Micro-kinetic modeling of NH3 decomposition on Ni and its application to solid oxide fuel cells

Several experimental studies have shown that, 1) the extent of the poisoning effect due to trace amounts of sulfur compounds in the fuel is lower if a SOFC is operated at a higher current density, and 2) the performance drop due to sulfur poisoning is much lower for Ni-GDC or Ni-ScSZ anodes when compared to Ni-YSZ anodes. This work presents a first principles numerical model that simulates experimental studies of sulfur poisoning on SOFC button cells. The exchange current densities for the electrodes are determined using sulfur-free polarization data for cells fueled by humidified mixtures of H 2 and N 2 . A detailed surface reaction model that predicts the fractional coverage of all adsorbed species at the three phase interface is coupled to the SOFC model and the sulfur coverage is used to alter the anode exchange current density. The resulting model predictions match experimental observations during both galvanostatic and potentiostatic operation. Our analysis shows that the observed lower performance drop at higher current density is due to the non-linear nature of the electrochemical rate equations, and that the lower impact of sulfur poisoning on Ni-GDC and Ni-ScSZ anodes (compared to Ni-YSZ anodes) is due to their higher electrochemical activity. Solid oxide fuel cell (SOFC) systems operating on natural gas or gasified coal can be significantly more energy efficient than today's combustion systems and can thus reduce CO 2 emissions. However, a major problem associated with the use of hydrocarbon fuels is the presence of sulfur containing compounds that are converted to H 2 S under SOFC operating conditions.1 The H 2 S present in the feed gas can readily deactivate conventional Ni cermet anodes. One can certainly have a desulfurization unit before the fuel cell stack to deal with the sulfur poisoning. However, this leads to increased system complexity and decrease in overall efficiency. Nevertheless, it is important to understand and quantify the sulfur tolerance of SOFC anodes in the event of sulfur breakthrough from the desulfurizer. Sulfur poisoning of Ni-YSZ anodes in SOFC is well studied experimentally for model fuel compositions consisting of ppm levels of H 2 S in mixtures of H 2 , H 2 O and an inert 2-4 as well as H 2 S in gas mixtures representative of natural gas or reformed natural gas.5-8 Most of the experimental literature deals with H 2 S concentrations well below 10 ppm and work that treats H 2 S concentrations higher than 20 ppm is rather limited. 2,5,8 On the other hand it is well known that natural gas and biogas can have substantially higher sulfur concentrations and that catalytic steam reforming of methane can be carried out on Ni even with 100 ppm H 2 S without losing all catalytic activity 9,10 e.g., a methane conversion (in model biogas) of ∼35% can be maintained using a supported Ni catalyst with 100 ppm H 2 S at 800• C. 10For an electrolyte supported cell Zha et al., 2 studied the cell performance for a wide range of H 2 S concentrations using DC polarization and electrochemical i...

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“…The forward reaction rate k f i for reaction i is calculated by a modified Arrhenius expression [22][23][24][25] …”

Section: Modeling Methodologymentioning

confidence: 99%

Sulfur Poisoning of SOFCs: A Model Based Explanation of Polarization Dependent Extent of Poisoning

Janardhanan

Monder

2014

J. Electrochem. Soc.

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“…To our knowledge, this is the first time that a comprehensive reaction mechanism has been developed for ethanol partial oxidation and reforming on Pt, using a hierarchical multiscale modeling approach [13][14][15][16][17]. The manuscript is organized as follows.…”

Section: Motivationmentioning

confidence: 99%

“…Thermodynamic consistency is an often overlooked, but important aspect in mechanism development [13,57,63,67]. Especially at or near equilibrium conditions, this may result in the calculation of incorrect reaction rates.…”

Section: Thermodynamic Consistencymentioning

confidence: 99%

“…Microkinetic modeling has been applied in the past for a variety of chemical processes such as methane partial oxidation and reforming [14,57,58], selective catalytic reduction of NO x [59,60], NO oxidation [61], ammonia decomposition [13,62], ethylene hydrogenation [63], and Water-Gas Shift [64][65][66], to name a few recent applications.…”

Section: Chapter 3: Surface Reaction Mechanism Developmentmentioning

confidence: 99%

“…The overall approach for surface reaction mechanism development was derived from microkinetic modeling concepts proposed by Dumesic and colleagues in the early 1990s [56], which was further extended by various researchers through hierarchical multiscale modeling [13][14][15][16][17]. Microkinetic modeling has been applied in the past for a variety of chemical processes such as methane partial oxidation and reforming [14,57,58], selective catalytic reduction of NO x [59,60], NO oxidation [61], ammonia decomposition [13,62], ethylene hydrogenation [63], and Water-Gas Shift [64][65][66], to name a few recent applications.…”

Section: Chapter 3: Surface Reaction Mechanism Developmentmentioning

confidence: 99%

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Microkinetic modeling and analysis of ethanol partial oxidation and reforming reaction pathways on platinum at short contact times

Koehle

Mhadeshwar

2012

Chemical Engineering Science

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2013iii ACKNOWLDGEMENTSThere are many people without whom this thesis would not have been possible. I must thank Ashish Mhadeshwar for the incredible amount of knowledge he imparted and the guidance he provided in order for this research to be conducted, shared at conferences and published. Many thanks to Doug Cooper, Bill Mustain and Ranjan Srivastava for standing in as advisors and providing support that was above and beyond expectations. I want to thank everyone that has become part of my UConn family, without whom this project would have been possible but much less fun:

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Thermocatalytic Ammonia Decomposition – Status and Current Research Demands for a Carbon‐Free Hydrogen Fuel Technology

2022

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Hydrogen storage materials and technologies are deemed as the cornerstone towards a world economy less reliant on, and ultimately independent of fossil resources. Ammonia is considered among the most efficient carbon‐free hydrogen carriers because of its relatively high gravimetric and volumetric hydrogen storage capacities and, equally important, ease of transport and storage. In addition, the well‐established chemical production of ammonia (preferably a green Haber‐Bosch process) would accelerate the immediate introduction of hydrogen into energy infrastructure. Thermocatalytic decomposition of ammonia yields clean, COx‐free hydrogen, but is energy intensive and currently performed with expensive Ru‐based catalysts. The development of more efficient catalysts based on more abundant and cheaper elements is indispensable for future wide‐scale industrial application. Therefore, the conceptualization of strategies and challenges for material developments, study and optimization of ammonia decomposition catalysts as well as their in situ/operando characterizations are pivotal aspects to be considered.

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Micro-kinetic modeling of NH3 decomposition on Ni and its application to solid oxide fuel cells

Cited by 54 publications

References 29 publications

Sulfur Poisoning of SOFCs: A Model Based Explanation of Polarization Dependent Extent of Poisoning

Sulfur Poisoning of SOFCs: A Model Based Explanation of Polarization Dependent Extent of Poisoning

Microkinetic modeling and analysis of ethanol partial oxidation and reforming reaction pathways on platinum at short contact times

Thermocatalytic Ammonia Decomposition – Status and Current Research Demands for a Carbon‐Free Hydrogen Fuel Technology

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