Review
Fundamentals of electro- and thermochemistry in the anode of solid-oxide fuel cells with hydrocarbon and syngas fuels

https://doi.org/10.1016/j.pecs.2013.10.001Get rights and content

Abstract

High fuel flexibility of solid-oxide fuel cells (SOFCs) affords the possibility to use relatively cheap, safe, and readily available hydrocarbon (e.g., CH4) or coal syngas (i.e., CO-H2 mixtures) fuels. Utilization of such fuels would greatly lower fuel cost and increase the feasibility of SOFC commercialization, especially for near-term adoption in anticipation of the long-awaited so-called “hydrogen economy”. Current SOFC technology has shown good performance with a wide range of hydrocarbon and syngas fuels, but there are still significant challenges for practical application. In this paper, the basic operating principles, state-of-the-art performance benchmarks, and SOFC-relevant materials are summarized. More in-depth reviews on those topics can be found in Kee and co-workers [Combust Sci and Tech 2008; 180:1207–44 and Proc Combust Inst 2005; 30:2379–404] and McIntosh and Gorte [Chem Rev 2004; 104:4845–65]. The focus of this review is on the fundamentals and development of detailed electro- and thermal (or simply, electrothermal) chemistry within the SOFC anode, including electrochemical oxidation mechanisms for H2, CO, CH4, and carbon, as well as the effects of carbon deposition and sulfur poisoning. The interdependence of heterogeneous chemistry, charge-transfer processes, and transport are discussed in the context of SOFC membrane-electrode assembly modeling.

Introduction

Solid-oxide fuel cells (SOFCs) are the most efficient devices known to convert the chemical energy of a fuel directly into electricity [1]. Research on SOFCs is vast, and their development has been long and continuous, primarily because these devices carry a set of attractive features [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. SOFCs (and other fuel cells) offer high conversion efficiencies and an environmentally friendly alternative to modern combustion-based systems, but their oxygen-ion-conducting electrolyte and high operating temperature (600–1000 °C) allows greater fuel flexibility than most other fuel cells (though materials selection and sealing can be an issue at very high temperatures).

The high fuel flexibility of SOFCs affords the possibility to use relatively cheap, safe, and readily available carbon-based fuels instead of hydrogen. Additionally, CO2 capture in SOFC-based systems can be achieved at lower cost and lower system complexity than in conventional combustion systems [18], [19], because the fuel and oxidizer in an SOFC are not in direct contact as in traditional combustion processes. This facilitates CO2 sequestration without the need for expensive gas-separation technologies.

Despite the apparent promise and advantages of SOFC as an energy-conversion technology, several obstacles must be overcome before SOFCs move beyond the early stages of commercialization. For example, improving efficiencies, finding catalysts with higher tolerances to fuel impurities, preventing deactivation of the anode as a result of carbon deposition when using carbon-based fuels, long-term operational reliability and durability, sealing problems, stack and system integration issues, and overall production costs.

A comprehensive review of the operating principles, performance, and current challenges associated with SOFCs utilizing hydrocarbon and syngas fuels can be found in Kee and co-workers [6], [7] and McIntosh and Gorte [20]. In this paper, we provide a further review of the research and developmental status of typical hydrocarbon- or coal-syngas-fueled SOFCs by focusing on the following topics:

  • fundamental mechanisms of electrode chemical and electrochemical reactions, specifically in the anode;

  • anode materials, particularly in the context of carbon deposition and tolerance to sulfur poisoning;

  • coupling of electro- and thermochemistry with transport in physics-based models of SOFC membrane-electrode assemblies.

A better understanding of these topics is essential for further improvement and optimization of SOFCs operating on hydrocarbon or coal-derived fuels.

Section snippets

Operating principles and materials

The electrochemical charge-transfer reactions and some of the thermochemical (reforming) reactions in an SOFC take place in the membrane-electrode assembly (MEA)1 as

Modeling of SOFC membrane-electrode assemblies

Detailed physics-based models are important for fuel cell development because they afford the opportunity to study each process independently, as well as how one process is connected to the others. It is difficult to study them experimentally in porous electrodes because of, among other challenges, physical access limitations to inner regions of the electrode. The fundamental conservation equations (e.g., momentum, energy, and species transport) that underlie all models are more or less the

Electrochemistry in SOFC anodes

The reaction mechanism describing chemistry and electrochemistry within an SOFC anode is extremely complex. The elementary reaction steps may include homogeneous gas-phase chemistry, heterogeneous surface reactions of ad/desorbing species, homogeneous surface dissociation and reactions between adsorbed species, and heterogeneous charge-transfer reactions. To complicate things further, reaction kinetics are governed by a host of phenomena spanning many length scales (e.g., continuum mass

Thermochemistry in SOFC anodes

Typical operating temperatures of SOFCs are high enough that homogeneous gas-phase chemistry within the anode and fuel channel should be considered with hydrocarbon fuels. Moreover, because the most common anode metal is nickel, there are abundant nickel catalysts to promote thermochemical reactions via heterogeneous chemistry. It is well known that nickel is an efficient catalyst for hydrocarbon cracking, and is highly susceptible to carbon deposition sometimes leading to the growth of carbon

Concluding remarks

The high fuel flexibility of solid-oxide fuel cells makes it possible to use relatively cheap, safe, and readily available hydrocarbon or coal syngas fuels, thereby increasing the feasibility of near-term SOFC commercialization for cleaner and more efficient conversion of these fuels to generate power. In the first part of this paper, the basic operating principles and performance of SOFCs with different fuels were summarized. Results from a wide range of studies indicate that SOFCs have

Acknowledgments

This work has been supported by an award from King Abdullah University of Science and Technology, grant number KUS-11-010-01, and a grant from the Tsinghua-Cambridge-MIT Low Carbon Energy University Alliance.

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