Nanomaterials for Renewable Energy: Solid Oxide Fuel Cells Group 11: Youmean Lee Ethan Licon Sokkwan Leong William Liang Figure 1: Solid Oxide Fuel Cells http://phys.org/news/2011-11-fuel-cells-hydrogen-tank.html

Outline Purpose Introduction Background Information Applications Comparison Between Other Fuel Cells Advantages Disadvantages Cost & Lifetime Targeting Structure Planar Tubular Experimental Structures Electrolyte-Supported vs. Electrode- Supported Advances Intermediate- and Low- Temperature SOFCs Operations Materials Anode Cathode Polarization Fuels Hydrocarbons Renewable Environmental Impact Summary References

Purpose The objective of this presentation is to discuss Solid Oxide Fuel Cells, including Advantages Disadvantages The Structure of Solid Oxide Fuel Cells How an Solid Oxide Fuel Cells Works Materials used in Solid Oxide Fuel Cells We will discuss the challenges and problems with current Solid Oxide Fuel Cell production. As well as the long term benefits and the importance of material science in the development of Solid Oxide Fuel Cells. Solid Oxide Fuel Cells provide a promising lead for green energy. If developed fully, it can bring major benefits to societies energy future. Figure 2: Cross section of SOFC http://www.us.schott.com/epackaging/english/glass/powder/fuel_cells.html

Introduction Fuel Cell: A device that oxidizes fuel to produce a continuous electric current. A fuel cell consists of an anode, a cathode, and an electrolyte. Solid Oxide Fuel Cell (SOFC): A fuel cell that uses a solid oxide or ceramic electrolyte that conducts oxygen ions. To conduct these ions, a SOFC operates at very high temperatures (600-1000oC). Figure 3: A stack of fuel cells http://www.ikts.fraunhofer.de/en/research_fields/energy_systems/materials_and_components/ceramic_energy_converters/sofc_stack_development.html

Introduction: What Makes SOFC Renewable? While the incoming fuel is still a hydrocarbon, the efficiency is about 45-60% The overall reaction also involves hydrogen and oxygen such that the main waste component is water. Hydrogen can be produced from renewable fuels and electrolysis while oxygen is readily available from the atmosphere. Figure 4: Animated representation of SOFC operation http://www.nasa.gov/vision/earth/technologies/18mar_fuelcell.html

Background Information and Applications The concept of the first fuel cell was derived in the early 19th century by Cornish chemist, Sir Humphry Davy. By the end of the 19th century German physicist, Walther Nernst, discovered that Y2O3 and ZrO2 could be added to other solid oxides to reduce their electrical resistivity. In the late 1950s, the development period for the SOFC began. Today, some applications for the SOFC include: Stationary power generation Mobile\transportation power Auxiliary power (power not used for propulsion) Figure 5A: Microscopic structure of a SOFC Figure 5B: Power generation from SOFC http://ceramics.org/ceramic-tech-today/with-low-temp-sofc-gains-like-this-why-stop-now

Applications Used in large and small stationary power generations for example: Planar: SOFCs with output of few kW are tested for smaller cogeneration applications, such as domestic combined heat and power The only fuel cell that has the potential for a wide range of applications: Considered for portable devices (500W battery chargers) Distributed generation power plants (100-500kW systems) Integrated with gas turbine to form large pressurized hybrid systems Figure 6: Portable SOFC system http://www.sciencedirect.com/science/article/pii/S0167273804004813

Comparison Between Other Fuel Cells Compared to other fuel cells, the SOFC operates at much higher temperatures, uses short hydrocarbons as fuel, and operates in a relatively high efficiency range. Types of fuel cell Electrolyte Operating T Fuel Oxidant Efficiency Alkaline (AFC) potassium hydroxide (KOH) 50-200oC pure hydrogen, or hydrazine O2/Air 50-55% Direct methanol (DMFC) polymer 60-200oC liquid methanol 40-55% Phosphoric acid (PAFC) phosphoric acid 160-210oC hydrogen from hydrocarbons and alcohol 40-50% Sulfuric acid (SAFC) sulfuric acid 80-90oC alcohol or impure hydrogen Proton-exchange membrane (PEMFC) polymer, proton exchange membrane 50-80oC less pure hydrogen from pure or methanol Molten carbonate (MCFC) molten salt such as nitrate, sulphate, carbonates... 630-650oC hydrogen , carbon monoxide, natural gas, propane, marine diesel CO2/O2/Air 50-60% Solid Oxide (SOFC) ceramic as stabilized zirconia and doped perovskite 600-1000oC natural gas or propane 45-60% Protonic ceramic (PCFC) Thin membrane of barium cerium oxide 600-700oC hydrocarbons Table 1: Comparison of different fuel cell types http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.476.6374&rep=rep1&type=pdf

Advantages High fuel flexibility; allows internal reforming High efficiency due to high thermal process (45-60% conversion efficiency; >90% in cogeneration) Low emission as carbon monoxide is converted to carbon dioxide Elimination of electrode loss maintenance and corrosion due to solidity of components in the cell. Tolerance of impurities Long-term stability Longer life span (40,000-80,000 hr) Relatively low cost Electrolyte is ductile; can be casted into various shapes No noise pollution due to modular and solid state construction Table 2: Advantages of SOFC plants Compared to other types of energy production, SOFC has much higher efficiency and is cleaner. The DEMOSOFC is a current project to create a SOFC plant in Turin, Italy. http://www.demosofc.eu/

Disadvantages The high operating temperature of the SOFC makes the cells take longer to start up SOFC can be expensive because the cell must be constructed of robust, heat-resistant materials, and must be shielded to prevent heat loss. Thermal expansion mismatches between materials causes difficulty in sealing, especially in planar configurations Sealing is done to prevent mixing of the fuel and oxidant This sealing is often design specific. Figure 7: Possible seal configurations http://www.netl.doe.go v/File%20Library/Rese arch/Coal/energy%20s ystems/fuel%20cells/p roceedings/OverviewSt evenson.pdf

Cost & Lifetime Targeting The majority of the cost can be explained by the complex manufacturing of the fuel cell’s electrode/electrolytic plates as well as the lower power density (W/cm2) and is therefore dependent on the stack size and area of the SOFC. The US Department of Energy recommends a factory cost of about $40/kW and has guidelines for the lifetime of SOFC: Table 3: Lifetime comparison of fuel cells Stationary Fuel Cells Transportation Fuel Cells Lifetime 40,000 hours (~ 4.5 yrs) 5,000 hours (~ 6 months)

Structure SOFC consists of a Fuel Cell Unit and a Hot Storage Tank/Supplementary Boiler Unit: Fuel Cell Unit: Generates electricity and recovers the exhaust heat as hot water simultaneously Hot Storage Tank/Supplementary Boiler Unit: Stores the recovered hot water in order to reuse the hot water Figure 8: Map of a SOFC system http://www.osakagas.co.jp/en/rd/fuelcell/sofc/sofc/system.html

Structure Table 5. Fuel Cell Unit Reformer Converts city gas, fuel, into hydrogen (H) and carbon monoxide (CO) Cell Stack Generates power using oxygen in the air and city gas (reformed to H and CO) Inverter Converts power generated in the cell stack into alternating current and supplies it to homes Exhaust Gas Heat Exchanger Recovers the heat generated during power generation as hot water Table 6: Hot Storage Tank/Supplementary Boiler Unit Hot Water Storage Tank Tank for storing hot water recovered by the exhaust gas heat exchanger Supplementary Boiler When there is not enough hot water in the Hot Water Storage Tank, the Supplementary Boiler is used to supply hot water Figure 9: Map of a SOFC system http://www.osakagas.co.jp/en/rd/fuelcell/sofc/sofc/system.html

Structure A single cell consists of four layers stacked together: Interconnect: electrically connects the anode of one cell to the cathode of another Electrolyte: dense material that provides pure ionic conductivity and physically keep separated fuel and oxidant Cathode: air electrode; positive electrode associated with reduction of the oxidant that gains electrons from the external circuit Anode: fuel electrode; negative electrode associated with fuel oxidation and release of electrons into the external circuit Figure 10: Cross section of layers of a tubular SOFC http://large.stanford.edu/courses/2013/ph240/rasooly2/

Traditional Structure: Planar Two types of SOFCs in terms of cell structure: planar and tubular Planar Cell is made into a flat disk, square, or rectangular plane Cells placed in series and connected by interconnected plates Fuel is fed through the anode and interconnect Air through the Cathode and interconnect Stack of anode, interconnect, cathode, and electrolyte stack are repeated Figure 11: Unit Stack of a Planar SOFC http://www.doitpoms.ac.uk/tlplib/fuel-cells/high_temp_sofc.php

Traditional Structure: Tubular In the tubular structure, the electrode is made into a long tube with porous walls Outside the electrode tube, the electrolyte and another electrode are wrapped around Cells connected in series through interconnect placed along the edge of the tube Advantages Rapid start-up and cool-down times Good for smaller applications Figure 12: Structure of a tubular SOFC http://www.aki.che.tohoku.ac.jp/~koyama/html/research/SOFC.html

Traditional Structures: Comparison of Planar and Tubular Higher in power area and volume Easier to manufacture More expensive Tubular Lower in power area and volume Harder to manufacture Cheaper Better long term stability Better sealing Table 7: Comparison of planar and tubular SOFC structures http://pubs.rsc.org/en/content/chapter/bk9781849736541-00026/978-1-84973-654-1#!divabstract

Experimental Structures Modified Planar Features a wave-like structure in a flat configuration The cells are said to be in “flat-configuration” when each solid oxide fuel cell is stacked on top of each other Shares the benefits of both tubular and planar cells Increased surface area allows rapid cooling and rapid starting The flat configuration allows for lower manufacturing costs Figure 13: Diagram of a Modified Planar Cell http://www.faqs.org/patents/imgfull/20150004522_07

Experimental Structures Monolithic Design laminates the air electrode, electrolyte, fuel, and interconnect Uses co-sintering to bring the stack together. Sintering is compacting and forming a solid mass using heat and pressure Avoid making the solids into a liquid Eliminates the need for high temperature seals in planar configurations Figure 14A: 3D View Monolithic Structure https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&ved=0ahUKEwi1wr3n-_DLAhVJOCYKHWFVD_8Q5TUICg&url=http%3A%2F%2Fwww.engj.org%2Findex.php%2Fej%2Farticle%2Fdownload%2F25%2F3&bvm=bv.118443451,d.eWE&psig=AFQjCNG4NH0TtANfU7Oo4EEs2ykzJbsPgQ&ust=1459721151386282 Figure 14B: 2D View of Monolithic Structure http://shodhganga.inflibnet.ac.in/bitstream/10603/8950/11/11_chapter%201.pdf

Experimental Structures Modified Tubular Shapes The biggest challenge faced by SOFC is the lengthy start up and cool down times Modified tubular designs increase the surface area of the tubular base design. This increases heat transfer and allows for a quicker start up and cool down Quicker start up and cool down allow for more applications of SOFC A disadvantage is that the modified tubular shapes may be more difficult to manufacuter Figure 15: Modified Tubular Shapes https://electrochem.org/dl/interface/wtr/wtr07/wtr07_p41-44.pdf

Structure: Electrolyte-Supported vs. Electrode-Supported The SOFC can be supported by either using the electrolyte or the electrodes (anode or cathode) The supporting electrolyte or electrodes (anode or cathode) will be thicker than the other layers This can be seen in Figure 16 The left SOFC is supported by the electrolyte The right SOFC is supported by the anode Electrolyte-supported cells are harder to produce Electrode-supported cells are now commonly used throughout industry Electrode-supported are typically thinner and have less internal resistance Electrode-supported can operate at slightly lower temperatures Figure 16: Electrolyte-supported SOFC (Left) compared with an Electrode-supported SOFC (Right) http://research.che.tamu.edu/groups/Seminario/Special%20Assigment/2012%20Solid%20oxide%20fuel%20cells%20&%20Solgel.pdf

Structure: Advanced Intermediate- and Low-Temperature SOFCs Current challenge with SOFC is lowering the operating temperature Lowering the operating temperature will decrease the cost of materials and operation The triple-phase boundary (where the electrodes and electrolyte come together) must be in electrical contact and exposed to the reactant There are two solutions to help achieve a low-temperature SOFC Ideal would be a porous substance with fine particle size for a high surface area (as seen in the figure) Reducing the thickness would also reduce the resistive loss Currently the thickness of the electrolyte is several hundred micrometers Target is to decrease the thickness to ten micrometers Figure 17: Comparison of large particle size (a) and small particle size (b) for the electrolyte http://www.doitpoms.ac.uk/tlplib/fuel-cells/sofc_electrode_materials.php

Operation Hydrogen containing fuel is fed into the anode of the fuel cell Example: Natural Gas Oxygen is fed into the cathode Step 1 On the Anode side, hydrogen containing fuel is combined with oxygen to produce carbon monoxide and hydrogen gas (CO and H2) and 2 e- This decreases the concentration of oxygen on the anode side, creating a demand across the electrolyte Figure 18: Operation of SOFC http://pubs.rsc.org/en/content/articlehtml/2003/cs/b105764m Figure 19: Operations of SOFC http://www.fuelcellmarkets.com/fuel_cell_markets/solid_oxide_fuel_cells_sofc/4,1,1,2503.html H2 + O2- H2O + 2e-

Figure 20: The O2- conversion within SOFC http://pubs.rsc.org/en/content/articlehtml/2003/cs/b105764m Operation Step 2 The free electrons travel around from the anode to the cathode to produce a current Oxygen is being used up on the Anode side, leading to a decreased concentration The decreased concentration pulls oxygen from the oxygen-rich cathode side through the electrolyte The oxygen-rich cathode side combines with the O2 to create O2(2-) The solid electrolyte is an oxygen ion conductor The electrolyte will only let oxygen ions through, thus the oxygen has to be ionized prior to traveling to the anode to react O2 + 4e- 2O2- Figure 21: Ionization of oxygen http://spmchemistry.onlinetuition.com.my/2013/10/formation-of-negative-ion.html

4e- + O2 2O2- H2 + O2- H2O + 2e- Operation Step 3: Anode Side The O2- Ion combines with freed H2 to produce water and 2e- on the anode (reaction below) 2 electrons are freed by the reactions The electrons will travel to the cathode side This flow of electrons produces a current Electrons flowing onto the cathode side recombine with oxygen to form oxygen ion The electrolyte will conduct oxygen ions only Figure 22: Diagram of SOFC Operation http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.476.6374&rep=rep1&type=pdf 4e- + O2 2O2- H2 + O2- H2O + 2e-

CO + O2- CO2 + 2e- Operation Step 4 On the anode side, CO is created through the incomplete combustion of CH4 This excess CO combines with O2- to make Carbon Dioxide and electrons Electrons travel around the wire to the cathode, producing a current Electrons deposited on the cathode combine with Oxygen to create oxygen ion Oxygen ion travels through the solid electrolyte to react more Figure 23: The CO + O2- converstion to CO2 http://pubs.rsc.org/en/content/articlehtml/2003/cs/b105764m CO + O2- CO2 + 2e-

CH4+ 2O22- 2H2O + CO2 + 8e- Operation Step 5 - Anode Uncombusted CH4 is sometimes left in the anode This uncombusted CH4 reacts with oxygen ion (O2-) to create Water, Carbon Dioxide, and Electrons The free electrons travel around to electron-deficient cathode side This flow of electron create a current Electrons entering the cathode recombine with Oxygen to form Oxygen ion Figure 24: Overall conversion within the cell http://pubs.rsc.org/en/content/articlehtml/2003/cs/b105764m CH4+ 2O22- 2H2O + CO2 + 8e-

Materials Requirements: Chemically and physically stable for oxidizing and/or reducing reactions Very high operating temperatures: 800-1000oC Chemically compatible with other components; absence of interface reaction/diffusion Proper conductivity Similar thermal expansion coefficients to other components Dimensional stability in the presence of chemical gradients Strong, yet easy to fabricate Figure 25: Simple scheme of a SOFC, with the state-of-the-art materials used as anode, cathode and electrolyte http://pubs.rsc.org/is/content/articlehtml/2010/ee/c0ee00166j

Materials: Anode Ceramic layer that must be porous for fuel flow towards the electrolyte Often Granular matter Commonly the thickest and strongest layer Must conduct oxygen ions, but not oxygen Cermet made up of nickel mixed with the ceramic used for the electrolyte (YSZ, ScSZ, GDC) is most commonly used The performance of the anode, in terms of minimal electrode polarization loss and minimal degradation during operation, depends strongly on its microstructure Conducting oxygen ions allows the flow of the ionized oxygen but not oxygen This is important because if diatomic oxygen is conducted, no electrons would need to flow to ionize the oxygen Figure 26: SEM of a Ceramic Anode http://www.nonmet.mat.ethz.ch/research/Functional_Ceramics/Single_Chamber_Solid_Oxide_Fuel_Cells

Materials: Electrolyte Dense ceramic layer that conducts oxygen ions Electron conductivity must be kept as low as possible to prevent losses from leakage currents High operating temperatures of SOFCs allow the kinetics of oxygen ion transport to be sufficient for good performance. As the operating temperature approaches the lower temperature, the electrolyte begins to have large ionic transport resistances and affect the performance Common Examples: Yttria-stabilized zirconia (YSZ), Scandia stabilized zirconia (ScSZ) Gadolinium doped ceria (GDC); highly conductive between 500 - 700oC The ability of electrolyte to hold a large content of oxygen vacancies makes them a good candidate of electric conductor Figure 27A: Yttria-stabilized zirconia (YSZ) http://pubs.rsc.org/en/Content/ArticleHtml/2005/JM/b41714 3h Figure 27B: Peroxide structure http://www.intechopen.com/books/perovskite-materials-synthesis- characterisation-properties-and-applications/designing-perovskite-oxides -for-solid-oxide-fuel-cells

Materials: Cathode Only noble metals or electronic conducting oxide due to the high operating temperature of the SOFC. Noble metals are extremely expensive and using them would be unsustainable The material depends on the application the fuel cell will be used in Common examples LASrMnO3 (Lanthanum strontium manganite, LSM) LaCaMnO3 (Lanthanum calcium manganite, LCM) Both are excellent options for operating temperatures above 800oC For lower temperatures (600-800oC) For increased performance, electrolyte materials (YSZ, SDC) can be mixed with perovskite electrode materials (LSM, LCM). The electrolyte material increases the amount active sites for electrochemical reactions, allowing good performance at lower temperatures Figure 28: Microscopic view of Lanthanum Strontium Manganite https://www.americanelements.com/lanthanum-strontium-manganite-lsm

Materials: Interconnect Interconnecting plates are used to allow the flow of hydrogen and oxygen to repeating units Only a few oxides can be used for this application Initially, doped CoCr2O4 was used as an interconnect Commonly used now is YCrO3 Glass interconnects with a LaCrO3 coating is also being developed The major difficulty (and cost) of the interconnect comes with sustaining the high temperatures of the SOFC Most work is dedicated to bringing down the operating temperature of SOFC. If this can be achieved, material cost across the board will decrease Figure 29: Cross section showing the layers of a SOFC http://www.doitpoms.ac.uk/tlplib/fuel-cells/high_temp_sofc.php

Anode Porosity characteristics of anode must be maintained at high temperature to allows the fuel to flow towards the electrolyte At the anode, hydrogen in the fuel is being electrochemically oxidized. The protons formed during oxidation also combine with oxygen ions, conducted through the electrolyte from the cathode, to form byproduct water vapor in the anode Besides porosity, the amount of reaction zones aka triple phase boundary(TPB) also plays an important role in the kinetics of the oxidation reaction Figure 30: Ni-YSZ example of an anode http://electrochemical.asmedigitalcollection.asme.org/article.aspx?articleid=1472416&journalid=123

Anode The relationship between the conductivity of the nickel/solid electrolyte cermet and nickel content at ambient temperature is shown on the figure: The threshold for electrical conductivity is about 30 vol% nickel < 30 vol% nickel, the conductivity of the cermet is similar to that of YSZ. > 30 vol% nickel, the conductivity is about 3 orders magnitude higher, corresponding to a change in mechanism of the nickel phase Nickel is also preferred over cobalt and precious metals due to its low cost Figure 31: Conductivity vs. nickel content http://pubs.rsc.org/en/content/articlehtml/2003/cs/ b105764m

Cathode Cathode must be highly conductive, stable and porous at high operating temperature At the cathode, oxygen reduction takes places; Oxygen combines with electrons from the cell externally, is reduced to oxygen ions through the following electrochemical reaction Like anode, conduction in porous electrodes occurs through TPB, whereas in MIECs, this also occurs through DPB Figure 32A: Example of a cathode; Lanthanum Nickel Cobalt Oxide Cathode Powder https://www.fuelcellmaterials.com/site/powders-and-pastes/close-out-powders Figure 32B: Schematic representation of conducting mechanism of cathode http://www.intechopen.com/books/perovskite-materials-synthesis-characterisation-properties-and-applications/designing-perovskite-oxides-for-solid-oxide-fuel-cells

Polarization Polarization is the loss of voltage output due to imperfections in materials, microstructure, and design in SOFC. Types of polarization include: Ohmic polarization - Resistance in materials lead to voltage reduction (V = IR) Concentration polarization - Occurs from inefficient transport of fuel to reaction sites Activation polarization - A result of the energy requirement of the chemical reaction at the anode and cathode To calculate the effect of polarization (Pe) on power output, the following equation is used: Figure 33: Calculation of Effect of Polarization on Power Output http://orgs.kettering.edu/altfuel/fcback.htm

CH4+ O2- CO + H2 + 2e- CH4+ 4O2- CO2 + 2H2O + 8e- Fuels: Hydrocarbon Requires a partial oxidation reformer to pre-process fuel. Examples: Gasoline, Diesel, Natural Gas Benefits of Hydrocarbons Infrastructure readily available Naturally stable for easy transport and storage Complex hydrocarbons will be able to give off more electrons, since they are able to undergo more reactions Methane is commonly used Figure 34: Methane (CH4) is commonly used to fuel SOFC CH4+ O2- CO + H2 + 2e- CH4+ 4O2- CO2 + 2H2O + 8e-

Fuels: Renewable H2 can be used as a fuel source Extremely volatile Costly to manufacture Very difficult to handle and transport Complete renewable Biogas (a combination of methane and carbon dioxide) extracted from farms can be used to power the fuel cell Sulfur must be extracted from the biogas prior to entering the SOFC Varying differences in the amount of methane and CO2 is a challenge for directly using biogas as a fuel source Biogas would have no carbon footprint All carbon produced would be negated Figure 35: Using carbon dioxide as a fuel source http://mypages.iit.edu/~smart/garrear/fuelcells.htm

Environmental Impact Even though SOFC runs on hydrocarbons, it can still be considered a green technology The main benefit from SOFC stems from the increased efficiency (45%-60%) Increased electricity output from the same amount of hydrocarbon The product gas reactions with O2- allow for the increased efficiency In combustion engines product gases do not react further Could be made to run of pure hydrogen Liquid hydrogen is extremely unstable Storage and Handling would be difficult in comparison to hydrocarbons According to the figure below, SOFC is the most efficient source of power generation that is based on non-renewable resources Figure 36: Comparison of SOFC efficiencies to other non-renewable power generation. https://fuelcellsworks.com/archives/2009/12/15/ational-institute-of-advanced-industrial-science-and-technology-aist-develops-micro-tubular-solid-oxide-fuel-cell-integrated-compact-modules-operable-at-low-temperatures/

Environmental Impact Emissions If biogas is used (methane and CO2), the net carbon emissions would be zero A net reduction of 2 million kg of CO2 per year Near-zero levels of NOx, SOx, and particulates Only emissions: steam, trace amounts of NOx, trace amounts of SOx, and a small amount of CO2 SOFC provides the lowest emissions of any non-renewable power generation Air Emissions SOx NOx CO Particles Organic compounds CO2 Fossil Fuelled Plant 12,740 18,850 12,797 228 213 1,840,020 SOFC system 32 846,300 Table 8: Environmental Impact of Fossil Fuel Plant vs SOFC System

Summary Solid Oxide Fuel Cells have the potential to provide major benefits as we work towards a more sustainable future. Its primary advantages are its high efficiency and low emissions. It can run on both hydrocarbons and hydrogen fuel. If powered by Hydrogen or Biogas, it is a carbon neutral energy source. A SOFC is composed of a cathode, electrolytic plate, an anode, and an interconnection between plates. Its primary disadvantage involves high operating temperature. This high operating temperature prevents the use of conventional materials. The material must withstand high temperatures and extra measures must be taken to avoid heat loss and mixing of gases. A SOFC is first heated to 600-1000oC so that the ceramic electrolytic plate conducts oxygen ions. Oxygen is ionized at the cathode. Oxygen ions diffuse through the electrolytic plate while hydrogen comes in via the anode. Together they oxidize the fuel. The electrons given off in the process then flow through an external circuit to do work. The electrons then enter the cathode and the cycle repeats. The most important material involved is the particular solid oxide used as the ceramic electrolytic plate, typically a variant of Yttria-stabilized zirconia (YSZ). Future research could focus on finding ceramic materials that begin to conduct ions at lower temperatures.

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