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Solid Oxide Electrochemical Systems: Material Degradation Processes and Novel Mitigation Approaches

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Solid Oxide Electrochemical Systems: Material Degradation Processes and Novel Mitigation Approaches materials Review Solid Oxide Electrochemical Systems: Material Degradation Processes and Novel Mitigation Approaches Michael Reisert, Ashish Aphale and Prabhakar Singh * Department of Materials Science and Engineering, University of Connecticut, Storrs, CT 06269, USA; [email protected] (M.R.); [email protected] (A.A.) * Correspondence: [email protected]; Tel.: +1-860-486-8379; Fax: +1-860-486-8378 Received: 2 August 2018; Accepted: 30 October 2018; Published: 2 November 2018 Abstract: Solid oxide electrochemical systems, such as solid oxide fuel cells (SOFC), solid oxide electrolysis cells (SOEC), and oxygen transport membranes (OTM) enable clean and reliable production of energy or fuel for a range of applications, including, but not limited to, residential, commercial, industrial, and grid-support. These systems utilize solid-state ceramic oxides which offer enhanced stability, fuel flexibility, and high energy conversion efficiency throughout operation. However, the nature of system conditions, such as high temperatures, complex redox atmosphere, and presence of volatile reactive species become taxing on solid oxide materials and limit their viability during long-term operation. Ongoing research efforts to identify the material corrosion and degradation phenomena, as well as discover possible mitigation techniques to extend material efficiency and longevity, is the current focus of the research and industrial community. In this review, degradation processes in select solid oxide electrochemical systems, system components, and comprising materials will be discussed. Overall degradation phenomena are presented and certain degradation mechanisms are discussed. State-of-the-art technologies to mitigate or minimize the above-mentioned degradation processes are presented. Keywords: corrosion; electrode poisoning; solid oxide; interconnect; electrode; oxide scale 1. Introduction 1.1. Solid Oxide Electrochemical Systems There is an ever-growing push to utilize alternative, clean forms of energy and deviate from a dependency on nonrenewable fossil fuels. One way this has been achieved is through the exploitation of electrochemical devices, which convert the chemical energy from fuels into electricity or vis-versa without any combustion of the fuels [1]. Some systems are currently integrated in electrical grids and used in automotive/aerospace applications as liquid or polymer electrolytes. However, a different variety is comprised of a solid metal oxide electrolyte and electrodes. These solid oxide electrochemical systems are favorable as they provide high energy conversion efficiencies, flexibility in design, and flexibility in fuel choice [2–4]. This review will discuss three particular solid oxide electrochemical systems: Solid oxide fuel cells, solid oxide electrolysis cells, and oxygen transport membranes, as well as related degradation processes associated with them. To achieve wide-scale industrial use of solid oxide electrochemical devices, the materials that comprise them and any material degradation processes must be well understood. Major degradation phenomena can occur at every system component. In this review, the solid–gas interactions which play a large role in system performance degradation will be discussed [5,6]. This includes poisoning of both fuel and air electrodes by volatilized species, as well as oxidation and Materials 2018, 11, 2169; doi:10.3390/ma11112169 www.mdpi.com/journal/materials Materials 2018, 11, 2169 2 of 16 metal loss, resulting from simultaneous exposure of metallic components to different gaseous species. KnowledgeMaterials 2018 of these, 11, x FOR phenomena PEER REVIEW and a review of the current state-of-the-art technology and2 of 15 novel mitigation approaches will be useful in prolonging material lifespans for efficient operation of solid oxidation and metal loss, resulting from simultaneous exposure of metallic components to different oxidegaseous electrochemical species. Knowledge systems. of these phenomena and a review of the current state-of-the-art technology and novel mitigation approaches will be useful in prolonging material lifespans for 1.1.1.efficient Solid Oxide operation Fuel of Cells solid oxide electrochemical systems. Fuel cells are open systems that can be continuously fueled. This makes fuel cells optimal for 1.1.1. Solid Oxide Fuel Cells grid applications, as they can be intermittently refueled to continuously provide electricity with no system replacementFuel cells are necessary open systems [7,8]. that They can operate be continuously more efficiently fueled. This than makes thermomechanical fuel cells optimal means for of energygrid production applications, as directas they energy can be conversionintermittently eliminates refueled to the continuously need for combustion provide electricity [4]. Fuel with cells no have beensystem designed replacement with various necessary materials, [7,8]. They specifically operate electrolytemore efficiently materials, than thermomechanical to yield a variety means of types. of A energy production as direct energy conversion eliminates the need for combustion [4]. Fuel cells have proton exchange membrane fuel cell (PEMFC), for example, utilizes a polymer electrolyte such as been designed with various materials, specifically electrolyte materials, to yield a variety of types. A Nafion® to foster protonic movement. A platinum catalyst is used as an anode to split the hydrogen proton exchange membrane fuel cell (PEMFC), for example, utilizes a polymer electrolyte such as fuel intoNafion protons® to foster and protonic electrons. movement. The electrons A platinum are forced catalyst to is anused external as an anode circuit to duesplit tothe the hydrogen electrically insulatingfuel into properties protons ofand the electrons. electrolyte, The electrons while the are protons forced areto an conducted external circuit through due the to the electrolyte electrically toward the cathode.insulating At properties the cathode, of the the el protonsectrolyte, are while reunited the protons with electronsare conducted that havethrough travelled the electrolyte the external circuit,toward as well the ascathode. oxygen At fromthe cathode, an oxidizing the protons gas flownare reunited to the with cathode. electrons This that results have travelled in the chemical the formationexternal of watercircuit, vapor, as well which as oxygen is filtered from outan oxidiz of theing cell gas as flown waste. to This the electrochemistrycathode. This results is thein the driving forcechemical behind fuelformation cell operation of water vapor, and has which been is adheredfiltered out to of in the developing cell as waste. new This types electrochemistry of fuel cells. is Onethe driving promising force varietybehind fuel of fuel cell celloperation is the and solid has oxide been fueladhered cell to (SOFC), in developing which new uses types a solid of fuel ceramic cells. electrolyte. This is advantageous as the ceramic electrolyte is very stable and offers a long operating One promising variety of fuel cell is the solid oxide fuel cell (SOFC), which uses a solid ceramic lifetime, whereas polymer electrolytes can dry out or flood if they are not hydrated in the precise electrolyte. This is advantageous as the ceramic electrolyte is very stable and offers a long operating amount,lifetime, at which whereas point polymer they lose electrolytes efficiency can or dry stop out working or flood altogetherif they are [not9]. Theyhydrated can in also the operate precise on a rangeamount, of hydrogen-based at which point fuels they likelose hydrocarbons,efficiency or stop whereas working PEMaltogether fuel cells[9]. They must can use also a pureoperate hydrogen on fuel sourcea range [ 4of]. hydrogen-based Solid oxide fuel fuels cells like operate hydrocarbons, at high temperatures whereas PEM byfuel reducing cells must an use oxidizing a pure hydrogen gas (usually air) atfuel the source cathode [4]. into Solid oxygen oxide fuel ions. cells Unlike operate PEM at fuelhigh cells, temperatures which reduce by reducing a fuel andan oxidizing move ions gas from anode(usually to cathode, air) at a the SOFC cathode moves into oxygen oxygen ions ions. from Unlike cathode PEM fuel toanode, cells, which where reduce they a meet fuel hydrogenand move ions to formions water from vapor anode as to wastecathode, and a releaseSOFC moves an electron oxygen to ions an external from cathode circuit. to Thisanode, is becausewhere they of themeet nature of thehydrogen solid electrolyte, ions to form which water is vapor typically as waste an oxygen-ion and release conducting an electron to yttria-stabilized an external circuit. zirconia This is (YSZ) because of the nature of the solid electrolyte, which is typically an oxygen-ion conducting yttria- ceramic [2,3]. The cathode, also known as the air electrode (AE), utilizes electronically conducting stabilized zirconia (YSZ) ceramic [2,3]. The cathode, also known as the air electrode (AE), utilizes or mixed ionically and electronically conducting, oxygen-reducing ceramics, which have a thermal electronically conducting or mixed ionically and electronically conducting, oxygen-reducing expansionceramics, coefficient which have that a closelythermal matchesexpansion the coefficient electrolyte that toclosel reducey matches thermal the stresseselectrolyte under to reduce dynamic operatingthermal conditions. stresses under A comparative dynamic operating schematic condit of theions. operating A comparative principles schematic of
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