Electrochemical modeling of ammonia-fed solid oxide fuel cells based on proton conducting electrolyte

An electrochemical model was developed to study the NH₃-fed and H₂-fed solid oxide fuel cells based on proton conducting electrolyte (SOFC-H). The modeling results were consistent with experimental data in literature. It is found that there is little difference in working voltage and power density between the NH₃-fed and the H₂-fed SOFC-H with an electrolyte-support configuration due to an extremely high ohmic overpotential in the SOFC-H. With an anode-supported configuration, especially when a thin film electrolyte is used, the H₂-fed SOFC-H shows significantly higher voltage and power density than the NH₃-fed SOFC-H due to the significant difference in concentration overpotentials. The anode concentration overpotential of the NH₃-fed SOFC-H is found much higher than the H₂-fed SOFC-H, as the presence of N₂ gas dilutes the H₂ concentration and slows down the transport of H₂. More importantly, the cathode concentration overpotential is found very significant despite of the thin cathode used in the anode-supported configuration. In the SOFC-H, H₂O is produced in the cathode, which enables complete fuel utilization on one hand, but dilutes the concentration of O₂ and impedes the diffusion of O₂ to the reaction sites on the other hand. Thus, the cathode concentration overpotential is the limiting factor for the H₂-fed SOFC-H and an important voltage loss in the NH₃-fed SOFC-H. How to reduce the concentration overpotentials at both electrodes is identified crucial to develop high performance SOFC-H.     

Mathematical modeling of ammonia-fed solid oxide fuel cells with different electrolytes

An electrochemical model was developed to study the ammonia (NH₃)-fed solid oxide fuel cells with proton-conducting electrolyte (SOFC-H) and oxygen ion-conducting electrolyte (SOFC-O). Different from previous thermodynamic analysis, the present study reveals that the actual performance of the NH₃-fed SOFC-H is considerably lower than the SOFC-O, mainly due to higher ohmic overpotential of the SOFC-H electrolyte. More analyses have been performed to study the separate overpotentials of the NH₃-fed SOFC-H and SOFC-O. Compared with the NH₃-fed SOFC-H, the SOFC-O has higher anode concentration overpotential and lower cathode concentration overpotential. The effects of temperature and electrode porosity on concentration overpotentials have also been studied in order to identify possible methods for improvement of SOFC performance. This study reveals that the use of different electrolytes not only causes different ion conduction characteristics at the electrolyte, but also significantly influences the concentration overpotentials at the electrodes. The model developed in this article can be extended to 2D and 3D models for further design optimization.     

Modeling of methane fed solid oxide fuel cells: Comparison between proton conducting electrolyte and oxygen ion conducting electrolyte

An electrochemical model was developed to study the methane (CH₄) fed solid oxide fuel cell (SOFC) using proton conducting electrolyte (SOFC-H) and oxygen ion conducting electrolyte (SOFC-O). Both the internal methane steam reforming (MSR) and water gas shift (WGS) reactions are considered in the model. Previous study has shown that the CH₄ fed SOFC-H had significantly better performance than the SOFC-O. However, the present study reveals that the actual performance of the CH₄ fed SOFC-H is considerably lower than the SOFC-O, partly due to higher ohmic overpotential of SOFC-H. It is also found that the CH₄ fed SOFC-H has considerably higher cathode concentration overpotential and lower anode concentration overpotential than the SOFC-O. The anode concentration overpotentials of the CH₄ fed SOFC-H and SOFC-O are found to decrease with increasing temperature, which is different from previous analyses on the H₂ fed SOFC. Therefore, high temperature is desirable for increasing the potential of the CH₄ fed SOFC. It is also found that there exist optimal electrode porosities that minimize the electrode total overpotentials. The analyses provided in this paper signify the difference between the CH₄ fed SOFC-H and SOFC-O. The model developed in this paper can be extended to 2D or 3D models to study the performance of practical SOFC systems.     

Catalytic dehydrogenation of ammonia borane in non-aqueous medium

Dehydrogenation of Ammonia Borane (NH₃BH₃, AB) catalyzed by transition metal heterogeneous catalysts was carried out in non-aqueous solution at temperatures below the standard polymer electrolyte membrane (PEM) fuel cell operating conditions. The introduction of a catalytic amount (∼2 mol%) of platinum to a solution of AB in 2-methoxyethyl ether (0.02–0.33 M) resulted in a rapid evolution of H₂ gas at room temperature. At 70 °C, the rate of platinum catalyzed hydrogen release from AB was the dehydrogenation rate which was 0.04 g s⁻¹ H₂ kW⁻¹.     

Thermodynamic analysis of ammonia fed solid oxide fuel cells: Comparison between proton-conducting electrolyte and oxygen ion-conducting electrolyte

A thermodynamic analysis has been performed to compare the theoretical performance of ammonia fed solid oxide fuel cells (SOFCs) based on proton-conducting electrolyte (SOFC-H) and oxygen ion-conducting electrolyte (SOFC-O). It is found that the ammonia fed SOFC-H is superior to SOFC-O in terms of theoretical maximum efficiency. For example, at a fuel utilization of 80% and an oxygen utilization of 20%, the efficiency of ammonia fed SOFC-H is 11% higher than that of SOFC-O. The difference between SOFC-H and SOFC-O becomes more significant at higher fuel utilizations and higher temperatures. This is because an SOFC-H has a higher hydrogen partial pressure and a lower steam partial pressure than an SOFC-O. In addition, an increase in oxygen utilization is found to increase the efficiency of ammonia fed SOFCs due to an increase in oxygen molar fraction and a reduction in steam molar fraction. With further development of new ceramics with high proton conductivity and effective fabrication of thin film electrolyte, the SOFC based on proton-conducting electrolyte is expected to be a promising approach to convert ammonia fuel into electricity.     

Diagnosis of contamination introduced by ammonia at the cathode in a polymer electrolyte membrane fuel cell

Contamination introduced by impurities from feed streams can impact polymer electrolyte membrane fuel cell performance dramatically. The presence of unwanted trace species, such as CO, H₂S, and NH₃, can adversely affect the function of a fuel cell. It has been reported that the major impact of CO and H₂S contamination on fuel cell performance is kinetic, while the effect of NH₃ contamination is speculated to be mainly membrane conductivity reduction. In this paper, the effect of NH₃ contamination from the cathode side was investigated. The mechanisms of NH₃ contamination were diagnosed based on degradation tests using electrochemical impedance spectroscopy and cyclic voltammetry. The contamination factors investigated included ammonia concentration, operating current, temperature, and relative humidity. The results show that the severity of the adverse effect caused by ammonia contamination was enhanced by increased ammonia concentration, decreased operating temperature, and decreased relative humidity. The performance decay induced by ammonia is attributable to reduced membrane/ionomer conductivity and ammonia adsorption on the catalyst surface, which blocks the active sites and hinders mass transfer.     

Analysis and performance assessment of NH3 and H2 fed SOFC with proton-conducting electrolyte

The present paper investigates the performance of a solid oxide fuel cell based on proton-conducting electrolyte (SOFC-H⁺) using one-dimensional steady-state model. The analysis covers a detailed electro-chemical model for H₂ and NH₃ fuels. The direct internal reforming of NH₃ is examined, and the effects of some operating parameters (e.g. temperature, pressure, fuel utilization and oxidant utilization) on the reversible cell potential are investigated. In addition, the overpotentials (including activation, ohmic and concentration) are calculated to study the irreversible behavior of the SOFC-H⁺ with some actual data operating conditions and material properties taken from the literature. In addition, effects of some operation and structural parameters on cell performance were examined. The present results indicate that the activation and the ohmic losses are considerable. The concentration overpotential at the anode side is negligible due to the fact that H₂O is produced at the cathode side. The maximum power density is calculated as 3212 and 3113 W/m² at 1073 K and 1 atm for the fuels of H₂ and NH₃. The results further show that H₂ provides better performance than NH₃ at the same partial pressure. Moreover, NH₃ is an excellent hydrogen carrier which is a potential candidate for SOFC-H⁺ due to its high hydrogen content and considerable cell performance.     

Proton-conducting fuel cells operating on hydrogen,ammonia and hydrazine at intermediate temperatures

Anode-supported proton-conducting fuel cell with BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) electrolyte and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) cathode was fabricated. Peak power densities of ∼420 and 135 mW/cm² were achieved, respectively, at 700 and 450 °C for a cell with 35 μm thick electrolyte operating on hydrogen fuel. The endothermic nature of the ammonia decomposition reaction, however, resulted in cell temperature 30–65 °C lower than the furnace when operating on ammonia. Accounting the cooling effect, comparable power density was achieved for the cell operating on ammonia and hydrogen at high temperature. At reduced temperature, the cell demonstrated worse performance when operating on ammonia than on hydrogen due to the poor activity of the anode towards NH₃ catalytic decomposition. By applying on-line catalytic decomposition products of N₂H₄ as the fuel, similar cell performance to that with NH₃ fuel was also observed.     

Recyclable hydrogen storage system composed of ammonia and alkali metal hydride

Ammonia (NH₃) reacts with alkali metal hydrides MH (M = Li, Na, and K) in an exothermic reaction to release hydrogen (H₂) at room temperature, resulting that alkali metal amides (MNH₂) which are formed as by-products. In this work, hydrogen desorption properties of these systems and the condition for the recycle from MNH₂ back to MH were investigated systematically. For the hydrogen desorption reaction, the reactivities of MH with NH₃ were better following the atomic number of M on the periodic table, Li < Na < K. It was confirmed that the hydrogen absorption reaction of all the systems proceeded under 0.5 MPa of H₂ flow condition below 300 °C.     

Performance of a micro-thermophotovoltaic power system using an ammonia-hydrogen blend-fueled micro-emitter

The potential of ammonia (NH₃)-hydrogen (H₂) blends as a carbon-free, green fuel in a 1–10 W micro-thermophotovoltaic (micro-TPV) device is evaluated experimentally. When NH₃–H₂ blends are used directly (without any modification) in a heat-recirculating micro-TPV configuration that has an installation of gallium antimonide (GaSb) photovoltaic cells and was developed for hydrocarbon fuel, low temperature on the micro-emitter outer surface is observed, generating a secondary flame at the micro-emitter outlet. Thus, the micro-TPV device has been modified to eliminate the secondary flame by enhancing the residence time of fed NH₃–H₂–air mixtures and uniform burning: a cyclone adapter for a fuel-air mixture supply system and a helical adapter for the fuel-air mixture upstream of the micro-emitter. Under optimized design and operating conditions, the micro-TPV device produces 5.2 W with an overall efficiency of 2.1% and an emitter efficiency of 37%, indicating the maximum temperature of the micro-emitter outer surface up to 1408 K. Thus, the feasibility of using NH₃–H₂ blends in practical micro power-generation devices has been demonstrated, implying the potential of partial NH₃ substitution to improve the safety of pure H₂ use with no carbon generation.     

Solid-oxide fuel cell operated on in situ catalytic decomposition products of liquid hydrazine

Hydrazine was examined as a fuel for a solid-oxide fuel cell (SOFC) that employed a typical nickel-based anode. An in situ catalytic decomposition of hydrazine at liquid state under room temperature and ambient pressure before introducing to the fuel cell was developed by applying a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) oxide catalyst. Catalytic testing demonstrated that liquid N₂H₄ can be decomposed to gaseous NH₃ and H₂ at a favorable rate and at a temperature as low as 20 °C and H₂ selectivity reaching values as high as 10% at 60 °C. Comparable fuel cell performance was observed using either the in situ decomposition products of hydrazine or pure hydrogen as fuel. A peak power density of ∼850 mW cm⁻² at 900 °C was obtained with a typical fuel cell composed of scandia-stabilized zirconia and La_0.8Sr_0.2MnO₃ cathode. The high energy and power density, easy storage and simplicity in fuel delivery make it highly attractive for portable applications.     

Ammonia‐fed solid oxide fuel cells for power generation—A review

Ammonia has been identified as a promising sustainable fuel and hydrogen source for solid oxide fuel cells (SOFC). This paper aims to provide a literature review on ammonia‐fed SOFCs. Both experimental studies and mathematical modeling investigations on NH₃‐fed SOFC are included and discussed. It is found that NH₃ is a technically feasible fuel for direct use in SOFCs and the performance of NH₃‐fed SOFC is comparable with that of the H₂ fed SOFC. Experimental study in literature also demonstrates that both oxygen ion‐conducting electrolyte (SOFC‐O) and proton‐conducting electrolyte (SOFC‐H) can be used in NH₃‐fed SOFC, as the amount of NO_x generated in a SOFC‐O is negligible. Fabricating thin film electrolyte and developing more reactive electrode materials are important to improve the performance of NH₃‐fed SOFCs. Mathematical models are useful design tools for understanding the coupled transport and reaction phenomena and for optimizing the SOFC performance. Thermodynamic and pioneering 1D electrochemical models have been developed, validated and demonstrated to be reliable by the present author. Copyright © 2009 John Wiley & Sons, Ltd.     

A fuel cell operating between room temperature and 250 °C based on a new phosphoric acid based composite electrolyte

A phosphoric acid based composite material with core-shell microstructure has been developed to be used as a new electrolyte for fuel cells. A fuel cell based on this electrolyte can operate at room temperature indicating leaching of H₃PO₄ with liquid water is insignificant at room temperature. This will help to improve the thermal cyclability of phosphoric acid based electrolyte to make it easier for practical use. The conductivity of this H₃PO₄-based electrolyte is stable at 250 °C with addition of the hydrophilic inorganic compound BPO₄ forming a core-shell microstructure which makes it possible to run a PAFC at a temperature above 200 °C. The core-shell microstructure retains after the fuel cell measurements. A power density of 350 mW/cm² for a H₂/O₂ fuel cell has been achieved at 200 °C. The increase in operating temperature does not have significant benefit to the performance of a H₂/O₂ fuel cell. For the first time, a composite electrolyte material for phosphoric acid fuel cells which can operate in a wide range of temperature has been evaluated but certainly further investigation is required.     

Hydrolysis of solid ammonia borane

Ammonia borane NH₃BH₃ is a promising hydrogen storage material by virtue of a theoretical gravimetric hydrogen storage capacity (GHSC) of 19.5 wt%. However, stored hydrogen has to be effectively released, one way of recovering this hydrogen being the metal-catalyzed hydrolysis. The present study focuses on CoCl₂-catalyzed hydrolysis of NH₃BH₃ with the concern of improving the effective GHSC of the system NH₃BH₃-H₂O. For that, NH₃BH₃ is stored as a solid and H₂O is provided in stoichiometric amount. By this way, an effective GHSC of 7.8 wt% has been reached at 25 °C. To our knowledge, it is the highest value ever reported. Besides, one of the highest hydrogen generation rates (HGRs, 21 ml(H₂) min⁻¹) has been found. In parallel, the increases of the water amount and temperature have been studied and the reaction kinetics has been determined. Finally, it has been observed that some NH₃ release, what is detrimental for a fuel cell. To summarize, high performances in terms of GHSCs and HGRs can be reached with NH₃BH₃ and since research devoted to this boron hydride is at the beginning we may be confident in making it viable in a near future.     

Direct ammonia solid oxide fuel cell based on thin proton-conducting electrolyte

Thin proton-conducting electrolyte with composition BaCe_0.8Gd_0.2O_3−δ (BCGO) was prepared over substrates composed of Ce_0.8Gd_0.2O_1.9 (CGO)-Ni by the dry-pressing method. Solid oxide fuel cells (SOFCs) were fabricated with the structure Ni-CGO/BCGO/Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO)-CGO. The performance of a single cell was tested at 600 and 650 °C, with ammonia directly used as fuel. The open circuit voltages (OCVs) were 1.12 and 1.1 V at 600 and 650 °C, respectively. The higher OCV may be due to both the compaction of the BCGO electrolyte (no porosity) and complete decomposition of ammonia. The maximum power density was 147 mW cm⁻² at 600 °C. Comparisons of the cell with hydrogen as fuel indicate that ammonia can be treated as a substitute liquid fuel for SOFCs based on a proton-conducting solid electrolyte.     

Fabrication and characterization of a Ba0.5Sr0.5Co0.8Fe0.2O3−δ—Gadolinia-doped ceria cathode for an anode-supported solid-oxide fuel cell

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and gadolinia-doped ceria (GDC) were synthesized via a glycine-nitrate process (GNP). A cubic perovskite of BSCF was observed by X-ray diffraction (XRD) at a calcination temperature above 950 °C. An anode-supported solid-oxide fuel cell was constructed from the porous NiO + YSZ as the anode substrate, the yittria-stabilized zirconia (YSZ) as the electrolyte, and the porous BSCF-GDC layer as the cathode with a GDC barrier layer. For the performance test, the maximum power density was 191.3 mW cm⁻² at a temperature of 750 °C with H₂ fuel and air at flow rates of 335 and 670 sccm, respectively. According to the AC-impedance data, the charge-transfer resistances of the electrodes were 0.10 and 1.59 Ω cm², and the oxygen-reduction and oxygen-ion diffusion resistances were 0.69 and 0.98 Ω cm² at 750 and 600 °C, respectively. SEM microstructural characterization indicated that the fuel cell as fabricated exhibited good compatibility between cathode and electrolyte layers.     

A new fuel cell using aqueous ammonia-borane as the fuel

Ammonia-borane (NH₃BH₃), as a source of protide (H⁻), is initially proposed to release its energy through a fuel cell (direct ammonia-borane fuel cell, DABFC). Cell performance has been elucidated in a 25 cm² laboratory cell constructed with an oxygen cathode and an ammonia-borane solution fed anode, where the catalyst layers are made of Vulcan XC-72 with 30 wt.% Pt. The potential is 0.6 V at the current density of 24 mA cm⁻², corresponding to power density >14 mW cm⁻² at room temperature. The direct electron transfer from protide (H⁻) in NH₃BH₃ to proton (H⁺) has been further proved by the open circuit potential and the cyclic voltammetry results, which show the possibility of improvement in the performance of DABFC by, for example, exploring new electrode materials.     

On-board hydrogen storage and production: An application of ammonia electrolysis

On-board hydrogen storage and production via ammonia electrolysis was evaluated to determine whether the process was feasible using galvanostatic studies between an ammonia electrolytic cell (AEC) and a breathable proton exchange membrane fuel cell (PEMFC). Hydrogen-dense liquid ammonia stored at ambient temperature and pressure is an excellent source for hydrogen storage. This hydrogen is released from ammonia through electrolysis, which theoretically consumes 95% less energy than water electrolysis; 1.55 Wh g⁻¹ H₂ is required for ammonia electrolysis and 33 Wh g⁻¹ H₂ for water electrolysis. An ammonia electrolytic cell (AEC), comprised of carbon fiber paper (CFP) electrodes supported by Ti foil and deposited with Pt-Ir, was designed and constructed for electrolyzing an alkaline ammonia solution. Hydrogen from the cathode compartment of the AEC was fed to a polymer exchange membrane fuel cell (PEMFC). In terms of electric energy, input to the AEC was less than the output from the PEMFC yielding net electrical energies as high as 9.7 ± 1.1 Wh g⁻¹ H₂ while maintaining H₂ production equivalent to consumption.     

The decreasing effect of ammonia enrichment on the combustion emission of hydrogen,methane, and propane fuels

In the present study, non-premixed combustion and NOx emission of H₂, NH₃, C₃H₈, and CH₄ fuels have been studied in a combustion test unit under lean mixture conditions (λ = 4) at 8.6 kW thermal capacity. Furthermore, the combustion and NOx emission of the H₂, C₃H₈, and CH₄ fuels have been investigated for various NH₃ enrichment ratios (5, 10, 20, and 50%) and excess air coefficients (λ = 1.1, 2, 3, and 4) at the same thermal capacity. The obtained results have been compared for each fuel. Numerical simulation results show that H₂ emits intense energy through the reaction zone despite the lowest fuel consumption in mass, among others, due to its high calorific value. Therefore, it has a higher flame temperature than others. At the same time, C₃H₈ has the lowest flame temperature. Besides, NH₃ has the shortest flame length among others, while C₃H₈ has the most extended flame form. The highest level of NOx is released from the NH₃ flame in the combustion chamber, while the lowest NOx is released from the CH₄. However, the lowest NOx emission at the combustion chamber exit is obtained in NH₃ combustion, while the highest NOx emission is obtained with H₂ combustion. It results from the shortest flame length of NH₃, short residence time, and backward NOx reduction to N₂ for NH₃. As for H₂, high flame temperature and relatively long flame, and high residence time of the products trigger NOx formation and keep the NOx level high. On the other hand, excess air coefficient from 1.1 to 2 increases NOx for H₂, CH₄, and NH₃ due to their large flame diameters, unlike propane. Then, NOx emission levels decrease sharply as the excess air coefficient increases to 4 for each fuel. NH₃ fuel also emits minimum NOx in other excess air coefficients at the exit, while H₂ emits too much emission. With NH₃ enrichment, the NOx emissions of H₂, CH₄, and C₃H₈ fuels at the combustion chamber exit decrease gradually almost every excess air coefficient apart from λ = 1.1. As a general conclusion, like renewable fuels, H₂ appears to be a source of pollution in terms of NOx emissions in combustion applications. In contrast, NH₃ appears to be a relatively modest fuel with a low NOx level. In addition, the high amount of NOx emission released from H₂ and other fuels during the combustion can be remarkably reduced by NH₃ enrichment with an excess air combustion.     

Monolithic micro-direct methanol fuel cell in polydimethylsiloxane with microfluidic channel-integrated Nafion strip

We demonstrate a monolithic polymer electrolyte membrane fuel cell by integrating a narrow (200 μm) Nafion strip in a molded polydimethylsiloxane (PDMS) structure. We propose two designs, based on two 200 μm-wide and two 80 μm-wide parallel microfluidic channels, sandwiching the Nafion strip, respectively. Clamping the PDMS/Nafion assembly with a glass chip that has catalyst-covered Au electrodes, results in a leak-tight fuel cell with stable electrical output. Using 1 M CH₃OH in 0.5 M H₂SO₄ solution as fuel in the anodic channel, we compare the performance of (I) O₂-saturated 0.5 M H₂SO₄ and (II) 0.01 M H₂O₂ in 0.5 M H₂SO₄ oxidant solutions in the cathodic channel. For the 200 μm channel width, the fuel cell has a maximum power density of 0.5 mW cm⁻² and 1.5 mW cm⁻² at room temperature, for oxidants I and II, respectively, with fuel and oxidant flow rates in the 50-160 μL min⁻¹ range. A maximum power density of 3.0 mW cm⁻² is obtained, using oxidant II for the chip with 80 μm-wide channel, due to an improved design that reduces oxidant and fuel depletion effects near the electrodes.