development. Thus, in the present study, a new anion conducting electrolyte based 5-cell direct type fuel cell stack operating with ammonia is developed. Furthermore, the stack and cell performances are studied at different operation conditions. The specific objectives of this study include (i) developing a new anion exchange membrane-based direct ammonia fuel cell, (ii) experimentally investigating the performance of a single cell as well as a 5-cell stack in terms of open circuit voltages, peak power densities, and thermodynamic exergy as well as energy efficiencies, (iii) analyzing the effect of varying humidifier temperature on the performance of the single cell arrangement as well as the 5-cell stack. 2. Experimental investigation In the present study, an anion exchange membrane is utilized as the solid electrolyte. The O2 and H2O molecules react electrochemically at the cathode to produce hydroxyl (OH) ions. This electrochemical reaction is ensued owing to the platinum black catalyst at the electrode. These anions formed, pass through the anion exchange membrane and react electrochemically with ammonia molecules at the fuel cell anode. Ammonia molecules react electrochemically with the hydroxyl ions at the anode that generates electron flow. The expected cathodic, anodic and the overall fuel cell reactions are given in Eqs. (1)–(3). Overall reaction:

3 2NH3 þ O2 ! N2 þ 3H2 O 2

ð1Þ

Anodic reaction:

2NH3 + 6OH !N2 + 6H2 O + 6e

ð2Þ

Cathodic reaction:

3 O þ 3H2 O þ 6e ! 6OH 2 2

ð3Þ

However, intermediate reactions producing species of Nads atoms have also been suggested in the literature. These species are suggested to form during the anodic electrochemical reaction that also deteriorate the fuel cell performance. A membrane with the functional group of quaternary ammonium is used in the present study (AMI-7001 Anion Exchange Membranes). The membrane specifications are listed in Table 1. The maximum allowable application of current density is limited to 500 A m2. A polymer structure comprising of polystyrene cross-linked with divinylbenzene is utilized to make the membrane. Activation of the membrane was achieved by immersion in 1 M KOH solution (Sigma Aldrich) for a period of 1 h. Gas diffusion layers with a coating of platinum black catalyst are used. The membrane is sandwiched between two such layers that have an active area of 14 cm2 (Platinum on Vulcan-Carbon Paper Electrode). The diffusion layers allow efficient gas diffusion between the membrane or layers of catalyst and flow channel

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Table 1 Anion exchange membrane specifications. Source: AMI-7001 Anion Exchange Membranes. Ionic form Functional group Maximum current density Maximum stable temperature Functionality Polymer structure Color Burst strength (Mullen)

Chloride Quaternary ammonium <500 A m2 90 °C Strong base anion exchange membrane Divinylbenzene cross linked with gel polystyrene Light yellow >80 psi

plates as they contain micro-level pathways. Platinum black coated carbon fiber paper diffusion layers are used the current study. The loading of the platinum black catalyst on the diffusion layers is 0.45 mg cm2. Further, 40% platinum on Vulcan Carbon support is used to fabricate the paper gas diffusion layers. They entail the properties of low electrical resistance, high strength and minimal electrochemical corrosion. Also, the carbon fibers utilized to fabricate the diffusion layers are associated with a high tensile strength and modulus. Table 2 lists the gas diffusion layer properties and specifications. In the present study, flow field plates are also utilized in the fuel cell assembly to aid the passage as well as diffusion of gases over the electrodes. Stainless steel plates are used as stack configurations require high strength. In the stack arrangement, the fuel cell assembly is compressed with high pressures by applying high torques through screws, nuts and washers. This is essential to avoid any leakage of reactant gases and minimize the separation distances between each component for the minimization of contact resistances. Hence, flow field plates constituting a stainless steel composition encompass adequate strength to withstand the exerted pressures. Further, they also provide resistance to corrosion. Specifically, for the case of ammonia, materials with high corrosion-resistance such as stainless steel are required to operate the fuel cell efficiently and continuously owing to high corrosive properties of ammonia. Deterioration of the flow channel plates adversely affects the performance of the fuel cell. In addition, to prevent any leakage of reactants from cell components, gaskets comprising of rubber material are used that separate all components of the cell. These gaskets also allow appropriate compression of the assembly. All cell components lie between two nonconducting end plates. An exploded view of the assembly of various fuel cell components for a single cell is depicted in Fig. 1(a). To develop a 5-cell stack, 5 such cells are placed in a series arrangement. The developed 5-cell stack is depicted in Fig. 1(b). Furthermore, Fig. 2 depicts a schematic of the utilized experimental setup. Ammonia gas is fed at the anode side of the fuel cell at a gage pressure of 1 Bar through a pressurized tank. The ammonia flow rate utilized in the present study is 1 mg/s. Furthermore, for the half-cell electrochemical cathodic reaction given by Eq. (3), molecules of water and oxygen are needed at the fuel cell cathode. These are provided via input of humid air. As depicted in Fig. 2,

Table 2 Platinum black coated gas diffusion layer specifications. Source: (Siddiqui and Dincer, 2018) Type of catalyst Air permeability Thickness Pt black loading Electrical resistivity through plane Permeability PTFE treatment

40% Pt on Vulcan 1 ± 0.6 cm3 cm2 s1 0.24 ± 0.025 mm 0.45 mg cm2 <12 mX cm2 80% 5 wt%

Fig. 1. (a) Components of the single cell fuel cell assembly, (b) Developed 5-cell direct ammonia fuel cell stack.

a humidifier and compressor are used to for the input of humidified air. A 1 L/min air flow rate is utilized. This amount of air is sufficient to provide a stoichiometric excess of oxygen molecules. To humidify the input air, a bubbler humidifier is utilized. Eqs. (2) and (3) depict the expected fuel cell cathodic and anodic half-cell electrochemical reactions. To study the fuel cell performance under varying humidifier temperatures, temperatures of 25 °C, 60 °C and 80 °C are utilized. With the utilized bubbler humidifier setup, a 50 ± 7% relative humidity is attained. The performance parameters of open circuit voltages, peak power densities, exergy and energy efficiencies are used to evaluate performance of the fuel cell at varying conditions. The electrochemical measurements are performed by a GAMRY Ref 3000 potentiostat (Gamry). The polarization curves are obtained for a single cell as well as a 5cell stack arrangement to analyse the performances. The key performance measures of a system include energy and exergy efficiencies. The energy and exergy efficiencies of the developed fuel cell are evaluated as follows:

g¼

_ WA _nNH3 LHV

ð4Þ

w¼

_ WA n_ NH3 ex

ð5Þ

_ A denotes the active where the power density is represented by W, cell area, LHV represents ammonia’s molar lower heating value, ex

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289

Fig. 2. Schematic of the experimental system used to investigate developed fuel cell under different operating conditions.

denotes the molar specific exergy of ammonia, n_ NH3 represents the number of moles of ammonia molecules consumed in the electrochemical reaction that can be determined as follows:

n_ NH3 ¼

JA ne F

ð6Þ

where J signifies the current density, ne represent the number of moles of electrons transferred per mole of ammonia and F denotes the Faraday’s constant. The energy and exergy efficiencies are evaluated for the developed fuel cell for power densities at the peak values. The further details on the exergetic aspects are available elsewhere (Ay et al., 2006; Demirel, 2013). 3. Results and discussion The performance of a single cell and 5-cell direct ammonia fuel cell stack is studied. The open circuit voltage obtained for single cell arrangement is 280 ± 8 mV at ambient atmospheric temperature and a bubbler humidifier temperature of 25 °C as can be observed from Fig. 3. The obtained voltage is lower than the Nernst value for ammonia fuel cells of 1.17 V. The low open circuit voltage has been attributed to various reasons by previous studies. The platinum black catalyst is deteriorated by ammonia molecules. Hence, reducing its catalytic activity. This results in low voltages as well as overall performance. The reason for deterioration has been proposed to be the high rate of nitrogen adsorption (Nads) on the layer of catalyst (Suzuki et al., 2012). In addition to this, some previous studies have indicated that Nads atoms entail a high adsorption energy on platinum. Due to this, platinum has been suggested as an unsuitable catalyst to be utilized for the electrooxidation of ammonia (de Vooys et al., 2001; Bunce and Bejan, 2011). Thus, other substances including iridium as well as ruthenium have been investigated in the literature. Iridium was reported to entail comparatively lower catalyst poisoning owing to a limited coverage for saturation of 20%. This is comparatively

lower than the normal transitional metals that entail around 50% saturation coverage. Platinum-iridium alloy was also found to entail higher oxidation current densities (Lópezde Mishima et al., 1998). In addition, studies were also conducted to investigate platinum-copper based alloys for electrochemical oxidation of ammonia. However, the electro-catalytic activity of platinumiridium was found to be comparatively more promising than platinum-cooper (Endo et al., 2004b). Another study on platinum-iridium, platinum-ruthenium and platinum-nickel was conducted to comparatively assess their electro-catalytic activities. Platinum-iridium and ruthenium alloys were confirmed to be better options than sole platinum catalyst (Endo et al., 2004a). Furthermore, the phenomenon of fuel cross over is also suggested to be an important factor that results in low open circuit voltages. Particularly, in fuel cells consisting of anion exchange membrane electrolytes, the fuel crossover phenomenon was found to be considerable (Suzuki et al., 2012). In case of ammonia molecules crossing over to the cathodic side, their oxidation at the cathodic platinum black catalyst can affect the voltages significantly. Studies investigating these phenomena should be conducted to determine their actual effects on fuel cell performances. Furthermore, the membrane thickness also plays an important role in determining the performance of ammonia fuel cells. Developing and utilizing membranes with lower thicknesses is expected to aid in achieving higher fuel cell performances. This can be primarily attributed to the reduction in the Ohmic losses. However, the fuel cross over phenomenon can be increased due to a decrease in membrane thickness. Hence, further studies need to be performed to determine the optimal membrane thicknesses that can reduce the Ohmic losses as well as keep the crossover to a minimum. For a single-cell arrangement, the power density at the peak value is found to be 6.4 ± 0.2 W m2 at a 25 °C of humidifying temperature as can be depicted from Fig. 4 as also reported in previous studies (Siddiqui and Dincer, 2019). The voltages as well as the power densities are found to rise as the humidifier temperatures are increased. For a 5-cell stack arrangement, the open circuit

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Fig. 3. Polarization curves for the developed single-cell fuel cell fed with ammonia in the gaseous form at different humidifying temperatures.

Fig. 4. Power density vs current density graphs for the developed single-cell fuel cell fed with ammonia in the gaseous form at different humidifying temperatures.

voltage is obtained as 1249 ± 37.5 mV. Furthermore, the power density at the maximum point value is found as 13.4 ± 0.4 W m2. The voltage and current density at this power density value are 23.4 ± 0.7 A m2 and 574.2 ± 17 mV respectively as can be observed from Figs. 5 and 6. The energy efficiency of the 5-cell direct ammonia fuel cell stack is evaluated as 52.5 ± 1.6% and the exergy efficiency is found to be 49.2 ± 1.5% at 25 °C of humidifying temperature and the maximum power density value. The humidifier temperature effects the temperature of the water molecules as well as the overall air input temperature. The developed fuel cell is studied at different temperatures of the humidifier for a single-cell arrangement as well as the 5-cell stack. The humidifier temperatures used in the current study to analyze their effect on the performance of the fuel cell are 25 °C, 60 °C and 80 °C. The polarization curves obtained for a single cell

arrangement at varying temperatures of the humidifier are shown in Fig. 3. Increasing humidifier temperatures are observed to marginally increase the open circuit voltages. The open circuit voltage increases from 280 ± 8 mV at a 25 °C humidifier temperature to 299 ± 8 mV at a 60 °C humidifier temperature as also reported in previous studies (Siddiqui and Dincer, 2018). Furthermore, at 80 °C humidifying temperature, 335 ± 10 mV of voltage is observed in the open circuit conditions. This is attributed to a rise in electrochemical rates of rates due to higher temperature of water molecules participating in the half-cell cathodic reactions. In addition, the maximum power density is also found to rise marginally from 6.3 ± 0.2 W m2 at a 25 °C to 7.1 ± 0.2 W m2 when the humidifier temperature is set at 80 °C. The voltage and current density values are obtained as 160.5 ± 4 mV and 40 ± 1.2 A m2 at the power density of the peak point of 6.3 ± 0.2 W m2. The maximum power

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Fig. 5. Polarization curves for the developed 5-cell fuel cell stack fed with ammonia in the gaseous form at different humidifying temperatures.

Fig. 6. Power density vs current density curves for a 5-cell fuel cell stack fed with ammonia in the gaseous form at different humidifying temperatures.

density is obtained at a voltage of 176.7 ± 5 mV and same current density when an 80 °C humidifier temperature is used. Hence, the humidifier temperature is not found to effect the limiting current density considerably for the single-cell arrangement. However, the higher peak power densities can be attributed to higher open circuit voltages observed at a higher humidifier temperatures. Moreover, for the 5-cell stack, the voltage rises from 1249 mV ± 37 mV at a 25 °C humidifier temperature to 1399 ± 42 mV at a 60 °C humidifier temperature. Further, at a higher humidifier temperature of 80 °C, 1582 ± 47 mV is found to be the voltage in open circuit conditions. In addition, the peak

power density of 13.4 ± 0.4 W m2 at 25 °C is found to increase to 17 ± 0.5 W m2 at an 80 °C humidifier temperature. A voltage of 730 ± 22 mV and a current density of 23.3 ± 0.7 A m2 are found at the maximum power density. The open circuit voltages and peak power densities for the 5-cell stack are not found to be five times the values obtained for a single cell. This can be attributed to higher polarization losses with increased number of cells. In case of a 5-cell stack, the Ohmic resistance can be a significant contributor to the lower performance as compared to a single-cell arrangement. In fuel cells, membranes are the cell components with the highest Ohmic resistances. Hence, anion exchange membranes with lower resistances can be developed. Furthermore,

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1 0.9

Energy efficiency

0.8 0.7

0.9

η at 25°C

ψ at 25°C

η at 60°C

ψ at 60°C

η at 80°C

ψ at 80°C

0.8 0.7 0.6

0.6

0.5

0.5

0.4

0.4

0.3

0.3 0.2

0.2

0.1

0.1

0 20

25

30

Current density (A m -2)

35

Exergy efficiency

292

0 40

Fig. 7. Variation of energy and exergy efficiencies of the 5-cell anion exchange membrane fuel cell stack with current density.

lower membrane thickness can be utilized to reduce the membrane Ohmic resistance. The energy and exergy efficiencies of the 5-cell stack at varying current densities are depicted in Fig. 7. The efficiencies are observed to decrease with increasing current densities. This is attributed to the increase in polarization losses with increasing current densities. Higher polarization losses occur as the current density through the fuel cell increases, hence, decreasing the efficiencies. The energy and exergy efficiencies at the peak power densities for different temperatures of the humidifier are shown in Fig. 8. At a humidifier temperature of 25 °C, the energy efficiency of the developed fuel cell stack is determined to be 52.5 ± 1.6% at the peak power density. In addition to this, when a higher temperature of 60 °C is used, the energy efficiency is found to increase to 59.4 ± 1.7%. Further, the energy efficiency is found to rise to 66.8 ± 2% when the humidifier temperature is raised to 80 °C. This can be due to a drop in polarization losses as the humidifier temperature is increased. Similar trends are observed in exergy efficiencies. At the maximum power density, the exergy efficiency is evaluated to be 49.2 ± 1.6% at a humidifier temperature of 25 °C. This is observed to increase to 55.6 ± 1.7% and 62.6 ± 2% at temperatures of 60 °C and 80 °C respectively. This can also be attributed to

lower polarization losses at higher humidifier temperatures that result in lower exergy destruction rates leading to higher exergy efficiencies. The performance of such fuel cells can be enhanced significantly if suitable catalyst materials are investigated that are compatible with ammonia. Furthermore, the phenomenon of adsorption of Nads atoms on platinum needs to be investigated further because it is a major reason for deterioration in performance of such type of fuel cells. Moreover, anion exchange membranes that minimize fuel crossover need to be investigated as fuel crossovers have been suggested to deteriorate the fuel cell performances considerably (Ahmed and Dincer, 2011). 4. Conclusions A new direct ammonia fuel cell stack utilizing a solid electrolyte in the form of an anion exchange membrane is made and experimentally investigated. For a single-cell test, the open circuit voltage is obtained as 280 ± 8 mV. In the case of a 5-cell stack, the voltage is found to be 1249 ± 37.5 mV in the open circuit conditions. Furthermore, the peak power densities are found to be 6.4 ± 0.2 W m2 and 13.4 ± 0.4 W m2 for a single-cell and a 5cell stack respectively. The fuel cell operation is investigated at varying humidifier temperatures and increasing temperatures are found to enhance the performances. The exergy efficiency and energy efficiencies of the developed fuel cell stack are evaluated to be 52.4 ± 1.6% and 49.3 ± 1.6% at the maximum point power densities. The fuel cell energy as well as exergy efficiencies are found to improve at higher humidifier temperatures. Conflict of interest The authors declared that there is no conflict of interest. References

Fig. 8. Energy and exergy efficiencies of the 5-cell anion exchange membrane fuel cell stack evaluated at peak power densities at varying humidifying temperatures.

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